Ammonium Ion Minisensors from Self-Assembled Bilayer Lipid Membranes Using Gramicidin as an Ionophore. Modulation of Ammonium Selectivity by Platelet-Activating Factor

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Abstract
The present paper reports the electrochemical investigation of ion transport through self-assembled bilayer lipid membranes on a metal support and represents a technology suitable for development of an ammonium ion minisensor. Gramicidin D was used as a channel-forming ionophore for selective conduction of ammonium ion through lipid bilayers composed of egg phosphatidylcholine. The ammonium ion sensor exhibited good mechanical stability and longevity (routinely over 48 h) and constant sensitivity and response to a given concentration of ammonium ion in solution. The use of stabilized metal-supported BLMs has allowed the electrochemical investigation of the reversibility of response to ammonium ions and of ionophore binding to lipid membranes. The effects of pH and some possible interferents were examined. Semisynthetic platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine, AGEPC) was found to enhance transport of ammonium ions by gramicidin and to reduce potassium interference, whereas the non-acetylated derivative of AGEPC did not exert any alterations in transport of ammonium ions. The present microfabricated ammonium sensor based on thin lipid film technology provides advantages of extremely fast response times (to millisecond speeds) to alterations of ammonium ion concentration (0.02−5 mmol L-1), high selectivity to ammonium ions in the presence of volatile amines, and the capability of analyzing small volumes of samples. A detection limit of ammonium ions of ∼1 × 10-6 M was attained using BLMs modified with PAF. Furthermore, a device can now simply and reliably be fabricated at low cost and therefore can be used as a disposable sensor.