Part:BBa_K4778013
Colloidal Gold Particles (after crosslinking)
Introduction
Belonging to the heterogeneous system, colloidal gold solution refers to the gold sol whose dispersed phase particle diameter is between 1-150 nm, and the color is orange-yellow to wine-red. The smallest colloidal gold (2 ~ 5nm) is orange-yellow, the medium-sized colloidal gold (10 ~ 20nm) is wine-red, and the larger particles of colloidal gold (30 ~ 80nm) are purplish red. Colloidal gold has a single light absorption peak (λ max) in the visible range, which is in the range of 510 ~ 550nm. With the size change of colloidal gold particles, λ max large colloidal gold is biased to long wavelength. On the contrary, λ max of small colloidal gold is biased to short wavelength. λ max of some colloidal gold listed in Table 1
Applications
1.By taking advantage of the fact that colloidal gold particles can be attached to proteins, we can link it to antibodies to establish direct or indirect immune colloidal gold staining. Colloidal gold particles linked to proteins can also be used as a probe to accurately locate biological macromolecules (proteins, antigens, and hormones, etc) on the cell surface and inside the cell.[1]
2. (1) Due to the Localized Surface Plasmon Resonance (LSPR) phenomenon, the plasma particles cluster to a certain extent and the reduced distance between the particles will enhance the plasmon resonance on their surface. As a result, the LSPR absorption summit will move and the color of the solution will change. In other words, when the distance between colloidal gold particles in the solution changes, the color of the solution will change accordingly.[2]
(2) Colloidal gold particles can form gold sulfur bonds with sulfhydryl groups, when the sulfhydryl group is modified at both ends of the DNA chain, you can connect colloidal gold particles through the DNA chain, by controlling the length of the DNA strand, we can achieve the control of the distance between colloidal gold particles, so as to achieve the color change of the solution.[3]
Our design
1. We want to reflect the change of the detection index visually through the change of the color of the colloidal gold solution. In our previous reaction step, the output was cas12a with trans-cleaving activity, so we chose single-stranded DNA to connect colloidal gold. Firstly, the color of the solution is changed through the connection of DNA strands. Secondly, when the single strand DNA is cleaved after the output of cas12a in the previous reaction, the color of the solution can be restored to different degrees.
(This approach could also be applied to other systems by designing the DNA strands that connect colloidal gold particles based on the properties of the enzymes that were output in the previous reaction.)
2.The colloidal gold solution we selected was itself wine-red. After experiments, we verified that the color of the colloid gold could be changed by connecting with 30nt(9nm) ssDNA.
Figure1(A: Before crosslinking, the solution color appears wine-red; B: After crosslinking, the solution color appears purple; C: (TEM photo) The colloidal gold particles in the solution before crosslinking are relatively dispersed; D: (TEM photo) The colloidal gold particles are aggregated together after crosslinking.)
3.We mixed the colloidal gold solutions before and after crosslinking in different proportions to simulate the possible solution color after cleaving of different amounts of cas12a output in the previous reaction. From our experimental results, it can be seen that the solution color did change.
Reference
[1]Yokota S. Preparation of colloidal gold particles and conjugation to protein A, IgG, F(ab')(2), and streptavidin. Methods Mol Biol. 2010;657:109-19. doi: 10.1007/978-1-60761-783-9_8. PMID: 20602210.
[2]Lv S, Du Y, Wu F, Cai Y, Zhou T. Review on LSPR assisted photocatalysis: effects of physical fields and opportunities in multifield decoupling. Nanoscale Adv. 2022 Apr 28;4(12):2608-2631. doi: 10.1039/d2na00140c. PMID: 36132289; PMCID: PMC9416914.
[3]Choi JH, Lim J, Shin M, Paek SH, Choi JW. CRISPR-Cas12a-Based Nucleic Acid Amplification-Free DNA Biosensor via Au Nanoparticle-Assisted Metal-Enhanced Fluorescence and Colorimetric Analysis. Nano Lett. 2021 Jan 13;21(1):693-699. doi: 10.1021/acs.nanolett.0c04303. Epub 2020 Dec 21. PMID: 33346665.
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