Lab-on-Paper with Dual Electrochemical/Colorimetric Detection for Simultaneous Determination of Gold and Iron

Abstract

A novel lab-on-paper device combining electrochemical and colorimetric detection for the rapid screening of Au(III) in the presence of a common interference, Fe(III), in industrial waste solutions is presented here. With dilute aqua regia (0.1 M HCl + 0.05 M HNO₃) as the supporting electrolyte, square wave voltammetry on paper provided a well-defined reduction peak for Au(III) at ∼287 ± 12 mV vs Ag/AgCl. Under the optimized working conditions, the calibration curve showed good linearity in the concentration range of 1−200 ppm of Au(III) with a correlation coefficient of 0.997. The limit of detection (LOD) of the proposed method is 1 ppm. Interferences from various cations were also studied. Fe(III) is the only metal that affects the electrochemical determination of Au(III) when present above a 2.5-fold excess concentration of that of the Au(III). To overcome this limitation, a colorimetric method was used to simultaneously detect Fe(III) as a screening tool. The procedure was then successfully applied to determine Au(III) in gold-refining waste solutions. The results are in agreement with those obtained from inductively coupled plasma−atomic emission spectrometry (ICP-AES).