Synthesis of Prolate-Shaped Au Nanoparticles and Au Nanoprisms

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Article Cite This: ACS Omega 2019, 4, 7874−7883

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Synthesis of Prolate-Shaped Au Nanoparticles and Au Nanoprisms and Study of Catalytic Reduction Reactions of 4‑Nitrophenol Soo Ik Park and Hyon-Min Song* Department of Chemistry, Dong-A University, Busan 604-714, South Korea

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S Supporting Information *

ABSTRACT: The growth into anisotropic one or two dimensions is important in plasmonic gold nanomaterials because extinction occurs along multiple axes and sometimes the resonance extends to the near-infrared region. The surfactant mixture of cetyltrimethylammonium bromide (CTAB) and Pluronic triblock copolymers has been recently demonstrated to be efficient anisotropic soft templates for the growth of noble metal nanomaterials. Seed-mediated growth of two types of anisotropic Au nanoparticles is achieved in this study. One is one-dimensional prolate-shaped Au nanoparticles with the average aspect ratios of 2.29 and 2.59, and the other is two-dimensional gold nanoprisms with the average edge length of 50.4 nm. These anisotropic structures are believed to be produced by the tendency of Pluronic copolymers to be micellized anisotropically at the elevated temperatures and by the preference for being lamellar mesophases in the phase diagrams when the concentration is highest. When prepared in the surfactant mixture of CTAB and L-64 (17.9%), Au nanoparticles containing spherical particles (27.9 nm) as the major products show the best catalytic performances in the reduction reactions of 4-nitrophenol.

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INTRODUCTION The shape of plasmonic metal nanoparticles (NPs) such as gold, silver, and copper is an important factor to determine the optical extinction of these materials. Resonance with the specific wavelength of light occurs on the surface of these NPs, and the shape along with the size and the surrounding medium determines these plasmon resonance modes. Gold nanomaterials draw broad interest from nanomedicine to plasmonics due to their relatively low toxicity and diverse resonance modes extending to the near-infrared region.1,2 The synthesis is also active, and designing anisotropic gold NPs can be assisted using micellar soft templates. Pluronic triblock copolymers have been known as early as the 1950s and recognized as one of the most popular polymer surfactants. They are composed of poly(ethylene oxide) (PEO)−poly(propylene oxide) (PPO)−PEO triblocks. They are biocompatible and accordingly they are used as the cell culture medium and for many biological purposes such as drug delivery vehicles.3 Pluronic copolymers are easily organized into amphiphilic micelles with hydrophobic PPO cores surrounded by hydrophilic PEO coronas. When another surfactant is added to amphiphilic copolymers, the mixture of block copolymers and surfactants tends to form anisotropic micelles by the strong interaction of charged surfactants on polymer micellar aggregates. 4 In fact, cetyltrimethylammonium bromide (CTAB) and Pluronic copolymers are not simply mixed, as the alkyl chains of CTAB and methyl groups in PPO cores are self-assembled into hydrophobic cores and the head groups of CTAB are placed in the interface between PPO cores and PEO coronas.5 The rheological study also proved the easy formation of micellar © 2019 American Chemical Society

self-assembly between CTAB and Pluronics with a sufficiently high concentration of Pluronics.6 With regard to the formation of anisotropic soft templates, Pluronic copolymers are organized into rod-like micelles or cylindrical micelles in hexagonal symmetry at the elevated temperature.7,8 Due to this dynamic soft nature of micelles according to the surrounding environment, Pluronic copolymers can be used in the shapecontrolled synthesis of metal NPs. Seedless preparation of anisotropic Pd nanorods was achieved at 65 °C in the surfactant mixture of CTAB and Pluronics,9 and a rare synthetic method of Pd@Au core−shell nanorods was studied in the mixture of CTAB and Pluronics.10 In addition, utilizing the ability of Pluronics as the metal-coordinating ligands, micron-scale anisotropic rods and plates were generated by the self-assembly into hexagonal or lamellar mesophases in the presence of Ag(I).11 A similar synthetic approach is extended to gold nanomaterials, and we discovered that prolate-shaped Au NPs were prepared with Pluronic F-127 (17.9%) and relatively small Au nanoprisms with an edge length of 50.4 nm were prepared with Pluronic L-64 (35.7%) in the seedmediated method. The synthesis was performed at 43 °C, which is not common in the synthesis of anisotropic gold NPs in aqueous medium. With the strong temperature sensitivity and with the ability for guiding the arrested growth within the micelles, the surfactant mixture of CTAB and Pluronics can serve as soft templates for various particle morphologies. The relatively pronounced catalytic activity was also seen for these Received: March 8, 2019 Accepted: April 18, 2019 Published: April 30, 2019 7874

