High Throughput Techniques for the Investigation of Cell–Material Interactions - Biointerfaces: Where Material Meets Biology - page 288

methyl iodide or similar. 107,126,195 The functionalisations described above
can easily be confirmed by infrared (IR) spectroscopy or X-ray photoelectron
spectroscopy (XPS). Terminal alkynes can also be immobilised onto pSi
through the use of either an anodic or cathodic electrografting. 194,196,197
Using a cathodic current, alkynes are attached directly onto the surface, still
retaining the triple bond functionality. Using an anodic current however, the
triple bond functionality is not retained, instead yielding an alkyl surface.
All of the examples listed above describe the modification of a pSi surface
to create uniform surface functionality; however, there are few reports to
date where this technique has been employed to generate chemical gradi-
ents on pSi. The preparation of gradients by electrografting is a relatively
straightforward method and uses the same electrochemical cell that is used
for the generation of pSi pore size gradients as described previously. Add-
itionally, the electrografting technique is arguably simpler and quicker in
terms of generating chemical gradients than many of the techniques de-
scribed in this chapter and hence, constitutes an ecient approach to
prepare substrate materials for a wide range of applications. Clements
et al. 108,198 generated chemical gradients of ethyl-6-bromohexanoate on
porous silicon substrates via electrografting. The chemical gradient was
prepared using an asymmetrical electrode placement, followed by end-
capping of unreacted silicon-hydride groups with methyl groups using a
parallel electrode placement. The resulting ester moieties were hydrolysed
and activated to produce a gradient of carboxylic acid functional groups.
After subsequent immobilisation of cyclic Arg-Gly-Asp- D -Phe-Lys
(c(RGDfK)), the surfaces were used to screen the extent of rat mesenchymal
stem cell (rMSC) attachment. Detailed methods and tips for the preparation
of electrografted and peptide density gradients are described below. Map-
ping of the surface chemistry was carried out by means of infrared micro-
scopy (IRm) and X-ray photoelectron spectroscopy (XPS). rMSC culture
studies showed that short-term cell attachment responded to the c(RGDfK)
density gradient present on the surface.
Thompson et al. 107 created gradients of methyl, pentyl acetate and decyl
groups on pSi substrates. Interestingly, gradients were generated using a
magnesium anode rather than the standard Pt electrode which lead to re-
duced oxidation of the surface. Subsequent cleavage of the pentyl acetate
groups to generate a hydroxy terminated surface allowed researchers to in-
vestigate the ethanol concentration in water samples.
Electrografting and peptide density gradient formation:

d n 3 r 4 n g | 7

.

1. Uniform pSi or gradient pSi substrates should be prepared as described
above.
2. Freshly etched pSi substrates were immediately transferred to an inert
reaction cell, brought under vacuum and then purged with argon.
Whilst under an argon atmosphere, 2 mL of alkyl halide solution was
transferred to the reaction cell and the cell was again sealed under an
argon atmosphere.