Concentration-Driven Fascinating Vesicle-Fibril Transition Employing

Aug 29, 2017 - In this article, anionic lipophilic dye merocyanine 540(MC540) and cationic surface-active ionic liquid (SAIL) 1-octyl-3-methylimidazol...
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Concentration-Driven Fascinating Vesicle-Fibril Transition Employing Merocyanine 540 and 1‑Octyl-3-methylimidazolium Chloride Rupam Dutta, Arghajit Pyne, Sangita Kundu, Pavel Banerjee, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB India S Supporting Information *

ABSTRACT: In this article, anionic lipophilic dye merocyanine 540(MC540) and cationic surface-active ionic liquid (SAIL) 1-octyl-3-methylimidazolium chloride (C8mimCl) are employed to construct highly ordered fibrillar and vesicular aggregates exploiting an ionic self-assembly (ISA) strategy. It is noteworthy that the concentration of the counterions has exquisite control over the morphology, in which lowering the concentration of both the building blocks in a stoichiometric ratio of 1:1 provides a vesicle to fibril transition. Here, we have reported the concentrationcontrolled fibril−vesicle transition utilizing the emerging fluorescence lifetime imaging microscopy (FLIM) technique. Furthermore, we have detected this morphological transformation by means of other microscopic techniques such as field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and cryogenic-transmission electron microscopy (cryo-TEM) to gain additional support. Besides, multiwavelength FLIM (MW-FLIM) and atomic force microscopy (AFM) techniques assist us in knowing the microheterogeneity and the height profile of the vesicles, respectively. We have replaced the SAIL, C8mimCl, by an analogous traditional surfactant, n-octyltrimethylammonium bromide (OTAB), and it provides a discernible change in morphology similar to that of C8mimCl, whereas 1-octanol is unable to exhibit any structural aggregation and thus reveals the importance of electrostatic interaction in supramolecular aggregate formation. However, the SAILs having the same imidazolium headgroup with different chain lengths other than C8mimCl are unable to display any structural transition and determine the importance of the correct chain length for efficient packing of the counterions to form a specific self-assembly. Therefore, this study reveals the synergistic interplay of electrostatic, hydrophobic, and π−π stacking interactions to construct the self-assembly and their concentration-dependent morphological transition.

1. INTRODUCTION Molecular self-assembly formation and the subsequent morphological transition is one of the most important areas of research and has drawn immense interest in recent years. Supramolecular self-assemblies1 cover a wide range of fascinating structures including giant vesicles,2−5 various nanostructures such as nanoflowers,6 and nanobelts2 and also one-dimensional structures such as microfibers,7 tubes, and rods.8 These self-assemblies have been extensively used in the fields of medicine, biotechnology,9 and drug delivery.10 Among these supramolecular assemblies, giant vesicles have drawn much attention because they can act as microreactors for silver and gold nanoparticle synthesis2,3 and can entrap carbon quantum dots (CQD) and also quantum dots (QDs) inside their hollow cavities.3,4 Besides, giant vesicles can act as artificial cells owing to the structural and dynamical similarities with the biological cell membrane;4 moreover, they show antimicrobial activity.11 To fabricate the above-mentioned self-assembled supramolecular aggregates, various routes have been employed such as hydrogen bonding, π-conjugation, metal coordination, key-lock combination, amphiphilic association, and ionic selfassembly (ISA).2,8,12,13 Among these techniques, the ISA strategy is of paramount interest because it is an easy, cheap, and flexible method with universal applicability.14,15 In this approach, two building blocks of opposite charges are © 2017 American Chemical Society

employed, and the electrostatic force of attraction between them plays a key role in the formation of versatile selfassembled aggregates.2,3,5,8,12 ISA is a very facile strategy even in the field of biology, where negatively charged DNA or RNA binds with oppositely charged proteins through the columbic force of attraction to form a virus assembly16 and chromatin fibers.17 Here, it is relevant to mention that dye molecules are one of the most important building blocks in this ISA strategy as a result of their easy availability, extended π-conjugation, regular shape, and optical applications.3,13,18 Several research groups have reported various self-assembled architectures using suitable dye and surfactant combinations.12,18,19 Faul and Antonietti have used a series of azo dyes with varying surfactant molecules to form dye−surfactant assemblies.12,13,18 Besides azo dyes, other dye molecules can also interact electrostatically with suitable oppositely charged building units.2,20−22 Moreover, cationic peptides14,23 as well as bile salts2,20 are also useful candidates for the ISA technique to form a supramolecular assembly. Very recently, polyoxometalates have also been used Received: June 23, 2017 Revised: July 29, 2017 Published: August 29, 2017 9811

