The invention provides dyes, biosensors, and methods for using the dyes and biosensors to detect selected target molecules. The biosensors have a binding domain and a dye, or a dye which is attached directly to the target of interest. Binding domains contemplated by the invention include biomolecules or fragments of biomolecules that interact with target molecules of interest and can be specific to a given conformational state or covalent modification of the molecule (e.g. phosphorylation). In one embodiment, the binding domain of a biosensor is a single chain variable fragment (scFv) with a dye of the invention linked to a CDR3 region. The invention also provides environmentally sensitive dyes useful for detecting changes in the binding, conformational change, or posttranslational modification of the selected target.