Lineare und nichtlineare dielektrische Spektroskopie am Relaxor-Ferroelektrikum Bleimagnesiumniobat

Relaxor-Ferroelektrika sind wegen ihrer möglichen technischen Anwendungen und vom Standpunkt der Grundlagenforschung als Beispiel für ungeordnete Systeme von wissenschaftlichem Interesse. Trotz zahlreicher experimenteller Untersuchungen sind die mikroskopischen Ursachen ihrer Eigenschaften aber nach wie vor ungeklärt. Im Rahmen dieser Arbeit wurde das Relaxor-Ferroelektrikum Bleimagnesiumniobat-Bleititanat (PMN-10PT) mittels linearer und nichtlinearer dielektrischer Spektroskopie untersucht. Durch Anregung mit starken elektrischen Wechselfeldern konnten mit der Methode des nichtresonanten dielektrischen Lochbrennens frequenzselektiv einzelne spektrale Bereiche aus dem verbreiterten Relaxationsspektrum herausgegriffen und deren Rückrelaxation separat verfolgt werden. Die experimentellen Ergebnisse zeigten, daß eine langlebige dynamische Heterogenität der dipolaren Reorientierung existiert. Durch ihr ausgeprägt nichtergodisches Verhalten zeigen Relaxor-Ferroelektrika starke Alterungseffekte. Die Untersuchung des Alterungsverhaltens der dielektrischen Suszeptibilität zeigte, daß ein Gedächtnis für die bei einer Alterungstemperatur eingenommene Konfiguration bestehen bleibt, sofern die Temperatur nach einer unvollständigen isothermen Alterung nur um einige Grad abgesenkt oder erhöht wurde.Außerdem wurde die induzierte Polarisation bei stochastischen dielektrischen Anregungen mit elektrischen Feldern, die in sehr guter Näherung ein weißes Rauschen darstellten, untersucht. Über die Bildung der Kreuzkorrelationsfunktion zwischen Feld und Polarisation konnte die Impulsantwortfunktion des Systems berechnet werden.Die experimentellen Ergebnisse am Relaxor-Ferroelektrikum PMN-10PT können sehr gut mit einem Modell eines ungeordneten Ferroelektrikums erklärt werden, dessen Domänenwände unordnungsbedingt an sogenannten Pinning-Zentren festhaften.

Pinning-Effekte kolumnarer Schwerionenspuren in Kuprat-Supraleiter-Dünnschichten

In this work the flux line dynamics in High-Temperature Superconductor (HTSC) thin films in the presence of columnar defects was studied using electronic transport measurements. The columnar defects which are correlated pinning centers for vortices were generated by irradiation with swift heavy ions at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt. In the first part, the vortex dynamics is discussed within the framework of the Bose-glass model. This approach describes the continuous transition from a vortex liquid to a Bose-glass phase which is characterized by the localization of the flux lines at the columnar defects. The critical behavior of the characteristic length and time scales for temperatures in the vicinity of this phase transition were probed by scaling properties of experimentally obtained current-voltage characteristics. In contrast to the predicted universal properties of the critical behavior the scaling analysis shows a strong dependence of the dynamic critical exponent on the experimentally accessible electric field range. In addition, the predicted divergence of the activation energy in the limit of low current densities was experimentally not confirmed.The dynamic behavior of flux lines in spatially resolved irradiation geometries is reported in the second part. Weak pinning channels with widths between 10 µm and 100 µm were generated in a strong pinning environment with the use of metal masks and the GSI microprobe, respectively. Measurements of the anisotropic transport properties of these structures show a striking resemblance to the results in YBCO single crystals with unidirected twin boundaries which were interpreted as a guided vortex motion effect. The use of two additional test bridges allowed to determine in parallel the resistivities of the irradiated and unirradiated parts as well as the respective current-voltage characteristics. These measurements provided the input parameters for a numerical simulation of the potential distribution in the spatially resolved irradiation geometry. The results are interpreted within a model that describes the hydrodynamic interaction between a Bose-glass phase and a vortex liquid. The interface between weakly pinned flux lines in the unirradiated channels and strongly pinned vortices leads to a nonuniform vortex velocity profile and therefore a variation of the local electric field. The length scale of these interactions was estimated for the first time in measuring the local variation of the electric field profile in a Bose-glass contact.Finally, a method for the determination of the true temperature in HTSC thin films at high dissipation levels is described. In this regime of electronic transport the occurrence of a flux flow instability is accompanied by heating effects in the vortex system. The heat propagation properties of the film/substrate system are deduced from the time dependent voltage response to a short high current density pulse of rectangular shape. The influence of heavy ion irradiation on the heat resistance at the film/substrate interface is studied.

