February 22, 2019
by Carol Lynn Alpert
Participating: Jessica, Peter, Abhijit, Abhilasha, Kil Ho, Faiz, Carol Lynn
Can we continue our quest beyond the April 30 end-date of our grant? The Optics and Communications teams have reserved enough funds to request a "no-cost extension," for another year of work, but the QDot team is running out of funds. During this discussion, Jessica agreed to request a supplement to keep the QDot team embedded in the collaborative effort. While, they haven’t yet solved all the challenges to making a photoswitchable quantum dot, they have made a lot of progress. We want them to keep working on it, so we can test it on the microscope in Georgia.
This month, Abby explored a possible method of detecting successful on/off photoswitching of QDot-DNA conjugates. STORM imaging requires that our QDots switch on and off intermittently, and our strategy is to use complementary single strands of DNA that can be triggered to hybridize and pull a gold nanoparticle close to the QDot, quenching it, or dehybridize into single strands, pulling the gold nanoparticle further away and allowing the QDot to resume fluorescing. The trigger here is an azobenzene molecule connected to the DNA strands. Abby focused her efforts on seeing if she could first study photoswitching in just the azobenze-DNA composite (azoDNA) alone, by measuring absorbance of UV light. Absorbance value readings should have been consistent, indicating whether the azoDNA is in hybridized or dehybridized mode. But the readings for each trial were inconsistent, and so that detection method proved disappointing. Next, Abby is considering using an intercalating dye which would fluoresce more brightly on hybridized DNA strands than on single strands. Read more here.
Meanwhile, Kil Ho explored the effect of increasing the thickness of the zinc sulfide (ZnS) shells around his QDots. He hoped that the thicker shells would increase DNA embedding and also enhance the fluorescence intensity emitted by the QDots. (He also hopes thicker shells will reduce the tendency of QDs to leak through the filters during the purification process with PBS buffer.) He succeeded in increasing shell size, DNA embedding, and fluorescence. Next he will investigate whether the thicker shells will also solve the filtering problem.
Kil Ho also investigated DNA hybridization between his single-strand DNA-QDot conjugates and complementary single-strand DNA attached to DNA hinges made by the Castro group at Ohio State. (The hinges served as a stand-in for the gold nanoparticles that will eventually be used.) Kil Ho's initial TEM image (see figure) did not reveal QDots bound to the hinges; so next time, he will try increasing the number of QDots in solution. Read more here.
Abhijit delivered a talk on the Kner Lab’s current research at the Photonics West conference in San Francisco in early February.
His work this month focused on finding the optimal distance between the interferometer and the camera in order to increase the signal-to-noise ratio. A shorter distance improves the signal-to-noise ratio but at the cost of some degree of resolution between two points.
After making the adjustments, Abhijit recorded the axial view of a 100nm fluorescent bead imaged at different focal planes (4 µm step size) across a 20 µm axial range. (This is called phase-shifting (or phase-stepped) digital holography.) Abhijit captured a sequence of interferograms that each indicate the optical phase relationships between light returned from the bead and a controlled reference beam of light. See more here.
In Boston, plans are underway for NanoDays with a Quantum Leap and the Finals of the national Quantum Matters™ Science Communication Competition, both occurring on April 6. We are also working with IBM to bring a replica of its 20-qubit quantum computer to the Museum for the month of April. And, last week Carol Lynn was inducted as a Fellow of the American Association for the Advancement of Science.
Our next meeting is scheduled for Wednesday, March 20, at noon.