Flexible and Linker-Free Enzymatic Sensors Based on Zinc Oxide Nanoflakes for Noninvasive L-Lactate Sensing in Sweat

We report on a highly sensitive, selective, and flexible enzymatic sensor based on Zinc oxide nanoflakes (ZnO NFs) for noninvasive L-lactate sensing in sweat. A single step sonochemical process was used to synthesize ZnO NFs on gold-coated flexible polyethylene terephthalate (PET) substrates. The lactate oxidase enzyme (LOx) immobilized on the as-synthesized ZnO-NFs-based sensing electrode provided a high level of selectivity for lactate sensing. The sensor was tested in the range of 10 pM–20 mM, which covers the entire physiological range and demonstrated the sensitivity of $11.76~\mu \text{A}$ /decade/cm2 and the limit of detection (LoD) of 1.26 nM. The presented sensor does not require a linker due to the high isoelectric point of ZnO-NFs yet demonstrated ~55 times better response compared to conventional gold electrodes with a linker. Our statistical analysis showed that the sensor also has high reproducibility.