Thermal effect of near-infrared lasers in single nanoparticle whispering-gallery-mode sensing in aqueous solution

Optical whispering-gallery mode (WGM) sensors have been developed for the detection of clinically relevant biomolecules, detection of materials in different phases and forms, and field-based sensing applications. The optical resonance inside a microsphere cavity, induced by total internal reflection that excites the WGM, and the small refractive index change caused by the loading of nano-scale particles, results in a detectable shift of the resonant dip. We confirmed this mode volume-dependent reactive sensing mechanism by using SiO2 nanoparticles and obtained the optimal microsphere radius by verifying the wavelength shift through controlling the size of the microsphere. By comparing the simulated and experimental results, the sensitivity of the resonant sensing is approaching the single nanoparticle regime. Then, a microfluidic system was built to investigate the influence of the resonant dip shift caused by the rise of the water temperature through the heating effect of near-infrared lasers. The results showed that the microfluidic system could greatly suppress the thermal effect even using a 1550 nm laser. Furthermore, when using the microfluidic system, the WGM sensing using the 1550 nm laser shows higher stability than that using the 852 nm laser. This work offers a possibility for the WGM sensing in aqueous solutions with the c-band lasers.