Recovery of expensive Pt/C catalysts from the end-of-life membrane electrode assembly of proton exchange membrane fuel cells

A simple and economical route using an environmental friendly solvent is reported for recovering catalyst from the end-of-life membrane electrode assembly. This precious component is recovered in the form of Pt/C and ionomer powder. The catalyst exhibited an electrochemical surface area of 42 m² g⁻¹ and remarkable stability, showcasing its potential for second life applications.

A sustainable approach for the recovery of EoL MEAs and possibility of pushing the recovered products back into the supply chain.

The drive for the decarbonization of the automotive industry owing to the adverse effects of global warming on both environment and human health has propelled the development of emission free, hydrogen-based fuel cell vehicles (FCVs). However, the complete diffusion of FCVs in fossil fuel-powered internal combustion (IC) engine dominated global vehicle market is limited by their high cost.^1,2 This is majorly due to the usage of Pt-based catalysts in electrochemical reactions generating power, which contributes ∼35–45% to the cost of FCVs.^3,4 The mass production of FCVs to cater to road transport as substitutes to IC-based engines will further increase the requirement of Pt. This is a bottleneck for the wide-scale applications of FCVs; besides being expensive, Pt has limited and geographically restricted natural reserves. Also, the mass employment of FCVs requires readiness with respect to the waste management strategies for end-of-life (EoL) components for the effective utilization of their precious components. This has boosted research on the recycling/recovering Pt from the EoL membrane electrode assemblies (MEAs) of fuel cells. Moreover, the efficient recycling of precious components from EoL MEAs presents sustainable waste management of fuel cell components as well as the development of cost-effective alternative technologies with the recycled components.Recovery strategies for the EoL MEAs of fuel cells are typically categorized into four classes: (i) high-temperature combustion processes, (ii) acid dissolution process, (iii) electrochemical process-based recovery route, and (iv) alcohol treatment method. All these techniques have been proved efficient for the recovery of Pt, but suffer from drawbacks towards effective implementation, hence presenting avenue for better recovery processes. The high-temperature combustion methods are most commonly employed for the Pt recovery from EoL MEAs. However, besides being energy intensive, this process also leads to the emission of toxic HF, arising from high fluorine-containing components, perfluorosulfonic acid (PFSA) membranes and polytetrafluoroethylene (PTFE) binders in MEAs.⁵ The acid dissolution technique for the recovery of Pt utilizes highly corrosive acids, such as HCl, H₂SO₄, HNO₃, aqua regia (HNO₃/HCl) and piranha acid (H₂SO₄/H₂O₂), requires specialized infrastructure and also suffers from hazardous emissions such as HCl vapour, Cl₂, NO_x and SO₂.^6,7 Electrochemical methods of the Pt recovery often employs corrosive or toxic electrolytes, which also lead to harmful emissions.^8,9 These recovery processes also result in the complete loss of the other expensive component, i.e., the Nafion membrane. The alcohol treatment process offers the possibility of membrane recovery; however, utilizes large quantity of alcohols making the process cost-intensive.¹⁰ Besides, this process results in the recovery of larger particle-sized Pt-based catalysts (>50 μm), making them unsuitable for direct catalytic applications.¹¹ Considering the limitations of current recovery technologies, the development of a simple and environmental benign route with the potential for the direct application of recovered products is imperative for the efficient utilization of natural resources and development of sustainable recovery centres. Sustainable EoL recovery technologies are also vital to support future Pt-based technologies and take away burden from limited Pt natural reserves.Herein, a low-temperature hydrothermal treatment is reported for the recovery of EoL MEAs of a proton exchange membrane fuel cell (PEMFC) in 50 : 50 v/v of water and isopropanol (IPA) solution under ambient pressure, as presented in Fig. 1.Open in a separate windowFig. 1Schematic representation of the recovery process of EoL MEA.The presence of the aqueous solution of IPA aids in the disruption of the bond between the fluorocarbon-containing Nafion membrane and the coated Pt/C catalysts, facilitating catalyst detachment.¹² Furthermore, the hydrothermal treatment in the aqueous solution of IPA results in the dissolution of the membrane,¹³ and subsequently Pt/C catalysts were recovered via a simple vacuum filtration technique. The Nafion membrane was recovered in the form of an ionomer-enriched filtrate solution, which can be used to prepare a new Nafion membrane via recasting.¹⁴ The recovered ionomer solution was further heat-treated for 12 h at 80 °C for the evaporation of volatile solvents to yield ionomer powder. This offers the possibility of reuse and desirable modification of the recovered Nafion membrane as powder, which can be dissolved in large number of solvents.¹⁵ Fig. 2 shows the Fourier transform infrared (FTIR) spectrum of the recovered ionomer enriched solution (IES-R) and fresh commercial 5 wt% Nafion dispersion. The spectra display molecular segments similar to commercial Nafion dispersion. The strong band in the region 3700–3000 cm⁻¹ is attributed to the O–H stretching of water. However, the shoulder peak at 2978 cm⁻¹ and sharp peak at 1640 cm⁻¹ correspond to hydrated H₃O⁺.¹⁶ The peak at 1236 cm⁻¹ can be attributed to the stretching vibrations of SO₃⁻ groups.¹⁷ The peaks in the range 1400–1000 cm⁻¹ can be ascribed to the CF₂ stretching vibrations.^16,17 The peak appeared at 963 cm⁻¹ ascribes the to C–O–C stretching modes, whereas the region between 800–500 cm⁻¹ mostly corresponds to C–F groups.^18,19 The FTIR spectroscopy results confirm the dissolution of the Nafion membrane from EoL MEAs, resulting in the formation of an ionomer-enriched solution. This also reveals the efficacy of the employed recovery route to reclaim the precious membrane component of EoL MEAs.Open in a separate windowFig. 2FTIR spectra of IES-R and commercial 5 wt% Nafion dispersion.The recovery of Pt/C catalysts and Nafion membrane from the EoL MEAs of PEMFC via the low temperature non-toxic aqueous alcohol-based route was further confirmed through X-ray diffraction studies. Fig. 3a shows the X-ray diffractogram of recovered ionomer powder (IP-R) obtained after the evaporation of solvents from the IES-R solution and Pt/C catalyst (Pt/C-R). The recovered ionomer powder showed the characteristic peak of Nafion at 16.6° and 39.9°, corresponding to the polyfluorocarbon chains of Nafion.^20,21 The X-ray diffractogram of Pt/C-R displays characteristic diffraction planes, corresponding to the face centered cubic (fcc) Pt lattice. The peaks at 39.8°, 46.3°, 67.5°, 81.4° and 85.9° correspond to the (111), (200), (220), (311) and (222) planes of the fcc Pt lattice. The average crystallite size of the recovered Pt was calculated to be 6.1 nm using the Scherrer equation utilizing the most abundant reflections of Pt (111). It is noteworthy to mention that much smaller crystallite sized Pt nanoparticles (NPs) were recovered in this study employing the non-toxic aqueous IPA solvent-based low temperature recovery route as compared to Pt NPs (30 nm) recovered using a corrosive, concentrated H₂SO₄ solution.²¹ A small hump at around 26° in the diffractogram of Pt/C-R corresponds to carbon support. The morphology and particle size distribution of the recovered catalyst were investigated via transmission electron microscopy (TEM). Fig. 3b shows the TEM image and the corresponding particle size distribution histogram of Pt/C-R. Spherical Pt NPs with an average particle size of 5.5 ± 0.9 nm were found to be uniformly distributed on the carbon support. This value is in close agreement with the crystallite size determined using the XRD data. The attainment of smaller sized Pt NPs substantiates the prospective of the recovered catalyst for direct applications in fuel cells signifying avenue for the efficient utilization of the recovered products from EoL MEAs. Furthermore, the total amount of recovered Pt/C was 0.74 g, which is 98.7% of the catalyst loading on untreated EoL CCM, revealing the efficacy of the recovery process. The Pt wt% in the recovered catalyst was determined via the thermogravimetric analysis under air atmosphere, and the metal content was found to be ∼17 wt% (Fig. S1^†). This value was used for electrochemical calculations.Open in a separate windowFig. 3(a) X-ray diffractogram of IP-R and Pt/C-R, and (b) TEM microgram and particle size distribution histogram of the Pt/C-R catalyst.The catalytic activity of the Pt/C-R catalyst for fuel cell applications was assessed via cyclic voltammetry (CV) in an acidic medium (0.5 M H₂SO₄). The performance was compared with the Pt/C-C catalyst to assess the applicability of the recovered catalyst in real time applications. Fig. 4a shows the cyclic voltammograms of Pt/C-C and Pt/C-R catalysts. Both the catalysts exhibited the characteristic hydrogen adsorption/desorption feature (0–0.3 V) of the active Pt surface. Pt/C-C exhibited a higher current density as compared to Pt/C-R, indicating superior catalytic activity. This is also evident from the electrochemical surface area (ECSA) calculation of the catalysts. The ECSA represents the active Pt sites available for participation in an electrochemical reaction. Hence, ECSA is a measure of catalytic activity and was calculated using the following equation.ECSA = Q_H/(0.21 × [Pt])where Q_H is the coulombic charge for hydrogen desorption in mC cm⁻², 0.21 is the coulombic charge required to oxidize a monolayer of H₂ in mC cm⁻², and [Pt] is the Pt loading on the working electrode in mg cm⁻². Pt/C-C exhibited higher ECSA (83 m² g⁻¹) as compared to Pt/C-R (42 m² g⁻¹), which is anticipated considering the derivation of the recovered catalyst from EoL MEA. However, Pt/C-R exemplified ∼50% ECSA of the fresh catalyst showcasing its potential for subsequent usage as a catalyst in fuel cells. It is also noted that Pt/C-R exhibited similar ECSA to that of the Pt catalyst prepared from the H₂PtCl₆ precursor obtained after the recovery process based on electrochemical dissolution in 1 M HCl (43 m² g⁻¹).^22,23 This highlights the advantage of the employed recovery technique as free from secondary processing such as dissolution of spent Pt and its redeposition/reduction to obtain re-usable catalyst for subsequent applications.Open in a separate windowFig. 4(a) CV plots of Pt/C-C and Pt/C-R catalyst, and (b) CV plots of the Pt/C-R catalyst before and after 5000 cycles of the stability test at a scan rate of 50 mV s⁻¹ in N₂ saturated 0.5 M H₂SO₄.The stability of the catalyst was also evaluated to access its potential for further applications. The stability of Pt/C-R was evaluated through measurement of changes in ECSA after subjecting the catalyst to 5000 potential cycles in the range of 0.6–1.0 V at the scan rate of 100 mV s⁻¹ in the N₂ saturated 0.5 M H₂SO₄ electrolyte. The CVs of Pt/C-R before and after the stability tests are presented in Fig. 4b. No significant suppression of hydrogen desorption peaks in the voltammograms were observed implying the minimal loss of ECSA. Pt/C-R retained ∼93% of its initial ECSA (42 m² g⁻¹vs. 39 m² g⁻¹) with a loss rate of 0.0006 m² g⁻¹ per cycle. The results indicate remarkable stability of the recovered catalyst exemplifying its potential for second life applications. The electrochemical results demonstrated the potential of the recovered catalyst for direct fuel cell applications. The practical viability of the Pt/C-R catalyst for fuel cell applications was assessed through single cell PEMFC performance. Fig. 5 shows the polarization and power density curves of the Pt/C-R catalyst. The cell exhibited an open circuit potential of 0.922 V and maximum power density of 134 mW cm⁻², revealing the functioning of the recovered catalyst in PEMFC. However, studies are in progress on improving the current density of the recovered catalyst for PEMFC applications.Open in a separate windowFig. 5Single cell polarization and power density curves of the Pt/C-R catalyst recorded at the cell temperature of 60 °C with humidified H₂ and air gases.To summarize, the study reports a simple, cost and energy effective, low temperature hydrothermal route utilizing an environmentally friendly aqueous IPA solvent for the recovery of two precious components of EoL MEAs of PEMFC, Pt/C catalyst and Nafion membrane. The molecular composition of the recovered ionomer enriched solution was investigated through FT-IR spectroscopic studies, and it was found to be similar to that of commercial Nafion dispersion, indicating the efficacy of the recovery route as well the recovered product for the utilization of precious components of EoL MEAs for second life applications. The recovered Pt/C exhibited well dispersed Pt NPs on the carbon support with average particle size of 5.5 ± 0.9, nm revealing the effectiveness of the recovery route in the attainment of smaller Pt NPs with prospective for direct applications. In addition, the process demonstrated a high Pt/C catalyst recovery rate of 98.7%. The electrochemical studies supported the potential of the Pt/C-R catalyst for fuel cell applications. Pt/C-R exhibited an ECSA of 42 m² g⁻¹ and remarkable stability with loss of only ∼7% of ECSA after 5000 cycles of stability tests, signifying their viability for fuel cell applications. Pt/C-R also showed ∼50% ECSA of a commercial, fresh Pt/C catalyst disclosing the avenue for the further utilization of the recovered catalyst for niche applications. Furthermore, a single cell PEMFC with the Pt/C-R catalyst demonstrated maximum power density of 134 mW cm⁻², showcasing the applicability of recovered Pt/C as an alternative and sustainable candidate for the development of cost effective PEMFCs. Also, the effective recovery of the Pt/C catalyst in the non-toxic solvent system at a low temperature under mild reaction conditions and utilizing simple equipment is an astounding advantage of this methodology. In addition, the recovery of the active ionomer along with Pt/C is another novel feature of the adopted recovery approach. Added to this two-fold advantage, the whole recovery process exemplifies a sustainable approach for both precious component recovery from EoL MEAs as well as the possibility of pushing the recovered products back into the supply chain.     

Highly dispersed Pt nanoclusters supported on zeolite-templated carbon for the oxygen reduction reaction

The formation of highly dispersed Pt nanoclusters supported on zeolite-templated carbon (PtNC/ZTC) by a facile electrochemical method as an electrocatalyst for the oxygen reduction reaction (ORR) is reported. The uniform micropores of ZTC serve as nanocages to stabilize the PtNCs with a sharp size distribution of 0.8–1.5 nm. The resultant PtNC/ZTC exhibits excellent catalytic activity for the ORR due to the small size of the Pt clusters and high accessibility of the active sites through the abundant micropores in ZTC.

Electrochemically synthesized highly dispersed Pt nanoclusters (PtNCs) stabilized by the nanocages of zeolite-templated carbon (ZTC) exhibit excellent electrocatalytic performance toward the oxygen reduction reaction.

