GaN-based platforms with Au-Ag alloyed metal layer for surface enhanced Raman scattering

J. L. Weyher, I. Dzięcielewski, A. Kamińska, T. Roliński, G. Nowak and R. Hołyst

Journal of Applied Physics 112, 114327 (2012)

The fabrication procedure of efficient surface enhanced Raman scattering (SERS) platforms is demonstrated based on reproducibly photo-etched GaN epitaxial layers covered by a thin Au-Ag layer and subjected to partial de-alloying. Using a gold-rich Au57Ag43 alloy (70/30 wt. %), it is possible to preserve about 19 at. % of silver in the layer, even after etching in nitric acid for up to 24 h. A large enhancement of the Raman signal from the test 4-mercaptobenzoic acid molecules attached to such porous Au-Ag metal layer is obtained due to the presence of a high percentage of Ag. For the optimal etching time, in the range of 6–24 h, an enhancement factor (EF) greater than 107 was obtained. The contribution of different features of our SERS platforms such as the size of pores, nano-roughness, and the chemical composition of the metal layer for the EF parameter are critically discussed.

Large-scale molecular dynamics verification of the Rayleigh-Plesset approximation for collapse of nanobubbles

R. Hołyst, M. Litniewski and P. Garstecki

Phys. Rev. E 82, 066309

We report large-scale (107 atoms in an 85-nm-wide container) molecular dynamics simulations of collapse of nanoscopic (5–12 nm in diameter) voids in liquid argon. During the collapse the pressure on the liquid side decreases, and this decrease propagates into liquid at the speed of sound. Despite the nonuniform profile of pressure in the liquid the solutions of the Rayleigh-Plesset equation compares well to the measured evolution of the radius of the void and the velocity of the interface. Evaporation of liquid into the void does not affect the dynamics appreciably.

Scale-dependent diffusion of spheres in solutions of flexible and rigid polymers: mean square displacement and autocorrelation function for FCS and DLS measurements

A. Ochab-Marcinek and R. Hołyst

Soft Matter, 2011, 7, 7366-7374

We present a theoretical description of diffusion of a sphere in polymer solution. The depletion layer around the sphere affects its motion and leads to scale-dependent diffusion. We propose the model of walking confined diffusion. Although it is different from anomalous diffusion, we show that the experimental data generated by this process may have features characteristic of anomalous diffusion. We give analytical formulas for the autocorrelation functions describing this type of motion in: i) dynamic light scattering experiments and ii) fluorescence correlation spectroscopy experiments. We compare our results to existing experimental data for polyethylene oxide, fd-virus, and F-actin solutions.

Highly reproducible, stable and multiply regenerated surface-enhanced Raman scattering substrate for biomedical applications

A. Kamińska, I. Dzięcielewski, J. L. Weyher, J. Waluk, S. Gawinkowski, V. Sashuk, M. Fiałkowski, M. Sawicka, T. Suski, S. Porowski and R. Hołyst

J. Mater. Chem., 2011,21, 8662-8669

We fabricated a Surface Enhanced Raman Scattering (SERS)-active surface based on photo-etched and Au-coated GaN. The highest enhancement factor (EF) in SERS and high reproducibility of spectra were obtained from surfaces covered with bunched nanopillars which were produced by relatively long defect-selective photo-etching. The surfaces exhibited SERS enhancements of the order of 2.8 × 106 for malachite green isothiocyanate (MGITC) and 2 × 106 for p-mercaptobenzoic acid (PMBA). These SERS enhancement factors were comparable to those of conventional SERS substrates, while the EF for MGITC was two orders of magnitude larger than the corresponding one reported for the SERS platform made on porous GaN. The standard deviation of the relative intensity of the 1180 cm−1 mode of MGITC was less than 5% for 100 randomly distributed locations across a single platform and less than 10% between different platforms. The SERS signal of MGITC at our GaN/Au surface (kept under ambient conditions) was extremely stable. We could not detect any peak shift or appreciable change of intensity even after three months. We used these surfaces to detect biological molecules such as amino acids and bovine serum albumin (BSA) at low concentration and with short detection time. We developed simple and effective cleaning procedures for our substrates. After cleaning, the same substrate could be used multiple times retaining the SERS activity. We are not aware of any other multiply regenerated SERS substrate which provides simultaneously such high stability with high enhancement, good uniformity, and high reproducibility.

