Perspective on the Future Role of Aptamers in Analytical Chemistry

Aptamer melting biosensors for thousands of signaling and regenerating cycles

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

Ferrocenyl Compounds as Alternative Redox Labels for Robust and Versatile Electrochemical Aptamer-Based Sensors

This study explores the potential of seven ferrocenyl (Fc) compounds with cross-linking groups as alternative redox labels to methylene blue (MB) for electrochemical aptamer-based (E-AB) sensors. The cross-linking efficiency, formal potential (E0'), and electrochemical durability of these compounds were evaluated. Compound Fc1a-X exhibited superior performance, characterized by efficient cross-linking, a moderate and pH-insensitive E0', and enhanced durability during repeated potential scans. The attachment of Fc1a-X, which includes a 3-carbon chain spacer and an N-hydroxysuccinimide-ester cross-linking group, to an amine-terminated monolayer on a Au electrode demonstrated high cross-linking efficiency, which is critical for achieving high sensitivity. The E0' of Fc1a-X attached to the aptamer monolayer was 0.14 V, which is within the optimal range of -0.2 to 0.2 V vs Ag/AgCl. Square wave voltammetry showed that the peak potential and current of Fc1a-X are pH-insensitive, which is critical for versatile use. In serum, Fc1a-X maintained stable peak current levels without a gradual decrease after an initial rapid decrease during the first 2 h with considerably less reduction over 12 h compared to MB. Using Fc1a-X as the redox label, an E-AB sensor effectively detected doxorubicin in serum, covering the clinical range. These findings suggest Fc1a-X as a promising candidate for developing robust, versatile, and sensitive E-AB sensors.

Immuno-Rolling Circle Amplification (Immuno-RCA): Biosensing Strategies, Practical Applications, and Future Perspectives

In the rapidly evolving field of life sciences and biomedicine, detecting low-abundance biomolecules, and ultraweak biosignals presents significant challenges. This has spurred a rapid development of analytical techniques aiming for increased sensitivity and specificity. These advancements, including signal amplification strategies and the integration of biorecognition events, mark a transformative era in bioanalytical precision and accuracy. A prominent method among these innovations is immuno-rolling circle amplification (immuno-RCA) technology, which effectively combines immunoassays with signal amplification via RCA. This process starts when a targeted biomolecule, such as a protein or cell, binds to an immobilized antibody or probe on a substrate. The introduction of a circular DNA template triggers RCA, leading to exponential amplification and significantly enhanced signal intensity, thus the target molecule is detectable and quantifiable even at the single-molecule level. This review provides an overview of the biosensing strategy and extensive practical applications of immuno-RCA in detecting biomarkers. Furthermore, it scrutinizes the limitations inherent to these sensors and sets forth expectations for their future trajectory. This review serves as a valuable reference for advancing immuno-RCA in various domains, such as diagnostics, biomarker discovery, and molecular imaging.

Glycan-Evocated Metallization for Amplification-Free Electrochemical Detection of Glycoproteins at Low Concentration Levels

Glycoproteins have become the most often screened tumor markers in the in vitro diagnostics. Although a large number of electrochemical methods have been proposed to sensitively detect glycoproteins, most of them involve the aid of laborious signal amplification. Herein, we report the use of glycan-evocated metallization (GlyMetal) for the amplification-free electrochemical detection of glycoproteins at low concentration levels. Briefly, the glycoproteins of interest are captured by an aptamer recognition layer, and then the glycans of targets are oxidized by NaIO4 to convert the 1,2-diol sites into aldehyde groups for the silver deposition-based metallization, followed by the electrochemical stripping assay of the deposited metallic silver for glycoprotein quantification via the established solid-state Ag/AgCl voltammetric process. As GlyMetal can enable the deposition of a large amount of metallic silver and a high signal-to-background ratio can be obtained for the solid-state Ag/AgCl voltammetric stripping assay, the developed GlyMetal-based electrochemical method is applicable to the amplification-free detection of glycoproteins. As a proof of concept, a detection limit of 1.65 pg/mL has been achieved for carcinoembryonic antigen (CEA) detection. In addition to the high selectivity, desirable results have been obtained with respect to the use of the method for CEA detection in serum samples. In consideration of the desirable simplicity, short assay time, and cost-effectiveness of the amplification-free approach, the GlyMetal-based electrochemical method shows great promise in the point-of-care detection of glycoproteins.

Label-free, real-time monitoring of cytochrome C drug responses in microdissected tumor biopsies with a multi-well aptasensor platform

Functional assays on intact tumor biopsies can complement genomics-based approaches for precision oncology, drug testing, and organs-on-chips cancer disease models by capturing key therapeutic response determinants, such as tissue architecture, tumor heterogeneity, and the tumor microenvironment. Most of these assays rely on fluorescent labeling, a semiquantitative method best suited for single-time-point assays or labor-intensive immunostaining analysis. Here, we report integrated aptamer electrochemical sensors for on-chip, real-time monitoring of cytochrome C, a cell death indicator, from intact microdissected tissues with high affinity and specificity. The platform features a multi-well sensor layout and a multiplexed electronic setup. The aptasensors measure increases in cytochrome C in the supernatant of mouse or human microdissected tumors after exposure to various drug treatments. Because of the sensor's high affinity, it primarily tracks rising concentrations of cytochrome C, capturing dynamic changes during apoptosis. This approach could help develop more advanced cancer disease models and apply to other complex in vitro disease models, such as organs-on-chips and organoids.

Facile Synthesis of Aptamer-Functionalized Polydopamine-Coated Magnetic Graphene Oxide Nanocomposites for Highly Efficient Purification of His-Tagged Proteins

Recombinant proteins hold significant importance in numerous disciplines. As the demand for expressing and purifying these proteins grows, the scientific community is in dire need of a simple yet versatile methodology that can efficiently purify these proteins. Aptamers as synthetic nucleic acid-based ligands with high affinity have shown promise in this regard, as they can capture targets through molecular recognition. In this study, novel aptamer-functionalized polydopamine-coated magnetic graphene oxide nanocomposites were facilely prepared, achieving an impressive average aptamer coverage density (45 nmol/mg). These nanocomposites exhibited a uniform structure and robust magnetic responsiveness. The findings indicated that they possess several advantages, such as rapid adsorption, substantial capacity (171.4 mg/g), and excellent reusability. Notably, due to the inherent properties of nucleic acids, the immobilized aptamer-magnetic beads can be utilized repeatedly with high purification efficiency. Finally, the nanocomposites were further employed to purify His-tagged proteins from actual samples. Remarkably, they were able to selectively and efficiently isolate His-tagged retinoid X receptor alpha protein from complex Escherichia coli lysate. The purified His-tagged retinoid X receptor alpha protein was analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. This confirmed the efficacy of developed nanocomposites, reinforcing their vast potential for purification of His-tagged recombinant proteins.