Platinum (Pt) is currently considered one of the best electrocatalysts for the oxygen reduction reaction (ORR), which occurs at the cathode of a fuel cell and is the key process determining the overall performance.^1–5 However, the high cost and scarcity of Pt limit its wide commercialization in this field. According to the US Department of Energy, the total Pt loading is required to be below 0.125 mg cm⁻², in contrast to a presently used Pt loading of 0.4 mg cm⁻² or more for fuel cell application.⁴ Therefore, reducing the Pt loading without loss or with an improvement of the cathode performance has received significant interest in electrocatalytic research for fuel cell systems.^6–10 In this regard, reducing the size of Pt particles to a nanocluster scale (size < 2 nm) and maximizing the Pt dispersion may offer an efficient way to achieve maximum utilization of the Pt electrocatalyst with appropriate consumption.^4,11–15The size of nanomaterials generally plays a critical role in controlling the physical and chemical properties for catalytic applications.^16–20 With a decrease in the particle size to the nanoscale, quantum size effects are induced, which alter the surface energy of the material due to unsaturated coordination and change in the energy level of the d orbital of metal atoms, leading to spatial localization of the electrons.^17–20 This size-induced effect on the electronic structures at the active sites modifies the capability of binding the reactant molecules in catalytic reactions, thereby altering the activity of the nanocatalyst.²⁰ When the particle contains a few to several dozens of atoms with sizes, ranging from sub-nanometer to 2 nm often termed as nanocluster that bridges nanoparticle and a single atom.²¹ However, the Pt single atom is not an appropriate electrocatalyst for the ORR in a fuel cell system as the fast four-electron (4e⁻) pathway for the reduction of O₂ to H₂O requires at least two neighboring Pt atoms.^22,23 Anderson''s group demonstrated that the ratio between the production of H₂O (product of 4e⁻ process) and H₂O₂ (2e⁻) in the ORR strongly depends on the number of atoms in the Pt cluster. Typically, it requires more than 14 atoms in a Pt cluster to produce H₂O efficiently through the 4e⁻ pathway of the ORR.²⁴ Therefore, Pt nanoclusters having more than a dozen atoms have proven to be highly efficient ORR electrocatalysts for fuel cell systems.^13–15 Upon decreasing the size of the nanoparticles to a nanocluster, the electronic state and structure are known to be changed, leading to an increase of the catalytic activity in the ORR. Therefore, it is highly desirable to synthesize a Pt nanocluster-based material as an ORR electrocatalyst with high catalytic performance. To date, several synthesis strategies, such as wet-chemical, atomic-layer deposition, and photochemical methods, have been applied for the preparation of well-dispersed Pt nanoclusters on different types of support, such as dendrimer, metal oxide, and carbon materials.^{13–15,25–31}An alternative approach to synthesize Pt nanocluster (PtNC) is the encapsulation of the cluster within nanosized pores, for example, by utilizing microporous (diameters less than 2 nm) carbon materials.³² Among the microporous carbons, zeolite-templated carbon (ZTC) has been attractive for supporting Pt clusters due to its ordered microporous structure.^33–37 ZTC is a potentially promising material as catalyst support as it offers the advantages of extremely large surface area and high electrical conductivity of graphene-like carbon frameworks constituting a three-dimensional (3D) interconnected pore structure.³⁶ Moreover, the micropores of ZTC can serve as nanocages for stabilization of the Pt nanoclusters. Coker et al. used Pt²⁺ ion-exchanged zeolite as a carbon template to synthesize Pt nanoparticles in ZTC with size in a range of 1.3 to 2.0 nm.³³ Recently, atomically dispersed Pt ionic species was synthesized via a simple wet-impregnation method on ZTC containing a large amount of sulfur (17 wt%).²³ Itoi et al. synthesized PtNC consisting of 4–5 atoms and a single Pt atom in ZTC using the organoplatinum complex.³⁷ Although these methods produced Pt nanoclusters with narrow size distribution and atomic dispersion, they required multi-step processes and/or high-temperature treatment (>300 °C). High-temperature treatment often induces the sintering of nanoclusters to aggregated clusters. Therefore, it is highly desirable to develop a simple and low-cost method for the preparation of PtNC supported on ZTC (PtNC/ZTC) for use as an efficient ORR electrocatalyst. The electrochemical reduction approach offers an alternate and efficient route for the synthesis of PtNC in the micropores of ZTC. The electrochemical method is one of the popular ways to prepare electrocatalysts because it is a simple single-step procedure and ensures electrical contact between the nanoparticles and the support.^38,39Herein, we report a facile electrochemical method for the formation of PtNC with a narrow size range of 0.8–1.5 nm supported on ZTC. The resultant PtNC/ZTC shows higher electrocatalytic activities towards ORR compared to that of commercial Pt/C. Here, ZTC plays two important roles: (i) it provides nanocages to stabilize the PtNC and (ii) it accelerates the ORR activity by enhancing the accessibility of active sites through its abundant micropores. Fig. 1a shows a schematic representation of the typical electrochemical synthesis of PtNC/ZTC. In the first step, ZTC was impregnated with a Pt-precursor dissolved in a water–ethanol mixture. As ZTC possess ordered micropores (Fig. S1a^†) with high Brunauer–Emmett–Teller (BET) surface area of 3400 m² g⁻¹ (vide infra), the uniform adsorption and anchorage of PtCl₆²⁻ ions into the micropores of ZTC was favored. After impregnating and drying, the resultant ZTC–PtCl₆²⁻ was mixed with water–ethanol and Nafion to make the ink for the preparation of the electrode. Using the prepared electrode, a potential of 0.77 V vs. reversible hydrogen electrode (RHE) (Fig. 1b) was applied followed by potential cycling between 1.12 to −0.02 V vs. RHE until the cyclic voltammogram was stabilized. The Pt content of PtNC/ZTC was determined to be ∼10 wt% (Fig. S2^†) by thermogravimetric analysis (TGA). The obtained PtNC/ZTC was electrochemically characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The cyclic voltammogram (Fig. 1c) after potential cycling in fresh KOH electrolyte shows the characteristic Pt peaks corresponding to hydrogen adsorption and desorption. The Nyquist plots (Fig. 1d) demonstrate that PtNC/ZTC has lower electrolyte resistance (42 Ω) than that of ZTC (70 Ω), implying an improvement in the conductivity of ZTC by the presence of PtNC. Due to the increase in the conductivity, PtNC/ZTC could facilitate the electron transfer more effectively than ZTC, enhancing its electrocatalytic activity.Open in a separate windowFig. 1(a) Illustration for the formation of PtNC/ZTC:Pt-precursor was impregnated into ZTC micropores, and then a potential (0.77 V vs. RHE) was exerted on the ZTC–PtCl₆²⁻ composite in a 0.1 M KOH solution to form PtNC/ZTC (b) Chronoamperometric response of ZTC–PtCl₆²⁻ at a constant potential of 0.77 V (vs. RHE) in 0.1 M KOH electrolyte. (c) Cyclic voltammogram of PtNC/ZTC in a fresh 0.1 M KOH at a scan rate of 20 mV s⁻¹. (d) Nyquist plots of ZTC and PtNC/ZTC in 0.1 M KOH. Fig. 2a and b show images from aberration-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF). The HAADF-STEM images exhibit the typical morphology of the final product (PtNC/ZTC) after electrochemical reduction. As shown in Fig. 2a, it is very clear that isolated PtNCs are uniformly dispersed in ZTC. These PtNCs have a homogeneous distribution with a narrow size range (0.8–1.5 nm, Fig. 2b). On further magnification, the STEM image shows a cluster-like structure of Pt (Fig. 2c). The STEM image of selected PtNC (Fig. 2d) reveals that it consists of ∼20 atoms. The number of atom content in PtNC was further determined by matrix-assisted laser-desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry using trans-2-[3-(4-test-butylphenyl)-2-methyl-2-propenylidene]malononitrile as the matrix.^40,41 As shown in Fig. S3,^† MALDI-TOF measurement produces a mass spectra with a predominant peak centered at ∼3700 Da corresponding to the Pt₁₉ cluster. The TEM image (Fig. S4 a and b^†) validates the formation of PtNC with an average size of 0.9 nm. In addition, the energy dispersive X-ray spectrometer (EDS) mapping images clearly shows the uniform dispersion of Pt nanocluster in ZTC (Fig. S4c^†). The X-ray powder diffraction (XRD) pattern (Fig. 2e) of PtNC/ZTC showed three broad peaks associated with small size metallic Pt corresponds to (111), (200), and (311) planes (Fig. 2e, inset), along with peaks of ZTC at 2θ = 7.8° and 14.9° corresponding to the ordered microporous structure. Along with the structural analysis, the porous texture of PtNC/ZTC was examined by Ar adsorption (Fig. 2f). PtNC/ZTC had a high BET surface area of 2360 m² g_ZTC⁻¹, which is 1.4 times lower than that of pristine ZTC (3400 m² g_ZTC⁻¹). The decrease in Ar adsorption capacity after the formation of PtNC in ZTC is interpreted as a result of the filling of ZTC micropores by PtNC. This micropore filling was confirmed in the pore size distributions of the pristine ZTC and the metal-loaded carbon (inset of Fig. 2f). The X-ray photoelectron spectroscopy (XPS) results reveal the signature of Pt in ZTC (Fig. S5^†). The elemental survey (Fig. S5a^†) shows the signature of C 1s, O 1s, F 1s (Nafion), and Pt 4f. The chemical nature of Pt in PtNC/ZTC was inspected by a detailed Pt 4f XPS analysis. The deconvoluted Pt 4f XPS spectra (Fig. S5b^†) reveals the presence of both metallic and ionic Pt species. The peaks observed at 71.0 (4f_7/2) and 74.2 (4f_5/2) eV correspond to metallic Pt whereas the other peaks positioned at 72.6 (4f_7/2) and 76.0 (4f_5/2) are attributed to Pt²⁺ and the peaks at 74.9 (4f_7/2) and 77.8 (4f_5/2) eV are attributed to Pt⁴⁺ originating from the surface oxidation of metallic Pt.⁴²Open in a separate windowFig. 2(a–d) Representative spherical aberration-corrected HAADF-STEM images of PtNC/ZTC at various magnifications. (e) XRD pattern of PtNC/ZTC and (f) Ar adsorption–desorption isotherms of ZTC and PtNC/ZTC. Inset in (e) shows a 30 times magnified high-angle region of XRD of PtNC/ZTC. Inset in (f) shows the pore size distributions of the ZTC and PtNC/ZTC.The formation of narrow sized PtNC by the electrochemical method can be ascribed to the stabilization of PtNC in the ZTC micropores, which serve as cages to impose a spatial limitation on the size of the Pt clusters. For comparison, Pt supported on ZTC was also prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction at high temperature (300 °C). The Pt obtained by this incipient wetness impregnation method shows the formation of Pt nanoparticles on the exterior surface of ZTC (PtNP/ZTC) (Fig. S6^†). The formation of larger Pt nanoparticles is due to the sintering at high temperature, showing that even ZTC micropores could not prevent the aggregation of PtNCs at high temperatures. Fig. 3 shows the electrochemical ORR activity of PtNC/ZTC using linear sweep voltammetry (LSV) technique on a rotating disc electrode (RDE) in a 0.1 M KOH solution saturated with O₂ at a scan rate of 5 mV s⁻¹. The ORR activity of ZTC (without PtNC) was measured for comparison as well. As shown in Fig. 3a, PtNC/ZTC exhibited higher diffusion limiting current density and higher positive onset and half-wave potential compared to ZTC alone, indicating that PtNC is the active center for the ORR. To investigate the effect of the Pt loading amount on the ORR activity, PtNC/ZTC with various Pt loadings, 2–20 wt%, was used for the measurement of LSV at 1600 rpm. With an increase in Pt content, both the onset and half-wave potential shifted towards more positive potential up to 10 wt% loading of Pt (Fig. 3a and S7^†). Upon further increase of loading of Pt on ZTC to 20 wt%, both the onset and half-wave potential of PtNC/ZTC shifted towards less positive potential along with a slight decrease in the diffusion limiting current density (Fig. 3a). The decrease in the ORR activity of PtNC/ZTC at high loading of Pt (20 wt%) was attributed to the decrease in the electrochemically active surface area (Fig. S8^†) and decrease in the specific surface area (Fig. S9^†). The STEM image clearly shows that the aggregated Pt clusters were formed on the exterior surface of ZTC at 20 wt% loading of Pt (Fig. S10c^†), blocking the accessibility of active sites. Therefore, PtNC/ZTC with the optimum loading of 10 wt% of Pt leads to superior ORR activity with a high positive onset potential of 0.99 V, which is similar to commercial Tanaka Pt/C (Pt/C-TKK) (Fig. 3b), and a half-wave potential of 0.87 V, which is ∼10 mV more positive than that of commercial Pt/C-TKK (0.86 V) (Fig. 3b). Compared to the case of PtNC/ZTC, both the onset and half-wave potential of PtNP/ZTC prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction with the same loading of Pt exhibited a less positive value (Fig. S11^†). The poorer activity of PtNP/ZTC is due to the blockage of active sites by larger PtNPs formed on the exterior surface of ZTC (Fig. S6^†).Open in a separate windowFig. 3(a) RDE ORR polarization curves of PtNC/ZTC with different mass loading of Pt. (b) Comparison of PtNC/ZTC (PtNC_10%/ZTC) with commercial Pt/C-TKK at the same loading of 40 μg_Pt cm⁻². (c) RDE ORR polarization curves of PtNC/ZTC at different rotation speeds. Inset in (c) shows the corresponding K–L plots at different potentials. (d) Represents the kinetic current density values of Pt/C-TKK and PtNC/ZTC at the potential of 0.8 V vs. RHE.To investigate the kinetics of the ORR activity of PtNC/ZTC, LSV measurements were performed with RDE at different rotating rates (Fig. 3c), and the kinetics was analyzed using a Koutecký–Levich (K–L) plot (Fig. 3c, inset). From Fig. 3c, it was observed that the current density increases with the increasing speed of rotation of the electrode, which is characteristic of a diffusion-controlled reaction. The corresponding linear K–L plots (Fig. 3c, inset) with a similar slope at different potentials reveal that the number of transferred electrons was ∼4, indicating that O₂ is directly reduced to OH⁻ and the ORR is dominated by the H₂O₂-free 4e⁻ pathway. To estimate the amount of produced peroxide ion, rotating ring-disc electrode (RRDE) measurement was performed and the produce peroxide ion calculated from RRDE curve was < 4% (Fig. S12^†). The kinetic current density (J_k) obtained from K–L plot at the potential of 0.8 V (Fig. 3d) for PtNC/ZTC (J_k = 50 mA cm⁻²) is 2.2 times higher than that of commercial Pt/C-TKK (J_k = 22 mA cm⁻²).As Pt-based electrocatalysts are known to be highly active in an acidic medium, the ORR activity of PtNC/ZTC in O₂-saturated 0.1 M HClO₄ was also evaluated by comparing it with that of commercial Pt/C-TKK with the same loading of Pt on the electrode surface using RDE at a scan rate of 5 mV s⁻¹. The PtNC/ZTC-based electrode exhibited ORR activity with an onset potential of 0.96 V (Fig. 4), which is close to that of Pt/C-TKK (0.98 V), and half-wave potentials of 0.84 V, which is 20 mV more positive than that of Pt/C-TKK (0.82 V). PtNC/ZTC showed a slightly higher diffusion-limiting current density of ∼5.9 mA cm⁻² (0.4–0.7 V) compared with that of the Pt/C-TKK catalyst (∼5.6 mA cm⁻²). The kinetics of the ORR in an acidic medium was further analyzed using RDE at different rotation rates (Fig. S13^†) and it was observed that the current density increases with the increasing speed of rotation of the electrode, as in the case of the alkaline medium. The number of electron involved and the amount of produced H₂O₂ estimated by RRDE measurement were ∼4 and < 5%, respectively (Fig. S14^†). The mass activity of PtNC/ZTC obtained using the mass transport corrected kinetic current at 0.8 V is 0.15 A mg⁻¹, which is 3.2 times higher than that of Pt/C-TKK (0.046 A mg⁻¹).Open in a separate windowFig. 4(a) RDE ORR polarization curves at 1600 rpm and (b) mass activity at 0.8 V of PtNC/ZTC and Pt/C-TKK in 0.1 M HClO₄.Furthermore, the methanol tolerance of PtNC/ZTC was assessed by intentionally adding methanol to the oxygen saturated electrolyte solution (both in alkaline and acidic media). The commercial Pt/C-TKK was used for comparison as well. The peak current densities for methanol oxidation with PtNC/ZTC were ∼2.8 and ∼3 times lower than that of Pt/C-TKK in alkaline (Fig. 5a) and acidic (Fig. 5b) media, respectively. These results indicate that PtNC/ZTC has much higher tolerance towards methanol than Pt/C-TKK does. This higher methanol tolerance of PtNC/ZTC can be attributed to the small size of the Pt cluster, which may not be sufficient to catalyze the oxidation of methanol efficiently, as the oxidation of methanol requires Pt ensemble sites.⁴³Open in a separate windowFig. 5ORR polarization curves of PtNC/ZTC and Pt/C-TKK in the absence (solid line) and presence (dotted line) of 0.1 M of CH₃OH at a rotation rate of 1600 rpm in (a) alkaline and (b) acid media.The durability of PtNC/ZTC was also investigated by the amperometric technique. The test was performed at a constant voltage of the half-wave potential in an O₂-saturated alkaline medium and at 0.7 V in an O₂-saturated acidic medium at a rotation rate of 1600 rpm (Fig. S15a and b^†). The durability of the PtNC/ZTC catalyst in the alkaline medium was higher than that of Pt/C-TKK, exhibiting a 30% decrease compared to a 40% decrease of Pt/C-TKK in 5.5 h of ORR operation (Fig. S15a^†). The higher durability of PtNC/ZTC compared to Pt/C-TKK in the alkaline medium may be due to the stabilization of PtNC by pore entrapment. In the acidic medium, however, PtNC/ZTC exhibited a 54% decrease in the initial current after 5.5 h of operation while a 33% decrease was observed in the case of Pt/C-TKK (Fig. S15b^†). The decrease in ORR activity in the acidic medium may be due to the leaching out of tiny Pt nanoclusters in acid electrolyte from the ZTC micropores. To understand the decrease in the ORR durability with time, STEM measurements of PtNC/ZTC after 5.5 h of ORR operation were performed. In the alkaline medium, the STEM image of post-ORR PtNC/ZTC shows a slight change in the size of PtNC (Fig. S15c^†) while the STEM image of PtNC/ZTC after ORR in the acidic medium exhibited sintering of PtNC into large particles with an average size of 30 nm (Fig. S15d^†), resulting in a decrease of the ORR activity. In the alkaline medium, the decrease in ORR activity with time may be due to the oxidation of the ZTC support in KOH.⁴⁴We attributed the excellent ORR activity of PtNC/ZTC to the interplay between the following: (1) the structure of the Pt cluster possessing a high ratio of surface atoms that benefits the surface reactions,^45–47 (2) the microporous 3D graphene-like structure of the ZTC support that enables easy access of O₂ and electrolyte molecules to the active sites,⁴⁸ and (3) the high conductivity and large accessible surface area of ZTC that facilitates the electron transfer.^49–51     

Size effect studies in catalysis: a simple surfactant-free synthesis of sub 3 nm Pd nanocatalysts supported on carbon

Supported Pd nanoparticles are prepared under ambient conditions via a surfactant-free synthesis. Pd(NO₃)₂ is reduced in the presence of a carbon support in alkaline methanol to obtain sub 3 nm nanoparticles. The preparation method is relevant to the study of size effects in catalytic reactions like ethanol electro-oxidation.

A simple surfactant-free synthesis of sub 3 nm carbon-supported Pd nanocatalysts is introduced to study size effects in catalysis.

A key achievement in the design of catalytic materials is to optimise the use of resources. This can be done by designing nanomaterials with high surface area due to their nanometre scale. A second achievement is to control and improve catalytic activity, stability and selectivity. These properties are also strongly influenced by size.^1–3 To investigate ‘size effects’ it is then important to develop synthesis routes that ensure well-defined particle size distribution, especially towards smaller sizes (1–10 nm).Metal nanoparticles are widely studied catalysts. In several wet chemical syntheses, NP size can be controlled using surfactants. These additives are, however, undesirable for many applications^4,5 since they can block active sites and impair the catalytic activity. They need to be removed in ‘activation’ steps which can negatively alter the physical and catalytic properties of the as-produced NPs. Surfactant-free syntheses are well suited to design catalysts with optimal catalytic activity⁶ but their widespread use is limited by a challenging size control.³Palladium (Pd) NPs are important catalysts for a range of chemical transformations like selective hydrogenation reactions and energy applications.^7–9 It is however challenging to obtain sub 3 nm Pd NPs, in particular without using surfactants.² Surfactant-free syntheses are nevertheless attracting a growing interest due to the need for catalysts with higher performances.^10–14Promising surfactant-free syntheses of Pd NPs were recently reported.^8,15 The NPs obtained in these approaches are in the size range of 1–2 nm and show enhanced activity for acetylene hydrogenation⁸ and dehydrogenation of formic acid.¹⁵ Enhanced properties are attributed to the absence of capping agents leading to readily active Pd NPs. The reported syntheses consist in mixing palladium acetate, Pd(OAc)₂, in methanol and the reduction of the metal complex to NPs occurs at room temperature. The synthesis is better controlled in anhydrous conditions to achieve a fast reaction in ca. 1 hour. Another drawback is that the synthesis must be stopped to avoid overgrowth of the particles. Therefore, a support material needs to be added after the synthesis has been initiated and no simple control over the NP size is achieved.^8,15In this communication a more straightforward surfactant-free synthesis leading to sub 3 nm carbon-supported Pd NPs in alkaline methanol at ambient conditions is presented. A solution of Pd(OAc)₂ in methanol undergoes a colour change from orange to dark, indicative of a reduction to metallic Pd, after ca. a day. However, only ca. 1 hour is needed with Pd(NO₃)₂, Fig. 1 and UV-vis data in Fig. S1.^† The fast reduction of the Pd(NO₃)₂ complex in non-anhydrous conditions is a first benefit of the synthesis presented as compared to previous approaches.Open in a separate windowFig. 1Pictures of 4 mM Pd metal complexes in methanol without or with a base (as indicated).For particle suspensions prepared with Pd(OAc)₂ or Pd(NO₃)₂ the NPs agglomerate and quickly sediment leading to large ‘flake-like’ materials. When the reduction of Pd(NO₃)₂ in methanol is performed in presence of a carbon support and after reduction the solution is centrifuged and washed in methanol, a clear supernatant is observed indicating that no significant amount of NPs are left in methanol. Transmission electron microscope (TEM) analysis confirms that NPs are formed and well-dispersed on the carbon support surface and no unsupported NPs are observed, Fig. 2a. Likely, the reduction of the NPs proceeds directly on the carbon support. However, the size of the NPs is in the range 5–25 nm, which is still a relatively large particle size and broad size distribution.Open in a separate windowFig. 2TEM micrographs of Pd NPs obtained by stirring 4 mM Pd(NO₃)₂ in methanol and a carbon support for 3 hours, (a) without NaOH and (b) with 20 mM NaOH. Size distribution histograms are reported in Fig. S4.^† The same samples after electrochemical treatments are characterised in (c) and (d) respectively. Size distribution histograms are reported in Fig. S7.^†Assuming a ‘nucleation and growth’ mechanism, the NPs should become larger over time.¹⁶ But the reaction is so fast that by stopping the reaction before completion, size control is not achieved and unreacted precious metal is observed, Fig. S2.^† To achieve a finer size control and more efficient use of the Pd resources, a base was added to the reaction mixture, e.g. NaOH.³ In alkaline media, the formation of Pd NPs is slower; it takes ca. 60 minutes to observe a dark colour for a 5 mM Pd(NO₃)₂ solution with a base/Pd molar ratio of 10 in absence of a support, Fig. 1.Also in alkaline methanol, the NPs agglomerate over time in absence of a support material. However, if the alkaline solution of Pd(NO₃)₂ is left to stir in presence of a carbon support the desired result is achieved, i.e. Pd NPs with a significantly smaller size and size distribution of ca. 2.5 ± 1.0 nm, Fig. 2b. The NPs homogeneously cover the carbon support and no unsupported NPs are observed by TEM suggesting that the NPs nucleate directly on the carbon surface. Furthermore, the supernatant after centrifugation is clear, indicating an efficient conversion of the Pd(NO₃)₂ complex to NPs, Fig. S3.^† Furthermore, there is no need for an extra reducing agent as in other approaches, for instance in alkaline aqueous solutions.⁹The benefits of surfactant-free syntheses of Pd NPs for achieving improved catalytic activity have been demonstrated for heterogeneous catalysis.^8,15 Surfactant-free syntheses are also well suited for electrochemical applications where fully accessible surfaces are required for fast and efficient electron transfer. Several reactions for energy conversion benefit from Pd NPs. An example is the electro-oxidation of alcohols,⁷ in particular ethanol¹⁷ (see also Table S1^†).Previous studies optimised the activity of Pd electrocatalysts by alloying,^18–20 by using different supports^17,21–23 or crystal structures.^24,25 Investigating NPs with a diameter less than 3 nm was challenging.^2,26,27 The surfactant-free synthesis method presented here allows to further study the size effect on Pd NPs supported on carbon (Pd/C) for electrocatalytic reactions.In Fig. 3, results for ethanol electro-oxidation in 1 M ethanol solution mixed with 1 M KOH aqueous electrolyte are reported based on cyclic voltammetry (CV) and chronoamperometry (CA) with Pd/C catalysts exhibiting 2 significantly different size distributions. The electrode preparation, the measurement procedure and the sequence of electrochemical treatments are detailed in the ESI.^† In order to highlight size effects, we compare geometric and Pd mass normalized currents (Fig. 3a and c) as well as the oxidation currents normalized to the Pd surface area (Fig. 3b).Open in a separate windowFig. 3Electrochemical characterisation of carbon supported Pd NPs with 5–25 nm (grey) and 2.5 nm (dark) size in 1 M KOH + 1 M ethanol aqueous electrolyte. (a) 2^nd CV before chronoamperometry (CA), (b) current normalised by the electrochemically active surface area of Pd, (c) CA recorded at 0.71 V vs. RHE after 50 cycles between 0.27 and 1.27 V.It is clearly seen that based on the geometric current density, the smaller Pd NPs exhibit significantly higher currents for ethanol oxidation than the larger NPs. To differentiate if this observation is a sole consequence of the different surface area, the electrochemically active surface area (ECSA) has been estimated based on “blank” CVs (without ethanol) recorded between 0.27 and 1.27 V vs. RHE in pure 1 M KOH aqueous electrolyte and integrating the area of the reduction peak at ca. 0.68 V, Fig. S5.^† As conversion factor, 424 μC cm⁻² was used.²⁸Using this method, the smaller NPs with a size around 2.5 nm exhibit an ECSA of 92 m² g⁻¹ whereas the larger NPs with a size in the range 5–25 nm exhibit an ECSA of 47 m² g⁻¹, consistent with a larger size. Normalising the ethanol electro-oxidation to these ECSA values instead of the geometric surface area, Fig. 3b, still indicates a size effect. It is clearly seen that the smaller Pd NPs exhibit higher surface specific ethanol oxidation currents, in particular at low electrode potentials. Furthermore, a clear difference in the peak ratios in the CVs is observed. The ratio in current density of the forward anodic peak (j_f, around 0.9 V) and the backward cathodic peak (j_b, around 0.7 V vs. SCE) is around one for the smaller NPs, whereas it is about 0.5 for the larger NPs. The forward scan corresponds to the oxidation of chemisorbed species from ethanol adsorption. The backward scan is related to the removal of carbonaceous species not fully oxidised in the forward scan. The higher j_f/j_b ratio therefore confirms that the smaller NPs are more active for ethanol electro-oxidation and less prone to poisoning, e.g. by formation of carbonaceous species that accumulate on the catalyst surface.^29,30 This observation is further supported by a chronoamperometry (CA) experiment, Fig. 3c, at 0.71 V performed after continuous cycling (50 cycles between 0.27 and 1.27 V at a scan rate of 50 mV s⁻¹). In the CA testing of the thus aged catalysts at 0.71 V, the ethanol oxidation current on the two catalysts starts at around the same values, however, its decay rate is significantly different. The Pd mass related oxidation currents for the smaller NPs are after 30 minutes almost twice as high (ca. 200 A g_Pd⁻¹) as for the larger ones (ca. 130 A g_Pd⁻¹), confirming that the small Pd NPs are less prone to poisoning. In particular a factor up to 4 in the Pd mass related ethanol oxidation currents after 1800 s of continuous operation is achieved compared to a recently characterised commercial Pd catalyst on carbon,²⁰ Table S1.^† Despite different testing procedure reported in the literature, it can be concluded from these investigations that the surfactant-free synthesis presented shows promising properties for electrocatalytic ethanol oxidation even after extended cycling.The extended cycling, however, has different consequences for the two catalysts. For the small (2.5 nm) NPs of the Pd/C catalyst, a massive particle loss, but only moderate particle growth is observed as highlighted in Fig. 2 (see also Fig. S6^†). TEM micrographs of the two Pd/C samples recorded before and after the complete testing (CVs and CAs, for details see Fig. S7^†) show that the catalyst with small Pd NPs exhibits a pronounced particle loss as well as a particle growth to ca. 6 nm probably due to sintering. By comparison, for the Pd/C catalyst with the large Pd NPs, no significant influence of the testing on particle size or particle density is apparent.     

