Design of Polymeric Stabilizers for Size-Controlled Synthesis of Monodisperse Gold Nanoparticles in Water

Design of Polymeric Stabilizers for Size-Controlled Synthesis ofMonodisperse 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, TheUniVersity of LiVerpool, Crown Street, LiVerpool, L69 3BX, United Kingdom, State Key Laboratory ofElectro-Analytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

Changchun, 130022, China, National Institute for Biotechnology and Genetic Engineering (NIBGE), JhangRoad 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 nanoparticleswith diameters below 5 nm using thioether- and thiol-functionalized polymer ligands. The particle size and sizedistribution 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 thatrelatively low molar mass ligands (∼2500 g/mol) gave rise to the narrowest particle size distributions. Particle uniformityand colloidal stability to changes in ionic strength and pH were strongly affected by the hydrophobicity of the ligandend group. “Multidentate” thiol-terminated ligands were produced by employing dithiols and tetrathiols as chain-transfer agents, and these ligands gave rise to particles with unprecedented control over particle size and enhancedcolloidal stability. It was found throughout that dynamic light scattering (DLS) is a very useful corroboratory techniquefor characterization of these gold nanoparticles in addition to optical spectroscopy and TEM.

IntroductionGold nanoparticles have a wide range of uses in modern

nanoscale science, and it is therefore important to understandand control their physical and chemical properties, which aregenerally size dependent.1,2Gold nanoparticles are commerciallyavailable in many forms, and numerous preparative methods aredocumented in the literature for particles from about 1 nm toseveral micrometers diameter.3-7 Nonetheless, only a handful ofstandard procedures are employed routinely to prepare goldparticles foramultitudeofapplications.Thesemethodsare reliableand simple to carry out and lead to uniform particles with anarrow size distribution in the desired range. The most widelyapplied procedures to obtain gold hydrosols are variations of theclassic Turkevich-Frens citrate reduction route.8,9 Most hy-drophobic (and some hydrophilic) particles are prepared byborohydride reduction in an organic solvent in the presence ofthiol capping ligands using either a two-phase liquid/liquid systemor a suitable single-phase solvent.10-19 The latter approach isusually employed for particles in the 1 to ca. 8 nm range. Gold

nanoparticles are useful in a broad range of applications,20-22

but practical limitations are apparent when monodispersity isrequired: for example, in electrochemical quantized capacitancecharging,21-23 single-electron transistor assembly,24 and ap-plications such as thermal gradient optical imaging.25 In manycases, monodispersefractions of particles must be prepared,usually in low yield following cumbersome size separationprocedures, suchassizeexclusionchromatography.26,27Moreover,

* 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.Langmuir2006, 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.Langmuir2003, 19,

4831-4835.(5) Jana, N. R.; Gearheart, L.; Murphy, C. J.Langmuir2001, 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, 6402-

6408.(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.

(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.Langmuir1998, 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.Nanotechnology2003, 14, R15-R27.

(22) Pellegrino, T.; Kudera, S.; Liedl, T.; Javier, A. M.; Manna, L.; Parak, W.J. Small2005, 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.Science1998,280, 2098-2101.

(24) Moriarty, P.Rep. Prog. Phys.2001, 64, 297-381.(25) Boyer, D.; Tamarat, P.; Maali, A.; Lounis, B.; Orrit, M.Science2002,

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.Langmuir2000, 16, 9912-

9920.

10.1021/la062623h CCC: $33.50 © xxxx American Chemical SocietyPAGE EST: 11Published on Web 11/16/2006

such fractionation methods do not necessarily lead to mono-disperse samples.28 The availability of a simple, robust protocolfor the gram-scale preparation of uniform MPCs below 5 nmwould thus be of broad practical value.

A number of research groups have investigated water-solublepolymers as stabilizing ligands for gold nanoparticles in water,particularly with the aim of achieving size-controlled nanoparticlesynthesis. For example, poly(ethylene oxide)-poly(propyleneoxide)-stabilized gold nanoparticles were prepared29 but foundto be quite polydisperse. Star-shaped poly(ethylene oxide)-block-poly(caprolactone) ligands have also been developed.30 Theseligands led to rather better control over particle size distributionsbut did not achieve monodisperse samples. Thiol-terminated poly-(ethylene glycol) monomethyl ether (MeO-PEG-SH) was foundto stabilize gold nanoparticles,19,31 and poly(N-isopropylacry-lamide)-monolayer-protected gold clusters (PNIPAMMPC)32

have also been produced. The MeO-PEG-SH route in particulargave rise to well-defined (although not monodisperse) particlesin water.19,31 Thiol-terminated polystyrene and poly(ethyleneglycol)-stabilized gold nanoparticles have also been synthesizedby “grafting to” approaches,33,34although in these cases the goldparticles were preformed and then postfunctionalized with thepolymer.

In general, there is an incomplete understanding of therelationship between structure and function for polymeric ligandsof this type, and preparation of more sophisticated structures, forexample, dendritic ligands,35 has not necessarily led to a greaterdegree of control over particle size and particle size distribution.Moreover, there is a real need to introduce complementarycharacterization methods to assess the degree of control overparticle size for the bulk sample: many studies rely exclusivelyon TEM measurements, often using a relatively small samplingarea. It is easy to significantly overestimate the monodispersityof a sample using TEM analysis as the sole means ofcharacterization.

Previously, we have shown that thioether-terminated poly-(methacrylic acid) (PMAA) ligands could be used to produceaqueous gold nanodispersions in one step with unprecedentedcontrol over particle size distributions in the 1-5 nm size range.15

In this new study we investigate in detail the effect of the ligandstructure on the particle size and particle size distribution andshow that a number of “design rules” can be formulated forpolymeric ligands of this type. As a result, we identify a modifiedpolymer architecture with a multidentate thiol headgroup whichleads to significantly smaller and more monodisperse particlesat a given ligand concentration. We also show that dynamic lightscattering (DLS) is a very useful complementary technique (inaddition to TEM and UV-vis spectroscopy) for assessing thesize and monodispersity of the bulk gold nanoparticle dispersions.The stability of these particles toward salt concentration and pHis strongly affected by relatively small changes in the polymerligand structure, and we show that certain systems are very stable

in both regards. Last, we show that the thioether-terminatedligands can be exchanged for biological species such as peptides.

Experimental Section

Chemicals.All chemicals were purchased from Aldrich and usedas received, unless otherwise described. Full details of the polymerligand synthesis can be found in the Supporting Information. Milli-Qwater (18.2 MΩ) was used in all experimental processes.

