Cyclodextrin-Assisted Two-Component Sonogel for Visual Humidity

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Cyclodextrin assisted two-component sonogel for visual humidity sensing Xudong Yu, Xiaoting Ge, Lijun Geng, Haichuang Lan, Jujie Ren, Yajuan Li, and Tao Yi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04401 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 12, 2017

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Cyclodextrin assisted two-component sonogel for visual humidity sensing Xudong Yu,*,a Xiaoting Ge,a Lijun Geng,a Haichuang Lan,b Jujie Ren,a Yajuan Li,a Tao Yi*,b a

College of Science and Hebei Research Center of Pharmaceutical and Chemical Engineering,

Hebei University of Science and Technology, Yuhua Road 70, Shijiazhuang 050080, PR China b

Department of Chemistry and Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, 220 Handan Road, Shanghai 200433, China

ABSTRACT: In this work, two naphthalimide based compounds, 1a and 1b, have been designed and synthesized. Both compounds can form stable two-component gels in n-propanol or n-butanol upon addition of α-CD (α-cyclodextrin) followed by sonication at room temperature. Interestingly, the 1a/α-CD gel is thixotropic and very sensitive to water. Addition of small amount of water induces rapid gel collapse, allowing further development of the gel as a visual relative humidity sensor. The specificity of the sensor has been confirmed by several approaches, including scanning electronic microscopy, fluorescence, FT-IR and 1H NMR experiments. The results show that α-CD acts as a junction for assembly of 1a or 1b through hydrogen bonding between hydroxyl and amide groups. Upon addition of water, α-CD interacts with the adamantane group of 1a via incomplete host-guest encapsulation, resulting in

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dissociation of the hydrogen bonding-assisted two-component assembly, accompanied by gel collapse.

Keywords: Humidity sensor, fluorescence, naphthalimide, organogel, host-guest interaction, ultrasound

1. Introduction The design of reliable sensors for monitoring relative humidity (RH) has been of great importance in technology and daily life, e.g., in agriculture, medicine, and biotechnology.1–4 Several techniques, including capacitive, conductive, and resistive methods, have been explored to detect humidity based on nanostructure materials such as carbon nanotubes, metal–organic frameworks (MOFs), metal oxide nanoparticles, and nanowire (NW) films.5–8 Several instruments such as hygrometer have been developed for RH detection in our daily life. However, those instruments generally need complicated design and circuity analysis and not convenient to carry. Therefore, detection of RH without equipment by naked-eye is required, especially in the harsh environment. However, easy and visual detection of RH without equipment is still a challenge. Searching for novel materials for sensing RH is thus a critical importance in the development of new humidity sensors. Recently, molecular gel formation and collapsing have been proved to be efficient and simple way for constructing visual sensors toward analytes such as gas, chiral compounds, ions, amides as well as biomolecules.9-29 By using fluorescent gels, we also constructed many kinds of aggregates for sensing analytes such as amines, ions and pressure. 30-34 However, very few woks have been focused on using gels for humidity sensing. Recently, Ayyappanpillai Ajayaghosh et al reported that organic assembly could response to water stimulus with fluorescent enhancement


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and emission wavelength changes, which could be used as a self-erasable security marker.35 Moreover, the multiple component gels based on host-guest interactions were found to be good candidates for constructing sensing system toward external stimuli.36-39 In previous work, we found that water was able to trigger gel collapsing in the β-cyclodextrin (β-CD) and adamantanebased two component organogel system, due to the formation of a host-guest encapsulation complex, which did not exist in the solvent of pure DMF.40 Therefore, we envision that by proper molecular design, such systems with host-guest interaction building blocks may be developed to create humidity sensors in gel tissue. To achieve this purpose, two points should be meet: 1) the gel should be stable in nonaqueous environment, and very sensitive to water molecules with outcomes of color, emission or phase changes for visually detection purpose; 2) the cyclodextrin should not interact with adamantane segment in gel state, but be necessary for the gel formation. Herein, a naphthalimide-based compound containing adamantane and cholesterol derivatives at two ends of the aromatic part linked by amides was designed (1a in Scheme 1). Another compound 1b with the sugar chain replacing adamantane was also synthesized as the reference compound. Those two compounds were able to form twocomponent stable gels with α-CD in n-propanol or n-butanol accelerated by sonication just at room temperature, while none of them could form gel by themselves. As expected, the gel formed by α-CD and 1a bearing adamantane group had thixotropic properties and was very sensitive to water molecule, which was further developed as a probe for visual humidity sensing.


