Micellar effect of surfactant on the aggregation pattern of a fluorescent dye in ultra-thin film

Abstract

This communication reports the effect of micelles on the molecular organisation of an anionic fluorescent dye Fluorescein Sodium salt (FSS) in Langmuir-Blodgett (LB) film. FSS forms non-fluorescent H-aggregates in LB film when interact with Cetyltrimethylammonium Bromide (CTAB) molecules below critical micellar concentration (CMC). While above CMC, LB films of FSS adsorbed on the micellar surface of CTAB, show predominant J-aggregated sites. Complex LB films of FSS are also prepared by allowing it to adsorb onto a preformed Langmuir monolayer of cationic amphiphile octadecylamine (ODA) at the air-water interface and then depositing it onto the solid substrate. Complex ODA-FSS LB films are also investigated and a comparison is made with the SA-CTAB-FSS complex films. Unlike SA-CTAB-FSS complex LB films, ODA-FSS LB films can not effectively control the non-fluorescent H-aggregated sites. The films are characterised by UV–vis absorption and fluorescence spectroscopic studies and in-situ Brewster Angle Microscopic (BAM) studies. Thus CTAB micelles play an important role to decrease the H-aggregated sites in the SA-CTAB-FSS complex LB films. This complex LB film can act as efficient fluorescence probe for several biological systems due to the absence of non-fluorescent H-aggregated sites.