A Novel Ionic Liquid-in-Oil Microemulsion Composed of Biologically

Article pubs.acs.org/JPCB

A Novel Ionic Liquid-in-Oil Microemulsion Composed of Biologically Acceptable Components: An Excitation Wavelength Dependent Fluorescence Resonance Energy Transfer Study Sarthak Mandal, Surajit Ghosh, Chiranjib Banerjee, Jagannath Kuchlyan, Debasis Banik, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India ABSTRACT: In this work we have reported the formulation of a novel ionic liquid-in-oil (IL/O) microemulsion where the polar core of the ionic liquid, 1ethyl-3-methylimidazolium n-butylsulfate ([C2mim][C4SO4]), is stabilized by a mixture of two nontoxic nonionic surfactants, polyoxyethylene sorbitan monooleate (Tween-80) and sorbitan laurate (Span-20), in a biological oil phase of isopropyl myristate (IPM). The formation of the microemulsion droplets has been confirmed from the dynamic light scattering (DLS) and phase behavior study. To assess the dynamic heterogeneity of this tween-based IL/O microemulsion, we have performed an excitation wavelength dependent fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G). The multiple donor−acceptor (D−A) distances, ∼15, 30, and 45 Å, obtained from the rise times of the acceptor emission in the presence of a donor can be rationalized from the varying distribution of the donor, C480, in the different regions of the microemulsion system. With increasing the excitation wavelength from 375 to 408 nm, the contribution of the rise component of ∼240 ps which results the D−A distance of ∼30 Å increases significantly due to the enhanced contribution of the C480 probe molecules closer to the acceptor in the ionic liquid pool of the microemulsion.

1. INTRODUCTION Microemulsions are a class of transparent, thermodynamically stable microheterogeneous systems comprised of two immiscible liquids, namely, polar and nonpolar solvents stabilized by an interfacial film of surfactant frequently in combination with a cosurfactant. Both the water-in-oil (W/O) and oil-in-water (O/ W) type of microemulsions are capable of solubilizing a wide variety of polar and nonpolar substances in their nanodomains.1−3 As a consequence these are now being extensively used as a microreactor for various organic and inorganic chemical reactions.4 Recent investigations have shown the utilization of microemulsion droplets to control the size and shape of various inorganic nanostructures synthesized in the core of the microemulsions.5,6 The microemulsion droplets are not only being used in the synthesis of nanomaterials7,8 but also used in the purification,9 extraction of biomolecules,9 and drug delivery.10,11 The change in the structural properties such as the size, shape, and interfacial rigidity of the microemulsions plays an important role in controlling the chemical reactivity12 of the reactants, morphology of the nanomaterials,13,14 and the release of the drug molecules from the microemulsion droplets.15,16 The droplet size of the microemulsions is usually controlled by a parameter called R which is equal to the value of the molar ratio of polar liquid (water in case of water-in-oil type of microemulsions) and surfactant. Extensive studies have been performed to understand the interaction and dynamics of the microemulsion droplets using various techniques such as smallangle neutron scattering (SANS), small-angle X-ray scattering © 2013 American Chemical Society

(SAXS), transmission electron microscopy, dynamic light scattering (DLS), NMR, fluorescence spectroscopy, conductance, and viscosity measurements.17−21 In a very recent study, Sen et al.22 have shown how fluorescence correlation spectroscopy (FCS) can be used as an efficient tool for the measurement of size distribution of the microemulsion droplets. The idea of nonaqueous microemulsions is not new and was demonstrated by a number of groups where the water-forming polar core has been replaced by several nonaqueous polar solvents such as methanol, acetonitrile, glycerol, ethylene glycol, and formamide.23−28 The past few years, however, have witnessed a growing interest in the development of ionic liquid based nonaqueous microemulsions where the polar core has been replaced by a room temperature ionic liquid (RTIL). The formation of ionic liquid containing nonaqueous microemulsions was first demonstrated by Han and co-workers.29 The microstructure of these microemulsions was subsequently studied by Eastoe et al. using SANS experiment.30 After that, extensive studies have been performed to synthesize and characterize new types of ionic liquid containing microemulsions for various applications.31−34 In a series of papers Gao et al.35,36 reported the temperature and water induced changes in the microstructural properties of different ionic Received: January 28, 2013 Revised: February 25, 2013 Published: February 27, 2013 3221

dx.doi.org/10.1021/jp4009515 | J. Phys. Chem. B 2013, 117, 3221−3231

The Journal of Physical Chemistry B

Article

phase behaviors at low pH ∼2.6 and in 0.9% NaCl salt concentration.53 Goto and co-workers52 have shown that the cell viability of the tween-80 surfactant-based IL/O microemulsions containing 4 wt % IL was over 80% compared to Dulbecco’s phosphate-buffered salines. It is also interesting to know that the toxicity of the ionic liquids is reduced in more saline water.64The effect of water in the IL/O microemulsions is extensively reported in literature.36 Therefore, ionic liquid with saline water can be used instead of only IL to enhance the biocompatibility of these IL/O microemulsions for pharmaceutical purposes. In the present work we have used isopropyl myristate as nontoxic biological oil and the mixtures of two biocompatible nonionic surfactants Tween-80 (TW-80) and Span-20 in 2:1 weight ratio to develop a new IL/O microemulsion. Initially we tried with a series of imidazolium ionic liquids, and finally we were successfully able to prepare the ionic liquid assisted microemulsion with [C2mim][CnSO4] (n = 4, 6, and 8) ionic liquids. Here it is important to note that the long chain alkyl sulphates of the imidazolium containing ionic liquids provide higher level of biodegradability.61 The biocompatibility of this microemulsion can further be enhanced by tuning the properties of ionic liquid. The viscosity and phase behavior of the pseudoternary microemulsion solution have been investigated in detail along with the size of the droplets using [C2mim][C4SO4] as a polar core. Recently, much interest has been aroused to encapsulate a wide variety of biological macromolecules such as, proteins, DNA and biopolymers inside the core of the microemulsions.65,66 The structural and conformational dynamics of such biological macromolecules inside the core of the microemulsions seems interesting and therefore can be effectively monitored by the fluorescence resonance energy transfer (FRET) process. Over the past few decades, FRET67−70 has been extensively used as a powerful optical tool for the determination of the distances between the donor and acceptor labeled at the two specific sites of a biological macromolecule and thus enabling us to provide an insight into their structures and dynamics.71,72 However, as per Förster theory, FRET is restricted to measure the distances up to the D−A separation of 80 Å only. Fleming and co-workers73−76 have suggested that in aggregated systems where multiple donors and acceptors are confined at a very short distance of separation the Förster theory in its standard form is found to be invalid. They have also put an effort to develop a new framework using a generalization of Fö rster theory for calculating the energy transfer rate in such systems. This is applicable to many of the light harvesting supramolecular systems in artificial photosynthesis where multistep FRET mechanisms are operative. Recently, major advances have occurred in understanding of the long-range conformational dynamics of the complex biological macromolecules with the development of quantum dot based FRET77 and nanometal based surface energy transfer (NSET) where the rate of energy transfer is found to be deviated from R−6 distance dependency.78,79 While substantial efforts have been made in the study of FRET inside various biologically resembled organized supramolecular assemblies including micelles, reverse micelles, microemulsions, and vesicles, so far, there is no study of FRET in a Tween surfactant-based IL/O microemulsion in spite of its widespread applications in the pharmaceutical industries. In the present work we have first characterized an

