SERS Active Surface Based on Au‐Coated Porous GaN

A. Kaminska, Weyherb, J. Waluk, S. Gawinkowski and R. Holyst

AIP Conference Proceedings 2010, 1267, 954

Sınce the discovery of the surface-enhanced Raman spectroscopy SERS effect on roughened silver electrode surfaces numerous promising substrates have been explored that can be used as efficient SERS-platform. In the last decade broad interest was focused on producing and using nano-structural forms (e.g. nano-columns) as active platforms for immobilization and examination of organic substances. Such platforms, covered by nano-size metal layers (e.g. Au, Ag, Cu), constitutes very sensitive bio-sensors on which different indıvıdual organic molecules can be examıned by SERS Despite the considerable progress that have been made towards improving and optimizing SERS substrates, the fabrication of reproducible SERS platform still remains a challenging task. In this respect a promising approached was described recently, namely the use of porous GaN3 covered by gold and sılver layers.

Reversible aggregation of X-Shaped bolaamphiphiles with partially fluorinated lateral chains at the air/water interface

P. Nitoń, A. Żywociński, R. Hołyst, R. Kieffer, C. Tschierske, J. Paczesny, D. Pociechad and E. Górecka

Chem. Commun., 2010,46, 1896-1898

Langmuir films of four X-shaped bolaamphiphiles were studied using surface pressure and Kelvin potential measurements, Brewster angle microscopy and X-ray reflectivity. The partially fluorinated bolaamphiphiles exhibit an unusual reversibility and reproducibility of Langmuir isotherms, and create very stable and well defined single- or triple layers which can be transferred to solid substrates.

Phase Transition in Salt-Free Catanionic Surfactant Mixtures Induced by Temperature

H. Li, S. A. Wieczorek, Xia Xin, T. Kalwarczyk, N. Ziebacz, T. Szymborski, R. Hołyst, J. Hao, E. Gorecka and D. Pociecha

Langmuir 2010, 26, 1, 34–40

Aggregate transitions in salt-free catanionic surfactant mixtures of tetradecyltrimethylammonium hydroxide (TTAOH)/fatty acid were investigated as a function of surfactant concentration and temperature. Lauric acid (LA), myristic acid (MA), and palmitic acid (PA) were chosen for the current study. The TTAOH/LA mixture exhibited rich phase behavior at room temperature. With increasing total surfactant concentration (cT), a bluish vesicular (Lαv) phase, an isotropic micellar (L1) phase, and a birefringent lamellar (Lα) phase were observed. Between the Lαv phase and the L1 phase, a narrow Lα′/L1 two-phase region was determined. With increasing temperature, a transition from the Lα phase to the L1 phase was induced at higher cT whereas at lower cT an opposite transition from the L1 phase to the Lαv phase was noticed. Thus surprisingly, we observed bilayer-to-micelle and micelle-to-bilayer transitions in the same catanionic surfactant system, both induced by the temperature increase. Replacing LA by MA and PA caused a continuous increase in the average Krafft point of the mixture. The Lαv-phase region and phase-separated region become larger. Moreover, a single L1-phase region was absent within the investigated temperature range.

Polymer-induced ordering and phase separation in ionic surfactants

E. Kalwarczyk, M. Gołoś, R. Hołyst and M. Fiałkowski

Journal of Colloid and Interface Science 2010, 342, 1, 93-102

We present a new method to induce phase separation in solutions of ionic surfactants. In this method, the phase separation is obtained either by addition of polyelectrolytes or nonionic polymers along with inorganic salt. As a result, the system separates into polyelectrolyte-rich (or nonionic polymer-rich) and surfactant-rich phase. Four types of the mixtures were investigated: (i) anionic surfactants and anionic polyelectrolytes, (ii) cationic surfactants and cationic polyelectrolytes, (iii) cationic surfactants and nonionic polymers, and (iv) anionic surfactants and nonionic polymers. We found that the addition of polyelectrolyte with the charge of the same sign as that of surfactant can induce the phase separation in a wide range of surfactant concentrations. The addition of nonionic polymers induces the phase separation only in solutions of cationic surfactants. Moreover, the addition of nonionic polymers induces the phase separation only for relatively high total content of polymer and surfactant in the mixture. We found however that the addition of inorganic salt to the mixture of cationic surfactant and nonionic polymer triggers the phase separation even for a small concentrations of surfactant. In our experiments, water as well as mixtures of water and polar solvents were employed as solvents. Based on the optical microscopy studies we found that the surfactant-rich phase represents hexagonal ordering.

