Ind. Eng. Chem. Res. 2010, 49, 2707–2715
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Synthesis of Gold Nanoparticles Coated with Polystyrene-block-poly(N-isopropylacrylamide) and Their Thermoresponsive Ultraviolet-Visible Absorbance Yunxia Liu, Weixia Tu, and Dapeng Cao* DiVision of Molecular and Materials Simulation, Key Lab of Nanomaterials, Ministry of Education, Beijing UniVersity of Chemical Technology, Beijing 100029, P.R. China
Poly(styrene-b-N-isopropylacrylamide) (PSt-b-PNIPAM) with a narrow molecular weight distribution is prepared by reversible addition-fragmentation transfer radical polymerization. The dithioester group at the chain end of PSt-b-PNIPAM is converted into thiol terminal group by LiB(C2H5)3H. Gold nanoparticles and the PSt-b-PNIPAM interact via the terminal thiol group (-SH). The size of the gold nanoparticles coated with copolymers can be easily manipulated by adjusting the molar ratio of HAuCl4/PSt-b-PNIPAM. Interestingly, the thermosensitive gold nanoparticles exhibit a sharp and reversible transparent-opaque transition between 25 and 40 °C. Moreover, the transition change is sensitive to the size of the gold nanoparticles and the macromolecular weight of the copolymer. The maximum wavelength of the surface plasmon band and the hydrodynamic size of the PSt-b-PNIPAM-Au micelles are sensitive to temperature, pH, and salt concentration. A considerable red shift from 524 to 534 nm in the plasmon band is observed in 0.9% NaCl aqueous solution, but no appreciable change in the band is observed in pure water when the temperature increases from 25 to 40 °C. With a decrease of the pH of the solution, the maximum wavelength of the surface plasmon band exhibits a red shift (from 520 to 534 nm). In addition, dynamic light scatting (DLS) reveals that the hydrodynamic size of coated gold nanoparticles exhibits a small change under alkaline and neutral (pH ) 7) conditions, but it gives a pronounced change in the acidic condition (from 350 to 280 nm) when the temperature increases from 25 to 40 °C. Furthermore, the UV-vis absorption spectrum clearly shows that the red shift of the thermosensitive gold nanoparticles is reversible. 1. Introduction Functionalized metal nanoparticles, especially gold nanoparticles have attracted great interest during the past decade owing to their unique physical and chemical properties. They have various potential applications in biomedical, electronic, and optical materials as well as in catalysis.1-3 Typically, an organic layer composed of a surfactant4,5 or a polymer6,7 is anchored at the interface to localize the nanoparticle and stabilize its properties. Recently, nanoparticles grafted with polymers have been extensively studied. In the “grafting-to” strategy to prepare polymer-protected gold nanoparticles, the grafted gold nanoparticles can be prepared in situ in a homogeneous solution consisting of HAuCl4 · xH2O and polymers end-capped with a thiol functionalized group8 or containing a disulfide unit.9 The “grafting-to” strategy combining with a controlled free radical polymerization technique is an especially useful synthetic route to prepare polymer-stabilized gold nanoparticles. Through the reversible addition-fragmentation chain transfer (RAFT) polymerization, polymers bearing a dithioester end group can be straightforwardly employed in the synthesis of grafted gold nanoparticles, with no need of prehydrolysis for the polymers to the thiolated ones. This is because a dithioester end group can be reduced to a thiol simultaneously when HAuCl4 is reduced by adding a reductant.10,11 RAFT polymerization can be applied to many monomers in a broad range of experimental conditions and provides an easy method to prepare polymers with complex structures.12-14 Gold nanoparticles stabilized by various polymers have been synthesized by this technique.10,15-17 * To whom correspondence should be addressed. E-mail: caodp@ mail.buct.edu.cn;
[email protected]. Fax: +8610-64427616.
