Ag3)-Thiolate Giant Cluster

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Photoproduced Fluorescent Au(I)@(Ag2/Ag3)‑Thiolate Giant Cluster: An Intriguing Sensing Platform for DMSO and Pb(II) Mainak Ganguly,† Chanchal Mondal,† Jayasmita Jana,† Anjali Pal,‡ and Tarasankar Pal*,† †

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, West Bengal, India Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, West Bengal, India

‡

S Supporting Information *

ABSTRACT: Synergistic evolution of fluorescent Au(I)@(Ag2/Ag3)-thiolate core−shell particles has been made possible under the Sun in presence of the respective precursor coinage metal compounds and glutathione (GSH). The green chemically synthesized fluorescent clusters are giant (∼600 nm) in size and robust. Among all the common water miscible solvents, exclusively DMSO exhibits selective fluorescence quenching (Turn Off) because of the removal of GSH from the giant cluster. Again, only Pb(II) ion brings back the lost fluorescence (Turn On) leaving aside all other metal ions. This happens owing to the strong affinity of the sulfur donor of DMSO for Pb(II). Thus, employing the aqueous solution containing the giant cluster, we can detect DMSO contamination in water bodies at trace level. Besides, a selective sensing platform has emerged out for Pb(II) ion with a detection limit of 14 × 10−8 M. Pb(II) induced fluorescence recovery is again vanished by I− implying a promising route to sense I− ion.

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INTRODUCTION

Dimethyl sulfoxide (DMSO) is not only a well-known organic solvent but also a natural product. DMSO has also been obtained in foods and beverages (wine, coffee, and tea) at the micromolar level.17,18 Dimethyl sulfide (DMS), the reductive product of DMSO, is inauspicious for foods or beverages owing to its offensive smell. In quality control, DMSO is measured as a potential source of DMS. Usually, a purge and trap-GC method has been employed for the determination of DMSO.19 For aqueous DMSO samples, an electrochemical enzymatic biosensing approach has been found as an analytical method.20−22 Again, Pirsa and Alizadeh have reported the fabrication of the DMSO selective gas sensor.23 Pb(II) is considered as one of the most toxic heavy metal cations. Ingestion of even very small amounts of Pb(II) creates numerous health problems such as anemia, memory loss, and slow nerve conduction velocity in children.24 The CDC (Centers of Disease Control) reports that ≥100 mg/L (0.48 mM) Pb concentration in the blood is noxious to children.25 Selective and quantitative detection of Pb(II) in trace levels in water at neutral pH is therefore an emergent issue. Design of fluorescent receptors for Pb(II) ion has attracted much

Metal nanoparticles with a size ≤2 nm, termed as metal clusters consisting of a few to several hundred metal atoms, have attracted much attention in recent years owing to their exceptional physical, electrical, and optical properties in bioconjugation, catalysis, nanodevices, imaging, and sensing.1−6 Among the various metal clusters reported to date, Au and Ag clusters warrant special attention because of their nobility, nontoxicity, and intense fluorescing behavior. Though Au clusters have been comprehensively investigated for the ease of preparation and chemical stability, fluorescent Ag clusters are coming in a big way as a promising probe due to stronger fluorescence than Au clusters.7,8 To date, stable fluorescent silver clusters synthesis is a great challenge. Only a few groups have reported stable fluorescent silver clusters using different synthetic tricks.9,10 Fluorescent Ag clusters are widely employed for sensing purposes. Not only hazardous metal ions [Hg(II),6 Cu(II),11 Cr(III),12 etc.] but also biothiols [cysteine,13 homocysteine, glutathione,14 etc.] are detected by using fluorescent silver clusters. Detection of recognition sites for target DNA strands and communication between these two components can also be carried out.15 Biosensing is one of the magnificent applications of fluorescent silver clusrers.16 © 2013 American Chemical Society

Received: October 5, 2013 Revised: December 18, 2013 Published: December 20, 2013 348

