Functionalized gold nanoparticles and nanoplatelets synthesized using plant extract and their potential for surface-enhanced Raman spectroscopy

Gold nanoparticles are increasingly used in biomedical fields due to their biocompatibility and their optical properties, such as tunable absorption of light and their efficiency for light to heat conversion, leading to recent developments in applications like photothermal therapy. In this work, gold nanoparticles and nanoplatelets were synthesized using an Aloe Vera leaf extract, without any additional surfactant, and functionalized using 16-mercaptohexadecanoic acid or L-Cysteine. The latest grafted nanoparticles and nanoplatelets were also further functionalized using Alexa Fluor NHS ester, that covalently binds to the amine group of L-Cysteine. They were fully characterized using Scanning/Transmission Electron Microscopy, UV-vis-NIR absorption and Raman spectroscopy The synthesized gold particles possess a large diversity of shapes: platelets with a triangular or hexagonal profile, tetrahedra, decahedra, icosahedra and bipyramids, but also polycrystals made of the assembly of two or three crystals. Their dimensions and proportions are correlated to the plant extract concentration used during the synthesis: the lower the concentration of the plant extract, the bigger the particles. Finally, the particles exhibit localized surface plasmon resonances correlated to their shapes. More, at the vicinity of the functionalized particles, we show that the Raman signal of 2,2’-bipyridine molecules is greatly enhanced, proving their potential use in SERS applications.