Design of Polymeric Stabilizers for Size-Controlled Synthesis of

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Langmuir 2007, 23, 885-895

885

Design of Polymeric Stabilizers for Size-Controlled Synthesis of Monodisperse Gold Nanoparticles in Water Zhenxin Wang,†,‡ Bien Tan,† Irshad Hussain,†,§ Nicolas Schaeffer,† Mark F. Wyatt,| Mathias Brust,† and Andrew I. Cooper*,† Centre for Nanoscale Science and Centre for Materials DiscoVery, Department of Chemistry, The UniVersity of LiVerpool, Crown Street, LiVerpool, L69 3BX, United Kingdom, State Key Laboratory of Electro-Analytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China, National Institute for Biotechnology and Genetic Engineering (NIBGE), Jhang Road Faisalabad, Pakistan, and EPSRC National Mass Spectrometry SerVice Centre (NMSSC), School of Medicine, Swansea UniVersity, Singleton Park, Swansea, SA2 8PP, United Kingdom ReceiVed September 7, 2006 A new methodology is described for the one-step aqueous preparation of highly monodisperse gold nanoparticles with diameters below 5 nm using thioether- and thiol-functionalized polymer ligands. The particle size and size distribution was controlled by subtle variation of the polymer structure. It was shown that poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) were the most effective stabilizing polymers in the group studied and that relatively low molar mass ligands (∼2500 g/mol) gave rise to the narrowest particle size distributions. Particle uniformity and colloidal stability to changes in ionic strength and pH were strongly affected by the hydrophobicity of the ligand end group. “Multidentate” thiol-terminated ligands were produced by employing dithiols and tetrathiols as chaintransfer agents, and these ligands gave rise to particles with unprecedented control over particle size and enhanced colloidal stability. It was found throughout that dynamic light scattering (DLS) is a very useful corroboratory technique for characterization of these gold nanoparticles in addition to optical spectroscopy and TEM.

Introduction Gold nanoparticles have a wide range of uses in modern nanoscale science, and it is therefore important to understand and control their physical and chemical properties, which are generally size dependent.1,2 Gold nanoparticles are commercially available in many forms, and numerous preparative methods are documented in the literature for particles from about 1 nm to several micrometers diameter.3-7 Nonetheless, only a handful of standard procedures are employed routinely to prepare gold particles for a multitude of applications. These methods are reliable and simple to carry out and lead to uniform particles with a narrow size distribution in the desired range. The most widely applied procedures to obtain gold hydrosols are variations of the classic Turkevich-Frens citrate reduction route.8,9 Most hydrophobic (and some hydrophilic) particles are prepared by borohydride reduction in an organic solvent in the presence of thiol capping ligands using either a two-phase liquid/liquid system or a suitable single-phase solvent.10-19 The latter approach is usually employed for particles in the 1 to ca. 8 nm range. Gold * Corresponding author. † University of Liverpool. ‡ Chinese Academy of Sciences. § NIBGE. | Swansea University. (1) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (2) Liz-Marzan, L. M. Langmuir 2006, 22, 32-41. (3) Goia, D. V.; Matijevic, E. Colloids Surf. A, Phys. Eng. Asp. 1999, 146, 139-152. (4) Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I. Langmuir 2003, 19, 4831-4835. (5) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782-6786. (6) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Calis, G. H. M.; Vandervelden, W. A. Chem. Ber. Recl. 1981, 114, 3634-3642. (7) Wilcoxon, J. P.; Provencio, P. P. J. Am. Chem. Soc. 2004, 126, 64026408. (8) Frens, G. Nat. Phys. Sci. 1973, 241, 20-22. (9) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 55. (10) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem.sEur. J. 1996, 2, 359-363.

nanoparticles are useful in a broad range of applications,20-22 but practical limitations are apparent when monodispersity is required: for example, in electrochemical quantized capacitance charging,21-23 single-electron transistor assembly,24 and applications such as thermal gradient optical imaging.25 In many cases, monodisperse fractions of particles must be prepared, usually in low yield following cumbersome size separation procedures, such as size exclusion chromatography.26,27 Moreover, (11) Bartz, M.; Kuther, J.; Nelles, G.; Weber, N.; Seshadri, R.; Tremel, W. J. Mater. Chem. 1999, 9, 1121-1125. (12) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (13) Fabris, L.; Antonello, S.; Armelao, L.; Donkers, R. L.; Polo, F.; Toniolo, C.; Maran, F. J. Am. Chem. Soc. 2006, 128, 326-336. (14) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; 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. (15) Hussain, I.; Graham, S.; Wang, Z. X.; Tan, B.; Sherrington, D. C.; Rannard, S. P.; Cooper, A. I.; Brust, M. J. Am. Chem. Soc. 2005, 127, 16398-16399. (16) Kanaras, A. G.; Kamounah, F. S.; Schaumburg, K.; Kiely, C. J.; Brust, M. Chem. Commun. 2002, 2294-2295. (17) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (18) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M. Acc. Chem. Res. 1999, 32, 397-406. (19) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696-12697. (20) Brust, M.; Kiely, C. J. Colloids Surf. A, Phys. Eng. Asp. 2002, 202, 175-186. (21) Parak, W. J.; Gerion, D.; Pellegrino, T.; Zanchet, D.; Micheel, C.; Williams, S. C.; Boudreau, R.; Le Gros, M. A.; Larabell, C. A.; Alivisatos, A. P. Nanotechnology 2003, 14, R15-R27. (22) Pellegrino, T.; Kudera, S.; Liedl, T.; Javier, A. M.; Manna, L.; Parak, W. J. Small 2005, 1, 48-63. (23) Chen, S. W.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098-2101. (24) Moriarty, P. Rep. Prog. Phys. 2001, 64, 297-381. (25) Boyer, D.; Tamarat, P.; Maali, A.; Lounis, B.; Orrit, M. Science 2002, 297, 1160-1163. (26) Sweeney, S. F.; Woehrle, G. H.; Hutchison, J. E. J. Am. Chem. Soc. 2006, 128, 3190-3197. (27) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. Langmuir 2000, 16, 99129920.

