Disposable Strip Biosensor for Visual Detection of Hg2+ Based on Hg2+-Triggered Toehold Binding and Exonuclease III-Assisted Signal Amplification

A dual-signal aptasensor based on cascade amplification for ultrasensitive detection of aflatoxin B1

Aflatoxin B1 (AFB1) is considered as a serious carcinogenic mycotoxin that was widely detected in grains and foods, and its sensitive analysis is of key importance to avoid the health threats for consumers. In this study, a dual-signal aptasensor based on cascade of entropy-driven strand displacement reaction (ESDR) and linear rolling circle amplification (LRCA) was fabricated for ultrasensitive determination of AFB1. At the sensing system, the complementary strand would be released after the aptamer combined with AFB1, which will bring about the functional domains exposed, triggering the subsequent ESDR. Meanwhile, the two strands that were outputted by ESDR would incur the downstream LRCA reaction to produce a pair of long strands to assist in the generation of fluorescence and absorbance signals. Under the optimized conditions, the proposed aptasensor could achieve excellent sensitivity (limit of detection, 0.427 pg/mL) with satisfactory accuracy (recoveries, 92.8-107.9 %; RSD, 2.4-5.0 %), mainly ascribed to the cascade amplification. Importantly, owing to the flexibility design of nucleic acid primer, this analytical method can be applied in monitoring various hazardous substances according to the specific requirements. Our strategy provides some novel insights at signal amplification for rapid detection of AFB1 and other targets.

Advances in colorimetric aptasensors for heavy metal ion detection utilizing nanomaterials: a comprehensive review

Heavy metal ion contamination poses significant environmental and health risks, necessitating rapid and efficient detection methods. In the last decade, colorimetric aptasensors have emerged as powerful tools for heavy metal ion detection, owing to their notable attributes such as high specificity, facile synthesis, adaptability to modifications, long-term stability, and heightened sensitivity. This comprehensive overview summarizes the key developments in this field over the past ten years. It discusses the principles, design strategies, and innovative techniques employed in colorimetric aptasensors using nanomaterials. Recent advancements in enhancing sensitivity, selectivity, and on-site applicability are highlighted. The review also presents application studies of successful heavy metal ion detection using colorimetric aptasensors, underlining their potential for environmental monitoring and health protection. Finally, future directions and challenges in the continued evolution of these aptasensors are outlined.

Heavy Metal Ions Detection Using Nanomaterials-Based Aptasensors

Heavy metals ions as metallic pollutants are a growing global issue due to their adverse effects on the aquatic ecosystem, and human health. Unfortunately, conventional detection methods such as atomic absorption spectrometry exhibit a relatively low limit of detection and hold numerous disadvantages, and therefore, the development of an efficient method for in-situ and real-time detection of heavy metal residues is of great importance. The aptamer-based sensors offer distinct advantages over antibodies and emerged as a robust sensing platform against various heavy metals due to their high sensitivity, ease of production, simple operations, excellent specificity, better stability, low immunogenicity, and cost-effectiveness. The nucleic acid aptamers in conjugation with nanomaterials can bind to the metal ions with good specificity/selectivity and can be used for on-site monitoring of metal ion residues. This review aimed to provide background information about nanomaterials-based aptasensor, recent advancements in aptamer conjunction on nanomaterials surface, the role of nanomaterials in improving signal transduction, recent progress of nanomaterials-based aptasening procedures (from 2010 to 2022), and future perspectives toward the practical applications of nanomaterials-based aptasensors against hazardous metal ions for food safety and environmental monitoring.

Electric Field-Induced Specific Preconcentration to Enhance DNA-Based Electrochemical Sensing of Hg2+ via the Synergy of Enrichment and Self-Cleaning

Efficient preconcentration is critical for sensitive and selective electrochemical detection of metal ions, but rapid specific enrichment with depressed absorption of interfering ions at the electrode is challenging. Here, we proposed an electric field-induced specific preconcentration to boost the analytical performance of DNA-based electrochemical sensors for Hg²⁺ detection. As for such preconcentration, a positive external electric field was first used to enrich Hg²⁺ at an electrode assembled with T-rich DNA, thus boosting T-Hg²⁺-T recognitions. The following applied inverse electric field strips the nonspecifically absorbed Hg²⁺ and other interfering ions, thus depressing matrix interferences via self-cleaning. Based on this principle, we designed a portable device to realize programmable control of electric fields; a T-Hg²⁺-T recognition-based electrochemical sensor was thus fabricated as a model platform to assess the feasibility of electric field-induced preconcentration. The experimental results revealed that such a strategy decreased the time of T-Hg²⁺-T-based recognition from 60 to 20 min and led to detection with better reproducibility by depressing the influence of free Hg²⁺ as well as interfering ions. This strategy offered Hg²⁺ detection limits of 0.01 pM─three-fold better than that without preconcentration─within 22 min. The proposed preconcentration strategy offers a new way to enhance the analytical performance of sensing at the solid-liquid interface.