It is well-known that an aqueous solution of poly(Nisopropylacrylamide) (PNIPAM) undergoes a thermally reversible phase separation.18,19 PNIPAM dissolves in water with a random coil conformation at room temperature, but it separates from the aqueous phase above 31-32 °C, a lower critical solution temperature (LCST). Recently, the welldefined PNIPAM has been synthesized by RAFT radical polymerization.20-23 Thermosensitive gold nanoparticles exhibit a sharp clear-to-opaque transition in water between 25 and 30 °C.24 Recently, more detailed investigations on the preparation and thermosensitive behavior of PNIPAMcoated gold nanoparticles were reported by Tenhu and co-workers.25,26 They studied the optical properties of the thermosensitive gold nanoparticles27 and the changes in the surface plasmon resonance (SPR) of gold nanoparticles coated by the homopolymer PNIPAM.25 The SPR of Au nanoparticles was also studied by Suzuki and Kawaguchi.28-30 They reported the synthesis of hybrid core/shell microgels, in which Au or Au/Ag nanoparticles were dispersed in the gel matrix, and the hybrid microgels exhibit multiple brilliant colors due to the thermosensitive properties of the microgels. Gold nanoparticles grafted with homopolymer have been extensively studied. In fact, thermosensitive gold nanoparticles also can be stabilized by the diblock copolymers. As pointed out in previous literature, the solution is influenced by added salt and changing the pH, because they can alter the polymerwater interaction.31 The addition of hydrophobic blocks to the PNIPAM would decrease the LCST of the homopolymer.32-34 Accordingly, in this work, we prepared the gold nanoparticles grafted with PSt-b-PNIPAM successfully and investigated the optical property of the gold nanoparticles changing with the size of the gold nanoparticles and the macromolecular weight
10.1021/ie901143p 2010 American Chemical Society Published on Web 02/15/2010
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Scheme 1. The Schematic Route to CEDB
of the copolymer. In addition, the thermosensitive behavior of gold nanoparticles was studied by UV-vis spectroscopy and dynamic light scattering (DLS) in the salt solution and different pH of the solvent. 2. Experimental Section 2.1. Materials. N-isopropylacrylamide (NIPAM, Acros, 99%); hydrogen tetrachloroaureate(III) hydrate (HAuCl4 · 2H2O, Au content: 52%, Beijing Chemical Reagent); 1.0 M solution of lithium triethylborohydride (LiB(C2H5)3H) in THF (Alfa, Aesar); tetrahydrofuran (THF, Beijing Chemical Reagent) were of analytical grade and used as received. 2,2′-Azobis-(isobutyronitrile) (AIBN, Tianjin, Bodi) was recrystallized from ethanol. Styrene (Tianjin, Fuchen) was distilled at reduced pressure prior to use. All other reagents were of analytical grade and used as received. 2.2. Synthesis of 2-Carboxylethyl Dithiobenzoate (CEDB). PSt-b-PNIPAM was synthesized by the RAFT polymerization starting from the preparation of a RAFT agent 2-carboxylethyl dithiobenzoate (CEDB), which was synthesized according to the method in the literature.35 Here the synthesis of CEDB was described briefly. Bromobenzene (10.5 mL, 0.1 mol) and magnesium (2.4 g, 0.1 mol) were mixed with 50 mL of dried tetrahydrofuran (THF). The mixture was stirred for 30 min or more and then heated to reflux for 1 h under a nitrogen atmosphere. Carbon disulfide (6.0 mL, 0.1 mol) was added slowly after the mixture was chilled with ice water. The reaction mixtures were kept at 0 °C for 3 h under stirring and then 1.0 M HCl was added. The solution was poured into ice water and diethyl ether was added. Dithiobenzoic acid was extracted into the ethereal layer. This washing process was repeated two or more times. After removal of the diethyl ether, the crude dithiobenzoic acid was obtained as a red-brown oil. Dithiobenzoic acid, 2-bromopropionic acid, and carbon tetrachloride were mixed and heated to reflux at 70 °C for 7 h under a nitrogen atmosphere. The resultant mixture was concentrated by a rotary evaporator and the residue was purified by column chromatography on silica gel with n-hexane as the eluent. 1H NMR: 1.90 (3H, CH-CH3); 5.35 (1H, CHCH3); 7.30-7.57 (3H, Ar-H); 7.95 (2H, σ-Ar-H of dithiobenzoate). The schematic route to CEDB is elucidated in Scheme 1. 2.3. Preparation of Dithiobenzoate-Terminated Polystyrene (PSt-SC(S)Ph) and Homopolymer PNIPAM by RAFT Polymerization. St (0.2 mol), AIBN (0.2 mmol), and CEDB (0.4 mmol) were dissolved in THF. The solution was added into a
