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Article Cite This: ACS Omega 2018, 3, 3744−3751
Self-Healing Hydrogel from a Dipeptide and HCl Sensing Sujay Kumar Nandi, Krishnendu Maji, and Debasish Haldar* Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, West Bengal, India S Supporting Information *
ABSTRACT: Ordered self-assembly of small organic molecules may induce novel properties in a supramolecular arrangement and can act as advance functional materials. This paper discusses the development of a new stimuliresponsive dipeptide hydrogelator containing L-phenylalanine and α-aminoisobutyric acid (Aib). The dipeptide Boc-Phe-AibOH, on addition with three equivalent of sodium hydroxide and water, transformed into a robust hydrogel. The transparent hydrogel is self-healing in nature. The instant gelation is highly selective toward sodium hydroxide and does not need any sonication or heating−cooling cycle. The thixotropic nature of the gel has been confirmed by rheological step-strain experiments at room temperature. Moreover, in the oil−water mixture, the compound exhibits phase-selective gelation. When the gel cylinder is cut into pieces, it does not conduct electricity, but once self-healing occurs, it conducts electricity. The diffusion of rhodamine 6G through the hydrogel indicates the dynamic nature. The hydrogel is highly sensitive toward HCl, that is, in the presence of HCl vapor, the gel becomes deformed.
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INTRODUCTION Precise organization, selective interaction, and molecular recognition are keys for the fabrication of multicomponent advance materials like supramolecular gels.1−9 The supramolecular gels contain mostly the liquid, immobilized by a cross-linked network of fibers made of self-organized building blocks.10−12 Because of the formation through noncovalent interactions such as the hydrogen bonding interaction, π−π stacking interaction, and ion pairing interactions between the building blocks, the supramolecular gels are reversible. The supramolecular gels can be stimuli responsive by introducing receptor unit in the gelator molecule. Many reported that supramolecular gels have excellent sensitivity to the different stimuli including ions, pH, enzyme, heat, light, ultrasound, and mechanical steering.13 Moreover, the supramolecular gels have high homogeneity, tunability, biocompatibility, and degradability.14 The supramolecular gels are used in many areas, from foods to electronics. These soft materials have wide applications as sensors for biologically important molecules and ions, as light-harvesting materials,15−19 as an efficient catalyst20 for optoelectronic systems,21 and as an external support for tissue repairing and tissue engineering as well as selective drug delivery.22−29 Different low molecular weight organic compounds including steroids,30 short peptides and peptide mimetics,31−34 urea derivatives,35−40 carbohydrates,41 dendrimers,42,43 and π-gelators44,45 have been used to form a stable supramolecular gel. However, most of these compounds may be obtained by a complex multistep synthesis. Also, most of these compounds are insoluble in water. Therefore, the exploration of simple stimuli-responsive low molecular weight © 2018 American Chemical Society
hydrogelators with diverse applications is highly important.46−48 Herein, we have synthesized a dipeptide (Figure 1) that can form stimuli-responsive, robust transparent hydrogel with
Figure 1. Chemical structure of dipeptide 1.
significant self-healing propensity. The hydrogelation is extremely selective with sodium hydroxide. The gel exhibits high stability and thixotropic behavior. The supramolecular hydrogel can be used as soft conducting materials. The hydrogel can be colored with dyes and can be crafted in any shape. The hydrogel is highly sensitive toward HCl vapor (Figure 1).
