**2. Application of Nanomaterials in Aptamer-Based Electrochemical Sensors**

In recent years, the successful synthesis of various nanomaterials has attracted significant attention. Their unique physical and chemical properties, including high surface area/volume ratio, high reactivity, size dependence, and high functionalization, make them widely used in chemical analysis, therapy, diagnosis, and food safety [20–24]. In addition to the binding activity of the aptamer, the reason why the electrochemical aptamer sensors can detect heavy metal ions as low as fM is largely attributed to the rational use of nanomaterials. Sensing platforms are formed by immobilizing aptamers on nanomaterial surfaces through intermolecular forces or catalyzing chemical reactions in sensors as sensitizers to enhance electrical signals and improve the specificity and sensitivity of sensors [25]. Due to their recognized properties, the coupling of aptamers on nanomaterials has great potential to form biosensor platforms.

Gold nanoparticles, carbon nanotubes, graphene, quantum dots, and metal-organic frameworks are common and basic nanomaterials used in E-apt sensors. These materials can be mixed or coupled with other substances to create new composite materials. They have shown many advantages such as high specific surface area, good biocompatibility, high electrical conductivity, high magnetic properties, and unique electro-optic and physicochemical properties [26–28]. In addition to being used as sensitizers, most nanomaterials in E-apt sensors are used to fix aptamers to form couplings with a better capture effect. Researchers often use couplings as probes to obtain better detection results. Figure 4 indicates how popular nanomaterials (shown as their microscopic morphology) are coupled with aptamers.

**Figure 4.** SEM images and combination modes of various nanomaterials.
