Scaling of activation energy for macroscopic flow in poly(ethylene glycol) solutions: Entangled – Non-entangled crossover

A. Wiśniewska, K. Sozański, T. Kalwarczyk, K. Królika, C. P. Stefan, A. Wieczorek, S. Jakieła, J. Enderlein and R. Hołyst

Polymer, Volume 55, Issue 18, 2014, Pages 4651-4657

We postulate an empirical scaling equation, which accurately describes flow of polymer solutions, complimenting the paradigm of length-scale-dependent viscosity. We investigated poly(ethylene glycol) aqueous solutions and observed an exponential dependence of viscosity on the hydrodynamic radius of a single coil Rh divided by the correlation length ξ. Properties of the system changed abruptly with the onset of chain entanglement at concentration corresponding to ξ = Rh. We propose a single equation valid for all the investigated systems, analyze the physical meaning of parameters appearing therein and discuss the impact of chain entanglement. Viscous flow is treated as an activated process, following the Eyring rate theory. We show that the difference of activation energy for flow between pure solvent and polymer solution, ΔEa, is a function of concentration, whose derivative has a discontinuity at the crossover concentration. For dilute PEG solutions ΔEa takes values of up to several kJ/mol and is proportional to the intrinsic viscosity. We successfully apply the scaling approach to the diffusive motion of a protein (aldolase) in solutions of 25 kg/mol PEO (concentrations of 2–20%), investigated by fluorescence correlation spectroscopy (FCS). A significant difference in the influence of crowding on translational and rotational motion of the protein is revealed.

Electrochemical pathway for the quantification of SERS enhancement factor

A. Sivanesan, W. Adamkiewicz, G. Kalaivani, A. Kamińska, J. Waluk, R. Hołyst and E. L. Izake

Electrochemistry Communications, 2014, 49, 103-106

This communication presents a new pathway for the more precise quantification of surface-enhanced Raman scattering (SERS) enhancement factor via deducing resonance Raman scattering (RRS) effect from surface-enhanced resonance Raman scattering (SERRS). To achieve this, a self-assembled monolayer of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4α-CoIITAPc) is formed on plasmon inactive glassy carbon (GC) and plasmon active GC/AuNP surface. The surfaces are subsequently used as common probes for electrochemical and Raman (RRS and SERRS) studies. The most crucial parameters required for the quantification of SERS substrate enhancement factor (SSEF) such as real surface area of GC/AuNPs substarte and the number of 4α-CoIITAPc molecules contributing to RRS (on GC) and SERRS (on GC/AuNPs) are precisely estimated by cyclic voltammetry experiments. The present approach of SSEF quantification can be applied to varieties of surfaces by choosing an appropriate laser line and probe molecule for each surface.

Implications of macromolecular crowding for protein–protein association kinetics in the cytoplasm of living cells

M. Tabaka, L. Sun, T. Kalwarczyk and R. Hołyst

Soft Matter, 2013, 9, 4386-4389

We present a method of prediction of in vivo association rate constants between proteins from in vitro rate constants. This model accounts for the changes in length-scale dependent viscosity of the cytoplasm of a living cell and interaction potential between proteins. We explain the unexpected results obtained for proteinprotein association rate constants in the cytoplasm of HeLa cells determined by Phillip et al. Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 1461.

Nanoscale transport of energy and mass flux during evaporation of liquid droplets into inert gas: computer simulations and experiments

R. Hołyst, M. Litniewski, D. Jakubczyk, M. Zientara and M. Woźniak

Soft Matter, 2013, 9, 7766-7774

We use molecular dynamics (MD) simulations of a two-component Lennard-Jones (LJ) fluid to analyze the energy flux from an inert gas to the interface of an evaporating liquid droplet. Using this analysis we derive an analytical equation for the radius of the droplet, R(t), as a function of time, t. The formula is valid for evaporation of droplets of any material or size into the gas characterized by the mean free path, λ, much larger than the molecular diameter, σ. We find linear dependence R(t) ∼ t, for high λ/R(t) ratios and standard law R2(t) ∼ t for small λ/R(t) ratios. We apply equation for R(t) to experimental results of evaporation of water micro-droplets into air and glyceroldiethylene glycol and triethylene glycol micro-droplets into the nitrogen gas evaporating in time from seconds to tens of minutes. The experimental results together with computer simulations span 12 orders of magnitude of evaporation times and more than 3 orders of magnitude of droplets’ radii. In the experiments the evaporation rate is governed by a very small difference in temperatures (from one tenth of mK to a few K) between the gas far from the droplet and evaporating liquid. From MD simulations we also obtain suitable boundary conditions for the energy flux at the interface, used in irreversible thermodynamics, and the accommodation coefficients used in kinetic models of evaporation.

