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Sol–gel films doped with fluorescein (FL) and phenol red (PR) dyes were evaluated for their responses to ammonia gas. In a steady state, the sensors respond to the gas-phase ammonia evolved from ammonium hydroxide solutions in the range of 5 × 10−6 to 0.05 M for FL-doped gels, and 10−4 to 1 M for PR-doped gels. The sensors have high selectivity for ammonia against larger amines and moderate selectivity against methyl and ethylamines. The FL-gel exhibits rapid and reproducible response to dry NH3 gas samples of 9–50 ppm, while PR-gel is insensitive to ammonia in this low concentration range. The life-times of the sensors are over a year.

A dye doped sol–gel for the sensing of hydrochloric acid in solution and/or gaseous phase is described. The sol–gel is obtained by acidic hydrolysis of tetraethoxysilane (TEOS) and phenyltriethoxysilane (Ph-TriEOS) in the presence of phenol red (PR) and further spin-coating onto glass slides. The sensitive response is based on an increase of the absorption band at 510 nm of phenol red entrapped in the sol–gel casting when exposed to HCl solution or gas, due to protonation of the dye. The detection limit of the sol–gel response to moisturized gaseous HCl is below 12 ppm, and its response to HCl in solution falls in the range of 0.01–6 M. The sol–gel coating has a response time of less than 40 s in steady-state, and life-time of more than a year. Weak acids such as acetic acid, benzoic acid, salicylate acid, citrate acid, and carbonic acid do not interfere the response. The responses in acid solutions are completely reversible. In the gaseous phase, response of HCl appears to be moisture sensitive.