Synchronous modulation of fluorescence and oxidase-mimicking activity in a Mn-based MOF nanozyme via post-synthetic metal exchange for Cu2+ sensing.

Copper ions (Cu2+) are essential micronutrients but become hazardous when accumulated in excess, necessitating reliable, sensitive, and field-deployable detection methods. Herein, we report a fluorescent Mn-based MOF nanozyme (Mn-TCPP) that exhibits intrinsic oxidase-like activity, capable of catalyzing the oxidation of non-fluorescent o-phenylenediamine (OPD) to fluorescent 2,3-diaminophenazine (DAP) with an emission peak at 560 nm, while the Mn-TCPP itself displays characteristic fluorescence at 655 nm. Upon exposure to Cu2+, the stronger affinity of Cu2+ toward the porphyrinic TCPP ligand displaces Mn2+ from the framework, forming Cu-TCPP. This metal-exchange process simultaneously quenches the 655 nm fluorescence of the MOF and attenuates its oxidase-mimicking activity, whereas free Cu2+ in solution independently catalyzes OPD oxidation, further enhancing the 560 nm emission. Leveraging this dual-response mechanism, we develop a ratiometric fluorescence sensing strategy for Cu2+ detection based on the intensity ratio F560/F655. The calibration curve follows a linear equation: F560/F655 = 0.201x-0.001 (0.2-10 μM, R2 = 0.985) and y = 0.002x+2.061 (10-100 μM, R2 = 0.994) with the limit of detection (LOD) of 81 nM (S/N = 3). The sensor exhibits excellent selectivity over competing metal ions, and the method reliability is validated by standard addition recovery experiments in real water and rice samples. Furthermore, the Mn-TCPP nanozyme is embedded into an agarose hydrogel matrix, enabling visual, instrument-free monitoring of Cu2+ contamination: increasing Cu2+ concentrations induce a discernible fluorescence color transition from orange to yellow under 365 nm illumination. This work presents a tunable MOF platform that exploits competitive targets synchronous modulation of fluorescence and enzyme-mimicking activity, providing a robust, selective, and field-deployable ratiometric sensor for Cu2+.