Nucleation and growth of glycine crystals on self-assembled monolayers on gold

Control of crystal morphology is critical in many pharmaceutical and food applications. Here we show that SAMs and mixed SAMs of rigid thiols on gold can serve as nucleation planes and modify the morphology of glycine crystals. Self-assembled monolayers (SAMs) and mixed SAMs of 4′-hydroxy-4-mercaptobiphenyl, 4-(4-mercaptophenyl)pyridine, and their mixed SAMs with 4′-methyl-4-mercaptobiphenyl were prepared on gold (111) surfaces and used as templates for the nucleation and growth of glycine crystals. Glycine nucleates in the α-glycine structure independent of hydroxy or pyridine surface concentration. The crystallographic planes corresponding to the nucleation surfaces, for the different SAM surfaces under study, were determined by interfacial angle measurements. For nucleation on 100% OH surfaces, the glycine crystallographic plane corresponding to the nucleation is {011}, whereas for the 0 and 50% OH surfaces, the crystallographic plane corresponding to the nucleation surface is a {h0l} face, probably {101}. For 25%, 75%, and 100% surface pyridine concentrations, the crystallographic planes corresponding to the nucleation are {010}, {121}, and {1105}, respectively. These differences are attributed to differences in H-bonding between glycine molecules in the nucleating layer and the SAM surface. As interfacial H-bonding increases, the dipoles of glycine molecules within the crystal become more perpendicular to the SAM surface. The direction of dipoles of glycine molecules that nucleated on a pyridine surface are not as close to the surface normal as those of molecules that nucleated on hydroxyl surface. This implies that the overall H-bonding interactions between the CO₂^- and NH₃⁺ groups of the glycine and the hydroxyl groups of the SAMs surface are stronger than those between the NH₃⁺ and the pyridine group.