Robust, Self-Healing, and Multistimuli-Responsive Supergelator for

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Robust, Self-Healing, and Multistimuli-Responsive Supergelator for the Visual Recognition and Separation of Short-Chain Cycloalkanes and Alkanes Tao Wang, Xudong Yu,* Yajuan Li, Jujie Ren, and Xiaoli Zhen College of Science, and Hebei Research Center of Pharmaceutical and Chemical Engineering, Hebei University of Science and Technology, Yuhua Road 70, Shijiazhuang 050080, P. R. China S Supporting Information *

ABSTRACT: In this study, we show that a novel kind of cholesterol-based gelator NPS containing pyridyl and naphthalimide units can visually discriminate cyclohexane/cyclopentane from hexane/pentane on the basis of distinct optical differences in the gel platform, which is not observed in solution. The effect of congeneric solvents on the gel properties, such as morphology, rheology, and stimuliresponsive properties, is also studied. Intriguingly, NPS can form self-supporting, self-healing, fluorescent, and highly visible transmittance gels in cyclohexane that can selectively and visually respond to picric acid. It is deduced that NPS adopted H-type aggregation mode in cyclohexane, and the gel exhibits a strong green emission, whereas, in hexane, J-type aggregates of NPS molecules are observed with yellow emission. Correlations between the gelation properties and Hansen solubility parameters indicate that the dispersion interactions are the main factor for the selective gelation of NPS toward shortchain alkanes. A comparison of Hansen solvent parameters indicated that a similar energetic weight of the hydrogen-bonding units is the major contribution for the strong and specific interaction between NPS and cyclohexane. Furthermore, we demonstrated that the NPS xerogel can selectively solidify cyclohexane in the single-phase liquid of solvent mixtures, exhibiting fast gelation, high separation efficiency (>92%), and easy recycling of gelator and liquids. To the best of our knowledge, herein, we report the first paradigm that molecular gel formation is developed to visually discriminate and separate organic analogues of solvents with similar polarity. KEYWORDS: molecular gel, self-healing, isomer recognition, fluorescence, stimuli-responsive material enantiomers through selective gel formations.37 Liu and coworkers studied the discrimination of isomeric naphthoic acids by selective gel shrinkage using a urea-based two-component gel;38 Tu demonstrated that the selective gel collapse can be developed to differentiate positional isomers.39 However, in these works, the analysts generally behave as additional elements to be added into the gel systems, wherein only organogelators and solvents are necessary. Moreover, very few works focused on the analogue separation and uptake from their mixtures on the basis of the molecular discrimination or recognition. The fast molecular separation with high selectivity and efficiency is of significant importance in the area of pollutant removal, material sensing, organic catalysis, and functional photoelectric materials.40,41 In particular, the discrimination and separation of solvent analogues with similar polarity and structure (e.g., propanol and isopropanol; hexane and cyclohexane; and CHCl3 and CHCl2) is an intriguing and

1. INTRODUCTION Supramolecular formation of low molecular mass organic gelators (LMOGs) with stimuli-responsive properties has attracted significant attention because of its potential application in biomaterials, tissue engineering, intelligent soft materials, and controlled release.1−15 Recently, designing robust gel with self-healing and self-supporting properties in noncovalent systems has attracted particular interest because of its merits of bioimitability, adaptability, and reversibility.16−21 To date, many kinds of self-healing LMOGs have been reported; however, most of them show poor robustness, exhibiting limited harvesting functions, such as sensors, pollutant removal, and photonics.22−27 The design of robust gels and the exploration of novel functions from them continue to be an interesting and challenging task. Visualized molecular recognition or discrimination in soft materials, especially LMOGs, is regarded as one of the fascinating tasks in supramolecular chemistry.28−36 Recently, gel formation, shrinkage, or collapse utilized for the discrimination of structural analogues on a macroscopic scale has begun to attract a level of attention. For example, Pandey et al. realized the visual-size recognition of N-tosylated amino acid © XXXX American Chemical Society

Received: November 28, 2016 Accepted: March 29, 2017 Published: March 29, 2017 A

DOI: 10.1021/acsami.6b15249 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Chemical Structure and Assembly Mode of NPS and the Gels of NPS (3 wt %) in Cyclohexane and Hexanea

a

Note: 3.5 and 2.4 nm were obtained from X-ray diffraction (XRD) spectra.

