Synthesis and catalytic activity of branched gold nanoparticles in AQUEOUS MEDIUM - Phan Ha Nu Diem

Vietnam Journal of Science and Technology 55 (5B) (2017) 227-235 SYNTHESIS AND CATALYTIC ACTIVITY OF BRANCHED GOLD NANOPARTICLES IN AQUEOUS MEDIUM Phan Ha Nu Diem 1, 2, * , Tran Thai Hoa 2 , Tran Thuc Binh 2 1 Dong Nai University, 4 Le Quy Don, Tan Hiep Ward, Bien Hoa city, Dong Nai 2 College of Sciences, Hue University, 77 Nguyen Hue, Hue city * Email: phannudiem@gmail.com Received: 26 September 2017; Accepted for publication: 7 October 2017 ABSTRACT In this article, a simple method for the preparation of multi–branched gold nanoparticles from an aqueous solution of silver seeds, cetyl-trimethylammonium bromide (CTAB), HAuCl4, and Pluronic F–127 was described. It was found that morphologies and sizes of gold nanostructures (AuNPs) depended strongly on such experimental parameters as concentrations of Pluronic F–127 and Au3+. The products were characterized by transmission electron microscopy (TEM). Interestingly, the multi – branched AuNPs were found to serve as an effective catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4. Kinetic data have been obtained from monitoring the concentrations of 4- NP and BH4‒ by UV‒vis spectroscopy. Keywords: multi – branched gold nano, Pluronic F-127, catalytic activity. 1. INTRODUCTION Gold is a noble metal that has been employed in several applications throughout the human history due to its properties. Actually, gold in the nanoscale is important investigation field for their unique and distinctive spectral properties. Gold nanoparticles (AuNPs) have stimulated a great interest in many areas such as catalysis [1 - 3], biological and chemical sensing [4 - 6], antimicrobial [7 - 8], electrochemical [9 - 10], surface-enhanced Raman spectroscopy [11 - 13], cancer therapy and drug delivery [4, 14]. The studies on the functionalization of spherical gold nanoparticles, growth of other shape- specific gold nanoparticles has also been an active area of research interest in the past few years Gold nanoparticles has been reported for several other particle morphologies, including sphere [10, 15], dendrites [12], rods [6,13]. The formation of these gold nanocrystals makes possible the examination of new optical properties associated with shape variations. While there are many studies on the controlled growth of gold nanocrystals using a wide range of preparation approaches, it is interesting to note that only recently have branched gold nanocrystals been reported. Chen et al. prepared bipod, tripod, and tetrapod gold nanocrystals using a large amount of capping surfactant cetyltri-methylammonium bromide (CTAB), HAuCl4, ascorbic acid, and Phan Ha Nu Diem, Tran Thai Hoa, Tran Thuc Binh 228 NaOH [16]. Hao et al. synthesized three tipped gold nanocrystals from an aqueous solution of bis-(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium, H2O2, sodium citrate, and HAuCl4 [17]. Sau et al. prepared branched gold nanoparticles from an aqueous solution of gold seeds, CTAB, HAuCl4, and ascorbic acid [18]. Branched gold nanoparticles can offer new insights about the relationships between the different particle morphologies and their corresponding surface plasmon resonance absorption characteristics, and they offer several advantages for catalysis. In the present work, we report a simple and relatively fast synthesis of branched gold nanocrystals by a combination of both surfactants CTAB and Pluronic block copolymer F-127 to finally produce a high yield of long branched gold nanoparticles. An atomic resolution observation of the surface defects of the tips of the branches by means of transmission electronmicroscopy (TEM) is also presented, showing the in situ surface reconstruction and proving the stability of certain facets. Moreover, the catalytic reduction of 4-nitrophenol (4-NP) to its amino derivative 4-aminophenol (4-AP), via sodium borohydride (NaBH4) is a well- studied model reaction catalyzed only in the presence of metal nanostructures. Therefore, the catalytic activity of the multi – branched AuNPs was evaluated by studying the catalytic in 4-nitrophenol reduction. 2. EXPERIMENTAL 2.1. Reagent and apparatus Cetyltrimethylammonium bromide (CTAB, CH3(CH2)15N(Br)(CH3)3, 99 %), sodium borohydride (NaBH4, 99 %), Tetrachloroauric(III) acid (HAuCl4.3H2O, 99.9 %) , L-ascorbic acid (C6H8O6, 99 %), trisodium citrate dihydrate (Na3(C6H5O7).2H2O, 99 %) were purchased from Sigma-Aldrich, 4-nitrophenol (4-NP) and silver nitrate (AgNO3, 99 %) was obtained from Merck. All the glassware was cleaned with soap and aqua regia (HCl:HNO3 in a 3:1 ratio by volume), and rinsed with twice distilled water prior to the experiments. All the chemicals were separately dissolved in twice distilled water. We recorded the UV-vis spectra to track the progress of the reaction as the volume of HAuCl4 solution was increased. The UV-Visible extinction spectra were taken by the UV-Vis spectrophotometer model V-630 (Jasco, Japan) and the microstructure of gold nanostar had been characterized by transmission electron microscopy (TEM) imaging (Model JEOLE-3432, Japan). 