Calcium Phosphate Mineralization beneath a Polycationic Monolayer at the Air-Water Interface

The self-assembly of the amphiphilic block copolymer poly(n-butylmethacrylate)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface was investigated at different pH values. Similar to Rehfeldt et al. (J. Phys. Chem. B 2006, 110, 9171), the subphase pH strongly affects the monolayer properties. Further experiments show that the formation of calcium phosphate beneath the monolayer can be tuned by adjusting the subphase pH and hence the monolayer charges and organization. After 12 h of mineralization at pH 5, the polymer monolayers are still transparent, but transmission electron microscopy (TEM) shows that very thin calcium phosphate fibers form, which aggregate into “cotton-balls” with diameters of ca. 20 nm. In contrast, after 12 hours of mineralization at pH 8, the polymer film is very slightly turbid and TEM shows dense aggregates with sizes between ca. 200 and 700 nm. The formation of calcium phosphate is further confirmed by IR, Raman, and energy dispersive X-ray spectroscopy (EDXS). The different calcium phosphate architectures can be assigned to effects of monolayer charge, which are different at different pH values. The study thus demonstrates that the effects of polycations should not be ignored if attempting to understand the colloid chemistry of biomimetic mineralization. It also shows that basic block copolymers are useful complementary systems to the much more commonly studied acidic block copolymer templates.