Ultrasensitive Detection of 17β-Estradiol (E2) Based on Multistep Isothermal Amplification

Tailored microenvironment of functional DNA enables highly robust blood estradiol tests

Monitoring estradiol (E2) levels in the blood is crucial for obtaining reliable hormone levels necessary for assessing overall reproductive and cardiovascular health. While existing techniques like mass spectrometry and chemiluminescence immunoassays offer high sensitivity and selectivity, they are often complex, costly, and suffer from issues, such as cross-reactivity and limited biostability. Herein, we present a DNA-based method for E2 detection in blood, which mitigates the matrix effect using engineered DNA microenvironments. We demonstrate that incorporating heparin significantly enhances the binding between aptamers and E2 using fluorescence and electrochemical assays. Additionally, the presence of non-charged polymer on electrode surface reduces the apparent dissociation constant by over 70-fold, enabling us to design assays that overcome the dynamic range limitations of surface binding assays. Our method proves reliable for monitoring E2 levels in clinical samples, including those from women undergoing hormone replacement therapy, assisted reproductive technology, and throughout the menstrual cycle. Overall, our work presents a promising approach for E2 testing in blood and underscores the critical role of the DNA local environment in target recognition within biological settings.

Ultra-sensitive detection of microRNAs using an electrochemical biosensor based on a PNA-DNA2 three-way junction nanostructure and dual cascade isothermal amplification

BACKGROUND

MiRNA expression profiles may serve as valuable biomarkers for early cancer diagnosis. However, the detection of miRNA remains greatly challenging due to its shorter length and lower abundance in cells. Electrochemical biosensors based on DNA nanostructures have attracted significant attention due to their improved capture efficiency, high sensitivity, and ease of miniaturization, but the complex construction process, non-specific interactions among DNA probes, and their low stability continue to severely restrict their widespread application in clinical diagnostics. Therefore, developing robust and sensitive methods for miRNA analysis is still of great value.

RESULTS

We describe an ultra-sensitive electrochemical biosensor for miRNA by integrating the PNA-DNA2 three-way junction (3WJ) nanostructure with target-recycling catalytic hairpin assembly (CHA) and hybridization chain reaction (HCR) dual cascade isothermal amplification. The target miRNA triggers CHA amplification, resulting in the generation of substantial quantities of double-stranded DNA (dsDNA) products. These dsDNA are subsequently captured by PNA probes immobilized on the electrode surface, leading to the formation of a densely packed layer of PNA-DNA2 3WJ nanostructure. Subsequently, the single-stranded termini of the two branches extending from 3WJ function as promoters to initiate the HCR, resulting in the formation of a layer of intricately intertwined branched long dsDNA molecules, which could adsorb substantial quantities of [Ru(NH3)6]3+, thereby significantly amplifying the electrochemical signal. This electrochemical biosensor exhibits exceptional sensitivity towards miRNA-21, achieving a detection limit as low as 2.9 aM while effectively discriminating single base mutations.

SIGNIFICANCE

This represents the first electrochemical system for miRNA detection by integrating the PNA-DNA2 3WJ with CHA-HCR dual cascade isothermal amplification. The robust, specific and ultrasensitive feature makes cancer cell miRNA monitoring possible, suggesting its great application prospect as a promising sensing platform for monitoring various miRNA biomarkers in cancer diagnostics.

Highly responsive cryogel based sensing platform by encapsulating programmed DNA for colorimetric detection of 17β-estradiol

Responsive hydrogels show obvious superiorities when being employed at detection owing to their polymeric networks and optical properties. Albeit important, the slow response of traditional hydrogels against targets limits their further applications, and enhancing the responsiveness of hydrogel is therefore imperative. Herein, a cryogel with hierarchical structures was designed through adjusting the hydrogel structure by the freezing process to improve the responsive properties, and fabricated for rapid analysis of 17β-estradiol (E2). The cryogel encapsulating the DNA hairpins rapidly adsorbed the complementary DNA isolated from the aptamer in the presence of E2 and activated an efficient hybridization chain reaction (HCR). After integration with a smartphone system, a portable colorimetric sensing platform was created for sensitive detection of E2 with a low detection limit of 0.5 nM. Our work provides a promising way for highly responsive hydrogel-based biosensor design, and broadens their applications in rapid and portable target detection.