DOI: 10.1021/acsomega.9b00647 ACS Omega 2019, 4, 7874−7883

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Scheme 1. Illustration for the Synthetic Procedures of Gold Nanoprisms and Prolate-Shaped Gold Nanoparticles

Figure 1. (a, b, d, e) TEM images of prolate-shaped gold NPs with the aspect ratio of 2.29 (Au1). (c) Electron diffraction pattern of the square region displayed in (b). (f) Electron diffraction pattern of the square region displayed in (e).

atures.8,14 When Pluronic F-127 (17.9%) was used, spherical micelles are assumed to be produced at 43 °C in which hydrophobic PPO cores were surrounded by the hydrophilic PEO coronas. Hydrophobic micellar cores are known to contain around 20% of aqueous solution and they are not perfectly dehydrated.15 Most of Au cations before the addition of Au seeds are assumed to reside inside the hydrophobic cores by complexation with ethylene glycol. The growth of prolateshaped gold NPs commences within the hydrophobic micellar cores in a seed-mediated method. With the increase of temperature, the micelles become larger due to the increase of hydrophobicity and to minimize the structural instability of micelles, they elongate further to be the prolate ellipsoidal shape.8 Therefore, NPs with the higher aspect ratio (2.59) were obtained at 43 °C. In fact, the neutron scattering study shows that Pluronic micelles prefer to be ellipsoidal shape to minimize the repulsion between micelles at short distances.16

Au NPs compared to CTAB-coated Au NPs. It is supposed that 4-nitrophenolate ions are concentrated near the surface of Au NPs, which is assisted by the hydrophobic and electrostatic interactions between 4-nitrophenolate ions and the selfassembled micelles constructed from the surfactant mixture of CTAB and Pluronics.

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RESULTS AND DISCUSSION Scheme 1 illustrates the process for the growth of gold nanoprisms and prolate-shaped gold NPs. In the phase diagram of Pluronics, isotropic spherical micelles are formed when the concentration is over the critical micelle concentrations, and with the increase of temperature or concentration of Pluronics, several lyotropic liquid crystalline phases are observed. Usually isotropic lamellar mesophases are found next to the spherical micelles,13 and crystalline cubic, hexagonal, and inverse hexagonal mesophases are observed at elevated temper7875


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Table 1. Experimental Conditions for the Synthesis of Au Nanomaterials

Au1 Au2 Au3 Au4 Au5 Au6 Au7

pluronic copolymers

concentration and the amount of HAuCl4·3H2O

F-127 (17.9%) F-127 (17.9%) L-64 (35.7%) L-64 (17.9%) L-64 (17.9%) L-64 (17.9%) none

0.036 M, 0.5 mL 0.036 M, 0.5 mL 0.036 0.036 0.036 0.036 0.036

M, M, M, M, M,

0.5 0.5 0.5 0.5 0.5

mL mL mL mL mL

shape and dimension of Au nanomaterials (width × height)

growth temperature (°C)

0

prolate shape, 25.6 nm (±3.42) × 57.9 nm (±9.46)

23

0

prolate shape, 27.0 nm (±4.31) × 69.2 nm (±16.0)

43

nanoprisms, edge length 50.4 nm (± 11.4) low symmetric nanoparticles mixture of nanoprisms, nanospheres and nanorods mixture of platelets and nanospheres mixture of nanospheres and nanorods

43 43 43 23 43

amount of AgNO3 (0.400 mM) (mL)

0 0.5 0 0 0

Figure 2. (a, b) TEM images of prolate-shaped gold NPs with the aspect ratio of 2.59 (Au2). (c, d) TEM images of gold nanoprisms with an average edge length of 50.4 nm (Au3).

