Gold nanoparticles (4-7 nm) were synthesized from tetraauric acid using various amino acids as reducing and capping agents. The gold nanoparticles were produced from the incubation of a AuCl4- solution with an amino acid at 80 degrees C for 20 min. Among the twenty amino acids tested, several amino acids produced gold nanoparticles. The color of the nanoparticle solutions varied with the amino acids used for the reduction. We adopted L-histidine as a reducing agent and investigated the effects of the synthesis conditions on the gold nanoparticles. The His and AuCl4- concentrations affected the size of the gold nanopartides and their aggregates. The pH of the reaction solution also affected the reaction yields and the shape of the gold nanoparticles. (C) 2014 Elsevier Inc. All rights reserved.