Synthesis of Gold Nanoparticles.A general procedure for thepreparation of polymer-stabilized gold nanoparticles in water isdescribed as follows. An aqueous solution of HAuCl4 (10 mL) wasadded to an aqueous polymer solution under vigorous stirring togive a final concentration of HAuCl4 of 0.5 mM. Each particlepreparation was repeated at four different polymer concentrations(0.006, 0.06, 0.6 and 6 mM)sthat is, four different particlepreparations were produced for each polymer ligand. Freshly preparedNaBH4 solution (1 mL, 50 mM) was added after mixing the gold/polymer solutions for 1 h. The reducing agent was added rapidlyin two aliquots (2× 0.5 mL). The reaction was allowed to continueovernight under uniform and vigorous stirring. The gold nanoparticleswere 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 weredialyzed overnight using a 96-well microplate dialyzer (10 000 g/mol,The Nest Group Inc., USA) in order to remove any last traces ofunreacted polymer ligand.

Ligand Exchange for DDT-PMAA-Stabilized Gold Nano-particles. The conditions used for ligand exchange of DDT-PMAA-stabilized 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 isdescribed 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 nanoparticleswere reacted with excess avidin followed by purification using aSephadex G-25 column (3 times). The unexchanged DDT-PMAAparticles, the peptide-stabilized gold nanoparticles, and the peptide-stabilized gold nanoparticles after reaction with avidin (20µL) wereloaded onto agarose gels (2% w/v in 1× TBE) and subjected toelectrophoresis at 100 V for 0.5 h.

UV-visible Absorption Spectroscopy. UV-visible spectra werecarried out using a microplate reader (µQuant, Bio-Tek Instruments).The aqueous gold nanoparticle solutions (200µL) were analyzed in96-well microplate at 25°C.

Transmission Electron Microscopy. Transmission electronmicroscopy (TEM) micrographs of the colloidal dispersions wereobtained using a JEM-2000EX/FX instrument operated at anaccelerating voltage of 200 kV. A high-resolution carbon-supportedcopper mesh was used to support the colloidal dispersions. Specimensfor inspection by TEM were prepared by evaporating a droplet ofthe filtered and dialyzed gold nanoparticle solutions onto a carbon-coated copper mesh grid directly from watersthat is, without solventexchange into an organic solvent as employed previously.15 Thediameter of each particle was quantified using ImagesJ software(1.34s, NIH, USA) to analyze the digitized photographic images foreach sample in the magnification range 200 000-500 000×. Ahistogram of the particle size distribution and the average particlediameter were obtained by measuring about 200 particles in arbitrarilychosen 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 cmoptical path cell. Each samplewas analyzed three times.

MALDI-TOFMS, Gold Clusters. 2,5-Dihydroxybenzoic acid(DHB) matrix was purchased from Fluka (Dorset, U.K.). Dowex50W-X8, 200µm, ion-exchange resin and ammonium acetate werepurchased from Sigma-Aldrich (Dorset, U.K.). Resin was loaded

(28) Akthakul, A.; Hochbaum, A. I.; Stellacci, F.; Mayes, A. M.AdV. Mater.2005, 17, 532-535.

(29) Sakai, T.; Alexandridis, P.Langmuir2005, 21, 8019-8025.(30) Filali, M.; Meier, M. A. R.; Schubert, U. S.; Gohy, J. F.Langmuir2005,

21, 7995-8000.(31) Shimmin, R. G.; Schoch, A. B.; Braun, P. V.Langmuir2004, 20, 5613-

5620.(32) Shan, J.; Nuopponen, M.; Jiang, H.; Kauppinen, E.; Tenhu, H.

Macromolecules2003, 36, 4526-4533.(33) Corbierre, M. K.; Cameron, N. S.; Lennox, R. B.Langmuir2004, 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.Macromolecules2005, 38, 8308-8315.

B Langmuir Wang et al.

with NH4+ ions as reported previously.36 HPLC-grade acetonitrile

(MeCN) and Milli-Q water were used where appropriate. The DHBmatrix 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 (10mg mL-1 in H2O), and 200µL was placed into a plastic, snap-topvial. Roughly 0.5 mg of NH4+-loaded resin was added, and thesolution was agitated via vortex mixer at slow speed for 20 s. Sampleand matrix solutions were mixed in a 1:10 ratio in a separate plasticvial. A 0.5µL amount of the final mixture was spotted onto a stainlesssteel sample plate and dried in a stream of cool air. MALDI-TOFMSdata were acquired using an Applied Biosystems Voyager DE-STRspectrometer (Framingham,MA),whichwasequippedwithanitrogenlaser (λ ) 337 nm). For the PTMP-PMAA-protected gold clusters,the instrument was operated in negative-ion, linear mode. Theaccelerating voltage was 25 kV, while the grid voltage was maintainedat 91%. The delay time was 450 ns, and laser fluence was attenuatedto just above the threshold of ionization. For the PTMP-PMAAligand itself (see Supporting Information) the instrument was operatedin positive-ion, reflectron mode. The accelerating voltage was 20kV, while the grid voltage was maintained at 65.5%. The delay timewas 150 ns. For all samples the laser was fired at a frequency of3 Hz, and spectra were accumulated in multiples of 25 laser shotswith 150 shots in total. Postacquisition processing of data wasperformed utilizing Data Explorer V4.0 software supplied by AppliedBiosystems.

Results and Discussion

Synthesis and Characterization of Polymer Ligands.Aseries of polymer ligands was synthesized by chain-transfermethods15 using thiols (or diol/multithiols) as the chain-transferagent (Scheme 1; see Supporting Information for details; TablesS1-S5; Figures S1-S5).

A feature of this methodology is that it leads to low molarmass oligomeric species with relatively narrow molecular weightdistributions (PDI< 1.5). Any unreacted free thiol was removedby polymer reprecipitation in a solvent which was a good solventfor the thiol chain-transfer agent. Using this methodology wewere able to produce a small library of polymeric ligand structures(Scheme 2) which varied in monomer type, end-group func-tionality, and molecular weight.