(±3.42) × 57.9 nm (±9.46) (width × height). Two regions of NPs were examined by their electron diffraction patterns (Figure 1b,e), and both display the same patterns of two zones of and , which were shown by rectangle and square red lines, respectively (Figure 1c,f). These two zones are characteristic of penta-twinned Au nanorods which are grown in a seed-mediated method.17,18 The structure of seeds are twinned structures, and the twinned seeds determine the structure of the final product. It is hard to prepare single crystalline Au nanomaterials, and the twin structures are widely observed even in 2 nm Au NPs. Another prolate-shaped gold NPs were prepared similarly, but the growth temperature was 43 °C (Figure 2a,b). The average size is 27.0 nm (±4.31) × 69.2 nm (±16.0) (width × height) with the aspect ratio of 2.59. The large size and the high aspect ratio are believed to be due to the increase of hydrophobicity with increasing temperature. The micellar core radius increases with increasing

When Pluronic F-64 (35.7%) was used, layer-by-layer lamellar mesophases were believed to be produced because the anisotropic lamellar phases are normally observed when the concentration of Pluronics is highest.13 Similar to the selfassembly of individual amphiphiles, the surfactant mixture of CTAB and Pluronic F-64 (35.7%) develops into the anisotropic soft templates with layer-by-layer phases, and the growth of gold nanoprisms proceeds in the region of hydrophobic lamellar layers. The relatively small size of nanoprisms is due to the size of lamellar micelles. Prolate-shaped gold NPs with the aspect ratio of 2.29 (Au1) were observed in transmission electron microscopy (TEM) images (Figure 1a,d). They were prepared in a seed-mediated method at room temperature with Pluronic F-127 (3.0 mL, 17.9%), CTAB (3.0 mL, 0.2 M), HAuCl4·3H2O (0.5 mL, 36 mM), and ascorbic acid (0.65 mL, 0.568 M). Table 1 summarizes the synthetic conditions for the synthesis of Au nanomaterials in this study. The average size is 25.6 nm 7876


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Figure 3. (a) TEM image, and (b) the simulation of Moiré fringes in (a). (c) the TEM image and (d) the simulation of Moiré fringes in (c).

Figure 4. (a) TEM image of low symmetry gold NPs when AgNO3 (0.5 mL, 0.400 mM) was added to Pluronic L-64 (17.9%, Au4). TEM images of the mixture of spheres, rods, and prisms synthesized with L-64 (17.9%) (b) at 43 °C (Au5) and (c) at 23 °C (Au6). (d) TEM image and (e) simulation of Moiré fringes of platelets of Au5.

temperature, and the large size of micelles prefers the ellipsoidal shape. Gold nanoprisms with an average edge length of 50.4 nm (±11.4) were prepared with Pluronic L-64 (35.7%, Figure 2c,d). The synthesis of nanoprisms was conducted at 43 °C. Generally, anisotropic gold NPs do not maintain shape stability

and change into spherical shape at elevated temperatures. Seed-mediated synthesis in the literature, which uses iodide ions as the shape directing agents, does not produce nanoprisms over 30 °C.19 However, the preference of Pluronic copolymers for being anisotropic micelles with the increase of temperature causes the formation of two-dimensional aniso7877


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Figure 5. Optical absorbance spectra of (a) prolate-shaped gold NPs with the aspect ratio of 2.29 (Au1, black line) and 2.59 (Au2, red line), (b) gold nanoprisms (Au3), (c) low symmetry Au NPs (Au4), and (d) the mixture of spheres, rods, and prisms synthesized at 43 °C with L-64 (17.9%, Au5, black line), and Au NPs synthesized at 23 °C with L-64 (17.9%, Au6, red line).

