A Low-Molecular-Weight Gelator Composed of Pyrene and Fluorene

Article Cite This: Langmuir XXXX, XXX, XXX-XXX

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A Low-Molecular-Weight Gelator Composed of Pyrene and Fluorene Moieties for Effective Charge Transfer in Supramolecular Ambidextrous Gel Samala Murali Mohan Reddy,†,‡ Pramod Dorishetty,§ George Augustine,∥ Abhijit P. Deshpande,§ Niraikulam Ayyadurai,∥ and Ganesh Shanmugam*,†,‡ †

Bioorganic Chemistry Laboratory and ∥Biochemistry & Biotechnology Laboratory, Council of Scientific and Industrial Research (CSIR)-Central Leather Research Institute (CLRI), Adyar, Chennai 600020, India ‡ Academy of Scientific and Innovative Research (AcSIR), CSIR-CLRI Campus, Adyar, Chennai 600020, India § Department of Chemical Engineering, Indian Institute of Technology Madras, Adyar, Chennai 600036, India S Supporting Information *

ABSTRACT: Charge-transfer (CT) gel materials obtained from low-molecular-weight (LMW) compounds through a supramolecular self-assembly approach have received fascinating attention by many researchers because of their interesting material property and potential applications. However, most of the CT gel materials constructed were of organogels while the construction of CT gels in the form of a hydrogel is a challenge because of the solubility issue in water, which considerably limits the use of CT hydrogels. Herein, for the first time, we report a new LMW gelator [Nα-(fluorenylmethoxycarbonyl)Nε-(δ-butyric-1-pyrenyl)-L-lysine, (FmKPy)], composed of two functional moieties such as fluorenylmethoxycarbonyl and pyrene, which not only parade both hydro and organo (ambidextrous) supramolecular gel formation but also exhibit CT ambidextrous gels when mixed with an electron acceptor such as 2,4,7-trinitro-9-fluorenone (TNF). This finding is significant as the established CT organogelator in the literature did not form an organogel in the absence of an electron acceptor or lose their gelation property upon the addition of the acceptor. CT between pyrene and TNF was confirmed by the color change as well as the appearance of the CT band in the visible region of the absorption spectrum. CT between FmKPy and TNF was supported by the solvent dilution method using tetrahydrofuran as the gel breaker and pyrene fluorescence quenching in the case compound containing pyrene and TNF. The morphology of FmKPy ambidextrous gels indicates the fibrous nature while the self-assembled structure is primarily stabilized by π−π stacking among fluorenyl and pyrenyl moieties and hydrogen bonding between amide groups. The FmKPy−TNF CT ambidextrous gel retains the fibrous nature; however, the size of the fibers changed. In FmKPy− TNF CT gels, TNF is intercalated between pyrene moieties in the self-assembled structure as confirmed by fluorescence quenching and powder X-ray diffraction. The FmKPy ambidextrous gel exhibits significant properties such as low minimum gelation concentration (MGC), thixotropic nature, pH stimuli response, and high thermal stability. Upon the addition of TNF, the FmKPy−TNF CT ambidextrous gel maintains all these properties except MGC which increased for FmKPy−TNF. Because pyrene-based LMW organogels have been developed widely for many applications while their hydrogels were limited, the current finding of the pyrene-based ambidextrous fluorescent gel with the CT property provides a wide opportunity to use FmKPy as a soft material maker and also for potential applications in fields like surface coating, three-dimensional printing, and so forth. stimuli-responsiveness,11−14 formation of higher order structures,7,15,16 and control over their physical and chemical properties.11,16−21 The LMW gels are mainly constructed through the supramolecular self-assembly using a bottom-up approach. These gels are three-dimensional networks formed through weak intermolecular noncovalent interactions such as π−π stacking, hydrogen bonding, van der Waals interactions,

1. INTRODUCTION Soft material (gel) built from low-molecular-weight (LMW) organic compounds (molar mass ≤ 3000) using self-assembly processes fascinated many researchers because of their potential applications as biosensors, drug carrier, optical sensors, tissue engineering, and so forth.1−8 Gels have attracted a significant amount of attention because they yield elastic and stiff soft materials with multifunctionalities.9,10 Although gels can be obtained from polymers, construction of gels using an LMW gelator (a compound used to make gel) is the choice of the researcher because of several advantages such as easy synthesis, © XXXX American Chemical Society

Received: October 3, 2017 Revised: November 1, 2017

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DOI: 10.1021/acs.langmuir.7b03453 Langmuir XXXX, XXX, XXX−XXX

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lysine not only provides a linker but also has inherent a small alkyl chain, which is hydrophobic and usually required for organogelation.38 Further, L-lysine offers new amide (−CO− NH−) bonds (Figure 1) upon coupling to pyrene butyric acid and Fmoc−Cl which could provide an additional driving force in the form of hydrogen bonding to stabilize the FmKPy selfassembly. Furthermore, the presence of −COOH in L-lysine may show a pH stimulus-responsive gelation because of its pHdependent ionic nature. Hence, it is believed that designed FmKPy has an inherent nature of inducing the major noncovalent intermolecular forces such as aromatic π−π stacking (pyrene and fluorene), hydrogen bonding (−CO− NH), and ionic (−COOH) and hydrophobic (alkyl chain) interactions, which are involved in the supramolecular gelation.