Photoelectrochemistry of Redox-Active Self-Assembled Monolayers Formed on n-Si/Au Nanoparticle Photoelectrodes

Controlling the chemistry of the electrode-solution interface is critically important for applications in sensing, energy storage, corrosion prevention, molecular electronics, and surface patterning. While numerous methods of chemically modifying electrodes exist, self-assembled monolayers (SAMs) containing redox-active moieties are particularly important because they are easy to prepare, have well-defined interfaces, and can exhibit textbook photoelectrochemistry. Here, we investigate the photoelectrochemistry of redox-active SAMs on semiconductor/metal interfaces, where the SAM is attached to the metal site instead of the semiconductor. n-Si/Au photoelectrodes were fabricated using a benchtop electrodeposition procedure and subsequently modified by immersion in aqueous solutions of (ferrocenyl)hexanethiol and mercaptohexanol. We explored the relevant preparation conditions, finding that after optimization, we were able to obtain canonical cyclic voltammetry for a surface-bound redox molecule that could be turned on and off using light. We then characterized the optimized electrodes under varying illumination intensities, finding that the heterogeneous electron transfer kinetics improved under higher illumination intensities. These results lay the foundation for future studies of semiconductor/metal/molecule interfaces relevant to sensing and electrocatalysis.

Rapid nanomolar detection of cocaine in biofluids by electrochemical aptamer-based sensor with low-temperature effect for drugged driving screening

Cocaine is one of the most abused illicit drugs, and its abuse damages the central nervous system and can even lead directly to death. Therefore, the development of simple, rapid and highly sensitive detection methods is crucial for the prevention and control of drug abuse, traffic accidents and crime. In this work, an electrochemical aptamer-based (EAB) sensor based on the low-temperature enhancement effect was developed for the direct determination of cocaine in bio-samples. The signal gain of the sensor at 10 °C was greatly improved compared to room temperature, owing to the improved affinity between the aptamer and the target. Additionally, the electroactive area of the gold electrode used to fabricate the EAB sensor was increased 20 times by a simple electrochemical roughening method. The porous electrode possesses more efficient electron transfer and better antifouling properties after roughening. These improvements enabled the sensor to achieve rapid detection of cocaine in complex bio-samples. The low detection limits (LOD) of cocaine in undiluted urine, 50% serum and 50% saliva were 70 nM, 30 nM and 10 nM, respectively, which are below the concentration threshold in drugged driving screening. The aptasensor was simple to construct and reusable, which offers potential for drugged driving screening in the real world.

Mg2+-promoted high-efficiency DNA conjugation on polydopamine surfaces for aptamer-based ochratoxin A detection

BACKGROUND

Surface immobilization of DNA is the foundation of a broad range of applications in biosensing and specific DNA extraction. Polydopamine (PDA) coatings can serve as intermediate layers to immobilize amino- or thiol-labelled molecules, including DNA, onto various materials through Michael addition and/or Schiff base reactions. However, the conjugation efficiency is limited by electrostatic repulsion between negatively charged DNA and PDA. Recently, it has been reported that polyvalent metal ions (such as Mg2+ and Ca2+) can mediate the adsorption of DNA on PDA surfaces. Inspired by this, in this work we aimed to exploit polyvalent metal ions to facilitate the conjugation of DNA on PDA.

RESULTS

Mg2+ was used to promote the conjugation of amino-terminated DNA complementary to ochratoxin A (OTA) aptamer (cDNA-NH2) on PDA-coated magnetic nanoparticles (Fe3O4@PDA). After the reaction, the unlinked cDNA-NH2 adsorbed on Fe3O4@PDA mediated by Mg2+ was removed with EDTA. In the presence of 20 mM Mg2+, the amount of covalently linked cDNA-NH2 increased approximately 11-fold compared to that in the absence of Mg2+. The resulting Fe3O4@PDA@cDNA conjugates exhibited superior hybridization capacity towards OTA aptamers, minimal nonspecific adsorption, and excellent chemical stability. The conjugates combined with fluorophore-labelled aptamers were employed for OTA detection, achieving a limit of detection (LOD) of 2.77 ng mL-1. To demonstrate versatility, this conjugation method was extended to Ca2+-promoted conjugation of cDNA-NH2 on Fe3O4@PDA nanoparticles and Mg2+-promoted conjugation of cDNA-NH2 on PDA-coated 96-well plates.

SIGNIFICANCE

The conjugation efficiency of DNA on PDA was significantly improved with the assistance of polyvalent metal ions (Mg2+ and Ca2+), providing a facile and efficient method for DNA immobilization. Due to the substrate-independent adhesion property of PDA, this method demonstrates versatility in DNA surface modification and holds great potential for applications in target extraction, biosensing, and other fields.

Precision-SELEX aptamer screening for the colorimetric and fluorescent dual-readout aptasensing of AFB1 in food

As nucleic acid-based affinity elements, aptamers have attracted significant attention for a wide range of analytical applications. Although several aflatoxin B1 (AFB1) aptamers have been identified, they are unsuitable for overcoming the unavoidable cross-reactions from interferents in complex food matrices due to their poor binding affinities and specificities. Herein, a novel precision-systematic evolution of ligands by exponential enrichment (P-SELEX) strategy through introducing the counter (matrix without target AFB1) and positive (with AFB1) screening steps was implemented to accurately identify AFB1 aptamers. A DNA aptamer A-42-2 at a 24-nt length was selected finally, which possessed nanomolar-level affinity of 5.55 nM, high specificity to other interferents, and strong anti-cross-reactivity ability for matrix components. Then, an A-42-2 aptamer-based ultra-sensitive colorimetric and fluorescent dual-readout aptasensor was fabricated for AFB1 detection in three kinds of complex food samples rich in starch without cross-reactions. The aptasensor displayed outstanding detection capacity with a wide liner range of 0.25-30 nM (1.95-234.4 μg/kg), while the detection limit for colorimetric measurement as low as 0.22 nM (1.72 μg/kg) and 0.048 nM (0.20 μg/kg) for fluorescent determination. P-SELEX is ideal for screening and applying aptamers in complex food matrices, creating more opportunities for the efficient and cost-effective development of high-quality aptamers and aptasensors for other targets.

Mg2+- or Ca2+-regulated aptamer adsorption on polydopamine-coated magnetic nanoparticles for fluorescence detection of ochratoxin A

It has been observed that polyvalent metal ions can mediate the adsorption of DNA on polydopamine (PDA) surfaces. Exploiting this, we used two divalent metal ions (Mg2+ or Ca2+) to promote the adsorption of fluorescence-labelled ochratoxin A (OTA) aptamers on PDA-coated magnetic nanoparticles (Fe3O4@PDA). Based on the different adsorption affinities of free aptamers and OTA-bound aptamers, a facile assay method was established for OTA detection. The aptamers adsorbed on Fe3O4@PDA were removed via simple magnetic separation, and the remaining aptamers in the supernatant exhibited a positive correlation with the OTA concentration. The concentrations of Mg2+ and Ca2+ were finely tuned to attain the optimal adsorption affinity and sensitivity for OTA detection. In addition, other factors, including the Fe3O4@PDA dosage, pH, mixing order, and incubation time, were studied. Finally, under optimized conditions, a detection limit (3σ/s) of 1.26 ng/mL was achieved for OTA. Real samples of spiked red wine were analysed with this aptamer-based method. This is the first report of regulating aptamer adsorption on the PDA surface with polyvalent metal ions for OTA detection. By changing the aptamers, the method can be easily extended to other target analytes.