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Scheme 1 The chemical structure of 1a and 1b 2. Experimental Section Materials All starting materials were obtained from commercial supplies and used without further purification. Cholesteryl chloroformate (99%) and 1-adamantanecarbonyl chloride was obtained from Sigma-Aldrich, Beijing. 4-Bromobenzaldehyde, 2- HOBt (N-Hydroxybenzotrizole, 98%), EDC·HCl (1-ethyl-3-(3-dimethyllaminopropyl carbodiie hydrochlide, 98%), 4-bromo-1, 8naphthalic anhydride, ethylenediamine, D-(+)-gluconic acid δ-lactone, CuCl, and 6aminocaproic acid were supplied from Shanghai Darui fine chemical Co. Ltd., Shanghai. Alcohols for gel test were dried by Mg and I2. Techniques FTIR spectra were recorded by using an IRPRESTIGE-21 spectrometer (Shimadzu). SEM images of the xerogels were obtained by using SSX-550 (Shimadzu) and FE-SEM S-4800


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(Hitachi) instruments. Samples were prepared by spinning the gels on glass slides and coating them with Au. NMR spectra were performed on a Bruker Advance DRX 400 spectrometer operating at 500/400 and 125/100 MHz for 1H NMR and

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C NMR spectroscopy, respectively.

The high-resolution mass spectra (HR-MS) were measured on a Bruker Micro TOF II 10257 instrument. Fluorescence spectra were collected on an Edinburgh instrument FLS-920 spectrometer with a Xe lamp as an excitation source. The X-ray diffraction pattern (XRD) was generated by using a Bruker AXS D8 instrument (Cu target; λ = 0.1542 nm) with a power of 40 kV and 50 mA. UV-Vis absorption spectra were recorded on a UV-vis 2550 spectroscope (Shimadzu). Sonication treatment of a sol was performed on a KQ-500DB ultrasonic cleaner (maximum power, 100 W, 40 KHz, Kunshan Ultrasound Instrument Co, Ltd., China). The humidity was tested by GEMlead hygrometer STH310. Rheological measurements were carried out on freshly prepared gels using a controlled stress rheometer (Malvern Bohlin GeminiHRnano). Cone and plate geometry of 40 mm diameter was employed throughout the dynamic oscillatory work. Gelation test of organic fluids The two-component gelator and the tested solvent were put in a septum-capped test tube and heated (150 ºC > T > 60 ºC, depending on the boiling point of the solvent) until the solid was dissolved. Then the sample vial was cooled to room temperature (heating-cooling process). 41-43 A Sonogel was obtained when the cooled sample was treated by ultrasound (0.31 W cm-2, 40 KHz) for 0.5-5 minites at 25 ºC. Qualitatively, gelation was considered successful if no sample flow was observed upon inverting the container at room temperature (i.e., the “inverse flow” method).


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3. Results and Discussion The synthesis detail of 1a and 1b was described in the Supporting Information (SI). 1a or 1b did not form any gels in the organic solvents either by classic heating-cooling process or triggered by ultrasound. Interestingly, the mixture of 1a or 1b with certain amount of α-CD (namely 1a/α-CD and 1b/α-CD, Table 1 and Table S1) could form stable gels in npropanol and n-butanol, respectively, when accelerated by ultrasound just at room temperature for several seconds. Even though β-CD also assisted the gelation process of 1b in n-butanol (with ratio of 1:1), the gel was not stable. However, 1a with β-CD could not form gels in test organic solvents and γ-CD did not assist the gelation process of neither 1a nor 1b. The results revealed that smaller CD was favorable for the formation of two-component gels. Both gels of 1a/α-CD or 1b/α-CD are thermal stable. Heating 1a or 1b in n-propanol or n-butanol, respectively, in a closed test tube to 100 °C, they could be dissolved completely. On the contrary, no transparent solution could be observed even heating gels of 1a/α-CD or 1b/α-CD up to 150 °C in a closed test tube, indicating the irreversible process of sonication triggered gelation. Intriguingly, the gel of 1a/α-CD (16.5 mg/mL of 1a with the molar ratio of 1a: α-CD from 1: 0.2 to 1: 1) showed distinct thixotropic properties. By shaking the gel, it lost most of its viscosity and transformed into a suspension, then after resting for less than 10 min, the gel could be recovered (Figure 1). Such gel-to-suspension-to-gel process could be repeated for many times. In contrast, the two-component gels of 1b/α-CD are very stable to the mechanical force. The above results revealed that the formation and property of these gels were highly relied on the interaction of 1a or 1b with α-CD promoted by sonication.


