Preparation of Silver Nanoparticles Loaded Photoresponsive

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Preparation of Silver Nanoparticles Loaded Photo-Responsive Composite Microgels and their Light-Controllable Catalytic Activity Shaoyang Li, Danli Lin, Jianfeng Zhou, and Liusheng Zha J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11724 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on March 2, 2016

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The Journal of Physical Chemistry

Preparation of Silver Nanoparticles Loaded PhotoResponsive Composite Microgels and their LightControllable Catalytic Activity Shaoyang Lia, Danli Linb, Jianfeng Zhoub,*, Liusheng Zhaa,b,* a. State Key Laboratory for Modication of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, 201620, People's Republic of China. b. Research Center for Analysis and Measurement, Donghua University, Shanghai, 201620, People's Republic of China.

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Abstract A new type of smart composite microgels, which are able to control the catalytic activity of their loaded silver nanoparticles by light, was designed and fabricated based on the idea of function transfer between their constituent components. First, the surfaces of monodisperse gold nanorods (AuNRs) with strong photothermal effect were coated with poly(N-isopropylacrylamide) (PNIPAM) hydrogel by seed precipitation polymerization to prepare the two-component composite microgels with core-shell structure (AuNR@PNIPAM microgels). Then, Ag+ ions coordinated into the shell of AuNR@PNIPAM microgels were in-situ reduced by sodium borohydride to produce silver nanoparticles (AgNPs) loaded three-component composite microgels (AuNR@(AgNPs/PNIPAM) microgels). The characterization results obtained by transmission electron microscopy show that the gold nanorod is located at the center of the threecomponent composite microgels and AgNPs with the average particle diameters of 6 to 10 nm are evenly distributed within its shell. The hydrodynamic diameters of the composite microgels, measured by dynamic light scattering before or after exposure of their aqueous dispersion to near-infrared (NIR) laser of 808 nm wavelength indicate that they have photo-responsive property. The AgNPs and AuNR inside AuNR@(AgNPs/PNIPAM) microgels hold their respective localized surface plasmon resonance (LSPR) optical property, and the longitudinal LSPR wavelength of the latter is blue-shifted with increasing content of the former. Moreover, the LSPR efficiency of the AgNPs and the longitudinal LSPR wavelength of the AuNR are capable of being changed in response to the NIR illumination, and the stimulus-responsive behavior is reversible. AuNR@(AgNPs/PNIPAM) microgels are able to be used as the smart microreactor for reducing 4-nitrophenol by NaBH4, and the reaction rate can be modulated by power density of the NIR light, demonstrating that the three-component composite microgels have light-controllable catalytic activity.


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1. Introduction It is well known that silver nanoparticles (AgNPs) of less than 10 nm size have catalytic activity for many organic reactions, such as the reduction reactions of nitro compounds and organic dyes, ethylene epoxidation and dehydrogenation reaction1,2. Comparing with other metal nanoparticles with catalytic activity, such as Pt and Pd nanoparticles3,4, AgNPs can be prepared more readily and inexpensively, so they have broad application prospect in catalysis5. However, like other nanoparticles, AgNPs are prone to self-aggregation to become big particles due to their high surface energy6, resulting in loss of their catalytic activity. Besides, the AgNPs as catalyst are hard to separate from products after reaction, so that the purity of the product is lowered, which would have a negative effect on its performance. Furthermore, if the AgNPs as catalyst are not recycled, the cost of the reaction product could be increased. Loading of AgNPs into some support not only avoids their self-aggregation but also helps in their recovery7, which may improve purity of the product and reduce its cost8. Therefore, the development of novel catalyst support has been the subject of intense research in chemical synthesis9, and has also attracted more and more attention in the field of material science10. Smart microgels, also called ‘stimuli responsive microgels’, are the colloidal hydrogel particles with the diameters ranging from 10 nm to 1000 nm, which are capable of undergoing volume phase transition in response to external stimuli, such as temperature, pH, light or magnetic field11. If AgNPs are loaded into smart microgels, not only the self-aggregation of AgNPs can be overcome thanks to their immobilization within the mcirogel network, but also the microgels can be recovered by facile filtration or centrifugal separation approach. More interestingly, their catalytic activity can be tuned by external stimuli, so the catalytic reaction rate is able to be controlled, which would benefit reducing the side reaction. For instance, when AgNPs were