100 mL flask with a magnetic bar. After being degassed by nitrogen atmosphere for 30 min, the flask was sealed and then placed in an oil bath at 80 °C under nitrogen atmosphere. After 24 h, the flask was cooled in an ice bath. The product was diluted with THF and the polymer was precipitated in methanol under stirring. After the purification of three precipitations with THF/methanol, PSt-SC(S)Ph was dried under vacuum at room temperature overnight. The molar mass and polydispersity (Mw/ Mn) of PSt-SC(S)Ph were measured by gel-permeation chromatography (GPC) with Mn ) 16253 and Mw/Mn ) 1.30. Homopolymer PNIPAM was synthesized by the route similar for that for PSt-SC(S)Ph, except for the concentration of N-isopropylacrylamide monomer being 0.2 mol, CEDB concentration being 0.1 mmol, AIBN concentration being 0.02 mmol, and the solvent dioxane volume was 10 mL. After polymerization, the excessive solvent was removed by rotary evaporation and the product was precipitated by the addition of diethyl ether. The PNIPAM was obtained with Mn ) 19825 and Mw/Mn ) 1.08. 2.4. Preparation of PSt-b-PNIPAM. PSt-SC(S)Ph, NIPAM, THF, and AIBN were added into a 100-mL flask with a magnetic bar. The solution was degassed by nitrogen atmosphere for 30 min. The reaction mixture was refluxed at 70 °C for 24 h under nitrogen atmosphere. After cooling the mixture with an ice bath, the excessive solvent was removed by rotary evaporation, and the product was precipitated by the addition of diethyl ether. The precipitation procedure was repeated three times with THF/diethyl ether. The powdery copolymer was dried for 24 h under vacuum at 40 °C. Two types of block copolymers (PStb-PNIPAM) were prepared with different feed ratios (Table 1). 2.5. Preparation of Gold Nanoparticles. All the gold nanoparticles were prepared in a rockered flask with different feed ratios (Table 2). The details of the preparation of PSt-bPNIPAM1-Au nanoparticles were described as an example below. A 0.1 mmol portion of HAuCl4 · 2H2O was dissolved in 10 mL of anhydrous THF. Then the copolymer dissolved in 10 mL of THF (the molar ratio copolymer/HAuCl4 · 2H2O ) 1/10) was added with a vigorous stir. The mixture was stirred for 30 min in an ice bath. Then, 1.2 mL of LiB(C2H5)3H (1.0 M solution in THF) was added dropwise for 2 min to the vigorously stirred solution. The mixture solution turned immediately purple and was stirred in an ice bath for a further 4 h. The resulting mixture was first separated by a centrifuge at 3000 rpm. THF was removed with a rotary evaporator. The solution was purified using dialysis by deionized water for 2 days. Finally, the aqueous solution was frozen and lyophilized. The color of gold nanoparticles was purple. The other PSt-b-PNIPAM-Au was prepared and purified using the same procedure except for the feed ratios. The schematic route is elucidated in Scheme 2. 2.6. Instrumentation and Characterization. Gel-Permeation Chromatography (GPC). GPC analysis was performed at 30 °C using GPC515-2410 of Waters Company in America
Table 1. RAFT Polymerization of PSt-b-NIPAM Condition and Molecular Weight Measured by GPC Method GPC sample
[AIBN] (mM)
[PSt] (mM)
[NIPAM] (M)
T (°C)
time (h)
Mn
Mw/Mn
PSt-b-PNIPAM-1 PSt-b-PNIPAM-2
0.02 0.02
0.1 0.1
0.2 0.1
80 80
24 24
39872 26671
1.39 1.35
Table 2. The Gold Nanoparticles Prepared by Different Feed Ratios and Their Diameter
PSt-b-PNIPAM (mmol) HAuCl4 · 2H2O (mmol) diameter (nm)
PbP1-Au6
PbP1-Au15
PbP1-Au30
PbP2-Au6
PbP2-Au15
PbP2-Au30
0.01 0.1 6.0 ( 1.0
0.01 0.2 15 ( 1.5
0.01 0.3 30 ( 2.5
0.01 0.1 6.0 ( 1.0
0.01 0.2 15 ( 1.5
0.01 0.3 30 ( 2.5
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Scheme 2. The Schematic Route to Gold Nanoparticles
Figure 1. 1H NMR spectrum of PSt-b-PNIPAM in CDCl3.
