Novel label-free and high-throughput microchip electrophoresis platform for multiplex antibiotic residues detection based on aptamer probes and target catalyzed hairpin assembly for signal amplification

Advances in novel biosensors in biomedical applications

Biosensors, devices capable of detecting biomolecules or bioactive substances, have recently become one of the important tools in the fields of bioanalysis and medical diagnostics. A biosensor is an analytical system composed of biosensitive elements and signal-processing elements used to detect various biological and chemical substances. Biomimetic elements are key to biosensor technology and are the components in a sensor that are responsible for identifying the target analyte. The construction methods and working principles of biosensors based on synthetic biomimetic elements, such as DNAzyme, molecular imprinted polymers and aptamers, and their updated applications in biomedical analysis are summarised. Finally, the technical bottlenecks and future development prospects for biomedical analysis are summarised and discussed.

An electrochemical impedimetric platform formed by a CNT@UiO-66 nanocomposite for quantitative analysis of oxytetracycline

Functional materials are considered one of the most critical factors in constructing high-performance electrochemical aptasensors in the sensing field. In this work, the microporous Zr-MOF UiO-66 (UiO = Universitetet i Oslo) is selected for assembly with carbon nanotubes (CNTs) to prepare a CNT@UiO-66 composite. The as-synthesized CNT@UiO-66 composite has a high surface area, excellent stability, good electrical conductivity, and abundant Zr(IV) sites, conferring it great potential for application in fabricating high-performance electrochemical aptasensors. It is gratifying that this electrochemical impedimetric aptasensor can detect trace oxytetracycline (OTC) from 0.01 to 0.7 pg mL-1 with a low limit of detection (LOD) of 1.48 fg mL-1. Meanwhile, this fabricated sensor based on CNT@UiO-66 has fine stability, excellent selectivity, and available reproducibility. In particular, the CNT@UiO-66-based aptasensor can quantitatively detect the OTC concentration in real samples.

Size-controlling preparation of covalent organic framework nanospheres for electrochemical impedimetric aptasensing of oxytetracycline

Covalent organic framework (COF) nanospheres with controlled size of ∼500 and ∼1100 nm were successfully prepared by adjusting the HOAc amount in the synthetic system. The as-synthesized COFs have large conjugate aromatic skeleton, excellent stability, abundant pore, and uniform morphology. These advantages of COFs are benefit for immobilizing aptamers to fabricate the targeted electrochemical aptasensor. The commonly used oxytetracycline (OTC) is an analytic model to explore the sensing performance of the COF-based aptasensor, indicating that the smaller COF (∼500 nm) is more conducive to acquiring the sensitive sensor than that of the larger COF (∼1100 nm). Moreover, the limitation of detection of the COF (∼500 nm)-based aptasensor is calculated to be 7.4 fg mL^-1 using the response impedance signal. Additionally, the aptamer-based biosensor has fine reproducibility, good stability, excellent specificity, and available usability even in real samples.

A competitive colorimetric aptasensor for simple and sensitive detection of kanamycin based on terminal deoxynucleotidyl transferase-mediated signal amplification strategy

We developed a rapid and sensitive colorimetric biosensor based on competitive recognition between kanamycin (KAN), magnetic beads-kanamycin (MBs-KAN) and aptamer and terminal deoxynucleotidyl transferase (TdT)-mediated signal amplification strategy. In the absence of KAN, aptamers recognize MBs-KAN. TdT can amplify oligonucleotides to the 3'-OH ends of aptamers, with biotin-dUTP being embedded in the long single stranded DNA (ssDNA). Then the assay produced visual readout due to the horseradish peroxidase (HRP)-catalyzed color change of the substrate after the linkage between biotin and streptavidin (SA)-HRP. In the presence of KAN, however, aptamers tend to bind free KAN rather than MBs-KAN. In this case, aptamers are isolated by magnetic separation, resulting in the failure of signal amplification and catalytic signals. This competitive colorimetric sensor showed excellent selectivity toward KAN with the limit of detection (LOD) as low as 9 pM. And recovery values were between 93.8 and 107.8% when spiked KAN in milk and honey samples.

Microfluidic tools for veterinary and zoonotic disease diagnostics

Animal disease diagnostics has linked as the cause and cure of any disease. It also plays a vital role in disease management and prevention. A small outbreak of disease can pose a threat to the entire animal community as we realized in corona pandemic. Thus, to ensure the overall welfare of animals and disease spread monitoring, the development of detection tools for veterinary diagnosis becomes essential. Currently, the animal disease diagnosis is relied on laboratory-based testing. There is a parallel necessity for rapid, reliable and low-cost diagnostic tests to be done by intervention of growing area such as microfluidic platform. Therefore, in this chapter, we have discussed about various microfluidic platform and their application for early diagnosis of veterinary disease. Followed by, we also lightened on future perspective of role of microfluidic in animal disease diagnostics.

