Phospholipid Langmuir-Blodgett nano-thin monolayers: Electrical response to cadmium ions and harmful volatile organic compounds

Biomolecules have the potential to be used for sensitive physico-chemical detection due to their high selectivity and unique properties. Because of such practical application, we have focused our attention on lipid-based systems. We are primarily interested in Langmuir-Blodgett (LB) nano-thin molecular monolayers of phospholipid dipalmitoyl-phosphatidyl-ethanolamine (DPPE). In our work, DPPE LB films of a thickness~3nm were inspected. Using complex electrical impedance spectroscopy (EIS) in the frequency range 0.1Hz–3MHz, the response of the DPPE LB monolayers to Cadmium ions (Cd2+), as well as to harmful vapors of volatile organic compounds (VOCs), was characterized. In the first case, DPPE LB films were deposited from an aqueous subphase on the surface of an electro-conductive glass substrate, and during the deposition they were modified with a very small amount of Cd2+ ions at a concentration of 1μg/L. The impedimetric response of DPPE LB monolayers to Cd2+ ions was studied by applying a micropipette-drop technique. In the second case, DPPE LB films were deposited on planar interdigital electrodes and their impedimetric response was assessed, when the films were in contact with vapors of VOCs, such as acetone, methanol, hexane, chloroform, ethanol, tetrachloromethane, and petroleum ether, at concentrations up to 0.8g/dm3. The results obtained in this work show that the combinations of phospholipid LB monolayers with the measurement techniques applied here (EIS and sensing through liquid electrolyte drop interface, as well as in-plane EIS studies by use of interdigital electrodes) are useful. They are promising for detection and quantification of the tested reagents (Cd2+ ions and VOCs), as based on the electrostatic interactions between them and DPPE LB monolayers. In particular, EIS demonstrates that DPPE phospholipid LB monolayers were responsive to VOCs whose molecules are polar.