liquid-in-oil (IL/O) microemulsions. These ionic liquid containing microemulsions have recently been extensively used as a nanoreactor to perform various chemical reactions with modified solubility, increased thermal stability, and effectiveness of the reactants.37,38 Very recently, our group have designed a new strategy to develop a number of ionic liquid-in-oil microemulsions for studying various photophysical and dynamical phenomena occurring inside the core of the microemulsions.39,40 Several other groups have explored the internal structure and dynamics of the ionic liquid-based microemulsions by means of various photophysical and dynamical techniques.33,41−43 The recent advances in the growing interests of ionic liquid based nonaqueous microemulsions have been documented in a several number of review articles.44−49 The efforts have been further made to prepare nontoxic biocompatible ionic liquid-based microemulsions that can be used in the pharmaceutical industries as a drug delivery media with improved solubility of the drug molecules which are poorly soluble or insoluble in water and most of the organic solvents.50−52 However for that purpose the ingredients of the microemulsions should be nontoxic in nature. While a number of water-based biocompatible microemulsions53−55 are known in literature and extensively studied as a drug delivery media, the ionic liquid-based biocompatible microemulsions are limited and therefore hoped to be a growing interest in the near future. Goto and co-workers50−52 have recently demonstrated the utilization of the biocompatible ionic liquid-in-oil microemulsions for the enhanced solubilization and stabilization of the sparingly water-soluble drug molecules. So far, most of the studies on ionic liquid-in-oil (IL/O) microemulsions have been carried out using TX-100 as surfactant and cyclohexane, toluene, or benzene as continuous oil phase which are not pharmaceutically acceptable components.29−36,41,42 In the present work, therefore, effort has been given to characterize an ionic liquid containing microemulsion comprised of pharmaceutically acceptable components by using dynamic light scattering and fluorescence spectroscopic techniques. Room temperature ionic liquids (RTILs), molten salts at ambient temperature, are essentially termed as designer solvents, as their cationic and anionic constituents can be tuned according to the desired properties that enables the dissolution of many sparingly soluble substrates.56 These are often used as a green substitutes of volatile organic solvents owing to their unique physicochemical properties such as nonvolatility, noniflamability, high thermal stability, low vapor pressure, wide liquidous range, and wide electrochemical window which allows their use in a wide range of applications.57−60 However, their applications for the pharmaceutical purposes are limited because of the question of whether ionic liquids are sufficiently biodegradable and nontoxic or not.61 Considering the tunable nature of the ionic liquids, it is however possible to design biocompatible, nontoxic ionic liquids for pharmaceutical applications. Recently, several groups have synthesized biodegradable ionic liquids of nontoxic inorganic and organic constituents.62,63 Such biodegradable nontoxic ionic liquids can be used for the development of novel IL/O microemulsions. However, for potential pharmaceutical applications, the system has to be stable at various physiological environments. Several earlier studies have indicated that nonionic surfactant (Tween-80)based water in oil microemulsions are stable with unaltered 3222



Article

2.3. Spectroscopic Techniques. UV−vis absorption and steady-state fluorescence spectra were recorded on a Shimadzu (model number UV 2450) spectrophotometer and a Hitachi (model number F 7000) spectrofluorimeter, respectively. The time-resolved emission spectra were recorded using TCSPC picosecond spectrometer. The details of this experimental setup have been described in our previous publication.80 In brief, picosecond diode lasers at 375 and 408 nm (IBH, UK, Nanoled) were used as light sources, and the signal was detected in magic angle (54.7°) polarization using Hamamatsu MCP PMT (3809U). The typical instrument response function is ∼100 ps in our system. The decays were analyzed using IBH DAS-6 decay analysis software. Femtosecond fluorescence decays have been collected using fluorescence up-conversion technique (FOG 100, CDP, Russia). The details of this experimental set up has been given in an earlier report.81 In brief, the samples were excited using the second harmonic (405 nm) of a mode-locked Tisapphire laser (Tsunami, Spectra physics). The fundamental beam (800 nm) was frequency doubled in nonlinear crystal (1 mm BBO, θ = 25°, ϕ = 90°) and used for the excitation. The sample was placed inside a 1-mm-thick rotating quartz cell. The fluorescence emitted from the sample was up-converted in a nonlinear crystal (0.5 mm BBO, θ = 38°, ϕ = 90°) using the fundamental beam as a gate pulse. The up-converted light is dispersed in a monochromator and detected using photon counting electronics. The instrument response function of the apparatus is 300 fs. The longer lifetime components obtained from TCSPC were kept fixed to analyze the femtosecond decays. The fluorescence quantum yield of C480 in the IL containing microemulsion (R = 0.1) solution in absence of acceptor was determined using reported82 quantum yield 0.66 of C480 in water at 25 °C as a secondary standard. The following equation was used for calculation:

ionic liquid containing Tween-based [C2mim][C4SO4]/Tween80/Span-20/IPM microemulsions and then performed an excitation wavelength dependent FRET from C480 to R6G inside the nanodomain of this microemulsion droplets to understand their dynamic heterogeneity. By varying the excitation wavelength we have probed different region of the microemulsion droplets as the location of the probe molecules in such microheterogeneous system varies depending upon their chemical properties. Since FRET is strongly dependent on the distance between the molecular centers of the donor and acceptor, it is possible to get an idea about the structural heterogeneity of such a self-assembled organized system by monitoring the excitation wavelength dependency of FRET parameters.

2. EXPERIMENTAL SECTION 2.1. Materials. Tween-80 (polyoxyethylene sorbitan monooleate), Span-20 (sorbitan laurate), and isopropyl myristate (IPM) were purchased from Sigma-Aldrich and used as received. The room temperature ionic liquid, 1-ethyl-3methylimidazolium n-butylsulfate ([C2mim][C4SO4] obtained from Kanto Chemicals (98% purity) was also used as received. Laser grade coumarin 480 (C480) and rhodamine 6G (R6G) were purchased from Exciton. The chemical structures of all of the materials have been given in Scheme 1. Scheme 1. Chemical Structures of R6G, C480 Dyes, RTIL [C2mim][C4SO4], and Nonpolar Solvent Isopropyl Myristate

ΦS A n2 (Abs)R = S × × S2 ΦR AR (Abs)S nR

(1)

where Φ represents quantum yield, Abs represents absorbance, A represents area under the fluorescence curve, and n is the refractive index of the medium. The subscripts S and R denote the corresponding parameters for the sample and reference, respectively. 2.4. Viscosity Measurements. Viscosities of the RTIL and RTIL-co solvent binary mixtures were measured using Brookfield DV-II+ Pro (viscometer) at 25 °C temperature. We have also determined the change of viscosity of RTIL with the change in temperature. The temperature was maintained constant by circulating water through the sample holder using a JEIO TECH Thermostat (RW-0525GS). 2.5. Dynamic Light Scattering Measurements. Dynamic light scattering (DLS) measurements were carried out to determine the droplet size of the microemulsion using Malvern Nano ZS instrument employing a 4 mW He−Ne laser (λ = 632.8 nm) and equipped with a thermostatic sample chamber. 2.6. Calculation of FRET Parameters. According to the Förster theory the rate of fluorescence resonance energy transfer (kFRET) can be calculated using the following equation:83