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such fractionation methods do not necessarily lead to monodisperse samples.28 The availability of a simple, robust protocol for the gram-scale preparation of uniform MPCs below 5 nm would thus be of broad practical value. A number of research groups have investigated water-soluble polymers as stabilizing ligands for gold nanoparticles in water, particularly with the aim of achieving size-controlled nanoparticle synthesis. For example, poly(ethylene oxide)-poly(propylene oxide)-stabilized gold nanoparticles were prepared29 but found to be quite polydisperse. Star-shaped poly(ethylene oxide)-blockpoly(caprolactone) ligands have also been developed.30 These ligands led to rather better control over particle size distributions but did not achieve monodisperse samples. Thiol-terminated poly(ethylene glycol) monomethyl ether (MeO-PEG-SH) was found to stabilize gold nanoparticles,19,31 and poly(N-isopropylacrylamide)-monolayer-protected gold clusters (PNIPAMMPC)32 have also been produced. The MeO-PEG-SH route in particular gave rise to well-defined (although not monodisperse) particles in water.19,31 Thiol-terminated polystyrene and poly(ethylene glycol)-stabilized gold nanoparticles have also been synthesized by “grafting to” approaches,33,34 although in these cases the gold particles were preformed and then postfunctionalized with the polymer. In general, there is an incomplete understanding of the relationship between structure and function for polymeric ligands of this type, and preparation of more sophisticated structures, for example, dendritic ligands,35 has not necessarily led to a greater degree of control over particle size and particle size distribution. Moreover, there is a real need to introduce complementary characterization methods to assess the degree of control over particle size for the bulk sample: many studies rely exclusively on TEM measurements, often using a relatively small sampling area. It is easy to significantly overestimate the monodispersity of a sample using TEM analysis as the sole means of characterization. Previously, we have shown that thioether-terminated poly(methacrylic acid) (PMAA) ligands could be used to produce aqueous gold nanodispersions in one step with unprecedented control over particle size distributions in the 1-5 nm size range.15 In this new study we investigate in detail the effect of the ligand structure on the particle size and particle size distribution and show that a number of “design rules” can be formulated for polymeric ligands of this type. As a result, we identify a modified polymer architecture with a multidentate thiol headgroup which leads to significantly smaller and more monodisperse particles at a given ligand concentration. We also show that dynamic light scattering (DLS) is a very useful complementary technique (in addition to TEM and UV-vis spectroscopy) for assessing the size and monodispersity of the bulk gold nanoparticle dispersions. The stability of these particles toward salt concentration and pH is strongly affected by relatively small changes in the polymer ligand structure, and we show that certain systems are very stable (28) Akthakul, A.; Hochbaum, A. I.; Stellacci, F.; Mayes, A. M. AdV. Mater. 2005, 17, 532-535. (29) Sakai, T.; Alexandridis, P. Langmuir 2005, 21, 8019-8025. (30) Filali, M.; Meier, M. A. R.; Schubert, U. S.; Gohy, J. F. Langmuir 2005, 21, 7995-8000. (31) Shimmin, R. G.; Schoch, A. B.; Braun, P. V. Langmuir 2004, 20, 56135620. (32) Shan, J.; Nuopponen, M.; Jiang, H.; Kauppinen, E.; Tenhu, H. Macromolecules 2003, 36, 4526-4533. (33) Corbierre, M. K.; Cameron, N. S.; Lennox, R. B. Langmuir 2004, 20, 2867-2873. (34) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ruhm, A.; Lennox, R. B. J. Am. Chem. Soc. 2001, 123, 10411-10412. (35) Kramer, M.; Perignon, N.; Haag, R.; Marty, J. D.; Thomann, R.; Lauth-de Viguerie, N.; Mingotaud, C. Macromolecules 2005, 38, 8308-8315.

Wang et al.

