Core-Size Dependent Fluorescent Gold Nanoclusters and

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Core-Size Dependent Fluorescent Gold Nanoclusters and Ultrasensitive Detection of Pb2+ Ion Dipankar Bain, Subarna Maity, Bipattaran Paramanik, and Amitava Patra ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03794 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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ACS Sustainable Chemistry & Engineering

Core-Size Dependent Fluorescent Gold Nanoclusters and Ultrasensitive Detection of Pb2+ Ion

Dipankar Bain, Subarna Maity, Bipattaran Paramanik#, and Amitava Patra* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India

*

Author to whom correspondence should be addressed. Electronic mail: [email protected]

Telephone: (91)-33-2473-4971. Fax: (91)-33-2473-2805 #

Present address; Department of Chemistry, New Alipore College, Kolkata 700053, India

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ABSTRACT

Gold nanoclusters (Au NCs) are new class of fluorescent nanomaterials with widespread applications in energy, water and healthcare. Here, we report a green synthesis of Au NCs with tunable emission wavelength from 590 nm to 510 nm in aqueous medium by core etching and ligand exchange method. Investigation reveals that the number of Au atoms present in the core of nanoclusters controls the emission wavelength. The quantum yield (QY) of nanoclusters increases from 0.57 to 3.15 % with changing core from Au12 to Au6. Time resolved spectroscopic study reveals that the emission with higher lifetime (>100 ns) originates from ligand to metal charge transfer (LMCT; S to gold core of NCs). It is demonstrated that the highly green emitting NCs (Au-510) are more sensitive than orange emitting NCs (Au-590) towards Pb2+. The detection limit of Pb2+ is found to be 10 nM which is much lower than allowed concentration of Pb2+ in drinking water. Thus, Au NCs based optical sensor is promising for the selective detection of Pb2+ in drinking water.

Keywords: Gold Nanoclusters, Core-etching, Tunable Emission, Metal ion, Sensing, limit of detection (LOD).


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INTRODUCTION Considerable attention has been paid on fluorescent metal nanoclusters (Au, Ag and Cu NCs) for their extraordinary features such as ultra small size, quantum confinement, high photo stability and biocompatibility.1-10 Nanoclusters are the ultrasmall nanoparticles having size less than 2 nm and it exhibits intense fluorescence instead of surface plasmon resonance band.6, 11-12 Another advantage of the ultrasmall NCs is nontoxic compared to quantum dots (QDs) and organic dye.4 Designing of size tunable, photo stable, highly fluorescent Au NCs remain one of the most challenging issues now-a-days.13-16 Most of the conventional thiolate protected Au NCs are synthesized by using two strategies.17-20 One strategy involves to the direct reduction of gold precursor in presence of reducing agent such as NaBH4 or macromolecular template like bovine serum albumin (BSA) protein. Xie and his coworkers have reported highly luminescent red emitting Au NCs using BSA as capping ligands as well as reducing agent.8 Like protein, DNA templates are also used as capping agents where several cytosine base pairs of DNA stabilize the nanoclusters.21-23 The second strategy involves the etching of larger size Au NPs (2 to 4 nm) by excess thiol ligands such as glutathione, and mercaptopropionic acid (MPA).24-26 Thiol containing amino acid and peptides are most commonly used capping ligands for aqueous medium synthesis because of their high solubility.27-29 Fang Huang and his coworkers have reported a synthesis method for luminescent Au NCs using pentapeptide as capping agent.30 Often, surface capping ligand plays vital role in controlling the stability, QY and lifetime of metal nanoclusters.31-32 Deng et al.33 have described a method of Au cluster synthesis to enhance the QY up to 65 % via the interaction of guest molecule with Au NCs. Dickson et al.7 have synthesized magic number and size tunable Au NCs such as Au5, Au8, Au13, Au23 and Au31 using PAMAM dendrimer as a surface capping ligands. Chang et al.26 have prepared tunable emission of Au NCs by core etching of larger Au NPs. Despite of various synthesis methods available in literature, however less attention has been given on describing the detailed understanding on formation of size tunable Au NCs and their photophysical properties.34 Recently, Au NCs was used for diverse application such as, aerobic oxidation,35 antimicrobial agent,36 cancer radiotherapy37 and catalytic reaction38. Here, we have developed a strategy for the synthesis of photostable, water soluble, highly fluorescent Au NCs. The Au NCs were synthesized in aqueous medium by ligand exchange as well as core etching technique. The as synthesized nanoclusters exhibit wavelength tunable 3 ACS Paragon Plus Environment


