Two Principles for Colorimetric Detections of Cr6+ Using Polyurethane Foam—Gold Nanoparticles Composite

Two principles of leaching and precipitating for colorimetric sensing of hexavalent chromium using a composite of polyurethane foam with gold nanoparticles (PUF-AuNPs) with a smartphone and image processing were reported. The PUF-AuNP composites were exposed to different concentrations of hydrobromic acid (HBr) before taking a one-shot image and processing it to obtain a color value with ImageJ for a sample with triplicate results. In 0.05 M HBr, the leaching of AuNPs by oxidation would provide a linear calibration with yellow value = 0.021[Cr6+, mM] + 0.078 (R2 = 0.998) for 0.5–5 mM with a limit of quantitation (LOQ) of 0.3 mM. In 1 M HBr, the amount of precipitate of Cr3+-bromide was proportional to the concentration of Cr6+ with a linear range of 2–20 mM and a green value = −2.5[Cr6+, mM] + 210 (R2 = 0.981) with a limit of detection (LOD) of 0.6 mM. The precision was lower than 4% RSD with a good accuracy by % relative error. Various techniques, such as FT-IR, SEM, TEM, and EDS, confirmed the presented principles, and PUF-AuNPs. The PUF was readily synthesized in a lab using simple apparatuses like plastic cups and stirrer rods. AuNPs sorbed on PUF were obtained from the reduction of hydrogen tetrachloroaurate (III), which would be obtained from a low-cost gold leaf. Preliminarily, two proposed colorimetric sensors were tested for Cr6+ assay in known aqueous samples with calculations in a calibration graph and the naked eye.