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.     

Correction: Ultrafast conversion of carcinogenic 4-nitrophenol into 4-aminophenol in the dark catalyzed by surface interaction on BiPO4/g-C3N4 nanostructures in the presence of NaBH4

Correction for ‘Ultrafast conversion of carcinogenic 4-nitrophenol into 4-aminophenol in the dark catalyzed by surface interaction on BiPO₄/g-C₃N₄ nanostructures in the presence of NaBH₄’ by Ahmed B. Azzam et al., RSC Adv., 2021, 11, 18797–18808. DOI: 10.1039/D1RA02852A.

The authors regret that some misleading statements were included in section 3.2.1 ‘Effect of initial concentration on 4-NP’. The corrected version of section 3.2.1 is presented below. There are no changes to Fig. 8 or its caption.Open in a separate windowFig. 8Effect of initial concentration on reduction rate of 4-NP (a); corresponding linear transform ln(A_t/A₀) = f(t) of the 4-NP reduction kinetics curves (b); comparison of the rate constant value for the reduction of the 4-NP over initial different concentration (c). ([4-NP] = 10–70 mg L⁻¹, [catalyst] = 0.5 g L⁻¹). 3.2.1 Effect of initial concentration of 4-NP The effect of the initial concentration of 4-NP on the catalytic efficiency rate using 50% BiPO₄/g-C₃N₄ catalyst was carried out by varying the concentration from 10 to 70 mg L⁻¹, and the obtained results are shown in Fig. 8a. Interestingly, 50% BiPO₄/g-C₃N₄ was able to reduce all 4-NP solutions at concentrations from 10 to 70 mg L⁻¹, reflecting the high efficiency of such a catalyst towards this 4-NP reduction. At lower concentrations, a superior constant rate was recorded due to the availability of a large number of catalytic sites per given amount of 4-NP moles. And vice versa, the higher the concentration, the lower the rate constant (Fig. 8b), due to the high competition of 4-NP molecules on the limited sites. In addition, the number of molecules adsorbed at the surface of the BiPO₄/g-C₃N₄ heterojunction increases with the increase in concentration of 4-nitrophenol and hence, the surface becomes saturated by 4-nitrophenol molecules. This leads to a decrease in concentration of BH₄⁻ ions approaching the surface of the BiPO₄/g-C₃N₄ heterojunction, hence lowering the rate of hydrogen transfer from BH₄⁻ ion to the 4-nitrophenol molecule.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Correction: High-detectivity perovskite-based photodetector using a Zr-doped TiOx cathode interlayer

Correction for ‘High-detectivity perovskite-based photodetector using a Zr-doped TiO_x cathode interlayer’ by C. H. Ji et al., RSC Adv., 2018, 8, 8302–8309.

The authors regret that the names of the authors are shown incorrectly in the original article. The corrected author list is as shown above.In addition, the authors regret that an incorrect version of Fig. 4b was included in the original article. The correct version of Fig. 4 is as shown below.Open in a separate windowFig. 4Dynamic characteristics of photocurrent response times using a laser diode at a light intensity of 650 μW cm⁻² at 525 nm. (a) Photocurrent response time under −0.1 V at a pulsed frequency of 1 MHz. (b) Cut-off frequency for the perovskite photodetector under −0.1 V.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Correction: Split-anion solvent extraction of light rare earths from concentrated chloride aqueous solutions to nitrate organic ionic liquids

Correction for ‘Split-anion solvent extraction of light rare earths from concentrated chloride aqueous solutions to nitrate organic ionic liquids’ by Mercedes Regadío et al., RSC Adv., 2018, 8, 34754–34763, DOI: 10.1039/c8ra06055j.

The authors regret that an incorrect figure caption was given for Fig. 5. The correct version is presented below.Open in a separate windowFig. 5Viscosity as a function of the temperature and the organic phase composition: (1) after loading 39 g L⁻¹ of REE in 20 v% Cy923 in [C101][NO₃], (2) pure [C101][NO₃], (3) 20 v% Cy923 in [C101][NO₃] and (4) pure Cy923.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

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.     

Correction: Three-dimensional directional nerve guide conduits fabricated by dopamine-functionalized conductive carbon nanofibre-based nanocomposite ink printing

Correction for ’Three-dimensional directional nerve guide conduits fabricated by dopamine-functionalized conductive carbon nanofibre-based nanocomposite ink printing’ by Shadi Houshyar et al., RSC Adv., 2020, 10, 40351–40364, DOI: 10.1039/D0RA06556K.

The authors regret that an incorrect version of Fig. 2 was included in the original article. The correct version of Fig. 2 is presented below.Open in a separate windowFig. 2(a) FTIR spectra of pure PCL and PCL printed with CNF and DA (40 and 100 μg mL⁻¹), where circles emphasize the OH peak (3700 cm⁻¹) of the carboxylated CNF and NH peak (1565 cm⁻¹) of dopamine. (b) Shear stress of the CNF and CNF + DA nanocomposite inks versus shear rate. (c) Viscosity versus shear rate of the prepared nanocomposite inks.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Characterization of biosurfactant lipopeptide and its performance evaluation for oil-spill remediation

Biosurfactant lipopeptide is a promising dispersant over varieties of chemical ones in oil-spill remediation. The toxicity, biodegradability and performance of the biosurfactant lipopeptide are studied in this paper.

Biosurfactant lipopeptide is a promising dispersant over varieties of chemical ones in oil-spill remediation.

Dispersants were globally applied to physico-chemically enhance the dispersion of oil in water and were assumed to stimulate oil biodegradation by indigenous microorganisms and to reduce the environmental impact of oil spills.^1,2 Since the 1960s,^3,4 chemical dispersants have been applied as an emergency response to oil spills in marine ecosystems,⁵ and have showed effectiveness at removing oil slicks from the coast.^3,6,7 However, most of the chemically synthesized dispersants are inherently toxic to various aquatic species and hardly biodegradable in the natural environment.^2,8 The application of chemical dispersants in the 2010 Gulf of Mexico oil spill also raised concerns regarding the toxicity and the potential environmental impact,^9,10 and caused a debate about the effectiveness of chemical dispersants on the rates of oil biodegradation.¹¹ Biosurfactants are promising dispersants in oil-spill remediation, owning to their environmentally friendly and biodegradable properties.¹² Chemical surfactants could be replaced with biosurfactants and this change would diminish the environmental impact of traditional dispersants.^8,13,14 Lipopeptide produced by microorganisms is one of the representative biosurfactants and has showed great potential applications in food,¹⁵ medicine,¹⁶ microbial enhanced oil recovery,¹⁷ and other fields.¹⁸ Nevertheless, the knowledge about the application of biosurfactant lipopeptide in marine oil-spill remediation is still limited.In the present work, the dispersion effectiveness, aquatic toxicity, biodegradability and environmental compatibility of the biosurfactant lipopeptide were determined using recognized standardized methods,^19–23 and the biosurfactant lipopeptide used as a bio-dispersant for marine oil-spill remediation were studied, which is, to the best of our knowledge, the first report about biosurfactant lipopeptide used in oil-spill remediation.The lipopeptide samples were isolated from cell-free broth of B. subtilis HSO121 at our laboratory.²⁴ The typical chemical structure of the lipopeptide used in the study was shown in Fig. 1 and its critical micelle concentration (CMC) was 8.69 × 10⁻⁵ mol L⁻¹.Open in a separate windowFig. 1Typical chemical structure of lipopeptides (a) and the surface tensions of lipopeptides respect to concentrations (b).Dispersion effectiveness (DE) of lipopeptides was examined at different surfactant-to-oil ratios (SORs), temperatures, pH values, and salinities. It indicated in Fig. 2 that DE of lipopeptides reached 70.23% at SORs of 1 : 10 (w/w) at 25 °C, pH 7 and the present of 3% NaCl (w/v). It should be noticed that the DE of lipopeptides was almost kept when SORs dropped to 1 : 250 w/w. The increase in DE with increasing SORs can be attributed to the generation of emulsions with smaller droplets and lower rising velocity.² Sharp drop off in DE was observed for lipopeptides when SORs below 1 : 500 (w/w), and DE value was 36.45% at an extreme SORs of 1 : 1250 (w/w). It had been reported that the abrupt decline for 80 : 20 lecithin : Tween 80 (w/w) surfactant happened when SORs below 1 : 100 v/v, from 77% (SORs 1 : 100 v/v) to 15% (SORs 1 : 200 v/v),²⁵ indicating a lower SOR in lipopeptides usage could reach its maximum effectiveness. Lipopeptides exhibited >70% DE values with temperatures ranged from 15 °C to 25 °C, and an increasing DE values when pH values raised, the largest DE was 77.45% at pH 11. DE of lipopeptides increased from 56.12% to 71.14% with increase in salinity. Higher DE at higher salinity was observed for anionic biosurfactants, which can be attributed to the electrostatic repulsion between polar head groups reduced by ions, and a close-packed arrangement of surfactant molecules at the oil–water interface were formed.²Open in a separate windowFig. 2The dispersion effectiveness (DE) of lipopeptides under different SORs (), temperatures (), pH values () and salinities ().Mortalities of zebrafish under different concentrations of different surfactants were shown in Fig. 3. It was evident that the toxicity of lipopeptides was far less than those of sodium dodecyl sulfate (SDS) and 3-(N,N-dimethyl palmityl ammonio) propane sulfonate (Betaine). The 24 h median lethal concentration (LC₅₀) values were calculated and showed in 26 in which an 96 h LC₅₀ of 1.9 mg L⁻¹ for Cyprinodon variegatus was reported. Low toxicities of lipopeptides on whiteleg shrimp and copepods were also evaluated that the 96 h LC₅₀ of lipopeptides from Bacillus sp. GY19 were 1050 mg L⁻¹ and 1174 mg L⁻¹, respectively.²⁷Open in a separate windowFig. 3Mortality of zebrafish (M_z) after a 24 h exposure to Betaine, SDS or lipopeptides.Ecotoxicity of tested surfactants to the zebrafish

Aggregation-induced emission enhancement (AIEE)-active tetraphenylethene (TPE)-based chemosensor for Hg2+ with solvatochromism and cell imaging characteristics

An aggregation-induced emission enhancement (AIEE)-active fluorescent sensor based on a tetraphenylethene (TPE) unit has been successfully designed and synthesized. Interestingly, the luminogen could detect Hg²⁺ with high selectivity in an acetonitrile solution without interference from other competitive metal ions, and the detection limit was 7.46 × 10⁻⁶ mol L⁻¹. Furthermore, the luminogen also showed interesting solvatochromic behavior and superior cell imaging performance.

