High-acidity optical sensors based on sol-gel-derived thin films

Optical sensors have been developed for high-acidity ([H+] = 1-9 M) measurements. The indicators Rhodamine B and Safranine T were entrapped in porous silica glass films prepared by the sol-gel method. The entrapped indicators were nonleachable and respond to high acid concentrations in a reversible manner. The influences of different experimental parameters such as the catalyst concentration, the water/tetraethoxy silane ratio, the amount of co-solvent, and the amount of surfactant on the chemical and mechanical stabilities of the sol-gel films were investigated. The optical sensors possess a short response time of <5 s and are stable for a period of at least 3 months. The relative standard deviation of the absorbance measurements is <2%. The interfering effect of different alkali, alkaline earth and transition metal salts on the response behavior of the sensors was studied.