Flexible Ureasil Hybrids with Tailored Optical Properties through

10268

Langmuir 2004, 20, 10268-10272

Flexible Ureasil Hybrids with Tailored Optical Properties through Doping with Metal Nanoparticles Victor I. Boev,†,‡ Jorge Pe´rez-Juste,† Isabel Pastoriza-Santos,† Carlos J. R. Silva,‡ M. de Jesus M. Gomes,‡ and Luis M. Liz-Marza´n*,† Departamento de Quı´mica Fı´sica, Universidade de Vigo, 36310, Vigo, Spain, and Departamento de Quı´mica, Universidade do Minho, 4710-057 Braga, Portugal Received May 4, 2004. In Final Form: August 20, 2004 Hybrid organic-inorganic nanocomposites containing uniform distributions of metal nanoparticles have been prepared by mixing a preformed nanoparticle colloid with the precursors of a ureasil, prior to the sol-gel transition. These nanocomposites possess not only high optical quality and optical features dictated by the size and shape of the nanoparticle dopants but also a high degree of flexibility, which can largely enhance the range of applications in practical devices. The deposition of a uniform silica shell on the nanoparticle surface prior to the sol-gel transition was found to be required to maintain the colloidal stability during the process and, thus, to retain the optical properties in the final nanocomposite material. This method can be readily extended to other materials, such as semiconductor and magnetic nanoparticles.

Introduction Due to their special surface properties, metal nanoparticles offer a great potential for applications in nonlinear optical devices, such as ultrafast optical switches and filters,1,2 and for uses as catalysts3 or sensors,4 among others. Although colloid chemistry is now sufficiently well developed to provide methods for the synthesis of metal nanoparticles with various composition, size, and shape, most of such methods yield dispersions in liquids, and it is not straightforward to transfer the nanoparticles into solid substrates with high optical quality while retaining the characteristic properties of single particles, i.e., preventing aggregation. To avoid this problem and to obtain a homogeneous distribution of nanoparticles within glasses and polymers, many approaches to the preparation of metal nanoparticle composites involve the in situ reduction of metal salts.5-7 However, these techniques suffer from a poor control of monodispersity, particle size, and especially particle shape and composition. Therefore, new procedures are being investigated for the incorporation of preformed (metal) nanoparticles within polymers and glasses.8 We have recently reported9-11 that, through the coating of metal nanoparticles with thin silica shells, * Author to whom correspondence should be addressed. E-mail: [email protected]. Fax: +34 986812556. † Universidade de Vigo. ‡ Universidade do Minho. (1) Hache, F.; Ricard, D.; Flytzanis, C. J. Opt. Soc. Am. B 1986, 3. (2) Bloemer, M. J.; Haus, J. W.; Ashley, P. R. J. Opt. Soc. Am. B 1990, 7, 790. (3) Braun, S.; Rappoport, S.; Zusmen, R.; Avnir, D.; Ottolenghi, M. Mater. Lett. 1990, 10, 1. (4) Wallace, J. M.; Rice, J. K.; Pietron, J. J.; Stroud, R. M.; Long, J. W.; Rolison, D. R. Nano Lett. 2003, 3, 1463. (5) Bharathi, S.; Fishelson, N.; Lev, O. Langmuir 1999, 15, 1929. (6) Hund, J. F.; Bertino, M. F.; Zhang, G.; Sotiriou-Leventis, C.; Leventis, N.; Tokuhiro, A. T.; Farmer, J. J. Phys. Chem. B 2003, 107, 465. (7) Wu, P.-W.; Cheng, W.; Martini, I. B.; Dunn, B.; Schwartz, B. J.; Yablonovich, E. Adv. Mater. 2000, 12, 1438. (8) Morris, C. A.; Anderson, M. L.; Stroud, R. M.; Merzbacher, C. I.; Rolison, D. R. Science 1999, 284, 622. (9) Kobayashi, Y.; Correa-Duarte, M. A.; Liz-Marza´n, L. M. Langmuir 2001, 17, 6375. (10) Pe´rez-Juste, J.; Correa-Duarte, M. A.; Liz-Marza´n, L. M. Appl. Surf. Sci. 2004, 226, 137. (11) Rodrı´guez-Gonza´lez, B.; Sa´nchez-Iglesias, A.; Giersig, M.; LizMarza´n, L. M. Faraday Discuss. 2004, 125, 133.

