Retracted Article: Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m⁻².

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m^−2.

It is well known that inkjet printing works as precise and versatile patterning method for printed electronics.^1,2 As for its advantages, it is already being exploited widespread for printing electronics.^3,4 Those merits are easily processed from solutions and conveniently used for air-stable electrodes.^5,6Light-emitting electrochemical cells (LECs) have emerged as an active layer,^7,8 arousing tremendous attention over the years.^9–11 Nevertheless, LECs are simpler than organic light-emitting diodes (OLEDs).^12,13 Due to its single layer architecture, low fabrication price and operating voltages, LECs are considered as a promising, next-generation, emissive thin-film technology.^14–16The most efficient device used to date for LECs are biscyclometalated iridium(iii) (Ir) complexes, because they have highly efficient and stable devices spanning the whole visible range.^17–19 However, avoiding the use of Ir is strongly desired because of its high cost and limited supply.^20–22 Till now, thiazole has worked as active component in a LECs system with a long-lived charge-separation molecule, without additional ions in its active layer.^23–26 As for its advantages, it can reduce recombination of charge carriers and facilitate carrier transfer.^27–29 More and more importance has been attached to thiazole compounds, especially bithiazole-based ones.^30–33 Therefore, these have aroused interest in the syntheses of bithiazoles.^34–36Based on our previous research,^37–41 the use of bithiazole ligands already explored by our group for photocatalytic technology has the advantage of an easy transformation of neutral complexes into charged ones by substitution on the N-atom of the thiazole moiety.^42,43 Furthermore, we demonstrate the fabrication of LEC devices by a combination of inkjet printing and spin coating, on A4 paper substrates for low cost, disposable and flexible conductive pattern.^44,45 Their corresponding material characteristics were closely investigated, a detailed report of the material properties of the resulting coatings, and an envisaged proof of concept application are disclosed in this paper. These fully printed devices demonstrate the potential upscaling of the fabrication of optoelectronic devices.Herein we report the design for a range of bithiazole derivatives 1a–1c (Fig. 1, their synthesis is shown in Scheme S1^†), and explore their properties. First of all, the UV-visible diffuse-reflectance (UV-vis DRS) spectra of those specimens at room temperature are displayed in Fig. 2 and the data are collected in Table S1.^† This enhancement was ascribed to the increase of aromatic rings, which has an intense absorption in the visible-light region. And their bandgap energies (E_g) are between 2.97–3.01 eV (Table S1^†), and are estimated from these absorption spectra according to the Kubelka–Munk method. Expanding the conjugated system and electron density with these donor groups, it can lead to a larger bathochromic shift of the absorption maximum.^46,47Open in a separate windowFig. 1Chemical structures of compounds 1a–1c described in this work.Open in a separate windowFig. 2UV-vis DRS spectra of 1a–1c.All of the three compounds are photoluminescent at room temperature, the relevant fluorescence peak maximum ranges from 360 to 398 nm (Fig. 3). They emit blue light when they are being excited. In agreement with Fig. 3, the presence of electron-releasing groups would shift the emission maximum. Among them, the most electron-donating carbazole unit would give the most red-shifted peak at 398 nm for 1c (Table S1^†). The PL quantum yield (ϕ_PL) studies show that both 1a and 1b are as high as 62 and 78%, while another one is 85% (Table S1^†). The data for their fluorescent quantum yields largely depends on interaction effect with molecules 1a–1c in the crystal packing. In this context, π–π intermolecular interactions can inhibit the fluorescence.⁴⁸Open in a separate windowFig. 3Typical PL spectra of 1a–1c.It may indicate that enlarging the conjugation length and electron density with these donor groups plays an important role in increasing the ϕ_PL.⁴⁹ Moreover, those with high ϕ_PL may be suitable for application as efficient light-emitting material in LECs.⁵⁰In addition, time-resolved measurements of the donor lifetime (τ) in 1a–1c were carried out, and their corresponding values are given in Table S1.^† The bithiazole derivatives 1a–1c showed long-lived singlet fluorescence lifetimes (τ_F), ranging between 1.04 and 9.54 ns (Table S1^†). This was on average more than double the fluorescence lifetime of the bithiazole starting material.⁵¹ These three samples possess relatively high decay time, as a result of their inhomogeneity.⁵² Compound 1c exhibited the longest fluorescence lifetime decay τ_F = 9.54 ns. The sensitivity of the emission to the polarity of the solvent is beneficial for an intramolecular charge transfer (ICT)-like emission.⁵³ This difference probably accounts for the high concentration of donor units in the bithiazole backbone, low rates of intersystem crossing to reactive triplet states.^48,54Consequently, the composite 1c was the best performing LEC and chosen for the below test. The EL spectra obtained for both devices were almost identical, with a main peak at 498 nm and other peak at 504 nm wavelength as illustrated in Fig. 4a. It may account for trapping, cavity, and self-absorption effects from within the LEC device 1c multilayer-structure. The spin-coated emission is quantified by the commission Internationale de L’Eclairage (CIE) coordinates of (0.28, 0.42), and a colour rendering index (CRI) of 65 (Table S2^†). While inkjet-printed light emission with CIE coordinates of (0.34, 0.43) and a CRI value of 83 was achieved for device 1c (Table S2^†). It exhibits good colour rendering indices (CRI > 70), this device exhibited a warm-white appearance, and the colour rendering is considered sufficient for indoor lighting applications.Open in a separate windowFig. 4Comparison of device characteristics with spin-cast and inkjet printer emitting layer for 1c: (a) electroluminescent spectra, (b) luminance vs. voltage, (c) current density vs. voltage, (d) efficiency vs. luminance behaviour.The time-dependent brightness and current density under constant biases of 2.9–3.3 V for device 1c are shown in Fig. 4b and c. Interestingly, the device with the inkjet-printed emitting layer (EML) produced a light output of around 3600 cd m⁻², whereas 4600 cd m⁻² was achieved with the spin-coated EML. In other words, the device with the inkjet printed EML obtained about 78.3% of brightness compared with the reference device. The differences between the two devices regarding current efficiency (J) and power efficiency (PE) were rather closed, as listed in the Table S2.^† A luminous efficiency much larger than 10 cd A⁻¹ was achieved for the inkjet-printed EML, which tended to be 73.9%, and is similar to the spin-coated emitting layer.The performance gap between the devices (Fig. 4d) can be attributed to differences in the height and surface roughness of the emitting layer.⁵⁵ Lateral sizing histograms (Fig. S11^†) show that the spin-coated EML possessed a thickness of 35 nm and a very smooth surface. The inferior performance of the inkjet-printed EML is caused by a less homogeneous surface morphology and an overall fatter layer.⁵⁶ The depth of the inkjet-printed EML ranged from 25 nm (pixel centre) to 35 nm (pixel edges). As a consequence, it is an inkjet deposition process and the related evaporation dynamics reduces the light output and efficiency, but also postpones the stability of the device in the system.^10,57We then shifted our attention to the turn-on kinetics, efficiency and long-term stability of the LEC device for 1c; the typical evolution of current density and light emission is presented in Fig. 5a and b, showing a slow turn-on followed by a progressive decay over time.Open in a separate windowFig. 5(a) Current density (closed symbols) and brightness (open symbols) versus time at 3 V for a device with the representative sample 1c; (b) EQE versus time at an applied voltage of 3 V.The build-up of the light output is synchronous with that of the current density. This time-delayed response is one of the striking features of the operation of an electrochemical cell and reflects the mechanism of device operation.²² The champion device in this set exhibited a peak power efficiency (PE) of 17.4 lm W⁻¹ at a luminance of 18 000 cd m⁻². Meanwhile, the current density began to decrease, possibly due to the electrochemical oxidation or electro corrosion that occurred on component 1c through accumulated electroinduced holes, and then it kept up a durative datum.⁵⁸ Apparently it exhibited outstanding long-term operation stability beyond 40 h (Fig. 5b).The time-dependent external quantum efficiency (EQE) of LEC for the representative substrate 1c are shown in Fig. 5b. It also exhibited similar temporal tendency in device efficiency. When a bias was applied on the LECs, the EQE quickly increased since balanced carrier injection was achieved by the formation of the doped layers.⁵⁹ After attaining the crest value, the device current was still rising while the EQE reduced little by little. It implied that both growing the doped layers and weakening of the EQE was resulting from exciton quenching near the bithiazlole core in persistently extended doped layers.⁶⁰ Doping-induced self-absorption was rather adaptable to the temporal roll-off in device efficiency. This configuration shows the best performance (EQE = 12.8%) compared to those devices prepared with the single compounds, which is given in Table S3.^† The reason may be that the steady-state recombination zone in 1c device was close to the central active layer.⁶¹ Besides it contains fluorenes on its side chains, constructing plane structure, benefiting from good electron injection abilities, leading to enhancing the EQE itself.⁶²The fabrication of inkjet-printed bithiazole interdigitated electrode (IDE) is illustrated in Fig. 6. All the prepared varieties of bithiazole-based ink have been found to be highly stable with nil or miniscule precipitation in over 180 days being stored on the shelf (Fig. 6a). After HCl treatment of the paper substrate, the prepared inks have filled within ink cartridges of a low-cost desktop printer Canon iP1188 (Fig. 6b). Fig. 6c shows the photograph of inkjet-printed conductive patterns.Open in a separate windowFig. 6(a) The representative ink based on sample 1c was stable for 180 days; (b) electrode printing using Canon iP1188 printer; (c) scale showing size of printed electrodes; (d) interdigitated electrode (inset shows small size printed pattern).In addition, the most attractive prospect of the bilayer device structure at this stage is the possibility for patterned emission for the creation of a static display. Fig. 7 presents a photograph of a small portion of a larger static display, with a resolution of 170 PPI, which repeatedly exhibits a message in the form of the word “LEC”. As shown in Fig. 7, the pixel array produced are fairly luminous. The EL spectra were collected from each pixel on the substrate, the emission peak wavelength was about 510 nm and the full width at half maximum was about 170 nm. The low variation in emission intensity in the different pixels implies a small variation in thickness of the solution-coated layers. More specifically, if we assume a minimum diameter of 20 μm for an inkjetted electrolyte droplet, and a smallest inter-droplet distance of 10 μm, we attain a pitch of 30 μm, which corresponds to a high display resolution of 850 PPI. Finally, we draw attention to the herein presented static-display LEC that comprises solely air-stabile materials, and that we routinely fabricate the bilayer stack under ambient atmosphere.Open in a separate windowFig. 7(a) The patterned light emission from a bilayer LEC, with the emission pattern defined by the selected positions of the ink jetted electrolyte droplets. (b) The droplet diameter and pitch were 50 and 150 μm, respectively, and the device was driven at V = 3 V. The scale bar measures 300 μm.     

Colour-tunable ultralong organic phosphorescence upon temperature stimulus

A new class of single-component molecular crystal with colour-tunable ultralong organic phosphorescence (UOP) was designed and synthesized through alkyl chain engineering. Forming a more rigid environment at 77 K, the colour-tunable UOP from yellow-white to blue-green is achieved through dual-emission of crystal and amorphous states.

A new class of single-component molecular crystal with colour-tunable ultralong organic phosphorescence (UOP) was designed and synthesized through alkyl chain engineering.

Ultralong phosphorescence, a kind of phosphorescence that can be observed by the naked eye after removing the excitation source, has received great attention in the fields of sensing,¹ displays,² imaging,³ anti-counterfeiting⁴ and so on during the past years. Unfortunately, it is limited to inorganic materials because of the weak spin–orbit coupling (SOC) and strong non-radiative transition of pure organic materials.⁵ Compared with inorganic materials, however, organic materials have some excellent merits, such as inexpensive cost, relative safety to the environment, and soft preparation conditions.⁶ The realization of UOP becomes very significant. As mentioned above, UOP can be achieved by promoting intersystem crossing (ISC) through enhancing SOC and suppressing non-radiative transitions. In view of these factors, various strategies such as polymerization,⁷ H-aggregation,⁸ crystallization,⁹ host–guest doping,¹⁰ and freezing conditions¹¹ have been explored to achieve UOP with the unremitting efforts of scientific researchers.Intelligent-response organic luminescent materials can change their luminescent properties such as colour, lifetime, intensity, etc. after being stimulated by the external factor, such as mechanical forces, temperature, pH, light, solvent, etc., which have caught great attraction. The sensitivity of luminescent molecule to excitation wavelength, temperature or oxygen can be applied in sensors, optical recording and so on.^12–14However, the luminescence of these materials with intelligent-response are mostly limited to fluorescence or room temperature phosphorescence (RTP) with relatively short lifetimes.^15–18 Few samples display persistent luminescent feature.¹⁹ Most organic compounds can only tune their afterglow properties by changing molecular side groups or multi-component doping.^20–23 Therefore, the development of single component UOP materials with intelligent response remains a challenge. Inspired by the alkyl-chain engineering²⁴ and freezing conditions, herein we speculate that temperature response UOP might be induced by controlling the activity of the alkyl chain to regulate non-radiative transition rate. By means of reducing the temperature to restrict the molecular motions at amorphous state, colour-tunable UOP with temperature-response can be realized by dual-emission of molecules at both amorphous and crystalline states.In our previous study, MCzT crystals showed yellow UOP at RT.²⁵ The crystals showed the same UOP at 77 K with that at RT (Fig. 1a). Here, 9-(2-((4,6-dimethoxy-1,3,5-triazin-2-yl)oxy)ethyl)-9H-carbazole (MTOD) was designed and via alkyl chain attaching carbazole with a triazine core. The target molecule was characterized by ¹H NMR and ¹³C NMR (Fig. S1–S3^†). Its melting point reaches 130 °C. (Fig. S4^†) MTOD achieved a lifetime of up to 860 ms under ambient conditions. Surprisingly, the UOP changed from yellow-white to blue-green after the removal of the UV-lamp for several seconds at 77 K (Fig. 1b), demonstrating colour-tunable property of UOP with a low temperature stimulus.Open in a separate windowFig. 1Molecular structures and UOP photographs of MCzT (a) and MTOD (b).In order to explore the reasons for the colour-tunable UOP of MTOD at low temperature. The photophysical properties of MTOD in the crystal state were first investigated under ambient conditions. As shown in Fig. 2a, the photoluminescence (PL) spectrum of MTOD shows two main emission peaks at 375 and 413 nm and a shoulder at 435 nm. From the lifetime decay profiles (Fig. S5a^†), it was confirmed that they were all assigned to the fluorescence. Notably, the crystals of MTOD presented yellow afterglow after turning off the UV lamp. From the delayed phosphorescence spectrum, the main emission of MTOD was located at 414, 556 and 600 nm with lifetimes of 681.91, 860.56 and 860.59 ms, respectively (Fig. 2b and S5b^†). Among these, the emission around 414 nm is assigned to triplet–triplet annihilation (TTA) fluorescence originating larger π–π overlaps of carbazole groups from the crystal data (Fig. S6^†).²⁶ Remaining two emission peaks are attributed to UOP emission. From the phosphorescence excitation-emission spectra of MTOD, UOP can be efficiently excited from 260 to 380 nm, with the optimal excitation at 360 nm (Fig. S7^†).Open in a separate windowFig. 2Photophysical properties of MTOD at room temperature and at 77 K. (a) Steady-state photoluminescence (PL, blue dashed line) and phosphorescence (red solid line) spectra at RT and 77 K. Inset: photographs taken after removing excitation. (b) Time-resolved phosphorescence decay of the emission bands at 556 and 600 nm at room temperature, respectively. (c) Time-resolved phosphorescence decay of the emission bands at 478, 564 and 612 nm at 77 K, respectively. (d) PL (blue dashed line) and phosphorescence (red solid line) spectra at molten state.Subsequently, we measured the PL and phosphorescence spectra of MTOD at 77 K (Fig. 2a). The steady-state PL spectrum showed four peaks at 361, 376, 410 and 436 nm, with little change compared with its corresponding spectrum at RT. However, a new phosphorescence peak appears at about 480 nm with an intense emission than others, which displays an ultralong lifetime of 2.5 s. Obviously, the new emission peak plays an indispensable role in the colour-changed UOP at 77 K. The yellowish-white UOP observed by naked eyes at 77 K is generated by the combination of three phosphorescence peaks. However, as time goes by, only the blue-green afterglow at about 480 nm due to the shorter lifetime of long-wavelength phosphorescence can be observed.To find out the origin of this new peak at 480 nm, the PL and phosphorescence spectra of MTOD at molten state were measured as amorphous emission. As shown in Fig. 2d, the PL peak is located at 380 nm and the phosphorescence spectrum shows a broad band with a main peak at 484 nm. It is suggested that the new phosphorescence emission peak at 77 K may be attributed to amorphous state. Taken together, we deduce that the colour-tunable property of the crystal is possibly due to the presence of an amorphous state at low temperature.The X-ray single crystal diffraction of MTOD crystal was taken to explore the mechanism of UOP at room temperature. Abundant intermolecular and intramolecular interactions (Fig. 3a, S6, S8 and S9a^†) in the crystals strongly restrict the torsional molecular configuration. The dihedral angle between the triazine and carbazole groups is about 75° (Fig. S10^†). In the crystal of MTOD, the single molecule is limited by multiple intermolecular interactions, including C–H⋯N (2.681 Å), π-H⋯π (2.791, 2.876 Å), π-H⋯N (2.703, 2.711 Å) (Table S3^†). The rich intermolecular interactions are beneficial to limit molecular motions to suppress non-radiative transitions of excited molecules, leading to UOP. However, the amorphous molecules around the crystals with weak restriction displayed negligible phosphorescence at room temperature due to the strong motions of alkyl chains.Open in a separate windowFig. 3Intermolecular interactions (a) at room temperature and (b) at 100 K.Comparatively, the single crystal of MTOD molecule at 100 K was measured in order to explore the colour-changing mechanism of UOP at low temperature (Fig. 3b and S9b^†). By comparison, MTOD crystals exhibit more intermolecular interactions at 100 K and the distance become shorter. Molecular conformation of MTOD changed slightly, the dihedral angle between triazine and carbazole changed from 74.95° to 75.32° (Fig. S10^†). These increased interactions can constrain the molecules more effectively and the stronger restriction of alkyl chain and carbazole will further suppress non-radiative transitions, resulting in the much longer phosphorescence lifetime of over 1.0 s at low temperature. Compared with the molecules in the crystalline state, freezing condition can provide a more rigid environment to minimize the movement of the alkyl chain, greatly reducing the non-radiative transition rate at amorphous state, resulting in the lifetime of short wavelength phosphorescence at 478 nm up to 2.5 s.According to the above results, the photophysical process of colour-tunable phosphorescence can be described by Jablonski diagram as shown in Fig. 4. Upon photoexcitation, both electrons in amorphous and crystalline molecules transforms to lowest singlet states (S₁). Then, the electrons in S₁ would further transform to the lowest triplet (T₁) through ISC. At room temperature, amorphous molecules show strong molecular motions to facilitate the non-radiative transitions. However, crystalline molecules due to closely arrangement can exhibit phosphorescence through radiative decay. At 77 K, both excitons in amorphous and crystal states are dominated by radiative transitions, leading to colour-tunable UOP.Open in a separate windowFig. 4(a) Jablonski diagram of the relevant photophysical processes illustrating amorphous and crystalline UOP process at room temperature (top) and 77 K (bottom). (b) Phosphorescence spectra of MTOD at different temperature and (c) corresponding coordinates in CIE.In view of the interesting luminescent phenomenon, we have investigated a series of phosphorescent spectra of MTOD crystals at different temperatures ranged from 183 to 273 K. As shown in Fig. 4b, as the temperature increases, the phosphorescence intensity of the amorphous molecules gradually decreases. The colour variations of the MTOD crystals in response to the environmental temperatures are shown in the Commission International de l’Eclairage (CIE) coordinate diagram (Fig. 4c). As the temperature was gradually changed from 183 to 273 K, the UOP changed from green to yellow with good linearity of the CIE coordinates. This demonstrated that MTOD crystals may have potential in low temperature sensing.In conclusion, we synthesized a colour-tunable single-component UOP compound through alkyl chain engineering. Combined the spectral and single crystal analyses, it is indicated that colour-tunable UOP comes from dual-emission of molecules at amorphous and crystalline states. Low temperature provides better rigid effect on UOP of amorphous molecules than crystals, resulting in the UOP colour changed from yellow-white to blue-green. More interestingly, red-shifted UOP of MTOD crystals with the increase of temperature can be achieved, demonstrating its potential for temperature sensing. This study will provide a platform for the design of single-component UOP molecules with tunable colour emission and broadens its application field.     

Mesoporous-silica-coated palladium-nanocubes as recyclable nanocatalyst in C–C-coupling reaction – a green approach

We report the straightforward design of a recyclable palladium-core–silica-shell nanocatalyst showing an excellent balance between sufficient stability and permeability. The overall process – design, catalysis and purification – is characterized by its sustainability and simplicity accompanied by a great recycling potential and ultra high yields in C–C-coupling reactions.

A green approach: in a single-step coating process a mesoporous silica shell was tailored onto palladium-nanocubes. Along with a PEG-matrix this core–shell-nanocatalyst could be recovered after C–C-coupling reactions and reused without any significant decrease in product yield.

In contrast to bulk materials, metal-nanocyrstals (NC) possess unique physical and chemical properties. Both, nano-scale and geometry can dictate their optical, electronic and catalytic behavior.^1–3 Consequently, nanomaterials have been implemented already in various fields like medicine,⁴ sensing,⁵ nano-electronics⁶ and organic synthesis.⁷ Smart strategies for a sustainable usage of limited resources such as precious metals and hydrocarbons are inevitable due to the consistently growing population and economy.^8–10 Generally, catalysis gives rise to novel and energy-saving synthetic routes. However, homogeneous catalysis is not widely used in industrial processes, due to the need of mostly toxic ligands and the costly purification along with a restricted reusability potential.^11–13 Heterogeneous catalysis based on the utilization of metal nanocrystals overcomes most of these limitations. Caused by its high surface-to-volume ratio, catalytic activities are drastically increased in comparison to bulk materials. However, the most common disadvantage in NC-based catalysis lies in the occurrence of aggregates during the reaction, which leads to a decrease of the catalytic active surface. Several studies were already presented in literature to delay that phenomenon using micelle-like- or core–shell-nanostructures as potential nanocatalytic systems.^14–20 However, these surfactants or shells are either potentially harmful for the environment or very step-inefficient to produce. Other approaches used the deposition of small NCs in a mesoporous support which offers a large active surface area.^21–24 In this context, it is crucial to find an adequate balance between permeability for small organic molecules and the overall stability of the nanocatalyst. Overcoming these obstacles can contribute to a sustainable supply of drugs and other organic substances.In the present work palladium-nanocubes (Pd-NCubes) were fabricated in aqueous solution using cetyltrimethyl-ammoniumbromide (CTAB) as surfactant. The procedure is adapted from a previously published study.^25,26 The respective transmission electron microscopy (TEM) image and the selected area electron diffraction (SAED) pattern of the as-obtained Pd-NCubes are depicted in Fig. S1.^† The SAED confirms the single-crystallinity of Pd-NCubes bound by {100}-facets. TE micrographs of Pd-NCubes revealed an average edge-length of (18 ± 2) nm (for histogram see Fig. S2^†). The formation of polyhedra and nanorods was found to be less than 1%.To the best of our knowledge, there is no procedure reported that showed the direct fabrication of a mesoporous silica (mSi) shell tailored on Pd-NCs. However, Matsuura et al. demonstrated a single-step coating approach of CTAB-capped gold-nanorods and CdSe/ZnS quantum dots obtaining a mesoporous silica shell.²⁷ The pores were determined to be 4 nm in width with 2 nm thick walls. Since the Pd-NCubes are covered by CTAB, already no surfactant exchange is necessary. Consequently, this procedure could be directly transferred to the as-obtained Pd-NCubes (∼10¹⁵ particles per L) of this study using tetraethyl orthosilicate (TEOS) as silica precursor in an alkaline solution. Here, CTAB serves as organic template for the formation of the mesoporous silica shell.TE micrographs of individual Pd-mSi-nanohybrids are depicted in Fig. 1 showing a spherical silica coating with a thickness of (17 ± 2) nm (for histogram see Fig. S3^†). The porosity is essential to ensure that vacant coordination sites on the palladium-core are present and accessible for catalysis. In contrast to other multistep approaches, pores are formed in situ with no additional etching step necessary.^28,29 This avoids the usage of harmful etching agents such as fluorides or ammonia.³⁰ The silica shell then served as platform for further surface modification using two different PEG-silanes (M_n = 5000 g mol⁻¹ and M_n = 20 000 g mol⁻¹). TE microscopy did not reveal any changes in the structure of the PEG functionalized Pd-mSi-nanohybrids opposed to the unfunctionalized nanocatalyst, since the contrast of polymer is too low (see Fig. S4^†). However, dynamic light scattering (DLS) measurements in diluted aqueous solutions proved an increased hydrodynamic radius with increasing molecular weight of the PEG-chain grafted onto the silica shell (see Fig. 2). These results provide evidence that only individual nanostructures are formed while no larger aggregates are present.Open in a separate windowFig. 1Exemplary TE micrographs of Pd-mSi-nanohybrids.Open in a separate windowFig. 2DLS results along the different stages of the hierarchal fabrication process of the nanocatalyst.The successful functionalization of the Pd-mSi-nanohybrid with PEG provides the dispersibility for the overall nanocatalyst in a PEG matrix. Due to its lack of toxicity and its simple recovery, caused by its melting point at ∼50 °C, PEG is considered as a “green” reaction medium.³¹ It has already been shown that PEG can act as suitable solvent for both homogeneous and heterogeneous catalysis.^32–34 Consequently, further experiments were performed using only the Pd-mSi-nanohybrid functionalized with PEG-silane having an average molecular weight of M_n = 5000 g mol⁻¹ (PEG-5k). For catalytic reactions, Pd-mSi-PEG-5k was dispersed in a PEG matrix (PEG-2000, M_n = 2000 g mol⁻¹) and charged into a Teflon centrifuge tube. Using only one tube for the reaction and the product separation avoids an additional transfer step and prevents any loss of the nanocatalyst between reaction cycles (see recycling Scheme 1). Here, the C–C-coupling between ethyl acrylate and p-iodoanisole served as model Heck-reaction to prove the catalytic activity of the designed catalyst (4.4 mol% overall Pd conc. equal to ca. 0.3 mol% surface-available Pd; conc. is determined by ICP-MS measurements, see Table S1^†). Sodium phosphate was used as base providing the largest product yield when compared to other bases, such as Na₂CO₃, K₂CO₃ and K₃PO₄. Once the catalysis was performed and the PEG-2000 was cooled down, diethyl ether was added to extract the product and separated from the reaction medium via centrifugation.Open in a separate windowScheme 1Recycling process of the Pd-mSi-PEG-5k-nanocatalyst and PEG-2000 as solvent after the Heck-reaction between ethyl acrylate and p-iodoanisole to form ethyl p-methoxycinnamate.To exclude any suspended compounds from the desired product, the mixture was passed through a PTFE-filter. Et₂O was removed in vacuo without the need of column chromatography. Comparing the ¹H-NMR spectra of the as-obtained product with the educts indicate a yield of 98% with only small amounts of PEG-2000 present (identified by the signal at ∼3.6 ppm, < 1 weight-%). The results show that both educts and the base Na₃PO₄ are able to diffuse through the mesoporous silica shell to the palladium core (see Fig. 3). A detailed ¹H-NMR signal assignment of the product is given in Fig. S5.^†Open in a separate windowFig. 3 ¹H-NMR spectra of the educts ethyl acrylate (top) and p-iodoanisole (center) and the product ethyl p-methoxycinnamate (bottom).After recovering the reaction mixture containing Pd-mSi-PEG-5k and PEG-2000, seven further Heck-reaction-cycles were conducted under the same conditions. Results obtained from ¹H-NMR and gravimetry indicate no significant decrease in catalytic activity (see ¹H-NMR spectra in Fig. 4). Along the eight Heck-reactions, product yields were determined between 94% and 99%. Only small traces of p-iodoanisole (identified by the signal at ∼6.75 ppm) were still present while ethyl acrylate could be fully removed in vacuo. The yields obtained after each cycle are displayed in Fig. 5. ICP-MS measurements of the catalysis product were performed to determine the palladium leaching out of the catalytic system. The results are displayed in Table S1^† indicating that leaching is strongly suppressed since the overall Pd-content in the product ranges from 0.3–5.7 ng, only. This corresponds to 0.002–0.044 ppm palladium with respect to the product mass. The data are in good agreement with the high product yields along the eight Heck-reactions. To trace the evolution of the nanocatalyst along the cycles, TE micrographs were taken after the 1^st and the 8^th Heck-cycle (see Fig. 6). It can be seen that the cubical structure of the palladium vanishes during the first reaction (left TEM image). Inside the silica shell spherical palladium particles were formed. An explanation for this rearrangement lies in the suggested Heck-mechanism.³⁵ Here, a Pd²⁺-species forms after the oxidative addition of the p-iodoanisole which can desorb from the Pd-NCube. Once the reductive elimination of the product occurs the Pd⁰-species is generated again that can re-deposit on the palladium-core.²¹ Since the spherical geometry possesses the lowest free surface energy, globules were eventually formed.³ The mesoporous silica shell is not affected significantly by the catalysis and the rearrangement of the palladium. The porosity appears to stay intact, enabling the penetration of further small organic molecules. Control experiments were performed to validate whether these observations are based on either a thermally or a chemically induced rearrangement process of the palladium core. Therefore, only the nanocatalyst was dispersed in PEG-2000 and heated at 110 °C for 24 h without any conducted catalysis reaction. TEM results showed no changes in the structure of neither the palladium core nor the silica shell (see Fig. S6^†). After the 8^th reaction, no core–shell-nanostructures could be detected anymore via TEM. Only small randomly shaped Pd-nanoparticles (≤20 nm) were found indicating a slow leaching of the palladium out of the silica shell. However, no larger aggregates were formed (see right TE micrograph in Fig. 6) which explains the continuously high catalytic activity.Open in a separate windowFig. 4 ¹H-NMR spectra of the product ethyl p-methoxycinnamate after the 1^st Heck-reaction (bottom) up to the 8^th reaction (top).Open in a separate windowFig. 5Conversions of ethyl p-methoxycinnamate obtained via NMR and gravimetry after each respective Heck-reaction (1–8).Open in a separate windowFig. 6TE micrographs of the Pd-mSi-PEG-5k-nanocatalyst taken after the 1^st (left) and the 8^th Heck-reaction (right).     