Hybrid Porous Crystalline Materials from Metal Organic Frameworks and Covalent Organic Frameworks

Two frontier crystalline porous framework materials, namely, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely explored owing to their outstanding physicochemical properties. While each type of framework has its own intrinsic advantages and shortcomings for specific applications, combining the complementary properties of the two materials allows the engineering of new classes of hybrid porous crystalline materials with properties superior to the individual components. Since the first report of MOF/COF hybrid in 2016, it has rapidly evolved as a novel platform for diverse applications. The state-of-art advances in the various synthetic approaches of MOF/COF hybrids are hereby summarized, together with their applications in different areas. Perspectives on the main challenges and future opportunities are also offered in order to inspire a multidisciplinary effort toward the further development of chemically diverse, multi-functional hybrid porous crystalline materials.

Review of microchip analytical methods for the determination of pathogenic Escherichia coli

Bacterial infections remain the principal cause of mortality worldwide, making the detection of pathogenic bacteria highly important, especially Escherichia coli (E. coli). Current E. coli detection methods are labour-intensive, time-consuming, or require expensive instrumentation, making it critical to develop new strategies that are sensitive and specific. Microchips are an automated analytical technique used to analyse food based on their separation efficiency and low analyte consumption, which make them the preferred method to detect pathogenic bacteria. This review presents an overview of microchip-based analytical methods for analysing E. coli, which were published in recent years. Specifically, this review focuses on current research based on microchips for the detection of E. coli and reviews the limitations of microchip-based methods and future perspectives for the analysis of pathogenic bacteria.

ASSURED Point-of-Need Food Safety Screening: A Critical Assessment of Portable Food Analyzers

Standard methods for chemical food safety testing in official laboratories rely largely on liquid or gas chromatography coupled with mass spectrometry. Although these methods are considered the gold standard for quantitative confirmatory analysis, they require sampling, transferring the samples to a central laboratory to be tested by highly trained personnel, and the use of expensive equipment. Therefore, there is an increasing demand for portable and handheld devices to provide rapid, efficient, and on-site screening of food contaminants. Recent technological advancements in the field include smartphone-based, microfluidic chip-based, and paper-based devices integrated with electrochemical and optical biosensing platforms. Furthermore, the potential application of portable mass spectrometers in food testing might bring the confirmatory analysis from the laboratory to the field in the future. Although such systems open new promising possibilities for portable food testing, few of these devices are commercially available. To understand why barriers remain, portable food analyzers reported in the literature over the last ten years were reviewed. To this end, the analytical performance of these devices and the extent they match the World Health Organization benchmark for diagnostic tests, i.e., the Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment-free, and Deliverable to end-users (ASSURED) criteria, was evaluated critically. A five-star scoring system was used to assess their potential to be implemented as food safety testing systems. The main findings highlight the need for concentrated efforts towards combining the best features of different technologies, to bridge technological gaps and meet commercialization requirements.

Establishment of a dual-aptasensor for simultaneous detection of chloramphenicol and kanamycin

Aptamers, as single-stranded DNA or RNA fragments, have been widely applied as the bio-recognition element for fabrication of flexible and reliable aptasensors to be used in food safety control, clinical therapy and diagnosis and environment monitoring fields. With increasingly fierce antibiotics resistance appearing as a worldwide problem, a highly efficient method is urgently needed to detect antibiotics residues in animal-sourced food. Herein, a simply operated aptasensor based on quantitative real-time PCR (qRT-PCR) was fabricated to realise the simultaneous detection of two antibiotics (i.e. chloramphenicol and kanamycin). The limit of detection (LOD) of 6.13 ng/mL for chloramphenicol and of 19.17 ng/mL for kanamycin of this dual-aptasensor were achieved. Actually, such LOD values were not as good as that of an aptasensor individually established for each antibiotic. The circular dichroism analysis suggested that in the dual-aptasensor, adjacent aptamers might disturb each other's binding with their respective target. Although certain detection sensitivity was lost, the dual-aptasensor could still fulfil the detection requirements, and more importantly, it would improve the detection efficiency. Finally, this dual-aptasensor was applied for detecting chloramphenicol and kanamycin in real spiked food samples, and results indicated good recovery rates. These results demonstrated this developed dual-aptasensor to be a promising highly efficient method with low cost for simultaneous detection of chloramphenicol and kanamycin residues in animal-sourced food samples.

Fluorescence assay of oxytetracycline in seawater after selective capture using magnetic molecularly imprinted nanoparticles

A comprehensive strategy for manufacturing a novel sorbent based on magnetic molecularly imprinted polymers (MMIPs) is addressed for selective capture of oxytetracycline from seawater. The novel MMIPs were synthesized by nano-Fe₃O₄ as sacrificial matrix and adsorption properties of the polymers demonstrate rapid adsorption kinetics, high adsorption capacity, and specificity towards oxytetracycline provided by the core-shell composite structure. After screening the critical parameters by multivariate optimization, a magnetic imprinting solid phase extraction method combined with fluorescence spectrophotometry (MMIP-SPE-FL) was constructed for sensitive determination of oxytetracycline in seawater samples. The results show a good linear response dependence on the spiking concentration of 3-100 μg L^-1, and a satisfactory limit of detection of 0.7 μg L^-1 after the MMIP-SPE preconcentration. Seven seawater samples from Jiaozhou bay were analyzed to give recoveries in the range of 89.75-107.65% with relative standard deviation values of less than 5.44% (n = 3).

