Strategies of Miniaturised Reference Electrodes Integrated in a Silicon Based “one chip” pH Sensor

Carbon Nanotube-Based Ion Selective Sensors for Wearable Applications

Wearable electronics offer new opportunities in a wide range of applications, especially sweat analysis using skin sensors. A fundamental challenge in these applications is the formation of sensitive and stable electrodes. In this article we report the development of a wearable sensor based on carbon nanotube (CNT) electrode arrays for sweat sensing. Solid-state ion selective electrodes (ISEs), sensitive to Na⁺ ions, were prepared by drop coating plasticized poly(vinyl chloride) (PVC) doped with ionophore and ion exchanger on CNT electrodes. The ion selective membrane (ISM) filled the intertubular spaces of the highly porous CNT film and formed an attachment that was stronger than that achieved with flat Au, Pt, or carbon electrodes. Concentration of the ISM solution used influenced the attachment to the CNT film, the ISM surface morphology, and the overall performance of the sensor. Sensitivity of 56 ± 3 mV/decade to Na⁺ ions was achieved. Optimized solid-state reference electrodes (REs), suitable for wearable applications, were prepared by coating CNT electrodes with colloidal dispersion of Ag/AgCl, agarose hydrogel with 0.5 M NaCl, and a passivation layer of PVC doped with NaCl. The CNT-based REs had low sensitivity (-1.7 ± 1.2 mV/decade) toward the NaCl solution and high repeatability and were superior to bare Ag/AgCl, metals, carbon, and CNT films, reported previously as REs. CNT-based ISEs were calibrated against CNT-based REs, and the short-term stability of the system was tested. We demonstrate that CNT-based devices implemented on a flexible support are a very attractive platform for future wearable technology devices.

Differential pH measurements of metabolic cellular activity in nl culture volumes using microfabricated iridium oxide electrodes

In this paper we describe a new approach to measure pH differences in microfluidic devices and demonstrated acidification rate measurements in on-chip cell culture systems with nl wells. We use two miniaturized identical iridium oxide (IrOx) thin film electrodes (20 micromx400 microm), one as a quasi-reference electrode, the other as a sensing electrode, placed in two confluent compartments on chip. The IrOx electrodes were deposited onto microfabricated platinum (Pt) electrodes simultaneously using electrodeposition. Incorporating the electrodes into a microfluidic device allowed us to expose each electrode to a different solution with a pH difference of one pH unit maintaining a confluent connection between the electrodes. In this configuration, we obtained a reproducible voltage difference between the two IrOx thin film electrodes, which corresponds to the electrode sensitivities of -70 mV/pH at 22 degrees C. In order to measure the acidification rate of cells in nl cell culture volumes we placed one IrOx thin film electrode in the perfusion channel as a quasi-reference electrode and the other in the cell culture volume. We obtained an acidification rate of 0.19+/-0.02 pH/min for fibroblast cells using a stop flow protocol. These results show that we can use two identical miniaturized microfabricated IrOx electrodes to measure pH differences to monitor the metabolic activity of cell cultures on chip. Furthermore, our approach can also be applied in biosensor or bioanalytical applications.