# Aptamer-Based Biosensors for the Analytical Determination of Bisphenol A in Foodstuffs
Biosensors are mainly composed of a recognition element that binds to the target analyte and of a transducer that transforms the biochemical interaction into a detectable and measurable signal. Among all biosensor types, aptamer-based biosensors, named aptasensors, are generally preferred because of their multiple advantages. The use of an aptamer as a recognition element is advantageous for several reasons: aptamers can be easily synthesized and modified to be adapted to a wide range of chemical derivativizations, and they can be re-used with consequent cost reduction. Moreover, aptasensors show high sensitivity and good reproducibility and are easy to use [39]. In our search for scientific works on BPA aptasensors, we noticed that few reviews discussed the issue of BPA aptasensors [39; 40], but none of them focused entirely on the application of this technology to the quantitative analysis of BPA in food. Therefore, the present review aims to overcome this lack with the goal of providing a useful tool to understand current state-of-the-art research and to approach this interesting research topic. Since the first anti-BPA aptamers were selected by [PERSON] et al. [43], many efforts to develop sensitive BPA aptasensors for their use in different matrices have been made. With its high affinity and selectivity, BPA-Apt-2 (Table 1) is the main adopted sequence in the building up of aptasensors, and despite its 63 nt length, no truncated sequence has been developed until 2017 from the initial aptamer. Starting from the predicted secondary structure of the aptamer, [PERSON] et al. [44] identified that BPA-Apt-3 (Figure 3 and Table 1) was able to bind to BPA with increased affinity and selectivity compared to BPA-Apt-2. In 2020, a similar truncation protocol was adopted by [PERSON] et al. [45], succeeding in obtaining two differently truncated sequences (Figure 4). Figure 2: Scheme of the general strategy for aptasensor development. Figure 4: The flow chart from BPA-Apt-2 to the two selected 38 and 12 nts truncated aptamers [45]. [Reprinted from Food Chem, 317, [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] High affinity truncated aptamers for ultra-sensitive colorimetric detection of bisphenol A with label-free aptasensor 126459–12467. Copyright 2020, with permission from Elsevier]. Figure 3: The 63 nts BPA-Apta-2 and the corresponding flow chart showing the process used to select the 24 nts truncated aptamer. Ten T residues at 5\({}^{\prime}\)-terminus of the initial sequence and of the corresponding truncated ones have been added. [Reprinted with permission from [PERSON]; [PERSON]; [PERSON]; [PERSON] Highly Sensitive Detection of Bisphenol A by Nano-Aptamer Assay with Truncated Aptamer. ACS Appl. Mater. Interfaces 2017, 9, 14889–14898. Copyright 2017, ACS American Chemical Society]. The comparison between the three-dimensional structure predicted for BPA-Apt-3 and that of BPA-Apt-5 shows the presence of the same stem, formed by the 5\({}^{\prime}\)-GGA and 3\({}^{\prime}\)-TCC, and a differently long loop containing a common AGCGT sequence in both the folded aptamers. These two works confirm that SELEX alone is unsatisfactory for obtaining an efficient and cost-effective aptamer. Therefore, further studies are necessary to identify the exact binding mode of BPA to its aptamers and/or to the shortened aptamer derivatives as well as the secondary structures adopted in solution by the aptamer(s) alone and in the BPA-aptamer complex(es). Indeed, only a few studies reported the molecular modeling of aptamers and their BPA complexes [44; 45], whereas in most cases, the stem-loop aptamer structures have been derived based on predictive DNA programs, and only in few cases they were confirmed by circular dichroism analysis. Notably, some authors mentioned the ability of BPA to induce the folding of BPA-Apt-2 (Table 1) into G-quadruplex(s) [47; 48; 49; 50], although this hypothesis requires further experimental evidence.