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. 2017 Nov 17;2(11):8086–8098. doi: 10.1021/acsomega.7b01560

Solvent Polarity-Dependent Behavior of Aliphatic Thiols and Amines toward Intriguingly Fluorescent AuAgGSH Assembly

Jayasmita Jana †, Teresa Aditya †, Yuichi Negishi ‡, Tarasankar Pal †,*
PMCID: PMC6645147  PMID: 31457357

Abstract

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Highly stable fluorescent glutathione (GSH)-protected AuAg assembly has been synthesized in water under UV irradiation. The assembly is composed of small Ag2/Ag3 clusters. These clusters gain stability through synergistic interaction with Au(I) present within the assembly. This makes the overall assembly fluorescent. Here, GSH acts as a reducing as well as stabilizing agent. The assembly is so robust that it can be vacuum-dried to solid particles. The as-obtained solid is dispersible in nonaqueous solvents. The interaction between solvent and the assembly provides stability to the assembly, and the assembly shows fluorescence. It is interesting to see that the behavior of long-chain aliphatic thiols or amines toward the fluorescent assembly is altogether a different phenomenon in aqueous and nonaqueous mediums. The assembly gets ruptured in water due to direct interaction with long-chain thiols or amines, whereas in nonaqueous medium, solvation of added thiols or amines becomes pronounced, which hinders the interaction of solvent with the assembly. However, the fluorescence of the assembly is always quenched with thiols or amines no matter what the solvent medium is. In aqueous medium, the fluorescence quenching by aliphatic thiol or amine becomes pronounced with successive decrease in their chain length, whereas in nonaqueous medium, the trend is just reversed with chain length. The reasons behind such an interesting reversal of fluorescence quenching in aqueous and nonaqueous solvents have been discussed explicitly. Again, in organic solvents, thiol or amine-induced quenched fluorescence is selectively recovered by Pb(II) ion without any alteration of excitation and emission maxima. This phenomenon is not observed in water because of the ruptured fluorescent assembly. The fluorescence recovery by Pb(II) and unaltered emission peak only in nonaqueous solvent unequivocally prove the engagement of Pb(II) with thiols or amines, which in turn revert the original solvent-supported stabilization of the assembly.

Introduction

Fluorescent gold (Au) and silver (Ag) nanoparticles (NPs) have created immense interest due to their optical properties and a wide range of applications like sensing,1,2 biolabeling,3 and imaging.4 Generally, metal nanoparticles of dimension <2 nm, categorized under clusters, show molecule-like behavior.5,6 They possess discrete energy states due to quantum confinement effects. These nanoparticles lying within sub-2 nm dimension show excellent photoluminescence property. However, the stability of the fluorescent particles especially for Ag clusters has always been a matter of concern. The similarity of dissociation energy with the excitation energy of Ag dimmers and the tendency toward oxidation as well as spontaneous aggregation caused problem for Ag clusters.7 Several strategies have been adopted to stabilize fluorescent clusters. One of them is the use of scaffolds, like amines, thiols, dendrimers, microemulsions, polymers, DNA, etc. together with reducing agents in the chemical reduction method.8−11 Another tactic is to apply the synergistic effect out of association with other metals. Fluorescent bimetallic AuAg NPs of different compositions are easy to synthesize in varied size regimes due to their similar lattice constants and isomorphous structures.12 As bimetallic AuAg NPs possess high quantum yield, they circumvent the usual weak fluorescence of the individual Ag or Au NPs. This intense emission is the key factor for different applications. The synergistic effect between Au and Ag stabilizes and enhances the fluorescence of AuAg NPs if manipulated judiciously. This synergism shows a pronounced effect on AuAg clusters (NC).13 Photophysical and photochemical properties of fluorophores differ greatly depending on the solvent polarity, refractive index, viscosity, etc.14 It is reported that the fluorescence intensity, lifetime, and quantum yield of fluorescent clusters and core–shell particles, e.g., Ag nanoclusters stabilized by poly(methyl methacrylate)/poly(methyl methacrylate)–poly(methacrylic acid) core/shell nanoparticles are improved in different organic solvents; however, the Stokes shift remains unaltered.15 Zhou et al. reported solvent-dependent photoluminescence properties of Au8 nanoclusters.16 For the fluorescing systems that contain metal (0)–metal (I)–ligand structures, there are two possibilities that either the fluorescence may arise from the metal (0) core or metal (I)–ligand system.6 Initial studies support for the charge and valence state of metal core-dependent fluorescence of the moieties. However, the metal (I)–ligand interactions are now considered as the contributing factors for the strong interaction between metal and ligand. A high surface-to-volume ratio has always been an important factor in this field of study. It has been observed that the clusters with the same number of metal atoms at the core but different protecting ligands show different emission property. Yuan et al.17 have shown that the use of different thiolated ligands (3-mercaptopropionic acid, 6-mercaptohexanoic acid, etc.) causes varied emissions from the Au25 cluster. The ligands with electron-rich functional groups and electropositivity of the metal core promote the fluorescence property of metal nanoclusters.6 Shang et al.18 have shown that for a fluorescent particle, Au(0)Au(I) ligand, the presence of Au(I) is an important moiety for fluorescence generation. Upon further reduction of the fluorescent particle by a strong reducing agent, borohydride, they have observed the decreased emission intensity. Again, Goswami et al.19 reported an aggregation-induced emission (AIE) from such a Au(0)–Au(I)–glutathione nanocluster. Another examlpe of the core–shell type fluorescent nanoparticle that showed AIE was reported by Zhao et al.20 In this case, glutathione-protected Au(I) is the core and SiO2 acts as the shell. The fluorescence of the NPs has shown to further increase with the incorporation of Cd(II) cations. However, further research is going on to explore the reasons behind the fluorescence of NPs of different morphologies.

