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Gold Nanoparticles Protected with pH and Temperature-Sensitive Diblock Copolymers Markus Nuopponen and Heikki Tenhu* Laboratory of Polymer Chemistry, UniVersity of Helsinki, PB 55, FIN-00014 HY, Finland ReceiVed NoVember 6, 2006. In Final Form: February 27, 2007 Aqueous dispersions of gold nanoparticles protected with a stimuli-sensitive diblock copolymer were studied as a function of pH and temperature. Poly(methacrylic acid)-block-poly(N-isopropylacrylamide), PMAA-b-PNIPAM, copolymer was synthesized using the RAFT technique. A one-pot method utilizing the dithiobenzoate functionalized polymer was used to prepare gold nanoparticles protected with PMAA-b-PNIPAM. The gold nanoparticles coated with block copolymers, with the PNIPAM block bound to the particle surface and PMAA as an outer block form stimuli-sensitive aggregates in water. The changes in the absorption maxima of the surface plasmon resonance, SPR, of the gold particles and in the size of the aggregates were investigated as a function of pH and temperature. pH was observed to affect the size of the aggregates, whereas the effect of temperature was moderate. However, a blue shift in the SPR was observed both with decreasing pH and increasing temperature. Whereas the PMAA blocks control the colloidal stability of the particles and their aggregates, the thermo-sensitive PNIPAM blocks have a noticeable effect on the polarity of the immediate surroundings of the particles.
Introduction Functionalized metal nanoparticles, especially gold nanoparticles have attracted great interest during the past decade due to their various potential applications as biomedical, electronic, and optical materials as well as in catalysis.1-5 Inspired by the synthetic advances in preparing alkanethiol-protected gold particles,6 nanoparticles grafted with polymers have also been extensively studied. In the “grafting-to” strategy to prepare polymer protected gold clusters, polymers end-capped with a thiol group or containing a disulfide unit have been used instead of alkanethiol ligands.7-10 The “grafting-to” strategy is an especially useful synthetic route to prepare polymer stabilized gold clusters, when combined with a controlled free radical polymerization technique which enables the control of the molecular weight and molecular weight distribution of the polymers. Reversible addition-fragmentation chain transfer (RAFT) polymerization mediated by thiocarbonylthio compounds is an effective and versatile process, applicable to a wide range of vinyl monomers without a need for protecting groups.11-14 In our previous work,15 we found that polymers bearing a dithioester * Corresponding author. Tel.: +358-9-19150334. Fax: +358-9-19150330. E-mail:
[email protected]. (1) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959-1964. (2) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18-52. (3) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (4) Daniel, M.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (5) Corti, C. W.; Holliday, R. J. Gold Bull. 2004, 37, 20-26. (6) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc. Chem. Comm. 1994, 801-802. (7) Teranishi, T.; Kiyokawa, I.; Miyake, M. AdV. Mater. 1998, 10, 596-599. (8) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696-12697. (9) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ruehm, A.; Lennox, R. B. J. Am. Chem. Soc. 2001, 123, 10411-10412. (10) Corbierre, M. K.; Cameron, N. S.; Lennox, R. B. Langmuir 2004, 20, 2867-2873. (11) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559-5562. (12) Le, T. P.; Moad, G.; Rizzardo, E.; Thang, S. H. WO 98/01478, 1998. (13) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2005, 58, 379-410. (14) McCormick, C. L.; Lowe, A. B. Acc. Chem. Res. 2004, 37, 312-325.
