Synthesis of Highly Fluorescent Silver Clusters on Gold(I) Surface

Article pubs.acs.org/Langmuir

Synthesis of Highly Fluorescent Silver Clusters on Gold(I) Surface Mainak Ganguly,† Anjali Pal,‡ Yuichi Negishi,§ and Tarasankar Pal*,† †

Department of Chemistry, and ‡Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721302, India Department of Applied Chemistry, Tokyo University of Science, Tokyo 1628601, Japan

§

S Supporting Information *

ABSTRACT: Evolution of fluorescence from a giant core−shell particle is new and synergistic, which requires both gold and silver ions in an appropriate ratio in glutathione (GSH) solution. The formation of highly fluorescent Ag2/Ag3 clusters on the surface of AuI assembly results in giant AuIcore−Ag0shell water-soluble microparticles (∼500 nm). Here, AuI acts as the template for the generation of fluorescent Ag clusters. The presence of gold under the synthetic strategy is selective, and no other metal supports such synergistic evolution. The core−shell particle exhibits stable and static emission (emission maximum, 565 nm; quantum yield, 4.6%; and stroke shift, 179 nm) with an average lifetime of ∼25 ns. The drift of electron density by the AuI core presumably enhances the fluorescence. The positively charged core offers unprecedented long-term stability to the microparticles in aqueous GSH solution.

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silver nanoclusters. The first example of strong, size-dependent fluorescence of Ag nanoclusters prepared by photoirradiation of AgO films was reported in 2001 by Dickson and co-workers.22 It has also been reported that silver clusters containing 2−8 atoms (Ag2−Ag8) display fluorescence behavior.22,23 Not only silver or gold nanoclusters but also a few AgI complexes are found to show luminescent property specially in their solid state because of the argentophilic interaction.24 The gold(I) complex with thiosemicarbazone is also known for its fluorescent property.25 An effect arising between two or more substances that produces an effect greater than the sum of their individual effects is called as synergism. The term is widely used for nanoparticle-induced catalysis.26 A nanoalloy or core−shell structure involving two metals (Au/Ag, Au/Pd, etc.) very often demonstrates the phenomenon of synergism. However, synergism is rarely reported in fluorescence. Zhang and Zhao showed27 the fluorescence of gold and platinum to be redshifted for the Au/Pt nanocomposite. A luminescent complex containing AgI, AuI, and hypercoordinated carbon has been reported by Jia and Wang28 from single-crystal X-ray diffraction (XRD). Fluorescing entities of noble metals (i.e., noble metal nanoclusters) are generally very small. Sometimes they are sub-nanometer in size.29 To the contrary, metal-enhanced fluorescence (MEF) demands aggregated larger particles to increase the fluorescence intensity of a fluorophore. Lukomska et al.30 have proven that large aggregated colloids enhance the fluorescence to a greater extent than the smaller colloids. In the

INTRODUCTION Metal nanoparticles (MNPs) bearing dimension >2 nm became interesting candidates in the field of research for their exceptional size- and shape-dependent catalytic, optical, electrical, magnetic, and redox chemical properties, which are quite improved and unexpected from their bulk counterparts.1−5 The continuous density of states of MNPs split into discrete energy levels by further lowering of the particle size when they meet the Fermi wavelength of electrons. As a result, surprising properties compared to nanoparticles result.6−9 To differentiate these ultrasmall (≤2 nm) particles, a new vocabulary “nanocluster” has now come into view. In other words, metal nanoclusters have become “molecular species”10 that might exhibit size-dependent fluorescence emission, owing to the interband transition of free electrons upon photoexcitation in the ultraviolet−visible (UV−vis) range.7 AuI−glutathione (GSH)11−14 is a well-known intermediate for the synthesis of monolayer-protected gold nanoclusters. Upon the addition of strong reducing agents, such as NaBH4 to AuI−GSH, the gold(I)−sulfur bonds are broken and a few gold atoms aggregate to form gold clusters through aurophilic interactions.15−17 Recently, Luo et al.18 have shown the significance of the aurophilic interaction of ultrabright Au0@ AuI−thiolate core−shell nanoclusters. Employing the similar protocol, silver nanoclusters have been produced.19 In comparison to the Au nanocluster, the Ag nanocluster shows intense fluorescence that augments its utility to a large extent. Since olden times, colloidal Ag particles have been used in classical photographic processes.20 Gas-phase and low-temperature matrix-isolated Ag nanoclusters have been found for the first time to exhibit discrete absorption and fluorescence profiles.21 Photoreduction, chemical reduction, and radiolytic approaches are normally adopted to prepare water-soluble © 2013 American Chemical Society

