Multi-Parameter Optical Fiber Sensing of Gaseous Ammonia and Carbon Dioxide

Ammonia (NH 3 ) and carbon dioxide (CO 2 ) are important breath biomarkers for diagnosis of renal and respiratory diseases.Simultaneous sensing of the two gases is useful for assessment of Helicobacter Pylori infection.This article introduces a multi-parameter optical fibre sensor by coating two different dyes, i.e., thymol blue and tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS) into one optical fibre sensing system for multi-sensing of the two gases.The dyes are separately coated on distal ends of a 2 × 2 optical fibre coupler using the sol-gel technique.The reflection spectrum contains distinctive absorption peaks of the two dyes in the wavelength domain and hence can be analysed using a single spectrometer.The absorbance at a wavelength of 608 nm corresponding to the absorption peak of the thymol blue exhibits an exponential decrease as the CO 2 concentration increases from 0 ppm to 60 000 ppm.The absorption bands at approximately 490 nm and 711 nm corresponding to the Soret band and Q-band of TPPS, respectively, show changes both in absorbance and central wavelengths in response to NH 3 concentration in the range from 0 ppm to 80 ppm.The limit of detection and response time for CO 2 and NH 3 are 637 ppm and 0.15 ppm, 86 s and 83 s, respectively.The proposed sensor demonstrates the capability of sensing NH 3 and CO 2 simultaneously in a complex gas atmosphere containing other chemicals such as methanol and acetone, which are representative of industrial or metabolic compounds that may confound measurements.