A General Method To Coat Colloidal Particles with Silica

A General Method To Coat Colloidal Particles with Silica

(Parte 1 de 3)

A General Method To Coat Colloidal Particles with Silica

Christina Graf,*,†,§ Dirk L. J. Vossen,†,‡ Arnout Imhof,† and Alfons van Blaaderen*,†,‡

Soft Condensed Matter and Biophysics, Debye Institute, Utrecht University,

Ornstein Laboratory, Princetonplein 5, 3584 C Utrecht, The Netherlands, and FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands

Received May 8, 2003

A general method to coat colloids with silica is described. The amphiphilic, nonionic polymer poly- (vinylpyrrolidone) (PVP) was adsorbed to various colloidal particles such as small gold colloids, gold-shell silica-core particles, small and large silver colloids, boehmite rods, gibbsite platelets, and positively or negatively charged polystyrene. After this functionalization the stabilized particles could be transferred to a solution of ammonia in ethanol and directly coated with smooth and homogeneous silica shells of variable thickness by addition of tetraethoxysilane in a seeded growth process. The length of the polymer used strongly influences the stability of the colloids and the homogeneity and smoothness of the initial silica coating. This method is especially useful for colloidal particles that cannot be covered directly with

SiO2 by a Stober-like growth process. Compared to methods known from the literature for the coating of such particles, this new method is faster and requires neither the use of silane coupling agents nor a precoating step with sodium silicate, which is poorly reproducible.

Introduction

Silica-coated colloidal particles are a class of materials widelyusedinmanyfieldsofcolloidandmaterialsscience. A wide variety of coating procedures has been developed for these samples. These surface coatings allow manipulation of the interaction potential and make it possible to disperse colloids in a wide range of solvents from very polartoapolar.Theso-calledStobergrowth1ofsilicashells by addition of tetraethoxysilane to solutions of seed particles in an ethanol/ammonia mixture yields smooth surfaces since the growth takes place on a molecular scale.2-4 If silica-coated particles are grown further by thisprocedure,thepolydispersityoftheparticlesdecreases with R-1, where R is the particle radius.4 This makes it possible to grow crystals of core shell particles even when the cores are polydisperse.

Silica colloids and silica-coated particles are often used as model particles to study phase behavior, rheology, and diffusion. Surface modifications of silica spheres have yielded systems with hard-core potentials,5 short-range attractive potentials,6 and Yukawa potentials.7

In addition, the silica layer allows for controlled placementofvariousdyes.8Suchdye-labeledparticlescan be used in quantitative real-space studies with confocal fluorescence microscopy9 or as tracer particles in, for example, fluorescence recovery after photobleaching (FRAP)10 or time-resolved phosphorescence anisotropy11 measurements.Moreover,thepossibilityoftheplacement ofdyemoleculeswithnanometerprecisionallowsthestudy of the local density of states in photonic applications.12

Interest in the use of silica-coated particles as building blocks for photonic crystals is increasing.13-16 Here the outer silica shell allows tuning of not only the interaction potential of the particles but also the optical properties of the crystal.

An outer silica coating also offers new possibilities for the shape control of a particle. Silica particles can be anisotropically deformed in a controlled way by ion beam irradiation.17 This method was successfully applied to silica-coated gold particles.17 Due to deformation of the silicashellitwaspossibletodeformsphericalgoldparticles into prolate colloids in a controlled way, while normally gold particles remain unchanged under these conditions.

There are many surfaces that can be directly coated with silica because of the significant chemical affinity of thesematerials,likeclayminerals,18hematite,19zirconia, and titania.20 However, many other surfaces can only be coated with the help of stabilizers, surfactants, silane couplingagents,orafastprecipitationfromawaterglass

* To whom correspondence should be addressed. E-mail: cgraf@phys-chemie.uni-wuerzburg.de (C.G.); a.vanblaaderen@ phys.u.nl (A.B.).

† Utrecht University. ‡ FOM Institute for Atomic and Molecular Physics. § Present address: Institut fur Physikalische Chemie, Univer- sitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany.

