GO@Polyaniline Nanorod Arrays Hierarchical Structureï¼A

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GO@Polyaniline Nanorod Arrays Hierarchical Structure# A photothermal agent with high photothermal conversion efficiency for Fast Near-Infrared Responsive Hydrogels Ping Fan, Zhe Fan, Fanglin Huang, Jintao Yang, Feng Chen, Zhengdong Fei, and Mingqiang Zhong Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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Industrial & Engineering Chemistry Research

GO@Polyaniline Nanorod Arrays Hierarchical Structure: A photothermal Agent with High Photothermal Conversion Efficiency for Fast NearInfrared Responsive Hydrogels

Ping Fan*, Zhe Fan, Fanglin Huang, Jintao Yang, Feng Chen, Zhengdong Fei， Mingqiang Zhong* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China Corresponding author, Tel: +86-517-88320856; Fax: +86-517-88320856

E-mail address: [email protected] (Ping Fan), [email protected] (Mingqiang Zhong)

Abstract Near-infrared (NIR) responsive hydrogels are of significance in remote control microvalves. However, the development and the application of NIR responsive hydrogels were either limited by high cost or limited by long response time (on the order of minutes) and modest volume change. To address this problem, in this work, a GO@PANI hybrid nanocomposite with nanorods arrays hierarchical structure was fabricated by dilute polymerization. The as-synthesized hierarchical GO@PANI nanosheets exhibited high NIR photothermal conversion efficiency (PTCE) (49% ） which was approximate 188% magnitude of that of GO sheets. Due to its high photothermal conversion efficiency, when GO@PANI was incorporated into temperature-responsive PNIPA, a highly responsive hydrogel with rapid and significant NIR-induced volume change (over 50%) were obtained. When this material was applied as a remote-controllable microvalve, it exhibits fast NIR response (~30s).

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Key words: graphene, polyaniline, photothermal conversion, microvalves, nearinfrared responsive hydrogels


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Introduction Hydrogel-based microvalves can control the flow direction and flux through their deformation induced by environment stimuli1, 2 and create new opportunities for developing smart microsystems. Normally, the “on–off” switch of fluidic channels is achieved by the volume change of such hydrogel induced by various environmental stimuli, such as light3, temperature4-6, pH7, 8, magnetic field9, electric field10, concentration12,

13.

11,

ionic and salt

Compared with other stimuli, NIR stimulation provides distinct

advantages such as remote activation, point-specific controllability, and tunable intensity of the irradiating light. Besides, NIR can penetrate human tissues with sufficient intensity and minimal damage. To achieve near-infrared (NIR) light responsive hydrogel systems, one effective way is immobilizing or entrapping NIR-absorbing materials such as noble metal particles14-16, carbon-based particles (carbon nanotubes(CNTs)17,

18

and

graphene

oxide nanosheets(GO)19-21), organic conductive polymerics (polypyrrole(PPY)22-24 and polyaniline(PANI)25), and some semiconductor particles26 into thermoresponsive hydrogel matrices such as poly(N-isopropylacrylamide) (PNIPA). In this case, once the NIR-absorbing materials in hydrogel matrix absorb NIR light, they can efficiently convert NIR light into heat. Then once the temperature is over the Lower Critical Solution Temperature (LCST) of PNIPA (32°C), the polymer chains collapse, leading to the composite hydrogel shrinking. The higher photothermal conversion efficiency (PTCE) of NIR-absorbing materials, the more heat could be released under NIR irradiation and a higher volume change could be observed.



Many studies have been done on NIR-absorbing materials. Noble metals (such as gold, silver), have been widely explored for photothermal ablation (PTA) therapy owing to their high optical extinction coefficients in the NIR wavelength range. However, their high preparation cost and low thermostability under long-term laser irradiation limits their applications in the field of actuators or microvalves. Thus, most NIR triggered hydrogel microvalve reported now adopt carbon-based materials or semiconductor materials as NIR absorbing materials 38. Although these carbon-based materials or semiconductor materials have a great potential application in NIR triggered hydrogel microvalve due to their low cost, they normally show low molar extinction coefficient27 and low PTCE. The NIR light-responsive hydrogels with these materials normally need long irradiation time(in the order of minutes) to achieve modest volume change19,

28.

