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

Cascade Fluorescent Sensors Based on Isothermal Signal Amplification for the Detection of Mercury and Silver Ions

In this study, novel fluorescent DNA biosensors for mercury (Hg2+) and silver (Ag+) ions were developed based on thymine (T)- and cytosine (C)-rich recognition elements in combination with exonuclease III and a mismatch-catalyzed hairpin assembly (MCHA)-based cascade isothermal signal-amplification strategy. In the presence of the respective target analytes, the recognition element terminals form so-called T-Hg2+-T or C-Ag+-C structures, resulting in cleavage by Exo III and the release of the trigger strand for MCHA. This binds to the H1 hairpin, which is fluorescently labeled with carboxyfluorescein (FAM) and tetramethylrhodamine (TAMRA), disrupting fluorescence resonance energy transfer between them and, thus, restoring FAM fluorescence, generating a strong signal at 520 nm. The linear range of the Hg2+ sensor is 0.5 to 3 pM, with a detection limit of 0.07 pM. The recovery range in actual spiked water samples is between 98.5% and 105.2%, with a relative standard deviation (RSD) ranging from 2.0% to 4.2%. The linear range of the Ag+ sensor is 10 to 90 pM, with a detection limit of 7.6 pM. The recovery range in actual spiked water samples is between 96.2% and 104.1%, with an RSD ranging from 3.2% to 6.3%. The cascade isothermal signal amplification strategy effectively enhances sensor sensitivity, while MCHA decreases the false-positive rate. The aptamer sensor exhibits high specificity, is resistant to interference, and can be used for the detection of Hg2+ and Ag+ in environmental water samples.

Rapid and sensitive determination of Piroxicam by N-doped carbon dots prepared by plant soot

Piroxicam (PX) as a nonsteroidal anti-inflammatory drug (NSAID) can be effectively used for anti-inflammatory and analgesia. However, overdoses may induce side effects such as gastrointestinal ulcers and headaches. Therefore, the assay of piroxicam has considerable significance. In this work, nitrogen-doped carbon dots (N-CDs) was synthesized for PX detection. The fluorescence sensor was fabricated by hydrothermal method with plant soot and ethylenediamine. The strategy exhibited a detection range of 6-200 μg/mL and 250-700 μg/mL with the limited detection of 2 μg/mL. The mechanism of the PX assay base on the fluorescence sensor was the process of electron transfer between the PX and N-CDs. The assay subsequently demonstrated could be successfully used in actual sample. The results indicated that the N-CDs could be a superior candidate nanomaterial for piroxicam monitoring in the healthcare product industry.

A graphene-oxide-based aptasensor for fluorometric determination of chloramphenicol in milk and honey samples utilizing exonuclease III-assisted target recycling and Nb.BbvCI-powered DNA walker cascade amplification

Herein, a graphene oxide (GO)-based fluorescence aptasensor was developed for the sensitive and selective detection of chloramphenicol (CAP), based on exonuclease III (Exo III)-assisted target recycling and Nb.BbvCI-driven DNA walker cascade amplification. Interactions between CAP, hairpin1(HP1), hairpin2 (HP2), and 3'-amino modified hairpin3 (HP3) labeled with carboxyfluorescein (FAM) and covalently coupled to GO enabled efficient CAP detection. CAP was quantitatively assayed by measuring fluorescence at excitation/emission wavelengths of 480/514 nm, resulting from the accumulation of released FAM. A good linear range of 1 fM to 1 nM and a limit of detection (LOD) of 0.875 fM (signal-to-noise (S/N)= 3) were achieved. This aptasensor can distinguish the CAP from interference antibiotics with good specificity and selectivity, even if the concentration of the interfering substance is ten-fold higher than the target concentration. Moreover, the developed fluorescence aptasensor was successfully applied for the detection of CAP in spiked milk and honey samples. Thus, this method is potentially applicable for assaying CAP in foods and provides a promising strategy for the development of fluorescence aptasensors for environmental sample analysis.

