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C: Physical Processes in Nanomaterials and Nanostructures

Engineering Atomically Precise Copper Nanoclusters with Aggregation Induced Emission Subarna Maity, Dipankar Bain, and Amitava Patra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09467 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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The Journal of Physical Chemistry

Engineering Atomically Precise Copper Nanoclusters with Aggregation Induced Emission

Subarna Maity, Dipankar Bain and Amitava Patra* School of Materials Sciences, 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

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The Journal of Physical Chemistry 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

ABSTRACT Controlled synthesis of atomically precise metal nanoclusters (MNCs) and fundamental understanding of their physical properties have emerged as an active area of research because of their potential applications in healthcare and energy-related materials. In the present study, atomically precise copper nanoclusters (Cu NCs) have been synthesized from non-luminescent plasmonic copper nanoparticles (Cu NPs) by core etching with excess reduced glutathione (GSH). Electrospray ionization (ESI) mass spectrometry confirms the formation of kinetically controlled, polydisperse Cu34-32(SG)16-13 NCs at room temperature and monodisperse Cu25(SG)20 NCs at elevated temperature (70 ºC). Cu34-32(SG)16-13 NCs exhibit weak red emission (625 nm) while Cu25(SG)20 NCs emit intense blue luminescence at 442 nm with 9.7 % quantum yield. Rational tuning of reaction temperature, pH, GSH concentration and reaction time are crucial for the composition and emission band tuning of atomically precise Cu NCs. Interestingly, Cu34-32(SG)1613

NCs exhibit an aggregation induced emission (AIE) with addition of a less polar solvent, ethanol

(EtOH) and the enhancement in the luminescence is attributed to the alteration in the excited state dynamics with the change in solvent polarity. The unique and low cost synthetic methodology of Cu NCs with interesting AIE property may open up new possibilities for their applications in the field of bioimaging, photocatalysis, photosensors and light emitting devices.

INTRODUCTION 2 ACS Paragon Plus Environment

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Atomically precise metal nanoclusters (MNCs) have gained significant research interest in the field of nanoscience owing to their versatile applications in catalysis, sensing, optoelectronics and biomedical science.1-10 The ultrasmall particles (diameter < 2 nm) are composed of several tens to hundreds of metal atoms with capping ligands decorating the surface of metal core.1, 11-13 As MNCs act as the bridge between the atomic regime and large metal nanoparticles (NPs), they exhibit unusual molecular like properties such as quantized charging, strong luminescence, unusual redox behavior and intrinsic magnetism.14-19 Luminescent MNCs along with ultrafine size and good biocompatibility are quintessential for widespread applications, when tailoring the size and structure of NCs becomes straight-forward by a universal and convenient synthetic method.1113, 20-23

Enormous effort has been given in creating the NCs family of divergent atomic specificity

with different capping ligands like thiols, phosphines, carbenes, DNAs, nucleosides, proteins etc. 12, 22-30Among

them, thiolate is the most functional candidate as they form very stable metal sulfur

covalent bonds.29 Significant attention has been paid on the synthesis of Au, Ag and their alloy MNCs in recent years.17, 31-35 The synthetic methodology of thiolate protected MNCs has diverged into two broad areas: one is bottom up and another is top down method.3 One pot, bottom up strategy involves a direct reduction of metal ligand complexes to the atomically precise NCs with the aid of suitable reducing agent and the evolution of NCs is a coupling between two equilibrated processes i.e. the reduction-assisted growth of mixed sized NCs and etching mediated size focusing.12, 21, 36-37 The delicate control over the rates of the two processes lead to the successful formation of monodispersed NCs.37-38 Diverse external factors like pH, solvent, temperature, functional groups in ligand and presence of hetero metal atom influence the atomic specificity of NCs to a great extent.33, 39-43 On the contrary, in top down method, metal salts are reduced and nucleated to the nanoparticles comprising thousands of metal atoms in their reduced state and the subsequent etching of them with excess ligand leads to the NCs with well defined formula.44-46 Being a two step processes, there is no competition between the nucleation of metal atoms and etching of the particles to the most stable NCs.20, 46 Considering the advantage, a number of Au and Ag NCs have been synthesized from polydisperse NPs through a core etching or digestive ripening process governed by temperature, excess ligand or excess metal salts.11, 22-23 In spite of that, very few reports have been come up with the production of atomically precise Cu NCs. Wang et al. has reported Cu NCs from copper nanocrystals by using ascorbic acid as reducing agent.47 3 ACS Paragon Plus Environment


Recently, ammonia based rapid etching has been described for the synthesis of luminescent Cu NCs.48 Susceptibility to aerial oxidation, lacking acquaintance of emission mechanism, poor control over emission tuning and mostly unavailability in convenient and universal synthesis methodology curtail the applications of Cu NCs.49-52 There is a pressing need to design stable nanostructures with tunable and intense luminescence guided by logical surface engineering, controlled aggregation or ion pairing of NCs.13, 53-55 Now-a-days aggregation induced emission (AIE) of MNCs opens up a new door towards the fabrication of novel photo luminescent materials.47,

49, 52, 56-58

Luo et al. have reported a strategy to induce aggregation of Au(I)-SG

complexes on the Au(0) core by changing solvent and cation where intense luminescence was generated.58 Lee and coworkers have demonstrated the bright emission from Au22 involving the rigidification of the gold shell by introduction of tetraoctylammonium bromide.59 Furthermore, self assembly driven by pH, temperature and mechanical strength significantly revamp the luminescence behavior of non-luminescent or weakly luminescent NCs.50, 56-57, 60 61-62 Controlled aggregation of NCs significantly influences the ligand-ligand, ligand-metal and metal-metal interactions in the NCs which in turn dictate the change in molecular motions.50, 63 Though the exact mechanism of AIE is not clear, most possibly the restriction in rotational and vibrational motions and consequent increment in radiative decay pathways prevail the boost in luminescence.49-51, 59 Herein, a top down method more specifically a digestive ripening of polydisperse Cu NPs is undertaken to design atomically precise Cu NCs with tunable emission. GSH capped Cu NPs are synthesized by direct reduction of copper salts with sodium borohydride. A weak red emitting Cu34-32(SG)16-13 NCs is achieved in core etching reaction with the excess GSH at room temperature. Whereas long reaction time (36 h) and higher temperature (70ºC) leads to a very stable, monodisperse, bright luminescent Cu25(SG)20 NCs following the ‘survival of most robust’ principle. Moreover, Cu34-32(SG)16-13 NCs display an aggregation induced emission enhancement. 36 fold increment in emission intensity and shifting of emission maxima from 625 nm to 597 nm are observed upon introduction of relatively less polar EtOH. The AIE property of Cu NCs may shed some light on the applications regarding photo sensors and light emitting devices. EXPERIMENTAL SECTION 4 ACS Paragon Plus Environment