Influence of nano-viscosity and depletion interactions on cleavage of DNA by enzymes in glycerol and poly(ethylene glycol) solutions: qualitative analysis

S. Hou, N. Ziebacz, T. Kalwarczyk, T. S. Kaminski, S. A. Wieczorek and R. Holyst

Soft Matter, 2011, 7, 3092-3099

Biochemical reactions in living systems take place in an environment crowded by various macromolecules and ligands. Therefore experimental data obtained in buffer do not reflect in vivo conditions. We have used glycerol poly(ethylene glycol) (PEG) 6000 and PEG 8 M solutions to investigate the influence of the crowded environment on cleavage of plasmid DNA by restriction enzyme HindIII. PEG 6000 solution can effectively slow down the cleavage process. However, neither PEG 8 M solution of the same viscosity as PEG 6000 solution nor glycerol solution of the same concentration as PEG 6000 solution slows the cleavage of DNA appreciably. The viscosity experienced by the biomolecules (here called nano-viscosity) and aggregation induced by the depletion interactions between DNA molecules in polymer solution (PEG 6000) are two factors responsible for slow cleavage of DNA. We have ruled out the change of pH and denaturation of HindIII as possible sources for the effect.

Formation and structure of PEI/DNA complexes: quantitative analysis

S. Hou, N. Ziebacz, S. A. Wieczorek, E. Kalwarczyk, V. Sashuk, T. Kalwarczyk, T. S. Kaminski and R. Holyst

Soft Matter, 2011,7, 6967-6972

Controlled formation of gene delivery complexes (DNA and a vector, usually a cationic polymer) is one of the key challenges in developing efficient gene delivery systems. The researchers focused their procedures on the ratio of vector to DNA, neglecting the influence of concentration on the complex formation process. In this study we show, by studying the association of polyethylenimine (PEI) and 66-base pair (bp) DNA fragments, that the concentration of the gene delivery system greatly influences the formation of PEI/DNA complexes even at a fixed PEI/DNA ratio. We find that the charge and the size of PEI/DNA complexes are increasing functions of their concentration even in a highly dilute regime of concentrations. The number of PEI/DNA molecules in a complex was calculated from the measured charge and electrophoretic mobility. We established a model, on the basis of Smoluchowski theory, to explain the relation between the concentration and the size of PEI/DNA complexes. We analyzed the structure of the complexes and found out that a large proportion of space in the PEI/DNA complexes is occupied by the solvent. This study indicates that the influence of concentration should be seriously considered in gene delivery studies, since large PEI/DNA complexes can be prepared by scaling up their concentration simultaneously without increasing the dosage of PEI.

Crossover regime for the diffusion of nanoparticles in polyethylene glycol solutions: influence of the depletion layer

N. Ziębacz, S. A. Wieczorek, T. Kalwarczyk, M. Fiałkowskia and R. Hołyst

Soft Matter, 2011,7, 7181-7186

The viscosity in soft matter systems is a scale dependent quantity. In polymer solutions the viscosity of nanoprobes of size R approaches the macroscopic viscosity when R exceeds the radius of gyration of the polymer, Rg. The nano to macroviscosity crossover occurs for R ∼ Rg. Here we analyze diffusion in a polymer (polyethylene glycol) solution of nanoparticles in the crossover regime. We report a scale dependent diffusion coefficient in this regime due to non-uniform viscosity in the depletion layer around particles. The phenomenological scaling of the slow diffusion coefficient as a function of probe size is compared to the same scaling for macroscopic viscosity as a function of polymer size.

