Recently, magnetic nanoparticles are finding an increased use in biomedical applications and research. Nanobeads are widely used for cell separation, biosensing and cancer therapy, among others. Due to their properties, nanowires (NWs) are gaining ground for similar applications and, as with all biomaterials, their cytotoxicity is an important factor to be considered before conducting biological studies with them. In this work, the cytotoxic effects of nickel NWs (Ni NWs) were investigated in terms of cell viability and damage to the cellular membrane. Ni NWs with an average diameter of 30-34 nm were prepared by electrodeposition in nanoporous alumina templates. The templates were obtained by a two-step anodization process with oxalic acid on an aluminum substrate. Characterization of NWs was done using X-Ray diffraction (XRD) and energy dispersive X-Ray analysis (EDAX), whereas their morphology was observed with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cell viability studies were carried out on human colorectal carcinoma cells HCT 116 by the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) cell proliferation colorimetric assay, whereas the lactate dehydrogenase (LDH) homogenous membrane fluorimetric assay was used to measure the degree of cell membrane rupture. The density of cell seeding was calculated to obtain a specific cell number and confluency before treatment with NWs. Optical readings of the cell-reduced MTT products were measured at 570 nm, whereas fluorescent LDH membrane leakage was recorded with an excitation wavelength of 525 nm and an emission wavelength of 580 - 640 nm. The effects of NW length, cell exposure time, as well as NW:cell ratio, were evaluated through both cytotoxic assays. The results show that cell viability due to Ni NWs is affected depending on both exposure time and NW number. On the other hand, membrane rupture and leakage was only significant at later exposure times. Both cytotoxic assessment assays showed an earlier cytotoxic effect in case of shorter NWs, with longer ones having a more marked toxicity, albeit with a delay in time. These findings demonstrate that different levels of biocompatibility can be obtained with specific doses and properties of Ni NWs and can serve as guideline for future experiments.

Οι μονοδιάστατες μεταλλικές νανοδομές, όπως νανοσύρματα και νανοσωλήνες, έχουν ελκύσει το ενδιαφέρον της επιστημονικής κοινότητας τα τελευταία χρόνια λόγο του σημαντικού ρόλου που παίζουν στην κατασκευή νανοσυσκευών, όπως ανιχνευτές, ηλεκτρονικά και οπτικά συστήματα και συστήματα αποθήκευσης πληροφορίας. Μια απλή και αποτελεσματική τεχνική για την κατασκευή μεγάλου αριθμού νανοσυρμάτων και νανοσωλήνων με μεγάλη αναλογία μήκους προς διάμετρο είναι η ονομαζόμενη «σύνθεση μήτρας», η οποία βασίζεται στην ηλεκτροχημική εναπόθεση μετάλλου μέσα στους πόρους κατάλληλου υλικού το οποίο λειτουργεί σαν μήτρα. Η πορώδης αλουμίνα είναι ένα υλικό που χρησιμοποιείται ευρέως για τον σκοπό αυτό λόγο της χημικής και μηχανικής της σταθερότητας και της αντοχής της σε υψηλές θερμοκρασίες. Επίσης, τα γεωμετρικά της χαρακτηριστικά μπορούν να ελέγχουν εύκολα κατά την διάρκεια της διαδικασίας παρασκευής της. Στην εργασία περιγράφεται η διαδικασία κατασκευής μεμβρανών πορώδους αλουμίνας με εξαγωνική κατανομή πόρων, ανοιχτών και στις δύο επιφάνειες και με συγκεκριμένα γεωμετρικά χαρακτηριστικά. Για να κατασκευασθούν οι μήτρες οι οποίες θα χρησιμοποιηθούν στην ηλεκτροεναπόθεση και για άλλες εφαρμογές εξήχθη η σχέση μεταξύ του ρυθμού ανάπτυξης της μεμβράνης πορώδους αλουμίνας και των παραμέτρων της ανοδίωσης (πυκνότητα ρεύματος, θερμοκρασία) για ανοδίωση σε υδατικό διάλυμα