Searching for fluorescent nanocrystals in aqueous solutions of 7-methoxycoumarin

The unexpected formation of nanocrystals in dilute dye solutions was revealed using the hydrophobic dye, 7-methoxycoumarin, the spectroscopic behavior of which has been frequently studied in water; comparison was made with the water-soluble, sulfonated analogue. A solution of the dye at 10−5 M concentration range was found to be a suspension of nanocrystals; at concentrations of 1 × 10−4 M, > 70% dye was in the form of nanocrystals. Surprisingly, the presence of these nanocrystals was difficult to detect by routine absorbance and fluorescence spectroscopy and was mainly noticeable by measurement of fluorescence quantum yield, as the nanocrystals were poorly emissive compared to dissolved dye molecules; electron microscopy and dynamic light scattering gave clear evidence for the presence of nanocrystals. This article illustrates the fact that the formation of nanocrystals, although abundant, may easily go unnoticed.