DNA aptamer folding on gold nanoparticles: from colloid chemistry to biosensors

Fluorogenic target competitors for developing label-free and sensitive folding-unswitching aptamer sensors

BACKGROUND

Aptamers have aroused tremendous applications in sensors, drug deliveries, diagnosis, and therapies. In particular, target-induced global structure switching of aptamers has been widely used to develop selective sensors. However, fluorophore and/or quencher modification, sequence elongation, and nano-interface adsorption are required to design such global structure-switching aptamer sensors (SSAS) in order to signal target binding events. Accordingly, these requirements make SSAS at a high cost and expense of sensors' sensitivity. In this aspect, efforts should be made to overcome these drawbacks of SSAS.

RESULTS

Herein, we tried to develop label-free folding-unswitching aptamer sensors (FUAS) by searching fluorogenic target competitors. Using adenine nucleoside/nucleotide as the proof-of-concept model targets, we screened out berberine (BER) from natural isoquinoline alkaloids (having rings comparable to targets) as the best fluorogenic target competitor. Binding of BER at the conserved nucleotides of intact aptamer foldings turned on this fluorogenic target competitor' fluorescence. Targets then competed with this fluorogenic target competitor over the same conserved nucleotides to cause its release in favor of a resultant fluorescence change. We found that the developed FUAS are much more sensitive than the previously reported SSAS. The FUAS were successfully applied to assays of ATP and adenosine deaminase in serums, and to screening of the adenosine deaminase's inhibitor, verifying the reliability and applicability of this FUAS platform in variant fields.

SIGNIFICANCE

We demonstrate that by designing fluorogenic target competitors, FUAS can be alternatively developed in a label-free manner and with a higher sensitivity than the previously developed SSAS. This work opens a new way to develop high-performance aptamer-based sensors. Furthermore, our developed FUAS should inspire more interest for wide applications incluidng target-triggered drug deliveries when therapeutic fluorogenic target competitors are used.

Colorimetric aptasensor based on self-screened aptamers and cascaded catalytic reaction for the detection of quarantine plant bacteria

Quarantine plant bacteria (QPB) are significant component of invasive alien species that result in substantial economic losses and serious environmental damage. Herein, a colorimetric aptasensor has been proposed based on the sandwich structure and the cascaded catalytic strategy for on-site detecting Xanthomonas hyacinthi, a type of QPB, in natural environments. The self-screened aptamer obtained through SELEX can bind to specific sites on the surface of viable organism with high affinity and specificity, which guarantees the selectivity of aptasensor. As an important part of the aptasensor, MIL-88-NH2(Fe) not only acts as a multifunctional carrier for both aptamers and glucose oxidase, but also catalyzes enzyme-like reaction because of specific surface area, amino and peroxidase-like activity. The present of Xanthomonas hyacinthi can trigger the formation of a sandwich structure and the occurrence of cascade catalytic reaction, enabling the detection with UV-Vis spectra and naked eyes. The proposed aptasensor presents a low detection limit of 2 cfu/mL and a wide linear range of 10 -107 cfu/mL. Compared to traditional detection methods for QPB, the reasonable design, high selectivity and convenience significantly improve the detection efficiency and contribute to environmental protection.

Dual-targeted TfRA4-DNA1-Ag@AuNPs: An innovative radiosensitizer for enhancing radiotherapy in glioblastoma multiforme

Radiation therapy (RT) is one of the most effective and widely used treatment methods for glioblastoma multiforme (GBM). However, its efficacy is often compromised by the inherent radioresistance of tumor cells, while the restrictive nature of the blood-brain barrier (BBB) specifically impedes the delivery of radiosensitizer. Thus, we constructed and characterized polyethylene glycol (PEG)-functionalized silver-gold core-shell nanoparticles (PSGNPs) targeting both BBB (TfRA4) and GBM (DNA1) (TDSGNPs). Afterwards, studies conducted both in vitro and in vivo were employed to assess the BBB penetration capabilities, abilities of GBM targeting and radiosensitization effect. Transmission electron microscope images of PSGNPs showed a core-shell structure, and the results of ultraviolet-visible absorption spectroscopy and dynamic light scattering displayed that TDSGNPs were successfully constructed with excellent dispersion properties. TDSGNPs could be specifically taken up by U87MG cells and the uptake peaked at 24 h. TDSGNPs combined with RT obviously increased the apoptosis proportion of the cells. It was shown by the in vitro and in vivo investigations that TDSGNPs could target U87MG cells after crossing the BBB, and further study revealed that TDSGNPs showed an uptake peak in the tumor sites after 3 h intravenous injection. The radiosensitization of TDSGNPs was better than that of the nanoparticles modified with single aptamers and the median survival of tumor-bearing mice was greatly extended. This study demonstrated that TDSGNPs could penetrate BBB to target GBM, functioning as a promising radiosensitizer for the targeted therapy of GBM.

Superlarge, Rigidified DNA Tetrahedron with a Y-Shaped Backbone for Organizing Biomolecules Spatially and Maintaining Their Full Bioactivity

A major impediment to the clinical translation of DNA tiling nanostructures is a technical bottleneck for the programmable assembly of DNA architectures with well-defined local geometry due to the inability to achieve both sufficient structural rigidity and a large framework. In this work, a Y-backbone was inserted into each face to construct a superlarge, sufficiently rigidified tetrahedral DNA nanostructure (called RDT) with extremely high efficiency. In RDT, the spatial size increased by 6.86-fold, and the structural rigidity was enhanced at least 4-fold, contributing to an ∼350-fold improvement in the resistance to nucleolytic degradation even without a protective coating. RDT can be mounted onto an artificial lipid-bilayer membrane with molecular-level precision and well-defined spatial orientation that can be validated using the fluorescence resonance energy transfer (FRET) assay. The spatial orientation of Y-shaped backbone-rigidified RDT is unachievable for conventional DNA polyhedrons and ensures a high level of precision in the geometric positioning of diverse biomolecules with an approximately homogeneous environment. In tests of RDT, surface-confined horseradish peroxidase (HRP) exhibited nearly 100% catalytic activity and targeting aptamer-immobilized gold nanoparticles showed 5.3-fold enhanced cellular internalization. Significantly, RDT exhibited a 27.5-fold enhanced structural stability in a bodily environment and did not induce detectable systemic toxicity.