Recent development of antibiotic detection in food and environment: the combination of sensors and nanomaterials

In recent years, the abuse of antibiotics has led to the pollution of soil and water environment, not only poultry husbandry and food manufacturing will be influenced to different degree, but also the human body will produce antibody. The detection of antibiotic content in production and life is imperative. In this review, we provide comprehensive information about chemical sensors and biosensors for antibiotic detection. We classify the currently reported antibiotic detection technologies into chromatography, mass spectrometry, capillary electrophoresis, optical detection, and electrochemistry, introduce some representative examples for each technology, and conclude the advantages and limitations. In particular, the optical and electrochemical methods based on nanomaterials are discussed and evaluated in detail. In addition, the latest research in the detection of antibiotics by photosensitive materials is discussed. Finally, we summarize the pros and cons of various antibiotic detection methods and present a discussion and outlook on the expansion of cross-scientific areas. The synthesis and application of optoelectronic nanomaterials and aptamer screening are discussed and prospected, and the future trends and potential impact of biosensors in antibiotic detection are outlined.Graphical abstract.

Application of Multiplexed Aptasensors in Food Contaminants Detection

The existence of contaminants in food poses a serious threat to human health. In recent years, aptamer sensors (aptasensors) have been developed rapidly for the detection of food contaminants because of their high specificity, design flexibility, and high efficiency. However, the development of high-throughput, highly sensitive, on-site, and cost-effective methods for simultaneous detection of food contaminants is still restricted due to multiple signal overlap or mutual interference and cross-reaction between different analytes with similar molecular structures. To overcome these problems, this Review summarizes some effective strategies from the articles published in recent years about multiplexed aptasensors for the simultaneous detection of food contaminants. This work focuses on the application of multiplexed aptasensors to simultaneously detect antibiotics, pathogens, and mycotoxins in food. These aptasensors mainly contain fluorescent aptasensors, electrochemical aptasensors, surface-enhanced Raman scattering-based aptasensors, microfluidic chip aptasensors, and paper-based multiplexed aptasensors. In addition, this Review also covers the application of nucleic acid cycle amplification and nanomaterial amplification strategies to improve the detection sensitivity. Finally, the limitations and challenges in the design of multiplexed aptasensor are also taken into account.

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.

Recent Advances and Applications of Microfluidic Capillary Electrophoresis: A Comprehensive Review (2017-Mid 2019)

Microfluidic capillary electrophoresis (MCE) is the novel technique resulted from the CE mininaturization as planar separation and analysis device. This review presents and discusses various application fields of this advanced technology published in the period 2017 till mid-2019 in eight different sections including clinical, biological, single cell analysis, environmental, pharmaceuticals, food analysis, forensic and ion analysis. The need for miniaturization of CE and the consequence advantages achieved are also discussed including high-throughput, miniaturized detection, effective separation, portability and the need for micro- or even nano-volume of samples. Comprehensive tables for the MCE applications in the different studied fields are provided. Also, figure comparing the number of the published papers applying MCE in the eight discussed fields within the studied period is included. The future investigation should put into consideration the possibility of replacing conventional CE with the MCE after proper validation. Suitable validation parameters with their suitable accepted ranges should be tailored for analysis methods utilizing such unique technique (MCE).

Aptamer-based fluorometric determination of chloramphenicol by controlling the activity of hemin as a peroxidase mimetic

A method for the aptamer-based determination of chloramphenicol (CAP) was developed by exploiting the peroxidase mimicking activity of hemin. The method includes two hemin-modified DNA probes termed P1 and P2. P1, which was modified at its 5' end with one hemin monomer, contains the CAP-binding sequence. The hybridization between P1 and P2 brings the two hemin monomers in close proximity, resulting in the formation of a hemin dimer with low peroxidase mimicking activity. The duplex structure was dehybridized in the presence of CAP. The formed hemin monomer featured a strong peroxidase mimicking activity and catalyzed the conversion of non-fluorescent tyramine into fluorescent dityramine by hydrogen peroxide. Fluorescence (with an excitation/emission maxima at 320 and 410 nm, respectively) increased linearly in the 0.1 ng mL-1 to 10 ng mL-1 CAP concentration range. The detection limit based on the 3σ/k criterion reached 0.07 ng mL-1. The proposed assay was successfully employed for CAP detection in (spiked) honey samples with recoveries of 94.3-117.2%. Given its high sensitivity and good stability, this method shows potential in providing a platform for antibiotic detection.

Chiral Interaction Is a Decisive Factor To Replace d-DNA with l-DNA Aptamers

Nucleic acid aptamers have been widely used in various fields such as biosensing, DNA chip, and medical diagnosis. However, the high susceptibility of nucleic acids to ubiquitous nucleases reduces the biostability of aptamers and limits their applications in biological contexts. Therefore, improving the biostability of aptamers becomes an urgent need. Herein, we present a simple strategy to resolve this problem by directly replacing the d-DNA-based aptamers with left-handed l-DNA. By testing several reported aptamers against respective targets, we found that our proposed strategy stood up well for nonchiral small molecule targets (e.g., Hemin and cationic porphyrin) and chiral targets whose interactions with aptamers are chirality-independent (e.g., ATP). We also found that the l-DNA aptamers were indeed endowed with greatly improved biostability due to the extraordinary resistance of l-DNA to nuclease digestion. With respect to other small-molecule targets whose interactions with aptamers are chirality-dependent (e.g., kanamycin) and biomacromolecules (e.g., tyrosine kinase-7), however, the proposed strategy was not entirely effective likely due to the participation of the DNA backbone chirality into the target recognition. In spite of this limitation, this strategy indeed paves an easy way to screen highly biostable aptamers important for the applications in many fields.

