Secondary coordination shell nitrogen modulation of Cu-N4 single-atom nanozymes: Unraveling catalytic mechanism and constructing portable dual-mode nitrite sensor.

Regulating the peripheral nitrogen microenvironment of single-atom nanozymes (SANs) is an effective strategy to boost their enzyme-mimetic catalytic performance, yet facile gradient modulation of peripheral nitrogen species remains a major challenge. Herein, we develop a melamine-assisted pyrolysis strategy to fabricate Cu-N/CB SANs with intact Cu-N4 coordination configuration and tunable peripheral graphitic nitrogen content. Adjusting melamine dosage drives the conversion of pyrrolic N to graphitic N in the second coordination shell, which delivers an electron-donating effect to central Cu sites, raises the proportion of low-valent Cu+, and greatly improves the intrinsic peroxidase (POD)-like activity. Peripheral graphitic nitrogen optimizes the electronic structure of Cu active sites, accelerates interfacial electron transfer and reduces the energy barrier of the rate-determining step during H2O2 decomposition. Two nitrite sensing platforms were constructed based on Cu-N/CB-2.5: a ratiometric colorimetric sensor and a portable hydrogel-smartphone visual detection system. Benefiting from strong anti-interference and favorable linear response, the dual-mode sensor achieves satisfactory spike recoveries in real sausage and pickle samples. This work develops a controllable synthetic route for peripheral nitrogen-modulated Cu single-atom nanozymes, clarifies the structure-activity relationship between peripheral nitrogen and POD-like activity, and offers a feasible strategy for designing high-performance nanozyme-based portable sensors.