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Supramolecular Gel with Response Towards Gaseous Acid and Organic Amine

Xinhua Cao , Qingqing Han , Aiping Gao , Guixia Wang

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (9) : 1538 -1549.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (9) : 1538-1549 DOI: 10.7536/PC200844
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Supramolecular Gel with Response Towards Gaseous Acid and Organic Amine
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Abstract

Supramolecular gels as important soft materials are formed by self-assembly of organic molecules under the driving force of non-covalent interactions, which can gel organic solvents or water. These materials possess both liquid and solid properties. This kind of materials with the advantages of stimulus response behavior and easy modification is widely used in many fields such as chemistry, biology, medicine and energy. Compared with the traditional small-molecule probes, supramolecular gels have many advantages for their application as chemical sensors. For example, the internal three-dimensional network structure of supramolecular gel material and its large contact area facilitate the rapid infiltration of analytes, and the change of gel state can be used as the output signal in the detection process. In addition, the xerogel film material also has a three-dimensional network structure, which also shows excellent detection performance in the detection of gaseous analytes. This work focuses on the application of supramolecular gels in the detection of gaseous acids and organic amines, as well as the designs and detection mechanisms of supramolecular gels, so as to provide reference for the construction of new supramolecular gels for the detection of gaseous acids and organic amines. Finally, the problems and prospects in the application of supramolecular gels are summarized and prospected.

Contents

1 Introduction

2 Mechanism of supramolecular gel response to gaseous acids and organic amines

3 Application of supramolecular gels for detection of gaseous acids and organic amines

3.1 Gaseous acid-responsive supramolecular gel

3.2 Organic amine-responsive supramolecular gel

3.3 Gaseous acid and organic amine-dual-responsive supramolecular gel

4 Conclusion and outlook

Graphical abstract

Keywords

gel / supramolecular / sensing / gaseous acid / organic amine

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Xinhua Cao, Qingqing Han, Aiping Gao, Guixia Wang. Supramolecular Gel with Response Towards Gaseous Acid and Organic Amine. Progress in Chemistry, 2021, 33(9): 1538-1549 DOI:10.7536/PC200844

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1 Introduction

Supramolecular gel is a three-dimensional network structure formed by self-assembly through non-covalent forces such as hydrogen bonding, π-π stacking and Van der Waals force, which can gelate organic solvents or water[1,2]. Supramolecular gels are sensitive to external stimuli due to the reversibility of these weak noncovalent forces[3⇓⇓~6]. At the same time, during the formation of supramolecular gels, small molecules can self-assemble into structures with different morphologies, including tubes, fibers and spheres[7,8]. Therefore, supramolecular gels are widely used in many fields, such as pollutant removal, light capture, drug release, catalysis, energy, functional materials, supramolecular chirality, chemical sensing and biological fields[9][10][11,12][13⇓~15][16⇓~18][19⇓⇓~22][23,24][25,26][27⇓~29].
Supramolecular gels have unique advantages in the application of chemical sensors. Compared with small molecule probes, the three-dimensional network structure and larger contact area of supramolecular gels facilitate the rapid entry of analytes into the gel system, and the gel system can detect analytes in three States of solution, gel and dry gel, and express multiple response signals such as color, light or gel state changes[30]. For analytes that cannot be distinguished by the fluorescence and color changes of small molecule fluorescent probes, the detection can be achieved by the change of the state of the supramolecular gel system. Most importantly, the gel system can be well made into a thin film material for detecting gas molecules[31]. Organic amines and gaseous acids are common chemical raw materials in industry. Even trace amounts of these materials can cause great harm to the environment and human health[32]. Organic amines (such as triethylamine, tripropylamine, ethylenediamine, aniline) and inorganic amines (such as ammonia) have great harm to people and the environment (such as the red groundwater incident in Hebei). Although these amines can be identified by odor, when the concentration is relatively low, the detection of these substances becomes difficult. Gaseous acids, such as formic acid, acetic acid, trifluoroacetic acid, hydrochloric acid and nitric acid, have certain corrosiveness, irritation and volatility, and their molecular weight is relatively small, so they are not easy to capture and detect. Therefore, when the concentration of these organic amines or gaseous acids is lower than the concentration that can be perceived by the human sense of smell, it is necessary to detect them with the help of functional organic amine and gaseous acid sensors.
In recent years, the supramolecular gel system has made a breakthrough in the detection of organic amines and gaseous acids. In this paper, the research progress and response mechanism of supramolecular gels responding to gaseous acids and organic amines in recent years are reviewed, and the applications and future research directions of supramolecular gels are summarized and prospected.

