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Tuning the Gelation Ability of Racemic Mixture by Melamine: Enhanced Mechanical Rigidity and Tunable Nanoscale Chirality Zhaocun Shen, Tianyu Wang,* and Minghua Liu* Beijing National Laboratory for Molecular Science (BNLMS), CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ABSTRACT: Understanding the relationship between molecular chirality of the gelators and the properties of the assembled supramolecular gels could be very important for developing novel functional soft matters. Although mixing the enantiomers with different molar ratios has been proved to be useful for modulating supramolecular assemblies, usually the racemates of different chiral molecules are not good gelators. In this study, the coassembly of the glutamic acid-based bolaamphiphile racemate and melamine was found to form hydrogels, while the assembly of the racemate only produced precipitates. Remarkably, the racemic hydrogels show lower CGC value, enhanced mechanical rigidity, and dual pHresponsive ability compared to the pure enantiomer hydrogels. The gelation properties, nanoscale chirality, and nanostructures of the racemic hydrogels can be regulated flexibly by changing molar ratios of different molecular building blocks.

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INTRODUCTION As the new low molecular weight gels assembled collaboratively from several different molecular building blocks, multicomponent supramolecular gels have attracted considerable attention recently.1−6 Because the gelation of multicomponent systems is dependent on the subtle collaboration of multiple noncovalent interactions from different molecular building blocks, the extensive research on multicomponent gels could be very helpful for further understanding the principles of supramolecular assembly in nature. Moreover, the excellent tunable properties and remarkable stimuli-responsive capabilities of multicomponent gels are also very useful for developing novel functional soft materials.7−12 However, making multicomponent supramolecular gels is still challenging. Especially in the case of hydrogels, only a few systems have been successfully developed.13−16 For multicomponent supramolecular assemblies with ordered nanostructures, whether they are obtained from the coassembly or orthogonal assembly, diverse influence factors, such as solubility, molecular chirality, hydrogen bonding, π−π stacking, and so on, from different molecular building blocks, have to work together in a complex manner. And the cooperation and confliction between these factors must be carefully considered. Notably, the gelation of supramolecular systems, no matter with single component or multicomponent, is largely dependent on their molecular chirality and supramolecular chirality. Therefore, many chiral molecules are good gelators,17−28 while only a few kinds of achiral molecules can form organogels or hydrogels.29−34 Similarly, the racemates are usually not good gelators;35−41 only a few racemic systems were found to form supramolecular gels.42−44 However, racemic mixtures could produce new modes of assemblies, of which the nanostructures are different from corresponding pure enantiomer assemblies, © 2014 American Chemical Society

and mixing enantiomers has been shown to be very useful for tuning the properties of supramolecular assemblies.45−47 Therefore, developing racemic gels with fine-tuning supramolecular chirality should be a valuable research topic. On the other hand, although a few racemic gels have been prepared, the multicomponent supramolecular gels coassembled by a racemate and another achiral functional molecular building block are still rarely fabricated.48,49 Actually, these multicomponent supramolecular gels may have more tunable nature and different properties. Thus, for the fine-tuning of supramolecular gels, in addition to modulating supramolecular chirality via the mixing of enantiomers, the noncovalent interactions between racemate and achiral molecule as well as the self-assembly of this achiral molecule could play very important roles. Moreover, these multicomponent systems containing both chiral enantiomers and achiral molecules also can provide unique supramolecular models for studying chirality at different scales. Previously, we have found that L-glutamic acid-based bolaamphiphile (L-HDGA) can self-assemble into helical nanotubes, and the coassembly of L-HDGA with melamine can form diverse nanotubes with varied length, diameter, and wall thickness.50 In this paper, we show the racemic hydrogels formed by HDGA racemate/melamine coassembly with unexpected properties. In particular, the hydrogelation properties of pure enantiomer, racemate, and racemate/melamine mixtures are totally different. Thus, pure enantiomer (L- or DHDGA) itself can self-assemble into left- and right-handed helical nanotubes and form hydrogels. However, the mixtures Received: July 16, 2014 Revised: August 19, 2014 Published: August 19, 2014 10772

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of equimolar L- and D-HDGA (racemate) only form precipitates with flat ribbon nanostructures. When melamine was added into those precipitates, nice racemic hydrogels containing longer nanoribbons formed. And the gelation properties, nanoscale chirality, and nanostructures of multicomponent assemblies, which show diverse forms, can be regulated flexibly by changing molar ratios of the three molecular building blocks (Scheme 1).

