Materials World Network : Heterogeneous Nucleation on Nanoporous Substrates

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This a joint research and education project between investigators at Johns Hopkins University in the US and Naomi Chayen at Imperial College in the United Kingdom, with a goal to study the heterogeneous nucleation of crystals of macromolecular species with diameters in the 1-10 nm range. Typical species with this mesoscale include proteins and macromolecules, and are distinguished from colloidal spheres (at larger scales) and molecules (at smaller scales) by a large degree of internal configurational entropy; for this reason, they are difficult to crystallize. The work here builds on an experimental observation made by Chayen that the nucleation rate of macromolecules on nanoporous substrates is greatly enhanced. The working hypothesis is that the nucleation rate enhancement is specifically linked to the idea that the entropy of crystallization of macromolecules is minimized on porous substrates when a highly random porosity with void size of order the macromolecule width is used. This idea competes with a published notion that the substrate curvature enhances crystallization, which we don?t think is relevant for macromolecules on porous substrates. A detailed study of the nucleation rate of macromolecules over porous substrates as it varies with supersaturation, temperature, surface chemistry, and pore size, is being performed using a new substrate material ? nanoporous gold made by dealloying of silver/gold alloys. Nanoporous gold is a random, uniform, mesoporous material whose mean pore size can be varied from 3-50 nm using only simple chemistry. Its surface is also easily chemically modified, and all these features together make it uniquely perfect for this kind of study. Nanoporous gold is also useful here because gold nanoparticles with equivalent curvatures can be made, and we can compare and contrast crystallization in solutions containing gold colloid to crystallization on porous substrates. The research effort supports an international collaboration between the Chayen group at Imperial College, London, specializing in protein crystallization and the Erlebacher group at Johns Hopkins University, Baltimore, which specializes in the synthesis and surface modification of nanoporous metals. By marrying these specializations, a fundamental study of heterogeneous crystal nucleation on nanoporous substrates will be efficiently pursued. The collaboration is structured with significant exchange of students and junior personnel. The results of this research will yield new understanding of the fundamentals of heterogeneous nucleation, with general applicability to protein and colloidal crystallization. Furthermore, the international collaboration will help to build networks in nucleation theory and experiment, bridging the disciplines of thin film metallurgy and solution phase crystallization, and involving undergraduates and graduate students all stages.