Template-dependent morphogenesis of oriented calcite crystals in the presence of magnesium ions.

The ability of biological systems to exert precise control over the shape, size, orientation, and hierarchical ordering of inorganic materials is of great interest to chemists and material scientists. Potential applications of this biological level of control to the development of new synthetic pathways and to the improvement of existing materials is beginning to be recognized. Biogenic calcium carbonate, one of the most abundant minerals formed by organisms, is a wonderful example of nature s precision. The elaborate and unique morphologies of biogenic calcium carbonates, which in the course of evolution are cleverly refined to accommodate diverse survival functions, have prompted extensive studies into the mechanism by which they form. The development of biogenic calcium carbonates was shown to be closely associated with proteins rich in acidic/hydroxy residues 4,8] and with divalent cations, such as magnesium ions. The formation of crystals with diverse morphologies has been reported by numerous in vitro studies, in which calcite growth was modified by the interaction with various organic and inorganic additives in solution. 12–20] Although these experiments demonstrated successfully that the morphology of calcite could be altered from its regular {104} cleavage rhombohedron form, the modified crystals were, in general, heterogeneous in their shapes, sizes, and orientations. In particular, calcite growth in the presence of Mg ions, in both synthetic and sedimentary environments, was shown to result in the formation of elongated crystals that express new pseudofacets that are roughly parallel to the c-axis. 13,21–24] This habit modification was traditionally explained by a facespecific mechanism involving interaction between the Mg ions and calcite surfaces parallel to the c-axis and recently by a step-specific mechanism involving differential incorporation of Mg ions into nonequivalent steps. It has been shown that such interactions between Mg ions and calcite lead to calcite rhombohedra developing characteristic, elongated, roughened seedlike shapes that depend on the ratio of Mg/Ca ions. The experimental data showed, however, that the sizes of Mg-ion-treated crystals, the extent of the elongation along the c-axis, the ratio between the {104} plane and the new Mg-ion-induced facets, and their precise {hkl} indices were nonuniform, even within the same experiment and under the same conditions. Herein, we report an approach to synthesize arrays of homogeneous calcite crystals in a variety of morphological forms. Crystal growth in the presence of an additive (Mg ions) is coupled with control over the oriented nucleation achieved by using functionalized selfassembled monolayers (SAMs) as nucleation templates. Experiments performed with a range of template/additive combinations demonstrated the formation of arrays of calcite crystals with highly uniform features (namely, size, shape, orientation, composition, and dynamics of growth), which are characteristic for each surface. We discuss possible mechanisms for this intriguing new phenomenon, which we have termed template-dependent crystal morphogenesis. The four surfaces used to control the oriented nucleation were Au films functionalized with thiols containing carboxylic acids of odd chain length (C15 CO2H), carboxylic acids of even chain length (C10 CO2H), alcohols (C11 OH), and sulfonic acids (C11 SO3H). These species were chosen to correspond to the functional residues of biomolecules commonly associated with biominerals. Although ordered functional groups on surfaces alone do not mimic biomolecules, they do provide insight into the importance of ordered arrays of functional groups that are often displayed on proteins repsonsible for the regulation of biomineral formation. Calcite crystals grown on the surfaces of SAMs in the absence of additives were normal {104} rhombohedra that selectively nucleated from different planes induced by the SAM/metal combination. 27] More than 90% of the crystals grown from the surface functionalized with C10 CO2H species nucleated from the (11l) crystallographic planes of calcite (l= 3–4) and more than 95% of crystals templated by the surface functionalized with C15 CO2H species nucleated from the (01l) planes (l= 2–5), as shown by X-ray diffraction and morphological analyses (Figure 1a,b). Interestingly, the surface functionalized with C11 SO3H species provided a mixture of crystals with the (106) and (1.0.12) nucleating planes in a ratio of 70:30 (Figure 1c). This latter observation suggests that the interfacial SO3H groups apparently conform to two different orientations/structures upon coordinating with the Ca ions (a possibility that is currently being investigated). More than 98% of the calcite crystals grown on the surfaces functionalized with C11 OH species selectively nucleated from the (104) plane (Figure 1d). The size and nucleation densities of the crystals grown on different templates are summarized in Table 1. Dramatic changes in the crystallization characteristics were observed upon addition ofMg ions to the solution. Small amounts of Mg ions (n 0.5, n=Mg/Ca ions (mol/mol)) induced significant homogenization of crystal sizes for three acid-terminated surfaces with no morphological changes (Table 1). The addition of Mg ions (n 0.5) to the crystallizing solution of the SAMs terminated by sulfonic acid groups favored the formation of crystals nucleating from the (106) crystallographic planes and the number of crystals nucleating from the (1.0.12) planes decreased dramatically. This result suggests that Mg ions are involved in the nucleation process, [*] Dr. Y.-J. Han, L. M. Wysocki, M. S. Thanawala, Dr. T. Siegrist, Dr. J. Aizenberg Bell Laboratories Lucent Technologies, 600 Mountain Avenue Murray Hill, NJ 07974 (USA) Fax: (+1)908-582-4868 E-mail: [email protected]