Controlling the size and shape of gold nanoparticles in fulvic acid colloidal solutions and their optical characterization using SERS

The size and shape of nanoparticles can be controlled by varying the pH and concentration of the fulvic acid (FA) colloidal solution. FA thin solid films with embedded Au nanoparticles have been fabricated, and their optical field enhancing properties tested in surface-enhanced Raman scattering (SERS). High-resolution transmission electron microscopy data indicated that the size of nanoparticles ranges from 4 to 15 nm for films obtained at pH 11 and 8. At pH 5, however, hexagonal and triangular particles of ∼50 nm and truncated triangles of ∼200 nm are produced with a high concentration of fulvic acid (250 mg L−1). The number and size of truncated triangles can be diminished by decreasing the concentration of FA in the colloidal solution. This unprecedented control of size and shape was preserved in thin solid FA films embedding the gold nanoparticles. The plasmon-assisted enhancing optical properties are demonstrated by SERS of 2-naphthalenethiol, an analyte with a well defined SERS spectrum.