Facile synthesis and size dependent visible light photo catalytic properties of bio-compatible silver nanoclusters

Accepted Manuscript Title: Facile synthesis and size dependent visible light photo catalytic properties of bio-compatible silver nanoclusters Authors: Tapas Goswami, Manjeet Singh, K. Mohan Reddy PII: DOI: Reference:

S0025-5408(18)31480-6 https://doi.org/10.1016/j.materresbull.2018.08.008 MRB 10129

To appear in:

MRB

Received date: Revised date: Accepted date:

14-5-2018 2-8-2018 3-8-2018

Please cite this article as: Goswami T, Singh M, Reddy KM, Facile synthesis and size dependent visible light photo catalytic properties of bio-compatible silver nanoclusters, Materials Research Bulletin (2018), https://doi.org/10.1016/j.materresbull.2018.08.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis and size dependent visible light photo catalytic properties of biocompatible silver nanoclusters Tapas Goswami*, Manjeet Singh and K. Mohan Reddy Department of Chemistry, University of Petroleum and Energy studies, Dehra Dun, Uttarakhand,

SC RI PT

248007, India *

Corresponding author: [email protected]

N

U

Graphical Abstract

M

A

Fe(OH)2+ Fe2+ + •OH

LUMO

D

Visible Light

e-

TE

Agm(SG)n h+

OH-

•OH

A

CC

EP

HOMO

H2O •OH

MO or RhB

Synthesis-A

Synthesis-B

Synthesis-C

Catalytic performance

Products (CO2+H2O)

Highlights

Facile synthetic methodology to prepare biocompatible silver nanoclusters ligated with

SC RI PT



glutathione. 

Controlled size and composition of AgNC was achieved and fully characterized.



Visible light photo catalytic performances AgNCs are observed to be size dependent.



Catalytic efficiency was found to be enhanced upon reduction in size of AgNCs in visible

A

N

U

photo-Fenton like processes.

M

Abstract

Recently tremendous research efforts have been carried out to synthesize metal nanocluster

D

owing to their wide applications in bio-imaging, nonlinear optical materials and catalysis. To

TE

explore such applications a precise control of size and composition of the metal nanoclusters are

EP

highly desirable. Here we report controlled synthesis and characterization of water soluble silver nanocluster ligated with glutathione. We have developed facile methodologies to prepare

CC

nanoclusters with distinct optical and catalytic properties. To achieve tunable size, optical property and chemical composition, here we mainly modulate the nature of metallic salt

A

precursor, reducing agent and reaction conditions. Catalytic properties of as synthesized nanoclusters are explored in visible light induced photo-Fenton like processes for the degradation of model water pollutants such as methyl orange (MO) and Rhodamine B (RhB). The present research work investigates the influence of size of silver nanoclusters (Ag NCs) on photo Fenton like processes under visible light.

Keywords: nanocluster, biosensor, glutathione, fluorescence, catalysis

1. Introduction

SC RI PT

Metal nanoclusters with precise chemical composition are very recently discovered [1-8]. It has drawn a considerable focus in the field of nano-science and technology, because of their promising wide range of application in bio-photonics, nonlinear optical materials, catalysis etc. [4, 9-13]. Owing to their ultrafine size, good bio-compatibility and excellent photo stability, metal clusters are possible potential replacement of commonly used semiconductor quantum dots,

organic

dye

molecules

in

biomedical

and

optoelectronic

industry

[13-18].

U

Photoluminescence property of the nanocluster is one of the enabling factors for the diverse

N

application [19-20]. Very recent experimental and theoretical study confirmed that fascinating optical properties are not only because of metal core (quantization effect), it could also be tuned

A

by surrounding ligand environment [21-27]. Now days there are wide range of recipes to

M

synthesize ligated nanocluster with tunable optical properties [28-31]. However it possesses severe limitations in their practical applicability due to very low quantum yield (QY). A lot of

D

effort has been devoted very recently on enhancing the photoluminescence efficiency of gold

TE

nanocluster by various methods. Recent studies suggested that drastic enhancement of photoluminescence could be obtained by controlling solvent or cation induced aggregation of metal-thiolate complex [32-37]. However, this methodology produced inhomogeneous emitter

EP

with poor photo-stability. Based on fundamental properties of electron correlation of noble metals and ligand-to-metal-metal charge transfer (LMMCT) relaxation, Lee and co-workers were

CC

able to enhance photoluminescence with ultrahigh efficiency [38]. But the drawback of their strategy was the poor stability of the nanocluster in aqueous medium. In very recent publications,

A

method of rigidification of ligand shell around metal core has been demonstrated and it has been shown that host-guest interaction is one of the key enabling factor in enhancing the photoluminescence of ligated nanocluster [39]. Based on the understanding of interaction of ligand with metal, an appropriate choice of reactant materials (ligand, metals, and reducing agents) is essential. Kinetics, thermodynamic parameters of reaction also play an important role in achieving reproducible desired products [40]. Therefore, it offers an urgent challenge to find a

novel reproducible strategy to design and fabricate water soluble, large quantum yield, homogenous emitter nanomaterials for practical biosensor application. Many previous synthetic routes have been reported for the synthesis of Ag and Au nanoclusters where polydispersity of the nanoclusters are major drawbacks [41-42]. Among them, most of the have

utilized

sophisticated

separation

techniques

like

polyacrylamide

gel

SC RI PT

methods

electrophoresis (PAGE) and cut-off filters to get desired size and composition of nanoclusters. In addition to controlling experimental parameters, the ligating agent must also be bio-compatible in view of the bio-sensor applications of synthesized nanocluster. Glutathione which is a natural tripeptide and it has been found to be a highly promising ligating agent in the development of nano-biosensor [15,16]. As it has been known that the silver nanoparticle is found to exhibit

U

cytotoxicity, toxicity of silver nanocluster stabilized by glutathione was investigated in the recent past [15, 43-44]. These previous studies have concluded that these silver nanoclusters stabilized

N

by glutathione showed no cytotoxicity. It has been reported that the strong metal-thiol co-

A

ordination in the glutathione stabilized nanocluster improved the biocompatibility [45].

M

Glutathione (GSH), a natural peptide being already present in cell gives the added advantage to the biocompatibility of the metal clusters. Glutathione ligand contains additional groups such as

D

–COOH, -NH2 to be bonded with drug molecules which makes it a good candidate for disease

TE

therapy [44].

