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Introduction

Chapter1

Chapter 1

Centre For Interdisciplinary Research, DYPU, Kolhapur 1

1.1 Introduction to Nanoscience, nanotechnology and Nanomedicine

In 1959, the physicist Richard Feynman shared his vision of what very

small things would look like and how they would behave. In a speech titled

“There’s Plenty of Room at the Bottom,” Feynman gave the first clue about

“nanoscience” during a conference of the American Physical Society at the

California Institute of Technology. Nanoscience is the study of the phenomena

and manipulation of materials at smaller scale such as atomic, molecular and

macromolecular scale, where properties vary appreciably from those at larger

scale [1]. It is a fabulous area where the frontier between fundamental science

and applied science becomes a part of exchange and innovation. In general,

nanoscience is the study of the behaviour of objects at a very small scale,

roughly 1 to 100 nanometres (nm). The size of atoms are a few tenths of a

nanometre in diameter and molecules are typically a few nanometre in size.

Nanometre is a magical point on the length scale, where the small man-made

devices meet the atoms and molecules of the natural world. Typically, one

nanometre means 10-9m. So, a nanometre is one billionth of a metre and is the

unit of length that is generally most appropriate for describing the size of

individual molecule. Nanometre objects are so small that one cannot see with

naked eye. The intermediary between the atom and solid, from large molecule

or the small solid objects to the strong relationship between surface and

volume is nothing but the nanoworld. The idea of the nanoworld is based on

the convergence of a real mix of scientific and technological domains which

once were separate. In fact, various scientific communities including every

single segment such as nanomaterials, micro and nanomachines, micro and

nanoelectronics, have their own paradigm. This is reason why there is

differentiation between innovations and industrial developments. However,

these fields are strongly interlinked. It is therefore, indispensable to make our

studies more interdisciplinary in order to enable us to understand the

nanoworld. Nanoscience provides scientists with a rich set of materials useful

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for probing the fundamental character of matter. These materials have typical

structures and tunable properties. This makes them helpful for several real

world applications with aid of nanotechnology.

Nanotechnology as a revolutionary new 21st century technology is

opening to impact almost every aspect of humanity. The term

“nanotechnology” is not a single technology or scientific discipline. It is a

multidisciplinary combination of physical, chemical, biological, engineering,

and electronic, aspects of materials and its applications in which the defining

characteristic is one of size. The development of disease diagnosis and

treatment is one of the highest impact areas where nanotechnology has

excellent potential and promise [2-3]. National Institutes of Health (NIH) have

defined the nanomedicine as applications of nanotechnology for disease

treatment, diagnosis, monitoring, and control of biological systems. At the

nanometer scales nanomedicine exploits the improved and novel physical,

chemical and biological characteristics of materials. The term ‘nanomedicine’

can also be considered as another implementation of nanotechnology in the

field of medical science and diagnostics. The research institutes named ‘The

National Cancer Institute’ and the ‘National Aeronautics & Space

Administration,’ USA are working to develop nanotechnology that can detect,

diagnose, and treat disease. Therefore, nanomedicine has the potential to

enable early detection and prevention, and to essentially improve diagnosis,

treatment and follow-up of diseases. Nanomedicines include organic materials

such as liposomes, micelles, dendrimers, fullerenes and carbon nanotubes or

inorganic such as magnetic nanoparticles (MNPs). This versatility in

composition and physicochemical properties opens the door for a wide range

of medical applications which greatly benefits the research and practice of

medicine [4]. As far as history of nanoparticles (NPs) research is concerned,

the use of these particles dates back to the 9th century in Mesopotamia when

artisans used these to generate a glittering effect on the surface of pots.

