A new ultra-fast and sensitive chemosensor based on NIR chromenylium-cyanine molecule modified with thiazole unit for the detection of Hg²⁺ in real samples

Since Hg²⁺ ion contamination constitutes a global threat, the necessity for a rapid, selective and sensitive sensing methods for Hg²⁺ ions has attracted growing research interest in recent years. Herein, we present the development of a new “ratiometric” and “turn-on” fluorescent probe NIR-12 having absorption and emission properties within the near-infrared (NIR) spectrum. The probe was constructed on a chromenylium cyanine platform decorated with 2-amino-5-bromothiazole for the purpose of efficiently detecting Hg²⁺ ions in an aqueous medium. Structural analysis of NIR-12 and its interaction with Hg²⁺ ions were confirmed by several spectrophotometric techniques. The non-fluorescent probe NIR-12 selectively interacted with Hg²⁺ ions, resulting in a ring opening on the platform. This process led to the generation of strong NIR absorption and emission signals, as well as a substantial color change. Consequently, this enabled the execution of colorimetric analyses. The probe responded to Hg²⁺ ions very rapidly within 10 s, and exhibited wide linear ranges (1.68-9.81 μM for UV-Vis and 0.85-42.98 μM for fluorescence methods) and remarkably low detection limits for Hg²⁺ ions (LOD) (0.057 μM (11.40 ppb) for UV-Vis, 0.277 μM (55.53 ppb) for fluorescence). The probe was also applied to real drinking water samples with high recovery ranges (95.13% - 106.62%). The application of smartphone-assisted analysis for Hg²⁺ ions was thoroughly investigated, demonstrating a linear range between 5.32 × 10⁻⁸ M and 9.49 × 10⁻⁶ M and a low LOD of 3.42 × 10⁻⁶ M.