Integrated multiplexed optical biosensors for rapid diagnostics

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The ability of light to act as a non-contact probe of its environment provides optical sensors with many attractive capabilities. Whilst various optical sensing technologies are already widely used for many different applications in the biomedical, environmental and industrial sector, this technology has not yet achieved its full potential. Optical micro-resonators represent the most sensitive class of label-free optical sensors. Light propagates multiple times around the inside of the resonator and any change to the surface properties will influence the resonant frequency, which can be measured. A biosensor can be made by coating the surface with a chemistry that will bind specifically to the molecules to be detected. The challenge of current micro-resonator sensors is that they need to be interrogated by using tunable lasers to measure the change of resonant frequency. This adds to cost and complexity and represent a significant barrier to integration. Our objective is to develop very sensitive and integrated multi-channel optical sensors that require only low cost and light sources and photodetectors. Over the past ten years we have investigated the application of a time-domain technique, namely phase shift cavity ring down spectroscopy (PS-CRDS) to bio-detection in optical micro-resonators. We make use of the fact that a surface binding event will also change the length of time that a photon remains in the cavity, which can be measured by detecting the signal phase shift when sinusoidally modulated light is injected. This requires modest (RF) modulation rates and a simple photodetector. In a series of experiments we have demonstrated that PS-CRDS in optical microcavities can be used to measure specific absorption and desorption of biomolecules, and that PS-CRDS can be implemented with a source that has a bandwidth spanning multiple resonance peaks, removing the need for a tunable laser. However, the next challenge, and the subject of this research program, is to implement multiplexed biosensing in an integrated array of micro-ring resonators, allowing multiple biomarkers to be detected. We must overcome significant challenges in terms of device design, modeling, fabrication, signal processing and packaging. We will design sensors for fabrication in silicon nitride and silica and will collaborate with experts in microfluidics and surface functionalization to develop suitable sample delivery and capture techniques. We will develop a scattering model to understand how surface absorption influences the signal, and by introducing signal processing techniques from telecommunications we will be implement a high degree of multiplexing. Working with domain experts we will validate our sensors by implementing a sensor array for the detection of liver and prostate cancers, water-borne pathogens and traumatic brain injury. This interdisciplinary program will provide high quality training to 17 graduate and undergraduate students over 5 years.