Nile Red Dye in Aqueous Surfactant and Micellar Solution - Langmuir

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Nile Red Dye in Aqueous Surfactant and Micellar Solution Indah Nurita Kurniasih, Hua Liang, Parveen Choudhary Mohr, Gaurang Ramesh Khot, Jürgen P. Rabe, and Andreas Mohr Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504378m • Publication Date (Web): 11 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Nile Red Dye in Aqueous Surfactant and Micellar Solution Indah Nurita Kurniasih,† Hua Liang,‡ Parveen Choudhary Mohr,†,* Gaurang-Ramesh Khot,# Jürgen P. Rabe,‡ and Andreas Mohr†,#,* † Institut für Chemie und Biochemie, Freie Universität Berlin, Takustr. 3, D-14195 Berlin, Germany, ‡ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstraße 15, D-12489 Berlin, Germany # Fakultät Technologie und Bionik, Hochschule Rhein-Waal, Marie-Curie-Straße 1, D-47533 Kleve, Germany

KEYWORDS: nile red − solvatochromic dye – surfactant micelles − micellar solubilization – microenvironment – self-assembly − π-π interactions – H-aggregates

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ABSTRACT: The solubilization behavior of nile red dye in aqueous surfactant and micellar solutions was studied by optical spectroscopic techniques, dynamic light scattering, and atomic force microscopy. Nile red exhibits considerable absorption in the submicellar concentration region. When dispersed in aqueous surfactant and/or micellar solution nile red molecules tend to form non-emissive dimers and/or H-type aggregates through π−π stacking interactions. This phenomenon may limit the use of nile red in solubilization studies. In presence of ionic SDS and CTAB micelles, the solubilization of nile red appears to take place primarily at the charged micellar surface within the interfacial region. Similarly, spectra in micellar solution of non-ionic Triton X-100 revealed that nile red dye penetrates the hydrophilic, interfacial poly(oxyethylene) region of the micelles but cannot reach the hydrophobic, innermost core. Our results therefore suggest that nile red dye must be chosen carefully when probing (micellar) hydrophobic environments and (micro)domains. INTRODUCTION Nile red has attracted increasing interest during the past decades, because of its highly emissive, positive solvatochromic properties compared to other dyes.1-9 The solvatochromic behavior of nile red in different homogenous solutions is attributed to a twisted intramolecular charge transfer (TICT) process, which depends exponentially on the local polarity of the medium.10 Due to its sensitivity to the local immediate environment, nile red has been advantageously applied to probe the local polarity in the study of various heterogenous systems, including micelles,11 reverse micelles,4 dendrimers,12-14 liposomes,15,16 proteins,17,18 zeolites,10 and ionic liquids.5,19 In drug delivery research, solubilization (encapsulation) of nile red has been discussed in terms of hydrophobic interactions between the dye molecules and the non-polar interior environment of


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the nanoparticulate scaffolds.20-25 Despite the widespread use of nile red as a hydrophobic probe, very little attention has been given to its aggregation and interfacial behavior. Only few studies concerning these aspects have been published so far, and accordingly the phenomena observed are not yet well understood. Varghese and Wagenknecht26 investigated the non-covalent selfassembly of (nile·red)-modified 2-deoxyuridine in water. They found that the absorption spectrum in methanol (570 nm, ε = 32.7) shifts to a shorter wavelength in water (540 nm, ε = 78.4). This unusual hypsochromical shift has been assigned to the formation of H-type aggregates. Similarly, H-aggregation of nile red dye was observed at high loading levels in aqueous solutions of sugar-PEG-based polymers27 and dendritic core-multishell nanoparticles.12 A recent study by Felbeck et al.28 focused on nile red loaded laponite hybrid materials. Spectroscopic results revealed that in aqueous solution high dye concentrations favor the formation of non-emissive H-type dimers. To gain deeper insight into the binding interactions and mechanisms involved further studies on nile red self-assembly and characterization of the supramolecular structures are required. Micellar solubilization of hydrophobic compounds is of major practical importance.29,30 Therefore, much attention has been given to both the extent of solubilization and the possible positions at which solubilization processes might occur. Spectral analysis allows us to determine micellar binding sites and to estimate quantitatively the local polarity around the probe. Because H-type dimers and aggregates are non-emissive,31 spontaneous self-assembly may limit the use of nile red in solubilization studies. Here, we report the spectroscopic properties of nile red dye in aqueous surfactant and micellar solutions (Figure 1). We have observed that in presence of surfactant nile red exhibits considerable absorption in the submicellar concentration region. The optical absorption spectra


