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Download as PDFHeterogeneous Crystallization on Engineered Surfaces
Surfaces play a vital role in heterogeneous crystallization; surface properties such as chemistry, roughness and shape impact nucleation significantly [1] [2] [3] [4] [5] [6] [7] . Kim et al. applied bi-functional self-assembled-monolayers (SAM) surfaces to crystallize the metastable form β-glycine as small as several hundred nanometers [8] [9] . Diao et al. showed that choosing different polymeric surfaces of diverse chemical functionalities and different shapes [10] [11] [12] can strongly influence the nucleation of aspirin. Our group is exploring effects of SAMs and nanopores of varied geometries on polymorphic outcome in crystallization of nano-sized crystals. These studies would lead to better understanding of the fundamental mechanism governing heterogeneous nucleation.
We designed and fabricated gold islands on silicon substrate using electron beam lithography, photolithography, and electron beam evaporation. The gold islands were coated with hydrophilic thiol SAMs while hydrophobic silane SAMs were cast on the remaining silicon surfaces. The bi-functional SAMs substrate allows formation of droplets and crystallization in droplets on hydrophilic islands. We have produced the second stable α-form glycine crystals of lateral dimension 286±69 nm, 391±82 nm, 652±145 nm, and 862±179 nm. The corresponding heights are 38±18 nm, 73±28 nm, 107±34 nm, and 152±49 nm. We have also proven that polymorphs of micro-sized crystals can be controlled by using different SAMs. Using interference lithography, we designed and fabricated nanopoles of different angles 30°-150°. The nanopoles imprint the surfaces of biocompatible polymer, which are then used as heteronucleants during crystallization of active pharmaceutical ingredients. Initial results indicate that in the presence of square nanopores, nucleation occurred twice as fast as in their absence and twenty times faster than in the absence of polymer. Analysis through powder X-ray diffraction and Raman spectroscopy of crystals grown on the films confirmed the presence of the metastable form of mefenamic acid (form II). We are investigating the effect of these geometric sites in promoting nucleation by constructing micro-sized pores on silicon wafers via photolithography. We use the patterned surfaces to monitor crystallization events in situ to define the role of the angle during heterogeneous crystallization.
Figure 1. Glycine nano-crystals form on 1-µm gold islands: Left image is under optical microscope. Right image is under AFM.
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