Flexible electrochemical sensor platform
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Bringing an electroanalytical assay from the laboratory to the level of a commercially or clinically viable product often requires substantial cost, effort and time. While the real value may lie in the assay and sensor development, it has to be bundled with instrumentation, packaging, and readout software in order to function as a single, handleable unit. To unburden the sensor developer by not having to design the architecture to support the sensor, a universal electrochemical sensor platform, the ecFlex, has been developed. It is made small (25 mm × 29 mm, 1.2 g) by limiting the number of electronic components, and portable through data transfer via Bluetooth Low Energy to either PC or smartphone, and battery operation. Universality was realized by implementing several electrochemical measurement techniques such as chronoamperometry, squarewave voltammetry and open-circuit potentiometry. The platform allows for different material selections for the integrated screen-printed electrodes, as well as the option of interfacing external electrodes. Applicability was secured using a flexible printed circuit as the substrate to allow conformity towards different applications, including wearable sensors. The voltammetric range was limited to ±720 with 60 mV increments and ±872 μA with 0.87 nA resolution, whereas potentiometric capabilities had a useable range of 10-300 mV with 310 μV resolution. Whereas the power consumption depends upon the electrochemical technique used, cell current passed and transmission intervals for telemetry, the sensor platform was able to run continuously for 14h under the conditions of open-circuit potentiometric mode with, and 0.5 s transmission intervals on a 25 mAh battery. The performance of one of the key components of electrochemical sensor applications, the reference electrode, was investigated in detail. Specifically, the long-term stability of five different screen-printed material systems in a phosphate buffer was characterized by the means of an automated open circuit potentiometric measurement setup. Among screen-printed Pt, Ag/Pd, Ag, and Ag/AgCl of atomic ratios 3:1 and 9:1, only 3:1 Ag/AgCl remained stable throughout 40 days of continuous operation, as determined by the deviation from their initial potential. Its stability was attributed to having a strong potential-determining reaction with the dissolved Cl in the solution, and having sufficient AgCl preventing full dissolution during the course of the experiment. As a tradeoff, the dissolved AgCl caused a local build-up of Cl¯ that yielded a higher drift (-0.2 mV day-1) than the 9:1 Ag/AgCl electrodes (-0.1 mV day-1). Thus, the 3:1 Ag/AgCl ink was selected as the reference electrode for the present sensor applications, and in one case also for the working and counter electrodes. The role of the reference electrode was further studied through finite element analysis. The particular case for space-saving applications using a combined reference and counter electrode, and a similarly sized working electrode was investigated. It was found that the polarization of the reference electrode could shift the working electrode potential so close to the reversible potential of the working electrode reaction that it severely affected its sensitivity and drift, compared to using a three-electrode system. More specifically, the high current density through the reference electrode caused a high enough deficiency of Cl¯ compared to the bulk solution that its potential, and correspondingly the potential of the working electrode, shifted positively. The working electrode was shifted so close to the redox potential of the electrode reaction that it kinetically limited the electrode, as well as adding contributions of the reverse reaction, resulting in a lowering of the analytical sensitivity. The continuous depletion of Cl¯ additionally caused a drift in the electrode potentials, and consequently the cell current. The effect was verified through experiment. One of the implications is that there may be sensor systems in the real world suffering a performance loss due to an insufficiently dimensioned counter/reference electrode. The simulated sensor response gave up to 14 % deviation from the ideal analytical signal. The sensor platform was demonstrated for analytical applications through the determination of capsaicinoid content in chili-derived products, and pH and Cl¯ quantification. A simplified measurement technique, coarsely-stepped cyclic squarewave voltammetry, was developed, demonstrating that more refined techniques that require more precise, and thus expensive equipment, such as cyclic voltammetry, might be redundant for certain applications. A rinsing protocol was developed that made the screen-printed carbon working electrode reusable for at least 20 measurements. Potentiometric sensors for measuring pH in the range 2-9 (± 0.43) with half-nernstian sensitivity (26 mV) was designed based on carbon electrodes containing a redox couple coated with graphene oxide. A solid-state reference electrolyte was employed to allow sample matrices of unknown Cl¯ content. Moreover, a reagentless Cl¯ sensor was made. The sensor was based around the technique of squarewave amperometry on a threeelectrode system where all electrodes were identical Cl¯ selective Ag/AgCl electrodes. The platform was able detect Cl¯ in the range pCl 3 to 0 (± 0.27). Overall, the work demonstrates a universal and portable platform compatible with different electrochemical detection principles and the determination of several analytes that are important to a range of applications, at a low material cost. The open platform allows research and commercial efforts to quickly transform from a proof-of-concept, to something more like a product, reducing both the cost and effort required for prototype design.
Has partsArticle 1: S. Søpstad, E. A. Johannessen, F. Seland, and K. Imenes (2018). Long-term stability of screen-printed pseudo-reference electrodes for electrochemical biosensors, Electrochim. Acta, vol. 287, pp. 29–36. doi: 10.1016/j.electacta.2018.08.045
Article 2: S. Søpstad, K. Imenes, and E. A. Johannessen (2019). Hybrid electrochemical sensor platform for capsaicin determination using coarsely stepped cyclic squarewave voltammetry, Biosens. Bioelectron., vol. 130, pp. 374–381. doi: 10.1016/j.bios.2018.09.036
Article 3: S. Søpstad, K. Imenes, and E. A. Johannessen (2019). Chloride and pH detection on flexible a electrochemical sensor platform, IEEE Sensors Journal ( Early Access ), doi: 10.1109/JSEN.2019.2944407
Article 4: S. Søpstad, K. Imenes, and E. A. Johannessen (2019). Analytical errors in biosensors employing combined counter/reference electrodes. Submitted to Results in Chemistry July 2019