An electrochemical biosensor, based on an amplification method using nanoparticles, is being developed for bacteria detection. Firstly, silica nanoparticles with different sizes were synthesized and studied in interaction with Escherichia coli cells for their potential toxicity, by atomic force microscopy and viability tests. A critical diameter in the range of 50-70 nm was found under which nanoparticles triggered drastic membrane damage even leading to cell lysis. Secondly, harmless nanoparticles were used in combination with polyelectrolytes as a transducer. A two steps spin coating protocol enabled (i) to immobilize a polyelectrolytes multilayer and (ii) to adsorb a 100 nm – NPs solution at 1g.L-1. Lastly, we plan to conjugate these nanoparticles with antibodies specific to Escherichia coli for their selective and sensitive detection through an electrochemical sensor.