1). Further study shows that NPS adopts the H-type aggregation mode in cyclohexane, whereas J-type aggregates of NPS molecules are found in hexane. Amazingly, organogelator NPS can form fluorescent, robust, self-healing, and highly transparent organogels in cyclohexane. The solvent effect on gel morphology, rheology, and the stimuli-responsive properties is also studied. To obtain a clear insight into these different gelation behaviors of NPS in short-chain alkanes, Hansen solvent parameters are also examined, and it is found that the dispersion interactions and similar energetic weight of the hydrogen-bonding components are the major factors contributing to the specific and strong interaction between NPS and cyclohexane. Amazingly, NPS xerogel as prepared from cyclohexane can selectively separate cyclohexane from short-chain alkane mixtures within minutes just at room temperature, which endows the fast, selective, repeatable, and efficient separation of cyclohexane from single-phase liquid of solvent mixtures.

challenging task, which is somewhat neglected in the literature.42 Cholesterol-based gels as one of the most studied gels have found extensive applications in chiral recognition, oil/water separation, and stimuli-responsive materials.43 For example, Fang et al. used cholesterol-based organogelator CDDE as template to prepare low-density materials, which can absorb organic liquids from oil/water mixtures.44 In previous works, we have also reported many kinds of functional steroid-based organogels that can behave as visual sensors toward analysts, such as humidity, amines, and ions.45−48 However, there are very few reports on their application in visual organic solvent discrimination that can further separate a specific solvent from a mixture, although organic solvents play important roles in the fields of chemistry, industry, and environment. To date, the visual and selective discrimination of organic solvents with similar polarity remains a rapidly growing area of interest. Because the solvent polarity has a significant effect on the solvatochromic phenomenon, selecting sovatochromic sensors toward special solvents is still a challenge. Herein, we report the design and synthesis of cholesterol-based organogelator NPS containing a naphthalimide unit as the electron acceptor and a pyridine unit as the electron donor (with lone pairs of electrons of N for donation when intermolecular interaction occurred rather than intramolecular interaction).45 NPS can selectively gel in short-chain alkanes among test organic solvents. Importantly, we demonstrate that n-hexane/n-pentane and cyclohexane/cyclopentane can be visually discriminated by obvious optical difference via gel-formation approach (Scheme

2. EXPERIMENTAL SECTION 2.1. Materials. All used chemical materials were obtained from commercial sources without further purification. Cholesteryl chloroformate (99%) was purchased from Sigma-Aldrich. Methyl L-lysinate dihydrochloride, aniline, phenol, 2-acetylpyridine, 4-bromoaniline, HOBt (N-hydroxybenzotrizole, 98%), EDC·HCl (1-ethyl-3-(3-dimethyllaminopropyl) carbodiimide hydrochloride 98%), diethylenetriamine, p-nitrophenol, 2,4-dinitrophenol, picric acid, and high-performance liquid chromatography (HPLC)-grade solvents for gelation tests were bought from Shanghai Darui Fine Chemical Co. Ltd. B


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Figure 1. (a) Transmittance spectra of NPS gel in cyclohexane and hexane. (b) Normalized fluorescence spectra of NPS solutions (10−5 M) and gels in cyclohexane and hexane (3 wt %), λex = 385 nm. (c) Normalized absorption spectra of NPS solution (10−5 M) and gels in cyclohexane and hexane. interaction parameter of the solvent, and δd, δp, and δh are the parameters of NPS. 2.4. Gelation Conditions. The solid for the gelation test was obtained from column chromatography. The NPS solid can form stable and transparent gels in both cyclohexane and cyclopentane at room temperature, by ultrasound treatment, or by heating−cooling process. The gel in hexane or pentane was formed by dissolving the NPS solid with sonication at room temperature (50 nm) compared to those of NPS in diluted solution, certifying the π−π stacking interaction of the fluorophores. The gels also exhibited different circular dichroism spectra (Figure S5). The positive peaks of the NPS gel in cyclohexane showed a blue shift compared to those of the NPS gel in hexane, indicating the different steric effect of solvents on the NPS chiral assembly. Further study showed that the NPS assembly can also be

