Fiber-optic pH sensor based on Poly(o-methoxyaniline)

This work describes the development of optical sensors for the measurement of pH. They work by measuring the diffuse reflectance of a conductive polymer, in the visible range of the spectrum. The pH sensitive material was poly(o-methoxyaniline) doped by p-toluene sulfonic acid (PoAnis/TSA), which was immobilized either on or within a polymeric structure. Two optodes were constructed. The first was assembled using a bifurcated bundle of borosilicate optical fibers. The sensing phase was a polymeric composite made of PoAnis/TSA and cellulose acetate, and was fixed at the common end of the bifurcate bundle. This optode allowed 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 variation from acid to basic, and 22 min for the reversal change. The sensor response depends on the ionic strength of the solution and on the 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 polyethylene. This optode yielded measurements in a working range from pH 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 sensors based on poly(o-methoxyaniline) offer a wide dynamic range but suffer from long response times and interferences from both cations and anions.