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typically show a maximum in the 450−500 nm range (compared to 593 nm in water), accompanied by an intense shoulder in the red region of the spectrum. We attribute the shift of the peak maximum to the formation of H-type aggregates. Interestingly, stirring dispersions of nile red within an aqueous solution of surfactant and/or micelles results in 100 nm-sized dyesurfactant (dye) aggregates. Our results may contribute to a more fundamental understanding of the interfacial behavior, which is needed for the successful application of nile red in nanoscience and nanotechnology. In the present study, commercially available surfactants have been selected because their physical and chemical properties are well documented in the literature.

Figure 1. Chemical structure of nile red and surfactants investigated herein. In parentheses, critical micelle concentration (CMC) values in water at 25°C.32 EXPERIMENTAL SECTION Materials. All chemicals were commercially available and of analytical grade. Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were purchased from Fluka.


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Polyoxyethylene (10) tert-octylphenyl ether (Triton X-100) was used as received from SigmaAldrich. Nile red was purchased from ABCR GmbH & Co.KG, Germany. Analytical-grade solvents were purchased from Sigma-Aldrich and Acros Organics companies. All chemicals were used without further purification. Water from a Millipore system (resistivity ~18 MΩ·cm-1) was used in all experiments and for preparation of all samples. UV/Vis and Fluorescence Spectroscopic Studies. Absorption spectra were acquired at 25°C using a Scinco S-3150 UV/VIS spectrophotometer (range: 187–1193 nm; resolution: 1024 points; thermostated cell holder for square 1cm cuvettes). Fluorescence emission spectra were taken with a Jasco FP−6500 spectrofluorimeter equipped with a thermostated cell holder (1 cm), a DC-powered 150W Xenon lamp, a Hamamatsu R928 photomultiplier, and a variable slit system. Emission spectra of nile red were recorded at 25°C from 575 to 800 nm after excitation at 550 nm. Both excitation and emission slits were set at 5 nm. Dynamic Light Scattering. DLS measurements were performed to determine the precise size of micelles and aggregates. Experiments were carried out on a Zetasizer Nano ZS analyzer with integrated 4 mW He–Ne laser, λ = 633 nm (Malvern Instruments Ltd, U.K.). This apparatus, which uses the backscattering detection (scattering angle θ = 173°) and an avalanche photodiode detector, is equipped with a Helium-Neon laser source (operating wavelength 633 nm; power 4.0 mW), and a thermostated sample chamber controlled by a thermoelectric peltier. Both solution viscosity (η = 0.89 mPa·s)33 and refractive index (n = 1.33)34 were assumed to be those of water. For all experiments disposable UV-transparent cuvettes (12.5×12.5×45 mm, Sarstedt AG & Co, Germany) were used.


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Atomic Force Microscopy (AFM). A droplet of sample aqueous solution was deposited on a freshly cleaved mica surface and spun off after 5 seconds. The surface was dried under a flux of nitrogen gas and then imaged by AFM in tapping-mode under ambient conditions, employing a Nanoscope 3a (Veeco, USA), using silicon cantilevers (Olympus, Japan) with a typical resonance frequency of 300 kHz and a spring constant of about 42 N⋅m-1.

Measurements in the submicellar concentration regime: Aqueous dispersions with different concentrations of surfactant were freshly prepared. Nile red (1 and 10 mg/mL) was added as a fine powder, and the mixtures were stirred for 2−3 days at room temperature. Insoluble solid residues of the dye were then removed by filtration through a 0.45 µm PTFE filter. The obtained solutions were analysed by UV/Vis and fluorescence spectroscopy. Aggregate size and shape were determined by dynamic light scattering and atomic force microscopy. Measurements in Aqueous Micellar Solutions: Aliquots of a nile red stock solution (1 mM in THF) were first transferred into glass vials. The solvent was carefully removed under reduced pressure leaving brick red thin solid films. Aqueous surfactant solutions with different concentrations of surfactant were then added to the sample vials. Similarly, spectra were obtained after 12 hours of stirring at room temperature. Because dissolution of the dye was incomplete, insoluble solid residues of the films were removed by filtration. For the studies in Triton X-100 solution (1.0 mM) the amount of dye was varied between 0.008 and 0.955 mg/mL. The filtered solutions (0.45 µm PTFE) were finally investigated by UV/Vis and fluorescence spectroscopy. Micelle/aggregate sizes were measured by means of dynamic light scattering and atomic force microscopy.


