Amphoterization of colloidal gold particles by capping with valine molecules and their phase transfer from water to toluene by electrostatic coordination with fatty amine molecules

The surface modification of colloidal gold particles with the amino acid va line is demonstrated. Self-assembly of valine on the gold particles is acco mplished in the aqueous phase, linkage with the gold particles possibly occ urring through covalent interaction of the amine group with the surface gol d atoms. Derivatization with the amino acid in this manner imparts amphoter icity to the gold particles, the particles being negatively charged at pH v alues greater than 6 (the isoelectric point of valine) and positively charg ed below this pH. The charge reversal on the gold particles is demonstrated by electrostatic self-assembly of the colloidal gold particles on glass (w hich is negatively charged at pH > 3) by immersion of the substrate alterna tely into the valine-capped gold solutions maintained at pH = 3.5 and 8.5. The phase transfer of the aqueous valine-capped gold colloidal particles by electrostatic linkage with fatty amine molecules present in toluene is als o described. The two-phase method described herein is extremely simple and results in hydrophobic gold particles which may be precipitated out of solu tion and redissolved in different nonpolar organic solvents without signifi cant variation in the particle size distribution. The electrostatically dri ven multilayer assembly of the valine-derivatized gold particles on glass s ubstrates, the phase transfer process, and the stability of the hydrophobiz ed gold particles have been investigated with a variety of techniques such as UV-vis, infrared and X-ray photoemission spectroscopies, ellipsometry, a nd thermogravimetry/differential thermal analyses.