Complete removal of external probe could thus in principle be achieved, while the good fluorescence properties of lucigenin should be preserved. Our attention was caught by the lucigenin derivative bis(3sulfopropyl)-9,9'-biacridine (SPBA), prepared by Werner et al. The presence of two charged sulphonate groups, its three-step synthesis, and Stern-Volmer constants only slightly lower than for lucigenin, made it an ideal candidate for testing. When prepared and tested, the measured fluorescence properties were found to be in agreement with the literature (