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Langmuir Scheme 1. Chemical Structures of MC540, C8mimCl, and OTAB

charge delocalization compared to the corresponding aliphatic analogue.3,5,8,37 Kumar et al. have discussed the interaction among three structurally different calixarenes and SAIL, 1decyl-3-methylimidazolium chloride (C10mimCl), in detail.38 In this context, it is important to note that various stimuli such as temperature,2,39,40 pH,40 redox, enzyme, light,41 and conjugation7 make a substantial impact on the morphological transitions. Thermoresponsive vesicle to nanofiber39 and nanobelt2 transitions are reported by the Dong and Yue groups, respectively. Joshi et al. have reported a conjugationtriggered fiber to spherical morphological transition.7 Nowadays, fluorescence lifetime imaging microscopy (FLIM) is extensively used as a technique complementary to conventional fluorescence intensity measurement, having several applications in the determination of molecular environment parameter, the molecular state of cells, and protein interactions via Förster resonance energy transfer (FRET).42−45 In this context, it is important to mention that the packing parameter (P) strongly controls the morphology of different molecular self-assemblies. The value of P provides a broad idea about various supramolecular aggregates.46−49 With this background, in the present study, using the ISA strategy we have shown the formation of two different supramolecular aggregates (fibril and vesicle) at different MC540/C8mimCl concentrations by maintaining the stoichiometric ratio at 1:1. We have further characterized the concentration-dependent morphological transition using FLIM, FESEM, TEM, and AFM techniques. Thereafter, we have employed various imidazolium SAILs (CnmimCl, n = 4, 6, 8, 10, 12, and 16) with different chain lengths as a counterion of MC540 and infer that the appropriate chain length of SAIL is required to obtain the morphological conversion. Moreover, the different types of interactions of MC540 with OTAB and 1octanol confirm the importance of electrostatic interaction in supramolecular aggregate formation.

extensively to prepare supramolecular hybrid nanomaterials.6,14,24,25 Merocyanine 540 (MC540) is an anionic, lipophilic, watersoluble fluorescent dye having a substituted thiobarbituric acid at one end and a substituted benzoxazole ring at the other end, and these two moieties are linked by a chain containing four conjugated methine groups.26 Hence, MC540 can be a promising building block for forming a supramolecular assembly with oppositely charged surfactants via the ISA strategy. Even though there are many reports emphasizing the use of different dye molecules in the ISA strategy, the efficiency of MC540 as a suitable candidate has remained undiscovered. MC540 shows fluorescent monomer and nonfluorescent dimer equilibrium and the relative contribution of the monomer and dimer forms varies in different homogeneous solvents,27,28 microheterogenous media such as micelles, 28,29 membranes,30,31 and protein solutions,27,28 and also polymer− surfactant aggregates.32 In general, with the decrease in the dielectric constant values of the neat solvents or microenvironments, the monomeric form of MC540 dominates.26 MC540 is very useful as a photosensitizer.33 However, the main importance of MC540 lies in the fields of biology and medicine because it is an important biological probe in membrane studies and it can selectively stain leukemic cells.26 In the past few years, room-temperature ionic liquids (RTILs) have drawn a great deal of attention because of their distinct physical properties such as wide liquidus range, thermal stability, low vapor pressure, and biocompatibility.34,35 Pandey and co-workers have reported that six-carbon-containing IL 1hexyl-3-methylimidazolium bromide (C6mimBr) can alter the physicochemical properties of an aqueous cationic surfactant and C6mimBr behaves partially as a cosolvent.34 On the other hand, a water-soluble imidazolium-based IL can interact with fluorophore 2-naphtholate and quenches its emission intensity, behaving as an electron acceptor.35 ILs can also control the fluorescent H-aggregate formation of cyanine dyes.36 In general, short-chain-containing ILs act as ordinary inorganic salts. However, long hydrophobic alkyl chains containing ILs show the properties of surfactants and are known as surface-active ionic liquids (SAILs). Generally, SAILs have a relatively low critical micelle concentration (CMC) and a greater positive

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Room-temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium chloride (BmimCl) was purchased from SRL (India) (extrapure), 1-hexyl-3-methylimidazolium chloride (C6mimCl) was purchased from TCI Chemicals 9812

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Figure 1. FLIM images of the aggregates formed at (a) 20 mM MC540/20 mM C8mimCl, (b) 10 mM MC540/10 mM C8mimCl, (c) 5 mM MC540/5 mM C8mimCl, (d) 2 mM MC540/2 mM C8mimCl, and (e) 1 mM MC540/1 mM C8mimCl. photodiode (SPAD) detectors are used to collect the fluorescence lifetime images. The images presented in this work are generated using polarized fluorescence transients and acquired using time-correlated single-photon-counting detection electronics (Becker & Hickl). The instrument response function or fwhm (full width at half-maximum) of this system is less than 100 ps. About 10 μL of the sample solution is placed on a slide and allowed to dry before taking all of the images. 2.2.2. Multiwavelength FLIM (MW-FLIM) Study. The MW-FLIM system can detect the fluorescence simultaneously in 16 wavelength channels. Therefore, the wavelength regions can be tuned depending on the situation. The light from one DCS-120 output is focused into the slit plane of the polychromator. The polychromator project a spectrum of fluorescence light on a 16-channel PMT tube inside a bh PML-16C multichannel detector. PML-16 delivers a timing pulse for every photon. Thus, the TCSPC module “routes” photons of different wavelengths into separate lifetime images, and the process does not involve a noticeable loss of photons. MW-FLIM has its own internal high-voltage generator. Thus, no external high-voltage power supply is required. It is controlled by the DCC-100 detector controller module that provides for the power supply, gain control, and overload shutdown. A detailed description of the instrumentation is contained in the Supporting Information.

