Rapid formation of coordination multilayers using accelerated self-assembly procedure (ASAP)

Layer-by-layer (LbL) assembly of multilayers on surfaces using metal-organic coordination between consecutive layers is a well-established method for multilayer construction. The basic scheme includes self-assembly of a ligand (anchor) monolayer on the surface, followed by alternate binding of metal ions and multifunctional ligand layers to form a coordination multilayer. Binding of the ligand repeat unit to form a new layer is commonly a slow process, taking typically overnight to complete. This renders the process of multilayer preparation exceedingly slow and, in many cases, impractical. Here we describe a method for LbL synthesis of self-assembled coordination multilayers denoted accelerated self-assembly procedure (ASAP), where binding of a full organic ligand layer occurs in ca. 1 min. In the new protocol a small volume of a dilute ligand solution is spread on the substrate surface and evaporated under natural convection conditions, leaving the surface covered with excess ligand. Extensive rinsing in pure solvent results in complete removal of unbound molecules from the surface, leaving only the new coordinated layer. ASAP is demonstrated here by the construction of two kinds of coordination multilayers, comprising mercaptoundecanoic acid-Cu(II) and bishydroxamate-Zr(IV). Multilayers prepared by ASAP and by the standard (overnight adsorption) procedure are compared using ellipsometry, contact-angle, and FTIR data, showing regular multilayer growth in both cases. However, the rapid binding associated with ASAP may lead to a different structure than the one reached after prolonged assembly. Study of the ASAP mechanism suggests that the fast ligand binding kinetics are attributed to a large increase of the local ligand concentration at the moving liquid front when the solvent evaporates on the surface.