In recent days, chemical-based molecular logic gates have attracted growing interest due to their fascinating significance in the development of molecular-scale electronic devices. These perform Boolean logic operations in response to chemical inputs. Different groups have reported molecular logic gates by using materials, such as nucleic acids,21 proteins,22 and organic molecules.23 There are certain methods, like luminescence, colorimetry, electrochemistry, etc., that are used in logic devices. However, photoluminescence is broadly considered for high sensitivity and a simple and fast response.24 The molecular logic gates are implemented depending on the sensing results (fluorescence turn off or turn on).

Here, in this work, we have reported the synthesis of an intriguingly fluorescent assembly from aqueous solution of glutathione (GSH), chloroauric acid, and silver nitrate in a particular ratio under UV irradiation. Again, the dried assembly, AuAgGSH, produced a stable dispersion in organic solvents with increased emissive behavior. We presume that electronic interactions between Ag(0) and Au(I) as well as Au(I) and GSH are responsible for the stability and fluorescence of the assembly. To prove this, we have performed a ligand exchange reaction and substituted GSH with different aliphatic thiols and amine ligands. We took this task as commercial aliphatic thiols and amines with different chain lengths are readily available. There are some reports regarding the effect of thiol ligands with different functionalities on the emission behavior of fluorescent NPs6 and monolayer formation on the metallic surface of a Ag/Cu bimetallic nanocluster.25 However, there is no report, as our knowledge goes, on the effect of ligands with the same functional groups but with varied chain lengths. To widen the studies on the ligand effect, long-chain amines are also considered here for facile but competitive amine–Ag interaction. During the ligand exchange process, we have found that the solvent as well as ligand chain length provides interesting results that relates to the stability of the as-synthesized fluorescent assembly. Fluorescence of the assembly increases when water is replaced by another solvent, but when long-chain amines or thiols are added individually or simultaneously to the assembly, taken in any solvent, the fluorescence is quenched. Here, we have discovered that the quenching trend is dependent on the chain length as well as the solvent, i.e., the dispersion medium. Interestingly, Pb(II) is the only metal ion that has been found to increase the quenched fluorescence of AuAgGSH in different organic solvents, whereas in water, Pb(II) cannot rescue the quenched fluorescence caused by thiols or amines. Capitalizing on this idea, IMPLICATION logic gates have been proposed in nonaqueous medium only.

Experimental Section

Materials and Instruments

All of the reagents were of AR grade. Triple distilled water was employed throughout the experiment. Chloroauric acid (HAuCl4), silver nitrate (AgNO3), glutathione (GSH), metal salts, long-chain thiol and amine compounds were obtained from Sigma-Aldrich. All of the solvents were purchased from Merck. All glass wares were cleaned with freshly prepared aqua regia, subsequently rinsed with distilled water, and dried well before use.

All UV–vis absorption spectra were recorded in an Evolution 201 spectrophotometer (Thermo Scientific). The absorbance was measured using a glass cuvette. At room temperature (25 °C), the fluorescence measurement was done with a Perkin-Elmer LS55 fluorescence spectrometer. Fluorescence lifetimes were measured with Easy life V (Optical Building Blocks Corporation) equipped with a 380 nm light-emitting diode excitation source. A nonlinear least squares (χ2) fit was tested to determine the fit of the decay rate to a sum of exponentials, and a visual inspection of the residuals and the autocorrelation function were used to determine the quality of the fit. The sample was taken in a quartz cuvette with a path length of 1 cm for fluorescence measurement. The VG Scientific ESCALAB MK II spectrometer (U.K.) equipped with Mg Kα excitation source (1253.6 eV) and a five-channeltron detection system were used for X-ray photoelectron spectroscopy (XPS) analysis. The samples were fridge-dried before XPS measurement. Transmission electron microscopy (TEM) analyses were done with an H-9000 NAR instrument (Hitachi) having an accelerating voltage of 300 kV. The samples were drop-casted onto a carbon-coated copper grid, and the grid was vacuum-dried before loading into the microscope. Matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) spectroscopy was performed by a VOYAGER-DE PRO (manufactured by Applied Biosystems). Sinapinic acid was used as the matrix during drop casting of the sample for analysis. Fourier transform infrared (FTIR) of the liquid samples were done using a Thermo-Nicolet continuum FTIR microscope. All of the experiments were repeated three times at room temperature and almost neutral medium (pH = 6.9).

