*2.2. Carbon Nanotubes*

Carbon nanotubes (CNTs) are a kind of carbon nanomaterial with unique electrical transmission properties and a large specific surface area, with excellent chemical, mechanical, and thermal stability, and direct binding with other molecules. Therefore, they have received great attention in biosensor design [37,38]. They can be thought of as cylindrical tubes made of one or more sheets of graphene rolled and folded. Nanotubes formed from a single sheet of graphene are called single-walled nanotubes (SWNT), with diameters between 0.4~2 nm, and multi-walled carbon nanotubes(MWCNTs) are 2~10 times larger [39]. Graphene contains SP2 hybrid carbon atoms, and nucleic acids can easily be attached to the surface of the nanotubes through π-π bonds, so CNTs are often used to carry ssDNA or RNA. In 2005, So et al. [40] constructed a biosensor using single-walled CNTs by using aptamer as a molecular recognition element for the first time. Since then, the potential of CNTs in sensing platforms has been gradually explored.

It is more common to modify electrodes with MWCNTs and then fix aptamers on the surface of MWCNTs. Zhu et al. [41] used carboxylic acid group functionalized MWCNTs to fix aptamers. Because the material has a large surface area and good charge transferability, the DNA attachment amount and sensor performance could be significantly improved. The sensor could detect Pb2+ in the range of 5.0 × <sup>10</sup>−11~1.0 × <sup>10</sup>−<sup>14</sup> M. Additionally, Zou et al. [42] covalently immobilized aptamers on carboxylic functionalized MWCNTs by EDC/NHS chemistry. CNTs can not only carry aptamers in sensors but also provide binding sites for the reactions between aptamers and other nanomaterials. Rabai et al. [43] modified the electrode surface with CNTs, then deposited AuNPs on the composite electrode. Finally, with the aptamer fixed, the sensor had a detection limit of 0.02 pM for Cd2+. He et al. [44] developed an electrochemical sensor with a Zn3(PO4)2 modified aptamer and found that the sensor with MWCNTs had higher sensitivity.

#### *2.3. Graphene*

Graphene is a two-dimensional carbon nanomaterial with SP2 hybrid connected carbon atoms tightly packed into a single layer honeycomb lattice structure. Due to its excellent electrical properties, graphene has attracted great interest in recent years. Graphene has a unique type of structure that also gives it features not found in many other nanomaterials; its large specific surface area makes it an ideal candidate for immobilizing large numbers of functionalized metal oxides and noble metal nanoparticles [45]. Exceptional carrier mobility offers great prospects for nanoscale applications, such as electronic devices and chemical/biological sensors. In addition, its high electron transfer efficiency and wide electrochemical window make graphene an ideal material for constructing a highly sensitive E-apt sensor [46]. In recent years, graphene as a REDOX molecule in electrochemical sensors has attracted great attention.

The most common use of graphene in sensors is to modify electrodes. Zhang et al. [47] developed an Hg2+ detector for a graphene-fixed aptamer probe with a detection limit of 5 pM. Using graphene-modified electrodes, the modified electrode surface can be reused. Similar to AuNPs, it is more likely that graphene will be coupled with other nanomaterials to form new composite materials for better modification. Hai et al. [48] modified the electrode with AuNPs coupled with graphene. The aptamer was self-assembled on the electrode, and the detection limit of Pb2+ was as low as 3.8 pM. Jiang et al. [49] designed a new nanometer composite material consisting of TiO2, AuNPs, and nitrogen-doped graphene. The composite exhibits excellent optical properties, increasing the exciton lifetime and improving the charge transfer photocurrent intensity to 18.2 times higher than that of original TiO2. Wang et al. [50] synthesized another new material modified electrode by using CS instead of AuNPs for the detection of Pb2+. The modified electrode showed good repeatability, stability, and specificity to other interfering metal ions. Li et al. [51] prepared a sensor for mercury ions based on a perylene-3, 4, 9, 10-tetracarboxylic acid/graphene oxide (PTCA/GO) and quercetin-copper(II) complex. They efficiently promoted the separation of photoexcited carriers and enhanced the photocurrent.

#### *2.4. Quantum Dots*

Quantum dots (QDs), also known as semiconductor nanocrystals, are mixtures of cadmium and selenium or tellurium with nanoscale clusters with diameters of 1–20 nm [52]. Due to their unique properties, such as high quantum yield, excitation-dependent emission, surface modification versatility, and long-term photostability [53,54], QDs have been used in many research fields, especially in nanoelectronics, optoelectronics, and biological analysis [55]. The physical size of the nanocrystals determines the wavelength of the emitted fluorescence, so multiple analyses can be performed using a single excitation source. With the rapid development of nanotechnology, such nanomaterials have been widely used to improve the sensitivity of sensors. When they are incorporated into the design of biosensors, they can be used as tags, as part of a signal sensor [56].

QDs are mainly used in electrochemical luminescence sensors and photoelectric electrochemistry sensors. When the aptamer reacts with the corresponding target, the brightness of the QDs will also change due to the resonance energy transfer, thus improving the sensitivity of the sensor. CdTe QDs are most frequently used in sensors; Shi et al. [57]

developed a new method for the detection of Pb2+ based on the sensitization effect of CdTe QDs. When the target is present, the labeled QDs close to the electrode surface produce a sensitization effect and the photocurrent intensity is enhanced. Feng et al. [58] modified CdTe QDs with MIL-53 and determined Hg2+ and Pb2+ simultaneously by the ECL method with good recovery. Except for CdTe QDs, there are many other types of QDs, such as CdS and nitrogen-doped graphene (NG) [59,60], and their principle of action is roughly the same. When the conformation of the aptamer changes, the distance between the QDs and the electrode alters, and the electrical signal is either enhanced or weakened. This property makes QDs widely used to design E-apt sensors for food and water analysis.
