A multi-stimuli-responsive organogel based on salicylidene Schiff base

Sensors and Actuators B 185 (2013) 389–397

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A multi-stimuli-responsive organogel based on salicylidene Schiff base Libin Zang, Hongxing Shang, Dayan Wei, Shimei Jiang ∗ State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012, PR China

a r t i c l e

i n f o

Article history: Received 18 February 2013 Received in revised form 25 April 2013 Accepted 6 May 2013 Available online 20 May 2013 Keywords: Salicylidene Schiff base Organogel Multi-responsive Enhanced fluorescence Zn2+ ion-responsive gel F− ion-responsive gel

a b s t r a c t A cholesterol-based salicylidene Schiff base derivative CDBHA (cholesterol 2-(3,5-di-tert-butyl2-hydroxybenzylideneamino)acetate) with excellent multi-stimuli-responsive properties has been designed and synthesized. CDBHA can readily self-assemble into gels with nanofiber structures in several kinds of organic solvents. Compared to their corresponding solutions, the generated gels exhibited a significantly enhanced emission. This typical aggregation-induced emission (AIE) property is a result of the cooperative formation of the J-aggregates and inhibition of the intramolecular rotation. In addition, CDBHA gels are intelligent. On one hand, gel–sol transitions together with noticeable fluorescence changes can be reversibly modulated by alternating rounds of cooling and heating. On the other hand, the gels show a highly selective dual-responsive behavior to Zn2+ through a gel–sol transition, a turnon of fluorescence and an efficient dual-responsive behavior to F− through gel–sol transition and color changes. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Low-molecular-mass organic gelators (LMOGs) have attracted great attention over the past decades for their application as templates, drug carriers, sensors, actuators, and other molecular devices [1–9]. Organogels are formed by assembly of LMOGs into entangled three-dimensional networks through weak intermolecular forces such as hydrogen bonding, ␲–␲ stacking, and van der Waals interactions. These weak intermolecular forces can be easily manipulated by external stimuli, allowing for the tuning of the physical properties of the gel. Recently, smart materials based on stimuli responsive LMOGs have become a new focus, owing to their potential applications in photo switches, sensors, molecular logic gates, and other functional materials [10–20]. To date, a great number of organogels have been reported that can respond to light [21,22], temperature [23], sound [24,25], mechanical stress [26], anions [27–29], metal ions [30,31], redox [32,33], proton [34]/pH [35] and small molecules [36]. Meanwhile, some multi-responsive LMOGs, which can achieve multiple functions in one material, have also been developed [37–39]. Though some successful multiresponsive LMOGs have been reported, it is still quite a challenge to design and synthesize new ones. As a kind of multi-stimuli-response molecule, salicylidene Schiff bases have recently received attention in particular because of their excellent photochromism, thermochromism, solvatochromism and their response to metal ions, anions as well as pH [40–42].

∗ Corresponding author. Tel.: +86 431 85168474; fax: +86 431 85193421. E-mail address: [email protected] (S. Jiang). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.022

However, only a few LMOGs based on salicylidene Schiff bases have been reported in the past few years. For example, Lu et al. reported two organogels based on cholesterol connected salicylideneaniline derivatives and focused on their gelation abilities, fluorescence, photochromism and thermochromism [43,44]. Dattaa and Bhattacharya synthesized a gallic acid derived salicylideneaniline gelator and proved the keto–enol-tautomerism in the gel by 1 H NMR [45]. Liu et al. developed metal-ion-mediated chiral twists and chiral recognition of amphiphilic Schiff-base organogels in their latest report [46]. To the best of our knowledge, organogels based on salicylidene Schiff bases with responsive properties to metal ions and anions have not been reported yet. In this paper, we have designed and synthesized an ALStype organogelator CDBHA, cholesterol 2-(3,5-di-tert-butyl-2hydroxybenzylideneamino)acetate. In general, the ALS gelators are comprised of an aromatic (A) functional group coupled with a steroidal (S) moiety through a flexible linker (L), and these type of gelators could display effective, and somewhat predictable gelation abilities [1–3]. In CDBHA, as shown in Scheme 1, component A is a salicylidene Schiff base unit, L is a glycine group, and S is a cholesterol moiety, and CDBHA is expected to be an effective gelator with multiple stimuli-responsive behaviors endowed by the salicylidene Schiff base unit. Indeed, CDBHA can readily self-assemble into gels with nanofibers structure in several kinds of organic solvents. The CDBHA gels not only exhibited enhanced fluorescence emission and thermochromism, but also displayed excellent multi-stimuliresponsive properties in the gel phase. CDBHA gels showed a significant and selective fluorescent response to Zn2+ and a colorimetric response to F− . The gelation and stimuli-responsive