2 Design and detection mechanism of supramolecular gels responsive to gaseous acids and organic amines

For supramolecular gels responding to gaseous acids, the gel factors usually contain proton binding sites, such as N, N-dimethylamino groups, or basic nitrogen atoms in the conjugated structure of the molecule.Uch as triphenylamine, benzimidazole, quinoxaline, benzoxazole, pyridine, acridine, quinoline and the like, which are introduced into the structure of the gel factor as proton binding sites to realize the sensitive response of the supramolecular gel to gaseous acid. The protonation of the gelling agent disrupts the intramolecular charge transfer, metal coordination and non-covalent interactions in the system, which induces changes in the color, fluorescence and gel state of the gel system. The energy of the highest occupied molecular orbital (HOMO) of the gelator responding to the organic amine is lower than that of the organic amine, which is beneficial to the electron transfer of the organic amine to the gelator, thereby inhibiting the original luminescence process of the gelator (PET or ICT) or promoting the metal coordination in the system[33]. Sometimes, the addition of organic amine can also cause the destruction of hydrogen bonds in the system to some extent, resulting in the change of gel state. For the gel system that can continuously respond to gaseous acid and organic amine, it usually reacts with gaseous acid first, and its fluorescence, color or gel state changes. When the organic amine is added, the gel system is restored to the initial state through the acid-base reaction in the system, and the change process can realize continuous detection of gaseous acid and organic amine and can be repeatedly used. The detection performance of the gelling factor mainly depends on its chemical structure, especially the detection performance of the gelling factor in solution mainly depends on the sensitivity and stability of the chromophore containing the action site to the analyte; In the gel state, the packing of the gelling factors is too tight for the analytes to enter the three-dimensional network in the system, which will lead to the unchanged gel state or the irreversible repetition of the gel-sol-gel process. For xerogels, too tight packing will also reduce the specific surface area and pore structure, which is not conducive to the entry of gaseous analytes into the network of xerogels and the interaction with gel factors, thus affecting the detection performance. The molecular structures referred to herein are shown in Table 1.

