Variable Growth and Characterizations of Monolayer-Protected Gold

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Interface-Rich Materials and Assemblies

Variable Growth and Characterizations of Monolayer Protected Gold Nanoclusters Based on Molar Ratio of Gold and Capping Ligands Seyyedamirhossein Hosseini, Nouf Alsiraey, Andrew Riley, Tykhon Zubkov, Trent Closson, Jesse W. Tye, Nataraju Bodappa, and Zhihai Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02623 • Publication Date (Web): 25 Nov 2018 Downloaded from http://pubs.acs.org on November 26, 2018

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Variable Growth and Characterizations of Monolayer Protected Gold Nanoparticles Based on Molar Ratio of Gold and Capping Ligands

Seyyedamirhossein Hosseini,1 Nouf Alsiraey,1 Andrew J. Riley,1 Tykhon Zubkov,1 Trent Closson,1 Jesse Tye,1 Nataraju Bodappa,2* Zhihai Li1*

1

Department of Chemistry, Ball State University, Muncie, IN 47306, U.S.A.

2

University of Bern, Department of Chemistry and Biochemistry, Freiestrasse 3, CH-3012

Bern, Switzerland

Keywords Gold nanoparticles, disintegration, molar ratio, particle size, STM, electrochemistry

Abstract Controlling the size of nanoscale entities is important because many properties of nanomaterials are directly related to the size of the particles. Gold nanoparticles represent classic materials and are of particular interest due to their potential application in a variety of fields. In this study, hexanethiol-capped gold nanoparticles are synthesized via the Brust-Schiffrin method. Synthesized nanoparticles were characterized by various analytical techniques such as transmission electron microscopy (TEM), scanning tunneling microscopy (STM), UV-Visible absorption spectroscopy (UV-Vis) and electrochemical techniques. We have varied the molar 1

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ratio of gold to the protecting agent (hexanethiol) to discover the effect of gold-to-hexanethiol ligand ratio on the size of gold clusters. The clear correlation between cluster size and molar ratio is found that the averaged cluster size decreases from 4.28 ± 0.83 nm to 1.54 ± 0.67 nm as the gold-to-ligand molar ratio changes from 1:1 to 1:9. In contrast to a recent report that thiolated gold nanoparticles are under spontaneous disintegration when they are assembled on a gold substrate, our STM experiments proved that these gold nanoparticles can form a stable monolayer or multiple layers on the platinum electrode without observing disintegration within 72 hours. Therefore, our STM experiments demonstrate that the disintegration behavior of gold nanoparticles is related to the type of ligands and the nature of substrate materials. In electrochemical experiments, these gold nanoparticles displayed an electrochemical quantized charging effect, making these nanoparticles useful in the device applications such as electrochemical or biological sensors.

1

INTRODUCTION

Gold nanoparticles possess distinct chemical and physical properties with a significant application in chemistry and biology.1-5 Nanoparticles are not a new field, as colloidal gold or gold nanoparticles (Au NPs) have been known with the initial investigations dating back to Faraday.6-8 However, the investigation of gold nanoparticles has never ceased because gold nanoparticles represent a classical model system to understand the structure, properties, and applications of nanomaterials.9-15 Chemical synthesis of gold nanoparticles has experienced a significant development, beginning with Turkevich who developed a well-accepted approach for the synthesis of gold nanoparticles in 1951, that led to the synthesis of 20 nm clusters.16 The next milestone improvement came from Burst-Schiffrin method that led to the synthesis of 1-3 nm, 2


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thiol capped gold nanoparticles.17 Meanwhile, numerous studies covering various syntheses of gold nanoparticles have been focusing on control over their size, shape, stability, and other applications.2,7,18-22

Monolayer-protected nanoparticles have attracted major interest recently not only because of their applications in many fields such as biology,5,23 medicine,24 and electrocatalysis,1 but also due to their unique electrical properties as nanoscale building blocks for constructing nanodevices and/or future molecular circuits.18,25 Nanoparticles with a core diameter of 1-5 nm behave qualitatively differently from molecules, large nanoparticles, and bulk metals.26 It has been observed that these monolayer protected clusters assembled at solid-electrolyte interfaces function as nanoscale capacitors and allow the quantized charging due to multivalent redox behavior.10,26-30 Recent studies show that the modulation current during these quantized coulomb charging processes can be monitored by electroanalytical and scanning tunneling microscopy/spectroscopy (STM/STS) techniques. The information obtained can be used to further explore the concepts of molecular switches, coulomb blockade and molecule-level electronic transistor-like behavior in an STM configuration.31,32

Except for the shape and composition, the size of the nanoparticles also plays an important role in the device applications mentioned above.26,33-37 For example, in an electrochemical environment, these gold nanoparticles can behave as nanoscale capacitors, and its capacitance is directly correlated to the cluster size.27,32,38 It is expected that the separation of the quantized charging peaks in both electrochemical cyclic voltammetry and STM scanning tunneling spectroscopy (STS) is substantially influenced by the size, i.e., the capacitance, of nanoparticles 3


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at electrode-electrolyte interfaces.26 Therefore, controlling the size of the nanoparticles is critical in the investigation of a quantized charging phenomena and in the study of electrochemical sensors. This motivated us to prepare a series of monolayer-protected gold nanoparticles with varied particle sizes. Several parameters affect the size and composition of monolayer protected nanoparticles such as ratio of capping agent-metal center,39,40 type, amount, and rate of addition of the reducing reagent,37,41 temperature,37 solvent,22 and so on. In this study, we employed the modified Brust-Schiffrin method and systematically varied the molar ratio of gold and ligand (hexanethiols) to discover how the variation of gold-ligand ratio affects the size of synthesized nanoparticles. After the gold nanoparticles were prepared, we characterized the nanoparticles by TEM to determine the cluster size. The prepared nanoparticles assembled on a platinum substrate were imaged by STM to discover the stability of these clusters at electrode-electrolyte interfaces. For device application, quantized electron charging and discharging into and out of these nanoparticles were monitored by electrochemical differential pulse voltammetry (DPV) technique.

