A tryptophan assay based on the glassy carbon electrode modified with a nano-sized tryptophan-imprinted polymer and multi-walled carbon nanotubes

Abstract

A new nanostructured molecularly imprinted polymer (nano-MIP), possessing tryptophan compatible cavities, was synthesized using 1-(allyloxy)-4-nitrobenzene as an innovative functional monomer. The polymer was synthesized in methanol using ethylene glycoldimethacrylate as a cross-linker agent via a precipitation polymerization procedure. In order to fabricate a tryptophan selective electrode, a thin layer of nano-MIP/multi-walled carbon nanotube (MWCNT) composite was deposited on a glassy carbon (GC) electrode. This was followed by coating of a Nafion film on the nanocomposite. The described electrode showed excellent recognition capability to tryptophan, compared to its non-imprinted polymer (NIP)-based counterpart. An oxidative differential pulse voltammetric signal of tryptophan, extracted previously in the modified electrode, was adopted as the analytical signal. It was demonstrated that the selectivity of the electrode was affected greatly by the MWCNT content of the nanocomposite. The effect of tryptophan extraction pH as well as its extraction time on the electrochemical signal was investigated and optimized. The developed electrode exhibited very high selectivity to the target analyte against tyrosine, known as its potential interfering agent. The current response of the electrode to tryptophan concentration was linear in the concentration range of 6.0 × 10⁻⁸–4.0 × 10⁻⁵ mol L⁻¹. The detection limit was also calculated to be equal to 7.1 nmol L⁻¹, based on 3S_b/m. The analytical method was successfully applied for the tryptophan assay in various real samples including human blood serum, milk and tryptophan related pharmaceutical samples.