January 17, 2018
by Kil Ho Lee
Toward "DNA-embedded Quantum Dots (CdSe/ZnS)"
Quantum Dots from Ocean Nanotech (CdSe/ZnS – emission: 600 nm) was used to optimize the procedure for coating additional ZnS layer.
Above method was adopted from a literature. However, the aggregation was observed as soon as Zinc source was added and increasing temperature further aggregated the nanoparticles. It was assumed that the ligand exchange was only partially achieved.
Figre 1. QDs in water (Left) after 3-MPA ligand exchange, (Right) after additional ZnS coating
|(1) Ligand exchange procedure was further optimized by increasing the amount of 3-MPA (pH 10) and the ligand/QD mixing time. Small aliquots were taken during the ligand exchange process and the particle stability was examined by centrifugation (low speed); if the particles were not stable, they formed a pallet. – Figure 1.(Left) (2) ZnS coating was previously done in a closed glass vial with high temperature controller. This time, it was done in a beaker on a hotplate. Previous trails suggested the “under-shooting” is better than “over-shooting”. – Figure 1.(Right)|
|UV-Vis spectrometer was used to examine the absorbance of QDs before and after coating the surface with additional ZnS layers. Additional ZnS layer on QDs did not cause a shift in wavelength. However, the overall absorbance increased. More studies is to be done to see if UV-Vis spectrometer results can be a good indicator for quantifying ZnS layers.|
Transmission Electron Microscopy was used to examine the size of QDs before and after the ZnS coating. Figure (A) shows TEM of QDs in water before ZnS coating. Figure (B) shows TEM of QDs after ZnS coating. ImageJ analysis was done to quantify the size of QDs. Figure (C) shows the normalized size distribution of QDs based on the ImageJ analysis. The average (and the modes) sizes of QDs in both cases were not statistically different with the average Feret's length of approximately 6.8 nm. However, it was noted that the size distribution of QDs after additional ZnS coating showed broader range with the largest QD size ~ 11 nm.
To ensure the succuess of ZnS coating procedure, we are currently working on coating QDs with excess Zn. If the additional ZnS coating procedure is working well, we expect to see a dramatic change in the size of QDs.
Once we confirm the ZnS coating on QDs surface, DNA embedding will be done. DNA strands will be added during the ZnS coating process.