hydrophobic interactions, charge-transfer (CT) interactions, and so forth. Among the noncovalent interactions, CT interactions22 between the electron donor and the acceptor in a two-component gel gains distinct importance as they can modulate the structure and mechanical properties of the gels by altering the CT pairs.23−25 To date, although several twocomponent gels with the CT property have been developed, most of them were organogels.26−29 Further, in most of the CT organogels, LMW compounds form organogels only in the presence of an acceptor molecule and do not act as an individual gelator.26−29 One of the possible reasons could be the requirement of extensive π-systems for CT which makes the gelator insoluble in water. Consequently, it is a great challenge to develop CT hydrogel material. To the best of our knowledge, hydrogels with the CT property are not explored except a recent report on a multistimuli-responsive CT hydrogel.30 Thus, it is necessary and beneficial to explore supramolecular hydrogels with intermolecular CT upon mixing with a donor/acceptor, depending on the gelator nature. Instead of hydrogels, developing an ambidextrous gel (gel forms in both water and organic solvents) with the CT property adds an advantage in the material property. For instance, the same gelator can be utilized for the construction of both the CT hydrogel and CT organogel. Such kind of a system has not been reported yet. In this study, we have designed and synthesized a new LMW compound Nα-(fluorenylmethoxycarbonyl)-Nε-(δ-butyric-1pyrenyl)-L-lysine (FmKPy) (Figure 1) to not only demonstrate

2. EXPERIMENTAL SECTION 2.1. Materials. Fmoc-L-lysine(Boc)-OH, 1-pyrene butyric acid, Nhydroxysuccinimide, tetrahydrofuran (THF), and 1,1,1,3,3,3-hexafluoroisopropanol were purchased from Sigma-Aldrich. N,N′-Dicyclohexylcarbodiimide was purchased from Avra Chemicals (India). HCl (37%) was obtained from Merck Millipore (India), whereas all other chemicals were purchased from local vendors. 2.2. Methods. 2.2.1. Synthesis and Characterization of FmKPy. An FmKPy gelator was synthesized using the previously reported method39,40 with a minor modification. The detailed synthesis and characterization has been provided in the Supporting Information. 2.2.2. Hydrogelation. The required amount of FmKPy was weighed in a glass vial, and an appropriate volume of sodium phosphate buffer (50 mM) with a different pH was added to obtain the desired concentration of the gelator (2.5 mM). Because of poor solubility of FmKPy, the solution was heated to 90 °C and cooled back to room temperature. To obtain hydrogelation, the solution was kept at room temperature for 60 min. Hydrogelation was confirmed by the conventional vial inversion method. 2.2.3. Organogelation. Organic solvents such as chloroform, dichloromethane, methanol, acetonitrile, dimethylformamide, dimethyl sulfoxide (DMSO), THF, hexane, toluene, benzene, and styrene were used to test the organogelation of FmKPy. Among these solvents, FmKPy showed organogelation only in aromatic solvents such as toluene, benzene, and styrene, whereas in other solvents, FmKPy either precipitated or was in a soluble form. Organogelation was induced by the addition of selected solvents to the preweighed FmKPy followed by sonication (5 min) and incubation at room temperature for 1 h. 2.2.4. CT Gel. TNF was dissolved in 50 mM sodium phosphate buffer (pH 7.4) as well as in toluene. An appropriate volume of TNF stocks was added to preweighed FmKPy to get the final molar ratio of 1:1. The concentration of CT gels of FmKPy−TNF was 1.9 wt %. Hydro- and organogelation were induced as stated above. 2.2.5. Circular Dichroism Measurements. Circular dichroism (CD) spectra were measured on a JASCO-715 spectropolarimeter (Jasco Inc, Japan) equipped with a Peltier cell holder. All measurements were carried out at 25 °C unless otherwise stated. The FmKPy (1.5 mg/ mL) gel was prepared similarly to a method described above, except that the gel was formed in a 1 mm rectangular quartz cell. The CD spectra were collected from 450 to 320 nm with a scan speed of 100 nm/min. Each spectrum was collected by averaging 3 individual scans, and the CD spectrum of buffer alone was subtracted from each spectrum. The parameters bandwidth, data pitch, and response were set to be 1 nm, 0.5 nm, and 1 s, respectively. 2.2.6. Fourier Transform Infrared Measurements. All Fourier transform infrared (FTIR) spectra were collected using a JASCO-4700 spectrometer. The FTIR spectra of gels were measured in the form of a xerogel state. Hydro- and organogels of FmKPy and FmKPy−TNF were placed on a ZnSe window and dried under ambient conditions. For FTIR of TNF, a conventional KBr pellet method was used. All FTIR spectra were collected at 8 cm−1 resolution, and each spectrum represents the average of 32 scans. The ZnSe window was used as the

Figure 1. Chemical structure of the FmKPy gelator and TNF. The highlighted part in FmKPy indicates the possible intermolecular interactions during the supramolecular self-assembly.

