Point-of-need detection of Pb2+ ions using a smartphone-enabled fluorescent paper sensor with modified calix[4]arene scaffold

In this study, we report the development of a novel fluorescence sensor based on calix[4]arene-crown[5] (C4CBy) functionalized with bansyl chloride as a fluorophore. The synthesized probe was characterized using various spectroscopic techniques, including MALDI-TOF-MS, FT-IR, ¹H NMR, ¹³C NMR, and 135 DEPT NMR. Emission spectroscopy analysis demonstrated that C4CBy exhibits high selectivity towards Pb²⁺ ions, leading to a significant enhancement in fluorescence intensity. This fluorescence enhancement is attributed to the ligand-to-metal charge transfer (LMCT) mechanism, with a linear response observed over a Pb²⁺ concentration range of 0–100 nM (R² = 0.9927) and a detection limit of 1.65 nM. A Job's plot analysis confirmed a 1:1 stoichiometric binding ratio between C4CBy and Pb²⁺ ions via fluorescence spectroscopy. To evaluate the practical applicability of C4CBy, the sensor was tested in real sample matrices, including drinking water, lake water, laboratory water, cow milk, buffalo milk, orange, lake soil, industrial soil, and farm soil samples, within a Pb²⁺ concentration range of 0–30 nM. Additionally, a disposable paper-based analytical device was developed, along with a smartphone-integrated portable sensing platform, providing a cost-effective, user-friendly, and highly selective approach for Pb²⁺ detection.