New One-Pot Technique to Introduce Charged Nanoparticles into a Lyotropic Liquid Crystal Matrix

E. Kalwarczyk, M. Paszewski, X. Xin, E. Gorecka, D. Pociecha, R. Holyst and M. Fialkowski

Langmuir 2011, 27, 7, 3937–3944

We present a new method to incorporate hydrophilic charged nanoparticles into the lyotropic liquid crystal (LLC) template. This method is based on the effect of the polymer-induced phase separation (PIPS) and consists of two steps. In the first step, the nanoparticles are mixed with a surfactant micellar solution. In the second step, upon addition of polymer, phase separation is induced and the LLC phase doped with the nanoparticles is formed. Columnar hexagonal and lamellar LLC templates are obtained with the PIPS method. The ordering of the LLC phase can be controlled by the amount of polymer added to induce phase separation. The method works both for the system of nonionic surfactants and polymers and ionic surfactants and polyelectrolytes. We demonstrate that the PIPS method enables the fabrication of the LLC templates doped with positively or negatively charged nanoparticles as well as with a mixture of oppositely charged nanoparticles in arbitrary proportions.

Self‐Assembly at Different Length Scales: Polyphilic Star‐Branched Liquid Crystals and Miktoarm Star Copolymers

G. Ungar, C. Tschierske, V. Abetz, R. Holyst, M. A. Bates, F. Liu, M. Prehm, R. Kieffer, X. Zeng, M. Walker, B. Glettner and A. Zywocinski

Advanced Functional Materials 2011, 21, 7, 296-1323

The diversity of phase morphologies observed recently in star‐branched liquid‐crystalline and polymeric compounds containing at least three immiscible segments is reviewed. Bolaamphiphiles and facial amphiphiles with rodlike aromatic cores, two end‐groups, and one (T‐shape) or two (X‐shape) chains attached laterally to the core, form numerous honeycomblike liquid‐crystal phases, as well as a variety of novel lamellar and 3D‐ordered mesophases. Molecular self‐organization is described in bulk phases and in thin films on solid and liquid surfaces, as well as in Langmuir–Blodgett films. The remarkably reversible formation of mono‐ and trilayer films is highlighted. In the bulk, T‐shaped “rod–coil” molecules without appended end‐groups form predominantly lamellar phases if the core is a straight rod, but the bent‐core variety forms hexagonal honeycombs. Furthermore, self‐assembly of “Janus”‐type molecules, is discussed. Also covered is the diversity of morphologies observed in miktoarm star terpolymers, i.e., polymers with three different and incompatible arms of well defined lengths. Similarities and differences are highlighted between the liquid‐crystal morphologies on the 3–15 nm scale and the polymer morphologies on the scale of 10–100 nm. A separate section is dedicated to computer simulations of such systems, particularly those using dissipative particle and molecular dynamics. Of special interest are the recently synthesised X‐shaped tetraphilic molecules, where two different and incompatible side‐chains are attached at opposite sides of the rodlike core. The tendency for their phase separation produces liquid‐crystal honeycombs with cells of different compositions that can be represented as a plane paved with different colored tiles. The independent variation of chain length and “color” creates the potential for developing a considerable range of complex new 2D and 3D soft nanostructures. Analogous X‐shaped rod–coil compounds with unequal side groups are also of considerable interest, forming tubular lyotropic structures capable of confining strings of guest molecules.

