Concentration-Driven Fascinating Vesicle-Fibril Transition Employing

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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 Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02136 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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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. E-mail: [email protected] Fax: 91-3222-255303 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 ionic self-assembly (ISA) strategy. It is noteworthy that the concentration of the counterions has the exquisite control over the morphology, as on lowering the concentration of both the building blocks at stoichiometric ratio 1:1, provides vesicle to fibril transition. Here, we have reported concentration-controlled 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 like field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and cryogenictransmission electron microscopy (cryo-TEM) to gain additional support. Besides, multiwavelength FLIM (MW-FLIM) and atomic force microscopy (AFM) techniques assist to know the microheterogeneity and the height profile of the vesicles, respectively. We have replaced the SAIL, C8mimCl by 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 aggregate and thus reveals the importance of electrostatic interaction in the supramolecular aggregate formation. However, the SAILs having the same imidazolium head group with different chain lengths other than C8mimCl are unable to display any structural transition and conclude the importance of correct chain length for efficient packing of the counterions to form a specific self-assembly. Therefore, this study reveals the synergic interplay of electrostatic, hydrophobic and π-π stacking interactions to construct the self-assembly and their concentration dependent morphological transition.

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1. Introduction Molecular self-assembly formation and subsequent morphological transition are one of the important areas of research and have drawn immense interest in recent years. Supramolecular self-assemblies1 cover a wide range of fascinating structures including giant vesicles,2-5 various nanostructures like nanoflower,6 nanobelt,2 and also one dimensional structures like microfibers,7 tubes, rods etc.8 These self-assemblies have extensive use in the field of medicine, biotechnology,9 drug delivery10 etc. Among these supramolecular assemblies, giant vesicles have drawn much attention as they can act as micro-reactor for silver and gold nanoparticle synthesis,2,3 can entrap carbon quantum dots (CQD) and also quantum dots (QDs) inside their hollow cavity.3,4 Besides, giant vesicles can act as artificial cells owing to the structural and dynamical similarities with biological cell membrane4 and moreover they show antimicrobial activity.11 To fabricate the above mentioned self-assembled supramolecular aggregates, various routes have been employed such as hydrogen bonding, πconjugation, metal coordination, key-lock combination, amphiphilic association, and ionic self-assembly (ISA).2,8,12,13 Among these techniques, ISA strategy is of paramount interest as it is an easy, cheap and flexible method with universal applicability.14,15 In this approach two building blocks of oppositely charges are employed and electrostatic force of attraction between them plays the key role in the formation of versatile self-assembled 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 columbic force of attraction to form 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 due to their easy availability, extended π-conjugation, regular shape, optical applications etc.3,13,18 Several research groups have reported various selfassembled 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 dyesurfactant 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 ISA technique to form supramolecular assembly. Very recently, polyoxometalates are also used extensively to prepare supramolecular hybrid nanomaterials.6, 14, 24, 25 Merocyanine 540 (MC540) is an anionic, lipophilic, water soluble 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 2 ACS Paragon Plus Environment

