The inner hydration in surfactant/cholesterol vesicles differs from the outer one: a spectroscopic investigation

Vesicles contain two aqueous regions: inner core and outer-to-bulk . It has remained an open question whether hydration behaviour in the inner core differs from the outer-to-bulk region, mostly owning to the inability of the conventional spectroscopic techniques to deconvolute the contribution from these two regions. We, using THz-FTIR spectroscopy (1.5-13.5 THz) experimentally probe the inner hydration of three differently charged surfactant/cholesterol vesicles composed of SDS, CTAB and Brij 30. Both dynamic light scattering (DLS) and atomic force microscopy (AFM) measurements affirm the transition from micelles to vesicles as cholesterol is added into surfactant solutions. FTIR measurements show that hydration behaviour changes significantly as micelles are converted into vesicles, the change been exclusively caused due to the formation of an inner core . Our measurements on the hydrogen bond stretch and librational motion of the inner hydration show distinct features compared to the overall hydration, which in turn is found to be surfactant type and cholesterol concentration dependent.

Development of a carbon dot and methylene blue NIR-emitting FLIM-FRET pair in niosomes for controlled ROS generation

Non-ionic surfactant vesicular systems (niosomes) are structurally similar to lipid vesicles, differing only in the bilayer composition. Herein we report a unique method to generate reactive oxygen species (ROS) utilizing a FLIM-FRET technique involving niosome-trapped yellow emissive carbon dots (YCDs) and methylene blue (MB) in aqueous medium under neutral conditions. Niosomes are biologically important because of their good stability and extremely low toxicity. Fluorescent CDs, emitting in the higher wavelengths on visible light excitation, are of incredible importance in bio-imaging and optoelectronics. Hence, we prepared nitrogen-containing YCDs from a single precursor, o-phenylenediamine, and explained their detailed photophysics upon incorporation into the niosomal bilayer. The YCDs are polarity sensitive, and are rotationally restricted in niosomes, which increases their fluorescence quantum yield from 29% (in water) to 91%. These YCDs are tactically employed to develop a near infrared (NIR) FRET pair with methylene blue (MB), which is a very well-known type-I and type-II photosensitizer. This FRET pair, which emits in the NIR region, is found to be an ideal system to generate ROS by excitation in the lower visible wavelengths. Interestingly, the ROS production by MB from the dissolved oxygen is enhanced inside the niosomes. The donor and the acceptor moieties in this unique NIR-emitting FRET pair display an unprecedented 300 nm Stokes shift. The findings could be influential in bio-imaging in the NIR region evading cellular autofluorescence and the controllably generated ROS can be further applied as a potential photodynamic therapeutic agent.

Lipophilic Vitamin E Diffusion through Bicontinuous Microemulsions

We studied the diffusion properties of lipophilic vitamin E (VE) through bicontinuous microemulsions (BME) using both electrochemical and fluorescence correlation spectroscopy (FCS) measurements. We investigated the effect of different composition ratios of micro-water and micro-oil phases in BMEs (W/O_BME). When we employed the BME with a lower W/O_BME value of 40/60 (oil-rich BME) as an electrolyte solution, we obtained a larger current response from VE at a fluorinated nanocarbon film electrode. Further voltammetric studies revealed that a higher VE diffusion coefficient was observed in the oil-rich BME. The FCS results also exhibited faster diffusion through the oil-rich BME, which played a significant role in accelerating the VE diffusion probably due to the widening of the micro-oil phase pathway in the BME. Moreover, the effect of increasing the VE diffusion was pronounced at the interface between the electrode surface and the BME solution. These results indicate that controlling the conditions of the BME as the measurement electrolyte is very effective for achieving superior electrochemical measurements in a BME.

