Chemistry Reference
In-Depth Information
(3D) space group) is limited by the 2D data obtained. Tilt-angle cryo-TEM
overcomes this limitation by imaging multiple sample orientations for single
particles, allowing the alignment and study of different crystallographic planes
relative to the incident electron beam, as shown in Figure 4.11. (Barauskas et
al., 2005; Sagalowicz et al., 2006, 2007). Control of the tilt-angle allows greater
distinction between different dispersed LLC particles (liposomes, cubosomes,
micellar cubosomes, and hexosomes), and recording images at two to fi ve dif-
ferent tilting angles allows suffi cient data to perform crystallographic analysis
to be obtained in order to differentiate between Pn3m or Im3m cubic sym-
metries, for example (Sagalowicz et al., 2006, 2007).
4.3.2.3 Scanning Electron Microscopy and Cryo-Field Emission
Scanning Electron Microscopy (Cryo-FESEM)
Unlike TEM, in SEM, an
electron beam is raster scanned across the surface of a sample, and the second-
ary electrons, or X-rays, are “back-scattered” from a conductive coating applied
to the sample surface. This method generates a lower resolution image (typi-
cally 10-100 nm) but allows the direct mapping of surface features and can
even be used for elemental analysis. As such, SEM allows surface morphologi-
cal characterization of dispersed LLC systems but suffers from many of the
sample preparation problems discussed earlier with respect to noncryogenic
TEM techniques and hence has been less widely applied.
Spicer et al. (2002) employed a spray-drying method to produce dry powder
cubosome precursors encapsulated with hydrophobically modifi ed starch.
SEM indicated uniform encapsulation of the monoolein by the starch and
interrelationship between powder particle size and cubosome particle size
distributions. Using the same method, cubic phase particles were used as tem-
plates for infi ltration of silica precursor solutions. Figure 4.12 shows the SEM
Figure 4.12
SEM image of a silica cube templated by coating a cubosome with a silica
sol, drying, and then dissolving the cubosomes (Spicer, 2005b).
Search WWH ::
Custom Search