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Scheme 1. Exclusive Fluorescence Quenching of Fluorescent Au(I)@(Ag2/Ag3)-Thiolate Particle by DMSO and Rebirth of Lost Fluorescence Selectively by Pb(II)a

a

Again, Pb(II) induced fluorescence is destroyed by Na2-EDTA. Sigma-Aldrich. Solvents were used after purification via distillation. All glassware was cleaned with freshly prepared aqua regia, rinsed with a plentiful amount of distilled water, and dried before use. UV−vis absorption spectra were recorded by a SPECTRASCAN UV 2600 digital spectrophotometer (Chemito). The fluorescence measurements were carried out employing a LS55 fluorescence spectrometer (Perkin-Elmer). Fluorescence images were obtained with an Olympus DP72 fluorescence microscope. Matrix assisted laser desorption ionization (MALDI) was done with a time-of-flight spectrometer (Perkin-Elmer). An electrospray-ionization mass spectrometer (Water Corporation, Milford, MA, USA) was used to obtain the ESI-MS spectrum. A VG Scientific ESCALAB MK II spectrometer (UK) equipped with a Mg Kα excitation source (1253.6 eV) and a five-channeltron detection system was involved for X-ray photoelectron spectroscopy (XPS) analysis. Particle morphology was recorded with a field emission scanning electron microscope (Supra 40, Carl ZEISS Pvt. Ltd.). Transmission electron microscopy (TEM) analyses were performed using an H-9000 NAR instrument (Hitachi) having an accelerating voltage of 300 kV. Synthesis of Au(I)@(Ag2/Ag3)-Thiolate Giant Cluster. A stock solution of 5 × 10−3 M GSH (reduced) was kept reserved by dissolving an appropriate amount of solid GSH in 100 mL of triple distilled water. In a 100 mL beaker, 34.5 mL of triple distilled water, 20 mL of 5 × 10−3 M freshly prepared GSH solution, 2.0 mL of 10−2 M HAuCl4, and 3.4 mL of 10−2 M AgNO3 were mixed thoroughly. Within a minute, a white curdy precipitate was obtained. The reaction mixture with white precipitate was stirred with a magnetic stirrer at 2400 rpm for 12 h under sunlight. During stirring, the temperature of the solution was kept constant at 30 °C using a water jacketed thermostat. Irradiation of the reaction mixture for ∼12 h caused a homogeneous faint yellow colored solution that was strongly fluorescent. Then, ∼60 mL of exposed solution was centrifuged at ∼15 000 rpm and the obtained yellow mass was washed several times with triple distilled water. Finally, the yellow mass was redispersed in ∼60 mL of triple distilled water and a highly fluorescent (λex = 400 nm, λem = 564 nm) solution was obtained. Further experimental evidence revealed that the fluorescent solution bears spherical giant (600 nm diameter) clusters of Au(I) core and (Ag2, Ag3) shell. In the rest of the article, this solution has been termed as AuAgF. The strategy is simple and highly reproducible. We always obtained the same fluorescent solution by repetitive manipulation of the same experimental protocol.

consideration because of their simplicity and rapidity in detection. Early reported Pb(II) receptors, however, suffer from a number of problems: (i) poor selectivity for Pb(II),26−28 (ii) workable in organic media,29−31 and (iii) long time for sensing (>10 min).32,33 Another desirable property for precise sensing is the ratiometric response of fluorescence to Pb(II).34,35 Employing fluorescence, selective determination of a trace amount of DMSO in water is not reported to the best of our knowledge. We, for the first time, demonstrate that the highly fluorescent Au(I)@(Ag2/Ag3)-thiolate core−shell particle loses its fluorescence selectively with the successive addition of DMSO. Other water miscible solvents (DMF, acetone, ethanol, methanol, isopropanol, etc.) greatly enhance the fluorescence of the particle. The bond between the sulfur and oxygen atom in DMSO is the intermediate of a polarized double bond and a dative bond.36 The S−O interaction offers an electrostatic aspect with significant dipolar character causing negative charge centered on oxygen. Such intense negatively charged oxygen in DMSO helps easy replacement of GSH from the positively charged giant clusters (evident from the zeta potential value). Substitution of large GSH by small DMSO is also favorable from steric ground. Oxygen in DMSO reduces the positive charge of giant clusters with substantial decrement of fluorescence as reported by Maretti et al.37 Among the water miscible solvents, only DMSO quenches fluorescence. Again, introduction of Pb(II) exclusively regenerates the lost fluorescence. Pb(II) binds DMSO owing to the strong affinity of Pb(II) with S. Then, the rebirth of fluorescence is noticed because GSH becomes free and takes its original position. Such Turn-Off-On fluorescence has been employed to design highly selective DMSO as well as Pb(II) sensor in solution (Scheme 1).

