Biomedical Engineering Reference
In-Depth Information
Figure 3.18
Magnetically confi ned impurity - containing
SWCNTs on the sidewall of the saw-tooth nickel
microstructure induced by a permanent magnet. Locally
confi ned impure SWCNTs were repeatedly concentrated in the
regions of highest magnetic-fl ux density. Reproduced with
permission from Ref. [41]; © 2007, Wiley-VCH Verlag GmbH
& Co. KGaA.
high-purity SWCNTs, due to the relatively low magnetic fi eld intensity employed
and crude control of the SWCNT solutions. Consequently, a microfl uidic device
for magnetophoresis was used for the purifi cation of SWCNTs, with highly
enhanced magnetic fl ux density gradient and precise fl uidic controls assisted by
microfl uidic manipulation techniques.
In Figure 3.18, SWCNTs containing the metal catalysts are shown trapped by
the highly enhanced magnetic fl ux density gradient across the microfl uidic
channel. Because the SWCNTs are too small to be visualized with conventional
microscopy, the analysis of SWCNT purifi cation should be assessed with indirect
methods such as TGA, TEM, SEM, and SQUID. Figure 3.19 shows two TEM
images of SWCNTs collected from the device, supporting the purifi ed SWCNTs
after a single - round purifi cation process of the microfl uidic magnetophoresis. The
relationship between the mass of the superparamagnetic materials and magne-
tized moments [107] allows an evaluation of the purifi ed SWCNTs compared to
the unpurifi ed samples [41]. As shown in Figure 3.20, the
M - H
curves of as-
prepared and purifi ed SWCNTs (ca. 10.0 mg) were measured using SQUID mag-
netometry at 300 K, and support the fact that purifi ed SWCNTs rarely contain metal
impurities. In these results, the saturation magnetic moment (
M
S
) should be cali-
brated to the net weight of SWCNTs because the purifi ed SWCNTs are mixed with
surfactant. The measured moment was 8.37
1 0
− 4
emu, where the sample weight
was 11.9 mg. The calibrated
M
S
(0.42 emu g
− 1
) was rather smaller than that of the
SWCNTs, albeit with 98.47
×
0.33% purity [107] . The practical
M
S
may be smaller
than as-calibrated, since the dilution factor (1/6) increases if the collected SWCNTs
are washed with methanol. When comparing these results with previously
published data [107], the purity of the purifi ed SWCNTs can be estimated as
±
