Editing Nanotechnology (section)

===Simple to complex: a molecular perspective===
{{Main|Molecular self-assembly}}

Modern [[chemical synthesis|synthetic chemistry]] can prepare small molecules of almost any structure. These methods are used to manufacture a wide variety of useful chemicals such as [[drug|pharmaceuticals]] or commercial [[polymer]]s. This ability raises the question of extending this kind of control to the next-larger level, seeking methods to assemble single molecules into [[supramolecular assembly|supramolecular assemblies]] consisting of many molecules arranged in a well-defined manner.

These approaches utilize the concepts of molecular [[self-assembly]] and/or [[supramolecular chemistry]] to automatically arrange themselves into a useful conformation through a [[Top-down and bottom-up#Nanotechnology|bottom-up]] approach. The concept of [[molecular recognition]] is important: molecules can be designed so that a specific configuration or arrangement is favored due to [[Noncovalent bonding|non-covalent]] [[intermolecular force]]s. The Watson–Crick [[base pair|basepairing]] rules are a direct result of this, as is the specificity of an [[enzyme]] targeting a single [[substrate (biochemistry)|substrate]], or the specific [[protein folding|folding of a protein]]. Thus, components can be designed to be complementary and mutually attractive so that they make a more complex and useful whole.

Such bottom-up approaches should be capable of producing devices in parallel and be much cheaper than top-down methods, but could potentially be overwhelmed as the size and complexity of the desired assembly increases. Most useful structures require complex and thermodynamically unlikely arrangements of atoms. Nevertheless, many examples of self-assembly based on molecular recognition in exist in [[biology]], most notably Watson–Crick basepairing and enzyme-substrate interactions.