This inevitably leads to low NIR-sensitivity. Therefore, to meet the

demand of fast NIR responsive microvalves, developing NIR-absorbing materials with high photothermal conversion efficiency but low cost is the first priority According to literature, when a beam of light shines on the surface of a substance, a series of possible physical phenomena such as scattering (including multiple refraction and reflection), absorption and luminescence, occur. And the relation of the intensity of incident light (Iincident), the intensity of scattering light (Iscatter) and the intensity of absorption light (labs) is approximately: lincident=labs+Iscatter. Since the light energy a material absorbed will eventually turn almost entirely into heat energy, if some structures are built and make the light multireflect and mutirefringence in/between them, the total NIR absorption of this material might be improved.


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Aware of this, some scientists start construct nanostructure in material, especially three-dimensional(3D) superstructure, to enhance the its PTCE. Hu’s group has prepared a flower-like CuS superstructures29, flower-like bismuth sulfur30 composed by their nanoparticles. Wu et al.

31

reported the hollow and hierarchical CuInS2

microspheres with close cavities and open cavities. Tan et al. 32 fabricated honeycomblike CuS mesostructures composed of packed CuS NPs. All their results confirmed that an increased PTCE could be obtained by constructing nanostructures. However, up to now the preparation of these nanostructures with NIR photothermal conversion properties usually need complicated and lengthy fabrication process. Besides, some nanostructure/superstructure were self-assembled through Van der waals’ force. Its intrinsic structure and shape might hardly be maintained during hydrogel polymerization and swelling/deswelling process upon NIR on/off. As one of carbon-based material, graphene is a NIR absorbing material with huge specific surface area. Inspired by above literatures, we thought maybe we can take the advantage of its huge specific surface area and build nanorods arrays on it to form a 3D nanocomposite particles and thus obtain an NIR-absorbing agent with improved PTCE. Under the guidance of such thought, in this paper, aniline which has high tendency to attach on graphene surface (π-π interaction) and can be polymerized to polyaniline(PANI) through simple method, was choose to construct the nanorods arrays on GO surface. The structure, morphology and photothermal conversion properties of obtained GO@PANI sheets were systematically investigated. On this basis, GO@PANI/PNIPA hydrogels were prepared and employed as a fast responsive NIR



light triggered microvalve.

Experimental section Materials N,N,N′,N′-tetramethylethylenediamine (TEMED) and ammonium persulphate (APS) were purchased from Aladdin Chemical reagent Co. Ltd. They were used without purification. Both N-Isopropylacrylamide (NIPAM) (Aladdin Chemical reagent Co. Ltd.) and N,N′-Methylenebis(acrylamide) (Bis) (Aladdin Chemical reagent Co. Ltd.) were purified by recrystallization before use. Aniline (Aladdin Chemical reagent Co. Ltd.) was distilled under reduced pressure. All water used in the synthesis and characterization was distilled twice.

Synthesis of GO@PANI Nanocomposites Firstly, GO was prepared according to Hummers method33, and the detailed preparation process has been described in the literature34. GO@PANI nanocomposites were synthesized by dilute polymerization. First, 9 mg GO were ultrasonic dispersed in aqueous HClO4 solution (1 mol/L) of 15 mL. 5 mL of ethanol was also added to prevent the mixture from being frozen at low temperature. After 30 min stirring at -10 °C, aniline monomer which was pre-refined and pre-refrigerated was added and stirred continually to form a uniform mixture. Then the oxidant, APS, which was pre-dissolved in 5 mL of aqueous HClO4 solution and pre-refrigerated was added rapidly into the mixture above (the molar ratio of aniline/APS is 1.5). Subsequently, the polymerization was carried out at -10°C for 24 h. Finally, a grandmother green precipitate was obtained.


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After separated, washed and dried, this precipitate was labeled as GO@PANI

Synthesis of GO@PANI/PNIPA Composite Hydrogels First, 300 mg of NIPA and 16 mg of Bis were dissolved in 10 mL GO@PANI aqueous suspension with 0.1 mg/mL, 0.5 mg/mL or 1 mg/mL concentration. The mixture was cooled down to 0oC and bubbled with N2 gas to remove oxygen. Then, 10 μL of TEMED and 25 μL of APS aqueous solution (10 wt%) were added rapidly. N2 gas was continuously bubbled until the system became sticky. After reacted at 0oC for 24 hours, a solidified composite hydrogel could be obtained. This obtained hydrogels were washed by deionized water to remove the excess monomer. The pure PNIPA hydrogels and GO/PNIPA hydrogels were also synthesized for comparation.