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EXPERIMENTAL SECTION

Materials. The bolaamphiphile (HDGA) containing L- or Dglutamic acid and alkyl spacer was synthesized by following the method previously reported.50 Melamine was purchased from Alfa Aesar and used as received. Milli-Q water (18.2 MΩ·cm) was used in all cases. Characterization. Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 FE-SEM with an accelerating voltage of 10 kV. Before SEM measurements, the samples on silicon wafers were coated with a thin layer of Pt to increase the contrast. The AFM images, without any image processing except flattening, were recorded by using the tapping mode on a Digital Instrument Nanoscope IIIa Multimode system (Santa Barbara, CA) with a silicon cantilever. FTIR spectra were recorded on a JASCO FT/IR-660 plus spectrophotometer with the resolution of 4 cm−1 at room temperature. Samples were first vacuum-dried and made into plates with KBr for FT-IR spectral measurements. X-ray diffraction (XRD) analysis was performed on a Rigaku D/Max-2500 X-ray diffractometer (Japan) with Cu Kα radiation (λ = 1.5406 Å), which was operated at a voltage of 40 kV and a current of 200 mA. Samples were cast on glass substrates and vacuum-dried for XRD measurements. Rheological studies were achieved on a Discovery DHR-1 rheometer (TA Instruments). The rheology experiments were performed at 20 °C using parallel plate geometry in a Peltier plate (40 mm diameter aluminum plates). Self-Assembly Experiments. For the typical coassembly of the racemate with melamine, 0.5 mg of L-HDGA, 0.5 mg of D-HDGA, and a certain amount of melamine were added into 1 mL of Milli-Q water, and the corresponding mixtures were heated up to form a transparent solution. When the solution was slowly cooled down to room temperature, white precipitates or gels formed, which could be different supramolecular assemblies depending on molar ratios between the racemate and melamine.

Scheme 1. (A) Molecular Structure of the Glutamic AcidBased Bolaamphiphile (HDGA); (B) Schematic Presentation of the Racemic Hydrogels Formed by the Coassembly of HDGA Racemate and Melamine;a (C) Gelation Properties of the (L + D)-HDGA Mixtures and the (L + D)-HDGA/Melamine Mixtures with Different ee Valuesb

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RESULTS AND DISCUSSION Coassembly of HDGA Racemate with Melamine. The bolaamphiphiles (HDGA) have two chiral glutamic acid head groups connected by alkyl chain via amide bonds. For the selfassembly of HDGA, except for the hydrophobic interactions from the packing of alkyl chains, the hydrogen bonding interactions can be from the amide groups as well as the carboxylic acid of the glutamic acid head groups. These noncovalent interactions render the chiral HDGA self-assemble into single-walled helical supramolecular nanotubes and form transparent hydrogels with the critical gelation concentration (CGC) coming to about 0.3% w/v. The supramolecular chirality of helical nanotubes is determined by molecular chirality of the bolas. L-HDGA molecules assemble into lefthanded single-walled helical supramolecular nanotubes in water, while the self-assembly of D-enantiomer in water can form right-handed nanotubes. On the other hand, the selfassembly of the racemic mixtures of HDGA (two enantiomers with 1:1 molar ratio) in water cannot form gels. Instead, only precipitates can be obtained. The SEM measurements show that the precipitates are flat ribbons (Figure 1a). No helical nanostructures with nanoscale chirality can be observed from the assembly of the racemic bolas. For making multicomponent racemic hydrogels, melamine was introduced into the systems to form coassemblies with HDGA racemate. Previously, we have found that the coassembly of L-HDGA and melamine with different molar ratios could produce diverse supramolecular nanotubes with fine-tuned length, diameter, and wall thickness.50 Although it is achiral, melamine has been found to form strong multiple hydrogen bonding and π−π stacking within different supramolecular assemblies. In the case of HDGA racemate, the