A Gold Nanoparticle-Based Cortisol Aptasensor for Non-Invasive Detection of Fish Stress

Cortisol is a key stress biomarker in humans and animals, including fishes. In aquafarming, stress monitoring using cortisol quantification can help to optimize aquaculture practices for welfare and productivity enhancement. However, most current methods for cortisol detection rely on invasive tissue sampling. In this work, we developed a gold nanoparticle (AuNP)-based cortisol sensor to address the demand of detecting picomolar ranges of cortisol from complex fish tank water matrices as a non-invasive alternative for more effective stress monitoring. We first identified a DNA aptamer with effective binding to cortisol and then conjugated the thiol-labelled aptamer to AuNPs together with a blocker molecule (CALNN) to form an Au-Apt-CALNN conjugate that is stable in fish tank water. The cortisol detection principle is based on magnesium chloride (MgCl2)-induced particle aggregation, where the cortisol-bound aptamer on the AuNPs folds into a tertiary structure and provides greater protection for Au-Apt-CALNN against MgCl2-induced aggregation due to steric stabilization. At an optimum MgCl2 concentration, the differential stability of particles with and without cortisol binding offers a limit of detection (LOD) of 100 pM for cortisol within a 35 min reaction. The aptasensor has been validated on recirculating aquaculture system (RAS) fish tank water samples by the HPLC method and was able to detect changes in water cortisol induced by two different stress paradigms. This on-site deployable and non-invasive sensor offers opportunities for more efficient and real-time fish stress monitoring for the optimization of aquaculture practices.

Formulation optimization of lyophilized aptamer-gold nanoparticles: Maintained colloidal stability and cellular uptake

Anti-nucleolin (NCL) aptamer AS1411 is the first anticancer aptamer tested in clinical trials. Gold nanoparticles (AuNP) have been widely exploited for various biomedical applications due to their unique functional properties. In this study, we evaluated the colloidal stability and targeting capacity of AS1411-funtionalized AuNP (AuNP/NCL-Apt) against MCF-7 breast cancer cell line before and after lyophilization. Trehalose, mannitol, and sucrose at various concentrations were evaluated to determine their cryoprotection effects. Our results indicate that sucrose at 10 % (w/v) exhibits the best cryoprotection effect and minimal AuNP/NCL-Apt aggregation as confirmed by UV-Vis spectroscopy and dynamic light scattering (DLS) measurements. Moreover, the lyophilized AuNP/NCL-Apt at optimized formulation maintained its targeting and cytotoxic functionality against MCF-7 cells as proven by the cellular uptake assays utilizing flow cytometry and confocal laser scanning microscopy (CLSM). Quantitative PCR (qPCR) analysis of nucleolin-target gene expression also confirmed the effectiveness of AuNP/NCL-Apt. This study highlights the importance of selecting the proper type and concentration of cryoprotectant in the typical nanoparticle lyophilization process and contributes to our understanding of the physical and biological properties of functionalized nanoparticles upon lyophilization.

Engineered aptamers for molecular imaging

Molecular imaging, including quantification and molecular interaction studies, plays a crucial role in visualizing and analysing molecular events occurring within cells or organisms, thus facilitating the understanding of biological processes. Moreover, molecular imaging offers promising applications for early disease diagnosis and therapeutic evaluation. Aptamers are oligonucleotides that can recognize targets with a high affinity and specificity by folding themselves into various three-dimensional structures, thus serving as ideal molecular recognition elements in molecular imaging. This review summarizes the commonly employed aptamers in molecular imaging and outlines the prevalent design approaches for their applications. Furthermore, it highlights the successful application of aptamers to a wide range of targets and imaging modalities. Finally, the review concludes with a forward-looking perspective on future advancements in aptamer-based molecular imaging.

Unraveling the Possibilities: Recent Progress in DNA Biosensing

Due to the advantages of its numerous modification sites, predictable structure, high thermal stability, and excellent biocompatibility, DNA is the ideal choice as a key component of biosensors. DNA biosensors offer significant advantages over existing bioanalytical techniques, addressing limitations in sensitivity, selectivity, and limit of detection. Consequently, they have attracted significant attention from researchers worldwide. Here, we exemplify four foundational categories of functional nucleic acids: aptamers, DNAzymes, i-motifs, and G-quadruplexes, from the perspective of the structure-driven functionality in constructing DNA biosensors. Furthermore, we provide a concise overview of the design and detection mechanisms employed in these DNA biosensors. Noteworthy advantages of DNA as a sensor component, including its programmable structure, reaction predictility, exceptional specificity, excellent sensitivity, and thermal stability, are highlighted. These characteristics contribute to the efficacy and reliability of DNA biosensors. Despite their great potential, challenges remain for the successful application of DNA biosensors, spanning storage and detection conditions, as well as associated costs. To overcome these limitations, we propose potential strategies that can be implemented to solve these issues. By offering these insights, we aim to inspire subsequent researchers in related fields.

A review of advances in aptamer-based cell detection technology

Since cells are the basic structural and functional units of organisms, the detection or quantitation of cells is one of the most common basic problems in life science research. The established cell detection techniques mainly include fluorescent dye labeling, colorimetric assay, and lateral flow assay, all of which employ antibodies as cell recognition elements. However, the widespread application of the established methods generally dependent on antibodies is limited, because the preparation of antibodies is complicated and time-consuming, and unrecoverable denaturation is prone to occur with antibodies. By contrast, aptamers that are generally selected through the systematic evolution of ligands by exponential enrichment can avoid the disadvantages of antibodies due to their controllable synthesis, thermostability, and long shelf life, etc. Accordingly, aptamers may serve as novel molecular recognition elements like antibodies in combination with various techniques for cell detection. This paper reviews the developed aptamer-based cell detection methods, mainly including aptamer-fluorescent labeling, aptamer-isothermal amplification assay, electrochemical aptamer sensor, aptamer-based lateral flow analysis, and aptamer-colorimetric assay. The principles, advantages, progress of application in cell detection and future development trend of these methods were specially discussed. Overall, different assays are suitable for different detection purposes, and the development of more accurate, economical, efficient, and rapid aptamer-based cell detection methods is always on the road in the future. This review is expected to provide a reference for achieving efficient and accurate detection of cells as well as improving the usefulness of aptamers in the field of analytical applications.