Incorporation of Carbon Nanotubes into a Lyotropic Liquid Crystal by Phase Separation in the Presence of a Hydrophilic Polymer

X. Xin, H. Li, S. A. Wieczorek, T. Szymborski, E. Kalwarczyk, N. Ziebacz, E. Gorecka, D. Pociecha and R. Hołyst

Langmuir 2010, 26, 5, 3562–3568

Single-walled carbon nanotubes (SWNTs) were incorporated into a lyotropic liquid crystal (LLC) matrix formed by n-dodecyl octaoxyethene monoether (C12E6) at room temperature through spontaneous phase separation induced by nonionic hydrophilic polymer poly(ethylene glycol) (PEG). The quality of SWNTs/LLC composite was evaluated by polarized microscopy observations and small-angle X-ray scattering (SAXS) measurements. The results obtained clearly indicated that SWNTs have been successfully incorporated into the LLC matrix up to a considerable high content without destroying the LLC matrix, although interesting changes of the LLC matrix were also induced by SWNTs incorporation. By varying the ratio of PEG to C12E6, the type of LLC matrix can be controlled from hexagonal phase to lamellar phase. Temperature was found to have a significant influence on the quality of SWNTs/LLC composite, and tube aggregation can be induced at higher temperature. When SWNTs were changed to multiwalled carbon nanotubes (MWNTs), they became difficult to be incorporated into LLC matrix because of an increase in the average tube diameter.

Single-Walled Carbon Nanotube/Lyotropic Liquid Crystal Hybrid Materials by a Phase Separation Method in the Presence of Polyelectrolyte

X. Xin, H. Li, E. Kalwarczyk, A. Kelm, M. Fiałkowski, E. Gorecka, D. Pociecha and R. Hołyst

Langmuir 2010, 26, 11, 8821–8828

We present a detailed study on the incorporation of single-walled carbon nanotubes (SWNTs) into lyotropic liquid crystals (LLC) by phase separation in the presence of polyelectrolytes. Two cases were studied in this work: (i) incorporation of SWNTs into the LLC phase formed by an anionic surfactant sodium dodecyl sulfate (SDS) in the presence of an anionic polyelectrolyte poly(sodium styrenesulfonate) (PSS); (ii) incorporation of SWNTs into the LLC phase formed by a cationic surfactant cetyltrimethylammonium bromide (CTAB) in the presence of a cationic polyelectrolyte poly(diallydimethylammonium chloride) (PDADMAC). The SWNTs/LLC composites were characterized by polarized optical microscopy (POM) observations and small-angle X-ray scattering (SAXS) measurements. In both systems, the surfactant phase was condensed into a hexagonal lattice by the polyelectrolyte within the investigated concentration range. Several factors that can influence the property of SWNTs/LLC composite were examined, including concentration of surfactants and polyelectrolytes and temperature. Aggregated SWNTs were not observed, indicating that SWNTs were well dispersed in the LLC phases. SAXS measurements showed the lattice parameter of the host LLC phase changed upon varying the mixing ratio of polyelectrolyte to ionic surfactant. The SWNTs/LLC hybrids showed considerable stability against temperature rise in both systems, and desorption of surfactant from SWNTs was not observed at higher temperature.

Size and Shape of Micelles Studied by Means of SANS, PCS, and FCS

J. Gapiński, J. Szymański, A. Wilk, J. Kohlbrecher, A. Patkowski and R. Hołyst

Langmuir 2010, 26, 12, 9304–9314

The hexaethylene glycol monododecyl ether (C12E6) micelles at concentrations up to 10% have been studied in their isotropic phase (10−48 °C) by means of small angle neutron scattering (SANS) and photon correlation spectroscopy (PCS). The SANS data obtained at low temperatures could be unequivocally interpreted as a result of scattering from a suspension of compact globular micelles with the shape of a triaxial ellipsoid or a short end-capped elliptical rod. Different models have been applied to analyze the SANS data obtained at higher temperatures: (i) elongated rod-like micelles with purely sterical interactions, (ii) compact globular micelles with a weak attractive potential, and (iii) globular micelles influenced by the critical phenomena in the whole temperature range studied. The good quality of the experimental data indicated model (i) as the best fit for our data. The diffusion coefficients obtained from the PCS measurements have been compared to the diffusion coefficients calculated for the rod-like micelles—results of the SANS data analysis. A good agreement was achieved using the solvent viscosity, in agreement with the theoretical predictions for sterically interacting globular colloidal particles. Finally, the SANS results obtained at 24 °C were compared to the micelle self-diffusion coefficients previously measured by means of fluorescence correlation spectroscopy (FCS) at this temperature. The good agreement obtained after scaling the data with solution viscosity supports the validity of the generalized Stokes−Einstein relation in sterically interacting systems: the product of the colloidal particle self-diffusion coefficient and the macroscopic viscosity remains constant in a broad range of concentrations. It has been concluded that the FCS technique in combination with simple viscosity measurements might serve as a tool for estimating the micellar size and shape.

