Jai -Pil Choi
4.1 Introduction
Nanotechnology is an emerging, interdisciplinary fi eld of both science and engineering that combines areas of physics, chemistry, biology, and material science. This technology allows scientists and engineers to design, fabricate, characterize, manipulate, and utilize materials that have well -defi ned structural features on the nanometer scale (1 -100 nm). At this scale, the physical and chemical properties of materials, so -called nanomaterials, can be different from those of bulk or of atoms/molecules having the same chemical composition. For example, when gold (Au) forms clusters consisting of 25 atoms (ca. 1.1 nm diameter), discrete molecular electronic states are developed, producing a HOMO -LUMO ( highest occupied molecular orbital -lowest unoccupied molecular orbital ) energy gap of 1.3 eV. In bulk Au, however, discrete electronic states merge into continuous bands, and no energy gap exists between an occupied conduction band and unoccupied valence band. This is a well -known example of the quantum -size effect and its physical or chemical effects are profound [1,2] .
Recently a wide variety of nanomaterials have been developed by synthetic methods that can reliably control both size and structure. The range of structure includes nanorods, nanowires, nanotubes, and nanoclusters, which encompasses nanoparticles, nanocrystals, and quantum dots. These nanomaterials are useful for developing chemical and/or biological sensors because of their high surfaceto -volume ratio and varied surface -dependent properties. Their large surface area per unit volume is an important factor in miniaturizing devices as well as increasing the performance and throughput of sensors. Their surface -dependent properties can be utilized to develop indirect sensing methods, whereby a change in a surface property induced by an analyte can be converted into a suitable response signal. In contrast, direct sensing methods are based on the intrinsic properties (e.g., spectroscopic, thermal, electrochemical) of the analyte. It is well known that the indirect approach dramatically expands the range of potential analytes and usually improves analytical performance, such as detection limit.