Calcium phosphate growth beneath a polycationic monolayer at the air–water interface: effects of oscillating surface pressure on mineralization

Abstract

The self-assembly of the amphiphilic block copolymer poly(butadiene)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface and the mineralization of the monolayers with calcium phosphate was investigated at different pH values. As expected for polyelectrolytes, the subphase pH strongly affects the monolayer properties. The focus of the current study, however, is on the effect of an oscillating (instead of a static) polymer monolayer on calcium phosphate mineralization. Monitoring of the surface pressure vs. mineralization time shows that the monolayer is quite stable if the mineralization is performed at pH 8. In contrast, the monolayer at pH 5 shows a measurable decrease of the surface pressure already after ca. 2 h of mineralization. Transmission electron microscopy reveals that mineralization at low pH under constant oscillation leads to small particles, which are arranged in circular features and larger entities with holes of ca. 200 nm. The larger features with the holes disappear as the mineralization is continued in favor of the smaller particles. These grow with time and form necklace-like architectures of spherical particles with a uniform diameter. In contrast, mineralization at pH 8 leads to very uniform particle morphologies already after 2 h. The mineralization products consist of a circular feature with a dark dot in the center. The increasing contrast of the precipitates in the electron micrographs with mineralization time indicates an increasing degree of mineralization vs. reaction time. The study therefore shows that mechanical effects on mineralization at interfaces are quite complex.

Crystal structure and chemical composition of biomimetic calcium phosphate nanofibers

“…The resulting fibers are morphologically very similar to those obtained previously (diameters below ca. 5 nm and lengths in the micrometer range) and to fibers reported in the literature, 27,28 but BF-TEM shows that mineralization from 20 mM solutions leads to more electron beam-resistant fibers and to a much higher number of fibers than mineralization from 2 mM solutions 13,14 ( Fig. 1).…”

“…To simplify the fiber synthesis we have, in contrast to our previous studies, 13,14 not buffered the reaction mixture, but used the same block copolymer and the same salt concentrations (2 mM) as in the previous work. Bright field transmission electron microscopy (BF-TEM) shows that this does not affect the outcome of the mineralization as in all cases fibers with diameters of a few nanometers form.…”

“…[28][29][30] We therefore infer that the fibers formed at 20 mM at the PnBMA 75 -PDMAEMA 147 block copolymer monolayers are chemically and crystallographically closely related to those reported previously. 13,14,[27][28][29][30] Fig . 2 shows high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDXS) data obtained from the fibers and nested structures.…”

“…[11][12][13][14][15][16][17][18][19] Likewise, the effects of (poly)cations on calcium phosphate and calcium carbonate mineralization have only recently begun to be addressed. 13,14,[19][20][21][22][23][24][25][26] According to these studies, there are indeed significant differences between (i) bulk and surface mineralization and between (ii) polyanion and polycationcontrolled calcium phosphate mineralization. Among others, poly(cation) monolayers at the air-water interface template the formation of intriguing fibers with diameters of only a few nanometers.…”

“…Among others, poly(cation) monolayers at the air-water interface template the formation of intriguing fibers with diameters of only a few nanometers. 13,14 As the fibers are many micrometers long and form extended structures, they could be interesting for the construction of hybrid biomaterials or as fillers for mechanical stabilization of composite biomaterials. Interestingly, the fibers resemble examples that have been made via precipitation from bulk solution using negatively charged polymer additives.…”