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loaded into thermo-responsive microgel, their catalytic activity could be controlled by temperature.12,13 However, exerting temperature stimulus on reaction system, i.e. changing reaction temperature, is a slow and inconvenient process, which would result in some side reaction. Therefore, it is a significant research subject to develop other AgNPs loading smart composite microgels to control their catalytic activity quickly and easily. Photo-responsive microgels undergo volume phase transition in response to light stimulus14. In comparison to change in temperature or pH, light illumination is easier to be exerted on stimulus responsive system, so that photo-responsive microgels are attracting increasing attention in recent years15-18. In term of their stimulus response mechanism, photo-responsive microgels are divided into two categories. The first one is typically fabricated basing on photo-responsive polymer with photoactive groups such as azobenzene, spirobenzopyran, triphenylmethane or cinnamonyl groups19 . Upon exposure to light illumination, this kind of photo-responsive microgels can undergo volume phase transition due to the structure or polarity variations of the photoactive groups of their constituent polymers. However, they usually respond to ultraviolet or visible light of shorter wavelength20, which has weak penetration and destructive effect on some chemical or biological substances. The second one is a composite system which is composed of the metal nanoparticles with photothermal effect and a thermo-responsive polymer network21. Its photo-responsive property stems from the function transfer between these two components, that is, the metal nanoparticles absorb light and convert it to heat, which induces a volume phase transition of the thermo-responsive polymer network. If gold nanorods (AuNRs) are used as the metal nanoparticles, the fabricated composite microgels are able to respond to near infrared (NIR) light, which has strong penetration and little destructive effect even on biological tissue22.


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In literature, the design and fabrication of advanced composite materials are mostly based on simple function combination of various involved components, rarely on function transfer between them. In this study, a new type of smart composite microgel consisting of three components, i.e. AgNPs, AuNR and thermo-responsive polymer network, was designed and fabricated for controlling catalytic activity of the AgNPs by NIR light basing on their function transfer. Concretely, in the composite system, AuNR can absorb NIR light and convert it to heat, and then the heat triggers phase transition of the polymer network, so the catalytic activity of the AgNPs can be controlled thanks to diffusion hindrance of the reactants within the network. Since NIR light illumination with strong penetration power and fast on-off switch is readily exerted on microgels dispersion as a reaction system, the photo-responsive composite microgels allow for facile controlling catalytic activity of their loaded AgNPs compared to previously reported AgNPs loaded thermo-responsive composite microgels. 2. Experimental Section 2.1 Materials Cetyltrimethylammonium bromide (CTAB) (purity 99%), sodium oleate (NaOL) (chemical purity), tetrachloroauric acid (HAuCl4) (analytical purity) and sodium borohydride (NaBH4) (purity 96%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Silver nitrate (AgNO3) (purity 99%) was purchased from Shanghai Fine Chemical Materials Institute. L-ascorbic acid (AA) (purity 99%) was purchased from Acros Organics. 2, 2'-azobis (2-amidinopropane hydrochloride) (AAPH) (purity 98%) and butenoic acid (BA) (purity 96%) were purchased from Sigma Aldrich. The above reagents were used without purification. N-Isopropylacrylamide (NIPAM) (purity 95%, TCI) was purified by recrystallization from a mixture of toluene and


