# A portable lateral flow distance-based paper sensor for drinking water hardness test

## TL;DR

A portable paper sensor is developed to semi-quantitatively detect drinking water hardness using a lateral flow method.

## Contribution

A novel lateral flow distance-based paper sensor is introduced for semi-quantitative detection of water hardness.

## Key findings

- The sensor quantifies Ca2+ and Mg2+ in ranges of 0–10 mmol L-1 and 4–20 mmol L-1 with recoveries from 95% to 108.9%.
- The sensor's performance is comparable to commercial kits for real water samples.
- The method offers convenience, low cost, and potential for practical application in water hardness testing.

## Abstract

Hardness is one of the basic parameters of water, and a high-level hardness of drinking water may be harmful to human health. Thus, it is very important to monitor drinking water hardness. In this work, a portable lateral flow distance-based paper sensor for the semi-quantitative detection of drinking water hardness is demonstrated. In the presence of Ca2+/Mg2+, the hydrogel can be formed via the chelation between sodium alginate and Ca2+/Mg2+, inducing a phase separation process. The viscosity change of the sodium alginate solution is directly related to the Ca2+/Mg2+ concentration and can be determined by the water lateral flow distance on test strips. The sensor successfully realizes the quantification of Ca2+ and Mg2+ in the range of 0–10 mmol L-1 and 4–20 mmol L-1, respectively. The recoveries are found varied from 95% to 108.9%. The water hardness is acceptable for drinking if the Cr values lies in the range of 0.259 to 0.419, and it is high with the Cr value above 0.595. Remarkably, the performance of the sensor is comparable with the commercial kit for real water samples, which avoids the subjective judgment. Overall, this method provides a portable approach for semi-quantitative detection of drinking water hardness with the merits of convenience and low cost, which shows great potential for the potential application.