Competitive fluorometric assay for the food toxin T-2 by using DNA-modified silver nanoclusters, aptamer-modified magnetic beads, and exponential isothermal amplification

EXPAR for biosensing: recent developments and applications

Emerging as a promising novel amplification technique, the exponential amplification reaction (EXPAR) offers significant advantages due to its potent exponential amplification capability, straightforward reaction design, rapid reaction kinetics, and isothermal operation. The past few years have witnessed swift advancements and refinements in EXPAR-based technologies, with numerous high-performance biosensing systems documented. A deeper understanding of the EXPAR mechanism has facilitated the proposal of novel strategies to overcome limitations inherent to traditional EXPAR. Furthermore, the synergistic integration of EXPAR with diverse amplification methodologies, including the use of a CRISPR/Cas system, metal nanoparticles, aptamers, alternative isothermal amplification techniques, and enzymes, has significantly bolstered analytical efficacy, aiming to enhance specificity, sensitivity, and amplification efficiency. This comprehensive review presents a detailed exposition of the EXPAR mechanism and analyzes its primary challenges. Additionally, we summarize the latest research advancements in the biomedical field concerning the integration of EXPAR with diverse amplification technologies for sensing strategies. Finally, we discuss the challenges and future prospects of EXPAR technology in the realms of biosensing and clinical applications.

Fluorescent DNA-Silver nanoclusters in food safety detection: From synthesis to application

In recent years, the conventional preparation of silver nanoclusters (AgNCs) has attracted much attention due to their ultra-small size, tunable fluorescence, easy-to-engineer, as well as biocompatible material. Moreover, its great affinity towards cytosine bases on single-stranded DNA has led to the construction of biosensors, especially aptamers, for a broad variety of applications in food safety and environmental protection. In past years, numerous researchers paid attention to the construction of AgNCs aptasensor. Therefore, this review will be an effort to summarize the synthetic strategy along with the influences of factors on synthesis, categorize the sensing mechanism of aptamer-functionalized AgNCs biosensors, as well as their specific applications in food safety detection including heavy metal, toxin, and foodborne pathogenic bacteria. Furthermore, a brief conclusion and outlook regarding the prospects and challenges of their applications in food safety were drawn in line with the developments in DNA-AgNCs.

Programmable DNA barcode-encoded exponential amplification reaction for the multiplex detection of miRNAs

Multiple analysis of miRNAs is essential for the early diagnosis and monitoring of diseases. Here, a programmable, multiplex, and sensitive approach was developed for one-pot detection of miRNAs by melting temperature encoded sequences and exponential isothermal amplification (E-EXPAR). In the presence of target miRNAs, the corresponding templates initiate the cycles of nicking and polymerization/displacement, generating numerous barcode strands with unique encoding sequences. Subsequently, generated barcode strands hybridize with fluorescent probes and quench the fluorophore by a triplet of G base through a photo-induced electron transfer mechanism. Finally, a melting curve analysis is performed to quantify miRNAs by calculating the rate of fluorescence change at the corresponding melting temperature. Based on this, miRNA-21, miRNA-9, and miRNA-122 were detected with the detection limits of 3.3 fM, 2.9 fM, and 1.7 fM, respectively. This E-EXPAR was also employed to simultaneously detect three miRNAs in biological samples, showing consistent results with RT-qPCR. Overall, this study provides a programmable and universal platform for multiplex analysis of miRNAs, and holds great promise as an alternative to the multiplex analysis in clinical diagnostics and prognostics for nucleic acid detection.

