February 26, 2018
by Carol Lynn Alpert
This is the month when we are reminded how much of our scientific progress depends on the reliability of products delivered to us by outside vendors. We are building a finely-tuned imaging instrument; we have little wiggle room for error. Sometimes it takes a lot of trouble-shooting to find out that vendor quality control is at fault. Peter, Jessica, Abby, Kil-Ho, and Carol Lynn conferred this month.
The Optics team has had to send the new high-precision Smaract microscope stage back to Germany to get the Y-axis actuators fixed. Not only that, but the adaptive optics mirror appears to be "on the fritz" (Peter's exact words), and they've had to order a new one. Instead of standing around twiddling their thumbs, the team is using their old scope to help a collaborating biologist take measurements of histone proteins in mouse oocytes. The eggs jiggle around like jello, so it was hard to get them fixed in the dish. The collaborator removed an outer layer of cells; that seemed to calm them down. The images are not yet available for posting.
Meanwhile, in Columbus, Abby and Kil Ho are dealing with a new batch of CdSe quantum dots - both red and green - just sent from a vendor - that seem to have become oxidized somewhere on the way to Columbus. They cost about $550 a batch, and the company promised to send replacements. In the meantime, there was just enough fluorescence remaining for Kil Ho and Abby each to make some progress this month: thery are getting closer to conjugation with DNA! If you want the blow-by-blow account, see their QDot Team blog posts. Here's a summary:
The DNA Embedding technique: This is Kil Ho's baby, and it's an attempt to embed single strand DNA within the ZnS shell as it grows around the QDot core. Last month’s attempt showed no measurable enlargement of core size and therefore no accumulation of ZnS, and the QDots also precipitated out of the aqueous environment. This month, however, Kil Ho was able to keep both pH and temperature more tightly controlled. He used a water bath rather than a hot plate to heat the solution to 90° C for the required hour. The bath provided more even heat distribution and better stability. Kil Ho was able to verify that a ZnS coating with embedded DNA had accumulated around the core: the average particle size increased and so did the average Zeta potential (a measurement of charge). Curiously, the coating elongated the cores in an axial manner rather than spherical. The next step is to verify DNA embedding by incubating the shell/core combos with DNA origami hinges. The Castro group will provide DNA hinges equipped with complementary DNA sequences at the nanoparticle binding site. Kil-Ho will use TEM and fluorescence microscopy to determine if the DNA-embedded QDs bind to the hinges.
Loops, Trains, Trails Technique: During January, Abby reported that she had achieved the loops-trains-tails aqueous transfer and low pH buffer exchange of organic quantum dots; however, it was accompanied by a disappointing loss of fluorescence. This month, she replicated the procedure, again with poor results, including phase separation and poor solubility. To avoid excess interference from the phase separations, she converted to a two component system using the completely miscible solvents pyridine and TMAOH, and leaving out the water. This, it turns out, is feasible because of the high solubility of phytochelatin-3 in pyridine.
It worked. The modified procedure turned out to be efficient and reproducible with minimal loss of fluorescence during the complete aqueous transfer. Abby recorded a negative zeta potential on the surface of the phytochelatin-3 coated particles, indicating the availability of carboxyl groups for conjugation with the amine terminal on DNA. She was able to use the standard EDC-sulfo NHS chemistry and the QDots maintained stability. They also maintained stability in the buffer conditions required for DNA origami conjugation.
And yet, they did NOT conjugate. Abby thinks perhaps the single DNA strands she’s using are too long, and thus have a more negative charge. She will test this by substituting an amine-terminated fluorescent dye molecule of the same length.
The Museum team had some great help refining our Quantum Dot Mystery Detective activity, from our Research Communication Lab students at the Center for Excitonics at MIT. The new version will debut at NanoDays on April 7. Also debuting there will be the Finals for our first ever Quantum Matters™ Science Communication Competition. Four finalists were notified last week and we are providing them with coaching next weekend and the following week.
Next Up: Phone Meeting on 3/30 at noon and April 10 at noon. Our annual reports are due at the end of April.