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Chemicals: Copper nitrate trihydrate [Cu(NO3)2.3H2O] was purchased from Loba Chemie. Reduced glutathione was obtained from SRL. Sodium hydroxide (NaOH), sodium borohydride (NaBH4), methanol (MeOH) and ethanol (EtOH) were purchased from Merck and EtOH was used after distillation. All solutions were prepared using Milli-Q water (18.2 MΩ). Synthesis of GSH capped Cu NPs: In a typical synthesis procedure, 60 mg Cu(NO3)2.3H2O was added to 50 mL methanol in a conical flask and the solution was allowed to cool in a ice bath for 10 min. Then, 307 mg GSH was added slowly under vigorous stirring to form Cu(I)-SG complexes. Next, freshly prepared, ice cooled aqueous solution of NaBH4 (100 mg in 10 mL) was added and instantly black precipitate of Cu NPs was formed. The mixture was allowed to stir in ice bath for 30 minute. Then, the mixture was centrifuged at 12000 rpm for 10 min and the black precipitate was washed with H2O:MeOH (1:3) solution. The washing process was repeated thrice and the precipitate was air dried. Then, the Cu NPs were dissolved in 10 mL HPLC water for further use. Synthesis of Cu34-32(SG)16-13 NCs: 2.5 mL Cu NPs solution was diluted up to 7 mL with HPLC water and 153.5 mg GSH was added at room temperature with vigorous stirring. After 5 min, pH of the medium was adjusted to 5.0 by adding 1 (M) NaOH solution. The solution was stirred for 30 min and then stored at 0 ºC for further experiment. Synthesis Cu25(SG)20 NCs: 2.5 mL Cu NP solution was diluted up to 7 mL with HPLC water and 307 mg GSH was added. After 5 min, pH of the medium was adjusted to 3.0 by adding requisite amount of 1 (M) NaOH solution. The solution was heated at 70 ºC for 36 hour resulting into a light blue colored solution of stable Cu NCs. The solution is centrifuged for 10 min at 5000 rpm and the supernatant was stored at 4 ºC for further use. The overall synthesis procedure is summarized in Scheme 1.

RT 30 min Ice Bath NaBH4 70 ºC Cu NPs

36 hr

Cu2+ GSH



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Scheme 1: Schematic illustration of the top-down synthesis of luminescent Cu NCs Instrumentation: The transmission electron microscopy (TEM) images were obtained by using a JEOL-JEM-2100F transmission electron microscope. Fourier-transform infrared (FTIR) spectroscopy was performed on a SHIMADZU made FTIR-8300 Spectrometer using KBr pellets. The XPS measurements were carried out using an Omicron Nanotechnology instrument. Thermo gravimetric analysis (TGA) was performed on a SDT Q Series 600 Universal VA.2E TA instrument. Zeta potential and hydrodynamic diameter measurements were carried out in a Malveron Zetasizer instrument. High resolution mass spectra were obtained from Waters micromass Q-TOF micro MS system by electrospray ionization (ESI) technique. Native polyacrylamide gel electrophoresis (PAGE) of the Cu NCs was performed in a Bio-rad, Miniprotein Tetra cell gel electrophoresis unit with 1 mm thick spacer. The resolving gel and staker gel are prepared by TGX stain free Biorad acrylamide resolver and staker solutions. The crude Cu NCs (10 µL Cu NCs in 6 vol % glycerol) was loaded onto the wells of staking gel. Tris-glycine buffer consisted of 192 mM glycine and 25 mM tris(hydroxymethylamine) was used. The electrophoresis was allowed to run for 4 h at a constant voltage of 100 V. During the gel electrophoresis, PAGE setup was kept at ice cold temperature. After successful run, the photographs of wet gel are captured under UV illumination (365 nm). Room temperature absorption spectra were recorded with a UV-vis spectrophotometer (Shimadzu) using a cuvette with a path length of 1 cm. The emission spectra of all of the samples were taken with a FluoroMax-P (HORIBA JobinYvon) luminescence spectrophotometer. For time correlated single photon counting (TCSPC) measurement, the samples were excited at 371 nm by Nano-LED-370. The fluorescence decays were collected on a Hamamatsu MCP photomultiplier. The following expression was used to analyze the experimental time-resolved fluorescence decays, P(t): n

P(t )  b    i exp (t /  i )

(1)

i

Here, n is the number of emissive species, b is the baseline correction, and  i and  i are the preexponential factor and the excited-state fluorescence decay time associated with the ith component.


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n

The average decay time,   was calculated from following equation.      i i i 1

(2) Where

 i =  i /   i and is the contribution of the decay component.64 The quantum yield (QY)

of Cu NCs was measured using quinine sulfate as reference dye using the following equation64 QYs = (Fs × Ar × ηs2 × QYr) / (Fr × As × ηr2)

(3)

Where, Fs and Fr are the integrated fluorescence emission of the sample and the reference respectively. As and Ar are the absorbance of the sample and the reference respectively at the excitation wavelength. QYs and QYr are the quantum yields of the sample and the reference (QYr = 0.53). The refractive indices of the solvents used for the preparation of the sample and reference are given by ηs (1.33) and ηr (1.33) respectively (here both solvents are water). RESULTS AND DISCUSSION Temperature directed synthesis of Cu NCs: In the temperature dependent digestive ripening process, GSH capped Cu NPs are used as precursor for the formation of NCs with high quantum yield and narrow size distribution by core etching. First, Cu2+ salts and capping ligand GSH are mixed in MeOH to promote the formation of Cu(I)-SG complexes.31 NaBH4 is used as reducing agent and a black precipitate is appeared instantaneously after adding this. The precipitate indicates the formation of Cu NPs via reductive decomposition of Cu(I)-SG complexes by NaBH4. The whole reaction is carried out in an ice bath to lower the reducing ability of NaBH4. The binding of protecting ligand GSH is investigated by Fourier transform infrared (FTIR) spectroscopy (Figure S1). A characteristic peak at 2525 cm-1 is present in the spectrum of free GSH which arises due to S-H bond stretching.23 The peak disappears owing to the formation of Cu-S covalent bond in Cu NPs. Transmission electron microscopy (TEM) images of NPs reveal that the particles are spherical and polydisperse with an average size 2.67 nm ± 0.05 nm (Figure 1A and 1B). The NPs exhibit a surface plasmon resonance (SPR) band at 650 nm in UV-Vis absorption spectrum (Figure 1C) which is well documented for metal nanoparticles.65 The black colored NPs do not have any luminescence when illuminated with 365 nm UV light (Inset of Figure 1C). The feeding ratio of Cu2+ and GSH was kept 0.25:1 in the reaction and the composition of the product is estimated by thermo gravimetric analysis (TGA) (Figure 1D). A loss of ~ 36 % mass is detected due to the 7 ACS Paragon Plus Environment


thermal dissociation of the surface ligands as gaseous products. The SG/Cu ratio is found to be 0.11 which essentially indicates very high Cu content in the core of NPs. The objective of the core etching process is to reduce the average core size of precursor NPs with excess thiol ligand to achieve atomically precise NCs. In presence of temperature, excess ligand and O2, the polydisperse particles are converted to a thermodynamically stable monodisperse NCs via a digestive ripening process following the “survival of the most robust” principle. In this methodology Cu NCs with divergent atomic specificity have been successfully synthesized with a subtle change in reaction condition.