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L-HDGA assembles into hydrogels with left-handed single-walled nanotubes (ref 50 and references therein). HDGA racemates coassemble into short ribbons and form precipitates. Melamine molecules connect the short ribbons to form longer nanoribbons, which intertwine with each other to gel the water. bL/D represents (L + D)-HDGA mixtures and L/D/M represents (L + D)-HDGA/ melamine mixtures. The molar ratio of (L + D)-HDGA/melamine equals to 1/6. Herein, the enantiomeric excess was used to evaluate the chirality effect. We define it ee = (ΦL − ΦD)/(ΦL + ΦD), in which ΦL and ΦD are the relative molar concentrations of L- and D-HDGA, respectively. G: stable gel; PG: partial gel; P: precipitate.

Remarkably, the multicomponent racemic hydrogels have many performance advantages comparing with the chiral pure enantiomer hydrogels. First of all, the multicomponent racemic hydrogelators even have lower critical gelation concentration (CGC) value than that of the pure enantiomer gelators. Second, at the same mass concentration, the storage modulus (G′) of the multicomponent racemic hydrogels can be 10 times higher than that of the chiral pure enantiomer hydrogels. Third, the multicomponent racemic hydrogels show good tunable properties in terms of nanostructures and nanoscale chirality compared to pure enantiomer hydrogels. 10773

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Figure 1. SEM images of the racemic assemblies containing HDGA racemate and different amounts of melamine. The concentration of HDGA racemate is 1 mg mL−1. The molar ratios of racemate/ melamine are 1/0 (a), 1/2 (b), 1/3 (c), 1/4 (d), 1/6 (e), and 1/12 (f).

coassembly with melamine is expected to form robust and elongated nanostructures, which may gel water. Indeed, when the HDGA racemate were mixed with melamine in different proportions, the precipitates transformed into hydrogels gradually. Most interestingly, the self-assembled nanostructures changed from short flat ribbons to microspheres and again to ultralong ribbons along with the increasing of melamine within the multicomponent systems (Figure 1). For the coassembly of racemate and melamine with molar ratio equal to 1/2, microspheres with diameter about 1−3 μm can be observed from the SEM image (Figure 1b), and the enlarged SEM image show that these microspheres are formed hierarchically by the aggregation of small nanosheets. When the molar ratio of melamine/racemate was further increased, wider ribbons would form together with the microspheres (Figure 1c). For racemate/melamine = 1/4, only long nanoribbons can be observed from the SEM measurement (Figure 1d), and the hydrogelation can be achieved by intertwining of these nanoribbons (Figure 1d, inset). The racemate/melamine = 1/6 assemblies could form much stronger hydrogels with CGC value coming to 0.24% w/v, and the SEM image of racemate/melamine = 1/6 hydrogels also shows much longer nanoribbons (Figure 1e). These results suggest that large amounts of melamine in the system could increase the length of assembled ribbons and form nice hydrogels. On the other hand, when the amount of melamine was further increased to racemate/melamine = 1/12, some irregular ribbons can be obtained (Figure 1f), indicating too much melamine may also destroy the multicomponent hydrogels. Morphologies and Nanoscale Chirality. The racemic hydrogels formed by the coassembly of HDGA racemate and melamine have great dynamic and tunable properties compared with the pure enantiomer hydrogels. Most importantly, for producing helical nanostructures, such multicomponent assemblies can form more uniform morphologies, while the morphologies of (L + D)-HDGA assemblies without melamine are always not homogeneous (Figure 2). For these studies, unequal amounts of L- and D-HDGA molecules were mixed to form different assemblies, and the corresponding morphologies were studied by SEM and AFM measurements. In the case of multicomponent assemblies, although the morphologies of racemate/melamine = 1/6 hydrogels have been detected as ultralong flat ribbons, the gelation properties, nanostructures, and nanoscale chirality of the assemblies can be tuned thoroughly from changing the molar ratio of L-HDGA/ D-HDGA, as shown in Scheme 1c and Figure 2. Interestingly,

