Electron donor solvent effects provide biosensing with quantum dots

A palmitate biosensor that uses the emission intensity of a semiconducting nanoparticle to report palmitate concentration is presented. This method uses electron transfer to quench the emission from a ZnS-coated CdSe nanoparticle. The fatty acid binding pocket of intestinal fatty acid binding protein is used to modulate the electron transfer properties of Ru(L)(NH3)4](PF6)2 (L = 5-maleimido-1,10-phenanthroline) that is covalently attached within this pocket. Once the metal-complex-modified protein is attached to ZnS-coated CdSe nanoparticles, palmitate addition excludes water from around the metal complex and increases the electron transfer from the metal complex to the valence band hole of the nanoparticle excited state. A 1.6-fold change in emission intensity is observed upon adding a saturated amount (500 nM) of sodium palmitate. The dissociation constant was calculated as 5 nM with a 1 nM lower limit of detection. Since palmitate does not alter the global conformation of intestinal fatty acid binding protein, palmitate-mediated changes in pocket solvation are suggested. This represents a new method in biosensor construction with semiconducting nanoparticles. Including previous conformation-dependent biosensors, there are thousands of potential analytes that can be detected with these strategies. Such biosensors will provide fluorescence contrast imaging reagents for small molecule analytes.