Stable and Uniform SERS Signals from Self-Assembled Two-Dimensional Interfacial Arrays of Optically Coupled Ag Nanoparticles

Densely packed interfacial nanoparticle films form spontaneously when aqueous Ag colloid is shaken with CH₂Cl₂ in the presence of a “promoter” such as 10^–4 mol dm^–3 tetrabutylammonium nitrate (TBA⁺NO₃^–), which induces rapid self-assembly of the nanoparticles at the liquid/liquid interface without adsorbing onto their surfaces. The particles within these reflective, metal-like liquid films (MeLLFs) are optically coupled and give strong SERS enhancement, similar to that obtained for the same colloid aggregated with optimized concentration of metal salt. However, unlike aggregated colloids their structure means they do not sediment out of solution so they give SERS spectra that are stable for >20 h) and have good uniformity (relative standard deviation in absolute intensity over 1 mm² array of 25 points was 1.1%). Since the films lie at the aqueous/organic interface they are open to adsorption of analytes from either of the phases and can be probed in situ to detect both water- and nonwater-soluble analytes. The detection limit for mercaptobenzoic acid (MBA) added to the organic layer was found to be <2 ppb. These materials therefore combine many of the best features of both patterned surfaces and metal colloids for quantitative SERS analysis.