Recent Trends and Future Perspectives of Emergent Analytical Techniques for Mercury Sensing in Aquatic Environments

Environmental emissions of mercury from industrial waste and natural sources, even in trace amounts, are toxic to organisms and ecosystems. However, industrial-scale mercury detection is limited by the high cost, low sensitivity/specificity, and poor selectivity of the available analytical tools. This review summarizes the key sensors for mercury detection in aqueous environments: colorimetric-, electrochemical-, fluorescence-, and surface-enhanced Raman spectroscopy-based sensors reported between 2014-2021. It then compares the performances of these sensors in the determination of inorganic mercury (Hg²⁺ ) and methyl mercury (CH₃ Hg⁺ ) species in aqueous samples. Mercury sensors for aquatic applications still face serious challenges in terms of difficult deployment in remote areas and low robustness, reliability, and selectivity in harsh environments. We provide future perspectives on the selective detection of organomercury species, which are especially toxic and reactive in aquatic environments. This review is intended as a valuable resource for scientists in the field of mercury sensing.

Self-enhanced luminol-based electrochemiluminescent hydrogels: An ultrasensitive biosensing platform for fusion gene analysis coupled with target-initiated DNAzyme motor

BCR/ABL fusion gene has been discovered as an important and reliable biomarker for early diagnosis of chronic myeloid leukemia (CML). Herein, a novel and switching electrochemiluminescence (ECL) biosensor was developed for ultrasensitive determination of the fusion gene based on the self-enhanced polyethyleneimine-luminol (PEI-Lum) hydrogels coupled with target-initiated DNAzyme motor. The facilely prepared PEI-Lum hydrogels could not only immobilize enormous luminol but shorten the distance of binary system, thus facilitating the mass and electron transfer efficiency of the sensing interface, so that the enhanced ECL signal was achieved. Moreover, the engineering DNA motor was powered by Mg²⁺-dependent DNAzyme for isothermal DNA signal amplification. As a result, the fabricated ECL biosensor enabled highly sensitive detection of BCR/ABL fusion gene with a broad linear range from 10.0 fM to 10.0 nM and a low detection limit of 3.75 fM (S/N = 3). Significantly, the developed biosensing method provides a potential tool for nucleic acid analysis in clinical diagnosis and a new avenue to design high-efficient ECL nanomaterials.

Visual Test Paper for on-Site Polychlorinated Biphenyls Detection and Its Logic Gate Applications

A visual detection method was proposed for polychlorinated biphenyls (PCBs) detection using lateral flow test paper as the sensing platform. The aptamer sequence was used to recognize the target 3,3',4,4'-tetrachlorobiphenyl (PCB77). The integration of Zn²⁺-dependent DNAzyme with toehold-mediated strand displacement reaction significantly improved the response signals. Gold nanoparticles were utilized as the signal tracers in the test paper, making the results visible directly by the naked eye. Under optimal conditions, the paper enables the visual detection of PCB77 as low as 10 pM without additional instrumentation. The assay displays a high selectivity for PCB77 against potential interfering molecules. The visual test paper is robust and has been applied to the detection of PCB77 in milk samples with good recovery and satisfactory accuracy. Using two different PCBs (PCB77 and PCB72) as inputs, we further fabricated OR and AND logic gates, which is conducive to the development of an intelligent detection strategy for PCBs monitoring. Given the attractive characteristics of disposability, low cost, logic operation, and intuitive output, the test paper shows great promise for on-site screening of PCBs in resource-limited areas.

Development of a gold-nanorod-based lateral flow immunoassay for a fast and dual-modal detection of C-reactive protein in clinical plasma samples

Fast and simple detection of C-reactive protein (CRP) is highly significant for the diagnosis and prognosis of inflammatory or infectious diseases. Lateral flow immunoassay has the advantages of rapid detection, simple operation and low cost, but it is usually limited by the quantitative ability and speed of data extraction. Herein, a gold-nanorod-based lateral flow immunoassay was developed to rapidly detect CRP by simultaneously monitoring the colorimetric and temperature signals. In this method, anti-CRP antibody-modified gold nanorods (GNRs) were designed as colorimetric and photothermal conversion probes. A mouse anti-CRP monoclonal antibody and goat anti-mouse IgG were used as test and control lines, respectively. Then, a lateral flow immunochromatographic strip was constructed by a sandwich-type method for detecting CRP by introducing antibody-modified GNRs, and this procedure needed less than 15 min. Finally, the detection signals can be directly observed by eyes and directly read using a thermal imager. The as-synthesized GNR showed high photothermal conversion efficiency (η = 39%) and strong localized surface plasmon resonance (LSPR) absorption. For CRP detection, the proposed immunochromatographic strip exhibited good specificity, high sensitivity, good linearity within the range of 50-10 000 ng mL-1 and a low limit of detection (LOD, 1.3 ng mL-1). This method was successfully applied for CRP detection in clinical plasma samples, and it correlated very well with the diagnostic kit of immunoturbidimetry (r = 0.96). The results indicated that the developed GNR-based immunochromatographic strip has immense potential for use as a rapid and cost-effective in vitro diagnostic kit.