A novel fluorescent probe for imaging the process of HOCl oxidation and Cys/Hcy reduction in living cells

A new on-off-on fluorescent probe, CMOS, based on coumarin was developed to detect the process of hypochlorous acid (HOCl) oxidative stress and cysteine/homocysteine (Cys/Hcy) reduction. The probe exhibited a fast response, good sensitivity and selectivity. Moreover, it was applied for monitoring the redox process in living cells.

A new on–off–on fluorescent probe, CMOS, was designed and applied to detect the process of HOCl oxidation and Cys/Hcy reduction.

Reactive oxygen species (ROS) are indispensable products and are closely connected to various physiological processes and diseases.¹ For instance, endogenous hypochlorous acid (HOCl) as one of the most important ROS, which is mainly produced from the reaction of hydrogen peroxide with chloride catalyzed by myeloperoxidase (MPO), is a potent weapon against invading pathogens of the immune system.^2,3 However, excess production of HOCl may also give rise to oxidative damage via oxidizing or chlorinating the biomolecules.⁴ The imbalance of cellular homostasis will cause a serious pathogenic mechanism in numerous diseases, including neurodegenerative disorders,⁵ renal diseases,⁶ cardiovascular disease,⁷ and even cancer.⁸ Fortunately, cells possess an elaborate antioxidant defense system to cope with the oxidative stress.⁹ Therefore, it is necessary and urgent to study the redox process between ROS and antioxidants biosystems.Fluorescence imaging has been regarded as a powerful visual methodology for researching various biological components as its advantages of high sensitivity, good selectivity, little invasiveness and real-time detection.^10,11 To date, amounts of small molecular fluorescent probes have been reported for detection and visualization of HOCl in vivo and in vitro.^12–22,29 The designed strategies of HOCl sensitive probes are based on various HOCl-reactive functional groups, such as p-methoxyphenol,¹³p-alkoxyaniline,¹⁴ dibenzoyl-hydrazine,¹⁵ selenide,¹⁶ thioether,¹⁷ oxime,¹⁸ hydrazide,¹⁹ hydrazone.²⁰ But, many of these probes display a delayed response time and low sensitivity. And, only few fluorescent probes can be applied for investigating the changes of intracellular redox status.²¹ Besides, it''s worth noting that most of the redox fluorescent probes rely on the organoselenium compounds.²² Even though these probes are well applied for detection of cellular redox changes, excessive organic selenium is harmful to organisms and the synthesis of organoselenium compounds is high requirement and costly. Additionally, almost all the reports have only investigated the reduction effects of glutathione (GSH) as an antioxidant in the redox events. While, there are the other two important biothiols, cysteine (Cys) and homocysteine (Hcy), which not only present vital antioxidants, but also are tightly related to a wide variety of pathological effects in biosystem, such as slowed growth, liver damage, skin lesions,²³ cardiovascular,²⁴ and Alzheimer''s diseases.²⁵ However, the fluorescent probes for specially studying internal redox changes between HOCl and Cys/Hcy are rarely reported. In this respect, a novel redox-responsive fluorescent probe, CMOS, was designed and synthesized in this work, and we hope that it can be a potential tool for studying their biological relevance in living cells.Based on literature research, the aldehyde group has excellent selectivity in identification of Cys/Hcy, and the thiol atom in methionine can be easily oxidized to sulfoxide and sulfone by HOCl.^26,27 Considering these two points, we utilized 2-mercaptoethanol to protect the 3-aldehyde of 7-diethylamino-coumarin as the recognition part of HOCl, meaning that two kinds of potential recognition moieties are merged into one site. Fluorescent probe CMOS can be easily synthesized by the acetal reaction in one step (Scheme S1^†). A control molecule CMOS-2 was also prepared by 3-acetyl-7-diethylaminocoumarin (CMAC) similarly. The structure of all these compounds have been convinced by ¹H NMR, ¹³C NMR, and HR-MS (see ESI^†).As shown in Scheme 1a, we estimated that both CMOS and CMOS-2 can be rapidly oxidized in the appearance of HOCl. The oxidation product CMCHO of CMOS, which has the aldehyde moiety, can further react with Cys/Hcy to obtain the final product CMCys and CMHcy, respectively. In contrast, the oxidation product CMAC of CMOS-2 cannot combine with Cys/Hcy or other biothiols anymore (Scheme 1b).Open in a separate windowScheme 1Proposed reaction mechanism of CMOS and CMOS-2 to HOCl and Cys/Hcy.In order to confirm our design concept, the basic photo-physical characteristics of CMOS, CMCHO, CMOS-2 and CMAC were tested (Table S1, Fig. S1^†). Under the excitation wavelength 405 nm, CMOS and CMOS-2 exhibited strong fluorescence centred at 480 nm in PBS buffer solution, while the fluorescence of CMCHO and CMAC was weak around this band. The emission properties of CMOS and CMCHO were also investigated at the excitation wavelength 448 nm under the same experimental conditions as well (Fig. S2^†). After careful consideration, we chose 405 nm as the excitation wavelength in the follow-up experiments in vitro and in vivo.Next, the sensitivity of CMOS and CMOS-2 to HOCl and Cys/Hcy were investigated. As we expected, both the CMOS and CMOS-2 exhibited good response to HOCl. The fluorescence intensity of CMOS and CMOS-2 decreased gradually with addition of NaOCl (Fig. 1a, S3a^†), indicating that the fluorescence was switched off obviously in the presence of HOCl. The variation of intensity displayed good linearity with concentration of HOCl in the range of 0–20 μM (R² = 0.993, Fig. S4^†), and the detection limit of CMOS to HOCl was calculated to be 21 nM (S/N = 3). Subsequently, when Cys/Hcy was added to the final solution in Fig. 1a, the fluorescence intensity increased gradually within 180 min (Fig. 1b, S5^†). However, the fluorescence cannot be recovered by addition thiols to the CMOS-2 solution with excess HOCl (Fig. S3b^†). These results indicate that the probe CMOS can response to HOCl and Cys/Hcy in a fluorescence on-off-on manner, and can be used for monitoring the redox process with high sensitivity.Open in a separate windowFig. 1(a) Fluorescence responses of CMOS (2 μM) to different concentrations of NaOCl (0–200 μM). (b) Fluorescence responses of the CMOS solution (2 μM) with HOCl (200 μM) to Cys/Hcy (5 mM). (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_ex = 405 nm).To further identify the recognizing mechanism of probe CMOS, high performance liquid chromatography (HPLC) and mass spectral (MS) analysis were used to detect the redox process. Initially, probe CMOS displayed a single peak with a retention time at 3.7 min (Fig. 2a, S6^†) while reference compound CMCHO produced a single peak with a retention time at 2.5 min (Fig. 2b, S7^†). Upon the addition of HOCl to the solution of CMOS, the peak at 3.7 min weakened while 2.5 min and 2.2 min appeared (Fig. 2c). According to corresponding mass spectra, the new main peak at 2.5 min is related to compound CMCHO (Fig. S8^†). The other new peak of 2.2 min corresponds to the compound C3, which can be predicted as an intermediate in the oxidation process (Fig. S8^†).²⁸ The addition of Cys to the solution of CMCHO also caused a new peak with a retention time at 2.1 min, which has been confirmed to be the thioacetal product CMCys (Fig. S9^†). The possible sensing mechanism is depicted in Fig. S10.^†Open in a separate windowFig. 2The reversed-phase HPLC with absorption (400 nm) detection. (a) 10 μM CMOS. (b) 10 μM CMCHO. (c) 10 μM CMOS in the presence of 50 μM HOCl for 30 s. (d) 10 μM CMCHO in the presence of 1 mM Cys for 30 min. (Eluent: CH₃CN containing 0.5% CH₃COOH; 100% CH₃CN (0–7 min), 0.5 ml min⁻¹, 25 °C; injection volume, 5.0 μL).To study the selectivity of CMOS towards HOCl, we performed fluorescence response to different reactive oxygen species (ROS), reactive nitrogen species (RNS) and reactive sulfur species (RSS). As shown in Fig. 3a, CMOS exhibited significant change of fluorescence intensity only in the presence of HOCl, while other ROS and RNS, such as singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), hydroxyl radical (HO·), superoxide anion (O₂⁻), nitric oxide (NO), tert-butylhydroperoxide (t-BuOOH) and tert-butoxy radical (t-BuOO·) had no obvious fluorescence emission changes. Additionally, RSS which are abundant in biological samples, showed no influence in this process under the identical condition. The detection of reducing process was also investigated. As displayed in Fig. 3b, only cysteine and homocysteine induced excellent fluorescence recovery towards other reducing materials, such as RSS and various amino acids. Furthermore, the selectivity of CMOS-2 was also studied in the same condition. As expected, CMOS-2 could selectively detect HOCl, and not alter fluorescence intensity under various kinds of biothiols (Fig. S11^†). Therefore, our design strategy for the on–off–on probe is confirmed by results obtained above, with which CMOS can be utilized for detecting the redox process between HOCl and Cys/Hcy with high selectivity.Open in a separate windowFig. 3(a) Fluorescence response of CMOS (2 μM) to different ROS, RNS and RSS (200 μM). Bars represent emission intensity ratios before (F₀) and after (F₁) addition of each analytes. (a) HOCl; (b) KO₂; (c) H₂O₂; (d) ¹O₂; (e) HO·; (f) t-BuOOH; (g) t-BuOO·; (h) NO₂⁻; (i) NO₃⁻; (j) NO; (k) GSH; (l) Cys; (m) Hcy; (n) Na₂S; (o) Na₂S₂O₃; (p) Na₂S₂O₈; (q) NaSCN; (r) DTT; (s) Na₂SO₃. (b) Fluorescence response of the solution added HOCl in (a) to different RSS and amino acids. Bars represent emission intensity ratios before (F₂) and after (F₃) addition of each analytes (5 mM). (a) Cys; (b) Hcy; (c) Na₂S; (d) Na₂S₂O₃; (e) Na₂S₂O₈; (f) NaSCN; (g) DTT; (h) Na₂SO₃; (i) Ala; (j) Glu; (k) Gly; (l) His; (m) Ile; (n) Leu; (o) Met; (p) Phe; (q) Pro; (r) Ser; (s) Trp; (t) Vc; (u) GSH. (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_ex/λ_em = 405/480 nm).Subsequently, the influence of pH on probe CMOS was measured. The fluorescence intensity of CMOS and CMCHO perform no significant variances in wide pH ranges (pH = 4–11, Fig. S12a^†). Fluorescence intensity changes could be observed immediately when HOCl was added into the solution of probe CMOS, especially in alkaline condition (Fig. 4a). Considering the pK_a of HOCl is 7.6,²⁹CMOS is responsive to both HOCl and OCl⁻. Alkaline condition was also benefit for the fluorescence recovery of CMOS from Cys/Hcy (Fig. S12b^†). It is reasonable to consider that thiol atom displays higher nucleophilicity in alkaline condition. From the stop-flow test, the UV-visible absorbance of probe CMOS sharply decreased at the wavelength of 400 nm (Fig. 4b). The response time was within 10 s and the kinetic of the reaction was fitted to a single exponential function (k_obs = 0.67 s⁻¹). The ability of instantaneous response is extremely necessary to intracellular HOCl detection.Open in a separate windowFig. 4(a) Fluorescence responses of CMOS to HOCl under different pH values. Squares represent emission intensity ratios after (F₁) and before (F₀) addition of 200 μM HOCl (λ_ex/λ_em = 405/480 nm). (b) Time-dependent changes in the absorption intensity of CMOS (1 μM) before and after addition of HOCl. (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_abs = 400 nm).With these data in hand, we next applied CMOS for fluorescence imaging of the redox changes with HOCl and Cys/Hcy in living cells. After incubation with 5 μM CMOS at 37 °C for 30 min, intense fluorescence was observed of the SKVO-3 cells in the optical window 425–525 nm (Fig. 5a and d), indicating the probe can easily penetrate into cells. Treating the cells with 100 μM NaOCl led to remarkable fluorescence quenching as the probe sensed the HOCl-induced oxidative stress (Fig. 5b and e). After 3 min, the cells were washed with PBS buffer three times, and added 5 mM Cys/Hcy for 1 h, respectively. Then the fluorescence was recovered obviously (Fig. 5c and f). Experimental results clearly declare that the probe CMOS was successfully used to detect the process of HOCl oxidative stress and Cys/Hcy reducing repair in living cells.Open in a separate windowFig. 5Fluorescence imaging of the process of HOCl oxidative stress and thiols repair in CMOS-labeled SKVO-3 cells. Fluorescence images of SKVO-3 cells loaded with 5 μM CMOS at 37 °C for 30 min (a and d). Dye-loaded cells treated with 100 μM NaOCl at 25 °C for 3 min (b and e). Dye-loaded, NaOCl-treated cell incubated with 5 mM Cys (c), 5 mM Hcy (f) for 1 h. Emission intensities were collected in an optical window 425–525 nm, λ_ex = 405 nm, intensity bar: 0–3900.     

Complement activation by gold nanoparticles passivated with polyelectrolyte ligands

Gold nanoparticles passivated by polyelectrolyte ligands are widely used to confer stability and biofunctionality. While nanoparticles and polyelectrolytes have been reported as activators, their ability to activate the complement system as hybrid polyelectrolyte-coated nanoparticles is poorly characterized. Here, we found that gold nanoparticles passivated by common polyelectrolytes activated the system differently. The surface area of AuNPs appeared to be a major determinant of complement activation level as it determined the amount of adsorbed polyelectrolytes. Although a moderate negative correlation between AuNP surface hydrophilicity and their activation level was observed, the surface charge and functional group of polyelectrolyte ligands also influenced the final complement activation level.

We reported that the surface area and hydrophilicity of polyelectrolyte-coated gold nanoparticles influence their complement activation, a biological response not well understood to date.

Nanomaterials may elicit various biological responses upon contact with blood, of which activation of the complement system in innate immunity is one of the earliest.^1–3 The complement system is a collection of over 40 soluble and membrane-bound proteins, which acts in any of three distinct enzymatic cascades: classical, lectin, and alternative leading to the formation of a C3 convertase complex, and its accompanying range of biological responses, including inflammation, opsonization and cytolysis.^2,4While these responses could lead to undesirable physiological responses from over-activation and rapid clearance of nanomaterials from circulation, the intricate link between complement activation and adaptive immunity^5–7 also presents opportunities for exploitation in immune-related applications such as vaccines development. Therefore, complement activation by nanomaterials in biomedical applications has attracted great attention recently.^3,8–10We previously reported complement activation by gold nanoparticles (AuNPs) of different shapes in their as-synthesized citrate and CTAB coatings.¹¹ Polyelectrolyte ligands are also widely used in the preparation of AuNPs not only to stabilize AuNPs against aggregation, but to also enhance their solubility and confer additional surface functionalities.^12–14 These polyelectrolytes with different functional groups have been reported as complement activators in both their particulate and planar surface-immobilized forms.^15–21 However, the ability of polymer-passivated AuNPs to activate the complement system and the underlying mechanism remain poorly understood, although our previous study demonstrated that passivating the surface of AuNPs by poly(ethylene glycol) (PEG) modulated the activation level.¹¹Herein, we stabilized different shapes of AuNPs with different polyelectrolytes and examined their levels of complement activation and underlying mechanism. This would provide rational guidelines on the use of polyelectrolytes to modulate complement activation. All polyelectrolytes used in this study are hydrophilic and widely used as delivery platforms^5,22 and surface passivating ligands of nanomaterials²³ (Scheme 1).Open in a separate windowScheme 1Schematic illustration of gold nanoparticles (AuNPs) core, polyelectrolytes and methods used in this study. AuNPs with spherical shape of 20 nm (Au20) and 40 nm (Au40) as well as rod-shape (AuNR) were used as AuNP cores. These AuNPs were passivated with various polyelectrolyte ligands, examined for their hydrophilicities and levels of complement activation.Spherical AuNPs of two diameters, 20 nm (Au20) and 40 nm (Au40), were synthesized by well-known citrate reduction method.²⁴ AuNPs of rod-like shape (AuNR) were synthesized by hexadecyltrimethylammonium bromide (CTAB)-mediated method²⁵ with the CTAB ligands subsequently replaced by citrate using previously established protocol.²⁶ All AuNPs were then passivated by different polyelectrolytes following previously established layer-by-layer protocols (see Experimental section, ESI^†). With this library of AuNPs, we sought to examine the effects of size and shape of AuNP core as well as polyelectrolyte shell on the level of complement activation.The TEM images showed that Au20 and Au40 were spherical and homogenous while AuNR was rod-shaped with dimensions of approximately 40 × 10 nm (Fig. 1a–c). Hydrodynamic diameters, D_h, (Fig. 1d) were consistent with sizes of 20.2, 41.8, and 10.6 × 40.2 nm obtained from TEM images for Au20, Au40, and AuNR respectively (Fig. S1b–e, ESI^†). The UV-Vis absorption spectra showed surface plasmon resonance (SPR) peaks of 523 nm and 530 nm for Au20 and Au40, respectively while AuNR had transverse peak of 509 nm and longitudinal peak of 800 nm (Fig. S1a, ESI^†).^27,28 All citrate-capped Au20, Au40 and AuNRs had similar zeta potentials of −30 mV, which became nearly neutral after passivated with poly(vinyl alcohol) (ζ_AuNP-PVA ≈ −5 mV) and polyamidoamine (ζ_AuNP-PAMAM ≈ −10 mV) (Fig. 1e). In contrast, surface passivation with poly(acrylic acid) (AuNP-PAA), poly(styrenesulfonate) (AuNP-PSS), and heparin (AuNP-Heparin) conferred AuNPs a more negative zeta potential of ≈−40 mV (Fig. 1e), while coating with poly(ethyleneimine) (AuNP-PEI) conferred AuNPs a positive surface charge of ≈+40 mV. The presence of polyelectrolytes on AuNPs was further confirmed by the increase in D_h of polyelectrolyte-passivated AuNPs (Fig. 1d).Open in a separate windowFig. 1Physical properties of library of AuNPs. TEM images of (a) citrate-capped Au20, (b) Au40, and (c) AuNR. (d) Hydrodynamic diameter, D_h, and (e) zeta potential, ζ, of citrate-capped AuNPs and polyelectrolyte-passivated AuNPs. Each data point represents mean ± standard deviation (SD) of triplicate experiments. Scale bar in the TEM images represents 50 nm.When these polyelectrolyte-passivated AuNPs were incubated in normal human serum, the generation of SC5b-9 as an endpoint biomarker showed complement activation regardless of the activation pathway.^3,4,8 Here, elevated levels of SC5b-9 were detected in all AuNPs-treated sera (Fig. 2), indicating activation of the complement system. Prior to surface passivation by polyelectrolytes, we observed an expected complement activation by AuNP-citrate, although the level was significantly lower than our positive control zymosan (a well-known complement activator derived from the wall of yeast cell). Also, Au40-citrate induced more SC5b-9 than Au20-citrate and AuNR-citrate (Fig. 2).Open in a separate windowFig. 2Detection of endpoint product of complement activation, SC5b-9, using ELISA kit. 1× PBS and zymosan (10 mg ml⁻¹) were used as negative (−) and positive (+) controls, respectively. Each data point represents the mean ± standard deviation of triplicate experiments.The level of complement activation was dependent on both the AuNP core and polyelectrolyte ligand. Amongst the polyelectrolytes, we observed the highest level of complement activation from PEI, comparable or even higher than zymosan (Fig. 2). This agreed with previously published results where positively charged polymers carrying primary amino groups were shown to interact strongly with complement proteins to activate the complement system.^18,21Between the three AuNP cores, Au40-PEI had the highest level of complement activation ([SC5b-9] = 1.80 μg ml⁻¹), followed by Au20-PEI ([SC5b-9] = 1.29 μg ml⁻¹) and AuNR-PEI ([SC5b-9] = 1.16 μg ml⁻¹), which had comparable levels of complement activation (Fig. 2). The same trend was true not only for AuNPs-PEI but also other polyelectrolyte-passivated AuNPs. While complement activation by nanomaterials have been shown to depend on their sizes and shapes,^8,29 the differences observed here were more likely attributed to the amount of PEI adsorbed on the surface of AuNPs, which was in turn dictated by their surface area, since PEI by itself has been shown to activate the complement system in concentration-dependent manner.¹⁸ Here, Au20 and AuNR had comparable surface areas (1256 and 1413 nm², respectively) and hence similar levels of complement activation, while Au40 had the largest surface area (5024 nm²), thus accounting for the highest level of complement activation (Fig. 2).Unlike PEI, AuNPs passivated by both PVA and PAA induced the lowest levels of SC5b-9 (Fig. 2). While PVA is widely known as a biocompatible ligand on AuNRs,²³ its complement activation was not totally avoided probably due to its nucleophilic hydroxyl groups,^16,17 similar to PEI with nucleophilic amine groups. However, its complement activation level was much lower, and was likely due to its near neutral surface charge which did not promote interaction with many negatively charged complement proteins unlike the positively charged PEI.Similarly, the highly negative charge AuNP-PAA due to high density of carboxyl groups did not promote their interaction with negatively charged complement proteins, thus inducing an equally low level of complement activation as AuNP-PVA. Therefore, PAA is one of the most widely used water-soluble polyelectrolyte, superabsorbent polymer as well as food additive. Nonetheless, PAA conjugated to IgG has been found to specifically interact with positively charged C1q complement protein to activate the classical pathway.^30,31 Hence, AuNP-PAA has been found to promote inflammation via activation with fibrinogen.³² In this study, the AuNP-PAA could have activated the complement system via the classical pathway due to their interactions with C1q complement protein as reported.^30,31Owing to its nature as an anticoagulant, heparin has been reported to minimize complement activation.^33–35 However, we found that AuNP-heparin still activated the complement system although their level of complement activation was comparable to PVA and PAA, and much lower than PEI, PSS and PAMAM.Interestingly, despite the presence of nucleophilic amine groups on PAMAM similar to PEI, AuNP-PAMAM activated the complement system at much lower levels compared to AuNP-PEI. PAMAM dendrimers were known to be generation-dependent complement activators with stronger complement activation observed in higher generations.¹⁵ Here, we used PAMAM of generation 2.0 with aminoethanol surface and reasoned that coupling of amine with hydroxyl group helped to reduce activation level of AuNP-PAMAM. In fact, the activation level of AuNP-PAMAM was between that of AuNP-PVA with hydroxyl groups, and AuNP-PEI with amine groups. Furthermore, since primary amine groups adsorbed more C3b, a major component of complement proteins in serum, than secondary or tertiary amino groups,²⁰ the lack of primary amine group in PAMAM also explained its weaker complement activation than PEI which possesses a mixture of primary, secondary, and tertiary amine groups.The amount of SC5b-9 induced by AuNP-PSS was comparable to that of AuNP-PAMAM (Fig. 2). PSS is a widely used polyelectrolyte building block in layer-by-layer assembly,³⁶ and we used it as an intermediate ligand to prepare AuNR-citrate (Fig. S2, ESI^†). PSS by itself interacted with complement proteins of the classical pathway to activate the complement system.¹⁹ Our results not only confirmed complement activation by PSS passivation but also highlight the potential side effects of block-copolymer containing PSS commonly used as drug delivery platform in activating the complement system.Since the hydrophilicity of nanoparticles has been shown to modulate non-specific protein adsorption³⁷ and dictate immune response,³⁸ we further examined for possible correlation between the hydrophilicity of polyelectrolyte-passivated AuNPs and their level of complement activation. We measured relative hydrophilicity of polyelectrolyte-passivated AuNPs by dye absorption using a hydrophilic dye Nile Blue which interacted with a hydrophilic moiety (see Experimental section, ESI). On mixing the dye with AuNPs passivated with different polyelectrolyte ligands, we determined the partitioning quotient (PQ) as the ratio of dye bound on nanoparticles surface to the amount of free dye. In a plot of PQ versus surface area of nanoparticles, the slope of this linear regression line represented relative surface hydrophilicity.³⁹We observed different levels of hydrophilic dye adsorption on all polyelectrolyte-passivated AuNPs after 3 h incubation, indicating differences in their hydrophilicities (Fig. S8–10, ESI^†). AuNPs-PEI was the least hydrophilic (Fig. 3) as determined from the smallest slope of the linear regression line (Table S1, ESI^†), while AuNPs passivated with PVA, PAA, and heparin are amongst the most hydrophilic as given by the larger slope of the PQ vs. nanoparticle surface area plot (Fig. 3 and Table S1, ESI^†).Open in a separate windowFig. 3Negative correlation between surface hydrophilicity and complement activation by polyelectrolyte-passivated (a) Au20, (b) Au40 and (c) AuNR. X scale bar was plotted as ln of the value of relative hydrophilicity.We also observed a negative correlation between surface hydrophilicity and complement activation level across all AuNPs regardless of shape or size (Fig. 3). However, this correlation was only moderate with Pearson correlation coefficient, r = −0.2730, −0.4101, and −0.5489 for Au20, Au40 and AuNR, respectively (Fig. 3). Since Moyano et al. reported a positive correlation between nanoparticle surface hydrophobicity and their immune response previously,³⁸ the similarity in our observations suggested that the complement system could be a potential mediator between surface hydrophilicity/hydrophobicity and downstream immune response. Nonetheless, our results also demonstrated the complexity of complement activation by engineered nanomaterials, as it was not dependent solely on any one physical property such as shape, size, surface charge, surface functional group or surface hydrophilicity, but the interplay of these properties to influence the complement proteins adsorption.In summary, AuNPs passivated with different polyelectrolyte ligands activated the complement system at different levels, as characterized by the presence of endpoint product of complement activation, SC5b-9. The surface area of AuNPs appeared as a major determinant of complement activation level as it determined the amount of adsorbed polyelectrolytes. Although a moderate negative correlation between AuNPs surface hydrophilicity and their activation level was observed, the surface charge and functional group of polyelectrolyte ligands also influenced the final complement activation level. These findings provide new insights to rational selection and design guidelines for the use of polyelectrolytes to either suppress complement activation and downstream immune response for nanoparticulate drug delivery systems or to enhance complement activation and immune response for vaccine development.     

A stable tunnel-type NaGe3/2Mn1/2O4 anode for Na-ion batteries

Tunnel-type NaGe_3/2Mn_1/2O₄ was fabricated for anode of sodium ion batteries, delivering a discharge capacity of 200.32 mAh g⁻¹ and an ultra-low potential platform compared with that of pure Na₄Ge₉O₂₀ (NGO). The results of X-ray photoelectron spectroscopy (XPS) demonstrate that Ge redox occurs, and partial substitution of Mn effectively improves the Na-storage properties compared to those of NGO.

We investigated tunnel-type NaGe_3/2Mn_1/2O₄; the main structure is Na₄Ge₉O₂₀. NaGe_3/2Mn_1/2O₄ electrodes as anodes for sodium ions batteries deliver a discharge capacity of 200.32 mAh g⁻¹ and satisfactory capacity retention after 50 cycles.

