Physiological sensing conducted in a liquid environment requires electrodes with long lifetime. The development of a robust ion-selective electrode-based biochip in a labon-a-chip platform is described. To compare electrode lifetime, which is driven by the transducer layer, electrochemical measurements were performed in a custom-made flow-cell chamber. The results of potentiometric measurement of cationic analytes demonstrate the electrodes to have a nearNernstian slope profile even after they are stored for almost a month in liquid medium. The electrodes also achieved H O amperometric sensitivity (1.25 and 3.32 μAmM cm for PEDOT:PSS and PEDOT:CaSO4 respectively) and lower detection limit (2.21 μM, 8.4 μM, 3.44 μM, for H , NH , Ca respectively) comparable to that of wire-type electrodes. Furthermore, the lifetime is dependent on the electrodeposition method of the conductive polymer, and the transducer layer must be modified to fit the analyte types. These results indicate that extended lifetime of microfabricated ion-selective electrodes in a multiplex format can be realized by optimizing the microfabricated electrode surface functionalization. © International Federation for Medical and Biological Engineering 2016.

Lab-on-a-chip (LOC) applications in environmental, biomedical, agricultural, biological, and spaceflight research require an ion-selective electrode (ISE) that can withstand prolonged storage in complex biological media [1][2][3][4] . An all-solid-state ion-selective-electrode (ASSISE) is especially attractive for the aforementioned applications. The electrode should have the following favorable characteristics: easy construction, low maintenance, and (potential for) miniaturization, allowing for batch processing. A microfabricated ASSISE intended for quantifying H + , Ca 2+ , and CO 3 2-ions was constructed. It consists of a noble-metal electrode layer (i.e. Pt), a transduction layer, and an ion-selective membrane (ISM) layer. The transduction layer functions to transduce the concentration-dependent chemical potential of the ion-selective membrane into a measurable electrical signal.The lifetime of an ASSISE is found to depend on maintaining the potential at the conductive layer/membrane interface [5][6][7] . To extend the ASSISE working lifetime and thereby maintain stable potentials at the interfacial layers, we utilized the conductive polymer (CP) poly(3,4-ethylenedioxythiophene) (PEDOT) 7-9 in place of silver/silver chloride (Ag/AgCl) as the transducer layer. We constructed the ASSISE in a lab-ona-chip format, which we called the multi-analyte biochip (MAB) (Figure 1).Calibrations in test solutions demonstrated that the MAB can monitor pH (operational range pH 4-9), CO 3 2-(measured range 0.01 mM -1 mM),and Ca 2+ (log-linear range 0.01 mM to 1 mM). The MAB for pH provides a near-Nernstian slope response after almost one month storage in algal medium. The carbonate biochips show a potentiometric profile similar to that of a conventional ion-selective electrode. Physiological measurements were employed to monitor biological activity of the model system, the microalga Chlorella vulgaris.The MAB conveys an advantage in size, versatility, and multiplexed analyte sensing capability, making it applicable to many confined monitoring situations, on Earth or in space. Biochip Design and Experimental MethodsThe biochip is 10 x 11 mm in dimension and has 9 ASSISEs designated as working electrodes (WEs) and 5 Ag/AgCl reference electrodes (REs). Each working electrode (WE) is 240 μm in diameter and is equally spaced at 1.4 mm from the REs, which are 480 μm in diameter. These electrodes are connected to electrical contact pads with a dimension of 0.5 mm x 0.5 mm. The schematic is shown in Figure 2.Cyclic voltammetry (CV) and galvanostatic deposition methods are used to electropolymerize the PEDOT films using a Bioanalytical Systems Inc. (BASI) C3 cell stand (Figure 3). The counter-ion for the PEDOT film is tailored to suit the analyte ion of interest. A PEDOT with poly(styrenesulfonate) counter ion (PEDOT/PSS) is utilized for H + and CO 3 2-, while one with sulphate (added to the solution as CaSO 4 ) is utilized for Ca 2+ . The electrochemical properties of the PEDOT-coated WE is analyzed using CVs in redox-acti...

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