An intelligent spectral identification approach for the simultaneous detection of endocrine-disrupting chemicals in aquatic environments

With the rapid progression of industrialization, the application and release of endocrine disruptors (EDCs), including bisphenol A (BPA), octylphenol and nonylphenol have significantly increased, presenting substantial health hazards. Conventional analytical techniques, such as high-performance liquid chromatography and gas chromatography-mass spectrometry, are highly sophisticated but suffer from complex procedures and high costs. To overcome these limitations, this study introduces an innovative spectral methodology for the simultaneous detection of multiple aquatic multicomponent EDCs. By leveraging chemical machine vision, specifically with convolutional neural network (CNN) models, we employed a long-path holographic spectrometer for rapid, cost-effective identification of BPA, 4-tert-octylphenol, and 4-nonylphenol in aqueous samples. The CNN, refined with the ResNet-50 architecture, demonstrated superior predictive performance, achieving detection limits as low as 3.34, 3.71 and 4.36 μg/L, respectively. The sensitivity and quantification capability of our approach were confirmed through the analysis of spectral image Euclidean distances, while its universality and resistance properties were validated by assessments of environmental samples. This technology offers significantly advantages over conventional techniques in terms of efficiency and cost, offering a novel solution for EDC monitoring in aquatic environments. The implications of this research extend beyond improved detection speed and cost reduction, presenting new methodologies for analyzing complex chemical systems and contributing to environmental protection and public health.

Metal-to-particle charge transfer invoked photoelectrochemistry on ferroelectric SrTiO3 for split-mode and high-throughput aptasensing

Split-mode aptasensing is highly desirable in photoelectrochemistry because of its distinctive advantages of high-throughput, avoided damage to biomolecules, and increased sensitivity and selectivity. However, the currently available photoelectrochemical (PEC) strategy conducible to split-mode aptasensing is still limited to the bioreaction mediated generation of photoactive species, in which a low photocurrent was usually attained, rending this strategy impotent for attaining high sensitivity. As a result, to explore new strategies that are amendable to highly efficient, split-mode PEC aptasensing are still challenging but demanding. Herein, ferrocyanide mediated metal-to-particle charge transition (MPCT) on ferroelectric strontium titanate (SrTiO3) was explored as an innovative signal transduction strategy and was validated for high-performance aptasensing. By taking the 17β-Estradiol (E2) as a model analyte, the recognition between the target (E2) and its aptamer anchored on the Fe3O4@Au (named as Fe3O4@Au/Apt) destroyed the beforehand formed assembly between the Fe3O4@Au/Apt and the ssDNA labeled liposome encapsulated with [Fe(CN)6]4- (named as DLL-FeCN), which resulted in the release of the beforehand encapsulated [Fe(CN)6]4- into solution. The released [Fe(CN)6]4- then coordinated onto the surface of SrTiO3 nanoparticles consisted photoelectrode, forming the MPCT process from metal ion (iron (II) in [Fe(CN)6]4-) to the conduction band (CB) of SrTiO3 for anodic photocurrent signal output. The detection achieved linear range of 1.0 pM-100 nM, with a detection limit of 0.3 pM for E2. Benefiting from the cooperative effects of the MPCT process and the ferroelectric polarization in bulk SrTiO3 for achieving highly efficient photocurrent generation capability, the developed split-type detection had the advantage of high sensitivity/selectivity and high throughput. This work not only opens up the MPCT process for innovative PEC sensing strategy but also blazes a new road for high performance PEC aptasensing.

Fluorescence and colorimetric analysis of β-estradiol based on aptamer assembled spherical nucleic acids

Detecting β-estradiol (E2) in environmental monitoring is a complex task due to its status as a significant environmental contaminant. The detection methods require precision, sensitivity, and the ability to be conducted on-site without expensive instrumentation. Herein, we developed a novel approach using E2 aptamer assembled spherical nucleic acids (SNAs), which combines the sensitivity of fluorescence and the simplicity of colorimetry. Initially, a fluorescein (FAM)-labeled DNA aptamer is attached to the surface of gold nanoparticles (AuNPs) through hybridization with thiol-labeled DNA, resulting in fluorescence quenching. However, when E2 is present, the aptamer specifically binds to it, displacing from the thiol-DNA and releasing from the AuNP's surface. This leads to the recovery of fluorescence, allowing for quantitative detection of E2 by measuring the increase in fluorescence signal. Additionally, E2 detection can also be achieved visually using ultraviolet light. For colorimetric analysis, we introduce another set of AuNPs modified with thiol-DNA complementary to the DNA remaining on the surface of the previous AuNPs. When E2 triggers the release of the aptamer, the DNA on both AuNPs hybridized to each other, causing the aggregation of AuNPs and resulting in a distinct color change from red to purple. The detection limits for fluorescence and colorimetric analyses are 1 nM and 5 nM, respectively. We successfully applied this biosensing strategy to determine E2 concentrations in tap water and serum samples. Furthermore, our assay exhibits high selectivity towards E2 over other estrogens. Overall, this innovative approach provides an effective and versatile method for convenient on-site monitoring of E2.