Dual enzyme-like activity of iridium nanoparticles and their applications for the detection of glucose and glutathione

Here we show that iridium nanoparticles (Ir NPs) functionally mimic peroxidase and catalase. The possible mechanism of intrinsic dual-enzyme mimetic activity of Ir NPs was investigated. Based on the excellent peroxidase-like activity of Ir NPs, a new colorimetric detection method for reduced glutathione (GSH) and glucose was proposed.

Iridium nanoparticles could functionally mimic peroxidase and catalase. The possible mechanism of intrinsic dual-enzyme mimetic activity of Ir NPs was investigated.

Recently, nanomaterials with inherent enzyme-like activity have attracted considerable attention due to their simple preparation, storage, and separation, as well as the low cost as compared with natural enzymes. Various nanomaterials have been shown to mimic the activity of oxidase, peroxidase, catalase or superoxide dismutase (SOD), ranging from metals,^1–10 metal oxides,^11–15 and metal coordination complexes^16–21 to carbon-based nanomaterials.^22–26 The ability of these nanomaterials to replace specific enzymes may offer new opportunities for enzyme-based applications. For example, nanozymes with oxidase-like or peroxidase-like activity have shown potential applications in biosensing and immunoassay, such as the detection of H₂O₂, glucose, antioxidants, antigens, antibodies and so on.²⁷ By using nanozymes with peroxidase-like activities, Wei and co-workers recently have developed novel sensor arrays to detect biothiols and proteins as well as discriminate cancer cells owing to the differential nonspecific interactions between the components of the sensor arrays and the analytes, providing a potential approach to discriminate versatile analytes.⁸ Nanomaterials with SOD-like or catalase-like activity have exhibited antioxidant activity, thus could protect aerobic cells from oxidative stress, showing potential application in inflammation therapy.^6,28–31 Also, nanozymes with high catalase-like activity was able to produce O₂ at the hypoxic tumor site, serving as efficient agents for cancer therapy.^23,32,33 Very recently, Zhang'' group has developed a simple and biocompatible platform to elevate O₂ for improving photodynamic therapeutic efficacy by combining the photosensitizer with Prussian blue nanomaterials.³⁴ Prussian blue could catalyze H₂O₂ to generate O₂, and then the photosensitizer transforms the O₂ to produce singlet oxygen (¹O₂) upon laser irradiation for cancer therapy. Besides, Qu et al. have found the porous platinum nanoparticles with catalase-like activity, which greatly enhanced radiotherapy efficacy and overcame the hypoxic tumor microenvironment.³⁵In this work, we demonstrate that Ir NPs exhibited both peroxidase-like and catalase-like activities. As shown in Scheme 1, Ir NPs can catalyze the decomposition reaction of H₂O₂ into oxygen and water, possessing potential applications in the cancer therapy as catalase mimics. On the other hand, the tiny Ir NPs with the average diameter of 2.4 nm exhibited high peroxidase-like catalytic activity. Furthermore, by using H₂O₂ as an intermediary, a simple and sensitive colorimetric detection method for GSH and glucose has been designed.Open in a separate windowScheme 1Schematic presentation for dual-enzyme mimetic activity of Ir NPs.Synthesis of Ir NPs were carried out by a simple chemical reduction process, in which sodium hexachloroiridate(iii) hydrate was used as precursor with ascorbic acid as a protecting agent and sodium borohydride as the reducing agent (see Experimental section in ESI^†). After heating and stirring at 95 °C for 15 min, the resulting homogeneous light brown Ir NPs dispersion was obtained with good stability and reproducibility. The obtained Ir NPs was thoroughly characterized by various methods. Transmission electron microscopy (TEM) images indicated that the as-prepared Ir NPs showed a narrow size distribution with the average diameter of ∼2.4 nm (Fig. 1A–C). UV-vis spectrum of Ir NPs showed an absorption peak at ∼280 nm (Fig. 1D), in agreement with the value reported earlier,²⁸ indicating the formation of Ir NPs. XPS spectra of Ir 4f in Ir NPs were presented in the Fig. S1.^† A pair of doublet peaks at 61.0 and 64.0 eV were observed, revealing that Ir in Ir NPs is mostly metallic Ir(0).³⁶ Furthermore, inductively coupled plasma-optical emission spectroscopy (ICP-OES) disclosed that the exact concentration of Ir NPs is 25 μg mL⁻¹.Open in a separate windowFig. 1(A and B) TEM images of Ir NPs at different magnification. (C) Size distribution histogram of Ir NPs. (D) UV-vis absorption spectrum of Ir NPs. The inset shows the photograph of Ir NPs dispersed in the aqueous solution.To investigate the peroxidase-like activity of Ir NPs, the peroxidase coupled assay was employed and the change in absorbance of reaction was monitored using a UV-vis absorbance spectrophotometer. As shown in Fig. 2A, exposure of Ir NPs to a colorless peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂ resulted in the fast oxidation of TMB to a blue product. However, no color change of the TMB substrate was observed only with Ir NPs or H₂O₂, indicating the intrinsic peroxidase-like activity of Ir NPs. Similar to HRP, the catalytic activity of Ir NPs is dependent on the pH, temperature and catalyst dosage. Fig. S2^† showed that the catalytic activity of Ir NPs was much higher in weakly acidic solution and reached its highest at pH 4.0, consistent with those reported for peroxidase-like NPs and HRP.^37–40 In the range of 20–80 °C, the maximum catalytic activity was obtained under 50 °C. For simplicity, we adopted pH 4.0 and room temperature (∼20 °C) for subsequent analysis of peroxidase-like activity of Ir NPs. When increasing the dosage of Ir NPs, the catalytic activities of Ir NPs clearly increased as shown in Fig. 2B. Interestingly, some small gas bubbles were observed in the tubes at the same time.Open in a separate windowFig. 2(A) The absorption spectra and digital photos of different colorimetric reaction systems: (a) TMB + H₂O₂, (b) TMB + Ir NPs, and (c) TMB + H₂O₂+Ir NPs. (B) Time-dependent absorbance changes at 652 nm of TMB reaction solutions catalyzed by the different concentrations of Ir NPs. (C) Time-dependent absorbance changes at 240 nm of 20 mM H₂O₂ catalyzed by the different concentrations of Ir NPs incubated in 0.1 M HAc–NaAc buffer (pH 9.0). (D) The effect of concentration of Ir NPs on the formation of hydroxyl radical with terephthalic acid as a fluorescence probe. Reaction condition: 0.1 M HAc–NaAc buffer (pH 6.0).In order to confirm which gas was produced and whether Ir NPs had the intrinsic catalase-like activity, the decomposition of H₂O₂ was further investigated by monitoring the changes of UV-vis absorbance at 240 nm under the basic conditions. As shown in Fig. 2C, the absorbance was obviously decreased as the dosage of Ir NPs increased, and many gas bubbles could be observed in the cuvette, indicating Ir NPs could catalyze the decomposition of H₂O₂ into O₂. Temperature and pH can also make a big effect on the catalase-like activity of Ir NPs. Under the basic conditions or higher temperature, much more and bigger gas bubbles were produced (Fig. S3^†), suggesting higher catalase-like activity of Ir NPs. Obviously, Ir NPs possessed intrinsic catalase-like activity, which could be regulated by adjusting the temperature and pH.The formation of ˙OH during the reactions was assessed to better understand the mechanism for the dual enzyme-like activity of Ir NPs. Terephthalic acid was adopted here as a fluorescence probe to trap ˙OH. As shown in Fig. 2D, in the absence of Ir NPs, terephthalic acid emitted blue. However, the fluorescence intensity was gradually decreased as the concentration of Ir NPs increased, suggesting that Ir NPs could consume ˙OH radicals rather than generate ones. The reactivity of Ir NPs appears to be different from that of the other peroxidase mimics, where ˙OH mediates the oxidation of organic substrate.As Ir NPs exhibited the peroxidase-like activity, we monitored the reaction of Ir NPs with H₂O₂ and TMB. The apparent steady-state kinetic parameters were determined by changing one substrate concentration while keeping the other substrate concentration constant. The value ε = 39 000 M⁻¹ cm⁻¹ (at 652 nm) for the oxidized product of TMB was used here to obtain the corresponding concentration term from the absorbance data. As shown in Fig. S4,^† we observed that the oxidation reaction catalyzed by Ir NPs followed the typical Michaelis–Menten behavior toward both substrates. The Michaelis–Menten constant (K_m) and the maximum initial velocity (V_m) given in Table S1^† were obtained by using Lineweaver–Burk plot (Fig. S4B and D^†). Compared with the Pd–Ir cubes,³⁸ Ir NPs presented a similar K_m for TMB and a very low K_m for H₂O₂, suggesting that Ir NPs have a higher affinity to H₂O₂. Moreover, Ir NPs presented larger V_m for both of TMB and H₂O₂, indicating the strong catalytic activity of Ir NPs. What is more important is that Ir NPs showed high stability after long-term storage. After five months of storage at room temperature, the peroxidase-like catalytic activity of Ir NPs maintained 96% (Fig. S5^†), significantly expanding their practical applications.On the basis of the high affinity and catalytic activity of Ir NPs to H₂O₂, the analytes that could consume or produce H₂O₂ could be detected indirectly by using the TMB as substrate.²⁷ Therefore, a simple colorimetric method was developed to detect GSH and glucose using Ir NPs. GSH, which plays an important role in many cellular processes including redox activities, signal transduction, detoxification, and gene regulation,^41,42 can consume H₂O₂ and result in the shallowing color of the Ir NPs-TMB-H₂O₂ system. As presented in Fig. 3A, the absorbance dropped sharply with the addition of GSH. And a linear relationship between the absorbance and logarithmic values of GSH''s concentrations was obtained in the range from 200 nM to 100 μM (Fig. 3B). The level changes of GSH have been linked to varieties of diseases, such as diabetes, psoriasis, liver damage and Parkinsons.^43,44 The proposed colorimetric biosensor provided a sensitive method to monitor GSH. Furthermore, because H₂O₂ is the main product of the glucose oxidase (GOx)-catalyzed reaction; glucose could be detected based on the combination of GOx. Fig. 3C shows typical glucose concentration–response curves with the linear range of 10 μM to 2 mM. According to the principle of S/N = 3, the calculated detection limit was 5.8 μM. Table S2^† was listed to compare the sensing performance of Ir NPs with other nanomaterials. Ir NPs are superior to other nanomaterials in lower detection limit and wider detection range for colorimetric determination of glucose.Open in a separate windowFig. 3(A) Dose–response curve for GSH detection at 652 nm. (B) Linear relationship between the absorbance and logarithmic values of GSH''s concentrations in the range from 200 nM to 100 μM. (C) Dose–response curve for glucose detection at 652 nm. (D) Determination of the selectivity of glucose detection (from left to right: blank, 0.3 mM glucose, 3 mM fructose, 3 mM maltose, 3 mM lactose and 3 mM sucrose). Inset: the color change with the different solutions. The error bars represents the standard deviation of four measurements.To explore the selectivity of above glucose sensor, 10 times concentration of control samples including fructose, maltose, lactose and sucrose were tested as shown in Fig. 3D. The color difference could be distinguished by the naked eye, suggesting the high selectivity of the biosensing system for glucose detection. Using this method, we detected glucose in 50-fold dilution fetal bovine serum to demonstrate the feasibility of this biosensor for practical applications, and the results are listed in Table S3.^† As can be seen, the recoveries of glucose fall in the range of 93.3–104% by using the standard addition method. The proposed biosensor was also applied for determining glucose concentrations in blood samples donated by healthy and diabetic persons (Fig. S6^†). According to the calibration curve, the concentration of glucose from different samples was 7.0 mM and 14.4 mM, which agrees well with that measured in the local hospital, 6.8 mM and 14.4 mM. Therefore, this colorimetric method is suitable and satisfactory for glucose analysis of real samples with high sensitivity and selectivity.In summary, Ir NPs synthesized by a simple chemical reduction process exhibited both of peroxidase-like and catalase-like activity. Moreover, the dual enzyme-like activity could be regulated by adjusting the temperature and pH. On the one hand, Ir NPs could consume ˙OH radicals exhibiting potential applications in the antioxidant therapeutics as antioxidant nanozymes. On the other hand, as peroxidase mimics, Ir NPs were successfully applied in the construction of colorimetric biosensors to detect GSH and glucose. This work will facilitate the utilization of intrinsic dual-enzyme activity and other catalytic properties of Ir NPs in analytical chemistry, biotechnology, and medicine.     

Facile synthesis of high-performance SiO2@Au core–shell nanoparticles with high SERS activity

This study proposes a facile and general method for fabricating a wide range of high-performance SiO₂@Au core–shell nanoparticles (NPs). The thicknesses of Au shells can be easily controlled, and the process of Au shell formation was completed within 5 min through sonication. The fabricated SiO₂@Au NPs with highly uniform size and SERS activity could be ideal SERS tags for SERS-based immunoassay.

This study proposes a facile and general method for fabricating a wide range of high-performance SiO₂@Au core–shell nanoparticles (NPs).

The design and controlled fabrication of Au nanocomposites have attracted extensive attention because of their outstanding chemical and optical properties and wide applications in various fields, such as catalysis,¹ drug delivery,² photothermal cancer therapy,³ sensing,⁴ and surface-enhanced Raman scattering (SERS).⁵ However, small Au nanocomposites tend to aggregate, which seriously affects their stability and usability. The combination of silica nanoparticles (SiO₂ NPs) and Au shells provides a good alternative to Au nanocomposites.^6,7 These SiO₂ NPs are ideal core materials due to their high stability, easy preparation, uniform spherical shape, and large particle size range.^8,9Many synthesis methods have been explored for the fabrication of SiO₂@Au core–shell NPs; these methods include electroless plating,¹⁰ self-assembly,¹¹ layer-by-layer synthesis,¹² and seed growth.¹³ The seed growth method is the most commonly used to coat the Au shell on the surface of the SiO₂ core and involves two steps: deposition of nucleus seeds on the functionalized SiO₂ surface and Au shell growth. Although this method is beneficial for the synthesis of nanostructures with narrow size distribution, it exhibits two major shortcomings. First, the surface of SiO₂ NPs must be functionalized with various organosilanes containing amino (–NH₂) or mercapto (–SH) groups for adsorption or deposition of metal seeds on the SiO₂ NPs before subsequent growth of Au shells.^14,15 However, full surface amino/mercapto modification is often difficult to achieve; in this regard, dense metal seed layer formation on the surface of SiO₂ NPs cannot be achieved, eventually affecting the uniform and complete Au shell coating. Second, the formation of complete Au shell on the SiO₂ NPs is frequently achieved using a slow-growth approach through slow or multiple addition of HAuCl₄ to the seed-coated SiO₂ NPs suspension containing reducing agents.^16,17 The application of these slow-growth methods is restricted by its complex procedure and time-consuming preparation. Thus, a facile method must be developed for synthesis of Au coated SiO₂ NPs with controllable Au shell, good dispersibility, and fast preparation.In this work, we report a sonochemically assisted seed growth method for facile synthesis of monodisperse SiO₂@Au core–shell NPs for the first time. Cationic polyethyleneimine (PEI) was used to form a cationic thin interlayer with numerous primary amine groups for easy adsorption of dense Au seeds on the silica surface and keeping the nanostructure stability during shell growth. Sonication was used instead of traditionally used mechanical stirring to shorten the reaction time. The entire reaction process for Au shell formation was completed within 5 min. Moreover, the thickness of the Au shell was easily controlled outside the silica cores of different sizes. To the best of our knowledge, the proposed method is the most convenient synthesis route for preparation of high-performance SiO₂@Au core–shell NPs to date. Our results further demonstrate that the fabricated SiO₂@Au NP could be an ideal SERS tag for SERS-based lateral flow immunoassay (LFA). The method was validated for detection of human immunoglobulin M (IgM) and showed a detection limit as low as 0.1 ng mL⁻¹. The details of the experiments including SiO₂@Au NPs preparation, SERS-based LFA strip preparation, SERS detection protocol, and sensitivity test were provided in the ESI^† section.The synthesis principle of monodisperse SiO₂@Au NPs is presented in Fig. 1a. SiO₂ NPs were first prepared by using a modified Stöber method as the core. The SiO₂ NPs were ultrasonically treated with PEI solution to form PEI-coated SiO₂ NPs (SiO₂@PEI). The positively charged PEI effectively attached to the negatively charged SiO₂ NPs and formed a stable polymer layer via electrostatic self-assembly. SiO₂–Au seed NPs were prepared by adsorbing small Au NPs (3–5 nm) on the PEI layer of SiO₂ NPs densely and firmly through covalent binding between the –NH₂ groups of PEI and Au NPs. Finally, monodisperse SiO₂@Au NPs were quickly obtained through the reduction of HAuCl₄ by hydroxylamine hydrochloride (NH₂OH·HCl) under the stabilization of PVP. The uniform Au shells outside the SiO₂ NPs were formed within 5 minutes through the isotropic growth of all Au seeds under sonication.Open in a separate windowFig. 1Synthesis principle of SiO₂@Au NPs (a). TEM images of (b) SiO₂ NPs, (c) SiO₂–Au seed NPs, (d) SiO₂@Au NPs and their corresponding elemental mapping in (g), (h), and (i) respectively. (e) HRTEM picture and (f) bright-field TEM image of a single SiO₂@Au NP.The morphology of the as-synthesized products in different stages were characterized through transmission electron microscopy (TEM). The as-prepared SiO₂ NPs were uniform in size and had a diameter of approximately 140 nm (Fig. 1b). After coating the SiO₂@PEI NPs with Au seeds, many small seeds homogeneously adhered to the surface of the silica core (Fig. 1c). The dense Au seeds acted as randomly oriented crystalline sites for subsequent seed-mediated growth of the Au shell. Fig. 1d and e show the low- and high-magnification TEM images of the final SiO₂@Au core–shell NPs, respectively. Continuous and rough edges were detected around the SiO₂@Au NPs. The HRTEM image (Fig. 1e) indicated that large adjacent Au NPs covered the entire surface of the SiO₂ NPs, forming a complete and rough Au shell. The average particle size increased from 140 nm to 190 nm after the Au shell formation, indicating that the thickness of the Au shell was approximately 25 nm. Additionally, the SEM images (Fig. S1^†) showed that the SiO₂@Au NPs were successfully fabricated on a large scale and exhibited a rough surface and uniform size. The elemental composition of SiO₂@Au NPs was also confirmed through X-ray mapping (Fig. 1f–i). The results indicated that a layer of Au shell was uniformly coated on the surface of the SiO₂ NPs. The zeta potentials of SiO₂, SiO₂@PEI, SiO₂–Au seeds, and SiO₂@Au NPs in aqueous solution were found to be −46.7, +41.9, −7.4, and −21.1 mV, respectively (Fig. S2^†). The significant change in the zeta potential revealed the successive completion of PEI coating, Au seed adsorption, and Au shell formation. Fig. 2a shows the typical XRD patterns of the as-synthesized SiO₂–Au seed (blue line) and SiO₂@Au NPs (red line). The specific XRD pattern of Au is characterized by five peaks positioned at 2θ values of 38.3°, 44.3°, 64.5°, 77.4°, and 81.6°, which correspond to the reflections of the (111), (200), (220), (311), and (222) crystalline planes of Au (JCPDS no. 04-0784), respectively.^18,19 The intensity of the diffraction peaks of SiO₂@Au NPs increased when the Au shells were coated. No peaks of SiO₂ and PEI were detected in the XRD pattern because of their amorphous form.²⁰Open in a separate windowFig. 2Typical XRD patterns (a) and UV-visible spectra (b) of the as-synthesized products. Fig. 2b illustrates the UV-vis spectra of the as-synthesized products dispersed in deionized water in different stages. SiO₂ and SiO₂@PEI NPs had no obvious absorption peaks in the UV-vis spectra (curves a and b). SiO₂–Au seed NPs displayed a clear absorption peak at about 568 nm (curve c), which confirms the formation of the Au seed layer onto the surface of SiO₂ NPs. As the Au shell formed, the UV-vis spectral peak obviously red-shifted, and the intensity increased significantly (curve d). This result could be due to the strong interaction between and the coupling of the large adjacent Au NPs of the Au shells outside the SiO₂ NPs.²¹The strategy for Au shell formation is essentially seed-mediated growth. Thus, the surface morphology of SiO₂@Au NPs can be easily controlled by adjusting the Au³⁺ concentration by using a constant amount of SiO₂–Au seed. Fig. 3a–d shows the representative TEM images of SiO₂@Au NPs synthesized with different concentrations of HAuCl₄ while the other parameters remained constant. As the concentration of the HAuCl₄ increased from 0.01 mM to 0.04 mM, the Au seeds absorbed outside the SiO₂ NPs gradually increased in size and finally intersected with each other and formed a continuous and Au shell of a different thickness.Open in a separate windowFig. 3TEM images of SiO₂@Au NPs synthesized with different HAuCl₄ concentrations: (a–d) 0.01, 0.02, 0.03, and 0.04 mM HAuCl₄. (e) UV-vis spectra of SiO₂@Au synthesized with different HAuCl₄ concentrations: curves (a–e) 0, 0.01, 0.02, 0.03, and 0.04 mM HAuCl₄ and the corresponding Raman spectra of DTNB (f). Fig. 3e shows the UV-vis spectra of the synthesized SiO₂–Au seed and SiO₂@Au NPs with different Au shell thicknesses. The absorption peak of the obtained products red shifted gradually from 568 nm to 700 nm, and the peak width became broader with increasing concentration of HAuCl₄. Thus, the absorption peak of SiO₂@Au NPs can also reflect the formation and thickness of the Au shell. Fig. 3f shows the SERS activity of SiO₂@Au NPs prepared with different HAuCl₄ concentrations. 5,5-Dithiobis-(2-nitrobenzoic acid) (DTNB) was used as Raman molecule because it contains a double sulfur bond, which can be chemically coupled to the Au shell to form Au–S chemical bond and could produce strong and concise SERS peaks located at 1062, 1148, 1331, and 1556 cm⁻¹.^22,23 Moreover, DTNB molecules can provide free carboxyl groups as sites to conjugate antibodies.²⁴ As shown in the Raman spectra in Fig. 3f, the SiO₂–Au seed showed fairly weak SERS ability (spectra a), whereas the SiO₂@Au NPs exhibited gradually enhanced SERS activity as the HAuCl₄ concentration increased (spectra b–d). However, the overgrowth of the Au shell decreased the SERS activity of SiO₂@Au NPs (spectra e). This phenomenon could be attributed to the fully continuous Au shell formation, which reduced the nanogaps and hot spots on the surface of SiO₂@Au NPs. Hence, we chose SiO₂@Au NPs prepared with 0.02 mM HAuCl₄ as the optimal material for SERS application because of their nearly complete Au shell and optimal enhancement effect.PEI can be self-assembled on the surface of SiO₂ NPs of any size under sonication conditions. Thus, our proposed PEI-assisted seed growth method is a general route for preparing monodisperse SiO₂@Au core–shell particles with different sizes, ranging from nanoscale to microscale levels. Fig. 4a–c shows the TEM images of single SiO₂–Au seed NPs with different sizes (70–300 nm), and Fig. 4d–f clearly shows their corresponding fabricated SiO₂@Au NPs, respectively. The TEM images of multiple SiO₂@Au NPs with different sizes are displayed in Fig. S3.^† All of the obtained SiO₂@Au NPs possess homogeneous nanostructure, uniform Au nanoshell, and good dispersity. We further examine the dependence of SERS activity on the SiO₂@Au NPs size up to 350 nm. Fig. S4^† shows a set of SERS spectra of DTNB (10⁻⁵ M) adsorbed on the SiO₂@Au NPs of different sizes. The SERS intensity presented in the figure is the average intensity from 10 spots for each sample. Obviously, all the SiO₂@Au NPs exhibited excellent SERS abilities, and the signal intensities were gradually enhanced as the particle size increased. In fact, the Au shells of SiO₂@Au NPs were made of large sized Au NPs. This experimental result indicates that the larger the size of the Au NPs of shell, the higher the SERS activity achieved.Open in a separate windowFig. 4(a–c) TEM images of single SiO₂–Au seed with different sizes: (a) 70, (b) 150, and (c) 300 nm and their corresponding fabricated SiO₂@Au NPs (d), (e), and (f), respectively.For the determination of the SERS sensitivity of the 80 nm SiO₂@Au NPs, a series of DTNB ethanol solution (with concentration ranging from 10⁻⁴ M to 10⁻¹¹ M) was prepared. Each tube of DTNB solution was mixed with 10 μL of SiO₂@Au NPs (1 mg mL⁻¹) and sonicated for 1 h. After separation and washing, the final precipitate was dropped on a Si chip and analyzed with Raman signals. The spectra and calibration curve of DTNB absorbed on the SiO₂@Au NPs are shown in Fig. 5a and b, respectively. The SERS signal significantly decreased as the concentration of DTNB decreased, and the main Raman peak at 1331 cm⁻¹ remained evident at DTNB concentrations as low as 10⁻¹⁰ M. Thus, the limit of detection (LOD) of DTNB is 10⁻¹⁰ M. These results indicate that the SiO₂@Au NPs have good potential to be active SERS substrate for greatly enhancing the SERS signal of molecules adsorbed on them.Open in a separate windowFig. 5(a) SERS spectra of DTNB measured with different concentrations on the SiO₂@Au NPs. (b) Calibration curve for DTNB at a concentration range of 10⁻⁴ M to 10⁻¹¹ M obtained using SERS intensity at 1331 cm⁻¹. The error bars represent the standard deviations from five measurements.Upon modification with Raman report molecules and detection antibodies, the monodisperse SiO₂@Au NPs must be efficient SERS tags for highly reproducible SERS immunoassays due to the integration of high SERS activity of the Au nanoshell and the homogeneity and stability of SiO₂ NPs (Fig. 6a). SERS-based LFA strip is a recently reported analytical technique to overcome the shortcomings of conventional lateral flow assay, such as poor sensitivity and semiquantitative ability on the basis of colorimetric analysis.^25–27 The fundamental principle of SERS-based strip is the use of functional SERS tags instead of Au NPs. High-sensitivity and quantitative detection can be achieved by Raman spectroscopy because the intensity of the SERS signal is directly proportional to the number of SERS tags on the test line.Open in a separate windowFig. 6(a) Synthesis route for SiO₂@Au SERS tags. (b) Schematic of SERS-based LFA strips for quantitative detection of human IgM. Fig. 6b represents the operating principle of the monodisperse SiO₂@Au NPs (80 nm) based SERS-LFA strip. Human IgM was selected as the model target antigen to explore the sensitivity of the proposed method. The representative SERS-LFA strip is composed of a sample loading pad, a conjugate pad, a NC membrane containing test line and control line, and an absorption pad. In our system, goat anti-human IgM antibody-labeled SiO₂@Au/DTNB NPs were dispensed onto the glass fiber paper as the conjugate pad, and the goat anti-human IgM antibody and donkey anti-goat immunoglobulin G (IgG) antibody were dispensed onto the NC membrane to form the test line and control line, respectively. When the sample solution containing the target human IgM passed through the conjugation pad, immunocomplexes (human IgM/SERS tags) were formed and continued migrating along the NC membrane until they reach the test line where they were captured by the previously immobilized anti-human IgM antibodies. Excess antibody-conjugated SiO₂@Au tags continued to migrate to the control line and were immobilized by the donkey anti-goat IgG antibody. Consequently, two dark bands appeared in the presence of the target human IgM (positive), whereas only the control line turned to a dark band in the absence of human IgM (negative). Quantitative detection of human IgM could be realized by detecting the SERS signal on the test line.Human IgM was diluted within 10 000 ng mL⁻¹ to 0.1 ng mL⁻¹ as the sample solution, and PBST solution (10 mM PBS, 0.05% Tween-20) was used as blank control. As shown in Fig. 7a, the color of SERS tags captured by the test line was visualized and gradually decreased with decreasing human IgM concentration. The LOD of colorimetric method for detection of human IgM was found to be 10 ng mL⁻¹. Quantitative analysis was also conducted by measuring the characteristic Raman signals of the SERS tags on the test lines, and the Raman spectra are displayed in Fig. 7b. The Raman spectra were analyzed by plotting the intensity at 1331 cm⁻¹ of DTNB as a function of the logarithm of the target human IgM concentration to generate a calibration curve (Fig. 7c). The LOD of the SERS-LFA strips based on the SiO₂@Au tags is 0.1 ng mL⁻¹, which was calculated as a 3 : 1 threshold ratio with respect to the blank control measurement. Using SiO₂@Au tags-based SERS LFA strip offers a 100-fold improvement in the detection limit compared with colorimetric analysis. Basing on these results, we demonstrated the high efficiency and great potential of monodisperse SiO₂@Au NPs as suitable SERS tags for SERS-based LFA strips. The specificity of the SERS-LFA strips was tested by a high concentration (1 μg mL⁻¹) of other proteins including human IgG and BSA. Fig. S5^† shows the result of the specificity test. IgG and BSA did not show significant interference signals both in visualization and Raman spectrum analyses, whereas 100 ng mL⁻¹ human IgM exhibited a strong signal. Hence, the SiO₂@Au tags-based SERS-LFA strip has good selectivity.Open in a separate windowFig. 7(a) Photographs of SERS-based LFA strips in the presence of different concentrations of human IgM. (b) SERS spectra measured in the corresponding test lines. (c) Plot of the Raman intensity at 1331 cm⁻¹ as a function of the logarithmic concentration of human IgM. The error bars represent the standard deviations from five measurements.In summary, this work proposes a sonochemically assisted seed growth method for facile synthesis of monodisperse SiO₂@Au core–shell NPs with a complete Au shell. This method is a general route for preparing SiO₂@Au particles with sizes ranging from nanoscale to microscale levels. High-performance SiO₂@Au NPs were obtained from the intermediate product (SiO₂–Au seed) within 5 min through sonication. The obtained SiO₂@Au NPs were highly uniform in size and shape and exhibited satisfactory SERS activity. Hence, these NPs could be ideal SERS tags for various SERS based immunoassays. The small SiO₂@Au NPs (80 nm) with light weight and good dispersibility were also successfully applied to SERS-based LFA strip for human IgM rapid detection, with limit of detection as low as 0.1 ng mL⁻¹. We expect that high-performance SiO₂@Au NPs SERS tags can be used for actual detection.     