Received: December 8, 2012 Revised: January 10, 2013 Published: January 12, 2013 2033

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(1253.6 eV) and a five-channeltron detection system. For ζ-potential measurement, a Malvern Nano ZS instrument employing a 4 mW He−Ne laser operating at a wavelength of 633 nm was used. Transmission electron microscopy (TEM) analysis was performed with a H-9000 NAR instrument, Hitachi, using an accelerating voltage of 300 kV. Synthesis of Fluorescent Solution. In a 10 mL beaker, 2 mL of 5 × 10−3 M aqueous GSH, 4 mL of triple-distilled water, 0.20 mL of 0.01 M HAuCl4, and 0.34 mL of 0.01 M AgNO3 were mixed. Immediately, a white turbidity appeared in the solution. Then, the solution was kept under ∼365 nm UV light for photoirradiation under vigorous stirring conditions. A transparent faint yellow solution (AuAgF1) was obtained after ∼10 h of UV irradiation.

context of MEF, MNPs behaved as nanoantennas frequently manipulating light and the light−matter interaction at the nanoscale. The radiative decay rate of the emitters placed in their near field31−35 can easily be influenced, owing to the existence of localized plasmon polaritons of MNPs. Two mechanisms have been anticipated for fluorescence enhancement: (a) localized surface plasmon resonance (LSPR) at the surface of the MNPs causing enhancement of the electromagnetic field and (b) coupling between the surface plasmon field of the metal and the molecular dipole of probe molecules.36 The radiating plasmon (RP) model suggests that the far-field radiation originated from scattering to be responsible for the observed fluorescence intensity.37 The extent of enhancement lies on the geometry of the metallic nanostructures. The size and shape possess different surface plasmonic modes, causing drastic enhancement of fluorescence signals at the “hotspots”.38 At a very close proximity (18 h) produces a deep yellow solution along with the successive red shifting of the fluorescence emission maximum. Furthermore, a decrease of fluorescence intensity as well as broadening of the fluorescence spectra are also noticed (Figure 2b) because of flocculation of the fluorescent Ag cluster shell. It is to be noted that, without photoirradiation, a white turbidity is obtained, which is due to the formation of a mixture of AuI−GSH and AgI−GSH species with no fluorescence property.11−17 Irradiation of the reaction mixture under visible light with a wavelength of 450, 550, and 650 nm also causes turbid nonfluorescing solution. However, we found only UV (∼365 nm) exposure to be necessary for obtaining fluorescent solution (Figure 3). To the best of our knowledge, it is the first report where Ag I clusters are synergistically produced exclusively on the AuI scaffold by the gift of a synthetic strategy under UV photoactivation. After drying the drop-casted fluorescent solution AuAgF1, a bright yellow fluorescence image (unlike the AgNF and AuNF solutions) is observed under the fluorescence microscope (Olympus DP72) with UV and blue light exposure. Green light does not show any distinct fluorescence image of the dried

AuAgF1 spots. It is noteworthy that, after drop-casting the solution on glass slides, most of the fluorescent particles are drifted toward the periphery of the spot, creating brighter circumference, owing to the surface tension of the solution bearing a giant particle. The surface energy of the particles is pivotal for such observation (Figure 4). However, when the solution was drop-casted onto a hydrophobic Bakelite sheet, a uniform distribution of fluorescent particles was observed, unlike what is observed on the hydrophilic glass slide (see Figure S2 of the Supporting Information). GSH is a typical electron donor. It is found from the literature11−14 that GSH reduces AuIII to AuI and the formation of AuI−SG takes place. The reaction of silver nitrate and GSH produces AgI−SG.19 Therefore, no change of the oxidation state of AgI occurs. This is observed because of the higher reduction potential of gold compared to silver. However, in our present work, we have irradiated the mixture of AuIII and AgI in GSH solution and obtained fluorescing AuAgF1 solution. From XPS analysis under freeze-drying conditions, we found that Au is always in the +1 oxidation state and Ag is always in the 0 oxidation state. The peak of binding energy at ∼85 eV corresponds to AuI 4f7/2.42 For the case of silver, the binding energy has been found to be 368.5 eV, which corresponds to Ag0 3d5/2.43 The little shift (0.5 eV) of the binding energy toward a higher energy is due to the AuIcore−Ag0shell structure,

Figure 2. (a) Fluorescence intensity of AuAgF1 solution (in days) at 565 nm (emission maximum). (b) Effect of UV exposure (in hours) on the fluorescence behavior of AuAgF1 solution. 2035

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Figure 4. (a) Optical and (b−d) fluorescence images of AuAgF1 solution observed with different light (b, UV; c, blue light; and d, green light). (e) Optical and (f) fluorescence images of the periphery of the spot on the glass slide observed with UV light. Fluorescence images of the (g) AuNF and (h) AgNF solutions observed with UV light.

