Competitive influence of the plasmon effect and energy transfer between chromophores and Ag nanoparticles on the fluorescent properties of indopolycarbocyanine dyes

Abstract

As illustrated with a vinylogous series of cationic symmetric indopolycarbocyanines, it is found out that adding Ag nanoparticles into their solutions had caused enhanced dye fluorescence at the constant absorption intensity. It has been shown that the enhanced fluorescence intensifies in response to the lengthened polymethine chain of indopolycarbocyanines, although the spectral overlap between Ag nanoparticles and polymethine dyes (PDs) decreases. A mathematical model has been proposed that incorporates the effect of plasmon nanoparticles on deactivation of excited state of indopolycarbocyanine molecules. It has been shown that the main reason of the growth of fluorescence intensity is an increase in fluorescence rate of the dye molecules in the presence of metal nanoparticles. It has been found out that Fӧrster resonance energy transfer (FRET) from dye molecules to Ag nanoparticles also affects fluorescence of indopolycarbocyanines. Herewith energy transfer is a competing process with respect to the fluorescence. The calculation showed that the FRET rate is higher for shorter molecules. This, in turn, leads to a decrease in the number of dye molecules that decay by fluorescence. For molecules of longer polymethine chain, the rate of energy transfer from the dye to nanoparticles has lower values. As a result, the fraction of fluorescent molecules is greater, which is manifested in the enhancement coefficient of fluorescence intensity.