A universal CRISPR-Cas14a responsive triple-sensitized upconversion photoelectrochemical sensor

It has recently been discovered that, like other members of the Cas family (12a and 13a), the clustered regularly interspaced short palindrome repeat CRISPR-Cas14a system not only mediates high-sensitivity detection with exceptionally strong gene editing ability but is also generally useful for DNA detection via fluorescence. Photoelectrochemical (PEC) sensors have been widely applied as efficient analytical tools. Measuring electrical signals is more cost-effective and the necessary equipment is more easily portable than fluorescence signal detectors, but their stability still needs to be improved. The high base resolution of CRISPR-Cas14a can compensate for such shortcomings. Therefore, electrical signals and fluorescence signals were combined, and the development of a universal CRISPR-Cas14a-responsive ultrasensitive upconversion PEC sensor is described in this paper. Moreover, strand displacement amplification (SDA) and a near-infrared (NIR) light source were utilized to further improve the stability and sensitivity of the photoelectric signals. At the same time, the modified working electrode (UCNPs-ssDNA-CdS@Au/ITO) on the three-electrode disposable sensor was used as the reporter probe, which cooperates with the trans-cleavage activity of Cas14a endonuclease. To verify the universality of this sensor, the UCNPs-Cas14a-based PEC sensor was applied for the detection of the small-molecule toxin T2 and protein kinase PTK7. Here, we report that the limit of detection of this reagent was within the fg range, successfully applied to the detection of T2 in oats and PTK7 in human serum. We propose that by combining PEC and CRISPR-14a, UCNPs-Cas14a-based PEC sensors could become powerful drivers for the extensive development of ultrasensitive, accurate and cost-effective universal sensors for detection and diagnosis.

A fluorescence aptasensor based on hybridization chain reaction for simultaneous detection of T-2 toxins and zearalenone1

It is extremely necessary to establish a rapid and high-throughput method to detect mycotoxins in food, because grains and cereals are greatly vulnerable to mycotoxins before and after harvest. In this study, we developed a portable aptasensor based on streptavidin magnetic microspheres (MMPs) and hybridization chain reaction (HCR) to simultaneously detect T-2 toxin and zearalenone (ZEN) in corn and oat flour. The MMPs compete with the aptamer for binding, which releases more H0 and triggers HCR with the H1 intermediate modified using 6-FAM and BHQ-1 and the unmodified H2. Subsequently, placing the HCR system corresponding to T-2 and ZEN in a constant-temperature fluorescence detector resulted in well-recovered fluorescence of the HCR products. T-2 and ZEN exhibited good fluorescence response in the dynamic range of 0.001-10 ng mL^-1 and 0.01-100 ng mL^-1 with detection limits of 0.1 pg mL^-1 and 1.2 pg mL^-1, respectively. In addition, this strategy achieved the selective detection of T-2 and ZEN in the spiked corn and oat flour samples. The results are also in good agreement with those obtained using commercial ELISA kits. This developed aptasensor with the characteristics of simple operation and portability has the application potential of establishing sensitive and portable field detection of various mycotoxins.

A switchable and signal-amplified aptasensor based on metal organic frameworks as the quencher for turn-on detection of T-2 mycotoxin

A simple and low-cost fluorescence aptasensor was developed for rapid and sensitive signal amplification detection of T-2 mycotoxin (T-2). Dual-terminal-fluorescein amidite (FAM)-labeled aptamer (D-aptamer) acted as a recognition element and signal indicator. The metal organic frameworks (MOFs) of N, N'-bis(2-hydroxyethyl)dithiooxamidato copper (II) (H₂dtoaCu) were as the quencher. The D-aptamer was initially adsorbed to the surface of H₂dtoaCu, leading to efficient quenching of the aptasensor. Upon addition of T-2, the D-aptamer underwent a conformation change to form the T-2/T-2 aptamer complex, which induced the signaling probe to be released from the H₂dtoaCu surface. Thus, the fluorescence intensity (FL) of the D-aptamer was recovered. Versus the single-terminal-FAM-labeled aptamer (S-aptamer), the D-aptamer showed a lower detection limit of 0.39 ng/mL. The aptasensor was also successfully applied to detect T-2 in corn and wheat samples with good recoveries.

