Charging and Discharging of Poly(m-aminophenylboronic Acid) Doped with Phytic Acid for Enzyme-Free Real-Time Monitoring of Human Sweat Lactate

Abstract

In this study, we realized a real-time and enzyme-free measurement of lactate in sweat in the same way as an enzyme-based amperometric method. A conductive polymer, which is based on polyaniline (PANI), was strongly coated on a glassy carbon electrode as a poly m-aminophenylboronic acid (PANI-PBA) membrane by drop-casting, which is a convenient method, owing to adhesive phytic acid (PA) molecules with negative charges included as a dopant. This polymer membrane had a functional structure with PBA in the PANI main chain, which expectedly induced electrical charges upon diol binding to lactate, owing to the formation of deprotonated boronate esters with negative charges. This indicates that PBA served as the self-dopant and as the site of binding to lactate. On the basis of the fundamental electrochemical characteristics such as the membrane resistance, the change in the current density of the PA-doped PANI-PBA electrode was quantitatively monitored with the change in lactate concentration from 1 to 300 mM under acidic conditions in real time, considering pH and interfering substances in sweat. Moreover, the sweat lactate concentration was determined to be ca. 60 mM using the PA-doped PANI-PBA electrode in a microfluidic system in measurements using sweat samples collected during exercise load. A change in current density induced a change in the density of charges in the capacitive PA-doped PANI-PBA membrane. This means that the detection mechanism for the change in the lactate concentration in sweat was based on repeated charging and discharging in the PA-doped PANI-PBA electrode.

Scheme 1. Schematic of Polymerization of m-Amino–PBA to PANI–PBA (Top) and Diol-Based Reversible Binding of Lactate and PBA (Bottom)

Figure 1

(a) Drop-casting. (b) Polymerization at 5 °C for 30 min. (c) Ultrasonic cleaning at 25 °C for 5 min. A rod-type electrode (glassy carbon) was used in (a–c). (d) Scheme of sweat lactate sensing. The microfluidic system was composed of two devices: the sensing device with the printed sheet electrode (carbon paste) coated with the PA-doped PANI–PBA membrane (Figures S1 and S2) and the sampling device (Figure S3).

Figure 2

Nyquist plot for changes in lactate concentrations (1–100 mM) measured with PA-doped PANI–PBA (a) and PA-doped PANI (b) coated on a rod-type glassy carbon electrode. The measurements were performed in 100 mM phosphate buffer with 300 mM NaClO₄ (pH 5.3) to maintain the ionic strength of Na⁺ even when a certain volume of a lactate solution was exchanged with the buffer at each lactate concentration. This is why, the solution resistance (R_s) remained constant. The frequency was varied from 100 kHz to 0.1 Hz.

Figure 3

Relative changes in membrane resistance (R_m) with changes in lactate, NaCl, and glucose concentrations (1–100 mM) measured with a PA-doped PANI–PBA electrode at pH 5.3 (a) and 7.4 (b). The error bars represent the standard error (n = 4).

Figure 4

(a) Change in current density (i) with change in lactate concentration (1–100 mM) measured with printed sheet electrode (carbon paste) coated with PA-doped PANI–PBA membrane in a microfluidic system. The buffer solution with or without lactate was allowed to flow within 0–200 or 200–400 s, respectively. (b) Change in charge (Q) with change in lactate concentration (1–300 mM). Q was calculated by the integration of the change in i with the reaction time on the basis of the data shown in (a). The error bars represent the standard error (n = 5).

Figure 5

(a) Change in current density (i) upon adding 10 mM NaCl, 100 mM lactate, the mixture, or buffer without NaCl and lactate at pH 5.3 measured with printed sheet electrode (carbon paste) coated with PA-doped PANI–PBA membrane in a microfluidic system. The measurement solutions were suctioned and allowed to flow at a rate of 300 μL/min through the sensing device. (b) Change in charge (Q) upon adding 100 mM lactate or a mixture of 10 mM NaCl and 100 mM lactate. Q was calculated by the integration of the change in i with reaction time on the basis of the data shown in (a). The error bars represent the standard error (n = 4).

Figure 6

(a) Change in current density (i) during exercise load measured with printed sheet electrode (carbon paste) coated with PA-doped PANI–PBA membrane in the microfluidic system. The pH 5.3 buffer solution was suctioned and allowed to flow at a rate of 5 μL/min through the sampling and sensing devices. (b) Change in charge (Q) with a logarithmic change in lactate concentration (1–300 mM), which was based on Figure 4b. Q estimated in (a), which was derived from sweat lactate, is indicated by a dotted line. The concentration of sweat lactate diluted in the microfluidic system was estimated to be approximately 15 mM. The error bars represent the standard error (n = 5).