Synthesis of Positively Charged Photoluminescent Bimetallic AuâAg

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Synthesis of positively charged photoluminescent bimetallic Au-Ag nanoclusters by double target sputtering method on a biocompatible polymer matrix Ryan D. Corpuz, Yohei Ishida, Mai Thanh Nguyen, and Tetsu Yonezawa Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02011 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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Synthesis of positively charged photoluminescent bimetallic AuAg nanoclusters by double target sputtering method on a biocompatible polymer matrix Ryan D. Corpuz, Yohei Ishida, Mai Thanh Nguyen and Tetsu Yonezawa* Division of Material Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan. *[email protected]

Abstract Herein we report a novel positively charged photoluminescent Au-Ag bimetallic nanocluster synthesized using 11-mercaptoundecyl-N,N,N-trimethylammonium bromide as capping ligand by means of “green” double target sputtering method in a biocompatible polymer matrix. The photoluminescent Au-Ag bimetallic cluster showed emission tunability from the blue to near infrared (NIR) regions with respect to change in composition.

Introduction Bimetallic nanocluster is a hot topic in research community due to its improved optical and catalytic properties typically attributed to synergistic effects1 when two types of atoms having distinct properties mixed and combined with each other to form electronically and geometrically stable nanoclusters. Among of the bimetallic clusters, Au-Ag bimetallic system is the most intensively investigated due to the noble characteristics of Au and Ag and the fact that these two elements with comparable sizes form homogeneous solution in wide range of composition.2-4

1 Environment ACS Paragon Plus

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Bimetallic Au-Ag photoluminescent nanoclusters are typically produced by utilizing capping and protecting ligand such as protein,5 DNA,6 and gelatin7 to control the number of atoms and its growth. To date these photoluminescent bimetallic clusters were only successfully synthesized by means of chemical reduction via galvanic exchange reaction8, coreduction,9 and anti-galvanic reaction methods10 and typically utilized toxic reducing reagents thus its utilization for biomedical applications is still subject for intensive researches and clinical trials due to purity and toxicity related issues. In this study, we introduced a green physical method in producing a bimetallic Au-Ag photoluminescent nanocluster utilizing a positively charged long thiol molecule, 11mercaptoundecyl-N,N,N-trimethylammonium bromide (MUTAB) as capping ligand by means of double target sputtering on a biocompatible liquid polymer matrix, polyethylene glycol (PEG). With this ligand, cationic fluorescent nanoclusters of coinage metals (Au, Ag, and Cu) were successfully obtained by sputtering process.11 Unlike the chemical synthesis, this method requires neither reducing agent nor tedious purification processes to produce near infrared (NIR) emissive bimetallic nanoclusters. NIR emissive nanoclusters has the advantage for its deep tissue penetrating photons,12 making it positively charge will make these photoluminescent nanoclusters adhere to cell selectively and thus it could potentially be used for targeted drug delivery and as selective photoluminescent probe to investigate cell activities owing to high affinity of proteins inside the cell for positively charged particles.13 In our previous report14 using the double target sputtering, we tried to tune the plasmon absorbance of Ag-Au alloy nanoparticles by manipulating the current of the metal targets. Consequently, we were able to correlate the observed absorbance of the resulting alloy to its corresponding composition. Inspired by this result, in this study we extended the double target sputtering method to synthesize photoluminescent Au-Ag bimetallic nanoclusters. Most of our studies regarding photoluminescent nanoclusters were limited only


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to monometallic ones such as the monometallic photoluminescent Au, Ag and Cu nanoclusters using various types of neutral, anionic and cationic thiol molecule ligands.11,15-18 Herein, we investigated the effect of varying the composition ratio of Ag/Au in synthesizing a photoluminescent Au-Ag bimetallic nanoclusters.

