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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Structure and Catalytic Activities of Gold Nanoparticles Protected by Homogeneous Polyoxyethylene Alkyl Ether Type Nonionic Surfactants Shiho Yada, and Tomokazu Yoshimura Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00142 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019
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Structure and Catalytic Activities of Gold Nanoparticles Protected by Homogeneous Polyoxyethylene Alkyl Ether Type Nonionic Surfactants Shiho Yada, and Tomokazu Yoshimura* Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoyanishi-machi, Nara 630-8506, Japan
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ABSTRACT
Gold nanoparticles were prepared in aqueous solutions containing four homogeneous polyoxyethylene (EO) alkyl ether type nonionic surfactants: octaoxyethylene dodecyl ether (C12EO8), methoxy-octaoxyethylene dodecyl ether (C12EO8OMe), ethoxy-octaoxyethylene dodecyl ether (C12EO8OEt), and trioxypropylene-octaoxyethylene dodecyl ether (C12EO8PO3). The sizes of obtained gold nanoparticles were almost independent of the terminal group in the EO surfactants; and the average sizes of nanoparticles prepared by surfactants with hydroxy, methoxy, ethoxy, and trioxypropylene terminal groups at [surfactant]:[Au+3] = 1:1 were 5.1 ± 1.2, 8.1 ± 1.4, 6.4 ± 2.1, and 8.6 ± 2.9 nm, respectively. The gold nanoparticles easily aggregated together according to the increasing hydrophobicity of hydroxy < methoxy ethoxy < trioxypropylene terminal groups. Highly stable dispersed nanoparticles were observed with hydroxy group in the EO terminal group. On the other hand, introducing hydrophobic moiety to the hydroxy group resulted in aggregated nanoparticles because of the interaction between the hydrophobic groups of protective agent for the gold nanoparticles. For the reduction reaction of p-nitrophenol and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging reaction, catalytic activities of the prepared gold nanoparticles decreased by the introduction of methoxy, ethoxy, or trioxypropylene to the hydroxy group of the EO type surfactant. Thus, a significant correlation was observed between the structure of gold nanoparticles and their catalytic activities.
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INTRODUCTION
Surfactants are extensively used as protective agents to stabilize metal nanoparticles. Esumi et al. had prepared metal nanoparticles by using cationic (hexadecylpyridinium chloride and hexadecyltrimethylammonium chloride), anionic (sodium dodecyl sulfate), and nonionic (polyoxyethylene nonylphenyl ether) surfactants; and investigated the effect of surfactant molecular structure on the nanoparticle properties. 1–3 When using cationic surfactants, the gold nanoparticles had a mean size of 20–30 nm. Sakai et al. reported that metal nanoparticles could be prepared by simply mixing a metal salt (HAuCl4 and AgNO3) solution with a Pluronic-type surfactant, i.e. a block copolymer consisting of polyoxyethylene and polyoxypropylene, in the absence of reducing agents and without external force.4
In recent years, the thermoresponsive assembly of gold nanoparticles also has been studied. 5–6 Iida et al. revealed that introduction of hydrophobic groups such as methyl, ethyl, and isopropyl to the terminal hydroxy group of polyoxyethylene chain of hexa(ethylene glycol)-undecanethiol could control the assembly temperature depending on the structure of terminal groups. 5 Yusa et al. revealed that the size of gold nanoparticles coated with poly(N-isopropylacrylamide) was significantly affected by temperature upon addition of the salt to the nanoparticle solution. 6
Gomez-Grana et al. prepared gold nanorods surrounded by cationic monomeric surfactant of hexadecyltrimethylammonium bromide and gemini surfactant of (oligooxa)alkanedyl-α,ωbis(dimethylhexadecylammonium) bromide. They characterized the precise thickness and density of the protective agents using transmission electron microscopy (TEM), small-angle Xray scattering (SAXS), and small-angle neutron scattering (SANS) techniques. 7 Maeta et al. reported that stable silver nanoparticles were obtained in polar and non-polar solvents by using
