Optical Strain Detectors based on Gold/Elastomer Nanoparticulated Films

The application of two different optical effects is demonstrated for the detection of strain applied to elastomeric films. On one hand, dense coatings made of silica-coated gold nanoparticles (Au@SiO 2 NPs), which are built up onto poly(dimethylsiloxane) (PDMS) elastomeric films, using the layer-by-layer (LbL) method, provide intense surface plasmon resonance (SPR) absorption. On the other hand, polystyrene spheres can be deposited as ordered monolayers to create patterned PDMS films with well-defined light diffraction. Both effects were used to monitor the structural damage of such PDMS films upon stretching, remaining both physical phenomena (absorption from the gold film and diffraction from the ordered structure) active for optical sensing applications in the early detection of structural damage in critical infrastructures. Introduction The assembly of nanocrystals into macroscopic thin films offers an exciting pathway for the construction of materials with specifically designed optical, electrical, and/or catalytic properties. Inasmuch as these materials, fabricated in the nanosize regime, are purported to possess advantageous properties, they provide new avenues for performance and functional gains in structures made with such nanomaterials which would be unthought of using conventional ones. In particular, nanoparticulated metallic films have been the focus of intense research, primarily due to the interest in their optical and electronic properties, and thereby offering a high potential for the development of novel electrical and optical sensors and catalysts. In fact, there is a growing need to enhance the function of these engineered structures beyond their present boundaries through the use of advanced materials. Therefore, gold nanoparticles (NPs) with diameters ranging from 10nm to 100nm, with unique optical and electronic properties, are attractive candidates for the construction of inorganic-organic hybrid structures, which can be applied in advanced spectroscopy, chemical and biosensor technology and microelectronic devices. One of the most attractive features of gold NPs is the observation of intense surface plasmon resonances (SPR), which lead to strong surface enhanced Raman scattering, as well as nonlinear optical properties. SPR arise from the coupling of conduction electrons with incident electromagnetic waves 11 and are especially relevant in noble metals such as gold and silver, since the resonances occur within the visible range, though the precise frequency depends on various scattering conditions at the NPs interface. For example, surface-enhanced Raman scattering (SERS) is a highly sensitive spectroscopic tool that can detect individual molecules adsorbed on silver or gold NPs. The SERS effect mainly arises from the enhancement of the local electromagnetic field in the vicinity of the metal surface, which is strongly coupled to the excitation of the surface plasmon. In the case of nanoparticulated metallic films, their functional properties are also governed by the size, shape, and size distribution of the metal NPs, in addition to the dielectric properties of the surrounding medium. Therefore, various techniques have been developed in recent years for the construction of metallic nanoparticulated films on flat surfaces. Strategies for the immobilization of Au NP layers onto surfaces include either physical deposition (e.g., spin-coating and spraying) or layer-by-layer (LbL) schemes. Some LbL approaches that have been demonstrated include covalent schemes, primarily through surface attachment of NPs via dithiol linkages 26 or mercaptosilanes, and offer a better control over the film thickness and density. Electrostatic binding has also been demonstrated, including the use of charged molecules for attaching NPs to the surface, as well as polyelectrolyte systems. 31-34 Gold NPs are usually preferred in these nanoparticulated films, because of their chemical stability and relatively simple preparation. Gold Bulletin 2007 • 40/1 Gold Bulletin 2007 • 40/1 7 changes in the dielectric constant of the medium between particles are considered to be major factors affecting the dipolar-dipolar interactions and SPR frequency. Dipole-dipole interactions have been demonstrated to play a key role in optics of metal NPs, and especially in the case of closely spaced NP films, as has been recently shown by Ung et al. for assemblies of silica-coated gold (Au@SiO 2 ) NPs on flat surfaces and by Caruso et al. for assemblies on spheres. In these systems, the interparticle separation determines the plasmon oscillation frequency. Therefore, there is a substantial interest in the design of Au nanoparticulated films with control over the spacing between NPs, particularly in the distance range smaller than the NP diameter, where the optical and electronic properties change substantially with spacing. Furthermore such Au@SiO 2 NPs can be further functionalized through the reactivity of the hydroxide groups onto the silica surface, thereby facilitating chemical manipulation of the particle surfaces. The early detection of structural damage in critical infrastructures is also a long-standing envisaged goal. In particular, a major limitation in the development of structural sensing systems is that they are incapable of detecting submicron-level damage due to their limited sensitivity at this scale. Following this goal, the use of photonic crystals in submicron damage detection has been recently proposed. The idea arises from the use of the spectral response of photonic lattices, very sensitive to geometrical modifications, to reveal the strain in an adherent substrate through the induced microdamage on the photonic crystal structure. The application of such systems requires a precise fabrication of the photonic crystals which to date, has been demonstrated In fact, the reactivity of sulphur-containing molecules (thiols, disulfides, etc) toward Au surfaces, enabling formation of a large variety of self-assembled monolayers, can be exploited for chemical manipulation of Au surfaces. It has also been of particular interest the formation of thin films with a high density of these metal NPs, which is often desired for catalytic and electronic applications. Thin films comprising a dense packing of gold NPs can be prepared by means of the adsorption of 4-(dimethylamino)pyridine gold NPs (DMAP-AuNP) onto preformed polyelectrolyte (PE) films. The formation of these dense nanoparticulated films was attributed to the reversible binding nature of the DMAP ligand and the nature of the PE films (poly-(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH)), which act as matrices for nanoparticle adsorption. PAH/PSS multilayer films loaded with DMAP-Au NPs have also been used as coatings on colloids, in order to modulate the optical response of colloidal crystals, as electrochemical sensors, or as nanostructured optically addressable capsules that can be irradiated with near-IR radiation for the controlled release of (bio)macromolecules. 37 The ability to construct nanoparticle-based thin films with tailored properties, as well as their ultimate application, is dependent on a fundamental understanding of the interactions involved. In these condensed, organized films, the gold NPs displaying very different dielectric properties are subjected to strong interactions. These interactions between neighbouring gold particles generate collective plasmon resonances, controlled by the long-range aggregation of the gold NPs within the same layer. A variable packing density and the corresponding interparticle distances, along with Figure 1 Schematic illustration of the synthetic process followed for preparation of functionalized PDMS films. Au@SiO 2 NPs are deposited onto either planar PDMS elastomeric films (A), or on PDMS films with a close packed honeycomb-like hole structure (B). Au Np deposition