No description available.

This research evaluated and demonstrated the disinfection efficiency of an electrochemical system for total coliform removal from wastewater effluents after secondary treatment. Four bench scale batch electrochemical cells were assembled and operated in the laboratory: the first electro-disinfection reactor was set with aluminum electrodes, the second with standard 316 stainless steel electrodes, the third one with titanium electrodes, and the fourth one with a standard 316 stainless steel cathode and a titanium anode. During the electro-disinfection process the water sample was placed on the reactor/disinfector to which direct current (DC) was charged. The results showed that total coliform counts in the treated water decreased significantly and that the characteristics of the effluent were highly improved, especially when stainless steel or titanium electrodes were employed. A bactericidal efficiency of 98.7 % or higher was achieved within a contact time of less than 15 min and a current density lower than 7.5 mA/cm2 when stainless steel electrodes were used, and a contact time of less than 5 min and a current density lower than 3.5 mA/cm2 when the stainless steel/titanium cell was utilized. Electrochlorination does not seem to be the predominant disinfective means of the process. Production of other short lived and more powerful killing substances such as H2O2, [O], •OH, and •HO2 provide the strong disinfecting action of the system within a short contact time. The bactericidal efficiency of the process generally increased with the current density and contact time, and the impact of these factors was much larger than that of salinity. The results obtained suggest that this electrochemical treatment is applicable to wastewater effluents. However, further investigation on the optimum operating conditions and a detailed comparative study of energy consumption by the electrochemical treatment system and the conventional methods are needed before constructing an industrial application system in the future. It is also indispensable to find out if halogenated hydrocarbons and other toxic compounds are produced during the process.

A kinetic study on polycrystalline platinum (Pt) in sulphuric acid is presented. An electrochemical kinetic mechanism of Pt oxide reduction and surface oxide structures are proposed. The reduction reaction was studied by cyclic voltammetry (CV) and various potential programs that combine sweep and hold periods by an assembled analog instrumentation. The reduction peak was studied under three surface conditions: same oxide coverage θ and same potential E, different θ and same E, and same θ but different E, to determine the influence of θ and E on the peak potential and peak shape. The double-layer charge measured previously by dynamic electrochemical impedance spectroscopy (dEIS) was used to correct the CV baseline. Differential-equation-based models as a function of θ and E were investigated to simulate the oxide reduction and oxidation, and estimate kinetic parameters. A simple mechanism combining desorption and multi-layer growth mechanisms showed good fit with both the spread-out oxidation peak and the sharp reduction peak. A microscopic surface oxide growth model was proposed to explain the surface oxides reduction mechanism. / Graduate

The ability to monitor the presence of analytes is of great importance both in industrial applications and physiological systems. Since the crucial recognition events of chemistry, biology, and materials science occur in a much smaller world, it is very difficult to gather this kind of information. Therefore much effort has been devoted to the detection of chosen molecules selectively and signalling this presence. This thesis highlighted the uniqueness and utility of both fluorescent sensor and electrochemical sensor to investigate biologically important species. The determination of copper(II) ion is very crucial to both environment and human health. To utilise the fluorescent sensors for recognition has plenty of advantages, such as high sensitivity, “on-off” switch ability and submillisecond temporal resolution. Naphthalimide based probes has always been the key point of the chemosensors due to its excellent photophysical properties. Therefore, the aim of the project is to investigate boronic acid receptor linked to the naphthalimide fluorophore for copper(II) detection. And the utility of boronic acid as binding site is one of the rare examples of fluorescent chemosensors for Cu2+ detection. Neutral molecules such as glutathione (GSH) play a crucial role in maintaining appropriate redox homeostasis in biological systems. We creatively use the chromophore of dicyanomethylene-4H-pyran(DCM) for the design of probe, due to its emission located at the red or near infra-red (NIR) region, which is particularly suitable for application in biological samples. GSH, the most abundant cellular thiol, is of great importance in cellular defence against toxins and free radicals. Therefore we developed a colorimetric and NIR fluorescence turn-on thiol probe containing DCM as the fluorophore and DNBS as the fluorescence quencher and recognition moiety. The interaction of ferrocene-boronic acid with fructose is investigated in aqueous 0.1 M phosphate buffer at pH 7, 8, and 9. Two voltammetric methods, (i) based on a dual-plate generator-collector micro-trench electrode (steady state) and (ii) based on square-wave voltammetry (transient), are applied and compared in terms of mechanistic resolution. A combination of experimental data is employed to obtain new insights into the binding rates and the cumulative binding constants for both the reduced ferrocene-boronic acid (pH dependent and weakly binding) and for the oxidised ferrocene-boronic acid (pH independent and strongly binding). Finally, a redox-activated fluorescence switch based on a ferrocene - fluorophore - boronic ester conjugate was investigated. The development of multifunctional systems that can integrate individual basic logic gates into combinational circuits has drawn much attention to smart materials. A novel electrochemically and fluorescence active boronic ester sensor molecule has been developed containing ferrocence and naphthalimide as the redox and fluorophore units. The solid state electrochemical characterisation of the compound was investigated in aqueous media and it indicates a direct interaction with fluoride anions. The fluorescence can also be modulated through photoinduced electron transfer (PET) by a redox process. An OFF-ON fluorescence response occurs when the ferrocene is oxidised by Fe3+. While in the presence of F-, the fluorescence enhancement was offset. Therefore, the combinations of iron (Fe3+ ) ions, sodium L-ascorbate, and fluoride (F-) ions can be used to produce a molecular system displaying INHIBIT logic gate, due to indirect fluorescence quenching.

