A reusable aptamer-based evanescent wave all-fiber biosensor for highly sensitive detection of Ochratoxin A

Aptamer melting biosensors for thousands of signaling and regenerating cycles

Due to their recognition abilities and inherent regenerability, aptamers have great potential in biosensing applications. However, effective signal transduction and regeneration strategies are still required. Herein, we develop a melting-based aptamer sensing strategy capable of homogeneous signaling with over 1000 regenerating cycles without significant deterioration of performance. Such melting aptasensors employ melting temperature changes upon target binding as signal readout, and the high temperature involved in the melting process regenerates the aptamers for reuse. This reversible biosensor is reagentless, affordable, and maintenance-free, thus accelerating the real-world applications of aptasensors in continuous monitoring, wearable sensors, unattended operation, and resource-limited areas.

Unpacking the packaged optical fiber bio-sensors: understanding the obstacle for biomedical application

A biosensor is a promising alternative tool for the detection of clinically relevant analytes. Optical fiber as a transducer element in biosensors offers low cost, biocompatibility, and lack of electromagnetic interference. Moreover, due to the miniature size of optical fibers, they have the potential to be used in microfluidic chips and in vivo applications. The number of optical fiber biosensors are extensively growing: they have been developed to detect different analytes ranging from small molecules to whole cells. Yet the widespread applications of optical fiber biosensor have been hindered; one of the reasons is the lack of suitable packaging for their real-life application. In order to translate optical fiber biosensors into clinical practice, a proper embedding of biosensors into medical devices or portable chips is often required. A proper packaging approach is frequently as challenging as the sensor architecture itself. Therefore, this review aims to give an unpack different aspects of the integration of optical fiber biosensors into packaging platforms to bring them closer to actual clinical use. Particularly, the paper discusses how optical fiber sensors are integrated into flow cells, organized into microfluidic chips, inserted into catheters, or otherwise encased in medical devices to meet requirements of the prospective applications.

Plasmonic immunosensors based on spoon-shaped waveguides for fast and on-site ultra-low detection of ochratoxin A in coffee samples

The high toxicity and occurrence of ochratoxin A (OTA) in grains and foods has been a growing concern due to the impacts on health and the economy in many countries. In this sense, simplified devices with high sensitivity and specificity for local monitoring are enthusiastically pursued. In this work, we report for the first time the detection of ochratoxin A in coffee samples using a spoon-shaped waveguide immunosensor. The biosensor was built with the surface of the spoon-shaped waveguide covered by a 60 nm layer of gold to enable the SPR phenomenon. The measurements indicated a linear relationship between the change in the SPR phenomenon values and the OTA concentration in the range from 0.2 ppt to 5 ppt. When analyzed in coffee samples, the biosensor was highly selective and did not suffer matrix interference. The developed biosensor represents a promising analytical device for coffee quality analyses, as it is portable, simple, and suitable for onsite detection of target analytes.

Recent progress in biosensor regeneration techniques

Biosensors are widely used in various applications, from medical diagnostics to environmental monitoring. Their widespread and continuous use necessitates regeneration methods to ensure cost-effectiveness and sustainability. In the realm of advancing human-centric bioelectronics for continuous monitoring, employing these sensors for real-time, in situ detection of biomarkers presents a considerable challenge. This mini-review examines diverse strategies utilized for the regeneration of biosensors, categorizing them based on their underlying mechanisms and discussing representative works. We explore methods ranging from surface engineering/re-functionalization, chemical treatments, allosteric regulation of bioreceptors, to manipulations of electric/magnetic fields, highlighting their working principles and exemplary studies. The advantages of each method, such as simplicity, high regeneration efficiency, and versatility, are discussed alongside their challenges, including degradation over cycles, limited applicability, and potential damage to sensors. As the demand for continuous and real-time biosensing escalates, the development of efficient and reliable regeneration strategies becomes essential. This mini-review offers an overview of the current landscape of biosensor regeneration, aiming to guide future research and innovations in this area.

Effect-Targeted Mapping of Potential Estrogenic Agonists and Antagonists in Wastewater via a Conformation-Specific Reporter-Mediated Biosensor

Wastewater treatment plants (WWTPs) are regarded as the main sources of estrogens that reach the aquatic environment. Hence, continuous monitoring of potential estrogenic-active compounds by a biosensor is an appealing approach. However, existing biosensors cannot simultaneously distinguish and quantify estrogenic agonists and antagonists. To overcome the challenge, we developed an estrogen receptor-based biosensor that selectively screened estrogenic agonists and antagonists by introducing rationally designed agonist/antagonist conformation-specific reporters. The double functional conformation-specific reporters consist of a Cy5.5-labeled streptavidin moiety and a peptide moiety, serving as signal recognition and signal transduction elements. In addition, the conformation recognition mechanism was further validated at the molecular level through molecular docking. Based on the two-step "turn-off" strategy, the biosensor exhibited remarkable sensitivity, detecting 17β-estradiol-binding activity equivalent (E2-BAE) at 7 ng/L and 4-hydroxytamoxifen-binding activity equivalent (4-OHT-BAE) at 91 ng/L. To validate its practicality, the biosensor was employed in a case study involving wastewater samples from two full-scale WWTPs across different treatment stages to map their estrogenic agonist and antagonist binding activities. Comparison with the yeast two-hybrid bioassay showed a strong liner relationship (r2 = 0.991, p < 0.0001), indicating the excellent accuracy and reliability of this technology in real applications.

Recent Progress in Functional-Nucleic-Acid-Based Fluorescent Fiber-Optic Evanescent Wave Biosensors

Biosensors capable of onsite and continuous detection of environmental and food pollutants and biomarkers are highly desired, but only a few sensing platforms meet the "2-SAR" requirements (sensitivity, specificity, affordability, automation, rapidity, and reusability). A fiber optic evanescent wave (FOEW) sensor is an attractive type of portable device that has the advantages of high sensitivity, low cost, good reusability, and long-term stability. By utilizing functional nucleic acids (FNAs) such as aptamers, DNAzymes, and rational designed nucleic acid probes as specific recognition ligands, the FOEW sensor has been demonstrated to be a general sensing platform for the onsite and continuous detection of various targets ranging from small molecules and heavy metal ions to proteins, nucleic acids, and pathogens. In this review, we cover the progress of the fluorescent FNA-based FOEW biosensor since its first report in 1995. We focus on the chemical modification of the optical fiber and the sensing mechanisms for the five above-mentioned types of targets. The challenges and prospects on the isolation of high-quality aptamers, reagent-free detection, long-term stability under application conditions, and high throughput are also included in this review to highlight the future trends for the development of FOEW biosensors capable of onsite and continuous detection.

