Single-particle impact electrochemistry in carbon nanopipettes: Probing the effect of nanodiamonds microstructures on...

Nanodiamonds (NDs) and their derivatives are promising peroxidase (POD)-mimicking nanozymes for biomedical applications due to their tunable surface chemistry and excellent biocompatibility. While modulating the sp²/sp³ hybridization ratio effectively tuned their catalytic performance, a comprehensive understanding of the structure-activity relationship between sp²/sp³ configuration and POD-like activity, particularly at the single-particle level, remains elusive. In this work, single-particle impact electrochemistry with carbon nanopipettes as probes was employed to investigate the POD-like activity of NDs and graphitized-NDs (G-NDs) with varying sp²/sp³ configurations and sizes. The results demonstrated that the catalytic activity varied with the hybridization degree, with the G-ND₈₀₀ featured with a moderate sp²/sp³ ratio achieved the highest catalytic kinetics, displayed a single-particle turnover frequency of (6.868 ± 0.139) × 10⁷ s^-1. This performance outperformed pristine NDs by 4.3 times and G-ND₁₆₀₀ by 2.3 times due to synergistic enhancement of H₂O₂ adsorption and electron transfer driven by surface sp² carbon content. Additionally, 30 nm NDs exhibited faster catalytic rates and stronger electron transfer compared to 5 nm NDs. Compared to traditional chromogenic assays, single-particle impact electrochemistry eliminated the influence of chromogenic reagent affinity to accurately measure the inherent catalytic kinetics and particle heterogeneity of nanozymes, particularly those with low activity. These findings established a clear structure-activity framework for rationally design of highly efficient carbon-based nanozymes.