Synthesis of Polymer-Stabilized Gold Nanoparticles.Effectof Polymer Structure.A series of six water-soluble polymerligands with different monomer repeat units was synthesized inorder to study the effect of the ligand chain structure on theaverage particle size and size distribution for the gold nano-dispersions. The six monomers studied were methacrylic acid(MAA), acrylic acid (AA), vinylpyrrolidone (VP), vinylsulfonicacid (VSA), hydroxyethyl acrylate (HEA), and poly(ethyleneglycol) (PEG) methacrylate (PEG-MA). The same thiol chain-transfer agent (DDT) was used in each case; as such, each ligandin the series was terminated with a dodecylthioether end group.15,37

The number-average molecular weight,Mn, for the six ligandswas found to be in the range 1500-4500 g/mol (see Table 1)with the exception of DDT-PVP, which exhibited a much highermolecular weight (Mn ) 37 320 g/mol) despite the fact that thiolchain-transfer agents have been used previously to prepare low

molar mass oligomeric species.38A commercially available linearpoly(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 DDT-PPEG-MA; see Scheme 2) gave rise to stable red-colored goldnanodispersions at a polymer concentration of 0.006 mM. Athigher polymer concentrations the relative performance of theligands varied markedly. Color images of the as-producedpolymer-stabilized gold nanodispersions are shown in Figure1a. In general, the color of nanodispersions changed from redto yellow when the polymer concentration was increased from0.006 to 6.0 mM, indicating that particles of different averagesizes were prepared in each case. The polymer ligand DDT-PVSA was an exception to this trend; at higher concentrationsof DDT-PVSA (0.6 and 6.0 mM), the gold nanodispersion turneddark blue/black and precipitation was observed (Figure 1a),indicating that the particles were not stable to aggregation withthis ligand. All of the stable nanodispersions were characterizedby TEM, UV-visible, and dynamic light scattering (DLS) (seeFigures 1-3 and Figures S6-S13). Overall, ligands DDT-PMAA(Mn ) 3220 g/mol) and DDT-PAA (Mn ) 2550 g/mol) gave riseto gold nanoparticles with the most narrow size distributionsover the polymer concentration range 0.006-6.0 mM. A seriesof typical UV-visible spectra for gold nanoparticles producedusing DDT-PAA (Mn ) 2550 g/mol) are shown in Figure 1b.The spectra vary strongly as the concentration of the polymerligand is changed and suggest that the average particle size isbelow 5 nm forall samples since larger particles would exhibita sharper and more intense plasmon absorption band close to525 nm.11,12Some of the spectra for particles produced at higherpolymer concentrations (6.0 mM) do not show a plasmon bandat 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 unprecedentedfor as-prepared samples.15The ability to prepare spectroscopically(36) Nordhoff, E.; Ingendoh, A.; Cramer, R.; Overberg, A.; Stahl, B.; Karas,

M.; Hillenkamp, F.; Crain, P. F.Rapid Commun. Mass Spectrom.1992, 6, 771-776.

(37) Li, X. M.; de Jong, M. R.; Inoue, K.; Shinkai, S.; Huskens, J.; Reinhoudt,D. N. J. Mater. Chem.2001, 11, 1919-1923.

(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.Biomaterials2001, 22, 3035-3044.

Scheme 1 Scheme 2

Design of Polymeric Stabilizers Langmuir C

distinct samples with diameters sub-5 nm suggests a narrowparticle size distribution for each sample. To confirm this byTEM, it was convenient to first isolate the particles from excesspolymer by filtration and dialysis (dialysis membrane molecularweight cutoff) 10 000 g/mol). This procedure avoids the needto phase transfer the particles into an organic solvent prior toTEM analysis.15 Characterization by TEM (Figure 2 and FigureS7) confirmed that the particles produced using ligands DDT-PMAA and DDT-PAA did indeed have very narrow sizedistributions as inferred from the optical spectra. Since sizenonuniformities can be easily underestimated by TEM due tosize segregation phenomena during sample preparation, largeand representative areas were imaged in order to support ourclaim of near monodispersity. The average particle sizes and

particle size distributions for all samples as estimated from TEMare summarized in Table 1. By contrast, gold nanoparticlesproduced using the commercially available PMAA ligand withno thioether end group were found to be larger (3-12 nm), even

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/PDIa 0.006 mM 0.06 mM 0.6 mM 6.0 mMc

DDT-PAA 2550/3490/1.37 5.3( 0.4 3.7( 0.25 2.7( 0.2 1.8( 0.2(6.5) (4.9) (3.8) (1.4)

DDT-PMAA 3220/3500/1.09 5.3( 0.7 4.0( 0.4 2.8( 0.3 1.7( 0.25(5.0) (3.6) (2.8) (1.5)

DDT-PVSA 870/1550/1.79 6.0( 1.5 3.6( 0.45 d d(5.5) (4.2)

DDT-PHEA 1810/2030/1.12 5.2( 1.0 4.6( 0.8 2.7( 0.45 2.0( 0.3(6.4) (4.3) (3.2) (2.8)

DDT-PPEG-MA 3470/4180/1.21 7.7( 1.9 4.6( 0.7 3.6( 0.7 2.7( 0.4(17.5) (4.2) (3.9) (3.0)

DDT-PVP 37 320/77 620/2.08 5.0( 1.4 4.7( 0.9 4.2( 0.7 2.6( 0.6(8.5) (5.6) (4.9) (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 usingsix different DDT-terminated polymer ligands at four differentpolymer concentrations. The control solutions were prepared usingcommercially available poly(methacrylic acid) (Mn ) 2000 g/mol)sthat is, a ligand which does not contain a thioether end group. (b)UV-visible spectra of DDT-PMAA-stabilized gold nanoparticlesobtained using a polymer concentration at 0.006 (black), 0.06 (red),0.6 (blue), and 6.0 mM (magenta).

Figure 2. TEM images of DDT-PAA-stabilized gold nanoparticlesobtained 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 sizedistribution histograms for samples a, b, c, and d, respectively.

D Langmuir Wang et al.

at higher polymer concentrations (6.0 mM), and exhibited a muchbroader particle size distribution (Figure S13).

To further corroborate the near monodispersity and sampleuniformity in these materials, bulk samples were also studiedusing dynamic light scattering (DLS).39Like TEM, this techniquealso has inherent limitations (e.g., limited measurement range)and should not in any case be expected to give particle diameterswhich are identical to the TEM observations. Nonetheless, amajor advantage of DLS is that it gives a bulk measurementsthat is, providing that there is no “settling” or precipitation, themethod avoids selective sampling as can occur in TEM. As suchDLS can serve as a very useful corroboratory technique incombination with TEM and UV-visible measurements. Figure3 shows a series of DLS spectra for the DDT-PAA-stabilizedgold nanoparticles shown in the TEM images (Figure 2) asproduced at the four different polymer concentrations. Thesespectra confirm that each sample exhibits a relatively mono-disperse and unimodal distribution of particle sizes. Moreover,using DLS it is possible to distinguish clearly between the foursamples and gain a rapid estimate (measurement time≈ 5 min)of the average particle size and the breadth of the particle sizedistribution. The DLS average particle size measurements are infact quite close to those measured by TEM. The average particlediameters determined by DLS for this sample were 1.4, 3.8, 4.9,and 6.5 nm, respectively, in comparison with TEM measurementsof 1.8, 2.7, 3.7, and 5.3 nm. The global correlation between TEMand DLS measurements for all samples is illustrated in FigureS8.