Noble metal nanocluster has drawn a considerable interest in nano-photocatalysis due to its nonmetallic semiconductor behavior [46]. It has been demonstrated that metal nanocluster with

EP

size less than 3 nm possesses very high reduction potentials which makes them efficient as catalyst in electron transfer reactions [47, 48]. To utilize the catalytic property more efficiently,

CC

facile synthesis and well defined size control is highly desirable [49-51]. Catalysis using silver nanoparticles have drawn a considerable attention as it is less expensive in comparison to the

A

other noble metals [49, 52-54]. Size dependent catalytic activity of silver nanoparticle has also been studied in hydrogentation of choloronitrobenzene [49]. Nanoclusters of silver have been recently used as enhanced catalyst in reduction processes of nitro group [48]. To evaluate the catalytic properties of silver nanocluster we have chosen photo Fenton like processes for the degradation of dyes which are common effluents of textile industry. This photo Fenton like process is selected for study as it has many advantages such as non-expensive, non-toxic, cost

effective and efficient for water treatment [55-56]. However it requires UV irradiation for the oxidation of organic molecules. Use of silver nanocluster in photo-Fenton like processes allows degrading organic molecule under visible light with less amount of Fenton reagent. In this report, we demonstrate the easy synthetic method to control the size, chemical

SC RI PT

composition and optical properties of silver nanocluster stabilized by non-toxic, bio-compatible ligand, glutathione. Size dependent visible light catalytic performance of silver nanoclusters prepared by different synthetic methodologies is thoroughly investigated. 2. Materials and methods 2.1 Materials

U

Reduced L-glutathione reduced (GSH), methanol, diethyl ether, silver nitrate (AgNO3), silver

N

lactate (C3H5AgO3), silver trifluoroacetate (CF3CO2Ag), triethyl amine, sodium borohydride (NaBH4), tetramethyl ammonium borohydride ((CH3)4NBH4) were purchased from Sigma-

A

aldrich and were used without further purification. For catalytic test Methyl orange, Rhodamine

M

B, FeCl3, H2O2 were purchased from MERCK and MOLYCHEM and used without further purification. MilliQ water was used for all experiments. All glassware used in this study was

D

washed with aqua regia and rinsed with MilliQ water.

TE

2.2 Synthesis and purification of nanoclusters ligated with glutathione Silver nanoclusters (Ag NCs) are synthesized according to the methodologies described

EP

elsewhere with suitable modification [41, 57-59]. The variable parameters used for different methodologies are nature of precursor silver salt, reaction medium, nature of reducing agent and

CC

agitation speed. Following three synthetic methodologies are used to synthesize and purify silver nanoclusters ligated with glutathione.

A

Synthesis-A

0.0503 gm (≈ 0.25 mmol) AgNO3 was dissolved in 50 ml deionized water and was taken in 100 mL three neck RB flask kept in an ice-bath. 0.308 gm (≈ 1 mmol) glutathione was added to silver nitrate solution. Cloudy white precipitate was formed. Solution was stirred for 40 min in an ice

bath and kept in dark and sealed container to ensure minimal exposure to atmosphere and sunlight. Aqueous ice cold solution of 12 mL 0.2 M sodium borohydride (0.094 gm) was added dropwise with vigorous stirring at 1200 rpm. Gradual colour changes from yellow to deep brown were

SC RI PT

observed. Solution was stirred for another 4 hours at 400 rpm in ice bath. Synthesis-B

In this synthesis, we followed the same methodology as described above with little modifications. The precursor salt we used here was silver trifluoroacetate in methanol. Cloudy yellowish white precipitate was observed when glutathione (301 mg) was added in the solution of silver trifluoro acetate (55 mg) in 40 mL methanol. Precipitate was dispersed by the dropwise

U

addition of triethyl amine (≈ 1 mL) and pH of the solution was raised to pH ≈ 9. Solution was

N

kept under agitation at 300 rpm for 3 hours period. Then 230 mg of tetramethyl ammonium

A

borohydride (TMAB) was added rapidly and kept under agitation for another 4 hours in an ice

M

bath. Synthesis-C

D

In this methodology, 301 mg glutathione in 40 mL methanol was stirred in an ice bath. Whitish

TE

yellow suspension was formed. 1ml triethyl amine was added dropwise to get a fine dispersion. 49.2 mg silver lactate was added to the solution and kept under constant stirring for 1 hour at 300

EP

rpm. After 3 hours of stirring 230 mg tetramethyl ammonium borohydride (TMAB) was added rapidly and kept under agitation for another 4 hours in an ice bath.

CC

The whole synthetic process was done in dark and closed photo reactor to minimize exposure with surrounding environment. The ligated silver nanocluster as synthesized by the above three

A

methods were concentrated to 5 mL by rotavapor at 40℃. It was then precipitated out by adding methanol in excess. Then it was centrifuged at 14500 rpm. The supernatant liquid was discarded and solid was dispersed in methanol. The process of precipitation and centrifugation was done three times by adding excess amount (25 mL) of diethyl ether and methanol. After purification, sample was dried and re-dispersed in methanol for further use. The suspension was stable for about six months without aggregation.

2.3 Characterization The synthesized nanoclusters were characterized by measuring the absorption spectra in UV-VIS spectrometer (UV1800-Shimadzu). Photoluminescence measurements were carried out using Photoluminescence spectrometer (ParkinElmer). Sizes and morphology of the nanoclusters were

SC RI PT

confirmed by high resolution transmission electron microscope (HRTEM) (Jeol electron microscope JEOL 2100). HRTEM images were taken by dropcasting a diluted ethanolic solution of the nanoclusters in methanol on carbon coated copper grid and dried under ambient laboratory condition. HRTEM measurements were carried out by using 200 kV acceleration voltages with point resolution 0.2 nm to minimize the beam induced damage of the nanocluster. Elemental analyses were carried out by Energy Dispersive X-ray Spectrometry (EDS) mapping using HR-

U

TEM. Chemical compositions of the synthesized nanoclusters were characterized by high

N

resolution electro spray ionization mass spectrometry (Bruker microTOF-Q-II).

A

2.4 Catalytic study of AgNC on photo-Fenton like processes

M

The photo catalytic degradation of Methyl orange (MO) and Rhodamine B (RhB) were carried out in a 100 mL RB flask placed on a magnetic stirrer in a photocatalytic reactor. The

D

temperature of the reactor was kept constant at 25 ℃ using high speed fan and by connecting the reaction vessel with chiller. Irradiation of the reactor was carried out by direct exposure into the

TE

solution with 200 W visible lights (Quartz Tungsten halogen lamp). A cut off filter was used in front of the halogen lamp to ensure the irradiation of only visible light and high reflector mount

EP

was used to increase the intensity of the light. In the model dye degradation experiments 50 ml of 10 ppm of the dye (Methyl orange and Rhodamine B) was taken in 100 ml flask. To the dye

CC

solution 10 ml of 60 ppm Fe3+ solution and 2 ml of 5 ppm Ag NCs were added. The mixture was stirred for 20 mins in dark to ensure adsorption-desorption equilibrium. Then 0.5 ml of H2O2 was added to above mixture and the pH of the reaction mixture were adjusted to 3.2, by adding

A

dropwise dilute H2SO4. pH of the reaction mixture was measured using Systronics Digital pH meter. To study the influence of silver nanoclusters, we performed the same photo Fenton like process (Fe3+/H2O2/Visible light) of dye degradation without using AgNCs. Both solutions were then directly placed under the direct exposure of tungsten halogen lamp with a fixed distance of 30 cm. The reaction mixture was taken out from the reactor at a regular time intervals and centrifuged at 14000 rpm to separate silver nanocluster. The UV-VIS spectrum of supernatant

solutions was analyzed using UV-visible spectrophotometer. The same catalytic test on MO and RhB using photo-Fenton like processes was performed by using different silver nanoclusters as prepared by above mentioned synthetic methods A, B and C. This allows us to investigate the effect of size on catalytic performance of AgNCs.

