Island Surfaces: Fabrication


Alexander Couzis Department of Chemical Engineering, City College of City University of New York

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Nanoisland patterned surfaces, in which functionalized nanodomains are surrounded by a matrix of secondary functionality, are scientifically and technologically important for their wide variety applications in biomimetic materials, templated nanocrystallization, induced reactions (as surface catalysts), etc. Islands terminated with particular groups can serve as direct sites for molecular recognition and can therefore be used in separations (e.g., as chromatographic supports). Designing the matrix to be nonadsorbing reduces the fouling of the surface and enhances the island activity for molecular arraying. For example, polyethylene glycol (PEG) terminations are particularly important for their resistance to nonspecific protein adsorption and thus are valuable in bioassaying, in which the receptors are often large molecules (ranging from a few nanometers in cross section for DNA to several nanometers for proteins). Corralling of these receptors into domains is not always effective because steric interaction limits the number of targets that can bind to the receptors. In this case, it is more advantageous to individually isolate the receptors onto nanoscale island sites, where only one receptor is anchored by the terminal group in the island. In addition, by anchoring the terminal group in recessed islands (pockets), the conformation of the binding site can be arranged for optimum accessibility of the target. For example, thiol functionalization of the island self-assembled monolayer (SAM) can be used to link a protein through disulfide linkages to cysteine residues. Another important potential application of nanoislands is for nanocrystallization. Due to their small size and unique electrical, photonic, and transport properties, nanoparticles have been the focus of much research intended to take advantage of their unique properties. The nanoparticulate form is a preferred route of delivery because of the ready passage of the particles through the circulatory system, and unimpeded transport across cell membranes. The ability to tailor the band gap structure of a semiconductor nanoparticle by controlling the particle size leads to useful technological applications. Recessed nanoislands can serve as vestibules for the crystallization of nanocrystals or the sequestering of nanosized particles such as liposomes. It is well recognized that chemically functionalized surfaces at the interface of a crystallizing solution can heterogeneously nucleate crystallites, sometimes with polymorph selectivity. Charged functionalizations are effective nucleators of ionic crystals as they attract oppositely charged counterions in the solution to the surface, which leads to the assembly of the incipient nucleus. Island patterned surfaces consisting of recessed nanodomains that have floors that are functionalized with active templates and that are surrounded by a matrix inert to crystallization can nucleate individual, nonagglomerating nanoparticles of controlled size. Particles covered with island cavities on their surfaces sequestering grown nanoparticles can subsequently be dispersed in an appropriate matrix (e.g., glass or polymers) to achieve a substantial collective response, avoiding the usual problems associated with nanoparticle agglomeration.