average edge length of nanoprisms in this study is 50.4 nm and the thickness is 13 nm. Moiré fringes in the local region within one Au nanopropeller was reported,27 but Moiré fringes of gold nanomaterials by the entire overlap of two nanoprisms such as in Figure 3 are rarely seen. There are two types of Moiré fringes in gold nanoprisms in this study. One is contributed by three lattice planes, which are composed of all {220} planes (Figure 3a). The other type is contributed by one lattice plane of (220) (Figure 3c). The Moiré fringe, as shown in Figure 3a, is rarely observed because it only occurs when the orientation of two superimposed nanoprisms are the same, and at the same time when the electron beam of TEM is perpendicular to the surface {111} planes of nanoprisms. Two nanoprisms are superimposed with the rotation of 64° (Figure 3b) and in this case, the sizes of two nanoprisms are different. As shown in Figure 3d, however, the slight rotation of 1.3° produces Moiré fringes from two nanoprisms with similar sizes. When Pluronic L-64 (17.9%) was used, the mixture of spheres, rods, and prisms was obtained (Au5, Figure 4b). Close examination reveals that faceted particles contain twin boundaries, but some particles are spherical plates without twin boundaries. Moiré fringes are also observed by (220) lattice planes (Figure 4d,e). Compared to Moiré fringes in nanoprisms in Figure 3c, the distance between the fringes is exceptionally small (6.7 Å, Figure 4e), which were produced by the rotation of 10.9° of (220) lattice planes. The same experiment as the synthesis of gold nanoprisms was conducted at the growth temperature of 23 °C with Pluronic L-64 (17.9%) and the plate NPs such as hexagonal prisms and triangular prisms were found along with spherical NPs as the major products (Au6, Figure 4c). The effect of AgNO3 was also investigated by adding AgNO3 (0.5 mL, 0.400 mM) to the reaction mixture containing Pluronic L-64 (17.9%) and by conducting the growth at 43 °C. Irregular shape particles with low symmetry were obtained (Au4, Figure 4a). The average end-to-end distance along the long axis is 26.1 nm (±4.5), and

tropic plates in this study. Gold nanoprisms are useful mostly for sensing applications, and they gain attention these days, as the electrons and holes at the hot spots are promising for the photothermal effect to increase local temperatures,20 or as contrast agents for optical coherence tomography.21 Diverse methods for the preparation of these platelet Au nanoprisms have been reported. The classic seed-mediated method using iodide ions produces gold nanoprisms with the edge length of 98 nm.19 Utilizing the ability of certain halide or metal ions to adsorb more strongly on certain facets, {111} planes in the case of nanoprisms, and to inhibit the growth perpendicular to those facets is the popular method to prepare gold nanoprisms. Organic solvents of acetonitrile and trioctylamine are also known to produce gold nanoprisms with an edge length of 28 nm in a seedless method.22 The seed-mediated photochemical method using a halogen lamp has been reported recently to produce gold nanoprisms with an edge length of 496 nm.23 In addition, the seedless method has been adopted using Pluronic copolymers to prepare macroscale hexagonal and triangular prisms with an edge length of over 100 nm.24 There are not many studies of obtaining relatively small size gold nanoprisms in aqueous medium. The importance of smaller size anisotropic gold nanomaterials is that it has stronger absorption than scattering in their extinction spectra,25 and if absorption takes place in the near-infrared region, it has applications in biological studies. Two superimposed gold nanoprisms with the same orientation generate Moiré fringes (Figure 3). Moiré fringes are well observed in the core−shell noble metal nanomaterials due to the similar face-centered cubic symmetry and also due to the small difference of cell parameters among them.26 However, Moiré fringes in superimposed gold nanomaterials are not commonly observed. High electron density prevents electron beam from penetrating through gold nanomaterials, and the preference for the faceted shapes due to the twinned structures also inhibit the correct alignment of NPs. The 7878


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Figure 6. Time-dependent absorption spectra for the catalytic reduction reactions of 4-nitrophenol according to the addition of (a) prolate-shaped gold NPs with the aspect ratio of 2.29 (Au1), (b) prolate-shaped gold NPs with the aspect ratio of 2.59 (Au2), (c) gold nanoprisms (Au3), (d) low symmetric Au NPs (Au4), (e) the mixture of spheres, rods, and prisms synthesized at 43 °C with L-64 (17.9%, Au5), and (f) Au NPs synthesized at 23 °C with L-64 (17.9%, Au6).