emission by controlling etching time. Decay dynamics reveals that different electronic structures and ligand to metal charge transfer pathways which are governing factors for changing the quantum yields (QY). The emission wavelength of Au NCs is found to be strongly dependent on the number of metal atoms in core. As the synthesis of Au NCs was carried out in aqueous phase therefore it can have potential applications in biology, imaging, and sensing. Here, the green emitting Au NCs is being used for selective optical probe for Pb(II) ion detection in presence of common and interfering cations such as Na+, K+, Mg2+, Ni2+, Cd2+, Fe3+, Mn2+, Ag+, Zn2+, Cr6+, Cu2+ and Hg2+. The detection limit of Pb2+ in water is found as low as 10 nM, which is much lower than allowed concentration of Pb2+ in drinking water, permitted by World Health Organization (WHO). Thus, Au NCs could be use as optical sensor for detection of Pb2+ in drinking water.

EXPERIMENTAL SECTION Chemicals: Tetrachloroauric acid trihydrate (HAuCl4.3H2O), 3-Mercaptopropionic acid (MPA), glutathione reduced (GSH), L-Cysteine, D-penicillamine, 2, 5-dihydrixybenzoic acid (DHB), αcyano-4-hydroxycinnamic acid (CHCA), and sinapinic acid (SA) were purchased from SigmaAldrich. Other chemicals such as tetrakis(hydroxymethyl)phosphonium chloride (THPC), sodium hydroxied (NaOH) were purchased from Merck. Sodium phosphate dibasic heptahydrate (Na2HPO4.7H2O) were obtained from Spectrochem and sodium dihydrogen phosphate dehydrate (NaH2PO4.2H2O) were received from Lobachemie. Metal nitrate salts were purchased from Sigma-Aldrich. Throughout the experiment high purity water (≈18.2 MΩ) was used. All the chemicals of highest purity grade were used without further purification. Synthesis of gold nanoclusters with tunable emission: The synthesis of fluorescent Au NCs with tunable core-size involves the following steps. Gold nanoclusters synthesis: In 10 mL HPLC water, 125 µL of 1(M) NaOH was added, then 3 µL of reducing agent THPC, (80% in H2O) was added to the solution and the solution was stirred for one minute. The aqueous solution of Au3+ (100 µL, 100 mM) was added to the reaction mixture. The color of the reaction mixture immediately turns into light brown color, indicating the formation of ultra small Au NCs. Then, after five minutes of continuous stirring, 50 µL of 0.1 M GSH was added to the as prepared NCs. The as prepared Au NCs shows very weak


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fluorescence intensity at 652 nm and this Au NCs was kept at 4°C for further use. The chemical structure of capping ligands GSH and MPA are given in Figure S1. Synthesis of luminescent gold nanoclusters with tunable emission: A series of Au NCs with tunable emission wavelengths was prepared from as prepared weak fluorescent Au NCs. After aging 12 hour at 4°C, 2 mL phosphate buffer (PBS, 50 mM) of pH 9 was added into 10 mL Au NCs with continuously stirring at dark. Then, 150 µL of MPA was added to the above reaction mixture for etching and the ligand exchange of Au NCs. Then, the reaction was left for continuous stirring and the above reaction was monitored regularly by using UV-vis absorption and PL spectroscopy. After 24 hour of progress, the as-prepared Au NCs exhibit emission at 590 nm with relatively higher fluorescence intensity. The emission maximum of Au NCs is slightly blue shifted after 48 hour of continuous stirring. Again, another 100 µL of MPA was added to the reaction mixture to achieve the faster growth. After 72 hour stirring, the Au NCs show emission at 545 nm with higher fluorescence intensity than Au NCs of 590 nm. Again, the nanocluster exhibit very bright emission at 510 nm after another 24 hour later. It is to be noted that the Au NCs do not exhibit any change in emission wavelength after 96 hour of etching time, which confirm the completion of the reaction under this condition. The as-prepared Au NCs were collected and purified by centrifugation at 12000 rpm (14,500 g force) for 10 minutes to remove excess capping ligands. The as prepared Au NCs were abbreviated as Au-652, Au-590, Au-545 and Au-510, respectively, depending on their corresponding emission wavelength (Scheme 1). The concentration of Au NCs is estimated using the molar extinction coefficient at 420 nm (ε420 = 112000 cm-1 M-1), using earlier report.39 The final concentration of all the nanoclusters solution is fixed to 1.66 x10-6 M.