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Hence, MC540 can be a promising building block to form supramolecular assembly with oppositely charged surfactants via ISA strategy. Even though there are many reports emphasizing the use of different dye molecules in 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 monomer and dimer form varies in different homogeneous solvents,27,28 microheterogenous media like micelles,28,29 membrane,30,31 protein solutions27,28 and also in polymer-surfactant aggregates.32 In general, with decrease in the dielectric constant values of the neat solvents or microenvironments, monomeric form of MC540 dominates.26 MC540 is very much useful as a photosensitizer.33 However, the main importance of MC540 lies in the field of biology and medicine as 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 due to their distinct physical properties such as wide liquidus range, thermal stability, low vapor pressure, biocompatibility.34,35 Pandey and coworkers have reported that six carbon containing IL 1-hexyl-3-methylimidazolium bromide (C6mimBr) can alter the physicochemical properties of aqueous cationic surfactant and C6mimBr behaves partly as a cosolvent.34 On the other hand, 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 chain containing ILs show the properties of surfactants and known as surface active ionic liquids (SAILs). Generally, SAILs have relatively low critical micelle concentration (CMC) and greater positive charge delocalization compared to the corresponding aliphatic analogue.3,5,8,37 Kumar et al. have discussed the interaction between three structurally different calixarenes and SAIL, 1-decyl-3-methylimidazolium chloride (C10mimCl) in detail.38 In this context, it is important to note that various stimuli, like temperature,2,39,40 pH,40 redox, enzyme, light,41 conjugation7 etc. have substantial impact on the morphological transitions. Thermo-responsive vesicle to nanofiber39 and nanobelt2 transitions are reported by the Dong and Yue groups, respectively. Joshi et al. have reported conjugation triggered fiber to spherical morphological transition.7 Nowadays, fluorescence lifetime imaging microscopy (FLIM) is extensively used as a complementary technique to conventional fluorescence intensity measurement, having several applications in the determination of molecular environment parameter, molecular state of cells, protein interactions via Förster resonance energy transfer (FRET) etc.42-45 In this context it is 3 ACS Paragon Plus Environment

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important to mention that 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 ISA strategy we have shown the formation of two different supramolecular aggregates (fibril and vesicle) at different MC540/C8mimCl concentrations keeping 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, 16) with different chain lengths as a counterion of MC540 and infers that appropriate chain length of SAIL is required to obtain the morphological conversion. Moreover, the different types of interactions of MC540 with OTAB and 1-octanol confirm the importance of electrostatic interaction in supramolecular aggregate formation. 2. Experimental section 2.1. Materials and Sample Preparation. Room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium chloride (BmimCl) is purchased from SRL (India) (extrapure), 1-hexyl-3-methylimidazolium chloride (C6mimCl) is purchased from TCI Chemicals (India) Pvt. Ltd., 1-decyl-3-methylimidazolium chloride (C10mimCl) is purchased from Sigma-Aldrich and other SAILs like 1-octyl-3methylimidazolium

chloride

(C8mimCl),

1-dodecyl-3-methylimidazolium

chloride

(C12mimCl) and 1-hexadecyl-3-methylimidazolium chloride (C16mimCl) are received from Kanto Chemicals (98% purity). n-octyltrimethylammonium bromide (OTAB) and spectroscopic grade 1-octanol are purchased from TCI Chemicals(India) Pvt. Ltd. and Spectrochem, respectively. Merocyanine 540(MC540), cetyl trimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB) are bought from SigmaAldrich. All these materials are used as received without further purifications. Milli-Q water is used to prepare all the solutions. MC540, C4mimCl (BmimCl), C6mimCl, C8mimCl, C10mimCl, C12mimCl, C16mimCl, OTAB, 1-octanol, CTAB and TTAB are weighed and dissolved in milli-Q water to prepare the solutions of required concentrations. The supramolecular structures are prepared by simple mixing of the oppositely charged building units followed by vigorous stirring to obtain uniform mixing of the solutions. The prepared solutions are kept at room temperature for two days before performing all the experiments. For FLIM and field emission scanning electron microscopy (FESEM) measurements, we have drop casted the above prepared samples on a glass slide whereas for transmission 4 ACS Paragon Plus Environment

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electron microscopy (TEM) measurement, the samples are 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 the measurements the samples are allowed to dry overnight for the complete evaporation of the solvent. However, for AFM study, a drop of the prepared solution is placed on a newly cleaved mica surface. Furthermore, the sample is 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.

Scheme 1. The chemical structures of MC540, C8mimCl and OTAB.