Experimental Section Materials. Au and Ag targets with 99.99% purity were obtained from Tanaka Precious Metals, Tokyo. Poly(ethylene glycol) 600 (PEG, Junsei) and 11-mercaptoundecyl-N,N,Ntrimethylammonium bromide (MUTAB, (Aldrich) were used as received. Preparation. Shown in Figure 1 is the schematic diagram of experimental double target sputtering set-up used in this study. Metal targets Au and Ag were utilized to produce the photoluminescent bimetallic nanocluster. In a usual sputtering process, under high vacuum the metal targets were simultaneously etched by ionized Ar gas which consequently generates unstable mixtures of Au and Ag particles. In this novel set up, the targets are oriented at an angle slightly facing each other which increase the probability of collision among etched particles to form bimetallic nanoclusters in the gas phase and on the surface of the polymer matrix initially mixed with MUTAB.


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Figure 1. Schematic diagram of experimental double target sputtering set up to produce bimetallic Au-Ag photoluminescent nanoclusters.

In a typical synthesis similar to the previous studies,11,14 the liquid polymer solvent used (PEG) was initially degassed at 100 ºC for 2 h to ensure removal of volatile components. 10.152 g of PEG was then mixed with 0.04 g MUTAB ligand followed with 15 min degassing prior to sputtering process. Before sputtering, the chamber was evacuated until the pressure reaches the 10-4 Pa value done at ambient temperature. Ar gas was then introduced in the chamber and gas exchange was done multiple times for 20 minutes. After which the current for each metal target was then set and pre-sputtering on aluminum foil was done for 10 min before the actual 15 min sputtering. Shown in Table 1 are the sputtering conditions used in this study. Plasmonic Au, Ag and bimetallic Au-Ag nanoparticles synthesized without MUTAB were designated as samples 1, 2 and 3 respectively, as also observed in our previous report.14 Monometallic nanoclusters Au and Ag synthesized with


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MUTAB on the other hand, were designated as samples 4 and 5 respectively, and the bimetallic nanoclusters synthesized with MUTAB under varied applied sputtering current are designated as samples 6–11.

Table 1. Sputtering conditions for the synthesis of Au-Ag bimetallic nanoclusters. Sample number 1 2 3 4 5 6 7 8 9 10 11

Applied sputtering current (mA) Au Ag 20 0 0 34 20 34 20 0 0 34 10 50 14 46 18 38 20 34 24 20 26 14

MUTAB

Absence

Presence

Characterization. Right after sputtering, UV-Vis extinction measurement was conducted using a 10 mm × 10 mm quartz cuvette cell mounted in a Shimadzu UV-1800 spectrophotometer. The same sample was then subjected for photoluminescent measurement using JASCO FP-6600 spectrofluorometer. TEM images of the sample were taken to determine the particle size and size distribution using JEOL JEM-2000FX operated at 200kV. TEM samples were prepared by carefully dropping nanocluster dispersion of PEG onto collodion-coated copper grids. After drying, the grids were then soaked into pure methanol for 30 min in order to remove the excess PEG and MUTAB and dried under vacuum overnight.


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The obtained PEG suspension was purified by the re-precipitation using acetonitrile as a poor solvent to remove PEG and excess MUTAB for further characterization. XPS was detected by JEOL JPS-9200 X-ray photoelectron spectroscopy device equipped with a monochromatic Al Kα source operating at 100 W under ultrahigh vacuum (~ 1.0 × 10–7 Pa) conditions. Binding energies were referenced to the C1s binding energy of the adventitious carbon contamination. XPS elemental quantification and ICP-AES (ICPE-9000 Shimadzu Corporation) elemental analysis were obtained to determine the elemental composition of bimetallic Au-Ag nanocluster. For ICP-AES measurements, standard gold and silver aqueous solutions were prepared with the following concentrations: 0 ppb (blank), 500 ppb, 1250 ppb, 2500 ppb, 5000 ppb and 10000 ppb, 6 ml of each solution was then mixed with 2 mL aqua regia (3 wt%) to generate a calibration curve. The samples with unknown concentrations initially re-precipitated using acetonitrile, purified by centrifugation (15000 rpm, 10 min, 2 times repetition) and vacuum dried were then diluted with 6 ml distilled water and mixed with 2 mL aqua regia (3 wt%) and concentrations were then automatically analyzed using the plotted standard calibration curve. The concentrations obtained were then converted to atomic % composition. STEM-HAADF (FEI TITAN III G2- Cubed, acceleration voltage 300 kV) and EDS elemental mapping were conducted to verify the existence of bimetallic Au-Ag nanocluster. For STEM observation, we used carbon-coated micro grids in order to obtain better images.