Group-targeting sulfonamides via an evanescent-wave biosensor based on rational designed coating antigen

In order to effectively monitor a wide variety of sulfonamides residues in the environment, group-targeting immunoassay based on the group-specific antibodies has attracted great attentions, which can realize the detection of a group of contaminants in environment as many as possible even the unrecognized ones. Indirect competitive immunoassay is generally adopted for small molecule detection however the rational design of immobilized coating antigen for improved recognition capability on the solid surface is far from enough. To cover the research gap, we proposed the design criteria of coating antigen for surface-based indirect competitive immunoassay based on the molecular docking. Taking the group-specific antibodies against sulfonamides (SA) as a proof-of-concept, a hapten with a linking arm with 3 methyl groups was selected to synthesize the coating antigen. Through surface immobilization of coating antigen, a portable biosensor for group-targeting immunoassay of sulfonamides was developed and demonstrated excellent performance with detection limits lower than 0.6 μg/L for four SA variants, and the cross-reactivities of 148-215 % relative to sulfadiazine. The recovery rates of SAs in liquid milk ranges from 87 to 97 %, which confirmed the application potential of this method in the determination of SAs. Its capability to measure total SAs in a simple and low-cost way would pave the way for a variety of application fields.

A review of Optical Point-of-Care devices to Estimate the Technology Transfer of These Cutting-Edge Technologies

Despite the remarkable development related to Point-of-Care devices based on optical technology, their difficulties when used outside of research laboratories are notable. In this sense, it would be interesting to ask ourselves what the degree of transferability of the research work to the market is, for example, by analysing the relation between the scientific work developed and the registered one, through patent. In this work, we provide an overview of the state-of-the-art in the sector of optical Point-of-Care devices, not only in the research area but also regarding their transfer to market. To this end, we explored a methodology for searching articles and patents to obtain an indicator that relates to both. This figure of merit to estimate this transfer is based on classifying the relevant research articles in the area and the patents that have been generated from these ones. To delimit the scope of this study, we researched the results of a large enough number of publications in the period from 2015 to 2020, by using keywords "biosensor", "optic", and "device" to obtain the most representative articles from Web of Science and Scopus. Then, we classified them according to a particular classification of the optical PoC devices. Once we had this sampling frame, we defined a patent search strategy to cross-link the article with a registered patent (by surfing Google Patents) and classified them accordingly to the categories described. Finally, we proposed a relative figure called Index of Technology Transference (IoTT), which estimates to what extent our findings in science materialized in published articles are protected by patent.

Evanescent Wave Optical-Fiber Aptasensor for Rapid Detection of Zearalenone in Corn with Unprecedented Sensitivity

Zearalenone (ZEN) is a common mycotoxin pollutant found in agricultural products. Aptamers are attractive recognition biomolecules for the development of mycotoxin biosensors. Even though numerous aptasensors have been reported for the detection of ZEN in recent years, many of them suffer from problems including low sensitivity, low specificity, tedious experimental steps, high-cost, and difficulty of automation. We report here the first evanescent wave optical-fiber aptasensor for the detection of ZEN with unprecedented sensitivity, high specificity, low cost, and easy of automation. In our aptasensor, a 40-nt ZEN-specific aptamer (8Z₃₁) is covalently immobilized on the fiber. The 17-nt fluorophore Cy5.5-labeled complementary DNA strand and ZEN competitively bind with the aptamer immobilized on the fiber, enabling the signal-off fluorescent detection of ZEN. The coating of Tween 80 enhanced both the sensitivity and the reproducibility of the aptasensor. The sensor was able to detect ZEN spiked-in the corn flour extract with a semilog linear detection range of 10 pM-10 nM and a limit of detection (LOD, S/N = 3) of 18.4 ± 4.0 pM (equivalent to 29.3 ± 6.4 ng/kg). The LOD is more than 1000-fold lower than the maximum ZEN residue limits set by China (60 μg/kg) and EU (20 μg/kg). The sensor also has extremely high specificity and showed negligible cross-reactivity to other common mycotoxins. In addition, the sensor was able to be regenerated for 28 times, further decreasing its cost. Our sensor holds great potential for practical applications according to its multiple compelling features.

Rapid screening of aptamers for fluorescent targets by integrated digital PCR and flow cytometry

In this paper, we report the development of a new strategy termed integrated digital PCR-fluorescence activated sorting based SELEX (IFS-SELEX) that enables rapid screening of aptamers against fluorescent targets. Initially, this strategy employs an integrated digital PCR system to amplify each sequence of a preliminarily enriched library, which is obtained by a traditional SELEX method, on the surface of polystyrene beads. Then, the as-prepared beads are incubated with the fluorescent target and then subjected to fluorescence-activated sorting. Since only those sequences with high binding affinity for the target are collected and sequenced, unnecessary analysis of ineligible sequences is avoided by this method, and the aptamer selection process is thereby greatly streamlined. As a proof-of-concept, we applied this strategy for the screening of aptamers against two fluorescent targets, i.e., ciprofloxacin (CFX) and thioflavin T (ThT), and successfully obtained corresponding sequences with low dissociation constants. The binding affinities of aptamers for ThT were well associated with the sorting regions defined in the fluorescence channel of the flow cytometry process. The experimental results demonstrated that the as-designed IFS-SELEX method can serve as a universal platform for rapid, facile, and efficient aptamer selection against various fluorescent targets.

