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

Ammonia (NH₃) and carbon dioxide (CO2) 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 CO₂ 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 NH₃ concentration in the range from 0 ppm to 80 ppm. The limit of detection and response time for CO₂ and NH₃ are 637 ppm and 0.15 ppm, 86 s and 83 s, respectively. The proposed sensor demonstrates the capability of sensing NH₃ and CO₂ 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.