colloidal stability is sufficiently enhanced so as to allow direct sol-gel processing and to obtain silica gels and glasses with well-defined optical properties. A drawback of such gels comes from their relatively poor mechanical properties, which make processing into real devices complicated, and thus, other substrates, such as organically modified silicates (ormosils), offer a much more attractive alternative for the preparation of optically active materials. Such hybrid systems are currently the material of choice for many protective transparent coatings, as well as for flexible optical components such as contact lenses, and thus, one can easily see that incorporation of optical functionalities can greatly expand the range of applications of these materials. Ureasils12,13 are a family of hybrid organic-inorganic materials with an internal structure defined by a silica network to which mixed oligopolyoxyethylene/oligopolyoxypropylene chains [POE, (OCH2CH2)n, POP, (OCHCH3CH2)n] are grafted through urea cross-links, as shown in Scheme 1. Thus, the silica matrix provides good mechanical resistance, while the mixed POE/POP chains provide a high degree of flexibility. We focused on the so-called ureasil U(600), for which the average repetition numbers are a + c ) 2.5 and b ) 8.5. Ureasils have advantages over other polymeric materials since they can be easily synthesized using the (low-temperature) solgel process, as well as over traditional silica gels and glasses, since they are more flexible and show a lower density, a higher refractive index, and a lower porosity. These organic-inorganic materials were initially devised for synthesis of highly luminescent polymers14-17 through the incorporation of doping ions, such as Eu3+, but they are also excellent candidates for the incorporation of (12) de Zea Bermu´dez, V.; Carlos, L. D.; Duarte, M. C.; Silva, M. M.; Silva, C. J. R.; Smith, R. J.; Assunc¸ a˜o, M.; Alca´cer, L. J. Alloys Compd. 1998, 21, 275. (13) de Zea Bermu´dez, V.; Carlos, L. D.; Alca´cer, L. Chem. Mater. 1999, 11, 569. (14) de Zea Bermu´dez, V.; Carlos, L. D.; Alca´cer, L. Chem. Mater. 1999, 11, 581. (15) Sa´ Ferreira, R. A.; Carlos, L. D.; de Zea Bermudez, V. Thin Solid Films 1999, 344, 476. (16) Carlos, L. D.; Messaddeq, Y.; Brito, H. F.; Sa´ Ferreira, R. A.; de Zea Bermudez, V.; Ribeiro, S. J. L. Adv. Mater. 2000, 12, 594. (17) Brankova, T.; Bekiari, V.; Lianos, P. Chem. Mater. 2003, 15, 1855.

10.1021/la048902r CCC: $27.50 © 2004 American Chemical Society Published on Web 10/02/2004

Ureasil Hybrids with Tailored Optical Properties

Langmuir, Vol. 20, No. 23, 2004 10269 Scheme 1

further functionalities if nanoparticles can be uniformly distributed within the hybrid matrix. In this paper, we show that metal nanoparticles coated with thin silica shells and dispersed in ethanol can be easily introduced within ureasils, yielding flexible nanocomposites that retain the well-defined optical properties (narrow absorption due to surface plasmons) of the precursor nanoparticle colloids. Since coating of nanoparticles with silica can be applied not only to metal nanoparticles, such as gold,18 silver,19,20 and their alloys,11 but also to semiconductor quantum dots21-24 or magnetic nanoparticles,25-27 this procedure shows an enormous potential for the synthesis of hybrids with a great variety of functionalities. The method permits full control of nanoparticle morphology (and properties) prior to gelation and, at the same time, ensures a homogeneous distribution within the final nanocomposite. We demonstrate here the uniform distribution of gold and silver nanospheres, as well as gold nanorods, through the conservation of their characteristic optical properties, which arise from welldefined plasmon absorbance bands in the visible and the near-infrared spectral regions.28 The choice of metal nanoparticles as dopants was motivated on one hand by the possible applications of the resulting hybrid nanocomposites and on the other hand by the possibility of monitoring any aggregation process spectroscopically, or even through the naked eye. It is well-known that noble metal nanoparticles display intense and well-defined absorption bands due to surface plasmon resonances29 and the specific position of the surface plasmon band is mainly sensitive to the chemical nature of the particles, their size and shape, the refractive index of the surrounding medium, and the separation from other metal particles.30 Therefore, the plasmon band can be used as a sensor with respect to the aggregation of the nanoparticles during the process. Additionally, by controlling the shape of gold nanocrystals, and specifically using nanorods of varying aspect ratio, the position of the plasmon band can be tuned so that the whole visible wavelength range can be covered. Experimental Section Tetrachloroauric acid (HAuCl4‚3H2O), 3-aminopropyl trimethoxysilane (APS), 3-isocyanatepropyltriethoxysilane (ICPTES), (18) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (19) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740. (20) Yin, Y.; Lu, Y.; Sun, Y.; Xia, Y. Nano Lett. 2002, 2, 427. (21) Correa-Duarte, M. A.; Giersig, M.; Liz-Marza´n, L. M. Chem. Phys. Lett. 1998, 286, 497. (22) Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem. B 2001, 105, 8861. (23) Rogach, A. L.; Nagesha, D. K.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676. (24) Wang, Y.; Tang, Z.; Liang, X.; Liz-Marza´n, L. M.; Kotov, N. A. Nano Lett. 2004, 4, 226. (25) Philipse, A. P.; van Bruggen, M. P.; Pathmamanoharan, C. Langmuir 1994, 10, 92. (26) Correa-Duarte, M. A.; Giersig, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 1998, 14, 6430. (27) Tartaj, P.; Serna, C. J. Am. Chem. Soc. 2003, 125, 15754. (28) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (29) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer-Verlag: Berlin, 1996. (30) Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B 2003, 107, 7312.

