Oxygen-vacancy-rich amorphous MnO2@MOF-808 nanozyme for dual-mode colorimetric and smartphone-assisted determination of Sr2+ and H2O2.

Abstract

The rapid and specific monitoring of toxic strontium (Sr2+) contamination is critically important yet challenging, as it typically relies on non-portable and expensive instrumentation. Herein, the crystal-phase dependence of MnO2 nanozymes was systematically investigated by in situ growth of five representative polymorphs, α-, β-, γ-, δ- and amorphous (A-) MnO2, onto a Zr-based MOF-808 scaffold. XPS and steady-state kinetics revealed that the dual peroxidase-like (POD) and oxidase-like (OXD) activities are proportional to the Mn3+/Mn4+ ratio and surface oxygen-vacancy density, both of which peaked in A-MnO2@MOF-808. The hierarchically porous MOF-808 framework reinforced the resistance of the amorphous shell to H2O2-induced reductive etching, supporting dual OXD and POD catalytic modes on the same nanozyme platform under independently optimized conditions. Capitalizing on its POD activity, a colorimetric H2O2 assay was first established (LOD = 0.3 μM), simultaneously providing a quantitative validation of the catalytic competence of the platform. For Sr2+, an OXD-coupled G-quadruplex (G4) aptamer strategy afforded an LOD of 0.9 nM with excellent specificity against a 50-fold excess of Ca2+. The platform was translated into a fully portable point-of-care device by integrating an ABTS-loaded sodium alginate hydrogel cap, a 3D-printed dark chamber and a custom WeChat mini-program ("Sr-Sense") for smartphone-based RGB analysis, achieving an on-site LOD of 7 nM, an assay time below 15 min and a unit cost of $0.187 per test. Validation in seven authentic freshwater samples, simulated seawater and simulated nuclear wastewater yielded recoveries fully comparable to ICP-MS, highlighting its strong potential for environmental, marine and nuclear-emergency monitoring.