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Silica gels with tailored, gold nanorod-driven optical functionalities
Applied Surface Science 226 (2004) 137–143
Silica gels with tailored, gold nanorod-driven optical functionalities Jorge Pe´rez-Juste, Miguel A. Correa-Duarte, Luis M. Liz-Marza´n* Departamento de Quı´mica Fı´sica, Universidade de Vigo, 36200 Vigo, Spain
Abstract The incorporation of metal nanoparticles with various size and shapes provides silica gels with optical functionalities arising from the characteristic optical properties of the starting nanoparticle dispersions. The uniform distribution of the preformed nanoparticles and their mutual separation within the gels are key factors to maintain single-particle properties. These requirements are achieved by coating the nanoparticles with thin silica shells, which greatly enhance their stability and ensure separation between metal nanoparticle cores. The great advantage of this procedure is its ability to use nanoparticles with tailored shapes and thus to expand the possible functionalities to be implemented. We show the incorporation of gold nanorods with varying aspect ratio, which allows to have well-defined absorption bands even in the near infrared. # 2003 Elsevier B.V. All rights reserved. PACS: 81.05.Y; 78.66.J Keywords: Nano-materials; Silica gels; Gold nanorods; Optical properties
1. Introduction Silica gels and glasses with optical functionalities are important for a number of technological applications, such as decorative coatings [1], catalysis [2], optical filters [3], non-linear optical materials [4], etc. and therefore such materials have been made for centuries. In fact, metal nanoparticles provide the famous Lycurgus cup with its very special optical behaviour [5]. However, most of the techniques used to dope glasses with metal nanoparticles are based on the reduction of metal salts within the glass precursor [6–8], and rely on a homogeneous distribution of the salt and a homogeneous reduction to achieve particles with a minimum size uniformity. Metal salt concen* Corresponding author. Fax: þ34-986-812556. E-mail address: [email protected] (L.M. Liz-Marza´n).
tration and temperature are normally the main parameters used to control the average particle size, and therefore this becomes hard to achieve. Alternatively, the sol–gel process can be carried out in the presence of preformed nanoparticles with the composition, morphology and properties suitable for the desired application [9,10]. In this way, not only spherical, but also nanoparticles with complex structures or specific shapes, such as alloys, core-shell particles or nanorods can be incorporated within the gel. However, it is mandatory in such cases that the colloidal stability of the nanoparticles is sufficient to keep them separated (non-aggregated) during the sol– gel transition, so that single-particle functionalities are preserved. We have recently devised a procedure that overcomes these difficulties by means of surface modification of the guest nanoparticles via deposition of a thin
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.11.013
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J. Pe´ rez-Juste et al. / Applied Surface Science 226 (2004) 137–143
silica shell, which preserves colloidal stability during the whole sol–gel transition [11]. Although initially the method was only demonstrated for spherical gold nanoparticles, its application to other compositions, such as CdS [12] or AuAg alloys [13] was also achieved. In this paper we demonstrate for the first time that the same procedure can be also applied to the sol–gel processing of anisometric nanoparticles, in this case gold nanorods with various aspect ratios, which expands the optical functionalities (well-defined surface plasmon bands) within the gels toward the near IR.
2. Experimental 2.1. Materials and methods Tetrachloroauric acid (HAuCl43H2O), cetyltrimethyl ammonium bromide (CTAB), ACS grade methanol, sodium silicate solution (Na2O(SiO2)3–5, 27 wt.% SiO2), ascorbic acid, sodium borohydride and silver nitrate were purchased from Aldrich. 3Mercaptopropyl trimethoxysilane (MPS) and tetramethoxysilane (TMOS) were purchased from Fluka. All reactants were used as received. Milli-Q water with a resistivity higher than 18.2 MO cm was used in all the preparations. UV-Vis spectra were measured with a HP 8453 diode array spectrophotometer in 1 cm path length cuvettes. Transmission electron microscopy (TEM) was performed on a JEOL JEM 1010 microscope operating at 100 kV. Samples for TEM were prepared by depositing a drop of either the nanorod colloid or the suspension of a ground piece of gel in ethanol, on a carbon-coated TEM copper grid.