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hexane (60:40, v/v). N, N'-Methylenebisacrylamide (MBA) (purity 95%, Sigma Aldrich) was purified by recrystallization from methanol. All water used in the synthesis and characterization was distilled twice. 2.2 Preparation of AuNR@PNIPAM composite microgels Monodisperse AuNRs were first synthesized using CTAB and NaOL as binary surfactants23 and purified according to our previous report24. Then, 300 µL of BA was added to 100 mL of 1.5 mM AuNRs aqueous dispersion at 30 °C. After 1 h, the dispersion was centrifuged at 7200 rpm for 50 min to remove excessive BA and the precipitation was redispersed in 10 mL water. Finally, 0.07 g NIPAM and 0.01 g MBA were added to BA modified AuNRs dispersion under magnetic stirring and heated to 70 °C under nitrogen purge, and their polymerization was initiated by addition of 300 µL of 50 mM AAPH. After 15 min, the produced dispersion was cooled to room temperature and purified by centrifugation at 7200 rpm for 50 min, and the precipitated AuNR@PNIPAM composite microgels were redispersed in equivalent volume of water. 2.3 Preparation of AuNR@(AgNPs/PNIPAM) composite microgels AuNR@(AgNPs/PNIPAM) composite microgels were prepared by in situ reduction of Ag+ ions coordinated into the shells of AuNR@PNIPAM microgels by NaBH4. Typically, 3 mg AgNO3 was dissolved in 75 mL water and the solution was mixed with the above purified AuNR@PNIPAM microgels aqueous dispersion. After stirring at room temperature under nitrogen purge for 30 min, 10 mL of fresh 26 mM NaBH4 aqueous was added to the dispersion dropwise. The reduction reaction proceeded at room temperature under stirring for 1 h. The


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obtained brown dispersion was centrifuged at 7200 rpm for 50 min to remove excessive reactants and free AgNPs outside the microgels, and then the precipitated AuNR@(AgNPs/PNIPAM) composite microgels were redispersed in water. Three AuNR@(AgNPs/PNIPAM) composite microgels with various AgNPs content were prepared by changing the feeding amount of AgNO3, as listed in Table 1. In addition, AgNPs/PNIPAM composite microgels without AuNR core were synthesized by our reported method13. Table 1. The synthesis recipe of composite microgels and some characterization results. AgNO3

AgNPs content

(mg)

(wt%)

AgNPs (nm)

(nm/nm)

Au@(Ag/PN)-01

2.2

15

10.0

336/173

7.3

Au@(Ag/PN)-02

3.4

26

6.5

365/171

8.6

Au@(Ag/PN)-03

4.6

29

8.6

333/185

5.8

Ag/PN

2.0

11

11.0

463/482

0.9

Samples

Average diameter of DH,Off/DH,On

SR

2.4 Characterization The ultraviolet-visible-near infrared (UV-vis-NIR) adsorption spectra before and after exposure to 808 nm NIR laser illumination were obtained from an UV-3600 spectrometer (Shimadzu, Japan) with temperature controller. A quartz cell with 1 cm optical path length was used. Transmission electron microscopy (TEM) images were taken with a JEM-2100F transmission electron microscope (JOEL, Japan) at an accelerating voltage of 200 kV. Diluted sample was dropped onto carbon coated copper grids and dried at room temperature. The average particle sizes of the AuNRs and AgNPs inside the microgels were statistically obtained by measuring the sizes of 50 particles on the TEM images by particle size analysis software. Thermal gravimetric analysis (TGA) was carried out on a TGA-7 analyzer (Perkin-Elmer, USA).


