STABILISATION OF CARBON NANOTUBE SUSPENSIONS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 2

SW CNTs

MWCNTs

Figure 1.1 Schematic representation of CNTs in the form or either single-walled
(SWCNTs) or multi-walled (MWCNTs) tubes.

They have high mechanical and tensile strength and high electrical
and thermal conductivity. They exhibit both semi-metallic and metallic
behaviour and have large surface areas. 3 They possess outstanding chemical
and thermal stability. 6 The interaction between cells and CNTs, and hence
their internalisation into cells, needs to be clariied to ascertain their future
potential as drug delivery systems. 7 Numerous studies have been conducted
using biocompatible CNTs whereby CNTs have undergone covalent or non-
covalent functionalisation rendering them soluble in aqueous media, and
hence biologically compatible. 8 Overall, the general consensus is that CNTs are
capable of crossing many types of cell membranes; however, the mechanism
by which this occurs is not clearly understood, and there are discrepancies
between different authors. CNTs have been shown to be internalised within
cells by using a simple tracking process of CNTs labelled with a luorescing
agent. 9
It was initially observed that CNTs were capable of penetrating the
cell membrane primarily via a passive and endocytosis process. This was
conirmed to occur depending on the cell type and CNT characteristics,
such as surface charge or the nature of the functional groups attached to the
CNTs. 7 A hypothesis of functionalised carbon nanotubes ( f -CNTs) acting as
“nanoneedles” (Fig. 1.2) was proposed on the basis of images obtained from
high-resolution transmission electron microscopy (TEM), which showed CNT
interaction with mammalian cells where the CNTs adopted a perpendicular
orientation towards the plasma membrane of the cells during the process of
internalisation within the cells. It has been shown that CNTs can passively
traverse numerous types of cell membranes via a translocation mechanism
termed the nanoneedle mechanism. 5,7 These nanoneedles are the tiniest of
needles that have the potential to channel therapeutic agents into tumour
cells.