Angew. Chem. Int. Ed.http://doi.org/fz46fv (2012)

Nanoparticle films have been used for the resistance-based sensing of small organic molecules. These sensors rely on the change of resistance that occurs when the distance between nanoparticles alters in the presence of analyte molecules. Biological species, such as proteins, are too large to penetrate the interparticle regions of the films, however, and biosensors that exploit resistance-based sensing have not been realized so far. Now, Justin Gooding and colleagues have made a biosensor in which the displacement of antibodies from the surface of nanoparticles changes the interparticle distance and hence the resistance of the film. First, gold-coated magnetic nanoparticles are functionalized with a self-assembled monolayer containing amine functions, followed by covalent attachment of the antibiotic, enrofloxacin. Anti-enrofloxacin IgM antibodies bind to enrofloxacin, yielding antibody-modified magnetic nanoparticles that assemble between two interdigitated electrodes. In the presence of enrofloxacin, some antibodies dissociate from the nanoparticles and the resistance of the films decreases. The biosensor has a response time of only 40 minutes and is able to detect enrofloxacin in a spiked solution of milk.