(i) an ambidextrous gelation with the CT property when mixed with an electron acceptor (2,4,7-trinitro-9-fluorenone, TNF) but also to display (ii) hydrogelation and (iii) organogelation in the absence of an acceptor, for the first time. In FmKPy, bifunctional moieties such as fluorenylmethoxycarbonyl (Fmoc) and pyrene (electron donor) butyric acid are linked by L-lysine through α- and ε-amino groups. Although it is difficult to design a molecule to form ambidextrous gels, the anticipation of FmKPy as an ambidextrous gelator is based on the individual significance of Fmoc and pyrene moieties, which are widely used in the development of hydro- and organogels, respectively.4,31−34 In addition to this, the capacity of inducing stable intermolecular π-stacking among pyrene and fluorene moieties is expected to support the self-assembly process. Pyrene was chosen as it is extensively used as an electron donor in several systems such as photo electronics,23,35 sensors,36 surfactants,37 CT organogels,26−29 and so forth, whereas Fmoc moiety was selected as it is easy to functionalize amino compounds and also commonly employed in peptide synthesis. It is noteworthy that, despite its wide applications, pyrene primarily displays only organogels. Meanwhile, in FmKPy, LB


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Langmuir background and subtracted from each spectrum. For solution measurements, FmKPy was dissolved in DMSO-d6 (100 mM). The solution was transferred to a fixed path length cell (50 mm) containing CaF2 windows. For baseline correction, the IR spectrum of neat DMSO-d6 was subtracted from the sample spectrum. The solution IR spectrum was collected by averaging of 512 scans. 2.2.7. Fluorescence Measurements. Fluorescence emission spectra were collected on a Carry (eclipse) fluorescence spectrophotometer. For gel-state measurements, FmKPy gels and FmKPy−TNF gels were prepared as mentioned above except that the quartz cell was used instead of a glass vial. For the solution state, the required amount of solid FmKPy was dissolved in DMSO and transferred to a 1 cm quartz cell. Emission spectra (360−700 nm) were collected by exciting FmKPy at 340 nm. Both the emission and excitation slit width was 5 nm. A temperature-dependent experiment was done with a Peltier equipped spectrometer. Each spectrum was measured after reaching a prefixed temperature. The excitation and emission slit widths were same as mentioned above. 2.2.8. High-Resolution Scanning Electron Microscopy. SEM images were taken using an FEI Quanta FEG 200 high-resolution SEM. Hydro- and organogels of FmKPy and FmKPy−TNF were placed on a thick aluminium foil and dried at ambient conditions followed by gold coating before measurements. 2.2.9. Powder XRD Measurements. Powder X-ray diffraction (PXRD) measurements of FmKPy in powder and xerogel forms were recorded with a Rigaku, MiniFlex (II) X-ray diffractometer [operating conditions: Cu Kα (λ1.54 mA), 30 kV, 15 mA]. The xerogel of FmKPy and FmKPy−TNF CT gels was prepared as detailed in SEM analysis, except that a glass plate was used instead of an aluminium plate and gold coating. The samples were scanned from 5° to 50° with a scan speed of 1° min−1. 2.2.10. Rheological Measurements. Rheology measurements were carried out with an Anton Paar MCR-502 rheometer operating in an oscillatory mode with a 25 mm parallel plate geometry at 25 °C. Preformed hydro- and organogels of FmKPy and FmKPy−TNF were transferred to the lower plate (0.5 mm gap) followed by a dynamic strain sweep at 0.01−100% strain with a constant frequency of 1 rad s−1 [to gain the linear viscoelastic region (LVR)]. All subsequent experiments have been done below the LVR. A dynamic frequency sweep was measured over a range of frequencies from 0.1 to 100 rad s−1 at 0.5% strain. Immediately following the frequency sweep, the thixotropic-cyclic experiment was performed for the gels. Here, a shear strain of 0.5−100% was applied to the gel, which was kept at 0.5% for 10 min and maintained for 2 min at 100% strain, and then the strain was released to 0.5% and maintained for 10 min. The cycle was repeated three times. Angular frequency was fixed at 1 rad s−1. Following the step strain sweep, the temperature sweep for the hydrogel was performed at 0.5% strain and 1 rad s−1 angular frequency. The temperature was increased from 25 to 93 °C followed by cooling (annealed) with a heating/cooling rate of 2 °C min−1. Following the temperature sweep, the angular frequency sweep was performed for the annealed gel (AG) at 0.5% strain. 2.2.11. UV−Visible Measurements. All absorption spectra were collected using an Inkarp (SICAN 2600) UV−visible spectrophotometer. The FmKPy hydrogel (0.15 wt %) and organogel (1 wt %) and CT gels of FmKPy−TNF (1:1, 20 mM:20 mM, total gel = 1.9 wt %) were prepared in a 0.1 cm quartz cell. For control, FmKPy (1.3 wt %, 20 mM) and TNF (0.6 wt %, 20 mM) alone were dissolved in DMSO and toluene, respectively. The absorbance spectra were recorded from 800 to 300 nm with a scan speed of 300 nm/min. 2.2.12. NMR Measurements. Temperature-dependent 1H NMR spectra were recorded on a Bruker 400 MHz spectrometer. Here, the FmKPy hydrogel was obtained using a phosphate buffer (pH 7.4) which is prepared using D2O instead of H2O. The concentration of the hydrogel was 0.15 wt %. The NMR spectra was measured after attaining the desired temperature and equilibrated for 10 min. 2.2.13. Cytotoxicity Study. 2.2.13.1. Cell Culture Maintenance. Mouse embryonic fibroblast (3T3) cell lines obtained from American Type Culture Collection were maintained in Dulbecco’s modified Eagle medium containing high glucose supplemented with 10% fetal

