p53 is a tumor suppressor whose detection in the body is highly valuable as marker for early cancer diagnosis and prognosis. In this report we have studied constructs based on gold nanoparticles (NPs) functionalized with specific anti-p53 antibodies and with a fluoresceine derivative, FITC. The interaction of surface plasmons on gold NPs few nm in size, with fluorophores bound within few nanometers from the surface, induces changes in the fluorophore excited state lifetime. This parameter follows linearly the p53 concentration in solutions up to 200-400 pM, depending on the size of the NP, with an uncertainty approximatley =25 pM. We have evaluated the specificity of the nanosensor for p53 by testing it against bovine serum albumin, beta-lactoglobulin and lysozyme solutions. The titration of total cell extracts from p53 positive or p53-null cells with the anti-p53 antibody decorated gold NPs indicates that this construct is promising for possible applications to in vivo screening.