Detection of sodium and potassium ions in human sweat using electrochemical system

Sweat contains a large number of biomarkers which can be used to indicate the healthy state of the body. Real-time detection of ion contents in sweat at trace levels has significant implications for health monitoring and medical diagnosis. In this work, an electrochemical detection system combining an electrochemical detection circuit and an electrochemical sensor with ion selectivity was developed. Based on the system sodium and potassium ions were detected at room temperature using the open circuit voltage-time (OCPT) method. The ion-selective sensors have high sensitivity (98.3 mV/decade for sodium ion detection and 99 mV/decade for potassium ion detection), and good stability. Miniaturized detection system makes it possible to detect sweat on wearable devices and holds great promise in the wearable field.

indicate the healthy state of the body.Real-time detection of ion contents in sweat at trace levels has significant implications for health monitoring and medical diagnosis.
In this work, an electrochemical detection system combining an electrochemical detection circuit and an electrochemical sensor with ion selectivity was developed.Based on the system sodium and potassium ions were detected at room temperature using the open circuit voltage-time (OCPT) method.The ion-selective sensors have high sensitivity (98.3 mV/decade for sodium ion detection and 99 mV/decade for potassium ion detection), and good stability.Miniaturized detection system makes it possible to detect sweat on wearable devices and holds great promise in the wearable field.

1.Introduction
Electrochemical sensors convert the recognition that occurs on the electrode surface into a detectable electrical signal, allowing quantitative analysis of the molecule under test.The electrochemical sensor has the advantages of short response time, real-time detection, ease of handling, miniaturization and excellent sensitivity [1][2].Human sweat is rich in health-related biomarkers [3][4].These biomarkers can provide critical information related to the body's metabolic dynamics, providing medical diagnosis from diseases such as cystic fibrosis [5] and kidney disorders [6].
In this paper, a self-developed electrochemical detection circuit and an electrochemical sensor were used to detect sodium and potassium ion in human sweat.The quantitative analysis of two ions on the same electrochemical sensor was successfully accomplished.These have far-reaching significance in realizing real-time electrochemical detection of human sweat biomarkers, and provide a label-free, real-time, and rapid method for the detection of human sweat biomarkers.

2.Materials and Reagents
A miniaturized electrochemical system was proposed, which included an FPGA detection circuit, a control unit (smart phone) and an ion-selective electrode, as shown in Figure 1.The ion-selective electrode is connected to the FPGA detection circuit by electrode connector, that enables the conversion of chemical signals to electrical signals.Reagents required for ion-selective sensor preparation include sputtered electrode(gold), sodium ion carrier X, valinomycin, sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Na-TFPB), di-n-octyl sebacate (DOS), polyvinyl chloride (PVC) , methanol and tetrahydrofuran (THF).
For the sodium ion-selective electrode, the sodium ion carrier X, Na-TFPB, PVC and DOS were sequentially dissolved in methanol at a certain ratio, and then the reagents were sonicated for 30 minutes to dissolve completely.The solutions prepared from the mixture of reagents was added dropwise to the surface of working electrode, and dried at room temperature to produce the sodium ion-selective electrode.
For the potassium ion-selective electrode, the valinomycin, Na-TFPB, PVC, and DOS were sequentially dissolved in THF in a certain ratio.and then the reagents were sonicated for 30 minutes to dissolve completely.The solutions prepared from the mixture of reagents was added dropwise to the surface of another working electrode, and dried at room temperature to produce the potassium ion-selective electrode.

Testing of sodium ion-selective sensors
In order to test the reliability, sensitivity and hysteresis of the sodium ion-selective sensors, the change of electrode potential was investigated at different concentrations of NaCl solution.Standard concentrations of NaCl solutions (8 mM, 16 mM, 32 mM, 64 mM and 128 mM) were added dropwise to the surface of sensors.Fig. 2a clearly indicates that the open-circuit potential of the ion sensor rises proportionally with increasing concentration.In contrast, the open-circuit potential alteration of the sensor with unmodified sensitive membrane exhibits no distinct pattern of change.The hysteresis of the sensors can be tested by adjusting the direction of concentration change on the test solution.In the same experiment the forward and reverse tests were run sequentially for different concentrations of the standard solution and the results are shown in Fig. 2b.In forward and reverse strokes, the same size input corresponds to a nearly identical output, which indicates an excellent reproducibility of sensors.The sensitivity of this sensors to different concentrations of sodium ions is further demonstrated.As shown in Fig. 2c, the sensitivity of sensors is 98.3 mV/decade, which is obviously larger compared with the theoretical value of 59.2 mV/decade.Meanwhile, the response signal shows an obvious linear relationship, indicating a good experimental stability.

Testing of potassium ion-selective sensors
As well, the effect of different concentrations of KCl solution on the electrode potential was also investigated to verify the response performance of the potassium ion-selective sensors.Firstly, standard concentrations of KCl solutions (1 mM, 2 mM, 4 mM, 8 mM, 16 mM and 32 mM) were added dropwise to the surface of sensors.As shown in Fig. 3a and Fig. 3c, the response potential exhibits a clear tendency of gradient increase with proportional increase in potassium ion concentration.And the sensitivity of sensors was 99 mV/decade for potassium ions.Response signals to interferences such as HSO4 -, OH -, Na + , and H + were tested to verify the selectivity of the sensor.The results showed that the signal responses of sensor to interferences were much lower than K + response.In addition, the hysteresis of potassium ion-selective sensors was also successfully validated.As shown in Fig. 3b, the experimental results indicated hysteresis had small effect on the sensors, and large hysteresis loops were not induced by changes in the concentration of the target substance.This also proves that the sensor has good sensitivity and accuracy.

4.Conclusion
In conclusion, we propose a miniaturized electrochemical sensor system that can detect sodium and potassium ions in human sweat.And the system has good sensitivity.Na + detection sensitivity up to 98.3 mV/decade and K + detection sensitivity up to 99 mV/decade.At the same time, the hysteresis of the sensors was successfully verified.This article also demonstrates the feasibility of simultaneous sodium and potassium ion detection at one sensor.

Figure 2 .
Figure 2. a) Detection of sodium ions by the sensors.b) Hysteresis detection of sensors for sodium ions.c) Relationship between sodium ions concentration and potential on the sodium ion-selective sensor(n=3).

Figure 3 .
Figure 3. a) Detection of potassium ions by the sensors.b) Hysteresis detection of sensors for potassium ions c) Relationship between potassium ions concentration and potential on the potassium ion-selective sensor(n=3).