Dr. Shengting Cui
Research Associate Professor
Ph.D., University of Virginia: Molecular Dynamics Study of the Sputtering of the Condensed Gas Solids.
Brief Bio
Dr. Cui earned his bachelor’s degree in geophysics from University of Science and Technology of China in 1981, and his Ph.D. in Engineering Physics and Materials Science from University of Virginia in 1989. His dissertation involved research into materials sputtering and desorption from solid surface and related electronic energy transfer processes in surfaces. In postdoctoral researches at Australian National University, Dr. Cui conducted molecular modeling computational investigation of fluid rheology under shear flow. And subsequently at MIT, Dr, Cui carried out research in supercritical water oxidation as an approach for the treatment of wastes and toxic materials. Dr. Cui moved to University of Tennessee in 1994 and joined the faculty of University of Tennessee in 1996. Dr Cui’s research interests include: Lubricant and materials performance under shear stress; confined thin films and friction modification; phase transition and complex structure formation in fluids; and fluid flow and biomolecular transport through micro- and nano-channels. Dr. Cui’s current research efforts involve developing nanofluidics approach for the next generation DNA sequencing technology to realize NIH’s goal of $1000/human genome. Dr. Cui is also working on developing nuclear fuel reprocessing technology and nuclear waste treatment. Dr. Cui has authored or co-authored more than 50 technical papers and contributed numerous technical presentations at national and international conferences.
Research Areas
Nuclear Fuel Reprocessing
Works in this area are aimed at developing an experimentally validated computational capability for understanding the complex processes governing the performance of solvent extraction devices used for separations in nuclear fuel reprocessing. These applications pose a grand challenge due to the three-dimensional, turbulent, reactive, multi-component, multi-phase nature of transport in solvent extraction devices. The figure below provides a schematic view of multi-scales involved in such a system.
Figure Caption: Illustration of multiscale phenomena involved in a typical centrifugal contactor from macroscopic to molecular level. Atomistic simulation is needed to investigate the metal ion extraction process through aqueous/organic interface to increase the extraction efficiency.
The liquid-liquid extraction system of interest consists of an aqueous phase which contains the metal ions to be extracted, an organic phase, and a specific extracting agent that mostly resides in the organic phase. The basic qualitative physical mechanism involved in the system is that the extractant molecules bind to the cations and form a complex at the aqueous-organic interface. The complex then migrates to the organic phase due to its high solubility in the organic phase effectively carrying the metal ion from the interface region into the bulk of the organic phase. This complex migration process involves many molecular level events including: The solvation of the ions and the extractant molecule in bulk and at the interface; the binding kinetics of the extractant and the cation; the diffusion of the extractant-cation complex from the interface to the organic phase; the phase equilibria between the complex at the interface and in bulk phases; the interface composition, etc. The entire process is concisely summarized in the following equation:
To gain an understanding to the molecular level events and their impact on the overall migration process, we have conducted extensive molecular dynamics simulation study to elucidate the uranyl ion extraction processes.
One crucial issue is the complex formation at the aqueous-organic interface. The interface provides a favorable environment for various complexes to form. The extractable charge-neutral species are more easily transferred to the organic phase, tipping the balance of equilibrium, so more extractable complexes form at the interface. We have found that increased nitric acid concentration enhances the extraction efficiency by increasing the formation of extractable charge-neutral complexes. The following figure illustrates the formation of the various intermediates and extractable complexes formed at the aqueous-organic interface.
Figure Caption: Snapshot at 16 ns of molecular dynamics simulation of uranyl extraction from aqueous to organic phase with 19,338 atoms in a periodic computational box. Periodicity means that the two part of the organic phase form one continuous phase. Left: Aqueous phase in the middle of the box contains: water, hydronium ions, uranyl, and nitrate; organic phase at left and right of the aqueous phase contains: TBP and dodecane. Right: Computational box with dodecane, butyl tails of TBP, and unassociated water removed for clarity. Top panel: zoom-in on selected molecular associations in the bulk of the phases. Bottom panel: the same as top panel but showing only the various complexes formed at or near the interfaces. Color: oxygen in red (or pink for the 3 butyl-oxygen on TBP); hydrogen in white; phosphorous in yellow; nitrogen in blue; uranium in green, and CHn (n=2, 3) groups in gray.
