• Development of bioanalytical lab-on-a-chip devices based on interferometric monolithic optoelectronic transducers (bioactivated optocouplers) for highly sensitive and label free assays suitable for Point of Care applications
Background and Technology
Miniaturized bioanalytical devices find applications in areas vital to the public interest such as personalized health care and detection of pathogens in food. Optical detection is advantageous to other types of sensing as the optical frequency regime of operation and the galvanic isolation of the transducer from the excitation and detection electronics suppress unwanted parasitic currents and signal drifts. In the area of optical biosensing devices, the particular category of planar interferometric waveguides is far more sensitive compared to free space optics. In an integrated interferometer the photons in the sensing waveguide probe the biomolecular adlayer hundreds or thousand times compared to a couple of times in reflectometric interference spectroscopy or ellipsometric and SPR methods.
The sensitivity enhancement and immunity to parasitics make planar waveguide based interference devices ideal for label free testing. The problem so far was the inability to monolithically integrate active optical components on the same chip. Our solution to this problem is interferometric silicon chips with monolithically integrated light emitting devices (LEDs). The LEDs employed are silicon avalanche diodes that emit white light when biased beyond their breakdown voltage. In the monolithic chips such LEDs are coupled to co-integrated silicon nitride monomodal waveguides that form Mach-Zehnder interferometers (MZI) through mainstream silicon technology. The silicon LEDs feed their white light to the interferometer and following the second Y junction the modulated output spectrum is monitored by a spectrometer. This Broad-Band Mach-Zehnder Interferometry configuration greatly enhances the spectral shifts compared to other type of single waveguide interferometers, like ring resonators. In a ring resonator the spectral shifts upon effective medium changes are inversely proportional to the wavelength derivative of the Ns/λ ratio, where Ns the resonator effective refractive index. In a MZI, with reference and sensing arm effective refractive indices equal to Nr and Ns, respectively, the spectral shifts are inversely proportional to the wavelength derivative of the (Nr-Ns)/λ ratio. The latter is about two orders of magnitude smaller than the former one, resulting in equally higher spectral shift sensitivities. Such a spectral shift enhancement in association with broad band nature of the spectral output makes possible the use of portable spectrometers as high resolution detectors, thus enabling high performance Point of Care determinations.