Photothermal Plasmonic Triggering of Au Nanoparticle Surface Radical Polymerization

Gold nanoparticle theranostic agents have dramatic potential in the fight against disease, particularly cancer, as multifunctional platforms combining biocompatibility, unique optical properties for detection/activation, and heat generation. In this vein, a new thiol-functionalized enediyne surfactant ligand was synthesized and coordinated to gold nanoparticles, as confirmed by the red-shift in the optical spectrum from A. = 520 to 529 nm upon ligand exchange. Raman spectra of the nanoparticle conjugate material show characteristic vibrations at 2192 (alkyne), 1582 (alkene), and 670 cm(-1) (C-S). The photoreactivity of the material is explored under two sets of photolysis conditions: solution, lambda(exc) = 514 nm, RT, t = 8 h; solid aggregate, lambda(exc) = 785 nm, T = -190 degrees C, t = 4 h. Under these conditions, exciting into the surface plasmon of the Au nanopartide substrate transfers heat to the organic ligand layer, initiating enediyne cyclization and generating surface radicals that lead to subsequent polymerization. New vibrational signatures arise in the alkyne (2170-1900 cm(-1)) and aromatic (1520-1200 cm(-1)) spectral regions, indicating the formation of highly conjugated species in the initial stages of the photoreaction. Prolonged irradiation results in the observation of a dense polymer coating in the TEM images, complete loss of observable molecular vibrations in the Raman spectra as a result of strong fluorescence, and a red-shift and broadening of the surface plasmon band in the electronic spectrum. Translation of this approach to nanorods and other architectures is also possible with carbon coatings clearly visible by TEM. The reported nanomaterial design represents a new approach to developing reactive biomedical agents for phototherapy applications, as well as a novel method toward carbonaceous coatings of nanoarchitectures.