Are Aptamers Really Promising as Receptors for Analytical Purposes? Insights into Anti-Lysozyme DNA Aptamers through a Multitechnique Study

Aptamers are recognition elements increasingly used for the development of biosensing strategies, especially in the detection of proteins or small molecule targets. Lysozyme, which is recognized as an important biomarker for various diseases and a major allergenic protein found in egg whites, is one of the main analytical targets of aptamer-based biosensors. However, since aptamer-based strategies can be prone to artifacts and data misinterpretation, rigorous strategies for multifaceted characterization of the aptamer-target interaction are needed. In this work, a multitechnique approach has been devised to get further insights into the binding performance of the anti-lysozyme DNA aptamers commonly used in the literature. To study molecular interactions between lysozyme and different anti-lysozyme DNA aptamers, measurements based on a magneto-electrochemical apta-assay, circular dichroism spectroscopy, fluorescence spectroscopy, and asymmetrical flow field-flow fractionation were performed. The reliability and versatility of the approach were proved by investigating a SELEX-selected RNA aptamer reported in the literature, that acts as a positive control. The results confirmed that an interaction in the low micromolar range is present in the investigated binding buffers, and the binding is not associated with a conformational change of either the protein or the DNA aptamer. The similar behavior of the anti-lysozyme DNA aptamers compared to that of randomized sequences and polythymine, used as negative controls, showed nonsequence-specific interactions. This study demonstrates that severe testing of aptamers resulting from SELEX selection is the unique way to push these biorecognition elements toward reliable and reproducible results in the analytical field.

Label-Free, Real-Time Monitoring of Cytochrome C Responses to Drugs in Microdissected Tumor Biopsies with a Multi-Well Aptasensor Platform

Functional assays on intact tumor biopsies can potentially complement and extend genomics-based approaches for precision oncology, drug testing, and organs-on-chips cancer disease models by capturing key determinants of therapeutic response, such as tissue architecture, tumor heterogeneity, and the tumor microenvironment. Currently, most of these assays rely on fluorescent labeling, a semi-quantitative method best suited to be a single-time-point terminal assay or labor-intensive terminal immunostaining analysis. Here, we report integrated aptamer electrochemical sensors for on-chip, real-time monitoring of increases of cytochrome C, a cell death indicator, from intact microdissected tissues with high affinity and specificity. The platform features a multi-well sensor layout and a multiplexed electronic setup. The aptasensors measure increases in cytochrome C in the supernatant of mouse or human microdissected tumors after exposure to various drug treatments. Since the aptamer probe can be easily exchanged to recognize different targets, the platform could be adapted for multiplexed monitoring of various biomarkers, providing critical information on the tumor and its microenvironment. This approach could not only help develop more advanced cancer disease models but also apply to other complex in vitro disease models, such as organs-on-chips and organoids.

Rationally Designed DNA-Based Scaffolds and Switching Probes for Protein Sensing

The detection of a protein analyte and use of this type of information for disease diagnosis and physiological monitoring requires methods with high sensitivity and specificity that have to be also easy to use, rapid and, ideally, single step. In the last 10 years, a number of DNA-based sensing methods and sensors have been developed in order to achieve quantitative readout of protein biomarkers. Inspired by the speed, specificity, and versatility of naturally occurring chemosensors based on structure-switching biomolecules, significant efforts have been done to reproduce these mechanisms into the fabrication of artificial biosensors for protein detection. As an alternative, in scaffold DNA biosensors, different recognition elements (e.g., peptides, proteins, small molecules, and antibodies) can be conjugated to the DNA scaffold with high accuracy and precision in order to specifically interact with the target protein with high affinity and specificity. They have several advantages and potential, especially because the transduction signal can be drastically enhanced. Our aim here is to provide an overview of the best examples of structure switching-based and scaffold DNA sensors, as well as to introduce the reader to the rational design of innovative sensing mechanisms and strategies based on programmable functional DNA systems for protein detection.

Nucleic Acid Aptamer-Based Biosensors: A Review

Aptamers, short strands of either DNA, RNA, or peptides, known for their exceptional specificity and high binding affinity to target molecules, are providing significant advancements in the field of health. When seamlessly integrated into biosensor platforms, aptamers give rise to aptasensors, unlocking a new dimension in point-of-care diagnostics with rapid response times and remarkable versatility. As such, this review aims to present an overview of the distinct advantages conferred by aptamers over traditional antibodies as the molecular recognition element in biosensors. Additionally, it delves into the realm of specific aptamers made for the detection of biomarkers associated with infectious diseases, cancer, cardiovascular diseases, and metabolomic and neurological disorders. The review further elucidates the varying binding assays and transducer techniques that support the development of aptasensors. Ultimately, this review discusses the current state of point-of-care diagnostics facilitated by aptasensors and underscores the immense potential of these technologies in advancing the landscape of healthcare delivery.

Aptamer-Protein Interactions: From Regulation to Biomolecular Detection

Serving as the basis of cell life, interactions between nucleic acids and proteins play essential roles in fundamental cellular processes. Aptamers are unique single-stranded oligonucleotides generated by in vitro evolution methods, possessing the ability to interact with proteins specifically. Altering the structure of aptamers will largely modulate their interactions with proteins and further affect related cellular behaviors. Recently, with the in-depth research of aptamer-protein interactions, the analytical assays based on their interactions have been widely developed and become a powerful tool for biomolecular detection. There are some insightful reviews on aptamers applied in protein detection, while few systematic discussions are from the perspective of regulating aptamer-protein interactions. Herein, we comprehensively introduce the methods for regulating aptamer-protein interactions and elaborate on the detection techniques for analyzing aptamer-protein interactions. Additionally, this review provides a broad summary of analytical assays based on the regulation of aptamer-protein interactions for detecting biomolecules. Finally, we present our perspectives regarding the opportunities and challenges of analytical assays for biological analysis, aiming to provide guidance for disease mechanism research and drug discovery.