in both regards. Last, we show that the thioether-terminated ligands can be exchanged for biological species such as peptides. Experimental Section Chemicals. All chemicals were purchased from Aldrich and used as received, unless otherwise described. Full details of the polymer ligand synthesis can be found in the Supporting Information. Milli-Q water (18.2 MΩ) was used in all experimental processes. Synthesis of Gold Nanoparticles. A general procedure for the preparation of polymer-stabilized gold nanoparticles in water is described as follows. An aqueous solution of HAuCl4 (10 mL) was added to an aqueous polymer solution under vigorous stirring to give a final concentration of HAuCl4 of 0.5 mM. Each particle preparation was repeated at four different polymer concentrations (0.006, 0.06, 0.6 and 6 mM)sthat is, four different particle preparations were produced for each polymer ligand. Freshly prepared NaBH4 solution (1 mL, 50 mM) was added after mixing the gold/ polymer solutions for 1 h. The reducing agent was added rapidly in two aliquots (2 × 0.5 mL). The reaction was allowed to continue overnight under uniform and vigorous stirring. The gold nanoparticles were separated from excess unreacted polymer ligand by filtration (three times) with a Vivaspin centrifugal filter (Vivascience, Hannover, Germany; 10 000 g/mol cutoff). Last, the particles were dialyzed overnight using a 96-well microplate dialyzer (10 000 g/mol, The Nest Group Inc., USA) in order to remove any last traces of unreacted polymer ligand. Ligand Exchange for DDT-PMAA-Stabilized Gold Nanoparticles. The conditions used for ligand exchange of DDT-PMAAstabilized gold nanoparticles with other ligands (i.e., dodecanethiol, 11-mercaptoundecanoic acid (MUA), and peptide CALNN (95%)/ CALNNGK(biotin)G(5%)) was different for each ligand and is described in the Supporting Information. Binding Studies with Avidin and Agarose Gel Electrophoresis. After ligand exchange with a mixture of peptides (CALNN (95%)/ CALNNGK(biotin)G(5%)), the peptide-stabilized gold nanoparticles were reacted with excess avidin followed by purification using a Sephadex G-25 column (3 times). The unexchanged DDT-PMAA particles, the peptide-stabilized gold nanoparticles, and the peptidestabilized gold nanoparticles after reaction with avidin (20 µL) were loaded onto agarose gels (2% w/v in 1 × TBE) and subjected to electrophoresis at 100 V for 0.5 h. UV-visible Absorption Spectroscopy. UV-visible spectra were carried out using a microplate reader (µQuant, Bio-Tek Instruments). The aqueous gold nanoparticle solutions (200 µL) were analyzed in 96-well microplate at 25 °C. Transmission Electron Microscopy. Transmission electron microscopy (TEM) micrographs of the colloidal dispersions were obtained using a JEM-2000EX/FX instrument operated at an accelerating voltage of 200 kV. A high-resolution carbon-supported copper mesh was used to support the colloidal dispersions. Specimens for inspection by TEM were prepared by evaporating a droplet of the filtered and dialyzed gold nanoparticle solutions onto a carboncoated copper mesh grid directly from watersthat is, without solvent exchange into an organic solvent as employed previously.15 The diameter of each particle was quantified using ImagesJ software (1.34s, NIH, USA) to analyze the digitized photographic images for each sample in the magnification range 200 000-500 000×. A histogram of the particle size distribution and the average particle diameter were obtained by measuring about 200 particles in arbitrarily chosen areas in the photograph. Dynamic Light Scattering. Dynamic light scattering (DLS) measurements were carried out with Zetasizer Nano ZS (Malvern, U.K.) instrument equipped with a 1 cm optical path cell. Each sample was analyzed three times. MALDI-TOFMS, Gold Clusters. 2,5-Dihydroxybenzoic acid (DHB) matrix was purchased from Fluka (Dorset, U.K.). Dowex 50W-X8, 200 µm, ion-exchange resin and ammonium acetate were purchased from Sigma-Aldrich (Dorset, U.K.). Resin was loaded

Design of Polymeric Stabilizers Scheme 1

with NH4+ ions as reported previously.36 HPLC-grade acetonitrile (MeCN) and Milli-Q water were used where appropriate. The DHB matrix solution was made to a concentration of 10 mg mL-1 in 1:1 (v/v) H2O/MeCN. A solution of PTMP-PMAA was prepared (10 mg mL-1 in H2O), and 200 µL was placed into a plastic, snap-top vial. Roughly 0.5 mg of NH4+-loaded resin was added, and the solution was agitated via vortex mixer at slow speed for 20 s. Sample and matrix solutions were mixed in a 1:10 ratio in a separate plastic vial. A 0.5 µL amount of the final mixture was spotted onto a stainless steel sample plate and dried in a stream of cool air. MALDI-TOFMS data were acquired using an Applied Biosystems Voyager DE-STR spectrometer (Framingham, MA), which was equipped with a nitrogen laser (λ ) 337 nm). For the PTMP-PMAA-protected gold clusters, the instrument was operated in negative-ion, linear mode. The accelerating voltage was 25 kV, while the grid voltage was maintained at 91%. The delay time was 450 ns, and laser fluence was attenuated to just above the threshold of ionization. For the PTMP-PMAA ligand itself (see Supporting Information) the instrument was operated in positive-ion, reflectron mode. The accelerating voltage was 20 kV, while the grid voltage was maintained at 65.5%. The delay time was 150 ns. For all samples the laser was fired at a frequency of 3 Hz, and spectra were accumulated in multiples of 25 laser shots with 150 shots in total. Postacquisition processing of data was performed utilizing Data Explorer V4.0 software supplied by Applied Biosystems.

Results and Discussion Synthesis and Characterization of Polymer Ligands. A series of polymer ligands was synthesized by chain-transfer methods15 using thiols (or diol/multithiols) as the chain-transfer agent (Scheme 1; see Supporting Information for details; Tables S1-S5; Figures S1-S5). A feature of this methodology is that it leads to low molar mass oligomeric species with relatively narrow molecular weight distributions (PDI < 1.5). Any unreacted free thiol was removed by polymer reprecipitation in a solvent which was a good solvent for the thiol chain-transfer agent. Using this methodology we were able to produce a small library of polymeric ligand structures (Scheme 2) which varied in monomer type, end-group functionality, and molecular weight. Synthesis of Polymer-Stabilized Gold Nanoparticles. Effect of Polymer Structure. A series of six water-soluble polymer ligands with different monomer repeat units was synthesized in order to study the effect of the ligand chain structure on the average particle size and size distribution for the gold nanodispersions. The six monomers studied were methacrylic acid (MAA), acrylic acid (AA), vinylpyrrolidone (VP), vinylsulfonic acid (VSA), hydroxyethyl acrylate (HEA), and poly(ethylene glycol) (PEG) methacrylate (PEG-MA). The same thiol chaintransfer agent (DDT) was used in each case; as such, each ligand in the series was terminated with a dodecylthioether end group.15,37 The number-average molecular weight, Mn, for the six ligands was found to be in the range 1500-4500 g/mol (see Table 1) with the exception of DDT-PVP, which exhibited a much higher molecular weight (Mn ) 37 320 g/mol) despite the fact that thiol chain-transfer agents have been used previously to prepare low (36) Nordhoff, E.; Ingendoh, A.; Cramer, R.; Overberg, A.; Stahl, B.; Karas, M.; Hillenkamp, F.; Crain, P. F. Rapid Commun. Mass Spectrom. 1992, 6, 771776. (37) Li, X. M.; de Jong, M. R.; Inoue, K.; Shinkai, S.; Huskens, J.; Reinhoudt, D. N. J. Mater. Chem. 2001, 11, 1919-1923.

