Reversible On–Off Luminescence Switching in Self-Healable

Article pubs.acs.org/Langmuir

Reversible On−Off Luminescence Switching in Self-Healable Hydrogels Yalan Yao, Yige Wang, Zhiqiang Li, and Huanrong Li* School of Chemical Engineering and Technology, Hebei University of Technology, GuangRong Dao 8, Hongqiao District, Tianjin 300130, People’s Republic of China S Supporting Information *

ABSTRACT: We present herein an easy way to prepare novel responsive hydrogels by simply doping lanthanide complexes into a polymer hydrogel, poly(2-acrylamido-2-methyl-1propanesulfonicacid) (PAMPSA). The resulting hybrid hydrogels can be readily processed into a range of shapes. Both the on−off luminescence switching and the healable properties are simultaneously achieved in the resulting responsive hybrid hydrogels. They exhibit effectively self-healing performance without any external stimulus and reversible “on−off” luminescence switching triggered by exposure to acid−base vapor. The key to this on−off luminescence switching behavior is that the protonation of the organic ligands compete with full coordination to Ln3+ and that incomplete coordination affects the luminescence yield. The high proton strength in the resulting hydrogels makes the doped lanthanide complexes unstable, and ammonia (or triethylamine) vapor can dramatically decrease the proton strength through neutralization, driving the full coordination of the ligand to Ln3+.

1. INTRODUCTION On−off switching of solid-state luminescence in the presence of suitable guest species or by external stimuli has caused a great amount of concern for fundamental research and practical applications.1−9 Lanthanide complexes are believed to be a promising candidate for developing luminescence switching systems because of their intriguing optical properties, such as sharp and intense emissions in visible and near-infrared regions, long decay times, and large Stokes shift.8−11 Luminescence switching of lanthanide complexes triggered by changes in pH,12,13 microenvironment,14,15 etc.16−18 were reported frequently. However, drawbacks, such as the difficulty of fabricating lanthanide complex film as well as lower thermal and photostabilities, hamper their application as luminescent switching devices. Doping lanthanide complexes in inorganic matrices is an ideal way to overcome these drawbacks. Europium(III) complex-doped zeolite films were reported to display interesting luminescence switching upon exposure to acid−base vapors.19,20 One of the disadvantages of such films is their rigidity, which makes them easily crack during applications. Soft materials, such as metallo-supramolecular polymers, show obvious advantages in optical device applications, owing to their properties, including flexibility and good processability, as compared to inorganic materials. An Eu-based metallo-supramolecular polymer prepared by selfassembly coordination polymerization showing unique vapoluminescence properties triggered by acid−base vapor has been reported by Higuchi et al.21 The luminescence switching system is robust and provides access to display devices of quick © XXXX American Chemical Society

response and good reversibility. However, the multi-step synthesis and purifying of the functional ligand constitute the major challenges for constructing a luminescence switching system by self-assembly coordination polymerization. Vaporresponsive luminescent films have recently been realized by simple doping proton-sensitive lanthanide complexes into poly(vinylpyrrolidone),22 and the luminescence color of the films can be facile tuned by changing the OH− concentration in the films. However, such materials showing luminescence switching property lack the ability of self-healing. Although selfhealable luminescent materials have appeared in the literature,23,24 there is rarely a report about on−off luminescence switching of self-healable materials. Wei et al.25 introduced a unique type of novel supramolecular hybrid hydrogel that undergoes reversible switchable sol−gel transition simultaneously and exhibits excellent self-healing properties. Herein, we describe a facile method to prepare the self-healable vapoluminescent hybrid hydrogels by simple doping lanthanide complexes [Eu(tta)3·2H2O or Tb(sal)3·2H2O, where tta is 2thenoyltrifluoroacetone and sal is salicylic acid] into a selfhealable polymer hydrogel, poly(2-acrylamido-2-methyl-1-propanesulfonicacid) (PAMPSA), and the luminescence of the resulting hybrid hydrogels can be reversibly switched using acid−base vapors. Received: August 19, 2015 Revised: October 28, 2015