(1) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (2) Philipse, A. P. Colloid Polym. Sci. 1988, 266, 1174. (3) Bogush, G. H.; Zukoski, C. F. J. Colloid Interface Sci. 1991, 142, 19. (4) van Blaaderen, A.; van Geest, J.; Vrij, A. J. Colloid Interface Sci. 1992, 154, 481. (5) van Helden, A. K.; Jansen, J. W.; Vrij, A. J. Colloid Interface Sci. 1980, 8, 312. (6) Jansen, J. W.; de Kruif, C. G.; Vrij, A. Chem. Phys. Lett. 1984, 107, 450. (7) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1989, 128, 121. (8) van Blaaderen, A.; Vrij, A. Langmuir 1992, 8, 2921.

(9) van Blaaderen, A.; Wiltzius, P. Science 1995, 270, 17. (10) van Blaaderen, A.; Peetermans, J.; Maret, G.; Dhont, J. K. G.

J. Chem. Phys. 1992, 96, 4591. (1) Lettinga, M. P.; van Zandvoort, M. A. M. J.; van Kats, C. M.;

Philipse, A. P. Langmuir 2000, 16, 6156. (12) Photonic Crystals and Light Localization in the 21st Century;

Kluwer Academic Publishers: Dordrecht, Boston, London, 2001; Vol. 563. (13) Garcia-Santamaria, F.; Salgueirino-Maceira, V.; Lopez, C.; Liz-

Marzan, L. M. Langmuir 2002, 18, 4519. (14) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524. (15) Velikov, K. P.; Blaaderen, A. v. Langmuir 2001, 17, 4779. (16) Moroz, A. Phys. Rev. B 2002, 6, 115109. (17) Snoeks, E.; van Blaaderen, A.; van Dillen, T.; van Kats, C. M.;

Brongersma, M. L.; Polman, A. Adv. Mater. 2000, 12, 1511. Roorda, S.; van Dillen, T.; Kooi, B.; de Hosson, J.; Graf, C.; van Blaaderen, A.; Polman, A. Submitted for publication in Adv. Mater. (18) Iler, R. K. U. S. Patent No. 2,885,366, 1959. (19) Ohmori, M.; Matijevic, E. J. Colloid Interface Sci. 1992, 150, 594. (20) Ryan, J. N.; Elimelech, M.; Baeseman, J. L.; Magelky, R. D. Environ. Sci. Technol. 2000, 34, 2000.

10.1021/la0347859 C: $25.0 © 203 American Chemical Society Published on Web 07/1/2003 solution,andmostofthesecoatingmethodsaremultistep processes.Forinstance,inproceduresusingmercapto-or aminosilanes as coupling agent gold colloids,21 silver colloids,2 and semiconductor nanocrystals23,24 could be coated with silica.

A critical step in many silica-coating procedures is the transferofcolloidsthatareonlystableinaqueoussolution toethanolwheretheclassicalStoberprocessisperformed. A widely used technique to achieve this transfer is the water glass process.18,21,25 First, colloidal particles are covered with a thin layer of sodium silicate in aqueous solution. If the particles are then sufficiently stabilized, theycanbetransferredintoethanolandbefurthercoated with a thicker silica layer using seeded growth. In this way boehmite rods could be coated with silica via a threestep coating procedure.25 Such a step was also necessary for the coating of gold and silver colloids via the use of silane coupling agents. A disadvantage of this method is that the growth of the initial shell with sodium silicate is strongly pH dependent and not very well controllable. The formation of such an initial silica coating on gold colloids requires reaction times between 24 h and several weeks14,21,26,27 before a sufficiently thick shell for transfer of the particles into ethanol is achieved.

In this paper we present a general, simple and fast method to coat colloids with silica. This method is based on the use of poly(vinylpyrrolidone) (PVP) as a coupling agent. This amphiphilic, nonionic polymer is widely used inscienceandtechnologyandadsorbsontoabroadrange of different materials such as metals (e.g., gold, silver, iron),metaloxides(kaolinite,TiO2,ironoxide,alumina),28 polystyrene,29 silica,30 graphite,31 and cellulose.32 It sta- bilizes colloidal particles in water and many nonaqueous solvents.InthisarticleweshowhowPVPcanbeadsorbed ontovariouscolloidswhichcanthenbedirectlytransferred into an ammonia/ethanol mixture where smooth and homogeneous silica coatings of variable thickness can be grown by addition of tetraethoxysilane (TES). Further, we show that the length of the PVP used plays an important role in the stability of the particles during the growth process and the smoothness and homogeneity of the silica shells obtained.

