Synthesis of Nanosized Gold-Silica Core-Shell Particles

Synthesis of Nanosized Gold-Silica Core-Shell Particles

(Parte 1 de 2)

Synthesis of Nanosized Gold-Silica Core-Shell Particles

Luis M. Liz-Marzan,*,† Michael Giersig,‡ and Paul Mulvaney*,§

Advanced Mineral Products Research Centre, School of Chemistry, University of Melbourne, Parkville, Victoria, 3052, Australia

Goldcolloidshavebeenhomogeneouslycoatedwithsilicausingthesilanecouplingagent(3-aminopropyl)- trimethoxysilane as a primer to render the gold surface vitreophilic. After the formation of a thin silica layer in aqueous solution, the particles can be transferred into ethanol for further growth using the Stober method. The thickness of the silica layer can be completely controlled, and (after surface modification) the particles can be transferred into practically any solvent. Varying the silica shell thickness and the refractive index of the solvent allows control over the optical properties of the dispersions. The optical spectra of the coated particles are in good agreement with calculations using Mie’s theory for core-shell particles.

Introduction

The preparation of ultrafine metal particles can be facilitated greatly by careful choice of the ligands or stabilizers used to prevent particle coalescence. For example, in aqueous solution, polymeric stabilizers are very efficacious dispersants,1 whereas, in organic media, long chain surfactants or chemically specific ligands are most commonly used.2 Alternatively, stabilization can be achieved through compartmentalization in micelles and microemulsions,3 while immobilization in glasses4 or solgels5 is the preferred technique when redox reactions of the particles with the matrix need to be avoided. More recently, LB films have been used as particle stabilizers,6 and electrodeposition7 of surfactant stabilized metal particles has been used to create ordered 2D crystals. These various techniques not only permit the synthesis of pure metal particles; they also allow the preparation of ultrafine alloys, mixed metal particles, and coated particles8aswellasparticleswithnonsphericalgeometries (e.g. rods or platelets).9

However, many of the stabilizers employed affect the solid state properties of the particles.1a,f To circumvent this problem it is necessary to find a stabilizer which not only prevents particle coalescence but also is chemically inertandopticallytransparent. Theseconditionsaremet by silica, a coating material used in a wide range of industrial colloid products ranging from paints10 and magnetic fluids,1 to high-quality paper coatings.12,13 To date, silica coating has not been applied to stabilize nanosized metal particles or to modulate their optical properties.

The silica-coating procedures reported in the literature14-17 generally involve surfaces with a significant chemical or electrostatic affinity for silica. However gold metalhasverylittleaffinityforsilicabecause,unlikemost other metals, it does not form a passivating oxide film in solution. Furthermore, there are usually adsorbed carboxylicacidsorotherorganicanionspresentonthesurface to stabilize the particles against coagulation. These stabilizers also render the gold surface vitreophobic.

Previous attempts to overcome this vitreophobic character involved the heterocoagulation of gold colloids on silica colloids dispersed in water, followed by extensive

† Permanent address: Dept. of Pure and Applied Chemistry,

PhysicalChemistrySection,VigoUniversity,Apdo.874,36200Vigo, Spain.

‡ Permanentaddress: Hahn-MeitnerInstitut,Abt.Kleinteilchenforschung, Glienickerstr. 100, Berlin, Germany.

§ Fax: 61-3-9344-6233.Phone: 61-3-9344-6486or6481.E-mail: p.mulvaney@chemistry.unimelb.edu.au.

X AbstractpublishedinAdvanceACSAbstracts,August1,1996.

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M. J. Am. Chem. Soc. 1979, 101, 7214. (c) Belloni, J. Curr. Opinion 1996, 2, 184. (d) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (e)Furlong,D.N.;Laukonis,A.;Sasse,W.H.F.;Sanders,J.V.J.Chem. Soc., Faraday Trans. 1 1984, 80, 571. (f) Mulvaney, P. Langmuir 1996, 12, 788. (2) (a) Lin, S.-T.; Franklin, M. T.; Klabunde, K. J. Langmuir 1986, 2, 259. (b) Hirai, H.; Aizawa, H.; Shiozaki, H. Chem. Lett. 1992, 1527. (c)Deshpande,V.M.;Singh,P.;Narasimhan,C.S.J.Chem.Soc.,Chem. Commun. 1990, 1181. (d) Bradley, J. S.; Hill, E. W.; Behal, S.; Klein, C.;Chaudret,B.;Duteil,A.Chem.Mater.1992,4,1234.(e)Liz-Marzan, L. M.; Lado-Tourino, I. Langmuir 1996, 12, 3585. (f) Brust, M.; Fink, J.;Bethell,D.;Schiffrin,D.J.;Kiely,C.J.Chem.Soc.,Chem.Commun. 1995, 1655. (g) Esumi, K.; Tano, T.; Torigoe, K.; Meguro, K. Chem. Mater. 1990, 2, 564. (h) Duteil, A.; Schmid, G.; Meyer-Zaika, W. J. Chem. Soc., Chem. Commun. 1995, 31. (i) Andrews, M. P.; Ozin, G. A. J. Phys. Chem. 1986, 90, 2929. (3) (a) Wilcoxon, J. P.; Williamson, R. L.; Baughman, R. J. J. Chem.

