Nucleation kinetics of calcium phosphates on polyelectrolyte multilayers displaying internal secondary structure

The understanding of the role of biological macromolecules in the nucleation of calcium carbonates and calcium phosphates is of primary importance for the development of biomimetic surfaces able to mimic the proteins used by living organisms to produce such composite materials. In this study we show how polyelectrolyte multilayers made from poly-L-lysine (PLL) as the polycation and from a mixture of two polyanions, poly-L-glutamic acid (PGA) and poly-L-aspartic acid (Pasp), at different ratios are able to affect the duration of the lag time preceding crystal growth at constant supersaturation. These polyelectrolyte multilayers appear as interesting candidates for biomimetic surfaces, since they display a content of beta-sheets that can be controlled by changing the PGA proportion in the polyanion mixture. The duration of the lag times preceding fast crystal growth is measured in situ with three complementary techniques, namely quartz crystal microbalance with dissipation QCM-D), optical waveguide lightmode spectroscopy (OWLS), and infrared spectroscopy in the attenuated total reflection mode (ATR-FTIR). The three techniques lead to comparable results, and ATR-FTIR spectroscopy allows us to demonstrate that the growing inorganic particles are calcium phosphates. The obtained mineral is most probably octacalcium phosphate or poorly crystalline hydroxyapatite, whatever the PGA content of the multilayer film. However, the duration of the lag time as well as the morphology of the obtained particles cannot be simply correlated with the beta sheet content of the multilayer film.