Recent Progress and Development of G-Quadruplex-Based Luminescent Assays for Ochratoxin A Detection

Detection of zearalenone in miscellaneous beans by functionalized SERS sensor based on sea urchin aptamer

In this work, a sea urchin gold nanoparticles-zearalenone aptamer- tetramethylrhodamine sensor was constructed. Sea urchin gold nanoparticles, prepared using the seed-mediated growth method, were used as Raman substrates. Nucleic acid aptamers were mainly used as specific recognition molecules. Zearalenone detection in miscellaneous beans was accomplished using the principle of conformational change in aptamer. In addition, we evaluated the linear range, sensitivity, and selectivity of our sensor. We observed that at the displacement of 814 cm-1, for Zearalenone concentrations of 0.01-60 ng/mL, the Raman signal intensity linearly correlated with the zearalenone concentration, with a limit of detection of 0.01 ng/mL, and recoveries of 91.7% to 108.3%. The optimum detection time was 30 min. Thus, our sensor exhibited great potential in zearalenone detection in food products.

A smartphone-assisted fluorescent sensing platform for ochratoxin A using Mn-doped CsPbBr3 perovskite quantum dots embedded in the mesoporous silica as a ratiometric probe

Food sources are susceptible to contamination with ochratoxin A (OTA), which is a serious threat to human health. Thus, the construction of novel, simple sensing platforms for OTA monitoring is of utmost need. Manganese-doped lead halide perovskite quantum dots encapsulated with mesoporous SiO2 (Mn-CsPbBr3 QDs@SiO2) were prepared here and used as a ratiometric fluorescent probe for OTA. Mn-CsPbBr3 QDs, synthesized at room temperature, exhibit dual emission with maximum wavelengths of 440 and 570 nm and, when embedded in the SiO2 layer, produce a stable and robust photoluminescence signal. By adding OTA to the probe, emission at 440 nm increases while emission at 570 nm decreases, so a ratiometric response is obtained. Experimental variables affecting the probe signal were studied and optimized and the mechanism of sensing was discussed. This ratiometric sensor demonstrated excellent selectivity and low detection limit (4.1 ng/ml) as well as a wide linear range from 5.0 to 250 ng/ml for OTA. A simple portable smartphone-based device was also constructed and applied for the fluorescence assay. With different OTA concentrations, the multicolor transition from pink to blue under a UV lamp led to simple visual and smartphone-assisted sensing of OTA by using a color analyzing application. Satisfactory recoveries in black tea, coffee, moldy fig and flour samples confirmed the reliability of the assay. The accuracy of the probe was proved by comparison of the results with high-performance liquid chromatography (HPLC).

The prevalence and concentration of ochratoxin A in meat and edible offal: A global systematic review and meta-analysis

The prevalence of ochratoxin A (OTA) in meat, edible offal, and meat products (MOP) was assessed through systematic review and meta-analysis. Four electronic databases were used to gather data from 1975 to September 15, 2022. Seventy-five articles comprising 8585 samples were identified and analyzed. The studies included in the analysis were conducted at a global level, with a predominant focus on Europe [72% (54/75)], Asia [13.33% (10/75)], Africa [13.33% (10/75)], and North America [1.33% (1/75)]. The overall prevalence of OTA in MOP was 39%. The highest and lowest prevalence percentages were recorded in Iraq (77%) and the USA (3%), respectively. Concerning food type, OTA prevalence was highest in the poultry gizzard (66%) and lowest in the cow liver (2%). The overall concentration of OTA in the MOP was 1.789 μg/kg. Poultry kidneys had the highest concentration of OTA (0.880-22.984 μg/kg), while pork had the lowest concentration (0.127-0.824 μg/kg). Conspicuous amounts of OTA contamination have been reported in fermented sausages. The lowest OTA concentration was found in Belgium (0.220 μg/kg) and the highest in Denmark (60.527μg/kg). These results can help food authorities minimize and control OTA in the MOP.

Advances in immunoassay-based strategies for mycotoxin detection in food: From single-mode immunosensors to dual-mode immunosensors

Mycotoxin contamination in foods and other goods has become a broad issue owing to serious toxicity, tremendous threat to public safety, and terrible loss of resources. Herein, it is necessary to develop simple, sensitive, inexpensive, and rapid platforms for the detection of mycotoxins. Currently, the limitation of instrumental and chemical methods cannot be massively applied in practice. Immunoassays are considered one of the best candidates for toxin detection due to their simplicity, rapidness, and cost-effectiveness. Especially, the field of dual-mode immunosensors and corresponding assays is rapidly developing as an advanced and intersected technology. So, this review summarized the types and detection principles of single-mode immunosensors including optical and electrical immunosensors in recent years, then focused on developing dual-mode immunosensors including integrated immunosensors and combined immunosensors to detect mycotoxins, as well as the combination of dual-mode immunosensors with a portable device for point-of-care test. The remaining challenges were discussed with the aim of stimulating future development of dual-mode immunosensors to accelerate the transformation of scientific laboratory technologies into easy-to-operate and rapid detection platforms.

