In - situ real-time measurement of adsorption and desorption of organic cations on a lipid monolayer by using a second harmonic generation technique

The adsorption and desorption dynamics of organic cations, Malachite Green (MG), on the anionic lipid monolayer at an air/water interface were investigated in real time by using the optical second harmonic generation (SHG) technique. We showed the average orientation of the adsorbed MG on the lipid monolayer to be almost constant during the adsorption process, which is necessary to get the MG adsorption dynamics from the observed SH field change. When we increased the subphase salt concentration after saturation of the MG adsorption, the SHG field exponentially dropped with time due to MG desorption. Both the adsorption and the desorption rates show linear proportions to the subphase MG concentration, which agrees with the theoretical consideration. This application of SHG for the real-time observation of the adsorption and desorption dynamics on a lipid monolayer can be complementarily used with the steady-state SHG measurement to study many relevant biological problems.