Screen-Printed Lactate Biosensor Based on a Lox-Immobilized MgO-Templated Carbon for Continuously Monitoring of Lactate Concentration in Sweat

Abstract

Lactate concentration in sweat is closely related to exercise intensity. Thus, real-time monitoring of lactate concentration is extremely important for improving athlete training and health management. There are many reports for monitoring the change in the lactic acid concentration in sweat over a long period of time using a wearable lactate biosensor 1,2). For practical application of a wearable lactate biosensor for long-term monitoring, it is important to suppress elution of the enzyme from the electrode by simple enzyme immobilization scheme. Recently, we have reported that long-term stability of a glucose oxidase (GOD)-immobilized electrode was dramatically improved by introducing a poly(glycidyl methacrylate) on a mesoporous carbon, namely MgO-templated carbon (MgOC), which binds to a GOD by coavalent bonding scheme. In this study, we applied the method mentioned above to a printed lactate biosensor. A lactate oxidase (LOx) was immobilized on the electrode by covalent bonding with a poly(glycidyl methacrylate)-modified carbon surface. Firstly, radicals were introduced on the MgOC surface by electron beam irradiation, and glycidyl methacrylate (GMA) having an epoxy group was polymerized according to the previously reported scheme3). We referred the poly(GMA)-immobilized MgOC was “GMgOC” in this study. GMgOC and polyvinylidene fluoride (PVdF) were dispersed in N-methyl pyrrolidone (NMP) to prepare a graft polymerized carbon ink for printing. A printed electrode tips were fabricated by screen printing. Then, an enzyme-immobilized electrode was prepared by modifying 1,2-naphthoquinone as a mediator and lactate oxidase as an enzyme. The stability of the enzyme-immobilized electrode was evaluated by 10 cycles of cyclic voltammetry. The measurement was performed in a 1 mol dm-3 phosphate buffer (pH 7.0) containing 25 mmol dm-3 lactic acid with a three-electrode system. A platinum wire as a counter electrode, and a sat. KCl/Ag/AgCl electrode as a reference electrode were used. The maximum current density was observed at about 0.13 V vs. Ag/AgCl. Figure 1 shows the maximum current of the cyclic voltammogram plotted against the number of cycles during multiple sweeps. In case of the GMgOC, the current value retained 95.7% after 10 cycles, but it was 68.4% in the case of unmodified MgOC. By performing graft polymerization on the MgOC surface to fabricate the electrode, no significant change in the current value was observed. These results indicated that LOx was stably immobilized on the GMgOC surface without degeneration. Acknowledgements This work was partially supported by JST-ASTEP Grant Number JPMJTS1513, JSPS Grant Number 17H02162 and Private University Research Branding Project (2017-2019) from Ministry of Education, Culture, Sports, Science and Technology, and Tokyo University of Science Grant for President's Research Promotion． References 1) W. Jia, A. J. Bandodkar, G. Valdés-Ramírez, J.R. Windmiller, Z. Yang,J. Ramírez ,G. Chan, J. Wang, Analytical Chemistry, 85 (2013), 6553. 2) R.A.Escalona-Villalpando,E.Ortiz-Ortega,J.P.Bocanegra-Ugalde,Shelley,D.Minteer,J.Ledesma-García, L.G. Arriaga, Journal of Power Sources, 412 (2019),496-504. 3) I. Shitanda ,T. Kato ,R. Suzuki ,T. Aikawa ,Y. Hoshi ,M. Itagaki , and S. Tsujimura , Bulletin of the Chemical Society of Japan, 93 (2019), 32-36． Figure 1