sensing of transition metal ions was developed by L. Fabbrizzi, among others. In chemosensing, the use of fluorophore connected to the receptor via a covalent spacer is now commonly referred to as fluorophores-spacer-receptor principle. In such systems, the sensing event is normally described as being due to changes in the photophysical properties of the chemosensor systems due to chelation induced enhanced fluorescence (CHEF), and photoinduced electron transfer (PET), mechanisms. In principle the two mechanisms are based on the same idea; the communication pathway is in the form of a through-space electron transfer from the electron rich receptors to the electron deficient fluorophores (through space). This results in fluorescence quenching (active electron transfer), and the emission from the chemosensor is 'switched off,' for both mechanisms in the absence of the analytes. However, upon forming a host–guest complex between the analyte and receptor, the communication pathway is broken and the fluorescence emission from the fluorophores is enhanced, or 'switched on'. In other words, the fluorescence intensity and quantum yield are enhanced upon analyte recognition. The fluorophores-receptor can also be integrated within the chemosensor. This leads to changes in the emission wavelength, which often results in change in colour. When the sensing event results in the formation of a signal that is visible to the naked eye, such sensors are normally referred to as colorimetric. Many examples of colorimetric chemosensors for ions such as fluoride have been developed. A pH indicator can be consider as a colorimetric chemosensors for protons. Such sensors have been developed for other cations, as well as anions and larger organic and biological molecules, such as proteins and carbohydrates. Design principles Chemosensors are nano-sized molecules and for application in vivo need to be non-toxic. A chemosensor must be able to give a measurable signal in direct response to the analyte recognition. Hence, the signal response is directly related to the magnitude of the sensing event (and, in turn concentration of the analyte). While the signalling moiety acts as a signal transducer, converting the recognition event into an optical response. The recognition moiety is responsible for binding to the analyte in a selective and reversible manner. If the binding sites are 'irreversible chemical reactions,' the indicators are described as fluorescent chemodosimeters, or fluorescent probes. An active communication pathway has to be open between the two moieties for the sensor to operate. In colorimetric chemosensors, this usually relies on the receptor