bovine serum (v/v), penicillin, and streptomycin, each 50 U/mL, and L-glutamine (2 mM). Cultured flasks were kept at 37 °C in a humidified 5% CO2 incubator. Once the cells reached the confluence, the culture medium was removed and rinsed twice with 1× phosphate buffered saline (PBS) pH 8. Subsequently, the confluent cell layers were trypsinized by adding trypsin/ethylenediaminetetraacetic acid solution and suspended in the culture medium. Cell viability and numbers were calculated with trypan blue [0.4% (w/v)] staining using a light microscope. 2.2.13.2. Proliferation Assay. Cytotoxicity assay was done in the corning’s cell costar 96-well plate to determine the number of viable cells after adding the hydrogel (15 mg/mL) at different volumes. The assay was performed as mentioned in the previously published protocols pertaining to cytotoxicity assay.41,42 Different volumes of the FmKPy hydrogel were suspended in culture media to obtain different concentrations of hydrogels (final volume: 100 μL) before seeding 2 × 105 cells/well, incubated at 37 °C with 5% CO2. The assay was performed in triplicates for each day to study the cell viability at different time intervals such as 12, 24, 48, and 52 h. At the end of every exposure, the medium was removed and rinsed with the 1× PBS buffer followed by the addition of MTT (thiazolyl blue tetrazolium bromide salt) solution (20 μL/well). After the completion of the exposure period, MTT solutions were removed, and tiny formazan crystals were solubilized by adding 200 μL of DMSO. Two internal controls were set up for each experiment consisting of cells and medium alone. The cell viability was calculated by the following formula and expressed in percentage viability.

Cell viability (%) = (O. D. of treated cells/O. D. of untreated cells) × 100

3. RESULTS AND DISCUSSION 3.1. Ambidextrous and CT Ambidextrous Gelation. Before the demonstration of the ambidextrous CT gel, first, we present the ambidextrous gelation of FmKPy alone. Gelation of FmKPy in aqueous buffer solutions and various organic solvents at different concentrations was tested by a conventional vial inversion method at room temperature, and it was found that FmKPy forms both hydro- and organogels (Figure 2A,B) with a minimum gelation concentration (MGC) of hydrogelation as low as 0.15 wt %. Hydrogelation of FmKPy was found to be pH-dependent, and it forms gels between pH 6.0 and 9.5 while it precipitates at pH below 6.0, which could be because of the change in the ionization state of −COOH moiety. Gelation was not tested above pH 9.5 as Fmoc-moiety is a base-labile. Among the organic solvents tested, FmKPy displayed organogelation only in aromatic solvents such as toluene, benzene, and styrene, whereas in other solvents, FmKPy either precipitated or was in a soluble form. The MGC of the organogel was found to be independent of tested solvents, and it was found to be 1 wt %. The small values of MGCs indicate that FmKPy is an effective ambidextrous gelator. The digital images clearly show the hydrogel and organogel (e.g., in toluene) which are opaque in nature (Figure 2A,B). Under UV light (365 nm), both gels were illuminated in blue color. In the presence of TNF, both the hydrogel (pH 7.4) and organogel remain stable, and the color of the gel turns to dark brown, which is characteristic of CT from pyrene to TNF. It is noteworthy that the neat solution of TNF and monomer of FmKPy was yellow and violet (under UV light), respectively, whereas a mixture of gelator and TNF was off white before forming the gel and CT (Figure S1). A similar color change has been previously used as a confirmatory test for CT, specifically for CT complexes containing pyrene and TNF.29,43 CT was C


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FmKPy, hydrogelation of the FmKPy−TNF complex was tested at different pH, and it was found that the hydrogel forms at pH ≥ 7, whereas below this pH, the complex not only fails to display gelation (as confirmed by the vial inversion method), but also no significant CT was observed as evidenced from the less-intense color change and illumination of the CT complex under UV light (Figure S3). It should be noted that the MGC of both CT gels of FmKPy−TNF increased to 2 wt % which could be because of the interference of TNF between FmKPy molecules in the self-assembly during the gelation (vide infra). 3.2. Characterization of the FmKPy Ambidextrous Gel. 3.2.1. Morphology and Rheology of the FmKPy Ambidextrous Gel. The morphological feature of the ambidextrous gel was explored by SEM analyses in the form of the xerogel. Figure 3A,B shows a fascia-like fibrous structure

Figure 2. Digital images of the FmKPy hydrogel (A) and organogel (B) in the absence and presence of TNF. Bottom: digital images depict the corresponding gels under the illumination of a UV lamp at 365 nm. (C) UV−visible spectra of FmKPy as monomer (black), hydrogel (0.15 wt %) (blue), and organogel (1 wt %) (pink), FmKPy−TNF (1:1 molar ratio) as CT hydrogel (red) and CT organogel (brown), and TNF alone (green). The concentration of the FmKPy monomer and TNF in DMSO and toluene, respectively, was 20 mM. The base-line shifts at the higher wavelength region for the gels especially for the CT hydrogel are most likely because of their opaque nature. Figure 3. Representative high-resolution scanning electron microscopy (HR-SEM) images of the xerogel obtained from the hydrogel (A,B) and organogel (C,D) of FmKPy.