end group synthesized through the RAFT polymerization can be employed in the synthesis of polymer-grafted gold nanoparticles in a straight forward manner, with no need of prehydrolysis of the polymers to the thiolated ones. This is because the dithioester end group can be reduced to a thiol simultaneously when reducing HAuCl4 into metallic gold.16,17 “Smart polymers” respond to external stimuli such as changes in temperature and pH.18 Owing to the above-mentioned progress in the controlled radical polymerization, it is relatively easy to synthesize block copolymers consisting of blocks with different responsive characters (e.g., temperature and pH-sensitive blocks). Thus, amphiphilic and double hydrophilic block copolymers combining two stimuli sensitive polymers have attracted much attention.19-21 Poly(N-isopropylacrylamide)22,23 (PNIPAM) is one of the most studied thermally responsive polymers, which exhibits a lower critical solution temperature (LCST) around 32 °C in water. However, recent studies have shown that hydrophobic/hydrophilic modification and surface-grafting affect the behavior of PNIPAM.24-26 It is also well-known that poly(methacrylic acid) (PMAA) can undergo a marked pH-induced conformational transition. At low pH, PMAA chains adopt a compact structure due to hydrophobic interactions.27,28 PNIPAM and PMAA are (15) Shan, J.; Nuopponen, M.; Jiang, H.; Kauppinen, E.; Tenhu, H. Macromolecules 2003, 36, 4526-4533. (16) Lowe, A. B.; Sumerlin, B. S.; Donovan, M. S.; McCormick, C. L. J. Am. Chem. Soc. 2002, 124, 11562-11563. (17) Duwez, A. S.; Guillet, P.; Colard, C.; Gohy, J. F.; Fustin, C. A. Macromolecules 2006, 39, 2729-2731. (18) Hoffman, A. S.; Stayton, P. S. Macromol. Symp. 2004, 207, 139-151. (19) Chen, G.; Hoffman, A. S. Nature 1995, 373, 49-52. (20) Schilli, C. M.; Zhang, M.; Rizzardo, E.; Thang, S. H.; Chong, Y. K.; Edwards, K.; Karlsson, G.; Mueller, A. H. E. Macromolecules 2004, 37, 78617866. (21) Mertoglu, M.; Garnier, S.; Laschewsky, A.; Skrabania, K.; Storsberg, J. Polymer 2005, 46, 7726-7740. (22) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163-249. (23) Aseyev, V. O.; Tenhu, H.; Winnik, F. M. AdV. Polym. Sci. 2006, 196, 1-85. (24) Xia, Y.; Burke, N. A. D.; Stoever, H. D. H. Macromolecules 2006, 39, 2275-2283. (25) Balamurugan, S.; Mendez, S.; Balamurugan, S. S.; O’Brien, M. J.; Lopez, G. P. Langmuir 2003, 19, 2545-2549. (26) Shan, J.; Chen, J.; Nuopponen, M.; Tenhu, H. Langmuir 2004, 20, 46714676.
10.1021/la063240m CCC: $37.00 © 2007 American Chemical Society Published on Web 04/13/2007
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the polymers used in the present investigation. Environmentally responsive polymers adsorbed or grafted to surfaces offer a possibility to vary surface properties in a controlled manner. Recently, environmentally responsive polymers have also been used to stabilize gold nanoparticles.29-36 In this work, a PMAA-b-PNIPAM block copolymer was synthesized by RAFT polymerization, thus having a dithiobenzoate chain end. This polymer was used to passivate the gold nanoparticles in a one-pot synthesis where the polymer is bound to the gold surface by a sulfur bridge. The properties of the product gold nanoparticles protected with stimuli-sensitive diblock copolymers can be modified by varying pH and temperature of the aqueous dispersions of the particles. In the present case, the dithiobenzoate end group was in the end of the PNIPAM block, and thus, the block copolymers were bound to the gold surfaces through this end, this leaving the PMAA blocks as the outer shell around the particles. The colloidal stability of the particles is correspondingly assumed to be governed by the PMAA blocks. The hypothesis was that colloidal stability of the particles may be controlled by adjusting the pH. On the other hand, thermoresponsive PNIPAM, which is directly bound to the metal surface, was expected to be capable of modulating the intimate