GC-MS metabolomics reveals metabolic differences of the farmed Mandarin fish Siniperca chuatsi in recirculating ponds aquaculture system and pond

Siniperca chuatsi is currently one of the most important economic farmed freshwater fish in China. The aim of this study was to evaluate the metabolic profile of recirculating ponds aquaculture system (RAS)-farmed S. chuatsi. Gas Chromatography-Mass Spectrophotometry (GC-MS) metabolomic platform was used to comprehensively analyze the effects of recirculating ponds aquaculture system (RAS) on the Mandarin fish S. chuatsi metabolism. Database searching and statistical analysis revealed that there were altogether 335 metabolites quantified (similarity > 0) and 205 metabolites were identified by mass spectrum matching with a spectral similarity > 700. Among the 335 metabolites quantified, 33 metabolites were significantly different (VIP > 1 and p < 0.05) between RAS and pond groups. In these thirty-three metabolites, taurine, 1-Hexadecanol, Shikimic Acid, Alloxanoic Acid and Acetaminophen were higher in the pond group, while 28 metabolites were increased notably in the RAS group. The biosynthesis of unsaturated fatty acids, lysosome, tryptophan metabolism were recommended as the KEGG pathway maps for S. chuatsi farmed in RAS. RAS can provide comprehensive benefits to the effects of Siniperca chuatsi metabolism, which suggest RAS is an efficient, economic, and environmentally friendly farming system compared to pond system.

An All-in-One Aptasensor Integrating Enzyme Powered Three-Dimensional DNA Machine for Antibiotic Detection

In this work, we have developed an all-in-one aptasensor based on an enzyme-driven three-dimensional DNA walker for antibiotic detection. To overcome the drawback of time-consuming methods, high-density substrate strands were anchored on the walking interface that accelerated the signal amplification efficiency. Such an all-in-one design integrated the functionality of target recognition, signal amplification, as well as signal output into a single probe. Upon addition of kanamycin, the activated walking strand moved along the track by the stepwise cleavage of a nicking enzyme, which resulted in the enhancement of the fluorescence intensity of the solution. Under the optimized conditions, the detection process was accomplished in 40 min with a low detection limit of 1.23 pM. This aptasensor was also applied in spiked milk samples with satisfactory recoveries of 97.76% to 105.33%, demonstrating an excellent stability and accuracy. Therefore, this all-in-one aptasensor shows great potential for applications in food safety.

The Growing Interest in Development of Innovative Optical Aptasensors for the Detection of Antimicrobial Residues in Food Products

The presence of antimicrobial residues in food-producing animals can lead to harmful effects on the consumer (e.g., allergies, antimicrobial resistance, toxicological effects) and cause issues in food transformation (i.e., cheese, yogurts production). Therefore, to control antimicrobial residues in food products of animal origin, screening methods are of utmost importance. Microbiological and immunological methods (e.g., ELISA, dipsticks) are conventional screening methods. Biosensors are an innovative solution for the development of more performant screening methods. Among the different kinds of biosensing elements (e.g., antibodies, aptamers, molecularly imprinted polymers (MIP), enzymes), aptamers for targeting antimicrobial residues are in continuous development since 2000. Therefore, this review has highlighted recent advances in the development of aptasensors, which present multiple advantages over immunosensors. Most of the aptasensors described in the literature for the detection of antimicrobial residues in animal-derived food products are either optical or electrochemical sensors. In this review, I have focused on optical aptasensors and showed how nanotechnologies (nanomaterials, micro/nanofluidics, and signal amplification techniques) largely contribute to the improvement of their performance (sensitivity, specificity, miniaturization, portability). Finally, I have explored different techniques to develop multiplex screening methods. Multiplex screening methods are necessary for the wide spectrum detection of antimicrobials authorized for animal treatment (i.e., having maximum residue limits).

Lab-on-a-Chip Systems for Aptamer-Based Biosensing

Aptamers are oligonucleotides or peptides that are selected from a pool of random sequences that exhibit high affinity toward a specific biomolecular species of interest. Therefore, they are ideal for use as recognition elements and ligands for binding to the target. In recent years, aptamers have gained a great deal of attention in the field of biosensing as the next-generation target receptors that could potentially replace the functions of antibodies. Consequently, it is increasingly becoming popular to integrate aptamers into a variety of sensing platforms to enhance specificity and selectivity in analyte detection. Simultaneously, as the fields of lab-on-a-chip (LOC) technology, point-of-care (POC) diagnostics, and personal medicine become topics of great interest, integration of such aptamer-based sensors with LOC devices are showing promising results as evidenced by the recent growth of literature in this area. The focus of this review article is to highlight the recent progress in aptamer-based biosensor development with emphasis on the integration between aptamers and the various forms of LOC devices including microfluidic chips and paper-based microfluidics. As aptamers are extremely versatile in terms of their utilization in different detection principles, a broad range of techniques are covered including electrochemical, optical, colorimetric, and gravimetric sensing as well as surface acoustics waves and transistor-based detection.