The Application of Hybridization Chain Reaction in the Detection of Foodborne Pathogens

Today, with the globalization of the food trade progressing, food safety continues to warrant widespread attention. Foodborne diseases caused by contaminated food, including foodborne pathogens, seriously threaten public health and the economy. This has led to the development of more sensitive and accurate methods for detecting pathogenic bacteria. Many signal amplification techniques have been used to improve the sensitivity of foodborne pathogen detection. Among them, hybridization chain reaction (HCR), an isothermal nucleic acid hybridization signal amplification technique, has received increasing attention due to its enzyme-free and isothermal characteristics, and pathogenic bacteria detection methods using HCR for signal amplification have experienced rapid development in the last five years. In this review, we first describe the development of detection technologies for food contaminants represented by pathogens and introduce the fundamental principles, classifications, and characteristics of HCR. Furthermore, we highlight the application of various biosensors based on HCR nucleic acid amplification technology in detecting foodborne pathogens. Lastly, we summarize and offer insights into the prospects of HCR technology and its application in pathogen detection.

Multiplexed SELEX for Sulfonamide Antibiotics Yielding a Group-Specific DNA Aptamer for Biosensors

The widespread use of sulfonamide (SA) antibiotics in animal husbandry has led to residues of SAs in the environment, causing adverse effects to the ecosystem and a risk of bacterial resistance, which is a potential threat to public health. Therefore, it is highly desirable to develop simple, high-throughput methods that can detect multiple SAs simultaneously. In this study, we isolated aptamers with different specificities based on a multi-SA systematic evolution of ligands by the exponential enrichment (SELEX) strategy using a mixture of sulfadimethoxine (SDM), sulfaquinoxaline (SQX), and sulfamethoxazole (SMZ). Three aptamers were obtained, and one of them showed a similar binding to all tested SAs, with dissociation constant (Kd) ranging from 0.22 to 0.63 μM. For the other two aptamers, one is specific for SQX, and the other is specific for SDM and sulfaclozine. A label-free detection method based on the broad-specificity aptamer was developed for the simultaneous detection of six SAs, with detection of limits ranging from 0.14 to 0.71 μM in a lake water sample. The aptasensor has no binding for other broad-spectrum antibiotics such as β-lactam antibiotics, quinolones, tetracyclines, and chloramphenicol. This work provides a promising biosensor for rapid, multiresidue, and high-throughput detection of SAs, as well as a shortcut for the preparation of different specific recognition elements required for the detection of broad-spectrum antibiotics.

Recent trends in aptamer-based nanobiosensors for detection of vascular endothelial growth factors (VEGFs) biomarker: A review

Vascular endothelial growth factor (VEGF) is a remarkable cytokine that plays an important role in regulating vascular formation during the angiogenesis process. Therefore, real-time detection and quantification of VEGF is essential for clinical diagnosis and treatment due to its overexpression in various tumors. Among various sensing strategies, the aptamer-based sensors in combination with biological molecules improve the detection ability VEGFs. Aptamers are suitable biological recognition agents for the preparation of sensitive and reproducible aptasensors (Apt-sensors) due to their low immunogenicity, simple and straightforward chemical modification, and high resistance to denaturation. Here, a summary of the strategies for immobilization of aptamers (e.g., direct or self-assembled monolayer (SAM) attachment, etc.) on different types of electrodes was provided. Moreover, we discussed nanoparticle deposition techniques and surface modification methods used for signal amplification in the detection of VEGF. Furthermore, we are investigating various types of optical and electrochemical Apt-sensors used to improve sensor characterization in the detection of VEGF biomarkers.

Solution and Gas-Phase Stability of DNA Junctions from Temperature-Controlled Electrospray Ionization and Surface-Induced Dissociation

DNA three-way junction (TWJ) structures transiently form during key cellular processes such as transcription, replication, and DNA repair. Despite their significance, the thermodynamics of TWJs, including the influence of strand length, base pair composition, and ligand binding on TWJ stability and dissociation mechanisms, are poorly understood. To address these questions, we interfaced temperature-controlled nanoelectrospray ionization mass spectrometry (TC-nESI-MS) with a cyclic ion mobility spectrometry (cIMS) instrument that was also equipped with a surface-induced dissociation (SID) stage. This novel combination allowed us to investigate the structural intermediates of three TWJ complexes and examine the effects of GC base pairs on their dissociation pathways. We found that two TWJ-specific ligands, 2,7-tris-naphthalene (2,7-TrisNP) and tris-phenoxybenzene (TrisPOB), lead to TWJ stabilization, revealed by an increase in the melting temperature (Tm) by 13 or 26 °C, respectively. To gain insights into conformational changes in the gas phase, we employed cIMS and SID to analyze TWJs and their complexes with ligands. Analysis of IM arrival distributions suggested a single-step dissociation of TWJs and their intermediates for the three studied TWJ complexes. Upon ligand binding, a higher SID energy by 3 V (2,7-TrisNP) and 5 V (TrisPOB) was required to induce 50% dissociation of TWJ, compared to 38 V in the absence of ligands. Our results demonstrate the power of utilizing TC-nESI-MS in combination with cIMS and SID for thermodynamic characterization of TWJ complexes and investigation of ligand binding. These techniques are essential for the TWJ design and development as drug targets, aptamers, and structural units for functional biomaterials.

Capture-SELEX for Chloramphenicol Binding Aptamers for Labeled and Label-Free Fluorescence Sensing

Chloramphenicol (CAP) is a potent antibiotic. Due to its side effects, CAP is currently banned in most countries, but it is still found in many food products and in the environment. Developing aptamer-based biosensors for the detection of CAP has interested many researchers. While both RNA and DNA aptamers were previously reported for CAP, they were all obtained by immobilization of the CAP base, which omitted the two chlorine atoms. In this work, DNA aptamers were selected using the library-immobilized method and free unmodified CAP. Three families of aptamers were obtained, and the best one named CAP1 showed a dissociation constant (Kd) of 9.8 μM using isothermal titration calorimetry (ITC). A fluorescent strand-displacement sensor showed a limit of detection (LOD) of 14 μM CAP. Thioflavin T (ThT) staining allowed label-free detection of CAP with a LOD of 1 μM in buffer, 1.8 μM in Lake Ontario water, and 3.6 μM in a wastewater sample. Comparisons were made with previously reported aptamers, and ITC failed to show binding of a previously reported 80-mer aptamer. Due to the small size and well-defined secondary structures of CAP1, this aptamer will find analytical applications for environmental and food monitoring.

High-Throughput Effect-Directed Monitoring Platform for Specific Toxicity Quantification of Unknown Waters: Lead-Caused Cell Damage as a Model Using a DNA Hybrid Chain-Reaction-Induced AuNPs@aptamer Self-Assembly Assay

A high-throughput cell-based monitoring platform was fabricated to rapidly measure the specific toxicity of unknown waters, based on AuNPs@aptamer fluorescence bioassays. The aptamer is employed in the platform for capturing the toxicity indicator, wherein hybrid chain-reaction (HCR)-induced DNA functional gold nanoparticle (AuNPs) self-assembly was carried out for signal amplification, which is essential for sensitively measuring the sub-lethal effects caused by target compounds. Moreover, the excellent stability given by the synthesized DNA nanostructure provides mild conditions for the aptamer thus used to bind the analyte. Herein, ATP was treated as a toxicity indicator and verified using lead-caused cell damage as a model. Under optimized conditions, excellent performance for water sample measurement was observed, yielding satisfactory accuracy (recovery rate: 82.69-114.20%; CV, 2.57%-4.65%) and sensitivity (LOD, 0.26 µM) without sample pretreatment other than filtration, indicating the method's simplicity, high efficiency, and reliability. Most importantly, this bioassay could be used as a universal platform to encourage its application in the rapid quantification of specific toxicity in varied sources of samples, ranging from drinking water to highly contaminated wastewater.