A TPE-based AIEE-active fluorescent sensor for Hg²⁺ was synthesized. Furthermore, it showed solvatochromism and cell imaging characteristics.

The design and synthesis of molecular sensors for the detection of metal ions, especially transition-metal ions, has attracted much attention.^1–3 Among all transition-metal ions, mercury (Hg²⁺) is identified as one of the most dangerous and ubiquitous heavy metals. Indeed, it is not biodegradable, and can cause extreme injury to the environment as well as human health.^4–8 Additionally, it can be accumulated through the food chain in the human body, consequently giving rise to several deleterious effects such as central nervous system defects, kidney damage, endocrine system disease and so on.^9–12 Although many governments around the world have adopted strict regulations to limit Hg²⁺ emission, the global Hg²⁺ pollution caused by human activities is still serious.^13,14 Therefore, it is highly desirable to develop a new method for the detection of Hg²⁺ with high selectivity and sensitivity.^15–17Most traditional fluorescent sensors suffer from a detrimental phenomenon called aggregation-caused quenching (ACQ), which usually results in a poor solid-state emission efficiency. Fortunately, in 2001, Tang et al. reported a fluorescent molecule named 1-methyl-1,2,3,4,5-pentaphenylsilole. Interestingly, the fluorescence emission of this luminogen was induced by aggregation, a phenomenon referred to as aggregation-induced emission (AIE).¹⁸ Subsequently, in 2002, Park et al. reported an interesting phenomenon named aggregation-induced emission enhancement (AIEE).¹⁹ In fact, both AIE and AIEE can achieve highly efficient emission in the solid state or aggregated state.^20–27 In the past years, AIE (or AIEE) phenomenon has attracted considerable research interest owing to the potential applications in a lot of fields, including bioimaging, fluorescence sensors, organic lighting emitting diode (OLED) devices and organic lasers.^28–33 Meanwhile, many stimuli-responsive materials have been reported, including photochromism, mechanochromism, and solvatochromism.^34–46 At present, the solvatochromism materials have been generally used in chemical and biological systems.⁴⁷ To date, many fluorescent chemosensors for Hg²⁺ have been reported. In contrast, the corresponding chemosensors with AIE or AIEE effect are rare, not to mention solvatochromic AIE or AIEE-active fluorescent sensors for Hg²⁺ with good cell imaging behavior. Indeed, preparing such multifunctional sensors is challenging and significative. In this paper, we reported an AIEE-active tetraphenylethene (TPE)-based fluorescent sensor (Scheme 1) for the detection of Hg²⁺. Furthermore, the luminogen also showed remarkable solvatochromism and good cell imaging characteristics.Open in a separate windowScheme 1The synthetic route of luminogen 1.To investigate the AIEE phenomenon of luminogen 1, we initially studied the UV-visible absorption spectra and photoluminescence (PL) spectra in acetonitrile–water mixtures with different water fractions (f_w). The results indicated that the absorption spectra exhibited level-off tails in the long wavelength region as the water content increased (Fig. S1^†). It is well-known that such tails are usually associated with the Mie scattering effect, which is the key signal of nano-aggregate formation.^48,49 As presented in Fig. 1, luminogen 1 showed weak fluorescence and the luminescence quantum yield (Φ) was as low as 1.35%. Interestingly, when the f_w in the acetonitrile solution was increased to 80%, an obvious emission band was formed, and a yellow-green fluorescence was observed. As the water content was increased to 90%, the emission intensity was further increased, and a bright yellow-green luminescence (Φ = 27.81%) with a λ_max at 545 nm could be observed.Open in a separate windowFig. 1(a) Fluorescence spectra of the dilute solutions of 1 (2.0 × 10⁻⁵ mol L⁻¹) in acetonitrile–water mixtures with different water content (0–90%). Excitation wavelength = 380 nm. (b) Changes the emission intensity of 1 at 545 nm in acetonitrile–water mixtures with different volume fractions of water (0–90%). (c) PL images of 1 (2.0 × 10⁻⁵ mol L⁻¹) in acetonitrile–water mixtures with different f_w values under 365 nm UV illumination.Clearly, water is a poor solvent of luminogen 1. As a result, the generation of the yellow-green emission can be attributed to the aggregate formation.^50–52 Moreover, as shown in Fig. 2, the nano-aggregates obtained were verified by dynamic light scatterings (DLS). Therefore, 1 is a typical AIEE-active fluorescent molecule, and its AIEE behavior is caused by the restricted intramolecular rotation. As shown in Fig. S2,^† solid-state compound 1 showed a strong green emission (Φ = 14.50%) with a λ_max at 497 nm, and the corresponding fluorescence lifetime is 2.66 ns (Fig. S3^†).Open in a separate windowFig. 2Size distribution curve of luminogen 1 (2.0 × 10⁻⁵ mol L⁻¹) in acetonitrile–water mixtures with f_w = 90%.On the other hand, the luminogen 1 also displayed interesting solvatochromism effect. As presented in Fig. 3, the absorption spectra and fluorescence spectra of luminogen 1 in different solvents were investigated, respectively. Obviously, the absorption spectra was barely affected by the polarity of solvents. However, the photoluminescence peaks were gradually red-shifted from 515 nm to 625 nm, and thus 1 exhibited remarkable solvatochromism behavior. Obviously, the molecular structure of luminogen 1 is distorted due to the presence of TPE unit, and the conjugation degree of molecule 1 is different in various solvents, and the intramolecular charge transfer (ICT) effect is possibly responsible for the solvatochromism behavior of 1.Open in a separate windowFig. 3(a) UV-Vis absorption spectra and (b) normalized fluorescence spectra (Excitation wavelength = 380 nm) of luminogen 1 in different solvents (2.0 × 10⁻⁵ mol L⁻¹). (c) Photographs of 1 under 365 nm UV illumination in different solvents: tol (toluene); THF (tetrahydrofuran); EA (ethyl acetate); BT (acetone); MeCN (acetonitrile); DMSO (dimethyl sulfoxide).Subsequently, the changes in the fluorescence of luminogen 1 induced by Hg²⁺ were investigated in acetonitrile (2.0 × 10⁻⁵ mol L⁻¹) at room temperature. In the fluorometric titration experiments, as shown in Fig. 4, the emission intensity significantly decreased when the Hg²⁺ concentration increased from 0 to 7.0 equivalents in acetonitrile. Meanwhile, the fluorescent color changed from orange-red to colorless, and followed by a plateau upon further titration (Fig. 5). Remarkably, the emission intensity of luminogen 1 was almost quenched completely. Furthermore, based on the titration experiments, the detection limit of luminogen 1 for Hg²⁺ on the basis of LOD = 3 × σ/B (where σ is the standard deviation of blank sample and B is the slope between the fluorescence intensity versus Hg²⁺ concentration) was 7.46 × 10⁻⁶ M (Fig. S4^†). Moreover, a good linear relationship could be obtained (R = −0.9933) and the quenching constant of luminogen 1 with Hg²⁺ was 1.9 × 10⁴ M⁻¹ (Fig. S5^†). On the other hand, the binding ratio of luminogen 1 for Hg²⁺ was established through Job''s plot and the results showed 1 : 1 stoichiometric complexation (Fig. S6^†). Next, the binding interactions between 1 and Hg²⁺ were further investigated by NMR in acetonitrile-d₃. As presented in Fig. 6, the signal of Ha from 9.42 ppm shifted to 9.62 ppm and the Hb changed from 8.43 ppm to 9.24 ppm. Besides, the Hc or Hd was slightly shifted for 0.04 ppm or 0.06 ppm, respectively. These consequences revealed that the N on the pyridine and the N on the pyrazine (near the N on the pyridine) are the most probable binding with Hg²⁺. Mass spectra were utilized to further demonstrate the binding mode of luminogen 1 toward Hg²⁺. The peak located at m/z = 820.0 was coincided well with the ensemble [1 + Hg²⁺ +2NO₃⁻ + Cl⁻]⁻, confirming the binding ratio of luminogen 1 for Hg²⁺ with 1 : 1 stoichiometry (Fig. S7^†).Open in a separate windowFig. 4Fluorescence titration spectra of luminogen 1 (2.0 × 10⁻⁵ mol L⁻¹) induced by Hg²⁺ (0–7.0 equiv.) in an acetonitrile solution. Excitation wavelength = 380 nm.Open in a separate windowFig. 5The emission intensity changes of luminogen 1 at 625 nm with different equivalents of Hg²⁺.Open in a separate windowFig. 6 ¹H NMR (acetonitrile-d₃, 400 MHz) spectra changes of luminogen 1 in the presence of Hg²⁺.Next, in order to study the selectivity behavior of luminogen 1 as a fluorescent sensor for Hg²⁺, other metal ions, such as Zn²⁺, Cd²⁺, Ba²⁺, Sr²⁺, Mn²⁺, Mg²⁺, Ca²⁺, Pb²⁺, Ni²⁺, Co²⁺, Cu²⁺, Al³⁺, Fe³⁺, Cr³⁺, Ag⁺ and K⁺ were also measured in acetonitrile under the same experimental conditions. The corresponding UV-Vis absorption spectra were shown in Fig. S8.^† Furthermore, as showed in Fig. 7, when these cations were added separately into the solution containing luminogen 1, no obvious fluorescence changes were observed. Indeed, as noticed in Fig. S9,^† no obvious interference was observed when Hg²⁺ (7.0 equiv.) was added with other ions (7.0 equiv.). These results indicated that luminogen 1 could be served as a highly selective fluorescent sensor for detection of Hg²⁺.Open in a separate windowFig. 7(a) Fluorescence spectra of luminogen 1 (2.0 × 10⁻⁵ mol L⁻¹) towards various cations including Zn²⁺, Cd²⁺, Hg²⁺, Ba²⁺, Sr²⁺, Mn²⁺, Mg²⁺, Ca²⁺, Pb²⁺, Ni²⁺, Co²⁺, Cu²⁺, Al³⁺, Fe³⁺, Cr³⁺, Ag⁺ and K⁺. Excitation wavelength = 380 nm. (b) Fluorescence photographs of luminogen 1 after addition of various metal ions (7.0 equiv.) under 365 nm light.Fluorescent probe is a powerful tool for optical imaging, which allows direct visualization of biological analytes.⁵³ Luminogen 1 was AIEE-active due to the restriction of intramolecular rotation in the aggregate state, which is beneficial for cell imaging. Indeed, the viability of HeLa cells incubated with luminogen 1 was evaluated by the standard MTT method (Fig. 8), and the result indicated that compound 1 exhibited low cytotoxicity. Next, cell imaging behavior of luminogen 1 was investigated using a confocal laser scanning microscopy (CLSM). HeLa cells were incubated with luminogen 1 (20 μM) for 30 min at 37 °C and the fluorescence images were obtained by CLSM.Open in a separate windowFig. 8The MTT assay of compound 1 for measuring cell viability.As presented in Fig. 9, an intense yellow-green fluorescence, which was consistent with the fluorescence of luminogen 1 in acetonitrile–water mixture with high water content (80% or 90%), was displayed inside the cells. This result indicated that luminogen 1 tended to aggregate inside the cells, and thus the bright yellow-green luminescence was clearly observed. Furthermore, the merged picture c demonstrated that picture a and picture b overlapped very well. These results indicated that luminogen 1 showed superior cell imaging performance.Open in a separate windowFig. 9Fluorescence images of HeLa cells incubated with luminogen 1 (20 μM) for 30 min at 37 °C: (a) bright field image; (b) fluorescence image; (c) merge image (a and b).In summary, a TPE-based fluorescent molecule was designed and synthesized. The luminogen exhibited obvious AIEE phenomenon. Moreover, luminogen 1 could be used as a highly selective fluorescence turn-off chemosensor for Hg²⁺, and the detection limit was 7.46 × 10⁻⁶ mol L⁻¹. Furthermore, 1 also displayed interesting solvatochromic behavior and superior cell imaging performance. Further studies on the design of new AIE or AIEE-active fluorescent chemosensors are in progress.     