also confirmed by UV−visible spectroscopic measurement where a broad band centered at ∼560 nm was observed for hydro- and organogels (Figure 2C) which is the characteristic CT band for the pyrene−TNF CT complex.29,43 It should be noted that UV−vis absorption spectra of both hydro- and organogels of FmKPy alone and their monomer form (in DMSO) did not show any CT band in the visible region (Figure 2C). This result indicates that CT occurs between FmKPy and TNF rather than FmKPy itself. CT interaction was supported by the reduction of CT band intensity for both gels upon the addition of THF (final volume = 5 v/v %) which effectively disturbs the gels (Figure S2). Under similar experimental conditions, the absorption spectrum of the FmKPy−TNF mixture in a neat THF solvent displays a weak CT band which supports that THF disfavors CT interaction between pyrene and TNF (Figure S2). A similar solvent addition test was previously conducted to confirm the CT in the gel where methanol was used to disturb the gel.23 Under UV light, the fluorescence of both CT gels was quenched (Figure 2A,B), indicating the interference of TNF in the stacking of pyrene moieties during the supramolecular selfassembly. Fluorescence quenching suggests44 that TNF most likely interacts with pyrenes in an intercalation mode (vide infra). In addition to the above experiments, steady-state fluorescence spectral measurements in the presence of TNF confirm the CT of FmKPy−TNF gels (vide infra). Similar to

for the hydrogel, whereas the organogel displays cross-linked nanofibrils (Figure 3C,D). The rigidity and mechanical properties of the ambidextrous gel were determined by measuring the rheological parameters. A typical amplitude sweep was studied to identify the LVR which allows us to set an appropriate parameter (below the LVR) for other rheological experiments (Figure S4). The LVR was found to be ∼3% strain. In a typical frequency sweep, the variation of both storage (G′) and loss (G″) moduli was measured as a function of frequency under a constant strain. In the present case, the result shows that G′ was higher than G″ (G′ > G″), with ΔG (G′ − G″) of 500 Pa (Figure 4A). The feature of G′ > G″ is characteristic of viscoelastic materials such as gels.45−47 The slight decrease in the ΔG at the higher frequency range suggests a small frequency dependence, which is observed for many supramolecular gels. Interestingly, the FmKPy hydrogel exhibits the thixotropic (gel → solution → gel) property where gel breaks down upon applying high strain followed by regelation upon release of the strain (step-strain).46,48 For example, a time sweep experiment was carried out with 0.5% (below the LVR) strain followed by 100% (above the LVR) strain where the gel is destroyed as evident from the modulus values (G′ < G″) (Figure 4B). After 2 min, recovery of the gel was noticed upon D


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Figure 4. Rheological properties of FmKPy hydrogel showing the dynamic frequency sweep (A) at 0.5% strain and continuous step strain sweep (B) followed by the temperature sweep and annealing the gelation (C) and the AG frequency sweep (D).

Figure 5. (A) Fluorescence spectra of FmKPy in DMSO (black line) and the normal hydrogel (at 25 °C, red line, and at 95 °C, blue line) and the AG from 95 to 25 °C (pink line). The inset picture shows the hydrogel at 90 °C with and without UV exposure. The concentration of FmKPy in DMSO and the hydrogel was 0.15 wt %. (B) CD spectra of the FmKPy hydrogel (25 °C, red line, and 90 °C, blue line) and the AG from 95 to 25 °C (pink line). (C) FTIR spectra of FmKPy in the form of the xerogel obtained from the hydro- and organogel, and as a monomer (DMSO-d6). (D) PXRD spectra of the xerogel of the hydro- and organogel of FmKPy and in a powder form.

release of the strain from 100 to 0.5% as evident from G′ being higher than G″ (G′ > G″). A similar effect has been observed for two consecutive cycles which confirm that the FmKPy hydrogel has a thixotropic nature. Temperature-dependent rheological experiments were also performed to explore the

thermal stability as well as the annealing property of the hydrogel.19 Accordingly, a temperature sweep was carried out with a 0.5% strain. As can be seen from Figure 4C, during heating, the FmKPy hydrogel melts at a thermal melting temperature (Tm) of 85 °C at which both moduli (G′ and G″) E