In terms of the high abundance and ready availability of sodium, sodium-ion batteries (SIBs) have been generally regarded as a better alternative to lithium-ion batteries for power stations.^1–4 Hard carbon is widely recognized as one of the most attractive and ideal anode materials for SIBs.^5,6 However, the potential required for sodium ions to insert into hard carbon is very close to that for sodium plating, resulting in sodium dendrites, which raise safety concerns.^7,8 Moreover, the reaction of electrode materials with sodium through alloying or conversion mechanisms always results in serious volume changes in the process of sodium insertion and extraction.⁹ Therefore, insertion-type transition-metal oxides as anodes have attracted much attention owing to their suitable operating potentials and minor volume expansion.^10,11 Recently, embedded titanium/vanadium/molybdenum based oxides with layered structures have been studied as anode materials for SIBs,¹² such as layered Na₂Ti₃O₇,¹³ tunnel Na₂Ti₆O₁₃ (ref. 14) and spinel Li₄Ti₅O₁₂.¹⁵ In addition, post-spinel structured materials have been proposed, which show ultra-stable cycle performances via highly reversible sodium-ion insertion/desertion through large-size tunnels. Recently, in Zhou''s group, NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄ (ref. 17) have been prepared and they have been shown to possess robust cycle lifetimes (more than 10 000 cycles) and discharge plateaus of 0.84 V and 0.7 V, respectively. Meanwhile, in our group, Na_0.76Mn_0.48Ti_0.44O₂ has been developed, which holds an initial discharge capacity of 103.4 mAh g⁻¹, shows a superb rate capability and retains 74.9% capacity after 600 cycles.¹⁸ The large radius of the redox active metal center could optimize the tunnel size and thus boosting the electrochemical performance. It is also a big challenge to find further suitable active centers for insertion-type transition metal oxides as anodes of SIBs. Besides, a host of published reports have said that germanium-based materials can be used as alloy anodes for SIBs with highly reversible sodium storage properties and satisfactory ionic/electronic conductivity.¹⁹ However, it is unclear to us whether Ge could act as an active center in a transition-metal oxide anodes.In this work, we fabricated a tunnel-type NaGe_3/2Mn_1/2O₄ (NGMO) material. When used as the anode of SIBs, it delivers a sustained discharge capacity of 200.32 mAh g⁻¹. Compared with NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄,¹⁷ NGMO delivers a lower safety voltage of 0.36 V. Pure Na₄Ge₉O₂₀ (NGO) as a comparative sample, only exhibits a capacity of 24.8 mAh g⁻¹, which is far inferior to that of NGMO. During discharge and charge process, reversible redox reactions around Ge center occur, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The introduction of Mn in the NGMO improves the reversibility of the Ge redox performance.The structure of NGMO was carefully characterized by XRD, and Rietveld refinement was performed as depicted in Fig. 1. The main Bragg peaks of NGMO could be assigned to space groups of P1(2) and I4₁/a(88), which were fitted to give lattice parameters of a = 10.56/15.04 Å, b = 11.18/15.04 Å, and c = 9.22/7.39 Å, and a volume of 811.2/1672.2 ų, respectively. Na₄Ge₉O₂₀ has a typical tunnel structure, which consists of polymerized Ge/MnO₄ tetrahedra connected with Ge/MnO₆ octahedra. Four Ge/MnO₆ octahedra are connected together by sharing edges to form a tetrameric (Ge/Mn)₄O₁₆ cluster. Each cluster is connected to six GeO₄ tetrahedra, and adjacent clusters are connected by GeO₄ tetrahedra. Na atoms are located in the channels and have elongated Na–O bonds.¹⁹ This highly stable crystal structure can effectively accelerate the migration of sodium ions.²⁰Fig. 2 shows the low and high magnification scanning electron microscopy (SEM) images of NGMO, which is composed of particles of different sizes from 1 to 3 μm; the larger particles are the result of sintering at high temperature. SEM images of NGO with different magnifications are given in Fig. S1,^† and show that the average particle size of NGO is 1 μm.Open in a separate windowFig. 1The Rietveld refinement spectra of NGMO.Open in a separate windowFig. 2(a) and (b) SEM images of NGMO at different magnifications.The morphology and fine structure were studied by transmission electron microscopy (TEM). Fig. 3a and b show the low magnification TEM images. It can be seen from the images that NGMO has an irregular sheet-like morphology with particle sizes from 250 nm to 2 μm. As shown in Fig. 3c, the lattice spacing of the (200) plane is 4.55 Å. In the SAED pattern of Fig. 3d, the red line corresponds to the (020) plane in NGMO, and the lattice spacing is 13.100 Å. These results clearly demonstrate that NGMO exhibits good crystallinity. The corresponding energy dispersive X-ray spectroscopy (EDS) results, and Raman and infrared spectra (IR) are provided in Table S1 and Fig. S2.^† The results indicate that the atomic ratio of Na : Ge : Mn is close to 1 : 1.5 : 0.5 and that there is little sodium loss. The Raman and IR peaks in the high frequency region are attributed to stretching vibrations of Ge–O–Ge and the peaks between 600 and 400 cm⁻¹ are attributed to the bending vibrations of Ge–O–Ge in NGMO.Open in a separate windowFig. 3(a) and (b) Low resolution TEM images, (c) a HRTEM image and (d) a SAED image of NGMO.Galvanostatic electrochemical measurements were evaluated in a voltage range of 0.05–2.0 V, with the current density of 20 mA g⁻¹. Fig. 4a and b show the discharge and charge profiles of NGMO and NGO, respectively. Because of the formation of a solid electrolyte interface (SEI) layer in the initial cycle, the electrochemical behaviour tends stabilize in the second cycle, so voltage profiles are given from the second cycle; the first cycles of the discharge–charge curves of NGMO and NGO materials are given in Fig. S3.^† It can be seen intuitively that both NGMO and NGO have low voltage platforms, while NGMO has the smaller polarization. In Fig. 4a, we notice a reversible voltage profile in the second cycle with discharge capacity of 200.32 mAh g⁻¹ for NGMO. Only NGMO has a flat potential platform and delivers an ultra-low plateau potential. It can be seen from Fig. 4b that NGO shows a capacity of 24.8 mAh g⁻¹, obvious polarization at the 20^th cycle and increased capacity due to the surface side-effect. Fig. 4c indicates that the capacity retention of the NGMO electrode after 50 cycles is 86.2%, which is superior to that of NGO; the coulombic efficiency of NGMO is also provided in Fig. S4.^† To further understand the redox reactions along with the discharge/charge process in NGMO, Fig. 4d displays the differential capacity versus voltage (dQ/dV) curve. The clear anodic peak at 0.33 V and cathodic peak at 0.81 V correspond well with the redox reactions of NGMO.Open in a separate windowFig. 4Electrochemical performance: (a) and (b) voltage profiles of NGMO and NGO, respectively; (c) cycling performance; (d) dQ/dV profile of NGMO. The current density was controlled at 20 mA g⁻¹ over a voltage range of 0.05–2.0 V.The electrochemical impedance spectra (EIS) of fresh and cycled electrodes of NGMO and NGO, with a frequency range of 0.01 Hz to 100 kHz, are shown in Fig. 5. From Fig. 5a, it is obvious that the charge-transfer resistance of the fresh NGMO electrode is lower than that of NGO. This indicates that the migration of charges in the NGMO material occurs more easily than in NGO, which also facilitates the shifting of ions on the surface and inside of the electrodes of NGMO. In Fig. 5b, NGMO electrode in its 10^th cycle exhibits a smaller charge-transfer resistance than both NGO and the NGMO fresh electrode, indicating that the surface of NGMO more readily forms a stable SEI film.¹⁸ The EIS results were fitted by the model shown in Fig. S5.^† The fitting results are provided in Table S2.^† The resistances of the fresh and cycled electrodes of NGMO and NGO are composed of an internal resistance (R_s), the resistance of the surface film (SEI) (R_f; a small semicircle in the high frequency region), the resistance of the charge transfer (R_ct; another opposite semicircle in the middle frequency region), and the Warburg resistance (W; an oblique line in the low frequency region).²¹ Both the fresh and cycled electrodes of NGMO deliver lower charge transfer resistance than NGO. Meanwhile, the transfer resistance of the cycled NGMO electrode is lower than that of the fresh electrode and its slope at low frequency is higher than that of the fresh one (Table S2^†). All these results show that NGMO has lower resistance and better electronic/ionic conductivity than NGO.Open in a separate windowFig. 5Nyquist plots of (a) fresh NGMO and NGO electrodes, and (b) NGMO and NGO electrodes after ten cycles.The evolution of the chemical valence states of the 150-times discharged electrodes were observed by XPS and SEM as provided in Fig. S6.^† It is generally clear that the electrode surface was covered with a thick SEI layer after discharging. Ar plasma etching was used to obtain the internal information. The Ge 3d core-level of the discharged NGMO electrode with and without etching is shown in Fig. S7.^† Before etching, the peaks of the Ge 3d core-level could be fitted to Ge¹⁺ and Ge²⁺.²² After etching, (Fig. S7c^†), the peaks at 30.8 eV and 30.2 eV were also associated with Ge¹⁺ and Ge²⁺, respectively. This result indicates that the valence of Ge decreases as a whole and that there is no obvious difference between the etched and non-etched samples. The reversible redox reactions of Ge remain stable even after cycling. Meanwhile, the Mn 2p core-level spectra are shown in Fig. S7d–f.^† For the Mn 2p core level, owing to the spin orbit coupling, the valence states of Mn comprise two couples including Mn³⁺ and Mn²⁺ (Fig. S7e and f^†). The binding energies of Mn³⁺ are 642.37 eV and 654.04 eV, and the binding energies of Mn²⁺ are 640.71 eV and 652.18 eV. Similarly, after discharging, the binding energies of Mn³⁺ are 642.40 eV and 654.06 eV, and those for Mn²⁺ are 640.69 eV and 652.20 eV, indicating that there are no changes in Mn binding energies before and after etching. This is in good agreement with results in previous reports.^23,24 All results also show that a thin SEI layer has been formed, favoring ions transfer on the repeatedly cycled electrode. It can be seen from the refined XRD results that NGMO consists of Ge⁴⁺, Mn²⁺ and Mn³⁺, and combined with XPS analysis, the results show that the valence states of Mn does not change for the discharged NGMO electrode. Ge displays electrochemical activity in NGMO, and Mn exhibits good chemical stability in the framework.     

Superhydrophilic fluorinated polyarylate membranes via in situ photocopolymerization and microphase separation for efficient separation of oil-in-water emulsion

A superhydrophilic modified fluorinated polyarylate membrane with high tensile strength was prepared by a combination of in situ photocopolymerization and microphase separation. The as-prepared membrane was successfully utilized for oil-in-water emulsion separation with high separation efficiency and high flux. Furthermore, the membrane displayed excellent antifouling performance and recyclability for long-term use.

We have developed a novel superhydrophilic FPAR membrane with high tensile strength by in situ photocopolymerization and microphase separation, which can effectively separate oil-in-water emulsions with high separation efficiency and flux.

Today, the ever-growing serious environmental pollution caused by oil-contaminated water from the daily life of people as well as from industries demands the search for novel materials and strategies to realize oil/water separation with high efficiency.^1–5 Traditional separation technologies such as gravity separation, centrifugation, skimming, sedimentation, and flotation are useful for most of the separation processes. Unfortunately, low separation efficiency, high energy consumption and complex equipment have restricted the application of these technologies to some extent.^6–8 Other than that, it may be very difficult for them to separate emulsified oil/water solutions.⁹ Therefore, desirable materials for effective separation of oil/water emulsions are urgently needed. As a result, filtration polymer membranes have been considered to be a suitable technology for separating various emulsions, but suffer from low flux, surface fouling and poor mechanical properties.^2,9Recently, significant interests have been attracted to the design and preparation of oil/water separation membranes with special wettability by a combination of rough structure and surface chemistry.^2,10–14 Typically, these polymer membranes may be classed into two types, polymer coated mesh membranes and polymer porous membranes.^3,15–22 For polymer coated mesh membrane, it requires a mesh as a support which is capable of improving mechanical properties and rendering a micro-scale porous structure.² For example, Tuteja and co-workers developed a superhydrophobic mesh membrane coated with a blend of cross-linked poly(ethylene glycol)diacrylate and fluorodecyl polyhedral oligomeric silsesquioxane, which was valuable for separation of oil/water emulsions with droplet sizes larger than 1 μm.²¹ PVDF has been acknowledged as one of the main materials for manufacturing polymer porous membranes for separation of oil/water emulsions through a phase-inversion process.^1,2 In 2014, a superhydrophilic and underwater superoleophobic poly-(acrylic acid)-grafted PVDF (PAA-g-PVDF) membrane was fabricated by a salt-induced phase-inversion approach and applied to oil-in-water emulsions, however, the tensile strength of this membrane was not more than 0.64 MPa, which limited their practical applications.²²Polyarylate, a family of high-performance polymers, noted for their strength, toughness, chemical resistance, and high melting points.^23–26 Recently, Livingston and co-workers have demonstrated the formation of crosslinked polyarylate microporous membranes which have great potential for applications in molecular separations.²⁷ In previous studies, our group developed a simple procedure to fabricate a superhydrophobic and superoleophilic porous polyarylate membrane which could effectively separate oil/water mixtures.²⁸ In this communication, we reported the fabrication of a novel superhydrophilic sodium acrylate modified fluorinated polyarylate (SFPAR) membrane for efficient separation of oil-in-water emulsion by a combination of in situ photocopolymerization and microphase separation. It was very exciting that the as-prepared SFPAR membrane exhibited prominent mechanical strength and outstanding water permeability. Furthermore, the membrane also displayed excellent underwater superoleophobicity, antifouling performance and recyclability for long-term use, which highlight its potential for practical applications. Fig. 1 shows the formation of a SFPAR membrane via in situ photocopolymerization for endowing with the hydrophilic property of FPAR (Scheme 1a–c), followed a microphase separation (Scheme 1d and e) for obtaining the SFPAR membrane with porous structure. The experiments are described in detail in the ESI.^† Here, in situ photocopolymerization was applied for getting hydrophilic FPAR, which have the following advantages: good dispersibility of the formed acrylate copolymer in the FPAR matrix, low reaction temperature, and shortening the preparation time of membrane. After a microphase separation and a drying process, a white membrane was obtained by peeling from a substrate (Scheme 1f).Open in a separate windowFig. 1Water contact angle of the FPAR membranes as function of sodium acrylate mass fraction (a), water contact angle of the SFPAR and FPAR membranes as function of time (the insets are photographs of water drops on the membrane surfaces) (b), underwater–oil contact angle (c and d) and dynamic underwater–oil-adhesion of the SFPAR membrane (e and f). The underwater–oil contact angle were measured with 4 μL hexadecane droplet.Open in a separate windowScheme 1Schematic of the formation of a SFPAR membrane via in situ photocopolymerization and a limited micro-phase separation. The fluorinated polyarylate (FPAR) was fabricated by interfacial polymerization of bisphenol AF, terephthaloyl chloride, and isophthaloyl chloride (Fig. S1a^†). The number-average molecular weight of the obtained PAR is 93 000 and the polydispersity index is 1.76 (Fig. S2^†). The experiments are described in detail in the ESI.^†One of the main purposes of in situ photocopolymerization is to improve the wettability of FPAR by introducing carboxylate salts (Fig. S1b^†). Fig. 1a shows water contact angle of the FPAR membranes as function of sodium acrylate mass fraction. The results indicated that the value of water contact angle on the FPAR surface significant decreased with the increase of the sodium acrylate content. After the sodium acrylate content exceeded ∼9.5 wt%, the contact angle tended to equilibrium, less than ∼1°, which formed a superhydrophilic modified FPAR membrane (SFPAR). To further examine the wettability of water on the FPAR membranes, the water contact angles of the FPAR and SFPAR membrane as function of time was also measured (Fig. 1b). The pure FPAR membrane had the initial water contact angel of approximately 96.2° and the value of water contact angel almost kept stable after 100 s, exhibiting good hydrophobicity. On the contrary, the approach to introducing carboxylate to FPAR caused a significant differences. The SFPAR membrane had the initial water contact angel of approximately 40°. Interestingly, the value of water contact angel of the SFPAR membrane rapidly decreased to ∼1° in less than 7 s, illustrating outstanding superhydrophilicity, which is caused by the introduction of carboxylate sodium and the porous structure of the SFPAR membrane. Furthermore, the underwater–oil contact angle (OCA) and dynamic underwater–oil-adhesion of the SPAR membrane were also studied (Fig. 1c and d). An oil droplet was lifted up and contacted the SPAR membrane surface under water (Fig. 1c and d). It was observed that the oil droplet remained spherical and the underwater–OCA of this membrane is ∼161.7°, demonstrating excellent underwater superoleophobicity. From Fig. 1e to Fig. 1f, the oil droplet was forced to adequately contact the membrance surface and then moved to the left. During the moving process, the spherical oil droplet had no obvious deformation, also showing that the SPAR membrane had excellent antiadhesion to oil.ATR-FTIR spectra of the SFPAR and FPAR membranes are shown in Fig. 2a. For ATR-FTIR spectrum of the SFPAR membrane, besides the corresponding absorption peak of FPAR, The characteristic stretching peaks were obviously shown at 2850–3000 cm⁻¹ and 1457 cm⁻¹, respectively, resulting from –CH₂– and –CH₃, and –O–CH₂– groups of the crosslinked acrylate copolymer prepared by in situ photocopolymerization. The peak at 1569 cm⁻¹ was the asymmetric CO²⁻ (salts) stretching vibration in –CO₂Na of the formed acrylate copolymer. Moreover, the peak at 1640 cm⁻¹, which was attributed to the stretching vibration of vinyl bond, was not observed from the ATR-FTIR spectrum of the SFPAR membrane, indicating that the monomers were polymerized.Open in a separate windowFig. 2ATR-FTTR spectra of the SFPAR and FPAR membranes (a) and the overall XPS spectra of the SFPAR and FPAR membranes (b).XPS was performed to examine the surface chemical composition. Fig. 2b exhibits the overall XPS spectra of the SFPAR and FPAR membranes. There were three signals on the surface of the FPAR membrane attributed to C, O and F element whose atomic percentage was approximately 66.0, 20.6, and 13.4%, respectively. In comparison with the XPS spectrum of FPAR, the new signal appearing in the spectrum of the SFPAR membrane was attributed to Na element. The percentage of Na was estimated to be approximately 4.5 wt%, higher than the bulk content (2.3 wt%), and the F content of SFPAR membrane was obvious decreased and the O content is increased after in situ photocopolymerization, indicating the obvious surface enrichment of sodium carboxylate groups in the SFPAR membrane. Fig. 3 displays SEM images of the surface and cross section of the FPAR and SFPAR membranes. Apparently, the morphologies of the SFPAR membrane are different from those of the FPAR which can be attributed to the thermodynamics instability and the non-solvent Induce phase separation. The FPAR surface is smooth (Fig. 3a), while the SFPAP surface is porous and has a great number of micro nano-scale pores (Fig. 3c) and the pore size and pore distribution were calculated by Nano Measurer 1.2 (Fig. S3a^†). For the cross of the FPAR and SFPAR membranes, the former is dense and few pores can be found (Fig. 3b). However, the latter is loose and possesses many inter-connected nano-scale channels with a diameter of 50–200 nm (Fig. 3d). Different from the reported superhydrophilic membranes made from semicrystalline PVDF,^22,29,30 the FPAR is amorphous. According to the XPS and SEM results above, the possible formation mechanism of SFPAR membranes with porous structure prepared by in situ photocopolymerization and phase separation can be described as follows. As can be seen from Scheme 1, the FPAR, the monomers (BA, SA and TEGDA) and photoinitiator are first dissolved in THF to form the homogenous viscous solution (Scheme 1a and b). After in situ photocopolymerization of BA, SA and TEGDA occurs at room temperature, the crosslinked polyacrylate containing sodium carboxylate groups come into being in the viscous solution (Scheme 1c). During the immersion process (Scheme 1d), with the extraction of THF by the coagulation bath, the blend matrix of the amorphous FPAR and the crosslinked polyacrylate will gradually shrink and solidify. Simultaneously, a microphase separation occurs in the blend matrix due to the crosslinking of polyacrylate and the thermodynamics instability. Furthermore, sodium carboxylate groups attached to the polyacrylate network can absorb enough water in the blend matrix, ultimately leading to the formation of the wet membrane containing water. During the drying process, water is evaporated from the wet membrane and the porous structure appears in the membrane because the solidification of FPAR matrix restricts the movement of the polyacrylate segments. Finally, the SFPAR membrane with porous structure is obtained after the blend matrix is fully dried at room temperature (Scheme 1e and f). The hydrophilic sodium carboxylate groups will enrich in the SFPAR membrane surface and the inner surface of the micro nano channel due to the driving forces of surface free energy and hydrophilicity/hydrophobicity interactions (Fig. S3b, ESI^†), which makes it possible for the preparation of oil–water separation membrane.Open in a separate windowFig. 3SEM images of the FPAR and SFPAR membranes: the surface (a) and cross section (b) of the FPAR membrane; the surface (c) and cross section (d) of the SFPAR membrane. The inset is high-magnification SEM image of the SFPAR membrane surface.In this work, oil–water separation of the SFPAR membrane was carried out with a vacuum driven filtration system at 0.07 MPa. Toluene-in-water emulsion was employed to evaluate the separation ability of the membrane and the droplet size distribution of the emulsion is in the range from ∼900 nm to ∼8 μm (Fig. S4, ESI^†). Fig. 4a illustrates a self-made separation device and the separation result of toluene-in-water emulsion (the separation experiments are described in detail in the ESI^†). Compared with the milky white feed emulsion (up), the filtrate (down) is colorless from the appearance. A noticable difference was observed between the feed and the filtrate by the optical microscopy images. There appear a great many droplets in the image of the feed before filtration, however, no droplet can be viewed for the filtrate. Furthermore, the characteristic peak of toluene for the filtrate is not observed from UV-VIS spectrometer (TU-1901, Beijing Purkinje General Instrument Co., Ltd, China) in comparison with the feed (Fig. S5, ESI^†), and the oil content in the filtrate is 54 ± 17 ppm measured by a total organic carbon analyzer, indicating that the as-prepared membrane can successfully separate oil/water emulsion with high efficiency. The other two emulsions also have good separation efficiency (Table S1, ESI^†).Open in a separate windowFig. 4The vacuum driven filtration system and separation results for toluene-in-water emulsion (a) and change of the flux and flux recovery in the separation of a toluene-in-water emulsion over five cycles (b).Taking advantage of the reported method,^22,29 the flux and the antifouling property of the membrane were measured by the vacuum driven filtration system. Continuous separation of the toluene-in-water emulsion lasts for approximately 30 hours over five cycles and the flux is detected every an hour and six points were taken down within each cycle. The SFPAR membrane is gently washed by using DI water to dispose of surface adsorbent. As shown in Fig. 4b, the flux has a slight decline from ∼3800 to ∼3600 L m⁻² h⁻¹ within one cycle. Nevertheless, the membrane can recover fully to the initial flux after it is washed by water. The results show that the SFPAR membrane possesses a high flux and an outstanding antifouling performance for long-term use. Further studies will focus on the regulation of the pore size of the SFPAR membrane and get the most proper selectivity and penetration. Moreover, as one of the important factors in practical application, the tensile strength of the membrane was also tested by a testing machine (Fig. S6, ESI^†). Due to the porous structure, the tensile strength of the SFPAR membrane is ∼6.02 MPa, less than that of the FPAR membrane (∼27.59 MPa). However, the SFPAR membrane still has high mechanical property compared with the reported hydrophilic modified PVDF oil/water separation membrane.^14,17In conclusion, we have developed a novel superhydrophilic modified FPAR membrane with porous structure by in situ photocopolymerization of acrylate monomers and subsequent microphase separation. The results of ATR-FTIR and XPS demonstrated that sodium carboxylate groups was immobilized in the FPAR membrane by in situ photocopolymerization. When the sodium acrylate content was beyond ∼9.5 wt%, the as-prepared SFPAR membrane exhibited prominent superhydrophilicity, underwater superoleophobicity, and water permeability. The SFPAR membrane could effectively separate oil-in-water emulsions with high separation efficiency and high flux. Significantly, the obtained membrane possessed a good antifouling property and could be recycled for long-time use. From a practical perspective, the SFPAR membrane had a higher mechanical strength than traditional hydrophilic polymeric membranes with similar permeation properties. Therefore, we anticipate that our membrane will have high potential in practical application for treating wastewater from the daily life and industries.     

Facile one-pot solvothermal preparation of two-dimensional Ni-based metal–organic framework microsheets as a high-performance supercapacitor material

We report a facile one-pot solvothermal way to prepare two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) using only Ni precursor and ligand without any surfactant. The Ni-MOFms exhibit good specific capacities (91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively) and long-term stability in 5000 cycles when used for a supercapacitor electrode.

Two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) were synthesized via a facial one-pot solvothermal approach and exhibited good specific capacities and excellent long-term stability when used for a supercapacitor electrode.

With the continuous growth of energy demand worldwide, high-performance, environmental-friendly, and low-cost energy storage devices have attracted extensive research interest.^1–3 Among them, supercapacitors are considered most promising because of their high power density, long lifespan, and fast charging/discharging speed.^4–6 To date, numerous materials have been explored for fabricating supercapacitors. Carbon materials have been usually used for electrical double-layer capacitors (EDLCs), including carbon fibers, carbon nanotubes, carbon spheres, carbon aerogels, and graphene,^7–12 while conducting redox polymers and transition metal oxides/hydroxides are widely explored as active materials for pseudocapacitance and battery-type electrodes.^13–16Metal–organic frameworks (MOFs), a porous crystalline material composed of metal nodes and organic linkers, have been widely applied in versatile fields including chemical sensors, catalysis, separation, biomedicine, and gained more and more attention in the area of energy storage.^17–25 Recently, two-dimensional (2D) MOFs have aroused great interest as a new kind of 2D materials.^26,27 Compared with traditional bulk MOFs, 2D MOFs possess distinctive properties, such as short ion transport distances, abundant active sites, and high aspect ratios, making them exhibit better performance than their bulk counterparts.^28–32 Bottom-up methods are generally adopted to prepare 2D MOFs with the addition of surfactants to control the growth of MOFs in a specific direction.^33–35 However, the use of surfactants inevitably blocks part of the active sites at the expense of the performance of materials. Therefore, it is highly necessary to explore and develop a direct solvothermal synthesis of 2D MOFs with the advantages of additive-free, simple operation, and easy scale-up.Herein, we report a facile one-pot solvothermal method to synthesize 2D Ni-based MOF microsheets (denoted as Ni-MOFms) by treating nickel chloride hexahydrate (NiCl₂·6H₂O, the metal precursor) together with the trimesic acid (H₃BTC, the ligand) in a mixed solvent of N,N-dimethylformamide (DMF), ethanol (EtOH) and H₂O. During the whole preparation process, only Ni precursor and the ligand are used while no surfactant is added. When used as active materials for a supercapacitor electrode, the obtained Ni-MOFms displayed excellent reversibility and rate performance. It also exhibited specific capacities of 91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively. Besides, they showed a good cycling performance in 5000 cycles with about 70% of the specific capacity and almost 100% of the coulombic efficiency maintained.Morphologies of the Ni-MOFms were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 1a and b, the Ni-MOFms were successfully fabricated via the facile one-pot solvothermal method with varying lateral sizes on the micron scale. Energy dispersive spectroscopy (EDS) mapping indicated that the obtained microsheets were mainly composed of C, O, and Ni. A trace amount of N was also observed, which could be attributed to the residual DMF in the mixed solvent (Fig. 1c). These elements were uniformly distributed throughout the whole microsheet. To measure the exact thickness of the Ni-MOFms, atomic force microscopy (AFM) was used. Fig. 1d showed that the thickness of the microsheet was about 58 nm. Considering the large lateral size, even such thickness could produce a relatively high aspect ratio, which is beneficial to the electrochemical performance.Open in a separate windowFig. 1(a) TEM image, (b) SEM image, (c) EDS mapping, and (d) AFM image and the corresponding height profile of the Ni-MOFms.The composition information of the Ni-MOFms was analyzed by X-ray diffraction (XRD) and the resulting diffraction pattern was shown in Fig. 2a. It was clear that the sample was a crystalline material. However, the exact structure was difficult to determine because no matching MOF structure has been found. Therefore, the structure of the Ni-MOFms was further confirmed by Fourier transform infrared spectroscopy (FT-IR). As shown in Fig. 2b, there was a sharp peak at 1721 cm⁻¹ for H₃BTC, which could be ascribed to the stretching vibration of C O in the nonionized carboxyl group.³⁶ For the Ni-MOFms, the peak at this location disappeared while four new peaks appeared. Bands at 1634 and 1557 cm⁻¹ were related to the asymmetric stretching vibration of carboxylate ions (–COO⁻) and peaks at 1433 and 1371 cm⁻¹ were the characteristic peaks of the symmetric stretching vibration of –COO⁻.^37,38 All these changes indicate that the ligand interacted well with the metal precursor.Open in a separate windowFig. 2(a) XRD pattern of the Ni-MOFms. (b) FT-IR spectra of H₃BTC and the Ni-MOFms.The chemical status and surface composition of the Ni-MOFms were further examined by X-ray photoelectron spectroscopy (XPS). From Fig. S1a^† we could see that the Ni-MOFms were composed of C, O, Ni, and N, which was consistent with the result of EDS mapping. High-resolution spectra of C 1s, Ni 2p, O 1s, and N 1s were shown in Fig. S1b–e.^† Characteristic peaks of C 1s at 288.27, 286.50, 285.85, and 284.80 eV were related to O C–OH, C–O, C–C, and C C, respectively, suggesting the presence of H₃BTC (Fig. S1b^†).³⁹ The Ni 2p spectrum showed two peaks at 873.32 and 855.77 eV, which could be ascribed to Ni 2p_1/2 and Ni 2p_3/2, respectively, together with two satellite peaks at 879.26 and 861.05 eV, verifying the existence of Ni²⁺ (Fig. S1c^†).⁴⁰ In the O 1s region, bands positioned at 532.94 and 531.40 eV could be ascribed to the adsorbed H₂O molecules on the surface of Ni-MOFms and typical metal–oxygen bonds, respectively, further corroborating the coordination between H₃BTC and Ni²⁺ (Fig. S1d^†).³⁹ Finally, the high-resolution spectrum of N 1s was also analyzed (Fig. S1e^†). There were two main peaks at 400.18 and 402.21 eV that could be ascribed to neutral amine and charged nitrogen, respectively,⁴¹ further proving the residual DMF on the Ni-MOFms surface.To explore the crucial factors in the formation process of the Ni-MOFms, the reaction time and temperature, the solvent, the ligand addition amount, and the ligand type were studied. As shown in Fig. S2,^† different crystalline materials were obtained at different reaction times. With the increase of reaction time, the material gradually changed from sphere to sheet. The reaction temperature is another crucial factor. At 120 °C, the material was amorphous and spherical. When the temperature rose, the crystal formed and appeared as microsheets (Fig. S3^†). The effect of solvent was illustrated in Fig. S4.^† Microsheets could not be synthesized in DMF or DMF with a small amount of EtOH. In the mixed solvent of DMF and H₂O, crystals could be prepared, indicating the vital role of H₂O. However, spheres existed in the products. Only when a mixture of DMF, EtOH, and H₂O with a certain proportion was used as the solvent, the Ni-MOFms could be obtained. Furthermore, we investigated the effect of the ligand addition amount. From Fig. 1 and S5^† we can see that the Ni-MOFms crystals formed when the molar ratio of Ni precursor and H₃BTC was 1 : 2 (Fig. 1). We speculated that ligands could simultaneously act as regulators to adjust the morphology of materials, avoiding the use of additional surfactants. When the ligand was replaced with 2-methylimidazole (2-MI) or terephthalic acid (H₂BDC), flower-like crystals rather than microsheets were obtained (Fig. S6^†), indicating the importance of the ligand type. Taking the above factors into account, we could finally determine the suitable conditions for preparing the Ni-MOFms (see the experimental section in ESI^†).The potential application of the Ni-MOFms in supercapacitors was first tested by cyclic voltammetry (CV) in 3 M KOH between 0 and 0.4 V (vs. saturated calomel electrode, SCE). As can be seen from Fig. 3a, all CV curves had similar shapes and the peak currents improved gradually as the scan rate increased, suggesting the good capacitive behavior of the Ni-MOFms electrode.⁴² When the scan rate was as high as 150 mV s⁻¹, redox peaks could still be observed, which indicated the excellent rate performance and kinetic reversibility.⁴³ Besides, as the scan rate went up from 10 to 150 mV s⁻¹, the reduction and oxidation peaks moved towards negative and positive potential, respectively, demonstrating the electrode polarization at large scan rates.⁴⁴Open in a separate windowFig. 3Electrochemical measurements of the Ni-MOFms. (a) CV curves at different scan rates. (b) GCD curves at various current densities and (c) corresponding specific capacities. (d) The EIS Nyquist plot at the bias potential of 0.4 V and the equivalent circuit model with the fitted plots (the red dots).The galvanostatic charge–discharge (GCD) behavior was further investigated to assess the coulombic efficiency and the specific capacity of the Ni-MOFms (see the ESI^† for detailed calculation method).^45,46 As shown in Fig. 3b, the shape of GCD curves was highly symmetric during charging and discharging, indicating that the coulombic efficiency of Ni-MOFms was almost 100% at various current densities. The specific capacities of 91.4, 78.4, 71.4, 64.0, and 60.0 C g⁻¹ were achieved at current densities of 2, 4, 6, 8, and 10 A g⁻¹ (Fig. 3c), respectively, demonstrating the excellent rate capability with about 65.6% of the specific capacity maintained from 2 to 10 A g⁻¹. The specific capacity at 2 A g⁻¹ was comparable with or even superior to that of some MOF materials reported in the literatures (Table S1^†).^47–50The kinetics of the electroanalytical process was then investigated by electrochemical impedance spectroscopy (EIS). Fig. 3d showed the Nyquist plot of Ni-MOFms from 0.01 to 100000 Hz and the corresponding equivalent circuit model (inset) with the fitted plots. CPE was the constant phase element related to the double layer capacity.⁵¹ The equivalent series resistance was denoted by R_s and its value obtained from the x-axis intercept was about 2.1 Ω, indicating the low resistance of the solution.⁴³R_ct represented the charge-transfer resistance at the interface of the electrode and electrolyte.⁵² For Ni-MOFms, the value of R_ct was up to 147.1 Ω, which could be attributed to the poor conductivity of MOF materials.The long-term stability of Ni-MOFms was also explored by charging–discharging at 10 A g⁻¹ for 5000 consecutive cycles. From Fig. 4 we could see that the specific capacity retention remained about 70% after 5000 cycles and the coulombic efficiency was maintained at almost 100% throughout the whole process. Furthermore, the inset in Fig. 4 exhibited that the GCD curves of the last 10 cycles were the same as the first 10 cycles, indicating excellent cycling stability.Open in a separate windowFig. 4Cycle property of Ni-MOFms at 10 A g⁻¹. Inset: GCD curves of the first 10 cycles (left) and the last 10 cycles (right).     