(A)

(B)

No of Particles

40 30 20 10 0

0.8

1

2 3 Diameter (nm)

100

(C)

Mass (%)

1.2

Absorbance

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

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4

(D) 36% loss

75

0.4

50 0.0 400

150

600 800 Wavelength (nm)

300 450o Temperature ( C)

600

Figure 1: TEM images of GSH capped Cu NPs with scale bar (A) 50 nm, 5 nm (inset) (B) 20 nm and particle size distribution curve (inset). (C) UV-Vis absorption spectrum of GSH capped Cu NPs in aqueous solution; inset shows the digital photograph of solid Cu NPs. (D) TGA curve of GSH capped Cu NPs. At room temperature (25ºC), when the Cu NPs solution is treated with GSH, black colored solution instantaneously turns in to light brown, indicating of the rapid consumption of NPs. The absorption peak at 650 nm due to SPR in NPs disappears suggesting the conversion to smaller 8 ACS Paragon Plus Environment

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NCs. (Figure 2A). A hump at 340 nm appears in the absorption spectrum resemblance of their molecular like behavior.18 The photo excitation maximum is centered at 360 nm which is in agreement with absorption spectrum (Figure 2B). The light brown solution Cu NCs shows a weak red emission upon excitation at 365 nm. Photoluminescence (PL) spectrum shows an emission maximum at 625 nm and the quantum yield is found to be 0.03 %. The as prepared NCs exhibit significantly large stokes shift of 265 nm. As the excess GSH induces the formation of NCs, the effect of GSH feeding ratio is monitored by PL measurements and 0.5 mmol is found to be sufficient for maximum chemical yield of Cu NCs (Figure S2A). The pH of the reaction has a dramatic effect on the core size reduction process and pH 5 is found to be optimum (Figure S2B). If the reaction is carried out at pH 3, insoluble precipitate is formed along with Cu NCs within 10 minutes of the initiation of reaction, whereas pH is higher than 5 (pH = 7, 9 and 11) results into a non luminescent product. In basic pH, the electrostatic repulsion among the negative surface charges of Cu NPs and GSH makes the interaction unfavorable. The measured Zeta potential is 12.1 mV which arises due to the de-protonated carboxyl groups on surface of GSH capped Cu NCs. The reaction is very fast and is initiated just after the addition of GSH. Reaction is continued to 30 minutes for complete formation of Cu NCs, as the luminescence intensity becomes nearly constant there after (Figure S3). Native polyacrylamide gel electrophoresis (PAGE) for the Cu NCs shows a broad band with red luminescence under UV illumination suggesting that the Cu NCs contain mixed sized of NCs with very close mass difference. Electrospray ionization (ESI) mass spectrometry being a soft ionization technique is employed to investigate the composition of NCs. The positive ion mode ESI mass spectrum of Cu NCs displays a series of peaks at m/z 1670, 1527, 1362, 1186 and 1033 (Figure 2C). Inset of Figure 2C shows the zoomed view of the mass spectrum with marked signature peaks. Precise composition of NCs is estimated by the mass spectrometric data coupled with isotopic simulations (Figure 2D and 2E). The peak a is assigned to [Cu32(SG)15 + 2Na+ + 2H+]4+ as the spacing between each peaks is found to be 0.25. Cu33(SG)13 has appeared at m/z 1526 with the composition as [Cu33(SG)13 + Na+ + 3H+]4+ (Figure S4). Fragment c is characterized as [Cu34(SG)15 + 2Na+ + 3H+]5+ as the isotopic pattern shows a spacing of 0.2. Peak d has a formula [Cu34(SG)16 + 2Na+ + 4H+]6+ and lowering of mass equal to 3 unit of GS- results into fragment e, [Cu34(SG)13 + 2Na+ + 4H+]6+. Therefore, the Cu NCs prepared at room temperature are the mixed sized NCs with formula Cu34-32(SG)16-13. The Cu34-32(SG)16-13 NCs undergo a 69.2 % mass loss in TGA analysis which corresponds to the mass of thiolates (Figure 9 ACS Paragon Plus Environment


S5). The calculated SG/Cu ratio is 0.465 which perfectly matches with the composition of Cu34(SG)16 NCs. As the course of reaction is very short and the rate is so fast, the ligand mediated etching does not result into a particular size instead a mixture of similar sized NCs has been evolved. Higher temperature or long reaction time may be worthwhile to attain particular sized NCs via core etching process.

Figure 2: (A) UV-Vis absorption spectrum, (B) photoexcitation (a) and photoluminescence spectra (b) of red emitting Cu NCs in aqueous solution. Inset shows wet gel after PAGE at room temperature under UV illumination (365 nm) (C) ESI-MS spectrum of Cu NCs and inset shows the zoomed view. (D) Isotopic patterns of peak d (experimental: green, calculated: black) and (E) peak e (experimental: red, calculated: black) with formula [Cu34(SG)16 + 2Na+ + 4H+]6+ and [Cu34(SG)13 + 2Na+ + 4H+]6+ respectively. Temperature of the core etching reaction is raised to 70ºC to achieve monodispersity. When GSH is added to the black NP solution, the instantaneous color change of the solution to light brown is similar to reaction at room temperature. If the reaction is continued at 70ºC, the light 10 ACS Paragon Plus Environment

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brown solution is gradually converted to insoluble light yellow aggregates. As time proceeds, the aggregates get dissolved and a light blue colored transparent solution of Cu NCs is obtained after 36 hour of initiation. Polymeric Cu(I)-SG complexes formed during reaction are removed by centrifugation. No peak at 650 nm in UV-vis absorption spectrum confirms the disappearance of SPR and consequent size reduction of the parent Cu NPs (Figure 3A). Two absorption peaks at 700 nm and 380 nm are distinct and completely different from parent Cu NPs and Cu34-32(SG)16-13 NCs formed at initial stage. The excitation maximum is centered at 380 nm which is in accordance to the absorption peak (Figure 3B). The light blue colored solution of Cu NCs has a bright blue emission when illuminated with a 365 nm UV source (Inset of Figure 3B). PL spectrum of Cu NCs shows an emission peak at 442 nm when excited with 375 nm excitation (Figure 3B). The stokes shift is found to be 62 nm and is significantly lower compared to the Cu34-32(SG)16 NCs. The as synthesized Cu NCs possess 9.7 % quantum yield when measured using quinine sulfate as reference dye. The PL maxima remain unchanged when excited with different wavelengths indicating the definite HOMO-LUMO gap (Figure S6). The GSH concentration and pH are found to be crucial towards the size selective conversion of NPs. PL measurements reveal that the optimized condition requires 1 mmol GSH which is 2 fold higher than that requires to obtain Cu3432(SG)16-13

