Development of colorimetric sensors based on gold nanoparticles for SARS-CoV-2 RdRp, E and S genes detection.

We present a fast, reliable and easy to scale-up colorimetric sensor based on gold nanoparticles (AuNPs) to detect the sequences coding for the RdRp, E, and S proteins of SARS-CoV-2. The optimization of the system (so-called "the sensor") includes the evaluation of different sizes of nanoparticles, sequences of oligonucleotides and buffers. It is stable for months without any noticeable decrease in its activity, allowing the detection of SARS-CoV-2 sequences by the naked eye in 15 min. The efficiency and selectivity of detection, in terms of significative colorimetric changes in the solution upon target recognition, are qualitatively (visually) and quantitatively (absorbance measurements) assessed using synthetic samples and samples derived from infected cells and patients. Furthermore, an easy and affordable amplification approach is implemented to increase the system's sensitivity for detecting high and medium viral loads (≥10 - 10 viral RNA copies/μl) in patient samples. The whole process (amplification and detection) takes 2.5 h. Due to the ease of use, stability and minimum equipment requirements, the proposed approach can be a valuable tool for the detection of SARS-CoV-2 at facilities with limited resources.