3 Gaseous acid and organic amine responsive supramolecular gel

3.1 Gaseous acid responsive supramolecular gel

In 2015, Lu et al. designed gelling factors 1-3 based on phenanthroimidazole derivatives, which can form organogels in different solvents and self-assemble into nanofibers (Figure 1)[34]. Trifluoroacetic acid (TFA) gas induces intramolecular charge transfer (ICT) through protonation of gel factor 1, which significantly quenches the fluorescence of nanofiber membranes. On the other hand, TFA also causes the luminescent colors of the nanofibers 2 and 3 to change from sky blue and blue to yellow and green. The decay time of the gel factor 1 in the nanofiber film and the spin-coated film to TFA saturated gas is 0.1 s and 1.9 s, respectively, and the detection limit of the fiber film to TFA is as low as 0.12 ppm, which is mainly due to the large volume ratio and large porosity of the nanofiber network, which is beneficial to enhancing the adsorption, accumulation and diffusion of gas molecules.
Lu et al. synthesized two new carbazole derivatives, gelling factors 4 and 5 (fig. 2)[35]. Under ultrasound stimulation, gelling factor 5 with tert-butyl group could gel cyclohexane, cyclopentanol and octane/cyclohexane (V/V = 2/1), while gelling factor 4 without tert-butyl group could not form organic gel, which indicated that tert-butyl played an important role in gel formation. The gel system adopts a self-assembly mode of a layered structure. The xerogel film formed by gelling factor 5 can emit strong blue light and can be used as a fluorescent sensing material for the detection of acid vapors such as TFA, HCl, HNO3, formic acid and acetic acid. The detection limits for TFA and HCl are 1.5 ppm and 5.2 ppm, respectively, and the response times are 1.61 s and 0.65 s, respectively. This shows that the gelling agent can be used to prepare a simple and efficient gaseous acid chemical sensor.
In 2016, Lu et al. Prepared novel cyanostyryl and benzimidazole functional compounds 6 and 7 (fig. 3)[36]. Under the induction of ultrasound, 6 and 7 can self-assemble into organic gels in some mixed solvents. 6 and 7 have aggregation-induced luminescence (AIE) properties, and their xerogel films can emit strong yellow light. The fluorescence of THF/water solutions of 6 and 7 with high water content can be significantly quenched by trifluoroacetic acid, and these hydrophobic dyes can be used to detect acids in the aqueous phase. At the same time, the nanofiber films of 6 and 7 can sensitively respond to TFA vapor, and the fluorescence of the films is rapidly quenched after contacting with TFA vapor, and the thinner the film is, the better the response performance is. For example, the detection limit and decay time of the nanofiber membrane 6 with a thickness of 0.21 mm for TFA gas are 0.33 ppm and 0.38 s, respectively; The detection limit and decay time of the thin film with a thickness of 1.73 nm are 8.0 ppm and 2.5 s, respectively. This study provides a new strategy for the preparation of novel and efficient gaseous acid fluorescent sensing materials.
In 2017, Lu et al. Synthesized a series of unconventional β-iminoketone gelling factors 8A ~ 15A and their corresponding difluoroborate complexes 8B ~ 15B (Fig. 4)[37]. The existence of halogen atoms in the substituents is of great significance to the gelation ability, and bromine-containing compounds 11A and 11B, in particular, show good gelation properties. The single crystal structure, PXRD pattern and the changes of electronic spectra in the dry gel and in the gel process prove that π-π, C-H … F and C-H … Br interactions are the driving forces for the formation of gel 15 B. The 10 A ~ 12 A in the solution state has the property of aggregation-induced luminescence, while in the gel state, it emits strong yellow light. Interestingly, the difluoroborate complexes 10 B ~ 12 B have strong fluorescence. In addition, the fluorescence of the nanofiber film of 11 B was significantly quenched when exposed to trifluoroacetic acid gas, and the decay time and detection limit were 0. 5 s and 0. 17 ppm, respectively. This work provides a new strategy for controlling the polarity of conjugated systems and designing unconventional π gel factors.
In 2017, Xue et al designed and synthesized a cyanostyrene derivative gel factor 16, which can form supramolecular organogels in cyclohexane, n-hexane and n-octane (Fig. 5)[38]. The gelling factor self-assembles into nanofibers in the gel phase, and these nanofibers can respond to mechanical forces and solvents, accompanied by significant changes in their fluorescence and color. The dilute solution of gel factor 16 was also sensitive to acid, and the emission peak at 472 nm was red-shifted to 535 nm after the addition of TFA. In addition, the film formed by the gel factor is also sensitive to TFA vapor, the emission color gradually changes from yellow to orange, and the detection limit for TFA is as low as 70 ppb. The system realizes sensitive response to mechanical force and gaseous acid gas.
Bhattacharya et al. reported a two-component hydrogel constructed from the donor perylhydrazone derivative 17 and the acceptor naphthalimide derivative (NDI) (fig. 6)[39]. At room temperature, 17 and NDI in water/DMF (99 ∶ 1, V/V) can instantaneously form a transparent hydrogel 17-NDI (1 ∶ 1) with a minimum gel concentration of 1.82 mmol/L. More interestingly, the gel system does not turn into a sol even when heated to 90 ° C, and forms a semitransparent gel at temperatures above 70 ° C. After cooling to room temperature, the thermotropic gel becomes a solution again. The gel-solution transition temperature can be regulated by the molar ratio between compound 17 and NDI. The newly prepared gel 17-NDI was able to respond to AgOTf (1.0 eq), immediately collapsing and turning into an orange solution. The reversible transition of the gel-sol-gel process can be achieved by deprotonation and protonation of the acidic hydrazide of compound 17 by the sequential addition of NaOH and HCl. A small amount of HCl (6 mol/L) can protonate the nitrogen atom in the pyridine group of compound NDI and form a brown solution. This thermotropic gel has potential applications in tissue engineering and drug delivery.