2.2. Synthesis of gold nanorods The synthesis of Au nanorods was performed through a modification of the procedure published by Nikoobakht and El-Sayed [14]. Details of the synthesis follow. A seed solution was prepared as follows: in a 20 mL vial, 5 mL of an aqueous solution of HAuCl4 (2:50 104 M) and CTAB (0.10 M) was prepared at room temperature; next, 0.3 mL of ice-cold, freshly prepared 0.01 M NaBH4 solution was added under vigorous stirring. The stirring was slowed down after 30 s and the Au sol was gently stirred for 15 min at 40–45 8C to ensure removal of excess NaBH4. For the growth of gold nanorods with various aspect ratios, different solutions were prepared containing 10 mL of growth solution (½HAuCl4 ¼ 5:00 104 M, ½CTAB ¼ 0:1 M, ½AgNO3 ¼ 4:00 105 M) at 25–30 8C. Next, 0.070 mL of 0.1 M ascorbic acid was added to each sample and mixed thoroughly. Finally, different volumes of seed solution were added: 0.006, 0.012, 0.024, 0.050 and 0.15 mL. The nanorods obtained had aspect ratios of 1.94, 2.35, 2.48, 3.08, and 3.21, respectively (see Fig. 1 and Table 1). 2.3. Silica coating of Au nanorods Silica coated Au nanorods (Au @ SiO2) were prepared as follows: after the synthesis of the nanorods, the excess CTAB was eliminated by centrifugation, removal of the supernatant and redispersion in pure water. To the clean nanorod dispersion (10 mL, ½Au ¼ 2:5 104 M), a freshly prepared aqueous solution of MPS (0.050 mL, 1 mM in ethanol) was added under vigorous magnetic stirring. The mixture of MPS and gold dispersion was allowed to stand for 15 min and then 0.2 mL of a 0.54 wt.% sodium silicate
Fig. 1. Representative TEM micrographs of CTAB stabilized Au nanorods with average aspect ratios of 1.94 (a), 2.35 (b), 2.48 (c), 3.08 (d), and 3.21 (e).
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Table 1 Dimensions, aspect ratio, and positions of the maxima of longitudinal plasmon resonances for the various nanorod samples at the different stages of the sol–gel process Aspect ratio
1.94 2.35 2.56 3.08 3.21
0.20 0.33 0.30 0.31 0.32
Length (nm)
50 44 39.3 31.6 25.8
5 2.4 2.5 2.7 3.7
Width (nm)
25.7 18.8 16.7 10 7.9
2.4 2.1 1.5 0.7 0.7
solution at pH 10–11 (adjusted with a cation exchange resin) was added, again under vigorous magnetic stirring. The resulting dispersion (pH 8:5) was then allowed to stand for 2 days, so that the active silica polymerised onto the primed gold particle surface. The thickness of the deposited silica shell was about 5–7 nm thick. 2.4. Sol–gel transition The method used to prepare silica gels loaded with Au nanorods was identical to that previously reported [11], which was based on the sol–gel literature [15]. In a typical preparation procedure, first 0.32 mL of TMOS was added to 1.33 mL of methanol. Next, 2 mL of the nanoparticle dispersion was added to the mixture of TMOS and methanol, then the final mixture was poured in a cuvette which was shielded with a rubber sheet to avoid unwanted evaporation. Upon several hours a rigid gel was obtained. The final molar ratio of TMOS, water and methanol was 1:50:15, assuming that 2 mL of colloid contains 2 mL of water.
