Metal ions play both functional and toxic role in biological systems. Many enzymes require metal cations as co-factors, for example, Se, Zn and Ca, are essential components of glutathione peroxidase, insulin and calmodulin, respectively. Yet excessive Cu and Zn have deleterious effects in that they contribute to the oxidative stress and inflammation in the central nervous system. The presence of metal ions in our environment, e.g. surface waters, also has important influence on our health. Therefore, monitoring the levels of metal ions in living oranisms and the environment is a key analytical issue in life sciences with links to clinical medicine and in pollution control. At present, monitoring metal ions is mainly based on measurement of average concentration, which enables detection of 1 ion in the sample consisting of few billions of other molecules. We propose developing a new approach for single molecule detection, which would allow finding 1 ion in one mole of other molecules. This step improvement in sensitivity can offer a new insight into the research on ion interactions on molecular level, as it removes the complexity associated with ensemble-averaged macroscopic measurements. The goal can be achieved by combining single molecule detection techniques, enabling observation of the volume as small as ~1 micrometer^3, with fluorescence technique based on counting single photons. The aim is to develop the sensor to be able to recognise a specific single metal ion by using an ion-selective molecular mechanism, namely fluorescence resonance energy transfer (FRET).