Results and Discussions Preparation of Au-Ag bimetallic nanoclusters. In all cases, stable dispersions were obtained (Figure S1). Without MUTAB, colored dispersions were obtained according to the plasmon absorption of Au and Ag. With MUTAB, less colored dispersions were obtained. Figure 2 shows the UV-Vis spectra of as synthesized samples with and without the addition


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of MUTAB (the corresponding color of these samples under sunlight are shown in Figure S1). 1 and 2 showed the typical plasmon absorbance of Au and Ag nanoparticles with extinction maximum around 535 nm and 435 nm respectively, similar to our previous report.14 These plasmonic absorbances are attributed to the collective oscillation of the conduction electrons as it interacts with the electromagnetic wave. With the addition of MUTAB, these plasmonic absorbances in the case of monometallic nanoclusters 4 and 5 were no longer observable as can be seen in the featureless extinction profile in the visible region for these particular samples. This phenomenon suggests the formation of photoluminescent nanoclusters which has particle sizes well below 2 nm wherein the particles started to behave somewhat like a molecule due to quantum size effects.11,20,21 For evaluation of the surface stabilization of cationic MUTAB, zeta potential measurement is very useful. However, as these particles were prepared in PEG, unfortunately, it is very difficult to measure their zeta potentials because of the high viscosity of PEG. Comparing with our previous results of fluorescent metal nanoclusters prepared by matrix sputtering in the presence of thiol compounds, it can be concluded that the small nanoclusters are stabilized

by

thiol

molecules

including

11-mercaptoundecanoic

mercaptohexanol,22 and MUTAB.11


acid,18,19

6-

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Figure 2. UV-Vis Extinction spectra of as synthesized samples without (1 – 3) and with (4 – 11) MUTAB. The sample numbers and sputtering conditions are summarized in Table 1.

3 shows the typical plasmon absorbance when Au-Ag bimetallic alloy nanoparticle is formed.14,23,24 It can be observed that the new extinction maximum around 480 nm blue shifted with respect to the maximum plasmon extinction of gold nanoparticle and red shifted with respect to the maximum extinction of silver nanoparticle. With the addition of MUTAB however, the plasmon absorbance of the Au-Ag alloy nanoparticle is no longer observable as can be seen in the extinction profile of 9. Similarly, samples 6, 7, 8, 10 and 11, synthesized at different Ag/Au current ratios with MUTAB ligand, also showed featureless extinction profile. This phenomenon is similar to the ones reported by other researchers for the formation of photoluminescent bimetallic Au-Ag nanocluster synthesized by means of chemical reduction.25-31


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Figure 3.TEM images and particle size distribution histogram of samples with MUTAB. (5) Ag@34mA, (7) Au@14mA-Ag@46mA, (9) Au@20mA-Ag@34mA, and (11) Au@26mAAg@14mA.

TEM observations. In order to understand these phenomena, we conducted particle size analysis for each sample using TEM. Upon checking the particle sizes and size distributions of the samples we found out that the average sizes of all samples are well below 2 nm as shown in Figure 3 and Figure S2. For the monometallic clusters 4 and 5 the average diameters were1.1 ± 0.3 nm and 1.4 ± 0.4 nm, respectively. On the other hand, the average diameters for bimetallic nanoclusters 6, 7, 8, 9, 10 and 11 are 1.5 ± 0.5 nm, 1.5 ± 0.4 nm, 1.3 ± 0.4 nm, 1.3 ± 0.3 nm, 1.2 ± 0.5 nm and 1.2 ± 0.3 nm, respectively.


