What controls the optical properties of DNA-linked gold nanoparticle assemblies?

A study aimed at understanding the factors that control the optical propert ies of DNA-linked Sold nanoparticle aggregates containing oligonucleotide l inkers of varying length (24-72 base pain) is described. In this system, si milar to 15 nm diameter Au particles modified with (alkanethiol)-12 base ol igomers are hybridized to a series of oligonucleotide linkers ranging from 24 to 72 base pairs (similar to 80-240 Angstrom) in length. Aggregated at r oom temperature, the various macroscopic nanoparticle assemblies have plasm on frequency changes that are inversely dependent on the oligonucleotide li nker length. Upon annealing at temperatures close to the melting temperatur e of the DNA the optical properties of the DNA-linked assemblies containing the longer linkers (48 and 72 base pairs) red-shift until they are similar to the assemblies containing the shorter linkers (24 base pairs). The pre- and postannealed DNA-linked assemblies were characterized by sedimentation rate, transmission electron microscopy, dynamic light scattering, and UV-v is spectroscopy which show that the oligonucleotide Linker length kinetical ly controls the size of the aggregates that are formed under the preanneale d conditions, thereby controlling the optical properties. Through the use o f small-angle X-ray scattering and electrodynamic modeling in conjunction w ith the techniques mentioned above, we have determined that the temperature -dependent optical changes observed upon annealing of the aggregates contai ning the longer oligonucleotides (48 and 72 base pairs) can be attributed t o aggregate growth through an "Ostwald ripening" mechanism (where larger ag gregates grow at the expense of smaller aggregates). This type of aggregate growth leads to the red-shift in plasmon frequency observed for the aggreg ates. Significantly, these experiments provide evidence that the optical pr operties of these DNA-linked nanoparticle assemblies are governed by aggreg ate size, regardless of oligonucleotide linker length, which has important implications for the development of colorimetric detection methods based on these nanoparticle materials.