key: cord-1014506-051dhfdq authors: Dai, Zongren; Xu, Xin; Wang, Yifan; Li, Mingfang; Zhou, Kaiming; Zhang, Lin; Tan, Yidong title: Surface plasmon resonance biosensor with laser heterodyne feedback for highly-sensitive and rapid detection of COVID-19 spike antigen date: 2022-03-06 journal: Biosens Bioelectron DOI: 10.1016/j.bios.2022.114163 sha: d4701fc820e70215acea4d152870f93cd357d785 doc_id: 1014506 cord_uid: 051dhfdq The ongoing outbreak of the COVID-19 has highlighted the importance of the pandemic prevention and control. A rapid and sensitive antigen assay is crucial in diagnosing and curbing pandemic. Here, we report a novel surface plasmon resonance biosensor based on laser heterodyne feedback interferometry for the detection of SARS-CoV-2 spike antigen, which is achieved by detecting the tiny difference in refractive index between different antigen concentrations. The biosensor converts the refractive index changes at the sensing unit into the intensity changes of light through surface plasmon resonance, achieving label-free and real-time detection of biological samples. Moreover, the gain amplification effect of the laser heterodyne feedback interferometry further improved the sensitivity of this biosensor. The biosensor can rapidly respond to continuous and periodic changes in the refractive index with a high resolution of 3.75 × 10(−8) RIU, demonstrating the repeatability of the biosensor. Afterwards, the biosensor is immobilized by the anti-SARS-CoV-2 spike monoclonal antibodies, thus realizing the specific recognition of the antigen. The biosensor exhibited a high sensitivity towards the concentration of the antigen with a linear dynamic range of five orders of magnitude and a resolution of 0.08 pg/mL. These results indicate that this principle can be used as a rapid diagnostic method for COVID-19 antigens without sample labelling. (Ω) = 2 [ 2 2 + 4 2 Ω 2 ] 1 2 ⁄ [4 2 2 2 Ω 2 + (4 2 2 − 4 2 Ω 2 ) 2 ] 1 2 ⁄ 160 where is the laser cavity decay rate, is the normalized pumping rate, is the decay rate 161 of the population inversion, represents the relaxation oscillation frequency of the laser. The simulation results based on the parameters of the microchip laser indicate that the closer solution are detected using the LHFI-based SPR biosensor. Figure 3a shows the real-time 200 responses of the biosensor to NaCl solutions with concentrations ranging from 0% to 1.0%. Calculating the average voltage and RI corresponding to the solution of each concentration, the 202 fitted curve indicates that the biosensor has a good linear response, as shown in Fig. 3b . As 203 evidence of the stability and repeatability of the biosensor, a continuous and periodic change in 204 pure water and 0.003% NaCl solution has been clearly detected (Fig. 3c) . Sensitivity and resolution, which are two of the most important parameters in RI sensing, are 206 calculated using the following formulas: where S is the sensitivity of RI sensing, is the resolution of RI sensing, and ∆ is the 210 response voltage difference in RI sensing between water and 0.003% NaCl solution 211 (n=nwater+0.000006), which is approximately 6.40 mV (Fig. 3c) . noise is the standard 212 deviation of the voltage corresponding to pure water, and its value is 0.04 mV (inset of Fig. 3c ). The sensitivity is calculated to be 1.07×10 6 mV/RIU and the noise-limited resolution is To confirm that the biosensor is successfully immobilized by the MAbs, we obtain the surface 243 analysis data of the biosensor by the X-ray photoelectron spectrometer (XPS) and the atomic 244 force microscopy (AFM). The elemental composition on the surface of the biosensor is 245 analyzed by XPS (Thermo Fisher, ESCALAB 250Xi ). The survey data shows that the atomic 246 percentage of N increased from 3.33% to 12.30% after modification. Figure 4a indicates the 247 comparison of the N 1s peak intensity before and after modification. Initially, the biosensor 248 exhibited no meaningful N 1s peak (black line in Fig. 4a) , whereas the N 1s peak was observed 249 in the modified sample (red line in Fig. 4a ). The increase in the intensity of the N 1s peak proves 250 the success of the surface modification. Next, we characterize the surface morphology of the 251 biosensor using AFM (Bruker, Dimension FastScan). The surface roughness (RMS) increases 252 slightly from ~1.26 nm to ~2.72 nm after modification ( Fig. 4b and Fig. 4c) . These results A novel surface plasmon resonance biosensor based on laser heterodyne feedback 299 interferometry has been successfully developed. A minor difference between the RI of water 300 and that of 0.003% sodium chloride solution can be distinguished in real time by the biosensor, 301 and the resolution is calculated to be 3.75×10 -8 RIU. Specifically, we apply the biosensor to 302 detect the SARS-CoV-2 spike antigen. After modification, the biosensor realizes the detection magnitude, from 0.01 ng/mL to 1000 ng/mL with a resolution as low as 0.08 pg/mL. Therefore, 305 the proposed highly-sensitive, real-time, and label-free biosensor can potentially provide a 306 reliable diagnostic platform and relieve the pressure on RT-PCR testing during the outbreak 307 period of COVID-19. Because the biosensor only has one optical path, it is susceptible to environmental 309 disturbances and fluctuations in laser power. Moreover, it needs to work efficiently in a 310 relatively stable environment, which affects its sensing performance. In future work, we would 311 like to reuse the sensing unit by chemical treatment, thus greatly reducing the cost and 312 fabrication time, and we will design a compensation optical path to further improve the 313 resolution of this biosensor. Microelectronic Engineering Advances in Optics and Photonics 2020. Coronavirus disease 2019: Situation report-52 Coronavirus disease 2019: Weekly epidemiological update on COVID-19 Applied Sciences. 9 The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.