Red-Emitting Fluorescence Sensors for Metal Cations: The Role of Counteranions and Sensing of SCN - in Biological Materials.

The spatiotemporal sensing of specific cationic and anionic species is crucial for understanding the processes occurring in living systems. Herein, we developed new fluorescence sensors derived from tetrapyrazinoporphyrazines (TPyzPzs) with a recognition moiety that consists of an aza-crown and supporting substituents. Their sensitivity and selectivity were compared by fluorescence titration experiments with the properties of known TPyzPzs (with either one aza-crown moiety or two of these moieties in a tweezer arrangement). Method of standard addition was employed for analyte quantification in saliva. For K ⁺ recognition, the new derivatives had comparable or larger association constants with larger fluorescence enhancement factors compared to that with one aza-crown. Their fluorescence quantum yields in the ON state were 18× higher than that of TPyzPzs with a tweezer arrangement. Importantly, the sensitivity toward cations was strongly dependent on counteranions and increased as follows: NO ₃ ^- < Br ^- < CF ₃ SO ₃ ^- < ClO ₄ ^- ≪ SCN ^- . This trend resembles the chaotropic ability expressed by the Hofmeister series. The high selectivity toward KSCN was explained by synergic association of both K ⁺ and SCN ^- with TPyzPz sensors. The sensing of SCN ^- was further exploited in a proof of concept study to quantify SCN ^- levels in the saliva of a smoker and to demonstrate the sensing ability of TPyzPzs under in vitro conditions.