"Electrochemistry, based on the study of an electrochemical reaction at the interface between an electrode and an electrolyte, is having a profound effect on the development of different fields of science and engineering including battery, fuel cell, electrochemical sensor, electrochromic display, electrodeposition, electroplating, electrophoresis, corrosion, and so on. The performance of the electrochemical reaction depends strongly on the nature of the employed electrode such as structure, chemical composition, and surface morphology. Nanomaterials, notable for their extremely small feature size (normally in the range of 1-100 nm), exhibit new properties which are different from those of bulk materials due to their small size effect. In past decade, nanomaterials have been widely used to develop new strategies for designing electrode and its surface morphology for electrocatalysis and electrochemical sensing applications. My work is aimed at exploring the application of low dimensional nanomaterials (nanotubes and nanoparticles) in electrocatalysis and electrochemical biosensors. Electrocatalysis plays an important role in energy and industrial applications. As one of the most attractive support materials for electrocatalyst, carbon nanotubes have been extensively reported to enhance the performance of various electrochemical catalytic reactions. In recent years, carbon nanotubes with a bamboo-like structure due to nitrogen doping have become a hot topic of increased interest in the field of electrocatalysis because of the unique bamboo shaped structure associated properties. In this work, bamboo shaped carbon nanotubes, synthesized by chemical vapor deposition method, were investigated for ethanol/methanol electro-oxidation, respectively. Small sized platinum nanoparticles (Pt NPs) were dispersed onto BCNT surface through an impregnation method. The role of nitrogen doping in the formation of bamboo shaped structure and its effect in the electrochemical performance of CNTs were discussed. The electrochemical studies showed that the as-prepared Pt/BCNTs electrocatalysts indeed exhibited a remarkable enhancement in catalytic activity for methanol/ethanol oxidation compared to that of the Pt/commercial CNT electrocatalysts. In order to further investigate the potential of using BCNTs as bioelectrocatalyst support materials, a hybrid organic-inorganic nanocomposite film of BCNTs/ploymer was constructed to immobilize an enzyme horseradish peroxidase (HRP) to examine the direct electrochemical behavior of the enzyme towards electrocatalysis process of H2O2. The results indicated that the immobilized HRP onto the film retains its good bioelectrocatalytic activity to H2O2. The defective sites on the BCNTs surface induced by nitrogen doping could help to promote the direct electron transfer between enzyme and the electrode. The BCNT/polymer film structure provides a vast array of new opportunities to use BCNTs as building units for bioelectrochemical and biomedical applications. Compared to carbon nanotubes, TiO2 nanotubes have much better biocompatibility and show greater potential as implant materials. The advantages of TiO2 nanotube array include high biocompatibility, good corrosion resistance in biological environments and highly ordered one dimensional nanotubular geometry. Herein, a well performing non-enzymatic electrochemical glucose biosensor by using CuO nanoparticle decorated TiO2 nanotube array electrode was developed. Well-aligned TiO2 nanotube arrays were successfully synthesized by electrochemical anodization. Highly uniform CuO nanoparticles were electrodeposited onto TiO2 nanotube arrays through a two-step method and used to electrocatalyze the glucose oxidation. The proposed electrode produced a high