Figure 5. XPS spectra for AuAgF1, AuNF, and AgNF solutions under freeze-dried conditions.

Figure 6. XPS spectra of the solution containing GSH, AgNO3, and HAuCl4 at pH 10 after UV irradiation.

elimination of the S−H stretching vibration47 of GSH (∼2530 cm−1) for the AuAgF1 solution. At higher pH, deep yellow virtually nonfluorescent solution is produced, originating from gold and silver precursors in the presence of GSH under UV light exposure. From XPS spectra of the deep yellow solution, it is found that gold and silver are both in a 0 oxidation state unlike fluorescent AuAgF1 (Figure 6). Figure 7 displays the SEM image of the particles present in AuAgF1 solution. The particles are all spherical with tight size distribution. Energy-dispersive analysis of X-rays (EDAX) spectra (Au/Ag ≪ 1), line mapping, and area mapping of a single particle speak in favor of the AuIcore−Ag0shell structure (see Figure S4 of the Supporting Information). TEM images (Figure 8) also reveal that the fluorescent particles represent the AuIcore−Ag0shell structure. The EDAX spectrum again supports the facts. With the increase of the precursor silver concentration (above AuIII/AgI = 1:0.5),

where Ag0 in the shell is strained in the presence of AuI in the core. Consequently, after the formation of AuI, further reduction to Au0 is stopped, while AgI is converted to Ag0. A spontaneous disproportionation of AuI is not thermodynamically allowed because of a huge negative reduction potential of E0[AuI/Au0]Cl−.44 When AuIII or AgI is individually irradiated in the presence of GSH for AuNF and AgNF solutions, the same redox phenomenon is observed; i.e., gold is in the +1 oxidation state, and silver goes to the 0 oxidation state. X-ray photoelectron spectroscopy (XPS) spectra reveal that a ∼163.9 eV (for S2p3/2, higher than reduced GSH)45 binding energy of all three solutions (AuAgF1, AuNF, and AgNF) is due to the formation of disulfide linkage.46 As a corollary of the reduction of the noble metal, GSH becomes oxidized to GSSG (oxidized) species, which renders stability to the cluster. Therefore, disulfide linkage is generated (Figure 5). Infrared (IR) spectra (see Figure S3 of the Supporting Information) also indicate the 2036

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Figure 7. (a) FESEM image, (b) EDAX and area mapping for the elements Au, Ag, and S, and (c) line mapping for the elements Au, Ag, and S of a single spherical particle of the drop-casted AuAgF1 solution.

It is reported48 that the sequence of the addition of gold(III) and silver(I) ions has a great role to form the core−shell (Aucore−Agshell) and inverted core−shell (Agcore−Aushell) structures. We have adopted three different sequences of addition: (1) A mixture of AuIII and AgI is prepared in aqueous GSH solution. Then, the mixture is exposed to UV light for 10 h (AuAgF1). (2) At first, AuIII and GSH are mixed together and exposed to UV light. After 5 h of UV exposure, AgI is introduced and again exposed to UV light for another 5 h (AuAgF2). (3) AgI and GSH are mixed together and exposed to UV light. After 5 h of UV exposure, AuIII is introduced and exposed to UV light further for 5 h (AuAgF3). In all three cases, we obtained normal core−shell [Aucore− Agshell] particles of ∼500 nm in size. Besides, for all of the cases, the same emission maximum is obtained with the same fluorescence intensity. AuAgF2 reaches the highest fluorescence intensity in 7 h. To attain the maximum emission intensity, AuAgF1 needs 10 h and AuAgF3 needs 15 h. The shape and

spherical core−shell particles start to develop. Further increment of the precursor AgI concentration encases the AuI core with a thicker Ag0 shell, and obviously, the particle size increases as expected. The Au/Ag ratio of 1:1.7 (AuAgF1) produces ∼500 nm core−shell particles with the highest fluorescence intensity. Surprisingly, when precursor AuIII/AgI > 2.5, the core−shell particle is not at all produced. Rather, assemblies of small particles are obtained. TEM images indicate the Aucore−Agshell nature of the particle. EDAX measurement (TEM as well SEM) in different regions indicates that the atomic percentage of silver is ∼24 times higher than that of gold. However, the precursor AgI is only 1.7 times higher than the precursor AuIII. It indicates that the gold is inside; i.e., gold is in the core, and silver is in the shell. EDAX analyses only justify the nature of the outer surface. Abundance of silver from EDAX indirectly supports the Aucore−Agshell nature of the particle. 2037