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EXPERIMENTAL SECTION

Materials and Instruments. All the reagents used throughout the experiment were of AR grade. Triple distilled water was employed during the experiment. Chloroauric acid (HAuCl4), silver nitrate (AgNO3), glutathione (GSH), oxidized glutathione (GSSG), lead nitrate, and all the solvents including DMSO were obtained from 349

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RESULTS AND DISCUSSION A highly fluorescent large (∼600 nm) Au(I)@(Ag2/Ag3)thiolate core−shell particle (AuAgF) has been synthesized from Ag(I) and Au(III) salts in the presence of glutathione (GSH) via a green chemical approach employing solar irradiation. The fluorescent solution is highly stable, and the emissive property remains unaltered with respect to intensity and λem for >3 months. XPS spectra reveal that gold and silver are in +1 and zero oxidation state, respectively. The TEM image indicates that Ag(0) is present on the Au(I) surface, producing a large Au(I)core-Ag(0)shell particle. Residual chloride probably offers stabilization for Au(I) species along with other anions. Again, ∼9 times higher silver concentration than gold (precursor [Ag(I)]:[Au(III)] = 1.7:1) on a single particle in energy dispersive X-ray analysis (EDAX) supports the core−shell nature, as EDAX analysis only depicts surface information (discussed later). Plenty of silver and trace gold obtained from the elemental mapping (Figure S1, Supporting Information) of a single particle also indicate the core−shell nature. It is reported that silver particles with 2−8 atoms bring unique fluorescence due to interband transition.38,39 MALDI mass analysis (Figure S2, Supporting Information) has confirmed the presence of tiny Ag2 and Ag3 clusters that are present on the surface of Au(I), making the large Au(I)core-Ag(0)shell particles as a whole fluorescent, and it is convenient to term the large core−shell fluorescent particles as giant clusters. As MALDI mass analysis is a hard ionization technique, we have also performed electrospray ionization mass spectral (ESI-MS) analysis that further supports Ag2 and Ag3 cluster formation (Figure S3, Supporting Information). No plasmon band of noble metals (Figure S4, Supporting Information)40 and the emission maximum at ∼565 nm (matches well with the emission peak of tiny Ag clusters)37,41 also speak in favor of the presence of a fluorescent cluster. Usually, fluorescent silver clusters are unstable. Maretti et al.37 have reported that fluorescent silver clusters gain stability by the positive environment. Our as-prepared silver clusters (zeta potential, +11.9 mV indicates the surface to be positively charged), located on a positively charged Au+ surface, are highly stable presumably due to the drift of electron density from silver. It is sunlight that contributes adequate activation energy for reducing Au(III) and Ag(I) to Au(I) and Ag(0), respectively, for the synthesis of fluorescent giant clusters. The reaction was carried out at a constant temperature (30 °C) condition under the Sun. Luo et al.42 have nicely depicted the formation of ultrabright Au(0)@Au(I)-thiolate core−shell particles by heating at ∼70 °C. However, heating our reaction mixture containing GSH, HAuCl4, and AgNO3 at such a temperature, we could not generate fluorescent giant clusters. To have a plausible mechanism for the formation of a Au(I)core-(Ag2/ Ag3)shell-thiolate giant cluster, we have independently exposed Au(III) and Ag(I) in presence of the GSH under the Sun at a constant temperature (30 °C). In both cases, reduction of metal ion has been observed but the evolution of fluorescent solution is not obtained. Au(III) is converted to Au(I), while Ag(I) is transformed to Ag(0) as found from XPS analysis (Figure S5, Supporting Information). The oxidized form of GSH, i.e., GSSG, remains present in solution as a corollary of reduction of metal ions.43 When Au(III), Ag(I), and GSH are simultaneously exposed, a Au(I)core-Ag(0)shell-thiolate giant cluster is formed. Due to higher reduction potential for gold than silver, Au(I)-SG is formed immediately and Ag(I) (present in a large

