Improved performance of printed electrochemical sensors via cold atmospheric plasma surface modification

All-solid-state printed electrochemical sensors based on solid-contact ion-selective electrodes (SC-ISE) have shown great potential for use in many portable applications ranging from healthcare to environmental monitoring. Despite the low cost and scalable manufacturability of these sensors, they often underperform in long-term applications due to the formation of a water layer between the ion-selective membrane (ISM) and the printed electrode. Here, we report a simple approach to minimize the water layer formation and increase the stability of the SC-ISE by improving the interfacial bonding between the ISM and printed carbon electrodes through cold atmospheric plasma (CAP) treatment. A systematic study with different processing gas flow rates and power conditions was performed to identify the effect of CAP treatment on printed carbon electrode surface modification and to assess the role of functionalized surfaces in improving the sensor performance. To achieve maximal functionality, plasma settings were optimized with a minimum plasma power of 300 W and an oxygen flow rate of 120 mL min(-1) which was further verified by surface characterization techniques including water contact angle measurements and Raman spectroscopy. X-Ray photoelectron spectra (XPS) of the plasma-treated (PT) electrode surfaces indicated the formation of functional groups including hydroxyl (-OH), carboxyl (-COOH), and carbonyl (-CO), all of which aided in maximizing surface energy and creation of strong covalent bonds between the printed carbon electrode and ISM. In addition, CAP treatment led to the decomposition/removal of impurities and the polymer binder from the surface of printed carbon which effectively increased the exposed uncontaminated graphene flakes on the surface of the electrodes. Finally, as a proof of concept, we demonstrated that the use of CAP treatment prior to the coating with a nitrate sensitive ISM resulted in improved chemical bonding between the layers and a significant reduction in the water layer formation, leading to enhanced long-term stability (7-fold improvement) as compared with that of the untreated electrode.