Langmuir, Vol. 23, No. 2, 2007 887 Scheme 2

molar mass oligomeric species.38 A commercially available linear poly(methacrylic acid) (PMAA) sample (Mn ) 2000 g/mol) sample with no thioether end group was also used as a control. All six thioether-terminated polymer ligands (DDT-PMAA, DDT-PAA, DDT-PVP, DDT-PVSA, DDT-PHEA, and DDTPPEG-MA; see Scheme 2) gave rise to stable red-colored gold nanodispersions at a polymer concentration of 0.006 mM. At higher polymer concentrations the relative performance of the ligands varied markedly. Color images of the as-produced polymer-stabilized gold nanodispersions are shown in Figure 1a. In general, the color of nanodispersions changed from red to yellow when the polymer concentration was increased from 0.006 to 6.0 mM, indicating that particles of different average sizes were prepared in each case. The polymer ligand DDTPVSA was an exception to this trend; at higher concentrations of DDT-PVSA (0.6 and 6.0 mM), the gold nanodispersion turned dark blue/black and precipitation was observed (Figure 1a), indicating that the particles were not stable to aggregation with this ligand. All of the stable nanodispersions were characterized by TEM, UV-visible, and dynamic light scattering (DLS) (see Figures 1-3 and Figures S6-S13). Overall, ligands DDT-PMAA (Mn ) 3220 g/mol) and DDT-PAA (Mn ) 2550 g/mol) gave rise to gold nanoparticles with the most narrow size distributions over the polymer concentration range 0.006-6.0 mM. A series of typical UV-visible spectra for gold nanoparticles produced using DDT-PAA (Mn ) 2550 g/mol) are shown in Figure 1b. The spectra vary strongly as the concentration of the polymer ligand is changed and suggest that the average particle size is below 5 nm for all samples since larger particles would exhibit a sharper and more intense plasmon absorption band close to 525 nm.11,12 Some of the spectra for particles produced at higher polymer concentrations (6.0 mM) do not show a plasmon band at all, indicating that most particles are below ca. 3 nm in size. The series of spectra obtained is well known for size-separated (fractionated) particles in the range below 5 nm but unprecedented for as-prepared samples.15 The ability to prepare spectroscopically (38) Torchilin, V. P.; Levchenko, T. S.; Whiteman, K. R.; Yaroslavov, A. A.; Tsatsakis, A. M.; Rizos, A. K.; Michailova, E. V.; Shtilman, M. I. Biomaterials 2001, 22, 3035-3044.

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Table 1: Effect of Monomer Type on Au Nanoparticles Produced Using DDT Thioether-Capped Vinyl Polymers particle diameter (nm)b polymer ligand

mol wt (g/mol) Mn/Mw/PDI

DDT-PAA

2550/3490/1.37

DDT-PMAA

3220/3500/1.09

DDT-PVSA

870/1550/1.79

DDT-PHEA

1810/2030/1.12

DDT-PPEG-MA

3470/4180/1.21

DDT-PVP

37 320/77 620/2.08

a

0.006 mM

0.06 mM

0.6 mM

6.0 mMc

5.3 ( 0.4 (6.5) 5.3 ( 0.7 (5.0) 6.0 ( 1.5 (5.5) 5.2 ( 1.0 (6.4) 7.7 ( 1.9 (17.5) 5.0 ( 1.4 (8.5)

3.7 ( 0.25 (4.9) 4.0 ( 0.4 (3.6) 3.6 ( 0.45 (4.2) 4.6 ( 0.8 (4.3) 4.6 ( 0.7 (4.2) 4.7 ( 0.9 (5.6)

2.7 ( 0.2 (3.8) 2.8 ( 0.3 (2.8) d

1.8 ( 0.2 (1.4) 1.7 ( 0.25 (1.5) d

2.7 ( 0.45 (3.2) 3.6 ( 0.7 (3.9) 4.2 ( 0.7 (4.9)

2.0 ( 0.3 (2.8) 2.7 ( 0.4 (3.0) 2.6 ( 0.6 (3.6)

a Mn ) number average molecular weight; Mw ) weight average molecular weight; PDI ) polydispersity index. b As estimated from TEM imaging; numbers in parentheses as measured by dynamic light scattering (DLS). c Polymer ligand concentration. d Did not form stable nanodispersion.