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After freeze drying under vacuum for ca. 24 h, the samples were heated from 25 to 900 °C with a heating rate of 10 °C/min in N2 purge. The hydrodynamic diameters (DH) of the composite microgels before and after exposure to 808 nm NIR laser illumination were measured via BI200SM dynamic light scattering (DLS) (Brookheaven, USA), which is equipped with a solid laser source (wavelength of 532 nm and output power of 100 mW) and a BI-9000AT digital autocorrelator. The scattering light was collected at 90°, and the CONTIN statistical method was used to convert the measured correlation data into a particle size distribution. The photothermal property of the AuNRs or AuNR@(AgNPs/PNIPAM) microgels was evaluated by measuring time dependent temperatures of their dispersion with the same concentration using an infrared thermal imaging camera. 2.5 mL of the AuNRs or the composite microgels aqueous dispersion with the same concentration in a quartz cell of 1 cm path length was illuminated by a 808 nm semiconductor laser with an adjustable power (0-15 W) (Shanghai Xilong Optoelectronics Technology Ltd. Co.), which was measured using a handheld optical power meter (LP-3, Beijing Physcience Opto-Electronics, China). The laser power density exposed on the sample is calculated by dividing its power by the light spot area(dia. of 1 cm). The temperature of the aqueous dispersion was recorded in real time by an infrared thermal imaging camera (A300, FLIR systems Inc.), which was calibrated by a thermocouple. 2.5 Photo-controlled catalytic reaction The photo-controlled catalytic activity of the AgNPs embedded inside AuNR@(AgNPs/PNIPAM) microgels were assessed through obtaining the reaction rate constants of AgNPs catalyzed reduction of 4-nitrophenol to 4-aminophenol upon exposure to 808 nm laser illumination of different power densities. 0.05 mL of the composite microgels


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dispersion were added to 2 mL of 0.09 mM 4-nitrophenol solution in a quartz cuvette with 1cm path length, and the mixing solution was incubated at 25 °C for 10 min. After it had been irradiated by 808 nm NIR laser of certain power density for 3 min, 0.5 mL of 0.5 M fresh NaBH4 aqueous solution was added to the cuvette, and the reducing reaction proceeded under the NIR light illumination. During the reaction process, the cuvette was discontinuously put into UV-vis spectrometer to measure the absorbance at 400 nm after certain reaction time. The each measurement took less than 10 s. The reaction rate constant was obtained in term of our previous report13. 3. Results and discussion 3.1 The fabrication and characterization of AuNR@(AgNPs/PNIPAM) composite microgels The design and fabrication of AuNR@(AgNPs/PNIPAM) composite microgels are schematically illustrated in Figure 1. In order to obtain the composite microgels with outstanding photo-responsiveness, the AuNRs of strong photothermal effect under 808 nm NIR laser illumination were first synthesized using CTAB and NaOL as binary surfactants, as described in our recent report24. A series of AuNRs with various aspect ratios were synthesized by changing the used amount of AgNO3 in the feed recipe, and their UV-vis spectra were measured, as shown in Figure S1 (a). Their typical TEM image is exhibited in Figure S1 (b), it can be found that the synthesized AuNRs have narrow size distribution, with its relative standard deviation less than 5%. Their photothermal property was evaluated under illumination of 808 nm NIR laser of 3 W/cm2 power density, and their time-dependent temperature was measured in real time using an infrared thermal imaging camera, as depicted in Figure S1 (c). Comparing Figure


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S1(a) and Figure S1(c), we can find that the AuNRs with the longitudinal localized surface plasmon resonance (LSPR) wavelength of 812 nm have the highest end temperature after identical illumination time, meaning that they have the strongest photothermal effect among our synthesized AuNRs. This is because their longitudinal LSPR wavelength is closest to laser wavelength, leading to the most efficient photothermal conversion25,26. Therefore, the AuNRs with longitudinal LSPR wavelength of 812 nm were chosen to fabricate AuNR@(AgNPs/PNIPAM) composite microgels. The appearance of the AuNRs aqueous dispersion is shown in Figure S1(d), and their average aspect ratio obtained from TEM image is 3.9. Then, the surfaces of the AuNRs were modified with BA for two purposes, that is, introducing carbon-carbon double bond on their surfaces and increasing their surface hydrophobicity, which are helpful for subsequent homogeneous encapsulation of crosslinked poly(N-isopropylacrylamide) (PNIPAM) on their surfaces27. After initiation by addition of AAPH at 70 °C, NIPAM and MBA in the reaction solution containing the BA-modified AuNRs were copolymerized, and the formed copolymers precipitated on the surfaces of the AuNRs to produce AuNR@PNIPAM composite microgels. The appearance of the microgels dispersion is shown in Figure S2 (a), and their TEM image is shown in Figure S2 (b). The composite microgels have a regular core-shell structure, and AuNR is located at their centers. Finally, when AgNO3 was added into AuNR@PNIPAM microgels aqueous dispersion, Ag+ ions diffused into their shells and coordinated with the amide groups of PNIPAM, resulting in uniform distribution of Ag+ ions within their polymer networks13. The Ag+ ions were in situ reduced to AgNPs as sodium borohydride was added, and AgNPs loaded AuNR@PNIPAM microgels, called AuNR@(AgNPs/PNIPAM) microgels, were obtained. Correspondingly, the color of the reaction solution gradually changed from wine red to claybank, as shown in Figure 2 (a). By changing


