Study of γ-cyclodextrin host-guest complex and nanotube aggregate by fluorescence correlation spectroscopy

Abstract

Fluorescence correlation spectroscopy (FCS) has been used to study the formation of large nanotube aggregates involving γ-cyclodextrin (γ-CD) and coumarin 153 (C153). It is observed that the length of a γ-CD:C153 nanotube aggregate is 770 nm. This is 480 times larger than the length of a 1:1 γ-CD:C480 complex (1.6 nm) and 950 times that of a γ-CD. This implies that 950 γ-CD units are noncovalently attached in the γ-CD:C153 aggregate. Binding constants (K_b) of both the dyes to γ-CD were obtained from the fluctuation in fluorescence intensity. The rate of association and dissociation are obtained from the inverse of τ_off and τ_on, respectively. The binding constant for the 1:1 γ-CD:C480 complex is 1000 M^-1. The burst integrated fluorescence lifetime (BIFL) histogram reveals presence of three distinct lifetime 1.8 ns (18%), 2.8 ns (69%), 3.2 ns (13%). These three lifetimes correspond to C153 present in bulk water and at the end and middle of the γ-CD:C153 nanotube aggregate, respectively. The lifetime of C480 in the 1:1 γ-CD:C480 complex is found to be 3.7 ns.