Evaluation of Ligand-Selector Interaction from Effective Diffusion Coefficient

A. Bielejewska, A. Bylina, K. Duszczyk, M. Fiałkowski and R. Hołyst

Anal. Chem. 2010, 82, 13, 5463–5469

We present an analytical technique for determination of ligand-selector equilibrium binding constants. The method is based on the measurements of effective molecular diffusion coefficient of the ligand during Poiseuille flow through a long (approximately 25 m), thin (0.254 mm ± 0.05 mm ID) capillary with and without the selector. The data are analyzed using the Taylor dispersion theory. Bovine Serum Albumin (BSA) and cyclodextrin (CD) were taken as model selectors. We have tested our method on the following selector-ligand complexes: BSA with warfarin, propranolol, noscapine, salicylic acid, and riboflavin, and cyclodextrin with 4-nitrophenol. The results are in good agreement with data from the literature and with our own results obtained within classical chromatography. This method works equally well for uncharged and charged compounds.

Reverse Vesicles from a Salt-Free Catanionic Surfactant System: A Confocal Fluorescence Microscopy Study

H. Li, X. Xin, T. Kalwarczyk, E. Kalwarczyk, P. Niton, R. Hołyst and J. Hao

Langmuir 2010, 26, 19, 15210–15218

We give a detailed confocal fluorescence microscopy study on reverse vesicles from a salt-free catanionic surfactant system. When tetradecyltrimethylammonium laurate (TTAL) and lauric acid (LA) are mixed in cyclohexane at the presence of a small amount of water, stable reverse vesicular phases form spontaneously. The reverse vesicular phases can be easily labeled with dyes of varying molecular size and hydrophobicity while the dyes are nearly insoluble in cyclohexane without reverse vesicles. This indicates the reverse vesicular phases can be good candidates to host guest molecules. With the help of a fluorescence microscope combined a confocal method, the features of these interesting reverse supramolecular self-assemblies were revealed for the first time. Because of the absence of electrostatic repulsions and hydration forces between adjacent vesicles, the reverse vesicles have a strong propensity to aggregate with each other and form three-dimensional clusters. The size distributions of both individual reverse vesicles and clusters are polydisperse. Huge multilamellar reverse vesicles with closely stacked thick walls (giant reverse onions) were observed. Besides the spherical reverse vesicles and onions, other supramolecular structures such as tubes have also been detected and structural evolutions between different structures were noticed. These interesting supramolecular self-assemblies form in a nonpolar organic solvent may serve as ideal micro- or nanoreaction centers for biological reactions and synthesis of inorganic nanomaterials.

Binary and graded evolution in time in a simple model of gene induction

M. Tabaka and R. Hołyst

Phys. Rev. E 2010, 82, 052902

We solve analytically the model of gene expression induction which consists of three steps: gene activation, gene products synthesis, and product degradation. The solution is given as a time-dependent probability distribution for gene products. Following the distribution in time from the inactive state of the gene to the stationary state we observe binary or graded response depending solely on the ratio r of the gene activation rate to the rate of the gene product degradation. If r1 the response is binary and the continuous transition from binary to graded response occurs between r=0.1 and r=1. Therefore, if binary response is observed during relaxation to steady state, then the activation rate constant must be smaller than the degradation rate constant.

Dynamic charge separation in a liquid crystalline meniscus

T. Szymborski, O. Cybulski, I. Bownik, A. Żywociński, S.A. Wieczorek, M. Fiałkowski, R. Hołyst and P.Garstecki

Soft Matter, 2009,5, 2352-2360

Oscillating electric fields can sustain a macroscopic and steady separation of electrostatic charges. The control over the dynamic charge separation (dyCHASE) is presented for the example of circular menisci of thin, free standing smectic films. These films are subject to an in-plane, alternating radial electric field. The boundaries of the menisci become charged and unstable in the electric field and deform into pulsating, flower-like shapes. This instability ensues only at frequencies of the electric field that are lower than a critical one. The critical frequency is a linear function of the strength of the electric field. Since the speed of electrophoretic drift of ions is also linearly related to the strength of the field, the linear relation between critical frequency and the amplitude of the field sets a characteristic length scale in the system. We postulate that dyCHASE is due to (i) electrophoretic motion of ions in the liquid crystalline (LC) film, (ii) microscopic separation of charges over distances similar in magnitude to the Debye screening length, and (iii) further, macroscopic separation of charges through an electro-hydrodynamic instability. Interestingly, the electrophoretic motion of ions couples with the macroscopic motion of the LC material that can be observed with the use of simple optical microscopy.