An ultrasensitive dual-mode stagey for 17β-estradiol assay: Photoelectrochemical and colorimetric biosensor based on a WSe2/TiO2-modified electrode coupled with nucleic acid amplification

BACKGROUND

The abuse of 17β-estradiol(E2) has aroused wide concern in environmental and biomedical fields, which severely affects the endocrine function of human and animals. Therefore, an ultrasensitive and accurate assay of E2 is critically important. Traditional chromatography or immunoassay techniques exhibited good sensitivity and selectivity, but expensive instruments and antibodies may pose cost and stability issues, as well as difficulties in meeting on-site detection requirements. Ultrasensitive, reliable, and on-site detection of E2 at trace level remains a challenge. Hence, developing a simple, ultrasensitive assay to simultaneously achieve accurate detection and rapid visual analysis of E2 is extremely crucial.

RESULTS

We developed a versatile dual-mode photoelectrochemical (PEC) and colorimetric biosensor based on isothermal nucleic acid amplification strategy for the ultrasensitive and accurate detection of E2. The method modified titanium dioxide (TiO2) with tungsten selenide (WSe2) nanoflowers to synthesize WSe2/TiO2 heterostructures as a substrate for signal amplification and nanoprobe modification. Isothermal nucleic acid amplification strategy has been proven to be a powerful tool for strong signal amplification. The presence of a target triggered the nucleic acid amplification reaction, and produced a large amount of tDNA that competed with G-quadruplex immobilized on the electrode surface. The remaining G-quadruplex/hemin catalyzed the 4-chloro-1-naphthol (4-CN) to form biocatalytic precipitation (BCP) and ABTS-H2O2 chromogenic reaction, thus, the dual-mode platform was capable of achieving PEC-colorimetric ultrasensitive detection based on the catalytic activity of G-quadruplex/hemin DNAzyme. Within optimal conditions, the dual-mode biosensor exhibited a remarkable detection limit as low as 0.026 pM.

SIGNIFICANCE

Benefiting from the superior performance of WSe2/TiO2 and the power signal amplification of isothermal nucleic acid amplification strategy, this aptasensor achieved the ultrasensitive detection of E2. The independent transmission paths of photoelectrochemical and colorimetric provide mutual support and flexible switching, significantly enhancing the overall sensitivity and accuracy of the detection strategy, which can meet the needs for E2 precise quantification and rapid on-site detection.

A liposome encapsulated methylene blue-mediated electrochemical and UV-visible dual mode split-type immunoassay for the detection of 17β-estradiol

Herein, we fabricated an electrochemical (EC) and UV-visible absorption (UV-vis) dual mode split-type immunoassay for the detection of 17β-estradiol (E2), which was mediated by liposome encapsulated methylene blue (MB@lip). MB molecule acted as the probe in the EC and UV-vis absorption dual mode detections, and its release was controlled by liposome. The competitive immune recognition was conducted between the E2 in the sample and E2 conjugated bovine serum protein (E2-BSA) adsorbed on the 96-wells plate in combining with E2 antibody labeled with MB@lip (E2-Ab/MB@lip). MB molecule could be released from the resulting immune composite of E2-BSA/E2-Ab/MB@lip in the presence of Triton X-100, and quantified by UV-vis and EC methods. The three-dimensional cross-linked reduced graphene oxide/Ti3C2 (3D-rGO/Ti3C2) aerogel was prepared through hydrothermal method, then complexed with the electroactive anthraquinone (AQ) and used as the electrode modified material. The AQ/3D-rGO/Ti3C2 composite had high surface area and provided abundant adsorption sites for MB, and the displacement/competitive behavior between AQ and MB could dexterously achieve the ratiometric EC detection of E2. In addition, the inherent blue color of MB allowed it to be analyzed by UV-vis absorption method. The proposed dual mode detection method exhibited broad linear ranges of 0.1 pg mL-1 to 50 ng mL-1 (by UV-vis) and 0.03 pg mL-1 to 50 ng mL-1 (by EC) for E2 detection, and the detection limits were 0.023 pg mL-1 (S/N = 3) and 8.0 fg mL-1 (S/N = 3), respectively. Moreover, the proposed immunoassay exhibited good practicability and was applied to monitor E2 in milk and serum successfully.

