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Eco-Friendly Solid-State Upconversion Hydrogel With Thermoresponsive Feature as the Temperature Indicator Changqing Ye, Jinsuo Ma, Shuoran Chen, Jie Ge, Wenting Yang, Qi Zheng, Xiaomei Wang, Zuoqin Liang, and Yuyang Zhou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06504 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 28, 2017

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The Journal of Physical Chemistry

Eco-Friendly Solid-State Upconversion Hydrogel with Thermoresponsive Feature as the Temperature Indicator Changqing Ye, Jinsuo Ma, Shuoran Chen, Jie Ge, Wenting Yang, Qi Zheng, Xiaomei Wang*, Zuoqin Liang, Yuyang Zhou Research Center for Green Printing Nanophotonic Materials, Institute of Chemistry, Biology and Materials Engineering, Suzhou University of Science and Technology, Suzhou 215009, China. E-mail: [email protected] KEYWORDS: upconversion; hydrogel; thermoresponsive ABSTRACT: By loading microemulsion containing both sensitizer and emitter into the treated eco-friendly hydrogel, a new solid-state material of triplet-triplet annihilation based upconversion (TTA-UC) material was firstly reported. This UC hydrogel has shown an excellent stable emission of the green-to-blue luminescence, owing to the honeycomb-like nanoporous structure of the hydrogel. Moreover, this emission of UC hydrogel was strongly depended on the temperature. The UC fluorescence intensity is close to the linear growth from 30oC to 60oC. This thermoresponsive upconversion hydrogel, which could be easily molded into various shapes, has provided a new perspective in manufacturing novel temperature-indicator and as well other applications in stabilizing air-sensitive species in aerated system.

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Introduction Upconversion (UC), an anti-stokes emission process which converts low-energy photons into high-energy photons, has attracted great attention for its potential applications in photocatalysis,16

photovoltaics,7 organic electroluminescent,8,9 biological imaging10 and photodynamic therapy11.

Conventional UC techniques based upon two-photon absorption12 and second-harmonic generation,13 suffer from some fundamental drawbacks,14 such as requirement of considerably high excitation intensities (MW·cm-2) and low UC quantum yield. Triplet-triplet annihilation based upconversion (TTA-UC) has been widely researched owing to its low excitation power density, readily tunable excitation emission wavelength, strong absorption ability and high UC quantum yield.15,16 TTA-UC system mainly consists of triplet sensitizer and acceptor. Upon excitation, the triplet sensitizer will achieve its singlet state, followed by intersystem crossing process to reach the triplet state. Then the energy will be transferred to the acceptor via triplet–triplet energy transfer (TTET). Two triplet state acceptors will annihilate by collision and populate the singlet excited state. This acceptor at singlet excited state could go back to the ground state with UC fluorescence.17,18 As we know, the triplet excited state of sensitizer and acceptor can be easily quenched by oxygen, resulting in a fairly low UC efficiency or even none. To avoid triplet state quenching, TTA-UC has been studied mostly in deoxygenated organic solvents.19-24 As a consequence, TTAUC shows certain restrictions for practical application in the ambient aqueous phase. For this reason, efficient TTA-UC in the ambient, oxygen-rich environment has aroused great attention and been explored by many researchers. For example, sensitizers and acceptors were embedded into rubbery polymers film with low glass transition temperatures, such as EO-EPI or the


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polyurethanes.25-27 Besides, some rigid polymer film and porous silica particles have been applied as TTA-UC mediums as well.28,29 Moreover, efficient TTA-UC emission under airsaturated condition was achieved in some supramolecular organogel nanofibers with the adaptive and cooperative accumulation of different donor−acceptor upconversion pairs.30 And core-shell microspheres, in which sensitizers and acceptors were encapsulated, could also produce upconversion luminescence in oxygen-rich environment.31-33 Upconversion efficiency in these mediums was actually lower than that in organic solvents. Therefore, a general strategy of preparing high-efficiency TTA-UC system in ambient air is necessary. For example, Duan et. al. have developed a solvent-free liquid photon UC system, by introducing branched alkyl chains to both of the donor and acceptor molecules, which ensured their good miscibility and insusceptibility of upconversion luminescence to oxygen (Φuc=1.5).30 Sripathyan et. al. have reported a gelated photochemical upconversion material comprised of quasi-solid organogel of DMDBS, which has an upconversion efficiency of 0.22.34 We have transformed an oil-in-water (O/W) microemulsion containing a bimolecular UC system doped with Palladium (II) tetraphenylporphyrin and 9,10-dinaphthylanthracene (i.e. upconverted O/W microemulsion) into a treated eco-friendly sodium polyacrylate (PAAS) hydrogel with honeycomb-like structure. PAAS is a kind of super-absorbent intelligent polymer material, which is promising for practical applications such as environmental protection,35 hygienic products36 and others. PAAS could be swelled to hydrogel in water and could be reusable after filtration because of its insolubility in either water or conventional organic solvents. Here we demonstrated that the O/W UC microemulsion could be easily absorbed in this treated honeycomb-like PAAS owing to the efficient interconnected network structure among the pores in this treated PAAS, and then turned into a solid-state UC hydrogel. This UC hydrogel could be easily molded into


