Green Synthesis and Reversible Dispersion of a Giant Fluorescent

Aug 2, 2013 - ABSTRACT: A water-soluble highly fluorescent silver cluster on Au(I) surface has been synthesized with green chemistry under sunlight...
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Green Synthesis and Reversible Dispersion of a Giant Fluorescent Cluster in Solid and Liquid Phase Mainak Ganguly,† Jaya Pal,† Sancharini Das,‡ Chanchal Mondal,† Anjali Pal,§ Yuichi Negishi,∥ and Tarasankar Pal*,† †

Department of Chemistry, ‡Department of Biotechnology, 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: A water-soluble highly fluorescent silver cluster on Au(I) surface has been synthesized with green chemistry under sunlight. The evolution of the silver cluster is synergistic, demanding gold and glutathione. The fluorescent Au(I)core− Ag(0)shell particles are huge in size and at the same time they are robust. That is why they become a deliverable fluorescing solid upon drying. Again, the giant particles run into common water miscible solvents. As a result, the fluorescence intensity increases to a great extent without any alteration of emission maxima. In this respect, acetone has been found to be the best-suited solvent. To have a universal applicability of the fluorescent clusters, the particles in the water pool of a reverse micelle have been prepared to transfer the particles into different water immiscible solvents. The comparatively lower fluorescence intensity of the particles has been ascribed to a space confinement effect. Finally, giant-cluster-impregnated yelloworange fluorescent polymer film and fluorescent cotton wool, as well as paper substrate, have been prepared. The antibacterial activity of the fluorescent particle has also been tested involving modified cotton wool and paper substrate for Gram-negative and -positive Escherichia coli and Staphylococcus aureus, respectively.

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Zhang et al.19 and Shen et al.20 have individually stated the photogeneration of fluorescent Ag nanoclusters using poly(Nisopropylacrylamide-acrylic acid-2-hydroxyethyl acrylate) and multiarm star polyglycerol-block-poly(acrylic acid) copolymer based microgel particles as scaffolds. Fluorescent Ag nanoclusters by common polyelectrolyte poly(methacrylic acid) as a template have also been reported by Shang et al.21 Silverexchanged zeolites using heat treatment22,23 and within glass using femtosecond laser irradiation24 are known in literature. Various other irradiation techniques are documented to prepare Ag clusters. Employment of γ-ray, electron, microwave, and polychromic irradiation for the preparation of Ag clusters is also demonstrated by several groups.25,26 All these tactics involve costly synthesis of macromolecular or dendrimeric templates. So water-soluble, robust, biocompatible, template-based synthesis of silver clusters is now a hot area of research. Water-soluble fluorescent silver cluster formation is very often reported, but reports of fluorescent clusters in organic medium are quite rare. Photoreduction to form few-atom silver clusters in different organic solvents using plolymeric template is reported by Diez et al.27 Blue, green, and red emissive silver nanoclusters are reported in various polar and apolar organic

INTRODUCTION

Thiolate-protected gold1 and silver2,3 nanoclusters are ultrasmall (≤2 nm) in size. The continuous density of states of metal nanoparticles get split into discrete energy levels by lowering the particle size, and they meet the Fermi wavelength of electrons, displaying intriguing property alterations in comparison to classical nanoparticles.4−8 The nanoclusters, a subgenre of nanoparticles, act as a missing link between atoms and nanocrystals with noteworthy contribution in basic and applied research. The employment of their molecule-like properties, such as quantized charging9,10 and luminescence,11−13 for application developments is promising day by day. Though gold nanoclusters have been vividly explored over the past decade, only several methods have been reported for synthesizing fluorescent silver nanoclusters due to poor stability. In comparison to the Au nanocluster, the Ag nanocluster shows intense fluorescence, augmenting its efficacy to different key applications. Since olden times, colloidal Ag particles have been employed in classical photographic practices.14 Poly(amidoamine) (PAMAM) dendrimer and DNA templates have been used by Dickson and co-workers to prepare water-soluble fluorescent Ag nanoclusters.15,16 Again, the effect of the variation of sequences of DNA templates on Ag nanoclusters has also been investigated.17,18 © 2013 American Chemical Society

