Evanescent wave fluorescence biosensors: Advances of the last decade

CRISPR/Cas12a powered air-displacement enhanced evanescent wave fluorescence fiber-embedded microfluidic biochip for nucleic acid amplification-free detection of Escherichia coli O157:H7

Simple yet ultrasensitive and contamination-free quantification of environmental pathogenic bacteria is in high demand. In this study, we present a portable clustered regularly interspaced short palindromic repeats-associated protein 12a (CRISPR/Cas12a) powered Air-displacement enhanced Evanescent wave fluorescence Fiber-embedded microfluidic Biochip (AEFB) for the high-frequency and nucleic acid amplification-free ultrasensitive detection of Escherichia coli O157:H7. The performance of AEFB was dramatically enhanced upon employing a simple air-solution displacement process. Theoretical assays demonstrated that air-solution displacement significantly enhances evanescent wave field intensity on the fiber biosensor surface and increases the V-number in tapered fiber biosensors. Consequently, light-matter interaction is strengthened, and fluorescence coupling and collection efficiency are improved, considerably enhancing sensitivity. By integrating the CRISPR biosensing mechanism, AEFB facilitated rapid, accurate, nucleic acid amplification-free detection of E.coli O157:H7 with polymerase chain reaction (PCR)-level sensitivity (176 cfu/mL). To validate its practicality, AEFB was used to detect E.coli O157:H7 in surface water and wastewater. Comparison with RT-PCR showed a strong linear relationship (R2 = 0.9871), indicating the excellent accuracy and reliability of this technology in real applications. AEFB is highly versatile and can be easily extended to detect other pathogenic bacteria, which will significantly promote the high-frequency assessment and early-warning of bacterial contamination in aquatic environments.

Functional Optical Fiber Sensors Detecting Imperceptible Physical/Chemical Changes for Smart Batteries

The battery technology progress has been a contradictory process in which performance improvement and hidden risks coexist. Now the battery is still a "black box", thus requiring a deep understanding of its internal state. The battery should "sense its internal physical/chemical conditions", which puts strict requirements on embedded sensing parts. This paper summarizes the application of advanced optical fiber sensors in lithium-ion batteries and energy storage technologies that may be mass deployed, focuses on the insights of advanced optical fiber sensors into the processes of one-dimensional nano-micro-level battery material structural phase transition, electrolyte degradation, electrode-electrolyte interface dynamics to three-dimensional macro-safety evolution. The paper contributes to understanding how to use optical fiber sensors to achieve "real" and "embedded" monitoring. Through the inherent advantages of the advanced optical fiber sensor, it helps clarify the battery internal state and reaction mechanism, aiding in the establishment of more detailed models. These advancements can promote the development of smart batteries, with significant importance lying in essentially promoting the improvement of system consistency. Furthermore, with the help of smart batteries in the future, the importance of consistency can be weakened or even eliminated. The application of advanced optical fiber sensors helps comprehensively improve the battery quality, reliability, and life.

Photonic-Plasmonic Coupling Enhanced Fluorescence Enabling Digital-Resolution Ultrasensitive Protein Detection

Assays utilizing fluorophores are common throughout life science research and diagnostics, although detection limits are generally limited by weak emission intensity, thus requiring many labeled target molecules to combine their output to achieve higher signal-to-noise. We describe how the synergistic coupling of plasmonic and photonic modes can significantly boost the emission from fluorophores. By optimally matching the resonant modes of a plasmonic fluor (PF) nanoparticle and a photonic crystal (PC) with the absorption and emission spectrum of the fluorescent dye, a 52-fold improvement in signal intensity is observed, enabling individual PFs to be observed and digitally counted, where one PF tag represents one detected target molecule. The amplification can be attributed to the strong near-field enhancement due to the cavity-induced activation of the PF, PC band structure-mediated improvement in collection efficiency, and increased rate of spontaneous emission. The applicability of the method by dose-response characterization of a sandwich immunoassay for human interleukin-6, a biomarker used to assist diagnosis of cancer, inflammation, sepsis, and autoimmune disease is demonstrated. A limit of detection of 10 fg mL-1 and 100 fg mL-1 in buffer and human plasma respectively, is achieved, representing a capability nearly three orders of magnitude lower than standard immunoassays.

Selection of regioselective DNA aptamer for detection of homocysteine in nondeproteinized human plasma

Small molecule-binding aptamers often suffer from high cross reactivity to structure analogues in biological samples, limiting their value for clinical diagnosis. Herein, we present a method to overcome this issue, by performing binding-inhibited organic reaction-based regioselective selection of aptamers against homocysteine (Hcy), which is a marker for diagnosing many disorders including stroke and Alzheimer's. This approach has led to isolation of a DNA aptamer that binds to the alkane thiol chain of Hcy with exceptional specificity against cysteine. It also binds with oxidized Hcy at weaker affinity. Using this new aptamer, we produced a reusable fluorescent optical fiber aptasensor for direct and validated detection of both free and total Hcy in nondeproteinized patient plasma in the diagnostic concentration range. The binding site-specific aptamer selection and optical-fiber-sensing strategy can expand the practical utility of aptamers in clinical diagnosis.