2.2. Preparation of Ionic Liquid-in-Oil Microemulsions. To prepare [C2mim][C4SO4]/Tween-80/Span-20/IPM ionic liquid-in-oil microemulsions at a fixed concentration (30 wt %) of surfactant mixture, first an appropriate amount of the mixture (TW-80 and Span 20 in 2:1 weight ratio) was dissolved in isopropyl myristate. The ionic liquid, [C2mim][C4SO4], was then gradually added to the Tween-80/Span-20/IPM aggregated system to formulate microemulsion droplets of varying ionic liquid content (R) until the solution becomes turbid.

kFRET 3223

6 1 1 ⎛ R0 ⎞ = A = 0⎜ ⎟ τD ⎝ RDA ⎠ τrise

(2)



Article

where τArise is the rise time of the acceptor emission in the presence of donor and τ0D is the lifetime of the donor in the absence of acceptor. RDA represents the distance between the molecular centers of the donor and acceptor and at a distance R0 where the efficiency of energy transfer is assumed to be 50%. R0 is called Förster distance which is calculated from the following equation: R 0 = 0.211[κ 2n−4Q DJ(λ)]1/6

(3)

where n is the refractive index of the medium, QD is the quantum yield of the donor in the absence of acceptor, κ2 is the orientation factor, and J(λ) is the spectral overlap between the emission spectrum of the donor and the absorption spectrum of acceptor. J(λ) is related to the normalized fluorescence intensity of the donor in the absence of acceptor (FD(λ)) and the extinction coefficient of the acceptor (εA(λ)) as follows:

Figure 1. Pseudoternary phase diagram of the [C2mim][C4SO4]/ Tween-80/Span-20/IPM microemulsion system.

constant at 30 wt %. Figure 2A represents the variation of the intensity-size distribution profile with increasing R value, that is, with increasing ionic liquid content. As we can see that the size of the microemulsion droplets increases linearly from 9 to 21 nm as we increase the R value from 0 to 0.1. At this point it is important to note that the size of these Tween based IL/O microemulsions are smaller in size than that observed for TX100 surfactant based IL/O containing microemulsions where the size of the microemulsions are found to be above 20−30 nm.34 In the DLS measurements we observed a narrow intensity-size distribution histogram in each case of increasing R values with the observed polydispersity index (PDI) in between 0.04 and 0.3. This indicates the formation of microemulsions with almost homogeneous distribution. Moreover, the size of the microemulsion droplets varies linearly with increasing ionic liquid content (Figure 2B). This implies that the droplets are spherical in nature. Multiangle dynamic light scattering method can be applied to further verify the spherical shape of the droplets as reported in literature.84 The bulk viscosity of the microemulsion solution is also found to be increased with increasing ionic liquid content. As we increase the R value from 0 to 0.1, the viscosity of the medium increases from 21 mPa·s to 31 mPa·s. In recent literatures, FRET is not only being used to understand the conformational dynamics of real biological systems but also to probe the structural dynamics of many biologically relevant supramolecular assemblies such as, micelles, reverse micelles, microemulsions, and vesicles where there is a distribution of donor−acceptor (D−A) distance instead of a single D−A distance.85−89 In spite of the complexity, such calculations of multiple D−A distances have considerable potential for the studies of multiple fluorophore organization in the self-assembled organized systems. In the present work the structural heterogeneity of the [C2mim][C4SO4]/Tween-80/Span-20/IPM microemulsion system has been assessed by monitoring the FRET at different excitation wavelengths. Before going to the details about the FRET parameters, we first want to shed light on the microenvironment of the [C2mim][C4SO4]/Tween-80/Span-20/IPM microheterogeneous systems as it is very necessary to establish the binding and location of the donor and acceptor inside the microemulsion droplets. Steady-state UV−vis absorption and fluorescence spectra of C480 and R6G were performed in the microemulsion systems and compared the results with that in simple oil phase (IPM) to get an idea about the binding and location of the probe molecules. The efficient loading of the probe molecules into the microemulsion droplets can be

∞

J (λ ) =

∫0 FD(λ)εA (λ)λ 4 dλ ∞

∫0 FD(λ)dλ

(4)

In eq 3 the values of κ always remain within the range 0−4. The zero value of κ2 indicates the mutually perpendicular orientation of the transition dipoles which further implies the forbidden of FRET process. However, in the present system as we observe the involvement of FRET, the value of κ2 must be greater than 0. In general the value of κ2 is taken as 2/3 for the random orientation of transition dipoles. Moreover, the calculated Förster distance is found to vary slightly in the whole range of values of κ2. Therefore in the present system we have used κ2 = 2/3 for the calculation of Förster distance. 2

3. RESULTS AND DISCUSSION As a preliminary part of the characterization of a pseudoternary microemulsion, the construction of its phase diagram is necessary to know the maximum solubility of the ionic liquid, [C2mim][C4SO4] in Tween-80/Span-20/IPM aggregated systems at different concentrations (weight fraction) of the surfactant mixture (Tween-80 and Span-20 in the weight ratio of 2:1). For that purpose we have prepared a series of solutions by varying the total surfactant concentrations ranging from 10 to 80 wt % in IPM. Now the maximum solubility of the ionic liquid was checked in each solution with the gradual addition of [C2mim][C4SO4] until the clear transparent solution becomes turbid. After each addition the solution was stirred well with vigorous shaking. The onset of turbidity of the solution was detected through the naked eye and then confirmed by UV−vis spectrophotometer. Figure 1 represents the pseudoternary phase diagram of [C2mim][C4SO4]/Tween-80/Span-20/IPM microemulsion system at 25 °C. The reason of choosing the mixture of two surfactants (Tween-80:Span-20 = 2:1) instead of using only one surfactant is to improve the solubility of ionic liquid so that we can increase the single phase region.50 The single phase region is found to be varied with the change in TW-80/Span-20 weight ratios in consistent with the earlier reports by Goto and co-workers.50−52 It is noteworthy to mention that the phase behavior of this microemulsion system does not change significantly with increasing alkyl chain length of the anionic constituent of the ionic liquid. The intensity size distribution of the microemulsion droplets were measured using dynamic light scattering (DLS) experiment. All measurements were carried out at 25 °C. The total surfactant concentration of the microemulsion system was kept 3224



Article

Figure 2. (A) Dynamic light scattering (DLS) intensity vs size distribution histogram of RTIL microemulsion. (B) Variation of diameter of droplets of the [C2mim][C4SO4]/Tween-80/Span-20/IPM microemulsion with RTIL content in terms of R value.

understood from the significant red shift in the absorption and emission maxima of the probe molecules from the nonpolar solvent IPM to the microemulsion system as shown in Figure 3. The emission spectra of C480 in IPM and in [C2mim][C4SO4]/Tween-80/Span-20/IPM microemulsion of varying R values have been shown in Figure 3b. In IPM, C480 exhibits an absorption maximum at 366 nm which is significantly redshifted ∼16 nm upon encapsulation into the microemulsion. This clearly indicates that C480 probe molecules partitioned

into the microemulsion phase. Moreover the acceptor molecule R6G is also found to be efficiently loaded into the microemulsion system as it is insoluble in the nonpolar solvent IPM. R6G being a cationic probe, the location of the probe molecule is expected to be in the ionic liquid core of the microemulsion. This can be supported from the fact that in RTIL containing microemulsion the emission maximum of R6G at 570 nm is excitation wavelength independent which implies almost uniform microenvironment surrounding the probe molecules. However, with the change in excitation wavelength from 375 to 408 nm the emission maxima of C480 are found to be redshifted ∼10 nm from 450 to 460 nm. Therefore it is possible for the donor molecules to be distributed in both polar and nonpolar phases. The emission spectra of C480 at different excitation wavelengths have been given in Figure 4. Excitation at 375 nm results in the preferential excitation of the probe molecules present in the relatively nonpolar phase, while red end excitation at ∼408 nm results the preferential excitation of the probe molecules present in the polar core of the microemulsion. The present microemulsion system therefore exhibits a greater extent of heterogeneity with respect to the

Figure 3. (a) Absorption spectra of C480 in IPM and in microemulsion (ME) (R = 0.1). (b) Steady-state fluorescence emission spectra of C480 in IPM (λex = 375 nm) and in microemulsion of varying R values (λex = 408 nm).