and Styragel HT3-HT5-HT6E columns connected in series. THF was used as the eluent at a flow rate of 1.0 mL/min. The numberaverage molecular weight (Mn), weight-average molecular weight (Mw), and Mw/Mn of the sample polymers were calibrated by standard polystyrene samples. Fourier Transform Infrared Spectroscopy (FT-IR). Fourier-transform infrared spectroscopy was recorded by Nicolet 8700 spectrometer from Thermo-fisher Company, U.S. The wavelength range was 500-4000 cm-1 and scan resolution was 4 cm-1. Samples were prepared by milling with potassium bromide (KBr) to form a very fine powder and then compressed into a thin pellet. Transmission Electron Microscopy (TEM). TEM measurements were performed on a JEM 3010 instrument operated at an accelerating voltage of 100 kV. A sample was prepared by placing a drop of gold nanoparticle solution on a carbon-coated copper grid followed by air-drying. Nuclear Magnetic Resonance (NMR). 1H NMR spectra was obtained with a Bruker AV 600 spectrometer operating at 500 MHz. The samples were analyzed in D2O at 25 °C. UV-visible Absorption Spectroscopy. UV-vis absorption spectroscopic measurements were performed with a UV-vis 2501 PC spectrophotometer with a 1.0 cm path length quartz cell at various temperatures. The temperature was changed from 20 to 50 °C with a heating rate of 0.5 °C/min. Nano Granularity Analyzer. The diameter of the gold nanoparticles was measured by ZETA-SIZER 3000HS Nano Granularity Analyzer. The samples were dissolved in THF and the concentration of the solution was 0.01-0.05 g/L. Dynamic Light Scattering (DLS). DLS measurements were conducted with a Brookhaven Instruments BI-200SM goniometer and a BI-9000AT digital correlator. The light source was a Lexel 85 argon laser (514.5 nm, power range 15-150 mW). The time correlation functions were analyzed with a Laplace inversion program (CONTIN). The block copolymer samples were prepared as 1% solutions in water. Aqueous gold dispersions were filtered through Millipore membranes (0.5 µm pore size) to remove dust and nondispersed particles. The temperature increased from 20 to 45 °C with a heating rate of 0.1 °C/min. 3. Results and Discussion 3.1. Synthesis of PSt-b-PNIPAM. Figure 1 shows the 1H NMR spectrum of PSt-b-PNIPAM. The resonances labels 1 (δ ) 7.95 ppm) and 2, 7 (δ ) 6.89-7.57 ppm) represent the aromatic protons of the phenyl group. The signals at 1.6 and 2.2 ppm represent the methylene protons and methine protons of polystyrene. The signals at 6.5, 3.9, and 1.05 ppm can be
Figure 2. FTIR spectrum of PSt-b-PNIPAM.