Polydiacetylene vesicles acting as colorimetric sensor for the detection of plantaricin LD1

The interaction of antimicrobial peptides with membrane lipids plays a major role in numerous physiological processes. In this study, polydiacetylene (PDA) vesicles were synthesized using 10, 12-tricosadiynoic acid (TRCDA) and 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). These vesicles were applied as artificial membrane biosensor for the detection of plantaricin LD1 purified from Lactobacillus plantarum LD1. Plantaricin LD1 (200 μg/mL) was able to interact with PDA vesicles by changing the color from blue to red with colorimetric response 30.26 ± 0.59. Nisin (200 μg/mL), used as control, also changed the color of the vesicles with CR% 50.56 ± 0.98 validating the assay. The vesicles treated with nisin and plantaricin LD1 showed increased infrared absorbance at 1411.46 and 1000-1150 cm^-1 indicated the interaction of bacteriocins with phospholipids and fatty acids, respectively suggesting membrane-acting nature of these bacteriocins. Further, microscopic observation of bacteriocin-treated vesicles showed several damages indicating the interaction of bacteriocins. These findings suggest that the PDA vesicles may be used as bio-mimetic sensor for the detection of bacteriocins produced by several probiotics in food and therapeutic applications.

A CRISPR-based and post-amplification coupled SARS-CoV-2 detection with a portable evanescent wave biosensor

The continuing pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, which causes coronavirus disease 2019 (COVID-19), has spread globally and its reliable diagnosis is one of the foremost priorities for protecting public health. Herein a rapid (<1 h), easy-to-implement, and accurate CRISPR-based evanescent wave fluorescence biosensing platform for detection of SARS-CoV-2 is reported. The collateral effect of Cas13a is combined with a universal autonomous enzyme-free hybridization chain reaction (HCR) by designing a cleavage hairpin reporter, which is cleaved upon target recognition, and hence releasing the initiator sequence to trigger the downstream HCR circuits. Detection of HCR assemblies is accomplished by first adsorbing to the desthiobiotin-modified optical fiber, followed by fluorescence emission induced by an evanescent field. Three Cas13a crRNAs targeting the genes of S, N and Orf1ab of SARS-CoV-2 are programmed to specifically target SARS-CoV-2 or broadly detect related coronavirus strains, such as MERS-CoV and SARS-CoV. The HCR amplification coupled Cas13a-based biosensing platform is capable of rapid detection of SARS-CoV-2 with attomolar sensitivity. This method is further validated by adding target RNA of SARS-CoV-2 in negative oropharyngeal swabs. The good discrimination capability of this technique demonstrates its promising potential for point-of-care diagnosis of COVID-19.

Optical Biosensors for Virus Detection: Prospects for SARS-CoV-2/COVID-19

The recent pandemic of the novel coronavirus disease 2019 (COVID-19) has caused huge worldwide disruption due to the lack of available testing locations and equipment. The use of optical techniques for viral detection has flourished in the past 15 years, providing more reliable, inexpensive, and accurate detection methods. In the current minireview, optical phenomena including fluorescence, surface plasmons, surface-enhanced Raman scattering (SERS), and colorimetry are discussed in the context of detecting virus pathogens. The sensitivity of a viral detection method can be dramatically improved by using materials that exhibit surface plasmons or SERS, but often this requires advanced instrumentation for detection. Although fluorescence and colorimetry lack high sensitivity, they show promise as point-of-care diagnostics because of their relatively less complicated instrumentation, ease of use, lower costs, and the fact that they do not require nucleic acid amplification. The advantages and disadvantages of each optical detection method are presented, and prospects for applying optical biosensors in COVID-19 detection are discussed.

A split aptamer (SPA)-based sandwich-type biosensor for facile and rapid detection of streptomycin

As antibiotic pollution is gaining prominence as a global issue, the demand for detection of streptomycin (STR), which is a widely used antibiotic with potential human health and ecological risks, has attracted increasing attention. Aptamer-based biosensors have been developed for the detection of STR in buffers and samples, however, the non-target signals due to the conformational variation of free aptamers possibly affect their sensitivity and stability. In this study, by introducing the STR-specific split aptamer (SPA), a sensitive evanescent wave fluorescent (EWF) biosensor is developed for the sandwich-type based detection of STR. The standard calibration curve obtained for STR has a detection limit of 33 nM with a linear range of 60-526 nM. This biosensor exhibited good selectivity, reliable reusability for at least 100 times measurements, and high recovery rates for spiked water samples; moreover, all detection steps are easy-to-operate and can be completed in 5 min. Therefore, it exhibits great promise for actual on-site environmental monitoring. Additionally, without introducing any other oligonucleotides or auxiliary materials, this SPA-based biosensing method shows potential as a simple, sensitive, and low-cost manner for the detection of other small molecular targets.

Reusable Cyclodextrin-Based Electrochemical Platform for Detection of trans-Resveratrol

A reusable sensor architecture, through the combination of self-assembled monolayers and cyclodextrin supramolecular interactions, is demonstrated for class recognition of hydrophobic analytes demonstrated with trans-resveratrol. The reloadable sensor is based on reversible immobilization of α-cyclodextrin on polyethylene glycol surface. α-cyclodextrins complexes with polyethylene glycols and causes the polymer chains to change their surface configuration. The reproducibility and stability of the sur-face, in the detection of nanomolar concentrations of trans-resveratrol, can be demonstrated by electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and Attenuated total reflectance-Fourier transform infrared spectroscopy. We propose that during sensor operation, α-cyclodextrin decouples from the poly-ethylene glycol surface to complex with trans-resveratrol in solution, and after use, the surface regeneration is conducted with a simple α-cyclodextrin soak. To test the nonspecific response, the sensor was also tested with trans-resveratrol spiked human urine.

Ultra-low pressure cellulose-based nanofiltration membrane fabricated on layer-by-layer assembly for efficient sodium chloride removal

Cellulose is a renewable, biodegradable, biocompatible, and sustainable material. A bamboo cellulose-based nanofiltration membrane (LBL-NF-CS/BCM) was prepared with a combination of layer-by-layer assembly and spraying methods. The chemical structure, morphology, and surface charge of the resultant LBL-NF-CS/BCM composite membranes were characterized based on Thermo Gravimetric Analysis (TGA), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy Scanning (XPS). The nanofiltration performance of the LBL-NF-CS/BCM composite membranes was evaluated using 500 ppm NaCl solutions under 0.3 MPa pressure. It was found that the LBL-NF-CS/BCM composite membranes had a rejection rate of about 36.11 % against a 500 ppm NaCl solution under the conditions tested, and membrane flux of about 12.08 L/(m² h) was reached. The combined layer-by-layer assembly and spraying provides a scalable and convenient process concept for nanofiltration membrane fabrication.

