Double-stranded DNAs (ds-DNAs) have been identified as efficient templates favoring the formation of fluorescent copper nanoparticles (Cu NPs). Herein, we have tried to synthesize fluorescent Cu NPs using single-stranded DNAs (ss-DNAs) as templates and to identify the critical DNA sequences. By comparing the results using homopolymer DNAs, hairpin DNAs, and pristine ss-DNAs as templates, we found that DNA thymine base plays a dominant role in producing red-emissive fluorescent Cu NPs on ss-DNA templates. The thymine-dependent growth of the fluorescent Cu NPs is confirmed by Hg2+ mediated T–T base pair in comparison with the other non-specific metal ions, which could be developed into a practical sensor for turn-on fluorescence detection of Hg2+ with a high selectivity. The mechanism is briefly discussed according the DNA sequence-dependent formation of fluorescent Cu NPs. This work demonstrates the sequence role in producing fluorescent Cu NPs that could serve as promising fluorescent nanoprobes in biosensing and DNA-hosted Cu nanomaterials.

A novel, facile sensor was further developed for microcystin-LR (MC-LR) determination by visible spectroscopy. Antibody-functionalized SiO₂-coated magnetic nanoparticles (Fe₃O₄@SiO₂) and aptamer-functionalized polydopamine nanospheres decorated with Cu nanoparticles (PDA/CuNPs) recognized specific sites in MC-LR and then the sandwich-type composites were separated magnetically. The Cu in the separated composites was converted to Cu²⁺ ions in solution and turn-on visible absorption was achieved after reaction with bis(cyclohexanone)oxaldihydrazone (BCO) (λ_max = 600 nm). There was a quantitative relationship between the spectral intensity and MC-LR concentration. In addition, under the optimum conditions, the sensor turns out to be a linear relationship from 0.05 to 25 nM, with a limit of detection of 0.05 nM (0.05 μg/L) (S/N = 3) for MC-LR. The sensitivity was dependent on the low background absorption from the off-to-on spectrum and label amplification by the polydopamine (PDA) surface. The sensor had high selectivity, which shows the importance of dual-site recognition by the aptamer and antibody and the highly specific color formed by BCO with Cu²⁺. The bioassay was complete within 150 min, which enabled quick determination. The sensor was successfully used with real spiked samples. These results suggest it has potential applications in visible detection and could be used to detect other microcystin analogs.