DNA-directed assembly of multifunctional nanoparticle networks using metallic and bioinorganic building blocks

Oligonucleotide-modified nanoparticles of gold or protein-encapsulated iron oxide (ferritin) were prepared respectively by covalent or streptavidin/biotin coupling, and reversibly assembled into interlinked networks by a two-step process involving targeted DNA hybridisation. In the first step, a 3′-biotin-terminated 12-base single-strand oligonucleotide was bound to a streptavidin-biotinylated ferritin conjugate to produce an oligonucleotide–protein complex, which was then mixed with Au nanoparticles capped with a 5′-mercapto-terminated 12-base non-complementary single-strand oligonucleotide. Double-stranded crosslinks between the oligonucleotide-modified Au and ferritin building blocks were then induced by addition of a half-complementary 24-base single-strand target oligonucleotide. TEM studies revealed the presence of disordered networks often several micrometres in size that consisted of closely associated Au nanoparticles and iron oxide ferritin cores. Corresponding uv-vis spectra showed a reduction in the intensity of the oligonucleotide 260 nm absorbance band, consistent with base-pairing and duplex formation, and a small but distinct 7 nm red shift for the Au surface plasmon resonance band. Heating the aggregates to 70 °C shifted the plasmon resonance peak back to an original value of 524 nm, and produced a marked increase in absorbance at 260 nm consistent with disassembly of the DNA duplex and solubilization of the Au nanoparticles. Thermal cycling of the Au-ferritin networks revealed a melting temperature for interparticle duplex hybridisation of 54 °C, and indicated that the assembly process was reversible.