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Cocondensation of Organosilica Hybrid Shells on Nanoparticle Templates: A Direct Synthetic Route to Functionalized Core-Shell Colloids Simon R. Hall, Sean A. Davis, and Stephen Mann* School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. Received July 12, 1999. In Final Form: October 4, 1999
Introduction Developments in nanotechnology demand building blocks with increasing structural and compositional complexity, which can be reproducibly self-assembled into functional materials. In this regard, nanoparticles with core-shell morphologies represent a new type of constructional unit consisting of two dissimilar compositional and structural domains. Such materials should have enhanced physical and chemical properties and a broader range of applications than their single-component counterparts. The synthesis of gold-silica core-shell nanoparticles has recently been reported.1-3 Colloidal gold, prepared by the citrate reduction method,4 can be rendered vitreophillic by replacement of surface-adsorbed citrate by amino-coupling of (3-aminopropyl)triethoxysilane to the metal surface. A thin silica coating or “subshell” is then deposited onto the primed surface so that the colloid can be transferred into ethanol for further shell growth via hydrolysis and condensation of tetraethoxysilane (TEOS) using the Stober process.5 Here we extend the compositional complexity of core-shell colloids by the single-step synthesis of a 7 nm thick organosilica corona around vitreophillic gold nanoparticles. We use cocondensation reactions of organotrialkoxysilanes and TEOS at various molar ratios to covalently link organo-functionalities, such as phenyl, allyl, mercapto, amino, cyano, perfluoro, or dinitrophenylamino moieties, both within and on the surface of the shell structure. Our method involves the direct synthesis of organosilica shells and is therefore significantly different from previous studies, which have focused on the postsynthetic silanization of silica colloids to prepare surface-functionalized materials. This approach not only reduces the number of processing steps but could provide a general route to shells consisting of hybrid inorganic-organic structures with bulk and surface properties (polarity, reactivity, porosity, etc.) that are compositionally controlled. Experimental Section HAuCl4 (Aldrich), trisodium citrate dihydrate (Aldrich), NH4OH (Sigma), and sodium silicate solution (Aldrich) were used as received. Tetraethoxysilane (TEOS), (3-aminopropyl)triethoxysilane, (3-aminopropyl)trimethoxysilane (APTMS), phenyltriethoxysilane, allyltrimethoxysilane, (3-mercaptopropyl)triethoxysilane, (3-cyanopropyl)triethoxysilane, and 3-(2,4-dinitro* Author for correspondence. Email:
[email protected]. (1) Liz-Marzan, L. M.; Philipse, A. P. J. Colloid Interface Sci. 1995, 176, 459. (2) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. J. Chem. Soc., Chem. Commun. 1996, 731. (3) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (4) Enustun, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85 (12), 3317. (5) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
phenylamino)propyltriethoxysilane were all purchased from Apollo Sci. Perfluorodecyltriethoxysilane was purchased from Fluorochem. Technical grade ethanol and double-distilled deionized water were used in all preparations. Silicic acid solution (“active silica”) was prepared by running a 27 wt % sodium silicate solution through a cationic exchange column (Duolite C225 Na, 14-52 mesh, from BDH Chemicals) which had been recharged by flushing with hot distilled H2O, followed by 2 M HCl(aq) and then cold distilled H2O. The resultant solution had a pH of between 4 and 5 and was immediately raised to a pH of 10 by addition of small amounts of the stock silicate solution. Colloidal gold was prepared via the citrate reduction method.4 To a flask containing 94 mL of boiling distilled deionized water, HAuCl4 solution (1 mL, 5 × 10-4 M ) was added, and the solution allowed to come to the boil. A freshly prepared solution of sodium citrate (5 mL, 3 × 10-2 M) was then added to the flask. After a few minutes, the solution turned from colorless to gray/blue and eventually to a deep wine-red color due to the formation of colloidal gold. The sol was left overnight to cool. Silica-coated gold colloids were prepared as follows. First, a surface primer layer with a pendent trimethoxysilane functionality was attached to the Au surface by adding 0.25 mL of a freshly prepared 1 mM aqueous solution of APTMS to 50 mL of the colloidal gold sol under vigorous stirring. The sol was left to stand for 15 min to achieve hydrolysis of the surface methoxysilane groups and formation of vitreophilic Au nanoparticles. Higher concentrations of APTMS resulted in darkening of the sol, followed by flocculation typically over a period of a few hours. Second, 2 mL of a silicic acid solution was added to the functionalized sol under vigorous stirring, and the mixture was allowed to stand for at least 36 h to produce an approximately 2 nm thick coating (“subshell”) of silica around the gold nanoparticles. The sol was then dialyzed against distilled deionized water for 24 h, to remove any excess silicate. A shell of organosilica was synthesized around the primed nanoparticles by cocondensation using the Stober process. Typically, 50 mL of the silica-coated colloidal gold was added to a mixture of 200 mL of ethanol and 1 mL of NH4OH (25%). A 0.15 mL portion of a freshly prepared organotrialkoxysilane/TEOS mixture, containing from 5 to 100 mol % organosilane, was added dropwise under vigorous stirring, and the sol left under mild magnetic stirring for at least 12 h at room temperature. The sols remained red in color, with no visible evidence for flocculation. The sol was then dialyzed for a second time against distilled deionized water for 24 h, to ensure no soluble silicate species remained in situ. Samples were characterizated by UV-vis, FTIR, and 1H NMR spectroscopies, transmission electron microscopy (TEM), and energy-dispersive X-ray analysis (EDXA). TEM samples were prepared by placing a drop of the sol onto a copper-coated, Formvar-covered 300 mesh carrier grid for 20 s, before being wicked away and air-dried.
