*3.1. Aflatoxins*

Aflatoxins are a widespread group of food toxins that are produced by *Aspergillus flavus* and *Aspergillus parasiticus* [92–95]. There are four main types of aflatoxins: B1, B2, G1, and G2, which are based on their fluorescence characteristics under UV light (blue or green) excitation and relative chromatographic mobility in thin-layer chromatography. Among the aflatoxins, AFB1 is considered the most toxic aflatoxin, and can cause cancers, such as hepatocellular carcinoma. Various 2D nanomaterial-based electrochemical biosensors have been constructed for detecting AFB1 in various matrixes [96–113]. Srivastava et al. have developed a series of functionalized GO nanocomposite-based electrochemical biosensors for profiling AFB1 in foods since they developed the first rGO-based AFB1 immunosensor through the covalent conjugation of the monoclonal anti-AFB1 antibodies onto an rGO modified indium tin oxide (ITO) electrode in 2013 [96–99]. Among these electrochemical biosensors, the functionalized GO/rGO-based nanocomposites are employed in different roles, such as catalysts, electroactive probes and immobilization platforms for improving the biosensing performance. For instance, benefiting from the highly crystalline properties of the rGO-Ni NPs sheets (Ni nanoparticle decorated rGO sheets) along with the excellent electro-catalytic properties, the rGO-Ni NPs-ITO-based AFB1 immunosensor exhibits high sensitivity (129.6 mA ng<sup>−</sup><sup>1</sup> mL cm<sup>−</sup>2), long term stability (up to 6 weeks) and low LOD (0.16 ng mL−1) [99]. Photoelectrochemical (PEC) biosensors have attracted grea<sup>t</sup> attention in the biological analytical field as the PEC method can obtain high sensitivity without expensive equipment. Recently, Hao et al. developed a dual channel self-reference PEC biosensor for detecting AFB1 through immobilization of the AFB1 aptamer onto cadmium telluride (CdTe) and the CdTe-GO modified ITO electrode (as shown in Figure 4) [104]. In this case, CdTe and CdTe-GO were used to generate an anodic photocurrent and cathodic photocurrent, respectively. The AFB1 aptamer was immobilized on the PEC active materials, CdTe and CdTe-GO, through a covalent reaction or physical absorption, respectively. In the presence of AFB1, the aptamer is released from the CdTe-GO surface, resulting in the recovery of the cathodic photocurrent, while the aptamer forms an aptamer-AFB1 complex on the CdTe surface, and the anodic photocurrent decreases further. Compared

to traditional PEC biosensors, the CdTe/CdTe-GO-based dual channel self-reference PEC biosensor can provide better precision and reliability, which is promising for detection of AFB1 in complex matrixes. Very recently, Peng et al. developed an AFB1 electrochemical aptasensor based on tetrahedral DNA nanostructures (TDNs) immobilized on three dimensionally ordered macroporous MoS2-AuNPs hybrids (3DOM MoS2-AuNPs) [107]. 3DOM MoS2-AuNPs can enhance the immobilization amount of TDNs and facilitate the movement of the electrons between the electrode surface and the redox probe. In combination with a HRP functionalized magnetic signal amplifier, the aptasensor achieves a good linear range (from 0.1 fg mL−<sup>1</sup> to 0.1 μg mL−1) and a LOD of 0.01 fg mL−1, which can be employed to detect AFB1 in grain products such as rice and wheat powder samples.

**Figure 4.** Schematic representation of the construction of the self-reference photoelectrochemical (PEC) biosensor for the detection of AFB1 (adapted from Hao et al. 2017 [104], Copyright 2017 American Chemical Society and reproduced with permission).