με περιεκτικότητα 0.3Μ οξαλικό οξύ. Επιπλέον, μελετήθηκε ο απαραίτητος χρόνος για την διάλυση του συμπαγούς διαχωριστικού στρώματος (barrier layer) της πορώδους αλουμίνας με την χρήση υδατικού διαλύματος 5%wt. φωσφορικού οξέος. Τέλος, στην εργασία αυτή μελετήθηκε η ανάπτυξη νανοσυρμάτων νικελίου σε πορώδης αλουμίνα με μέση διάμετρο πόρων 240nm. Για την καλύτερη κατανόηση των μηχανισμών ανάπτυξης των νανοσυρμάτων μελετήθηκε η σχέση της κρυσταλλογραφικής δομής των νανοσυρμάτων με την εφαρμοζόμενη τάση κατά την ανάπτυξη τους με την χρήση της τεχνικής dc ηλεκτροεναπόθεσης. Κατά την μελέτη αυτή αναπτύχθηκαν μονοκρυσταλλικά νανοσύρματα νικελίου προσανατολισμένα κατά την διεύθυνση [110] και πολυκρυσταλλικά νανοσύρματα με ισχυρό προσανατολισμό κατά την διεύθυνση [111]. / One-dimensional metallic nanostructured materials, like nanowires and nanotubes, have attracted extensive attention in recent years because of their importance in the fabrication of nanometer-scale devices such as sensors, electronics, and optics and information storage systems. A simple and effective technique to fabricate large number of metallic nanowires and nanotubes with high aspect ratio is the so called “template synthesis”, which involves electrochemically depositing metal into nanopores of a suitable material used as a template. Porous alumina membrane is a commonly used material for this purpose because of its chemical and mechanical stability and durability at high temperatures. Moreover, its geometrical characteristics are easily controlled during the fabrication process. In this work the fabrication process of free- standing porous alumina membranes with highly organized hexagonal structure and with specific geometrical characteristics is described. In order to fabricate templates for use in electrodeposition and other applications the dependence relation between the thickness growth rate and the anodization parameters (current density and temperature) for anodization in aqueous solution of 0.3M oxalic acid was derived. Furthermore, the time needed for barrier layer dissolution of porous alumina membrane with the use of hydrate solution of 5%wt. phosphoric acid. Finally, nickel nanowires were fabricated inside porous alumina membranes with mean pore diameter of 240nm. For better understanding of the nanowires growth mechanism the dependence of the crystal structure of nickel nanowire fabricated with dc electrodeposition from the applied voltage was studied. As a result, single crystal nickel nanowires oriented along [110] and polycrystalline nickel nanowires with a strong orientation along [111] have been fabricated.

One dimensional (1-D) metallic nanostructures (e.g. nanowires, nanorods) have stimulated great interest recently as important building blocks for future nanoscale electronic and electromechanical devices. In this thesis work, gold and nickel nanowires with various diameters were successfully fabricated, and two dedicated platforms, based on (1) a novel micro mechanical device (MMD) assisted with a quantitative nanoindenter and (2) a TEM-AFM sample holder system, were developed and adopted to perform in situ tensile tests inside SEM and TEM on samples with diameter ranging from a few nanometers to hundreds nanometers. Size-dependent mechanical behavior and different fracture mechanisms of gold nanowires had been revealed and discussed. In addition, we discovered cold welding phenomenon for ultrathin gold nanowires (diameter ∠ 10nm), which is anticipated to have potential applications in the future bottom-up integration of metallic 1-D nanostructures and next-generation interconnects for extremely dense logic circuits.

