Self-enhancement lateral flow immunoassay for COVID-19 diagnosis

Equipment-free colorimetric-based lateral flow immunoassay (LFIA) is the most convenient and popular tool for various applications, including diagnostic tools requiring high sensitivity for the detection of pathogens. Thus, improvements and developments of LFIA are constantly being reported. Herein, we enriched the sensitivity of LFIA using the gold enhancement principle, emphasizing needlessly complicated apparatus, only one step for the strip test operation, and typical time incubation (15 min) process. Self-enhanced LFIA was then executed for subsequent flows by overlapping the additionally enhanced pad composed of gold ions and reducing agent on the conjugate pad and the sample pad. Self-enhanced LFIA was performed to detect SARS-CoV-2 antigens in saliva. The obtained result depicted that the achieved sensitivity was up to tenfold compared with that of conventional LFIA by visual measurements. The detection limits of self-enhanced LFIA detecting nucleocapsid protein antigens in the saliva sample was 0.50 and 0.10 ng/mL employed by naked eye detection and calibration curve-based calculation, respectively. When the proposed device was applied to 207 human saliva samples, the diagnostic performance presented a 96.10 % sensitivity and 99.23 % specificity. This self-enhanced LFIA could be implemented in large-scale production and demonstrates higher sensitivity with effortless use, which meets the requirements for point-of-care testing and on-field mass screening.

Automated summary

In this study, the sensitivity of equipment-free colorimetric-based lateral flow immunoassay (LFIA) was enhanced using the gold enhancement principle. The self-enhanced LFIA, which only requires one step of operation and has a typical time incubation of 15 minutes, was used for the detection of SARS-CoV-2 antigens in saliva. The results showed a tenfold increase in sensitivity compared to conventional LFIA by visual measurements, with a detection limit of 0.50 ng/mL and 0.10 ng/mL using naked eye detection and calibration curve-based calculation, respectively. When applied to 207 human saliva samples, the self-enhanced LFIA demonstrated a sensitivity of 96.10% and a specificity of 99.23%. This method can be used for point-of-care testing and on-field mass screening due to its higher sensitivity and effortless use.