ABOUT QUANTUM DOT-BASED

BIOSENSOR

STRUCTURE AND OPERATION

This system consists in a lab-on-a-chip where our sample is a nasopharyngeal swab in saline

solution. This sample is deposited in the sample spot and let the sample go over all the microfluidic

length. Finally the sample ends in a spot where we previously set our molecule quantum dot-

antibody(QDs-Ab) solution and the two solutions mix, QDs-Ab binded with the SARS-CoV-2 spike

protein will be excited with an UV led and the photoreceptor will capture the fluorescence and analyze

the sample in order to deliver a COVID positive or negative result [figure 1].

In addition, for greater sensitivity, we will add a second microfluidic channel and receiving two different

signals (also corresponding calibration curve), these can be compared through calculations of ΔI.

Figure 1. Diagram of COVID-19 lab-on-a-chip device procedure. A sample is deposited in the sample

spot and goes over all the microfluidic length. Finally the sample ends in a spot with QDs-Ab solution

and the two solutions mix to finally be detected by a photobiosensor.

RECOGNITION ELEMENT

SARS-COV-2 structure contains four important proteins: envelope, nucleocapsid, matrix and spike.

This last one is highly immunogenic and it is a major transmembrane protein, because of that reason

it is best suited to be used as a diagnostic antigen. It also exhibits amino acid sequence diversity

exclusive of SARS-COV-2.

Seo, et.al. in 2020 used SARS-CoV-2 spike antibody immobilized onto a Field-Effect Transistor-

Based Biosensor and they verified the performance of the antibody by enzyme-linked immunosorbent

assay (ELISA). Their results prove that the antibody bound specifically to the SARS-CoV-2 spike

protein therefore is suitable in the detection of SARS-CoV-2.

The SARS-Cov 2 Spike S2 monoclonal antibodies were first produced in mice by injecting an

immunogenic fragment of the S2 subunit of SARS-CoV 2 to generate murine monoclonal antibodies,

which are later purified from the cell culture with a protein affinity column. Now they are produced in

in-vitro rabbit cell cultures.

Figure 2. Antibody binding to coronavirus spike protein. Molecular model of an antibody (blue)

binding to the spike (S) protein (red) of the new coronavirus SARS-CoV-2. Gaernet (2020).

QUANTUM DOT SIGNAL

The variation of photoluminescence (PL) spectra in CdSe/ZnS quantum dots (QDs) at the

conjugation to biomolecules, in this case, two types of CdSe/ZnS QDs with different CdSe

core sizes (5.4 and 6.4 nm) and emissions (605 and 655 nm) were studied before and after

the conjugation to anti–Interleukin-10 (IL-10) and anti-Pseudo rabies virus (PRV) ABs. PL

spectra varies essentially in bioconjugated QDs: the PL intensity decreases on 10–50% and

the PL high energy spectral shift appears (Figures 3. 1b, c and 4b, c). Simultaneously, the

full width at half maximum (FWHM) of PL bands increases and the PL band shape becomes

asymmetric with essential high energy tails (Figures 3. 1b, c and 4b, c).

The outcome we are expecting in case of a positive diagnosis for SARS-CoV 2 is a decrease

in the photoluminescence intensity as described before.

VIRUS CONCENTRATION ESTIMATE

Research by Henan University carried out practical field samples using sixty human throat

swab samples where QDs-LFIA and real-time PCR were compared and it shows that all

positive samples with low real-time PCR were detected by QDs- LFIAS with a high accuracy.

A matter of fact, compared with real-time PCR, the QDs-LFIAS had an accuracy of 95%,

while that of the commercial influenza A antigen rapid diagnostic test kit (colloidal gold) was

56.7%.

Figure 8. Results of the research by Henan University “DLS data of QDs and conjugated QDs-Ab.

(a) Carboxyl-functionalized QDs. (b) Antibody conjugated QDs. Average hydrodynamic size of

CdSe/ZnS QDs was 42.86 nm and this size increased to 109.5 nm after conjugation with

antibodies. (c) Fluorescence intensity vs concentration graphic” [3].

Figure 9. (A) Specificity tests of QDs-LFIAS. (B) Fluorescence intensity scans at different

concentrations of influenzaA virus subtypes. Shows QDs-LFIAS could detect all the subtypes of

influenza A virus used but none of other type antigens. B shows QDs-LFIAS could detect the

subtypes of influenza A virus with high sensitivity.

Due to this information, an estimate of the relationship of the intensity of fluorescence with

the concentration of the virus SARS-COV2 could be made.

CONCLUSIONS

Through this research, we develop an-idea of an ultrasensitive, rapid and low cost lateral

flow immune sensor for SARS-COV2. A QDs-LFIAS method, which rapidly analyzed the

sample through one step. We estimate that QD-LFIAS could detect spike protein antibodies

with high sensitivity and specificity. This was more sensitive than that of traditional point-of-

care testing methods. The specificity and reproducibility were shown to be good. Owing to

previous studies we know that real patient samples demonstrate that the QDs-LFIAS had

higher accuracy, and detection of nasal-pharyngeal swab samples makes it more rapid and

efficient for identification of viral infection and improves patients management.

REFERENCE

1) Giwan S. et. al. (2020). Rapid Detection of COVID-19 Causative Virus (SARS-

CoV-2) in Human Nasopharyngeal Swab Specimens Using Field-Effect

Transistor-Based Biosensor. American chemical society.

2) Gaertner, J. (2020). Antibody binding to coronavirus spike protein, illustration.

Science photo library.

3) Figure 8. PL spectra of three non-conjugated 605N (a) and three bio-conjugated

605-PRV (b) and 605-IL-10 (c) QD ensembles.

4) Feng, W., et. al. (2016) Ultra Sensitive Detection of Influenza A Virus Based on

Cdse/Zns Quantum Dots Immunoassay. SOJ Biochemistry.