During the COVID-19 pandemic, the development of sensitive and rapid techniques for detection of viruses have become vital. Surface-enhanced Raman scattering (SERS) is an appropriate tool for new techniques due to its high sensitivity. SERS materials modified with short-structured oligonucleotides (DNA aptamers) provide specificity for SERS biosensors. Existing SERS-based aptasensors for rapid virus detection are either inapplicable for quantitative determination or have sophisticated and expensive construction and implementation. In this paper, we provide a SERS-aptasensor based on colloidal solutions which combines rapidity and specificity in quantitative determination of SARS-CoV-2 virus, discriminating it from the other respiratory viruses.
Development of sensitive techniques for rapid detection of viruses is on a high demand. Surface-enhanced Raman spectroscopy (SERS) is an appropriate tool for new techniques due to its high sensitivity. DNA aptamers are short structured oligonucleotides that can provide specificity for SERS biosensors. Existing SERS-based aptasensors for rapid virus detection had several disadvantages. Some of them lacked possibility of quantitative determination, while others had sophisticated and expensive implementation. In this paper, we provide a new approach that combines rapid specific detection and the possibility of quantitative determination of viruses using the example of influenza A virus.
Biosensors combining the ultrahigh sensitivity of surface-enhanced Raman scattering (SERS) and the specificity of nucleic acid aptamers have recently drawn attention in the detection of respiratory viruses. The most sensitive SERS-based aptasensors allow determining as low as 104 virus particles per mL that is 100-fold lower than any antibody-based lateral flow tests but 10–100-times higher than a routine polymerase chain reaction with reversed transcription (RT-PCR). Sensitivity of RT-PCR has not been achieved in SERS-based aptasensors despite the usage of sophisticated SERS-active substrates. Here, we proposed a novel design of a SERS-based aptasensor with the limit of detection of just 103 particles per ml of the influenza A virus that approaches closely to RT-PCR sensitivity. The sensor utilizes silver nanoparticles with the simplest preparation instead of sophisticated SERS-active surfaces. The analytical signal is provided by a unique Raman-active dye that competes with the virus for the binding to the G-quadruplex core of the aptamer. The aptasensor functions even with aliquots of the biological fluids due to separation of the off-target molecules by pre-filtration through a polymeric membrane. The aptasensor detects influenza viruses in the range of 1·103–5·1010 virus particles per ml.
Biosensors based on the effect of surface-enhanced Raman scattering obtained on silver nanoclusters modified with DNA aptamers allow viruses to be detected with high sensitivity. However, measurements in biological media are complicated by the nonspecific sorption of biomolecules on silver. Conditions for preparing samples of biological fluids that allow the nonspecific sorption of biomolecules to be nullified are studied.
Gold nanoparticles (AuNPs) are popular labels for colorimetric detection of various analytes, involving proteins, nucleic acids, viruses, and whole cells because of their outstanding optical properties, inertness, and modification variability. In this work, we present an improved approach for enhancement of color intensity for DNA membrane microarrays based on seed-mediated growth of AuNP labels. Biotin-labeled DNA is hybridized with capture oligonucleotide probes immobilized on the microarrays. Then biotin is revealed by a streptavidin–AuNP conjugate followed by the detection of AuNPs. Optimization of seed-mediated enlargement of AuNPs by the reduction of tetrachloroauric acid with hydroxylamine made it possible to change the coloring of specific spots on the microarrays from pink to a more contrasting black with minor background staining. Mean size of the resulting AuNPs was four times larger than before the enhancement. Adjusting the pH of HAuCl4 solution to 3.5 and use of a large excess of hydroxylamine increased the signal/background ratio by several times. The method’s applicability was demonstrated for quantification of a short oligonucleotide of 19 bases and full-length TEM-type β-lactamase genes of 860 bp responsible for the development of bacterial resistance against β-lactam antibiotics. Improved protocol for AuNP enlargement may be further transferred to any other membrane-based assays of nucleic acids with both instrumental and visual colorimetric detection.
Nucleic acid aptamers are artificial recognizing molecules that are capable of specific binding to a wide variety of targets. Aptamers are commonly selected from a huge library of oligonucleotides and improved by introducing several mutations or modular constructions. Although aptamers hold great promise as therapeutic and diagnostic tools, no simple approach to improve their affinity has been suggested yet. Our recent analysis of aptamer–protein complexes revealed that aptamer affinity correlates with the size of an amino acid sidechain in the protein interface that was explained by efficient dissipation of the energy released during complex formation. G-quadruplex-based thrombin aptamers are not involved in the described dependence. Moreover, aptamers to the same thrombin site have 100-fold differences in affinity. Here we focused on a detailed analysis of the nucleic acid interface of thrombin–aptamer complexes. High affinity of the aptamers was shown to correlate with the solvent accessibility of the apolar part of recognizing loops. To prove the concept experimentally, these loops were modified to enhance contact with the solvent. Dissociation rates of the aptamer–thrombin complexes were drastically slowed due to these modifications. In full correspondence with the energy dissipation hypothesis, the modifications improved both the stability of the G-quadruplexes and affinity to thrombin. The most evident effect was shown for unstable Na+-coordinated G-quadruplexes. These data are of high interest for a directed improvement of aptamers introducing unnatural modifications into the ‘hot spot’ residues.
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