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Scheme 1. Schematic representation of the synthesis of Au NCs with tunable emission via core etching as well as ligand exchange method CHARACTERIZATION The optical absorption spectra were taken with a UV-vis spectrophotometer (Shimadzu) using a cuvette with a path length of 1 cm at room temperature. The emission spectra of all of the samples were taken with a Fluoro Max-P (HORIBA Jobin Yvon) luminescence spectrophotometer. The transmission electron microscopy (TEM) was performed on a JEOLJEM-2100F transmission electron microscopy system at an accelerating voltage of 200 kV. The TEM samples were prepared by drop casting of Au NCs solution onto a copper grid covered by a carbon film. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (Bruker Daltonics Autoflex II TOF/TOF) were used to determine the mass of the Au NCs. A pulse laser of 337 nm was used and a saturated matrix solution (CHCA, DHB and SA) was selected the MALDI-TOF experiment. The X-ray photoelectron spectroscopy (XPS) (Omicron Nanotechnology instrument) study was carried out to determine the binding energy. Raman spectra were recorded by using Horiba Jobin Yvon, (T64000 model) instrument by exciting the samples at 632 nm laser beam. Fourier-transform infrared (FTIR) spectroscopy analyses were performed on a SHIMADZU made FTIR-8300 spectrometer using KBr pellets. The zeta potential was measured with a Malveron Zetasizer instrument. High resolution mass spectra were carried out on Q-Tof micro MS system by electron spray ionization (ESI) technique. Cyclic voltammogram analysis was performed with a CHI 600D series instrument with a Pt working electrode, a Pt wire auxiliary electrode, and Ag/AgCl reference electrode in ethanol solution with tetrabutylammonium perchlorate as the supporting electrolyte. The solution of Au NCs is added in the supporting electrolyte. For the time correlated single photon counting (TCSPC) analysis, the samples were excited at 375 nm using a picosecond NANO-LED IBH 375L instrument. The typical full width at half-maximum (FWHM) of the system response using a liquid scatter was about 90 ps. The fluorescence decays were collected on a Hamamatsu MCP photomultiplier. Equation (1) is used to analyze the experimental time-resolved fluorescence decays40, P(t): n

P (t ) = b + ∑ α i exp ( − t / τ i )

(1)

i


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Here, n is the number of emissive species, b is the baseline correction (“DC” offset), and αi and τi are, respectively, the pre-exponential factor and the excited-state fluorescence decay time associated with the ith component. The average decay time, < τ >, is calculated from following equation.40 n

< τ >= ∑ β iτ i

(2)

i =1

Where β i = α i / ∑ α i and β i is the contribution of the decay component. The QY of Au NCs were obtained by comparison with reference dye, quinine sulphate (in water), using the following equation.40 QYs = (Fs × Ar × ηs2 × QYr) / (Fr × As × ηr2)

(3)

Where, Fs and Fr are the integrated fluorescence emission of the sample and the reference. As and Ar are the absorbance at the excitation wavelength of the sample and the reference. QYs and QYr are the quantum yields of the sample and the reference, respectively. Quinine sulphate were used as a reference dye (QYr= 53%) to obtained the QYs of Au NCs. The refractive indexes (ηr and ηs) of the solvent are 1.33 for both the reference and sample because water is used for both cases. A local made UV exposure equipped with UV light source 365 nm was used to obtain the picture under the UV exposure. All the experimental analyses were carried out at room temperature.

RESULTS AND DISCUSSIONS Structural Analysis Wavelength tunable Au NCs with varying the core atoms were synthesized in aqueous medium by etching of weakly fluorescent Au NCs using the top-down approach (Scheme 1). In this green synthesis route, we have synthesized orange, yellow, green emitting nanoclusters by core etching as well as ligand exchange of weakly fluorescent Au-652 NCs where MPA ligand acts as an etching agent. Depending upon the etching time, the Au NCs exhibit emission at different wavelengths. Details investigation reveals that the emission wavelength of Au NCs strongly depends on the metal core as well as chemical environment of nanoclusters. TEM was used to investigate the shape and the size distribution of particles. Figure 1A depicts the TEM images of Au-652 and it is seen from the TEM image that the particles are



spherical shape and polydisperse in nature. The average size of Au-652 is found to be 1.9 ± 0.5 nm.

Figure 1. TEM images of (A) Au-652, (B) Au-590 and (C) Au-510 and insets show the corresponding particle size distribution.

Again, TEM analysis of Au-590 was carried out to investigate the growth of nanoclusters during the core etching of Au-652 NCs (Figure 1B). The size of the Au-590 is calculated from particle size distribution and is found to be 1.7 ± 0.5 nm. As the size of NCs decreases from 1.9 nm to 1.7 nm, therefore successful core etching occurs in presence of MPA. The core etching of Au-652 was continued for 96 hour for size focusing synthesis of NCs. Moreover, the TEM image of Au-510 shows that the NCs are polydisperse and the average size is 1.4 ± 0.5 nm (Figure 1C). TEM analysis reveals that the core size of all the particles is < 2 nm, which is well matched with previous report.29,

41

Thus, the as-prepared particles are nanoclusters not nanoparticles.

