A miniaturized electrochemical system with a novel polyelectrolyte reference electrode and its application to thin layer electroanalysis

Miniaturized liquid junction reference electrodes with short- and long-term stabilities were fabricated by using photopolymerization of a patterned polyelectrolytic hydrogel, poly diallyldimethylammonium chloride (pDADMAC). Internal and external solutions were separated by the polyelectrolytic hydrogel barrier, through which Cl− ions carried the ionic current. The impedance spectrum of the developed hydrogel-based salt bridge was flat at 40 kΩ, indicating negligible capacitance. Most of thin film-type reference electrodes patterned on microfluidic chips suffer from a short lifespan and complicated structure, which seriously antagonize the advantages of electrochemical detection. The novel reference electrode system using a polyelectrolyte salt bridge on a microfluidic chip maintained a reproducible potential of 19.3 ± 6 mV versus a commercial Ag/AgCl reference electrode over 30 h. Anionic interferents such as Br−, I−, and S2− ions had no influence on the performance of the reference electrode. In particular, the dependence on the Cl− ion concentration was substantially suppressed by separating the Ag/AgCl wire from the sample solution with the salt bridge. A 10 μm-thick cell with a polyelectrolyte salt bridge on a microfluidic chip showed the typical voltammetric behavior of thin layer cells that was confirmed by cyclic voltammetry (CV). This chip-based thin layer cell was exploited to determine the concentration of dopamine by differential pulse voltammetry (DPV) as well. A linear current response was observed down to 5 μM dopamine and the limit of detection was calculated to 2 μM.