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DOI: 10.1021/acs.langmuir.5b03102 Langmuir XXXX, XXX, XXX−XXX

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2. RESULTS AND DISCUSSION The polymer hydrogel PAMPSA was prepared by simply heating AMPSA aqueous solution at 80 °C without the need of initiators and cross-linkers as well as stirring.26 The polymerization of the monomer was verified by proton nuclear magnetic resonance (1H NMR) (Figure S1 of the Supporting Information). The self-healable responsive hybrid hydrogels were immediately achieved when PAMPSA (1 g) was simply mixed with an appropriate amount of lanthanide complexes dissolved in ethanol and the optional amount was 0.2 mol. The resulting hybrid hydrogels are denoted as Eu(tta)3@PAMPSA and Tb(sal)3@PAMPSA, respectively. It has been reported that the sample of PAMPSA possesses flexibility, highly effective self-healing, and excellent mechanical property as a result of the formation of multiple dynamic interchain hydrogen bonding.26 Similar to PAMPSA, after the addition of lanthanide complexes, the lanthanide complex-containing hydrogels, Eu(tta)3@ PAMPSA and Tb(sal)3@PAMPSA, could also be modulated into various shapes, as illustrated in Figure 1a. Both of the

ability of the material. After the addition of ethanol solution of lanthanide complexes, the elongation at break decreases from 1200 to 935 and 958%, for Eu(tta)3@PAMPSA and Tb(sal)3@ PAMPSA, respectively (Figure 2a). Figure 2b shows the stress−

Figure 2. (a) Stress−strain curves of samples and (b) stress−strain curves of the healed samples (after healing for ∼32 h).

strain curves of healed samples. After 32 h of self-healing, for healed PAMPSA, the self-healing properties can be maintained, while the elongation at break (350%) decreases to ∼29% of the original value. After mixing ethanol solution of lanthanide complexes in PAMPSA, the elongation at break of both healed hybrid hydrogels only displays a slightly decrease, with 25% recovery of the original elongation values at break. The decrease of the elongation at break for the lanthanide complex-containing hydrogels compared to pristine PAMPSA shown in Figure 2 can be attributed to the coordination between −SO2O− and the lanthanide ion, which might be detrimental to the formation of the multiple hydrogen bonds between the oxygen atoms of the −SO2O− group and the hydrogen atoms of the amide group. This can be verified by the Fourier transform infrared (FTIR) spectra of the samples (Figure 3). The band at 1650 cm−1 in the spectrum of PAMPSA is assigned to the CO stretching vibration of the amide group (amide I). The amide II band attributed to the

Figure 1. (a) Photographs of flexible self-healing hybrid hydrogels (with a water content of ∼47 wt %) and illustration of the self-healing process of hybrid hydrogels at (b) room temperature, 298 K, and (c) LN2, 77 K.

hybrid hydrogels still exhibit effective self-healing properties. The hybrid hydrogels merged together automatically without any external stimulus after the samples were cut into two pieces and then placed closely together along the cut line for 30 min. The healed hybrid hydrogels could not be forced to split along the dimly visible cut lines (Figure 1b). Moreover, the hybrid hydrogels also showed good self-healing ability upon exposure to liquid nitrogen (LN2, 77 K), although the hybrid hydrogels were somewhat hardened but recovered self-healing properties after volatilization (Figure 1c). The excellent self-healing properties may benefit from the interaction between PAMPSA and lanthanide complexes, such as hydrogen bonding and competitive coordination between lanthanide ions and the −SO2O− group, as well as electrostatic attractions. PAMPSA is readily deprotonated in aqueous solution; therefore, the electrostatic interactions are not negligible, which also contribute to its self-healing properties. A continuous cross-linking gel matrix is evident in the scanning electron microscopy (SEM) micrographs (Figure S2 of the Supporting Information). The hybrid gels of Eu(tta)3@ PAMPSA and Tb(sal)3@PAMPSA present an unordered crosslinking morphology. In addition, tensile tests were performed to evaluate mechanical and self-healing properties of the hybrid hydrogels. Tensile tests were accomplished on the repaired samples after 32 h of self-healing. For original PAMPSA, the elongation at break exceeds 1200%, indicating good stretch-

