Department of Physics
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Browsing Department of Physics by Subject "03 Chemical Sciences"
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- High-Precision Megahertz-to-Terahertz Dielectric Spectroscopy of Protein Collective Motions and Hydration DynamicsCharkhesht, Ali; Regmi, Chola K.; Mitchell-Koch, Katie R.; Cheng, Shengfeng; Vinh, Nguyen Q. (American Chemical Society, 2018-06-21)The low-frequency collective vibrational modes in proteins as well as the protein-water interface have been suggested as dominant factors controlling the efficiency of biochemical reactions and biological energy transport. It is thus crucial to uncover the mystery of the hydration structure and dynamics as well as their coupling to collective motions of proteins in aqueous solutions. Here, we report dielectric properties of aqueous bovine serum albumin protein solutions as a model system using an extremely sensitive dielectric spectrometer with frequencies spanning from megahertz to terahertz. The dielectric relaxation spectra reveal several polarization mechanisms at the molecular level with different time constants and dielectric strengths, reflecting the complexity of protein-water interactions. Combining the effective-medium approximation and molecular dynamics simulations, we have determined collective vibrational modes at terahertz frequencies and the number of water molecules in the tightly bound and loosely bound hydration layers. High-precision measurements of the number of hydration water molecules indicate that the dynamical influence of proteins extends beyond the first solvation layer, to around 7 Å distance from the protein surface, with the largest slowdown arising from water molecules directly hydrogen-bonded to the protein. Our results reveal critical information of protein dynamics and protein-water interfaces, which determine biochemical functions and reactivity of proteins.
- The meniscus on the outside of a circular cylinder: From microscopic to macroscopic scalesTang, Yanfei; Cheng, Shengfeng (Academic Press – Elsevier, 2019-01-01)We systematically study the meniscus on the outside of a small circular cylinder vertically immersed in a liquid bath in a cylindrical container that is coaxial with the cylinder. The cylinder has a radius R much smaller than the capillary length, κ-1, and the container radius, L, is varied from a small value comparable to R to ∞. In the limit of L≪κ-1, we analytically solve the general Young-Laplace equation governing the meniscus profile and show that the meniscus height, Δh, scales approximately with Rln(L/R). In the opposite limit where L≫κ-1,Δh becomes independent of L and scales with Rln(κ-1/R). We implement a numerical scheme to solve the general Young-Laplace equation for an arbitrary L and demonstrate the crossover of the meniscus profile between these two limits. The crossover region has been determined to be roughly 0.4κ-1≲L≲4κ-1. An approximate analytical expression has been found for Δh, enabling its accurate prediction at any values of L that ranges from microscopic to macroscopic scales.
- Transparent and Sprayable Surface Coatings that Kill Drug-Resistant Bacteria within Minutes and Inactivate SARS-CoV-2 VirusBehzadinasab, Saeed; Williams, Myra D.; Hosseini, Mohsen; Poon, Leo L. M.; Chin, Alex W. H.; Falkinham, Joseph O. III; Ducker, William A. (American Chemical Society, 2021-11-24)Antimicrobial coatings are one method to reduce the spread of microbial diseases. Transparent coatings preserve the visual properties of surfaces and are strictly necessary for applications such as antimicrobial cell phone screens. This work describes transparent coatings that inactivate microbes within minutes. The coatings are based on a polydopamine (PDA) adhesive, which has the useful property that the monomer can be sprayed, and then the monomer polymerizes in a conformal film at room temperature. Two coatings are described (1) a coating where PDA is deposited first and then a thin layer of copper is grown on the PDA by electroless deposition (PDA/Cu) and (2) a coating where a suspension of Cu2O particles in a PDA solution is deposited in a single step (PDA/Cu2O). In the second coating, PDA menisci bind Cu2O particles to the solid surface. Both coatings are transparent and are highly efficient in inactivating microbes. PDA/Cu kills >99.99% of Pseudomonas aeruginosa and 99.18% of methicillin-resistant Staphylococcus aureus (MRSA) in only 10 min and inactivates 99.98% of SARS-CoV-2 virus in 1 h. PDA/Cu2O kills 99.94% of P. aeruginosa and 96.82% of MRSA within 10 min and inactivates 99.88% of SARS-CoV-2 in 1 h.
- Tunable Gap Plasmons in Gold Nanospheres Adsorbed into a pH-Responsive Polymer FilmJao, Chih-Yu; Robinson, Hans D.; Samaimongkol, Panupon (Elsevier, 2019-06)Hypothesis Plasmon nanorules are exquisitely sensitive distance sensors that are based on the electromagnetic interaction between metal nanoparticles and surfaces. We hypothesize that nanorulers can act as quantitative probes of processes such as particle aggregation and adsorption, and deploy them to investigate particle adsorption onto stimulus-responsive polymer films. While such systems have previously been qualitatively investigated with plasmon nanorulers, our quantitative analysis should provide deeper insights. Experiment Gold nanospheres are adsorbed from solution onto pH-responsive, amine-rich polyelectrolyte multilayer (PEM) films that are either directly deposited on a gold substrate or onto an intermediate self-assembled monolayer (SAM) of charged thiols. Fitting the optical scattering spectrum to a full-wave calculation, we quantify the sphere-substrate gap distance with good accuracy. Findings We find that the gold spheres partially embed into the PEMs rather than ride on top of them, and that although the amount of actuation of the spheres afforded by tuning the pH is well controlled, it is significantly smaller than the corresponding thickness changes in unstrained films. Further, the presence of a SAM below the PEM increases the amount of polymer in the PEM, except for the thickest and most highly charged films, where the SAM instead appears to displace from the area below the nanospheres.