3. Results and discussion 3.1. Silica coating Although most of the synthetic methods used for the preparation of metal nanorods suffered from limitations, either in the amount of material [16,17] or in the yield of nanorods, as compared to nanospheres [18,19], Nikoobakht and El-Sayed recently reported a variation of the so-called seed-mediated method [14] which affords the synthesis of relatively large amounts
lmax CTAB
SiO2 shell
SiO2 gel
632.5 653.7 679.2 718.3 737.9
637.2 659.6 693.3 725.4 754.6
641.2 665.4 695.8 737.9 751.7
of nanorods with very little contamination by nanospheres, and variable aspect ratio. The modification of the aspect ratio was accomplished in the original publication through a variation in the concentration of AgNO3, although the specific role of this salt was not disclosed. Since it is not clear whether or in which way the Ag atoms are incorporated within the rods, we varied the amount of seed solution while maintaining a constant AgNO3 concentration, which allowed us to prepare five different samples with aspect ratios ranging from 1.94 up to 3.21. Representative TEM images of the various nanorods are shown in Fig. 1, where it can be easily observed that variations in the aspect ratio are not due to longer, but rather to thinner (and even shorter) rods, which actually facilitates the further processing into the gels. Details on the average length and width in the different samples are provided in Table 1. For the deposition of silica shells on the nanorods, we employed a modification of the method previously devised for the coating of Au nanospheres [20]. The modification of the method mainly consists of the use of a mercaptosilane, rather than an aminosilane, due to the stronger affinity for Au complexation, so that it is able to displace adsorbed CTAB. Additionally, careful cleaning of the nanorod dispersion to remove as much dissolved CTAB as possible is absolutely essential, since otherwise there is nucleation of free silica in solution, which is still observed in some extent, as shown in Fig. 2a. However, homogeneous coating of all rods can be invariably observed for all samples. 3.2. Sol–gel processing The stability of the nanorods during the intermediate steps of the sol–gel process is of paramount importance to maintain their individuality within
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Fig. 2. Representative TEM micrographs of silica-coated Au nanorods with average aspect ratios of 1.94 (a) and 3.08 (b).
the final gels. As was previously observed for Au [11] and CdS [12] nanospheres, when the silica coating step was skipped, we invariably observed coagulation before the gel was formed. Coagulation could be observed with the naked eye through a color change, but it was even clearer when the process was followed spectroscopically (see Fig. 3). It can be observed that the longitudinal plasmon band blue-shifts and drops in intensity, while the transverse plasmon band red-shifts and gets slightly enhanced, long before gel formation. This means that the coagulation of the rods leads to aggregates with smaller aspect ratios and larger sizes than the original rod diameters, which eventually get large enough to sediment, as shown in the inset of Fig. 3.
A similar experiment performed on an Au @ SiO2 colloid formed from the same nanorods produces no visual changes at all, and only a small change in the spectrum, basically due to dilution. This results in a gel with uniform colour arising from the separate nanorods, homogeneously distributed in its interior. Examples of the morphology of the gels are shown in Fig. 4, where it is clear that the nanorods are well separated within the porous structure of the silica gel. This separation allows to basically maintain the optical properties of individual nanorods within the final gel, as discussed in the following section. Additionally, it should be noted that no basic or acid catalyst is added in the procedure outlined above, so that the conditions for gel formation are very mild,
Fig. 3. Spectral evolution of a CTAB-stabilized gold nanorod colloid upon mixing with TMOS in methanol. Total time: 36 min. The inset shows photographs of gels formed using silica-coated (left) and CTAB stabilized (right) Au nanorods.
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Fig. 4. Representative TEM micrographs of silica gels containing Au at SiO2 nanorods with average aspect ratios of 1.94 (a) and 3.08 (b).
thus favouring uniform structures. It was observed though, that addition of a small amount of sodium citrate to the mixtures notably accelerated the formation of the gels. 3.3. Optical properties The optical properties of metal nanorods have been discussed in detail [14,15,21,22]. Although they are based on the collective oscillation of conduction electrons (surface plasmons), the main difference with metal nanospheres is that two oscillation modes are possible, one parallel (longitudinal mode) and another perpendicular (transverse mode) to the long axis of the rod. This implies that two absorption bands are present in the UV-Vis spectrum, with the transverse band located at wavelengths close to the characteristic band of nanospheres of similar diameter, and the longitudinal band at higher wavelengths, which can even reach the near IR for long rods. The actual position of the longitudinal resonance is not directly determined by the length of the nanorods, but rather by their aspect ratio, which is also confirmed through calculations using Mie theory [21,23]. Additionally, the intensity of the longitudinal band is much higher than that of the transverse band, and thus this is the main functionality provided to the gels through doping with nanorods. The maximum position of the longitudinal resonances for the CTAB-Au nanorods used in this work ranged from 630 up to 740 nm (see Table 1 for details). In Fig. 5 we compare the spectra of the five different samples at the three main stages of the sol–gel process.
