Recent advances towards point-of-care devices for fungal detection: Emphasizing the role of plasmonic nanomaterials in current and future technologies

Fungal infections are a significant global health problem, particularly affecting individuals with weakened immune systems. Moreover, as uncontrolled antibiotic and immunosuppressant use increases continuously, fungal infections have seen a dramatic increase, with some strains developing antibiotic resistance. Traditional approaches to identifying fungal strains often rely on morphological characteristics, thus owning limitations, such as struggles in identifying several strains or distinguishing between fungal strains with similar morphologies. This review explores the multifaceted impact of fungi infections on individuals, healthcare providers, and society, highlighting the often-underestimated economic burden and healthcare implications of these infections. In light of the serious constraints of traditional fungal identification methods, this review discusses the potential of plasmonic nanoparticle-based biosensors for fungal infection identification. These biosensors can enable rapid and precise fungal pathogen detection by exploiting several readout approaches, including various spectroscopic techniques, colorimetric and electrochemical assays, as well as lateral-flow immunoassay methods. Moreover, we report the remarkable impact of plasmonic Lab on a Chip technology and microfluidic devices, as they recently emerged as a class of advanced biosensors. Finally, we provide an overview of smartphone-based Point-of-Care devices and the associated technologies developed for detecting and identifying fungal pathogens.

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).

Emerging Trends of Gold Nanostructures for Point-of-Care Biosensor-Based Detection of COVID-19

In 2019, a worldwide pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged. SARS-CoV-2 is the deadly microorganism responsible for coronavirus disease 2019 (COVID-19), which has caused millions of deaths and irreversible health problems worldwide. To restrict the spread of SARS-CoV-2, accurate detection of COVID-19 is essential for the identification and control of infected cases. Although recent detection technologies such as the real-time polymerase chain reaction delivers an accurate diagnosis of SARS-CoV-2, they require a long processing duration, expensive equipment, and highly skilled personnel. Therefore, a rapid diagnosis with accurate results is indispensable to offer effective disease suppression. Nanotechnology is the backbone of current science and technology developments including nanoparticles (NPs) that can biomimic the corona and develop deep interaction with its proteins because of their identical structures on the nanoscale. Various NPs have been extensively applied in numerous medical applications, including implants, biosensors, drug delivery, and bioimaging. Among them, point-of-care biosensors mediated with gold nanoparticles (GNPSs) have received great attention due to their accurate sensing characteristics, which are widely used in the detection of amino acids, enzymes, DNA, and RNA in samples. GNPS have reconstructed the biomedical application of biosensors because of its outstanding physicochemical characteristics. This review provides an overview of emerging trends in GNP-mediated point-of-care biosensor strategies for diagnosing various mutated forms of human coronaviruses that incorporate different transducers and biomarkers. The review also specifically highlights trends in gold nanobiosensors for coronavirus detection, ranging from the initial COVID-19 outbreak to its subsequent evolution into a pandemic.

Gold nanoparticle-mediated fluorescence immunoassay for rapid and sensitive detection of Ochratoxin A

In this work, a fluorescence immunoassay based on horseradish peroxidase-labeled IgG (HRP-IgG)-modified gold nanoparticle (AuNP@HRP-IgG) probe was established for detection of ochratoxin A (OTA). Through the catalysis of HRP, the dopamine (DA) and 1,5-dihydroxynaphthalene (DHA) can rapidly generate azamonardine fluorescence compound (AFC) with intense yellow fluorescence. Large amounts of AFC can be formed within 4 min, which led to fluorescence enhancement at 545 nm. This new method displayed high sensitivity with a limit of detection (LOD) of 0.18 ng/mL and a linear range of 0.78-200 ng/mL for OTA. Meanwhile, the recoveries of OTA in corn samples were 101.41% - 113.45%. Due to the universality of the probe and the rapidity of signal output, the fluorescence immunoassay allowed rapid and sensitive detection of targets.

Competitive ELISA based on pH-responsive persistent luminescence nanoparticles for autofluorescence-free biosensor determination of ochratoxin A in cereals

