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Near-Infrared Light-Responsive Poly(N-isopropylacrylamide)/ Graphene Oxide Nanocomposite Hydrogels with Ultrahigh Tensibility Kun Shi, Zhuang Liu, Yun-Yan Wei, Wei Wang, Xiao-Jie Ju, Rui Xie, and Liang-Yin Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08609 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 25, 2015

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ACS Applied Materials & Interfaces

Near-Infrared Light-Responsive Poly(Nisopropylacrylamide)/Graphene Oxide Nanocomposite Hydrogels with Ultrahigh Tensibility Kun Shi,† Zhuang Liu,*,† Yun-Yan Wei,† Wei Wang,† Xiao-Jie Ju,† Rui Xie,† and Liang-Yin Chu*,†,‡ †

School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R.

China ‡

State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu,

Sichuan 610065, P. R. China KEYWORDS Hydrogels; Graphene oxide; Stimuli-responsive materials; Near-infrared light-response; Ultrahigh tensibility

1 Environment ACS Paragon Plus


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ABSTRACT Novel near-infrared (NIR) light-responsive poly(N-isopropylacrylamide)/graphene oxide (PNIPAM-GO) nanocomposite hydrogels with ultrahigh tensibility are prepared by incorporating sparse chemical crosslinking of small molecules with physical crosslinking of graphene oxide (GO) nanosheets. Combination of the GO nanosheets and thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) polymeric networks provides the hydrogels with excellent NIR light-responsive property.

The ultrahigh tensibility of PNIPAM-GO

nanocomposite hydrogels is achieved by simply using very low concentration of N,N'methylenebisacrylamide (BIS) molecules as chemical crosslinkers to generate relatively homogeneous structure with flexible long polymer chains and rare chemically-crosslinked dense clusters. Moreover, the oxidized groups of GO nanosheets enable to form hydrogen bond interaction with the amide groups of PNIPAM chains, which could physically crosslink the PNIPAM chains to increase the toughness of the hydrogel networks. The prepared PNIPAM-GO nanocomposite hydrogels with ultrahigh tensibility exhibit rapid, reversible and repeatable NIR light-responsive property, which are highly promising for fabricating remote light-controlled devices, smart actuators and artificial muscles, and so on.


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INTRODUCTION Smart hydrogels are three-dimensional networks composed of crosslinked hydrophilic polymer chains that can dramatically change their volume or other properties in response to various external stimuli such as temperature,1,2 light,3,4 pH,5,6 electric-field7,8 or certain chemicals.9-11 Due to such stimuli-responsiveness, they show remarkable potential for many applications, such as smart sensors/actuators,12-14 “on/off” switches,15-17 drug delivery vehicles,6,18,19

artificial

muscles,4,20,21

tissue

engineering

scaffolds,19,22

and

chem/bioseparation platforms.17,23 Among the stimuli-responsive hydrogels, near-infrared (NIR) light-responsive hydrogels are of particular interests, because NIR light stimulus can be easily and remotely controlled with adjustable intensity and wavelength, as well as used as rapid and precise "on/off" trigger, as compared with other stimuli.24 Moreover, NIR light enables to penetrate human tissues well without harm.25 NIR light-responsive hydrogels allowing NIR light-induced swelling/shrinking,26,27 or bending/unbending,28,29 which create opportunities for transforming NIR light signal into mechanical motion, show great prospects in many applications, such as smart microvalves,30,31 smart actuators,32,33 cell scaffolds,27 and controlled release systems.34,35

Generally, excellent mechanical properties such as high

tensibility are indispensable in many practical applications such as NIR light controlled microvalves, artificial muscles and actuators.

Therefore, development of NIR light-

responsive smart hydrogels with high tensibility is of great interests and significant importance. Typically, NIR light-responsive hydrogels are fabricated by combining photo-thermal

inorganic components such as graphene nanosheets,27,30,33 gold nanoparticles,35,36 carbon nanotubes32 and ferroferric oxide (Fe3O4) nanoparticles,37 with thermo-responsive polymers such as poly(N-isopropylacrylamide) (PNIPAM). These strategies can effectively produce the NIR light-responsive property of smart hydrogels. However, most of the hydrogels are



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featured with poor tensibility because of the heterogeneous polymeric networks, which contain a amount of crosslinked dense clusters that resulted from the highly chemical crosslinking by small molecules.38 To improve the mechanical properties of the hydrogels, several strategies have been developed to prepare robust hydrogels with high tensibility, such as nanocomposite hydrogels,39,40 double-network hydrogels,41,42 slide-ring hydrogels,43,44 Tetra-PEG hydrogels and hydrophobic modified hydrogels.45-47

All the strategies can

significantly improve the mechanical properties including the tensibility of hydrogels, but they cannot help to offer the NIR light-responsive properties due to the absence of lighresponsive domains.

Recently, NIR light-responsive smart hydrogels with both high

tensibility (~900%) and NIR light-responsive ability have been reported using clay nanoparticles as crosslinkers and graphene oxide (GO) as photo-thermal heat source.48,49 However, the physical crosslinking of such hydrogels networked by clay nanoparticles is unstable and even can be destroyed in certain environments. For example, the hydrogen bonds between PNIPAM chains and clay nanoparticles in the clay-crosslinked hydrogels become too weak to hold the crosslinked networks in polar solvents,50 such as dimethylsulfoxide (DMSO) solution (as shown in Figure S1, Supporting Information). Alternatively, the inorganic clay nanoparticles as crosslinkers can be decomposed when immersed in hydrofluoric acid (HF), resulting in the damage of the clay-based hydrogel networks.51 Therefore, it still remains a challenge to fabricate chemically stable NIR lightresponsive smart hydrogels with high tensibility. In this study, we report on a novel type of NIR light-responsive hydrogels with both ultrahigh tensibility and excellent chemical stability, which are prepared by incorporating sparse chemical crosslinking of small molecules with physical crosslinking of GO nanosheets. Without any inorganic clay crosslinkers, our hydrogels are extremely stable in both polar solvent and strong acid.

Combination of PNIPAM with GO nanosheets provides the


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hydrogels with excellent NIR light-responsive property. The ultrahigh tensibility of the proposed NIR light-responsive hydrogels is achieved by using N,N'-methylenebisacrylamide (BIS) molecules as chemical crosslinkers with very low concentration to avoid the formation of highly-crosslinked dense clusters.

