AMMONIUM ION MINISENSORS FROM SELF-ASSEMBLED BILAYER-LIPID MEMBRANES USING GRAMICIDIN AS AN IONOPHORE - MODULATION OF AMMONIUM SELECTIVITY BY PLATELET-ACTIVATING-FACTOR

The present paper reports the electrochemical investigation of ion tra nsport through self-assembled bilayer lipid membranes on a metal suppo rt 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 compos ed 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 s olution, The use of stabilized metal-supported BLMs has allowed the el ectrochemical investigation of the reversibility of response to ammoni um ions and of ionophore binding to lipid membranes, The effects of pH and some possible interferents were examined, Semisynthetic platelet- activating factor (PAF; -O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylchol ine, AGEPC) was found to enhance transport of ammonium ions by gramici din and to reduce potassium interference, whereas the non-acetylated d erivative of AGEPC did not exert any alterations in transport of ammon ium ions, The present microfabricated ammonium sensor based on thin li pid film technology provides advantages of extremely fast response tim es (to millisecond speeds) to alterations of ammonium ion concentratio n (0.02-5 mmol L(-1)), high selectivity to ammonium ions in the presen ce of volatile amines, and the capability of analyzing small volumes o f samples, A detection limit of ammonium ions of similar to 1 x 10(-6) M was attained using BLMs modified with PAF. Furthermore, a device ca n now simply and reliably be fabricated at low cost and therefore can be used as a disposable sensor.