Self-assembly induced hydrogelation approach as novel means of selective and visual sensing toward picric acid

Applied Surface Science 487 (2019) 473–479

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Self-assembly induced hydrogelation approach as novel means of selective and visual sensing toward picric acid ⁎

Xudong Yu , Jiangbo Guo, Pan Peng, Fengjuan Shen, Yajuan Li, Lijun Geng, Tao Wang

T

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College of Science, Hebei Research Center of Pharmaceutical and Chemical Engineering, Hebei University of Science and Technology, Yuhua Road 70, Shijiazhuang 050080, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydrogel Picric acid Sugar derivatives Isomers Hydrogen bonding

Picric acid is a kind of dangerous and toxic nitroaromatic explosive chemicals and causes great concern in safety, health and environment issues. However, the visual and selective sensing of picric acid in pure water is still a challenge in recent years. In this work, two novel isomeric sugar-based derivatives denoted as NGs (NG1 and NG2) with different terminal groups (pyridyl segments) have been designed and synthesized. We demonstrate the preparation of picric acid assisted supramolecular hydrogels driven by hydrogen bonding interaction, which endows the visual and direct recognition for picric acid via fast and selective gelation approach without expensive equipment. The binding mechanism of NGs with PA both in solutions and gels and the isomeric effect on the gelation properties are studied in detail by several techniques. At last, NG2 based test strips are also developed to detect tiny amount of PA in water in a contact mode.

1. Introduction Picric acid (PA or TNP) is regard as a highly dangerous nitroaromatic explosive chemical and highly toxic environmental substance [1–4]. Therefore, selective detection of PA arises increasing attention in both global security and environmental issue [5]. To date, various materials including metal clusters, MOFs (Metal-Organic Frameworks), carbon nanoparticles, polymers, QDs (Quantum Dots), have been utilized to prepare PA sensors [6–9]. Several techniques are also developed for selective detection of PA such as Raman spectrometry, X-ray diffraction, as well as gas chromatography–mass spectrometry. However, straightforward, selective and visual sensing toward PA without expensive equipment is still a challenge. Fluorescent low-molecular-weight gels constructed by non-covalent interactions have such properties that display a significant response to the external physical and chemical stimuli [10–14]. Therefore, fluorescent assemblies and the resulting gels have been shown very useful sensors for the detection of various analytes via gel formation, gel collapse and gel shrinkage approaches [15–17]. In particular, due to the structural sensitivity and dedicate balance of non-covalent interactions, the analyte assisted gelation approach has the merits of high selectivity and the potential for analyte removal with phase separation. In previous reports, there are several reports that the as prepared gels or xerogels are developed to be good sensing materials toward PA. For

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example, S. Nath et al. have designed tetrazole-based organogels that response to PA with gel-sol transition [18]; Tarasankar Pal's group has reported that the metal-organic hydrogel showed selective color response to PA based on gel-to-precipitate phase transition [19]; T. Yi et al. have demonstrated the coated organogel on test strips that could detect PA in aqueous solution [20]. However, the selective and visual sensing of PA is still a challenge due to the similar electron nature and structure of phenolic analogues. Additionally, these gels are usually formed in organic solvents rather than water, and PA behaves as an additional element. Therefore, PA as a positive factor to assist gel formation is very rare in the literatures, which has potential not only in PA detection but also in the aspect of PA removal from environment. In previous works, we have dedicated to design and synthesis a series of organogels and hydrogels that could sense both chemical and physical stimuli [21–28]. Recently, we have found that the specific hydrogen-bonding interaction between -OH of PA and pyridyl group could direct the spontaneous formation of 3D matrix gel with a longrange ordered pattern in cyclohexane based on cholesterol derivative NPS (naphthalimide-pyridyine-cholesterol) [29]. Inspired by the above works, herein, to acquire the selective and visual PA sensing in pure water solution and further study the host-guest interaction mechanism, two sugar-based naphthalimide derivatives denoted as NGs (NG1 and NG2) with different pyridyl segments are designed and characterized (Scheme 1). Supramolecular assembly of PA and NG1 or NG2 can lead

