Ion-selective electrodes:: Polyaniline modification and anion recognition

A novel arrangement for polymeric membranes used in ion-selective electrodes is presented. The surface of a conventional plasticized poly(vinyl chloride) (PVC) membrane was coated with a chemically generated film of a conducting polymer, polyaniline (PANI). The effect of PANI on the anionic potentiometric response of PVC membranes derived from calix[4]pyrrole and tridodecylmethylammonium chloride (TDDMACI) is reported. Five different types of electrodes with the following membrane compositions were studied: (1) PVC/PANI (a reference experiment), (2) PVC + TDDMACI (as anion-exchanger)/PANI, (3) PVC + calix[4]pyrrole (as neutral carrier)/PANI, (4) PVC + calix[4]pyrrole + TDDMACI (uncoated membrane) and (5) PVC + calix[4]pyrrole + TDDMACl/PANI. Hydrophilicity of the uncoated and PANI-coated membrane surfaces was assessed by measurements of water contact angle, theta. In the protonated state, the neat PANI is more hydrophilic, theta = 49 degrees, than in its deprotonated base form, theta = 94 degrees. After surface deposition of thin PANI film, increased potentiometric sensitivity of the membranes, especially towards hydrophilic anions (F-, H2PO4-), was achieved. The distinct differences in the character of calix[4]pyrrole (neutral carrier) and TDDMACI (anion-exchanger) to sense anionic species were retained and became more pronounced. The PANI film may be regarded as a novel kind of an electrode modifier for anion-sensing. The role of the PANI film in the formation of the potentiometric response is discussed. It is proposed that the PANI film acts as an anion-exchanger. The transport of anions through a thin PANI film is also affected by the character of anions and by pH.