cetyltrimethylammonium bromide (CTAB), ascorbic acid, sodium borohydride (NaBH4), silver nitrate (AgNO3), and sodium silicate solution (Na2O(SiO2)3-5, 27 wt% SiO2) were purchased from Aldrich, while O,O′-bis(2-aminopropyl)- polypropylene glycolblock-poly(ethylene glycol)-block-polypropylene glycol-500, (Jeffamine ED-600), and 3-mercaptopropyl trimethoxysilane (MPS) were purchased from Fluka. Pure grade ethanol (Scharlab) and Milli-Q water with a resistivity higher than 18.2 MΩ cm were used in all the preparations. UV-visible spectra of liquid samples were measured with a HP 8453 diode array spectrophotometer in 1 cm path length quartz cuvettes, while those of solid composites were measured directly with a Cary 5000 UV-visible-NIR spectrophotometer using a special sample holder. Spherical Ag nanoparticles (30 nm diameter) were synthesized according to the seeding method described by Lu et al..31 for core-shell particles. Briefly, ∼10-nm Ag seeds were prepared by the fast reduction of a 100-mL solution containing 0.1 mM AgNO3 and 0.3 mM sodium citrate by 1 mL of 50 mM NaBH4. The seed particles were grown by adding 3 mL of the seed colloid to a growth solution containing 0.5 mL of 10 mM AgNO3, 1 mL of 100 mM ascorbic acid, and 20 mL of 50 mM CTAB, followed by dropwise addition of 0.1 mL of 1 M NaOH. The mixture was further stirred for 6 h. Spherical gold nanoparticles (15 nm) were prepared by boiling 5 × 10-4 M HAuCl4 in the presence of 1.6 × 10-3 M sodium citrate, for 15 min.32 Au nanorods were prepared through a modification of the seeding method published by Nikoobakht and El-Sayed.33 Briefly, a seed solution was prepared mixing 5 mL of an aqueous solution of HAuCl4 (0.250 mM) and CTAB (0.10 M) with 0.3 mL of icecold, freshly prepared 10 mM NaBH4 solution. For the growth of gold nanorods, different amounts of the seed solution (0.03 and 0.06 mL) were added to a reaction mixture containing 0.1 mL of 50 mM HAuCl4, 0.080 mL of 5 mM AgNO3, 0.075 mL of 100 mM ascorbic acid, and 10 mL of 0.1 M CTAB. The aspect ratio of the rods increased as the amount of seed solution added increased. UV-vis spectroscopy and TEM analysis of the Au nanorods revealed average aspect ratios of 2.13 (length, 42.3 ( 4.9 nm; width, 19.8 ( 2.9 nm) and 2.99 (length, 35.7 ( 4.7 nm; width, 12.0 ( 1.6 nm). Silica coating was carried out by the method previously reported.18 To the metal colloid (10 mL, [M] ) 2.5 × 10-4 M) were added, in turn, APS (0.050 mL, 1 mM) and sodium silicate solution (0.2 mL, 0.54 wt%, pH 10-11) under vigorous magnetic stirring. The resulting dispersion (pH ≈ 8.5) was then allowed to stand for 2 days, resulting in the deposition of 5-7-nm thick silica shells. In the case of Ag spheres and Au rods, MPS was used instead of APS and removal of excess CTAB by centrifugation was required prior to MPS addition. Metal nanoparticle-doped ureasils were prepared by mixing stoichiometric (1:2) amounts of vacuum-dried Jeffamine ED-600 (535 µL, 0.936 mmol) and ICPTES (464 µL, 1.87 mmol) in a closed glass vessel for 10 min under stirring at ca. 200 rpm so that the rapid uncatalyzed reaction between amino and isocyanate groups forming polyurea linkages takes place.13 Subsequently, 500 µL of 4.1 M NH4OH was added dropwise (as a catalyst for the sol-gel process) and the mixture stirred to homogeneity. Although initially the mixture was turbid, it cleared up within 20-30 min of stirring. Finally, 800 µL of the Au@SiO2 colloid in ethanol was added. The mixture was poured either in a polycarbonate cuvette (10 mm path length) or in a Teflon mold and covered with Parafilm. Under these conditions, the formation of the gel was completed within 5 h, and then the Parafilm was (31) Lu, L.; Wang, H.; Zhou, Y.; Xi, S.; Zhang, H.; Hu, J.; Zhao, B. Chem. Commun. 2002, 144. (32) Turkevich, J.; Garton, G.; Stevenson, P. C. J. Colloid Sci., Suppl. 1 1954, 26. (33) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15 1957.