SC RI PT

3. Results and discussion Here we report the tunable optical properties, size and composition of the nanocluster ligated with glutathione. We exploit the covalent nature of precursor salt, strength of the reducing agent and the other reaction conditions to develop methodologies to get a nearly monodisperse silver nanocluster with desired properties.

U

Comparisons of optical absorption and emission spectrum of glutathione stabilized silvernanocluster are shown in Fig.1. Nanocluster as prepared by synthesis A and C shows a broad

N

band absorption centered at 520 nm while blue shifted band centered at 450 nm was found in

A

silver nanocluster synthesized by synthesis-B. Syntheses of nanoclusters were closely monitored

M

by recording the UV-VIS spectrum of the reaction mixture at every 20 mins after the addition of reducing agent (Fig. S1). The reaction mixture was kept under agitation until we get a steady absorption spectrum. Silver nanocluster as synthesized by our methods were kept dispersed in

A

CC

EP

TE

after a month of storage.

D

methanol and found to be very stable as it shows negligible change in the absorption spectrum

Figure 1 UV-VIS spectra of silver nanoclusters as synthesized by synthesis A, B and C. Inset figure shows change in photoluminescence property upon changing the synthetic methodology. Photoluminescence property is one of the enabling factors for diverse applications of ligated silver nanocluster. Photoluminescence spectra of silver nanoclusters from synthesis A, B, C

SC RI PT

dissolved in water were collected using 350 nm as an excitation source. Inset in Fig. 1 shows the photoluminescence spectrum of glutathione stabilized silver nanocluster in ppm concentration. We observed no fluorescence when only aqueous solution of glutathione was excited with 350 nm. Synthesis B was found to produce a strongly blue emitting nanocluster species while synthesis A and C produces red emitting species. Optical properties are found to be highly dependent on reaction conditions like nature of precursor salt and the strength of the reducing

U

agent.

N

Size of the nanocluster is one of the vital factors in determining the fluorescence property because of the quantum confinement affect. However recent study confirms that besides size,

A

metal-ligand, metal-metal interactions are some of the decisive factor in modulating the emission

M

property. Size and morphology of the nanoclusters are confirmed using HR-TEM analysis as shown in Fig. 2. Size of the nanocluster was found to be smaller when it was synthesized using

D

silver lactate (synthesis- C) as precursor salt. Average size of 5 nm and 2.5 nm was found in case

TE

of silver nanocluster as prepared using silver nitrate (Synthesis A) and silver trifluoro acetate (Synthesis B) as precursor salts respectively. The particles are barely observable when same nanocluster was prepared by using silver lactate (Synthesis C) as precursor salt due to very small

EP

size of the nanoclusters. It is evident from Fig. 2 that bonding nature of precursor silver salt is

A

CC

one of the vital factors in tuning the size of the ligated nanoclusters.

Figure 2 HRTEM image of glutathione stabilized silver nanocluster prepared by different synthetic methods A, B and C.

Stability of the nanocluster was also confirmed by HRTEM analysis as it did not show any beam damage over time during HRTEM measurements. Selected area electron diffraction (SAED) pattern of silver nanocluster as synthesized by synthesis-A is shown in Fig. S2(a). This informs the crystalline nature of silver nanocluster sample. Electron diffraction pattern of silver nanocluster as synthesized by synthesis-B (Fig. S2(b)) also shows bright spots in the diffraction

SC RI PT

fringes, which indicate the crystalline nature of the nanoclusters. In contrast, nanoclusters as synthesized by synthesis- C, are found to be amorphous like as evident from Fig. S2(c). Therefore we can say that besides reduction of size, morphology of the nanocluster was also found to be modulated by varying the synthetic methodology. S

C BF

S

N

U

BFC

Ag

Fig. 3(a)

keV

Fig. 3(b)

keV

S

EP

TE

BFC

D

M

A

Ag

A

CC

Ag

Fig. 3(c)

keV

Figure 3 Energy Dispersive X-ray Spectrometry (EDS) mapping using HR-TEM of silver nanocluster from Synthesis A, B and C. EDS maps of BF (bright field image) image of selected area, S and Ag are shown in the figure. Representative EDAX spectrums are shown for each cluster.

Quantitative elemental concentrations are analyzed by measuring the Energy Dispersive X-ray Spectrometry (EDS) spectrum using HRTEM at a specific region of nanometer scale of sample as shown in Fig. 3. The results reveal that Ag atoms are surrounded by glutathione molecules containing C, S and N as shown in Fig. 3 (a), (b) and (c). The elemental composition of different

SC RI PT

AgNCs with weight percentages have been listed in supplementary table-1. The mechanistic approach of formation of metal nanoclusters were well demonstrated by Burst et al. [58,59]. Similar mechanism is also applicable in our present study. Formation of silver nanocluster is proposed to be followed in two steps as shown below: (i) Silver glutathione complex formation

U

Ag salt + GSH → Ag1m (SG)n (silver − thiolate complex)

N

Where m and n are the number of silver atoms and glutathione (GSH) molecules.

A

(ii) Reduction of Ag(I) to Ag(0):

M

Ag1m (SG)n + TMAB → Ag 0m (SG)n In the first step of mechanism silver thiolate complex is formed and over the time size of

D

the silver thiolate complex grows through aurophilic interaction (metal-metal interaction)

TE

and silver-sulphur interactions. It is possible to control the number of silver atoms and thiol ligand in silver thiolate complex by changing the number of free silver ions in the

EP

solution. Number of free silver ions in the solution is controlled by using more covalent silver salt. There might be another possibility of formation of covalent colloidal silver

CC

nanoparticle when silver lactate is used as precursor salt. Subsequently colloidal silver nanoparticle thus formed results in a smaller silver thiolate complex. In the next step reducing agent reduces the Ag1 to Ag0. Mild reducing agent like TMAB slows down the

A

nucleation process and reduces the growth rate of silver glutathione complex and hence it allowed us to modulate the composition and size of the nanocluster. It is found that the method of direct core reduction produces nearly monodisperse nanoclusters. While strong reducing agent such as sodium borohydride is found to produce a poly-disperse cluster size and composition [40,60-61]. Particle size and optical property (both absorption and emission) did not show any direct systematic correlation. This may be due to the more

complex interactions of ligand and metal-core. These interactions lead to ligand to metal charge transfer consequently shifts may be observed in absorption and emission maximum in various optical measurements.

Mass spectrometry has been proven as highly desirable tool in analysing the exact

SC RI PT

chemical composition of nanocluster. Tsukuda et al. has contributed significantly in exploration of mass spectrometry tool in deducing the exact composition of several nanocluster materials [29]. It is very important to discover the exact composition of the nanocluster as it determines the optical and catalytic property which can lead to the possible practical application of this type of nanocluster.

(a)

D

M

A

N

U

(b)

Figure 4 High resolution mass spectrum of glutathione stabilized silver nanocluster from synthesis

TE

A, B and C. The inset figure in (b) and (c) are expanded view of mass spectra.

