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A Multiple Stimuli-Responsive Fluorescent Sensor from Citric Acid and 1-(2-Aminoethyl)piperazine Wen Yang, Fangbing Liu, Ran Li, Xiaoji Wang, and Wentao Hao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17894 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018

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ACS Applied Materials & Interfaces

A Multiple Stimuli-Responsive Fluorescent Sensor from Citric Acid and 1-(2-Aminoethyl)piperazine Wen Yang, Fangbing Liu, Ran Li, Xiaoji Wang, Wentao Hao* Department of Polymer Materials and Engineering, Hefei University of Technology, Hefei 230009, P. R. China E-mail: [email protected] KEYWORDS: Multi-responsive; fluorescent; sensor; supramolecular; ionic network ABSTRACT: Multi-responsive fluorescent supramolecular materials are quite interesting for that they combine the multi-responsiveness of supramolecules and the high-sensitivity of fluorescent materials. Different from the multi-responsive supramolecular materials based on host-guest interaction, in this report a supramolecular ionic network was fabricated by 1-(2-aminoethyl) piperazine (AEPZ) and citric acids via ionic interactions. Despite that there is none conventional chromophores, the obtained supramolecular ionic material can emit strong fluorescence. Most interestingly, the thin film of this supramolecular ionic material can change its fluorescent intensity in responding to four external stimuli, including humidity, triethylamine, acetic acid and temperature. Beneficial to the supramolecular ionic structure, this multi-responsive fluorescent sensor is self-healable. It is convinced that a new route has been opened to prepare the multi-responsive fluorescent sensors.

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INTRODUCTION Stimuli-responsive fluorescent materials have gained more and more attention in the area of chemical and biochemical sensors.1-3 To date, stimuli-responsive fluorescent systems in responding to single external stimulus such as pH,4-6 temperature7,8 and humidity9,10 have been widely studied. However, the preparation of multiple stimuli-responsive fluorescent systems in responding to three or more stimuli is less common because it requires incorporating several different stimuli-responsive groups and fluorescent unit into the stimuli-responsive system simultaneously.11,12 For polymers that are connected via covalent bonds, the synthetic routes are usually complicated,13,14 and therefore noncovalent interactions have been studied in the development of multi-responsive fluorescent systems recently. Supramolecular materials, which were fabricated through various noncovalent bonds, such as host-guest interaction have been reported as multi-responsive fluorescent sensors.15,16 For example, Huang and co-workers selected a supramolecular polymer network with multiresponsive cross-linkers to fabricate the fluorescent sensor.15 The fluorescent intensity could respond to four types of stimuli, including potassium cation, chloride anion, pH and heating. Liu and co-workers prepared multi-responsive system by integrating several different types of crosslinkers into a single supramolecular assembly.16 Benefited from the dynamic nature of noncovalent interactions, multi-responsive supramolecular materials are easily prepared from diverse sets of starting materials. However, to our knowledge, nearly all the multi-responsive fluorescent supramolecular materials were based on host-guest interactions. It is highly demanded to explore the potential of the other types of noncovalent interactions in fabricating novel multi-responsive fluorescent sensors. Recently, a type of supramolecular ionic material composed from multi-cationic and multi-anionic molecules via ionic interactions has attracted much attention.17,18 Wathier and


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Grinstaff reported the first supramolecular ionic network by using a phosphonium dication and a tetraanion, ethylenediaminetetraacetate.17 Mecerreyes and co-workers developed another type of supramolecular ionic networks using the commercially available small molecules, and therefore provided an attractive method for constructing supramolecular structures. It is known that the ionic interactions are sensitive to several external stimuli, such as monofunctional acids and amines, moisture etc. However, the multi-responsive fluorescent sensors based on supramolecular ionic networks had not been reported yet. It might be because the traditional chromophores are conjugated compounds with rigid structure, whereas the supramolecular ionic networks with rigid molecules are always crystal-like,18 and hardly respond to the external gaseous stimuli actively. 1-(2-aminoethyl)piperazine (AEPZ) is a tertiary amine containing compound that has been widely applied in synthesis of non-conventional fluorescent polymers.19-21 The fluorescence of this kind of polymers is thought to be originated from the tertiary amine groups.22,23 As the relaxation of tertiary amine groups was tightly restricted, the polymers are able to emit strong fluorescence. Therefore, it is reasonably speculated that the AEPZ molecules might be able to function as chromophore in supramolecular ionic materials. Here in this report, we used citric acid and AEPZ as raw material to fabricate a fluorescent supramolecular ionic material by simply mixing the two compounds in aqueous solutions and then drying out. It is interesting that this supramolecular ionic material is able to emit fluorescence without conventional chromophores. Most importantly, the fluorescence intensity of this supramolecular ionic material is actively changeable in responding to humidity, triethylamine, acetic acid and temperature. To the best of our knowledge, it is the first report on a multi-responsive fluorescent sensor based on ionic interactions and composed from non-conjugated compounds.