Chemiluminescent organic nanophotosensitizer for a penetration depth independent photodynamic therapy

Photodynamic therapy initiated by external photoexcitation is a clinically-approved therapeutic paradigm, but its practical application has been severely hindered by the shallow penetration of light. Here, we describe a penetration-independent PDT modality using a chemiluminescent organic nanophotosensitizer, which is activated by hydrogen peroxide instead of external photoexcitation.

A chemiluminescent organic nanophotosensitizer activated by hydrogen peroxide was fabricated for a potential penetration depth-independent photodynamic therapy.

Photodynamic therapy (PDT) performed with the cooperation of a photosensitizer, molecular oxygen and light has become a minimally non-invasive therapeutic paradigm in clinics for the treatment of various diseases such as psoriasis, vitiligo and cancer.^1–4 In general, exposing photosensitizers to suitable light, generates very toxic reactive oxygen species (mainly, single oxygen, ¹O₂) that kill tumour cells. Near-infrared light is preferred as external light to activate PDT owing to its considerably deeper penetration into tissue as compared to ultraviolet or visible light.^5,6 However, advances in PDT have been severely confined to superficial lesions for decades as all these external light-based phototherapies, including PDT, suffer from rapid attenuation of external light in tissue.⁷ Recently, Cerenkov radiation has been used as external light to break the depth dependency of PDT.^7–9 Although conceptually impressive, the expensive radiation source and inevitable DNA damage induced by ionizing radiation remain major limitations for practical applications. Therefore, it is highly desirable to develop novel better PDT with the penetration depth-independent feature.Unlike external light sources, internal light sources, such as fibre-optic light sources, could address the penetration issue and have thus been proposed as an alternative solution to activate PS, but it brings invasive problems.¹⁰ In this case, another intriguing internal light arising from chemiluminescence has been proposed for penetration depth independent PDT. In this paradigm, chemiluminescence, which occurs when a specific chemical (such as luminol) is mixed with an appropriate oxidizing agent (such as hydrogen peroxide, H₂O₂), could activate the adjacent photosensitizer to proceed to PDT. Moreover, elevated H₂O₂ levels have been found in several types of cancer cells compared to that in normal cells, potentially affording H₂O₂-activatable PDT with good selectivity. However, only few chemiluminescent PDT systems have been reported.^11–14 Moreover, these chemiluminescent PDT systems usually require coupling with additional PS to activate PDT by energy transfer, complicating the system with reduced reproducibility. In addition, to match the absorption of near-infrared absorptive PS (such as chlorin e6) for efficient energy transfer, quantum dots were usually employed to red-shift bioluminescence,^10,15 which not only complicates the system but also raises the potential of metal-induced toxicity issues.Herein, we fabricated a novel chemiluminescent NPs (C NPs) as a nanophotosensitizer for penetration depth independent PDT in tumour cells and bacteria (Scheme 1). This concise chemiluminescent NPs consisted of luminol and horseradish peroxidase (HRP), which is activated by hydrogen peroxide instead by external photoexcitation. In H₂O₂-rich conditions, C NPs exhibited remarkably enhanced ¹O₂ production compared to the luminol/HRP mixture. Finally, C NPs was demonstrated as a powerful nanophotosensitizer for efficient PDT in tumour cells and bacteria without external photoexcitation, becoming a promising platform for the future design of efficient PDT at high tissue depth.Open in a separate windowScheme 1Schematic illustration of the NPs structure and penetration depth-independent PDT.C NPs were prepared by the self-assembly of PLGA, luminol, HRP, and DSPE-mPEG2000 via a nanoprecipitation method (Scheme 1).¹⁶ The detailed preparation process is presented in the ESI.^† Briefly, the PLGA was dissolved in acetonitrile. Luminol and HRP were dissolved in an aqueous solution, which was then added into the previous PLGA acetonitrile solution. This mixed solution was added dropwise to an aqueous solution of DSPE-PEG2000. After gently stirring for 4 h at room temperature, the remaining organic solvent and free molecules were removed by ultrafiltration. The designed C NPs could be stored in an aqueous solution up to 2 months without any eye-observed precipitation (Fig. 1a), indicating their excellent stability. Transmission electron microscopy (TEM) results reveal the spherical morphology of the C NPs with high monodispersity (Fig. 1b), while dynamic light scattering (DLS) indicated their average hydrodynamic diameter with a value of about 70 nm (Fig. 1c).Open in a separate windowFig. 1(a) Photograph of C NPs in an aqueous solution. (b) TEM image of C NPs. Scale bar: 500 nm. (c) Representative DLS of C NPs.The production of reactive oxygen species (ROS, e.g., ¹O₂) in NPs or luminol (L) + HRP was experimentally confirmed by the photodegradation of anthracene-9,10-diyl-bis-methylmalonate (ADMA) in the presence of H₂O₂ and molecular oxygen.^17,18 After the addition of H₂O₂, the characteristic absorbances (260, 358, 378 and 399 nm) of ADMA dispersed in L + HRP solutions gradually decreased with prolonged time (Fig. 2a), indicating the inefficient production of ¹O₂. In sharp contrast, the absorbance of ADMA decreased remarkably in a mixture of ADMA and NPs after the addition of H₂O₂ (Fig. 2b). This is a solid evidence to illustrate that C NPs can be activated by H₂O₂ rather than external photoexcitation to perform PDT. Notably, within 2 min, the NPs almost totally consumed the AMDA, while the L + HRP only showed negligible consumption (Fig. 2c), which unambiguously demonstrated much more efficient production of ¹O₂ by C NPs. This is reasonable because L and HRP within NPs are close together, which is favourable for chemiluminescence. Despite the origin of ¹O₂ production during the C NP-induced chemiluminescence, the above-mentioned results clearly indicate that C NPs can efficiently produce ¹O₂ under H₂O₂ activation, which shows tremendous potential in vivo PDT in high tissue depth.Open in a separate windowFig. 2The absorption spectra of a C NPs (a) or L + HRP (b) and ADMA mixture before and after addition of H₂O₂. The spectra were obtained every 2 min after addition of H₂O₂. Rapidly decreased characteristic absorbance at 260 nm of ADMA within 2 min confirms the efficient ¹O₂ production of C NPs in the presence of H₂O₂. (c) Kinetic curves of the ADMA consumption of L + HRP and C NPs.To verify the PDT effects, we utilized calcein-AM (living cell) and propidium iodide (PI, dead cell) cell viability kits to distinguish the dead cells from living ones (Fig. 3).^19,20 After incubation with H₂O₂, H₂O₂ + HRP, and H₂O₂ + HRP alone, HeLa cells showed comparable cellular viability to the blank group (control) without any treatments, which demonstrate the resistance of HeLa cells toward H₂O₂, H₂O₂ + HRP, and H₂O₂ + HRP. Without the addition of H₂O₂, both L + HRP and C NP-incubated HeLa cells exhibited negligible cytotoxicity, suggesting low dark-cytotoxicity of L + HRP and C NPs. After the addition of H₂O₂, C NP-incubated HeLa cells exhibited strong cytotoxicity relative to the mixed solution of L + HRP.Open in a separate windowFig. 3Live/dead assay of HeLa cells. Green colour represents live cells, and red colour represents dead cells.Furthermore, we evaluated the antibacterial ability of NPs using Gram-negative bacteria, namely E. coli. As shown in Fig. 4a, all groups treated with C NPs, L + HRP, and H₂O₂ alone showed a tiny difference as compared to the blank group (control) without any treatment, which means that C NPs, L + HRP, and H₂O₂ have no obvious influence on E. coli. After the addition of H₂O₂, a huge decrease in C NPs + H₂O₂ was observed, while only a small decrease in L + HRP + H₂O₂, indicating a stronger antibacterial ability of C NPs. The antibacterial efficiency of NPs + H₂O₂ was determined to be 70% (Fig. 4b), which is approximately 10-fold stronger than L + HRP + H₂O₂ (7%). These results clearly demonstrated the strong antibacterial ability of C NPs.Open in a separate windowFig. 4(a) Colony-forming units (CFU) for E. coli treated with NPs before and after addition of H₂O₂ on LB agar plate. The treated E. coli diluted 10⁰ to 10⁻³, 6 microliters of each dilution were inoculated to solid LB media, the E. coli were grown at 37 °C for 12 h. (b) Antibacterial activity of NPs before and after addition of H₂O₂.In summary, we have fabricated a chemiluminescent organic nanophotosensitizer, namely C NPs, which could be activated by H₂O₂ instead by external photoexcitation. The C NPs show a very strong ¹O₂ generation ability in the presence of H₂O₂. The tumour cells and bacteria explanation clearly demonstrate that C NPs could be used as a powerful nanophotosensitizer for potential penetration depth-independent PDT. Our results provide an attractive platform for the future design of a powerful photosensitizer, which can expand the application scope of PDT.     

Correction: Noninvasive target CT detection and anti-inflammation of MRSA pneumonia with theranostic silver loaded mesoporous silica

Correction for ‘Noninvasive target CT detection and anti-inflammation of MRSA pneumonia with theranostic silver loaded mesoporous silica’ by Hao Zhang et al., RSC Adv., 2016, 6, 5049–5056.

The authors regret that an incorrect version of Fig. 1 was included in the original article. The correct version of Fig. 1 is presented below.Open in a separate windowFig. 1(A) SEM image of PEGylated SLS NPs; inset: high-resolution TEM image highlighting the anchored Ag NPs. (B) XPS result of the SLS NPs and silver element. (C) DLS and zeta-potential profiles of the SLS NPs pre- and post-PEGylation.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Continuous syntheses of carbon-supported Pd and Pd@Pt core–shell nanoparticles using a flow-type single-mode microwave reactor

Continuous syntheses of carbon-supported Pd@Pt core–shell nanoparticles were performed using microwave-assisted flow reaction in polyol to synthesize carbon-supported core Pd with subsequent direct coating of a Pt shell. By optimizing the amount of NaOH, almost all Pt precursors contributed to shell formation without specific chemicals.

Continuous syntheses of carbon-supported Pd@Pt core–shell nanoparticles were performed using flow processes including microwave-assisted Pd core–nanoparticle formation.