Corresponding authors. E-mail addresses: [email protected] (X. Yu), [email protected] (T. Wang).

https://doi.org/10.1016/j.apsusc.2019.05.126 Received 3 January 2019; Received in revised form 25 February 2019; Accepted 12 May 2019 Available online 14 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Scheme 1. a) The chemical structures of NG1 and NG2; b) the sensing targets including PA and its congeners; c) the suspension-to-gel transition of NG/PA hybrids in different gelation approach. Note: H-C process, heating-cooling process.

geometry of 25 mm was utilized for the experiments. CD (circular dichroism) data of the hydrogels and solutions were obtained on a MOS450 spectropolarimeter. 1H NMR and 13C NMR spectra were performed on a Bruker Advance DRX 400 spectrometer operating at 500 and 125 MHz respectively. Fluorescence spectra of the solutions, gels, and titrations were acquired on an Edinburgh Instruments FLS-920 spectrometer (excitation source: a Xe lamp). UV–vis absorption spectra were collected on a UV–vis 2550 spectroscope (Shimadzu). Sonication treatment of the suspensions was performed in a KQ-500DB ultrasonic cleaner (maximum power, 100 W, 40 kHz, Kunshang Ultrasound Instrument Co., Ltd., China).

to the formation of 3D matrix hydrogels with red color originated from yellow suspensions, which endows the visual and selective sensing toward PA among tested relevant species. The terminal modification effect of pyridyl segment of NGs on the gelation and responsive properties are also compared and studied. Moreover, the gelation processes and sensing mechanisms in both solutions and gels are investigated in detail. At last, NG2 assembly-based test strips are also developed to visual sense PA in aqueous solutions in aw contact mode. 2. Experimental section 2.1. Materials

3. Results and discussion All organic intermediates were obtained from commercial suppliers and used directly. δ-Gluconolactone was provided from Alfa Aesar. Ethylenediamine, 2-(Methylamino)Pyridine, 4-Bromo-1, 8-naphthalic anhydride, 4-picolylamine, (95%)and other reagents were bought from Shanghai Darui fine chemical Co. Ltd. The organic solvents were purchased from Tianjin Hengxing Chemical Preparation Co. Ltd.

The synthesis procedure and characterization of the isomers including NG1 and NG2 were supplied in supporting information. Our first attempt was to study the fluorescence changes of NG1 or NG2 (diluted solutions) in water upon the addition of PA and its analogues including 2,4-dinitrophenol, 4-nitrophenol, phenol, aniline, 4-nitroaniline, 4-Br-anilne, 2-hydroxybenzaldehyde, 4-hydroxybenzaldehyde, 4-Br-benzaldehyde. In aqueous solution (10−5 mol/L), both NG1 and NG2 exhibited green emission color with maximum emission peak positioned at 537 nm and 544 nm respectively, which were shown in Figs. 1a, S1 and S2. In the presence of PA, 2,4-dinitrophenol or 4-nitrophenol (60 equiv.), significant fluorescent quenching was observed, indicating the quenching effect of nitro group on fluorescence. The observations suggested that NG1 or NG2 was not efficient sensor for selective sensing PA in diluted aqueous solution. To determine the binding ability of NGs with PA, the fluorescent titration experiments were performed. Upon the gradually addition of PA (0–72 equiv.) into the solution of NG1, the fluorescence quenched regularly (Fig. 2), and the job plot experiment revealed the stoichiometric molecular ratio of NG1 and PA was 1:1 (Fig. S3). The binding constant of NG1 with PA in 1:1 stoichiometry was calculated to be 1.71 × 103 M−1 according to our previous literatures (Fig. S4), and the detect limit was