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Langmuir become equal (G′ = G″). Similarly, upon cooling, the FmKPy solution started reforming the gel (AG) at around 80 °C with G′ being higher than G″ (G′ > G″). This experiment confirms the thermally reversible nature of the supramolecular hydrogel formed by FmKPy. It should be noted that the magnitudes of both moduli (G′ and G″) were higher compared to the normal gel (Figure 4C). Mainly, G′ and ΔG (7000 Pa) of the AG increased 14 and 8 times, respectively, compared to that of the normal gel (vide supra) as observed from the dynamic frequency sweep (Figure 4D). These results suggest that the AG has more rigidity and elasticity than the normal gel. The difference in rheological properties between the AG and the normal gel is consistent with SEM images where different fibrillar structures were observed for the AG and normal FmKPy hydrogels (Figure S5). Unlike the normal gel of FmKPy, a typical compact fibril morphology was observed for its AG. It is to note that annealing is expected to produce a well-ordered self-assembly in self-assembling molecules/gelators because of the removal of internal stresses. By contrast, the rapid addition of solvents to gelators induces a less-ordered selfassembled structure compared to slow processes such as annealing. Similar to the hydrogel, the frequency sweep and a continuous step strain sweep were performed on the organogel and both experiments confirm the viscoelastic nature and the thixotropic property (Figure S6). 3.2.2. Molecular Arrangement of FmKPy in the Supramolecular Gel. To explore the molecular arrangement and the driving forces involved in the stabilization supramolecular selfassembly of FmKPy in the ambidextrous gel, fluorescence, CD, FTIR, and PXRD were performed. The fluorescence and electronic CD spectroscopy was used to investigate the intermolecular aromatic π−π interactions involved in the gel at a wet gel form. As can be seen from Figure 5A, the fluorescence spectrum (excited at the pyrene excitation wavelength) of the hydrogel was quenched compared to the monomer (DMSO solution) where a typical fluorescence pattern of pyrene (380, 398, and 418 nm) was observed. The fluorescence quenching is known for pyrene when it undergoes aggregation,49 and hence the data confirm the stacking of pyrene moieties through intermolecular π−π interactions during the gelation. Interestingly, when the temperature of the hydrogel was increased to 95 °C (Figures 5A and S7), pyrene showed a drastic enhancement in the excimer emission peak at 480 nm, whereas fluorescence was quenched while cooling back to room temperature.50 The former result indicates that FmKPy prefers to be as a dimer at a higher temperature while it is favored to be in the supramolecular selfassembly at a lower temperature,49,50 which was confirmed by the formation of gel upon cooling (AG). It should be noted that the emission spectrum of the AG is different from that of the normal gel, indicating that FmKPy molecules most likely reorganize during cooling and hence form a highly ordered selfassembled structure than the normal gel. This interpretation is consistent with SEM images of the normal gel and the AG where AG exhibited a typical fibrillar nature than the normal gel (Figure S5). To understand the molecular packing further, temperature-dependent 1H NMR spectra were recorded during heating followed by cooling. However, it is difficult to obtain any meaningful conclusion with respect to the molecular arrangement, as none of the protons of FmKPy appeared in its gel state at 25 °C before heating and after annealing (Figure S8). However, these NMR experiments supported the Tgel of 85 °C where complete proton signals of FmKPy appeared and

disappeared during heating and cooling, respectively. In the case of the organogel, unlike the hydrogel, the fluorescence spectrum of the organogel shows both emission and excimer peaks, which indicates the extensive π-stacking of pyrene in the organogel (Figure S9).49 Temperature-dependent fluorescence and 1H NMR experiments were not carried out because of the irreversibility nature of the organogel. CD has been previously used to detect the supramolecular chirality induced by LMW compounds during self-assembly.51,52 Accordingly, CD spectra of FmKPy were measured to probe the long-range intermolecular orientations of pyrene stacking at different temperatures.51,52 At 25 °C, the FmKPy hydrogel showed an induced CD with a positive maximum around 385 and 350 nm (Figure 5B), which is characteristic of the self-assembly of pyrene chromophore in the hydrogel.51,52 Upon increasing the temperature to 90 °C, complete loss of the CD signal was observed, which indicates that the long-range interaction between pyrene moieties was disrupted and thereby induced a gel → solution transition.51 Upon cooling (AG), the CD signal at 385 nm appeared, indicating the self-assembly of pyrene moieties that lead to regelation (Figure 5B). The difference in the CD intensity indicates that the efficiency of molecular arrangements of FmKPy in the normal gel is different from that in the AG. Although CD was measured up to 310 nm because of high absorbance of Fmoc-moieties (250−310 nm), an enhanced CD signal close to 310 nm suggests that fluorene moieties were involved in the intermolecular interaction through π-stacking, which could support the molecular selfassembly. FTIR was used to ascertain whether hydrogen bonding interaction contributes to the stability of the supramolecular self-assembly of FmKPy. Close inspection of FTIR spectra of the xerogel of the FmKPy hydrogel shows a carbamate48 and amide carbonyl stretching frequencies at 1670 and 1647 cm−1 which are shifted to lower frequency compared to the monomer (in DMSO-d6) where 1715 and 1660 cm−1, respectively, were observed (Figure 5C). Similar to the hydrogel, FTIR of the organogel also displayed a similar lowfrequency shift for carbamate and amide carbonyl stretching frequencies to 1689 and 1639 cm−1, compared to the monomer. These spectral changes are attributed to the participation of both carbonyl groups in the intermolecular hydrogen bonding interactions. The difference in the carbonyl stretching frequencies between hydro- and organogel indicates that the strength of the hydrogen bonding varies between them. It should be noted that the carbonyl group from the −COOH group is overlapped on the free carbamate carbonyl group from Fmoc-moiety in the monomeric form.48 To confirm the aromatic stacking between FmKPy, the PXRD pattern was collected in the form of the xerogel. From the pattern (Figure 5D), it was found that the diffraction (2θ) peaks appeared at 26.48 (d = 0.34 nm) and 24.74 (d = 0.35 nm) for the hydrogel and organogel, respectively, support the distance of π−π stacking between aromatic (pyrene and fluorene) moieties.46,53 3.3. Characterization of the FmKPy−TNF CT Ambidextrous Gel. 3.3.1. Morphology and Molecular Arrangement of FmKPy−TNF in the CT Ambidextrous Gel. SEM images of the xerogel of FmKPy−TNF CT hydrogel and organogel were analyzed to understand the influence of TNF in the morphology of supramolecular FmKPy gels. As can be seen from Figure 6A,B, the xerogel of hydrogels shows a fascia-like fibrous structure which is similar to what was observed for FmKPy hydrogels without TNF, but with decreased thickness of the fibers (Figures 3A & 6A). The formation of a similar F


