Tailoring polythiophene cation-selective optodes for wide pH range sensing

A challenge in turn-on mode operating optodes application is elimination of pH sensitivity. The polyoctylthiophene based optodes response mechanism is not involving hydrogen ions exchange, thus paves the way for optodes applicable for sensing in wide pH range. We report on nanoptodes that can be used both in alkaline and in acidic pH range inaccessible for classical systems using pH sensitive dyes as transducers. The proposed sensors offer 6 orders of magnitude broad, linear dependence of emission intensity on logarithm of analyte (K+ or Ca2+) concentration, in turn on mode. However, the slopes of calibration plots are to some extent dependent on solution pH and oxygen presence/absence. It is shown that this effect is resulting from changes of solution redox potential maintained mainly by dissolved oxygen. The solution redox potential affects the oxidation state of the polymer and thus amount of the neutral, fluorescent form of polyoctylthiophene, ultimately affecting performance of the sensor especially in acidic pH. Tailoring composition of polyoctylthiophene optodes, including hydrogen binding compound in the polymer phase, effectively diminishes the effect of pH change on sensitivity of proposed optodes, as shown on model examples of potassium and calcium sensors. Thus polyoctylthiophene based nanosensors show equally high sensitivity for analyte cations concentration change both in acidic (pH=4) and alkaline (pH=9.2) media.