# Non-Invasive Hydration Monitoring with a Graphene Dual Sweat Sensor
## 4 Conclusions  
In this study, a graphene-based dual-sensing electrochemical sensor for rapid and accurate sweat sodium and potassium detection was developed to address the limitations of conventional hydration monitoring techniques. By harnessing graphene's unique physicochemical properties, the sensor achieved a near-ideal Nernstian response (61.93 mV/decade for sodium and 61.21 mV/decade for potassium) and demonstrated high sensitivity, selectivity, and stability across a targeted detection range of 0.1 mM to 100 mM, making it suitable for sweat monitoring. Sweat samples were collected from a trained athlete, and the concentrations of sodium and potassium were measured using the dual-sensing sensor, ICP-OES, and commercial analyzers. The experimental outcomes demonstrated that the sensor exhibited outstanding performance in regards to sweat sodium and potassium detection, statistically indistinguishable from ICP-OES (\(p>0.05\)) and out-performing commercial analyzers. Requiring only 30 \(\upmu\)L of the sample--ten times less than the commercial alternative--no sample preparation, and producing result in 30 s, these features further corroborate the graphene dual sensor's practical utility. The sensor's rapid response time, minimal sample requirement, and non-invasive nature position it as an attractive tool for real-time hydration monitoring, with potential applications in sports science, clinical diagnostics, and personalized health management. Future studies should include the investigation of the sensor's stability in regards to varying temperatures and humidity levels under long-term storage after fabrication. The capability of the sensor could also be further expanded to include the detection of other electrolytes or biomarkers, with potential anti-doping applications by targeting illegal substances in biofluids such as saliva, tears, or urine. Testing with larger, diverse cohorts will also help to establish individualized hydration thresholds and validate the sensor's performance across varied environmental and physiological conditions, further solidifying its role in real-time health monitoring. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15094970/s1](https://www.mdpi.com/article/10.3390/app15094970/s1)]([https://www.mdpi.com/article/10.3390/app15094970/s1](https://www.mdpi.com/article/10.3390/app15094970/s1)), Table S1: Performance comparison of our graphene dual-sensor against existing technologies; Table S2: Quantification of sodium and potassium on the graphene dual-sensor, with comparison to other methods of quantification; Figure S1: Comparison of our work, bare screen-printed electrode's reference electrode and another commercial reference electrode against a commercial reference electrode stored in 3 M KCl with varying concentrations of sodium chloride added; Figure S2a-d: Bland-Altman plots; Table S3: [PERSON]'s d value. References [18; 19; 27; 33; 34; 35; 36] are cited in the supplementary materials. [PERSON], [PERSON] and [PERSON] conceptualized the project. [PERSON], [PERSON], [PERSON] and [PERSON] performed the measurements. [PERSON], [PERSON], [PERSON] and [PERSON] conducted the analysis and discussion. [PERSON], [PERSON], [PERSON] and [PERSON] led the manuscript writing, with the assistance of all co-authors. [PERSON] and [PERSON] managed the administration and finances of the project. A.H.C.N. supervised the project. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Research Foundation, Prime Minister's Office, Singapore, under its Medium-Sized Center Program and Central GAP (Project No. TAP2502020-05-02), and by the Singapore Ministry of Education under its Research Center of Excellence award to the Institute for Functional Intelligent Materials, National University of Singapore (Project No. EDUNC-33-18-279-V12). Not applicable. Verbal informed consent was obtained from the volunteer.