sensitivity of 239.9 ìA mM-1 cm-2 and a low detection limit of 0.78 ìM with good stability, reproducibility, selectivity and fast response time, suggesting its potential to be developed as a low-cost nano-biosensor for glucose measurements in human fluids. The final work of this thesis presents a simple sandwich-type electrochemical impedance immunosensor with antitoxin heavy-chain-only VH (VHH) antibodies labeled gold nanoparticles as the amplifying probe for detecting Clostridium difficile toxins. Gold nanoparticles (Au NPs) with diameter of ~13-15 nm were synthesized and characterized by transmission electron microscopy and UV-vis spectra. The electron transfer resistance of the working electrode surface was used as parameter in the measurement of the biosensor. With the increase of the concentration of toxins from 1pg/mL to 100 pg/mL, a linear relationship was observed between the relative electron transfer resistance and toxin concentration. In addition, the detection signal was enhanced due to the amplification effect. This proposed method achieved a limit of detection for TcdA and TcdB as 0.61 pg/mL and 0.60 pg/mL, respectively. The pilot study with spiked clinical stool samples showed promising results, indicating the designed biosensor has a great potential in clinical applications."

No description available.

In this thesis, the synthesis and characterization of nitrogen-doped graphene (NG) and NG-Co3O4 composites are described. Moreover, the effect of airborne contamination and nitrogen doping on the capacitance of graphene was investigated. Firstly, nitrogen-doped thermally expanded graphene oxide (NtGO) was prepared by a facile thermal expansion and hydrothermal doping process. The thermal expansion process plays a vital role in improving the electrochemical performance of N-doped graphene by preventing its aggregation and improving its conductivity. The specific capacitance of NtGO is 270 F g-1 at a discharge current density of 1 A g-1 and the capacitance retention is 97 % after 2000 cycles at this current density. Secondly, a hierarchical electrode structure, consisting of cobalt oxide and nitrogen-doped graphene foam (NGF), has been fabricated with the aim of achieving enhanced charge storage performance. The Co3O4/NGF electrode shows an enhanced charge-storage performance, attributed to the 3D hierarchical structure and the synergistic effect of Co3O4 and NGF. The present study shows that specific capacitances as high as 451 F g-1 can be obtained, indicating that high-performance electrochemical capacitors can be made using electrode materials with advanced structures. Thirdly, a study of the differences between the capacitance of freshly exfoliated highly ordered pyrolytic graphite (HOPG, sample denoted FEG), HOPG aged in air (denoted AAG) and aged in an inert atmosphere (hereafter IAG) is presented in this work. Electrochemical impedance spectroscopy shows the FEG possesses a higher intrinsic capacitance (6.0 µF cm-2 at the potential of minimum capacitance) than AAG (4.3 µF cm-2) and IAG (4.7 µF cm-2). This change in capacitance is correlated with other physical changes of the sample, and attributed to contamination due to airborne hydrocarbons. Finally, the effect of N-doping of graphene prepared by chemical vapour deposition is investigated. The differential capacitance of PG and NG was measured by a microinjection-micromanipulator system. The quantum capacitance of PG and NG was calculated and discussed. The increase in differential capacitance with nitrogen-doping and the growth of the quantum capacitance of NG suggest that the increased capacitance of many electrodes of electrochemical capacitors is primarily due to the modification of the electronic structure of the graphene by the N dopant.

