On-chip photonic label-free biosensors

The development of a highly integrated optical biosensor is expected to significantly impact on the performances and on the throughput of biochemical assays, with applications in the field of pharmaceutical research, point-of-care diagnostic, food-borne pathogens screening and safety. This dissertation studies the development of a label-free on-chip biosensor for the selective detection of Aflatoxin-M1 from milk content. We detail the design and the realization of two types of multiplexed sensors. They are based on the silicon photonics technology and operates in liquid ambient at wavelengths in the near-visible and near-infrared spectra. Most of this work is focused on the first type of sensor,which is based on a whispering-gallery-mode resonator. In particular, we analyze microdisk, microring and wedge resonator structures, studying the sensitivity and the quality factor of each. The appeal of these structure is given by the low detection limit that can be achieved in a footprint of few tens of microns per side. The second sensor type is based on a spectrally resolved asymmetric Mach-Zehnder interferometer. In this case, the high level of folding permitted by the use of high refractive-index-contrast materials enables the fabrication of sensitive interferometers in a reasonable footprint. The experimental characterization of the bulk refractometric sensing of the devices is performed in continuous flow, using a dedicated microfluidic flow-cell in PDMS. This characterization assesses the high resolution of both device types, which are able to resolve variations in the refractive index of the liquids with a limit of detection down to 10^-􀀀6 refractive index units (RIU). The selective superficial sensing is also evaluated, implementing a biorecognition functional layer with DNA-aptamers. The assay of buffered solutions containing Aflatoxin-M1 molecules confirms that the devices under test are suitable biosensors, with specific detection limits down to about 100 pg/ml􀀀 for the Mach-Zehnder interferometer, and slightly larger for the microring resonator. A procedure for the regeneration of the sensor has been optimized, enabling reproducible sensing up to nine times. The detection of the receptor-ligand binding in real-time enabled the study of the kinetics of the binding reaction, and we measured for the first time the kinetic rate constants of the antiaflatoxin aptamers of our sensors.