# Non-Invasive Hydration Monitoring with a Graphene Dual Sweat Sensor
## 1 Introduction
A screen printed electrode (SPE) was selected as the platform, as it is a versatile and cost-effective approach to the electrochemical detection of sodium and potassium in sweat. This method leverages the high sensitivity, selectivity, and rapid response times [21] of SPEs--attributes enhanced by its simple fabrication methods and miniaturized systems. Specifically, the electrochemical detection technique used in this paper comprises open circuit potentiometry (OCP); this method involves detecting the potential difference between the working electrode and the reference electrode, providing accurate and reliable measurements of ion concentrations. Ag/AgCl is the common material of choice for the reference electrode on SPEs due to its ability to maintain a stable potential via the redox reaction (Equation (1)). Although AgCl displays low solubility, the chloride ion can leach, disrupting the equilibrium of the redox reaction due to the loss of the chloride ion. This is particularly critical for sweat analysis due to the presence of a high electrolyte concentration, including that of chloride, which may exacerbate the disruption. A junction layer could be fabricated to overcome this challenge [22]. The layer will serve as a barrier that allows for interaction between the sample solution and the reference electrode, enabling the flow of current necessary for the electrochemical cell to function, while preventing the loss of chloride ions. \[\mathrm{AgCl(s)+e^{-}\rightleftharpoons Ag(s)+Cl^{-}} \tag{1}\]  
Theoretically, based on the Nernst equation (Equation (2)) [19], a sensor working under standard conditions (25 \({}^{\circ}\)C, \(n\) = 1) is predicted to exhibit a shift of 59.2 mV per 10-fold increase in ion concentration (mV/ decade). This value, known as the Nernstian response, serves as a benchmark to assess whether the developed sensor achieves optimal sensitivity and accuracy. \[E=E^{0}+\frac{RT}{nF}\mathrm{ln}\bigg{[}\frac{\mathrm{oxidized}}{\mathrm{ reduced}}\bigg{]} \tag{2}\]  
However, achieving this response requires the electrochemical sensor to be functionalized for effective ion detection, enhancing its specificity and selectivity towards the target analyte. Common materials employed for working electrodes include gold, platinum, carbon, and graphene. Among these, graphene-based nanomaterials have emerged as highly promising candidates due to their unique and adjustable physicochemical properties, biocompatibility, and extensive surface area, which facilitate the immobilization of bio-recognition elements, amplifying detection sensitivity [23]. Regarding the electrochemical sensor design, graphene's exceptional electrical conductivity, combined with its capacity for controlled functionalization through sp\({}^{2}\) hybridized carbon atoms, makes it ideal for efficient electron transfer in electrochemical sensors [24]. This paper proposes a graphene-based dual-sensing sensor designed to quantify sodium and potassium concentrations in human sweat within 30 s using a minimal sample volume. We conducted a comparative analysis of our sensor's performance against a commercial analyzer and the inductively coupled plasma atomic emission spectroscopy (ICP-OES) technique. Our findings demonstrate that the graphene dual-sensor delivers reliable results, comparable to those obtained via ICP-OES. This study also attempts to undertake a cross-validation with an alternative technique to corroborate the concentrations measured by such sensors.