Figure 1. (a) Optical image of gold nanodispersions formed using six different DDT-terminated polymer ligands at four different polymer concentrations. The control solutions were prepared using commercially available poly(methacrylic acid) (Mn ) 2000 g/mol)s that is, a ligand which does not contain a thioether end group. (b) UV-visible spectra of DDT-PMAA-stabilized gold nanoparticles obtained using a polymer concentration at 0.006 (black), 0.06 (red), 0.6 (blue), and 6.0 mM (magenta).

distinct samples with diameters sub-5 nm suggests a narrow particle size distribution for each sample. To confirm this by TEM, it was convenient to first isolate the particles from excess polymer by filtration and dialysis (dialysis membrane molecular weight cutoff ) 10 000 g/mol). This procedure avoids the need to phase transfer the particles into an organic solvent prior to TEM analysis.15 Characterization by TEM (Figure 2 and Figure S7) confirmed that the particles produced using ligands DDTPMAA and DDT-PAA did indeed have very narrow size distributions as inferred from the optical spectra. Since size nonuniformities can be easily underestimated by TEM due to size segregation phenomena during sample preparation, large and representative areas were imaged in order to support our claim of near monodispersity. The average particle sizes and

Figure 2. TEM images of DDT-PAA-stabilized gold nanoparticles obtained using polymer concentration of (a) 0.006, (b) 0.06, (c) 0.6, and (d) 6.0 mM; (e, f, g, and h) the corresponding particle size distribution histograms for samples a, b, c, and d, respectively.

particle size distributions for all samples as estimated from TEM are summarized in Table 1. By contrast, gold nanoparticles produced using the commercially available PMAA ligand with no thioether end group were found to be larger (3-12 nm), even

Design of Polymeric Stabilizers

Langmuir, Vol. 23, No. 2, 2007 889

Table 2: Effect of Polymer Molecular Weight on Au Nanoparticles Produced Using DDT-PMAA Ligands particle diameter (nm)b polymer ligand DDT-PMAA1 DDT-PMAA2 DDT-PMAA3 DDT-PMAA4 DDT-PMAA5 DDT-PMAA6

mol wt (g/mol) Mn/Mw/PDI 13 500/18 800/1.4 8610/11 100/1.29 7000/9540/1.36 3640/4520/1.24 3220/3500/1.09 2490/2730/1.10

a

0.006 mM

0.06 mM

0.6 mM

6.0 mMc

7.6 ( 2.3 (17.9) 5.2 ( 1.5 (9.1) 5.5 ( 0.9 (5.6) 4.9 ( 0.8 (6.5) 5.3 ( 0.7 (5.0) 5.0 ( 0.5 (6.5)

5.2 ( 1.0 (7.0) 5.0 ( 1.2 (6.6) 4.3 ( 0.65 (4.9) 3.8 ( 0.6 (5.7) 4.0 ( 0.4 (3.6) 3.7 ( 0.3 (5.6)

4.2 ( 0.9 (5.4) 4.0 ( 0.75 (5.4) 3.1 ( 0.4 (3.6) 2.9 ( 0.4 (3.1) 2.8 ( 0.3 (2.8) 2.7 ( 0.25 (3.5)

2.8 ( 0.6 (2.4) 2.7 ( 0.45 (3.2) 2.4 ( 0.4 (3.0) 2.1 ( 0.4 (2.5) 1.7 ( 0.25 (1.5) 1.8 ( 0.2 (1.9)

a Mn ) number average molecular weight; Mw ) weight average molecular weight; PDI ) polydispersity index. b As estimated from TEM imaging; numbers in parentheses as measured by dynamic light scattering (DLS). c Polymer ligand concentration.

Figure 3. DLS spectra for DDT-PAA-stabilized gold nanoparticles obtained using polymer concentrations of 0.006 (black), 0.06 (red), 0.6 (blue), and 6.0 mM (magenta). The average particle diameters determined by DLS are 6.5, 4.9, 3.8, and 1.4 nm, respectively.

at higher polymer concentrations (6.0 mM), and exhibited a much broader particle size distribution (Figure S13). To further corroborate the near monodispersity and sample uniformity in these materials, bulk samples were also studied using dynamic light scattering (DLS).39 Like TEM, this technique also has inherent limitations (e.g., limited measurement range) and should not in any case be expected to give particle diameters which are identical to the TEM observations. Nonetheless, a major advantage of DLS is that it gives a bulk measurements that is, providing that there is no “settling” or precipitation, the method avoids selective sampling as can occur in TEM. As such DLS can serve as a very useful corroboratory technique in combination with TEM and UV-visible measurements. Figure 3 shows a series of DLS spectra for the DDT-PAA-stabilized gold nanoparticles shown in the TEM images (Figure 2) as produced at the four different polymer concentrations. These spectra confirm that each sample exhibits a relatively monodisperse and unimodal distribution of particle sizes. Moreover, using DLS it is possible to distinguish clearly between the four samples and gain a rapid estimate (measurement time ≈ 5 min) of the average particle size and the breadth of the particle size distribution. The DLS average particle size measurements are in fact quite close to those measured by TEM. The average particle diameters determined by DLS for this sample were 1.4, 3.8, 4.9, and 6.5 nm, respectively, in comparison with TEM measurements of 1.8, 2.7, 3.7, and 5.3 nm. The global correlation between TEM and DLS measurements for all samples is illustrated in Figure S8. (39) Andreescu, D.; Sau, T. K.; Goia, D. V. J. Colloid Interface Sci. 2006, 298, 742-751.