Aptamer-integrated nucleic acid circuits for biosensing: Classification, challenges and perspectives

Owing to their high programmability and modularity, autonomous enzyme-free nucleic acid circuits are attracting ever-growing interest as signal amplifiers with potential applications in developing highly sensitive biosensing techniques. Besides nucleic acid input, the biosensing scope of aptamer-integrated nucleic acids could be further expanded to non-nucleic targets by integrating nucleic acid circuits with aptamers-a class of functional oligonucleotides with binding capabilities toward specific targets. By coupling upstream target recognition with downstream signal amplification, aptamer-integrated nucleic acid circuits enable aptasensors with increased sensitivity and enhanced performances, which may act as powerful tools in various fields including environment monitoring, personal care, clinical diagnosis, etc. In designing aptamer-integrated nucleic acid circuits, smart integration between aptamer and nucleic acid circuits plays a crucial role in developing reliable circuits with good performances. To date, although there are plenty of published researches adopting aptamer-integrated nucleic acid circuits as amplifiers in biosensing systems, deep discussion or systematic review on rational design strategies for aptamer-integrated nucleic acid circuits is still lacking. To fill this gap, rational aptamer-nucleic acid circuits integration modes were classified and summarized for the first time based on reviewing the state of art of existing aptamer-integrated nucleic acid circuits. Moreover, theoretical updates in nucleic acid circuits designs and major challenges to be overcome in developing highly sensitive aptamer-integrated nucleic acids based biosensing systems are discussed in this review.

Fiber-Optic Skew Ray Sensors

The evanescent fields along multimode fibers are usually relatively weak. To enhance the sensitivity of the resulting sensors, skew rays have been exploited for their larger number of total internal reflections and their more comprehensive spread over the fiber surface. The uniform distribution of light-matter interactions across the fiber surface facilitates high sensitivity through an increased interaction area, while mitigating the risk of laser-induced coating-material damage and photobleaching. Power-dependent measurements are less susceptible to temperature effects than interferometric techniques, and place loose requirements on the laser source. This review highlights the key developments in this area, while discussing the benefits, challenges as well as future development.

Triple functional small-molecule-protein conjugate mediated optical biosensor for quantification of estrogenic activities in water samples

Establishing biosensors to map a comprehensive picture of potential estrogen-active chemicals remains challenging and must be addressed. Herein, we describe an estrogen receptor (ER)-based evanescent wave fluorescent biosensor by using a triple functional small-molecule-protein conjugate as a signal probe for the determination of estrogenic activities in water samples. The signal probe, consisting of a Cy5.5-labelled streptavidin (STV) moiety and a 17β-estradiol (E₂) moiety, acts simultaneously as signal conversion, signal recognition and signal report elements. When xenoestrogens compete with the E₂ moiety of conjugate in binding to the ER, the unbound conjugates are released, and their STV moiety binds with desthiobiotin (DTB) modified on the optical fiber via the STV-DTB affinity interactions. Signal probe detection is accomplished by fluorescence emission induced by an evanescent field, which positively relates with the estrogenic activities in samples. Quantification of estrogenic activity expressed as E₂ equivalent concentration (EEQ) can be achieved with a detection limit of 1.05 μg/L EEQ by using three times standard deviation of the mean blank values and a linear calibration range from 20.8 to 476.7 μg/L EEQ. The optical fiber system is robust enough for hundreds of sensing cycles. The biosensor-based determination of estrogenic activities in wastewater samples obtained from a full-scale wastewater treatment plant is consistent with that measured by the two-hybrid recombinant yeast bioassay.

Multitag-Regulated Cascade Reaction: A Generalizable Ultrasensitive MicroRNA Biosensing Approach for Cancer Prognosis

Ultrasensitive PCR-free microRNA (miR) analysis based on biosensors with enzyme-free nucleic acid amplification and reusable surface has great clinical significance in cancer prognosis. However, building such a biosensing strategy has long been challenging due to uncontrollable miR-triggered cascade amplifiers and insufficient sensing surface regeneration capability. To meet the challenge, for the first time, a general approach, named enzyme-free multitag-regulated cascade reaction (MCR), is developed to fabricate reliable trace miR biosensors. As a proof of concept, miR let-7a is detected on an evanescent wave fluorescent optical-fiber biosensing platform. The size and morphology of well-formed MCR assemblies (∼1 μm in length) are characterized by atomic force microscopy. This MCR method achieves a 30 000-fold improved sensitivity (detection limit 0.8 fM) compared to the MCR-free system and can detect abnormal urinary miR levels in lung cancer patients. Moreover, the biosensor is robust enough to be reused for over 100 cycles, which greatly reduces the cost of single detection. In sum, MCR is developed as a generalizable ultrasensitive miR biosensing approach for cancer prognosis, which opens a broad field for facile enzyme-free biosensing applications by nucleic acid assembling regulation.

A Novel Graphene Oxide-Based Aptasensor for Amplified Fluorescent Detection of Aflatoxin M1 in Milk Powder

In this paper, a rapid and sensitive fluorescent aptasensor for the detection of aflatoxin M1 (AFM1) in milk powder was developed. Graphene oxide (GO) was employed to quench the fluorescence of a carboxyfluorescein-labelled aptamer and protect the aptamer from nuclease cleavage. Upon the addition of AFM1, the formation of an AFM1/aptamer complex resulted in the aptamer detaching from the surface of GO, followed by the aptamer cleavage by DNase I and the release of the target AFM1 for a new cycle, which led to great signal amplification and high sensitivity. Under optimized conditions, the GO-based detection of the aptasensor exhibited a linear response to AFM1 levels in a dynamic range from 0.2 to 10 μg/kg, with a limit of detection (LOD) of 0.05 μg/kg. Moreover, the developed aptasensor showed a high specificity towards AFM1 without interference from other mycotoxins. In addition, the technique was successfully applied for the detection of AFM1 in infant milk powder samples. The aptasensor proposed here offers a promising technology for food safety monitoring and can be extended to various targets.

