Molecularly Imprinted Polymer Modified Laser-Engraved Graphene Electrode for the Ultra-Sensitive and Specific Detection of Ascorbic Acid
Authors: Yao Handong , Zhang Yuan , Xu Jiaqiang 
Published: August 2024

Sensitive and accurate detection of small molecule targets is critical for a variety of applications, including environmental monitoring, food safety and medical diagnostics. Electrochemical sensors with the advantages of high sensitivity, accuracy, convenience and low-cost make them well suited for these applications. Moreover, the combination of electrochemical sensors with micro-and nanofabrication technology offers the possibility of integrated and intelligent sensing systems.
Figure 1. (a) The SEM characterization of PPD/LEG electrode, (b) the FT-IR of LEG and PPD/LEG electrode, (c) the Nyquist plots of LEG and PPD/LEG electrode, (d) the specificity, (e) linear relationship and (f) sensitivity of integrated electrochemical microsensor.
In general, the ultimate performance of an electrochemical sensor relies on the high efficiency of bio/chemical reactions as well as transport of the analytes by diffusion onto the electrode. Accordingly, the desired electrode materials should permit very fast mass transport rates and possess the capability of specific capture of target analytes. Laser-etched graphene (LEG) electrodes featured with the advantages of porous three-dimensional network structure, large specific surface area, and high conductivity, are considered as an ideal sensing element for sensitive molecules detection, but cannot recognize specific targets. Molecularly imprinted polymers (MIPs) are low-cost and easy to modify on electrochemical sensing surfaces. It can be used to form molecular recognition sites by synthesizing target templates in polymers, giving the modified electrodes the ability to specifically recognize target molecules, which is an ideal modification material for electrochemical sensor platforms.
Herein, we propose an integrated electrochemical microsensor sensing ultra-low concentration of ascorbic acid (AA) molecules by using MIP modified LEG as electrode material. The main characteristics of the detection system are as follows: 1) LEG electrode enables electrochemical sensing strategy with simple preparation and high sensitivity, 2) MIP poly-o-phenylenediamine (PPD) membrane on the surface of graphene provides the desirable property for highly selective AA molecular recognition, 3) the integrated PPD/LEG electrode allows to detect AA molecules in a ultra-low concentration range which is superior than that of other electrochemical detection techniques.
The surface morphology of PPD/LEG electrode is characterized by using scanning electron microscopy (SEM, shown in Figure 1a), from which LEG shows three-dimensional network porous structure and the nature of graphene. In addition, the electropolymerized PPD membrane exhibits a rough and porous surface which would provide the large surface area for the adsorption of target AA molecules at the modified electrode. The formation of PPD/LEG electrode is further examined by Fourier Transform Infrared Spectroscopy (FT-IR Spectrometer) in Figure 1b. The polymerization of PPD membrane on the surface of the LEG electrode is confirmed by the appearance of two absorption peaks at 3385 cm-1 and 3360 cm-1, which are related to the stretching vibration of the -NH2 from the PPD. As shown in the Nyquist plots (Figure 1c), the semicircle in the high-frequency region is described as an intrinsic electron-transfer resistance in the electrode materials. The identical semicircles at high-frequency region of two electrodes indicate nearly equal electron transfer resistance (Rct) of LEG and PPD/LEG. The low-frequency region of the Nyquist plot is described as surface diffusion control, and the increase in the linear slope represents an enhanced surface diffusion control process at the electrodes demonstrating the lower impedance of the PPD/LEG electrode. Figure 1d verifies the specific recognition of AA molecules by the PPD/LEG electrode in PBS mixed with 1 mM of AA, dopamine (DA), uric acid (UA) and acetaminophen (AP) molecules. It can be seen that compared with the LEG electrode, the DPV response of the PPD/LEG electrode to DA, UA and AP molecules is significantly reduced, while the response to AA molecules is significantly enhanced. Furthermore, the ability of the integrated electrochemical microsensor to detect ultra-low concentrations of AA molecules in the 10 nM to 100 nM range is shown in Figure 1e, with a LOD of 2 nM (Figure 1f).
In conclusion, we present an integrated electrochemical microsensor that can specifically detect AA molecules with ultra-high sensitivity. The LEG electrode with the modification of PPD membrane can effectively realize the specific recognition and accurate detection of AA molecules in mixed solutions. This integrated electrochemical sensing method is expected to be applied to the selective detection of multiple small molecule targets in complex environments.
Key words: Laser-engraved Graphene Electrodes, Molecularly Imprinted Polymers, Integrated Electrochemical Microsensor, Ultra Sensitivity Detection.

Figure 1