Doping Silver Increases the Au38(SR)24 Cluster Surface Flexibility

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Doping Silver Increases the Au (SR) Cluster Surface Flexibility Bei Zhang, and Thomas Burgi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12690 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 17, 2016

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The Journal of Physical Chemistry

Doping Silver Increases the Au38(SR)24 Cluster Surface Flexibility Bei Zhang, Thomas Bürgi* Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland ABSTRACT: Multiple Ag atoms were doped inside Au38(SCH2CH2Ph)24 nanoclusters using the metal exchange method for the first time for the synthesis of AgxAu38-x(SCH2CH2Ph)24. MALDI TOF mass spectrometry revealed the time dependence of the synthesis. Cluster species with different numbers of Ag atoms (different x) migrate differently on a chromatography (HPLC) column, which allows one to isolate cluster samples with narrowed distribution of exchanged metal atoms. The enantiomers of selected AgxAu38-x(SCH2CH2Ph)24 samples (average x=6.5 and 7.9) have been separated by HPLC. Doping changes the electronic structure, as is evidenced by the significantly different CD spectra. UV-vis spectra of the doped sample also show diminished features. The temperature required for complete racemization follows Au38> AgxAu38-x (x=6.5)> AgxAu38-x (x=7.9). To our surprise, the racemization of AgxAu38-x(SCH2CH2Ph)24 (x= 7.9) occurred even at 20 ˚C. Racemization involves a rearrangement of the staple motifs at the cluster surface. The results therefore show an increased flexibility of the cluster with increasing silver content. The weaker Ag-S bonds compared to Au-S, is proposed to be at the origin of this observation. The experimentally determined activation energy for the racemization is ca. 21.5 kcal/mol (x=6.5) and 19.5 kcal/mol (x=7.9), compared to 29.5 kcal/mol for Au38(SCH2CH2Ph)24, suggesting no complete metal-S bond breaking in the process.

INTRODUCTION Thiolate protected gold nanoclusters with the size range 18 MΩ) was used. Synthesis and separation of AgxAu38-x(SCH2CH2Ph)24. AgxAu38-x(SCH2CH2Ph)24 was synthesized by metal exchange of AgSCH2CH2Ph and Au38(SCH2CH2Ph)24. Step 1. Racemic Au38(SCH2CH2Ph)24 was synthesized by thermal etching of polydispersed Aun(SG)m nanoclusters.35 0.5 mmol HAuCl4∙3H2O and 2.0 mmol L-(–)-glutathione were mixed in acetone (20 mL) at room temperature under vigorous stirring for 20 min. The mixture was then cooled to 0 °C in an ice bath. An ice-cooled solution of NaBH4 (5 mmol, dissolved in 6 mL of water) was rapidly added to the mixture under vigorous stirring for 20 min. Black Aun(SG)m nanoclusters precipitate were obtained. The clear acetone solution was decanted from the mixture and 6 mL of water was added to dissolve the Aun(SG)m clusters. A solution of Aun(SG)m (200-300 mg, dissolved in 6 mL of nanopure water) was mixed with 0.3 mL of ethanol, 2 mL of toluene, and 2 mL of PhC2H4SH. The diphase solution was heated to and maintained at 80 °C under air atmosphere, the Aun(SG)m clusters were found to transfer from the water phase to the organic phase in less than 10 min. The thermal process was allowed to continue for 40 h at 80 °C. Over the long time etching process, the initial polydisperse Aun nanoclusters were finally converted to monodisperse Au38(SCH2CH2Ph)24 clusters. The organic phase was dried and washed with methanol to remove excess thiol. Then the Au38(SCH2CH2Ph)24 nanoclusters were isolated by size exclusion chromatography (SEC), the purity of which was confirmed by MALDI and UV(Figure S1).