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RESULTS AND DISCUSSION The dipeptide was synthesized by traditional solution phase methods, purified, and well characterized by nuclear magnetic Received: February 28, 2018 Accepted: March 22, 2018 Published: April 3, 2018 3744
DOI: 10.1021/acsomega.8b00358 ACS Omega 2018, 3, 3744−3751
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ACS Omega resonance spectroscopy (both 1H NMR and 13C NMR), FT-IR spectroscopy, and mass spectrometry (MS) analysis. A detailed description of the synthesis and characterization of dipeptide 1 is given in the Supporting Information. Briefly, tBoc-protected phenylalanine was coupled with Aib-OMe using a standard coupling reagent N,N′-dicyclohexylcarbodiimide (DCC) and 1hydroxybenzotriazole (HOBT) followed by hydrolysis to obtain 1 in good yield. The signals at δ 7.26 to 7.17 in its 1H NMR spectrum are assigned to the aromatic protons of phenylalanine. The signals at δ 8.05 have been assigned to the NH of Aib. The signal at m/z 373.41 in the ESI-mass spectrum further confirmed the formation of the dipeptide. The dipeptide, Boc-Phe-Aib-OH, when mixed with 3 equiv of sodium hydroxide in water and shaken for 30 s, due to the effect of mixing transformed to a stable transparent hydrogel (Figure 2). The instant hydrogel formation is extremely
Figure 3. Gelation studies with dipeptide 1 analogues (A) Boc-TrpAib-OH, (B) Boc-Tyr-Aib-OH, (C) Boc-Phe-Gly-OH (D) Boc-PheAib-OH, (E) Boc-Phe-Ala-OH, and (F) Boc-Aib-Phe-OH. Hydrogel forms only in Boc-Phe-Aib-OH.
Rheology provides insights about the tertiary structure (type of network) presence in the gel.33 The rheological data were obtained using a freshly prepared NaOH-sensitive hydrogel. The elastic response of the hydrogel (G′, storage modulus) and the viscous response (G″, loss modulus) were examined as a function of shear strain at 20 °C, and 10 rad s−1 frequency. For NaOH-responsive supramolecular hydrogel, the G′ (storage modulus) was approximately an order of magnitude larger than the G″ (loss modulus). This indicates the elastic nature of the hydrogel which is common for physical cross-link (Figure 4). The G′ and G″ have not crossed each other, which indicates that the hydrogel is stable and rigid.
Figure 2. Hydrogelation studies of peptide 1 in the presence of (A) LiOH, (B) NH4OH, (C) Na2CO3, (D) NaCl−LiOH mixture, (E) Na2CO3−LiOH mixture, (F) NH4OH−NaCl mixture, (G) KOH, (H) NaOH, and (I) K2CO3. Hydrogel forms only in the presence of sodium and hydroxide ion. Peptide (10 mg/mL, 1.0 w/v %) was used.
sensitive to sodium hydroxide only. Other bases such as KOH, LiOH, K2CO3, Na2CO3, and NH4OH fail to form hydrogels under same conditions (Figure 2). The dipeptide, Boc-Phe-Aib-OH, forms gel in the mixture of NH4OH and NaCl; Na2CO3 and LiOH; and NaCl and LiOH (Figure 2). This indicates that not only sodium but also hydroxide ions have a significant role in the supramolecular hydrogel formation. The minimum gelation concentration for peptide 1 is 4 mg/mL. The supramolecular hydrogel formation has been confirmed by an inverted vial test.49,50 More excitingly, the hydrogel formation does not require additional stimuli like sonication and heating−cooling cycles. We have also tested the gelation under the same condition with analogues of dipeptide 1 such as Boc-Tyr-Aib-OH, Boc-TrpAib-OH, Boc-Phe-Ala-OH,29 Boc-Phe-Gly-OH,24 and Boc-AibPhe-OH. However, they have failed to form such a kind of hydrogel (Figure 3). The transparent hydrogels can be kept at room temperature for 3 months. The hydrogel has shown an endotherm on heating at 67.8 °C (Supporting Information, Figure S1). Different rheological studies have been performed to know the visco-elastic properties of the NaOH-responsive hydrogel.51
Figure 4. Dynamic rheology of hydrogel contained 1.0% (w/v) gelator; frequency sweep experiment at 20 °C at a constant strain (0.1%).