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washed with Et2 O (3 × 50 mL) and the organic phase was extracted with a saturated solution of NaHCO3 (2 × 50 mL). The aqueous phases were acidified with a dilute solution of KHSO4 to pH 2–3 and extracted with AcOEt (3 × 100 mL). The solution was washed with water and dried over anhydrous Na2 SO4 . After filtration and evaporation, a white crystalline material was obtained (12.00 g, 68.5 mmol) with 98% yield. 1 H NMR (500 MHz, CDCl3 /Me4 Si): 10.911 (br s, 1H), 6.753 and 5.067 (s, 1H), 4.00–3.87 (m, 2H), 1.454 (s, 9H). Anal. Calcd. for C7 H13 NO4 : C, 47.99; H, 7.48; N, 8.00. Found: C, 48.35; H, 7.365; N, 7.80.

Scheme 1. The synthetic route to organogelator CDBHA.

behaviors of CDBHA was investigated thoroughly, and the details are presented as below. 2. Experimental 2.1. Materials All the materials for synthesis and spectra were purchased from commercial suppliers and used without further purification. All solvents and reagents used in the spectroscopic studies were analytical grade. The metal ions were Zn(OAc)2 ·2H2 O, Cu(OAc)2 , Ni(OAc)2 ·4H2 O, Co(OAc)2 ·4H2 O, FeCl3 ·6H2 O, Mn(OAc)2 ·4H2 O, Cd(OAc)2 ·2H2 O, AgNO3 , KOAc, NaOAc, CaCl2 . All anions were in the form of tetrabutylammonium (TBA) salts. 2.2. Instrumentation and methods 1 H NMR (TMS) was recorded on a Bruker UltraShield 500 MHz spectrometer and a MERCURY “300BB” 300 MHz spectrometer. Mass spectrum was measured on Thermo Scientific ITQ 1100TM GC/MSn. Elemental analyses were carried out with a vario MICRO cube elementar. Scanning electron microscopy (SEM) pictures of the xerogels were carried out on a JEOL JSM 6700F field emission scanning electron microscope, and the samples were sputtered with a layer of gold prior to imaging. The xerogels for the SEM were obtained by spontaneous evaporation of gels at room temperature. UV–vis absorption spectra were taken on a Shimadzu 3100 UV–VIS–NIR recording spectrophotometer using a 2 nm slit width. The fluorescence spectra were scanned with a Shimadzu RF-5301PC spectrofluorophotometer. The absorption spectra of thin gel film were prepared on quartz plates, and the spectra of gel phase were measured in 1 mm quartz cells. The other spectra, such as titration experiments, were observed in 1 cm quartz cells. The fluorescence quantum yield was measured by a method used before (quinine sulfate as standard) [40]. LSCM (laser scanning confocal microscope) graphs were performed with a FV1000 confocal laser-scanning fluorescent microscope (Olympus, Japan).