With these Au NPs as catalysts, reduction reactions of 4nitrophenol to 4-aminophenol were studied in the presence of aqueous NaBH4 solution. This reaction is now a universal catalytic reaction for evaluating catalytic activities of the newly synthesized metal NPs in the fastest and easiest way. It is known that without metal catalysts, aqueous NaBH4 alone cannot reduce 4-nitrophenol.29 The general concepts of the catalytic reduction reaction of 4-nitrophenol by nanosize metal NPs are summarized in the recent review.30 Without possessing good or excellent hydrogen hosting properties, Au catalyzes this reduction reaction quite efficiently and also differently from Pt, Pd, and Ir. The formation of the nitroso compound as the intermediate rather than the direct formation of phenylhydroxylamine is the major difference in the reaction mechanism between Au and the other metals.31 The catalytic reaction using Au NPs in this study completes mostly within 5 min (Figure 6). The ratio between the number of moles of NaBH4 and 4-nitrophenol is 500, which is the same in the study with CTAB-coated Au NPs32 and similar to those in other studies. In the case of prolate-shaped Au NPs as the catalysts, the rate constants of 0.0093 s−1 (Au1, aspect ratio 2.29) and 0.0080 s−1 (Au2, aspect ratio 2.59) were found (Figure 7, Table 2). The faster rate constant in Au1 is believed due to the smaller size of Au NPs. In fact, this reduction reaction is sensitive to the size of the catalysts and also to the surfactants surrounding the catalysts.33 Relatively large Au nanoprisms (Au3) show slow reaction kinetics with the rate constant of 0.0060 s−1 and the rate constant of the reaction using low symmetry Au NPs (Au4) as catalysts is 0.0178 s−1. The faster reaction in Au4 is believed not from the effect of silver, but it is due to the irregularity of the shape and the smaller size compared to prolate Au NPs. A strong reducing agent such as NaBH4 causes another homogeneous nucleation

they are much smaller than the normal gold nanostars or branched-shape gold NPs. For these Au nanomaterials, optical absorbance was measured in a standard quartz cuvette with a 1 cm cell path length. In prolate-shaped gold NPs with the aspect ratio of 2.29 (Au1, Figure 5a, black line), two plasmon resonance modes are observed with the transverse mode at 529 nm and the longitudinal mode at 675 nm. The relatively weak longitudinal mode is due to the small aspect ratio, and the absorbance from spherical shape NPs also affects stronger absorbance at 529 nm. Usually, the longitudinal modes are stronger than the transverse modes, therefore anisotropic gold nanorods with high aspect ratios show intense longitudinal resonances. In prolate-shaped gold NPs with the aspect ratio of 2.59 (Au2, Figure 5a, red line), a slight red-shift of the longitudinal mode was observed at 696 nm, whereas the transverse mode appears at 530 nm. The shoulder peak at 597 nm derives from the minor mixture of nanoprisms and spherical nanoplates as is shown in TEM images (Figure 2a,b). Gold nanoprisms with the edge length of 50.4 nm show the absorbance peaks at 538 and 611 nm (Au3, Figure 5b). When the size of gold nanoprisms is large, longitudinal modes are red-shifted. The small size of gold nanoprisms in this study is assumed to produce two resonance modes close to each other. Irregular low symmetric Au NPs show two resonance peaks at 567 and 680 nm (Au4, Figure 5c). The peak at 567 nm appears due to the transverse mode across the center of Au NPs and the peak at 680 nm is due to the longitudinal mode across the long axis of NPs.28 The mixture of prisms, rods, and spheres (Au5 and Au6, Figure 5d) produce peaks at 530 nm and around 645 nm. The weak peak at 645 nm is due to the longitudinal modes of platelets. 7879


