The evolution of pathogens has increased the demand for a sensing and detection platform, capable of qualifying constituents in real time. Whispering Gallery Mode Resonators provide an ideal biochemical sensing platform due to their low cost, high sensitivity, and low impact on the analyte. These resonators have high quality factors and possess the ability to detect minute changes in the local environment, as the light traveling on the surface of the resonator, when at resonance interacts with the surrounding medium for interaction lengths on the order of ~10-100cm’s . These changes in physical properties are captured through shifts of the resonance wavelength, resonance dip intensity, and/or quality factor. In this work, we provide our design of a 3-D printed microfluidic cell that is compatible with our taper and sphere coupling scheme developed from our previous work. Initially, the baseline performance of the resonator fluidic system was established by measuring the resonance wavelength shift due to refractive index change from water to phosphate buffered saline (PBS). Next, we showcase our biofunctionalization procedure and measure the accumulation of pathogens, such as E. Coli and Influenza A, on the resonator’s surface. The presence of these biological analytes results in small changes in the resonator’s diameter and refractive index, which manifests in real time as a red shift of the resonance wavelength on the picometer scale. Finally, we develop the foundation for a silicon integrated circuit chip resonator system, resulting in a further reduction of our system’s footprint.
This paper discusses ongoing research at Lawrence Livermore National Laboratory (LLNL) that investigates the effectiveness of a whispering gallery mode micro-resonator as a biosensor. Whispering Gallery mode resonators have properties such as ultrahigh quality factors (Q factors up to 1011), very high power density, and small mode volume that make them suitable for sensing applications1. In this work, silica microspheres (spheres on the order of 250μm) are used as resonators. These resonators are coupled to a tapered optical fiber connected to an infrared laser. Using critical coupling techniques, resonant wavelengths (wavelengths of zero power transmission) are produced. The resonant wavelengths of the coupled system are dependent upon properties of the microsphere such as diameter and index of refraction. Conjugation of biological organisms to the sphere causes a small change in these properties and thus creates a shift in resonant wavelengths (free spectral range) which can be characterized and used as a sensor. This paper will discuss microsphere and taper fabrication, the tested functionalization process, and the effect conjugation has on the microsphere Q factors. Future work includes real time analysis of biological organism conjugation and bringing the sensor down to the chip sized scale.
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