A “wrap-and-wrest” mechanism of fluorescence quenching of CdSe/ZnS quantum dots by surfactant molecules

E. Kalwarczyk, N. Ziębacz, T. Kalwarczyk, R. Hołyst and M. Fiałkowski

Nanoscale, 2013, 5, 9908-9916

We identified a mechanism of fluorescence quenching of CdSe/ZnS quantum dots (QDs) coated with two organic layers, octadecylamine and an amphiphilic polymer containing COOH groups, by nonionic polyoxyethylene-based (C12Ensurfactants. The surfactant molecules by themselves do not affect the fluorescence of the QDs. For the quenching to occur, “wrapping” of the QDs by a bilayer of the surfactant molecules is necessary. The formation of the bilayer causes an irreversible detachment (“wresting”) of the ligand molecules, accompanied by the creation of quenching sites on the QD surface. Due to its two-stage nature, we refer to the quenching mechanism as the “wrap-and-wrest” mechanism. The adsorption of the surfactant on the QD surface is a relatively slow process, occurring within minutes or hours. Such long quenching times allowed monitoring surfactant adsorption progress in real time. The fluorescence signal decays exponentially, and the decay time is inversely proportional to the surfactant concentration in solution.

A “nano-windmill” driven by a flux of water vapour: a comparison to the rotating ATPase

P. Nitoń, A. Żywociński, M. Fiałkowski and R. Hołyst

Nanoscale, 2013, 5, 9732-9738

We measure the frequency of collective molecular precession as a function of temperature in the ferroelectric liquid crystalline monolayer at the water–air interface. This movement is driven by the unidirectional flux of evaporating water molecules. The collective rotation in the monolayer with angular velocities ω ∼ 1 s−1 (at T = 312 K) to 10−2 s−1 (at T = 285.8 K) is 9 to 14 orders of magnitude slower than rotation of a single molecule (typically ω ∼ 109 to 1012 s−1). The angular velocity reaches 0 upon approach to the two dimensional liquid-to-solid transition in the monolayer at T = 285.8 K. We estimate the rotational viscosity, γ1, in the monolayer and the torque, Γ, driving this rotation. The torque per molecule equals Γ = 5.7 × 10−8 pN nm at 310 K (γ1 = 0.081 Pa s, ω = 0.87 s−1). The energy generated during one turn of the molecule at the same temperature is W = 3.5 × 10−28 J. Surprisingly, although this energy is 7 orders of magnitude smaller than the thermal energy, kBT (310 K) = 4.3 × 10−21 J, the rotation is very stable. The potential of the studied effect lies in the collective motion of many (>1012) “nano-windmills” acting “in concerto” at the scale of millimetres. Therefore, such systems are candidates for construction of artificial molecular engines, despite the small energy density per molecular volume (5 orders of magnitude smaller than for a single ATPase).

Transport of Mass at the Nanoscale during Evaporation of Droplets: the Hertz–Knudsen Equation at the Nanoscale

M. Zientara, D. Jakubczyk, M. Litniewski and R. Hołyst

J. Phys. Chem. C 2013, 117, 2, 1146–1150

The applicability of the Hertz–Knudsen equation to the evolution of droplets at the nanoscale was investigated upon analysis of existing molecular dynamics (MD) simulations ( HolystPhys. Rev. Lett. 2008100, 055701; Yaguchi; J. Fluid Sci. Technol. 20105, 180–191; Ishiyama; Phys. Fluids 200416, 2899–2906). The equation was found satisfactory for radii larger than ∼4 nm. Concepts of the Gibbs equimolecular dividing surface and the surface of tension were utilized in order to accommodate the surface phase density and temperature profiles, clearly manifesting at the nanoscale. The equimolecular dividing surface was identified as the surface of the droplet. A modification to the Tolman formula was proposed in order to describe surface tension for droplet radii smaller than ∼50 nm. We assumed that the evaporation coefficient for a system in and out of equilibrium may differ. We verified that this difference might be attributed to surface temperature change only. The empirical dependencies of the evaporation coefficient and the surface tension for a flat interface, of liquid Ar in Ar gas at equilibrium, at the nanoscale, upon temperature was taken from existing MD data. Two parametrizations of the Hertz–Knudsen equation were proposed: (i) one using the off-equilibrium condensation coefficient and the effective density and (ii) another one using the effective density and the temperature at the interface. The second parametrization leads to an approximate solution of the Hertz–Knudsen equation requiring no free parameters. Such a solution is suitable for experimental use at the nanoscale if only the temperature of the droplet (core) can be measured.

