An Approach to Fabricate Compact Gold Nanoparticles Film with the

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An Approach to Fabricate Compact Gold Nanoparticles Film with the Assistance of a Surfactant Huan Min, Jun Zhou, Xiaojue Bai, linlin Li, Kai Zhang, Tieqiang Wang, Xuemin Zhang, Yunong Li, Yonghua Jiao, Xuan Qi, and Yu Fu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00255 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017

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An Approach to Fabricate Compact Gold Nanoparticles Film with the Assistance of a Surfactant Huan Min,a# Jun Zhou, c# Xiaojue Bai,a Linlin Li,a Kai Zhang,a Tieqiang Wang,a Xuemin Zhang,a Yunong Li,a Yonghua Jiao,b* Xuan Qi,a* Yu Fua*

a

College of Sciences, Northeastern University, Shenyang 110819, P. R. China

b

College of Life and Health Sciences, Northeastern University, Shenyang 110819, P. R. China

c

School of Materials Science and Engineering, Key Laboratory for Anisotropy and Texture of

Materials, Ministry of Education, Northeastern University, Shenyang 110819, P. R. China

Corresponding author *E-mail: [email protected], [email protected] and [email protected]

Author Contributions #

These authors contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT: We report a facile method to fabricate a compact Au nanoparticles film with the assistance of surfactants. First, the dodecanethiol-coated Au nanoparticles were floated on the surface of the toluene/acetonitrile mixture solvent and adjusted to an expanded dispersion by changing the mixture ratio. The silicone oil was then added as a surfactant to compress the floating nanoparticles from the original loose status to a closely-packed arrangement which produced a compact nanoparticle film. The relationship of the compressed film area with the silicone oil concentration was plotted and compared with the surface tension curve of silicone oil. The results were quite consistent suggesting that the surface location of the surfactant induced the nanoparticles compression. The resulting nanoparticle film was uniform and sufficiently robust to be transferred to the solid substrate. Moreover, it could be applied to catalyze the reduction of 4-nitrophenol. Our study indicated that utilization of surfactants to compress the well-dispersed nanoparticles on the liquid surface is a simple, fast and adaptable method to fabricate compact nanoparticle films with great promise for future applications.

KEYWORDS: Nanoparticles film, Surfactant, Surface, Self-assembly


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Introduction

The assembly of nanoparticles into two-dimensional films is a powerful tool to manipulate nanoparticles and create collective properties.1-3 Up to now, various methods have been developed to prepare nanoparticles film4-6 including the use of a liquid surface as assembly platform.7,8 The main advantage of liquid surface over the solid surface is that it offers sufficient nanoparticle mobility and allows them to spontaneously adjust to the equilibrium or quasiequilibrium status during assembly. That is beneficial because it reduces the defects of the nanoparticle arrangements and produces a uniform and continuous film. In addition, while the film fabricated on the liquid surface must be transferred to the solid substrate prior to application, there is no restriction on the size, shape and morphology of the substrates, which will extend their applications. Currently, there are essentially two approaches to assemble nanoparticles on the liquid surface: self-assembly and directed-assembly.9,10 Self-assembly is a spontaneous process that relies on the delicate equilibrium between the nanoparticles and the environment. Therefore, self-assembly of nanoparticles into a uniform film is quite challenging, and it normally requires elaborate designs with a strict control on the experimental conditions. On the other hand, directed-assembly offers an external force to help nanoparticle assembly which considerably reduces the fabrication difficulty.11


