Transformation of an Aqueous Micellar Phase to a Bilayer of Gemini Surfactants on Gold Nanoparticles: A Steady-State and Time-Resolved Fluorescence and Fluorescence Anisotropy Study by Tuning the Precise Locations of Probes

The present work reports the transformation of aqueous micelles of gemini surfactants (12-4-12,2Br-or 12-8-12,2Br-) to a bilayer of surfactants on the surface of goldnanoparticles (AuNPs) capped with citrate during their in situsynthesis. The synthesized AuNPs are characterized using surfaceplasmon resonance (SPR), X-ray diffraction (XRD), X-rayphotoelectron spectroscopy (XPS), dynamic light scattering(DLS), scanning transmission electron microscopy (STEM), andhigh-resolution transmission electron microscopy (HR-TEM)techniques. The transformation of micelles to a surfactant bilayerwith time is understood with the help offluorescence measure-ments such as steady-state and time-resolvedfluorescence andfluorescence anisotropy, probed by tuning the precise locations oftwofluorophores, Coumarin-480 (C-480) and rhodamine 6G (Rh6G), present in the micelles. As the formation of AuNPs initiates,the dyes get relocated from micelles to a site near the NP surface, which results influorescence quenching and a decrease in lifetimedue to the nanomaterial surface energy transfer (NSET) from the donor dye to the acceptor AuNPs. The results indicated thatRh6G lies close to the head groups of the surfactant in micelles. Fast segmental/tumbling motions of Rh6G in micelles/bilayers areprimarily responsible for the decay of anisotropy to zero. Lateral diffusion is responsible for slow rotational relaxation. With thegrowth of rod/needle-shaped NPs, the average lifetime and rotational relaxation time increase with an increase influorescenceintensity due to the transfer of dye molecules from the NP surface to the interior of the bilayer. A significant change in the weightageof the slow component of C-480 compared to that of Rh6G with the formation of the bilayer supports that the former dye is locateddeep inside the bilayer with an equal contribution to depolarization from both the rotational motions at equilibrium. The excitationwavelength-dependent rotational relaxation time of Rh6G in the bilayer supports the prolate ellipsoidal shape of AuNPs surroundedby the bilayer. This study can help understand the mechanism of drug loading in forming metal NP-based hybrid drug delivery systems in an aqueous micellar solution of gemini surfactants.