2

EXPERIMENTAL

2.1

Synthesis of hexanethiol protected gold nanoparticles

Sodium tetrachloroaurate (III) dihydrate (NaAuCl4∙2H2O, crystalline 99.99%, Alfa Aesar), 1hexanethiol (C6H13SH, 97%, Aldrich), sodium borohydride (sodium tetrahydridoborate, 99%, Aldrich),

tetra-n-octylammoniumbromide (98%, Aldrich), toluene (Fisher Scientific) and

ethanol (95%, Fisher Scientific), chloroform (99.8%, Fisher Scientific), acetonitrile (99%, Fisher Scientific) were all used as received.

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Hexanethiol-protected gold nanoparticles were synthesized using the Brust-Schiffrin method with slight modification.17,30 Briefly, gold salt (NaAuCl4) was dispersed in water-toluene heterogeneous mixture under surfactants so that gold ions can be transferred from aqueous (aq) to organic (org) phase (step 1 in Scheme 1), and ligand molecules (1-hexanethiol) were added. The addition of thiol (step 2) causes the reduction of Au(+3) to Au(+1) and produce [TOA][Au(+1)Br2] complex [TOA = Tetraoctylammonium] and/or (RS-Au(+1)-SR)n polymers depending on the ratio of RSH/[AuCl4]-.42-45 And subsequently, ice cold NaBH4 was added to reduce gold ion (Au1+) to gold atoms (Au0) and to form monolayer protected gold nanoparticles, see Scheme 1.

Au3+(aq) (3) NaBH4

(1) surfactant/toluene

Au0

Au3+(org)

(2) C6H13SH

Au0n (nano-clusters)

Scheme 1. Reactions and schematic procedure of the synthesis of monolayer protected gold nanoparticles based on the Brust-Schiffrin method.

In a typical procedure, 0.31 g NaAuCl4∙2H2O was dissolved in 10 ml of distilled water and transferred gradually to a solution containing of 0.5 g of tetra-n-octylammonium bromide and 20 ml of toluene under rigorous stirring for 30 min. Synthesis was followed by separating organic phase from aquatic phase and addition of appropriate amount (Table S1) of hexanethiol ligands to organic phase (gold solution) under vigorous stirring. The orange color of the solution fades within 10-30 min depending on the gold-thiol ratio. Subsequently, 0.38 g NaBH4 was dissolved 5


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in 5 mL of cold Millipure water (4 C ̊ ), and added to the organic phase (4 ̊C) in dropwise manner where a dark-brown or almost black solution was instantly obtained. The organic phase was isolated and rewashed three times with Millipore water in order to ensure that all extra ions are removed. Extra toluene was then removed with the application of rotary evaporation under embient conditions until a black paste was obtained. The black product was re-dissolved in 30 mL of ethanol and stirred overnight. The ethanol-gold suspension was separated with centrifugation (10 min, 12000 rpm, 4˚C) and the solid phase was subsequently washed three times with a 1:1 acetonitrile-ethanol mixture. The final product was dispersed in chloroform and was subjected to TEM and UV-Vis studies.

2.2

Characterization of the prepared monolayer-protected nanoparticles

Hexanethiol-protected gold nanoparticles were first characterized by transmission electron microscopy (TEM, JEOL JEM-1400, 120 kV) to study cluster shape as well as size distribution for each sample.46 The samples were deposited on the carbon film coated copper grids (Electron Microscopy Sciences, Carbon Film 400 Mesh-Cu). To have reliable data, many samples are prepared by different students, and hundred of TEM images were acquired from different experiments. As a normal practice, for each image, TEM stage was shifted randomly by 100 µm laterally. Cluster sizes were calculated by averaging the diameters of a cluster measured in two perpendicular direction and statistical histograms were built to determine the averaged particle size as well as to examine the size distribution. Optical properties of the nanoparticles were studied by UV-Visible absorption spectroscopy (Agilent, see the spectra in Supporting Information). In a typical experiment, 200 L of the sample was diluted with an adequate 6


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volume of chloroform to produce an optical density of 0.35-0.5 in order to be able to compare the light absorptivity of samples. Gold nanoparticles were assembled onto the Pt (111) substrate and imaged with STM to test their stability during formation of a monolayer and its relation to a particle size. The STM experiments were carried out with a Molecular Imaging Pico-SPM.47 The STM tips were electrochemically etched from a platinum/Iridium (80:20) wire (0.25 mm diameter). STM tip etching took place in an electrolyte solution containing 1M CaCl2 and 0.25 M HCl using a 55 V square mode DC voltage with a glassy carbon rod serving as a counter electrode (cathode) and a Pt/Ir wire as an anode. All STM images were recorded in the air at room temperature in constant current mode and are presented as top views.

2.3

Electrochemistry

Differential pulse voltammetry (DPV) were carried out with an Autolab PGSTAT128N potentiostat (Metrohm, U.S.A. Inc.) In a typical electrochemical experiment, a lab-built 3electrode cell was used with a Pt coil as a counter electrode and Ag/AgNO3(0.01M)/0.1M TBAPF6/CH3CN as a reference electrode. Freshly polished, polycrystalline Pt disk was used as working electrode. For DPV measurements, we used a pulse width of 60 ms, the pulse height of 50 mV, pulse period of 200 ms and a scan rate of 20 mV/s. In a typical DPV experiment, we used a gold nanoparticles concentration of  0.1 mM dissolved in 2:1 toluene and acetontrile containg 0.1 M TBAPF6. For STM measurements, a single crystal platinum disc (Mateck GmbH, Germany, 2 mm height and 10 mm diameter) was annealed for 5 min. in a hydrogen flame and then immediately quenched in a Milli-Q water bath (saturated with Ar). Finally, the platinum disk was dried with a continuous stream of dry argon. During electrochemical measurement, oxygen was removed by purging with solvent-saturated high purity argon. 7