(India) Pvt. Ltd., 1-decyl-3-methylimidazolium chloride (C10mimCl) was purchased from Sigma-Aldrich, and other SAILs such as 1-octyl-3methylimidazolium chloride (C8mimCl), 1-dodecyl-3-methylimidazolium chloride (C12mimCl), and 1-hexadecyl-3-methylimidazolium chloride (C16mimCl) were received from Kanto Chemicals (98% purity). n-Octyltrimethylammonium bromide (OTAB) and spectroscopic grade 1-octanol were purchased from TCI Chemicals (India) Pvt. Ltd. and Spectrochem, respectively. Merocyanine 540(MC540), cetyltrimethylammonium bromide (CTAB), and tetradecyltrimethylammonium bromide (TTAB) were purchased from Sigma-Aldrich. All of these materials were used as received without further purification. Milli-Q water was used to prepare all of the solutions. MC540, C4mimCl (BmimCl), C6mimCl, C8mimCl, C10mimCl, C12mimCl, C16mimCl, OTAB, 1-octanol, CTAB, and TTAB were weighed and dissolved in Milli-Q water to prepare solutions of the required concentrations. The supramolecular structures were prepared by simple mixing of the oppositely charged building units followed by vigorous stirring to obtain uniform mixing of the solutions. The prepared solutions were kept at room temperature for 2 days before performing all of the experiments. For FLIM and field emission scanning electron microscopy (FESEM) measurements, we have drop cast the above prepared samples on a glass slide whereas for transmission electron microscopy (TEM) measurement the samples were prepared by blotting a carbon-coated (50 nm carbon film) Cu grid (300 mesh, electron microscopy science) with a drop of solution, and for all measurements the samples were allowed to dry overnight for the complete evaporation of the solvent. However, for the AFM study, a drop of the prepared solution was placed on a newly cleaved mica surface. Furthermore, the sample was spin-coated and air-dried overnight before imaging. The sample preparation for cryo-TEM is discussed in the Supporting Information in detail. The chemical structures of the materials are shown in Scheme 1. 2.2. Instrumentation. We have performed FLIM, multiwavelength FLIM (MW-FLIM), FESEM, TEM, cryo-TEM, AFM, Fourier transform infrared spectroscopy (FTIR), and steady-state absorption and zeta potential (ζ) measurements. 2.2.1. Fluorescence Lifetime Imaging Microscopy (FLIM) Measurement. The DCS 120 confocal laser scanning FLIM system (Becker & Hickl DCS-120) equipped with a Zeiss inverted optical microscope is used to take fluorescence lifetime images of the samples. The fluorescence lifetime is determined with a polarized dual-channel confocal scanning instrument (Becker & Hickl DCS-120) that is joined to an output port of the microscope, and a galvodrive unit (Becker & Hickl GDA- 120) controls it. The DCS-120 is equipped with a polarizing beam splitter, and two single-photon avalanche

3. RESULTS AND DISCUSSION 3.1. Composition and Morphologies of Aggregates for MC540/C8mimCl System. Herein, we have utilized the concept of ISA to prepare different fascinating supramolecular assemblies in aqueous solution using negatively charged cyanine dye MC540 and positively charged SAIL C8mimCl by varying the concentrations of both the building units from 1 to 20 mM by retaining the 1:1 stoichiometry of the counterions. The concentration of SAIL C8mimCl is taken up to 20 mM, which is well below the cmc of C8mimCl (∼100 mM),50 and thus within this concentration range, C8mimCl is mainly present as a monomer. On the other hand, MC540 shows monomer−dimer equilibrium in water, and the corresponding monomer, dimer, and polyaggregate absorption peaks appear at ∼542, ∼502, and ∼456 nm, respectively, at ∼10−6 M dye concentration.26,31 The increment of the dye concentration from 1 μM to ∼10 mM lowers the monomer peak intensity whereas the dimer peak is not affected 9813