A turn-on unlabeled colorimetric biosensor based on aptamer-AuNPs conjugates for amyloid-β oligomer detection

Amyloid-β oligomers (AβO) have been identified as core biomarkers for early diagnosis of Alzheimer's disease (AD). For the first time, a "turn-on" unlabeled colorimetric aptasensor based on aptamer-polythymine (polyT)-polyadenine (polyA)-gold nanoparticles (pA-pT-apt@AuNPs) was developed for highly sensitive and specific detection of amyloid-β_1-40 oligomers (Aβ40-O). In this system, polyA sequence could preferentially anchor onto AuNPs surface as well as reduce the non-specific adsorption, and the aptamer could form upright conformation for the specific recognition of Aβ40-O. The aggregation of pA-pT-apt@AuNPs was induced by MgCl₂. However, the addition of Aβ40-O enabled the aptamer fold adaptively upon recognition and aptamer-Aβ40-O complex formed surrounding AuNPs, effectively stabilizing pA-pT-apt@AuNPs against salt-induced aggregation, therefore the color of pA-pT-apt@AuNPs solution still retained red. Based on this principle, the proposed aptasensor exhibited high sensitivity with the limit of detection of 3.03 nM and a linear detectable range from 10.00 nM to 100.0 nM. The superb sensitivity was achieved via the optimization of the length of polyA and polyT spacer. This pA-pT-apt@AuNPs based colorimetric aptasensor provides a rapid, cost-effective, highly sensitive detection method for Aβ40-O, which is valuable for the early diagnosis of AD.

Integration of phosphate functionalized Pt/TiO2 and Ru(bpy)32+ sensitization for ultrasensitive assay of adenosine deaminase activity on a novel split-typed PEC aptasensor

To date, it is still a challenge for high-performance photoelectrochemical (PEC) assay of low-abundance adenosine deaminase (ADA) in fundamental research and clinical diagnosis. Herein, phosphate-functionalized Pt/TiO₂ (termed PO₄^3-/Pt/TiO₂) was prepared as ideal photoactive material to develop a split-typed PEC aptasensor for detection of ADA activity, coupled by a Ru(bpy)₃²⁺ sensitization strategy. We critically studied the effects of the PO₄^3- and Ru(bpy)₃²⁺ on the detection signals, and discussed the signal-amplified mechanism. Specifically, hairpin-structured adenosine (AD) aptamer was splited into single chain via ADA-induced catalytic reaction, and subsequently hybridized with complementary DNA (cDNA, initially coating on magnetic beads). The in-situ formed double-stranded DNA (dsDNA) was further intercalated by more Ru(bpy)₃²⁺ to amplify the photocurrents. The resultant PEC biosensor showed a broader linear range of 0.05-100 U L^-1 and a lower limit of detection (0.019 U L^-1), which can fill the blank for analysis of ADA activity. This research would provide some valuable insights for building advanced PEC aptasensors in ADA-related research and clinical diagnosis.

A ratiometric self-powered aptasensor for simultaneous detection of cortisol and progesterone based on spatially resolved tri-channel photofuel cell

In this work, a self-powered sensor was proposed for simultaneous detection of two typical steroid hormones, namely cortisol (COR) and progesterone (P4). A tri-channel photofuel cell (PFC) consisting of three spatially resolved SnS₂@SnO₂ photoanodes and one Pt cathode was designed to generate the electricity to drive the sensing process under the control of a multiplex switch. Among three photoanodes, one served as the control, while the other two were modified with COR-binding or P4-binding aptamer to respond specifically to the COR or P4 target. The ratios of the inhibited PFC output from aptamer-immobilized photoanodes to the reference signal from the control photoanode were utilized for simultaneous detection of COR and P4. The results showed that the developed self-powered sensor exhibited broad concentration ranges toward targets, with COR concentration ranging from 1 nM to 1000 nM and P4 concentration ranging from 1 nM to 500 nM. The detection limits for COR and P4 were calculated to be 0.88 nM and 0.52 nM, respectively. Moreover, the proposed sensing platform demonstrated high selectivity, good reproducibility, and high stability. Finally, the sensor was successfully applied to the simultaneous determination of COR and P4 in a human female serum sample.

Aptamer-based CRISPR-Cas powered diagnostics of diverse biomarkers and small molecule targets

CRISPR-Cas systems have been widely used in genome editing and transcriptional regulation. Recently, CRISPR-Cas effectors are adopted for biosensor construction due to its adjustable properties, such as simplicity of design, easy operation, collateral cleavage activity, and high biocompatibility. Aptamers' excellent sensitivity, specificity, in vitro synthesis, base-pairing, labeling, modification, and programmability has made them an attractive molecular recognition element for inclusion in CRISPR-Cas systems. Here, we review current advances in aptamer-based CRISPR-Cas sensors. We briefly discuss aptamers and the knowledge of Cas effector proteins, crRNA, reporter probes, analytes, and applications of target-specific aptamers. Next, we provide fabrication strategies, molecular binding, and detection using fluorescence, electrochemical, colorimetric, nanomaterials, Rayleigh, and Raman scattering. The application of CRISPR-Cas systems in aptamer-based sensing of a wide range of biomarkers (disease and pathogens) and toxic contaminants is growing. This review provides an update and offers novel insights into developing CRISPR-Cas-based sensors using ssDNA aptamers with high efficiency and specificity for point-of-care setting diagnostics.

Capture-SELEX: Selection Strategy, Aptamer Identification, and Biosensing Application

Small-molecule contaminants, such as antibiotics, pesticides, and plasticizers, have emerged as one of the substances most detrimental to human health and the environment. Therefore, it is crucial to develop low-cost, user-friendly, and portable biosensors capable of rapidly detecting these contaminants. Antibodies have traditionally been used as biorecognition elements. However, aptamers have recently been applied as biorecognition elements in aptamer-based biosensors, also known as aptasensors. The systematic evolution of ligands by exponential enrichment (SELEX) is an in vitro technique used to generate aptamers that bind their targets with high affinity and specificity. Over the past decade, a modified SELEX method known as Capture-SELEX has been widely used to generate DNA or RNA aptamers that bind small molecules. In this review, we summarize the recent strategies used for Capture-SELEX, describe the methods commonly used for detecting and characterizing small-molecule-aptamer interactions, and discuss the development of aptamer-based biosensors for various applications. We also discuss the challenges of the Capture-SELEX platform and biosensor development and the possibilities for their future application.

