Volume 50 Number 20 11 March 2014 Pages 2549–2684
ChemComm Chemical Communications www.rsc.org/chemcomm
ISSN 1359-7345
COMMUNICATION Hucheng Zhang, Jianji Wang et al. Functional dialkylimidazolium-mediated synthesis of silver nanocrystals with sensitive Hg2+-sensing and efficient catalysis
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Cite this: Chem. Commun., 2014, 50, 2565
Functional dialkylimidazolium-mediated synthesis of silver nanocrystals with sensitive Hg2+-sensing and efficient catalysis†
Received 23rd October 2013, Accepted 29th December 2013
Hucheng Zhang,* Lu Ye, Xingang Wang, Fangfang Li and Jianji Wang*
DOI: 10.1039/c3cc48121b www.rsc.org/chemcomm
Thiol-functionalized dialkylimidazolium bromide was synthesized, and used as a stabilizer to prepare monodisperse silver nanocrystals through a facile one-pot aqueous approach. It is shown that the multipurpose ligands play a vital role in improving the performance of nanocrystals. The mechanisms of dialkylimidazolium-mediated promotion were analyzed on the basis of the physicochemical properties of the specifically designed ligand.
Noble metal nanocrystals (NCs) have received ever intensive attention over the past decade owing to their distinct physicochemical properties that are totally different from those of the bulk counterparts, and exhibit broad applications in many fields, such as catalysis,1–5 fluorescence,5–8 sensing,8–10 imaging,7,11–13 analysis,8,13–15 optoelectronic devices,16–18 antimicrobial technology,19 therapeutics and diagnostics,20,21 etc. Undoubtedly, Ag NCs are one of the most widely studied subjects in metallic nanoscience, and a number of different synthesis strategies have been developed in water, organic solvents, liquid– liquid or liquid–solid diphase systems. It has been demonstrated that well-controlled sizes and shapes of Ag NCs can be generated by choosing suitable stabilizers and reductants, and sometimes foreign additives, with notable examples including Cl , Br , S2 and Fe3+, are required for some synthesis protocols.22–24 Among them, the ligands are of key importance, and accordingly various ligands have been used for the preparation of Ag NCs, involving carboxylates,22,25 polymers,23,24,26 alkylthiols,27,28 biomolecules,29,30 and so on. However, these investigations focused mainly on the role of ligands in shapeand structure-controlled synthesis, as well as in preventing nanocrystals from aggregation, and paid little attention to the effect of the distinct properties of ligands on the performance of Ag NCs to date. This can be attributed to three possible reasons: (i) the morphology in most situations exert more effects on the properties of Ag NCs than Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China. E-mail: [email protected], [email protected]; Fax: +86 3733326336; Tel: +86 3733325805 † Electronic supplementary information (ESI) available: Details of all experimental procedures. See DOI: 10.1039/c3cc48121b
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ligands, (ii) the commercial ligands are available over a wide range, (iii) the performance of nanocrystals can often be determined by the trial-and-error approach. In this regard, it still remains a great challenge to realize goal-directed design and synthesis of the functionalized ligands, and further produce Ag NCs with improved performance. The 1,3-dialkylimidazolium salt, 1-(10-mercaptodecyl)-3-methylimidazolium bromide ([HS-C10mim]Br), was synthesized and employed as a ligand for the preparation of Ag NCs in this work. The specific choice was prompted by the characteristics of the functional ligand:31–35 (i) great affinity between the Ag surface and thiol groups; (ii) excellent compatibility of the 1,3-dialkylimidazolium salt with a great deal of compounds through Van der Waals forces, hydrogen bonds, and p–p interactions; (iii) easy electrostatic adsorption on the surface of [HS-C10mim]+-capped Ag NCs; (iv) strong hydrophobic interactions among the alkyl side chains; (v) controllable nucleation of Ag NCs in the presence of Br . Therefore, in addition to the properties of Ag NCs, the studies have shown that 1,3-dialkylimidazolium bromide can contribute to the better performance of Ag NCs. Highly toxic mercury is a widely spread environmental pollutant, and its accumulation in body could cause serious damage to the nervous and endocrine systems. Although many detection methods have been developed, the ever