Plasmon-resonant gold nanorods have outstanding potential as multifunctional agents for image-guided therapies.
Nanorods have large absorption cross sections at near-infrared (NIR) frequencies, and produce two-photon
luminescence (TPL) when excited by fs-pulsed laser irradiation. The TPL signals can be detected with single-particle
sensitivity, enabling nanorods to be imaged in vivo while passing through blood vessels at subpicomolar concentrations.
Furthermore, cells labeled with nanorods become highly susceptible to photothermal damage when irradiated at
plasmon resonance, often resulting in a dramatic blebbing of the cell membrane. However, the straightforward
application of gold nanorods for cell-specific labeling is obstructed by the presence of CTAB, a cationic surfactant
carried over from nanorod synthesis which also promotes their nonspecific uptake into cells. Careful exchange and
replacement of CTAB can be achieved by introducing oligoethyleneglycol (OEG) units capable of chemisorption onto
nanorod surfaces by in situ dithiocarbamate formation, a novel method of surface functionalization. Nanorods with a
dense coating of methyl-terminated OEG chains are shielded from nonspecific cell uptake, whereas nanorods
functionalized with folate-terminated OEG chains accumulate on the surface of tumor cells overexpressing their cognate
receptor, with subsequent delivery of photoinduced cell damage at low laser fluence.