NCs (Figure S7A). Optimum pH is found to be 3 for convenient preparation of blue

emitting NCs (Figure S7B). Native PAGE of as obtained blue emitting Cu NCs displays one exclusive band in the gel which emits blue luminescence under UV illumination (Inset of figure 3B). The exact molecular formulas are determined by ESI mass spectrometry in positive ion mode. The characteristic isotopic patterns of each fragment are well matched with the calculated data. A series of peaks are observed at m/z 1288, 1226, 1098 and 969 (Figure 3C). Inset of Figure 3C shows the zoomed view of the mass spectrum with assigned signature peaks. The peak a (Inset of figure 3C) is assigned to the formula [Cu25(SG)20 + 6H+]6+ and the experimental and simulated isotopic patterns confirm the assignment (Figure 3D). The sodium adducts of Cu25(SG)20 NCs with charge +8 appear at m/z 969 (fragment d) and is assigned as [Cu25(SG)20 + Na+ + 7H+]8+. The zoomed in view shows the peaks have a spacing of 0.125 which is in accordance to the assignment (Figure 3E). The fragment b suffers a subsequent loss of one Cu and one GS- from Cu25(SG)20 and have characterized as [Cu24(SG)19 + 6H+]6+. Another intense peak, fragment c with m/z 1098 arises due to one Cu elimination from parent Cu25(SG)20. The corresponding calculated and observed isotopic 11 ACS Paragon Plus Environment


simulations are compared in Figure S8. TGA analysis suggests a 79.5 % mass loss due to loss of thiolates with increasing temperature. The calculated SG/Cu ratio, 0.8 is in good agreement with the formula Cu25(SG)20 obtained from ESI-MS (Figure S9). Hereby, the mass spectrometric observations along with isotope patterns and TGA analysis confirm the formation of Cu25(SG)20 NCs and reduction in the core size at 70ºC. Zeta potential of the particles is found to be slightly positive (4.07 mV) as the surface have –NH3+ groups at the acidic pH.

Figure 3: (A) UV-Vis absorption spectrum, (B) photoexcitation (a) and photoluminescence spectra (b) of blue emitting Cu NCs in aqueous solution. Inset shows the digital photographs of Cu NCs with daylight, under UV illumination (365 nm) and wet gel after PAGE at room temperature under UV illumination (365 nm) (C) ESI-MS spectrum of Cu NCs obtained at 70 º C and inset shows the zoomed view. (D) Experimental and calculated isotopic distributions of peak a (experimental: green, calculated: black) and (E) peak d (experimental: red, calculated: black) with formula [Cu25(SG)20 + 6H+]6+ and [Cu25(SG)20 + Na+ + 7H+]8+ respectively. 12 ACS Paragon Plus Environment

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TEM images of both Cu NCs reveal that the particles are spherical and the core size is reduced during the etching reaction starting from Cu NPs with an average dimension < 2 nm (Figure 4). The particles of Cu34-32(SG)16-13 NCs are not well separated and show a sign of aggregation (Figure 4A and 4B) while the particles of Cu25(SG)20 NCs are well separated (Figure 4C and 4D). Binding of the excess GSH with Cu core is examined by FTIR spectroscopy. The SH stretching vibration of GSH at 2525 cm-1 is absent in the spectrum of both Cu NCs, assuring the formation of Cu-S bond with excess GSH (Figure S10). The oxidation states of Cu in the core of Cu NCs are investigated by XPS spectroscopy (Figure 4E). The peaks at 931.0 and 950.9 eV are assigned for Cu 2p3/2 and 2p1/2 of Cu (0). No sign of satellite peak at 942 eV indicating the absence of Cu (II) in the Cu core. Electron spin resonance measurements also reveal the absence of Cu (II) in the Cu NCs (Data is not shown). The binding energy difference between Cu (0) and Cu (I) is 0.1 eV, thus the precise amount of Cu (0) and Cu (I) cannot be estimated.36 As the Cu NCs are derived from NPs precursor, the amount of Cu (0) is thought to be higher than Cu (I). Another two peaks at 162.3 and 163.5 eV are assigned as S 2p3/2 and 2p1/2 respectively indicating Cu-S covalent bond formation with GSH (Figure S11).

(A)

(B) (E)

Cu 2p 3/2

(b)

Cu 2p 1/2

20 nm

(D)

(C)

(a)

Cu 2p 3/2

Cu 2p 1/2

930

940 950 Binding Energy (eV)

Figure 4: TEM images of Cu34-32(SG)16-13NCs with scale bar (A) 50 nm and (B) 20 nm and Cu25(SG)20 NCs with scale bar (C) 20 nm and (D) 10 nm . The particles are indicated with yellow circles. (E) XPS spectra for Cu 2p of (a) Cu34-32(SG)16-13 and (b) Cu25(SG)20 NCs. 13 ACS Paragon Plus Environment


Analysis reveals that temperature has a profound effect in the ligand mediated core etching reaction. Polydisperse Cu34-32(SG)16-13 NCs with weak red emission formed at mild etching conditions (i.e. room temperature and 30 min time) is relatively less stable, probably the kinetically controlled product. The elevated temperature (70ºC) and prolonged time (36 h) lead to the blue emitting monodisperse Cu25(SG)20 NCs which is very stable and the thermodynamically controlled one. As soon as the reaction initiates at 70 ºC, Cu34-32(SG)16-13 NCs is formed in situ and also gets decomposed within 30 min which is evident from the PL monitoring of reaction medium (Figure 5). The zoomed in view in the inset of Figure 5 shows the depletion of red emission from Cu3432(SG)16-13 13

NCs with time. The excess GSH in medium coordinates on the surface of Cu34-32(SG)16-

NCs and forms a yellow colored insoluble aggregate (Inset of Figure 5). While the reaction

proceeds at 70ºC, the insoluble aggregates are converted into monodisperse NCs. The enhancement in temperature is essential in order to overcome the energy barrier of the stable Cu25(SG)20 NCs formation.

400

f e a b c

PL Intensity (a.u.)

PL Intensity (a.u.)

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

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d 550



700

700

Figure 5: PL monitoring of the core etching reaction at 70ºC and zoomed view shows the depletion of red emitting Cu34-32(SG)16-13 NCs at (a) 10 min, (b) 30 min, (c) 6 h, (d) 12 h, (e) 24 h (blue line) and (f) 36 h (red line). Inset shows the insoluble aggregates formed during reaction. However, the NCs show very weak luminescence even after 36 h when the etching reaction is carried out at 50 ºC (Figure S12). The excess GSH concentration plays a crucial role to dictate the composition of NCs. The SG/Cu ratio in poly disperse Cu34-32(SG)16-13 NCs shows a value 14 ACS Paragon Plus Environment

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0.465 (Figure S5). The increment in the ligand content in NCs than the parent NPs indicates the excess GSH coordination with the NPs surface and consequent core etching. In case of monodisperse Cu25(SG)20 NCs, the ratio of SG/Cu further increases to 0.8 which is confirmed from TGA analysis (Figure S9). The very high content of GS- in Cu25(SG)20 NCs supports the excess GSH binding to the in-situ generated Cu34-32(SG)16-13 NCs. Thus, the excess GSH concentration is important toward the thermodynamically stable Cu25(SG)20 NCs formation. The shifting of emission maxima from 625 nm to 442 nm is in the agreement with the size reduction of the Cu core. We have succeeded to isolate two atomically precise Cu NCs with tunable emission which is dependent on the temperature of core etching reaction. Aggregation induced emission of Cu NCs: A strategy based on controlled assembly of NCs is designed to improve the emission intensity and tuning the emission color. Though Cu34-32(SG)1613

