Deposition from dopamine solutions at Ge substrates: an in situ ATR-FTIR study

Deposition from dopamine (DA) solutions at germanium (Ge) model substrates was monitored under stationary conditions using surface sensitive in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. ATR-FTIR spectra of the interfacial organic layer formed upon contact of TRIS buffered aqueous DA solutions to a Ge internal reflection element (IRE) showed conveniently strong diagnostic IR absorption bands, which were increasing with deposition time up to at least 6 h. Comparison of IR spectra of unreacted pristine DA, surface reacted, and bulk reacted material confirmed chemical reactions of DA to a polymerizate according to the literature. The found IR bands could be assigned to aromatic as well as C-O single bond moieties. The kinetic courses of the diagnostic band integrals showed an initial increase and saturation of the deposition after around 300 min, which could be empirically represented by an exponential damping function revealing a rather small kinetic constant. Highest deposition levels were found at pH = 8.5 (TRIS buffer or NaOH) in contrast to pH = 6.2, where no deposition occurred. Minor deposition was found in the presence of salt or at ZnSe instead of Ge due to the absence of reactive hydroxyl groups. The concentration dependence of DA deposition showed an initial increase and a saturation beginning at around 0.4 mg/mL (0.0022 M), where around 50 nm thick films featuring granular surface morphologies are formed. The adsorbed species are suggested to be smaller bulk reacted DA polymerizate particles with reactive end groups. Rinsing the formed films by pure TRIS buffer resulted in a time dependent release of deposited organic material by ≈23%, which could be represented by an exponential decay function. A saturation of the release after around 100 min and a larger kinetic constant compared to deposition could be determined. The released material is suggested to be larger aggregated bulk reacted DA polymerizate particles loosely bound to the surface by weak interaction forces.