FIBEROPTIC PH SENSOR-BASED ON POLY(O-METHOXYANILINE)

This work describes the development of optical sensors for the measure ment of pH. They work by measuring the diffuse reflectance of a conduc tive polymer, in the visible range of the spectrum. The pH sensitive m aterial was poly(o-methoxyaniline) doped by p-toluene sulfonic acid (P oAnis/TSA), which was immobilized either on or within a polymeric stru cture. Two optodes were constructed. The first was assembled using a b ifurcated bundle of borosilicate optical fibers. The sensing phase was a polymeric composite made of PoAnis/TSA and cellulose acetate, and w as fixed at the common end of the bifurcate bundle. This optode allowe d measurements in the pH range from 4.9 to 10.5 with a precision of +/ -0.01 pH. The response time for 90% change was 5.0 min for a full pH v ariation from acid to basic, and 22 min for the reversal change. The s ensor response depends on the ionic strength of the solution and on th e supporting electrolyte used to adjust the ionic strength. The second optode was built using two cables of optical fibers, each one having a single plastic fiber with 1.0 mm diameter. The two cables were fixed in a Perspex block, together with the pH sensitive layer. The sensing phase of this optode consisted of a film of PoAnis/TSA adsorbed on po lyethylene. This optode yielded measurements in a working range from p H 2.0-10.0 and with relatively faster response time for 90% change for a variation from basic to acid (5.0 min). It was verified that pH sen sors based on poly(o-methoxyaniline) offer a wide dynamic range but su ffer from long response times and interferences from both cations and anions. (C) 1997 Elsevier Science B.V.