Rapid, high-sensitivity detection of biomolecules using dual-comb biosensing

Rapid, sensitive detection of biomolecules is important for biosensing of infectious pathogens as well as biomarkers and pollutants. For example, biosensing of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is still strongly required for the fight against coronavirus disease 2019 (COVID-19) pandemic. Here, we aim to achieve the rapid and sensitive detection of SARS-CoV-2 nucleocapsid protein antigen by enhancing the performance of optical biosensing based on optical frequency combs (OFC). The virus-concentration-dependent optical spectrum shift produced by antigen-antibody interactions is transformed into a photonic radio-frequency (RF) shift by a frequency conversion between the optical and RF regions in the OFC, facilitating rapid and sensitive detection with well-established electrical frequency measurements. Furthermore, active-dummy temperature-drift compensation with a dual-comb configuration enables the very small change in the virus-concentration-dependent signal to be extracted from the large, variable background signal caused by temperature disturbance. The achieved performance of dual-comb biosensing will greatly enhance the applicability of biosensors to viruses, biomarkers, environmental hormones, and so on.

Fabrication of waveguide directional couplers using 2-photon lithography

Advances in 2-photon lithography have enabled in-lab production of sub-micron resolution and millimeter scale 3D optical components. The potential complex geometries are well suited to rapid prototyping and production of waveguide structures, interconnects, and waveguide directional couplers, furthering future development and miniaturization of waveguide-based imaging technologies. System alignment is inherent to the 2-photon process, obviating the need for manual assembly and allowing precise micron scale waveguide geometries not possible in traditional fused fiber coupler fabrication. Here we present the use of 2-photon lithography for direct printing of multi-mode waveguide couplers with air cladding and single mode waveguide couplers with uncured liquid photoresin cladding. Experimental results show reproducible coupling which can be modified by selected design parameters.

SERS immuno- and apta-assays in biosensing/bio-detection: Performance comparison, clinical applications, challenges

Surface Enhanced Raman Spectroscopy is increasingly used as a sensitive bioanalytical tool for detection of variety of analytes ranging from viruses and bacteria to cancer biomarkers and toxins, etc. This comprehensive review describes principles of operation and compares the performance of immunoassays and aptamer assays with Surface Enhanced Raman scattering (SERS) detection to each other and to some other bioassay methods, including ELISA and fluorescence assays. Both immuno- and aptamer-based assays are categorized into assay on solid substrates, assays with magnetic nanoparticles and assays in laminar flow or/and strip assays. The best performing and recent examples of assays in each category are described in the text and illustrated in the figures. The average performance, particularly, limit of detection (LOD) for each of those methods reflected in 9 tables of the manuscript and average LODs are calculated and compared. We found out that, on average, there is some advantage in terms of LOD for SERS immunoassays (0.5 pM median LOD of 88 papers) vs SERS aptamer-based assays (1.7 pM median LOD of 51 papers). We also tabulated and analyzed the clinical performance of SERS immune and aptamer assays, where selectivity, specificity, and accuracy are reported, we summarized the best examples. We also reviewed challenges to SERS bioassay performance and real-life application, including non-specific protein binding, nanoparticle aggregation, limited nanotag stability, sometimes, relatively long time to results, etc. The proposed solutions to those challenges are also discussed in the review. Overall, this review may be interesting not only to bioanalytical chemist, but to medical and life science researchers who are interested in improvement of bioanalyte detection and diagnostics.

Advances in Portable Heavy Metal Ion Sensors

Heavy metal ions, one of the major pollutants in the environment, exhibit non-degradable and bio-chain accumulation characteristics, seriously damage the environment, and threaten human health. Traditional heavy metal ion detection methods often require complex and expensive instruments, professional operation, tedious sample preparation, high requirements for laboratory conditions, and operator professionalism, and they cannot be widely used in the field for real-time and rapid detection. Therefore, developing portable, highly sensitive, selective, and economical sensors is necessary for the detection of toxic metal ions in the field. This paper presents portable sensing based on optical and electrochemical methods for the in situ detection of trace heavy metal ions. Progress in research on portable sensor devices based on fluorescence, colorimetric, portable surface Raman enhancement, plasmon resonance, and various electrical parameter analysis principles is highlighted, and the characteristics of the detection limits, linear detection ranges, and stability of the various sensing methods are analyzed. Accordingly, this review provides a reference for the design of portable heavy metal ion sensing.

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.

Molecular testing devices for on-site detection of E. coli in water samples

Escherichia coli (E. coli) cells are present in fecal materials that can be the main source for disease-causing agents in water. As a result, E. coli is recommended as a water quality indicator. We have developed an innovative platform to detect E. coli for monitoring water quality on-site by integrating paper-based sample preparation with nucleic acid isothermal amplification. The platform carries out bacterial lysis and DNA enrichment onto a paper pad through ball-based valves for fluid control, with no need of laboratory equipment, followed by loop-mediated isothermal amplification (LAMP) in a battery-operated coffee mug, and colorimetric detection. We have used the platform to detect E. coli in environmental water samples in about 1 h, with a limit of quantitation of 0.2 CFU/mL, and 3 copies per reaction. The platform was confirmed for detecting multiple E. coli strains, and for water samples of different salt concentrations. We validated the functions of the platform by analyzing recreational water samples collected near the Atlantic Ocean that contain different concentrations of salt and bacteria.

Seven-layer analysis model of an optical waveguide excitation fluorescence microscopy

In this paper, an optical waveguide evanescent field fluorescence microscopy is studied. Based on Maxwell's equation, a seven-layer theoretical analysis model is developed for the evaluation of an optical waveguide excitation fluorescence microscopy. The optical waveguide excitation fluorescence microscopy structure is systematically and comprehensively analysed at the wavelengths of 488, 532 and 646 nm for fluorescent dyes. The analysis results provide some useful suggestions, which will be beneficial to the research of an optical waveguide evanescent field fluorescence microscopy.