Interestingly, from the TEM analysis it is clear that the core size of Au NCs significantly decreases due to core etching in presence of MPA ligands. The size of Au NCs decreases from 1.9 nm to 1.4 nm, this contraction in core size is responsible owing to the change of number of gold atom in metal core. Furthermore, the energy dispersive X-ray (EDX) corresponding to Au590 NCs depicts the distinctive peak of Au and S, indicating the NCs are capped by thiol end of capping ligands (Figure S2). MALDI-TOF analysis has been carried out to investigate the number of gold atoms present at the core of metal nanoclusters. The MALDI spectrum of Au-652 shows a hump at m/z 4867 Da. Due to the low resolution of mass spectra, we tentatively assign the peak as [Au15(GSH)6+3Na+H]4+ (Figure 2A). However, in presence of MPA, the MALDI peak is shifted 8 ACS Paragon Plus Environment

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from 4867 Da to 3870 Da within 24 hour, which confirms the formation of Au-590 NCs via the core etching of Au-652 NCs (Figure S3). In the MALDI spectrum of Au-590 shows a hump at m/z 3870. Due to the low resolution of mass spectra, we tentatively assign the peak as [Au12(GSH)4(MPA)2+3Na+H]4+. With increasing the etching time, the core of the gold cluster is etched from fifteen atoms (Au15) to twelve atoms (Au12). Furthermore, six more Au atoms and two glutathione ligands are etched when the etching time continues from 24 hour to 96 hour. Figure 2B exhibits MALDI peak of Au-510 at 2058 Da after 96 hour etching time in presence of excess MPA where the peak is tentatively assigned to [Au6(GSH)2(MPA)2+2Na+4H]6+. 4

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Figure 2. MALDI-TOF spectra of Au NCs (A) Au-652 and (B) Au-510

The shifting of MALDI peak from 4867 Da to 2058 Da confirms the core etching of Au652 in presence of MPA ligand. The Au-510 nanoclusters consist of six gold atoms in its core capped by two glutathione and two MPA ligands. This kind of MALDI hump not distinctive peak is quite common for gold nanocluster in aqueous phase synthesis.42 Interestingly, Au-510 NCs exhibit single peak in its MALDI spectrum which confirms the formation of highly pure single NCs rather than the formation mixed nanoclusters. Furthermore, the existence of single peak illustrates the controlled size focusing of Au NCs in presence excess MPA (Figure 2B). Small molecule like MPA facilitates the etching process of nanoclusters. Furthermore, it reveals from the MALDI analysis that core size of Au NCs successfully changes after etching by MPA ligands. Noteworthy, the MALDI mass spectra is very much consistent with the size focusing



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and conversion of weakly fluorescent Au NCs to highly fluorescent Au NCs. In contrary, some other thiol ligands were used instead of MPA, such as cysteine, D-penicillamine but only MPA is suitable for core etching. Possibly less steric hindrance of MPA molecule is suitable for effective etching of Au-652 NCs. FTIR experiment was carried out to investigate the binding mode of capping ligands to gold atom of Au NCs. In this synthesis, both the capping ligands GSH and MPA are supposed to bind with the gold core through Au-S covalent bond formation, which is confirmed from FTIR experiment. It is clearly seen from FTIR spectra that both the GSH and MPA show a peak at around 2525 cm-1 which appears due to S-H bond vibration in pure ligands (Figure S4A). Noteworthy, after complete formation of Au NCs, the S-H bond vibration peak is completely disappeared owing to formation of stable Au-S covalent bond. Again, there is no other prominent change in the fingerprint region which further confirms that both the capping ligands GSH and MPA bind to Au atom only through S-H binding site (Figure S4A). FTIR spectra of GSH capped Au NCs without addition of MPA reveals that there is no peak around 2525 cm-1 due to S-H bond (Figure S4B). Furthermore, we have investigated metal-ligand bond from Raman spectrum analysis. Raman studies were performed using green emitting Au-510; Raman peak for Au-S bond appears below 400 cm-1. In Raman spectrum, a strong hump is found at around 232 cm-1 which is due the presence of Au-S bond formation in nanoclusters (Figure S5).43 The oxidation state of gold atom in Au NCs has very important role in its optical properties. The oxidation states of gold atom in nanoclusters are determined from XPS analysis. Figure 3A depicts the fully scanned spectrum of Au, S, C and O of Au-510 NCs.41,