Experimental Section

Materials. Tetraethoxysilane (TES, g98.0%) was obtained from Fluka. Poly(vinylpyrrolidone) with average molar masses of360kg/mol(PVP-360),of40kg/mol(PVP-40),andof10kg/mol (PVP-10) was purchasedfrom Sigma-Aldrich.Poly(vinylpyrrolidone) with an average molar mass of 3.5 kg/mol (PVP-3.5) was obtained from Acros Organics. Ethanol (p.a.), acetone (p.a.), ammonia (29.3 wt % NH3 in water), and hydrofluoric acid (38- 40%) were purchased from Merck. All chemicals were used as received. Water used in the described reactions and for cleaning of the glassware was obtained from a water purification system, WATER PRO PS from Labcono, and had a measured resistivity of2M ¿.

Syntheses. The general procedure to coat colloids with silica consist of two steps: adsorption of PVP and growth of the silica shell after transfer of the particles to ethanol. An outline of the synthesis is shown in Figure 1.Synthesis of the Colloids. Small gold colloids of 7 and 19 nm radius were synthesized according to the standard sodium citrate reduction method33,34 and were not further purified. Silica particles (228 nm radius) with a 38 nm thick gold shell were prepared as described in ref 14 and purified by repeated sedimentation and redispersion in water. Small silver particles of 13 nm radius were synthesized by a modifiedpolyolprocess:35Silvernitratewasreducedinethylene glycolinthepresenceofPVP-10.Afterthesynthesis,theparticles were separated from ethylene glycol by addition of acetone (500 mL of acetone/75 mL of reaction mixture) and subsequent centrifugation at 600g as described in ref 35. Next, the supernatant was removed and the particles were redispersed in ethanol,againcentrifugedat600g,andredispersedinasolution of ammonia in ethanol (4.2 vol % ammonia (29.3 wt % NH3 in water) in ethanol). This solution (c ) 0.31 g/L) could be directly used in the silica-coating step (see below). Large silver colloids of 320 nm radius were synthesized by reducing silver nitrate with ascorbic acid in the presence of the polymeric stabilizer gum arabic36 and purified by repeated sedimentation and redispersioninwater.Boehmiterodsandgibbsiteplateletswere prepared from aqueous aluminum oxide solutions by hydrothermal treatment at 85 °C (gibbsite) and at 150 °C (boehmite) andpurifiedbydialysisagainstdemineralizedwaterasdescribed in refs 37 and 38.

Cationic polystyrene spheres were prepared by surfactantfree emulsion polymerization as described in ref 39 and anionic sulfate stabilized polystyrene spheres as described in ref 40.

(21) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (2) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740. (23) Correa-Duarte, M. A.; Giersig, M.; Liz-Marzan, L. M. Chem.

Phys. Lett. 1998, 286, 497. (24) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P.

Science 1998, 281, 2013. (25) van Bruggen, M. P. B. Langmuir 1998, 14, 2245. (26) Makarova,O.V.;Ostafin,A.E.;Miyoshi,H.;Norris,J.R.;Meisel,

D. J. Phys. Chem. B 1999, 103, 9080. (27) Hall, S. R.; Davis, S. A.; Mann, S. Langmuir 2000, 16, 1454. (28) Pattanaik, M.; Bhaumik, S. K. Mater. Lett. 2000, 4, 352. (29) Smith, J. N.; Meadows, J.; Williams, P. A. Langmuir 1996, 12, 3773. (30) Esumi, K.; Matsui, H. Colloid Surf., A: Physicochem. Eng. Asp. 1993, 80, 273. (31) Otsuka, H.; Esumii, K. J. Colloid Interface Sci. 1995, 170, 113. (32) Kotelnikova, N. E.; Panarin, E. F.; Kudina, N. P. Zh. Obshch. Khim. 1997, 67, 335.

(3) Enustun, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 317. (34) Frens, G. Nature (London), Phys. Sci. 1973, 241, 20. (35) Silvert,P. Y.; Herrera-Urbina,R.; Duvauchelle,N.; Vijayakrishnan, V.; Elhsissen, K. T. J. Mater. Chem. 1996, 6, 573. (36) Velikov,K.P.;Zegers,G.E.;vanBlaaderen,A.Langmuir2003, 19, 1384. (37) Wierenga, A. M.; Lenstra, T. A. J.; Philipse, A. P. Colloid Surf., A: Physicochem. Eng. Asp. 1998, 134, 359.

