Extraordinary Waveguide-Enhanced Optical Microfiber Sensor for Operando Electrocatalysis Studies

Optical detection and sensing have been widely applied to electrochemical systems, and their cutting-edge technology is creating current trends in operando characterization. The mass transfer at the electrode–electrolyte interface induces not only the electron exchange but also the changes in optical properties such as dielectric constants, resulting in detectable absorption or resonance signals. However, light–matter interactions are limited due to the inherently short optical path length of the interface. Here, we report the ultrasensitive detection of electrocatalytic processes enhanced by waveguide-engineered modal interference. We show that, by modulating the microfiber diameter so that the group phase velocity of beating modes approaches equalization, the substantially enhanced refractive index sensitivity enables accurate capture of chemical dynamics near the electrode surface, presenting a clear "eye diagram" related to the methanol oxidation reaction during operando studies.