Capillary electrophoresis-based immunoassay and aptamer assay: A review

Over the last two decades, the group of techniques called affinity probe CE has been widely used for the detection and the determination of several types of biomolecules with high sensitivity. These techniques combine the low sample consumption and high separation power of CE with the selectivity of the probe to the target molecule. The assays can be defined according to the type of probe used: CE immunoassays, with an antibody as the probe, or aptamer-based CE, with an aptamer as the probe. Immunoassays are generally divided into homogeneous and heterogeneous groups, and homogeneous variant can be further performed in competitive or noncompetitive formats. Interacting partners are free in solution at homogeneous assay, as opposed to heterogeneous analyses, where one of them is immobilized onto a solid support. Highly sensitive fluorescence, chemiluminescence or electrochemical detections were typically used in this type of study. The use of the aptamers as probes has several advantages over antibodies such as shorter generation time, higher thermal stability, lower price, and lower variability. The aptamer-based CE technique was in practice utilized for the determination of proteins in biological fluids and environmentally or clinically important small molecules. Both techniques were also transferred to microchip. This review is focused on theoretical principles of these techniques and a summary of their applications in research.

An electrochemical aptasensor for lead ion detection based on catalytic hairpin assembly and porous carbon supported platinum as signal amplification

A novel electrochemical aptasensor for lead detection based on catalytic hairpin assembly and PtNPs@PCs as signal amplification.

An aptamer-based fluorometric zearalenone assay using a lighting-up silver nanocluster probe and catalyzed by a hairpin assembly

An enzyme-free fluorometric assay is described for the determination of zearalenone (ZEN). The method combines (a) catalyzed hairpin assembly (CHA), (b) ultrahigh fluorescent light-up G-rich DNA sequences in proximity to silver nanoclusters (Ag NCs), and (c) the use of aptamers (Apt). In the presence of ZEN, the inhibit sequence (Inh) is released from the aptamer-trigger sequence (Apt-T) via the binding of ZEN and the aptamer of Apt-T. The free Apt-T acts as a switch that opens the hairpins H1 and H2 to generate H1-H2 complex. The released Apt-T is available to trigger the next round of CHA between H1 and H2. Finally, the hybridization between H1 and the Ag NCs probe (P) causes the G-rich sequence to be in close proximity to the dark Ag NCs encapsulated by P. This leads to highly efficient lighting up of the Ag NCs and the production of amplified fluorescence with excitation/emission peaks at 575/628 nm. Under the optimized conditions, a linear correlation was observed with concentrations ranging from 1.3 pg mL^-1 to 100 ng mL^-1, and the limit of detection was 0.32 pg mL^-1 (at S/N = 3). The method was successfully validated by analyzing maize and beer for levels of ZEN after a simple sample preparation procedure. Graphical abstractSchematic of the assay. The inhibit sequence (Inh) is released from aptamer-trigger sequence (Apt-T) via binding of ZEN and aptamer. The free Apt-T triggers catalyzed hairpin assembly (CHA).G-rich DNA is in proximity to silver nanoclusters (Ag NCs) and fluorescence intensity increases to detect ZEN.

An ultrasensitive microchip electrophoresis chemiluminescence assay platform for detection of trace biomolecules

An ultrasensitive microchip electrophoresis chemiluminescence (MCE-CL) assay platform based on separation assisted cascade signal amplification was developed for detection of trace biomolecules. In this work, the aptamer was used as a target probe to bind target molecule and triggering cascade signal amplification reaction. The horseradish peroxide labeled DNA (HRP-DNA) was used as signal probe, utilizing nucleic acid hybridization and exonuclease cutting technology realized ultrasensitive detection of biomolecules on the MCE-CL assay platform. Taking gamma interferon (IFN-γ) as a model analyte, the linear range for IFN-γ detection is 8.0 × 10^-15-1.0 × 10^-8 M, the detection limit is 1.6 fM, which is six orders magnitude lower than that of without signal amplification. The proposed method was successfully applied for the quantification of IFN-γ in human plasma samples. It was demonstrated that the MCE-CL assay platform was quick, sensitive, and highly selective. It may serve as a tool for clinical analysis of IFN-γ to assist in the diagnosis of disease.

Enrichment-Stowage-Cycle Strategy for Ultrasensitive Electrochemiluminescent Detection of HIV-DNA with Wide Dynamic Range

Sensitive detection of human immunodeficiency virus DNA (HIV-DNA) is essential for timely diagnosis and cure of the illness. Herein, a novel "enrichment-stowage-cycle" strategy was proposed to fabricate a multiple amplified electrochemiluminecence (ECL) biosensor for HIV-DNA detection. On the basis of the enrichment role of magnetic nanobeads, assembly role of copolymer nanospheres and strand displacement amplification (SDA), the processes were named as "enrichment", "stowage", and "cycle", respectively. The method employed electrochemiluminescent nanospheres (ENs) as signal labels by assembling three layers of CdSe/ZnS quantum dots (QDs) onto the surface of copolymer nanospheres. Compared to QDs, the same concentration of ENs can the enhance the ECL intensity by about 11.3-fold. SDA could further amplify the signals by about 3.77-fold, possessing high sensitivity for low-abundant biomarkers detection. The integration of magnetic separation improved detection specificity and stability, making the method possible for practical application. On the basis of magnetic separation, ENs and SDA, the ECL biosensor realized ultrasensitive detection of 39.81 fM HIV-DNA, which was more sensitive than other HIV-DNA analytical methods, with a wide dynamic range of 0.05 pM to 50 nM. The successful detection of HIV-DNA in complex samples with good sensitivity and accuracy indicated its potential utilization in early judgment of diseases and fabrication of signal amplification platforms.