An accurate and sensitive competitive enzyme-linked immunosorbent assay (ELISA) based on persistent luminescence nanoparticles Zn₂GeO₄:Mn²⁺, Eu³⁺ (ZGME) was developed for detecting ochratoxin A (OTA), a powerfully toxic mycotoxin usually found in grains. As a signal output element of autofluorescence-free biosensors, ZGME can be integrated into ELISA with glucose oxidase (GOx)-binding OTA molecules due to its excellent pH-responsive persistent luminescence. In the absence of OTA, the OTA-GOx conjugate was captured by the anti-OTA monoclonal antibody (anti-OTA mAb) pre-coated on the 96-well plate. The results indicate a decrease in the pH value of the solution, which triggered the quenching of ZGME luminescence due to GOx-dependent gluconic acid production. The presence of OTA inhibited the binding of OTA-GOx on the plate, thus decreasing the production of gluconic acid and increasing the persistent luminous intensity of ZGME. Under the optimized concentrations of anti-OTA mAb and OTA-GOx, quantitative determination of OTA was achieved by plotting the increase or decrease in persistent luminescence intensity of ZGME at 535 nm. In this study, the linear range was from 0.1 μg L^-1 to 63 μg L^-1, and the limit of detection (LOD) was as low as 0.045 μg L^-1. In five food samples (corn grit, brown rice, soybean, rice, and wheat), the results exhibited good stability and repeatability, with a recovery range from 81.3% to 94.4% and a relative standard deviation (RSD) of less than 4.2%. Hence, the established method provides a sensitive, accurate, and autofluorescence-free approach for the determination of OTA in different grain samples.

Detection of ochratoxin A using a "turn-on" fluorescence assay based on guanine quenching of the aptamer

Ochratoxin A (OTA) is a common mycotoxin with high carcinogenicity; therefore, it is crucial to establish a simple, rapid, and sensitive method for its detection. In this study, we developed a "turn-on" fluorescence assay for detecting OTA based on guanine quenching of the aptamer. The method uses fluorescein (FAM) fluorophore to label the complementary strand of the OTA aptamer, Fc-DNA. In the absence of OTA, the Fc-DNA hybridizes with the aptamer to form a double strand. Due to the occurrence of photo-induced electron transfer (PET), the FAM fluorescence signal is quenched as the FAM on the Fc-DNA approaches the guanine of the aptamer at the 5' end. When OTA is present, the aptamer binds to it and thus, is unable to hybridize with Fc-DNA to form a double strand; the FAM fluorescence signal is restored as FAM moves away from the guanine of the aptamer. The assay achieved OTA detection at a detection limit of 28.4 nM. The application of the original guanine of the aptamer as the quenching agent helps avoid the complex designing and labeling of the aptamer, which ensures the high affinity of the aptamer for OTA. Meanwhile, this "turn-on" detection mode helps avoid potential false-positive results as in the "turn-off" mode and improves the assay's sensitivity. Additionally, the method has good selectivity and can be used to detect OTA in traditional Chinese medicine. This method provides a simple, low-cost, and rapid method for OTA detection.

One-step immunoassay based on switching peptides for analyzing ochratoxin A in wines

A one-step immunoassay is presented for the detection of ochratoxin A (OTA) using an antibody complex with switching peptides. Because the switching peptides (fluorescence-labeled) were able to bind the frame region of antibodies (IgGs), they were dissociated from antibodies immediately when target analytes were bound to the binding pockets of antibodies. From the fluorescence signal measurements of switching peptides, a quantitative analysis of target analytes, via a one-step immunoassay without any washing steps, could be performed. As the first step, the binding constant (KD) of OTA to the antibodies was estimated under the continuous flow conditions of a surface plasmon resonance biosensor. Then, the optimal switching peptide, among four types of switching peptides, and the reaction condition for complex formation with the switching peptide were determined for the one-step immunoassay for OTA analysis. Additionally, the selectivity test of one-step immunoassay for OTA was carried out in comparison with phenylalanine and zearalenone. For the application to the one-step immunoassay to detect OTA in wines, two types of sample pre-treatment methods were compared: (1) a liquid extraction was carried out using chloroform as a solvent with subsequent resuspension in phosphate-buffered saline (total analysis time < 1 h); (2) direct dilution of the wine sample (total analysis time < 0.5 h). Finally, the direct dilution method was found to be effective for the one-step immunoassay based on the switching peptide assay for OTA in wines with a markedly improved total analysis time (< 0.5 h). Additionally, the assay results were compared with commercial lateral flow immunoassay.

Multifunctional Smart ZnSe-Nanostructure-Based Fluorescent Aptasensor for the Detection of Ochratoxin A

Herein, we present a comprehensive investigation of rationally designed zinc selenide (ZnSe) nanostructures to achieve highly negatively charged ZnSe nanostructures. A Microwave-assisted hydrothermal synthesis method was used to synthesize three types of ZnSe nanostructures, i.e., nanorods, µ-spheres and nanoclusters, as characterized by a zeta potential analyzer, X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and BET, which were labeled as type A, B and C. Three different solvents were used for the synthesis of type A, B and C ZnSe nanostructures, keeping other synthesis conditions such as temperature, pressure and precursors ratio constant. Based on two heating time intervals, 6 and 9 h, types A, B and C were further divided into types A6, A9, B6, B9, C6 and C9. ZnSe nanostructures were further evaluated based on their fluorescent quenching efficiency. The maximum fluorescence quenching effect was exhibited by the ZnSe-B6 type, which can be attributed to its highly negative surface charge that favored its strong interaction with cationic dye Rhodamine B (Rh-B). Further, the optimized ZnSe-B6 was used to fabricate an aptasensor for the detection of a food-based toxin, ochratoxin-A (OTA). The developed aptasensor exhibited a limit of detection of 0.07 ng/L with a wide linear range of 0.1 to 200 ng/L.

