Toxicology of Carbon Nanotubes - A Review

A large number of nanoparticles are present in the environment in which some are unintentionally produced; ultra fine particles or intentionally produced engineered nanoparticle (ENPs). The carbon based ENPs include singlewalled and multi walled carbon nanotubes (SWCNTs and MWCNTs), spherical fullerenes and dendrimers. Among all ENPs, the carbon based ENPs are attracting much attention for potential biomedical applications, such as biosensors design, drug design, drug delivery, tumor therapy and tissue engineering, because of their electronic, optical and mechanical properties. The pristine CNTs are inert in nature so it needs to be functionalized to make it reactive. The functionalization appends different functional group e.g. C=O, C–O, –OH and –COOH to CNTs, which make it dispersible and suitable for different applications. The biocompatibility of these functionalized CNTs and their composite has to be tested before real time applications in the biological system. Determining the toxicity of CNT is the most persistent questions in nanotechnology. Inconsistent reports on toxicity of CNTs often appear in the literature and a mechanistic explanation of the reported toxicity remains incomprehensible. Results from various scientific tests on cells have so far proven confusing, with some results indicating it to be highly toxic and others showing no signs of toxicity. Several toxicity mechanisms have been proposed for CNTs including interruption of trans membrane electron transfer, disruption/penetration of the cell envelope, oxidation of cell components, and production of secondary products such as dissolved heavy metal ions or reactive oxygen species (ROS).Toxicity of a CNT sample is dependent on its composition along with its geometry and surface functionalization. Several studies have suggested that wellfunctionalized CNTs are safe to animal cells, while raw CNTs or CNTs without functionalization show severe toxicity to animal or human cells at even moderate dosage.