2.3.2. Preparation of cholesterol 2-aminoacetate hydrochloride (CAAHC) CAAHC was also synthesized by the previous method [48]. a: 1.75 g (10.0 mmol) of Boc-glycin and 3.87 g (10.0 mmol) of cholesterol were dissolved in 100 mL of dichloromethane. The solution was maintained at 0 ◦ C in an ice bath. 2.06 g (10.0 mmol) of dicyclohexylcarbodimide (DCC) and 0.20 g (1.65 mmol) of N,Ndimethylaminopyridine (DMAP) were then added, and the reaction mixture was stirred for 6 h at 0 ◦ C. After reaction, the mixture was filtered and the filtrate was washed with 0.001 mol L−1 hydrochloric acid (50 mL × 3), 0.001 mol L−1 sodium hydroxide aqueous solutions (50 mL × 3) and pure water (50 mL × 3). The organic layer was evaporated to dryness. The residue was purified by a silica gel column eluting with THF/n-hexane (1:4, v/v) to give cholesteryl Boc-glycinate (3.35 g, 6.2 mmol) in 61.6% yield as a white solid. b: 2.45 g (4.5 mmol) of cholesteryl Boc-glycinate was dissolved in 100 mL of dichloromethane and then bubbled with dry HCl gas for 1 h. The mixture was filtered, and the residue was washed with dichloromethane and dried in a vacuum to give the desired product (1.95 g, 4.1 mmol) with 90% yield as a white crystal. 1 H NMR (500 MHz, CDCl3 /Me4 Si): 5.368 (1 H, alkenyl), 4.637 (m, 1 H, oxycyclohexyl), 3.978 (2H, CH2 CO), 2.325 (d, 2H, CH2 , J = 8.0 Hz), 0.67–1.85 (42H, m, cholesteryl protons). Anal. Calcd. for C29 H50 ClNO2 : C, 72.54; H, 10.50; N, 2.92. Found: C, 71.85; H, 10.548; N, 2.76. 2.3.3. Preparation of cholesterol 2-(3,5-di-tert-butyl-2-hydroxybenzylideneamino) acetate (CDBHA) A 0.81 g (1.7 mmol) sample of CAAHC was suspended in 25 mL of ethanol. In order to dissolve CAAHC absolutely, 0.37 mL (2.7 mmol) TEA was added. Then 0.40 g (1.7 mmol) of 3,5-di-tert-butyl-2hydroxybenzaldehyde was added to the system and the mixture was refluxed for 5 h. During the reaction, the solution was clear and yellow. After the reaction, the system was cooled to room temperature and a yellow gel was formed. The gel was then filtered and washed with 15 mL of cold ethanol. The yellow product was recrystallized in ethanol. CDBHA as yellow xerogel (0.67 g, yield 60.9%) was obtained. 1 H NMR (300 MHz, CDCl3 /Me4 Si): 13.348 (s, 1H), 8.375 (s, 1H), 7.399 (d, J = 2.7 Hz, 1H), 7.096 (t, J = 2.7 Hz, 1H), 5.386 (d, J = 4.2 Hz, 1H), 4.707 (m, 1H), 4.334 (s, 2H), 2.368 (d, J = 7.5 Hz, 2H), 0.82–2.08 (m, 56H), 0.678 (s, 3H). Anal. Calcd. for C44 H69 NO3 : C, 80.07; H, 10.54; N, 2.12. Found: C, 80.24; H, 10.657; N, 2.09. MS: m/z = 658.76.

2.3. Synthesis

2.4. Gelation test

2.3.1. Preparation of Boc-Gly-OH Boc-Gly-OH was synthesized by the method which had been reported previously [47]. A solution of (Boc)2 O (22.75 g, 104.0 mmol, 50 mL of dioxane) was added dropwise to an aqueous solution of glycine (5.23 g, 69.8 mmol) and NaOH (4.17 g, 104.0 mmol, 100 mL H2 O and 50 mL dioxane) in 30 min. The solution was stirred for another 24 h at room temperature. Then the dioxane was evaporated. The remaining aqueous solution was

The gelation properties of CDBHA were tested by a previously reported method [49]. In a typical procedure, a certain amount of the samples was mixed with the corresponding organic solvent (1 mL) in a sealed test tube, and then the mixture was heated until the solid was completely dissolved. If the mixture was able to turn into a gel when cooled to room temperature spontaneously, this kind of gel is a thermo-reversible gel. If the hot solution was able to form into a gel immediately with ultrasound treatment, the gel can

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Table 1 Gelation ability of CDBHA in various solvents.a Solvent

Observations

Solvent

Observations

Ethanol Isopropanol 1-Propanol Isobutyl alcohol 1-Butanol acetonitrile 1-Pentanol 1-Hexanol 1-Octanol Chloroform Dichloromethane

T gel, S gel (7.0) T gel, S gel (11) T gel, S gel (12) T gel, S gel (17) T gel (23) S gel (6.5) S S S S S

Toluene Xylene Dimethylformamide Ethyl acetate 1,4-Dioxane tetrahydrofuran Ethyl ether n-Hexane Cyclohexane Dimethyl sulfoxide Methanol

