Device-Free Colorimetric Sensing of Lead in Water by Kinetically Trapping Lead(II) Rhodizonate into a Solid-solution of Cellulose Paper

Anthropogenic lead pollution remains a serious environmental concern due to the high incidence of human exposure worldwide and its severe health impact, particularly to children. We report a device-free colorimetric sensor for lead(II) ions in water based on the microdrop detection of the lead(II) rhodizonate complex trapped into a solid-solution of cellulose paper. This allows for the selective colorimetric detection of Pb²⁺ with a qualitative and quantitative detection limit of ~ 1 ppm. Comparative studies with Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺, Cd²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Fe²⁺, Hg²⁺, Co²⁺, Ce²⁺, Mn²⁺, Cr²⁺, Cr²⁺, and Au³⁺ show high detectability for Pb²⁺ due to the unique charge transfer band present in the kinetically trapped Pb-RA complex. We demonstrated that without the solid-solution matrix, the lead(II) rhodizonate complex polymerizes into an extended solid. The mechanism and kinetics of Pb²⁺ detection were studied by UV-visible spectroscopy, XRD, transmission electron microscope (TEM), and dynamic light scattering analysis (DLS). This device-free sensing method for Pb²⁺ is highly portable and suitable for applications on the field.