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the used amount of AgNO3, a series of AuNR@(AgNPs/PNIPAM) microgels were prepared, as listed in Table 1. Their TEM images are shown in Figure 2 (b)~(d), respectively. It can be found that the AgNPs with the average particle diameters ranging from 6 to 10 nm were evenly distributed within their shells. As the feeding amount of AgNO3 increases, the number of AgNPs inside the shells is increased, which is evidenced by the AgNPs weight content measured by TGA, as exhibited in Table 1. For comparison, AgNPs loading PNIPAM microgels (AgNPs/PNIPAM microgels) in the absence of AuNR seeds were prepared in term of our previous report13, and their TEM image is shown in Figure S3.

Figure 1. Schematic for fabricating AuNR@(AgNPs/PNIPAM) microgel.

(a)

(b)


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(c)

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(d)

Figure 2. Typical optical image of AuNR@(AgNPs/PNIPAM) microgels dispersion (a) and TEM images of Au@(Ag/PN)-01(b), Au@(Ag/PN)-02(c), and Au@(Ag/PN)-03(d). 3.2 Photo-responsive property of AuNR@(AgNPs/PNIPAM) composite microgels Two

approaches

have

been

used

to

evaluate

photo-responsive

property

of

AuNR@(AgNPs/PNIPAM) microgels. The first one is using DLS to measure the DH of AuNR@(AgNPs/PNIPAM) microgels before and after their exposure to 808 nm laser illumination of 3 W/cm2. Figure 3 illustrates the results of the sample Au@(Ag/PN)-02. After exposure to the illumination for 4 min, its DH decreased from 365 nm to 171 nm, indicating that the composite microgels underwent light induced volume phase transition. In addition, as shown in Figure 3, a cycle of shrinkage and swelling could be triggered by switching the irradiation on and off with 4 min interval. The same behavior was found for other AuNR@(AgNPs/PNIPAM) microgels with different AgNPs content, such as Au@(Ag/PN)-01 and Au@(Ag/PN)-03, as shown in Table 1. If the same illumination was exerted on AgNPs/PNIPAM microgels dispersion with identical concentration, its DH was a little elevated. These results imply that AuNR@(AgNPs/PNIPAM) microgels have reversible photo-responsiveness.


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400 Off 350

300

DH /nm

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


250

200 On 150 0

1

2

3

Cycle times Temperature / ℃

Figure 3. Hydrodynamic diameters of Au@(Ag/N)-02 measured by DLS under switching 808 nm laser irradiation on and off with 4 min interval. The second approach to assess photo-responsiveness of AuNR@(AgNPs/PNIPAM) microgels is measuring their UV-vis-NIR spectra before and after their exposure to 808 nm laser illumination

of

3

W/cm2.

Figure

4

illustrates

the

UV-vis-NIR

spectra

of

AuNR@(AgNPs/PNIPAM) microgels with different AgNPs contents, which were obtained by measuring their diluted aqueous dispersion before illumination of the NIR laser, as well as the ones of AuNRs, AuNR@PNIPAM microgels and AgNPs/PNIPAM microgels for comparison. Due to anisotropy of AuNRs, there are two peaks in their UV-vis-NIR spectra (seeing curve (a) in Figure 4), usually called longitudinal and transverse LSPR ones, respectively. When they are encapsulated by crosslinked PNIPAM to generate AuNR@PNIPAM microgels, their longitudinal LSPR peak is a little blueshifted. However, little change occurs for their transverse LSPR peak, which is ascribed to the fact their longitudinal LSPR wavelength is much more sensitive to the environment variation surrounding the nanorods than their transverse LSPR wavelength26. When AgNPs are loaded into AuNR@PNIPAM microgels, the longitudinal LSPR peak of their embedded AuNRs is further blueshifted, and the shifting magnitude is gradually


