Solid-contact polymeric membrane electrodes with detection limits in the subnanomolar range Original Research Article

Ion-selective electrode (ISE) membranes with a solid contact rather than a traditional liquid inner contact are anticipated to give lower detection limits because of diminished ion fluxes. Often, however, ill-defined solid contacts give rise to instabilities and interferences by oxygen gas. Here, a poly(methyl methacrylate)/poly(decyl methacrylate) (MMA/DMA) copolymer as membrane matrix was solvent-cast on a layer of poly(n-octyl)thiophene (POT) deposited on Au as internal contact. The resulting Ca2+- and Pb2+-selective electrodes were not sensitive to O2 and no evidence was found for the presence of a water film between membrane and internal contact. On the other hand, a water film was present when electropolymerized poly(n-octyl)thiophene was used. In the absence of a conducting polymer, O2 induced a significant change in the EMF but no clear evidence was found for a water film between POT and the MMA/DMA membrane. The lower detection limit of the new Au-POT-MMA/DMA solid-contact electrodes was somewhat better than that observed with a corresponding liquid-contact ISE and reached subnanomolar levels. Importantly, the response times for the solid-contact ISE are, at low concentrations, much faster than for the liquid-contact ISE.