Affinity peptide-based magnetic micromotors for rapid, selective, ultra-sensitive, and reliable quantification of SARS-CoV-2 spike protein in microliter-scale nasopharyngeal and plasma samples from infected patients

Monitoring respiratory viruses is paramount to reducing the spread of the virus and associated costs, especially during the winter months. Here, we present magnetically propelled MoS₂/Ni micromotors modified with a rhodamine B-labelled peptide that binds to the receptor-binding domain (RBD) on the S1 subunit of the spike protein (S). The approach uses an "on-the-fly" OFF-ON fluorescence method to detect the SARS-CoV-2 virus in two types of samples: nasopharyngeal and plasma samples, both types previously tested by molecular methods, and plasma samples additionally analyzed by serological assays. The micromotors move through the sample, inducing enhanced fluid mixing and increasing the number of peptide-spike protein biorecognition events, yielding an efficient operation using just 1 μL of sample. The adsorbed peptide on MoS₂ (OFF state) desorbs from the micromotors, due to interaction rhodamine B-labelled peptide-spike protein resulting in an increase in target-dependent fluorescence intensity in the solution measured using fluorescence microscopy (ON state). The approach takes just 1 min, displaying a linear range of 0.006-50 ng/mL with an excellent limit of detection of 1.8 pg/mL. The good agreement between the main infection indicators from nasopharyngeal and plasma samples, as measured by molecular and serological tests in the hospital, and the protein S concentration, as measured by our micromotor approach, demonstrates the applicability of the method for the rapid diagnosis of respiratory infections, opening new avenues for the diagnosis of respiratory infections. While the potency of the micromotor approach is demonstrated here, novel integration approaches are also envisioned in the near future.