First ratiometric fluorescent hydrochar bulk from Lavandula multifida: A sustainable sensor for dose-dependent mercury detection

In this work, we report for the first time a simple, green, and economical one-step hydrothermal carbonization (HTC) route for synthesizing highly fluorescent hydrochar bulk (HC) from the aqueous extract of Lavandula multifida (Lamiaceae). The hydrochars prepared at 180 degrees C, 240 degrees C, and 300 degrees C were extensively characterized using TEM, FTIR, XRD, UV-Vis, and photoluminescence spectroscopy to elucidate the influence of temperature on their morphological, microstructural, and optical properties. Among the synthesized materials, the Lm 180 hydrochar exhibited the strongest fluorescence emission which was attributed to its richer content of oxygenated surface functional groups and organic chromophores. Excitation-dependent photoluminescence revealed the coexistence of multiple fluorophores, giving rise to blue, yellow, and green emission bands. Leveraging these unique optical features, Lm 180 was explored as a fluorescent probe for heavy-metal ion detection. Remarkably, it showed high selectivity for Hg2+ over Pb2+, Cd2+, Mn2+, Ni2+, and Co2+, displaying a dual-mode detection mechanism: a fluorescence turn-off response at 436 nm at low Hg2+ concentrations (1-10 nM) and a turn-on response at 530 nm at higher concentrations (1-10 mu M). This dual emission behavior enabled the development of a robust ratiometric fluorescent sensor based on the F436/F530 ratio, achieving excellent linearity (R2 = 1.00) and an ultralow limit of detection (LOD) of 0.21 nM Overall, this study demonstrates that Lavandula multifida hydrochar, produced via a sustainable one-step process, is a highly effective and low-cost ratiometric fluorescent sensor for trace mercury detection in aqueous solutions, with strong potential for application in real environmental samples.