increasing health concerns still call for more sensitive ion-sensors with high selectivity to monitor the trace amounts of Hg2+.36–42 This work demonstrates that the task-specific ligand-capped Ag NCs have great sensitivity and selectivity for the Hg2+ detection, and also high catalytic activity in 4-nitrophenol reduction in aqueous solutions. In aqueous solution of [HS-C10mim]Br, the synthesis of Ag NCs was performed by the reduction of AgNO3 by NaBH4 at 298 K. Fig. 1A shows the TEM image of Ag NCs prepared using AgNO3 and [HS-C10mim]Br in equimolar amounts. The Ag NCs appear to be spherical in shape, and have an average diameter of 4.9 0.4 nm which falls within the size range of quantum dots with a high surface–volume ratio. Fig. 1B presents the selected-area electron diffraction (SAED) pattern of Ag NCs, and these diffraction rings could be indexed to face-centered cubic silver. The high-resolution TEM image obtained from a selected individual nanocrystal gives
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Fig. 1 (A) TEM images of [HS-C10mim]+-stabilized Ag nanocrystals. (B) The SAED pattern of Ag NCs. (C) UV-vis absorption spectrum of Ag nanocrystals in water. The insets in (A) and (C) are respectively the high-resolution TEM image of a single Ag nanocrystal and the sample picture in natural light.
clearly resolved lattice fringes, and the lattice spacing is calculated to be about 0.236 nm, which matches the [111] plane of silver metal. This lattice-fringe pattern extends over the entire nanocrystal, implying that the particle is made of a single crystal. The UV-vis absorption spectrum of Ag NCs was recorded in the aqueous suspension (Fig. 1C). It is observed that the peak centers at 420 nm with a strong absorption, and exhibits a typical localized surface plasmon resonance (LSPR) feature of Ag NCs. Further, the measurement of dynamic light scattering indicates good monodispersity of Ag NCs with an average hydrodynamic diameter of 6.6 nm in water (Fig. S1, ESI†). The presence of Ag, Br, S and N elements in the Ag NCs was confirmed by the energy-dispersive X-ray spectrum, and the component appeared as expected (Fig. S2, ESI†). Experimentally, it is demonstrated that the facile one-pot aqueous approach reported herein is highly robust and reproducible, and the resulting nanocrystals have excellent stability in aqueous solution with a shelf-time longer than 12 months at room temperature. In the synthesis strategy, [HS-C10mim]Br plays a dual role of both a stabilizer of Ag NCs and a provider of Br . However, Ag NCs are not synthesized by this route if Br is substituted for NO3 , or the alkyl side chain for a shorter one than the octyl chain. The as-synthesized colloidal Ag NCs can be directly used to detect Hg2+. First, Ag NCs were hybridized with rhodamine 6G (Rh6G) in aqueous solution, and the composite sensors were easily formed by the strong adsorption of Rh6G on [HS-C10mim]+-capped Ag NCs,43 in which Rh6G acts as the ion-sensing indicator. Then, different concentrations of Hg2+ were introduced into the sensing system, and the pH value of the resulting sample is about 2.0. The sensor sensitivity was evaluated by fluorescence spectrometry. Because of the fluorescence quenching effect of Ag NCs toward the adsorbed Rh6G, as shown in Fig. 2A, the weakest fluorescence intensity was recorded in the aqueous solution of Rh6G–Ag nanocomposite. The fluorescence intensity gradually increases with increasing Hg2+ concentration (c), and a good linear relationship of the relative fluorescence intensity versus Hg2+ concentration ranging from 1.0 10 13 to 1.0 10 4 mol L 1 is determined with a high correlation coefficient (R2 = 0.997), and follows
2566 | Chem. Commun., 2014, 50, 2565--2568
Fig. 2 (A) The fluorescence spectrum of Rh6G–Ag nanocomposite at different concentrations of Hg2+ (mol L 1) at pH = 2.0: (a) 0; (b) 1.0 10 13; (c) 1.0 10 12; (d) 1.0 10 11; (e) 1.0 10 10; (f) 1.0 10 9; (g) 1.0 10 8; (h) 1.0 10 7; (i) 1.0 10 6; (j) 1.0 10 5; and (k) 1.0 10 4; where the inset shows the dependence of relative fluorescence intensity at 552 nm on a negative logarithm of the Hg2+ concentration. (B) The fluorescence response of Rh6G–Ag nanocomposite sensor toward different metal ions at the concentration of 1.0 10 4 mol L 1. (C) The dynamic equilibrium of a free [HS-C10mim]+ ion with an Ag nanocrystal, and the proposed Hg2+-sensing mechanism for the [HS-C10mim]+-stabilized silver nanocrystals.
