(Invited) Mediator-Free Electron-Transfer on Flexible and Patternable Biointerface for Sweat Glucose and Surface Electromyography Monitoring

Abstract

Introduction Wearable electrochemical biosensors are widely applied to detect biochemical molecules related to physiology and metabolism in biofluids such as sweat, tear and saliva.[1] These electrochemical biosensors are mainly based on enzyme-electrochemical reactions with high specificity at enzyme-electrode interfaces to transduce the molecular signal to an electrical signal. [2] Therefore, electron transfer (ET) at the enzyme-electrode interface is the key step for such wearable electrochemical biosensors.[3] Most current reported wearable electrochemical biosensors often employed extra electron mediators to enhance the enzyme-electrochemical reactions and to achieve efficient ET.[4] However, the use of electron mediators not only increases the complexity of biosensors but also leads to low selectivity and instability due to the cross-reaction and the leakages of electron mediators, respectively.[5] Therefore, the realization of direct ET at enzyme-electrode interfaces without using electron mediators to develop a wearable electrochemical biosensor with high selectivity and sensitivity for biochemical molecules monitoring is highly appreciated. Method Conductive CNT-EVA film was prepared by well dispersing CNT in xylene and then mixed through being sonicated to form a homogeneous ink. Then CNT-EVA ink was casted into a glass substrate and dried at room temperature to remove the solvent totally, to prepare flexible CNT-EVA films. The enzymatic bioelectrodes were prepared by dropping 50 μL aliquot of an enzyme/reagent mixture containing HRP (usually 0.25 mg mL-1), GOx (2.5 mg mL-1), and glutaraldehyde (2.5%) onto the flexible CNT-EVA film surface and dried at 4 °C for 2 h.All electrochemical measurements were carried out by an electrochemical analyzer CHI660D (Shanghai Chenhua, China), a three-electrode system constituted with a CNT-EVA work electrode, an Ag|AgCl|sat. KCl reference electrode and a Pt wire counter electrode was utilized. Surface electromyography (sEMG) signal was captured by a homemade eight-channel EMG amplifier with a gain of 4000, and then sampled by a data converter (DAQ-6341, National Instruments, Austin, TX, USA) at a sample rate of 1000 Hz with a 16-bit resolution. Results and Conclusions A flexible, conductive and hierarchical meso/macro porous carbon nanotubes (CNT) - ethylene-vinyl acetate copolymer (EVA) film is constructed as the sensing substrate in a facile route. Thanks to its well-formed three-dimensional nanoporous structure, direct electron transfer-type of bioelectrocatalysis is successfully realized on the glucose oxidase-horseradish peroxidase bienzyme functionalized CNT-EVA film for glucose monitoring without any mediator. The as-fabricated biosensors show excellent selectivity to glucose with a super-high sensitivity of 270 ± 10 μA mM-1 cm-2. To the best of the authors' knowledge, this might be the highest sensitivity of the flexible enzymatic biosensor for glucose monitoring demonstrated so far. This flexible CNT-EVA films also are utilized for monitoring the sEMG with high signal-to-background sensitivity due to its high electrical conductivity as well as good conformability to the skin. On-body experiments illustrated the capability of the proposed flexible biosensor for simultaneously monitoring sweat glucose and sEMG, guiding the way for measuring multiplex physiological signals for health monitoring, early diagnosis, and clinical research in future.