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1.2 Nanomedicine as a Potential Platform for Therapeutic and Diagnostic

Purposes

Now days, research into the rational delivery and targeting of

therapeutic, pharmaceutical, and diagnostic agents is at the forefront of

projects in nanomedicine. These therapeutic and diagnostic treatments include

the identification of precise targets such as cells and receptors related to

specific clinical conditions and choice of the appropriate nanocarriers to

achieve the required responses while minimizing the side effects. In

nanomedicine, the survey of primary problems related to the biocompatibility

of nanomaterials has been initiated both theoretically and experimentally. The

complicated issues related to the future approval of nanomedical materials

have been discussed by the US Food and Drug Administration. It is seen that

provisions are being made for our society to develop nanomedicine for better

health of human. Fundamental mechanism of nanomedicine must be fully

investigated, clinical trials and validation procedures must be strictly

conducted, before nanomedicine can be used in clinics. For instance, it is

promising that some biological entities, like proteins, DNA and other bio-

polymers, might be directly used for biosensor applications, nevertheless some

serious issues, like biocompatibility and robustness, may obstruct the

development of these efforts [5].

Recently, the field of magnetic nanomedicine is growing rapidly, in a

broad range of applications including cell separation, biosensing, studies of

cellular function, as well as a variety of potential medical and therapeutic uses

[6]. In magnetic nanomedicine, MNPs are usually utilized which consists of

only a single magnetic species. The dimensions of these NPs make them

perfect candidates for nanoengineering of surfaces and the production of

functional nanostructures. Such surface functionalization of NPs facilitates

their use in biomedicine. Some important biomedical applications of MNPs

are discussed below.

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1.2.1 Cancer hyperthermia therapy

Fig.1.1 Hyperthermia treatment for tumour destruction [7].

The concept of hyperthermia dates back to more than 4000 years ago

when heating was already mentioned as a potential treatment for some

diseases in the advanced cultures of the old Egypt. Recently, hyperthermia has

received great awareness which suggest a potential application in cancer

therapy. Especially, the use of MNPs as heat mediators looks promising in the

development of novel thermotherapy treatments, particularly, in combination

with conventional cancer therapies, including surgery, radio and

chemotherapy. Basically hyperthermia means abnormally high body

temperature. This can be caused as part of treatment, by an infection or by

exposure to heat [8]. Hyperthermia therapy is a type of treatment in which

body tissue is exposed to high temperatures to damage and kill cancer cells or

to make cancer cells more sensitive to the effects of radiation and certain

anticancer drugs. Fig.1.1 shows hyperthermia treatment for tumour

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destruction. The important feature of magnetic hyperthermia is that energy is

absorbed from the alternating magnetic field by the MNPs and transformed

into heat by means of one or combination of the generation of eddy currents in

a material of low electrical resistivity, reversal of the magnetization inside a

magnetic material and rotation of the magnetic material relative to its

surroundings. Specific absorption rate (SAR) plays significant role for the safe

use of MNPs in cancer hyperthermia therapy or in magnetic fluid

hyperthermia (MFH). The SAR is defined as a measure of the rate at which

energy is absorbed by the body when exposed to a radio frequency (RF)

electromagnetic field.

1.2.2 Magnetic resonance Imaging (MRI)

Fig. 1.2 MRI imaging made possible using MNPs [9].

MRI is a noninvasive imaging modality capable of providing high-

resolution anatomical images. It is a broadly used method mainly for the

diagnosis of soft tissue or cartilage pathologies, allowing the differentiation

between malignant and healthy tissues. In vivo molecular imaging has been

identified by the National Cancer Institute of the United States of America as

an astonishing prospect for studying diseases non-invasively at the molecular

level. Iron oxide NPs are excellent MRI contrast agents [10-12] because of

their transverse relaxation property with excellent sensitivity, as compared to

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other MRI agents such as chelates of paramagnetic ions like Gd3+−diethylene

triamine pentaacetic acid (Gd3+−DTPA). The aim of this technique is to

visualize molecular characteristics of physiological or pathological processes

in living organisms before they manifest in the form of anatomic changes

without invasive procedures. Fig. 1.2 represents schematic of in vivo MRI

imaging using MNPs.

Mainly, MRI is based on the principle that protons align and precess

along an applied magnetic field. When transverse radiofrequency pulse

applied, these precessed protons are perturbed from the magnetic field

direction. The subsequent process, through which the pulsing field is turned

off to allow protons to return to their original state, is referred as relaxation.