Comparative Analysis of Viscosity of Complex Liquids and Cytoplasm of Mammalian Cells at the Nanoscale

T. Kalwarczyk, N. Ziȩbacz, A. Bielejewska, E. Zaboklicka, K. Koynov, J. Szymański, A. Wilk, A. Patkowski, J. Gapiński, Hans-Jürgen Butt and R. Hołyst

Nano Lett. 2011, 11, 5, 2157–2163

We present a scaling formula for size-dependent viscosity coefficients for proteins, polymers, and fluorescent dyes diffusing in complex liquids. The formula was used to analyze the mobilities of probes of different sizes in HeLa and Swiss 3T3 mammalian cells. This analysis unveils in the cytoplasm two length scales: (i) the correlation length ξ (approximately 5 nm in HeLa and 7 nm in Swiss 3T3 cells) and (ii) the limiting length scale that marks the crossover between nano- and macroscale viscosity (approximately 86 nm in HeLa and 30 nm in Swiss 3T3 cells). During motion, probes smaller than ξ experienced matrix viscosity: ηmatrix ≈ 2.0 mPa·s for HeLa and 0.88 mPa·s for Swiss 3T3 cells. Probes much larger than the limiting length scale experienced macroscopic viscosity, ηmacro ≈ 4.4 × 10–2 and 2.4 × 10–2 Pa·s for HeLa and Swiss 3T3 cells, respectively. Our results are persistent for the lengths scales from 0.14 nm to a few hundred nanometers.

Aggregation and Layering Transitions in Thin Films of X‐, T‐, and Anchor‐Shaped Bolaamphiphiles at the Air–Water Interface

P. Nitoń, A. Żywociński J. Paczesny, M. Fiałkowski, R. Hołyst B. Glettner R. Kieffer, C. Tschierske, D. Pociecha and E. Górecka

Chemistry - A European Journal 2011, 17, 21, 5861-5873

Aggregation in Langmuir films is usually understood as being a disorderly grouping of molecules turning into chaotic three‐dimensional aggregates and is considered an unwanted phenomenon causing irreversible changes. In this work we present the studies of 11 compounds from the group of specific surfactants, known as bolaamphiphiles, that exhibit reversible aggregation and, in many cases, transition to well‐defined multilayers, which can be considered as a layering transition. These bolaamphiphiles incorporate rigid π‐conjugated aromatics as hydrophobic cores, glycerol‐based polar groups and hydrophobic lateral chains. Molecules of different shapes (X‐, T‐, and anchor) were studied and compared. The key property of these compounds is the partial fluorination of the lateral chains linked to the rigid cores of the molecules. The most interesting feature of the compounds is that, depending on their shape and degree of fluorination, they are able to resist aggregation and preserve a monolayer structure up to relatively high surface pressures (T‐shaped and some X‐shaped molecules), or create well‐defined trilayers (X‐ and anchor‐shaped molecules). Experimental studies were performed using Langmuir balance, surface potential and X‐ray reflectivity measurements.

Ionic polarization of liquid-liquid interfaces; dynamic control of the rate of electro-coalescence

T. Szymborski, P. M. Korczyk, R. Hołyst and P. Garstecki

Appl. Phys. Lett. 2011, 99, 094101

Electrostatic forces are the strongest interactions in soft matter physics, yet they are usually screened by ions present in the solutions. Here we demonstrate that the extent of ionic polarization of liquid-liquid interfaces can be dynamically controlled via appropriate choice of the strength and frequency of the external electric field. Results of microfluidic experiments on electro-coalescence of droplets of aqueous solutions of salt provide guidelines for optimization of the process.
Project operated within the Foundation for Polish Science (FPS) Team Program (2008-1/1) co-financed by the EU European Regional Development Fund. T.S. thanks the Ministry of Science for support (IP2010 042270). R.H. thanks the FPS for support within the TEAM program (2008-2/2).

Ionic Strength-Controlled Deposition of Charged Nanoparticles on a Solid Substrate

K. Winkler, M. Paszewski, T. Kalwarczyk, E. Kalwarczyk, T. Wojciechowski, E. Gorecka, D. Pociecha, R. Holyst and M. Fialkowski

J. Phys. Chem. C 2011, 115, 39, 19096–19103

There is experimental evidence that in solutions of low ionic strength, charged nanoparticles (NPs) adsorbing onto an oppositely charged substrate cannot form dense monolayers due to the electrostatic repulsion. Here, we investigated the adsorption onto negatively charged substrates occurring in solutions containing positively charged NPs and salt for a wide range of ionic strengths. We found that the salt added in high concentration, above 2M, stabilizes the solution and the NPs adsorb on the substrate to form dense coatings characterized by the surface coverage of about 50%. In this regime, the adsorption rate was found to grow with the square of the NP concentration. This effect of salt on the deposition process provides a facile method of coating of solid substrates with NP monolayers. The density of such monolayers depends on and can be easily controlled by the deposition time, as well as by the salt and NP concentration in the plating solution.