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Figure 8. TEM image of (I) AuNF and (II) AgNF. (III) TEM image [a, b, c, d1 (AuAgF1), and e] of the fluorescent solutions obtained with variable HAuCl4 and AgNO3 (added simultaneously) ratios after photoirradiation. TEM image of d2 (AuAgF2) and d3 (AuAgF3) where precursor salts are introduced at different time intervals, and their ratio remains the same as in d1. (IV) Selected area electron diffraction (SAED) and (V) EDAX of AuAgF1.

Figure 9. Fluorescence spectra of the pale yellow solution with the variation of (a) silver keeping the gold concentration constant, (b) gold keeping the silver concentration constant, (c) concentration of gold and silver keeping the ratio of gold and silver constant, and (d) GSH keeping gold and silver constant. Fluorescence spectra of AuAgF1 solution recorded with (e) variable excitation energy and (f) variable temperature conditions. The inset represents the profile of the decrease of fluorescence by heating and the recovery of fluorescence by cooling the hot solution.

core−shell nature of the particle virtually remain unaltered, indicating the habitual particle growth, which is not affected by the sequence of the addition of the constituents. In other

words, all of the solutions (AuAgF1, AuAgF2, and AuAgF3) contain similar core−shell particles with GSH and GSSG capping, but their growth kinetics differs. 2038

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The precursor concentration ratio of AgI/AuIII = 1.7:1 shows a maximum fluorescence intensity. A further increase of the AgI concentration in the reaction mixture causes quenching of the fluorescence intensity because of the presence of unreduced AgI as a result of spin−orbit coupling associated with a heavy atom effect.49 Keeping the ratio constant, an increased concentration of the two metals causes a decrease of the fluorescence intensity after a certain limit. The optimum precursor gold(III) and silver(I) concentrations are 3.33 × 10−4 and 5.66 × 10−4 M, respectively. If the AgI/AuIII ratio is increased to 3:1, a gray mass at the bottom of the exposed solution is obtained with a significant decrement of fluorescence intensity of the solution. The GSH concentration is also crucial in this context. The ratio of GSH/AgI/AuIII = 5:1.7:1 shows the highest fluorescence intensity. An increase or decrease of the GSH concentration results in the reduction of the fluorescence intensity in either case. A lower GSH concentration causes the evolution of the plasmon band of metal. To observe a maximum fluorescence intensity for the exposed solution, an excitation wavelength of 385 nm is suitable. Above and below that excitation wavelength, the fluorescence intensity is decreased. It is noteworthy that, with the variation of the excitation wavelength, the emission maximum is not altered. It indicates monodispersity of the particle50 produced by this UV photoactivation technique. When the temperature of the solution is increased, the emission intensity is reduced mainly because of Brownian motion. Now, when the solution is cooled, the fluorescence intensity is increased. However, the cooling effect cannot bring back the intensity of the original solution, owing to the destruction of some fluorescent Ag2 and Ag3 clusters by heat (heating causes aggregation) (Figure 9). GSH denotes the ubiquitous tripeptide GSH (γ-Glu-CysGly).51 GSH is an assembly of three amino acids: (1) glycine, (2) cysteine, and (3) glutamic acid. Instead of GSH, the introduction of individual amino acid components of GSH as well as a mixture of all three components does not produce pale yellow fluorescent solution. In the case of cysteine, we have found pale yellow and deep yellow solutions and orange white precipitate at different stages of UV irradiation. However, fluorescent solution is not obtained (Figure 10). The pH of the reaction mixture (containing GSH, silver nitrate, and chloroauric acid) that produces highly fluorescent solution (AuAgF1) after photoirradiation is 5.5 ± 0.3. Lowering the pH (9.0) using NaOH and subsequent UV exposure causes a remarkable decrease in the fluorescent intensity of the solution. At lower pH, a black precipitate is thrown at the bottom of the faint yellow solution. Consequently, hydrosol gradually destabilizes with the gradual decrease of pH. Black precipitation indicates protonation of the capping agent and, hence, destabilization of the micrometer size particles. At higher pH, the fluorescing Ag2 and Ag3 formations are not observed because of the agglomeration in the presence of plentiful OH−, and deep yellow solution results. It is worth noting that, in both of the cases (higher and lower pH), the core−shell is not produced. The change of pH from 5.5 to lower and higher sides causes peak broadening and a decrease in emission intensity (Figure 11). The fluorescence comes from the smaller clusters, which are present as the shell. However, the Aucore−Agshell particles as a whole are large in size, and fluorescence comes only from very small Ag2 and Ag3 clusters located on the AuI surface. The giant particles are purified by repeated centrifugation and washing

