The PhD has studied gold nanoparticles (NPs) at the liquid-liquid interface (LLI). Some of the key aspects of the work are listed below. • Centrifugation as a novel and efficient method for adsorbing a controlled number of NPs to the LLI. • Controlled and tuneable 2D interparticle separation at the LLI as evidenced by the plasmon ruler. • Reversible adsorption of 16 nm gold nanoparticles at the water-DCE interface. • Centrifugation as a novel method to form an ultra-concentrated NP aqueous phase. • Controlled and tuneable 3D inter-particle separation in water by ultra-concentration. • The ultra-concentrated NP solution has some 'record-breaking' physical properties - e.g. densities in excess of 4.5 gcm-3; optical densities in excess of 70,000; and active surface areas of in excess of 70 m2/mL. • Ultra-concentration is also demonstrated as an efficient purification and NP size-separation technique with efficiencies in excess of 99.9% and 99.5%, respectively. • Close-packed NPs at the LLI are also demonstrated to be an efficient sensing platform through surface enhanced Raman spectroscopy. • The platform benefits from: fmole detection limits; extremely facile, quick and cheap assembly; applicability to a wide range of target analytes; both hydrophilic and hydrophobic detection capabilities simultaneously. • Through evaporation of the organic phase, the platform is also demonstrated to be able to identify and estimate concentrations of airborne analytes. • Finally, 1,8-diaminonaphthalene and its analogues are demonstrated to be extremely efficient mercury reporters when combined with SERS at the LLI. • A dramatic increase in SERS intensity is observed in the presence of mercury - though the exact reason for such an increase is still under investigation, some potential mechanisms are provided. • As with other analytes, NPs at the liquid-air interface demonstrate airborne mercury detection capabilities and this is demonstrated for the first time using SERS.