Recent Advances in (Bio)Chemical Sensors for Food Safety and Quality Based on Silver Nanomaterials

There is a continuing need for tools and devices which can simplify, quicken and reduce the cost of analyses of food safety and quality. Chemical sensors and biosensors are increasingly being developed for this purpose, reaping from the opportunities provided by nanotechnology. Due to the distinct electrical and optical properties of silver nanoparticles (AgNPs), this material plays a vital role in (bio)sensor development. This review is an analysis of chemical sensors and biosensors based on silver nanoparticles with application in food and beverage matrices. It consists of academic research published from 2015 to 2020. The paper is structured to separately explore the designs of two major (bio)sensor classes: electrochemical (including voltammetric and impedimetric sensors) and optical sensors (including colourimetric and luminescent), with special focus on the type of silver nanomaterial and its role in the sensor system. The review indicates that diverse nanosensors have been developed, capable of detecting analytes such as pesticides, mycotoxins, fertilisers, microorganisms, heavy metals, and various additives with exceptional analytical performance. Current trends in the design of such sensors are highlighted and challenges which need to be overcome in the future are discussed.

Novel colorimetric aptasensor based on unmodified gold nanoparticle and ssDNA for rapid and sensitive detection of T-2 toxin

A novel colorimetric aptasensor based on unmodified gold nanoparticle (AuNPs) and single-strand DNA (ssDNA) aptamer was developed for the rapid and sensitive detection of T-2 toxin. In the absence of T-2, the AuNPs were wrapped by the aptamer to avoid the salt-induced aggregation and the solution remains red. In the presence of T-2, the aptamer was bound with T-2 and released from the surface of AuNPs, resulting in the aggregation of AuNPs under proper salt solution and the color change from red to purple-blue. The aptasensor exhibited a high sensitivity and selectivity for the detection of T-2. The range of linearity and detection limit were 0.1 ng/mL-5000 ng/mL (0.21435 nM-10717.5 nM) and 57.8 pg/mL (0.124 nM), respectively. The aptasensor developed here was applicable to assay T-2 in wheat and corn samples. These results implied that the colorimetric aptasensor was potentially useful in food detection.

Recent progress in mycotoxins detection based on surface-enhanced Raman spectroscopy

Mycotoxins are toxic compounds naturally produced by certain types of fungi. The contamination of mycotoxins can occur on numerous foodstuffs, including cereals, nuts, fruits, and spices, and pose a major threat to humans and animals by causing acute and chronic toxic effects. In this regard, reliable techniques for accurate and sensitive detection of mycotoxins in agricultural products and food samples are urgently needed. As an advanced analytical tool, surface-enhanced Raman spectroscopy (SERS), presents several major advantages, such as ultrahigh sensitivity, rapid detection, fingerprint-type information, and miniaturized equipment. Benefiting from these merits, rapid growth has been observed under the topic of SERS-based mycotoxin detection. This review provides a comprehensive overview of the recent achievements in this area. The progress of SERS-based label-free detection, aptasensor, and immunosensor, as well as SERS combined with other techniques, has been summarized, and in-depth discussion of the remaining challenges has been provided, in order to inspire future development of translating the techniques invented in scientific laboratories into easy-to-operate analytic platforms for rapid detection of mycotoxins.

A competitive immunoassay based on engineered magnetic/fluorescent nanoparticles and biolayer interferometry-based assay for T-2 toxin determination

For the first time a competitive immunoassay was developed by employing T-2 antibody-functionalized magnetite nanoparticles and T-2 toxin-conjugated fluorescent quantum dots (QDs). Free T-2 and the T-2-modified QDs compete for binding to antibody-modified magnetic beads; the magnetic beads collected by magnetic separation were subjected to fluorescence intensity analysis (with excitation/emission wavelengths at 460/616 nm). This competitive immunoassay for T-2 toxin determination was applied both in a microcentrifuge tube and on a 96-well plate. The dynamic range of the immunoassay is 1-100 ng mL^-1, the limit of detection (LOD) is 0.1 ng mL^-1, and determination was completed in about 40 min and 30 min in the microcentrifuge tube and 96-well plate, respectively. Moreover, the biolayer interferometry (BLI) technique was employed for T-2 determination for the first time, in which the conjugate of T-2 toxin and bovine serum albumin (BSA) was immobilized on the sensors before detection. Its average recovery of T-2 toxin from barley sample ranged from 82.00 to 123.33%, and the relative standard deviation (RSD) was between 9.42 and 15.73%. The LOD of the BLI-based assay is 5 ng mL^-1, and it only takes 10 min to finish the determination. Graphical abstract.

