*3.2. Ochratoxin*

Ochratoxin A (OTA) is the major mycotoxin of the ochratoxin group, which are produced primarily by fungi (e.g., *Aspergillus ochraceus, Penicillium verrucosum* and *Aspergillus niger*) [114–117]. OTA has strong nephrotoxicity, and is the main etiological agen<sup>t</sup> responsible for human Balkan endemic nephropathy (BEN) and associated urinary tract tumors. In addition, high concentrations of OTA has certain hepatotoxicity. During the last 5 years, several 2D nanomaterial-based electrochemical biosensors including immunosensors and aptasensors have also been developed for sensing OTA [118–131]. For instance, a series of aptasensors based on rGO-AuNP nanocomposites have been constructed by Wang's group [118–120]. The rGO-AuNP nanocomposites have well-dispersity and controllable surface coverage of AuNPs on the rGO sheet, which can be employed as an excellent signal amplified platform for an impedimetric aptasensor and/or an efficient nanocarrier for the CdTe QD (cadmium telluride quantum dot)-based amperometric aptasensor. As a typical example, a label free electrochemical aptasensor was successfully fabricated for ultrasensitive detection of OTA through using the CdTe QDs modified graphene/AuNPs nanocomposite (GAu/CdTe) as a signal amplifier. The as-proposed label-free amperometric aptasensor exhibits a wide linear range from 0.2 pg mL−<sup>1</sup> to 4 ng mL−<sup>1</sup> and a low LOD (0.07 pg mL−1), which has grea<sup>t</sup> potential in various applications, such as food safety monitoring and clinical diagnosis [120]. Bulbul et al. developed a non-enzymatic nanocatalyst-based amperometric aptasensor for OTA detection through immobilization of the OTA aptamer on the GO-modified electrode and the electro-oxidation of a nanoceria (nCe) tag [121]. In this case, GO was used as an electrode material for facilitating the electron transport and enhancing the electrochemical response because it has high conductivity and peroxidase-like activity. In particular, the synergistic effect between the catalase activity of nCe and the peroxidase like activity of GO increases the OTA detection sensitivity significantly. The LOD of as-proposed amperometric aptasensor is calculated to be 0.1 nmol <sup>L</sup>−1, which is below the European Union regulatory limits of OTA (such as 5 μg kg−<sup>1</sup> in raw cereal grains, 3 μg kg−<sup>1</sup> in products derived from cereals, and 2 μg kg−<sup>1</sup> in grape juice). The analytical reliability of the amperometric aptasensor has been demonstrated by the detection of OTA in spiked corn samples. Recently, Wang et al. constructed a ratiometric electrochemical aptasensor for OTA detection through assembly of a methylene blue (MB)-modified OTA aptamer (MB-aptamer) on the MoS2 nanosheet/AuNP (MoS2-AuNP) nanocomposite-decorated gold electrode through the host-guest recognition of β-cyclodextrin (β-CD) (as shown in Figure 5) [128]. After interaction with OTA, the MB-aptamer was disassembled because of G-quadruplex formation, leading to a decrease in the peak current of MB. Whereas the free ferrocenecarboxylic acid was recognized by β-CD and produced signals in the current, resulting in the "ratiometric" effect. With the combination of high electrocatalytic activity of MoS2-AuNP nanocomposites and the recognition capability of β-CD, the as-proposed ratiometric electrochemical aptasensor possesses satisfactory superiority in terms of detection range (from 0.1 nmol L−<sup>1</sup> to 50 nmol <sup>L</sup>−1), sensitivity (a LOD of 0.06 nmol <sup>L</sup>−1), and accuracy (6.5% of the relative standard deviation (RSD)). The practicability of the aptasensor was successfully demonstrated by detecting OTA in red wine samples.

**Figure 5.** Schematic representation of the fabrication of the ratiometric electrochemical aptasensor for OTA detection based on nanocomposites of gold nanoparticle and MoS2 nanosheets with β-CD-SH (thiolated β-CD) (adapted from Wang et al. 2018 [128], Copyright 2018 Elsevier Ltd. and reproduced with permission).

#### *3.3. Mycotoxins Produced by Fusarium*

The 2D nanomaterial-based electrochemical biosensors have also been developed for detecting other *mycotoxins* produced by *Fusarium* including deoxynivalenol (DON), fumonisin 1 (FB1), and zearalenone (ZEN) [132–137]. Shi et al. developed an aptasensor for sensitive FB1 detection by using the dual amplification of AuNPs and graphene/thionine nanocomposites (GSTH) [132]. GSTH served as electrochemical probes, which exhibit a strong electrochemical signal because the graphene has excellent conductivity and a large surface area for immobilizing a large amount of thionine molecules. The as-prepared aptasensor has a six orders of magnitude linear range with a LOD of 1 pg mL−1. Lu et al. fabricated an electrochemical immunosensor based on a graphene nanocomposite for rapid and sensitive detection of two mycotoxins, DON and FB1 by using correspondent anti-toxin antibodies (as shown in Figure 6) [134]. In this case, the disposable SPCE was used as a sensing platform, which was modified by AuNPs and polypyrrole (PPy)-electrochemical rGO (PPy/ErGO) nanocomposite film. The film exhibits effective anti-toxin antibody immobilization capacity, enhanced electrical conductivity, and biocompatibility. The current signal of PPy/ErGO-SPCE is much better than that of PPy/rGO-SPCE. Benefiting from the excellent electrochemical response and effective antibody immobilization, the immunosensor exhibits good sensitivity, with a LOD of 4.2 ng mL−<sup>1</sup> for FB1 and 8.6 ng mL−<sup>1</sup> for DON. The immunosensor can be used for simultaneous detection of multiple co-contaminant mycotoxins individually in the practical samples (e.g., corn extracts) because it shows low matrix interference even in co-existing toxin environments. Very recently, Jiang et al. constructed a facile electrochemical immunosensor based on thin-layer MoS2 and thionin (MoS2-Thi) composites for the sensitive and rapid detection of zearalenone (ZEA) in human biofluids (as shown in Figure 7) [136]. The as-prepared MoS2-Thi nanocomposites were employed as excellent electrochemical probes, as well as an efficient anti-ZEA antibody loading platform because MoS2 retains the electrochemical activity of Thi, and has a large surface area. The MoS2-Thi-based electrochemical immunosensor has good ZEA detection performance including a wide linear range (0.01 to 50 ng mL−1), low LOD (0.005 ng mL−<sup>1</sup> ZEA in both the plasma and urine), excellent selectivity, rapid responding time (20 min), acceptable stability (retained more than 85% detection capability at 4 ◦C for 10 days) and good practicability (detection of ZEA in real human biofluids).

**Figure 6.** (**A**) Schematic representation of fabrication of the immunosensor and (**B**) detection of mycotoxins (adapted from Lu et al. 2016 [134], Copyright 2016 Elsevier Ltd. and reproduced with permission).

**Figure 7.** Schematic representation of the electrochemical immunosensor based on MoS2-Thi composites for the rapid detection of ZEA in biofluids (adapted from Jiang et al. 2019 [136], Copyright 2019 Elsevier B.V. and reproduced with permission).

#### **4. Detection of Algal Toxins**