Point-of-Care Biosensing of Urinary Tract Infections Employing Optoplasmonic Surfaces Embedded with Metal Nanotwins

We report the synthesis of gold nanotwins (Au NTs) on a solid and transparent glass substrate which in turn has been employed for the selective optoplasmonic detection of Escherichia coli (EC) bacteria in human urine for the point-of-care diagnosis of urinary tract infections (UTIs). As compared to the single nanoparticle systems (Au NPs), the Au NTs show an enriched localized surface plasmon resonance (LSPR) due to the enhancement of the electric field under electromagnetic irradiation, e.g., photon, which helps in improving the limits of detection. For this purpose, initially a simple glass surface has been coated with Au NPs, with the help of the linker 3-aminopropyl-triethoxysilane - APTES. The surface has been linked further with another Au NP with the help of the 1,10-alkane-dithiol linker with two thiol ends, which eventually leads to the development of the optoplasmonic surface with Au NTs and an enhanced LSPR response. Subsequently, the EC specific aptamer has been chemically immobilized on the surface of Au NTs with the blocking of free sites via bovine serum albumin (BSA). Remarkably, Raman spectroscopy unfolds a 7-fold increase in the peak intensities with the Au NTs on the glass surface as compared to the surface coated with isolated Au NPs. The enhancement in the LSPR response of glass substrates coated with Au NTs and the EC specific aptamer has been further utilized for the selective and sensitive detection of UTIs. The results have been verified with the help of UV-visible spectroscopy to establish the utility of the proposed sensing methodology. An extensive interference study with other bacterial species unveils the selectivity and specificity of the proposed optoplasmonic sensors toward EC with a detection range of 5 × 10³ to 10⁷ CFU/mL. Intuitively, the method is more versatile in a sense that the sensor can be made specific to any other pathogens by simply changing the design of the aptamer. Finally, a low-cost, portable, and point-of-care optoplasmonic transduction setup is designed with a laser light illumination source, a sample holder, and a sensitive photodetector for the detection of UTIs in human urine.

Colorimetric detection of viral RNA fragments based on an integrated logic-operated three-dimensional DNA walker

Timely and accurate detection of virus is crucial for preventing spread of disease and early treatment of the infected cases. Herein we design an integrated logic-operated three-dimensional DNA walker for colorimetric detection of viral RNA fragments, by taking SARS-CoV-2 as an example. The DNA walker is composed of small amounts of dually-blocked walking strands and large amounts of dual-stem-loop track strands on gold nanoparticles. The walking strand contains a swing arm domain and a DNAzyme domain blocked at both sides of catalytic core, while the track strand contains a substrate domain located at the peripheral larger loop. Only the presence of both ORF1ab and N RNA fragments can fully de-block the walking strand, which then continuously hybridizes with track strands and cleaves them by DNAzyme-catalyzed hydrolysis. As the cleavage of track strands from long-stranded, double stem-loop structure to short-stranded, linear sequence, the DNA walker shows much lowered stability due to decreased negative charge density and diminished steric repulsion, which then gets aggregated at high salt concentration, accompanied by a visible color change. The colorimetric DNA walker detects RNA fragments down to 1 nM, responds dual viral genes in a "AND" logic way, and shows high specificity to target sequence. It can further detect large nucleic acids containing ORF1ab and N sequences, and reach 200 copies/mL detection limit by coupling a simple upstream amplification of sample. The method may provide a convenient way for reliable detection of viral RNA.

A Fiber-Based SPR Aptasensor for the In Vitro Detection of Inflammation Biomarkers

It is widely accepted that the abnormal concentrations of different inflammation biomarkers can be used for the early diagnosis of cardiovascular disease (CVD). Currently, many reported strategies, which require extra report tags or bulky detection equipment, are not portable enough for onsite inflammation biomarker detection. In this work, a fiber-based surface plasmon resonance (SPR) biosensor decorated with DNA aptamers, which were specific to two typical inflammation biomarkers, C-reactive protein (CRP) and cardiac troponin I (cTn-I), was developed. By optimizing the surface concentration of the DNA aptamer, the proposed sensor could achieve a limit of detection (LOD) of 1.7 nM (0.204 μg/mL) and 2.5 nM (57.5 ng/mL) to CRP and cTn-I, respectively. Additionally, this biosensor could also be used to detect other biomarkers by immobilizing corresponding specific DNA aptamers. Integrated with a miniaturized spectral analysis device, the proposed sensor could be applied for constructing a portable instrument to provide the point of care testing (POCT) for CVD patients.

Detection of SARS-CoV-2 virus via dynamic light scattering using antibody-gold nanoparticle bioconjugates against viral spike protein

Mass testing for the diagnosis of COVID-19 has been hampered in many countries owing to the high cost of genetic material detection. This study reports on a low-cost immunoassay for detecting SARS-CoV-2 within 30 min using dynamic light scattering (DLS). The immunosensor comprises 50-nm gold nanoparticles (AuNPs) functionalized with antibodies against SARS-CoV-2 spike glycoprotein, whose bioconjugation was confirmed using transmission electron microscopy (TEM), UV-Vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), and surface-enhanced Raman scattering spectroscopy (SERS). The specific binding of the bioconjugates to the spike protein led to an increase in bioconjugate size, with a limit of detection (LOD) 5.29 × 10³ TCID₅₀/mL (Tissue Culture Infectious Dose). The immunosensor was also proven to be selective upon interaction with influenza viruses once no increase in size was observed after DLS measurement. The strategy proposed here aimed to use antibodies conjugated to AuNPs as a generic platform that can be extended to other detection principles, enabling technologies for low-cost mass testing for COVID-19.

DNA-Crosslinked 2D Photonic Crystal Hydrogels for Detection of Adenosine Actuated by an Adenosine-Binding Aptamer

There is a need to develop versatile sensing motifs that can be used to detect a variety of chemical targets in resource-limited settings, for example, at the point of care. While numerous sensing technologies have been developed toward this effort, these technologies can be overly complex and require a skilled technician, extensive sample preparation, or sophisticated instrumentation to use, limiting their generalizability and application in resource-limited settings. Here, we report a novel sensing motif that utilizes DNA-crosslinked two-dimensional photonic crystal (2DPC) hydrogels. These hydrogel sensors contain a DNA aptamer recognition group that binds a target analyte. As proof of concept, we fabricated 2DPC hydrogels using a well-studied adenosine-binding aptamer. This adenosine aptamer is duplexed with a partially complementary strand and forms responsive crosslinks in the hydrogel polymer network. When adenosine is introduced, aptamer-adenosine binding occurs, breaking the DNA crosslinks and causing the hydrogel to swell. This in turn increases the particle spacing of an embedded 2DPC array, shifting the 2DPC Bragg diffraction. Thus, adenosine concentration can be monitored through 2DPC Bragg diffraction measurements. A linear range of 20 μM to 2 mM was observed. The detection limits were calculated to be 13.9 μM in adenosine-binding buffer and 26.7 μM in fetal bovine serum. This reported sensing motif has a readout that is simple and rapid and requires minimal equipment. We hypothesize that this sensing motif is generalizable and that other sensors can be easily fabricated by simply exchanging the aptamer that serves as a molecular recognition group.

