Abstract.Gold nanoparticles (AuNPs) can oxidize glucose, producing hydrogen peroxide and gluconic acid, which are the same products as those generated by glucose oxidase (GOx). In this regard, AuNPs are a nanozyme. Herein, a new colorimetric method is developed to understand the surface chemistry of gold nanoparticles for this oxidation reaction. The color of nanoceria is changed to yellow by the hydrogen peroxide generated glucose oxidation. Using this assay, we find that adsorption of small molecules such as citrate does not deactivate AuNPs, while adsorption of polymers including serum proteins and high molecular weight polyethylene glycol inhibits glucose oxidation. In addition to glucose, AuNPs can also oxidize galactose. Therefore, this reaction is unlikely to be directly useful for glucose detection for biomedical applications. On the other hand, AuNPs might serve as a general oxidase for a broad range of substrates. The glucose oxidation reaction is slower at lower pH. Since the reaction generates an acid product, glucose oxidation becomes slower as the reaction proceeds. The effects of temperature, AuNP size, and reaction kinetics have been systematically studied. This work provides new insights regarding the surface chemistry of AuNPs as a nanozyme.3

An ideal colloidal system should be highly stable in a diverse range of buffer conditions while still retaining its surface accessibility. We recently reported that dispersing citrate-capped gold nanoparticles (AuNPs) in polymers such as polyethylene glycol (PEG) can achieve such a goal due to contributions from depletion stabilization. Since AuNPs can weakly adsorb PEG to exert steric stabilization and the remaining citrate can impart charge stabilization, the extent of the contribution of depletion stabilization is unclear. In this work, we aim to dissect the contribution of each stabilizing factor. This is achieved by coating AuNPs with a layer of thiolated compound, which inhibits the adsorption of PEG and also allows the control of surface charge. We found that depletion stabilization alone was insufficient to stabilize AuNPs at room temperature. However, when working together with other stabilization mechanisms, ultrahigh stability can be achieved. The size of both AuNPs and PEG was systematically varied and the trend was compared with theoretical calculations. Finally, we report the importance of the surface chemistry of commercial AuNPs.