44

The

photoemission is originated from the core 4f level of Au, which is further expanded in figure 3B. Au NCs shows a characteristic twin XPS peak between 80-90 eV, which is assigned as 4f of Au. The lower energy peak at around 83 - 84 eV is due to 4f7/2 and the higher energy peak at around 87 - 88 eV is due to 4f5/2 of Au, respectively. These two peaks could be further deconvoluted into two distinctive peaks at binding energies of 83.68 eV and 84.20 eV (blue and magenta curve) for 4f7/2. The 4f5/2 peak is deconvoluted into two distinctive peaks 87.39 eV and 87.88 eV (wine and orange curve). The higher binding energies (84.20 and 87.88 eV) are assigned for Au+ and the lower energies (83.68 and 87.39 eV) are due to presence of Au0.45 Therefore, the coexistence of both Au+ and Au0 is a special characteristic of Au NCs. Usually in Au NCs, Au0 occurs in metal core protected by Au+ and surface capping ligands.46 10 ACS Paragon Plus Environment

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Figure 3. X-ray photoelectron spectra of Au NCs (A), Au 4f (B), S 2p (C) and C 1s (D)

To analyze the binding nature of thiol moiety (-SH) of capping ligands with Au clusters, we have deconvoluted the asymmetric curve of S 2p into two distinctive peaks (Figure 3C). The higher binging energy at around 163.62 eV is assigned for 2p1/2 which is due to S-C bond present in the capping ligand itself. Again, the small hump at around 162.65 eV is for 2p3/2 which is due to Au-S covalent bond formation, as previously reported.47 Two distinguished peaks are observed for C1s element with binding energy 288.5 eV and 284.5 eV (Figure 3D). The higher binding energy (288.5 eV) is due to O-C=O bond and the lower energy (284.5 eV) is due to C-C bond of capping ligands.47 Steady State Spectroscopy The initial growth mechanism of NCs is investigated from UV-visible absorption spectroscopy. The absorption profile of Au-652 NCs with varying etching time in presence of MPA ligands at different time intervals is given in Figure S6A. It is clearly seen from the absorption spectra that there is no prominent change within 10 hour of etching. However, the emission wavelength of the nanoclusters changes from 652 nm to 632 nm within initial 10 hour. 11 ACS Paragon Plus Environment


The 20 nm blue shifting indicates that the growth of nanoclusters begins within 10 hour due to core etching and ligand exchange (Figure S6B). It is observed that the absorption hump at 335 nm is disappeared when the etching time increases from 24 hour to 48 hour. Notably, a new absorption peak is observed at around 410 nm after 48 hour etching time (Figure 4,b curve). The absorption band at 410 nm is due to d-sp intra band electronic transition, which further indicates the formation of molecular like ultra small Au NCs.48-49 However, initially the etching of NCs is very slow therefore, 100 µL MPA is added to the reaction mixture after 48 hour which facilitates the growth of the reaction (Figure 4, b to d curves). As the reaction proceeds from 48 hour to 96 hour, the absorption peak is shifted from 410 nm to 386 nm with significant enhancement in absorbance (Figure 4). This 24 nm blue shifting in the absorption profile confirms formation of Au NCs with varying core atoms (inset of figure 4). The spacing between the discrete energy states increases with decreasing in core-size, which causes the blue shifting in absorption bands. The full width half maxima (FWHM) of the absorption spectra decreases with etching time (inset of figure 4). The mixture of different size nanoclusters is formed initially and then the size distribution decreases after long time etching with MPA, which suggest size focusing synthesis of NCs. The formation of gold nanoparticles (Au NPs) is ruled out as no surface plasmon resonance (SPR) peak corresponds to 520 nm is observed during the experiment.50-51 0.75 24 nm

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Figure 4. Absorption spectra of Au NCs (a) 24 hour, (b) 48 hour, (c) 72 hour and (d) 96 hour and. Inset shows the shifting of absorption peak with increasing etching time from 48 hour to 96 hour. Figure 5 depicts the emission spectra of all the synthesized NCs such as Au-652, Au-590, Au-545 and Au-510, respectively after excitation at 375 nm. The inset of figure 5 shows digital 12 ACS Paragon Plus Environment

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photographs of Au NCs in the presence of daylight and under the excitation of UV light at 365 nm. The weakly fluorescent Au NCs exhibit emission at 652 nm with very low QYs 5.89 × 10-4 % (Figure 5A).20 Three other highly fluorescent Au NCs were prepared from weakly fluorescent Au-652 NCs by etching metal core with MPA.

Figure 5. Emission spectra of nanoclusters (A) Au-652, (B) Au-590, (C) Au-545 and (D) Au510 and inset shows the corresponding nanoclusters solution under day light and UV light (365 nm excitation).