Elektrorheologie homogener Systeme

Zusammenfassung: Thema dieser Arbeit ist der Einfluss von elektrischen Feldern auf die Viskosität nematischer Flüssigkristalle sowie isotroper Polymerlösungen.Für zwei thermotrope nematische Flüssigkristalle, 4-(trans-4'-n-Pentylcyclohexyl)-benzonitril (PCH-5) und 4-n-Pentyl-4'-cyanobiphenyl (5CB), wurde die Viskosität mit elektrorheologischen Messungen in Abhängigkeit von Feldstärke und Scherrate bestimmt. Alle Messwerte kommen bei Auftragung gegen E?/? auf einer Masterkurve zu liegen. Die Anpassung dieser Daten mit einer auf der Theorie von Ericksen und Leslie basierenden Gleichung liefert gleichzeitig die Scherviskositäten ? und ? sowie die Rotationsviskosität ? in guter Übereinstimmung mit Literaturwerten. Elektrorheologische Messungen stellen somit eine Methode zur Bestimmung dieser Viskositätskoeffizienten für Nematen dar, wenn diese eine ausreichend große positive dielektrische Anisotropie aufweisen. Bei den isotropen Polymerlösungen ließ sich der elektrorheologische Effekt des Systems Decalin/Polystyrol im Bereich der Entmischungstemperatur klären. Während im Ein-Phasen-Gebiet nur eine geringe Viskositätsänderung im E-Feld aufgrund elektrohydrodynamischer Strömungen zu beobachten ist, tritt unterhalb der Entmischungstemperatur ein deutlicher, aber nicht reproduzierbarer ER-Effekt auf. Dieser ist auf einen Einfluss des E-Felds auf die Zwei-Phasen-Morphologie zurückzuführen.Für das System Diethylbenzol/Isopentylcellulose wurde ein negativer elektrorheologischer Effekt auf die Dissoziation von assoziierten Polymermolekülen zurückgeführt. Die Dissoziation erfolgt aufgrund der Wirkung des elektrischen Feldes auf freie Ladungen.