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various shapes, such as cuboid or ball, which is promising in manufacturing various kinds of complicated optical components. Moreover, it was found out that this emission of UC hydrogel was strongly depended on its temperature. All these features have shown a great potential application in novel optical temperature-indicator as well. Experimental Section Materials. All reagents and solvents were used as received without further purification. 9,10diphenylanthracene (DPA) was purchased from Aldrich. Palladium (II) tetraphenylporphyrin (PdTPP) was synthesized as previously reported. Acrylic acid, potassium persulfate, N,Nmethylene bisacrylamide, Tween-20 was purchased from Aldrich or Acros Chemical Co. Toluene, benzonitrile, ethanol and chloroform were analytical reagent. Deionized water was used in the experiments throughout. Preparation of UC O/W microemulsion. Mixed solutions of PdTPP (0.12 mM) and DPA (36 mM) in toluene were prepared prior to use. 19 mL deionized water was added into a flask, then the flask was vacuumed and back-filled with nitrogen for several times and degassing for 45 min. After that, 9 mL Tween-20 was inserted into the flask via syringe, followed by 2 mL PdTPP/DPA UC solution. The mixed solution was stirred under nitrogen atmosphere to form a homogeneous microemulsion. Finally, a pink microemulsion was obtained and sealed for further experiment. Preparation and measurement of porous PAAS. PAAS (sodium polyacrylate) absorbent was synthesized with PIT (Phase Inversion Temperature) method. Sodium hydroxide (2.32 g) was completely dissolved in 30 mL deionized water. Acrylic acid (5 mL) was added slowly with an ice-water bath. Then potassium peroxydisulfate (KPS, 0.015 g) was added as the initiator,


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followed by a cross-linking agent N, N'-methylene-bis-(acrylamide) (BIS, 0.0013 g). The mixture was magnetic stirred at 70oC and kept reacting for 3h. After the polymerization was finished, a raw PAAS was obtained as a transparent elastic solid. Specific alcohol leaching treatment to form the honeycomb-like porous structure was as follow: raw PAAS absorbent resin (1 g) was put into 500 mL deionized water and soaked until the resin fully swelled to hydrogel. Then absolute ethanol was poured in quantity and stirred rapidly. After standing for 20 minutes, the hydrogel began to dehydrate and some white flocculus appeared on surface. The water was poured out and large amounts of absolute ethanol was added again. This process was repeated a couple of times until the hydrogel was entirely turned into white solid by draining away almost all the water from it. The porous PAAS was dried and kept sealed before further experiments and measurements. Water absorbent rate (µ) is the volume of absorbed water per unit time at unit mass of the resin. The prepared dry resins were put into deionized water and soaked until fully swelling. It was calculated with the following equation, where △V represents the volume of water absorbed by the resin, m is the mass of the resin, t is the time.

(1) In order to obtain the morphologies of hydrogels, the swelled PAAS hydrogel was prefrozen in the liquid nitrogen, then was freeze-dried into a test sample for the micro morphology characterization. A field-emission scanning electron microscope (FEI Quanta 250) was used, after sputtering the samples with a thin layer of gold.