The thixotropic behavior of the hydrogel was confirmed by rheological measurement. Under external stimuli, a thixotropic gel can disintegrate to solution or a quasiliquid state and can regain its original shape after the removal of the applied force. To figure out the thixotropic property of the NaOH-responsive hydrogel, rheological step-strain experiments52,53 have been done at 20 °C under varying strain. At first, gels were placed at a constant strain of 0.1%, which shows that G′ is higher than G″ (step 1, Figure 5). Finally, the strain increased from 0.1 to 50% and was continued for few minutes. This shows the complete 3745
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studied by AFM. Interestingly, the AFM shows the presence of twisted helical fibers (Figure 6c). The dipeptide contains a LPhe residue. Helical twisted gel fibers are common for chiral gelators, as the molecular chirality is generally translated to the supramolecular level. The height profile diagram (Figure 6c inset) of hydrogel fibers (line 1 marked in Figure 6c) suggests that the distinct fibers are closely packed. The 3D image (Figure 6d) exhibits that the twisted fiber surface is very rough. The peptide 1 in methanol−water (1:1) solution exhibits an absorption band at 290 nm responsible for π−π* transition. The intensity of the band gradually increases with the increasing NaOH concentration (Figure 7a). The emission spectrum of peptide 1 in methanol−water (1:1) solution also shows that with the increasing NaOH concentration both bands at 365 nm for the aromatic ring and 422 nm responsible for molecular stacking are increasing (Figure 7b). The NaOHresponsive peptide 1 hydrogel exhibits blue emission upon excitation at 366 nm (Figure 7c). Furthermore, the NaOH-responsive aggregation of peptide 1 was investigated by circular dichroism (CD) spectroscopy with a JASCO J-815-150S instrument at 25 °C. For this experiment, a 1 cm path length quartz cell cuvette was used. The CD spectra of peptide 1 (1 × 10−5 M, in methanol−water (1:1) solution) show negative Cotton effect at 214 and 230 nm (Supporting Information, Figure S2). This indicates an antiparallel sheet-like structure for peptide 1. With the addition of NaOH, the intensity of the band decreases and a new band appeared at 208 nm. Titration of peptide 1 in CD3OD (90%, 10% D2O) with increasing NaOH amount was recorded by 1H NMR spectroscopy. This provides clear evidence of the NaOH peptide 1 interaction. The Aib NH proton disappeared, and Phe α proton signal of peptide 1 shifted gradually toward downfield with the addition of NaOH, which suggests the interaction between NaOH and peptide 1 (Supporting Information, Figure S3). Xerogel was grinded with the help of a mortar and pastel for 1 min to obtain a fine powder. Then, the powder sample was placed in a glass sample holder, and then, the holder was placed into the Rigaku X-ray diffractometer (C 3000) with a parallel beam optics attachment horizontally to measure the data. The powder X-ray diffraction (PXRD) of peptide 1 xerogel indicates a crystalline structure and shows multiple diffractions at 2θ = 9.04 (9.772 Å), 18.12 (4.891 Å), 22.28 (3.986 Å), 27.90 (3.194 Å), 29.05 (3.071 Å), 32.37 (2.763 Å), 33.90 (2.642 Å), 34.80 (2.575 Å), 35.69 (2.513 Å), 36.83 (2.438 Å), and 39.90° (2.257 Å) (Supporting Information, Figure S4). The strongest scattering peak at 9.04° (2θ) corresponds to a second-order diffraction and antiparallel structure.25 Finally, the dipeptide has also been characterized by a single-crystal X-ray diffraction experiment. From X-ray crystallography, the asymmetric unit contains one molecule of dipeptide 1. The ORTEP diagram (Figure 8a) of dipeptide 1 exhibits that the backbone of peptide have a kink-like conformation (ϕ1 = −82.90° and ψ1 = 128.00°). The individual subunits are themselves regularly interlinked through multiple O−H···O and N−H···O intermolecular hydrogen-bonding interaction and hence form a supramolecular sheet along the crystallographic b direction (Figure 8b). The hydrogen bonding parameters of peptide 1 are also listed in the Supporting Information Table S1. The supramolecular sheet-like structures are also stabilized by π−π stacking interactions (shortest C−C distance is 4.03 Å) between Phe aromatic rings. A β-sheet-like arrangement of the gelator molecule both in the gel state as well as solid state
Figure 5. Thixotropic data of the NaOH responsive hydrogel contained 1.0% (w/v) gelator, obtained by step-strain measurements at 20 °C at a constant frequency (0.1 rad s−1).
