Development of a selective and sensitive colour reagent for gold and silver ions and its application to desktop...

Desktop scanners can be favorable alternatives to sophisticated spectrophotometers for the assessment of analytes in complex real samples. Distinctively, our method has been thoroughly investigated, optimized, validated and successfully applied to the assessment of silver and gold in complex real samples, applying syringal rhodanine (SR) as a novel specifically tailored chromogenic reagent and using a desktop scanner as a versatile sensor. Maximum colour absorbance was obtained in the presence of cetylpyridinium chloride (CPC) and cetyltrimethylammonium chloride (CTAC) for silver and gold chelates, respectively. For each metal ion, two ternary complexes were formed depending on the SR concentration with stoichiometries of 1 : 1 : 1 and 1 : 2 : 3 (Ag-SR-CPC) and 1 : 2 : 3 and 1 : 3 : 4 (Au-SR-CTAC), respectively. The methods adhered to Beer's law for 0.15-2.5 and 0.15-2.25 μg mL^-1 with detection limits of 0.0089 and 0.0163 μg mL^-1 for silver and gold, respectively. The molar absorptivities were 3.63 × 10⁴ and 6.15 × 10⁴ L mol^-1 cm^-1 at 550 nm and 554 nm, with Sandell's sensitivity indexes of 0.0029 and 0.0032 μg cm^-2, respectively. The method was successfully applied to the assessment of silver and gold in a wide range of complex environmental samples.