Two independent relaxation processes, longitudinal relaxation (T1 - recovery)

and transverse relaxation (T2 - decay), are utilized to generate a bright and

dark MR image, respectively. Local variations in relaxation, corresponding to

image contrast, arise from proton density as well as the chemical and physical

nature of the tissues within the specimen. Upon accumulation in tissues,

superparamagnetic (SPM) NPs act typically as T2 contrast agents to provide a

dark image and the contrast enhancement is proportional to the magnetization

magnitude.

The accessibility of a integrated magnetic resonance marker gene to

image gene expression could be particularly important in examining gene

therapy, in which exogeneous genes are introduced to ameliorate a genetic

defect or to add an additional gene function to cells, and construction and

testing of such vectors is currently going on. The desired strategy can also be

used to image endogenous gene expression during development and

pathogenesis of disease. With advances in establishing transgenic mouse

models, an animal line might be grown with an imaging marker gene under

the direct of a given promoter under study, so that the promoter activity can be

directly visualized. The work opens an exciting opportunity for developing

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additional and complementary policies to image gene expression in deep

organs by MRI [11, 13]. The MRI technique is able to identify apoptosis at an

early stage in the process and has the benefits over other techniques such as

magnetic resonance spectroscopy (MRS) and radionuclide techniques, which

is able to detect apoptotic regions with relatively high spatial resolution. The

ferrite NPs label is extremely insightful to MR detection and is also relatively

non-toxic. These NPs have been granted for clinical use as a blood pool agent

for MRI [7]. Most of the MNPs have a relatively short blood half-life and their

primary application is for imaging of liver, spleen and the GI tract. Surface

functionalized MNPs having long blood circulation times, however, can

demonstrate very helpful for imaging of the vascular compartment (magnetic

resonance angiography), imaging of lymph nodes, perfusion imaging, receptor

imaging and target specific imaging.

1.2.3 Drug Delivery

Another most promising application with the rapid development of

nanotechnology, colloidal NP-based drug carriers have been emerging as

effective tools for drug delivery in cancer therapy. A drug delivery system

(DDS) is defined as a system in which the bioactive agent (drug) is integrated

with a non-active agent (carrier) in such a way that the drug is released from

the carrier in a predetermined manner, at a constant rate in what is known as

zero-order release, in a cyclic manner, or in response to an external trigger

such as a change in pH, ionic strength or temperature of the medium

[15-17]. Fig. 1.3 shows the site-targeting specificity of particulate drug

delivery systems using MNPs. The first preclinical experiments using

magnetic albumin microspheres loaded with doxorubicin for cancer treatment

in rats were reported by Widder et al. [18].

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Fig.1.3 Schematic represents the site-targeting specificity particulate

drug delivery systems [14].

There are many different kinds of macromolecular structures designed

for drug delivery systems, such as micelles, [19-21] liposomes NPs [21-23],

dendrimers [24] and polymers [25]. In these methods, the drug is entrapped,

attached, adsorbed, or encapsulated into or onto nano-matrices [26]. Ideally,

they could bear on their surface or in their bulk a pharmaceutical drug that

could be driven to the target organ and released there. For these applications,

the surface chemistry, charge and size of the magnetic particles are

particularly important and strongly affect both the blood circulation time as

well as bioavailability of the particles within the body [27]. In addition,

magnetic properties and internalization of particles depend strongly on the

size of the magnetic particles [28]. For instance, following systemic

administration, larger NPs with diameters bigger than 200 nm are generally

sequestered by the spleen as a result of mechanical filtration and are

ultimately take out by the cells of the phagocyte system, resulting in decreased

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blood circulation times. SPM NPs of narrow size range are easily produced

and functionalised with different polymers, providing convenient, readily

targetable MRI agents. Because of the large surface area to volume ratio, the

MNPs tend to agglomerate and adsorb plasma proteins. The body’s

reticuloendothelial system (RES), mainly the kupffer cells in the liver,

typically take up these NPs due to the hydrophobic surface. Surface coverage

by amphiphilic polymeric surfactants such as poloxamers, poloxamines and

polyethylene glycol (PEG) derivatives over the NPs importantly increases the

blood circulation time by minimizing or eliminating the protein adsorption to

the NPs [11, 29].

