New fluorescent quinolinium dyes — applications in nanometre particle sizing

Bromide, iodide and tetraphenylborate quaternary salts of four new highly fluorescent dyes have been produced by the reaction of the 6-methoxyquinoline heterocyclic nitrogen base with 3-bromo-1-propanol, methyl iodide and methyl bromide with one of the dyes having its counter-ion exchanged for the tetraphenylborate ion. Three of the dyes, unlike the base, are readily water soluble, the tetraphenylborate salt only slightly water soluble and all are fluorescent over a broad pH range. The dyes have been characterised in terms of their water solubility, their steady-state fluorescence spectra and their bi-exponential fluorescence lifetimes, which are all found to be in the range 10‐30 ns. We have found that the cationic dyes readily bind to negatively charged colloidal silica particles and because of the suitably long fluorescence lifetimes we have been able to determine the particle size of Du Pont’s SM30 colloidal silica using timeresolved fluorescence anisotropy. The average particle diameter, determined using three of the dyes, was 6.8 a 0.6 nm, which compares well with Du Pont’s mean value of 7 nm. # 1999 Elsevier Science Ltd. All rights reserved.