ACS Nano http://doi.org/gcgtd9 (2017)
The assembly of colloidal particles is usually attained as a result of random aggregation, but this approach prevents the formation of highly uniform aggregates. A better, if more challenging, approach would be to counteract the Brownian motion in the solution phase to direct the aggregation of nanoparticles. As an effort in this direction, Thrift et al. now report a methodology to make uniform nanoparticle assemblies in which the Brownian fluctuations in solution are tamed in virtue of the presence of a long-range electrokinetic interaction.
The researchers seed gold nanoparticles functionalized with lipoic acid ligands on top of a silicon electrode. Applying a voltage bias in the solution creates an electrostatic potential that attracts other functionalized particles from the solution phase towards the seeded nanoparticles. The attractive force is larger in the direction parallel to the surface. In this way, most of the aggregation occurs in the horizontal direction. As the particles gets close to one another, a chemical reaction between the lipoic acid ligands occurs in the presence of a carbodiimide molecule. As a result, an anhydride linker forms, which keeps the nanoparticles at a constant distance of 0.9 nm. Notably, Thrift et al. can follow the chemical reaction by surface-enhanced Raman spectroscopy (SERS), as the gap space benefits from the highest electromagnetic field intensity. Through their methodology, the researchers demonstrate a uniform SERS signal enhancement of 10% over an area of 1 mm2.