Label-Free Detection of the Receptor-Binding Domain of the SARS-CoV-2 Spike Glycoprotein at Physiologically Relevant Concentrations Using Surface-Enhanced Raman Spectroscopy

Abstract

Surface-enhanced Raman scattering (SERS) spectroscopy is a surface- or cavity-enhanced variant of Raman scattering spectroscopy that allows the detection of analytes with a sensitivity down to single molecules. This method involves the use of SERS-active surfaces or cavities capable of concentrating incident radiation into small mode volumes containing the analyte. Here, we have engineered an ultranarrow metal–dielectric nano-cavity out of a film of the receptor-binding domain (RBD) of SARS-CoV-2 spike (S) glycoprotein and a silver surface, held together by interaction between reduced protein sulfhydryl groups and silver. The concentration of light in this nano-cavity allows the label-free recording of the characteristic Raman spectra of protein samples smaller than 1 pg. This is sufficient for the ultrasensitive detection of viral protein antigens at physiologically relevant levels. Moreover, the protein SERS signal can be increased by several orders of magnitude by coating the RBD film with a nanometer-thick silver shell, thereby raising the cavity Q-factor. This ensures a sub-femtogram sensitivity of the viral antigen detection. A simple theoretical model explaining the observed additional enhancement of the SERS signal from the silver-coated protein is proposed. Our study is the first to obtain the characteristic Raman and SERS spectra of the RBD of S glycoprotein, the key SARS-CoV-2 viral antigen, directly, without the use of Raman-reporter molecules. Thus, our approach allows label-free recording of the characteristic spectra of viral antigens at concentrations orders of magnitude lower than those required for detecting the whole virus in biological media. This makes it possible to develop a high-performance optical detection method and conformational analysis of the pathogen and its variants.

Diagnostics and analysis of SARS-CoV-2: current status, recent advances, challenges and perspectives

“…Many of the technologies described above were developed and designed to detect RBD to diagnose SARS-CoV-2. For example, Raman spectroscopy, [390][391][392][393] electrochemical methods, [394][395][396] localized surface plasmon resonance (LSPR) 397 and lateral ow tomography. 333,398 Recently, Yang et al reported a high-affinity SARS-CoV-2 Omicron variant RBD-binding aptamer (SCORe) that binds to the Omicron BA.1 and BA.2 variant RBD with affinity at the nM level.…”

Miniaturized Raman Instruments for SERS-Based Point-of-Care Testing on Respiratory Viruses

“…However, such procedures cannot be used in the POC testing setup. Comparatively, SERS can provide quantitative data with excellent specificity, sensitivity, and multiplex detection capacity, and has previously been employed for POC settings [ 128 ].…”

Nanogap Plasmon Resonator: An Analytical Model

“…Several groups have demonstrated the achievement of strong electric fields thanks to plasmonic nanoparticles/nanostructures deposited on a metallic film serving as a mirror [10][11][12][13][14][15][16]. Furthermore, a great number of research groups have studied the coupling of plasmonic nanoparticles with nanogaps or subnanogaps [9,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. These various groups have theoretically and experimentally investigated great enhancements of the electromagnetic field concerning the coupling of these plasmonic nanoparticles with nanogaps (or subnanogaps) [35][36][37][38][39][40][41][42][43][44].…”

“…A strong motivation for the investigation of gap plasmons is the effect of surface-enhanced Raman scattering (SERS), which relies on plasmonic enhancement to enable the identification of trace molecules captured within gaps [54]. SERS is extremely important for medical diagnostics, for instance for cancer detection, imaging, and therapy, drug delivery, and the quantitative control of biomarkers, including glycated proteins and cardiovascular biomarkers, and viruses [34,[55][56][57][58][59]. For instance, Sarychev et al demonstrated a sensitive detection of less than 1 pg of a biological molecule (the spike glycoprotein of SARS-CoV-2) by employing a nanometric cavity formed by an Ag film and a layer of a receptor-binding domain of this glycoprotein [34].…”