S S S S S S S P P P I

a T gel, thermo-reversible gel; S gel, ultrasound-induced gel; S, soluble; P, precipitate; I, insoluble. Numbers in parentheses represent the minimal gelation concentration (mg/mL). The MGCs were all measured in S gel except 1-butanol.

be thought of as a ultrasound-induced gel. Gelation was considered to have occurred after a homogeneous substance was obtained that exhibited no gravitational flow, which is denoted as G (gel). In the cases when solution and solid-like gel coexisted within a system, it was denoted as PG (partial gel). While S (solution) stood for only solution, and P (precipitation) stood for precipitation. But in some cases, where the sample could not dissolve even after the boiling point of the solvent was reached, it was marked as I (insoluble). MGC (minimum gel concentration) was described as the minimum amount of gelator needed for the formation of a homogeneous gel at room temperature, and it was measured by decreasing the concentration of hot solution gradually. The gel–sol transition temperature Tg was measured with the “tube inversion” method in a water bath.

The morphologies of xerogels obtained from different solvents were investigated by SEM (Fig. 2, Fig. S3). The images demonstrated that the organogelator molecules in the gel phase were self-assembled into 1-D nanofibers with the width ranging from 30 to 100 nm, which further cross-linked into 3-D networks. Moreover, xerogel morphologies prepared by the thermo and ultrasonic approaches were similar, and the morphologies were not affected by concentrations (Fig. 3). However, the amorphous state of CDBHA, prepared by slow evaporation of dichloromethane solution, had no fiber network structures. These results suggested that the long fiber network structures were crucial for forming the gel of CDBHA.

3. Results and discussion

CDBHA in the gel phase and dilute solution were investigated by UV–vis absorption spectroscopy. Three strong peaks at 222 nm, 263 nm and 332 nm along with a quite weak peak at 430 nm could be observed in the ethanol solution (Fig. 4). As we know, there are two tautomers of salicylidene Schiff base derivatives: the enol (–OH) form and keto (–NH) form. Generally, these two tautomers are at equilibrium and the keto form usually has a longer absorption wavelength than the enol form in the UV–vis absorption spectra [51–53]. Therefore, the weak peak at 430 nm could be regarded as the keto form absorption, which formed due to the weak solvatochromism in ethanol [54,55]. In addition, these peaks in solution did not shift with concentration (Fig. S4). However, compared to the spectrum in ethanol solution, a red-shift of 4 nm (332 nm to

3.1. Gelation properties and self-assembled characteristics CDBHA was synthesized by the procedure shown in Scheme 1, and the detailed experiment is presented in the experimental section. The complete 1 H NMR and MS spectra can be seen in the Supporting Information (Fig. S1-S2). The gelation ability of CDBHA in twenty two different organic solvents was examined by the heating-and-cooling method or heating-and-sonication method. Correspondingly, thermo-reversible gels (designated as T gel) or ultrasound-induced gels (designated as S gel) could be obtained. Though CDBHA was soluble in many solvents, it could only form gels in lower alcohol solvents and acetonitrile. As shown in Table 1, CDBHA was insoluble in methanol, however it could form both T and S gels in ethanol, isopropanol, 1-propanol and isobutyl alcohol. The ultrasound-induced gels were also reversible after the heating-and-sonication process. Furthermore, the minimum gel concentration (MGC) at room temperature increased as the alkyl chain of the alcohol increased. Particularly, in 1-butanol we can only obtain a thermo-reversible gel at the test concentration other than S gel. Cooling of a thermally dissolved acetonitrile solution of CDBHA only resulted in partial gels; however, when ultrasound irradiation was introduced into the hot system, it could form a homogeneous gel in pure acetonitrile. In order to investigate the thermotropic behavior of the gels, we took the S gels of CDBHA in ethanol and acetonitrile as samples and studied the relationship between the gel–sol transition temperature (Tg ) and the concentration of CDBHA (Fig. 1). The results showed that the Tg increased as the gelator concentration increased, and the Tg in acetonitrile was always higher than it was in ethanol at the same concentration. The difference in Tg was attributed to the different solubility of CDBHA in the two solvents. Since CDBHA had lower solubility in acetonitrile than in ethanol, thus CDBHA had higher Tg in acetonitrile at the same concentration [50].