Correction: iTRAQ-based quantitative proteomic analysis for identification of biomarkers associated with emodin against severe acute pancreatitis in rats

Correction for ‘iTRAQ-based quantitative proteomic analysis for identification of biomarkers associated with emodin against severe acute pancreatitis in rats’ by Hong Xiang et al., RSC Adv., 2016, 6, 72447–72457.

The authors regret that Fig. 2–4 were shown incorrectly in the original article. An incorrect section of the SAP group in the MPO-immunohistochemical staining (Fig. 2A) and HE staining (Fig. 3) experiments was used in error. In addition, Fig. 4 has been revised to show the zymogen granule, in order to better represent the ultrastructure of the pancreas. The correct versions of Fig. 2–4 are shown below.Open in a separate windowFig. 2Emodin down-regulated the MPO protein expression in pancreas of SAP rats. (A) Effect of emodin on MPO-immunopositive area (brown) staining of pancreatic tissue in SAP rats by immunohistochemical detection. (B) Effect of emodin on MPO-immunopositive area (red) staining of pancreatic tissue in SAP rats by immunofluorescence detection. Images are presented at 200× magnification. The data are presented as the mean ± SD, n = 6. **P < 0.01 versus SO; ^#P < 0.05 versus SAP, ^##P < 0.01 versus SAP.Open in a separate windowFig. 3Emodin improved pancreatic histopathology of SAP rats. Effect of emodin on H&E staining of pancreatic tissue in SAP rats. Images are presented at 200× magnification. The data are presented as the mean ± SD, n = 6. **P < 0.01 versus SO; ^#P < 0.05 versus SAP, ^##P < 0.01 versus SAP.Open in a separate windowFig. 4Emodin attenuated cellular structure changes in pancreas of SAP rats. Representative images of the cells’ ultrastructure in the SO (A), SAP (B), 60 mg kg⁻¹ emodin (C), 30 mg kg⁻¹ emodin (D) and 15 mg kg⁻¹ emodin (E) groups. Images are presented at 25 000× magnification.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Correction: Facile synthesis of CuS mesostructures with high photothermal conversion efficiency

Correction for ‘Facile synthesis of CuS mesostructures with high photothermal conversion efficiency’ by Lianjiang Tan et al., RSC Adv., 2015, 5, 35317–35324. DOI: 10.1039/C5RA01835H.

The authors wish to correct Fig. 6c–e in the original version of this paper as it unfortunately contained incorrect images. The corrected version of Fig. 6c–e is provided below as Fig. 1. The authors confirm that these errors do not affect the scientific findings and conclusions of the paper, and sincerely apologize for the errors and any confusion.Open in a separate windowFig. 1Fluorescence images of HeLa cells before and after irradiation by 980 nm laser with the power density of 0.5 W cm⁻² over a period of 5 min in the absence (c and d) and presence (e and f) of the CuS mesostructures with the concentration of 0.3 g L⁻¹. The living cells were labeled by calcein AM (green emission), and the dead cells were labeled by propidium iodide (red emission). The scale bar represents 100 mm.     

Correction: Influence of Cu doping on the visible-light-induced photocatalytic activity of InVO4

Correction for ‘Influence of Cu doping on the visible-light-induced photocatalytic activity of InVO₄’ by Natda Wetchakun et al., RSC Adv., 2017, 7, 13911–13918, DOI: 10.1039/C6RA27138C.

The authors regret errors in Fig. 4, ​,7,7, and 9 in the previously published article. The corrections for the errors in the article are described as follows:Open in a separate windowFig. 4Kubelka–Munk absorbance spectra and band gaps (insets) of the pure InVO₄ (a) and 1.0 mol% Cu-doped InVO₄ (b) samples.Open in a separate windowFig. 7Schematic of the charge migration and separation on Cu-doped InVO₄.(1) The diffuse reflectance spectra of pure InVO₄ and 1.0 mol% Cu-doped InVO₄ are shown in Fig. 4. The absorption margin of 1.0 mol% Cu-doped InVO₄ was shifted to a longer wavelength, indicating a decrease in the band gap with respect to pure InVO₄. The absorption margins of the pure InVO₄ and 1.0 mol% Cu-doped InVO₄ samples were 505 nm and 510 nm, corresponding to band gaps of 2.51 eV and 2.45 eV, respectively (Fig. 4a and b).(2) The band edge positions of the conduction band (CB) and the valence band (VB) of InVO₄ can be calculated by the following equation: E⁰_CB = χ − E^C − 0.5E_g,¹ where χ is the electronegativity of the semiconductor, E^C is the energy of free electrons on the hydrogen scale of 4.5 eV, E_g is the band gap of InVO₄, and the χ value of InVO₄ is 5.74 eV.² The E_g value of InVO₄ evaluated from the UV-vis DRS analysis was about 2.51 eV. The valence band energy (E_VB) can be calculated by the following equation:³E_VB = E_CB + E_g, where E_CB is the conduction band energy. Based on the equation above, the calculated CB and VB edge potentials of InVO₄ were −0.02 eV and 2.49 eV, respectively. Now, we are in a position to discuss the photocatalytic mechanism of Cu-doped InVO₄ for MB degradation (Fig. 7). In the photocatalysis process, when the absorbed photon energy (hν) equals or exceeds the band gap, the Cu-doped InVO₄ generates electron–hole (e⁻/h⁺) pairs. In that case, the generated electrons from the valence band can be transferred to the conduction band of InVO₄. Since the CB edge potential of InVO₄ (−0.02 eV) is higher than the standard redox potential, E⁰(O₂/O₂˙⁻) = −0.33 V vs. NHE at pH 7, this suggests that the electrons in the CB of InVO₄ cannot reduce O₂ to the superoxide radical ion (O₂˙⁻). In addition, the VB of InVO₄ (2.49 eV) is higher than the standard redox potential, E⁰(OH⁻/OH˙) = 1.99 V vs. NHE at pH 7. This indicates that the photogenerated holes in the valence band of InVO₄ can oxidize the hydroxyl ion (OH⁻) or water (H₂O) to form the hydroxyl radical (OH˙).(3) Due to the contradiction between the scavenging test and the proposed photocatalytic mechanism, Fig. 9 was removed from the original article.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Insight into trifluoromethylation – experimental electron density for Togni reagent I

3,3-Dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole represents a popular reagent for trifluoromethylation. The σ hole on the hypervalent iodine atom in this “Togni reagent” is crucial for adduct formation between the reagent and a nucleophilic substrate. The electronic situation may be probed by high resolution X-ray diffraction: the experimental charge density thus derived shows that the short intermolecular contact of 3.0 Å between the iodine and a neighbouring oxygen atom is associated with a local charge depletion on the heavy halogen in the direction of the nucleophile and visible polarization of the O valence shell towards the iodine atom. In agreement with the expectation for λ3-iodanes, the intermolecular O⋯I–C_aryl halogen bond deviates significantly from linearity.

The experimentally observed electron density for the “Togni reagent” explains the interaction of the hypervalent iodine atom with a nucleophile.

