<p>A novel approach is reported&#xa0;to quantify CuO NPs in aqueous samples by using digital image colorimetry (DIC), which is a rapid, low-cost, and facile method. We have used a colour recogniser application on a smartphone to register the gradual colour changes experimented, under <i>λ</i>_ex = 365&#xa0;nm radiation, by a dansyl-based chemosensor immobilised on cellulose paper, after interacting with aqueous dispersions of CuO NPs with concentrations between 5 and 100&#xa0;µg L⁻¹. The low limits of detection (LOD) and quantification (LOQ) (10.5 and 34.9&#xa0;µg L⁻¹, respectively) show a relevant sensitivity for this simple method, which also displays a good precision and accuracy (analytical recovery, 99 ± 1%). A comparison of the results similarly obtained for the modified paper through diffuse reflectance (DR) revealed worse detection parameters (LOD = 100.0&#xa0;µg L⁻¹ and LOQ = 333.4&#xa0;µg L⁻¹). In addition, fluorescence spectroscopy (<i>λ</i>_em = 520&#xa0;nm) demonstrated the ability of the free chemosensor (1.26&#xa0;µg L⁻¹ in 20:80 (v/v) water–ethanol solutions) for the quantification of CuO NPs (LOD = 30&#xa0;µg L⁻¹ and LOQ = 101&#xa0;µg L⁻¹), without improvement of the results obtained with DIC on the modified papers. The dissolved free chemosensor demonstrated selectivity towards CuO NPs in the presence both of metal ions common in aqueous samples as K⁺, Na⁺, Ca²⁺, Mg²⁺, Fe³⁺, or Al³⁺ and of other usual nanomaterials as those of Cu, CdSe, TiO₂, and ZnO NPs.</p> Graphical Abstract <p></p>