Trends in sensor development toward next-generation point-of-care testing for mercury

Mercury is one of the most common heavy metals and a major environmental pollutant that affects ecosystems. Since mercury and its compounds are toxic to humans, even at low concentrations, it is very important to monitor mercury contamination in water and foods. Although conventional mercury detection methods, including inductively coupled plasma mass spectrometry, atomic absorption spectroscopy, and gas chromatography-mass spectrometry, exhibit excellent sensitivity and accuracy, they require operation by an expert in a sophisticated and fully controlled laboratory environment. To overcome these limitations and realize point-of-care testing, many novel methods for direct sample analysis in the field have recently been developed by improving the speed and simplicity of detection. Commonly, these unconventional sensors rely on colorimetric, fluorescence, or electrochemical mechanisms to transduce signals from mercury. In the case of colorimetric and fluorescent sensors, benchtop methods have gradually evolved through technology convergence to give standalone platforms, such as paper-based assays and lab-on-a-chip systems, and portable measurement devices, such as smartphones. Electrochemical sensors that use screen-printed electrodes with carbon or metal nanomaterials or hybrid materials to improve sensitivity and stability also provide promising detection platforms. This review summarizes the current state of sensor platforms for the on-field detection of mercury with a focus on key features and recent developments. Furthermore, trends for next-generation mercury sensors are suggested based on a paradigm shift to the active integration of cutting-edge technologies, such as drones, systems based on artificial intelligence, machine learning, and three-dimensional printing, and high-quality smartphones.

Ultrasensitive fluorometric biosensor based on Ti3C2 MXenes with Hg2+-triggered exonuclease III-assisted recycling amplification

Herein a fluorometric bioanalysis platform for mercury(ii) (Hg²⁺) detection was developed using two-dimensional MXenes (Ti₃C₂) as fluorescence quencher and Hg²⁺-induced exonuclease III-assisted target recycling strategy for efficient signal amplification.

AuNanostar@4-MBA@Au Core-Shell Nanostructure Coupled with Exonuclease III-Assisted Cycling Amplification for Ultrasensitive SERS Detection of Ochratoxin A

The "turn-on" mode surface-enhanced Raman scattering (SERS) aptasensor for ultrasensitive ochratoxin A (OTA) detection was developed based on the SERS "hot spots" of AuNanostar@4-MBA@Au core-shell nanostructures (AuNS@4-MBA@Au) and exonuclease III (Exo III)-assisted target cycle amplification strategy. Compared with conventional gold nanoparticles, AuNS@4-MBA@Au provides a much higher SERS enhancement factor because AuNS exhibits a larger surface roughness and the lightning rod effect, as well as an excellent electromagnetic field between the AuNS core and the Au shell, which contribute to the superstrong SERS signal. Meanwhile, Exo III-assisted target cycle amplification can be used as an effective method for the further amplified detection of OTA. Additionally, the utilization of streptavidin magnesphere paramagnetic particles offers a green, economical, and facile technology for the accumulation and separation of the signal probe AuNS@4-MBA@Au from solution. All these factors lead to a significant enhancement of detectable signals and superhigh sensitivity. As a result, the limit of detection as low as 0.25 fg mL^-1 could be achieved, which was lower than that in the other reported literatures on SERS methods for OTA detection as we know. The developed SERS aptasensor also provides a promising tool for foodstuff detection.

Comparative evaluation of 15-minute rapid diagnosis of ischemic heart disease by high-sensitivity quantification of cardiac biomarkers

A rapid clinical diagnosis may efficiently improve the survival rate and prognosis of patients with ischemic heart disease (IHD). Therefore, a one-step, rapid and inexpensive analysis for the quick diagnosis of IHD was investigated in the present study. Consecutive patients who were subjected to myoglobin, cardiac troponin I and creatine kinase-MB isoenzyme assessment at the Emergency Department of Shenzhen Second People's Hospital (Shenzhen, China) between December 2017 and March 2018 prior to treatment were screened. Clinically applicable disposable strips were employed for quantification of the cardiac biomarkers. The analytical performance of the strips was evaluated by receiver operating characteristic (ROC) curves and compared with the traditional chemiluminescence immunoassay (CLIA) method. The data of 391 participants were collected. At the baseline, 57 patients were diagnosed with IHD and 334 patients were diagnosed with other diseases. The area under the ROC curve (AUC) of the CLIA model was 0.787 (95% CI, 0.709-0.865) with a specificity of 76.7% and a sensitivity of 71.9%. At the optimal cutoff value of -1.867, the negative and the positive predictive value were 94.1 and 34.5%, respectively. The AUC of the disposable strip model was 0.792 (95% CI, 0.729-0.855). At the cutoff value of -1.820 or below, the negative predictive value was 94.9%, the positive predictive value was 28.9%, the specificity was 66.8% and the sensitivity was 79.0%. The P-value of the ROCs was 0.858, indicating no statistically significant difference between the two assay methods. The cost of the disposable strip was 50% of that of the CLIA method and it took only 25% of the time that was required for the quantification of the three cardiac markers by CLIA. In conclusion, the disposable strip provides a platform for point-of-care testing and may be an easy, rapid, reliable and cost-saving method for the diagnosis of IHD.

