Determination of Tryptophan at Carbon Nanomaterials Modified Glassy Carbon Sensors: A Comparison

The effect of surface modification of glassy carbon (GC) by single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs) and graphene has been studied for the determination of tryptophan (Trp). The surface morphology of the unmodified and modified sensors was characterized by Field Emission Scanning Electron Microscopy(FE-SEM), Electrochemical Impedance Spectroscopy (EIS), square wave voltammetry (SWV) and cyclic voltammetry (CV). The unmodified GCE exhibited an oxidation peak for Trp at 846 mV at pH 7.4, whereas after the modification with SWCNTs/GCE the oxidation peak shifted to less potential at 685 mV. At MWCNTs/GCE and Graphene/GCE, a well-defined anodic peak was observed at 669 mV and 769 mV, respectively for the oxidation of Trp. Under optimum conditions, a linear calibration curve was observed at unmodified GCE in the concentration range of 10–100 μ M of Trp with a detection limit of 190 nM. At SWCNTs/GCE, Graphene/GCE and MWCNTs/GCE the detection limits observed were 15.5 nM, 10.3 nM and 4.6 nM, respectively. The oxidation of Trp involved equal number of proton and electron at all the sensors, which was confirmed by pH studies. A comparison of limit of detection indicated that the modified MWCNTs/GCE sensor exhibited best catalytic response for the sensitive determination of Trp.