Crystallization in Patterns: A Bio-Inspired Approach
Original Entry by Michelle Borkin, AP225 Fall 2009
J. Aizenberg, Advanced Materials, 2004, 16, 1295-1302.
Biomineralization, Artificial Crystallization, Self-assembled monolayers (SAMs), Nucleation
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.