Circularly polarized light modulated supramolecular self-assembly for an azobenzene-based chiral gel

UV light-triggered trans-to-cis isomerization of azobenzene usually results in the collapse of a self-assembly system owing to the breaking of molecular planarity. Interestingly, two totally opposite self-assembly trends have been detected when a C_2v-symmetric chiral gelator was irradiated by a circularly polarized light (CPL) with specific handedness, indicating that CPL could become a powerful tool in modulating the assembly behaviour of the photo-responsive system.

An unconventional supramolecular self-assembly triggered by left-handed circularly polarized light breaks the traditional knowledge of azobenzene photoisomerization.

Modulating the self-assembling process and its nano-architecture by external stimuli has long been a challenging topic in supramolecular self-assembly.¹ Research ranging from the fabrication of responsive supramolecules to the dynamic control of self-assembly behaviors has promoted broad and interesting applications in nanotechnology,² electronics,³ tissue engineering⁴ and biomedical fields.⁵ Numerous exterior factors could affect self-assembly behaviors, such as temperature, solvent, magnetic field and light irradiation.⁶ Among these stimuli, light is considered to be remotely and accurately controlled, quickly switched and easily focused⁷ thus has attracted great interest in the construction of photo-responsive assemblies and devices.Mainstreams of photo-responsive systems commonly focus on natural light. With further studies, investigations have been extended to polarized light, streams of photons with either right- or left-handed spin, which can transfer integer photonic spin to molecules. Such properties have endowed CPL as the inherent chiral light and has been regarded as the possible source of chiral information in living organisms.⁸ As exciting examples, CPL-driven absolute asymmetric (AAS) and mirror-symmetry breaking (MSB) have been actively investigated in recent years;⁹ nanoparticles with chiral structures or helical arrangements could be generated by the CPL irradiation.¹⁰ By comparison, the effect of CPL handedness on small molecule self-assembly is still with less exploration, especially those with photo-responsive capacities. In general, trans-azobenzene based gelators have strong self-assembly capacities owing to their favourable planar and symmetric structures. UV-light irradiation-induced trans-to-cis transition normally will break such molecular symmetry and lead to the collapse of self-assembled structures. Interestingly, in this study, we find that the introduction of CPL with specific handedness breaks this traditional idea (Fig. 1a). For an azobenzene-based chiral gel with C_2v-symmetric structure, right-CPL promotes the collapse of gel; by contrast, left-CPL triggers the formation of a new self-assembled structure, and macroscopic gel is well maintained. This unconventional finding affords a new pathway to the fabrications of photo-responsive devices.Open in a separate windowFig. 1(a) Illustration of different supramolecular self-assembly modulated by CPL handedness, right-CPL promotes the collapse of ordered structure, but left-CPL triggers the formation of new helical structure. (b) Chemical structure of 4,4′-azobenzene-linked dipeptide gelator, abbreviated to Azo-DF.Chemical structure of our chiral gelator, 4,4′-azobenzene-linked l-aspartate-l-phenylalanine methyl ester (abbreviated to Azo-DF), is shown in Fig. 1b. The 4,4′-di-substituted azobenzene, which supplies a C_2v-symmetric skeleton, functions as a photo-responsive group. While two l,l-DF dipeptides are linked to the two sides of azobenzene owing to their satisfactory self-assembly capacities.¹¹ Synthesis process and characterization data of Azo-DF are described in the ESI.^† An additional control experiment and the possible self-assembly modes calculated from quantum chemical calculation indicated that π–π stacking between adjacent phenyl groups or between azo-benzene groups might be the driving force for the supramolecular self-assembly of Azo-DF (Fig. S3 and S4 in ESI^†). Owning to the excellent gelation capacity in chloroform/methanol (v/v = 1 : 3),¹² our following research mainly focused on this mixed solvent (Table S1 in ESI^†). The CPL pumping platform was consisted of an LED-UV (365 nm) light torch (SCOUT UVFLUXS-3W, 20 mW cm⁻²), a Glan–Thompson prism (200–900 nm) and a quarter-wave plate (Fig. 2a). Details on the CPL pumping platform are described in Part S4 in ESI.^† Because of subtle changes under 3 mW cm⁻² (distance (D): 45 cm) light irradiation and too swift response to catch under 10 mW cm⁻² (D: 10 cm) irradiation, 5 mW cm⁻² (D: 32.5 cm) was regarded as the optimal light intensity in the following experiments. Meanwhile, the light intensities of left- and right-CPL were identical (Fig. S5b and c in ESI^†).Open in a separate windowFig. 2(a) Photo of the CPL pumping system. F1: Glan–Thompson prism; F2: quarter-wave plate; s: gelator sample. (b) UV-vis spectra of Azo-DF in chloroform (0.05 mg ml⁻¹) before (dotted line) and after (solid lines) CPL illumination for 30 min. (c) Gel–sol transition of Azo-DF in chloroform/methanol (v/v = 1 : 3, 2 mg ml⁻¹), the gel collapsed under right-CPL irradiation, compared to only few solvent squeezing out from the gel under left-CPL irradiation. (d–f) Scanning electronic microscopy (SEM) images of Azo-DF xerogel before (d) and after 3 h irradiation of right-CPL (e) or left-CPL (f). (g and h) Circular dichroism (CD) spectra of Azo-DF gel in chloroform/methanol (v/v = 1 : 3, 2 mg ml⁻¹) before and after the irradiation of right-(g) or left-CPL (h). (λ: 365 nm, 5 mW cm⁻²).First, the photo-responsiveness of Azo-DF triggered by right- or left-CPL was detected by UV-vis absorption spectra. Azo-DF gelator was dissolved in chloroform at a low concentration of 0.05 mg ml⁻¹, in which Azo-DF was in isolated state and supramolecular self-assembly would not happen. Initially, the UV-vis spectrum of Azo-DF consisted of a strong UV band with a maximum absorption peak at 326 nm, which could be ascribed to the π–π* transition and corresponds to the vibrational structure of the typical trans-azobenzene (Fig. 2b). After irradiation of right- or left-CPL, two well separated bands in the UV region (λ_max ∼ 290 nm) and visible region (λ_max ∼ 440 nm) increased, representing the π–π* and n–π transition of cis-azobenzene structure, respectively.¹³ The photo-stationary state was reached after 30 min right- or left-CPL irradiation, revealing that the typical trans- to cis-transformation of azobenzene in Azo-DF had indeed happened. It is worth noting that the amount of range caused by CPL handedness were almost equal within the error range. A consistent tendency was observed when the individual azo-benzene was irradiated by the left- or right-CPL (Fig. S6 in ESI^†).Then the gelator concentration was increased to 2 mg ml⁻¹ in chloroform/methanol (v/v = 1 : 3), allowing the gel formation. The distinct gel–sol transition degree caused by the CPL handedness on macroscopic gel was observed. As shown in Fig. 2c, when the gel was exposed to the right-CPL irradiation, the gel showed an obvious tendency to loose and collapse after 3 h irradiation, remaining some fluffy aggregates in the 30 μL solvents. By comparison, Azo-DF gel maintained stable in the gel state after 3 h of left-CPL irradiation, only a few solvents (12 μL) were squeezed out. A similar solvent squeezed out situation of azobenzene gel, where a stable layered superstructure after UV light irradiation was claimed to be responsible, was reported by Jeong.¹⁴ Scanning electronic microscopy (SEM) were utilized to observe the gel morphology before and after 3 h of right- or left-CPL irradiation, respectively (Fig. 2d–f). Many long and regular ribbon-like fibres were observed. These fibres intertwined with each other and formed a dense three-dimensional (3D) network, revealing strong self-assembly capacities and linear packing pattern of Azo-DF. Large-scaled fibre distribution and enlarged view of a single fibre are shown in Fig. S7 in ESI.^† After 3 h of right-CPL irradiation, the ribbon-like fibres were divided into numerous short stubs that scattered and piled up randomly, corresponding to the collapse of the gels. While after 3 h left-CPL irradiation, the intertwined ribbon-fibres were well-preserved. The arrangement of fibres became slightly loose, which explained why there some solvents were squeezed out.Circular dichroism (CD) spectroscopy was used to monitor this self-assembly process (Fig. 2g and h). Initially, there are two weak peaks at 260 and 330 nm in the CD spectrum, which could be attributed to the molar chirality of two dipeptide arms carried by Azo-DF gelator itself. Such weak peaks were consistent with the morphology observed by SEM, where no evidential helical structure was detected. When the Azo-DF gel was irradiated with right-CPL, fluctuations around the baseline were observed in the CD spectra. But with the left-CPL irradiation, the CD spectrum (Fig. 2h) shows negative and positive cotton effect bands at 314 and 385 nm, respectively. These bands enhanced remarkably after 3 hours of the left-CPL irradiation, indicating the appearance of new helical structure during self-assembly process under the left-CPL irradiation,¹⁵ although such helical structure was not a common scenario.X-ray powder diffraction (XRD) was implemented to further reveal the stacking structures of the xerogel. Fig. 3a shows XRD spectrum of Azo-DF xerogel before CPL irradiation, multiple sharp peaks indicated long-range molecular packing and high crystallinity of the gel structure.¹⁶ After r-CPL irradiation for 3 h, the measured XRD signals sharply diminished to bottom line (Fig. 3b), which indicated that the trans-to-cis transition of the azobenzene obviously decreased the planar symmetry of the gelators, destroying the highly ordered stacking of the gelators. This phenomenon were well consistent with the morphological changes observed by SEM. By comparison, for the gel sample after left-CPL illumination for 3 h, the crystal order was well maintained except some slight decreases in the XRD intensity. In addition, several new diffraction peaks are observed in Fig. 3c. Specifically, the peak A (2θ = 7.81°) splits into two bands, while two new peaks (B′ and C′) are found at 2θ = 1.66°, 7.90°. In wide-angle region (2θ = 18–25°) the position of diffraction peak appears a shift,¹⁶ while the peak D splits into two peaks and the peak E decreases remarkably. These data clearly suggested that new crystalline state came into being after left-CPL illumination, moreover, the decomposition of the self-assembled structure might be largely postponed by left-CPL.Open in a separate windowFig. 3XRD spectra of Azo-DF xerogels before and after 3 h irradiation of right- or left-CPL, light intensity: 5 mW cm⁻².Unambiguous evidence for CPL handedness-dependent supramolecular self-assembly was provided by atomic force spectroscopy (AFM). Optimization experiments indicated that the Azo-DF could form highly ordered self-assembled pattern at a concentration of 0.5 mg ml⁻¹ in chloroform. So we prepared AFM sample by dripping one drop of Azo-DF solution (0.5 mg ml⁻¹, chloroform) before and after right- or left-CPL irradiation onto freshly cleaved mica, respectively. As shown in Fig. 4a, the initially self-assembled pattern of Azo-DF presents as numerous short nanofibers evenly distributed in three directions, the average length, width and height of these nanofibers are 2.4 μm, 80 nm and 25 nm, respectively. Upon the irradiation of right-CPL for 2 h, the three-direction short nanofibers are replaced by some long and flattened ribbons with average length of more than 10 μm and height of 30 nm (Fig. 4b). After 3 h right-CPL irradiation, the self-assembled structure was hard to be detected (Fig. 4c). Such obvious change could be reasonably attributed to the weak self-assembled capacity of the gelator in a cis-azobenzene form, which dominated the configuration of the gelator. In comparison, when Azo-DF was irradiated by left-CPL for 2 h (Fig. 4d), rather than decomposition, those short nanofibers showed a strong tendency to aggregate, the fiber length and width were estimated to be 3.8 μm and 40 nm. A detailed enlarged view (Fig. 4e) of 1 μm shows that these nanofibers intertwisted with each other to form a rope like structure. More interestingly, when the Azo-DF solution was exposed to left-CPL for 3 h (Fig. 4f), a dendritic structure was observed featured with an aggregate center over 150 nm height and numerous branches of nearly 50 nm height. The aggregation difference resulted from the CPL handedness was also monitored by dynamic light scattering (DLS) measurement (Fig. S8 in ESI^†). The distinct self-assembly difference was well consistent with the above gel morphological changes. At the same time, the subsequent fiber growth upon left-CPL irradiation further demonstrated a special staking model of Azo-DF gelators where the supramolecular self-assembly was successfully inverted by exchanging the handedness of CPL. A nearly reversed CPL-modulated self-assembly trend was detected by using Azo-d,d-DF (Fig. S9 in ESI^†). Considering Azo-l,l-DF and Azo-d,d-DF were not completely mirror symmetric owing to the four chiral centers, this result provided an auxiliary proof that the distinct self-assembly behaviors of the gelators were caused by CPL handedness. Compared to natural light, the introduction of CPL significantly emphasized the quantum characteristic of light orientation,^8a where the delicate balance between the isomerization of azobenzene and the chiral preference of the dipeptide played an important role in the dissociation or supramolecular self-assembly of Azo-DF gelators.^13b,17 The possible mechanism is described in Part S6 in ESI.^†Open in a separate windowFig. 4AFM images of the self-assembled morphologies and the corresponding section profiles of Azo-DF in chloroform (0.5 mg ml⁻¹) before (a) and after 2 h (b) or 3 h (c) of right-CPL irradiation; (d–f) Azo-DF after 2 h (d and e) or 3 h (f) of left-CPL irradiation.In conclusion, we observed an unconventional chiral effect: azobenzene-based chiral gelator showed a collapse tendency under the right-CPL irradiation, by contrast, intensive self-assembly emerged when the gelator was irradiated with left-CPL. Such CPL handedness-triggered self-assembly difference might arise great interest in the potential of CPL sources in controlling the supramolecular self-assembly of chiral molecules or polymers as well as for exploring their roles in the fabrication of function materials, moreover, inspiring rethought of some vital chemical and biological processes from the unique perspective of “dynamic molecular chirality”.     

Microbubble flows in superwettable fluidic channels

The control of bubble adhesion underwater is important for various applications, yet the dynamics under flow conditions are still to be unraveled. Herein, we observed the wetting dynamics of an underwater microbubble stream in superwettable channels. The flow of microbubbles was generated by integrating a microfluidic device with an electrochemical system. The microbubble motions were visualized via tracing the flow using a high-speed camera. We show that a vortex is generated in the air layer of the superaerophilic surface under laminar conditions and that the microbubbles are transported on the superaerophilic surface under turbulent conditions driven by the dynamic motion of the air film. Furthermore, microbubbles oscillated backward and forward on the superaerophobic surface under turbulent conditions. This investigation contributes to our understanding of the principles of drag reduction through wettability control and bubble flow.

Microbubble flows inside a superwettable channel revealed underwater superwetting phenomena under flow conditions, contributing to the understanding of real-world environmental wetting systems.

Nature offers us ideas for the design of materials with superwettability.¹ In superwettable systems, the wetting of air underwater has generated interest recently.^2–6 For example, penguin feathers are superaerophilic, with an air layer forming on the surface underwater, which allows penguins to swim in the sea with small amounts of drag.^2,3 Inspired by this, researchers have theoretically and/or experimentally studied the influence of wettability on drag reduction underwater.^4–6 In addition, fish scales are superaerophobic, which offers the idea of designing no-bubble adhesion electrodes that demonstrate high and stable oxygen evolution reaction performance.^7,8 However, despite the development of superwettable materials for the controllable adhesion of air and/or bubbles underwater,⁹ the wetting dynamics of bubbles under flow conditions, which we must consider in real environments, have not been investigated.Herein, we generated microbubble flows parallel to superwettable substrates inside a microfluidic device^10,11 and studied the wetting dynamics through integrating an electrochemical setup¹² with a microfluidic device, as shown in Fig. 1. The bubbles were formed through the electrolysis of water (see the ESI^†). Two platinum plates were used: one as the working electrode and the other as the counter electrode. To increase the electrical conductivity, 2.0 mM K₂SO₄ was added to the water. The bath water–vapor interfacial tension, γ_LV, was 71.9 ± 3.6 mN m⁻¹ (n = 15) and the pH of the water was 7.8. We applied a current of ∼0.25 mA cm⁻² to generate microbubbles with a diameter of 463.9 ± 245.1 μm (n = 120). The microfluidic device was generated using a 3D printer and connected to a water-flow generator (see the ESI^† for the dimensions of the device). The microbubbles generated around the electrodes moved in the direction of the water flow and the coated substrates were placed parallel to the flow.Open in a separate windowFig. 1A schematic illustration of the microfluidic device with an electrochemical setup. We generated a flow of microbubbles and investigated the influences of coating wettability and flow type on the microbubbles dynamics via high speed camera observations. Scale bar: 10 mm.We used the microbubbles as tracers and analyzed their flow as well as that of the water (i.e. microbubble image velocimetry), as shown in Fig. 2. We controlled the Reynolds number, Re = 4Q(πDν)⁻¹ (Q is the flow rate of the water, D is the tube diameter, and ν is the kinetic viscosity of the water). Laminar flow was obtained at Re = 79.21 and turbulent flow was obtained at Re = 396.06 (Fig. 2A). Under laminar flow conditions, the flow speed was nearly constant and the flow direction was close to perpendicular to the substrate (φ ≈ 0, where φ is the angle between the microbubble direction of movement and the width direction of the substrate) in all areas; this behavior was time-independent (Fig. 2B and C). Under turbulent flow conditions, the flow speed was not constant, and the flow direction was unstable (φ fluctuated between −180 and 180°) in all areas. We confirmed that the separation of flow did not occur, at least during the observation period, since the flow direction was parallel to the superwetting microfluidic device.Open in a separate windowFig. 2The flow conditions of the microbubbles. We created laminar and turbulent flows through altering the Reynolds number. (A) Flow velocimetry of the microbubbles under laminar (left) and turbulent (right) flow conditions; scale bar: 10 mm. (B) Velocity and flow direction profiles of microbubbles over the flow area under laminar (left) and turbulent (right) flow conditions. (C) Average velocity and flow direction fluctuations with time under laminar (left) and turbulent (right) flow conditions.We then prepared substrate coatings with superaerophilicity and superaerophobicity. Superaerophilic substrates were fabricated according to our previous study.¹² Concisely, a glass plate was dip-coated with a mixture of zinc oxide micro-tetrapod powder for surface roughening and polydimethylsiloxane for aerophilization. Superaerophobic surfaces were prepared through modifying a glass substrate with hydroxy groups using an aqueous potassium hydroxide solution.¹³ The wettability of the superaerophobic surfaces in relation to bubbles was confirmed via measuring the underwater bubble contact angle (θ); the results are shown in Fig. 3. We calculated the adhesion forces of bubbles, F_adh = πl²γ_LV(1 + cos θ)/4,¹⁴ where l is the bubble–solid adhesion length. On the superaerophilic surface, the adhesion force was 3.2 × 10³ μN, and on the superaerophobic surface the force was 4.37 μN for 6 μL bubbles.Open in a separate windowFig. 3Wettability of the coatings. (A) schematic illustration of the measurement of the underwater air contact angle. The contact behavior of 6 μL microbubbles underwater on superaerophilic (B) and superaerophobic (C) surfaces.In Fig. 4, we observed air film formation on superaerophilic surfaces under laminar and turbulent flow conditions. As we have previously shown, when microbubbles are vertically deposited on superaerophilic surfaces, a uniform air layer is formed.¹⁰ In the present study, under both laminar and turbulent flow conditions, a uniform air layer formed on the superaerophilic surfaces, but the air layers grew non-uniformly with the deposition of microbubbles owing to Rayleigh–Taylor instability¹⁴ (Fig. 4A and B). In all five independent observations, the shape of the air layer was non-uniform; thus, the flow of microbubbles influenced the shape of the air layer. However, bubbles with l = 4–7 mm formed on the surfaces under both laminar and turbulent flow conditions.Open in a separate windowFig. 4Microbubble deposition behavior on a superaerophilic surface under laminar (A) and turbulent (B) flow conditions. The top and bottom parts of the images are the initial and time-aged stages, respectively. (C) The vortex motion of microbubbles on deposited air films under laminar flow conditions. (D) Microbubbles transported on a superaerophilic surface under turbulent conditions driven by the dynamic motion of the air film. (E) and (F) Schematic representations of the microbubble behavior from (C) and (D), respectively. All scale bars: 10 mm.After aging for 1000 s, a continuous air film formed on the superaerophilic surfaces under turbulent conditions. However, the shape was unstable and changed with time (Fig. 4D). In Fig. 4C and E, we observe the formation of a vortex on the hemispherical air film under laminar flow conditions (see Movie S1^†). This phenomenon is interesting because under laminar flow conditions a vortex should not be generated (Fig. 2A); this cannot be explained using Bernoulli''s theorem¹⁵ and the generation of a vortex suggests the separation of flows, which works to decrease flow resistance at the interface. Vortex generation may be due to the coalescence of microbubbles with the air layer, causing a change in the curvature of the hemispherical air film. This, in turn, would result in a change in the Laplace pressure of 2Δκγ_LV, where Δκ is the change in curvature. There is a fluctuation in the vertical force torque to generate the vortex, and the force should be balanced by a Kutta–Joukowski force in the form of 2γ_LV dκ/dt ≈ ρΓU, where ρ is the density of flows, Γ is the vortex constant, and U is the velocity of the constant laminar flow.¹⁶In Fig. 4D and F, we observe that microbubbles on the air film were transported as the shape of the air film dynamically changed to a wave-like nature; however, the microbubbles and air film did not coalesce (see Movie S2^†). This indicates that a thin water layer exists between the microbubbles and the air film to prevent coalescence, whereas microbubbles are trapped on the air film by the buoyancy force of the microbubbles, which ≈(Δρ)Ωg, where Δρ is the difference in densities between a bubble and water, Ω is the volume of a microbubble, and g is gravitational acceleration.We then observed the dynamics of the microbubbles on the superaerophobic surfaces (Fig. 5). As we have previously shown, when microbubbles are vertically deposited on superaerophobic surfaces, they are uniformly deposited on the surface and have a spherical shape.¹² Under both laminar and turbulent flow conditions, microbubbles were deposited on the superaerophobic surfaces with spherical shapes but with non-uniform deposition (Fig. 5A and B). We then observed the motion of bubbles in contact with the superaerophobic surfaces. Under laminar flow conditions, microbubbles adhering to the surface moved in the direction of the flow (Fig. 5C and Movie S3^†). In contrast, turbulent flow conditions caused the microbubbles to oscillate backward and forward (Fig. 5D and Movie S4^†). The velocimetry profiles in Fig. 5E and F confirm that the bubble motion is linear in time under laminar flow, but it varies under turbulent flow (with the velocity periodically becoming negative). Despite the periodic negative velocity under turbulent flow conditions, the bubbles go forwards in the flow direction, which is not due to the laminar boundary but because the turbulent flow has more positive components than negative ones. This is because the length of positive motion under turbulent flow conditions increases with the size of the bubbles, obeying Newton''s viscosity law.¹⁷ Thus, we confirmed that the motion of bubbles on superaerophobic surfaces is influenced by the flow conditions. The bubble motion distance on superaerophobic surfaces increased with bubble diameter.Open in a separate windowFig. 5Microbubble deposition behavior on a superaerophobic surface under laminar (A) and turbulent conditions (B). The top and bottom parts of the images are the initial and time-aged stages, respectively. (C) Linear motion of the microbubbles on the surface under laminar flow. (D) The oscillating motion of microbubbles on the surface under turbulent flow. (E) Motion distance and (F) velocity analysis of microbubbles under laminar (left) and turbulent (right) conditions for different bubble diameters (2R). All scale bars: 10 mm.     

Metallo-supramolecular polymers derived from benzothiadiazole-based platinum acetylide complexes for fluorescent security application

Metallo-supramolecular polymers with the incorporation of benzothiadiazole-substituted organoplatinum moiety have been successfully constructed. The designed monomer displays intense fluorescence signals, which are severely quenched upon the supramolecular polymerization process. On–off switching of fluorescence can be further exploited for data security materials in response to the chemical stimuli. Accordingly, the resulting supramolecular polymers can be regarded as a novel and efficient candidate toward information processing applications.

Metallo-supramolecular polymers with the incorporation of benzothiadiazole-substituted organoplatinum moiety have been successfully constructed.

