# Recent Development of Aptamer Sensors for the Quantification of Aflatoxin B1
## 2 Colorimetric, Electrochemical, and Fluorescent Aptamer Sensors to Monitor AFB1
### Electrochemical Aptamer Sensor for AFB1  
Mo and colleagues have prepared a probe for the detection of aflatoxin B1, which used graphene oxide and an aflatoxin B1 aptamer-modified multi-anodized anodized aluminum (Figure 5) [22]. First, the aptamer was attached onto porous anodized aluminum nanopores through covalent attachment. Subsequently, GO was supported on the surface of the porous anodized aluminum through the \(\pi\)-\(\pi\)  
Figure 4: Illustration of the strategy for detecting AFB1 based on a catalytic DNA circuit and gold nanoparticles. Reproduced from [20]. Copyright 2016, Royal Society of Chemistry. stacking interaction between the aptamer and GO. By introducing negatively charged graphene oxide, as well as the aptamers, the nanochannels became more negatively charged. Meanwhile, the increase of negative charge of the nanochannels resulted in an increase of steric hindrance. These two effects led to a decrease in Fe(CN)\({}_{6}\)[3]- through the nanochannel. When aflatoxin B1 was present, the aptamer formed a complex with aflatoxin B1, and the graphene oxide was then pulled away from the surface of the nanochannel. A decrease of the charge density and steric hindrance were, thus, observed leading to an increase in Fe(CN)\({}_{6}\)[3]- in the nanochannel. Consequently, an increase in the current signal was observed. As shown in Figure 6B, the increased current signal had a good linear relationship with the concentration of aflatoxin B1 in the range of \(1.0\times 10^{-6}\) -\(2.0\times 10^{-5}\) g\(\cdot\)L\({}^{-1}\). The probe had an excellent detection limit for aflatoxin B1, as low as \(1.3\times 10^{-7}\) g\(\cdot\)L\({}^{-1}\). It can be seen from Figure 6C that the probe had good selectivity for aflatoxin B1. Therefore, the probe offers potential possibilities for simple, efficient, and highly sensitive detection of mycotoxins. [PERSON] et al. developed a probe for the quantitation of AFB1, which employed aptamers as the recognition element, where telomerase and Exo III were used as signal enhancement elements [23]. Single-strand DNA, on the surface of gold nanoparticles, extended in the presence of telomerase. As a consequence, telomerase amplification expanded the signal response range of the electrochemical aptamer probes, which were used in the signal-off mode. The aptamer formed double-strand DNA upon the presence of AFB1. Subsequently, the DNA duplexes were cleaved by Exo III, through recycling digestion, and released, resulting in an enhanced signal. With two rounds of signal amplification, the electrochemical aptamer probe could be successfully applied to detect trace amounts of AFB1. The detection limit of this probe was as low as \(6.0\times 10^{-15}\) g\(\cdot\)L\({}^{-1}\). Furthermore, the probe had excellent selectivity, due to its excellent binding ability with AFB1. The design-based dual-cycle amplification strategy greatly improved not only the detection range, but also the sensitivity of the probe. Compared to traditional methods, the probe exhibited a good sensitivity to AFB1. On the basis of dual-signal amplification strategies, the ultrasensitive detection of AFB, as well as other mycotoxins, can be achieved by changing the sequence of the aptamer. Figure 5: (**A**) Schematic representation of the strategy for the detection of AFB1. (**B**) Linear plot of current change versus the concentration of AFB1. (**C**) The current change of other toxins and AFB1. Reproduced from [22]. Copyright 2018, Elsevier. An electrochemical aptasensor combined with horseradish peroxidase for sensing AFB1 was described by [PERSON] et al. [24]. As shown in Figure 7, the aptasensor was comprised of two parts: A DNA tetrahedron, which was immobilized to the three-dimensional mesoporous structure of MoS\({}_{2}\)-AuNPs, as a recognition moiety; and horseradish peroxidase-modified Fe\({}_{3}\)O\({}_{4}\), which was used as a signal amplification unit.