# Aptamer-Based Biosensors for the Analytical Determination of Bisphenol A in Foodstuffs
## 4 Detection Techniques Applied in the Analysis of Real Food Samples
### Electrochemilunnescence-Based Atrasensor  
Electrochemiluminescence (ECL) is a light emission process that occurs in a redox reaction of electrogenerated reactants. This detection technique shows several advantages over other methods, including a high signal-to-noise ratio and measurements with minimal or no background signal. Consequently, different ECL-sensing approaches have been recently employed to develop new, simple, low cost, and portable biosensors, including aptasensors [71; 72]. In this frame, [PERSON] et al. [54] developed a chemiluminescent nanomaterial NaYF\({}_{4}\):Yb,Er/Mn UCNPs and used it in an ECL-based aptasensors. In this strategy a thiolated BPA aptamer was covalently immobilized on a gold nanoparticle (AuNPs)-modified electrode and paired with complementary DNA labeled with NaYF4:Yb,Er/Mn UCNPs. The formation of the aptamer/cDNA-NaYF4:Yb,Er/Mn UCNPs double helix on the AuNPs/apt electrode generated a strong and stable chemiluminescent signal. The binding of BPA to the coated aptamer allowed for the release of conjugated NaYF4:Yb,Er/Mn UCNPs-DNA, and consequently the electrochemiluminescent signal decreased.  
[PERSON] et al. [59] used [Ru(phen)\({}_{3}\)]\({}^{2+}\) as the indicator of the ECL signal. The aptamer coated on a gold electrode was hybridized with the complementary DNA to form dsDNA.  
Figure 6: The sensing mechanism developed by [PERSON] et al. [55]. The GCE/AuNPs/Apt WE reacted with complementary DNA conjugated with biotin to form a dsDNA on the surface of WE. AuNP/HRP/Avidin was bound through biotin/avidin recognition. Horseradish-peroxidase (HRP) catalyses the hydroquinone HQ) oxidation (BQ), in presence of H\({}_{2}\)O\({}_{2}\). The oxalate hydroquinone comes back to HQ recovering electrons from the electrode, thus causing the signal amplification. In presence of BPA, Aptamer-BPA complex is formed, and the complementary DNA is released, thus decreasing the catalytic peak current. [Reprinted from J. Electroanal. Chem., 781, [PERSON]; [PERSON]; [PERSON] A dual-signaling strategy for ultrasensitive detection of bisphenol A by aptamer-based electrochemical biosensor, 265–271, Copyright 2016, with permission from Elsevier].  
[Ru(phen)\({}_{3}\)]\({}^{2+}\) intercalated into the grooves of dsDNA; therefore, high ECL intensity was detected from the electrode surface. The binding of BPA to the aptamer disrupted the dsDNA, and [Ru(phen)\({}_{3}\)]\({}^{2+}\) was released from the electrode surface, leading to a decrease in the ECL signal.  
[PERSON] et al. [73] used hybridization chain reaction (HCR) technology to realize the binding site of the electrochemuluminescent [Ru(phen)\({}_{3}\)]\({}^{2+}\) on the indium-tin-oxide (ITO) working electrode (Figure 7). In particular, cDNA was coated on the surface of the electrode and then hybridized with BPA aptamer to form dsDNA. In the presence of BPA, the aptamer was released, and the free cDNA triggered the formation of long dsDNA by HCR. Therefore, [Ru(phen)\({}_{3}\)]\({}^{2+}\) could intercalate the long dsDNA, causing an enhancement of the electrochemical signal.  
Very low LOD values have been obtained in the last two cases (Table 2). However, it should be carefully considered that these chemuluminescent materials are associated with the use of heavy and toxic metals. In their use on a broader scale, they have the disadvantage of producing polluting waste.