10270

Langmuir, Vol. 20, No. 23, 2004

Boev et al.

perforated to allow for slow drying of the gels, either at room temperature or within an oven at 35°C for 48 h. Doping of ureasils with Ag@SiO2 nanoparticles using the protocol described above leads to dissolution of the silver cores due to oxidation into the soluble Ag(NH3)2+ complex, resulting in formation of hollow silica spheres and loss of color.19 To overcome this problem, citric acid (0.22 M) was added as a catalyst (instead of ammonia), promoting a change of pH in the ureasilicate sol from 8 to 3.5. Drying was carried out under the same conditions as for gold nanoparticles.

Results and Discussion A key step during the synthesis of the nanocomposite hybrids is the mixture of the precursor (ureapropyltriethoxysilane, UPTS, formed through reaction of Jeffamine and ICPTS) with the nanoparticle colloid in ethanol (see Experimental Section) since the chemical nature of the medium in which the nanoparticles are dispersed markedly varies and aggregation inevitably occurs if the nanoparticle surface properties are not properly engineered. We tested several metal colloids stabilized in ethanol using different stabilizers and found that only those coated with homogeneous silica shells remained stable (judging from the optical features) upon mixture with UPTS. Such a stabilizing effect of silica shells has been exploited in the past for numerous studies that would have been basically impossible using other aqueous metal colloids, such as chemical charging34 or growth of porous silica shells.35 A similar effect was found during “standard” sol-gel processing9 since the chemical nature of the coating was identical to that of the gel being formed. The structure of the ureasils is defined by the molecular sequence shown in Scheme 1,13 with organic spacers between siloxane units, which most likely requires that the silica-coated particles reside mainly in the proximity of the siloxane groups and are, thus, well separated from each other so that an additional degree of stabilization is gained with respect to pure silica. To obtain a uniform distribution of nanoparticles within the gel, the use of a catalyst is important so as to increase the gelation rate. As indicated in the Experimental Section, ammonia was used for samples with Au nanoparticles but citric acid was used instead for silver composites since these would get oxidized and completely dissolved by ammonia, as reported earlier.19 In experiments where no catalyst was used, slow aggregation and partial sedimentation of Au@SiO2 was observed. The versatility of the method is demonstrated here through the incorporation not only of nanoparticles of different metals (gold and silver) but also of nanoparticles with different shape (spheres and rods), which allows us to provide the nanocomposites with optical features in a wavelength range of over 400 nm. Transmission electron micrographs representative of the different samples studied are shown in Figure 1, and it can be observed that the particles are rather monodisperse, even in the case of nanorods, and that they are homogeneously coated with thin silica shells. The coating procedure was based on a method previously published18 and is briefly described in the Experimental Section. The actual appearance of the undoped and doped hybrids is shown in Figure 2, clearly demonstrating that, while the undoped ureasil is completely colorless and transparent (absorbance lower than 0.07 for λ > 425 nm and optical path length of 7 mm), samples doped with silver and gold (34) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B 1999, 103, 6770. (35) Nooney, N. I.; Dhanasekaran, T.; Chen, Y.; Josephs, R.; Ostafin, A. E. Adv. Mater. 2002, 14, 529.