2.2. Equipment SEM images of the powders and xerogels were obtained using SSX550 (Shimadzu) and FE-SEM S-4800 (Hitachi) instruments. Samples were prepared by spinning the gels (For the concentration of NGs: 25 mg/mL, molecular ratio of NG and PA = 1:1) on glass slides followed by coating them with Au. TEM experiments were performed on a JEOL JEM2011 apparatus operating at 200 kV. The samples were prepared by putting the gel on a carbon-coated copper grid and freezedrying it for 48 h. The X-ray diffraction (XRD) experiments of precipitate and xerogels were performed using a Bruker AXS D8 instrument (Cu target; λ = 0.1542 nm). The xerogels were obtained by vacuum freeze-drying of corresponding hydrogels. Rheological measurements of hydrogels were performed on freshly prepared gels using a controlled stress rheometer (Malvern Kinexus Ultra+), and a vertebral plate 474

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was transferred to pyridyl segment when pyridyl segment was bonded with PA through hydrogen bonding interaction, leading to hypochromic effect and fluorescence quenching of NG1. Having seen the sensing effect in diluted solution, we further tested the response of assembling NGs with PA and its analogues at higher concentrations (25 mg/mL). Both NG1 and NG2 did not dissolve in water even at elevated temperature at higher concentrations. Interestingly, in the presence of PA (NG1: 25 mg/mL, molecular ratio of NG1 and PA: from 1:0.6 to 1:2), the NG1 suspension with yellow color transformed to red gel directly at room temperature treated by sonication (< 2 min). Only suspension was observed without PA via different gelation pathways such as heating-cooling or sonication treatment. Moreover, no hydrogel was obtained in the mixtures of NG1 with other PA analogues, as shown in Fig. 3. When the 4-(aminomethyl) pyridyl of terminal group was changed to 2-(aminomethyl)pyridyl, NG2/PA hydrogel (NG2: 25 mg/mL, molecular ratio of NG2 and PA: from 1:1 to 1:3) should be acquired by heating-cooling process, and NG2 also had high gelation selectivity toward PA among the test species (Fig. 3b) via heating-cooling process. It should be noted that no NG2 hydrogel formed by heating-cooling process without PA, whereas, NG2 hydrogel could be also obtained when the heating suspensions (90 °C) was put into thermostat (25 °C) following by sonication, which was called TS-gel. The above results clearly demonstrated that both NG1 and NG2 assembly expressed selectively response toward PA over other test species, which showed obvious color and phase changes (suspension to gel). Considering the high selectivity of NGs toward PA in assembly process, the assembly nature was further investigated by UV–vis and fluorescence experiments. As shown in Fig. 4a, the solution of NG1/PA hybrid exhibited two absorption peaks at 428 and 356 nm, which was rationally ascribed to the ICT (intermolecular charge transfer) process of naphthalimide and PA group respectively. In gel state, the peaks became broader, which was in accordance with the red color of the hydrogel, and such effect might be assigned to the π-π donor-acceptor interactions between intermolecular naphthalimide-PA rings, which was not observed in solutions where single molecule existed [29]. The broad emission peak in the fluorescence spectra centred at 562 nm displayed 25 nm red shift in compared with what was seen in the corresponding solution, certifying the π-π stacking interactions of fluorophore cores (Fig. 4b). SEM images were utilized to trace the phase changes in macroscopic level (Fig. 5). The SEM image of NG1 suspension showed irregular ribbon-like surface in macro scale. In the presence of PA, the NG1/PA hybrid gel was comprised of three-dimensional fibers with pores in nano scale, which was typical structure of low molecular-weight gel. The image of NG2 powder appeared micro sheet structure, while the