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quenched compared to the corresponding gels in the absence of TNF (Figure 7). Such quenching evidences the nonradiative CT interactions between pyrene and TNF. This result further confirms the CT in the FmKPy−TNF ambidextrous gel, in addition to UV−vis absorption data as discussed previously (Figure 2C). The fluorescence quenching result also suggests that TNF moiety is intercalated between pyrenes in the supramolecular self-assembly and thereby decreases the emission of the excimer peak which is associated with the dimer of pyrene moieties.50,54 A similar fluorescence quenching has been previously observed for the self-assembly of several co-assembly systems consisting of a pyrene-based compound and acceptor aromatic molecules including TNF.30 The intercalation of TNF was supported by PXRD where a characteristic d-spacing peak of 0.3 nm (2θ = 26.48) has been observed for aromatic π−π stacking interactions in both hydro- and organogels (Figure 8), although the excimer (due to pyrene stacking) was quenched in the presence of TNF. This result, together with fluorescence, confirms that TNF is intercalated between pyrene moieties and maintains the aromatic stacking interactions to stabilize the supramolecular self-assembly in the presence of TNF in both hydro- and organogels. Similar to FmKPy ambidextrous gels, FTIR spectra of both FmKPy CT gels show the Fmoc-carbonyl stretching frequency at 1697 cm−1 which is shifted to lower frequency compared to the FmKPy monomer. This result suggests that the involvement of hydrogen bonding in the supramolecular self-assembly of FmKPy gels is maintained upon the addition of TNF in the CT gels. The contribution of carbonyl frequency associated with TNF in the FTIR spectrum of both FmKPy CT gels are identified at 1734 cm−1, whereas the band at ∼1660−

Figure 6. Representative HR-SEM images of the xerogel obtained from CT hydrogel (A,B) and CT organogel (C,D) containing FmKPy and TNF.

morphology with a decreased size suggests that TNF has some influence on the supramolecular self-assembly of FmKPy in the hydrogelation. Similarly, although the xerogel of organogels displays a fibrillar nature, it is shorter in length and higher in thickness compared to that of FmKPy organogels without TNF (Figure 6C,D). Again, this result suggests that TNF had some influence on the supramolecular self-assembly of FmKPy in the organogelation. Meanwhile, an excimer peak in the fluorescence spectra of FmKPy−TNF CT hydrogel and organogel drastically

Figure 7. Fluorescence spectra of FmKPy hydrogel (A) and organogel (B) and the corresponding CT hydrogel and organogel containing TNF. The concentration of FmKPy hydro- and organogels were 1.3 wt %, and the total concentration of both CT gels was 1.9 wt % (1:1 molar ratio). (C) FTIR spectra of FmKPy−TNF in the form of xerogels obtained from CT hydro- and organogels, and as TNF (KBr). (D) PXRD spectra of xerogels of CT hydro- and organogels of FmKPy−TNF and in the powder form. G


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Figure 8. Rheological properties showing the CT hydrogel of the dynamic frequency sweep (A) at 0.5% strain and continuous step strain sweep (B) followed by the temperature sweep and annealing the gelation (C) and the AG frequency sweep (D).

Figure 9. Rheological properties showing the CT organogel of the dynamic frequency sweep (A) at 0.5% strain and the continuous step strain sweep (B).

1665 cm−1 is attributed to the amide carbonyl group formed between pyrene and lysine (Figure 7C). 3.3.2. Rheology of FmKPy−TNF CT Ambidextrous Gels. The mechanical property of FmKPy−TNF CT ambidextrous gels was also analyzed by following Rheological parameters. As can be seen from Figure 8, a typical frequency sweep experiment for FmKPy−TNF CT hydrogels shows a characteristic observation of G′ being higher than G″ (G′ > G″) for viscoelastic materials such as gels.45−47 Similar to FmKPy hydrogels, a slight decrease in the ΔG at a higher frequency range suggests small frequency dependence (Figure 8A). Interestingly, FmKPy−TNF CT hydrogels also exhibit the thixotropic (gel → solution → gel) property as evidenced from the step-strain experiment.46,48 For example, the gel was destroyed after applying 100% strain which is evident from the modulus values (G′ < G″), and regelation occurred upon release of the strain from 100 to 0.5% as evident from G′ being higher than G″ (G′ > G″). A similar effect has been observed for two consecutive cycles which confirm that the FmKPy− TNF CT hydrogel retains the thixotropic nature (Figure 8B).