Synthesis of the Fluorescent Assembly

The glutathione-mediated Ag(0)Au(I) assembly was synthesized by irradiation of the mixture of AgNO3, GSH, and HAuCl4 under UV light. Typically, 0.5 mL of 10–2 M HAuCl4 solution was added to 8 mg of GSH, followed by 1.5 mg of AgNO3. The mixture was mixed very well and dissolved into 15 mL of water. Then, the colorless solution was subjected to UV (wavelength = 365 nm) irradiation for 10 h. After 10 h, a yellow solution was obtained. This solution showed fluorescence at 580 nm when excited at 390 nm. The as-synthesized solution was vacuum-dried to obtain a yellow solid product.

The as-obtained yellow solid was dissolved in 15 mL of ethanol directly, and the solution was also fluorescent (λex = 390 nm and λem = 580 nm). Dioctyl sulfosuccinate sodium salt (AOT, 0.5 mg) was dissolved in 15 mL of heptane, followed by the as-synthesized yellow solid to attain complete dissolution of the solid in n-heptane. This produced a fluorescent solution (λex = 390 nm and λem = 574 nm).

Effect of Pb(II) on the Thiol or Amine Assembly in Different Solvents

Thiol or amine solution (1.6 × 10–4 M) was added to 0.0132 mg/mL fluorescent assembly, and the final volume was adjusted to 3 mL with solvent. This mixture was shaken gently and kept at room temperature. After 24 h, the spectra were measured.

To see the effect of Pb(II) on the fluorescence of the solution, 1.6 × 10–4 M Pb(II) solution was added to the thiol or amine-added solution of the assembly prepared as per the above-mentioned method (thiol or amine concentration remained 1.6 × 10–4 M, and concentration of the assembly remained 0.0132 mg/mL). The final volume of the solution was kept at 3 mL. After 6 h of addition, the spectra were recorded.

