A novel approach for the selective analysis of l-lysine in untreated human serum by a co-crosslinked l-lysine-α-oxidase/overoxidized polypyrrole bilayer based amperometric biosensor

An amperometric biosensor based on an l-lysine-α-oxidase (LO) layer immobilized by co-crosslinking onto the surface of an overoxidized polypyrrole modified Pt electrode (Pt/oPPy) and able to analyse l-lysine (Lys) in untreated human serum is described. The sensing electrode has been characterised and a proper enzyme kinetics optimisation permits to use a low specific enzyme as LO from Trichoderma viride for the selective biorecognition of Lys in the presence of other interferent amino acids; a kinetics study of LO evidenced also the allosteric behaviour of this enzyme, a kinetic feature which was never reported before for this enzyme. The biosensor showed a sensitivity of 0.11 μA/mM mm2, linear responses up to 4 mM and a limit of detection of 2 μM; the within-a-day coefficients of variation for replicate (n = 5) were 0.92% and 1.35% at 4 mM and 0.2 mM Lys levels, respectively. The permselective behaviour of Pt/oPPy modified electrode assured an interference- and fouling-free determination of Lys even in untreated serum samples. The determination of Lys in human serum from healthy donors gave Lys levels in good agreement with the expected values so that the use of the proposed biosensor appears promising in the relevant clinical fields.