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metal exchange method; (b) HPLC separation of different doped clusters and (c) racemization study. Synthesis a: 1 mg AgSCH2CH2Ph was added to the solution of Au38(SCH2CH2Ph)24 (1 mg, dissolved in 1 mL). 0.2 mL of reaction suspension was taken at 0 h, 3 h, 8 h, 24 h and 48 h, respectively, filtered, dried and washed with methanol and characterized by UV and MALDI. Synthesis b: 1 mg AgSCH2CH2Ph was added to the solution of Au38(SCH2CH2Ph)24 (1 mg, dissolved in 0.8 mL) for 24 h. Synthesis c: Two racemic AgxAu38-x(SCH2CH2Ph)24 samples were synthesized. The first sample (sample A) was obtained by adding 2 mg AgSCH2CH2Ph to Au38(SCH2CH2Ph)24 solution (2mg, dissolved in 2mL toluene) and stirring for 20h. The second sample (sample B) was obtained by adding 2.0 mg AgSCH2CH2Ph to Au38(SCH2CH2Ph)24 solution (2.0 mg, dissolved in 2 mL toluene) and stirring for 22 h. After cleaning, 1.8 mg A and 1.7 mg B were collected. Step 4. Isolation of AgxAu38-x(SCH2CH2Ph)24 was tested in HPLC. Sample from synthesis b was used directly for separation. Two fractions were collected and characterized by MALDI. Step 5. AgxAu38-x(SCH2CH2Ph)24 sample A and B from synthesis c were used directly for HPLC enantioseparation. Two fractions were collected, dried immediately and stored in fridge to reduce racemization. The distribution of doped clusters with different Ag numbers was determined by MALDI. Characterization. UV−vis spectra were recorded on a Varian Cary 50 spectrometer. Quartz cuvette of 10 mm path length was used (solvents: methylene chloride and toluene). CD spectra were recorded on a JASCO J-815 CD spectrometer. Full spectra were recorded in toluene or dichloromethane. For time-course measurements, the chiroptical response at 303 nm (sample A) and 355 nm (sample B) were followed in 10 s intervals at different temperatures (20, 30, 40, 50 and 60 °C for sample A, 20, 30, 35, 40, and 50 °C for sample B, JASCO Peltier element, toluene, 5 mm path length), and the average signal of toluene at the corresponding temperature over 5 min was subtracted.

Step 2. Synthesis of AgSCH2CH2Ph was performed according to a recent protocol.25 0.04 mmol of AgNO3 was dissolved in 0.2 mL H2O and 1 mL ethanol. A mixture of 0.88 mmol PhSCH2CH2SH, 2.86 mmol tri-ethylamine and 1.45 mL ethanol was added to the solution under vigorous stirring. A yellowish white precipitate was formed immediately. After 30 min, the reaction mixture was centrifuged and cleaned by ethanol/H2O to remove the PhSCH2CH2SH.

HPLC. Chiral HPLC was performed on a JASCO 20XX HPLC system equipped with a Phenomenex Lux Cellulose-1 column (100 Å, 250 mm × 4.6 mm), and the eluting analytes were detected with a JASCO 2077plus UV detector (485 nm). The analytes were dissolved in toluene and eluted with n-hexane:2-propanol (80:20) at a flow rate of 2.5 mL/min. The separation temperature was set at 20 °C for sample A and 18 °C for sample B and the sample for HPLC isolation test. The collected fractions were dried and inject in HPLC to determine the enantiomeric excess (ee).

Step 3. Three batches of AgxAu38-x(SCH2CH2Ph)24 were synthesized by metal exchange between AgSCH2CH2Ph and Au38(SCH2CH2Ph)24 for different studies: (a) time dependence of the AgxAu38-x(SCH2CH2Ph)24 formation in

Racemization. A stock solution in toluene was made and stored in fridge for the first enantiomer collected in HPLC (0.77 mg/mL, ee 83.70% for sample A and 1.06 mg/mL, ee 46.36% for sample B) and split for racemiza-

