Three-layer composite magnetic nanoparticle probes for DNA

A method for synthesizing composite nanoparticles with a gold shell, an Fe₃O₄ inner shell, and a silica core has been developed. The approach utilizes positively charged amino-modified SiO₂ particles as templates for the assembly of negatively charged 15 nm superparamagnetic water-soluble Fe₃O₄ nanoparticles. The SiO₂-Fe₃O₄ particles electrostatically attract 1-3 nm Au nanoparticle seeds that act in a subsequent step as nucleation sites for the formation of a continuous gold shell around the SiO₂-Fe₃O₄ particles upon HAuCl₄ reduction. The three-layer magnetic nanoparticles, when functionalized with oligonucleotides, exhibit the surface chemistry, optical properties, and cooperative DNA binding properties of gold nanoparticle probes, but the magnetic properties of the Fe₃O₄ inner shell.