Engineering constructed of high selectivity dexamethasone aptamer based on truncation and mutation technology

Various biosensors based on aptamers are currently the most popular rapid detection approaches, but the performance of these sensors is closely related to the affinity of aptamers. In this work, a strategy for constructed high-affinity aptamer was proposed. By truncating the bases flanking the 59 nt dexamethasones (DEX) original aptamer sequence to improve the sensitivity of the aptamer to DEX, and then base mutation was introduced to further improve the sensitivity and selectivity of aptamers. Finally, the 33 nt aptamer Apt-M13 with G-quadruplex structures was obtained. The dissociation constant (K_d) was determined to be 200 nM by Graphene oxide (GO)-based fluorometry. As-prepared Apt-M13 was used for a label-free colorimetric aptamer sensor based on gold nanoparticles, the LOD was 3.2-fold lower than the original aptamer described in previous works. The anti-interference ability of DEX analogs is also further improved. It indicates that truncation technology effectively improves the specificity of the aptamer to DEX in this work, and the introduction of mutation further improves the affinity and selectivity of the aptamer to DEX. Therefore, the proposed aptamer optimization method is also expected to become a general strategy for various aptamer sequences.

Structural Insights into the Mechanism of High-Affinity Binding of Ochratoxin A by a DNA Aptamer

A 36-mer guanine (G)-rich DNA aptamer (OBA36) is able to distinguish one atomic difference between ochratoxin analogues A (OTA) and B (OTB), showing prominent recognition specificity and affinity among hundreds of aptamers for small molecules. Why OBA36 has >100-fold higher binding affinity to OTA than OTB remains a long-standing question due to the lack of high-resolution structure. Here we report the solution NMR structure of the aptamer-OTA complex. It was found that OTA binding induces the aptamer to fold into a well-defined unique duplex-quadruplex structural scaffold stabilized by Mg²⁺ and Na⁺ ions. OTA does not directly interact with the G-quadruplex, but specifically binds at the junction between the double helix and G-quadruplex through π-π stacking, halogen bonding (X-bond), and hydrophobic interaction. OTB has the same binding site as OTA but lacks the X-bond. The strong X-bond formed between the chlorine atom of OTA and the aromatic ring of C5 is the key to discriminating the strong binding toward OTA. The present research contributes to a deeper insight of aptamer molecular recognition, reveals structural basis of the high-affinity binding of aptamers, and provides a foundation for further aptamer engineering and applications.

Label free detection of auramine O by G-quadruplex-based fluorescent turn-on strategy

Auramine o (AO) is a synthetic dye used in paper and textile industries. Although it has been an unauthorized food additive in many countries due to its toxic and carcinogenic possibility, its illegal uses have been detected in certain food products such as pasta, semolina and spices and also in pharmaceuticals. The presence of AO in food products should be monitored, therefore, to minimize the negative health effects on consumers. In this study, a simple, highly sensitive and selective label free detection method was investigated for AO by G-quadruplex-based fluorescent turn-on strategy. The optimum fluorescent detection assay was achieved with a specific G-quadruplex DNA sequence, c-myc, at 400 nM in Tris-HCl buffer at pH 7.4. The linearity of fluorescence intensity depending on AO concentration ranged from 0 to 0.07 µM and LOD and LOQ were 3 nM and 10 nM, respectively. The G-quadruplex-based detection assay was highly specific for AO as compared to other two synthetic food colorings and successfully applied to determine AO in pasta, bulgur and curry powder with recoveries in the range from 70.33% to 106.49%. This G-quadruplex-based label free detection assay has a significant potential to be used in the detection of AO in food products.

Simultaneous determination of zearalenone and ochratoxin A based on microscale thermophoresis assay with a bifunctional aptamer

Nowadays, contamination of various mycotoxins in crops and their products exposes increasing risks to human health. Efficient determination methods are urgently needed. Herein, a bifunctional aptamer and a simple aptasensor based on microscale thermophoresis assay (MST) were constructed for the first time for simultaneous determination of two mycotoxins, i.e. zearalenone (ZEN) and ochratoxin A (OTA). The bifunctional aptamer was engineered by splicing a ZEN aptamer and an OTA aptamer with a linker according to the structure analysis of aptamers. The binding mechanism of the bifunctional aptamer to ZEN and OTA were revealed basing on the molecular docking studies. The MST assay proved that the bifunctional aptamer showed high affinity and specificity towards ZEN and OTA. Furthermore, a bifunctional aptamer-based MST-aptasensor was developed for simultaneous detection of ZEN and OTA in corn oil sample. The MST-aptasensor provided a limit of detection (LOD) of 0.12 nM, with satisfactory recoveries of 93.31-104.19% and excellent selectivity, indicating that the bifunctional aptamer and MST-aptasensor had great potential in practical applications.