2.2. Instrumentation. We have performed FLIM, multiwavelength FLIM (MW-FLIM), FESEM, TEM, Cryo-TEM, AFM, fourier transformed infrared spectroscopy (FTIR), 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 an inverted optical microscope of Zeiss is used to take the fluorescence lifetime images of the samples. Fluorescence lifetime is determined with a polarized dual channel confocal scanning instrument (Becker & Hickl DCS-120) which is joined with an output port of the microscope and a galvo-drive unit (Becker & Hickl GDA- 120) controls it. The DCS-120 is equipped with a polarizing beam splitter and two single photon avalanche photodiode (SPAD) detectors are used to collect the fluorescence lifetime images. The images presented 5 ACS Paragon Plus Environment

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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 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 the images. 2.2.2. Multi-Wavelength 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 the 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, TCSPC module ‘routes’ photon of different wavelengths into separate lifetime images and the process does not involve 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 which provides for power supply, gain control and overload shutdown. The detailed description on instrumentation section is discussed in the Supporting Information. 3. Results and Discussion 3.1. Composition and the 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 keeping 1:1 stoichiometry of the counterions. The concentration of the 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 monomer. On the other hand, MC540 shows a 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 Increment of the dye concentration from 1 µM to ~10 mM lowers the monomer peak intensity whereas the dimer peak is not affected significantly except 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 like dimer, trimer, and tetramer etc.26 So, from the above discussion it is quite clear that the monomer/dimer ratio of 6 ACS Paragon Plus Environment

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MC540 is no longer same in the concentration range 1-20 mM of dye. There is a good number of reports on the importance of choosing 1:1 stoichiometric ratio of the counterions in ISA strategy and they reveal that at this condition complex formation takes place in a highly cooperative fashion and the resultant product comes out as precipitate from the solution which can be ascribed to strong coulombic force of attraction.2,3, 8,18 The morphologies of the supramolecular aggregates at different counterion concentrations are characterized using 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 provides vesicular aggregates with a broad size distribution. Furthermore, at 5 mM concentration of both the counterions there appears some fibrillar assemblies along with spherical morphology. Again, 1 and 2 mM MC540/ C8mimCl combination yields completely fibrillar morphology with dense packing. Apparently at both the concentrations the morphology is similar with only the thicknesses of the fibrils are greater at 2 mM concentration compared to 1 mM.

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 (e) 1 mM MC540/1 mM C8mimCl 7 ACS Paragon Plus Environment

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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 corresponding to 20 mM MC540/20 mM C8mimCl, exhibit the lifetime distribution with peak maximum ranging from 750 ps to 850 ps, shown in Figure 2a. With lowering the concentration of the dye/SAIL pair to 10 and 5 mM resulting in the lowering of the lifetime of MC540 to 460 ps to 550 ps and 460 ps to 530 ps ranges, respectively and shown in Figure 2b,c. Therefore, from these observations it is quite understandable that the environment surroundings the dye MC540 is changing and becoming less restricted. Further lowering the concentrations of the counter ions to 2 and 1 mM keeping the molar ratio at 1 unveils the significant reduction in rigidity as well as microheterogeneity around MC540 as the lifetime distribution histogram shifts to lower lifetime region at around 300 ps to 370 ps and 290 ps to 330 ps, respectively and the distributions are relatively less broad, shown in Figure 2d, e. In this context it is important to mention that fluorescence lifetime is independent of the concentration and fluorescence intensity of the fluorophore. Moreover, fluorescence lifetime is sensitive to the fluorophore structure, temperature, polarity, viscosity; fluorescence quencher etc.42 Hence, the effect of fluorophore concentration is overshadowed by the change in the microenvironment around the fluorophore. Thus we can conclude that at higher dyeSAIL concentration (i.e., 20 mM and 10 mM) vesicular aggregates form with higher lifetime and 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. (a) Intensity(a.u.)