Figure 4. Overlap between the steady-state fluorescence emission spectra of C480 at different excitation wavelength 375 nm (−■−) and 408 nm (−▲−) with the absorption spectra of R6G (−) in microemulsion (R = 0.1). 3225



Article

Figure 5. Fluorescence emission spectra of (i) donor C480 (25 μM), (ii) C480 (25 μM) + R6G (40 μM), and (iii) acceptor R6G (40 μM) in RTIL microemulsion (ME) at the excitation wavelength of (a) 375 nm and (b) 408 nm.

extent of increase in the efficiency of FRET with an increase in the excitation wavelength can be accounted for by the dominated fluorescence intensity of the non-FRET donor. This is an obvious fact as there are only a few microemulsion droplets in the system which carries both the donor and the acceptor together.42 In the time-resolved measurements the efficiency of energy transfer processes are commonly monitored by the decrease in fluorescence lifetime of the donor in the presence of an acceptor. However in this case we did not observe any change in the time-resolved decay profiles of the donor in the absence and presence of acceptor in our picoseconds experimental setup (Figure 6 and Table 2). This observation is however not surprising in the study of FRET from different coumarin donors to R6G acceptor in such microheterogeneous systems.42,86−88 It has been suggested that, as the donor emission is dominated by the unquenched non-FRET donor resulting from the low possibility of the coexistence of both donor and acceptor in the same microemulsion droplet, no shortening of donor lifetime is observed. In such cases the convenient way to directly look into the timescale of occurring FRET is by monitoring the decay of acceptor in the absence and presence of donor. However, we have to be careful about the contribution of the donor and direct excitation of the acceptor while taking into account the change in the decay profile of the acceptor which arises only from the FRET processes.83 In that case monitoring the rise time of the acceptor emission which is only observed in the presence of donor is the right choice to determine the rate of fluorescence resonance energy transfer.83 Figure 7 represents the overlay of the emission spectra of R6G obtained in the absence and presence of donor at different excitation wavelengths. The appearance of rise time in the acceptor emission in presence of donor is evident from this figure. The time-resolved decays have been collected at the emission wavelength of 575 nm where the contribution of emission from the quenched donors is negligible. However to be confirmed about the fact that the rise time does not originate from the contribution of donor emission we have collected the emission decay of only C480 in the ME with the absence of acceptor at this wavelength (575 nm), and surprisingly we found no rise component in the decay (Figure 8). In the absence of donor (C480), R6G exhibits single exponential decay having the time constant of ∼4.40 ns with no rise component and also invariant with the change in excitation wavelength. This observation is consistent with that expected from the steady-state measurements. However a

reported [pmim][BF4]/TX-100/benzene microemulsion where the probe molecule, C480, exhibits red edge excitation shift (REES) of ∼6 nm upon changing the excitation wavelength from 375 to 435 nm.42 Now, the occurrence of effective FRET from the donor C480 to the acceptor R6G at both the excitation wavelength of 375 and 408 nm is evident from the decrease in the steady state fluorescence intensity of the donor in presence of acceptor and subsequent increase in the fluorescence intensity of the acceptor in presence of donor (Figure 5). FRET is strongly dependent on the distance between the molecular centers of the donor and acceptor. The greater the distance between donor and acceptor, the lesser is the efficiency of FRET. As a matter of fact the efficiency of FRET is found to be excitation wavelength dependent. Experimentally it has been observed that upon excitation at 375 nm the steady-state emission intensity of C480 is reduced by 28%, whereas excitation at 408 nm leads to the reduction of fluorescence intensity by ∼34%. This reduction of fluorescence intensity is ascribed as the steady-state efficiency of FRET which is calculated from the equation FDA (5) F The variation of the efficiency of FRET with the change in excitation wavelength is clearly observed from Table 1. The E=1−

Table 1. Energy Transfer Parameters for the C480−R6G Pair in the Microemulsion System system C480 in ME (R = 0.1) C480 in ME (R = 0.1) a

λex (nm)

λmax em of C480 (nm)

ΦD

J(λ)a (M−1 cm−1 nm4)

R0a (Å)

εsa (%)

375

450

0.72

8.85 × 1014

45.58

28

408

460

0.72

1.16 × 1015

47.68

34

Experimental error of ±10%.

comparatively higher efficiency of FRET at the excitation wavelength of 408 nm is an outcome of the higher degree of spectral overlap between the emission spectra of donor and the absorption spectrum of the acceptor at this wavelength as shown in Figure 4. Since the emission spectra of donor, C480, exhibit significant red edge excitation shift (REES), the overlap integral (J(λ)) is found to be increased ∼31% as the excitation wavelength increases from 375 to 408 nm. However, the lesser 3226



Article

Figure 6. Picosecond decays of the donor C480 (25 μM) with (i) 0 (μM) and (ii) 40 (μM) acceptor R6G in RTIL microemulsion at (a) λex = 375 nm, λem = 450 nm, and (b) λex = 408 nm, λem = 460 nm.

Table 2. Picosecond Decay Parameters of C480 (Donor) Emission in the Presence and Absence of R6G (Acceptor) system

λex

λmax em

τ1/ns (a1)

τ2/ns (a2)

⟨τav⟩a/ns

C480 (25 μM) in ME (R = 0.1) C480 (25 μM) + R6G (40 μM) in ME (R = 0.1) C480 (25 μM) in ME (R = 0.1) C480 (25 μM) + R6G (40 μM) in ME (R = 0.1)

375

450

2.16 (0.32)

4.08 (0.68)

3.47

375

450

2.16 (0.32)

4.08 (0.68)

3.47

408

460

1.92 (0.28)

4.43 (0.72)

3.73

408

460

1.86 (0.25)

4.35 (0.75)

3.73

a

Experimental error of ±5%. Figure 8. Time-resolved fluorescence decay of only C480 (25 μM) in the microemulsion showing the absence of any rise component in the decay profile while being monitored at the emission wavelength of 575 nm.

distinct rise component which is the measure of FRET from C480 to R6G is clearly observed in the emission decays of R6G with the addition of donor (Figure 7). The time constants obtained from the exponential fitting of the decays have been given in Table 3. The observed time constants of the rise components ∼240 ps and ∼3300 ps are attributed to the FRET which is occurring from C480 probe molecules located at the different regions of the microemulsion droplets to R6G. The greater the magnitude of the time constant of rise component, the longer is the distance between donor and acceptor. Since the conventional surfactants forming micelles are smaller in size compared to the microemulsions, the FRET process is expected to be relatively fast in the micellar systems than microemulsions. Bhattacharyya et al.86−88 extensively studied the FRET behavior of different coumarin-

