Flexible synthesis of high-purity plasmonic assemblies
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CT The self-assembly of nanoparticles has attracted a vast amount of attention due to the ability of the nanostructure to control light at the sub-wavelength scale, along with consequent strong electromagnetic field enhancement. However, most approaches developed for the formation of discrete assemblies are limited to a single and homogeneous system, and incorporation of larger or asymmetrical nanoparticles into assemblies with high purity remains a key challenge. Here, a simple and versatile approach to assemble nanoparticles of different sizes, shapes, and materials into various discrete homo- or hetero-structures using only two complementary deoxyribonucleic acid (DNA) strands is presented. First, surface functionalisation using DNA and alkyl-polyethylene glycol (PEG) enables transformation of as-synthesised nanoparticles into readily usable plasmonic building blocks for self-assembly. Optimisation of the DNA coverage enables the production of different assembly types, such as homo- and hetero-dimers, trimers and tetramers and core-satellite structures, which are produced in high purity using electrophoresis purification. The approach is extended from purely plasmonic structures to incorporate (luminescent) semiconductor nanoparticles for formation of hybrid assemblies. The deposited assemblies form a high yield of specific geometrical arrangements, attributed to the van der Waals attraction between particles. This method will enable the development of new complex colloidal nanoassemblies for biological and optical applications.
KEYWORDS deoxyribonucleic acid (DNA) self-assembly, electrophoretic purification, nanoparticle assemblies, colloidal stability
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Introduction
A key challenge in nanoscience is the precise arrangement of individual nanocrystals, of different materials, into specific assemblies on the nanoscale [1–3]. This challenge becomes even more demanding for asymmetric nanoparticles [4]. The successful implementation of methods to achieve this promises to open pathways for the formation of functional nanoscale superstructures. Over the past decade developments in colloidal chemistry have allowed the synthesis of nanocrystals of different materials, shapes and sizes. With these as building blocks, the potential number of combinations in material, nanocrystal spatial arrangement and geometry is immense. The assembly of metallic nanoparticles into discrete superstructures is motivated by the possibility of controlling electromagnetic fields at the nanoscale via coupling of the nanoparticle plasmon resonances. The resultant electromagnetic enhancement (in the near-field) and interparticle separation dependant spectral response have been exploited for surface-enhanced Raman spectroscopy [5], fluorescence enhancement [6], chemical sensors [7], and the development of therapeutic and imaging agents [8, 9]. Due to the vast number of different ways of assembling particles, many assembly geometries can be potentially achieved, including homo-, hetero- dimers/trimers/tetramers along with core–satellite assembl
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