Tris(2-ethylhexyl)phosphine oxide as an effective solvent mediator for constructing a serotonin-selective membrane electrode

A series of solvent mediators containing a phosphoryl (PO) group, such as tris(2-ethylhexyl)phosphate, bis(2-ethylhexyl) 2-ethylhexylphosphonate, 2-ethylhexyl bis(2-ethylhexyl)phosphinate, and tris(2-ethylhexyl)phosphine oxide, were used to construct serotonin-selective membrane electrodes. We found that replacing the alkoxy groups attached to phosphorus atoms in PO groups with alkyl groups strengthened the response of the electrode to serotonin, suppressing remarkably interference from inorganic cations, such as Na+. Thus, an electrode combining tris(2-ethylhexyl)phosphine oxide with an ion-exchanger, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate, gave a detection limit of 9 × 10−6 M with a slope of 55.2 mV per concentration decade in physiological saline containing 150 mM NaCl and 10 mM NaH2PO4/Na2HPO4 (pH 7.4). This is the best detection limit of any serotonin-selective electrode developed to date. The selectivity of this electrode for serotonin was over 103 times that for inorganic cations, such as Na+ and K+, and lipophilic quaternary ammonium ions, such as acetylcholine and (C2H5)4N+. Using the electrode, we measured the amount of serotonin released from platelets and found that the results agreed well with those obtained by a conventional fluorimetric assay of serotonin.