A fluorescent amplification strategy for high-sensitive detection of 17 β-estradiol based on EXPAR and HCR

Environmental endocrine disruptors in the environment and food, especially 17 β-estradiol (E2), are important factors affecting the growth and development of organisms. In this research, we constructed a fluorescence strategy for two-step amplification that combined two currently popular methods, exponential amplification reaction (EXPAR) and hybridization chain reaction (HCR). E2 competed with the complementary DNA (cDNA) to bind the aptamer modified on the magnetic beads. The free complementary strand in the supernatant was used as a trigger sequence to activate EXPAR, producing a large amount of short single-stranded DNA (ssDNA). The amplified ssDNA can trigger the second HCR amplification, producing many long double-stranded DNA (dsDNA) analogues. According to the principle of fluorescence resonance energy transfer, the carboxyfluorescein (FAM) signals in H1 and H2 hairpins were quenched by black hole quencher (BHQ-1). After the addition of E2 and initiation of amplification, the initially quenched fluorescent signal would be restored. This strategy with a detection limit of 0.37 pg mL^-1 (S/N = 3) showed a good linear relationship in the range of 0.4-800 pg mL^-1. In addition, the recovery rates of the method for milk and water samples were 98.55%-116.95% and 92.32%-107.00%, respectively. This is the first report of the combined detection of EXPAR and HCR, providing a reference for rapid and highly sensitive detection using multiple isothermal amplification methods.

CRISPR-Cas9 Triggered Two-Step Isothermal Amplification Method for E. coli O157:H7 Detection Based on a Metal-Organic Framework Platform

Escherichia coli O157:H7 has been reported as an important pathogenic bacteria causing serious infection and economic loss. However, detection of Escherichia coli O157:H7 needs improvement, given its current complexity and sensitivity. Herein, we attempt to build a fluorescence sensing method to detect Escherichia coli O157:H7 with easy operation and high efficiency. The target virulence gene sequences are recognized and cleaved by the CRISPR-Cas9 system, and trigger strand displacement amplification and rolling circle amplification. After amplification reactions, massive products can hybridize with the probes, the fluorescence of which are quenched based on a metal-organic framework platform, leading to the fluorescence recovery at typical excitation/emission wavelengths of 480/518 nm. This method exhibits high sensitivity with the detection limit at 4.0 × 10¹ CFU mL^-1 and a wide range from 1.3 × 10² CFU mL^-1 to 6.5 × 10⁴ CFU mL^-1. Meanwhile, this assay also shows significant specificity and applies to practical samples with high accuracy. Therefore, our method would have great potential application in bacterial detection, food safety monitoring, or clinical diagnostics.

Ultrasensitive competitive detection of patulin toxin by using strand displacement amplification and DNA G-quadruplex with aggregation-induced emission

A novel sensitive assay was established by using strand displacement amplification (SDA) and DNA G-quadruplex with aggregation-induced emission (AIE) for the detection of patulin (PAT) toxin. The complementary DNA (cDNA) of the aptamer and PAT competed for binding to aptamer-modified magnetic beads. The cDNA was obtained by magnetic separation and used as a primer in SDA to produce a large amount of G-base single-stranded DNA (ssDNA). They can form the G-quadruplex to be combined with the AIE of TTAPE dye, which features a special combination of G-quadruplex that amplify the fluorescent signals. This work can reach a lower detection limit of 0.042 pg mL^-1 with a wide linear range of 0.001-100 ng mL^-1 for PAT detection than other methods. The results also showed good recoveries of 97.8%-104% and 101.7%-105.3% in spiked apple and grape juices, respectively. The assay used for the detection of PAT exhibits high sensitivity and good specificity. It also provides a stable and reliable platform for detecting other small-molecule toxins.