Electrodeposition for preparation of efficient surface-enhanced Raman scattering-active silver nanoparticle substrates for neurotransmitter detection

M. Siek, A. Kaminska, A. Kelm, T. Rolinski, R. Holyst, M. Opallo, J. Niedziolka-Jonsson

Electrochimica Acta Volume 89, 1 February 2013, Pages 284-291

A stable and efficient surface-enhanced Raman scattering (SERS) substrate for neurotransmitter and cholinergic neurotransmission precursor detection was obtained by silver nanoparticle (AgNP) electrodeposition onto tin-doped indium oxide (ITO) using cyclic voltammetry. The size and surface coverage of the deposited AgNPs were controlled by changing the scan rate and the number of scans. The SERS performance of these substrates was analyzed by studying its reproducibility, repeatability and signal enhancement measured from p-aminothiophenol (p-ATP) covalently bonded to the substrate. We compared the SERS performance for samples with different Ag particle coverage and particle sizes. The performance was also compared with a commercial substrate. Our substrates exhibited a SERS enhancement factor of around 107 for p-ATP which is three orders of magnitude larger than for the commercial substrate. Apart from this high enhancement effect the substrate also shows extremely good reproducibility. The average spectral correlation coefficient (Γ) is 0.96. This is larger than for the commercial substrate (0.85) exhibiting a much lower SERS signal intensity. Finally, the application of our substrates as SERS bio-sensors was demonstrated with the detection of the neurotransmitters acetylcholine, dopamine, epinephrine and choline, the precursor for acetylcholine. The intensive SERS spectra observed for low concentrations of choline (2 × 10−6 M), acetylcholine (4 × 10−6 M), dopamine (1 × 10−7 M) and epinephrine (7 × 10−4 M) demonstrated the high sensitivity of our substrate. The high sensitivity and fast data acquisition make our substrates suitable for testing physiological samples.

Evaporation of freely suspended single droplets: experimental, theoretical and computational simulations

R. Hołyst, M. Litniewski, D. Jakubczyk, K. Kolwas, M. Kolwas, K. Kowalski, S. Migacz, S. Palesa and M. Zientara

Rep. Prog. Phys. 76 034601, 2013

Evaporation is ubiquitous in nature. This process influences the climate, the formation of clouds, transpiration in plants, the survival of arctic organisms, the efficiency of car engines, the structure of dried materials and many other phenomena. Recent experiments discovered two novel mechanisms accompanying evaporation: temperature discontinuity at the liquid–vapour interface during evaporation and equilibration of pressures in the whole system during evaporation. None of these effects has been predicted previously by existing theories despite the fact that after 130 years of investigation the theory of evaporation was believed to be mature. These two effects call for reanalysis of existing experimental data and such is the goal of this review. In this article we analyse the experimental and the computational simulation data on the droplet evaporation of several different systems: water into its own vapour, water into the air, diethylene glycol into nitrogen and argon into its own vapour. We show that the temperature discontinuity at the liquid–vapour interface discovered by Fang and Ward (1999 Phys. Rev. E 59 417–28) is a rule rather than an exception. We show in computer simulations for a single-component system (argon) that this discontinuity is due to the constraint of momentum/pressure equilibrium during evaporation. For high vapour pressure the temperature is continuous across the liquid–vapour interface, while for small vapour pressures the temperature is discontinuous. The temperature jump at the interface is inversely proportional to the vapour density close to the interface. We have also found that all analysed data are described by the following equation: da/dt = P1/(a + P2), where a is the radius of the evaporating droplet, t is time and P1 and P2 are two parameters. P1 = −λΔT/(qeffρL), where λ is the thermal conductivity coefficient in the vapour at the interface, ΔT is the temperature difference between the liquid droplet and the vapour far from the interface, qeff is the enthalpy of evaporation per unit mass and ρL is the liquid density. The P2 parameter is the kinetic correction proportional to the evaporation coefficient. P2 = 0 only in the absence of temperature discontinuity at the interface. We discuss various models and problems in the determination of the evaporation coefficient and discuss evaporation scenarios in the case of single- and multi-component systems.