Magnetic drug targeting utilizing MNPs as carriers is a promising

cancer treatment avoiding the side effects of conventional chemotherapy. Iron

oxide NPs covered by starch derivatives with phosphate groups, which bound

mitoxantrone, have been used as chemotherapy. Alexiou et al. have reported

that a strong magnetic field gradient at the tumor location induces

accumulation of the NPs [30]. Electron microscope investigations show that

the magnetic fluid can be enriched in tumor tissue and tumor cells. The

attachment of drugs to MNPs can be used to reduce drug doses and potential

side effects to healthy tissues and the costs associated with drug treatment.

1.3 Magnetic nanoparticles

Magnetic materials are those materials that show a response to

an applied magnetic field. The magnetic properties of NPs are estimated by

several factors, including the the type and the degree of defectiveness of the

crystal lattice, chemical composition, the particle shape and size, the

morphology (for structurally inhomogeneous particles) the interaction of the

particle with the surrounding matrix and the neighbouring particles. By tuning

the NPs size, shape, composition and structure, one can control to an extent

the magnetic characteristics of the material based on them. Synthesis of NPs

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with nearly equal in size and chemical composition cannot always be

controlled therefore; the properties of nanomaterials of the same type can be

markedly different [31].

Fig.1.4 MNPs in various shapes.

In past, the magnetic characteristics of a material consisting of a

nonmagnetic solid dielectric matrix with MNPs (3±10nm) distributed in the

matrix were described in 1980 [32]. These particles are abundant in nature and

also found in many biological objects [33]. Generally, the magnetic

nanomaterials used in nanomedicine fall into three categories: zero

dimensional nanomaterials such as nanospheres; one-dimensional

nanomaterials such as nanowires and nanotubes; and two-dimensional

nanomaterials such as nanodisc, thin films etc. Fig. 1.4 shows MNPs in

various shapes. The nanospheres, nanorods, nanowires and nanotubes are

usually called NPs among which nanorods, nanowires and nanotubes show

high aspect ratio. Magnetic thin films are frequently used in the development

of high sensitivity and high accuracy magnetic sensors and biochips, which

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are vital for the identification of biological entities bound with MNPs. For

application in biomedicine in most cases these MNPs are suspended in

appropriate carrier liquids, forming magnetic nanofluid, also called

ferrofluids. In some of the biomedical applications, magnetic thin films are

often fabricated into magnetic biosensors or biochips, by etching them into

certain patterns to perform specific functions.

1.3.1 Features and required properties of MNPs for nanomedicine

The MNPs show remarkable phenomena such as high saturation field,

superparamagnetism, extra anisotropy contributions or shifted loops after field

cooling, high field irreversibility. The occurrence of these properties arises

from finite size and surface effects that dominate the magnetic behaviour of

individual NPs [34]. According to several studies reported in the literature for

MNPs; superparamagnetism is seeming to be preferred for solving the

problem in the field of nanomedicine. MNPs should be biocompatible or non-

toxic. Amongst the various physico-chemical properties water dispersibility

and colloidal stability are the important requisites in the field of

nanomedicine.

MNPs are preferred to be sufficiently small (10–50 nm). Frenkel et al.

[35] was the first to predict that a particle of ferromagnetic material, below a

critical particle size (<15 nm for the common NPs ), which consist of a single

magnetic domain, i.e. a state of uniform magnetization at any field. In

addition, the particles in this size range are rapidly removed through

extravasations and renal clearance besides avoiding the capillary embolism.

The dipole-dipole interaction among the particles depends on critical radius

(rc). Hence, the dipolar interactions become extremely small when the particle

size becomes smaller. This will help to minimize particle aggregation when

the field is applied. MNPs must have a high saturation magnetization because

the movement of the particles in the blood can be controlled with a moderate

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or changing external magnetic field and the particles can be moved close to

the targeted pathologic tissue.