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3

RESULTS AND DISCUSSION

Size analysis of prepared monolayer protected gold nanoparticles After the monolayer protected gold nanoparticles were prepared according to the Brust-Schiffrin method with slight modification (Experimental Section), then, the solution containing gold nanoparticles was washed with water, ethanol, and then acetonitrile, and finally was dispersed in chloroform for characterization by techniques including TEM, STM, UV-Vis, mass spectrometry (see UV and Mass Spectra in Supporting Information) and for electrochemical studies. Gold nanoparticles were synthesized by varying the molar ratio of gold-to-hexanethiol ligands to discover the effect of the ratio on the particle size of the nanoparticles. Figure 1 shows typical TEM images of nanoparticles prepared by using 1:9, 1:6, 1:3, and 1:1 molar ratio of gold to hexanethiol, respectively.

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a

b

c

d

Figure 1. TEM images of gold nanoparticles prepared by using different gold-to-hexanethiol molar ratio: (a) gold:hexanethiol = 1:9; (b) gold:hexanethiol = 1:6; (c) gold:hexanethiol = 1:3; (b) gold:hexanethiol = 1:1.

To obtain the statistical information on how the molar ratio of metal to ligand affects the particle size, we have synthesized many samples using a standard procedure (see Experimental Section) while only changing the ratio of ligand to gold. The molar ratio of NaAuCl4∙2H2O to hexanethiol was changed from 1:1, to 1:3, to 1:6, to 1:9. The analysis of these samples synthesized by different students shows a clear size dependence on the Au:ligand ratio. The particle sizes were determined by measuring the particle diameter in two different directions and then averaging the 9


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two diameter values. The gold nanoparticles were also characterized by Matrix-assisted laser desorption/ionization (MALDI) Mass spectrometry (MS). Figure S2 shows the spectra of gold nanoparticles prepared by using gold:thiol molar ratio of 1:6. The intensity peak of about 32,000 m/Z corresponds to Au144(C6H13S)60 structure with an approximate particle size of about 2 nm.48-50 This size value is consistent to the particle size determined by TEM (Table 1). To accurately determine the particle size and composition, and further investigate the structural motif and morphologies, Electrospray ionization mass spectrometry (ESI-MS), X-ray power diffraction analyzes, and laser desorption ionization mass spectrometry (LDI-MS) need to be performed which is not yet carried out in this

current study.51-58 The averaged diameter values (particle size) were compiled into a table and a statistical histogram was constructed for each of the four specific molar ratios. The histograms were fitted to a Gaussian equation to determine the probable particle size. The histograms in Figure 2A, 2B, 2C shows the most probable particle sizes of 1.04 ±0.04 nm, 1.98 ±0.02 nm, and 2.78 ± 0.04 nm from the Gaussian fit for Au:ligand ratio of 1:9, 1:6, and 1:3, respectively. The most probable particle size from the Gaussian fit peak and the averaged (mean) particle size are shown in Table 1 for comparison (median size values are also shown in Table 1 for each ratio). It is found that when the molar ratio of Au:ligand is equal to 1:6, the average particle size (2.14 ± 0.65 nm) is close to the center of the Gaussian size distribution fit, 1.98 nm, which is the frequently reported particle size for alkanethiol protected gold clusters. When the Au:ligand (hexanethiol) ratio was changed to 1:9, i.e., there are more hexanethiol ligands in the reacting solution compared with a ratio of 1:6, the particle size decreases. This indicates that more ligands around the gold metal (atoms) can prevent gold from growing into larger clusters. Similarly, when the Au:ligand ratio was changed to 1:3, which means, fewer ligands in the reacting solution, the average particle size increases to 3.38 nm (the most probable size from Gaussian fit 10


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is 2.79 nm, see Table 1). However, when, the Au:ligand ratio became 1:1, the particle size changed dramatically. There is a majority of clusters having a size of 5.4 nm, but there is also a large number of clusters having much larger sizes, distributing broadly from 10 nm to 38 nm (Figure 2D). That indicates when the relative concentration (percentage) of ligand decreases to a certain degree, the gold clusters tend to accumulate and grow into large clusters. Literature supports this hypothesis. According to Hosteler and Murray et al, when the reductant was delivered slowly, the very small particles are formed initially, then, the depletion of thiol concentration leads to a progression of larger cores.34. In other words, the low percentage of ligands is insufficient to cap monolayer-covered clusters with well-protected ligands, so that some clusters might be able to integrate into larger clusters, indicated by a blue arrow in Figure 2D. This explains why there is a wide range of large clusters. Further changing the Au:ligand ratio to 3:1, which decreases the ligand concentration, makes it impossible to form monolayerprotected gold nanoparticles in our experiments. Basically, irregular bulk-like large gold clusters were formed and this “failed” experiments were not shown in this paper. These results are in good consistency with previous report by Chen et al. where a U-Like relationship was observed for the correlation of the particle size and thiol/gold ratio. It is noted that in the present work, thiol-gold ratio is still in the right side of the curve where enhancement of the thiol amount results in reduction of nanoparticle size.59 The statistical data on the cluster size and molar ratio are compiled into Table 1.

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A

B

Au to Thiol ratio: 1:9

60

40

Au to thiol ratio: 1:6

30 Count

Count

50 40

50

30

20

20 10

0

C

1

2 3 Particles size (nm)

60

0

4

D


50

2 Particles size (nm)


50 40

40 Counts

20

3

50

Au to thiol ratio: 1:1, The group of small size only

40 30 20 10 0

30

30

2

3

4 5 6 Particles size (nm)

7

8

20 10

10 0

1

Count

10

Count

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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0 1

2

3 4 5 Particles size (nm)

6

7

0

5

10

15

20

25

30

35

40

Particles size (nm)

Figure 2. Statistical histograms of particle sizes and Gaussian fit for gold:ligand (hexanethiol) ratios equals to 1:9 (A), 1:6 (B), 1:3 (C), and 1:1 (D). When the ratio changes to 1:1, clusters tend to form large clusters corresponding to the wide tail in the distribution histogram indicated by a blue arrow. Inset (D) is the Gaussian fit of the left part of histogram D – distribution of the smaller size clusters.