Received: June 28, 2013 Revised: August 1, 2013 Published: August 2, 2013 10945

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solvents with polystyrene-block-poly(methacrylic acid) block copolymer as supportive medium by Diez et al.27 Amazing resemblance in spectroscopic properties is depicted between these clusters and organic dyes, where charge-transfer prevails. Inverted or reverse micelles prepared from surfactant (charged or neutral), oil, and a polar solvent are transparent, homogeneous, and thermodynamically stable solutions.28,29 It is well-known that many surfactant molecules form aggregates as soon as their concentration goes beyond a certain critical value. The aggregate that forms in water or other highly polar, protic solvents with polar head groups pointing outward is named as micelle.29 To the contrary, reverse micelles refer to an aggregates in a bulk nonpolar solvent (e.g., n-heptane) with the polar ends pointing inward.30 When water is introduced in such a reverse micelle, a microemulsion originates. It is actually a nanometer-sized water droplet (“water pool”) encased by a layer of the surfactant. The mostly used surfactant for microemulsions is sodium dioctyl sulfosuccinate (AOT). In nheptane, the radius of the water pool of the AOT microemulsions is proved to be ∼2w0 (in angstroms) (w0 = ratio of the number of water and the AOT molecules).30 Different photophysical processes are very often studied in micelles and reverse micelles.31 Noble metal nanoparticles are known for their antibacterial activity. It is also revealed that the biocidal property markedly depends on the morphology of the particle.32 Several mechanisms have been found for the antimicrobial activities of silver nanoparticles, including their inhibition of a large range of bacterial proteins, enzymes, and DNA; cell membrane perforations; and alteration of selective permeability. As a result of bacteriostatic and bactericidal effects,33,34 a vast range of common pathogens, including bacteria, fungi, and viruses, have been targeted. Silver nanoparticles and their composites have been used widely to generate antimicrobial surfaces on textiles, fibers, and polymers. Besides, they are utilized for wound and burn dressings,35 microbial resistant catheters,36 surgical masks and tools,37 tissue engineering scaffolds, etc.38 Again, for antimicrobial paints,39 water purification, and effluent treatment membranes,40 the antibacterial property is gaining importance. In our present report, a synergistic evolution of fluorescent silver clusters using gold as a template has been reported for the first time via a green chemical approach under solar light irradiation using glutathione as well as S-lactoylglutathione as the reducing and capping agents. We have observed appreciable fluorescence enhancement without any change in emission maxima while the as prepared giant particle, Au(I)core−(Ag2/ Ag3)shell, traverses into homogeneous water−organic solvents system from a body of water. Noble metal clusters that exhibit intense fluorescence are generally ultrasmall4 in size, while our synthesized fluorescent particles are huge in size. So, it is convenient to term them as giant clusters. Again, we have synthesized the same fluorescent particles in a water pool of reverse micelles using dioctyl sulfosuccinate sodium salt (AOT) in heptane. Upon vacuum drying, the fluorescent particles in AOT become redispersible in different water immiscible organic solvents. Furthermore, we have also prepared giant-particle-impregnated fluorescing yellow cotton wool and paper substrates. Common Whatman 41 filter paper and S & S paper substrate (with no background fluorescence) as well as cotton showed efficient antibacterial activity.

Article

EXPERIMENTAL SECTION

Materials and Instruments. All the reagents were of AR grade. Triple distilled water was used throughout the experiment. Chloroauric acid (HAuCl4), silver nitrate (AgNO3), glutathione (GSH), (R)-S-lactoylglutathione, poly(methyl methacrylate) (PMMA), AOT, and 2,7-dichlorofluorescein diacetate (DCFH-DA) were obtained from Sigma-Aldrich. All the solvents were purchased from Merck. Whatman 41 filter paper was purchased from Whatman Ltd. (Maidstone, England). Paper with no background fluorescence was obtained from Schleicher and Schuell, Inc. All glass wares were cleaned with freshly prepared aqua regia, subsequently rinsed with copious amount of distilled water, and dried well before use. UV−vis absorption spectra were recorded in a SPECTRASCAN UV 2600 digital spectrophotometer (Chemito). Reflectance spectra were measured using diffuse reflectance spectra (DRS) mode with a Cary model 5000 UV−vis−NIR spectrophotometer. The fluorescence measurement was carried out at room temperature using a LS55 fluorescence spectrometer (Perkin-Elmer). Fluorescence images were taken with a fluorescence microscope (Olympus DP72). Particle morphology was examined using a field emission scanning electron microscope (Supra 40, Carl ZEISS Pvt. Ltd.). X-ray photoelectron spectroscopy (XPS) analysis was carried out with a VG Scientific ESCALAB MK II spectrometer (UK) equipped with Mg Kα excitation source (1253.6 eV) and a five-channeltron detection system. For ζpotential measurement, a Malvern Nano ZS instrument emsploying a 4 mW He−Ne laser operating at a wavelength of 633 nm was used. Circular dichroism spectra were measured with a circular dichroism spectrophotometer (JASCO International Co. Ltd., Tokyo, Japan). TEM analyses were performed with an H-9000 NAR instrument (Hitachi) using an accelerating voltage of 300 kV. Thermal analyses were performed by a thermogravimetric and differential thermal analyzer (Perkin-Elmer Pyris Diamond TG−DTA). Synthesis of Fluorescent Solution. A curdy, white turbidity is observed as soon as 2.0 mL of 10−2 M HAuCl4 and 3.4 mL of 10−2 M AgNO3 were added simultaneously to 60 mL of 16 × 10−4 M aqueous glutathione. Then the solution was kept under sunlight for photoirradiation for ∼12 h with vigorous stirring. A yellow solution is obtained with high fluorescence intensity (λem = 564 nm). Synthesis of Fluorescent Cotton Wool and Paper Substrate. The curdy turbidity of Ag(I)−glutathione3 is prepared by adding 5.1 mL of 10−2 M AgNO3 to 90 mL of 16 × 10−4 M glutathione. In a 250 mL beaker, 46 mg of cotton wool was submerged in 3 mL of 10−2 M HAuCl4. After aging for 2 h, 90 mL of Ag(I)−glutathione solution was introduced into the beaker containing cotton wool. It was kept for ∼15 h under sunlight with gentle shaking. The solution was then aged in undisturbed condition for ∼3 days. The cotton wool was removed from solution and dried in air to obtain yellow fluorescent cotton wool. The same procedure was performed to prepare yellowish fluorescent paper substrate by employing Whatman 41 filter paper substrate of 4 cm × 3 cm size in lieu of cotton wool. Antibacterial Assay by the Disk Diffusion Method. Nutrient agar plates were prepared aseptically and allowed to solidify. Grampositive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacterial strains were collected from B. K. Roy Research Center, Peerless Hospital, Kolkata, India, and allowed to grow in nutrient broth (HiMedia) for 4 h (start of their log phase of growth). Bacterial suspensions of 50 μL were collected from each of the culture and spread over a nutrient agar plate. The modified 46 mg of cotton wool (area of contact 14 cm2) and paper substrate (area of contact 12 cm2) along with uncoated sterilized cotton wool and paper substrate were placed in nutrient agar plates of both types bacterial cultures. The plates were incubated for 24 h at 37 °C and the zones of inhibition were observed. Antibacterial Assay by Colony Counting. Fluorescent cotton wool was tested for their antibacterial activities against the Gramnegative bacteria E. coli and the Gram-positive bacteria S. aureus. The bacterial strains were collected from B. K. Roy Research Center, Peerless Hospital, Kolkata, India. To test the antibacterial activity, a conventional process was followed. The bacterial strains were 10946