AuNP-based biosensors for the diagnosis of pathogenic human coronaviruses: COVID-19 pandemic developments

The outbreak rate of human coronaviruses (CoVs) especially highly pathogenic CoVs is increasing alarmingly. Early detection of these viruses allows treatment interventions to be provided more quickly to people at higher risk, as well as helping to identify asymptomatic carriers and isolate them as quickly as possible, thus preventing the disease transmission chain. The current diagnostic methods such as RT-PCR are not ideal due to high cost, low accuracy, low speed, and probability of false results. Therefore, a reliable and accurate method for the detection of CoVs in biofluids can become a front-line tool in order to deal with the spread of these deadly viruses. Currently, the nanomaterial-based sensing devices for detection of human coronaviruses from laboratory diagnosis to point-of-care (PoC) diagnosis are progressing rapidly. Gold nanoparticles (AuNPs) have revolutionized the field of biosensors because of the outstanding optical and electrochemical properties. In this review paper, a detailed overview of AuNP-based biosensing strategies with the varied transducers (electrochemical, optical, etc.) and also different biomarkers (protein antigens and nucleic acids) was presented for the detection of human coronaviruses including SARS-CoV-2, SARS-CoV-1, and MERS-CoV and lowly pathogenic CoVs. The present review highlights the newest trends in the SARS-CoV-2 nanobiosensors from the beginning of the COVID-19 epidemic until 2022. We hope that the presented examples in this review paper convince readers that AuNPs are a suitable platform for the designing of biosensors.

Development of a graphene oxide nanosheet and double-stranded DNA structure based fluorescent “signal off” aptasensor for ochratoxin A detection in malt

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A fluorescence “signal off” aptasensor was developed for OTA detection in malt.

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It was based on the fluorescence quenching of GO nanosheets on FAM fluorophores labeled on cDNA.

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An OTA concentration-dependent quenched fluorescence was measured for OTA quantitation.

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The aptasensor exhibited outstanding sensitivity with a LOD of 11 pg/mL and a wide dynamic range.

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The feasibility and reliability was verified by determining OTA in malt samples.

A “signal off” fluorescent aptasensor based on graphene oxide (GO) nanosheet and double-stranded DNA structure was fabricated for OTA detection. In the absence of OTA, the aptamer and its complementary DNA (cDNA) formed double-stranded conjugates that could coexist with GO, presenting fluorescence responses. Then, the presented OTA was captured by the aptamers, causing the release of cDNA-FAM probes. The free probes were adsorbed by GO, leading to an OTA concentration-dependent fluorescence quenching through fluorescence resonance energy transfer. Under optimum conditions, the fluorescent aptasensor exhibited outstanding sensitivity with a LOD of 11 pg/mL and a wide dynamic range of 0.04–30 ng/mL. The selectivity of the aptasensor was confirmed against other four mycotoxins, and the feasibility and reliability were verified by determining OTA in the spiked malt samples with satisfactory recovery of 95.50%-112.05%. This aptasensing platform could be adapted to measure other mycotoxins by replacing the aptamer sequences for food safety.

Adsorption-desorption nano-aptasensors: fluorescent screening assays for ochratoxin A

In this study, a FRET-based fluorescent aptasensor for the detection of ochratoxin A (OTA) was optimized based on the quenching efficiency of single-walled carbon nanotubes (SWCNTs) and the binding affinity of aptamers. OTA aptamers were conjugated with quantum dots and adsorbed to the surface of both acid-modified and unmodified SWCNTs. The maximum fluorescence quenching efficiency of the SWCNTs were compared. Acid-modified SWCNTs (amSWCNTs) have moderate quenching efficiency, providing an optimal sensitivity for qualitative fluorescence-enhancement biosensor assays. The binding parameters of the QD-modified OTA aptamers (1.12.2 and A08min) on the surface of amSWCNTs were compared. Based on our results, the A08min aptamer is a better candidate for OTA detection. Using the A08min aptamer, the SWCNT method had a limit of detection (LOD) of 40 nM. The amSWCNT method had a significantly lower LOD of 14 nM. Turn-on fluorescent nano-aptasensors are emerging as an effective diagnostic tool for simple detection of mycotoxins. Nanocomplexes designed for the detection of mycotoxins in solution and paper-based tests have proven to be useful.

