In this study, nanoparticles were used to enhance bioethanol production in syngas fermentation by Clostridium ljungdahlii. Six types of nanoparticles were tested: palladium on carbon, palladium on alumina, silica, hydroxyl-functionalized single-walled carbon nanotubes, alumina, and iron(III) oxide. Of these, silica nanoparticles at a concentration of 0.3 wt% were the best at enhancing gas-liquid mass transfer. The hydrophilic surfaces of silica nanoparticles were modified with hydrophobic functional groups such as methyl and isopropyl. Methyl-functionalized silica nanoparticles were better than unmodified and isopropyl-functionalized silica nanoparticles at enhancing mass transfer. The dissolved concentrations of CO, CO2, and H-2 were enhanced by 272.9%, 200.2%, and 156.1%, respectively, by using methyl-functionalized silica nanoparticles. The use of methyl-functionalized silica nanoparticles at a concentration of 0.3 wt% during syngas fermentation by C. ljungdahlii led to significant increases in the levels of biomass, ethanol, and acetic acid production (34.5%, 166.1%, and 29.1%, respectively). (c) 2014 Elsevier Ltd. All rights reserved.