Fig. 5. UV-Vis spectra of the different nanorod samples stabilized with CTAB (top), coated with thin silica shells (middle), and after sol–gel processing (bottom). The line labels are the same for all graphs.
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The first stage corresponds to the nanorods stabilized with CTAB in water, prior to the addition of MPS. The second stage corresponds to the rods coated with thin silica shells (see Fig. 2), dispersed in water. Finally, the third stage corresponds to the rigid hydrogels, when the sol–gel transition has been completed. The spectra were normalized to a maximum absorbance of 1 to simplify the comparison. The spectra of the gels are more noisy because the nanorod concentration was noticeably lower and light scattering higher. The positions of all maxima are listed in Table 1. One first observation from Fig. 5 is that there is an excellent correlation between the positions of the maxima in the three different situations, with a consistent red shift upon silica coating, and a further shift upon gel formation. The first red shift is due to an increase of the local refractive index around each Au nanorod [20], while the second red shift is due to an increase of the average refractive index of the surrounding medium after formation of the gel [11]. It is also remarkable that the corresponding shifts are much larger in the longitudinal band than those of the single plasmon band of spherical nanoparticles, showing that this system is more sensitive to refractive index changes, which has also been observed for silver nanoprisms [24,25]. There is also a relative increase of intensity at higher energies, which is an indication of an increase in light scattering [20] arising from the formation of mesopores within the gel structure. This was confirmed by TEM (Fig. 4) and nitrogen adsorption as reported elsewhere [11,12].
4. Conclusions In this paper, we have demonstrated that silicacoated gold nanorods with various aspect ratios can be easily incorporated within transparent silica gels. Both UV-Vis absorption spectra and transmission electron microscopy show that there is no aggregation of the metal nanoparticles during the sol–gel transition, so that the optical properties of the starting colloid are basically retained in the gel, except for a shift of the plasmon band due to refractive index changes. This represents a generalization of this sol–gel technique for the homogeneous incorporation
of anisometric nanoparticles, thus providing the rigid gel with tailored optical functionalities.
Acknowledgements The authors are indebted to Benito Rodrı´guezGonza´ lez from CACTI (Universidade de Vigo) for assistance with TEM measurements. This work has been supported by the Spanish Ministerio de Ciencia y Tecnologı´a (Project no. BQU2001-3799) and Xunta de Galicia (Project no. PGIDIT).
References [1] W.A. Weyl, Coloured Glasses, Society of Glass Technology, Sheffield, 1951. [2] S. Braun, S. Rappoport, R. Zusmen, D. Avnir, M. Ottolenghi, Mater. Lett. 10 (1990) 1. [3] M.R. Bo¨ hmer, A.R. Balkenende, T.N.M. Bernards, M.P.J. Peeters, M.J. van Bommel, E.P. Boonekamp, M.A. Verheijen, L.H.M. Krings, Z.A.E.P. Vroon, in: H.S. Nalwa (Ed.), Handbook of Advanced Electronic and Photonic Devices, Academic Press, San Diego, 2001. [4] S. Tamil-Selvan, T. Hayakawa, M. Nogami, Y. Kobayashi, L.M. Liz-Marza´ n, Y. Hamanaka, A. Nakamura, J. Phys. Chem. B 106 (2002) 10157. [5] P. Mulvaney, MRS Bull. 26 (2001) 1009. [6] H. Kozuka, S. Sakka, Chem. Mater. 5 (1993) 222. [7] J.-H. Gwak, S.-J. Kim, M. Lee, J. Phys. Chem. B 102 (1998) 7699. [8] L. Armelao, R. Bertoncello, M. De Dominicis, Adv. Mater. 9 (1997) 737. [9] C.A. Morris, M.L. Anderson, R.M. Stroud, C.I. Merzbacher, D.R. Rolison, Science 284 (1999) 622. [10] M.L. Anderson, C.A. Morris, R.M. Stroud, C.I. Merzbacher, D.R. Rolison, Langmuir 15 (1999) 674. [11] Y. Kobayashi, M.A. Correa-Duarte, L.M. Liz-Marza´ n, Langmuir 17 (2001) 6375. [12] M.A. Correa-Duarte, Y. Kobayashi, R.A. Caruso, L.M. LizMarza´ n, J. Nanosci. Nanotechnol. 1 (2001) 95. [13] B. Rodrı´guez-Gonza´ lez, A. Sa´ nchez-Iglesias, M. Giersig, L.M. Liz-Marza´ n, Faraday Discuss. 125 (2003) 133. [14] B. Nikoobakht, M.A. El-Sayed, Chem. Mater. 15 (2003) 1957. [15] C.J. Brinker, G.W. Scherer, Sol–gel Science, Academic Press, San Diego, 1990. [16] B.M.I. van der Zande, M.R. Boehmer, L.G.J. Fokkink, C. Scho¨ enenberger, Langmuir 16 (2000) 451. [17] Y.-Y. Yu, S.-S. Chang, C.-L. Lee, C.R.C., J. Phys. Chem. B 101 (1997) 6661. [18] N.R. Jana, L. Gearheart, C.J. Murphy, Chem. Commun. (2001) 617.