Nanomaterial-based aptamer biosensors for ochratoxin A detection: a review

Ochratoxin A (OTA) is a widely distributed mycotoxin that often contaminates food, grains and animal feed. It poses a serious threat to human health because of its high toxicity and persistence. Therefore, the development of an inexpensive, highly sensitive, accurate and rapid method for OTA detection is imperative. In recent years, various nanomaterials used in the establishment of aptasensors have attracted great attention due to their large surface-to-volume ratio, good stability and facile preparation. This review summarizes the development of nanomaterial-based aptasensors for OTA determination and sample treatment over the past 5 years. The nanomaterials used in OTA aptasensors include metal, carbon, luminescent, magnetic and other nanomaterials. Finally, the limitations and future challenges in the development of nanomaterial-based OTA aptasensors are reviewed and discussed.

Gold nanoparticle-based colorimetric aptasensor for rapid detection of multiple mycotoxins in rice

A novel, simple and rapid colorimetric aptasensor for multiple mycotoxins (ochratoxin A (OTA) and aflatoxin B₁ (AFB₁)) detection was developed using unmodified gold nanoparticles (AuNPs). In the work, the high affinities of OTA and AFB₁ aptamers were employed as the recognition elements for the colorimetric determination of OTA and AFB₁. In the absence of mycotoxins, the sulfhydryl-modified aptamers were directly adsorbed to the AuNP surface through Au-S bonds, further prohibiting the aggregation induced by a high concentration of salt, and the solutions remain red. In the presence of mycotoxins, the corresponding aptamer-target complexes were formed and the corresponding aptamers were detached from the surface of AuNPs, leading to the aggregation of AuNPs under the optimal salt solution and a color change. By spectroscopic quantitative analysis and visual analysis, the LODs of OTA and AFB₁ were down to 0.005 ng mL^-1 and 0.07 ng mL^-1, respectively. Furthermore, the colorimetric aptasensor showed a high specificity in the presence of other interfering mycotoxins and metal ions. Finally, the developed aptasensor was applicable to detect OTA and AFB₁ in rice samples with satisfactory performance. Our strategy has great potential for the rapid and sensitive detection of OTA and AFB₁ for on-site analysis.

Establishment of a Chemiluminescence Immunoassay Combined with Immunomagnetic Beads for Rapid Analysis Of Ochratoxin A

BACKGROUND

Ochratoxins A (OTA), as the harmful mycotoxins, are toxic secondary metabolite produced by several species of Aspergillus and Penicillium. Hence, it is of vital important to establish a sensitive method to detect OTA in various grains.

OBJECTIVE

Herein a novel highly sensitive chemiluminescence immunoassay was developed to determination of ochratoxin A in wheat, corn, and poultry feed.

METHODS

The immunomagnetic beads were used as solid phase carrier and separator in the competitive chemiluminescence method, the monoclonal antibodies anti-OTA coated on immunomagnetic beads were applied as the capturing antibody, in which OTA would compete with Ochratoxin A-alkaline phosphatase (OTA-ALP) in binding with OTA antibodies.

RESULTS

The proposed method exhibited good linearity (R2 ≥ 0.999), adequate OTA recovery rate (83.60% to 102.50%), and good repeatability. The sensitivity of the proposed method was 2.05 pg/mL. In addition, the method was highly selective, it did not have cross-reaction with other mycotoxin (e.g., AFB1, DON, ZEN). Furthermore, the developed method was applied in the analysis of wheat, corn and two animal feeds successfully, the results obtained by the chemiluminescence method showed the high correlation with those obtained by HPLC method (correlation coefficient 0.9958) when testing each extract by both methods.

CONCLUSIONS

This strategy shows great potential application for other toxic and harmful mycotoxin detection.

HIGHLIGHTS

A novel highly rapid and sensitive chemiluminescence immunoassay was established to quantitation for ochratoxin A.

An ultra-sensitive optical aptasensor based on gold nanoparticles/poly vinyl alcohol hydrogel as acceptor/emitter pair for fluorometric detection of digoxin with on/off/on strategy

A novel nanobiosensor was prepared by aptamer and gold nanoparticles conjugate in poly vinyl alcohol hydrogel for sensitive detection of digoxin in human plasma samples. The developed nanobiosensor was characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, and dynamic light scattering instrument. In this sensor the hydrogel acted as a fluorescent probe. The fluorescence intensity of the hydrogel was quenched by aptamer stabilized gold nanoparticles as energy acceptor. Upon addition of digoxin, the aptamer/drug complex was formed and the fluorescence of the hydrogel was restored because of destabilization and aggregation of gold nanoparticles in the presence of salt. The affecting parameters on the nanobiosensor performance were assessed and under the optimized conditions the external and in plasma calibration curves were linear in the 10-1000 ng L^-1 digoxin concentration range with detection limits of 2.9 and 3.1 ng L^-1, respectively. The relative standard deviations for 5 replicate determinations of 50, 250, and 500 ng L^-1 of digoxin, were 7.3, 5.1, and 3.8%, respectively. This nanofluoroprobe was successfully applied for determination of digoxin in spiked plasma samples without any pretreatment procedure.