A Facile Aptasensor for Instantaneous Determination of Cadmium Ions Based on Fluorescence Amplification Effect of MOPS on FAM-Labeled Aptamer

Analytical performance and efficiency are two pivotal issues for developing an on-site and real-time aptasensor for cadmium (Cd²⁺) determination. However, suffering from redundant preparations, fabrications, and incubation, most of them fail to well satisfy the requirements. In this work, we found that fluorescence intensity of 6-carboxyfluorescein(FAM)-labeled aptamer (FAM-aptamer) could be remarkably amplified by 3-(N-morpholino)propane sulfonic acid (MOPS), then fell proportionally as Cd²⁺ concentration introduced. Importantly, the fluorescence variation occurred immediately after addition of Cd²⁺, and would keep stable for at least 60 min. Based on the discovery, a facile and ultra-efficient aptasensor for Cd²⁺ determination was successfully developed. The sensing mechanism was confirmed by fluorescence pattern, circular dichroism (CD) and intermolecular interaction related to pK_a. Under the optimal conditions, Cd²⁺ could be determined rapidly from 5 to 4000 ng mL^-1. The detection limit (1.92 ng mL^-1) was also lower than the concentration limit for drinking water set by WHO and EPA (3 and 5 ng mL^-1, respectively). More than a widely used buffer, MOPS was firstly revealed to have fluorescence amplification effect on FAM-aptamer upon a given context. Despite being sensitive to pH, this simple, high-performance and ultra-efficient aptasensor would be practical for on-site and real-time monitoring of Cd²⁺.

Construction of DNA Biosensors for Mercury (II) Ion Detection Based on Enzyme-Driven Signal Amplification Strategy

Mercury ion (Hg²⁺) is a well-known toxic heavy metal ion. It is harmful for human health even at low concentrations in the environment. Therefore, it is very important to measure the level of Hg²⁺. Many methods, reviewed in several papers, have been established on DNA biosensors for detecting Hg²⁺. However, few reviews on the strategy of enzyme-driven signal amplification have been reported. In this paper, we reviewed this topic by dividing the enzymes into nucleases and DNAzymes according to their chemical nature. Initially, we introduce the nucleases including Exo III, Exo I, Nickase, DSN, and DNase I. In this section, the Exo III-driven signal amplification strategy was described in detail. Because Hg²⁺ can help ssDNA fold into dsDNA by T-Hg-T, and the substrate of Exo III is dsDNA, Exo III can be used to design Hg²⁺ biosensor very flexibly. Then, the DNAzyme-assisted signal amplification strategies were reviewed in three categories, including UO₂²⁺-specific DNAzymes, Cu²⁺-specific DNAzymes and Mg²⁺-specific DNAzymes. In this section, the Mg²⁺-specific DNAzyme was introduced in detail, because this DNAzyme has highly catalytic activity, and Mg²⁺ is very common ion which is not harmful to the environment. Finally, the challenges and future perspectives were discussed.

Microwave-assisted synthesis of fluorescent silicon quantum dots for ratiometric sensing of Hg (II) based on the regulation of energy transfer

The rapid and sensitive detection of Hg²⁺ is highly required to protect the environmental safety and human healthy. In the present work, a ratiometric fluorescent sensing platform, consisting of silicon quantum dots (SiQDs), Rox-labelled DNA (Rox-DNA), and Exonuclease III (Exo III), is developed for the accurate detection of Hg²⁺. As for fluorescent probe, we report the first use of glutathione as reduction reagent for the microwave synthesis of SiQDs, achieving the facile (using a house-hold microwave oven) and rapid (within 8 min) synthesis. Such SiQDs show pH-independent spectra and reversible fluorescent behavior with temperature. Moreover, experimental results revealed that the electrostatic interaction-induced aggregation of Rox-DNA and SiQDs facilitated the occurring of energy transfer (ET). And detection principle based on the regulation of ET between Rox and SiQDs with Exo III was designed for analysis. ET effect resulted in the fluorescent fading of Rox while that of SiQDs kept stable. For analysis, the addition of Hg²⁺ led to the formation of double-stranded Rox-DNA via T-Hg²⁺-T. Exo III would cut these double-stranded DNA to release Rox and Hg²⁺, thereby impeding the ET effect and recovering the fluorescent of Rox. Such SiQDs/Rox-DNA/Exo III ratiometric fluorescent sensing platform exhibited a linear response concentration range of 0.02 nM-10 nM with a detection limit of 0.01 nM. It was successfully used to analyze the water and soil samples. The reliability was validated by ICP-MS. Our work should promote the practical application of ratiometric fluorescent assay.