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[23] C.F. Bohren, D.F. Huffman, Absorption and Scattering of Light by Small Particles, Wiley, New York, 1983. [24] I. Pastoriza-Santos, L.M. Liz-Marza´ n, Nano Lett. 2 (2002) 903. [25] J.C. Riboh, A.J. Haes, A.D. McFarland, C.R. Yonzon, R.P. Van Duyne, J. Phys. Chem. B 107 (2003) 1772.
Silica gels with tailored, gold nanorod-driven optical functionalities Jorge Pe´rez-Juste, Miguel A. Correa-Duarte, Luis M. Liz-Marza´n* Departamento de Quı´mica Fı´sica, Universidade de Vigo, 36200 Vigo, Spain
Abstract The incorporation of metal nanoparticles with various size and shapes provides silica gels with optical functionalities arising from the characteristic optical properties of the starting nanoparticle dispersions. The uniform distribution of the preformed nanoparticles and their mutual separation within the gels are key factors to maintain single-particle properties. These requirements are achieved by coating the nanoparticles with thin silica shells, which greatly enhance their stability and ensure separation between metal nanoparticle cores. The great advantage of this procedure is its ability to use nanoparticles with tailored shapes and thus to expand the possible functionalities to be implemented. We show the incorporation of gold nanorods with varying aspect ratio, which allows to have well-defined absorption bands even in the near infrared. # 2003 Elsevier B.V. All rights reserved. PACS: 81.05.Y; 78.66.J Keywords: Nano-materials; Silica gels; Gold nanorods; Optical properties
1. Introduction Silica gels and glasses with optical functionalities are important for a number of technological applications, such as decorative coatings [1], catalysis [2], optical filters [3], non-linear optical materials [4], etc. and therefore such materials have been made for centuries. In fact, metal nanoparticles provide the famous Lycurgus cup with its very special optical behaviour [5]. However, most of the techniques used to dope glasses with metal nanoparticles are based on the reduction of metal salts within the glass precursor [6–8], and rely on a homogeneous distribution of the salt and a homogeneous reduction to achieve particles with a minimum size uniformity. Metal salt concen* Corresponding author. Fax: þ34-986-812556. E-mail address: [email protected] (L.M. Liz-Marza´n).
tration and temperature are normally the main parameters used to control the average particle size, and therefore this becomes hard to achieve. Alternatively, the sol–gel process can be carried out in the presence of preformed nanoparticles with the composition, morphology and properties suitable for the desired application [9,10]. In this way, not only spherical, but also nanoparticles with complex structures or specific shapes, such as alloys, core-shell particles or nanorods can be incorporated within the gel. However, it is mandatory in such cases that the colloidal stability of the nanoparticles is sufficient to keep them separated (non-aggregated) during the sol– gel transition, so that single-particle functionalities are preserved. We have recently devised a procedure that overcomes these difficulties by means of surface modification of the guest nanoparticles via deposition of a thin
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.11.013
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silica shell, which preserves colloidal stability during the whole sol–gel transition [11]. Although initially the method was only demonstrated for spherical gold nanoparticles, its application to other compositions, such as CdS [12] or AuAg alloys [13] was also achieved. In this paper we demonstrate for the first time that the same procedure can be also applied to the sol–gel processing of anisometric nanoparticles, in this case gold nanorods with various aspect ratios, which expands the optical functionalities (well-defined surface plasmon bands) within the gels toward the near IR.