Characterization The as-prepared samples were characterized by Atomic Force Microscopy (AFM) (Veccco Nanoman VS) in tapping mode, Transmission Electron Microscopy (TEM) (JEM-1230 operating at 120 kV), Scanning Electron Microscope (SEM) (Hitachi S4700), X-ray Photoelectron Spectroscopy(XPS) (X’Pert PRO diffractometer) equipped with Cu radiation (36 kV, 30 mA), Fourier Transform Infrared Spectroscopy(FTIR) (Nicolet 6700 FT-IR) and Raman spectrum (LabRAM HR UV800).

Measurements of photothermal performance Suspensions with different GO concentration and GO@PANI concentration were



placed in the 0.5 mL quartz cell. Then the quartz cell was irradiated by a NIR laser. The source of irradiation was an 808 nm laser (0.5 W) with the spot size of 1 cm long and 0.5 cm wide. The suspension temperature was measured using a digital thermometer (with an accuracy of 0.1°C). To explore the photo-thermal reversibility of GO@PANI/PNIPA composite hydrogel, the hydrogel was irradiated with NIR laser for 10 min (LASER ON), and then NIR laser was switch off (LASER OFF) and let the hydrogel naturally cool down to room temperature. This cycle was repeated five times. To investigate the ability of the composite hydrogel as a microvalves in the fluidic channels, A small piece of GO@PANI/PNIPA cylindrical hydrogel was inserted into a quartz tube with an inner diameter of 2mm. Prussian blue solution was infused from the top of the quartz tube and its downward flow is blocked by the hydrogel. When the composite hydrogel was irradiated by NIR light, it shrank and allowed the liquid flow out. This process was observed and photographed by an optical microscope.


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Results and discussion The morphology of PANI is affected by reaction parameters such as solution acidity, aniline concentration, and reaction temperature35. In this paper, a dilute aniline (less than 0.05 M) and a low reaction temperature (-5℃) were adopt to obtain aligned nanorods arrays. The morphology of GO@PANI was shown in Figure 1. The AFM and TEM image of GO were also shown in Figure 1 for comparation. It could be seen that GO nanosheets exhibit as crumpled transparency nanosheets with the thickness of ∼1 nm. These facts fit well with the characteristics of a fully exfoliated GO nanosheet36. While from TEM image of obtained GO@PANI sheets, it could be seen that although GO@PANI also exhibited a 2D nanosheet morphology, many dark spots could be observed on its surface (Figure 1c-1d). SEM image (Figure 1e-1f) showed that the dark spots in TEM were caused by the nanorod-like structure. The length of these nanorods was about 100 nm and the average diameter of the nanorods was about 20nm with a narrow size distribution. These nanorods stood on both sides of GO uniformly and led GO@PANI showed a freestanding sheet-like morphology with a thickness of ∼200 nm.



Figure 1 AFM image of GO (a), TEM image of GO (b), TEM image of GO@PANI(c-d) and SEM image of GO@PANI(e-f)

In order to confirm the chemical structure of these nanorods, XPS was performed (Figure 2). Figure 2a was the full XPS spectra of the GO and GO@PANI. Compared with that of GO, tiny amounts of nitrogen (4.86%) was detected on GO@PANI sample. The C1s XPS spectrum of GO was consisted by four components: The C in C═C bonds


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(284.6 eV), the C in C−O bonds (286.7 eV), the C in C═O bonds (287.8 eV), and the C in O−C═O bonds (289.1 eV). However, in the C1s XPS spectrum of GO@PANI, although these four components also presented, the intensity of the peaks attributed to C−C bonds and C−O bonds were much smaller than those in GO. Furthermore, a new component ascribed to C in C−N bonds (285.2 eV) appeared and dominated in the C1s XPS spectrum of GO@PANI. Further examination of the N1s spectrum of GO@PANI (Figure 2d) revealed there existed two different chemical states of nitrogen: The N in N–H bonds (400.0 eV) and the N in C–N bonds (401.5 eV). It also showed that the concentration of the N–H bonds was almost two times of that of the C–N bond. All the XPS results suggested that the nanorods might be PANI nanorods.