3.2 Organic amine responsive supramolecular gel

In 2015, Xue et al. Designed a two-component gel system 18, which can form a gel in an organic solvent through intermolecular hydrogen bonding (Fig. 7)[40]. The system shows very weak fluorescence in the solution state, and emits strong blue fluorescence when the sol becomes gel, and the emission intensity is increased by 100 times. More importantly, the two-component gel film can be used as a sensor for the quantitative detection and identification of aliphatic and aromatic organic amine vapors. Aromatic amine vapor can quench the fluorescence of the gel film, and the fluorescence can be recovered after exposure to air, indicating the reversible sensing behavior of the gel film. On the other hand, after the strong blue fluorescence of the two-component gel system was quenched by aliphatic amine vapor, the blue fluorescence could not be restored due to the destruction of intermolecular hydrogen bonds. The detection limits for aniline and n-butylamine are 1. 6 ppb and 3. 4 ppm, respectively, and the corresponding response times are 0. 7 s and 0. 04 s, respectively.
In the same year, Yi et al. Designed and synthesized a fluorescent gel factor 19 of naphthalimide (Fig. 8)[41]. Under the induction of ultrasound (less than 2 min), the gelation factor can realize the transformation from precipitation to gel. During the gelation process, the fluorescence intensity of gelation factor 19 decreased gradually and showed a linear relationship with the ultrasonic time. The obtained gel 19 could be used for the detection of aliphatic amines and aromatic amines. For example, the addition of propylamine results in a significant increase in the fluorescence intensity of gel 19, accompanied by a gel to sol transition, with a detection limit of 130 nmol/L for propylamine. When the gel 19 was treated with an aromatic amine such as aniline, the fluorescence of the gel was quenched, but there was no gel collapse. Hydrogen nuclear magnetic resonance spectroscopy, small angle X-ray scattering, scanning electron microscopy and spectroscopic experiments were used to study the sensing mechanism of the gel system to propylamine. When propylamine was added to the gel, the alkenyl group in the molecule 19 changed from trans to cis, resulting in a gel-sol transition. However, aromatic aniline molecules are more likely to insert into the gel network through hydrogen bonding and charge transfer interactions, maintaining their gel state.
Lu et al. designed new D-A-D type β-diketone difluoroborate complex gel factors 20-22 (fig. 9)[42]. 21 and 22 without a conventional gelling group can gel a mixed solvent containing 1,4-dioxane by balancing the π-π interaction. Interestingly, the fluorescence emissions of 21 and 22 are in the near infrared region. 21 and 22 are the narrowest band gap small molecular weight gel factors reported to date. The xerogel films prepared from 21 and 22 can be used as sensors for the detection of gaseous aliphatic amines with high performance. For example, after exposure to saturated n-butylamine and n-propylamine vapors for 30 s, the emission of the film at 710 nm is completely quenched, but a new emission peak at 560 nm appears due to the decomposition of the difluoroborate diketone complex.
Based on the results of quantum chemistry calculations, Xue et al. reported a dicyano-modified oligo (p-styrene) fluorescent gel factor 23, whose lower HOMO energy makes it useful as a sensing material for the detection of organic amine vapors (fig. 10)[43]. Both the wet gel and the dry gel nanofiber films of the gelling factor can generate strong emission light. The fluorescence of the xerogel film can be rapidly and effectively quenched when exposed to low concentrations of aromatic amine vapors. The detection limits of aniline, N-methylaniline and N, N-dimethylaniline were 21 ppb, 5 ppb and 11 ppb, respectively, and the corresponding response times were 1. 6 s, 2.0 s and 0. 5 s, respectively. In contrast, xerogel films are less sensitive to aliphatic amines due to the fact that the HOMO energy level of the gel-factor fluorophore is intermediate between that of aromatic and aliphatic amines. Therefore, the sensing film material has high selectivity to the aromatic amine.
Our research group designed and synthesized a new gel factor 24 and 25 containing bis-naphthalimide group (Figure 11)[44]. The gel factor 25 can form a gel in a mixed solvent of DMF and DMSO/H2O(10/1,v/v). The gel 25 formed in the DMSO/H2O(10/1,v/v) mixed solvent has the ability to identify o-phenylenediamine, m-phenylenediamine, and p-phenylenediamine. Meanwhile, the gel factor 25 can sensitively and selectively detect the p-phenylenediamine, and the detection limit is 8.96×10-8mol/L. This study will provide a new idea for the application of supramolecular self-assembled materials in the detection of organic amines.
Our research group also designed and synthesized 4-isopropoxy borate modified naphthalimide gelation factors 26 and 27 (Figure 12), which can respond to organic amines in solution or gel state[45]. In the presence of organic amine, the blue fluorescence of the gel factor is quenched, but it is restored after the addition of acid. Taking triethylamine (Et3N) as an example, the binding constant (K) between gel factor 26 and Et3N is 3.75×103L/mol, and the detection limit for Et3N is 4.77×10-7mol/L. The detection mechanism of the gelling factor for organic amine is that the HOMO electron of the organic amine is transferred to the HOMO of the gelling factor, and the original ICT luminescence process is blocked. This study expands the application of supramolecular self-assembled materials in the field of sensors and provides a new method for the detection of organic amines.
Yu et al. prepared low molecular weight hydrogels and polymer hydrogels based on amphiphilic tripyridine molecules 28 through various supramolecular interactions (fig. 13)[46]. Based on the hydrogen bonding interaction between — COOH and 28 in the hydrogel, the polymer hydrogel can generate stable tripyridyl radicals by irradiating the polymer hydrogel with light or heat, and the process is accompanied by a color transition from white to dark purple. Electron paramagnetic resonance (EPR) and ultraviolet-visible (UV-Vis) absorption spectroscopy experiments proved that free radicals were induced by light or heat. The free radical hydrogel has the properties of self-healing, stretchability and self-support, and can be molded into different macrostructures. The formation of tripyridyl radicals in the gel system promotes intermolecular hydrogen bonding and π-π interactions. Based on the change of reversible hydrogen bonding interaction, the free radical gel can selectively make a reversible color change to ammonia and stretching reaction.
Our research group designed and synthesized a series of 3-hydroxynaphthalimide gel factors 29 ~ 31 (Fig. 14)[47]. The gelling factors 30 and 31 are capable of gelling certain single or mixed solvents and self-assembling to form hydrophobic microribbons or nanofiber structures. Interestingly, these compounds exhibit solvatochromic properties, and compound 31 has the ability to colorimetrically detect organic amines with a detection limit of 2.23×10-6mol/L for triethylamine. At the same time, compound 31 can distinguish aromatic amines from aliphatic amines by fluorescence changes in both solution and gel States. These compounds showed specific and dependent fluorescence behavior in the pH range of 3 ~ 11. Compound 29 is capable of real-time quantitative monitoring of intracellular pH. This work provides a new window for sensitive detection of organic amines and accurate monitoring of intracellular pH changes by self-assembled materials.
Eychm Eychmüller et al. reported a multifunctional platform for enzyme coated CdTe QD gels for biocatalysis and fluorescence detection of dopamine[48]. Mercaptosuccinic acid (MSA) -coated CdTe QD hydrogels can be formed in phosphate buffer (PB,50 mmol·L-1) with a pH value of 6.5 to 7.1, and are suitable for a variety of enzymes. At the same time, the mesopores with diameters of 10-50 nm obtained in the gel system can immobilize the enzyme. Tyrosinase (TRS) can be encapsulated in hydrogels and still retain many of its properties in solution, such as high enzymatic activity towards catechol. Due to the quantum confinement effect and high enzymatic activity of the hydrogel, the hydrogel can be used as a biosensor for dopamine, and the detection limit for the dopamine is 5.0×10-8mol/L, and the detection range is 5.0×10-5~1.0×10-3mol/L. Xerogels made from this gel are still sensitive to dopamine and can be reused for six weeks. This study provides a new method for the construction of enzyme-based biosensors and portable sensing devices.