Effect of Polymer Molecular Weight. Having established that the carboxylic acid-based monomers AA and MAA gave rise to the most promising ligands, we next investigated the influence of ligand molecular weight. A series of DDT-PMAA ligands was synthesized with six different molecular weights (referred to hereafter as DDT-PMAA1 through DDT-PMAA6 from highest to lowest Mn). The DDT-PMAA ligand system was chosen since this polymer gave rise to relatively narrow particle size distributions and because it proved easier to achieve good molecular weight control with the methacrylate monomer in comparison with the DDT-PAA system, which also produced comparably monodisperse particles (cf., Tables S2 and S3). As before, the gold particle size was found to decrease with increasing polymer concentration for all of six ligands synthesized (DDTPMAA1-DDT-PMAA6; Table 2). Characterization by TEM, UV-visible spectroscopy, and DLS indicated that all of the polymer molecular weights studied gave rise to size-controlled gold nanoparticles with relatively narrow particle size distributions. It was clear, however, from the combined characterization data that the lowest molecular weight polymer ligand (DDTPMAA6, Mn ) 2490 g/mol; Figure 4) gave the best overall control of the gold particle size distribution. A direct comparison of the TEM and DLS data for the various molecular weight ligands is given in Figures S14-S18. In addition to the effect of molecular weight on sample monodispersity, it was also apparent that the average particle size decreased somewhat at a given polymer ligand concentration as the molecular weight of the ligand was decreased (Figure S19). It is important to note here that the gold particles were synthesized using a constant molar concentration of the ligandss that is, the amount of ligand was adjusted to take account of the number-average molecular weight, Mn. As such, the series of samples was prepared with (approximately) the same molar ratio of polymer ligand chains to gold in each case. Effect of Hydrophobic End Group. We next focused on the influence of the hydrophobic thioether end group on particle synthesis. A series of six low molar mass PMAA polymers was synthesized (MAT-PMAA, PropT-PMAA, PentT-PMAA, HTPMAA, DDT-PMAA, and ODT-PMAA) with increasingly hydrophobic thioether end groups (i.e., C2-C18). We targeted the same molecular weight in each case (around 2500 g/mol), although it was not possible to achieve identical Mn for every sample using this chemistry (see Table 3), probably because the various thiols in the series have slightly different chain-transfer constants.40,41 Again, a constant molar concentration of ligand was used in each particle preparation, adjusting for Mn in each case. The first ligand in this series (MAT-PMAA) has a carboxylic acid end group which mimics the polymer repeat unit structure and can be considered to be essentially hydrophilic. By contrast, (40) Harrisson, S.; Davis, T. P.; Evans, R. A.; Rizzardo, E. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 4421-4425. (41) Henriquez, C.; Bueno, C.; Lissi, E. A.; Encinas, M. V. Polymer 2003, 44, 5559-5561.

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Figure 4. Full characterization data for DDT-PMAA-stabilized gold nanoparticles (low molar mass DDT-PMAA, Mn ) 2490 g/mol) obtained at four different polymer concentrations; TEM images for samples prepared at (a) 0.006, (b) 0.06, (c) 0.6, and (d) 6.0 mM polymer; (e, f, g, and h) the corresponding particle size distribution histograms for samples a, b, c, and d, respectively; (i) UV-visible for same series of particles 0.006 (black), 0.06 (red), 0.6 (blue), and 6.0 mM (magenta); (j) DLS spectra for series of particles, color coded as in i. The average particle diameters determined by DLS are 6.5, 5.6, 3.5, and 1.9 nm, respectively. Table 3: Effect of End-Group Hydrophobicity on Au Nanoparticles Produced Using Thioether-Capped PMAA Ligands particle diameter (nm)b polymer ligand

mol wt (g/mol) Mn/Mw/PDI

MAT-PMAA PropT-PMAA PenT-PMAA HT-PMAA DDT-PMAA ODT-PMAA

1780/2060/1.16 1960/2340/1.19 2300/2670/1.16 2180/2550/1.17 2490/2730/1.10 2980/3480/1.17

a

0.006 mM

0.06 mM

0.6 mM

6.0 mM c

5.2 ( 2.8 (9.3) 4.1 ( 1.7 (6.6) 4.9 ( 1.1 (5.8) 4.6 ( 0.7 (5.8) 5.0 ( 0.5 (6.5) 4.5 ( 0.3 (5.6)

5.0 ( 2.2 (7.5) 3.2 ( 0.9 (5.0) 3.5 ( 0.8 (4.9) 3.5 ( 0.4 (5.1) 3.7 ( 0.3 (5.6) (4.2)d

3.6 ( 1.0 (6.5) 2.4 ( 0.6 (2.1) 2.6 ( 0.45 (3.3) 2.8 ( 0.36 (3.6) 2.7 ( 0.25 (3.5) 1.9 ( 0.2 (2.6)

2.2 ( 0.65 (2.0) 1.9 ( 0.6 (1.6) 1.7 ( 0.35 (2.4) 1.7 ( 0.3 (2.6) 1.8 ( 0.2 (1.9) 1.7 ( 0.2 (2.3)e

a Mn ) number average molecular weight; Mw ) weight average molecular weight; PDI ) polydispersity index. b As estimated from TEM imaging; numbers in parentheses as measured by dynamic light scattering (DLS). c Polymer ligand concentration. d Sample formed large aggregates on TEM grid; hence, no size data by TEM; see text and Figure S23b. e ODT-PMAA concentration ) 1.5 mM; this more hydrophobic ligand was not fully soluble at 6.0 mM.