Results and Discussion By judicious modification of the experimental procedure, we were able to prepare a range of organosilica coreshell metallic colloids by direct cocondensation of TEOS and organotrialkoxysilane precursors onto the surface of vitreophilic gold nanoparticles. A variety of different organic moieties could be covalently linked into and upon the silica shell at moderate TEOS:organosiloxane molar ratios to produce sols that were often stable for several months (Table 1). In each case, absorption spectra showed a slight increase in intensity of the Au plasmon absorption band at around 525 nm. This is consistent with effects on absorption previously observed for silica-coated gold particles and is due to an increase in the local refractive index around the core-shell particles.3 In some samples, a shoulder was observed in the spectra at 658 nm due to minor amounts of flocculation in the sol. TEM images showed a relatively monodisperse sol of discrete silica-coated gold particles (Figure 1). Each
10.1021/la9909143 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999
Notes
Langmuir, Vol. 16, No. 3, 2000 1455 Table 1. Stability and Shell Thickness for Gold-Organosilica Core-Shell Colloidsa
functionality phenyl allyl mercapto amino cyano DNPA perfluoro b
5 mol % *** ** ** *** ** ***
10 mol % *** * ** ** *** *
20-60 mol %
shell thicknessb (nm)
**
6.29 6.35 7.57 8.09 6.46 7.81 8.38
a Stable to flocculation for (/) 1 day, (**) 5 days, (***) 2 months. Mean thickness from TEM measurements.
Figure 1. TEM images of Au/organosilica core-shell colloids: (a) close packed array of gold nanoparticles with aminofunctionalized silica shells, and (b) similar colloid but with phenyl-functionalized silica shells.
particle consisted of a single, electron-dense Au core with uniform size (mean ) 17.7 ( 1.7 nm) surrounded by a homogeneous silica shell, 7.3 ( 1 nm in thickness. Particle size measurements showed a narrow size distribution regardless of the organosiloxane incorporated, presumably due to the high degree of control achieved in the citrate-reduction Au synthesis and base-catalyzed Stober process. The thickness of the organo-functionalized silica shell, appeared to be largely independent of the organotrialkoxysilane used in association with TEOS (Table 1). The homogeneity of the samples was increased by extensive dialysis of the sols to remove unreacted silicic acid and organosilanes before and after the Stober-induced cocondensation reaction, respectively, and by optimizing the time of the initial reaction of the silanol-primed Au nanoparticles with silicic acid. In general, 36 h were required for complete surface coverage of the functionalized nanoparticles with an approximately 2 nm thick
silica subshell. These colloids, unlike primed particles with no silica subshell or colloids reacted for less than 36 h, did not flocculate upon transfer into the ethanol/water/ NH4OH Stober mixture, presumably because van der Waals attractive forces between the silica-coated gold cores are minimized. Longer reaction times than 36 h produced irregular subshells and hence ill-defined core-shell particles after cocondensation. Incorporation of the covalently linked organic groups into the silica shell was confirmed by various methods. 1H NMR spectra of each of the colloidal dispersions showed no observable signals for the organic groups, although these resonances, in particular an intense phenyl 1H signal at 7.56 ppm in phenyltriethoxysilane (PTES) and dinitrophenylamino functionalities, were observed for solutions of the organosilanes prepared at reactant concentrations and lower in the absence of Au nanoparticles. The data suggested that effectively all the organosilane molecules are condensed onto the particles and incorporated as organosiloxane linkages in the silica shell. The presence of phenyl groups in the silica corona of core-shell colloids prepared from mixtures of TEOS and 20, 40, or 60 mol % PTES was confirmed by FTIR spectroscopy, which showed aromatic CdC vibrations at ca. 1450, 1500, 1580, and 1600 cm-1 that were also present in pure PTES. Significantly, the absorption bands increased in intensity for colloids functionalized with increasing molar amounts of PTES. The vibrational spectra also showed a weak Si-OH band at 955 cm-1 as well as Si-O-Si vibrations at around 1114 and 1003 cm-1 that were consistent with a shell of partially and fully condensed siloxane centers. These bands were present in all of the organo-functionalized core-shell colloids. For colloids with thiol-containing shells, FTIR spectra showed weak R-SH vibrations at 2590-2550 cm-1; this was consistent with EDXA data on single nanoparticles imaged in the TEM which showed characteristic peaks for Au, Si, and S.6 A covalently linked organic chromophore was detected in the core-shell particles by UV-vis spectroscopy. In addition to the Au absorption band, high-intensity bands at 354 and 421 nm were observed for colloids functionalized with shells containing dinitrophenylamino