Lawsone (2-hydroxy-1,4-naphthoquinone) as a sensitive cyanide and acetate sensor

Lawsone (2-hydroxy-1,4-naphthoquinone), the essential component of henna, is used as a sensitive colorimetric and electrochemical sensor for anions such as cyanide, acetate, fluoride and dihydrogen phosphate (DHP) in acetonitrile. These anions cause a color change in Lawsone's solution from yellow to orange-red. Other anions, such as chloride, bromide, iodide, perchlorate, do not show a significant change. Cyanide changes the color of Lawsone in water:acetonitrile (95:5) solution significantly while acetate causes a slight color change. On the other hand, hydrogen sulfate anion fades the yellow color of the solution. Job's plots show a 1:1 host:guest complex in acetonitrile for cyanide, acetate and fluorides and 1:2 for DHP. In water/acetonitrile the stoichiometry is 1:1 for host:anion. The binding constants show strong binding with cyanide and acetate in both solvent systems.