Chemistry Reference
In-Depth Information
2.4 Analysis of Nanoscale Structure
2.4.1 Electron Microscopy
Perhaps more than any other method, electron microscopy is the technique that
opened up the nanoworld for closer inspection, and as such equipment has
become increasingly standard in many laboratories, the field of ''nano-
technology'' has become dominant.
32-34
In the simplest use of electron
microscopy to image the nanostructure of gel-phase materials, a thin layer of
gel is first allowed to dry on a substrate (either under ambient conditions or
in vacuo). The sample is coated under vacuum with a (ca. 2 nm) metallic layer
and then imaged by scanning electron microscopy (SEM). The SEM image
obtained in this way therefore represents a dried and treated sample. Usually,
the network structure of the gel collapses onto itself during drying to yield a
xerogel (if collapse does not occur, the structure is referred to as an aerogel). It
should be noted that structural changes other than collapse may also occur
during drying, however, it is often assumed that such effects are minor - at least
when comparing related families of gelators. Transmission electron microscopy
(TEM) can also be applied to gel imaging, although it is often necessary to
apply a heavy-metal staining agent to enhance image contrast.
A range of different gel morphologies can be observed using SEM.
35
In general
terms, transparent gels often exhibit nanoscale structuring, whilst opaque gels,
which scatter light, have larger microscale features. Typically, assembled
supramolecular polymer nanofibres are observed. Other types of ''one-dimen-
sional'' objects such as tapes/ribbons have also been reported (Figure 2.5).
However, it should be noted that generally, the observed nanostructure diam-
eters are much larger than the molecular diameter. This is a consequence of the
fact that molecular-scale fibrils often bundle together in some way to generate
larger nanofibres. However, the mechanism by which molecular-scale fibrillar
objects hierarchically assemble into the nanoscale fibres observed by electron
microscopy is often somewhat unclear - although in some cases, detailed models
to help understand this process can be developed (see Section 2.5.4).
36
In some cases, the chirality inherent at the molecular level is transcribed into
the nanoscale assembled objects and can be directly observed by electron
microscopy - usually in the form of helicity.
37
Indeed, in early work on gels, the
two enantiomers of lithium 12-hydroxystearate were shown by transmission
electron microscopy to assemble into left- or right-handed helical fibres, de-
pending on the molecular-scale chiral information (Figure 2.6). Remarkably,
this work was reported as early as the 1960s, and as such predates most of
supramolecular chemistry and the genesis of ''nanochemistry''.
38
It is worth
noting in passing that there is lots of precedent for studying gels within the
grease/lubrication literature, in which such materials have been applied for over
100 years,
39
and some of this has often been overlooked by academic chemists.
Cryo-electron microscopy techniques are used to try and minimise disruption
to the self-assembled network on drying.
40
A rapid freezing step is used in an
attempt to limit thermal motion of the self-assembled gelator network. Freezing
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