Time-resolved spectral observations of cadmium-enriched cadmium sulfide nanoparticles and the effects of DNA oligomer binding

We measured the steady-state and time-resolved fluorescence spectral proper ties of cadmium-enriched nanoparticles (CdS-Cd2+). These particles displaye d two emission maxima, at 460 and 580 nm. The emission spectra were indepen dent of excitation wavelength. Surprisingly, the intensity decays were stro ngly dependent on the observation wavelength, with longer decay times being observed at longer wavelengths. The mean lifetime increased from 150 to 37 0 ns as the emission wavelength was increased from 460 to 650 nm. The wavel ength-dependent lifetimes were used to construct the time-resolved emission spectra, which showed a growth of the long-wavelength emission at longer t imes, and decay-associated spectra, which showed the longer wavelength emis sion associated with the longer decay time. These nanoparticles displayed a nisotropy values as high as 0.35, depending on the excitation and emission wavelengths. Such high anisotropies are unexpected for presumably spherical nanoparticles. The anisotropy decayed with two correlation times near 5 an d 370 ns, with the larger value probably due to overall rotational diffusio n of the nanoparticles. Addition of a 32-base pair oligomer selectively que nched the 460-nm emission, with less quenching being observed at longer wav elengths. The time-resolved intensity decays were minimally affected by the DNA, suggesting a static quenching mechanism. The wavelength-selected quen ching shown by the nanoparticles may make them useful for DNA analysis. (C) 2000 Academic Press.