RNA-mediated fluorescent colloidal CdSe nanostructures in aqueous medium – analysis of Cd2+ induced folding of RNA associated with morphological transformation (0D to 1D), change in photophysics and selective Hg2+ sensing
The present manuscript reports the synthesis of fluorescent RNA-mediated CdSe QDs with the wurtzite phase having an average size of 4 nm, as estimated from TEM analysis. XRD, XPS, SAED, HRTEM and Raman analyses clearly evidenced the formation of CdSe. These particles fluoresce in the visible range with a fluorescence quantum efficiency of about 22% and an average fluorescence lifetime (〈τ〉) of 106 ns. The presence of excess Cd2+ induced the folding of RNA upon aging to produce encapsulated CdSe nanoneedles having dimensions (average length × width at tip × width at the centre) of 42.2 × 4.2 × 6.8 nm, which upon further aging results in their stacking. The transformation from QDs to nanoneedles is also evidenced by about four-fold increase in rotational correlation time from 17.7 ns for the fresh sample to 68.0 ns for the aged sample and is associated with an increase in 〈τ〉 to 116 ns. I–V measurements demonstrate the rectifying behavior with a significant increase (≥6 times) in current upon illumination compared to that observed in the dark. The folding of RNA was clearly evidenced by CD, IR, 1H and 31P NMR spectroscopy. It possibly results in the transformation of RNA from its B- to A-form involving conversion of the ribose conformation from C2′-endo to the thermodynamically stable C3′-endo. The as-synthesized QDs were found to be selective for the detection of highly toxic Hg2+ in the presence of other heavy metals (Pb2+, Ni2+, Co2+, Cu2+ and Mn2+) at 1 nM.