A magnetic nanoscale metal–organic framework (MNMOF) as a viable fluorescence quencher material for ssDNA and for the detection of mercury ions via a novel quenching–quenching mechanism

A novel fluorescent biosensor has been designed and synthesized comprising a magnetic nanoscale metal–organic framework (MNMOF) functionalized with fluorescein amidite (FAM)-labeled ssDNA. It exhibits good sensitivity and selectivity for Hg(ii) cations over other co-existing metal ions. MNMOF was fabricated by a one-pot synthetic method and it was successfully characterized with various techniques such as UV-visible spectroscopy, fluorescence spectroscopy, Fourier-transform infrared (FT-IR) spectrometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The FAM-labeled ssDNA was adsorbed onto the surface of MNMOF through π–π stacking and electrostatic interactions, which resulted in the partial quenching of its fluorescence intensity (65%). Upon the subsequent addition of Hg(ii) ions, the fluorescence intensity was further quenched at 52%, due to the re-adsorption of dsDNA onto the surface of MNMOF. Thus, the FAM-labeled ssDNA showed a drastic decrease in fluorescence intensity with Hg(ii). This quenching–quenching mechanism led to a linear response in the fluorescence intensity to Hg(ii) concentration (R(2) = 0.934) with a low detection limit of 8 nM. The specific merits of MNMOF make it an ideal platform for mercury sensor applications.