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.