Controlled growth of MOF crystals under microgravity mimicking conditions using microfluidic environments

Abstract

The classical nucleation theory states that crystallization processes generally occur via monomer-by-monomer addition, a mechanism that leads to the most stable solid phase, i.e. to the thermodynamic product. However, there are systems that prefer to form less stable phases that do not lay in the global minima of Gibbs energy. These phases require less energy to form, but at the end, they can be also guided to the most stable phase via successive phase transformations that can be triggered, for example, by changes in the solvent composition. This research theory, also known as mesoscale assembly, is expected to provide new insights for a better understanding of crystallization processes; a result that would enable unprecedented control during crystals growth. The main goal of this project is to study the mesoscale assembly of soft porous crystalline networks such as peptide-based metal-organic frameworks (MOFs). We focus on these materials as their crystallization pathway is especially dependent on the solvent composition. Additionally, the host group has recently demonstrated the control synthesis of peptide-based MOFs in microfluidic devices due to a control diffusion of reagents1. Accordingly, herein, we will study the mesoscale assembly of a MOF formed by the coordination of the tripeptide glycine-L-histidine-glycine (GHG) to Cu2+, hereafter called CuGHG, via the control diffusion of a solvent and a non-solvent across a polymeric membrane. To achieve this goal, we have used a double-layer microfluidic device in our investigations fabricated with polydimethylsiloxane (PDMS), vide infra. I observed that after doing a controlled diffusion of a solvent (e.g. water) and a non-solvent (e.g. ethanol) through the PDMS membrane of the double-layer microfluidic device and into a microreaction chamber containing an aqueous solution of GHG and Cu2+, a phase separation process occurs that leads to the formation of highly concentrated droplets. Eventually, these droplets coalesce resulting after sometime into a CuGHG crystal. This result is of particular importance as it demonstrates that MOFs can also follow mesoscale assembly before its crystallization occurs.