A “halogen bond” denotes a short contact between a nucleophile acting as electron density donor and a (mostly heavy) halogen atom as electrophile;^1,2 halogen bonds are a special of σ hole interactions.^3–5 Such interactions do not only play an important role in crystal engineering;^6–11 rather, the concept of a nucleophile approaching the σ hole of a neighbouring atom may also prove helpful for understanding chemical reactivity.The title compound provides an example for such a σ hole based reactivity: 3,3-dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole, 1, (Scheme 1) commonly known as “Togni reagent I”, is used for the electrophilic transfer of a trifluoromethyl group by reductive elimination. The original articles in which the application of 1¹² and other closely related “Togni reagents”¹³ were communicated have been and still are highly cited. Trifluoromethylation is not the only application for hypervalent iodine compounds; they have also been used as alkynylating¹⁴ or azide transfer reagents.^15,16 The syntheses of hypervalent iodanes and their application in organofluorine chemistry have been reviewed,¹⁷ and a special issues of the Journal of Organic Chemistry has been dedicated to Hypervalent Iodine Catalysis and Reagents.¹⁸ Recently, Pietrasiak and Togni have expanded the concept of hypervalent reagents to tellurium.¹⁹Open in a separate windowScheme 1Lewis structure of the Togni reagent, 1.Results from theory link σ hole interactions and chemical properties and indicate that the electron density distribution associated with the hypervalent iodine atom in 1 is essential for the reactivity of the molecule in trifluoromethylation.²⁰ Lüthi and coworkers have studied solvent effects and shown that activation entropy and volume play relevant roles for assigning the correct reaction mechanism to trifluoromethylation via1. These authors have confirmed the dominant role of reductive elimination and hence the relevance of the σ hole interaction for the reactivity of 1 in solution by ab initio molecular dynamics (AIMD) simulations.^21,22 An experimental approach to the electron density may complement theoretical calculations: low temperature X-ray diffraction data of sufficient resolution allow to obtain the experimental charge density and associate it with intra- and inter-molecular interactions.^23–25 Such advanced structure models based on aspherical scattering factors have also been applied in the study of halogen bonds.^26–30 In this contribution, we provide direct experimental information for the electronic situation in Togni reagent I, 1; in particular, we analyze the charge distribution around the hypervalent iodine atom.Excellent single crystals of the title compound were grown by sublimation.^§ The so-called independent atom model (IAM), i.e. the structure model based on conventional spherical scattering factors for neutral atoms, confirms the solid state structure reported by the original authors,¹² albeit with increased accuracy. As depicted in Fig. 1, two molecules of 1 interact via a crystallographic center of inversion. The pair of short intermolecular O⋯I contacts thus generated can be perceived as red areas on the interaction-sensitive Hirshfeld surface.³¹Open in a separate windowFig. 1Two neighbouring molecules of 1, related by a crystallographic center of inversion. The short intermolecular I⋯O contacts show up in red on the Hirshfeld surface³² enclosing the left molecule. (90% probability ellipsoids, H atoms omitted, symmetry operator 1 − x, 1 − y, 1 − z).The high resolution of our diffraction data ^¶ for 1 allowed an atom-centered multipole refinement^33,34 and thus an improved model for the experimental electron density which takes features of chemical bonding and lone pairs into account. Fig. 2 shows the deformation density, i.e. the difference electron density between this advanced multipole model and the IAM in the same orientation as Fig. 1.^|| The orientation of an oxygen lone pair (blue arrow) pointing towards the σ hole of the heavy halogen in the inversion-related molecule and the region of positive charge at this iodine atom (red arrow) are clearly visible. Single-bonded terminal halides are associated with one σ hole opposite to the only σ bond, thus resulting in a linear arrangement about the halogen atom. Different geometries and potentially more than a single σ hole are to be expected for λ3-iodanes such as our target molecule, and as a tendency, the resulting halogen bonds are expected to be weaker than those subtended by single-bonded iodine atoms.³⁵ In agreement with these theoretical considerations, the closest I⋯O contacts in 1 amount to 2.9822(9) Å. This distance is significantly shorter than the sum of the van-der-Waals radii (I, 1.98 Å; O, 1.52 Å (ref. 36)) but cannot compete with the shortest halogen bonds between iodine and oxygen^37,38 or iodine and nitrogen.^39–41Fig. 1 and ​and22 show that the C_aryl–I⋯O contacts are not linear; they subtend an angle C10–I1⋯O1′ of 141.23(3)° at the iodine atom. On the basis of theoretical calculations, Kirshenboim and Kozuch³⁵ have suggested that the split σ holes should be situated in the plane of the three substituents of the hypervalent atom and that halogen and covalent bonds should be coplanar. Fig. 3 shows that the C_aryl–I⋯O interaction in 1 closely matches this expectation, with the next oxygen neighbour O1′ only 0.47 Å out of the least-squares plane through the heavy halogen I1 and its three covalently bonded partners C1, O1 and C10.Open in a separate windowFig. 2Deformation density for the pair of neighbouring molecules in 1; the dashed blue and red arrows indicate regions of opposite charge. (Contour interval 0.10e Å⁻³; blue lines positive, red lines negative, green lines zero contours, symmetry operator 1 − x, 1 − y, 1 − z).Open in a separate windowFig. 3A molecule of 1, shown [platon] along O1⋯C1, and its halogen-bonded neighbour O1ⁱ. Symmetry operator 1 − x, 1 − y, 1 − z.The Laplacian, the scalar derivative of the gradient vector field of the electron density, emphasizes local charge accumulations and depletions and it allows to assess the character of intra- and inter-molecular interactions. A detailed analysis of all bonds in 1 according to Bader''s Atoms In Molecules theory⁴² is provided in the ESI.^‡ We here only mention that the electron density in the bond critical point (bcp) of the short intermolecular I⋯O contact amounts to 0.102(5)e Å⁻³; we are not aware of charge density studies on λ3-iodanes, but both the electron density and its small positive Laplacian match values experimentally observed for halogen bonds involving O and terminal I in the same distance range.⁴³The crystal structure of 1 necessarily implies additional contacts beyond the short halogen bond shown in Fig. 1 and ​and2.2. The shortest among these secondary interactions is depicted in Fig. 4: it involved a non-classical C–H⋯F contact with a H⋯F distance of 2.55 Å.Open in a separate windowFig. 4C–H⋯F contact in 1; additional information has been compiled in the ESI.^‡ Symmetry operators i = 1 − x, 1 − y, 1 − z; ii = 1 − x, 1 − y, −z.The topological analysis of the experimental charge density reveals that this non-classical C–H⋯F hydrogen bond and all other secondary contacts are only associated with very small electron densities in the bcps. Table S8 in the ESI^‡ provides a summary of this analysis and confirms that the I⋯O halogen bond discussed in Fig. 1 and ​and22 represents by far the most relevant intermolecular interaction.The relevance of this halogen bond extends beyond the crystal structure of 1: Insight into the spatial disposition of electrophilic and nucleophilic regions and hence into the expected reactivity of a molecule may be gained from another electron-density derived property, the electrostatic potential (ESP). The ESP for the pair of interacting molecules in 1 is depicted in Fig. 5.Open in a separate windowFig. 5Electrostatic potential for a pair of molecules in 1 mapped on an electron density isosurface (ρ = 0.5e Å⁻³; program MoleCoolQt^44,45). Fig. 5 underlines the complementary electrostatic interactions between the positively charged iodine and the negatively charged oxygen atoms. One can easily imagine to “replace” the inversion-related partner molecule in crystalline 1 by an incoming nucleophile.The ESP tentatively obtained for a single molecule in the structure of 1 did not differ significantly from that derived for the inversion-related pair (Fig. 5), and even the results from theoretical calculations in the gas phase for an isolated molecule²⁰ are in good qualitative agreement with our ESP derived from the crystal structure. In the absence of very short contacts, polarization by neighbouring molecules only has a minor influence on the ESP. The experimentally observed electron density matches the proven reactivity for the title compound, and we consider it rewarding to extend our charge density studies on related hypervalent reagents.     

Correction: A sensitive OFF–ON–OFF fluorescent probe for the cascade sensing of Al3+ and F− ions in aqueous media and living cells

Correction for ‘A sensitive OFF–ON–OFF fluorescent probe for the cascade sensing of Al³⁺ and F⁻ ions in aqueous media and living cells’ by Lingjie Hou et al., RSC Adv., 2020, 10, 21629–21635, DOI: 10.1039/D0RA02848G.

The authors regret that an incorrect version of Fig. 4 was included in the original article. The correct version of Fig. 4 is presented below.Open in a separate windowFig. 4The ESI-MS spectrum of Al³⁺–HNS complex.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Rutin-modified silver nanoparticles as a chromogenic probe for the selective detection of Fe3+ in aqueous medium

The development of nanoprobes for selective detection of metal ions in solution has attracted great attention due to their impact on living organisms. As a contribution to this field, this paper reports the synthesis of silver nanoparticles modified with rutin in the presence of ascorbic acid and their successful use as a chromogenic probe for the selective detection of Fe³⁺ in aqueous solution. Limits of detection and quantification were found to be 17 nmol L⁻¹ and 56 nmol L⁻¹, respectively. The sensing ability is proposed to proceed via an iron-induced nanoparticle growth/aggregation mechanism. A practical approach using image analysis for quantification of Fe³⁺ is also described.

The use of rutin-modified silver nanoparticles for selective detection and sensitive quantification of Fe³⁺ in aqueous solution is described.