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Measurements in Cosolvent-Water Mixtures: Spectra were obtained in mixtures of water and dioxane. All mixtures were prepared in terms of volume ratios. A calibration curve was prepared for determining the solubilization capacity of nile red in micellar solution. For this purpose, a nile red stock solution (1.0 mM) was freshly prepared in a 70% dioxane-water mixture. Aliquots were then taken to form 1.25, 2.5, 5.0, 10, 15, 20, and 25 µM sample solutions. After temperature equilibration samples were analyzed by UV/Vis spectroscopy. Absorption maxima were recorded at 25°C between 300 and 900 nm.

RESULTS AND DISCUSSION Nile Red in Aqueous Surfactant Solutions. The effect of ionic surfactants (SDS and CTAB) on aqueous dispersions of nile red was first studied at submicellar concentrations by means of steady-state absorption spectroscopy. In a typical experiment, nile red powder was added to the surfactant solutions and the resulting mixtures were vigorously stirred for 2−3 days at room temperature. The samples were filtered and investigated spectrophotometrically as described in the Experimental Section. We found that even when the micelle-forming surfactant concentration is below its critical micelle concentration, the optical absorption spectra show a well-defined maximum in the 450−500 nm range, accompanied by an intense shoulder in the red region of the spectrum (Figure 2). This unusual spectroscopic behavior prior to micelle formation and the appearance of a new peak in the absorption spectrum suggest strong binding interactions between the dye and/or between the dye and the surfactant molecules. Hence, it is certainly conceivable that such an interaction might lead to the formation of dye-surfactant (dye) aggregates in water held together by π-π stacking and hydrophobic interactions.


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Figure 2. (A) The effect of surfactants on aqueous dispersions (1 mg/mL) of nile red. Absorption spectra (a)−(e) in 0.88, 1.76, 3.52, 5.63, and 7.04 mM SDS. (B) The CTAB concentrations were 0.07, 0.26, 0.52, 0.83, and 1.03 mM, respectively. All spectra were recorded at 25°C. The CMCs


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of SDS and CTAB in water are reported to be between 8.1−8.4 and 0.92−1.00 mM, respectively.32 Similar experiments as those described above were also performed on Triton X-100 solutions. Results from the spectroscopic measurements are shown in Figure 3. In presence of 0.05 mM Triton X-100 (0.19−0.21×CMC) nile red exhibited absorption maxima at 499 and 484 nm for samples containing 1 and 10 mg/mL of nile red, respectively. Most striking, the absorption bands are again accompanied with red shifted shoulder bands at around 550 nm. Furthermore, from the spectra it can be seen that the absorbance considerably decreases as the amount of nile red is increased. Interestingly, a slight hypsochromic shift of 15 nm is observed for the absorption band. Again, this suggests strong binding interactions between the dye and/or the surfactant molecules at submicellar concentration. The aggregates, however, are held together most likely by associative π−π stacking interactions of the condensed aromatic ring system of nile red. The inset of Figure 3 shows the corresponding fluorescence emission spectra. In line with expectation, the two nile red sample solutions were nearly non-emissive in water. On the contrary, in presence of 1.0 mM Triton X-100 (3.7−4.2×CMC) a relatively intense fluorescence is observed with an emission maximum at 633 nm. This wavelength clearly indicates that micellar bound dye molecules are exposed to a less polar environment. In micellar solution of Triton X-100 the UV/Vis spectrum shows a well-defined absorption band with a maximum at 549 nm (Figure 3). From this absorption spectrum it can be inferred that the solubilization of nile red takes place primarily within the hydrophilic poly(oxyethylene) layer of the micelle. The effective dielectric constant of the microenvironment is lower than that of bulk water (λmax=593 nm; ε = 78.4) and approximates the polarity of poly(ethylene-glycol) PEG 400 (λmax=550 nm; ε =12.4).35


