Table 1.

Plasmonic biosensors for the detection of SARS-CoV-2

Biosensing technique	Material selection and design	Biomarker	Limit of detection
Dual-functional plasmonic photothermal Biosensor [29]	Two-dimensional AuNI chips were fabricated using the self-assembly process of thermal dewetted Au nanofilm. The thickness of magnetron-sputtered Au nanofilms on the BK7 glass surface was within the range of 5 to 5.2 nm	RdRp-COVID (SARS-CoV-2 RNA)	0.22 ± 0.08 pM
Nanoplasmonic biosensor [47]	An Au-TiO2-Au nano-cup array chip, fabricated by the replica molding process, with a drop of water on top of it, was used as the sensor chip on the silicon oxide wafer. the thicknesses of Au and Ti on the nano-cap array were 70 nm and 10 nm, respectively	Spike protein of SARS-CoV-2	30 virus particles in one step
Near-infrared plasmonic biosensor [48]	integrating two-dimensional (2D) Van der Waals heterostructures, including tellurene and carboxyl-functionalized molybdenum disulfide layers, with transparent indium tin oxide film	SARS-CoV-2 spike (S) glycoprotein	Sensitivity = 8.4069 × 104 deg/RIU
N gene-targeted antisense oligonucleotide capped plasmonic nanoparticles (naked-eye detection) [49]	Four ASOs sequences used to cap AuNPs were selected according to their closely target following position, binding disruption energies, and binding energies. Mixing all ASO-capped AuNPs, which resulted in the formation of Au-ASOmix, increased the sensitivity of the gold nanoparticles for the detection of SARS-CoV-2 RNA	SARS-CoV-2 N gene (nucleocapsid phosphoprotein gene)	0.18 ng/μL
Toroidal plasmonic metasensor [50]	A mixture of 0.1 M of reactant buffer with 50 μL of purified spike S1 antibody was utilized to conjugate SARS-CoV-2 Spike S1 antibody with the NHS activated gold nanoparticles. The average diameter of AuNPs was around 45 nm. For dissolving the immunoreagents, both bovine serum albumin and a phosphate buffer solution were used. The functionalized AuNPs were dispersed on the sensor surface to enhance the binding events.	SARS-CoV-2 spike protein	4.2 fmol
Colorimetric biosensor based on localized surface plasmon resonance (LSPR) [51]	AuNPs were used in these biosensors because of their surface chemistry and biocompatibility. By using gold nanoparticles in the colorimetric method, the color changes from red to blue in a colloidal suspension because of LSPR coupling among the AuNPs, which were functionalized with antibodies	SARS-CoV-2 spike, envelope, and membrane proteins	Ct = 36.5 (the limit of detection of this colorimetric biosensor was reported based on the real-time PCR cycle threshold (Ct))
Gold nano spikes in an opto-microfluidic chip (based on localized surface plasmon resonance) [53]	1000 Å of Gold was electrodeposited on a 50 Å of chromium layer on the glass substrates. These gold nano spikes were functionalized by immersing them in the thiol mixture. 1:1 solution of 10 mM NHS (N-Hydroxysuccinimide) and 40 mM of EDC (1-Ethyl-3-(3-dimethyl aminopropyl) carbodiimide) was used to activate the surface gold nano spikes	SARS-CoV-2 spike protein	∼ 0.08 ng/mL (∼ 0.5 pM)
Single-step and washing-free immunoassay for the detection of SARS-CoV-2 by photonic resonator absorption microscopy (PRAM) [54]	the linear grating period of the PC-based biosensor was 380 nm. Its grating depth was 97 nm, etched into a glass substrate using the reactive ion etching method. This granting was next coated with a 98.5 nm thick TiO2 layer. By employing the AC + DC assay, SARS-CoV-2 IgG proteins activated the functionalized AuNPs in solution, which triggered the binding of activated AuNPs to the PC. To activate the PC surface, oxygen plasma treatment was used. Subsequently, a coating of the recombinant COVID-19 spike protein was applied on the PC's surface to capture COVID-19 IgG. Finally, COVID-19 IgG was incubated with secondary antibody functionalized AuNPs	SARS-CoV-2 IgG	limit of detecting = 26.7 ± 7.7 pg/mL limit of quantification = 32.0 ± 8.9 pg/mL