Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 69

adsorption of the ions at polar sites in the lipid head group region and to the
resulting variations of the head group orientation and lipid packaging
density. At the macroscopic scale, these effects manifest themselves in a
variation (here a decrease) of the phase transition temperature.
In summary, the results discussed in this section demonstrate the appli-
cation of electrokinetic measurements in combination with FRAP for ana-
lyzing the impact of charging and ion-specific interactions on phase
transitions in sBLMs. The methodology can be analogously applied to
practically more relevant membrane compositions and, thus, contribute to
clarifying the influence of these processes on membrane formation, stabil-
ity, structure, and fluidity.

d n 3 r 4 n g | 5

3.4 Summary
Electrokinetic measurements provide a versatile tool for analyzing the
charge and structure of soft biopolymer films in contact with aqueous so-
lutions. Recent progress was achieved in interpreting the electrokinetics of
complex systems by the development of sophisticated theories on the elec-
trohydrodynamics of diffuse soft interfaces, and with merging of experi-
mental techniques for the concomitant determination of surface
conductivity and film swelling. The fruitful options arising from these
various developments are illustrated here with selected case studies. In
particular, we provide insights into the experimental strategies and theore-
tical procedures for investigating the interrelations between the electric
charging and structure of stimuli-responsive coatings, and for evaluating the
key ionization characteristics, composition, and cross-linking degree of
biohybrid hydrogels. Finally, we demonstrate the suitability of combined
electrokinetic and FRAP measurements to understand the variations in
phospholipid bilayers fluidity through interactions with electrolyte ions.
These examples cover only a small range of biomaterials and processes that
can be studied by means of electrokinetics. By way of further illustration,
and when combined with other analytical methods such as reflectrometric
interference spectroscopy, 58,59 reflectometry 60 or Fourier transform infrared
spectroscopy, the methodology outlined here enables the in situ investi-
gation of adsorption-desorption processes and the analysis of the impact of
charge on the secondary structure of biopolymers at interfaces. Future de-
velopments will include the analysis of electrokinetics and surface con-
ductivity of soft films under dynamic ion-transport conditions. Such
measurements could extend our understanding on the binding and release
of charged analytes, e.g., proteins, nanoparticles or heavy metal ions, to/from
soft polymer materials. With the increasing complexity of the systems
studied, the field would also benefit from molecular approaches that take
into account ion-specific characteristics, spatial location, and chemical
environment of ionisable groups within the film and local fluctuation of
polymer chains.

.