Control of the size and distribution of gold nanoparticles by unmodified cyclodextrins

We report and discuss how gold nanoparticles were synthesized by the reduction of hydrogen tetrachloroaurate(III) trihydrate by sodium citrate in the presence of unmodified alpha-cyclodextrin (CD), beta-CD, and gamma-CD. Gold nanoparticles were immobilized on poly(diallyldimethylammonium) chloride (PDDA) modified glass slides to enable AFM measurements. The particle size was dependent upon the type and concentration of cyclodextrin used as well as the sodium citrate concentration. An increase in the cyclodextrin concentration effected a shift of the particle size range from 12-15 to 4-6 nm with uniform particle size distribution. The homogeneity of the synthesized gold nanoparticles was also evident from transmission electron micrographs (TEMs). Synthesis of gold nanoparticles by the reduction of hydrogen tetrachloroaurate(III) trihydrate by sodium borohydride in the presence of cyclodextrins also reduced the particle size from 6-8 to 2-4 nm. The consecutive particle growth due to the mutual coalescence between nanoclusters and their neighboring free gold atoms was limited in the presence of CDs. FT-Raman, FT-IR spectroscopy, and mass spectrometry (MS) indicated that the synthesis procedure exhibited no effect on the cyclodextrins. There was no evidence that gold nanoparticles were included in the CD cavities.