Direct writing of optical microresonators in a lab-on-a-chip for label-free biosensing.

Whispering gallery mode (WGM) resonators are promising optical structures for microfluidic label-free biosensors mainly due to their high sensitivity, but from a practical point of view they present numerous constraints that make their use in real laboratory diagnosis application difficult. Herein we report on a monolithic lab on a chip fabricated by a hybrid femtosecond laser micromachining approach, for label-free biosensing. It consists of a polymer WGM microresonator sensor integrated inside a glass microfluidic chip, presenting a refractive index change sensitivity of 61 nm per RIU. The biosensing capabilities of the device have been demonstrated by exploiting the biotin-streptavidin binding affinity, obtaining a measurable minimum surface density increase of 67 × 103 molecules per μm2.