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A representative directe-assembly is Langmuir-Blodgett (LB) or Langmuir-Schaefer (LS) technique.12,13 Typically, nanoparticles in a volatile organic solvent are spread over a water surface in a special trough. After evaporation of the organic solvent, an external force is exerted by a barrier on water surface to compress the floating nanoparticles into a closely-packed film, which is then deposited onto a solid substrate through vertical-dipping (LB) or horizontal-lifting (LS) techniques. Versus self-assembly, the LB/LS technology is more suitable for the fabrication of large-area nanoparticle films. However, the fabrication LB/LS film usually requires special instruments, and the preparation process is tedious including a thorough cleaning of the LB trough, plotting surface-pressure isotherms, etc. These challenges derive from the utilization of the barrier as the source of pressure to push the nanoparticles. It demands precise measurement and control on the surface tension, which relies exclusively on the instrument and preparation procedure. Thus, it would be intriguing to develop a new way to replace the barrier and compress the nanoparticles on the surface for producing high-quality nanoparticle films. The addition of surfactants can also put pressure to the nanoparticles on the liquid surface. The surfactant has a strong inclination to occupy the liquid surface and reduce the surface tension, which could compress the floating objects on liquid surface together. So far, the surfactants are usually used to fabricate polystyrene microsphere films.14,15 Firstly, the microspheres are floated on the water surface by slowly injecting toluene. Then, a trace of surfactant such as sodium lauryl sulfate is added after the organic solvents evaporate off. Once the surfactant is added, the surface seems to be immediately solidified. Simultaneously, the original scattered domains of


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polystyrene microspheres are squeezed into one piece to produce a large-scale and uniform film. The fabrication process is fast, simple and practical. It is widely used in the fabrication of twodimensional colloidal crystals. However, its application in the fabrication of nanoparticles films remains limited. Inspired by the use of surfactants in the fabrication of colloidal crystals film, we propose a new method to prepare nanoparticles film with the assistance of a surfactant (Scheme 1). The Au nanoparticles were 5 nm in diameter with 1-dodecanethiol (DDT) ligands and they were chosen as the building block because of their popularity in both basic and applied research. First, the Au nanoparticle solution in hexane was dispersed on the surface of the toluene/acetonitrile mixture. By adjusting the ratio of the mixture, these nanoparticles were immobilized on the surface and formed a uniform but flexible monolayer. Then, a small amount of silicone oil surfactant was added. By the influence of surfactant, the dispersed nanoparticles were immediately compressed to generate a robust film. The generated film could be transferred to the solid substrate and used for catalyzing the reduction of 4-nitrophenol. Thus, the use of surfactants to compress the dispersed nanoparticles on the surface is a facile and practical strategy for fabricating nanoparticle films, which shows a great potential in nanoscience and nanotechnology.


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Scheme 1 Schematic of the preparation and transfer of Au nanoparticles film via surfactants.

Experimental Section

Materials: Chloroauric acid, sodium borohydride, 1-dodecanethiol, silicone oil (Mw=14,000), and 4-nitrophenol (4-NP) were purchased from Alfa Aesar. All chemical reagents were used without any further purification.

Preparation of Au nanoparticles: The Au nanoparticles were prepared according to the literature.16 First, 95 µL aqueous solution (50 mM HAuCl4 with the same molar amount of HCl) was added to 9.5 g water in a 50-mL glass vial followed by addition of 425 µL NaBH4/NaOH aqueous solution (50 mM both NaBH4 and NaOH) at once. The vial was under vigorous stirring for 1 min and then heated in boiling water for 2.5 min. Then, 5.0 g acetone was added to the resulting solution and mixed by hand. Next, 5 g hexane solution containing 0.1 µL DDT was


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added with shaking by hand for 1 min. Finally, the nanoparticles were transferred from aqueous solution to organic phase resulting in the DDT-coated Au nanoparticles.

Fabrication and transfer of Au nanoparticles film: The 12 mL toluene and 6 mL acetonitrile were mixed in a 40-mm by 20-mm glass dish. Then, 1.4 mL of the Au nanoparticle solution in hexane was added dropwise to the mixture to generate an expanded dispersion on mixture solution surface. After that, 10 µL of toluene in silicone oil (1.2 mg/ ml) was added to the mixed solution, and the nanoparticles were compressed and immobilized beside the wall to form a robust film. To transfer the film, a quartz substrate was fixed on a stepper motor at 165° (the angle between quartz substrate and liquid surface). It was dipped into the mixture carefully and slowly, and then pulled out at a rate of 20 µm/s. For the mixed solvent in other conditions, the volume ratios of toluene and acetonitrile ranged from 4:1 to 1:0 and were chosen to prepare the solvent with various polarity values. The other experimental conditions were consisted the above details.

