In this study, we designed a fluorescence resonance energy transfer system containing gold nanorods (AuNRs) and fluorescein (FAM) for the detection of hepatitis B virus DNA sequences. AuNRs were synthesized according to the seed-mediated surfactant-directed approach, and the surface of the AuNRs was wrapped with a thin layer of cetyltrimethylammonium bromide (CTAB), resulting in the AuNRs being positively charged. When FAM-tagged single-stranded DNA (FAM-ssDNA) was added into the AuNRs suspension, it was adsorbed onto the surface of the positively charged AuNRs and formed a FAM-ssDNA-CTAB-AuNRs ternary complex, the resulting structure led to a fluorescence resonance energy transfer (FRET) process from FAM to AuNRs and the fluorescence intensity of FAM was consequently quenched. When complementary target DNA was added to the FAM-ssDNA-CTAB-AuNRs complex solution, a further decrease in fluorescence intensity was observed because of an increased FRET efficiency. Under optimal conditions, the decline of the fluorescence intensity of FAM (ΔF) was linear with the concentration of the complementary DNA from 0.045 to 6.0 nmol L(-1) and the detection limit was as low as 15 pmol L(-1) (signal/noise ratio of 3). When this fluorescent DNA sensor was used to detect the polymerase chain reaction product of hepatitis B virus gene extracted from a positive real sample, a positive response was obtained. Impressively, the biosensor exhibits good selectivity, even for single-mismatched DNA detection.
In this work, we describe the fabrication of an ultrasensitive electrochemical biosensor for the determination of the mercury(II) ion (Hg 2+ ) in aqueous solution. The biosensor is based on a glassy carbon electrode (GCE) modified with multi-walled carbon nanotubes (MWCNTs) and gold nanoparticles (Au-NPs). Probe DNA (HS-DNA) was then covalently linked to the surface of the Au-NPs/MWCNTs via Au-S bonds. In the presence of Hg 2+ , the probe DNA can hybridize with the target DNA to form duplex DNA via the strong and specific binding of Hg 2+ by two DNA thymine bases (T-Hg 2+ -T). Differential pulse voltammetry was employed to investigate the hybridization process and measure the changes in peak current intensity of intercalated adriamycin in the presence of different concentrations of Hg 2+ . The increase in peak current intensity of adriamycin was linear with an increase in the concentration of Hg 2+ in the range from 0.1 to 20 nmol L À1 with a detection limit of 0.03 nmol L À1 (S/N ¼ 3) under the optimal conditions. In addition, the biosensor exhibits good selectivity, stability and reproducibility.
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