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Figure 1. (A) Scheme showing synergistic evolution; (B) absorption (a), excitation (b) and fluorescence (c) spectra; and (C) fluorescence decay profile of aqueous 2.5 mL fluorescent solution.

Scheme 1. Synergistic Evolution of Highly Fluorescent Ag2 and Ag3 Clusters on Au(I) Surface by Solar Light Irradiation of Ag(I)−Glutathione and Au(I)−Glutathionea

a

A lack of irradiation or the absence of any one component [Ag(I)−glutathione and Au(I)−glutathione] produces a nonfluorescent species.

transferred in nutrient agar plates and incubated at 37 °C for overnight growth. The bacterial colonies from respective agar plates were inoculated in separate nutrient broth medium (HiMedia) at an initial optical density (OD at 660 nm) of 0.1 and incubated at 37 °C for 4 h (0.3 OD at 660 m, beginning of the log phase of bacterial growth). The strains were collected from liquid cultures and washed aseptically with saline water (NaCl, 0.145 M, pH 6.5) two times. The bacterial cells were resuspended in saline water to yield cultures of 121 cfu mL−1 for E. coli and 78 cfu mL−1 for S. aureus. The fluorescent-particlecoated 48-mg cotton wool fabric was suspended in the bacterial cultures of 11.5 cfu mL−1 for E. coli and 7 cfu mL−1 for S aureus, incubated at 37 °C, stirred at 180 rpm for 4 h, along with a negative

control of bacterial culture without any cotton wool and positive control of bacterial culture with sterilized uncoated cotton wool. Samples were collected from each of the cultures after different time intervals (0, 1, and 3 h) to spread over a nutrient agar plate and incubated at 37 °C for 24 h. The viable bacterial cells were counted in terms of cfu mL−1. The ratio of the number of colony forming units from the culture exposed to coated cotton wool (N) and the number of colony forming units from the control bacterial culture (N0) indicated the survival fraction (N/N0) after the exposure to giant fluorescent particle immobilized cotton wool. 10947

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Figure 2. (A) SEM image and line mapping for the elements gold, silver, and sulfur for a single particle; (B) TEM image (a) and EDAX analysis (b); (C) optical (a) and fluorescence (b) image; and (D) XPS spectra under freeze-drying condition for gold (a), silver (b), and sulfur (c).

Figure 3. Effect of successive addition of (A) Au(III) and (B) Ag(I) into 2.5 mL of aqueous fluorescent solution. (C) Plot of I0/I vs [Au(III)] and [Ag(I)].

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Figure 4. XPS of the black precipitate obtained after the addition of Au(III) to the presynthesized aqueous fluorescent solution.

Figure 5. (A) Absorption spectra and (B) fluorescence spectra when different silver salts (AgX, X = NO3−, ClO4−, SO42‑, CO32‑, and CH3COO−) are added during the synthesis of aqueous fluorescent solution (2.5 mL).

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RESULTS AND DISCUSSION Yellow-colored highly fluorescent water-soluble Au(I)core− Agshell giant particles with an emission maximum of 564 nm have been obtained via a green chemical approach under sunlight. The ratio of Au(III):Ag(I):glutathione needs to be 1:1.7:5 to produce the most intense solution in almost neutral pH, and the fluorescence image indicates that the fluorescent particles exhibit yellow-orange coloration. Different signatures in absorption and excitation spectra support cluster formation. Lifetime measurement of the solution implies that three different components, 5.3 ns (24%), 0.3 ns (46%), and 75.5 ns (30%), are there, with an average lifetime of 24 ns (Figure 1). Moreover, the fluorescence property of such a solution is remarkably stable without any decrement of fluorescence intensity or change in emission maximum for months together

at room temperature. A trial experiment with Au(III) or Ag(I) alone in GSH does not generate the fluorescing species, indicating the need for both gold and silver in the reaction mixture. Both of them are crucial to obtain fluorescent silver clusters, so a unique synergism is observed here (Scheme 1). SEM, elemental line mapping, TEM images, EDAX from TEM images, fluorescence images, and XPS data are presented as Figure 2 for confirmation of the structure of the fluorescent particles. SEM images indicate that the fluorescent particles are spherical and large in size. TEM images also corroborates huge, spherical core−shell particles, and for the present case, they are Au(I)core−Ag(0)shell in nature. We have to have [Ag]/[Au] = 1.7 as definitive precursor concentrations for evolution and stabilization of the particles under consideration. But EDAX analysis of the as prepared fluorescent particle reveals that, at 10949