To see the general trend more clearly, we plotted the particle size as a function of the gold-toligands molar ratio (Figure 3). Both the average size (Figure 3A, black) and probable size from the Gaussian fit (Figure 3A, red) increase when gold:thiol molar ratio changes from 0.111 (gold:thiol = 1:9), to 0.166 (gold:thiol = 1:6), to 0.333 (gold:thiol = 1:3), and to 1.00 (gold:thiol = 1:1). It is difficult to use a simple model to fit the data because there are only four data points 12


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available. But one sees that the general trend is that when the gold-to-thiol ratio increases, the cluster size become larger for both the average size and probable size from the Gaussian fit. This result is consistent with the discovery by Hostetler and Murray et al. who found the mean size of gold nanoparticles can be adjusted by the change of gold:dodecanethiolate ratio as well as the temperature and reduction rate,34 indicating the general trend for the size-dependent on gold:ligand ratio is effective for both the long and short alkanethiol molecules. Laemmerhofer et al. reported that gold nanocluster size changes as a function of ligand (citrate):HAuCl4 ratio – the cluster size decreases when ligand ratio increases. The two trend lines in Fig. 3A are slightly different from Laemmerhofer results, which could be related to the chemical property of softer citrate ligands versus more actively (strongly) interacting thiol ligands.33 The different trend lines also indicate the different kinetics involved in the nanocluster/nanoparticle growth (the solvent effect here is neglected in this comparison). Though the size dependence on the molar ratio of ligand and gold are mostly investigated using alkanethiol26 as capping ligands with the majority studies focusing on the long alkanethiol such as dodecanethiols,7,22 this molar ratio effect on size is also applicable to other ligands such as polymers,36 benzylthiol.22 In addition to the molar ratio of reactants, other parameters also play a role in determining the particle size, such as reducing agent,34 synthesis temperature,37 solvent,22 etc. In general, fast addition of the reductant cold solution (low temperature) produces small size particles.2 Average and median sizes, determined from hundreds of clusters from different samples and experiments, are compiled into Table 1. Meanwhile, it is interesting to plot all the individual sizes from hundreds of measurements as a function of the gold-to-thiol molar ratio (Figure 3B). Figure 3B shows that though the averaged size has a clear dependence on the molar ratio of reactant components, for the individual particle,

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the particle size can vary within the certain size range, and some individual particle size can be outliers from the group.

5

A

8

Averaged size Guassian Fit peak

Particle size in diameter (nm)

6 Particle size in diameter (nm)

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4 3 2 1 0 0.0

0.2

0.4

0.6

0.8

6

4 Number of particles 181 136 200 200

2

0 0.0

1.0

B

Molar ratio of Au-to-thiol

0.2

0.4

0.6

0.8

1.0

Molar ratio of Au-to-thiol

Figure 3. Particle sizes in diameter change as a function of gold:ligand (hexanethiol) molar ratio. (A) Averaged sizes of clusters are plotted as a black square with standard deviation (green line), and probable sizes from the Gaussian fit are plotted as a red circle. (B) Plotting of individual cluster sizes versus the gold-to-thiol molar ratio, and the legend shows the numbers of nanoparticles analyzed.

Table 1: The particle size of prepared nanoparticles with different gold:hexanethiol molar ratio.

Au-to-Thiol molar ratio

1:1

1:3

1:6

1:9

Average (nm)

4.28 ± 0.83

3.34 ± 1.00

2.14 ± 0.65

1.54 ± 0.67

Size from peak of Gaussian fit

4.30 ± 0.05

2.79 ± 0.04

1.98 ± 0.02

1.04 ± 0.04

Median (nm)

5.52

3.09

2.02

1.28

Stability study of monolayer-protected gold nanoparticles on Pt by STM

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To further characterize the surface morphology of the prepared nanoparticles, we assembled the nanoparticles on single crystal Pt metal surface and imaged the gold nanoparticles using scanning tunneling microscopy (STM) techniques. It was recently reported that gold nanoparticles are not stable and experience spontaneous disintegration while assembled on a gold surface.9 Our own STM experiments also confirmed that thiolate-protected gold nanoparticles were unstable on the gold surface (Figure S4) when we assembled them on a gold single crystal electrode. Figure S4 shows that the disintegration of gold NPs on a gold substrate can take place with and without STM tip scanning. But the tip scanning, the external stimulant, can accelerate the disintegration process (Figure S4). In that disintegration experiment, we used NPs capped with hexane mono- and dithiol mixtured ligands. Initially, we assume that the disintegration could be caused by the interexchange of ligands between the NP and gold substracte as illustrated in Figure S5. However, Albrecht et al reported that NPs protected by hexanethiol (monothiol) also disintegrate in air or in mesitylene solvent, and suggested that the disintegration is attributed to the redistribution of processes of capping ligangs between NP and the substrate.9 This could be due to the strong chemical interaction between thiol groups of capped ligands and substrate gold atoms. To avoid the dynamic instability of gold nanoparticles on the gold metal surface, in this study we chose to assemble hexanethiol-protected gold nanoparticles on a platinum (Pt) substrate. The assembly of gold NPs on Pt have been employed for the quantized charging study using scanning tunneling spectrascopy (STS)10,28 The Pt(111) single crystal was prepared according to the well-established procedure as described in the experimental section. This treatment creates an atomically flat Pt(111)-(1x1) surface with atomically high steps as Fig. 4A shows. To assemble nanoparticles on Pt for STM imaging, the hexanethiol protected Au nanoparticles were subjected to ligand place exchange process using 4-(4 pyridyl)thiophenols.10 15


