On-chip label-free biosensing based on active whispering gallery mode resonators pumped by a light-emitting diode.

Biosensing based on whispering-gallery mode (WGM) resonators has been continuously studied with great attention due to its excellent sensitivity guaranteeing the label-free detection. However, its practical impact is insignificant to date despite notable achievements in academic research. Here, we demonstrate a novel practical platform of on-chip WGM sensors integrated with microfluidic channels. By placing silicon nanoclusters as a stable active compound in micro-resonators, the sensor chip can be operated with a remote pump and readout, which simplifies the chip integration and connection to the external setup. In addition, silicon nanoclusters having large absorption cross-section over broad wavelength range allow active sensing for the first time with an LED pump in a top-illumination scheme which significantly reduces the complexity and cost of the measurement setup. The nano-slot structure of 25 nm gap width is embedded in the resonator where the target bio-molecules are selectively detected with the sensitivity enhanced by strongly confined mode-field. The sensitivity confirmed by real-time measurements for the streptavidin-biotin complex is 0.012 nm/nM, improved over 20 times larger than the previously reported WGM sensors with remote readout.