developed to discriminate cyclopentane from pentane by using a similar strategy (Figure S6). Such a difference of NPS between cycloalkanes and alkanes inspired us to examine the assembly effect of NPS in congeneric solvents. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the NPS xerogel in cyclohexane showed a flower structure at the macro level, which was composed of cross-linked nanospheres (Figure S7). By contrast, NPS aggregated into a multilayer sheet structure in hexane. The above results presented that the congeneric solvents influenced both the optical properties of gels and the aggregation of NPS molecules in the macro level (Figure S8). XRD patterns were also investigated to observe the different aggregation structures of the two gels (Figure 2). The XRD pattern of the NPS xerogel from hexane exhibited a peak at 4.2 nm, which was nearly double the length of a single molecule, indicating a dimer structure. The NPS xerogel from cyclohexane showed a peak at 3.5 nm with a shorter d value, suggesting the denser structure of the NPS molecules with dimers. The NPS xerogel from hexane showed peaks at 4.2, 2.1, and 1.4 nm, which was just with the value ratio of 1:1/2:1/3, indicating the lamellar order structure of molecules. The patterns of the NSP xerogel from cyclohexane gave the same result. IR spectra of the two xerogels and the power were also studied. As shown in Figure S9, they showed highly similar spectra. The stretching band at 3359 cm−1 of these samples was attributed to the NH vibrations, and the vibration of CO showed peaks at 1547, 1582, and 1655 cm−1, indicating the intermolecular hydrogen-bonding interactions. Generally speaking, the gel formation in a particular solvent can be considered to be a balance between dissolution and selfassembly of a gelator. Therefore, Hansen solvent parameters (polar interaction parameter, δp; dispersion interaction parameter, δd; hydrogen interaction parameter, δh) were calculated and analyzed to further understand the correlation between the gelation and organic solvents. As seen from Figure 3, and Tables S3 and S4, the parameters for NPS were calculated: δd = 17.9 MPa1/2, δp = 0.06 MPa1/2, and δh = 0.14 MPa1/2. The gelation solvents were dominated by short-chain alkanes in a region of high δd but lower δh and δp. This meant that the gelation ability highly relied on the establishment of the dispersion force. Benzene and other solvents with lower polarity behaved as a good solvent for NPS, which were in the region of lower δp. With the increased δh and δp of organic solvents, NPS showed insoluble properties in solvents such as D


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were 0.2 and 1.8 MPa1/2, respectively. On the contrary, δh values of both hexane and pentane were zero. Such contrasts suggested that a weaker hydrogen-bonding force might be the major factor contributing to the specific and strong interaction between NPS and cyclohexane. Both of the gels in hexane and cyclohexane transformed to a sol-like state triggered by strong shaking, which reformed to gel state after staying for minutes. Intriguingly, the NPS gel in cyclohexane can display such a self-healing property even at concentrations as low as 0.7 wt %. Rheological experiments were carried out to characterize the gels and thixotropic properties. In Figure 4a,b, the storage moduli of the gels (1.5 wt %) were much higher than the loss moduli, supporting the characteristics of true gels. From the dynamic strain sweep measurements, the linear viscoelastic region of the NPS gel in cyclohexane showed a wider range compared to that of the gel in hexane, and the flowing (gel-to-sol transition) point was as high as 69.5% in the strain. The recovery experiments of the gels by imposing an alternating strain were performed as shown in Figure 4c,d. When a constant strain of 100% from 200 to 400 s was imposed, the gel became a sol (G′ < G″). The gel reformed after about 3 min when the strain decreased to 0.1%, and the recovery value was almost 100%. The NPS gel in hexane displayed similar but weaker recovery properties after cycles of the tests. Interestingly, the gel in cyclohexane (1.5 wt %) was sufficiently strong that it could be molded into self-sustaining and self-supporting geometrical shapes, including column, ellipse, and circle (Figure 5a−e). Also, the gel could be developed to support coins and slides. The self-healing