surroundings of the particles. Recently it was shown by Shan et al.37 that optical properties of a monolayer of gold nanoparticles on water surface are in a most delicate manner affected by the conformation of PNIPAM chains bound to the particles. The amount of water close to the particle surface, and thus the polarity (dielectricity) of the medium can be changed by changing the PNIPAM conformation. In the present case, the aim was to find out whether the thermal collapse of PNIPAM at its demixing temperature may be observed as a shift in the surface plasmon resonance band of the gold particle. To sum up, the purpose of this report is to elucidate how the inter and intra particle properties of the gold entities can be altered by changing pH (this affecting the degree of aggregation of the particles) and temperature (changing the polarity of the microenvironment of the particles). Both stimuli will be shown to affect, e.g., the optical properties of the particles. Experimental Section Materials. The monomer tert-butylmethacrylate (t-BMA, Aldrich) was distilled under reduced pressure prior to use. N-isopropylacrylamide (NIPAM, Polysciences, Ins.) was recrystallized twice from benzene. Azobis(isobutyronitrile) (AIBN, Fluka) was recrystallized from methanol. Dioxane (Lab-Scan, Analytical Sc.) and tetrahydrofuran (THF) (Lab-Scan, HPLC) used for the synthesis of gold nanoparticles were distilled before use. Benzyl chloride (Aldrich), elemental sulfur (Aldrich), sodium methoxide (Fluka), R-methylstyrene (Aldrich), carbon tetrachloride (Merck), alumina (Merck), hydrogen tetrachloroaureate (III) hydrate (HAuCl4‚xH2O, Au content (27) Eliassaf, J.; Silberberg, A. Polymer 1962, 3, 555-564. (28) Olea, A. F.; Thomas, J. K. Macromolecules 1989, 22, 1165-1169. (29) Nath, N.; Chilkoti, A. J. Am. Chem. Soc. 2001, 123, 8197-8202. (30) Mangeney, C.; Ferrage, F.; Aujard, I.; Marchi-Artzner, V.; Jullien, L.; Ouari, O.; Rekaie, E. D.; Laschewsky, A.; Vikholm, I.; Sadowski, J. W. J. Am. Chem. Soc. 2002, 124, 5811-5821. (31) Raula, J.; Shan, J.; Nuopponen, M.; Niskanen, A.; Jiang, H.; Kauppinen, E. I.; Tenhu, H. Langmuir 2003, 19, 3499-3504. (32) Zhu, M.; Wang, L.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656-2657. (33) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 12016-12024. (34) Luo, S.; Xu, J.; Zhang, Y.; Liu, S.; Wu, C. J. Phys. Chem. B 2005, 109, 22159-22166. (35) Fustin, C. A.; Colard, C.; Filali, M.; Guillet, P.; Duwez, A. S.; Meier, M. A. R.; Schubert, U. S.; Gohy, J. F. Langmuir 2006, 22, 6690-6695. (36) Zheng, P.; Jiang, X.; Zhang, X.; Zhang, W.; Shi, L. Langmuir 2006, 22, 9393. (37) Shan, J.; Chen, J.; Nuopponen, M.; Viitala, T.; Jiang, H.; Peltonen, J.; Kauppinen, E.; Tenhu, H. Langmuir 2006, 22, 794-801.
Scheme 1. Block Copolymer PNIPAM-b-PMAA Synthesized by RAFT Polymerizationa
a
Mn PNIPAM ) 13900 g/mol, Mn PMAA ) 11600 g/mol
52%, Fluka), and 1.0 M solution of lithium triethylborohydride (LiB(C2H5)3H) in THF (Aldrich) were used as received. All other solvents with highest purity were used as received. The water used for all the measurements was purified and deionized in an Elgastat UHQPS purification system. Synthesis of Poly(t-BMA)-b-PNIPAM. Polymer was synthesized using RAFT technique where cumyl dithiobenzoate38 was used as a RAFT agent. The synthetic routes of the RAFT agent and PNIPAM have been described earlier.39 First, tert-butyl methacrylate was polymerized in dioxane. The reaction mixture (AIBN 1.3 mM, cumyl dithiobenzoate 12.3 mM, t-BMA 6.0 M, dioxane 5.6 mL) was degassed by three freeze-pump-thaw cycles before heating in a thermostated oil bath at 70 °C for 46 h. After the polymerization, the poly(t-BMA) was isolated by precipitating in a water-methanol (1:4) mixture. The molar mass of poly(t-BMA) was Mn ) 19 400 and PDI ) 1.15, measured by SEC. Poly(t-BMA) was isolated and used as a macro RAFT-agent in the polymerization of NIPAM. Block copolymerization (AIBN 0.9 