Label-free probes using DNA-templated silver nanoclusters as versatile reporters

DNA-templated silver nanoclusters (DNA-AgNCs) have demonstrated pervasive applications in analytical chemistry recently. As a way of signal output in DNA-based detection methods, DNA-AgNCs have prominent advantages: first, the recognition and synthesizing sequences are naturally integrated in one DNA probe without any chemical modification or connection; second, the emissive wavelength of DNA-AgNCs can be adjusted in a wide range by employing different sequences; third, DNA-AgNCs can be utilized for producing not only fluorescence, also electrochemiluminescence and electrochemical signals. Besides, they also show potential applications for cell imaging, and are considered to be one of the most ideal nanomaterials for in-vivo imaging due to their ultra-small particle size. In this review, a brief and comprehensive introduction of DNA-AgNCs is firstly given, then label-free probes using DNA-AgNCs are classified and summarized, lastly concluding perspectives are provided on the defects and application potentials.

Electrochemical lead(II) biosensor by using an ion-dependent split DNAzyme and a template-free DNA extension reaction for signal amplification

A voltammetric biosensor for lead(II) (Pb²⁺) is described that is based on signal amplification by using an ion-dependent split DNAzyme and template-free DNA extension reaction. The Pb²⁺-dependent split DNAzyme was assembled on gold nanoparticles (Au@Fe₃O₄), and this nanoprobe then was exposed to Pb²⁺ which causes the split-off of DNAzymes to release primers containing 3'-OH groups (S₁ and S₂). The template-free DNA extension reaction triggers the generation of long ssDNA nanotails, which then can bind the free redox probe N,N'-bis(2-(trimethylammonium iodide)propylene)perylene-3,4,9,10-tetracarboxyldiimide (PDA⁺) via electrostatic adsorption. Hence, the concentration of PDA⁺ in solution is reduced. Therefore, less free PDA⁺ can be immobilized on a glassy carbon electrode modified with electrodeposited gold nanoparticles (depAu) to produce an electrochemical signal, typically measured at ∼0.38 V (vs. SCE) for quantitation of Pb²⁺. The use of a Pb²⁺-dependent split DNAzyme avoids the usage of a proteinic enzyme. It also increases the sensitivity of the sensor which has a lower detection limit of 30 pM of Pb²⁺. Graphical abstract Novel electrochemical biosensor based on the amplification of ion-dependent split DNAzyme and template-free DNA extension reaction for trace detection of Pb²⁺.

Fluorescence Polarization Immunoassay for the Determination of T-2 and HT-2 Toxins and Their Glucosides in Wheat

T-2 and HT-2 toxins and their main modified forms (T-2 glucoside and HT-2 glucoside) may co-occur in cereals and cereal-based products. A fluorescence polarization immunoassay (FPIA) was developed for the simultaneous determination of T-2 toxin, HT-2 toxin and relevant glucosides, expressed as sum. The developed FPIA, using a HT-2-specific antibody, showed high sensitivity (IC50 = 2.0 ng/mL) and high cross-reactivity (100% for T-2 toxin and 80% for T-2 and HT-2 glucosides). The FPIA has been used to develop two rapid and easy-to-use methods using two different extraction protocols, based on the use of organic (methanol/water, 90:10, v/v) and non-organic (water) solvents, for the determination of these toxins in wheat. The two proposed methods showed analytical performances in terms of sensitivity (LOD 10 µg/kg) recovery (92-97%) and precision (relative standard deviations ≤13%), fulfilling the criteria for acceptability of an analytical method for the quantitative determination of T-2 and HT-2 toxins established by the European Union. Furthermore, the methods were then validated in accordance with the harmonized guidelines for the validation of screening methods included in the Regulation (EU) No. 519/2014. The satisfactory analytical performances, in terms of intermediate precision (≤25%), cut-off level (80 and 96 µg/kg for the two methods) and rate of false positives (<0.1%) confirmed the applicability of the proposed methods as screening method for assessing the content of these toxins in wheat at the EU indicative levels reported for T-2 and HT-2 toxins.