Metal ions are key species in nature due to their essential functions in living organisms.^1,2 On the other hand, heavy metals as well as essential metals at abnormally high levels are toxic.² Iron, for instance, in addition to its popular use in industry and construction, is essential to the human body and active in biological processes. Although the trivalent form of iron is particularly important for oxygen transport in blood and the mitochondrial respiratory chain, high levels of this cation are associated with important pathologies.^3,4 The detection of metal ions in aqueous solution is traditionally performed by methods including atomic absorption spectrometry,⁵ electrochemical measurements,^6,7 and inductively coupled plasma techniques,⁸ among others. However, these techniques have important drawbacks, notably the need for sophisticated instrumentation, in addition to being time-consuming and requiring laborious procedures. To overcome these issues, the development of chromogenic and fluorogenic chemosensors for the selective detection of metal-targets has attracted great attention, especially due to the possibility of fast, sensitive and non-expensive analysis.^9,10 In the last decade, nanoscaled materials have been reported as selective probes for metal ions, including Fe³⁺.^11–24Silver nanoparticles (AgNPs) are of particular interest because of the affordable price of starting materials, ease of controlling size and morphology, possibility to functionalize their surface with organic molecules, and optical properties that enable detection of a variety of analytes via simple UV-vis spectroscopy and digital image analysis. Furthermore, applications of AgNPs are also biotechnologically relevant due to the possibility of green synthetic protocols, including the use of plant extracts,²⁵ natural sources,²⁶ glycerol,²⁷ among others.Flavonoids are secondary metabolites naturally found in fruits and other vegetables with relevant roles due to their nutritional, pharmaceutical and medicinal properties.²⁸ Because of their adequate structural features, flavonoids are candidates to be employed in the synthesis of AgNPs.²⁶ Rutin (RU), a sugar-based flavonoid, may be employed as reducing agent in the synthesis of AgNPs along with a stabilizer such as polyvinylpyrrolidone (PVP)²⁶ or used as crude plant extract component.^29,30This paper reports the use of RU-modified AgNPs (RU-AgNPs) as a chromogenic probe for Fe³⁺ in aqueous medium in the presence of ascorbic acid (AA). Sensing ability of RU-AgNPs for the selective detection of Fe³⁺ toward other metal cations was investigated with UV-vis spectroscopy analysis. These data and transmission electronic microscopy (TEM) results allowed a mechanistic proposal involved in the selective detection of Fe³⁺ by RU-AgNPs. Furthermore, a practical approach based on correlation of images of solutions obtained with a conventional smartphone and chemometrics was employed for a simpler quantification of Fe³⁺ in aqueous medium.Initially, the order of reagent combination was investigated in the synthesis of RU-AgNPs. Concentration of RU ranged from 0.10 to 0.50 mmol L⁻¹, while concentrations of other components were fixed at 0.20 mmol L⁻¹ AgNO₃, 0.10 mmol L⁻¹ AA and 0.10 M NaOH. Water was used as solvent in all cases. Narrower surface plasmon resonance (SPR) bands were obtained from adding a solution of AA and NaOH to a solution containing RU and AgNO₃ (Fig. 1a) against the addition of RU, AA, and NaOH to AgNO₃ solution (Fig. S1a – ESI^†), or the addition of RU and NaOH to a solution of AA and AgNO₃ (Fig. S1b^†). RU-AgNPs obtained from the condition presented in Fig. 1a are small (4.1 nm average diameter) and considerably polydisperse (standard deviation of 4.7 nm), however, presenting only one population (Fig. 1b and S2^†). A study of the influence of RU concentration (0.10 to 0.50 mmol L⁻¹) on the stability of RU-AgNPs over time indicated that 0.10 mmol L⁻¹ RU generates more stable RU-AgNPs (Fig. S3^†). Next, a study on the influence of pH indicated that RU-AgNPs are only stable under strong alkaline conditions (pH 12.5 or higher) (Fig. S4^†).Open in a separate windowFig. 1(a) UV-vis analysis of RU-AgNPs under strong alkaline condition (pH > 12.5) based on the order of adding reagents (RU, 0.10 to 0.50 mmol L⁻¹; AgNO₃, 0.20 mmol L⁻¹; AA, 0.10 mmol L⁻¹; NaOH, 0.10 mmol L⁻¹); (b) TEM image of RU-AgNPs under the selected condition.The ability of RU-AgNPs to sense metal cations was investigated by both naked-eye and UV-vis spectroscopy analysis (Fig. 2). The separate addition of 10 μmol L⁻¹ of several metal cations (Fe³⁺, Co²⁺, Zn²⁺, Sr²⁺, Cu²⁺. Al³⁺, Ba²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Mg²⁺, Hg²⁺, Cu⁺ and Cr³⁺) to solutions of RU-AgNPs (prepared according to Fig. 1a) indicated that only Fe³⁺ induces a significant colorimetric change in the final aspect of solution after 50 minutes (Fig. 2a).Open in a separate windowFig. 2Naked-eye (a) and UV-vis (b) analysis of RU-AgNPs in absence (control, C) and presence of 10 μmol L⁻¹ of selected metal cations (Fe³⁺, Co²⁺, Zn²⁺, Sr²⁺, Cu²⁺. Al³⁺, Ba²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Mg²⁺, Hg²⁺, Cu⁺ and Cr³⁺) after 50 min; (c) calibration curve obtained by the addition of different amounts (0 to 10 μmol L⁻¹) of Fe³⁺ to solutions of RU-AgNPs; (d) TEM image of RU-AgNPs after addition of Fe³⁺ (10 μmol L⁻¹). In all experiments, RU-AgNPs were prepared in the presence of ascorbic acid.The results presented in Fig. 2a are consistent with the UV-vis spectroscopy analysis (Fig. 2b). Co²⁺ ions also induce some change in the system, however at a considerably smaller extension than Fe³⁺. In this study, AA played a crucial role in the selective detection of Fe³⁺ by the referred nanoprobe. In the absence of AA, Co²⁺ (mainly) as well as other cations induce stronger colorimetric and spectral changes (Fig. S5a and b,^† respectively) in the analysis of solutions of RU-AgNPs when compared to the system containing AA. This selectivity may arise from two possible reasons: (i) preservation of RU by avoiding its oxidation in the reduction of Ag⁺ ions; (ii) coordination of ascorbate anion to cations other than Fe³⁺.Interaction of RU-AgNPs and Fe³⁺ (10 μmol L⁻¹) stabilizes after approximately 40 minutes (Fig. S6^†). Next, a calibration curve was built from the direct relationship between the absorbance at 396 nm and the concentration of Fe³⁺ (Fig. 2c), presenting a good correlation (R² = 0.9929). The limits of detection and quantification were found to be 17 nmol L⁻¹ and 56 nmol L⁻¹, respectively, which is very satisfactory.^13,14 The influence of other cations in the detection of Fe³⁺ was investigated by UV-vis spectroscopy. Fig. S7^† clearly demonstrates that there are only small changes when a second cation (30 μmol L⁻¹) is added together with Fe³⁺ to the RU-AgNPs solution.Mechanistically, the detection of Fe³⁺ by RU-AgNPs in aqueous medium proceeds via a growth/aggregation-combined process. This proposal is first evidenced by UV-vis analysis due the suppression of SPR band (Fig. 2b), a behavior consistent with the literature.^31,32 Interestingly, TEM analysis clearly shows a growth in AgNPs size after addition of Fe³⁺ to the solution (Fig. 2d), resulting in a final single AgNPs population with average diameter of 14.7 nm ± 8.9 nm. Due to the strong alkaline medium, the main specie responsible for the behavior of NPs is likely to be Fe(OH)₃. A schematic illustration of the mechanism involving aggregation of RU-AgNPs induced by the addition of Fe³⁺ is presented in Fig. 3. AgNPs are initially formed by adsorption of anionic RU to the silver surface via deprotonated 5-hydroxychromen-4-one moiety. This is supported by the literature³³ and confirmed by alteration in the 1800–1500 cm⁻¹ region of the RU infrared spectra before and after coordination with silver (Fig. S9^†). Afterwards, the addition of Fe³⁺ induces the formation of a coordination complex through an anionic catechol group, in which at least 2 : 1 ligand–Fe³⁺ stoichiometry is required for an aggregated effect.Open in a separate windowFig. 3Mechanistic proposal for growth/aggregation of RU-AgNPs in the presence of Fe³⁺. Insert: binding model for RU-AgNPs.Due to increasing interest in image processing as an analytical tool for many purposes,^34,35 Multiple Linear Regression was employed to verify the capacity of the RU-AgNPs to probe Fe³⁺ standards at distinct concentrations, as presented in Fig. 4. The curve was obtained by plotting the color absorbances RGB-based values versus the concentrations of Fe³⁺ standards after RU-AgNPs interaction. Predicted iron is a vector based on RGB values that were then extracted from the filtered images and inserted in the equation described by Beer–Lambert law in order to generate the absorbances for the construction of the analytical curve. A linear behavior between the predicted response and the measured concentrations was observed (R² = 0.9806).Open in a separate windowFig. 4Calibration curve for Fe³⁺ analysis showing the predicted iron (RGB) vs. iron(iii) concentration (1 to 8 μmol L⁻¹). Adjusted R² = 0.9806.Regression coefficients of the calibration model (using the R, G and B channels simultaneously) obtained by MLR method are shown in eqn (1):[Fe³⁺] = 44.5R − 4.9G − 16.4B + 5.11where Fe³⁺ concentration is the dependent variable (Predicted Iron), 5.1 is the intercept (β₀), 44.5, −4.9 and −16.4 are the regression coefficients of the independent variables (R, G and B channels, respectively).It is possible to observe an interesting performance of the method using the three RGB color channels allied with MLR to quantify the Fe³⁺ content, which presented reasonable deviations in its responses considering that it is simple and low-cost. The good linearity is similar to other colorimetric methods such as sodium determination in seawater and coconut water (R² > 0.91) by Moraes and coauthors,³⁶ or iron(ii) in simulated seawater (R² = 0.9993) by Gasparotto et al.³⁷Second order regression was applied to the dataset to obtain a better adjustment, resulting in an adjusted R-squared of 0.9955. RGB values were then inserted in eqn (2) in order to generate the construction of the correlation:[Fe³⁺] = 14.4R + 30.7G − 23.4B − 398RG − 348RB − 0.8BG − 978R² + 695G² + 26.9B² + 4.52This paper reports the use of AgNPs functionalized with RU as nanoprobes for selective detection and sensitive quantification of Fe³⁺ in aqueous solution. The synthesis of RU-AgNPs is reproducible, easily performed and requires no stabilizer agent other than RU. AA has a crucial role in the selectivity by either the avoidance of oxidation of RU by silver and/or coordination of ascorbate with other cations. The literature brings relevant examples of chromogenic and fluorogenic chemosensors for selective detection of Fe³⁺ in solution. Many of these artificial organic receptors present high selectivity and relevant limits of detection, requiring, however, very specific reagents and laborious synthetic procedures.^38–41 Metal-based nanoparticles have emerged as potential probes for detection of Fe³⁺.^11–16 Although effective in Fe³⁺ sensing, the synthesis of these nanoprobes require some toxic reagents, such as NaBH₄ or PVP, use plant extracts, which may lead to some drawbacks, such as the understanding of the sensing mechanism. In contrast, our method is based on commercially available, nontoxic, low-cost reagents. Fe³⁺ sensing performed satisfactorily in the 1–10 μmol L⁻¹ range, and the limit of detection obtained with this method (17 nmol L⁻¹) is comparable to the most sensitive methods reported in literature. A mechanism for the detection of Fe³⁺ by RU-AgNPs involves a combined growth/aggregation of the NPs. There is a still limited number of nanoscaled systems reported as being selective and sensitive in the detection of Fe³⁺, which reinforces the relevance of the method reported herein. The linearity range obtained by both UV-vis spectroscopy and image analysis comprises the maximum of residual Fe³⁺ in drinking water according to the European and US legislations.^15,42     

Correction: Composition design,electrical properties,and temperature stability in (1 − x)K0.44Na0.56Nb0.96Sb0.04O3-xBi0.45La0.05Na0.5ZrO3 lead-free ceramics

Correction for ‘Composition design, electrical properties, and temperature stability in (1 − x)K_0.44Na_0.56Nb_0.96Sb_0.04O₃-xBi_0.45La_0.05Na_0.5ZrO₃ lead-free ceramics’ by Jian Ma et al., RSC Adv., 2018, 8, 29871–29878.

