Curcumin in Reverse Micelle: An Example to Control Excited-State Intramolecular Proton Transfer (ESIPT) in Confined Media

In this Article, we focused on the modulation of the photophysical properties of curcumin, an anti-cancer drug, in aqueous and nonaqueous reverse micelles of AOT in <i>n</i>-heptane using steady-state and time-resolved fluorescence spectroscopy. The instability of curcumin is a common problem which restricts its numerous applications like Alzheimer disease, HIV infections, cystic fibrosis, etc. Our study reveals that curcumin shows comparatively higher stability after encapsulation into the interfacial region of the reverse micelle. To get a vivid description of the microenvironment, we added hydrogen-bond-donor (HBD) as well as no<i>n</i>-hydrogen-bond-donor (NBD) core solvents. For experimental purposes, we used water, ethylene glycol (EG), glycerol (GY) as HBD solvents and <i>N</i>,<i>N</i>-dimethyl formamide (DMF) as a NBD solvent. With increasing amount of core solvents, irrespective of HBD or NBD, the fluorescence intensity and lifetime of curcumin increase with remarkable red-shift inside the reverse micelle. This is attributed to the modulation of the nonradiative rates associated with the excited-state intermolecular hydrogen bonding between the pigment and the polar solvents. We obtained a high partition constant at <i>W</i>₀ = 0 (<i>W</i>₀ = [core solvent]/[AOT]) which is certainly due to the hydrogen bonding between the negatively charged sulfonate group of AOT and hydroxyl groups of curcumin. Steady-state anisotropy and time-resolved results give an idea about the microenvironment sensed by the curcumin molecules. The red-shift of emission spectra, increase in the value of <i>E</i>_T(30), as well as the increase in the fluorescence lifetime were interpreted as being caused by the partition of the probe between the micellar interface and the polar core solvent. Indeed, we show here that it is possible to control the excited state intramolecular proton transfer (ESIPT) process of curcumin by simply changing the properties of the AOT reverse micelle interfaces by choosing the appropriate polar solvents to make the reverse micelle media

Spontaneous Transition of Micelle–Vesicle–Micelle in a Mixture of Cationic Surfactant and Anionic Surfactant-like Ionic Liquid: A Pure Nonlipid Small Unilamellar Vesicular Template Used for Solvent and Rotational Relaxation Study

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

An Understanding of the Modulation of Photophysical Properties of Curcumin inside a Micelle Formed by an Ionic Liquid: A New Possibility of Tunable Drug Delivery System

The present study reveals the modulation of photophysical properties of curcumin, an important drug for numerous reasons, inside a micellar environment formed by a surfactant-like ionic liquid (IL-micelle) in aqueous solution. Higher stability of the drug inside IL-micelle in the absence and presence of a simple salt (sodium chloride) as well as considerably large partition coefficient (<i>K</i>_p = 8.59 × 10³) to the micellar phase from water make this system a well behaved drug loading vehicle. Remarkable change in fluorescence intensity with a strong blue-shift implies the gradual perturbation of intramolecular hydrogen bond (H-bond) present within the keto–enol group of curcumin along with considerable formation of intermolecular H-bond between curcumin and the headgroup of surfactant-like IL. Very fast nonradiative decay channels in curcumin mainly caused by the excited state intramolecular proton transfer (ESIPT) are thus depleted remarkably in the presence of IL-micelle of reduced polarity and as a result of restricted rotational and vibrational degrees of freedom when bound to the micelle. Moreover, time-resolved results confirm that not only the keto–enol group of curcumin is playing here but also the phenolic hydroxyl groups are also responsible for such modulation in photophysical properties. From a thermodynamic point of view, our system shows good correlation with its stability parameters (higher binding constant with very less hydrolytic degradation rate ∼1%) and higher negative value of binding enthalpy of interaction (−Δ<i>H</i>) than total free energy change (−Δ<i>G</i>) implies that the nature of binding interaction is enthalpy driven not entropy alone. Summarizing all the above observations, we have concluded that the modulation of the intramolecular proton transfer is due to the presence of both intermolecular proton transfer as well as strong hydrophobic interaction between curcumin and the IL-micelle

Pluronic Micellar Aggregates Loaded with Gold Nanoparticles (Au NPs) and Fluorescent Dyes: A Study of Controlled Nanometal Surface Energy Transfer

In this work we have reported the controlled synthesis of gold nanoparticles into the surface cavities of P123 micellar assemblies together with the fluorescent dye molecules and investigated nanometal surface energy transfer (NSET) from confined donor dye to metal nanoparticles. The formation of hybrid spherical assemblies of P123 combined with fluorescent dyes and gold nanoparticles has been confirmed from HR-TEM, DLS, UV–vis, and fluorescence spectroscopic studies. The observed steady state as well as time-resolved fluorescence quenching of the confined micellar dyes present in the close proximity of gold nanoparticles which are attached to the surface of micellar assemblies, indicates efficient surface energy transfer from dye to gold (Au) nanoparticles. Since the NSET process is strongly dependent on the distance between donor dye and acceptor nanoparticles, successful applications of NSET require the perfect control over their relative location. Herein, we investigate the utilization of nanoparticles embedded self-assemblies of P123 for controlled NSET by tuning the precise location of donor dyes. Through the nanoencapsulation of the different fluorophore having different location inside P123 micelles, we have shown the corona region of P123 micelles as a perfect place for NSET and the core region as a barrier for NSET. Additionally, we have investigated the microenvironment of the confined micellar probe molecules in presence and absence of nanoparticles. This study further reveals that when the system changes from normal micelles to nanoparticles loaded hybrid micelles, unlike the probes C480 and C153, the anionic probe C343 undergoes a change in its location indicating the modulation of the properties of micelles in presence of nanoparticles