13


increased with the loaded AgNPs amount. The phenomenon may be attributed to the plasmonic coupling between AuNR core and its neighboring AgNPs. The LSPR peak of the loaded AgNPs, positioned at ca. 400 nm, appears in the UV-vis-NIR spectra of AuNR@(AgNPs/PNIPAM) microgels. Its position, i.e. the LSPR wavelength of the AgNPs, is not changed with the AgNPs content, but its intensity is enhanced with the increasing content. As shown in Figure 4, only the LSPR peak of AgNPs appears in the UV-vis-NIR spectrum of AgNPs/PNIPAM microgels. Due to difference of their LSPR behaviors, the aqueous dispersion appearances of AuNRs, AuNR@PNIPAM microgels, AuNR@(AgNPs/PNIPAM) microgels and AgNPs/PNIPAM microgels are notably different, as displayed in Figure 4.

1.8 (a) (b) (c) (d) (e) (f)

1.5

Absorbance / a.u.

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

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1.2 0.9 0.6 0.3 0.0 300

400

500

600

700

800

900

1000 1100

Wavelength / nm

Figure 4. UV-vis-NIR spectra of diluted dispersions with the same concentration of ca. 0.3 mg/mL for (a) AuNR, (b) Au@PN, (c) Au@(Ag/PN)-01, (d) Au@(Ag/PN)-02, (e) Au@(Ag/PN)-03 and (f) Ag/PN (inset shows the optical images of (a)-(f)). The UV-vis-NIR spectra of sample Au@(Ag/PN)-02 after their exposure to 808 nm laser illumination with different power densities for different times are shown in Figure 5 (a) to (c). It can be seen from these figures that, with illuminating time extending, the LSPR peak of the AgNPs is gradually enhanced and the longitudinal LSPR peak of the AuNR is simultaneously


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blueshifted, irrespective of power density of the illumination. The two changes finally reach an equilibrium state after some illuminating time. If the absorbance at ca. 400 nm is plotted against the illuminating time, as depicted in Figure 5(d), one notes that the time for reaching the equilibrium state is reduced with increasing power density of the laser illumination. The changes of both the absorbance at ca. 400 nm and the longitudinal LSPR wavelength are confirmed to be reversible from the UV-vis-NIR spectra measured under switching the illumination on and off, as shown in Figure S4. These results again indicate that AuNR@(AgNPs/PNIPAM) microgels possess reversible photo-responsiveness.

(b) 1.6

0 min 5 min 10 min 15 min 20 min 25 min

Absorbance / a.u.

1.4 1.2 1.0 0.8 0.6

1.6

1.2 1.0 0.8 0.6

0.4

0.4

0.2

0.2

0.0 300

400

500

600

700

800

900

0 min 3 min 5 min 8 min 10 min 20 min

1.4

Absorbance / a.u.

(a)

0.0 300

1000 1100

400

500

Wavelength / nm

600

700

800

900

1000 1100

Wavelength / nm

(d)

(c)

1.60 0 min 1 min 2 min 3 min 4 min 5 min 10 min 20 min

1.4 1.2 1.0 0.8 0.6 0.4

1.55

A400nm / a.u.

1.6

Absorbance / a.u.

