A dual-mode sensing platform based on novel Schiff base compounds for zinc ion detection and HeLa cell imaging via downconversion and NIR-excited upconversion luminescence.

We successfully synthesized two novel siloxane-containing Schiff base compounds, pBSIA and oBSIA, via a one-step process. These compounds exhibited remarkable upconversion luminescence properties, efficiently converting near-infrared (NIR) light into blue emission. Based on oBSIA, we developed a dual-mode sensing platform that integrates downconversion fluorescence and NIR-excited upconversion luminescence, enabling Zn detection and NIR-excited imaging of HeLa cells. Experimental results demonstrated that the oBSIA-Zn complex exhibited a nearly 30-fold enhancement in fluorescence intensity and over 500 % increase in absolute quantum yield compared to uncomplexed oBSIA. This ligand-enhanced fluorescence mechanism supports both qualitative and quantitative Zn detection. Furthermore, the probe exhibited low cytotoxicity and excellent stability, facilitating efficient penetration into living HeLa cells and enabling noninvasive upconversion imaging under NIR excitation. This approach effectively addresses the limitations of conventional fluorescence imaging techniques. The developed Schiff base compounds serve as a powerful tool for fluorescence sensing in biological systems. They also offer new opportunities for designing NIR upconversion bioactive molecules with potential applications in biosensing and bioimaging.