MicroLED biosensor with colloidal quantum dots and smartphone detection

A fluorescence sensor with the capability for spatially multiplexed measurements utilizing smartphone detection is presented. Bioconjugated quantum dots are used as the fluorescent tag and are excited using a blue-emitting microLED (µLED). The 1-dimensional GaN µLED array is butt-coupled to one edge of the glass slide to take advantage of total internal reflection fluorescence (TIRF) principles. The bioassays on the top surface of the glass waveguide are excited and the resultant fluorescence is detected with the smartphone. The red, green, and blue channels of the digital image are utilized to spectrally separate the excitation light from the fluorescence for analysis. Using a biotin-functionalized glass slide as proof of principle, we have shown that streptavidin conjugated quantum dots can be detected down to a concentration of 8 nM.

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.

Plasmonic Modes and Fluorescence Enhancement Coupling Mechanism: A Case with a Nanostructured Grating

The recent development and technological improvement in dealing with plasmonic metasurfaces has triggered a series of interesting applications related to sensing challenges. Fluorescence has been one of the most studied tools within such a context. With this in mind, we used some well characterized structures supporting plasmonic resonances to study their influence on the emission efficiency of a fluorophore. An extended optical analysis and a complementary investigation through finite-difference time-domain (FDTD) simulations have been combined to understand the coupling mechanism between the excitation of plasmonic modes and the fluorescence absorption and emission processes. The results provide evidence of the spectral shape dependence of fluorescence on the plasmonic field distribution together with a further relationship connected with the enhancement of its signal. It has made evident that the spectral region characterized by the largest relative enhancement closely corresponds to the strongest signatures of the plasmonic modes, as described by both the optical measurements and the FDTD findings.

Liquid-Core Hydrogel Optical Fiber Fluorescence Probes

This paper first reports a liquid-core hydrogel optical fiber fluorescence probe. It is composed of a liquid core, a high-refractive-index hydrogel fiber core, and a low-refractive-index hydrogel fiber cladding, which is completely different from many existing optical fiber fluorescence probes. The sensing solution with sensitive materials is sealed as a liquid core, and it can sufficiently react with small-molecule targets penetrating through the hydrogel fiber cladding and core, thus inducing variations in the fluorescence signals. These fluorescence signals can be localized and transmitted within the probe and finally collected and quantified for target detection. This proposed probe can be simply and rapidly fabricated and reused, and it was proven to have high sensitivity, accuracy, and selectivity in practical applications. Therefore, this liquid-core hydrogel optical fiber fluorescence probe will enable a novel sensing platform for small-molecule analyte detection that faces on-site detection challenges.

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.

Universal optothermal micro/nanoscale rotors

Rotation of micro/nano-objects is important for micro/nanorobotics, three-dimensional imaging, and lab-on-a-chip systems. Optical rotation techniques are especially attractive because of their fuel-free and remote operation. However, current techniques require laser beams with designed intensity profile and polarization or objects with sophisticated shapes or optical birefringence. These requirements make it challenging to use simple optical setups for light-driven rotation of many highly symmetric or isotropic objects, including biological cells. Here, we report a universal approach to the out-of-plane rotation of various objects, including spherically symmetric and isotropic particles, using an arbitrary low-power laser beam. Moreover, the laser beam is positioned away from the objects to reduce optical damage from direct illumination. The rotation mechanism based on opto-thermoelectrical coupling is elucidated by rigorous experiments combined with multiscale simulations. With its general applicability and excellent biocompatibility, our universal light-driven rotation platform is instrumental for various scientific research and engineering applications.

Fast dopamine detection based on evanescent wave detection platform

A compact evanescent wave detection platform (EWDP) is developed for the detection of fluorescence gold nanoclusters. The EWDP employs a simple optical system and a Si-based photodetector SOP-1000 assembly to improve the optical efficiency and detection sensitivity. A microfluidic sample cell is also used to decrease the amount of analyte to 200 μL (The volume of sample cell is really about 30 μL). On this basis, we design a strategy for detecting dopamine (DA) based on the photoinduced electron transfer (PET) quenching mechanism. By introduction of tyrosinase (TYR) during the detection, the testing time is shortened to 1 min. The fluorescence emission signal decreased dramatically and the quenching ratio (F₀-F)/F₀ is linearly related to the concentration of DA in the range of 0.03-60 μM with a detection limit of 0.03 μM. Additionally, this detection platform has potential applications for DA fast detection in the microsamples.

A sandwich-based evanescent wave fluorescent biosensor for simple, real-time exosome detection†

Exosomes are regarded as a promising biomarker for the noninvasive diagnosis and treatment of diseases. The value of exosomes for medical research has promoted the search for a fast, efficient, and sensitive detection method. This study reported a sandwich-based evanescent wave fluorescent biosensor (S-EWFB) for exosome detection. A two-step strategy was implemented to take advantages of the simple binding of fluorescent probes with exosomes via the hydrophobic interaction between the cholesteryl and phospholipid bilayer membrane, as well as real-time detection on an evanescent wave liquid-solid interface based on CD63 aptamer-specific capture to form an exosome@fluorescence probe/aptamer sandwich structure. The one-to-many connection between exosomes and signal molecules and the aptamer-modified evanescent wave optical fiber detection platform reduced the detection limit of exosomes to 7.66 particles/mL, with a linear range of 47.5-4.75 × 10⁶ particles/mL. The entire detection process was simple, rapid, and real-time and lasted about 1 h while requiring no separation and purification. Additionally, this platform showed excellent surface regeneration capability and exhibited good performance during the analysis of tumor and non-tumor-derived exosomes.