Figure 1. Transmission electron micrographs of Ag and Au nanoparticles of different shapes used for doping of ureasils. (a) 30 nm Ag spheres, (b) 15 nm Au spheres, (c) Au nanorods with aspect ratio ) 2.13, and (d) Au nanorods with aspect ratio ) 2.99.

Figure 2. (a) Photographs of monoliths made of (from left to right) undoped ureasil and ureasil doped with Ag nanospheres, Au nanospheres, and Au short and long nanorods. (b, c) Photographs of ureasil disk-shaped films doped with a low concentration of Au@SiO2 nanospheres (b) and short nanorods (c). Transparency, color uniformity, and flexibility can be clearly seen.

spheres (30 and 15 nm diameter, respectively) and short gold rods (aspect ratio ) 2.13) display characteristic yellow, red, and blue colors due to intense absorption bands centered at 428, 526, and 677 nm, respectively (see Figure 3 below). However, for the hybrid nanocomposite containing longer Au nanorods (aspect ratio ) 2.99) the color is brownish, indicating that the main (longitudinal) plasmon absorption band is located in the NIR (λmax ) 761 nm). While the samples in Figure 2a were obtained through polymerization within a polycarbonate cuvette (1 cm path length), those shown in Figure 2b and c were prepared by using a Teflon container with a diameter of 45 mm as a mold, so as to obtain thin disks and demonstrate the flexibility of the nanocomposites. In all these samples, the ethanol used as solvent for the nanoparticle dispersions was allowed to evaporate completely at room temperature, which caused the formation of transparent gels. Further drying at 35 °C led to a shrinkage of approximately 30%.

Ureasil Hybrids with Tailored Optical Properties

Figure 3. Normalized UV-vis-NIR spectra of Ag (yellow) and Au nanospheres (red), as well as short (blue) and long (brown) Au nanorods coated with silica, dispersed in ethanol (solid lines), and within a solid ureasil monolith ca. 6.5 mm thick (dashed lines).

Once the samples have been dried and stored at room temperature, all properties studied remain unchanged for periods of several months. Two important observations should be stressed regarding the pictures shown in Figure 2. First of all, the colors are very uniform within each sample, and they are basically identical (to the naked eye) to those of the original colloids in ethanol, thereby confirming that the optical functionalities of the doped hybrids are fully determined by those of the nanoparticle dopants. This homogeneity of the nanoparticle distribution within the samples was further confirmed by systematically measuring spectra on various spots of the samples, using a mask with a 1-mm diameter pinhole. In all cases, the measured spectra were identical within experimental error (see Supporting Information). Additionally, the nanocomposites are fully transparent, which means that the contribution of light scattering is very low as compared to the absorption of the dispersed nanoparticles, even though the concentration is rather low in these examples (of the order of 1 mM). The optical properties were further studied through UV-vis-NIR spectroscopy, as shown in Figure 3 for the four samples described. It is obvious that the spectra of the solid monoliths perfectly resemble those of the starting alcosols, though slight variations of the peak positions are observed, as predicted by Mie theory.28 The peaks are consistently red-shifted in the ureasils, which is due to a refractive index increase when changing the dispersion medium from ethanol (n ) 1.359) to the ureasil (n ) 1.508).36 In the case of nanospheres, the shift is of just 5-7 nm, while for nanorods, the transverse plasmon band shows a similar shift but the longitudinal one shifts as much as 20 and 25 nm for the nanorods with aspect ratios of 2.13 and 2.99, respectively. This longer shift for elongated particles is in accordance with predictions using Mie theory and was observed when rods were dispersed in solvents with varying refractive index.28 These relatively small spectral variations confirm the nonaggregated nature of the nanoparticles within the nanocomposite material. It is also remarkable that the shapes of the spectra are basically identical, which demonstrates the optical quality of these materials, meaning that the scattering contribution from the substrate is negligible. (36) The refractive index of the ureasil U(600) was measured with an ellipsometer using a laser beam at a wavelength of 638 nm. Since the thickness of the sample was larger than 5 mm, bulk behavior was assumed.

Langmuir, Vol. 20, No. 23, 2004 10271

Figure 4. UV-visible spectra of solid ureasil monoliths containing Au nanospheres with various concentrations, as indicated. Optical path length is 3 mm.

