Parallel comparative studies on composition-dependent peroxidase-like catalytic activity of ultrasmall ferrite nanoparticles

Ferrite nanoparticles, known for their enzyme-like catalytic activity, have gained significant attention as innovative nanozymes for various catalysis medicine applications. However, the relationship between catalytic activity and ultrasmall ferrite nanoparticle composition remains unclear, which hinders the development of ferrite-based nanozymes with high catalytic performance. Here, we have synthesized a series of ultrasmall ferrite nanozymes for studying their composition dependent peroxidase (POD)-like activity. Initially, their size and surface charge were regulated to assess their impact on POD-like activity. The results indicate that smaller ferrite nanozymes with a negative charge exhibited superior activity when using TMB as the substrate. Subsequently, we examined the ultrasmall ferrite nanozymes with the same size and surface charge but different compositions (CoFe₂O₄, MnFe₂O₄, and γ-Fe₂O₃), and comprehensively investigated the effect of composition on POD-like activity. The results show that the POD-like activity is closely related to the composition of the ultrasmall ferrite nanozymes and the activity order towards TMB is found to be CoFe₂O₄ > MnFe₂O₄ > γ-Fe₂O₃. By comparing the catalytic performance of nanoparticles with different compositions, the influence of composition on their activity is elucidated. Furthermore, we determined that the optimal pH and temperature for the POD-like catalytic activity of ultrasmall CoFe₂O₄ nanozyme were pH = 4-4.5 and 30 °C. Under these optimal catalytic conditions, the ultrasmall CoFe₂O₄ nanozymes exhibited a higher POD-like activity, resulting in increased tumor cell staining intensity. This suggests that ultrasmall CoFe₂O₄ nanozymes may serve as a viable alternative to horseradish peroxidase for immunohistochemical staining applications. This work provides experimental evidence for designing efficient ultrasmall ferrite catalysts for nanozyme catalysis medicine applications.