Enzymes are biocatalysts that play an important role in living systems. However, they are expensive, difficult to store, laborious to produce, and are easily denatured in external environments from varying temperature, pH, and chemical stressors [1, 2]. These drawbacks critically limit their practical uses .
To overcome the above limitations, nanozymes have been developed as a new alternative to enzymes [1, 4]. Nanozymes are nanomaterials that possess an intrinsic enzyme-like activity and have advantages such as stability in external environments, reasonable costs, and good catalytic activity [1, 2, 5–9].
Since the discovery of the unique peroxidase-like activity of Fe3O4 magnetic nanoparticles (NPs) by Yan’s group in 2007, numerous researchers around the world have gained an interest in nanozymes made of metal nanomaterials . Metal nanomaterials, including gold (Au) NPs, platinum NPs, iron oxide NPs, cerium oxide NPs, manganese oxide NPs, and copper oxide NPs began to be used as nanozymes [5, 6, 10–15]. Most of them have exhibited enzyme-like activities like that of peroxidases, catalases, oxidases, and superoxide oxidases [1–3, 5, 6, 10, 14–22].
Among the various metal nanomaterials, Au NPs have attracted the most attention because of their outstanding catalytic properties and advantages [23–35]. Au NPs can be synthesized easily and are stable [36, 37]. In addition, the physical/chemical properties of Au NPs can be controlled by controlling their size and shape [24, 26, 33, 35]. These NPs are also highly biocompatible and are easy to functionalize [24, 30, 35, 38–42]. However, gold is not cheap, and the catalytic reactions depend on the surface area of the NPs. Fine-sized NPs can be cost-effective, but are difficult to separate for reuse [1, 3, 8–10, 17, 43, 44]. The development of a structure that uses small amounts of gold to acquire a large surface area, while still being easily separated, might prove to be a highly useful material in the field of nanozymes. A nanostructure where Au NPs are assembled onto a silica (SiO2) sphere core was developed by the Halas group in 1998 . The chemical and optical properties of the Au NP-embedded SiO2 structure can be changed by simply controlling the diameter of the core and layered nanoparticles [46–51]. The NP-embedded SiO2 nanostructures are also cost-effective since only small amounts of expensive Au NPs are embedded onto the silica core, and they are easily separated from the reaction solution with the SiO2 core.
Au NP-assembled SiO2 nanostructures have been investigated in various fields [44, 50, 52–62] and have been found to have many merits due to the combined properties of Au NPs and of the silica core, simultaneously utilizing the outstanding and unique features of Au NPs as well as the inert and versatile feature of SiO2 [53, 55, 56, 63]. In addition, the absorbance spectra of the nanostructures can be tuned across the visible and infrared regions by controlling the size of the Au NPs [38, 39, 64, 65]. Due to these properties, the Au NP-assembled SiO2 nanostructures have broad applications. While there are many possible approaches for the control of the size and density of Au NPs on a template surface, a method of synthesis for those nanostructures has not yet been established [46, 48–50, 64, 65]. For this reason, the low density and non-uniform morphology of Au NPs on nanostructures remain a considerable challenge [46, 48–50]. There is, therefore, a need for the development of an improved method for preparing Au NP-assembled silica nanostructures.
Our group recently developed SiO2@Au nanostructures in which Au NPs were densely immobilized on the surface of a SiO2 nanosphere . For this nanostructure, the SiO2 nanosphere was used as a template and the Au NPs were uniformly and densely introduced on it using the seed-mediated growth method (Fig. 1). SiO2@Au nanostructures have enhanced separation and re-dispersion properties and are more stable during surface modification than Au NPs. Also, they have shown potential as effective nanozymes. In this study, besides introducing dense and uniform Au NPs onto the SiO2 nanospheres, we also developed a facile method to very precisely control the size of Au NPs on the SiO2 surface and investigated their optical and catalytic characteristics by controlling the size of the Au NPs. Furthermore, various factors affecting the peroxidase-like activity of SiO2@Au NPs were also studied.