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Figure 6. (A) Structure of S-lactoylglutathione. (B) Absorption (a), fluorescence (b), and lifetime (c) spectra of the fluorescent aqueous solution obtained with S-lactoylglutathione. (C) CD spectra of the aqueous solution of S-lactoylglutathione, S-lactoylglutathione with Au(III) and Ag(I) just after mixing, and S-lactoylglutathione with Au(III) and Ag(I) after sunlight exposure. (D) Optical (a) and fluorescence (b) image of the drop-cast fluorescent solution on a glass slide obtained from S-lactoylglutathione.

performed ESI mass analysis, a much softer ionization technique43 than MALDI, to check for the formation of any larger clusters. The spectrum does not support the formation of larger clusters and residual Ag salt. To the contrary, formation of Ag2, Ag3 clusters is further supported by ESI-MS analysis (Figure S2, Supporting Information). To confirm the fact that fluorescence is coming from silver clusters located on the Au(I) surface, we have done a control experiment. None of the reagents (GSH, AgNO3, HAuCl4) exhibit fluorescence. Again, GSH, GSH/Au(III), and GSH/ Ag(I) independently do not show any fluorescence after sunlight exposure, but a mixture of GSH/Au(III)/Ag(I) with the reported proportion, after sunlight irradiation, produces a pale yellow solution that exhibits strong fluorescence. The absence of any plasmon band due to silver or gold supports the formation of clusters, as reported by different groups.3 MALDI mass analysis has confirmed the formation of Ag2, Ag3 clusters responsible for fluorescence. Different groups have demonstrated that silver clusters having two to eight atoms are uniquely fluorescing.3,15 Moreover, the emission maximum of our fluorescent solution is ∼564 nm, which has been reported for Ag2 clusters by Maretti et al.44 They also explained that such fluorescing silver clusters need a positively charged environment for stabilization. In our case, the in situ produced Au(I) serves the purpose of stabilization. Mechanistically, it can be spelt out that the drift of electron density toward Au(I) core is mainly responsible for the generation of the long-term stability for fluorescent silver clusters. Actually, they decorate the Au(I) surface as a shell. Owing to this, the huge core−shell particle as a whole becomes fluorescing, and fluorescence microscopic images display large yellow-orange fluorescing spherical particles. It is found that addition of extra Au(III) or Ag(I) to the already prepared fluorescent solution causes the reduction of fluorescent intensity. Interestingly, the decrease of fluorescence is more drastic with Au(III) than Ag(I). This may be due to the higher charge density of Au(III) and a heavy ion effect. Upon addition of Au(III) or Ag(I) into the fluorescing solution, a

Figure 7. Fluorescence spectra of the fluorescing solution in different water miscible organic solvents.

[Ag]/[Au] > 9, gold is distinctly covered by silver clusters. Note that EDAX provides only surface information. Furthermore, elemental mapping also detected a trace amount of gold and an abundance of silver and sulfur. The result is an outcome of reproducible data for more than nine replicate measurements. Yellow-orange large fluorescing particles are also observed under fluorescence microscopy after the asprepared solution is drop-cast and dried on a glass slide. XPS analysis under freeze-drying condition indicates that Au is in the +1 oxidation state while Ag is in the zero oxidation state. The peak of binding energy at ∼84.8 and 88.5 eV corresponds to Au(I)4f7/2 and Au(I)4f5/2, respectively.41 For the case of silver, the peak of binding energy has been found to be 368.06 and 374.08 eV, representing Ag(0)3d5/2 and Ag(0)3d3/2 respectively.42 The XPS measurement rules out the presence of Ag+ species with the fluorescing particles. Moreover, the exposed solution does not produce any turbidity with NaCl solution, which authenticates the absence of Ag+ ion in the fluorescing solution. MALDI mass analysis (Figure S1, Supporting Information) has confirmed the formation of Ag2, Ag3 clusters. We have also 10950

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Figure 8. Schematic representation of the formation of fluorescent solution in different organic solvent by an AOT-mediated route. (A) Fluorescence image (under blue light exposure), (B) optical image, (C) fluorescence decay profile, and (D) TEM image of fluorescent particles obtained from heptane solution. (E) Fluorescence spectra and (F) the corresponding bar diagram of fluorescent solution in different water immiscible organic solvents.