Figure 3. FTIR spectra of (a) PAMPSA, (b) Eu(tta)3@PAMPSA, and (c) Tb(sal)3@PAMPSA. B

DOI: 10.1021/acs.langmuir.5b03102 Langmuir XXXX, XXX, XXX−XXX

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Figure 4. (a) Photographs of hybrid hydrogels during the exposure to NH3−HCl vapors upon UV light (λex = 365 nm), changes before and after exposure to NH3 vapor in luminescence (b) excitation spectra monitored at 612 nm and (c) emission spectra excited at 380 nm of Eu(tta)3@ PAMPSA (black) and Eu(tta)3@PAMPSA−NH3 (red), changes before and after exposure to NH3 vapor in luminescence (d) excitation spectra monitored at 544 nm and (e) emission spectra excited at 330 nm of Tb(sal)3@PAMPSA (black) and Tb(sal)3@PAMPSA−NH3 (green), and (f) responses of luminescent intensity at 612 nm of Eu(tta)3@PAMPSA during HCl−NH3 exposure cycles.

bending of the N−H bond appears at 1557 cm−1. The bands at 1458 and 1217/1032 cm−1 are assigned to the stretching of the C−N bond (amide III) and asymmetric/symmetric stretching of the −SO2O− group, respectively.27 The coordination of Eu3+ ions to the oxygen atoms in the −SO2O− groups can be verified by the hypochromatic shift of bands at 1217/1032 cm−1 (1217−1240 and 1032−1086 cm−1), as shown in Figure 3b. Upon addition of Tb(sal)3·2H2O to PAMPSA, the shift of the bands corresponding to the −SO2O− group can be observed in the spectrum of Tb(sal)3@PAMPSA and is also due to the coordination between −SO2O− and Tb3+ ions. In addition, the presence of the lanthanide complexes can be straightforwardly detected by the FTIR spectra. After the addition of Eu(tta)3· 2H2O to PAMPSA, the appearance of the band at 1614 cm−1 and the band at 832/558 cm−1 in the spectrum of b is attributed to the CO and −CF3 groups of tta, respectively, which demonstrates the successful formation of Eu(tta)3@ PAMPSA. As with Tb(sal)3@PAMPSA, the band assigned to the symmetric stretching vibration of the carboxylic bond belonging to sal occurs at 1460 cm−1, which demonstrates that we have successfully developed Tb(sal)3@PAMPSA. Both of the hybrid hydrogels exhibit no emissions under ultraviolet (UV) excitation extraordinarily (Figure 4a), which is out of our expectation because Eu(tta)3·2H2O and Tb(sal)3· 2H2O emit bright red and green light when illuminated with UV light (Figure S3 of the Supporting Information). However, interestingly, we found that the photoluminescence of the hybrid hydrogels can be switched on after exposure to base vaporm such as ammonia (NH3). Eu(tta)3@PAMPSA emits a red luminescence, and Tb(sal)3@PAMPSA is also shown to have bright green luminescence, after treatment with NH3 vapor when illuminated with an UV lamp. When these hydrogels are exposed to a HCl-gas-enriched environment for 5 min, the red or green luminescence disappears to the naked

