In situ ppb-level dissolved gas detection with micro-gas-cavity-based fiber photoacoustic spectrometer

In situ and continuous monitoring of the gas concentration can provide valuable insights into the cell viability and allow accurate concentration control of signaling gas molecule for disease therapy. However, current techniques are mostly benchtop instrument for off-line dissolved gas analysis or require the cell labelling disturbing the cell normal activities. To achieve in-situ, fast and sensitive dissolved gas detection, a miniature fiber photoacoustic spectrometer integrating the gas cell, gas-liquid separator and analyzer into a single micro-gas-cavity at the fiber tip is developed here. The gas-liquid interface directly separates at the gas from the liquid and simultaneously acts as an acoustic sensitive element that detects the locally-generated photoacoustic waves from excited gas molecules in the micro-gas-cavity. The flexible gas-liquid interface forms a Fabry-Perot cavity with the fiber tip end fact and reads out the acoustic wave induced deformation with high-resolution fiber-optic interferometry. A noise-equivalent pressure level of ~ 270 μPa/Hz^1/2 is achieved, which allows sensitive dissolved acetylene with a concentration down to 500 ppb and a response time ~ 10 s. Temporal evolutions of the carbon dioxide concentration for the normal and cancer cells at different glucose levels and the algae cells at different photosynthesis conditions are recorded in situ and continuously. The proposed photoacoustic spectrometer with microscale size, high-sensitivity, and fast response provides an all-optical technique for in-situ and continuous dissolved gas monitoring in diverse areas ranging from biological science to medical treatment.