Mixed Mode Fracture Behaviour of Piezoelectric Materials

Piezoelectrics present an interactive electromechanical behaviour that, especially in recent years, has generated much interest since it renders these materials adapt for use in a variety of electronic and industrial applications like sensors, actuators, transducers, smart structures. Both mechanical and electric loads are generally applied on these devices and can cause high concentrations of stress, particularly in proximity of defects or inhomogeneities, such as flaws, cavities or included particles. A thorough understanding of their fracture behaviour is crucial in order to improve their performances and avoid unexpected failures. Therefore, a considerable number of research works have addressed this topic in the last decades. Most of the theoretical studies on this subject find their analytical background in the complex variable formulation of plane anisotropic elasticity. This theoretical approach bases its main origins in the pioneering works of Muskelishvili and Lekhnitskii who obtained the solution of the elastic problem in terms of independent analytic functions of complex variables. In the present work, the expressions of stresses and elastic and electric displacements are obtained as functions of complex potentials through an analytical formulation which is the application to the piezoelectric static case of an approach introduced for orthotropic materials to solve elastodynamics problems. This method can be considered an alternative to other formalisms currently used, like the Stroh’s formalism. The equilibrium equations are reduced to a first order system involving a six-dimensional vector field. After that, a similarity transformation is induced to reach three independent Cauchy-Riemann systems, so justifying the introduction of the complex variable notation. Closed form expressions of near tip stress and displacement fields are therefore obtained. In the theoretical study of cracked piezoelectric bodies, the issue of assigning consistent electric boundary conditions on the crack faces is of central importance and has been addressed by many researchers. Three different boundary conditions are commonly accepted in literature: the permeable, the impermeable and the semipermeable (“exact”) crack model. This thesis takes into considerations all the three models, comparing the results obtained and analysing the effects of the boundary condition choice on the solution. The influence of load biaxiality and of the application of a remote electric field has been studied, pointing out that both can affect to a various extent the stress fields and the angle of initial crack extension, especially when non-singular terms are retained in the expressions of the electro-elastic solution. Furthermore, two different fracture criteria are applied to the piezoelectric case, and their outcomes are compared and discussed. The work is organized as follows: Chapter 1 briefly introduces the fundamental concepts of Fracture Mechanics. Chapter 2 describes plane elasticity formalisms for an anisotropic continuum (Eshelby-Read-Shockley and Stroh) and introduces for the simplified orthotropic case the alternative formalism we want to propose. Chapter 3 outlines the Linear Theory of Piezoelectricity, its basic relations and electro-elastic equations. Chapter 4 introduces the proposed method for obtaining the expressions of stresses and elastic and electric displacements, given as functions of complex potentials. The solution is obtained in close form and non-singular terms are retained as well. Chapter 5 presents several numerical applications aimed at estimating the effect of load biaxiality, electric field, considered permittivity of the crack. Through the application of fracture criteria the influence of the above listed conditions on the response of the system and in particular on the direction of crack branching is thoroughly discussed.

Mode coupling for precise measurements of the electronic g-factor of hydrogen-like ions in Penning traps

The g-factor is a constant which connects the magnetic moment $vec{mu}$ of a charged particle, of charge q and mass m, with its angular momentum $vec{J}$. Thus, the magnetic moment can be writen $ vec{mu}_J=g_Jfrac{q}{2m}vec{J}$. The g-factor for a free particle of spin s=1/2 should take the value g=2. But due to quantum electro-dynamical effects it deviates from this value by a small amount, the so called g-factor anomaly $a_e$, which is of the order of $10^{-3}$ for the free electron. This deviation is even bigger if the electron is exposed to high electric fields. Therefore highly charged ions, where electric field strength gets values on the order of $10^{13}-10^{16}$V/cm at the position of the bound electron, are an interesting field of investigations to test QED-calculations. In previous experiments [H"aff00,Ver04] using a single hydrogen-like ion confined in a Penning trap an accuracy of few parts in $10^{-9}$ was obtained. In the present work a new method for precise measurement of magnetic the electronic g-factor of hydrogen-like ions is discussed. Due to the unavoidable magnetic field inhomogeneity in a Penning trap, a very important contribution to the systematic uncertainty in the previous measurements arose from the elevated energy of the ion required for the measurement of its motional frequencies. Then it was necessary to extrapolate the result to vanishing energies. In the new method the energy in the cyclotron degree of freedom is reduced to the minimum attainable energy. This method consist in measuring the reduced cyclotron frequency $nu_{+}$ indirectly by coupling the axial to the reduced cyclotron motion by irradiation of the radio frequency $nu_{coup}=nu_{+}-nu_{ax}+delta$ where $delta$ is, in principle, an unknown detuning that can be obtained from the knowledge of the coupling process. Then the only unknown parameter is the desired value of $nu_+$. As a test, a measurement with, for simplicity, artificially increased axial energy was performed yielding the result $g_{exp}=2.000~047~020~8(24)(44)$. This is in perfect agreement with both the theoretical result $g_{theo}=2.000~047~020~2(6)$ and the previous experimental result $g_{exp1}=2.000~047~025~4(15)(44).$ In the experimental results the second error-bar is due to the uncertainty in the accepted value for the electron's mass. Thus, with the new method a higher accuracy in the g-factor could lead by comparison to the theoretical value to an improved value of the electron's mass. [H"af00] H. H"affner et al., Phys. Rev. Lett. 85 (2000) 5308 [Ver04] J. Verd'u et al., Phys. Rev. Lett. 92 (2004) 093002-1

