Crystallization in Patterns: A Bio-Inspired Approach
Original Entry by Michelle Borkin, AP225 Fall 2009
J. Aizenberg, Advanced Materials, 2004, 16, 1295-1302.
Many examples in nature can be found of biomineralization in which inorganic salts are assembled to form "functional minearlized tissues". This process occurs in very specific environments and is controlled by cells and various macromolecules. The research presented in this paper is a study of these processes and how to apply them to artificially produce crystals in a "bottom-up" approach. Conventional crystal production techniques take a "top-down" approach: grow one large single crystal and then cut-it-down into pieces meeting the correct size, orientation, etc. requirements. In a "bottom-up" approach, the growth of the crystals are governed by their initial physical and chemical conditions to produce the desired crystal. Examples in nature, as shown in Figure 1, are more complex than contemporary manufacturing technology can produce. Creating an effective approach is of great interest to the materials science industry.
This paper presents strategies for artificially mimicing natural "bottom-up" approaches to crystallization. The experimental set-up's presented attempt to incorporate the following features: crystal nucleation is governed by membranes, crystal properties are adjusted by ionic and soluable "organic growth modifiers", crystals have precise predetermined patterns, and crystallization can occur through a "transformation of a transient amorphous phase". The new experimental approaches presented are able to control during the crystallization process at the micrometer scale the transfer of mass across the surface, the molecular structure, and the sites of nucleation.
The key to growing the artificial crystals is to create a template with nucleation sites that will determine where growth occurs and possibly how it grows. Experimental techniques that have focused on mimicking crystal nucleation as governed by membranes use molecular assemblies (e.g. Langmuir monolayers, self-assembled monolayers (SAMs), surfactant aggregates, etc.) to pattern nucleation. Alternatively, experimental techniques that have focused on mimicking organic growth modifiers or ions/proteins in solutions use various additives to focus calcium carbonate precipitation into patterned crystalline structures. The research presented in this paper focused on SAMs since then allow one to both precisely pick the sites of nucleation, but also control some of the crystal growth orientation and patterning.
SAMs (self-assembled monolayers) are a self-organizing layer of amphiphilic molecules, in this case along a solid-liquid interface, in which the "head" group is attracted to the solid substrate and the hydrophobic "tail" end sticks-out into the solution. The self-attracting head group makes a tightly packed single molecule layer on the substrate. The research in this paper used SAMs of <math>\omega</math>-terminated alkanethiols since they will easily form crystaline patterns on a metal substrate and they are easy to chemically control. Also, the functional terminal ends were chosen from biological molecules that have been observed in nature to control calcium carbonate growth.
Two lithographic techniques were used to create nucleation templates for the SAMs. The first technique, called microcontact printing (<math>\mu</math>CP), is shown in Figure 2(a). An array of nucleation sites is created by using an elastomeric stamp "inked" with a layer of thiol with the desired pattern for the SAMs. The second technique, called topographically assisted assembly, is shown in Figure 2(b). A metal layer is evaporated onto the surface of another metal through a stencil thus creating the desired template for the SAMs. In this case nucleation occurs at the disorder regions in the SAMs where the different metals meet. Both techniques concluded in the upside-down suspension of the prepared substrate in a calcium carbonate solution for crystal growth. Also, as show in Figure 2(c), as soon as nucleation starts at specific sites than an ion flux is induced in these regions resulting in depleting the surrounding solution. Examples of the highly uniform and controlled outcome can be seen in Figure 3.
Finally, a third experimental technique was created to specifically mimic echinoderms (the brittle star for example, Figure 1(a)). This strategy involves an "amorphous-to-crystalline" transition: macromolecules are deposited on a patterned surface thus conforming to the shape with nucleation occurs at intracellular sites. A sketch of the experimental set-up as well as the outcome are shown in Figure 4.