An electrochemiluminescent assay for high sensitive detection of mercury (II) based on isothermal rolling circular amplification

Engineered Electrochemiluminescence Biosensors for Monitoring Heavy Metal Ions: Current Status and Prospects

Metal ion contamination has serious impacts on environmental and biological health, so it is crucial to effectively monitor the levels of these metal ions. With the continuous progression of optoelectronic nanotechnology and biometrics, the emerging electrochemiluminescence (ECL) biosensing technology has not only proven its simplicity, but also showcased its utility and remarkable sensitivity in engineered monitoring of residual heavy metal contaminants. This comprehensive review begins by introducing the composition, advantages, and detection principles of ECL biosensors, and delving into the engineered aspects. Furthermore, it explores two signal amplification methods: biometric element-based strategies (e.g., HCR, RCA, EDC, and CRISPR/Cas) and nanomaterial (NM)-based amplification, including quantum dots, metal nanoclusters, carbon-based nanomaterials, and porous nanomaterials. Ultimately, this review envisions future research trends and engineered technological enhancements of ECL biosensors to meet the surging demand for metal ion monitoring.

Recent Achievements in Electrochemical and Optical Nucleic Acids Based Detection of Metal Ions

Recently nucleic acids gained considerable attention as selective receptors of metal ions. This is because of the possibility of adjusting their sequences in new aptamers selection, as well as the convenience of elaborating new detection mechanisms. Such a flexibility allows for easy utilization of newly emerging nanomaterials for the development of detection devices. This, in turn, can significantly increase, e.g., analytical signal intensity, both optical and electrochemical, and the same can allow for obtaining exceptionally low detection limits and fast biosensor responses. All these properties, together with low power consumption, make nucleic acids biosensors perfect candidates as detection elements of fully automatic portable microfluidic devices. This review provides current progress in nucleic acids application in monitoring environmentally and clinically important metal ions in the electrochemical or optical manner. In addition, several examples of such biosensor applications in portable microfluidic devices are shown.

Recent advances in biological detection with rolling circle amplification: design strategy, biosensing mechanism, and practical applications

Rolling circle amplification (RCA) is a simple and isothermal DNA amplification technique that is used to generate thousands of repeating DNA sequences using circular templates under the catalysis of DNA polymerase.

Colorimetric aptasensor for the detection of mercury based on signal intensification by rolling circle amplification

Techniques that are sensitive to detect mercury ion (Hg²⁺) are very important, due to its serious threat to public health and food security. In this work, a colorimetric aptasensor was fabricated for the detection of Hg²⁺ based on rolling circle amplification (RCA). The aptamer was immobilized onto the microplate and hybridized with its complementary strand (cDNA₁) which linked with a primer for triggering the RCA reaction of circular template. The successfully RCA process led to the formation of long ssDNA chains on the microplate, which created many hybridized DNA fragments for bio-cDNA₂. The tagged amount of horseradish peroxidase (HRP) was enhanced through the avidin/biotin binding between avi-HRP and bio-cDNA₂. In the addition of TMB-H₂O₂, HRP was catalyzed and generated an optical signal. However, in the presence of target, Hg²⁺ specifically and preferentially bound with aptamer and formed a strong and stable T-Hg²⁺-T complex, which led to the release of cDNA₁ and HRP cluster. Consequently, the optical signal decreased. Our results showed that the limit of detection (LOD) of this system was 1.6 nM with excellent specificity, and that the detection signals were enhanced by up to 18 times under RCA conditions when compared with detections without RCA. This method has been successfully used to detect Hg²⁺ in water samples with a recovery of 98%-105.74%.

Recent Advances in Electrochemiluminescence Sensors for Pathogenic Bacteria Detection

Pathogenic bacterial contamination greatly threats human health and safety. Rapidly biosensing pathogens in the early stage of infection would be helpful to choose the correct drug treatment, prevent transmission of pathogens, as well as decrease mortality and economic losses. Traditional techniques, such as polymerase chain reaction and enzyme-linked immunosorbent assay, are accurate and effective, but are greatly limited because they are complex and time-consuming. Electrochemiluminescence (ECL) biosensors combine the advantages of both electrochemical and photoluminescence analysis and are suitable for high sensitivity and simple pathogenic bacteria detection. In this review, we summarize recent advances in ECL sensors for pathogenic bacteria detection and highlight the development of paper-based ECL platforms in point of care diagnosis of pathogens.

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.

Ultrasensitive Biosensor for Detection of Mercury(II) Ions Based on DNA-Cu Nanoclusters and Exonuclease III-assisted Signal Amplification

This paper describes a novel method for label-free mercury(II) ion detection based on exonuclease III-induced target signal recycling amplification using double-stranded DNA templated copper nanoclusters. The synthesized DNA-Cu nanoclusters were used with exonuclease III loop amplification technology for ultra-high sensitivity detection of mercury(II) ions, which were detected by significantly decreased fluorescence intensity. Under the optimal experimental conditions, there was a clear linear relationship between Hg²⁺ concentration in the range of 0.04 to 8 nM and fluorescence intensity. The detection limit for Hg²⁺ was 4 pM. In addition, the interference of other metal ions on the mercury(II) ion detection was also studied. To confirm the application of the fluorescent sensor, it was applied to determine the concentrations of mercury(II) ions in tap water, and the results showed that the method can be used to detect mercury(II) ions in water samples successfully.