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Fig. 1. a) Photos of NG1 solutions (10−5 M) upon the addition of PA and tested congeners. 1) NG1 solution; 2) phenol (Ph); 3) 4-nitro-phenol (4-NPh); 4) 2,4dinitro-phenol (2,4-DN-Ph); 5) aniline (An); 6) 4-nitroaniline (4-N-An); 7) 4-Branilne (4-Br-An); 8) 2-hydroxybenzaldehyde (2-HBA); 9) 4-hydroxybenzaldehyde (4-HBA); 10) 4-Br-benzaldehyde (4-Br-BA); 11) PA. b) The fluorescence intensities of NG1 (10−5 M) and NG1 with tested congeners (60 equiv.).

determined to be 5.2 × 10−8 mol/L [28]. The binding of NG2 with PA gave the similar results both in selectivity and titration experiments (Figs. S5, S6), and the binding constant was calculated to be 1.50 × 103 M−1. In order to further understand the influence of PA on the photophysical properties of NG1, UV–vis spectra NG1 with different amount of PA was measured and the results were shown in Fig. 2b. It was seen that the maximum absorption peak of NG1 at 438 nm gradually blue shifted to 422 nm with the increasing amount of PA. Generally, the ICT process of naphthalimide groups is resulted from the electron push-pull effect from the electron donor of N atom to naphthalic anhydride group as electron acceptor. [30] Herein, the electron

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Fig. 2. a) Fluorescent titrations of NG1 (10−5 M) with PA from 0 to 72 equiv.; b) normalized UV–vis spectra of NG1 and PA with different ratios (concentrations of NG1: 10−5 M), Note: R = mole ratio. 475

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Fig. 3. The gelation photos of NG1 (a, 25 mg/mL) and NG2 (b, 25 mg/mL) in the presence of PA and its analogues (1 equiv. of NG). 1) Phenol; 2) 4-dinitro-phenol; 3) 2,4-dinitro-phenol; 4) aniline; 5) 4-nitroaniline; 6) 4-Br-anilne; 7) 4-hydroxybenzaldehyde; 8) 2-hydroxybenzaldehyde; 9) 4-Br-benzaldehyde; 10) PA.

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Fig. 4. a) UV–vis spectra of NG1/PA hybrid gel (NG1: 25 mg/mL, molar ratio of NG1 and PA was 1:1) and solutions (NG1: 10−5 M, the molar ratio of NG1 and PA was 1:1); b) the corresponding fluorescent spectra of NG1 (10−5 M), NG1/PA hybrid gel and solutions.

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Fig. 5. SEM images of NG suspensions and NG/PA xerogels (NG: 25 mg/mL, molar ratio of NG 1 and PA was 1:1). a) NG1 suspensions; b) NG1/PA hybrid hydrogel; c) NG2 suspensions; d) NG2/PA hybrid hydrogel. Scale bar: a) 3 μm, b) 10 μm, c) 1 μm, d) 4 μm.

corresponding hybrid hydrogel obtained by heating-cooling process showed folded structure. The regular morphologies of these gels suggested the strong interaction between NGs and PA through specific

hydrogen bonding interaction between pyridyl and -OH segments, which had the ability to direct the co-assembly between NGs and PA. CD experiment is an efficient way to study the hierarchical self476

