Ultrasensitive quantification of Hg²⁺ in real samples using colorimetric, fluorimetric and smartphone-based...

Mercury is one of the most toxic heavy metal, posing severe threats to environmental safety and human health due to its persistence, bioaccumulation, and detrimental neurological and systemic effects. Therefore, the development of fast, sensitive, selective, and on-site detection methods for Hg²⁺ remains an urgent priority. Herein, we present a phenylthiourea-derived indoline-fused chromenylium-cyanine chemodosimeter (INIR2), engineered as a "turn-on" near-infrared (NIR) fluorescent and colorimetric probe for Hg²⁺ recognition. Upon coordination with Hg²⁺, INIR2 undergoes a specific chemical transformation that results in a distinct color change and a remarkable fluorescence enhancement in the NIR region (λ_em = 766 nm) within 10 s. The probe exhibited ultralow detection limits (UV-Vis: 1.13 × 10^-8 M (2.27 ng/mL); fluorescence: 1.83 × 10^-8 M (3.67 ng/mL)) and outstanding selectivity among a wide range of competing metal ions. INIR2 also demonstrated robust performance across diverse matrices, including environmental waters (tap, drinking and lake) and food samples (corn and rice), achieving near-quantitative recoveries. Most importantly, the analysis of Hg²⁺ from micromolar to nanomolar levels (LOD: 2.10 × 10^-8 M) was performed using a smartphone-based RGB application among molecular turn-on probes. This method has significant potential for cost-effective and real-time monitoring of Hg²⁺ in the field, even at ultra-trace levels. Cellular studies confirmed low cytotoxicity and selective fluorescence activation in Hg²⁺-treated cells, further highlighting its bioimaging capability. These findings demonstrate that INIR2 is a highly sensitive and versatile tool for instantaneous and on-site mercury detection, with promising applications in environmental monitoring, food safety and biomedical research.