Ortsaufgelöster Nachweis von Elementspuren mittels Laserablation und Resonanzionisations-Massenspektrometrie und Untersuchungen zur Laserablations-Dynamik

Im Rahmen dieser Arbeit wurde eine neue Methode für einen empfindlichen und isotopenselektiven Elementnachweis entwickelt. Unter Einsatz von Laserablation geschieht der Probenaufschluß direkt und mit einer Ortsauflösung von unter 30 m. Hierzu wurde ein hochauflösendes MALDI-TOF-Massenspektrometer, welches üblicherweise für biochemische Fragestellungen eingesetzt wird, mit einem spektroskopischen Aufbau zur resonanten Ionisation von Elementgehalten modifiziert. Die Methode ist somit insbesondere für die Untersuchung von Elementspuren in Festkörperproben mit mikroskopischer Struktur konzipiert. Methodische Entwicklungsarbeiten wurden anhand des Elements Gadolinium durchgeführt. Durch die Verwendung gepulster Farbstofflaser stehen ausreichend hohe Laserfelder zur Verfügung, um unabhängig von Hyperfeinstruktur und Isotopieverschiebung Übergänge aller Isotope im Rahmen des Resonanzionisations-Verfahrens zu sättigen. Darauf konnte eine Isotopenverhältnisanalyse mit einer Genauigkeit im Prozentbereich verwirklicht werden. Verschiedene Anregungsleitern wurden untersucht, und mit elementspezifischen Resonanzüberhöhungen bis zu zwei Größenordnungen über dem nicht-resonant gebildeten Untergrund konnte eine Nachweiseffizienz von über 10-4 (entsprechend sub-fg/g-Niveau) erzielt werden. Dazu wurden Simulationsrechnungen zum atomaren Sättigungsverhalten in starken resonanten Laserfeldern durchgeführt. Erste Anwendungen des Laserablationsverfahrens waren Proben kosmologischer Herkunft. Der physikalische Prozeß der Laserablation bei Metallen wurde unter Hochvakuum-Bedingung systematisch in Abhängigkeit der Laserfluenz untersucht. In der ablatierten Plasmaphase erwies sich der Neutralanteil als besonders geeignet für geschwindigkeitsselektive Laserionisations-Messungen. Eine bimodale Struktur wurde beobachtet, bestehend aus einer thermischen und einer schockwellen-induzierten Komponente. Der ionische Anteil der ablatierten Dampfphase konnte über variable elektrische Feldpulse untersucht werden. Laserablation unter Atmosphärenbedingung wurde an einem beschichteten Messingtarget untersucht. Dabei wurde die Entstehung von permanenten Oberflächenstrukturen beobachtet, welche sich durch Nichtgleichgewichts-Prozesse in der Dampfphase erklären lassen.

Multiscale fabrication of functional materials for life sciences

Regenerative medicine claims for a better understanding of the cause-effect relation between cell behaviour and environment signals. The latter encompasses topographical, chemical and mechanical stimuli, electromagnetic fields, gradients of chemo-attractants and haptotaxis. In this perspective, a spatial control of the structures composing the environment is required. In this thesis I describe a novel approach for the multiscale patterning of biocompatible functional materials in order to provide systems able to accurately control cell adhesion and proliferation. The behaviour of different neural cell lines in response to several stimuli, specifically chemical, topographical and electrical gradients is presented. For each of the three kind of signals, I chose properly tailored materials and fabrication and characterization techniques. After a brief introduction on the state of art of nanotechnology, nanofabrication techniques and regenerative medicine in Chapter 1 and a detailed description of the main fabrication and characterization techniques employed in this work in Chapter 2, in Chapter 3 an easy route to obtain accurate control over cell proliferation close to 100% is described (chemical control). In Chapter 4 (topographical control) it is shown how the multiscale patterning of a well-established biocompatible material as titanium dioxide provides a versatile and robust method to study the effect of local topography on cell adhesion and growth. The third signal, viz. electric field, is investigated in Chapter 5 (electrical control), where the very early stages of neural cell adhesion are studied in the presence of modest steady electric fields. In Chapter 6 (appendix) a new patterning technique, called Lithographically Controlled Etching (LCE), is proposed. It is shown how LCE can provide at the same time the micro/nanostructuring and functionalization of a surface with nanosized objects, thus being suitable for applications both in regenerative medicine in biosensing.

