Integrating SPR-ellipsometry and electrochemical measurements for performance evaluation of label-free thiophene-based biosensor

Abstract

The surface plasmon resonance reflectance changes measured with a circularly polarized ellipsometry and an electrochemical impedance spectroscopy were identified to be able to characterize the critical roles of biomolecules for vastly different biological functions and processes. Throughout the course of this study, interferon-gamma (IFN-γ) was chosen as the biomarker to test and to verify the performance of this newly developed system for Tuberculosis detection. The interactions of IFN-γ with immobilized anti-IFN-γ antibody at various concentrations were interrogated both optically and electrochemically. A semi-conductive linker bis-thiophene was thiolated to ensure the cross-linked monoclonal human IFN-γ antibody got self-assembled onto the gold thin film and form a label-free biosensor. The functional features of the bis-thiophene coated-gold film were characterized by cyclic voltammetry and impedance spectroscopy methods. The association of IFN-γ to the bis-thiophene bridging units via antibody-antigen interactions provided the basis for ultrasensitive detection of IFN-γ by tracking the conformation changes in surface-bound protein molecules. The phase shift can be attributed to the average thickness and the real-time index of refraction of the protein layer in different protein layer. Experimental results obtained by impedance spectroscopy and by phase-interrogation SPR showed linear dynamic range. Our experimental results verified that an increase in the concentration of the IFN-γ usually accompanied by phase increase in SPR and an impedance decrease in EIS. These results indicated that our newly developed integrated biosensing system can potentially provide new insight into various conjugate phenomena and interfacial processes for observing molecular conformation changes.