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polyoxyethylene octylamine. 8 Ray et al. prepared highly concentrated gold nanoparticles at room temperature from hydrogen tetrachloroaureate (III) hydrate (HAuCl4·3H2O) using block copolymer P85 (EO26PO39EO26) in aqueous solution. 9 Dey et al. revealed that when hyperbranched polymer were added to gold nanoparticles stabilized by citric acid, the nanoparticles self-assembled and the self-assembly could be controlled by changing the number of branch ends of the hyperbranched polymer and the particle size. 10 Sabir et al. showed that the morphology and size of gold nanoparticles could be easily changed by adjusting the ratio of mixtures of triblock copolymers (L31 and F68) and Au (III). 11
Compared to their bulk counterparts, metal nanoparticles generally exhibit different electromagnetic, chemical, thermal, and optical properties because of the increased surface area and surface free energy. 12 For example, while gold is generally considered to be very stable chemically in the bulk, Haruta et al. found that gold nanoparticles exhibited high catalytic activities in several reactions. 13 It was reported for the first time by Pal et al. 14 and Esumi et al.15 that gold nanoparticles do not participate in the reduction reaction of p-nitrophenol or the radical scavenging activity of 1,1-diphenyl-2-picrylhydrazyl (DPPH). Rather, it only acts as a catalyst to lower the activation energy of the reaction, which was made clear by many researchers. 16, 17 The nanocomposites of gold with titanium (IV) oxide, 18 dendrimer, 19–20 and biopolymer 21–23 have also been revealed to exhibit high catalytic activities and radical scavenging activities.
Our research group was the first to study homogeneous polyoxypropylene-polyoxyethylene (PO-EO) type nonionic surfactants. We have reported the syntheses and solution properties of homogeneous PO-EO alkyl ether type nonionic surfactants24–26 and alkoxy group-modified EO alkyl ether type nonionic surfactants27 with single EO and PO chain length distributions, in
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which a PO chain or an alkoxy group was introduced to the terminal hydroxy group of EO chain of homogeneous EO type surfactant. These surfactants have excellent properties such as adsorption at the air/water interface and aggregation in aqueous solution compared with the corresponding EO type surfactants, despite their complex hydrophobic-alkyl-chain/hydrophilicEO-chain/hydrophobic-PO-chain structures. Our previous research revealed the usefulness of these surfactants in a variety of industrial fields, including as a protective agent for gold nanoparticles. However, little has been reported about the properties of gold nanoparticle prepared by using these alkyl ether type nonionic surfactants.
In this study, gold nanoparticles were prepared with a homogeneous EO alkyl ether type nonionic surfactant (octaoxyethylene dodecyl ether, C12EO8), two homogeneous alkoxy groupmodified EO alkyl ether type nonionic surfactants (alkoxy (methoxy, and ethoxy)octaoxyethylene dodecyl ether, C12EO8OR, R = Me and Et), and a homogeneous PO-EO alkyl ether type nonionic surfactant (trioxypropylene-octaoxyethylene dodecyl ether, C12EO8PO3). Figure 1 shows the structures of these nonionic surfactants. The nanoparticle structure and their catalytic activities were correlated with each other for the various protective agents. The effects of different terminal groups of EO chain and surfactant concentrations on the catalytic activities of the gold nanoparticles were also studied.
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C12H25 (OCH2CH2)8 OH C12EO8
C12H25 (OCH2CH2)8 OCnH2n+1 C12EO8OR
C12H25 (OCH2CH2)8 (OCH2CH)3 OH CH3 C12EO8PO3 Figure 1. Structures of homogeneous EO type nonionic surfactants C12EO8, C12EO8OR (R = Me (CH3) or Et (C2H5)), and C12EO8PO3.
EXPERIMENTAL SECTION Materials
The homogeneous EO type nonionic surfactant (C12EO8) was supplied by Nikko Chemicals Co., Ltd. (Tokyo, Japan), and used as received. The homogeneous alkoxy-EO type nonionic surfactants (C12EO8OMe, C12EO8OEt) and the homogeneous PO-EO type nonionic surfactant (C12EO8PO3) were synthesized according to our previous report. 24 Tetrachloroauric acid tetrahydrate (HAuCl4·4H2O) was obtained from Tanaka Kikinzoku Kogyo KK (Tokyo, Japan). p-Nitrophenol was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), and sodium borohydride and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical were obtained from FUJIFILM Wako Pure Chemical Co., Ltd. (Osaka, Japan). The gold nanoparticle solutions were prepared using Merck KGaA Milli-Q Plus water (resistivity = 18.2 MΩ cm, Darmstadt, Germany).