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Preparation and measurement of UC hydrogel. The porous PAAS was put into the prepared O/W UC microemulsion and soaked about 48 hours. The microemulsion was well absorbed and the porous PAAS was fully swelling to UC hydrogel. The hydrogel was kept in the atmospheric environment before further characterization and measurements. The UV-visible absorption spectra were recorded on a Hitachi U-3500 recording spectrophotometer. Steady-state fluorescence and experiments were measured on an Edinburgh FLS 920 fluorophotometer equipped with a time-correlated single-photon counting (TCSPC) card. The excitation source for the upconversion was a diode-pumped solid-state continuous laser (532 nm). The power of the laser was measured with photodiode detector and the diameter of the 532 nm laser spot was approximately 3 mm. For the upconversion experiments, the samples were tested immediately in air atmosphere. The upconverted fluorescence was observed with an UVvis spectrometer (PG2000 Pro, Ideaoptics Technology Ltd., China). A 532 nm optical filter was used to get rid of the scattered laser peak. The upconversion emission photographs were taken by a Sony digital camera. TTA-UC efficiency (ΦUC) can be calculated by Eq. (2), where subscript ‘‘s’’ and ‘‘r’’ represent sample and reference (Rhodamine B in EtOH was used as standard, ΦF = 65%), respectively. Φr is the fluorescence quantum yield of reference, A is the absorbance, F is the integrated fluorescence intensity and η is the refractive index of solvents. The equation is multiplied by a factor of 2, accounting for the fact that two absorbed photons are required to produce one upconverted photon.

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 A  F  η  ΦUC = 2Φr  r  s  s   As   Fr  ηr 


(2)

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Results and Discussion PdTPP was chosen as the sensitizer which showed Soret absorption band at 412 nm and Qband at 521 nm. Upon irradiation of Q-band, PdTPP emitted phosphoresces around 650 nm under nitrogen atmosphere (Figure 1a). Since triplet excited state energy matched with PdTPP, DPA was appropriate to be the emitter for the TTA-UC system. In toluene, DPA gave a maximal absorption at 374 nm, which showed no absorption at 532 nm. The intense blue fluorescence was observed when DPA was excited (Figure 1b).

Figure 1. Normalized excitation and emission spectra of (a) PdTPP and (b) DPA in toluene. Firstly, mixed solutions of PdTPP (0.12 mM) and DPA (36 mM) were prepared. The UC microemulsion liquid was obtained with toluene as oil phase dispersed in water (Figure 2a). Then the PAAS hydrogel prepared by solution polymerization showed high swelling ability in


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the presence of hydrophilic groups. Water absorbency of the PAAS is about 450 mL/g, which showed an excellent absorption performance. Figure 2b presented a transparent hydrogel fully absorbed with water. However, the raw PAAS hydrogel can hardly swell in the viscous microemulsion, unlike in purified water. Therefore, the key point of the further treatment of the raw PAAS to make it easier to absorb the microemulsion was essential. The microstructures of the PAAS before and after the treatment were compared under scanning electron microscopy. SEM images in Figure 2d,e showed a flow fold surface of the raw PAAS with some large and small pits scattered on it. But there was no inner porous structure in nano or micro scale. As a contrast, after the alcohol leaching process, the raw PAAS hydrogel has a three-dimensional cross-linked, porous, honeycomb-like structure (Figure 2f,g). It’s noteworthy that water could be removed from hydrogel effectively to form an inner porous structure of hydrogel, when one solvent that could both be miscible with water and not destroy any structure of the hydrogel was added in the hydrogel. This porous, honeycomb-like structure was resulting from the washing process by ethanol which could remove the low molecular weight polymers. The treated PAAS with porous, honeycomb-like structure showed excellent absorption performance. The water absorbent rate (125 mL/h) is almost twice larger than the raw PAAS hydrogel (75 mL/h). Meanwhile, the treated hydrogel can absorb the microemulsion at the absorbent rate about 11 mL/h, while the raw PAAS can barely absorb the microemulsion. Since the hydrogel’s swelling and shrinking capacity are mainly dependent on the capillary force, the diffusion rate of microemulsion in hydrogel developed significantly larger than in the polymer network. What’s more, the honeycomb-like interconnection among the pores could form a reticular network and shorter diffusion paths, which could facilitate the absorption of the viscous microemulsion.


