Calligonum comosum stabilized fluorescent silver nanocluster-based dual optical sensing for aluminum and copper ions

Calligonum comosum-capped silver nanoclusters (CC-AgNCs) were synthesized through a simple, one-pot green route and developed as a dual optical probe for ratiometric sensing of Al(III) and "turn-off" sensing of Cu(II) in aqueous media. In this work, phytochemicals from Calligonum comosum simultaneously reduce Ag(I) and cap the resulting ultrasmall clusters, affording a stable, biocompatible, and dispersed fluorescent nanoclusters that exhibit a large Stokes shift, with a strong visible emission band at 488 nm and a weak band at 388 nm under UV excitation. The Calligonum comosum capping agent provides a rich coordination site for metal-ion chelation, offering a significant opportunity for sensing applications. TEM and DLS analyses confirm their diameter to be mainly in the 1-2 nm range. Upon interaction with Al(III), the optical response of the CC-AgNCs becomes strongly ratiometric: the emission at 488 nm is efficiently quenched, whereas the band at 388 nm is markedly enhanced with Al(III) concentrations over a useful analytical range. The calibration plot at 388 nm is highly linear with R² = 0.99, the LOD for Al(III) with the CC-AgNC probe is 1.085 nM, and the LOQ is 3.61 nM, demonstrating very high sensitivity. The Benesi-Hildebrand equation affords large binding constants of Al(III) at the CC-AgNCs. Kinetic measurements of the ratiometric response show a stable plateau within about 100 s, enabling rapid analysis without a long incubation period. In contrast, Cu(II) induces a pronounced "turn-off" of the 488 nm emission. A linear decrease in the fluorescence intensity is observed between 0 and 1.2 µM Cu(II) with R² = 0.99, from which the LOD and the LOQ were calculated to be 4.39 nM and 13.3 nM, respectively. The analytical applicability of CC-AgNCs was validated using real water samples. Overall, the CC-AgNCs offer a sustainable, cost-effective, and highly sensitive fluorescent platform for ratiometric detection of Al(III) and Cu(II) (quenching), combining green synthesis, rapid response, good reversibility, and successful application to real samples including mineral and tap water samples.