Metal-organic framework (MOF) nanozymes share the similar features with metalloenzymes including well-defined catalytic active sites and versatile functionalities which have drawn great attention. However, most of the peroxidase-like MOF nanozymes perform the highest activity under quite acid conditions inhibiting their applications. Herein, we developed a nanoconfinement effect-inspired strategy onto a model MOF nanozyme, PCN-222-Fe nanoparticle (NP) (PCN=Porous Coordination Network), which is constructed by Zr6 clusters and Fe-TCPP (TCPP=Tetrakis(4-carboxyphenyl)porphyrin). Poly(acrylic acid) (Mw=2 kDa, PAA) was utilized to modify the nanochannels of PCN-222-Fe NPs through the coordination between Zr6 clusters of NPs and carboxyl groups of PAA. The abundant carboxyl groups not only donate plenty of protons but also confine them inside the nanochannels through the deprotonation and protonation process. About a four to five-fold activity increase for a colorimetric substrate 3,3',5,5'-tetramethylbenzidine dihydrochloride (TMB) and a fluorescent substrate as well, Amplex Red, at psychological pH was realized due to the lowering of microenvironment pH near the catalytic active centers (i.e., Fe-TCPP) which was evidenced by a pH-responsive fluorescent test. Considering the activity increase at psychological pH, PCN-222-Fe NPs were employed to construct cascade system in combination with glucose oxidase (GOx), and a 4-fold increase of the cascade output was achieved. Moreover, we demonstrated that this strategy could help improve the detection of biomolecules including H2O2, glucose, and ascorbate. We speculate that this proton-confinement strategy will shed light on modulation of nanozymes' activity further expanding their applications in the field of biosensing and others.

Engineering psychological pH active nanozymes via nanoconfinement effect

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