A simple aptamer-based fluorescent aflatoxin B1 sensor using humic acid as quencher

This work described a fluorometric and aptamer-based assay for aflatoxin B1 (AFB1). Aptamer-modified carbon dots (DNA-CDs) were first synthesized as fluorescence probes, then reacted with humic acid (HAs) which acted as quencher of the blue fluorescence of the CDs. It was found that HAs can readily adsorb ssDNA aptamers due to the presence of a rich surface chemistry (quinoidal units, aromatic rings and sugar moieties). This resulted in quenching of the fluorescence of the CDs (with excitation/emission peaks at 360/450 nm), probably due to π interactions. If the nanoprobe was reacted with AFB1, the DNA-CDs detached from the HAs and fluorescence was restored. Under optimized experimental conditions, the assay had a linear response in the 0.1-0.8 ng mL^-1 AFB1 concentration range, with a low limit of detection of 70 pg mL^-1.

Exonuclease I-assisted fluorescent method for ochratoxin A detection using iron-doped porous carbon, nitrogen-doped graphene quantum dots, and double magnetic separation

In this paper, a fluorescent method was developed for ochratoxin A (OTA) detection that uses iron-doped porous carbon (MPC) and aptamer-functionalized nitrogen-doped graphene quantum dots (NGQDs-Apt) as probes. In this method, the adsorbance of the NGQDs-Apt on the MPC due to a π-π interaction between the aptamer and the MPC results in the quenching of the fluorescence of the NGQDs-Apt. However, since OTA interacts strongly with the aptamer, the presence of OTA leads to the detachment of the NGQDs-Apt from the MPC, resulting in the resumption of fluorescence from the NGQDs-Apt. When exonuclease I (Exo I) is also added to the solution, this exonuclease specifically digests the aptamer, leading to the release of the OTA back into the solution. This free OTA then interacts with another MPC-NGQDs-Apt system, inducing the release of more NGQDs into the solution, which enhances the fluorescent intensity compared to that of the system with no Exo I. Utilizing this behavior of OTA in the presence of NGQDs-Apt, it was possible to detect concentrations of OTA ranging from 10 to 5000 nM, with a limit of detection of 2.28 nM. Our method was tested by applying it to the detection of OTA in wheat and corn samples. This method has four advantages: (1) the magnetic porous carbon is easy to prepare, its porosity enhances its loading capacity for NGQDs, it highly efficiently quenches the fluorescence of the NGQDs, and its magnetic properties facilitate the separation of the MPC from other species in solution; (2) applying double magnetic separation decreases the background signal; (3) Exo I digests the free aptamer effectively, which allows the resulting free OTA to induce the release of more NGQDs-Apt, ultimately enhancing the fluorescent signal; and (4) the proposed method presented high sensitivity and a wide linear detection range. This method may prove helpful in food safety analysis and new biosensor development (achieved by using different aptamer sequences to that used in the present work). Graphical abstract Exonuclease I (Exo I)-assisted fluorescent method for ochratoxin A (OTA) detection using magnetic porous carbon (MPC), nitrogen-doped graphene quantum dots (NGQDs), and double magnetic separation.

Nucleic acid functionalized fiber optic probes for sensing in evanescent wave: optimization and application

Nucleic acid functionalized evanescent wave fiber optic (EWFO) biosensors have attracted much attention due to their remarkable advantages in both device configuration and sensing performance. One critical technique in EWFO biosensor fabrication is its surface modification, which requires (1) minimal nonspecific adsorption and (2) high-quality DNA immobilization to guarantee satisfactory sensing performances. Focusing on these two requirements, a series of optimizations have been conducted in this work to develop reliable DNA-functionalized EWFO probes. Firstly, the surface planeness of EWFO probes were found to be greatly improved by a novel HF/HNO₃ mixture etching solution. Both atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were conducted to investigate the morphological structures and surface chemical compositions. Besides, EWFO sensing performances adopting moderate immobilization of irrelevant DNA were investigated for optimization purposes. Furthermore, a split aptamer based sandwich-type EWFO sensor was developed using adenosine (Ade) as the model target (LOD = 25 μM). To the best of our knowledge, this study is the first case to focus on the optimization of etching solution compositions in the fabrication of combination tapered fibers, which provides experimental basis for the understanding of the silica-etching mechanism using HF/HNO₃ mixture solution and may further inspire related researches.

Reliable DNA-functionalized optic probes for sensing in evanescent wave have been developed based a series of optimizations on the etching solution and immobilization chemistry.

Rapid detection of cocaine using aptamer-based biosensor on an evanescent wave fibre platform

The rapid detection of cocaine has received considerable attention because of the instantaneous and adverse effects of cocaine overdose on human health. Aptamer-based biosensors for cocaine detection have been well established for research and application. However, reducing the analytic duration without deteriorating the sensitivity still remains as a challenge. Here, we proposed an aptamer-based evanescent wave fibre (EWF) biosensor to rapidly detect cocaine in a wide working range. At first, the aptamers were conjugated to complementary DNA with fluorescence tag and such conjugants were then immobilized on magnetic beads. After cocaine was introduced to compete against the aptamer-DNA conjugants, the released DNA in supernatant was detected on the EWF platform. The dynamic curves of EWF signals could be interpreted by the first-order kinetics and saturation model. The semi-log calibration curve covered a working range of 10-5000 µM of cocaine, and the limit of detection was approximately 10.5 µM. The duration of the full procedure was 990 s (16.5 min), and the detection interval was 390 s (6.5 min). The specified detection of cocaine was confirmed from four typical pharmaceutic agents. The analysis was repeated for 50 cycles without significant loss of sensitivity. Therefore, the aptamer-based EWF biosensor is a feasible solution to rapidly detect cocaine.