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Table 1 Gelation properties of 1a (25 mg/mL) and 1a/α-CD (with molar ratio of 1:1) Solvent

H-C (1a)

U (1a)

U (1a/α-CD)

H-C (1a/α-CD)

1,4-dioxane

S

I

I

S

n-propanol

P

I

G

I

isopropanol

P

I

I

P

methanol

P

I

I

P

ethanol

P

I

I

P

n-butanol

P

I

I

P

tert-butanol

P

I

I

P

2-butanol

P

I

I

P

acetone

P

I

I

P

THF

P

I

I

P

chloroform

P

I

I

P

glycol monomethyl ether

P

I

I

P

DMSO

P

I

P

P

Note: S, solution; I, insoluble; G, gel; P, precipitate, U: ultrasound; H-C: heating-cooling process.


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Figure 1. a) Photos of the suspension-to-gel transformation of 1a/α-CD in propanol triggered by ultrasound, b) the thixotropic and humidity responsive properties of 1a/α-CD organogel in propanol. It is well known that CD encapsulation complexes are difficult to form in organic solvents.44 To examine the binding mechanism of 1a and 1b with α-CD, 1H NMR experiments were performed. Considering the commonly used NMR solvents and the solubility of 1a, 1b and α-CD, DMSO-d6 was chosen to study their dynamic interaction. It was obvious that there were no obvious changes in the steroid or adamantane groups in the 1H NMR spectra of both 1a and 1b upon addition of α-CD and treatment with ultrasound (Figure S1), indicating no production of encapsulation complexes between α-CD and 1a or 1b. However, significant changes in the NMR of 1b and α-CD were observed for the proton signals of amide and hydroxyl groups (Figures 2 and 3). The OH protons (Ha, Hb and Hc) of α-CD obviously weakened when bonded with 1b, especially, the peaks of Ha and Hb were both shifted upfield from 5.50 and 5.43 to 5.16 and 4.96 ppm (Ha’and Hb’), respectively, and Hc downfield shifted from 4.47 to 4.63 ppm (Hc’) (Figure 2a). Simultaneously, the up field shift of amide protons (H6, H7, H8 and H9) of 1b were also observed (Figure 3a). Those changes revealed the strong hydrogen bonding interactions between 1b and OH groups of α-CD. This intermolecular interaction in 1a/α-CD in DMSO-d6 was weaker than that of 1b from the NMR signal change. The OH protons (a, b and c) of α-CD slightly weakened in 1a/α-CD (Figure 2b). Notably, the signal at around 7.69 ppm (2 protons integrals) belonging to H7 and H4 of 1a separated with slightly downfield shift of H7 due to the formation of hydrogen bonding between 1a and α-CD (Figure 3b). At last, slightly upfield shift of the aromatic proton of H2 of naphthalimide for both 1a (from 6.871 to 6.868 ppm) and 1b (6.896 to 6.891 ppm) were observed upon the addition of α-CD followed by sonication treatment (Figure


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3), which indicated the minor environmental changes of the naphthalimide group of 1a or 1b after interaction with α-CD. The formation of intermolecular hydrogen bandings between 1a and α-CD was further confirmed by the comparison of the FT-IR spectra and XRD diffraction of powder 1a and xerogel of 1a/α-CD. As shown in Figure S2, after combining 1a with α-CD, the O–H stretching vibration of α-CD changed from 3409 to 3332 cm−1, whereas the C=O vibration of amides of 1a shifted from 1689, 1643 cm−1 to 1683, 1635 cm-1, respectively, confirming hydrogen bonding interactions between 1a and α-CD. XRD data indicated that the gel (1a/α-CD) had a more ordered structure compared with that of the precipitate of 1a (Figure S3). The results suggested that α-CD molecules may serve as junctions in the polymerization gel network of 1a or 1b aggregates via hydrogen bonding interactions. Fluorescence spectra were examined to further check the hydrogen bonding interactions between α-CD and 1a. Due to the existence of naphthalimide fluorophore, 1a solution has an emission at 520 nm (10−4 M), which was red shifted with the increase of the concentration (Figure S4). The fluorescence of 1a/α-CD gel (16.5 mg/mL 1a, with molar ratio of 1:1) showed a significant increase in intensity compared with that of 1a solution of the same concentration, together with a 2 nm red shift (from 539 to 541 nm, Figure 4). Moreover, dilution of the 1a/α-CD gel showed almost no change in fluorescence intensity, whereas the fluorescence intensity of the solution of 1a obviously increased with the decrease of the concentration (Figure S5). This indicated that the interaction between 1a and α-CD restrained the concentration induced fluorescent quenching of 1a due to the formation of hydrogen bonds between α-CD and amide groups of 1a.


