December 7, 2017
by Carol Lynn Alpert
All hands were on board for today's phone meeting. (We had no November meeting because of Thanksgiving breaks.)
Abhijit posted the first Optics Team blog! He included lots of pix of the microscope set-up as new parts are machined and assembled. Here is a close-up of the area beneath the sample stage.
Abhijit is currently setting up all laser emission pathways to the sample and from the sample to the camera. A new laser (551nm, yellow) is on order. Abhijit worked with the stage supplier Smaract and they were able to get his Micromanager software to talk to Smaract and the camera. He can now exploit all the stage functionality required. Our first images will start coming in sometime in January!
In Ohio, success! Kil Ho reported success using 3-MPA ligands to transfer Ocean Nano red CdSe/ZnS core/shell combos from organic (oily) to aqueous (water) solution, while also preserving their absorbance and fluorescence peaks. He vortexed the solution overnight, avoiding any sonication which can damage the QD surfaces.
This successful transfer to water means Kil Ho is ready to try DNA conjugation, adding more ZnS coating mixed with DNA. After he's solved that step, they'll move on to smaller Ocean Nano green quantum dots, which are less stable, but required for the interaction with gold nanoparticle quenching.
Abby is still struggling with all three aspects of the "Loops, Trains, Trails" DNA conjugation method: (1) preserving QD fluorescence through growth of ZnS shells; (2) aqueous transfer; and (3) DNA conjugation.
(1) She wasn't able to replicate her prior success using the SILAR method to add ZnS shell layers to the CdSe cores. She was using an existing batch of homemade CdSe cores, and has concluded that fresh batches need to be made for every experiment to avoid oxidative damage.
(2) Since she had no core/shell combos to proceed to the aqueous transfer stage, Abby decided to test the transfer stage using stable red Ocean Nano core/shell combos. After learning she needed to heat the solution to about 80°C, she was able to take them successfully through the intermediated pyridine transfer step with almost no loss of fluorescence. However, the next aqueous transfer step - attaching phytohelatin-3 (PC3) ligands in a high-pH buffer solution - failed. Was the concentration of that buffer solution too high? The QD/shells did not completely dissolve in water; they settled over time. Additional QD/shells underwent aggregation and sedimentation during dialysis. While the resulting QD/shells were fluorescent, they did not show a clear dissolution or absorption peak in solution. Abby called the author of this published technique and got an exact product # for the pH buffer solution to see if that will fix the problem when she tackles it again.
(3) Abby also worked on the DNA conjugation phase, using the lab's micelle-coated QDs in aqueous solution, which are pretty stable. But they were too big to work well with the DNA hinges. Then, she tried working with red QDs from Ocean Nano, transfering them to water by coating them with 3-MPA (as Kil Ho did) instead of the more desirable PC. But this failed as well, with the QDs precipitating out immediately when she added the low pH buffer necessary for DNA conjugation. The same thing happened when starting with commercial QDs that were already in aqueous solution. Next time she will use a buffer with a higher "physiological pH" of 7.2. The drawback is that this will impede the efficiency of the EDH chemistry for conjugating the DNA. So, what she really wants to do is to get working the SILAR transfer step of attaching PC3 ligands to the core/shells. These PC3 ligands are hardier than 3-MPA, because they offer more attachment sites for the DNA hinges.
The Communications Team is thrilled that the Blog is up and running and have made several posts about our work as well as posting monthly phone meeting notes. And Abhijit's entry into the QSTORM blogosphere has delighted us.
We'll be back in 2018. Until then, happy loops, trains, and trails.