Enhancing Fenton-like biocatalysis of CuPd bimetallic nanozymes by alkylamine ligand self-assembled monolayers.

The synergistic effect of copper and palladium makes CuPd bimetallic nanozymes potential candidates for tumor biocatalysis. However, their practical catalytic activity is closely related to their surface modification and structure. In this study, CuPd nanozymes were prepared by a one-step reduction method using various surface ligands (decamethylamine (DA), dodecamethylamine (DDA), hexadecamethylamine (HDA), and octadecamethylamine (ODA)) with different alkylamine lengths in order to study the effects of surface self-assembled monolayers (SAMs) formed by ligands. TEM, XPS, and XRD results showed that all four nanozymes were similar in their structures, sizes, and compositions. However, the as-synthesized CuPd@DA and CuPd@HDA presented higher Fenton-like activity due to the lower steric hindrance caused by ordered and thinner SAMs. In contrast, both CuPd@DDA and CuPd@ODA exhibited weak biocatalytic activity for the thicker and stacked SAMs because of the long length of alkylamine ligands. In vitro anti-tumor cytotoxicity evaluation showed lower IC₅₀ values for CuPd@DA (104.6 μg mL^-1) and CuPd@HDA (118.6 μg mL^-1), and higher values for CuPd@DA and CuPd@HDA, demonstrating the biocatalytic differences caused by surface SAMs. This study provides unique insights for optimizing the biocatalytic effect of bimetallic nanozymes in the perspective of surface SAMs.