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The resulted Au clusters were deposited onto the Pt(111) surface via self-assemble method by placing the Pt disk electrode into (19 M) cluster solution for 1 hour. Here, the exchanged pyridyl molecules on clusters surface act as a rigid linker between the Pt substrate and cluster particle and avoid a close contact of the Au core to the Pt surface. After the gold nanoparticles were deposited onto the surface, the cluster-modified crystal was characterized by STM at room temperature and in the ambient condition. Figs. 4A and B shows landscape and zoomed preview of STM images of a Pt surface covered with a low density of hexanethiol-protected gold nanoparticles respectively. As can be seen clearly, monolayer-protected gold clusters are not evenly distributed on the surface. Meanwhile, one can also identify “striped lines” in the STM images, which might be caused by the interaction between ligand molecules and the STM probe. For curiosity, we also created high-density structures of hexanethiol-protected gold nanoparticles on Pt(111). These multiple layers of gold nanoparticles show “worms-like” features (Fig. 4C). The cause of the worm-like features could be that these continuously dispersed monolayerprotected clusters interact and accumulate (interdigitate) with each other. The close contact of these nanoparticles might cause the overlap of the soft ligand shells (hexanethiol molecules), making them the polymer-like or worm-like features under STM investigation. Our STM experiments prove that, though thiolated gold nanoparticles are unstable on a gold electrode, these clusters are indeed stable on Pt surface as there is no evidence of disintegration after 72 hours. In this cluster disintegration experiment, we selected the linker molecule, in this case 4-(4 pyridyl)thiophenol which acts as rigid backbone to avoid the contact of cluster on to the substrate. Therefore, our experiment proves that careful choice of linkers and the type subsrate can avoid the disintigration of the nanoparticles on a specific substrate. This observation may

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stimulate further experimental and theoretical investigation to elucidate the mechanism of such a particle disintegration reported by Albrecht et al.9

Figure 4. (A) Large scale STM image of a low density assembly of hexanethiol-protected gold nanoparticles (gold:thiol molar ratio = 1:6) on Pt(111) substrate; (B) High-resolution STM image of a low density assembly (A) of hexanethiol-protected gold nanoparticles on Pt(111) substrate; (C) Multilayered high density of hexanethiol-protected gold nanoparticles assembled on Pt(111).

Electrochemical characterization of monolayer-protected gold nanoparticles Monolayer-protected nanoparticles have many applications in nanoelectronics, molecular devices, and electrochemical sensors. These applications often involve interfacial charge transport processes at electrode-electrolyte interfaces. In this work, we extended our study to electrochemical applications in terms of electrochemically quantized charging of nanoscale capacitors. Figure 5 shows an electrochemical differential pulse voltammogram (DPV) of a platinum electrode recorded using hexanethiol protected Au clusters dissolved in 0.1 M TBAPF6 in toluene + acetonitrile (2/1 volume/volume). Over a wide potential range from –2.2 V to 1.2 V, quantized double layer (QDL) single electron charging features are clearly discerned as

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pronounced peaks superimposed on the capacitive background charging of the electrode (red dashed line in Figure 5).

Figure 5. Electrochemical differential pulse voltammogram (DPV) of monolayer-protected gold nanoparticles in 0.1 M TBAPF6 in toluene/acetonitrile (2/1 volume/volume). Wok electrode: polycrystalline Pt electrode, Reference electrode: Ag/AgCl, counter electrode: Pt wire. Red dashed curve shows the response of bare Pt electrode in solution in absence of gold nanoparticles.

Compared with traditional cyclic voltammetry techniques, differential pulse voltammetry can substantially reduce the capacitive background current, thus enhance the visibility of the quantized double layer charging (QDLC) response.10,31,32,38 Such a QDLC response is due to the collective quantized (one-electron) charging of the gold nanoparticles.27,30,32,38 This experiment proves that the charging effect of such a sub attofarad-level capacitance can be monitored using an electrochemical technique at room temperature. Furthermore, the capacitance C of the monolayer-protected nanoparticles can be calculated from the spacing of QDLC peaks (0.26 V) 18


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based on equation E = e/C (with electron charge e), which gives rise to a cluster capacitance of  0.62 aF.60 Hicks and Murray et al showed that in addition to particle size and monolayer thickness (ligand length), the measured capacitance of nanoparticles can vary if different electrolytes with different dielectric constant were used.30,60 The measured capacitance (0.62 aF) in our experiments is well consistent with the capacitance value (0.63 aF for haxanethiol protected nanoparticles) reported by Hicks and Murray et al when measurements in the same electrolyte is compared.30 The QDLC redox potential (Figure 5), multiple charging states and the capacitance of NPs agree with the pioneering works by Quinn and Murray et al.27,32,61,62 Our QDLC experiments only exhibit 13 charging states, but Quinn’s data demonstrated the 15 charging states due to the wider electrochemical potential window applied.62

4

SUMMARY AND CONCLUSIONS

We have synthesized a series of monolayer protected gold nanoprticles through the application of the modified Brust-Schiffrin method and characterized them with different techniques including TEM, STM and UV-Vis. The molar ratio of gold:ligand has been systematically varied from 1:1, to 1:3, 1:6, and 1:9 and it was discovered that the particle size decreases continuously when the ratio changes, i.e., the particle size becomes smaller when the relative percent of ligand (thiol) increases. When the gold-to-ligand ratio increases, gold atoms tend to form larger clusters, and eventually make it impossible to form monolayer-protected “nanoscale” clusters when the gold-to-thiol ratio is higher than 3:1. We postulate that gold atoms form bulk clusters if there is not enough ligand available to cover (protect) the nanoparticle surface. This observation is in good agreement with the observation that for the ratio of 1:1, some larger clusters formed due to the lack of capping ligands. Furthermore, we also investigated the electrochemical application of 19


