# Functionalized Carbon-Based Electrochemical Sensors for Food and Alcoholic Beverages Safety
## 3 Carbon Nanomaterial-Based Electrochemical Sensors
### CDs  
In 2004, [PERSON] et al. discovered that CDs can present fluorescence during the process of electrophoresis [56]. CDs are defined as zero-dimensional nanoparticles having a size of less than 10 nm that are mainly composed of carbon elements. CDs have been widely used in electroluminescence, medical imaging, environmental monitoring, chemical analysis, photocatalysis, and energy conversion due to their colorful optical properties, good water solubility, low toxicity, environmental friendliness, abundant raw source, low cost, and good biocompatibility [60; 61]. Regarding CDs' preparation, a variety of methods, including arc discharge, laser ablation, electrochemical oxidation, acidic oxidation, microwave, ultrasonic, calcination, hydrothermal, template synthesis, and hydrosol condensation polymerization, have been reported [62; 63; 64]. Interestingly, CDs can be functionalized easily with other materials, such as organic, polymeric, and inorganic species, via reacting with surface carboxylic acid moieties. In the following section, we illustrate the applications of CD-based materials in the development of electrochemical sensors for H\({}_{2}\)O\({}_{2}\), dopamine (DA), triclosan, glucose, acetaminophen, Cu\({}^{2+}\) ions, uric acid, etc. Acting as a neuromodulator of ionotropic synapses, DA sets a threshold for the striatal activity involved in many diseases and drug addiction. In addition, DA is available as an intravenous medication acting on the sympathetic nervous system. Hence, the determination of DA in vivo/vitro becomes increasingly important in practice [65; 66]. As shown in Figure 3a, [PERSON] et al. [67] prepared a new type of N-doped CDs by a one-step microwave irradiation method. The N-doped CDs obtained by this method had a highly sensitive electrochemical response to DA, with a linear range of 0.05-8 uM and a detection limit of 1.2 nM. Moreover, in contrast to traditional detection methods requiring complex pretreatment and producing a large quantity of organic waste, the developed electrochemical sensing detection can be performed by directly adding DA to the detection system for qualitative and quantitative analysis. [PERSON] et al. [68] designed a new type of DA sensor (Figure 3b) based on a Au@CDs-CS/glassy carbon electrode (GCE), which has high sensitivity and excellent performance stability and can suppress the background interference currents from ascorbic acid (AA) and uric acid (UA). Under the optimum experimental conditions, the linear range of 0.01-100.0 uM and the detection limit of 0.1 nM (S/N = 3) were obtained. [PERSON] and co-workers [69] reported that CDs and chitosan were combined to construct a composite electrode. It was found that the signal was significantly higher than that of the bare electrode. Moreover, the composite electrode has higher sensitivity for the detection of triclosan, and also shows good results for the detection of actual samples such as toothpaste and gargle daily water, with the linear range of 10-1.0 mM and the detection limit of 0.92 nM (Figure 3c). As shown in Figure 3d, [PERSON] et al. [70] used CDs-Chitosan (CS)/hemoglobin composite membrane for electrochemical detection of H\({}_{2}\)O\({}_{2}\), which showed good sensitivity and robust usability. The linear current response for H\({}_{2}\)O\({}_{2}\) was from 1 to 118 uM with a detection limit of 0.27 uM at the signal-to-noise ratio of 3, and the apparent Michaelis-Menten constant was 0.067 mM. Their work showed that loading metal nanoparticles can further significantly improve the sensitivity and selectivity of the electrochemical sensors, presenting a superior detection performance to the pure approach. Based on the easy modification of CDs, the rich carboxyl or amino functional groups on their surface are conducive to the grafting of small organic molecules for the detection of special chemicals. For example, [PERSON] et al. [71] selected nitrogen-doped carbon dots (N-CDs) containing amino groups as the raw electrode due to their good conductivity and redox performance.