Detection of SARS-CoV-2 RNA by plasmonic-magnetic nanorobots

By Dr. Priyom Bose, Ph.D.Reviewed by Benedette Cuffari, M.Sc.Feb 16 2022

The coronavirus disease 2019 (COVID-19) pandemic has been caused by the rapid outbreak of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The COVID-19 pandemic has significantly affected the global healthcare sector and economy.

Although the rapid development of COVID-19 vaccines has significantly reduced mortality and hospitalization rates, the emergence of new SARS-CoV-2 variants due to mutations and the silent spread of the infection among asymptomatic individuals has threatened their efficacy.

Study: Plasmonic-magnetic nanorobots for SARS-CoV-2 RNA detection through electronic readout. Image Credit: Marcin Janiec / Shutterstock.com

COVID-19 diagnostics

Scientists around the world have continued to develop improved COVID-19 diagnostic kits for the rapid screening of individuals infected with SARS-CoV-2 in an effort to promote the early isolation of these individuals. This early identification and quarantining of COVID-19-positive individuals would prevent further transmission of SARS-CoV-2, particularly in those who remain asymptomatic throughout the course of their infection.

Current testing for COVID-19 is predominantly based on quantitative reverse transcription-polymerase chain reaction (RT-PCR) assays. Despite their utility, RT-PCR requires bulky instrumentations and elaborate procedures, thereby limiting the quick and mass screening of SARS-CoV-2-infected individuals in most parts of the world.

Although serological lateral-flow assays serve as a point-of-care (POC) tool, this technique is not optimally sensitive in confirming the presence of SARS-CoV-2 at an early stage of the infection.

An overview of nanorobots

Micro/nanorobots are small-scale artificial machines that have demonstrated their potential in a wide range of applications including microsurgery, sensing, targeted delivery, disruption of biofilms, and environmental remediation. In fact, several studies have successfully developed nanorobotics to improve the analytical functioning of biosensors.

Bioreceptor-functionalized micro/nanorobots are in constant motion, which enables microscale fluid mixing and ‘on-the-fly’ target capturing, which improves binding kinetics between biomolecules. Recent reports indicate that these tiny machines can efficiently capture biological targets and isolate cells, nucleic acids, and proteins.

About the study

In a new Applied Materials Today study, researchers discuss the development of a simple and effective COVID-19 detection assay with plasmonic-magnetic nanorobots via an electronic readout platform. To this end, ferric oxide (Fe₃O₄)/gold (Au)/ silver (Ag) nanoparticles were used as the building blocks of nanorobots.

Fe₃O₄/Au/Ag nanoparticles were synthesized by a sequential chemical reduction method. The formation of hierarchically structured Fe₃O₄/Au/Ag nanoparticles was initiated by the formation of Au on the Fe₃O₄ nanoparticles, which enabled nucleation and growth of high-density Ag particles.

The researchers then fabricated nanorobots by assembling the Fe₃O₄/Au/Ag nanoparticles into rod-shaped microaggregates, which permits well-regulated propulsion and navigation under the influence of a transversal rotating magnetic field. Taken together, the nanorobots consist of a Fe₃O₄ nanoparticle backbone, with Au and Ag nanoparticles on the outer surface.

The scientists then designed single-stranded deoxyribonucleic acid (ssDNA) probes at a length of 20 mer to target the SARS-CoV-2 nucleocapsid phosphoprotein (N) gene. The viral ribonucleic acid (RNA) target was detected by differentiating between the different electrostatic properties of the ssDNA probe and its hybridized duplex.

Upon incubation, ssDNA probes preferentially attached to the Ag surface of nanorobots through the van der Waals force between nucleobases and nanoparticles. The probes were then magnetically navigated in the assay solution.

The nanorobot moved by a tumbling motion in the magnetic field, thus demonstrating its precise maneuverable speed and directionality. The maximum programmed random directional propulsion speed achieved in this study was 8.9 µm s−1, with a 3.6 body length of s−1.

After hybridization of ssDNA with complementary target RNAs, the nucleobases trapped within the negatively charged helical phosphate backbones caused electrostatic repulsion between hybridized duplex and citrate-coated Fe₃O₄/Au/Ag nanorobots. Thereafter, the preloaded ssDNA probes were released and washed out by assay solution.  

The quantification of target RNA was carried out through a screen-printed electrode (SPE), where a small neodymium magnet could effectively detect Fe₃O₄/Au/Ag nanorobots on the electrode surface. RNA was quantified based on a remnant probe by analyzing its oxidative peaks. A reduction in the oxidative peak intensity was proportional to the concentration of detected SARS-CoV-2 RNA.

The Ag surface of the nanorobot served as the probe transport and release site upon hybridization, which was analyzed by hyperspectral dark-field microscopic imaging analysis. The authors explained that the magnetic actuation of probe DNA-tagged nanorobots facilitated a specific binding reaction with the complementary nucleic acid.

The automated random swarming mode of the nanorobot enabled fluid mixing and molecular binding kinetics with a 1.7-fold higher sensing signal as compared to the typical vortex shaking method.

The selectivity of the assay was determined using single-base mismatch and non-complementary sequences. In the future, the sensitivity could be enhanced by the combination of electrode modification or nucleic acid amplification techniques.  

Conclusions

In the current study, the authors presented a simple and efficient plasmonic-magnetic nanorobot for the detection of SARS-CoV-2 RNA. They further highlighted the versatility of their method to detect various nucleic acids, thereby demonstrating the potential utility of this nanorobot as a powerful diagnostic tool for other infectious diseases, forensic analyses, and environmental toxins.