mM, macroRAFT 5.6 mM, NIPAM 2.7 M) was performed in dioxane (41 mL) at 70 °C for 48 h. Polymer was precipitated from chloroform into a water-methanol (1:4) mixture. Purification was repeated by dissolving polymer in chloroform and precipitating in water/methanol. The molar mass (25 600 g/mol) and polydispersity (1.18) were measured using 1H NMR and SEC. Preparation of Poly(Methacrylic Acid-block-N-Isopropylacrylamide). Poly(MAA-b-NIPAM) was prepared by hydrolyzing the poly(t-BMA-b-NIPAM) under acidic condition. Polymer was dispersed in a 4.0 M HCl solution. The reaction mixture was heated at 80 °C for 24 h. Then, the solution was dialyzed in water for several days. The 1H NMR spectrum of the resulting poly(t-BMAb-NIPAM) in D2O showed the disappearance of the tert-butyl resonance at δ ) 1.40 ppm. Hydrolysis of PNIPAM to poly(acrylamide) was not observed. The structure of poly(MAA-bNIPAM) is presented in Scheme 1. Preparation of Gold Nanoparticles. To a vigorously stirred yellow solution of 0.1 mmol (37.9 mg) of HAuCl4‚xH2O in 10 mL of anhydrous THF was added 0.01 mmol (260 mg) of polymer in 10 mL of methanol (the molar ratio polymer/ HAuCl4‚xH2O ) 1/10. The mixture was stirred for 30 min in an ice bath. Then, 1.0 mL of 1.0 M solution of LiB(C2H5)3H 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. Purification of nanoparticles was performed using centrifugation and dialysis. The aqueous dispersion was frozen and lyophilized. Instrumentation and Characterization. The molar masses (Mn) and molar mass distributions (Mw/Mn) were determined using Waters SEC equipment with Styragel columns, a Waters 2410 refractive index detector and Waters 2487 UV detector. THF was used as an eluent, flow rate being 0.8 mL/min, and the calibration was carried out with polystyrene standards (Showa Denko). 1H NMR spectra of the polymers were measured at ambient temperature with a 200 MHz Varian Gemini 2000 spectrometer using deuterated chloroform, methanol or D2O as a solvent. UV-vis spectra (400-800 nm) were recorded on a Shimadzu UV-1601PC spectrophotometer. Gold particle dispersions were filtered through Millipore membranes (0.45 µm pore size) prior to (38) Moad, G.; Chiefari, J.; Chong, Y. K.; Krstina, J.; Mayadunne, R. T. A.; Postma, A.; Rizzardo, E.; Thang, S. H. Polym. Int. 2000, 49, 993-1001. (39) Ganachaud, F.; Monteiro, M. J.; Gilbert, R. G.; Dourges, M.; Thang, S. H.; Rizzardo, E. Macromolecules 2000, 33, 6738-6745.
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Scheme 2. One-step Synthesis of Au-PNIPAM-PMAA with a Gold Core, a PNIPAM Inner Shell, and a PMAA Corona
measurements to remove dust. The temperature was varied from 20 to 70 °C with a heating rate of 0.2 °C/min. DLS measurements were conducted with a Brookhaven Instruments BI-200SM goniometer and a BI-9000AT digital correlator. Light source was a Lexel 85 Argon laser (488 nm power range of 15-150 mW) or a He/Ne laser (Spectra Physics SP127-35, ∼20 mW) operating at 632.8 nm. Time correlation functions were analyzed with a Laplace inversion program CONTIN. Dispersions were filtered through Millipore membranes (0.45 µm pore size) prior to measurements to remove dust. Hydrodynamic radii (Rh) of the aggregates were measured using DLS at 20 and 50 °C. Dispersions with particle concentration 0.2 mg/mL were equilibrated for 30 min before the measurements. The amount of polymer on the gold surface was determined by thermogravimetric analysis using a Mettler Toledo TGA 850, in a flowing nitrogen atmosphere. The temperature ranged from 20 to 800 °C. Bright-field TEM was performed on a FEI Tecnai 12 transmission electron microscope operating at an accelerating voltage of 120 kV. The well-dispersed particles were placed on a nickel grid by dropcasting. Thermal transitions of dilute (2.0 mg/mL) aqueous solutions of particles were measured with a Microcal VP-DSC. Temperature interval was from 10 to 65 °C and heating rates varied from 0.5 to 1.5 °C min-1.