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EXPERIMENTAL SECTION Materials. Citric acid, triethylamine, and acetic acid were purchased from Sinopharm Reagents Co., Ltd. 1-(2-aminoethyl)piperazine (AEPZ) was purchased from Aladdin Reagents Co., Ltd. All materials were used as received. Characterization. FTIR analysis was performed on Nicolet 67, Thermo Nicolet. Thermal transition was measured on Q2000, TA instruments. UV absorption was tested on UV-2550, Shimazu. WAXD measurements were performed on D/MAX2500V, Rigaku. Fluorescence measurements were performed on F-2700, Hitachi, and Fluorolog Tau-3, Horbita Jobin Yvon equipped with low-temperature measuring system from Janis Research. Preparation of Supramolecular Ionic Material. Typically, 1.98 g (0.01 mol) citric acid and 1.29 g (0.01 mol) AEPZ were placed in a glass vessel and then added in 1 mL deionized water. The solution was vigorously stirred for 30 min at room temperature. The solution of citric acid and AEPZ was coated onto a quartz plate evenly and then dried in a vacuum oven for 24 hours at 60 °C. After that, a thin film of supramolecular ionic material was obtained. Thickness of the obtained film was about 1.0 mm. Experimental Methods. The humidity sensing experiments were done in the following procedure. The thin film of this supramolecular ionic material was fixed on the quartz plate, and then dried at 60 °C for 30 min in oven. The film sample was kept in a desiccator. A drop of deionized water was injected into the bottom of a quartz cuvette at the room temperature. The cuvette was sealed quickly. After reaching equilibrium, the film sample on quartz plate was placed into a quartz cuvette, and then the fluorescence spectrum of the film was recorded once a


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minute. The triethylamine sensing experiments and the acetic acid sensing experiments were done in the similar procedure. RESULTS AND DISCUSSION As shown in Figure 1, AEPZ and citric acid were mixed together in aqueous solutions. After evaporation of water, the desired supramolecular ionic material was obtained. The supramolecular ionic material from AEPZ and citric acid was golden brown in color. It could emit strong blue fluorescence when irradiated by the UV light. The fluorescence was thought to be originated from the tertiary amines on the AEPZ molecules. In the previous reports, it was demonstrated that the fluorescence of tertiary amines could be enhanced as their motion was suppressed.22,23 For example, the tertiary amines being fixed on the branching points can emit stronger fluorescence than those dangling on the side chains. The enhanced fluorescence was attributed to the reduced collisional relaxation. In the supramolecular ionic material, the motion of tertiary amines was greatly suppressed because the AEPZ molecules were tightly immobilized (Figure S5). Subsequently, the supramolecular ionic material from AEPZ and citric acid can emit strong fluorescence.


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Figure 1. Synthesis of supramolecular ionic material from AEPZ and citric acid. Through mixing the AEPZ and citric acid in aqueous solutions, then drying out, the supramolecular ionic material was obtained. The fluorescence spectrum and picture was recorded under UV irradiation of 365 nm.


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Figure 2. FTIR spectrum (a), DSC thermogram (b), WAXD patterns (c) and fluorescent spectrum (d) of supramolecular ionic material composed of AEPZ and citric acid. The Inset in Figure 2b shows the appearance of obtained supramolecular ionic material, which is gold-brown in appearance and feels like dough. The poly(amido amine) in Figure 2c is synthesized from AEPZ and N,N’-bismethyleneacrylamide (MBA) via Michael addition reaction. FTIR, DSC, WAXD were used to characterize the supramolecular ionic material composed from AEPZ and citric acids. In the FTIR spectrum shown in Figure 2a, the broad band at 3416 cm-1 was attributed to the stretching vibration of -OH groups from citric acid. The two bands at 1716 cm-1 and 1577 cm-1 were assigned to hydrogen-bonded carbonyl groups and carboxylate anions, respectively.24 In brief, the FTIR results confirmed the proton transferring reactions, which were essential to the formation of the supramolecular ionic material. Compared