2.2 Experimental process 2.2.1. Synthesis of silver seed Silver seeds nanoparticles (≈ 4 nm) were synthesized by the reduction of a volume of 5 mL of 0.5 mM silver nitrate solution was added to 5 mL of aqueous solution containing 0.5 mM trisodium citrate. Concurrently, 0.3 mL of freshly ice-cooled 10 mM sodium borohydride solution was rapidly added into the vortex of the mixture solution. After this addition, the solution suffered an immediate color change to dark green. Stirring was stopped after 60 s. The silver seed solution was kept under static conditions for several hours, for at least 2h to allow the degradation of excess borohydride prior to be used, and Synthesis and catalytic activity of branched gold nanoparticles in aqueous medium 229 preserved from light in order to avoid photo-degradation. This seed was used within 2 h after preparation but in no case more than 5 h. 2.2.2. Synthesis of branched gold nanoparticles Multi – branched gold nanoparticles were synthesized by slight modifications of the seed mediated growth method reported by Mayoral et al. [19]. A growth solution containing 10 mL of 1.25 mM CTAB and 5 mL of 0.25 mM HAuCl4, forming an orange-red mixture. Gold nanostar were prepared by the addition 10 mL of 0.125 mM Pluronic F-127 to the growth solution and let under vigorous stirring for 5 minutes, followed by the addition of 0.7 mL of 1 M ascorbic acid and the solution color turned colorless, indication the reduction of Au 3+ to Au + species. Finally, 12.5 µL of silver seed solution was added. The colorless solution also slowly turned blue. The finished nanoparticles were then stirred for at least 30 minutes at 30 o C. The gold nanostars was centrifuged for 20 min at 2000 rpm. Finally, the residue was re-dispersed in an appropriate amount of twice distilled water for further analysis 3. RESULTS AND DISCUSSION 3.1. Influence of the Pluronic F-127 concentration Pluronic F-127 is formed by 2 lateral chains of poly(ethylene oxide) (PEO) and a central chain of the poly(propylene oxide) (PPO) (Figure 1). The anisotropic growth in the presence of the block copolymer is due to the hydrophobic behavior of the central chain of polyoxypropylene. Additionally, the PEO units contribute to the reduction of gold increasing the efficiency of the synthesis [20]. Figure 1. Structure of CTAB (a) and basic structure of Pluronic copolymers (b) [20]. In this experiment, AuNPs were carried out with 12.5 µL of silver seed solution that is kept constant and in the presence of increasing Pluronic F-127 concentrations 0.025, 0.05, 0.1, 0.15 and 0.2 mM. Corresponding UV-Vis spectra showed the λmax at 638, 820, 980 nm, and two datums exceed the limit of this UV-Vis spectrophotometer (Figure 2a). When the samples used were prepared using a higher concentration of Pluronic F-127, which was found to favor the formation of a large average size of AuNPs. The major surface plasmon resonance (SPR) peaks displayed a constant red-shift from 638 nm to 820 nm and 980 as the size of the AuNPs increased, corresponding TEM image shown in Figure 2b. It has been demonstrated that Pluronic F-127 plays a key role as simultaneous capping and reducing agent in the formation of the long branches in gold nanopstructures as shown in Figure 3 [19]. The hydrophobic PPO heads are preferentially absorbed onto the gold surfaces forming a compact CTAB Pluronic bilayer. Figure 3 depicts formation of anisotropic gold structures in the presence of the copolymer and CTAB. Iqbal et al. [20] reported that the combination of CTAB and the Pluronic F-127 yielded branched gold nanoparticles with increased aspect ratio. Mayoral Phan Ha Nu Diem, Tran Thai Hoa, Tran Thuc Binh 230 and co-workers used this combination of co-surfactants to obtain long branched gold nanostructures [21]. Figure 2. UV-Vis spectra (a) and TEM images of branched Au nanostructures synthesized at different concentration of Pluronic F-127: 0.05, 0.1, 0.15 and 0.2 mM (b). Figure 3. Schematic cartoon showing nanomaterials obtained. (a) Original gold nanoparticle coated with a CTAB bilayer; (b) resulting branched nanoparticle stabilized with CTAB and F127 [19]. The hydrophobic tails of the CTAB bind to the relatively hydrophobic PPO blocks of the Pluronic F-127, leading to the association of CTAB with the Pluronic F-127, thus forming stable surfactant-polymer complexes (Figure 3b). The use of more stable polymer-surfactant complexes by the addition of Pluronic F-127 increases the yield and the anisotropy degree of the resultant gold nanostars and increments their size. Consequently, we select 0.1 mM is the optimum Pluronic F-127 concentration for following experiments. Synthesis and catalytic activity of branched gold nanoparticles in aqueous medium 231 3.2. Influence of Au 3+ concentration In this section are included all the syntheses carried out with constant CTAB and Pluronic F-127 concentration while varying Au 3+ concentration: 0.5, 1.0 and 2.0 mM. The UV-Vis spectra in Figure 4a show when the Au 3+ concentration is gradually increased, the λmax at 527 nm and 879 nm for Au 3+ concentration of 1.0 mM, the intensity of