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Figure 2. (a) Digital photo of O/W UC microemulsion; (b) digital photo of transparent untreated hydrogel fully absorbed with water; (c) digital photo of solid-state UC hydrogel; (d,e) The SEM images showed the untreated raw PAAS hydrogel with a flow fold surface, on which some large and small pits scattered; (f,g) The SEM images showed the treated PAAS with porous, honeycomb-like structure; Scale bar: (d,f) 50 µm; (e,g) 40 µm. Hereby, we successfully modify a conventional PAAS into a honeycomb-like nanoporous, super-absorbent polymer by a very simple way. This honeycomb-like PAAS has proved to be an excellent candidate material as the solid carrier in TTA-UC, which can efficiently swell the O/W UC microemulsion and transform into a solid-state UC hydrogel (Figure 2c). Moreover, the hydrogel could be easily molded into various shapes, such as ball (Figure 3). With the diode pumped solid-state laser (λex = 532 nm, 50 mW/cm2) as excitation light source, the sensitizer (PdTPP) was excited and achieved at its triplet excited state because of the heavy-atom effect of palladium (Pd), then the triplet excited state of acceptor (DPA) was populated via TTET. Finally


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the upconverted fluorescence of DPA could be produced via TTA. Therefore, the green-to-blue upconverted emission around 457 nm was successfully observed in air atmosphere through a 532 nm filter (Figure 3b). It can also be observed that the UC microemulsion was swelled and distributed homogeneously in this honeycomb-like PAAS. The upconverted emission was uniform, when the laser was irradiated at different part of the hydrogel (Figure S2).

Figure 3. (a) The UC hydrogel could be easily molded into various shapes like balls. (b) With the diode pumped solid-state laser (λex = 532 nm, 50 mW/cm2) as excitation light source, the green to blue upconverted emission at around 457 nm was successfully observed in air atmosphere through a notch filter at 532 nm, 60oC.


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Figure 4. (a) The spectra of UC hydrogel at different temperatures from 30oC to 60oC in air atmosphere. The excitation source was a diode laser at 532 nm and 50 mW/cm2. (b) The integrated area of the spectra peak represented the total energy of the UC emission (400-525 nm) and the phosphorescence (650-850 nm) were compared at different temperatures. Amazingly, we further found out that this emission of UC hydrogel was strongly depended on its temperature. The luminescence spectra of the UC hydrogel at various temperatures from 30oC to 60oC were carefully studied in Figure 4a. It was shown that the UC emission peak raised as well as the temperature increased, while the phosphorescence peak of the sensitizer was descending. For further study of the relationship between the emission intensity and temperature, the integrated emission intensities of UC fluorescence band (400-525 nm) and phosphorescence band (650-850 nm) were plotted as a function of temperature (Figure 4b). The UC fluorescence


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intensity is close to the linear growth from 30oC to 60oC, which were also in accordance with the the fitting equation (IUC=0.3478T-9.111).

Figure 5. (a) The upconversion intensity of the UC hydrogel in the heating−cooling cycles (λex=532nm; the integrated UC intensity represented the total energy of the spectra peak from 400 nm to 525 nm). (b) Digital photo of the emission of UC hydrogel at 30oC (orange red) and 60oC (blue). (c) The temperature-dependent upconversion efficiency of the UC hydrogel in air (532 nm, 50 mW/cm-2).


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This thermoresponsive UC emission can be repeated reversibly between 30oC and 60oC for many cycles, suggesting the reversibility of the UC hydrogel (Figure 5a). As the temperature rose, the red phosphorescence faded and the blue upconversion fluorescence showed again, which showed a promising application as the temperature-indicator. The luminance hue changes of the hydrogel between blue and red could be observed by the naked eyes during cooling process (Figure 5b). Besides, the upconversion efficiency of the hydrogel was also dependent on the temperature. The upconversion efficiency could rise to 5.6% at 60oC (Figure 5c). This upconversion emission was surprisingly stable. The upconversion efficiency could remained almost unchanged under exposure to air for more than 25 days (Figure S3). It is to note that such stable upconversion efficiency is not observable for the PdTPP/DPA pair in aerated DMF solution, in the absence of the PAAS hydrogel. These observations indicate that the quenching effect by dissolved molecular oxygen is effectively prevented by the PAAS hydrogel. This interesting thermoresponsiveness feature of the UC hydrogel owed to the character of the O/W UC microemulsion. The nonionic surfactant used as the emulsifier in the O/W microemulsion could be reversibly converted into a W/O (water-in-oil) microemulsion. At first, the nanoscale droplets of oil containing PdTPP/DPA pair could be dispersed in a water-rich continuous phase at room temperature. Since the upconversion is diffusion controlled, the molecular collision was limited in aqueous phase which led to inefficiency triplet energy transfer. Consequently, strong stokes emission from 3PdTPP phosphorescence at 700-850 nm was observed, and the upconversion emission was not much high at 30oC. As the temperature increased, the microemulsion converted into the state of the nanoscale droplets of water dispersing in an oil-rich continuous phase, in which the soluble upconversion pairs (PdTPP/DPA) could freely collide with each other. Then the TTET process became efficient, and