Reusable smartphone-facilitated mobile fluorescence biosensor for rapid and sensitive on-site quantitative detection of trace pollutants

Increasing exposure to toxic pollutants highlights the need for their sensitive detection technologies that can be rapidly adapted and deployed in various settings. Optical biosensors are an excellent solution due to their outstanding features. However, the sophisticated and expensive optical design limits their scalability and actual application. Herein, an innovative reusable smartphone-facilitated mobile fluorescence biosensor (s-MFB) was built through integrating miniaturized all-fiber optical system and microfluidic system with smartphone. An asymmetric Y-shaped fiber optic coupler (Y-FOC) is constructed for simultaneous transmission of excitation light and the collected fluorescence. In particular, the incidence rays are introduced into the fiber bio-probe at a specific angle through the single-mode fiber of the Y-FOC, which enhances the evanescent wave field and the number of total internal reflections. The s-MBF showed a LOD for free Cy5.5 of 0.1 nM. Combining indirect competitive immunoassay with the s-MFB, this new assay, which achieve quantitative detection of bisphenol A and norfloxacin in 15 min with high sensitivity and reusability, substantially reduces the complexity and improves the scalability of trace pollutants detection. The adjunctive smartphone application allows on-site real-time quantitative detection, automated interpretation of reporting results, and early-warning of pollution accidents.

Simultaneous and sensitive determination of Escherichia coli O157:H7 and Salmonella Typhimurium using evanescent wave dual-color fluorescence aptasensor based on micro/nano size effect

The simultaneous and sensitive determination of two common pathogenic bacteria, Escherichia coli O157:H7 (E. coli O157:H7) and Salmonella Typhimurium (S. Typhimurium) was achieved using evanescent wave dual-color fluorescence aptasensor and the fiber nanoprobe through combining the micro/nano size and time-resolved effect. Two fluorescence labeled aptasensors, Cy3-apt-E and Cy5.5-apt-S, were regarded as biorecognition elements and signal reporters for E. coli O157:H7 and S. Typhimurium, which were alternatively excited by evanescent waves originated from 520 nm to 635 nm excitation lights, respectively. The fiber nanoprobe with in-situ etched nanopores was used for distinguishing free aptasensors and aptasensors bound to pathogenic bacteria based on the limited penetrated depth of evanescent wave and the significant size difference of bacteria and nanopore. The E. coli O157:H7 and S. Typhimurium were directly and simultaneously quantitated in less than 35 min without the requirement of the complex immobilization of biorecognition molecules and bacteria enrichment/separation processes. The limits of detection of E. coli O157:H7 and S. Typhimurium were 340 CFU/mL and 180 CFU/mL, respectively. The satisfied recovery rate of real samples testing verified the feasibility and accuracy of the proposed method. Our strategy not only greatly simplifies the detection and identification process of multiple pathogenic bacteria, but also is easy to extend as a universal technology for sensitive determination of other bacteria using their respective biorecognition molecules.

State-of-the-Art Optical Devices for Biomedical Sensing Applications—A Review

Optical sensors for biomedical applications have gained prominence in recent decades due to their compact size, high sensitivity, reliability, portability, and low cost. In this review, we summarized and discussed a few selected techniques and corresponding technological platforms enabling the manufacturing of optical biomedical sensors of different types. We discussed integrated optical biosensors, vertical grating couplers, plasmonic sensors, surface plasmon resonance optical fiber biosensors, and metasurface biosensors, Photonic crystal-based biosensors, thin metal films biosensors, and fiber Bragg grating biosensors as the most representative cases. All of these might enable the identification of symptoms of deadly illnesses in their early stages; thus, potentially saving a patient’s life. The aim of this paper was not to render a definitive judgment in favor of one sensor technology over another. We presented the pros and cons of all the major sensor systems enabling the readers to choose the solution tailored to their needs and demands.

Smartphone-based optical spectroscopic platforms for biomedical applications: a review [Invited]

Rapid advancements in smartphone technology have enabled the integration of many optical detection techniques that leverage the embedded functional components and software platform of these sophisticated devices. Over the past few years, several research groups have developed high-resolution smartphone-based optical spectroscopic platforms and demonstrated their usability in different biomedical applications. Such platforms provide unprecedented opportunity to develop point-of-care diagnostics systems, especially for resource-constrained environments. In this review, we discuss the development of smartphone systems for optical spectroscopy and highlight current challenges and potential solutions to improve the scope for their future adaptability.

Coupling of silver nanoparticle-conjugated fluorescent dyes into optical fiber modes for enhanced signal-to-noise ratio

We present the optical coupling of the silver nanoparticles (AgNPs)-conjugated dye molecule into fiber optical modes for detecting fluorescence with the enhanced signal-to-noise (S/N) ratio. This near field coupling of the excited state of organic dye (FAM) molecules into the fiber multimodes occurs by immobilizing them on the exposed surface of fiber core, permitting the coupled light to be guided along the fiber for detection. This fiber based scheme is the first attempt to single out the fluorescence using fiber modes not for carrying excitation light but only for collecting emission light via the dye-fiber coupling. The emission-selective coupling into fiber modes turns out to be effective in reducing the unwanted background noise arising from both the false detection of excitation light and bulk autofluorescence. This scheme differs from the previously reported fluorescence sensors based on waveguides where guided modes at λ_ex excite dye molecules via their evanescent fields. In addition, the local fields enhanced by AgNPs in close proximity to FAM molecules on the fiber core surface increase the rates of dye excitation and radiative decay/AgNP supported surface plasmon coupled emission. While focusing on demonstrating the proof-of-concept of the scheme presented, we obtain the maximum of 4.2-fold enhancement of the signal-to-noise (S/N) ratio in detecting fluorescence as compared to a conventional fluorescence detection scheme. The results presented in the fiber-based scheme may find an application where high S/N ratio fluorescence based biochemical assay is required in a small-sized device with remote sensing capability.