Figure 9. Schematic representation of the formation of a fluorescent film in the PMMA matrix. (A) Digital image of fluorescent PMMA film on a glass slide and (B) digital image of fluorescent PMMA film after removing the film from the glass slide. (C) optical and (D) fluorescent image (under blue light exposure) of the fluorescent PMMA film.

fluorescent solution after the addition of metal ion) is much steeper for gold than silver (Figure 3). Furthermore, spin−orbit coupling of heavy metals as mentioned already may also be a good reason for the decrease of fluorescence as reported.45 It is interesting to mention in this context that addition of excess Au(III) produces a white, curdy precipitate, and aging for 1 day gives rise a black precipitate. XPS analysis also finds an

white turbidity appears in both the cases, indicating the formation of Au(I)/Ag(I)−GSH complex. Thus, the capping agent of the fluorescent core−shell structure is removed, causing a diminution in the stability of the fluorescing solution. Hence, a decrease of fluorescence intensity of the solution is understandable. As a consequence, I0/I (I0 = fluorescence intensity of fluorescent solution; I = fluorescence intensity of 10951

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Figure 10. (A) FIB image of the fluorescent paper substrate under low (a) and high (b) resolution. (B) TEM image of fluorescent paper substrate under low (a) and high (b) resolution. (C) FESEM image of fluorescent paper substrate under low (a), medium (b), and high (c) resolution.

Figure 11. (A) low (a) and high (b) resolution FESEM image. (B) TEM image of treated cotton wool. (C) Absorption spectra (obtained from DRS measurement) of the fluorescent (treated) and untreated cotton wool.

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and Lucas.47 The binding energy for Ag3d5/2 is shifted toward higher energy (368.8 eV), indicating that silver is in the zero oxidation state and the Ag2/Ag3 clusters are destroyed with the decrement in fluorescence intensity (Figure 4). We also synthesized fluorescent solution by employing different silver salts (with different counteranions) in the presence of glutathione and HAuCl4. We have found the highest degree of fluorescence intensity when AgClO4 is introduced. It is probably due to larger size of ClO4−, which exerts a weak ligand field effect. Thus, the counteranion, ClO4−, adds an insignificant effect on Ag+. As a result, Ag+ is free to react, exhibiting highly fluorescent solution. Large-sized ClO4− being attached to positively charged (ζ-potential value +11.9 mV) giant clusters may also contribute further rigidity with increased emission intensity, as reported by Szabó and Marek.48 Here, it is worth noting that water insoluble silver carbonate can also produce the same type of fluorescent solution with unaltered emission maxima (Figure 5 and Figure S3, Supporting Information). Not only glutathione but optically pure (R)-S-lactoylglutathione also has been used to produce a yellow-colored solution with high fluorescence intensity with emission maximum at 575 nm. Lifetime measurement of the solution implies that three different components, 5.2 ns (28%), 0.37 ns (45%), and 70.6 ns (27%), are there, with an average lifetime of 21 ns (Figure 6). So, for the case of glutathione and S-lactoylglutathione (where sulfur is attached to the chiral carbon via a carbonyl group), the fluorescence behavior of green chemically produced core−shell giant clusters remains the same. The aqueous solution of asymmetric S-lactoylglutathione exhibits a band with a maximum at ∼275 nm in the circular dichroism (CD) spectra. It is eliminated just after addition of Au(III) and Ag(I), indicating the decrease of chirality. Of course, in the fluorescent solution, the band for chiral S-lactoylglutathione is absent, indicating the loss of its asymmetric nature (Figure 6). The

Figure 12. (A) Optical image (a), fluorescence image after UV exposure (b), and fluorescence image after blue light exposure (c) of the untreated paper substrate. Optical image (a1), fluorescence image after UV exposure (b1), and fluorescence image after blue light exposure (c1) of the fluorescent (treated) paper substrate. (B) Optical image (a), fluorescence image after UV exposure (b), and fluorescence image after blue light exposure (c) of the untreated cotton wool. Optical image (a1), fluorescence image after UV exposure (b1), and fluorescence image after blue light exposure (c1) of the fluorescent (treated) cotton wool.

appreciable amount of Au(III) along with Au(I).46 The broadened band for sulfur in the black precipitate corroborates the formation of gold sulfide. Peaks at 84.8 and 89.0 eV are due to Au−S7/2 and Au−S5/2, respectively, as reported by Johnston

Figure 13. (A) Figures showing the fabricated sample holder for solid-state fluorescence measurement. (B) Fluorescence spectral profile obtained using Whatman 41 paper substrate (treated and untreated) posted in the sample holder. (C) Absorption spectral profile (obtained from DRS measurement) obtained using Whatman 41 paper substrate (treated and untreated). 10953

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Figure 14. (A) XPS spectra of fluorescent cotton wool (left) and fluorescent paper substrate (right) for gold (a), silver (b), and sulfur (c). (B) TG− DTG−DTA of the fluorescent cotton wool (a), untreated cotton wool (a1), fluorescent paper substrate (b), untreated paper substrate (b1).

