Biomedical Engineering Reference
In-Depth Information
the core of the almost spherical aggregates, termed
micelles
. This shields
the hydrophobic components of the surfactant molecules from the aqueous
environment and positions the hydrophilic regions towards the outer surface
of the micelles. The outermost part of the micelle is hence composed of these
hydrophilic groups which maintain the solubility of the aggregates in an
aqueous environment.
15
Surfactants have the ability to suspend individual CNTs by distributing the
charges over the graphitic surface and by modifying the particle-suspending
medium interface, which prevents their re-aggregation over longer periods
of time.
16
They provide an additional repulsive force (electrostatic and
steric) which reduces the surface energy and alters the rheological surface
properties, which in turn contribute to enhancing suspension stability.
13
Micelles are increasingly being employed as solubilising and stabilising
agents for nanoparticles, such as CNTs, for two reasons. Firstly, they act to
stabilise and hence disperse the inherently hydrophobic CNTs, but they also
reduce their high toxicity.
17
Sodium dodecyl sulphate (SDS) is an example of
a traditional surfactant, one of the most widely used and extensively studied
surfactants; however, it only produces stable CNT suspensions at very high
concentrations, and SDS itself has raised concern regarding toxicity issues.
18
Phospholipids are natural amphiphiles that occur in the cell membrane.
They are therefore biocompatible and pose signiicantly less risk than other
non-biocompatible surfactants. It has been found that lysophospholipids
(Fig. 1.3),
or single-tailed phospholipids, can form supramolecular complexes
with SWCNTs and offer unsurpassed solubility for SWCNTs compared with
other surfactants such as SDS.
19
A comparison of SWCNT solubility with four
different pure phospholipids - lysophosphatidylcholine (LPC), dimyristoyl
phosphatidylcholine (PC), 1,2-dioleoylphosphatidylglycerol (PG) and 1,2-
dipalmitoylphosphatidylethanolamine (PE) - in a phosphate buffered saline
(PBS) solution showed complete solubilisation of CNTs by LPC following one
hour of bath sonication.
11
In the same paper, by Wu
et al
.,
11
a comparison of
SWCNT solubility in LPC, lysophosphatidylglycerol (LPG) and SDS solutions
revealed LPC to show superiority over the other two lipid agents in dispersing
CNTs in PBS. The authors attributed this to LPC's possessing a bulkier head
group for interaction with water and a longer acyl chain for binding to
SWCNTs. Furthermore, the experimental data in this article revealed that LPC
exhibited enhanced binding afinity for SWCNTs compared with LPG and that
single-chain phospholipids showed exceptional solubilisation of SWCNTs
while double-chained phospholipids were ineffective.
11
It has been recently
shown that the binding of lysophospholipids onto CNTs is dependent on the
charge and geometry of the lipids and the pH of the solvent and is not affected
by the temperature of the solvent.
12
Additionally, it has been demonstrated
that solubilising SWCNTs with lysophospholipids is more effective than
solubilising them with nucleic acids, including both single-stranded (ss)
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