Dual-Network Poly(vinyl alcohol)/Sodium Alginate Hydrogel Photonic Crystals Films for Visual Sensing

Poly(vinyl alcohol) (PVA)/sodium alginate (SA) dual-network hydrogels were prepared via the freeze–thaw method. Tensile testing, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and fiber-optic spectroscopy were employed for material characterization, and multiple metal ions were screened to optimize mechanical properties and ionic conductivity. The results showed that, at PVA:SA mass ratio of 2:1, the hydrogel achieved 163% elongation and 0.18 MPa tensile strength. Ca2+ crosslinking formed an enhanced structure with mechanical properties of 170% elongation and 0.21 MPa strength, and ionic conductivity (0.69 S/m). Combined with colloidal photonic crystal (PC) templates, the PVA/SA-PC film exhibited an inverted opal structure, showing sensitive color response (green to red) and diffraction red-shift toward Ca2+. The conductive film could power a small bulb, demonstrating potential for portable visual sensing applications.