Calcium ion concentration detected by structural changes of a Millipore DOPH artificial membrane

The surface structure of an artificial membrane composed of a Millipore filter and a synthetic lipid analogue, dioleylphosphate (DOPH), immersed in KCl solution changes when the CaCl2 concentration reaches a critical value. Using a photomultiplier mounted on the eyepiece of a stereoscopic microscope the structural change can be measured quantitatively as an intensity variation of reflected light of the membrane surface. Calcium ions decreased the intensity to about 50% of the original hydrophilic state whose solution was only KCl. However, the degree of structure change was dependent on adsorbed DOPH density, solution temperature and pH value. On the other hand, using the light reflectance curves obtained from the structural observations, an unknown CaCl2 concentration in a KCl solution can be determined. Since the reflectance curve shifts widely with various densities of DOPH adsorbed in the Millipore filter, it is possible to measure the CaCl2 concentration over a wide range. The detection range was from 0.1 to 50 mM CaCl2.