DATA: THE NEW CORPORATE RESOURCE - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 9

Pluronics, or poloxamers, are commercially available block copolymers
that are extensively used in drug delivery. These consist of three polymeric
blocks, poly(ethylene oxide) x -poly(propylene oxide) y -poly(ethylene oxide) z
(PEO-PPO-PEO), with the central block being the hydrophobic one, forming
the core of the micelle, and the PEO being the hydrophilic block, forming a
hydrated micellar corona responsible for the biocompatibility of the polymers
and the prolonged in vivo circulation time of these systems. 24 Eficient use
of pluronics in the preparation of stable suspensions of individual SWCNTs
and MWCNTs was shown by TEM images. This was found to be true also
for polymer solutions of concentrations lower than the critical micellar
concentration (CMC) and at temperatures below the critical micellar
temperature (CMT); in fact it was found that the presence of SWCNTs
affected the value of the polymer CMT, suggesting that a new type of hybrid
between CNTs and pluronics is formed. 25 Differential scanning calorimetry
(DSC) studies revealed that SWCNTs form larger aggregates compared with
those formed by the polymer alone, while MWCNTs form smaller aggregates
than both SWCNTs and the polymer alone, because the small diameter of the
SWCNTs does not induce perturbation of the dynamics of polymeric assembly.
Therefore, the structure of the system is very similar to that of the original
micelle but with an elongated form. While the bigger diameter of MWCNTs
does not allow formation of the micelle as the core diameter, it is smaller than
the diameter of CNTs, and therefore the polymer adsorbs to the MWCNTs
forming a different type of aggregate. 25 These indings are further conirmed
by spin probe electron paramagnetic resonance (EPR) data, suggesting that
the formation of micelles in the presence of an SWCNT-polymer hybrid is
suppressed and the composite nanostructure dominates the system. 26
A similar approach to the stabilisation of CNTs in an aqueous environment
was applied by Wang et al. , 24 who used the triblock copolymer poly(ethylene
glycol)-poly(acrylic acid)-poly(styrene) (PEG-PAA-PS) (Fig. 1.5a). The
rationale behind the choice of this polymer is based on the fact that the PS end
can interact with the hydrophobic sidewalls of the nanotubes, while the PEG
end will stabilise the complex in the aqueous environment. The PAA core has
been introduced to allow cross-linking of the polymer once the CNT-polymer
complex is formed; this was identiied as a way to improve in vivo stability,
as previously prepared CNT-pluronic complexes were found to undergo
polymer displacement by blood proteins. 27 The so-called SWCNT PEG-eggs
showed eficient water dispersion and improved in vivo stability, and at the
same time the cross-linked coating did not diminish the CNTs' intrinsic near-
infrared (NIR) luorescence, which could be exploited in in vivo imaging, and
the complex did not show acute cytotoxicity. 27,28