eye. After subsequent exposure of the non-luminescent hydrogels to NH3 vapor for 5 min, the red luminescence is again recovered, as seen by the naked eye (Figure 4a). As seen from panels b−e of Figure 4, before exposure to NH3 vapor, the excitation spectra of both hybrid hydrogels show comparably weak absorption between 200 and 480 nm and no notable characteristic emission peaks are observed in the emission spectra, especially for Tb(sal)3@PAMPSA. The absolute quantum yield for both samples is too low to be determined using an integrating sphere according to the reported procedure.9 In contrast, the excitation spectrum shows a broad band ranging from 250 to 450 nm after treatment with NH3 vapor, which is assigned to the absorption of tta. The emission spectrum of Eu(tta)3@PAMPSA displays five sharp lines, ascribed to the 5D0 → 7FJ transitions of Eu3+ (J = 0, 1, 2, 3, and 4). It is dominated by the 5D0 → 7F2 band at 612 nm, which is responsible for the red emission color shown in Figure 4a. The broad band ranging from 250 to 400 nm in the excitation spectrum of Tb(sal)3@PAMPSA after exposure to NH3 vapor is due to the absorption of sal (Figure 4d). The emission spectrum excited at 330 nm shows four characteristic peaks at 488, 545, 581, and 620 nm after exposure to NH3 vapor, which can be attributed to the 5D4 → 7FJ (J = 6, 5, 4, and 3) transitions of the Tb3+ ion. In addition, the absolute quantum yield of Eu(tta)3@PAMPSA and Tb(sal)3@PAMPSA is increased to 0.23 and 0.17, respectively, upon exposure to NH3 vapor. From the viewpoint of device applications, one of the most important factors is the reversibility. To examine the reversibility, the emission intensity changes at 612 nm of Eu(tta)3@PAMPSA were monitored upon alternate exposure to NH3 vapor and HCl gas over 5 NH3−HCl cycles (Figure 4f). Eu(tta)3@PAMPSA displays good reversibility after 10 times. In addition, the reversible switching with a high contrast can be used for erasable writing, using ammonia as the ink. C

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Langmuir Information can be written on the hybrid hydrogels. The text is readable under UV light. Furthermore, the materials are reusable, because the text can be erased by brief exposure to hydrochloric acid (Figure S4 of the Supporting Information). No luminescence for Eu(tta)3@PAMPSA and Tb(sal)3@ PAMPSA before ammonia vapor treatment can be attributed to the high proton strength arising from the counterion H+ of the sulfonic group in PAMPSA.19,28,29 It is known that protonation of diketonates and carboxylates competes with full coordination to Eu3+ and Tb3+ and that incomplete coordination affects the luminescence yield. Upon exposure to NH3 vapor, the protons in the polymer hydrogel are neutralized by NH3 vapor, driving the full coordination of the ligand to Ln3+ (Figure 5). As a

Figure 6. Self-healing process of hybrid hydrogels after exposure to NH3 vapor under an UV lamp (λex = 365 nm).

Tb(sal)3@PAMPSA−NH3 can also merge together with slight pressing, as shown in Figure 6c. It is noteworthy that the couple hydrogels can be stretched by hand with a gentle force. These results suggest that the hybrid hydrogels after treatment with NH3 vapor also display effective self-healing ability.

3. CONCLUSION In summary, we have demonstrated the reversible and effective on−off luminescence switching in lanthanide complex-containing hydrogels with good self-healing properties and good mechanical properties. These hybrid hydrogels are facile to prepare and can be readily processed into a range of shapes. The formation of multiple dynamic interchain hydrogen bonding in the PAMPSA hydrogel was believed to be responsible for the good ability of healable and mechanical properties.26 The facile changes of the proton strengths by alternatively treating the resulting hydrogels with ammonia and chloride acid vapors are responsible for the effective reversible on−off luminescence switching. We believe that these remarkable features, such as the good self-healability, effective reversible on−off luminescence switching, and good mechanical properties, allow the hybrid hydrogels to find practical usage in fields such as smart devices and sensors.

Figure 5. Luminescence switching mechanism of the hybrid hydrogels.

