# Non-Invasive Hydration Monitoring with a Graphene Dual Sweat Sensor
## 3 Results and Discussion
### Practical Application  
Sweat samples collected from a participant were analyzed to test the dual-sensing sensor's practical use. Results from ICP-OES, the dual-sensing sensor, and a commercial analyzer (Table S2) were plotted, with an error bar set at 10%, based on the ICP-OES results as a reference (Figure 6a,b). While the relative standard deviation (RSD) for all samples tested using ICP-OES was less than 5%, an additional 5% RSD was factored in to account for errors during sample preparation for ICP-OES. Thus, the error bar was set at 10% as a tolerance threshold when comparing the results of ICP-OES against the dual-sensing sensor and the commercial analyzer. The graphs show that the dual-sensing sensor demonstrated less variation from ICP-OES for both sodium and potassium detection. This result was further confirmed with statistical analysis. One-way repeated measures ANOVA and a Tukey test for post hoc analysis were performed using Origin Pro to obtain results regarding the sodium concentration. The Tukey test revealed that there is no statistically significant difference (\(\alpha\) = 0.05) when comparing ICP-OES against the dual-sensing sensor (\(p\) = 0.38) and the dual-sensing sensor against the commercial analyzer (\(p\) = 0.37). However, there is a statistically significant difference when comparing ICP-OES against the commercial analyzer (\(p\) = 0.04). Similarly, statistical analysis was performed for potassium results using Friedman ANOVA, as the data do not present a normally distributed curve. The results revealed that there are no statistically significant differences between the three methods (\(\alpha\) = 0.05): ICP-OES vs dual-sensing sensor (\(p\) = 0.87), dual-sensing sensor vs commercial analyzer (\(p\) = 0.23), and ICP-OES vs commercial analyzer (\(p\) = 0.76). These results suggest that the dual-sensing sensor is reliable and that the results are comparable to those for ICP-OES. The results were further analyzed using Bland-Altman plots, with a 95% confidence interval, to assess the agreement between the methods (Figure S2a-d), and Cohen's d was calculated to quantify the effect sizes of the differences (Table S3). While both analyses suggest that the differences between ICP-OES and the commercial analyzer are small, in practical terms, they do indicate that the dual-sensing sensor has a slight edge over the commercial analyzer, displaying a bias closer to zero on the Bland-Altman plot and exhibiting a smaller Cohen's d value. While there are sensors developed for the detection of sodium and potassium, and some have used actual sweat samples in their experiments [19, 27], it is not usual to observe the use of a second analytical technique for cross-validation, further proving that the results received via their sensor are accurate or comparable to established detection techniques. In addition to providing more accurate results, the dual-sensing sensor only requires a single-step measurement with about 30 \(\upmu\)L of the sample as compared to the constraints for the commercial analyzer, which requires one device for each ion and utilizes 300 \(\upmu\)L of each sample for reliable measurements. While the concentration of potassium seems to fluctuate as the intervals progress (Figure 6d), the differences are less than 2 mM, and the concentrations are well within the typical range. With that, we conclude that the sample collected was not contaminated. As with many other papers that compared sweat sodium concentration during exercise [11, 19, 28], the participant showed an increase in sweat sodium concentration as  
Figure 6: Variation and trends of sweat sodium and potassium concentrations. (**a**) Variation of sweat sodium concentration between all three methods of detection (legend: black circle—ICP-OES; blue triangle—commercial sensor; green square—graphene dual sensor). Error bar was set at 10%. (**b**) Variation of sweat potassium concentration between all three methods of detection (legend: black circle—ICP-OES; blue triangle—commercial sensor; green square—graphene dual sensor).