Gold Micro-Flowers: One-Step Fabrication of Efficient, Highly Reproducible Surface-Enhanced Raman Spectroscopy Platform

K. Winkler, A. Kaminska, T. Wojciechowski, R. Holyst and M. Fialkowski

Plasmonics volume 2011, 6, Article number: 697

We present a new method enabling simultaneous synthesis and deposition of gold micro-flowers (AuMFs) on solid substrates in a one-pot process that uses two reagents, auric acid and hydroxylamine hydrochloride, in aqueous reaction mixture. The AuMFs deposited onto the substrate form mechanically stable gold layer of expanded nanostructured surface. The morphology of the AuMFs depends on and can be controlled by the composition of the reaction solution as well as by the reaction time. The nanostructured metallic layers obtained with our method are employed as efficient platforms for chemical and biological sensing based on surface-enhanced Raman spectroscopy (SERS). SERS spectra recorded by such platforms for p-mercaptobenzoic acid and phage lambda exhibit enhancement factors above 106 and excellent reproducibility.

Dynamic Supramolecular Polymers Based on Benzene‐1,3,5‐tricarboxamides: The Influence of Amide Connectivity on Aggregate Stability and Amplification of Chirality

P. J. M. Stals, J. C. Everts, R. de Bruijn, I. A. W. Filot, M. M. J. Smulders, R. Martin-Rapun, E. A. Pidko, T. F. A. de Greef, A. R. A. Palmans and E. W. Meijer

Chem. Eur. J., 2010, 16, 810-821

N‐Centred benzene‐1,3,5‐tricarboxamides (N‐BTAs) composed of chiral and achiral alkyl substituents were synthesised and their solid‐state behaviour and self‐assembly in dilute alkane solutions were investigated. A combination of differential scanning calorimetry (DSC), polarisation optical microscopy (POM) and X‐ray diffraction revealed that the chiral N‐BTA derivatives with branched 3,7‐dimethyloctanoyl chains were liquid crystalline and the mesophase was assigned as Colho. In contrast, N‐BTA derivatives with linear tetradecanoyl or octanoyl chains lacked a mesophase and were obtained as crystalline compounds. Variable‐temperature infrared spectroscopy showed the presence of threefold, intermolecular hydrogen bonding between neighbouring molecules in the mesophase of the chiral N‐BTAs. In the crystalline state at room temperature a more complicated packing between the molecules was observed. Ultraviolet and circular dichroism spectroscopy on dilute solutions of N‐BTAs revealed a cooperative self‐assembly behaviour of the N‐BTA molecules into supramolecular polymers with preferred helicity when chiral alkyl chains were present. Both the sergeants‐and‐soldiers as well as the majority‐rules principles were operative in stacks of N‐BTAs. In fact, the self‐assembly of N‐BTAs resembles closely that of their carbonyl (CO)‐centred counterparts, with the exception that aggregation is weaker and amplification of chirality is less pronounced. The differences in the self‐assembly of N‐ and CO‐BTAs were analysed by density functional theory (DFT) calculations. These reveal a substantially lower interaction energy between the monomeric units in the supramolecular polymers of N‐BTAs. The lower interaction energy is due to the higher energy penalty for rotation around the PhNH bond compared to the PhCO bond and the diminished magnitude of dipole–dipole interactions. Finally, we observed that mixed stacks are formed in dilute solution when mixing N‐BTAs and CO BTAs.

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