Results and Discussion

When aqueous mixture of glutathione (GSH) and silver nitrate are kept under ∼365 nm UV irradiation, it produces a feebly fluorescent water miscible material (WAgGSH). Again, the mixture of chloroauric acid and GSH, under UV light irradiation, does not produce a significantly fluorescent product (WAuGSH) under the experimental conditions (Figure S1, Supporting Information). However, addition of chloroauric acid to the mixture of GSH together with silver nitrate, under UV irradiation for 10 h, generates highly fluorescent material. The assembly in water is denoted as WAuAgGSH. Then, the as-synthesized solution is dried under vacuum to obtain a yellow powder (AuAgGSH) that readily dissolves in common organic solvents, like ethanol, methanol, acetone, dimethyl formamide, etc. AuAgGSH can be deliberately delivered into water miscible organic solvents, like heptanes, toluene, etc., with the assistance of dioctyl sulfosuccinate sodium salt (AOT). Here, GSH is used as both a reducing and stabilizing agent. Under UV irradiation, GSH reduces Ag(I) to Ag(0) and then stabilizes the Ag(0) particle. Dickson et al. have previously reported the synthesis of fluorescent Ag clusters.26 However, stability related issues have always been a concern as the Ag nanoclusters are prone to aggregation and also the excitation energy of Ag clusters is quite close to their dissociation energy.27 In the present case, Au(I), produced by GSH reduction, provides stability to Ag(0) particles by withdrawing electron density from Ag(0). Maretti et al.28 demonstrated that the positive surface of Au(I) helps to stabilize Ag(0) nanoclusters. This is a kind of synergism where the resultant fluorescence is much enhanced than the sum of the fluorescence of the individual component. Explicitly, GSH can reduce Au(III) to Au(I) and the synergistic effect between Au(I) and Ag(0) makes the overall system stable as well as highly fluorescent. The oxidized form of GSH remains as GSSG in the system.29 The oxidation states of the species are confirmed from XPS peaks. Peaks at 368.20 and 374.2 eV represent binding energy for Ag(0) 3d5/2 and Ag(0) 3d3/2, respectively, and at 84.5 and 88.1 eV represents Au(I) 4f7/2 and Au(I) 4f5/2, respectively (Figure 1). From XPS, we find that Ag is in 0 oxidation state and Au is in +1 oxidation state. The peak at 162.5 eV shows the presence of S 2p3/2. Further deconvolution shows peaks at 161.7 and 162.8 eV, indicating the presence of Ag–S–R-like interaction30 and Au–S interaction,31 respectively (Figure S2, Supporting Information). In FTIR spectra, the absence of characteristic −S–H band indicates M–S interaction (Figure S2, Supporting Information).32 Mass analysis is another important characterization technique.33 We performed MALDI-TOF analysis that shows the presence of Ag3 and Ag2 clusters in the assembly (Figure S2, Supporting Information). MALDI-TOF spectra suggests that Ag(0) and Au(I) collocate on each other. Also, absorption spectra suggest the presence of Ag2/Ag3 moiety.34 A report by Chakraborty et al.35 suggests that with the increase in the number of atoms in the bimetallic cluster core, the pronounced absorption peaks at the visible region increases. However, the as-synthesized fluorescent assembly containing Ag(0) and Au(I) does not show significant peak, indicating an insignificant plasmon band of the overall moiety. Hence, Ag2/Ag3 clusters were considered. Further investigation is warranted in this respect. The emissive nature of Ag arises due to the transfer of electron density from the submerged and quasicontinuum 5d band to the lowest unoccupied conduction band of Ag clusters. This is a kind of an interband transition.27 Hence, the fluorescence comes from the Ag2/Ag3 cluster and these clusters are stabilized through synergism with Au(I). WAuAgGSH shows fluorescence at 580 nm upon excitation at 390 nm (Figure 1). The excitation peak arises at 391 nm (Figure S1, Supporting Information) corresponding to emission at 580 nm, showing the appropriateness of the chosen excitation wavelength. Another point to be noted is that the emission peak position does not change to a great extent with respect to variation in excitation wavelength (Figure S1, Supporting Information). This may indicate that only one fluorescing species is present in the system.36 Again, from the mass analysis data, we have found that two types of Ag clusters are present in the system. The emission peak is quite broad, indicating the fact that overall emission is the overlapped emission from each species.37 The quantum yield (φ) value has been found to be 4.98% when measured with respect to standard quinine sulfate solution. WAuAgGSH particles do not possess any characteristic absorption peak in the visible region (Figure 1D). WAuAgGSH particles possess an average lifetime of 4.67 ns (Figure S3, Supporting Information). The TEM image shows that the average particle dimension is ∼560 nm (Figure 1). Figure S4, Supporting Information shows the particle size distribution plot.

Figure 1.

Figure 1

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(A) Broad range XPS spectrum of WAuAgGSH. Narrow range XPS spectra, indicating elemental (B) Ag and (C) Au for WAuAgGSH. Measurements are done under fridge drying condition. (D) Absorption and emission spectra of WAuAgGSH. λex = 390 nm, room temperature (RT) (E) TEM image of WAuAgGSH.

Fluorescence of WAuAgGSH depends not only on the synergistic collaboration of Au(I) and Ag(0) species but also on the suitable stabilizing ligand, GSH. An attempt of the ligand exchange process affects the stability of the fluorescent moiety and causes quenching of fluorescence. Certain long-chain aliphatic amine and thiol compounds are utilized in this process thinking of the ease of interaction of thiols and amines with Au and Ag moiety, respectively.38 The thiol compounds used are 1-decanethiol [CH3(CH2)9SH], 1-dodecanethiol [CH3(CH2)11SH], 1-tetradecanethiol [CH3(CH2)13SH], and 1-hexadecanethiol [CH3(CH2)15SH], and the amines are 1-decylamine [CH3(CH2)9NH], 1-dodecylamine [CH3(CH2)11NH], 1-tetradecylamine [CH3(CH2)13NH], and 1-hexadecylamine [CH3(CH2)15NH]. When thiols (Figure 2A) and amines (Figure 2B) are added individually, the solution turns slightly turbid with the increase in the chain length and the inherent fluorescence of WAuAgGSH gets quenched (Figure S5, Supporting Information). As the used amines or thiols are smaller than GSH, they diffuse toward the metal surface causing rupture of the stabilized system.

Figure 2.

Figure 2

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Fluorescence spectral profile of WAuAgGSH when (A) long-chain aliphatic thiols are added individually, (B) long-chain aliphatic amines are added individually, and (C) long-chain aliphatic amine and thiols of the same chain length are introduced together. λex = 390 nm, RT. [Thiol/amine] = 1.6 × 10–4 M, [WAuAgGSH] = 0.0132 mg/mL, total volume of solution = 3 mL TEM image of (D) CH3(CH2)9SH–WAuAgGSH and (E) CH3(CH2)9NH–WAuAgGSH.

