Gold-Coated Quantum Dots

Scientists have devised a way to make gold-coated quantum dots that retain the desirable optical properties of both the gold and the nanocrystal. Such gold-coated quantum dots will allow researchers to perform both fluorescence and plasmonic imaging with a single tag. But making them is harder than it sounds because gold can snuff out the quantum dot's fluorescence. Xiaohu Gao and Yongdong Jin of the University of Washington, Seattle, overcame that barrier by leaving a gap between the quantum dot and its glittering shell (Nat. Nanotechnol., DOI: 10.1038/nnano.2009.193). They coat lipid-stabilized, polyethylene glycol-coated quantum dots with a layer of poly-l-histidine, which serves as a template for gold deposition. The thickness of the lipid-PEG layer determines the size of the gap, which can be tuned with nanometer precision by adding polyelectrolyte bilayers of polyallylamine hydrochloride and sodium polystyrene sulfonate before adding the peptide and gold. The nanoparticles' gold shells are 2–3 nm thick. TEM images of the nanoparticles show a clear gap between the quantum dot and its shell because the organic layers are not dense enough to be seen with transmission electron microscopy.