Application in catalysis: Prior to catalysis, the Au nanoparticle film was deposited on the quartz substrate and was cleaned by plasma treatment. First, the vacuum drying process at room temperature was necessary for Au nanoparticles films before plasma treatment. The quartz substrate with Au nanoparticles film was then placed in the cavity of plasma cleaner. After that, the sample under plasma bombardment (700 V, 15 mA, 10.5 w) was maintained for 30 s to remove the capped nanoparticles ligands, which occupied the active sites on the surface of


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nanoparticles resulting in a low catalytic efficiency. Finally, the film was immersed in the solution of 0.1 M ascorbic acid for about 10 minutes and then rinsed repeatedly with deionized water to reduce the gold atoms that may be oxidized in the plasma treatment. In the catalysis, 10 mL of 10-4 M solution of 4-nitrophenol was mixed with 10 mL of 0.1 M aqueous solution of NaBH4 in a glass beaker under gentle stirring at room temperature. The Au nanoparticle film was immersed into the solution and then the reaction was monitored by UV-vis spectroscopy. To monitor the reaction, a 2-mL aliquot was withdrawn from the reaction every 10 min and dispersed back into the reaction immediately after UV-vis measurement.

Characterization: The transmission electron microscopy (TEM) images were obtained with a Hitachi H-7650 electron microscope. Ultraviolet−visible (UV) spectra were measured using custom-made setup consisting of a collimated beam of a fiber-coupled tungsten−bromine lamp (Ocean Optics) coupled to a spectrometer (Ocean Optics, Maya 2000PRO). Atomic force microscopy (AFM) was performed by Broker’s Dimension Icon in SanAsyst mode. The surface tension was measured by optical contact angle measurement instrument (KRUSS DSA100). The plasma cleaning used a Harrick plasma cleaner PDC-32G-2.

Results and Discussion

Preparation of Au nanoparticles: It is well known that the nanoparticle assembly depends on an extremely delicate equilibrium between various factors. One of the most critical factor is the nanoparticle property. This includes not only the measurable properties like shape and size, but


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also the properties that are more difficult to characterize like charge, conformation of the ligands, etc. Normally, all of these properties are determined by the synthesis method. Therefore, it is necessary to prepare the nanoparticles first.

The Au nanoparticle in our study was synthesized according to the literature.16 HAuCl4 was reduced by NaBH4 in an aqueous solution followed by extraction into a DDT solution in hexane. This synthesis is reproducible, surfactant-free and convenient for the successive assembly. The resulting nanoparticles were negatively charged and coated with the alkyl chains of DDT. Detail Au nanoparticle characterization is shown in Figure 1. The average diameter of prepared nanoparticles was 4.39±0.63 nm and the UV-Vis absorbance peak was at 518 nm resulting from the surface plasma resonance of Au nanoparticles.

Figure 1 The UV-vis spectrum and TEM image of the Au nanoparticles.

Dispersion of Au nanoparticles on a liquid surface: The DDT-coated Au nanoparticles could be dispersed on the surface of toluene and acetonitrile mixture. In the mixture, toluene was a


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good solvent for nanoparticles. Toluene could disperse Au nanoparticles well thanks to its low polarity and consequent good solvation of the alky chains on nanoparticles surface. On the contrary, acetonitrile was a poor solvent for its high polarity and poor miscibility with the alkyl chains, which usually resulted in an uncontrollable aggregation of the Au nanoparticles. Thus, adjusting the ratio of two solvents could regulate the dispersion status of the nanoparticles. At a high volume ratio of toluene (e.g., 4:1 or even pure toluene), there were no Au nanoparticles dispersed on the liquid surface. Instead, a pink solution was observed suggesting that the toluene solvation of the alkyl chains on nanoparticles surface of was so strong that the nanoparticles were completely dissolved into the solution. At a high volume ratio of acetonitrile (e.g., 0.5:1 or even pure acetonitrile), the Au nanoparticles were aggregated into irregular fragments on mixture surface as shown in Figure 2a. TEM result showed that the nanoparticles were closely packed, but randomly and continuously distributed (Figure 2c). Such a structure could be attributed to the solvophobic effect between alkyl chains and acetonitrile, which floated the nanoparticles on mixture surface and restricted their motion. When the toluene and acetonitrile were mixed at an appropriate ratio (toluene/acetonitrile was around 2/1), the Au nanoparticles could form an expanded dispersion on mixture surface as shown in Figure 2b. In contrast to the aggregation seen with pure acetonitrile, nanoparticles dispersed on the surface of mixed solvent uniformly. They spread as much as possible.