fluorescent clusters display much higher fluorescence intensity than in water. It is known that with the decrease of solvent polarity an increase of the fluorescence intensity with a blue shift in emission maxima takes place.53 A careful observation reveals that with the increase of the dielectric constant(ε)/ solvent polarity, fluorescence intensity is enhanced with unaltered emission maxima. It indicates the stability of Ag2/ Ag3 clusters on the surface of Au(I) to be pivotal for fluorescence increment. The silver clusters are so robust, being embedded on the Au(I) surface, that the emission maximum is almost indifferent to solvent polarity. Solvents with higher polarity contribute higher stabilization of the positively charged core−shell particles. As a consequence, the fluorescent particles exhibit high fluorescence intensity in methanol (possessing high dielectric constant), where oxygen is in sp3 hybridization. The emissive nature of Ag clusters is due to the journey of electrons from the submerged and quasi-continuum 5d band to the lowest unoccupied conduction band of Ag clusters (an interband transition).4 It is facilitated by the positively charged environment.44 Again, the sp2 oxygen of acetone (with inherent affinity toward the positively changed clusters) explains the supremacy of acetone even with its comparatively low dielectric constant. The strong silver−

partial breakage of the C−S bond and the formation of the S−S bond [broad low intense band for π(S−S)−σ*(S−S) transitions] along with the production of fluorescent clusters may provide an excuse of the reduction of chirality of the molecule.49−51 Again, the yellow color of the fluorescent solution may be evidence for the formation of elemental sulfur responsible for long-term stability. Retention of the yellow coloration of the fluorescent solution after addition of CS2 (in which elemental sulfur is highly soluble) and the synthesis of such a yellow-colored fluorescent solution in CS2 (described later) speak against such inference. A recent theoretical study52 demonstrates that glutathione is very much prone to intramolecular hydrogen bonding (unlike cystein) with noble metals, which may convert the moiety as a whole to achiral just after the addition of noble metals in glutathione. Such specialty of glutathione is also responsible for the formation of fluorescent particles, unlike cysteine, at least in our experimental conditions. To investigate the effect of solvents, we have redispersed fluorescent core shell particles in various water miscible solvents, i.e., ethanol (ε = 24.5), methanol (ε = 32.7), acetone (ε = 20.7), acetonitrile (ε = 37.5), and 2-propanol (ε = 18.2), along with water (ε = 80.1) (Figure 7). In all the solvents, giant 10954

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some extent broad in the AOT-mediated route. It is important to mention in this context that just by vacuum drying the fluorescent heptane solution it becomes redispersible in different organic solvents (choloform, carbon disulfide, diethyl ether, heptanes, hexane, tetrahydrofuran, and toluene). Tetrahydrofuran and hexane are found to have the highest emission intensity, and chloroform exhibits the lowest intensity after the redispersion of the fluorescent particles in the AOT matrix in a different water immiscible solvent. But in any case, no alteration of the fluorescence band position is noticed, unlike the reports by Diez et al.27 It indicates that our yellowcolored fluorescing Ag2/Ag3 particles are very stable and robust in nature, probably due to the Au(I) support. According to the report of Eastoe et al.,30 the radius of the water pool (w0 = 10) of the AOT microemulsions in heptane in the reverse micelle system that we have used to generate fluorescent particle is ∼200 nm. Again, the obtained fluorescent particle in AOT/ heptane is ∼200 nm, unlike the giant clusters in normal water. So, the size of the water pool has a great role to tune the size of the fluorescent particle. In this context, it should be mentioned that in the reverse micelle the comparatively smaller particles exhibit to some extent lesser fluorescence intensity. This is due to the size confinement in the reverse micelle microreactors hindering the accumulation of a sufficient amount of Ag2/Ag3 clusters on the Au(I) surface (Figure 8). Furthermore, PMMA is dissolved in toluene containing fluorescent particles and spread over the glass slide homogeneously. As a result, a fluorescent thin film on the glass slide is produced. A pale yellow, highly fluorescent film is obtained by removing it from the slide. It exhibits a nice yellow emission by UV or blue light exposure (Figure 9). The extraordinary stability of the giant fluorescent clusters has prompted us to produce fluorescing cotton wool and fluorescing paper substrate for myriad applications. Gentle shaking of chloroauric acid soaked cotton wool and paper substrate in Ag(I)−SG solution under sunlight produces the fluorescing-microparticle-impregnated cotton wool and paper substrate. FESEM image reveals that on the cellulose fibers of the paper substrate, fluorescent particles cling to and decorate the fibers uniformly. A TEM image of the nanoparticlemodified paper substrate shows closely packed spherical particles on the paper surface. Again focus ion beam (FIB) analysis corroborates the TEM observation. Unlike the unmodified part of the paper substrate, microparticle-modified paper substrate appear to have a closely placed macaroni-like arrangement due to the core shell nature of the particles (Figure 10). Line mapping (Figure S5, Supporting Information) on the coated surface of the paper substrate indicates the abundance of silver and sulfur. No signature for gold is obtained from the mapping, indicating deeply buried gold underneath silver and GSH, while EDX elemental mapping provides surface information. For cotton wool also, closely placed particles are obtained by the FESEM analysis on the surface of cotton wool fiber. The TEM image also supports the core−shell nature. From the absorption spectrum, it is found that a broad peak at ∼300 nm is generated from the wellpressed modified cotton wool (made as a pellet), unlike the unmodified one (Figure 11). Fluorescence microscopic image reveals that both the paper substrate and cotton wool are highly fluorescent, emitting yellow color with UV as well as blue light excitation. In this context, it should be mentioned that ordinary paper substrate

Figure 15. Antibacterial activity of the fluorescent cotton wool in agar plate for E. coli (a) and S. aureus (b). Antibacterial activity of the fluorescent paper substrate using agar plate for E. coli (a1) and S. aureus (b1).

