Carbon Nanotubes: An Emerging Drug Carrier for Targeting Cancer Cells

During recent years carbon nanotubes (CNTs) have been attracted by many researchers as a drug delivery carrier. CNTs are the third allotropic form of carbon-fullerenes which were rolled into cylindrical tubes. To be integrated into the biological systems, CNTs can be chemically modified or functionalised with therapeutically active molecules by forming stable covalent bonds or supramolecular assemblies based on noncovalent interactions. Owing to their high carrying capacity, biocompatibility, and specificity to cells, various cancer cells have been explored with CNTs for evaluation of pharmacokinetic parameters, cell viability, cytotoxicty, and drug delivery in tumor cells. This review attempts to highlight all aspects of CNTs which render them as an effective anticancer drug carrier and imaging agent. Also the potential application of CNT in targeting metastatic cancer cells by entrapping biomolecules and anticancer drugs has been covered in this review. 1. Introduction After the discovery of the third allotropic form of carbon fullerene in 1991, Sumio Iijima identified a new structural form of this allotrope, the cylindrical fullerene and named them as carbon nanotubes (CNTs) [1]. CNTs are graphene sheets rolled into a seamless cylinder that can be open ended or capped, having a high aspect ratio with diameters as small as 1nm and a length of several micrometers [2]. Depending on the number of sheets rolled into concentric cylinders, there are two broad categories of CNTs, namely, single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) (Figure 1). SWCNTs are made up of single graphene layer wrapped into a hexagonal close-packed cylindrical structure whose diameter varies from 0.4 to 3.0？nm and length ranges from 20 to 1000？nm and are held together by Vander Waals forces, which makes them easily twistable and more pliable [3]. SWCNTs are produced by the electric arc [4], laser ablation [5], chemical vapor deposition (CVD) [6], and gas-phase catalytic processes (HiPco or high-pressure CO conversion) [7]. Figure 1: Carbon nanotube: (a) single walled carbon nanotube and (b) multiwalled carbon nanotube. MWCNTs consist of several coaxial cylinders, each made of a single graphene sheet surrounding a hollow core. The outer diameter of MWCNTs ranges from 2 to 100？nm, while the inner diameter is in the range of 1–3？nm, and their length is 1 to several m [8]. Electric arc [9] and chemical vapor deposition (CVD) [10, 11] are the main techniques for their production. Owing to the sp2 hybridization in MWCNTs, a delocalized electron cloud