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microheterogeneity around MC540 as the lifetime distribution histogram shifts to a lower lifetime region at around 300−370 and 290−330 ps, respectively and the distributions are relatively less broad, as shown in Figure 2d,e. In this context, it is important to mention that the fluorescence lifetime is independent of the concentration and fluorescence intensity of the fluorophore. Moreover, the fluorescence lifetime is sensitive to the fluorophore structure, temperature, polarity, viscosity, and fluorescence quencher.42 Hence, the effect of the fluorophore concentration is overshadowed by the change in the microenvironment around the fluorophore. Thus, we can conclude that at higher dye-SAIL concentration (i.e., 20 mM and 10 mM) vesicular aggregates form with a higher lifetime and a broad distribution owing to the better packing of the counterions, resulting in a more rigid and heterogeneous environment around the probe MC540. Similarly, at lower dye-SAIL concentrations (i.e., 1 and 2 mM) packing between the counterions is relatively less efficient and more homogeneous as the lifetime distribution appears at the lower time domain with less broadness. However, we have performed the FLIM experiments for MC540 and C8mimCl at different concentration ratios along with 1:1 stoichiometry, and the details of the concentrations of the counterions and the morphology of the aggregates are tabulated in Table S1 (Supporting Information). It is clear from the observation of the morphologies of the aggregates that the concentration of the counterions is more important than the ratio of the same. Moreover, it is found that the concentration of C8mimCl is more dominant over MC540 in controlling the morphology of the aggregates. At lower C8mimCl concentrations (1−4 mM), only fibrillar morphology is present, whereas at higher C8mimCl concentrations (8−20 mM), vesicular aggregates solely exist for the MC540 concentration ranging from 1 to 20 mM. However, at moderate concentrations of C8mimCl (5−7 mM) both fibrillar and vesicular aggregates are observed. On the other hand, it is quite clear from the tabulated data that the concentration of MC540 also has control of the morphology of the aggregates, though this is less significant than that of C8mimCl. For further confirmation of these morphological transitions by varying the concentrations of the counterions, we have employed the FESEM technique. The images of distinct supramolecular aggregates at dye-SAIL concentrations 2, 5, and 20 mM are depicted in Figure 3i (a−f). These images are well correlated with the FLIM images shown before. Notably, for the 20 mM dye-SAIL pair the vesicular aggregates are present (Figure 3i(a,b)) and at 5 mM concentration rodlike fibrillar aggregates are observed along with some spherical morphology (Figure 3i(c,d)). On further lowering the concentrations of the pair, the supramolecular morphology appears as a highly ordered dense fibril with no trace of spherical aggregates (Figure 3i(e,f)). To obtain more assurance about the structural transition of these highly ordered supramolecular self-assemblies, TEM analysis is executed and shown in Figure 3ii(a−f). TEM images also provide direct evidence to characterize the morphological transformation. Via this technique, we have also found spherical aggregates at 20 mM MC540/20 mM C8mimCl (Figure 3ii(a,b)). Moreover, mixtures of spherical vesicles and rodlike fibrils and entire fibrillar aggregates are also present in 5 and 2 mM dye-SAIL pairs, respectively (Figure 3ii(c,f)). Therefore, the above-mentioned instrumental techniques are sufficient enough to justify the structural transition of the supramolecular

significantly except a few nanometer hypsochromic shift. This type of spectral observation is the result of self-stacking of the monomeric form of MC540 to develop higher-order aggregates such as dimers, trimers, and tetramers.26 Therefore, from the above discussion it is quite clear that the monomer/dimer ratio of MC540 is no longer the same in the concentration range of 1−20 mM dye. There are a good number of reports on the importance of choosing a 1:1 stoichiometric ratio of the counterions in the ISA strategy, and they reveal that under this condition complex formation takes place in a highly cooperative fashion and the resultant product comes out as a precipitate from the solution, which can be ascribed to a strong Coulombic force of attraction.2,3,8,18 The morphologies of the supramolecular aggregates at different counterion concentrations are characterized using the FLIM technique. The FLIM images indicate that there is a prominent morphological transition taking place from spherical vesicular aggregates to fibrils in decreasing the concentrations of the dye−SAIL pair, which is shown in Figure 1a−e. A careful observation unveils that 20 mM MC540/20 mM C8mimCl and 10 mM MC540/10 mM C8mimCl provide vesicular aggregates with a broad size distribution. Furthermore, at a 5 mM concentration of both counterions there appear some fibrillar assemblies along with spherical morphology. Again, a 1 and 2 mM MC540/C8mimCl combination yields completely fibrillar morphology with dense packing. Apparently at both concentrations the morphology is similar to only the thickness of the fibrils, which is greater at 2 mM concentration than at 1 mM. Herein, we have attempted to monitor this structural transition for the first time using efficient lifetime distribution histograms, shown in Figure 2a−e. Vesicular aggregates

Figure 2. Lifetime distribution histograms of the aggregates formed at (a) 20 mM MC540/20 mM C8mimCl, (b) 10 mM MC540/10 mM C8mimCl, (c) 5 mM MC540/5 mM C8mimCl, (d) 2 mM MC540/2 mM C8mimCl, and (e) 1 mM MC540/1 mM C8mimCl.

corresponding to 20 mM MC540/20 mM C8mimCl exhibit the lifetime distribution with the peak maximum ranging from 750 to 850 ps, as shown in Figure 2a. Lowering the concentration of the dye/SAIL pair to 10 and 5 mM results in the lowering of the lifetime of MC540 to 460−550 and 460− 530 ps ranges, respectively, as shown in Figure 2b,c. Therefore, from these observations it is quite understandable that the environment surroundings dye MC540 is changing and becoming less restricted. Further lowering the concentrations of the counterions to 2 and 1 mM by keeping the molar ratio at 1 unveils the significant reduction in rigidity as well as 9814

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Figure 3. (i) FESEM images of 20 mM MC540/20 mM C8mimCl (a, b), 5 mM MC540/5 mM C8mimCl (c, d), and 2 mM MC540/2 mM C8mimCl (e,f). (ii) TEM images of 20 mM MC540/20 mM C8mimCl (a, b), 5 mM MC540/5 mM C8mimCl (c, d), and 2 mM MC540/2 mM C8mimCl (e, f).