Meanwhile, GO nanosheets can also physically

crosslink the PNIPAM chains by hydrogen bonds and enable effectively dissipate the strain energy in the deformation process of hydrogels. The proposed PNIPAM-GO nanocomposite hydrogels with excellent NIR light-responsive property and ultrahigh tensibility are highly promising for many applications, such as responsive valves, smart actuators and artificial muscles.

EXPERIMENTAL SECTION Materials N-isopropylacrylamide (NIPAM, purchased from Sigma-Aldrich) was purified by recrystallization. N,N’-methylene-bis-acrylamide (BIS), potassium persulfate (KPS, K2S2O8) and N,N,N’,N’-tetramethyl-ethylenediamine (TEMED) were all purchased from Chengdu Kelong Chemicals and used without further purification. Raw graphite (325 mesh), used to synthesize graphene oxide (GO), was purchased from XF NANO Ltd.. All other chemicals used for synthesizing graphite oxide were of analytical grade and used without further treatment. Pure water (18.2 MΩ at 25 °C) from a Milli-Q Plus water purification system (Millipore) was used throughout the experiments.

Synthesis of GO GO was synthesized according to the Hummers method.52 Firstly, for pretreatment, graphite (3 g), K2S2O8 (2.5 g) and P2O5 (2.5 g) were added to concentrated H2SO4 (12 mL) in the flask. The mixture was heated in oil bath (80 oC) with stirring for 4.5 h. After cooling to room



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temperature, the mixture was diluted with pure water (500 mL) and stirred overnight. Filtering and washing were carried out together to remove excess acid. Then, the cake layer was dried until there was no moisture. Secondly, for oxidization, KMnO4 (15 g) was added extremely slowly to concentrated H2SO4 (120 mL) with the treated graphite powder, and the temperature of the mixture was kept under 20 oC all the time. The resulting mixture was heated at 35 oC in oil bath and stirred for 2 h. After that, it was then diluted with pure water (250 mL) in ice bath and stirred for 2 h. The mixture was further diluted with pure water (700 mL), and H2O2 (30%, 20 mL) was added gradually until the mixture turned yellow. The mixture was then washed with large amounts of deionized water and HCl solution (1 L, 10%) by centrifugation and dialyzed against pure water for one week with refreshing water twice a day. Finally, GO suspension was obtained by the treatment of ultrasonication.

Preparation of Hydrogels PNIPAM-GO nanocomposite hydrogels were prepared with NIPAM as monomer, GO nanosheets as additives, BIS as chemical crosslinker, KPS as initiator and TEMED as an accelerator at 20 oC. Typically, NIPAM (1.695 g, 15 mmol), BIS (0.231 mg, 0.0015 mmol) and KPS (0.027 g) was dissolved in the GO suspension (10 mL) of desired concentration at 0 o

C. Next, TEMED (40 µL) was added to the reaction solution, and then the solution was

treated by sonication in an ice bath for 3 min. Subsequently, the resulting mixture was transferred into a glass tube and kept for polymerization at 20 oC for 24 h. In the experiments, the GO concentration in the mixture solution was typically 1.0, 2.0, 3.0 and 4.0 mg mL-1. The as-prepared PNIPAM-GO nanocomposite hydrogels were designated as ‘GOx-y’, in which ‘x’ representing the mass concentration of GO (mg mL-1), and ‘y’ indicating the percentage molar ratio of BIS to NIPAM in the hydrogel preparation (%). For example, a code of GO2-0.01 meaned that the PNIPAM-GO hydrogel was prepared with the


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concentration of GO nanosheets being 2 mg mL-1 and the molar ratio of BIS to NIPAM being 0.01%.

To investigate the effect of BIS concentration, PNIPAM-GO nanocomposite

hydrogels with different BIS concentrations were also prepared according to the abovementioned method and coded as GO2-1, GO2-0.1 and GO2-0.001 respectively. GO0-0.01 and GO0-1 hydrogels were also prepared as the control samples by the above-mentioned method but without any GO nanosheets.

SEM and TEM Characterization of Hydrogels The microstructures of freeze-dried hydrogels were observed by field emission scanning electron microscope (FESEM, JSM-7500F, JEOL). To prepare freeze-dried samples, the swollen hydrogels in deionized water were rapidly frozen in liquid nitrogen for 15 min, and then freeze-dried by a freeze drier (FD-1C-50, Beijing BoYiKang) at -48 oC for 48 h. Transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN) was employed to study the dispersion morphology of GO nanosheets in the PNIPAM-GO nanocomposite hydrogels. To prepare the sample for TEM observation, ultra-thin lamellas of GO2-0.01 hydrogels were first cut with cryosection system (Leica FC6), and then collected on copper grids and subsequently dried at room temperature.

Mechanical Property Tests The tensile mechanical property of the as-prepared hydrogels were measured by a tensile machine (EZ-LX, Shimadzu) at 25 oC. The dimension of the rod-like samples was 5 mm in diameter and 30 mm in length. The gauge length in the tensile tests was 10 mm and a stretch rate of 100 mm min-1 was used. The water contents of the hydrogels for tensile property tests were all about 85 %. The tensile stress was defined as the force applied on the deformed hydrogel divided by the real-time characteristic cross-sectional area of the deformed hydrogel.



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The Young’s modulus (E) of the hydrogel at a certain tensile strain was calculated by the slope of the stress-strain curve at the corresponding point. Dynamic rheological tests of the as-prepared hydrogels were performed on a rheometer (TA-DHR-2) using the clamp plates of 25 mm diameter at 25 oC. The as-prepared slab hydrogel samples with 2 mm thickness were under compression with a constant distance of 1 mm during measurement to ensure that the hydrogels were always under compression. Frequency-sweep tests at constant shear strain (γ = 0.001) were carried out over the frequency range 0.01 to 100 rad sec-1.

Thermo-Responsive Property Tests The hydrogel samples washed and swollen by excess of pure water for over one week were cut into discs with thickness of 5 mm. The thermo-responsive equilibrium volume change behaviors of the hydrogel discs were recorded in the range from 15 to 45 oC by a digital camera.

Before each measurement, the hydrogel samples had been kept at each

predetermined temperature for 8 h to ensure reaching the equilibrium state. To measure the dynamic thermo-responsive deswelling behaviors of hydrogels, the hydrogel discs swollen in cool water at 25 oC were transferred abruptly into hot water at 55 oC, and their volume were recorded by a digital camera every 5 min. At least three specimens were measured to obtain statistic data for each thermo-responsive test. The equilibrium swelling ratios are defined as VT/V45 = (dT/d45)3, where VT and V45 represent the equilibrium volumes of hydrogels at T oC and 45 oC respectively; dT and d45 are the diameters of hydrogels at T oC and 45 oC respectively. The dynamic volume-deswelling ratios are defined as Vt/V0 = (dt/d0)3, where Vt and V0 represent the volumes of hydrogels at time t and at beginning, respectively; dt and d0 are the diameters of hydrogels at time t and at beginning, respectively.