Ultrasensitive colorimetric aptasensor for Hg2+ detection using Exo-III assisted target recycling amplification and unmodified AuNPs as indicators

Facile and ultrasensitive detection of Hg²⁺ in water environment remains challenging. Exonuclease III (Exo-III)-assisted target recycling is one of the most popular amplification strategies. Although the magnesium (II) ions are widely acting as cofactors of Exo-III, we recognized that Mg²⁺ cofactors would strongly disturb the charge distribution on citrate-stablized gold nanoparticles (in the general sense, unmodified AuNPs) surface, thus generate false positive colorimetric signals. To address this issue, we first put forward the view that the cobalt (II) ions can function as the Exo-III cofactor and successfully construct a novel label-free colorimetric aptasensor for facile and ultrasensitive detection of Hg²⁺ using Hg²⁺-triggered Exo-III-assisted signal amplification and unmodified AuNPs as indicators. A hairpin-looped DNA probe was rationally designed with thymine-rich recognition termini and specifically recognized trace Hg²⁺ by a stable T-Hg²⁺-T structure. A blue-to-red color change of AuNPs with the addition of Hg²⁺ provided the quantitative detection of Hg²⁺ with a limit of detection of 0.2 nM and a linear working range from 0.5 nM to 5.0 nM. The whole testing time for one assay was approximately 40 min. Real water samples, even containing Hg²⁺ at 1 nM, could be determined by the aptasensor with recovery rates from 97% to 103%.

Thymine-Hg2+-thymine coordination chemistry induced entropy driven catalytic reaction to form Hemin/G-quadruplex-HRP-mimicking DNAzyme for colorimetric and visual determination of Hg2

A sensitive and visible colorimetric strategy was proposed for Hg²⁺ detection by thymine-Hg²⁺-thymine (T-Hg²⁺-T) coordination chemistry and entropy driven catalytic reaction. The entropy driven catalytic reaction is induced by T-Hg²⁺-T coordination chemistry, resulting the releasing of G-riched sequence. Hemin/G-quadruplex-HRP-mimicking DNAzyme can be formed with the help of hemin, catalyzing TMB to TMB⁺ with a color change from colorless to blue. The sensitivity of this strategy can be reached to 2 pM, which is significantly improved by entropy driven catalytic reaction. In addition, entropy driven catalytic reaction provides a more reliable and accurate results. This method shows great promise for on-site analysis and in-house diagnosis of Hg²⁺ in water.

Comparison of three sample addition methods in competitive and sandwich colloidal gold immunochromatographic assay

Immunochromatographic assays (ICAs) are mainstream point-of-care diagnostic tools in disease control, food safety, and environmental monitoring. However, the important issue pertaining to the influence of sample addition methods on the detection performance of ICAs has not been addressed, and related information is still lacking. Herein, we selected the well-accepted gold nanoparticles (AuNPs) as visual labels. AuNP-based ICA was then used to explore the effects of three sample addition methods (i.e., dry, wet, and insert) on the analytical performance of ICAs by using competitive and sandwich models. Under optimized conditions, the competitive ICA with clenbuterol as an analyte showed a negligible difference (p > 0.05) in the detection performance of the three methods in ideal phosphate buffered saline solution. However, the wet method demonstrated the worst performance in pork samples (p < 0.05). The sandwich ICA strip with human chorionic gonadotropin as an analyte revealed the significantly different analytical performances of the three approaches in phosphate buffer (PB) solution and spiked serum (p < 0.05). Two independent linear correlations were observed with the increase in target concentration. However, for the wet method in the PB solution and serum, the first linear correlation was at a relatively narrow target concentration range, and the second linear correlation was at a wider concentration range compared with those for the dry and insert methods. Our findings demonstrated that sample addition methods slightly influence competitive ICAs (p > 0.05) but remarkably affect sandwich ICAs (p < 0.05). We believe that this study can further explain the differences in detection results for the same target analyte in actual ICA detection. The results may serve as a reference in the rational selection of the appropriate sample addition method for succeeding ICA works.

Simple and accurate visual detection of single nucleotide polymorphism based on colloidal gold nucleic acid strip biosensor and primer-specific PCR

Single nucleotide polymorphism (SNP) was associated with many human diseases, therefore, SNP detection was important for early diagnosis and clinical prognosis. Herein, a simple and accurate method for visual detection SNP sites (A/A, G/G, A/G) in CYP1A1 gene related to cancers based on colloidal gold nucleic acid strip biosensor and primer-specific polymerase chain reaction (PCR) was established. This method could directly distinguish SNP sites on strip biosensor by introducing twice PCR amplifications. The second PCR (primer-specific PCR) was performed using specific product of the first PCR as template, thus this twice PCR could reduce non-specific amplification greatly and obtain target product. In addition, single-strand or double-strand DNA (ssDNA or dsDNA) was accurately produced by introducing mismatched base at the 3' end of forward primers in primer-specific PCR. The designed strip biosensor could only combine with the ssDNA, thus visual detection of SNP could be achieved within 10 min by color difference of a pair of strips. 61 human blood samples by this method were identical with those of pyrosequencing. This method had the advantages of rapid, visual and low-cost and was expected to be applied in medical diagnosis.