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Figure 3. UV absorption of nile red in aqueous Triton X-100 solutions. Inset: corresponding fluorescence emission spectra of the dye (λexc.= 550 nm). Measurements were performed below and above the CMC on solutions containing (a) 0.1 mg/mL of nile red in 1.0 mM Triton X-100, (b) 1 mg/mL of nile red in 0.05 mM Triton X-100, and (c) 10 mg/mL of nile red in 0.05 mM Triton X-100. Size and Shape of Dye-Surfactant(Dye) Aggregates in Aqueous Surfactant Solutions. Structural aspects were further investigated using dynamic light scattering (DLS) technique and atomic force microscopy (AFM). Similarly, experiments were performed in 1.0 mM SDS (0.12×CMC) and 0.05 mM Triton X-100 (0.19−0.21×CMC) solutions. Prior to analysis, nile red powder was suspended in aqueous surfactant solution to form mixtures of 1 and 10 mg/mL, respectively. The suspensions were stirred for 2−3 days at room temperature, and then filtered to obtain clear and red-coloured solutions. The aggregate size distributions determined by DLS are shown in Figure 4. A broad particle size distribution was found in both SDS and Triton X-100 solutions. The intensity weighted curves


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typically show single broad peaks with hydrodynamic diameters ranging from 100 to 200 nm. For samples prepared from a 1 mg/mL dispersion, the Z-average diameters were found as 165±3 (PDI=0.191) and 153±1 nm (PDI=0.151) in 1.0 mM SDS and 0.05 mM Triton X-100, respectively. Similarly, diameters determined for 10 mg/mL dispersions were found to be 137±1 (PDI=0.245) and 189±4 nm (PDI=0.164) in 1.0 mM SDS and 0.05 mM Triton X-100, respectively. A comparison of the intensity distribution profiles with the corresponding volume distributions suggests a bimodal size distribution with about similar aggregate size. As seen in Figure 4, peak shoulders are clearly visible for all samples. In addition, a tail towards the upper size end was found in the number-averaged profiles, again indicating a bi- or multimodal size distribution. Size distribution data by intensity, volume, and number as well as Z-average and PDI values are presented in Table S1 of the Supporting Information (SI). AFM was applied to confirm the aggregate morphology. Figure 5 shows aggregates isolated from a 10 mg/mL suspension of nile red in 1.0 mM SDS solution. On mica, the aggregates have a typical length of around 120 nm, which is slightly smaller than the average hydrodynamic diameter determined by DLS (137±1 nm). This result is consistent with the general observation that the hydrodynamic diameter equals the length of the long axis established by AFM.


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Figure 5. AFM height image of nile red aggregates deposited on a mica surface. Aggregates were isolated from a 10 mg/mL suspension of nile red in 1.0 mM of SDS (0.12×CMC). Further experiments were performed on aggregates isolated from a 1 mg/mL suspension of nile red in 0.05 mM (0.19−0.21×CMC) Triton X-100 solution. To gain information on the internal aggregate structure, the sample was subjected to ultrasonic treatment for 20 minutes, followed by a 3-steps manipulation with an AFM tip. On mica, the aggregates were first imaged by AFM in tapping mode, thereafter manipulated with an AFM tip in contact mode, and finally imaged again by tapping mode, in order to check whether they were stable individual entities.36−38 Figure 6 shows the AFM height images and the corresponding height profiles across a (dye-surfactant) aggregate. After ultrasonic treatment, the aggregates decomposed to (multi) layer structures. During the manipulation process the top layer in the center was cut and moved by the AFM tip to the right side of the image. As shown in Figure 6, the height of the (newly formed) layer on the


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right side of the image was around 1.4 nm, whereas the height in the center reduced from 3.4 to 2.2 nm. We attribute a single layer to a parallel arrangement of self-assembled dye molecules. Considering the length (long dimension) of nile red molecules,39 a single layer is approximately 1.1 nm in thickness. Before tip manipulation the center was composed of three layers (~3.3 nm in height), while the edge was comprised of only one layer (~1.1 nm). In fact, this fits very well with the height profile shown in Figure 6a. During manipulation the top layer was moved by the tip, thus leaving 2 layers (~2.2 nm) in the center. After manipulation, the top layer had a height of 1.4 nm, slightly larger than the single layer width of self-assembled nile red molecules (1.1 nm). We attribute this to a faintly bending of the layer after manipulation.

