A novel ultra-sensing composed Langmuir–Blodgett membrane for selective calcium determination in aqueous solutions

A novel sensing membrane to determine concentration of calcium in aqueous solutions is demonstrated. The multilayer organized membranes were formed from octadecylamine and calcium chelator 1,2-bis-(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid via Langmuir–Blodgett (LB) technique. The structure and composition of membrane enable the conversion of calcium bound by the chelator to the metal hydroxide and its accumulation in the film, thus maintaining the availability of ligands for continuous binding. The formation of Ca(OH)2 crystals in the film was verified by optical microscopy. Major analytical parameters were determined by means of quartz crystal microbalance. The logarithm of the sensor's response was a linear function of calcium concentrations ranging from 10−8 to 10−1 mol L−1, the response time was 10 s. The repeatability of the response determined as relative standard deviation and measured by five membranes for each of eight calcium standards was 4.2 ± 0.3%. The membrane can be applied to the calcium determination in solution of 0.1 M NaCl at neutral pH and is selective with respect to other biologically relevant cations. The electrochemical impedance and surface plasmon resonance spectroscopies were used to demonstrate the applicability of the membrane to different registration methods.