Interfacial Accessibility in MnO2-Based Nanozyme Catalysis

Colloidal manganese oxides are redox-active materials for chromogenic oxidation reactions, but their apparent catalytic response is difficult to rationalize using single dry-state descriptors such as Brunauer-Emmett-Teller (BET) surface area or formal Mn oxidation state. Here, Li- and Sr-directed MnO2-based colloids, including Fe-containing analogues, were compared under identical H2O2-independent TMB oxidation conditions. In the Li-series, the highest responses were obtained for the proton-exchanged H-forms of the calcined samples L6 and L9, with L9 showing the strongest response within the present sample library; this enhancement was associated with Mn-O framework reorganization and mesopore evolution rather than with delithiation alone. In the Sr-series, proton exchange was accompanied by extensive Sr depletion, structural evolution toward a more defective H-form, pore opening, lower-temperature reducibility, and an increased TMB oxidation response, although the best Sr-derived samples remained less active than L9. Fe was retained during postsynthetic treatment but did not act as an independently dominant catalytic center. The L9-based streptavidin conjugate retained colorimetric function in a proof-of-concept immunoassay. These results establish a route-resolved, interface-centered framework in which cation removal, preceding thermal history, Mn-O framework evolution, mesoporosity, reducibility, and colloidal accessibility are considered together.