EP

To determine the chemical composition of the nanocluster soft ionization technique like electrospray ionization (ESI) is used in high resolution mass spectrometry. It allows us to

CC

analyze intact cluster mass and therefore exact compositions of nanoclusters could be inferred. All mass spectra data were collected using Bruker microTOF-Q-II high resolution mass spectrometer and are shown in Fig.4. Sample used for collecting the mass spectra was dissolved

A

in 1:1 methanol/water mixture in ppm concentration. Mass spectra were collected in the m/z range 50 to 3000 Dalton and it was operated in negative ionization mode. Several ion species present in as synthesized nanocluster samples are identified by analyzing the respective mass spectrum. As shown in Fig. 4(a) wide range of abundant fragment ion peaks are observed in the mass range of 1900 to 3000 Da. These fragment ion peaks could be assigned to

(c)

[Ag16(SG)7], [Ag31(SG)19], [Ag32(SG)19] with negative charge state of 2, 4 and 5 (where SG refers to glutathione molecule). Besides larger fragment ions, it also reveals the presence of other smaller fragment ions species. This indicates the polydispersity nature in size and composition of the nanocluster synthesized by synthesis-A.

SC RI PT

Major fragment ion species found in the mass spectra of modified silver nanocluster as synthesized by synthesis-B are in the m/z range of 500 to 1600 Da. These mass peaks could be assigned to [Ag(SG)]-, [Ag2(SG)2]-, [Ag3(SG)2]-, [Ag8(SG)5]2-, [Ag9(SG)6]2-. Expanded view of the mass spectrum in the mass range of 1000 to 28000 Da as shown in inset Fig. 4(b) reveals the presence of Ag8-9&12 ligated with glutathione.

Mass spectra analysis of nanocluster from synthesis C reveal that most abundant species present

U

are with mass range of 500 to 1000 Da. It shows the presence of [Ag7(SG)4] and [Ag8(SG)5] with

N

negative charge state of 2. However, besides smaller fragment ions, expansion of mass spectra in the mass range of 1300 to 3000 Da as shown in the inset Fig. 4(c) showed the presence of large

M

heavier ions as indicated in the Fig. 4.

A

fragment ions with very low abundance. These mass peaks are attributed to other possible

Silver nanocluster found in the lower mass range with molecular formula Ag2, Ag3 as shown in

D

our mass spectra, are more often resultant of fragmentation of heavier ionic species. As discussed

TE

by Bakr et al. [62], small silver nanocluster with few metal atoms in the metal core can also be produced due to other environmental factors such as temperature [63], pH [57,63] and solvents

EP

[63]. It has been shown that these smaller nanocluster species also play a role in tuning the optical properties [62,63].

CC

A similar assignment of mass peaks and determination of chemical structure has been reported in previous publications [57,65-67]. Chemical structure of Ag32(SG)19 were identified through ESI

A

mass spectrum in negative 4 and 5 charge states by Griffith et al. [65]. Exact chemical structure of several other silver nanocluster ligated with glutathione have been also identified using ESI mass spectrometry. Immense effort has been focused recently on synthesis of smaller nanocluster containing few silver atoms in metal core with high monodispersity. Silver nanocluster with few silver atoms in the metallic core (Ag7) was first identified mass spectrometrically by Jin et al. [68]. Ag7-8 clusters were also prepared by Pradeep et al. recently for enhanced catalytic

applications [69]. Ag9, Ag8, Ag11 are also recently discovered and it shows distinct optical properties [66,70]. The chemical structures of the silver nanocluster stabilized by glutathione as identified in recent publications are tabulated in table-1. References 65, 66 68, 71 66 57

[Ag11(SG)7] [Ag12(SG)6], [Ag12(SG)9], [Ag16(SG)7], [Ag16(SG)9], [Ag16(SG)11] [Ag29(SG)22] [Ag31(SG)19], [Ag32(SG)19] [Ag35(SG)18] [Ag75(SG)44] [Ag16(SG)7], [Ag31(SG)19], [Ag32(SG)19], [Ag(SG)], [Ag2(SG)2], [Ag3(SG)2], [Ag8(SG)5], [Ag9(SG)6]

70 66 72,73 57, 65 67 74 Present paper

A

N

U

SC RI PT

Chemical structure of silver nanocluster [Ag2(SG)1], [Ag3(SG)2], [Ag2(SG)2] [Ag7(SG)4] [Ag8(SG)5], [Ag9(SG)6] [Ag10(SG)6], [Ag13(SG)9], [Ag14(SG)10], [Ag15(SG)11], [Ag16(SG)11]

M

Table 1 List of previously reported silver nanocluster ligated with glutathione and their identified chemical structure using mass spectrometry.

D

Identification of molecular structures of our synthesized nanoclusters is in good agreement with

TE

previously reported mass spectrum of silver nanocluster ligated with glutathione. As number of metal atoms is reduced in metallic core of nanocluster, because of more effect of quantum confinement, it could exhibit exciting properties. It is expected that smaller silver nano-cluster

EP

will show blue emission. But recent study shows that it is not always `true, as for example Ag9 and Ag15 shows blue emission in contrast to Ag7 and Ag11 [57,66,69-70]. The reason behind this

CC

could be highly complex interaction between metal, ligand and the surrounding environment. Several efforts have been made in recent times to theoretically fit the experimental absorption

A

and emission spectrum of several nanoclusters with accurate chemical composition [24,26,57, 75]. Despite the fact that it is a useful method to gain insight of photo physical processes, currently no generic theoretical calculations exist and difficult to fit experimental absorption and fluorescence of spectra of nanoclusters due to their complex nature and huge number of parameters. We have compared our experimental absorption and fluorescence spectra with previously reported theoretical calculation of nanoclusters with similar chemical compositions.

Recently Yau et al. determined optical properties of Ag32(SG)195- by performing TDDFT calculation on Ag32(SH)206- [26]. In their calculation Ag32 core and –SH ligands were used instead of glutathione to make the calculations manageable. The calculated absorption spectrum was found to have a peak at 475 nm. In the other method a metal core based on Ag31(SCH3)19 model as suggested by Bertorelle et al. were used to theoretically calculate absorption spectrum

SC RI PT

of Ag31 and Ag32 [24, 57]. A broad absorption spectra centered at 500 nm which was assigned to the ligand to metal charge transition was found. Theoretical absorption spectra were also calculated by Day et al. and Yau et al. using Ag15(SCH3)11 model [24, 26]. A gradual slope and a broad absorption band at 400-500 nm was observed. In our synthesis (a) most abundant peaks were assigned to Ag31 and Ag32, it shows a broad absorption band around 500 nm which is in

U

close agreement with theoretically calculated absorption band.

