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

National Science Centre Poland (NCN), NCN 2018/29/B/ST7/02552

Microfluidics provide unique possibilities to control tiny volumes of liquids and their composition. In this work we combined a microfluidic system with 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 volumes down to picoliters with an exceptionally high refractive index 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 the influence of flow rate and injection volume on the response, and that the orientation of the cavity versus the flow direction has only a minor impact on the results. Finally, we have supported the system by band electrodes making it possible to induce redox reactions in the microchannel to detect the flowing products of the reactions optically. 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. The capability for monitoring the products was shown for a standard redox probe, potassium ferricyanide, as well as for the neurotransmitter dopamine. This work shows 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.