Materials with nanoscopic dimensions not only have potential technological applications in areas such as device technology and drug delivery but also are of fundamental interest in that the properties of a material can change in this regime of transition between the bulk and molecular scales. In this article, a relatively new method for preparing nanomaterials, membrane-based synthesis, is reviewed. This method entails synthesis of the desired material within the pores of a nanoporous membrane. Because the membranes used contain cylindrical pores of uniform diameter, monodisperse nanocylinders of the desired material, whose dimensions can be carefully controlled, are obtained. This "template" method has been used to prepare polymers, metals, semiconductors, and other materials on a nanoscopic scale.

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This paper reviews a relatively new method for preparing nanomaterials:  membrane-based synthesis. This method entails the synthesis of the desired material within the pores of a nanoporous membrane. Because the membranes employed contain cylindrical pores of uniform diameter, monodisperse nanocylinders of the desired material, whose dimensions can be carefully controlled, are obtained. These nanocylinders may be either hollow (a tubule) or solid (a fibril or nanowire). We call this approach the “template” method because the pores in the nanoporous membranes are used as templates for forming the desired material. This template method is a very general approach; it has been used to prepare nanotubules and fibrils of polymers, metals, semiconductors, carbons, and other materials.

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Membranes containing cylindrical metal nanotubules that span the complete thickness of the membrane are described. The inside radius of the nanotubules can be varied at will; nanotubule radii as small as 0.8 nanometer are reported. These membranes show selective ion transport analogous to that observed in ion-exchange polymers. Ion permselectivity occurs because excess charge density can be present on the inner walls of the metal tubules. The membranes reject ions with the same sign as the excess charge and transport ions of the opposite sign. Because the sign of the excess charge on the tubule can be changed potentiostatically, a metal nanotubule membrane can be either cation selective or anion selective, depending on the potential applied to the membrane.

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Nanochemistry is an emerging subdiscipline of the chemical and materials sciences that deals with the development of methods for synthesizing nanoscopic bits of a desired material and with scientific investigations of the nanomaterial obtained.12 Nanomaterials have numerous possible commercial and technological applications including use in electronic, optical, and mechanical devices,1-5 drug delivery,6 and bioencapsulation.7 In addition, this field poses an important fundamental philosophical question: How do the properties (e.g., electronic, optical, magnetic, Finally, the tubular or fibrillar nanostructures synthesized within the pores can be freed from the template membrane and collected. Alternatively, an ensemble of nanostructures that protrude from a surface like the bristles of a brush can be obtained (Figure 2B). Charles R. Martin obtained his B.S. in chemistry from Centre College of Kentucky in 1975. He did graduate work in the Department of Chemistry at the University of Arizona, with Henry Freiser, and postdoctoral work at the University of Texas, with Allen J. Bard. Martin began his academic career in 1981 in the Department of Chemistry at Texas A&M University. In 1990 he moved to the Department of Chemistry at Colorado State University. Professor Martin's research interests are in nanomaterials, electrochemistry, electronic conductive polymers, and polymer films for membrane-based chemical separations.

Polymeric membranes that contain a collection of monodisperse gold nanotubules, with inside diameters of molecular dimensions (less than 1 nanometer), were used in a simple membrane-permeation experiment to cleanly separate small molecules on the basis of molecular size. For example, when such a membrane was presented with an aqueous feed solution containing pyridine (molecular weight 79) and quinine (molecular weight 324), only the smaller pyridine molecule was transported through the nanotubules and into a receiver solution on the other side of the membrane.