Compact Peptoid Molecular Brushes for Nanoparticle Stabilization

Controlling the interfaces and interactions of colloidal nanoparticles (NPs) via tethered molecular moieties is crucial for NP applications in engineered nanomaterials, optics, catalysis, and nanomedicine. Despite a broad range of molecular types explored, there is a need for a flexible approach to rationally vary the chemistry and structure of these interfacial molecules for controlling NP stability in diverse environments, while maintaining a small size of the NP molecular shell. Here, we demonstrate that low-molecular-weight, bifunctional comb-shaped, and sequence-defined peptoids can effectively stabilize gold NPs (AuNPs). The generality of this robust functionalization strategy was also demonstrated by coating of silver, platinum, and iron oxide NPs with designed peptoids. Each peptoid (PE) is designed with varied arrangements of a multivalent AuNP-binding domain and a solvation domain consisting of oligo-ethylene glycol (EG) branches. Among designs, a peptoid (PE5) with a diblock structure is demonstrated to provide a superior nanocolloidal stability in diverse aqueous solutions while forming a compact shell (∼1.5 nm) on the AuNP surface. We demonstrate by experiments and molecular dynamics simulations that PE5-coated AuNPs (PE5/AuNPs) are stable in select organic solvents owing to the strong PE5 (amine)-Au binding and solubility of the oligo-EG motifs. At the vapor-aqueous interface, we show that PE5/AuNPs remain stable and can self-assemble into ordered 2D lattices. The NP films exhibit strong near-field plasmonic coupling when transferred to solid substrates.

Microwave-Assisted Synthesis of Silver Nanoparticles for Multimode Colorimetric Sensing of Multiplex Metal Ions and Molecular Informatization Applications

Plasmonic materials have been widely used in chemo/biosensing and biomedicine. However, little attention has been paid to the application of plasmonic materials in terms of the transition from molecular sensing to molecular informatization. Herein, we demonstrated that silver nanoparticles (AgNPs) prepared through facile and rapid microwave heating have multimode colorimetric sensing capabilities to different metal ions (Cr³⁺, Hg²⁺, and Ni²⁺), which can be further transformed into interesting and powerful molecular information technology (massively parallel molecular logic computing and molecular information protection). The prepared AgNPs can quantitatively and sensitively detect Cr³⁺ and Hg²⁺ in actual water samples. The AgNPs' multimode-guided multianalyte sensing processing was further investigated to construct a series of basic logic gates and advanced cascaded logic circuits by considering the analytes as the inputs and the colorimetric signals (like color, absorbance, wavelength shift) as the outputs. Moreover, the selective responses and molecular logic computing ability of AgNPs were also utilized to develop molecular cryptosteganography for encrypting and hiding some specific information, which proves that the molecular world and the information world are interconnected and use each other. This research not only opens the door for the transition from molecular sensing to molecular informatization but also provides an excellent opportunity for the construction of the "metaverse" of the molecular world.

The Interaction between DNA and Three Intercalating Anthracyclines Using Electrochemical DNA Nanobiosensor Based on Metal Nanoparticles Modified Screen-Printed Electrode

The screen-printed electrodes have gained increasing importance due to their advantages, such as robustness, portability, and easy handling. The manuscript presents the investigation of the interaction between double-strand deoxyribonucleic acid (dsDNA) and three anthracyclines: epirubicin (EPI), idarubicin (IDA), and doxorubicin (DOX) by differential pulse voltammetry on metal nanoparticles modified by screen-printed electrodes. In order to investigate the interaction, the voltammetric signals of dsDNA electroactive bases were used as an indicator. The effect of various metal nanomaterials on the signals of guanine and adenine was evaluated. Moreover, dsDNA/PtNPs/AgNPs/SPE (platinum nanoparticles/silver nanoparticles/screen-printed electrodes) was designed for anthracyclines–dsDNA interaction studies since the layer-by-layer modification strategy of metal nanoparticles increases the surface area. Using the signal of multi-layer calf thymus (ct)-dsDNA, the within-day reproducibility results (RSD%) for guanine and adenine peak currents were found as 0.58% and 0.73%, respectively, and the between-day reproducibility results (RSD%) for guanine and adenine peak currents were found as 1.04% and 1.26%, respectively. The effect of binding time and concentration of three anthracyclines on voltammetric signals of dsDNA bases were also evaluated. The response was examined in the range of 0.3–1.3 ppm EPI, 0.1–1.0 ppm IDA and DOX concentration on dsDNA/PtNPs/AgNPs/SPE. Electrochemical studies proposed that the interaction mechanism between three anthracyclines and dsDNA was an intercalation mode.

Stimuli-Responsive Three-Dimensional DNA Nanomachines Engineered by Controlling Dynamic Interactions at Biomolecule-Nanoparticle Interfaces

Stimuli-responsive nanomachines are attractive tools for biosensing, imaging, and drug delivery. Herein, we demonstrate that the orientation of macromolecules and subsequent dynamic interactions at the biomolecule-nanoparticle (bio-nano) interfaces can be rationally controlled to engineer stimuli-responsive DNA nanomachines. The success of this design principle was demonstrated by engineering a series of antibody-responsive DNA walkers capable of moving persistently on a three-dimensional track made of DNA functionalized gold nanoparticles. We show that drastically different responses to antibodies could be achieved using DNA walkers of identical sequences but with varying number or sites of modifications. We also show that multiple interfacial factors could be combined to engineer stimuli-responsive DNA nanomachines with high sensitivity and modularity. The potential of our strategy for practical uses was finally demonstrated for the amplified detection of antibodies and small molecules in both buffer and human serum samples. Unlike many DNA-based nanomachines, the performance of which could be significantly hindered by the matrix of serum, our system shows a matrix-enhanced sensitivity as a result of the engineering approach at the bio-nano interface.