R6G pairs in micelles and reverse micelles by monitoring the rise time of the acceptor emission. They have shown that the time scales of FRET in bigger micelles of a pluronic triblock copolymer (P123) are slower compared to that in the smaller micelles of sodium dodecyl sulfate (SDS). In SDS micelles the FRET process is so fast that in the picosecond setup they did not observe any rise time of the acceptor (R6G) emission in the presence of donor (coumarin 153). In the present work we have shown multiple time scale of FRET resulting from the different distribution of the donors with respect to the acceptor which is present in the ionic liquid pool of the microemulsion

Figure 7. Picosecond time-resolved fluorescence decays of the acceptor R6G (40 μM) with (i) 0 μM C480 and (ii) 25 μM C480 in RTIL microemulsion at the excitation wavelength of (a) 375 nm and (b) 408 nm. λem = 575 nm. 3227



Article

Table 3. Picosecond Decay Parameter of R6G (40 μM, λem = 575 nm) in the Presence and Absence of C480 (25 μM) system

λex (nm)

Absence of Donor C480 375 575 408 575 Presence of Donor C480 375 575 408 575

R6G (40 μM) in ME (R = 0.1)

R6G (40 μM) + C480 (25 μM) in ME (R = 0.1) a

λem (nm)

τ1/psa (a1)

τ2/psa (a2)

τ3/psa (a3) 4500 (1.00) 4500 (1.00)

240 (−0.02) 240 (−0.13)

3300 (−2.53) 3300 (−1.11)

5450(3.55) 5450 (2.24)

Experimental error of ±5%

decays we have detected an ultrafast component of ∼3.5 ps (Table 4) which is responsible for the ultrafast FRET that occurs inside the polar core of the microemulsion droplets. However, the relative contribution of this ultrafast component is very less suggesting that very few number of donor molecules are present in the core of the microemulsions. The donor−acceptor distances calculated from the rise time of the acceptor as discussed in the Experimental Section have been given in Table 5. All of these calculated distances are

droplets. Multiple time scale of FRET in this microemulsion originates from the significant heterogeneity of the system. In the Tween-80/Span-20/IPM aggregated systems without ionic liquids, the donor molecules are distributed in both the polar and the nonpolar regions. In this case the donors and acceptors are separated by surfactant chains resulting in a FRET of multiple picosecond time scales. As the ionic liquid content increases the probe molecules are shifted to the polar core of the microemulsion which results an increase in contribution of the faster time scale of FRET. This can be understood from the fact that the contribution of the fast component of ∼240 ps increases significantly as we increase the excitation wavelength from 375 nm to 408 nm (see Table 3). This is basically due to the increased contribution of the donor present near the polar ionic liquid core where the acceptor molecules are actually located resulting in a shorter donor−acceptor distance at the excitation wavelength of 408 nm. In the ionic liquid pool of the microemulsions the FRET becomes very fast because of the presence of both the donor and acceptor at the close proximity to each other. However from the picosecond time-resolved decay analysis, we were unable to detect the ultrafast rise component of the acceptor which arises from the very fast FRET process occurring from the donor to the acceptor of having very small distance of separation. The ultrafast component of FRET was, therefore, detected using a femtosecond up-conversion setup. The ultrafast decay curves of the acceptor (R6G) recorded at the excitation and emission wavelength of 408 and 575 nm, respectively, in the presence and absence of donor have been shown in Figure 9. From the satisfied fitting of the

Table 5. Donor−Acceptor Distances in the RTIL Microemulsion, [C2mim][C4SO4]/TW80-Span20 (2:1)/ IPM, Calculated from the Rise of Acceptor Emission system [C2mim][C4SO4]/Tween-80/Span-20/ IPM microemulsion

λex (nm) 375 408

a

τFRET (ps) 3.5, 240, 3300 3.5, 240, 3300

RDAa (Å) 14, 29, 45 15, 30, 47

Experimental error of ±10%.

significantly smaller than the size of the microemulsion suggesting the occurrence of FRET from the donor to the acceptor present within the same microemulsion droplet. As the ionic acceptors reside in the polar core of the microemulsion with more or less uniform microenvironment, the distances signify the distribution of the donor molecules in the different regions of the microemulsion. The smallest distance ∼15 Å is attributed to the energy transfer occurring from the donor C480 to the acceptor R6G, while both the molecules are present inside the pool of the microemulsion. The much longer D−A distances ∼45 Å correspond to the distance between C480 located in the relatively less polar microenvironment probably at the outer periphery of the microemulsion and the R6G present in the polar core of the microemulsion.

4. CONCLUSION In summary we have successfully demonstrated the formulation of an ionic liquid containing Tween-based nonaqueous microemulsion with biologically acceptable components and investigated FRET from a molecular donor to the molecular acceptor embedded into the same microemulsions system. To date, there is no extensive experimental study on FRET in such

Figure 9. Femtosecond transients (λex = 408 nm, λem = 575 nm) of R6G (40 μM) in the presence of (a) 0 μM C480 and (b) 25 μM C480 in RTIL microemulsion.

Table 4. Femtosecond Decay Parameters of Acceptor R6G (40 μM) Emission in the Presence and Absence of C480 (25 μM). λex = 408 nm

a

system

λex (nm)

λem (nm)

τ1/psa (a1)

τ2/psb (a2)

τ3/psb (a3)

τ4/psb (a4)

R6G (40 μM) in ME (R = 0.1) R6G (40 μM) in ME (R = 0.1) + C480 (25 μM)

408 408

575 575

70 (0.12) 3.5 (−0.04)

240 (−0.16)

3300 (−0.22)

4500 (0.88) 5450 (1.42)