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NIR Light-Responsive Property Tests The laser system (DS2-11312-110) used for NIR light-responsive tests generated NIR light with wave-length of 808 nm. To investigate the temperature and volume change of the hydrogels responding to NIR light, the swollen hydrogel discs with thickness of 4 mm were exposed to NIR light at power density of 0.58 W cm-2 in air at initial temperature of 20 oC. The temperature change of hydrogel discs was recorded in real time by an infrared camera (E40, Flir system), and the average temperature of the hydrogel discs was calculated by the matched software (FLIR tools+). At the same time, a digital camera was used to record the dimension change of the hydrogel samples every 2 min. To investigate the NIR light-responsive reversibility of the proposed PNIPAM-GO nanocomposite hydrogels, the swollen GO3-0.01 hydrogel discs with thickness of 4 mm were exposed under NIR light (0.58 W cm-2) in air at 20 oC for 0.5 h, and reswelled in pure water at 20 oC for 24 h repeatedly. The dimension change of the hydrogel discs was recorded by a digital camera from the second shrinking-swelling cycle. The dynamic volume change ratios are all defined as Vt/V0 = (dt/d0)3, in which Vt and V0 are the volumes of hydrogels at time t and at beginning (t=0) respectively.

Test of Remote NIR Light-Controlled Property of Hydrogel Switch To investigate the NIR remote controlling ability of the PNIPAM-GO nanocomposite hydrogels, the hydrogel disc was applied as a smart electrical switch that controlled by the NIR light. The hydrogel electrical switch was made by inserting two parallel steel needles (200 µm in diameter) into the GO4-0.01 hydrogel disc. The steel needles and the aluminium sheets were kept separated at the initial state. During the process of NIR remote controlling on the electric circuit, the DC regulated power supply with output voltage of 4 V was turned on, and then the hydrogel disc was exposed under NIR light. The process of NIR remote



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controlling on the hydrogel switch and the the real-time current value on the display of the power supply was recorded by a digital camera.

RESULTS AND DISCUSSION Fabrication Strategy The fabrication strategy of the proposed PNIPAM-GO nanocomposite hydrogel is schematically illustrated in Figure 1.

By free radical polymerization (Figure 1a,b), the

PNIPAM chains of the as-prepared hydrogels are chemically crosslinked via the covalent bonds created by quite small quantity of BIS molecules, and physically crosslinked based on the hydrogen bond interactions between the oxygen-containing groups on the GO nanosheets and amide groups of PNIPAM chains.38,49 Due to the chemical crosslinking interaction between polymer chains, the proposed PNIPAM-GO nanocomposite hydrogels are strongly stable in strong acid or polar solvent, such as in HF or DMSO solution (Figure S2, Supporting Information). Such PNIPAM-GO nanocomposite hydrogels exhibit ultrahigh tensibility due to their homogenous polymeric networks with rare chemically-crosslinked dense clusters. With such an internal architecture, the applied load on the hydrogel could be distributed over a large fraction of long PNIPAM chains with uniform strain distribution. Furthermore, the strain can also be dissipated by the GO nanosheets since the nanosheets could twirl to parallel to the strain axis in the polymeric networks during elongation (Figure 1c).53 Because graphene can absorb NIR light and convert it into heat with high efficiency,54 the PNIPAM-based nanocomposite hydrogels containing GO nanosheets as NIR lightinduced heat sources possess excellent NIR light-responsive performance. Thus, the phase transition of the proposed PNIPAM-GO nanocomposite hydrogel can be controlled remotely by NIR light irradiation, and it is completely reversible via turning NIR light on and off (Figure 1d).


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Morphological Analyses of PNIPAM-GO Nanocomposite Hydrogels Scanning electron microscope (SEM) micrographs show that the microstructures of freezedried PNIPAM-GO hydrogels are significantly dependent on the content of GO nanosheets and the degree of chemical crosslinking (Figure 2). The micropores within the freeze-dried PNIPAM-GO hydrogels result from the ice crystals formed in the corresponding swollen hydrogels acting as templates for pore generation.55 For the GO0-0.01 hydrogel prepared without any GO nanosheets, because of the long PNIPAM chains between the rare chemically-crosslinked points, water inside the swollen GO0-0.01 hydrogel exists in a interconnected state at the microcosmic level, resulting in mesh-like structures inside the GO0-0.01 hydrogel (Figure 2a).

With the addition of GO nanosheets, the number of

interconnected pores of hydrogels obviously decreases, and the PNIPAM-GO nanocomposite hydrogels with larger GO content show less interconnected pores in the network structures (Figure 2b-e). The results indicate that the GO nanosheets, which are dispersed well in the hydrogel networks (see a typical TEM image in Figure S3, Supporting Information), participate in the formation of the network structures via forming physical interaction force with PNIPAM chains. Compared with GO2-0.01 hydrogel (Figure 2c), the GO2-1 hydrogel with much higher degree of chemical crosslinking exhibits a honeycomb-like structure with nearly dense cell walls (Figure 2f), due to the high crosslinking density and the short PNIPAM chains.

Highly Tensible Characteristics of PNIPAM-GO Nanocomposite Hydrogels The PNIPAM-GO nanocomposite hydrogels with low degree of chemical crosslinking exhibit extraordinary mechanical properties (Figure 3). Conventional PNIPAM hydrogels (GO0-1) are too brittle to sustain compression (Figure 3a), slicing with a blade (Figure 3b), or



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elongation (Figure 3c) (Supplementary Movie S1).