1.3.2 Surface functionalization of magnetic NPs in nanomedicine

Surface functionalization of MNPs with suitable material is a vital task

for application in biomedicine. The efficiency of coating material and the

attachment of particular ligands to the particles will resist to agglomeration,

evading of biological clearance and superior targeting. The magnetic particles

should be stable in water at pH = 7.4 i. e. in a physiological conditions. With

small sized particles precipitation due to gravitation forces can be avoided

which is an important issue in biomedicine.

1.4 Introduction to the Perovskite La0.7Sr0.3MnO3 (LSMO)

Perovskite structured transitional-metal oxides compounds with

composition ABO3 have a great attention in various research fields and have

been known as materials with a variety of interesting properties, such as

electrical transport, magnetic, dielectric, and optical properties. In the past few

decades synthesis of strontium-doped lanthanum manganites, LSMO, have

gained much attention due to their fascinating properties and intriguing

applications in technical as well as biomedicine [36]. The chemical

composition of LSMO is La1−xSrxMnO3, where x indicates doping level.

Recently, nanomagnetic materials of LSMO, are used in magnetic recording

media, ferrofluids, medical diagnostic, catalysis, magnetic refrigeration,

bioprocessing, miniaturized magnetic sensor applications, drug delivery

system, etc.

A group of half-metallic ferromagnetic materials, such as LSMO, are

of interest in biomedical application due to the tuned transition temperature or

Curie temperature (Tc). The wide range of Tc from 10-107 ºC can be

tuned by different divalent metal ions doping into La site. Mainly, the physical

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properties of the manganite perovskites, can be tuned by controlling the

doping level x.

1.4.1 Crystal structure

The crystal structure of La1−x SrxMnO3 was studied by Jonker et al. at

the end of the 1940 [37-38]. LSMO is an oxide with perovskite crystal

structure.

Fig. 1.5 Crystal structure of perovskite LSMO manganites [39].

One unit cell of LSMO is as shown in Fig. 1.5. Present work deal with

manganites, where the A and B-site are occupied by La/Sr and Mn-ions,

respectively, i.e. ABMnO3. The manganese atoms are surrounded by an

octahedron of oxygen atoms. The oxygen atoms are ionized to O2-, lanthanum

to La3+, and strontium to Sr2+. The manganese atoms are either ionized to

Mn3+ or to Mn4+ depending on the doping level of x which results in the 3d

shell of the manganese atoms being filled with either 4 or 3 electrons.

Generally, the mixed-valence oxides can be considered as solid solutions

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between end members LaMnO3 and SrMnO3 with formal valence states (La3+

Mn3+ O2-3) Sr2+ Mn4+ O2-

3 and, leading to mixed-valence compounds. (The

electronic configurations of Mn3+ and Mn4+ are 3d4 and 3d3, respectively) [38].

By knowing the electrons in the 3d shell of the manganese atoms, electronic

and magnetic properties can be studied. The substitution of divalent metal ion

(Sr2+) in the lanthanum at A-site creates the mixed valencies of the

manganese ions in B-site (Mn 3+, Mn 4+) and significantly increases the

electrical conductivity and magnetization of compound due to double

exchange mechanism.

In LSMO crystal, it was found that ferromagnetism is related to the

simultaneous existence of Mn3+ and Mn4+ ions which appears in the range

of 0.1<x<0.5 [40-41]. Both of the end members are antiferromagnetic and

insulating, whereas when x= 0.3 the solid-solutions are both ferromagnetic

and conducting. It means the manganite perovskites are known to undergo

phase transformation from the ferromagnetic metal to paramagnetic insulator

and this phase transformation depends on the phase composition of the

sample, internal stress, and the structural defects. TC for LSMO composition

which had studied in present work is maximum around x = ⅓ and for <rA>

≈1.24Å and here the Mn-O-Mn bond angle is 166.3º where rA radius of cation

on A side [42-43].

1.4.2 Phase diagram of LSMO

Fig. 1.6 represents the phase diagram of La1-x SrxMnO3 obtained from

Sr composition dependence of temperature. The rich phase diagram of La1-

xSrxMnO3 which is sometimes considered as complex with various magnetic

phases which may either be conductive or insulative can be attributed to the

competition between the charge, spin, orbital and the lattice. These materials

show paramagnetic and semiconducting or insulating behaviour at high

temperatures. When the temperature is decreased, a transition from the

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insulating paramagnetic state to ferromagnetic metallic state occurs at

the Tc.

