Unraveling Plasmon-Enhanced Reactive Oxygen Species Generation through Ultrafast Light

Reactive oxygen species (ROS) generation through gold nanorods (AuNRs) excited by 812 nm centered, 85 femtosecond (fs)-pulsed laser irradiation was investigated through a rhodamine B degradation assay. The initial rate of rhodamine B fluorescence intensity degradation is determined by the rate of ROS generation, but at later time points the laser irradiation induced deformation of AuNRs reduces the rate of rhodamine B degradation. For different AuNR preparations that all had a localized surface plasmon resonance (LSPR) mode at around 800 nm but differed in size, the initial rate of rhodamine B fluorescence intensity decrease follows a trend predicted by the simulated peak near-field intensities and absorption efficiencies except for the smallest AuNRs with dimensions of 30 by 7 nm. The initial rate of ROS generation exhibits a power law dependence on the fluence. The reshaping of the AuNRs on longer timescales also depends on the fluence. For 2.3 mJ/cm² the establishment of a stable regime is observed, where an initial reshaping of the AuNRs decreases the spectral overlap between longitudinal plasmon resonance and excitation wavelength so that the absorbed energy is insufficient to induce further structural changes but still allows for ROS generation. For a fluence of 3.9 mJ/cm² the AuNR plasmon spectrum almost completely detunes from the excitation wavelength, resulting in a further reduction of ROS generation. AuNR reshaping and ROS generation also depend on the surface passivation of the AuNRs. Intriguingly, a lipid-coating was observed to provide a relative stabilization of the AuNRs when compared with polyethyleneglycol (PEG) or cetyltrimethylammonium (CTAB) surface chemistries and still allow for ROS generation.