Light-sheet skew rays enhanced U-shaped fiber-optic fluorescent immunosensor for Microcystin-LR

A novel U-shaped fiber-optic evanescent-wave fluorescent immunosensor was designed that exploits light-sheet excitation of skew rays in a passive fiber for sensitive microcystin-LR (MC-LR) detection in real-time. In particular, a light sheet comprising a thin plane of light can be concentrated into exciting the optimum ray group, resulting in enhanced interaction between light and fluorophores. Meanwhile, skew rays excited by transmitting light into an optical fiber with an angle offset allow a much higher number of total-internal-reflections with increased interaction length along the fiber interface, which strengthens the light-matter interactions. Under the optimal angle offset, the proposed evanescent wave fluorescent immunosensor is the first demonstration of integrating light-sheet skew rays and a U-shaped fiber-optic probe for enhanced sensitivity. The results show that fluorescence sensitivity of the U-shaped fiber-optic probe with light-sheet skew rays excitation is 16 times higher than that of collimated skew rays excitation. Combined with this newly designed light-sheet skew rays enhanced U-shaped fiber-optic fluorescent immunosensor, a sensitive and real-time MC-LR detection method was established based on the indirect competitive immunoassay principle. Real environmental water samples spiked with MC-LR were determined by the immunosensor with recovery rates between 85% and 112%. The present system could be an alternative tool for the on-site environmental monitoring, in-field food safety assurance and clinical diagnostics. It also advances the fiber-optic sensors field in terms of experimental design.

Nanoscale Affinity Double Layer Overcomes the Poor Antimatrix Interference Capability of Aptamers

Poor antimatrix interference capability of aptamers is one of the major obstacles preventing their wide applications for real-sample detections. Here, we devise a multiple-function interface, denoted as a nanoscale affinity double layer (NADL), to overcome this bottleneck via in situ simultaneous target enrichment, purification, and detection. The NADL consists of an upper aptamer layer for target purification and sensing and a lower nanoscale solid-phase microextraction (SPME) layer for sample enrichment. The targets flowing through the NADL-functionalized surface are instantly million-fold enriched and purified by the sequential extraction of aptamer and SPME. The formation of the aptamer-target complex is greatly enhanced, enabling ultrasensitive detection of targets with minimized interference from the matrix. Taking the fiber-optic evanescent wave sensor as an example, we demonstrated the feasibility and generality of the NADL. The unprecedented detection of limits of 800, 4.8, 40, and 0.14 fM were, respectively, achieved for three representative small-molecule targets with distinct hydrophobicity (kanamycin A, sulfadimethoxine, and di-(2-ethylhexyl) phthalate) and protein target (human serum albumin), corresponding to 2500 to 3 × 10⁸-fold improvement compared to the sensors without the NADL. Our sensors also showed exceptionally high target specificity (>1000) and tunable dynamic ranges simply by manipulating the SPME layer. With these features comes the ability to directly detect targets in diluted environmental, food, and biological samples at concentrations all well below the tolerance limits.

Simple, rapid, and sensitive on-site detection of Hg2+ in water samples through combining portable evanescent wave optofluidic biosensor and fluorescence resonance energy transfer principle

Rapid and sensitive detection of Hg²⁺ in the environment and drinking water is vital because of its non-degradability, bioaccumulation, and high toxicity. Herein, we report a portable evanescent wave optofluidic biosensor (EWOB) for simple sensitive detection of Hg²⁺ using fluorescence labeled poly-A DNA strand (CY-A14) and quencher labeled poly-T DNA strand (BQ-T14) as signal reporter and biorecognition element, respectively. Both CY-A14 and Hg²⁺ can competitively bind with BQ-T14 based on DNA hybridization and the specifical binding of Hg²⁺ and T bases of DNA to form T-Hg²⁺-T mismatch structure, respectively. Higher concentration of Hg²⁺ lead to less CY-A14 bound to BQ-T14 and thus a higher fluorescence intensity. The influence of several key environmental factors on Hg²⁺ biosensor, such as pH, temperature, and ionic strength, was investigated in details because they were essential for practical applications of Hg²⁺ biosensor. Under optimal conditions, a detection cycle for a single sample, including the measurement and regeneration, was less than 10 min with a Hg²⁺ detection limit of 8.5 nM. The high selectivity of the biosensor was showed by evaluating its response to various potentially interfering metal ions. Our results clearly demonstrated that the portable EWOB could serve as a powerful tool for rapid and sensitive on-site detection of Hg²⁺ in real water samples. The EWOB is also potentially applicable to detect other heavy metal ions or small molecule targets for which DNA/aptamers could be applied as specific biosensing probes.