Effect of Polymer Molecular Weight.Having established thatthe carboxylic acid-based monomers AA and MAA gave rise tothe most promising ligands, we next investigated the influenceof ligand molecular weight. A series of DDT-PMAA ligandswas synthesized with six different molecular weights (referredto hereafter as DDT-PMAA1 through DDT-PMAA6 from highestto lowestMn). The DDT-PMAA ligand system was chosen sincethis polymer gave rise to relatively narrow particle sizedistributions and because it proved easier to achieve goodmolecular weight control with the methacrylate monomer incomparison with the DDT-PAA system, which also producedcomparably monodisperse particles (cf., Tables S2 and S3). Asbefore, the gold particle size was found to decrease with increasingpolymer concentration for all of six ligands synthesized (DDT-PMAA1-DDT-PMAA6; Table 2). Characterization by TEM,UV-visible spectroscopy, and DLS indicated that all of thepolymer molecular weights studied gave rise to size-controlledgold nanoparticles with relatively narrow particle size distribu-tions. It was clear, however, from the combined characterizationdata that the lowest molecular weight polymer ligand (DDT-PMAA6, Mn ) 2490 g/mol; Figure 4) gave the best overallcontrol of the gold particle size distribution. A direct comparisonof the TEM and DLS data for the various molecular weightligands is given in Figures S14-S18. In addition to the effectof molecular weight on sample monodispersity, it was alsoapparent that the average particle size decreased somewhat at agiven polymer ligand concentration as the molecular weight ofthe ligand was decreased (Figure S19).

It is important to note here that the gold particles weresynthesized using a constantmolarconcentration of the ligandssthat is, the amount of ligand was adjusted to take account of thenumber-average molecular weight,Mn. As such, the series ofsamples was prepared with (approximately) the same molar ratioof polymer ligand chains to gold in each case.

Effect of Hydrophobic End Group.We next focused on theinfluence of the hydrophobic thioether end group on particlesynthesis. A series of six low molar mass PMAA polymers wassynthesized (MAT-PMAA, PropT-PMAA, PentT-PMAA, HT-PMAA, DDT-PMAA, and ODT-PMAA) with increasinglyhydrophobic thioether end groups (i.e., C2-C18). We targetedthe same molecular weight in each case (around 2500 g/mol),although it was not possible to achieve identicalMn for everysample using this chemistry (see Table 3), probably because thevarious thiols in the series have slightly different chain-transferconstants.40,41 Again, a constant molar concentration of ligandwas used in each particle preparation, adjusting forMn in eachcase. The first ligand in this series (MAT-PMAA) has a carboxylicacid end group which mimics the polymer repeat unit structureand can be considered to be essentially hydrophilic. By contrast,

(39) Andreescu, D.; Sau, T. K.; Goia, D. V.J. Colloid Interface Sci.2006, 298,742-751.

(40) Harrisson, S.; Davis, T. P.; Evans, R. A.; Rizzardo, E.J. Polym. Sci., PartA: Polym. Chem.2002, 40, 4421-4425.

(41) Henriquez, C.; Bueno, C.; Lissi, E. A.; Encinas, M. V.Polymer2003, 44,5559-5561.

Table 2: Effect of Polymer Molecular Weight on Au Nanoparticles Produced Using DDT-PMAA Ligands

particle diameter (nm)b

polymer ligand mol wt (g/mol)Mn/Mw/PDIa 0.006 mM 0.06 mM 0.6 mM 6.0 mMc

DDT-PMAA1 13 500/18 800/1.4 7.6( 2.3 (17.9) 5.2( 1.0 (7.0) 4.2( 0.9 (5.4) 2.8( 0.6 (2.4)DDT-PMAA2 8610/11 100/1.29 5.2( 1.5 (9.1) 5.0( 1.2 (6.6) 4.0( 0.75 (5.4) 2.7( 0.45 (3.2)DDT-PMAA3 7000/9540/1.36 5.5( 0.9 (5.6) 4.3( 0.65 (4.9) 3.1( 0.4 (3.6) 2.4( 0.4 (3.0)DDT-PMAA4 3640/4520/1.24 4.9( 0.8 (6.5) 3.8( 0.6 (5.7) 2.9( 0.4 (3.1) 2.1( 0.4 (2.5)DDT-PMAA5 3220/3500/1.09 5.3( 0.7 (5.0) 4.0( 0.4 (3.6) 2.8( 0.3 (2.8) 1.7( 0.25 (1.5)DDT-PMAA6 2490/2730/1.10 5.0( 0.5 (6.5) 3.7( 0.3 (5.6) 2.7( 0.25 (3.5) 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 nanoparticlesobtained using polymer concentrations of 0.006 (black), 0.06 (red),0.6 (blue), and 6.0 mM (magenta). The average particle diametersdetermined by DLS are 6.5, 4.9, 3.8, and 1.4 nm, respectively.

Design of Polymeric Stabilizers Langmuir E

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

gold particle size distribution. This is evident from examinationof the TEM images for the particles (see Figures S20-S24), butthe effect is perhaps illustrated most simply by comparison ofthe DLS spectra for samples prepared using PMAA ligands withdifferent end groups (Figure 5). The polymer with the mosthydrophilic end group, 2-mecapto acetic acid (MAT-PMAA),gave rise to particles with a very broad size distribution (seeFigure 5a, Figure S20). The particle size distribution for samplesprepared at all four polymer concentrations was observed todecrease significantly as the length and hydrophobicity of theend 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 wasalso apparent from TEM images for the samples (Figures S21-

Figure 4. Full characterization data for DDT-PMAA-stabilized gold nanoparticles (low molar mass DDT-PMAA,Mn ) 2490 g/mol) obtainedat 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 ofparticles 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 averageparticle 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/PDIa 0.006 mM 0.06 mM 0.6 mM 6.0 mMc