Nanofluidics Detection of DNA and Biomolecules
NIH’s long-term goal is “to cut the cost of whole-genome sequencing to $1,000 or less, which would enable the sequencing of individual human genomes as part of medical care. The ability to sequence each person’s genome cost-effectively could give rise to more individualized strategies for diagnosing, treating and preventing disease. Such information could enable doctors to tailor therapies to each person’s unique genetic profile.”
One approach to realizing this goal is to discriminate DNA bases through detecting the electric current in the confinement of a nanopore. This approach is inspired by early experiment using hemolysin pore, in which ionic current difference was observed depending on the presence of differing type of nucleotide segment. Microscopically, ionic current conduction in nanopore is associated with net migration of ionic species, which is modulated by the interaction between ions and DNA.
Our study shows that because of the high negative charge density of the DNA chain, co-ions are largely excluded from nanopore. On the other hand, positively charged counter-ions are attracted to the charges on the DNA which slows down their migration through nanopore. This, in addition to the blockade of the ions by DNA, significantly affects ionic current conduction through nanopore.
The basic mechanism emerging is that the interaction between ions and DNA modulates the ionic current. This interaction is in turn strongly influenced by the molecular characteristics of the bases which affect the accessibility of the ions to the phosphate groups of the DNA. The transient binding of the ions to the phosphate groups considerably slows down the migration of the ions through the pore, causing the current variation. Interested readers are referred to our recent publication [S. T. Cui, Phys. Rev. Lett. 98, 138101 (2007)].
Selected Awards and Honors
1997 Lockheed Martin/ORNL Technical Achievement Award
1992 Leopold Schepp Foundation Fellowship
Professional Activities
Membership: Society of Rheology; American Chemical Society; American Institute of Physics
Selected Publications
S. T. Cui, “Counterion-Hopping along the Backbone of Single-Stranded DNA in Nanometer Pores: A Mechanism for Current Conduction,” Phys. Rev. Lett. 98, 138101 (2007).
S. T. Cui, J. W. Liu, M. E. Selvan, D. J. Keffer, B. J. Edwards, W. V. Steele, “A Molecular Dynamics Study of a Nafion Polyelectrolyte Membrane and the Aqueous Phase Structure for Proton Transport”, J. Phys. Chem. B, 111, 2208 (2007).
J. W. Liu, M. E. Selvan, S. T. Cui, B. J. Edwards, D. J. Keffer, W. V. Steele, “Molecular-level Modeling of the structure and wetting of Electrode/Electrolyte interfaces in hydrogen fuel cells,” J. Phys. Chem. C 112, 1985 (2008).
S. T. Cui, “Electrostatic potential in cylindrical dielectric media using the image charge method,” Molecular Physics 104, 2993-3001 (2006).
S. T. Cui, “Molecular self-diffusion in nanoscale cylindrical pores and classical Fick’s law predictions”, J. Chem. Phys. 123, 054706 (2005).
S. T. Cui, “Molecular dynamics study of single-stranded DNA in aqueous solution confined in a nanopore”, Mol. Phys. 102, 139 (2004).
S. T. Cui, C. McCabe P. T. Cummings, H. D. Cochran, “Molecular dynamics study of the nano-rheology of n-dodecane confined between planar surfaces”, J. Chem. Phys. 118, 8941 (2003).
S. T. Cui, P. T. Cummings, and H. D. Cochran, “Molecular Simulation of the Transition from Liquid-like to Solid-like Behavior in Complex Fluids Confined to Nanoscale Gaps”, J. Chem. Phys. 114, 7189 (2001).
J. D. Moore, S. T. Cui, H. D. Cochran, and P. T. Cummings, “A Molecular Dynamics Study of a Short Polyethylene Melt. I. Steady Shear”, J. Non-Newt. Fluid Mech. 93, 83 (2000).
S. Salaniwal, S. T. Cui, P. T. Cummings, and H. D. Cochran, “Self-Assembly in a Dichain Surfactant/Water/Carbon Dioxide System via Molecular Simulation. I. Structural Properties of Surfactant Aggregates”, Langmuir 17, 1773 (2001).
Contact Dr. Cui
442 Dougherty Engineering Bldg.
Department of Chemical and Biomolecular Engineering
University of Tennessee
Knoxville, TN 37996-2200
Tel: (865) 974-4820
Email: scui@utk.edu