Gold nanorod-enhanced two-photon excitation fluorescence of conjugated oligomers for two-photon imaging guided photodynamic therapy

Plasmon enhanced optical properties achieved by using metal nanoparticles are both fundamentally important and appealing for many biological and photonic applications. Although metal-enhanced onephoton excitation fluorescence and two-photon absorption have been previously demonstrated, up to now there are few reports on the overall two-photon excitation fluorescence enhancement by plasmon resonance. Conjugated polymers and oligomers are known to display large two-photon absorption cross-sections. Here we report plasmon-enhanced two-photon excitation fluorescence and singlet oxygen generation of a conjugated oligomer obtained by using gold nanorods in a core-shell structure. The separation between the metal core and the conjugated oligomer molecules was adjusted by varying the thickness of the spacer from 8.5 to 45 nm to achieve an optimized enhancement effect. The one- and two-photon excitation fluorescence intensities of the conjugated oligomer were found to be strongly dependent on the silica shell thickness. The optimum two-photon excitation fluorescence enhancement occurs at a shell thickness of 15 nm with an enhancement factor of 14.2. These core-shell nanostructures also exhibit significantly improved singlet oxygen generation capability under two-photon excitation. The application of these nanoparticles as effective imaging agents for two-photon cell imaging and as nano-photosensitizers for two-photon photodynamic therapy with improved efficiency has been demonstrated on HepG2 cells using the XTT assay. The remarkable two-photon excitation fluorescence enhancement property of these nanoparticles makes them promising agents for two-photon imaging guided phototherapy.