BIOMEDICAL APPLICATIONS I: DELIVERY OF DRUGS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 28

stacking interactions take place with the graphitic sidewalls of CNTs, while
hydrophilic groups are exposed to the aqueous solution. Surfactants such as
sodium dodecyl sulfate (SDS) were found to be very effective in dispersing
SWCNTs. 115 Similarly, water-soluble polymers such as polyvinylpyrrolidone
(PVP) wrapped around the surface of SWCNTs facilitated the nanotubes'
dissolution in aqueous phases. 114
Even though water dispersibility of CNTs is extremely important and re-
aggregation of the tubes should be minimised, the above-mentioned procedures
do not always allow an effective, stable incorporation of additional bioactive
molecules. Therefore, such non-covalent functionalisation does not guarantee
eficient DDS. An exception in this sense is represented by a recent work by
Park and collaborators, 75 who designed a “trivalent” amphiphilic polymer
by preparing a polymer bearing both hydrophobic and hydrophilic residues
and thus able to disperse CNTs in water, and by subsequently integrating a
polyethylene glycol (PEG)-based copolymer useful as a solubilising agent
and as protection against protein adsorption (the so-called anti-biofouling
effect, which is often encountered during in vivo studies). Doxorubicin, an
antitumour agent was then condensed to nanotubes. The positively charged
amino group of the drug interacted with the carboxylic functions of the
polymer, while its aromatic rings stabilised the π - π stacking interactions
at the surface of CNTs. Non-covalent functionalisation with polyethylene
glycol 116 or the block copolymer Pluronic F127 76 was also used to adsorb the
same drug (DOX) onto SWCNTs and MWCNTs, respectively, but noticeably
different results might induce us to think that further mechanistic evaluation
should disclose whether the cell internalisation of CNTs is also affected by
drug-nanotube complexes. With a similar procedure, it was observed that
short ibres of CNTs were able to increase both delivery and absorption of
erythropoietin (EPO), thereby identifying key factors to improve oral delivery
of drug proteins. 77
2.3. “DEFECT” FUNCTIONAL IS ATION AT THE TIPS AND
SIDEWALLS
Besides non-covalent procedures, CNTs can also be cut and functionalised
simultaneously by simply treating them with oxidising agents such as HNO 3 ,
KMnO 4 /H 2 SO 4 , O 2 , K 2 Cr 2 O 7 /H 2 SO 4 or OsO 4 , thereby making them soluble
in polar organic solvents, acids and water without the aid of sonication,
surfactants or any other means. 59 - 61 After this treatment, it is possible to
use the carboxylic acid groups and the carboxylated fractions introduced by
oxidisation treatment, to further functionalise the nanotubes via amidation,