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a

OH

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b

c

α−CD

α−CD 1a 1a/α−CD

O *

1b 1b/α−CD

O H HO

a

b

a b

*

OH

n

c

a'

5.5

b'

c'

5.0 4.5 Chemical Shift (ppm)

4.0

5.5

5.0 4.5 4.0 Chemical Shift (ppm)

3.5

Figure 2. Partial 1H NMR spectra of 1a or 1b with α-CD in DMSO-d6. a) 1b, α-CD and 1b/αCD (with molar ratio of 1:1); b) 1a, α-CD and 1a/α-CD (with molar ratio of 1:1). All the samples were treated with sonication for 5 min.


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a 5

3

1b 1b/α−CD

1 7

8.5

9,8 4

2

6

8.0 7.0 Chemical Shift (ppm)

b

8,9

6.9

7 1a/α-CD+H2O 4

1a/α-CD 3

5

1

9 8 7,4

2

6

1a 8.6

8.4

8.2

8.0 7.8 6.95 Chemical Shift (ppm)

6.90

6.85

Figure 3. Partial 1H NMR spectra of 1a or 1b with α-CD in DMSO-d6 in down field range. a) 1b and 1b/α-CD (with molar ratio of 1:1); b) 1a and 1a/α-CD (with molar ratio of 1:1), the samples were treated with sonication for 5 min.


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800k 1a/α-CD gel 1a solution

700k 600k Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500k 400k 300k 200k 100k 0 450

500

550 600 650 Wavelength (nm)

700

750

Figure 4. Fluorescence emission spectra of 1a solution (16.5 mg/mL, 1.6 × 10-2 M) and 1a/α-CD gel (16.5 mg/mL 1a, with molar ratio of 1:1). SEM images were obtained to study the co-assembly properties of 1a and 1b with αCD. As seen in Figure S6, 1a precipitated from n-propanol with a solid knot structure, indicating dense aggregates of molecules, which was transformed to a pore structure after sonication. A similar morphology transition was also observed in the SEM images of 1b in propanol (Figures S7a-S7c), revealing an effect of ultrasound on assembly of 1a and 1b alone. Regular and straight nanoribbons with length on the micro scale were observed in the gel of 1a/α-CD (Figure 5a and 5b), which was different from that observed for the pure α-CD assembly that had a porous film structure after sonication (Figure S8). Similarly, the 1b/α-CD gel was composed of entangled nanoribbons (Figure S7d). The results indicated that co-assembly of 1a or 1b with α-CD was prone to form a 1D fibrous structure accelerated by ultrasound, which was favorable for gelation.


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Figure 5. SEM images of 1a/α-CD aggregates. a) 1a/α-CD xerogel (16.5 mg/mL 1a, with molar ratio of 1:1); b) magnification image of a); c) collapsing xerogel of 1a/α-CD triggered by humidity (54%); d) collapsing xerogel of 1a/α-CD (16.5 mg/mL 1b, 0.8 eq. of CD, 0.3 mL) upon the addition of water (20 µL). Scale bar for a, b, c and d is 40, 5, 1 and 1 µm, respectively.