Results and Discussion Scheme 2 shows the approach to prepare the diblock copolymer coated gold nanoparticles. RAFT polymerization of t-BMA using cumyl dithiobentzoate as a RAFT agent was conducted in dioxane at 60 °C for 48 h. The molecular weight and molecular weight distribution of poly(t-BMA) were characterized by SEC, resulting in Mn ) 19 400 and Mw/Mn ) 1.15. The relatively low polydispersity indicated the controlled character of the polymerization. Next, poly(t-BMA) was used as a macroRAFT agent in the synthesis of PNIPAM, also conducted in dioxane which is a common solvent for both polymers. We did not study the kinetics of the polymerization since controlled polymerizations of PNIPAM using cumyl dithiobenzoate have been studied in detail by Ganachaud et al.39 The average degree of polymerization of PNIPAM block was determined by 1H NMR to be 123 monomer units. Molecular weight distribution Mw/Mn of the block copolymer was 1.18 according to SEC analysis. The tert-butyl ester groups of poly(t-BMA)135-b-PNIPAM123 were hydrolyzed using an excess of HCl by refluxing in dioxane for 24 h. The complete hydrolysis was evidenced by the disappearance of the tert-butyl resonance at δ ) 1.40 ppm in the 1H NMR spectrum. NIPAM repeating units were not affected by this treatment. The one-pot synthesis utilizing RAFT polymers has turned out to be a method of choice to prepare polymer-protected gold nanoparticles. The size of the gold core decreases when increasing the molar ratio of polymer repeating unit/HAuCl4. Here, the molar ratio of PMAA135-b- PNIPAM123 /HAuCl4 1:10 was used. Gold particles were purified using centrifugation and dialysis. The shapes of the gold cores were taken as truncated octahedra
Figure 1. HRTEM. The mean diameter was 2.7 ( 0.4 nm. The footprint of the polymer was estimated to be 2.2 nm2 /polymer chain. Scale bar in 20 nm.
as suggested by Whetten.40 The total mass loss of the polymer chains bound to the gold cores was determined by TGA. The sizes of the particles were measured using electron microscopy (>50 particle measured), an image is shown in Figure 1. Welldispersed particles were drop-casted on a hydrophobic nickel grid, and correspondingly, the slight aggregation of the particles seen in the micrograph may be due to solvent evaporation and is not necessarily the real situation in solution. The particles have reasonably narrow size distributions, the mean diameter being 2.7 ( 0.4 nm. The density of polymer chains was calculated in terms of the surface area of the corresponding gold core divided by the number of polymer chains bound to a surface and was estimated to be 2.2 nm2/polymer chain. Considering the estimate of the Rg for PNIPAM with a molar mass 13 000 g/mol (12 nm) by Yim et al. one may conclude that the chains bound to the surfaces are in the overlapping regime.41 Number of polymer chains per particle is remarkably lower that in previous studies,8,15,42 being just 15 chains/core (Table 1). On the other hand, the amount of organic material on the particles measured by TGA is approximately the same as in our previous investigations where polymers with lower molecular weights were used.15 Bigger polymer chains have a larger footprint area on the surface of the gold core and a lower number of polymer chains is needed to protect particles. The distance between the grafted polymer chains is, however, much shorter than their radius of gyration and one may thus assume them to be compelled to adopt a brush conformation. A diblock copolymer shell consisting of PMAA and PNIPAM blocks was used to protect the gold nanoparticles in order to create particles that respond to changes in both pH and temperature. The solubility of the PMAA block in water depends on the pH of the medium. With lowering pH carboxylic groups of the PMAA blocks get protonated and the polymer becomes less soluble in the aqueous medium. On the other hand, demixing temperature, Tdem, of the PNIPAM block can be altered by a hydrophilic block and in the present case Tdem increases upon (40) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. AdV. Mater. 1996, 8, 428-433. (41) Yim, H.; Kent, M. S.; Satija, S.; Mendez, S.; Balamurugan, S. S.; Balamurugan, S.; Lopez, C. P. J. Polym. Sci., Part B 2004, 42, 3302-3310. (42) Hostetler, M. J.; Wingate, J. E.; Zhong, C.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30.