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strong anti-Stokes emission from 1DPA at 400-525 nm could also be obtained at low power density (50 mW/cm2). However, this increase also had a limit. When the temperature increased over 60oC, the swelled hydrogel may be dehydrated slowly to a white solid and the transmittance of hydrogel reduced. Therefore, the emission intensity was sharply reduced when the temperature exceeded 60oC. To prove the TTA-UC mechanism, dependence on the power density of incident light was investigated. Figure 6a shows steady-state luminescence spectra of the UC hydrogel obtained at varied incident laser power. With increasing incident excitation power from 12.31 mW/cm2 to 64.84 mW/cm2, a rapid enhancement of the upconversion fluorescent could be observed. TTAUC threshold excitation intensity Ith is another important parameter characterizing the TTA-UC processes.30 In general, TTA-UC shows a quadratic incident light power dependence of emission intensity in the weak-annihilation limit, which consequently turns into first-order dependence in the saturated annihilation regime.37,38 The Ith value is experimentally determined as the intersection point of these two lines. Figure 6b presents a double logarithm plot for the upconverted emission intensity of the hydrogel as a function of incident light power density (λex = 532 nm). The dash lines in Figure 6b were the fitting results at different power density regimes. At the lower-power regime under Ith, a slope of 2.02 was observed, whereas it changed to 1.19 (near to 1) at higher-power density. It provided decisive experimental evidence for TTAbased photon UC in this hydrogel system. It is noteworthy that the crossover threshold was observed at a relatively low power density around 23.98 mW/cm2, which is comparably lower than those reported for the solvent-free polymer systems.30,39,40


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Figure 6. (a) Steady-state luminescence spectra of the UC hydrogel obtained at varied incident laser power (from 12.31 mW/cm2 to 64.84 mW/cm2), 60oC. A notch filter at 532 nm was used to remove the scattered incident light. (b) A double logarithm plot for the UC emission intensity of the hydrogel as a function of incident light power density (λex=532 nm). The dash lines were the fitting results at different power density regimes. The dependence of upconversion intensity on laser power density switching from quadratic to linear provided further decisive experimental evidence for strong TTA-based upconversion of this hydrogel system.


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Conclusions In summary, a new solid-state material of triplet-triplet annihilation based upconversion was developed by loading both sensitizer and emitter into the PAAS hydrogel with three-dimensional porous honeycomb-like structure. This eco-friendly soft hydrogel has endowed this solid-state UC material an excellent forming property, which could be easily molded into various shapes. This UC hydrogel containing PdTPP/DPA microemulsion can populate the green-to-blue upconversion luminescence in the atmospheric environment, which has shown a good upconversion luminescence stability. Moreover, this emission of UC hydrogel was strongly depended on the temperature. The upconversion fluorescence intensity is close to the linear growth from 30oC to 60oC, which were also in accordance with the decreased phosphorescence intensity. Therefore, this temperature-sensitive hydrogel has great potential in manufacturing some novel temperature-indicator applications. We believe that this new strategy for TTA-UC materials will greatly facilitate the promising applications upconversion materials in some practical optical devices. Moreover, it also provides a new perspective in environmentally friendly hydrogel research because it not only offers a useful and versatile methodology to develop photon-upconverting soft materials but also a surprising potential of blocking molecular oxygen. ASSOCIATED CONTENT Supporting Information The molecular structures, photos of the upconversion hydrogel, time-dependent upconversion efficiency and the stress-strain curve of the upconversion hydrogel were shown. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

ACKNOWLEDGMENTS The author would like to acknowledge Prof. Weibang Lv and Prof. Xiaohua Zhuang from the Center for Nanocomposite Materials, Suzhou Institute of Nano-tech and Nano-bionics, CAS for their kindly help in the mechanical measurement. The authors are grateful to National Natural Science Foundation of China (51303122, 51273141, 51673143, 51603141), Natural Science Foundation of Jiangsu Province- Excellent Youth Foundation (BK20170065), Natural Science Foundation of Jiangsu Province (BK20130262, BK20160358, BK20150285), Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (17KJA430016), Qing Lan Project, Excellent Innovation Team in Science and Technology of Education Department of Jiangsu Province, the Priority Academic Program Development of Education Department of Jiangsu Province (PAPD) and Collaborative Innovation Center for the financial supports. REFERENCES (1)

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