3.3 Gaseous acid and organic amine continuous response type supramolecular gel

Mahapatra et al. reported that two terpyridine derivatives (32A and 32b) can be used as gelling factors and antennas for phototrapping systems to sensitize lanthanide (III) ions (fig. 15)[49]. 32A and 32b can be coordinated with lanthanide ions (Tb (III) or Eu (III)) in a solution or gel state, and emit the characteristic green or red light of Tb (III) or Eu (Ⅲ) by energy transfer from 32A and 32b to the lanthanide ions. The emission behavior of the gel can be adjusted by adjusting the stoichiometric ratio of Eu (Ⅲ) and Tb (Ⅲ) in the gel system. A near white light emitting gel with CIE (0.31, 0.35) color coordinates was obtained by doping Eu (Ⅲ) -doped red light emitting gel with [IrIII(F2ppy)2(biimid)]PF6. Photofunctional composite films can be constructed by doping PMMA polymer with gels with different emission behaviors. The hybrid film has low transmittance in the ultraviolet region and high transmittance in the visible region, and has potential application value in anti-ultraviolet glass and window film. The hybrid gel 32b has stimulus-responsive properties to TFA and triethylamine (TEA) gas. The emission of the gel changes from white light to blue light, showing an obvious transition from gel to solution, which is due to the protonation of the nitrogen atom of the terpyridine ligand and the dissociation of the coordination interaction between the Ln (Ⅲ) ion and the ligand. The sol may reform into a gel upon exposure to the vapor of triethylamine. Thin-layer chromatographic plates coated with a white light emitting gel film can also respond to hydrochloric acid and ammonia solutions and show significant fluorescence changes.
Maji et al. Designed an amphiphilic tri-branched terpyridine derivative, Gel Factor 33 (fig. 16)[50]. The gel 33 can selectively recognize the Zn2+ due to the steric recombination of the terpyridine complex with the Zn2+, and the emission at 425 nm is significantly enhanced (10-fold). The detection limit of gel factor 33 for Zn2+ is 6.25×10-7mol/L(0.18 ppm), which is lower than the acceptable limit of 5 ppm for Zn2+ in drinking water recommended by the World Health Organization (WHO). At room temperature, gelling factor 33 can complex Zn2+ gelled water/MeOH mixed solution, emit blue light and self-assemble into micron-sized flakes. In the presence of Tb (Ⅲ) and Eu (Ⅲ), an organic gel 33 can be obtained in a CHCl3/THF solution, which emits green and red light, respectively. By adjusting the molar ratio between lanthanide ions (Tb (Ⅲ) and Eu (Ⅲ)) and 33, the fluorescence emission of gel 33 can be controlled. The gel 33 exhibits mechanical, thermal, and chemical reaction behavior. Under the stimulation of ultrasound or heating, the process of gel-sol-gel transition can be observed. The response of the gel 33 to TFA is followed by a further response to TEA, accompanied by a change in the emitted light and the state of the gel. In the TFA atmosphere, the protonation of the pyridine N atom in molecule 33 leads to the cleavage of the coordination bond, which in turn leads to the gel-sol transition and weakens the original fluorescence intensity of 33.
Xue et al. Designed an L-phenylalanine derivative, Gel Factor 34 (fig. 17)[51]. UV-Vis absorption spectra showed that the gel was formed in the H-aggregation mode, and there was a strong exciton coupling between the fluorophores. Because that cyano group in the molecule inhibit the fluorescence quenching dure the self-assembly process, the gel and xerogel systems of the gel system emit strong green fluorescence. In addition, the xerogel film with strong green fluorescence can be used as a double chemical sensor to quantitatively and rapidly detect volatile acids and organic amine vapors, and has the characteristics of short response time and low detection limit due to large specific surface area and high fluorescence quenching efficiency. The detection limits for aniline and trifluoroacetic acid are 796 ppt and 25 ppb, and the response times are 1.6 s and 1.1 s, respectively.