Table 1. Antibacterial Activity Assay of the Fluorescent Cotton Wool against E. coli and S. aureus duration of treatment (h) T0 T1 T3 T0 T1 T3

cfu mL−1 (×106)

N/N0

E. coli 11.5 100 6 3.7 0.8 0.09 S. aureus 7 100 3 5.3 2 1.4

reduction in viability (%) 0 96.29 99.91 0 94.7 98.6

nitrogen affinity reduces the fluorescence intensity of the Ag clusters in the case of acetonitrile to some extent. The compromise among solvent polarity, chemical interaction, and the electron density of the coordinating atom of the solvents proves acetone to be the best-suited solvent with 6 times larger emissive property than water. It is to be noted that water with a high enough ε value is also not an appropriate solvent for fluorescence study, due to an intermolecular hydrogen bond reported by different groups,54 as depicted in the comparative account in Figures 7 and S4 (Supporting Information). To prepare the fluorescing species in water immiscible organic solvents, we have introduced 0.1 M AOT in heptane. In the 6 mL AOT/heptane solution, we have added glutathione and aqueous chloroauric acid as well as silver nitrate to obtain the same final concentrations of all three ingredients, as mentioned in the Experimental Section for preparing fluorescent hydrosol. After shaking, the heptane−water phase separation changes to a homogeneous solution with the formation of reverse micelle with aqueous microreactors. Then the solution is put under the sun for ∼15 h to obtain the fluorescent particles in the water pools of the reverse micelles. The pale yellow solution becomes highly fluorescent. The emission maximum is also ∼570 nm, but the peak is to 10955

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results, it can be depicted that fluorescent cotton wool is marginally less effective for Gram-positive bacteria than Gramnegative bacteria within the initial 3 h of incubation. Giant-particle-modified paper substrate as well as cotton wool was separately incubated in 30 mL of 0.05 M DCFH-DA solutions for 3 h. Conversion of nonfluorescent DCFH to highly fluorescent 2′,7′-dichlorofluorescein (DCF) was not observed, which speaks against the formation of free radicals and reactive oxygen species (ROS).58 Consequently, the antibacterial activity is due to the direct contact of Au(I)core− Ag(0)shell with the bacterial cell. It was found from previous studies that noble metal nanoparticles generally change bacterial membrane permeability and cause several membrane proteins to be nonfunctional by attaching on their active site.59 The cell membrane of Gram-negative bacteria (e.g., E. coli) is made up of dense lipopolysaccharides, while Gram-positive bacteria (e.g., S. aureus) possesses an additional layer of peptidoglycons, which may be the possible reason for marginal differences in reduction of viability between this two types of bacteria. In a nutshell, we have synthesized highly stable giant fluorescent clusters under sunlight. They are so robust that they are transformed into a solid powder that is redispersible in different water miscible solvent systems without any significant change in the emission maximum. The highly stable fluorescent clusters can also be synthesized in water immiscible solvent by employing AOT. Furthermore, the creation of fluorescent film, cotton, and paper substrates (with efficient antibacterial activity) has been accomplished by exploiting the giant cluster. So, widespread applications of the fluorescent material have been focused on in the paper. Several experimental results (MALDI, ESI mass, absorption,3 and emission spectra44) clearly demonstrate the presence of Ag2 and Ag3 clusters to be the root cause of fluorescence. TEM and EDAX spectra at different sites support the formation of Au(I)core−Ag(0)shell. Of course, the thickness of the silver shell is small. As a result, the contrast in the image is not very prominent. But careful examination certainly reveals the presence of the silver shell in the TEM image. By point-topoint sweeping the focus of the microscope, we have always obtained exclusively such giant particles. The SEM image (Figure S7, Supporting Information) at low resolution shows a good number of large particles. Again, emission spectra (that matches well for Ag2−Ag3 clusters44) reveal that the large particles are not the resultant effect of aggregation of tiny silver clusters. Moreover, aggregation causes a red shift60 of the emission maxima and gradually destroys the fluorescence of the clusters.43 This is not at all found in our cases, supporting the robustness of the fluorescing species. Mass spectra also support our proposition, i.e., the presence of fluorescent Ag2−Ag3 clusters. The only organic compound we have used is GSH. GSH and oxidized glutathione (may be produced during giant cluster synthesis) are completely nonfluorescent in nature. The synthetic protocol is synergistic. Without the employment of gold during the synthesis, no fluorescent particles are obtained. This supports that gold has a pivotal role for the synthesis and stabilization of silver clusters. So, the possibility of agglomeration of Ag clusters with some organics as a source of fluorescence is ruled out. Point-to-point elemental mapping and EDAX analyses (only collect surface information) indicate a trace amount of gold and abundant silver [though we have introduced Ag(I) only 1.7 times higher in concentration than Au(III) in the reaction mixture] in the fluorescent particles. All

