Rational design of Ag@Fe₃O₄ core-shell nanozymes with peroxidase-mimic activity: shell thickness-dependent kinetics...

The rational design of nanozymes with tunable catalytic properties is critical for advancing colorimetric sensing. This study demonstrates that the activity and substrate specificity of Ag@Fe₃O₄ core-shell nanozymes can be precisely controlled by engineering the Fe₃O₄ shell thickness. Variants with diameters of 170, 200, and 300 nm were synthesized to evaluate their peroxidase-like activity, which relies on a synergistic Fenton-like and plasmonic mechanism. Kinetic analysis revealed a strong structure-function relationship. The 200 nm variant showed the highest affinity for H₂O₂ (Michaelis-Menten constant, K_m  = 18.91 mM). Conversely, the 300 nm variant displayed exceptional affinity for 3,3',5,5'-tetramethylbenzidine (TMB) (K_m  = 0.0224 mM), significantly surpassing natural horseradish peroxidase. This unique specificity enabled the sensitive colorimetric detection of dopamine via competitive inhibition. These findings establish that architectural tuning of the nanozyme shell is a powerful strategy for creating substrate-specific catalysts for advanced sensing platforms.