NCs is weakly luminescent at ambient temperature, it shows very intense orange color upon

freezing temperature (< 0ºC). The intense luminescence fades away again at room temperature showing the reversible nature of the emission enhancement process (Figure 6A). The increase in intensity may be attributed to the restriction on the rotational and vibrational motion of surface ligands in frozen state.59 A controlled aggregation of NCs can make them a promising candidate towards the formation of light emitting devices.50 In general, aggregation of NCs was originated by two different approaches: solvent and cation induced aggregation.58 The extent of aggregation is controlled by employing water-ethanol solvent mixture with volume fraction (fv) of EtOH up to 90 %. The comparatively less polar solvent, ethanol (dielectric constant, ε=25.1) greatly influences the surface landscapes in the NCs. In aqueous medium, the negative surface charges of colloidal Cu34-32(SG)16-13 NCs impart good solubility and stability due to the formation of hydration shell around them. With increasing EtOH content in medium, the hydration shell gets destroyed and the surface charges become neutralized. The lowering of negative surface charges is evident from the systematic reduction in zeta potential value upon introduction of EtOH (Figure S13). Due to charge neutralization, the particles suffer a lack of stability and come closer as a consequence of higher intra and inter NCs Cu(I)-Cu(I) cuprophilic interaction. A clouding in the NCs solution arises owing to the formation of large aggregates and TEM images of NCs shows aggregated structure in water-EtOH solvent mixture (Figure 6B). The hydrodynamic diameter of the NCs increases from 2.34 nm to 191.35 nm upon increasing EtOH (fv = 50 %) (Figure S14).



(a)

(B) (C)

(b)

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(D) x 36

a

500

600 Wavelength (nm)

Normalized PL Intensity

(A)

PL Intensity (a.u.)

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700 500

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(a) (b) (c) (d) (e) (f)

f

a

600 Wavelength (nm)

(E)

700

Figure 6: (A) Digital photographs of Cu34-32(SG)16-13 under 365 nm UV illumination (a) at ambient temperature and (b) upon freezing (Temperature < 0 ºC), (B) TEM image of aggregates with scale bar 100 nm and 50 nm (inset) (C) Corresponding digital photographs with fv (a) 0, (b) 20, (c) 40, (d) 60, (e) 80 and (f) 90 % under daylight and UV (365 nm) illumination, (D) PL spectra of Cu3432(SG)16-13

and (E) Shifting of emission maxima in water-EtOH mixture with fv = 0 to 90 %.

The solvent induced aggregation has altered the luminescence properties of Cu34-32(SG)16-13 NCs to a great extent. The photoemission intensity increases sharply with increasing EtOH content and the change in emission is clearly visible from the digital photographs of Cu34-32(SG)16-13 NCs under UV (365 nm) illumination (Figure 6C). The hump at 340 nm in absorption profile of Cu34-32(SG)1613

remains unaffected with increasing amount of EtOH, indicating no change in the NCs core upon

aggregation (Figure S15). No absorption peak is observed in NIR region as there is no charge transfer involving Cu (II). Upon addition of EtOH with fv = 90 %, the emission intensity increases 36 times than NCs in aqueous solution (Figure 6D). In addition to that, a 28 nm blue shifting of emission maxima from 625 nm to 597 nm is observed with the increase in EtOH content (Figure 6E). The Cu34-32(SG)16-13 NCs in aqueous medium exhibits an average lifetime 115.9 ns when 16 ACS Paragon Plus Environment

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excited with 371 nm wavelength. The large stoke shift (265 nm) and near microsecond scale lifetime suggest that the emission arises mainly from phosphorescence involving a ligand to metal charge transfer (LMCT) or ligand to metal-metal charge transfer (LMMCT) via S atoms of GSH.59 The following radiative decay from a metal centered triplet state to ground state (S0) is the origin of photoemission. Upon formation of aggregates, the lifetime increases to 239.5 ns when fv= 40 % (Figure 7A). In addition to the emission enhancement, the shifting of emission maxima with the increase in degree of aggregation is due to the alternation in charge transfer with the change in solvent polarity. In the native Cu34-32(SG)16-13 NCs, after a very fast inter system crossing from S1 to T1, it comes down to another low lying triplet state T2. As the T2 state possesses significant charge transfer characteristics, it is very stable in polar aqueous medium. The succeeding relaxation from T2 to ground state via phosphorescence gives rise to the emission at 625 nm with low quantum yield. In higher EtOH content, the perturbation due to water becomes less and the neutralized surface charge restricts the charge transfer to the metal core. As a consequence, the T2 state becomes destabilized in the less polar media and the radiative decay directly occurs from high lying triplet state T1, resulting into the reinforcement in intensity and blue shifting in emission maxima.59 The non radiative decay pathways like molecular rotations and vibrations are reduced with the lowering in the perturbation from solvent and an elevation in photoluminescence lifetime is observed. In a nutshell, the tuning of emission maxima and the enhancement of emission intensity of an weakly emissive Cu34-32(SG)16-13 NCs was accomplished by changing the solvent polarity only.



(B) Counts (in log)

(A) Counts (in log)

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b

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3

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S1

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20 Time (ns)

30

(C)

Sn S1

T1 High QY

S0

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Low QY

S0

Cu34-32(SG)16-13

Cu25(SG)20

Figure 7: (A) Time resolved decay curves of Cu34-32(SG)16-13 NCs in water-EtOH mixture with fv (a) 0 % and (b) 40 %. (B) Time resolved decay curve of Cu25(SG)20 NCs in water-EtOH mixture with fv (a) 0 % and (b) 50 %. (C) Schematic diagram of decay dynamics and origin of emission in Cu34-32(SG)16-13 and Cu25(SG)20 NCs. On the other hand, monodisperse Cu25(SG)20 NCs exhibit a very bright blue emission in aqueous solution. The solvent induced aggregation aids only two fold enhancement of luminescence intensity in water-EtOH mixture with fv = 75% (Figure S16). Additionally, the emission maxima remain unaltered upon increasing amount of EtOH. The effect of change in polarity of dispersing medium on the luminescence of Cu25(SG)20 NCs is negligible. High tolerance of the luminescence towards the change in polarity is attributed to two factors. As the synthetic medium is acidic, the –COOH groups on the surface are not de-protonated and the –NH2 groups exist as –NH3+ which is evident from positive zeta potential. Hence, the incoming solvent EtOH has negligible effect on the surface charge. Secondly, the origin of luminescence in Cu25(SG)20 NCs is very different from the former one. The NCs possess an average excited state 18 ACS Paragon Plus Environment