2. Experimental 2.1. Materials and methods Tetrachloroauric acid (HAuCl43H2O), cetyltrimethyl ammonium bromide (CTAB), ACS grade methanol, sodium silicate solution (Na2O(SiO2)3–5, 27 wt.% SiO2), ascorbic acid, sodium borohydride and silver nitrate were purchased from Aldrich. 3Mercaptopropyl trimethoxysilane (MPS) and tetramethoxysilane (TMOS) were purchased from Fluka. All reactants were used as received. Milli-Q water with a resistivity higher than 18.2 MO cm was used in all the preparations. UV-Vis spectra were measured with a HP 8453 diode array spectrophotometer in 1 cm path length cuvettes. Transmission electron microscopy (TEM) was performed on a JEOL JEM 1010 microscope operating at 100 kV. Samples for TEM were prepared by depositing a drop of either the nanorod colloid or the suspension of a ground piece of gel in ethanol, on a carbon-coated TEM copper grid.
2.2. Synthesis of gold nanorods The synthesis of Au nanorods was performed through a modification of the procedure published by Nikoobakht and El-Sayed [14]. Details of the synthesis follow. A seed solution was prepared as follows: in a 20 mL vial, 5 mL of an aqueous solution of HAuCl4 (2:50 104 M) and CTAB (0.10 M) was prepared at room temperature; next, 0.3 mL of ice-cold, freshly prepared 0.01 M NaBH4 solution was added under vigorous stirring. The stirring was slowed down after 30 s and the Au sol was gently stirred for 15 min at 40–45 8C to ensure removal of excess NaBH4. For the growth of gold nanorods with various aspect ratios, different solutions were prepared containing 10 mL of growth solution (½HAuCl4 ¼ 5:00 104 M, ½CTAB ¼ 0:1 M, ½AgNO3 ¼ 4:00 105 M) at 25–30 8C. Next, 0.070 mL of 0.1 M ascorbic acid was added to each sample and mixed thoroughly. Finally, different volumes of seed solution were added: 0.006, 0.012, 0.024, 0.050 and 0.15 mL. The nanorods obtained had aspect ratios of 1.94, 2.35, 2.48, 3.08, and 3.21, respectively (see Fig. 1 and Table 1). 2.3. Silica coating of Au nanorods Silica coated Au nanorods (Au @ SiO2) were prepared as follows: after the synthesis of the nanorods, the excess CTAB was eliminated by centrifugation, removal of the supernatant and redispersion in pure water. To the clean nanorod dispersion (10 mL, ½Au ¼ 2:5 104 M), a freshly prepared aqueous solution of MPS (0.050 mL, 1 mM in ethanol) was added under vigorous magnetic stirring. The mixture of MPS and gold dispersion was allowed to stand for 15 min and then 0.2 mL of a 0.54 wt.% sodium silicate
Fig. 1. Representative TEM micrographs of CTAB stabilized Au nanorods with average aspect ratios of 1.94 (a), 2.35 (b), 2.48 (c), 3.08 (d), and 3.21 (e).
