Electrochemistry in an Optical Fiber Microcavity - Optical Monitoring of Electrochemical Processes in Picoliter Volumes

In this work, we demonstrate a novel method for multi-domain analysis of properties of analytes in volumes as small as picoliter, combining electrochemistry and optical measurements. A microcavity in-line Mach-Zehnder interferometer (µIMZI) obtained in a standard single-mode optical fiber using femtosecond laser micromachining was able to accommodate a microelectrode and optically monitor electrochemical processes inside the fiber. The interferometer shows exceptional sensitivity to changes in optical properties of analytes in the microcavity. We show that the optical readout follows the electrochemical reactions. Here, the redox probe (ferrocenedimethanol) undergoing reactions of oxidation and reduction changes the optical properties of the analyte (refractive index and absorbance) that are monitored by the µIMZI. Measurements have been supported by numerical analysis of both optical and electrochemical phenomena. On top of a capability of the approach to perform analysis in microscale, the difference between oxidized and reduced forms in the near-infrared can be clearly measured using the µIMZI, which is hardly possible using other optical techniques. The proposed multi-domain concept is a promising approach for highly reliable and ultrasensitive chemo- and biosensing.