A molecular sensor or chemosensor is a molecular structure (organic or inorganic complexes) that is used for sensing of an analyte to produce a detectable change or a signal. The action of a chemosensor, relies on an interaction occurring at the molecular level, usually involves the continuous monitoring of the activity of a chemical species in a given matrix such as solution, air, blood, tissue, waste effluents, drinking water, etc. The application of chemosensors is referred to as chemosensing, which is a form of molecular recognition. All chemosensors are designed to contain a signalling moiety and a recognition moiety, that is connected either directly to each other or through a some kind of connector or a spacer. The signalling is often optically based electromagnetic radiation, giving rise to changes in either (or both) the ultraviolet and visible absorption or the emission properties of the sensors. Chemosensors may also be electrochemically based. Small molecule sensors are related to chemosensors. These are traditionally, however, considered as being structurally simple molecules and reflect the need to form chelating molecules for complexing ions in analytical chemistry. Chemosensors are synthetic analogues of biosensors, the difference being that biosensors incorporate biological receptors such as antibodies, aptamers or large biopolymers. Chemosensors describes molecule of synthetic origin that signal the presence of matter or energy. A chemosensor can be considered as type of an analytical device. Chemosensors are used in everyday life and have been applied to various areas such as in chemistry, biochemistry, immunology, physiology, etc. and within medicine in general, such as in critical care analysis of blood samples. Chemosensors can be designed to detect/signal a single analyte or a mixture of such species in solution. This can be achieved through either a single measurement or through the use of continuous monitoring. The signalling moiety acts as a signal transducer, converting the information (recognition event between the chemosensor and the analyte) into an optical response in a clear and reproducible manner. Most commonly, the change (the signal) is observed by measuring the various physical properties of the chemosensor, such as the photo-physical properties seen in the absorption or emission, where different wavelengths of the electromagnetic spectrum are used. Consequently, most chemosensors are described as being either colorimetric (ground state) or luminescent (excited state, fluorescent or phosphorescent). Colorimetric chemosensors give rise to changes in their absorption properties (recorded using ultraviolet–visible spectroscopy), such as in absorption intensity and