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synthesized nanoparticles in terms of quantized single electron charging at electrode-electrolyte interfaces. Quantized double layer charging (QDL) responses were observed and capacitance of monolayer-protected nanoparticles was calculated. It is recently reported that monolayerprotected gold nanoparticles are not stable and experience spontaneous disintegration on a gold electrode.9 Our own experiments confirmed that gold nanoparticles completely disintegrated into mono-atomically high gold islands on a Au(111) electrode within a few hours. These disintegration phenomena cause some concerns on the applications of these nanoparticleson metal surfaces. However, the STM and electrochemical experiments in this paper show that gold nanoparticles are stable on a platinum surface, which indicates that the instability and disintegration of nanoparticles are related to the properties of substrate electrode materials. Therefore, our studies provide useful information on the choice of substrate for device applications of nanoparticles and may promote further investigation on exploring the mechanism and fundamental reason of the instability or disintegration of monolayer-protected nanoparticles on certain metal surfaces.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]. E-mail: [email protected] Present Addresses (S.H.) Department of Chemistry, Indiana University of Indiana, Bloomington, IN 47405

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(N.B.) State Key Laboratory of Physical Chemistry of Solid Surfaces, MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

ACKNOWLEDGEMENTS Z. L. acknowledges the financial support from Indiana Academy of Science Senior Research Grant and Ball State University ASPiRE Junior Faculty Awards. B.N. acknowledges the support of National Research Program (NRP 62, project number 406240_126108) of the Swiss National Science Foundation (SNSF) and the University of Bern. T.Z. acknowledges the NSF MRI grant DBI-1126196 for the purchase of the Transmission Electron Microscope for Ball State University. Authors acknowledge Yifei Chen from Indiana Academy for Science, Mathematics, and Humanities for helping with nanoparticles synthesis, and acknowledge Kelsi Goshinski and Cody Leasor for imaging samples with scanning probe microscopy.

REFERENCES (1) Li, Y. X.; Cox, J. T.; Zhang, B. Electrochemical Responses and Electrocatalysis at Single Au Nanoparticles. J. Am. Chem. Soc. 2010, 132, 3047-3054. (2) Daniel, M. C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293-346. (3) Yamazoe, S.; Koyasu, K.; Tsukuda, T. Nonscalable Oxidation Catalysis of Gold Clusters. Acc. Chem. Res. 2014, 47, 816-824. (4) Jahn, W. Review: Chemical Aspects of the Use of Gold Clusters in Structural Biology. J. Struct. Biol. 1999, 127, 106-112. (5) Yang, C.; Uertz, J.; Yohan, D.; Chithrani, B. D. Peptide Modified Gold Nanoparticles for Improved Cellular Uptake, Nuclear Transport, and Intracellular Retention. Nanoscale 2014, 6, 12026-12033. (6) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. Optical Properties of Gold and Silver Nanoclusters Investigated by Liquid Chromatography. J. Chem. Phys. 2001, 115, 998-1008. (7) Wilcoxon, J. P.; Abrams, B. L. Synthesis, Structure and Properties of Metal Nanoclusters. Chem. Soc. Rev. 2006, 35, 1162-1194. 21


Langmuir 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

(8) Horikoshi, S.; Serpone, N. Introduction to Nanoparticles. Microwaves in Nanoparticle Synthesis, First Edition, Wiley-VCH Verlag Gmbh & Co. KGaA 2013. (9) Pensa, E.; Albrecht, T. Controlling the Dynamic Instability of Capped Metal Nanoparticles on Metallic Surfaces. J. Phys. Chem. Lett. 2018, 9, 57-62. (10) Bodappa, N.; Fluch, U.; Fu, Y. C.; Mayor, M.; Moreno-Garcia, P.; Siegenthaler, H.; Wandlowski, T. Controlled Assembly and Single Electron Charging of Monolayer Protected Au-144 Clusters: An Electrochemistry and Scanning Tunneling Spectroscopy Study. Nanoscale 2014, 6, 15117-15126. (11) Krishnadas, K. R.; Baksi, A.; Ghosh, A.; Natarajan, G.; Pradeep, T. Manifestation of Geometric and Electronic Shell Structures of Metal Clusters in Intercluster Reactions. ACS Nano 2017, 11, 6015-6023. (12) Krishnadas, K. R.; Baksi, A.; Ghosh, A.; Natarajan, G.; Som, A.; Pradeep, T. Interparticle Reactions: An Emerging Direction in Nanomaterials Chemistry. Acc. Chem. Res. 2017, 50, 1988-1996. (13) Pensa, E.; Cortes, E.; Corthey, G.; Carro, P.; Vericat, C.; Fonticelli, M. H.; Benitez, G.; Rubert, A. A.; Salvarezza, R. C. The Chemistry of the Sulfur-Gold Interface: In Search of a Unified Model. Acc. Chem. Res. 2012, 45, 1183-1192. (14) Hakkinen, H. The Gold-Sulfur Interface at the Nanoscale. Nat. Chem. 2012, 4, 443-455. (15) Yao, Q. F.; Chen, T. K.; Yuan, X.; Xie, J. P. Toward Total Synthesis of ThiolateProtected Metal Nanoclusters. Acc. Chem. Res. 2018, 51, 1338-1348. (16) Turkevich, J.; Stevenson, P. C.; Hillier, J. A Study of the Nucleation and Growth Processes in the Synthesis of Colloidal Gold. Discuss. Faraday Soc. 1951, 55-75. (17) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of Thiol-Derivatized Gold Nanoparticles in a 2-Phase Liquid-Liquid System. J. Chem. Soc., Chem. Commun 1994, 801-802. (18) Morita, T.; Lindsay, S. Determination of Single Molecule Conductances of Alkanedithiols by Conducting-Atomic Force Microscopy with Large Gold Nanoparticles. J. Am. Chem. Soc. 2007, 129, 7262-7263. (19) Jin, R. C.; Zeng, C. J.; Zhou, M.; Chen, Y. X. Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities. Chem. Rev. 2016, 116, 1034610413. (20) Chakraborty, I.; Pradeep, T. Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles. Chem. Rev. 2017, 117, 8208-8271. (21) Bootharaju, M. S.; Chaudhari, K.; Pradeep, T. Real Time Plasmonic Spectroscopy of the Interaction of Hg2+ with Single Noble Metal Nanoparticles. RSC Adv. 2012, 2, 1004810056. (22) Vitale, F.; Fratoddi, I.; Battocchio, C.; Piscopiello, E.; Tapfer, L.; Russo, M. V.; Polzonetti, G.; Giannini, C. Mono- and Bi-Functional Arenethiols as Surfactants for Gold Nanoparticles: Synthesis and Characterization. Nanoscale Res. Lett. 2011, 6, 103(1-9). (23) Saha, K.; Agasti, S. S.; Kim, C.; Li, X. N.; Rotello, V. M. Gold Nanoparticles in Chemical and Biological Sensing. Chem. Rev. 2012, 112, 2739-2779. (24) Giljohann, D. A.; Seferos, D. S.; Daniel, W. L.; Massich, M. D.; Patel, P. C.; Mirkin, C. A. Gold Nanoparticles for Biology and Medicine. Angew. Chem. Int. Ed. 2010, 49, 3280-3294. 22


