Reagentless and reversible maltose biosensors are demonstrated using ZnS coated CdSe (CdSe@ZnS) nanoparticle emission intensities. This method is based on electron transfer quenching of unimolecular proteinâ CdSe@ZnS nanoparticle assemblies, which is provided by a protein-attached Ru^II complex. This Ru^II complex is presumed to reduce a valence band hole of the CdSe@ZnS excited state by tunneling through the ZnS overcoating. The Ru^II complex mediated quenching of CdSe@ZnS nanoparticle emission was only decreased 1.2-fold relative to the CdSe nanoparticle systems. While four different Ru^II complex attachment sites provided different amounts of nanoparticle emission quenching (1.20 to 1.75-fold decrease), all of these attachment sites yielded maltose-dependent intensity changes (1.1 to 1.4-fold increase upon maltose addition). Maltose dissociation constants for these four biosensing systems range from 250 nM to 1.0 µM, which are similar to the maltoseâ maltose binding protein dissociation constant that these sensors are based on. The increased fluorescence intensity was found to only occur in the presence of maltose. Furthermore, the ability of these reagentless proteinâ nanoparticle assemblies to perform maltose biosensing reversibly is demonstrated with the addition of α-glucosidase. Three 50 µM maltose additions after α-glucosidase addition showed increases of 2.2 µM, 600 nM, and 150 nM maltose. This result demonstrates a fluorometric method for examining α-glucosidase activity. Using maltose binding protein to control Ru^II complex interactions with CdSe@ZnS nanoparticle surfaces provide a novel class of highly fluorescent, photostable biosensors that are selective for maltose.