Exploring the Ligand Preferences of the PHD1 Domain of Histone Demethylase KDM5A Reveals Tolerance for Modifications of the Q5 Residue of Histone 3

Understanding the ligand preferences of epigenetic reader domains enables identification of modification states of chromatin with which these domains associate and can yield insight into recruitment and catalysis of chromatin-acting complexes. However, thorough exploration of the ligand preferences of reader domains is hindered by the limitations of traditional protein-ligand binding assays. Here, we evaluate the binding preferences of the PHD1 domain of histone demethylase KDM5A using the protein interaction by SAMDI (PI-SAMDI) assay, which measures protein-ligand binding in a high-throughput and sensitive manner via binding-induced enhancement in the activity of a reporter enzyme, in combination with fluorescence polarization. The PI-SAMDI assay was validated by confirming its ability to accurately profile the relative binding affinity of a set of well-characterized histone 3 (H3) ligands of PHD1. The assay was then used to assess the affinity of PHD1 for 361 H3 mutant ligands, a select number of which were further characterized by fluorescence polarization. Together, these experiments revealed PHD1's tolerance for H3Q5 mutations, including an unexpected tolerance for aromatic residues in this position. Motivated by this finding, we further demonstrate a high-affinity interaction between PHD1 and recently identified Q5-serotonylated H3. This work yields interesting insights into permissible PHD1-H3 interactions and demonstrates the value of interfacing PI-SAMDI and fluorescence polarization in investigations of protein-ligand binding.

Waveguide-Based Fluorescent Immunosensor for the Simultaneous Detection of Carbofuran and 3-Hydroxy-Carbofuran

Carbofuran (CBF) is an efficient and broad-spectrum insecticide. As testing indicators for water quality and agricultural products, CBF and its metabolite 3-hydroxy-carbofuran (3-OH-CBF) are regulated by many countries. The detection of CBF and 3-OH-CBF is of great importance for the environment and human health. However, an immunosensor detection method for the simultaneous analysis of CBF and 3-OH-CBF remains unavailable. Herein, we report a waveguide-based fluorescent immunosensor for detecting CBF and 3-OH-CBF, synchronously. The immunosensor is based on a broad-spectrum monoclonal antibody with high binding affinity against CBF and 3-OH-CBF. The linear detection ranges for CBF and 3-OH-CBF are 0.29-2.69 and 0.12-4.59 μg/L, with limits of detection of 0.13 μg/L for CBF and 0.06 μg/L for 3-OH-CBF, respectively. The whole detection process for each cycle is approximately 30 min. The results show a good application prospect for the rapid detection of CBF and 3-OH-CBF in water or agricultural products.

Microbial biosensors for recreational and source waters

Biosensors are finding new places in science, and the growth of this technology will lead to dramatic improvements in the ability to detect microorganisms in recreational and source waters for the protection of public health. Much of the improvement in biosensors has followed developments in molecular biology processes and coupling these with advances in engineering. Progress in the fields of nano-engineering and materials science have opened many new avenues for biosensors. The adaptation of these diverse technological fields into sensors has been driven by the need to develop more rapid sensors that are highly accurate, sensitive and specific, and have other desirable properties, such as robust deployment capability, unattended operations, and remote data transfer. The primary challenges to the adoption of biosensors in recreational and source water applications are cost of ownership, particularly operations and maintenance costs, problems caused by false positive rates, and to a lesser extent false negative rates, legacy technologies, policies and practices which will change as biosensors improve to the point of replacing more traditional methods for detecting organisms in environmental samples.

Molecular Imprinted Polymers Coupled to Photonic Structures in Biosensors: The State of Art

Optical sensing, taking advantage of the variety of available optical structures, is a rapidly expanding area. Over recent years, whispering gallery mode resonators, photonic crystals, optical waveguides, optical fibers and surface plasmon resonance have been exploited to devise different optical sensing configurations. In the present review, we report on the state of the art of optical sensing devices based on the aforementioned optical structures and on synthetic receptors prepared by means of the molecular imprinting technology. Molecularly imprinted polymers (MIPs) are polymeric receptors, cheap and robust, with high affinity and selectivity, prepared by a template assisted synthesis. The state of the art of the MIP functionalized optical structures is critically discussed, highlighting the key progresses that enabled the achievement of improved sensing performances, the merits and the limits both in MIP synthetic strategies and in MIP coupling.

Reusable optofluidic point-of-care testing platform with lyophilized specific antibody for fluorescence detection of cholylglycine in serum

A reusable optofluidic point-of-care testing platform (OPOCT) was successfully constructed through integrating evanescent wave fluorescence technology into an all-fiber-based optofluidic system. The compact design of the OPOCT allows it to be portable and suitable for on-site sensitive determination of biomarkers in serum without complicated and costly procedures. The sensitivity of 90.9 pM for antibody determination is observed thanks to the high transmission efficiency of excitation light and fluorescence in the OPOCT. The affinity constant between cholylglycine (CG) and anti-CG antibody was quantified using this platform based on the proposed theory. Using the lyophilized fluorescence-labeled specific antibody and reusable fiber optic biosensor, the OPOCT is applied to the one-step sensitive determination of CG in serum, which eliminates the dearth associated with liquid reagent handling, disposable biosensors, and user intervention. A limit of detection of 0.025 μg/mL for CG is obtained, which is far more than adequate for meeting diagnostic requirements. The matrix effect of serum samples on the evanescent wave-based optofluidic biosensor can be effectively reduced by simple dilution of serum samples. The performance of the OPOCT also compared favorably with that of a commercial turbidimetric inhibition immunoassay through analyzing multiple serum samples. This platform is ready to expand to measure any other biomarker by using its specific antibody. The simplicity, sensitivity, cost-effectiveness, and robustness of the OPOCT enable the early diagnosis of disease and making a timely clinical decision. Graphical abstract .