amount) causes aggregation of Au(I)-SG due to the electrolytic effect. Then, Ag(I) is adsorbed onto Au(I)-SG species and sunlight irradiation helps the reduction of Ag(I) to Ag(0) in the GSH martix. Au(I) at the core bestows long-term stability to the Ag(0) (in the form of Ag2 and Ag3 fluorescent clusters) anchored to the surface by drifting electron density from the silver (similar to the proposition of Maretti et al.37). Thus, large particles are evolved with strong fluorescence. The particles have been characterized by different spectroscopic studies. The exact architecture of the particles is difficult to ascertain, as they are in solution. Moreover, chloride, GSH, GSSG, and Ag(0) clusters cojointly stabilize the Au(I) core. Thus, only the molar ratio cannot provide the exact architecture of the shell. Moreover, silver clusters are very small having a larger surface area. Thus, they can cover large gold spheres. An abundance of silver and a minute amount of gold in EDAX and elemental mapping in the AuAgF solution confirm the fact. Again, we have synthesized a fluorescent cluster by solar irradiation not only in water but also in different water−DMSO molar ratios keeping gold, silver, and GSH concentration unaltered. Increased DMSO concentration renders decreased fluorescence with gradual red-shifted emission peaks (Figure 1A). Besides, the color of the solution changes from faint

Figure 1. (A) Fluorescence spectral profile (λex = 400 nm) of fluorescent particle synthesized green chemically using Au(III), Ag(I), and GSH in different water−DMSO molar ratios. (B) Digital image of the fluorescent solution at different water−DMSO molar ratios. Condition: [Ag(I)] = 5.2 × 10−4 M, [Au(III)] = 3 × 10−4 M, [GSH] = 1.47 × 10−3 M.

yellow to deep yellow and finally to red with the decrease of [H2O]/[DMSO] molar ratio, as depicted in Figure 1B. This observation is unique for DMSO and has encouraged us to design a fluorescence based DMSO (highly miscible in water) sensor to detect its contamination in water. Addition of different water miscible solvents in aqueous fluorescent AuAgF solution causes a dramatic increase in fluorescence except DMSO. In this context, it is mentionable that giant clusters do not show any virtual change of emission maxima in different solvents unlike the report of Diez et al.44 pronouncing the robust nature of the particle regarding fluorescence behavior. Water with a high dielectric constant 350

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Figure 2. I. (A) Fluorescent spectral profile (λex = 400 nm) of 0.5 mL of AuAgF aqueous solution in 2 mL of different water miscible solvents. (B) Bar diagram indicating the fluorescent intensity of 0.5 mL of AuAgF aqueous solution in 2 mL of different water miscible solvents. II. Fluorescent spectral profile (λex = 400 nm) indicating the effect of DMSO concentration on the fluorescence of 0.5 mL of AuAgF aqueous solution: (A) DMSO/ H2O (molar ratio) ≤ 0.01; (B) DMSO/H2O (molar ratio) ≥ 0.01; (C) extent of fluorescence quenching with DMSO concentration.

intensity of aqueous AuAgF solution by ∼34 times. In other words, among the different water miscible solvents, only DMSO causes efficient quenching of fluorescence of the aqueous AuAgF solution. To the best of our knowledge, it is the first report where the fluorescent cluster has been shown to lose its fluorescence in the presence of DMSO. Here, DMSO has been employed as a reagent and not as a solvent. Such observation has prompted us to design a highly selective as well as sensitive DMSO sensor. If a trace amount of DMSO contaminates water, one can detect it selectively at ease by

contributes significantly to intermolecular hydrogen bonds that reduce emissive behavior by facilitating energy loss. Figure 2(I) represents the change in the fluorescence intensity when 0.5 mL of aqueous AuAgF solution is added to different 2 mL of water miscible solvents. The increase in fluorescence (with respect to water) has been recorded reproducibly as follows: acetone ∼4.8 times, acetonitrile ∼3.8 times, ethanol ∼3.6 times, isopropanol ∼3.4 times, methanol ∼4.2 times, and DMF ∼3.5 times increase in fluorescence with respect to pure water. To the contrary, water miscible DMSO reduces the fluorescence 351