Figure 2. SEM and AFM images of the (L + D)-HDGA assemblies and multicomponent (L + D)-HDGA/melamine assemblies with different HDGA ee values; (a, h) ee = 100%; (b, i) ee = 50%; (c, j) ee = 25%; (d, k) ee = 0%; (e, l) ee = −25%; (f, m) ee = −50%; (g, n) ee = −100%. The molar ratio of (L + D)-HDGA/melamine is fixed at 1/6, and the total mass concentration of (L + D)-HDGA is 2 mg mL−1.

hydrogels would be obtained when melamine was added into the precipitates formed by (L + D)-HDGA mixtures, while precipitates would be obtained when melamine was added into the hydrogels formed by (L + D)-HDGA mixtures with ee values greater than 75% or less than −75% (Scheme 1c). For the ee value equal to 100%, the coassembly of melamine with LHDGA would produce multiwalled nanotubes (Figure 2a). Once small amount of D-HDGA was introduced into the system to form multicomponent assemblies (ee = 50%), the supramolecular nanotubes changed into uniform left-handed nanotwists (Figure 2b). When more and more D-HDGA was 10774

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Figure 3. X-ray diffraction patterns (A) and FT-IR spectra (B) of the L-HDGA assemblies, racemate assemblies, and multicomponent assemblies containing HDGA racemate and different amounts of melamine. (a) L-HDGA; (b) HDGA racemate; (c) racemate/melamine = 1/2; (d) racemate/ melamine = 1/4; (e) racemate/melamine = 1/6; (f) racemate/melamine = 1/12; (g) melamine.

nanostructures could also exist within the multicomponent racemic assemblies, and melamine may form ordered assemblies between different HDGA MLM layers.51,52 To get further insight into the molecular packing and noncovalent interactions within the multicomponent racemic hydrogels, the FT-IR spectra of these assemblies were measured, as shown in Figure 3B. For the chiral L-HDGA assemblies, the asymmetric and symmetric CH2 stretching vibrations at 2920 and 2851 cm−1 suggest the all-trans conformation and ordered packing of alkyl chains.53,54 The stretching vibration of the N−H at around 3333 cm−1 suggests the amide hydrogen bonding, which can also be confirmed by amide I and amide II bands, appearing at around 1643 and 1543 cm−1, respectively. The CO stretching vibrations at 1733 and 1678 cm−1 indicate the multiple hydrogen bonds between carboxylic acid groups. Different from the FT-IR spectra of chiral L-HDGA assemblies, the FT-IR spectra of the racemic HDGA assemblies show the asymmetric and symmetric CH2 stretching vibrations, which appear at 2918 and 2850 cm−1, respectively, suggesting more closely packed alkyl chains. In contrast, the amide I peak and the CO stretching vibration peak from the carboxylic acid shift to higher wavenumbers, indicating the multiple hydrogen bonding interactions between the bolas with opposite molecular chirality are relatively weak. Therefore, comparing with the case of chiral enantiomer assemblies, the hydrophobic interactions between the alkyl chains have been enhanced within the racemic HDGA assemblies. While for the aggregation of hydrophilic head groups with opposite molecular chirality, the hydrogen bonding interactions could be reduced. For the FT-IR spectra of the multicomponent racemic hydrogels, such as racemate/melamine = 1/6, the CO stretching vibration of free carboxylic acid disappears completely, indicating that all the carboxylic acid groups of HDGA molecules have already formed a complex with melamine upon gelation.50,55−57 In addition, the protonation of the triazine ring nitrogen can be proved from the changes in the CN vibration of melamine, suggesting interaction between melamine and HDGA. On the other hand, the relatively small changes on the peaks of CH2 stretching vibration and amide I suggest that melamine does not destroy the monolayer lipid membranes (MLM) formed by the assembly of HDGA. Coassembly Mechanism. Based on these results and our previous works, the possible mechanism for forming multicomponent racemic hydrogels can be proposed, as shown in