I/I0 = 3.493 + 0.188 log c, where I0 and I are the fluorescence intensities at 552 nm in the absence and presence of the metal ion, respectively. The detection limit is 1.0 10 13 mol L 1, at least three orders of magnitude smaller than that reported previously in the literature.38,39 The selectivity of the composite sensors was further examined by fluorescence spectrometry in the presence of other metal ions. At low metal ion concentrations, no interference is observed in the Hg2+ detection. As the metal ion concentration is increased up to 1.0 10 4 mol L 1, the turn-on fluorescence intensity of Hg2+ is at least 8 times more than that of other metal ions (Fig. 2B), a well-accredited
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value for the quantitative analysis of Hg2+. Evidently, the method is sensitive and reliable, and can be applied successfully to detect ultra low concentrations of Hg2+ in aqueous solutions. The specific Hg2+-sensing mechanism results mainly from antigalvanic reduction (AGR), in which the nanometer-sized noble metal can reduce more reactive metal ions in solution.44 In this regard, Ag atoms are oxidized by the added Hg2+, and the resulting mercury deposits and forms an alloy coating on the surface of Ag NCs. Because the composition variation leads to the change in nanocrystal properties, the LSPR peak at 420 nm blue shifts slightly, and attenuates in absorption intensity (Fig. S3, ESI†). At the same time, the fluorescence intensity of Rh6G in the composite sensor increases linearly with increasing Hg2+. As the Hg2+ concentration reaches 1.0 10 3 mol L 1, the LSPR absorption peak disappears due to full AGR, and the solution for Hg2+-sensing changes from light pink to a colourless liquid. The AGR principle gives a straightforward illustration to enlighten us about the sensing mechanism, but fails to address several crucial questions: (i) how do Hg2+ cations cross the positively charged barriers of ligand shells outside Ag NCs, (ii) how AGR can be realized in the ultra low Hg2+ concentration, and (iii) why the composite sensors respond selectively to Hg2+ rather than Cu2+ that are also reduced by Ag NCs.44 Therefore, the unusual sensing mechanism should be further discussed from the roles played by the functionalized ligands. [HS-C10mim]+ ions in the sensing system could fall into three categories: the anchored ones on the surface of Ag NCs, the intercalated ones in ligand shells, and the freely diffused ones in aqueous solution. The dynamic light scattering measurement31 has shown that the free [HS-C10mim]+ ions can intercalate into ligand shells by strong hydrophobic and p–p interactions among them, and the intercalated [HS-C10mim]+ ions are in a dynamic equilibrium with the free ligand ions. The thiophilic affinity of Hg2+ makes the S atoms in the free [HS-C10mim]+ ions to discriminate the metal ions, and capture them efficiently in solution (Fig. 2C). By means of such dynamic equilibrium, the free [HS-C10mim]+ ions entrap Hg2+ into ligand shells, and condense them on the surface of Ag NCs. Because the functionalized [HS-C10mim]+ ions bind preferentially with Hg2+ and enrich them in ligand shells, Ag NCs exhibit superior reactivity of AGR even at low Hg2+ concentrations, as a result, the sensitivity and selectivity of the composite sensors are dramatically improved in the aqueous solution. The typical pseudo-homogeneous catalysis of noble nanoparticles has advantages of efficient activity and high selectivity under mild reaction conditions, and is of particular interest in reactions including water splitting, hydrogenation, and coupling reactions, etc.45–47 Here, the catalytic activity of Ag NCs was assessed by the reduction of p-nitrophenol in aqueous solution at 298 K.48–50 It has been known that the Langmuir–Hinshelwood model can be used to analyze the reduction of p-nitrophenol (Nip) by NaBH4, in which both reactants must be adsorbed onto the surface of metal nanoparticles to react. As BH4 is added into the reaction system, the aqueous solution represents a strongly alkaline solution, and converts Nip into a p-nitrophenolate anion. Therefore, both reactants are preferred to be adsorbed on the positively charged Ag NCs, and the electrostatic adsorption of [HS-C10mim]+ can promote the reaction process. However, experimental results of the control show that the reduction proceeds very slowly if the reaction mixture contains the