Metal-Enhanced Fluorescence Study in Aqueous Medium by Coupling Gold Nanoparticles and Fluorophores Using a Bilayer Vesicle Platform

Gold nanoparticles (AuNPs) display excellent plasmonic properties, which are expected to assist fluorescence enhancement for dyes, and the phenomenon is known as "metal-enhanced fluorescence" (MEF). In this study, we demonstrate AuNP-induced MEF for a modified bipyridine-based construct 4-(pyridine-2-yl)-3H-pyrrolo[2,3-c]quinoline (PPQ) when it binds with biologically important Zn2+. Importantly, this phenomenon is observed under aqueous conditions in a biocompatible bilayer vesicle platform. When PPQ binds with Zn2+ to form the complex in the presence of appropriate AuNPs, MEF is evident once compared with the fluorescence intensity in the absence of AuNPs. Among the three different sizes of AuNPs used, the enhancement is observed with an average diameter of 33 nm, whereas 18 and 160 nm do not show any enhancement. A possible mechanism is ascribed to the radiating plasmons of the AuNPs, which can couple with the emission frequencies of the fluorophore under a critical distance-dependent arrangement. We witness that the enhancement in fluorescence is accompanied with a reduction in lifetime components. It is proposed that the mechanism may be predominantly derived from the enhancement of an intrinsic radiative decay rate and partly from the localized electric field effect. Overall, this work shows a rational approach to design fluorophore-metal configurations with the desired emissive properties and a basis for a useful nanophotonic technology under biological conditions.

Establishing Structure Property Relationship in Drug Partitioning into and Release from Niosomes: Physical Chemistry Insights with Anti-Inflammatory Drugs

Understanding the physical chemistry underlying interactions of drugs with delivery formulations is extremely important in devising effective drug delivery systems. The partitioning and release kinetics of diclofenac sodium and naproxen from Brij 30 and Triton X-100 niosomal formulations have been addressed based on structural characterization, partitioning energetics, and release kinetics, thus establishing a relationship between structures and observed properties. Both the drugs partition in nonpolar regions of TX-100 niosomes via stacking of aromatic rings. The combined effects of interactions of the drugs with polar head groups and the rigidity of the niosome vesicles determine entry and partitioning of drugs into niosomes. The observed slower rate of release of the drugs from the drug encapsulated niosomes of TX-100 than those of Brij 30, suggest stable complexation of drugs in the nonpolar interior of the former. No release of drugs from the niosomes was observed until 24 h even upon varying pH conditions without SDS. However, SDS in drug loaded niosomes led to release of drugs in as early as 6 h. The sustained pattern of in vitro release kinetics of the drugs thus observed from our niosomal preparations suggest these vesicular systems to be promising for pharamaceutical applications as potential drug delivery vehicles.

How does the surface charge of ionic surfactant and cholesterol forming vesicles control rotational and translational motion of rhodamine 6G perchlorate (R6G ClO₄)?

The rotational dynamics and translational diffusion of a hydrophilic organic molecule, rhodamine 6G perchlorate (R6G ClO4) in small unilamellar vesicles formed by two different ionic surfactants, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), with cholesterol have been investigated using fluorescence spectroscopic methods. Moreover, in this article the formation of vesicle using anionic surfactant, SDS at different cholesterol-to-surfactant molar ratio (expressed by Q value (Q = [cholesterol]/[surfactant])) has also been reported. Visual observation, dynamic light scattering (DLS) study, turbidity measurement, steady state fluorescence anisotropy (r0) measurement, and eventually microscopic images reveal the formation of small unilamellar vesicles in aqueous solution. Also, in this study, an attempt has been made to observe whether the cationic probe molecule, rhodamine 6G (R6G) experiences similar or different microenvironment in cholesterol-SDS and cholesterol-CTAB assemblies with increase in cholesterol concentration. The influence of cholesterol on rotational and translational diffusion of R6G molecules has been investigated by monitoring UV-vis absorption, fluorescence, time-resolved fluorescence anisotropy, and finally fluorescence correlation spectroscopy (FCS) measurements. In cholesterol-SDS assemblies, due to the strong electrostatic attractive interaction between the negatively charged surface of vesicle and cationic R6G molecules, the rotational and diffusion motion of R6G becomes slower. However, in cholesterol-CTAB aggregates, the enhanced hydrophobicity and electrostatic repulsion induces the migration of R6G from vesicle bilayer to aqueous phase. The experimental observations suggest that the surface charge of vesicles has a stronger influence than the hydrophobicity of the vesicle bilayer on the rotational and diffusion motion of R6G molecules.

