Integrated optical circuits and dielectrophoresis: Towards bacterial sensing applications

Developing real-time, label-free and portable bio-sensors for water monitoring is crucial for the general well-being. Glass integrated photonics is a promising technology to employ due to its bio-compatibility, robustness and high sensitivity. This project is dedicated to the development of an optical bio-sensor based on a principle of interferometry or on absorption and scattering effects and uses electrokinetic phenomena in alternating current to capture bacteria sensitive to pollution. The compatibility of the electrodes with the glass and the absence of surface functionalization make it a perfectly suitable solution for reliable and low-cost sensors. In this study, the photonic circuit, implemented by silver-sodium ion-exchange on glass, is co-integrated using standard photolithography processes with aluminum electrodes designed to optimize dielectrophoresis. Charged polystyrene latex beads (Aldrich, CLB9) dispersed in deionized water are used to model the bacteria. The sensor is exposed to the sample either through a microfluidic channel made of polydimethylsiloxane (PDMS) or by directly depositing a drop on the sensitive surface. Various alternating current signals are applied on the electrodes and their effects on the capture of the micro-spheres are evaluated. A home-made Python script is also developed for the analysis of the trapping recorded via an optical microscope. We found that a frequency in the range of 50 - 200 kHz is needed to maximize the trapping of the beads with an applied voltage of at least 20 Vpp. We also observed a variation in the optical losses of the guided optical signal at a wavelength of 1550 nm by selectively controlling the trapping. The response times of the sensor following alternating trapping and de-trapping of the beads are of the order of minutes with relative signal variations greater than 8% between these two states.