Theoretical and numerical study of the laser-plasma ion acceleration

The laser driven ion acceleration is a burgeoning field of resarch and is attracting a growing number of scientists since the first results reported in 2000 obtained irradiating thin solid foils by high power laser pulses. The growing interest is driven by the peculiar characteristics of the produced bunches, the compactness of the whole accelerating system and the very short accelerating length of this all-optical accelerators. A fervent theoretical and experimental work has been done since then. An important part of the theoretical study is done by means of numerical simulations and the most widely used technique exploits PIC codes (“Particle In Cell'”). In this thesis the PIC code AlaDyn, developed by our research group considering innovative algorithms, is described. My work has been devoted to the developement of the code and the investigation of the laser driven ion acceleration for different target configurations. Two target configurations for the proton acceleration are presented together with the results of the 2D and 3D numerical investigation. One target configuration consists of a solid foil with a low density layer attached on the irradiated side. The nearly critical plasma of the foam layer allows a very high energy absorption by the target and an increase of the proton energy up to a factor 3, when compared to the ``pure'' TNSA configuration. The differences of the regime with respect to the standard TNSA are described The case of nearly critical density targets has been investigated with 3D simulations. In this case the laser travels throughout the plasma and exits on the rear side. During the propagation, the laser drills a channel and induce a magnetic vortex that expanding on the rear side of the targer is source of a very intense electric field. The protons of the plasma are strongly accelerated up to energies of 100 MeV using a 200PW laser.

Stochastische Modelle für nichtlineare Antwortfunktionen in ungeordneten Systemen

In dieser Arbeit werden nichtlineare Experimente zur Untersuchung der Dynamik in amorphen Festkörpern im Rahmen von Modellrechnungen diskutiert. Die Experimente beschäftigen sich mit der Frage nach dynamischen Heterogenitäten, worunter man das Vorliegen dynamischer Prozesse auf unterschiedlichen Zeitskalen versteht. Ist es möglich, gezielt 'langsame' oder 'schnelle' Dynamik in der Probe nachzuweisen, so ist die Existenz von dynamischen Heterogenitäten gezeigt. Ziel der Experimente sind deshalb sogenannte frequenzselektive Anregungen des Systems. In den beiden diskutierten Experimenten, zum einen nichtresonantes Lochbrennen, zum anderen ein ähnliches Experiment, das auf dem dynamischen Kerreffekt beruht, werden nichtlineare Antwortfunktionen gemessen. Um eine Probe in frequenzselektiver Weise anzuregen, werden zunächst einer oder mehrere Zyklen eines oszillierenden elektrischen Feldes an die Probe angelegt. Die Experimente werden zunächst im Terahertz-Bereich untersucht. Auf dieser Zeitskala findet man phonon-ähnliche kollektive Schwingungen in Gläsern. Diese Schwingungen werden durch (anharmonische) Brownsche Oszillatoren beschrieben. Der zentrale Befund der Modellrechnungen ist, daß eine frequenzselektive Anregung im Terahertz-Bereich möglich ist. Ein Nachweis dynamischer Heterogenitäten im Terahertz-Bereich ist somit durch beide Experimente möglich. Anschliessend wird das vorgestellte Kerreffekt-Experiment im Bereich wesentlich kleinerer Frequenzen diskutiert. Die langsame Reorientierungsdynamik in unterkühlten Flüssigkeiten wird dabei durch ein Rotationsdiffusionsmodell beschrieben. Es werden zum einen ein heterogenes und zum anderen ein homogenes Szenario zugrundegelegt. Es stellt sich heraus, daß wie beim Lochbrennen eine Unterscheidung durch das Experiment möglich ist. Das Kerreffekt-Experiment wird somit als eine relativ einfache Alternative zur Technik des nichtresonanten Lochbrennens vorgeschlagen.