Here, two prominent effects are observed, (a) with the increase in the chain length thiol or amine, the quenching of fluorescence of the assembly becomes less prominent, and (b) an aliphatic thiol of certain chain length shows higher quenching ability than an amine of comparable chain length. Thiol and Au(I) interaction is predominant, and thus the existing synergistic interaction between Au(I) and Ag(0) is lost. It may be argued that still there are Ag clusters that can show fluorescence but the natural tendency of Ag clusters toward aggregation may be the reason of fluorescence quenching, but when amine is added individually, although the WAuAgGSH system is ruptured, amine can prevent the aggregation of Ag clusters. So the extent of quenching is less with amine. To justify our conclusion, we have simultaneously added thiol and amine to the WAuAgGSH solution. For this, we have chosen the pair of [CH3(CH2)nSH] and [CH3(CH2)nNH] when n = 9, 11, 13, and 15, respectively, for thiol and amine so that the effect of same chain length can be visualized (Figure 2C). We have found that the quenching efficiency of thiols and amines are in the order of thiol > thiol + amine > amine. When thiol is present in the solution, it diffuses directly toward the Au(I) surface but amine is there to prevent the aggregation of Ag particles. To see the effect prominently, we have used thiols and amines of comparable chain length. TEM image has been taken for CH3(CH2)9SH–WAuAgGSH and CH3(CH2)9NH–WAuAgGSH systems. From TEM (Figure 2D,E) images, we see that the large WAuAgGSH particles get converted to small aggregated particles. Also, the quenching depends on the chain length. A thiol or amine compound with shorter chain length causes pronounced quenching. It is assumed that the thiols or amines diffuse toward the metal surface by penetrating the GSH layer. So, with the increase in size, the diffusion decreases which in turn causes prominent quenching. Lifetime values of CH3(CH2)9SH–WAuAgGSH and CH3(CH2)9NH–WAuAgGSH systems have been calculated to be 1.26 and 1.65 ns, respectively (Table S1, Supporting Information).

Now, CH3(CH2)9SH–WAuAgGSH and CH3(CH2)9NH–WAuAgGSH systems were subjected to further investigation. When different cations [Cr(III), Co(II), Cd(II), Cu(II), Fe(III), Hg(II), Ni(II), and Zn(II)] are added to two solutions, we find that no cation can restore the lost fluorescence. However, addition of Pb(II) selectively showed enhanced fluorescence of WAuAgGSH. This enhancement may be due to the Pb(II)–GSH interaction that increases positive environment around the Ag(0)Au(I) system. However, the quenching of the fluorescence of WAuAgGSH in the presence of long-chain aliphatic thiol and amine compounds cannot be recovered as the long-chain aliphatic thiol and amine permanently disrupts the stable fluorescing GSH protected Ag(0)Au(I) system (Figure 3). The disruption of Au(I)–Ag(0) synergism in the presence of thiol or amine is further confirmed by XPS studies of CH3(CH2)9SH–WAuAgGSH and CH3(CH2)9NH–WAuAgGSH. The shift in the Au(I) peaks from 84.5 and 88.1 to 83.7 and 87.4 eV, respectively, indicates that the Au is in zero oxidation state rather than in +1 state39 in CH3(CH2)9SH–WAuAgGSH. In this case, the Ag clusters undergo aggregation due to lack of a stabilizing factor.40 XPS peaks appear at lower regions, i.e., 367.03 and 373.02 eV for Ag(0) 3d5/2 and Ag(0) 3d3/2, respectively. For CH3(CH2)9NH–WAuAgGSH, the peaks of Au appeared at 84.06 and 87.6 eV for Au(0) 4f7/2 and Au(0) 4f5/2, respectively. However, the peaks for Ag(0) appeared at slightly higher region [367.7 and 373.6 eV for Ag(0) 3d5/2 and Ag(0) 3d3/2, respectively] than that in thiol-capped WAuAgGSH (Figure S5, Supporting Information).

Figure 3.

Figure 3

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Fluorescence spectral profile of (A) CH3(CH2)9SH–WAuAgGSH and (B) CH3(CH2)9NH–WAuAgGSH systems in the presence of different cations. λex = 390 nm, RT. [Thiol/amine] = 1.6 × 10–4 M and [cation] = 1.6 × 10–4 M, [WAuAgGSH] = 0.0132 mg/mL, total volume of solution = 3 mL. Cations were added individually to the CH3(CH2)9SH–WAuAgGSH and CH3(CH2)9NH–WAuAgGSH systems.

