Two-dimensional metal nanoparticle arrays are normally constructed at liquid-oil interfaces by modifying the surfaces of the constituent nanoparticles so that they self-assemble. Here we present a general and facile new approach for promoting such interfacial assembly without any surface modification. The method use salts that have hydrophobic ions of opposite charge to the nanoparticles, which sit in the oil layer and thus reduce the Coulombic repulsion between the particles in the organic phase, allowing the particles to sit in close proximity to each other at the interface. The advantage of this method is that because it does not require the surface of the particles to be modified it allows nonmetallic particles including TiO2 and SiO2 to be assembled into dense interfacial layers using the same procedure as is used for metallic particles. This opens up a route to a new family of nanostructured functional materials.

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique, which allows quantitative detection of chemical species with molecular specificity and single-molecule sensitivity. These useful properties can be further combined with...

Self-assembly at water-oil interfaces has been shown to be a cheap, convenient and efficient route to obtaining densely-packed layers of plasmonic nanoparticles which have small interparticle distances. This creates highly...

elegant and effective way to obtain 2D NP arrays is through bottom-up self-assembly of colloidal particles at the interface between two immiscible liquids. [5][6][7][8] We are interested in preparing interfacial NP arrays using methods which cause minimum perturbation of the NP surface chemistry so that the chemical properties of the constituent particles are retained in the array. In particular, it is important to avoid modifying the surfaces of particles through adsorption of strongly bound molecular passivating layers because these will interfere with the functionality of the array, e.g., by preventing intimate contact between the surface and reactants in catalysis or hindering the close approach of target analytes to the surface which is essential for surface-enhanced Raman spectroscopy (SERS). [9,10] The ideal method for interfacial assembly would be one in which the initial colloidal suspension is simply shaken with pure nonaqueous solvent and the particles spontaneously migrate to the interface. Unfortunately, while adsorption of solid NPs to liquidliquid interfaces (LLIs) is highly favorable, since it significantly lowers the energy of the interface, this is counterbalanced by electrostatic repulsion between NPs. This means that NPs seldom spontaneously migrate to LLIs in significant numbers when aqueous colloids are simply mixed with highly immiscible oils. [11] Conventionally, films may be formed by functionalizing the surfaces of charged colloidal NPs with charge-neutral organic "modifiers" to reduce repulsion but this means that the surface of the NPs is covered in a layer of strongly adsorbed organic capping agents, which is undesirable for the reasons outlined above. [12,13] Alternatively, less polar co-solvents such as acetone and ethanol may be added, which reduce the ability of the aqueous phase to solvate ionic compounds and again significantly weaken the electrostatic repulsion between NPs. [14,15] Although this method can be readily applied to assemble various types of NPs, the loss of surface charge also leads to unwanted aggregation of the particles and the formation of structural defects in the product NP array. [16] We have previously shown a third approach to induce selfassembly of colloidal NPs into defect-free NP arrays, which is based on adding amphiphilic salts to the biphasic system. [10,17,18] Self-assembly of colloidal nanoparticles at water-oil interfaces offers an efficient way to construct multi-dimensional arrays. Self-assembly is generally induced by using adsorbing molecular "modifiers" or co-solvents to remove the nanoparticles' surface charge. Here, it is shown that cetyltrimethylammonium bromide (CTAB), which is commonly used in bulk quantities in colloidal synthesis, can induce self-assembly of negatively charged colloidal nanoparticles at water-oil interfaces, even at sub-micromolar levels but it does this by providing charge screening rather than removing the surface charge. Since this is a physical effect, CTAB can promote assembly of nanoparticles regardless of their ...