Phys.1993,98,93.(b)Petit,C.;Lixon,P.;Pileni,M.P.J.Phys.Chem. 1993, 97, 12974. (c) Barnickel, P.; Wokaun, A.; Sager, W.; Eicke, H. F. J. Colloid Interface Sci. 1992, 148, 80. (4) Doremus, R. H. J. Chem. Phys. 1965, 42, 414. (5) Akbarian, F.; Dunn, B. S.; Zink, J. I. J. Phys. Chem. 195, 9, 3892. (6) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (7) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408.

(8) (a) Papavassiliou, G. C. J. Phys. F 1976, 6, L103. (b) Marignier,

J.; Belloni, J.; Delcourt, M.; Chevalier, J. Nature 1985, 317, 344. (c) Sermon,P.A.;Thomas,J.M.;Keryou,K.;Millward,G.R.Angew.Chem., Int.Ed.Engl.1987,26,918.(d)Henglein,A.;Mulvaney,P.;Holzwarth, A.; Sosebee, T. E.; Fojtik, A. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 754. (e) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1992, 96, 10419. (f) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (g) Toshima, N.; Harada, M.; Yamazaki, Y.; Kiyotaka, A.J.Phys.Chem.1992,96,9927.(h)Bradley,J.S.;Hill,E.W.;Chaudret, B.; Duteil, A. Langmuir 1995, 1, 693. (i) Liz-Marzan, L. M.; Philipse, A. P. J. Phys. Chem. 195, 9, 15120. (9) (a) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 1, 3285. (b) Tanori, J.; Duxin, N.; Petit, C.; Lisiecki, I.; Veillet, P.; Pileni, M. P. Colloid Polym. Sci. 1995, 273, 886. (10) Furlong, D. N. In The Chemistry of Colloidal Silica; Bergna, H.

E., Ed.; Advances in Chemistry Series 234; American Chemical Society: Washington, DC, 1994; p 535. (1) James, R. O.; Bertucci, S. J.; Oltean, G. L. U.S. Patent No. 5252441, 1993. (12) Payne, C. C. In The Chemistry of Colloidal Silica; Bergna, H.

E., Ed.; Advances in Chemistry Series 234; American Chemical Society: Washington, DC, 1994; p 581. (13) Wilson, I. V. U.S. Patent 2643048, 1953. (14) Iler, R. K. U.S. Patent No. 2,885366, 1959. (15) Ohmori, M.; Matijevic, E. J. Colloid Interface Sci. 1992, 150, 594. (16) (a) Philipse, A. P.; van Bruggen, M. P. B.; Pathmamanoharan,

C. Langmuir 1994, 10, 92. (b) Philipse, A. P.; Nechifor, A. M.; Pathmamanoharan, C. Langmuir 1994, 10, 4451. (17) Chang, S.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1994, 116, 6739.

S0743-7463(96)0187-4 C: $12.0 © 1996 American Chemical Society growthinethanol.18 Thisresultedinamixtureoflabeled and unlabeled silica particles, with a rather low concentration of the labeled ones. The approach presented here is fundamentally different and involves modifying the particlesurfacetomakeitvitreophilic. Thesimplestway to do this is using silane coupling agents as surface primers.19 Virtually all the studies on silane coupling agents focus on the adsorption of the silanol groups onto different surfaces to generate functionalized surfaces, whilehereweusethemtoformahydratedsilicamonolayer bonded to the metal substrate. A brief report on the preparationofsilica-coatedgoldhasappeared;20wediscuss here in detail the preparation and optical properties of silica-coated gold particles.

Experimental Section

(3-Aminopropyl)trimethoxysilane (APS), tetraethoxysilane (TES),3-(trimethoxysilyl)propylmethacrylate(TPM),andsodium silicate solution (Na2O(SiO2)3-5,2 7w t%S iO2) were purchased from Aldrich. HAuCl4 (Sigma), trisodium citrate dihydrate

(Normapur), and NH4OH (Rhone-Poulenc, 28%) were used as received. Technical grade ethanol and Milli-Q water were used in all the preparations.