It has been observed that emission energies of nanoclusters did not show any systematic change

N

with particle size, no correlation could be drawn with a theoretical model [76]. Theoretical

A

photoluminescence spectra simpler nanocluster model Ag25(SH)18- has been theoretically

M

determined very recently [77]. As shown in the previously reported density of state calculation on Ag32 and Ag15, absorption of 400 nm (~3.1 eV) would lead to the emission at 680 nm (~ 1.8

D

eV) [25-26]. These results are in close agreement with our experimental results as we have also observed fluorescence peak centered at 650 nm with excitation at 350 nm. No general

TE

mechanism is currently known that could explain the emission and absorption for all the nanoclusters. More systematic theoretical and experimental study is thus required to unravel the

EP

structure property correlation of ligated silver nanocluster. Marked suppression of abundance of larger fragment ion species are observed in modified silver

CC

nanoclusters which are synthesized by other method B and C. As shown in Fig. 4(b) and (c) mass spectra of the modified silver nanocluster contains a narrow range of fragment ions distribution

A

which indicates the nearly monodisperse in size and composition. These mass spectra results also validate our TEM results (Fig. 2), as it could be observed that size of the clusters are reduced upon reduction of ion distribution in the respective mass spectra. Therefore in this recent study, we were able to synthesize nanocluster species with desired molecular structure by modulating the reaction condition. This control in size and composition of the nanocluster could be

explained by control of free ionic silver ions to form thiolate complex and slow rate of nucleation growth. Degradation of methyl orange and rhodamine B in visible light driven photo-Fenton like processes

SC RI PT

As prepared, Ag NCs were used to investigate the catalytic property in visible photo Fenton like processes for the degradation of Methyl Orange (MO) and Rhodamine B (RhB). AgNC catalyzed visible light photo Fenton like (Ag NCs/Fe3+ / H2O2 /visible light) were performed at acidic pH 3 where hydrogen peroxide served as a source of •OH. The Ag NCs were found to have a significant influence on the efficiency of photo Fenton like processes, which enhances the degradation in the case of MO dye and RhB dye. It is found that the photo Fenton like processes

U

with Fe3+ ions in presence Ag NCs shows higher degradation efficiency compared to the process

N

in absence of AgNCs. The influence of pH and the concentration of Fe3+, H2O2 on photo Fenton like processes has already been reported. The optimum experimental conditions were reported to

A

be as pH = 3, Fe3+ = 10 ppm, H2O2 = 10 ppm [55]. The same experimental conditions are used in

M

the present research work to investigate the catalytic performance of AgNCs on photo Fenton like processes for degradation of model dyes.

D

The absorbance at 500 nm for methyl orange and 550 nm for rhodamine B, corresponding to the

TE

maximum absorbance was selected to analyze the concentration variation of dye solution with visible light photo irradiation. Degradation efficiency was measured by calculating

× 100 =

× 100 where A, A0 and C, C0 are final and initial absorbance and concentrations. Fig. 5(a) and

EP

𝐶 𝐶0

𝐴 𝐴0

CC

(b) shows the degradation of methyl orange and rhodamine B under visible light irradiation at different time intervals. As shown in Fig. 5(a) and (b), rate of degradation of dye enhances under visible light when smaller cluster, synthesized by Synthesis C was used in photo Fenton like

A

process. Gradual enhancement of catalytic performance was observed when size of the nanocluster reduces from 5 nm (Synthesis A) to 2.5 nm (Synthesis B) to ~1 nm (Synthesis C). The influence of silver nanocluster in the photo Fenton like process could be understood by comparing degradation of dye without AgNCs.

(a)

SC RI PT

(b)

Figure 5(a) Degradation of methyl orange with visible light irradiation (b) Degradation of Rhodamine B with visible light irradiation. “F” denotes Fenton like reagent (Fe3+/H2O2), AgNC

U

(A) denotes silver nanocluster as synthesized by synthesis A.

N

As shown in the above results, more efficient catalytic activity was achieved by silver

A

nanocluster which was synthesized by synthesis C where size of the nanocluster was very small

M

(barely visible in HRTEM). The change in spectrum of methyl orange and Rhodamine B using AgNC as synthesized by synthesis C in Fenton like processes under visible light irradiation is

D

shown in Fig. 6 (a) and (b). To understand the effect of nanocluster, spectrums are compared with only Fenton like processes under same experimental conditions. As shown in Fig. 6 (a),

TE

100% decolorization of MO dye was achieved in 40 min of visible light irradiation using AgNC (C) as a catalyst whereas the same was achieved in 90 min without nanocluster. Complete

EP

degradation of Rhodamine B was observed, when dye sample was analyzed after 40 mins of visible light irradiation in presence of Fenton reagent (Fe3+/H2O2) as shown in Fig. 6(b).

CC

Rhodamine B was not completely degraded (84%) after 120 min of irradiation without using

A

AgNCs as catalyst.

(b)

SC RI PT

(a)

Figure 6(a) UV-VIS spectra of Methyl Orange (MO) at different time intervals. In the figure (MO+F) denotes spectra of mixture methyl orange and Fenton like reagent (Fe3+/H2O2),

U

MO+F+AgNC (C) denotes spectra of mixture of methyl orange, Fenton like reagent (Fe3+/H2O2)

N

and silver nanocluster synthesized by synthesis C. (b) UV-VIS spectra of Rhodamine B (RhB) at

A

different time intervals.

M

Silver nanocluster catalyzed visible photo-Fenton like degradation of dye is proposed as follows [55,56] :

AgNCs

 hv  AgNCs

(e



CB

 h

 VB

)

. Because of the high negative reduction potential

TE

equation:

D

The electron-hole pair is generated when Ag NC is illuminated with visible light as shown in

of quantum clusters and formation of free electron in CB, it will rapidly reduce Fe 3+ into Fe2+ 

Fe2+ + •OH. More hydroxyl

EP

along with the formation of •OH by the reaction as Fe(OH)2+

radicals are generated from H2O2 which was used in the reaction as a source of OH radicals. H2O2 +

hv



2 •OH. Hydroxyl radical could also be generated from H2O2 by combining the

CC

free electron from AgNC through equation H2O2 + e−→ OH− + •OH. Further hydroxyl radicals

A

are generated from water present in the dye solution by breakdown of water molecules and hydroxyl ions through reaction with hole, H2O

h

 vb

  

H2O + •OH,

h

 vb

+ OH-  •OH.

Hydroxyl radical are then oxidizes dye molecule into CO2 and H2O. Table 2 Catalytic efficiency of AgNCs (C) and Fenton-like processes under visible light

Experimental Condition

MO

RhB

% Degradation

Time taken for complete degradation (min)

AgNCs/Fe3+/H2O2/visible light

100

40

Fe3+/H2O2/visible light

100

90

AgNCs/Fe3+/H2O2/visible light

100

Fe3+/H2O2/visible light

84

SC RI PT

Model dye

40

120

U

Table-2 summarizes the various parameters characterizing the catalytic performance of AgNC

N

(C) with respect to degradation of dyes using photo Fenton like processes. Thus it was observed that quantum clusters of silver play a vital catalytic role and efficiency depends on the size of the

A

nanocluster. The nanocluster synthesized by our facile methodology may also be exploited for

M

solar light induced organic reactions and bio sensing applications.

D

Conclusions

TE

We have successfully demonstrated a facile method of controlled synthesis of silver nanoclusters ligated with bio compatible glutathione for visible light photocatalytic and biosensing applications point of view. We have also reported control over experimental parameters to

EP

modulate the size and morphology of AgNCs. Mass spectrometry analysis indicates the notable reduction in nuclearity and size distribution and hence we could modify silver nanocluster from

CC

highly polydispese to nearly monodisperse in size and composition. Covalent character of precursor salt in the synthesis plays an important role in determining the size and chemical

A

composition of desired glutathione stabilized silver nano-cluster. We were able modulate the optical property by suitable change in reaction condition and nature of the reducing agent. Catalytic performances of the as synthesized nanocluster are observed to be size dependent. As the size of the silver nanocluster is reduced catalytic activity is also found to be enhanced in our present study on visible photo-Fenton like processes on the degradation of two model dyes (Methyl orange and Rhodamine B). The present study will lead to discovery of new rational

design of efficient nano-catalyst which could drastically reduce the quantity of catalyst required for degradation of toxic industrial effluent under solar light. The synthesized biocompatible silver-nanoclusters can also be very much useful for bio-sensor, optical storage, and water

SC RI PT

purification applications.

