Abstract
This work reports the process of sensor development, optimization, and characterization before the transition to on-body measurements can be made. Sensors using lactate oxidase as a sensing mechanism and tetrathiafulvalene as a mediator were optimized for sporting applications. Optimized sensors show linear range up to 24 mM lactate and sensitivity of 4.8 μA/mM which normalizes to 68 μA*cm−2/mM when accounting for surface area of the sensor. The optimized sensors were characterized 3 different ways: using commercially available reference and counter electrodes, using printed reference and counter electrodes, and using a printed reference electrode with no counter electrode. Sensors intended for measuring sweat must be selective in the presence of sweat constituents. Thus, in addition to traditional characterization in pH 7.0 buffer, we characterized sensor performance in solutions intended to approximate sweat. Sensor performance in pH 7.0 buffer solution was not reflective of sensor performance in artificial sweat, indicating that further characterization is necessary between sensor measurement in pH 7.0 buffer and on-body measurements. Furthermore, we performed enzyme activity measurements and sensor measurements concurrently in five different salts individually, finding that while NH4Cl and MgCl2 do not affect enzyme activity or sensor performance in physiologically relevant ranges of salt concentration, NaCl concentration or KCl concentration decreases enzyme activity and sensor current. On the other hand, CaCl2 induced a nonlinear change in sensor performance and enzyme activity with increasing salt concentration.
Highlights
Amperometric enzymatic sensors can be made at relatively low cost with a high sensitivity, making them ideal for commercial scale production, especially since the limited lifetime of enzymes dictates that enzymes are most appropriate for inexpensive disposable sensor applications
By optimizing mediating and enzyme layer components of the working electrode, we report linear range up to 24 mM lactate and sensitivity up to 4.8 μA/mM which normalizes to 68 μA*cm−2/ mM when accounting for surface area, which is suitable for sporting applications in both sensitivity and linear range
The working electrode (WE) employs the enzyme lactate oxidase (LOX) as the sensing mechanism immobilized by the polymer chitosan with carbon nanotubes (CNTs)
Summary
Amperometric enzymatic sensors can be made at relatively low cost with a high sensitivity, making them ideal for commercial scale production, especially since the limited lifetime of enzymes dictates that enzymes are most appropriate for inexpensive disposable sensor applications. The sensing layers on the working electrode transducer are fabricated using the enzyme lactate oxidase (LOX) immobilized in a polymer network, typically chitosan. Printing is a high-throughput additive process with the potential to reduce cost of production, leading to inexpensive products which are necessary in the case of disposable sensors such as enzymatic sensors. By optimizing mediating and enzyme layer components of the working electrode, we report linear range up to 24 mM lactate and sensitivity up to 4.8 μA/mM which normalizes to 68 μA*cm−2/ mM when accounting for surface area, which is suitable for sporting applications in both sensitivity and linear range. The ability to mimic in vivo characterization would allow us to produce a calibration curve that mimics sensor performance on the body in real sweat conditions. Until each component of sweat is investigated and accounted for alongside a lactate sensor, current lactate sensors can at best be labeled indicators of aerobic or anaerobic metabolism, but cannot give accurate lactate sweat concentrations
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