Hydroxyl functionalization of single-walled carbon nanotubes causes inhibition to the bacterial denitrification process

Functionalized groups are often attached to the surface of single-walled carbon nanotubes (SWNTs) to improve their outstanding characteristics for more extensive applications. However, the potential impacts of SWNTs modified by hydroxyl groups (SWNTs-OH) on denitrifying microorganisms are unclear. In this study, the effect of SWNTs-OH on denitrification was investigated by the use of Paracoccus denitrificans as a model denitrifying microbe. The presence of 50 mg/L of SWNTs-OH, compared with the pristine SWNTs, was found to remarkably decrease the denitrification efficiency from 99.3% to 75.0%. The investigation of the mechanisms showed that SWNTs-OH inhibited the key enzymes responsible for glycolysis due to the increased properties of the dispersibility, the combinative potential with enzyme proteins, the possibility to interact with membrane, and the generation of reactive oxygen species. The metabolism of P. denitrificans utilizing carbon source (glucose) was therefore severely disturbed, and subsequently the growth of bacteria and the generation of electron donor (NADH) for denitrification were declined. Further studies revealed that SWNTs-OH also decreased the activity of nitrate reductase. It seems that the release of SWNTs-OH into the environment will cause severe disturbance of nitrogen cycle in biosphere.