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Figure 3. (A−E) Fluorescent spectral profile (λex = 400 nm) showing DMSO induced fluorescence quenching of fluorescent Au(I)@(Ag2/Ag3)thiolate particles in different water miscible solvents at different DMSO, solvent molar ratios. (F) A comparative account showing DMSO induced fluorescence quenching of fluorescent Au(I)@(Ag2/Ag3)-thiolate particles in different water miscible solvents.

monitoring the fluorescence quenching of a giant Au(I)@(Ag2/ Ag3)-thiolate cluster. Even when DMSO molecules present in water are 4.09 × 10−5 times the number of water molecules, it can be easily detected by employing a Au(I)@(Ag2/Ag3)-thiolate giant cluster. With the increase of DMSO concentration, the fluorescence of giant clusters is monotonously decreased. Such a scenario is found until DMSO:H2O [molar ratio] = 0.01:1 [Figure 2II(A)]. DMSO detection by our prescribed tactic via fluorescence quenching can be made possible in water when the molar concentration of water is 2.4 × 104 to 1 × 102 (a vast range) times higher than DMSO and the lower detection limit (LOD) is 178 ppm. It is astounding to state that a further increase of DMSO concentration [DMSO/H2O (molar ratio) > 0.01] results in fluorescence enhancement along with a gradual red shift. When DMSO/H2O (molar ratio) = 0.08, the fluorescence intensity reaches its maximum with ∼29 nm red shift. Again, it decreases

with the further increase of DMSO level in water [Figure 2II(B)]. Fluorescent Au(I)@(Ag2/Ag3)-thiolate particles in AuAgF solution are highly robust in nature. They can be obtained as a deliverable solid after vacuum drying. Such a solid can also be dispersed in different water miscible solvents to obtain fluorescent solution in varied solvent systems. Of course, the fluorescence intensity is hugely enhanced in various nonaqueous water miscible solvents (Figure 3). Again, addition of DMSO to such solutions monotonously decreases the fluorescence without any anomalous behavior as obtained for water beyond DMSO/solvent (molar ratio) = 0.01. This phenomenon broadens the utility of Au(I)@(Ag2/Ag3)-thiolate to detect contamination of DMSO not only in water but also in various non-aqueous miscible solvents (acetone, acetonitrile, ethanol, methanol, isopropanol, etc.). However, when the number of solvent molecules is more than 250 times that of 352

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Figure 4. Fluorescence spectral profile (λex = 400 nm) showing the effect of temperature on (A) aqueous AuAgF and (B) DAuAgF solution [DMSO:H2O (molar ratio) = 8.29 × 10−4:1 with 0.5 mL of AuAgF solution]. (C) Temperature dependent fluorescence intensity alteration with respect to the fluorescence at room temperature (30 °C) of AuAgF and DAuAgF solution during heating as well as cooling.

Figure 5. (A) Fluorescence spectral profile (λex = 400 nm) showing the effect of different metal ions on DAuAgF solution. (B) Bar diagram showing the effect of different metal ions on DAuAgF solution. Condition: DAuAgF solution - DMSO:H2O (molar ratio) = 0.01:1 with 0.5 mL of AuAgF solution, [Mn+] = 9.5 × 10−4 M.

