Preparation and Characterization of Au Colloid Monolayers

The design and initial characterization of two-dimensional arrays of colloidal Au particles are reported. These surfaces are prepared by self-assembly of 12 nm diameter colloidal Au particles onto immobilized polymers having pendant functional groups with high affinity for Au (i.e., CN, SH, and NH₂). The polymers are formed by condensation of functionalized alkoxysilanes onto cleaned quartz, glass, and SiO₂ surfaces. The assembly protocol is carried out completely in solution: cleaned substrates are immersed in methanolic solutions of organosilane, rinsed, and subsequently immersed in aqueous colloidal Au solutions. Two-dimensional arrays spontaneously form on the polymer surface. The resulting substrates have been characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), and surface-enhanced Raman scattering (SERS). TEM data show that the particles are spatially separated but close enough to interact electromagnetically (small spacing compared to λ). The UV-vis data show that collective particle surface plasmon modes are present in the 650-750 nm region, suggesting that these assemblies are SERS-active. This is indeed the case, with enhancement factors of roughly 10⁴. Au colloid monolayers possess a set of features that make them very attractive for both basic and applied uses, including uniform roughness, high stability, and biocompatibility.