Preparation, Characterization and Application of Noble-Metal Colloids Stabilized By Chiral Molecules

Gold or platinum colloids stabilized with chiral compounds having a hydroxyl or amino group and multiple carboxylic acids (CHMCA) were prepared by a modified citric-acid method in which L-, D- or DL-tartaric acid, L-, D- or DL-malic acid were used instead of citric acid.  Controlling the molar ratio to metal and concentration of CHMCAs and pH of the reaction mixture during the colloid preparation process, CHMCA-stabilized gold or platinum colloids, without giving precipitates even when stored for long time, was successfully prepared.  

  The CHMCA-stabilized gold colloids showed a photoabsorption peak at arou  nd 520 nm, corresponding to the particle size of a few tens nanometer, and assigned to the surface plasmon resonance (SPR) and a peak in the circular dichromic spectra in the wavelength region of the SPR absorption though CHMCAs have no absorption there (Fig. 1).  These results suggest that CHMCA-modified metal nanoparticles, i.e., "chiral surface plasmon resonance (cSPR)" depending on adsorption rotation direction of circularly polarized light.  Application of the gold and platinum colloids loaded onto e.g., titania particles, to catalysis and photocatalysis using cSPR is discussed.