Figure 3. Solubility data for NPS (3 wt %) presented in Hansen space. Green: gel; blue: solution; red: precipitate or insoluble; pink: coordinate of NPS. Unit of δp, δd, and δh: MPa1/2.

acetone and short-chain alcohols. Moreover, the distinct optical properties of DTA gel in cyclohexane and hexane promoted us to examine the value difference of Hansen parameters of shortchain alkanes. The radii of interaction sphere of NPS with cyclohexane and cyclopentane were 2.36 and 3.57 MPa1/2, respectively, which are smaller than those of NPS with hexane and pentane. The value of δh for cyclohexane and cyclopentane

Figure 4. (a) Dynamic strain sweep measurement of NPS gels (1.5 wt %) with angular frequency at 10 rad/s. (b) Frequency dependency of G′ and G″ for gels with strain at 0.1%. (c, d) Recovery test for NPS gels in cyclohexane and hexane, with alternating strain amplitudes of 100 and 0.1%. E


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Figure 5. a−c) Self-sustaining films of NPS gels (1.5 wt %). (d−f) Sustaining gel films of NPS gels (4 wt %). (f−j) Healing experiment of the gel (4 wt %): (f) gel block; (g) gel after being cut in half; (h) contacted pieces; (i, j) healing gel. (k−m) Gels carrying weights of 20, 50, and 100 g, respectively.

Figure 6. Specific and selective gelation and separation process of cyclohexane using NPS xerogel in mixed solvents. (a) In mixed solvents of cyclohexane and hexane (400 μL, v/v = 1:1) and (b) in mixed solvents of short-chain alkanes, including pentane (150 μL), hexane (150 μL), heptane (150 μL), octane (150 μL), and cyclohexane (200 μL).

properties of the gels at the macro level are identified as their ability to repair the damage spontaneously just like biological tissues. When the gel concentration increased to 4 wt %, it showed excellent self-healing and self-supporting properties. As

seen in Figure 5f−g, when the cut pieces of gel blocks contacted with each other, they join together instantly and merge into a continuous block within 15 min. The gel could support weights ranging from 0 to 100 g without losing its F


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Figure 7. Proposed gelation processes and aggregation difference of NPS in cyclohexane and hexane, and the illustration of NPS xerogel for cyclohexane uptake from solvent mixtures.

and hexane; (2) The similar energetic weight of the hydrogen bonding units (δh) is the major factor contributing to the strong, reversible, and specific interaction between NPS and cyclohexane. Finally, on the basis of the solvent recognition properties of the NPS gel, we demonstrated that the solvent that selectively formed gels just at room temperature could be employed to uptake cyclohexane from complex solvent mixtures with high efficiency and reusability. The room-temperature gelation and the remarkable selfhealing properties prompted us to further investigate the responsive properties of the NPS gel in cyclohexane toward chemical stimuli. The robust gel was proved to be a highperformance sensor toward gases of HCl and NH3. When the gel was exposed to HCl gas, the fluorescence quenched completely within hours, together with gel-to-sol changes, which suggested the fast acid−base interaction of the pyridyl unit and H+ proton. Subsequently, the gel reformed upon exposure to NH3 vapor at room temperature without heat stimuli (Figures 8a and S11). Such cycles of breaking and reformation of the gel controlled by HCl and NH3 vapors could be repeated for at least six times, indicating the high reversibility and the strong noncovalent interaction of the NPS assembly in cyclohexane. SEM and 1H NMR measurements were also carried out to examine the phase changes. The flower structure of the NPS gel transformed to nanospheres when exposed to HCl gas (Figure S12a). Notably, the reformed gel triggered by NH3 gas has a porous sheet structure, which was not reverted to the initial state (Figure S12b). As seen from Figure S13, upon the addition of 3 equiv of HCl, the Hs of naphthalimide and pyridyl segments all became broadened, which suggested the acid−base interaction of the pyridyl segment and H+. Upon the further addition of 3 equiv of NH3, most of the Hs of the aromatic ring appeared again. As potential application for visual and selective sensing, we also test the sensing properties of NPS toward TNP (2,4,6trinitrophenol, picric acid) and its derivatives, such as phenol, 2,4-dinitrophenol, aniline, p-nitrophenol, and 4-bromoaniline, in both the solution and a self-assembly gel. The addition of 20 equiv of TNP to the NPS solution (10−5 M) resulted in 98% quenching of fluorescence intensity, whereas other tested derivatives caused the change of emission color from blue to yellow (Figures 8b and S14). The high selectivity of NPS toward TNP in solution inspired us to examine the responsive properties of NPS gel toward TNP with visual signal outputs. As expected, upon coating TNP (1 equiv) or its derivatives on the gel surface, similar fluorescence emission changes were also observed compared to those of the solution (Figures S15 and

