Engineering Sensitivity: An Optical Optimization of Ring Resonator Arrays for Label-Free Whole Bacterial Sensing
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The quick, reliable, and sensitive detection of bacterial contamination
is desired in areas such as counter bioterrorism, medicine, and food/water
safety as pathogens such as E. coli can cause harmful effects with the
presence of just a few cells. However, standard high sensitivity techniques
require laboratories and trained technicians, requiring significant time and
expense. More desirable would be a sensitive point-of-care device that could
detect an array of pathogens without sample pre-treatment, or a continuous
monitoring device operating without the need for frequent operator
Optical microring resonators in silicon photonic platforms are particularly promising as scalable, multiplexed refractive index sensors for an integrated biosensing array. However, no systematic effort has been made to optimize the sensitivity of microrings for the detection of relatively large discrete analytes such as bacteria, which differs from the commonly considered cases of fluid or molecular sensitivity. This work demonstrates the feasibility of using high finesse microrings to detect whole bacterial cells with single cell resolution over a full range of potential analyte-to-sensor binding scenarios. Sensitivity parameters describing the case of discrete analyte detection are derived and used to guide computational optimization of microrings and their constituent waveguides, after considering a range of parameters such as waveguide dimension, material, modal polarization, and ring radius. The sensitivity of the optimized 2.5 µm radius silicon TM O-band ring is experimentally demonstrated with photoresist cellular simulants. A multiplexed optimized ring array is then shown to detect E. Coli cells in an experimental proof of concept.