Dual Sample Preconcentration for Simultaneous Quantification of Metal Ions Using Electrochemical and Colorimetric Assays

A paper-based method for heating preconcentration (PAD-HP) has been developed for the determination of Pb²⁺, Cd²⁺, Fe³⁺, and Ni²⁺. The design of our heating system was evaluated for dual quantification of ions using electrochemical and colorimetric methods simultaneously. The PAD-HP was used to detect Pb²⁺ and Cd²⁺ by anodic stripping voltammetry and to detect Fe³⁺ and Ni²⁺ by colorimetric reactions. Assay conditions were optimized by evaluating performance when changing the concentration of the colorimetric reagent, eluent volume, electrolyte concentration, and electrochemical parameters. Limits of detection (LOD) were determined to be 0.97 and 2.33 μg L^–1 for Pb²⁺ and Cd²⁺ (via voltammetry) and 0.03 and 0.04 mg L^–1 for Fe³⁺ and Ni²⁺ (via colorimetric assay), respectively. The relative standard deviations for assays were in the range of 5.76 to 10.12%. We observed that the PAD-HP method significantly enhanced the signal of all metals ions (14–100-fold, depending on the metal) in comparison to paper-based devices that did not use a heating preconcentration system. This PAD-HP method was successfully applied to the determination of metals ions in samples of drinking water, tap water, pond water, and wastewater. These results suggest that our approach can provide a convenient strategy to monitor aqueous samples for heavy metals with high sensitivity and selectivity.