January 16, 2018
by Abhilasha "Abby" Dehankar
Happy new year, everyone! Following is my update for the first call in 2018!
Quantum dot synthesis
Prior trial for growing multiple (~10-15) shells on Cadmium selenide core led to growth of two distinct populations indicating an unsuccessful growth. Therefore, I had decided to revisit the complete synthesis procedure. In all the previous trial, CdSe cores optimized to green color in Winter lab were used as precursors for future shell growth as opposed to that used in the reference paper. According to literature, best ZnS shell growth was obtained for CdSe cores with Cd:Se ratio of 1:10, which is much lower as compared to Winter lab CdSe cores, Cd:Se=1:2. Thus, to avoid any complications related to CdSe cores for robust ZnS shell growth it was decided to synthesize the exact cores used in the reference paper. In the current reporting period, green CdSe cores from reference were synthesized. The absorbance and fluorescence spectrum of the synthesized cores (top) are represented in the figure to the left and can be compared with the desired CdSe cores for the core (bottom). Despite good fluorescence as synthesized, the optimized purification procedure could not produce consistent quantum yield cores indicating a sensitive surface. Furthermore, observed fluorescence peak wavelength of the cores is considerably blue shifted at the operating conditions indicating incomplete growth. As a result, subsequent shell growth process could not be tested. Future experiments would focus on producing robust cores followed by shell growth.
Biocompatible QDs and DNA conjugation:
Successful transfer of organic QDs to water requires an efficient transfer of its organic ligands to pyridine in the first step, followed by ligand exchange with PC3 (diagram). The hot pyridine method for the first step described during the prior meeting showed excellent reproducibility for the commercial red CdSe/ZnS QDs. Ligand exchange of pyridine to PC3 is triggered by increment of the solution above pH 10 using Tetramethylammonium Hydroxide (TMAOH). The aqueous transfer performed using newly ordered TMAOH was successful. These results were confirmed through absorption, fluorescence, long term storage and high speed centrifugation. In addition, these biocompatible PC3 coated QDs demonstrated successful transfer to low pH (pH 5) as well as physiological pH (pH 7) buffers that are involved in DNA conjugation chemistries. However, these steps were accompanied by fluorescence decrease. Therefore, future experiments would focus on reproducing these results while maintaining a high quantum yield for the QDs.
EDC chemistry was explored for DNA conjugation of the QDs above. The final conjugated displayed solubility at high magnesium salt concentrations required for DNA origami conjugates, hinting towards a successful conjugation. This final product however yet remains to be tested with DNA origami hinges for conjugation confirmation.