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free [HS-C10mim]+ ions. The restrained action of free [HS-C10mim]+ results probably due to the interactions of the imidazole ring with both reactants instead of the S atom in the [HS-C10mim]+ ion, which holds back the reactants to access the catalytic sites on Ag NCs. To achieve the optimal catalytic activity, the Ag NCs were separated by centrifugation to remove the intercalated and free [HS-C10mim]+ ions, and the resulting sediment was immediately redispersed in water as the catalyst. In the presence of the purified Ag NCs, the reaction kinetics in aqueous solution was analyzed by monitoring the changes in UV/vis spectra at different time intervals (Fig. 3A). After an induction period of 20 s, the absorption intensity of Nip at 401 nm decreased gradually with the progress of the reaction. Meanwhile, a new absorption peak of the product of p-aminophenol (Amp) appeared at 302 nm with increasing absorption intensity. Considering NaBH4 in excessive doses, the reaction should be of the pseudo-first-order with respect to the Nip concentration, and a linear
Fig. 3 (A) UV/vis spectral evolution of p-nitrophenol reduced by NaBH4 using Ag NCs as the catalyst in aqueous solutions at 298 K. (B) Dependence of ln(At/A0) on reaction time for the pseudo-first-order reaction kinetics. (C) The Langmuir–Hinshelwood mechanism for the reduction of p-nitrophenol on Ag nanocrystals with sparsely anchored ligands.
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relationship of ln(At/A0) versus reduction time is shown in Fig. 3B, where At and A0 are the Nip absorbance at time t and t = 0, respectively. From the linear plot, the apparent rate constant (kapp) was determined to be 1.25 10 2 s 1, and the activity parameter, a ratio of kapp to the mass of the catalyst, was estimated to be 2834 s 1 g 1. To our knowledge, this is the highest activity parameter for the catalytic reduction of Nip in aqueous solution reported in the literature. The positively charged ligand shell and the good compatibility of dialkylimidazolium ions with inorganic and organic compounds are advantageous to the recognition and adsorption of both reactants on Ag NCs. Assuming that the diffusion of the reactants and the desorption of the products are fast steps, the rate-determining step is the surface reaction of the adsorbed Nip with hydrogen species on Ag NCs. The heavy ligand shell outside Ag NCs is the physical barrier, unfavourable for catalysis, and the removal of intercalated ligands from Ag NCs could generate as many active sites as possible for the access of reactants. The surface potential of Ag NCs after centrifugal separation is about four times less than the as-prepared NCs. This implies that most of the ligands around the silver core are the intercalated dialkylimidazolium ions, because the electrostatic repulsion and steric hindrance among ligands prevent more dialkylimidazolium ions to anchor onto the silver core. As a result, the employed catalyst is sparsely covered by the anchored ligands, and has more exposed internal crystal faces with high accessibility. The almost bare Ag NCs are able to markedly lower the reaction energy barrier of the rate-determining step, and efficiently promote the catalytic reduction. In conclusion, the unique performance of Ag NCs, besides their size, shape, and morphology, are strongly dependent on the ligand characteristics. The functionalized dialkylimidazolium ions provide a facile green route to prepare monodisperse Ag NCs, and are able to perform the role of both stabilizer and mediator in the synthesis of Ag NCs, captor and enricher in the detection of Hg2+, or of activator and inhibitor in the reduction of Nip. This work bridges the ligand properties with the nanocrystal performance, and puts up a sensible choice to harvest multifunctional nanoparticles with enhanced performance using specifically designed ligands. The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21073055), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1061).