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tion at different temperatures. Prior to and after heating, a CD and UV-vis spectrum was recorded for comparison. The samples were heated to 20, 30, 40, 50 and 60 °C (sample A) and 20, 30, 35, 40, and 50 °C (sample B) for 30 min. The CD response was monitored in situ at the wavelength which showed the highest intensity in (303 nm for sample A and 355 nm for sample B. Signal of solvent at corresponding temperature was subtracted for the plots of ln(ee) versus time. RESULTS AND DISCUSSION Time dependent synthesis. Metal exchange between AgSCH2CH2Ph and Au38(SCH2CH2Ph)24 was applied to synthesize AgxAu38-x(SCH2CH2Ph)24 nanoclusters. MALDI TOF mass spectrometry was used to follow the composition of the reaction mixture at the reaction time 0 h, 3 h, 8 h, 24 h and 48 h (Figure 1). AgxAu38-x alloy was obtained by metal exchange. Clear shift to lower masses of the MALDI peak can be observed after longer reaction time, which indicates higher Ag dopant number. In the MALDI spectra of the 3 h and 8h samples, peaks can unambiguously be assigned to AgxAu38-x(SCH2CH2Ph)24 species due to the large mass difference between Ag and Au (Δm/z = 89.1). The peaks below 9500 (m/z) are due to fragments. The average number of incorporated silver atoms increased from 2.0 to 3.2, respectively, for the 3h and 8h samples. Similarly, the maximum number of Ag atoms incorporated increased from 7 and 9. The AgxAu38x(SCH2CH2Ph)24 peaks at 24 h and 48 h, however, overlap partially with the fragmentation peaks. Apart from the obviously increased Ag dopant number, the MALDI peaks (especially at lower masses) cannot be assigned to doped cluster. AgxAu38-x(SCH2CH2Ph)24 clusters with less Ag dopant were therefore synthesized for clarity.

Figure 1. MALDI TOF spectra (positive mode) of AgxAu38x(SCH2CH2Ph)24 nanoclusters at the reaction time 0 h (black), 3 h (red), 8 h (blue), 24 h (pink) and 48 h (olive). The highest dopant number is marked at the corresponding peak for 3 h and 8 h. Fragmentation peaks of the AgxAu38x(SCH2CH2Ph)24 nanoclusters can be observed with the mass difference ∆m/z = 137.2 and 1366.8, which corresponding to – SR and Au4(SR)4. The fragmentation peaks at 24h and 48h

were not observed due to the low laser intensity applied in the measurements. Average dopant number was calculated according to n=(∑ni*Ai)/∑Ai, in which A stands for the corresponding peak area in MALDI spectra.

HPLC Separation. As is shown in Figure 1, the sample contains species with different numbers of silver dopant atoms. The diversity of AgxAu38-x(SCH2CH2Ph)24 species increases if we consider the isomers of each doped cluster (with silver atoms doped at different positions). Isolation of AgxAu38-x(SCH2CH2Ph)24 with different dopant number were studied in the first place. AgxAu38-x(SCH2CH2Ph)24 sample with the silver dopant number from 5 to 15 was synthesized (Figure S2a). HPLC separation of the sample gave two broad bands corresponding to clusters with different handedness (see below). Two fractions were taken from the second band (Figure S2b), with the retention time 70 – 90 min and 90 – 110 min respectively. A clear difference of cluster composition was shown in the MALDI spectra of the two fractions (Figure S2a). AgxAu38x(SCH2CH2Ph)24 with smaller dopant number was eluted faster in the method applied (first sample with the dopant number 5 - 9 and the second sample with the dopant number from 6 - 14). However, it remains a challenge to isolate the individual AgxAu38-x(SCH2CH2Ph)24 with the current method. Enantioseparation and Chrial Properties. In the HPLC chromatogram of parent nanocluster two bands were observed, which correspond to the two enantiomers of Au38(SCH2CH2Ph)24.9 The two bands of AgxAu38-x(SCH2CH2Ph)24 were therefore separated for further study. In order to investigate the sensitivity of Au38(SCH2CH2Ph)24 properties to the Ag dopant numbers, two AgxAu38-x(SCH2CH2Ph)24 samples (sample A and sample B) with close reaction time were synthesized. The two fractions of both AgxAu38-x(SCH2CH2Ph)24 samples were collected respectively over several HPLC runs (Figure 2). A narrow retention time range was chosen for the collection due to the slight overlap between the two peaks. The average silver number of the first fraction was determined from MALDI spectra (Figure 2), which is 6.5 for sample A and 7.9 for sample B. The second fractions showed similar silver number, however the average number deviates slightly from the first fraction (5.4 for sample A and 5.8 for sample B), which can be ascribed to the narrow collection range in HPLC separation. UV-vis spectra of the two fractions were similar in both samples respectively with the main absorption bands at 445 nm, 512 nm and 610 nm (Figure 2). However, the characteristic features of Au38(SCH2CH2Ph)24 diminishes with increased Ag dopant number. Comparison between the first fractions of both samples shows slight differences, sample A showing a more pronounced band at 412 nm. CD spectra of the two fractions in both samples gives mirror

