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March 4, 2003

Polymers can Force Calcite to Form via Amorphous Mineral Precursors — and Synchrotron X-ray Studies can Reveal the Details

E. Dimasi¹, V.M. Patel², M. Sivakumar², M.J. Olszta², Y.P. Yang², and L.B. Gower^2
1Brookhaven National Laboratory, Upton, NY; ²University of Florida, Gainesville, FL

Scientists at Brookhaven National Laboratory and the University of Florida have used synchrotron x-ray scattering to monitor how mineral crystals and films form at an organic boundary surface, a process related to biological mineral formation. Beneath an organic monolayer, calcium carbonates crystallize rapidly from solution into calcite, but the addition of polymers produces an amorphous thin film and delays the conversion to crystalline calcite. The x-ray measurements reveal that the amorphous thin film forms through kinetic mechanisms rather than the previously assumed templating mechanism, where the atomic positions of organic functional groups match the lattice spacing of the mineral crystal.

In a process called biomineralization, some living organisms incorporate insoluble mineral compounds into their biological structures, creating biominerals that are usually hard but much less brittle than the inorganic (geological) minerals. Such biominerals are all the more fascinating because of the wealth of minerals having the same composition but different crystal structures used by animals. For example, calcium carbonates (CaCO₃) are abundant geologically but mainly as the stable minerals calcite and aragonite. But numerous marine invertebrates incorporate a broader range of calcium carbonates into their shells, such as the metastable vaterite and unstable amorphous hydrated calcium carbonates.

To study the biomineralization mechanisms, we have assembled organic molecules on a supersaturated CaCO₃ solution. The cations (Ca²⁺) in solution are attracted to the negatively charged monolayer of organic molecules at the surface, creating an ion-rich region that eventually crystallizes into one or more crystal forms of CaCO₃ (figure 1). Up to now, this process had never before monitored by a quantitative structural probe.

For the first time, we have determined the cation binding by analyzing the x-ray reflectivity from the initially assembled film. We found that, in the presence of polyacrylic acid – a soluble polymer which may mimic the action of biomolecules – the number of calcium cations bound to the surface layer was 80% less than in the case without polymer. Thus, the acidic macromolecules affect biomineralization primarily by reducing the concentration of cations at the monolayer. Instead, the charged polymer concentrates the ions to form a metastable amorphous layer, from which they can begin to crystallize against the overlying organic monolayer.

Without an appropriate organic template, this polymer-induced precursor initiates the formation of calcite homogeneously in solution, and, in some cases, we noticed that the amorphous precursor has a liquid-like consistency. This observation suggests that it may be possible for living organisms to “mold” biomineral crystals through a precursor mechanism. The organic substrate therefore controls the liquid-like or solid nature of the precursor material, a mechanism that may allow the fabrication of synthetic “biomimetic,” or biomineral-inspired, materials.

At beamline X22B, we have measured both x-ray reflectivity – sensitive to the surface-normal density profile of the film – and grazing-incidence diffraction – probing in-plane structure – during the 20-hour biomineralization process. The time series of reflectivity curves (figure 2-a) displays interference patterns showing that a thick film grows beneath the organic monolayer. Fitting a structural model to this data indicates that the film has a density comparable to amorphous, hydrated CaCO₃ phases, half as dense as the anhydrous crystalline forms.

During the 20-hour biomineralization process, diffraction patterns within the layer plane (figure 2-b) show that the organic monolayer retains a tightly packed, two-dimensional structure, but no crystalline diffraction from the underlying mineral is observed.

These x-ray measurements provide important new information: Biomineralization can occur through an amorphous precursor, even when no structural alignment between the organic monolayer and the mineral is present. Also, simple variations in polymer concentration and supersaturation have pronounced effects on mineralization rate. These results are in contrast to suggestions in the literature, which have relied, up to now, upon assumptions regarding the alignment of crystals to organic monolayer templates. Our study shows that in-situ structural probes are increasingly important in illuminating the mechanisms of biomineralization.

BEAMLINE
X22B

FUNDING
U.S. Department of Energy
National Science Foundation
National Institutes of Health

PUBLICATION
E. DiMasi (BNL), V. M. Patel, M. Sivakumar, M. J. Olszta, Y. P. Yang, and L. B. Gower (UFL), “Polymer-Controlled Growth Rate of an Amorphous Mineral Precursor Film Nucleated at a Fatty Acid Monolayer,” Langmuir 18 (2002) 8902.

FOR MORE INFORMATION
Elaine DiMasi
BNL Physics Department
Upton, NY
Email: dimasi@bnl.gov
Web: http://www.solids.bnl.gov/~dimasi/bones