Beyond canonical PROTAC: biological targeted protein degradation (bioTPD)

Targeted protein degradation (TPD) is an emerging therapeutic strategy with the potential to modulate disease-associated proteins that have previously been considered undruggable, by employing the host destruction machinery. The exploration and discovery of cellular degradation pathways, including but not limited to proteasomes and lysosome pathways as well as their degraders, is an area of active research. Since the concept of proteolysis-targeting chimeras (PROTACs) was introduced in 2001, the paradigm of TPD has been greatly expanded and moved from academia to industry for clinical translation, with small-molecule TPD being particularly represented. As an indispensable part of TPD, biological TPD (bioTPD) technologies including peptide-, fusion protein-, antibody-, nucleic acid-based bioTPD and others have also emerged and undergone significant advancement in recent years, demonstrating unique and promising activities beyond those of conventional small-molecule TPD. In this review, we provide an overview of recent advances in bioTPD technologies, summarize their compositional features and potential applications, and briefly discuss their drawbacks. Moreover, we present some strategies to improve the delivery efficacy of bioTPD, addressing their challenges in further clinical development.

An aptamer array for discriminating tetracycline antibiotics based on binding-enhanced intrinsic fluorescence

Tetracyclines are a class of antibiotics with a similar four-ringed structure. Due to this structural similarity, they are not easily differentiated from each other. We recently selected aptamers using oxytetracycline as a target and focused on an aptamer named OTC5, which has similar affinities for oxytetracycline (OTC), tetracycline (TC), and doxycycline (DOX). Tetracyclines exhibit an intrinsic fluorescence that is enhanced upon aptamer binding, allowing convenient binding assays and label-free detection. In this study, we analyzed the top 100 sequences from the previous selection library. Three other sequences were found to differentiate between different tetracyclines (OTC, DOX, and TC) by the selective enhancement of their intrinsic fluorescence. Among them, the OTC43 aptamer was more selective for OTC with a limit of detection (LOD) of 0.7 nM OTC, OTC22 was more selective for DOX (LOD 0.4 nM), and OTC2 was more selective for TC (0.3 nM). Using these three aptamers to form a sensor array, principal component analysis was able to discriminate between the three tetracyclines from each other and from the other molecules. This group of aptamers could be useful as probes for the detection of tetracycline antibiotics.

Microneedle-based transdermal detection and sensing devices

Microneedles have been expected for the construction of next-generation biosensors towards personalization, digitization, and intellectualization due to their metrics of minimal invasiveness, high integration, and favorable biocompatibility. Herein, an overview of state-of-the-art microneedle-based detection and sensing systems is presented. First, the designs of microneedle devices based on extraction mechanisms are concluded, corresponding to different geometries and materials of microneedles. Second, the targets of equipment-assisted microneedle detections are summarized, as well as the objective significance, revealing the current performance and potential scenarios of these microneedles. Third, the trend towards highly integrated sensors is elaborated by emphasizing the sensing principles (colorimetric, fluorometric and electronic manner). Finally, the key challenges to be tackled and the perspectives on future development are discussed.

Detection of β-amyloid peptide aggregates by quartz crystal microbalance based on dual-aptamer assisted signal amplification

β-amyloid peptide (Aβ) aggregates are regarded as a typical neuropathology hallmark for the diagnosis of Alzheimer's disease (AD). Aβ₄₀ aggregates include soluble oligomers (Aβ₄₀O) and insoluble fibrils (Aβ₄₀F). Both of them can simultaneously bind to two different kinds of its aptamer (Apt1 and Apt2). As a mass-sensitive sensing platform, quartz crystal microbalance (QCM) converts changes in mass on the Au chip surface into frequency shift. Here, a dual-aptamer assisted Aβ₄₀ aggregates assay was developed. Taking Aβ₄₀O detection as an example, Apt2 was modified on the surface of Au chip by Au-S bond. Subsequently, the solution consisted of Aβ₄₀O and gold nanoparticles-Apt1 (AuNPs-Apt1) were injected into the QCM chamber. As a result, Aβ₄₀O was specifically recognized and captured by Apt2. AuNPs-Apt1 were also combined on the surface of the Au chip because Aβ₄₀O can simultaneously bind to Apt1. Then, a significant frequency shift occurred because of the large weight of AuNPs. Similarly, this procedure can be used to detect Aβ₄₀F. This QCM biosensor was able to detect Aβ₄₀O with a range of 0.2-10 pM with a detection limit of 0.11 pM, while the linear range for Aβ₄₀F was 0.1-10 pM with a detection limit of 0.02 pM. This QCM biosensor was simple and highly sensitive, which provided a new method for Aβ₄₀ aggregates detection.

Aptamer-based Advances in Skin Cancer Research

Cancer diseases have been one of the biggest health threats for the last two decades. Approximately 9% of all diagnosed cancers are skin cancers, including melanoma and non-melanoma. In all cancer cases, early diagnosis is essential to achieve efficient treatment. New solutions and advanced techniques for rapid diagnosis are constantly being sought. Aptamers are single-stranded RNA or DNA synthetic sequences or peptides, which offer novel possibilities to this area of research by specifically binding selected molecules, the so-called cancer biomarkers. Nowadays, they are widely used as diagnostic probes in imaging and targeted therapy. In this review, we have summarized the recently made advances in diagnostics and treatment of skin cancers, which have been achieved by combining aptamers with basic or modern technologies.

DNA Aptamer Selected against Esophageal Squamous Cell Carcinoma for Tissue Imaging and Targeted Therapy with Integrin β1 as a Molecular Target

Esophageal cancer, especially esophageal squamous cell carcinoma (ESCC), poses a serious threat to human health. It is urgently needed to develop recognition tools and discover molecular targets for early diagnosis and targeted therapy of esophageal cancer. Here, we developed several DNA aptamers that can bind to ESCC KYSE410 cells with a nanomolar range of dissociation constants by using cell-based systematic evolution of ligands by exponential enrichment (cell-SELEX). The selected A2 aptamer is found to strongly bind with multiple cancer cells, including several ESCC cell lines. Tissue imaging displayed that the A2 aptamer can specifically recognize clinical ESCC tissues but not the adjacent tissues. Moreover, we identified integrin β1 as the binding target of A2 through pull-down and RNA interference assays. Meanwhile, molecular docking and mutation assays suggested that A2 probably binds to integrin β1 through the nucleotides of DA16-DG21, and competitive binding and structural alignment assays indicated that A2 shares the overlapped binding sites with laminin and arginine-glycine-aspartate ligands. Furthermore, we engineered A2-induced targeted therapy for ESCC. By constructing A2-tethered DNA nanoassemblies carrying multiple doxorubicin (Dox) molecules as antitumor agents, inhibition of tumor cell growth in vitro and in vivo was achieved. This work provides a useful targeting tool and a potential molecular target for cancer diagnosis and targeted therapy and is helpful for understanding the integrin mechanism and developing integrin inhibitors.