Effect of surface chemistry on bio-conjugation and bio-recognition abilities of 2D germanene materials

The interest of the scientific community for 2D graphene analogues has been recently focused on 2D-Xene materials from Group 14. Among them, germanene and its derivatives have shown great potential because of their large bandgap and easily tuneable electronic and optical properties. With the latter having been already explored, the use of chemically modified germanenes for optical bio-recognition is yet to be investigated. Herein, we have synthesized two germanene materials with different surface ligands namely hydrogenated germanene (Ge-H) and methylated germanene (Ge-Me) and used them as an optical platform for the label-free biorecognition of Ochratoxin A (OTA), a highly carcinogenic food contaminant. It was discovered that firstly the surface ligands on chemically modified germanenes have strong influence on the intrinsic fluorescence of the material; secondly they also highly affect both the bio-conjugation ability and the bio-recognition efficiency of the material towards the detection of the analyte. An improved calibration sensitivity, together with superior reproducibility and linearity of response, was obtained with a methylated germanene (Ge-Me) material, indicating also the better suitability of the latter for real sample analysis. Such research is highly beneficial for the development and optimization of 2D material based optical platforms for fast and cost-effective bioassays.

Unravelling the Aptamer-Analyte Interaction Dynamics through Fluorescence Quenching in Graphene Quantum Dots (GQDs) Based Homogeneous Assays

Graphene quantum dots (GQDs) are used here as a biosensing platform for the recognition of the major food contaminant ochratoxin A (OTA), with a fluorescently labelled DNA aptamer (FAM OTA aptamer) functioning as the biorecognition element. The detection principle lies in the formation of noncovalent interactions between the FAM OTA aptamer and the GQD surface, and the consequent fluorescence quenching. The further change in the fluorescence signal, induced by the formation of the FAM OTA Aptamer/OTA conjugate during the detection step, could then be correlated to the presence and concentration of the target analyte. Upon tuning the concentration of GQDs, a switch in the biorecognition mechanism occurred. Specifically, while a lower GQD concentration (0.060 mg/mL) resulted in a restoration of the fluorescence intensity upon incubation with OTA, a higher GQD concentration (0.150 mg/mL) provided a further quenching of the final fluorescence intensity. Upon further calibration study, it was discovered that the latter mechanism provided a better option in terms of linearity of response, detection limit and selectivity.

A label-free aptasensor for turn-on fluorescent detection of ochratoxin A based on aggregation-induced emission probe

A novel label-free fluorescence aptasensor used for the detection of ochratoxin A (OTA) is presented in this study. When aggregated on the surface of DNA aptamer, aggregation-induced emission (AIE) fluorescence probe presents turn-on fluorescence property. The method proposed in this article was based on an AIE probe, 4, 4-(1E,1E)-2, 2-(anthracene-9, 10-diyl) bis (ethene-2, 1-diyl) bis (N, N, N-trimethylbenzenaminium iodide) (DSAI). With OTA present, the aptamer will combine with OTA and the conformation of the aptamer will switch to an antiparallel G-quadruplex from the initial random coil, which obstructs its digestion by Exo I. After the solution is added with DSAI, DSAI will aggregate on the surface of the aptamer/OTA complex and produces a strong emission. In the range of 5 to 500 ng · ml^-1 OTA concentrations, the fluorescence increases with a linear logarithm relationship. The detection limit is 1.9 ng · ml^-1. This method was used to detect OTA in spiked real samples, with recoveries and RSDs in the range of 92.2% to 106.3%, and 2.7% to 5.2%, respectively.

Gold Nanoparticles Adsorb DNA and Aptamer Probes Too Strongly and a Comparison with Graphene Oxide for Biosensing

Using fluorescently labeled DNA oligonucleotides and nanomaterials for developing biosensors has been extensively reported for gold nanoparticles (AuNPs) and graphene oxide (GO) among others. These materials have vastly different affinities and mechanisms for interacting with DNA, and their analytical performance is likely to be different. In this work, we used several DNA sequences and, respectively, adsorbed them on AuNPs and GO to quench fluorescence. Different from previous work, we used KCN to fully dissolve the AuNPs to calculate the percentage of the desorbed DNA due to the complementary DNA (cDNA) and aptamer target. The desorbed probe DNA from the AuNPs was less than 5% for all of the targets including DNA, adenosine, Hg²⁺, and lysozyme, indicating a very strong DNA adsorption affinity. Desorption of DNA was achieved by adding HEPES buffer, NaCl, and As(III), but such desorption was attributed to the adsorption of these molecules or ions by the AuNPs instead of their interaction with the adsorbed DNA. For GO, more probes desorbed with addition of target analytes but so did nonspecific desorption by random DNA and proteins. In summary, AuNPs are unlikely to be a good surface for developing biosensors relying solely on the desorption of probe DNA, while for GO the main problem is nonspecific desorption.