In the next step, AuAgGSH is dissolved into weakly polar organic solvent ethanol and a fluorescent solution is produced. AuAgGSH in ethanol (now termed as EAuAgGSH, quantum yield = 9.08%) shows fluorescence at 580 nm when excited at 390 nm (Figure 4A). The measured average lifetime is 5.36 ns (Figure S6, Supporting Information). It was reported that with the decrease in the solvent polarity, the fluorescence intensity increases along with a blueshift of the emission peak.41 However, in the present case, we find no alteration of the emission peak. The Au(I) core is so stable and Ag(0) clusters remain strongly anchored on Au(I) that eventually the emission maximum remains unaltered even in different solvent systems. Different dipolar interactions within the excited state fluorophore and the surrounding molecules play the main role. The local environment formed by the scaffold in the solvent caused bright fluorescence. The excited state charge transfer is considered to be highly dependent on solvent polarity.42,43 XPS studies show that the oxidation state of the moiety does not change with solvent. Peaks at 368.2 and 374.4 eV represent binding energy for Ag(0) 3d5/2 and Ag(0) 3d3/2, respectively, and at 84.1 and 88.07 eV represent Au(I) 4f7/2 and Au(I) 4f5/2, respectively (Figure S7, Supporting Information). The TEM image shows interconnected particles with a diameter of ∼100 nm for EAuAgGSH (Figure 4B). This is a kind of solvent-induced aggregation.29

Figure 4.

Figure 4

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(A) Absorption and emission spectra of EAuAgGSH. λex = 390 nm, RT. (B) TEM image of EAuAgGSH.

When long-chain thiols or amines (Figure 5A) are added to EAuAgGSH individually, we see that the amine-induced quenched fluorescence is less prominent than thiol-induced fluorescence, as observed in aqueous medium. However, the thiol or amine compound with longer hydrocarbon chain shows better quenching unlike that in water. Such phenomenon is attributed to solvation energy. Solvation of aliphatic thiols and amines is favored in nonpolar solvent. Also, it worth mentioning that with the increase in the hydrocarbon chain, the solvation of thiol and amine compound increases in nonpolar solvents. In our case, the fluorescing assembly in nonpolar solvents get stabilized by the surrounding solvent molecules, as evidenced by the increased fluorescence intensity. Once thiol or amine is introduced in the system, assembly–solvent interaction is interrupted and quenching of fluorescence is observed. The fluorescence quenching becomes pronounced with thiol and amine of longer hydrocarbon chain. It is further observed that on addition of both thiol and amine of comparable chain length at a time causes more quenching than that which occurs due to their individual effect on the EAuAgGSH system. Lifetime values of CH3(CH2)9SH–EAuAgGSH and CH3(CH2)9NH–EAuAgGSH systems have been calculated to be 1.91 and 2.96 ns, respectively (Table S1, Supporting Information). TEM images (Figure 5B,C) of [CH3(CH2)15SH]–EAuAgGSH and [CH3(CH2)15NH]–EAuAgGSH show that in the presence of long-chain thiol and amine compounds, the NPs disintegrate. The proper solvent-coordinated structure becomes loosely aggregated. As [CH3(CH2)15SH] and [CH3(CH2)15NH] cause maximum quenching, [CH3(CH2)15SH]–EAuAgGSH and [CH3(CH2)15NH]–EAuAgGSH solutions were taken for further studies. On addition of certain metals to [CH3(CH2)15SH]–EAuAgGSH and [CH3(CH2)15NH]–EAuAgGSH, it is found that Pb(II) selectively brings back the lost fluorescence (Figure 6). Pb(II) has an inherent binding affinity toward thiol compounds. In the presence of Pb(II), thiol compounds interact with Pb(II) and the solvent–Ag cluster interaction is restored. However, in the previous case, we have seen that the quenched fluorescence of WAuAgGSH cannot be recovered as the fluorescent system is destroyed due to ligand exchange. Thus, we find that the quenching of the fluorescence of EAuAgGSH is not due to destruction of the GSH layer but rather due to the disruption of electronic interaction between solvent and Ag cluster. It has been observed that thiol or amine-added EAuAgGSH shows very weak emission (λem = 580 nm) upon excitation at 390 nm, whereas strongly fluorescing WAuAgGSH and EAuAgGSH particles also emit at 580 nm, when excited at 390 nm. As the fluorescence peak remains unaltered, it is concluded that the EAuAgGSH particles are not ruptured, as it happened for WAuAgGSH upon the addition of thiol or amine (Figure S8, Supporting Information). This may be due to the fact that the thiol or amine becomes too bulky in association with the solvent molecules, so they cannot approach the fluorescent assembly in ethanol medium. Also, from XPS studies of CH3(CH2)15SH–EAuAgGSH and CH3(CH2)15NH–EAuAgGSH, we see that the peaks for Au(I) remain unaltered, indicating that there is no direct interaction between thiol or amines with the fluorescent assembly (Figure S7, Supporting Information). It should be mentioned that in the presence of Pb(II), the fluorescence of EAuAgGSH is enhanced by ∼1.1-fold.

Figure 5.

Figure 5

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(A) Fluorescence spectral profile of EAuAgGSH in the individual presence of long-chain thiols and amines, λex = 390 nm, RT. [Thiol/amine] = 1.6 × 10–4 M. TEM image of (B) CH3(CH2)15SH–EAuAgGSH and (C) CH3(CH2)15NH–EAuAgGSH.