clearly observed, and they are characteristic signals of PNIPAM block. This is also supported by the FTIR spectrum in Figure 2, in which the N-H stretching (3500 cm-1), CdO stretching (1650 cm-1), and second amide N-H stretching (1550 cm-1) attributable to PNIPAM are clearly visible in the spectrum of the PSt-b-PNIPAM. On the other hand, the aromatic ring stretching (1454 cm-1) and stretching of five adjacent H atoms in the benzene ring (699 cm-1) can be attributed to PS. The FTIR results clearly show that PNIPAM and PS are bound to each other. The signals of the CdS (1081 cm-1) and COO-H (3000 cm-1) also can be seen in the spectrum of PSt-b-PNIPAM. 3.2. Synthesis of Gold Nanoparticles. Scheme 3 shows the temperature-responsive PSt-b-PNIPAM-Au core-shell micelles. The PSt-b-PNIPAM-Au chains have a good solubility in THF and the PSt blocks turn inside when they dissolve in water. The resultant PSt-b-PNIPAM-Au micelles composed of PNIPAM shell and PSt core, the gold nanoparticles are collapsed on the PSt core. When the temperature is above the LCST, the PNIPAM blocks collapse and form PNIPAM loops. The diameters of the gold nanoparticles are measured by Nano Granularity Analyzer using THF as solvent. The results are shown in Table 2, in which the smaller nanoparticles can be obtained by decreasing the ratio of the copolymer and the gold solution. The diameter of the gold nanoparticles is also characterized by TEM. Figure 3 presents TEM images of (a) PbP1-Au6, (c) PbP1-Au15, (e) PbP1-Au30 and their size distributions (PbP1-Au6, PbP1-Au15, PbP1-Au30 mean the PStb-PNIPAM1 micelles grafted with the 6, 15, and 30 nm gold nanoparticles, respectively). In Figure 3a, the gold nanoparticles are prepared at molar ratio of Au/PbP ) 10/1, the mean diameter of the gold nanoparticles is 6 nm, and the size distribution is narrower (Figure 3b). When Au/PbP increases to 20/1 (Figure 3c], the size of gold nanoparticles localized on the micelle increases obviously and the size distribution is broader than the
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Scheme 3. The Temperature Responsive Core Shell Micelles Grafted with the Gold Nanoparticles
PbP1-Au6 (Figure 3d). When Au/PbP is 30/1, the gold nanoparticles have a very large average size and distribution (Figure 3e,f). Figure 3g,I is the image of PbP1-Au6 micelle and Figure 3h is its relevant size distribution. It can be seen from Figure 3I that the micelles contain a PSt core (gray) and PNIPAM shell (French gray), and gold nanoparticles (the dark dots) are grafted on the PSt-b-PNIPAM micelle region. The other PSt-b-PNIPAM micelles and their distributions are similar to the PbP1-Au6 micelles. The PSt-b-PNIPAM-Au micelles in water are stable and no precipitation is observed at room temperature for 1 month. 3.3. Transmittance Change of the Polymer and Gold Nanoparticles. Gold nanoparticles coated with the copolymer show remarkable temperature sensitivity in their optical transmittance switching property at 524 nm. Figure 4 shows the typical plots of the temperature-dependent optical transmittance of the thermosensitive gold nanoparticles at 524 nm. The concentrations of all the solutions are 5 × 10-4 g/mL. It can be seen from Figure 4 although the gold is coated by the same copolymer, the transition of 6 nm gold nanoparticles (PbP1Au6) occurs at 25-31 °C, while that of 15 and 30 nm gold nanoparticles (i.e., PbP1-Au15 and PbP1-Au30) shifts to 26-35 °C. Thermosensitive gold nanoparticle of 6 nm (i.e., Au6) has a narrower transparent-to-opaque transition (6 °C) than Au15 (9 °C) and Au30 (9 °C). The optical transmittances of the homopolymer (PNIPAM) and the block copolymer (PSt-bPNIPAM1) have been investigated in Figure 4. The transmittance-temperature curve for the block copolymer (10 °C) is narrower than that of the pure homopolymer (15 °C). The onset occurs at 28 °C and completes at 38 °C for block copolymer, whereas it starts at 31 °C and completes at 46 °C with residual 10% transmittance for pure homopolymer PNIPAM. The reason is that the hydrophobic block of the polystyrene in the copolymer enhances the hydrophobic property when the temperature is above the LCST. For the thermosensitive gold nanoparticles, when the temperature is lower than LCST, the transmittance is about 80%. At high temperatures, the solution becomes completely opaque with transmittance