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Optical properties of Au-Ag bimetallic nanolucsters. From the careful observation of TEM images, it can be inferred that the featureless extinction profiles exhibited by the samples are an indication also of the formation of emissive bimetallic Au-Ag nanoclusters as can be deduced from its corresponding particle sizes. To verify if this observation is correct we irradiated the samples with UV light. Shown in Figure S3 and S4 are the images under UV light irradiation. It can be observed that 1, 2 and 3 did not emit as expected for plasmonic nanoparticle whereas the emission color for nanoclusters synthesized with MUTAB is red for 4, 10 and 11, blue for 5, 6 and 7, and yellow-orange for 8 and 9. We then measured the photoluminescence spectra of each sample.

Figure 4. Photoluminescence spectra: (a) excitation and (b) emission of obtained photoluminescent monometallic nanoclusters (4 and 5), and bimetallic nanoclusters (6–11). Sample numbers and sputtering conditions are summarized in Table 1.

Excitation

wavelengths: 373 nm (4), 355 nm (5), 377 nm (6), 371 nm (7), 365 nm (8), 397 nm (9), 353 nm (10), and 364 nm (11).

The obtained dispersions were stable up to 4 months after preparation. Although these clusters have some tendency to aggregate in a long term but after this period, they showed fluorescence. Figure 4 shows the photoluminescence spectra of samples 4–11. Monometallic


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Au nanocluster 4 has emission in the red region with maximum peak located at 678 nm whereas monometallic Ag nanocluster 5 showed blue emission with maximum peak located at 431 nm. It could be observed that with respect to monometallic gold nanocluster, the bimetallic clusters 6, 7, 8, 9, 10 and 11 are blue shifted with emission peaks located at 444 nm, 452 nm, 621 nm 639 nm, 656nm and 661 nm, respectively. Quantum yield is sometimes enhanced by Ag doping to Au nanoclusters.28-31 Unfortunately, we could not obtain accurate quantum yield values due to the weak photoluminescence intensity that is not high to allow relevant measurement, but the peak intensities of Au-Ag bimetallic nanoclusters of 7 – 11 are larger than monometallic ones. Our bimetallic nanoclusters also shows Ag boost of quantum yields. Digital images of the nanolucster dispersions (Figures S3 and S4) also suggest that phenomenon. Theoretically using the spherical Jellium model, it has been predicted in the case of monometallic cluster that as the number of atoms decrease, a blue shift in emission is usually to be expected.32 In the present case however wherein we are already considering two distinct metal atoms such theoretical prediction is not observed, in fact, with slight decrease in average particle size, we found that the emission peak shifted towards the red region which led us to hypothesized that there are other factors contributing to this behavior other than the particle size of the nanoclusters produced. Similar result was reported by Andolina et al.33 in the case of emissive bimetallic Au-Cu nanocluster wherein there is no correlation between the decrease in particle size and the shift in emission peaks. In the field of Au-Ag photoluminescent bimetallic nanoclusters synthesized by chemical reduction, there are reports wherein the blue shift in emission peak position correlates with the increase in Ag composition.27,28 Thus, in order to verify if this is also the case in our present study, we attempted to determine the composition of 4–11 using XPS and ICP-AES analyses.


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Table 2. Composition measured of Au-Ag bimetallic nanoclusters prepared by sputtering in the presence of MUTAB by XPS and ICP-AES with respect to average diameter and emission peak energy. Sample Number

Current ratio (mA : mA) Au : Ag

4 5 6 7 8 9 10 11

20:0 0:34 10:50 14:46 18:38 20:34 24:20 26:14

XPS (atomic %) Au Ag 100 0 0 100 15 85 31 69 41 59 56 44 72 28 79 21

ICP-AES (atomic %) Au Ag 100 0 0 100 16 84 31 69 39 61 52 48 72 28 83 17

Average Emission diameter energy (nm) (eV) 1.1 ± 0.3 1.4 ± 0.4 1.5 ± 0.5 1.5 ± 0.4 1.3 ± 0.4 1.3 ± 0.3 1.2 ± 0.5 1.2 ± 0.3

1.82 2.88 2.79 2.74 1.99 1.94 1.92 1.89

Table 2 tabulates the summary of the atomic compositions obtained using XPS and ICP-AES. As can be seen from Table 2 both measurements were in good agreement with each other. It can also be observed that with increasing Ag/Au current ratio, there is a corresponding increase in Ag composition and increase in the position of emission peak energy which may suggest that this blue shift in emission peak is probably related with the increase in Ag/Au composition ratio in these bimetallic nanoclusters.