Figure 1. Diagram of the general procedure for the coating of colloids with silica. In the first step poly(vinylpyrrolidone) is adsorbed onto the colloidal particles. Then these stabilized particlesaretransferredintoasolutionofammoniainethanol. A silica shell is grown by consecutive additions of tetraethoxysilane.

6694 Langmuir, Vol. 19, No. 17, 2003 Graf et al.

Coating with Silica. The reaction vessels used in the silicacoating step were cleaned with hydrofluoric acid (8 vol %) and after that rinsed several times with water. All reaction steps with gold or silver colloids were carried out under exclusion of light.

Functionalizion with PVP. The molar mass of the PVP used in this step depends on the size of the particles used and is given inTable1.Theconcentrationandamountsofthecolloidalsolution are also given in Table 1.

TheamountofPVPwascalculatedtoprovidethecolloidswith about 60 PVP molecules per nm2 surface, independent of the molar mass of the PVP (Table 1). The PVP was dissolved in waterbyultrasonicationofthesolutionfor15min.Subsequently, the PVP and the colloidal solutions were mixed under stirring (600 rpm in the case of the gold and silver colloids and 100 rpm in the case of the boehmite rods and the gibbsite platelets.). To guarantee that adsorption was complete, the reaction mixture was stirred for 24 h at room temperature.

Thepolystyreneparticleswerealreadyinanethanolsolution.

Theseparticlesweresedimented,thesupernatantwasremoved, andasolutionofPVPinethanolwasadded.Becauseofthelarge size of the PVP (PVP-360) used for these particles, the chosen PVP concentrations were lower in relation to the total surface area. These solutions were directly used for the silica-coating step.

TransferintoEthanol.TotransferthePVP-stabilizedparticles into ethanol, the solutions were centrifuged or sedimented (see Table 1) and the supernatant was removed.

Synthesis of the Silica Shell. The sediments were redispersed in a solution of ammonia in ethanol (4.2 vol % ammonia (29.3 wt

%NH3inwater)inethanol).Inthecaseofthepolystyrenespheres, ammonia (29.3 wt % NH3 in water) was added to the PVP polystyrenesolutions.ImmediatelyafterthisaTESsolution(10 vol % in ethanol) was added under stirring (600 rpm in the case of the gold and silver colloids and 100 rpm in the case of the boehmite rods and the gibbsite platelets). In the case of the boehmite rods the TES solution was added in four steps over 6 h(first51.25íLofTESwasadded,after2hagain51.25íL,after 4 h 102.5 íL, and after6h2 05 íL). The total amount of TES added was chosen depending on the desired thickness of the silica shell. The reaction mixtures were then stirred for another 12 h.

Characterization. TEM. Samples for transmission electron microscopy (TEM) were prepared by dipping copper 400-mesh carrier grids covered with carbon-coated Formvar films in the dispersions (c 0.5-5 g/L). The boehmite, gibbsite, and polystyreneparticleswereimagedwithaPhilipsTecnai20FEG high-resolution transmission electron microscope (HRTEM) operatedat200keV.AllothercolloidswereimagedwithaPhilips Tecnai12transmissionelectronmicroscopeoperatedat120keV.

The size of the colloids was determined with the measurement tools of the Analysis software provided with the Philips Tecnai microscopes by binarization of the images, separation of the particlesbystandardimageanalysistechniques,andestimation of the mean particle radius from the measured surface area. A diffraction grating was used to calibrate the magnification.

SEM.Scanningelectronmicroscopy(SEM)wasusedtostudy the positively and negatively charged polystyrene spheres and the silica-coated gold shell silica core particles. Samples were prepared by dropping a dilute dispersion (c 0.05-0.005 g/L) on a silicon substrate (used as received). Colloidal crystals of silica-coated gold-shell silica-core particles were obtained by putting an ethanolic solution of these particles (c ) 1.43 g/L) on asiliconwaferanddryinginair.Themeasurementswerecarried out with a Philips XL30SFEG scanning electron microscope.

EDX.Todeterminetheelementalcompositionoftheparticles studied during electron microscopy measurements, an energydispersive X-ray (EDX) detector from EDAX was used together withtheHRTEMorSEM.Forthemeasurementsincombination with the HRTEM, the same samples were used as for the TEM imaging. For the measurements in combination with the SEM, diluted colloidal dispersions (c 5 mg/L) were dropped onto an aluminumsubstrate(cleanedwithacetone)togivewell-separated particles on the surface.

Results and Discussion

(Parte 1 de 3)

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