In-Solution Molecular Recognition Comparison of Aptamers against the Herbicide Atrazine

Atrazine is a common herbicide that is widely used to control weed growth in both agricultural and residential settings. It has been shown to act as an endocrine disruptor that affects aquatic organisms. Rapid and low-cost monitoring methods for atrazine is the first step to mitigate its widespread persistency. Aptamers are small synthetic oligonucleotides that can assume a 3D structure to act as the molecular recognition element for a specific target of interest. Two different atrazine binding aptamers (R12.23 Trunc. and R12.45 Trunc.) have been identified from the same library design but with fundamentally different in vitro selection methodologies. While the R12.23 Trunc. has been utilized in immobilized biosensing platforms, it is unclear if in-solution-based applications would be suitable for both atrazine binding aptamers. This study provides the first insight of comparative in-solution binding profiles of the two atrazine binding aptamers. Based on our results, this information will be useful for future biosensing platform development utilizing the two aptamers.

Advances in biosensors for the detection of ochratoxin A: Bio-receptors, nanomaterials, and their applications

Ochratoxin A (OTA) is a class of mycotoxin mainly produced by the genera Aspergillus and Penicillium. OTA can cause various forms of kidney, liver and brain diseases in both humans and animals although trace amount of OTA is normally present in food. Therefore, development of fast and sensitive detection technique is essential for accurate diagnosis of OTA. Currently, the most commonly used detection methods are enzyme-linked immune sorbent assays (ELISA) and chromatographic techniques. These techniques are sensitive but time consuming, and require expensive equipment, highly trained operators, as well as extensive preparation steps. These drawbacks limit their wide application in OTA detection. On the contrary, biosensors hold a great potential for OTA detection at for both research and industry because they are less expensive, rapid, sensitive, specific, simple and portable. This paper aims to provide an extensive overview on biosensors for OTA detection by highlighting the main biosensing recognition elements for OTA, the most commonly used nanomaterials for fabricating the sensing interface, and their applications in different read-out types of biosensors. Current challenges and future perspectives are discussed as well.

A nanozyme-based cascade colorimetric aptasensor for amplified detection of ochratoxin A

Colorimetric assays have been widely developed for the detection of toxin ochratoxin A (OTA), but most of them suffer from moderate sensitivity when they are adopted for the detection of trace OTA in a complicated food matrix. For the purpose of overcoming this issue, an innovative cascade reaction-based colorimetric aptasensor was developed for the achievement of high sensitivity. The biotin-labelled OTA aptamer was immobilized onto streptavidin magnetic beads by means of the biotin-streptavidin reaction. With OTA binding to its aptamer, the structural switching of the aptamer results in the release of the alkaline phosphatase-labelled oligonucleotide, which is partially complementary to the aptamer. Following the magnetic separation, the cascade reaction is initiated through the enzymatic conversion of ascorbic acid-2-phosphate into ascorbic acid. Subsequent to that, the generated ascorbic acid reduces MnO2 nanosheets to Mn2+ ions, accordingly destroying the oxidase-mimicking activity of MnO2 nanosheets. In consequence, it is not possible to oxidize 3,3',5,5'-tetramethylbenzidine (TMB), a substrate for oxidase, with Mn2+ for the production of the blue colour product (TMB Ox). With the increasing amount of OTA, a colour change occurs from blue to colourless. The cascade reaction has the potential of greatly amplifying the detection signal, together with remarkably improving the sensitivity, making this colorimetric sensor a universal and promising platform for the highly sensitive detection of mycotoxins in the field of public food safety monitoring.

Electrochemical Immuno- and Aptasensors for Mycotoxin Determination

Modern analysis of food and feed is mostly focused on development of fast and reliable portable devices intended for field applications. In this review, electrochemical biosensors based on immunological reactions and aptamers are considered in the determination of mycotoxins as one of most common contaminants able to negatively affect human health. The characteristics of biosensors are considered from the point of view of general principles of bioreceptor implementation and signal transduction providing sub-nanomolar detection limits of mycotoxins. Moreover, the modern trends of bioreceptor selection and modification are discussed as well as future trends of biosensor development for mycotoxin determination are considered.

