Polarity-sensitive (solvatochromic) probes are useful tools to investigate on subtle biochemical processes which occur at membrane and subcellular level (e.g., the uptake and cell targeting of a functionalized nanoparticle). In this paper, we describe the development of a new solvatochromic biosensor. The probe is structured as a "push-pull" system, bearing an electron-rich naphtyl ring and an electron poor benzothiazene group, conjugated to a central coumarin core. These features confer interesting spectroscopic and solvatochromic properties to the fluorophore. The dye was derivatized with a versatile sulfonyl chloride functional group. We also demonstrated that solvatochromic properties are nearly maintained when the fluorophore is covalently linked to proteins usually employed for nanoparticle coatings (bovine serum albumin or streptavidin). Finally, we proved the efficiency of our polarity-sensitive probe by obtaining a statistically significant difference between the fluorescence signals of fluorophore-labeled streptavidin before and after addition of BSA-labeled biotin.