Scaling form of viscosity at all length-scales in poly(ethylene glycol) solutions studied by fluorescence correlation spectroscopy and capillary electrophoresis

R. Holyst, A. Bielejewska, J. Szymański, A. Wilk, A. Patkowski, J. Gapiński, A. Żywociński, T. Kalwarczyk, E. Kalwarczyk, M. Tabaka, N. Ziębacza and S. A. Wieczoreka

Phys. Chem. Chem. Phys. 2009,11, 9025-9032

We measured the viscosity of poly(ethylene glycol) (PEG 6000, 12 000, 20 000) in water using capillary electrophoresis and fluorescence correlation spectroscopy with nanoscopic probes of different diameters (from 1.7 to 114 nm). For a probe of diameter smaller than the radius of gyration of PEG (e.g.rhodamine B or lyzozyme) the measured nanoviscosity was orders of magnitude smaller than the macroviscosity. For sizes equal to (or larger than) the polymer radius of gyration, macroscopic value of viscosity was measured. A mathematical relation for macro and nanoviscosity was found as a function of PEG radius of gyration, Rg, correlation length in semi-dilute solution, ξ, and probe size, R. For R < Rg, the nanoviscosity (normalized by water viscosity) is given by exp(b(R/ξ)a), and for R > Rg, both nano and macroviscosity follow the same curve, exp(b(R/ξ)a), where a and b are two constants close to unity. This mathematical relation was shown to equally well describe rhodamine (of size 1.7 nm) in PEG 20 000 and the macroviscosity of PEG 8 000 000, whose radius of gyration exceeds 200 nm. Additionally, for the smallest probes (rhodamine B and lysozyme) we have verified, using capillary electrophoresis and fluorescence correlation spectroscopy, that the Stokes–Einstein (SE) relation holds, providing that we use a size-dependent viscosity in the formula. The SE relation is correct even in PEG solutions of very high viscosity (three orders of magnitude larger than that of water).

Evaporation into vacuum: Mass flux from momentum flux and the Hertz-Knudsen relation revisited

R. Hołyst and M. Litniewski

J. Chem. Phys. 2009, 130, 074707

We performed molecular dynamics simulations of liquid film evaporation into vacuum for two cases: free evaporation without external supply of energy and evaporation at constant average liquid temperature. In both cases we found that the pressure inside a liquid film was constant, while temperature decreased and density increased as a function of distance from the middle of the film. The momentum flux in the vapor far from the liquid was equal to the liquid pressure in the evaporating film. Moreover the pseudopressure (stagnation pressure) was found to be constant in the evaporating vapor and equal to the liquid pressure. The momentum flux and its relation to the pressure determined the number of evaporating molecules per unit time and as a consequence the mass evaporation flux. We found a simple formula for the evaporation flux, which much better describes simulation results than the commonly used Hertz–Knudsen relation.

Challenges in thermodynamics: Irreversible processes, nonextensive entropies, and systems without equilibrium states

R. Hołyst

Pure and Applied Chemistry 2009, 81, 10, 1719-1726

Recent works on evaporation and condensation demonstrate that even these sim-plest irreversible processes, studied for over 100 years, are not well understood. In the caseof a liquid evaporating into its vapor, the liquid temperature is constant during evaporationand the evaporation flux is governed by the heat transfer from the hotter vapor into the colderliquid. Whether liquid evaporates into its own vapor or into the vacuum, the irreversible path-way in the process goes through a number of steps which quickly lead to the steady-state con-ditions with mechanical equilibrium in most parts of the system—the fact overlooked in allprevious studies. Even less is known about general rules which govern systems far from equi-librium. Recently, it has been demonstrated that a work done in an irreversible process canbe related to the free energy difference between equilibrium states joined by the process.Finally, a real challenge in thermodynamics is a description of living systems since they donot have equilibrium states, are nonextensive, (i.e., they cannot be divided into subsystems),and cannot be isolated. Thus, their proper description requires new paradigms in thermo -dynamics.

From complex structures to complex processes: Percolation theory applied to the formation of a city

A. Bitner, R. Hołyst and M. Fiałkowski

Phys. Rev. E 2009, 80, 037102

We investigate the morphology of the spatial pattern resulting from the division of land into the parcels that is observed in the centers of the cities, by analyzing the distribution function of the parcel areas. A simple model based on a two-dimensional bond percolation is employed to mimic the process of the formation of the city. The model reproduces the empirical distribution of the parcel areas that is found to exhibit the power law with the exponent τ=2.0. We argue that the city emerges from a collection of separated settlements in a process that can be described as a structural phase transition.

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