# Aptamer-Based Biosensors for the Analytical Determination of Bisphenol A in Foodstuffs
## 4 Detection Techniques Applied in the Analysis of Real Food Samples
### Electrochemical Aptasensors
The electron flow was hampered by both physical hindrance to the passage of the redox probe and by a repulsion charge with the negatively charged backbone of the aptamer-BPA complexes. An increment of BPA concentration increased the formation of closed aptamer gates and subsequently reduced the number of open tunnels, decreasing the electrochemical signal (Figure 5b, panels BC). All these strategies are simple and cost-effective. However, the use of an external probe presents the same limitations, especially for repetitive detections, due to possible electrodeposition of contaminants on the electrode. Nonetheless, the necessity to add reagents during the analysis could further increase the variability of results. A singular and simple DNA- and enzyme-based BPA electrochemical aptasensor was developed by [PERSON] et al. [68]. A commercial screen-printed gold electrode (SPGE) was first functionalized with both the BPA aptamer and its complementary strand, with the latter extended at the 3\({}^{\prime}\)-end with a polyA tract. The use of the terminal deoxynucleotidyl transferase (TdT) in the presence of dT triphosphate led to the addition of T residues at the 3\({}^{\prime}\)-end of the aptamer coated on the electrode. Then, polyT at 3\({}^{\prime}\)-end of the aptamer and the polyA at the 3\({}^{\prime}\)-end of the complementary sequence hybridized, forming a bridge on the surface of the electrode. This negatively charged physical barrier hampered the access of redox mediators to the electrode surface, producing a weak current signal. On the other hand, in the presence of BPA, the aptamers were sequestered. The flow of events which led to the formation of bridged duplexes was disrupted, and an increase in the current signal occurred. Consequently, with respect to the cases described above, the measured electrochemical parameter increases after the binding of BPA to the immobilized aptamer. However, aside from the external redox probe, other reagents (TdT and dT) should be added in the system that may further contribute to the variability of the results in repetitive detections. Very sensitive electrochemical aptasensors, which also include the use of external redox probes, have been recently built up by [PERSON] et al. [69] and by [PERSON] et al. [60]. In the first case, an increase in sensitivity has been obtained by immobilizing a highly conductive polipyrrole (PPY), which was immobilized on a GCE. Firstly, the surface of the GCE was covered with AuNPs. Then, a BPA aptamer was immobilized on the electrode surface in the presence of BPA. Finally, the pyrrole was electropolymerized on the surface of the resulting electrode. After the removal of BPA, the functionalized GCE was used for analytical determinations. The latter case relied upon the use of aptamer-modified magnetic Fe\({}_{3}\)O\({}_{4}\)/Au nanoparticles to obtain signal amplification and a catalytic effect on the reduction of Ag\({}^{+}\) to Ag\({}^{0}\). A functionalized aptamer-gold electrode was used to capture the BPA in solution. The aptamer-modified magnetic Fe\({}_{3}\)O\({}_{4}\)/Au nanoparticles solution was then dropped onto the electrode, and the BPA molecules on the gold-electrode could attach to many aptamers on Fe\({}_{3}\)O\({}_{4}\)/Au/Apt nanoparticles. As a result of the increment of its concentration in the tested solution, the BPA amount captured on the aptamer-modified electrode rose, increasing the sequestered aptamer-modified Fe\({}_{3}\)O\({}_{4}\)/Au nanoparticles. The Fe\({}_{3}\)O\({}_{4}\)/Au nanoparticles were detached from the electrode and transferred to a solution containing Ag\({}^{+}\) ions. As a result, they catalyzed the reduction of Ag\({}^{+}\) and then captured Ag\({}^{0}\). Finally, the electrode was used in a three-electrode cell. A positive-going differential pulse potential scan was applied to oxidate the Ag\({}^{0}\) to the AgCl, and the magnitude of the stripping anodic signal of the Ag\({}^{0}\) was related to the concentration of the BPA.