Cu2O facet controlled reactivity for peroxidase-like activity

Abstract

Replicating the enzymatic surface microenvironment in vitro is challenging, but constructing an analogous model could facilitate our understanding of surface effects and aid in developing an efficient bioinspired catalytic system. In this study, we generate five unique Cu₂O morphologies: cubic (c-Cu₂O), octahedral (o-Cu₂O), rhombododecahedral (r-Cu₂O), cuboctahedral (co-Cu₂O), and spherical (s-Cu₂O) with an average edge length between 0.59 and 0.61 μm. By precisely controlling crystal growth on distinct surface planes, with surface energies in the order γ_{100} < γ_{111} < γ_{110}, we achieve a wide spectrum of shape variations during the evolution of these structures. The variations in surface morphology are a consequence of differences in the exposure of low-index facets, such as {100}, {111}, and {110}, leading to varying numbers of unsaturated copper sites at the surface. The peroxidase-like activity was investigated by altering the Cu₂O surface structures in the reduction of H₂O₂ using chromogenic substrates like TMB/ABTS. The activity catalyzed by various Cu₂O surfaces (r-Cu₂O, o-Cu₂O, and c-Cu₂O) followed the typical Michaelis–Menten kinetics, with K_m values ranging from 0.096 to 0.120 mM for TMB and 1.57 to 2.90 mM for H₂O₂ as substrates, respectively. Compared to native peroxidase enzymes (with K_m values of 0.434 mM for TMB and 3.70 mM for H₂O₂ under identical conditions), these Cu₂O catalysts exhibited a higher affinity. In general, the reactivity order observed was as follows: co-Cu₂O ≈ r-Cu₂O > o-Cu₂O > c-Cu₂O > s-Cu₂O. Mechanistically, the H₂O₂ reduction on the surface produces hydroxyl radicals that undergo H-abstraction from TMB, where the latter was found to be the rate-determining step. Both kinetic and DFT studies have unveiled that the heightened reactivity of r-Cu₂O can be attributed to its higher proportion of {110} planes, which contain a higher number of dangling (unsaturated) Cu atoms that facilitate H₂O₂ decomposition. Additionally, they exhibit sufficiently low energy barriers (TS2: 0.70 eV) that enable OH radicals to efficiently oxidize TMB molecules compared to other morphologies.