Depending upon the etching time, the Au NCs exhibit emission at different wavelengths. After 24 hour of etching, Au NCs display emission at around 590 nm (orange color) with QYs 0.57% (Figure 5B). Again, the emission band is shifted from 590 nm to 545 nm with relatively high QYs 2.40% when the etching time is increased from 24 hour to 72 hour (Figure 5C). Furthermore, after 96 hour etching time, Au NCs exhibit emission at around 510 nm with very high QYs 3.15 % (Figure 5D). The PL intensity of Au NCs increases as etching time increases from 24 to 96 hour (Figure S7). The Au NCs exhibit tunable PL with high QYs in the wavelength ranging from 590 to 510 nm. This 80 nm blue shifting (590 nm to 510 nm) in PL with changing the etching time from 24 hour to 96 hour indicates the change of number of gold 13 ACS Paragon Plus Environment


atoms in the core of Au NCs (Figure 6). The shifting in emission maxima is due to the reduction of core size in NCs as well as due to change of chemical environments of i.e. capping ligands.17

1.2 Normalized PL Intensity

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24 hours 48 hours 72 hours 96 hours

0.9 0.6 0.3 0.0 450

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Figure 6. The shifting of PL maxima of Au NCs with increasing the reaction etching time from 24 hour to 96 hour. A large Stokes shift is observed in the PL emission of both the as prepared Au-590 and Au-510 NCs. The Stokes shift in Au-590 is found to be 255 nm whereas the Stokes shift is reduced to 124 nm in Au-510 (Figure S8). The significant change in Stokes shift (255 nm to 124 nm) indicates the change in core of NCs during the core etching process. This large Stokes shift is very much consistent with ligand to metal charge transfer in excited state.52 The optical band gap of Au-510 is 1.42 eV, obtained from a plot of absorbance versus energy (Figure S9).53 Furthermore, we also calculated the band gap of the same Au-510 using cyclic voltammetry analysis (Figure S10). The first oxidation and reduction lie at -0.24 and +1.09 V, respectively.54 The electrochemical band gap is found to 1.33 eV which is in a good agreement with measured optical band gap.11 The HOMO-LUMO band gap of Au-510 is larger than Au25(SR)18 (Eg ~1.3 eV), indicating that the smaller clusters exhibit higher band gap.11 This optical and electrochemical band gap studies reveal the molecular like features of Au NCs. The fluorescence excitation spectrum of Au-510 exhibit maximum peak at 400 nm (Figure S11) which is well matched with the absorption maximum at 386 nm (Figure 4, d curve). In addition, to investigate the excitation dependent emission property of Au NCs, we have excited Au-510 NCs in the 14 ACS Paragon Plus Environment

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wavelength region 300-460 nm (Figure S12). There was no change in the emission maximum while changing the excitation wavelength of Au-510 from 300 to 460 nm, confirms the distinctive HOMO-LUMO band gap of Au NCs.7, 39 In a control experiment, we synthesized Au cluster without excess MPA (100 µL MPA is not added after 48 hour) and the reaction was kept for stirring at dark. Noteworthy, the emission of the NCs changes from 590 nm to 557 nm even after seven days and the absorption hump was found to be 398 nm (Figure S13). This implies that the reaction is completed only in presence of excess thiol. Time-Resolved Spectroscopy

Luminescence lifetime of Au NCs was measured to understand the excited state dynamics ( λex = 375 nm, λem = 590 nm and 510 nm) (Figure 7). Analysis of decay dynamic reveals whether the emission is originated from (i) the electronic transitions of gold core or due to the (ii) ligand to metal charge transfer (LMCT; S→ noble metal).46,

55-58

Generally, the

nanoscale lifetimes exhibited by Au nanocluster are due to different electronic transitions such as sp to sp and sp to d.

1000

Counts (log)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a 100 b 10 0

800

1600

2400

3200

4000

Time (ns) Figure 7.Time-resolved decay curves of (a) Au-590 and (b) Au-510 (λex 375 nm).

On the other hand, several Au NC shows longer life time (> 100 ns) which implies the existence of ligand to metal charge transfer. The average lifetime of orange emitting Au-590 is found to be 222 ns with the components of 32 ns (44%), 217 ns (40%) and 757 ns (16%). However, the average lifetime of Au-510 is found to be 163 ns with the components of 29 ns (56%), 201 ns (31%) and 655 ns (13%) (Table S1). The shortening of lifetime of Au-510 NCs 15 ACS Paragon Plus Environment


