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Fluorescent nanocrystals have been employed as stable fluorophores to tag proetins cells and oligonucleotides. A more unusual approach is to use the nanocrystals to render silica microspheres fluorescent (Image 1). Large fluorescent micropsheres exhibit whispering gallery modes: the sphere acts as a cavity which suppresses or enhances particular emission wavelengths, causing the emission envelope to break up into a series of very sharp peaks (see spectrum). The resonance wavelengths depend on the exact particle size and refractive index mismatch with the environment. This forms the basis for a sensor for unlabelled oligonucleotides [1]. Single strand DNA is chemisorbed to the silica microsphere. Adsorption of the complementary DNA strand leads to a small red-shift in the spectra. Detection down to attomole levels are routinely possible [2].
[1] E. Nuhiji et al. , Small 3, 1408-14 (2007).
[2] E. Nuhiji et al., Anal. Chem. (submitted)
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