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NG2 and PA. The d value at 2.74 nm was closed to twice as that of NG2/PA molecular length, revealing a dimer structure. Similar structure was also seen in the XRD spectra of NG1/PA hydrogel (d = 3.0 nm). The hydrogen interaction was further certified by 1H NMR experiments. Fig. 7a showed the downfield 1H NMR shifts of protons of both NG1 and NG2 that corresponded to naphthalimide and pyridyl segments. Upon the addition of equimolar amount of PA, the signals of pyridyl segments of NG1 expressed obvious downfield shifts from 7.26 and 8.46 ppm to 8.02 and 8.82 ppm respectively. Whereas, no obvious changes were observed for the protons of naphthalimide. The protons of pyridyl of NG2 also gave similar 1H NMR spectra changes (Fig. 7b). These implied the existence of strong hydrogen bonding interactions between NGs and PA groups, and the pyridyl group was the most reasonable binding site for the hydrogen bonding interaction with PA. The position of N of pyridyl group in NGs not only impacted the assembly properties and gelation pathways as discussed above, but also had high impact on the mechanical properties of the hydrogels. The mechanical properties and isomeric effect of the terminal pyridyl groups on the rheological properties were seen from Fig. 8. From the dynamic strain sweep measurement and time sweep experiment data, the values of G' (elastic storage modulus) were much higher than that of G" (loss modulus) for both NG1/PA and NG2/PA hydrogels, indicating the solid-like property. Notably, the G' of NG1/PA hydrogel with value around 8229 Pa was much greater than that of NG2/PA hydrogel with G' around 662 Pa from the dynamic strain sweep experiments, indicating the stronger interactions between NG1 and PA. On the other hand, the strain for the flowing part (gel to suspension transition) of NG2/PA hydrogel (> 100% strain) was higher than that of NG1/PA hydrogel (6.9% strain), suggesting the “hard” properties of NG1/PA hydrogel and “soft” properties of NG2/PA hydrogel. By employing the TS-gel of NG2, it could be also developed to visually sense PA among test samples by selective gel-to-suspension transition, which was accompanied by significant fluorescence quenching (Fig. 9). It should be noted that the 4-nitrobaniline that bonded NG2 with hydrogen bonding interaction of -NH and -OH was unable to permeate the gel networks, which was rationally ascribe to the stronger hydrogen bonding interactions between self-assembling NG2 molecules than that of NG2 with PA. Moreover, the intake of PA by human body might cause many diseases such as dermatitis, gastritis and liver damage, and it was toxic when the amount of TNT in drinking water was above 2 ppb. The sensing ability with fluorescence off approach of NG assembly made it possible to detect PA in aqueous solution. The test strip of NG2 was prepared by coating the hydrogel on the filter paper and dying it under vacuum to remove water. The aqueous solution with different concentrations was then written to the surface of

NG2 TS-gel NG1/PA hydrogel NG2/PA hydrogel

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assembly constructed by small molecules. When the heating solution of NG2 (25 mg/mL) was treated by sonication, an unstable hydrogel was formed, which was called TS-gel. The effect of PA on the assembly of NGs was studied as shown in Fig. 6. The TS-gel of NG2 had very weak CD signal positioned at 492 nm, which was rational assigned to the sugar induced chirality of naphthalimide-pyridyl group. Conversely, the NG1/PA hybrid hydrogel revealed a wide negative and magnified valley at around 675 nm that represented the sugar induced chiral peak of naphthalimide-pyridyl-PA groups, and was in accordance with the red color of the hydrogel. Unlike that of NG1, the NG2/PA hybrid hydrogel exhibited positive peak at 489 nm. The results implied that the hydrogen bonding interaction between naphthalimide-pyridyl and PA groups not only directed the formation of hydrogels, but also promoted the chirality amplification in the assembly process. To gain further insight into the driving force for the assembly of NGs and PA, XRD and 1H NMR spectra were also examined. The XRD spectra of all samples were shown in Fig. S7. For the aggregates of NG1 and NG2, the peaks observed at 1.67 and 1.37 nm were assigned to the molecule lengths of NG1 and NG2 respectively. The peaks of NG2 TSxerogel centred at 1.51, 0.75, 0.38 nm were exactly in the ratio of 1:1/ 2:1/4, revealing the layer structure of the assembly, and 1.5 nm was rationally assigned to the d value of NG2 with an extended structure. When bonded with PA, the XRD pattern of NGs displayed obvious changes. In the case of NG2/PA hydrogel, a novel lamellar structure with d spacing of 2.74, 1.37, 0.69 (ratio of values: 1:1/2:1/4) was observed, strongly proving the hydrogen bonding interaction between

Fig. 7. 1H NMR (500 MHz) spectra of both NG1 (a) and NG2 (b) in the presence of PA (1 equiv.) in DMSO-d6. 477

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affect the geometry and direction of intermolecular hydrogen bonding interactions, resulting in different molecular self-assembly processes. Notably, the different chiral magnification effect of PA on the NGs in hydrogels is also observed, which might be ascribed to the high breaking on the symmetry of aggregations and directivity of hydrogen bonding. We believe that the visual sensing of PA using a self-assembled fluorescent potential gelator will pave a new way for the design of novel visual sensors and stimuli-responsive soft materials.

