Reactive fluorescent dyes compositions and methods of using same are disclosed. Squaraine nucleus, Nile Red nucleus, benzodioxazole nucleus, coumarin nucleus or aza coumarin nucleus dyes are disclosed having thiol-reactive groups. Squaraine nucleus, Nile Red nucleus, benzodioxazole nucleus, coumarin nucleus or aza coumarin nucleus dyes are disclosed that exhibit a fluorescence emission of at least about 575 nm. Biosensors are disclosed having a binding protein and a squaraine nucleus, Nile Red nucleus, benzodioxazole nucleus, coumarin nucleus or aza coumarin nucleus.