Fig. 1.6 Phase diagram of La1-xSrxMnO3. AFM, PM, PI, FM, FI and CI

represent antiferromagnetic metal, paramagnetic metal, paramagnetic

insulator, ferromagnetic metal, ferromagnetic insulator and spin canted

insulator states, respectively, Tc is the Curie temperature and TN is the

Neel temperature [44].

The fundamental mechanism by which LSMO present ferromagnetic

properties is due to long-range interactions between parallel aligned atomic

moments that result in a spontaneous net magnetization even in the absence of

an external field. Ferromagnetic materials undergo a phase transition from a

high-temperature phase that does not have a macroscopic magnetization

(atomic moments are randomly aligned resulting in a paramagnetic phase) to

a low-temperature phase that does.

The occurrence of the ferromagnetic phase and the antiferomagnetic

phases which appears on the phase diagram is partly due to the coexistence of

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the antiferomagnetic superexchange interactions [45]. The antiferromagnetic

superexchange interaction is the type of interaction between the t2g spins and

the electronic anisotropy which originates from the orbital ordering of the

conduction eg electrons. The parent compound of LSMO, i.e. undoped

LaMnO3 show all manganese atoms take on the Mn3+ configuration, with three

electrons in the t2g band and one electron in the eg band. In this particular case,

electrons are not able to move between ions because of strong Coulomb

repulsion between them, and LaMnO3 is Mott insulator. Some manganese ions

can transform into Mn+4 to compensate by replacing the La3+ to Sr2+. In this

doped state, the eg band is empty which opens the possibility for electrons to

move from the Mn3+ ion to an Mn4+ ion.

Fig. 1.7 Representation of double exchange mechanism for LSMO [46].

The double exchange mechanism in LSMO is schematically

represented in Fig. 1.7 [47] it offered an explanation that remains the core of

the understanding of magnetic oxides. Researchers noted that, the two

configurations of manganese in doped Manganese oxides are degenerate and

connected by the so-called double-exchange matrix element. In this

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mechanism, due to the strong Coulomb repulsion, no d-orbital possibly

occupied by more than one electron, and due to Hunds-rule coupling, all

electron spins on a known Mn ion should be ferromagnetically aligned. The

electrons in the t2g state form a core spin Sc with a magnitude 3/2, while the

electron in the eg state is able to move freely if definite conditions are

satisfied. In this case when an electron is on a site i, its spin have to remain

parallel with the related Sc, and to move to position j, its spin must also be

parallel with the Sc associated with the orbital. Therefore, in a ferromagnetic

state with aligned spins throughout the material, the electrons will generally

be delocalized. This phenomenon is known as “double exchange mechanism”

[48]. Consequently the strontium-doped lanthanum manganates have a

particularly complex phase diagram due to these interactions [49] (see Fig.

1.7).

1.4.3 Electrical properties of LSMO NPs

At low doping, LSMO and mixed-valence manganites in general are

insulators at 27ºC and above. One of the significant character of these

compounds is that the metallic behaviour may be induced by increasing the

relative amount of Mn (IV). It can be achieved either by substitutional doping,

i.e. three-valence lanthanides are partly replaced with two-valence alkaline

earths or, to some extent, by annealing in air or oxygen. From ~20% to ~50%

of Mn (IV), the materials show metallic behaviour at room temperature. At

lower temperatures the metallic state can be stable already with 10% Mn

(IV) doping. A fascinating phase comes out at ~50 to ~60% Mn (IV)

according to the occupied eg orbitals become ordered along the (001) planes to

give itinerant electrons within these planes and canted A-type

antiferromagnetic superexchange coupling between the planes [50]. Support

for this 2D metallicity coexisting with anti-ferromagnetism comes from

investigations of the thermal hysteresis of magnetization and resistivity in the

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range 0.5 ≤ x ≤ 0.6 and 0.4 ≤ x ≤ 0.85 [51]. It is interesting to note that the

interaction between pairs of Mn3+ and Mn4+ ions through an oxygen atom is

responsible for metal to insulator transition in the LSMO. The mixed

conductivity found in this structure appears to play a key role in the

remarkable catalytic activity of this compound.

