Influence of hydrophobization of the polyazulene ion-to-electron transducer on the potential stability of calcium-selective solid-contact electrodes

We introduce here electrosynthesized polyazulene (PAz) as a new ion-to-electron transducer material in solid-contact ion-selective electrodes (SC-ISEs). It is shown that the neutral form of PAz possesses high hydrophobicity (water contact angle: 126 +/- 14 degrees) due to its aromatic carbon structure. This makes PAz attractive as a solid-contact (SC) material in SC-ISEs in combination with its high redox capacitance which is three times higher than for PEDOT-PSS. We have studied the electrochemical stability of PAz in direct contact with aqueous electrolyte solutions and applied it as the SC layer buried under a calcium-selective plasticized poly(vinyl chloride) ion-selective membrane (ISM). It is demonstrated that the hydrophobization of the PAz solid-contact at E <= 0.2 V significantly improved the potential stability of the calcium-selective SC-ISEs which showed a potential drift of only 0.5 mVh(-1) when the ISMs were for the first time contacted with 0.1 M CaCl2 for 24 h. Simultaneous determination of the potential stability and the water uptake with the FTIR-ATR spectroscopy could not clearly prove the correlation between low water content at the PAz/substrate interface and the good potential stability obtained with the pre-hydrophobized SC-ISEs. This is due to the relatively uneven surface morphology of the PAz film making the evanescent standing wave of the IR beam to sense water not only in the PAz transducer layer, but also in the ISM. Hence, it is not possible to distinguish between the distribution of water in the ISM and the PAz solid-contact layer. The new SC material presented here was also characterized with cyclic voltammetry, electrochemical impedance spectroscopy and scanning electron microscopy. (C) 2014 Elsevier B.V. All rights reserved.