Abstract
We describe the optimization of a flexible printed electrochemical sensing platform to monitor sodium ion (Na+), ammonium ion (NH4+), and lactate in human sweat. We used previously reported material systems and adapted them to scalable fabrication techniques. In the case of potentiometric Na+ and NH4+ sensors, ion-selective electrodes (ISEs) required minimum optimization beyond previously reported protocols, while a reference electrode had to be modified in order to achieve a stable response. We incorporated a carbon nanotube (CNT) layer between the membrane and the silver/silver chloride (Ag/AgCl) layer to act as a surface for adsorption and retention of Cl−. The resulting reference electrode showed minimal potential variation up to 0.08 mV in the solutions with Cl concentration varying from 0.1 mM to 100 mM. Increasing the ionophore content in the NH4+ ISE sensing membrane eliminated an offset in the potential readout, while incorporating CNTs into the sensing membranes had a marginal effect on the sensitivity of both Na+ and NH4+ sensors. Na+ and NH4+ sensors showed a stable near-Nernstian response with sensitivities of 60.0 ± 4.0 mV and 56.2 ± 2.3 mV, respectively, long-term stability for at least 60 min of continuous operation, and selectivity to Na+ and NH4+. For the lactate sensor, we compared the performance of the tetrathiafulvalene mediated lactate oxidase based working electrode with and without diffusion-limiting polyvinyl chloride membrane. The working electrodes with and without the membrane showed sensitivities of 3.28 ± 8 A/mM and 0.43 ± 0.11 μA/mM with a linear range up to 20 mM and 30 mM lactate, respectively.
Highlights
Non-invasive detection of biomarkers in sweat is of great interest for assessing how the body responds to physical activity and for clinical diagnostics
Increasing the ionophore content in the NH4+ ion-selective electrodes (ISEs) sensing membrane eliminated an offset in the potential readout, while incorporating carbon nanotube (CNT) into the sensing membranes had a marginal effect on the sensitivity of both Na+ and NH4+ sensors
The potential is set between the working and reference electrode to facilitate the electrochemical reaction between the enzyme lactate oxidase present in the working electrode and lactate in sweat
Summary
Non-invasive detection of biomarkers in sweat is of great interest for assessing how the body responds to physical activity and for clinical diagnostics. While the reported work advances the important aspects of sensor development, sensors that meet all the requisites of the commercially viable system are yet to be demonstrated Such requisites include fast response, sensitivity to physiologically relevant concentrations of analyte, batch to batch reproducibility, stable performance for the duration dictated by the application, specificity, i.e., insensitivity to other analytes present in sweat, long-term storage, and ideally no calibration requirement. To mitigate the diffusion of the chloride ions, we incorporated a carbon nanotube (CNT) layer between the membrane and the Ag/AgCl layer to act as a surface for the adsorption and retention of chloride ions This effective modification led to a stable, reproducible reference electrode. The membrane ensures that the sensor response is unaffected by the motion of sweat since diffusion within the membrane is significantly lower than the external diffusion. in our previous work, we showed that biologically relevant concentrations of sodium, potassium, and calcium ions present in sweat affect the enzyme activity and, the sensitivity of the lactate sensors. The membrane reduces the relative variation of the sensor’s sensitivity with respect to variations of the enzyme activity. The 3 mm diameter electrodes with and without the membrane showed sensitivities of 3.28 μA/mM and 0.43 μA/mM with a linear range up to 20 mM and 30 mM lactate, respectively
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