Method

About HCR

 The HCR is a reaction performed by an initiator and two types of hairpin DNA (Fig.1). In the presence of Initiator, H1 and H2 open and form double strands which results in linear elongation if the complex. In the absence of the initiator, no reaction occurs and the secondary structure of the hairpin DNA remains (1). As a characteristic of HCR, the reaction will proceed if there is a small amount of target that is collected individually.

Mechanism of aggregation of gold nanoparticles

 Single-stranded DNA (ssDNA) prevent AuNP from aggregation by electrostatically adsorbing to the surface of AuNP (Fig.2). On the contrary, the exposure of the phosphate skeleton makes it difficult for negatively charged Double-stranded (dsDNA) to adhere to AuNP (2). So AuNP is easy to aggregate. Whether AuNP aggregate or not can be observed as color difference (Fig.3) .

 The solution looks red because it absorbs blue light when nanoparticles are dispersed, and looks blue when they are aggregated. The principle of seeing color is that some wavelengths are absorbed and the unabsorbed light enters the eye as reflected light and is recognized as color. For example, The sea is blue because water molecules absorb red light and its complementary color, blue light, remains.

A series of flows

 We will explain the flow of our ideas in this section (Fig.4).

 First, saliva is collected and put into a test solution. Saliva contains a lot of coronaviruses and is easy to collect from the human body (6).The test solution contains an aptamer-initiator complex, two types of hairpin DNA, and gold nanoparticles. Even if a small amount of coronavirus is present in the saliva, the spike protein with the aptamer, and the single strand on the aptamer side binds to the spike protein to an initiator.

 Then, triggered by the initiator, DNA elongation by HCR occurs. The double-stranded DNA extended by HCR causes agglomeration of gold nanoparticles, changing the color of the solution from red to blue (7). You can see the result by this color change.

 Without the new coronavirus, there would be no reaction with aptamers, no initiators would be generated. Therefore, HCR is not started and no color change occurs.

 You can confirm the result only by putting your saliva into the solution and seeing its color change.

Reference

(6)To, Kelvin Kai-Wang, et al. "Consistent detection of 2019 novel coronavirus in saliva." Clinical Infectious Diseases (2020).

(7)Xu, Chengnan, Yibin Ying, and Jianfeng Ping. "Colorimetric aggregation assay for kanamycin using gold nanoparticles modified with hairpin DNA probes and hybridization chain reaction-assisted amplification." Microchimica Acta 186.7 (2019): 448.