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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 (e) 1 mM MC540/1 mM C8mimCl 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 counter ions 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 counter ions 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 are solely exist for 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 has also the control on 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 FESEM technique. The images of distinct supramolecular aggregates at the dye-SAIL concentrations 2, 5 and 20 mM are depicted in Figure 3.i (a-f).These images are well correlated with the FLIM images shown before. Notably, at 20 mM dye-SAIL pair the vesicular aggregates are present Figure 3.i(a, b) and at 5 mM concentrations rod-like fibrillar aggregates are observed along with some spherical morphology, Figure 3.i(c,d). On further lowering the concentrations of the pair, the supramolecular morphology appears as highly ordered dense fibril with no trace of spherical aggregates, Figure 3.i(e, f). To obtain more assurance about the structural transition of these highly ordered supramolecular self-assembly, TEM analysis is executed and shown in Figure 3.ii(a-f). TEM images also provide direct evidence to characterize the morphological transformation. In this technique we have also found spherical aggregates at 20 mM MC540/20 mM C8mimCl, Figure 3.ii(a,b). Moreover, mixtures of spherical vesicle and rod-like fibril as well as entire fibrillar aggregates are also present at 5 and 2 mM dye-SAIL pair, respectively. Figure 3.ii(c,f). Therefore, the above mentioned instrumental techniques are sufficient enough to justify the structural transition of the supramolecular assemblies and also the obtained results are in a good agreement with each other. 9 ACS Paragon Plus Environment

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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); 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); 2 mM MC540/2 mM C8mimCl (e,f). Here, we have employed a more sophisticated technique, cryo-TEM to gain the more detailed information about the morphological transformation of the supramolecular aggregates and are shown in Figure 4.

(a)

(b)

(c)

Figure 4. Cryo-TEM images of (a) 20 mM MC540/20 mM C8mimCl (b) 5 mM MC540/5 mM C8mimCl (c) 2 mM MC540/2 mM C8mimCl. Additionally, AFM is employed as an attractive tool to observe image of the vesicular aggregates, constructed at 20 mM MC/20 mM C8mimCl, shown in Figure S1(Supporting Information). Here, we have found vesicles with average height around ~6 nm. Height profile of a single vesicle is taken along the dotted line, 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 has analysed the change in lifetime distribution of a fluorescent probe molecule inside a single vesicle at different emission wavelengths using multiwavelength FLIM (MW-FLIM) technique. This technique provides important information regarding the solvation of probe molecule in a single vesicle, 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, substantial amount of heterogeneity is present. We have mentioned earlier that the lifetime distribution of the vesicular assembly exhibits a broad nature which is the 11 ACS Paragon Plus Environment

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consequence of environmental heterogeneity around the fluorophore. Therefore, we are very much interested to perform the MW-FLIM technique to get finer details about the structural heterogeneity of the vesicular aggregates. In this method, data is collected by simultaneous use of several detector channels and thus we obtain FLIM images with different lifetime distributions at 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 nm to 769 nm 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 regimes and 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 analyse the MW-FLIM images employing lifetime distribution at 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 ps to 230 ps in the wavelength region 606 nm to 619 nm i.e., at the blue end of emission wavelength. However, in moving towards the red end of the emission spectrum, fluorescence lifetime increases and at the extreme red end of the emission wavelength i.e., in the 756 nm to 769 nm region there is significant increase in the fluorescence lifetime ranging from ~570 ps to 820 ps is observed which clearly indicates that the dye is getting more solvated and stabilised at the red end of the emission measurement.


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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 at four different emission wavelength regions: (i) 606-619, (ii) 656-669, (iii) 706-719, and (iv) 756-769 nm using excitation wavelength of 488 nm. The upper panel represents the lifetime images, and the lower panel represents the intensity images and (b) the corresponding lifetime distribution of vesicles collected in four different wavelength regions. 3.2. Driving forces for the formation of different supramolecular aggregates. We have discussed that supramolecular transition of MC540/C8mimCl pair is highly concentration dependent and these concentrations (1 to 20 mM) are much less than that of the CMC (~100 mM) of C8mimCl.Thus the critical aggregation concentration (CAC) of this dyeSAIL assembly is lower than the CMC of C8mimCl owing to the greater free enthalpy of binding for dye and SAILs than the SAILs themselves. Here, we have chosen MC540 and C8mimCl as the building blocks for the formation of supramolecular assembly, where both the molecules have long hydrophobic chain, delocalised π-electron cloud and a charged head group and 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 synergic interplay of various interactions between the building blocks. 13 ACS Paragon Plus Environment