Xue et al. Synthesized two N-dodecyl-L-phenylmalonamide functionalized cyanovinyl acridine derivatives gel factors 35 and 36 via succinyl and glutaryl linkages (fig. 18)[52]. These two compounds can form gels at low concentrations in some solvents and show fluorescence enhancement effects. Although the molecular structures of the two compounds are different only by one methylene group, they have different gelling abilities and microscopic self-assembly structures. In addition, the low HOMO energy level of the gel factor and the presence of acridine groups at the acid binding sites make the uniform fibrous film prepared from gel 35 an effective sensor for the quantitative detection of aromatic amines and volatile acid vapors. The detection limits for TFA, HCl and acetic acid are 1.26 ppm, 0.12 ppm and 0.5 ppm, respectively, and the response time for TFA is 1.1 s. The detection limits for aniline, N-methylaniline, N, N-dimethylaniline and phenylhydrazine were 0. 33 ppb, 0.48 ppb, 1.38 ppb and 3. 8 ppb respectively, and the response time for aniline was 0. 53 s. The sensitive detection of gaseous acids and organic amines by the thin film material is mainly due to the fact that the π-π stacking orientation facilitates exciton diffusion and induces enhanced fluorescence quenching efficiency in long nanofibers.
Our group designed a thiophene derivative hybrid gel system 37 containing iridium complex and EuCl3·6H2O, which achieved dual response to volatile acid and organic amine vapor (Figure 19)[53]. A stable hybrid organogel 37-Ir-Eu can be obtained by adding an iridium complex and a EuCl3·6H2O. The results of IR spectra show that the hydrogen bonds still exist in the system even if the iridium complex, EuCl3·6H2O, sodium hydroxide and CF3COOH are added. The luminescent properties of the hybrid organogel 37-Ir-Eu can be converted between the cyan light and the red light by adding sodium hydroxide and CF3COOH. The 37-Ir-Eu xerogel thin film prepared in the presence of sodium hydroxide can rapidly and reversibly respond to volatile acids (such as CF3COOH, formic acid, acetic acid and propionic acid) and organic amine vapors (such as Et3N, tripropylamine and ethylenediamine), in addition to ammonia. These response processes can be repeated many times. The results of contact angle experiments showed that the surface morphology and internal composition of the xerogel film changed in response to trifluoroacetic acid and triethylamine. The results of NMR experiments showed that a new complex was formed between the iridium complex and the EuCl3·6H2O in the acid-base response process. This work provides a method for the preparation of multifunctional supramolecular materials for the detection of volatile acids and organic amine vapors.
Our research group designed and synthesized a novel gelling factor 38 containing quinoline group, which can obtain stable organogels in solvents such as ethanol, acetonitrile, n-hexane, petroleum ether and DMSO (Figure 20)[54]. The gelling factors 38 can self-assemble into self-assembled structures ranging from superhydrophilic to superhydrophobic with different surface wettability. Compared with the organogels formed in the other four solvents, the organogels formed in acetonitrile can emit stronger light, and the emission wavelength is red-shifted by 70 nm compared with the corresponding solution state. The theoretical calculation further verifies the difference of fluorescence emission of the gel factor 38 in the solution state and the gel state. With the addition of TFA and TEA alternately, the maximum fluorescence emission wavelength of the acetonitrile solution of gel factor 38 can be reversibly switched between 382 nm and 458 nm, and the detection limit for TFA is 5. 68 nmol/L. The xerogel 38 formed in acetonitrile showed a more interesting fluorescence emission behavior in response to TFA and TEA. The fluorescence emission of xerogel 38 showed a blue shift from 494 nm to 383 nm after exposure to TFA, and a red shift from 383 nm to 436 nm after further exposure to TEA.
Our group designed a novel cyanostyrene gelling factor 39, which can form stable organogels in n-hexane, petroleum ether, DMSO and acetonitrile (Figure 21)[55]. Field emission scanning electron microscopy (FESem) was used to observe the formation of nanofibers and microspheres in the self-assembly system. The results of UV-Vis and IR spectra show that the molecular self-assembly process adopts J-aggregation mode, and hydrogen bonding and π-π stacking interaction are the main driving forces for the formation of organic gels. Compared with the acetonitrile solution with a gel factor of 39, the fluorescence emission in the gel state has a red shift of 43 nm. Interestingly, compound 39 can respond sensitively to volatile acids and then to organic amine gases, accompanied by significant fluorescence and color changes. The detection limit and the association constant (K) of compound 39 in acetonitrile for TFA are 1.03×10-8mol/L and 6.85×104L/mol, respectively, and for TEA are 1.27×10-8mol/L and 4.27×104L/mol, respectively. The color of solution 39 reversibly changes from yellow to colorless to yellow during titration. The fluorescence of xerogel 39 was quenched after 7 s contact with volatile acid and recovered within 15 s after further contact with volatile organic amine. The detection limit of the xerogel membrane 39 for TFA gas is 3.2 ppb. Organogel39 show sensitive response to TFA and TEA in that solution, gel and dry gel States, and exhibit rapid changes in fluorescence, gel state and color.