inversion of G′ and G″, indicating the complete break of the hydrogel (step 2, Figure 5). Again the strain was decreased from 50 to 0.1% and continued for few minutes and gel restoration kinetics were observed (step 3, Figure 5). This was continued for several cycles and confirmed the self-healing behavior of the NaOH-responsive hydrogel. The morphology of the NaOH-responsive hydrogel was investigated by the field emission scanning electron microscopic studies (FE-SEM).54 The FE-SEM images of xerogel exhibit rod-like fibrillar network structures. The polydisperse fibrils are highly entangled to form the network structure (Figure 6a). Energy-dispersive X-ray spectroscopy (EDS)
Figure 6. (a) FE-SEM image of the xerogel exhibiting a fibrillar structure of NaOH-responsive peptide 1 hydrogel. (b) EDS exhibits the presence of sodium in gel fibers. (c) AFM image of the xerogel showing that the hydrogel fibers are twisted in nature (inset: height profile plot). (d) 3D image of a twisted fibrillar network.
studies have been done to know the alkali metal content in the xerogel matrix. The EDS confirms the sodium content (Figure 6b). To determine the topology of fibrillar aggregates, atomic force microscopic (AFM) experiments of the hydrogel were done. The NaOH-responsive peptide 1 hydrogel was sliced and cast on a microscopic glass coverslip and dried by slow evaporation. The microscopic glass slide was then allowed to dry under vacuum at room temperature for 3 days and 3746
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Figure 7. (a) Absorption spectra of peptide 1 with gradual addition of NaOH show increasing intensity of the band at 290 nm. (b) Emission spectra of peptide 1 with gradual addition of NaOH. (c) Hydrogel exhibits blue emission upon excitation at 366 nm.
Figure 9. (a) Gel cylinder suspended in air by holding in hand. (b) 100 mL conical flask on top of hydrogel cylinders showing the stability and weight tolerance of the bar. (c) Hydrogel bars are strong enough to accept several grams of weight. (d) Alternate organization of rhodamine 6G-doped and undoped hydrogel bars. (e) Self-healing of few pieces of hydrogel blocks. (f) Bar regenerates from three small pieces of hydrogel suspended by two glass stoppers. (g) Electric conductivity experiment using a gel cylinder form by merging gel blocks, self-healing, and regeneration of the electric circuit. Figure 8. (a) ORTEP diagram of peptide 1. (b) Intermolecular hydrogen bond-mediated supramolecular sheet-like structure of peptide 1 along the crystallographic b direction.
very dynamic in nature. The dye rhodamine 6G can disperse through the gel block. Moreover, the hydrogels have significant self-healing properties. When a block of gel was cut into two pieces and then placed together, the pieces merged.56 Also, several small pieces of hydrogels could be merged into a continuous stable self-supporting bar. These fused bars could be suspended in air by holding one side. Inside the hydrogel, there exists a dynamic equilibrium showing the formation of new self-assembled fibers as well as dissociation of mature fibers. This dynamic equilibrium allows the regeneration of new fibers at the fusion interface, which is important for the selfhealing and repairing process. The fusion of a rhodamine 6Gdoped hydrogel bar with an undoped hydrogel bar shows the diffusion of rhodamine 6G through an undoped hydrogel bar.