added into the systems to form multicomponent racemic assemblies (ee = 0%), the left-handed nanotwists gradually changed into flat ribbons (Figure 2d). On the other hand, when the amount of D-HDGA in the system was further increased to outpace the amount of L-HDGA, the nanotwists with righthandedness can be observed from SEM measurements (Figure 2e,f). Thus, for the multicomponent assemblies containing bola enantiomers and achiral melamine, the tuning of the supramolecular chirality should also follow the “majority rule” principle. In addition, the pure D-HDGA (ee = −100%) could also form supramolecular nanotubes from the coassembly with melamine (Figure 2g). Particularly, for the multicomponent assemblies, the gelation properties and nanostructures show successive change depending on the ee values of the mixture, and the SEM images show that the morphologies of these multicomponent assemblies are always uniform under different ee values. In contrast, for the assemblies containing just L- and D-HDGA but without melamine, only when the ee values are equal to 100%, 0%, and −100%, uniform morphologies can be achieved (Figure 2h,k,n). However, whenever there are unequal amount of L- and DHDGA in the systems, the SEM images of the assemblies always show the mixture of different nanostructures. For example, in the case of mixing HDGA enantiomers with the ee value equal to 25%, both left-handed helical ribbons and flat ribbons can be observed from the SEM measurements (Figure 2j). In this context, due to the strong multiple hydrogen bonds and π−π interactions from melamine, the multicomponent assemblies containing (L + D)-HDGA and melamine can have more uniform nanostructures. XRD and FT-IR. For understanding the molecular packing and nanostructures of the multicomponent racemic hydrogels, the X-ray diffraction analyses were carried out for different assemblies (Figure 3A). In general, the self-assembly of chiral bolaamphiphiles would form monolayer lipid membranes (MLM).50 The XRD pattern shows that the layer distance of the monolayer lipid membranes (MLM) of HDGA is at about 2.38 nm, which is close to the length of HDGA molecules. Interestingly, the XRD pattern of racemic HDGA assemblies also shows organized layer structures with the layer distance of about 2.44 nm. Thus, the basic patterns of monolayer lipid membranes (MLM) from HDGA assemblies could keep constant from the assembly of equivalent amount of enantiomers. Moreover, the XRD patterns of the multicomponent racemic hydrogels also show nice layered structures with larger layer distance. These results indicate that the MLM 10775