Recent advances of molecularly imprinted polymer-based sensors in the detection of food safety hazard factors

With increasing economic globalization, food safety is becoming the most serious concern in the food production and distribution system. Food safety hazard factors (FSHFs) can be categorized into chemical hazards, biological hazards and physical hazards, with the detection of the former two having fascinated interdisciplinary research areas spanning chemistry, material science and biological science. Molecularly imprinted polymer (MIP) -based sensors overcome many limitations of traditional detection methods and provide opportunities for efficient, sensitive and low-cost detection using smart miniaturized equipment. With highly specific molecular recognition capacity and high stability in harsh chemical and physical conditions, MIPs have been used in sensing platforms such as electrochemical, optical and mass-sensitive sensors as promising alternatives to bio-receptors for food analysis. In this systemic review, we summarize recent advances of MIPs and MIP-based sensors, such as popular monomers, usual polymerization strategies, fresh modification materials and advanced sensing mechanisms. The applications of MIP-based sensors in FSHF detection are discussed according to sensing mechanisms, including electrochemistry, optics and mass-sensitivity. Finally, future perspectives and challenges are discussed.

Methods for field measurement of antibiotic concentrations: limitations and outlook

The growing prevalence of antibiotic resistance poses an increasingly serious threat to human health. Although an important driver of antibiotic resistance is the continuous exposure of bacteria to sublethal concentrations of antibiotics in natural environments, antibiotic pollutants are not currently tracked globally or systematically. This limits the international capacity to address the rise of antibiotic resistance at its source. To address this lack of data, the development of methods to measure antibiotic concentrations on-site is essential. These methods, ideally, must be sensitive to sublethal concentrations of antibiotics and require minimal technical expertise. Furthermore, factors such as cost, selectivity, biosafety and the ability to multiplex must be evaluated in the context of field use. Based on these criteria, we provide a critical review of current methods in antibiotic detection and evaluate their adaptability for use on-site. We categorize these methods into microbiological assays, physical and chemical assays, immunoassays, aptasensors and whole-cell biosensors. We recommend continued development of a dipstick or microfluidics approach with a bacterial promoter-based mechanism and colorimetric output. This technique would incorporate the advantageous aspects of existing methods, maximize shelf-life and ease-of-use, and require minimal resources to implement in the field.

Self-Assembly of Functional Nucleic Acid-Based Colorimetric Competition Assay for the Detection of Immunoglobulin E

In this work, we have developed a simple and rapid colorimetric assay for the detection of immunoglobulin E (IgE) using functional nucleic acids (FNAs) and a solid-phase competition enzyme-linked immunosorbent assay (ELISA). The FNAs including aptamer of recombinant IgE, G-quadruplex and its complementary fragments were immobilized on 96-well microplates to achieve recognition and detection of IgE in biological samples. The G-quadruplex DNAzyme catalyzed 2,2'-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid (ABTS)-hemin-H2O2 system was used to improve the sensitivity of colorimetric assay. In the presence of IgE, the hairpin structure and G-quadruplex would be destroyed, resulting in the inactivation of DNAzyme and subsequent reduction of its absorbance. This cost-effective approach detected IgE in the linear range from 5.0 pg/mL to 500 ng/mL, with the limit of detection (LOD) of 2.0 pg/mL, under optimal conditions. Moreover, the developed method was successfully applied to the rapid detection of IgE in human urine, indicating a great potentiality of this approach in clinical diagnosis and other biomedical applications.

Multiplex detection of quality indicator molecule targets in urine using programmable hairpin probes based on a simple double-T type microchip electrophoresis platform and isothermal polymerase-catalyzed target recycling

Recently, it has been crucial to be able to detect and quantify small molecular targets simultaneously in biological samples. Herein, a simple and conventional double-T type microchip electrophoresis (MCE) based platform for the multiplex detection of quality indicator molecule targets in urine, using ampicillin (AMPI), adenosine triphosphate (ATP) and estradiol (E2) as models, was developed. Several programmable hairpin probes (PHPs) were designed for detecting different targets and triggering isothermal polymerase-catalyzed target recycling (IPCTR) for signal amplification. Based on the target-responsive aptamer structure of PHP (Domain I), target recognition can induce PHP conformational transition and produce extension duplex DNA (dsDNA), assisted by primers & Bst polymerase. Afterwards, the target can be displaced to react with another PHP and initiate the next cycle. After several rounds of reaction, the dsDNA can be produced in large amounts by IPCTR. Three targets can be simultaneously converted to dsDNA fragments with different lengths, which can be separated and detected using MCE. Thus, a simple double-T type MCE based platform was successfully built for the homogeneous detection of multiplex targets in one channel. Under optimal conditions, the assay exhibited high throughput (48 samples per hour at most, not including reaction time) and sensitivity to three targets in urine with a detection limit of 1 nM (ATP), 0.05 nM (AMPI) and 0.1 nM (E2) respectively. The multiplex assay was successfully employed for the above three targets in several urine samples and combined the advantages of the high specificity of programmable hairpin probes, the excellent signal amplification of IPCTR, and the high through-put of MCE which can be employed for screening in biochemical analysis.