<p>The particles of the Pt metal were well dispersed on carbon nanotubes when EG was used and in isopropanol poor dispersion was observed and no further investigation was done on them. The platinum wt% on the supports observed from EDS was 21.8, 19.10 and 16.74wt% for Pt/EMWCNT, Pt/LPGCNT and Pt/ commercial CNT respectively. Pt/LPGMWCNT showed high electro-catalytic activity of 2.48 mA and active surface area of 76 m2/g, toward oxygen reduction, observed from cyclic voltammogram in iv sulfuric acid. Pt/LPGMWCNT also showed better tolerance toward methanol, however it was not highly active towards methanol, and hence the methanol oxidation peak current observed between 0.75 and 08 potential was the smallest. In this study a wide range of instruments was used to characterize the properties and behavior of Platinum nanoparticles on multi-wall carbon nanotubes. To add to the already mentioned, Scanning electrochemical microscopy (SEM), proton induced x-ray emission (PIXE), scanning electrochemical microscopy (SECM) and Brunauer-Emmett Tellar (BET) were also used.</p>

Existe una demanda creciente de dispositivos de análisis multianalito, con aplicaciones potenciales en los campos de la biomedicina y biotecnología, así como en el ámbito industrial y ambiental. Para el desarrollo de estos dispositivos resulta esencial un buen control espacial durante la etapa de inmovilización de las biomoléculas de interés; cada una de ellas debe ser depositada de forma precisa sobre la superficie del sensor (por ejemplo, un transductor amperométrico), evitando solapamientos que puedan comprometer la especificidad del sistema. El objetivo de esta tesis es desarrollar diferentes métodos de patterning para la inmovilización selectiva de biomoléculas. El primer método consiste en la electrodeposición selectiva de nanopartículas de oro biofuncionalizadas para el desarrollo de biochips. Se trata de un método de patterning controlado electroquímicamente, en el que las nanopartículas de oro se modifican en primer lugar recubriéndolas con diversos enzimas y a continuación se electrodepositan selectivamente sobre la superficie de un electrodo. Como parte de esta metodología, se prepararon nanopartículas de oro biofuncionalizadas utilizando tres estrategias diferentes: a través del enlace dativo oro-tiol, por adsorción directa o mediante interacción electrostática siguiendo la técnica layer-by-layer (capa por capa). Para la funcionalización de las nanopartículas de oro se emplearon distintas biomoléculas, como los enzimas peroxidasa de rábano (HRP), glucosa oxidasa (GOX) y albúmina de suero bovino (BSA), y finalmente oligonucleótidos modificados con moléculas fluorescentes y grupos tiol. Las nanopartículas biofuncionalizadas fueron caracterizadas mediante técnicas de espectroscopía UV-visible, microscopía electrónica de transmisión (TEM) y medida del potencial zeta. Mediante espectroscopía UV-visible se observó un pico de resonancia de plasmón característico de las nanopartículas modificadas, relacionado con la estabilidad de la preparación. La medida del potencial zeta permitió la caracterización de las nanopartículas de oro modificadas capa por capa con polímero redox y enzimas. También se estudiaron los cambios en el potencial zeta de nanopartículas modificadas con BSA a distintos valores de pH. Tras la preparación de las partículas biofuncionalizadas, se llevaron a cabo estudios fundamentales de electrodeposición de nanopartículas de oro modificadas con BSA y un polímero redox, con el fin de analizar el efecto de varios parámetros: potencial aplicado, tiempo de deposición, distancia entre los electrodos, superficie del electrodo auxiliar y pH del medio. Para estudiar el comportamiento electrocatalítico de las nanopartículas modificadas una vez electrodepositadas, se llevaron a cabo experimentos utilizando coloides de oro modificados con HRP y GOX. A continuación se empleó esta metodología para el desarrollo de biochips, utilizando dos configuraciones diferentes. En la primera, se electrodepositaron nanopartículas de oro funcionalizadas con GOX y HRP y modificadas con un polímero redox sobre la superficie de un chip de electrodos interdigitados (IDE), consiguiendo eliminar por completo las repuestas no específicas. En la segunda configuración, las partículas se modificaron con una capa adicional de polímero redox, comprobando de nuevo la ausencia total de respuestas no específicas después de la electrodeposición. Esta método de patterning es genérico y puede utilizarse para la producción de diversos biochips. El segundo método de patterning también está basado en el control electroquímico, y consiste en la modificación de los electrodos con monocapas autoensambladas electroactivas cuya funcionalidad es modulable en función del potencial aplicado. En esta metodología, la monocapa electroactiva contiene grupos acetal que pueden ser desprotegidos selectivamente mediante la aplicación de un potencial en zonas específicas de la superficie del electrodo. De esta manera quedan expuestos en la superficie grupos aldehído activos, que pueden ser fácilmente conjugados con aminas primarias presentes en las biomoléculas de interés. Los enzimas GOX y HRP se usaron como proteínas modelo para