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Langmuir, Vol. 23, No. 10, 2007 5355 Table 1. Molecular Characteristics of the Polymer and the Gold Nanoparticles
a
d
sample
Mn,polyma (g mol-1)
Au-(PNIPAM123-b-PMAA135)15
25600 1
Mw/Mnb
PNIPAM mol %c
1.18
40
b
Au/PNIPAM/PMAA wt %d 29/39/32
c
1
Determined by SEC using calibration with PS standards and H NMR. Determined by SEC. Determined with H NMR spectroscopy. Thermogravimetric analysis.
Figure 3. DLS size distribution of Au_PNIPAM_PMAA in 0.2 mg/mL aqueous dispersion in (from left to right) pH 5.0 (solid), 7.0 (dashed), and 9.0 (dotted) at 20 °C. Figure 2. UV-vis spectra as a function of pH. Turbidity increases as pH is decreased; pH decreases from bottom to top (pH 8, 7, 6, and 5). The surface plasmon resonance band undergoes a blue shift (534-525 nm) when pH is decreased.
the deprotonation of the PMAA block. It may be expected that the demixing is even totally suppressed when PMAA forms a hydrophilic corona and covers sufficiently the surface of the aggregate. The effect of pH on gold nanoparticles was monitored by optical spectroscopy (Figure 2). pH was adjusted by adding HCl or NaOH to aqueous dispersions. NaCl was used to keep the salt concentration constant (5 mM) and nullify salt effects. Absorbance of the sample increases as pH decreases as could be expected, indicating that the dispersibility of the particles decreases as PMAA is protonated. Finally, at pH 4, particles precipitate from water. Decreasing the pH of the solvent was accompanied by a blue shift in the surface plasmon resonance (SPR). When pH was lowered from 8 to 5, the λmax shifted around 10 nm to lower wavelengths. The surface plasmon absorption is an optical property for metallic nanoparticles due to an extensive electronic correlation and corresponds to a collective excitation of conduction electrons relative to the ionic core.43,44 The SPR band of nanoparticles is also dependent on the dielectric constant of the medium.45-47 On the basis of the same reasoning as in the previous work by Shan et al.,37 one can suggest that the number of water molecules in the proximity of the metal particles decreases with decreasing pH. This would be a natural consequence of the decreased solubility of PMAA, especially at pH 5. On the other hand, we are still able to observe the SPR band of the individual particles (no coupling) even at pH 5, this indicating that the gold cores are separated from each other. One needs to take extra care while analyzing these results, however. It has been pointed out by Liz-Marzan et al.48 that increasing scattering may result in (43) Kreibig, U.; Vollmer, M., Eds. Optical Properties of Metal Clusters; Springer Series in Material Science 25; Springer-Verlag: Berlin, 1995. (44) Liz-Marzan, L. M. Langmuir 2006, 22, 32-41. (45) Mulvaney, P. Langmuir 1996, 12, 788-800. (46) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564-570. (47) Ghosh, S. K.; Nath, S.; Kundu, S.; Esumi, K.; Pal, T. J. Phys. Chem. B 2004, 108, 13963-13971. (48) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 43294335.