ODT-PMAA has a C18 end group which is comparable in length with the PMAA chain itself. The total Mn for the polymer was estimated as 2980 g/mol by GPCsthat is, approximately 31 repeat units of methacrylic acid and 9 repeat units of “ethylene” in the ODT end group. As such, one might expect that the end group in ODT-PMAA could play a significant mechanistic role, and this structure is perhaps best considered as a low molar mass thioether-linked diblock copolymer. As for all the polymer ligands reported here thus far, the average gold particle size was found to have a strong dependence on polymer concentration for the six ligands in this “end-group series” (see Table 3). Overall, the combined results from TEM, UV-visible spectroscopy, and DLS indicate clearly that increasing the hydrophobicity of the thioether end group leads to a greater degree of control over the resulting

gold particle size distribution. This is evident from examination of the TEM images for the particles (see Figures S20-S24), but the effect is perhaps illustrated most simply by comparison of the DLS spectra for samples prepared using PMAA ligands with different end groups (Figure 5). The polymer with the most hydrophilic end group, 2-mecapto acetic acid (MAT-PMAA), gave rise to particles with a very broad size distribution (see Figure 5a, Figure S20). The particle size distribution for samples prepared at all four polymer concentrations was observed to decrease significantly as the length and hydrophobicity of the end group was increased from propyl- (PropT-PMAA) to pentyl(PenT-PMAA, Figure 5b) to heptyl- (HT-PMAA, Figure 5c) and dodecyl-thioether (DDT-PMAA, Figure 5d). This effect was also apparent from TEM images for the samples (Figures S21-

Design of Polymeric Stabilizers

Langmuir, Vol. 23, No. 2, 2007 891 Scheme 3

Figure 5. DLS spectra for nanoparticles stabilized using PMAA with four different thioether end groups of increasing hydrophobicity: (a) MAT-PMAA, (b) Pent-PMAA, (c) HT-PMAA, and (d) DDTPMAA. Samples were synthesized at four different polymer concentrations in each case: 0.006 (black), 0.06 (red), 0.6 (blue), and 6.0 mM (magenta). In general, the particle size distribution decreases with increasing end-group hydrophobicity in the ligand at all concentrations studied.

S23). The influence of end-group structure appears to be somewhat greater than the influence of ligand molecular weight over the range 1500-3000 g/mol, and we therefore ascribe these changes in particle size primarily to an end-group effect. Moreover, the most monodisperse particles were observed with the most hydrophobic thiols (DDT, ODT), which produced the highest molar mass ligands in the seriessthat is, the hydrophobicity trend is in opposition to the ligand molecular weight variation described in the previous section. Interestingly, particles prepared using the polymer with the most hydrophobic end group, ODT-PMAA, at a concentration of 0.06 mM formed large aggregated “supraparticles”42 with diameters greater than 30 nm when imaged by TEM (Figure

S24b) but not at the other polymer concentrations which were used (i.e., 0.006, 0.6, 1.5 mM). We believe that these large spheres are formed as a result of the drying process involved in the sample preparation since they were only observed by TEM and there was no evidence for such aggregates in either DLS or UV-visible measurements for this sample (Figures S24h and S24j). Indeed, DLS and UV-visible spectroscopy suggest that ligand ODT-PMAA gives rise to gold nanodispersions with comparably narrow size distributions to those observed with ligand DDT-PMAA. This further highlights the advantage of using a range of complementary characterization methods to analyze these materials. In summary, it is evident that control over particle size distribution is enhanced when the length of the hydrophobic end group is increased for PMAA ligands of Mn ≈ 1800-3000 g/mol, at least up to C18 hydrophobic chain lengths. The nature of the end group was also found to have a pronounced effect on the stability of the particles toward changes in the ionic strength of the aqueous solution (see section below). Effect of End-Group Denticity. The chain-transfer methodology used to generate the thioether-terminated ligands discussed thus far is readily adapted to produce ligands which contain both thioether and primary thiol functionalitiessthat is, polymer ligands with a multidentate binding capacity for gold. For example, a series of PMAA ligands was synthesized using dithiols (EDT, MES, NDT; Scheme 3) and a tetrathiol (PTMP; Scheme 3) as the chain-transfer agent. While one might expect all of the available thiol groups in these molecules to participate in chain-transfer reactions, NMR, GPC, and MALDI-TOF data suggest that, on average, only one thiol in each of these molecules reacts in this way. This differential reactivity effect has been exploited previously to generate thiolfunctionalized diblock copolymers using dithiol chain-transfer agents.43,44 The characterization data for our materials suggest strongly that the dominant “average” structure contains just one thioether-PMAA linkage per molecule, as shown in Scheme 3sthat is, a “macrothiol” is formed.44 It is not, however, possible to exclude entirely the possibility of small amounts of product where two or more thiols are converted into PMAA thioethers. These ligand structures all contain a mixture of thioether and thiol units and hence have sulfur “denticities” per chain ranging from two (EDT-PMAA and NDT-PMAA; one thioether + one thiol) to three (MES-PMAA; two thioethers + one thiol) up to four (PTMP-PMAA; one thioether + three thiols). As before, each multidentate ligand was evaluated for the synthesis of gold nanoparticles at four different polymer concentrations in the range 0.006-6.0 mM for the first three polymers (EDT-PMAA, NDTPMAA, and MES-PMAA). For PTMP-PMAA, four different (42) Hussain, I.; Wang, Z. X.; Cooper, A. I.; Brust, M. Langmuir 2006, 22, 2938-2941. (43) Teodorescu, M.; Draghici, C. Polym. Bull. (Berlin) 2006, 56, 359-368. (44) Nair, C. P. R.; Sivadasan, P.; Balagangadharan, V. P. J. Macromol. Sci., Pure Appl. Chem. 1999, A36, 51-72.

