(Invited) towards Continuous Health Monitoring Platforms By Enzyme-Free Electrochemical Sensors and Triboelectric Sensors for Noninvasive Lactate Detection

Abstract

Emerging wearable devices with non-invasively biosensing technics have drawn considerable attention to continuously monitor several metabolites in body fluids, such as a tear, saliva, and sweat, to diagnose human health conditions. Most importantly, wearable sensors could offer unique possibilities for online, real-time and non-invasive monitoring of health compared to traditional invasive biosensors. Among lots of analytes, lactic acid concentration in the human body exhibits a high relationship with several diseases such as acute heart diseases, hypoxia, muscle fatigue, meningitis, and cystic fibrosis, and it could also cause muscle pain in athletes. To further boost up the reproducibility and reliability of wearable biosensors to detect lactate concentration levels from human sweat, Ni-based layered double hydroxide (LDH) with various secondary transition metals (Fe and Co) was proposed as electrocatalysts in an enzyme-free electrochemical lactate sensor. According to the mechanism of lactate oxidation on the transition metal-based electrocatalyst, secondary transition metal of Co could serve as the active site for lactate oxidation and facilitate the adsorption of OH- in the alkaline electrolyte. To further increasing the active surface area for enhancing the sensitivity of Ni-based LDH, ZIF-67 derived NiCo LDH was synthesized as the electrocatalyst for non-enzymatic lactate detection. Co-based ZIF-67 served as self-sacrificial templates to fabricate hierarchically structural NiCo LDH with uniform porosity and high electrochemically active surface area to achieve outstanding electrocatalytic performance for lactate sensing. After optimizing the particle size of ZIF-67 and transformation times, ZIF-67 derived NiCo LDH reached the ultrahigh sensitivity of 83.98 μA mM- 1 cm- 2 at an applied potential of 0.55 V (vs. Ag/AgCl KCl sat’d) in the concentration range from 2 to 26 mM. On the other hand, pioneering works in biosensors for lactate detection in sweat has been encountered major challenges such as noble material usage, immobile power supply, and complicated circuit connection to realize the compact sustainable sensing systems. To solve these restrictions, herein, the self-powered molecular imprinted polymers based triboelectric sensor (MIP-TES) was designed to offer a multifunctional noninvasive approach for specific and simultaneous lactate detection. Free-standing PVDF/graphene flexible electrode modified poly(3-aminophenyl boronic acid) imprinted lactate molecule demonstrated the change of the surface properties afterlactate adsorption. MIP-modified electrode revealed the selective lactate sensing over non molecular imprinted polymers (NIP) electrode through the superior and stable signal change with variation of lactate concentration in human sweat. Moreover, MIP modified lactate sensor was further introduced in the triboelectric nanogenerator system to harvest mechanical energy from contact and separation into electrical output. The more adsorbed lactate led to lower energy barriers and decreasing electrical potential when detecting higher lactate concentration. Self-power triboelectric lactate sensor could directly power the number of LED lights without an external energy supply. Eventually, it was validated the feasible application of wearable sensors on human skin. After introducing noninvasive enzyme-free biosensors and triboelectric sensors, an innovatively continuous non-invasive health monitoring platforms can be achieved for practical applications, especially in the areas of home medical examination and wearable personal biosensors.