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The DNA-based biosensor utilises a thymine/guanine(T/G)-rich ODN-4 scaffold with 4',6-diamidino-2-phenylindole(DAPI) as a fluorescent emissary to monitor mercury/lead(Hg(II)/Pb(II)) ions simultaneously. Key to its bifocal detection capability is the twin unbound cytosine(C) bases strategically bridging the G-quadruplex and T-rich sequences, enabling their synergistic interplay. It facilitates the recognition of Hg(II)/Pb(II) ions, characterised by high specificity, and effectively mitigates interference from silver(Ag(I)). The G-quadruplex, guided by the C bases, induces a conformational transition in T-Hg(II)-T complexes, resulting in intense fluorescence. Pb(II) causes a spatial shift in the G-quadruplex, relaxing the T-Hg(II)-T base pairs and attenuating the fluorescence signal. The ODN-4 exhibits a robust, linear correlation with Hg(II) concentration (4.09 nmol/L to 1000 nmol/L) and Pb(II) concentration (3.22 nmol/L to 5 µmol/L). Recovery rates in milk, tap water, and rice water specimens with both ions validate method accuracy (Hg(II): 95.19% to 104.68%, Pb(II): 98.20% to 103.46%). It holds promising prospects for practical food analysis.

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A double signal amplification method was developed for sensitive detection of Hg2+ based on exonuclease III (Exo III) and Polymerase Chain Reaction (PCR). In the presence of Hg2+, the ineffective primers could bind with helper DNA to form dsDNA by T-Hg(II)-T mismatch for the first signal amplification. Then, the ineffective primers were digested by Exo III to effective primers which initiate PCR reaction for the second signal amplification. This conversion from ineffective to effective primers for triggering PCR reaction has not been reported for the detection of Hg2+. Through the double signal amplification strategy, the sensitivity of this proposed method was significantly improved with the limit of detection 1.46 nM. With the specific T-Hg(II)-T recognition, the selectivity of this new method was satisfactory. And the recoveries were between 92.3 % and 109.0 %. These results suggested that the proposed method was reliable to detect Hg2+ in water samples.

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A catalytic cleavage strategy was developed for the fluorometric determination of Hg(II). The method is based on the use of a Mg(II)-dependent split DNAzyme. Fluorophore labeled hairpins were conjugated to gold nanoparticles (AuNPs) upon which fluorescence is quenched. Thymine-Hg(II)-thymine (T-Hg(II)-T) interaction causes the two DNA sequences to form an entire enzyme-strand DNA (E-DNA). The E-DNA bind to the hairpins on the AuNPs to form a Mg(II)-dependent DNAzyme structure. The circular cleavage of hairpins results in a signal amplification and in the recovery of fluorescence. The assay has a limit of detection (LOD) as low as 80 pM of Hg(II). This LOD is comparable to those obtained with other amplification strategies. The method was successfully applied to the determination of Hg(II) in Chinese herbs (Atractylodes macrocephala Koidz). Graphical abstract Schematic of a catalytic cleavage strategy based on Mg(II)-dependent split DNAzyme for fluorometric determination of Hg(II).

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A method was developed for the determination of mercuric ion Hg(II). It is based on hybridization chain reaction (HCR) and surface-enhanced Raman scattering (SERS). Raman signal DNA and streptavidin were self-assembled on gold nanoparticles as a novel signal nanoprobe (AuNP-sDNA). A thymine-mercury(II)-thymine structure was immobilized on magnetic beads (MBs). The HCR makes use of two hairpin probes that are initiated by the trigger DNA to form a stable nicked dsDNA structure (MB-TS-hDNAs). A large number of the binding sites is provided to connect the signal nanoprobe. The stable sandwich structure (MB-TS-hDNA/AuNP-sDNA) was isolated by applying a magnetic field and used in the amplification step. In this way, Hg(II) can be determined sensitively after multiple signal amplification. The SERS signal, measured at 1499 cm-1, increases linearly in the 0.1 pM to 10 nM Hg(II) concentration range, and the limit of detection is 0.08 pM (at an S/N ratio of 3). The method was applied to the detection of Hg(II) in spiked environment water samples, with recoveries ranging from 96 to 119%. Graphical abstract Schematic of a method based on the use of a stable T-Hg(II)-T structure and a self-assembled nanoprobe. It was applied to the trace Hg(II) detection based on hybridization chain reaction (HCR) and surface-enhanced Raman scattering (SERS).

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In recent years, it has increased the number of works focused on the development of novel nanoparticle-based sensors for mercury detection, mainly motivated by the need of low cost portable devices capable of giving fast and reliable analytical response, thus contributing to the analytical decentralization. Methodologies employing colorimetric, fluorometric, magnetic, and electrochemical output signals allowed reaching detection limits within the pM and nM ranges. Most of these developments proved their suitability in detecting and quantifying mercury (II) ions in synthetic solutions or spiked water samples. However, the state of art in these technologies is still behind the standard methods of mercury quantification, such as cold vapor atomic absorption spectrometry and inductively coupled plasma techniques, in terms of reliability and sensitivity. This is mainly because the response of nanoparticle-based sensors is highly affected by the sample matrix. The developed analytical nanosystems may fail in real samples because of the negative incidence of the ionic strength and the presence of exchangeable ligands. The aim of this review is to critically consider the recently published innovations in this area, and highlight the needs to include more realistic assays in future research in order to make these advances suitable for on-site analysis.

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