The detection of Li+ is currently in demand for both biomedical and industrial applications. We here report the functionalization of 4 nm Au particles with a 1,10-phenanthroline ligand that binds selectively to Li+. The ligand binds to Li+ by forming a 2:1 ligand-metal complex, causing Au nanoparticles to aggregate. Au nanoparticle aggregation causes a shift in the extinction spectrum with a concomitant color change, providing a useful optical method of detecting Li+ in aqueous solution.