The authors regret that an incorrect version of Fig. 4 was included in the original article. The correct version of Fig. 4 is presented below.Open in a separate windowFig. 1FE-SEM surface images of (1 − x)K_0.44Na_0.56Nb_0.96Sb_0.04O₃-xBi_0.45La_0.05Na_0.5ZrO₃ ceramics with (a) x = 0, (b) x = 0.020, (c) x = 0.040, (d) x = 0.060.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Ni(ii)-based coordination polymers for efficient electrocatalytic oxygen evolution reaction

The exploration of highly efficient, stable and cheap water oxidation electrocatalysts using earth-abundant elements is still a great challenge. Herein, alkaline-stable cationic Ni(ii) coordination polymers (Ni-CPs) were successfully obtained under hydrothermal conditions, which could stabilize the incorporation of Fe(iii) to form Fe-immobilized Fe@Ni-CPs. The newly developed Ni-based CPs were used for the first time as an effective electrocatalyst for the oxygen evolution reaction in strong alkaline media.

An alkaline-stable cationic Ni(ii) coordination polymer showed remarkable oxygen evolution reaction (OER) catalytic activity due to capturing Fe ions.

The oxygen evolution reaction (OER) plays a vital role in energy storage and conversion applications due to energy issues and the need for sustainable development.^1–4 Because of the sluggish kinetics of the OER, excellent electrocatalysts are required to work in acidic or strong alkaline environments.^5,6 Noble metal-based catalysts like IrO₂ and RuO₂ with high efficiency showed excellent OER catalytic activity.⁷ However, noble metal with high-cost and scarcity are impractical for scale-up applications. Currently, to substitute these precious metal-based materials, transition-metal-based (Fe, Co, Ni and so on) and metal-free (e.g., N, P and S) hybrid materials have been extensively developed.^8–10 For example, metal–organic frameworks (MOFs) such as ZIF-8/67 derived metal–carbon composite materials exhibit promising electrocatalytic performance.^11,12 Unfortunately, the pyrolysis process destroys the framework completely and causes agglomeration of metals, resulting in a decreased number of active sites. Therefore, to explore highly efficient and low-cost OER catalysts that can be directly used in the OER without calcination are desired, including complexes and MOFs.Recently, much of transition bimetallic materials showed excellent electrocatalytic activity.^13–15 However, a handful of examples such as Fe–Co-MOFs or Co–Ni-MOFs have been explored due to instability, poor conductivity and harsh synthesis conditions.^16–20 Notably, the disadvantages of MOFs have limited their usage in the potential OER. Therefore, it is urgent to develop cheap, stable and active OER catalysts to replace the precious metals. However, this is still a great challenge. Coordination polymers (CPs) with a low-dimensional framework similar to MOFs are constructed by metal ion and organic ligands with potential active sites and functional groups, exhibiting wide applications in sensing, photoluminescence and photocatalysis.^21–26 However, rare examples of CPs have been directly explored in the OER. For Fe/Ni-based bimetal electrocatalysts, a novel strategy involves doping Fe(iii) into a functional Ni-based CPs, which could enhance the electrocatalytic OER activities.Herein, we report the hydrothermal synthesis of a Ni-based CPs as a high-performance OER electrocatalyst in strong alkaline solutions (Scheme 1). The blue crystals of [Ni(bp)₃·(H₂O)₂]·(bp)·(ClO₄)₂ (bp = 4,4′-biprydine) were obtained upon the reaction of bp ligands with NiClO₄·6H₂O under hydrothermal systems. Chemical stability tests displayed that Ni-CPs could retain their original framework in water or even a strong alkaline (pH = 14) solution after 12 hours (Fig. S1^†), which is rarely reported for most transition metal CPs. The thermogravimetric analysis (TGA) showed that there is a significant change at about 110 °C due to the weight loss of the partial guest (Fig. S2^†). Interestingly, the cationic Ni-CPs successfully captured the Mohr''s salt (ammonium iron(ii) sulfate) by taking advantage of the post-synthetic strategy. The obtained Fe@Ni-CPs exhibited a high-efficient OER activity under strong alkaline conditions.Open in a separate windowScheme 1Illustration of the synthesis process for Fe doped Ni coordination polymers for the OER.Single-crystal X-ray diffraction analysis revealed that the Ni-CPs crystallized in the C2/c space group (Table S1^†). The obtained Ni coordination polymer was the isostructural compound reported by Talham,²⁷ but their packing modes were distinctly different (Fig. S3–S5^†). It also had a similar coordination environment to the railroad-like double chains synthesized by Yaghi.²⁸ The parallel chains were occupied by 4,4′-bpy, perchlorate and water molecules. In the Ni-CPs, most of the phenyl rings adopted the face-to-face mode. There were evident π⋯π interactions between the adjacent 4,4′-biprydine (Fig. 1a). In addition, there were strong intermolecular hydrogen bonds between 4,4′-bpy and perchlorate (strong Cl–O⋯C amongst adjacent layers) (Fig. 1a). The weak reaction increased the high density of the framework and protected the coordination bonds against external guest attacking. These chains and guests were further packed with a three-dimensional structure along the c-axis (Fig. 1b).Open in a separate windowFig. 1(a) The weak reaction between chains in Ni CPs, showing the Cl–O⋯C (green and black dashed line) and π⋯π interactions (pink dashed line) between the adjacent perchlorate and 4,4′-biprydine, respectively; (b) the packing view of Ni CPs along the c-axis. Cl in green, Ni in pale blue, N in blue, O in red and C in black. H atoms and partial guest molecules were omitted for clarity.The unique cationic Ni-CPs framework has the potential to immobilize some counterpart ions. To demonstrate this, Mohr''s salts were investigated. Most strikingly, the color slowly changed from blue to green in an aqueous solution, given by the optical image (Fig. 2a and b), which not only indicated that Ni-CPs captured Mohr''s salts via ion-exchange, but also suggested an alteration in the valence of Fe ions. It was possible that Fe²⁺ may have been further oxidized to Fe³⁺ under the O₂ and water environment when we prepared the Fe@Ni-CPs (4Fe²⁺ + 2H₂O + O₂ = 4Fe³⁺ + 4OH⁻). The PXRD pattern showed that the frameworks remained unchanged after doping with Fe ions (Fig. S1^†). From the transmission electron microscopy (TEM) images (Fig. 2c), after immobilization, the morphology of the Fe@Ni-CPs was still level and smooth; no ring-like patterns arose corresponding to the selected area for electron diffraction (SAED) (Fig. 2d), indicating that no bulk Fe particles formed during ion-exchange. This was further demonstrated using high resolution TEM (HRTEM) (Fig. 2e), in which there was no lattice fringe of crystallized Fe. The well distribution of C, N, O, Cl, Fe, and Ni in Fe@Ni-CPs was demonstrated by elemental mapping (Fig. 2f–l). Energy-dispersive X-ray spectroscopy (EDX) also agreed well with the above mapping data (Fig. S6^†). In addition, the Fe³⁺ uptake was 4.1 wt%, as determined by inductively coupled plasma atomic emission spectroscopy (ICP). These results showed Fe ions to have been successfully immobilized by the Ni-CPs.Open in a separate windowFig. 2(a–b) The optical image of Ni-CPs and Fe@Ni-CPs; (c) TEM images of Fe@ Ni-CPs; (d–e) the corresponding SAED and HRTEM pattern. (f–l) Element mapping of C, N, O, Cl, Fe, and Ni in Fe@Ni-CPs.The X-ray photoelectron spectroscopy (XPS) survey spectrum of the Fe@Ni-CPs also showed the presence of C, N, O, Cl, Fe, and Ni elements (Fig. 3a). The Fe 2p high resolution XPS spectrum exhibited peaks at 725 eV and 711 eV (Fig. 3b), further indicating the presence of the Fe³⁺ oxidation state. This could be explained by the transformations of Fe²⁺ to Fe³⁺ during the ion-exchange process. Similarly, in the Ni 2p spectra (Fig. 3c), two main peaks located at 855.8 eV and 873.5 eV can be ascribed to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2, respectively. These peaks are associated with two shakeup satellite peaks, indicating that Ni still remained in a divalent state. The Cl 2p and N 1s region could be corresponded to the ClO₄⁻ and biprydine, respectively (Fig. S7^†). The O 1s spectrum (Fig. 3d) was divided into two peaks at 531.7 eV and 533.2 eV, which could be assigned to the OH group from filled H₂O molecules and partial ClO₄⁻, respectively.Open in a separate windowFig. 3(a) XPS survey spectrum of the Ni-CPs; XPS spectra of the Ni-CPs in the (b) Fe 2p, (c) Ni 2p, and (d) O 1s regions.The above Ni-CPs with Fe doping encouraged us to investigate its electrocatalytic application in oxygen evolution reaction. To study the electrocatalytic activity of Fe@Ni-CPs for the OER, linear sweep voltammetry (LSV) was performed in a strong alkaline solution (pH = 14) for Fe@Ni-CPs@GC (fresh samples coated on glassy carbon electrode with Nafion binder). Fe@Ni-CPs@GC directly acted as working electrodes and showed good OER activity with an onset potential of 1.52 V (Fig. 4a), overpotential of 368 mV at 10 mA cm⁻², and a Tafel slope of 59.3 mV dec⁻¹ (Fig. 4b). These OER performances are close to some reported MOFs catalysts (Table S2^†) and even better than commercial benchmark OER catalysts like RuO₂ working at the same condition (Fig. S8^†). In contrast, the electrocatalytic OER activities of the pristine Ni-CPs@GC without Fe incorporation displayed much worse activity. The onset potential, overpotential (at 10 mA cm⁻²), and the Tafel slope reached 1.62 V, 458 mV, and 96.8 mV dec⁻¹, respectively (Fig. 4a and b). In addition, Fe@Ni-CPs showed a strong durability during the OER process. The chronoamperometric response of Fe@Ni-CPs displayed a slight anodic current attenuation within 12 h due to the peeling of samples during the evolution of a large amount of O₂ gas (Fig. S9^†). Furthermore, LSV of Fe@Ni-CPs showed negligible changes after OER tests for 12 h (Fig. S10^†). These results indicate that the Fe-doped Ni-CPs with more active sites could serve as an excellent candidate for OER in strong alkaline conditions.Open in a separate windowFig. 4(a) OER polarization curves and (b) Tafel plots of various electrocatalysts in a 1 M KOH aqueous solution; (c) linear relationship of the current density at 1.1 V (vs. RHE) vs. scan rates for Fe@Ni-CPs@GC and Ni-CPs@GC; (d) EIS of Fe@Ni-CPs and Ni-CPs electrode.When Fe(iii) was introduced, the resulting Fe@Ni-CPs catalysts greatly improved OER catalytic performance. There were dynamic collisions between Fe³⁺ ions and Ni-CPs, which allowed for more accessible catalytic active sites compared to the Ni-CPs. Particularly, Fe(iii) doping can contribute to the adsorption and reaction of OH⁻ groups in OER process.²⁹ As a result, Fe@Ni-CPs enhanced charge transfer under an apt electronic environment of the mixed Fe⋯Ni systems. In addition, electrochemical impedance spectrum and double-layer capacitance (C_dl) of the Fe@Ni-CP were also studied. The C_dl of Fe@Ni-CPs was confirmed to be 269.7 μF cm⁻² (Fig. 4c and S11^†), which is higher than that of Ni-CPs (C_dl = 174.2 μF cm⁻²) (Fig. 4c and S12^†). The semicircular diameter in EIS of Fe@Ni-PCP was smaller than that of Ni-CPS (Fig. 4d). These results further showed that Fe@Ni-CPs were more effective in enlarging the catalytically active surface area, conductivity and synergistic effects between Fe and Ni in comparison to Ni-CPs coated on electrodes.In conclusion, a new alkaline-stable cationic Ni(ii) coordinated polymers was synthesized under hydrothermal conditions. The Ni CPs could quickly interact with Mohr''s salt. Interestingly, the Ni CPs could act as a unique oxidation matrix to realize the transformation of Fe²⁺ to Fe³⁺ during the ion-exchange process. Furthermore, the resulting Fe@Ni-CPs electrode, for the first time, showed an excellent electrocatalytic activity for OER in strong alkaline media. This study provides a new avenue to explore stable coordinated polymers by incorporating the low-cost and high-activity transition metal, Fe, which will substitute the rare noble metals used in energy-related research.     