DMSO, no virtual quenching in fluorescence is noticed. Thus, sensitivity is decreased in non-aqueous organic solvent. With the increase of temperature of AuAgF solution, the fluorescence intensity is decreased gradually due to enhanced Brownian motions45 that assist energy loss through dynamic quenching by the solvent molecule. Again, cooling almost completely brings back the lost fluorescence. Little loss of fluorescence after cooling of the preheated solution is because of the permanent loss of a few clusters by irreversible aggregation. For the case of DAuAgF solution [DMSO:H2O (molar ratio) = 8.29 × 10−4:1 with 0.5 mL of AuAgF solution], increase of temperature causes a dramatic loss of fluorescence. However, cooling of the preheated solution does not bring back the lost fluorescence quantitatively. A large extent of fluorescing behavior is lost permanently by heating that facilitates irreversible exchange of GSH by DMSO (Figure 4).

To the DAuAgF solution [DMSO:H2O (molar ratio) = 0.01:1 with 0.5 mL of AuAgF solution], different metal ions have been added. It has been found that exclusively Pb(II) brings back the fluorescence (PbAuAgF). Other metal ions are virtually indifferent on the DMSO induced quenched fluorescence. This observation has prompted us to design a selective Pb(II) sensor (Figure 5). To the DAuAgF solution, successive addition of Pb(II) causes gradual increment of fluorescence. Up to 9.5 × 10−4 M Pb(II) concentration, such enhancement of fluorescence is observed. Further increase of Pb(II) concentration decreases the fluorescence via spin orbit coupling associated with the heavy metal ion effect that accelerates intersystem crossing processes.46 A linear correlation is obtained in the range 0−20 μM, and the lower detection limit is 2 × 10−7 M (Figure 6). 353

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Figure 6. (A) Fluorescence spectral profile (λex = 400 nm) showing the effect of Pb(II) concentration on DAuAgF solution. (B) Degree of fluorescence enhancement with Pb(II) concentration and linear detection range (inset) for Pb(II) detection. Condition: DAuAgF solution DMSO:H2O = 0.01:1 with 0.5 mL of AuAgF solution.

By centrifugation of the fluorescent PbAuAgF solution, a yellow precipitate (YPPT) is obtained. The YPPT was washed several times with triple distilled water to remove unbound Pb(II). An aqueous suspension of YPPT exhibits strong fluorescence. Addition of Na2-EDTA to the aqueous suspension of YPPT destroys its fluorescence completely. It indicates that Pb(II) is a member of YPPT. EDAX and elemental mapping (Figure S6, Supporting Information) also confirm Pb along with Ag, Au, and S in YPPT. Thus, YPPT is different from core−shell particles in AuAgF solution. GSH has a crucial role for silver cluster formation. A positively charged large particle with a fluorescent silver surface makes room for DMSO and quenches its fluorescence owing to the removal of capped GSH. Again, Pb(II) binds with DMSO selectively and GSH finds a way to get back to the original position, causing regeneration of fluorescent PbAuAgF solution. In the YPPT, Pb(II) bound DMSO is attached with silver clusters and Au(I) producing an amoeboid shaped fluorescent entity. To the YPPT, further addition of DMSO causes significantly less quenching than that of AuAgF, indicating that Pb(II) creates a barrier for DMSO to interact. To the aqueous suspension of YPPT, addition of Na2EDTA destroys the fluorescence completely as Pb(II) forms a complex with EDTA (high formation constant, 2 × 10−18). Now, DMSO is set free to quench the fluorescence of giant fluorescent clusters as observed for DAuAgF solution. It is mentionable here that Na2-EDTA has no effect on the fluorescence of AuAgF solution (Figure 7).