A quadratic isothermal amplification fluorescent biosensor without intermediate purification for ultrasensitive detection of circulating tumor DNA

Circulating tumor DNA (ctDNA) is an auspicious tumor biomarker released into the bloodstream by tumor cells, offering abundant information concerning cancer genes. It plays a crucial role in the early diagnosis of cancer. However, due to extremely low levels in body fluids, achieving a simple, sensitive, and highly specific detection of ctDNA remains challenging. Here, we constructed a purification-free fluorescence biosensor based on quadratic amplification of ctDNA by combining nicking enzyme mediated amplification (NEMA) and catalytic hairpin assembly (CHA) reactions. After double isothermal amplification, this biosensor achieved an impressive signal amplification of nearly 107-fold, enabling it to detect ctDNA with ultra-sensitivity. And the detection limit of this biosensor is as low as 2 aM. In addition, we explored the influence of human serum on the performance of the biosensor and found that it showed favorable sensitivity in the presence of serum. This biosensor eliminates the need for an intermediate purification step, resulting in enhanced sensitivity and convenience. Thus, our purification-free fluorescent biosensor exhibits ultra-high sensitivity when compared to other biosensors and has the potential to serve as an effective diagnostic tool for early detection of cancer.

Cyclic Multiple Primer Generation Rolling Circle Amplification Assisted Capillary Electrophoresis for Simultaneous and Ultrasensitive Detection of Multiple Pathogenic Bacteria

Efficient, accurate, and economical detection of pathogenic bacteria is crucial in ensuring food safety and preventing foodborne illnesses. How to fulfill the highly sensitive and simultaneous detection of multiple trace pathogenic bacteria is a big challenge. In this work, capillary electrophoresis coupled with a cyclic multiple primer generation rolling circle amplification (cyclic MPG-RCA) was studied for highly sensitive and simultaneous detection of three kinds of pathogenic bacteria. The cyclic MPG-RCA was based on a carefully designed clover-shaped DNA probe, in which three "leaves" corresponded to three types of aimed pathogenic bacteria: Shigella dysenteriae (S. dysenteriae), Salmonella enterica subsp. enterica serovar Typhi (S. Typhi), and Vibrio parahaemolyticus (V. parahaemolyticus). Under the optimal experimental conditions, the limits of detection (S/N = 3) of this method for bacterial target DNA were 11.4 amol·L-1 (S. dysenteriae), 4.88 amol·L-1 (S. Typhi), and 14.9 amol·L-1 (V. parahaemolyticus), and the conversion concentrations for the target bacteria were 10 colony-forming units (CFU)·mL-1 (S. dysenteriae), 3 CFU·mL-1 (S. Typhi), and 12 CFU·mL-1 (V. parahaemolyticus). This method had been applied to the detection of tap water samples with good results, which proved that it could be used as an effective tool for trace pathogenic bacteria monitoring in foods, environments, and medicines.

Detection of Escherichia coli by capillary electrophoresis assisted by large volume sample stacking and nicking endonuclease signal amplification

Efficient, accurate and economical detection of pathogenic bacteria is crucial in ensuring food safety and preventing foodborne illnesses. In this study, a capillary electrophoresis coupled laser-induced fluorescence assay (CE-LIF) was developed for the detection of Escherichia coli (E. coli) by detecting its specific DNA. The CE-LIF was assisted by both online enrichment and offline amplification to improve the detection sensitivity of bacterial DNA. Here the online amplification was performed by large volume sample stacking (LVSS), while the offline amplification was nicking endonuclease signal amplification (NESA). Under the optimal experimental conditions, the detection limit of bacterial target DNA was 2.5 fM, and the conversion concentration of E. coli was 3 CFU · mL-1. The method had been applied to the detection of commercially available skim milk samples with good results, which proved that it could be used as an effective tool for food and environmental bacteria monitoring.

