Environmental Engineering Reference
In-Depth Information
mass spectrometry, elemental analysis and X-ray spectroscopy. Solid-phase
synthesis greatly facilitates the purification of products and has become the
backbone of modern combinatorial chemistry, but the characterization of
products bound to the solid supports is more difficult. Solid-state NMR spec-
troscopy is one possible way to monitor a reaction, but in general, the products
can only be fully analyzed after cleavage from the support. When working with
reactions on monolayers, the problems of monitoring the reaction, determining
the products and estimating the yield become quite significant. The extremely
small quantities involved render most analytical tools useless, and very often, a
combination of techniques is necessary to prove the structure on the surface.
Monolayers on gold nano particles/colloids (20 nm size range) have been used
as models for 2D SAMs [33] and their reactivity studied by NMR spectroscopy
in solution. However, as these small particles are highly curved, it is not always
straightforward to extrapolate yields from nano particles to planar surfaces.
One of the advantages of SAMs on smooth, reflective surfaces, is that
reactions on these monolayers can be studied by a wide range of techniques
including infrared spectroscopy [34], infrared spectroscopic ellipsometry
(IRSE) [35], scanning electron microscopy [18], contact angle measurements
[36], atomic force microscopy (AFM) [37], surface plasmon resonance [38],
ellipsometry [39], low-angle X-ray reflectometry [40], surface acoustic wave
and acoustic plate mode devices [41], X-ray photoelectron spectroscopy [39],
sum frequency spectroscopy [42], quartz crystal microbalance [43], electroche-
mical methods [44], confocal and optical microscopes [45], secondary ion mass
spectrometry (SIMS) [46] and near-edge X-ray absorption fine structure
(NEXAFS) [47]. Details of these techniques are discussed elsewhere [21].
In practice, IR spectroscopy, ellipsometry and XPS are the techniques most
widely used to study chemical transformations, whereas AFM is particularly
useful to image-patterned surfaces. The introduction of fluorescent tags and
their detection using (confocal scanning) fluorescence microscopy is widely
used to study the attachment of labeled biomolecules to a substrate. The
quantitative analysis can be quite difficult, however, instead of determining
the yield while the molecules are in the monolayer, it is also possible to cleave
the products from the solid support and analyze the molecules 'off-line'. Using
very sensitive analytical tools, even the tiny amounts of material cleaved from
substrates can be characterized. Butler et al. [48] measured the efficiency of
phosphoramidite-based oligonucleotide synthesis on surface tension arrays
using capillary electrophoresis of cleaved products.
3.2.5 Surface Modification of SAMs
The modification of surface properties through the selection of the appropriate
terminal functional group in the monolayer has led to the development of an
emerging research field 'surface organic chemistry', where the aim is to control