Microchip electrophoresis based aptasensor for multiplexed detection of antibiotics in foods via a stir-bar assisted multi-arm junctions recycling for signal amplification

Microchip electrophoresis (MCE) was a good available method for high-throughput and rapid detecting chemical pollutants in food samples. However, many of the reported MCE assays involve complex design of microchip, laborious operation and poor universality which limited its promotion in multiple antibiotics' detection. Herein, a multiplexed aptasensor was developed based on a universal double-T type microchip to one-step and simultaneously detect several antibiotics within 3 min using chloramphenicol (CAP) and kanamycin (Kana) as representatives. Besides, a novel stir-bar assisted DNA multi-arm junctions recycling (MAJR) strategy was designed for transducing and amplifying the signal. The brief detection mechanism was as following: the added CAP and Kana can specifically react with their aptamer probes on the stir-bar and produce different single-stranded DNA primer, respectively. Afterwards, the primers can trigger MAJR to form a lot of three- and four-arm DNA junctions corresponding to different targets. The DNA multi-arm junctions can be easily separated and detected by MCE for quantification. Moreover, the stir-bar can facilitate phase separation and obviously eliminate matrix interference in food. The assay was successfully applied in milk and fish samples, showing excellent selectivity and sensitivity with a detection limits of 0.52 pg mL^-1 CAP and 0.41 pg mL^-1 Kana (S/N = 3). Thus, the assay holds a great potential application for screening of antibiotics in food.

A fluorometric aptamer method for kanamycin by applying a dual amplification strategy and using double Y-shaped DNA probes on a gold bar and on magnetite nanoparticles

A simple and highly sensitive fluorometric method is described for the determination of the antibiotic kanamycin (Kana) in food. Dual signal amplification is accomplished by making use of double Y-shaped aptamer DNA probes acting as a capture probes and signal amplification probes. The DNA probes were immobilized on a gold bar and on a magnetic bar, respectively. On addition of Kana, the Y-shaped aptamer probe captures Kana and then is disassembled to release two single-stranded DNAs. These trigger target recycling and HCR between the two bars simultaneously. As a result, many long duplex DNA chains are formed in the supernatant. After pulling out the bars and adding the fluorescent intercalating probe SYBR Green I, strong fluorescence (with excitation/emission peaks at 497/525 nm) is induced. The use of such double Y-shaped DNA probes obviously overcomes the unspecific signal amplification by HCR which increases selectivity and sensitivity. This is due to the fact that the hairpin of HCR is separated in being present in different arms of the Y-shaped probe. Under the optimal conditions, the assay has a limit of 0.45 pg·mL^-1 for Kana. It was applied to analyze spiked milk, fish and pork samples. Graphical abstract The scheme represents a sensitive fluorometric aptamer-based method to detect kanamycin (Kana). It is making use of a double stirring bar-assisted dual amplification strategy with zero background. Abbreviations: apt: aptamer, AuNPs: gold nanoparticles, HCR: hybridization chain reaction.

Construction of Ce-MOF@COF hybrid nanostructure: Label-free aptasensor for the ultrasensitive detection of oxytetracycline residues in aqueous solution environments

Porous organic framework (COF) nanomaterials have drawn increasing attention and showed promising potential in the applications of various fields. Nevertheless, its applications in biosensing or biomedical fields are still in the early stage. In this work, we designed and synthesized a series of nanohybrids of COF and Ce-based metal organic framework (Ce-MOF) for the first time as label-free bioplatforms for a sensitive electrochemical aptasensor to detect oxytetracycline (OTC). A novel kinds of Ce-MOF@COF hybrids were prepared by adding different dosages of COF, into the preparation system of Ce-MOF, for which COF was synthesized using melamine and cyanutic acidmonomers through polycondensation (represented by MCA). Basic characterizations revealed that Ce-MOF@MCA nanohybrids not only remained their orignal crystal and chemical structure and features, such as different Ce species containing in Ce-MOF (Ce³⁺ and Ce⁴⁺), various functional amino-groups of MCA, and individual frameworks, but also showed a large specific surface area and interpenetrated morphologies. As a result, the Ce-MOF@MCA hybrid with high content of MCA exhibited high bioaffinity toward the OTC-targeted aptamer, further leading to the incremental detection effect for OTC detection. Among different hybrid-based aptasensors, the Ce-MOF@MCA-based one with an MCA dosage of 500 mg exhibited the lowest limit of detection at 17.4 fg mL^-1 within a wider linearity of the OTC concentration within 0.1-0.5 ng mL^-1. Additionally, the fabricated aptasensor displayed excellent analytical performance with great reproducibility, high selectivity and stability, and acceptable applicability for detecting OTC in various aqueous solutions, including milk, wastewater, and urine samples. This new Ce-MOF@MCA hybrid will become an excellent aptasensors platform for detecting various analytes, such as antibiotics, heavy metal ions, or cancer markers, and it have shown the promissing application potentials in the fields of biomedicine, food safety and environmental monitoring.