Recent advances in cascade isothermal amplification techniques for ultra-sensitive nucleic acid detection

Nucleic acid amplification techniques have always been one of the hot spots of research, especially in the outbreak of COVID-19. From the initial polymerase chain reaction (PCR) to the current popular isothermal amplification, each new amplification techniques provides new ideas and methods for nucleic acid detection. However, limited by thermostable DNA polymerase and expensive thermal cycler, PCR is difficult to achieve point of care testing (POCT). Although isothermal amplification techniques overcome the defects of temperature control, single isothermal amplification is also limited by false positives, nucleic acid sequence compatibility, and signal amplification capability to some extent. Fortunately, efforts to integrating different enzymes or amplification techniques that enable to achieve intercatalyst communication and cascaded biotransformations may overcome the corner of single isothermal amplification. In this review, we systematically summarized the design fundamentals, signal generation, evolution, and application of cascade amplification. More importantly, the challenges and trends of cascade amplification were discussed in depth.

Integrated multifunctional nanoplatform for fluorescence detection and inactivation of Staphylococcus aureus

Foodborne illness caused by Staphylococcus aureus (S. aureus) has posed a significant threat to human health. Herein, an integrated multifunctional nanoplatform was developed for fluorescence detection and inactivation of S. aureus based on cascade signal amplification coupled with single strand DNA-template copper nanoparticles (ssDNA-Cu NPs). Benefiting from reasonable design, one-step cascade signal amplification was achieved through strand displacement amplification combined with rolling circle amplification, followed by in-situ generation of copper nanoparticles. S. aureus detection could be performed through naked eye observation and microplate reader measurement of the red fluorescence signal. The multifunctional nanoplatform had satisfactory specificity and sensitivity, achieving 5.2 CFU mL^-1 detection limit and successful detection of 7.3 CFU of S. aureus in spiked egg after < 5 h of enrichment. Moreover, ssDNA-Cu NPs could eliminate S. aureus to avoid secondary bacterial contamination without further treatment. Therefore, this multifunctional nanoplatform has potential application in food safety dtection.

Novel method in emerging environmental contaminants detection: Fiber optic sensors based on microfluidic chips

Recently, human industrial practices and certain activities have caused the widespread spread of emerging contaminants throughout the environmental matrix, even in trace amounts, which constitute a serious threat to human health and environmental ecology, and have therefore attracted the attention of research scholars. Different traditional techniques are used to monitor water pollutants, However, they still have some disadvantages such as high costs, ecological problems and treatment times, and require technicians and researchers to operate them effectively. There is therefore an urgent need to develop simple, inexpensive and highly sensitive methods to sense and detect these toxic environmental contaminants. Optical fiber microfluidic coupled sensors offer different advantages over other detection technologies, allowing manipulation of light through controlled microfluidics, precise detection results and good stability, and have therefore become a logical device for screening and identifying environmental contaminants. This paper reviews the application of fiber optic microfluidic sensors in emerging environmental contaminant detection, focusing on the characteristics of different emerging contaminant types, different types of fiber optic microfluidic sensors, methodological principles of detection, and specific emerging contaminant detection applications. The optical detection methods in fiber optic microfluidic chips and their respective advantages and disadvantages are analyzed in the discussion. The applications of fiber optic biochemical sensors in microfluidic chips, especially for the detection of emerging contaminants in the aqueous environment, such as personal care products, endocrine disruptors, and perfluorinated compounds, are reviewed. Finally, the prospects of fiber optic microfluidic coupled sensors in environmental detection and related fields are foreseen.

Recent progress in the development of DNA-based biosensors integrated with hybridization chain reaction or catalytic hairpin assembly

As isothermal, enzyme-free signal amplification strategies, hybridization chain reaction (HCR) and catalytic hairpin assembly (CHA) possess the advantages such as high amplification efficiency, excellent biocompatibility, mild reactions, and easy operation. Therefore, they have been widely applied in DNA-based biosensors for detecting small molecules, nucleic acids, and proteins. In this review, we summarize the recent progress of DNA-based sensors employing typical and advanced HCR and CHA strategies, including branched HCR or CHA, localized HCR or CHA, and cascaded reactions. In addition, the bottlenecks of implementing HCR and CHA in biosensing applications are discussed, such as high background signals, lower amplification efficiency than enzyme-assisted techniques, slow kinetics, poor stability, and internalization of DNA probes in cellular applications.

