# Recent Development of Aptamer Sensors for the Quantification of Aflatoxin B1
## 2 Colorimetric, Electrochemical, and Fluorescent Aptamer Sensors to Monitor AFB1  
### Colorimetric Aptamer Sensor for AFB1  
[PERSON] and coworkers developed a low-cost, as well as sensitive, probe for the colorimetric determination of microccystin-LR (MC-LR), funnisin B1, aflatoxin B1, as well asochratoxin A via DNA-induced graphene oxide (GO) self-assembly and magnetic separation [18]. As shown in Figure 2, the probe consisted of two platforms, where one was comprised of four allochroic dyes, which included methyl violet (MV), thymolphthalein (TP), malachite green carbinol base (MGCB), and phenolphthalein (PP) for four different analytes, as well as assistant DNA probe 1 adsorbed on the GO. The other platform was comprised of assistant DNA probe 2 adsorbed on the surface of the GO, which was modified with Fe\({}_{3}\)O\({}_{4}\). Assistant DNA probes 1 and 2 were partially hybridized to the aptamer, upon mixing the aptamer with the two platforms in aqueous media. Subsequently, the two GO assemblies formed immediately in the presence of the aptamer. The assembly was easy to separate, by magnet, in the absence of the target. After the recognition of various targets, the reaction system was subjected to magnetic separation. Subsequently, pH value was employed to control the order of release of the dyes from the GO. Upon addition of acidic water into the supernatant, the MGCB, as well as the MV, molecules, which were adsorbed on the surface of the GO, were released as the allochroic dyes had changed to hydrophilic. A mixture of green (MGCB) and purple (MV) was observed in the supernatant. On account of good linearity between the analyte and the dye, the color signals attributed to MGCB and MV were related to the contents of AFB1 and MC-LR, respectively. The PP, as well as TP, which were adsorbed on the GO, were released with the addition of basic water into the precipitate. A mixture of pink and blue was observed in the color of the precipitate. This probe can be used to rapidly detect target through simple magnetic separation, as well as the adjustment of the pH of the aqueous media. By using a similar strategy, different targets could be detected simultaneously, by using various respective dyes and different ssDNA. [PERSON] et al. designed an ultra-sensitive colorimetric probe which was capable of detecting aflatoxin B1. The probe consisted of two split DNAzyme halves, as well as an aptamer (see Figure 3) [19]. When AFB1 was absent, the split probes had a capability for peroxidase-mimicking activity, accompanied by a change of color in the catalytic reaction. AFB1 was specifically recognized by the aptamer, enabling the dissociation of the aptamer-DNAzyme complex. Therefore, the catalytic ability of DNAzyme was reduced, owing to the splitting of the DNAzyme halves. The color of the solution containing ABTS changed from colorless to green. However, the AFB1 aptamer was capable of binding to AFB1 when AFB1 was present, which led to the splitting of a hemin, which then could not bind to the DNAzymes. Therefore, the color of the solution was colorless and did not change. As a consequence, the colorimetric signal decreased as the concentration increased from \(1.0\times 10^{-7}\) to \(1.0\times 10^{-2}\) g\(\cdot\)L\({}^{-1}\)  
Figure 2: Schematic representation of the detection mechanism of the probe for monitoring microccystin-LR, funninis B1, aflat B1, and ochratoxin A using a colorimetric assay. Reproduced from [18]. Copyright 2018, American Chemical Society. The oxide ABTS decreased when AFB1 was present. The detection limit of the probe was as low as \(1.0\times 10^{-7}\) g-L\({}^{-1}\). The novel probe exhibited good performance for distinguishing AFB1 from other mycotoxins and could be used for the quantification of AFB1 in ground corn samples. [PERSON] et al. developed an enzyme-free sensor for the determination of aflatoxin B1 by utilizing a catalytic DNA circuit to amplify the colorimetric signal (Figure 4) [20], where the colorimetric signal can be visualized without any other equipment.