Affinity Biosensors for Detection of Mycotoxins in Food

This chapter reviews recent achievements in methods of detection of mycotoxins in food. Special focus is on the biosensor technology that utilizes antibodies and nucleic acid aptamers as receptors. Development of biosensors is based on the immobilization of antibodies or aptamers onto various conventional supports like gold layer, but also on nanomaterials such as graphene oxide, carbon nanotubes, and quantum dots that provide an effective platform for achieving high sensitivity of detection using various physical methods, including electrochemical, mass sensitive, and optical. The biosensors developed so far demonstrate high sensitivity typically in subnanomolar limit of detection. Several biosensors have been validated in real samples. The sensitivity of biosensors is similar and, in some cases, even better than traditional analytical methods such as ELISA or chromatography. We believe that future trends will be focused on improving biosensor properties toward practical application in food industry.

Chimeric DNA/LNA-based biosensor for the rapid detection of African swine fever virus

African swine fever (ASF) virus is a DNA virus responsible for a severe haemorrhagic fever in pigs, which (still in the absence of vaccination strategies) results in high mortality rates. Herein, we present a biosensor-based method for the detection of ASF viral DNA in the blood of pigs. The biosensor exploits a single-strand DNA probe with locked nucleic acid nucleotides (LNA) substitutions as the complementary recognition element for the conserved region of vp72 gene of ASF virus. The biosensor was calibrated using qPCR-quantified ASF viral DNA extracted from the blood of pigs experimentally infected with the virulent Italian isolate 49/08, genotype I. Globally, the proposed biosensor showed good sensitivity and specificity, with the limits of detection (LOD) and quantification (LOQ) being 178 and 245 copies/μL of genomic ASF viral DNA, respectively. The reversible nature of the interaction between the DNA/LNA probe and the target DNA sequence granted multiple rapid analyses, with up to 40 analyses per single surface possible, and a single test requiring approximately 5 min. When applied to non-amplified DNA extracts from the blood of field-infected pigs, the assay discriminated between ASFV-infected and ASFV non-infected animals, and allowed the rapid quantification of ASF viral DNA, with values falling in the range 373-1058 copies/μL of genomic ASFV DNA. In this range, excellent correlation was observed between the results of this biosensor and OIE-approved qPCR. This method represents a promising screening assay for preliminary ASF diagnosis, having the major advantages in the relative rapidity, ease-of-use, the reusability of the sensing surface, and low cost per single test.

Fluorescence method for quickly detecting ochratoxin A in flour and beer using nitrogen doped carbon dots and silver nanoparticles

In this paper, a FRET (Forster resonance energy transfer) based fluorescence method was developed for the quickly detection of ochratoxin A (OTA) in agricultural products (e.g., flour and beer). A highly fluorescent nitrogen doped carbon dots (CD) were served as energy donor, the DNA and MCH (6-mercapto-1-hexanol) modified Ag nanoparticles were served as energy acceptor in the FRET system. OTA can be detected in a concentration range between 10 and 5000 nM, the limit of detection is 8.7 nM. This method has three advantages: (1) an enhanced fluorescent intensity can be acquired by utilizing the nitrogen doped CD synthesized by one-step approach without sophisticated modification of nanoparticles; (2) OTA detection was accomplished quickly (less than 30 min) by using MCH as assistant molecule; (3) an extended OTA detection linear range was acquired, which may facilitate the OTA detection in real agricultural samples, and is helpful for solving food safety problems.

An Optical Biosensor-Based Quantification of the Microcystin Synthetase A Gene: Early Warning of Toxic Cyanobacterial Blooming

The monitoring and control of toxic cyanobacterial strains, which can produce microcystins, is critical to protect human and ecological health. We herein reported an optical-biosensor-based quantification of the microcystin synthetase A (mcyA) gene so as to discriminate microcystin-producing strains from nonproducing strains. In this assay, the mcyA-specific ssDNA probes were designed in silico with an on-line tool and then synthesized to be covalently immobilized on an optical-fiber surface. Production of fluorescently modified target DNA fragment amplicons was accomplished through the use of Cy5-tagged deoxycytidine triphosphates (dCTPs) in the polymerase chain reaction (PCR) method, which resulted in copies with internally labeled multiple sites per DNA molecule and delivered great sensitivity. With a facile surface-based hybridization process, the PCR amplicons were captured on the optical-fiber surface and were induced by an evanescent-wave field into fluorescence emission. Under the optimum conditions, the detection limit was found to be 10 pM (S/N ratio = 3) and equaled 10³ gene copies/mL. The assay was triumphantly demonstrated for PCR amplicons of mcyA detection and showed satisfactory stability and reproducibility. Moreover, the sensing system exhibited excellent selectivity with quantitative spike recoveries from 87 to 102% for M. aeruginosa species in the mixed samples. There results confirmed that the method would serve as an accurate, cost-effective, and rapid technique for in-field testing of toxic Microcystis sp. in water, giving early information for water quality monitoring against microcystin-producing cyanobacteria.

Evanescent wave aptasensor for continuous and online aminoglycoside antibiotics detection based on target binding facilitated fluorescence quenching

The biosensors capable for on-site continuous and online monitoring of pollutants in environment are highly desired due to their practical importance and convenience. The group specific detection of pollutants is especially attractive due to the diversity of environmental pollutants. Here we devise an evanescent wave aptasensor based on target binding facilitated fluorescence quenching (FQ-EWA) for the online continuous and group-specific detection of aminoglycoside antibiotics (AMGAs). In FQ-EWA, a fluorophore labeled DNA aptamer selected against kanamycin was used for both the target recognition in solution and signal transduction on optical fiber of EWA. The aptamers form multiple-strand complex (M-Apt) in the absence of AMGAs. The binding between AMGA and the aptamer disrupts M-Apt and leads to the formation of AMGA -aptamer complex (AMGA-Apt). The photo-induced electron transfer between the fluorophore and AMGA partially quenches the fluorescence of AMGA-Apt. The structure-selective absorption of AMGA-Apt over M-Apt on the graphene oxide further quenches the fluorescence of AMGA-Apt. Meanwhile, the unbound aptamers in solution assemble with the unlabeled aptamers immobilized on the fiber to form M-Apt. The amount of M-Apt on the fiber is inversely proportional to the concentration of AMGAs, enabling the signal-off detection of AMGAs from 200nM to 200μM with a detection limit of 26nM. The whole detection process is carried out in an online mode without any offline operation, providing a great benefit for system automation and miniaturization. FQ-EWA also shows great surface regeneration capability and enables the continuous detection more than 60 times.

