Alizarin Red S-Functionalized Polyurethane Coatings as Passive Colorimetric Interfaces for Visual Heavy-Metal Screening in Simulated Seawater

This study investigates Alizarin Red S (ARS)-functionalized polyurethane (PU) coatings as passive colorimetric interfaces for preliminary visual screening of selected heavy-metal ions in simulated seawater. ARS was incorporated into a hydrophilic PU matrix and deposited on glass substrates. UV–visible spectroscopy and ATR-FTIR were used to analyze the optical and vibrational features of the PU–ARS system, while DLS provided comparative information on the colloidal behavior of the liquid precursor formulations before film formation. After 48 h exposure to simulated seawater (ASTM D1141-98, pH 8.2) containing Pb2+, Cd2+, Ni2+, or Hg2+ at 5 × 10−4 M, the coatings developed ion-dependent color changes quantified in the CIE L*a*b* color space. Under these single-ion conditions, the total color difference followed the order Pb2+ > Cd2+ > Ni2+ > Hg2+, with approximate ΔE values of 27, 19, 16, and 13, respectively. Saline pre-immersion experiments up to 72 h, followed by 48 h exposure to metal-ion solutions, indicated that the coatings retained macroscopic readability and optical responsiveness under the tested conditions. The results support the use of PU–ARS coatings as proof-of-concept passive interfaces for visual heavy-metal screening in marine-like media. Further studies including calibration, detection limits, selectivity, interference, leaching assessment, and validation in real seawater will be required before analytical implementation.