Figure 2

XPS full spectra of GO and GO@PANI(a)、C1s spectra of GO (b)、C1s spectra of GO@PANI (c) and N1s of GO@PANI (d)



Figure 3

FTIR spectra of GO, PANI and GO@PANI

The chemical structure of the nanorods were further confirmed by FTIR. Figure 3 showed the FTIR spectra of GO, GO@PANI and PANI. The FTIR spectrum of GO was typical for GO 37, 38. The characteristic peaks included the O−H stretching peak at 3232 cm−1, C═O vibration peak at 1704 cm−1, C═O stretching peak at 1704 cm−1, C−O stretching peak at 1025.9 cm−1 and aromatic C−H peak at 867.8 cm−1 39 . However, these GO characteristic peaks were illegible in the spectrum of GO@PANI. Instead, peaks attributed to PANI could be observed. These peaks included the absorption peaks located at 1565.9 cm−1 (the aromatic C═C stretching of the quinonoid ring) , 1484.9 cm−1 (the aromatic C═C stretching of the benzenoid ring), 1307.5cm-1 (the C–N stretching of the secondary aromatic amine), 1118.5 and 806 cm−1 (the aromatic C–H out-of-plane deformation vibration)

40, 41.

Compared with the FTIR spectra of pure

PANI, these characteristic peaks of PANI have an obvious red shift in the spectrum of


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the GO@PANI nanoparticles. This phenomena should be ascribed to π-π interaction and hydrogen bonding between GO and the PANI backbone42. These strong interactions might guarantee the stability of the intrinsic structure and shape of GO@PANI during subsequent PNIPA polymerization and swelling/deswelling process.

Figure 4

Raman spectra of (a) GO and (b) GO@PANI nanocomposite

Raman spectra were also used to investigate the structure of GO@PANI, as shown in Figure 4. Two characteristic peaks at 1335 cm−1 (D-band) and 1598 cm−1 (G-band) were observed in both GO and GO@PANI spectra, which correspond to graphitic and diamond structures, respectively. The ID/IG values of the GO and GO@PANI were 1.023 and 1.035 respectively, which suggest that PANI did not hybridize the GO nanosheet and the electronic structure of GO was not changed by the PANI attachment. In the Raman spectrum of GO@PANI nanocomposites, it could be observed that apart from the D and G bands of GO, three new peaks belong to PANI appeared. They were C=C stretching vibration of quinoid ring (1501 cm-1), C–H bending of the quinoid ring (1173 cm-1) and bending vibration of substituted benzene ring (805 cm-1)43, 44. As shown in Figure 1, in GO@PANI, the PANI nanorods arrays were uniformly



covered on GO surface. These PANI nanorods arrays might serve as laser-cavity mirrors, which made the light multi-reflect and multi-refringence in/between them (as demonstrated in Figure 5). This might result in improved multi-absorption ability for NIR and an improved PTCE of GO@PANI. To confirm the effectiveness and benefit of this PANI nanorods arrays structure, the photothermal effect of GO@PANI nanocomposites was investigated by monitoring the temperature of an aqueous solution of GO@PANI nanocomposites (2 mg/mL) under NIR laser irradiation (808 nm, 0.5 W/cm2) and compared with that of GO aqueous solution with same concentration under same irradiation source. As can be seen from Figure 6, the temperature of GO@PANI nanocomposites solution increased 21.2℃ within 7 min. In contrast, the solution temperature of GO (2 mg/mL) and water only increased 14.3℃ and 6℃ within 7 min respectively. This indicated that GO@PANI nanosheets indeed had an excellent photothermal conversion effect.

Figure 5 Schematic representation of GO@PANI nanorod arrays hierarchical structure serving as laser-cavity mirrors for 808 nm laser


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Figure 6 Temperature variation of GO and GO@PANI aqueous dispersions (2mg/mL) with 2% PVA as stabilizer, as a function of irradiation time in a procedure of laser-on for 10 min and then laser-off (water was also irradiated for the control).

