Preparation of a novel optical sensor for low pH values using agarose membranes as support

A simple and effective method for the preparation of a new generation of optical sensors based on optically transparent agarose membranes was developed. An optical pH sensor for low pH values was accordingly fabricated by immobilization of Congo Red on epoxy-activated agarose. In an optimized coupling pH of 10 and a dye concentration of 10−5 mol L−1, a sensitive sensor with long term stability was obtained.It was found that the acidity constant of Congo Red in the immobilized form is smaller than in solution. The sensor responds rapidly to the pH changes in a range of 0.5–5 with a response time of 3.0 min and a reproducibility better than 0.30% (R.S.D). The sensor was mounted in a flow cell and successfully applied for on-line pH monitoring. No evidence of leaching of the dye or any signal drift was observed.