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This article reports the first fluorescent distance-based paper device coupled with an evaporating preconcentration system for determining trace mercury ions (Hg2+) in water.

A paper-based method for heating preconcentration (PAD-HP) has been developed for the determination of Pb 2+ , Cd 2+ , Fe 3+ , and Ni 2+ . 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 Pb 2+ and Cd 2+ by anodic stripping voltammetry and to detect Fe 3+ and Ni 2+ 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 Pb 2+ and Cd 2+ (via voltammetry) and 0.03 and 0.04 mg L −1 for Fe 3+ and Ni 2+ (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.

We report for the first time the development of a distance-based paper sensor for a simple, inexpensive, instrument-free, and portable determination of chloride ions. Our analysis reaction is based on the oxidative etching of silver nanoparticles (AgNPs) to form AgCl in the presence of Cl- and H2O2. H2O2 reacts with AgNPs in the channel of the paper device and Cl- in the sample forming a white precipitate (AgCl) where the white color band length is proportional to the Cl- concentration. Quantification of Cl- is achieved by measuring the length of the white color band using a ruler printed on the side of the channel. Under optimal conditions, the distance-based paper sensor was characterized by a working range of 25-1000 mg L-1 (R2 = 0.9954) and the naked eye detection limit (LOD) was 2 mg L-1 (0.08 μg). Our sensor was found to be reproducible with a relative standard deviation of less than 4.51% (n = 10). The levels of Cl- in real water samples measured using our proposed device were within the error of the values measured using traditional tests but without the need for any external instrumentation. Therefore, our proposed method presents acceptable accuracy, precision, and high selectivity for point of need monitoring of Cl- in real water samples.