Silica gels with tailored, gold nanorod-driven optical functionalities

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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.

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], [7], [8], and rely on a homogeneous distribution of the salt and a homogeneous reduction to achieve particles with a minimum size uniformity. Metal salt concentration 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 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.

Section snippets

Materials and methods

Tetrachloroauric acid (HAuCl4·3H2O), 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. 3-Mercaptopropyl 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  cm was used in all the preparations.

UV-Vis spectra were measured with a HP 8453

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 of nanorods with very little contamination by nanospheres, and variable aspect ratio. The modification of the aspect ratio

Conclusions

In this paper, we have demonstrated that silica-coated 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

Acknowledgements

The authors are indebted to Benito Rodrı́guez-González from CACTI (Universidade de Vigo) for assistance with TEM measurements. This work has been supported by the Spanish Ministerio de Ciencia y Tecnologı́a (Project no. BQU2001-3799) and Xunta de Galicia (Project no. PGIDIT).

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