Facile synthesis of hollow urchin-like gold nanoparticles and their catalytic activity
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The galvanic replacement reaction between Ag nanoparticles (NPs) and HAuCl4 followed by addition of ascorbic acid led to the formation of AuNPs sharing both urchin-like and hollow structures. The AgNPs took as sacrificial templates to guide the hollow structure and the intermediates provided rough surface and active sites for the further deposition of AuNPs, which originated from the reduction of excess HAuCl4 by ascorbic acid. These unique structured AuNPs presented excellent optical properties and great advantages in catalysis applications.
KeywordsHollow structure Urchin-like structure Gold nanoparticles Catalytic activity
Nanostructured gold nanoparticles (AuNPs) have revealed great potential applications in many fields owing to their unique optical and catalytic properties [1, 2, 3, 4, 5, 6]. Among them, urchin-like or branched gold particles with a rough surface are of great significance. The high surface area endows them excellent performance in catalysis, surface plasmon resonance (SPR) , surface-enhanced Raman spectra (SERS) , electronic devices, and biological applications [1, 2, 9].In particular, owing to the existence of a large electromagnetic field enhancement at the tips of branched particles, a strong SERS activity can be detected with intensity over 107 and a relatively high reproducibility . In addition, the urchin-like structure leads to large SPR shift from 500 to 800 nm and tunable SPR enhancement light scattering and absorption, making them novel and highly effective contrast agents for in vivo cancer diagnosis and therapy . Many strategies emerge for the construction of size and branch-controlled urchin-like gold particles , mainly through seeding growth approach [12, 13, 14], where the preferential deposition of atoms on certain facets is dominant . Besides, various molecules are adapted as stabilizer or capping agent to guide the formation of urchin-like structures, such as thiol-terminated molecules , cetyltrimethylammonium bromide , and sodium dodecyl sulfate .
On the other hand, the hollow metal nanostructures present many potential applications in catalysts, drug delivery, optical imaging, and nano-reactors [17, 18]. In the hollow nanosphere system, SPR peak locations shift over a region of more than 100 nm due to changes of shell thickness , rendering them great potential application in optical sensors . As for the generation of NPs with hollow interiors, the sacrificial template approach is the most adaptable one [18, 19, 20, 21]. Generally, AgNPs have always been taken as sacrificial templates and their reaction with HAuCl4 or other metal salts leads to the formation of metal NPs with voids [17, 22, 23, 24, 25, 26, 27, 28], in which the morphology of the AgNP seeds directly controls that of the resulting metal shells. These hollow-structured materials have been explored for biomedical and catalytic applications. An improved performance has already been achieved for their use as contrast enhancement agents for both optical coherence tomography and photoacoustic tomography [22, 26].
In this paper, we provided a facile method to prepare gold particles sharing both hollow interior and urchin-like shell. Such a unique nanostructure facilitates the access of species to the Au surface and improves their catalytic performance .
Deionized water was used in all the experimental processes (18.0 MΩ cm−1). Hydrogen tetrachloroaurate (III) (Au ≥ 47.8 % on metals basis), silver nitrate, polyvinylpyrrolidone (PVP), sodium borohydride were bought from Sinopharm Chemical Reagent Co. Ltd., ascorbic acid, sodium citrate, and glucose were from Beijing Chemical Works and all these above chemicals were used as received. 4-Nitrophenol (4-NP, Shanghai Chemical Reagent Co. Ltd.) was purified before use.
Synthesis of silver seeds
Typically, 4.5 mg of AgNO3 was dissolved in 25 mL of water and preheated at 70°C for 10 min. After that, the temperature was increased to 110°C followed by addition of 1.5 mL of sodium citrate (1 % in mass) to initiate the reduction of AgNO3. After 1.5 h, the colloidal Ag solution was cooled to room temperature.
The AgNPs in (ethylene) glycol (EG) system were prepared based on the previous report : 0.025 g of AgNO3 and 0.10 g of PVP were dissolved in 10 mL of EG at room temperature. After that, the solution was heated at 160°C in an oil bath for 3 h under stirring.