Figure 4. Cryo-TEM images of (a) 20 mM MC540/20 mM C8mimCl, (b) 5 mM MC540/5 mM C8mimCl, and (c) 2 mM MC540/2 mM C8mimCl.

an average height of around ∼6 nm. The height profile of a single vesicle is taken along the dotted line, as shown in the image. We have already mentioned the various important utilities of the emerging and efficient FLIM technique in biology. Very recently, our group analyzed the change in the lifetime distribution of a fluorescent probe molecule inside a single vesicle at different emission wavelengths using a multiwavelength FLIM (MW-FLIM) technique. This technique provides important information regarding the solvation of a

assemblies, and also the obtained results are in good agreement with each other. Here, we have employed a more sophisticated technique, cryo-TEM, to gain more detailed information about the morphological transformation of the supramolecular aggregates, as shown in Figure 4. Additionally, AFM is employed as an attractive tool to observe the image of the vesicular aggregates, constructed at 20 mM MC540/20 mM C8mimCl, as shown in Figure S1 (Supporting Information). Here, we have found vesicles with 9815

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Figure 5. (a) Multiwavelength FLIM measurement of a single vesicle composed of 20 mM MC540 and 20 mM C8mimCl. Images are collected in four different emission wavelength regions: (i) 606−619, (ii) 656−669, (iii) 706−719, and (iv) 756−769 nm using an excitation wavelength of 488 nm. The upper panel represents the lifetime images, and the lower panel represents the intensity images. (b) Corresponding lifetime distribution of vesicles collected in four different wavelength regions.

regimes, and a biexponential fitting equation is utilized to fit the lifetime decays. The presence of dynamic heterogeneity inside a single vesicle is provided by the FLIM images, where the probe molecules are distributed in different regions. Thus, it is important to analyze the MW-FLIM images employing a lifetime distribution in different emission wavelength regions shown in Figure 5b. The lifetime distribution histogram suggests that the lifetime of MC540 is sharp and lies in between 150 to 230 ps in the wavelength region of 606 to 619 nm, i.e., at the blue end of the emission wavelength. However, in moving toward the red end of the emission spectrum, the fluorescence lifetime increases, and at the extreme red end of the emission wavelength, i.e., in the 756 to 769 nm region, there is a significant increase in the fluorescence lifetime ranging from ∼570 to 820 ps, which clearly indicates that the dye is getting more solvated and stabilized at the red end of the emission measurement. 3.2. Driving Forces for the Formation of Different Supramolecular Aggregates. We have discussed that the

probe molecule in a single vesicle, the autofluorescent property of various cells, and so forth.44 It is well known that the properties of a single vesicle differ significantly from the ensemble average measurements, and even in a single vesicle, a substantial amount of heterogeneity is present. We have mentioned earlier that the lifetime distribution of the vesicular assembly exhibits a broad nature that is a consequence of the environmental heterogeneity around the fluorophore. Therefore, we are very interested in using the MW-FLIM technique to get finer details about the structural heterogeneity of the vesicular aggregates. In this method, data is collected by the simultaneous use of several detector channels, and thus we obtain FLIM images with different lifetime distributions in different emission wavelength regimes. Here we have used 488 nm as the excitation wavelength for MC540, and FLIM images are collected in four different emission wavelength regimes starting from 606 to 769 nm, as shown in Figure 5a. We have monitored the lifetime values at a particular position of a single vesicle (diameter ∼6 μm) in each of the four wavelength 9816