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Importantly, the nanoparticles dispersion was flexible. When air flow was used to disturb the surface, the film would change its shape with the flow. Moreover, after withdrawing the air flow, the shape of film could recover immediately (as shown in supporting information SV1). Under the same conditions, the aggregates with pure acetonitrile would break into pieces under air flow (as shown in supporting information SV2). Another difference was that the color of the expanded dispersion changed from violet to pink. This indicated that the localized surface plasmon resonance (LSPR) of the Au nanoparticles blue-shifted, implying the arrangement of the nanoparticles in the dispersion became looser. Figure 2d showed that the nanoparticles were loosely and disorderedly dispersed in agreement with the color variation. These experiments suggested that toluene and acetonitrile reached equilibrium. The solvophobic effect of the acetonitrile floated the nanoparticles on the surface while the solvation of toluene made the nanoparticles mobile on the surface.


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Figure 2 Photographs of the films at a high acetonitrile volume ratio (a) and at an appropriate volume ratio (b) without the silicone oil as well as the corresponding TEM images (c and d)

Formation of a compact Au nanoparticle film: These results showed that by adjusting the ratio of the mixtures, the Au nanoparticles could form a uniform dispersion on the liquid surface. However, because the arrangement of nanoparticles on the surface of mixed solvent was loose, the resulting dispersion was vulnerable during transfer onto the solid substrate. The disturbance of the transfer could fragment the film or interfere with the location of the film on the substrate. Obviously, these films were not suitable for further applications. Nanoparticles should be packed in a condensed way to obtain a robust film that can survive the transfer. A feasible approach to compact the nanoparticles is using surfactants because they can occupy the surface.


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In this study, silicone oil—a moderate surfactant—was used to reduce the surface tension of solvent and compress Au nanoparticles on mixture surface, which would change the arrangement of nanoparticles. The dispersion of Au nanoparticles was fabricated on a mixture of toluene and acetonitrile with a 2:1 ratio. The silicone oil was then added by injecting toluene solutions under the surface with a syringe. Before addition of the silicone oil, the Au nanoparticles occupied almost the entire mixture surface, and the coverage ratio of the monolayer was close to 1 (Fig. 3a). Once the silicone oil was added, the film shrank immediately as shown in Fig. 3b-c and SV3. The film was immobilized on the side of the petri dish, and the shape could be maintained for days. Moreover, the film could survive slight disturbances, and hence could transfer to the solid substrate without remarkable damage as expected. To verify the effect of the silicone oil, a quantitative relationship between the surface tension and the silicone oil concentration (the final concentration in the mixture solvents, not the concentration of the addition solution) was plotted as shown in Figure 3d. The variation of the surface tension with the concentration fitted perfectly into the variation law of the critical micelle concentration (CMC), indicating that the silicone oil behaved as a typical surfactant in the toluene/acetonitrile mixture. Meanwhile, the relationship between the coverage ratio (the area ratio of the films after and before the addition) and the silicone oil concentration was plotted as well (Figure 3d). Upon comparing the two curves, it was found that the two curves were quite consistent, which suggested shrinkage of the film was directly related to the surface tension of the solution and the surfactant. In the initial stage, silicone oil molecule mainly dissolved in