Synthesis of hollow urchin-like gold nanoparticles
Four milliliters of Ag colloid was centrifuged at 4,000 rpm for 5 min and the concentrated Ag deposites were dropped into 1 mL of HAuCl4 aqueous solution (2.94 mM). Right after that, 1 mL of ascorbic acid (10 mM) was added. The solution turned dark blue in a short period of time. Five minutes later, 400 μL of PVP (50 mM in monomer concentration) was mixed with the product solution to avoid the possible aggregation in the further purification process. A control experiment was conducted with the same conditions in the absence of Ag seeds. The morphology of the products was investigated by a high-resolution transmission electron microscopy (HRTEM, JEOL-2100F) operated at 200 kV and scanning electron microscopy (SEM, XL-30 ESEM FEG). The optical spectra of the samples were recorded on a UV2400PC UV–vis spectrometer.
Catalytic reaction for degradation of 4-NP
Three hundred microliters of as-prepared solution was concentrated to a total of 10 μL by centrifugation at 6,000 rpm for 4 min. After removal of the supernatant, the isolated AuNPs were immediately added to a freshly prepared reaction mixture (3 mL) containing 4-NP (0.15 mM) and NaBH4 (5.5 mM). The optical property of the reaction system was analyzed by using a UV–vis spectroscopy (UV-2550) at every 5-min interval.
Electrocatalytic oxidation of glucose
A piece of indium–tin–oxide (ITO)-coated glass (1 × 5 cm) was firstly pre-cleaned with acetone and immersed into a solution of H2O2: NH3·H2O/H2O (1:1:5 v/v) for 30 min. After that, the ITO glass was rinsed with deionized water and dried at 40°C.
The cleaned ITO glass was treated with aqueous 3-aminopropyltriethoxysilane (0.25 wt%) for 15 min. After washing, the glass was immersed in a purified hollow urchin-like AuNPs solution for 30 min, washed with water, and dried in air.
The electrocatalytic oxidation of glucose was performed on a CHI 852C electrochemical workstation (CH Instruments, Chenhua Co., Shanghai, China). A conventional three-electrode cell was utilized with a saturated Ag/AgCl electrode as the reference electrode, a platinum plate as the counter electrode, and the Au-loaded ITO glass as the working electrode. A potential scan in the range of −0.6–0.85 V with a scan rate of 50 mV s−1 was implemented to explore the electrochemical behavior of glucose in stirred 0.1 M NaOH aqueous solutions containing 1.33 mM glucose.
Results and discussion
The selection of reductant appears significant for the generation of well-structured hollow urchin-like AuNPs. If NaBH4 was applied instead of ascorbic acid, no uniform urchin-like products were observed (Fig. 3c). Only small NPs with average diameter of 7 nm are distributed around the as-obtained hollow gold structures. This indicates that a very fast reaction (NaBH4 is a stronger reductant than ascorbic acid) does not benefit the further deposition of gold on the preexisting hollow gold surface.
Considering this reaction a first-order reaction, the rate constant is determined by the slope of the linear fit of −ln(Ct/C0) versus time, where Ct/C0 represents the ratio of 4-NP concentration at time t and 0 as calculated based on their corresponding absorbance intensity in the kinetic UV–vis spectra. The rate constants are 0.124 and 0.073 min−1 for the reaction by using hollow urchin-like and spherical AuNPs as catalysts, respectively (insets in Fig. 9a, b). Following the previous discussion on the catalyst, a relatively large surface permits the oxidation and reduction reaction occurring together while a small particle size is essential to keep its high activity . The hollow urchin-like AuNPs fit this point well. The sharp tips and thin shell not only render them high activity, but keep linked to avoid separating the two half reactions. Therefore, a higher catalytic activity was achieved for the hollow urchin-like AuNPs.
In summary, we successfully synthesized hollow urchin-like gold nanoparticles by using silver nanoparticles as sacrificial seeds followed by reduction of excessive HAuCl4 with ascorbic acid. This unique structure makes them excellent application in optics and catalysis fields. Similar strategies will be adopted for generation of other hollow urchin-like metal nanoparticles to dig their wider application.
This work was supported by National Natural Science Foundation of China (grant no. 21103018) and Jilin Provincial Science and Technology Development Foundation (grant no. 201101010).
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.
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