Aptamers used for biosensors and targeted therapy

Aptamers are single-stranded nucleic acid sequences that can bind to target molecules with high selectivity and affinity. Most aptamers are screened in vitro by a combinatorial biology technique called systematic evolution of ligands by exponential enrichment (SELEX). Since aptamers were discovered in the 1990s, they have attracted considerable attention and have been widely used in many fields owing to their unique advantages. In this review, we present an overview of the advancements made in aptamers used for biosensors and targeted therapy. For the former, we will discuss multiple aptamer-based biosensors with different principles detected by various signaling methods. For the latter, we will focus on aptamer-based targeted therapy using aptamers as both biotechnological tools for targeted drug delivery and as targeted therapeutic agents. Finally, challenges and new perspectives associated with these two regions were further discussed. We hope that this review will help researchers interested in aptamer-related biosensing and targeted therapy research.

Optical aptasensing of mercury(II) by using salt-induced and exonuclease I-induced gold nanoparticle aggregation under dark-field microscope observation

An optical method for determination of Hg(II) is described that exploits the aggregation of gold nanoparticles (AuNPs) under dark-field microscope (DFM) observation. This assay is based on the use of a Hg(II)-specific aptamer, AuNPs modified with complementary DNA strands, and exonuclease I (Exo I). In the absence of Hg(II), the added dsDNA prevents salt-induced aggregation of the green-colored AuNPs. If Hg(II) is added, the aptamer will capture it to form T-Hg(II)-T pairs, and the complementary strand is digested by Exo I. On addition of a solution of NaCl, the AuNPs will aggregate. This is accompanied by a color change from green to orange/red) in the dark-field image. By calculating the intensity of the orange/red dots in the dark-field image, concentration of Hg(II) can be accurately determined. The limit of detection is as low as 36 fM, and response is a linear in the 83 fM to 8.3 μM Hg(II) concentration range. Graphical abstract Schematic representation of a colorimetric assay for Hg(II) based on the use of a mercury(II)-specific aptamer, gold nanoparticles modified with complementary DNA strands, and exonuclease I.

L-Cysteine modified gold nanoparticles for tube-based fluorometric determination of mercury(II) ions

A fluorescent probe is described for detection of mercury(II) ion by using L-cysteine-modified gold nanoparticles (Cys-AuNP). These were fabricated by a tube-based redox reaction where Cys acts as both the reducing reagent and capping ligand. The Cys-AuNP display red fluorescence, with excitation/emission peaks at 373/625 nm. Owing to the high-affinity of the Hg(II)-Au(I) interaction and the Hg(II)/carboxy or amino group interaction, the presence of Hg(II) cause selective quenching the fluorescence, while other metal ions do not give such an effect. Based on these findings, a method was designed for the determination of Hg(II) that has attractive figures of merit. These include a low limit of detection (1.3 nM), a wide detection range (from 2 nM to 30µM), and excellent specificity. The method was applied to Hg(II) screening in (spiked) tap and river water, and it gave satisfactory results. Graphical abstract Schematic representation of the application of L-cysteine modified gold nanoparticles (Cys-AuNP) for qualitative and quantitative detection of mercury(II) ions. Based on the interaction between Cys-AuNP and mercury(II) ion to quench the red fluorescence of Cys-AuNP, the target mercury(II) can in turn be determined by a fluorometric method.

Fluorometric determination of mercury(II) by using thymine-thymine mismatches as recognition elements, toehold binding, and enzyme-assisted signal amplification

A highly sensitive fluorometric method is described for the determination of mercury(II) ions. It is based on (a) the use of a DNA probe containing thymine-thymine mismatches that are employed as Hg(II) recognition elements, (b) subsequent toehold binding, and (c) endocuclease-assisted signal amplification. Target recycling is triggered by exonuclease III. This produces a large amount of ssDNA (defined as primer). Then, the generated primer-initiated strand displacement reaction with the help of polymerase and nicking endonuclease releases the free fluorophore-labelled probe. Under excitation at 532 nm, the fluorescent probe displays emission with a peak at 582 nm. The sensitivity of this method is improved by introduction of nicking endonuclease. The working range of the assay extends from 20 pM to 10 nM, and the detection limit is as low as 6 pM of Hg(II). Graphical abstract Schematic presentation of the fluorometric method for determination of mercury(II). By using a special structure of thymine-thymine mismatches, target-induced toehold binding and enzyme-assisted signal amplification strategy were employed. This method is selective and good performance in real sample application.