was confirmed from the crystal structure matching with the PXRD data (Figure S4). The NaOH-responsive hydrogel formed by peptide 1 at low concentration (4 wt %) is very stable. Different self-supporting geometrical shape can be crafted from the gel.55 By holding, one side a gel bar can be suspended in air (Figure 9a). The NaOH-responsive hydrogels formed by peptide 1 low concentration (4 wt %) are strong enough to tolerate several grams of weight (Figure 9b,c). Even, one can slice a big gel block into several small pieces (Figure 9d).56 The hydrogel is 3747
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ACS Omega This exhibits the dynamic exchange of dissolved molecules at the interface (Figure 9d−f). This is further established by electric conductivity experiments.57 The conductivity of a hydrogel was measured by an ONKTON PC-2700 conductivity meter. The cylindrical electrodes which are separated by 1 cm were used. The NaOH-responsive hydrogel block formed by peptide 1 is highly conducting (Supporting Information, Figure S5). Conductance of NaOH added water = 253.60 μS/cm. Conductance of the gel = 194.20 μS/cm. Therefore, the conductivity decreased from Figure S5c to S5d, but the decrease is quite small. As the gels were formed, the mobility of the ions decreased and as a result of that conductivity decreased. Connection of the battery, LED, and gel block complete the circuit and shows the flow of electricity. However, chopping the gel block into two pieces disrupts the electric circuit and exhibits no flow of electricity. However, when joining the pieces together, because of self-healing, the gel pieces merged into a continuous gel block and conductivity of the circuit is regained (Figure 9g). The selectivity of peptide 1 to form NaOH-responsive hydrogels without any heating, stirring, or sonication makes it an interesting candidate in the separation of trace amounts of water from the organic solvent.58 The worm-like micelles of peptide 1 have been formed, which convert to invertable materials in multiple equivalents of NaOH, as one would need to screen the charge effectively to form a “gel”. Hence, the need for Na as the packing will depend on the packing which will be affected by the counterion as a normal surfactant. If there is diesel−water mixture, one can separate the water from diesel by the addition of peptide 1 and NaOH and forming hydrogel (Supporting Information, Figure S6). The diesel can be separated by decantation or filtration. Moreover, the hydrogel can be used as an online HCl gas sensor. This is highly important for environmental pollution analysis, automotive industry, and chemical industry. To demonstrate the HCl sensing application, a block of hydrogel was charged with HCl gas. HCl gas was generated in situ by the reaction between NaCl and H2SO4 (2NaCl + H2SO4 = Na2SO4 + 2HCl). A hydrogel cylinder was made by the dipeptide 1 (1.0% (w/v)) at room temperature. All electrical circuits were prepared by the inclusion of a gel cylinder. Then, the HCl gas was passed over the gel cylinder using a glass tube. On contact with HCl gas, the gel block deformed (Figure 10, Supporting Information Video S1). However, the gel is not responding to others like H2S, SOCl2, and CO2. The process of HCl gas detection can be upgraded to an on-line process for safetyrelated applications in industries which use or produce large amounts of HCl.
Figure 10. (a) Cylindrical hydrogel bar. (b) Gel cylinders in contact with HCl gas. (c,d) Gel cylinders are deformed by sensing HCl gas.
novel properties is promising for soft conducting materials and HCl sensors.
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EXPERIMENTAL SECTION General. Chemicals were procured from Sigma chemicals. Synthesis. The reported peptide was synthesized by traditional solution-phase methodology using racemisationfree fragment condensation strategy. The C-terminus was protected as a methyl ester, and the tertiarybutoxycarbonyl group was used for N-terminal protection. Coupling was mediated by N,N′-dicyclohexylcarbodiimide/1-hydroxybenzotriazole (DCC/HOBT). The products were purified by column chromatography using silica (100−200 mesh size) gel as the stationary phase and n-hexane−ethylacetate 9:1 as the eluent. The compounds were characterized by 400 MHz 1H NMR spectroscopy, 13C NMR spectroscopy, FT-IR spectroscopy, and MS. Synthesis of Boc-Phe(1)-Aib(2)-OMe 2. The compound was synthesized following the previous report by Pramanik and coworkers.59 1 H NMR (500 MHz, CDCl3, δ in ppm): 7.26−7.27 (m, 2H, phenyl ring protons), 7.21−7.22 (m, 2H, phenyl ring protons), 7.22−7.20 (m, 1H, phenyl ring proton), 6.48 (s, 1H, Aib NH), 5.22−5.19 (s, 1H, Phe NH), 4.22 (m, 1H, Phe CαH), 3.70 (s, 3H, −OCH3), 2.95−2.90 (m, 2H, Phe CβH), 1.44 (s, 6H, Aib CαH), 1.41 (s, 9H, BOC−CH3). 