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lifetime of 3.03 ns when excited with 371 nm light. Relatively small stokes shift (62 nm) and nanosecond scale lifetime indicate that fluorescence is the key source of emission in Cu25(SG)20 NCs and there is no involvement of triplet state (Figure 7B). Upon introduction of EtOH with fv = 50 %, the excited state lifetime remains unchanged suggesting that the change in polarity affects surface charges only (Figure 7C). The Cu core has no significant interference from the change in solvent polarity; hence the fluorescence from S1 remains unaffected. As the solvent induced aggregation alters the rotational and vibrational motions of ligands on surface, not the metal core, thus the aggregation has no considerable effect on the fluorescence of Cu25(SG)20 NCs. The overall process of emission enhancement and alteration in excited state are schematically represented in Figure 7C. The AIE properties make the Cu NCs interesting in the ever growing field of phosphors and light emitting devices. CONCLUSIONS Our work is an account of digestive ripening of polydisperse Cu NPs to synthesize water soluble atomically precise Cu NCs without involving any ligand exchange or interfacial synthesis method. Red emitting Cu34-32(SG)16-13 NCs is an outcome of core etching of GSH anchored Cu NPs in mild conditions. Whereas stable Cu25(SG)20 NCs with an intense blue emission is obtained at 70ºC temperature. Temperature and GSH concentration play an important role towards the synthesis of Cu NCs with different composition and properties. Moreover, a strategy of solvent induced aggregation is designed to attain intense and tunable emission. The enhancement in the luminescence of Cu34-32(SG)16-13 NCs is attributed to the alteration in the excited state dynamics with the change in solvent polarity. Together with the AIE property, the atomically precise Cu3432(SG)16-13

NCs will enable the versatile applications in light emitting devices, photosensors.

Having intense blue fluorescence, Cu25(SG)20 NCs are powerful candidates in the diverse field of applications in sensing, bioimaging, photovoltaics and optoelectronics.

AUTHOR INFORMATION 19 ACS Paragon Plus Environment


Corresponding author * E-mail: [email protected] ASSOCIATED CONTENT Conflict of interest: The authors declare no competing financial interest. Supporting Information: FTIR spectra of GSH and GSH protected Cu NPs, PL monitoring of room temperature etching, time evolution of PL spectra, Isotopic patterns of Cu34-32(SG)16-13 NCs, TGA curve of Cu34-32(SG)16-13 NCs, Excitation dependent PL spectra, PL monitoring of etching at 70ºC, Isotopic patterns of Cu25(SG)20 NCs, TGA curve of Cu25(SG)20 NCs, FTIR spectra of Cu NCs, XPS spectra for S 2p, PL spectra of Cu NCs obtained at 50ºC and 70ºC, Change of Zeta potential, Change in DLS size of Cu34-32(SG)16-13 NCs, Absortion spectra of Cu34-32(SG)16-13 NCs in water-EtOH solvent mixture, PL spectra of Cu25(SG)20 NCs in water-EtOH solvent mixture. The supporting information is available free of charge on the ACS publication website at DOI: Acknowledgements: SM and DB thank CSIR for awarding fellowship. We thank Professor Tapan Kanti Paine, School of Chemical Sciences, IACS for his fruitful scientific discussion.

REFERENCES 20 ACS Paragon Plus Environment

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1.

Jin, R.; Zeng, C.; Zhou, M.; Chen, Y., Atomically Precise Colloidal Metal Nanoclusters

and Nanoparticles: Fundamentals and Opportunities. Chem. Rev. 2016, 116, 10346-10413. 2.

Kundu, S.; Patra, A., Nanoscale Strategies for Light Harvesting. Chem. Rev. 2017, 117,

712-757. 3.

Chakraborty, I.; Pradeep, T., Atomically Precise Clusters of Noble Metals: Emerging Link

between Atoms and Nanoparticles. Chem. Rev. 2017, 117, 8208-8271. 4.

Wang, Z.; Chen, B.; Rogach, A. L., Synthesis, Optical Properties and Applications of

Light-Emitting Copper Nanoclusters. Nanoscale Horiz. 2017, 2, 135-146. 5.

Paramanik, B.; Bain, D.; Patra, A., Making and Breaking of DNA-Metal Base Pairs: Hg2+

and Au Nanocluster Based Off/On Probe. J. Phys. Chem. C 2016, 120, 17127-17135. 6.

Maity, S.; Bain, D.; Bhattacharyya, K.; Das, S.; Bera, R.; Jana, B.; Paramanik, B.; Datta,

A.; Patra, A., Ultrafast Relaxation Dynamics of Luminescent Copper Nanoclusters (Cu7L3) and Efficient Electron Transfer to Functionalized Reduced Graphene Oxide. . J. Phys. Chem. C 2018, 122, 13354-13362. 7.

Shang, L.; Dong, S.; Nienhaus, G. U., Ultra-Small Fluorescent Metal Nanoclusters:

Synthesis and Biological Applications. Nano Today 2011, 6, 401-418. 8.

Wu, Z.; Jiang, D.-e.; Mann, A. K. P.; Mullins, D. R.; Qiao, Z.-A.; Allard, L. F.; Zeng, C.;

Jin, R.; Overbury, S. H., Thiolate Ligands as a Double-Edged Sword for CO Oxidation on CeO2 Supported Au25(SCH2CH2Ph)18 Nanoclusters. J. Am. Chem. Soc. 2014, 136, 6111-6122. 9.

Wei, W.; Lu, Y.; Chen, W.; Chen, S., One-Pot Synthesis, Photoluminescence, and

Electrocatalytic Properties of Subnanometer-Sized Copper Clusters. J. Am. Chem. Soc. 2011, 133, 2060-2063. 10.

Das, N. K.; Ghosh, S.; Priya, A.; Datta, S.; Mukherjee, S., Luminescent Copper

Nanoclusters as a Specific Cell-Imaging Probe and a Selective Metal Ion Sensor. J. Phys. Chem. C 2015, 119, 24657-24664. 11.

Udaya Bhaskara Rao, T.; Pradeep, T., Luminescent Ag7 and Ag8 Clusters by Interfacial

Synthesis. Angew. Chem., Int. Ed. 2010, 49, 3925-3929. 12.

Muhammed, M. A. H.; Aldeek, F.; Palui, G.; Trapiella-Alfonso, L.; Mattoussi, H., Growth

of In Situ Functionalized Luminescent Silver Nanoclusters by Direct Reduction and Size Focusing. ACS Nano 2012, 6, 8950-8961.



13.

Yuan, X.; Luo, Z.; Zhang, Q.; Zhang, X.; Zheng, Y.; Lee, J. Y.; Xie, J., Synthesis of Highly

Fluorescent Metal (Ag, Au, Pt, and Cu) Nanoclusters by Electrostatically Induced Reversible Phase Transfer. ACS Nano 2011, 5, 8800-8808. 14.

Chen, Y.; Yang, T.; Pan, H.; Yuan, Y.; Chen, L.; Liu, M.; Zhang, K.; Zhang, S.; Wu, P.;

Xu, J., Photoemission Mechanism of Water-Soluble Silver Nanoclusters: Ligand-to-Metal-Metal Charge Transfer vs Strong Coupling between Surface Plasmon and Emitters. J. Am. Chem. Soc. 2014, 136, 1686-1689. 15.

Jin, R., Atomically Precise Metal Nanoclusters: Stable Sizes and Optical Properties.

Nanoscale 2015, 7, 1549-1565. 16.

Cantelli, A.; Guidetti, G.; Manzi, J.; Caponetti, V.; Montalti, M., Towards Ultra-Bright

Gold Nanoclusters. Eur. J. Inorg. Chem. 2017, 2017, 5068-5084. 17.

