CRYSTAL GROWTH & DESIGN
Mesoporous Calcite by Polymer Templating Miles G. Page,† Nadine Nassif,*,†,‡ Hans G. Börner,† Markus Antonietti,† and Helmut Cölfen*,† Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany, and Laboratoire Chimie de la Matie`re Condense´e de Paris, UMR 7574 CNRS - UniVersite´ Pierre et Marie Curie, 12 rue CuVier, 75005 Paris, France
2008 VOL. 8, NO. 6 1792–1794
ReceiVed September 18, 2007; ReVised Manuscript ReceiVed March 19, 2008
ABSTRACT: Layered calcite crystals with internal 10-50 nm pores are precipitated from aqueous solution containing a polymer-peptide conjugate that aggregates into pore-forming templates. The zwitterionic peptide sequence behaves as the “binding block” of a doublehydrophilic block copolymer, facilitating templating, nucleation inhibition, and stabilization of amorphous precursor particles. The formation of pores in inorganic crystals is of considerable interest especially due to high surface areas for catalysis, adsorption, storage, semiconductor activity, membranes, etc.1 High surface area mesoporous materials are well-known for example in transition metal oxides such as titania,2 zirconia,3 and in silica.4,5 They are usually synthesized via hydrolysis of a metal alkoxide precursor and templating of lyotropic surfactant or block copolymer phases, typically followed by calcination to obtain the final crystalline material.6 However, many technologically interesting minerals including calcium carbonate (CaCO3) are not available via alkoxide precursors and can only be precipitated from their ions in solution. Such minerals are typically obtained as dense, crystalline powders or single crystals, and thus the important question arises if minerals formed from their ionic precursors can also be templated in the mesoporous size range. Direct templating has been reported using hydrogels,7 and recently Lu et al.8 and Meldrum et al.9 observed CaCO3 surface porosity, directly templated by polymer latex particles of a few hundred nanometers. Internal pores appeared to be excluded in these cases, as might be expected due to high lattice energy of the calcite crystal. Experimental evidence has also emerged10 for mesoporous structures in CaCO3 under certain conditions. Growth proceeding via a mesocrystal type mechanism, that is, formation of superstructures by directed aggregation of initially stabilized nanoparticles,11,12 allows for the possibility of pore formation during recrystallization from (e.g.) amorphous calcium carbonate13,14 (ACC) to more dense, stable polymorphs, or by partially ordered aggregation15 leading to voids in aggregated nanocrystals. We now extend these concepts to show for the first time that, by inducing growth via nanocrystals stabilized by a meso-sized templating species, a direct templating effect can be achieved in the mesoporous size range. The present approach overcomes the problem of template exclusion due to the high crystal lattice energy, yielding mesoporous calcite crystal grains whose pores display rhombohedral calcite morphology. Double-hydrophilic block copolymers (DHBCs)16,17 efficiently mimic properties of those biomolecules found in CaCO3-based organisms (shells, sea urchin spines, or algae exoskeletons) that are presumed to control morphology and structure of the crystals. DHBCs consist of a solvating block and a binding block that can stabilize, in solution, nanoparticles that act as building units in a superstructure formation process, thus directing precipitation of minerals from aqueous solution. Control in functionality compared with native proteins is, however, rather limited. * Corresponding authors. (H.C.) Tel: +49-331-567-9513. Fax: +49-331-5679502. E-mail:
[email protected]. (N.N.) Tel: +33-1-4427-6552. Fax: +331-4427-6539. E-mail:
[email protected]. † Colloid Chemistry, Max-Planck-Institute of Colloids and Interfaces. ‡ Equipe Mate´riaux du Vivant, LCMCP, UPMC-EPHE-CNRS.
A peptide-polymer conjugate was synthesized incorporating a peptide-segment,18 arginine-glycine-aspartic acid (RGD), as the binding block, and poly(ethylene oxide) (PEO) as the solvating block. The amino acid sequence-defined peptide segment extends the DHBC approach by allowing tuneable and targeted interactions, with the possibility to adjust precisely specific interactions with, for example, surfaces,19 themselves,20,21 or biosystems.22,23 The RGD triad is the most commonly investigated adhesion sequence found in cell-binding domains of extracellular adhesive proteins such as fibronectin. Reciprocally, integrins on the surface of cells bind to the RGD-domain, allowing adhesion to other surfaces.24 The sequence is composed of three amino acids, with a basic (arginine) and an acidic (aspartic acid) group, both connected via a flexible, nonfunctional glycine moiety. It has an isoelectric point of pH 8.25, with a broad region of zero net charge between pH 6-10 and a net charge of