Recent Progress in Optical Sensors for Biomedical Diagnostics

In recent years, several types of optical sensors have been probed for their aptitude in healthcare biosensing, making their applications in biomedical diagnostics a rapidly evolving subject. Optical sensors show versatility amongst different receptor types and even permit the integration of different detection mechanisms. Such conjugated sensing platforms facilitate the exploitation of their neoteric synergistic characteristics for sensor fabrication. This paper covers nearly 250 research articles since 2016 representing the emerging interest in rapid, reproducible and ultrasensitive assays in clinical analysis. Therefore, we present an elaborate review of biomedical diagnostics with the help of optical sensors working on varied principles such as surface plasmon resonance, localised surface plasmon resonance, evanescent wave fluorescence, bioluminescence and several others. These sensors are capable of investigating toxins, proteins, pathogens, disease biomarkers and whole cells in varied sensing media ranging from water to buffer to more complex environments such as serum, blood or urine. Hence, the recent trends discussed in this review hold enormous potential for the widespread use of optical sensors in early-stage disease prediction and point-of-care testing devices.

Development of a handheld dual-channel optical fiber fluorescence sensor based on a smartphone

In order to meet the needs of on-site, accurate, fast, and remote detection, we design a smartphone-based handheld dual-channel optical fiber fluorescence sensor (DOFFS), which is composed of a semiconductor laser for exciting fluorescence signals, a smartphone with a dual-bandpass filter for collecting fluorescence signals, a fiber coupler for transmitting light, and batteries for laser power supply. All the components are integrated into a 3D printed shell, on the side of which there are two fiber flanges used for fiber probe connection. The fluorescence signals of green and red quantum dots modified on the fiber probes can be captured by the smartphone camera and calculated by a self-developed Android application. The comparisons of single-channel and dual-channel fluorescence signals with pH show that the performance of the sensor is good. The proposed sensor not only can simultaneously detect dual-channel signals for fast detection needs, but it also is handheld with a small size of 79×57×154mm³ and inner power supply, and the fiber probes can be easily replaced, supporting remote and on-site applications. It is a potential tool for many occasions in many fields.

Portable and automated fluorescence microarray biosensing platform for on-site parallel detection and early-warning of multiple pollutants

The portable and automated fluorescence microarray biosensing platform (FMB) that employed a compact hybrid optical structure, microfluidics, and microarray biosensors was constructed for on-site parallel detection of multiple analytes. In the FMB, a hybrid optical structure that composed of a 1 × 4 single mode fiber optic coupler, four fiber optic switches, a single-multi mode fiber optic bundle coupler was at the first time developed for the transmission of the excitation light and the collection and transmission of multi-channel fluorescence signals. Through the control of fiber optic switches, the parallel fluorescence assay of four channels could be achieved using only one excitation light and one photodiode detector on the basis of the time-resolved effect. This optical design not only greatly increased the efficiency of light transmission and fluorescence collection and detection sensitivity of the FMB, but also allows the miniaturization and portability of the whole system because of few optical separation elements used and no requirement of rigorous optical alignment. Taking Microcystin-LR (MC-LR), 2,4-D, atrazine (ATZ), and bisphenol A (BPA) for example, the application potential of the FMB to rapidly and parallelly detect four typical pollutants in real water with high sensitivity and specificity was demonstrated. The limits of detection of MC-LR, 2,4-D, ATZ, and BPA were 0.04 μg/L, 0.09 μg/L, 0.02 μg/L, and 0.03 μg/L, respectively. The FMB could also achieve early-warning of pollutants thanks to its ability of rapidity, high-frequency, and multiple-analyte detection. The FMB has significant implications as a multiplexable, portable, rapid, and quantitative detection platform for pollution accidents and water quality management to satisfy the increasing demands of alerting and protecting civilians.

Point-of-Care Strategies for Detection of Waterborne Pathogens

Waterborne diseases that originated due to pathogen microorganisms are emerging as a serious global health concern. Therefore, rapid, accurate, and specific detection of these microorganisms (i.e., bacteria, viruses, protozoa, and parasitic pathogens) in water resources has become a requirement of water quality assessment. Significant research has been conducted to develop rapid, efficient, scalable, and affordable sensing techniques to detect biological contaminants. State-of-the-art technology-assisted smart sensors have improved features (high sensitivity and very low detection limit) and can perform in a real-time manner. However, there is still a need to promote this area of research, keeping global aspects and demand in mind. Keeping this view, this article was designed carefully and critically to explore sensing technologies developed for the detection of biological contaminants. Advancements using paper-based assays, microfluidic platforms, and lateral flow devices are discussed in this report. The emerging recent trends, mainly point-of-care (POC) technologies, of water safety analysis are also discussed here, along with challenges and future prospective applications of these smart sensing technologies for water health diagnostics.

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.

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.

A novel dual-color total internal reflection fluorescence detecting platform using compact optical structure and silicon-based photodetector

A novel dual-color total internal reflection fluorescence detecting platform (DTP) was developed for the simultaneous detection of two fluorescence dyes. The DTP employed a simple and compact optical system and a Si-based photodetector SOP-1000 assembly to improve the optical efficiency and detection sensitivity. In the optical system, a single-multimode fiber optic coupler was applied for the transmission of two wavelength excitation lights and the collection and transmission of two wavelength fluorescence signals, and a fiber optical switch controlled two wavelength excited lights to alternatively enter into the combined tapered fiber optic probe and two wavelength fluorescence dyes were alternatively excited. The simultaneous and sensitive detection of two wavelength fluorescence signals was achieved by one photodetector SOP-1000 based on the time-resolved effect of the fiber optical switch. The DTP demonstrated its sensitivity of 50 fW light intensity, and the limit of detection of Cy5.5 and Pacific Blue dye (PB) were 0.05 nM and 2.1 nM, respectively. This miniaturized and integrated DTP with high sensitivity, simple optical structure, negligible cross-talk, and cost-effectiveness could be a potential alternative to the conventional dual-color fluorescence detecting systems. It could also be a powerful component of a μ-TAS system for highly sensitive dual-color fluorescence detection.

