We compared the effect of different oxidizing agents on purification, functionalization and opening of double-walled carbon nanotubes. The oxidative treatments were realized in nitric acid solutions at different concentrations (3 M or 15 M), in a mixture of two oxoacids (conc. HNO 3 /conc. H 2 SO 4) or in sulphuric acid solutions of KMnO 4 or K 2 Cr 2 O 7. Most of these treatments were very efficient for purification (removal of residual catalytic metal nanoparticles and/or of disorganized carbon) but also caused secondary reactions such as shortening of the nanotubes, creation of functionalized amorphous carbon deposits and covalent functionalization of the outer wall. Secondary treatments were undertaken in order to remove functionalized carbon deposit by washing with sodium hydroxide solutions or by heat treatment in air. A partial filling in solution was obtained with uranyl nitrate, in order to evidence the opening of carbon nanotubes. Effects of purification and filling treatments were characterized both qualitatively by TEM and HRTEM, AFM and Raman spectroscopy, and quantitatively by elemental chemical analysis and chemical titrations.

Different effects of multi-walled carbon nanotubes (MWCNTs) on the freshwater diatom Nitzschia palea were examined. MWCNTs used in this study (MWCNT) were dispersed either by sonication without (MWCNT sonicated) or with a realistic concentration (10 mg L(-1)) of Natural Organic Matter (MWCNT+NOM). A pocket-size device was designed to distinguish shading effect (using MWCNT suspensions as external filters) from total exposure effect of MWCNT sonicated and MWCNT+NOM on benthic algae. This study demonstrates that cell division was strongly inhibited after a 48 h exposure to MWCNT+NOM compared to MWCNT sonicated. This device did not yield a quantifiable contribution of shading to growth inhibition of MWCNT sonicated and below 10 mg L(-1) of MWCNT+NOM. In all cases, neither lethal effects nor drops in photosynthetic quantum yield were observed. After a 6-d exposure, a complete growth recovery was observed for all conditions except at the highest concentration of MWCNT+NOM. Different microscopic approaches using carbohydrates markers revealed the strong affinity between MWCNT and extracellular polymeric substances (EPS) produced by N. palea. These seem to constitute a defensive mechanism against MWCNT.