**2.3 Carbon-based nanobiosensors**

Carbon is one of the most special elements in the material science world. It can be used in many areas thanks to its different atomic array versions, such as graphene, graphite, fullerenes, and nanotubes. Two general approaches, such as top-down (laser ablation, chemical ablation, electrochemical, and sonication) and bottom-up (ultrasonication, solvothermal, electrochemical, hydrothermal, and microwave methods), have been used for synthesizing carbon dots, carbon nanotubes, carbon nanorods, and carbon fibers. Due to its ability to improve electrical, mechanical, physical, and chemical properties, it was used in various fields. In recent years, various studies have been carried out on carbon-based nanobiosensors in biosensor applications [40]. Sreekanth et al. [41] studied the detection of cadmium metal in water with multi-walled carbon nanotube enhanced nanobiosensors. Heavy metals, especially cadmium, are harmful to both nature and humans and pose a serious threat to human health. In this study, a nanobiosensor was developed to detect cadmium (Cd) metal with a DNA-assisted electrochemical technique. In the study, the glassy carbon electrode (GCE) was decorated with a multi-walled carbon nanotube, and dsDNA was immobilized on the carbon nanotube decorated GCE. Furthermore, heavy metal detection was examined by using differential pulse

voltammetry (DPV) analysis. In the presence of Cd (II) ions, dsDNA interacts with Cd to form ssDNA. ssDNA binds with ethyl green (EG), and this provides a noticeable change in reduction peak current. Higher reduction peak currents are observed at increasing Cd concentrations. The developed nanobiosensor has demonstrated the potential of multi-walled carbon nanotubes in nanobiosensor applications with its ability to detect Cd at a limit of detection (2 nM) and sensitivity (5 nA nM−1). In addition, Ballen et al. focused on the development of cantiveler biosensors to detect the presence of cadmium. In their studies, they developed urease, (GO), and urease/ GO-based nanobiosensors and investigated the properties of nanobiosensors for the detection of Cd. The urease nanobiosensor has a detection limit of 0.03776 ppb, while the GO/urease nanobiosensor has a more advanced detection limit of 0.01831 ppb [42]. In another study, Taşaltın et al. studied a nanobiosensor enhanced with rGO synthesized by an ultrasonic microwave assisted method for propamocarb pesticide detection. It has been reported that the developed biosensor in the study has superior properties such as high selectivity (101.1 μAμM−1 cm−2), rapid response (1 min), a wide linear range (1–5 μM), and a low detection limit (0.6 μM) of pesticide [43]. In another study, Elugoke et al. [44] fabricated a novel electrochemical biosensor based on a modified electrode with carbon quantum dots and CuO nanocomposite for the detection of dopamine using square wave voltammetry (SWV). The electrochemical results showed that the carbon quantum dots-CuO nanocomposite-based biosensor exhibited a low LOD of 25.4 μM in a wide linear range from 1 to180 μM. Furthermore, it was proposed that the sensing mechanism was based on the negative charge of the oxygen-containing functionalities on the modified electrode, which attracted the positively charged analyte. With a similar approach, Gaidukevic et al. [45] prepared a sensitive electrochemical rGO-based biosensor for the determination of dopamine in the presence of malonic acid and P2O5 additives. Experimental results showed that the proposed biosensor exhibited high sensitivity of 28.64 μA μM−1 cm−2 and a low LOD value of 0.11 μM for the detection of dopamine. Additionally, it was reported that the sensing mechanism of the redox reaction of analyte was changed due to the change from reversible to irreversible transition. The biosensing mechanism of the redox reaction of analyte was changed due to the change from a reversible to an irreversible transition. In addition, the electrochemical process was a phenomenon occurring in the surface adsorption-controlled reaction.