comprobar la versatilidad de esta técnica. Su aplicabilidad para la fabricación de biochips se demostró con medidas amperométricas y medidas en tiempo real mediante resonancia de plasmón de superficie combinado con electroquímica (eSPR). La tercera metodología es también un sistema de patterning controlado electroquímicamente, pero en este caso se utiliza la inmovilización del 4,4-bipiridil como base para la creación de biochips. Se sintetizaron moléculas de 4,4-bipiridil funcionalizadas con grupos carboxílicos, que fueron caracterizadas electroquímicamente y a continuación conjugadas con las biomoléculas de interés para la creación de biochips. La selectividad de estos sistemas se demostró colorimétricamente, obteniéndose niveles mínimos de respuesta inespecífica. Por último, el cuarto de los métodos de patterning desarrollados está basado en la técnica de fotolitografía. Los enzimas glucosa oxidasa y sarcosina oxidasa se depositaron selectivamente junto con un polímero redox sobre la superficie de electrodos interdigitados utilizando un proceso de lift off, consiguiendo eliminar por completo las señales cruzadas o cross-talk. Como parte de esta metodología se optimizaron varios procedimientos de inmovilización de las biomoléculas, con el fin de seleccionar la estrategia más adecuada. También se llevaron a cabo ensayos con diferentes reactivos para eliminar la adsorción inespecífica. Finalmente, el sistema optimizado fue aplicado sobre IDEs fabricados mediante fotolitografía. Los sensores de glucosa y sarcosina respondieron de forma selectiva a sus respectivos sustratos, con ausencia total de cross-talk. La presente tesis está estructurada en 7 capítulos. En el Capítulo I se exponen las bases del desarrollo de biochips, métodos de patterning con control electroquímico, otros métodos de patterning selectivo y las técnicas de fotolitografía, así como un resumen de la tesis. El Capítulo 2 y 3 describe la síntesis de coloides de oro, la modificación con biomoléculas, los estudios de estabilidad y los estudios fundamentales de electrodeposición de las nanopartículas de oro modificadas sobre la superficie de los electrodos. En el Capítulo 4 se muestra la aplicación de la electrodeposición de nanopartículas de oro biofuncionalizadas para la creación de biochips. El Capítulo 5 describe la inmovilización selectiva de biomoléculas mediante la desprotección electroquímica de monocapas autoensambladas electroactivas. En el Capítulo 6 se muestra la síntesis, caracterización e inmovilización selectiva de derivados de 4,4- bipiridil funcionalizados con HRP. El Capítulo 7 describe el patterning selectivo en la escala micrométrica de dos oxidasas sobre un chip de electrodos interdigitados mediante fotolitografía. Finalmente, el Capítulo 8 resume las conclusiones y el trabajo futuro. / There is an increasing demand of multianalyte sensing devices having potential applications in biomedical, biotechnological, industrial and environmental fields. A good spatial control during biomolecule deposition step is strictly necessary; each biomolecule has to be precisely deposited on the surface of the relevant sensor (eg., an amperometric transducer), avoiding mixing that can compromise the biosensor specificity. The aim of this thesis is to develop different patterning methods for the selective immobilization of biomolecules. The first method is selective electrodeposition of biofunctionalized Au nanoparticles for biosensor arrays. This is an electrochemically controlled patterning method where the Au nanoparticles modified by the enzymes initially and later the enzyme modified Au nanoparticles were electrodeposited selectively on the electrode surface. As a part of this methodology, initially biofunctionalized Au nanoparticles were prepared using three different approcahes. One is Au-thiol dative bonding, the second is direct adsorption and finally electrostatic layerby- layer approach. Different biomolecules like horse radish peroxidase(HRP), glucose oxidase (GOX), bovine serum albumin(BSA), and finally fluorescence labelled oilgonucleotide thiols were used to attch to the Au nanoparticles. Biofunctionalized Au nanoparticles were characterized by different techniques like zeta sizer, UV-Vis spectroscopy, transmission electron microscopy (TEM). UV-Vis spectroscopy showed the successfull modification of Au nanoparticles with a characterstic surface plasmon peak related to the stability. By using zeta sizer, layer-by-layer modification of the Au nanoparticles with redox polymer and enzymes were characterized successfully. Changes of the Au nanoparticles modified with BSA was characterised at different pH s by using the zeta sizer. After the preparation of biofunctionalized particles, some fundamental studies were done with electrodeposition of Au nanoparticles modified with medically important BSA, redox polymer to see how different parameters like potential, time of deposition, interelectrode distance, counter electrode sized, pH, effect the electrodeposition. As a part of these fundamental studies Au colloids modified with HRP and GOX were deposited for studying the electrocalaytic behaviour of the enzymes on the Au nanoparticles after electrodeposition. Later this