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found in the dye/1-octanol combination, and this additionally confirms the significance of the coulombic attraction between the building blocks. 3.2.2. Hydrogen Bonding Interaction. There are several literature reports dealing with urea addition to a supramolecular self-assembly to destroy the hydrogen bonding interactions existing between the building blocks.3,8 We have also added 2 M urea to our system and found that both the supramolecular assemblies, i.e., fibrils, and vesicles are still present. This observation indicates that here hydrogen bonding is not playing the governing role in the formation of the assemblies. To form any supramolecular self-assembly, the choice of the building blocks is very important, and here it also appears that MC540 and C8mimCl are suitable counterions where the concentrations of both the materials have exquisite control over the morphological transition. To get better insight into the structural importance of SAIL C8mimCl, we have carried out two types of control experiments. We can vary the hydrophilic imidazolim headgroup of C8mimCl by an aliphatic cationic analogue as well as the number of carbon atoms in the hydrophobic tail of C8mimCl. 3.2.3. Hydrophobic Effect. In the first study, we are interested in varying the hydrophobic chain length of SAIL C8mimCl, keeping the same hydrophilic headgroup. Therefore, we have substituted it with CnmimCl (n = 4, 6, 10, 12, 16), i.e., C4mimCl, C6mimCl, C10mimCl, C12mimCl, and C16mimCl, in order to investigate the effect of hydrophobicity in the supramolecular aggregate transition. When C8mimCl is replaced by C4mimCl, there is no signature of any ordered aggregates in the FLIM image, no precipitate comes out, and the solution remains homogeneous. Here also we have used the same five concentrations (i.e., 1, 2, 5, 10, and 20 mM) by keeping the molar ratio at 1. Similarly for C6mimCl and C10mimCl at the above-mentioned five concentrations at a molar ratio of 1, no morphological transition is found. Because the CMC of C12mimCl is ∼12.5 mM,52 we have performed FLIM analysis for 10, 5, and 2 mM concentrations of the MC540/C12mimCl pair, and the images are shown in Figure S3a−c. The images indicate that for a 10 mM dye-SAIL pair there exist vesicular aggregate and in moving to 5 and 2 mM concentrations an ellipsoid kind of morphology is developing along with vesicular aggregates. However, the gross morphology remains almost intact, and no such vesicle to fibril transition is observed in lowering the concentration. However, for the MC540/C16mimCl pair we have used 0.75 and 0.5 mM concentrations because the CMC of C16mimCl is ∼1 mM.52 Here, we have observed dense fibril or microtubular aggregates at both concentrations, and the images are depicted in Figure S4a,b. The above observations indicate that a very short hydrophobic chain ([C4mim]+ and [C6mim]+) cannot provide sufficient hydrophobicity to form any supramolecular aggregates. However C10mimCl has a longer chain length than C8mimCl, it is also unable to show any specific morphology. On the other hand, for C12mimCl and C16mimCl there is the formation of some self-assembled aggregates at different concentrations of the dye-SAIL pair. However, no such morphological transition takes place for [C12mim]+ and [C16mim]+, and this implies that the correct hydrophobic chain length is required to observe the transformation of morphology as a function of the concentration of the counterions. 3.2.4. Effect of Headgroups. In the second study, we have investigated the effect of the hydrophilic imidazolium head-

supramolecular transition of the MC540/C8mimCl pair is highly concentration-dependent and these concentrations (1 to 20 mM) are much smaller than that of the CMC (∼100 mM) of C8mimCl. Thus, the critical aggregation concentration (CAC) of this dye-SAIL assembly is lower than the CMC of C8mimCl owing to the greater free enthalpy of binding for dye and SAILs than for the SAILs themselves. Here, we have chosen MC540 and C8mimCl as the building blocks for the formation of the supramolecular assembly, where both molecules have long hydrophobic chains, delocalized π-electron clouds, and charged headgroups; therefore, the possible forces of interaction involved in the supramolecular assemblies are the following: electrostatic, hydrogen bonding, hydrophobic, and π−π stacking. Therefore, at this point, it is quite reasonable that several factors are acting together to form the aggregates and a detailed structural investigation is required to understand the synergistic interplay of various interactions between the building blocks. 3.2.1. Electrostatic Interaction. In the ISA strategy, the electrostatic interactions between the oppositely charged counterions play a key role in the formation of supramolecular aggregates. Here, we have used anionic fluorophore MC540 and cationic SAIL C8mimCl to prepare the self-assembly, and it is quite obvious that the coulombic force of attraction will dominate. To elucidate the effect of electrostatic interactions and to gain molecular-level information about the structural change in the self-assembly, we have employed FTIR spectroscopy as a powerful technique. The FTIR spectra of C8mimCl, 2 mM MC540, 20 mM MC540, and vesicle and fibrillar aggregates are depicted in Figure S2a,b. The symmetric (νsym) and antisymmetric (νasym) stretching vibrations of CH2 appear at 2854 and 2924 cm−1, respectively, and indicate that the alkyl chains are in the gauche conformation.3,51 However, after aggregate formation with MC540, there is no alteration in the CH2 stretching frequencies (both νsym and νasym) and it assures that CH2 remains in the same conformation even after complex formation. On the other hand, the band located at 1569 cm−1 corresponding to the C N of C8mimCl is shifted to 1581 and 1595 cm−1 as a result of vesicle and fibril formation, respectively, and confirms the C8mimCl contribution in the electrostatic interaction. Moreover, we have found that the sulfonate symmetric (νsym) and antisymmetric (νasym) stretching vibrations for 20 mM MC540 appear at 1118 and 1166 cm−1, respectively, and these characteristic bands undergo red shifts to 1100 and 1161 cm−1, respectively, in the vesicular aggregate. Again, for 2 mM MC540 these sulfonate stretching frequencies, νsym and νasym, show vibrational bands at 1116 and 1174 cm−1, respectively, and the fibrillar aggregates display the corresponding bands at 1123 and 1180 cm−1, respectively. Therefore, the above discussion dictates the significant impact of the sulfonate moiety in the electrostatic interaction with C8mimCl as well as in the ISA strategy. Besides, the zeta potential measurement of C8mimCl, 20 mM MC540, and vesicular aggregates provides values of 35 ± 2, −36 ± 4, and −0.002 + 0.0005 mV, respectively. Therefore, the zeta potential measurement further supports the involvement of the electrostatic interaction to form the supramolecular architectures. To realize the real impact of positively charged imidazolium cation C8mimCl in the electrostatic interaction with anionic MC540, we have substituted C8mimCl with 1octanol in all five of the above-mentioned concentrations at a stoichiometric ratio of 1:1. However, no ordered aggregates are 9817