Capture-SELEX of DNA Aptamers for Estradiol Specifically and Estrogenic Compounds Collectively

Estrogenic compounds such as estrone (E1), 17β-estradiol (E2), and 17α-ethynylestradiol (EE2) are serious environmental contaminants due to their potent biological activities. At least six selections were previously reported to obtain DNA aptamers for E2, highlighting its environmental importance. A careful analysis revealed that the previous aptamers either are too long or do not bind optimally. Herein, a series of new aptamers were obtained from the capture-SELEX method with dissociation constants down to 30 nM as determined by isothermal titration calorimetry (ITC). Two aptamers were converted to structure-switching fluorescent biosensors, which achieved a limit of detection down to 3.3 and 9.1 nM E2, respectively. One aptamer showed similar binding affinities to all the three estrogens, while the other aptamer is more selective for E2. Both aptamers required Mg²⁺ for binding. The proposed sensors were successfully applied in the determination of E2 in wastewater. Moreover, comparisons were made with previous aptamers based on primary sequence alignment and secondary structures. Among previously reported truncated aptamers, ITC showed binding only in one of them. The newly selected aptamers have the combined advantages of small size and high affinities.

Ultrasensitive Automatic Detection of Small Molecules by Membrane Imaging of Single Molecule Assays

Determination of trace amounts of targets or even a single molecule target has always been a challenge in the detection field. Digital measurement methods established for single molecule counting of proteins, such as single molecule arrays (Simoa) or dropcast single molecule assays (dSimoa), are not suitable for detecting small molecule, because of the limited category of small molecule antibodies and the weak signal that can be captured. To address this issue, we have developed a strategy for single molecule detection of small molecules, called small molecule detection with single molecule assays (smSimoa). In this strategy, an aptamer is used as a recognition element, and an addressable DNA Nanoflower (DNF) attached on the magnetic beads surface, which exhibit fluorescence imaging, is employed as the output signal. Accompanied by digital imaging and automated counting analysis, E2 at the attomolar level can be measured. The smSimoa breaks the barrier of small molecule detection concentration and provides a basis for high throughput detection of multiple substances with fluorescence encoded magnetic beads.

Dual-Mode Electrochemical Competitive Immunosensor Based on Cd2+/Au/Polydopamine/Ti3C2 Composite and Copper-Based Metal-Organic Framework for 17β-Estradiol Detection

Herein, a dual-mode electrochemical competitive immunosensor was constructed for the detection of 17β-estradiol (E2) based on differential pulse voltammetry (DPV) and chronoamperometry (i-t). During the immune recognition process, the E2 antibody (E2-Ab) was immobilized on the Cd²⁺/Au/polydopamine/Ti₃C₂ (Cd²⁺/Au/pDA/Ti₃C₂) composite-modified electrode; then, the E2-conjugated bovine serum albumin (E2-BSA) was labeled with a copper-based metal-organic framework (Cu-MOF) and competed with E2 in combining the E2-Ab. The Cu-MOF was not only an electroactive species but also possessed good electrocatalytic activity toward H₂O₂. Thus, E2 could be quantified according to the peak current change of the Cu-MOF in DPV curve or the variation of H₂O₂ reduction current. For DPV quantification, Cd²⁺ was introduced as an internal reference in this case, and a highly reproducible ratio readout was obtained. The as-prepared dual-mode E2 electrochemical immunosensor showed good linear relationship in the ranges of 1 pg mL^-1-10 ng mL^-1 (DPV) and 10 pg mL^-1-10 ng mL^-1 (i-t), and the detection limits were 0.47 and 5.4 pg mL^-1 (S/N = 3), respectively. Furthermore, the dual-mode electrochemical immunosensor exhibited good practicability in real sample analysis.

