Carbon nanotubes with improved biocompatibility prepared by template synthesis, which combines catalyst-free chemical vapor deposition (CVD) and chemical doping process

Abstract

Carbon nanotubes (CNTs) due to their unique electrical, thermal, chemical and mechanical properties have been extensively studied in terms of both fundamental and practical applications including electronics, energy storage, catalysis, fuel cells, solar cells, molecular separation, sensors, biosensors, and drug delivery. The diameter and morphology of CNTs are determined to be a key factor to govern the properties of CNT and extensive research has been directed to the growth of CNT structures with controllable dimensions. However, the presence of additional doping elements into CNT structure including nitrogen, boron, and phosphor also could considerable alter their conductivity, biocompatibility and chemical properties. In this work, we present several synthetic approaches to prepare multi-walled carbon nanotubes (MWCN) with controlled dimensions, with catalyst, catalyst-free and chemically doped with selected element (N). Our method is based on chemical vapour deposition (CVD) using nanoporous alumina (PA) as a template. The catalyst free carbon nanotubes (0-CNTs) and nitrogen doped (N-CNTs) were prepared in custom designed system set out with an ultrasonic generator, by the pyrolysis of liquid aerosol of toluene along with pyridine, over nanoporous alumina. The synthesis of 0-CNT/PA and N-CNT/PA composites with controllable nanotube dimensions such as diameters (30-150 nm), length, (2-20 μm), and chemistries (nitrogen doped) is demonstrated. The doped CNTs and un-doped CNTs have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy, and Raman spectroscopy. The results suggest that nitrogen groups were incorporated into the CNTs graphitic sheets, which decrease the graphitic sp2 content to less crystalline structure compared to un-doped CNTs. We observed an increasing trend in solubility of prepared CNTs with additional doping element N: catalyst free doped CNTs> catalyst free un-doped CNTs>catalyst CNTs, as a result of an increase in hydrophilicity. Hence, we predict that these catalyst-free grown N-CNTs will show significantly improved biocompatibility with minor cytotoxicity as a result of their structural properties (diameter and length), absence of catalyst and high level of solubility with less tendency of conglomeration compared to catalyst based MWCNTs or SWCNTs produced using conventional fabrication methods. These prepared N-CNT has potential to be applied as biocompatible CNT for number of biomedical and pharmaceutical applications.