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Nile Red in Aqueous Micellar Solution. Micellar solubilization is usually treated in terms of the phase separation model, in which a distinction is being made between the bulk aqueous solution and the micellar pseudo-phase.40-45 Apart from bulk water, micelles are treated as a separate phase. Thus, small (polar) guest molecules will distribute between both the aqueous and the micellar pseudo-phase. Nile red dissolves in a wide range of solvents and solvent mixtures but it exhibits very poor solubility in water (98%) of small, dye-loaded, micelles. Due to the solubilization of nile red dye, the micelle size increases slightly from 5.7 nm in neat surfactant solution to almost 10 nm in the presence of guest molecules. As can be seen from the UV/Vis spectra in Figure 7a, the maximum wavelength was found to be constant (λmax= 553 nm) within the concentration range studied (≤0.016 mg/mL). Dye loaded micelles tend to form aggregates upon solubilization of nile red (Figures 8). For dye concentrations ≥0.032 mg/mL, Triton X-100 micelles disappeared and only aggregates with intensity-average mean diameters ranging from 150 to 450 nm were found (Table S2, Figures S2−S4). A comparison of the intensity distribution profiles with the corresponding volume distributions suggests a bimodal, or rather multimodal size distribution. Most likely, this type of aggregation is driven by hydrophobic interactions. The penetration of nile red molecules into the hydrated polyoxyethylene corona will certainly decrease the hydrophilicity of the micellar surface. Like Triton X-100 micelles the aggregates


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serve as hosts and can accommodate nile red molecules. As mentioned above, the fluorescence intensity reached a maximum at 0.064 mg/mL. Up to this dye concentration the average distance between solubilized guest molecules is expected to be large. As a result, π−π interactions are less favoured and solubilized nile red molecules primarily exist as monomers (Figure 8). On the contrary, at high dye concentrations solubilized dye molecules may exist in an equilibrium mixture of monomers and non-emissive H-type dimers.28 Indeed, a gradual decrease in the fluorescence intensity was observed with increasing concentration of nile red (>0.064 mg/mL). Again, fluorescence quenching is caused by face-to-face stacking of the aromatic rings, whereby the degree of quenching usually acts as an indicator of intermolecular π−π interactions of the chromophores. However, only a limited amount of dye can be accommodated by the aggregates. Once the saturation level of dye content is reached, H-aggregation becomes prominent (Figure 8). In fact, a large blue-shift of the maximum wavelength of the absorption band was observed for high loading levels of nile red (see Inset, Figure 7a). As observed for the fluorescence intensity, the aggregate size reached a maximum at 0.064 mg/mL of nile red (Figure S2). With further increase in dye concentration (>0.064 mg/mL), the aggregate size decreased to reach a steady state value around 150 nm (PDI< 0.2). This phenomenon, which was also observed for dendritic core-multishell nanoparticles,13 can be attributed, most likely, to an optimal, thermodynamically favoured size for the nile red loaded aggregates. In summary, our findings clearly show that apart from spectral response also aggregate size is strongly dependent on the amount of solubilized nile red. Due to incomplete solubilization of the nile red films, the concentrations of the filtered samples are lower than those of the aliquots used for the film preparation (0.008−0.955 mg/mL). An estimate of the real dye concentrations was made from a standard curve. The calibration curve is


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based on Beer-Lambert’s law and was obtained by measuring the absorbance of various concentrations of nile red in a 70% dioxane-water mixture. As shown in Figure S5 of the SI, the molar absorptivity at λmax=551 nm was found to be (33100±100) M-1·cm-1. The true dye concentrations were estimated by applying the relationship c=A/ε·l, where A is the measured absorbance at λmax, ε is the wavelength-dependent molar absorptivity, l is the path length, and c is the true dye concentration. As expected, sample solutions exhibited nile red concentrations typically 20−50 times lower than those noted in the Experimental Section. The [dye]/[Triton X100] molar ratio was typically about 1/(≤800). Indeed, only small amounts of dye molecules were accommodated by the hosts. By means of the same calibration curve we finally estimated the solubilization capacity, i.e. the amount of solubilized dye per amount of surfactant employed. Similarly, for nile red loaded micelles and aggregates very small solubilization capacities were found. Typical values obtained are in the range of 0.7−2.1

mg g NR/ TX-100.

Further details can be

found in the SI. Probing hydrophilic and hydrophobic microdomains. In aqueous solution micelles exhibit a polarity (dielectric constant) gradient between the highly hydrated micellar surface and the hydrophobic core.30,45,47-51 As a consequence, solubilized nile red molecules are exposed to a less polar environment. To probe its surrounding environment in micelles, UV/Vis and fluorescence spectra were recorded in micellar solutions of SDS, CTAB, and Triton X-100. Figure 9a shows that in the presence of Triton X-100 the absorption spectrum of nile red is 14−23 nm blue-shifted in comparison to those in SDS and CTAB solutions. Similarly, the wavelength of the maximum emission band exhibits a blueshift in the range of 9−13 nm. These data clearly indicate that nile red experiences a less polar environment in Triton X-100 micelles than in micellar assemblies of charged SDS or CTAB.


