Off-On-Off Cascade Recognition of Cyanide, Mercury, and Aluminum Using N/5-Monosubstituted Rhodanines

This study aims to synthesize N- and 5-monosubstituted rhodanine derivatives as ion-sensing organics and investigate their sensing abilities. Following an easy and green approach to synthesis, the anion-sensing properties of the rhodanines were studied using colorimetric detection and spectroscopic methods. As a result of studies, rhodanines are found to be highly solvent-controlled colorimetric and fluorescent cyanide, mercury, and aluminum sensors. The stoichiometry of the interaction between CN^- and both probes was determined to be 1:1 using Job's plot analysis. The binding constants (K_s) of CN^- to 5-arylRh and -arylRh were calculated to be 3.25 × 10⁴ and 7.07 × 10⁴ M^-1, respectively, demonstrating their high affinity for cyanide ions. The limits of detections for the 5-arylRh and -arylRh were also determined as 356 and 617 nM, respectively. In addition to detecting CN^-, 5-arylRh also serves as a specific turn-off sensor for mercury and aluminum when cyanide and hydroxide are present. This enables the fluorescence intensity to be toggled on/off by alternating the addition of CN^-/OH^- and Hg²⁺/Al³⁺. Furthermore, the LOD values for Hg²⁺ and Al³⁺ with 5-arylRh-CN^- and 5-arylRh-OH^- were determined to be 414 nM and 1.35 μM, respectively. Furthermore, the turn-on binding mechanisms of 5-arylRh and -arylRh with cyanide ions were elucidated, and the experimental band gap (highest occupied molecular orbital/least unoccupied molecular orbital) energy values corroborated the proposed mechanism. Additionally, the interaction mechanism of the probes with CN^- was further investigated by using the ¹H NMR technique. Collectively, these findings suggest that 5-arylRh, -arylRh, and 5-arylRh-CN^- hold promise as selective and sensitive candidate sensors for CN^-, Hg²⁺, and Al³⁺ ions.