In Situ Visualizing Oxidase-Mimicking Activity of Single MnOOH Nanotubes with Mie Scattering-Based Absorption Microscopy

Imaging the catalytic activity at the single-particle level can greatly promote the screening and rational design of highly efficient nanozymes, but conventional techniques are based on ensemble analysis. Here, we present a new absorption microscopy for in situ visualizing oxidase-mimicking activity of single MnOOH nanotubes. The particle with a size more than 700 nm roughly equally scatters all wavelengths of visible light via Mie scattering, and the scattering light is collected by dark-field optical microscopy. When the particles absorb a single color of the scattering light, each individual nanoparticle shows its complementary color, enabling a form of absorption microscopy that we name Mie scattering-based absorption microscopy. We find that MnOOH nanotubes can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to generate polyTMB nanowires at their tips. There are multiple active sites on the surface of the individual nanotube, and the nanozyme activity shows a large heterogeneity as well as pH-dependent characteristic.