Continuous flow syntheses have attracted attention as a powerful method for organic, nanomaterial, and pharmaceutical syntheses because of various features that produce benefits in terms of efficiency, safety, and reduction of environmental burdens.^1–7 Advances of homogeneous heating and mixing techniques in continuous flow reactors have engendered further developments for precise reaction control, which is expected to create innovative materials through combination with multiple-step flow syntheses.Microwave (MW) dielectric heating has been recognized as a promising methodology for continuous flow syntheses because rapid or selective heating raises the reaction rate and product yield.^8–18 For the last two decades, most MW apparatus has been batch-type equipped with a stirring mechanism in a multi-mode cavity. Therefore, conventionally used MW-assisted flow reactors have been mainly of the modified batch-type. Results show that the electromagnetic field distribution can be spatially disordered, causing inhomogeneous heating of the reactor.^19–25 Improvements of reactors suitable for flow-type work have been studied actively in recent years to improve their energy efficiency and to make irradiation of MW more homogeneous.^26–37We originally designed a MW flow reactor system that forms a homogeneous heating zone through generation of a uniform electromagnetic field in a cylindrical single-mode MW cavity.^26,30 The temperatures of flowing liquids in the reactor were controlled precisely via the resonance frequency auto-tracking function. Continuous flow syntheses of metal nanoparticle, metal-oxide, and binary metal core–shell systems with uniform particle size have been achieved using our MW reactor system.^26,38,39 Furthermore, large-scale production necessary for industrial applications can be achieved through integration of multiple MW reactors.³⁰Carbon-supported metal catalysts are widely used in various chemical transformations and fine organic syntheses. Particularly, binary metal systems such as Pd@Pt core–shell nanoparticles have attracted considerable interest for electro-catalysis in polymer electrolyte membrane fuel cells (PEMFC) because of their enhanced oxygen reduction activity compared to a single-use Pt catalyst. Binary metal systems also contribute to minimization of the usage of valuable Pt.^40–51 Earlier studies of carbon-supported Pd@Pt syntheses involved multiple steps of batch procedures such as separation, washing and pre-treatment of core metal nanoparticles, coating procedures of metal shells, and dispersion onto carbon supports. Flow-through processes generally present advantages over batch processes in terms of simplicity and high efficiency in continuous material production.We present here a continuous synthesis of carbon-supported Pd and Pd@Pt core–shell nanoparticles as a synthesis example of a carbon-supported metal catalyst using our MW flow reactor. This system incorporates the direct transfer of a core metal dispersion into a shell formation reaction without isolation. Nanoparticle desorption is prevented by nanoparticle synthesis directly on a carbon support. The presence of protective agents that are commonly used in nanoparticle syntheses, such as poly(N-vinylpyrrolidone), can limit the chemical activity of the catalyst. Nevertheless, this system requires no protective agent. Moreover, this system is a simple polyol synthesis that uses no strong reducing agent. It therefore imposes little or no environmental burden. For this study, the particle size and distribution of metals in Pd and Pd@Pt core–shell nanoparticles were characterized using TEM, HAADF-STEM observations, and EDS elemental mapping. From electrochemical measurements, the catalytic performance of Pd@Pt core–shell nanoparticles was evaluated.A schematic view of the process for the continuous synthesis of carbon-supported Pd@Pt core–shell nanoparticles is presented in Fig. 1. Details of single-mode MW flow reactor are described in ESI.^† We attempted to conduct a series of reactions coherently in a flow reaction system, i.e., MW-assisted flow reaction for the synthesis of carbon supported core Pd nanoparticles with subsequent deposition of the Pt shell. Typically, a mixture containing Na₂[PdCl₄] (1–4 mM) in ethylene glycol (EG), carbon support (Vulcan XC72, 0.1 wt%), and an aqueous NaOH solution were prepared. This mixture was introduced continuously into the PTFE tube reactor placed in the center of the MW cavity. Here, EG works as the reaction solvent as well as the reducing agent that converts Pd(ii) into Pd(0) nanoparticles. The MW heating temperature was set to 100 °C with the flow rate of 80 ml h^−l, which corresponds to residence time of 4 s. The carbon-supported Pd nanoparticles were transferred directly to the Pt shell formation process without particle isolation. The dispersed solution was introduced into a T-type mixer and was mixed with a EG solution of H₂[PtCl₆]·6H₂O (10 mM). The molar ratio of Pd : Pt was fixed to 1 : 1. Subsequently, after additional aqueous NaOH solution was mixed at the second T-mixer, the reaction mixture was taken out of the mixer and was let to stand at room temperature (1–72 h) for Pt shell growth.Open in a separate windowFig. 1Schematic showing continuous synthesis of carbon-supported Pd and Pd@Pt core–shell nanoparticles. The Pd nanoparticles were dispersed on the carbon support by MW heating of the EG solution. The solution was then transferred directly to Pt shell formation.Rapid formation of Pd nanoparticles with average size of 3.0 nm took place at the carbon-support surface during MW heating in the tubular reactor (Fig. 2a). Most of the Pd(ii) precursor was converted instantaneously to Pd(0) nanoparticles and was well dispersed over the carbon surface. Fig. 2b shows the time profile of the outlet temperature and applied MW power during continuous synthesis of carbon-supported Pd nanoparticles. The solution temperature rose instantaneously, reaching the setting temperature in a few seconds. This temperature was maintained with high precision (±2 °C) by the continuous supply of ca. 18 W microwave power. No appreciable deposition of metal was observed inside of the PTFE tube. It is noteworthy that Pd of 98% or more was supported on carbon by heating for 4 s at 100 °C from ICP-OES measurement. Our earlier report described continuous polyol (EG) synthesis of Pd nanoparticles as nearly completed with 6 s at 200 °C.³⁹ The reaction temperature in polyol synthesis containing the carbon was considerably low, suggesting that selective reduction reaction occurs on the carbon surface, which is a high electron donating property.Open in a separate windowFig. 2(a) TEM image of carbon-supported Pd nanoparticles synthesized using the MW flow reactor. The average particle size was 3.0 nm. (b) The time profile of the temperature at the reactor outlet and applied microwave power during continuous synthesis of carbon-supported Pd nanoparticles. Na₂[PdCl₄] = 2 mM, NaOH = 10 mM.The concentrations of Na₂[PdCl₄] precursor and NaOH affect the Pd nanoparticle size. Results show that the Pd particle size increased as the initial concentration of Na₂[PdCl₄] increased (Fig. S1a and b^†). Change of NaOH concentration exerted a stronger influence on the particle size. Nanoparticles of 12.3 nm were observed without addition of NaOH, whereas 2.6 nm size particles were deposited at the concentration of 20 mM (Fig. S1c and d^†). The higher NaOH concentration led to instantaneous nucleation and rapid completion of reduction. The Pd nanoparticle surface is equilibrated with Pd–O⁻ and Pd–OH depending on the NaOH concentration. The surface is more negative at high concentrations of NaOH because of the increase of the number of Pd–O⁻, which inhibits the mutual aggregation and further particle growth. Furthermore, to control the Pd nanoparticle morphology, we conducted synthesis by adding NaBr, which has been reported as effective for cubic Pd nanoparticle synthesis.⁵² However, because reduction of the Pd precursor derives from electron donation from both the polyol and the carbon support, morphological control was not achieved (Fig. S2^†). That finding suggests that morphological control is difficult to achieve by adding surfactant agents to the polyol.For Pt shell formation, carbon supported Pd nanoparticles (3.0 nm average particle size) were mixed with H₂[PtCl₆]·6H₂O solution with the molar ratio of Pd : Pt = 1 : 1. Then additional NaOH solution was mixed. As described in earlier reports,³⁹ alkaline conditions under which base hydrolysis and reduction of [PtCl₆]²⁻ to [Pt(OH)₄]²⁻ takes place are necessary for effective Pt shell formation. It is noteworthy that the added Pt precursor was almost entirely supported on carbon within 24 h in cases where an appropriate amount of additional NaOH (5 mM) was mixed by the second T-mixer (Fig. 3a). However, for 10 mM, nucleation and growth of single Pt nanoparticles were enhanced in place of core–shell formation. Consequently, a mixture of Pd@Pt and single Pt nanoparticles was formed on the carbon support (Fig. 3b). Very fine Pt nanoparticles were observed in the supernatant solution.Open in a separate windowFig. 3(a) Time profiles of residual ratio of Pt in the mixed solutions. Horizontal axis was left standing time. Carbon-support in the mixed solution after added the Pt precursor was precipitated by centrifugation. The supernatant solution was measured by ICP-OES. Concentrations of additional NaOH were 0, 5, and 10 mM. (b) TEM image of carbon-supported Pd@Pt core–shell nanoparticles. The synthesis conditions of Pd nanoparticles were Na₂[PdCl₄] (2 mM) and NaOH (10 mM). The molar ratio of Na₂[PdCl₄] : H₂[PtCl₆]·6H₂O was 1 : 1, and additional NaOH concentration was 10 mM. After left standing for 72 h, the mixture of Pd@Pt and single Pt nanoparticles (1–2 nm) was formed on carbon-support. Fig. 4a portrays a TEM image of carbon supported Pd@Pt core–shell nanoparticles. The average particle size of Pd@Pt core–shell nanoparticles was 3.6 nm after being left to stand for 24 h: larger than the initial Pd nanoparticles (3.0 nm). Fig. 4b shows the HAADF-STEM image of Pd@Pt core–shell nanoparticles supported on carbon. The core–shell structure of the particles can be ascertained from the contrast of the image. The Z-contrast image shows the presence of brighter shells over darker cores. Actually, the contrast is strongly dependent on the atomic number (Z) of the element.⁵³ The Z values of Pt (Z = 78) and Pd (Z = 46) differ considerably. Therefore, the image shows the formation of Pd@Pt core–shell structure with the uniform elemental distribution. Elemental mapping images by STEM-EDS show that both Pd and Pt metals were present in all the observed nanoparticles (Fig. 4c). Based on the atomic ratio (Pd : Pt = 49 : 51), they show good agreement with the designed values. The Pt shell thickness was estimated as about 0.6 nm, which corresponds to 2–3 atomic layer thickness of Pt encapsulating the Pd core metal, indicating good agreement with Fig. 4b image. For an earlier study, uniform Pt shells were formed by dropwise injection of the Pt precursor solution because the Pt shell growth rate differs depending on the crystal plane of the Pd nanoparticle.⁴⁶ For more precise control of shell thickness in our system, the Pt precursor solution should be mixed in multiple steps.Open in a separate windowFig. 4(a) TEM image and (b) HAADF-STEM image of carbon-supported Pd@Pt core–shell nanoparticles and the line profile of contrast. (c) Elemental mapping image of carbon-supported Pd@Pt core–shell nanoparticles, where Pd and Pt elements are displayed respectively as red and green. The EDS atomic ratio of Pd : Pt was 49 : 51. The synthesis conditions of Pd nanoparticles were Na₂[PdCl₄] (2 mM) and NaOH (10 mM). The molar ratio of Na₂[PdCl₄] : H₂[PtCl₆]·6H₂O was 1 : 1. The concentration of additional NaOH were 5 mM. It was left standing for 24 h.A comparison of the catalytic performance of the carbon-supported Pd@Pt core–shell and Pt nanoparticles is shown in Fig. S3.^† For this experiment, carbon-supported Pt nanoparticles with Pt 2 mM were prepared as a reference catalyst using a similar synthetic method. The initial Pt mass activities of the carbon-supported Pd@Pt and Pt nanoparticles were, respectively, 0.39 and 0.24 A mg_Pt⁻¹, improving by the core–shell structure. In addition, durability tests for carbon-supported Pd@Pt nanoparticles show that the reduction rate of Pt mass activity after 5000 cycles was only 2%. The catalytic activities of carbon-supported Pd@Pt nanoparticles were superior in terms of durability, suggesting that the Pt shell was firmly formed.     

A hybrid nanoparticle/alkoxide ink for inkjet printing of TiO2: a templating effect to form anatase at 200 °C

A reactive ink (Ink 1) containing Ti(OPrⁱ)₄ in PrⁱOH with dimethoxyethan as a kinetic stabiliser deposits TiO₂ by inkjet printing. A hybrid ink (Ink 2) consists of Ink 1 with the addition of anatase NPs, which act as seeds for the formation of anatase TiO₂ at 200 °C. Printing of anatase on PET is also reported.

Novel hybrid nanoparticle/alkoxide inks for the inkjet printing of low temperature anatase TiO₂.

TiO₂, particularly in the form of anatase thin films, continues to attract significant interest, due to applications such as photocatalysis.¹ Anatase thin films are typically prepared by sol–gel or chemical vapour deposition (MOCVD or ALD) techniques.^2,3 As-deposited thin films from sol–gel synthesis are found to be amorphous and require annealing at >350 °C in order to form anatase. CVD techniques require sophisticated equipment, and although plasma enhanced CVD deposition of anatase at 100 °C has recently been reported,⁴ most CVD preparations of anatase require either a high substrate temperature (>300 °C) or high temperature annealing step.⁵Inkjet printing is an alternative method for thin-film deposition and has the advantages of being both low-cost and low-waste, as well as enabling direct patterning.^6,7 A major challenge in inkjet printing is the formulation of a stable ink with the correct chemical and rheological properties. To date, most published methods for inkjet printing of anatase have used colloidal TiO₂ solutions.⁸ As-deposited films are amorphous and require annealing at >375 °C to convert the printed films to anatase.⁹ Suspensions of pre-formed anatase nanoparticles have also been used as inks; these inks often require the addition of high molecular weight stabilisers in order to keep the nanoparticles in suspension. A high-temperature (>350 °C) processing step is therefore required in order to remove the stabiliser and to sinter the nanoparticles to form a continuous film.¹⁰ In some cases a lengthy heat-treatment is required after each printing pass before a final high temperature annealing stage.¹¹As an alternative to inks containing pre-formed TiO₂, we present a series of reactive inks that contain a metalorganic Ti precursor that reacts under ambient laboratory conditions with atmospheric moisture and surface OH groups to form TiO₂. The aim of this work is to avoid the need for high temperature treatments and thus to develop a process for printing anatase TiO₂ that is compatible with temperature-sensitive, flexible substrates such as polyethylene terephthalate (PET).^12,13Ti(OPrⁱ)₄ has been chosen as the reactive component of the ink: as it is readily available at relatively low cost, and has been extensively studied as a precursor for deposition of TiO₂ by sol–gel techniques.¹⁴ Ti(OPrⁱ)₄ reacts with H₂O (and with Si–OH groups on the surface of a glass substrate) by a sequence of protonolysis/condensation reactions to form TiO₂:In addition to optimising rheological properties, an ink formulation that is stable enough to have an acceptable shelf-life and not to cause blockages in the printer is required. The ink formulation however needs to be reactive enough to produce TiO₂ once printed under ambient conditions. PrⁱOH was chosen as the carrier solvent as it is chemically compatible with Ti(OPrⁱ)₄ and it also has compatible rheological properties for inkjet printing. However, a solution of Ti(OPrⁱ)₄ in PrⁱOH reacts within seconds with small traces of H₂O to form insoluble TiO₂, and this behaviour results in rapid blocking of the printer head. A variety of glycol ethers as kinetic stabilisers has therefore been investigated:These glycol ethers are Lewis bases with the ability to coordinate to Ti(OPrⁱ)₄, resulting in kinetic stabilisation by blocking access of H₂O to vacant coordination sites at Ti.¹⁵ In optimising the ink formulation, we aimed to achieve a printable ink with: a reasonable level of Ti loading, a rate of reaction with ambient atmosphere that is compatible with printing multiple passes in rapid succession, and a convenient level of shelf-stability. Ti(OPrⁱ)₄ concentrations from 0.05 M to 0.15 M, and glycol ether: Ti(OPrⁱ)₄ ratios from 2.5 : 1 up to 10 : 1 have been investigated.DME was identified as the optimum kinetic stabiliser: as its boiling point (85 °C) is close to that of PrⁱOH (83 °C), resulting in evaporation of both carrier solvent and stabiliser at similar rates post-deposition. The complimentary evaporation rates minimises surface tension driven Marangoni effects due to compositional changes, and results in good uniformity of deposited material.¹⁶ The optimised formulation for the stabilised Ti(OPrⁱ)₄ ink (designated as Ink 1) is given in Fig. 1), indicating good ink stability.Optimised Ti(OPrⁱ)₄ ink formulation (Ink 1); carrier solvent PrⁱOH

Poly(fluorenone-co-thiophene)-based nanoparticles for two-photon fluorescence imaging in living cells and tissues

Conjugate polymer nanoparticles (CPNs) were constructed based on poly(fluorenone-co-thiophenes) (PFOTs) synthesized through a direct arylation polymerization (DArP) approach. Results demonstrate that the developed novel CPNs have potential applications in two-photon fluorescence imaging of both cells and tissues.

Novel conjugate polymer nanoparticles (CPNs) based on poly(fluorenone-co-thiophenes) (PFOTs) were constructed for two-photon cell and tissue fluorescence imaging.

Fluorescence imaging has been widely applied in biological studies on subcellular microenvironments and tissues to develop disease diagnostic methods and clinical treatment.^1–3 The emerging two-photon fluorescence imaging approach has demonstrated some advantages over traditional single-photon fluorescence imaging, including deeper penetration depth, less photo-damage, reduced self-absorption and background signal due to autofluorescence, etc.^4–6 Several two-photon absorption (TPA) materials have been reported thus far, such as organic molecules, quantum dots, metal complexes, carbon quantum dots, graphene quantum dots, et al.,^7–11 both small-molecule fluorescent probes and nano-sized imaging agents fabricated by CPs are attractive and versatile materials for studying biological systems, and their size, composition, surface ligands, optical properties are important for their application.^12–14 So, developing new TPA materials remains challenging but is needed to improve fluorescence quantum yield, biocompatibility, photostability, and ease of preparation compared to existing TPA probes.Conjugated polymers (CPs) with extended π-conjugation structure have been studied as promising fluorescent probes over the past two decades. Due to their effective absorption and fluorescence, high photostability, signal amplification effect, and excellent biocompatibility, they have been widely implemented in biosensing, drug delivery, and imaging.^15–19 For instance, CPs-based nanoparticles with two-photon excitation character were applied as high contrast cell imaging probes by Xu et al.^20,21 It was reported that synthesized chromophores with donor–acceptor (D–A) structures, which tend to exhibit relatively large two-photon absorption cross-section, and thus, enhanced TPA properties by the charge transfer effect.^22,23 Wu et al. fabricated CPNs with polyfluorene derivative that were characterized by cross sections values up to 2.0 × 10⁵ GM.²⁴ Recently, Schanze et al. reported anionic conjugated polyelectrolytes (CPEs), such as PPE-SO₃⁻, with moderate two-photon absorption cross-sections in the near-infrared (NIR) region for two-photon fluorescence cell imaging.²⁵ Their work disclosed the potentials of CPEs and CPNs as TPA fluorescent materials in tissue imaging.Herein, we developed novel PFOT-based nanoparticles to exploit their single- and two-photon fluorescence properties for imaging applications in living cells and tissues (Scheme 1).Open in a separate windowScheme 1Illustration of PFOT nanoparticles as two-photon imaging probes.Direct arylation polymerization (DArP) has emerged as a simple and atom-economic method for polymer synthesis compared to traditional metal-catalyzed coupling polymerization, as shown in Fig. 1a.²⁶ In this work, 4 novel D–A type CPs composed of fluorenone and thiophene moieties were synthesized via the DArP method, as displayed in Fig. 1b and c. All polymers are soluble in typical organic solvents, such as chloroform, THF, methanol, acetone, and DMF. PFOT-2 and PFOT-3 are slightly soluble in water due to the increased length of their hydrophilic side chains. Under optimized experimental conditions, we acquired PFOT-1, PFOT-2, PFOT-3, and PFOBT with reasonable Mn of 1.99, 2.57, 5.14, and 4.76 kg mol⁻¹ respectively.Open in a separate windowFig. 1(a) Comparison of traditional coupling reactions and direct arylation polymerization, X is halogen. (b) Designed synthetic route of novel poly(fluorenone-co-thiophene) conjugated polymer. (c) Acquired novel poly(fluorenone-co-thiophene) (PFOT) conjugated polymers.Photophysical properties of CPs are affected by their main chain conjugated structure, molecular weight, side chain structure, solvent, and etc. For the fluorenone-based copolymers, it has been reported that the intermolecular CO⋯HC (aromatic) hydrogen bonds (H-bonds) would exert influence on absorption maximum position.^27,28 In addition, side chains of the CPs could also have impact on the conformation of the backbone and the aggregation state of the polymers. Photophysical properties of the obtained four CPs (PFOT-1, PFOT-2, PFOT-3, and PFOBT) were studied. UV-visible absorption and fluorescence emission spectra of the four CPs in chloroform are shown in Fig. 2a. The absorption of all the four polymers in chloroform features a strong band in the UV range and a broader and weaker band in the visible range. PFOT-1 and PFOBT show UV absorption with two maxima at 287 and 265 nm, that are assigned, respectively, to cisoid and transoid conformations of the conjugated backbone, while PFOT-2 and PFOT-3 only demonstrate one peak at about 305 nm, suggesting that the cisoid conformation dominates which enhance the polymer aggregation. Strong and wide fluorescence emission bands of the 4 polymers can be observed with the full width at half maxima (FWHM) at about 200 nm (Fig. 2b). Polymers with single thiophene in their repeat units, such as PFOT-1, PFOT-2, and PFOT-3, demonstrated red-shifted emission maximum wavelengths (595 to 630 nm) compared to that of the polymer PFOBT (555 nm), which contains bithiophene in its repeat units. We believe that PFOBT is in a less aggregated state than all other three PFOT polymers, leading to a blue-shifted absorption and emission band. It has been reported that D–A type CPs usually demonstrate lower bandgaps and large Stokes shifts due to the intramolecular charge transfer (ICT).²⁹ As the spectra shown, PFOT-1, PFOT-2, PFOT-3, and PFOBT CPs exhibit large Stokes shifts of 220, 260, 250, and 170 nm respectively, which is in favour of diminishing the reabsorption effect. Quantum yields (QYs) of PFOT-1, PFOT-2, PFOT-3, and PFOBT CPs in CHCl₃ were determined to be 0.081, 0.193, 0.376, and 0.432, respectively, via the reference method with coumarin 6 in ethanol as a reference. Then, PFOT-2, PFOT-3, and PFOBT were selected for further imaging studies.Open in a separate windowFig. 2Absorption (a) and fluorescence emission spectra (b) of PFOT-1 (red), PFOT-2 (yellow), PFOT-3 (blue), and PFOBT (green) polymers in CHCl₃, λ_ex = 380 nm. The concentration of all polymers is 10 μM in chloroform.For bio-imaging, efficient internalization of probes by cells is an essential step. In order to improve the biocompatibility of the CPs, CPNs were fabricated via the nanoprecipitation method using polymers and Pluronic-F127 as a surfactant and encapsulation matrix, as shown in Fig. 3a. The diameters of CPNs were characterized by dynamic light scattering (DLS) for three times with the average diameters range from 45.1 to 142.3 nm († revealed the core–shell morphology of the CPNs, probably formed by the hydrophobic inner cores and hydrophilic outer shells. This core–shell morphology of the CPNs helps with improving their stability in aqueous environment, and demonstrates their potential application as fluorescent probes and drug nano-vehicles. In Fig. 3b, compared to the corresponding CPs in solutions, fluorescence emission spectra of the four CPNs revealed red-shifts. This may be ascribed to the internal stress of the particles that increases with the formation of CPNs, which leads to variation in the energy band structures, incremental overlap of the electron wave function, narrowing of energy band-gap, and red-shift of the emission peak length.³⁰ Also, it has been reported that aggregated particles fabricated via nanoprecipitation method usually exhibit red-shifted emission compared to their non-aggregated forms, which can be ascribed to the bending, kinking, and interchain interactions of polymer backbones in the aggregation.^31,32 Cellular cytotoxicity assay of the PFOT CPNs towards HeLa cells displayed no apparent cytotoxicity with a concentration up to 200 μg mL⁻¹ as shown in Fig. 4a. The low cytotoxicity and efficient endocytosis (Fig. S4^†) demonstrated a good biocompatibility of the synthesized CPNs. Also, the photostability assay of PFOT-2 NPs, PFOT-3 NPs, and PFOBT NPs was carried out via a photobleaching experiment. As shown in Fig. 4b and c, fluorescence of PFOT-2 NPs, PFOT-3 NPs, PFOBT NPs, and LysoTracker Green remained at 55%, 54%, 54%, and 8% respectively, after 20 times scans, suggesting that the CPNs exhibited stronger photostability than the small molecular probe LysoTracker Green. Our results also show that the CPs and CPNs are stable at pH ranging from 5.0–7.4, which cover the biological pH ranges of tumor tissues, late endosomes and lysosomes, normal tissues and blood (Fig. S6^†).Open in a separate windowFig. 3(a) Fabrication of CPNs via nanoprecipitation method. Absorption (b) and fluorescence emission (c) spectra of PFOT-1 (red), PFOT-2 (yellow), PFOT-3 (blue), and PFOBT (green) NPs in water, λ_ex = 480 nm (b).Characterization of PFOT series CPNs

Self-assembled microrings of Au nanoparticle and Au nanorod clusters formed at the equators of Janus particles

A method for fabricating polymer Janus particles with microring structures at their equators has been developed. This method allows gold nanoparticles and nanorods to be aligned and densely packed along the microrings.

A method for fabricating polymer Janus particles with metal nanoparticle microring structures at their equators has been developed.

