A critical challenge in nanoparticle (NP) surface functional-ization is to label the NP surface with a single copy of a functional group or to display multiple, unique molecules on the NP surface with control of the orientation and intermoleculardistance. Recently, a few elegant strategies have been developed to obtain nanoparticles with stoichiometric control of the number of attached ligands. These methods include the use of gel electrophoresis to isolate gold nanoparticles bearing discrete numbers of DNA oligonucleotides, micrometer-sized beads with a large surface area to minimize the contacts between small nanoparticles to create monofunc-tional DNA-nanoparticle conjugates, an ordered monolayer coating to create polar singularities on the nanoparticle surface, and a stepwise surface-encoding protocol to assemble symmetric and asymmetric nanoclusters. Nevertheless, the challenge of achieving a single NP with multiple molecules arranged at spatially addressable locations on the particle surface still remains. By transforming the symmetric surface of a spherical nanoparticle into an asymmetric surface, control over the functionalization can be achieved.
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