Metallo-supramolecular polymers (MSPs), which represent polymeric assemblies constructed by reversible metal–ligand coordination, are considered as an important class of organic/inorganic hybrid supramolecular materials. Owing to the incorporation of metallic complexes in the polymer chain, these macromolecular metal-containing systems not only possess the properties of traditional organic polymers (viscosity, processability, etc.), but also exhibit redox, optical, electrochromic, catalytic and magnetic properties.^1–5 In addition, due to the incorporation of reversible and weak recognition moieties on the supramolecular polymeric backbones, MSPs show unique stimuli-responsive characters, which is essential to develop environment-adaptable materials.^6–9The structure of MSPs and their physical properties can be elaborately regulated by the coordination ligand motifs.^10,11 Terpyridines and their structural analogs are the most popular chelating end-groups to form MSPs, ascribed to their capability to complex with a variety of transition metals.^10,12–16 Furthermore, the linkage on the polytopic ligand is also of crucial importance, since it dictates the structure arrangement and physicochemical properties of the targeted MSPs assemblies.¹⁰ Up to now, a variety of π-conjugated organic chromophores have been incorporated into the backbone of MSPs.^17–22 In stark contrast, the employment of π-conjugated organometallic units as the linkages has been far-less exploited.²³In this work, we sought to attain this objective, and construct a new type of MSPs with the involvement of platinum acetylide linkage. The monomeric structure is showed in Scheme 1: terpyridine moieties were incorporated on both sides of the monomer, in which the rigid benzothiadiazole-functionalized dinuclear platinum(ii) acetylide moiety serves as the linkage unit. Herein, the platinum(ii) acetylide derivatives were chosen as the linkages based on the following two considerations. First, platinum(ii) acetylide unit features the intriguing photo-physical properties such as larger stokes shifts and higher photoluminescence quantum yields, which are primarily arised from the overlapping of d-orbitals of the transition metal with p-orbitals of the alkyne ligands.^24–26 As a result, it endows the resulting supramolecular polymers with the fascinating optical and electrochromic properties.^27–31 Second, the rigid linkage incorporated in the monomer structure restrains the tendency for cyclization. The reduced critical polymerization concentration (CPC) value promotes the linear supramolecular polymerization process.^32–35 In the meantime, due to the dynamic properties of metal–ligand interactions, the resulting supramolecular polymer was anticipated to possess the stimuli-responsive properties, which display the potential applications as the smart materials.^36–39Open in a separate windowScheme 1Schematic representation for the formation of metallo-supramolecular polymer 1.The synthetic route for monomer is quite straightforward. As shown in Scheme S1,^† Sonogashira coupling reaction between terpyridine and benzothiadiazole-functionalized dinuclear platinum(ii) acetylide moieties was employed as the key step to construct the designed monomer. All of the synthetic compounds were fully characterized with NMR and ESI-MS spectra (Fig. S1–S4, ESI^†). The introduction of dinuclear platinum(ii) acetylide unit is expected to achieve low critical polymerization concentration (CPC) value for the supramolecular polymerization process. As a consequence, it facilitates to fabricate fully rigid supramolecular polymers with intriguing optical and electrochromic properties.Non-covalent complexation between the terpyridine-contained monomer and metal ion Zn²⁺ was first investigated via the spectroscopic measurements. For the monomer itself, it exhibits two absorption bands with the maximum wavelength located at 358 nm and 477 nm, respectively (Fig. 1a). The high-energy absorption at 358 nm is mainly derived from the π–π* intraligand transitions. For the low-energy absorptions at 477 nm, it could be ascribed to the interplay between the π-conjugated benzothiadiazole acetylene ligand and the transition metal Pt²⁺.²⁸ With the gradual addition of Zn²⁺ to the monomer in CHCl₃/CH₃OH (2 : 1, v/v), an obvious decrease absorbance at 370 nm was observed (Fig. 1a), indicating the transformation from free terpyridine species to the metal–terpyridine complex.³⁸ Upon excitation of monomer at 420 nm, the relative quantum yield is determined to be 3.5%, while the emission lifetime is 1.3 ns (Fig. S7^†). As shown in Fig. 1b, with the stepwise addition of the Zn²⁺, the fluorescence of monomer gradually decreases, and reaches the minimum at 602 nm when the ratio of monomer/Zn²⁺ achieves to 1.0. Also, the 1 : 1 complexation was validated by the ITC experiment (Fig. S5^†).Open in a separate windowFig. 1The intensity of changes (a) absorbance at λ = 370 nm and (b) emission intensity at λ = 602 nm upon addition of Zn(OTf)₂. Insets: arrow shows (a) UV/Vis absorption and (b) emission spectral changes of monomer (5.0 × 10⁻⁵ M) upon addition of Zn(OTf)₂.As widely documented, Zn²⁺ could complex with terpyridine to form two kinds of metallic complex species, Zn(tpy)₂²⁺ and Zn(tpy)²⁺.²² However, UV/Vis and fluorescence measurements could not distinctly distinguish such types of complex. In this regard, ¹H NMR titration measurements were employed to get further insights into the exchange kinetics of Zn²⁺–terpyridine complexes. As shown in Fig. 2, when the feed-ratio of Zn²⁺/monomer is below 1.0 (Fig. 2a–d), the terpyridine proton H₁ exhibits the remarkable upfield shift from 8.74 to 7.80 ppm due to the shielding effect, while both H₃ and H₅ exhibit obviously downfield shifts (H₃: from 7.92 to 8.17 ppm; H₅: from 8.60 to 8.95 ppm). Upon adding 1.0 equivalent Zn²⁺, the original uncomplexed terpyridine signals totally disappeared, suggesting the formation of dimeric Zn(tpy)₂²⁺. However, once Zn²⁺/monomer ratio is further increased, the newly signals evolved and gradually strengthen, suggesting the formation of Zn(tpy)²⁺.³⁸Open in a separate windowFig. 2Partial ¹H NMR spectra (300 MHz, CDCl₃: CD₃OD (2/1, v/v)): (a) monomer, (b–f) gradual titration of Zn²⁺ into monomer.After confirming the non-covalent complexation behavior between Zn²⁺and monomer in relatively low concentration as the monomeric state, the supramolecular polymerization process was further studied via concentration-dependent ¹H NMR measurement. According to Fig. S8,^† for the 1 : 1 mixture of monomer and Zn(OTf)₂ at 2.0 mM, the aromatic protons on terpyridine moiety show one set of well-defined sharp signals, implying the dominance of oligomers (Fig. S8a^†). In sharp contrast, the broadening of all signals at high concentration of monomer suggests the formation of supramolecular polymer (Fig. S8e^†). Such conclusion could be also validated by two-dimensional diffusion order spectrum experiment (DOSY), which is a convenient and efficient technique to monitor the size variation of the dynamic aggregations. When the monomer concentration increased from 0.5 to 30.0 mM, the measured diffusion coefficients of metallo-supramolecular polymer 1 decreased remarkably from 2.81 × 10⁻¹¹ to 8.91 × 10⁻¹² m² s⁻¹ (Fig. S9^†), implying the formation of large-sized supramolecular polymeric assemblies at high concentration.Next, capillary viscosity measurements were performed to study the macroscopic properties of the resulting supramolecular assemblies. All of the viscosity studies were performed in DMSO containing 0.05 M tetrabutylammonium hexafluorophosphate to exclude the polyelectrolyte effect.³⁹ As shown in Fig. 3a, the monomer shows the comparably shallow curve for the specific viscosities. In sharp contrast, the specific viscosity of 1 : 1 mixture of Zn²⁺ and monomer changes exponentially as a function of monomer concentration, revealing the formation of high-molecular-weight supramolecular polymer.Open in a separate windowFig. 3(a) Specific viscosities of metallo-supramolecular polymer 1 (red) and the corresponding monomer(blue), (b) double logarithmic plots of specific viscosity of metallo-supramolecular polymer 1versus monomer concentration.The double logarithmic plots of versus specific viscosity concentration were further obtained for 1 (Fig. 3b), which displays a clear slope change. In the low concentration range, the slope value was determined to be 1.03, which indicates the presence of oligomers with the constant size. Remarkably, a sharp rise in the viscosity was observed when the concentration exceeds the critical polymerization concentration value (around 4.0 mM), and the slope value was estimated up to 1.40. Such phenomenon is in highly consistent with the aformentioned DOSY and ¹H NMR results, suggesting the formation of large supramolecular polymeric assemblies at high concentration.Stimuli-responsive properties of the resulting supramolecular polymer were further exploited, by manipulating the dynamic property of Zn²⁺–terpyridine recognition motif. It is well known that 1,4,7,10-tetraazacyclododecane (cyclen) exhibits higher affinity toward Zn²⁺ than terpyridine unit. With the addition of cyclen, the decrease of Zn²⁺/tpy absorption bands (λ = 477 nm), together with the increase of fluorescence intensity (λ = 602 nm), suggest that cyclen competitively complexes with terpyridine to destroy the Zn²⁺/terpyridine interactions (Fig. 4a, and S10^†). Such process could also be validated by capillary viscosity experiments, which is shown in Fig. 4b. With the stepwise addition of cyclen, the specific viscosity of the metallo-supramolecular polymer decreases and gradually levels off, suggesting the disassembly of metallo-supramolecular polymer. However, with the further addition of Zn²⁺, metallo-supramolecular polymer could be recovered. The conclusion could be manifested by the reappearance of Zn²⁺/tpy absorption band, the decrease of fluorescence intensity (Fig. S10^†), and the restore of the original ¹H NMR signals (Fig. S11^†). Hence, the successive addition of the competitive ligand cyclen and Zn²⁺ provide a possible method to manipulate the reversible assembly/disassembly of the metallo-supramolecular polymer.Open in a separate windowFig. 4(a) UV/Vis absorption spectral changes of metallo-supramolecular polymer 1 upon the successive addition of cyclen and Zn(OTf)₂. (b) Viscosity changes upon adding cyclen to metallo-supramolecular polymer 1.As mentioned above, the fluorescence intensity of resulting supramolecular polymers could be regulated by stepwise addition of cyclen and Zn²⁺. By taking advantage of the “on/off” fluorescent switching properties, we sought to explore their potential applications for data security. In detail, the solution of metallo-supramolecular polymer 1 was doped into polylactic acid (PLA) and formed a bright orange film (1.2 cm × 1.0 cm). The fluorescence intensity of resulting film is relatively lower, owing to the existence of Zn²⁺/terpyridine metal–ligand interactions. The resulting film could be used as a drawing board for the applications in document security, by employing the solution of cyclen as a magic ink (Fig. 5). In detail, when a letter “Z” was written on the surface of the drawing board, there have almost no changes for the film under daylight. However, the patterned security feature of “Z” encrypted with the solution of cyclen could be readily visualized under UV light, which could be ascribed to the disassembly of Zn²⁺–terpyridine complexes. Moreover, the letter “Z” could also be wiped by immerging the film into the Zn²⁺ solution and the drawing board could be recovered.Open in a separate windowFig. 5Left was irradiated by day light, and right was irradiated by 365 nm. (a) Metallo-supramolecular polymer 1 is doping in the PLA film, (b) cyclen solution is further coated on film, (c) the film is recovered with Zn(OTf)₂ solution.In summary, we have constructed the rigid metallo-supramolecular polymer, by using metal–ligand interactions as the non-covalent connecting bonds. Benzothiadiazole-functionalized dinuclear platinum(ii) acetylide moiety was incorporated, rendering fascinating photo-physical properties to the resulting monomer and supramolecular polymeric assemblies. ¹H NMR, UV/Vis, ITC, DOSY, and viscosity measurements were employed to investigate the supramolecular polymerization process. Additionally, the metallo-supramolecular polymer 1 could be reversibly assembled and disassembled upon adding the competitive ligands. By taking advantage of the fluorescence “on/off” switch process, the resulting supramolecular polymer could be used as fluorescent security materials. Therefore, the current work provides a convenient and efficient approach to fabricate supramolecular polymer toward smart information processing materials.^40,41     

Electronic energy levels of porphyrins are influenced by the local chemical environment

Self-assembled islands of 5,10,15,20-tetrakis(pentafluoro-phenyl)porphyrin (2HTFPP) on Au(111) contain two bistable molecular species that differ by shifted electronic energy levels. Interactions with the underlying gold herringbone reconstruction and neighboring 2HTFPP molecules cause approximately 60% of molecules to have shifted electronic energy levels. We observed the packing density decrease from 0.64 ± 0.04 molecules per nm² to 0.38 ± 0.03 molecules per nm² after annealing to 200 °C. The molecules with shifted electronic energy levels show longer-range hexagonal packing or are adjacent to molecular vacancies, indicating that molecule–molecule and molecule–substrate interactions contribute to the shifted energies. Multilayers of porphyrins do not exhibit the same shifting of electronic energy levels which strongly suggests that molecule–substrate interactions play a critical role in stabilization of two electronic species of 2HTFPP on Au(111).

Self-assembled islands of 5,10,15,20-tetrakis(pentafluoro-phenyl)porphyrin (2HTFPP) on Au(111) contain two bistable molecular species that differ by shifted electronic energy levels.

The packing of two-dimensional self-assembled monolayers is facilitated by molecule–surface and molecule–molecule interactions. At low coverage, molecules with repulsive intermolecular interactions can adsorb at preferred surface-adsorption sites,^1–4 while at higher concentrations the preferred surface site may be sacrificed to increase density and maximize intermolecular interactions.^3–6 Surface adsorption can change the conformational structure of a molecule to maximize these interactions resulting in a conformational shape not found in bulk crystals or solvated molecules.^4,7–10 Rational manipulation of shape and local environment can fine tune the electronic properties of the adsorbate.Porphyrins consist of four pyrrole-like moieties connected with methine bridges. The high level of conjugation, with 18 π electrons in the shortest cyclic path, allow porphyrins to strongly absorb light in the visible region, 400–700 nm.^11–13 Porphyrins are known as the color of life molecule.^14,15 Heme, an iron–porphyrin complex, is responsible for transporting oxygen through the bloodstream and gives red blood cells their bright color.¹⁶ Chlorophyll, a partially hydrogenated porphyrin, is responsible for the green color in plants and plays a critical role in photosynthesis.¹⁶ The strong interaction with light, overall chemical stability, and diversity through functionalization makes pyrrolic macrocycles ideal candidates for use in photovoltaics,^17–21 chemical sensors,^22–25 catalysis,^26–28 and molecularly based devices.^29–32The existence of bistable switchable states is the first step towards use of porphyrins in applications of molecular memory storage or binary devices.³³ Pyrrolic macrocycles have been shown to exhibit bistable switching behavior on surfaces including induced conformational switching,^34–36 orientational flip-flopping,^37–41 and tautomerization.^42–46 Examples include subphthalocyanine arrays that adsorb with a shuttlecock shape and exhibit scanning tunneling microscopy (STM) induced reversible orientational switching on Cu(100),³⁷ and current from the STM tip that has been used to induce hydrogen tautomerization in naphthalocyanine on an NaCl bilayer on Cu(111).⁴²Confinement to two dimensions through adsorption of 2HTFPP onto a surface creates a complex heterogeneous environment with influences from the substrate and neighboring molecules. In this study, we characterize the local density of states of 2HTFPP adsorbed on Au(111), and found that the local chemical environment can cause the energies of molecular orbitals to shift. This creates an environment where chemically identical molecules exist as two species on the surface.The sample was prepared using an Au(111) substrate on mica (Phasis) that was cleaned through several rounds of annealing and argon sputtering. Images were collected at ultra-high vacuum and 77 K with a low-temperature scanning tunnelling microscope (LT-STM, Scienta Omicron) under constant current mode. The tips were mechanically etched 0.25 mm Pt80/Ir20 wire (NanoScience Instruments). The sample was prepared at room temperature by depositing 1 mM solution of 2HTFPP dissolved in dichloromethane via pulsed-solenoid valve in a vacuum chamber onto the clean gold or 2HTFPP dissolved in dimethylformamide via drop-casting and annealing to 200 °C.⁴⁷The electronic structure of 2HTFPP, neutral TFPP, and dianionic TFPP were calculated using density functional theory (DFT). The structures were optimised using the B3LYP^48,49 density functional approximation and the 6-311+G(d) basis set. A harmonic vibrational frequency analysis confirmed the structure was in a local minimum on the ground state potential energy surface without imaginary frequencies. All calculations were performed on Gaussian 16 Rev. A.03.⁵⁰ Simulated STM images were computed using the Tersoff and Hamann approximation^51,52 as done by Kandel and co-workers.^53,54 The tunneling current was modeled as a function of tip position, and the surface was modeled as a featureless and constant density of states to reproduce a constant current experiment. Through the Tersoff and Hamann approximation the tunneling current leads to information on the local density of states at a specific energy and a discrete location.Pulse-depositing 2HTFPP in vacuum via a pulsed-solenoid valve resulted in ordered islands of the adsorbate, Fig. 1. The formation of close-packed islands, despite submonolayer coverage, indicates attractive intermolecular interactions between adsorbates. 2HTFPP has a packing density of 0.64 ± 0.04 molecules per nm² with a 1-molecule unit cell and rectangular packing.Open in a separate windowFig. 1(a–c) Unoccupied and (d–f) occupied electron states of 2HTFPP. All scale bars are 5 nm. (e) Two bistable species, diffuse square outlined in green and a double-dot feature outlined in white. (f) 2HTFPP appears as double-dot features. The white box indicates the 1-molecule, rectangular unit cell. Images taken with a tunneling current of 5 pA.Changing the polarity of the bias voltage of the STM sample allows unoccupied (positive voltage) or occupied (negative voltage) molecular orbitals to be imaged along with the topography of the surface. The unoccupied electron states, Fig. 1a–c, have an even distribution of electron density across the molecule and appear as diffuse squares. As the magnitude of the bias voltage is increased, a larger number of electronic energy levels are included in the measurement. Increasing the magnitude of the negative bias voltage from −0.25 V to −1.5 V, Fig. 1d and f, results in a change in contrast from a diffuse square to a bright double-dot feature. Imaging at −0.75 V, an intermediate voltage between these two energy levels, we observe two species—a mixture of diffuse squares and double-dot features, Fig. 1e. These species are differentiated from one another by having the same ground state molecular orbitals at shifted energies. If all the molecules on the surface were equivalent, we would expect to image the same molecular orbitals of each molecule at each bias voltage. At −0.75 V we do not observe the same orbital shape for each molecule, but we observe influence of the HOMO−2 and HOMO−3 energy levels in approximately 60% of that adsorbed 2HTFPP. Interaction with the local chemical environment causes the energy levels to be shifted enough to be included at a bias voltage with a smaller magnitude.We investigated the possibility that the two species were chemically different. If a gold atom formed a complex with the porphyrin, we would expect the displacement of the inner pyrrolic hydrogens. It was also possible that the presence of the STM tip was altering the adsorbates. Smykalla et al. reported that the STM tip can reversibly remove and replace the pyrrolic hydrogens which changes the shape and contrast of the porphyrin in the STM images.⁵⁵ Deng and Hipps reported a small shift in orbital energies of 0.04 eV for vapor-deposited metallo-porphyrins on Au(111) due to the height of the tip.⁵⁶ We observed a march larger shift of orbital energies and used DFT to calculate shapes of the occupied and unoccupied molecular orbitals of gas phase 2HTFPP, TFPP with the inner pyrrolic hydrogens removed, dianionic TFPP with the inner pyrrolic protons removed, and Au-TFPP with a gold atom coordinated to the porphyrin, Fig. 1 in ESI.^† We used the electronic structure calculations to make theoretical STM images. We found that the double-dot feature was only present in the occupied orbitals of 2HTFPP, Fig. 2. Theoretical STM images of Au-TFPP, Fig. 2 in ESI,^† does not show the double-dot feature. This suggests that intact 2HTFPP molecules make up the close-packed islands, and both detected species are chemically equivalent. The shift in electronic energy levels is due to the heterogeneous local chemical environment.Open in a separate windowFig. 2DFT, theoretical STM images, and comparison with experimental STM images of 2HTFPP. HOMO and HOMO−1 have a diffuse square shape, while HOMO thru HOMO−3 exhibit the double dot feature. Both of these shapes were experimentally observed at −0.75 V.The packing of adsorbates is a balance between molecule–molecule and molecule–substrate interactions. We observed various packing densities and unit cells that were dependent on sample preparation. Preparation with the pulsed-solenoid valve at room temperature created a 1-molecule unit cell with a packing density of 0.64 ± 0.04 molecules per nm², Fig. 1. Annealing the sample to 200 °C resulted in a 1-molecule unit cell with a packing density of 0.38 ± 0.03 molecules per nm², Fig. 4. Annealing altered both the adsorption sites and intermolecular ordering of 2HTFPP, and we no longer observed two electronic species on the surface. Annealing to 200 °C may remove excess solvent and provide the energy needed for the adsorbed species to leave the preferred adsorption site and maximize intermolecular interactions. This suggests that the local chemical environment plays a critical role in the creation of two bistable species. The surface of Au(111) undergoes herringbone reconstruction with packing of the surface gold atoms.^57,58 The surface consists of both face-centered cubic packing (fcc) and hexagonally-close packed (hcp) gold atoms with strained transition areas. The three different domains create a heterogeneous surface electronic structure.⁵⁹ To investigate the relationship of species 1 and species 2 with the underlying substrate we characterized the images using Fast Fourier Transform (FFT). FFT converts an image from real space to frequency space and can reveal longer-range ordering of molecular assemblies.Open in a separate windowFig. 42HTFPP after annealing to 200 °C. The unit cell has dimensions of 1.6 nm × 1.6 nm and the monolayer has a packing density of 0.38 molecules per nm². The bright double-dot features are doubly stacked porphyrins, the start of a second layer. The image was taken with a bias voltage of −10.0 mV and a tunneling current of 10 pA.In order to focus the FFT on one species at a time, white circles were placed over each double dot feature, Fig. 3a and b. FFT were taken of the images with the circles to determine the packing of the adsorbates, Fig. 3c and d. Hexagonal ordering as well as longer-range rectangular packing can be seen in the FFT, Fig. 3c, of the locations of the double-dot features in the image showing two species of 2HTFPP, Fig. 3a. We observed rectangular packing of 2HTFPP when the magnitude of the bias voltage was large enough for all of the adsorbed porphyrins to exhibit bright double-dot features, Fig. 3b and d. The presences of hexagonal packing for one species, but not the overall self-assembled island suggests that the adsorption site on the underlying gold herringbone reconstruction is playing a role in the shift of electronic energy levels. We did not observe the two species after annealing to 200 °C, suggesting that solvent and intermolecular interactions may influence the location of the adsorption site on Au(111). We also observed that the double-dot features in Fig. 3a were more likely to exist around molecular defects and edges of the close-packed island suggesting that intermolecular interactions also play a role in the shift of the electronic energies of the molecular orbitals.Open in a separate windowFig. 3(a) Circles placed over each 2HTFPP molecule with bright double dot features at −0.75 V and (b) −1.5 V (c) is FFT of (a) showing hexagonal order (d) is FFT of (b) showing rectangular order.Pulse deposition with a solenoid valve into vacuum has been shown to create meta-stable molecular motifs due to the rapid evaporation of solvent.⁵³ We planned to directly compare room-temperature pulse deposition with drop-casting but were unable to resolve drop-cast porphyrins prior to annealing. Drop-casting followed by annealing resulted in an alternate packing of 0.21 molecules per nm², Fig. 3 in ESI.^† Despite the fluorinated phenyl group inverting the polarity of the ring, we observe π stacking of pentafluorophenyl rings after pulse-depositing and annealing to 200 °C, Fig. 4. Although this is unexpected, Blanchard et al. have previously observed face-to-face π stacking in the crystal structure of pentafluorophenyl substituted ferrocenes.⁶⁰Several molecules on top of the self-assembled island, the start of a second layer, appear with bright double-dot features, Fig. 4. We did not observe the coexistence of two electronic species in the second layer of 2HTFPP.In conclusion, we observed two bistable species of 2HTFPP on Au(111) with non-degenerate electronic energy levels. DFT and theoretical STM images suggest that the two species are both 2HTFPP and not chemically altered through adsorption or interaction with the STM tip. The species with the affected electronic energy levels appear near molecular vacancies and exhibit longer-range hexagonal packing indicating that the two species are created through molecule–substrate and molecule–molecule interactions. Annealing to 200 °C decreases the packing density and alters the molecule–molecule and molecule–surface interactions. Adding a second layer of adsorbate only exhibits one electronic species which strongly suggests that the local chemical environment plays a critical role in the shift of the electronic energies of the molecular orbitals of 2HTFPP.     

Viscosity effect on the strategic kinetic overgrowth of molecular crystals in various morphologies: concave and octapod fullerene crystals

A kinetic overgrowth allowing organic molecular crystals in various morphologies is induced by temperature-dependent viscosity change of crystallization solution. By this strategy, concave cube and octapod fullerene C₇₀ crystals were successfully obtained by antisolvent crystallization (ASC). The structural analysis of fullerene C₇₀ crystals indicates that the morphological difference is the result of kinetic processes, which reveals that viscosity, the only variable that can change dynamics of solutes, has a significant influence on determining the morphology of crystals. The effect of solvent viscosity in the stage of crystal growth was investigated through time-dependent control experiments, which led to the proposal of a diffusion rate-based mechanism. Our findings suggest morphology control of organic crystals by diffusion rate control, which is scarcely known compared to inorganic crystals. This strategic method will promote the morphology controls of various organic molecular crystals, and boost the morphology–property relationship study.

A kinetic overgrowth allowing organic molecular crystals in various morphologies is induced by temperature-dependent viscosity change of crystallization solution.