Correction: Highly porous core–shell chitosan beads with superb immobilization efficiency for Lactobacillus reuteri 121 inulosucrase and production of inulin-type fructooligosaccharides

Correction for ‘Highly porous core–shell chitosan beads with superb immobilization efficiency for Lactobacillus reuteri 121 inulosucrase and production of inulin-type fructooligosaccharides’ by Thanapon Charoenwongpaiboon et al., RSC Adv., 2018, 8, 17008–17016.

The authors regret that Fig. 9 in the original article was displayed incorrectly. The correct version is shown below.Open in a separate windowFig. 9Batch reusability of INU-CSBs for IFOS synthesis. Reaction condition: 10 U mL⁻¹ of biocatalysts were incubated with 200 g L⁻¹ sucrose in acetate buffer pH 5.5, 40 °C and 2 h per batch.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

In situ approach of cementite nanoparticles encapsulated with nitrogen-doped graphitic shells as anode nanomaterials for Li-ion and Na-ion batteries

Novel Fe₃C nanoparticles encapsulated with nitrogen-doped graphitic shells were synthesized by floating catalytic pyrolysis. Due to the short synthesis time and controllable pyrolytic temperature, the diameters of Fe₃C core nanoparticles ranged from 5 to 15 nm (Fe₃C@NGS900 prepared at 900 °C) and the average thickness of N-doped graphitic shells was ∼1.2 nm, leading to their high electrochemical performance: specific capacity of 1300 mA h g⁻¹ at current density 0.2 A g⁻¹, outstanding rate capability of 939 mA h g⁻¹ at 3 A g⁻¹, improved initial coulombic efficiency (Fe₃C@NGS900: 72.1% vs. NGS900 (pure graphitic shells): 52%) for lithium ion batteries (LIBs), and impressive long-term cycle performance (1399 mA h g⁻¹ maintained at 3 A g⁻¹ after 500 cycles for LIBs; 214 mA h g⁻¹ maintained at 1 A g⁻¹ after 500 cycles for sodium ion batteries).

Novel Fe₃C nanoparticles encapsulated with nitrogen-doped graphitic shells were synthesized by floating catalytic pyrolysis.