However, the PNIPAM-GO

nanocomposite hydrogels with low degree of chemical crosslinking are highly stretchable, behaving like rubbers. For example, GO3-0.01 hydrogel could withstand high compression (Figure 3d) and slicing with a blade (Figure 3e) without any damage or breakage, which also could recover after unloading the stress at room temperature (Supplementary Movie S2). Compared with conventional PNIPAM hydrogels without GO nanosheets, the prepared GO21 hydrogels with high degree of chemical crosslinking are also failed to sustain elongation (Figure 3f). However, the GO2-0.01 hydrogel with low degree of chemical crosslinking exhibits ultrahigh tensibility to against extensive stretching even after knotting (Figure 3g,h, Supplementary Movie S3). Because the GO2-1 hydrogel are chemically crosslinked with high degree, a large amount of chemically-crosslinked micro-areas act as stress-failure points in the hydrogel networks.38 For the proposed PNIPAM-GO nanocomposite hydrogels with low degree of chemical crosslinking, flexible linear PNIPAM bridge chains connecting the rare crosslinked micro-areas are assumed to have relatively uniform and large length. The flexible PNIPAM chains could exhibit a natural coil state if no load is applied and be elongated under stress until they become straight. The stress can be dissipated effectively to the large number of flexible long PNIPAM chains. Meanwhile, the GO nanosheets enable to dissipate the strain energy via twirling to parallel to the strain axis in the polymeric network during elongation, because of the two-dimension structure of GO nanosheets and the strong hydrogen bond interaction between GO and PNIPAM chains.53,56 So, the PNIPAM-GO nanocomposite hydrogels with low degree of chemical crosslinking exhibit ultrahigh tensibility. Further, the mechanical tensile stress experiments of PNIPAM-GO nanocomposite hydrogels with different degrees of chemical crosslinking and different contents of GO nanosheets are performed by using a tensile machine (Figure 4). The rod-like hydrogel


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samples with 5 mm in diameter and 30 mm in length are fixed by clamps of the tensile machine, and the stretch rate is 100 mm min-1. The nanocomposite hydrogel is ultrahighly tensile; as a result, the characteristic dimension of the cross-sectional area becomes much smaller than the initial one after the hydrogel deformation.

Therefore, to avoid large

deviation, the tensile stress is defined as the force applied on the deformed hydrogel (F) divided by the real-time characteristic cross-sectional area of the deformed hydrogel (A), where, A = A0×L0/L from the fact of volume constant, and A0 is the initial cross-sectional area of the hydrogel, L0 is the initial gauge length of the hydrogel, and L is the real-time gauge length of the deformed hydrogel. The tensile strain is real-timely recorded by the tensile machine.

Because lower degree of chemical crosslinking in hydrogels results in rarer

crosslinked dense microclusters and longer linear PNIPAM chains, the PNIPAM-GO nanocomposite hydrogels become more tensible with decreasing the degree of chemical crosslinking (Figure 4a). The elongation ratio at break of the GO2-0.01 hydrogels can be as large as 2800%.

When the degree of chemical crosslinking is reduced to 0.001%, the

elongation ratio at break of the GO2-0.001 hydrogels is 3800%, but the Young’s modulus and breaking strength of GO2-0.001 hydrogel are quite small. Because the chemical crosslinkers are too less to tie the PNIPAM chains effectively, the hydrogel networks are very soft. With physical crosslinking formed by the interaction between GO nanosheets and PNIPAM chains, the tensile stress of the GOx-0.01 hydrogels are much higher than that of the GO0-0.01 hydrogels without GO nanosheets (Figure 4b). For the GOx-0.01 hydrogels with different contents of GO nanosheets at the tensile strain of 1000%, the Young’s modulus increases with increasing the GO content (Figure S4), which implies that the GO nanosheets acting as physical crosslinkers in these hydrogels can effectively toughen the GOx-0.01 hydrogels by dissipating the strain energy in the polymeric networks. Moreover, the elastic modulus G′ values of GOx-0.01 hydrogels are always larger than the viscous modulus G″ values over the



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entire frequency range (Figure S5), which confirm the formation of the PNIPAM-based hydrogels even with very low concentration of chemical crosslinkers. With increasing the GO content, the elastic modulus G′ of the hydrogel increases at low frequency region, which also confirms that the GO nanosheets have a mechanical enhancement on the PNIPAM-GO nanocomposite hydrogels. The deformation of the prepared hydrogels is almost completely reversible, which supports that the PNIPAM-GO nanocomposite hydrogels have good elasticity (Figure S6). However, when increasing the content of GO nanosheets, the fracture strain of hydrogels decreases (Figure 4b). It has been reported that the enhancement of the hydrogen bonds between GO nanosheets and PNIPAM chains, which increases the crosslinking density in hydrogel networks, results in reduce of the extensibility of hydrogels.49 Even so, the GO4-0.01 hydrogel with the highest GO content in this work still has ultrahigh tensibility (~1900%), which is even much larger than that of the graphene-clayPNIPAM hydrogels (typically < 1100%).49

The results show that the PNIPAM-GO

nanocomposite hydrogels designed and fabricated with our strategy (Figure 1) are featured with ultrahigh tensibility.

Responsive Properties of PNIPAM-GO Nanocomposite Hydrogels The PNIPAM hydrogel and PNIPAM-GO nanocomposite hydrogels show significant thermoresponsive characteristics (Figure 5). Particularly, when chemically crosslinked with low degree, the hydrogels exhibit outstanding responsive swelling ratio (Figure 5a,b) and rapid response rate (Figure 5c) in responding to environmental temperature change.

All the

PNIPAM-base hydrogels with or without GO nanosheets have the volume phase transition temperature (VPTT) at ~32 oC (Figure 5b), which implies that the addition of GO nanosheets does not influence the VPTT.

For the GO0-0.01 hydrogel, the thermo-responsive

equilibrium-swelling ratio in responding to temperature changing from 45 oC to 15 oC is as


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high as 6900% (Figure 5a,b). It is well known that the thermo-responsive equilibrium swelling ratio of hydrogels increases with decreasing the crosslinkage.57 When the degree of chemical crosslinking is fixed as 0.01%, with increasing the content of GO nanosheets, the physical crosslinkage of PNIPAM-GO nanocomposite hydrogels increases; as a result, the thermo-responsive equilibrium swelling ratio of PNIPAM-GO nanocomposite hydrogel decreases (Figure 5a,b). Compared with that of GO2-0.01 hydrogel, the thermo-responsive equilibrium-swelling ratio of GO2-1 hydrogel significantly decreases due to the remarkable increase of chemical crosslinkage. The PNIPAM-based hydrogels with or without GO nanosheets at low degree of chemical crosslinking have much more rapid thermo-responsive rate than that of the GO2-1 hydrogel with high degree of chemical crosslinking (Figure 5c). The flexible linear long PNIPAM chains in the networks shrink very fast because few crosslinked restriction exists for their response.

