Development of hard water sensor using Fluorescence Resonance Energy   Transfer
  A method is presented for the sensing of water hardness by determining the concentration of calcium and magnesium in water, based on Fluorescence resonance energy transfer (FRET) process. The principle of the proposed sensor is based on the change of FRET efficiency between two laser dyes Acriflavine (Acf) and Rhodamine B (RhB) in presence of permanent hard water components (CaCl2 and MgCl2). Nanodimensional clay platelet laponite was used to enhance the efficiency of the sensor. 
Dibyendu Dey, D. Bhattacharjee, S. Chakraborty, Syed Arshad Hussain
cond-mat.mtrl-sci