Paper-based analytical device for zinc ion quantification in water samples with power-free analyte concentration

Insufficient sensitivity is a general issue of colorimetric paper-based analytical devices (PADs) for trace analyte detection, such as metal ions, in environmental water. This paper demonstrates the colorimetric detection of zinc ions (Zn²⁺) on a paper-based analytical device with an integrated analyte concentration system. Concentration of Zn²⁺ ions from an enlarged sample volume (1 mL) has been achieved with the aid of a colorimetric Zn²⁺ indicator (Zincon) electrostatically immobilized onto a filter paper substrate in combination with highly water-absorbent materials. Analyte concentration as well as sample pretreatment, including pH adjustment and interferent masking, has been elaborated. The resulting device enables colorimetric quantification of Zn²⁺ in environmental water samples (tap water, river water) from a single sample application. The achieved detection limit of 0.53 μM is a significant improvement over that of a commercial colorimetric Zn²⁺ test paper (9.7 μM), demonstrating the efficiency of the developed analyte concentration system not requiring any equipment.