Split aptamer remodeling-initiated target-self-service 3D-DNA walker for ultrasensitive detection of 17β-estradiol

DNA walker machines, as one of the dynamic DNA nanodevices, have attracted extensive interest in the field of analysis due to their inherent superiority. Herein, we reported a split aptamer remodeling-initiated target-self-service 3D-DNA walker for ultrasensitive, specific, and high-signal-background ratio determination of 17β-estradiol (E2) in food samples. Two split probes (STWS-a and STWS-b) were rationally designed that can undergo structural reassembled to serve as walking strands (STWS) under the induction of the target. Meanwhile, an intact E6-DNAzyme region was formed and activated at the tail of STWS. The activated E6-DNAzyme then continuously drives the 3D-DNA walker for signal amplification and specific detection of E2. Under optimal conditions, the proposed DNA walker-based biosensor exhibited excellent linearity in the range of 1 pM to 50 nM with a low limit of detection (LOD) of 0.28 pM, and good precision (2.7%) for 11 replicate determinations of 1 nM of E2. Furthermore, the developed DNA walker-based biosensor achieved excellent sensitive analysis of E2 in the complex food matrix with recoveries of 95.6-106.5%. This newly proposed split aptamer-based strategy has the advantages of ultrasensitive, high signal-to-background ratio, and high stability. Noteworthy, the successful operation of the DNA walker initiated by the split aptamer expands the principles of DNA walker design and provides a universal signal amplification platform for trace analysis.

Split aptamer regulated CRISPR/Cas12a biosensor for 17β-estradiol through a gap-enhanced Raman tags based lateral flow strategy

It is significant to exploit the full potential of CRISPR/Cas based biosensor for non-nucleic-acid targets. Here, we developed a split aptamer regulated CRISPR/Cas12a and gap-enhanced Raman tags based lateral flow biosensor for small-molecule target, 17β-estradiol. In this assay, one split aptamer of 17β-estradiol was designed to complement with crRNA of Cas12a so that the trans-cleavage ability of CRISPR/Cas12a can be regulated by the competitive binding of 17β-estradiol and split aptamers. Through integration of the signal amplification ability of CRISPR/Cas12a and the ultra-sensitive gap-enhanced Raman tags based lateral flow assay, a visible-SERS dual mode determination of 17β-estradiol can be established. 17β-estradiol can be visibly recognized as low as 10 pM and accurately quantified with a detection limit of 180 fM by SERS signals, which is at least 10³-fold lower than that of the previous immunoassay lateral flow strategies. Our assay provides a novel perspective to develop split aptamer regulated CRISPR/Cas12a coupling with SERS lateral flow strips for ultrasensitive and easy-to-use non-nucleic-acid targets detection.

Highly-sensitive and simple fluorescent aptasensor for 17 b-estradiol detection coupled with HCR-HRP structure

As an important kind of environmental endocrine disruptors, 17 β -Estradiol (E2) plays a major role in affecting the growth of human including sexual characters, pregnancy system, etc. In the modern society, with the threat of abuse in breeding, it is imperative to design sensitive methods for detecting low concentration of E2 in environment. In this work, we constructed a highly sensitive and simple fluorescent aptasenor for detecting E2 via amplification of hybridization chain reaction (HCR) and horseradish peroxidase (HRP). Through the competitions between complementary strand (cmDNA) and E2 to E2 aptamer modified on magnetic beads, the unbound cmDNA would be collected and captured by polystyrene microspheres to induce HCR which brought abundant biotin sites. Subsequently, benefit from the excellent catalytic performance of streptavidin-horseradish peroxidase (SA-HRP), the highly sensitive fluorescence signals could be obtained in low concentration of E2. Under the optimal conditions, the prospered method for E2 detection was shown a good liner range from 1 to 100 pg/mL, with the lower detecting limit of 0.2 pg/mL compared with previous work. In addition, the recovery rates tested in the real samples of milk and water were 99.20%-108.06% and 91.07%-106.13%. In all, the assay may provide a perspective way for highly sensitive detection for various contaminants in the real samples.

Programmable design of isothermal nucleic acid diagnostic assays through abstraction-based models

Accelerating the design of nucleic acid amplification methods remains a critical challenge in the development of molecular tools to identify biomarkers to diagnose both infectious and non-communicable diseases. Many of the principles that underpin these mechanisms are often complex and can require iterative optimisation. Here we focus on creating a generalisable isothermal nucleic acid amplification methodology, describing the systematic implementation of abstraction-based models for the algorithmic design and application of assays. We demonstrate the simplicity, ease and flexibility of our approach using a software tool that provides amplification schemes de novo, based upon a user-input target sequence. The abstraction of reaction network predicts multiple reaction pathways across different strategies, facilitating assay optimisation for specific applications, including the ready design of multiplexed tests for short nucleic acid sequence miRNAs or for difficult pathogenic targets, such as highly mutating viruses.