3.2. UV–vis absorption spectra and thermochromism

Fig. 1. Plots of Tg versus the concentrations of CDBHA in ethanol and acetonitrile.

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Fig. 2. SEM images of the xerogels from the S gel (25 mg/mL) in ethanol (a), acetonitrile (b), and the T gel (25 mg/mL) in ethanol (c) and the partial gel (25 mg/mL) in acetonitrile (d).

336 nm) could be detected in the gel phase (Fig. 4), which indicated the formation of J aggregates of the salicylidene Schiff base moiety in the gel phase. The J aggregates adopt a head-to-tail stacking mode, and as a result, the absorption band red-shifts and the flourescence becomes enhanced [56]. Interestingly, the absorbance in the range of 400–490 nm (keto form) became stronger in the gel phase. This implied that the percentage of keto form increased in the gel phase, in accordance with previous reports [43,45]. In other words, aggregation increased the amount of the keto form. Dattaa and Bhattacharya proved the enol–keto tautomerism associates with the sol–gel transition in a salicylideneaniline gel by temperature dependent 1 H NMR spectroscopy [45]. The result of the UV–vis absorption spectra in our work proved a similar process with a simple strategy. The CDBHA gel also exhibited a significant thermochromic property. When CDBHA gel in ethanol was cooled to 77 K, the absorption peak of the gel in the range of 400–490 nm disappeared (Fig. S5). This suggested that the keto form structure transformed to the enol

form at low temperature [57]. Meanwhile, the yellow gel became colorless (Fig. 5). And such a color change could be reversibly converted many times. This phenomenon could also be observed in the xerogels (Fig. S6), and it proved that the aggregated gel fibers themselves had thermochromic properties. Generally speaking, the “close-packed-structure” of salicylidene Schiff base facilitates the thermochromism; simultaneously, the “open-structure” is favorable to photochromism [58–60]. Therefore, the experimental result suggested that the salicylidene Schiff base part of the CDBHA in the gel phase might form a closely packed structure. 3.3. Enhanced fluorescence emission in the gel phase Nowadays, growing attention has been drawn to the design and synthesis of gels with light emitting properties, because they are potential candidates for optoelectronic devices and light harvesting materials. While CDBHA almost had no fluorescence in solution (313 K), a significant enhancement of yellow–green emission under

Fig. 3. SEM images of the xerogels from the S gel in ethanol with different concentrations: 10 mg/mL (a), 25 mg/mL (b), 40 mg/mL (c).

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3.4. Metal ions responsive properties

Fig. 4. UV–vis spectra of CDBHA in ethanol solution (100 ␮M) and gel phase (10 mg/mL).

UV irradiation was observed after gelation (298 K) and the fluorescence of the gel was further enhanced at low temperature (77 K) (as shown in Fig. 5). When the gel was converted to solution after heating, the fluorescence became weak again. In this way, the fluorescence intensity can be reversibly modulated by alternating rounds of cooling and heating. Such unique photoluminescence properties are known as aggregation-induced enhanced emission (AIEE), and the inhibition of intramolecular rotation plays an important role in this AIEE process [61]. In our case, CDBHA formed J-aggregates are cross-linked to solid-like 3D network in the gel phase, so the AIEE of CDBHA resulted from the cooperative formation of the Jaggregates and inhibition of the intramolecular rotation [44]. As shown in Fig. 6a, the maximum emission peak was at 516 nm. The confocal microscopic image of the CDBHA gel is given in Fig. 6b, which reveals a dense 3-D network of fluorescent fibers.

