Harmful bacteria are the most common cause of food- and waterborne illnesses. Infection often leads to bloody diarrhoea, and occasionally to kidney failure. Several strains of the bacteria Escherichia coli produce a powerful toxin which causes serious illness. Food and water can be contaminated with other bacteria like Salmonella, Coliform, Pseudomonas, etc. Hence, it has become important to rapidly detect and identify infectious bacteria. Colloidal luminescent semiconductor nanocrystals or quantum dots (QDs) have elicited a great deal of interest in the biosensing community due to their unique fluorescent properties. Here ZnS : Mn2+ QDs are synthesised and biofunctionalised with chitosan. They are attached to the anionic cell wall of E. coli bacteria and different properties of this compound system are studied. These nanocrystals may offer cost effective and quicker alternative to detect single bacterium compared to other conventional methods. Several characterization methods are discussed. The particle size calculated is approximately 8–10 nm. The blue shift of PL peak has been observed after the bacteria get attached.