Ultrasensitive detection of ochratoxin A using aptasensors

Regarding teratogenic, carcinogenic, and immunotoxic nature of ochratoxin A (OTA), selective and sensitive monitoring of this molecule in food samples is of great importance. In recent years, various methods have been introduced for detection of OTA. However, they are usually time-consuming, labor-intensive and expensive. Therefore, these parameters limited their usage. The emerging method of detection, aptasensor, has attracted more attention for OTA detection, due to distinctive advantages including high sensitivity, selectivity and simplicity. In this review, the new developed aptasensors for detection of OTA have been investigated. We also highlighted advantages and disadvantages of different types of OTA aptasensors. This review also takes into consideration the goal to find out which designs are the most rational ones for highly sensitive detection of OTA.

Facile screening of potential xenoestrogens by an estrogen receptor-based reusable optical biosensor

The apparent increase in hormone-induced cancers and disorders of the reproductive tract has led to a growing demand for new technologies capable of screening xenoestrogens. We reported an estrogen receptor (ER)-based reusable fiber biosensor for facile screening estrogenic compounds in environment. The bioassay is based on the competition of xenoestrogens with 17β-estradiol (E₂) for binding to the recombinant receptor of human estrogen receptor α (hERα) protein, leaving E₂ free to bind to fluorophore-labeled anti-E₂ monoclonal antibody. Unbound anti-E₂ antibody then binds to the immobilized E₂-protein conjugate on the fiber surface, and is detected by fluorescence emission induced by evanescent field. As expected, the stronger estrogenic activity of xenoestrogen would result in the weaker fluorescent signal. Three estrogen-agonist compounds, diethylstilbestrol (DES), 4-n-nonylphenol (NP) and 4-n-octylphenol (OP), were chosen as a paradigm for validation of this assay. The rank order of estrogenic potency determined by this biosensor was DES>OP>NP, which were consistent with the published results in numerous studies. Moreover, the E₂-protein conjugate modified optical fiber was robust enough for over 300 sensing cycles with the signal recoveries ranging from 90% to 100%. In conclusion, the biosensor is reusable, reliable, portable and amenable to on-line operation, providing a facile, efficient and economical alternative to screen potential xenoestrogens in environment.

A sensitive and disposable indium tin oxide based electrochemical immunosensor for label-free detection of MAGE-1

MAGE-1 (MAGE, for melanoma antigen), was identified by virtue of its processing and cell surface expression as a tumor-specific peptide bound to major histocompatibility complexes which was reactive with autolytic T cells. 3-Glycidoxypropyltrimethoxysilane (3-GOPS) is frequently employed for the preparation of dense heterometal hybrid polymers which are used, e.g., for hard coatings of organic polymers and contact lens materials in the optical industry. In this study, we have improved a new immunological biosensor with indium tin oxide (ITO). Then, Anti-MAGE-1 antibody was covalently immobilized with 3-GOPS which formed a self-assembled monolayers (SAMs) on modified ITO electrodes. Analytical characteristics such as square wave voltammetry, linear determination range, repeatability, reproducibility and regeneration of biosensors are determined. All characterization steps are monitored by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV). The developed biosensor has wide determination range (0.5fg-15fg/mL). To investigate long shelf life of the fabricated biosensor, the immunosensors were stored at 4°C for periods ten weeks. Futhermore, binding kinetics of MAGE1 to antiMAGE-1 is monitored by single frequency technique in real time. Additionally, Kramer's-Kronig transform was used to understand whether the impedance spectra of biosensor system are affected from the variation that occurred because of external factor. Morphological characteristics of constructed biosensor were observed by scanning electron microscopy. Real human serum samples were also analyzed by the proposed biosensor, successfully. A commercial ELISA kit was also used as a reference method to validate the results obtained by the biosensor. Finally, this biosensor was tried in real blood sample and that showed it could be utilized in clinical applications. This biosensor can be preferred due to it has a wide linear range and it can be prepared easily.

Isoelectric Bovine Serum Albumin: Robust Blocking Agent for Enhanced Performance in Optical-Fiber Based DNA Sensing

Surface blocking is a well-known process for reducing unwanted nonspecific adsorption in sensor fabrication, especially important in the emerging field where DNA/RNA applied. Bovine serum albumin (BSA) is one of the most popular blocking agents with an isoelectric point at pH 4.6. Although it is widely recognized that the adsorption of a blocking agent is strongly affected by its net charge and the maximum adsorption is often observed under its isoelectric form, BSA has long been perfunctorily used for blocking merely in neutral solution, showing poor blocking performances in the optical-fiber evanescent wave (OFEW) based sensing toward DNA target. To meet this challenge, we first put forward the view that isoelectric BSA (iep-BSA) has the best blocking performance and use an OFEW sensor platform to demonstrate this concept. An optical-fiber was covalently modified with amino-DNA, and further coupled with the optical system to detect fluorophore labeled complementary DNA within the evanescent field. A dramatic improvement in the reusability of this DNA modified sensing surface was achieved with 120 stable detection cycles, which ensured accurate quantitative bioassay. As expected, the iep-BSA blocked OFEW system showed enhanced sensing performance toward target DNA with a detection limit of 125 pM. To the best of our knowledge, this is the highest number of regeneration cycles ever reported for a DNA immobilized optical-fiber surface. This study can also serve as a good reference and provide important implications for developing similar DNA-directed surface biosensors.

