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.

Measurement of infrared spectroscopy has emerged as a significant challenge for carbon materials due to the sampling problem. To overcome this issue, in this work, we performed measurements of IR spectra for carbon materials including C₆₀, C₇₀, diamond powders, graphene, and carbon nanotubes (CNTs) using the photoacoustic spectroscopy (PAS) technique; for comparison, the vibrational patterns of these materials were also studied with a conventional transmission method, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, or Raman spectroscopy. We found that the IR photoacoustic spectroscopy (IR-PAS) scheme worked successfully for these carbon materials, offering advantages in sampling. Interestingly, the profiles of IR-PAS spectra for graphene and CNTs exhibit negative bands using carbon black as the reference; the negative spectral information may provide valuable knowledge about the storage energy, production, structure, defect, or impurity of graphene and CNTs. Thus, this approach may open a new avenue for analyzing carbon materials.

The limited stability and aggregation of colloidal nanoparticles are typically major issues in catalysis. In this work, a highly efficient polystyrene based nano-silver containing polymer (PS-AgNPs) has been synthesized as a catalyst for the reduction of 4-nitrophenol. Entanglements of polystyrene chains restrict the aggregation of AgNPs in the polymer side chains and give rise to a stable nano-silver domain size. The domain size of nano-silver in both low Ag content PS-AgNPs (L-PS-AgNPs) and high Ag content PS-AgNPs (H-PS-AgNPs) is around 15 nm (in films). No intensified Ag aggregation occurs as the Ag concentration increases. Based on kinetic measurements of the catalytic model reaction of 4-nitrophenol, the linear relationship between the rate constant k_app and different Ag concentrations in L-PS-AgNPs was extended by two orders of magnitude of the Ag concentration for the first time. These results indicated that AgNPs in L-PS-AgNPs were homogeneously distributed and exhibited excellent dispersion stability. Additionally, the catalytic performance of H-PS-AgNPs was comparable to a microreactor including the core of AgNPs/SiO₂ and polymer shell. In this work, H-PS-AgNPs based on molecular entanglements were relatively simple as compared to the core/shell design of microreactors, which provided a new strategy to confine the size domain of metal nanoparticles immobilized on polymers.

Facilely synthesized peroxidase-like nanozymes with high catalytic activity and stability may serve as effective biocatalysts. The present study synthesizes peroxidase-like nanozymes with multinuclear active sites using two-dimensional (2D) metal-organic framework (MOF) nanosheets and evaluates them for their practical applications. A simple method involving a one-pot bottom-up reflux reaction is developed for the mass synthesis of (Cu-S)_n MOF 2D nanosheets, significantly increasing production quantity and reducing reaction time compared to traditional autoclave methods. The (Cu-S)_n MOF 2D nanosheets with the unique coordination of Cu(I) stabilized in Cu-based MOFs demonstrate impressive activity in mimicking natural peroxidase. The active sites of the peroxidase-like activity of (Cu-S)_n MOF 2D nanosheets were predominantly verified as Cu(I) rather than Cu(II) of other Cu-based MOFs. The cost-effective and long-term stability of (Cu-S)_n MOF 2D nanosheets make them suitable for practical applications. Furthermore, the inhibition of the peroxidase-like activity of (Cu-S)_n MOF nanosheets by glutathione (GSH) could provide a simple strategy for colorimetric detection of GSH against other amino acids. This work remarkably extends the utilization of (Cu-S)_n MOF 2D nanosheets in biosensing, revealing the potential for 2D (Cu-S)_n MOFs.