Size and surface coating control the reactive oxygen species (ROS) production from ZnO quantum dots

An innovative study aimed at understanding the influence of the size and of the surface coating of ZnO quantum dots (QDs) on their ability to produce reactive oxygen species (ROS) was undertaken. ZnO QDs with diameters varying between 3.5 and 5.0 nm were dispersed in water using aminosilanes (3-aminopropyltrimethoxysilane (APTMS), N-(2-aminoethyl)aminopropyltrimethoxysilane (AEAPS), or N1-(2-aminoethyl)-N2-[3-(trimethoxysilyl)propyl]-1,2-ethanediamine (AETPE)) to yield positively-charged nanoparticles. Zwitterionic- or negatively-charged ZnO QDs were prepared from aminosilanes-capped dots by reaction with propane sultone and succinic anhydride, respectively. Aqueous dispersions of these QDs were found to produce ROS, namely hydroxyl (OH*) and superoxide (O2*-) radicals and singlet oxygen (1O2), upon light activation. Results obtained demonstrate that ROS-production is associated with the size and the surface functionalization of the nanocrystals. A remarkable increase of ROS production is observed when decreasing the diameter of the dots from 5.0 to 3.5 nm. On the contrary, by varying the surface ligand from APTMS to AEAPS and AETPE, the ROS production decreased indicating that amine groups at the periphery of the dots can scavenge ROS produced from the ZnO core. Collectively, data obtained provide new insights of the toxicological impacts of ZnO QDs that can be exploited for engineering ZnO nanoparticles for intended applications (bio-imaging or therapy).