Our group also prepared a novel tetrazole-modified naphthalimide derivative of fluorescent gel factor 40, which can form a stable gel in acetone/water (1/1, V/V) (fig. 22)[56]. Gel factor 40 adopts a J-type aggregation mode in the process of self-assembly, and its fluorescence emission has a red shift of 153 nm, which is the largest red shift of naphthalimide gel factor reported so far. Scanning electron microscopy (SEM) showed the formation of microribbons during the self-assembly process. The self-assembly system can selectively detect Cu2+ and K+, and the corresponding detection limits are 5.65×10-7mol/L and 1.73×10-7mol/L, respectively. Although the gelling factor contains a basic tetrazolium group, the acid does not quench its fluorescence. Interestingly, the self-assembled system can continuously respond to organic amines and gaseous acids in three different States, such as solution, gel and xerogel. The detection limits of TEA and TFA in solution are 7.66×10-5mol/L and 2.01×10-7mol/L, respectively. The detection limit of the xerogel film 40 for TEA gas is 0.19 ppm, and the response time is 15 s. This study will provide a new idea for the construction of multifunctional self-assembled systems for the detection of metal ions, organic amines and gaseous acids.
Our research group designed a new multifunctional azobenzene derivative gelling factor 41, which has gelling effect on solvents such as n-hexane, petroleum ether, DMSO, ethanol and acetonitrile (Figure 23)[57]. Interestingly, Gel Factor 41 has a colorimetric detection capability for sensitive detection of Hg2+, Cu2+, Fe3+, and volatile acids and organic amine gases. At the same time, the organic gel 41 in acetonitrile also exhibits detection performance through a change in its color or gel state. More importantly, the xerogel film can continuously detect volatile acids and organic amine gases for at least 7 cycles, and the detection limit for TFA is as low as 0.0848 ppb, corresponding to a response time of 2 s. The sensing mechanism was investigated by 1H NMR, FESEM, HRMS and UV-vis absorption spectroscopy. The molecule 41 undergoes an intramolecular cyclization reaction upon encountering a Hg2+, losing a molecule of H2S. Under the coordination of Fe3+ and Cu2+ with — N = N — in the molecule, the changes of absorption spectra and color of the gel system were induced. In the case of the volatile acid, the volatile acid is bound to the N, N-dimethylamino group in molecule 41. This study opens up a new way for the preparation of supramolecular self-assembled gels for the detection of various metal ions and trace volatile acids in the environment.
Yu et al. prepared a novel naphthalimide derivative gel factor 42 containing cholesteric groups, which can be used to visually distinguish between cyclohexane/cyclopentane and n-hexane/pentane (fig. 24)[58]. Interestingly, the gel 42 has self-supporting, self-healing properties and is capable of responding to picric acid with high selectivity and visibility. Gel factor 42 shows strong green fluorescence in cyclohexane due to its H-type aggregation, while it shows yellow fluorescence in hexane due to its J-type aggregation. The relationship between gelling properties and Hansen solubility parameter indicates that the dispersion interaction is the main factor for the tendency of gelling factor 42 to gel short chain alkanes. The comparison of Hansen solvent parameters showed that the similar hydrogen bond parameters between gel factor 42 and cyclohexane and cyclopentane were one of the reasons for the specific binding of compound 42 with cyclohexane and cyclopentane. In addition, the xerogel 42 can selectively and rapidly gel cyclohexane in a mixed solvent, the separation efficiency is as high as 92%, and the gel and the liquid are easy to recover. The gel formed in cyclohexane can quickly respond to HCl gas, and the fluorescence of the gel is quenched when it turns into a solution. When the solution is again placed in an ammonia atmosphere, the solution can form a gel again with fluorescence recovery. This reversible response process can be repeated 6 times. Scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) measurements showed that the protonation and deprotonation of pyridine groups in the molecule were the main reasons for the gel-sol interconversion.