Transcription-Based Amplified Colorimetric Thrombin Sensor Using Non-Crosslinking Aggregation of DNA-Modified Gold Nanoparticles

Gold nanoparticles (AuNPs) have been employed as colorimetric biosensors due to the color difference between their dispersed (red) and aggregated (blue) states. Although signal amplification reactions triggered by structural changes of the ligands on AuNPs have been widely used to improve measurement sensitivity, the use of ligands is limited. In this study, we designed a AuNP-based signal-amplifying sandwich biosensor, which does not require a conformational change in the ligands. Thrombin was used as a model target, which is recognized by two different probes. In the presence of the target, an extension reaction occurs as a result of hybridization of the two probes. Then RNA synthesis is started by RNA polymerase activation due to RNA promoter duplex formation. The amplified RNA drives aggregation or dispersion of the AuNPs, and a difference of the color if the AuNP solution is observed. As this detection system does not require a conformational change in the ligand, it can be generically applied to a wide range ligands.

Recent Advances in Nucleic Acid Modulation for Functional Nanozyme

Nanozymes have the potential to replace natural enzymes, so they are widely used in energy conversion technologies such as biosensors and signal transduction (converting biological signals of a target into optical, electrical, or metabolic signals). The participation of nucleic acids leads nanozymes to produce richer interface effects and gives energy conversion events more attractive characteristics, creating what are called “functional nanozymes”. Since different nanozymes have different internal structures and external morphological characteristics, functional modulation needs to be compatible with these properties, and attention needs to be paid to the influence of nucleic acids on nanozyme activity. In this review, “functional nanozymes” are divided into three categories, (nanozyme precursor ion)/ (nucleic acid) self-assembly, nanozyme-nucleic acid irreversible binding, and nanozyme-nucleic acid reversible binding, and the effects of nucleic acids on modulation principles are summarized. Then, the latest developments of nucleic acid-modulated nanozymes are reviewed in terms of their use in energy conversion technology, and their conversion mechanisms are critically discussed. Finally, we outline the advantages and limitations of “functional nanozymes” and discuss the future development prospects and challenges in this field.

Investigating the effect of 6-mercaptohexanol on the performance of a biosensor based on nanosurface energy transfer between gold nanoparticles and quantum dots

Nanosurface energy transfer (NSET)-based sensors have been widely developed using various pairs of nanomaterials including gold nanoparticles (AuNPs) and quantum dots (QDs). However, a low signal to background ratio is one of the most important problems that researchers are continually trying to solve. Herein, we present a 6-mercaptohexanol (MCH) modified MCH/DNA-Au-QD sensor for the detection of nucleic acids and MUC1. Interestingly, an unexpected effect of MCH was found in enhancing the fluorescence recovery ratio, therefore yielding a higher signal to background ratio. Through further investigation, we perceive the enhancement as a result of lowering of the NSET efficiency between free DNA-AuNPs and free DNA-QDs, which arises from the stretching of adsorbed DNA on the surface of AuNPs. The employment of MCH endowed the sensor with a wider linear range from 5 nM to 120 nM and a relatively lower LOD of 1.19 nM in nucleic acid detection, outperforming the original DNA-Au-QD sensor. Furthermore, the application of the sensor can be further extended to MUC1 detection. This study offers a better understanding of the NSET process between QDs and AuNPs and also initiates a new approach for the performance optimization of analogous NSET-based sensors.

Review of optical sensing and manipulation of chiral molecules and nanostructures with the focus on plasmonic enhancements [Invited]

Chiral molecules are ubiquitous in nature; many important synthetic chemicals and drugs are chiral. Detecting chiral molecules and separating the enantiomers is difficult because their physiochemical properties can be very similar. Here we review the optical approaches that are emerging for detecting and manipulating chiral molecules and chiral nanostructures. Our review focuses on the methods that have used plasmonics to enhance the chiroptical response. We also review the fabrication and assembly of (dynamic) chiral plasmonic nanosystems in this context.

Colorimetric aptasensor for on-site detection of oxytetracycline antibiotic in milk

Oxytetracycline (OTC), one of the largely used antibiotic in veterinary practice has been banned due to its potential side effects. Development of a field applicable and affordable kit to detect OTC will help to eliminate such milk from human consumption. An aptamer has been designed (27 nt; K_d = 29.2 ± 19.4 nM) through rational truncation. OTC interacts with this aptamer in G rich regions as confirmed by molecular modelling and circular dichroism spectroscopy. To develop a lateral flow based aptasensor, OTC was conjugated with a 7 kDa carrier protein to immobilize onto the nitrocellulose membrane. Using 0.125 µM aptamer-gold conjugate, assay could visually detects upto 5 ng/mL of OTC in spiked milk within 10 mins [Limit of quantitation (LOQ)-0.254 ± 1.62 ng/mL; permissible limit 100 ng/mL]. It showed no cross reactivity with components of milk and data correlated with analysis done through HPLC.

Single-stranded DNA probe paired aptasensor with extra dye binding sites to enhance its fluorescence response in the presence of a target compound

The purpose of this study is to investigate the possibility of improving the performance of a DNA binding dye water quenching based aptasensor without changing or truncating the aptamer. To demonstrate the possibility of increasing the change in fluorescence of the aptasensor by pairing it with a suitable ssDNA probe, three ssDNA probes (probe 1, 2, and 3) were employed and the fluorescence from the bound dyes was measured. This showed that ssDNA probe 2 created the most additional binding sites. By varying the target compound concentration (0, 0.05, 0.5, 5, 50, and 500 mg L⁻¹ 4-n-nonylphenol), the corresponding change in the fluorescence signal of the unpaired and ssDNA probe paired aptasensors were measured and compared over a range of emission wavelengths. The response of all three ssDNA probe paired aptasensors showed good fit (R² = 0.88–0.92) to a logarithmic response. The sensitivity of the aptasensor paired with ssDNA probe 2 was improved by ∼60%, whereas that of the aptasensor paired with ssDNA probe 3 was only improved by a marginal ∼3%. This study is a demonstration of using an appropriate ssDNA probe to increase the number of binding sites and hence the performance of a DNA binding dye and water quenched aptasensor. It is a possibility that can be extended to similar aptasensors without having to change or truncate the aptamer.

Principle of an ssDNA paired aptasensor where extra dye binding sites are created to enhance its fluorescence response.