than Au-590 NCs owes to higher contribution (56%) of the faster component in Au-510 NCs. The longer lifetime components (757 ns and 655 ns) illustrated its origin from triplet to singlet transition. Because few photo excited NCs go to low-lying triplet state from excited singlet state via intersystem crossing (ISC).59-61 Again, for further analysis of PL emission, the radiative and the nonradiative rate constant is calculated for both Au-590 and Au-510 NCs. The calculated radiative rates are found to 2.56 × 104 s-1 and 1.93 × 105 s-1 for Au-590 and Au-510 NCs, respectively. The radiative rate of Au-510 is almost eight times higher than Au-590. The control growth eliminates the surface trap states from Au-510 which causes the high radiative rate in Au510 than Au-590. Again, the nonradiative rates are estimated and found to be 4.47 × 106 s-1 and 5.94 × 106 s-1, respectively (Table S1). Interestingly, the nonradiative constants are almost same, which indicates the nonradiative relaxations are same for both NCs. To understand the surface charge of as-prepared Au NCs, we have measured the zeta potential of Au-510 and it was found to be -10.2 mV (Figure S14). The negative zeta potential indicates the existence of carboxylate ion of GSH and MPA ligands in NCs. The advantages of high lifetime, and large stokes shift of Au NCs may be useful for metal ion detection. Detection of Pb2+ Ion

Fluorescent metal nanoclusters are widely used for optical sensor in different biological systems.62 Therefore, significant attention has been given for heavy metal ion sensing such as Cd2+, Hg2+, Pb2+ and Cu2+.48,

63-68

The photoluminescence (PL) responses of both green and

orange emitting NCs are being used to analyze for metal ion sensing. A significant PL quenching (75%) is found in presence of 4 x 10-7 M Pb2+ for green emitting Au NCs (Figure 8A). On the other hand, Na+, K+, Mg2+, Ni2+, Cd2+, Fe3+, Mn2+, Ag+, Zn2+ Cr6+ ions do not significantly affect the PL signal of Au-510 NC (except Cu2+ and Hg2+). The 24% PL quenching of Au NC is observed for both Cu2+ and Hg2+ ions, which is found to be much lower than the PL quenching caused by Pb2+. Noteworthy, only 16% PL quenching is observed by salt mixture (which contains all metal salt except Pb2+). Therefore, in the salt mixture 16% PL quenching is due to the presence of Cu2+ and Hg2+ ion. Furthermore, figure 8B displays the corresponding histogram plot of PL-quenching caused by individual metal ion. From the histogram, it is clearly seen that the green emitting Au NC exhibit extreme selectivity towards Pb2+ even in presence of interfering metal ions.


Page 16 of 28

Blank + Na K+ 2+ Mg 2+ Ni 2+ Cu 2+ Cd 3+ Fe

6

3x10

6

2x10

2+

1.2

(A)

Mn + Ag 2+ Zn 6+ Cr 2+ Hg Salt Mixture 2+ Pb

6

(B)

1.0 0.8 I0/I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PL Intensity (a.u)

Page 17 of 28

0.6 0.4

1x10

0.2 0 450

500

550

600

650

700

0.0 Blank

Wavelength (nm)

+ + 2+ 2+ 2+ 2+ 3+ 2+ + 2+ 6+ 2+ 2+ Na K Mg Ni Cu Cd Fe Mn Ag Zn Cr Hg Salt Pb mixture

Figure 8. (A) PL quenching of Au-510 NCs in presence of different metal ions and (B)

histograms of PL quenching of Au-510 NC in presence of different ions and their mixture, concentration of metal ion is 4×10-7 M, error bars represents the standard deviation based on three replicated experiments.

Again, the figure 9A exhibits the PL-quenching of Au-510 NC with increasing the concentration of Pb2+. The limit of detection (LOD) is calculated from the slop of the plot of I/I0 versus concentration of Pb2+ (Figure 9B). The LOD value is estimated according to the assumption of LOD equal to 3SD/S, where SD is standard deviation of intercept and S is the slope of calibration curve.69 Interestingly we obtained very low LOD, 10 nM which is much lower than earlier reports (Table S2).65, 70-71 However, in case of orange emitting NCs, 34 % PL quenching is achieved by Pb2+ (Figure S15). Again, the LOD for orange emitting Au NCs is found to be 46 nM (Figure S16). Therefore, the higher QYs of green emitting NCs are found to be better optical sensor towards Pb2+ than orange emitting NCs. The probable mechanism for Pb2+ sensing by Au NC is due to the metalophilic interaction between Pb2+ (5d10) and Au+ (5d10).72 To further understand the PL quenching of Au NCs caused by Pb2+, the decay dynamic by TCSPC analysis is important. The lifetime of Au-510 decreases from 163 ns to 38 ns in presence of Pb2+, which is around 76% lifetime quenching (Figure 9C). This drastic change in lifetime confirms due to energy transfer from Au-510 NC in presence of Pb2+.73



6

1.8x10

3.0

(A)

a

(B)

6

1.5x10

2.5

6

1.2x10

5

9.0x10

F0/F

PL Intensity (a.u)

i

2.0 1.5

5

6.0x10

1.0

5

3.0x10

0.0 450

0.5 500

550

600

650

0

Wavelength (nm)