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ChemComm Chemical Communications www.rsc.org/chemcomm
ISSN 1359-7345
COMMUNICATION Hucheng Zhang, Jianji Wang et al. Functional dialkylimidazolium-mediated synthesis of silver nanocrystals with sensitive Hg2+-sensing and efficient catalysis
ChemComm
Published on 03 January 2014. Downloaded by University of Illinois at Chicago on 27/10/2014 15:53:40.
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Cite this: Chem. Commun., 2014, 50, 2565
Functional dialkylimidazolium-mediated synthesis of silver nanocrystals with sensitive Hg2+-sensing and efficient catalysis†
Received 23rd October 2013, Accepted 29th December 2013
Hucheng Zhang,* Lu Ye, Xingang Wang, Fangfang Li and Jianji Wang*
DOI: 10.1039/c3cc48121b www.rsc.org/chemcomm
Thiol-functionalized dialkylimidazolium bromide was synthesized, and used as a stabilizer to prepare monodisperse silver nanocrystals through a facile one-pot aqueous approach. It is shown that the multipurpose ligands play a vital role in improving the performance of nanocrystals. The mechanisms of dialkylimidazolium-mediated promotion were analyzed on the basis of the physicochemical properties of the specifically designed ligand.
Noble metal nanocrystals (NCs) have received ever intensive attention over the past decade owing to their distinct physicochemical properties that are totally different from those of the bulk counterparts, and exhibit broad applications in many fields, such as catalysis,1–5 fluorescence,5–8 sensing,8–10 imaging,7,11–13 analysis,8,13–15 optoelectronic devices,16–18 antimicrobial technology,19 therapeutics and diagnostics,20,21 etc. Undoubtedly, Ag NCs are one of the most widely studied subjects in metallic nanoscience, and a number of different synthesis strategies have been developed in water, organic solvents, liquid– liquid or liquid–solid diphase systems. It has been demonstrated that well-controlled sizes and shapes of Ag NCs can be generated by choosing suitable stabilizers and reductants, and sometimes foreign additives, with notable examples including Cl , Br , S2 and Fe3+, are required for some synthesis protocols.22–24 Among them, the ligands are of key importance, and accordingly various ligands have been used for the preparation of Ag NCs, involving carboxylates,22,25 polymers,23,24,26 alkylthiols,27,28 biomolecules,29,30 and so on. However, these investigations focused mainly on the role of ligands in shapeand structure-controlled synthesis, as well as in preventing nanocrystals from aggregation, and paid little attention to the effect of the distinct properties of ligands on the performance of Ag NCs to date. This can be attributed to three possible reasons: (i) the morphology in most situations exert more effects on the properties of Ag NCs than Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China. E-mail: [email protected], [email protected]; Fax: +86 3733326336; Tel: +86 3733325805 † Electronic supplementary information (ESI) available: Details of all experimental procedures. See DOI: 10.1039/c3cc48121b
This journal is © The Royal Society of Chemistry 2014
ligands, (ii) the commercial ligands are available over a wide range, (iii) the performance of nanocrystals can often be determined by the trial-and-error approach. In this regard, it still remains a great challenge to realize goal-directed design and synthesis of the functionalized ligands, and further produce Ag NCs with improved performance. The 1,3-dialkylimidazolium salt, 1-(10-mercaptodecyl)-3-methylimidazolium bromide ([HS-C10mim]Br), was synthesized and employed as a ligand for the preparation of Ag NCs in this work. The specific choice was prompted by the characteristics of the functional ligand:31–35 (i) great affinity between the Ag surface and thiol groups; (ii) excellent compatibility of the 1,3-dialkylimidazolium salt with a great deal of compounds through Van der Waals forces, hydrogen bonds, and p–p interactions; (iii) easy electrostatic adsorption on the surface of [HS-C10mim]+-capped Ag NCs; (iv) strong hydrophobic interactions among the alkyl side chains; (v) controllable nucleation of Ag NCs in the presence of Br . Therefore, in addition to the properties of Ag NCs, the studies have shown that 1,3-dialkylimidazolium bromide can contribute to the better performance of Ag NCs. Highly toxic mercury is a widely spread environmental pollutant, and its accumulation in body could cause serious damage to the nervous and endocrine systems. Although many