In-situ synthesis of zeolitic imidazolate framework-8 on an optical microbubble cavity for enhanced ethanol sensing.

Optical whispering-gallery-mode microcavities have been applied for ultra-sensitive detection of physical parameters and nano-scale analytes due to strong light-matter interactions. Surface treatments to enhance mode confinement or increase analyte interaction with the optical field are effective methods to improve the sensing performance. In this work, an optical microbubble cavity fabricated via heat-and-pull and arc discharge techniques is functionalized with zeolitic imidazolate framework-8 (ZIF-8), a subclass of metal-organic frameworks (MOFs), for concentration sensing of ethanol solutions. The bubble-shaped structure forms an internal microfluidic channel to sustain the analyte solution, which provides an excellent platform for trace detection. By growing a multilayer of MOFs on the inner surface of the microcavity, the sensitivity is significantly enhanced, with distinct behaviors observed when detecting ethanol solution concentrations lower or higher than 0.1%. Compared to a pure silica microbubble cavity, the sensitivity is enhanced by 47 times and 7 times at lower and higher concentrations, respectively. The knowledge gained on sensitivity enhancement of microcavities induced by surface functionalization will guide the combinations of novel materials and optical sensors for superior detection demands.