Formation of Highly Metastable β Glycine by Confinement in Inkjet Printed Droplets

Inkjet printing was used to deposit picoliter droplets of aqueous glycine solution onto glass or aluminum substrates and it was found that the volume constraint on crystal growth resulted in formation of the highly metastable beta form. When the droplet volume increased to 0.1 mu L or greater, a mixture of the alpha and beta forms was seen in a coffee-ring formation. When the droplet volume increased to 10 mu L, needle-like beta crystals formed in the bulk of the ring and bipyramidal alpha crystals on the outer edge at the air-water interface. The beta form was stable under ambient conditions for at least 6 weeks. The observations suggest that glycine nucleates to, and remains stable as, the beta form when constrained in small volumes, because on the nanometer scale the beta form is thermodynamically stable. As the droplet volume increases, and constraint on crystal growth is reduced, the alpha form becomes thermodynamically stable, and so phase transformation occurs, probably facilitated by a solvent-mediated mechanism. Inkjet printing picoliter droplets may offer a new route to isolation of metastable polymorphs.