Development of dual-color total internal reflection fluorescence biosensor for simultaneous quantitation of two small molecules and their affinity constants with antibodies

A novel dual-color total internal reflection fluorescence biosensor (DTB) was successfully developed for the simultaneous detection of two small molecules based on a simple optical structure and the time resolved effect of fiber optic switch. The DTB employed a single-multi mode fiber optic coupler instead of a sophisticated confocal optical system for the transmission of two excitation lights and dual-color fluorescence, and a photodiode detector instead of photomultiplier for the simultaneous detection of dual-color fluorescence. The compact optical design of DTB improved its optical transmission efficiency and detection sensitivity because of no requirement of numerous optical separation elements and rigorous optical alignment. The DTB was applied for the simultaneous detection of 2,4-Bisphenol-A (BPA) and 2,4-Dichlorophenoxyacetic acid (2,4-D) using one bifunctional fiber optic bio-probe modified by two hapten-protein conjugates. When the mixture of Cy5.5 labeled anti-2,4-D antibody and Pacific Blue dye labeled anti-BPA antibody was introduced over the surface of the bio-probe, they bound with their respective hapten-protein conjugate immobilized onto the bio-probe. Based on the time-resolved effect of fiber optic switch, two fluorescence dyes were alternatively excited by 635 nm and 405 nm laser lights and simultaneously detected by one photodiode detector. Taking indirect competitive immunoassay principle, BPA and 2,4-D were simultaneously detected using the DTB with high sensitivity, accuracy, and rapidity. The quantitation of affinity constants between small molecules and their antibodies was also achieved based on the proposed theory. The DTB provides a flexible and powerful platform for simultaneously sensitive quantitation of multiple targets and the affinity constants of biomolecular interactions.

Optical Biosensors for Label-Free Detection of Small Molecules

Label-free optical biosensors are an intriguing option for the analyses of many analytes, as they offer several advantages such as high sensitivity, direct and real-time measurement in addition to multiplexing capabilities. However, development of label-free optical biosensors for small molecules can be challenging as most of them are not naturally chromogenic or fluorescent, and in some cases, the sensor response is related to the size of the analyte. To overcome some of the limitations associated with the analysis of biologically, pharmacologically, or environmentally relevant compounds of low molecular weight, recent advances in the field have improved the detection of these analytes using outstanding methodology, instrumentation, recognition elements, or immobilization strategies. In this review, we aim to introduce some of the latest developments in the field of label-free optical biosensors with the focus on applications with novel innovations to overcome the challenges related to small molecule detection. Optical label-free methods with different transduction schemes, including evanescent wave and optical fiber sensors, surface plasmon resonance, surface-enhanced Raman spectroscopy, and interferometry, using various biorecognition elements, such as antibodies, aptamers, enzymes, and bioinspired molecularly imprinted polymers, are reviewed.

Multivalent Antibody-Nanoparticle Conjugates To Enhance the Sensitivity of Surface-Enhanced Raman Scattering-Based Immunoassays

Multivalent immunoprobes can improve the sensitivity of biosensors because increased valency can strengthen the binding affinity between the receptor and target biomolecules. Here, we report surface-enhanced Raman scattering (SERS)-based immunoassays using multivalent antibody-conjugated nanoparticles (NPs) for the first time. Multivalent antibodies were generated through the ligation of Fab fragments fused with Fc-binding peptides to immunoglobulin G. This fabrication method is easy and fast because of the elimination of heterologous protein expression, high degrees of antibody modifications, and covalent chemical ligation steps. We constructed multivalent antibody-NP conjugates (MANCs) and employed them as SERS immunoprobes. MANCs improved the sensitivity of SERS-based immunoassays by 100 times compared to standard antibody-NP conjugates. MANCs will increase the feasibility of practical SERS-based immunoassays.

Universal and reusable hapten/antibody-mediated portable optofluidic immunosensing platform for rapid on-site detection of pathogens

A universal and reusable hapten-antibody-mediated portable optofluidic immunosensing platform (OIP) was developed for rapid on-site detection of pathogens. By using Escherichia coli O157:H7 (E. coli O157:H7) and bisphenol A-Bovine serum albumin (BPA-BSA)/anti-BPA antibody as a model pathogen and a mediated hapten-antibody, respectively, a novel immunoassay mechanism was proposed to detect pathogens. The BPA-BSA-modified immunosensor and E. coli O157:H7 were initially saturated with anti-BPA antibodies (mouse IgG) and anti-E. coli O157:H7 antibodies (mouse IgG), respectively. Then, the fluorescence-labeled secondary antibodies (goat anti-mouse IgG antibody) were incubated with E. coli O157:H7 with their antibodies. Next, the mixture was introduced into the immunosensor surface bound to the anti-BPA antibodies. A high concentration of E. coli O157:H7 in the sample reduced the number of fluorescence-labeled secondary antibodies bound to the immunosensor surface, thus resulting in the detection of low fluorescence signals. Under optimized conditions, the hapten-antibody-mediated OIP system exhibited a detection limit of 8 cfu/mL E. coli O157:H7 after concentrating 100 times by using centrifugation, and a test cycle, including prereaction, detection, and regeneration, was less than 1 h. The robustness of the hapten-carrier protein-modified immunosensor surface allowed multiple pathogen immunoassays. The proposed strategy demonstrated good recovery, precision, and accuracy through the evaluation of the spiked water samples. We expect that the new platform can be readily used for the detection of other pathogens in a variety of application fields ranging from environmental monitoring and food safety to medical diagnosis.

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).