Transmissionelectronmicroscopy(TEM)wascarriedoutwith a Philips CM10 microscope, and particle size distributions were measured from several TEM negatives of each sample. UV- visible spectra were measured with a Hitachi 150-20 spectrophotometer. Refractive indices of solvents were measured at 589.3 nm using an Abbe refractometer.

Particle synthesis. The procedure comprises several independent steps, which means that it can be interrupted at any stage of the preparation, depending on the aim for which the particles are being prepared. The standard preparation is as follows:

Agoldsol(500mL,5 10-4MHAuCl4)ispreparedaccording tothestandardsodiumcitratereductionmethod.21 Thismethod produces a stable, deep-red dispersion of gold particles with an average diameter of around 15 nm and 10% polydispersity. A freshly prepared aqueous solution of APS (2.5 mL, 1 mM) is addedto500mLofthegoldsolundervigorousmagneticstirring. The mixture of APS and gold dispersion is allowed to stand for 15 min to ensure complete complexation of the amine groups with the gold surface. A solution of active silica is prepared by loweringthepHofa0.54wt%sodiumsilicatesolutionto10-1 by progressive addition of cation exchange resin (Dualite C225- Na 14-52 mesh, from BDH Chemicals, converted into the acid form by repeated washing with HCl and water). Twenty milliliters of active silica is then added to 500 mL of the surfacemodified gold sol, again under vigorous magnetic stirring. The resulting dispersion (pH 8.5) is then allowed to stand for at least one day, so that the active silica polymerizes onto the gold particlesurface. Thesilicashellthicknessisabout2-4nmafter 24 h.

The particles can then be transferred into ethanol if further growth or chemical modification of the silica layer is intended. At this point, thicker shells can be grown via the Stober method. To 500 mL of the sol in 1:4 water/ethanol containing particles with a silica shell 4 nm thick is added 0.3 mL of TES and 2 mL ofammonia. Thesolutionisallowedtostandfor12hundermild magnetic stirring, and additions are repeated until the desired shell thickness is attained. The amount of TES to be added can be calculated from the initial and final particle sizes, by means of the following formula, where Vh refers to molar volume, Rtot is the final radius, and RAu is the radius of the starting colloid.

If the particles are to be dispersed in low-polarity solvents, they can be coated with a second silane coupling agent, such as TPM.2 The coating is performed by addition of excess TPM to thealcosol(inourcase0.5mLofTPMto500mLofalcosol,which contains about 0.5 g/L SiO2), stirring at room temperature for 45 min, slowly distilling 100 mL of the solvent for 1 h, and final cleaning of the excess TPM by repeated centrifugation at 3000 rpmandredispersioninpureethanol. TheTPM-coatedparticles are stable in organic solvents with low polarity, such as ethanol and toluene, as well as in ethanol/toluene mixtures.

Results and Discussion

In section I we consider the preparation and characterization of the coated particles, and in section I the optical properties of the coated particles are compared with the predictions of modified Mie theory.

I. Synthesis. Avarietyofthioalkanederivativesmay be used to stabilize gold colloids,7 but aminoalkanes also complexstronglywithgoldmetal.23 Themethodreported hereinvolvestheprimer(3-aminopropyl)trimethoxysilane (APS). One monolayer of APS is allowed to adsorb onto the gold colloid surface. During this process, the previously adsorbed (negatively charged) citrate groups are displaced by APS molecules, with the silanol groups believedtobepointingintosolution,assketchedinFigure 1. This is driven by the large complexation constant for gold amines. Hydrolysis of the surface-bonded siloxane moietiestoformsilanetriolsoccurswithinminutes,while condensationis much slower,especiallyat low concentrations.19 At pH 7, there is ionization of the silane triols (their isoelectric point is pH 2-3), and this ensures that there is adequate negative surface charge on the gold sol during stabilizer exchange to maintain sol stability. In the second step, active silica (sodium silicate solution at a pH just low enough to allow for a slow polymerization of silicate groups) is added to the dispersion, which promotes the formation of a thin, dense, and relatively homogeneoussilicalayeraroundtheparticles,14usingthe silanolgroupsasanchorpoints. Atthisstage,theparticles can be transferred into ethanol and the silica layer

thickness can be increased in a controlled way.