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Table 1 Dimensions, aspect ratio, and positions of the maxima of longitudinal plasmon resonances for the various nanorod samples at the different stages of the sol–gel process Aspect ratio
1.94 2.35 2.56 3.08 3.21
0.20 0.33 0.30 0.31 0.32
Length (nm)
50 44 39.3 31.6 25.8
5 2.4 2.5 2.7 3.7
Width (nm)
25.7 18.8 16.7 10 7.9
2.4 2.1 1.5 0.7 0.7
solution at pH 10–11 (adjusted with a cation exchange resin) was added, again under vigorous magnetic stirring. The resulting dispersion (pH 8:5) was then allowed to stand for 2 days, so that the active silica polymerised onto the primed gold particle surface. The thickness of the deposited silica shell was about 5–7 nm thick. 2.4. Sol–gel transition The method used to prepare silica gels loaded with Au nanorods was identical to that previously reported [11], which was based on the sol–gel literature [15]. In a typical preparation procedure, first 0.32 mL of TMOS was added to 1.33 mL of methanol. Next, 2 mL of the nanoparticle dispersion was added to the mixture of TMOS and methanol, then the final mixture was poured in a cuvette which was shielded with a rubber sheet to avoid unwanted evaporation. Upon several hours a rigid gel was obtained. The final molar ratio of TMOS, water and methanol was 1:50:15, assuming that 2 mL of colloid contains 2 mL of water.
3. Results and discussion 3.1. Silica coating Although most of the synthetic methods used for the preparation of metal nanorods suffered from limitations, either in the amount of material [16,17] or in the yield of nanorods, as compared to nanospheres [18,19], Nikoobakht and El-Sayed recently reported a variation of the so-called seed-mediated method [14] which affords the synthesis of relatively large amounts
lmax CTAB
SiO2 shell
SiO2 gel
632.5 653.7 679.2 718.3 737.9
637.2 659.6 693.3 725.4 754.6
641.2 665.4 695.8 737.9 751.7
of nanorods with very little contamination by nanospheres, and variable aspect ratio. The modification of the aspect ratio was accomplished in the original publication through a variation in the concentration of AgNO3, although the specific role of this salt was not disclosed. Since it is not clear whether or in which way the Ag atoms are incorporated within the rods, we varied the amount of seed solution while maintaining a constant AgNO3 concentration, which allowed us to prepare five different samples with aspect ratios ranging from 1.94 up to 3.21. Representative TEM images of the various nanorods are shown in Fig. 1, where it can be easily observed that variations in the aspect ratio are not due to longer, but rather to thinner (and even shorter) rods, which actually facilitates the further processing into the gels. Details on the average length and width in the different samples are provided in Table 1. For the deposition of silica shells on the nanorods, we employed a modification of the method previously devised for the coating of Au nanospheres [20]. The modification of the method mainly consists of the use of a mercaptosilane, rather than an aminosilane, due to the stronger affinity for Au complexation, so that it is able to displace adsorbed CTAB. Additionally, careful cleaning of the nanorod dispersion to remove as much dissolved CTAB as possible is absolutely essential, since otherwise there is nucleation of free silica in solution, which is still observed in some extent, as shown in Fig. 2a. However, homogeneous coating of all rods can be invariably observed for all samples. 3.2. Sol–gel processing The stability of the nanorods during the intermediate steps of the sol–gel process is of paramount importance to maintain their individuality within
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Fig. 2. Representative TEM micrographs of silica-coated Au nanorods with average aspect ratios of 1.94 (a) and 3.08 (b).
the final gels. As was previously observed for Au [11] and CdS [12] nanospheres, when the silica coating step was skipped, we invariably observed coagulation before the gel was formed. Coagulation could be observed with the naked eye through a color change, but it was even clearer when the process was followed spectroscopically (see Fig. 3). It can be observed that the longitudinal plasmon band blue-shifts and drops in intensity, while the transverse plasmon band red-shifts and gets slightly enhanced, long before gel formation. This means that the coagulation of the rods leads to aggregates with smaller aspect ratios and larger sizes than the original rod diameters, which eventually get large enough to sediment, as shown in the inset of Fig. 3.
A similar experiment performed on an Au @ SiO2 colloid formed from the same nanorods produces no visual changes at all, and only a small change in the spectrum, basically due to dilution. This results in a gel with uniform colour arising from the separate nanorods, homogeneously distributed in its interior. Examples of the morphology of the gels are shown in Fig. 4, where it is clear that the nanorods are well separated within the porous structure of the silica gel. This separation allows to basically maintain the optical properties of individual nanorods within the final gel, as discussed in the following section. Additionally, it should be noted that no basic or acid catalyst is added in the procedure outlined above, so that the conditions for gel formation are very mild,
Fig. 3. Spectral evolution of a CTAB-stabilized gold nanorod colloid upon mixing with TMOS in methanol. Total time: 36 min. The inset shows photographs of gels formed using silica-coated (left) and CTAB stabilized (right) Au nanorods.