Conflicts of interest There are no conflicts to declare. Acknowledgements

This work has been supported by UPES, Dehradun. Thanks to Dr. Rajaram Bal from IIP

U

Dehradun for HRTEM characterization. The authors acknowledge Dr. Debasis Banerjee and Mr.

N

Jagadish Das from IIT Roorkee for their support in HRMS measurement.

A

References

M

[1] R. Jin, Quantum sized, thiolate-protected gold nanoclusters, Nanoscale 2 (2010) 343–362. [2] P. Liu, R. Qin, G. Fu and N. Zheng, Surface Coordination Chemistry of Metal

D

Nanomaterials, J. Am. Chem. Soc. 139 (2017) 2122–2131. [3] I. Chakraborty, T. Pradeep, Atomically Precise Clusters of Noble Metals: Emerging Link

TE

between Atoms and Nanoparticles, Chem. Rev. 117 (2017) 8208–8271. [4] L. Shang, S. Dong and G. U. Nienhaus, Ultra-small fluorescent metal nanoclusters:

EP

Synthesis and biological applications, Nanotoday 6 (2011) 401-418. [5] A. Mathew and T. Pradeep, Noble metal clusters: Applications in energy, environment and

CC

biology, Part. Part. Syst. Charact. 31 (2014) 1017–1053. [6] Z. Lei, X.-K. Wan, S.-F. Yuan, J.-Q. Wang and Q.-M. Wang, Alkynyl-protected gold and gold–silver nanoclusters, Dalton Trans. 46 (2017) 3427–3434.

A

[7] S. Yamazoe, S. Takano, W. Kurashige, T. Yokoyama, K. Nitta, Y. Negishi and T. Tsukuda, Hierarchy of bond stiffnesses within icosahedral-based gold clusters protected by thiolates, Nat. Commun. 7 (2016) 10414-10420. [8] R. Jin, C. Zeng, M. Zhou and Y. Chen, Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities, Chem. Rev. 116 (2016) 10346–10413.

[9] L. Shang, N. Azadfar, F. Stockmar, W. Send, V. Trouillet, M. Bruns, D. Gerthsen, G. U. Nienhaus, One‐Pot Synthesis of Near‐Infrared Fluorescent Gold Clusters for Cellular Fluorescence Lifetime Imaging, Small 7 (2011) 2614–2620. [10] J. Yu, S. Choi, & R. M. Dickson, Shuttle-Based Fluorogenic Silver-Cluster Biolabels, Angew. Chem. Int. Ed. 48 (2009) 318–320.

SC RI PT

[11] W. Chen, in Silver Nanoparticles: Synthesis, Uses and Health Concerns, ed. I. Armentano and J. M. Kenny, Nova Science Publishers, Inc., 2013.

[12] S. M. Copp, D. E. Schultz, S. Swasey and E. G. Gwinn, Atomically Precise Arrays of Fluorescent Silver Clusters: A Modular Approach for Metal Cluster Photonics on DNA Nanostructures, ACS Nano 9 (2015) 2303–2310. [13] Ž. Sanader, M. Krstić, I. Russier-Antoine,

V. Bonačić-Koutecký, Two-photon absorption of ligand-protected Ag15

U

Antoine and

F. Bertorelle, P. Dugourd, P.-F. Brevet, R.

N

nanoclusters. Towards a new class of nonlinear optics nanomaterials, Phys. Chem. Chem. Phys. 18 (2016) 12404-12408.

A

[14] C. J. Lin, C. H. Lee, J. T. Hsieh, H.-H. Wang, J. K. Li, J. L. Shen, W. H. Chang, H. I.

M

Yeh, W. H. Chang, Synthesis of fluorescent metallic nanoclusters toward biomedical application: recent progress and present challenges, J. Med. Biol. Eng. 29 (2009) 276-283.

D

[15] X. Le Guéve, C. Spies, N. Daum, G. Jung and M. Schneider, Highly Fluorescent Silver

TE

Nanoclusters Stabilized by Glutathione: A Promising Fluorescent Label for Bioimaging, Nano Res. 5 (2012) 379-387. [16] L. Polavarapu, M. Manna, Q. H. Xu, Biocompatible glutathione capped gold clusters as

EP

one- and two-photon excitation fluorescence contrast agents for live cells imaging, Nanoscale 3 (2011) 429-434.

CC

[17] F. Wen, Y. Dong, L. Feng, S. Wang, S. Zhang, X. Zhang, Horseradish peroxidase functionalized fluorescent gold nanoclusters for hydrogen peroxide sensing, Anal. Chem. 83

A

(2011) 1193-1196.

[18] X. L. Guevel, N. Daum, M. Schneider, Synthesis and characterization of human transferrin-stabilized gold nanoclusters, Nanotechnology 22 (2011) 275103-09. [19] J. Zheng, P. R. Nicovich, R. M. Dickson, Highly Fluorescent Noble-Metal Quantum Dots, Annu. Rev. Phys. Chem. 58 (2007) 409–431.

[20] C. Zhou, M. Long, Y. Qin, X. Sun & J. Zheng, Luminescent Gold Nanoparticles with Efficient Renal Clearance, Angew. Chem. Int. Ed. 50 (2011) 3168–3172. [21] P. N. Day, R. Pachter and K. A. Nguyen, A Theoretical Investigation of the Structure and Optical Properties of a Silver Cluster in Solid Form and in Solution, J. Phys. Chem. A 121 (2017) 326–333.

SC RI PT

[22] K. L. Dimuthu M. Weerawardene, Emilie Brigitte Guidez, and Christine M. Aikens, Photoluminescence Origin of Au38(SR)24 and Au22(SR)18 Nanoparticles: A Theoretical Perspective, J. Phys. Chem. C 121 (2017) 15416–15423.

[23] P. N. Day, K. A. Nguyen and R. Pachter, J. Chem. Theory Comput., 2010, 6, 2809–2821. [24] P. N. Day, R. Pachter, K. A. Nguyen and T. P. Bigioni, Linear and Nonlinear Optical Response in Silver Nanoclusters: Insight from a Computational Investigation, J. Phys.

U

Chem. A 120 (2016) 507–518.

N

[25] B. A. Ashenfelter, A. Desireddy, S. H. Yau, T. Goodson III and T. P. Bigioni, Fluorescence from Molecular Silver Nanoparticles, J. Phys. Chem. C 119 (2015) 20728–

A

20734.

M

[26] S. H. Yau, B. A. Ashenfelter, A. Desireddy, A. P. Ashwell, O. Varnavski, G. C. Schatz, T. P. Bigioni and T. Goodson, Optical Properties and Structural Relationships of the Silver

D

Nanoclusters Ag32(SG)19 and Ag15(SG)11, J. Phys. Chem. C 121 (2017) 121, 1349–1361.

