Cu2O facet controlled reactivity for peroxidase-like activity

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 mu m. By precisely controlling crystal growth on distinct surface planes, with surface energies in the order gamma({100}) < gamma({111}) < gamma({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 approximate to 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.