A study of langmuir-blodgett films of valinomycin

Abstract

The deposition and characterisation of Langmuir-Blodgett (LB) films containing the ionophore valinomycin are described. In particular, the thesis concentrates on two specific mixed LB systems incorporating the ionophore, namely, arachidic acid/valinomycin and L-a-phosphatidic acid dipalmitoyl (DPPA)/valinomycin. The extremely sensitive spectroscopic technique of attenuated total reflection (ATR) Fourier transform infrared (FTRIR) spectroscopy is used to investigate the interaction of these ultra-thin LB structures with aqueous solutions containing potassium ions. It is shown that LB layers of pure valinomycin do not complex with potassium ions and that, in order for complexation to occur, the ionophore must be mixed with a secondary component. The formation of the valinomycin-potassium (VM/K(^+)) complex in the arachidic acid/vahnomycin system is demonstrated and the effects of the mole-fraction of arachidic acid, and of the potassium ion concentration upon complexation are described. The IR studies also reveal profound structural changes in the fatty acid matrix upon complexation, and the important result that dissociation of the VM/K(^+) complex does not occur in this mixed system. However, it is shown that if the fatty acid molecule is replaced by the phosphohpid molecule, DPPA, then both formation and dissociation of the complex occur. The results, however, indicate that this system is unstable with loss of the LB film into the aqueous solution during immersion. A number of attempts to eliminate this problem are described. The fabrication and characterisation of ion-selective-field-effect transistors (IS-FETs) are also reported. One of the aims of the research is to develop a potassium- ion sensor, and with this in mind, the deposition of LB films onto the gate surface of the ISFET is demonstrated. The K(^+)-response of the LB film coated devices is described and the results interpreted in terms of the IR evidence.