Figure 10. Fluorescence spectra of the photoirradiated solutions containing precursor salts (HAuCl4 and AgNO3) showing the effect of individual component of GSH. Conditions: GSH concentration, 1.65 × 10−3 M; concentration of individual components of GSH, 1.65 × 10−3 M; AuIII concentration, 3.33 × 10−4 M; and AgI concentration, 5.66 × 10−4 M.

with triple-distilled water. Again, from the TEM analysis, we have found that only large core−shell particles and fluorescence microscopy has revealed the giant fluorescent particles. The evolution of the giant fluorescent particle is synergistic. The time evolution of fluorescence includes two exponential components similar to other observations,52 as shown Figure 12. After 10 h of UV irradiation, it is found for AuAgF1 and AuAgF2 that ∼55% of particles possesses a lifetime of 48.17 ns (slow component) and 45% of particles has a lifetime of 3.5 ns (fast component). However, for AuAgF3, 71% of particles is with a short lifetime. It indicates that, after completion of the formation of fluorescent species in solution, the percentage of particles having a higher lifetime is increased. The average lifetimes calculated by τ = (A1τ1 + A2τ2)/(A1 + A2) of the three solutions AuAgF1, AuAgF2, and AuAgF3 are 24.9, 22.3, and 12.05 ns, respectively, after 10 h of UV exposure. To the pale yellow fluorescent solution, the addition of sodium borohydride destroys the fluorescence property with the evolution of the plasmon band (reddish yellow coloration) of Au0 and Ag0, along with the rupture of the core−shell structure. When the solution is aged for ∼6 days after NaBH4 addition, the solution turns pale yellow and the core−shell moiety is again generated. Probably, the AuIcore−Ag0shell structure reappears as a consequence of slow decomposition of sodium borohydride with time and because GSH is present in a plentiful amount in this reaction mixture. However, the fluorescence property is not restored because Ag2 and Ag3 clusters become agglomerated irreversibly as soon as borohydride is introduced (Figure 13). Again, the controlled addition of sodium cyanide (cyanide dissolution test)53 to the ∼6-day-old yellow solution could not fetch red-colored 2039

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Figure 11. (a) TEM micrographs of particles obtained from the aqueous solution containing GSH, silver nitrate, and chloroauric acid after ∼365 nm light exposure at different pH values. (b) UV−vis and (c) fluorescence spectra of the aqueous solution containing GSH, silver nitrate, and chloroauric acid after irradiation with ∼365 nm light at different pH values. Conditions: GSH concentration, 1.65 × 10−3 M; AuIII concentration, 3.33 × 10−4 M; and AgI concentration, 5.66 × 10−4 M.

Figure 12. (a) Fluorescence spectra and (b) lifetimes of AuAgF1, AuAgF2, and AuAgF3 solutions.

solution. This authenticates the presence of AuI in the core and not Au0.48 To unequivocally but indirectly prove the synergistic effect of gold and silver, the effect of other metal ions were tested: in one case, silver was substituted, and in the other case, gold was substituted, by a number of other metal ions. It is to be noted that, in either case, fluorescent solution is not generated (Figure 14). The synergistic effect is a well-announced vocabulary in catalysis for dramatic improvement in catalytic activity, selectivity, stability, etc., especially with bimetallics.54 However, altogether, a new example of synergism is provided for the fluorescence phenomenon, hitherto unknown, where gold and silver produce giant fluorescent AuIcore−(Ag2/Ag3)shell particles in the micrometer size. The synergistic evolution of a fluorescent Ag2/Ag3 cluster as the shell in the presence of a AuI core may unveil new insight for the basic understanding of fluorescent clusters. The drift of electron density from the fluorescent shell to the core presumably enhances its fluorescence and as a whole stability of the microparticles.41 The mechanism of synergism is still a point of research. The structural change by bimetallic systems inducing the modification of lattice parameters, the dilution of one type of

