Achieving superior results in chemical and biological sensing using plasmon-enhanced stimulated Raman scattering requires (i) the creation of a wide variety of hot spots and (ii) the formation of an efficient way to deliver target molecules to places with intense electromagnetic fields. Therefore, for advanced SERS-sensing we developed porous three-dimensional (3D) wedge-shaped nanoarchitectures based on close-packed multilayers of gold nanoparticles (Au NPs) with porosity 38-70% and nanoparticle sizes of 25–7 nm; the structures were produced by pulsed laser deposition in an argon atmosphere. Designed SERS substrates are characterized by 3D spatially distributed high-density pores with a size of 30–2 nm (plasmonic hot spots). The presence of 3D pores in the structure leads to an increase in the total surface area and allows the analyte molecules to diffuse deep into the materials, which ensures the collection of more molecules.
In this work, we demonstrate the effective enhancement of the Raman scattering of Rhodamine 6G molecules (within the concentration range 10-5 - 10−10M) under resonant excitation by radiation λex=488.0, 514.5 nm, which is outside the region of surface plasmon resonance (SPR) excitation in 3D Au nanostructures (λSPR= 570-720 nm). Raman enhancement mechanisms include a combination of several effects: molecular resonance; the electromagnetic effect; and non-resonant chemical amplification. Electromagnetic amplification dominates in the areas of hot spots, located mainly within the internal pores of the material; excitation of the gold nanoparticles surrounding the pore occurs as a result of re-emission (luminescence) of the laser excitation by rhodamine 6G molecules as a result of the optical frequency down-conversion process. An analysis of the dependence of the Raman intensity on the structural parameters of the porous layers of Au NPs made it possible to establish that the greatest signal amplification is observed for the material with the highest density of hot spots.
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