The concentration of the nanoparticles within the hybrid material can be easily adjusted simply by centrifugation of the nanoparticle colloid and redispersion in the appropriate amount of ethanol prior to the mixture with UPTS. In this way, the optical density can be easily tuned for the different applications. As an example, UV-visible spectra of nanocomposites containing Au@SiO2 nanospheres with concentrations ranging from 0.01 up to 0.09 wt% are shown in Figure 4. The only effect of the increased concentration is a higher absorbance at all wavelengths, which clearly indicates that in all cases the nanoparticles are homogeneously distributed within the gel. Photographs of the corresponding monoliths are available as Supporting Information. Regarding the stability of the composite, we have tested the dissolution of either the ureasil matrix or the embedded nanoparticles in different solvents (water, ethanol, N,Ndimethylformamide, toluene, heptane, and cyclohexane) and we found that there is no chemical degradation with any of them. Soaking in polar solvents (water, ethanol, DMF, and toluene) leads to reversible swelling, while with nonpolar solvents (heptane and cyclohexane) swelling does not occur, which is directly related to the miscibility of the Jeffamine precursor with the various solvents. To avoid fracture during drying, this should be carried out slowly. Chemical degradation was only observed when the nanocomposite was immersed in concentrated (1 M) sulfuric acid or in aqua regia for periods of several hours. While sulfuric acid attacks the organic polyoxyethylene chains, the silica skeleton seems to remain undamaged as the sample keeps its shape after drying unchangeable. Treatment with aqua regia leads to break-up of the ureasilicate xerogel and dissolution of the metal nanoparticles. Thermal stability of ureasils has been reported up to 250 °C.13 Although we have also observed a similar stability, there is a certain sensitivity to temperature, both in undoped and doped ureasils, which is mainly reflected in a strong increase of absorbance below 500 nm when the nanocomposites are treated at temperatures above 100 °C for periods of several hours. This effect, which is visually observed as a gradual yellow coloration, is exemplified in Figure 5, where spectra for an undoped sample, as well as one doped with Au@SiO2 spheres, are shown after heating in a standard oven for 2 h. This effect, which we have systematically observed but has not been previously reported, is most probably related to oxidation of the oxyethylene groups since a yellow coloration upon heating can also be observed in pure Jeffamine. Such thermal changes should be taken into account when devising applications for these systems.

10272

Langmuir, Vol. 20, No. 23, 2004

Boev et al.

(low turbidity), and high flexibility of the hybrid nanocomposites can be very useful in practical applications, such as colored coatings, selective filters, or colored contact glasses, though some color changes have been observed upon heating above 100 °C. Additionally, the procedure presented here can be readily extended to the incorporation of other metals, as well as other types of nanoparticles, which will be reported elsewhere.

Figure 5. UV-visible spectra of undoped ureasil (dashed lines) and ureasil doped with Au nanospheres (solid lines) after thermal treatment for 2 h at the temperatures indicated.

Conclusions We have demonstrated that silica-coated metal nanoparticles of various shapes can be mixed with the precursors of ureasilicates without loss of their individuality, thus yielding nanocomposites with a homogeneous distribution of nonaggregated particles, through a basecatalyzed sol-gel transition. The optical properties of the composites closely resemble those of the corresponding starting sols, which allows for tuning of their color by choosing nanoparticles with the appropriate size and shape. The well-defined and uniform colors, optical quality

Acknowledgment. The authors are indebted to Ana Sa´nchez-Iglesias for synthesizing silica-coated gold nanospheres. Stefano Chiussi and Pı´o Gonza´lez are thanked for measuring the refractive index of ureasil U(600). This work was partly supported by the Spanish Ministerio de Ciencia y Tecnologı´a and FEDER (Project No. MAT2004-02991), the Direccio´n Xeral de Investigacio´n e Desenvolvemento, Xunta de Galicia, (Project No. PGIDIT03TMT30101PR), and Fundac¸ a˜o para a Cieˆncia e Tecnologia (Project No. POCTI/FIS/10128/01), Portugal. V.I.B. thanks FCT for research grant SFRH/BD/3188/ 2000. Supporting Information Available: UV-visible-NIR spectra of ureasil nanocomposites containing Ag nanospheres, as well as Au nanospheres and nanorods, measured at multiple spots and photographs of ureasil monoliths doped with various concentrations of Au nanospheres. This material is available free of charge via the Internet at http://pubs.acs.org. LA048902R