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Acknowledgement We thank for the financial support by National Natural Science Foundation of China (21401040, 21301047, 21771051), Natural Science Foundation of Hebei Province (No. B2014208160, B2014208091, B2016208115), Young Talent Plan of Hebei Province and High-level Talent Project of Hebei Province (No. 2016002014), Key Foundation of Hebei Province Department of Education Fund (ZD2016059), Youth Foundation of Hebei Education Department (QN2017072), Excellent Youth Funding of Hebei Province (B2018208112).

Fig. 9. The gel photos of NG2 TS-gel (25 mg/mL) upon the addition of different PA analogues (2 equiv.). From left to right: 1) phenol; 2) 4-dinitro-phenol; 3) 2,4-dinitro-phenol; 4) aniline; 5) 4-nitroaniline; 6) 4-Br-anilne; 7) 4-hydroxybenzaldehyde; 8) 2-hydroxybenzaldehyde; 9) 4-Br-benzaldehyde; 10) PA.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.05.126.

Fig. 10. Photos of gel coated test strips of NG2 TS-hydrogel aggregates toward different concentrations of PA in aqueous solutions in dark (irradiated by 365 nm). For a) 10−6 M, b): c) 10−5 M, for d): 10−4 M.

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4. Conclusion In conclusion, a novel approach for the visual, straightforward and selective recognition of PA among test analogues have been established through controllable gelation pathways from isomeric naphthalimidebased fluorescent derivatives. Such approach and specific hydrogen bonding interaction between pyridyl group and PA may be relevant for designing novel kind of visual PA sensors. It is presented that the hydrogen bonding between naphthalimide derivatives and PA plays a vital role for directing the aggregations of NGs into fibrous networks, leading to formation of 3D matrix hydrogel. In addition, the isomeric effect on the hydrogelation and stimuli-responsive properties is further studied in detail. The results show that the terminal group of pyridyl segments 478

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Xudong Yu is a professor in the College of Science, Hebei University of Science and Technology. He received his doctoral degree from the department of chemistry, Fudan University, China in 2011. His current research interest is focused on the design and assembly of organogel and sensor. Jiangbo Guo graduated in Hebei University of Science and Technology and currently is a master student in the College of Science, Hebei University of Science and Technology. His current research lies in the covalently assembly of multi-functional mesoporous hybrids and metallogels. Pan Peng graduated in Hebei University of Science and Technology and currently is a master student in the College of Science, Hebei University of Science and Technology. His current research lies in the supramolecular chemistry and hydrogels. Fengjuan Shen a professor in the College of Science, Hebei University of Science and Technology. He received his doctoral degree from the department of chemistry, Hebei University, China in 2007. His current research interests include organic synthesis and fluorescent sensors. Yajuan Li is a professor in the College of Science, Hebei University of Science and Technology. She received his doctoral degree from the department of chemistry, Tong Ji University, China in 2011. His current research interest is focused on multi-functional lanthanide mesoporous hybrid materials with photoluminescent and sensing properties. Lijun Geng is a professor in the College of Science, Hebei University of Science and Technology. He received his doctoral degree from the department of chemistry, Hebei University, China in 2004. His current research interests include stimuli-responsive gels, and visual sensors. Tao Wang a professor in the College of Science, Hebei University of Science and Technology. His current research interests include functional organogels and metallogels.

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