A novel approach for the design of environmental sensors based on fluorescence interference contrast (FLIC) of semiconductor nanocrystals near a reflecting silicon surface was investigated. The fluorescence intensity of the Hydrophobic CdSeS nanocrystals adsorbed from a 1 mM toluene solution onto a poly(2-vinyl pyridine) (P2VP) brush layer was studied. It was found that the fluorescence intensity of the CdSeS nanocrystals adsorbed onto the P2VP brush strongly depended on the surrounding solvent. It was observed that the solvent-induced switching of the fluorescence intensity was completely reversible and reproducible. This approach was found to provide advantages of extreme instrumental simplicity, high spatial precision of the measurements and a fast signal response.