Because morphology directly affects the properties of crystals, morphology control of crystals has been one of the major research subjects in chemistry. While the strategies for the morphology control of inorganic metal crystals have been established quite well in the solution phase, primarily through surface chemistry guiding crystallization to occur only on non-passivated crystal planes or through kinetic overgrowth,^1–3 the absence of such strategic methods controlling the morphology of organic molecular crystals restricts their potential. Thus, enormous efforts have been devoted to exploring new approaches to control the morphology of molecular crystals through solvent^4,5 and temperature controls.^6,7As frequently demonstrated from inorganic metal crystals, kinetic overgrowth can be a prominent method to obtain molecular crystals in various shapes. The kinetic overgrowth induces preferred growth at specific sites resulting in morphologies that are not available thermodynamically, as demonstrated from concave cubes and octapods.^8–13 The kinetic overgrowth usually occurs with a concentration gradient around the seed crystals, which can be achieved by regulating the diffusion rate (V_diff) of solutes and crystal growth rate (V_grow). Therefore, it is important to understand and develop kinetic overgrowth process for organic molecular crystals, which can correspond to the well-established surface chemistry for inorganic metal crystals.Considering the importance of concentration gradient near seed crystals for kinetic overgrowth, among various crystallization methods, antisolvent crystallization (ASC) process in which supersaturation and nucleation are induced by the injection of antisolvent to which target molecules have a low solubility (Scheme 1),^14–18 is an ideal method for kinetic overgrowth of molecular crystals since it has many variables that can change micro-environment near seed crystals. In addition, fullerenes are good target molecules because obvious results are expected when kinetic product is successfully contrasted to well-known thermodynamic morphologies such as tubes, rods, and even polyhedrons,^19–23 as Yang et al. and Ariga et al. showed.^24–26 In this case, kinetically favorable fullerene concave cubes could be formed by involving sonication²⁴ and controlling solvent ratio,^25,26 and clearly contrasted with fullerene cube crystals. However, the origin and detailed mechanism of these kinetic overgrowths are still veiled, which prevents further application.Open in a separate windowScheme 1Schematic illustration of traditional ASC process. Target molecules are effectively crystallized via solvation shell mechanism.In this regard, the influence of temperature on kinetic overgrowth of organic molecular crystals should be investigated not only because of its contribution for thermodynamic versus kinetic reaction control in many branches of chemistry,^27–29 but also because it can alter the behavior of molecules significantly during ASC, especially by regulating solution viscosity. Therefore, in this study, we aim to investigate the effect of viscosity upon temperature change for kinetic overgrowth of fullerene crystals. Herein, we show that kinetic overgrowth can be induced by controlling temperature in ASC. Kinetically overgrown fullerene C₇₀ crystals, in concave cube and octapod shapes, are successfully obtained at low temperature, while only cube crystals are obtained at higher temperature. These results originate from increased solution viscosity, which causes slow diffusion of C₇₀ molecules to seed crystals, and consequent kinetic overgrowth. Diffusion rate-based mechanism of inorganic metal nanocrystals is successfully applied to this strategic morphology control.All the fullerene crystallization has been performed by ASC method. Isopropanol (IPA), an antisolvent, is added to C₇₀ solution in mesitylene to obtain cube-shaped C₇₀ crystals at room temperature. After 3 h, black precipitates have been separated from the solution by filtration for characterization. Well-defined faces, edges, and vertices of C₇₀ cubes are confirmed using a scanning electron microscope (SEM), which agrees well with previous report (Fig. 1a).¹⁹ To investigate the effect of growth temperature on the morphology, ASC of C₇₀ has been performed at lower temperature. When the growth temperature is lowered from RT to −16 °C using a refrigerator and −78 °C using dry ice bath, concave cube-shaped and unprecedented octapod C₇₀ crystals are obtained, respectively (Fig. 1b and c). The average sizes of the C₇₀ cube, concave cube, and octapod are 2.0 μm, 2.5 μm, and 1.7 μm, respectively (Fig. S1^†). The resulting crystals show a high degree of homogeneity in their morhpologies.Open in a separate windowFig. 1C₇₀ crystals prepared by ASC at different conditions. (a) Cubes at 25 °C, (b) concave cubes at −16 °C, and (c) octapods at −78 °C. (d) Crystallization condition and obtained morphology of product (scale bar: 2 μm).Because these morphologies are well-known kinetically overgrown products for inorganic metal nanocrystals,^8–13 we have checked the viscosity of crystallization solution at each temperature, which is directly related to V_diff of C₇₀ molecules by the Stokes–Einstein equation.³⁰ The apparent viscosity of the solution at the shear rate of 1000 Hz increases from 2.05 cp to 6.40 cp at −16 °C and further to 218.36 cp at −78 °C (Fig. 1d), which indicates dramatic decrease of V_diff of C₇₀ molecules may occur. From these results, it can be assumed that this morphology difference comes from the slow diffusion of C₇₀ molecules, which is induced by high viscosity at low temperature.The crystal structures of each product have been examined by powder X-ray diffraction (Fig. 2a). For C₇₀ cube crystals, the overall diffraction pattern including the intense peak from (100) plane with d-spacing of 10.56 Å indicates a simple cubic structure as known from previous results.¹⁹ Importantly, the XRD patterns of C₇₀ concave cube and octapod crystals are the same as cube crystals, which implies that the concave cube and octapod morphologies are the results of kinetic crystal overgrowth from cube crystals.²⁵Open in a separate windowFig. 2(a) X-ray diffraction patterns and (b) Fourier transform infrared spectra of C₇₀ cubes (black), concave cubes (blue), octapods (red), and pristine powder (green).Furthermore, to examine the effect of solvent inclusion on the crystal morphologies,²² the intercalated molecules have been analyzed using Fourier-transform infrared spectroscopy (Fig. 2b). Four representative peaks of mesitylene (2717, 2841, 2901, 3019 cm⁻¹) are observed equally from all three types of C₇₀ crystals.²⁵ This result suggests that the intercalation of solvent is irrelevant to the morphology decision. The influence of intercalated mesitylene on the rotational motion of fullerene is also investigated by Raman spectroscopy, and no peak shift also indicates that the intercalated mesitylene does not affect the motion of fullerene molecules in crystal lattice (Fig. S2^†).²⁵ Therefore, we conclude that morphological diversity of C₇₀ crystals is not originated from common structural difference, but from the crystal overgrowth induced by diffusion rate change induced by viscosity change.The overall mechanism of morphology control during ASC via viscosity control can be proposed as follows (Fig. 3). When antisolvent is injected into C₇₀ solution of mesitylene, emulsion droplets containing C₇₀ and mesitylene are formed instantaneously, followed by the diffusion of mesitylene out to continuous phase consisted of antisolvent (Fig. 3a).³¹ As a result, supersaturation and nucleation occur, and seed crystals are formed. To examine if the morphology decision is made at the nucleation stage or growth stage, the seed crystals obtained at 25 °C, −16 °C, and −78 °C are identified by SEM to confirm their cube-shaped morphology (Fig. 3b–d and Fig. S3^†). Therefore, the morphological difference must be induced at the crystal growth step, rather than the nucleation step, which is also on the line of seed-mediated kinetic overgrowth mechanism.Open in a separate windowFig. 3(a) Schematic illustration of temperature-independent nucleation of C₇₀ immediately after the injection of antisolvent, and the SEM images of seed crystals prepared at (b) 25 °C, (c) −16 °C, and (d) −78 °C (scale bar: 500 nm). (e) Schematic illustration of diffusion rate-dependent growth from cubic seed crystals.After nucleation, the reaction system goes through metastable states, where crystals continue to grow and kinetic overgrowth starts to play. All the mesitylene emulsion droplets are broken and release C₇₀ nucleates. Then, C₇₀ molecules remained in the continuous phase at the stage of nucleation are attached to the nucleates. In this stage, the competition between V_diff of C₇₀ molecules and V_grow plays a key role in the determination of crystal morphology, where V_diff is controlled effectively by viscosity (Fig. 3e). At 25 °C, the viscosity of solution is quite small, so C₇₀ molecules can easily diffuse from the bulk solution to the seed crystals (V_diff ≫ V_grow). In this condition, the concentration of C₇₀ around seed crystals is maintained almost same, hence no preferential overgrowth occurs, resulting in seed crystal morphology-retained cube crystals.¹⁹ Whereas, C₇₀ molecules cannot diffuse rapidly at −78 °C due to the high viscosity of the solution (V_diff ≪ V_grow). In this regime, the concentration gradient of solute around seed crystals is generated because of the fast consumption of C₇₀ molecules near seed crystals with slow refill from the bulk solution. Such a concentration gradient induces preferential attachment of distant solutes to the sites possessing the highest reactivity, vertices for cubic crystals, as known for the inorganic metal crystals.^8–13 Eventually, octapod-shaped C₇₀ crystals are formed. The kinetic overgrowth that frequently results in anisotropic crystal growth at specific sites^32–34 rather than thermodynamic isotropic growth resulting in simply bigger crystals was supported from time-dependent SEM images of C₇₀ crystals at −78 °C (Fig. 4) showing petals that are preferentially and continuously grown out of cube crystals. In contrast, an isotropic growth only with a size increase from cubic seed has been observed in the case of the growth at 25 °C (Fig. S4 and S5^†), which indicates thermodynamic crystal growth at low viscosity.Open in a separate windowFig. 4Time-dependent SEM images of C₇₀ octapod crystals obtained at −78 °C. Growth time for each image is (a) 0 min, (b) 1 min, (c) 5 min, and (d) 30 min, respectively (scale bar: 1 μm).To verify if this morphology control is directly related with viscosity change rather than temperature change itself, control experiments using less viscous solvent at the same temperature have been conducted, finding no morphology changes. When acetone is used as antisolvent instead of IPA, no morphological change is observed even at the growth temperature of −78 °C, (Fig. 5 and S6^†) and this result owes to the low viscosity of acetone even at low temperature.³⁵ In other words, the addition of acetone to mesitylene solution does not cause a dramatic viscosity change, which implies the importance of the selection of good solvent and antisolvent for the successful morphology control by ASC process. On the other hand, the use of other alcohols (ethanol, 1-propanol, and 1-butanol having viscosity of 5.26, 6.52, and 7.53 cp at −16 °C and 30.0, 79.8, and 132.0 cp at −78 °C, respectively) show clear kinetic overgrowth at low temperature (Fig. S7^†). Other than the viscosity change inducing morphology control, there is another important property change, i.e. solubility change upon temperature change (Fig. S8^†) to be considered, which requires further studies in the future.Open in a separate windowFig. 5C₇₀ cubes prepared using acetone as antisolvent at (a) 25 °C, (b) −16 °C, and (c) −78 °C (scale bar: 2 μm).     

The synthesis of highly active carbon dot-coated gold nanoparticles via the room-temperature in situ carbonization of organic ligands for 4-nitrophenol reduction

Due to the serious pollution issue caused by 4-nitrophenol (4-NP), it is of great importance to design effective catalysts for its reduction. Here, a novel and simple strategy was developed for the synthesis of carbon dot-decorated gold nanoparticles (AuNPs/CDs) via the in situ carbonization of organic ligands on AuNPs at room temperature. The enhanced adsorption of 4-NP on CDs via π–π stacking interactions provided a high concentration of 4-NP near AuNPs, leading to a more effective reduction of 4-NP.

Due to the serious pollution issue caused by 4-nitrophenol (4-NP), it is of great importance to design effective catalysts for its reduction.

With the rapid development of the global economy and the continuous progress of the society, environmental problems, especially water pollution caused by nitroaromatic compounds, have become increasingly serious during the last decade.^1,2 As one of the toxic phenolic pollutants, 4-nitrophenol (4-NP) is usually found in the wastewater discharged from many chemical industries, which causes severe environmental issues and also a serious toxic effect on the living organisms.^3–6 Hence, 4-NP has been classified as a priority toxic pollutant by the US Environmental Protection Agency.⁷ In contrast, 4-aminophenol (4-AP), a product of 4-NP, is less toxic and an important intermediate that can be applied in many fields.⁸ Therefore, developing a novel method to effectively convert hazardous 4-NP to nontoxic 4-AP is urgent for solving the environmental issues.⁹It has been reported that 4-NP can be reduced to 4-AP by borohydride ions (BH₄⁻) enabled by catalysts, which plays an important role in this process. Among various catalysts, noble-metal materials (especially Au) have been demonstrated to be effective catalysts and are still the main catalysts used for 4-NP reduction.^10–13 In any heterogeneous catalysis process, the catalytic reaction must occur on the surface of the catalyst.^14–16 Therefore, the adsorption of reagents on the catalyst surface is a vital process before the chemical reaction. Recently, the efficient adsorption of reagents with aromatic rings on π-rich supports has been proved based on π–π interactions.^17–19 As an aromatic compound, 4-NP is also a π-rich molecule in nature, and its adsorption on a carbon-based catalyst by these π–π stacking interactions has also been demonstrated.^20,21 By decorating metal nanoparticles (MNPs) on these supports, the catalytic activity of MNPs toward the reduction of 4-NP to 4-AP by NaBH₄ has also been effectively enhanced via a synergistic effect.^22,23 Based on this, various π-rich carbon materials like carbon fibers,²⁴ carbon nanotubes,²⁵ graphene oxide,²⁶ reduced graphene oxide,²⁷ and graphitic carbon nitride²⁸ have been utilized as supports to prepare metal/carbon-based catalysts with enhanced catalytic activity. However, these methods suffer from drawbacks such as a multiple-step preparation process, prior functionalization, high cost and low yields, limiting their wide practical applications.^29,30 More importantly, as described previously, the reduction of 4-NP mainly occurs on the AuNP surface;³¹ thus, we can speculate that anchoring smaller π-rich carbon materials on the AuNP surface may lead to higher catalytic activity for the 4-NP reduction. Carbon dots (CDs) with the structure of sp² carbons^32,33 can adsorb 4-NP via π–π stacking, with the additional advantages of excellent stability and smaller size.³⁴ Thus, the catalytic activity of AuNPs for 4-NP reduction can be improved by the surface decoration of CDs, which, however, has not been reported before.Herein, a convenient and simple method was proposed to synthesize CD-decorated AuNPs (AuNPs/CDs) via the room-temperature in situ carbonization of cetylpyridinium chloride monohydrate (CPC) pre-adsorbed on AuNPs as an organic ligand. This AuNP/CD hybrid was demonstrated to have higher catalytic activity for the reduction of 4-NP, which was about 2.7-fold higher than that of AuNPs. The improved catalytic activity of AuNPs/CDs can be attributed to the synergistic effect between AuNPs and CDs.According to previous research, CPC can be carbonized to carbon dots in the presence of NaOH.^35,36 Inspired by this, we used CPC as both the capping and reducing agent to synthesize AuNPs in the presence of NaOH, and the synthesis process is displayed in Fig. 1A. After adding HAuCl₄ into an aqueous solution of CPC, yellow suspended solids were generated quickly (Fig. 1A(a) and (b)), which should be attributed to the formation of CPC-Au(i) between AuCl₄⁻ and CPC. Fig. 1B shows the UV-vis spectra of the aqueous solution of CPC before and after the addition of AuCl₄⁻. The obvious absorption peak at 340 nm (green) should be due to the generated CPC-Au(i). The generated Au(i), confirmed by the XPS analysis results (Fig. 1D), can originate from the reduction of Au(iii) by CPC because there are no extra reducing agents present in this system. After adding NaOH into the above-mentioned CPC-Au(i) solution at room temperature (Fig. 1A(b) and (c)), the color of the solution gradually changed to dark red (Fig. 1A(c)). The UV-vis spectrum of the above-mentioned mixture solution was then recorded. The absorbance between 280 and 450 nm proved the formation of CDs due to the carbonization of CPC under alkaline conditions;³⁵ moreover, the obvious absorption peak at 520 nm (Fig. 1C (red)), corresponding to the surface plasmon resonance (SPR) of AuNPs,³⁷ indicated the formation of AuNPs in this process. The formation of AuNPs could be attributed to the further reduction of CPC-Au(i) after the addition of NaOH. XPS analysis was also performed to investigate the oxidation state of the Au species in the process of AuNP synthesis. The high-resolution Au 4f XPS spectra shown in Fig. 1D and E indicate the presence of different Au species in these two samples. For CPC-Au(i) (Fig. 1D), the four evident peaks at 84.8, 88.4 eV and 87.4, 91.1 eV were assigned to the Au 4f_7/2 and Au 4f_5/2 signals,³⁸ respectively, indicating the co-existence of nonmetallic Au⁺ and Au³⁺ in this sample.³⁹ However, for AuNPs/CDs (Fig. 1E), two typical binding energies at 86.8 eV and 83.1 eV, attributed to the binding energies of metallic Au(0),⁴⁰ were clearly observed, indicating the successful reduction of Au(iii) and/or Au(i) to Au(0) in the presence of NaOH.Open in a separate windowFig. 1(A) Diagram for the synthesis of AuNP/CD nanocomposites at room temperature. (B and C) The UV-vis absorption spectra of CPC (black), CPC-Au(i) (green), CDs (blue) and AuNPs/CDs (red) in aqueous solutions, respectively. The high-resolution XPS spectra of Au 4f in CPC-Au(i) (D) and AuNPs/CDs (E). Fig. 2A displays the TEM image of the synthesized AuNPs/CDs. From the image, it can be seen that the monodispersed AuNPs/CDs are spherical, with an average size of 3.5 ± 0.8 nm and a standard deviation of the particle sizes of 22.9% (Fig. S1^†). This relatively small size should be attributed to the use of CPC as a capping agent, which can effectively control the growth of AuNPs. Fig. 2B shows the HRTEM image of AuNPs/CDs; the characteristic lattice spacing of 2.06 Å can be attributed to the (200) plane of face-centered cubic (fcc) gold,⁴¹ which also indicates the successful synthesis of Au nanocrystals under these conditions. In addition, the presence of CDs on the AuNP surface can be clearly observed (Fig. 2B and C), and the Raman peak of AuNPs/CDs between 1100 and 1800 cm⁻¹ (Fig. S2^†) also indicates the presence of carbon dots on AuNPs.⁴²Fig. 2D shows the XRD patterns of CDs (black) and AuNPs/CDs (red). There is no diffraction peak for CDs, indicating the amorphous structure of CDs. However, the obvious peaks at 38.9°, 44.8°, 65.1° and 78.1° (red) for AuNPs/CDs, assigned to the diffraction from the (111), (200), (220) and (311) planes, respectively, of fcc Au crystals,⁴³ also prove the successful formation of Au nanocrystals and agree well with the results of TEM. Fig. 2E displays the FT-IR spectra of AuNPs/CDs (red) and CDs (black). Similar groups can also be found on both CDs and AuNPs/CDs, indicating the presence of CDs on the AuNP surface. In a word, the results described above not only prove the successful synthesis of AuNPs, but also the structure of CDs decorated on the AuNP surface based on the in situ carbonization of CPC at room temperature.Open in a separate windowFig. 2(A) TEM and (B) HRTEM images of AuNPs/CDs. (C) The structure of AuNPs/CDs. (D) XRD profiles and (E) FT-IR spectra of CDs (black) and AuNPs/CDs (red).The reduction of 4-NP to 4-AP with an excess amount of NaBH₄ was carried out to quantitatively evaluate the catalytic properties of AuNPs/CDs. In the absence of the catalyst, a strong absorbance peak at 400 nm was observed for the mixture of 4-NP and NaBH₄, which was attributed to the 4-NP ions under alkaline conditions.⁴⁴ After adding the catalysts CDs and AuNPs/CDs into the reaction system, the reduction process was monitored by measuring the time-dependent absorption spectra of the mixed reaction solution. As shown in Fig. 3A, the nearly unchanged absorption even after one hour when only CDs were present as the catalyst indicates the non-active nature of CDs for 4-NP reduction. However, when we used AuNPs/CDs as the catalyst, the absorbance intensity of 4-NP at 400 nm decreased quickly as the reaction time was extended, and this was accompanied by the appearance of an absorbance peak of 4-AP at 300 nm (Fig. 3B), indicating the higher catalytic activity of AuNPs/CDs for the reduction of 4-NP to 4-AP by NaBH₄. This result also indicates that the reduction of 4-NP should be attributed to the catalysis of AuNPs in AuNPs/CDs due to the non-active CDs. It should be noted that the reduction of 4-NP by NaBH₄ could be completed within 10 minutes, with the observation of fading and ultimate leaching of the yellow-green color of the reaction mixture in the aqueous solution. However, a longer reaction time (more than 18 minutes) was required to achieve the full reduction of 4-NP under similar conditions with AuNPs alone (synthesized with trisodium citrate; see ESI^† for preparation details) as the catalyst (Fig. 3C). Fig. 3D shows the absorption changes of the solutions at 400 nm in the presence of different catalysts, and the quicker decrease in absorption indicates the higher catalytic activity of AuNPs/CDs than that of AuNPs. Since the concentration of BH₄⁻ was constant during the catalytic reaction and much higher than that of 4-NP, the rate constants could be evaluated by the pseudo-first-order kinetics using ln(C_t/C₀) = −kt, where k is the apparent first-order rate constant; its value estimated directly from the slope of the straight line can be used to evaluate the catalytic activity of a catalyst.⁴⁵ As shown in Fig. 3E, a good linear relationship of ln(C_t/C₀) versus the reaction time (t) is observed for the two catalysts; the rate constant (k) values were calculated as 1.83 × 10⁻⁴ min⁻¹, 0.11 min⁻¹ and 0.30 min⁻¹ for CDs, AuNPs and AuNPs/CDs, respectively. These results clearly demonstrated the higher catalytic activity of the AuNPs/CDs composites, which was about 2.7-fold higher than that of AuNPs. In addition, the AuNPs/CDs catalyst exhibited better/comparable catalytic activity compared to/to that of previously reported modified gold catalysts in terms of the time needed for the reduction reaction and rate constant (Table S1^†), demonstrating its potential applications in catalysis.Open in a separate windowFig. 3The UV-vis absorption spectra for the reduction of 4-NP by CDs (A), AuNPs/CDs (B) and AuNPs (C). The plots of absorbance (D) and ln(C_t/C₀) (E) versus reaction time for the catalytic reduction of 4-NP based on different catalysts. (F) The mechanism for the enhanced catalytic activity of AuNPs/CDs for 4-NP reduction.Based on the experimental results and analysis mentioned above, the enhanced catalytic activity of AuNPs/CDs for 4-NP reduction should be attributed to the presence of CDs on AuNPs, which leads to a synergistic effect between AuNPs and CDs. This synergistic effect plays an active part in catalysis and therefore enhances the catalytic activity of AuNPs/CDs, which can be explained as follows: as a π-rich molecule, 4-NP can be adsorbed onto the surface of CDs via π–π stacking interactions.^20,23 In addition, the adsorption of 4-NP on AuNPs/CDs can be demonstrated by the decreased absorbance of the 4-NP solution after adding AuNPs/CDs in the presence of NaOH instead of NaBH₄ (Fig. S3^†). This endows AuNPs/CDs with anchoring sites for the adsorption of 4-NP, and such strong adsorption results in a higher concentration of 4-NP near the surface of AuNPs,²⁰ leading to efficient contact between them and therefore speeding up the catalytic process (Fig. 3F(I)). In contrast, for AuNPs without the presence of CDs, 4-NP must collide with AuNPs by chance and must remain in contact for the catalysis to proceed; or else, 4-NP will pass back into the solution and the reaction cannot occur (Fig. 3F(II)) until it reaches the AuNP surface again.⁴⁶ This can also be demonstrated by the increased reaction rate constant for the 4-NP reduction with more catalysts present in the reaction system (Fig. S4^†). In addition, due to the excellent electron acceptor and donor properties of CDs,^47,48 which can obtain/give excess electrons from/to AuNPs (Fig. 3F), like a reservoir of electrons, the electron transfer from CDs to AuNPs increases the local electron concentration and maintains AuNPs in an electron-enriched state,⁴⁹ facilitating the uptake of electrons by the 4-NP molecules.In summary, a nanocomposite of AuNPs/CDs was successfully synthesized via the room-temperature in situ carbonization of CPC as a capping agent for AuNPs. As a catalyst for 4-NP reduction, this composite showed superior catalytic performances to AuNPs, which could be rationally attributed to the enhanced adsorption of 4-NP on CDs providing a high concentration of 4-NP near AuNPs for a more effective reduction of 4-NP. This work not only offers an attractive catalyst material for 4-NP reduction, but also opens an exciting new avenue for the rational design and development of CD/metal nanostructure hybrids for various applications.     

Photoelectrochemical photocurrent switching effect on a pristine anodized Ti/TiO2 system as a platform for chemical logic devices

We report here the effect of the photoelectrochemical photocurrent switching (PEPS) observed on highly-ordered pristine anodized Ti/TiO₂ for the first time. At negative potential bias, blue irradiation gives cathodic photocurrent, whereas anodic photocurrent was observed for ultraviolet irradiation. We believe this phenomenon is due to the electron pathway provided by Ti³⁺ defect states.

We report here the effect of the photoelectrochemical photocurrent switching (PEPS) observed on highly-ordered pristine anodized Ti/TiO₂ for the first time.

Titanium dioxide, being one of the most studied materials, still draws much attention from researchers.^1,2 It is considered to be a very promising material due to its high chemical stability, nontoxicity, and its unique properties. Due to stable and robust photoactivity, titania is widely used in the design of solar cells³ and photocatalytic applications.⁴ In addition to the fact that titanium dioxide occurs in several crystalline modifications, it can also be obtained in various forms, such as, for example, nanotubes,⁵ nanofibers,⁶ and nanosheets.⁷ The photocatalytic performance of TiO₂ is highly dependent on crystallinity,⁸ phase content, form, and preparation method.⁹ It was reported that highly ordered arrays of TiO₂ nanotubes are characterized by short charge transport distance and little carrier transport loss.⁵ Therefore, electrochemically fabricated TiO₂ nanotube arrays are preferable compared to random non-oriented titania.¹⁰ Great varieties of photoelectrochemical behaviour can be achieved by doping¹¹ and surface modification.^12,13An interesting feature has recently been demonstrated for highly ordered arrays of TiO₂ nanotubes obtained by double stepwise electrochemical anodization of a titanium foil (Ti/TiO₂). Together with our colleagues observed that localized illumination of Ti/TiO₂ surface in water solution triggers proton flux from irradiated area.¹⁴ The photocatalytic activity of TiO₂ is based on photogenerated electron–hole pairs. Under the electric field of Ti/TiO₂ Schottky junction and due to upward surface band bending, efficient spatial charge separation occurs, and photoexcited holes (h⁺) reach TiO₂ – solution interface. The h⁺, which is a strong oxidizing agent, can react with water, and a pronounced pH gradient arises due to water photolysis. Thus, titanium dioxide can be used to trigger local ion fluxes, and proton release is associated with anodic photocurrent. The use of the light-pH coupling effect to control pH-sensitive soft matter was previously demonstrated.^15,16 Complementary species, H⁺ and OH⁻, annihilating when occurring simultaneously, extend chemical arithmetic with subtraction operation opening way to pure chemical calculations.¹⁷ Ion fluxes consideration as information transducers in solution were proposed¹⁸ and performing simple logic operations was demonstrated.¹⁹ This phenomenon opens perspectives to biomimetic information processing and developing effective human–machine interfaces.²⁰Photoelectrodes using light and potential as inputs and yielding photocurrents are being considered as the basis for logic devices. In this way, optical computing compatible with existing silicon-based devices may be performed.Logic operations are described by Boolean algebra operating with truth values denoted 0 (false) and 1 (true). Elementary logical operations are modelled by logic gates producing single binary output from multiple binary inputs and physically implemented by some switch. As for photoelectrode based information processing, the photoelectrochemical photocurrent switching (PEPS) effect is utilized. This effect is that under appropriate external polarization or/and illumination by light with appropriate photon energy, switching between anodic and cathodic photocurrent may be observed for n-type semiconductors and the opposite for p-type.^21,22Without further modification, this effect was observed for a very limited number of materials, such as bismuth orthovanadate, lead molybdate, V–VI–VII semiconductors, and some others. To show this effect, the majority of semiconductors require electronic structure perturbation creating new electron pathways. A convenient solution is specific modifier adsorption onto the semiconductors'' surface, providing a sufficient level of electronic coupling. Photoelectrodes made of nanocrystalline TiO₂ modified by cyanoferrate,^13,23 and ruthenium²⁴ complexes, thiamine, folic acid,²⁵ and carminic acid²⁶ demonstrated PEPS behavior.Surprisingly, we observed the PEPS effect on non-modified Ti/TiO₂ obtained by anodation of Ti plates.Highly ordered arrays of anatase Ti/TiO₂ were obtained. Crystallinity was proved by XRD (Fig. S1a^†). Fig. 1a shows a SEM image of TiO₂ nanotube arrays obtained as described above. According to SEM image, an average pore diameter is ca. 60 nm. As reported, highly ordered TiO₂ nanotubes possess a short charge transport distance and little carrier transport loss. Therefore, highly ordered TiO₂ nanotube arrays fabricated by electrochemical anodization of titanium may exhibit some enhanced capacity of electron transfer than non-oriented ones of random mixture.¹⁰Open in a separate windowFig. 1(a) SEM image of the TiO₂ nanotubes array. The inset shows cross-section view. (b) Scheme of a cell for photocurrent measurements experiment, CE – counter electrode, RE – reference electrode, WE – working electrode.According to Mott–Schottky analysis, at potential bias more positive than −0,697 V vs. Ag/AgCl reference electrode upwards band bending occurs (Fig. S2^†). Heat treatment in a nonoxidizing atmosphere leads to Ti³⁺ formation. Appearance of Ti³⁺ self-doping was proved by EDX analysis (Fig. S1b^†). It was previously reported that Ti³⁺ introduces gap states which act as recombination centers and pathways for electron transfer.^27–29 Ti³⁺ species in reduced TiO₂ introduce a gap state between valence and conduction bands.^27,28We studied dependence of photocurrent on applied potential. Ultraviolet irradiation (365 nm) gave positive photocurrent for all potentials studied in range from −0.6 V to 0.6 V vs. Ag/AgCl reference electrode (Fig. S3^†). The photocurrent increases as the potential becomes more positive, but eventually saturates. The dependence of the current on the potential under blue irradiation (405 nm) had a different character. Sigmoid function with inflection point at 0–0.2 V was observed for blue light.It should be noticed that photocurrent plotted against time on Fig. 2–4 as well as against potential on Fig. S3^† is ΔI = I_{under illumination} − I_{in darkness}. Steady state current values were used for calculations.Open in a separate windowFig. 2Photocurrent curves under chopped irradiation by (a) 365 nm UV LED, (c) 405 nm blue LED at applied potential bias +300 mV vs. Ag/AgCl, and corresponding scheme of electron pathway at +300 mV polarization under irradiation by (b) 365 nm UV LED and (d) 405 nm blue LED.Open in a separate windowFig. 3Photocurrent curves under chopped irradiation by (a) 365 nm UV LED, (c) 405 nm blue LED at applied potential bias −300 mV vs. Ag/AgCl, and corresponding scheme of electron pathway at −300 mV polarization under irradiation by (b) 365 nm UV LED and (d) 405 nm blue LED.Open in a separate windowFig. 4(a) XOR logic realized on negatively polarized (−0.3 V) pristine Ti/TiO₂ by two source irradiation, input A – UV light (365 nm), input B – blue light (405 nm); blue light gives anodic photocurrent, UV – cathodic photocurrent. The current, significantly different from the dark one, is taken as output 1, otherwise – 0. When irradiated by blue and UV light simultaneously, anodic and cathodic current compensate each other, and no total photocurrent observed. Thus output 0, when both inputs are 1 (b) OR logic realized on positively polarized (+0.3 V) non-modified Ti/TiO₂ by two sources of irradiation. Irradiation by any of them, blue or UV, gives anodic photocurrent.At +300 mV vs. Ag/AgCl irradiation by both blue and ultraviolet light give anodic photocurrent (Fig. 2a and c). The UV-irradiation (λ = 365 nm, 5 mW cm⁻²) excites electron directly to the conduction band (CB) of TiO_2, which is further transferred to conducting titanium support (Fig. 2b). When Ti/TiO₂ electrode in thermodynamic equilibrium with electrolyte, an upward surface band bending occurs at the semiconductor–liquid junction. This phenomenon obstructs electron injection from the conduction band into the electrolyte and forces electron drift to conducting substrate. The fast and steady photocurrent production/extinction upon light on/off indicates efficient charge separation and low recombination.Blue light (λ = 405 nm, 70 mW cm⁻²) is characterized by lower energy than UV-irradiation, which is not sufficient to excite the electron to CB. But electron excited by blue light can be trapped by Ti³⁺ located close to the conduction band and transferred to conduction support from these levels (Fig. 2c). An initial current spike following by an exponential decrease suggesting a fast recombination process. It should be also noticed than when irradiation is switched off photocurrent ‘overshoots’ as the remaining surface holes continue to recombine with electrons.At more negative potential (−300 mV vs. Ag/AgCl, for example) applied to non-modified anodized Ti/TiO₂ photoelectrode, we observed anodic photocurrent during irradiation by UV light (Fig. 3a) whereas blue irradiation gave anodic photocurrent (Fig. 3c). Excitation within bandgap by UV-irradiation leads to cathodic photocurrent (Fig. 3b). In the case of irradiation by blue light, electron trapping by Ti³⁺ occurs in the same manner as at +300 mV polarization. But at negative polarization, the energy landscape is such that electron transport to electron donor in solution is preferable (Fig. 3d). As a result, cathodic current occurs.Thereby, photoelectrode activity of non-modified anodized Ti/TiO₂ can be switched from anodic to cathodic and vice versa by applying various potentials and various photon energies. This is the effect of photoelectrochemical photocurrent switching.Thereby, when Ti/TiO₂ is irradiated simultaneously by blue and UV light being negatively polarized, competition between cathodic and anodic photocurrents occurs. Returning to Boolean logic, the PEPS effect allows us to perform annihilation of two input signals and implement optoelectronic XOR logic gate. XOR logic operation outputs true (1) only when input values are different and yield zero otherwise.It is necessary to assign logic values to input and output signals to analyse the system based on Ti/TiO₂ PEPS effect in terms of Boolean logic. Logical 0 and 1 are assigned to off and on states of the LEDs, respectively. Different wavelengths (365 and 405 nm) correspond to two different inputs of the logic gate. In the same way, we can assign logic 0 to the state when photocurrent is not generated and logic 1 to any nonzero photocurrent intensity irrespectively on its polarization (cathodic or anodic). Fig. 4 demonstrates how different types of Boolean logic are realized by irradiation of Ti/TiO₂. Light sources are denoted here as inputs, UV light – A and blue light – B. If the corresponding light source is switched ON and illuminates photoelectrode Ti/TiO₂, this input is ‘1’, otherwise, it''s ‘0’. The photocurrent is read as output. It''s considered to be ‘1’ if significantly differs from dark value and ‘0’ otherwise.At −300 mV vs. Ag/AgCl, pulsed irradiation with UV diode (365 nm, 5 mW cm⁻²) results in anodic photocurrent, which is consistent with electron excitation to CB and transfer to conducting support. Irradiation with blue LED (405 nm) gives cathodic photocurrent due to electron capture by Ti³⁺ states following by transferring to electron acceptor in solution. Simultaneous irradiation with two LEDs with adjusted intensity yields zero net current as anodic and cathodic photocurrents compensate effectively (Fig. 4a).At positive potentials, pulsed irradiation with UV diode gives anodic photocurrent pulses, as well as the blue one. It is interesting to note that when two sources of light are simultaneously irradiated, the photocurrents created by each of them individually do not summarize. At +300 mV, photocurrent output under the influence of two light inputs (365 nm and 405 nm) follows OR logic giving positive output if at least one of inputs is positive (Fig. 4b). Fig. 5 demonstrates the reconfigurable logic system which characteristics can be changed via an appropriate polarization of the photoelectrode regarded as programming input. Two irradiation sources are considered as inputs. OR/XOR logic is realized depending on programming input.Open in a separate windowFig. 5A reconfigurable logic system based on non-modified Ti/TiO₂. Light sources are inputs. The choice between XOR and OR function is determined by programming input of potential bias. At +300 mV OR logic is realized, at −300 – XOR logic. Corresponding truth table is presented.In summary, PEPS effect on modified nanocrystalline TiO₂ was previously discussed a lot.^13,23–26 In this work we report the same phenomenon for pristine anodized Ti/TiO₂ system. Due to substructure of Ti/TiO₂ system, it shows characteristic response to various range of illumination, including visible range and polarization. The Ti/TiO₂ system is a simple and robust model of chemical logic gates. Suggested mimicking of logic functions in aqueous solutions allows further integration of element into communication with living objects¹⁶vs. intrinsically associated photooxidation and degradation, but rather activation for needed function.³⁰     

Protein-activated transformation of silver nanoparticles into blue and red-emitting nanoclusters

Proteins are very effective capping agents to synthesize biocompatible metal nanomaterials in situ. Reduction of metal salts in the presence of a protein generates very different types of nanomaterials (nanoparticles or nanoclusters) at different pH. Can a simple pH jump trigger a transformation between the nanomaterials? This has been realized through the conversion of silver nanoparticles (AgNPs) into highly fluorescent silver nanoclusters (AgNCs) via a pH-induced activation with bovine serum albumin (BSA) capping. The BSA-capped AgNPs, stable at neutral pH, undergo rapid dissolution upon a pH jump to 11.5, followed by the generation of blue-emitting Ag₈NCs under prolonged incubation (∼9 days). The AgNPs can be transformed quickly (within 1 hour) into red-emitting Ag₁₃NCs by adding sodium borohydride during the dissolution period. The BSA-capping exerts both oxidizing and reducing properties in the basic solution; it first oxidizes AgNPs into Ag⁺ and then reduces the Ag⁺ ions into AgNCs.