MAT-PMAA 1780/2060/1.16 5.2( 2.8 (9.3) 5.0( 2.2 (7.5) 3.6( 1.0 (6.5) 2.2( 0.65 (2.0)PropT-PMAA 1960/2340/1.19 4.1( 1.7 (6.6) 3.2( 0.9 (5.0) 2.4( 0.6 (2.1) 1.9( 0.6 (1.6)PenT-PMAA 2300/2670/1.16 4.9( 1.1 (5.8) 3.5( 0.8 (4.9) 2.6( 0.45 (3.3) 1.7( 0.35 (2.4)HT-PMAA 2180/2550/1.17 4.6( 0.7 (5.8) 3.5( 0.4 (5.1) 2.8( 0.36 (3.6) 1.7( 0.3 (2.6)DDT-PMAA 2490/2730/1.10 5.0( 0.5 (6.5) 3.7( 0.3 (5.6) 2.7( 0.25 (3.5) 1.8( 0.2 (1.9)ODT-PMAA 2980/3480/1.17 4.5( 0.3 (5.6) (4.2)d 1.9( 0.2 (2.6) 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 TEMgrid; hence, no size data by TEM; see text and Figure S23b.e ODT-PMAA concentration) 1.5 mM; this more hydrophobic ligand was not fullysoluble at 6.0 mM.

F Langmuir Wang et al.

S23).The influenceofend-groupstructureappears tobesomewhatgreater than the influence of ligand molecular weight over therange 1500-3000 g/mol, and we therefore ascribe these changesin particle size primarily to an end-group effect. Moreover, themost monodisperse particles were observed with the mosthydrophobic thiols (DDT, ODT), which produced thehighestmolar mass ligands in the seriessthat is, the hydrophobicitytrend is in opposition to the ligand molecular weight variationdescribed in the previous section.

Interestingly, particles prepared using the polymer with themost hydrophobic end group, ODT-PMAA, at a concentrationof 0.06 mM formed large aggregated “supraparticles”42 withdiameters greater than 30 nm when imaged by TEM (Figure

S24b) but not at the other polymer concentrations which wereused (i.e., 0.006, 0.6, 1.5 mM). We believe that these large spheresare formed as a result of the drying process involved in thesample preparation since they were only observed by TEM andthere was no evidence for such aggregates in either DLS orUV-visible measurements for this sample (Figures S24h andS24j). Indeed, DLS and UV-visible spectroscopy suggest thatligand ODT-PMAA gives rise to gold nanodispersions withcomparably narrow size distributions to those observed withligand DDT-PMAA. This further highlights the advantage ofusing a range of complementary characterization methods toanalyze these materials.

In summary, it is evident that control over particle sizedistribution is enhanced when the length of the hydrophobic endgroup is increased for PMAA ligands ofMn≈ 1800-3000 g/mol,at least up to C18 hydrophobic chain lengths. The nature of theend group was also found to have a pronounced effect on thestability of the particles toward changes in the ionic strength ofthe aqueous solution (see section below).

Effect of End-Group Denticity.The chain-transfer methodologyused to generate the thioether-terminated ligands discussed thusfar is readily adapted to produce ligands which contain boththioether and primary thiol functionalitiessthat is, polymerligandswithamultidentatebindingcapacity forgold.Forexample,a series of PMAA ligands was synthesized using dithiols (EDT,MES, NDT; Scheme 3) and a tetrathiol (PTMP; Scheme 3) asthe chain-transfer agent.

While one might expect all of the available thiol groups inthese molecules to participate in chain-transfer reactions, NMR,GPC, and MALDI-TOF data suggest that, on average, only onethiol in each of these molecules reacts in this way. This differentialreactivity effect has been exploited previously to generate thiol-functionalized diblock copolymers using dithiol chain-transferagents.43,44 The characterization data for our materials suggeststrongly that the dominant “average” structure contains just onethioether-PMAA linkage per molecule, as shown in Scheme3sthat is, a “macrothiol” is formed.44It is not, however, possibleto exclude entirely the possibility of small amounts of productwhere two or more thiols are converted into PMAA thioethers.

These ligand structures all contain a mixture of thioether andthiol units and hence have sulfur “denticities” per chain rangingfrom two (EDT-PMAA and NDT-PMAA; one thioether+ onethiol) to three (MES-PMAA; two thioethers+ one thiol) up tofour (PTMP-PMAA; one thioether+ three thiols). As before,each multidentate ligand was evaluated for the synthesis of goldnanoparticles at four different polymer concentrations in the range0.006-6.0 mM for the first three polymers (EDT-PMAA, NDT-PMAA, and MES-PMAA). For PTMP-PMAA, four different

(42) Hussain, I.; Wang, Z. X.; Cooper, A. I.; Brust, M.Langmuir2006, 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.

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

Scheme 3

Design of Polymeric Stabilizers Langmuir G

concentrations in the range 0.006-0.6 mM were used since thesaturation concentration for this more hydrophobic ligand wasaround 0.6 mM. As for all other ligands studied, the averageparticle size decreased significantly as the polymer concentrationwas 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 overthe gold particle size distribution (Table 4). The broadest particlesize distributions in this series were obtained with ligand EDT-PMAA (Figure S25). The particle size distributions for this ligandwere comparable to (though slightly narrower than) those obtainedfor 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 (seeabove) that a significant hydrophobic end group is beneficial inthese ligands. Indeed, the particles obtained using ligand NDT-PMAA (which also contains one thioether and one thiol) appearedto be more monodisperse than those prepared with EDT-PMAAat all four polymer concentrations (cf., Figures S25 and S26).Ligand MES-PMAA gave rise to significantly more monodisperseparticles (Figure S27) than EDT-PMAA, perhaps because thisligand has both a higher “sulfur denticity” (three versus two) anda more hydrophobic end group than EDT-PMAA. It was certainlyevident that EDT-PMAA produced much more monodisperseparticles than the C3-C7 monodentate thioether ligands. This“tridentate” MES-PMAA ligand also gave more monodisperseparticles than the more hydrophobic “bidentate” ligand, NDT-PMAA (Figure S26), perhaps suggesting that ligand denticityoutweighs the effect of end-group hydrophobicity in this case.