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To quantitatively estimate the local polarity at the micellar binding site, UV/Vis and fluorescence spectra (Figures S5, SI) were further recorded in mixtures of dioxane and water and compared with those in surfactant solutions. Cosolvent-water mixtures can be taken as model systems to probe the local environment of dyes and drugs in micelles.32,50,52 We followed the protocol50 previously described by Kumbhakar et al. who investigated the solubilization locus of coumarin 153 (C153) in Triton X-100 micelles. Briefly, steady-state fluorescence spectra of C153 were first measured in a series of ethanol water-mixtures. Observed changes in the emission maxima were then plotted against the so-called orientational polarizability …

…

− 1 −1 − (1) ∆ = 2 + 1 2 + 1 where ε is the dielectric constant and n is the refractive index of the medium. The first term

accounts for the spectral shifts due to both the reorientation of the solvent dipoles and to

the redistribution of the electrons in the solvent molecules. The second term accounts for only the redistribution of the electrons. The difference of these of these two terms accounts for the spectral shifts due to reorientation of the solvent molecules, and hence the term orientation polarizability. From the calibration curve thus obtained the dielectric constant of the probe environment was estimated to be 22.4. Based on this result, it was concluded that C153 preferably resides in the palisade layer of the micelle. In the present study, we used mixtures of dioxane and water. Because the solvents are miscible in all proportions, mixtures cover a broad range of polarity from 2.21 in neat dioxane to 78.4 in water.53 Solvent mixtures were prepared in terms of volume ratios. Thus, for dioxane-water mixtures the dielectric constant and refractive index values were calculated applying the relations48


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= ∙ + ! ∙ ! (2)

= ∙ + ! ∙ ! (3)

where f represents the volume fractions of the cosolvents used. Figure 9b illustrates the dependence of the emission maxima of nile red (# $ in cm-1) against the orientation polarizability (∆f) for different dioxane-water mixtures. Over a limited range of solvent compositions (10−80% dioxane) the fluorescence maxima showed a linear dependence on ∆f (R2≥0.990). Plotting the emission maxima values initially obtained from the spectra in micellar solution (Figure 9a) on the same straight line allows the estimation of the effective dielectric constant in the vicinity of the dye. As can be seen from the regression curve in Figure 9b, perpendicular lines were drawn to the ∆f–axis, thus yielding individual values of 0.30, 0.29, and 0.27 for SDS, CTAB, and Triton X-100 micelles, respectively. The plot of the orientation polarizability (Figure 9b) clearly demonstrates a less polar environment for solubilized nile red in Triton X-100 micelles. Data were finally correlated to corresponding dielectric constant values using the above relationship for ∆f. Together with the assumption that the refractive index is similar to that of water the dielectric constants were estimated to be 24.6, 18.7, and 13.9 in SDS, CTAB, and Triton X-100 solution, respectively. It is inferred from the present result that in presence of Triton X-100 nile red molecules penetrate into the poly(oxyethylene) region (palisade layer) of the micelle. On the contrary, in micellar solutions of ionic SDS and CTAB nile red preferably resides completely at the aqueous micellar surface. As apparent from Figure 9b, the wavelength of the emission maximum remained practically unchanged in the presence of aggregates (≥0.032 mg/mL). This indicates that irrespective of size, shape, and structure of the host solubilized guest molecules are exposed to the same polar microenvironment (∆ε≤0.1).


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. 1,0

A

TX-100 (632 nm)

SDS (576 nm)

Absorbance

. 0,8

. 3,0 . 2,5

CTAB (567 nm)

. 2,0 . 0,6 . 1,5 . 0,4 CTAB (641 nm)

0,2 .

SDS (645 nm)

. 1,0 . 0,5

TX-100 (553 nm)

. 0,0

. 0,0

450

1,60 . νem (104 X cm-1)

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Fluorescence Intensity (a.u.)