a strong increase in absorbance at shorter wavelengths, promoting the blue shift of the SPR and weakening the intensity of the plasmon band. In the present case, however, the absorbance at short wavelengths (at the baseline) decreases with decreasing pH and the intensity of the SPR band does not decrease. This observation strengthens the conclusion of the decreasing polarity of the particle surroundings with decreasing pH. The hydrodynamic sizes of the particles in pure water were measured by dynamic light scattering. As is shown in Figure 3, the mean diameter of the scattering objects is around 100 nm at room temperature and pH 7. The finding is not surprising, knowing that the size distributions measured by DLS for the homopolymer PNIPAM are typically broad. So far we have not been able to measure the size of individual particles by light scattering. However, the aggregates are colloidally stable and individual gold cores remain mostly separated as shown above. As pH is increased, the dissociated PMAA outer layer stretches the polymers and the size of the aggregate increases slightly. On the other hand, at pH 5 aggregates take more compact structure than at pH 7. Addition of salt did not affect the size distributions. The effect of temperature on particles was measured by DLS and UV. When solutions at pH 7 were heated up to 50 °C, the average size of the aggregates decreased only slightly, from 102 to 88 nm. A similarly small change of dimension was observed also under alkaline conditions, this indicating that the dimensional change is not due to the hydrogen bonding but to the restricted mobility of the PNIPAM blocks. The finding is in line with our previous observations on crew-cut PS-PNIPAM micelles.49 At pH 5 the aggregates start to agglomerate with each other and the scattering intensity increased. The same effect was also observed by turbidity (absorbance) measurements; see Figure 4. The macroscopic agglomeration taking place at low pH and elevated temperature is irreversible, which is contrary to the known completely reversible phase transition of pure aqueous PNIPAM. Typical phase transition of pure PNIPAM, which takes place over a narrow range of temperatures was not observed by either measurement method. At neutral or basic conditions, the electrically charged PMAA corona shelters the particles keeping them dispersible in water and stretching the PNIPAM blocks, in this way preventing noticeable conformational changes. NMR (49) Nuopponen, M.; Ojala, J.; Tenhu, H. Polymer 2004, 45, 3643-3650.
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Figure 4. Turbidity curves of the Au_PNIPAM_PMAA 1.0 mg/ mL in water at two different pH’s (7.0 ) 0 and 5.0 ) 9).
Figure 5. Thermogram at pH 5, heating rate 1 °C min-1. Particle concentration 2 mg/mL.
measurements on PNIPAM-PMAA gels with a significant PNIPAM content have shown that the polymers lose their LCST behavior in basic medium but regain it at lower pHs.50 At pH 5, where the solubility of protonated PMAA is lowered, the reason for the absence of the PNIPAM thermal transition is different.51 It should be noted here that at room temperature and pH 5 the aggregates are clearly contracted, the average Rh being around 60 nm (around 100 nm at pH 7), see Figure 3. It has been shown previously by Burova et al.52 that at low pH, hydrogen bonding between PNIPAM and PMAA gives rise to the formation of hydrophobic complexes. The complexation of two blocks leads to compact structures in which PNIPAM at least partially loses its capability to collapse upon heating. To test the above conclusion on the limited capability of PNIPAM chains to collapse upon heating, the particle dispersions were further analyzed by microcalorimetry and by measuring the temperature dependence of the SPR band. At high pH, the phase transition was hardly observed even by microcalorimetry. Figure 5 displays the thermogram of particles at pH 5. Two weak but distinct transitions may be observed. The first transition at low temperature, with the heat capacity peak maximum at 19 °C corresponds to PNIPAM in a hydrophobic environment or to be more specific, enthalpy change of PNIPAM partially complexed with PMAA. Peak is in this case much wider than the peak of pure PNIPAM, and the transition enthalpy was suppressed (1.2 kJ/mol per repeating unit) compared to that of pure PNIPAM (50) Diez-Pena, E.; Quijada-Garrido, I.; Barrales-Rienda, J. M.; Schnell, I.; Spiess, H. W. Macromol. Chem. Phys. 2004, 205, 438-447. (51) Zhang, J.; Peppas, N. A. Macromolecules 2000, 33, 102-107. (52) Burova, T. V.; Grinberg, N. V.; Grinberg, V. Y.; Kalinina, E. V.; Lozinsky, V. I.; Aseyev, V. O.; Holappa, S.; Tenhu, H.; Khokhlov, A. R. Macromolecules 2005, 38, 1292-1299.
