Laser-Induced Graphene Electrodes Modified with a Molecularly Imprinted Polymer for Detection of Tetracycline in Milk and Meat

Abstract

Tetracycline (TC) is a widely known antibiotic used worldwide to treat animals. Its residues in animal-origin foods cause adverse health effects to consumers. Low-cost and real-time measuring systems of TC in food samples are, therefore, extremely needed. In this work, a three-electrode sensitive and label-free sensor was developed to detect TC residues from milk and meat extract samples, using CO₂ laser-induced graphene (LIG) electrodes modified with gold nanoparticles (AuNPs) and a molecularly imprinted polymer (MIP) used as a synthetic biorecognition element. LIG was patterned on a polyimide (PI) substrate, reaching a minimum sheet resistance (R_sh) of 17.27 ± 1.04 Ω/sq. The o-phenylenediamine (oPD) monomer and TC template were electropolymerized on the surface of the LIG working electrode to form the MIP. Surface morphology and electrochemical techniques were used to characterize the formation of LIG and to confirm each modification step. The sensitivity of the sensor was evaluated by differential pulse voltammetry (DPV), leading to a limit of detection (LOD) of 0.32 nM, 0.85 nM, and 0.80 nM in buffer, milk, and meat extract samples, respectively, with a working range of 5 nM to 500 nM and a linear response range between 10 nM to 300 nM. The sensor showed good LOD (0.32 nM), reproducibility, and stability, and it can be used as an alternative system to detect TC from animal-origin food products.

Keywords: antibiotic residue; electrochemical sensor; flexible; laser-induced graphene; meat; milk; molecularly imprinted polymer; tetracycline.

Figure 1

Optimization of the sensor fabrication steps: (A) for electrode cleaning, cyclic voltammetry curves showing that the increasing number of cyclic voltammetry cycles (up to 20) leads to a stable curve; (B) cyclic voltammetry curves for electrodeposition of AuNPs, where the blue and red-colored curves show before and after electrodeposition of AuNPs, respectively; (C) electropolymerization of MIP, where cyclic voltammetry curves show that the increasing number of cycles (up to 30 cycles) leads to a stable curve; (D) the rebounding of the tetracycline on the cavities takes up to 25 min until the saturation of the cavities.

Figure 2

Optimization of electrode fabrication parameters for 10 cm/s CO₂ speed: (A) resistance of LIG electrodes fabricated with different CO₂ powers; (B) respective sheet resistance.

Figure 3

SEM images at 5 kV extraction and acceleration voltage, and at 7.2 mm working distance with scale bar of 20 µm, for (A) bare PI, (B) LIG electrode fabricated with 10 cm/s speed and 4.5 W power, and (C) LIG electrode fabricated with 10 cm/s speed and 8.6 W power. (D) High resolution at 3 µm showing the graphite network.

Figure 4

Raman spectra for different laser powers at a speed of 10 cm/s: (a) 4.5 W, (b) 6.0 W, (c) 8.6 W, and (d) 9.0 W.

Figure 5

Cyclic voltammetry curves obtained after each modification step of LIG electrode.

Figure 6

Sensitivity test of the sensor: (A) the DPV curves for different TC concentration in acetate buffer; (B) the calibration curves extracted from the DPV curves heights (inset: linear range of the sensors).

Figure 7

Time stability test of the MIP electrochemical sensors.