Experimental error of ±10%. bExperimental error of ±5%. 3228



Article

(7) Destree, C.; Ghijsen, J.; Nagy, J. B. Preparation of Organic Nanoparticles using Microemulsions: Their Potential Use in Transdermal Delivery. Langmuir 2007, 23, 1965−1973. (8) Zhang, S.; Zhao, Y. Facile Preparation of Organic Nanoparticles by Interfacial Cross-Linking of Reverse Micelles and Template Synthesis of Subnanometer Au−Pt Nanoparticles. ACS Nano 2011, 5, 2637−2646. (9) Mazzola, P. G.; Lopes, A. M.; Hasmann, F. A.; Jozala, A. F.; Penna, T. C. V.; Magalhaes, P. O.; Rangel-Yagui, C. O.; Pessoa, A. Liquid−liquid Extraction of Biomolecules: An Overview and Update of the Main Techniques. J. Chem. Technol. Biotechnol. 2008, 83, 143− 157. (10) Trivedi, R.; Kompella, U. B. Nanomicellar Formulations for Sustained Drug Delivery: Strategies and Underlying Principles. Nanomedicine 2010, 5, 485−505. (11) Lawrencea, M. J.; Rees, G. D. Microemulsion-based Media as Novel Drug Delivery Systems. Adv. Drug Delivery Rev. 2000, 45, 89− 121. (12) Cabaleiro-Lago, C.; García-Río, L.; Hervella, P. The Effect of Changing the Microstructure of a Microemulsion on Chemical Reactivity. Langmuir 2007, 23, 9586−9595. (13) Sharma, S.; Pal, N.; Chowdhury, P. K.; Sen, S.; Ganguli, A. K. Understanding Growth Kinetics of Nanorods in Microemulsion: A Combined Fluorescence Correlation Spectroscopy, Dynamic Light Scattering, and Electron Microscopy Study. J. Am. Chem. Soc. 2012, 134, 19677−19684. (14) Fathi, H.; Kelly, J. P.; Vasquez, V. R.; Graeve, O. A. Ionic Concentration Effects on Reverse Micelle Size and Stability: Implications for the Synthesis of Nanoparticles. Langmuir 2012, 28, 9267−9274. (15) Podlogar, F.; Gašperlin, M.; Tomšič, M.; Jamnik, A.; Rogač, M. B. Structural Characterization of Water-Tween 40/Imwitor 308Isopropyl Myristate Microemulsions using Different Experimental Methods. Int. J. Pharm. 2004, 276, 115−128. (16) Trotta, M. Influence of Phase Transformation on Indomeihacin Release from Microemulsions. J. Controlled Release 1999, 60, 399−405. (17) Paul, B. K.; Moulik, S. P. Microemulsions: An Overview. J. Dispersion Sci. Technol. 1997, 18, 301−367. (18) Micellar Solutions and Microemulsions: Structure, Dynamics and Statistical Thermodynamics; Chen, S. H., Rajagopalan, R., Eds.; Springer-Verlag: New York, 1990. (19) Fayer, M. D.; Levinger, N. E. Analysis of Water in Confined Geometries and at Interfaces. Annu. Rev. Anal. Chem. 2010, 3, 89−107. (20) Pieniazek, P. A.; Lin, Y.-S.; Chowdhary, J.; Ladanyi, B. M.; Skinner, J. L. Vibrational Spectroscopy and Dynamics of Water Confined inside Reverse Micelles. J. Phys. Chem. B 2009, 113, 15017− 15028. (21) Bhattacharyya, K. Solvation Dynamics and Proton Transfer in Supramolecular Assemblies. Acc. Chem. Res. 2003, 36, 95−101. (22) Pal, N.; Verma, S. D.; Singh, M. K.; Sen, S. Fluorescence Correlation Spectroscopy: An Efficient Tool for Measuring Size, SizeDistribution and Polydispersity of Microemulsion Droplets in Solution. Anal. Chem. 2011, 83, 7736−7744. (23) Ray, S.; Moulik, S. P. Dynamics and Thermodynamics of Aerosol OT-aided Nonaqueous Microemulsions. Langmuir 1994, 10, 2511−2515. (24) Friberg, E.; Podzimek, M. A Non-aqueous Microemulsion. Colloid Polym. Sci. 1984, 262, 252−253. (25) Das, K. P.; Ceglie, C.; Lindman, B. Microstructure of Formamide Microemulsions from NMR Self-diffusion Measurements. J. Phys. Chem. 1987, 91, 2938−2946. (26) Shirota, H.; Horie, K. Solvation Dynamics in Nonaqueous Reverse Micelles. J. Phys. Chem. B 1999, 103, 1437−1443. (27) Correa, N. M.; Pires, P. A. R.; Silber, J. J. A.; El Seoud, O. Real Structure of Formamide Entrapped by AOT Nonaqueous Reverse Micelles: FT-IR and 1H NMR Studies. J. Phys. Chem. B 2005, 109, 21209−21219. (28) Setua, P.; Chakraborty, A.; Seth, D.; Bhatta, M. U.; Satyam, P. V.; Sarkar, N. Synthesis, Optical Properties, and Surface Enhanced

a nonaqueous IL/O microemulsion system which can be used as a drug delivery system for many sparingly water-soluble drug molecules. The size and shape of the microemulsion droplets have been characterized by using dynamic light scattering measurements. The present study indicates that the [C2mim][C4SO4]/Tween-80/Span-20/IPM microemulsion droplets are spherical in nature as their size varies linearly with the increasing ionic liquid content. Moreover a narrow intensity size distribution histogram is observed in each R value indicating almost uniform distribution of the droplets. Excitation wavelength dependent FRET study from C480 to R6G allows us to probe different regions of such a Tween based ionic liquid containing microemulsion. The observation of multiple time scale of FRET can be explained from the different distribution of the donor molecules inside the different regions of the microemulsion as confirmed from the significant red edge excitation shift (REES) of the emission maxima. The temperature and the presence of a small amount of water are known to affect the size of the microemulsion droplets for a number of ionic liquid containing microemulsions. Since the efficiency of FRET is strongly dependent upon the D−A distances, further studies of FRET showing the effect of temperature and water in this novel microemulsion will be interesting and are therefore currently being pursued in our laboratory.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 91-3222-255303. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS N.S. is thankful to Council of Scientific and Industrial Research (CSIR), Government of India, for generous research grants. S.M. and S.G. are thankful to CSIR for research fellowship. C.B. and J.K. are thankful to UGC for research fellowship. We acknowledge the instrumental facility of National Centre for Ultrafast Processes (NCUFP), Chennai, India, for femtosecond fluorescence up-conversion experiments. We are also thankful to Dr. C. Selvaraju, Mr. T. Senthil Kumar, and Mr. R. Suresh for their cooperation in recording femtosecond fluorescence lifetime decays.

■

REFERENCES

(1) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Water Core within Perfluoropolyether-Based Microemulsions Formed in Supercritical Carbon Dioxide. Science 1996, 271, 624−626. (2) Eastoe, J.; Gold, S.; Rogers, S.; Wyatt, P.; Steytler, D. C.; Gurgel, A.; Heenan, R. K.; Fan, X.; Beckman, E. J.; Enick, R. M. Designed CO2-Philes Stabilize Water-in-Carbon Dioxide Microemulsions. Angew. Chem., Int. Ed. 2006, 45, 3675−3677. (3) Liu, Y.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Switchable Surfactants. Science 2006, 313, 958−960. (4) Holmberg, K. Organic Reactions in Microemulsions. Eur. J. Org. Chem. 2007, 5, 731−742. (5) Pileni, M. P. The Role of Soft Colloidal Templates in Controlling the Size and Shape of Inorganic Nanocrystals. Nat. Mater. 2003, 2, 145−150. (6) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, Ch. J. Recent Advances in the Liquid-phase Syntheses of Inorganic Nanoparticles. Chem. Rev. 2004, 104, 3893−3946. 3229



