A Self-Assembled G-Quadruplex/Hemin DNAzyme-Driven DNA Walker Strategy for Sensitive and Rapid Detection of Lead Ions Based on Rolling Circle Amplification.

Herein, a sensitive biosensor is constructed based on a novel rolling circle amplification (RCA) for colorimetric quantification of lead ion (Pb2+). At the detection system, GR5 DNAzymes are modified on the surface of an immunomagnetic bead, and Pb2+ is captured by the aptamer, inducing the disintegration of the GR5 DNAzyme and the release of the DNA walker. After the introduction of the template DNA, T4 DNA ligase, and phi29 DNA polymerase, an RCA is initiated for the sensitivity improvement of this method. Moreover, a G4-hemin DNAzyme is formed as a colorimetric signal, owing to its peroxide-like activity to catalyze the TMB-H2O2 substrate. Under the optimized conditions, the limit of detection (LOD) of this fabricated biosensor could reach 3.3 pM for Pb2+ with a concentration in the range of 0.01-1000 nM. Furthermore, the results of real samples analysis demonstrate its satisfactory accuracy, implying its great potential in the rapid detection of heavy metals in the environment.

Metal-Polydopamine Framework as an Effective Fluorescent Quencher for Highly Sensitive Detection of Hg(II) and Ag(I) Ions through Exonuclease III Activity.

In this paper, we propose a metal-polydopamine (MPDA) framework with a specific molecular probe which appears to be the most promising approach to a strong fluorescence quencher. The MPDA framework quenching ability toward various organic fluorophore such as aminoethylcoumarin acetate, 6-carboxyfluorescein (FAM), carboxyteramethylrhodamine, and Cy5 are used to establish a fluorescent biosensor that can selectively recognize Hg2+ and Ag+ ions. The fluorescent quenching efficiency was sufficient to achieve more than 96%. The MPDA framework also exhibits different affinities with ssDNA and dsDNA. In addition, the FAM-labeled ssDNA was adsorbed onto the MPDA framework, based on their interaction with the complex formed between MPDA frameworks/ssDNA taken as a sensing platform. By taking advantage of this sensor, highly sensitive and selective determination of Hg2+ and Ag+ ions is achieved through exonuclease III signal amplification activity. The detection limits of Hg2+ and Ag+ achieved to be 1.3 and 34 pM, respectively, were compared to co-existing metal ions and graphene oxide-based sensors. Furthermore, the potential applications of this study establish the highly sensitive fluorescence detection targets in environmental and biological fields.

Fluorometric determination of lead(II) and mercury(II) based on their interaction with a complex formed between graphene oxide and a DNAzyme.

The authors have designed a DNAzyme where graphene oxide (GO) interacts with the ssDNA stem loop region. The DNAzyme strand and substrate strand are hybridized and bind to the surface of GO which act as a signal reporter, while GO act as a strong quencher. The presence of Pb(II) ion disturbs the GO-DNAzyme complex and causes internal cleavage of the DNAzyme complex. On addition of Thioflavin T (ThT) as a quadruplex inducer, fluorescence intensity (best measured at excitation/emission peaks of 425/490 nm) is strongly enhanced. Subsequent addition of Hg(II) to ThT/G-quadruplex complex decreases fluorescence because the G-quadruplex is unwinding to form a T-Hg(II)-T dsDNA system. Therefore, the change in fluorescence intensity of ThT is directly correlated to the concentration of Pb(II) and Hg(II). As a result, the assay is highly selective and sensitive. The limits of detection are 96 pM for Pb(II) and 356 pM for Hg(II). Moreover, the method was applied to the detection of the two ions in spiked real samples and gave satisfactory results. Graphical abstract A label free sensitive and selective "on-off" fluorescent assay for detection of Pb(II) and Hg(II) based on graphene oxide -DNAzyme complex with fluorogenic dye thioflavin T. The limits of detection are 96 pM (Pb2+) and 356 pM (Hg2+).