Salicylidene Schiff bases can provide coordination sites for forming stable metal complexes with many kinds of metal ions [62–64]. The metal ions binding properties of CDBHA were investigated both in solution and the gel state. Firstly, the absorption and fluorescence spectra titration experiments with different metal ions were measured in ethanol. The absorption spectra rarely changed after adding Cd2+ , Ag+ , K+ , Na+ and Ca2+ (Fig. S7), which implied that there were almost no interactions between these metal ions and CDBHA. In contrast, the absorption spectra displayed an intense variation upon addition of Zn2+ , Cu2+ , Ni2+ , Co2+ , Fe3+ and Mn2+ (Fig. 7, Fig. S8), and the absorption spectra could reach saturation within adding 5 equiv of these ions. This suggested that CDBHA could form strong metal complexes with Zn2+ , Cu2+ , Ni2+ , Co2+ , Fe3+ and Mn2+ . In addition, the Benesti–Hildebrand plot (Fig. 7) showed that the composition of the complex had a 1:1 ratio of CDBHA and Zn2+ , and the binding constant was calculated to be 2.35 × 104 M−1 [65]. In the fluorescence spectra (Fig. 8), the solution of CDBHA almost had no fluorescence. However, the addition of Zn2+ caused a significant blue–green emission (Ф = 0.196), while adding other metal ions did not lead to any fluorescence emission. An interesting fact was that the Cd2+ , which usually acts as an interfering substance in the detections of Zn2+ [66], did not respond to CDBHA. These results showed that CDBHA had an excellent selective fluorescent response to Zn2+ in solution. The effect of metal ions binding during the gelation process was performed by repeating the gelation experiment of CDBHA in ethanol in the presence of various metal ions. Five equiv. of various metal ions were added to a preformed CDBHA gel and the mixture was heated until a clear solution was obtained, then followed by cooling to room temperature. As shown in Fig. 9, Cd2+ , Ag+ , K+ , Na+ , and Ca2+ , which barely had any interactions with CDBHA as proved by the absorption spectra, neither disturbed the formation

Fig. 5. Photographs of the sol at 313 K, yellow gel at 298 K and colorless gel at 77 K of CDBHA (10 mg/mL, in ethanol). Right bottles were irradiated by 365 nm UV light.

Fig. 6. (a) Emission spectrum of CDBHA in gel phase (10 mg/mL, in ethanol, ex = 398 nm). (b) LSCM image of CDBHA gel.

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Fig. 7. (a) Absorption spectra of CDBHA (50 ␮M, in ethanol) in the presence of different concentrations of Zn2+ (0–5 equiv); (b) Benesi–Hilderbrand plot of CDBHA with Zn(OAc)2 .

results of absorption titration spectra in solution. Thus, the metal ions which could form strong complexes with CDBHA could inhibit the formation of gels. The reason for this is that the binding behavior of the salicylidene Schiff base moiety with metal ions destroyed the necessary weak interactions of gelator molecules in the gel, which resulted in the collapse of the gel. Particularly, among all kinds of metal ions we had tried, only Zn2+ caused a remarkable blue–green fluorescence emission. These results clearly demonstrated that CDBHA also had the selective fluorescent response to Zn2+ in gel phase. To sum up, CDBHA gel showed a highly selective dual-responsive property to Zn2+ through a gel–sol transition and a fluorescent turn-on change. 3.5. Anion responsive properties

Fig. 8. Fluorescence spectra change of CDBHA (50 ␮M, in ethanol) upon the addition of various metal ions (5 equiv of Zn2+ , Cu2+ , Ni2+ , Co2+ , Fe3+ , Mn2+ , Cd2+ , Ag+ , K+ , Na+ , Ca2+ . ex = 398 nm).

of gels nor caused any color change. However, gels could not be restored in the presence of Zn2+ , Cu2+ , Ni2+ , Co2+ , Fe3+ and Mn2+ , even when the samples were irritated with ultrasound or left over one day. In the meantime, the color of the solutions changed accordingly. These responses in the gel states were consistent with the

Salicylidene Schiff base derivatives are an excellent class of anion receptors through hydrogen bonding or deprotonation of the OH group [67–69]. However, the anion binding properties in their gels have rarely been investigated. This makes the study of the anion responsive properties of the CDBHA gel meaningful. It is interesting to note that CDBHA could form gels both in the aprotic solvent acetonitrile and protic solvent ethanol. The anion binding properties of CDBHA toward a number of selected anions (F− , Cl− , Br− , I− , AcO− , and H2 PO4 − ) were examined in acetonitrile and ethanol, respectively.

Fig. 9. Color (up) and fluorescent (bottom, under 365 nm light) changes of CDBHA gel (15 mg/mL, in ethanol) to various metal ions (5 equiv).