Yu, Y.; Luo, Z.; Chevrier, D. M.; Leong, D. T.; Zhang, P.; Jiang, D.-e.; Xie, J.,

Identification of a Highly Luminescent Au22(SG)18 Nanocluster. J. Am. Chem. Soc. 2014, 136, 1246-1249. 18.

Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R., Correlating the Crystal

Structure of A Thiol-Protected Au25 Cluster and Optical Properties. J. Am. Chem. Soc. 2008, 130, 5883-5885. 19.

Patra, A.; Bain, D.; Maity, S., Opportunities and Challenges in Energy and Electron

Transfer of Nanoclusters Based Hybrid Materials and Their Sensing Applications. Phys. Chem. Chem. Phys. 2018, DOI: 10.1039/C8CP06188B. 20.

Khatun, E.; Ghosh, A.; Ghosh, D.; Chakraborty, P.; Nag, A.; Mondal, B.; Chennu, S.;

Pradeep, T., [Ag59(2,5-DCBT)32]3-: A New Cluster and a Precursor for Three Well-known Clusters. Nanoscale 2017, 9, 8240-8248. 21.

Cao, Y., et al., Evolution of Thiolate-Stabilized Ag Nanoclusters from Ag-Thiolate Cluster

Intermediates. Nat. Commun. 2018, 9, 2379. 22.

Chen, Y.; Zeng, C.; Kauffman, D. R.; Jin, R., Tuning the Magic Size of Atomically Precise

Gold Nanoclusters via Isomeric Methylbenzenethiols. Nano Lett. 2015, 15, 3603-3609. 23.

Bain, D.; Paramanik, B.; Patra, A., Silver(I)-Induced Conformation Change of DNA: Gold

Nanocluster as a Spectroscopic Probe. J. Phys. Chem. C 2017, 121, 4608-4617. 24.

Xie, J.; Zheng, Y.; Ying, J. Y., Protein-Directed Synthesis of Highly Fluorescent Gold

Nanoclusters. J. Am. Chem. Soc. 2009, 131, 888-889. 22 ACS Paragon Plus Environment

Page 22 of 27

Page 23 of 27 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


25.

Wang, Y.; Chen, T.; Zhuang, Q.; Ni, Y., One-Pot Aqueous Synthesis of Nucleoside-

Templated Fluorescent Copper Nanoclusters and Their Application for Discrimination of Nucleosides. ACS Appl. Mater. Interfaces 2017, 9, 32135-32141. 26.

Chakraborty, P.; Nag, A.; Paramasivam, G.; Natarajan, G.; Pradeep, T., Fullerene-

Functionalized Monolayer-Protected Silver Clusters: [Ag29(BDT)12(C60)n]3- (n = 1-9). ACS Nano 2018, 12, 2415-2425. 27.

Tero, T.-R.; Malola, S.; Koncz, B.; Pohjolainen, E.; Lautala, S.; Mustalahti, S.; Permi, P.;

Groenhof, G.; Pettersson, M.; Häkkinen, H., Dynamic Stabilization of the Ligand-Metal Interface in Atomically Precise Gold Nanoclusters Au68 and Au144 Protected by meta-Mercaptobenzoic Acid. ACS Nano 2017, 11, 11872-11879. 28.

Bain, D.; Paramanik, B.; Sadhu, S.; Patra, A., A Study into the Role of Surface Capping

on Energy Transfer in Metal Cluster-Semiconductor Nanocomposites. Nanoscale 2015, 7, 2069720708. 29.

Kang, X.; Chong, H.; Zhu, M., Au25(SR)18: the Captain of the Great Nanocluster Ship.

Nanoscale 2018, 10, 10758-10834. 30.

Nag, A.; Baksi, A.; Krishnapriya, K. C.; Gupta, S. S.; Mondal, B.; Chakraborty, P.;

Pradeep, T., Synergistic Effect in Green Extraction of Noble Metals and Its Consequences. Eur. J. Inorg. Chem. 2017, 2017, 3072-3079. 31.

Yao, Q.; Chen, T.; Yuan, X.; Xie, J., Toward Total Synthesis of Thiolate-Protected Metal

Nanoclusters. Acc. Chem. Res. 2018, 51, 1338-1348. 32.

Mishra, D.; Lobodin, V.; Zhang, C.; Aldeek, F.; Lochner, E.; Mattoussi, H., Gold-Doped

Silver Nanoclusters with Enhanced Photophysical Properties. Phys. Chem. Chem. Phys. 2018, 20, 12992-13007. 33.

Paramanik, B.; Patra, A., Fluorescent AuAg Alloy Clusters: Synthesis and SERS

Applications. J. Mater. Chem. C 2014, 2, 3005-3012. 34.

Negishi, Y.; Nobusada, K.; Tsukuda, T., Glutathione-Protected Gold Clusters Revisited:

Bridging the Gap between Gold(I)-Thiolate Complexes and Thiolate-Protected Gold Nanocrystals. J. Am. Chem. Soc. 2005, 127, 5261-5270. 35.

Oh, E.; Delehanty, J. B.; Field, L. D.; Mäkinen, A. J.; Goswami, R.; Huston, A. L.;

Medintz, I. L., Synthesis and Characterization of PEGylated Luminescent Gold Nanoclusters Doped with Silver and Other Metals. Chem. Mater. 2016, 28, 8676-8688. 23 ACS Paragon Plus Environment


36.

Gao, X.; Lu, Y.; Liu, M.; He, S.; Chen, W., Sub-Nanometer Sized Cu6(GSH)3 Clusters:

One-Step Synthesis and Electrochemical Detection of Glucose. J. Mater. Chem. C 2015, 3, 40504056. 37.

Yu, Y.; Chen, X.; Yao, Q.; Yu, Y.; Yan, N.; Xie, J., Scalable and Precise Synthesis of

Thiolated Au10–12, Au15, Au18, and Au25 Nanoclusters via pH Controlled CO Reduction. Chem. Mater. 2013, 25, 946-952. 38.

Goswami, N.; Yao, Q.; Chen, T.; Xie, J., Mechanistic Exploration and Controlled Synthesis

of Precise Thiolate-Gold Nanoclusters. Coord. Chem. Rev. 2016, 329, 1-15. 39.

Feng, J.; Chen, Y.; Han, Y.; Liu, J.; Ma, S.; Zhang, H.; Chen, X., pH-Regulated Synthesis

of Trypsin-Templated Copper Nanoclusters with Blue and Yellow Fluorescent Emission. ACS Omega 2017, 2, 9109-9117. 40.

Bain, D.; Maity, S.; Paramanik, B.; Patra, A., Core-Size Dependent Fluorescent Gold

Nanoclusters and Ultrasensitive Detection of Pb2+ Ion. ACS Sustainable Chem. Eng. 2018, 6, 23342343. 41.

Wang, S.; Meng, X.; Das, A.; Li, T.; Song, Y.; Cao, T.; Zhu, X.; Zhu, M.; Jin, R., A 200-

Fold Quantum Yield Boost in the Photoluminescence of Silver-Doped AgxAu25-x Nanoclusters: The 13 th Silver Atom Matters. Angew. Chem., Int. Ed. 2014, 53, 2376-2380. 42.

