This paper describes a performance of precise control of shell thickness in silica-coating of Au nanoparticles based on a sol-gel process, and an investigation into X-ray imaging properties for the silica-coated Au (Au/SiO(2)) particles. The Au nanoparticles with a size of 16.9 +/- 1.2 nm prepared through a conventional citrate reduction method were used as core particles. The Au nanoparticles were silica-coated with a sol-gel reaction using tetraethylorthosilicate (TEOS) as a silica source, sodium hydroxide (NaOH) as a catalyst, and (3-aminopropyl) trimethoxysilane (APMS) as a silane coupling agent. An increase in TEOS concentration resulted in an increase in shell thickness. Under certain concentrations of Au, H(2)O, NaOH, and APMS, the Au/SiO(2) particles with silica shell thickness of 6.0-61.0 nm were produced with varying TEOS concentration. Absorption peak wavelength of surface plasmon resonance of the Au/SiO(2) colloid solution depended on silica shell thickness, which agreed approximately with the predictions by Mie theory. The as-prepared colloid solution could be concentrated up to an Au concentration of 0.19 M with salting-out and centrifugation. The concentrated colloid solution showed an X-ray image with high contrast, and a computed tomography value for the colloid solution with an Au concentration of 0.129 M was achieved 1329.7 +/- 52.7 HU. (C) 2011 Elsevier Inc. All rights reserved.