Protein capping can trigger nanoparticle to nanocluster transformation at elevated pH.

Noble metal nanomaterials, especially silver (Ag) and gold (Au), have witnessed exceptional research exploration in the last couple of decades from both fundamental and application perspectives.¹ These nanomaterials mainly exist in two distinct size regimes with unique optical characteristics. Ultra-small nanoclusters (NCs) (size typically <3 nm) contain only a handful of atoms (few to hundred), while relatively large nanoparticles (NPs) may comprise thousands of atoms. NPs may display strong extinction (absorption or scattering) spectra in the UV-vis region but are generally non-fluorescent.² In contrast, metal nanoclusters (MNCs) exhibit bright emission but not so noteworthy absorption spectra.^3,4 The distinct optical characteristics of the two nanomaterials have been exploited in various applications. For example, metal nanoparticles (MNPs) are extensively used in photothermal therapy⁵ and imaging,⁶ while NCs are more suited in fluorescence imagining⁷ and sensing⁸ applications. A facile transformation between the two nanomaterials could enable us to combine the complementary optical properties in a single system. Moreover, the kinetics of transformation can provide insights on various intermediate processes like dissolution, etching and digestive ripening etc.^9–11Silver nanoparticles (AgNPs) and nanoclusters (AgNCs) are of particular interest, as it not only possess the intriguing physicochemical properties of MNPs and MNCs, but also feature unique properties pertaining to silver.^3,12,13 For example, metallic silver has been well known for its capability to prevent infection since the ancient times, while recent studies revealed that ultrasmall AgNCs exhibit even superior antibacterial properties towards a broad spectrum of bacteria.^13,14 Moreover, due to superior plasmonic properties and bright fluorescence, AgNPs and AgNCs are preferred over other metal nanomaterials.^15,16The fluorescence properties of AgNCs mainly be attributed to the quantum confinement effect or surface ligand effect.¹⁷ The strong fluorescence generally arises from the electronic transition between occupied d band and states above the Fermi level (sp bands) or the electronic transition between highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).¹⁸ Several reviews have been devoted for the fundamental understanding of the fluorescence origin of AgNCs.^17,19 Recently, it was demonstrated that aggregation-induced emission (AIE) may also contribute to the luminescence pathway of MNCs.^19,20 The origin of AIE from MNCs could be attributed to the restriction of intramolecular vibration and rotation of ligand on the surface of MNCs after aggregation, which facilitates the radiative energy relaxation via inhibiting of non-radiative relaxations.^21,22Protein capping is quite common for obtaining both NPs^23,24 and NCs.^25–29 Serum proteins, bovine serum albumin (BSA) and human serum albumin (HSA) are the most popular among trials with different proteins.^26–29 BSA is a large protein which provides steric stabilization to the MNCs with its various functional group like –OH, –NH₂, –COOH, –SH.^25,30 The disulfide bond of BSA may have strong interaction with the MNCs where sulfur may be covalently bonded to the MNCs core.^24,31 The nanomaterials are synthesized within the protein template at very different pHs. AgNPs are obtained from the reduction of silver salts at neutral pH (6–8),²⁴ whereas the same process at a higher pH (>11) leads to AgNCs.^30,32 The protein capping itself may reduce Ag⁺; AgNCs are formed without any external reducing agent.^25,33 However, an external reducing agent may change the nature and kinetics of the NCs significantly.³⁰Thus, the influence of pH on the protein structure may govern the selective synthesis of AgNPs or AgNCs. BSA can achieve several conformations – N (native), B (basic), A (aged) and U (unfolded) as the pH of the medium gradually changes from neutral to highly alkaline.^34,35 It may be possible that a specific type of nanomaterial is stable within a particular conformation dictated by the pH of the medium. Hence, by simply changing the pH, we may expect a significant modulation of the morphology of the nanomaterial. Herein, we applied this concept to show an effortless transformation from AgNP to AgNC. Although BSA template is exceptionally popular in the preparation of both AgNPs and AgNCs, however, to the best of our knowledge, no report is available on the conversion from AgNP to AgNC within the protein capping.The BSA-capped AgNPs (BSA-AgNPs) were first synthesized at a neutral pH (pH = 6) using sodium borohydride reduction (see ESI^†). The AgNPs show a sharp surface plasmon resonance (SPR) band at 415 nm (Fig. 1a) and have uniform diameters of 12.5 ± 1.5 nm (Fig. 1b). The AgNPs are quite stable at this pH with no apparent change in the SPR band even after 15 days (Fig. S1^†).Open in a separate windowFig. 1(a) UV-vis spectrum and (b) TEM image of BSA-protected AgNPs synthesized at pH 6. The insets show the appearance of the AgNP solution under regular and UV light (left panel), and size distribution histogram (right panel).However, when the BSA-AgNPs were treated with NaOH to elevate the pH to 11.5, we observed a remarkable decrease in the SPR band at 415 nm and a color change from dark to light brown within 2 h of the pH jump (Fig. 2a). The observations indicate the dissolution of AgNPs, which was further confirmed from the TEM images taken quickly (∼10 min) after the NaOH treatment (Fig. S2^†). Heterogeneous distribution of AgNPs was obtained with sizes varying from 2.6 nm to 17 nm, which is in sharp contrast to the uniform AgNPs before the addition NaOH (cf.Fig. 1b). Upon further incubation (6 h), the light brown color gradually faded to light yellow with a further decrease in the SPR band absorbance (Fig. S3^†).Open in a separate windowFig. 2Time evaluation of the (a) UV-visible and (b) emission spectra (λ_ex = 370 nm) of BSA-capped AgNPs after enhancement of the pH from 6 to 11.5 (by addition of NaOH at t = 0). The inset shows the snapshot of the final blue-emitting AgNC solution under normal and UV lights. (c) TEM of the blue-emitting AgNCs along with the HRTEM image and the analyzed size distribution in the inset. (d) MALDI-mass spectra of the BSA protein and BSA-capped blue-emitting AgNCs.Interestingly, the solution also develops distinct fluorescence with a maximum at ∼460 nm after the addition of NaOH (Fig. 2b). The fluorescence intensity gradually grows up upon incubation, and finally, an intense blue fluorescence was developed within ∼9 days. The final NaOH-treated AgNP solution appears to be light yellow under normal light and blue-fluorescent when viewed under a hand-held UV lamp (Fig. 2b, inset). The blue-emitting AgNCs exhibit a single band excitation spectrum with a maximum at 372 nm (Fig. S4^†).TEM image of the optimized NCs (after 9 days incubation at 37 °C) exclusively reveals uniform AgNCs of ∼2.10 ± 0.28 nm diameter without any trace of large NPs (Fig. 2c). The mass of the BSA-capped AgNCs (67 375 Da) was shifted by 845 Da from that of native BSA (66 530 Da) (Fig. 2d). Thus, the new species should correspond to Ag₈ cluster. The characteristics of the blue-AgNCs were quite similar to the human serum albumin (HSA)-protected blue-AgNCs, directly prepared from silver salt.³³ However, the formation time of those AgNCs was significantly less (∼10 h) than the present method (∼9 days).³³ Thus, the initial dissolution process, although quite fast, may have a crucial role in the kinetics of the protein-protected NCs. When we performed a similar pH jump experiment on a citrate-stabilized AgNP,³⁶ the extinction spectrum of the AgNPs showed much less variation compared to the BSA-AgNPs. Instead of a strong decrease, the SRP band showed a red-shift with an extended tail indicating aggregation rather than dissolution of NPs (Fig. S5^†).Furthermore, a red-emitting cluster was generated when an external reducing agent, sodium borohydride (NaBH₄), was added during the dissolution process. NaBH₄ was added after ∼11 min of the NaOH addition when the SPR band of BSA-AgNP was already decreased by half (Fig. 3a). The SPR band (λ_max = 415 nm) of AgNP continues to diminish similarly before and after the addition of NaBH₄ (Fig. S3^†). Thus, NaBH₄ may not have any significant effect on the dissolution process of AgNP. However, it has a strong impact on the modulation of the fluorescence; a new fluorescence band was developed at ∼650 nm within a much shorter duration (1 h) (Fig. 3b). The solution exhibits a bright-red fluorescence under a UV lamp (Fig. 3b, inset) with a quantum yield of 3.5%.Open in a separate windowFig. 3Early time evolution of (a) UV-visible and (b) emission spectra (λ_ex = 370 nm) of the BSA-protected AgNPs upon subsequent treatments with NaOH (pH 11.5) and NaBH₄ at t = 0 and 11 min, respectively. The decrease of the SPR band at 415 nm and a concomitant increase of the fluorescence band at ∼650 nm indicates dissolution of the AgNPs and formation of the red-emitting cluster. The inset shows the visuals of the AgNCs formed after 1 h under normal light and UV light. (c) TEM of the red-emitting AgNCs along with the HRTEM image and the analyzed size distribution in the inset. (d) MALDI-mass spectra of the BSA protein and BSA-capped red-emitting AgNCs.TEM measurements of the red-emitting species show homogeneous distribution of AgNCs with ∼2.25 ± 0.25 nm diameter (Fig. 3c). MALDI-mass experiment further assigned the red-emitting species as Ag₁₃ cluster (Fig. 3d). The excitation spectrum (λ_em = 650 nm) displays two distinct peaks at 370 nm and 470 nm, which match closely to the reported excitation peaks of the Ag_13–15 clusters within BSA/HSA capping (Fig. S4^†).^30,32,33 Moreover, the fluorescence decay of the red-emitting-AgNCs converted from AgNP almost matches with those prepared directly from AgNO₃; both display very similar average lifetimes (0.95 ns vs. 0.89 ns) (Fig. S6 and Table S1^†).³⁷Another important observation is that the red-emitting AgNCs have only transient stability at 37 °C. With further incubation, the absorbance at ∼470 nm (characteristic excitation peak of the red-emitting cluster) reduces and the absorbance at 370 nm (excitation peak of the blue-emitting cluster) increases simultaneously (Fig. 4a). The red-emission at 650 nm also decreases gradually with a concomitant increase of a blue emission band at 465 nm (Fig. 4b). Thus, both absorption and emission measurements clearly indicate transformation of red- to blue-emitting clusters which takes up to ∼15 days for completion. The solution finally becomes light yellow and exhibits a bright blue fluorescence under UV light similar to the blue-emitting cluster obtained earlier from the AgNP in the absence of NaBH₄. Interestingly, other characteristics of the regenerated blue-emitting AgNCs (converted from Ag₁₃NCs) also match quite nicely with the directly prepared blue-AgNCs (converted from AgNPs in the absence of NaBH₄). The size of this blue cluster was 2.04 ± 0.12 nm, which is similar to the previously obtained direct blue-emitting cluster (2.10 ± 0.28) (Fig. 4c). Furthermore, MALDI-mass measurement reveals that both the blue-emitting clusters may have the same composition, Ag₈ (Fig. 4d). In addition to this, the average lifetime (0.53 ns) of the blue-emitting AgNCs synthesized from AgNP agrees well to the average lifetime (0.40 ns) of the blue-emitting AgNCs converted from the red-emitting AgNCs (Fig. S7 and Table S2^†). However, the quantum yield (23%) of blue-emitting AgNCs, converted from red-emitting AgNCs, was higher than the quantum yield (18%) of the blue-emitting AgNCs, converted from AgNPs. Since, the emission characteristics of the blue and the red-emitting clusters nearly matches with earlier report, we expect that silver may be present in the zero oxidation state as determined in those studies.^30,31Open in a separate windowFig. 4Transformation of red-emitting to blue-emitting cluster: (a) UV-visible and (b) emission spectra (λ_ex = 370 nm) showing transformation of the BSA-protected red-emitting Ag₁₃NCs (obtained at 1 h) to blue-emitting AgNC upon prolonged incubation. Red and blue arrows respectively denote the decrease/increase of the red/blue cluster absorbance and emission intensity with time. The inset (b) shows a magnified wavelength region in 580–720 nm of the emission spectra. (c) TEM image of the blue-emitting silver nanocluster while its inset shows HRTEM image with size histogram of corresponding silver nanocluster. (d) MALDI-mass spectra of BSA and BSA-containing blue-emitting silver nanocluster synthesized from Ag₁₃NCs.Moreover, the atomic composition of the NCs can be also be estimated from the Jellium model using the equation^38,39E_em = E_Fermi/N^0.33where E_Fermi is the Fermi energy of the metal (Ag), E_em is the emission energy of the MNCs and N is the number of atoms constituting a MNC. Using the model equation, the number of silver atoms for the blue-emitting AgNCs can be predicted as 8.45 (∼8) Ag atoms which is a good agreement with our MALDI data (8 Ag atoms). However, the theoretical calculation estimated as N ∼ 24 for the red-emitting AgNCs, which is not in agreement with the MALDI data (13 Ag atoms). This is because of the well-known deviation of the Jellium model for higher number of Ag atoms in AgNCs because of increase in the electronic screening effects and the harmonic distortion in the potential energy well.¹⁹Although the red-emitting cluster is not very stable at the experimental condition (pH 11.5, 37 °C), it may be easily stabilized by lowering the temperature or pH. The fluorescence intensity of the red-emitting and blue-emitting cluster kept at 4 °C, was almost preserved for more than 15 days (Fig. S8^†). On the other hand, lowering the pH to 6, also inhibits the red to blue-cluster transformation (Fig. S9^†). The observations indicate that the red-blue transformation has a moderate activation barrier and the conversion may be governed by the change in the structure of the protein in the alkaline condition. Acidification of the solution can stop the transformation of the protein conformation and inhibits the process.From these observations, we may conclude that the conversion from NPs to NCs occurs in two steps. First, a rapid dissolution of AgNP occurs in the alkaline medium. The kinetics of the dissolution process can be monitored through a time-dependent decrease of the SPR band and the time constant was found to be ∼13 min (Fig. S10^†). Dissolution of AgNPs is an important issue and assumed to be the leading cause of toxicity of AgNPs in biological mediums.⁴⁰ The dissolution is commonly favored at a low pH but drastically inhibited at high pH.⁴¹ The swift dissolution of the BSA-protected AgNP observed here at a high pH (11.5) is unprecedented. Thus, the BSA capping may have an active role in the dissolution process. We comprehend that the oxidation power of protein may be activated in the basic medium.Organothiols (R-SH) are known to promote dissolution of AgNPs; R-SH progressively reacts with Ag atoms to form RS-Ag complex.⁴² Since cysteine is also an organothiol, it is expected to play an essential role in the dissolution of AgNPs. Gondikas et al. showed that excess cysteine could favor the dissolution process of AgNPs, whereas another amino acid, serine (S–H bond is replaced by O–H bond), has no effect.⁴³ Zang and coworkers showed that only the isolated or reduced cysteine in a protein has a dominant role in the dissolution of NPs.⁴⁴ Although BSA contains as many as 35 cysteine residues; 34 of them are involved in S–S bond formation and only a single cysteine is present in free form (S–H). Hence, the dissolution of AgNPs at neutral pH may be negligible.Most proteins rich in sulfur-containing residues (cysteine and methionine) may degrade in alkaline solution. Florence reported that about 5 of 17 S–S bridges in BSA may be cleaved in the presence of 0.2 M NaOH.⁴⁵ Thus, at higher pH, some disulfide bonds may be cleaved and more cysteine residues may participate in the dissolution of BSA-capped AgNPs.In the second step, Ag⁺ ions generated from the dissolution of AgNPs, can be reduced either by the protein capping itself or by an external reducing agent to form NCs (Scheme 1). The tyrosine residues may be responsible for the reduction of the metal ions to NCs.^25,33 At a pH, higher than the pKa (10.46) of tyrosine, the reduction capability of tyrosine is enhanced by deprotonation of the phenolic group.^25,33,46 Moreover, the addition of a strong reducing agent (e.g., NaBH₄) may lead to a faster reduction, which favors quicker nucleation and growth of Ag atoms forming the bigger NCs (Ag₁₃NCs). However, the large Ag₁₃NCs may not be adequately stabilized by the protein conformation at that condition and hence may transform into the more stable blue-emitting Ag₈NCs.Open in a separate windowScheme 1Schematic representation of the transformation of the BSA-capped AgNPs to blue- and red-emitting AgNCs.The conformation change of the protein capping during the conversion was also supported by the circular dichroism (CD) measurements (Fig. S11^†). The formation of AgNPs results in a negligible change in the protein conformation (Table S3^†). However, the formation of red Ag₁₃ cluster results in a substantial modification in the BSA conformation. The α helix content reduces from 57% to 49%, whereas coil randomness increases from 17% to 21% without a major change in the β sheet. Interestingly, blue-emitting Ag₈ cluster perturbed the conformation of the BSA to a much larger extent (Table S3^†). As the cysteine disulfide bond has a direct role on maintaining the folded conformation of BSA, its breaking may change the protein conformation. The addition of NaOH induces breaking of S–S bond, which leads to formation of AgNCs with subsequent change in protein secondary structure.In conclusion, we report an unprecedented fast dissolution of AgNPs through activation of the protein (BSA) capping by elevating the pH of the medium to 11.5. At higher pH, the disulfide bonds may be cleaved, and the free cysteine may activate the dissolution process. The protein capping also plays a crucial role in the formation of fluorescent nanocluster after the completion of the dissolution process. Thus, we explored multiple roles of the BSA capping – (1) a stable capping agent at neutral pH to stabilize the AgNPs (2) activates the dissolution process probably via oxidative dissolution of the AgNPs (3) adsorbing the nascent silver ions within its scaffold and (4) finally reducing them to fluorescent nanocluster.     

Renewable juglone nanowires with size-dependent charge storage properties

Inspired by the biological metabolic process, some biomolecules with reversible redox functional groups have been used as promising electrode materials for rechargeable batteries, supercapacitors and other charge-storage devices. Although these biomolecule-based electrode materials possess remarkable beneficial properties, their controllable synthesis and morphology-related properties have been rarely studied. Herein, one dimensional nanostructures based on juglone biomolecules have been successfully fabricated by an antisolvent crystallization and self-assembly method. Moreover, the size effect on their electrochemical charge-storage properties has been investigated. It reveals that the diameters of the one dimensional nanostructure determine their electron/ion transport properties, and the juglone nanowires achieve a higher specific capacitance and rate capability. This work will promote the development of environmentally friendly and high-efficiency energy storage electrode materials.

Renewable juglone nanowires have been successfully fabricated, and their size effect on electrochemical charge-storage properties has been investigated.

Currently, the development of high-performance electrochemical energy-storage materials and devices is attracting intensive interest. Conventional electrode materials involving transition metal compounds,^1–4 elementary substances,^5–8 and conductive polymers,^9–12 with superior charge storage properties have been widely investigated. However, the poor biocompatibility, rising prices and depletion issues limit their sustainable applications due to their intrinsic material properties.¹³ Thus, exploring naturally abundant and renewable charge-storage materials with promising electrochemical performance is of great significance.In the biological system, its metabolic process mainly relies on ions transport and energy exchanges of redox-active biomolecules with special functional groups such as carbonyl groups, carboxyl groups, and pteridine centres.¹⁴ Due to their abundance, sustainability, environmental benignity, these renewable and nature-derivable biomolecules with well-defined charge-storage behaviors are ideal alternatives to conventional electrode materials for the next-generation green energy-storage devices.^15–17 For instance, biomolecules such as lignin,¹⁸ melanin,¹⁹ riboflavin,²⁰ juglone²¹ and humic acid²² have been demonstrated as promising electrode materials for the rechargeable batteries, supercapacitors and other charge-storage devices. Although these biomolecule-based electrode materials possess remarkable beneficial properties, they are still confronted with several serious problems of poor conductivity and high electrochemical reaction impedance.²³ For some conventional inorganic and organic active electrode materials, decreasing their size has been demonstrated to be effective strategies to enhance the electrochemical reaction kinetics by exposing more active sites to electrolytes and conductive agent.^8,24,25 These results have strongly motivated us to develop biomolecule-based nanostructures, and investigated their size-correlative charge storage behavior.^26,27Herein, one dimensional (1D) nanostructures based on juglone, a renewable redox-active biomolecule which can be derived from matured fruits of black walnut and the green peel of juglandaceae, have been successfully fabricated by an antisolvent crystallization and self-assembly method.^28–30 The size effect on the electrochemical charge-storage properties of these biomolecule-based 1D nanostructures have been investigated. It reveals that the electronic/ionic transport properties and charge-storage performance can be modulated by the size of self-assembled 1D nanostructures, and the samples with smaller diameter realize the higher specific capacitance and rate capability. Our work will provide insights for the development of high-performance biomolecule-based green energy-storage materials and devices.Juglone, also called 5-hydroxy-1,4-naphthalenedione, is a nature-derivable biomolecule, and displays a well-defined redox behavior in acetonitrile due to its quinone groups (Fig. 1a and b).³¹ As organic molecule inherently, it is soluble in organic solvent and difficult to dissolve in water, so its nano-architectures could be condonably fabricated by an antisolvent crystallization strategy (Fig. 1c) and would carry out stably for charge storage in an aqueous electrolyte.^29,30 Firstly, the juglone-biomolecule-based 1D nanostructures with different size were synthesized. The juglone micropillars with a mean diameter around 12 μm were prepared by directly recrystallizing a water/acetonitrile mixed solution of juglone at room temperature (Fig. 2a and d). Compared to commercially available raw juglone materials (Fig. S1^†), the juglone micropillars could increase its charge storage performance, but its relative large size would still confine its contact with electrolyte, and thus remarkably reduce the reaction kinetics.³² To further improve the potential reaction kinetics, the juglone microwires with an average diameter about 1 μm (Fig. 2b and e) and juglone nanowires with a mean diameter about 550 nm (Fig. 2c and f) were fabricated at room temperature using the reprecipitation method.³³ In the specific synthesis process, a high-concentration juglone acetonitrile solution is injected into water, which is a poor solvent for juglone molecules. Under stirring, juglone started to crystallize within a few seconds owing to its poor solubility in the water/acetonitrile mixture solvent and self-assembled into 1D nanostructures, and this procedure could be resulted from the π–π interaction.²⁹ It can be found that the diameter of 1D juglone materials is adjustable by tuning the concentration of juglone in acetonitrile, and the higher concentration of juglone acetonitrile solution yields the smaller size of 1D juglone materials. The insert panels show the percentage of juglone micropillar, microwire, nanowire with different diameter coverage (Fig. 2d–f).Open in a separate windowFig. 1(a) Chemical structural formula of juglone biomolecules which can be derived from the bark of black walnuts. (b) Juglone redox activity verified in a mixed solution of acetonitrile/deionized water by a three-electrode system using Pt foils as both the counter and working electrodes, Ag/AgCl as the reference electrode, and 2.3 M H₂SO₄ as the electrolyte. (c) Schematically illustration of the fabrication of the juglone nanowire/microwire.Open in a separate windowFig. 2SEM images of juglone 1D nanostructures at different magnification. (a and d) Juglone micropillar, (b and e) juglone microwire, (c and f) juglone nanowire. The insert images in panels (d–f) are corresponding mathematical statistic results of juglone samples with different diameter.Generally, the covalent bond, hydrogen bond, van der Waals forces, electrostatic forces, surface tension forces/dewetting, and π–π stacking interactions are considered as the effective factors in the self-assembly procedure of organic compounds.^34–36 The juglone biomolecule has a α-naphthol backbone with two carbonyl groups, which may induce the molecules self-assembly by the π–π interactions.^29,37–39 Fourier Transform Infrared Spectroscopy (FTIR) of such samples was utilized to examine the ingredient of these samples. As shown in Fig. S2,^† all the samples show similar characteristic peaks, the peak at 1640 cm⁻¹ is attributed to the stretching vibration of carbonyl groups, which is the main reversible redox center presented in juglone molecules. Meanwhile, Raman spectra also corroborated the results (Fig. S4^†). The crystal structures of these samples are further characterized by X-ray diffraction (XRD) as shown in Fig. S3.^† No impurity peak is observed in these patterns, demonstrating that these samples have the same crystal structure. And the peak intensity of juglone nanowire is stronger than juglone microwire and micropillar, suggesting that juglone nanowires have high crystallinity and relatively uniform crystal size.³⁰To investigate the redox behavior of 1D juglone micro/nanostructures, the cyclic voltammetry (CV) measurements were firstly performed (Fig. 3a), and the result reveals that these samples show a superior reversible redox performance, implying a potential application as energy storage materials. Meanwhile, compared with the CV curves of raw juglone, juglone micropillar and juglone microwire, the juglone nanowire exhibits the strongest redox peak, suggesting that the juglone nanowire-based electrode has a better charge storage behavior. To further evaluate the electrochemical performance of these samples, galvanostatic charge–discharge (GCD) measurements were carried out (Fig. 3b). First, juglone nanowire shows a higher specific capacity than that of microwire and micropillar. Besides, the plot of juglone 1D-based electrodes exhibit symmetric triangular shape with one pair of charge–discharge voltage plateau, and the voltage plateau of juglone micropillar, microwire and nanowire appears at 0.28 V, 0.26 V, 0.22 V, which is roughly consistent with the results of CV curves. Furthermore, it further reveals that the 1D nanostructure with smaller size exhibits higher specific capacity during the scan rates increasing from 10 to 200 mV s⁻¹ (Fig. 3c). In detail, at a low scan rate of 10 mV s⁻¹, the specific capacity delivered by juglone nanowire, juglone microwire, juglone micropillar are 389 F g⁻¹, 375 F g⁻¹, 116 F g⁻¹, respectively, and these samples possess the specific capacities of 232 F g⁻¹, 205 F g⁻¹, 80 F g⁻¹, when the scan rate are enhanced to 200 mV s⁻¹. The general perception is that nanomaterials with higher specific surface areas usually possess better electrochemical performance.⁴⁰ Obviously, the higher specific capacitance and superior rate capability are achieved by juglone nanowire electrode.Open in a separate windowFig. 3Electrochemical performance of the juglone samples with different diameter. (a) CV curves of juglone micropillar, microwire, nanowire electrodes in the potential range of −0.1 to 0.7 V (vs. Ag/AgCl) with a scan of 50 mV s⁻¹, (b) galvanostatic charge–discharge curves of juglone electrodes with different diameter at 2 A g⁻¹. (c) Statistical study of the rate performance conducted at each scan rate based on the cyclic voltammetry capacity of five electrodes as one batch. (d) Impedance phase angle as a function of frequency for juglone micropillar, microwire, nanowire electrodes. Inset is the Nyquist plot of them. (e) Correlation between the diameter and the specific capacity of the juglone samples at various scan rates.Then, the reaction kinetics of juglone with different sizes were investigated by electrochemical impedance spectroscopy (EIS) (Fig. 3d and S5^†). The crooked curve at high frequency denotes the interface resistance, involving contact and charge transfer resistances, while the low-frequency line represents ion diffusion resistance. The interface resistances of juglone nanowire, microwire, micropillar, raw material electrodes are 3.45 Ω, 3.80 Ω, 3.95 Ω, 4.02 Ω. The plot of the phase angle against the frequency reveals the characteristic frequency f₀ at the phase angle of −45° is 7.46 Hz for juglone nanowire-based electrode, and it is relatively higher than that for juglone microwire (7.36 Hz), micropillar (7.25 Hz) and raw material (6.48 Hz). Accordingly, the time constant t₀ (t₀ = 1/f₀) that represents the minimum time to discharge ≥50% of all the energy from the electrode was 0.134 s for juglone nanowire, whereas 0.136 s, 0.138 s, 0.154 s were required for juglone microwire, micropillar and raw material. The low charge transfer resistance, and short time constant validated the excellent charge and discharge capability of the juglone nanowire-based electrode.⁴¹In addition, for solid-state diffusion of H⁺ in electrode materials, the mean diffusion time is proportional to the square of the diffusion path length according to the following equation:t ≈ L²/D_H⁺1where L is the diffusion length and D_H⁺ the diffusion constant.^42,43 It has been found that the diffusion pathway will be shortening by nanostructuring of electrode materials when the diffusion constant D is same,⁴⁴ indicating that the smaller size nanostructures of juglone facilitate rapid ion/electron transport. Thus, 1D juglone nanowires possess higher specific capacitance and show slower capacity decay during the increased scan rates (Fig. 3e).Furthermore, the CV and GCD for juglone nanowire-based electrodes were tested under various scan rates and current densities. The CV profiles show that the redox activity can be maintained very well when increasing the scan rate from 10 to 100 mV s⁻¹ (Fig. 4a). Moreover, the GCD curves collected at different current densities exhibit symmetric triangular shape with one pair of charge–discharge voltage plateau, which is similar to the results of CV test (Fig. 4b and c). And the specific capacity is 342 F g⁻¹ at a current density of 2 A g⁻¹, it is approximately equivalent to the 345 F g⁻¹ at a scan rate of 20 mV s⁻¹. Next, the specific capacitance of our juglone nanowire is further compared to those of the reported conventional electrode materials.^45–51 As shown in Fig. 4d, the capacity of N/rGO (reduced graphene oxide doping with nitrogen) is 138 F g⁻¹, and our juglone nanowire have about 2-fold higher specific capacity than N/rGO, 1.5-fold higher than Fe₃O₄/rGO (154 F g⁻¹).Open in a separate windowFig. 4(a) CV curves of the juglone nanowire electrode at different scan rates from 10 mV s⁻¹ to 100 mV s⁻¹. (b) Charge and discharge curves in the current density range from 2 A g⁻¹ to 10 A g⁻¹. (c) Various specific capacity of the juglone nanowire electrode when increasing the scan rate from 10 mV s⁻¹ to 200 mV s⁻¹. (d) Specific capacity of juglone nanowire in comparison with different materials.     

