Effects of Polythiophene Surface Structure on Adsorption
and Conformation of Bovine Serum Albumin: A Multivariate and Multitechnique
Study
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Abstract
Protein interactions with surfaces
of promising conducting polymers
are critical for development of bioapplications. Surfaces of spin-cast
and postbaked poly(3-alkylthiophenes), regiorandom P3BT, and regioregular
RP3HT are examined prior to and after adsorption of model protein,
bovine serum albumin, with time-of-flight secondary ion mass spectrometry,
atomic force microscopy, and X-ray photoelectron spectroscopy. The
multivariate method of principal component analysis applied to ToF-SIMS
data maximizes information on subtle differences in surface chemistry:
PCA reveals alkyl side chains and conjugated backbones, exposed for
RP3HT and P3BT, respectively. Phase imaging AFM shows semicrystalline
microstructure of RP3HT and amorphous morphology of P3BT films. A
cellular-like pattern of proteins adsorbed on RP3HT develops with
coverage to more uniform overlayer, observed always on P3BT. The amount
of adsorbed protein, determined by XPS as a function of BSA concentration
(up to 10 mg/mL), is ∼21% lower for RP3HT than P3BT (up to
1.1 mg/m<sup>2</sup>). Although PCA differentiates protein from polythiophene,
relative protein surface composition evaluated from ToF-SIMS saturates
rather than increases with amount of adsorbed BSA from XPS. This reflects
ToF-SIMS sensitivity to outermost layer of proteins, enabling multivariate
analysis of protein conformation or orientation. PCA distinguishes
between amino acids characteristic for external regions of BSA adsorbed
to P3BT and RP3HT. These amino acids are identified for P3BT and RP3HT
as hydrophilic and hydrophobic, respectively, by relative hydrophobicity
of amino acid side chains. Alternative identification with BSA domains
fails, pointing to substrate-induced changes in conformation and degree
of denaturation rather than orientation of adsorbed protein