Ionochromic optical sensor of amino-functionalized alginate brush grating

Rapid and portable detection of heavy metal ions is essential for environmental monitoring and water safety. In this study, a Cu(II)-responsive optical sensor based on a one-dimensional amino-functionalized alginate (Alg-NH₂) polymer brush grating was developed by combining photolithographic patterning with surface-initiated polymer grafting on silicon substrates. The Alg-NH₂ brush layer provides abundant coordination sites for metal ions, while the periodic grating structure enables optical amplification of structural changes through diffraction. Upon exposure to Cu(II), coordination between Cu(II) ions and the amino/carboxyl groups of Alg-NH₂ induces contraction of the hydrated polymer brush layer, leading to simultaneous changes in the grating spacing and effective refractive index. As a result, the diffraction peak exhibits a large red shift from 449 to 660 nm as the Cu(II) concentration increases from 1 to 4 mg/L, accompanied by a visible color transition from blue to pink. The sensor demonstrates a rapid response time of less than 25 s and good reversibility over 10 adsorption-desorption cycles. Quantitative analysis reveals sensitivities up to 53.5 nm L/mg for Cu(II), with a detection limit of 0.28 mg/L. Interference experiments show that most divalent metal ions produce minor diffraction shifts, although Pb(II) exhibits comparable responses due to its strong coordination affinity with alginate. These results demonstrate that integrating ion-responsive biopolymer brushes with diffraction gratings provides a promising strategy for developing rapid, label-free optical sensors for heavy metal monitoring in water environments.