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solvent due to the low concentration (far below the CMC) and had little effect on the surface tension and the nanoparticles on the surface. With the concentration increasing, silicone oil began to from micelles in the solvent (c > 0.21 mg/ml) and some molecules accessed to the surface, which lowered the surface tension and compressed the nanoparticles into a more compact arrangement. Finally, when silicone oil concentration increased to 1.06 mg/mL, the micelle was arriving to a saturated status along with the maximum surface silicone oil molecules, which led to the greatest driving force to compress the nanoparticles. The above results and discussions demonstrated that silicone oil in mixture solvents played a surfactant role, which could efficiently regulate the surface tension of solvent and coverage ratio, thus affecting the arrangement of nanoparticles on the surface. In addition to area, the color of the film also experienced a change from purplish red to purple with increasing concentration of silicone oil. Figure 4 showed that the maximum absorption wavelength (λmax) of the film red-shifted from 554 nm to 565 nm as concentration increased from 0.01 mg/mL to 1.06 mg/mL. When the concentration increased to 5.3 mg/mL, the λmax remained constant. The red-shift of λmax was attributed to the decrease in the interparticle spacing,17,18 This implied the interparticle spacing was reduced with increasing silicone oil concentration. At the CMC (1.06 mg/mL), the film was not compressed any further in agreement with the variations in the film area.


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Figure 3 The photo images of the films before (a) (the coverage ratio was closed to 1) and after addition of the silicone oil with final concentrations of 0.21 mg/mL (b) and 1.06 mg/mL (c) with a volume ratio of 2:1. The plots of the surface tension (black) and the coverage ratio (red) v.s. the silicone oil concentration (d).


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Figure 4 The UV-vis spectra of the Au nanoparticle films in the presence of silicone oil with different concentrations. The inset shows the plot of λmax v.s. silicone oil concentration.

Figure 5 showed TEM images of the films prepared with different concentrations of silicone oil. Compared to disarranged distribution in the absence of silicone oil, the Au nanoparticles were relatively orderly with 0.01 mg/ml silicone oil with many voids (Figure 5a). With increasing silicone oil concentration, the void spaces were filled with more nanoparticle coverage (Figure 5b). Only a few defects were observed when the concentration reached 1.06 mg/mL. The nanoparticles almost fully covered the surface as a monolayer as shown in Figure 5c. Further concentration increased lead to nanoparticle overlapping and the formation of a multilayer film (Figure 5d).


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Figure 5 TEM images of the films fabricated with different concentrations of silicone oil: (a) 0.01 mg/mL; (b) 0.21 mg/mL; (c) 1.06 mg/mL; and (d) 5.3 mg/mL.

Transfer of the Au nanoparticle film to a solid substrate and applications in catalysis: The prepared Au nanoparticle film could be transferred to the solid substrate via a simple dip into the solution followed by pull out. Macroscopically, the film on the substrate was intact (Figure 6a). Microscopically, the nanoparticles were arranged closely and uniformly in the film (seen in situ AFM in Figure 6b). These observations indicated that the compact Au nanoparticle mononlayer on the solvent could be transferred in pristine form to the solid surface to fabricate a high-quality nanoparticle film.

The resulting Au nanoparticle film demonstrated outstanding catalytic properties.19 Prior to catalysis experiments, the DDT ligands on the nanoparticles surface were removed by plasma


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treatment to facilitate access of the reactant molecules to the Au surface. After that, the intact film and nanoparticle arrangement were maintained. However, the surface of the film became hydrophilic suggesting that the hydrophobic alkyl chains of DDT were etched out. The color of the film also changed from purple to bluish-purple. This was attributed to the variation of the reflective index from organic ligands to air. The classic reaction of 4-nitrophenol (4-NP) reduced to 4-aminophenol (4-AP) helped evaluate the catalytic performance of the film.20 This reaction was

typically monitored

via UV-vis

spectroscopy because nitrophenolate ions

4-

nitrophenolateions and 4-AP display absorption bands centered at 400 nm and 300 nm, respectively (presented in Figure 4c). There was a gradual decrease in the 4-nitrophenolateion band and a gradual increase in the intensity of the 4-AP band during reaction monitoring. This monitored the catalytic conversion of 4-NP to 4-AP.