Like Pb(II), Ag(I) as well as Hg(II) has a strong affinity to sulfur of DMSO. However, excess Ag(I) ion destabilizes the fluorescent silver clusters by aggregation while Hg(II) causes amalgam formation with silver.47 Thus, Ag(I) and Hg(II) could not bring back the lost fluorescence caused by DMSO. The fluorescence property is only restored by employing Pb(II). Tricomponent lifetime data [5.3 ns (24%), 0.3 ns (46%), and 75.5 ns (30%)] with an average lifetime of 24 ns in the AuAgF solution changes to a bicomponent one [3.19 ns (39%) and 74.9 ns (61%)] with an average lifetime of 46.92 ns in DAuAgF [DMSO:H2O (molar ratio) = 8.29 × 10−4:1 with 0.5 mL of AuAgF solution] solution. It is an uncommon case because static quenching maintains the lifetime intact and dynamic quenching decreases the lifetime. In the present case, the increment of lifetime can be explained by the decrease of radiative dacay rate.48 The average lifetime is again decreased (35.22 ns) in an aqueous suspension of YPPT having intense fluorescence (Figure S7, Supporting Information). The fluorescence of an aqueous suspension of YPPT is quenched dramatically by iodide (I−). I− plays the same role as that of EDTA for removal of Pb(II) as PbI2, making DMSO free to quench the fluorescence. Excellent selectivity among 11 types of anions (F−, Cl−, Br−, I−, NO3−, CO32−, HCO3−, IO3−, SO42−, CrO42−, and S2O32−) makes YPPT a promising candidate for iodide sensing. Au25(SG)18 has been reported as a fluorescent iodide sensor.49 However, I− detection involving a fluorescent silver cluster is completely novel (Figure 8). It is important to note that addition of S2− ion to the YPPT causes drastic quenching of fluorescence, as S2− ion removes Pb(II) efficiently (due to the strong affinity to sulfur) as well as GSH (due to the higher penetrating power of S2−). Such S2− capping of silver clusters also causes aggregation of the clusters along with the effect of free DMSO killing the fluorescence of the solution (Figure S8, Supporting Information). A TEM image reveals that the large fluorescent spherical particles (∼600 nm) in AuAgF solution are Aucore-Agshell in nature. With the addition of DMSO, the large particles are split into a small sphere with a substantial decrease in fluorescence in DAuAgF. In the fluorescent YPPT, tape-like morphology is obtained. A large amount of silver in EDAX analyses reveals that silver is present in large quantity at the surface (Figure 9A).8 Fluorescence microscopic images also display bright, dull, and again bright fluorescence for AuAgF, DAuAgF, and YPPT, respectively (Figure 9B). XPS spectra of the AuAgF solution under freeze-drying conditions demonstrate peaks at 368.07 and 374.07 eV for silver. These binding energies match to

Figure 7. Fluorescence spectral profile (λex = 400 nm) showing the effect of Na2-EDTA on AuAgF and YPPT. 354

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Figure 8. (A) Fluorescence spectral profile (λex = 400 nm) showing the effect of different anions on aqueous solution of YPPT. (B) Bar diagram displaying the degree of fluorescence alteration of aqueous solution of YPPT in the presence of different anions. Condition: YPPT obtained from PbAuAgF solution [DMSO:H2O (molar ratio) = 0.01:1 with 0.5 mL of AuAgF solution + 9.5 × 10−4 M Pb(II)], [anion] = 9.5 × 10−4 M.

Figure 9. (A) TEM image of (a1) AuAgF, (b1) DAuAgF, and (c1) YPPT; EDAX spectra of (a2) AuAgF, (b2) DAuAgF, and (c2) YPPT. (B) Fluorescence microscopic image of (a) AuAgF, (b) DAuAgF, and (c) YPPT after being drop-cast on a glass slide under UV light exposure. (C) XPS spectra for (a) silver, (b) gold, (c) sulfur, and (d) lead under freeze-drying conditions for AuAgF, DAuAgF, and YPPT. Condition: DAuAgF DMSO:H2O = 0.01:1 with 0.5 mL of AuAgF solution, YPPT obtained from PbAuAgF solution (DMSO:H2O = 0.01:1 with 0.5 mL of AuAgF solution + 9.5 × 10−4 M [Pb(II)]).

clusters as reported by Maretti et al.37 Added Pb(II) binds DMSO removing the core−shell particles and GSH/GSSG caps Au(I)@(Ag2/Ag3) particles again in PbAuAgF solution. This phenomenon causes some aggregation of Ag2/Ag3 clusters. Further shift of binding energies toward a higher value (369.09 and 375.10 eV, respectively) as well as

Ag(0)3d5/2 and Ag(0)3d3/2, respectively. In the DAuAgF solution, the energies are shifted toward higher values (368.66 and 374.57 eV, respectively), implying that Ag(0) possesses more electron density being capped with DMSO. This increment of electron density, i.e., more negative environment, destroys the fluorescence intensity of silver 355

