A SERS-active microneedle array for rapid and minimally invasive lactic acid detection.

Background: During intense exercise, anaerobic metabolism predominantly produces energy in the body, resulting in lactic acid (LA) accumulation, which contributes to muscle fatigue and soreness and may also impair neurological and cardiovascular functions. In endurance sports, the lactate threshold (LT) is a key indicator of an athlete's capacity to clear and utilize LA, directly influencing athletic performance and endurance. Therefore, LA detection is crucial for assessing the physical condition of both athletes and the general population, as well as for optimizing training programs.

Results: A set of surface-enhanced Raman scattering (SERS)-active microneedle (MN) arrays, developed by integrating gold nanoshells (GNSs), 4-mercaptobenzeneboronic acid (4-MPBA) and lactate oxidase (LOD) onto a MN array in turn, were inserted into skins to sense LA. Then mice in different physiological states and under different exercise intensities were used to verify the feasibility of the SERS-active MN array, respectively. After swimming for 3 and 6 min, the LA concentration of forelimb and hindlimb ISF of normal mice increased from 1.94 ± 0.33 mM and 2.41 ± 0.67 mM to 2.71 ± 0.28 mM and 8.12 ± 1.05 mM, and 6.43 ± 3.79 mM and 13.85 ± 2.51 mM, respectively, and the LA concentration of forelimb and hindlimb ISF of fasting mice increased from 2.97 ± 0.26 mM and 2.84 ± 0.23 mM to 3.62 ± 0.66 mM and 7.25 ± 1.40 mM, and 5.32 ± 1.99 mM and 13.07 ± 1.05 mM, respectively.

Significance: This study is the first to identify differences in LA production between fasting and normally fed animals. Furthermore, fasting led to an increase in baseline LA levels, though the rate of LA accumulation during exercise was lower compared to normal feeding conditions. With further optimization, this method would become a safer and more effective tool for exercise training and rehabilitation guidance for both athletes and the general population.