A novel aptasensor based on DNA hydrogel for sensitive visual detection of ochratoxin A

Hydrogel-integrated sensors for food safety and quality monitoring: Fabrication strategies and emerging applications

Food safety is a global issue in public hygiene. The accurate, sensitive, and on-site detection of various food contaminants performs significant implications. However, traditional methods suffer from the time-consuming and professional operation, restricting their on-site application. Hydrogels with the merits of highly porous structure, high biocompatibility, good shape-adaptability, and stimuli-responsiveness offer a promising biomaterial to design sensors for ensuring food safety. This review describes the emerging applications of hydrogel-based sensors in food safety inspection in recent years. In particular, this study elaborates on their fabrication strategies and unique sensing mechanisms depending on whether the hydrogel is stimuli-responsive or not. Stimuli-responsive hydrogels can be integrated with various functional ligands for sensitive and convenient detection via signal amplification and transduction; while non-stimuli-responsive hydrogels are mainly used as solid-state encapsulating carriers for signal probe, nanomaterial, or cell and as conductive media. In addition, their existing challenges, future perspectives, and application prospects are discussed. These practices greatly enrich the application scenarios and improve the detection performance of hydrogel-based sensors in food safety detection.

A G-quadruplex-assisted target-responsive dual-mode aptasensor based on copper nanoclusters synthesized in situ in a DNA hydrogel for ultrasensitive detection of ochratoxin A

Developing ultrasensitive sensing platforms for trace ochratoxin A (OTA) in food safety is still challenging. Herein, we presented a novel dual-mode sensing strategy for fluorescence and colorimetric detection of OTA by combining the target-responsive hemin-encapsulated and copper nanoclusters (CuNCs) functionalized DNA hydrogel. Through simple assembly and in situ synthesis methods, fluorescence CuNCs are synthesized and modified on the 3D hydrophilic network structure of DNA cross-linked. OTA specifically recognized by Apt-linker can control the collapse of hydrogel, resulting in the fluorescence quenching of CuNCs and release of coated hemin. Interestingly, OTA could trigger Apt-linker conformational changes to form G-quadruplex structures, allowing the released hemin to form G-quadruplex/hemin DNAzyme via self-assembly. Fluorescence signal amplification could be achieved through further fluorescence quenching of CuNCs caused by DNAzyme-catalyzed hydrogen peroxide (H2O2) because of the peroxidase activity of DNAzyme. Simultaneously, DNAzyme could catalyze the H2O2-mediated oxidation of TMB to provide colorimetric signal. Thereafter, the DNA-CuNCs hydrogel exhibited low detection limits of 3.49 pg/mL in fluorescence mode and 0.25 ng/mL in colorimetric modality. Real sample analyses of foodstuffs showed satisfactory results, providing prospective potential for monitoring mycotoxin contaminant.

Strategy of functional nucleic acids-mediated isothermal amplification for detection of foodborne microbial contaminants: A review

Foodborne microbial contamination (FMC) is the leading cause of food poisoning and foodborne illness. The foodborne microbial detection methods based on isothermal amplification have high sensitivity and short detection time, and functional nucleic acids (FNAs) could extend the detectable object of isothermal amplification to mycotoxins. Therefore, the strategy of FNAs-mediated isothermal amplification has been emergingly applied in biosensors for foodborne microbial contaminants detection, making biosensors more sensitive with lower cost and less dependent on nanomaterials for signal output. Here, the mechanism of six isothermal amplification technologies and their application in detecting FMC is firstly introduced. Then the strategy of FNAs-mediated isothermal amplification is systematically discussed from perspectives of FNAs' versatility including recognition elements (Aptamer, DNAzyme), programming tools (DNA tweezer, DNA walker and CRISPR-Cas) and signal units (G-quadruplex, FNAs-based nanomaterials). Finally, challenges and prospects are presented in terms of addressing the issue of nonspecific amplification reaction, developing better FNAs-based sensing elements and eliminating food matrix effects.