This latter hypothesis is supported by the results observed forthe “tetradentate” ligand, PTMP-PMAA, which behaved quitedifferently both in terms of phase behavior during reaction andin terms of the nanoparticles that were produced. First, an opaquewhite solution was observed immediately upon addition of thepolymer ligand to the AuHCl4 solution and prior to addition ofthe NaBH4reducing agent. This effect was observed at all polymerconcentrations (0.006-0.6 mM) and suggests that the Au(III)species was reduced to an insoluble Au(I) thiolate polymer45 bythe thiol-containing PTMP-PMAA ligand, as demonstratedpreviously for thiol ligands such asp-HSCH2(C6H4)C(CH3)3.46

This behavior was also observed for other ligands such as DDT-PMAA but only at significantly higher polymer concentration(>0.3 mM). The fact that PTMP-PMAA gives rise to this effecteven at the lowest polymer concentration (0.006 mM) may suggestthat phase separation occurs as a result of the multidentate natureof the PTMP-PMAA ligand which can, unlike the other threethiol-containing ligands, react with more than one gold species.Upon addition of the reducing agent, NaBH4, the opaque milkysolutions became rapidly transparent and assumed the yellow/orange/red colors observed for other ligands at these polymer

concentrations. However, inspection of the UV-visible spectrafor the PTMP-PMAA preparations reveals significant differencesin comparison to the other PMAA ligand structures studied (Figure6). At equivalent molar polymer concentrations, the PTMP-PMAA ligand leads to particles which exhibit UV-visible spectrawith a much less pronounced plasmon band at∼520 nm (cf.,Figures 4i and 6). At the higher polymer concentrations inparticular (0.2 and 0.6 mM), the plasmon band has disappearedentirely. In general, the UV-visible spectra are consistent withthis multidentate ligand forming significantly smaller particlesat a given polymer concentration. This was confirmed by bothTEM and DLS analysis (Figures 7 and 8). Figure 7 shows theTEM analysis for particles produced using PTMP-PMAA at0.006, 0.06, and 0.2 mM, respectively. These particles are bothsmaller and more monodisperse than any of the samples preparedusing other ligands (cf., Tables 1-4). For example, the particlesprepared using PTMP-PMAA at 0.006 mM had an averagediameter as estimated by TEM of 3.7( 0.2 nm compared with5.0( 0.5 nm for particles produced using DDT-PMAA6 at thesame concentration (Table 2). Similarly, smaller and moremonodisperse particles were observed with this ligand at 0.06and 0.2 mM (Figure 7, Table 4). At a PTMP-PMAA concentrationof 0.6 mM, however, it was not possible to observe the particlesby TEM under the analysis conditions employed for these samples.This suggested an average cluster size of<1 nm, which isconsistent with the UV-visible spectrum for particles producedat this concentration (Figure 6). DLS analysis corroborates allof these observations. By DLS, the particles produced at 0.006,0.06, and 0.2 mM PTMP-PMAA concentrations have narrowsize distributions (Figure 8) and average diameters of 4.7, 3.1,and 1.8 nm, respectively. The DLS measurement for the particlesproduced at 0.6 mM PTMP-PMAA is suggestive of an averageparticle size of<1 nm (Figure 8), although it must be noted thatthe DLS instrument used to calculate these size distributions isnot suitable for sizing particles with diameter< 1 nm and the

(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. B2003, 107, 7434-7440.

Table 4: Effect of End-Group “Denticity” on Au Nanoparticles Produced Using Thioether-Capped PMAA Ligands

particle diameter (nm)b

polymer ligand mol wt (g/mol)Mn/Mw/PDIa 0.006 mM 0.06 mM 0.6 mM 6.0 mMc

EDT-PMAA 1810/2130/1.17 4.5( 0.5 (6.1) 3.3( 0.5 (4.2) 2.8( 0.3 (3.9) 1.8( 0.25 (2.0)NDT-PMAA 1990/2160/1.09 4.7( 0.6 (5.6) 3.5( 0.45 (4.9) 2.5( 0.25 (3.0) 1.7( 0.25 (2.1)MES-PMAA 1510/1860/1.23 4.0( 0.3 (5.0) 3.4( 0.3 (4.1) 2.7( 0.2 (3.2) 1.8( 0.2 (2.3)PTMP-PMAA 1990/2240/1.13 3.7( 0.2 (4.7) 2.8( 0.15 (3.1) 1.9( 0.1 (1.8)d 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.

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

H Langmuir Wang et al.

precise shape of the distribution shown in Figure 8 for this sample(particularly sub-1 nm) should not be overinterpreted. MALDI-TOFMS characterization was carried out for this sample innegative-ion mode, and the results are shown in Figure 9. Thespectrum shows a broad mass of signals ranging fromm/zvaluesbetween about 5 and 17 kDa. This is broadly consistent withLDI-MS data reported for thiolate monolayer protected Au75

clusters,47 although the high molar mass “tail” for our PTMP-

PMAA-stabilized sample extends to lower mass values. Thismay suggest that our clusters are somewhat smaller or may bea function of the analysis conditions. The data are consistent,however, with the absence of>100 gold atom cores and thesubnanometer cluster size suggested by UV, DLS, and TEM.The fine structure of the MALDI distribution (see Figure S31)shows a multitude of peaks separated bym/z ) 32, which maysuggest loss of Au/Au-S moieties from the main fragment ion.47

In general, the size distributions for the particles producedusing PTMP-PMAA were narrower than those observed usingother ligands in this study. This was most evident for particlessynthesized at 0.006 mM PTMP, where a very monodispersesample was observed which formed ordered hexagonal arrayson the TEM grid (Figure 7a). Lower magnification TEM imagingconfirmed that this monodispersity was a feature of the bulkmaterialsthat is, the majority of the sample was composed ofexceptionally monodisperse particles, as illustrated in FiguresS28 and S29. The very narrow size distribution observed byDLS (black line, Figure 8) further supports this interpretation.

Salt and pH Stability of Polymer-Stabilized Gold Nano-particles.The stability of aqueous gold nanodispersions towardvariables such as salt concentration and pH is an importantconsideration for a number of practical applications. In general,classical citrate-stabilized particles exhibit very poor electrostaticstability and may form aggregates when just small quantities ofsalt are added (∼10 mM).48,49 We investigated the stability ofa series of PMAA-stabilized gold particles toward both saltconcentration and pH. Figure 10 shows an optical image of aseries of PMAA-stabilized gold particle solutions with additionof NaCl up to a maximum concentration of 1.5 M. For particlesstabilized with thioether-terminated ligands (PropT-PMAA-ODT-PMAA) the stability of the preparations toward saltconcentration was found to increase significantly as the hydro-phobicity of the thioether end group was increased. For example,the PropT-PMAA samples were destabilized to form bluesolutions at 0.5 M NaCl, whereas the ODT-PMAA ligand gaverise to particles which were completely stable over the wholeconcentration range (see also corresponding UV data in FigureS32). We ascribe this effect primarily to the end group ratherthan variations in total ligand molecular weight, even though themolar masses for this series of ligands were not all identical.This is supported by the fact that ligand DDT-PMAA5, for

(47) Balasubramanian, R.; Guo, R.; Mills, A. J.; Murray, R. W.J. Am. Chem.Soc.2005, 127, 8126-8132.