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have not been studied for the catalytic reduction reactions of 4nitrophenol. The dispersity is also thought to affect the catalytic activities, as the layered alignment of prolate-shaped Au NPs provides spatial confinement for Au NPs and 4nitrophenolate ions. The normalized rate constant was also calculated by dividing the rate constant with the number of moles of 4-nitrophenol and is shown in Table 2. In the case of Au6, the normalized rate constant is 2.933 × 103 min−1 mmol−1 and it is quite a high value among Au NPs and other metal NPs prepared in the chemical method. For identifying the differential effect between the surfactant mixture of CTAB and Pluronics and each surfactant, control experiments were conducted without adding Au nanomaterials in the catalytic reduction reactions. Raw 4-nitrophenol in aqueous medium (2.0 mL, 0.27 mM) shows the absorption peak at 318 nm (Figure 8a), whereas the peak from 4nitrophenolate ions appears at 400 nm with the addition of NaBH4 (0.4 mL, 0.675 M, Figure 8b).33 The intensity of the peak at 400 nm hardly decreases with time, but the reduction reaction commences with the addition of CTAB (10 μL, 0.2 M, Figure 8c). This is quite strikingly intriguing since not many studies show the catalytic activities of CTAB toward the reduction reactions of 4-nitrophenol.35 New CTAB chemicals from Sigma-Aldrich and from DaeJung Chemicals (Seoul, South Korea) were tested, and display similar activities toward the reduction reactions. CTAB alone forms micelles and micellar catalysis was explained by the change of redox potentials of the substrate on the surface of CTAB micelles,36 such that the pKa of acids is lowered near CTAB micelles.37 However, it is known that CTAB as the stabilizer of metal NPs has negative effects on the catalytic activities by the inhibition of the movement of 4-phenolate ions to the metal surfaces with its cationic micellar structures.33 For the comparison of the surfactant effect, we prepared CTAB-stabilized mixture of gold nanospheres and nanorods in a similar method without adding Pluronics and studied their catalytic activities (Au7, Figure 9). The size of CTAB-stabilized nanospheres is 27.0 ± 3.6 nm, which is measured in the scanning electron microscopy (SEM) image (Figure 9a) and is comparable to Au5 (31.8 ± 3.7 nm) and Au6 (27.9 ± 3.9 nm). Absorbance spectrum indicates the major peak of 531 nm and the minor broad peak centered at 826 nm, which is derived from the longitudinal resonance from gold nanorods (Figure 9b). The rate constant was obtained from the successive decline of intensity at 400 nm in timedependent absorption measurements (Figure 9c), and it shows

Figure 7. Plots of ln(Ct/C0) against reaction time for the reduction of 4-nitrophenol in the presence of Au nanomaterials.

Table 2. Rate Constants for the Catalytic Reduction Reactions of 4-Nitrophenol by Au Nanomaterials Au1 Au2 Au3 Au4 Au5 Au6 Au7

k (s−1)

k (min−1)

k (min−1 mmol−1)

0.0093 0.0080 0.0060 0.0178 0.0261 0.0264 0.00167

0.558 0.480 0.360 1.068 1.566 1.584 0.100

1033 889 667 1978 2900 2933 186

of Ag nanoparticles from AgNO3 and during this process NaBH4 is consumed and the catalytic reaction is affected with the lower concentration of NaBH4. Ag NPs prepared in situ during the catalytic reaction show better catalytic activities than Au, Pd, and Cu NPs.34 Rather than the nature of each metal, the dispersity from the irregular shapes of Ag NPs affects this catalytic enhancement, whereas the aggregated states of Au, Pd, and Cu NPs prepared in situ by NaBH4 show poor catalytic activities.34 In Au5 and Au6, which were prepared in the presence of CTAB and L-64 (17.9%), fastest catalytic reactions are achieved with the rate constants of 0.261 and 0.264 s−1, respectively. The average particle size of spherical Au NPs in Au5 is 31.8 (±3.7) nm and the average size in Au6 is 27.9 (±3.9) nm. It is not certain whether the size, or the shape, or the dispersibility of Au5 and Au6 enhances the catalytic activities. The surrounding environment, which affects the interaction of the 4-hydroxylphenolate ion with metal surfaces, needs to be considered. Metal NPs surrounded by the surfactant mixture of CTAB and Pluronics

