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 valine is demonstrated. Self-assembly of valine on the gold particles is accomplished in the aqueous phase, linkage with the gold particles possibly occurring through covalent interaction of the amine group with the surface gold atoms. Derivatization with the amino acid in this manner imparts amphotericity to the gold particles, the particles being negatively charged at pH values greater than 6 (the isoelectric point of valine) and positively charged below this pH. The charge reversal on the gold particles is demonstrated by electrostatic self-assembly of the colloidal gold particles on glass (which is negatively charged at pH > 3) by immersion of the substrate alternately 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 also described. The two-phase method described herein is extremely simple and results in hydrophobic gold particles which may be precipitated out of solution and redissolved in different nonpolar organic solvents without significant variation in the particle size distribution. The electrostatically driven multilayer assembly of the valine-derivatized gold particles on glass substrates, the phase transfer process, and the stability of the hydrophobized gold particles have been investigated with a variety of techniques such as UV-vis, infrared and X-ray photoemission spectroscopies, ellipsometry, and thermogravimetry/differential thermal analyses.