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Figure 5. Correlation between the emission energies and the atomic percent of Ag determined by XPS.

Figure 5 shows the relationship between the composition and the position of maximum emission peaks. It can be observed that with increasing atomic (%) of Ag, the emission maxima shifted towards high energy which is in good agreement with the results obtained by Negishi et al.34 due to continuous modulation in electronic structure upon addition of Ag atoms. In the present case, we found a linear relationship between Ag atomic composition and peak energy in low energy region when the Ag atomic composition is below ~60 at% of Ag, then, some hypsochromic effect can be observed at ~60 at% of Ag. And another linear relationship between emission peak energy and Ag atomic composition in the high energy region when Ag atomic composition is above ~60 at% of Ag. Emission peak positions of Au-Ag bimetallic nanoclusters are varied widely in the previous reports. In many cases, Au-Ag bimetallic nanoclusters show blue shift of emission maximum compared to Au monometallic nanoclusters as indicated above.27,28 Our results also show its blue shift. But in some cases, red shift of emission maximum was also


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reported.26,30 Bovine serum albumin-protected Au-Ag bimetallic nanoclusters show longer emission maximum than the both monometallic nanoclusters.26 However, in many cases, compositions of bimetallic nanoclusters were not varied widely because the chemical preparation of Au-Ag bimetallic nanoclusters with various compositions is enough complicated. Although we have not yet well understood this hypsochromic jump of emission peak energy of Au-Ag bimetallic clusters at ~60 at% of Ag, our results shown in Figure 5 are quite reasonable because all peak maxima are observed between those of the both monometallic emission peak maxima. Further investigation of this point will be reported in due course.

Structural consideration of Au-Ag bimetallic nanoclusters. In order to understand the bonding and formation of Au-Ag bimetallic nanoclusters, XPS analysis at Au 4f and Ag 3d regions was conducted to determine the charge states.

Table 3. Summary of Binding energies at Au 4f and Ag 3d regions. Binding Energy (eV) Sample Au 4f Ag 3d 7/2 5/2 5/2 3/2 84.8 88.8 4 366.8 372.8 5 83.9 87.6 367.6 373.6 6 83.8 87.4 367.5 373.5 7 83.7 87.3 367.4 373.4 8 83.6 87.2 367.2 373.2 9 83.4 87.0 366.8 372.5 10 83.1 86.7 366.6 372.3 11

Table 3 shows the summary of the position of binding energies at Au 4f and Ag 3d regions of the photoluminescent monometallic nanoclusters (4 and 5) and bimetallic Au-Ag nanoclusters (6–11) (corresponding XPS spectra and plot of binding energy with respect to


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composition ratio are shown in Figure S5). With respect to the Au 4f region, the position of the peak of 4 (84.8 eV and 88.8 eV for Au 4f7/2 and Au 4f5/2 respectively) lie between the binding energies of neutral bulk Au (84.0 eV and 88 eV at Au 4f7/2 and Au 4f5/2 respectively) and Au(I) complexes (~86 eV and ~90 eV at Au 4f7/2 and Au 4f5/2 respectively)35 suggesting that 4 is mainly consist of positively charge Au atoms. The peak positions of bimetallic nanoclusters 6-11 on the other hand, are in the lower binding energies with respect to the bulk gold which could suggests that the Au atoms for these samples are negatively charged. With regard to the Ag 3dregion on the other hand, the peak position of monometallic Ag nanocluster 5 (366.8 eV and 372.8 eV for Ag 3d5/2 and Ag 3d3/2 respectively) is in a much lower binding energy in comparison with that of bulk Ag (368.2 eV and 374.2 eV at Ag 3d5/2 and Ag 3d3/2 respectively) and closely match with the binding energy of Ag(I) complexes (366.8 eV and 372.8 eV at Ag 3d5/2 and Ag 3d3/2 respectively)36 suggesting that the nanocluster most probably contains positively charge Ag atoms. Correspondingly, the bimetallic nanoclusters 6–11 have binding energies located at lower energies with respect to the bulk Ag, suggesting that the Ag atoms of these nanoclusters are mostly positively charge also. In our previous report,19 using single target sputtering for the synthesis of monometallic Au nanocluster, the peaks of the binding energy at Au4f7/2 and Au4f5/2 are located at 85.5 eV and 89.2 eV respectively which suggest the existence of Au(I) species. In the present case, although 4 has main peaks at 84.8 eV and 88. 8 eV at Au 4f7/2 and Au 4f5/2 respectively which are typical binding energy reported for Au25(SR)18 nanoclusters with a core-shell structure,37 close examination of the spectra reveals that other secondary peaks in Au 4f7/2 and Au 4f5/2 regions with almost similar peak location with that of our previous report19 were observable, suggesting that the nanocluster might be mixtures of various sizes as expected from TEM images. The peak position of 5 on the other hand (366.8 eV and 372.8 eV at Ag 3d5/2 and Ag 3d3/2 respectively), suggests the dominance of Ag(I) species34,38-42for this sample. In the case


