Label free detection of SARS CoV-2 Receptor Binding Domain (RBD) protein by fabrication of gold nanorods deposited on electrochemical immunosensor (GDEI)

Coronavirus Disease 2019 (COVID-19) pandemic has shown the need for early diagnosis to manage infectious disease outbreaks. Here, we report a label free electrochemical Fluorine-Doped Tin Oxide (FTO) Immunosensor coupled with gold nanorods (GNRs) as an electron carrier for ultrasensitive detection of the Receptor Binding Domain (RBD) of SARS CoV-2 Spike protein. The RBD gene was cloned, and expressed in-house with confirmed molecular weight of ∼31 kDa via Sodium Dodecyl Sulphate-Polyacrylamide Gel Electrophoresis (SDS-PAGE) and Matrix-Assisted Laser Desorption/Ionization-Time of Flight (MALDI-TOF). RBD antibodies (Ab) were generated to be used as a bioreceptor for sensor fabrication, and characterized using SDS-PAGE, Western Blot, and Enzyme-Linked Immunosorbent Assay (ELISA). GNRs were fabricated on the electrode surface, followed by immobilization of RBD Ab. The conjugation steps were confirmed by UV-Vis Spectroscopy, Dynamic Light Scattering (DLS), Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), Cyclic Voltammetry (CV), and Differential Pulse Voltammetry (DPV). The fabricated electrode was further optimized for maximum efficiency and output. The detection limit of the developed electrode was determined as 0.73 fM for RBD antigen (Ag). Furthermore, the patient nasopharyngeal samples were collected in Viral Transport Media (VTM), and tested on the sensor surface that resulted in detection of SARS CoV-2 within 30 s, which was further validated via Reverse Transcription-Polymerase Chain Reaction (RT-PCR). Moreover, the immunosensor showed good repeatability, storage stability, and minimal cross reactivity against Middle East Respiratory Syndrome (MERS) spike protein. Along with ease of fabrication, the electrodes show future miniaturization potential for extensive and rapid screening of populations for COVID-19.