Highly sensitive detection of prostate cancer specific PCA3 mimic DNA using SERS-based competitive lateral flow assay

Accurate analysis of prostate cancer specific biomarkers plays an important role in the early diagnosis of prostate cancer. Traditional colorimetric lateral flow assay (LFA) has the limitations of low detection sensitivity and qualitative or semiquantitative detection. In this study, we developed a novel surface-enhanced Raman scattering (SERS)-based competitive LFA for the rapid and highly sensitive quantitative evaluation of prostate cancer antigen 3 (PCA3) mimic DNA. Herein, the competitive hybridization interaction with capture DNA between target PCA3 mimic DNA and reporter DNA-labeled SERS nanotags results in a change in the amount of SERS nanotags on the test line. The quantitative analysis of target PCA3 mimic DNA was realized by monitoring the Raman peak intensity of SERS nanotags on the test line. The limit of detection of PCA3 mimic DNA was estimated to be 3 fM, which is about three orders of magnitude more sensitive than that of a commercially available kit. By combining the outstanding characteristics of the well-established SERS-based competitive strategy and LFA platform, our design has strong potential for the early diagnosis of prostate cancer and other diseases.

Autocatalytic DNA circuit for Hg2+ detection with high sensitivity and selectivity based on exonuclease III and G-quadruplex DNAzyme

Utilizing G-quadruplex as the signal report probe, an ultrasensitive and label-free autocatalytic DNA circuit for Hg²⁺ detection on the basis of exonuclease III (Exo III)-assisted cascade signal amplification has been proposed. In the absence of Hg²⁺, the hairpin A and the DNA1 cannot hybridize due to the thymine-thymine (T-T) mismatches. Therefore, hairpin probes with the 3'-protruding terminus can be resistant to Exo III digestion, preventing the G-rich sequence to be released. In the presence of Hg²⁺, the combination of the DNA1 with the 3' end-extruding hairpin A via T-Hg²⁺-T coordination chemistry triggers the digestion reaction of Exo III, leading to the release of the DNA1 and the sequence with domains c, d, and e. Both of the DNA1 and the sequence with domains c, d, and e can combine with other hairpin probes and activate another round of the cleavage reaction. The produced G-rich sequence can form G-quadruplex structure by binding with N-Methyl mesoporphyrin IX (NMM). The biosensor exhibits excellent selectivity and high sensitivity for Hg²⁺. The linear range of this biosensor is from 10 fM to 100 nM, and the linear equation can be expressed as: F₆₁₀ = 1.3 × 10⁵ Lg C + 7.40 × 10⁴ (R² = 0.998), in which F₆₁₀ is the fluorescence intensity at 610 nm, C represents the Hg²⁺ concentrations, and Lg is the logarithm of 10. The detection limit is 10 fM. The biosensor is robust and can be applied to the detection of Hg²⁺ in water samples. By substituting the target-recognition elements, this sensing system can also be used for the detection of other metal ions.

Bio-Recognition in Spectroscopy-Based Biosensors for *Heavy Metals-Water and Waterborne Contamination Analysis

: Microsystems and biomolecules integration as well multiplexing determinations are key aspects of sensing devices in the field of heavy metal contamination monitoring. The present review collects the most relevant information about optical biosensors development in the last decade. Focus is put on analytical characteristics and applications that are dependent on: (i) Signal transduction method (luminescence, colorimetry, evanescent wave (EW), surface-enhanced Raman spectroscopy (SERS), Förster resonance energy transfer (FRET), surface plasmon resonance (SPR)); (ii) biorecognition molecules employed (proteins, nucleic acids, aptamers, and enzymes). The biosensing systems applied (or applicable) to water and milk samples will be considered for a comparative analysis, with an emphasis on water as the primary source of possible contamination along the food chain.

Paper-based point-of-care test with xeno nucleic acid probes

Bridging the unmet need of efficient point-of-care testing (POCT) in biomedical engineering research and practice with the emerging development in artificial synthetic xeno nucleic acids (XNAs), this review summarized the recent development in paper-based POCT using XNAs as sensing probes. Alongside the signal transducing mode and immobilization methods of XNA probes, a detailed evaluation of probe performance was disclosed. With these new aspects, both researchers in synthetic chemistry / biomedical engineering and physicians in clinical practice could gain new insights in designing, manufacturing and choosing suitable reagents and techniques for POCT.

A dandelion-like liposomes-encoded magnetic bead probe-based toehold-mediated DNA circuit for the amplification detection of MiRNA

The development of facile and sensitive miRNA quantitative detection methods is a central challenge for the early diagnosis of miRNA-related diseases. Herein, we propose a strategy for a liposome-encoded magnetic bead-based DNA toehold-mediated DNA circuit for the simple and sensitive detection of miRNA based on a toehold-mediated circular strand displacement reaction (TCSDR) coupled with a personal glucometer (PGM ). In this strategy, a glucoamylase-encapsulated liposomes (GELs)-encoded magnetic bead (GELs-MB) probe is designed to integrate target binding, magnetic separation, and signal response. Upon sensing the target miRNA-21, a GELs-MB probe-based toehold-mediated circular strand displacement reaction (TCSDR) was initiated with the help of fuel-DNA, constructing a DNA circuit system, and realizing target recycling amplification and the disassembly of the liposomes. The disassembled liposomes were finally removed via magnetic separation, and the encapsulated glucoamylase was liberated to catalyze amylose hydrolysis with multiple turnovers to glucose for a PGM readout. Benefiting from target recycling amplification initiated by the toehold-mediated DNA circuit and the liposome multiple-label amplification, a small quantity of target miRNA-21 can be transformed into a large glucose signal. The strategy realized the quantification of miRNA-21 down to a level of 0.7 fM without enzymatic amplification or precise instrumentation. Moreover, the high-density GELs-MB probe allows the sensitive detection of miRNA-21 to be accomplished within 1.5 h. Furthermore, this strategy exhibits the advantages of specificity and simplicity, since a toehold-mediated strand displacement reaction, magnetic separation and portable PGM were used. Importantly, this strategy has been demonstrated to allow the high-confidence quantification of miRNA. Therefore, with the advantages of low cost, ease of use, portability, and sensitivity, the reported method holds great potential for the early diagnosis of miRNA-related diseases.

