Continuous monitoring of amino acids and related compounds with poly(3-methylthiophene)-coated cylindrical carbon fiber microelectrodes

Abstract

The use of cylindrical carbon fiber microelectrodes (CFMEs) modified with poly(3-methylthiophene) (P3MT) films as amperometric microsensors for continuous monitoring of some amino acids such as tyrosine (Tyr), tryptophan (Trp), L-dopa, and related compounds such as tyramine and ascorbic acid, as well as small peptides such as Trp–Ala and Tyr–Gly, is discussed. Important practical advantages with respect to conventional glassy carbon (GC) electrodes, and also with respect to P3MT-coated GC electrodes of conventional size are demonstrated. The electrocatalytic ability of the modified surface allows the possibility of applying moderate potentials for the amperometric detection of the above mentioned compounds. Flow-injection experiments carried out using a phosphate buffer solution of pH 7.0 as the carrier, and an applied potential of +0.8 V showed that no cleaning or regeneration pretreatment was needed when working with the same polymer modified electrode during the whole working day. The effect of the presence of acetonitrile or methanol in the flowing solution was evaluated. A flow-injection method with amperometric detection was developed for the determination of L-dopa in pharmaceutical preparations. Finally, the modified microelectrodes have also shown suitable for amperometric detection in liquid chromatography (LC). A mobile phase composed of 5 : 95 (v/v) methanol : phosphate buffer solution of pH 7.0 allowed a good separation of mixtures of ascorbic acid, L-dopa, Tyr, tyramine and Trp, with detection limits of around 160 pmol. Furthermore, the possibility of detection of Tyr- and Trp-containing oligopeptides was also demonstrated.