RGB-readable dithizone–polyurethane coatings for colorimetric screening of hazardous metal ions in synthetic seawater

Colorimetric solid-state sensors for saline media must retain their chromophores within the sensing phase, remain optically readable, and operate reliably in high-ionic-strength matrices. In this work, PU–DTZ, a polyurethane (PU) coating containing dithizone (DTZ), was deposited on glass slides and coupled to a low-power RGB light-to-frequency (L2F) optoelectronic module housed in a simulated buoy configuration. Rheological analysis showed that DTZ incorporation did not substantially impair the processability of the waterborne PU formulation. Spectroscopic and morphological characterization collectively supported DTZ incorporation and optical activity within the polymer matrix but provided no evidence of covalent bonding between DTZ and PU. In ASTM D1141–98 synthetic seawater, separate single-metal exposures were performed at nominal total concentrations ranging from 5 × 10 −8 M to 5 × 10 −4 M. After the fixed 24 h exposure period, the lowest tested concentrations showing apparent discrimination from the matrix blank were 5 × 10 −6 M for Hg 2+ and 5 × 10 −5 M for Pb 2+ , Cd 2+ , and Ni 2+ . These values represent protocol-specific operational discrimination thresholds and should not be interpreted as analytically validated LODs. Exploratory PCA was used only to visualize condition-level RGB response patterns and not for predictive classification. The prototype demonstrates the feasibility of coupling a DTZ-loaded PU coating with a compact RGB L2F reader for alarm-level screening in saline media. However, dye leaching, long-term coating stability, response kinetics, mixed-ion selectivity, and ion-specific matrix effects were not assessed. Further work should address lower operational thresholds, replicate-level statistics, calibration, coating reproducibility, and validation in real seawater against reference elemental analysis before quantitative applications.