possesses faint fluorescence while the treated fluorescence is strongly yellow fluorescing (Figure 12). Like the fluorescent cotton wool, fluorescent paper substrate also possesses a broad absorption band ranging from 280 to 410 nm, unlike untreated paper substrate (obtained from absorbance mode of DRS spectra). The absorbance is comparatively lower for fluorescent paper substrate than for fluorescent cotton wool, implying less clinging of fluorescent particles on the paper surface. To obtain the fluorescence spectral profile from the treated and untreated paper substrate, laboratory-made sample holders were used. A steel sample holder for solid state (with paper substrate) fluorescence studies has been fabricated, as shown in Figure 13a. It is used simply in place of cuvettes, as its dimensions matches those of 1-cm fluorescence cuvettes. Only a steel plate with an open window (2.6 cm × 0.9 cm) and two screws sandwich the paper substrate for front side excitation in the sample holder. A somewhat broad fluorescence spectrum is obtained from the fluorescent paper substrate with emission maxima at 583 nm (Figure 13). The untreated paper substrate also bestows a faint peak at this region. By background subtraction, we find a spectral profile similar in nature to that of fluorescent hydrosol with a 19 nm red shift and inhomogeneity broadening.55 S & S paper substrate that contains no background fluorescence has also been employed, and the similar fluorescence spectrum is obtained, as depicted in Figure S6 (Supporting Information). Again, XPS analyses for the fluorescent-particle-treated cotton wool and paper substrate show the presence of both Au(I) and Ag(0) along with a broad peak for sulfur. The broadening of the peaks in the XPS spectra (Figure 14A) is owing to the inhomogeneity in the sulfur environment associated with sample charging being anchored on paper substrate and cotton wool, as reported by Unsworth et al.56 From the DTA plot, a broad endotherm is found for the untreated cotton wool from 302.6 to 389.3 °C, with a maximum at 358.1 °C. Out of this endotherm, a sharp polyester melt peak and cotton wool decomposition as well as polyester decomposition appeared. In the case of treated cotton wool, an exotherm at the maximum of 346.5 °C is obtained in lieu of an endotherm for the untreated cotton wool, indicating a dominating amorphous-to-crystalline transition, as reported by Salkar et al.57 The TG plot implies a new peak at 215 °C due to melting and decomposition of glutathione and oxidized glutathione for the treated cotton wool. Besides, the TG plot indicates the increase of thermal stability of the cotton wool treated with fluorescent particles. Of course, thermal stability is increased for treated paper substrate also. But alteration is only minute for paper substrate, unlike cotton wool, indicating a larger amount of microparticles anchored in cotton wool than paper substrate (Figure 14B). Paper substrate and cotton wool impregnated with fluorescing giant particles show antibacterial activity against both Gram-positive (S. aureus) and Gram-negative (E. coli) bacterial strains. Clear zones around the fluorescent cotton wool and paper substrate are observed in the case of the disk diffusion assay method (Figure 15). The presence of fluorescent-microparticle-impregnated cotton wool in bacterial culture causes reduction in bacterial number for both Grampositive and -negative bacteria, and the results are summarized in Table 1. It is observed that the fluorescent cotton wool shows 99.91% reduction in viability of Gram-negative bacteria within 3 h of incubation, while 98.6% is found in case of Grampositive bacteria within the same duration. From the above 10956

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the experimental findings support that silver, being present at the surface of the giant particle, covers gold. To quantify the fluorescence intensity of the fluorescent solution, quantum yield has been calculated using quinine bisulfate (1 N H2SO4) as reference. A quantum yield of 6.1% is found for the fluorescent aqueous solution (and many times higher in organic solvent) containing giant clusters, implying the strong fluorescing nature of Au(I)−Ag2/Ag3 giant clusters.

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CONCLUSIONS Robust and giant fluorescent clusters are evolved by a green chemical approach under solar light irradiation. Though the fluorescent particles as a whole are huge in size, the main fluorescing entities are the tiny Ag2/Ag3 clusters located on the Au(I) surface. The synthesis has been proved to be synergistic. The particles traverse into solid and liquid medium without any remarkable change in fluorescence spectral profile. Thus, the particles with fluorescence become promising for cell imaging and nanophotonics. The presence of silver clusters makes the as-synthesized product very efficient for antibacterial activity for both Gram-positive and -negative bacteria. Potential applications include wound-burn dressing, microbial resistant catheters, surgical masks and tools, tissue engineering scaffolds, and food packaging, but the giant-particle-treated cotton wool and paper substrate may also be used for a paper-substratebased sensor for rapid and portable biochemical detection owing to their strong and robust fluorescence property.

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ASSOCIATED CONTENT

S Supporting Information *

MALDI-mass, ESI mass, bar diagram, elemental mapping, fluorescence spectra, and SEM imaging. 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 Authors are thankful to Prof. A. Gedanken of Israel for FIB analysis. The authors are also thankful to the UGC, DST, NST, and CSIR, New Delhi, India, and the IIT Kharagpur for financial assistance. Again, the authors are thankful to Ms. Isozaki of Tokyo University of Science, Tokyo, Japan, for XPS measurement.

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REFERENCES

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