Revealing the Active Site of Gold Nanoparticles for the Peroxidase-Like Activity: The Determination of Surface Accessibility

Ching-Ping Liu,Po-Yen Yu,Ching-Feng Su,Po-Wei Lee,Kuan-Chung Chen

doi:10.3390/catal9060517

Abstract

Despite the fact that the enzyme-like activities of nanozymes (i.e., nanomaterial-based artificial enzymes) are highly associated with their surface properties, little is known about the catalytic active sites. Here, we used the sulfide ion (S2−)-induced inhibition of peroxidase-like activity to explore active sites of gold nanoparticles (AuNPs). The inhibition mechanism was based on the interaction with Au(I) to form Au2S, implying that the Au(I) might be the active site of AuNPs for the peroxidase-like activity. X-ray photoelectron spectroscopy (XPS) analysis showed that the content of Au(I) on the surface of AuNPs significantly decreased after the addition of S2−, which might be contributed to the more covalent Au–S bond in the formation of Au2S. Importantly, the variations of Au(I) with and without the addition of S2− for different surface-capped AuNPs were in good accordance with their corresponding peroxidase-like activities. These results confirmed that the accessible Au(I) on the surface was the main requisite for the peroxidase-like activity of AuNPs for the first time. In addition, the use of S2− could assist to determine available active sites for different surface modified AuNPs. This work not only provides a new method to evaluate the surface accessibility of colloidal AuNPs but also gains insight on the design of efficient AuNP-based peroxidase mimics.

Highlights

Owing to their potential biological importance, the development of nanomaterials as enzyme mimics has received a great of interest recently [1,2]
The X-ray photoelectron spectroscopy (XPS) measurements showed that the content of Au(I) on the surface of AuNPs significantly decreased after the addition of S2−, which might be contributed to the more covalent Au–S bond in the formation of Au2 S
Since the catalytic activity of AuNPs was size-dependent [13,14,15] and mainly contributed by the content of Au rather than those surface coating molecules, the size of all kinds of AuNPs should be carefully controlled during the preparation

Summary

Introduction

Owing to their potential biological importance, the development of nanomaterials as enzyme mimics has received a great of interest recently [1,2]. Nanomaterial-based artificial enzymes (i.e., nanozymes) exhibit several advantages, such as low cost, easy preparation, high stability and robustness. Metal, metal oxide and carbon-based nanomaterials possess several kinds of enzyme-like activities and have been applied in biosensing and immunoassays [3] as well as biomedical applications [4]. Despite the rapid progress achieved with nanozymes in terms of development and application, the relatively low catalytic efficiency of nanozymes still greatly limits further practical applications as compared to natural enzymes. Enzyme-like activities are intrinsic properties of nanozymes, but they can be regulated by the manipulation of aspects such as the size, shape, morphology, surface modification, and composition [1,2]. As for surface modification, it presents a dilemma between the colloidal stability and catalytic activities of the nanozymes [5]. Surface modification is indispensable in preventing the aggregation of colloidal

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Conclusion