I don't understand which is the mechanism behind the visual reaction? I mean how the thiolated probe, the gold nanoparticle and the genomic DNA interact so that you see a color change?
the thiol ~SH ~ group reacts with GNP as well as to DNA through vander walls force .So GNP may act as a indicator for the binding by color change.
Thanks forma answer, in this case you have thiolated oligonucleotides ( probes), the genomic DNA (dsDNA) and the gold nanoparticles. I hace read that the thiolated oligonucleotides interactuar covalently with the nanoparticles through a interacción between the thiol group and the citrate groups in the surface of the nanoparticles. I dont understand why when you add the complementary dsDNA to the nanoparticles with the thiolated oligonucleotides you ser a change in the color from red to blue.
The reason you can see the change of the color from red to blue is decreasing of the distance between the gold nanoparticles. If you use your GNPs conjugated with oligonucleotides and add specific ds or ssDNA, you assert the nanoparticles closer to each other and thus there is a change in plasmon resonance. You see it with the naked eye in the form of a color change from red to blue. If there is no specific DNA in the probe, the color of the GNPs should be still red.
I hope that I helped you?
Change in color normally indicates size-change of the gold nanoparticle (blue-shift: smaller; red-shift: larger). As the DNA-thiol should replace the citrate and bind directly to the gold surface you would intuitively think you would get a slight red-shift if anything. But the DNA might intact with the light and affect the plasmon resonance of the gold nanoparticle which could alter the color independently of change in particle size (eg. search for gold nanoparticles used as DNA hybridization sensors). I would check if you see change in the hydrodynamic size (before and after modification) by either dynamic light scatter (DLS) or NanoSight (video-tracking) and compare that with a spectroscopic analysis (UV-Vis).
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