A multifunctional polyacrylamide/chitosan hydrogel for dyes adsorption and metal ions detection in water

Removing noxious dyes and detecting excessive metal ions in water are both effective means to prevent damage from contaminants and ensure water safety. The emphasis problems were addressed by preparation a polyacrylamide chitosan (PAAM/CS) hydrogel. Polyacrylamide (PAAM) provides overall mechanical strength to carry loads and facilitate circulation, chitosan (CS) provides adsorption positions with high adsorption capacity. Which made that PAMM/CS hydrogel efficiently performed sorption of xylenol orange (XO). As the functional dye, XO binds to PAAM/CS and confers colorimetric properties on PAAM/CS hydrogels. XO sorbed hydrogel realized fluorescence dual-signal detection of Fe3+ and Al3+ in water. The significant swelling and adsorption potency of the hydrogel, combined with the dual-signal detection capability of XO sorbed hydrogel, make this hydrogel a versatile material for environmental applications.

Fluorescence Resonance Energy Transfer Aptasensor of Ochratoxin A Constructed Based on Gold Nanorods and DNA Tetrahedrons

Ochratoxin A (OTA) contamination of corn has received significant attention due to the wide distribution and high toxicity of OTA. The maximum residue limit standard of OTA in corn has been established by the Chinese Government and other unions. Nanoparticle-based fluorescence resonance energy transfer (FRET) assays are promising methods for the sensitive and fast detection of OTA. However, satisfactory detection sensitivity is commonly achieved with complicated signal amplification processes or specific nanoparticle morphologies, which means that these assays are not conducive to fast detection. This study proposes a simple and novel strategy to improve the sensitivity of FRET aptasensors. In this strategy, a DNA tetrahedron was first used in gold nanorod-based FRET aptasensors. DNA tetrahedron-modified gold nanorods are used as fluorescent acceptors, and Cy5-modified complementary sequences of the OTA aptamer are used as fluorescent donors. The aptamers of OTA are embedded in the DNA tetrahedrons, and FRET occurs when the aptamers hybridize with the Cy5-modified complementary sequences. The aptamer-integrated DNA tetrahedron modified on the surface of gold nanorods acts as an anchor, thus avoiding the crowding and entanglement of aptamers. Due to the competitive combination between the OTA aptamers and complementary sequences, the greater the amount of OTA, the less the amount of Cy5-modified complementary sequences that bind with the aptamers and the less the amount of Cy5 that is quenched. Thus, the fluorescence intensity is positively related to the OTA concentration. In this study, in the concentration range of 0.01-10 ng/mL, the fluorescence intensity was found to be linearly related to the logarithmic concentration of OTA. The limit of detection was calculated to be 0.005 ng/mL. The specificity of the developed biosensor was demonstrated to be efficient. The accuracy and stability of the developed aptasensor were also tested, and the method exhibited good performance in real samples.

Phage long tail fiber protein-immobilized magnetic nanoparticles for rapid and ultrasensitive detection of Salmonella

There is an urgent need to develop fast and sensitive detection methods for foodborne pathogens. But the conventional culture method that typically requires 2-3 days is not ideal for the rapid analysis. Food samples demonstrate a great challenge for direct detection due to the complex matrix. Hence, we present a new method based on the phage long-tail-fiber proteins (LTF4-a) immobilized magnetic nanoparticles (MNPs) for specific separation and concentration of Salmonella. The LTF4-a-MNP was prepared via the coupling of recombinant LTF4-a with MNPs and used to isolate and enrich Salmonella cells from contaminated food samples. The captured material was further integrated with the direct PCR program for accurate detection of Salmonella. Our study successfully established a new method for detecting contaminated food samples of Salmonella, the overall approach took no more than 3 h, which allowed a detection limit of 7 CFU/mL, demonstrating a promising alternative to the immunomagnetic separation method by replacing antibodies or aptamers, that is compatible with downstream analysis.