Label-free nano/bio-sensors based on whispering gallery mode (WGM) resonators have been continuously studied over the past decade in virtue of their high sensitivity attained from high quality factor and small mode volume. However, there is still considerable gap between its academic achievements and practical applications which require not only high sensitivity but also low cost and ease of use.
In this research, WGM resonator sensors integrated with microfluidic circuits are demonstrated in simple practical form released from strict coupling scheme of tapered fibers, which have long been considered as an essential element but a main hurdle for real applications. By taking advantage of large absorption cross-section of silicon nano-crystals, pump light exposed from the top of silicon-rich nitride (SRN) resonators can efficiently induce light emission spectrum peaks along the resonant modes which are detected by a spectrometer through free space optics. The shift of emission peaks revealing nanoparticles into measurable domain is around 5 times enhanced by nano-slot structure which strongly increases light-matter interaction by tightly confining the mode field in it. The quality factor of the resonators measured in aqueous channels is 7000 corresponding to 0.1 nm of full width at half maximum comparable to the resolution limit of the spectrometer, 0.05 nm. The real-time sensing experiments for the specific binding process of streptavidin to biotin pre-processed on the resonator slot surface showed 60 pM detection limit and 2.14 x 10^14 affinity constant attained from analyzing the binding kinetics by Langmuir model, which corresponds well to the theoretical value.