TE

[27] C. T. Yuan, C. A. Lin, T. N. Lin, W. H. Chang, J. L. Shen, H. W. Cheng and J. Tang, Probing the photoluminescence properties of gold nanoclusters by fluorescence lifetime correlation spectroscopy. J. Chem. Phys. 139 (2013) 234311-5. Y. Negishi, N. K. Chaki, Y. Shichibu, R. L. Whetten, T. Tsukuda, Origin of Magic

EP

[28]

Stability of Thiolated Gold Clusters:  A Case Study on Au25(SC6H13)18, J. Am. Chem. Soc.

CC

129 (2007) 11322−11323.

[29] Y. Negishi, K. Nobusada, T. Tsukuda, Glutathione-Protected Gold Clusters Revisited: 

A

Bridging the Gap between Gold(I)−Thiolate Complexes and Thiolate-Protected Gold Nanocrystals, J. Am. Chem. Soc. 127 (2005) 5261−5270.

[30] Y. Negishi, Y. Takasugi, S. Sato, H. Yao, K. Kimura, T. Tsukuda, Magic-Numbered Aun Clusters Protected by Glutathione Monolayers (n = 18, 21, 25, 28, 32, 39):  Isolation and Spectroscopic Characterization, J. Am. Chem. Soc. 126 (2004) 6518−6519.

[31] Ding-Fei He, Yang Xiang, Xu Wang, Xue-Feng Yu, Microwave-heating synthesis and sensing applications of bright gold nanoclusters, Materials Research Bulletin 46, (2011) 2418-2421. [32] Z. Luo, X. Yuan, Y. Yu, Q. Zhang, D. T. Leong, J. Y. Lee, J. Xie, From AggregationInduced Emission of Au(I)–Thiolate Complexes to Ultrabright Au(0)@Au(I)–Thiolate

SC RI PT

Core–Shell Nanoclusters, J. Am. Chem. Soc. 134 (2012) 16662−16670.

[33] X. Jia, J. Li, E. Wang, Cu Nanoclusters with Aggregation Induced Emission Enhancement, Small 9 (2013) 3873−3879.

[34] X. Jia, J. Li, E. Wang, Supramolecular Self-Assembly of Morphology-Dependent Luminescent Ag Nanoclusters, Chem. Commun. 50 (2014) 9565−9568.

[35] Z. Wu, J. Liu, Y. Gao, H. Liu, T. Li, H. Zou, Z. Wang, K. Zhang, Y. Wang, H. Zhang, B.

U

Yang, Assembly-Induced Enhancement of Cu Nanoclusters Luminescence with

N

Mechanochromic Property, J. Am. Chem. Soc. 137 (2015) 12906−12913. [36] X. Jia, X. Yang, J. Li, D. Li, E. Wang, Stable Cu Nanoclusters: From an Aggregation-

A

Induced Emission Mechanism to Biosensing and Catalytic Applications, Chem. Commun.

M

50 (2014) 237−239.

[37] N. Goswami, Q. Yao, Z. Luo, J. Li, T. Chen, J. Xie, Luminescent Metal Nanoclusters

D

with Aggregation-Induced Emission, J. Phys. Chem. Lett. 7 (2016) 962−975.

TE

[38] K. Pyo, V. D. Thanthirige, K. Kwak, P. Pandurangan, G. Ramakrishna, D. Lee, Ultrabright Luminescence from Gold Nanoclusters: Rigidifying the Au(I)–Thiolate Shell, J. Am. Chem. Soc. 137 (2015) 8244−8250.

EP

[39] H. H. Deng, X. Q. Shi, F. F. Wang, H. P. Peng, A. L. Liu, X. H. Xia and W. Chen, Fabrication

of

Water-Soluble,

Green-Emitting

Gold

Nanoclusters

with

a

65%

CC

Photoluminescence Quantum Yield via Host–Guest Recognition, Chem. Mater. 29 (2017) 1362–1369.

A

[40] Z. Wu, M. A. MacDonald, J. Chen, P. Zhang, R. Jin, Kinetic Control and Thermodynamic Selection in the Synthesis of Atomically Precise Gold Nanoclusters, J. Am. Chem. Soc. 133 (2011) 9670−9673.

[41] N. Cathcart, P. Mistry, C. Makra, B. Pietrobon, N. Coombs, M. Jelokhani-Niaraki, V. Kitaev, Chiral Thiol-Stabilized Silver Nanoclusters with Well-Resolved Optical Transitions

Synthesized by a Facile Etching Procedure in Aqueous Solutions, Langmuir 25 (2009) 5840- 5846. [42] A. Desireddy, B. E. Conn, J. Guo, B. Yoon, R. N. Barnett, B. M. Monahan, K. Kirschbaum, W. P. Griffith, R. L. Whetten, U. Landman, T. P. Bigioni, Ultrastable Silver Nanoparticles, Nature 501 (2013) 399-402. J. Yang, N. Xia, X. Wang, X. Liu, An Xu, Z. Wu and Z. Luo, One-pot one-cluster

SC RI PT

[43]

synthesis of fluorescent and bio-compatible Ag14 nanoclusters for cancer cell imaging, Nanoscale, 7 (2015) 18464-18470. [44]

Z. Luo, K. Zheng and J. Xie Engineering ultrasmall water-soluble gold and silver nanoclusters for biomedical applications, 50 (2014) 5143-5155.

[45]

A. Sangsuwan, H. Kawasaki, Y. Matsumura, and Y. Iwasaki, Antimicrobial Silver

[46]

N

Bioconjugate Chem., 27 (2016) 2527–2533.

U

Nanoclusters Bearing Biocompatible Phosphorylcholine-Based Zwitterionic Protection,

Y. Zhu, H. Qian and R. Jin, Catalysis opportunities of atomically precise gold

N. K. Chaki, J. Sharma, AB Mandle, IS Mulla, R. Pasricha, K. Vijayamohanan Size

M

[47]

A

nanoclusters, J. Mater. Chem., 21 (2011) 6793-6799

dependent redox behavior of monolayer protected silver nanoparticles (2-7 nm) in

A Leelavathi, T U Bhaskara Rao, T Pradeep, Supported quantum clusters of silver as

TE

[48]

D

aqueous medium. Phys Chem Chem Phys, 6 (2004) 1304- 1308.

enhanced catalysts for reduction, Nanoscale Research Letters 6, (2011), 123-132. [49]

Y. Chen, C. Wang, H. Liu, J. Qiu, X. Bao, Ag/SiO2: a novel catalyst with high activity

EP

and selectivity for hydrogenation of chloronitrobenzenes. Chem Commun, 0 (2005) 5298-5300.

L. Wang, S. Ouyang, B. Liu, R. Yang, T. Wang, S. Wang, One-pot synthesis of

CC

[50]

octahedral platinum nanoclusters with enhanced electrocatalytic activities, Mater. Res.

A

Bull. 61 (2015) 357-362.

[51]

V. Madurai Ramakrishnan, M. Natarajan, A. Santhanam, V. Asokan, D. Velauthapillai, Size controlled synthesis of TiO2 nanoparticles by modified solvothermal method towards effective photo catalytic and photovoltaic applications, Mater. Res. Bull. 97 (2018) 351-360.

[52]

KI Shimizu, A. Satsuma A: Selective catalytic reduction of NO over supported silver catalysts–practical and mechanistic aspects. Phys. Chem. Chem. Phys. 8 (2006) 26772695.

