AbstractA SiO₂-coumarin nanohybrid was investigated for its Cu(II) sensing performance in aqueous media, and in comparison with the Cu(II)-selective coumarin used alone. Fluorescence of both coumarin itself and the nanohybrid, λ_ex/λ_em 435/481 nm, was selectively quenched by Cu(II) when tested against a range of multivalent cations. The nanohybrid had enhanced Cu(II) sensing properties when compared to the coumarin including (i) improved limit of detection from μM-level (0.48 μM) of Cu(II) using coumarin alone to nM-level (0.033 μM) and (ii) an extended linear detection range of 0.033–260 μM (0.0005–4.1 mg/mL) Cu(II) compared to 0.48–55 μM for the coumarin itself. The lower limit of detection and extended range were achieved with a smaller amount of coumarin and no traces of organic solvents used to help coumarin dissolution. Characterization suggested that under applied test conditions at pH = 5, SiO₂ nanoparticles with negative surface charges adsorbed coumarin and then (when present) Cu(II) ions. The SiO₂-coumarin nanohybrid was then applied for the determination of Cu(II) levels in aqueous soil extracts reaching over 94% recovery rates when used against the standard soil analysis method by inductively coupled plasma mass spectrometry (ICP-MS). Graphical Abstract