Research

I am interested in the intersection of chemistry, biology, and computer science. My first research focus was the synthesis of semiconductor nanoparticles, working at UC Berkeley. Since then, my research has shifted to studying dynamic nanoscale systems in general and developing tools to do so from computer science and chemistry. These topics includes nanoscale self-assembly, single molecule imaging, and liquid electron microscopy. Along with these, I have become very interested in image analysis and computer vision. In the future, I am interested in translating my knowledge and skills into medical and clinical applications working at Stanford University.



Projects

Dynamic Electron Microscopy

Liquid electron miscropy enables us to finally peer into a dynamic world of particles and atoms. Currently, the technique limited to metallic materials that can withstand the bombardment with electrons and produce high contrast images. However, I believe this is the first step to finally seeing detailed biological systems in action.

We used this technique to study a variety of new phenomena. We measured piconewton forces generated by Van der Waals and magnetic dipole interactions between nanoparticles over 1-7 nm length scales. These interactions led to assembly of nanoparticles into organized chains and lattices. When templated by circular liquid droplets, we could produce nanoparticle rings 50-100nm in diameter. I also analyzed the growth and etching of crystals. In our study published in Science magazine, we demonstrated that preferential etching along surfaces with high curvature can be used to generate nanoparticles with new types of geometries, like tetrahexahedrons.

Protein labeling with Nanoparticles

Working at the Molecular Foundry at Lawrence Berkeley National Laboratory, I designed fluorescent nanoparticles to be bright, stable probes to visualize the motion of proteins inside cells.


Proteins are incredibly dynamic: they move around within cells and for many, internal moving parts are critical for their function. In this project, we were interested in kinesin, a motor protein that transports vesicles along microtubules via two feet and a hand over hand motion. I optimized the nanoparticle synthesis to produce two particles of different sizes with two different colors that were small, bright, and had narrow emission linewidths. These two colors would be used to track each head of a single kinesin independently. I coated the nanoparticles in amphiphilic polymer so they would be soluble inside the cell and attached them to engineered kinesin via the SNAP-tag labeling system. The nanoparticle I developed were 3x brighter than the commercially available ones and significantly improved the imaging in vivo.

Image Analysis and Segmentation

During my microscopy work, I have developed several custom algorithms and applications for image segmentation and analysis. These are particularly suited for electron microscopy videos and for segmenting images, tracking objects, and analyzing object properties such as velocities, geometric relations, and growth processes.

The low signal to noise ratio and fluctuations in background intensity make electron microscopy images particularly challenging for accurate image segmentation. I developed a successful method that uses laplacian of gaussian filters, that when convolved with the image, detect nanoparticle shaped blobs. By using a range of filter sizes in a scale space approach, I was able to quickly capture particles at multiple size scales, avoiding extensive preprocessing, and distinguish overlapping particles. My latest project is applying gaussian fitting from super-resolution flourescence microscopy to obtain tracking of nanoparticle motion with sub-nm accuracy.

Undestanding Stress Sensing Tetrapod Crystals

The Alivisatos lab developed tetrapod shaped nanoparticles capable of sensing tensile stress. When embedded in a polymer matrix, stress and strain applied to the matrix could be measured via a change in fluorescence emission wavelength from the particles. I used image analysis techniques to better understand this effect, studying electron microscopy images of the embedded particles. We found that the particles tended to aggregate within the polymer, and the sensing effected depended on how tightly they were packed together. Ultimately, we were able to use this technology to measure the deformation of an artificially respired chicken lung.


Publications

  1. Quantum dot luminescent concentrator cavity exhibiting 30-fold concentration
    ND Bronstein, Y Yao, L Xu, E O’Brien, AS Powers, VE Ferry, AP Alivisatos, ...
    ACS Photonics 2 (11), 1576-1583, 2015
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  2. Single-particle mapping of nonequilibrium nanocrystal transformations
    X Ye, MR Jones, LB Frechette, Q Chen, AS Powers, P Ercius, G Dunn, ...
    Science 354 (6314), 874-877, 2016
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  3. Mechanisms of local stress sensing in multifunctional polymer films using fluorescent tetrapod nanocrystals
    SN Raja, D Zherebetskyy, S Wu, P Ercius, A Powers, ACK Olson, DX Du, ...
    Nano letters 16 (8), 5060-5067, 2016
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  4. Tracking nanoparticle diffusion and interaction during self-assembly in a liquid cell
    AS Powers, HG Liao, SN Raja, ND Bronstein, AP Alivisatos, H Zheng
    Nano letters 17 (1), 15-20, 2016
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  5. Covalent Protein Labeling and Improved Single Molecule Optical Properties of Aqueous CdSe/CdS Quantum Dots
    SM Wichner, VR Mann, AS Powers, MA Segal, M Mir, JN Bandaria, ...
    ACS nano , 2017
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  6. Characterizing Photon Reabsorption in Quantum Dot-Polymer Composites for Use as Displacement Sensors
    MA Koc, SN Raja, LA Hanson, SC Nguyen, NJ Borys, AS Powers, S Wu, ...
    ACS nano 11 (2), 2075-2084, 2017
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  7. Liquid Cell TEM Study of Nanoparticle Diffusion and Interaction in Liquids
    AS Powers, HG Liao, H Zheng
    Proceedings of Microscopy and Microanalysis 22 (S3), 742-743