Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 65

d n 3 r 4 n g | 5

Surface conductivity, K s , and the dimensionless Donnan potential, y D ,
of a starPEG-heparin hydrogel film (thickness B 600 nm) 45 in 0.1 mM
KCl solution at different pH values (a). The film was prepared at a
molecular ratio starPEG to heparin of 3, i.e., in the case of a quantitative
reaction of the starPEG, a conversion of 12 of the 24 carboxyl groups of
the heparin would occur. The experimental surface conductivity data
were reproduced by the theory (solid line). The dashed and dotted lines
show the contribution of K 1 ions and H 3 O 1 ions to the overall surface
conductivity. The simulation data on the right (b) illustrate the impact
of the cross-linking degree on the surface conductivity of a starPEG
hydrogel

Figure 3.3

film under similar conditions. For further details see

Zimmerman et al. 45
Reprinted with permission from R. Zimmermann, S. Bartsch, U.
Freudenberg, C. Werner. Electrokinetic analysis to reveal composition
and structure of biohybrid hydrogels, Anal. Chem., 2012, 84, 9592-9595.
Copyright 2012 American Chemical Society.

.

the heparin as well as the pH-dependent pattern of charge compensation.
For pH o 5, the magnitude of K s is determined by the number and mobility
of counter-ions that neutralize the charge of the strongly acidic sulfate
groups. In the absence of the less acidic carboxyl groups, a plateau would
arise at neutral and weak alkaline pH (see curve for the molar ratio of
starPEG to heparin of g ¼ 6 in Figure 3.3b, corresponding to a complete
conversion of the heparin carboxyl groups during the gel formation).
Therefore, the increase of the surface conductivity above pH 6 un-
ambiguously results from the ionization of non-converted carboxyl groups in
the film. To illustrate the possible range of variations of K s with varying
cross-linking degree, Figure 3.3b depicts simulation results for different
molar ratios of starPEG and heparin. In the case of highly cross-linked
hydrogels (g ¼ 6), all heparin carboxyl groups are involved in gel formation
and, as mentioned above, K s remains nearly constant in the neutral pH
range. In contrast, the dashed line in Figure 3.3b represents the upper
boundary of K s increase due to the ionization of carboxyl groups. Weakly
cross-linked hydrogel films 46 would show an increase in K s somewhat below
this hypothetical curve. The surface conductivity further reflects the