Solvation Dynamics and Rotational Relaxation Study Inside Niosome, A Nonionic Innocuous Poly(ethylene Glycol)-Based Surfactant Assembly: An Excitation Wavelength Dependent Experiment

Excitation wavelength dependence of solvation and rotational relaxation dynamics has been investigated inside niosome, a biologically stable, nontoxic to our body, multilamellar vesicle system, by using steady state and time-resolved fluorescence spectroscopy to explore the heterogeneity of such a system. Red edge excitation shifts (REES) of 7 nm for Coumarin-153 (C-153) and 11 nm for C-480 were observed with change in λ_ex. Average solvation dynamics is composed of two types of slow components and one fast component. There are two distinct restricted regions, one at the bilayer headgroup region and the other on the two extreme surfaces, which are responsible for the slow components. An unaltered fast component is reported for the segmental chain dynamics of poly(ethylene glycol) (PEG) located at the headgroup region of niosome. The trend in λ_ex dependence obtained for C-153 is found to be similar to that obtained for C-480. Such hindered solvation is attributed to the presence of a strong H-bonding environment of water molecules in the headgroup region, and movement of these highly bound water molecules along with a hydrated oxyethyelene moiety control the observed slow relaxation

Dynamics of Solvation and Rotational Relaxation of Coumarin 480 in Pure Aqueous-AOT Reverse Micelle and Reverse Micelle Containing Different-Sized Silver Nanoparticles Inside Its Core: A Comparative Study

In this work, we have synthesized different-sized silver nanoparticles in an aqueous-AOT reverse micellar system under the same condition by choosing different reduction processes. We chose two different reducing agents, glucose (mild) and sodium borohydride (strong). In the glucose reduction process, we obtained smaller size nanoparticles in comparison to the nanoparticles obtained in the borohydride reduction process under the same condition. Solvation dynamics study showed that reverse micellar aggregated structures were present after the nanoparticles' formation in a perturbed state. Nanoparticles inside the reverse micellar core were responsible for this perturbation. Larger size nanoparticles were triggering larger perturbation than the smaller size nanoparticles. These changes in perturbation were also reflected clearly in solvation dynamics and rotational relaxation measurements

Photoinduced Electron Transfer in an Imidazolium Ionic Liquid and in Its Binary Mixtures with Water, Methanol, and 2-Propanol: Appearance of Marcus-Type of Inversion

The photoinduced electron transfer (PET) reaction has been investigated in a room temperature imidazolium ionic liquid (RTIL), 1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO₄]) and also in [Emim][EtSO₄]–co-solvents mixtures from <i>N</i>,<i>N</i>-dimethyl aniline (DMA) to different Coumarin dyes using steady state and time-resolved fluorescence quenching measurements. We have used water and methanol and 2-propanol as the cosolvents of RTILs for the PET study. On going from neat ionic liquid to the RTIL–co-solvents mixtures the electron transfer rate has been largely enhanced. In neat RTIL as well as in [Emim][EtSO₄]–co-solvents mixtures, a Marcus type of inversion in the PET rate have been observed

Photophysics and Photodynamics of 1′-Hydroxy-2′-acetonaphthone (HAN) in Micelles and Nonionic Surfactants Forming Vesicles: A Comparative Study of Different Microenvironments of Surfactant Assemblies

The effect of different microenvironments inside various biomimicking supramolecular assemblies of ionic (SDS/CTAB) and nonionic (TX100) micelles and nonionic surfactants (Tween-80/PEG-6000) forming vesicles (niosome) on the photophysical and rotational dynamical properties of 1′-hydroxy-2′-acetonaphthone (HAN) have been studied using steady-state and time-resolved fluorescence spectroscopy. Enhanced fluorescence intensity with a significant blue shift and longer emission lifetime of the caged tautomers of HAN indicate modulation of photophysics of HAN upon encapsulation in both micellar assemblies and the niosome system. The binding constant and free energy change for the complexation of HAN with micelles and niosome demonstrate a comparative study on the binding efficiency of the different assemblies depending on the nature of microenvironments toward HAN. The enhancement in the steady-state anisotropy in niosome solutions compared with that in pure aqueous solution indicates that HAN is located inside the motionally restricted bilayer region of niosome. The fluorescence quenching experiment further reveals the probable location of HAN in micelles and niosome. In TX100 micelles, the obtained lifetime values are 417 ps and 1.63 ns for the caged tautomers, whereas in the comparatively more rigid and confined environment provided by niosome those values are 444 ps and 2.5 ns. The rotational relaxation time constants for the caged tautomers in niosome are also found to be higher than those in micelles. The observed difference in binding ability of the different assemblies is due to the difference in the extent of water penetration and different extent of rigidity around the fluorophore

Designing a New Strategy for the Formation of IL-in-Oil Microemulsions

Due to the increasing applicability of ionic liquids (ILs) as different components of microemulsions (as the polar liquid, the oil phase, and the surfactant), it would be advantageous to devise a strategy by which we can formulate a microemulsion of our own interest. In this paper, we have shown how we can replace water from water-in-oil microemulsions by ILs to produce IL-in-oil microemulsions. We have synthesized AOT-derived surface-active ionic liquids (SAILs) which can be used to produce a large number of IL-in-oil microemulsions. In particular, we have characterized the phase diagram of the [C₄mim]­[BF₄]/[C₄mim]­[AOT]/benzene ternary system at 298 K. We have shown the formation of IL-in-oil microemulsions using the dynamic light scattering (DLS) technique and using methyl orange (MO), betaine 30, and coumarin-480 (C-480) as probe molecules