Page 22 of 26

Page 23 of 26 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

Langmuir

(25) Pradhan, S.; Sun, J.; Deng, F. J.; Chen, S. W. Single-Electron Transfer in Nanoparticle Solids. Adv. Mater. 2006, 18, 3279-3283. (26) Chen, S. W.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Gold Nanoelectrodes of Varied Size: Transition to Molecule-Like Charging. Science 1998, 280, 2098-2101. (27) Laaksonen, T.; Ruiz, V.; Liljeroth, P.; Quinn, B. M. Quantised Charging of Monolayer-Protected Nanoparticles. Chem. Soc. Rev. 2008, 37, 1836-1846. (28) Albrecht, T.; Mertens, S. F. L.; Ulstrup, J. Intrinsic Multistate Switching of Gold Clusters through Electrochemical Gating. J. Am. Chem. Soc. 2007, 129, 9162-9167. (29) Li, Z.; Liu, Y. Q.; Mertens, S. F. L.; Pobelov, I. V.; Wandlowski, T. From Redox Gating to Quantized Charging. J. Am. Chem. Soc. 2010, 132, 8187-8193. (30) Hicks, J. F.; Templeton, A. C.; Chen, S. W.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaf, T. G.; Whetten, R. L. The Monolayer Thickness Dependence of Quantized Double-Layer Capacitances of Monolayer-Protected Gold Clusters. Anal. Chem. 1999, 71, 3703-3711. (31) Su, B.; Girault, H. H. Redox Properties of Self-Assembled Gold Nanoclusters. J. Phys. Chem. B 2005, 109, 23925-23929. (32) Murray, R. W. Nanoelectrochemistry: Metal Nanoparticles, Nanoelectrodes, and Nanopores. Chem. Rev. 2008, 108, 2688-2720. (33) Han, S. T.; Zhou, Y.; Xu, Z. X.; Roy, V. A. L.; Hung, T. F. Nanoparticle Size Dependent Threshold Voltage Shifts in Organic Memory Transistors. J. Mater. Chem. 2011, 21, 14575-14580. (34) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Alkanethiolate Gold Cluster Molecules with Core Diameters from 1.5 to 5.2 nm: Core and Monolayer Properties as a Function of Core Size. Langmuir 1998, 14, 17-30. (35) Hinterwirth, H.; Kappel, S.; Waitz, T.; Prohaska, T.; Lindner, W.; Lammerhofer, M. Quantifying Thiol Ligand Density of Self-Assembled Monolayers on Gold Nanoparticles by Inductively Coupled Plasma-Mass Spectrometry. ACS Nano 2013, 7, 1129-1136. (36) Shimmin, R. G.; Schoch, A. B.; Braun, P. V. Polymer Size and Concentration Effects on the Size of Gold Nanoparticles Capped by Polymeric Thiols. Langmuir 2004, 20, 5613-5620. (37) Perala, S. R. K.; Kumar, S. On the Mechanism of Metal Nanoparticle Synthesis in the Brust-Schiffrin Method. Langmuir 2013, 29, 9863-9873. (38) Chen, S. W.; Murray, R. W. Electrochemical Quantized Capacitance Charging of Surface Ensembles of Gold Nanoparticles. J. Phys. Chem. B 1999, 103, 9996-10000. (39) Thanh, N. T. K.; Maclean, N.; Mahiddine, S. Mechanisms of Nucleation and Growth of Nanoparticles in Solution. Chem. Rev. 2014, 114, 7610-7630. (40) Chaudhuri, R. G.; Paria, S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2012, 112, 2373-2433. (41) Chen, T. K.; Fung, V.; Yao, Q. F.; Luo, Z. T.; Jiang, D. E.; Xie, J. P. Synthesis of Water-Soluble [Au-25(SR)18] - Using a Stoichiometric Amount of NaBH4. J. Am. Chem. Soc. 2018, 140, 11370-11377. 23


