A microfluidic system for analysis of electrochemical processing using a highly sensitive optical fiber microcavity

Microfluidics provide unique possibilities to control tiny volumes of liquids and their composition. To effectively benefit from the advantages of microfluidic solutions they need to be supported by interrogation subsystems, at best also matching the miniature scale. In this work we combined with a microfluidic system a Microcavity in-line Mach-Zehnder Interferometer (µIMZI) induced in the side surface of a single-mode optical fiber using a femtosecond laser micromachining. The µIMZI shows capability for investigating optical properties of as small as picoliter volumes with an exceptionally high sensitivity. Here we report numerical analysis and experimental results that show that when the µIMZI is incorporated with the microfluidic system the measurements can be performed with sensitivity exceeding 14,000 nm/RIU which is similar to measurements done under static conditions. In a flow injection system we show that a certain amount of liquid and flow rate are required to effectively exchange the liquid in the microcavity, while orientation of the cavity versus the flow direction has a minor impact on the exchange. Finally, we have supported the system by band electrodes making it possible to induce redox reactions in the microchannel and optical detection of flowing products of the reactions. It has been found that thanks to the high sensitivity of the µIMZI the products of the reactions can be clearly detected both electrochemically and optically even when the only part of the flowing redox probe is oxidized at the band electrode. This work proves that the proposed solution may offer highly sensitive optical measurements, even when the chemical reactions are not effective in the whole volume of the system.