Preparation of Gold Nanoparticles
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The gold nanoparticles were prepared by the chemical reduction of a metal salt and nonionic surfactant mixtures with sodium borohydride. In a typical experiment, 2.0 mL of freshly prepared 10.0 mmol/L HAuCl4·4H2O was added to 17.2 mL of nonionic surfactant solutions of various concentrations (1.16, 4.65, and 9.30 mmol/L), and the solutions were stirred for 30 min. Then, 0.8 mL of 250 mmol/L freshly prepared ice-cold sodium borohydride was quickly added to the solutions and stirred for 30 min. The final concentration of gold was 1.0 mmol/L, and the final concentration ratio of surfactant to gold was 1:1, 4:1, and 8:1. We used the gold nanoparticles without purification, and analyzed them by UV-visible spectroscopy (UV-vis, UV– 2400PC, Shimadzu Corporation, Kyoto, Japan), cryogenic transmission electron microscopy (cryo-TEM, JEOL JEM-2100F(G5), Tokyo, Japan), and SAXS. SAXS was conducted on the instrument installed at the BL40B2 beamline in SPring-8 (Hyogo, Japan). The X-ray wavelength was 0.7 Å, and the sample-to-detector distance was 2.0 m.
Reduction Reaction of p-Nitrophenol
In a standard quartz cuvette with a 1 cm path length, 0.3 mL of 1.0 mmol/L gold nanoparticle solution, 0.3 mL of 10 mmol/L p-nitrophenol solution, and 1.4 mL of pure water were combined and stirred. Then, to the mixed solution was added 1.0 mL of 150 mmol/L ice-cold sodium borohydride solution. The absorption spectra at 400 nm were recorded against time at 25 °C, and the decrease in peak intensity was followed. The final concentrations of gold nanoparticles, pnitrophenol, and sodium borohydride were 0.10, 1.0, and 50 mmol/L, respectively.
DPPH Radical Scavenging Reaction
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DPPH was dissolved in a 80 vol% ethanol-20 vol% tris buffer mixture. A 1.0-cm quartz cuvette was added with 2.0 mL of DPPH solution (0.3 mmol/L, pH 7.4), and settled in the holder of the UV-vis instrument. Then, UV-vis measurement was started immediately. The absorbance at 523 nm was recorded in time at 25 °C. As the DPPH radical was scavenged by an antioxidant and transformed to 1,1-diphenyl-2-picrylhydrazyne (DPPH-H) (Scheme 1), the color of the solution turned from purple to yellow, which was detected by the decay in absorbance at 523 nm. 18, 28–30
N
N O2N
N
NO2
Tris buffer (pH 7.4) AuNP
O2N
NH
NO2
NO2
NO2 Scheme 1. Radical scavenging reaction investigated in this study.
The scavenging reaction of DPPH radical (DPPH ·) by antioxidant AH is represented by the following formula.
DPPH∙ + AH → DPPH–H + A∙
(1)
The generated radical A · is stable, its side reactions with additional DPPH · or itself (as shown by the following formulae) can be ignored. 31, 32
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DPPH∙ + A∙ → DPPH–A
(2)
A∙ + A∙ → A–A
(3)
RESULTS AND DISCUSSION
Characterization of Gold Nanoparticles Protected by Surfactants Addition of sodium borohydride to the mixed solutions of homogeneous EO type nonionic surfactants (C12EO8, C12EO8OMe, C12EO8OEt, and C12EO8PO3) and tetrachloroauric acid tetrahydrate changed the color of solution from pale yellow to wine red. Figure 2 shows the UVvis spectra of gold nanoparticles prepared with [surfactant]:[Au+3] = 1:1, 4:1, and 8:1. For the nanoparticles protected by C12EO8, the peak of absorption band based on surface plasmon was observed at 523–527 nm. In the cases of C12EO8OMe, C12EO8OEt, and C12EO8PO3, the peak absorption was red-shifted compared with that of C12EO8. At any [surfactant]:[Au+3] ratio, the width of the absorption band became broader in the order of C12EO8 < C12EO8OMe < C12EO8OEt
