Electrochemical Detection of Ascorbic Acid in Finger-Actuated Microfluidic Chip

Abstract

The traditional quantitative analysis methods of ascorbic acid (AA), which require expensive equipment, a large amount of samples and professional technicians, are usually complex and time-consuming. A low-cost and high-efficiency AA detection device is reported in this work. It integrates a three-electrode sensor module prepared by screen printing technology, and a microfluidic chip with a finger-actuated micropump peeled from the liquid-crystal display (LCD) 3D printing resin molds. The AA detection process on this device is easy to operate. On-chip detection has been demonstrated to be 2.48 times more sensitive than off-chip detection and requires only a microliter-scale sample volume, which is much smaller than that required in traditional electrochemical methods. Experiments show that the sample and buffer can be fully mixed in the microchannel, which is consistent with the numerical simulation results wherein the mixing efficiency is greater than 90%. Commercially available tablets and beverages are also tested, and the result shows the reliability and accuracy of the device, demonstrating its broad application prospects in the field of point-of-care testing (POCT).

Keywords: 3D printing; ascorbic acid; electrochemical sensor; microfluidic chip; micropump.

Figure 1

(A) Scheme of silver/carbon screen-printed electrode (SPE). (a) Insulating layer, (b) carbon track/electrode, (c) silver track/electrode and (d) polyethylene terephthalate (PET) substrate. (B) Scheme of 3D-printed molds and PDMS blocks. (C) Schematic of the integrated microfluidic chip and additional details (reference values: d1 = d2 = d3 = 1–1.5 mm, D1 = 20 mm, D2 = 10 mm, L1 = 0.2 mm, L2 = 0.3 mm, L3 = 1.2 mm, h1 = 1.5–3 mm, h2 = 0.5 mm, h3 = 0.2 mm).

Figure 2

Images of SPE and SEM micrographs of the carbon electrode. (A) Overall structure morphology of the SPE. (B,C) Low-magnification and high-magnification SEM images of the carbon electrode. (D) Raman spectrum of carbon electrode.

Figure 3

(A) CV responses of SPE at different scan rates (10–1000 mV/s) in 2500 µM of AA solution. (B) The plot of the anodic current versus the square root of increasing scan rates (10–1000 mV/s), R² = 0.98597. The experiments were carried out at a volume of 40 μL sample on the electrode surface. (C) Nyquist plots in the frequency range from 0.01 Hz to 100 kHz. Inset is Randles equivalent circuit.

Figure 4

(A) The 50 sweep segments CV curve in PBS at a scan rate of 50 mV s⁻¹. (B) Comparative DPV responses of SPE with 250 µM and without AA. (C) DPV responses of SPE with increasing concentration of AA (10–5000 µM), and inset is a partial enlargement of 10–250 µM (n = 3). (D) The calibration plot of current vs. concentration of AA (n = 3).

Figure 5

Amperometric response of SPE to successive addition of 250 µM of AA, Na⁺ ions (250 µM), glucose (1 mM), CA (250 µM) and then AA (250 µM) twice at a potential of +0.4 V.

Figure 6

Numerical analysis of concentration distributions in the (A) xy plane of mixing channel and (B) yz plane of outlet; (C) effect of the microchannel height on the mixing efficiency.

Figure 7

(A) DPV responses of on-chip detection and off-chip detection with increasing concentration of AA (bottom to top: 50 µM, 100 µM, 150 µM, 200 µM, 250 µM) (n = 3). (B) The calibration plot of current vs. concentration of on-chip detection and off-chip detection (n = 3).