Molecular Machines Useful for the Design of Chemosensor

Nature is filled with ions and molecules and we are frequently required to measure the concentration of selected ions and small organic molecules in both in vivo and in vitro processes. One possible approach is a direct application of biomaterials such as proteins, biomembranes, etc. created by nature after appropriate modifications. This approach has made a mark: for example, one can consider “biosensors” as a representative and successful system. However, this system, borrowing ideas from nature, is frequently hampered by their inherent boundary conditions such as poor solubility in organic media, instability at high temperature, degradation and denaturation, lack of broad specificity, etc., because they were created for nature’s own purpose and not for use in our research. Thus, the artificial molecular design toward man-made receptors which show high affinity and high selectivity comparable with naturally-made systems has long been a dream for scientists and has recently become a very active area of endeavor. However, one should notice that this research field is just one part in the design of a total sensing system: even though an artificial receptor precisely recognizes a selected ion or molecule, it is still useless unless the guest-binding event can be read out as a convenient physical signal. To exploit a total sensing system, therefore, the binding event must be transduced to some changes in a molecular system and eventually converted to some physical signals: that is, a total sensing system consists of three different components — (1) an ion or molecule recognition site, (2) a signal conversion site, and (3) a signal reading-out site.