methodology was applied for creating biosensor arrays by using two different approaches. In the first approach, GOX and HRP functionalized redox polymer modified Au nanoparticles were electrodeposited successfully on an interdigitated electrode (IDE) array with complete absence of non-specific response. In the second approach the particles were modified with an extra redox polymer layer and proved that there is complete absence of nonspecific response after electrodeposition. Moreover, this patterning methodology is generic and can be used for production of different biochips. The second method is another electrochemically controlled patterning method where the electrodes were immobilized with self assembled monolayers with electroactive functionalities which can be tunable with potentials. In this methodology, electroactive self-assembled monolayer contains an active ligand aldehyde which can be readily conjugated to the primary amine group of the biomolecule is protected in the form of acetal. Later when a active potential was applied to the underlying electrode surface, the acetal functionality is deprotected to reveal the aldehyde functionality which was further conjugated to the biomolecule. Two enzymes GOX, HRP were used as model proteins to prove the versatility of this technique. Amperometric as well as real time measurements proved the selective applicability of this technique for creation of biosensor arrays. The third methodology is also an electrochemically controlled patterning methodology where the special advantage of the electrochemically-controlled immobilization of the 4,4-bipyridyl was taken as base for the creation of biosensor arrays. In this methodology, carboxylic acid functionalised 4,4, bipyridyl molecules were synthesized and characterized by electrochemistry. Later the biomolecules were conjugated to these special molecules for the creation of sensor arrays. Proof of selectivity was shown using colourimetrically with minimal non-specific response. Finally in the fourth method which is based on the photolithography technique, two different oxidases GOX & SOX were patterned along with redox polymer selectively on an IDE array using the lift off process with complete absence of cross-talk. As a part of this methodology, different immobilization methods were optimized initially for checking the best optimisation strategy. Later different reagents were tried to optimise the best reagent that prevents the non-specific adsorption. Later this optimised system was applied on the pholithographically created IDE array. Sarcosine and glucose sensors responded selectively to their substrates with complete absence of cross talk. This thesis is structured in 7 chapters. Chapter 1 establishes to basics of the biosensor arrays, electrochemically controlled patterning methods, other selectively patterned methods, photolithography and summary of this thesis. Chapter 2 describes about the gold colloid synthesis, modification with the biomolecules, stability studies. Chapter 3 decribes fundamental studies of the electrodeposition of the functionalised Au nanoparticles on the electrode surface. Chapter 4 describes the application of the electrodeposition of the protein functionalised Au nanoparticles for the creation of biosensor arrays. Chapter 5 describes the selective immobilization of biomolecules through electrochemical deprotection of electroactive self-assembled monolayers. Chapter 6 describes the synthesis, characterization and selective immobilization of HRP functionalized 4,4-bipyridyl derivatives. Chapter 7 describes the selective microscale protein patterning of two oxidases on an IDE array through photolithography. Finally chapter 8 summarizes the conclusions and the future work.

This thesis is to study on the preparation of iron nano-particles by electrochemical method in aqueous solution. The resultant particles are stabilized as a colloidal suspension by the use of cationic surfactants. The advantages include those high yield, low cost, and simple control of particle size by adjustment of the current density. It is revealed that current density, distance between electrodes, temperature, and surfactant concentration of aqueous solution play important roles on the preparation of nano-particles. The morphology, structure, composition, and optical properties of nano-particles are studied by Ultraviolet-Visible spectrophotometer (UV/Vis spectrophotometer), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The effect of particle size on the magnetic properties of nano-particles has been studied using superconducting quantum interference device (SQUID). According to the experimental results, the greater imposed current density is applied, the smaller the particle size is obtained. The absorption spectra of the particles exhibit that the characteristic peak of surface plasmon band is at 264 nm. The maghemite (£^-Fe2O3) phase is clearly confirmed by X-ray diffraction and TEM analysis. From the hysteresis loop studies, particles are paramagnetic at room temperature and they exhibited super-paramagnetic phenomenon. They become ferromagnetic at low temperature. The increase of the coercive force is due to the reduction of thermal vibration.