moieties (Figure 2). These bands are due to the ortho- and parachromophoric NO2 groups, respectively, of the phenyl ring and indicate that that there was no disintegration or loss of functionality of the organic group upon cocondensation and integration into the shell structure. FTIR data for this sample showed aromatic vibrational bands between 1450 and 1600 cm-1. There appears to be a limit to the amount of organic functionality that could be incorporated via cocondensation into the silica shell. Shells produced from a mixture of TEOS and 5 or 10 mol % organotrialkoxysilane were generally stable with respect to flocculation in the final sol. Phenyl-functionalized silica shells, however, could be prepared as stable sols from starting compositions containing up to 60 mol % of organosiloxane. Higher levels produced a characteristic blue-gray coloration, indicative of colloidal flocculation. One possible explanation is that under these conditions the silica shell is structurally unstable because it contains relatively few fully condensed Q4 siloxane linkages due to the large numbers of Tn organosiloxane linkages [Tn ) RSi(OSi)m(OH)3-m, m ) (6) EDXA data for mercapto-functionalized gold-silica core shell nanoparticles: Au LR1 (9.713 keV), Au Lβ1 (11.443 keV), Au Lβ2 (11.585 keV), Si KR1 (1.74 keV), Si KR2 (1.739 keV), Si Kβ1 (1.836 keV), S KR1 (2.307 keV), S KR2 (2.308 keV), and S Kβ1 (2.464 keV). Cu KR1 (8.048 keV), Cu KR2 (8.028 keV), Cu Kβ1 (8.905 keV) from the sample grid.
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Notes
Figure 2. UV-vis spectrum for core-shell nanoparticles prepared with a silica corona containing covalently linked dinitrophenylamino chromophoric residues. The dye moieties exhibit characteristic absorption bands at 421 and 354 nm. Inset shows the Au plasmon absorption at 525 nm.
1-3] that originate from the organosilane precursor. A similar effect has been reported for the use of cocondensation in the template-directed synthesis of organofunctionalized silicas with long-range hexagonal mesostructure.7 Corresponding TEM studies of core-shell colloids prepared in the presence of 80 mol % PTES showed that the Au nanoparticles were only partially or not covered at all by silica, consistent with this interpretation. Conclusions The synthesis of monodisperse gold-organosilica coreshell particles has been achieved by base-catalyzed cocondensation of tetraethoxysilane and organotriethoxysilane precursors in the presence of surface-primed Au sol. The metallic nanoparticles are effective templates provided that they are sufficiently vitreophilic, which was achieved by priming the Au nanoparticles with a monolayer of an amino-functionalized trimethoxysilane, followed by coupling of silicic acid to the pendent silanol groups to produce a nanometer thick subshell of silica. Subsequent overgrowth of the inorganic-organic phase by surface-directed cocondensation was successful in producing organosilica shells with covalently linked phenyl, allyl, mercapto, amino, cyano, perfluoro, or dinitrophenylamino moieties, as shown by a range of techniques, including 1H NMR, FTIR, and UV-vis spectroscopies and EDX analysis. High levels of organic moieties could be achieved with inorganic-organic hybrids containing phenyl groups. Vrij et al.8 and Mulvaney et al.,9 have shown that silica deposited by the Stober process is microporous, suggesting that the organic moieties covalently linked into the bulk
phase of the silica shell should be accessible to external reagents for use in catalysis10,11 and adsorption processes.12 Moreover, the incorporation of organic chromophores should have potential use in optical applications.13 The hydrophobic nature of many of the incorporated organic functionalities should also increase the stability of the colloids, as demonstrated by the phenyl-functionalized core-shell particles. In general, these hybrid organicinorganic materials might have uses as nano reaction vessels with or without the gold-core template, which can be removed by dissolution to leave a hollow intact silica shell.9 The synthetic procedure could also be extended to other nanoparticle templates, for example, semiconducting or magnetic cores could be coated with tailored functionalized silica shells. Work in this area is ongoing. Acknowledgment. We thank the EPSRC and the Leverhulme Trust for financial support. LA9909143 (7) Hall, S. R.; Fowler, C. E.; Lebeau, B.; Mann, S. J. Chem. Soc., Chem. Commun., 1999, 201. (8) Van Blaaderen, A.; Vrij, A. J. Colloid. Interface Sci. 1993, 156, 1. (9) Giersig, M.; Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Adv. Mater. 1997, 9, 570. (10) Lim, M. H.; Blanford, C. F.; Stein, A. Chem. Mater. 1998, 10, 467. (11) Clark, J. H.; MacQuarrie, D. J. J. Chem. Soc., Chem. Commun. 1997, 853. (12) Bambrough, C. M.; Slade, R. C. T.; Williams, R. T.; Burkett, S. L.; Sims, S. D.; Mann, S. J. Colloid Interface Sci. 1998, 201, 220. (13) Fowler, C. E.; Lebeau, B.; Mann. S. J. Chem. Soc., Chem. Commun. 1998, 825.
