Effect of Intense Hot-Spot-Specific Local Fields on Fluorescein Adsorbed at 3D Porous Gold Architecture: Evolution of SERS Amplification and Photobleaching under Resonant Illumination ^†

Plasmonic nanostructures with a high density of confined areas with high local electromagnetic fields (hot spots) are sine qua nonto increase the efficiency of surface-enhanced Raman spectroscopy (SERS). These nanostructures can be used both to identify biological molecules and to monitor photochemical reactions occurring on the metal surface. In this work, using the method of pulsed laser deposition, three-dimensional (3D) porous wedge-shaped arrays of gold nanoparticles (Au NPs) were obtained with structural parameters varying along the substrate, such as film thickness, porosity, nanoparticles size, and the distance between them. The resulting arrays were structures with a regularly changing density of hot spots along the substrate, in which the enhancement of the electromagnetic field strength is due to the geometric parameters of the nanostructure.By analyzing the evolution of fluorescence and Raman scattering of fluorescein molecules adsorbed on the surface of porous gold under illumination at 532 nm, the processes in the region of extreme values of the electromagnetic field of surface nanostructures was studied. A correlation has been established between the amplification of optical signals and the structural features of the surface. A correlation between SERS and fluorescence signals indicates the predominant contribution of hot spots to the electromagnetic amplification of optical signals. The observed time evolution of the fluorescence and SERS intensity of fluorescein can be explained by the combination of molecular photodegradation, the reconstruction of the hot spot architecture due to local heating, and potent relocation of analyte molecules outside the area of measurement owing to the effects of thermal gradients.