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Sanli Faez

Assistant professor of physics at Utrecht University My current scientific activities are mainly fo[...]

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Assistant professor of physics at Utrecht University My current scientific activities are mainly focused on sensing the motion of elementary charges in thermal or electronic transport with optical techniques. I studied Physics at the Sharif university of technology and got my master of science in nanotechnology from university of Twente. My master and PhD projects were focused on wave propagation in random media and Anderson localization. I got my PhD in 2011 from the university of Amsterdam and then moved to the Max Planck institute for the science of light as a post-doc. There I devised a new scheme for coherent coupling between single organic molecules and dielectric waveguides. In August 2013, I came back to the Netherlands and pursued the idea of optically tracing single electrons with single molecule spectroscopy at Leiden institute of Physics. During my stay at Leiden, I also developed nanocapillary electrophoretic tracking (nanoCET) to study rapid changes in the mobility of single nanoparticles. Since August 2015, I joined the group physics of light in complex systems (nanoLINX) at the department of physics and astronomy at Utrecht university. Our group is also part of the Debye institute for nanomaterials science. At Utrecht, I am expanding the nanoCET technique to study single charge transfer on mobile nanoparticles at ambient conditions.

  • Debye Institute for Nanomaterials Science - Utrecht University
    Padualaan 8, NL-3584 CH Utrecht, The Netherlands
  • Department of Physics and Astronomy, Faculty of Science - Utrecht University
    Buys Ballot building - Princetonplein 5, NL-3584 CC Utrecht, The Netherlands

Sanli Faez's Self-Journal


Electrophoresis is the underlying mechanism for a broad range of essential analysis techniques in colloid science, biochemistry, and biotechnology. The electrophoretic mobility (drift velocity in a viscous medium under application of external electric field) of proteins and other macromolecules is a very sensitive indicator of their internal charge state, their interaction with the surrounding environment, and their hydrodynamic radius. As such, it has been used for sorting a mixture of macromolecules or studying their conformational changes due to chemical reactions or physical adsorption to other molecules. Capillary- and gel- electrophoresis are two of the most widely used methods of measuring electrophoretic mobility in ensembles of molecules.

These methods, however powerful, are too slow for following the temporal dynamics of a kinetic reaction. In the existing devices, the smallest measured sample volume (in the nanolitre range) is limited by the optical detection sensitivity. This minimal volume still contains millions of molecules and each molecules proceeds on a stochastic and unsynchronized path throughout the reaction. While statistical methods can be used to uncover the reaction rates for simple processes, this is a daunting task for multi-step reactions.

In an ensemble measurement, the observation of intermediate states is often impossible because of averaging over unsynchronized and inhomogeneous processes. This is why single molecule studies have played a very central role in uncovering the intermediate steps of biomolecular and catalytic reactions. Single particle measurements are not prone to disturbance by impurities because these event can be singled out in data analysis. Furthermore, such measurements can be repeated as many times as necessary for collecting reliable and informative statistics. In other words, when the physicists’ granular view of a reaction on a single particle is reconstructed by a direct measurement, the characteristic kinetics and thermodynamics of the reaction is no more obscured by material inhomogeneities or stochastic dynamics over the metastable intermediate states. Important chemical reactions, such as photo-catalysis or enzyme-catalysis, involve multiple steps of largely differing timescales. Furthermore, their reaction rates can be heterogeneous within an ensemble of catalyst particles and can even vary over time for a single catalyst particle. In principle, there is no fundamental barrier for studying a full chain of chemical reactions on a single molecule or nanoparticle through monitoring its electrophoretic mobility, IF (and that is indeed a big IF) one could track the single molecule motion BOTH on the shortest necessary timescales AND as long as the reaction is complete.

Actually, in a glorified experiment more than a century ago, Robert Millikan and Harvey Fletcher mastered measuring the drift velocity of a floating charged particle continuously to a level that they could set an accuracy record for the value of the elementary charge. The challenge we still face is how to perform that measurement on a single particle or even a single molecule, rapidly enough for resolving the reaction steps. The significance of reaching this goal is self-evident: if a researcher can monitor the charge (or the electrophoretic mobility) of a single solute rapidly enough, she or he will be able to study kinetic interactions such as ionization, hydrolysis, or charge transfer at the single particle level. In such a measurement, one can directly "track" the intermediate steps of a reaction, which are smeared out in bulk experiments because each molecule follows its own pathway stochastically.

In this issue, I comment on a handful of experimental reports that, in my view, together make a convincing case that monitoring chemical reactions on mobile nanoparticles and single molecules is within experimental reach. I start with Millikan's 1911 article in the Physical Review where he presented his (or their, more on this later) first measurements of charges on a single oil droplet. Next comes the anti-Brownian (ABEL) trap developed by Cohen and Moerner which teaches out how to shrink the size of the probed particle to the nanometer scale and to perform the experiment inside dense fluids. In this 100-year fast forward, I skip all the remarkable developments around trapping single atoms and single electrons in vacuum inside quadropole traps with the excuse that those are passive, while Millikan and Cohen trap particles in their setups by using an active (visual) feedback. Another recent experiment by Beunis et al, performed in a passive optical trap, reconfirms elementary charge sensitivity of drift velocity measurements now in dense inorganic fluids, albeit on micrometer size dielectric particles. On a parallel token, I take a beautiful demonstration of sensitivity of capillary electrophoresis to elementary charge differences, here in ensembles of proteins, in the group of Whitesides. The fringe contrast of these charge ladders, limited mostly by the finite size of the measured sample, testifies to sufficient durability of charges in such biomolecules well below the elementary charge fluctuation. These results also signify the importance of long-duration measurements for monitoring charge fluctuations. Short duration of mobility measurement, due to photobleaching, and coarse time resolution, due to the limited photon budget, are the two major limitation of the ABEL trap for monitoring such chemical reactions that I envision. This is inherent to any technique that is based on fluorescence detection. I conclude this issue with one experimental report on particle tracking based on elastic light scattering and a feasibility study for performing equivalent of Millikan's experiment with small nanoparticles and even some macromolecules.

Sanli Faez
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The Isolation of an Ion, a Precision Measurement of its Charge, and the Correction of Stokes's Law

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