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Figure 6. UV−visible absorption spectra of 2 mM MC540, 20 mM MC540, fibrillar and vesicular solutions. (The inset shows the individual absorption spectra of 2 mM MC540, fibril, and vesicle solutions.)

fibrils are present with few vesicles and also two different lifetime distributions justify the simultaneous existence of two distinct morphologies. One distribution is around ∼165 to 330 ps, and the other one is around ∼440 to 950 ps, with the former having a higher intensity. Again, the complete fibrillar morphology, which forms at 2 mM MC540/2 mM OTAB, exhibits a further reduction in lifetime to 130−370 ps with a sharp lifetime distribution. In summary, we can tell that vesicular aggregates are heterogeneous with higher lifetime values, and in moving toward fibrillar aggregates, the lifetime of MC540 inside the self-assembly diminishes and also the heterogeniety is relatively smaller here. Therfore, similar to C8mimCl, here OTAB is also forming vesicles with MC540, which provides a more rigid environment for MC540 compared to the fibril. FESEM images at 20, 5, and 2 mM MC540/OTAB further confirm the similar morphologies obtained in FLIM, i.e., vesicles, fibril−vesicle mixtures, and completely fibrils, respectively, as depicted in Figure S7a−f. We have performed FLIM measurements using merocyanine 540 (MC540) and cationic surfactants cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB). We have observed the formation of some aggregates with no specific shapes, and also we have not found any morphological transition on changing the concentration of the counterions. Therefore, the hydrophobic chain length is also important in observing the structural transformation in the conventional surfactants. Earlier, we mentioned that MC540 shows monomer−dimer equilibrium and the relative proportion of each form is highly dependent on the dielectric constant of the solvent or microenvironment. Notably, a decrease in the polarity of the medium increases the monomer/dimer ratio because the monomer form is more dominant in a less-polar medium.26−28,31 Here, we have observed the UV−visible absorption spectra of 2 and 20 mM MC540 along with the solutions of fibrillar and vesicular aggregates by taking the clear solution (removing the precipitate) plotted in Figure 6. At a 20 mM concentration of MC540, the absorbance value is very high and the dimer peak gets saturated. However, at 2 mM MC540 the monomer and dimer peaks appear at ∼530 and ∼497 nm, respectively, with a monomer/dimer peak ratio of ∼0.74. The fibrillar aggegates give rise to monomer and dimer peaks at around ∼538 and ∼501 nm, having a monomer/dimer peak ratio of ∼0.92. Thus, from this viewpoint it is clear that fibrillar

group in the morphological transition while keeping the hydrophobic chain length fixed. For this purpose, we have chosen OTAB, where the π-electron cloud containing the aromatic imidazolium moiety is substituted by the aliphatic trimethylammonium moiety, having no π-electron involvement. In this study, we have also chosen the same five concentrations, i.e., 1, 2, 5, 10, and 20 mM, of the MC540/ OTAB pair for convenient comparison with C8mimCl because the CMC of OTAB is ∼130 mM.53 Li and co-workers have reported the interaction between an anionic photoresponsive azo dye and cationic surfactant OTAB.54 Interestingly, in this study we have found that at 20 and 10 mM the MC540/OTAB pair forms vesicles with diameter ranging from 1 to 4 μm, as shown in Figure S5a,b. Again, at a 5 mM concentration of the dye-surfactant pair the morphology is mainly fibrillar with small traces of vesicular aggregates (Figure S5c). At 2 and 1 mM concentrations of the building block pair, complete fibrillar aggregates are present with dense packing (Figure S5d,e). Therefore, these observations in the FLIM image provide the information indicating that the structural transition takes place on lowering the concentration of the MC540/OTAB pair, similar to that of MC540/C8mimCl. However, a close look at the morphologies reveal that there is some difference between the fibrillar aggregates, formed by 2 mM MC540/2 mM C8mimCl and 2 mM MC540/2 mM OTAB. Therefore, we can conclude that eight carbons containing a hydrocarbon chain and a cationic hydrophilic headgroup are essential in SAIL or the surfactant to witness the concentration-dependent structural transition in the presence of anionic dye MC540. In the presence of an equimolar concentration of MC540, surfactant OTAB shows a similar type of morphological transition without having a delocalized π-electron such as in C8mimCl. Still, we cannot rule out the effect of π−π interactions between the imidazolium cation of C8mimCl and aromatic moieties as some difference in the morphology with the dye/surfactant pair is observed. The lifetime distribution histograms of the MC540 dye inside the vesicle, a mixture of vesicles and fibrils, and in completely fibrillar aggregates are shown in Figure S6a−c corresponding to 20, 5, and 2 mM MC540/OTAB concentrations, respectively. For 20 mM MC540/20 mM OTAB, the lifetime distribution is quite broad, ranging from 350 to 1275 ps, indicating that heterogenity is present inside the vesicle. Whereas for 5 mM MC540/5 mM OTAB, mostly 9818

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parameter is very useful in predicting, explaining, and rationalizing the different self-assembled aggregates.

