Ion-selective self-assembly of Au nanoparticles is described. The Oxytricha telomere DNA hairpin 5'-d(G4T4G4) immobilized on 13-nm Au nanoparticles forms a supramolecular assembly via dG-quartets, as determined by the color change and by scanning electron microscopy (SEM). The aggregation is ion-dependent and selective for Na+ ions. K+ is less efficient, while Li+ and Cs+ do not drive the aggregation. This work is the first effort to explore the use of secondary structures of DNA (quadruplexes) for producing self-assemblies of Au nanoparticles. It can be utilized to generate novel devices and materials, potentially useful for sensing and other applications in medicinal or engineering science.