Article

Raman Scattering of Silver Nanoparticles in Nonaqueous Methanol Reverse Micelles. J. Phys. Chem. C 2007, 111, 3901−3907. (29) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R.; Yan, D. D. Microemulsions with Ionic Liquid Polar Domains. Phys. Chem. Chem. Phys. 2004, 6, 2914−2916. (30) Eastoe, S.; Gold, S. E.; Rogers, A.; Paul, T.; Welton, R. K.; Heenan, I. G. Ionic Liquid-in-Oil Microemulsions. J. Am. Chem. Soc. 2005, 127, 7302−7303. (31) Li, N.; Gao, Y. A.; Zheng, L. Q.; Zhang, J.; Yu, L.; Li, X. W. Studies on the Micropolarities of bmimBF4/TX-100/Toluene Ionic Liquid Microemulsions and Their Behaviors Characterized by UV− Visible Spectroscopy. Langmuir 2007, 23, 1091−1097. (32) Gao, Y. A.; Han, S. B.; Han, B. X.; Li, G. Z.; Shen, D.; Li, Z. H. TX-100/Water/1-Butyl-3-methylimidazolium Hexafluorophosphate Microemulsions. Langmuir 2005, 21, 5681−5684. (33) Falcone, R. D.; Correa, N. M.; Silber, J. J. On the Formation of New Reverse Micelles: A Comparative Study of Benzene/Surfactants/ Ionic Liquids Systems Using UV−Visible Absorption Spectroscopy and Dynamic Light Scattering. Langmuir 2009, 25, 10426−10429. (34) Pramanik, R.; Rao, V. G.; Sarkar, S.; Ghatak, C.; Setua, P.; Sarkar, N. Ionic Liquid Containing Microemulsions: Probe by Conductance, Dynamic Light Scattering, Diffusion-Ordered Spectroscopy NMR Measurements, and Study of Solvent Relaxation Dynamics. J. Phys. Chem. B 2011, 115, 2322−2330. (35) Gao, Y.; Li, N.; Hilfert, L.; Zhang, S.; Zheng, L.; Yu, L. Temperature-Induced Microstructural Changes in Ionic Liquid-Based Microemulsions. Langmuir 2009, 25, 1360−1365. (36) Gao, Y. A.; Li, N.; Zheng, L. Q.; Bai, X. T.; Yu, L.; Zhao, X. Y. Role of Solubilized Water in the Reverse Ionic Liquid Microemulsion of 1-Butyl-3-methylimidazolium Tetrafluoroborate/TX-100/Benzene. J. Phys. Chem. B 2007, 111, 2506−2513. (37) Gayet, F.; El Kalamouni, C.; Lavedan, P.; Marty, J. D.; Brûlet, A.; Lauth-de Viguerie, N. Ionic Liquid/Oil Microemulsions as Chemical Nanoreactors. Langmuir 2009, 25, 9741−9750. (38) Chen, Z.; Yan, F.; Qiu, L.; Lu, J.; Zhou, Y.; Chen, J.; Tang, Y.; Texter, J. Sustainable Polymerizations in Recoverable Microemulsions. Langmuir 2010, 26, 3803−3806. (39) Rao, V. G.; Ghosh, S.; Ghatak, C.; Mandal, S.; Brahmachari, U.; Sarkar, N. Designing a New Strategy for the Formation of IL-in-Oil Microemulsions. J. Phys. Chem. B 2012, 116, 2850−2855. (40) Rao, V. G.; Mandal, S.; Ghosh, S.; Banerjee, C.; Sarkar, N. Ionic Liquid-in-Oil Microemulsions Composed of Double Chain Surface Active Ionic Liquid as a Surfactant: Temperature Dependent Solvent and Rotational Relaxation Dynamics of Coumarin-153 in [Py][TF2N]/[C4mim][AOT]/Benzene Microemulsions. J. Phys. Chem. B 2012, 116, 8210−8221. (41) Sasmal, D. K.; Mojumdar, S. S.; Adhikari, A.; Bhattacharyya, K. Deuterium Isotope Effect on Femtosecond Solvation Dynamics in an Ionic Liquid Microemulsion: An Excitation Wavelength Dependence Study. J. Phys. Chem. B 2010, 114, 4565−4571. (42) Adhikari, A.; Das, D. K.; Sasmal, D. K.; Bhattacharyya, K. Ultrafast FRET in a Room Temperature Ionic Liquid Microemulsion: A Femtosecond Excitation Wavelength Dependence Study. J. Phys. Chem. A 2009, 113, 3737−3743. (43) Porada, J. H.; Mansueto, M.; Laschat, S.; Stubenrauch, C. Microemulsions with Novel Hydrophobic Ionic Liquids. Soft Matter 2011, 7, 6805−6810. (44) Eastoe, J.; Hollamby, M. J.; Hudson, L. Recent advances in nanoparticle synthesis with reversed micelles. Adv. Colloid Interface Sci. 2006, 128, 5−15. (45) Greaves, T. L.; Drummond, C. J. Ionic liquids as Amphiphile Self-assembly Media. Chem. Soc. Rev. 2008, 37, 1709−1726. (46) Qiu, Z.; Texter, J. Ionic Liquids in Microemulsions. Curr. Opin. Colloid Interface Sci. 2008, 13, 252−262. (47) Hao, J.; Zemb, T. Self-assembled Structures and Chemical Reactions in Room-Temperature Ionic Liquids. Curr. Opin. Colloid Interface Sci. 2007, 12, 129−137. (48) Acharya, D. P.; Hartley, P. G. Progress in Microemulsion Characterization. Curr. Opin. Colloid Interface Sci. 2012, 17, 274−280.

(49) Zech, O.; Thomaier, S.; Kolodziejski, A.; Touraud, D.; Grillo, I.; Kunz, W. Ionic Liquids in MicroemulsionsA Concept to Extend the Conventional Thermal Stability Range of Microemulsions. Chem. Eur. J. 2010, 16, 783−786. (50) Moniruzzaman, M.; Tahara, Y.; Tamura, M.; Kamiya, N.; Goto, M. Ionic liquid-assisted Transdermal Delivery of Sparingly Soluble Drugs. Chem. Commun. 2010, 46, 1452−1454. (51) Moniruzzaman, M.; Kamiya, N.; Goto, M. Ionic Liquid-based Microemulsion with Pharmaceutically Accepted Components: Formulation and Potential Applications. J. Colloid Interface Sci. 2010, 352, 136−142. (52) Moniruzzaman, M.; Tamuraa, M.; Taharaa, Y.; Kamiyaa, N.; Goto, M. Ionic liquid-in-Oil Microemulsion as a Potential Carrier of Sparingly Soluble Drug: Characterization and Cytotoxicity Evaluation. Int. J. Pharm. 2010, 400, 243−250. (53) Saha, R.; Rakshit, S.; Mitra, R. K.; Pal, S. K. Microstructure, Morphology, and Ultrafast Dynamics of a Novel Edible Microemulsion. Langmuir 2012, 28, 8309−8317. (54) Kogan, A.; Gati, N. Microemulsions as Transdermal Drug Delivery Vehicles. Adv. Colloid Interface Sci. 2006, 123, 369−385. (55) Papadimitriou, V.; Pispas, S.; Syriou, S.; Pournara, A.; Zournpanioti, M.; Sotiroudis, T. G.; Xenakis, A. Biocompatible Microemulsions Based on Limonene: Formulation, Structure, and Applications. Langmuir 2008, 24, 3380−3386. (56) Hallett, J. P.; Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chem. Rev. 2011, 111, 3508− 3576. (57) Ranke, J.; Stolte, S.; Stormann, R.; Arning, J.; Jastorff, B. Design of Sustainable Chemical Products−The Example of Ionic Liquids. Chem. Rev. 2007, 107, 2183−2206. (58) El-Abedin, S. Z.; Endres, F. Ionic Liquids: The Link to HighTemperature Molten Salts? Acc. Chem. Res. 2007, 40, 1106−1113. (59) Dupont, J. From Molten Salts to Ionic Liquids: A “Nano” Journey. Acc. Chem. Res. 2011, 44, 1223−1231. (60) Rogers, R. D.; Seddon, K. R. Ionic Liquids–Solvents of the Future? Science 2003, 302, 792−793. (61) Garcia, M. T.; Gathergood, N.; Scammells, P. J. Biodegradable ionic liquids Part II. Effect of the anion and toxicology. Green Chem. 2005, 7, 9−14. (62) Pernak, J.; Chwala, P.; Syguda, A. Roomtemperature Ionic LiquidsNew Choline Derivatives. Pol. J. Chem. 2004, 78, 539−546. (63) Weaver, K. D.; Kim, H. J.; Su, J.; MacFarlane, D. R.; Elliot, G. D. Cyto-toxicity and Biocompatibility of a Family of Choline Phosphate Ionic Liquids Designed for Pharmaceutical Applications. Green Chem. 2010, 12, 507−513. (64) Latała, A.; Nędzi, M.; Stepnowski, P. Toxicity of imidazolium ionic liquids towards algae. Influence of salinity variations. Green Chem. 2010, 12, 60−64. (65) Van Horn, W. D.; Ogilvie, M. E.; Flynn, P. F. Reverse Micelle Encapsulation as a Model for Intracellular Crowding. J. Am. Chem. Soc. 2009, 131, 8030−8039. (66) Zhou, J.; Wei, C.; Jia, G.; Wang, X.; Feng, Z.; Li, C. Formation and Stabilization of G-Quadruplex in Nanosized Water Pools. Chem. Commun. 2010, 46, 1700−1702. (67) Stryer, L.; Haugland, R. P. Energy Transfer: A Spectroscopic Ruler. Proc. Natl. Acad. Sci. U.S.A. 1967, 58, 719−725. (68) Chen, J.; Zeng, F.; Wu, S.; Zhao, J.; Chen, Q.; Tong, Z. Reversible Fluorescence Modulation through Energy Transfer with ABC Triblock Copolymer Micelles as Scaffolds. Chem. Commun. 2008, 5580−5582. (69) Yoo, S. I.; Bae, S. H.; Kim, K.; Sohn, B. Nanostructures of Diblock Copolymer Micelles for Controlled Fluorescence Resonance Energy Transfer. Soft Matter 2009, 5, 2990−2996. (70) Saini, S.; Srinivas, G.; Bagchi, B. Distance and Orientation Dependence of Excitation Energy Transfer: From Molecular Systems to Metal Nanoparticles. J. Phys. Chem. B 2009, 113, 1817−1832. (71) Wong, K. F.; Bagchi, B.; Roosky, P. J. Distance and Orientation Dependence of Excitation Transfer Rates in Conjugated Systems: Beyond the Förster Theory. J. Phys. Chem. A 2004, 108, 5752−5763. 3230