Submitted by Rafael Santana (rafael.silvasantana@ufpe.br) on 2017-07-10T18:06:04Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Dissertação_Cecília.pdf: 3721060 bytes, checksum: cd2515bc852d6c7bb0fadaeef5673702 (MD5) / Made available in DSpace on 2017-07-10T18:06:04Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Dissertação_Cecília.pdf: 3721060 bytes, checksum: cd2515bc852d6c7bb0fadaeef5673702 (MD5) Previous issue date: 2016-01-22 / CNPQ / Este trabalho tem como objetivo analisar propriedades magnéticas de arranjos ordenados de nanofios de níquel depositados em membranas porosas de alumina. Um estudo sobre a preparação das amostras, relatando as etapas de formação da membrana e posterior eletrodeposição dos fios via corrente alternada, é feito inicialmente. Na sequência, características morfológicas e estruturais são observadas via microscopia eletrônica de varredura e mapas de energia dispersiva, de onde foi possível constatar que os fios possuíam diâmetros entre 30 nm e 40 nm e comprimentos entre 100 nm e 550 nm. As análises das propriedades magnéticas das amostras são baseadas nas curvas de histerese obtidas via magnetometria de amostra vibrante para diferentes direções de campo aplicado e de temperatura. O caráter de anisotropia magnética uniaxial que esses sistemas de nanofios possuem é observado através destas curvas, bem como o efeito que a interação dipolar entre os fios causa nas propriedades magnéticas do sistema. O laço de histerese nos fornece parâmetros magnéticos como coercividade e remanência e uma análise sobre seus comportamentos em função de parâmetros geométricos e morfológicos dos fios, direção de campo aplicado e temperatura são realizadas e comparadas com os resultados reportados na literatura. / This work aims to analyze magnetic properties of nickel nanowires arrays electrodeposited on porous anodic alumina membrane. A study of sample preparation is reported initially and then morphological and structural features are observed via scanning electron microscopy, from which we observe wires with diameters between 30 nm and 40 nm and lengths between 100 nm and 550 nm. The analysis of the magnetic properties are based on the hysteresis curves for different directions of applied field and temperature. The uniaxial magnetic anisotropy is observed as well the effect of the dipolar interaction between the wires. These curves also provides the magnetic parameters such as coercivity and remanence and we analyze their behavior as a function of geometric and morphological parameters, applied field direction and temperature, comparing with the results reported in the literature.

There is an increasing interest for the use of nanostructures as potential tools in areas that include biology and medicine, for applications spanning from cell separation to treatments of diseases. Magnetic nanoparticles (MNPs) have been the most widely studied and utilized nanostructures in biomedical applications. Despite their popularity, the regular shape of MNPs limits their potential for certain applications. Studies have shown that magnetic nanowires (MNWs), due to their high-­-aspect ratio and specific magnetic properties, might provide improved performance for some biomedical applications. As a consequence, MNWs have received increasing attention from researchers in the last years. However, as with any other nanostructure intended for biomedical applications, rigorous studies must be carried out to determine their potential toxicity and adverse effects before they can be successfully incorporated in clinical applications. This work attempts to elucidate the cytotoxic effects of nickel NWs (Ni NWs) in human fibroblasts by measuring cell viability under different parameters. Ni NWs of three different lengths (0.86 ± 0.02 μm, 1.1 ± 0.1 μm and 6.1 ± 0.6 μm) were fabricated by electrodeposition using porous aluminum oxide (PAO) membranes as templates. Energy dispersive X-­-Ray analysis (EDAX) and X-­-Ray diffraction (XRD) were used for the chemical characterization of the Ni NWs. Their physical characterization was done using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) imaging. MTT assays were performed to assess cell viability of human fibroblasts in the presence of Ni NWs. NW length, NW/cell ratio and exposure time were changed throughout the experiments to elucidate their effects on cell viability. The results showed that NWs length has a strong effect on internalization and cytotoxicity. Smaller NWs showed higher toxicity levels at earlier times while longer NWs had stronger effects on cell viability at later times. NW/cell ratio did not seem to have a very strong effect at low concentrations. However, at high concentration (1000 NW/cell) significant loss of cell viability was observed, with the effects becoming stronger at later times. Other factors such as cell surface area, presence of oxide layer on NWs, and the cytotoxicity of Ni salts, were also studied and found to affect cell viability. For our knowledge, this is the first systematic study done in human fibroblasts wi-­-38 using ferromagnetic NWs; where the toxic effects of equivalent amounts of Ni in its salt and in its NW form are compared. It is also the first study to provide insights of the interaction between wi-­-38 cells and Ni NWs. The results of this study complement and enrich previous cytotoxicity studies of Ni NWs. This work aims at providing a more comprehensive understanding of the interaction between NWs and biological systems. Despite the advancements, further studies will be required to fully understand the factors affecting NW cytotoxicity. Only when we understand the underlying mechanisms, will we be able to design suitable nanostructures for biomedical applications.