detection methods have been developed, the ever increasing health concerns still call for more sensitive ion-sensors with high selectivity to monitor the trace amounts of Hg2+.36–42 This work demonstrates that the task-specific ligand-capped Ag NCs have great sensitivity and selectivity for the Hg2+ detection, and also high catalytic activity in 4-nitrophenol reduction in aqueous solutions. In aqueous solution of [HS-C10mim]Br, the synthesis of Ag NCs was performed by the reduction of AgNO3 by NaBH4 at 298 K. Fig. 1A shows the TEM image of Ag NCs prepared using AgNO3 and [HS-C10mim]Br in equimolar amounts. The Ag NCs appear to be spherical in shape, and have an average diameter of 4.9 0.4 nm which falls within the size range of quantum dots with a high surface–volume ratio. Fig. 1B presents the selected-area electron diffraction (SAED) pattern of Ag NCs, and these diffraction rings could be indexed to face-centered cubic silver. The high-resolution TEM image obtained from a selected individual nanocrystal gives
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Fig. 1 (A) TEM images of [HS-C10mim]+-stabilized Ag nanocrystals. (B) The SAED pattern of Ag NCs. (C) UV-vis absorption spectrum of Ag nanocrystals in water. The insets in (A) and (C) are respectively the high-resolution TEM image of a single Ag nanocrystal and the sample picture in natural light.
clearly resolved lattice fringes, and the lattice spacing is calculated to be about 0.236 nm, which matches the [111] plane of silver metal. This lattice-fringe pattern extends over the entire nanocrystal, implying that the particle is made of a single crystal. The UV-vis absorption spectrum of Ag NCs was recorded in the aqueous suspension (Fig. 1C). It is observed that the peak centers at 420 nm with a strong absorption, and exhibits a typical localized surface plasmon resonance (LSPR) feature of Ag NCs. Further, the measurement of dynamic light scattering indicates good monodispersity of Ag NCs with an average hydrodynamic diameter of 6.6 nm in water (Fig. S1, ESI†). The presence of Ag, Br, S and N elements in the Ag NCs was confirmed by the energy-dispersive X-ray spectrum, and the component appeared as expected (Fig. S2, ESI†). Experimentally, it is demonstrated that the facile one-pot aqueous approach reported herein is highly robust and reproducible, and the resulting nanocrystals have excellent stability in aqueous solution with a shelf-time longer than 12 months at room temperature. In the synthesis strategy, [HS-C10mim]Br plays a dual role of both a stabilizer of Ag NCs and a provider of Br . However, Ag NCs are not synthesized by this route if Br is substituted for NO3 , or the alkyl side chain for a shorter one than the octyl chain. The as-synthesized colloidal Ag NCs can be directly used to detect Hg2+. First, Ag NCs were hybridized with rhodamine 6G (Rh6G) in aqueous solution, and the composite sensors were easily formed by the strong adsorption of Rh6G on [HS-C10mim]+-capped Ag NCs,43 in which Rh6G acts as the ion-sensing indicator. Then, different concentrations of Hg2+ were introduced into the sensing system, and the pH value of the resulting sample is about 2.0. The sensor sensitivity was evaluated by fluorescence spectrometry. Because of the fluorescence quenching effect of Ag NCs toward the adsorbed Rh6G, as shown in Fig. 2A, the weakest fluorescence intensity was recorded in the aqueous solution of Rh6G–Ag nanocomposite. The fluorescence intensity gradually increases with increasing Hg2+ concentration (c), and a good linear relationship of the relative fluorescence intensity versus Hg2+ concentration ranging from 1.0 10 13 to 1.0 10 4 mol L 1 is determined with a high correlation coefficient (R2 = 0.997), and follows
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Fig. 2 (A) The fluorescence spectrum of Rh6G–Ag nanocomposite at different concentrations of Hg2+ (mol L 1) at pH = 2.0: (a) 0; (b) 1.0 10 13; (c) 1.0 10 12; (d) 1.0 10 11; (e) 1.0 10 10; (f) 1.0 10 9; (g) 1.0 10 8; (h) 1.0 10 7; (i) 1.0 10 6; (j) 1.0 10 5; and (k) 1.0 10 4; where the inset shows the dependence of relative fluorescence intensity at 552 nm on a negative logarithm of the Hg2+ concentration. (B) The fluorescence response of Rh6G–Ag nanocomposite sensor toward different metal ions at the concentration of 1.0 10 4 mol L 1. (C) The dynamic equilibrium of a free [HS-C10mim]+ ion with an Ag nanocrystal, and the proposed Hg2+-sensing mechanism for the [HS-C10mim]+-stabilized silver nanocrystals.
