Potentiometric chemical sensors from lignin–poly(propylene oxide) copolymers doped by carbon nanotubes

Abstract

Hardwood and softwood lignins obtained from industrial sulphite and kraft and laboratory oxygen-organosolv pulping processes were employed in co-polymerization with tolylene 2,4-diisocyanate terminated poly(propylene glycol). The obtained lignin-based polyurethanes were doped with 0.72 w/w% of multiwall carbon nanotubes (MWCNTs) with the aim of increasing their electrical conductivity to the levels suitable for sensor applications. Effects of the polymer doping with MWCNTs were assessed using electrical impedance (EIS) and UV-Resonance Raman (UV-RR) spectroscopy. Potentiometric sensors were prepared by drop casting of liquid polymer on the surface of carbon glass or platinum electrodes. Lignin-based sensors displayed a very low or no sensitivity to all alkali, alkali-earth and transition metal cations ions except Cr(VI) at pH 2. Response to Cr(VI) values of 39, 50 and 53 mV pX⁻¹ for the sensors based on kraft, organosolv and lignosulphonate lignins, respectively, were observed. Redox sensitivity values close to the theoretical values of 20 and 21 mV pX⁻¹ for organosolv and lignosulphonate based sensors respectively were detected in the Cr(III)/Cr(VI) solutions while a very low response was observed in the solutions containing Fe(CN)₆^3−/4−. Conducting composite lignin-based polyurethanes doped with MWCNTs were suggested as being promising materials for Cr(VI)-sensitive potentiometric sensors.