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

Abstract

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 gold nanoparticles (AuNPs) capped with citrate during their in situ synthesis. The synthesized AuNPs are characterized using surface plasmon resonance (SPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), scanning transmission electron microscopy (STEM), and high-resolution transmission electron microscopy (HR-TEM) techniques. The transformation of micelles to a surfactant bilayer with time is understood with the help of fluorescence measurements such as steady-state and time-resolved fluorescence and fluorescence anisotropy, probed by tuning the precise locations of two fluorophores, 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 in fluorescence quenching and a decrease in lifetime due to the nanomaterial surface energy transfer (NSET) from the donor dye to the acceptor AuNPs. The results indicated that Rh6G lies close to the head groups of the surfactant in micelles. Fast segmental/tumbling motions of Rh6G in micelles/bilayers are primarily responsible for the decay of anisotropy to zero. Lateral diffusion is responsible for slow rotational relaxation. With the growth of rod/needle-shaped NPs, the average lifetime and rotational relaxation time increase with an increase in fluorescence intensity due to the transfer of dye molecules from the NP surface to the interior of the bilayer. A significant change in the weightage of the slow component of C-480 compared to that of Rh6G with the formation of the bilayer supports that the former dye is located deep inside the bilayer with an equal contribution to depolarization from both the rotational motions at equilibrium. The excitation wavelength-dependent rotational relaxation time of Rh6G in the bilayer supports the prolate ellipsoidal shape of AuNPs surrounded by 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.

Ag and Au Nanoparticles as Color Indicators for Monomer/Micelle–Nanoparticle Interactions

“…In this work, we exploit the surface adsorption of Gemini surfactants on tiny Ag/Au nanoparticles (NPs) to understand the solid–liquid interface adsorption as well as micelle formation by using the surface plasmon resonance (SPR) of Au/Ag NPs as an indicator. Although dyes are the common color indicators for tracing the micellar properties, , the use of Ag and Au NPs as color indicators provides an additional advantage to understand the adsorption of the monomer/micelle on the solid–liquid interface.…”

“…Ag/Au NPs depict the absorbance due to SPR in the visible region, which is highly sensitive to the colloidal nature of NPs. It is significantly affected by the surfactant adsorption on the NP surface that induces a strong influence on SPR, which in turn is reflected by the variation in intensity as well as wavelength. − Thus, both latter parameters act as strong indicators of the surfactant–NP interactions as well as micelle formation behavior of the surfactant in the presence of NPs. These properties are important to understand the appropriate applicability of Ag/Au NPs in complex media where NPs frequently encounter the presence of surface-active molecules and their aggregated assemblies. , We demonstrate this by choosing a series of highly surface-active Gemini surfactants whose micelle formation and solid–liquid interfacial behavior significantly depend on the head group as well as hydrocarbon tail modifications.…”

Role of Gemini Surfactants with Variable Spacers and SiO2 Nanoparticles in ct-DNA Compaction and Applications toward In Vitro/In Vivo Gene Delivery

“…The rotational relaxation time or correlation time calculated after recording the fluorescence anisotropy decays gives valuable information about the rigidity of the microenvironment, different rotational motions of the probe, and the motional dynamics in the system itself in which the probe is located that is responsible for depolarization. , Here, the time-resolved fluorescence anisotropies ( r ( t )) of DAPI in all studied systems have been calculated using eq , and the corresponding decay curves are obtained after fitting to eq. .…”

“…It is expected that the dynamics should occur in the microsecond timescale for such a big complex. 120,121 This slow rotational motion could be associated with the ct-DNA double helix's segmental motion. 80,81 However, the data show that the time constant for this motion decreases with increasing concentration of a surfactant, i.e., with the compaction of ct-DNA, which is quite surprising.…”

Compaction of Calf Thymus DNA by a Potential One-Head-Two-Tail Surfactant: Properties of Nanomaterials and Biological Testing for Gene Delivery

“…SiO 2 NPs with a diameter of ∼100 nm are found to be effective for the compaction of DNA. Reportedly, hydrophobicity of surfactants 23,45,46 plays an important role in promoting DNA compaction, and 1-2 types of surfactants encourage a greater extent of hydrophobicity to the NPs surface compared to 2-2 and 3-2 types on an equal chain length basis. 47 In the present work, the ability of DDAB surfactant to compact ct-DNA has been compared with that by a cationic conventional surfactant, DTAB, and a Gemini surfactant, 12-6-12 (in order to indicate bromide ions as counterions, hereafter it is presented as 12-6-12,2Br − ) with equal hydrocarbon tail length (Scheme 1) in the presence of SiO 2 NPs.…”