Unfolding and refolding of a protein by cholesterol and cyclodextrin: a single molecule study

Cholesterol induced unfolding of a globular protein, human serum albumin (HSA), and β-cyclodextrin induced refolding of the unfolded protein is demonstrated in this study.

Dynamics in cytoplasm, nucleus, and lipid droplet of a live CHO cell: time-resolved confocal microscopy

Different regions of a single live Chinese hamster ovary (CHO) cell are probed by time-resolved confocal microscopy. We used coumarin 153 (C153) as a probe. The dye localizes in the cytoplasm, nucleus, and lipid droplets, as is clearly revealed by the image. The fluorescence correlation spectroscopy (FCS) data shows that the microviscosity of lipid droplets is ~34 ± 3 cP. The microviscosities of nucleus and cytoplasm are found to be 13 ± 1 and 14.5 ± 1 cP, respectively. The average solvation time (<τs>) in the lipid droplets (3600 ± 50 ps) is slower than that in the nucleus (<τs> = 750 ± 50 ps) and cytoplasm (<τs> = 1100 ± 50 ps). From the position of emission maxima of C153, the polarity of the nucleus is estimated to be similar to that of a mixture containing 26% DMSO in triacetin (η ~ 11.2 cP, ε ~ 26.2). The cytoplasm resembles a mixture of 18% DMSO in triacetin (η ∼ 12.6 cP, ε ∼ 21.9). The polarity of lipid droplets is less than that of pure triacetin (η ~ 21.7 cP, ε ~ 7.11).

Solvation dynamics of biological water in a single live cell under a confocal microscope

Time-resolved confocal microscopy has been applied to study the cytoplasm and nucleus region of a single live Chinese hamster ovary (CHO) cell. To select the cytoplasm and the nucleus region, two different fluorescent probes are used. A hydrophobic fluorescent dye, DCM, localizes preferentially in the cytoplasm region of a CHO cell. A DNA binding dye, DAPI, is found to be inside the nucleus of the cell. The locations of the probes are clearly seen in the image. Emission maxima of the dyes (DCM in cytoplasm and DAPI in the nucleus) are compared to those of the same dyes in different solvents. From this, it is concluded that the polarity (dielectric constant, ε) of the microenvironment of DCM in the cytoplasm is ~15. The nucleus is found to be much more polar with ε ≈ 60 (as reported by DAPI). The diffusion coefficient (and hence viscosity) in the cytoplasm and the nucleus was determined using fluorescence correlation spectroscopy (FCS). The diffusion coefficient (D(t)) of the dye (DCM) in the cytoplasm is 2 μm(2) s(-1) and is ~150 times slower than that in bulk water (buffer). D(t) of DAPI in the nucleus (15 μm(2) s(-1)) is 30 times slower than that in bulk water (buffer). The extremely slow diffusion inside the cell has been ascribed to higher viscosity and also to the binding of the probes (DCM and DAPI) to large biological macromolecules. The solvation dynamics of water in the cytoplasm (monitored by DCM) exhibits an average relaxation time [τ(sol)] of 1250 ± 50 ps, which is about 1000 times slower than in bulk water (1 ps). The solvation dynamics inside the nucleus (studied using DAPI) is about 2-fold faster, [τ(sol)] ≈ 775 ps. The higher polarity, faster diffusion, and faster solvation dynamics in the nucleus indicates that it is less crowded and less restricted than the cytoplasm.