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


1 W/cm2 2 W/cm2 3 W/cm2

1.50

1.45

1.40

0.2 0.0 300

1.35 400

500

600

700

800

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1000 1100

0

5

10

15

20

25

NIR irradiate time/min

Wavelength / nm


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Figure 5. Illuminating time dependent UV-vis-NIR spectra of Au@(Ag/PN)-02 measured under 808 nm laser illumination of different power densities ((a): 1W/cm2, (b): 2W/cm2, (c): 3W/cm2) and the plots of their absorbances at ca. 400 nm versus illumination time (d). In order to understand the mechanism of the above observed photo-responsiveness of AuNR@(AgNPs/PNIPAM) microgels, the temperature of their dispersions with the same concentration upon exposure to 808 nm NIR illumination with power density of 3 W/cm2 for 5 min were measured in real-time using an infrared thermal imaging camera, as shown in Figure S5. For comparison, the temperatures of pure water and AuNRs, AuNR@PNIPAM microgels and AgNPs/PNIPAM microgels aqueous dispersion under the same condition were measured. The plots of their temperatures against the illuminating time are illustrated in Figure 6. We can find from the figure that, other than pure water and AgNPs/PNIPAM microgels, AuNR@PNIPAM microgels and AuNR@(AgNPs/PNIPAM) microgels were heated and their temperatures were markedly raised with illumination time. After 5 min illumination, the end temperature of AuNR@(AgNPs/PNIPAM) microgels is lowered with their loaded AgNPs content increasing, which may be because the NIR light exerted on the microgels is partly scattered by the AgNPs inside their shells, resulting in reduction of the light absorbed by the centered AuNR. Another reason could be that the difference between the longitudinal LSPR wavelength of the AuNR and the NIR irradiation wavelength is increased with the AgNPs content, as above described, leading to decreasing photothermal conversion efficiency25. When the AuNRs centered at AuNR@(AgNPs/PNIPAM) microgels absorbed the NIR light, the produced heat caused their temperatures to rise. If the temperatures surpassed the volume phase transition temperature (VPTT, 37℃, measured by DLS, as shown in Figure S6) of the PNIPAM hydrogel layer on the AuNRs, the microgels shrank, behaving as photo-responsive.


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60 H2O Ag/PN AuNR Au@PN Au@(Ag/PN)-01 Au@(Ag/PN)-02 Au@(Ag/PN)-03

50

Temperature / °C

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


40

37 °C

30

20 0

60

120

180

240

300

Illumination time / s

Figure 6. The plots of the temperatures of pure water and AuNRs, AgNPs/PNIPAM microgels, AuNR@PNIPAM microgels and AuNR@(AgNPs/PNIPAM) microgels dispersions with the same concentration (ca. 0.3 mg/mL) against the illumination time. 3.3 Photo-controlled catalytic activity of AuNR@(AgNPs/PNIPAM) microgels The reduction of 4-nitrophenol by an excess amount of NaBH4 to 4-aminophenol was used as a model reaction to investigate photo-controlled catalytic activity of AuNR@(AgNPs/PNIPAM) microgels. Since trace amounts of the composite microgels were needed to catalyze the reduction reaction (ca. 6 × 10-3 mg/mL), their UV-vis absorption in the range of 275-500 nm could be ignored. The kinetics of this reaction was monitored by measuring UV-vis spectra as a function of time, as illustrated in Figure 7. The peak at ca. 400 nm for 4-nitrophenate ions produced immediately after addition of NaBH4 into 4-nitrophenol solution is gradually weakened as a function of time. Furthermore, it can be observed clearly from the appearances of the reaction system that the addition of the composite microgels (sample Au@(Ag/PN)-02) caused the fading and ultimate bleaching of the yellow color of the reactant, as displayed in Figure 7. The extent of the reactant conversion, the ratio of the concentration, ct, of 4-nitrophenol at time, t, to its initial value, c0, at t = 0 (ct/c0), can be directly obtained by the ratio of the respective absorbance At/A0.


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1.4

0s 43 s 86 s 129 s 171 s 213 s 256 s 298 s 340 s 383 s 425 s 468 s

1.2

Absorbance / a.u.