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Fig. 4. Representative TEM micrographs of silica gels containing Au at SiO2 nanorods with average aspect ratios of 1.94 (a) and 3.08 (b).
thus favouring uniform structures. It was observed though, that addition of a small amount of sodium citrate to the mixtures notably accelerated the formation of the gels. 3.3. Optical properties The optical properties of metal nanorods have been discussed in detail [14,15,21,22]. Although they are based on the collective oscillation of conduction electrons (surface plasmons), the main difference with metal nanospheres is that two oscillation modes are possible, one parallel (longitudinal mode) and another perpendicular (transverse mode) to the long axis of the rod. This implies that two absorption bands are present in the UV-Vis spectrum, with the transverse band located at wavelengths close to the characteristic band of nanospheres of similar diameter, and the longitudinal band at higher wavelengths, which can even reach the near IR for long rods. The actual position of the longitudinal resonance is not directly determined by the length of the nanorods, but rather by their aspect ratio, which is also confirmed through calculations using Mie theory [21,23]. Additionally, the intensity of the longitudinal band is much higher than that of the transverse band, and thus this is the main functionality provided to the gels through doping with nanorods. The maximum position of the longitudinal resonances for the CTAB-Au nanorods used in this work ranged from 630 up to 740 nm (see Table 1 for details). In Fig. 5 we compare the spectra of the five different samples at the three main stages of the sol–gel process.
Fig. 5. UV-Vis spectra of the different nanorod samples stabilized with CTAB (top), coated with thin silica shells (middle), and after sol–gel processing (bottom). The line labels are the same for all graphs.
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The first stage corresponds to the nanorods stabilized with CTAB in water, prior to the addition of MPS. The second stage corresponds to the rods coated with thin silica shells (see Fig. 2), dispersed in water. Finally, the third stage corresponds to the rigid hydrogels, when the sol–gel transition has been completed. The spectra were normalized to a maximum absorbance of 1 to simplify the comparison. The spectra of the gels are more noisy because the nanorod concentration was noticeably lower and light scattering higher. The positions of all maxima are listed in Table 1. One first observation from Fig. 5 is that there is an excellent correlation between the positions of the maxima in the three different situations, with a consistent red shift upon silica coating, and a further shift upon gel formation. The first red shift is due to an increase of the local refractive index around each Au nanorod [20], while the second red shift is due to an increase of the average refractive index of the surrounding medium after formation of the gel [11]. It is also remarkable that the corresponding shifts are much larger in the longitudinal band than those of the single plasmon band of spherical nanoparticles, showing that this system is more sensitive to refractive index changes, which has also been observed for silver nanoprisms [24,25]. There is also a relative increase of intensity at higher energies, which is an indication of an increase in light scattering [20] arising from the formation of mesopores within the gel structure. This was confirmed by TEM (Fig. 4) and nitrogen adsorption as reported elsewhere [11,12].
4. Conclusions In this paper, we have demonstrated that silicacoated gold nanorods with various aspect ratios can be easily incorporated within transparent silica gels. Both UV-Vis absorption spectra and transmission electron microscopy show that there is no aggregation of the metal nanoparticles during the sol–gel transition, so that the optical properties of the starting colloid are basically retained in the gel, except for a shift of the plasmon band due to refractive index changes. This represents a generalization of this sol–gel technique for the homogeneous incorporation
of anisometric nanoparticles, thus providing the rigid gel with tailored optical functionalities.
Acknowledgements The authors are indebted to Benito Rodrı´guezGonza´ lez from CACTI (Universidade de Vigo) for assistance with TEM measurements. This work has been supported by the Spanish Ministerio de Ciencia y Tecnologı´a (Project no. BQU2001-3799) and Xunta de Galicia (Project no. PGIDIT).
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