A dispersive solid phase extraction adsorbent based on aptamer modified chitosan nanofibers for zearalenone separation in corn, wheat, and beer samples

Highly selective separation of trace bio-toxins in food samples has long been a hot topic pursued by analytical chemists. In this paper, chitosan nanofibers prepared by freeze-drying were modified with aptamers for dispersive solid phase extraction (dSPE) of trace zearalenone. The morphology of achieved chitosan nanofibers was found to be uniform and continuous, and the length was at the micron level with about a 400 nm diameter. The immobilization capacity of the aptamer was as high as 10.1 μg on 5 mg chitosan nanofibers with good stability and repeatability, owing to the high specific surface area of nanofibers. The aptamer modified chitosan nanofibers (Apt-CNFs) showed specific selectivity to zearalenone with a selectivity coefficient of 2.65 compared to the scrambled oligonucleotide functionalized CNFs, and the selectivity factors over other analogs and reference compounds were from 1.57 to 50.0. After the optimization of extraction conditions, the Apt-CNF based dSPE was coupled with high-performance liquid chromatography for zearalenone monitoring, and a good linear range of 0.06-10.0 μg L-1 was achieved with a detection limit of 18.0 ng L-1. The spiking recoveries of 101-108%, 100-110%, and 98.3-101% were achieved for trace zearalenone in corn, wheat, and beer samples, respectively. The residual zearalenone was detected in corn and wheat with a content of 0.365 and 0.0775 μg g-1, respectively.

Interfacing DNA with Gold Nanoparticles for Heavy Metal Detection

The contamination of heavy metals (e.g., Hg, Pb, Cd and As) poses great risks to the environment and human health. Rapid and simple detection of heavy metals of considerable toxicity in low concentration levels is an important task in biological and environmental analysis. Among the many convenient detection methods for heavy metals, DNA-inspired gold nanoparticles (DNA-AuNPs) have become a well-established approach, in which assembly/disassembly of AuNPs is used for colorimetric signaling of the recognition event between DNA and target heavy metals at the AuNP interface. This review focuses on the recent efforts of employing DNA to manipulate the interfacial properties of AuNPs, as well as the major advances in the colorimetric detection of heavy metals. Beginning with the introduction of the fundamental aspects of DNA and AuNPs, three main strategies of constructing DNA-AuNPs with DNA binding-responsive interface are discussed, namely, crosslinking, electrostatic interaction and base pair stacking. Then, recent achievements in colorimetric biosensing of heavy metals based on manipulation of the interface of DNA-AuNPs are surveyed and compared. Finally, perspectives on challenges and opportunities for future research in this field are provided.

A review on nanostructure-based mercury (II) detection and monitoring focusing on aptamer and oligonucleotide biosensors

Heavy metal ion pollution is a severe problem in environmental protection and especially in human health due to their bioaccumulation in organisms. Mercury (II) (Hg²⁺), even at low concentrations, can lead to DNA damage and give permanent harm to the central nervous system by easily passing through biological membranes. Therefore, sensitive detection and monitoring of Hg²⁺ is of particular interest with significant specificity. In this review, aptamer-based strategies in combination with nanostructures as well as several other strategies to solve addressed problems in sensor development for Hg²⁺ are discussed in detail. In particular, the analytical performance of different aptamer and oligonucleotide-based strategies using different signal improvement approaches based on nanoparticles were compared within each strategy and in between. Although quite a number of the suggested methodologies analyzed in this review fulfills the standard requirements, further development is still needed on real sample analysis and analytical performance parameters.

Gold nanoparticle aptamer assay for the determination of histamine in foodstuffs

The development of a gold nanoparticle aptamer assay is persued for rapid and sensitive determination of histamine in foodstuffs, which could be deployed for on-site use. The assay is based on a histamine-specific aptamer and gold nanoparticles and the salt-induced aggregation of the particles in the presence of histamine indicated by the color change from red to blue. Gold nanoparticle size, salt type, and concentration as well as aptamer concentration were optimized, and using optimum conditions, a limit of detection of 8 nM (~ 0.05 mg/kg) was obtained. Finally, the aptamer AuNP assay was applied to the determination of histamine in quality control fish samples. The histamine levels of these samples had previously been determined using HPLC and commercial ELISA kits by numerous independent laboratories and a good correlation was obtained. The developed AuNP assay is rapid, sensitive, and reproducible. Graphical abstract.

Selection of aptamers targeted to food-borne pathogenic bacteria Vibrio parahaemolyticus

Vibrio parahaemolyticus (Vp) is a common marine halophilic food-borne pathogen, mainly found in seafood and food with a high salt content. Gastrointestinal reactions such as diarrhea, headache, vomiting, nausea, and abdominal cramps may occur after eating food infected with Vp. This study aimed to screen for high-affinity aptamers that specifically recognize Vp. A high-affinity modified aptamer screening kit was used to rapidly screen aptamers of the food-borne Vp. The first round of screening involved release of target aptamers from the microspheres. The "false-positive" aptamers were eliminated after specific binding to and elution of Vp in the second round. The second round of screening of the aptamers involved polymerase chain reaction (PCR), and the abundance of a sequence was determined using next-generation sequencing. Nine high-affinity aptamer sequences were obtained, and the first eight modified aptamer sequences were derived using a cloud-based intelligent software of the American AM Biotech Co. Escherichia coli (E. coli) was used as a control, and aptamer ID 12 with the highest affinity for Vp was selected using real-time PCR. According to the principle of color change caused by nano-gold condensing under salt induction, Salmonella, Listeria monocytogenes (L. monocytogenes), and E. coli were used as counter-screening bacteria, and the aptamer ID12 was combined with nano-gold. The results showed that aptamer ID12 has strong specificity for Vp. Based on these findings, this study developed a simple, innovative, and rapid method for screening Vp aptamers.

Practical aptamer-based assay of heavy metal mercury ion in contaminated environmental samples: convenience and sensitivity

Due to heavy metals' magnified pollution from their accumulation in the ecosystem, practical detection of ultra-low concentration of heavy metals in environmental sample is of great significance for environmental supervision and maintenance of people's health. Herein, a practical and sensitive assay of heavy metal mercury was developed by visually observing (or spectrum detecting) the change of cationic gold nanoparticles (AuNPs), which is directly caused by mercury ion induced hybridization between non-canonical base pairs. In this assay, signal probe's response was direct rather than the indirect salt induction, thus avoiding the defect of salt-induced indirect response. It makes the analysis more sensitive. The results showed that the response of 8.2 × 10^-8 M Hg²⁺ could be observed with naked eye and the detection limit of Hg²⁺ in spectrometric determination was 4.9 × 10^-11 M, which is more than one order of magnitude lower than that from indirect response pattern of signal probe. In addition, high specificity of the affinity chemistry for T-Hg-T renders the assay to be highly selective. Compared with the results of cold vapor atom adsorption spectroscopy (CVAAS), this analysis has good reliability for the detection of mercury. The results fully indicate that the developed assay is an ideal alternative for online detection of heavy metal mercury in environmental pollution samples.