Development of instrumental and sensory analytical methods of food obtained by traditional and emerging technologies

The consumer demand for natural, minimally processed, fresh like and functional food has lead to an increasing interest in emerging technologies. The aim of this PhD project was to study three innovative food processing technologies currently used in the food sector. Ultrasound-assisted freezing, vacuum impregnation and pulsed electric field have been investigated through laboratory scale systems and semi-industrial pilot plants. Furthermore, analytical and sensory techniques have been developed to evaluate the quality of food and vegetable matrix obtained by traditional and emerging processes. Ultrasound was found to be a valuable technique to improve the freezing process of potatoes, anticipating the beginning of the nucleation process, mainly when applied during the supercooling phase. A study of the effects of pulsed electric fields on phenol and enzymatic profile of melon juice has been realized and the statistical treatment of data was carried out through a response surface method. Next, flavour enrichment of apple sticks has been realized applying different techniques, as atmospheric, vacuum, ultrasound technologies and their combinations. The second section of the thesis deals with the development of analytical methods for the discrimination and quantification of phenol compounds in vegetable matrix, as chestnut bark extracts and olive mill waste water. The management of waste disposal in mill sector has been approached with the aim of reducing the amount of waste, and at the same time recovering valuable by-products, to be used in different industrial sectors. Finally, the sensory analysis of boiled potatoes has been carried out through the development of a quantitative descriptive procedure for the study of Italian and Mexican potato varieties. An update on flavour development in fresh and cooked potatoes has been realized and a sensory glossary, including general and specific definitions related to organic products, used in the European project Ecropolis, has been drafted.

Quantum effects and dynamics in hydrogen-bonded systems: a first-principles approach to spectroscopic experiments

Computer simulations have become an important tool in physics. Especially systems in the solid state have been investigated extensively with the help of modern computational methods. This thesis focuses on the simulation of hydrogen-bonded systems, using quantum chemical methods combined with molecular dynamics (MD) simulations. MD simulations are carried out for investigating the energetics and structure of a system under conditions that include physical parameters such as temperature and pressure. Ab initio quantum chemical methods have proven to be capable of predicting spectroscopic quantities. The combination of these two features still represents a methodological challenge. Furthermore, conventional MD simulations consider the nuclei as classical particles. Not only motional effects, but also the quantum nature of the nuclei are expected to influence the properties of a molecular system. This work aims at a more realistic description of properties that are accessible via NMR experiments. With the help of the path integral formalism the quantum nature of the nuclei has been incorporated and its influence on the NMR parameters explored. The effect on both the NMR chemical shift and the Nuclear Quadrupole Coupling Constants (NQCC) is presented for intra- and intermolecular hydrogen bonds. The second part of this thesis presents the computation of electric field gradients within the Gaussian and Augmented Plane Waves (GAPW) framework, that allows for all-electron calculations in periodic systems. This recent development improves the accuracy of many calculations compared to the pseudopotential approximation, which treats the core electrons as part of an effective potential. In combination with MD simulations of water, the NMR longitudinal relaxation times for 17O and 2H have been obtained. The results show a considerable agreement with the experiment. Finally, an implementation of the calculation of the stress tensor into the quantum chemical program suite CP2K is presented. This enables MD simulations under constant pressure conditions, which is demonstrated with a series of liquid water simulations, that sheds light on the influence of the exchange-correlation functional used on the density of the simulated liquid.

