# 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  
An electrochemical sensor measures an electrical parameter (potential, current, impedance, etc.) whose value can be related to the target analyte concentration in solution. Generally, it consists of a three-electrode electrochemical cell in which the working electrode (WE) constitutes the key element for analyte detection (Figure 5a). WE can be \"directly\" or \"indirectly\" functionalized with an aptamer that is able to selectively recognize a specific analyte, resulting in an electrochemical aptasensor. Direct functionalization involves the aptamer being covalently linked to a specific material that covers the WE, whereas in the indirect WE functionalization, the aptamer hybridizes with its complementary sequence, which, in turn, is somehow linked to the WE. Regardless, the formation of an aptamer-analyte complex lacks any detectable electrochemical signal, and, in Faradaic electrochemical aptasensors, a redox probe (methylene blue, ferrocene, horse radish peroxidase, etc.) needs to be employed. In accordance with the adopted sensing strategy, the redox probe can be conjugated to the aptamer or included in a specific step of the WE functionalization process. Alternatively, the redox probe can be externally used, adding it into the test solution ([Fe(CN)6]\({}^{3-4/4}\), Ag\({}^{+/0}\), etc.). Different working electrodes for BPA recognition were prepared and used in a conventional three electrode cell in which a Ag/AgCl (in a saturated KCl solution) and a Pt wire are generally used as reference and counter electrodes, respectively. [PERSON] et al. [47] coated a glassy carbon electrode (GCE) with graphene oxide (GO) and gold nano particles (AuNPs). (Figure 5b). [PERSON] et al. [53] built an electrochemical aptasensor by functionalizing a gold electrode with multiwall carbon nanotubes/gold nanoparticles (f-MWCNTs/AuNPs) to realize a nanocomposite film on which the aptamer was immobilized. [PERSON] et al. [46] built a very sensitive electrochemical aptasensor by coating a boron-doped diamond (BDD) layer with AuNPs and by modifying the latter with  
Figure 5: (**a**). Schematic representation of a three-electrode cell. The difference in potential is measured between the working electrode (WE, orange bar) and the reference electrode (RE, light green). The current is equal in absolute value but of opposite in sign between the WE and the counter electrode (CE, green). Compared to a two-electrode cell, in a three-electrode cell the current and the potential are decoupled, led to a more accurate measurement of these cell parameters. (**b**). The sensing mechanism using the [PERSON] et al. WE. In absence of BPA, the aptamers are unfolded and the exchange of electrons between the WE and the ferrycianide probe is allowed. After the formation of BPA-aptamer complexes the exchange of the electrons with the probe was unpaired, resulting in a decrease of the peak current. [Reprinted from J. Food Chem., 162, [PERSON]; [PERSON]; [PERSON]; [PERSON] An electrochemical aptasensor based on gold nanoparticles dotted graphene modified glassy carbon electrode for label-free detection of bisphenol A in milk samples. 34-40. Copyright 2014, with permission from Elsevier]. aptamers. [PERSON] et al. [50] developed a GCE coated with silver nanoparticles/carbon nanofiber composite (AgCNFs) and functionalized it with an aptamer via a disulfide bond between the aptamer and silver nanoparticles. In all these cases the anti-BPA aptamers covalently linked on the WE produced tunnels on the surface of the electrode that acted as gates for the redox probe ([Fe(CN)\({}_{6}\)]\({}^{3}\)-/+). In the absence of BPA, the gates were opened, and the passage of electrons to the electrode surface was allowed. Upon the addition of BPA, the conformation of aptamer changed, and the gates became closed.