Coordination-induced structural changes of DNA-based optical and electrochemical sensors for metal ions detection

Metal ions play a critical role in human health and abnormal levels are closely related to various diseases. Therefore, the detection of metal ions with high selectivity, sensitivity and accuracy is particularly important. This article highlights and comments on the coordination-induced structural changes of DNA-based optical, electrochemical and optical-electrochemical-combined sensors for metal ions detection. Challenges and potential solutions of DNA-based sensors for the simultaneous detection of multiple metal ions are also discussed for further development and exploitation.

A Universal Paper-Based Electrochemical Sensor for Zero-Background Assay of Diverse Biomarkers

This paper describes a universal paper-based electrochemical sensing platform that uses a paper modified with signal molecule-labeled DNA and a screen-printed electrode along with target recognition solutions to achieve the detection of multiple types of biomarkers. These assays rely on the target-induced synthesis of Mg²⁺-dependent DNAzyme for catalyzing the cleavage of substrate DNA from paper, which have been demonstrated by using microRNA recognition probe for miR-21, a phosphorylated hairpin probe for alkaline phosphatase, and a DNA aptamer for carcinoembryonic antigen assays, respectively. Taking advantages of the high specific target-triggered polymerization/nicking and DNAzyme-catalyzed signal amplification, the present assays enable highly sensitive and selective detection of these targets with zero-background. These assays can also be applied to detect target in spiked serum samples, demonstrating the potential for point-of-care detection of clinical samples.

An aptamer-based hook-effect-recognizable three-line lateral flow biosensor for rapid detection of thrombin

In this paper, a three-line LFB was successfully developed by adding a thrombin line to a conventional two-line LFB for the detection of thrombin in a wide range of human serum. We introduced a thrombin line between the test line and the control line. The concentration of thrombin in the sample was quantitatively related to the signal formation on the three lines of the LFB. We can make use of signal on three lines to quantitative determinate the thrombin by data processing. The detection range of thrombin concentrations measured in 10 min was 1 nM to 100 μM and the LOD was 0.85 nM. Our approach paves way for rapid and sensitive thrombin detection and a superior device for testing in a wide range of physiological concentrations, which also can be used in other hook-effect-limited aptamers or antibodies based sandwich LFBs, and has a high accuracy even within the range of the hook-effect.

Fluorometric determination of mercury(II) via a graphene oxide-based assay using exonuclease III-assisted signal amplification and thymidine-Hg(II)-thymidine interaction

A highly sensitive and selective fluorometric method is described for determination of mercury(II). It is based on (a) the use of graphene oxide (GO) acting as a quencher of the fluoresence of the carboxy-fluorescein (FAM), and (b) of Hg(II)-triggered cleavage of the newly formed nucleic acid sequences harbored blunt 3'-hydroxyl termini by exonuclease III (Exo III) that leads to signal amplification. Two DNA probes are used, viz. a capture probe (CP) and a help probe; HP) that is partially complementary. In the absence of Hg(II), the FAM-labeled hairpin (signal probe, SP) is adsorbed onto the surface of GO via π-stacking interactions. CP blocks the release of the HP for binding to SP. This results in quenching of the green fluorescence of the label. Upon addition of Hg(II), the linear structure of CP is converted to a hairpin structure due to the formation of thymidine-Hg(II)-thymidine duplexes. HP is released from the CP/HP hybrids, and this causes SP to be released from from GO and fluorescence to be recovered. The signal is strongly amplified by using Exo III-assisted targeting and recycling of HP. Hence, Hg(II) can be detected via the strong increase in fluorescence. The method has a linear response in the 0.1 to 30 nM Hg(II) concentration range and a 10 pM detection limit. It was applied to the determination of Hg(II) in three (spiked) Chinese medicines. Graphical abstract Schematic representation of fluorescence sensing strategy for Hg²⁺ by using graphene oxide as a quencher and exonuclease III-assisted signal amplification.

