Field-deployable fluorescent probe for rapid assessment of palladium and hydrazine contamination in real samples

The widespread use of palladium-based catalysts and the extensive application of hydrazine in industrial and agricultural processes have led to increasing environmental and biological concerns, necessitating the development of sensitive, selective, and field-deployable detection strategies. Herein, we report a reaction-based, dual-responsive fluorescent probe (HBI-S) capable of selectively detecting Pd²⁺ and hydrazine through well-separated emission channels. The probe was rationally designed by integrating an ESIPT-active hydroxybenzothiazole fluorophore, an indolinium electron-accepting unit, and a dimethylthiocarbamate reactive trigger. In its initial state, HBI-S exhibits weak fluorescence due to effective PET quenching. Upon Pd²⁺-induced cleavage of the dimethylthiocarbamate moiety, ESIPT and ICT processes are restored, generating a strong emission at 672 nm. In contrast, hydrazine induces both dimethylthiocarbamate removal and intramolecular spiro-cyclization, resulting in an intense cyan fluorescence centred at 475 nm. The probe demonstrates excellent sensitivity, with detection limits of 1.52 μM for Pd²⁺ and 2.80 μM for N₂H₄, along with rapid response times (<2 min), high selectivity, and neutral or physiological pH tolerance. The sensing mechanisms were validated by ¹H NMR, TOF-MS, and TD-DFT calculations. Practical applicability was further confirmed through portable test strips, smartphone-assisted analysis, and successful visualization of Pd²⁺ and N₂H₄ in soil, water, food matrices, hydrazine vapor, and living cells. Collectively, HBI-S provides a robust and versatile platform for real-time environmental monitoring of Pd²⁺ and hydrazine contamination.