The modulation of the photophysical properties of curcumin inside two different types of microenvironments provided by nonionic surfactant forming micelles and vesicles (niosomes) has been investigated using steady state and time-resolved fluorescence spectroscopy. The formation of small unilamellar Tween-20/cholesterol niosomes with narrow size distribution has been successfully demonstrated by means of dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. Our results indicate that niosomes are a better possible delivery system than the conventional surfactants forming normal micelles to suppress the level of degradation of curcumin. The enhanced fluorescence intensity along with the significant blue-shift in the emission maxima of curcumin upon encapsulation into the hydrophobic microenvironments of micelles and niosomes is a consequence of the reduced interaction of curcumin with the water molecules. We found that the more rigid and confined microenvironment of niosomes enhances the steady state fluorescence intensity along with the fluorescence lifetime of curcumin more than in micelles. The rigidity of the niosome membrane which arises basically due to the presence of cholesterol molecules increases the level of interaction between curcumin and the oxoethylene units of Tween-20 molecules. It is also possible for the hydroxyl groups of the cholesterol moieties to form intermolecular hydrogen bonds with curcumin to perturb nonradiative deactivation mechanism through excited state intramolecular hydrogen atom transfer (ESIHT).

The micelle-vesicle-micelle transition in aqueous mixtures of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and the anionic surfactant-like ionic liquid 1-butyl-3-methylimidazolium octyl sulfate, [C4mim][C8SO4] has been investigated by using dynamic light scattering (DLS), transmission electron microscopy (TEM), surface tension, conductivity, and fluorescence anisotropy at different volume fractions of surfactant. The surface tension value decreases sharply with increasing CTAB concentration up to ∼0.38 volume fraction and again increases up to ∼0.75 volume fraction of CTAB. Depending upon their relative amount, these surfactants either mixed together to form vesicles and/or micelles, or both of these structures were in equilibrium. Fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), incorporated in this system at different composition of surfactant indicates the formation of micelle and vesicle structures. The apparent hydrodynamic diameter of these large multilamellar vesicles is about ∼200 nm-300 nm obtained by DLS measurement and finally confirmed by TEM micrographs. The large multilamellar vesicles are transformed into small unilamellar ones by sonication using a Lab-line instruments probe sonicator with a diameter of ∼90-125 nm. To investigate the heterogeneity, solvent, and rotational relaxation of coumarin-153 (C-153) have been investigated in these unilamellar vesicles by using picosecond time-resolved fluorescence spectroscopic technique. The solvation dynamics of C-153 in these vesicles is found to be biexponential with average time constant ∼580 ps. This indicates the slow relaxation of water molecules in the surfactant bilayer. In accordance with solvation dynamics, fluorescence anisotropy analysis of C-153 in unilamellar vesicles also indicates hindered rotation compared to bulk water.

The formation of stable unilamellar vesicles which hold great potential for biological as well as biomedical applications has been reported in the aqueous mixed solution of a surface active ionic liquid (SAIL), 1-hexadecyl-3-methylimidazolium chloride ([C16mim]Cl) and cholesterol. To make a comparison we have also shown the formation of such stable vesicles using a common cationic surfactant, benzyldimethylhexadecylammonium chloride (BHDC) which has a similar alkyl chain length but different headgroup region to that of [C16mim]Cl. It has been revealed from dynamic light scattering (DLS), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), and other optical spectroscopic techniques that the micelles of [C16mim]Cl and BHDC in aqueous solutions transform into stable unilamellar vesicles upon increasing concentration of cholesterol. We find that, as the concentration of cholesterol increases, the solvation and rotational relaxation time of C153 in [C16mim]Cl/cholesterol solution as well as in BHDC/cholesterol solution gradually increases indicating a significant decrease in the hydration behavior around the self-assemblies upon micelle-vesicle transition. However, the extent of increase in solvation and rotational relaxation time is more prominent in the case of [C16mim]Cl/cholesterol solutions than in the BHDC/cholesterol system. This indicates that [C16mim]Cl/cholesterol vesicular membranes are comparatively less hydrated and more rigid than the BHDC/cholesterol vesicular bilayer.

In this work, we have investigated the composition dependent anomalous behavior of dimethyl sulfoxide (DMSO)-water binary mixture by collecting the ultrafast solvent relaxation response around a well known solvation probe Coumarin 480 (C480) by using a femtosecond fluorescence up-conversion spectrometer. Recent molecular dynamics simulations have predicted two anomalous regions of DMSO-water binary mixture. Particularly, these studies encourage us to investigate the anomalies from experimental background. DMSO-water binary mixture has repeatedly given evidences of its dual anomalous nature in front of our systematic investigation through steady-state and time-resolved measurements. We have calculated average solvation times of C480 by two individual well-known methods, among them first one is spectral-reconstruction method and another one is single-wavelength measurement method. The results of both the methods roughly indicate that solvation time of C480 reaches maxima in the mole fraction of DMSO XD = 0.12-0.17 and XD = 0.27-0.35, respectively. Among them, the second region (XD = 0.27-0.35) is very common as most of the thermodynamic properties exhibit deviation in this range. Most probably, the anomalous solvation trend in this region is fully guided by the shear viscosity of the medium. However, the first region is the most interesting one. In this region due to formation of strongly hydrogen bonded 1DMSO:2H2O complexes, hydration around the probe C480 decreases, as a result of which solvation time increases.

In this work we have investigated the anomalous behavior of DMSO-water binary mixtures using 2,2'-bipyridine-3,3'-diol (BP(OH)2) as a microenvironment-sensitive excited-state-intramolecular-double-proton-transfer (ESIDPT) probe. Here we present results on the UV-vis absorption and fluorescence properties of BP(OH)2 in the binary solutions. DMSO-water binary mixtures at various compositions are an intriguing hydrogen bonded system, where DMSO acts to diminish the hydrogen bonding ability of water with the dissolved solutes. As a result, we observe unusual changes in the photophysical properties of BP(OH)2 with increasing DMSO content in complete correlation with the prior simulation and experimental results on the solvent structures and dynamics. The fluorescence quantum yield and fluorescence lifetime of BP(OH)2 depend strongly on the DMSO content and become maximum at very low mole fraction (∼0.12) of DMSO. The anomalous behavior at this particular region likely arises from the enhanced pair hydrophobicity of the medium as demonstrated by Bagchi and co-workers (Banerjee, S.; Roy, S.; Bagchi, B. J. Phys. Chem. B 2010, 114, 12875-12882). In addition we have also shown the utilization of BP(OH)2 as a potential Zn(2+)-selective fluorescent sensor in a 1:1 DMSO-water binary mixture useful for biological applications. We observed highly enhanced fluorescence emission of BP(OH)2 selectively for binding with the Zn(2+) metal ion. Moreover, the fluorescence emission maximum of BP(OH)2-Zn(2+) is significantly blue-shifted with a reduced Stokes shift due to the inhibition of the ESIDPT process of BP(OH)2 through strong coordination.