aggregates provide a less-polar environment for MC540 because both the monomer and dimer peaks are red-shifted with increases in the corresponding monomer−dimer peak ratio. However, for the vesicular aggregates, the environment surrounding MC540 is more hydrophobic because the monomer−dimer peak ratio is ∼1 with the monomer peak maximum being almost the same at 537 nm and the dimer peak maximum being more red-shifted to 505 nm. Therefore, the UV−visible spectral observations further support the information obtained from FLIM, FESEM TEM, and cryo-TEM images. 3.3. Influence of Packing Parameter in Molecular SelfAssembly and Structural Transition. Lifetime distribution histograms and UV−visible spectra imply that fibrils and vesicles provide a more hydrophobic and more rigid environment for MC540 compared to that for free MC540. Moreover, this rigidity and confinement are much higher for vesicular rather than fibrillar aggregates. Israelachvili and co-workers have introduced the term molecular packing parameter (P) to shed light on the self-assembly phenomenon.46 Packing parameter P = v/a0lc is a balance of three geometric factors, where v and lc are the volume and length of the hydrophobic surfactant chain, respectively. The surface area of the polar headgroup at the CMC is represented by a0.47−49 For P < 1/3, spherical micelles form; for 1/3 < P < 1/2, rodlike or cylindrical micelles are the favorable morphology and bilayers with a spontaneous curvature (vesicles) form for 1/2 < P < 1.47 The surface area of the polar headgroups of the surfactants depend on two opposing factors. In the hydrocarbon−water interface, there is hydrophobic attraction between the hydrophobic chains, and on the other hand, a repulsive force appears as a result of the close proximity of the similarly charged hydrophilic headgroups. Because of the presence of these two opposing factors, the effective headgroup area per molecule at the surface is not the ordinary geometrical area; instead, an equilibrium parameter appears from various thermodynamic considerations. We have shown in the UV−visible spectra that MC540 is not present in the same monomer−dimer form at 2 and 20 mM concentrations. Along with monomer and dimer peaks, at 422 nm MC540 exhibits a broad absorption band due to the Haggregates. These H-aggregates are nonfluorescent and form because of the face-to-face stacking of the monomeric form of the dye.55 For lower concentrations of the dye-SAIL pair, fibrillar morphology forms through head to head or edge to edge packing and grows along one direction. Segota et al.56 have reported that the attraction between the counterions decreases the effective area of the headgroups of the building blocks. Therefore, at a higher dye-SAIL concentration of 20 mM, the aggregation number is higher, i.e., greater numbers of building blocks are involved, and it develops a greater force of attraction between the counterions and further lowers the effective headgroup area of the building blocks, resulting in an increase in the packing parameter (P) value. Notably, when the fibrillar aggregates achieve their critical size, they fold up to form vesicles. Therefore, at higher P (in between 1/2 and 1), the vesicular morphology dominates, having a higher curvature than fibrillar aggregates have. It is also reported that P > 1/2 requires a small headgroup area and a long hydrophobic tail part.56 Here also for vesicular aggregates, the headgroup area is decreasing as a result of the higher electrostatic force of attraction, and the increase in hydrophobicity is evidenced from UV−visible absorption measurements. Therefore, the packing

4. CONCLUSIONS Molecular self-assembled aggregates are ubiquitous in nature, and the constituent building blocks satisfy some precise characteristics to follow the ISA technique. This study focuses on a concentration-responsive remarkable fibril−vesicle structural change using MC540 and C8mimCl. Along with FLIM, FESEM, TEM, and cryo-TEM studies, FTIR spectra elicit the importance of the electrostatic interaction between the cationic imidazolium moiety of C8mimCl and the anionic sulfonate group of MC540 in the formation of supramolecular aggregates. Thereafter, a systematic study is performed to investigate the effect of the hydrophilic headgroup and the hydrophobic tail part of the SAILs in controlling this concentration-dependent aggregate transition. Traditional surfactant OTAB has the same chain length as does SAIL C8mimCl but differs in the headgroup and can also exhibit a similar type of morphological transformation. On the other hand, C4mimCl, C6mimCl, C10mimCl, C12mimCl, and C16mimCl, having the same headgroup but different hydrophobic chain lengths compared to that of C8mimCl, are unable to show a concentrationdependent vesicle−fibril transition. Therefore, this study successfully addresses the effect of hydrophobicity along with the electrostatic interaction, i.e., the structural importance of the building blocks in constructing different supramolecular assemblies. Detailed investigations with the aid of different instruments further confirm the involvement of electrostatic, hydrophobic, and π−π interactions on the development of the self-assembled aggregates. The packing parameter (P) also supports the above morphological transformation with the change in the concentrations of the counterions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b02136. Instrumental section; tabular form of morphology of the supramolecular aggregates at different concentration ratios of MC540 and C8mimCl; AFM images of vesicles; FTIR spectra of the building blocks and the aggregates; FLIM images of MC540/C12mimCl, MC540/C16mimCl, MC540/OTAB; and FESEM images of MC540/OTAB (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Nilmoni Sarkar: 0000-0002-8714-0000 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.S. is grateful to SERB, Department of Science and Technology (DST), Government of India, for generous research grants. R.D., A.P., S.K., and P.B. are grateful to CSIR for their research fellowships. 9819

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