Toward DNA-embedded Quantum Dots III
May 13, 2018
by Kil Ho Lee
CdSe/ZnS with 3-mercaptopropionic acid (3-MPA) ligand is soluble in aqueous environment. DNA embedding procedure involves the additional ZnS shell formation, which the ssDNA molecules are physically entrapped within ZnS layers. Previously, the success of ZnS shell formation on 3-MPA coaged CdSe/ZnS QDs were demonstrated (refer to Toward DNA-embedded Quantum Dots II). Based on TEM images and the size distribution alaysis, the formation of additional ZnS shell resulted in the increase in QD size from ~ 7 nm to 12 nm. This result validated the feasibility of the technique used.
DNA embedded QDs were stable in water and the visual and TEM iamge inspections did not show a sign of aggregationl; the aggregation of QDs is an indication that the surface is not well passivated with ligands and/or DNA strands. Also, based on our previous experience, the insufficient DNA passivation can cause nanoparticles to aggregate in a buffer (i.e. 1M Mg buffer).
To further validate the prescecne of DNA embedded in the newly formed ZnS shell, we characterized QDs using Total Internal Reflection Fluorescnece (TIRF) microscopy.
TIRF microscopy and QDs stability
First, 3-MPA coated QDs in Mg buffer was examined using TIRF microscopy. (Figure 1) The emission fo QDs used here is at 600 nm. QDs were excited at 561 nm and, as shown in Figure 1., photons emitted by QDs in Mg buffer were detected.
Figure 1. TIRF microscopy of 3-MPA coated QDs in Mg buffer
Next, DNA embedded QDs in water and Mg buffer were examined using TIRF microscopy. (Figure 2)
Figure 2. TIRF microscopy of DNA embedded QDs in water and Mg buffer
TIRF microscopy showed DNA embedded QDs in water was stable. However, they were aggregated in Mg buffer. This results sgugested the presence of QDs with partially embedded DNA; DNA embedded QDs in Mg buffer showed both aggregated QDs and individually dispersed QDs.
We expect the stability of QDs to be higher with increasing DNA packing on QD surface. Currently, we are working on achieving more densely packed DNA surface.