Abstract
Water hardness (WH) is a useful parameter for testing household water, such as drinking, cooking, and washing water. Many countries around the world use pipeline water in their houses, but there is a need to monitor the WH because hard water has a negative impact on appliances. Currently, WH is often measured using chemical dye-based WH indicators, and these techniques require expensive equipment, and trained personnel. Therefore, a low-cost and simple measurement method has been desired. Here, we report LOTUS-W, which consists of a luciferase, Nanoluc, a yellow fluorescent protein Venus, and a Ca2+/Mg2+ detection domain of human centrin 3. The binding of Ca2+/Mg2+ to this indicator changes the conformation of human centrin 3, and induces bioluminescence resonance energy transfer (BRET) from Nanoluc to Venus, which changes its emission spectrum about 140%. The dissociation constants of LOTUS-W for Ca2+/Mg2+ are approximately several mM, making it suitable for measuring WH in the household water. With this indicator in combination with a smartphone, we have demonstrated that it is possible to evaluate WH easily and quickly. This novel indicator has the potential to be used for measuring not only household water but also water used in the food industry, etc.
Highlights
Natural water contains several minerals, such as calcium, magnesium, potassium, phosphate, and carbonate
water hardness (WH) is determined based on the total concentration of Ca2+ and Mg2+ in parts per million or milligrams per liter [2,3]
WH is classified into soft water (60 milligrams per liter (mg/L) or less), moderately hard water (60–120 mg/L), hard water (120–180 mg/L), and very hard water by the World
Summary
Natural water contains several minerals, such as calcium, magnesium, potassium, phosphate, and carbonate. These minerals are generally added when natural water passes through soils and rocks containing large mineral deposits. Water with high concentrations of polyvalent cations (>+1), such as. Ca2+ and Mg2+ , is called hard water [1]. Other ions such as Al3+ , Fe2+ , Sr2+ , Zn2+ , and Mn2+ are associated with water hardness (WH). Their concentrations are much lower than Ca2+ and. WH is classified into soft water (60 mg/L or less), moderately hard water (60–120 mg/L), hard water (120–180 mg/L), and very hard water (over 181 mg/L) by the World
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