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525

600 675 Wavelength (nm)

750

B TX-100micelles (632 nm) TX-100aggregates (633 nm)

. 1,58

CTAB (641 nm)

. 1,56

SDS (645 nm)

. 1,54 . 1,52 . 0,26

. 0,28

. 0,30

. 0,32

−function 1

2(∆f) −1 Polarity

∆ =

2 + 1

−

2 2 + 1

Figure 9. (A) Absorption and fluorescence emission spectra (λexc=550 nm) of nile red (0.008 mg/mL) in micellar solutions of 50 mM SDS (6.0−6.2×CMC), 5 mM CTAB (5.0−5.4×CMC), and 1 mM Triton X-100 (3.7−4.2×CMC). Spectra with aggregates present are shown in Figure 7. (B) Plot of the emission maxima (#% in cm-1) in different dioxane-water mixtures against the orientation polarizability ∆f. Data points marked with full circles correspond to either the spectra in micellar solution or the samples with aggregates present.


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The above results are consistent with our previous study where supramolecular aggregates of sugar-poly(ethylene.glycol).(PEG)-based copolymers were explored for their drug encapsulation properties in buffered aqueous solution.27 By means of UV/Vis and fluorescence spectroscopy we could show that the solubilization of nile red took place preferably within the hydrophilic PEG corona. A similar result was found for dendritic core-multishell architectures build up from polar, hyperbranched polyglycerolamine cores and a double shell structure of linear hydrophobic alkyl chains and hydrophilic monomethyl poly(ethylene glycol) (mPEG) units.13 Like multilayer vesicles or liposomes CMS composites have an onion-like structure with clearly separated hydrophilic and hydrophobic domains. Although CMS nanoparticles provide an inner hydrophobic region through the linked aliphatic chains, solubilized nile red resides preferably within the outer polar shell.13 Recently, time-resolved fluorescence studies on polygylcerol-based architectures have shown a distribution of nile red dye within the nanotransporter shells.54 Apart from the interfacial region and the outer hydrated mPEG shell nile red molecules were shown to penetrate into the apolar region, i.e. most likely into the layer of mPEG chains devoid of water as well as the layer of aliphatic chains. Notwithstanding this, it is apparent from the above results that nile red might be of limited use for probing lipophilic (micellar) environments and (micro)domains. In other words, nile red does not behave as a model hydrophobic probe.

Hydrogen bonding interactions may play an important role in the micellar binding of nile red dye. The palisade layer of Triton X-100 micelles is mainly composed of the oxyethylene units of the surfactant head and a large number of water molecules.50,51 Water molecules are hydrogen bonded either among themselves or to the surfactant head groups. In the present study, nile red molecules were found to reside inside the palisade layer. We assume that solubilized nile red molecules can participate in hydrogen-bond formation. In this context it is worth mentioning a


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previous study by Cser et al.55 who have shown that nile red dye forms H-bonds with solvent molecules. For example, in dichloromethane in the presence of small amounts of fluorinated alcohol absorption spectra of nile red were bathochromically shifted due to hydrogen bonding interactions. Hydrogen bonding of nile red with alcohol can occur both in the ground and excited state.55 Fluorescence lifetime was found to decrease with increasing hydrogen bond donating power of the solvent. The role of electrostatic and/or hydrogen bonding interactions is currently under investigation for non-ionic micellar systems. To investigate the binding status in micelles time-resolved fluorescence spectroscopy is the method of choice. CONCLUSION The solubilization behavior of nile red dye in aqueous surfactant and micellar solutions was studied by UV/Vis and fluorescence spectroscopy, dynamic light scattering, and atomic force microscopy. From the solubilization studies described herein it can be inferred that the nile red probe does not penetrate the hydrophobic part of micelles. Instead, nile red solubilisation occurs preferably at the micellar surface or within the polar exterior regions. We therefore conclude that nile red must be chosen carefully when probing hydrophobic (micellar) environments and (micro)domains. Spectral analysis allows us to determine micellar binding sites and to estimate quantitatively the local polarity around the probe. Because of its quickness and ease, we recommend, as a first step, a spectroscopic study of the (micro)polarity when characterizing and evaluating a potential dye/drug carrier system. In submicellar aqueous solution nile red dye exhibits considerable absorption. Successive addition of the dye to the surfactant solutions causes a blue-shift, thus indicating the formation of H-aggregates. In micellar solution of Triton X-100 nile red dye is completely solubilized in the