40 80 120 160 2+ Concentration of Pb (nM)

200

(C)

Counts (log)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

Au-510 NC 2+ Au-510 NC+Pb

1000 a 100 b 10 0

700

1400

2100

2800

3500

Time (ns)

Figure 9. (A) PL quenching of Au-510 NC with varying Pb2+ metal with varying concentrations

from 0 nM to 190 nM (a to i), (B) Plot of I/I0 versus concentration of Pb2+ (0 nM to 190 nM), and (C) Time-resolved decay curves of (a) Au-510 and (b) Au-510 in presence of Pb2+ (λex 375 nm). Furthermore, TEM analysis is performed to further establish the interaction of Pb2+ with thiol capped Au NCs, (Figure 10A). The ultra small Au-510 become aggregated in presence of Pb2+ and form larger size non luminescent NCs, (inset of figure 10A). The EDX corresponding to Au-Pb2+ nanocomposites depicts the distinctive peak of Au, S and Pb, which suggest that the aggregation of Au NCs occurs in presence of Pb2+ (Figure 10B).


Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. (A) TEM image of Au-510 NC in presence of Pb2+, inset shows the enlarged view of

aggregated NCs (B) corresponding energy dispersive X-ray (EDX) spectra. Again, Au NCs is found to a promising optical probe for selective detection of Pb2+ both in highly alkaline and acidic medium (Figure S17). Moreover, to check the real sensing applications, we used pond water and river water for Pb2+ sensing. Our analysis reveals than that no Pb2+ is present in pond water (Figure S18A). On the other hand, the PL intensity of Au NCs is significantly quenched in presence of river water. The calculated I/I0 is 0.65, which is significantly low which confirms the presence of Pb2+ in the river water (Figure S18B). The estimated concentration of Pb2+ in the river water is found to be 88 nM which is higher than the allowed concentration of Pb2+ in drinking water. CONCLUSION

In summary, we report a facile synthesis route of Au NCs with tunable emission wavelength ranging from 590 to 510 nm in aqueous medium. In this aqueous phase synthesis, bright fluorescent Au NCs were prepared from a weakly fluorescent Au NCs via core etching as well as ligand exchange method. Depending upon the etching time, the emission wavelength of Au NCs varies from 590 nm to 510 nm. The number of gold atoms in each metal nanocluster was determined using MALDI analysis, which is very sensitive to its optical property. Furthermore, the lifetime measurement reveals that the fluorescence of Au NCs is originated 19 ACS Paragon Plus Environment


from ligand to metal charge transfer (LMCT) through S-Au bonds. The as prepared Au NCs exhibits long lifetime (>100 ns), high Stokes shift (255 nm to 124 nm) and biocompatibility. We demonstrated the application of highly luminescent Au NCs for Pb2+ sensing in presence of Na+, K+, Mg2+, Ni2+, Cu2+, Cd2+, Fe3+, Mn2+, Ag+, Zn2+ Cr6+ and Hg2+. Furthermore, Au NCs is an efficient sensor for Pb2+ detection in real sample. The Au NCs based optical probe can detect extremely low concentration of Pb2+ (LOD=10 nM) in drinking water, which is much lower than the maximum contaminant level for Pb2+. Thus, the as-prepared Au NCs has potential sensing capability to detect the presence of Pb2+ in drinking water. Supporting Information

Chemical structures of Glutathione reduced and 3-Mercaptopropionic acid, Energy dispersive X-ray (EDX) spectrum, MALDI-TOF spectrum, FTIR spectra, Raman spectrum, Absorption spectra and PL spectra, The PL intensity of Au NCs with increasing the reaction etching time, Stokes shift of fluorescent nanoclusters, Absorption spectrum of Au-510 is plotted in energy (eV) axis, Cyclic voltammogram, Excitation spectra, Emission spectra at different excitation wavelengths, Control experiment, Time-Resolved fluorescence data (Table S1), Zeta potential, Nanomerials based sensor for the detection of toxic ions (Table S2), PL spectra of Au NCs in presence of different metal ions and histogram plot, PL quenching of Au NC in presence of Pb2+ metal ion and calibration curve, PL spectra of Au NCs in presence of Pb2+ at pH 3 and pH 9, detection of Pb2+ in pond water and river water using Au-510 NCs. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS

“DAE-SRC Outstanding Investigator Award” is gratefully acknowledged for financial support. DB, SM and BP thank CSIR for awarding fellowship. We thank IACS for support throughout our experiment.


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Synopsis

Synthesis of wavelength tunable fluorescent gold nanoclusters by chemical etching process for the selective detection of Pb2+ ion


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Core-Size Dependent Fluorescent Gold Nanoclusters and

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