I/I0 = 3.493 + 0.188 log c, where I0 and I are the fluorescence intensities at 552 nm in the absence and presence of the metal ion, respectively. The detection limit is 1.0 10 13 mol L 1, at least three orders of magnitude smaller than that reported previously in the literature.38,39 The selectivity of the composite sensors was further examined by fluorescence spectrometry in the presence of other metal ions. At low metal ion concentrations, no interference is observed in the Hg2+ detection. As the metal ion concentration is increased up to 1.0 10 4 mol L 1, the turn-on fluorescence intensity of Hg2+ is at least 8 times more than that of other metal ions (Fig. 2B), a well-accredited
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value for the quantitative analysis of Hg2+. Evidently, the method is sensitive and reliable, and can be applied successfully to detect ultra low concentrations of Hg2+ in aqueous solutions. The specific Hg2+-sensing mechanism results mainly from antigalvanic reduction (AGR), in which the nanometer-sized noble metal can reduce more reactive metal ions in solution.44 In this regard, Ag atoms are oxidized by the added Hg2+, and the resulting mercury deposits and forms an alloy coating on the surface of Ag NCs. Because the composition variation leads to the change in nanocrystal properties, the LSPR peak at 420 nm blue shifts slightly, and attenuates in absorption intensity (Fig. S3, ESI†). At the same time, the fluorescence intensity of Rh6G in the composite sensor increases linearly with increasing Hg2+. As the Hg2+ concentration reaches 1.0 10 3 mol L 1, the LSPR absorption peak disappears due to full AGR, and the solution for Hg2+-sensing changes from light pink to a colourless liquid. The AGR principle gives a straightforward illustration to enlighten us about the sensing mechanism, but fails to address several crucial questions: (i) how do Hg2+ cations cross the positively charged barriers of ligand shells outside Ag NCs, (ii) how AGR can be realized in the ultra low Hg2+ concentration, and (iii) why the composite sensors respond selectively to Hg2+ rather than Cu2+ that are also reduced by Ag NCs.44 Therefore, the unusual sensing mechanism should be further discussed from the roles played by the functionalized ligands. [HS-C10mim]+ ions in the sensing system could fall into three categories: the anchored ones on the surface of Ag NCs, the intercalated ones in ligand shells, and the freely diffused ones in aqueous solution. The dynamic light scattering measurement31 has shown that the free [HS-C10mim]+ ions can intercalate into ligand shells by strong hydrophobic and p–p interactions among them, and the intercalated [HS-C10mim]+ ions are in a dynamic equilibrium with the free ligand ions. The thiophilic affinity of Hg2+ makes the S atoms in the free [HS-C10mim]+ ions to discriminate the metal ions, and capture them efficiently in solution (Fig. 2C). By means of such dynamic equilibrium, the free [HS-C10mim]+ ions entrap Hg2+ into ligand shells, and condense them on the surface of Ag NCs. Because the functionalized [HS-C10mim]+ ions bind preferentially with Hg2+ and enrich them in ligand shells, Ag NCs exhibit superior reactivity of AGR even at low Hg2+ concentrations, as a result, the sensitivity and selectivity of the composite sensors are dramatically improved in the aqueous solution. The typical pseudo-homogeneous catalysis of noble nanoparticles has advantages of efficient activity and high selectivity under mild reaction conditions, and is of particular interest in reactions including water splitting, hydrogenation, and coupling reactions, etc.45–47 Here, the catalytic activity of Ag NCs was assessed by the reduction of p-nitrophenol in aqueous solution at 298 K.48–50 It has been known that the Langmuir–Hinshelwood model can be used to analyze the reduction of p-nitrophenol (Nip) by NaBH4, in which both reactants must be adsorbed onto the surface of metal nanoparticles to react. As BH4 is added into the reaction system, the aqueous solution represents a strongly alkaline solution, and converts Nip into a p-nitrophenolate anion. Therefore, both reactants are preferred to be adsorbed on the positively charged Ag NCs, and the electrostatic adsorption of [HS-C10mim]+ can promote the reaction process. However, experimental results of the control show that the reduction proceeds very slowly if the reaction mixture contains the
