A zwitterionic squaraine dye with a large Stokes shift for in vivo and site-selective protein sensing.

A novel squaraine dye (SQ) exhibits improved fluorescence response toward protein detection by incorporation of a zwitterionic structure. With the aid of a dansylamide (DNSA) substituent, the new probe (DNSA-SQ) exhibits remarkable selectivity in binding to site I (a specific substructure in protein).