A FRET-based dual-color evanescent wave optical fiber aptasensor for simultaneous fluorometric determination of aflatoxin M1 and ochratoxin A

A dual-color fluorescence resonance energy transfer (FRET) based aptasensor is described for simultaneous determination of the mycotoxins aflatoxin M1 (AFM1) and ochratoxin A (OTA). Aptamers against AFM1 and OTA were labeled with two fluorophores with different excitation wavelengths (Cy5.5; 675 nm; and Alexa 405; 401 nm), respectively. They were used as the signalling probes. A compact dual-color evanescent wave all-fiber detection system with two lasers (635 nm; red; and 405 nm; purple) was used for the simultaneous collection of two-wavelength fluorescence signals. The hybridization of labeled aptamers with complementary sequences (Q-cDNA) labeled with a dark quencher (BHQ₃ or dabcyl) causes fluorescence to be strongly reduced because of the fluorescence resonance energy transfer. In the presence of AFM1 and OTA, they bind to their respective aptamer and result in the dissociation of double stranded DNA, which induce fluorescence recovery. Under the optimum conditions, AFM1 and OTA can simultaneously and selectively be determined ranged from 1 ng·L^-1 to 1 mg·L^-1. The detection limits of AFM1 and OTA are 21 and 330 ng·L^-1, respectively (S/N = 3). The FRET-based dual-color detection scheme was applied to the simultaneous detection of AFM1 and OTA in milk with good recovery, precision, and accuracy. Graphical abstract Aptamers against AFM1 and OTA were labeled with two fluorophores with different excitation wavelengths (Cy5.5; 675 nm; and Alexa 405; 401 nm) and then used as signalling probes. A FRET-based aptasensor is described for simultaneous determination of AFM1 and OTA using dual-color evanescent wave system with two lasers (635 nm; red; and 405 nm).

An aptamer-based fluorescence bio-sensor for chiral recognition of arginine enantiomers

In this study, a novel aptamer - based fluorescence bio-sensor (aptamer-AuNps) was developed for chiral recognition of arginine (Arg) enantiomers based on aptamer and gold nanoparticles (AuNps). Carboxyfluorescein (FAM) labeled aptamers (Apt) were absorbed on AuNps and their fluorescence intensity could be significantly quenched by AuNps based on fluorescence resonance energy transfer (FRET). Once d-Arg or l-Arg were added into the above solution, the aptamer specifically bind to Arg enantiomers and released from AuNps, so the fluorescence intensity of d-Arg system and l-Arg system were all enhanced. The affinity of Apt to l-Arg is tighter to d-Arg, so the enhanced fluorescence signals of l-Arg system was stronger than d-Arg system. What's more, the enhanced fluorescence were directly proportional to the concentration of d-Arg and l-Arg ranging from 0-300 nM and 0-400 nM with related coefficients of 0.9939 and 0.9952, respectively. Furthermore, the method was successfully applied to detection l-Arg in human urine samples with satisfactory results. Eventually, a simple "OR" logic gate with d-Arg &l-Arg as inputs and AuNps aggregation state as outputs was fabricated, which can help us understand the chiral recognition process deeply.

PVP-coated gold nanoparticles for the selective determination of ochratoxin A via quenching fluorescence of the free aptamer

This paper describes an aptamer/gold nanoparticle-based assay for ochratoxin A (OTA) detection. This assay is based on the use of an aptamer labeled with carboxyfluorescein (FAM) at its 5'-end and gold nanoparticles (AuNPs) that act as quenchers of fluorescence. When OTA is absent in the system, the fluorescently labeled aptamers are adsorbed on the surface of AuNPs. The fluorescence signal of the fluorescein-labeled OTA aptamer generated is quenched by the fluorescence resonance energy transfer effect of AuNPs. When OTA is present in the system, the fluorescently labeled aptamer binds to OTA and forms a folded structure, which can resist the adsorption of AuNPs. Thus, the fluorescent signal can be retained. The detection limit of this sensing platform is 5 nM, and the linear detection range is 10-1000 nM (R2 = 0.994). The procedure was validated by the quantitation of OTA in spiked ginger powder samples and were found to be free of interference by the sample matrix. The recoveries and the relative standard deviation varied from 89.0% to 117.8% and from 1.9% to 6.3%, respectively.