Because of the fast development of portable electronic devices and hybrid electric vehicles, lithium ion batteries (LIBs)^1–8 and sodium ion batteries (NIBs)^9–16 with high energy/power density, good cycling performance, and lack of memory effects are in ever-increasing need. Due to the low theoretical capacity of carbon materials (graphite: 372 mA h g⁻¹),^8,17–22 optimizing the morphology of graphitic electrode materials has been important to improve specific capacity.^1–7,23,24 On the other hand, chemical doping (e.g., N, S, B, P) is an effective strategy to raise their specific capacity by increasing conductivity or active sites for Li⁺ or Na⁺ storage.^3,4,25–28 Additionally, metallic compounds (e.g., Fe₃C) have been also introduced into improving electrochemical performance of carbon anodes, because such materials are proposed to activate some components for reversible transformation of the solid electrolyte interface (SEI) and further benefit reversible capacity.^13,16,29,30 Unfortunately, most of them have been prepared by complex methods including tedious synthetic steps or long-time annealing,^6,17–19 from which, it is hard to prepare Fe₃C particles with desirable small sizes.^8,13,15,16 Thus, developing appropriate Fe₃C/C electrode materials still requires further research.In this work, Fe₃C nanoparticles encapsulated with nitrogen-doped graphitic shells (Fe₃C@NGS) were in situ approached from floating catalytic pyrolysis. Due to the short annealing time of the pyrolysis, Fe₃C@NGSs was prepared with controllable sizes. Furthermore, the in situ approach led the graphitic shells just grew on the surface of Fe₃C core nanoparticles, which improved electron transfer between the cores and the shells. Thus, such nanoparticles might be a superb electrode material towards high performance applications of LIBs and NIBs.For preparing the Fe₃C@NGSs with controllable sizes, floating catalytic pyrolysis was carried out to shorten synthetic time: the gas mixture was introduced into quartz pipe furnace, which was set at 700–1100 °C. For a typical experiment, nitrogen (flow rate: 80 L h⁻¹), acetylene (10 mL min⁻¹) and ammonia (100 mL min⁻¹) gases were embedded into iron pentacarbonyl held at 10 °C to form the gas mixture. After the pyrolysis, Fe₃C@NGS was collected at the other end of the quartz pipe. The details of materials characterization and electrochemical measurements can be found in ESI.^†As shown in transmission electron microscope (TEM) images of the prepared Fe₃C@NGSs (Fig. 1a–c and S1a of ESI^†), the metallic cores (dark section) are encapsulated with their shell (light section). The XRD results (Fig. 1d) shows the metallic core nanoparticles are Fe₃C. Thus, the sample prepared at 900 and 1100 °C have been marked with Fe₃C@NGS900 and Fe₃C@NGS1100, respectively. However, the sample prepared at 700 °C, which has been marked with FN@NGS700, has been oxidized in air at room temperature, because of its poor graphitic layers. Compared with XRD pattern of Fe₃C@NGS900, those peaks of Fe₃C@NGS1100 are much sharper indicating much bigger ferrous cores of Fe₃C@NGS1100. According to the TEM and high resolution TEM (HRTEM) images for Fe₃C@NGS900 (Fig. 1a–c, S1a, and b^†), the diameter of the core nanoparticles is ranged from 5–15 nm and the average thickness of their shells is ∼1.2 nm, respectively. Moreover, the spacing of the lattice fringes (Fig. S1a and b^†) is ∼0.34 nm corresponding to the characteristic (002) peak of graphite implying high graphitization of those shells.^25,28,31 In the HRTEM images, every Fe₃C cores is found to be a single crystal and encapsulated with the graphitic shell. The boundary between Fe₃C cores and N-doped graphitic shells is continuous and distinguished, indicating Fe₃C cores are tightly encapsulated with the graphitic shell.Open in a separate windowFig. 1(a) TEM images of core-shells nanoparticles (FN@NGS700) prepared at 700 °C, TEM and HRTEM (inset) images of nanoparticles (Fe₃C@NGS900) prepared at 900 °C (b) and nanoparticles (Fe₃C@NGS1100) prepared at 1100 °C (c) and (d) XRD patterns of core-shells nanoparticles prepared at different temperatures.Fe₃C@NGS samples have been also analyzed by X-ray photoelectron spectroscopy (XPS), which suggests Fe₃C@NGS samples contain Fe, C, O and N atoms (FN@NGS700: C content of 16.3 wt%, Fe content of 54.6 wt%, N content of 1.6 wt% and O content of 27.5 wt%; Fe₃C@NGS900: C content of 26 wt%, Fe content of 67 wt%, N content of 2 wt% and O content of 5 wt%; Fe₃C@NGS1100: C content of 33.7 wt%, Fe content of 52 wt%, N content of 2.3 wt% and O content of 7 wt%). According to XPS results of Fe₃C@NGS900, the weight ratio of Fe₃C cores and graphitic shells is 3.39 : 1. Additionally, the element contents of Fe₃C@NGS samples are different due to their morphology and structure (Fig. 1). Higher O content of FN@NGS700 is because its ferrous cores have been oxidized, and thick-walled graphitic shells (∼3 nm) of Fe₃C@NGS1100 leads to its higher C content, as XPS can only measure element contents of surface of samples.The electrochemical properties of the Fe₃C@NGS-based electrodes have been shown in Fig. 2 and ​and3.3. Since FN@NGS700 sample has been completely oxidized, electrochemical properties of ferrous oxide has not been measured. The cyclic voltammetry (CV) curves of Fe₃C@NGS900-based electrode show details of possible lithium storage process. Besides the similar peak (0.5 V) in the initial cycle for the SEI formation, two reduction peaks are located at 0.7 and ∼1.5 V, corresponding to the reduction of some SEI components (Li⁺ insertion). During the Li⁺ extraction process, the corresponding oxidation peaks is found to shift from ∼1.7 to ∼1.9 V. The almost overlapped oxidation peaks demonstrate good reversibility and cycling stability of core–shell Fe₃C@NGS.¹⁵ As shown in Fig. 2b, the galvanostatic discharge–charge (GDC) profiles of Fe₃C@NGS900 in a voltage range of 0.005–3 V (vs. Li⁺/Li) exhibits the typical shape of Fe₃C@NGS-based anodes, and Fe₃C@NGS900 delivers initial charge and discharge capacities of 1246.7 and 1729.1 mA h g⁻¹, respectively. The initial columbic efficiency (CE) reaches up to 72.1% which is higher than 52% of pure graphitic shells.³ For Fe₃C@NGSs electrode, the phenomenon of capacity increment is related to growing reversibly SEI film via the decomposition of electrolyte due to the catalysis of Fe₃C.^16,29 From the second cycle, the shape of the discharge profiles changes with respect to that of the first cycle, which may be due to the modification of the SEI film.¹²Open in a separate windowFig. 2Electrochemical performance of prepared nanoparticles as anodes for LIBs: CV profile (a) of Fe₃C@NGS900 at a scan rate of 0.1 mV s⁻¹ between 0.01–3 V vs. Li⁺/Li for the 1st–2nd charge/discharge cycles; (b) the galvanostatic charge–discharge profiles of Fe₃C@NGS900; (c) charge–discharge cycling performance of Fe₃C@NGS core–shell nanoparticles at different current densities from 0.2 to 3 A g⁻¹ at room temperature; (d) cycling performance of Fe₃C@NGS at 3 A g⁻¹.Open in a separate windowFig. 3Electrochemical performance of the prepared nanoparticles as anodes for NIBs: (a) CV profile of Fe₃C@NGS900 at a scan rate of 0.1 mV s⁻¹ between 0.01–2.5 V vs. Na⁺/Na for the 1st–3rd charge/discharge cycles. (b) The galvanostatic charge–discharge profiles of Fe₃C@NGS900; (c) cycling performance of Fe₃C@NGS at a current density of 1 A g⁻¹, and the corresponding columbic efficiency.The excellent rate capability of Fe₃C@NGS-based anodes have been investigated by testing charge/discharge at current densities of 0.2, 0.5, 1 and 3 A g⁻¹ for every 5 cycles (Fig. 2c). At the corresponding rates, the reversible capacities are 1300, 1101, 1062 and 939 mA h g⁻¹ for Fe₃C@NGS900; 925, 721, 690 and 663 mA h g⁻¹ for Fe₃C@NGS1100, indicating the smaller size of the Fe₃C nanoparticles might enhance their electrochemical capability. Compared with reported pure graphitic shells (NGS900 prepared by removing Fe₃C cores of Fe₃C@NGS900),³ the core–shell nanoparticles (Fe₃C@NGS900) exhibits impressive rate performances (NGS900: 760 mA h g⁻¹ at 0.5 A g⁻¹, 620 mA h g⁻¹ at 1 A g⁻¹ and 340 mA h g⁻¹ at 5 A g⁻¹), which might be caused by Fe₃C, as a good conductor of electricity, can effectively improve electrical conductivity of carbon electrode material.¹³ Calculated from eqn (1) of ESI,^† specific capacity of Fe₃C cores of Fe₃C@NGS900 can be evaluated (1199 mA h g⁻¹ at 0.5 A g⁻¹ and 1193 mA h g⁻¹ at 1 A g⁻¹, respectively). Based on the conversion mechanism for lithium storage, if possible, Fe₃C can store only 1/6 Li per unit (∼26 mA h g⁻¹),¹⁵ which is negligible regarding to the high capacity of ∼1300 mA h g⁻¹. The specific capacity of Fe₃C is larger than what it should be, which might be resulted from the pseudocapacity on the interface between the material and the electrolyte.¹⁶ For evaluating N doping structure in the graphitic shells, Fe₃C@NGS samples have been prepared with different percent of doping content at 900 °C by introducing ammonia with different flow rates (0, 30, 100 or 500 mL min⁻¹). As a result, Fe₃C@GS900 prepared without ammonia has graphitic shells without N-doping leading to its poor electrochemical performance: at current densities of 0.2, 0.5, 1 and 3 A g⁻¹, its reversible capacities are 575, 492, 458 and 402 mA h g⁻¹ (Fig. S4^†); the performances of Fe₃C@NGS900 (ammonia flow rate: 100 mL min⁻¹; content of N: 2 wt%) and Fe₃C@NGS900A (ammonia flow rate: 30 mL min⁻¹; content of N: 1.5 wt%) are similar; the sample prepared with ammonia flow rates of 500 mL min⁻¹ (FN@NGS900B) was violent oxidized to ferrous oxide in the air. Hence, N doping structure in graphitic shells has been confirmed to enhance diffusion.The long-term cycling performance of these two electrodes also has been investigated in Fig. 2d. The Fe₃C@NGS-based anode exhibits a favorable reversible capacity, which can reach 1399 mA h g⁻¹ after 500 discharge/charge cycles at 3.0 A g⁻¹, showing high capacity retention with CE of ∼100%. The capacities of Fe₃C samples increases with cycle number rising (from 120 to 450), which might be attributed to the pseducapacity presented by the Fe₃C.¹⁶To better study the kinetic properties of Fe₃C@NGS900 and Fe₃C@NGS1100, Fig. S3 of ESI^† shows the Nyquist plots and equivalent circuit obtained from electrochemical impedance spectroscopy (EIS) measurements. Here, R_s represents the ohmic resistance of the battery. Constant phase element (CPE) represents the double layer capacitive reactance between the electrode materials and the electrolyte. The semicircles and straight lines correspond to the electrochemical polarization impedance (R_p) and Warburg resistance (W), respectively.³² Both the fitted R_s value (5.526 Ω) and R_p value (20.826 Ω) for Fe₃C@NGS900 electrode is much lower than that Fe₃C@NGS1100 electrode (R_s: 8.501 Ω; R_p: 43.665 Ω), indicating the superior redox kinetics in the Fe₃C@NGS900 composite.In order to study the electrochemical properties of the Fe₃C@NGSs electrode as anodes for NIBs, CV analysis has been carried out at a scanning rate of 0.1 mV s⁻¹ between 0.005 and 2.5 V vs. Na⁺/Na. As shown in Fig. 3a, there are two irreversible reduction peaks around 2.02 V and 0.83 V found during the initial cathodic scan, which could be ascribed to the interaction of Na ions with specific functional groups and the decomposition of electrolyte along with the formation of SEI film on the electrode surfaces.^33,34 For the subsequent cycles, the peak at 2.02 V disappears and the peak at 0.83 V shifts to 0.60 V. For the anodic scan, the main oxidation peak ranging from about 0.47 V to 1.28 V is supposed to be the desodiation reactions.³⁵Fig. 3b depicts the GDC curves of the Fe₃C@NGS900-based electrode for the 1st–5th cycle at current density of 0.1 A g⁻¹. The large irreversible capacity in the 1st cycle is attributed to the SEI formation and the irreversible insertion of sodium ion with a relatively large ionic radius.^22,23 Following the first cycle, the charge–discharge curves become more linear which exhibits a higher and more stable CE indicating a stable SEI layer formed in the first cycle.Meanwhile, cycling performance of Fe₃C@NGS-based anodes for NIBs have been shown in Fig. 3c. In the extended cycling test at 1 A g⁻¹, a reversible capacity 214 mA h g⁻¹ of Fe₃C@NGS900 electrode is still maintained after 500 cycles, which is ∼2 times the capacity delivered by the Fe₃C@NGS1100 indicating excellent cycling stability of Fe₃C@NGS900. Thus, Fe₃C@NGS-base anodes holds great potential as a promising candidate compared with other carbonaceous anode materials for NIBs (113 mA h g⁻¹ at 1 A g⁻¹ (modified PFR/C),³⁶ 188.6 mA h g⁻¹ at 0.1 A g⁻¹ after 300 cycles (S/C),³³ 150 mA h g⁻¹ at 1 A g⁻¹ after 200 cycles (S/graphene)³⁵).It is noticed that the excellent electrochemical performance of the prepared Fe₃C@NGSs is apparently ascribed to their novel structure. First, because of in situ growing graphitic shells on the surface of Fe₃C during floating catalytic pyrolysis, contact between Fe₃C cores and graphitic shells effectively increases, leading to lower contact resistance and faster electron transfer between the cores and the shells, comparing with traditional ferrous/carbon composites generated by multistep carbonization approach.^8,13,15,16 Second, due to floating catalytic pyrolysis, sizes of Fe₃C core nanoparticles have been under control (Fe₃C@NGS900: 5–15 nm vs. Fe₃C@NGS1100: 15–40 nm). Smaller size of Fe₃C core nanoparticles might increase surface of Fe₃C nanoparticles, leading to distinguished improvement of their electrochemical performance (Fig. 2 and ​and3),3), due to active sites for Li⁺ or Na⁺ storage rising. For comparison, sizes of reported Fe₃C composites have been listed: Fe₃C@C: 60 nm nanoparticles encapsulated with 4 nm carbon shells;⁸ Fe₃C@PC: 29 ± 5 nm nanoparticles embedded in 300 nm porous carbon;¹⁶ Fe₃C/C: 300 nm;¹³ Fe@Fe₃C/C: 28–58 nm Fe@Fe₃C nanoparticles.¹⁵ Third, introducing Fe₃C to electrode material can promote the reversible formation/decomposition of the SEI film, causing improvement of initial CE (72.1% vs. 52%) for LIBs, due to the catalysis function of Fe₃C.^16,29 Fourth, due to in situ N-doping (∼2 wt%) during their floating catalytic pyrolysis, such defects of the graphitic shells might offer lots channels for fast diffusion of electrolyte and Li⁺/Na⁺ into those nanoparticles. Fifth, such prepared core–shell nanoparticles have ultra thin-walled graphitic shells (∼1.2 nm shown in Fig. 1d), which shorten the diffusion route of ions and electrolyte. All above confirm Fe₃C@NGS900 has novel structure towards the electrochemical applications, compared with the reported works: Fe@Fe₃C/C sample was prepared by sol–gel and carbonization approach, and whether its pure Fe cores was good for Li⁺ storage was doubtful;¹⁵ the Fe₃C@C nanoparticles were prepared with no doping carbon shells;⁸ the graphitization of carbon structure was doubtful, when ferrous/carbon composites were prepared by polymerization-carbonization of iron phthalocyanine¹³ or hydrothermal method-carbonization.¹⁶ Thus, the Fe₃C@NGS900 performs better (1300 mA h g⁻¹ at current density of 0.2 A g⁻¹; 939 mA h g⁻¹ at 3 A g⁻¹) than many reported ferrous/carbon composite anode materials for LIBs (0.2 A g⁻¹: <382 (Fe@Fe₃C/C),¹⁵ ∼480 (Fe₃C/C),⁸ 787.9 (Fe₂O₃/C),⁵ ∼850 (Fe₃O₄/C),¹⁹ 873 (N,S/C),³⁶ and 881 (Fe₃O₄/C)⁹ mA h g⁻¹; 3 A g⁻¹: ∼300 (Fe₂O₃@C),¹⁸ ∼370 (FeS@C)²¹ and ∼612 (Fe₂O₃/C)¹⁴ mA h g⁻¹).