Compared with the GO0-0.01 hydrogel, all the GOx-0.01 PNIPAM-GO

nanocomposite hydrogels exhibit slightly slower response rate to the temperature (Figure 5c), because the hydrogen bond interactions between GO nanosheets and PNIPAM polymer chains restrict the PNIPAM chains to shrink flexibly to certain degrees. With increasing the content of GO nanosheets, the thermo-responsive rate of PNIPAM-GO nanocomposite hydrogels decreases firstly and then increases, which may be attributed to the heterogeneous microstructure of hydrogels caused by high content of GO nanosheets.57,58 Taking the GO0-0.01 hydrogel as a control sample, the PNIPAM-GO nanocomposite hydrogels with chemical crosslinking degree of 0.01% show excellent NIR light-responsive performances (Figure 6). It is well known that GO nanosheets have high absorbance of the NIR light and could convert it into heat effectively.54

Therefore, the PNIPAM-GO

nanocomposite hydrogels combining GO nanosheets and thermo-responsive PNIPAM networks exhibit excellent NIR light-responsive properties. As a control sample, the GO0-



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0.01 hydrogel without GO nanosheets does not respond to NIR light with the optical power density of 0.58 W cm-2, exhibiting no volume change within 20 min (Figure 6a). While, the PNIPAM-GO nanocomposite hydrogels with chemical crosslinking degree of 0.01% have significant volume decrease exposed under NIR light (Figure 6b-f). The GO nanosheets in the polymeric networks absorb the NIR light and convert it into heat, which causes the local temperature increase of the nanocomposite hydrogels (Figure 6b-e).

As a result, the

PNIPAM chains respond to the temperature increase across the VPTT and the nanocomposite hydrogels shrink significantly to extrude the water in the polymeric networks (Figure 6f). The heat exchange occurs at the edge of the hydrogel with the extruded water, and thus results in the uneven distribution of temperature in the area of the hydrogel sample (Figure 6b-e). Consequently, the average temperatures of the hydrogels at different times marked with dashed circles in Figure 6 are measureed and shown in Figure 7. When the PNIPAMGO nanocomposite hydrogels are exposed under NIR light, the temperatures of them can increase across the VPTT (32 oC) within different time periods. The more the content of GO nanosheets in the hydrogel network is, the faster the temperature increment rate. For example, the temperature of GO4-0.01 hydrogel increases to about 49 oC after exposed under NIR light within 4 min, and it is faster than that of GO3-0.01 hydrogel, which increases to about 41 oC within 4 min. While, the temperature of the GO0-0.01 hydrogel is always less than 25 oC even irradiated by NIR light for 20 min (Figure 7a). All the hydrogels exposed under NIR light enable to eventually reach a equilibrium temperature at last, due to the balance between the photo-thermal effect of GO nanosheets and the heat dissipation from the hydrogels to the external environment. So, the GO0-0.01 hydrogel does not have any volume change under NIR light, because the temperature is always below the VPTT. However, the PNIPAM-GO hydrogels exhibit significant volume change under NIR light, because the temperatures increase to above the VPTT. With larger content of GO nanosheets, the PNIPAM-GO


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nanocomposite hydrogels exhibit faster volume shrinkage rate (Figure 7b), because the larger content of GO nanosheets in hydrogels can more effectively and rapidly generate photothermal heat for accelerating the hydrogel shrinkage.

Such NIR light-induced volume

shrinkage of the PNIPAM-GO nanocomposite hydrogels is not only significant and rapid, but also reversible and repeatable. Repeatability of NIR light-responsive function is investigated by repeatedly exposing the GO3-0.01 hydrogel under NIR light in air and replacing it in water at 20 oC for cycles (Figure 8). Once exposed under NIR light in air, the GO3-0.01 hydrogel exhibits a rapid shrinking response behavior; when replaced in water at 20 oC without NIR light irradiation, the hydrogel slowly reswells to the equilibrium swollen state within 24 h. The results show that the NIR light-responsive behaviors of the PNIPAM-GO nanocomposite hydrogels are reversible and reproducible.

Application Demonstration of PNIPAM-GO Nanocomposite Hydrogels Due to ultrahigh tensible and excellent NIR light-responsive properties, the proposed PNIPAM-GO nanocomposite hydrogels are highly promising for developing soft systems in the soft robotic fields, such as smart actuators, artificial muscles, remote light-controlled devices and so on. As a demonstration, the PNIPAM-GO nanocomposite hydrogel is applied as a remote controlled switch (Figure 9).

To suppress interference in the test, the

nanocomposite hydrogels are demonstrated as nonconductor (Figure S7), which are attributed to the uniform dispersion and discontinuity of GO nanosheets in the hydrogel networks. A GO4-0.01 hydrogel disc inserted with two parallel steel needles is designed as an actuator for remote controlled switch of an electric circuit for the nanocomposite hydrogels are unconductive both in swollen and shrunken states (Figure S7). When the NIR light is turned off, the two parallel steel needles are out of touch with the aluminium sheets in the electric circuit (Figure 9a1). So, the electric circuit as well as the light-emitting diode (LED) light is



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turned off (Figure 9a2). Once exposed under the NIR light, the GO4-0.01 hydrogel disc shrinks rapidly to allow the two parallel steel needles contacting with the aluminium sheets in the electric circuit, resulting in the suddenly increased conductivity of the electric circuit (Figure S8), and thus the LED light is turned on immediately (Figure 9b, Supplementary Movie S4).

CONCLUSION In summary, a novel NIR light-responsive PNIPAM-GO nanocomposite hydrogel with ultrahigh tensibility has been successfully developed. The excellent NIR light-responsive property, which is obtained from the combination of thermo-responsive PNIPAM with GO nanosheets, is rapid, reversible and repeatable. The ultrahigh tensibility of the proposed NIR light-responsive hydrogels is achieved by incorporating sparse chemical crosslinking of small molecules with physical crosslinking of GO nanosheets. The sparse chemical crosslinking of small molecules ensures strong chemical stability and ultrahigh tensibility of the hydrogels; while the physical crosslinking of GO nanosheets further increases the toughness of the nanocomposite hydrogel networks. Such a combination of NIR light-responsive property, large responsive swelling ratio, rapid response rate and ultrahigh tensibility, along with an easy method of fabrication, makes the proposed PNIPAM-GO nanocomposite hydrogels ideal candidates in many applications, such as remote light-controlled devices, smart actuators and artificial muscles.

ASSOCIATED CONTENT Supporting Information The results about the chemical stability of PNIPAM-clay, PNIPAM-clay-GO nanocomposite hydrogels and PNIPAM-GO nanocomposite hydrogels; TEM image, Young’s modulus,


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rheological behavior, reversible tensile behavior, and electrical conductivity of the PNIPAMGO nanocomposite hydrogels; and conductivity variation of the electrical circuit when the NIR light-responsive PNIPAM-GO nanocomposite hydrogels act as swith; Movies of mechanical properties of GO0-1 and GO3-0.01 hydrogels; Movie of the tensibility of GO2-1 and GO2-0.01 hydrogels; Movie of electrical switch controlled by near-infrared light. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (L.-Y. Chu); [email protected] (Z. Liu). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge support from the National Natural Science Foundation of China (21490582, 21322605) and the State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).