Continuos Flow Single Cell Separation into Open Microwell Arrays

A novel design based on electric field-free open microwell arrays for the automated continuous-flow sorting of single or small clusters of cells is presented. The main feature of the proposed device is the parallel analysis of cell-cell and cell-particle interactions in each microwell of the array. High throughput sample recovery with a fast and separate transfer from the microsites to standard microtiter plates is also possible thanks to the flexible printed circuit board technology which permits to produce cost effective large area arrays featuring geometries compatible with laboratory equipment. The particle isolation is performed via negative dielectrophoretic forces which convey the particles’ into the microwells. Particles such as cells and beads flow in electrically active microchannels on whose substrate the electrodes are patterned. The introduction of particles within the microwells is automatically performed by generating the required feedback signal by a microscope-based optical counting and detection routine. In order to isolate a controlled number of particles we created two particular configurations of the electric field within the structure. The first one permits their isolation whereas the second one creates a net force which repels the particles from the microwell entrance. To increase the parallelism at which the cell-isolation function is implemented, a new technique based on coplanar electrodes to detect particle presence was implemented. A lock-in amplifying scheme was used to monitor the impedance of the channel perturbed by flowing particles in high-conductivity suspension mediums. The impedance measurement module was also combined with the dielectrophoretic focusing stage situated upstream of the measurement stage, to limit the measured signal amplitude dispersion due to the particles position variation within the microchannel. In conclusion, the designed system complies with the initial specifications making it suitable for cellomics and biotechnology applications.

Conductive Polymer Composites

In recent years, nanotechnologies have led to the production of materials with new and sometimes unexpected qualities through the manipulation of nanoscale components. This research aimed primarily to the study of the correlation between hierarchical structures of hybrid organic-inorganic materials such as conductive polymer composites (CPCs). Using a bottom-up methodology, we could synthesize a wide range of inorganic nanometric materials with a high degree of homogeneity and purity, such as thiol capped metal nanoparticles, stoichiometric geomimetic chrysotile nanotubes and metal dioxide nanoparticles. It was also possible to produce inorganic systems formed from the interaction between the synthesized materials. These synthesized materials and others like multiwalled carbon nanotubes and grapheme oxide were used to produce conductive polymer composites. Electrospinning causes polymer fibers to become elongated using an electric field. This technique was used to produce fibers with a nanometric diameter of a polymer blend based on two different intrinsically conducting polymers polymers (ICPs): polyaniline (PANI) and poly(3-hexylthiophene) (P3HT). Using different materials as second phase in the initial electrospun polymer fibers caused significant changes to the material hierarchical structure, leading to the creation of CPCs with modified electrical properties. Further study of the properties of these new materials resulted in a better understanding of the electrical conductivity mechanisms in these electrospun materials.

New biosensor applications of surface plasmon and hydrogel optical waveguide spectroscopy

Rapid and sensitive detection of chemical and biological analytes becomes increasingly important in areas such as medical diagnostics, food control and environmental monitoring. Optical biosensors based on surface plasmon resonance (SPR) and optical waveguide spectroscopy have been extensively pushed forward in these fields. In this study, we combine SPR, surface plasmon-enhanced fluorescence spectroscopy (SPFS) and optical waveguide spectroscopy with hydrogel thin film for highly sensitive detection of molecular analytes.rnrnA novel biosensor based on SPFS which was advanced through the excitation of long range surface plasmons (LRSPs) is reported in this study. LRSPs are special surface plasmon waves propagating along thin metal films with orders of magnitude higher electromagnetic field intensity and lower damping than conventional SPs. Therefore, their excitation on the sensor surface provides further increased fluorescence signal. An inhibition immunoassay based on LRSP-enhanced fluorescence spectroscopy (LRSP-FS) was developed for the detection of aflatoxin M1 (AFM1) in milk. The biosensor allowed for the detection of AFM1 in milk at concentrations as low as 0.6 pg mL-1, which is about two orders of magnitude lower than the maximum AFM1 residue level in milk stipulated by the European Commission legislation.rnrnIn addition, LRSPs probe the medium adjacent to the metallic surface with more extended evanescent field than regular SPs. Therefore, three-dimensional binding matrices with up to micrometer thickness have been proposed for the immobilization of biomolecular recognition elements with large surface density that allows to exploit the whole evanescent field of LRSP. A photocrosslinkable carboxymethyl dextran (PCDM) hydrogel thin film is used as a binding matrix, and it is applied for the detection of free prostate specific antigen (f-PSA) based on the LRSP-FS and sandwich immunoassay. We show that this approach allows for the detection of f-PSA at low femto-molar range, which is approximately four orders of magnitude lower than that for direct detection of f-PSA based on the monitoring of binding-induced refractive index changes.rnrnHowever, a three dimensional hydrogel binding matrix with micrometer thickness can also serve as an optical waveguide. Based on the measurement of binding-induced refractive index changes, a hydrogel optical waveguide spectroscopy (HOWS) is reported for a label-free biosensor. This biosensor is implemented by using a SPR optical setup in which a carboxylated poly(N-isoproprylacrylamide) (PNIPAAm) hydrogel film is attached on a metallic surface and modified by protein catcher molecules. Compared to regular SPR biosensor with thiol self-assembled monolayer (SAM), HOWS provides an order of magnitude improved resolution in the refractive index measurements and enlarged binding capacity owing to its low damping and large swelling ratio, respectively. A model immunoassay experiment revealed that HOWS allowed detection of IgG molecules with a 10 pM limit of detection (LOD) that was five-fold lower than that achieved for SPR with thiol SAM. For the high capacity hydrogel matrix, the affinity binding was mass transport limited.rnrnThe mass transport of target molecules to the sensor surface can play as critical a role as the chemical reaction itself. In order to overcome the diffusion-limited mass transfer, magnetic iron oxide nanoparticles were employed. The magnetic nanoparticles (MNPs) can serve both as labels providing enhancement of the refractive index changes, and “vehicles” for rapidly delivering the analytes from sample solution to an SPR sensor surface with a gradient magnetic field. A model sandwich assay for the detection of β human chorionic gonadotropin (βhCG) has been utilized on a gold sensor surface with metallic diffraction grating structure supporting the excitation of SPs. Various detection formats including a) direct detection, b) sandwich assay, c) MNPs immunoassay without and d) with applied magnetic field were compared. The results show that the highly-sensitive MNPs immunoassay improves the LOD on the detection of βhCG by a factor of 5 orders of magnitude with respect to the direct detection.rn