[53]

T. Mitsudome, S. Arita, H. Mori, T. Mizugaki, K. Jitsukawa, K. Kaneda, Supported

Silanols. Angew Chem Int Ed. 47 (2008) 7938-7940. [54]

SC RI PT

Silver-Nanoparticle-Catalyzed Highly Efficient Aqueous Oxidation of Phenylsilanes to

T. Mitsudome, Y. Mikami, H. Mori, S. Arita, T. Mizugaki, K. Jitsukawa, K. Kaneda, Supported silver nanoparticle catalyst for selective hydration of nitriles to amides in water. Chem. Commun. 0 (2009) 3258-3260.

[55]

L. Gomathi Devi, S. Girish Kumar, K. Mohan Reddy, C. Munikrishnappa, Photo degradation of Methyl Orange an azo dye by Advanced Fenton Process using zero valent

U

metallic iron: Influence of various reaction parameters and its degradation mechanism,

[56]

N

Journal of Hazardous Materials 164, (2009), 459-467.

Chin Kui Cheng & Zi Ying Kong & Maksudur R. Khan Photocatalytic-Fenton

M

Soil Pollut 226, (2015), 327-339.

A

Degradation of Glycerol Solution over Visible Light-Responsive CuFe2O4, Water Air

[57] F. Bertorelle, R. Hamouda, D. Rayane, M. Broyer, R. Antoine, P. Dugourd, L. Gell, A.

D

Kulesza, R. Mitric, V. Bonacic-Koutecky, Synthesis, characterization and optical properties

5637-5643.

TE

of low nuclearity liganded silver clusters: Ag31(SG)19 and Ag15(SG)11, Nanoscale 5 (2013)

[58] M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, Synthesis of thiol-

EP

derivatised gold nanoparticles in a two-phase Liquid–Liquid system, J. Chem. Soc., Chem. Commun. 0 (1994) 801−802.

CC

[59] M. Brust, J. Fink, D. Bethell, D. Schiffrin, C. Kiely, Synthesis and reactions of functionalised gold nanoparticles, J. Chem. Soc., Chem. Commun. 0 (1995) 1655-1656.

A

[60] A. Ghosh, T. Udayabhaskararao, T. Pradeep, One-Step Route to Luminescent Au18SG14 in the Condensed Phase and Its Closed Shell Molecular Ions in the Gas Phase, J. Phys. Chem. Lett. 3 (2012) 1997−2002.

[61] Y. Yu, Z. Luo, Y. Yu, J. Y. Lee, J. Xie, Observation of Cluster Size Growth in CODirected Synthesis of Au25(SR)18 Nanoclusters, ACS Nano 6 (2012) 7920−7927.

[62] C. P. Joshi, M. S. Bootharaju and O. M. Bakr, Tuning Properties in Silver Clusters, J. Phys. Chem. Lett. 6 (2015) 3023–3035. [63] Q.-Q. Xu, X.-Y. Dong, R.-W. Huang, B. Li, S.-Q. Zang, T. C. W. Mak, A thermochromic silver nanocluster exhibiting dual emission character, Nanoscale, 7 (2015) 1650-1654. [64] I. Díez, M. Pusa, S. Kulmala, H. Jiang, A. Walther, A. S. Goldmann, A. H. E. Müller, O.

SC RI PT

Ikkala, R. H. A. Ras, Color Tunability and Electrochemiluminescence of Silver Nanoclusters, Angew. Chem. Int. Ed. 48 (2009) 2122-2125.

[65] J. Guo, S. Kumar, M. Bolan, A. Desireddy, T. P. Bigioni, W. P. Griffith, Mass Spectrometric Identification of Silver Nanoparticles: The Case of Ag32(SG)19, Anal. Chem. 84 (2012) 5304-5308.

[66] X. Yuan, M. I. Setyawati, A. Shu Tan, C. Nam Ong, D. Tai Leong and J. Xie, Highly

U

luminescent silver nanoclusters with tunable emissions: cyclic reduction–decomposition

N

synthesis and antimicrobial properties, NPG Asia Mater. 5 (2013) e39. [67] M. S. Bootharaju, V. M. Burlakov, T. M. D. Besong, C. P. Joshi, Lina G. AbdulHalim,

A

David M. Black, R. L. Whetten, A. Goriely, and O. M. Bakr, Reversible Size Control of

M

Silver Nanoclusters via Ligand-Exchange, Chem. Mater. 27 (2015) 4289−4297. [68] Z. K. Wu, E. Lanni, W. Q. Chen, M. E. Bier, D. Ly and R. C. Jin, High Yield, Large

D

Scale Synthesis of Thiolate-Protected Ag7 Clusters, J. Am. Chem. Soc. 131 (2009) 16672–

TE

16674.

[69] T. U. Rao, T. Pradeep, Luminescent Ag7 and Ag8 Clusters by Interfacial Synthesis, Angew. Chem. Int. Ed. 49 (2010) 3925 –3929.

EP

[70] A. Baksi, M. S. Bootharaju, X. Chen, H. Häkkinen, T. Pradeep, Ag11(SG)7: A New Cluster Identified by Mass Spectrometry and Optical Spectroscopy, J. Phys. Chem. C 118

CC

(2014) 21722-21729.

[71] N. Cathcart and V. Kitaev, Silver Nanoclusters: Single-Stage Scaleable Synthesis of

A

Monodisperse Species and Their Chirooptical Properties, J. Phys. Chem. C 114 (2010) 16010–16017.

[72] A. Baksi, S. R. Harvey, G. Natarajan, V. H. Wysocki and T. Pradeep, [Au25(SR)18]22- : A noble metal cluster dimer in the gas phase, Chem. Commun. 52 (2016) 3805—3808.

[73] X. Yuan, Z. Luo, Q. Zhang, X. Zhang, Y. Zheng, J. Yang Lee, and J. Xie, Synthesis of Highly Fluorescent Metal (Ag, Au, Pt, and Cu) Nanoclusters by Electrostatically Induced Reversible Phase Transfer, ACS Nano 5 (2011) 8800−8808. [74] I. Chakraborty, T. Udayabhaskararao and T. Pradeep, High temperature nucleation and growth of glutathione protected ∼Ag75 clusters, Chem. Commun. 48 (2012) 6788–6790. Ailing Han, Lin Xiong, Sijia Hao, Yayu Yang, Xia Li, Guozhen Fang, Jifeng Liu, Yong

SC RI PT

[75]

Pei, and Shuo Wang, Highly Bright Self-Assembled Copper Nanoclusters: A Novel Photoluminescent Probe for Sensitive Detection of Histamine, Anal. Chem. doi: 10.1021/acs.analchem.8b01384. [76]

J. Zheng, C. W. Zhang, R. M. Dickson, Highly Fluorescent, Water- Soluble, SizeTunable Gold Quantum Dots. Phys. Rev. Lett. 2004, 93,077402.

K. L. Dimuthu M. Weerawardene, and C. M. Aikens, Origin of Photoluminescence of

U

[77]

N

Ag25(SR)18‾ Nanoparticles: Ligand and Doping Effect, J. Phys. Chem. C, 122 (2018)

A

CC

EP

TE

D

M

A

2440–2447.