# Aptamer-Based Biosensors for the Analytical Determination of Bisphenol A in Foodstuffs
## 4 Detection Techniques Applied in the Analysis of Real Food Samples
### Optical Atpasensors  
In these devices, the binding of BPA to its specific aptamer is transduced in an optical signal consisting of a change of a specific light parameter, such as intensity, phase, color, or polarization. [PERSON] et al. [51] produced a surface-enhanced Raman-scattering (SERS) aptasensor functionalizing AuNPs with thiolate probe DNAs and then coating the resulting DNA-AuNPs with a silver shell to enhance the Raman signal. Finally, the 18 nt sequence (CACCTGACCACCGCG) of the probe DNAs on Au/ AgNPs was hybridized with Cy3-labeled aptamers. The closeness of Cy3 at the 5\({}^{\prime}\)-end of the aptamer to the silver shell enhanced the SERS signal. Cy3-labeled aptamers were released from the nanoprobe surface after their binding to BPA, causing a decrease in the SERS signal. Another SERS-based aptasensor was developed by [PERSON] et al. [56]. They used a gold nanoparticle-nanorod hetero-assembled material to obtain amplifications of the Raman scattering signals. In this strategy, gold nanoparticles were functionalized with the aptamer, whereas the nanorods were functionalized with the complementary DNA. The two types of nanomaterials were then assembled via the formation of dsDNA, resulting in a new material that produced a high Raman scattering signal. In the presence of BPA, the material disassembled, producing a decrease in the Raman signal. [PERSON] et al. [62] built up a sensing device composed of a Liquid Crystal (LC) coated with a BPA aptamer and then used polarized light microscopy to monitor the changes in the LC structure after the binding of BPA to the aptamer. [PERSON] et al. [75] used a detection strategy based on the inner filter effect (IFE) on Cadmium Tellured Quantum Dots (CdTe QDs) caused by non-conjugated AuNPs. BPA aptamers could bind to AuNPs without changing the IFE of AuNPs on CdTe QDs,  
Figure 8: A schematic representation of the capacitive aptasensor by [PERSON] et al. [65]. [Reprinted from Biosensors and Bioelectronics, 89, [PERSON]; [PERSON] A highly sensitive and specific capacitive aptasensor for rapid and label-free trace analysis of Bisphenol A (BPA) in canned foods 1059–1067, Copyright 2017, with permission from Elsevier]. whereas CdTe QDs fluorescence was restored after the binding of BPA to the aptamer coating on the AuNPs. Recently, a series of different, simple, and efficient colorimetric methods for BPA-quantification in foodstuffs has also been developed. [PERSON] et al. [76] exploited the ability of the cationic polymer poly(diallyldimethylammonium chloride) (PDDA) to induce the aggregation of AuNPs when added to a solution containing dispersed AuNPs. In the absence of BPA, the addition of an appropriate quantity of PDDA to a solution of dispersed AuNPs and BPA aptamers left the color solution unchanged, because of the electrostatic interactions between PDDA and the aptamer. In the presence of BPA, the aptamer was sequestered, and the released PDDA caused AuNP aggregation. The switch between free and aggregate forms of AuNPs caused a BPA-concentration-dependent color change in the solution. Similar colorimetric methods were developed by [PERSON] et al. [45] and by [PERSON] et al. [67]. They conjugated differently truncated BPA aptamers to AuNPs, which underwent electrolyte-induced aggregation in the presence of very low levels of BPA. [PERSON] et al. [77] described a direct aptamer Fluorescence Anisotropy method for BPA detection by using a single tetramethylrhodamine (TMR)-labeled 35 nt aptamer probe (5'-CCGCTGGTGGTGGCG-T-(TMR)AGGGCGGCG-3'). The binding of BPA to the aptamer generated a BPA-concentration-dependent decrease in the fluorescence signal. More recently, [PERSON] et al. [78] developed a new fluorescence-based aptasensor combining a dual-labeled DNA substrate and a DNA circuit for signal amplification. A catalytic Mg\({}^{2+}\)-dependent DNAzyme was formed through a synergistical DNA hybridization strategy. The process started with the binding of BPA to the aptamer that produced the release of a DNA segment from a DNA/aptamer double helix.