Figure 8. Time-dependent absorption spectra of (a) 4-nitrophenol, (b) 4-nitrophenol after the addition of NaBH4 (0.4 mL, 0.675 M), (c) the mixture of CTAB (10 μL, 0.2 M), NaBH4 (0.4 mL, 0.675 M), and 4-nitrophenol (2.0 mL, 0.27 mM). 7880


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Figure 9. (a) SEM image and (b) optical absorbance spectrum of the mixture of CTAB-coated gold nanospheres and nanorods (Au7). (c) Timedependent absorption spectra of catalytic reduction reactions of 4-nitrophenol using Au7 as catalysts. (d) Plot of ln(Ct/C0) against reaction time for CTAB only and Au7.

a significant decrease (k = 0.00167 s−1, Figure 9d) compared to those of gold NPs prepared in the surfactant mixture of CTAB and Pluronics. The rate constant of the catalytic reaction with CTAB (0.2 M) alone is also obtained (k = 0.00105 s−1, Figure 9d) and there is a slight difference of catalytic activities between CTAB only (10 μL, 0.2 M) and CTAB-coated gold NPs. CTAB alone is readily organized into micelles, but in the presence of ascorbic acid the micellar structure is believed to be disrupted by pH change, and the original micellar catalysis is interrupted. Surfactants affect the redox potentials of metal NPs and the enhanced catalytic activities are due to the alteration of the redox potentials of metal NPs by surfactants.38 It is also known that surfactants provide additional stability to metal NPs by removing the contaminated or poisoned metal surfaces after the catalytic reaction.39 In some redox reactions, the inverse micelles are known to enhance catalytic activities better than the normal micelles.40 Hydrophobic interactions between the substrate and the micellar hydrophobic areas of amphiphilic surfactants play the major roles for catalytic enhancements. The fast catalytic reactions by Au NPs surrounded by the surfactant mixture of CTAB and Pluronics in this study are contributed significantly by the hydrophobic interaction between 4-hydroxylphenolate ions and the self-assembled hydrophobic region of CTAB and Pluronics. During this interaction, 4-hydroxylphenolate ions, which are the precursors after the addition of NaBH4, are concentrated near the surface of Au NPs. The structure of Pluronic L-64 is EO13PO30EO13, and that of F-127 is EO100PO65EO100. Due to the large hydrophilic coronas in F-

127, 4-hydroxylphenolate ions are hardly in direct contact with the surface of metal NPs in a short time, and this affects the slower catalytic reactions in Au1 and Au2.

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CONCLUSIONS

Prolate-shaped gold NPs with the aspect ratios of 2.29 and 2.59 and triangular gold nanoprisms with the edge length of 50.4 nm were prepared in a seed-mediated method using the surfactant mixture of CTAB and Pluronics. Anisotropic NPs with a higher aspect ratio of 2.59 was prepared at 43 °C, which is extraordinary from the tendency of gold nanomaterials to be isotropic with the increase of temperature. In addition, the synthesis of gold nanoprisms with a relatively small edge length is interesting, as the method described in this study provides a simple approach without using certain additives or foreign ions for the preparation of gold nanoprisms in aqueous medium. The method requires more refinement as the products include other shapes of gold nanomaterials. However, Au nanomaterials surrounded by the surfactant mixture of CTAB and Pluronics show pronounced catalytic effects for the reduction of 4-nitrophenol, in which the best performance was achieved with 28 nm spherical particles. Considering the diversity afforded by many different molecular weights of Pluronics, soft templates generated from CTAB and Pluronics are thought to be applied for the anisotropic growth of other similar noble metal nanomaterials. 7881


