# Non-Invasive Hydration Monitoring with a Graphene Dual Sweat Sensor
## 3 Results and Discussion
### Sensor Characterization: Electrochemical Performance
Reproducibility was evaluated by conducting one round of measurements with five identically fabricated sensors (Figure 5b,d). The descriptive statistics of these results (Table 1) showed a minimal variance within one sensor and between the five sensors. Hence, it can be concluded that the dual-sensing sensor is suitable for repeated measurement for at least 20 readings without recalibration. The low variability between sensors also enhances the reliability, as it demonstrates consistent performance across fabrication batches. To evaluate selectivity, 30 \(\upmu\)L of each of calcium chloride, magnesium chloride, and ammonium chloride, each at a concentration of 10 mM, were dispensed onto the sensor and analyzed. Potassium chloride was added to assess for interference on the sodium-detecting working electrode, and sodium chloride was added to assess for interference on the potassium-detecting working electrode. Some changes in the potential signal were observed upon the addition of the ions. To better understand how this impacts the sensor, the quantification of differences was converted to concentration (mM) using the logarithmic calibration equation in Figure 5b,d. As presented in Table 2, the effect of each interfearant (ion) on the sensor showed a maximum change of 0.3 mM. One possible cause for this minute change could be correlated from the similar deviation found between the selectivity tests and that gradient obtained from repeated measurements and the sensor-to-sensor variation, as seen in Table 1. The other cause could potentially be due to the difference in the base conductivity of the ions producing the difference in the signal seen in the potentiometry measurements [25,26]. If true, this would suggest that the change in signal \begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{2}{c}{**Sodium**} & \multicolumn{2}{c}{**Potassium**} \\ \hline
**(mV/decade)** & **Repeatability** & **Reproducibility** & **Repeatability** & **Reproducibility** \\ \hline Slope & 59.5 & 61.2 & 59.3 & 61.3 \\ Std. Dev. & 0.5 & 0.9 & 1.2 & 1.0 \\ \hline \end{tabular}
\end{table} 
Table 1: Average and standard deviation of slope in mV/decade of repeatability, and reproducibility tests for sodium and potassium sensors in mV/decade (\(t=5\)). Figure 4: Stability of sodium and potassium sensors. **(a)** Sodium selective electrode subjected to 40 mM of sodium chloride and potassium selective electrode subjected to 10 mM of potassium chloride over a period of 1 h; **(b)** reference electrode with Nafion PVB-NaCl coverage measured against a commercial reference electrode stored in 3 M KCl, with varying concentrations of sodium chloride added. A comparison of our work against a bare SPE reference electrode and another commercial reference electrode is shown in Figure S1. during the selectivity test was not due to the poor selectivity of the sensor but to the variability and intrinsic properties of the ions. To mimic the physiological electrolyte environment in sweat, a mixed solution of sodium chloride, potassium chloride, ammonium chloride, and calcium chloride, each \begin{table}
\begin{tabular}{c c c} \hline \hline
**Slectivity Tests** & \
\begin{tabular}{c} **Sodium Selective** \\ **Electrode Response (mM)** \\ \end{tabular} &
\begin{tabular}{c} **Potassium Selective** \\ **Electrode Response (mM)** \\ \end{tabular} \\ \hline
10 mM NaCl & - & \textless{}0.1 \\
10 mM KCl & \textless{}0.1 & - \\
10 mM NH4 Cl & \textless{}0.1 & \textless{}0.1 \\
10 mM CaCl\({}_{2}\) & \textless{}0.2 & \textless{}0.1 \\
10 mM MgCl\({}_{2}\) & \textless{}0.2 & \textless{}0.1 \\
10 mM mix * & 9.7–10.8 ** & 9.6–10.6 ** \\ \hline \hline \end{tabular}  
* Mixed solution with 10 mM NaCl, KCl, NH\({}_{4}\)Cl, CaCl\({}_{2}\), and MgCl\({}_{2}\). ** Received data from five different sensors.  
\end{table} 
Table 2: Selectivity tests with the respective sensors, with estimations of the voltage response using the regression in Figure 3b,d. Figure 5: Repeatability and reproducibility tests of sodium and potassium ion sensors.