The PTCE of GO@PANI nanosheet was measured following a previous method45, 46.

The aqueous solution of GO@PANI nanosheet (2mg/mL) was irradiated by the laser

(808 nm, 0.5 W) until the temperature reached a steady state (~10 min). The laser was then switch off and the temperature reduction of the aqueous solution was recorded in order to get the heat transfer rate from the aqueous solution of GO@PANI nanosheets to the environment (Figure 6). The PTCE value (η) was calculated using eqn (1):

η=

ℎ𝑆(𝑇𝑀𝑎𝑥 ― 𝑇𝑆𝑢𝑟𝑟) ― 𝑄𝐷𝑖𝑠 𝐼(1 ― 10 ― 𝐴808)

Where TMax is the equilibrium temperature, TSurr is ambient temperature of the surrounding. QDis expresses the rate of heat input due to light absorption by the solvent. I is the laser power (500 mW), A808 is the absorbance of GO@PANI nanosheets at an excitation wavelength of 808 nm, h is the heat transfer coefficient and S is the surface area of the container. The value of hS is derived according to eqn (2):

𝜏𝑠 =

𝑚𝐷𝐶𝑝 ℎ𝑆



where τs is the sample system time constant. mD and Cp are the mass and heat capacity of solution, respectively. The time constant for heat transfer of the system was determined to be τs=149s for GO@PANI and τs=163s for GO respectively as shown in Figure 7 according to the plot of cooling time versus negative natural logarithm of the temperature driving force (θ) obtained from the cooling stage. Then the photothermal conversion efficiency (PTCE) of GO@PANI and GO were calculated to be 49% and 26% respectively. The PTCE of GO@PANI was not only nearly twice as much as that of GO, but also higher to that of many NIR-absorbing nanoparticles reported previously47.

Figure 7 Linear fitting of -ln(θ) versus time obtained from the cooling period in Figure 6. (θ is defined as (T −Tsur)/ (Tmax−Tsur), where T is instant temperature at time t, Tsur is the ambient temperature, and Tmax is the maximum temperature of the sample)


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Figure 8 Photos of diameter variations of GO/PNIPA, GO@PANI/PNIPA composite hydrogel and pure PNIPA hydrogel vs. different NIR irradiation time

Figure 9 △d/d0 of hydrogels vs. different NIR irradiation time (△d is the diameter change, d0 is the initial diameter)

The as prepared GO@PANI nanosheets were then incorporated into PNIPA in order to obtain NIR responsive hydrogel. In order to investigate the NIR light



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responsiveness of the hydrogels, the diameter change of the composite hydrogels were photographed at different irradiation time (Figure 8). According to these images, the △d/d0 values of the hydrogels were calculated and plotted as a function of the irradiation time (Figure 9). It could be seen that the △d/d0 value of PNIPA hydrogel was almost unchanged

during

whole

irradiation

time,

while

both

GO/PNIPA

and

GO@PANI/PNIPA exhibited NIR response and volume shrinking. Compared with GO/PNIPA hydrogel at same photothermal particles concentrate, GO@PANI/PNIPA composite hydrogel exhibited faster responsive rate and larger volume change. And hydrogels with higher photothermal particle concentration exhibited higher volume change (improved responsive rate) under NIR irradiation, which suggest that the degree of valve opening (i.e., the shrinkage of the hydrogels) could be tuned by adjusting the concentration of GO or GO@PANI. When the concentration of GO@PANI was 1mg/mL, the hydrogel exhibit large volume shrinking(＞50%) in 6 min. Due to its fast response and large volume shrinking under NIR irradiation, GO@PANI/PNIPA hydrogel was fabricated as a NIR responsive liquid microvalve to control the flow in a fluidic channel (Figure 10). It could be seen that without NIR irradiation, the microvalve was in “OFF” state. Even after 10 min, the composite hydrogel valve still completely blocked the flow in the fluidic channel. However, when the microvalve was irradiated by NIR, only half min after irradiation, the composite hydrogel started to shrink. The microvalve was in “ON” state and some liquid flowed pass through the microvalve. 1 min later, about 0.5mL liquid had flowed out. And almost all liquid flowed out in 5 min under NIR irradiate.