REFERENCES (1) Hu, Z. B.; Zhang, X. M.; Li, Y. Control of Protein-Ligand Recognition Using a StimuliResponsive Polymer. Science 1995, 269, 472-474. (2) Kim, Y. S.; Liu, M. J.; Ishida, Y.; Ebina, Y.; Osada, M.; Sasaki, T.; Hikima, T.; Takata, M.; Aida, T. Thermoresponsive Actuation Enabled by Permittivity Switching in an Electrostatically Anisotropic Hydrogel. Nat. Mater. 2015, DOI: 10.1038/NMAT4363.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(3) Kloxin, A. M.; Kasko, A. M.; Salinas, C. N.; Anseth, K. S. Photodegradable Hydrogels for Dynamic Tuning of Physical and Chemical Properties. Science 2009, 324, 59-63. (4) Takashima, Y.; Hatanaka, S.; Otsubo, M.; Nakahata, M.; Kakuta, T.; Hashidzume, A.; Yamaguchi, H.; Harada, A. Expansion-Contraction of Photoresponsive Artificial Muscle Regulated by Host-Guest Interactions. Nat. Commun. 2012, 3, 1270-1277. (5) Angelos, S.; Yang, Y. W.; Patel, K.; Stoddart, J. F., Zink, J. I. pH-Responsive Supramolecular Nanovalves Based on Cucurbit[6] uril Pseudorotaxanes. Angew. Chem. Int. Ed. 2008, 47, 2222-2226. (6) Zhang, S. Y.; Bellinger, A. M.; Glettig, D. L.; Barman, R.; Lee, Y. A. L.; Zhu, J. H.; Cleveland, C.; Montgomery, V. A.; Gu, L.; Nash, L. D.; Maitland, D. J.; Langer, R.; Traverso, G. A pH-Responsive Supramolecular Polymer Gel as an Enteric Elastomer for Use in Gastric Devices. Nat. Mater. 2015, DOI: 10.1038/NMAT4355. (7) Osada, Y.; Okuzaki, H. & Hori, H. A Polymer Gel with Electrically Driven Motility. Nature 1992, 355, 242-244. (8) Yang, C.; Wang, W.; Yao, C.; Xie, R.; Ju, X. J.; Liu, Z.; Chu, L. Y. Hydrogel Walkers with Electro-Driven Motility for Cargo Transport. Sci. Rep. 2015, 5, 13622-13631. (9) Miyata, T.; Asami, N.; Uragami, T. A Reversibly Antigen-Responsive Hydrogel. Nature 1999, 399, 766-769. (10) Liu, Z.; Liu, L.; Ju, X. J.; Xie, R.; Zhang, B.; Chu, L. Y. K+-Recognition Capsules with Squirting Release Mechanisms. Chem. Commun. 2011, 47, 12283-12285. (11) Ikeda, M.; Tanida, T.; Yoshii, T.; Kurotani, K.; Onogi, S.; Urayama, K.; Hamachi, I. Installing Logic-Gate Responses to a Variety of Biological Substances in Supramolecular Hydrogel-Enzyme Hybrids. Nat. Chem. 2014, 6, 511-518.


Page 20 of 36

Page 21 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Sidorenko, A.; Krupenkin, T.; Taylor, A.; Fratzl, P.; Aizenberg, J. Reversible Switching of Hydrogel-Actuated Nanostructures into Complex Micropatterns. Science 2007, 315, 487-490. (13) Ma, M. M.; Guo, L.; Anderson, D. G.; Langer, R. Bio-Inspired Polymer Composite Actuator and Generator Driven by Water Gradients. Science 2013, 339, 186-189. (14) Yao, C.; Liu, Z.; Yang, C.; Wang, W.; Ju, X. J.; Xie, R.; Chu, L. Y. Poly(Nisopropylacrylamide)-Clay Nanocomposite Hydrogels with Responsive Bending Property as Temperature-Controlled Manipulators. Adv. Funct. Mater. 2015, 25, 29802991. (15) He, X.; Aizenberg, M.; Kuksenok, O.; Zarzar, L. D.; Shastri, A.; Balazs, A. C.; Aizenberg, J. Synthetic Homeostatic Materials with Chemo-Mechano-Chemical SelfRegulation. Nature 2012, 487, 214-218. (16) Kumacheva, E. The Catalytic Curtsey. Nat. Mater. 2012, 11, 665-666. (17) Shastri, A.; McGregor, L. M.; Liu, Ya.; Harris, V.; Nan, H.; Mujica, M.; Vasquez, Y.; Bhattacharya, A.; Ma, Y.; Aizenberg, M.; Kuksenok, O.; Balazs, A. C.; Aizenberg, J.; He, X. An Aptamer-Functionalized Chemomechanically Modulated Biomolecule Catchand-Release System. Nat. Chem. 2015, 7, 447-454. (18) Liu, L.; Wang, W.; Ju, X. J.; Xie, R.; Chu, L. Y. Smart Thermo-Triggered Squirting Capsules for Nanoparticle Delivery. Soft Matter 2010, 6, 3759-3763. (19) Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; Müller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Emerging Applications of Stimuli-Responsive Polymer Materials. Nat. Mater. 2010, 9, 101-113. (20) Islam, M. R.; Li, X.; Smyth, K.; Serpe, M. J. Polymer-Based Muscle Expansion and Contraction. Angew. Chem. Int. Ed. 2013, 52, 10330-10333.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(21) Iamsaard, S.; Aßhoff, S. J.; Matt, B.; Kudernac, T.; Cornelissen, J. J. L. M.; Fletcher, S. P.; Katsonis, N. Conversion of Light into Macroscopic Helical Motion. Nat. Chem. 2014, 6, 229-335. (22) Seliktar, D. Designing Cell-Compatible Hydrogels for Biomedical Applications. Science 2012, 336, 1124-1128. (23) Nagase, K.; Kobayashi, J.; Okano, T. Temperature-Responsive Intelligent Interfaces for Biomolecular Separation and Cell Sheet Engineering. J. R. Soc. Interface 2009, 6, S293S309. (24) Huang, X. H.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods. J. Am. Chem. Soc. 2006, 128, 2115-2120. (25) Weissleder, R. A Clearer Vision for in Vivo Imaging. Nat. Biotechnol. 2001, 19, 316317. (26) Charati, M. B.; Lee, I.; Hribar, K. C.; Burdick, J. A. Light-Sensitive Polypeptide Hydrogel and Nanorod Composites. Small 2010, 6, 1608-1611. (27) Li, W.; Wang, J. S.; Ren, J. S.; Qu, X. G. 3D Graphene Oxide-Polymer Hydrogel: NearInfrared Light-Triggered Active Scaffold for Reversible Cell Capture and On-Demand Release. Adv. Mater. 2013, 25, 6737-6743. (28) Wang, E.; Desai, M. S.; Lee, S. W. Light-Controlled Graphene-Elastin Composite Hydrogel Actuators. Nano Lett. 2013, 13, 2826-2830. (29) Ji, M. Y.; Jiang, N.; Chang, J.; Sun, J. Q. Near-Infrared Light-Driven, Highly Efficient Bilayer Actuators Based on Polydopamine-Modified Reduced Graphene Oxide. Adv. Funct. Mater. 2014, 24, 5412-5419.