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micellar pseudo-phase. At low guest concentration, dye molecules are emissive and primarily exist as monomers. An increase in dye concentration favors the formation of dimers and H-type aggregates through π−π stacking interactions. Due to their non-emissive behavior, spontaneous self-assembly may limit the use of nile red in solubilisation studies. Upon dye solubilization Triton X-100 micelles tend to aggregate as the hydrated polyoxyethylene corona becomes more hydrophobic. Both spectral response and aggregate size are strongly dependent on the amount of dye. Nile red exhibits interesting interfacial properties. The phenomena observed certainly deserve further attention. ASSOCIATED CONTENT Supporting Information. Schematic representation of micelles in aqueous solution. Size distribution data and profiles. Fluorescence emission spectra of nile red in different dioxanewater mixtures. Calibration curve of nile red dye in a 70 percent dioxane-water mixture. Determination of solubilisation capacities. Table of selected physical constants of common solvents. Long-wavelength visible absorption maxima values and corresponding transition energies for nile red dye in different solvents. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A.M.). *E-mail: [email protected] (P.C.M.). Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT Financial support from Deutsche Forschungsgemeinschaft (DFG) through SFB 765 is gratefully acknowledged. REFERENCES (1) Greenspan, P.; Fowler, S. D. Spectrofluorometric studies of the lipid probe, nile red. J. Lipid Res. 1985, 26, 781–789. (2) Deye, J. F.; Berger, T. A.; Anderson, A. G. Nile Red as a solvatochromic dye for measuring solvent strength in normal liquids and mixtures of normal liquids with supercritical and near critical fluids. Anal. Chem. 1990, 62, 615–622. (3) Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. 1994, 94, 2319–2358. (4) Datta, A.; Mandal, D.; Pal, S. K.; Bhattacharyya, K. Intramolecular Charge Transfer Processes in Confined Systems. Nile Red in Reverse Micelles. J. Phys. Chem. B 1997, 101, 10221–10225. (5) Fletcher, K. A.; Storey, I. A.; Hendricks, A. E.; Pandey, Sh.; Pandey, Si. Behavior of the solvatochromic

probes

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dye,

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Red

and

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pyrenecarbaldehyde within the room-temperature ionic liquid bmimPF6. Green Chemistry 2001, 3, 210–215. (6) Yablon, D. G.; Schilowitz, A. M. Solvatochromism of Nile Red in Nonpolar Solvents. Appl. Spectroscopy 2004, 58, 843–847.


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(15) Chattopadhyay, A.; Raghuraman, H.; Mukherjee, S. Membrane localization and dynamics of Nile Red: Effect of cholesterol. Biochim. Biophys. Acta 2007, 1768, 59–66. (16) Sloniec, J.; Schnurr, M.; Witte, C.; Resch-Genger, U.; Schröder, L.; Hennig, A. Biomembrane Interactions of Functionalized Cryptophane-A: Combined Fluorescence and 129Xe NMR Studies of a Bimodal Contrast Agent. Chem. Eur. J. 2013, 19, 3110–3118. (17) D. L. Sackett, D. L.; Wolff, J. Nile red as a polarity-sensitive fluorescent probe of hydrophobic protein surfaces. Anal. Biochem. 1987, 167, 228-234. (18) Hawe, A.; Sutter, M.; Jiskoot, W. Extrinsic Fluorescent Dyes as Tools for Protein Characterization. Pharm. Res. 2008, 25, 1487–1499. (19) Carmichael, A. J.; Seddon, K. R. Polarity study of some 1-alkyl-3-methylimidazolium ambient-temperature ionic liquids with the solvatochromic dye, Nile Red. J. Phys. Org. Chem. 2000, 13, 591–595. (20) Gillies, E. R.; Fréchet, J. M. J. A new approach towards acid sensitive copolymer micelles for drug delivery. Chem. Commun. 2003, 14, 1640–1641. (21) Gillies, E. R.; Jonsson, T. B.; Frechet, J. M. J. Stimuli-Responsive Supramolecular Assemblies of Linear-Dendritic Copolymers. J. Am. Chem. Soc. 2004, 126, 11936–11943. (22) Jang, C.-J.; Ryu, J.-H.; Lee, J.-D.; Sohn, D.; Lee, M. Synthesis and Supramolecular Nanostructure of Amphiphilic Rigid Aromatic-Flexible Dendritic Block Molecules. Chem. Mater. 2004, 16, 4226–4231. (23) Jiang, J.; Tong, X.; Zhao, Y. A New Design for Light-Breakable Polymer Micelles. J. Am. Chem. Soc. 2005, 127, 8290–8291.


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Table of Contents Graphic


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Nile Red Dye in Aqueous Surfactant and Micellar Solution - Langmuir

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