Carbon nanotubes (CNTs) are promising to be the next generation of viable tools for bioapplications. Further advances in such bioapplications may depend on improved understanding of CNTs physical and chemical properties as well as control over their biocompatibility. Herein we performed a systematic study to show how acid oxidation treatment changes CNTs physical and chemical properties and leads to improved CNTs biocompatibility. Specifically, by incubating CNTs in a strong acid mixture we created a user-defined library of CNTs samples with different characteristics as recorded using Raman energy dispersive X-ray spectroscopy, atomic force microscopy, or solubility tests. Systematically characterized CNTs were subsequently tested for their biocompatibility in relation to human epithelial cells or enzymes. Such selected examples are building pertinent relationships between CNTs biocompatibility and their intrinsic properties by showing that acid oxidation treatment lowers CNTs toxicity providing feasible platforms to be used for biomedical applications or the next generation of biosensors. (C) 2012 Elsevier B. V. All rights reserved.