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of bimetallic nanoclusters 6–11, Negishi et al.34 reported similar scenario wherein Au and Ag atoms are in its negative and positive charge states respectively for photoluminescent bimetallic (Au25-xAgx)(SR)18 nanoclusters. Thus this clearly indicates that samples 6–11 are bimetallic nanoclusters wherein the Au atoms which is more electronegative than Ag atoms assume negative charge and the Ag atoms assume positive charge and were covalently bonded to form a stable Au-Ag bimetallic nanoclusters.34 To confirm the formation and existence of Au-Ag bimetallic nanoclusters we investigated the samples using STEMHAADF and conducted EDS elemental mapping and quantification.

Figure 6. Representative STEM-HAADF image and EDS mapping of sample 8 (Au@18mAAg@38mA).

Figure 6 and Figure S6 show the STEM-HAADF images and the corresponding elemental mapping of samples 6-11. It is evident from these figures that the bimetallic nanoclusters were formed as can be deduced from its corresponding elemental distributions. Quantification of the elements (Table S1) showed that the compositions of the bimetallic nanoclusters 6-11 obtained by STEM-EDS were in good agreement with the results acquired in XPS and ICP-AES analyses shown in Table 2.


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Conclusions In conclusion, we successfully synthesized an NIR emitting positively charged photoluminescent

bimetallic

Au-Ag

nanoclusters

using

11-mercaptoundecyl-N,N,N-

trimethylammonium bromide as capping ligand by means of novel double target sputtering set-up which could have potential applications in biomedical fields. Results showed tunability of emission from blue to NIR region with respect to change in nanocluster composition. High resolution HAADF-STEM and elemental mapping in tandem with XPS analysis evidently showed the formation of bimetallic nanoclusters. This finding is the first physical synthesis showing an obvious correlation of nanocluster composition to the observed emission. The results presented here is therefore important for the synthesis of high purity photoluminescent bimetallic nanoclusters.

Acknowledgment TY thanks a financial support from Murata Foundation. YI acknowledges financial support from Building of Consortia for the Development of Human Resources in Science and Technology, Ministry of Education, Culture, Sports, Science and Technology, Japan. RDC thanks JICA, Japan for a financial support to his stay in Sapporo, Japan.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.xxxxx. Digital images of the Au-Ag nanocluster dispersions under sunlight; TEM images and size distributions of Au-Ag nanoclusters prepared with MUTAB; digital images of the Au-Ag nanocluster dispersions under UV light irradiation; XPS spectra and binding energies of Au 4f and Ag 3d of Au-Ag nanoclusters; STEM-HAADF images and elemental mapping of Au-


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Ag nanoclusters; elemental compositions of Au-Ag nanoclusters obtained by STEM-EDS analysis. (PDF)

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TOC

A cationically charged photoluminescent bimetallic Au-Ag nanoclusters with tunable NIR emission synthesized by a green double target sputtering technique.


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Synthesis of Positively Charged Photoluminescent Bimetallic AuâAg

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