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broadening of the fluorescence spectral profile supports the fact. Again, 84.59 and 88.19 eV binding energies in AuAgF solution indicate Au(I)4f7/2 and Au(I)4f7/2.8 They remain virtually unaffected (84.25 and 87.80 eV, respectively) in DAuAgF solution. However, in the PbAuAgF solution, appreciable shift of peak positions toward higher binding energy sites implies gold to be in the +3 oxidation state. Now, Pb to some extent fulfills the task of Au of electron drifting to some extent. Increased binding energies [139.83 and 144.78 eV correspond to Pb(II)4f7/2 and Pb(II)4f5/2, respectively] of Pb(II) contribute a support for the fact.50 Sulfur is in the +2 oxidation state in all three cases (∼163.6 eV binding energy is for S(II)2P3/2) (Figure 9C).8 Along with the absorption spectra of AuAgF, absorption spectra of DAuAgF, YPPT, and YPPT +Na 2 -EDTA have also been presented in Figure S4 (Supporting Information). In any case, the plasmon band of metals is not observed. It indicates that throughout the experiment clusters are not converted to nanocrystals. TEM image, EDAX, XPS analysis [Figure 9A(a1), A(a2), and C], and elemental mapping (Figure S1, Supporting Information) prove that, in the AuAgF solution, the particles are Au(I)core-Ag(0)shell in nature. The as produced Ag(0) cluster covers Au(I), forming stable core−shell aggregates, and GSH/ GSSG helps to stabilize them. Thus, Ag(I)-thiolate as well as Au−Ag species are not detected from mass spectral analyses. Only the presence of GSH, GSSG, Ag2, and Ag3 is prominent from mass spectral data. The presence of core−shell particles was observed largely from TEM analysis. The stabilization of Au(I)core in solution phase is presumably due to the presence of thiolate and anions like Cl− and NO3−. Moreover, Ag(0)shell of the giant cluster further helps the stabilization by donating electron cloud to gold core. As a result, Ag(0) in turn (in the form of a fluorescent Ag2/Ag3 cluster) gets stability.37 EDAX analyses have only pointed out the abundance of silver because EDAX collects largely the surface information and could not obtain sufficient information about gold at the core. This indirectly supports the formation of Au(I)core-Ag(0)shell particle. Usual metal nanoparticles are known good fluorescence quenchers due to Förster resonance energy transfer and lossy surface waves.51 Large aggregated particles52,53 have been established to be important candidates for enhancement of fluorescence of some plasmophore (radiates into the far-field with increased emission intensity) owing to a favorable scattering cross section.54,55 On the other hand, fluorescent clusters, missing link between atom and nanoparticle, are generally ultrasmall in size. It is amazing that we have synthesized microparticles that are with intense yellowish orange colored fluorescence (quantum yield 6% employing quinine bisulfate in 1 N H2SO4 as reference). DMSO infested selective fluorescence quenching of AuAgF and Pb(II) induced reincarnation are the main focus in the piece of article.

out of these novel materials in a lot of important pastures such as immunoassays, fluorescence imaging, and so on.

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ASSOCIATED CONTENT

S Supporting Information *

SEM image, elemental mapping, MALDI, ESI, fluorescence, UV−vis spectra, and lifetime decay profile. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the UGC, DST, NST, and CSIR, New Delhi, India, and the IIT Kharagpur for financial assistance.

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REFERENCES

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CONCLUSIONS Green chemically synthesized, highly fluorescent Au(I)core(Ag2/Ag3)shell-thiolate particle has been found to be an intriguing platform for the detection of a trace amount of DMSO and Pb(II) via a Turn Off-On mechanism in aqueous medium. Pb(II) induced fluorescence can further be employed for I− sensor owing to the selective quenching of fluorescence. Given the potential applications of fluorescent stable silver clusters in several fields, we believe that the work illustrated herein may also serve as a footing for designing and appliances 356

dx.doi.org/10.1021/la403848z | Langmuir 2014, 30, 348−357

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