The recent development of plasmonic nanomaterials has revealed the interaction between metallic nanostructures and light. Microring structures composed of metallic nanoparticles (NPs) are particularly interesting since they exhibit a strong electromagnetic reaction due to plasmonic coupling among neighboring NPs and the plasmon polariton resonance of the microring structure.^1,2 This is important for realizing optical metamaterials, and microring arrays of metallic NPs on 2D substrates have been fabricated using state-of-art nanoscale lithographic techniques.^3,4 However, it is still challenging to produce 3D arrays of microring structures comprising metallic NPs.Some approaches have been reported to successfully produce 3D metal NP clusters in the form of colloidal metamaterials, or so-called “metafluids”. The production of 3D metal NP clusters has been achieved by controlling the electrostatic and hydrophobic interactions among NPs.^5–7 Manna and co-workers studied optical magnetism in silver (Ag) NP clusters prepared by assembling the NPs on thiol-terminated silica particles.⁸ Formation of Au NP cluster shells on core–shell particles with amino-terminated polybutadiene shells and polystyrene cores has also been reported; strong plasmonic coupling and near infrared (NIR) absorption was observed.⁹ Thus, three-dimensionally symmetric metal NP clusters have been successfully prepared on spherical particles. However, fabrication of metal microring structures comprising metal NPs, which have intrinsically asymmetric structures, has not been realized yet in colloidal materials.Recent developments in polymer particle preparation techniques has allowed the formation of various nanostructured particles including patchy,¹⁰ striped,¹¹ core–shell¹² and Janus structures.^13,14 In particular, Janus polymer colloids have attracted considerable attention due to their potential as pigments in electronic paper,¹⁵ optical switches,¹⁶ anisotropic micrometers¹⁷ and other applications.¹⁸ Recently, we have developed a method for fabricating Janus and core–shell particles by simple evaporation of a volatile organic solvent from a polymer blend solution containing water as a poor solvent.¹⁹ After solvent evaporation, the solubility of the polymer decreases with increasing content of the poor solvent, and the polymer eventually precipitates in the form of spherical particles containing two separated phases. When the interfacial tension between the two blended polymers is balanced, Janus type phase separated particles are selectively obtained using this self-organizing precipitation (SORP) process.²⁰In previous work, polymer stabilized inorganic NPs were incorporated into such Janus particles.^21,22 Janus type phase separated particles have an interface between the two polymers and water. A three-phase contact line is formed at the equator of the particles, leading to energetic instability. If suitable nanoparticles were incorporated at the equator, they could be aligned at the interface and act as compatibilizers between the water and the two polymer domains.Here, we report the preparation of Au NPs and nanorods (Au NRs) covered with amino-terminated polystyrene (PS-NH₂) and amino-terminated polybutadiene (PB-NH₂) prepared by ligand exchange. These are incorporated into PS/PB polymer blended particles having a Janus type phase separated structure prepared by SORP. The effect of Au NP surface ligands on the composite particles is discussed.PB (M_n = 50 kg mol⁻¹, M_w/M_n = 1.06), PS-NH₂ (M_n = 25 kg mol⁻¹, M_w/M_n = 1.04) and PB-NH₂ (M_n = 41.5 kg mol⁻¹, M_w/M_n = 1.4) were purchased from Polymer Source Inc. Co. Ltd. (Montreal, Canada). PS (M_w = 29.3 kg mol⁻¹) was purchased from Aldrich, United States. Tetrahydrofuran (THF/GR) was purchased from WAKO Chemical Industries, Co. Ltd., Japan. An aqueous dispersion of Au NPs (diameter = 20 nm) was purchased from BBI solutions, Inc. An aqueous dispersion of Au NRs (short axis = 25 nm, long axis = 100 nm) was synthesized using a method reported in the literature.²³The fabrication process for polymer-stabilized Au NPs, Au NRs and composite Janus particles is shown in Scheme 1. PS-NH₂ or PB-NH₂ was dissolved in THF to prepare a 10 mg mL⁻¹ solution. The same amounts of the respective solutions were mixed to prepare PS-NH₂ and PB-NH₂ mixed solutions. 5 mL of an aqueous dispersion of Au NPs was mixed with 5 mL of PS-NH₂, PB-NH₂, and PS-NH₂/PB-NH₂ solutions, and the mixed solution was sonicated for 5 min to exchange the ligands from citrate to polymers. Saturated NaCl aq. was mixed into the opaque dispersion after sonication, and the THF dispersion of ligand-exchanged Au NPs was then separated from the water phase. Polymer-stabilized Au NPs were collected from the THF phase by centrifugation (12 000 rpm, 15 min) and dried at room temperature. Polymer-stabilized Au NRs were also prepared using the same procedure.Open in a separate windowScheme 1Schematic illustration of preparation of polymer stabilized Au NPs and composite Janus particles.UV-Vis spectra of the THF dispersions of Au NPs and Au NRs were measured using a UV-Vis-NIR spectrometer (V-760DS, Jasco, Japan). Fourier transform infrared (FT-IR) spectra of dried Au NPs and Au NRs were measured using a FT-IR spectrometer (FT/IR-6100, Jasco, Japan) equipped with an attenuated total reflection (ATR) unit. Thermogravimetric (TG) analysis was performed using Thermo plus Evo2, RIGAKU, Japan. The shapes of the NPs and NRs were observed using transmission electron microscopy (TEM, H-7650, Hitachi, Japan).0.5 mL of THF solutions of PS and PB were mixed with 0.1 mL of a THF dispersion of Au NPs whose optical density was 0.8 at λ = 530 nm. 1 mL of membrane filtered water was added to the mixed THF dispersion in a glass bottle at a rate of 1 mL min⁻¹ with stirring. After stirring was stopped, the THF was evaporated by immersing the bottle in a water bath at 40 °C for 12 h. The aqueous dispersion of composite particles was annealed at 100 °C for 1 h in a microwave heater (Analytic Jena, Germany). The interior structure of the composite particles was observed using TEM after staining the PB moieties with OsO₄. For sample preparation, 0.1 mL of 2 wt% OsO₄ aq. was added to 1 mL of dispersion and allowed to stand for 2 h. After staining, the composite particles were collected by centrifugation (12 000 rpm, 15 min). The sample was washed using membrane filtered water (3 cycles of washing and centrifugation) and finally, one drop of the aqueous dispersion of stained composite particles was placed on a Cu grid with an elastic carbon membrane. After drying at room temperature, the interior structure of the composite particles was imaged using TEM. Fig. 1(a) shows a typical TEM image of Au NPs@PS-NH₂/PB-NH₂. Monodisperse Au NPs covered with polymer shells can be seen. In order to confirm ligand exchange, UV-Vis and FT-IR ATR spectra of the original Au NPs, Au NPs@PS-NH₂, Au NPs@PB-NH₂ and Au NPs@PS-NH₂/PB-NH₂ were measured (Fig. 1(b)–(d)). In the UV-Vis absorption spectrum of the original Au NPs, a broad plasmonic resonance around 530 nm (Fig. 1(b)(i)) is clearly observed. On the other hand, absorption peaks at 260 nm attributed to the aromatic moiety in polystyrene (Fig. 1(b)(ii)) and two peaks attributed to double bonds of 3,4-addition and 1,2-addition in polybutadiene (Fig. 1(b)(iii)) appear. Furthermore, all of the absorption peaks present for these samples (i)–(iii) are observed in the case of Au NPs@PS-NH₂/PB-NH₂. These results imply that Au NPs were amino-terminated polymers selectively modified by ligand exchange. This is also confirmed by the FT-IR spectra. There are no clear absorption peaks observed for the original Au NPs, but peaks attributed to aromatic (iv), and alkyl chains (v) are seen in the case of Au NPs@ PS-NH₂, and peaks attributed to alkyl chains (iv) and double bonds (vii) are observed in the case of Au NPs@ PB-NH₂ (Fig. 1(c) and (d), respectively). All of these peaks appear in the spectrum of Au NPs@PS-NH₂/PB-NH₂. These results confirm the modification of Au NPs with amino-terminated polymers. This is also supported by the TG measurement results shown in the ESI (Fig. S1^†).Open in a separate windowFig. 1TEM image of Au NPs@PS-NH₂/PB-NH₂ (a), UV-Vis spectra (b) and FT-IR spectra (c), (d) of original Au NPs, Au NPs@PS-NH₂, Au NPs@PB-NH₂ and Au NPs@PS-NH₂/PB-NH₂, respectively.To check the effect of surface polymer ligands on the distribution of Au NPs in the composite particles, Janus particles containing PS, PB and polymer-stabilized Au NPs were prepared using SORP. Fig. 2(a) shows a TEM image of a Janus particle comprising PS, PB and Au NPs@PS-NH₂. The dark gray region represents the PB domain and the brighter gray region represents the PS domain. The small dark spots are Au NPs and are visible only in the PS domain. Same as case, Au NPs@PS-NH₂, Au NPs located only at the PB domain of Janus particles comprised of PS, PB and Au NPs@PB-NH₂. These results indicate that the location of Au NPs can be controlled by changing the surface polymer ligands, as has been previously reported.²¹Open in a separate windowFig. 2Schematic illustration of mixed polymers and NPs, TEM images and model of composite Janus particles of PS/PB/Au NPs@PS-NH₂ (a), PS/PB/Au NPs@PB-NH₂ (b) and PS/PB/Au NPs@PS-NH₂/PB-NH₂ (c).On the other hand, when both types of polymers were present on the surface of the Au NPs, the NPs did not disperse in the two different phases, and most of them spontaneously self-assembled in the form of a ring at the three-phase contract line among PS, PB and water (Fig. 2(c)). It is noteworthy that Au NPs seldom assemble at the interface between PS and PB in Janus particles. Russell et al. reported that metal NPs have intrinsic amphiphilic properties and stabilize interfaces between hydrophobic and hydrophilic materials. Due to the amphiphilic nature of Au NPs, they are naturally present at the surface of the Janus particles. Since both PS and PB are immobilized at the surface of the Au NPs, they have equal affinity to both phases, and the interface between PS and PB is energetically stable for them. The spacing between the Au NPs is almost constant since steric hindrance by the polymer chains on the surface prevents them from aggregating.Based on these findings, Au NRs were also employed to form a ring structure. Fig. 3(a) shows a TEM image of Au NRs modified with PS-NH₂ and PB-NH₂ using the same procedure as that for Au NPs. These modified Au NRs were mixed with a THF solution of PS and PB, and Janus particles were prepared by SORP. Before thermal annealing, the Au NRs were randomly located inside the Janus particles (See ESI, S2^†). On the other hand, after thermal annealing, the Au NRs were aligned along the equator of the Janus particles, forming a ring structure as in the case of Au NPs.Open in a separate windowFig. 3Schematic illustration of mixed polymers, NPs and TEM images of Au NRs@PS-NH₂/PB-NH₂ (a) and PS/PB/Au NPs@PS-NH₂/PB-NH₂ after (b) annealing.There are two kinds of alignment of Au NRs at the interface: one along the equator (head-to-tail), and the other perpendicular to that (side-by-side). Most NRs are aligned side-by-side, in order to maximize the packing density at the equator. Since Au NRs have high anisotropy (aspect ratio ∼4), four times fewer can be accommodated for the head-to-tail arrangement. Since the 1D alignment of Au NRs has an important effect on plasmonic resonance,^24,25 controlling this alignment in a confined geometry may represent a way to tune the plasmonic properties of Au NRs.     

Facile synthesis of hollow hierarchical Ni@C nanocomposites with well-dispersed high-loading Ni nanoparticles embedded in carbon for reduction of 4-nitrophenol

Hollow hierarchical Ni@C nanocomposites with highly dispersed Ni nanoparticles (NPs) embedded in well-graphitized carbon matrix have been synthesized by solid-state pyrolysis of simple, well-defined organic–inorganic layered nickel hydroxide. The integration of highly dispersed Ni NPs, high Ni NPs content (up to ∼88.01 wt%), well-graphitized carbon as well as strong Ni/carbon interaction in the Ni@C make them display excellent catalytic activity and stable magnetic recyclability toward the reduction of 4-nitrophenol by NaBH₄.

Hollow hierarchical Ni@C nanocomposites with highly dispersed Ni nanoparticles (NPs) embedded in well-graphitized carbon matrix have been synthesized by solid-state pyrolysis of simple, well-defined organic–inorganic layered nickel hydroxide.

The development of more efficient and stable catalysts has been an increasingly important goal for chemists and materials scientists for both economic and environmental reasons. As efficient and cost-effective non-noble metal, Ni NPs have attracted much attention owing to their potential applications in electronic or optical materials, as well as in catalysis.^1–3 In general, the overall performance of Ni NPs depends heavily upon the size, shape and dispersion. It is widely accepted that a higher catalytic activity can be achieved by increasing the surface area of the specific active phase of the catalysts through reducing the size of the corresponding catalytic particles.^4–7 However, due to their high surface area to volume ratio, Ni NPs are typically unstable and tend to sinter into larger species, especially at high metal contents, which results in a dramatic decrease in catalytic activity and selectivity. It is thus highly important to prepare stable Ni NPs with uniform dispersion and narrow size distribution to promote their catalytic activities.Over the past decades, embedding of Ni NPs inside the porous supports has been proved to enhance catalytic activity and impede nanoparticle sintering.^8–15 The advantages of carbon supports with respect to conventional oxidic supports, like silica and zirconia, involve the high specific surface area, high stability, intrinsic high electrical conductivity, as well as easy recovery of metal active phases from spent catalysts by burning away carbon. To date, various methods for preparing carbon-supported Ni NPs have been developed.^16–22 In particular, incipient wetness impregnation and coprecipitation are commonly used methods to prepare these supported catalysts. It is well known that traditional carbons (including activated carbon, carbon nanotubes, graphene and carbon nanofibers) are poor in functional groups and a complex functionalization pre-treatment (such as acid oxidation, ionic liquid linking or polymer wrapping) is always required. In addition, the excess reducing regents are also needed for the reduction of Ni NPs. Although a high dispersion of Ni NPs on porous carbon can be achieved by employing this methods, this tedious post-synthetic method renders instable catalysts with dispersed Ni NPs on the external surface or near pore mouths and Ni NPs still have a tendency to be sintered, especially at high metal loading or high temperature, resulting in a significant loss of reactivity because of the relatively weak interaction between supported metal and carbon supports. Therefore, methods for the large-scale fabrication of carbon supported Ni NPs without using any reducing agents and high resistance toward sintering at high a metal loading level or a high temperature are still needed.Recently, precursor-controlled pyrolysis of polymer frameworks have proven an attractive template-free, one-step approach toward creating novel metal–carbon hybrids with uniformly distributed metal NPs in a carbon matrix.^23–26 Taking advantage of this category of the systems, we demonstrate a facile synthetic route to fabricate hollow hierarchical Ni@C nanocomposite by one-step solid-state pyrolysis of a simple and inexpensive organic–inorganic layered nickel hydroxides. In this facile procedure, cheap commercially available nickel nitrate hexahydrate and sodium salicylate are used as starting materials, and during the pyrolysis process the interlayer salicylate anions act as carbon source and reducing agent without the need for any external agent or surface modification. In the composites, well-crystallized Ni NPs with high loading content (88.01 wt%) are uniformly embedded in the well-graphitized carbon matrix. The as-prepared Ni@C nanocomposites show excellent catalytic activity toward the reduction of 4-NP by NaBH₄ and can be easily magnetically recovered and reused for several cycles.The hierarchical layered nickel hydroxides intercalated with salicylate anions (LNHs–Sal) precursor was prepared in large quantities through a simple urea decomposition method in water (details of the experimental are given in the ESI^†). The X-ray diffraction (XRD) pattern (Fig. S1^†) of precursor exhibits the typical pattern of a layered structure with very well-ordered (00l) basal peaks at low angle, confirming the successful intercalation of salicylate anions.²⁷ The scanning electron microscopy (SEM) image (Fig. S2^†) displays the hierarchical LNHs–Sal microsphere (∼1.5 μm in diameter) composed of numerous frizzy nanoflakes intercrossing with each other. EDX spectra (Fig. S3^†) shows the presence of Ni, O, C (with additional Pt signals arising from the Pt coating). Based on the elemental analysis, the chemical composition of the LNHs–Sal can be written as Ni(OH)_1.676(C₇H₅O₃)_0.324·0.279H₂O (anal calcd: C 19.95, H 2.816%. Found: C 19.93, H 2.821%).The LNHs–Sal was directly carbonized at 500 °C for 2 h under N₂ flow. Fig. 1a shows the typical SEM image of the product, which displays large-scale particles. Interestingly, we find that the configuration of the hollow flower-like morphology was perfectly maintained after thermal treatment (Fig. 1a and b), indicative of the high thermal stability of a self-assembled three-dimensional nanostructure. EDX result shows that the hollow flower-like nanocomposites consisted of nickel, carbon and oxygen (Fig. S4^†). As revealed by transmission electron microscopy (TEM) image (Fig. 1c), it can be seen that shell thickness of hollow Ni@C nanocomposites is ∼200 nm, close to the lateral size of LNHs–Sal nanoflakes. XRD pattern of products is shown in Fig. 1d, three peaks appeared at 44.51, 51.85, 76.45° can be attributed to the (111), (200) and (220) planes, respectively, of fcc-Ni (JCPDF: 04-0850),²⁸ suggesting the successful in situ reduction of the Ni ions to a metallic state after carbonization. No peaks of impurities could be detected from this pattern, indicating the high purity of the materials.Open in a separate windowFig. 1(a) and (b) SEM images, (c) TEM image, and (d) XRD of Ni@C nanocomposites.High resolution transmission electron microscopy (HRTEM) of the Ni@C nanocomposites are shown in Fig. 2. Although no characteristic XRD pattern of graphitic carbon is observed directly (Fig. 1d), well-graphitized layers and lattice fringes are observed in the surroundings of Ni NPs (Fig. 2b and c), suggestive of the nature of graphitic carbon, which is also confirmed by the Raman spectra (Fig. S5^†). Notable, the sizes of the Ni NPs are limited to ∼9 nm, which is well consistent with the XRD results estimated by Debye–Scherrer equation. The Ni NPs are homogeneously dispersed within the well-graphitized carbon matrix without aggregation even if the Ni content is up to ∼88.01 wt%, which was evaluated by the result of the TG under air (Fig. S6^†), indicating an excellent confinement that can avoid sintering of the Ni NPs (Fig. 2c). Such superior confinement can be attributed to the in situ simultaneous formation of Ni NPs and carbon matrix during the high-temperature thermal treatment, where the Ni NPs formed were firmly locked within the carbon matrix.^29,30 The lattice distance of 0.203 nm (Fig. 2d), corresponding to the (111) planes of the fcc metallic nickel phase, is in good accordance with the unit cell given by XRD.Open in a separate windowFig. 2(a) TEM image, (b)–(d) HRTEM image of Ni@C nanocomposites.X-ray photoelectron spectroscopy (XPS) measurement was performed to investigate the specific surface composition and chemical environment of the representative Ni@C nanocomposites. Two principal peaks in the Ni 2p XPS spectrum (Fig. 3a) centered at 852.9 and 870.2 eV confirmed the metallic state of Ni in Ni@C.^31–33 While Ni 2p doublet with the binding energies of 856.3 and 874.3 eV, may be attributed to the Ni–O–C on the surface of Ni nanoparticles, indicates the formation of strong interaction between the Ni NPs and the carbon matrix.^34,35 In the expanded image of the C 1s peak (Fig. 3b), the high-resolution C 1s core-level spectrum could be deconvoluted into three peaks centered at ∼284.9, ∼285.9, and ∼289.2 eV, which was indexed to C C/C–C, C–OH and C–O–Ni, respectively.^36–38 The sharpest peak corresponding to C C/C–C indicated that most of the carbon atoms were in the form of a conjugated honeycomb lattice. The O 1s spectrum is deconvoluted into two peaks (Fig. 3c), and they can be assigned to C–O–Ni (530.2 eV) and surface C–OH (532.1 eV).³⁹ These results indicate that strong Ni–O–C interaction was formed, which is very important for preventing the aggregation of the Ni NPs and making the Ni NPs hard to detach from the carbon support (improving the stability). Meanwhile, the external surfaces of Ni@C nanocomposites are extensively functionalized with C–OH groups without any steps of modification, which are derived from the functional groups of salicylate anions and further confirmed by FTIR spectroscopy (Fig. S7^†) allowing the Ni@C nanocomposites to be dispersed in water to a certain extent. The Brunauer–Emmett–Teller (BET) surface area of the Ni@C nanocomposites was measured to be 106.11 m² g⁻¹, and the pore size was about 12.0 nm (Fig. 3d). This should offer a sufficient interface to facilitate to the mass transfer and facilitates the catalytic activity.Open in a separate windowFig. 3XPS spectra of Ni@C nanocomposites: (a) Ni 2p core levels, (b) C1 core level, and (c) O1 core level; (d) nitrogen adsorption–desorption isotherm Ni@C nanocomposites (inset is pore size distributions).The catalytic performance of the Ni@C nanocomposites for the reduction of 4-NP to 4-AP with excess of NaBH₄ were evaluated, of which the results are show in Fig. 4. In the absence of the catalyst, the peak remained unaltered with time, indicating that the reduction reaction did not occur. With the Ni@C catalyst added, the reduction reaction did proceed. The reaction kinetics could be monitored easily from the time-dependent absorption spectra, which showed the successive intensity decrease of the absorption peak at 400 nm, ascribed to nitro compounds, and the concomitant development of a new peak at 300 nm corresponding to 4-AP, the reduction product of 4-NP (Fig. 4a). In the reduction process, as excess NaBH₄ was used, the BH₄⁻ concentration can be considered as a constant throughout the reaction. The ratio of C_t and C₀, where C_t and C₀ are 4-NP concentrations at time t and 0, respectively, was measured from the relative intensity of the respective absorbances, A_t/A₀. The rate constant k can be determined from the linear plot of ln(C_t/C₀) versus reduction time was in seconds. As expected, a good linear correlation of ln(C_t/C₀) versus time was obtained (Fig. 4b), whereby a kinetic reaction rate constant k over the weight of catalyst is estimated to be 6.43 s⁻¹ g⁻¹, which is larger than those reported previously (40–44 The high activity arises from the synergistic effect of carbon matrix and embedded Ni NPs, explained as follows: (1) carbon layers has high adsorption ability towards 4-NP via π–π stacking interactions. This provides a high concentration of 4-NP near to the Ni nanoparticles embedded in carbon matrix, leading to highly efficient contact between them; and (2) the synergistic effect of Ni nanocrystals and well-graphitized carbon matrix, facilitating the uptake of electrons by 4-NP molecules. As shown in Fig. 4c and d, the catalysts can be successfully recycled by an external magnet after the catalytic reduction and reused in 10 successive reactions with a conversion of 87%, further suggesting an excellent stability and long life, which make the products be attractive materials for various potential applications.Open in a separate windowFig. 4(a) UV-vis spectra of the reduction of 4-NP in aqueous solution using (Ni@C) nanocomposites, (b) the relationship between ln(C_t/C₀) and reaction time, (c) magnetic separation and recycling of Ni@C nanocomposite, and (d) conversion of 4-NP in ten successive cycles of reduction.Comparison of reaction rate constant of Ni@C nanocomposites with other catalysts reported in literatures

The TiO2 topotactic transformation assisted trapping of an atomically dispersed Pt catalyst for low temperature CO oxidation

Atomically dispersed Pt catalysts are synthesized on TiO₂ with high activity and strong high temperature resistance by loading Pt in the process of converting the NH₄TiOF₃ precursor to TiO₂ by a topotactic transformation process. The atomically dispersed Pt catalyst displayed high catalytic activity for the low temperature CO oxidation reaction.