(48) Sakura, T.; Takahashi, T.; Kataoka, K.; Nagasaki, Y.Colloid Polym. Sci.2005, 284, 97-101.

(49) Levy, R.; Wang, Z. X.; Duchesne, L.; Doty, R. C.; Cooper, A. I.; Brust,M.; Fernig, D. G.ChemBioChem2006, 7, 592-594.

Figure 7. TEM images of PTMP-PMAA-stabilized gold nano-particles obtained from polymer concentration at (a) 0.006, (b) 0.06,and (c) 0.2 mM; (d, e, and f) histograms of the correspondingdistribution of particle sizes of a, b, and c, respectively.

Figure 8. DLS spectra of PTMP-PMAA-stabilized gold nanopar-ticles obtained from polymer concentration at 0.006 (black), 0.06(red), 0.2 (blue), and 0.6 mM (magenta). The average particlediameters determined by DLS are 4.7, 3.1, and 1.6 nm, respectively,for the three higher concentrations. Particles produced at 0.6 mMare too small to be sized reliably by DLS (see text).

Figure 9. Negative-ion mode MALDI-TOFMS spectrum for PTMP-PMAA-protected gold clusters.

Design of Polymeric Stabilizers Langmuir I

example, did not lead to comparably stable dispersions despitehaving a slightly higher molecular weight than ODT-PMAA.The thiol-containing bidentate and tetradentate ligands, NDT-PMAA and PTMP-PMAA, also gave rise to particle dispersionswhich were completely stable over the whole salt concentrationrange. It is not clear at this stage whether this results fromdifferences in the mode of binding to the gold surface or the factthat these ligands also have a substantial hydrophobic end group.It should be noted, however, that NDT-PMAA gave rise tosignificantly more salt-tolerant dispersions than DDT-PMAA(Figure 10), despite having a slightly less hydrophobic end groupand somewhat lower average molecular weight (cf., Tables 3and 4). A similar albeit less dramatic trend was observed for theeffect of pH (Figure 11; Figures S33-S34). All of the particlepreparations were stable over the pH range 4-12, but only theparticles prepared with the most hydrophobic thioether-terminatedligand, ODT-PMAA, and the two thiol-containing ligands, NDT-PMAA and PTMP-PMAA, were found to be stable at pH 13.

The ability to replace the polymer ligands in these monodispersegold particles with other functionalities such as biomolecules isimportant for a number of applications, and again salt/pH tolerance

is critical in such applications. Previously, it was shown thatDDT-PMAA-stabilized particles could be monofunctionalizedwith peptide ligands.49 Figure 12 shows an agarose gel-electrophoresis binding study with the protein avidin for DDT-PMAA-stabilized gold particles before and after exchange witha mixture of the peptide CALNN (95%) and CALNNGK(biotin)G(5%).49 This demonstrates that it is possible to functionalize theDDT-PMAA particles with peptides and suggests that a rangeof other biofunctionalization reactions should be possible. Assuch, these gold nanoparticles might be developed as biolabels,for example, for ultrasensitive detection and imaging methodsin bioanalytical screens.

Discussion.From the various data described above, a numberof basic “design rules” can be formulated for polymeric ligandsof this type. (i) PMAA and PAA are the most effective polymericstabilizers in the group studied. (ii) The resulting particle sizeis dependent on polymer concentration over the range 0.006-6.0 mM, with higher polymer concentrations leading to smallerparticles. (iii) At a given molar concentration, lower molar massDDT-PMAA chains give rise to smaller and more monodisperseparticles. (iv) PMAA ligands with more hydrophobic end groupsgive rise to somewhat more monodisperse particles, at least upto ODT-PMAA (C18). These particles are also much more stabletoward NaCl concentration and elevated pH. (v) PMAA ligandswith free thiol groups lead to monodisperse particles, and in thecase of the multidentate ligand PTMP-PMAA, the particles aresomewhat smaller and more monodisperse than those obtainedwith any other ligand studied in this series. These particles, too,are very stable toward NaCl and changes in pH.

In combination, these “rules” have allowed us to produce arange of ligands which are significantly more effective andversatile than those reported in our preliminary communication.15

The precise physical explanation of these various effects is moredifficult at this stage, although some trends may be rationalizedby comparison with previous studies.

For the DDT-capped ligands, PMAA and PAA were found tobe the most effective polymers with the DDT-PVSA ligand inparticular showing incompatibility with this synthetic route. DDT-PAA ligands (Figures 2 and 3) gave somewhat narrower particlesize distributions (7-9% standard deviation by TEM) than theDDT-PMAA ligand series (10-14% standard deviation) at allconcentrations studied possibly because of its higher water

Figure 10. Optical image of gold nanodispersions at a range ofNaCl concentrations (up to 1.5 M). The red solutions are stable; blueindicates aggregation.

Figure 11. Optical image of gold nanodispersions over the pHrange 4-14. The red solutions are stable; purple indicates aggregation.

Figure 12. Agarose gel electrophoresis of the DDT-PMAA-stabilized particles (0.06 mM, lane 2), peptide-stabilized goldnanoparticles (lane 3), and peptide-stabilized gold nanoparticles afterreaction with avidin (lane 1). Gold nanoparticles (20µL in1 × TBE)were loaded onto agarose gels (2% w/v in 1× TBE) and subjectedto electrophoresis at 100 V for 0.5 h.

J Langmuir Wang et al.

solubility. Both of these carboxylic-acid-containing polymers,DDT-PAA and DDT-PMAA, gave rise to much narrower particlesize distributions than ligands DDT-PHEA, DDT-PPEG-MA,and DDT-PVP, which led to standard deviations in the range15-28% (Table 1; Figures S10-S12). PAA and PMAA arepolyfunctional molecules bearing a carboxylic acid group oneach monomer and, as such, have the potential to stabilize theseparticles by a combination of both steric and electrostaticmechanisms. Both PAA50,51and PMAA52(without thioether endgroups)havebeenusedpreviouslyasstabilizers for thepreparationof stable nanodispersions of materials such as cerium oxide,50

nickel ferrite,51and zinc oxide.52The general decrease in particlesize with increasing ligand concentration observed for all of thepolymers studied is well documented for a number of ligandsystems.