Pre-equilibrium biosensors as an approach towards rapid and continuous molecular measurements

Almost all biosensors that use ligand-receptor binding operate under equilibrium conditions. However, at low ligand concentrations, the equilibration with the receptor (e.g., antibodies and aptamers) becomes slow and thus equilibrium-based biosensors are inherently limited in making measurements that are both rapid and sensitive. In this work, we provide a theoretical foundation for a method through which biosensors can quantitatively measure ligand concentration before reaching equilibrium. Rather than only measuring receptor binding at a single time-point, the pre-equilibrium approach leverages the receptor's kinetic response to instantaneously quantify the changing ligand concentration. Importantly, by analyzing the biosensor output in frequency domain, rather than in the time domain, we show the degree to which noise in the biosensor affects the accuracy of the pre-equilibrium approach. Through this analysis, we provide the conditions under which the signal-to-noise ratio of the biosensor can be maximized for a given target concentration range and rate of change. As a model, we apply our theoretical analysis to continuous insulin measurement and show that with a properly selected antibody, the pre-equilibrium approach could make the continuous tracking of physiological insulin fluctuations possible.

Recent Advances in Electrochemical Sensing Strategies for Food Allergen Detection

Food allergy has been indicated as the most frequent adverse reaction to food ingredients over the past few years. Since the only way to avoid the occurrence of allergic phenomena is to eliminate allergenic foods, it is essential to have complete and accurate information on the components of foodstuff. In this framework, it is mandatory and crucial to provide fast, cost-effective, affordable, and reliable analysis methods for the screening of specific allergen content in food products. This review reports the research advancements concerning food allergen detection, involving electrochemical biosensors. It focuses on the sensing strategies evidencing different types of recognition elements such as antibodies, nucleic acids, and cells, among others, the nanomaterial role, the several electrochemical techniques involved and last, but not least, the ad hoc electrodic surface modification approaches. Moreover, a selection of the most recent electrochemical sensors for allergen detection are reported and critically analyzed in terms of the sensors’ analytical performances. Finally, advantages, limitations, and potentialities for practical applications of electrochemical biosensors for allergens are discussed.

Label-free and Dye-free Fluorescent Sensing of Tetracyclines Using a Capture-Selected DNA Aptamer

Tetracyclines are a group of important antibiotics with a common four-ring scaffold. While most tetracyclines are currently used only in animals, their leaching into the environment and residues in food have caused health concerns. Aptamers are an attractive way to detect tetracyclines, and all previously reported aptamers for tetracyclines were obtained by immobilizing target molecules. In this work, we selected a few DNA aptamers by immobilizing the DNA library using oxytetracycline as the target. We obtained new aptamers with no overlapping sequences compared to the previously reported ones, and a representative sequence named OTC5 had a dissociation constant of 147 nM measured by isothermal titration calorimetry. Similar binding affinities were also observed with tetracycline and doxycycline. Because tetracyclines are fluorescent and their fluorescence intensity was enhanced by binding to the aptamers, a label-free and dye-free fluorescent biosensor was developed with a detection limit of 25 nM oxytetracycline. The sensor was able to detect targets in milk after extraction. Fluorescence polarization measurement showed that this aptamer is insensitive to sodium concentration but requires magnesium. Finally, a strand-displacement biosensor was designed, and it has a detection limit of 1.2 μM oxytetracycline.

Boronate-Affinity Cross-Linking-Based Ratiometric Electrochemical Detection of Glycoconjugates

Despite the widespread application of the boronate-affinity cross-linking (BAC) in the separation, enrichment, and sensing of glycoconjugates, it remains a huge challenge to integrate the BAC into the selective electrochemical detection of glycoconjugates due to the poor selectivity of the BAC. Herein, we demonstrate a BAC-based ratiometric electrochemical method for the simple, low-cost, and highly sensitive and selective detection of glycoconjugates. Briefly, the methylene blue (MB)-tagged nucleic acid aptamer is exploited as the recognition element to selectively capture target glycoconjugate, to which a large number of ferrocene (Fc) tags are subsequently labeled via the BAC between the phenylboronic acid (PBA) group and the cis-diol site of the oligosaccharide chains on the captured targets. Using the MB tag as the internal reference and the Fc tag as the reporter of the target capture, the dual-signal output enables the ratiometric detection. Due to the presence of a high density of the cis-diol sites on a glycoconjugate, sufficiently high sensitivity can be obtained even without using any amplification strategies. Using glycoprotein mucin 1 (MUC1) as the model target, the signal ratio (I_Fc/I_MB) exhibits good linearity over the range from 0.05 to 50 U/mL, with a detection limit of 0.021 U/mL. In addition to the high sensitivity and selectivity, the results of the analysis of MUC1 in serum samples are acceptable. By virtue of its simplicity, cost-effectiveness, and high robustness and reproducibility, this BAC-based ratiometric electrochemical method holds great promise in the highly sensitive and selective detection of glycoconjugates.

Effects of Nucleic Acid Structural Heterogeneity on the Electrochemistry of Tethered Redox Molecules

The cation condensation-induced collapse of electrode-bound nucleic acids and the resulting change in the electrochemical signal is a useful tool to predict the structure and redox probe location of heterogeneous structures of surface-tethered DNA probes─a common architecture employed in the development of electrochemical sensors. In this paper, we measure the faradaic current of an appended redox molecule at the 3' position of the nucleic acid using cyclic voltammetry before and after nucleic acid collapse for various nucleic acid architectures and heterogeneous mixtures on the same electrode surface. The voltammetric peak current change with collapse correlates with the proximity of the redox molecules from the surface. For stem-loop probes, the terminal methylene blue is initially held closer to the surface, such that inducing collapse, by reducing the dielectric permittivity of the interrogation solution, results in a ∼30% increase in current. However, when incorporating pseudoknot probes that hold methylene blue further away from the electrode surface, the current change is much larger (∼120%), indicating a larger conformation change. Upon a 50:50 ratio of the two, we observe a change in current that relates to the ratiometric distribution of the probe used to make the surfaces. Additionally, using cyclic voltammetry, we find that the change between diffusion-limited and diffusion-independent peak currents is dependent upon the distinct structural characteristics of DNA probes on the surface (stem-loop or pseudoknot), as well as the ratios of different DNA probes on the surface. Thus, we demonstrate that the heterogeneous nature of DNA probes governs the corresponding electrochemical signals, which can lead to a better understanding on how to predict the structures of functional nucleic acids on electrode surfaces and how this affects surface-to-surface variability and electrochemical response.

