Functional Film Coated Optical Micro/Nanofibers for High-Performance Gas Sensing

One of the challenges for optical micro/nanofiber (MNF) gas sensors is reliably measuring small volumes of samples with low detection limit and fast response. Herein, we introduce a concept of utilizing functional film coated optical MNFs for gas sensing. As a proof-of-concept, when crystal violet (CV) doped Nafion film was used for humidity sensing, the sensor exhibited nearly linear response to humidity in the range of 30%–100% relative humidity (RH), with a response time of ~150 ms and a resolution of 0.3% RH. Also, highly sensitive ammonia sensing was demonstrated by using bromothymol blue (BTB) doped polyvinyl alcohol (PVA) film, achieving a detection limit down to 30 ppb. This concept enhances the sensors’ stability by enclosing the MNF completely with polydimethylsiloxane (PDMS) microflow cell and functional film rather than suspending MNFs in a conventional large volume flow chamber. The higher sensitivity and lower detection limit are attributed to the gas-induced color changes in the pH indicator doped polymer film which improve evanescent absorption significantly. The shorter response time is due to the drastically reduced diffusional pathlength of gas by the thin layer of polymer film. Compared with the delicate surface coating technique, its simple fabrication procedure and low production cost boost the development fiber-optic sensing technology. We anticipate that the study is a starting point for practical applications of the MNF sensors.