A molecular weight trend was observed for the DDT-PMAAligand series, with somewhat smaller and more monodispersegold particles being produced at lower polymer molecular weights.This effect was weak (Figure S19) but reproducible at all ligandconcentrations. Shimmin et al. predicted that, in the small particlelimit, a weak increase in particle size will be observed for larger(i.e., higherMw) passivant molecules because they require moretime to diffuse and passivate the growing nanoparticle.31 Thisprediction was not supported by data for Me-PEG-SH ligandsof various molecular weights,31but our observations are broadlyconsistent with this hypothesis. An alternative hypothesis, putforward by the same authors,31 is that heteroatom-containingpolymers such as PEG (and by analogy PMAA/PAA) act as a“net” which assists in particle nucleation by more rapidlysupplying gold atoms to small, unstable gold clusters. In thiscase larger polymers might enhance the effect by being largernets.31 Our data for DDT-PMAA ligands do not suggest that thisis the main effect, although it is possible that both Au-atomsequestration and ligand diffusion play opposing roles in thesystem with ligand diffusivity (for lowerMw species) dominatingin this case.

To our knowledge there are no systematic studies on the effectof end-group hydrophobicity on particle size or stability for ligandsof this type, and hence, it is difficult to fully rationalize theincreased particle monodispersity that was observed for thePMAA ligands with more hydrophobic ends groups (C12, C18).It is known, however, that alkanethiols physisorb onto Au(111)surfaces through van der Waals interactions and that thisphysisorption enthalpy depends on the alkyl chain length.53 Thephysisorption enthalpy per CH2 group was found to be of theorder of 6.1 kJ/mol, which implies that for alkanethiols longerthanabout14carbonatoms thephysisorptionenthalpymayexceedthe chemisorption enthalpy.53Our PMAA ligands produced usingmonofunctional alkanethiol chain-transfer agents contain thio-ether,37 not thiol, end groups. Dialkyl sulfides bind less stronglyto Au, having bond strengths near 60 kJ mol-1 as compared withthe 130 kJ mol-1 value typical of chemisorbed alkanethiol-Aubonds.53,54Unlike alkanethiols, where both physisorbed (∼60 kJmol-1) and chemisorbed (∼120 kJ mol-1) forms have beenobserved, dialkyl sulfides do not readily chemisorb to Au(111)and only physisorption (∼60 kJ mol-1) occurs.53,54On the basisof these previous observations, it is conceivable that the PMAA

ligands with longer hydrophobic alkyl end groups physisorb morestrongly to the growing Au particles and that this causes theenhanced control over particle size distribution.

These PMAA ligands gave rise to gold particles which wereextremely stable over a broad range of ionic strength and pH.Gold nanoparticles stabilized electrostatically (e.g., using citrateligands) may be destabilized by even small changes in ionicstrength and mild changes in physiological conditions.48 PEG-thiol ligands, by contrast, are assumed to act as steric stabilizersand are thus unaffected by changes in ionic strength.19,48 BothPAA and PMAA are weak polyelectrolytes and can impart bothsteric and electrostatic stabilization to the particles. At high pHone might expect the carboxylate anions to contribute anelectrostatic stabilization.55 At low pH the electrostatic contribu-tion will be reduced because the acid groups are protonated.55

It was observed (Figures 10 and Figure 11) that the PMAAligands with longer alkyl end groups gave rise to significantlymore stable nanodispersions at high ionic strengths and elevatedpH. We cannot explain this phenomenon unequivocally, but itis possible that the longer alkyl chains lead to strongerphysisorption on the particle (see above) and that this preventsdisplacement of the ligands from the particle surface at highionic strength and elevated pH. The fact that the chemisorbed(and presumably nondisplaceable) thiol-terminated ligands NDT-PMAA and PTMP-PMAA also lead to similarly stable particlesis consistent with this hypothesis.

The multidentate ligand PTMP-PMAA gave rise to Au particleswhich were quite distinct from those obtained with the otherligands studied here. In particular, the particles were much smallerat a given ligand concentration and more monodisperse.Remarkably narrow size distributions were observed at a ligandconcentration of 0.006 mM (Figure 7a; Figures S28-30). At thisstage we cannot fully rationalize this effect, but it is clear thatthe structure of the PTMP-PMAA ligand differs in a number ofrespects. First, one might expect the tetradentate end group tohave a larger footprint19and hence for each PTMP-PMAA ligandto passivate an increased area on the Au particle surface at agiven molar coverage. Second, it is possible that the stickingprobability for this ligand is enhanced and that this affects thekinetics of passivation. Third, the end group, if multiply boundto the Au surface, may help to stabilize very small Au clusters,which is consistent with the marked decrease in particle size thatwas observed with this ligand at a given polymer concentration.

ConclusionsA simple protocol for the preparation of near-monodisperse

gold hydrosols in the small size regime below 5 nm has beendeveloped. The particle size is controlled by varying theconcentration, structure,and “denticity”of thestabilizingpolymer.We believe that this new protocol will replace previous methodswhenever precise size control and monodispersity are required.

Acknowledgment. We thank EPSRC (EP/C511794/1) andthe Royal Society for a Royal Society Research Fellowship (toA.I.C.). I.H. thanks NIBGE and the Ministry of Science andTechnology, Government of Pakistan, for financial support. Wethank Prof. D. Schiffrin and Dr. J. Weaver for helpful discussions.

Supporting Information Available: Experimental details,details of polymer synthesis, TEM images, DLS spectra, UV-visibleabsorption spectra, and MALDI-TOFMS data. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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(50) Sehgal, A.; Lalatonne, Y.; Berrett, J. F.; Morvan, M.Langmuir2005, 21,9359-9364.

(51) Chen, D. H.; He, X. R. Synthesis of nickel ferrite nanoparticles by sol-gel method.Mater. Res. Bull.2001, 36, 1369-1377.

(52) Tang, E. J.; Cheng, G. X.; Ma, X. L.; Pang, X. S.; Zhao, Q.Appl. Surf.Sci.2006, 252, 5227-5232.

(53) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G.J. Phys. Chem.B 1998, 102, 3456-3465.

(54) Pedersen, D. B.; Duncan, S.J. Phys. Chem. A2005, 109, 11172-11179.(55) Vamvakaki, M.; Billingham, N. C.; Armes, S. P.; Watts, J. F.; Greaves,

S. J.J. Mater. Chem.2001, 11, 2437-2444.

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