Nielsen, Steven O.

Permanent URI for this collectionhttps://hdl.handle.net/10735.1/3158

Dr. Stephen Nielsen joined the faculty of the Department of Chemistry in 2005. The overarching theme of his research is the theoretical description and molecular dynamics simulation of interfaces. His primary focus is on the curvature-dependent properties of interfaces. Major projects include the peptide solubilization of carbon nanotubes, the behavior of nanoparticles in fluids and at fluid Interfaces, and the free energy of phase transformations between different lipid self-assemblies. In addition, he develops new simulation tools including multiscale modeling algorithms and serves as head of the Nielsen Research Group.

Learn more about Professor Nielsen's work at his Department of Chemistry, Research Group and Research Explorer pages.

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Browsing Nielsen, Steven O. by Title

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    Effects Of Spherical Fullerene Nanoparticles On A Dipalmitoyl Phosphatidylcholine Lipid Monolayer: A Coarse Grain Molecular Dynamics Approach
    (Royal Society of Chemistry, 2012-07-30) Chiu, Chi-cheng; Shinoda, W.; DeVane, R. H.; Nielsen, Steven O.; Nielsen, Steven O.
    The effect of carbon-based nanoparticles (CNPs) on biological systems is currently of great concern. Yet, few experimental techniques are capable of directly imaging and probing the energetics of such nano-bio systems. Here, we use coarse grain molecular dynamics simulations to study spherical fullerene molecules interacting with dipalmitoyl phosphatidylcholine (DPPC) lipid membranes. Using free energy calculations we show that all the tested fullerene molecules can spontaneously diffuse into both a lipid bilayer and a lipid monolayer. In addition, we establish that large fullerene molecules tend to partition preferentially into bilayers, which affects the lipid monolayer-to-bilayer transition during the respiration cycle. Our results identify a possible CNP perturbation to the function of the pulmonary monolayer membrane and suggest a potential pathway for CNP entry into the body through lung inhalation.
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    A Guiding Potential Method for Evaluating the Bending Rigidity of Tensionless Lipid Membranes from Molecular Simulation
    (2013-07-18) Kawamoto, Shuhei; Nakamura, Takenobu; Nielsen, Steven O.; Shinoda, Wataru; Nielsen, Steven O.
    A new method is proposed to estimate the bending rigidity of lipid membranes from molecular dynamics simulations. An external cylindrical guiding potential is used to impose a sinusoidal deformation to a planar membrane. The bending rigidity is obtained from the mean force acting on the cylinder by calibrating against a discretized Helfrich model that accounts for thermal fluctuations of the membrane surface. The method has been successfully applied to a dimyristoyl phosphatidylcholine bilayer simulated with a coarse-grained model. A well-converged bending rigidity was obtained for the tension-free membrane and showed reasonable agreement with that obtained from the height fluctuation spectrum.
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    Hydrophilic Directional Slippery Rough Surfaces for Water Harvesting
    (Amer Assoc for the Advancement of Science) Dai, Xianming; Sun, Nan; Nielsen, Steven O.; Stogin, Birgitt Boschitsch; Wang, Jing; Yang, Shikuan; Wong, Tak-Sing; 0000-0001-5050-2867 (Dai, X); 308247739 (Dai, X); Dai, Xianming; Nielsen, Steven O.
    Multifunctional surfaces that are favorable for both droplet nucleation and removal are highly desirable for water harvesting applications but are rare. Inspired by the unique functions of pitcher plants and rice leaves, we present a hydrophilic directional slippery rough surface (SRS) that is capable of rapidly nucleating and removing water droplets. Our surfaces consist of nanotextured directional microgrooves in which the nanotextures alone are infused with hydrophilic liquid lubricant. We have shown through molecular dynamics simulations that the physical origin of the efficient droplet nucleation is attributed to the hydrophilic surface functional groups, whereas the rapid droplet removal is due to the significantly reduced droplet pinning of the directional surface structures and slippery interface. We have further demonstrated that the SRS, owing to its large surface area, hydrophilic slippery interface, and directional liquid repellency, outperforms conventional liquid-repellent surfaces in water harvesting applications.