Nuopponen and Tenhu
Figure 6. The λmax of the surface plasmon resonance as a function of temperature at two pH’s (7.0 ) 0 and 5.0 ) 9).
(5.5-7.5 kJ/mol).26,53 This kind of enthalpy decrease due to polymer complexation has been reported previously.52 The second transition in the thermogram (0.9 kJ/mol) with the onset temperature 45 °C, corresponds to the dehydration of aggregates. At elevated temperature water molecules bound to the surface of the aggregates are released. This occurrence can also be macroscopically observed as aggregates starting to coagulate. Absorption spectra of the dispersions were measured as a function of temperate at pH 5 and 7 to find out if the thermal transition of PNIPAM affects the SPR band. Particles at pH 7 remain soluble even at elevated temperature and absorbance even slightly decreases upon heating, showing good dispersibility of the particles. However, a clear blue shift as a function of temperature was observed. The shift of λmax to lower wavelengths upon heating (Figure 6) also confirms the occurrence of the conformational change of PNIPAM blocks. Upon heating PNIPAM layer turns hydrophobic squeezing out water molecules from the surroundings of the Au core. This process decreases the dielectric constant of the particle surroundings and leads to the observed blue shift in the SPR. On the other hand, at pH 5 the complexation of PNIPAM and PMAA blocks makes the surroundings of the gold core more hydrophobic, indicated as lower λmax value already at 20 °C (525 nm compared to 529 nm). Complexation of the blocks also restricts any further conformational changes at the very core of the particles. Thus, no significant blue shift is detected at low pH upon heating. This study shows the usefulness of block copolymer protected gold particles. Block copolymers make it possible to protect particles in a way that the outer layer colloidally stabilizes the aggregates while the inner layer modulates the polarity of the immediate surroundings of the gold core. Previously, we have prepared amphiphilic gold particles and shown that the optical properties of the Langmuir films of these may be altered by changing the surface pressure and/or temperature.37 Now we have in hand a way to modulate the properties of the films also by changing pH. In future work, changing the lengths of PNIPAM and PMAA blocks (or their order: PNIPAM might as well be the outer block) will also help in affecting the dimensional changes and aggregation behavior of the grafted particles. PMAA was selected as a corona owing to its pH-induced conformational transition. Thus, thermoresponsiveness of the gold particles changed considerably as pH was varied, and at low pH the complexation between PMAA and PNIPAM almost completely suppressed conformational changes of the inner PNIPAM core.
Conclusions Gold nanoparticles coated with stimuli-responsive PMAAb-PNIPAM copolymers were prepared in a convenient one-pot synthesis. For this, a diblock copolymer was first prepared by (53) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352-4356.
Gold Nanoparticles
RAFT polymerization. The dithiobenzoate functionalized polymer carrying the functionality at the PNIPAM end of the chain was mixed with auric acid in a common solvent and the polymergrafted particles were obtained by adding a reductant. The particles with an inner PNIPAM block close to the gold surface and a PMAA corona disperse in water and build up stimuli-sensitive aggregates. Properties of the aggregates can be adjusted by changing pH or temperature of the aqueous medium. The water dispersibility of the particles was found to be determined by the PMAA blocks, and it was strongly dependent on the pH of water. The effect of temperature on the size of aggregates was just moderate, but temperature very much affected the optical properties of the particles. Though it seems clear that in the
Langmuir, Vol. 23, No. 10, 2007 5357
present case the thermal contraction of PNIPAM is limited under the conditions studied, it is still effective and can well be used to modulate the properties like dielectricity of the surroundings of the gold nanoparticles. Overall, it has been shown that the association and optical properties of the gold nanoparticles grafted with smart polymers18 can be widely varied by pH and temperature. Acknowledgment. The work was supported by the Finnish Funding Agency for Technology and Innovation (TEKES) and ESPOM Graduate School (Electrochemical Science and Technology of Polymers and Membranes including Biomembranes). LA063240M
