December 7, 2017
by Abhilasha "Abby" Dehankar
Quantum dot synthesis
Previously, ~4 monolayers of Zinc Sulfide (ZnS) shell was developed on the CdSe cores. These QDs were not stable against ligand exchange to pyridine. Therefore, to increase the robustness of these particles, it was decided to develop additional (~10-15) ZnS monolayers. Unlike previous trials, recent effort to synthesize multiple ZnS layers, did not depict the decrease in CdSe core size on first TOP activation (Refer to figure: TOP assisted SILAR). Furthermore, addition of just two layers clearly indicated formation of two population sizes. Thus, this trial could not achieve the desired CdSe/ZnS QDs. Following figure consists of the Fluorescence and Absorption spectrum of CdSe cores and 2 ZnS monolayer coated QDs. The growth of two fluorescence peak for CdSe/ZnS as compared to a single peak for CdSe cores represents inefficient bimodal growth. The absorbance peak widening for CdSe/ZnS also hints towards presence of multiple trap states. Therefore, future efforts would be directed towards better understanding the synthesis flaws through deeper literature search and re-trial of synthesis.
Biocompatible QDs and DNA conjugation:
Loops-trains-tails procedure is being used for making organic QDs biocompatible. The pathway for aqueous transfer of QDs is illustrated in the diagram above. Previous trials for exchanging the native ligands of QDs with pyridine was extremely inefficient as observed from precipitation of QDs over time as well incomplete dissolution. As a result, ligand exhchange with the compact peptide (phytochelatin-3, PC3) also did not produce desired results. Therefore, experiments in this reporting period focused mostly on optimization of pyridine ligand exchange. Amongst all the tested procedures, ligand exchange using hot pyridine (80C) provided best results.
In the aqueous transfer process, QDs in pyridine are mixed with excess PC3 dissolved in water and the ligand exchange is triggered by addition of a base (tetramethyl ammonium hydroxide). During the current reporting period, experiments for aqueous transfer did not show complete dissolution of QDs in water and was accompanied by sedimentation over time. Literature indicates that following four factors greatly influence a robust PC3 coating:
Moving ahead, factors influencing the ligand exchange to PC3 would be investigated.
Furthermore, DNA conjugation was tested on QD encapsulating micelles and MPA-coated QDs as a control to simultaneously optimize the future steps loops-trains-tails biocompatible QDs. Since, all these particles are carboxyl group terminated after aqueous transfer, a standard EDC-sulfo NHS chemistry would be used for DNA conjugation. This process involves buffer exchange of carboxyl terminated QDs to low (pH~5)/physiological pH (pH~7) buffers. MPA-coated QDs as opposed to PC3-coated QDs, cannot survive low pH (~5). Therefore, these QDs cannot provide a good control for our experiments.
Micelle encapsulated QDs on the other hand are extremely robust to all required pH conditions and were conjugated to DNA. Post DNA conjugation, they were tested with DNA origami hinges for specific binding which would determine a successful conjugation. As shown in the figure below, the conjugation site for the micelle is supposed to be at the lower arm of the DNA hinge. Although many specific bindings were observed (yellow circles structures), the trial turned out to be inconclusive becayse of follwoing reasons:
As a result, micelle also did not provide a good control. Therefore, future experiments for DNA conjugation optimization would focus mostly on either smaller micelles or PC3 coated QDs.