# Functionalized Carbon-Based Electrochemical Sensors for Food and Alcoholic Beverages Safety
## 2 Electrochemical Techniques  
Electrochemical sensors have become a widespread analysis technique with the merits of low cost, high accuracy, fast response, and good reliability, and have drawn substantial attention for the qualitative or/and quantitative analysis of foods and alcoholic beverages [28; 29]. The design of high-quality electrochemical sensors needs a full understanding of their basic detection principles. Figure 1a shows that samples without pretreatment can be first added into the electrochemical cell. Then, upon applying electricity, the electrochemical sensor converts the physical and chemical response into recognizable electrochemical response signals. Through analyzing the correlation between the detected substances and corresponding electronic signals, the contaminants in foods and alcoholic beverages can be quickly identified. As shown in Figure 1b, a typical three-electrode electrochemical system includes a working electrode (WE), a reference electrode (RE) (e.g., Ag/AgCl), and a counter electrode (CE) (e.g., carbon rod, platinum). Recently, functional carbon-based nanomaterials such as CDs, CNTs, graphene oxide (GO), and porous carbon were selected as candidates for the working electrode (WE) due to their excellent conductivity and durability [30; 31]. Of note, the selectivity, linearity, sensitivity, stability, detection limit, dynamic range, and response time of the resultant electrochemical sensors can be dramatically improved by further regulating the composition and the structure of adopted carbon nanomaterials [32; 33; 34]. Additionally, versatile electrochemical analysis methods are also required to be established for efficiently extracting the unique information from different analytes in food and alcoholic beverages. Specifically, the current-type (generating an amperometric current), potential-type (generating a potentiometric potential), conductivity-type (measurably altering the conductive properties of a medium between electrodes), and impedimetric-type (measuring impedance through an electrochemical impedance spectroscopy method) are the four types of golden protocols [35; 36; 37; 38]. In practice, cyclic voltammetry (CV), linear sweep voltammetry (LSV), differential pulse voltammetry (DPV), and square wave voltammetry (SWV) (Figure 1c) are most commonly selected according to the properties of the detected substance [39; 40; 41; 42]. So, it is necessary to understand the working principles of those electrochemical analysis methods, which is helpful for more rationally and accurately analyzing the contaminants in food and alcohol beverages. First, CV is a commonly used electrochemical analysis method operated by scanning the response current one or more times in a triangular waveform under varyingly applied voltage. The reversibility of electrode reaction, intermediate, phase boundary adsorption, phase transformation, and the mechanism of coupled reactions can be acquired and judged according to the presented shape of CV curves [43]. Second, LSV is another electrochemical analysis method in which a linear change in voltage is applied on the electrode; that is, the potential changes linearly with the applied voltage recording the current on the working electrode at the same time. The peak current measured in  
Figure 1: Principle and test methods of electrochemical sensors. (**a**) The detection principle of an electrochemical sensor. (**b**) The basic configuration of an electrochemical sensor. (**c**) The diagrams of cyclic voltammetry (CV), linear sweep voltammetry (LSV), differential pulse voltammetry (DPV), and square wave voltammetry (SWV) curves. the LSV curve has a linear relationship with the concentration of the measured analytes, which is the basis of quantitative analysis [44]. Third, DPV is an electrochemical method that involves the superposition of a linearly increasing voltage with a rectangular pulse of constant amplitude. As a result of the rapid development of electronic circuits, pulse voltammetry has been widely used in the field of electrochemical analysis.