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with the FTIR spectrum of hyperbranched poly(amido amine) (HPAMAM) synthesized from AEPZ and N,N’-bismethyleneacrylamide via Michael addition reaction (Figure S8), there were no bands located at 1640 cm-1 and 1530 cm-1 corresponding to the amide groups, indicating that there was no chemical reaction occurred during preparation of this supermolecular ionic material. This supramolecular ionic material was based on the non-covalent interactions. The DSC thermogram shown in Figure 2b indicated that there was a transition for this supramolecular ionic material. The corresponding temperature was about -8.7 °C, much lower than that of HPAMAM, which was about 13 °C.25 It suggested that this supramolecular ionic material had stronger adaptability than the covalently bonded HPAMAM. WAXD result showed that there was a broad diffraction peak for this supramolecular ionic material, just like that of HPAMAM (Figure 2c). It demonstrated that this supramolecular ionic material had an amorphous structure similar to the HPAMAM. Moreover, the diffraction peak of this supramolecular ionic material was located almost in the same position as that of the HPAMAM, indicating that this supramolecular ionic material shared some structural similarity with the HPAMAM. Fluorescence spectra of this supramolecular ionic material were shown in Figure 2d. Interestingly, the emission peaks progressively shifted to longer wavelengths as the excitation wavelength increased. It was just like that of HPAMAM reported before.19 This supramolecular ionic material not only shared structural similarity with the HPAMAM, but also similar fluorescent properties. Despite that this supramolecular ionic material shares similar structural features and fluorescent property with the HPAMAMs, these two compounds are much different. The supramolecular ionic material from AEPZ and citric acid is based on non-covalent interactions.


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Nevertheless, the HPAMAMs are synthesized through Michael addition polymerization and the repeating units are connected by covalent bonds. Moreover, the ionic interactions are weakened in responding to humidity, acids, amines and temperature. Therefore, the supramolecular ionic material can be multi-responsive because the AEPZ, in addition to the tertiary amines, can retrieve their mobility. However, the covalently bonded HPAMAMs only have limited responsive ability. Furthermore, the supramolecular ionic material can exhibit its responsiveness in solid state, while the HPAMAMs show pH sensitivity only in solutions. Humidity detection is of great importance in power substation, textile, food and medicine, etc.26 However, the reported multi-responsive fluorescent supramolecular materials based on host-guest interactions were not able to detect the humidity.15,16 In this work, the fluorescent supramolecular ionic material from APEZ and citric acid was found to be very sensitive to the humidity. In a typical test, the completely dried thin film of this supramolecular ionic material was placed into a quartz cuvette with a drop of water at the bottom. Fluorescence response of the thin film was recorded as a function of time. The results were shown in Figure 3a. Within 14 min, the normalized fluorescence intensity proportionally dropped 80% and then reached equilibrium. In one of the previous reports, it took about 150 min to reach equilibrium for the hydrogel film containing fluorescent dye.27


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Figure 3. Fluorescence responses of the supramolecular ionic material to different stimuli, (a) humidity; (b) triethylamine; (c) acetic acid; (d) temperature. (Excitation wavelength = 400 nm) Note: to make the diagrams readable, some of the fluorescence records were omitted. For details, please refer to the supporting information. Moreover, the reversibility of fluorescence response of the suparmolecular ionic material was tested. At first, the sample film was dried at 60 °C for 30 min and then the fluorescence emission was quickly recorded. It was the spectrum at “dry” state. After the measurement, the sample film was sealed in a quartz cuvette with a drop of water and equilibrated at room temperature for 30 min. The fluorescence emission of the sample film was then recorded again. It was the spectrum at “wet” state. The tests were repeated 5 times and the results were shown in


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Figure S10. As seen, the fluorescence response to humidity of this supramolecular ionic material was completely recoverable. In each cycle, the normalized fluorescence intensity completely recovered to 1.0 in the “dry” states; while it decreased to ~0.2 in the “wet” states. The responsive ability of this supramolecular ionic material is exciting. In the previous report,28 a quantum-dots based sensor showed a limited response range from 0.75 to 0.55. The real-time detection of volatile organic amines via low-cost strategies is highly demanded in public health, food safety, and environmental protection.29 Meanwhile, the detection of organic acid vapor by simple methods is also preferred.30 However, only a few solid films could simultaneously respond to the volatile amine and acid.31 In this work, it was exciting to find that the fluorescent supramolecular ionic material could respond to volatile organic amine and organic acid vapor at the same time. During the test, a typical volatile organic amine, triethylamine, was chosen as the model compound. The supramolecular ionic material film was sealed in a quartz cuvette with a drop of triethylamine. The test results were shown in Figure 3b. As indicated, the fluorescence intensity decreased 40% at the equilibrium state, better than the previous report.31 The result of organic acid test was shown in Figure 3c. During the test, the acetic acid was used. It could be found that the fluorescence of this supramolecular ionic material film dropped gradually, but the responsiveness was poorer. At equilibrium state, the decrement of fluorescence intensity was about 20%, less than that in amine-sensitivity test, which was about 40%. It might be attributed to that the vapor pressure of acetic acid is lower than that of triethylamine. According to the literatures, the vapor pressure of acetic acid at 25 °C is 15.43 mmHg (2.057 KPa),32 while that for triethylamine at the same temperature is 67.75 mmHg (9.033 KPa).33