Multiplexed electrochemical aptasensor for antibiotics detection using metallic-encoded apoferritin probes and double stirring bars-assisted target recycling for signal amplification

Simultaneous and sensitive detection of various antibiotic residues in one sample is essential to evaluation of food safety status. Herein, a multiplexed electrochemical aptasensor for multiplex antibiotics detection, with kanamycin (KANA) and ampicillin (AMP) as representative analytes, was designed by using metal ions encoded apoferrtin probes and double stirring bars-assisted target recycling for signal amplification. The encoded probes were prepared by apoferritin loading Cd²⁺ and Pb²⁺ ions and labeling with duplex DNAs (aptamers corresponding to KANA and AMP hybrid with its complementary DNA sequence), respectively. In the presence of KANA and AMP, the targets can recurrently react with the probes on the bars, and then replace a lot of Apo-Mencoded signal tags into supernatant. The peak currents of Cd²⁺and Pb²⁺from the tags corresponding with the concentrations of KANA and AMP were detected by square wave voltammetry in one run. As a result, KANA and AMP can be detected simultaneously within the range from 0.05 pM to 50 nM. And the detection limits were 18 fM KANA and 15 fM AMP (S/N = 3). The assay was testified to detect KANA and AMP residues with consistent results of ELISA in samples, e.g. milks and fishes. The assay was highly-sensitive, selective, cost-effective and easy-to-operate due to Apo-M encoded probes with high loading capacity of signal source substances. Moreover, double stirring bar-assisted target recycling, which was enzyme-free and could overcome matrix interference, was fabricated for signal amplification. Thus, the assay showed potential advantages for sensitively screening of antibiotic residues in foods.

A colorimetric aptasensor for the antibiotics oxytetracycline and kanamycin based on the use of magnetic beads and gold nanoparticles

An aptamer based assay is presented for the determination of the antibiotics oxytetracycline (OTC) and kanamycin (KAN). Magnetic beads were applied for separation, and gold nanoparticles (AuNPs) for signal amplification. DNA aptamers against OTC and KAN were firstly designed. After specific recognition events, the aptamer sequences were released from the surface of magnetic beads and the remaining DNA probes captured horseradish peroxidase (HRP) modified AuNPs. Subsequently, 3,3',5,5'-tetramethylbenzidine and o-phenylenediamine are catalytically oxidized by HRP, and the generated colorimetric responses can reflect the concentrations of OTC (at 370 nm) and KAN (at 450 nm), respectively. Experimental results demonstrate that the method is highly sensitive with the detection limit as low as 1 ag mL^-1 for OTC and KAN. An extremely wide linear range (over 11 orders of magnitude) is achieved. The high selectivity is attributed to the high affinity between aptamer and the substrate. The results of real sample tests also verify that the method is promising for antibiotics analysis in the applications of food monitoring and clinical diagnosis. Graphical abstract Schematic presentation of a colorimetric assay for antibiotics based on aptamer-modified magnetic beads and horseradish peroxidase modified gold nanoparticles. Colorimetric responses result from the enzymatic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) and o-phenylenediamine (OPD), respectively.

A Novel AS1411 Aptamer-Based Three-Way Junction Pocket DNA Nanostructure Loaded with Doxorubicin for Targeting Cancer Cells in Vitro and in Vivo

Active targeting of nanostructures containing chemotherapeutic agents can improve cancer treatment. Here, a three-way junction pocket DNA nanostructure was developed for efficient doxorubicin (Dox) delivery into cancer cells. The three-way junction pocket DNA nanostructure is composed of three strands of AS1411 aptamer as both a therapeutic aptamer and nucleolin target, the potential biomarker of prostate (PC-3 cells) and breast (4T1 cells) cancers. The properties of the Dox-loaded three-way junction pocket DNA nanostructure were characterized and verified to have several advantages, including high serum stability and a pH-responsive property. Cellular uptake studies showed that the Dox-loaded DNA nanostructure was preferably internalized into target cancer cells (PC-3 and 4T1 cells). MTT cell viability assay demonstrated that the Dox-loaded DNA nanostructure had significantly higher cytotoxicity for PC-3 and 4T1 cells compared to that of nontarget cells (CHO cells, Chinese hamster ovary cell). The in vivo antitumor effect showed that the Dox-loaded DNA nanostructure was more effective in prohibition of the tumor growth compared to free Dox. These findings showed that the Dox-loaded three-way junction pocket DNA nanostructure could significantly reduce the cytotoxic effects of Dox against nontarget cells.

Recent Microdevice-Based Aptamer Sensors

Since the systematic evolution of ligands by exponential enrichment (SELEX) method was developed, aptamers have made significant contributions as bio-recognition sensors. Microdevice systems allow for low reagent consumption, high-throughput of samples, and disposability. Due to these advantages, there has been an increasing demand to develop microfluidic-based aptasensors for analytical technique applications. This review introduces the principal concepts of aptasensors and then presents some advanced applications of microdevice-based aptasensors on several platforms. Highly sensitive detection techniques, such as electrochemical and optical detection, have been integrated into lab-on-a-chip devices and researchers have moved towards the goal of establishing point-of-care diagnoses for target analyses.