Direct synthesis of covalent triazine-based frameworks (CTFs) through aromatic nucleophilic substitution reactions

Despite extensive efforts, only three main strategies have been developed to synthesize covalent triazine-based frameworks (CTFs) thus far. We report herein a totally new synthetic strategy which allows C–C bonds in the CTFs to be formed through aromatic nucleophilic substitution reactions. The as-synthesized CTF-1 and CTF-2 exhibited photocatalytic water splitting activity comparable to the CTFs made using ionothermal or Brønsted acid-catalyzed polymerization. Interestingly, CTF-2 distinguished itself by its two-photon fluorescence (emission at ∼530 nm under irradiation at either 400 nm or 800 nm).

Triazine-based frameworks (CTFs) were synthesized through a new synthetic strategy in which C–C bonds were formed through aromatic nucleophilic substitution reaction.

Covalent organic networks (CONs), usually porous and physicochemically stable light-weight materials,¹ can accommodate, interact with, and discriminate molecules, which provides CONs with many potential applications in catalysis,^1i,2 gas separation/storage,^1b,1i,2a,3 chemical sensing,⁴ drug delivery,⁵ and photovoltaic light harvesting.^1b,2a,2b,6 As a subclass of CONs, covalent triazine-based frameworks (CTFs) have attracted researchers'' increasing attention worldwide. CTFs represent a class of functionalized polymers based on the trimerization and subsequent oligomerization of aromatic dinitrile monomers in the presence of a catalyst. In brief, there have been three main strategies to synthesize CTFs from aromatic dinitrile monomers: (1) polymerization⁷ of aromatic dinitrile monomers mediated by ZnCl₂ (ionothermal process), or catalyzed by strong Brønsted acid; (2) Friedel–Crafts reaction using 2,4,6-trichloro-1,3,5-triazine (cyanuric chloride);⁸ and (3) nickel-catalyzed Yamamoto-type Ullmann cross-coupling of 2,4,6-tris-(4-bromo-phenyl)-[1,3,5]triazine.⁹ While the ZnCl₂-mediated ionothermal reaction is still the most widely used method, the high reaction temperature (400 °C) and long reaction time (40 h) limited its practical application. Although variations of the method to reduce the reaction time have been reported, the difficulty of completely removing residual ZnCl₂ remained as another obstacle.^7b The Brønsted acid method allows the polymerization to occur at ambient temperature, but it can only be applied to certain kinds of aromatic dinitrile monomers. An important environmental issue should not be overlooked in using aromatic dinitrile monomers, especially at large scale, as the dinitrile monomers were often made using over stoichiometric amounts of toxic cyanides, which is of neither economical nor ecological interest. Friedel–Crafts alkylation could work even without any addition of solvent, and is time-efficient, but residues of activating and/or bulking agents need to be removed and recovered upon framework formation. In addition, this approach is only limited to rigid and sterically demanding building blocks. Very recently, Baek''s group¹⁰ described a synthesis of CTFs at 400 °C through P₂O₅-mediated condensation of aromatic amide, which can be considered as a variation of ionothermal method of nitriles since P₂O₅ is known to catalyze the dehydration of amides into nitriles. Impressively, the approach that Jin, Tan and co-workers¹¹ recently reported was by way of condensation of aldehyde and amidine under mild reaction conditions.Herein, we are reporting a new synthetic strategy in which C–C bonds in the CTFs were formed through aromatic nucleophilic substitution reactions under mild reaction condition (refluxing in toluene). To the best of our knowledge, this is the first time that aromatic nucleophilic substitution was used for C–C bond formation in CTFs, and our method is different in many ways from the previously reported strategies. The as-synthesized CTFs exhibited photocatalytic water splitting activity comparable to the CTFs made using previously reported methods.While electrophilic substitution reactions are common for aromatic compounds, only some aryl halides can undergo substitution reactions with strong nucleophiles,¹² and some of such reactions have been used in covalent organic frameworks (COFs) preparation.¹³ We therefore envisioned that the chlorines in cyanuric chloride could be displaced by a strong nucleophilic reagent, for example, phenyllithium, and such a reaction could be used to prepare CTFs. To test the feasibility of this idea, we carried out a model reaction of phenyllithium (PhLi) and cyanuric chloride, which, we delightly found, gave 2,4,6-triphenyl-1,3,5-triazine (TriPh-triazine) in an isolated yield of 87% (Scheme 1 and ESI^†).Open in a separate windowScheme 1Reaction of phenyllithium and cyanuric chloride.Under very similar reaction conditions employed in the model reaction, two covalent triazine frameworks, CTF-1 and CTF-2 were successfully synthesized in gram-scale through aromatic nucleophilic substitution reaction of cyanuric chloride with para-dilithiumaromatic reagents in yields of 88% and 96%, respectively. The para-dilithiumaromatic reagents (1,4-dilithiumbenzene and 4,4′-dilithiumbiphenyl) used in the CTFs preparation were obtained through reactions of para-diiodoaromatic reagents with n-butyllithium, a widely used reagent in organic synthesis (caution: a highly reactive reagent and standard operating procedure should be strictly followed) (Scheme 2 and ESI^†). Albeit aromatic nucleophilic substitution reactions of cyanuric chloride was previously employed in the preparation COFs through the formation of C–X (X = N, S, O) bonds,¹⁴ such reactions have not been used to prepare COFs through C–C bond formation.Open in a separate windowScheme 2Synthesis of CTF-1 and CTF-2.CTF-1 and CTF-2 were characterized by Fourier transform infrared (FTIR) spectroscopy, solid-state ¹³C cross polarization magic angle spinning nuclear magnetic resonance spectroscopy (¹³C CP/MAS NMR), X-ray photoelectron spectroscopy (XPS) analysis, thermogravimetric (TG) analysis, field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscopy (FE-TEM), powder X-ray diffraction (PXRD) analysis, ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) and solid-state photoemission spectroscopy.The solid-state ¹³C NMR spectra of CTF-1 and CTF-2 (Fig. 1A) showed the presence of sp²-hybridized carbon of the triazine unit (∼170 ppm) as well as peaks of the aromatic rings (150–120 ppm).^7d In the FTIR spectrum of CTF-1 (Fig. 1B) (CTF-2, see ESI, Fig. S12^†), the disappearance of characteristic C–Cl band of cyanuric chloride at 850 cm⁻¹,⁸ together with the appearance of vibrational bands of triazine units at 1510 cm⁻¹ (C–N stretching) and 1360 cm⁻¹ (benzene ring breathing) implied the formation of CTFs.¹⁵Open in a separate windowFig. 1(A) Solid-state ¹³C CP/MAS NMR and repeating units of CTFs; (B) FTIR of CTF-1; (C) XPS survey spectra of CTFs; (D) TGA of CTFs; (E) PXRD of CTFs.In the XPS spectra of the CTFs, the appearance of the peaks with binding energies of 286.2 eV and 398.4 eV, assigned to C 1s and N 1s of the triazine (C–N C) ring, respectively, together with the peak at ∼284.3 eV, assigned to C 1s of the aromatic carbons,^7d,16 implied the existence of the triazine and aromatic units. The ratios of C_triazine : C_phenyl in the XPS spectra of CTF-1 and CTF-2 were 1 : 2.55 and 1 : 5.71, respectively, which were close to their theoretical values of 1 : 3 and 1 : 6 (Fig. 1C, S6 and S7 in ESI^†). The O 1s peaks seen in Fig. 1C are from physically adsorbed oxygen or water molecules which are usually found in carbon-based materials.^9b,17 Scarcely any Cl or I peaks were found in the XPS spectra of CTF-1 and CTF-2 (Fig. 1C and Tables S1 and S2, ESI^†), which indicated the completeness of aromatic nucleophilic substitution of cyanuric chloride with the corresponding aryl dilithium reagents. TG analysis shows that both CTF-1 and CTF-2 started decomposition at ∼300 °C and experienced ∼60% weight loss at 1000 °C in nitrogen atmosphere (Fig. 1D). Extended porous network structures with branched morphologies for the as-synthesized CTFs were observed by using FE-SEM (Fig. S19, ESI^†) and TEM images (Fig. S19, ESI^†). Powder X-ray diffraction (PXRD) patterns of the CTFs showed no distinct sharp peaks, but curved broad peaks at around 2θ = ∼19–20° (Fig. 1E) were observed, declaring their amorphous nature. No long-range crystallographic order for both CTFs indicated that the pore structures of the resulted CTFs maybe formed from unordered stacking and curling of the products. Such PXRD results are expected as the reaction was carried out under refluxing in toluene with vigorous stirring – the conditions unfavourable for the formation of crystalline products.The porous properties of the both CTFs were evaluated by nitrogen adsorption–desorption measurement at 77 K. Both samples exhibited the type IV adsorption isotherm character,¹⁸ typical for materials with mesoporous porosity (Fig. S8, ESI^†). The BET surface areas of CTF-1 and CTF-2 are ca. 40–100 m² g⁻¹, and their pore diameters were calculated to be around 15–24 nm by Barrett–Joyner–Halenda (BJH) method (Table S3, ESI^†). The surface areas of the CTFs synthesized using our method are larger than those of the CTFs obtained though Brønsted-acid method (5–8 m² g⁻¹),^7d but smaller than that of the CTF-1 obtained through ionothermal (ZnCl₂) process (791 m² g⁻¹).^7b In the ionothermal (ZnCl₂) process, zinc chloride might act as template and present in the as-synthesized CTFs products, removing the zinc chloride from the as-synthesized CTFs products could potentially result in higher porosity.¹⁹The UV-Vis diffuse reflectance spectra (UV-Vis DRS) of CTF-1 and CTF-2 (Fig. S10, ESI^†) showed the intrinsic absorptions in the range of visible light, which are attributed to π → π* electron transition of C_sp² and N_sp² in the polymeric networks.^7d The Kubelka–Munk transformed reflectance spectra,²⁰ as the mathematical equivalent transformation of UV-Vis DRS, showed band gaps of 2.62 eV and 2.90 eV of CTF-1 and CTF-2 (Fig. 2A), respectively.Open in a separate windowFig. 2(A) Kubelka–Munk transformed reflectance spectra of CTFs (inset: optical images of CTFs under visible light); (B) water-splitting reaction catalyzed by the two CTFs.Theoretical calculation (DFT) indicated that the top levels of the valence band (VB) of the CTFs were more positive than the redox potential of O₂/H₂O (1.23 V vs. NHE), while the bottom levels of their conduction band (CB) were more negative than the redox potential of H⁺/H₂ (0 V vs. NHE) (Fig. S9, ESI^†), which implied that the CTFs could be used in photocatalytic water splitting, like those described in previous work.^7d,21 The photo-catalytic water splitting abilities (hydrogen evolution reaction, HER) of the as-synthesized CTFs were thus evaluated in comparison with those of previously reported results under visible light (>420 nm) irradiation with triethanolamine (TEOA) as a photogenerated hole scavenger (ESI^†). Both CTFs showed water splitting ability under visible light (>420 nm), with CTF-2 possessing a slightly better catalytic activity than CTF-1 (Fig. 2B). Nonetheless, significant improvement was obviously needed on the photocatalytic activity toward water splitting for either CTFs.In addition to the absorption property and photo-catalytic water splitting abilities (HER) of the as-synthesized CTFs, the emission property of the CTFs was also evaluated. Both CTFs exhibits similar fluorescence spectra under identical higher-energy irradiation (400 nm, violet), as shown in Fig. 3A and B.Open in a separate windowFig. 3(A) Solid-state single photo fluorescence spectra (A) (400 nm excitation) and two-photo fluorescence spectra (B) (800 nm excitation) (the intensities of incident light on CTFs at 400 nm and 800 nm are the same, respectively).Interestingly, CTF-2 was found to display distinct two-photo-excited fluorescence (TPF) under lower-energy irradiation using a femtosecond laser pulse (800 nm, infrared) (red curve in Fig. 3B). The observation of two-photon excited fluorescence property, or up-conversion phenomenon, of CTF-2 implied that CTF-2 has a higher two-photo absorption cross section. The difference in the two-photo absorption cross section between CTF-1 and CTF-2 could possibly be attributed to their structural difference. As described in a previous report,²² structural features including effectiveness of conjugation, increased conjugation length, good planarity and strong electron donating ability were all critical to enhance the two-photon absorption cross sections. While the aromatics-linked triazine frameworks in repetitive structures made the both CTFs maintain good planarity, the effectively conjugated aryl units are longer in the biphenyl-linked CTF-2. The molecular force field (MM2)²³ calculation (ESI^†) indicated that the charge distributions of triazine rings (electron deficient parts) in the two CTFs were almost the same (C_triazine[+0.318]) (ESI^†), but the electron rich portions (aryl rings) of CTF-1 and CTF-2 hold different charge distributions (CTF-1 C_aromatic [−0.035] and CTF-2 C_aromatic [−0.041]), which could partially explain why CTF-2 has a higher two-photo absorption cross section value. The up-conversion phenomenon of CTF-2 implies that CTF-2 could be excited to the same excited state by either absorbing one photo under higher-energy irradiation (400 nm) or absorbing two photons under lower-energy irradiation (800 nm). As lower-energy irradiation (800 nm) provides a better penetration in scattering or absorbing media,²⁴ especially in organic and biological materials,²² CTF-2 could possibly be used in non-destructive photoimaging and optical memory.²⁵     

Enzymatic dephosphorylation-triggered self-assembly of DNA amphiphiles

In this work, phosphorylated lipid-conjugated oligonucleotide (DNA-lipid-P) has been synthesized to develop an enzyme-responsive self-assembly of DNA amphiphiles based on dephosphorylation-induced increase of hydrophobicity. Since elevated ALP level is a critical index in some diseases, ALP-triggered self-assembly of DNA amphiphiles shows promise in disease diagnosis and cancer treatment.

Enzymatic dephosphorylation-triggered self-assembly of DNA amphiphiles is developed by integrating enzymatic dephosphorylation-induced increase of hydrophobicity and intermolecular aggregation of lipid-conjugated oligonucleotides.

Lipid-conjugated oligonucleotides are DNA amphiphiles that can self-assemble into lipid-based DNA micelles. As such, lipid tails act as a hydrophobic core and DNA act as a hydrophilic corona.^1–3 Because of their advantages of facile preparation, programmable design, small size (<100 nm) and biocompatibility, lipid-based DNA micelles show potential in the imaging of intracellular targets (mRNA and small molecules) and drug delivery.^4–6To further improve their potential for better drug delivery and disease diagnosis, stimuli-responsive control of the change of morphology or assembly/disassembly of DNA micelles has attracted enormous attention in practical applications. It was reported that photo-irradiation and nucleic acid hybridization have been used to trigger assembly/disassembly or the change of morphology of DNA micelles.^7–10 For example, Jin et al. developed stability-tunable DNA micelles by using photo-controllable dissociation of an intermolecular G-quadruplex.⁷ The intermolecular parallel G-quadruplexes were introduced into lipid-based DNA micelles to lock the whole structure, resulting in enhanced structural stability against disruption by serum albumin. However, photo-controlled release of complementary DNA blocks the formation of G-quadruplexes and thus leads to the dissociation of micelles by the existence of serum albumin. In addition, Chien et al. reported stimuli-responsive programmable shape-shifting DNA micelles by controlling geometric structure and electrostatics.⁸ DNA hybridization and dissociation change the geometric structure and electrostatics of hydrophilic moiety, leads to the conversion of DNA assemblies between spherical and cylindrical structures.In spite of these advances, enzyme-responsive regulation of self-assembly of DNA amphiphiles has scarcely reported. Since the hydrophobicity of lipid tail plays a critical role in the aggregation of DNA micelles, we speculated that the self-assembly of lipid-conjugated oligonucleotides can be regulated by controlling the hydrophobicity of lipid tails. Carry this idea forward, we noted that ALP is a hydrolase that removes phosphate groups from nucleic acids or proteins.^11,12 Remarkably, ALP has been widely employed to convert hydrophilic phosphorylated small molecules to hydrophobic dephosphorylated products for molecular imaging, disease diagnosis and cancer therapy.^13–16 Herein, therefore, enzymatic dephosphorylation-triggered self-assembly of DNA amphiphile is reported. As shown in Fig. 1, DNA-lipid-P is composed of four segments: DNA, linker, lipid and phosphate groups. Four negatively charged phosphate groups at lipid terminus decrease their hydrophobicity. Therefore, DNA-lipid-P exhibits weak self-assembly. However, ALP converts DNA-lipid-P to DNA-lipid by removing phosphate groups. The newly generated DNA-lipid shows greater hydrophobicity compared to DNA-lipid-P thus enables self-aggregation in aqueous solution.Open in a separate windowFig. 1Schematic illustration of enzymatic dephosphorylation-triggered self-assembly of DNA amphiphile.Lipid-conjugated oligonucleotides are amphiphiles which compose of two segments: hydrophobic lipid tails and hydrophilic oligonucleotides. Generally, lipid-conjugated oligonucleotides can self-assemble into aggregated DNA nanostructures, for example, DNA micelles, in aqueous solution by intermolecular hydrophobic interaction. To investigate whether the hydrophobicity affects the self-assembly of lipid-conjugated oligonucleotides, a series of lipid phosphoramidites with different length of alkyl chains (six, nine, twelve and fifteen) at the lipid tail were conjugated with oligonucleotide on a DNA synthesizer. The obtained lipid-conjugated oligonucleotides named as C6-DNA, C9-DNA, C12-DNA and C15-DNA, respectively (Fig. 2a). High-performance liquid chromatography (HPLC) is a universal tool to assess the hydrophobicity of DNA by comparing their retention times. Greater retention time indicates the stronger hydrophobicity. As shown in Table S2,^† the retention time of DNA, C6-DNA, C9-DNA, C12-DNA and C15-DNA is 10.0, 19.1, 23.8, 27.3 and 30.8 minutes, respectively, suggesting that DNA with longer alkyl chains has stronger hydrophobicity. The result is consistent with the previous report.¹⁷ Next, the self-assembly of these lipid-conjugated oligonucleotides was investigated by agarose gel electrophoresis and dynamic light scattering (DLS) assays. As shown in Fig. 2b, only C15-DNA shows a tailed nucleic acids band which is belongs to the self-assembled nanostructure. In addition, results of DLS assays exhibit that the particle size of 10 μM C6-DNA, C9-DNA, C12-DNA and C15-DNA in buffer solution is 2.7 nm, 4.2 nm, 6.5 nm and 28.2 nm, respectively (Fig. 2c). Besides, the morphology of self-assembled C15-DNA micelles was visualized with atomic force microscopy (AFM) and the result shows the spherical nanostructure with diameter of 36.8 ± 6.1 nm (Fig. S7^†). Both evidences support the self-assembly of C15-DNA in buffer solution. In another word, the self-assembly of lipid-conjugated oligonucleotides into DNA micelles is hydrophobicity-dependent. A greater hydrophobicity indicates a stronger tendency of aggregation.Open in a separate windowFig. 2Hydrophobicity-dependent self-assembly of lipid-conjugated oligonucleotides. (a) Chemical structures of lipid-conjugated oligonucleotides with different length of alkyl chains at the terminus of lipid tail. (b) 1% agarose gel electrophoresis analysis of 1 μM TAMRA-labeled C6-DNA, C9-DNA, C12-DNA and C15-DNA. C15-DNA shows a tailed band in agarose gel which can be attributed to the formation of aggregated micellar nanostructure. (c) DLS size analysis of C6-DNA, C9-DNA, C12-DNA and C15-DNA in buffer solution. The average size of C6-DNA, C9-DNA, C12-DNA and C15-DNA in buffer solution is 2.7 nm, 4.2 nm, 6.5 nm and 28.2 nm, respectively.Next, we further synthesize DNA-lipid-P by solid-phase synthesis and phosphoramidite chemistry. In a previous literature, we developed a novel lipid phosphoramidite in which two DMT-protected hydroxyl groups was modified at the terminus of lipid tails thus enables further chemical phosphorylation during DNA synthesis.¹⁷ As shown in Fig. 3, linker and lipid phosphoramidites were successively conjugated at the 5′-terminus of DNA, followed by coupling with chemical phosphorylation reagent. After deprotection and purification, DNA-lipid-P was obtained and characterized by mass spectrum. As shown in Fig. S8,^† the calculated molecular weight of DNA-lipid-P is 7789.8 Da, and the observed molecular weight is 7792.4 Da. The mass error is 2.6 Da (0.03%) which is within the mass error tolerance (0.03%), suggesting the successful synthesis of DNA-lipid-P. As such, DNA-lipid was also successfully synthesized with high purity (>98%) (Fig. S9^†).Open in a separate windowFig. 3Solid-phase synthesis route of DNA-lipid-P.Having confirmed the successful synthesis of DNA-lipid-P, we further investigate the self-assembly of DNA-lipid and DNA-lipid-P in buffer solution. Nile red, a fluorescent dye that exhibits significant fluorescence in hydrophobic media, but negligible emission in aqueous solution, was used to determine the encapsulation of guest molecules to further assess the formation of the micellar structure.^7,18 Nile red (1 μM) were incubated with various concentrations of DNA, DNA-lipid or DNA-lipid-P and the corresponding fluorescence spectroscopies were recorded. As shown in Fig. 4, both DNA (Fig. 4b) and DNA-lipid-P (Fig. 4c) show weaker fluorescence emission at 630 nm, even the concentration was upper to 10 μM. However, DNA-lipid (Fig. 4d) exhibits a bright fluorescence emission at 630 nm, suggesting the formation of hydrophobic core. After calculation, the critical micelle concentration (CMC) of DNA-lipid is 0.36 μM (Fig. 4e). The remarkable difference of CMC between DNA-lipid and DNA-lipid-P indicates that enzymatic conversion of DNA-lipid-P to DNA-lipid could trigger the spontaneous intermolecular aggregation.Open in a separate windowFig. 4Characterizations of self-assembly of DNA-lipid-P and DNA-lipid. (a) The chemical structures of DNA-lipid and DNA-lipid-P. Fluorescence spectroscopies of Nile red-encapsulated DNA (b), DNA-lipid-P (c) and DNA-lipid (d) in buffer solution. The concentration of Nile red is 1 μM. (e) Fluorescence intensity of Nile red-encapsulated DNA, DNA-lipid-P and DNA-lipid at 630 nm. The CMC of DNA-lipid is 0.36 μM, and the CMC of DNA-lipid-P is larger than 10 μM.Next, ALP was used to convert DNA-lipid-P to DNA-lipid (Fig. 5a). As shown in Fig. 5b (black and red lines), the retention time of DNA-lipid and DNA-lipid-P is 26.5 and 20.9 minutes, respectively, indicates that the phosphorylation of lipid tail indeed decreases the hydrophobicity of lipid-conjugated oligonucleotides. Then, ALP was incubated with DNA-lipid-P (10 μM) at 37 °C for ten minutes and then 75 °C for five minutes to deactivate ALP, followed by subjected to fluorescence measurements. As shown in Fig. 5b (pink line), after the treatment of ALP (2 U), the DNA peak of DNA-lipid-P at 20.9 minutes disappeared; instead, a new DNA peak at 26.5 minutes was observed. Mass spectrum analysis indicates that the molecular weight of newly generated DNA peak is 7474.2 Da, which is consistent with the calculated molecular weight of DNA-lipid (7470.9 Da) (Fig. S10^†). In a word, ALP enables enzymatic dephosphorylation of DNA-lipid-P; and the generated DNA-lipid has greater hydrophobicity than DNA-lipid-P thus facilitates the controllable self-assembly into DNA micelles.Open in a separate windowFig. 5Enzymatic dephosphorylation of DNA-lipid-P. (a) Schematic of ALP-induced conversion of DNA-lipid-P to DNA-lipid. (b) HPLC chromatograms of DNA-lipid-P (black line), DNA-lipid (red line), and DNA-lipid-P treated with ALP (0.1 U (blue line), 1 U (green line) and 2 U (pink line)) in buffer solution.Encouraged by the ALP-induced dephosphorylation of DNA-lipid-P to DNA-lipid, we further assess whether ALP enables activatable self-assembly of DNA-lipid-P. As shown in Fig. 6a, DNA-lipid-P (1 μM) shows weak self-assembly in buffer solution. However, DNA-lipid (1 μM) exhibits obvious aggregation band in gel electrophoresis assay. After incubation with ALP (1 U), DNA-lipid-P + ALP group also shows tailed band which suggests the formation of aggregated nanostructures. In addition, results of Nile red-encapsulated fluorescence experiments also support the conclusion of ALP-activate self-assembly of DNA-lipid-P (Fig. 6b). Therefore, ALP-induced enzymatic dephosphorylation triggers the self-assembly of DNA-lipid-P.Open in a separate windowFig. 6Enzymatic dephosphorylation-triggered self-assembly of DNA-lipid-P. (a) 1% agarose gel electrophoresis analysis of DNA-lipid-P (lane 1), DNA-lipid (lane 2) and DNA-lipid-P treated with ALP (1 U) (lane 3). (b) Fluorescence spectroscopies of Nile red-encapsulated DNA-lipid, DNA-lipid (ALP), DNA-lipid-P or DNA-lipid-P (ALP).In summary, enzymatic dephosphorylation-triggered self-assembly of DNA amphiphile is developed by integrating enzymatic dephosphorylation-induced increase of hydrophobicity and intermolecular aggregation of lipid-conjugated oligonucleotides. The strategy may also suitable for many other amphiphiles. Since elevated ALP level is a critical index in some diseases and even cancers, we believe that ALP-triggered self-assembly of DNA-lipid-P shows potential in disease diagnosis and cancer therapy.