(18) Liz-Marzan, L. M.; Philipse, A. P. J. Colloid Interface Sci. 1995, 176, 459. (19) Plueddermann, E. P. Silane Coupling Agents, 2nd ed.; Plenum

Press: New York, 1991. (20) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. J. Chem. Soc.,

(2) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1989, 128, 121. (23) Puddephatt,R.J.TheChemistryofGold;Elsevier: Amsterdam, 1978.

VTES ) VhTESVhAu

VhSiO

Figure 1. Sketch of the surface reactions involved in the formationofathinsilicashelloncitrate-stabilizedgoldparticles.

InfluenceofAPSConcentration. TheAPSconcentration used in the standard preparation (5 10-6 M) supplies thesystemwithca.1300APSmoleculespergoldparticle. Assuming a surface coverage of 40 Å2 per APS molecule24

and taking the specific surface area of gold SAu (cm2 L-1) as the added amount of APS is close to one monolayer (but still slightly below it), and it has been observed to be sufficient for the preparation of homogeneous coatings. At this APS concentration, the dispersion remains stable for days and no clear change in the absorption spectrum is observed. After longer periods of time, some black precipitate is observed (often present in the citratestabilized sol too), while the rest of the sol retains the original red color. When more than one monolayer is added, bridging flocculation occurs over minutes to days depending on the APS concentration. This can easily be monitoredthroughtheshapeoftheUV-visiblespectrum ofthedispersion(seeFigure2). Thisbridgingflocculation can even be observed visually; initially the color of the dispersion becomes dark brown, and finally total precipitation occurs. The structure of the APS monolayer at the particle surface is thought to be uniform, with the amino groups complexed to gold surface atoms and the silane groups facing the solvent (see Figure 1). There may be some surface condensation of the silane groups whenthecoverageofAPSisclosetoamonolayer,butthis would not significantly affect the affinity toward silicate ions.

Role of Active Silica. Two parameters are important for the first silica coating: pH and silicate concentration. As indicated by Iler,14 the pH should be kept between 8 and10,sothatthesolubilityofthesilicatespeciespresent in the solution is reduced, and polymerization/precipitationmustoccuratasufficientratetohomogeneouslycoat the particles but still slow enough to avoid the formation ofsilicanuclei. Atthesametime,thesilicateconcentration alsoplaysaroleindeterminingthecoatingrate. Wefind thatalargeexcessofsilicate([SiO2]/[Au] 10)isnecessary to achieve a visible silica layer in a rather short period of time. With a silicate concentration of 0.021 wt %, a 2-4 nm layer is formed after 24 h. In Figure 3, transmission electronmicrographstakenatvariousstagesofthecoating process are shown. After 18 h in the active silica solution the silica layer is not perfectly homogeneous, but practically the entire surface of all the gold particles appears to be coated. One day later, the shell perfectly covers all theparticles;however,theparticlesurfaceisrough. This roughness is due to the oligomeric nature of active silica. Direct adsorption of silica oligomers takes place as aging proceeds. Finally after 5 days a further increase in silica layer thickness is observed, but small silica particles also nucleate out of the solution.

To avoid coprecipitation of silica nuclei, dialysis can be performed at several stages of the coating process (MW cutoff ) 12 0, 10 mL of Au-silica sol, 5 L reservoir of distilledwater)asawaytoremoveunreactedsilicateions from the solution. With thin coatings (even after short term dialysis), partial aggregation of the colloid takes place, as indicated by an increased absorbance at longer wavelengths and a decreased plasmon band.25,26 On the other hand, when the layer is thick, the dialysis is not very useful because small silica particles are already present in the dispersion and they cannot be removed by dialysis. This means that the preparation of aqueous dispersions of gold with thin silica shells that are free of silica particles is hard to achieve.

(24) Boerio, F. J.; Armogan, L.; Cheng, S. Y. J. Colloid Interface Sci. 1980, 73, 416.

(25) Turkevich,J.;Garton,G.;Stevenson,P.C.J.ColloidSci.,Suppl. 1 1954, 26. (26) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 545.

Figure 2. UV-visible spectra of sodium citrate-stabilized, 15 nm diameter gold colloids 1 day after addition of different amounts of APS.

Figure 3. Transmission electron micrographs of 15 nm gold particles coated with thin silica layers: (a, top) 18 hours after additionofactivesilica;(b,center)42hafteraddition;(c,bottom) 5 days after addition. The silica shell keeps on growing, but eventuallysmallsilicaparticlesalsonucleateoutofthesolution.

Synthesis of Nanosized Gold-Silica Core-Shell Particles Langmuir, Vol. 12, No. 18, 1996 4331

(Parte 1 de 2)

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