From Microscale to Nanoscale Shadow Electrochemiluminescence Microscopy.

Electrochemiluminescence (ECL) has emerged as a promising chemistry-based imaging technique, yet its potential in optical nano-imaging is still in the early stages. We report the label-free imaging of microobjects and nanoobjects with a sharp negative optical contrast. The shadow ECL images obtained are the optical signatures of the blocking of the electron-transfer reaction convoluted by the chemical reactivity and local diffusional hindrance of the ECL reagents by the object deposited on the electrode. Here, we address first the critical aspect of the smallest spherical nanoparticle imaged by shadow ECL microscopy. By investigating the imaging limits of shadow ECL, we demonstrate its capability to image single nanoparticles down to 50 nm. This work uncovers the sensitivity of the method by not only showing the smallest particles imaged by shadow ECL to date, but also, counterintuitively, a contrastive behavior of the single insulating particles with increased electrochemical rates at the insulator-electrolyte-electrode triple point. Finally, the practical utility of this shadow ECL method is illustrated by imaging complex samples such as densely packed particle assemblies or microbial Bacillus subtilis spores. It also opens new avenues for single-molecule and shadow ECL nano imaging of catalytic and biological systems.