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3.2.1. Electrostatic Interaction. In ISA strategy, the electrostatic interactions between the oppositely charged counterions particularly play the key role in the formation of supramolecular aggregates. Here, we have used anionic fluorophore MC540 and cationic SAIL C8mimCl to prepare self-assembly and it is quite obvious that coulombic force of attraction will dominate. To elucidate the effect of electrostatic interactions and to gain molecular level information about the structural change of the self-assembly, we have employed FTIR spectroscopy as a powerful technique. The FTIR spectra of C8mimCl, 2 mM MC540, 20 mM Mc540, 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 assuring that CH2 is remaining in the same conformation even after the complex formation. On the other hand, the band located at 1569 cm-1 corresponds to the C=N of C8mimCl is shifted to 1581 cm-1 and 1595 cm-1 due to vesicle and fibril formation, respectively and confirms the contribution C8mimCl in electrostatic interaction. Moreover, we have found that the sulfonate symmetric (νsym) and antisymmetric (νasym) stretching vibrations for 20 mM MC540 appears at 1118 and 1166 cm-1, respectively and these characteristic bands undergo red-shift to 1100 and 1161 cm-1, respectively in 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 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 the values as 35±2 mV, -36±4 mV and -0.002+0.0005 mV, respectively. Therefore, zeta potential measurement further supports the involvement of electrostatic interaction to form the supramolecular architectures. To realize the real impact of positively charged imidazolium cation C8mimCl in electrostatic interaction with anionic MC540, we have substituted C8mimCl by 1-octanol in all the above mentioned five concentrations, at stoichiometric ratio 1:1. However, no ordered aggregates are found in dye/1-octanol combination and this additionally confirms the significance of coulombic attraction between the building blocks.


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3.2.2. Hydrogen bonding interaction. There are several literature reports, dealing with urea addition to a supramolecular selfassembly 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 as well as vesicles are still present. This observation concludes that here hydrogen bonding is not playing the governing role in the formation of the assemblies. To form any supramolecular self-assemblies, choice of the building blocks are very much important and here also it appears that MC540 and C8mimCl are suitable counterions where concentration of both the materials have exquisite control over the morphological transition. To get better insight on the structural importance of the SAIL C8mimCl, we have carried out two types of control experiments. We can vary the hydrophilic imidazolim head group 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 to vary the hydrophobic chain length of the SAIL C8mimCl, keeping the same hydrophilic head group. Therefore, we have substituted it by 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 as homogeneous. Here also we have used the same five concentrations (i.e., 1 mM, 2 mM, 5 mM, 10 mM, 20 mM ) keeping the molar ratio at 1. Similarly for C6mimCl and C10mimCl at the above mentioned five concentrations at molar ratio 1, no morphological transition is found. As the CMC of C12mimCl is ~12.5 mM,52 we have performed the FLIM analysis for 10 mM, 5 mM and 2 mM concentrations of MC540/ C12mimCl pair and the images are shown in Figure S3a-c.The images indicate that at 10 mM dye-SAIL pair, there is existence of vesicular aggregate and in moving to 5 and 2 mM concentrations 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 concentrations. However, for MC540/C16mimCl pair we have used 0.75 mM and 0.5 mM concentrations as the CMC of C16mimCl is ~1 mM.52 Here, we have observed dense fibril or microtubular aggregates at both the concentrations and the images are depicted in Figure S4a,b. The above observations indicate that very short hydrophobic chain ([C4mim]+ and [C6mim]+) cannot provide 15 ACS Paragon Plus Environment

