A composite optical waveguide (OWG) gas sensor was fabricated by coating a film of a pH indicator dye (immobilized Congo red (CR) on crosslinking polyvinyl alcohol (PVA), CR-PVA) on two thin guiding layers of the optical waveguide (composite OWG). The pH indicator dye was synthesized using CR and crosslinking PVA. The dye changes its color from red to blue in the presence of an acidic gas such as hydrogen chloride (HCl) and strongly absorbs the evanescent wave in the CR-PVA film. The sensor detected 62 parts per billion (ppb) of HCl reversibly, and its response time was 3 s. The present sensor is selective to HCl gas, and its characteristic features include simple structure and simple fabrication.
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