Page 22 of 36

Page 23 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30) Zhu, C. H.; Lu, Y.; Peng, J.; Chen, J. F.; Yu, S. H. Photothermally Sensitive Poly(Nisopropylacrylamide)/Graphene Oxide Nanocomposite Hydrogels as Remote LightControlled Liquid Microvalves. Adv. Funct. Mater. 2012, 22, 4017-4022. (31) Hou, C. Y.; Zhang, Q. H.; Wang, H. Z.; Li, Y. G. Functionalization of PNIPAAm Microgels Using Magnetic Graphene and Their Application in Microreactors as Switch Materials. J. Mater. Chem. 2011, 21, 10512-10517. (32) Zhang, X. B.; Pint, C. L.; Lee, M. H.; Schubert, B. E.; Jamshidi, A.; Takei, K.; Ko, H.; Gillies, A.; Bardhan, R.; Urban, J. J.; Wu, M.; Fearing, R.; Javey, A. Optically-and Thermally-Responsive Programmable Materials Based on Carbon Nanotube-Hydrogel Polymer Composites. Nano Lett. 2011, 11, 3239-3244. (33) Fusco, S.; Sakar, M. S.; Kennedy, S.; Peters, C.; Bottani, R.; Starsich, F.; Mao, A.; Sotiriou, G. A.; Pané, S.; Pratsinis, S. E.; Mooney, D.; Nelson, B. J. An Integrated Microrobotic Platform for On-Demand, Targeted Therapeutic Interventions. Adv. Mater. 2014, 26, 952-957. (34) Yan, B.; Boyer, J. C.; Habault, D.; Branda, N. R.; Zhao, Y. Near Infrared Light Triggered Release of Biomacromolecules from Hydrogels Loaded with Upconversion Nanoparticles. J. Am. Chem. Soc. 2012, 134, 16558-16561. (35) Yang, X. J.; Liu, X.; Liu, Z.; Pu, F.; Ren, J. S.; Qu, X. G. Near-Infrared Light-Triggered, Targeted Drug Delivery to Cancer Cells by Aptamer Gated Nanovehicles. Adv. Mater. 2012, 24, 2890-2895. (36) Huang, X. H.; Neretina, S.; El-Sayed, M. A. Gold Nanorods: From Synthesis and Properties to Biological and Biomedical Applications. Adv. Mater. 2009, 21, 4880-4910. (37) Zhu,

C.

H.;

Lu,

Y.;

Chen,

J.

F.;

Yu,

S.

H.

Photothermal

Poly(N-

isopropylacrylamide)/Fe3O4 Nanocomposite Hydrogel as a Movable Position Heating Source under Remote Control. Small 2014, 10, 2796-2800.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(38) Naficy, S.; Brown, H. R.; Razal, J. M.; Spinks, G. M.; Whitten, P. G. Progress Toward Robust Polymer Hydrogels. Aust. J. Chem. 2011, 64, 1007-1025. (39) Haraguchi, K.; Takehisa, T. Nanocomposite Hydrogels: a Unique Organic-Inorganic Network Structure with Extraordinary Mechanical, Optical, and Swelling/De-swelling Properties. Adv. Mater. 2002, 14, 1120-1124. (40) Xia, L. W.; Xie, R.; Ju, X. J.; Wang, W.; Chen, Q. M.; Chu, L. Y. Nano-Structured Smart Hydrogels with Rapid Response and High Elasticity. Nat. Commun. 2013, 4, 2226-2236. (41) Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. Double-Network Hydrogels with Extremely High Mechanical Strength. Adv. Mater. 2003, 15, 1155-1158. (42) Sun, J. Y.; Zhao, X. H.; Illeperuma, W. R. K.; Chaudhuri, O.; Oh, K. H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. G. Highly Stretchable and Tough Hydrogels. Nature 2012, 489, 133-136. (43) Okumura, Y.; Ito, K. The polyrotaxane gel: A topological gel by figure-of-eight crosslinks. Adv. Mater. 2001, 13, 485-487. (44) Imran, A. B.; Esaki, K.; Gotoh, H.; Seki, T.; Ito, K.; Sakai, Y.; Takeoka, Y. Extremely Stretchable Thermosensitive Hydrogels by Introducing Slide-Ring Polyrotaxane CrossLinkers and Ionic Groups into the Polymer Network. Nat. Commun. 2014, 5, 5124-5131. (45) Sakai, T.; Matsunaga, T.; Yamamoto, Y.; Ito, C.; Yoshida, R.; Suzuki, S.; Sasaki, N.; Shibayama, M.; Chung, U. I. Design and Fabrication of a High-Strength Hydrogel with Ideally Homogeneous Network Structure from Tetrahedron-like Macromonomers. Macromolecules 2008, 41, 5379-5384. (46) Kamata, H.; Akagi, Y.; Kariya, Y. K.; Chung, U. I.; Sakai, T. “Nonswellable” Hydrogel Without Mechanical Hysteresis. Science 2014, 343, 873-875.


