A pH sensor can help understand chemical conditions of solutions, such as precise cell culture medium monitoring in real time. High-quality whispering-gallery-mode (WGM) microresonators have been utilized for surface sensing and are mainly based on the tracking of refractive index changes occurring within a wavelength range from their wall surface. This high sensitivity, reaching up to 10-5 RIU (~2.5 nm/RIU and measured at a femtometer resolution) leads to a broad range of applications, especially for biosensing purposes through the monitoring of molecular binding events. Here, we study the deposition of thin layers of poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) hydrogels inside a whispering gallery mode (WGM) microbubble resonator (MBR), fabricated inline with a silica capillary. The generation 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, resulting in hydrogel formation only in the cavity. The capillary is then rinsed and tested with varying pH solutions. The swelling ability of these gels is directly proportional to the pH of samples brought into contact with the cavity, leading to physical modifications of the WGM coupling properties. We show the preliminary results obtained for the polymerization and characterization of these gels in microbubbles and present the related signal shifts observed for several pH values. We also discuss the gel kinetics over time and investigate practical uses such as reversible and tunable detection of small pH changes.

Optical Whispering Gallery Mode (WGM) microresonators have shown great promise in sensing applications. Various efforts have been made to package WGM sensors in order to enhance their robustness for field applications. Previously, polydimethylsiloxane (PDMS) and other low index polymers have been employed for packaging of WGM sensors. However, the long curing time of PDMS and the rigidity of polymers pose other difficulties, thereby limiting their performance. Hydrogels, with shorter polymerization time and increased flexibility, transparency and biocompatibility, offer a great alternative to current packaging materials. The flexibility and optical transparency of hydrogels, in conjunction with the capability to functionalize them for specific applications, provide superior functionality and improved stability of WGM sensors. Herein we propose the use of laponite nanoclay and N, N, dimethylacrylamide (DMAA) based hydrogel for the packaging of WGM sensors. Microbubble resonators were used for the demonstration of hydrogel-based packaging. The hydrogel was synthesized by mixing DMAA in exfoliated nanoclay suspension at different concentrations with sodium persulfate as an initiator and Tetramethylethylenediamine (TEMED) as an accelerator. The microbubble resonators were packaged on 3D-printed chips and were characterized through their transmission spectra.