*3.4. Photoelectric Electrochemistry*

PEC is a kind of photoelectric analysis technology developed gradually based on electrochemistry in recent years. It is a highly sensitive and fast analysis method. PEC converts chemical energy into electrical energy by using light as the excitation source, and the photocurrent generated is used as the detection signal. It has the advantages of low background noise and high sensitivity and has been widely used in chemical synthesis, catalysis, and biological analysis [94].

In the process of PEC sensor construction, photoactive materials play an indispensable role, and their properties directly determine the performance of the sensor [95]. Various materials such as TiO2, ZnO, CdSe, and CdS are used to manufacture PEC sensors. ZnO nanomaterials are a kind of N-type semiconductor and have attracted extensive attention due to their advantages of low cost, good chemical stability, and excellent electrical and optical properties. Cao et al. [55] modified ZnO nanosheets on an ITO electrode, and then modified CdS nanoparticles on the surface of ZnO nanosheets to form a CdS/ZnOsensitized structure through continuous ion layer adsorption and the reaction of Cd2+ and S2−. Then, CdSe QDs were introduced into the sensing system through a hybridization reaction, forming a double co-sensitization structure, realizing high selectivity, high sensitivity, and high stability detection of Pb2+. Niu et al. [96] designed a ZnO and Reduced graphene oxide (ZnO-RGO) nanocomposite as a photoactive material, adding AuNPs to further enhance the electrical conductivity. Moreover, AuNPs can anchor the aptamer and its complementary chain to form a double chain structure in which MB can amplify the current response. When the sensor captures Cd2+, the aptamer and its complementary chain break, and MB is separated from the electrode surface, reducing the photocurrent response, resulting in a detection limit of 1.8 × <sup>10</sup>−<sup>12</sup> mol/L. Niu also designed ZnO-TiO2 nanocomposites as photoactive substrates and covered them with gold nanochains. One part of the aptamer was connected to the gold nanochain, and the other part was coupled to graphite-like Carbon Nitride (G-C3N4). When Cd2+ was detected by the aptamer sensor, the aptamer formed a stable hairpin structure, and the signal sensitizer G-C3N4 was closer to the electrode, making the changes in the photocurrent signal more sensitive. The detection limit was 1.1 × <sup>10</sup>−<sup>11</sup> mol/L, slightly lower than that of the first method. Both sensors have significant specificity because the aptamer usually reacts only after contact with its corresponding target, resulting in changes in secondary structure. Niu [97] developed a PEC sensor for Pb2+, again using AuNPs as a fixed aptamer, and CdS-TiO2 as a photoactive material, The difference is quercetin-copper(II) complex as intercalator and electron donor. The detection limit was 1.6 × <sup>10</sup>−<sup>12</sup> mol/L, which was satisfactory.

#### *3.5. Electrochemiluminescence*

Electrochemiluminescence (ECL) has received considerable interest in the development of an ultrasensitive detection technique in recent years. It works by bringing the system or component of the electric biomass into an excited state through electron transfer and then returning it from the excited state to the ground state to produce a chemiluminescence phenomenon. It combines the advantages of both electrochemical and chemiluminescent biosensors, with relatively low cost, simplicity, rapidity, and high selectivity [98,99].

Ruthenium (II) tris (bipyridine) (Ru(bpy)3 2+) and its derivatives remain the most popular ECL reagents due to their recyclability, high quantum yields, and suitability in different pH levels, It interacts with the electrode surface to produce a strong ECL signal, usually by using hairpin DNA to interact with Ru(bpy)3 2+ away from the electrode surface, thereby reducing the ECL signal [100]. According to this principle, Strand Displacement Amplification (SDA) is also widely used in the construction of ECL sensors. SDA was proposed and improved in 1992, relying on the combination of the strand-displacing polymerase and the nicking endonuclease to generate an exponential accumulation of single-stranded DNA (ssDNA) [101]. Zhu et al. [102] prepared a sensor for As3+ based on SDA technology. By using polydopamine nanospheres (PDANS) as inhibitors, hairpin DNA was constrained by PDANS and the SDA process was inhibited. Ru(bpy)3 2+ as an ECL probe could diffuse the ITO electrode surface and generate a strong ECL response. However, the presence of As3+ makes hairpin DNA no longer constrained and triggers the SDA process with the help of polymerase and incisor endonuclease to generate dsDNA, which interacts with Ru(bpy)3 2+ to form the dsDNA- Ru(bpy)3 2+ complex. Due to electrostatic repulsion, it is difficult for the complex to approach the ITO electrode surface, resulting in a low ECL response, the detection limit is 1.2 × <sup>10</sup>−<sup>3</sup> ppb. This strategy has also been applied to the detection of Cd2+ [103]. The difference is that Xu et al. used magnetic Fe3O4-GO nanosheets to constrain hairpin DNA, and the detection limit was 1.1 × <sup>10</sup>−<sup>4</sup> ppb. Ma et al. [104] did not use an SDA reaction for DNA amplification but hybridized a ruthenium complex with an aptamer and its complementary chain. The combination of Hg2+ and T-T mismatch induced adaptive folding and compression of the aptamer, keeping the Ruthenium complex away from the electrode and weakening the ECL signal. Li et al. [59] functionalized Ru(bpy)3 2+ with 3-aminopropyltriethoxysilane and mixed it with silica nanoparticles and graphene quantum dots to form an ECL composite material for mercury ion detection, which has good detection performance.

In addition to Ru(bpy)3 2+, the ECL resonance energy transfer (RET) of QDs and precious metal nanoparticles (such as AuNPs) is also considered to be a sensitive and reliable analysis technique, but their ECL intensity is usually lower than Ru(bpy)3 2+. Wang et al. [105] synthesized cadmium sulfide QDs doped with lanthanum ions and designed an ECL sensor based on the QDs and AuNPs. The surface plasmon resonance of AuNPs enhanced the strength of ECL. Secondly, in the presence of Hg2+, oligo-base pairs change from linear chains to hairpins. The realized ECL quenched, and finally, after incubation with TB, produced a strong and stable transfer, which resulted in the eventual recovery of the ECL signal. The detailed process is shown in Figure 8.

**Figure 8.** Fabrication of the ECL-RET aptasensor for Hg2+ and TB, respectively based on the resonance energy transfer between CdS:La QDs film and AuNPs; (**A**) Preparation of aptamer probe; (**B**) Electrode surface modification and sensor reaction principle. Reprinted with permission from ref. [105]. Copyright 2019 Elsevier.



**Table1.**Applicationoftheelectrochemicalsensorinthedetectionofheavymetalions.

324


*Foods* **2022**, *11*, 1404

*Foods* **2022**, *11*, 1404