Page 24 of 36

Page 25 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(47) Tuncaboylu, D. C.; Sari, M.; Oppermann, W.; Okay, O. Tough and Self-Healing Hydrogels Formed via Hydrophobic Interactions. Macromolecules 2011, 44, 4997-5005. (48) Zhang, E. Z.; Wang, T.; Hong, W.; Sun, W. X.; Liu, X. X.; Tong, Z. Infrared-Driving Actuation

Based

on

Bilayer

Graphene

Oxide-Poly(N-isopropylacrylamide)

Nanocomposite Hydrogels. J. Mater. Chem. A 2014, 2, 15633-15639. (49) Zhang, E. Z.; Wang, T.; Lian, C. X.; Sun, W. X.; Liu, X. X.; Tong, Z. Robust and Thermo-Response Graphene-PNIPAm Hybrid Hydrogels Reinforced by Hectorite Clay. Carbon 2013, 62, 117-126. (50) Eberhardt, E. S.; Raines, R. T. Amide-Amide and Amide-Water Hydrogen Bonds: Implications for Protein Folding and Stability. J. Am. Chem. Soc. 1994, 116, 2149-2150. (51) Haraguchi,

K.;

Xu,

Y.

J.;

Li,

G.

Molecular

Characteristics

of

Poly(N-

isopropylacrylamide) Separated from Nanocomposite Gels by Removal of Clay from the Polymer/Clay Network. Macromol. Rapid Commun. 2010, 31, 718-723. (52) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (53) Lian, C. X.; Lin, Z. M.; Wang, T.; Sun, W. X.; Liu, X. X.; Tong, Z. Self-Reinforcement of PNIPAm-Laponite Nanocomposite Gels Investigated by Atom Force Microscopy Nanoindentation. Macromolecules 2012, 45, 7220-7227. (54) Acik, M.; Lee, G.; Mattevi, G.; Chhowalla, M.; Cho, K.; Chabal, Y. J. Unusual InfraredAbsorption Mechanism in Thermally Reduced Graphene Oxide. Nat. Mater. 2010, 9, 840-845. (55) Kang, H. W.; Tabata, Y.; Ikada, Y. Fabrication of Porous Gelatin Scaffolds for Tissue Engineering. Biomaterials 1999, 20, 1339-1334.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(56) Fan, J. C.; Shi, Z. X.; Lian, M.; Li, H.; Yin, J. Mechanically Strong Graphene Oxide/Sodium Alginate/Polyacrylamide Nanocomposite Hydrogel with Improved Dye Adsorption Capacity. J. Mater. Chem. A 2013, 1, 7433-7443. (57) Okajima, T.; Harada, I.; Nishio, K.; Hirotsu, S. Kinetics of Volume Phase Transition in Poly(N-isopropylacrylamide) Gels. J. Chem. Phys. 2002, 116, 9068-9077. (58) Ju, X. J.; Chu, L. Y.; Zhu, X. L.; Hu, L.; Song, H.; Chen, W. M. Effects of Internal Microstructures of Poly(N-isopropylacrylamide) Hydrogels on Thermo-Responsive Volume Phase-Transition and Controlled-Release Characteristics. Smart Mater. Struct. 2006, 15, 1767-1774.


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FIGURES

Figure 1. Schematic illustration of fabrication process and performance mechanism of the proposed PNIPAM-GO nanocomposite hydrogels. a) GO nanosheets are homogeneously dispersed in the monomer solution. b) The PNIPAM-GO nanocomposite hydrogels are formed by both chemical and physical crosslinking, in which the PNIPAM chains are chemically crosslinked by BIS, and the hydrogen bond interactions between GO nanosheets and PNIPAM chains result in the physical crosslinking. c,d) The PNIPAM-GO hydrogels exhibit ultrahigh tensibility (c) and reversible NIR light-responsive property (d).



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Figure 2. SEM images of PNIPAM and PNIPAM-GO nanocomposite hydrogels. a) GO00.01; b) GO1-0.01; c) GO2-0.01; d) GO3-0.01; e) GO4-0.01; f) GO2-1. Scale bars are 10 µm.


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Figure 3. Mechanical characteristics of the PNIPAM-GO nanocomposite hydrogels and the contrasted samples. a-c) Optical images showing that GO0-1 samples are too brittle to sustain compression (a), slicing with a blade (b) or elongation (c). d,e) Optical images showing that GO3-0.01 samples can withstand high levels of deformation by a high compression (d), slicing with a blade (e). f-h) Compared with that GO2-1 sample fails to sustain elongation (f), the GO2-0.01 sample exhibits ultrahigh tensibility to against extensive stretching (g) or even after knotting (h) at room temperature.



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Figure 4. Typical stress-strain curves of the PNIPAM-GO nanocomposite hydrogels with different degrees of chemical crosslinking (a) and different contents of GO nanosheets (b).


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Figure 5. Temperature-responsive characteristics of PNIPAM-GO nanocomposite hydrogels. a) Optical images showing the hydrogels in swollen states at 15 oC (VPTT) in pure water. Scale bar is 10 mm. b) Temperature dependence of the equilibrium swelling ratios of the PNIPAM-GO nanocomposite hydrogels. c) Dynamic thermo-responsive volume shrinking behaviours of the PNIPAM-GO nanocomposite hydrogels after the external temperature jumping abruptly from 25 oC to 55 oC (equilibrated in water at 25 oC when t = 0).



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Figure 6. NIR light-responsive characteristics of the PNIPAM-GO nanocomposite hydrogels. a-e) Infrared thermal images of GO0-0.01 (a), GO1-0.01 (b), GO2-0.01 (c), GO3-0.01 (d), and G04-0.01 (e) hydrogels exposed under NIR light for different time periods. f) Optical images of the NIR light-responsive volume change of GO4-0.01 hydrogel. The power density of the NIR light is 0.58 W cm-2 at 20 oC. The dashed lines in (a-e) mark the outlines of the hydrogels.


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Figure 7. NIR light-responsive characteristics of the PNIPAM-GO nanocomposite hydrogels. a) Average temperature variation of the hydrogels exposed under NIR light for different time periods. b) Dynamic volume-deswelling behaviours of the hydrogels after exposed under NIR light. The power density of the NIR light is 0.58 W cm-2 at 20 oC.



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Figure 8. Dynamic volume change of GO3-0.01 hydrogel by repeatedly exposing it under NIR light for 0.5 h in air and replacing it in water for 24 h at 20 oC. The power density of the NIR light is 0.58 W cm-2 at 20 oC.


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Figure 9. Application demonstration of PNIPAM-GO nanocomposite hydrogels for remote NIR light-controlled switches. a) The GO4-0.01 hydrogel inserted with two parallel steel needles are swollen (a1), and the electric circuit is open with no NIR light exposure (a2). b) The shrinkage of the GO4-0.01 hydrogel induced by the NIR light makes the steel needles contacting with the aluminium sheets, resulting in the close of the electric circuit (b1), and the LED light is switched on (b2). The power density of the NIR light is 0.58 W cm-2 at 20 o C.



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TOC FIGURE


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