Fluorometric aptamer based assay for ochratoxin A based on the use of exonuclease III

This study describes an aptamer based assay for the mycotoxin ochratoxin A (OTA). The method is based on the use of an OTA-specific aptamer, exonuclease (Exo) III, SYBR Gold as a fluorescent probe, and a complementary strand that specifically combines with the aptamer. In the presence of OTA, the aptamer and OTA hybridize, thereby resulting in the formation of ssDNA, which is not digested by Exo III. Intense fluorescence is observed after addition of SYBR Gold (best measured at excitation/emission wavelengths of 495/540 nm). Fluorescence increases linearly with the log of the OTA concentration in the range from 8 to 1000 ng·mL^-1. The detection limit is 4.7 ng·mL^-1. The assay was applied to the determination of OTA in diluted [2%(v/v)] red wine, and recoveries and RSDs ranged between 93.5% and 113.8%, and between 3.2% and 5.7%, respectively. Graphical abstract In the presence of ochratoxin A (OTA), specific combinations of aptamer and OTA may occur and result in DNA double strands being untied, which avoids being digested by Exo III. Intense fluorescence is observed after SYBR Gold addition.

Fluorometric aptamer assay for ochratoxin A based on the use of single walled carbon nanohorns and exonuclease III-aided amplification

The authors describe an aptamer based assay for the food mycotoxin ochratoxin A (OTA). It is based on the use of exonuclease III (Exo III) which assists in signal amplification, and of single-walled carbon nanohorns (SWCNHs) which act as quenchers of fluorescence. The detection scheme employs a hairpin probe (HP) and a signal probe (SP) labeled with carboxyfluorescein (FAM) at its 5'-end. The fluorescence of intact SPs (best measured at excitation/emission wavelengths of 495/518 nm) is quenched by SWCNHs. The HP contains the OTA-specific aptamer sequence and is partially complementary to the SP. After addition of OTA, the aptamer binds OTA and thus exposes a single-stranded sequence that can hybridize with the SP. Exo III digests the SP to liberate the free fluorophore labels. The damaged SPs no longer are adsorbed by the SWCNHs so that fluorescence is no longer quenched. The method has a detection range that is linear from 10 nM to 1000 nM (with a correlation coefficient of 0.997). The limit of detection (LOD), calculated on the basis of a signal to noise ratio of 3, is 4.2 nM. The procedure was validated by the quantitation of OTA in spiked real samples and were found to be free of interference by the sample matrix. Recoveries ranged from 93.8 to 113.0% in beer and from 92.0 to115.9% in red wine. Graphical abstract After adding ochratoxin A (OTA), the aptamer region in hairpin probe (HP) combined with OTA and thus exposed a single-stranded sequence to hybridize with signal probe (SP). Exonuclease III (Exo III) digested SP to liberate the free fluorophore (FAM).

Highly sensitive colorimetric aptasensor for ochratoxin A detection based on enzyme-encapsulated liposome

A simple, low-cost, and sensitive liposome-based colorimetric aptasensor has been developed to detect ochratoxin A (OTA). Specifically, a dumbbell-shaped probe was designed, including magnetic beads (MBs), double-stranded DNA (dsDNA), and enzyme-encapsulated liposome. The dsDNA formed by the hybridization between OTA aptamer and its complementary probe. And the dsDNA was used to contact the MBs and the enzyme-encapsulated liposome. In the presence of OTA, the aptamer preferred to combine with OTA to form G-quadruplex, resulting in the release of the detection probe and the enzyme-encapsulated liposome. Each liposome contained a large amount of HRP. Thus, when the liposome was lysed by adding the mixed solution of 3,3',5,5'-tetramethylbenzidine (TMB) and H₂O₂, a large number of HRP were released. HRP could catalyze the H₂O₂-mediated oxidation of TMB and hence resulted in the color change from colorless to blue with the OTA concentration varying, and this variation can be observed by naked eyes easily. The result showed that the absorption intensity at 652 nm enhanced with the increase of OTA concentration ranging from 0.05 to 2.0 ng mL^-1, and the limit of detection was calculated to be 0.023 ng mL^-1 (S/N = 3). The developed colorimetric aptasensor has been applied to detect OTA concentration in corn samples with satisfied results.

Bioinspired recognition elements for mycotoxin sensors

Mycotoxins are low molecular weight molecules produced as secondary metabolites by filamentous fungi that can be found as natural contaminants in many foods and feeds. These toxins have been shown to have adverse effects on both human and animal health, and are the cause of significant economic losses worldwide. Sensors for mycotoxin analysis have traditionally applied elements of biological origin for the selective recognition purposes. However, since the 1970s there has been an exponential growth in the use of genetically engineered or synthetic biomimetic recognition elements that allow some of the limitations associated with the use of natural receptors for the analyses of these toxins to be circumvented. This review provides an overview of recent advances in the application of bioinspired recognition elements, including recombinant antibodies, peptides, aptamers, and molecularly imprinted polymers, to the development of sensors for mycotoxins based on different transduction elements. Graphical abstract Novel analytical methods based on bioinspired recognition elements, such as recombinant antibodies, peptides, aptamers, and molecularly imprinted polymers, can improve the detection of mycotoxins and provide better tools than their natural counterparts to ensure food safety.