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sufficient hydrophobicity to form any supramolecular aggregates. C10mimCl having the longer chain length to that of C8mimCl also unable to show any specific morphology. On the other hand, for C12mimCl and C16mimCl there is formation of some self-assembled aggregates at different concentrations of dye-SAIL pair. However, there is no such morphological transition takes place for [C12mim]+ and [C16mim]+ and this implies that correct hydrophobic chain length is required to observe the transformation of morphology as a function of concentration of the counterions. 3.2.4. The effect of head groups. In the second study, we have investigated the effect of hydrophilic imidazolium head group in the morphological transition, keeping the hydrophobic chain length fixed. For this purpose, we have chosen OTAB, where the π-electron cloud containing aromatic imidazolium moiety is substituted by aliphatic trimethyl ammonium moiety, having no π-electron involvement. In this study, we have also chosen the same five concentrations i.e., 1 mM, 2 mM, 5 mM, 10 mM and 20 mM of MC540/OTAB pair for convenient comparison with C8mimCl, as the CMC of OTAB is ~130 mM.53 Li and co-workers have reported the interaction between anionic photo-responsive azo dye and cationic surfactant OTAB.54 Interestingly in this study, we have found that at 20 mM and 10 mM MC540/OTAB pair form vesicles with diameter ranging from 1-4 µm, shown in Figure S5a,b. Again, at 5 mM concentration of dye-surfactant pair the morphology is mainly fibrillar with small traces of vesicular aggregates, Figure S5c. While 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 that structural transition takes place on lowering the concentration of 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 come up with the conclusion that eight carbon containing hydrocarbon chain and cationic hydrophilic head group is essential to be present in SAIL or surfactant to witness the concentration dependent structural transition in presence of anionic dye MC540. In presence of equimolar concentration of MC540, surfactant OTAB is showing similar type of morphological transition without having delocalised π-electron like 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. 16 ACS Paragon Plus Environment

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The lifetime distribution histograms of MC540 dye inside vesicle, mixture of vesicles and fibrils, and in completely fibrillar aggregates are shown in Figure S6a-c, corresponding to 20 mM, 5 mM and 2 mM, MC540/OTAB concentrations, respectively. For 20 mM MC540/20 mM OTAB, the lifetime distribution is quite broad, ranging from 350 ps to 1275 ps, indicates that hetrogenity is present inside the vesicle.Whereas for 5 mM MC540/5 mM OTAB, mostly 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 ps to 330 ps and the other one is around ~440 ps to 950 ps with former one having higher intensity. Again, the complete fibrillar morphology, forms at 2 mM MC540/2 mM OTAB, exhibits further reduction in lifetime to 130 ps to 370 ps with sharp lifetime distribution. In summary, we can tell that vesicular aggregates are heterogeneous with higher lifetime value and in moving towards fibrillar aggregates, lifetime of MC540 inside the self-assembly diminishes and also the heterogeniety is relatively less here. Therfore, similar like C8mimCl, here OTAB is also forming vesicles with MC540, which is providing more rigid environment to MC540 compared to fibril. FESEM images at 20 mM, 5 mM and 2 mM MC540/OTAB pair further confirm the similar morphologies, obtained in FLIM i.e., vesicle, fibril-vesicle mixture, and completely fibril, respectively and depicted in Figure S7a-f. We have performed FLIM measurements

using

Merocyanine

540

(MC540)

and

cationic

surfactants

Cetyl

trimethylammonium bromide (CTAB), 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 counter ions. Therefore, the hydrophobic chain length is also important for the conventional surfactants to observe the structural transformation. Earlier, we have 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, decrease in polarity of the medium, increases the monomer/dimer ratio as monomer form is more dominant in 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 20 mM concentration of MC540, the absorbance value is very high and the dimer peak gets saturated. Whereas, at 2 mM MC540 concentration the monomer and dimer peaks appear ~530 nm and ~497 nm, respectively with monomer/dimer peak ratio ~0.74. The fibrillar aggegates give rise to monomer and dimer peaks at around 17 ACS Paragon Plus Environment

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~538 nm and around ~501 nm having monomer/dimer peak ratio ~0.92. Thus, from this viewpoint it is clear that fibrillar aggregates provide less polar environment to MC540 as both the monomer and dimer peaks are red-shifted with increase in the corresponding monomerdimer peak ratio. However for the vesicular aggregates, the environment surrounding MC540 is more hydrophobic as the monomer-dimer peak ratio is ~1 with the monomer peak maximum is almost same at 537 nm and dimer peak maximum is 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.