Taylor Dispersion Analysis in Coiled Capillaries at High Flow Rates

A. Lewandrowska, A. Majcher, A. Ochab-Marcinek, M. Tabaka and R. Hołyst

Anal. Chem. 2013, 85, 8, 4051–4056

Taylor Dispersion Analysis (TDA) has been performed for analytes moving at high flow rates in long, coiled capillaries. A thin injection zone of the analyte is stretched by the flow and final distribution of concentration of the analyte at the end of the capillary has the Gaussian shape. The high flow rates in coiled capillary generate vortices. They convectively mix the analyte across the capillary. This mixing reduces the width of the Gaussian distribution several times in comparison to the width obtained in a straight capillary in standard TDA. We have determined an empirical, scaling equation for the width as a function of the flow rate, molecular diffusion coefficient of the analyte, viscosity of the carrier phase, internal radius of the cylindrical capillary, and external radius of the coiled capillary. This equation can be used for different sizes of capillaries in a wide range of parameters without an additional calibration procedure. Our experimental results of flow in the coiled capillary could not be explained by current models based on approximate solutions of the Navier–Stokes equation. We applied the technique to determine the diffusion coefficients of the following analytes: salts, drugs, single amino acids, peptides (from dipeptides to hexapeptides), and proteins.

Fractal trace of earthworms

K. Burdzy, R. Hołyst, and Ł. Pruski

Phys. Rev. E 87, 052120 – May 2013

We investigate a process of random walks of a point particle on a two-dimensional square lattice of size n×n with periodic boundary conditions. A fraction p20% of the lattice is occupied by holes (p represents macroporosity). A site not occupied by a hole is occupied by an obstacle. Upon a random step of the walker, a number of obstacles, M, can be pushed aside. The system approaches equilibrium in (nlnn)2 steps. We determine the distribution of M pushed in a single move at equilibrium. The distribution F(M) is given by Mγ where γ=1.18 for p=0.1, decreasing to γ=1.28 for p=0.01. Irrespective of the initial distribution of holes on the lattice, the final equilibrium distribution of holes forms a fractal with fractal dimension changing from a=1.56 for p=0.20 to a=1.42 for p=0.001 (for n=4,000). The trace of a random walker forms a distribution with expected fractal dimension 2.

Structural evolution of reverse vesicles from a salt-free catanionic surfactant system in toluene

H. Li, X. Xin, T. Kalwarczyk, R. Hołyst J. Chen and J. Hao

Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 436, 2013, Pages 49-56

A detailed study of the reverse vesicles formed by a salt-free catanionic surfactant system in toluene was carried out by confocal fluorescence microscopy observations, dye-solubilizing tests, UV–vis measurements and cryo-TEM observations. When tetradecyltrimethyl ammonium laurate (TTAL) and lauric acid (LA) were mixed in the binary solution of water and toluene, reverse vesicular phase formed spontaneously. The reverse vesicular phase is quite stable and can be labeled with fluorescent dyes for subsequent confocal fluorescence microscopy observations. However, the reverse vesicles were found to have smaller sizes (<1 μm) and undergo structural evolutions in a much shorter time scale compared to those formed by the same surfactant mixture in cyclohexane, as shown in a previous report [19]. With extended observation time, interesting intermediate structures were observed including onions, sheets and cellular networks. The structural evolution pathways were deduced and the possibly influencing factors were discussed. Dye-solubilizing tests showed the ability of the reverse vesicles to accommodate dye molecules is smaller compared to those in cyclohexane. Besides, cryo-TEM observations were applied to probe the morphology of the reverse vesicles.