Principles and Applications of Nucleic Acid Strand Displacement Reactions

Dynamic DNA nanotechnology, a subfield of DNA nanotechnology, is concerned with the study and application of nucleic acid strand-displacement reactions. Strand-displacement reactions generally proceed by three-way or four-way branch migration and initially were investigated for their relevance to genetic recombination. Through the use of toeholds, which are single-stranded segments of DNA to which an invader strand can bind to initiate branch migration, the rate with which strand displacement reactions proceed can be varied by more than 6 orders of magnitude. In addition, the use of toeholds enables the construction of enzyme-free DNA reaction networks exhibiting complex dynamical behavior. A demonstration of this was provided in the year 2000, in which strand displacement reactions were employed to drive a DNA-based nanomachine (Yurke, B.; et al. Nature 2000, 406, 605-608). Since then, toehold-mediated strand displacement reactions have been used with ever increasing sophistication and the field of dynamic DNA nanotechnology has grown exponentially. Besides molecular machines, the field has produced enzyme-free catalytic systems, all DNA chemical oscillators and the most complex molecular computers yet devised. Enzyme-free catalytic systems can function as chemical amplifiers and as such have received considerable attention for sensing and detection applications in chemistry and medical diagnostics. Strand-displacement reactions have been combined with other enzymatically driven processes and have also been employed within living cells (Groves, B.; et al. Nat. Nanotechnol. 2015, 11, 287-294). Strand-displacement principles have also been applied in synthetic biology to enable artificial gene regulation and computation in bacteria. Given the enormous progress of dynamic DNA nanotechnology over the past years, the field now seems poised for practical application.

Highly Sensitive Aptasensor for Trace Arsenic(III) Detection Using DNAzyme as the Biocatalytic Amplifier

A highly sensitive fluorescence biosensing system was designed for the detection of trace amounts of arsenic(III) (As³⁺) based on target-triggered successive signal amplification strategy. The specific recognition between the target As³⁺ and the aptamer sequence results in the release of the blocking DNA to trigger the subsequent signal amplification steps. Exonuclease III (Exo III)-mediated DNA recycling digest process is introduced into the sensing system to generate numerous Mg²⁺-dependent DNAzymes. After magnetic separation, the active DNAzyme with multiple turnovers could catalyze the continuous cleavage of the fluorophore-quencher-functionalized substrate strands, thus yielding a significantly amplified fluorescence signal for target detection. Due to the synergetic signal amplification of Exo III and DNAzyme, the fluorescent biosensor exhibits ultrasensitivity for As³⁺ monitoring, with a detection limit of 2 pM. Our established biosensor also displays excellent selectivity toward the target As³⁺ and has been successfully applied to the determination of As³⁺ in water samples with satisfactory accuracy. This sensing platform can be developed as a universal approach for the fast, sensitive, and accurate detection of aptamer-binding molecules.

Colorimetric and visual mercury(II) assay based on target-induced cyclic enzymatic amplification, thymine-Hg(II)-thymine interaction, and aggregation of gold nanoparticles

A colorimetric biosensor and visual test is described for the determination of mercury(II). It relies on the specific thymine-Hg(II)-thymine (T-Hg²⁺-T) interaction which induces a cyclic amplification process (caused by the enzyme exonuclease III) and the aggregation of gold nanoparticles. These results in a color change from red to violet. Under optimized conditions, this colorimetric assay (best performed at 524 nm) has a detection limit as low as 0.9 nM with a detection range over 4 orders of magnitude (from 1 nM to 10 μM). Graphical abstract Schematic of a colorimetric method for determination of mercury ions (Hg²⁺) based on the thymine-Hg²⁺-thymine interaction-triggered cyclic enzymatic amplification and aggregation of gold nanoparticles with the aid of exonuclease III (Exo III).

A lateral flow assay for copper(II) utilizing catalytic and stem-loop based signal amplification

A DNAzyme-based catalytic and stem-loop based amplification scheme is used in a Cu(II)-specific lateral flow assay (LFA). Three test lines with given cut-off value on the test strip are set as the signal indicating zone for semiquantitative analysis by the number of red color bands that appear after lateral flow. The colored bands are generated by accumulation of gold nanoparticles. Four detection ranges can be visualied: (a) 0-2 ng·mL^-1 (= negative); 2-50 ng·mL^-1; 50-200 ng·mL^-1 and > 200 ng·mL^-1 of Cu(II) (= positive). The visual detection limit is thus considered as being 2 ng·mL^-1 which is much lower than the U.S. EPA limit in drinking water (1.25 μg·mL^-1). The highly specific DNAzyme, the strong multiple-turnover catalytic target recycling property and highly efficient amplification strategy warrant the high specificity, sensitivity and rapidity of this LFA. Conceivbly, this detecton scheme can be extended to other metal ions by proper choice of the ion-specific DNAzyme. Graphical abstract Schematic presentation of a semiquantitative lateral flow test strip for Cu²⁺ analysis by three visual cut-off value test lines using catalytic and stem-loop based signal amplification strategy.

