Templates can be defined as patterns that are used to reproduce a particular shape accurately. In chemistry, a template can be the structure of a compound that serves for the production of another compound with a complementary shape. Template molecules are used in many areas of chemistry, especially in molecular imprinting [1, 2], in supramolecular chemistry  and in the field of catalytic antibodies . In general, molecular imprinting consists of the copolymerization of functional and cross-linking monomers in the presence of a template molecule. Subsequently, the template molecule can be removed, revealing well-defined binding sites in which the arrangement of functional groups is particularly suited for binding the template molecule or other structurally related molecules. The templating step in a molecular imprinting process can be carried out in solution or on surfaces, leading to a great variety of materials with recognition properties . In supramolecular chemistry, templates are generally used as a driving force for bringing together molecular components. Such components may subsequently react and go on to form products in which the template molecule can remain entwined or from which the template can easily be removed. The use of templates in supramolecular chemistry has led to the development of interesting, new receptor molecules, and also to the construction of very complex molecular architectures that would otherwise have been impossible. In the field of catalytic antibodies, template molecules are "injected" into living organisms that are able to produce antibodies capable of specifically binding to the template. Such antibodies can subsequently be employed to catalyze reactions in which the transition state is analogous in structure to the original template molecule. Although templates are generally thought of as single molecules or ionic species, much larger self-assembled structures, held together by weak forces, such as hydrogen bonding, -stacking or van derWaals interactions, can also function in a similar manner. Examples of self-assembled systems that have been used as templates are the self assembled fibrillar network (SAFIN) structures of gels of low molecular-mass organic gelators (LMOGs) , vesicles [7, 8], organic crystals [9, 10], and even larger entities such as red blood cells  and virus cells . All of these particular systems have been employed as templates for a process known as transcription . With this process the morphology of the organic template can be transferred to an inorganic material, giving rise to otherwise unattainable, microscopically detailed inorganic structures. The process of transcription consists of several steps (Figure 1). First of all, the organic template is brought into contact with inorganic precursor molecules or small particles of the inorganic material that will ultimately be formed. This step takes place in solution, in the presence of a catalyst, and leads to the deposition of the inorganic material on the inner or outer surface of the organic template. The material obtained at this point is an organic-inorganic hybrid that may present interesting properties as such or from which the template can be removed, leading to a purely inorganic material whose morphology is directly related to the organic template. Template removal can be achieved by heat treatment , microwave irradiation , or washing with organic solvents [11, 15]. The procedure of template removal by heat treatment, also known as calcination, is the most common and was first introduced by Kawahashi and Matijevi-during their search to produce hollow spheres of yttrium compounds . This procedure consists of placing the organicinorganic hybrid material in a furnace and exposing it to very intense heat (up to 500°C) under aerobic conditions, thereby burning away/decomposing the organic template. Microwave irradiation is a more recent procedure for template removal; it relies on the heat generated by microwave irradiation and is therefore a more rapid process than calcination . Removing the organic template by dissolving it with an appropriate solvent would seem to be the simplest and quickest method for obtaining the transcribed, inorganic material. However, at times, residues of organic material remain entrapped in the inorganic structures, yielding impure products and making this method of template removal generally unsuitable. In this chapter, the transcription of LMOG gels into discrete, inorganic structures will be discussed, whereas the formation of continuous structures in which for example templated channels and cavities are present (e.g., macroporous silica or zeolites) will not be considered. Due to the non-covalent nature of the aggregates that constitute LMOG gel templates, they can present a very wide variety of morphologies: fibrous, tubular, ribbon-like, lamellar, and hollow spherical. Similarly, surfactant molecules can also self-assemble into a variety of shapes and, together with organic crystals and biomaterials, they can give rise to interesting transcribable structures. However, it is not the aim of this book to discuss these systems in detail, and relevant reviews can be found in the literature [6, 16]. In Section 2, the chemical reactions involved in the sol-gel chemistry necessary for the transcription process are presented and the interactions between inorganic precursors and various catalysts, mainly with regards to the formation of silica-based products, are described. In Section 3, the important choice of the gelator molecule for successful transcription of the template structure is discussed. The importance of positively charged moieties and hydrogen-bonding groups, covalently attached to the gelator molecule or non-covalently incorporated into the network, is explained. The great variety of inorganic shapes obtained from gel transcription and the relationship between such shapes and the properties of the gelator molecules that generate them will be presented in Section 4. The concepts presented will be illustrated with examples from the recent literature, and the first actual application of a transcribed inorganic structure will be discussed along with future perspectives for these materials in Section 5.
|Title of host publication||Molecular Gels|
|Subtitle of host publication||Materials with Self-Assembled Fibrillar Networks|
|Number of pages||37|
|ISBN (Print)||1402033524, 9781402033520|
|Publication status||Published - 2006|
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)