Self-Assembly of Gold Nanoparticles into 2D Arrays Induced by Bolaamphiphilic Ligands

J. Paczesny, M. Wójcik, K. Sozański, K. Nikiforov, C. Tschierske, A. Lehmann, E. Górecka, J. Mieczkowski and R. Hołyst

J. Phys. Chem. C 2013, 117, 45, 24056–24062

We performed synthesis and investigated the self-assembly properties of gold nanoparticles (NPs) with covalently attached bolaamphiphilic ligands (B-AuNPs). The judiciously designed coating rendered the NPs amphiphilic and induced their self-assembly. The B-AuNPs formed ordered two-dimensional structures over large areas upon simple drop-casting. The films exhibited an uncommon and applicable topography, consisting of densely packed rings of inner diameter of around 30 nm, with the B-AuNPs at the rim and an empty interior. We introduced and proved experimentally an explanation of how the structures were formed. The model involved elements of geometric packing and ligand reorganization. Upon contact with the hydrophilic surface, ligands rearranged at the surface of the metallic cores of the B-AuNPs so that the bolaamphiphilic moieties (constituting ca. 50% of the coating) were in proximity to the surface, while the hexanethiol moieties moved away from it. The described mechanism is of general relevance for the design of functional NPs capable of self-assembly.

Activation Energy for Mobility of Dyes and Proteins in Polymer Solutions: From Diffusion of Single Particles to Macroscale Flow

K. Sozański, A. Wiśniewska, T. Kalwarczyk, and R. Hołyst

Phys. Rev. Lett. 111, 228301 – November 2013

We measure the activation energy Ea for the diffusion of molecular probes (dyes and proteins of radii from 0.52 to 6.9 nm) and for macroscopic flow in a model complex liquid—aqueous solutions of polyethylene glycol. We cover a broad range of polymer molecular weights, concentrations, and temperatures. Fluorescence correlation spectroscopy and rheometry experiments reveal a relationship between the excess of the activation energy in polymer solutions over the one in pure solvent ΔEa and simple parameters describing the structure of the system: probe radius, polymer hydrodynamic radius, and correlation length. ΔEa varies by more than an order of magnitude in the investigated systems (in the range of ca. 115kJ/mol) and for probes significantly larger than the polymer hydrodynamic radius approaches the value measured for macroscopic flow. We develop an explicit formula describing the smooth transition of ΔEa from the diffusion of molecular probes to macroscopic flow. This formula is a reference for the quantitative analysis of specific interactions of moving nano-objects with their environment as well as active transport. For instance, the power developed by a molecular motor moving at constant velocity u is proportional to u2exp(Ea/RT).

Fluorescence correlation spectroscopy analysis for accurate determination of proportion of doubly labeled DNA in fluorescent DNA pool for quantitative biochemical assays

S. Hou, L. Sun, S. A. Wieczorek, T. Kalwarczyk, T. S. Kaminski and R. Holyst

Biosensors and Bioelectronics, Volume 51, 15 January 2014, Pages 8-15

Fluorescent double-stranded DNA (dsDNA) molecules labeled at both ends are commonly produced by annealing of complementary single-stranded DNA (ssDNA) molecules, labeled with fluorescent dyes at the same (3′ or 5′) end. Because the labeling efficiency of ssDNA is smaller than 100%, the resulting dsDNA have two, one or are without a dye. Existing methods are insufficient to measure the percentage of the doubly-labeled dsDNA component in the fluorescent DNA sample and it is even difficult to distinguish the doubly-labeled DNA component from the singly-labeled component. Accurate measurement of the percentage of such doubly labeled dsDNA component is a critical prerequisite for quantitative biochemical measurements, which has puzzled scientists for decades. We established a fluorescence correlation spectroscopy (FCS) system to measure the percentage of doubly labeled dsDNA (PDL) in the total fluorescent dsDNA pool. The method is based on comparative analysis of the given sample and a reference dsDNA sample prepared by adding certain amount of unlabeled ssDNA into the original ssDNA solution. From FCS autocorrelation functions, we obtain the number of fluorescent dsDNA molecules in the focal volume of the confocal microscope and PDL. We also calculate the labeling efficiency of ssDNA. The method requires minimal amount of material. The samples have the concentration of DNA in the nano-molar/L range and the volume of tens of microliters. We verify our method by using restriction enzyme Hind III to cleave the fluorescent dsDNA. The kinetics of the reaction depends strongly on PDL, a critical parameter for quantitative biochemical measurements.

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