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    Modifying the Electronic Properties of Single-Walled Carbon Nanotubes using Designed Surfactant Peptides
    (2012-05-25) Samarajeewa, D. R.; Dieckmann, Gregg R.; Nielsen, Steven O.; Musselman, Inga H.; Nielsen, Steven O.; Musselman, Inga H.
    The electronic properties of carbon nanotubes can be altered significantly by modifying the nanotube surface. In this study, single-walled carbon nanotubes (SWCNTs) were functionalized noncovalently using designed surfactant peptides, and the resultant SWCNT electronic properties were investigated. These peptides have a common amino acid sequence of X(Valine) 5(Lysine) 2, where X indicates an aromatic amino acid containing either an electron-donating or electron-withdrawing functional group (i.e. p-amino-phenylalanine or p-cyano-phenylalanine). Circular dichroism spectra showed that the surfactant peptides primarily have random coil structures in an aqueous medium, both alone and in the presence of SWCNTs, simplifying analysis of the peptide/SWCNT interaction. The ability of the surfactant peptides to disperse individual SWCNTs in solution was verified using atomic force microscopy and ultraviolet-visible-near-infrared spectroscopy. The electronic properties of the surfactant peptide/SWCNT composites were examined using the observed nanotube Raman tangential band shifts and the observed additional features near the Fermi level in the scanning tunneling spectroscopy dI/dV spectra. The results revealed that SWCNTs functionalized with surfactant peptides containing electron-donor or electron-acceptor functional groups showed n-doped or p-doped altered electronic properties, respectively. This work unveils a facile and versatile approach to modify the intrinsic electronic properties of SWCNTs using a simple peptide structure, which is easily adaptable to obtain peptide/SWCNT composites for the design of tunable nanoscale electronic devices.
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    Nested Sampling of Isobaric Phase Space for the Direct Evaluation of the Isothermal-Isobaric Partition Function of Atomic Systems
    (American Institute of Physics) Wilson, Blake A.; Gelb, Lev D.; Nielsen, Steven O.; Wilson, Blake A.; Gelb, Lev D.; Nielsen, Steven O.
    Nested Sampling (NS) is a powerful athermal statistical mechanical sampling technique that directly calculates the partition function, and hence gives access to all thermodynamic quantities in absolute terms, including absolute free energies and absolute entropies. NS has been used predominately to compute the canonical (NVT) partition function. Although NS has recently been used to obtain the isothermal-isobaric (NPT) partition function of the hard sphere model, a general approach to the computation of the NPT partition function has yet to be developed. Here, we describe an isobaric NS (IBNS) method which allows for the computation of the NPT partition function of any atomic system. We demonstrate IBNS on two finite Lennard-Jones systems and confirm the results through comparison to parallel tempering Monte Carlo. Temperature-entropy plots are constructed as well as a simple pressure-temperature phase diagram for each system. We further demonstrate IBNS by computing part of the pressure-temperature phase diagram of a Lennard-Jones system under periodic boundary conditions.
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    Supramolecular and Biomacromolecular Enhancement of Metal-Free Magnetic Resonance Imaging Contrast Agents
    (Royal Society of Chemistry) Lee, Hamilton; Shahrivarkevishah, Arezoo; Lumata, Jenica L.; Luzuriaga, Michael A.; Hagge, Laurel M.; Benjamin, Candace E.; Brohlin, Olivia R.; Parish, Christopher R.; Firouzi, Hamid R.; Nielsen, Stephen O.; Lumata, Lloyd L.; Gassensmith, Jeremiah J.; 0000-0003-2176-925X (Hamilton, L); 0000-0002-5420-1954 (Shahrivarkevishah, A); 0000-0002-4650-3768 (Lumata, JL)); 0000-0001-6128-8800 (Luzuriaga, MA); 0000-0003-1064-6991 (Hagge, LM); 0000-0002-9211-718X (Benjamin, CE); 0000-0003-3226-6711 (Brohlin, OR); 0000-0003-4537-5992 (Firouzi, HR); 0000-0003-3390-3313 (Nielsen, SO); 0000-0002-3647-3753 (Lumata, LL); 0000-0001-6400-8106 (Gassensmith, JJ); Lee, Hamilton; Shahrivarkevishah, Arezoo; Lumata, Jenica L.; Luzuriaga, Michael A.; Hagge, Laurel M.; Benjamin, Candace E.; Brohlin, Olivia R.; Parish, Christopher R.; Firouzi, Hamid R.; Nielsen, Stephen O.; Lumata, Lloyd L.; Gassensmith, Jeremiah J.