Radial Flow Assay Using Gold Nanoparticles and Rolling Circle Amplification to Detect Mercuric Ions

A novel colorimetric assay employing oligonucleotide-conjugated gold nanoparticle (AuNP probes) and rolling circle amplification (RCA) was developed for simple detection of mercuric ions (Hg²⁺). The thymine-Hg²⁺-thymine (T-Hg²⁺-T) coordination chemistry makes our detection system selective for Hg²⁺. In the presence of Hg²⁺, the thymine 12-mer oligonucleotide is unable to act as a primer for RCA due to the formation of T-Hg²⁺-T before the RCA reaction. However, in the absence of Hg²⁺, DNA coils as RCA products are generated during the RCA reaction, and is further labeled with AuNP probes. Colorimetric signals that depend on the amount of DNA coil-AuNP probe complexes were generated by drop-drying the reaction solution on nitrocellulose-based paper. As the reaction solution spread radially because of capillary action, the complexes formed a concentric red spot on the paper. The colorimetric signals of the red spots were rapidly measured with a portable spectrophotometer and determined as the ΔE value, which indicates the calculated color intensity. Our assay displays great linearity (detection limit: 22.4 nM), precision, and reproducibility, thus demonstrating its utility for Hg²⁺ quantification in real samples. We suggest that our simple, portable, and cost-effective method could be used for on-site Hg²⁺ detections.

Rapid and label-free strategy for the sensitive detection of Hg2+ based on target-triggered exponential strand displacement amplification

A rapid and label-free strategy for sensitive detection of Hg²⁺ is developed based on target-triggered exponential strand displacement amplification (eSDA).

A label-free electrochemical sensor for detection of mercury(II) ions based on the direct growth of guanine nanowire

A simple, sensitive and label-free electrochemical sensor is developed for detection of Hg(2+) based on the strong and stable T-Hg(2+)-T mismatches. In the presence of Mg(2+), the parallel G-quadruplex structures could be specifically recognized and precipitated in parallel conformation. Therefore, the guanine nanowire was generated on the electrode surface, triggering the electrochemical H2O2-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). In this research, a new method of signal amplification for the quantitative detection of Hg(2+) was described based on the direct growth of guanine nanowire via guanine nanowire. Under optimum conditions, Hg(2+) was detected in the range of 100 pM-100 nM, and the detection limit is 33 pM. Compared to the traditional single G-quadruplex label unit, this electrochemical sensor showed high sensitivity and selectivity for detecting Hg(2+).

Fidelity quantification of mercury(ii) ion via circumventing biothiols-induced sequestration in enzymatic amplification system

A fidelity quantification of mercury(ii) ion based on nucleic acids amplification is developedviacircumventing biothiols-induced sequestration.

Isothermal Amplification of Nucleic Acids

Isothermal amplification of nucleic acids is a simple process that rapidly and efficiently accumulates nucleic acid sequences at constant temperature. Since the early 1990s, various isothermal amplification techniques have been developed as alternatives to polymerase chain reaction (PCR). These isothermal amplification methods have been used for biosensing targets such as DNA, RNA, cells, proteins, small molecules, and ions. The applications of these techniques for in situ or intracellular bioimaging and sequencing have been amply demonstrated. Amplicons produced by isothermal amplification methods have also been utilized to construct versatile nucleic acid nanomaterials for promising applications in biomedicine, bioimaging, and biosensing. The integration of isothermal amplification into microsystems or portable devices improves nucleic acid-based on-site assays and confers high sensitivity. Single-cell and single-molecule analyses have also been implemented based on integrated microfluidic systems. In this review, we provide a comprehensive overview of the isothermal amplification of nucleic acids encompassing work published in the past two decades. First, different isothermal amplification techniques are classified into three types based on reaction kinetics. Then, we summarize the applications of isothermal amplification in bioanalysis, diagnostics, nanotechnology, materials science, and device integration. Finally, several challenges and perspectives in the field are discussed.

Functional nucleic acid-based sensors for heavy metal ion assays

Heavy metal contaminants such as lead ions (Pb(2+)), mercury ions (Hg(2+)) and silver ions (Ag(+)) can cause significant harm to humans and generate enduring bioaccumulation in ecological systems. Even though a variety of methods have been developed for Pb(2+), Hg(2+) and Ag(+) assays, most of them are usually laborious and time-consuming with poor sensitivity. Due to their unique advantages of excellent catalytic properties and high affinity for heavy metal ions, functional nucleic acids such as DNAzymes and aptamers show great promise in the development of novel sensors for heavy metal ion assays. In this review, we summarize the development of functional nucleic acid-based sensors for the detection of Pb(2+), Hg(2+) and Ag(+), and especially focus on two categories including the direct assay and the amplification-based assay. We highlight the emerging trends in the development of sensitive and selective sensors for heavy metal ion assays as well.