Due to the large excess of NaBH4 relative to 4-NP, the reaction was fitted with pseudo-firstorder kinetics. Figure 4d showed the reaction kinetics through a plot of the absorption intensity at 400 nm vs. the reaction time as well as the fitted with the first order rate equation. The determined rate constant is Kapp=LN(C0/Ct), where Ct is the concentration of 4-NP at a time t, and C0 is the concentration of 4-NP at the beginning. The complete conversion was achieved within 90 min with a Kapp=6.0*10-4 s-1 as presented in the inset of Figure 4d and the conversion efficiency was as high as 98%. The excellent catalytic performance of the film may be attributed


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to the compact array of the Au nanoparticles, which allowed effective contact with the reactants and catalysis of the reaction.

Figure 6 (a) images of the films deposited on the substrates, (b) AFM image of film deposited on the substrate after plasma cleaning.

Figure 7 (a) Time evolution of the UV absorption spectra showing the gradual decrease in the 4nitrophenolate ions (peak at 400 nm) and the gradual production of 4-AP (peak at 300 nm). (b) Kinetic trace of the absorbance at 400 nm during the reaction of 4-NP and the best fit of a first


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order equation to the experimental data (red line). The inset in d shows the plot of LN(C0/Ct) vs. time (min).

Conclusion

We developed a method to fabricate a compact nanoparticle film via a silicone oil surfactant to compress the well-dispersed nanoparticles on the solvent surface. The resulting film could be transferred to the solid substrates and may be further applied in many fields such as catalysis. This fabrication method is simple, rapid and facile. There was no need for special or expensive instruments, no requirements for demanding preparation conditions, and no limitations towards substrates. The method is quite adaptable.

One of the major challenges in nanoparticle assembly is batch-to-batch variation. Nanoparticles synthesized under the same conditions often cannot be assembled with the same assembly conditions because of this reason. Our methods allow nanoparticles from different batches to be used because we could adjust the mixture ratio to regulate the nanoparticles and reach desired dispersion status. We could then adjust the concentration of the surfactant to obtain the desired film. None of these adjustments required any instrument characterization. Naked-eye observations could completely monitor the reaction. The synthesized Au nanoparticles were somewhat different from batch-to-batch. However, only simple adjustments still allowed us to create compact films. We also tried Au nanoparticles prepared via other methods. For example, d < 2 nm Au nanoparticles were synthesized with a large excess of stabilizers.21 The desired


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monolayer film could still be created through dispersion on the solvent surface and the addition of surfactants indicating the universality of the method. This approach is a promising alternative to the fabrication of nanoparticle films. It has great potential in optical devices, catalysis, sensors, etc.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (Grant no. 21404021, 21503037, 51601032), the Doctoral Scientific Research Foundation of Liaoning Province (20141013, 201501149), Fundamental Research Funds for the Central Universities (N150504004, N150504005, N140502001, N140503001, N160504002) and the Open Project of the State Key Laboratory of Supra molecular Structure and Materials (sklssm201705).

Supporting Information Available: Supporting Information includes the videos of the change of the Au nanoparticles dispersion after the addition of silicone oil and the response of the films fabricated from two different solvents ratio under the disturbance of air flow. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Pileni, M. P. Self-assembly of inorganic nanocrystals: Fabrication and collective intrinsic properties. Accounts Chem Res 2007; 40: 685-93. (2) Prasad, B. L. V.; Sorensen, C. M.; Klabunde, K. J. Gold nanoparticle superlattices. Chem Soc Rev 2008; 37: 1871-83.


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(20) Christophe Deraedt, L. S., Sylvain Gatard, Roberto Ciganda,; Ricardo Hernandez, J. R. a. D. A. Sodium borohydride stabilizes very active gold nanoparticle catalysts. ChemComm 2014; 50: 14194-6. (21) Martin, M. N.; Li, D. W.; Dass, A.; Eah, S. K. Ultrafast, 2 min synthesis of monolayerprotected gold nanoclusters (d < 2 nm). Nanoscale 2012; 4: 4091-4.


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An Approach to Fabricate Compact Gold Nanoparticles Film with the

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