shape (Figure 5k−m). Such self-supporting properties are reported rarely in the literature. Unlike NPS in cyclohexane, the gel in hexane could not be molded to any self-supporting blocks. Interestingly, we found that NPS was suitable for selectively gelation of cyclohexane from single-phase solvent mixtures. For example, when NPS solid (5 mg) was added into the solvent mixture of hexane (200 μL) and cylcohexane (200 μL), it took about 6 h to gelate cyclohexane, and hexane floated on the gel surface with the liquid state. Interestingly, NPS xerogel (5 mg) as prepared from cyclohexane by freeze-drying was so loose and light that can selectively and rapidly gelate cyclohexane within 5 min in the solvent mixture of cyclohexane and hexane (Figure 6a). Furthermore, cyclohexane and hexane could be easily separated by dumping, and the NPS solid could be recycled via simple distillation. Further study showed that cyclohexane could be also selectively separated within 10 min from complex mixtures, including cyclohexane, pentane, hexane, heptane, octane, and cyclohexane, by using the NPS xerogel (Figure 6b). In addition, the reusability and efficiency of the NPS xerogel for cyclohexane separation were determined in a 100 mL bottle containing 20 mL solvent mixture. It was clearly seen that the gelation efficiency for cyclohexane (mg/m0) was above 92% after several cycles (Figure S10). Although there are many reports about the oil/water separation in two-phase solvent mixtures through the gel formation approach,40 herein, we report the first paradigm that the xerogel assembly can also be developed to selectively and efficiently separate specific organic solvent just in single-phase organic solvent mixtures. From the above experiments and discussions, our findings could be summarized as follows (Figure 7): First, NPS can be spontaneously self-assembled into ordered structures, such as cross-linked nanospheres and sheet structure through noncovalent interactions, such as π−π stacking, hydrogen bonding, and hydrophobic interaction in special solvents. The strong dispersed interaction between NPS and short-chain alkanes was the main factor for the stable and selective gel formation of NPS in short-chain cycloalkanes and alkanes. Second, we presented that NPS can form robust, self-healing, transparent, and green emissive gels in cyclohexane just at room temperature, whereas only an opaque gel with yellow emission was formed in hexane by the heating−cooling process followed by sonication. The optical differences in both transparency and emission color endowed the visual discrimination of cyclohexane and hexane through the gel formation approach. Such differences were mainly attributed to two reasons: (1) NPS molecules adopted different aggregation types in cyclohexane G


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S20). The XRD pattern of the xerogel of NPS with TNP showed two peaks with d values of 5.4 and 2.7 nm (value ratio, 2:1), suggesting a lamellar and regular structure of the twocomponent aggregates (Figure S21). The above results indicated that the hydrogen-bonding interaction between the pyridyl group of NPS and OH of TNP directed the spontaneous formation of fibrous structure with a long-range ordered pattern, which restricted the rotations of molecules, resulting in a red gel. Because the sensing and detection of explosives especially TNP is a major security concern in environmental safety, our finding would be meaningful for constructing novel selective and visual sensors toward explosives.55,56

4. CONCLUSIONS In conclusion, we have presented for the first time that small cycloalkanes can be visually discriminated with alkanes by a gelation approach. More importantly, the NPS xerogel exhibited an effective (>92%) and fast (

Robust, Self-Healing, and Multistimuli-Responsive Supergelator for

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