Liu, C.; Li, G.; Pang, G.; Jin, R., Toward Understanding the Growth Mechanism of

Aun(SR)m Nanoclusters: Effect of Solvent on Cluster Size. RSC Adv. 2013, 3, 9778-9784. 43.

Lin, Y.-J.; Chen, P.-C.; Yuan, Z.; Ma, J.-Y.; Chang, H.-T., The Isomeric Effect of

Mercaptobenzoic Acids on the Preparation and Fluorescence Properties of Copper Nanoclusters. Chem. Commun. 2015, 51, 11983-11986. 44.

Dhanalakshmi, L.; Udayabhaskararao, T.; Pradeep, T., Conversion of Double Layer

Charge-Stabilized Ag@Citrate Colloids to Thiol Passivated Luminescent Quantum Clusters. Chem. Commun. 2012, 48, 859-861. 45.

Shichibu, Y.; Negishi, Y.; Tsunoyama, H.; Kanehara, M.; Teranishi, T.; Tsukuda, T.,

Extremely High Stability of Glutathionate-Protected Au25 Clusters Against Core Etching. Small 2007, 3, 835-839. 46.

Muhammed, M. A. H.; Verma, P. K.; Pal, S. K.; Kumar, R. C. A.; Paul, S.; Omkumar, R.

V.; Pradeep, T., Bright, NIR-Emitting Au23 from Au25: Characterization and Applications Including Biolabeling. Chem. Eur. J. 2009, 15, 10110-10120. 24 ACS Paragon Plus Environment

Page 24 of 27

Page 25 of 27 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


47.

Jia, X.; Li, J.; Wang, E., Cu Nanoclusters with Aggregation Induced Emission

Enhancement. Small 2013, 9, 3873-3879. 48.

Deng, H.-H.; Li, K.-L.; Zhuang, Q.-Q.; Peng, H.-P.; Zhuang, Q.-Q.; Liu, A.-L.; Xia, X.-

H.; Chen, W., An Ammonia-Based Etchant for Attaining Copper Nanoclusters with Green Fluorescence Emission. Nanoscale 2018, 10, 6467-6473. 49.

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

with Aggregation-Induced Emission. J. Phys. Chem. Lett. 2016, 7, 962-975. 50.

Wu, Z.; Liu, J.; Gao, Y.; Liu, H.; Li, T.; Zou, H.; Wang, Z.; Zhang, K.; Wang, Y.; Zhang,

H.; et al. Assembly-Induced Enhancement of Cu Nanoclusters Luminescence with Mechanochromic Property. J. Am. Chem. Soc. 2015, 137, 12906-12913. 51.

Zheng, K.; Yuan, X.; Kuah, K.; Luo, Z.; Yao, Q.; Zhang, Q.; Xie, J., Boiling Water

Synthesis of Ultrastable Thiolated Silver Nanoclusters with Aggregation-Induced Emission. Chem. Commun. 2015, 51, 15165-15168. 52.

Kang, X.; Wang, S.; Song, Y.; Jin, S.; Sun, G.; Yu, H.; Zhu, M., Bimetallic Au2Cu6

Nanoclusters: Strong Luminescence Induced by the Aggregation of Copper(I) Complexes with Gold(0) Species. Angew. Chem., Int. Ed. 2016, 55, 3611-3614. 53.

Pyo, K.; Thanthirige, V. D.; Yoon, S. Y.; Ramakrishna, G.; Lee, D., Enhanced

Luminescence of Au22(SG)18 Nanoclusters via Rational Surface Engineering. Nanoscale 2016, 8, 20008-20016. 54.

Bootharaju, M. S.; Kozlov, S. M.; Cao, Z.; Shkurenko, A.; El-Zohry, A. M.; Mohammed,

O. F.; Eddaoudi, M.; Bakr, O. M.; Cavallo, L.; Basset, J.-M., Tailoring the Crystal Structure of Nanoclusters Unveiled High Photoluminescence via Ion Pairing. Chem. Mater. 2018, 30, 27192725. 55.

Deng, H.-H.; Shi, X.-Q.; Wang, F.-F.; Peng, H.-P.; Liu, A.-L.; Xia, X.-H.; Chen, W.,

Fabrication of Water-Soluble, Green-Emitting Gold Nanoclusters with a 65% Photoluminescence Quantum Yield via Host-Guest Recognition. Chem. Mater. 2017, 29, 1362-1369. 56.

Liu, Y.; Yao, D.; Zhang, H., Self-Assembly Driven Aggregation-Induced Emission of

Copper Nanoclusters: A Novel Technology for Lighting. ACS Appl. Mater. Interfaces 2018, 10, 12071-12080. 57.

Su, X.; Liu, J., pH-Guided Self-Assembly of Copper Nanoclusters with Aggregation-

Induced Emission. ACS Appl. Mater. Interfaces 2017, 9, 3902-3910. 25 ACS Paragon Plus Environment


58.

Luo, Z.; Yuan, X.; Yu, Y.; Zhang, Q.; Leong, D. T.; Lee, J. Y.; Xie, J., From Aggregation-

Induced Emission of Au(I)-Thiolate Complexes to Ultrabright Au(0)@Au(I)-Thiolate Core-Shell Nanoclusters. J. Am. Chem. Soc. 2012, 134, 16662-16670. 59.

Pyo, K.; Thanthirige, V. D.; Kwak, K.; Pandurangan, P.; Ramakrishna, G.; Lee, D.,

Ultrabright Luminescence from Gold Nanoclusters: Rigidifying the Au(I)-Thiolate Shell. J. Am. Chem. Soc. 2015, 137, 8244-8250. 60.

Han, A.; Xiong, L.; Hao, S.; Yang, Y.; Li, X.; Fang, G.; Liu, J.; Pei, Y.; Wang, S., Highly

Bright Self-Assembled Copper Nanoclusters: A Novel Photoluminescent Probe for Sensitive Detection of Histamine. Anal. Chem. 2018, 90, 9060-9067. 61.

Ai, L., Jiang, W.; Liu, Z.; Liu, J.; Gao, Y.; Zou, H.; Wu, Z.; Wang, Z.; Liu, Y.; Zhang, H.;

et al. Engineering a Red Emission of Copper Nanocluster Self-Assembly Architectures by Employing Aromatic Thiols as Capping Ligands. Nanoscale 2017, 9, 12618-12627. 62.

Kang, X.; Wang, S.; Zhu, M., Observation of a New Type of Aggregation-Induced

Emission in Nanoclusters. Chem. Sci. 2018, 9, 3062-3068. 63.

Nag, A.; Chakraborty, P.; Bodiuzzaman, M.; Ahuja, T.; Antharjanam, S.; Pradeep, T.,

Polymorphism of Ag29(BDT)12(TPP)43− Cluster: Interactions of Secondary Ligands and Their Effect on Solid State Luminescence. Nanoscale 2018, 10, 9851-9855. 64.

Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer, 2006.

65.

Pedersen, D. B.; Wang, S., Surface Plasmon Resonance Spectra of 2.8 ± 0.5 nm Diameter

Copper Nanoparticles in Both Near and Far Fields. J. Phys. Chem. C 2007, 111, 17493-17499.

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