The thixotropic properties of the 1a/α-CD gel were studied by rheological experiments and fluorescence spectra. At molar ratios of 1:0.2–1:1, the 1a/α-CD gel (16.5 mg/mL of 1a) displayed thixotropic properties (Figure 1 and S9). Notably, the thixotropic properties were highly dependent on the ratio of α-CD. With ratios of 1a: α-CD < 1:1, gelation and thixotropic properties were gradually lost, indicating that excess α-CD was unfavorable for gelation (Figure S8). As a typical example, the mechanical properties of the 1a/α-CD


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gel at a molar ratio of 1:1 was studied. Initially, the storage modulus (Gʹ) of 1a/α-CD gel was found to be higher than the loss modulus (Gʹʹ) in the experiments of both frequency and strain sweeps (Figure 6a, 6b), indicating the good gel state of 1a/α-CD in n-propanol. Figure 6c revealing the recovery properties of 1a/α-CD gel. With the strain at 0.1%, the gel kept stable from 0 to 200 s (Gʹ > Gʹʹ); when the strain was increased to 100%, the gel collapsed to a sol (Gʹʹ > Gʹ). Furthermore, by decreasing the strain to 0.1%, the gel reformed, and a quick recovery of about 65% origin modulus values was observed. Such gel-to-sol then sol-to-gel transition controlled by strain could be repeated by several circles. The results certified the formation of a thixotropic gel by 1a and α-CD via dynamic hydrogen bonding interactions. Fluorescence measurements, as a simple approach, were performed to examine the cycles. When the gel 1a/α-CD was collapsed by simple shaking, the fluorescence intensity was enhanced by a factor of 1.12 compared with that of the original gel. The reversible fluorescence switch induced by shaking and rest could be repeated efficiently for several cycles with differences less than 6% (Figure 6d).


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10000

10000

a

b

100

G' G'' 10

10 1

G' G''

0.1

10 1

100

0.01

100

0.01

Angular Frequency (rad/s) 3000

Intensity

1000

G G

0

10

2500

2.6k

2000

2.4k

1500

Collapsing gel

2.2k gel

2.0k

'

0

2

200

400

600

800

1000 1200

0 500

4

6

8

10

Cycles

500

''

100

gel collasping by shaking collasping by humidity

d

1000

10

1

Shear strain (100%)

c

100

0.1

Intensity

1000

Modulus (Pa)

Modulus (Pa)

1000

Modulus (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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550

600

650

Wavelength (nm)

Time (s)

Figure 6. Oscillatory rheology measurements of gel of 1a/α-CD (16.5 mg/mL, with molecular ratio of 1:1). Plot of storage modulus (G’) and loss modulus (G’’) with frequency at 0.01% strain (a), shear stress at 10 rad sec-1 (b), and the recovery test for 1a/α-CD gel, by applying alternating strain amplitudes of 0.1% and 100% for 100 s (c). (d) The fluorescent spectra of gel 1a/α-CD (16.5 mg/mL, with molar ratio of 1:1), collapsing gel triggered by shaking and humidity (RH = 54%); inset is the fluorescence intensity changes at 537 nm of gel and sol of 1a/α-CD (16.5 mg/mL) vs. the cycle times of shaking. Since gel formation was highly dependent on hydrogen bonding interactions between 1a and α-CD, introduction of water to the two component system was expected to trigger partial hostguest interaction of α-CD and the adamantane group of 1a due to the high binding constant of αCD and adamantane in water. As anticipated, the two-component gel of 1a/α-CD was very


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sensitive to water molecules. For example, when 5 µL water was coated on the gel surface (5 mg/300 µL), the gel quickly collapsed in a few minutes. The fluorescence intensity of the collapsed gel was enhanced by a factor of 1.3 compared with that of the original gel (Figure 6d). The sensitivity and thixotropic properties of 1a/α-CD prompted us to investigate the possibility of the two-component gel for humidity sensor. As shown in Figure 7a and 7b, exposure of the gel to air in an open test tube at different humidity levels caused gradual gel collapse. The required time for complete collapse could be tuned from 43 to 105 min over a RH range of 40%– 70%. Interestingly, a linear relationship of collapse time as a function of RH was observed, which allowed quantitative and visual humidity sensing. When the gel was coated on the surface of a slide, the collapse time was significantly shortened, which might be attributed to the increased interface contact area between gel and water. For example, only 43 min was required for complete gel collapse at RH = 54%, compared with 80 min in a tube (Figure 7c). The encapsulating interaction between α-CD and the adamantane group of 1a after addition of water was confirmed by 1H NMR experiments. As seen in Figure 3b and S10, addition of water induced a pronounced upfield shift of –CH2 signals from 1.950 to 1.939 ppm in the adamantane, and the –CH signal of α-CD disappeared, indicating the complex formation between α-CD and the adamantane group of 1a. Moreover, signals for H1, H2 and H5 of naphthalimide and amide protons of H7 and H8 also all shifted downfield, while H3, H6 and H9 shifted up field, suggesting the formation of stronger hydrogen bonding interaction between 1a and water than that of α-CD and 1a. The above results revealed that water triggered the dissociation of hydrogen bonding interaction between 1a and CD, and accelerated the partially encapsulation between αCD and adamantane unit of 1a.