Utilization of unmodified gold nanoparticles for label-free detection of mercury (II): Insight into rational design of mercury-specific oligonucleotides

Colorimetric detection of mercury (II) with the use of DNA oligonucleotides and unmodified gold nanoparticles (AuNPs) as indicators has been extensively studied. This study provides in-depth insights into the rational design of mercury-specific oligonucleotides (MSO) in the biosensing system. The leftover bases of MSO, as a result of the formation of T-Hg²⁺-T base pairs, can adsorb on the AuNPs and hinder their aggregation at concentrations of salt. This phenomenon was directly verified by the changes in particle sizes characterized by dynamic light scattering for the first time. Based on these findings, we proposed a rational design for the MSO with approximately 20-fold improvement in detection sensitivity. The detection limit of the proposed assay decreased to 15nM with a linear working range from 50nM to 300nM for Hg²⁺. The cross-reactivity against eight other metal ions was negligible compared with the response to Hg²⁺. Considering the diverse applications of AuNPs with oligonucleotides, this study can serve as a good reference and provides important implications in sensing and DNA-directed nanoparticle assembly.

Conformational Changes of Enzymes and Aptamers Immobilized on Electrodes

Conformation constitutes a vital property of biomolecules, especially in the cases of enzymes and aptamers, and is essential in defining their molecular recognition ability. When biomolecules are immobilized on electrode surfaces, it is very important to have a control on all the possible conformational changes that may occur, either upon the recognition of their targets or by undesired alterations. Both enzymes and aptamers immobilized on electrodes are susceptible to conformational changes as a response to the nature of the charge of the surface and of the surrounding environment (pH, temperature, ionic strength, etc.). The main goal of this review is to analyze how the conformational changes of enzymes and aptamers immobilized on electrode surfaces have been treated in reports on biosensors and biofuel cells. This topic was selected due to insufficient information found on the actual conformational changes involved in the function of these bioelectrochemical devices despite its importance.

Voltammetric Aptasensor Based on Magnetic Beads Assay for Detection of Human Activated Protein C

Aptamers are defined as new generation of nucleic acids, which has recently presented promising specifications over to antibodies. An increasing number of electrochemical studies related to aptamer-based sensors, so-called aptasensors have been introduced in the literature. Herein, the interaction between human activated protein C (APC) and its cognate DNA aptamer (DNA APT) was performed at the surface of magnetic beads (MBs), followed by voltammetric detection using disposable graphite electrodes (PGEs).

A qPCR aptasensor for sensitive detection of aflatoxin M1

Aflatoxin M1 (AFM1), one of the most toxic mycotoxins, imposes serious health hazards. AFM1 had previously been classified as a group 2B carcinogen [1] and has been classified as a group 1 carcinogen by the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) [2]. Determination of AFM1 thus plays an important role for quality control of food safety. In this work, a sensitive and reliable aptasensor was developed for the detection of AFM1. The immobilization of aptamer through a strong interaction with biotin-streptavidin was used as a molecular recognition element, and its complementary ssDNA was employed as the template for a real-time quantitative polymerase chain reaction (RT-qPCR) amplification. Under optimized assay conditions, a linear relationship (ranging from 1.0 × 10(-4) to 1.0 μg L(-1)) was achieved with a limit of detection (LOD) down to 0.03 ng L(-1). In addition, the aptasensor developed here exhibits high selectivity for AFM1 over other mycotoxins and small effects from cross-reaction with structural analogs. The method proposed here has been successfully applied to quantitative determination of AFM1 in infant rice cereal and infant milk powder samples. Results demonstrated that the current approach is potentially useful for food safety analysis, and it could be extended to a large number of targets.

A fluorescent aptasensor based on a DNA pyramid nanostructure for ultrasensitive detection of ochratoxin A

Analytical techniques for detection of ochratoxin A (OTA) in food products and blood serum are of great significance. In this study, a fluorescent aptasensor was developed for sensitive and specific detection of OTA, based on a DNA pyramid nanostructure (DPN) and PicoGreen (PG) dye. The designed aptasensor inherits characteristics of DPN, such as high stability and capacity for PG loading. PG, as a fluorescent dye, could bind to double-stranded DNA (dsDNA). In the absence of OTA, the pyramid structure of DPN remains intact, leading to a very strong fluorescence emission. Because of higher affinity of aptamer for its target relative to its complementary strand, upon addition of target, the pyramid structure of DPN is disassembled, leading to a weak fluorescence emission. The presented aptasensor showed high specificity toward OTA with a limit of detection (LOD) as low as 0.135 nM. Besides, the designed sensing strategy was successfully utilized to recognize OTA in serum and grape juice with LODs of 0.184 and 0.149 nM, respectively.

Evanescent wave fluorescence biosensors: Advances of the last decade

Biosensor development has been a highly dynamic field of research and has progressed rapidly over the past two decades. The advances have accompanied the breakthroughs in molecular biology, nanomaterial sciences, and most importantly computers and electronics. The subfield of evanescent wave fluorescence biosensors has also matured dramatically during this time. Fundamentally, this review builds on our earlier 2005 review. While a brief mention of seminal early work will be included, this current review will focus on new technological developments as well as technology commercialized in just the last decade. Evanescent wave biosensors have found a wide array applications ranging from clinical diagnostics to biodefense to food testing; advances in those applications and more are described herein.

A novel fluorescent aptasensor based on gold and silica nanoparticles for the ultrasensitive detection of ochratoxin A

Analytical approaches for the detection and quantitation of ochratoxin A (OTA) in blood serum and food products are high in demand. In this study, a fluorescent aptamer-based sensor (aptasensor) is developed for the selective and sensitive detection of OTA, based on a complementary strand of aptamer (CS) and two types of nanoparticles, gold nanoparticles (AuNPs) and silica nanoparticles (SNPs) coated with streptavidin. The fabricated aptasensor inherits the characteristics of SNPs, as enhancers of fluorescence intensity; AuNPs, such as large surface area and unique optical properties; and high affinity of the aptamer toward its target compared to its CS. In the absence of OTA, no FAM and biotin-labeled CS is in the environment of the SNPs coated with streptavidin, which leads to no fluorescence emission. In the presence of the target, an FAM and biotin-labeled CS-SNPs coated with streptavidin conjugate is formed, thus resulting in a very strong fluorescence emission. The designed fluorescent aptasensor exhibits high selectivity toward OTA with a limit of detection (LOD) as low as 0.098 nM. Furthermore, the fabricated aptasensor was successfully applied for the detection of OTA in grape juice and serum with LODs of 0.113 and 0.152 nM, respectively.