Similar to the FmKPy hydrogel, temperature-dependent rheological experiments were also performed to explore the thermal stability as well as the annealing property of the hydrogel. This experiment was studied based on our initial assessment of regelation by the vial inversion method. As can be seen from Figure 8C, during heating, the FmKPy−TNF CT hydrogel also melts at the same temperature (Tgel = 85 °C) as that of FmKPy, indicating that TNF did not influence the thermal property of the CT hydrogel. Further, upon cooling, the CT solution also started reforming the gel (AG) at around 80 °C with G′ being higher than G″ (G′ > G″). This experiment confirms that the thermally reversible nature of the supramolecular hydrogel formed by FmKPy is also retained in the presence of TNF. However, the magnitudes of both moduli (G′ and G″) were slightly reduced compared to that of the normal gel (Figure 4C) which is different from the FmKPy hydrogel where both moduli were increased significantly. The reduced magnitude of G′ and G″ indicates that the AG of FmKPy−TNF has less rigidity and elasticity than the normal gel. A typical frequency sweep (Figure 8D) for AG confirms the H


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Figure 10. Proposed molecular packing in the FmKPy ambidextrous gel (left) and in the CT ambidextrous gel (right).

4. CONCLUSIONS

viscoelastic nature of the AG and also indicates small frequency dependence at a higher frequency range as manifested from a decrease in the ΔG. Similar to the hydrogel, the FmKPy−TNF CT organogel also displayed a viscoelastic nature as G′ being higher than G″ during the frequency sweep (Figure 9A) and thixotropic property (Figure 9B). Unlike the FmKPy organogel, ΔG (G′ − G″) remains the same over a range of frequency studied. This observation shows that the viscoelastic nature of the FmKPy organogel extends to a higher frequency range in the presence of TNF. 3.4. Proposed Molecular Packing of FmKPy and the FmKPy−TNF CT Self-Assembled Structure. On the basis of the spectroscopic and PXRD data obtained, we propose a model (Figure 10) for the supramolecular self-assembly of FmKPy where pyrene and fluorene moieties are stacked through π−π interactions with a distance of ∼0.34 nm. This model also shows the possible hydrogen bonding interactions between amide and carbamate carbonyl groups as well as hydrophobic interactions between alkyl chains. On the basis of this molecular arrangement, we also suggest a model for the CT gel where TNF could interact with pyrene in an intercalation mode (Figure 10). This type of orientation promotes the CT from pyrene to TNF. Although the distance between pyrenes could increase because of TNF intercalation, the hydrogen bonding among carbamate and amide groups and fluorene πstacking would not be affected because of the presence of the flexible chain as shown in Figure 5. Consequently, all driving forces such as π-stacking, hydrogen bonding, and hydrophobic chain might have been preserved in the CT gel containing FmKPy and TNF. 3.5. Cell Viability. Pyrene-based compounds have been used as a fluorescence dye to signal the presence of ions55 and small molecules.56 Importantly, they are utilized to detect nucleic acids57,58 and proteins59 in complex biological fluids. Hence, we evaluated the cell viability of FmKPy at different concentrations (below and after the MGC) at pH 7.4 using MTT assay to explore the possibility of using FmKPy in biological applications in the future. The results showed that cell viability depends on the concentration of FmKPy as the percentage of live cells decreased with increasing the concentration of FmKPy (Figure S10). At low concentration (0.1 wt %, below the MGC), cells not only are viable but also showed proliferation (∼20% on day 3), whereas cells are viable at the gelation concentration (0.2 wt %, above MGC), indicating the low cytotoxicity of the FmKPy hydrogel at the low gelation concentration.

Herein, we engineered and demonstrated a CT supramolecular ambidextrous fluorescence gel system using a gelator (FmKPy) where two versatile moieties such as pyrene and Fmoc and an acceptor TNF molecule are coupled through a natural amino acid L-lysine for the first time. The engineered FmKPy gelator not only exhibited efficient ambidextrous gelation (as evidenced by the low MGC) but also showed other significant properties such as high thermal stability of hydrogels at the low MGC, pH stimuli responsiveness, thixotropic nature, and temperatureinduced excimer formation. The FmKPy−TNF CT ambidextrous gel maintains pH stimuli responsiveness, thixotropic nature, and high thermal stability. The fibrils formed through the supramolecular self-assembly of FmKPy in FmKPy and FmKPy−TNF gels are stabilized by aromatic π-stacking, hydrogen bonding, and hydrophobic interactions. Because pyrene-based compounds in the form of organogels have been developed widely for many applications while their hydrogels were limited, the current finding of pyrene-based ambidextrous fluorescence gels with CT and cell viability provides a wide opportunity to use FmKPy for diverse applications. The successful demonstration of FmKPy molecules as an efficient gelator provided a lead to construct several pyrene-based compounds by a simple substitution of Fmoc-moiety with other potential functional groups, which is in progress.

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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.7b03453.

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Supporting Information contains synthesis and characterization of FmKPy and additional information (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91 44 24437223. ORCID

Abhijit P. Deshpande: 0000-0002-8971-0345 Niraikulam Ayyadurai: 0000-0002-7333-6344 Ganesh Shanmugam: 0000-0003-1096-4201 I


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The manuscript was written through contribution from all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Council of Scientific and Industrial Research (CSIR) under a XII five-year plan project (STRAIT CSC 0201). CLRI communication number is 1232. We thank the Director, CSIR-CLRI, for his support and permission to publish this work. We thank R. Chandrasekar, Senior Technical Officer, CATERS, CSIR-CLRI, for his help in MALDI measurement. One of the authors, S.M.M.R., thanks CSIR for financial support in the form of Senior Research Fellowship.

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A Low-Molecular-Weight Gelator Composed of Pyrene and Fluorene

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