Article

(72) Beljonne, D.; Curutchet, C.; Scholes, G. D.; Silbey, R. J. Beyond Förster Resonance Energy Transfer in Biological and Nanoscale Systems. J. Phys. Chem. B 2009, 113, 6583−6599. (73) Scholes, G. D.; Jordanides, X. J.; Fleming, G. R. Adapting the Förster Theory of Energy Transfer for Modeling Dynamics in Aggregated Molecular Assemblies. J. Phys. Chem. B 2001, 105, 1640−1651. (74) Jordanides, X. J.; Scholes, G. D.; Fleming, G. R. The Mechanism of Energy Transfer in the Bacterial Photosynthetic Reaction Center. J. Phys. Chem. B 2001, 105, 1652−1669. (75) Scholes, G. D.; Fleming, G. R. On the Mechanism of Light Harvesting in Photosynthetic Purple Bacteria: B800 to B850 Energy Transfer. J. Phys. Chem. B 2000, 104, 1854−1868. (76) Krueger, B. P.; Scholes, G. D.; Fleming, G. R. Calculation of Couplings and Energy-Transfer Pathways between the Pigments of LH2 by the ab Initio Transition Density Cube Method. J. Phys. Chem. B 1998, 102, 5378−5385. (77) Pons, T.; Medintz, I. L.; Wang, X.; English, D. S.; Mattoussi, H. Solution-Phase Single Quantum Dot Fluorescence Resonance Energy Transfer. J. Am. Chem. Soc. 2006, 128, 15324−15331. (78) Yun, C. S.; Javier, A.; Jennings, T.; Fisher, M.; Hira, S.; Peterson, S.; Hopkins, B.; Reich, N. O.; Strouse, G. F. Nanometal Surface Energy Transfer in Optical Rulers, Breaking the FRET Barrier. J. Am. Chem. Soc. 2005, 127, 3115−3119. (79) Mandal, S.; Ghatak, C.; Rao, V. G.; Ghosh, S.; Sarkar, N. Pluronic Micellar Aggregates Loaded with Gold Nanoparticles (Au NPs) and Fluorescent Dyes: A Study of Controlled Nanometal Surface Energy Transfer. J. Phys. Chem. C 2012, 116, 5585−5597. (80) Hazra, P.; Chakrabarty, D.; Sarkar, N. Solvation Dynamics of Coumarin 153 in Aqueous and Non-aqueous Reverse Micelles. Chem. Phys. Lett. 2003, 371, 553−562. (81) Dhenadhayalan, N.; Selvaraju, C. Role of Photoionization on the Dynamics and Mechanism of Photoinduced Electron Transfer Reaction of Coumarin 307 in Micelles. J. Phys. Chem. B 2012, 116, 4908−4920. (82) Jones, G., II; Jackson, W. R.; Choi, C.-Y.; Bergmark, W. R. Solvent effects on emission yield and lifetime for coumarin laser dyes. Requirements for a rotatory decay mechanism. J. Phys. Chem. 1985, 89, 294−300. (83) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006; Chapters 9, 13, 14, and 15. (84) Wu, C.; Unterforsthuber, K.; Lilge, D.; Lüddecke, E.; Horn, D. Determination of particle size distribution by the analysis of intensityconstrained multi-angle photon correlation spectroscopic data. Part. Part. Syst. Charact. 1994, 11, 145−149. (85) Ghatak, C.; Rao, V. G.; Pramanik, R.; Sarkar, S.; Sarkar, N. The Effect of Membrane Fluidity on FRET Parameters: An Energy Transfer Study inside Small Unilamellar Vesicle. Phys. Chem. Chem. Phys. 2011, 13, 3711−3720. (86) Sahu, K.; Ghosh, S.; Mondal, S. K.; Ghosh, B. C.; Sen, P.; Roy, D.; Bhattacharyya, K. Ultrafast Fluorescence Resonance Energy Transfer in a Micelle. J. Chem. Phys. 2006, 125, 044714−044721. (87) Ghosh, S.; Dey, S.; Adhikari, A.; Mandal, U.; Bhattacharyya, K. Ultrafast Fluorescence Resonance Energy Transfer in the Micelle and the Gel Phase of a PEO−PPO−PEO Triblock Copolymer: Excitation Wavelength Dependence. J. Phys. Chem. B 2007, 111, 7085−7091. (88) Mondal, S. K.; Ghosh, S.; Sahu, K.; Mandal, U.; Bhattacharyya, K. Ultrafast Fluorescence Resonance Energy Transfer in a Reverse Micelle: Excitation Wavelength Dependence. J. Chem. Phys. 2006, 125, 224710−224718. (89) Mondol, T.; Rajdev, P.; Makhal, A.; Pal, S. K. Interaction of an Antituberculosis Drug with a Nanoscopic Macromolecular Assembly: Temperature-Dependent Forster Resonance Energy Transfer Studies on Rifampicin in an Anionic Sodium Dodecyl Sulfate Micelle. J. Phys. Chem. B 2011, 115, 2924−2930.

3231


A Novel Ionic Liquid-in-Oil Microemulsion Composed of Biologically

Recommend Documents