Bioactive Conducting Polymers for Optimising the Neural Interface - Biointerfaces: Where Material Meets Biology - page 197

extent of doping) of the conducting polymer through deconvolution of
characteristic elemental peaks. Combining XPS with time-of-flight secondary
ion mass spectrometry provides a powerful combination of analytical tech-
niques that allows determination of the presence of biological species on or
within the top 1-2 nm of the materials surface.

d n 3 r 4 n g | 2

TIP: Before chemical examination using XPS, samples should be cleaned with
a suitable media (e.g., alcohol, water) to remove contamination that remain
on the surface after synthesis, handling or are adsorbed from air. A typical
contaminant adsorbed from air is carbon (E B ¼ 285 eV) and its high concen-
tration skews the results obtained for other elements. Binding energies for
double bonded carbon that is present in the polymer structure is E B ¼ 285.7 eV
and deconvolution of the carbon peak allows for determination of contri-
butions of each component to the total amount of detected carbon. A typical
procedure to remove carbon contamination from polymers is sonication in
alcohol for 5 min (or other adequate solvent) following by sonication in ultra-
pure water for 5 min and drying/purging with nitrogen (providing that both
alcohol and water do not interact with or dissolve the polymer).

Another useful technique for analysing elemental composition, although
providing less detailed information than XPS and also probing to a greater
depth, is energy-dispersive X-ray spectroscopy (EDS) imaging offered by
scanning and transmission electron microscopes. EDS has the advantage of
spatially mapping chemical composition, however this method is limited
in its resolution and accuracy (detection limit of 0.1-0.5 wt%) compared
to XPS.
Since charge transfer is dominantly a surface based phenomenon, it is
also important to characterise the surface morphology of conducting poly-
mers. The surface morphology of a conducting polymer film is dependent
upon choice of dopant (and non-dopant inclusions) as well as the electro-
deposition parameters. 7 Analysis of surface topography is most commonly
conducted using scanning electron microscopy, which can provide a quali-
tative analysis of surface morphology and roughness at the nanoscale.
Techniques such as optical profilometry and atomic force microscopy
(AFM) can be used to give more detailed, quantitative analyses of surface
properties. Optical profilometry uses light-based probes to scan the surface
and determine its roughness with resolution in the micro- to nanoscale. AFM
characterises surfaces properties by scanning a material surface with a nano-
probe and measuring the probe-surface interactions. In addition to topo-
graphy and roughness, AFM is capable of spatial mapping of elastic modulus
and electrical conductivity at the nanoscale.

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8.2.3.2 Electrochemical Properties
There are several key electrical properties which require characterisation
in order to evaluate the electrical performance of a conducting polymer.