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EXPERIMENTAL SECTION Pluronic copolymers and other chemicals were bought from Sigma-Aldrich. Optical absorbance spectra were measured using a JASCO V-770 spectrophotometer with a 1 cm cell path length and with a UV/vis bandwidth of 2.0 nm. JAC ultrasonic 1505 (150 W, 40 kHz) was used for sonication. TEM images were obtained with a JEOL JEM 2010FX operating at 200 kV. SEM images were obtained with a JEOL JSM 6700F with an operating voltage of 5.0 kV. Preparation of Gold Nanomaterials. The mixture of CTAB (3.0 mL, 0.2 M) and Pluronic L-64 (3.0 mL, 35.7%) was vortex-mixed for 15 s, followed by sonication at 30 °C for 10 min. HAuCl4·3H2O (0.5 mL, 0.036 M) was added and the mixture was vortex-mixed for 15 s, followed by sonication at 30 °C for 10 min. Ascorbic acid (0.65 mL, 0.568 M) was added and the mixture was vortex-mixed for 15 s. The seeds were prepared by the literature method using NaBH4 as the reducing agent.12 The seeds (50 μL) were added to the growth solution and the mixture was placed in an oven operating at 43 °C for 1 h. Gold nanoprisms (sample name Au3) were collected by centrifugation at 4000 rpm for 3 min for TEM sample preparation or they were dispersed asprepared without centrifugation for the catalytic reactions. The mixture of prisms, rods, and spheres (sample name Au5) was obtained when Pluronic L-64 (3.0 mL, 17.9%) was used. Sample Au6 was prepared similar to Au5, except for the growth temperature at 23 °C instead of 43 °C. With the other conditions being the same for making these mixtures, the addition of AgNO3 (0.5 mL, 0.400 mM) produced low symmetric irregular shape NPs (sample name Au4). When the other experimental conditions are the same, Pluronic F-127 (17.9%) instead of Pluronic L-64 produced prolate-shaped gold NPs with the aspect ratio of 2.59 (±0.584) when the growth was conducted in the oven at 43 °C (sample name Au2) and those with the aspect ratio of 2.29 (±0.426) when the growth was conducted at 23 °C (sample name Au1). For the comparison of the stabilization environment on the catalytic activities, the CTAB-coated mixture of gold nanospheres and nanorods (sample name Au7) was prepared with CTAB (6.0 mL, 0.2 M), HAuCl4·3H2O (0.5 mL, 0.036 M), ascorbic acid (0.65 mL, 0.568 M), and the seeds (50 μL) in a similar method to Au3. Summarized synthetic conditions for each gold NP is shown in Table 1. Catalytic Reduction Reactions of 4-Nitrophenol. The reduction of 4-nitrophenol to 4-aminophenol was performed in the presence of Au NPs and aqueous NaBH4 solution, and this catalytic reaction was studied in a standard quartz cell with a path length of 1 cm. Aqueous NaBH4 solution (0.4 mL, 0.675 M) was added to aqueous 4-nitrophenol solution (2.0 mL, 0.27 mM). Then, 10 μL of aqueous Au NPs, which was dispersed in CTAB and Pluronics in the as-prepared state without centrifugation, was introduced into the reaction mixture, while the absorption spectra were recorded every 30 s in the range from 250 to 450 nm at 25 °C.

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edge length of 50.4 nm, and the mixture of spheres, rods, and prisms; SEM images of the mixture of CTAB-coated gold nanospheres and nanorods (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-51-200-7257. Fax: +82-51-200-7259. ORCID

Hyon-Min Song: 0000-0001-9856-9868 Funding

The authors acknowledge the support from Dong-A University. Notes

The authors declare no competing financial interest.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00647. TEM images of prolate-shaped gold NPs with the aspect ratios of 2.29 and 2.59, gold nanoprisms with the average 7882


ACS Omega

Article

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