A glassy carbon electrode modified with reduced graphene oxide and gold nanoparticles for electrochemical aptasensing of lipopolysaccharides from Escherichia coli bacteria

An electrochemical aptasensor is described for the voltammetric determination of lipopolysaccharide (LPS) from Escherichia coli 055:B5. Aptamer chains were immobilized on the surface of a glassy carbon electrode (GCE) via reduced graphene oxide and gold nanoparticles (RGO/AuNPs). Fast Fourier transform infrared, X-ray diffraction and transmission electron microscopy were used to characterize the nanomaterials. Cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy were used to characterize the modified GCE. The results show that the modified electrode has a good selectivity for LPS over other biomolecules. The hexacyanoferrate redox system, typically operated at around 0.3 V (vs. Ag/AgCl) is used as an electrochemical probe. The detection limit is 30 fg·mL^-1. To decrease the electrochemical potential for detection of LPS, Mg/carbon quantum dots were used as redox active media. They decrease the detection potentialto 0 V and the detection of limit (LOD) to 1 fg·mL^-1. The electrode was successfully used to analyze serum of patients and healthy persons. Graphical abstractSchematic representation of the modification of reduced graphene oxide gold nanoparticles with aptamer chains to immobilize on the glassy carbon electrode surface for electrochemical detection of lipopolysaccharides.

Noble Metal Nanoparticles-Based Colorimetric Biosensor for Visual Quantification: A Mini Review

Nobel metal can be used to form a category of nanoparticles, termed noble metal nanoparticles (NMNPs), which are inert (resistant to oxidation/corrosion) and have unique physical and optical properties. NMNPs, particularly gold and silver nanoparticles (AuNPs and AgNPs), are highly accurate and sensitive visual biosensors for the analytical detection of a wide range of inorganic and organic compounds. The interaction between noble metal nanoparticles (NMNPs) and inorganic/organic molecules produces colorimetric shifts that enable the accurate and sensitive detection of toxins, heavy metal ions, nucleic acids, lipids, proteins, antibodies, and other molecules. Hydrogen bonding, electrostatic interactions, and steric effects of inorganic/organic molecules with NMNPs surface can react or displacing capping agents, inducing crosslinking and non-crosslinking, broadening, or shifting local surface plasmon resonance absorption. NMNPs-based biosensors have been widely applied to a series of simple, rapid, and low-cost diagnostic products using colorimetric readout or simple visual assessment. In this mini review, we introduce the concepts and properties of NMNPs with chemical reduction synthesis, tunable optical property, and surface modification technique that benefit the development of NMNPs-based colorimetric biosensors, especially for the visual quantification. The “aggregation strategy” based detection principle of NMNPs colorimetric biosensors with the mechanism of crosslinking and non-crosslinking have been discussed, particularly, the critical coagulation concentration-based salt titration methodology have been exhibited by derived equations to explain non-crosslinking strategy be applied to NMNPs based visual quantification. Among the broad categories of NMNPs based biosensor detection analyses, we typically focused on four types of molecules (melamine, single/double strand DNA, mercury ions, and proteins) with discussion from the standpoint of the interaction between NMNPs surface with molecules, and DNA engineered NMNPs-based biosensor applications. Taken together, NMNPs-based colorimetric biosensors have the potential to serve as a simple yet reliable technique to enable visual quantification.

An in-tube aptamer/gold nanoparticles coated capillary solid-phase microextraction for separation of adenosine in serum and urine samples

As an endogenous nucleoside, adenosine was significant for the diagnosis and treatment of some diseases, such as schizophrenia. However, due to the complicated matrix interference, it was difficult to monitor trace or ultra-trace adenosine directly in bio-samples. In this contribution, a novel in-tube SPME technique based on aptamer/Au nanoparticles coated open tubular fused-silica capillary was established to separate and enrich adenosine in bio-samples with high affinity. Therefore, a uniform and dense AuNPs layer was coated on the inner surface of the open tubular capillary, and then adenosine aptamer was immobilized on AuNPs with a high capacity of 2.44 μg per 27-cm capillary. As a result, the capillary shown high selectivity to adenosine with a selectivity factor of 14.4 when compared with the scrambled aptamer/AuNPs coated capillary. Also, the extraction amount of adenosine was 2.8-24.8 times higher than those of its structural analogs and contrast, such as guanosine, uridine, cytidine, thymidine, and toluic acid. After the optimization of extraction conditions, the aptamer/AuNPs coated in-tube SPME-HPLC method was developed for the adenosine assay with the linear range of 0.002-0.100 μg mL^-1 and the detection limit of 0.45 ng mL^-1. Subsequently, the approach was applied for trace adenosine monitoring in human serum and urine samples. It showed a strong performance of reducing matrix interference and improving sensitivity, and the spiking recoveries of 89.9-92.6% and 91.1-94.5% were achieved respectively.

Detachable nanoladders: A new method for signal identification and their application in the detection of ochratoxin A (OTA)

A highly sensitive fluorescence turn-off biosensor for the detection of ochratoxin A (OTA) was developed based on graphene oxide (GO) and an aptamer-induced detachable nanoladders. In this assay, two types of ssDNA (H1 and H2) were involved in the assembly of the DNA nanoladders, in which H1 was labeled with fluorophore, and H2 was the OTA binding aptamer. Self-assembly of the DNA nanoladders with the addition of GO weakened its adsorption and the fluorescence intensity remained strong. In the presence of OTA, the aptamer was specifically recognized and an aptamer-OTA complex was formed, leading to the detached of DNA nanoladders. With the addition of GO, the released H1 was adsorbed on the GO surface, thus efficiently quenching the fluorescence signal (turning off). The detection limit of OTA in this assay was 4.59 nM. To improve the sensitivity of this strategy, we creatively developed an alternative strategy to replace the sturdy nanoladders with frail nanoladders. More precisely, the sequences of H1 had mismatched bases, which, when hybridized with H2 could be used to create long non-perfect complementary nanoladders. For the mismatched bases-based frail nanoladders, it was easier for OTA to bind its aptamer sequence, thus enabling a more thorough and faster detachment of the nanoladders, along with a greater degree of fluorescence quenching. The detection limit for OTA was estimated to be 0.1 nM. The biosensors we developed were sensitive, specific, enzyme-free, cost-effective and can be applied in red wine samples spiked with known concentration of OTA.