The main focus of this thesis work has been the study of domain wall (DW) dynamics in disordered cylindrical nanomagnets. The study attempts to accurately quantify the stochasticity associated with driven (temperature/magnetic field/spin-torque) DW kinetics. Our results as summarized below, are particularly relevant with regard to the technological advancement of DW based magnetoelectronic devices. 1. Temperature dependent noise measurements showed an exponential increase in noise mag-nitude, which was explained in terms of thermally activated DW depinning within the Neel-Brown framework. The frequency-dependence of noise also indicated a crossover from nondiffusive kinetics to long-range diffusion of DWs at higher temperatures. We also observed strong collective depinning, which must be considered when implementing these nanowires in magnetoelectronic devices. 2. Our noise measurements were sensitive enough to detect not only the stochasticity in DW propagation (diffusive random walk) but also their nucleation in the presence of magnetic field down to a single DW unit inside an isolated single Ni nanowire. Controlled injection and detection of individual DWs is critical in designing DW based memory devices. 3. The spectral slope of noise was observed to be sensitive to DWkinetics that reveals a creep-like behavior of the DWs at the depinning threshold, and diffusive DW motion at higher spin torque drive. Different regimes of DW kinetics were characterized by universal kinetic exponents. Noise measurements also revealed that the critical current density and DW pinning energy can be significantly reduced in a magnetically coupled vertical ensemble of nanowires. This was attributed to strong dipolar interaction between the nanowires. Our results are particularly important in view of recent proposals for low power consumption magnetic storage devices that rely on DW motion. In all our experiments, the critical magnetic field/current density, required to set the DWs in duffusive kinetics, were found to be much smaller than the reported values for nanostrips. This could be attributed to the circular cross section of nanowires, where massless DWs results in the absence of Walker breakdown and hence in zero critical current density. At present the contribution from the non-adiabaticity, which acts as an effective field and can reduce the crit- ical current density, can not be denied. The main di±culty in quantifying the non-adiabatic spin-torque is that not only does it contain contributions due to non-adiabatic transport but also due to spin-relaxation provided by magnetic impurities or the sources for spin-orbit scattering. Fortunately, in cylindrical nanomagnet, non-adiabaticity does not affect the DW motion. There- fore, cylindrical NWs may be promising candidate for future magnetic storage devices. However, a systematic experimental study of DW dynamics in cylindrical nanomagnets is lacking. In chapter 7, silver nanowires (AgNWs) are shown to be stabilized in fcc or hcp crystal structure, depending on the electrochemical growth conditions. The AgNWs stabilized in hcp crystal structure are shown to exhibit exotic structural properties i.e. ultra low noise level, thermally driven unconventional structural phase transformation, and time dependent structural relaxation. Ultra noise level makes hcp AgNWs suitable for application in nanoelectronics and the structural transformation may be exploited for use in smart materials. Though time resolved transmission electron microscopy and noise measurements provide some understanding of the hcp AgNWs formation, the precise growth mechanism is still not clear. Future scope of the work The results in this thesis provide the groundwork for a good understanding of stochastic DW kinetics in isolated as well as ensemble of magnetic nanocylinders. Some extensions to this work that would help expand and strengthen the results, are listed below; 1. In all the nanocylinders used for our experiments the source of stochasticity in DWkinetics were randomly distributed structural defects. For a controlled injection and detection of DWs between the voltage probes, it would be of great importance to fabricate artificial notches (pinning centers) in the NW. These notches can be fabricated either by using nano-indentation or by a focussed ion beam. 2. To investigate whether DWs in different parts of the nanowire exhibit spatio-temporal correlation, a simultaneous detection of DWkinetics (through noise measurement) between different volage probes needs to be done. If the propagation time of DWs scales with the distance between the voltage probes, we can be confident of our velocity measurement. Then, by recording the DWvelocity as function of eld/current for nanowire (or nanostrip) absence (or presence) of the Walker breakdown can be probed. This would be a significant result for future spintronic devices. With an accurate determination of velocity even non- adiabaticity parameter may be calculated and one can see its effect on DW dynamics. 3. A complete understanding of sustained avalanches at finite magnetic fields, characterized by a high spectral exponent (a>¸ 2:5) in an ensemble of nanowires is still lacking. Per- forming a controlled experiment on a single nanowire, by varying the number of nanowires in the alumina matrix, one can study the chaotic dynamics of DWs in the ensemble in very accurate manner. All the experiments on AgNWs were performed on ensembles. The large change in a as well as noise magnitude in hcp AgNWs could arise from stress relaxation due to the presence of an insulating matrix or structural relaxation, determined by the nanowire growth kinetics. To resolve this issue, time and temperature dependent noise measurements should be performed on single nanowire stabilized in both hcp and fcc crystal structure.

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