Colorimetric monitoring of rolling circle amplification for detection of H5N1 influenza virus using metal indicator

A new colorimetric method for monitoring of rolling circle amplification was developed. At first H5N1 target hybrids with padlock probe (PLP) and then PLP is circularized upon the action of T4 ligase enzyme. Subsequently, the circular probe is served as a template for hyperbranched rolling circle amplification (HRCA) by utilizing Bst DNA polymerase enzyme. By improving the reaction, pyrophosphate is produced via DNA polymerization and chelates the Mg(2+) in the buffer solution. This causes change in solution color in the presence of hydroxy naphthol blue (HNB) as a metal indicator. By using pH shock instead of heat shock and isothermal RCA reaction not only the procedure becomes easier, but also application of HNB for colorimetric detection of RCA reaction further simplifies the assay. The responses of the biosensor toward H5N1 were linear in the concentration range from 0.16 to 1.20 pM with a detection limit of 28 fM.

Target-responsive dumbbell probe-mediated rolling circle amplification strategy for highly sensitive Hg2+ detection

A novel label-free amplified fluorescent sensing scheme based on target-responsive dumbbell probe-mediated rolling circle amplification (D-RCA) has been developed for sensitive and selective detection of mercuric ions.

Electrochemical uranyl cation biosensor with DNA oligonucleotides as receptor layer

The present study aims at the further development of the uranyl oligonucleotide-based voltammetric biosensor, which takes advantage of strong interaction between UO2(2+) and phosphate DNA backbone. Herein we report the optimization of working parameters of previously elaborated electrochemical DNA biosensor. It is shown that the sensor sensitivity is highly dependent on the oligonucleotide probe length and the incubation time of sensor in a sample solution. Consequently, the highest sensitivity was obtained for 10-nucleotide sequence and 60 min incubation time. The lower detection limit towards uranyl cation for developed biosensor was 30 nM. The influence of mixed monolayers and the possibility of developing a non-calibration device were also investigated. The selectivity of the proposed biosensor was significantly improved via elimination of adenine nucleobases from the DNA probe. Moreover, the regeneration procedure was elaborated and tested to prolong the use of the same biosensor for 4 subsequent determinations of UO2(2+).

Sensitive electrochemical monitoring of nucleic acids coupling DNA nanostructures with hybridization chain reaction

Methods based on metal nanotags have been developed for metallobioassay of nucleic acids, but most involve complicated labeling or stripping procedures and are unsuitable for routine use. Herein, we report the proof-of-concept of a novel and label-free metallobioassay for ultrasensitive electronic determination of human immunodeficiency virus (HIV)-related gene fragments at an ultralow concentration based on target-triggered long-range self-assembled DNA nanostructures and DNA-based hybridization chain reaction (HCR). The signal is amplified by silver nanotags on the DNA duplex. The assay mainly consists of capture probe, detection probe, and two different DNA hairpins. In the presence of target DNA, the capture probe immobilized on the sensor sandwiches target DNA with the 3' end of detection probe. Another exposed part of detection probe at the 5' end opens two alternating DNA hairpins in turn, and propagates a chain reaction of hybridization events to form a nicked double-helix. Finally, numerous silver nanotags are immobilized onto the long-range DNA nanostructures, each of which produces a strong electronic signal within the applied potentials. Under optimal conditions, the target-triggered long-range DNA nanostructures present good electrochemical behaviors for the detection of HIV DNA at a concentration as low as 0.5 fM. Importantly, the outstanding sensitivity can make this approach a promising scheme for development of next-generation DNA sensors without the need of enzyme labeling or fluorophore labeling.

Amplified fluorescence detection of mercury(II) ions (Hg2+) using target-induced DNAzyme cascade with catalytic and molecular beacons

In this work, a fluorescent sensing strategy was developed for the detection of mercury(II) ions (Hg(2+)) in aqueous solution with excellent sensitivity and selectivity using a target-induced DNAzyme cascade with catalytic and molecular beacons (CAMB). In order to construct the biosensor, a Mg(2+)-dependent DNAzyme was elaborately designed and artificially split into two separate oligonucleotide fragments. In the presence of Hg(2+), the specific thymine-Hg(2+)-thymine (T-Hg(2+)-T) interaction induced the two fragments to produce the activated Mg(2+)-dependent DNAzyme, which would hybridize with a hairpin-structured MB substrate to form the CAMB system. Eventually, each target-induced activated DNAzyme could catalyze the cleavage of many MB substrates through true enzymatic multiple turnovers. This would significantly enhance the sensitivity of the Hg(2+) sensing system and push the detection limit down to 0.2 nM within a 20 min assay time, much lower than those of most previously reported fluorescence assays. Owning to the strong coordination of Hg(2+) to the T-T mismatched pairs, this proposed sensing system exhibited excellent selectivity for Hg(2+) detection, even in the presence of 100 times of other interferential metal ions. Furthermore, the applicability of the biosensor for Hg(2+) detection in river water samples was demonstrated with satisfactory results. These advantages endow the sensing strategy with a great potential for the simple, rapid, sensitive, and specific detection of Hg(2+) from a wide range of real samples.