Ultrasensitive and simple fluorescence biosensor for detection of the mecA gene of Staphylococcus aureus by using an exonuclease III-assisted cascade signal amplification strategy

In this work, a label-free fluorescence biosensor for ultrasensitive and simple detection of the mecA gene of Staphylococcus aureus was proposed by using an exonuclease III (Exo III)-assisted cascade signal amplification strategy. The 3' end-extruding hairpin probe (HP) acted as the target recognition element and the caged G-quadruplex was used as the signal reporter. Without the mecA gene, the HP probe cannot be digested by Exo III, as the G-rich sequences are blocked in the stem of the HP probe. In the presence of the mecA gene, the hybridization of the mecA gene with the 3' end-extruding HP probe triggers the digestion reaction of Exo III, liberating the mecA gene and the mecA gene analogue. Both the released mecA gene and the mecA gene analogue can hybridize with other HP probes and activate another round of the cleavage reaction. Consequently, the released free G-quadruplex is "lit up" by N-methylmesoporphyrin IX (NMM), displaying a dramatically enhanced fluorescence intensity. This sensing platform showed a high sensitivity towards the mecA gene with a detection limit as low as 2.4 fM without any labelling, immobilization, or washing steps. The designed sensing system also exhibits excellent selectivity for the mecA gene in the presence of other interfering DNA sequences. Furthermore, the presented biosensor is robust and has been successfully applied for the detection of the mecA gene in a real food sample with satisfactory results. Owing to its simplicity, cost-effectiveness and ultrasensitivity, our proposed sensing strategy provides a promising platform for the detection of other genes by substituting the target-recognition element.

Microchip electrophoresis based multiplexed assay for silver and mercury ions simultaneous detection in complex samples using a stirring bar modified with encoded hairpin probes for specific extraction

It is crucially important to rapidly, simultaneously, and sensitively determine trace amounts of heavy metal ions in complex samples. Herein, a stirring bar modified with two kinds of encoded hairpin DNA probes (H₀ and H₀') was used in a multiplexed strategy allowing for specific extraction of Hg²⁺ and Ag⁺ coupled to microchip electrophoresis (MCE) separation and LED induced fluorescence (LIF) detection. The extraction step utilizes stir bars, which are functionalized with designed hairpin DNA probes (H₀ with TT and H₀' with CC mismatches in stems). This allows the specific capture of Hg²⁺ and Ag⁺ through CAg⁺C and THg²⁺T interactions. These complexes are then enzymatically degraded by the action of exonuclease III (Exo III). The ions released during this enzymatic reaction can initiate a new cycle of interactions with hairpin structures and enzymatic reactions and so on. This cyclic step is specific to the presence of Hg²⁺ and Ag⁺ and represents the first round of amplification of the presence of the selected ions. The resulting single strand DNAs on the stirring bars after enzymatic degradation were used in the second step as primers to trigger the catalytic hairpin assembly (CHA) in the presence of a couple of hairpin structures in solution. Such a reaction allows producing duplexes that can be monitored by MCE-LIF. The fluorescence intensity of CHA products (I_P) increased and that of hairpin DNAs (I_R) decreased with the increase of target concentrations. The signal ratios (I_P/I_R and I_P'/I_R') consisted of targets. The assay was employed for Hg²⁺ and Ag⁺ detection in several mediums including water, milk, and fish samples with complex matrices. The results showed that the assay could avoid matrix interference to increase the sensitivity. Therefore, the multiplexed assay was ideal to simultaneously and quickly detect metal ions in complex samples.

Enhancing the sensitivity of colorimetric lateral flow assay (CLFA) through signal amplification techniques

Colorimetric lateral flow assay (CLFA) is one of a handful of diagnostic technologies that can be truly taken out of the laboratory for point-of-care testing without the need for any equipment and skilled personnel. Despite its simplicity and practicality, it remains a grand challenge to substantially enhance the detection sensitivity of CLFA without adding complexity. Such a limitation in sensitivity inhibits many critical applications such as early detection of significant cancers and severe infectious diseases. With the rapid development of materials science and nanotechnology, signal amplification techniques that hold great potential to break through the existing detection limit barrier of CLFA have been developed in recent years. This article specifically highlights these emerging techniques for CLFA development. The rationale behind and advantages and limitations of each technique are discussed. Perspectives on future research directions in this niche and important field are provided.

Aptamer-based sensor for quantitative detection of mercury (II) ions by attenuated total reflection surface enhanced infrared absorption spectroscopy

A sensing platform based on the attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) technique and immobilized aptamer has been proposed herein for the selective detection of mercury ions (Hg²⁺). In the proposed platform, 5' thiolated 32-mer DNA probes with methylene blue at the 3' end were immobilized on a thin gold (Au) surface layer. Following Hg²⁺ ions interacting with T bases of the aptamer, T-Hg-T bonds are formed; resulting in a hairpin-shaped formation of the DNA and a detectable change in the IR absorbance of the sensing interface. Notably, the background noise produced by external molecules (e.g., water, non-specific binding molecules and bulk solution) is reduced to a negligible level by means of the ATR detection mode. It is shown that the proposed sensor has a linear response (R² = 0.986) with high sensitivity and good selectivity over the Hg²⁺ range of 0.01 μM-50 μM.

Programmable DNA switches and their applications

DNA switches are ideally suited for numerous nanotechnological applications, and increasing efforts are being directed toward their engineering. In this review, we discuss how to engineer these switches starting from the selection of a specific DNA-based recognition element, to its adaptation and optimisation into a switch, with applications ranging from sensing to drug delivery, smart materials, molecular transporters, logic gates and others. We provide many examples showcasing their high programmability and recent advances towards their real life applications. We conclude with a short perspective on this exciting emerging field.