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Scheme 1b. The homochiral HDGA enantiomer self-assembles into monolayer lipid membranes (MLM) and form singlewalled helical supramolecular nanotubes in water through multiple hydrogen bonds and hydrophobic interactions. While the HDGA racemate also forms MLM layers with achiral molecular packing, which hierarchically assemble into short flat ribbons and cannot form hydrogels. For the assembly of HDGA racemate, the hydrogen bonding from the head groups become weak due to the opposite molecular chirality, but hydrophobic interactions from the alkyl spacers become strong, which support the monolayer lipid membranes (MLM) nanostructures from racemic assemblies. In the case of multicomponent racemic hydrogels containing HDGA racemate and melamine, the melamine molecules can interact with the carboxylic acid groups of HDGA and connect different HDGA MLM layers. Moreover, these melamine molecules can form ordered assemblies between different HDGA MLM layers. The hydrogen bonding between melamine and HDGA, and the hydrogen bonding as well as the π−π interaction among different melamine molecules, can largely enhance and elongate the assemblies and form multicomponent racemic hydrogels. Dual pH Response. For multicomponent racemic hydrogels containing HDGA racemate and melamine, the complex noncovalent interactions between HDGA and melamine not only can produce the uniform nanostructures but also can offer interesting stimuli-responsive ability to the multicomponent racemic hydrogels. Thus, the gel−sol transformation can be triggered by both acid and base (Figure 4a). When the pH value of the systems was changed to 3 or 10, the hydrogels would become precipitates or solution. In the case of high pH value, this process is reversible. However, although L-HDGA hydrogels also can transform into solution at higher pH value, this single-component chiral hydrogels show no good response to acid. Mechanical Enhancement. Most importantly, comparing with the single-component hydrogels formed by the selfassembly of pure enantiomer in water, the multicomponent racemic hydrogels have been found to be “stronger” soft materials, which could be very useful for further applications. Figure 4 shows oscillatory rheology data that reports on the mechanical rigidity of L-HDGA gels (L-HDGA = 4.8 mg mL−1) as well as the multicomponent racemic gels formed by mixing HDGA racemate with melamine (L-HDGA + D-HDGA + melamine = 4.8 mg mL−1). In a typical frequency sweep experiment, the variation of storage modulus (G′) and loss modulus (G″) was monitored as a function of applied angular frequency under a constant strain 0.1%. Figure 4b shows the linear viscoelastic frequency sweep responses of these hydrogels. Both the storage modulus (G′) and the loss modulus (G″) exhibited a slight frequency dependence in the frequency range of 0.1−100 rad s−1 with G′ an order of magnitude larger than G″, indicating that the hydrogels are typical elastic rather than viscous soft matter.58−60 In addition, time sweep experiments were performed that monitor the variation of dynamic moduli (G′ and G″) versus time after gelation at 20 °C, as shown in Figure 4c. The modulus values keep constant within a long period of time, indicating the stability of the hydrogel networks. Surprisingly, comparing with pure enantiomer hydrogels, the multicomponent racemic hydrogels showed much better mechanical rigidity. In this case, the storage modulus (G′) of the multicomponent hydrogels (20 kPa) can be 10-fold greater than that of hydrogels prepared from pure enantiomer (2 kPa).

Figure 4. (a) Reversible gel−sol transitions of the multicomponent racemic hydrogels triggered by both acid and base. Storage modulus G′ and loss modulus G″ for L-HDGA hydrogels and multicomponent racemic hydrogels (4.8 mg mL−1) at 20 °C as a function of oscillation frequency (b) and time (c) with a frequency of 1 Hz. A constant strain of 0.1% is applied.

Since the total mass concentration of the multicomponent racemic hydrogels is equal to that of pure enantiomer hydrogels, the mechanical rigidity is really outstanding. Considering the HDGA racemate even cannot form hydrogels, the unexpected mechanical properties of the multicomponent hydrogels should be dependent on the hydrogen bonding between melamine and HDGA racemate as well as the π−π interactions among melamine molecules. For the racemic multicomponent hydrogels, melamine can work as “glue”, which can connect different assemblies to form robust nanostructures.

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CONCLUSIONS Multicomponent racemic supramolecular hydrogels formed from coassembly of the glutamic acid-based bolaamphiphile racemate and achiral melamine were investigated. These multicomponent hydrogels show very nice tunable properties 10776

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and enhanced mechanical rigidity. In this case, hydrogels with diverse supramolecular chiral nanostructures and mechanical properties can be prepared, and the applications of these new soft matters in the field such as molecular recognition and separation can also be expected. On the other hand, nature has produced many sophisticated soft matters by using complex noncovalent interactions. For making artificial functional soft matters, our results demonstrated that properly designed network of noncovalent interactions from different molecular building blocks could build the assemblies with very nice mechanical properties. Moreover, for tuning the nanoscale chirality by mixing enantiomers, an additional achiral molecular building block with hydrogen bonding and π−π interactions could be very helpful for the presentation of chiral information.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (M.L.). *E-mail [email protected] (T.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Research Development Program (2013CB834504), the National Natural Science Foundation of China (Nos. 91027042, 21321063, and 21227802), “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA09020102, XDB12020200), and the Fund of the Chinese Academy of Sciences.

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