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 SiC>2 surfaces. The assembly protocol is carried

The self-assembly of monodisperse gold and silver colloid particles into monolayers on polymer-coated substrates yields macroscopic surfaces that are highly active for surface-enhanced Raman scattering (SERS). Particles are bound to the substrate through multiple bonds between the colloidal metal and functional groups on the polymer such as cyanide (CN), amine (NH(2)), and thiol (SH). Surface evolution, which can be followed in real time by ultraviolet-visible spectroscopy and SERS, can be controlled to yield high reproducibility on both the nanometer and the centimeter scales. On conducting substrates, colloid monolayers are electrochemically addressable and behave like a collection of closely spaced microelectrodes. These favorable properties and the ease of monolayer construction suggest a widespread use for metal colloid-based substrates.

Ag-coated Au colloidal particles have been prepared by reduction of Ag+ in the presence of preformed Au colloids. The composition of the Au100-x Ag x particles was varied from x = 0 to 80. SERS spectra of pyridine, p-nitroso-N,N‘-dimethylaniline (p-NDMA), and trans-1,2-bis(4-pyridyl)ethylene (BPE) have been obtained with these colloids. At monolayer Ag coverages (x < 10), the optical spectra of Ag-coated Au particles are indistinguishable from uncoated Au particles. However, the SERS behavior of aggregated colloids with 647.1 nm excitation is extremely dependent upon the Ag:Au ratio. Very small amounts of Ag (x ≤ 5) lead to an increase in SERS intensity, but further increases lead to complete loss of signal. For p-NDMA and pyridine, these data can be explained by Ag inhibition of adsorbate-induced aggregation. The initial increase in SERS intensity results from production of smaller aggregates that exhibit a surface plasmon band in better alignment with the excitation wavelength; higher ratios of Ag eliminate aggregation and all SERS enhancement. For BPE, the same Ag-induced loss of SERS is observed, even though each of the Au100-x Ag x colloidal solutions is clearly aggregated by BPE adsorption. This finding suggests that submonolayers of Ag modulate specific chemical interactions between the Au and BPE that are responsible for SERS.

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The effect of colloidal Au particle aggregation on surface-enhanced Raman scattering (SERS) spectra was probed by SERS filtration experiments. In this approach, SERS and optical spectra were recorded for trans-1,2-bis(4-pyridyl)ethylene (BPE)-aggregated solutions of colloidal Au filtered through straight-channel membranes with successively smaller diameters. This allowed the overall SERS intensity to be factored into aggregate size-dependent contributions. Experiments were carried out as a function of adsorbate concentration (0.5-2.5 µM BPE) and initial particle size (12-50 nm diameter colloidal Au). The key findings are as follows: (i) under conditions of minimal aggregation, appreciable SERS intensity derives from aggregates with effective diameters less than 200 nm; (ii) the amount of aggregant clearly controls the average aggregate size; and (iii) similarly aggregated solutions based on different diameter colloidal Au particles give different distributions of aggregates. These studies provide an insight into the dynamics of colloidal Au aggregation, suggest a procedure for signal optimization in colloid SERS experiments, and set the stage for controlled surface confinement of SERS-active particle clusters.