Langmuir 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

(42) Yu, C. H.; Zhu, L. L.; Zhang, R. C.; Wang, X. L.; Guo, C. C.; Sun, P. C.; Xue, G. Investigation on the Mechanism of the Synthesis of Gold(I) Thiolate Complexes by NMR. J. Phys. Chem. C 2014, 118, 10434-10440. (43) Li, Y.; Zaluzhna, O.; Xu, B. L.; Gao, Y.; Modest, J. M.; Tong, Y. Y. J. Mechanistic Insights into the Brust-Schiffrin Two-Phase Synthesis of Organo-ChalcogenateProtected Metal Nanoparticles. J. Am. Chem. Soc. 2012, 134, 6498-6498. (44) Goulet, P. J. G.; Lennox, R. B. New Insights into Brust-Schiffrin Metal Nanoparticle Synthesis. J. Am. Chem. Soc. 2010, 132, 9582-9584. (45) Uehara, A.; Booth, S. G.; Chang, S. Y.; Schroeder, S. L. M.; Imai, T.; Hashimoto, T.; Mosselmans, J. F. W.; Dryfe, R. A. W. Electrochemical Insight into the Brust-Schiffrin Synthesis of Au Nanoparticles. J. Am. Chem. Soc. 2015, 137, 15135-15144. (46) Hosseini, S.; Farsi, H.; Moghiminia, S.; Zubkov, T.; Lightcap, I. V.; Riley, A.; Peters, D. G.; Li, Z. H. Nickel Tungstate (NiWO4) Nanoparticles/Graphene Composites: Preparation and Photoelectrochemical Applications. Semicond. Sci. Technol. 2018, 33, 055008(1-12). (47) Hosseini, S.; Madden, C.; Hihath, J.; Guo, S. Y.; Zang, L.; Li, Z. Single Molecule Charge Transport and Electrochemical Gating in Redox-Active Perylene Diimide Junctions. J. Phys. Chem. C 2016, 120, 22646-22654. (48) Jensen, K. M. O.; Juhas, P.; Tofanelli, M. A.; Heinecke, C. L.; Vaughan, G.; Ackerson, C. J.; Billinge, S. J. L. Polymorphism in Magic-Sized Au-144(SR)60 Clusters. Nat. Commun. 2016, 7, 11859(1-8). (49) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Nanocrystal Gold Molecules. Adv. Mater. 1996, 8, 428-433. (50) Kurashige, W.; Niihori, Y.; Sharma, S.; Negishi, Y. Precise Synthesis, Functionalization and Application of Thiolate-Protected Gold Clusters. Coord. Chem. Rev. 2016, 320, 238-250. (51) Yao, Q. F.; Feng, Y.; Fung, V.; Yu, Y.; Jiang, D. E.; Yang, J.; Xie, J. P. Precise Control of Alloying Sites of Bimetallic Nanoclusters Via Surface Motif Exchange Reaction. Nat. Commun. 2017, 8, 1555(1-11). (52) Yao, Q. F.; Fung, V.; Sun, C.; Huang, S. D.; Chen, T. K.; Jiang, D. E.; Lee, J. Y.; Xie, J. P. Revealing Isoelectronic Size Conversion Dynamics of Metal Nanoclusters by a Noncrystallization Approach. Nat. Commun. 2018, 9, 1979(1-11). (53) Yao, Q. F.; Yuan, X.; Fung, V.; Yu, Y.; Leong, D. T.; Jiang, D. E.; Xie, J. P. Understanding Seed-Mediated Growth of Gold Nanoclusters at Molecular Level. Nat. Commun. 2017, 8, 927(1-11). (54) Cleveland, C. L.; Landman, U.; Schaaff, T. G.; Shafigullin, M. N.; Stephens, P. W.; Whetten, R. L. Structural Evolution of Smaller Gold Nanocrystals: The Truncated Decahedral Motif. Phys. Rev. Lett. 1997, 79, 1873-1876. (55) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; GutierrezWing, C.; Ascensio, J.; JoseYacaman, M. J. Isolation of Smaller Nanocrystal Au Molecules: Robust Quantum Effects in Optical Spectra. J. Phys. Chem. B 1997, 101, 7885-7891.

24


Page 24 of 26

Page 25 of 26 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

Langmuir

(56) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Crystal Structures of Molecular Gold Nanocrystal Arrays. Acc. Chem. Res. 1999, 32, 397-406. (57) Tsunoyama, H.; Nickut, P.; Negishi, Y.; Al-Shamery, K.; Matsumoto, Y.; Tsukuda, T. Formation of Alkanethiolate-Protected Gold Clusters with Unprecedented Core Sizes in the Thiolation of Polymer-Stabilized Gold Clusters. J. Phys. Chem. C 2007, 111, 41534158. (58) Negishi, Y.; Nakazaki, T.; Maloa, S.; Takano, S.; Niihori, Y.; Kurashige, W.; Yamazoe, S.; Tsukuda, T.; Hakkinen, H. A Critical Size for Emergence of Nonbulk Electronic and Geometric Structures in Dodecanethiolate-Protected Au Clusters. J. Am. Chem. Soc. 2015, 137, 1206-1212. (59) Chen, T. K.; Luo, Z. T.; Yao, Q. F.; Yeo, A. X. H.; Xie, J. P. Synthesis of Thiolate-Protected Au Nanoparticles Revisited: U-Shape Trend between the Size of Nanoparticles and Thiol-to-Au Ratio. Chem. Commun. 2016, 52, 9522-9525. (60) Guo, R.; Georganopoulou, D.; Feldberg, S. W.; Donkers, R.; Murray, R. W. Supporting Electrolyte and Solvent Effects on Single-Electron Double Layer Capacitance Charging of Hexanethiolate-Coated Au-140 Nanoparticles. Anal. Chem. 2005, 77, 2662-2669. (61) Hicks, J. F.; Miles, D. T.; Murray, R. W. Quantized Double-Layer Charging of Highly Monodisperse Metal Nanoparticles. J. Am. Chem. Soc. 2002, 124, 13322-13328. (62) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. Electrochemical Resolution of 15 Oxidation States for Monolayer Protected Gold Nanoparticles. J. Am. Chem. Soc. 2003, 125, 6644-6645.

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Variable Growth and Characterizations of Monolayer-Protected Gold

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