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Figure 2. Characterization of the two AgxAu38-x(SCH2CH2Ph)24 nanoclusters (sample A and sample B); top left: HPLC chromatogram of the enantioseparation of both samples, the two fractions collected for each cluster were denoted as AF1, AF2, BF1 and BF2; top right: MALDI spectra of the collected fractions, the maximum and minimum dopant number are marked with lines; bottom left: UV-vis spectra of the collected fractions; bottom right: comparison of the doped and parent Au38 CD spectra.

images respectively with clear signals observed between 280 and 700 nm (303 (+), 359 (-), 427 (+), 540 (+) and 618 (-) nm for sample A, fraction 1 and 301 (+), 355 (-), 427 (+), 522 (+), and 619 (-) nm for sample B fraction 1, Figure 2). CD spectra of AgxAu38-x(SCH2CH2Ph)24 samples show significant differences. Particularly, the CD spectra of the doped cluster are less structured than the parent cluster, revealing the strong influence of Ag dopant on the Au38(SCH2CH2Ph)24 electronic structure. The concentration-independent anisotropy factors g = ΔA/A = θ[mdeg]/(32980 × A) were calculated for the first fractions of the two AgxAu38-x(SCH2CH2Ph)24 samples over the spectral range and scaled with the ee determined by HPLC (Figure S3). The maximum anisotropy factor found is about 7 × 10-4 (433 nm) and 5.7 × 10-4 (356 nm) for sample A and sample B and the value for Au38(SCH2CH2Ph)24 is 4 × 10-3.9 The significant decrease of chiroptical responses of Au38(SCH2CH2Ph)24 after Ag doping can be explained by the coexistence of different doped nanoclus-

ters and their isomers. This is in consistence with the decrease of anisotropy factor from sample A (average silver number 7.9) to sample B (average silver number 6.5). In our previous work, Pd doping inside Au38 also gave rise to weak anisotropy factor.20 Racemization. A stock toluene solution of sample A fraction 1 was prepared and split into 5 aliqots which were then subject to thermal treatment at 20, 30, 40, 50 and 60 ˚C for 30 min respectively. The CD response at 303 nm was followed during the time course (Figure 3). At a first glance, after 30 min complete racemization of sample A was achieved at 60 ˚C (also shown in the CD spectra after heating, Figure S4), which is much lower than the 80 ˚C required for Au38(SCH2CH2Ph)24. A considerable decrease of CD response had already occurred in the heating process at 30 ˚C. The UV-vis spectra of the solution after heating resemble the starting AgxAu38-x(SCH2CH2Ph)24 nanoclusters (Figure S4), which confirmed the stability of the clusters against decomposition under the conditions

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Figure 3. Racemization experiment with AgxAu38-x(SCH2CH2Ph)24 nanoclusters; left: CD response of AF1 at 303 nm as a function of time at different temperatures; right: CD response of BF1 at 355 nm as a function of time at different temperatures.

of racemization. It can be concluded that silver doping lowered the racemization temperature of Au38(SCH2CH2Ph)24. Since the racemization process involves a rearrangement of the staples on the cluster surface, it can be concluded that silver doping increases the flexibility of the metal – sulfur interface. In order to investigate the racemization behavior of nanoclusters with higher silver dopant numbers, similar experiment was carried out for the toluene solution of sample B fraction 1 at 20, 30, 35, 40 and 50 ˚C, and the detection wavelength was set at 355 nm. The racemization is surprisingly fast with more than 10% inversion at 20 ˚C after 30 min. This suggests that the racemization of the sample took place during the HPLC separation at 18 ˚C. This explains the intensity observed in the chromatogram between the two main peaks (Figure 2). This intensity arises from clusters undergoing racemization within the column. Complete

Figure 4. Eyring plot ln(k/T) vs 1/T for the racemization of 19 Au38(SCH2CH2Ph)24 nanoclusters, AgxAu38-x(SCH2CH2Ph)24 nanoclusters AF1 and BF1.