Microneedle Aptamer-Based Sensors for Continuous, Real-Time Therapeutic Drug Monitoring

The ability to continuously monitor the concentration of specific molecules in the body is a long-sought goal of biomedical research. For this purpose, interstitial fluid (ISF) was proposed as the ideal target biofluid because its composition can rapidly equilibrate with that of systemic blood, allowing the assessment of molecular concentrations that reflect full-body physiology. In the past, continuous monitoring in ISF was enabled by microneedle sensor arrays. Yet, benchmark microneedle sensors can only detect molecules that undergo redox reactions, which limits the ability to sense metabolites, biomarkers, and therapeutics that are not redox-active. To overcome this barrier, here, we expand the scope of these devices by demonstrating the first use of microneedle-supported electrochemical, aptamer-based (E-AB) sensors. This platform achieves molecular recognition based on affinity interactions, vastly expanding the scope of molecules that can be sensed. We report the fabrication of microneedle E-AB sensor arrays and a method to regenerate them for multiple uses. In addition, we demonstrate continuous molecular measurements using these sensors in flow systems in vitro using single and multiplexed microneedle array configurations. Translation of the platform to in vivo measurements is possible as we demonstrate with a first E-AB measurement in the ISF of a rodent. The encouraging results reported in this work should serve as the basis for future translation of microneedle E-AB sensor arrays to biomedical research in preclinical animal models.

Recent Progress on Highly Selective and Sensitive Electrochemical Aptamer-based Sensors

Highly selective, sensitive, and stable biosensors are essential for the molecular level understanding of many physiological activities and diseases. Electrochemical aptamer-based (E-AB) sensor is an appealing platform for measurement in biological system, attributing to the combined advantages of high selectivity of the aptamer and high sensitivity of electrochemical analysis. This review summarizes the latest development of E-AB sensors, focuses on the modification strategies used in the fabrication of sensors and the sensing strategies for analytes of different sizes in biological system, and then looks forward to the challenges and prospects of the future development of electrochemical aptamer-based sensors.

Systematically Modulating Aptamer Affinity and Specificity by Guanosine-to-Inosine Substitution

Aptamers are widely used in small molecule detection applications due to their specificity, stability, and cost effectiveness. One key challenge in utilizing aptamers in sensors is matching the binding affinity of the aptamer to the desired concentration range for analyte detection. The most common methods for modulating affinity have inherent limitations, such as the likelihood of drastic changes in aptamer folding. Here, we propose that substituting guanosine for inosine at specific locations in the aptamer sequence provides a less perturbative approach to modulating affinity. Inosine is a naturally occurring nucleotide that results from hydrolytic deamination of adenosine, and like guanine, it base pairs with cytosine. Using the well-studied cocaine binding aptamer, we systematically replaced guanosine with inosine and were able to generate sequences having a range of binding affinities from 230 nM to 80 μM. Interestingly, we found that these substitutions could also modulate the specificity of the aptamers, leading to a range of binding affinities for structurally related analytes. Analysis of folding stability via melting temperature shows that, as expected, aptamer structure is impacted by guanosine-to-inosine substitutions. The ability to tune binding affinity and specificity through guanosine-to-inosine substitution provides a convenient and reliable approach for rapidly generating aptamers for diverse biosensing applications.

Selection of Aptamers for Sensing Caffeine and Discrimination of Its Three Single Demethylated Analogues

With the growing consumption of caffeine-containing beverages, detection of caffeine has become an important biomedical, bioanalytical, and environmental topic. We herein isolated four high-quality aptamers for caffeine with dissociation constants ranging from 2.2 to 14.6 μM as characterized using isothermal titration calorimetry. Different binding patterns were obtained for the three single demethylated analogues: theobromine, theophylline, and paraxanthine, highlighting the effect of the molecular symmetry of the arrangement of the three methyl groups in caffeine. A structure-switching fluorescent sensor was designed showing a detection limit of 1.2 μM caffeine, which reflected the labeled caffeine concentration within 6.1% difference for eight commercial beverages. In 20% human serum, a detection limit of 4.0 μM caffeine was achieved. With the four aptamer sensors forming an array, caffeine and the three analogues were well separated from nine other closely related molecules.

Investigating the Influences of Random-Region Length on Aptamer Selection Efficiency Based on Capillary Electrophoresis-SELEX and High-Throughput Sequencing

For aptamer selection, the random-region length of an ssDNA library was generally taken in a relatively arbitrary fashion, which may lead to failure for unsuitable target binding. Herein, we coupled high-efficiency capillary electrophoresis (CE)-SELEX and high-throughput sequencing (HTS) to investigate the influences of random-region length. First, one round of selection against programmed cell death-ligand 1 (PD-L1) was performed using ssDNA libraries with random-region lengths of 15, 30, 40, and 60 nt, respectively. A good correlation was observed between candidates' random-region lengths and dissociation constant (K_d), in which the longer sequences presented higher affinity, and the picked Seq 60-1 after one round notably presented a similar affinity toward a reported aptamer through eight rounds. Molecular dynamics (MD) simulation suggested, for PD-L1, the long sequence could supply more noncovalent bonds including hydrogen bonds, electrostatic interactions, and hydrophobic interactions to form a stable protein/aptamer complex. Besides, four other proteins with selective binding performances validated the importance of random-region length. To further investigate how random-region length affects the selection efficiency, a mixed library with random-region lengths ranging from 10 to 50 nt was employed for six rounds of selection against Piezo2. Sequence variations were tracked by HTS, showing the preferential evolution and PCR uncertainty with even higher impact were the main causes. This study suggested random-region length plays a crucial factor, and a mixed library with different random-region sequences can be a worthy choice for increasing the speed of high-affinity aptamer selection. Moreover, the PCR process should be given particular attention in aptamer selection.

Toward the Development of Rapid, Specific, and Sensitive Microfluidic Sensors: A Comprehensive Device Blueprint

Recent advances in nano/microfluidics have led to the miniaturization of surface-based chemical and biochemical sensors, with applications ranging from environmental monitoring to disease diagnostics. These systems rely on the detection of analytes flowing in a liquid sample, by exploiting their innate nature to react with specific receptors immobilized on the microchannel walls. The efficiency of these systems is defined by the cumulative effect of analyte detection speed, sensitivity, and specificity. In this perspective, we provide a fresh outlook on the use of important parameters obtained from well-characterized analytical models, by connecting the mass transport and reaction limits with the experimentally attainable limits of analyte detection efficiency. Specifically, we breakdown when and how the operational (e.g., flow rates, channel geometries, mode of detection, etc.) and molecular (e.g., receptor affinity and functionality) variables can be tailored to enhance the analyte detection time, analytical specificity, and sensitivity of the system (i.e., limit of detection). Finally, we present a simple yet cohesive blueprint for the development of high-efficiency surface-based microfluidic sensors for rapid, sensitive, and specific detection of chemical and biochemical analytes, pertinent to a variety of applications.

