Biomedical Engineering Reference
In-Depth Information
The grafting of oxygen-containing groups using plasma processes has
been extensively reported. Hydroxyl, carbonyl and carboxyl groups were
grafted onto CNTs via O
2
and CO
2
RF plasma in order to form polar
groups that could improve the overall adhesion of CNTs to a polyamid
matrix (Bubert et al., 2003). Oxygen concentration of up to 14% was
determined by XPS, which was considered by the authors as equivalent to a
complete saturation of the outer CNT surface. The efficiency of oxygen
grafting in function of plasma frequency (RF or
μ
-wave) was determined
μ
through contact angle measurement, showing that
-wave plasma treatment
leads to better wettability (Chirila et al., 2005). Other plasma processes were
proposed to readily functionalize CNTs with oxygenated groups such as air-
atmospheric pressure dielectric barrier discharge (Okpalugo et al., 2005) or
air
-wave oven-generated plasma (HojatiTalemi et al., 2009).
Zschoerper et al. (2009) have characterized Ar/O
2
and Ar/H
2
ORF
plasma-treated CNTs by XPS. To overcome the limitations of XPS analysis
in distinguishing between functional groups with similar binding energies,
alcohol, keto/aldehyde and carboxyl groups were tagged using derivatiza-
tion techniques with fluorine-containing reagents. Trifluoroacetic anhydride
(TFAA) was used for the derivatization of alcohol groups, (trifluoromethyl)
phenylhydrazine (TFMPH) for keto/aldehyde groups and trifluoroethanol
for carboxyl groups through reactions carried out in the saturated vapor
phase. Despite the fact that the total oxygen content is almost identical,
variations in different functional group concentration are observed in the
function of treatment parameters such as pressure or treatment time. Based
on this work, CNT bucky papers were modified using Ar/O
2
plasma and
thereafter melt-mixed into polycarbonate (Po¨ tschke et al., 2009). Carboxylic
acid and ester groups were formed on the CNT surface, allowing better
macrodispersion and better phase adhesion to the matrix, as shown by
morphological investigations.
A flexible amperometric biosensor based on O
2
RF plasma-functionalized
CNTs films on polydimethylsiloxane substrates has been developed (Lee
et al., 2009). The plasma-treated samples presented better glucose response
than non-treated ones, showing their potential application as biosensors.
The O
2
RF plasma treatment was also found to improve the sensing
potential of CNTs to detect NO
2
(Ionescu et al., 2006) or to clean CNT
surfaces by removing amorphous carbon (Rawat et al., 2006). In fact, it has
been observed that purification of carbon nanotube powder can be attained
after CNT treatment in glow discharges (RF or
μ
-wave). Amorphous
carbon domains are eliminated and the impurities are removed by ion
bombardment and irradiation in an O
2
RF plasma (Xu et al., 2007).
However, it was also reported that the average diameter of multi-walled
CNTs decreases with treatment duration. Figure 14.10 shows a three-step
model proposed to explain this decrease. First, ion bombardment causes the
μ