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SEM images were acquired to study macro changes in the two-component aggregates triggered by water. Long ribbons of 1a/α-CD were shortened and some short bundles of fibers were observed when the gel was collapsed at RH = 54% (Figure 5c). Upon addition of 20 µL water, the collapsed gel was mainly composed of short fibers on the nanoscale (Figure 5d). The results suggested that humidity induced disaggregation of the long ribbons into short ones, resulting in gel collapse. With addition of sufficient water, the assembly of 1a with α-CD formed short and bundled nanofibers via partial encapsulation of adamantane and α-CD segments. In contrast, the 1b/α-CD gel, lacking the adamantane group, was very stable and tolerant to water.

Figure 7. a) Linear relationship of gel 1a/α-CD (with molar ratio of 1:1) collapsing time as a function of relative humidity at 25 °C, inset: 2 mL of HPLC tube was used for the humidity test; b) photo of the hygrometer STH310 used in the test; c) from left to right: photos of gel of 1b/αCD; staying at 54% RH after 10, 20, 30 and 43 min, Scale bar: 1 cm.


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From the results, the two-component gel formation and humidity sensing can be speculated to involve the following processes: 1) Ultrasound accelerated intermolecular hydrogen bonding between 1a or 1b with α-CD, that is, α-CD has been inserted into 1a or 1b aggregates as linkers to assist formation of the gel network. Before sonication, the solubility of both 1a and 1b is poor in organic solvent (Table 1), which means that the molecules tend to aggregate to three-dimensional powder due to strong intermolecular interactions of 1a or 1b itself (see Figure S6 and S7). Using α-CD as hydrogen bonding junctions can weaken the intermolecular interactions between homo-molecules of 1a or 1b in some certain directions and promote one-dimensional aggregation into fiber structures. However, larger CDs, such as β-CD or γ-CD, or larger amounts of α-CD, inhibited ordered assembly of 1a or 1b aggregates. Although many organic and inorganic compounds serve as linkers to enhance gel stability and confer thixotropic properties, this is the first example of a CD acting as a junction to trigger gel formation at room temperature via hydrogen bonding interactions rather than an inclusion effect; 2) gel collapse of 1a/α-CD occurred upon addition of water because water molecules favored partial inclusion of α-CD and adamantane groups, destroying the hydrogen bonding interactions. Moreover, the interaction of 1a and α-CD was not as strong as that of 1b with α-CD probably due to the larger steric hindrance of adamantane group in 1a than that of the sugar chain in 1b. Consequently, the 1a/α-CD gel was metastable with thixotropic properties, and exposure to water molecules was accompanied by phase and fluorescence changes when the junctions were damaged.

4. Conclusion


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This work demonstrates a unique approach to construction of stimuli-responsive two-component gels using α-CD as hydrogen bonding junctions. The driving force for the formation of gels is ascribed to the fast interaction of α-CD (as a proton donor) and amide derivative (as a proton acceptor) through hydrogen bonding interactions triggered by ultrasound, producing dynamically stable gels. Several experiments revealed that α-CD could serve as the junction point for the gelation. This interaction is disrupted by water molecules due to the partial complex formation between α-CD and adamantane groups. Herein, using α-CD as hydrogen bonding junction point has following two advantages: 1) the hydrogen bonding interaction between α-CD and 1a promotes the one-dimensional aggregation of 1a for gelation; 2) the interaction change of α-CD and 1a from hydrogen bonding to encapsulation endows the gel with water responsive properties. The room temperature gelation, thixotropic properties and the fluorescent assembly of the twocomponent gel enable effective, simple and visual relative humidity sensing. Such a strategy would be highly relevant for construction of smart materials in a specific and controllable manner.

ASSOCIATED CONTENT Supporting Information. Synthesis details and additional spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] (X. Yu); [email protected], Fax: (+86) 21-55664621 (T. Yi). ACKNOWLEDGMENT The authors thanks for the financial support by NNSFC (21401040, 21301047, 51373039), Natural Science Foundation of Hebei Province (No.

B2014208160, B2014208091,

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Cyclodextrin-Assisted Two-Component Sonogel for Visual Humidity

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