Dual CHA-mediated high-efficient formation of a tripedal DNA walker for constructing a novel proteinase-free dual-mode biosensing strategy

DNA walkers have been recognized as a type of powerful signal amplification tool for biosensors, but how to adopt a proper strategy to increase their amplification efficiency is still highly desirable. Herein we design a dual-catalytic hairpin assembly (CHA)-mediated strategy for the high-efficient formation of a tripedal Mg²⁺-dependent DNAzyme (MNAzyme)-DNA walker, and thus develop a novel proteinase-free dual-mode biosensing method for the kanamycin (Kana) antibiotic assay. The first CHA is initiated by a target-biorecognition reaction, which can produce the DNA walker and also induce the target recycling. The second CHA is initiated by a special base sequence designed as a one-half substrate of the MNAzyme. Upon the first CHA-triggered DNA walking at a magnetic bead (MB) track, this "pseudo-target" sequence can be released to induce another CHA-cycle for the formation of the same DNA walker. Meanwhile, the other one-half substrate strand exposed on the MB surface will trigger the quantitative hybridization chain reaction (HCR)-assembly of a G-quadruplex DNAzyme (G-DNAzyme)-enriched double-stranded DNA polymer. So the enzymatic reaction of G-DNAzymes enabled the convenient colorimetric and photoelectrochemical dual-mode signal transduction of the method. Due to the dual-CHA facilitation to the tripedal and three-dimensional DNA walking and synergetic signal amplification of HCR, this method exhibits very low detection limits of 9.4 and 0.55 fg mL^-1, respectively. In combination with its wide linear range, automated manipulation, and excellent selectivity, repeatability and reliability, the proposed method is expected to be used for the convenient semiquantitative screening and accurate determination of possible antibiotic residues in complicated matrices.

A signal-off photoelectrochemical aptasensor for ultrasensitive 17β-estradiol detection based on rose-like CdS@C nanostructure and enzymatic amplification

Carbon-coated cadmium sulfide rose-like nanostructures (CdS@C NRs) were prepared via a facile solvothermal approach and used as the photoelectrochemical (PEC) sensing platform for the integration of functional biomolecules. Based on this, a novel "signal-off" PEC aptasensor mediated by enzymatic amplification was proposed for the sensitive and selective detection of 17β-estradiol (E2). In the presence of E2, alkaline phosphatase-modified aptamer (ALP-apta) were released from the electrode surface through the specific recognition with E2, which caused the negative effect on PEC response due to the decrease of ascorbic acid (AA) produced by the ALP in situ enzymatic catalysis. The developed PEC aptasensor for detection of E2 exhibited a wide linear range of 1.0-250 nM, with the low detection limit of 0.37 nM. This work provides novel insight into the design of potential phoelectroactive materials and the application of signal amplification strategy in environmental analysis field.

Functional nucleic acid biosensors utilizing rolling circle amplification

Functional nucleic acids regulate rolling circle amplification to produce multiple detection outputs suitable for the development of point-of-care diagnostic devices.

Recognition and Selectivity Analysis Monitoring of Multicomponent Steroid Estrogen Mixtures in Complex Systems Using a Group-Targeting Environmental Sensor

The same class of steroid estrogen mixtures, coexisting in the environment of 17β-estradiol, estrone (E1), and ethinyl estradiol (EE2), have strong ability to disrupt the human endocrine system and are seriously prejudicial to the health of the organism and environmental safety. Herein, a highly sensitive and group-targeting environmental monitoring sensor was fabricated for a comprehensive analysis of multicomponent steroid estrogens (multi-SEs) in complex systems. This breakthrough was based on the highly sensitive photoelectrochemical response composite material CdSe NPs-TiO₂ nanotube and highly group-specific aptamers. The optimized procedure exhibited not only high sensitivity in a wide range of concentrations from 0.1 to 50 nM, indeed, the minimum detection limit was 33 pM, but also strong resistance to interference. The affinity and consistent action pockets of this sensor enable selective detection of multi-SEs in complex systems. It subsequently was applied for the analysis of multi-SEs from three real samples in the environment including medical wastewater, river water, and tap water to provide a means to clarify the fate of multi-SEs in the process of migration and transformation. This monitoring sensor has a brilliant application prospect for the identification and monitoring of the same class of endocrine-disrupting chemical mixtures in environmental complex systems.