major SPR peaks almost twice and the peak became narrower, indicating an increase of number of AuNPs in the colloidal solution and the uniformity of shape and size compared with those at 0.5 and 2.0 mM. Corresponding TEM images in Figure 4b show that the average sizes of multibranched Au nanostructures were about 77, 84 and above 120 nm for Au 3+ concentration of 0.5, 1.0 and 2.0 mM, respectively. The results showed that by increasing Au 3+ concentration, the number of AuNPs was not increased but it seemed to favor the formation of a large mean size. The multibranched AuNPs became more evident, as observed by TEM in Figure 4b. So we select 1.0 mM as the optimum Au 3+ concentration. Figure 4. UV-Vis spectra (a) and TEM images of branched Au nanostructures synthesized at different of Au 3+ concentration: 0.5, 1.0 and 2.0 mM (b). 3.3. Application of multibranched AuNPs for Catalytic Reduction of 4-Nitrophenol A potential application of metal nanoparticles is the catalysis of certain reactions that would otherwise not occur. In this report we have chosen the reduction of 4-NP to 4-AP as a model system [22,23] in order to evaluate the catalytic activity of multibranched AuNPs. The reduction of 4-NP by NaBH4 is thermodynamically feasible but kinetically restricted in the absence of a catalyst [24]. When addition of NaBH4 to a 4-NP solution changes the pH from acidic to highly basic so the color of the solution changes from light yellow to intense yellow due to formation of the Phan Ha Nu Diem, Tran Thai Hoa, Tran Thuc Binh 232 4-nitrophenolate ion [22]. Accordingly, the absorption peak shifts from 318 to 400 nm (Figure 5a). The catalytic process of this reaction was monitored by UV−Vis spectroscopy. This reduction is unfeasible in the presence of the strong reducing agent (NaBH4) without the nanoparticle catalyst. Absorption intensity at 400 nm for the 4-NP ion remained unaltered even after 90 min. as illustrated in Figure 5b. Figure 5. (a) UV−Vis spectra of 4-NP and 4-NP + NaBH4 solution, (b) The reduction does not proceed without a catalyst, (c) Enlarging the (b). Upon addition of multibranched AuNPs to solution in the presence of NaBH4, 4-NP accepts electrons from the donor borohydride and the catalytic reduction of 4-NP to 4-AP takes place rapidly at the AuNPs surfaces [25]. Figure 6. The catalytic activity of multibranched AuNPs was measured by a UV–Vis spectroscopy at different time intervals. Figure 6 illustrates the reduction reaction of 4-NP, observed at different time intervals using AuNPs as the catalyst. In the presence of multibranched AuNPs and NaBH4 the 4-NP was reduced, and the intensity of the absorption peak at 400 nm gradually decreased with time and after ∼ 20 min it fully disappeared. In the meantime, a new absorption peak appeared at 298 nm and progressively increased in intensity (Figure 6). This new peak is attributed to the typical absorption of 4-AP. The UV−Vis spectra in Figure 6a showed two isosbestic points at 280 and 314 nm. This result suggests that the catalytic reduction of 4-NP exclusively yielded 4-AP, without any other side products [25]. Synthesis and catalytic activity of branched gold nanoparticles in aqueous medium 233 3.4. Mechanism of reduction reaction of 4-NP to 4-AP in the presence of NaBH4 The mechanism of reduction reaction of 4-NP to 4-AP via NaBH4 in the presence of AuNPs can be explained as follows: Borohydride ions adsorb on the surface of the AgNP nanoparticles and transfer a surface- hydrogen species to the surface of the nanoparticles. Concomitantly, 4-NP molecules are adsorbed on the surface of the nanoparticles. Moreover, the adsorption, desorption equilibriums and diffusion of reactants to the nanoparticles are considered to be fast. The reduction of 4-NP, which is the rate-determining step, occurs due to the reaction of adsorbed 4-NP with the nanoparticles surface-bound hydrogen atoms (Figure 7). When the product, 4-AP, desorbs leaving free metal surface, the catalytic cycle can begin again. Figure 7. The proposed reduction of 4‐NP to 4‐AP catalyzed by gold nanostars in the presence of NaBH4 [26]. 4. CONCLUSIONS In summary, the combination of CTAB and the Pluronic F-127 yielded stable complexes that act as co-directing agents and favor the anisotropic growth of gold nanostructures in the form of multi-branched nanoparticles with diameter in the range of 80 – 120 nm. The importance of the existence of polymeric coatings on both AuNPs surfaces and the surfaces they interact with in stabilizing nanoparticles against deposition. Co-surfactants reported in this study stabilize AuNPs and kept them in aqueous solutions for more than a month. The AuNPs sizes can be controlled by varying Au 3+ and Pluronic F-127 concentrations. These nanoparticles could be very useful in catalytic applications, the catalytic activity of multi-branched nanoparticles thus prepared was investigated in the reduction reaction of 4-NP to 4-AP in the presence of NaBH4. Furthermore, polymer-bound AuNPs could be integrated into various biosensor applications and worth studying in the future. 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