13C NMR (125 MHz, CDCl3, δ in ppm): 174.17, 170.44, 156.31, 136.86, 129.54, 128.26, 126.95, 80.20, 56.43, 56.40, 52.65, 38.54, 28.32, 24.76; mass spectral data TOF-MS m/z: [M + Na]+ = 387.49, [M + K]+ = 403.44, [M-Boc + Na]+ = 287.51, [M-Boc + H]+ = 265.79, Mcal = 364.44; FT-IR (cm−1): 3369, 3312, 1729, 1680, 1540. Synthesis of Boc-Phe(1)-Aib-(2)-OH 1. Peptide 1 was synthesized following the previous report by Pramanik and co-workers.59 1 H NMR (400 MHz, DMSO-d6, δ in ppm): 12.39−12.16 (b, 1H, acid OH), 8.05 (s, 1H, Aib NH), 7.26−7.17 (m, 5H, phenyl ring protons), 6.76 (s, 1H, Phe−NH), 4.20 (m, 1H, Phe CαH), 2.96−2.91 (m, 1H, Phe CβH), 2.74−2.68 (m, 1H, Phe CβH), 1.36 (s, 6H, Aib CβH), 1.29 (s, 9H, BOC −CH3). 13C NMR (100 MHz, CDCl3, δ in ppm): 175.48, 170.88, 155.10, 138.10, 129.31, 127.94, 126.13, 77.98, 55.36, 54.92, 37.19, 28.14, 24.77; mass spectral data TOF-MS m/z: [M + Na]+ =
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CONCLUSIONS In conclusion, we have discussed stimuli-sensitive dipeptidebased hydrogel at room temperature. The peptide Boc-PheAib-OH 1 forms a parallel sheet-like structure. The gelation is extremely selective for sodium hydroxide only. The dipeptide hydrogel has high stability and load bearing capacity. The hydrogel shows self-healing properties and thixotropic nature. The dipeptide exhibits phase-selective gelation in oil−water mixtures. The diffusion of rhodamine 6G through the hydrogel shows the dynamic nature of the hydrogel. The self-healing gel is conductive. When the gel matrix is broken, it does not conduct electricity, but once self-healing occurs, it conducts electricity. The hydrogel is highly sensitive toward HCl vapor. These findings show that the self-healing hydrogel with its 3748
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ACS Omega 373.41, [M-Boc + Na]+ = 273.39, [M-Boc + H]+ = 251.71, Mcal = 350.41; FT-IR (cm−1): 3374, 3297, 1717, 1677, 1638, 1563. NMR Spectroscopy. All NMR experiments were performed on a JEOL 400 MHz or Bruker 500 MHz spectrometer. Compound concentrations were in the range 1−10 mm in CDCl3 and DMSO-d6. For NMR titration, peptide 1 was dissolved in CD3OD (90%, 10% D2O), and NaOH solution in D2O was gradually added and was recorded by 1H NMR spectroscopy. FT-IR Studies. Solid-state FT-IR spectra were obtained with a PerkinElmer spectrum RX1 spectrophotometer by the KBr disk technique. Absorption Spectroscopy. All absorption spectra were recorded on a PerkinElmer UV/Vis spectrometer (LAMBDA 35) using 1 cm path length quartz cell. Fluorescence Spectroscopy. All fluorescence spectra were recorded on a PerkinElmer fluorescent spectrometer (LS 55) using 1 cm path length quartz cell. Slit widths 2.5/2.5 were used. Mass Spectrometry. Mass spectra were recorded on a Waters Corporation Q-Tof Micro YA263 high-resolution mass spectrometer using positive-mode electrospray ionization. Field Emission Scanning Electron Microscopy. FE-SEM has been used to know the morphologies of the reported compounds. A drop of sample solution was placed on a clean microscopic glass slide and dried by slow evaporation. The materials were gold-coated, and the micrographs were taken in an FE-SEM apparatus (JEOL scanning microscope-JSM6700F). Atomic Force Microscopy. The topology of the hydrogel was investigated by AFM. A slice of gel was placed on a clean microscopic glass coverslip and dried by slow evaporation. The sample was then allowed to dry under vacuum at room temperature for 3 days. Images of the xerogel were taken by semicontact-mode using an NTMDT instrument, model no. AP-0100. Powder X-ray Diffraction Study. The PXRD pattern was carried out in a Rigaku X-ray diffractometer (C 3000) with a parallel beam optics attachment. The instrument was operated at a 45 kV voltage and 200 mA current and was calibrated with a standard silicon sample. The wavelength of the X-ray source is 1.5418 Å (Kα value of Cu). Xerogel was grinded for 1 min to obtain a fine powder sample with the help of a mortar and pastel. Then, the powder sample was subjected to place in a glass sample holder, and then the holder was placed into the instrument horizontally to measure the PXRD data. CD Spectroscopy. CD experiments of the reported peptides in 1:1 methanol to water mixture have been done with a JASCO J-815-150S instrument at 25 °C. For this experiment, a 1 cm path length quartz cell cuvette was used. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) experiments were performed by a METTLER DSC instrument under N2 gas. The sample (30 mg) was placed in the sample holder for the DSC experiment. Gelation. The peptide 1 (BOC-Phe-Aib-OH) (4 mg, 1 equiv) and an excess of NaOH (3 equiv) were mixed in 1 mL of water at room temperature and shaken for 30 s, the mixing has an effect and instant hydrogel formed. Rheological Experiments. To understand the mechanical strengths and thixotropic behavior of the hydrogel, we performed rheological measurements on an MCR 102 rheometer (Anton Paar, modular compact rheometer) using a steel parallel plate geometry with a 40 mm diameter at 20 °C.
To control the temperature accurately during the experiment, the rheometer is attached to a Peltier circulator thermo cube. The storage modulus (G′) and loss modulus (G″) of the hydrogels have been recorded using that experimental setup. From the dynamic strain sweep experiment, this hydrogel is stable up to 6.6% strain. This type of low strain tolerance is quite common for peptide-based hydrogel. In step-strain experiment, generally higher strain than the ruptured strain value has been used. Because the gel is stable up to 6.6% strain value, 50% strain (around 8 times higher) was used for the stepstrain measurement. Conductivity Experiments. The electrical conductivity of a hydrogel was measured by an ONKTON PC-2700 conductivity meter. The electrical conductivity of a solution or a gel is measured by determining the resistance of the solution or gel between two flat or cylindrical electrodes separated by a fixed distance. Here, we have used cylindrical electrodes which are separated by 1 cm. A mixture of dipeptide 1 [1.0% (w/v)] and NaOH (3 equiv) in water was dipped into the electrodes, and the data were recorded. HCl Sensing Experiments. For the HCl-triggered experiment, HCl gas was generated with the reaction between NaCl and H2SO4. A hydrogel cylinder was made by the dipeptide 1 [1.0% (w/v)] at room temperature. All electrical circuits were prepared by the inclusion of a gel cylinder. Then, the HCl gas was passed over the gel cylinder using a glass tube. X-ray Crystallography. X-ray diffraction quality the white colored crystal of the dipeptide 1 was obtained by slow evaporation of methanol−water solution. Intensity data have been collected with Mo Kα radiation using a Bruker APEX-2 CCD diffractometer. Data were processed by Bruker SAINT package, and the structure solution and refinement were done using SHELX97. The nonhydrogen atoms were refined with anisotropic thermal parameters. The data have been deposited at the Cambridge Crystallographic Data Centre with reference CCDC 1583322.
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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/acsomega.8b00358. Synthesis and characterization of peptides, 1H NMR, 13C NMR, and Figures S1−S8 (PDF) Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected] (D.H.). ORCID
Debasish Haldar: 0000-0002-7983-4272 Author Contributions
S.K.N. has synthesized the compound. S.K.N. and K.M. have carried out the experimental work. D.H. has done the analysis and wrote the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the CSIR, India, for financial assistance (project no. 02(0206)/14/EMR-II). This work is also 3749
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ACS Omega
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supported by CSIR, India (fellowship to S.K.N.). K.M. thanks IISER Kolkata for fellowship.
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DOI: 10.1021/acsomega.8b00358 ACS Omega 2018, 3, 3744−3751
