# Non-Invasive Hydration Monitoring with a Graphene Dual Sweat Sensor
## 3 Results and Discussion  
### Sensor Characterization: Electrochemical Performance  
To investigate the dynamic range, the graphene dual-sensing sensor was subjected to varying concentrations of NaCl and KCl, from 0.1 mM to 100 mM, via microfluidic control in tenfold steps. Figure 3a,c present the results obtained for sodium and potassium selective electrode, respectively. The detection range of both ion selective electrodes were then regressed using a semi-logarithmic scale, where the x-axis of the concentration is of a \(\log_{10}\) scale and thus, y = a + \(\log_{10}\)(x). As seen in Figure 3b, the sodium selective electrode is calculated to achieve approximately 61.93 mV/decade, R\({}^{2}\) = 0.9897, and the potassium selective electrode (Figure 3d) achieved 61.21 mV/decade, R\({}^{2}\) = 0.9997, both exhibiting a near-Nernstian response. This indicates that the dual-sensing sensor is reliable for the detection of sweat sodium and potassium levels within the physiological concentration range. The stability of the sodium and potassium selective electrodes was also assessed by subjecting the electrodes to extensive exposure to the respective ions for which they are selective. As seen in Figure 4a, the variation over time for the sodium and potassium selective electrode is found to be within 1 mV of fluctuation across the span of 1 h. In addition, an extensive stability test was conducted for the reference electrode by measuring it against a commercial external reference electrode probe, where the concentration of sodium chloride in the solution in which the electrodes were immersed increased to 1M, and this level was maintained for over one hour. If the reference electrode is sensitive to changes in chloride concentration, this would result in potential drift, even at constant concentrations, and further shifts as the concentration changes. Figure 2: Schematics of sweat collection experimental procedure. The procedure subjected the participant to three exercise intervals. Each interval started with a 30 min endurance run, followed by a 15 min seated rest before sweat was collected from the forearm, upper arm, and chest, with each sample labeled sequentially. Immediately after the collection, the participant started the next interval. The reference electrode with the junction layer showed few changes when the concentration was brought to 0.1 mM, 1 mM, and 10 mM (Figure 4b). A shift of about 4 mV was observed when the concentration was increased to 100 mM, and it subsequently decreased by 4 mV when the concentration was brought to 1M, stabilizing at around 92 mV--comparable to the initial values (91 mV) observed at lower concentrations. The signal remained stable, with slight fluctuation between 91 mV and 94 mV, for over 100 min. A variation of less than 10 mV was observed between the lowest and highest concentration, and stability was maintained for more than 100 min, indicating that this junction layer effectively stabilizes the reference electrode. This result demonstrate that the Nafion PVB-NaCl-covered reference electrode is able to return to a stable level quickly after the injection of sodium chloride, as well as its ability to maintain a stable voltage level in the presence of excess ions, a critical characteristic required for the ion-selective sensor to fulfill its role as a potential constant reference. Figure 3: Range of detection for sodium and potassium. (**a**) Response curve, voltage vs time plot (V-t), showing the change in voltage as the concentration of sodium varies from 0.1 mM to 100 mM; (**b**) semi-logarithmic regressed curve for sodium from 0.1 mM to 100 mM, R\({}^{2}\) = 0.9897; (**c**) response curve, voltage vs time plot (V-t), showing the change in voltage as the concentration of potassium varies from 0.1 mM to 100 mM; (**d**) semi-logarithmic regressed curve for potassium from 0.1 mM to 100 mM, R\({}^{2}\) = 0.9997. The repeatability of the dual-sensing sensor was evaluated by repeating five rounds of measurements on one sensor (Figure 5a,c).