Laser-induced, Surface Plasmon-enhanced Two-photon Excitation for Efficient Chemical Functionalization of Nanostructured Gold Surfaces

Functionalized gold nanostructures with efficient, broadband absorption properties are of interest for a variety of biomedical applications. In this study, we report a plasmon-enhanced functionalization methodology that results in selective surface conjugation of a fluorescent probe via two-photon excitation under visible-wavelength laser irradiation. The fluorescent probe was designed to incorporate a thiolated 4-piperidinyl-1,8-naphthalimide (SNaph) entity, carrying a photolabile 6-nitroveratryl (NV) protecting group, straightforwardly synthesized in a few steps in good yield. Efficient plasmon-enhanced photodeprotection of the NV-group, followed by thiol-gold bond formation on gold nanoisland substrates was recorded upon exposure to 650 nm laser light, supported by confocal laser scanning microscopy (CLSM). Photolysis of the labile NV-group, thereby efficiently exposing the free mercapto group of the thiolated 1,8-naphthalimide, was recorded upon UV-irradiation at 350 nm, whereas no cleavage occurred at 650 nm. No conjugation occurred on featureless, gold-plated flat substrates under the same conditions. Surface plasmon-enhanced two-photon excitation at plasmonic hotspots in the absorber layer thus resulted in selective and efficient conjugation of the fluorescent probe to the structured surfaces. The resulting laser-assisted approach introduces the advantage of accomplishing selective molecular functionalization at plasmonic hotspots, owing to the combined, simultaneous effects of long-wavelength deprotection and spontaneous conjugation.