racemization of sample B fraction 1 requires heating at 50 ˚C for around 20 min (Figure S4). The AgxAu38nanoclusters survived temperature x(SCH2CH2Ph)24 treatment without decomposition, as is evident by the conserved UV-vis spectra after heating (Figure S4). This indicates the high sensitivity of the Au38(SCH2CH2Ph)24 surface flexibility to the Ag dopant number. The linearity of the plot of ln(ee) vs. time shows that the reaction is of first-order kinetics. Rate constants were determined from a linear fit of the initial slope (Figure 4). Activation parameters ΔH⧧, ΔS⧧ and ΔG⧧ were derived from the Eyring analysis (Table 1). The activation barrier for the racemization of AgxAu38-x(SCH2CH2Ph)24 sample A and sample B, respectively is 21.5 and 19.5 kcal/mol. This value is much lower than the one found for Au38(SCH2CH2Ph)24 (29.5 kcal/mol). This is consistent with the experimental observation of lower temperature for the racemization of silver doped Au38(SCH2CH2Ph)24 nanoclusters. In the AgxAu38-x(SCH2CH2Ph)24 (x ≤ 5) obtained from Brust method, Ag atoms were reported to be at the biicosahedral Au23 core surface of cluster.30 In the present study we used a different method for the doping. Therefore, in principle, the silver atoms might also exchange within the staple. In order to get a hint on this we analyzed the fragmentation patterns in the MALDI spectra. The loss of Au4(SR)4 unit was suggested by the fragmentation peak of Au34(SR)20- (9342 Da)35 and Au21(SR)14- (6055 Da)36 in the MALDI spectra of Au38(SR)24 and Au25(SR)18. Zeng et al proposed a stepwise fragmentation mechanism of the staple motifs for the MALDI fragmentation in Au25(SR)18.37 MALDI spectra of sample A and sample B resemble Au38(SCH2CH2Ph)24 cluster, with no coexistence of AgxAu4-x (x=1,2,3 and 4) species observed (Figure S5). This indicates that by metal exchange method, Ag atoms are also incorporated in the core surface. In sample A and

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sample B, we can speculate the coexistence of Au-S and Ag-S bond on the surface. The weaker Ag-S bond contributes to the higher flexibility. The activation energy of AgxAu38-x(SCH2CH2Ph)24 nanoclusters is lower than the typical binding energy of Au-S (50 kcal/mol)38-42 and Ag-S (40 kcal/mol).43-44 It can be assumed that the complete metal-sulfur breaking is not involved in the racemization of AgxAu38-x(SCH2CH2Ph)24 nanoclusters.

The Swiss National Science Foundation (grant number 200020_152596).

Table 1. Activation parameters for the thermal inversion of Au38(SCH2CH2Ph)24, AgxAu38-x(SCH2CH2Ph)24 nanoclusters nanoclusters AF1 and BF1.

MALDI-TOF, matrix-assisted laser desorption coupled with time-of-flight mass spectrometer; CD, Circular dichroism; HPLC, High-performance liquid chromatography.

Sample

ΔH⧧ (kcal -1 mol )

ΔS⧧

ΔG⧧

Ea

(cal K-1 -1 mol )

(kcal -1 mol )

(kcal -1 mol )

Au38

28.8

9.7

25.6

29.5

AF1

20.9

-8.1

23.4

21.5

BF1

18.9

-11.3

22.4

19.5

In summary, we presented an efficient metal exchange method for the incorporation of Ag into Au38 nanocluster, which showed time dependence and improved efficiency. Using HPLC separation we were able to isolate AgxAu38-x samples with narrower distribution of dopant number. This is based on the observation that species with different x migrate slightly differently on the HPLC column. Both enantiomers of AgxAu38-x nanocluster were obtained by HPLC separation. Ag doping gives rise to significant changes in UV-vis and CD spectra in comparison with Au38, which can be related with the change in electronic structure. Silver doping lowers the racemization temperature required for AgxAu38-x. Clusters with higher Ag content show racemization already at room temperature, which reflects the high flexibility of metal-sulfur interface. Sensitivity of the racemization temperature to Ag dopant number can be ascribed to the number of weak Ag-S bond.

ASSOCIATED CONTENT Supporting Information MALDI, UV-vis, CD measurements results described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Funding Sources

ACKNOWLEDGMENT BZ thanks The 201306340012).

China

Scholarship

Council

(No.

ABBREVIATIONS

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Doping Silver Increases the Au38(SR)24 Cluster Surface Flexibility

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