pH-responsive hydrogels embedded in hollow-core optical resonators.

Whispering-gallery-mode (WGM) microresonators are typically studied for surface (bio)chemical sensing, mainly relying on small refractive index changes occurring within a nanometer range from their walls surface. This high sensitivity, reaching up to 10 refractive index unit (RIU, ∼2.5 nm/RIU and measured at a femtometer resolution) leads to broad ranges of applications, especially for biosensing purposes through the monitoring of molecular binding events. In this article, we investigate the gelling of thin layers of poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) hydrogels inside a WGM microbubble resonator (MBR), fabricated from a silica capillary. The formation of such layers is achieved by withdrawing a liquid solution of 25% PVA/PAA in pure water into the MBR and locally heating the microbubble region, leading to hydrogel formation only in the WGM cavity. The capillary is then rinsed and tested under varying pH solutions. The swelling ability of these hydrogels is proportional to the pH of samples brought into contact with the cavity, leading to physical deformations of the layers consequently leading to changes in the WGM resonance condition. We show the preliminary results obtained for the gelling and characterization of these thin layers in microbubble resonators and present the related wavelength shifts observed for several pH values. We discuss the kinetics and practical uses, such as reversibility and tunable detection of small pH changes.