Advanced schemes for surface plasmon resonance and plasmon-enhanced fluorescence biosensors

Advanced optical biosensor platforms exploiting long range surface plasmons (LRSPs) and responsive N-isopropylacrylamide (NIPAAm) hydrogel binding matrix for the detection of protein and bacterial pathogen analytes were carried out. LRSPs are optical waves that originate from coupling of surface plasmons on the opposite sites of a thin metallic film embedded between two dielectrics with similar refractive indices. LRSPs exhibit orders of magnitude lower damping and more extended profile of field compared to regular surface plasmons (SPs). Their excitation is accompanied with narrow resonance and provides stronger enhancement of electromagnetic field intensity that can advance the sensitivity of surface plasmon resonance (SPR) and surface plasmon-enhanced fluorescence spectroscopy (SPFS) biosensors. Firstly, we investigated thin gold layers deposited on fluoropolymer surface for the excitation of LRSPs. The study indicates that the morphological, optical and electrical properties of gold film can be changed by the surface energy of fluoropolymer and affect the performance of a SPFS biosensor. A photo-crosslinkable NIPAAm hydrogel was grafted to the sensor surface in order to serve as a binding matrix. It was modified with bio-recognition elements (BREs) via amine coupling chemistry and offered the advantage of large binding capacity, stimuli responsive properties and good biocompatibility. Through experimental observations supported by numerical simulations describing diffusion mass transfer and affinity binding of target molecules in the hydrogel, the hydrogel binding matrix thickness, concentration of BREs and the profile of the probing evanescent field was optimized. Hydrogel with a up to micrometer thickness was shown to support additional hydrogel optical waveguide (HOW) mode which was employed for probing affinity binding events in the gel by means of refractometric and fluorescence measurements. These schemes allow to reach limits of detection (LODs) at picomolar and femtomolar levels, respectively. Besides hydrogel based experiments for detection of molecular analytes, long range surface plasmon-enhanced fluorescence spectroscopy (LRSP-FS) was employed for detection of bacterial pathogens. The influence of capture efficiency of bacteria on surfaces and the profile of the probing field on sensor response were investigated. The potential of LRSP-FS with extended evanescent field is demonstrated for detection of pathogenic E. coli O157:H7 on sandwich immunoassays . LOD as low as 6 cfu mL-1 with a detection time of 40 minutes was achieved.rn