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Summary form only given. Direct monitoring of primary molecular binding interactions without the need for secondary reactants would markedly simplify and expand applications of high-throughput label-free detection methods. We have developed the Interferometric Reflectance Imaging Sensor (IRIS) for label-free, high throughput, high sensitivity and dynamic detection of molecular binding on a solid surface. IRIS has demonstrated protein-protein binding and DNA-protein binding in real time, label-free, and in a high-throughput format with exquisite sensitivity (~10 pg/mm2) and reproducibility [1,2] as well as label-free measurements of DNA hybridization kinetics  and viral detection . Recently, we have significantly advanced IRIS beyond the original tunable-laser configuration  and implemented a multi-LED discrete wavelength system that allows for high spatial resolution imaging with the demonstrated ability to detect single nanoscale particles . To detect and size pathogens, IRIS shines light from multi-color LED sources sequentially on nanoparticles bound to the sensor surface, which consists of a silicon dioxide layer on top of a silicon substrate. Interference of light reflected from the sensor surface is modified by the presence of particles producing a distinct signal that reveals the size of the particle. We have successfully detected 35 nm and 50 nm radius particles and H1N1 viruses (illustrated in the conceptual picture) with accurate size discrimination . The device has a very large surface area and can capture the telltale interferometric responses, in parallel, of up to a million nanoparticles. Size discrimination allows for reducing the noise resulting from many smaller particles that may be present in the target solution that may bind to the sensor indiscriminately. IRIS has a great advantage in that we can employ both polarization and pupil function engineering to characterize the shape, size and orientation of particles with res- - olution beyond the classical diffraction limit.