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Table 4: Effect of End-Group “Denticity” on Au Nanoparticles Produced Using Thioether-Capped PMAA Ligands particle diameter (nm)b polymer ligand EDT-PMAA NDT-PMAA MES-PMAA PTMP-PMAA

mol wt (g/mol) Mn/Mw/PDI 1810/2130/1.17 1990/2160/1.09 1510/1860/1.23 1990/2240/1.13

a

0.006 mM

0.06 mM

0.6 mM

6.0 mMc

4.5 ( 0.5 (6.1) 4.7 ( 0.6 (5.6) 4.0 ( 0.3 (5.0) 3.7 ( 0.2 (4.7)

3.3 ( 0.5 (4.2) 3.5 ( 0.45 (4.9) 3.4 ( 0.3 (4.1) 2.8 ( 0.15 (3.1)

2.8 ( 0.3 (3.9) 2.5 ( 0.25 (3.0) 2.7 ( 0.2 (3.2) 1.9 ( 0.1 (1.8)d

1.8 ( 0.25 (2.0) 1.7 ( 0.25 (2.1) 1.8 ( 0.2 (2.3) e

a Mn ) number average molecular weight; Mw ) weight average molecular weight; PDI ) polydispersity index. b As estimated from TEM imaging; numbers in parentheses as measured by dynamic light scattering (DLS). c Polymer ligand concentration. d PTMP-PMAA concentration ) 0.2 mM. e PTMP-PMAA concentration ) 0.6 mM. Particles were too small for TEM analysis and below the reliable DLS size cutoff.

concentrations in the range 0.006-0.6 mM were used since the saturation concentration for this more hydrophobic ligand was around 0.6 mM. As for all other ligands studied, the average particle size decreased significantly as the polymer concentration was increased. In general, characterization by TEM, UV-visible, and DLS indicated that these multidentate ligands gave good (and in the case of PTMP-PMAA unprecedented) control over the gold particle size distribution (Table 4). The broadest particle size distributions in this series were obtained with ligand EDTPMAA (Figure S25). The particle size distributions for this ligand were comparable to (though slightly narrower than) those obtained for ligand PropT-PMAA (Figure S21), which also contains a C3 end-group structure but contains just one sulfur (i.e., thioether) functionality per chain. This further supports the hypothesis (see above) that a significant hydrophobic end group is beneficial in these ligands. Indeed, the particles obtained using ligand NDTPMAA (which also contains one thioether and one thiol) appeared to be more monodisperse than those prepared with EDT-PMAA at all four polymer concentrations (cf., Figures S25 and S26). Ligand MES-PMAA gave rise to significantly more monodisperse particles (Figure S27) than EDT-PMAA, perhaps because this ligand has both a higher “sulfur denticity” (three versus two) and a more hydrophobic end group than EDT-PMAA. It was certainly evident that EDT-PMAA produced much more monodisperse particles than the C3-C7 monodentate thioether ligands. This “tridentate” MES-PMAA ligand also gave more monodisperse particles than the more hydrophobic “bidentate” ligand, NDTPMAA (Figure S26), perhaps suggesting that ligand denticity outweighs the effect of end-group hydrophobicity in this case. This latter hypothesis is supported by the results observed for the “tetradentate” ligand, PTMP-PMAA, which behaved quite differently both in terms of phase behavior during reaction and in terms of the nanoparticles that were produced. First, an opaque white solution was observed immediately upon addition of the polymer ligand to the AuHCl4 solution and prior to addition of the NaBH4 reducing agent. This effect was observed at all polymer concentrations (0.006-0.6 mM) and suggests that the Au(III) species was reduced to an insoluble Au(I) thiolate polymer45 by the thiol-containing PTMP-PMAA ligand, as demonstrated previously for thiol ligands such as p-HSCH2(C6H4)C(CH3)3.46 This behavior was also observed for other ligands such as DDTPMAA but only at significantly higher polymer concentration (>0.3 mM). The fact that PTMP-PMAA gives rise to this effect even at the lowest polymer concentration (0.006 mM) may suggest that phase separation occurs as a result of the multidentate nature of the PTMP-PMAA ligand which can, unlike the other three thiol-containing ligands, react with more than one gold species. Upon addition of the reducing agent, NaBH4, the opaque milky solutions became rapidly transparent and assumed the yellow/ orange/red colors observed for other ligands at these polymer (45) Alsaady, A. K. H.; Moss, K.; McAuliffe, C. A.; Parish, R. V. J. Chem. Soc., Dalton Trans. 1984, 1609-1616. (46) Huang, T.; Murray, R. W. J. Phys. Chem. B 2003, 107, 7434-7440.

Figure 6. UV-visible spectra of PTMP-PMAA-stabilized gold nanoparticles obtained from polymer concentration at 0.006 (black), 0.06 (red), 0.6 (blue), and 6.0 mM (magenta).

concentrations. However, inspection of the UV-visible spectra for the PTMP-PMAA preparations reveals significant differences in comparison to the other PMAA ligand structures studied (Figure 6). At equivalent molar polymer concentrations, the PTMPPMAA ligand leads to particles which exhibit UV-visible spectra with a much less pronounced plasmon band at ∼520 nm (cf., Figures 4i and 6). At the higher polymer concentrations in particular (0.2 and 0.6 mM), the plasmon band has disappeared entirely. In general, the UV-visible spectra are consistent with this multidentate ligand forming significantly smaller particles at a given polymer concentration. This was confirmed by both TEM and DLS analysis (Figures 7 and 8). Figure 7 shows the TEM analysis for particles produced using PTMP-PMAA at 0.006, 0.06, and 0.2 mM, respectively. These particles are both smaller and more monodisperse than any of the samples prepared using other ligands (cf., Tables 1-4). For example, the particles prepared using PTMP-PMAA at 0.006 mM had an average diameter as estimated by TEM of 3.7 ( 0.2 nm compared with 5.0 ( 0.5 nm for particles produced using DDT-PMAA6 at the same concentration (Table 2). Similarly, smaller and more monodisperse particles were observed with this ligand at 0.06 and 0.2 mM (Figure 7, Table 4). At a PTMP-PMAA concentration of 0.6 mM, however, it was not possible to observe the particles by TEM under the analysis conditions employed for these samples. This suggested an average cluster size of