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Figure10 Liquid microvalve fabricated by the GO@PANI/PNIPA. The photographs show the microvalve without NIR irradiation at 0 min (a), 10 min(b) and with NIR irradiation at 0 min(c), 26 sec (d), 1min (e), 5 min(f)

The stability and repeatability of swelling-shrink transition of GO@PANI/PNIPA hydrogel is also investigated. It could be seen from Figure11 that by repeating the on/off cycle of the NIR laser irradiation, the hydrogel temperature varied from 16.9 °C (lower than LCST) to 35.5 °C (higher than LCST). Moreover, the decrease of temperature elevation was less than 3.0% after five cycles of irradiation. These results suggest the light-induced volume changes in GO@PANI/PNIPA microvalve were fully reversible and repeatable.



Figure 11 Temperature changes of GO@PANI/PNIPA with 1 mg/mL GO@PANI concentration as a function of heating−cooling cycle.

Conclusion In summary, arrays of PANI ultrafine nanorods supported on two-dimensional GO sheets (GO@PANI) were fabricated by dilute polymerization. The as-synthesized hierarchical 3D GO@PANI nanosheets exhibited high NIR photothermal conversion efficiency (PTCE) (~49%). By combining this photothermal GO@PANI sheets with temperature-responsive PNIPA hydrogel, a composite hydrogel with fast NIRresponsive time and large NIR-induced volume change could be obtained. By adjusting the concentration of the GO@PANI and NIR irradiation time, the NIR-induced shrinkage of the hydrogels could be tuned effectively. Furthermore, the NIR-induced shrinkage of the hydrogels was completely fully reversible and repeatable. This NIRresponsive hydrogel could be utilized as a NIR-remote-trigger microvalve in fluidic channel. Due to the high PTCE of GO@PANI, the obtained hydrogel could shift from “OFF” state to “ON” state in half minute under NIR irradiation source (0.5W). This material potentially has many applications as actuators, such as in micro


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electromechanical systems, microfluidic devices.

Acknowledgments This study was financially supported by the Natural Science Foundation of Zhejiang Province (LY17E030006) and National Natural Science Foundation of China (51303158).

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For Table of Contents Only

NIR on


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AFM image of GO (a), TEM image of GO (b), TEM image of GO@PANI(c-d) and SEM image of GO@PANI(e-f)



XPS wide spectra of the GO and GO@PANI(a)、C1s spectra of GO@PANI (b)、C1s spectra of GO (c) and N1s of GO@PANI (d)


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FTIR spectra of GO, PANI and GO@PANI



Raman spectra of (a) GO and (b) GO@PANI nanocomposite


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Schematic representation of GO@PANI nanorod arrays hierarchical structure serving as laser-cavity mirrors for 808 nm laser



Temperature variation of GO and GO@PANI aqueous dispersions (2mg/mL) with 2% PVA as stabilizer, as a function of irradiation time in a procedure of laser-on for 10 min and then laser-off (water was also irradiated for the control)


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Time constant for heat transfer from the system, calculated with the linear time data from the cooling period versus the negative natural logarithm of driving force temperature. (θ = (T −Tsur)/ (Tmax−Tsur), where T is the temperature at different time of the cooling period, Tsur is the surrounding environment temperature, and Tmax is the equilibrium temperature under light irradiation)



Photos of diameter variations of GO/PNIPA, GO@PANI/PNIPA composite hydrogel and pure PNIPA hydrogel vs. different NIR irradiation time


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△d/d0 of hydrogels vs. different NIR irradiation time (△d is the diameter change, d0 is the initial diameter)



Liquid microvalve fabricated by the GO@PANI/PNIPA. The photographs show the microvalve before (a, b) and after (c-f) exposure to NIR irradiation (808 nm, 0.5 W). (without irradiation at 0 min (a) and 10 min(b); with irradiation at 0 min(c), 26 sec (d), 1min (e) and 5 min(f))


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Temperature changes of GO@PANI/PNIPA with 1 mg/mL GO@PANI concentration as a function of heating−cooling cycle



For Table of Contents Only 82x40mm (300 x 300 DPI)


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GO@Polyaniline Nanorod Arrays Hierarchical Structureï¼A

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