Self-assembled mycolic acid antigens on gold electrode as an impedimetric immunosensing platform for active tuberculosis

Abstract

Tuberculosis (TB) is well recognized as one of the most life-threatening infectious diseases in the world with about 2 million deaths and 8 million new cases per year. It is also the major cause of death in Human Immunodeficiency Virus/ Acquired Immune Deficiency Syndrome (HIV/AIDS) co-infected individuals, for whom no fast, affordable accurate TB test is available. There is therefore an urgent need for the rapid diagnosis of TB especially in Sub-Saharan Africa where the population is most burdened by the HIV/AIDS pandemic. In this study, an electrochemical impedimetric immunoassay of TB is proposed that is based on the detection of surrogate marker anti- mycolic acid (MA) antibodies that was previously shown not to be affected by patients co-infected with human immunodeficiency virus (HIV). It uses mycolic acid (MA) antigens integrated into a self-assembled monolayer (SAM) of N-(2-mercaptoethyl) octadecanamide (MEODA) on a gold electrode (Au). The integrity and properties of the Au-MEODA-MA layer were first investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). EIS data showed that Au-MEODA and Au-MEODA-MA behave as microelectrode arrays, with pinholes acting as the microelectrodes that permit electron transport between a redox-active probe in solution and the underlying gold surface. The average radii of the pinholes (ra) and half the distance between the centres of the neighbouring pinholes (rb), were estimated from EIS using the pore size model and discussed. Anti-MA antibodies present in a patient with active tuberculosis strongly interacted with Au-MEODA-MA showing a quite compact and stable bio-complex structure that is virtually defect-free. The electrochemical impedimetric properties associated with the ability of the Au-MEODA-MA to discriminate between TB positive and negative human sera are also discussed. This thesis proves that the Au-MEODA and Au-MEODA-MA electrodes, as well as the MA–anti-MA antibody interactions, are characterized with time-constant dispersion occurring mainly along the area of the electrode (2-D distribution) with some contributions from the axis normal to the plane of the electrode surface (3-D distribution), typical of microstructures with grain/grain boundary phases. These crucial physico-electrochemical insights into the behaviour of surface-confined MA-antibody complexes could provide a useful basis for the design and development of potential impedimetric immunosensors for the diagnosis of active tuberculosis. Copyright