Figure 6. UV-visible absorption spectra of 2 mM MC540, 20 mM MC540, fibrillar and vesicular solutions. (Inset shows the individual absorption spectra of 2 mM MC540, fibril and vesicle solutions) 3.3. Influence of packing parameter in molecular self-assembly and structural transition. Lifetime distribution histograms and UV-visible spectra infer that fibrils and vesicles provide more hydrophobic as well as rigid environment to MC540 compared to free MC540. Moreover, this rigidity as well as confinement is quite higher for vesicular 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 parameterP ( ), is a balance of three geometric factors where and are the volume and length of

the hydrophobic surfactant chain, respectively. Surface area of the polar head group at the 18 ACS Paragon Plus Environment

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CMC is represented by .47-49 For P< 13, spherical micelles form; for 13 < < 12, rodlike or cylindrical micelle is the favourable morphology and bilayers with a spontaneous curvature (vesicles) form for 12 < < 1.47 Surface area of the polar head groups 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, repulsive force appears due to the close proximity of the similarly charged hydrophilic head groups. Due to the presence of these two opposing factors, the effective head group area per molecule at the surface is not the ordinary geometrical area rather an equilibrium parameter appears from various thermodynamic considerations. We have shown in the UV-visible spectra that MC540 is not present in the same monomerdimer form at 2 mM and 20 mM concentrations. Along with monomer and dimer peaks, at 422 nm MC540 exhibits a broad absorption band due to the H-aggregates. These Haggregates are non-fluorescent and form due to the face to face stacking of the monomeric form of dye.55 For lower concentrations of 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 counter ions decrease the effective area of the head groups of the building blocks. Therefore at higher dye-SAIL concentration i.e., at 20 mM, the aggregation number is higher i.e., greater numbers of building blocks are involved and it develops greater force of attraction between the counterions and it further lowers the effective head group area of the building blocks, resulting in increase in packing parameter (P) value. Notably, when the fibrillar aggregates achieve their critical size, they fold up to form vesicle. Therefore, at higher P (in between 12 and 1) value vesicular morphology dominate having higher curvature than fibrillar aggregates. It is also reported that, > 12 requires small head group area and long hydrophobic tail part.56 Here also for vesicular aggregates, head group area is decreasing due to higher electrostatic force of attraction and increase in hydrophobicity is evidenced from UV-visible absorption measurements. Therefore, packing parameter is very much useful to predict, explain and rationalize the different self-assembled aggregates. 4. Conclusion In conclusion, molecular self-assembled aggregates are ubiquitous in nature and the constituent building blocks satisfy some precise characteristics to follow ISA technique. This study focuses on concentration-responsive remarkable fibril-vesicle structural change using MC540 and C8mimCl. Along with FLIM, FESEM, TEM and Cryo-TEM studies, FTIR 19 ACS Paragon Plus Environment

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spectra elicit the importance of electrostatic interaction between the cationic imidazolium moiety of C8mimCl and anionic sulfonate group of MC540 in the formation of supramolecular aggregates. Thereafter, a systematic study is performed to investigate the effect of hydrophilic head group and the hydrophobic tail part of the SAILs in controlling this concentration dependent aggregate transition. Traditional surfactant OTAB has the same chain length to that of SAIL C8mimCl but differ in head group and can also exhibit the similar type of morphological transformation. On the other hand, C4mimCl, C6mimCl, C10mimCl, C12mimCl and C16mimCl, having the same head group, but different hydrophobic chain length to that of C8mimCl are unable to show concentration dependent 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 counter ions. Supporting Information Instrumental Section, tabular form of Morphology of the supramolecular aggregates at different concentration ratios of MC540 and C8mimCl, AFM images of vesicle, FTIR spectra of the building blocks and the aggregates, FLIM images of MC540/ C12mimCl, MC540/ C16mimCl, MC540/ OTAB and FESEM images of MC540/ OTAB are provided in the Supporting Information.

Acknowledgement N.S. is thankful 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 thankful to CSIR for their research fellowships.

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Concentration-Driven Fascinating Vesicle-Fibril Transition Employing

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