consequence of this, the characteristic emission color of both hybrid hydrogels is “switched on”. Further treatment with HCl gas increases the acidity of the hydrogel, and the emission color is thus “switched off” again. The proposed mechanism can be further supported by the following observations: (1) The pH of the aqueous solution of PAMPSA (0.1 g/mL) was measured, and a pH ∼ 1 was shown. (2) The solution of lanthanide complexes containing hydrogels displayed an increase in pH from ∼1 to ∼10 after treatments with NH3 vapor and an repeat decrease in pH from ∼10 to ∼1 after exposure to HCl gas (Figure S5 of the Supporting Information). Furthermore, this mechanism can be further supported by FTIR spectra. The CO stretching vibrations of tta at 1614 cm−1 are shifted to ca. 1556 cm−1 when the hybrid hydrogel was treated with NH3 vapor (Figure S6 of the Supporting Information), indicating the effective coordination of the ligand tta to Eu3+ ions. Similar shift occurs in Tb(sal)3@PAMPSA (Figure S7 of the Supporting Information). In addition to NH3 vapor, this luminescence switching is also observed when other basic substances, such as triethylamine (Et3N) vapor, are used (Figure S8 of the Supporting Information). Furthermore, we also examined the self-healing process of NH3-treatment hybrid hydrogels. The test was carried out by cutting both hybrid hydrogels with different shapes into two pieces, and subsequently, the fresh surfaces were placed together through external light pressing. For 30 min, the two pieces healed as a single one through self-healing and no obvious damage to the healed area was observed. The healed hybrid hydrogels were strong enough to carry a 1 kg weight without damage, indicating that the dynamic hydrogen bond was not obviously decomposed by NH3 vapor (panels a and b of Figure 6). In addition, both Eu(tta)3@PAMPSA−NH3 and

4. EXPERIMENTAL SECTION 4.1. Materials. 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) was purchased from Aladdin and used without further purification. The typical lanthanide complexes, including Eu(tta)3· 2H2O and Tb(sal)3·2H2O, were prepared by refs 30 and 31. 4.2. Preparation of Eu(tta)3@PAMPSA and Tb(sal)3@PAMPSA. Specifically, the synthesis of hybrid hydrogels was performed in two steps from commercially available AMPSA, by adding it into the distilled water to obtain a homogeneous solution (50 wt %) (Figure S9a of the Supporting Information) and then heating the solution directly at 80 °C for 25 min to obtain PAMPSA (Figure S9b of the Supporting Information). The resulting lanthanide complex-containing hydrogels are subsequently achieved when doping an ethanol solution of lanthanide complexes [Eu(tta)3·2H2O or Tb(sal)3·2H2O] into PAMPSA evenly and slowly (panels e and f of Figure S9 of the Supporting Information). The hybrid gels were then placed in an oven and maintained at 30 °C for a moment to remove excess ethanol solution. The lanthanide complex-containing hydrogels could be modulated into different shapes. 4.3. Characterization. The 1H NMR spectra were recorded on a Bruker Biospin AG AVANCE400 spectrometer operating at 400 MHz, using D2O as the solvent. SEM has been carried out on a Nova Nano SEM450 at an acceleration voltage of 15 kV. The FTIR spectra of dry hybrid hydrogels were recorded by Bruker Vector 22. The steady-state luminescence spectra was measured on an Edinburgh Instrument FS920P spectrometer, with a 450 W xenon lamp as the steady-state D

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Langmuir excitation source, a double excitation monochromator (1800 lines mm−1), an emission monochromator (600 lines mm−1), and a Hamamatsu RMP928 photomultiplier tube. The absolute quantum yield was determined using an integrating sphere (150 mm diameter, BaSO4 coating) from Edinburgh Instruments according to the reported procedure.9 All spectra were obtained at room temperature. For investigation of the luminescent switching, the hybrid gel blocks were cylindrical of an average radius of 0.5 and 0.2 cm in height. The hybrid hydrogels were exposed to HCl gas and NH3 vapor for 5 min, respectively. For investigation of the self-healable ability, samples were cut into two separate pieces using a scissor and the cut faces were pressed together for 30 min. Tensile properties were determined according to CMT6104. The measurements were carried out at a crosshead speed of 30 mm/min. The dimension of the sample is 25 × 13 × 2.5 mm3. Before the tensile tests, the healed samples were placed at room temperature for ∼32 h to reach a steady state. Before pH measurements, the samples must be dispersing in distilled water to obtain a homogeneous solution and the values of pH can be measured by the pH test paper.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03102. Additional experimental details and figures as described in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-22-60203674. Fax: 86-22-60204294. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Key Basic Research Program (2012CB626804), the National Natural Science Foundation of China (20901022, 21171046, 21271060, and 21236001), the Tianjin Natural Science Foundation (13JCYBJC18400), the Natural Science Founda tion of Hebei Province (B2013202243), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1059), and the Educational Committee of Hebei Province (2011141 and LJRC021) is gratefully acknowledged.

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