Figure 6.

Figure 6

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Fluorescence spectral profile of (A) CH3(CH2)17SH–EAuAgGSH and (B) CH3(CH2)15NH–EAuAgGSH systems in the presence of different cations. λex = 390 nm, RT. [Thiol/amine] = 1.6 × 10–4 M and [cation] = 1.6 × 10–4 M, [EAuAgGSH] = 0.0132 mg/mL, total volume of solution = 3 mL. Cations were added individually to the CH3(CH2)9SH–EAuAgGSH and CH3(CH2)9NH–EAuAgGSH systems.

This experiment is further continued for water immiscible nonpolar solvent n-heptane. AuAgGSH is dispersed in n-heptane with the aid of AOT, and a fluorescent solution is generated. This is denoted as HAuAgGSH. Particles emit at 574 nm when excited at 390 nm (quantum yield = 7.01%) (Figure 7A). The observed fluorescence spectra cannot be correlated with the solvent polarity, but rather the presence of reverse micelle AOT is the main factor for the observed spectra.44 They have an average lifetime of 3.93 ns (Figure S9, Supporting Information). The TEM image (Figure 7B) shows that the particles are ∼100 nm in diameter and a network-like structure is observed. This is due to solvent-induced coordination. However, XPS studies show that oxidation states remain +1 and 0 for Au and Ag, respectively (Figure S10, Supporting Information). Peaks are found at 368.5 and 374.7 eV, representing binding energy for Ag(0) 3d5/2 and Ag(0) 3d3/2, respectively, and at 84.3 and 88.1 eV, representing Au(I) 4f7/2 and Au(I) 4f5/2, respectively.

Figure 7.

Figure 7

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(A) Absorption and emission spectra of HAuAgGSH. λex = 390 nm, RT. (B) TEM image of HAuAgGSH.

Intriguing fluorescence of HAuAgGSH particles is drastically quenched in the presence of long-chain aliphatic thiols and amines. In this case also, amines show lower quenching than thiols. However, variation of chain length of the thiols shows that the increase in the chain length of thiols causes a pronounced quenching effect (Figure 8). We assume that solvation energy is that factor that works behind this behavior, as in the case of ethanol. Thiol or amine compounds with a long hydrocarbon chain are easily dissolved in nonpolar n-heptane. Longer hydrocarbon chain will show higher nonpolarity and thus higher solvation. Thus, longer thiol or amine caused more effective quenching of fluorescence. Again, as thiols or amines becomes highly solvated, they cannot diffuse toward fluorescent moiety. Hence, the assembly remains undisturbed and the quenching is mainly caused due to interruption of solvent–assembly interaction in the AOT/heptanes system. Lifetime values of CH3(CH2)9SH–EAuAgGSH and CH3(CH2)9NH–EAuAgGSH systems have been calculated to be 1.51 and 1.32 ns, respectively (Table S1, Supporting Information). TEM images (Figure 8B,C) of [CH3(CH2)15SH]–HAuAgGSH and [CH3(CH2)15NH]–HAuAgGSH show that in the presence of long-chain thiol and amine, the NPs become loosely aggregated. As [CH3(CH2)15SH] and [CH3(CH2)15NH] cause maximum quenching, [CH3(CH2)15SH]–HAuAgGSH and [CH3(CH2)15NH]–HAuAgGSH solutions were taken for further studies. The XPS studies of [CH3(CH2)15SH]–HAuAgGSH and [CH3(CH2)15NH]–HAuAgGSH reveal that the oxidation state of Au and Ag remains the same, i.e., +1 for Au and 0 for Ag (Figure S10, Supporting Information). In this case also, the quenched fluorescence of [CH3(CH2)15SH]–HAuAgGSH and [CH3(CH2)15NH]–HAuAgGSH can be selectively triggered to turn on response in the presence of Pb(II) ion in the system (Figure 9).

Figure 8.

Figure 8

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(A) Fluorescence spectral profile of HAuAgGSH in the individual presence of long-chain thiols and amines. λex = 390 nm, RT. [Thiol/amine] = 1.6 × 10–4 M. TEM image of (B) CH3(CH2)15SH–HAuAgGSH and (C) CH3(CH2)15NH–HAuAgGSH.

Figure 9.

Figure 9

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Fluorescence spectral profile of (A) CH3(CH2)17SH–HAuAgGSH and (B) CH3(CH2)15NH–HAuAgGSH systems in the presence of different cations. λex = 390 nm. [Thiol/amine] = 1.6 × 10–4 M and [cation] = 1.6 × 10–4 M, [HAuAgGSH] = 0.0132 mg/mL, total volume of solution = 3 mL. Cations were added individually to the CH3(CH2)9SH–HAuAgGSH and CH3(CH2)9NH–HAuAgGSH systems.