atom by another on the metal surfaces, the transfer of electronic charge between different metal atoms, and surface decoration and formation of boundaries between two metal phases are very often attributed for such a situation.55,56 In the present study, the core AuI present in the giant nanocluster becomes pivotal for ejecting Ag2 and Ag3 (responsible for fluorescence) from the silver shell. Reduction of both gold and silver ions in aqueous GSH solution caused the evolution of giant particles, and eventually, the particles become fluorescent because of the fluorescing small fluorescent silver clusters on the AuI core. The nanoparticle surface is proven to be important for the evolution and survival of Ag2 clusters as reported by Maretti et al.41 We found that under UV light exposure, AuIII and AgI are reduced individually to AuI and Ag0 in the presence of aqueous GSH solution. Again, we obtained AuI and Ag0 from the reaction mixture containing both AuIII and AgI in GSH solution upon UV exposure. Several studies41,57−59 reveal that an excess positive charge is important to stabilize the silver clusters. Therefore, the Au+ surface supports the generation and stabilization of Ag2 and Ag3 clusters in the present case. A comparable binding energy of Ag−Ag and Ag−Au may also be 2040

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for the evolution of fluorescent silver clusters, both gold and silver are essential. The absence of any one of them does not generate the fluorescent silver cluster in our study. It is a phenomenon of synergism for the evolution of large AuIcore− Ag2/Ag3 shell particles.

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CONCLUSION We have reported a completely new water-soluble and highly stable fluorescent AuIcore−Ag0shell particles in the micrometer size regime in aqueous GSH solution as a result of UV photoactivation. The shell structure is composed of wellrecognized fluorescent Ag2/Ag3 clusters, which encase the AuI core. Then, fluorescence brilliance emerges out of the giant (∼500 nm) and robust core−shell microparticles in dispersion, where synergism of gold−silver is authenticated. Here, AuI behaves as an intriguing template, which helps to generate fluorescent Ag2/Ag3 on its surface. The fluorescence is presumably enhanced because of the electron acceptor AuI core and fluorescing silver clusters. Further in depth experimentation can only show the architectural view of the microparticle. The new fluorescent AuIcore−Ag0shell microparticle may prove to be an intriguing functional material with potential applications in imaging and nanophotonics. The outstanding long-term stability, excellent water solubility, and fluorescence of such microparticles would bring newer ideas for key applications.

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ASSOCIATED CONTENT

S Supporting Information *

MALDI mass spectra (Figure S1), optical and fluorescence images (Figure S2), IR spectra (Figure S3), and SEM image with elemental mapping (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 13. Effect of the addition of NaBH4 to the AuAgF1 solution. (a) Fluorescence spectra with (I) 0 M, (II) 2 × 10−4 M, (III) 5 × 10−4 M, (IV) 8 × 10−4 M, and (V) 10−3 M NaBH4. (b) Information obtained for AuAgF1 solution, while the NaBH4 concentration is 10−3 M. Results for (I) instantaneous addition and (II) after ∼6 days of aging.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

a factor for the long-term stability of fluorescent Ag2 clusters on the AuI surface with partial charge transfer.60 The core−shell particle is further stabilized by GSH in solution. In other words,

Notes

The authors declare no competing financial interest.

Figure 14. (a) Comparative fluorescence intensity of the reaction mixtures with different metal salts employed individually with AuIII in GSH solution after photoactivation. Conditions: GSH concentration, 1.65 × 10−3 M; other metal concentration, 5.66 × 10−4 M; and AuIII concentration, 3.33 × 10−4 M. (b) Comparative fluorescence intensity of the reaction mixtures with different metal salts employed individually with AgI in GSH solution after photoactivation. Conditions: GSH concentration, 1.65 × 10−3 M; other metal concentration, 3.33 × 10−4 M; and AgI concentration, 5.66 × 10−4 M. 2041

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ACKNOWLEDGMENTS The authors are thankful to the UGC, DST, NST, BRNS, and CSIR, New Delhi, India, and the IIT, Kharagpur, India, for financial assistance. The authors are also thankful to Isozaki of Tokyo University of Science, Tokyo, Japan, for XPS measurement.

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