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free [HS-C10mim]+ ions. The restrained action of free [HS-C10mim]+ results probably due to the interactions of the imidazole ring with both reactants instead of the S atom in the [HS-C10mim]+ ion, which holds back the reactants to access the catalytic sites on Ag NCs. To achieve the optimal catalytic activity, the Ag NCs were separated by centrifugation to remove the intercalated and free [HS-C10mim]+ ions, and the resulting sediment was immediately redispersed in water as the catalyst. In the presence of the purified Ag NCs, the reaction kinetics in aqueous solution was analyzed by monitoring the changes in UV/vis spectra at different time intervals (Fig. 3A). After an induction period of 20 s, the absorption intensity of Nip at 401 nm decreased gradually with the progress of the reaction. Meanwhile, a new absorption peak of the product of p-aminophenol (Amp) appeared at 302 nm with increasing absorption intensity. Considering NaBH4 in excessive doses, the reaction should be of the pseudo-first-order with respect to the Nip concentration, and a linear
Fig. 3 (A) UV/vis spectral evolution of p-nitrophenol reduced by NaBH4 using Ag NCs as the catalyst in aqueous solutions at 298 K. (B) Dependence of ln(At/A0) on reaction time for the pseudo-first-order reaction kinetics. (C) The Langmuir–Hinshelwood mechanism for the reduction of p-nitrophenol on Ag nanocrystals with sparsely anchored ligands.
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relationship of ln(At/A0) versus reduction time is shown in Fig. 3B, where At and A0 are the Nip absorbance at time t and t = 0, respectively. From the linear plot, the apparent rate constant (kapp) was determined to be 1.25 10 2 s 1, and the activity parameter, a ratio of kapp to the mass of the catalyst, was estimated to be 2834 s 1 g 1. To our knowledge, this is the highest activity parameter for the catalytic reduction of Nip in aqueous solution reported in the literature. The positively charged ligand shell and the good compatibility of dialkylimidazolium ions with inorganic and organic compounds are advantageous to the recognition and adsorption of both reactants on Ag NCs. Assuming that the diffusion of the reactants and the desorption of the products are fast steps, the rate-determining step is the surface reaction of the adsorbed Nip with hydrogen species on Ag NCs. The heavy ligand shell outside Ag NCs is the physical barrier, unfavourable for catalysis, and the removal of intercalated ligands from Ag NCs could generate as many active sites as possible for the access of reactants. The surface potential of Ag NCs after centrifugal separation is about four times less than the as-prepared NCs. This implies that most of the ligands around the silver core are the intercalated dialkylimidazolium ions, because the electrostatic repulsion and steric hindrance among ligands prevent more dialkylimidazolium ions to anchor onto the silver core. As a result, the employed catalyst is sparsely covered by the anchored ligands, and has more exposed internal crystal faces with high accessibility. The almost bare Ag NCs are able to markedly lower the reaction energy barrier of the rate-determining step, and efficiently promote the catalytic reduction. In conclusion, the unique performance of Ag NCs, besides their size, shape, and morphology, are strongly dependent on the ligand characteristics. The functionalized dialkylimidazolium ions provide a facile green route to prepare monodisperse Ag NCs, and are able to perform the role of both stabilizer and mediator in the synthesis of Ag NCs, captor and enricher in the detection of Hg2+, or of activator and inhibitor in the reduction of Nip. This work bridges the ligand properties with the nanocrystal performance, and puts up a sensible choice to harvest multifunctional nanoparticles with enhanced performance using specifically designed ligands. The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21073055), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1061).
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