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Fig. 10. Absorption spectra change of CDBHA (100 ␮M, in acetonitrile): (a) in the presence of different concentrations of F− , (b) upon the addition of various anions (30 equiv).

In acetonitrile, CDBHA changed a lot upon the addition of fluoride, and the absorption spectra would reach saturation after adding 15 equiv of TBAF (tetrabutylammonium fluoride) (Fig. 10a); however, other anions only caused little change in the absorption spectra even after adding twice as many as fluoride ions (Fig. 10b). This result indicated that CDBHA had a selective response to fluoride in acetonitrile solution. There is only one active site (OH group) of CDBHA, which could respond to fluoride, and this kind of interaction between the OH and fluoride has been studied in our previous work. It must be that the OH group of CDBHA forms hydrogen bonds with fluoride, which played a key role in the response of the anion [67]. In sharp contrast, it was interesting to find that the absorption spectra of CDBHA in ethanol was retained without any change, even after adding the same amount of TBAF and other anions (Fig. S9). It seems that the presence of protic solvent disfavors the interaction between fluoride and CDBHA. It is well known that there is a strong

interaction between fluoride and protic solvents, which could weaken the interaction between fluoride and the OH group of CDBHA [70,71]. This kind of solvent effect could also be observed in the gel phase. The addition of various anions including fluoride did not disturb the formation of gels and did not produce any colour change (Fig. S10). However, as shown in Fig. 11, in acetonitrile the addition of TBAF to CDBHA gel immediately produced a gel-to-sol transition in the interphase region. The gel was totally disrupted to yield a red solution after 2 h. In addition, the same amount of AcO− only disrupted the gel partially and the addition of other anions did not lead to decomposition of the gel under the same conditions (Fig. S11). The presence of fluoride not only changed the color of the system but also disrupted the preformed gel to a solution through slow diffusion. The results indicated that the CDBHA gel is very sensitive to fluoride, with a gel–sol transition and obvious color changes that can be sensed by the naked eye.

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Fig. 11. Transition of CDBHA gel (10 mg/mL, in acetonitrile) upon addition of 15 equiv of TBAF on the top surface of the gel.

4. Conclusions A cholesterol-based ALS type CDBHA based on salicylidene Schiff base has been developed. CDBHA showed excellent gelling capability in various solvents and performed multi-responsive properties in the gel phase. Thermochromism of the gels was observed which could be explained as the enol–keto tautomerism. Although CDBHA is non-luminescent in solution, the 1D nanostructures based on nanofibrous gels emit strong yellow green fluorescence, showing typical aggregation-induced emission (AIE) characteristics, which is ascribed to a combination of inhibition of the intramolecular rotation and the formation of J-aggregates. The fluorescence intensity can be reversibly modulated by alternating cooling and heating. More importantly, CDBHA can act as an intelligent gel with gel–sol transitions as well as noticeably fluorescent changes that can be reversibly modulated by alternating rounds of cooling and heating. The CDBHA gel shows a highly selective dual-responsive property to Zn2+ through a gel–sol transition and a change in fluorescent turn-on. Interestingly, the organogel of CDBHA could allow a two channel fluoride response by sol–gel transition and color changes. These significant properties also make this new gelator have more potential applications as a multifunctional material. Acknowledgements This work was supported by the National Basic Research Program of China (2012CB933800). In addition, we thank Matthew Brooks from West Virginia University for his contribution during his stay in Jilin University supported by 111 Project (B06009) and West Virginia Graduate Student Fellowships. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.05.022. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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Biographies Libin Zang received his BSc degree in chemistry from Jilin University, Changchun, China, in 2008. He is currently working toward a PhD in polymer chemistry and physics in Jilin University. His main research interests include chemical sensors, stimuli-response supramolecular materials and molecular logic gates. Hongxing Shang received her BSc degree in chemistry from Jilin University in 2011. Currently she is a graduated student of the College of Chemistry in Jilin University. Dayan Wei received her BSc degree in chemistry from Jilin University in 2011. Currently she is a graduated student of the College of Chemistry in Jilin University. Shimei Jiang is a professor of Chemistry in Jilin University. She received her PhD degree from Jilin University in 1998. Her main research interest is on the stimuliresponse supramolecular system and nano-materials.