Salmonella and Escherichia coli are foodborne pathogens responsible for several illnesses worldwide. Such bacteria, in small amount are not dangerous for human health but, when the food and the water are poorly managed, they can multiply and spread causing foodborne diseases. Standard methods of detecting these bacteria in contaminated food samples, are labour-intensive and time-consuming. Therefore, one of the challenges in food industry is the development of methods for the rapid detection of low level of Salmonella and Escherichia coli. Immunosensors, based on the interaction antibody-antigen, have proved to be powerful and reliable tools in the detection and monitoring of an ever-increasing number of contaminants in food and water. Conventional antibodies immobilization procedures, based on covalent and non-covalent interactions, require chemical treatments, modification of the surface and does not ensure a correct orientation of the antibodies. To face this issue, at the Physics Department of University of Naples “Federico II”, a Photochemical Immobilization Technique (PIT) was proposed in the last decade. PIT is a simple and rapid procedure able to steer antibodies in a convenient orientation of the Fab region once immobilized onto gold surface. In this thesis, PIT was used to realize a QCM-based immunosensor for the detection of Salmonella Typhimurium in chicken meat. The simple protocol led to a limit of detection (LOD) less than of 10^0 CFU/mL with a pre-enrichment step lasting only 2 h at 37 °C. The reliability of the proposed immunosensor was demonstrated through the validation of the experimental results with ISO standardized culture method. Then, for the first time PIT was used to functionalize gold electrodes in order to develop an impedimetric immunosensor for the detection of Escherichia coli in drinking water. The immunosensor exhibited a limit of detection of 3 x 10^1 CFU/mL, with no need for pre-concentration and pre-enrichment steps. In both cases, the LODs were comparable or even better than other techniques used to quantify these bacteria and a negligible or slight cross-reaction was observed with other bacterial species. Although not suitable for accurate quantitative analysis, the immunosensors proposed in this thesis lend themselves as very attractive devices for “on-off” qualitative analysis of contaminated food and water. These results demonstrated that PIT is effective proved even on commercial cheap electrodes and this is a major achievement since in most situations careful surface treatments are required to get an effective sensor response.