The mechanism of Pb(II)-induced fluorescence recovery has been experimentally verified. Primarily, it is considered that due to preferential binding of thiols or amines with the added Pb(II) ions, the prevalent solvent–thiol or solvent–amine interaction is disrupted. Under this circumstance, the EAuAgGSH/HAuAgGSH–solvent interaction is re-established in the nonaqueous medium only as thiol or amines become engaged to bind Pb(II) ion. This has been further verified bringing Na2EDTA into the system. Na2EDTA is a well-known chelating agent that easily binds Pb(II) in solution, and thus the effect of Pb(II) is masked. This let us to conclude that removal of quencher thiols or amines by Pb(II) is the cause of recovery of fluorescence in the nonaqueous medium. However, in aqueous medium, the thiol or amine-mediated quenching of fluorescence of the assembly is due to the complete rupture of the fluorescent assembly. Hence, in aqueous medium, Pb(II) ion cannot restore the fluorescent assembly and thus becomes ineffective. It is worth mentioning that the fluorescence of the assembly increases in the presence of Pb(II) ion when thiol or amine is absent. The reason behind this phenomenon is stable Pb–GSSG interaction that protects fluorescent assemblies from aggregation due to Pb(II)-bound interparticle electrostatic repulsion.

So we conclude that the AuAgGSH behaves in the same way in nonaqueous solvents. When long-chain aliphatic thiol or amine compounds are added to these systems, the solvent–Ag cluster interaction is disturbed. This causes fluorescence quenching. During the experiment, we have also found that the exchange of ligand GSH with aliphatic thiol or amine causes quenching in fluorescence of HAuAgGSH as well as EAuAgGSH and the lost fluorescence is selectively recovered by Pb(II) ions. This is schematically shown in Figure 10. This result is consistent with the Boolean logic operation IMPLICATION (Supporting Information) with more than one input signal. A binary IMPLICATION logic operation (Figure 11), represented by the situation in which the output is 1 when both the inputs are absent or both the inputs are present but output is 0 when either of the inputs are present, can be implemented using HAuAgGSH or EAuAgGSH as the gate and individual aliphatic thiol or amine and Pb(II) as binary inputs. IMPLICATION gates are noncommutative and cannot be expressed by the negation of one of the other basic gates.45,46 However, for WAuAgGSH, such a feasible molecular logic gate cannot be constructed on the basis of the response of the inputs.

Figure 10.

Figure 10

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Schematic representation of the effect of different solvents on the fluorescent assembly.

Figure 11.

Figure 11

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Fluorescence spectral profile for IMPLICATION logic operation on (A) EAuAgGSH using (a) CH3(CH2)15SH and Pb(II) as inputs and (b) CH3(CH2)15NH and Pb(II) as inputs and (B) HAuAgGSH using (a) CH3(CH2)15SH and Pb(II) as inputs and (b) CH3(CH2)15NH and Pb(II) as inputs. Truth table for logic operations on (C) EAuAgGSH and (D) HAuAgGSH. (E) Electronic equivalent circuitry of IMPLICATION logic gate.

Different interactions between the as-synthesized fluorescent assembly and the ligands cause the difference in construction of molecular logic gates. Hence, this is an important work from the academic as well as experimental point of view.

Conclusions

In summary, we have successfully explored the evolution of fluorescent AuAgGSH assembly and then examined the effect of aliphatic thiols and amines of varied chain lengths on the fluorescence property of the assembly in different solvents. It has been proven that destruction and retention of the fluorescent assembly happens in water and organic solvents, respectively, with added thiols and amines. Furthermore, thiol and amine chain length-dependent fluorescence quenching of the assembly has been reported. The quenching of fluorescence in nonaqueous medium is explained considering the disruption of the existing electronic interaction between the solvent and fluorescent assembly. For the nonaqueous systems, quenched fluorescence is recovered bringing Pb(II) ions in the system. Then, using thiol or amine and Pb(II) as two inputs we have constructed the IMPLICATION molecular logic gate. However, only in water, such ligand exchange actually occurs and that ruptures the fluorescent system permanently without leaving a way for fluorescence recovery, so no useful molecular logic gate can be constructed in water medium. Thus, the nature of solvent-controlled application of the fluorescent assembly is proved.

Acknowledgments

The authors are thankful to the UGC, DST, NST, and CSIR, New Delhi, India, and the IIT Kharagpur for financial assistance. The authors are also thankful to Dr. Mainak Ganguly for his valuable suggestions.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01560.

  • MALDI-mass spectra, XPS spectra, fluorescence decay profile, schematic diagram (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao7b01560_si_001.pdf (5.2MB, pdf)

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Supplementary Materials

ao7b01560_si_001.pdf (5.2MB, pdf)

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