Atomically dispersed Pt catalysts are deposited on the rough surface of TiO₂, which is synthesized via topotactic transformation from a NH₄TiOF₃ mesocrystal.

Supported noble metal catalysts are widely used in industrial processes on account of their high activity and/or selectivity for many key chemical reactions.¹ Usually, the noble metals are finely dispersed on a support to give a high specific surface area to effectively use the catalytically active component and increase the amount of active sites.^2,3 Atomically dispersed metal/metal oxide catalysts have attracted widespread interest in diverse research areas, such as chemistry, material science and environmental science.⁴ Due to their low-coordination, unsaturated atoms often function as active sites in catalytic processes, suggesting that downsizing the particles or clusters to single atoms is ideal for catalytic reactions.⁵ Single atoms tend to aggregate and grow into clusters or nanoparticles during the catalytic reaction processes due to the significant surface free energy increases with decreases in particle size.⁶ Although several methods have been explored for the preparation of atomically dispersed noble-metal catalysts in the past decade, the fabrication of stable atomically dispersed noble metal catalysts is still a great challenge.⁷ It is well known that the interaction between single atoms and supported substrate is essential for stabilizing active single atoms.³ Many previous reports on oxide supported metal clusters show that surface defects of the carriers can be used as anchor points for metal clusters or even single atoms.⁸ Besides noble metal atoms as the active sites, metal oxide carriers can also play an important role in the catalytic process. The interaction between oxide carrier and metal atoms can change the electronic properties of metal atoms, so it is of great significance to the activity and selectivity of the catalysts,⁹ especially in the catalytic oxidation of CO.^2,10 The different properties of the catalyst may be caused by different coordination environments around the single atom metal center, which is similar to the so-called “support effect” in traditional heterogeneous catalysis.Herein, we report the synthesis of atomically dispersed Pt catalysts on TiO₂ supports during a topotactic transformation process of NH₄TiOF₃ mesocrystals. Topotactic transformation is a useful method for preparing crystals with a required morphology through conversion from a precursor or mother crystal, in which the crystal orientations of the precursors and the target crystals have a certain topotactic correspondence.¹¹ NH₄TiOF₃ is a typical mesocrystal, which exists in similar structures to anatase TiO₂ with an average lattice mismatch of only 0.02%.¹² The NH₄TiOF₃ mesocrystals can be topotactically transformed into TiO₂ mesocrystals by either washing with aqueous H₃BO₃ or calcination at high temperature.^13–15 The topotactic transformation can be applied to anchor the single atom via atom trapping by the rough surface,¹⁶ crystalline defects and active vacancies.⁸As illustrated in Fig. 1, NH₄TiOF₃ mesocrystals were firstly impregnated with H₂PtCl₆ solution for a period of time, resulting in adsorption of [PtCl₆]²⁻ ions on the surface and an internal pore of NH₄TiOF₃ mesocrystals. Then, NH₄TiOF₃ began to form hollow anatase TiO₂ with ordered arranged nanothorns by topotactic transformation under an aqueous H₃BO₃ environment, in which the rough surface of TiO₂ can prevent the aggregation of Pt atoms and result in the formation of highly dispersed Pt single atoms on the TiO₂ substrate (Fig. S1^†). We compared the effects of topotactic transformation, noble metal loading content and calcination. Pt/TiO₂-T catalysts were prepared with various Pt loading contents of 0.1%, 0.5%, 1% and 3%. The above samples were calcined in N₂ at 450 °C for 4 h and the final products were labeled as Pt/TiO₂-TC (experimental details are given in the ESI^†).Open in a separate windowFig. 1Schematic diagram of the synthesis of Pt/TiO₂-TC.The dispersion and configuration of atomically dispersed Pt catalysts ware characterized by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), which can identify the heavy atoms in the actual catalyst. A panoramic SEM image of the NH₄TiOF₃ precursors clearly showed the well-defined uniform NH₄TiOF₃ nanobricks with an average length of 400 nm and a thickness of 70 nm (Fig. 2A). After topotactic transformation and calcination, the resulting Pt/TiO₂ turned rough on the surface and became a mesoporous hollow structure, as shown in Fig. 2B. From common TEM images, shown in Fig. 2C (also see Fig. S2^†), we clearly see that the sample is a hierarchical structure composed of regularly arranged nanocrystals. The inset of Fig. 2C shows a lattice spacing of 0.35 nm, which can be assigned to the (101) plane of TiO₂ nanocrystals. No Pt cluster or particle can be observed on the surface in the normal TEM observation, suggesting that the resulting Pt is highly dispersed and has a sub-nanometer size. In order to verify that Pt atoms have been successfully deposited on TiO₂, we performed atomic-resolution HAADF-STEM observations of the 1% Pt/TiO₂-TC. Large amounts of marked bright points show that individual Pt atoms (marked by the red circles) uniformly dispersed on the surfaces of the TiO₂ nanocrystals (Fig. 2D). Examination of different areas showed that only Pt single atoms existed in the sample 1% Pt/TiO₂-TC (Fig. S3^†). Fig. 2D clearly shows that each Pt atom (red circles) occupies the lattice position of a Ti atom. The statistical size distributions of Pt in 1% Pt/TiO₂-TC and 3% Pt/TiO₂-TC are 0.9 nm and 1.09 nm (Fig. S4^†), respectively, indicating that only subnanometer clusters and single atoms of Pt are formed on the TiO₂ substrate after the topotactic transformation and calcination. Energy dispersive spectroscopy (EDS) shows that Pt is uniformly dispersed on the surface of TiO₂ (Fig. 2E).Open in a separate windowFig. 2SEM images of (A) NH₄TiOF₃ mesocrystals and (B) 1% Pt/TiO₂-TC catalysts. (C) A TEM image of 1% Pt/TiO₂-TC; the inset shows the crystal lattice of anatase TiO₂. (D) High-resolution HAADF-STEM images of 1% Pt/TiO₂-TC. (E) EDS elemental mapping of a single 1% Pt/TiO₂-TC crystal.The structures of Pt/TiO₂-TC catalysts with certain Pt contents were analyzed by using powder X-ray diffraction (XRD) in order to assess the impact of Pt addition upon the nanoparticles. XRD analysis shows the structure transformation process from the NH₄TiOF₃ (Fig. S5^†) to TiO₂ (Fig. 3A). All the diffraction peaks are attributed to the anatase TiO₂. No Pt diffraction peaks were detected in these samples, indicating the presence of highly dispersed clusters or single atoms on TiO₂. Even after 450 °C of calcination in N₂ for 4 h, the catalysts exhibited no Pt diffractions, suggesting that the calcination did not cause Pt atom aggregation. The N₂ adsorption/desorption isotherms display that the BET specific areas of the 1% Pt/TiO₂-TC and 3% Pt/TiO₂-TC were 130.06 m² g⁻¹ and 132.14 m² g⁻¹, respectively (Fig. 3B).Open in a separate windowFig. 3(A) XRD patterns of Pt/TiO₂-TC catalysts with different Pt loading content. (B) N₂ adsorption–desorption isotherms and pore size distributions (inset) of Pt/TiO₂-TC catalysts. (C) O 1s XPS spectra and (D) Pt 4f XPS spectra for Pt/TiO₂ catalysts.X-ray photoelectron spectroscopy (XPS) analysis was carried out to evaluate the surface composition and valence states of the Pt/TiO₂ catalyst. The representative XPS survey scan spectrum indicates the existence of Ti, O, and Pt elements (Fig. S6^†). Ti 2p spectra at 458.6 and 464.3 eV belong to the Ti 2p_3/2 and Ti 2p_1/2 peaks of Ti⁴⁺ (Fig. S7^†).¹⁷ The binding energy of Ti 2p did not change after calcination. Fig. 3C shows O 1s core-level XPS spectra of Pt/TiO₂-T and Pt/TiO₂-TC; the catalysts contained two kinds of O species. The main peak centered at 530 eV is considered to be the oxygen band of Ti–O–Ti that can be assigned to the lattice oxygen of bulk TiO₂ and the shoulder peak at 531.35 eV can be ascribed to the surface OH species (Ti–OH) which could be correlated with an oxygen vacancy.¹⁸ It is obvious that the Pt/TiO₂-TC catalyst has more surface OH groups than the Pt/TiO₂-T catalyst. It is reported that surface OH groups are formed through water dissociation on oxygen vacancies or on metal surfaces by water–oxygen interaction.¹⁹ After calcination at 450 °C, peak shifts occurred and the two peaks corresponding to O 1s core-level XPS spectra were 529 and 531.6 eV, respectively, indicating that electron transfer occurred. The oxidation state of Pt in the catalyst is shown in Fig. 3D. The spectra collected for the 1% Pt/TiO₂-TC and 3% Pt/TiO₂-TC catalysts show two peaks at the Pt 4f edge with binding energies of 70.2 and 73.7 eV, which are assigned to the 4f_7/2 and 4f_5/2 states of Pt⁰, respectively. For 3% Pt/TiO₂-TC, the Pt 4f bimodal peak shows a downshift by 0.2 eV in binding energy. Deconvolution analysis reveals that there are two additional peaks at 71.4 and 75.8 eV, in addition to the two peaks related to Pt⁰, which can be attributed to the same spin-orbital split of Pt²⁺. The spectra collected for the 1% Pt/TiO₂-T and 3% Pt/TiO₂-T samples also show four peaks at the Pt 4f edge, in which the binding energies of 70.6 and 73.8 eV are assigned to the 4f_7/2 and 4f_5/2 states of Pt⁰, and the binding energies of 72.5 and 76 eV are assigned to the 4f_7/2 and 4f_5/2 states of Pt²⁺.²⁰ For Pt/TiO₂-T and Pt/TiO₂-TC catalysts, the binding energies of Pt²⁺ 4f_7/2 and Pt⁰ 4f_5/2 decreased by 0.3 eV and 0.8 eV, respectively, which indicates that the surface reconstruction occurred during the calcination pre-treatment and enhanced the interaction between the metal and the support. The peak area of Pt⁰ increased obviously after calcination, which may be the reason for the decomposition of the oxidation state Pt at a high temperature.²¹ The existence of Pt²⁺ suggests a strong interaction between Pt and O, which is of benefit for the thermal stability of a Pt single atom, and also propitious for the charge transfer during CO oxidation.^4b According to the XPS spectra, it is inferred that the actual content of Pt in the catalyst is very low (Table S1^†) and is favorable for atom-level dispersion.CO oxidation was chosen as a probe reaction to study the catalytic performance of single Pt atom supported on TiO₂ because such a reaction is highly sensitive to the chemical environment of the metal centers. Fig. 4 shows the catalytic performance of catalysts with different Pt loading contents. The data show that a CO oxidation reaction onset at near 50 °C and a total convention at near 130 °C, indicating an excellent catalytic performance for low temperature CO oxidation. Controlled experiments were carried out with catalysts prepared by non-topotactic transformation and non-calcined methods (Fig. S8^†). Even using a high-performance commercial TiO₂ photocatalyst, P25, as the supporting substrate, the resulting Pt/TiO₂–P25 catalyst also shows a much lower activity on CO oxidation in comparison with Pt/TiO₂-TC (Fig. S9^†). All of the samples of Pt/TiO₂-T (non-calcined) and Pt/TiO₂-NC (non-topotactic transformed) did not show good catalytic activity for CO oxidation. We summarize the CO oxidation reaction temperature in Table S2,^† in which T100 denotes the temperature at which 100% of CO was converted into CO₂ and T50 is the temperature required for a 50% CO conversion. The T50 and T100 of 1% Pt/TiO₂-TC were 108 °C and 130 °C, respectively, which are lower than those all other catalysts. The atomically dispersed Pt/TiO₂ catalysts show a higher catalytic performance in the CO oxidation reaction in comparison with previously reported single atom catalysts, such as Pt/La–Al₂O₃,¹⁶ Pt/θ-Al₂O₃ (ref. 10a) and Pd/La-γ-Al₂O₃ (ref. 10b) (Table S3^†). To illustrate the high activity of Pt/TiO₂-TC, Arrhenius plots are depicted in Fig. 4B. The corresponding Arrhenius plots of the CO reaction rate ln(TOF) show an approximate linear relation versus 1/T for the CO oxidation reaction. The apparent activation energy (E_a) is ∼10 kJ mol⁻¹, much lower than for Pt/Al₂O₃ or Pt/CeO₂ (E_a = 90–100 kJ mol⁻¹).²² 1% Pt/TiO₂-TC has the smallest apparent activation energy, which is one of the main reasons that it shows the highest catalytic activity. The TOFs with different Pt SACs are summarized in Open in a separate windowFig. 4(A) CO conversion and (B) corresponding Arrhenius plots of the reaction rate ln(TOF) versus 1/T for the CO oxidation reaction using Pt/TiO₂-TC catalysts with different Pt loading content.Specific rates and TOFs of Pt/TiO₂ catalysts compared with reported Pt SACs catalysts^a

Correction: Dipyrrolyl-bis-sulfonamide chromophore based probe for anion recognition

Correction for ‘Dipyrrolyl-bis-sulfonamide chromophore based probe for anion recognition’ by Namdev V. Ghule et al., RSC Adv., 2014, 4, 27112–27115, DOI: 10.1039/C4RA04000G.

The authors regret that an incorrect version of Fig. 1 was included in the original article. The correct version of Fig. 1 is presented below.Open in a separate windowFig. 1Color changes of receptor DPBS in chloroform upon addition of 5 equiv. of F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻, ClO₄⁻ and AcO⁻ (tetrabutylammonium salts).The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Mesoporous PbO nanoparticle-catalyzed synthesis of arylbenzodioxy xanthenedione scaffolds under solvent-free conditions in a ball mill

A protocol for the efficient synthesis of arylbenzodioxy xanthenedione scaffolds was developed via a one-pot multi-component reaction of aromatic aldehydes, 2-hydroxy-1,4-naphthoquinone, and 3,4-methylenedioxy phenol using mesoporous PbO nanoparticles (NPs) as a catalyst under ball milling conditions. The synthesis protocol offers outstanding advantages, including short reaction time (60 min), excellent yields of the products (92–97%), solvent-free conditions, use of mild and reusable PbO NPs as a catalyst, simple purification of the products by recrystallization, and finally, the use of a green process of dry ball milling.

An efficient one-pot multicomponent protocol was developed for the synthesis of arylbenzodioxy xanthenedione scaffolds using mesoporous PbO nanoparticles as reusable catalyst under solvent-free ball milling conditions.

Recently, the ball milling technique has received great attention as an environmentally benign strategy in the context of green organic synthesis.^1a The process of “ball milling” has been developed by adding mechanical grinding to the mixer or shaker mills. The ball milling generates a mechanochemical energy, which promotes the rupture and formation of the chemical bonds in organic transformations.^1b Subsequently, detailed literature^1c and books on this novel matter have been published.^2a,b Several typical examples include carbon–carbon and carbon–heteroatom bond formation,^2c organocatalytic reactions,^2d oxidation by using solid oxidants,^2e dehydrogenative coupling, asymmetric, and peptide or polymeric material synthesis, which have been reported under ball milling conditions.^2e Hence, the organic reactions using ball milling activation carried out under neat reaction environments, exhibit major advantages,^2f including short reaction time, lower energy consumption, quantitatively high yields and superior safety with the prospective for more improvement than the additional solvent-free conditions and clear-cut work-up.^3–5On the other hand, the organic transformations using metal and metal oxide nanoparticles⁶ are attracting enormous interest due to the unique and interesting properties of the NPs.^7,8,9a Particularly, PbO NPs^9b provide higher selectivity in some organic reactions^9c and find applications in various organic reactions, like Paal–Knorr reaction,¹⁰ synthesis of diethyl carbonate,¹¹ phthalazinediones,¹² disproportionation of methyl phenyl carbonate to synthesize diphenyl carbonate,¹³ the capping agent in organic synthesis, and selective conversion of methanol to propylene.¹⁴ In addition, the PbO NPs are also used in many industrial materials.^15,16However, till date, PbO NPs have not been explored in MCRs leading to biologically important scaffolds. Among others, the xanthene scaffolds¹⁷ are one of the important heterocyclic compounds¹⁸ and are extensively used as dyes, fluorescent ingredients for visual imaging of the bio-molecules, and in optical device technology because of their valuable chemical properties.¹⁹ The xanthene molecules have conjointly been expressed for their antibacterial activity,²⁰ photodynamic medical care, anti-inflammatory drug impact, and antiviral activity. Because of their various applications, the synthesis of these compounds has received a great deal of attention.²¹ Similarly, vitamin K nucleus^22,23 shows a broad spectrum of biological properties, like anti-inflammatory, antiviral, antiproliferative, antifungal, antibiotic, and antipyretic.^24a As a consequence, a variety of strategies^24b have been demonstrated in the literature for the synthesis of xanthenes and their keto derivatives, like rhodomyrtosone-B,^25a rhodomyrtosone-I,^25b and BF-6 ^25c as well as their connected bioactive moieties. Few biologically active xanthene scaffolds are shown in (Fig. 1).Open in a separate windowFig. 1Some biologically important xanthenes and their keto derivatives.Due to the significance of these compounds, the synthesis of xanthenes and their keto derivatives using green protocols is highly desirable. Reported studies reveal that these scaffolds are synthesized by three-component condensations using p-TSA²⁶ and scolecite²⁷ as catalysts. However, these methods suffer from the use of toxic acidic catalysts like p-TSA, long reaction times (3 h), harsh refluxing²⁶ or microwave reaction conditions,²⁷ and tedious work-up procedures. The previously reported methods for the synthesis of xanthenediones are shown in Scheme 1.Open in a separate windowScheme 1Previous protocol for the synthesis of xanthenedione derivatives.Herein, we report an economical and facile multicomponent protocol, using ball milling, for the synthesis of 7-aryl-6H-benzo[H][1,3]dioxolo[4,5-b]xanthene-5,6(7H)-dione using PbO NPs as a heterogeneous catalyst (Scheme 2). The PbO NPs are non-corrosive, inexpensive, and easily accessible.Open in a separate windowScheme 2General reaction scheme of PbO NP-catalyzed synthesis of the xanthenedione scaffolds under ball milling conditions.In our protocol,²⁸ the PbO NPs were initially prepared by mixing sodium dodecyl sulphate (2.5 mmol) and sodium hydroxide (10 mL, 0.1 N) with an aqueous methanolic solution of lead nitrate (2 mmol) under magnetic stirring at 30 °C by continuing the reaction for 2 h. Then, the obtained white polycrystalline product was filtered, washed with H₂O, and dried at 120 °C, followed by calcination at 650 °C for 2 h. During this step, the white PbO NPs turned pale yellow in colour. Eventually, the synthesized PbO was then characterized by spectroscopic and analytical techniques.The powder X-ray diffraction (XRD) pattern revealed the crystalline nature of the PbO NPs as the diffraction peaks corresponding to (131), (311), (222), (022), (210), (200), (002), and (111) crystal planes were identified (Fig. 2). The XRD outline of the synthesized PbO NPs was further established for the formation of space group Pca2₁ ²⁹ with a single orthorhombic structure (JCPDS card number 76-1796). The sharp diffraction peaks indicated good crystallinity, and the average particle size of the PbO NPs was estimated to be 69 nm, as calculated using the Debye–Scherer equation.Open in a separate windowFig. 2The powder XRD pattern of PbO NPs.The surface morphology of the PbO NPs was analyzed by scanning electron microscopy (SEM), and the SEM image revealed the discrete and spongy appearance of the PbO NPs (Fig. 3).Open in a separate windowFig. 3The SEM image of PbO NPs.Moreover, the elemental composition obtained from energy dispersive X-ray (EDX) analysis confirmed that the material contains Pb and O elements, and no other impurity was present (Fig. 4).Open in a separate windowFig. 4The EDAX spectrum of crystalline PbO NPs.The transmission electron microscopy (TEM) image shown in Fig. 5 indicated the formation of orthorhombic crystallites of PbO with several hexagon-shaped particles. The dark spot in the TEM micrograph further confirmed the synthesis of PbO NPs, as the selected area diffraction pattern associated with such spots reveals the occurrence of the PbO NPs in total agreement with the X-ray diffraction data (Fig. 6). The average size of the PbO nanocrystals by TEM was approximated to be around 20 nm.Open in a separate windowFig. 5The TEM image of nanocrystalline PbO NPs.Open in a separate windowFig. 6The SAED image of nanocrystalline PbO NPs.The Fourier transform infrared (FT-IR) spectrum (ESI, S6^†) of the PbO NPs displayed peaks at 575, 641, and 848 cm⁻¹, which corresponds to the Pb–O vibrations. Furthermore, the absorption band at ∼3315 cm⁻¹ was due to the presence of the hydroxyl group (–OH) in the NPs.The N₂ adsorption–desorption isotherms of the PbO nanoparticles shown in Fig. 7 was consistent with type IV adsorption–desorption isotherms with H1 hysteresis corresponding to the cylindrical mesoporous structure. Moreover, the surface area, pore-volume, and BJH pore diameter were found to be 32.0 m² g⁻¹, 0.023 cm³ g⁻¹, and 30.9 Å, respectively.Open in a separate windowFig. 7BET surface area and pore size of nanocrystalline PbO catalyst.The catalytic activity of the synthesized PbO NPs was tested in a one-pot multicomponent synthesis of arylbenzodioxoloyl xanthenedione derivative under ball milling condition according to the reaction scheme 2a, with 3,4-dimethoxybenzaldehyde (166.2 mg, 1.0 mmol), 3,4-methylenedioxyphenol (138.0 mg, 1.0 mmol), and 2-hydroxy-1,4-naphthoquinone (174.0 mg, 1.0 mmol) as reactants. The reaction conditions, the ball milling parameters (speed, time, and ball to solids ratio), and the PbO nanocatalyst amount were first optimized to produce the highest yield using experimental design as shown in

Thiophene-containing tetraphenylethene derivatives with different aggregation-induced emission (AIE) and mechanofluorochromic characteristics

Four thiophene-containing tetraphenylethene derivatives were successfully synthesized and characterized. All these highly fluorescent compounds showed typical aggregation-induced emission (AIE) characteristics and emitted different fluorescence colors including blue-green, green, yellow and orange in the aggregation state. In addition, these luminogens also exhibited various mechanofluorochromic phenomena.