Nucleic Acid Identity, Structure, and Flexibility Affect the Electrochemical Signal of Tethered Redox Molecules upon Biopolymer Collapse

We demonstrate that cation condensation can induce the collapse of surface-bound nucleic acids and that the electrochemical signal from a tethered redox molecule (methylene blue) upon collapse reports on nucleic acid identity, structure, and flexibility. Furthermore, the correlation of the electrochemical signal and structure is consistent with theoretical considerations of nucleic acid collapse. Changes in solution dielectric permittivity or the concentration of trivalent cations cause the structure of nucleic acids to become more compact due to an increase in attractive electrostatic interactions between the charged biopolymer backbone and multivalent ions in the solution. Consequently, the compaction of nucleic acids results in a change in the dynamics and location of the terminally appended redox marker, which is reflected in the faradaic current measured using cyclic voltammetry. In comparison to ssDNA, nucleic acid duplexes (dsDNA, DNA/peptide nucleic acid, and dsRNA) require nucleic-acid-composition-specific solution conditions for the collapse to occur. Moreover, the magnitude of current increase observed after the collapse is different for each nucleic structure, and we find here that these changes are dictated by physical parameters of the nucleic acids including the axial charge spacing and the periodicity of the helix. The work here aims to provide quantitative and predicative measures of the effects of the nucleic acid structure on the electrochemical signal produced from distal-end appended redox markers. This architecture is commonly employed in functional nucleic acid sensors and a better understanding of structure-to-signal correlations will enable the rational design of sensitive sensing architectures.

Oligonucleotide aptamers: Recent advances in their screening, molecular conformation and therapeutic applications

Aptamers are single stranded oligonucleotides with specific recognition and binding ability to target molecules, which can be obtained by Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Aptamers have the advantages of low molecular weight, low immunogenicity, easy modification and high stability. They play promising role in promoting food safety, monitoring the environment and basic research, especially in clinical diagnosis and therapeutic drugs. To date, great achievements regarding the selection, modifications and application of aptamers have been made. However, since it is still a challenge to obtain aptamers with high affinity in a more effective way, few aptamer-based products have already successfully entered into clinical use. This review aims to provide a thorough overview of the latest advances in this rapidly developing field, focusing on aptamer screening methods for different targets, the structure of the interaction between aptamers and target substances, and the challenges and potential of current therapeutic aptamers.

Sensitive Electrochemical Detection of Microcystin-LR in Water Samples Via Target-Induced Displacement of Aptamer Associated [Ru(NH3)6]3

In this study, we demonstrate the successful development of an electrochemical aptamer-based sensor for point-of-use detection and quantification of the highly potent microcystin-LR (MC-LR) in water. The sensor uses hexaammineruthenium(III) chloride ([Ru(NH₃)₆]³⁺) as redox mediator, because of the ability of the positively charged (3+) molecule to associate with the phosphate backbone of the nucleic acids. We quantitatively measure the target-induced displacement of aptamer associated, or surface confined, [Ru(NH₃)₆]³⁺ in the presence of MC-LR. Upon the addition of MC-LR in the water, surface-confined [Ru(NH₃)₆]³⁺ dissociates, resulting in less faradaic current from the reduction of [Ru(NH₃)₆]³⁺ to [Ru(NH₃)₆]²⁺ Sensing surfaces of highly packed immobilized aptamers were capable of recording decreasing square wave voltammetry (SWV) signals after the addition of MC-LR in buffer. As a result, SWV recorded substantial signal suppression within 15 min of target incubation. The sensor showed a calculated limit of detection (LOD) of 9.2 pM in buffer. The effects of interferents were minimal, except when high concentrations of natural organic matter (NOM) were present. Also, the sensor performed well in drinking water samples. These results indicate a sensor with potential for fast and specific quantitative determination of MC-LR in drinking water samples. A common challenge when developing electrochemical, aptamer-based sensors is the need to optimize the nucleic acid aptamer in order to achieve sensitive signaling. This is particularly important when an aptamer experiences only a small or localized conformational change that provides only a limited electrochemical signal change. This study suggests a strategy to overcome that challenge through the use of a nucleic acid-associated redox label.

Recent advances in Surface Plasmon Resonance (SPR) biosensors for food analysis: a review

Food safety is the prime area of concern that builds trust. With the prevailing advancements, it has become facile to ensure safety in almost all aspects. Technology has grown from tedious lab techniques to modern chromatographic techniques and immunoassays, progressed with more precise and rapid sensing through the advent of Biosensors. Biosensors provide an automated technology by presenting superfast, nondestructive and cost-effective detection in food analysis. SPR biosensor is an optical biosensor known for its versatility and has wider applications in food testing and analysis. It has an optical system for excitation and interrogation of surface plasmons, and a biomolecular recognition element to detect and seize the target analyte present in a sample. The optical signal detects the binding analyte, on the recognition element, which results in a change in refractive index at the surface and modifies the surface plasmons' propagation constant. SPR aids in label-free detection of various components such as adulterants, antibiotics, biomolecules, genetically modified foods, pesticides, insecticides, herbicides, microorganisms and microbial toxins in food and assures safety. The distinct advancements of SPR in food analysis have been found and discussed. The review also provides knowledge on the advantages and the key challenges encountered by SPR.

Construction of Aptamer-Based Molecular Beacons with Varied Blocked Structures and Targeted Detection of Thrombin

A kind of blocked aptamer-functionalized molecular beacon (MB) was designed as fluorescence sensors to detect thrombins by binding-induced "turn on" structural transformation. Three MBs named MB(8 + 8), MB(15 + 8), and MB(15 + 6) consisted of two single-stranded oligonucleotides. One long single-stranded oligonucleotide (abbreviated as SS) contained a thrombin aptamer sequence and was modified with a fluorescence group and quenching group on each end side. Another short single-stranded oligonucleotide (written as cDNA) was partially complementary to the long SS. It was interesting to find that the complementary sequence length of cDNA greatly influenced the structure of the MBs. The construction of MB experiments proved that MB(8 + 8) and MB(15 + 8) could form the quenching MBs but MB(15 + 6) could not. MB(8 + 8) was composed of a SS strand paired with a complementary cDNA(8 + 8), which was called one-to-one combination, while MB(15 + 8) was two-to-two combination and MB(15 + 6) was one-to-two combination. When the ratio of SS and cDNA (15 + 8) was 1:1, the quenching efficiency reached maximum. But with the molar ratio of SS and cDNA(8 + 8) increasing, the quenching efficiency increased continuously. Under the optimal conditions that we studied, the detection limit of thrombin by MB(8 + 8) and MB(15 + 8) was 0.19 and 1.2 nM, respectively. In addition, the assay proved to be selective, and the average recovery of thrombin detected by MB(8 + 8) and MB(15 + 8) in diluted serum was 95.4 and 94.5%, respectively.