| Summary: | The Schiff base ligand H<sub>2</sub>SB derived from the condensation of <i>N</i>-(2-aminobenzyl)-5-(dimethylamino)naphthalene-1-sulfonamide with 4-formyl-3-hydroxybenzoic acid has been investigated as a chemosensor for the detection of CdSe-Cys QDs in water samples. We immobilized H<sub>2</sub>SB onto cellulose paper by forming an amide bond, which results from the condensation of a carboxylic acid and an amine. Three dominant signals located around 270, 330, and 420 nm in the diffuse reflectance spectrum of the H<sub>2</sub>SB-modified paper demonstrated its immobilization. A linear decrease can be observed in the absorbance of the 270 nm band with the increase of the CdSe-Cys QDs concentration from 100 ppb to 2 ppm. The LOD and LOQ show values of 245 and 815 ppb, respectively. An interaction via metal–ligand coordination between CdSe-Cys QDs and H<sub>2</sub>SB has been demonstrated with <sup>1</sup>H NMR, ATR-FTIR, and UV-Vis spectroscopy.
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