4 Conclusion and prospect

Some progress has been made in the detection of gaseous acids and organic amines by supramolecular gels, but there are still some problems: (1) The sensitive detection of weak acidic volatile acids and weak basic organic amines by supramolecular gels. At present, supramolecular gel is mainly applied to the detection of strong acidic volatile acids and strong basic organic amines, but it is difficult to detect weak acidic volatile acids or weak basic organic amines. Therefore, how to design supramolecular gels sensitive to volatile weak acids and weak basic organic amines is an important research direction in the future. And (2) that supramolecular gel intelligently detect volatile acids and organic amine. The process of detecting volatile acids and organic amines in supramolecular gels is too complex, and the detection results can not be identified by naked eyes, especially in quantitative detection, which often requires special instruments. In addition, in the detection of gaseous acids and organic amines, the thickness of the film material made of supramolecular gel is often difficult to homogenize, but the thickness of the film material is directly related to its gas detection performance, but there is no systematic research in this area. (3) The expansion of supramolecular gel materials in practical applications. Although these supramolecular gels have good detection performance under laboratory conditions, there are few studies on the application of supramolecular gels in practical environment. At present, these application systems still have some shortcomings, such as simple system, poor stability, low selectivity and high preparation cost, which greatly limit their practical application. Therefore, the following measures can be taken to solve the above problems: firstly, in the design of functional groups, fluorescent groups which are more sensitive to gaseous acids and organic amines should be introduced into the gel factor, and at the same time, the groups which respond to gaseous acids should have a certain alkalinity, so as to realize the response to weakly acidic gaseous acids; Second, the group responsive to the organic amine preferably has a lower HOMO energy. By carrying out interdisciplinary research, introducing a variety of control methods, optimizing the preparation process of the corresponding thin film materials, especially combining with polymer thin film materials.It is expected to prepare supramolecular gel functional materials with intelligent response, rapid detection, stable performance and wide application, and realize the application of supramolecular gel in the detection of gaseous acids and organic amines.

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Funding

National Natural Science Foundation of China(U1704164)

Training Project for Youth Backbone Teachers in College and Universities of Henan Province(2018GGJS127)

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