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1.0 0.8 0.6 0.4 0.2 0.0 250

300

350

400

450

500

Wavelength / nm

Figure 7. UV-vis spectra of the reaction solution of 4-nitrophenol and NaBH4 after addition of Au@(Ag/PN)-02 (ca. 6×10-3 mg/mL) measured at different reaction time (inset shows the color change of the reaction solution at different reaction time). As the concentration of NaBH4 was much higher than that of 4-nitrophenol as given in the experimental section, a first-order reaction kinetics with regard to the latter reactant can be used to obtain the apparent rate constant, kapp, of the reaction in the case of low conversion12. As evidenced in Figure S7, linear relationships between ln[ct/c0] and the reaction time, t, are present at low conversions under the illumination of 808 nm NIR laser with different power densities (PD). The kapp as a function of PD was obtained from the plots at different power densities, as depicted in Figure 8. It can be found from the figure that the variation of kapp with PD can be divided into three stages. When the PD of the illumination was lower than 2 W/cm2, the heat from photothermal effect of the AuNR is not enough to drive the microgels to undergo volume phase transition. Under this case the AgNPs entrapped within their shell layer were able to contact the reactants completely for the catalytic reduction reaction. Therefore, the kapp is increased with PD of the irradiation due to rise of the reaction temperature inside the microrgels. However, as PD of the irradiation was increased from 2 to 4 W/cm2, the kapp dropped. This is


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because the shell layer of AuNR@(AgNPs/PNIPAM) microgels shrank markedly when the heat from the AuNR was enough to drive them to undergo volume phase transition, slowing down the diffusion of the reactants into the layer. In addition, the result reveals that the effect of reaction temperature on kapp is overcompensated by the diffusional barrier12. If the PD was further increased, the kapp was strikingly elevated again. As the composite microgels continued to shrink, some of the entrapped AgNPs might be exposed toward the aqueous medium, causing more reactants to contact AgNPs. In addition, since the effect of temperature on the kapp is presumably predominant over that of diffusional barrier, the temperature increase resulting from the photothermal effect of the AuNR could lead to its rise with the PD. The above results imply that the catalytic activity of AuNR@(AgNPs/PNIPAM) microgels can be controlled by the PD of 808 nm laser illumination exerted on the reaction system.

0.014 0.012 0.010

kapp / s-1

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0.008 0.006 0.004 0.002 0

1

2

3

PD / Wcm

4

5

6

-2

Figure 8. Plot of the kapp versus power density (PD) of 808 nm laser illumination exerted on the reaction system. 4. Conclusions


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In summary, a novel type of AgNPs loaded photo-responsive composite microgel basing on the AuNR with strong photothermal effect as core and thermo-responsive PNIPAM hydrogel as shell was prepared by seed precipitation copolymerization of NIPAM and MBA using the BAmodified AuNRs as seeds, followed by in situ reduction of Ag+ ions by NaBH4. The TEM characterization results verify that the AuNR is located at the center of the composite microgel and AgNPs with the average particle diameter in the range of 6~10 nm are evenly distributed within its shell. The hydrodynamic diameters of the composite microgels measured before or after exposure of their aqueous dispersion to NIR laser of 808 nm wavelength confirm that they have photo-responsive property. Correspondingly, the LSPR efficiency of their loaded AgNPs and the longitudinal LSPR wavelength of their loaded AuNR are capable of being changed in response to the NIR illumination, and the stimulus-responsive behavior is reversible. When the composite microgels were used as the catalysts for reducing 4-nitrophenol by NaBH4 in aqueous solution, their catalytic activity can be controlled by power density of the NIR laser exerted on the reaction system. ASSOCIATED CONTENT Supporting Information. Additional characterization details of UV-vis-NIR spectra, TEM images, appearance image, temperature images and linear relationships between ln[ct/c0] and the reaction time. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: 0086-21-67792824


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*E-mail: [email protected] Phone: 0086-21-67792047 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are grateful to National Natural Science Foundation of China (51373030; 21444002) for financial support. References

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Table of Contents


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Schematic for Preparation of Silver Nanoparticles Loaded Photo-Responsive Composite Microgels and their Light-Controllable Catalytic Activity. 30x18mm (300 x 300 DPI)


Preparation of Silver Nanoparticles Loaded Photoresponsive

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