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In the previous reports, the temperature responsiveness test of multi-responsive supramolecular ionic materials was inevitably performed in solutions.15,16 There was rare report on supramolecular ionic material in the solid state that could actively respond to the temperature.34 However, this supramolecular ionic material showed sensitivity to temperature in the solid state (Figure 3d). The logarithm of fluorescence intensity increased almost linearly as the temperature decreased from 293 K to 253 K. Below that temperature, the increment of fluorescence intensity slowed down. The increase of fluorescence intensity could be attributed to that the motion of tertiary amines was greatly suppressed as the temperature dropped. Moreover, the dynamic dissociation of electrostatic pairs was also suppressed at low temperatures.35

Figure 4. Demonstration to self-healing ability of the supramolecular ionic material from AEPZ and citric acid. From (a) to (d), the supramolecular ionic material healed at 60 °C within 20 min; from (e) to (g), the supramolecular ionic material healed at room temperature within one hour. The supramolecular materials are naturally self-healable because they are based on the reversible non-covalent interactions.36-41 However, the multi-responsive material with self-


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healing ability was rarely reported. 42 In this work, the self-healing ability of this supramolecular ionic material was tested. As indicated in Figure 4a-d, the scratched sample diminished the scar within 20 min at 60 °C. Moreover, the completely severed sample could be recombined within 1 hour at room temperature, and the healed sample could bear a 20 gram weight when bridging across two supporting poles (Figure 4e-g). The self-healing performance of this supramolecular ionic material was good. The reversible ionic interactions between the AEPZ molecules and citric acids contributed a lot to the self-healing ability of this supramolecular ionic material.18 CONCLUSION In summary, a multi-responsive fluorescent supramolecular ionic material was fabricated from commercially available, non-conjugated small molecules. This supramolecular ionic material is able to emit strong fluorescence under UV irradiation. Interestingly, it can sensitively respond to four types of signals, including humidity, volatile organic amine, organic acid and temperature. Moreover, this multiple stimuli-responsive material is self-healable. To our knowledge, it is the first report on multi-responsive fluorescent supramolecular ionic materials able to detect diverse signals. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ××××/××××. UV-Vis absorption spectrum, Fluorescence spectrum, Optical picture, TGA diagrams, FTIR spectrum and detailed results of responsiveness test (PDF). AUTHOR INFORMATION


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Corresponding Author *Email: [email protected]. Tel & Fax: +86-551-62901450. ORCID Wentao Hao: 0000-0002-3473-0197; Wen Yang: 0000-0003-2531-7533; Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We greatly appreciate the financial support of the Anhui Provincial Natural Science Foundation (No. 1508085ME107) and the National Natural Science Foundation of China (No. 21204016). REFERENCES (1) Wang H.; Ji X.; Li Z.; Huang F. Fluorescent Supramolecular materialic Materials. Adv. Mater. 2017, 29, 1606117. (2) Peng H.-Q.; Sun C.-L.; Niu L.-Y.; Chen Y.-Z.; Wu L.-Z.; Tung C.-H.; Yang Q.-Z. Supramolecular materialic Fluorescent Nanoparticles Based on Quadruple Hydrogen Bonds. Adv. Funct. Mater. 2016, 26, 5483-5489. (3) Ma X. Tian H. Stimuli-Responsive Supramolecular materials in Aqueous Solution. Acc. Chem. Res. 2014, 47, 1971-1981. (4) Wang Y.; Zhou K.; Huang G.; Hensley C.; Huang X.; Ma X.; Zhao T.; Sumer B. D.; De Berardinis R. J.; Gao J. A Nanoparticle-Based Strategy for the Imaging of a Broad Range of


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Multiple Stimuli-Responsive Fluorescent Sensor ... - ACS Publications

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