Figure 2.

Cyclam-based fluorescent probes for sulfur anions and reactive nitroxyl species. The Cu²⁺ complex 1 functions as a turn-on probe for HS⁻ 1, with fluorescence enhancement thought to arise due to demetallation of 1 by HS⁻ [68]. The cyclam-functionalised carbon dot system 2 (CD = carbon dot) functions as a probe first for Cu²⁺ (which quenches the intrinsic fluorescence of the CD) and then, using the resultant Cu²⁺ complex, for S²⁻ (which rescues the fluorescence emission, presumably by stripping copper from the macrocycle) [69]. The Cu²⁺-cyclam complexes 3 and 4 function as selective probes for nitroxyl (HNO) for applications in near-infrared fluorescence biological sensing [70,71].