Mechanistically regulated dual-mode fluorescence switching in a naphthalenyl-calix[4]arene platform for differential detection of Zn2+, Al3+, and 2,4,6-trinitrophenol

A naphthalenyl-functionalized Schiff-base calix[4]arene (NCX) was developed as a mechanistically regulated fluorescence sensing platform exhibiting dual-mode optical responses toward multiple analytes. The probe displayed distinct fluorescence turn-on behaviors in the presence of Al3+ and Zn2+ ions, with emission maxima at 468 nm and 500 nm, respectively, enabling differential signal discrimination. In contrast, the preformed NCX:Zn2+ complex exhibited pronounced fluorescence quenching upon interaction with 2,4,6-trinitrophenol (TNP), attributed to competitive binding-induced restoration of the photoinduced electron transfer (PET) process. Spectroscopic investigations indicated that metal coordination suppresses PET and restricts C=N isomerization, resulting in chelation-enhanced fluorescence, while subsequent interaction with electron-deficient nitroaromatic species reactivates non-radiative decay pathways. The sensor demonstrated high sensitivity with limits of detection of 36.6 nM for Al3+, 6.17 nM for Zn2+, and 0.78 nM for TNP. Binding studies revealed association constants of 3.15 × 103 M−1 and 9.51 × 104 M−1 for Al3+ and Zn2+, respectively, with a 1:1 binding stoichiometry confirmed by Job's plot, TOF-MS, and 1H NMR titration analyses. The NCX platform exhibited rapid response kinetics (<5 s), excellent stability over one month, and high selectivity under competitive conditions. Its practical applicability was demonstrated through successful detection of the target analytes in real river water and soil samples. This study presents a competitive binding-regulated fluorescence switching strategy associated with PET modulation, offering a versatile approach for differential multi-analyte detection in complex environmental systems.