Four thiophene-containing AIE-active TPE derivatives were synthesized. Furthermore, these luminogens exhibited various mechanofluorochromic phenomena.

High-efficiency organic fluorescent materials have attracted widespread attention due to their potential applications in organic light-emitting devices and fluorescent switches.^1–8 Meanwhile, smart materials sensitive to environmental stimuli have also aroused substantial interest. Mechanochromic luminescent materials exhibiting color changes under the action of mechanical force (such as rubbing or grinding) are one important type of stimuli-responsive smart materials, which can be used as pressure sensors and rewritable media.^9–18 Bright solid-state emission and high contrast before and after grinding are very significant for the high efficient application of mechanochromic fluorescence materials.^19–28 However, a majority of traditional emissive materials usually exhibit poor emission efficiency in the solid state due to the notorious phenomenon of aggregation caused quenching (ACQ), and the best way to solve the problem is to develop a class of novel luminescent materials oppositing to the luminophoric materials with ACQ effect. Fortunately, an unusual aggregation-induced emission (AIE) phenomenon was discovered by Tang et al. in 2001.²⁹ Indeed, the light emission of an AIE-active compound can be enhanced by aggregate formation.^30–32 Obviously, it is possible that AIE-active mechanochromic fluorescent compounds can be applied to the preparation of high-efficiency mechanofluorochromic materials. Numerous luminescent materials exhibiting mechanochromic fluorescent behavior have been discovered up to now.³³ Whereas, examples of fluorescent molecules simultaneously possessing AIE and mechanofluorochromic behaviors are still limited, and the exploitation of more AIE-active mechanofluorochromic luminogens is necessary. Organic solid emitters with twisted molecular conformation can effectively prevent the formation of ACQ effect, thus exhibiting strong solid-state luminescence. Tetraphenylethene is a highly twisted fluorophore. Meanwhile, it is also a typical AIE unit, which can be used to construct high emissive stimuli-responsive functional materials.^34–37The design and synthesis of novel organic emitters with tunable emission color has become a promising research topic at present. Only a limited number of organic fluorescent materials with full-color emission have been reported to date.^38,39 For example, in 2018, Tang et al. reported six tetraphenylpyrazine-based compounds. Interestingly, in film states, these luminogens exhibited different fluorescence colors covering the entire visible range, and this is the first example of realizing full-color emission based on the tetraphenylpyrazine unit.⁴⁰ It is still an urgent challenge to develop novel organic luminophors with tunable emission color basing on the same core structure.In this study, four organic fluorophores containing tetraphenylethene unit were successfully synthesized (Scheme 1). Introducing the thiophene and carbonyl units into the molecules possibly promoted the formation of weak intermolecular interactions such as C–H⋯S or C–H⋯O interaction, which was advantageous to the exploitation of interesting stimuli-responsive fluorescent materials. Indeed, all these compounds showed obvious AIE characteristics. Furthermore, these luminogens emitted a series of different fluorescent colors involving blue-green, green, yellow and orange in the aggregation state. In addition, these luminogens also exhibited reversible mechanofluorochromic phenomena involving different fluorescent color changes.Open in a separate windowScheme 1The molecular structures of compounds 1–4.To investigate the aggregation-induced properties of compounds 1–4, the UV-vis absorption spectra of 1, 2, 3 and 4 (20 μM) in DMF–H₂O mixtures of varying proportions were studied initially (Fig. S1^†). Obviously, level-off tails were obviously observed in the long-wavelength region as the water content increased. This interesting phenomenon is generally associated with the formation of nano-aggregates.⁴¹ Next, the photoluminescence (PL) spectra of 1–4 in DMF–H₂O mixtures with various water fraction (f_w) values were explored. As shown in Fig. 1, almost no PL signals were noticed when a diluted DMF solution of luminogen 1 was excited at 365 nm, and thus almost no fluorescence could be observed upon UV illumination at 365 nm, and the corresponding absolute fluorescence quantum yield (Φ) was as low as 0.04%. However, when the water content was increased to 50%, a new blue-green emission band with a λ_max at 501 nm was observed, and a faint blue-green fluorescence was noticed under 365 nm UV light. As the water content was further increased to 90%, a strong blue-green emission (Φ = 30.81%) could be observed. Furthermore, as shown in Fig. S2,^† the nano-aggregates (f_w = 90%) obtained were confirmed by dynamic light scattering (DLS). Therefore, the compound 1 with bright blue-green emission caused by aggregate formation showed typical AIE feature.Open in a separate windowFig. 1(a) Fluorescence spectra of the dilute solutions of compound 1 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with different water contents (0–90%). Excitation wavelength = 365 nm. (b) Fluorescence images of 1 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with different f_w values under 365 nm UV light.Similarly, as can be seen in Fig. 2–4, compounds 2–4 also showed obvious aggregation-induced green emission, aggregation-induced yellow emission, and aggregation-induced orange emission, respectively. When the water content was zero, the quantum yields of compounds 2–4 were 0.04%, 0.05% and 0.46%, respectively, while as the water content increased to 90%, the corresponding quantum yields of compounds 2–4 also increased to 30.67%, 45.57% and 26.53%, respectively. Hence, luminogens 2–4 were also AIE-active species. In addition, as shown in Fig. 5, the DFT calculations for the compounds 1–4 were performed. The calculated energy gaps (ΔE) of four compounds were 3.6178416 eV (compound 1), 3.276084 eV (compound 2), 3.3073755 eV (compound 3) and 3.0766347 eV (compound 4) respectively. Therefore, the various numbers and the various kinds of the substituents had slight effects on their molecular orbital energy levels of 1–4.Open in a separate windowFig. 2(a) Fluorescence spectra of the dilute solutions of compound 2 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with different water contents (0–90%). Excitation wavelength = 365 nm. (b) Fluorescence images of 2 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with different f_w values under 365 nm UV light.Open in a separate windowFig. 3(a) Fluorescence spectra of the dilute solutions of compound 3 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with different water contents (0–90%). Excitation wavelength = 365 nm. (b) Fluorescence images of 3 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with different f_w values under 365 nm UV light.Open in a separate windowFig. 4(a) Fluorescence spectra of the dilute solutions of compound 4 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with different water contents (0–90%). Excitation wavelength = 365 nm. (b) Fluorescence images of 4 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with different f_w values under 365 nm UV light.Open in a separate windowFig. 5(a) HOMO and LUMO frontier molecular orbitals of molecule 1 based on DFT (B3LYP/6-31G*) calculation. (b) HOMO and LUMO frontier molecular orbitals of molecule 2 based on DFT (B3LYP/6-31G*) calculation. (c) HOMO and LUMO frontier molecular orbitals of molecule 3 based on DFT (B3LYP/6-31G*) calculation. (d) HOMO and LUMO frontier molecular orbitals of molecule 4 based on DFT (B3LYP/6-31G*) calculation.Subsequently, the mechanochromic fluorescent behaviors of compounds 1–4 were surveyed by solid-state PL spectroscopy. As shown in Fig. 6, the as-synthesized powder sample 1 exhibited an emission band with a λ_max at 444 nm, corresponding to a blue fluorescence under 365 nm UV light. Intriguingly, a new blue-green light-emitting band with a λ_max at 507 nm was observed after the pristine solid sample was ground. After fuming with dichloromethane solvent vapor for 1 min, the blue-green fluorescence was converted back to the original blue fluorescence. Therefore, luminogen 1 exhibited reversible mechanochromic fluorescence feature. Furthermore, this reversible mechanofluorochromic conversion was repeated many times by grinding-exposure without showing signs of fatigue (Fig. 10).Open in a separate windowFig. 6(a) Solid-state PL spectra of compound 1 before grinding, after grinding, and after treatment with dichloromethane solvent vapor. Excitation wavelength: 365 nm. Photographic images of compound 1 under 365 nm UV light: (b) the as-synthesized powder sample. (c) The ground sample. (d) The sample after treatment with dichloromethane solvent vapor.Open in a separate windowFig. 10Repetitive experiment of mechanochromic behavior for compound 1.Similarly, as evident from Fig. 7–9, luminogens 2–4 also exhibited obvious mechanofluorochromic characteristics. Moreover, the repeatabilities of their mechanochromic behaviors were also satisfactory (Fig. S3^†). Hence, all the compounds 1–4 showed reversible mechanofluorochromic phenomena involving different fluorescent color changes, and the various numbers of the substituents could effectively influence the mechanofluorochromic behaviors of 1–4. Obviously, luminogen 3 or 4 after grinding exhibited more red-shifted fluorescence in comparison with that of the corresponding luminogen 1 or 2 after grinding.Open in a separate windowFig. 7(a) Solid-state PL spectra of compound 2 before grinding, after grinding, and after treatment with dichloromethane solvent vapor. Excitation wavelength: 365 nm. Photographic images of compound 2 under 365 nm UV light: (b) the as-synthesized powder sample. (c) The ground sample. (d) The sample after treatment with dichloromethane solvent vapor.Open in a separate windowFig. 8(a) Solid-state PL spectra of compound 3 before grinding, after grinding, and after treatment with dichloromethane solvent vapor. Excitation wavelength: 365 nm. Photographic images of compound 3 under 365 nm UV light: (b) the as-synthesized powder sample. (c) The ground sample. (d) The sample after treatment with dichloromethane solvent vapor.Open in a separate windowFig. 9(a) Solid-state PL spectra of compound 4 before grinding, after grinding, and after treatment with dichloromethane solvent vapor. Excitation wavelength: 365 nm. Photographic images of compound 4 under 365 nm UV light: (b) the as-synthesized powder sample. (c) The ground sample. (d) The sample after treatment with dichloromethane solvent vapor.In order to further explore the possible mechanism of mechanofluorochromism of 1–4, the powder X-ray diffraction (PXRD) measurements of various solid states of 1–4 were carried out. As depicted in Fig. 11, the pristine solid powder 1 showed many clear and intense reflection peaks, suggesting its crystalline phase. However, after the pristine powder sample was ground, the sharp and intense diffraction peaks vanished, which indicated the crystalline form was converted to the amorphous form. Interestingly, when the ground solid sample was fumigated with dichloromethane solvent vapor for 1 min, the corresponding sample powder exhibited the PXRD pattern of the initial crystalline form. Meanwhile, the structural transformations of the solid samples of 2–4 were similar to that of 1 (Fig. S4–S6^†). Obviously, the morphological changes of solid samples of 1–4 from crystalline state to amorphous state and vice versa could be attributed to the reversible mechanical switching in compounds 1–4, and the mechanofluorochromic phenomena observed in 1–4 were related to the morphological transition involving the ordered crystalline phase and the disordered amorphous phase.Open in a separate windowFig. 11XRD patterns of compound 1: unground, ground and after treatment with dichloromethane solvent vapor.Fortunately, single crystals of compounds 1 and 2 were obtained by slow diffusion of n-hexane into a trichloromethane solution containing small amounts of 1 or 2. As shown in Fig. 12 and ​and13,13, the molecular structures of 1 and 2 exhibited a twisted conformation due to the existence of tetraphenylethene unit. Meanwhile, some weak intermolecular interactions, such as C–H⋯π interaction (d = 2.866 Å) for 1, π⋯π interaction (d = 3.371 Å) for 1, C–H⋯S interaction (d = 2.977 Å) for 2, and π⋯π interaction (d = 3.189 Å) for 2, were observed. These weak intermolecular interactions gave rise to a loose packing motif of 1 or 2, which indicated their ordered crystal packings might readily collapse upon exposure to external mechanical stimulus. Therefore, their solid-state fluorescence could be adjusted by mechanical force.Open in a separate windowFig. 12The structural organization of compound 1.Open in a separate windowFig. 13The structural organization of compound 2.In summary, four fluorescent molecules containing thiophene and tetraphenylethene units were successfully designed and synthesized in this study. All these compounds showed obvious AIE characteristics. Furthermore, these luminogens emitted various fluorescence colors involving blue-green, green, yellow and orange in the aggregation state. Meanwhile, these luminogens basing on the same core structure also exhibited reversible mechanofluorochromic phenomena involving different fluorescent color changes. The results of this study will be beneficial for the exploitation of novel luminophors with full-color emission.     

A julolidine-fused anthracene derivative: synthesis,photophysical properties,and oxidative dimerization

We describe the synthesis and characterization of a julolidine-fused anthracene derivative J-A, which exhibits a maximum absorption of 450 nm and a maximum emission of 518 nm. The fluorescent quantum yield was determined to be 0.55 in toluene. J-A dimerizes in solution via oxidative coupling. Structure of the dimer was characterized using single crystal X-ray diffraction.

A julolidine fused anthracene derivative with unique photophysical and redox properties was presented.

Julolidine¹ is a popular structural subunit in various fluorescent dyes (Chart 1).² The restricted motion and strong electron-donating capability of the fused julolidine moiety are quite effective for improving the photophysical properties. For instance, julolidine-fused fluorophores normally display desirable photophysical characteristics, such as high quantum yield, red-shifted absorption and emission, and good photostability. Recently, julolidine derivatives have been widely exploited in various applications such as sensing,³ imaging,⁴ and nonlinear optical materials.⁵ Several julolidine dyes have been used in dye-sensitized solar cells due to their large π-conjugated system and the promising electron donating property.⁶Open in a separate windowChart 1The structure of julolidine, J-A and DAA.In this paper, we report a julolidine-fused anthracene derivative J-A, which exhibits attractive photophysical properties not observed in DAA, a dimethyl-amino substituted analogue. Both the absorption and emission of J-A show a dramatic red-shift (ca. 74 and 131 nm, respectively), compared with the unmodified anthracene (Fig. 2). The fluorescence quantum yield of J-A was determined to be 0.55 in toluene, while the emission of DAA was completely quenched. The observed spectral properties were rationalized by DFT calculations. In addition, we found that J-A was stable in the solid state, but reactive in solution. J-A dimer was formed through oxidative coupling at the para-position of the N-atom in a dichloromethane solution under air atmosphere. The structure of the dimerized product was characterized using single crystal X-ray diffraction, which unambiguously reveals the structural feature of the julolidine-fused anthracene compound. Preparation of J-A is shown in Scheme 1. Detailed synthesis and characterizations are provided in the ESI.^†Open in a separate windowScheme 1Synthetic route of J-A.Open in a separate windowFig. 2(A) Absorption spectra of J-A, AN, and DAA (1 × 10⁻⁴ mol L⁻¹ in dichloromethane); (B) emission spectra of J-A, AN, and DAA (1 × 10⁻⁵ mol L⁻¹ in dichloromethane). Excitation wavelength: 350 nm. 1 cm cuvette was used in both of the experiments. Inset: visualized fluorescence in solution was shown. ¹H-NMR signals of J-A shift to the high-field significantly, compared with DAA (Fig. 1), which indicates that the fused structure of J-A facilitates electron delocalization from the nitrogen atom to the anthracene moiety, and thus resulting in a stronger shielding effect. In the case of DAA, however, electron delocalization from the dimethyl amino group to the anthracene core is essentially inhibited due to steric hindrance, which will explain the fact that it displays a spectral feature similar to that of the unmodified anthracene.Open in a separate windowFig. 1Comparison of the ¹H-NMR spectrum between J-A and DAA in CDCl₃. Partial resonance signals in aromatic region are shown.Fusing with julolidine will exert significant effects on the photophysical properties of anthracene. The absorption and fluorescence spectra of J-A, DAA, and anthracene are shown in Fig. 2. The maximum absorption of J-A is 450 nm, which displays a red-shift of about 70 nm compared with the unmodified anthracene. J-A emits green light (^maxλ_em = 518 nm, Φ = 0.55), while anthracene emits blue light (^maxλ_em = 380 nm, Φ = 0.22). In contrast, the absorption of DAA essentially overlaps with that of anthracene, with only a minor red shift of ca. 10 nm, but its fluorescence is quenched significantly (Fig. 2). This spectral feature indicates that the dimethyl amino group is electronically separated from the anthracene moiety in the ground state, a result in good accordance with the ¹H-NMR data shown above. The quenched fluorescence of DAA may result from the photo-induced electron transfer⁷ from the lone pair of the nitrogen atom to the anthracene moiety in the excited state.The observed photophysical properties of J-A were reproduced by DFT calculations. The HOMO and LUMO orbitals are evenly distributed over the anthracene moiety and the N-atom in the julolidine, indicating the existence of a conjugated structure. The HOMO–LUMO transition (f = 0.10) corresponds to the absorption band at 450 nm. The sharper absorption at 390 nm can be assigned to the HOMO to LUMO + 1 transition. In contrast, the HOMO and LUMO orbitals of DAA resemble those of anthracene, because the dimethyl-amino group is orthogonal to the conjugated π-system (Fig. 3).Open in a separate windowFig. 3Molecular orbitals of J-A and DAA calculated at the B3LYP/6-31G(d) level of theory (iso value = 0.02). Orbital energies were given in parentheses. Excitation energies were computed by TD-DFT at the same level. Values in parentheses represent the oscillator strengths (f).J-A is stable in the solid state, but reactive in solution. The cyclic voltammetry (CV) diagram of J-A shows an irreversible oxidation potential at 0.007 V (vs. Fc/Fc⁺), indicating that J-A is easy to be oxidized (Fig. S3^†). Single crystals suitable for X-ray diffraction study were obtained by slow evaporation of a dichloromethane solution of J-A under air atmosphere. To our surprise, instead of J-A, X-ray data discloses the formation of a dimeric product (Scheme 2) of J-A. We hypothesized that the dimeric compound 5 formed via oxidative coupling reaction, a mechanism well-documented for the dimerization of the dimethylaniline compounds.⁸ The ¹H-NMR spectrum of 5 is distinct from that of J-A. All protons of the anthracene part (b′–d′) appear as a group of multiplet resonance signals (6.93–7.04 ppm) (Scheme 2). In addition, mass spectrometric analysis indicates that two hydrogen atoms were removed after the dimerization of J-A. Compound 5 exhibits a maximum absorption at 460 nm and a very weak emission (^maxλ_em = 530 nm, Φ = 0.03, Fig. S1^†). Two quasi-reversible oxidation waves were identified in the CV diagram of 5 at −0.010 V and 0.135 V (vs. Fc/Fc⁺), respectively (Fig. S5^†).Open in a separate windowScheme 2Oxidative dimerization of J-A. Inset: partial ¹H-NMR of 5 is shown.X-ray structure of 5 is shown in Fig. 4. The two connected anthracene planes are found to be orthogonal to each other with a dihedral angle of 90.17°, as a result of steric repulsion. Specifically, the bond length of N–C3 is 1.389 Å, which is similar to those of the other reported julolidine compounds (1.359–1.393 Å), while significantly shorter than that of the dimethyl-amino anthracene (1.433 Å).⁹ This result testifies the presence of electron delocalization between the fused julolidine nitrogen and the anthracene π-plane, which is in good agreement with the DFT calculations (Fig. 3). However, the two anthracene π-planes connected by the single bond (C4–C5, 1.489 Å) might not exhibit electron delocalization because of the orthogonal conformation. The fused julolidine ring-i and -ii are symmetric to each other, and both of them adopt an “envelope” conformation. The fused julolidine is nonplanar (bond angle, C3–N–C2, 115.73°, C3–N–C1, 115.92°, C1–N–C2, 113.81°). 5 is closely packed in the crystal (Fig. 4B), and no intercalated solvent molecules were observed. The closest distance between two adjacent anthracene planes is 3.922 Å, indicating a weak π–π stacking. Detailed crystal data are summarized in Table S5.^†Open in a separate windowFig. 4(A) Single crystal X-ray structure of 5. (B) View along a-axis.In summary, we report the synthesis and characterizations of a julolidine-fused anthracene derivative J-A, which demonstrates significantly red-shifted absorption (^maxλ_ab = 450 nm) and emission (^maxλ_em = 518 nm, Φ = 0.55), compared with the unmodified anthracene. The photophysical properties of J-A also contrast dramatically with a dimethyl-amino analogue DAA, which were rationalized by DFT calculations. In addition, J-A could be transformed into 5, a dimeric product, whose single crystal X-ray structure unambiguously confirmed the structural feature of the julolidine-fused anthracene.