Nanoscale Magnetic Resonance Imaging with Nitroxide Spin Labeling

Hoang Long Nguyen will share a fast novel, image-reconstruction protocol using Bayesian Markov-chain Monte Carlo methods he and his research team developed that obtains the electrons’ coordinates with error bars.

***Lunch will be served***

Imaging individual electron-spin labels attached to a single protein or protein complex with atomic precision would be a powerful tool for structural biology.Electron spin resonance from individual electron spins has been detected and imaged previously using magnet-tipped cantilevers [1] and nitrogen-vacancy centers in diamond [2], but these experiments required extraordinarily long signal-averaging times — 13 hr/point in Ref.1 and 40 min/point in Ref.2 [3].A per-spin sensitivity sufficient, in principle, to quickly detect single electron spins has been demonstrated in an experiment in which nuclear magnetic resonance was observed as a force acting on a high- compliance microcantilever carefully prepared with an integrated nanomagnet tip [4].Here we present protocols designed to enable these cantilevers to, in practice, (1) mechanically detect electron spin resonance from individual nitroxide-based spin probes in reasonable detection times of 1 minute/point and (2) reconstruct with atomic precision the location of the individual spins probes from the detected signal map.

To detect electron-spin resonance we will observe electron spin magnetization as a change in the spin-force gradient acting on a magnet-tipped cantilever [5].This approach has the advantage of allowing us to observe the average, Curie-law magnetization of the electron spins instead of requiring us to detect electron-spin fluctuations as in previous experiments [1, 2].The spin-force- gradient shifts the mechanical oscillation frequency of the cantilever.A signal map is acquired by scanning the cantilever in (x, y) and an image-reconstruction protocol is applied to obtain an estimate of the sample’s electron-spin density.

We have developed a fast novel, image-reconstruction protocol using Bayesian Markov-chain Monte Carlo methods, obtains the electrons’ coordinates with error bars.A surprising finding is that a three dimensional image can be obtained from two dimensional data, accelerating the data acquisition by a factor of 32 to 64.Simulations employing currently-achievable signal-to- noise ratios indicate that the algorithm can reconstruct the (x, y, z) locations of individual buried electron spins to within a resolution of just a few A ̊ in experiments requiring under a day of signal averaging.

[1]  Rugar, D., Budakian, R., Mamin, H. J. & Chui, B. W. Single spin detection by magnetic resonance force microscopy.Nature 430, 329 – 332

(2004).

[2]  Grinolds, M. S. et al.Subnanometre resolution in three-dimensional magnetic resonance imaging of individual dark spins.Nature Nanotech

9, 279 – 284 (2014).

[3]  Yacoby, A. Personal Communication (2015).

[4]  Longenecker, J. G., Mamin, H. J., Senko, A. W., Chen, L., Rettner, C. T., Rugar, D. & Marohn, J. A. High-gradient nanomagnets on

cantilevers for sensitive detection of nuclear magnetic resonance.ACS Nano 6, 9637 – 9645 (2012).[PMCID:PMC3535834].

[5]  Moore, E. W., Lee, S.-G., Hickman, S. A., Wright, S. J., Harrell, L. E., Borbat, P. P., Freed, J. H. & Marohn, J. A. Scanned-probe detection of

electron spin resonance from a nitroxide spin probe.Proc.Natl.Acad.Sci.U.S.A. 106, 22251 – 22256 (2009).[PMCID:PMC2799694].

Hoang Long Nguygen

Hoang Long Nguyen is a Ph.D. candidate in the Department of Chemistry and Chemical Biology at Cornell University.His research interests include nanoscale characterization of novel materials and biological samples.As a graduate student in Professor John Marohn’s lab, Hoang has focused on developing a scanning probe technique to obtain three dimensional images at sub- nanometer resolution using mechanically detected magnetic resonance.Through a combination of experimental work, theory and simulation, Hoang has developed a new protocol to image individual nitroxide spin probes commonly attached to biological molecules, with the potential to achieve angstrom resolution.Outside of research and teaching, Hoang has been engaged in multiple extracurricular activities, including outreach and the shared governance at Cornell.During his time at Cornell, Hoang co-led a workshop called “Juice from Juice:Berry Solar Cells” at the annual Expanding Your Horizons Conference.This work- shop aimed to educate middle school girls about solar energy, and to encourage them to pursue a career in science, technology, engineer and mathematics.In the last four years, Hoang served as a member and chair of the Graduate and Professional Student Assembly Finance Commission at Cornell.