Super-Resolution Imaging of Corrosion by Single Molecule Fluorescence Microscopy

Scientists and researchers have been studying corrosion since the founding of modern chemistry. A routine schematic in every general chemistry textbook shows individual electrons and metal ions undergoing reactions, diffusion, and dissolution at a pair of spatially-distinct electrodes — the cathode and anode — at a metal/solution interface. Yet, the correlation between the anode and cathode over both space and time has never been directly measured at the molecular scale. Physicists, electrochemists, and engineers give vastly different answers spanning orders of magnitude on the distance cathodic electrons can be detected away from anodic sites.

Temporal and spatial limitations of the techniques traditionally used are not able image corrosion in situ at the molecular level. Single molecule fluorescence microscopy provides an ideal way to detect in-situ corrosion at the molecular scale. Individual fluorescent molecules present in a sample can be observed at the highest resolution by a highly sensitive camera. Desired chemical process can be studied by selecting dyes that undergo specific fluorogenic reactions. Heterogeneities hidden in the conventional experiments can be determined by observing one molecule at a time. Millisecond time resolutions and sub-diffraction limited, three-dimensional spatial resolutions at ~10 nm can be achieved in-situ for single molecules.

We have demonstrated that redox-sensing molecules that have become fluorescent either by receiving an electron at the cathode or a metal ion at the anode can be detected at the single molecule scale and can quantify corrosion rates. We are now pursuing super-resolution analysis of the dyes to determine the locations of both the cathode and anode at ~10’s nm spatial and ~1’s ms temporal resolutions. Aspects of optical and sample design to achieve super-resolutions will be discussed, along with our spatiotemporal findings for iron and steel, and future prospects for expanding our technique to real world conditions.