    Many contrast agents for magnetic resonance imaging are based on gadolinium, however side effects limit their use in some patients. Organic radical contrast agents (ORCAs) are potential alternatives, but are reduced rapidly in physiological conditions and have low relaxivities as single molecule contrast agents. Herein, we use a supramolecular strategy where cucurbit[8]uril binds with nanomolar affinities to ORCAs and protects them against biological reductants to create a stable radical in vivo. We further overcame the weak contrast by conjugating this complex on the surface of a self-assembled biomacromolecule derived from the tobacco mosaic virus.
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    Towards an Understanding of Structure-Nonlinearity Relationships in Triarylamine-based Push-Pull Electro-Optic Chromophores: The Influence of Substituent and Molecular Conformation on Molecular Hyperpolarizabilities
    (Royal Soc Chemistry, 2014-02-17) Wu, Jingbo; Wilson, Blake A.; Smith, Dennis W., Jr.; Nielsen, Steven O.; 2012052347‏ (Smith, DW); Smith, Dennis W., Jr.; Nielsen, Steven O.
    We calculated the second-order hyperpolarizability (beta) of a series of triarylamine (TAA) based donor-bridge-acceptor (D-π-A) push-pull type nonlinear optical (NLO) chromophores with different electron donor moieties and the same thiophene π-bridge and dicyanovinyl electron acceptor using a time-dependent Hartree-Fock (TDHF) approach within the software package MOPAC 2012. NLO chromophores with various quantities and positions of methoxy groups in the TAA donor moiety were investigated. The relationship between NLO properties and the electronic or geometric structures of the TAA donor subunit is discussed through the calculation results. Both substituent and conformational effects affect the delocalization of the nitrogen lone pair into the aryl rings, leading to a dramatic influence on the nonlinear optical properties. Introduction of methoxy groups at the ortho positions of the TAA moiety has a larger influence on the molecular hyperpolarizability and dipole moment than the introduction of methoxy group at the para or meta positions. Our calculation results demonstrate how to improve the NLO properties of TAA based chromophores while meeting practical device requirements.
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    Ultrafast Pulsed Laser Induced Nanocrystal Transformation in Colloidal Plasmonic Vesicles
    (Wiley-VCH Verlag) Karim, Mohammad R.; Li, Xiuying; Kang, Peiyuan; Kang, Peiyuan; Randrianalisoa, J.; Ranathunga, Dineli; Nielsen, Steven O.; Qin, Zhenpeng; Qian, Dong; 0000-0003-3406-3045 (Qin, Z); 295272933 (Qian, D); Karim, Mohammad R.; Li, Xiuying; Ranathunga, Dineli; Nielsen, Steven O.; Qin, Zhenpeng; Qian, Dong
    Plasmonic vesicle consists of multiple gold nanocrystals within a polymer coating or around a phospholipid core. As a multifunctional nanostructure, it has unique advantages of assembling small nanoparticles (< 5 nm) for rapid renal clearance, strong plasmonic coupling for ultrasensitive biosensing and imaging, and near-infrared light absorption for drug release. Thus, understanding the interaction of plasmonic vesicles with light is critically important for a wide range of applications. In this paper, a combined experimental and computational study is presented on the nanocrystal transformation in colloidal plasmonic vesicles induced by the ultrafast picosecond pulsed laser. Experimentally observed merging and transformation of small nanocrystals into larger nanoparticles when treated by laser pulses is first reported. The underlying mechanisms responsible for the experimental observations are investigated with a multiphysics computational approach featuring coupled electromagnetic/molecular dynamics simulation. This study reveals for the first time that combined nanoparticle heating and laser-enhanced Brownian motion is responsible for the observed nanocrystal merging. Correspondingly, laser fluence, interparticle distance, and presence of water are identified as the most important factors governing the nanocrystal transformation. The guidelines established from this study can be employed to design a host of biomedical and nanomanufacturing applications involving laser interaction with plasmonic nanoparticles.

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