1.4.4 Magnetic properties

Fig.1.8 The different magnetic effects occurring in MNPs [52].

Magnetism in the material is an important issues to study the

application of magnetic materials in biomedicine. Magnetism occurs due to

which a material exerts either attractive or repulsive force on another.

However, the underlying principles and mechanisms that explain then

magnetic phenomenon are complex and restrained, and their understanding

has eluded scientists until relatively recent times. Generally, fundamental

source of magnetic force is movement of electrically charged particles.

Therefore, magnetic behaviour of a material can be traced to the structure of

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atoms. Electrons in atoms show planetary motion, they revolve around the

nucleus. This orbital motion and its own spin cause separate magnetic

moments, which contribute to the magnetic behavior of materials. In this way

the material can respond to a magnetic field. However, the manner in which a

material responds depend much on its atomic structure, and determines

whether a material will be strongly or weakly magnetic.

The macroscopic magnetic properties of materials are a consequence of

magnetic moments associated with individual electrons. Some of these

concepts are relatively complex and related with some quantum-mechanical

principles. The LSMO shows different magnetic properties, including

ferromagnetic, antiferromagnetic, canted antiferromagnetic and paramagnetic.

The pure compound without Sr doping i.e. LaMnO3 of manganites are

antiferromagnetic in nature. Further, Sr doping in parent compound implies

variation in magnetic properties of LSMO.

Fundamental changes in the magnetic structure of ferromagnetic,

ferrimagnetic, and even antiferromagnetic materials when sizes are

significantly reduced for the nanoscale range can be observed in the two most

important effects, i.e., finite-size and highest-surface. The microscopic origin

of magnetic properties of MNPs in matter lies in the orbital and spin motions

of electrons, whose spin and angular momentum are associated with a

magnetic moment. In case of MNPs especially interesting as the NP size is

comparable to the size of a magnetic domain. These results divided in two

significant types of magnetic behaviour in NPs, namely single magnetic

domain ferromagnetic NPs and SPM NPs. The interaction between the

magnetic moments of atoms from the same material causes magnetic order

below a certain critical temperature. Similar to bulk ferromagnets, an array of

single domain MNPs can exhibit hysteresis in the magnetization versus field

dependence. The magnetization at which all the moments are aligned in both

instances is referred to as the saturation magnetization (Ms). The bulk MNPs

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are composed of small regions, called magnetic domains, within which there

is an alignment of the magnetic moments. In a bulk magnetic material, the

magnetization increases in response to the field via domain wall nucleation

and rotation as well as the rotation of the magnetization vector away from the

easy axis of magnetization. If the size or volume of the material is reduced, as

in the case of MNPs, a situation in which just one domain is reached occurs

and the magnetic properties are no longer similar to bulk materials.

1.4.4.1 Single Domain Theory and Superparamagnetism

The magnetic properties of magnetic materials for monodomain or

single domain particles in analogy to paramagnetic systems are studied within

the framework the so-called SPM theory [53]. Superparamagnetism is a

property by which magnetic materials may show behaviour similar to

paramagnetism at temperatures below the Curie or the Neel temperature.

Superparamagnets consist of individual (single) magnetic domains of

elements (or compounds) that have ferromagnetic properties in bulk. A

material is SPM if it is made of very small single-domain non-interacting

magnetic grains dispersed in some non-magnetic medium [54]. Kittel in 1946

established theoretical predictions concerning energetic stability of a single

magnetic domain, [55] and defining certain critical size of a particle (usually

nanometers for ferromagnets) in smaller particle formation of a single

ferromagnetic domain is preferred.

As in a ferromagnetic material, multiple magnetic domains exist which

may result the balance between the exchange interaction energy that favours

the parallel alignment of neighboring atomic moments (thereby forming

magnetic domains), and the magnetostatic interaction energy that tries to

break them into smaller domains oriented antiparallel to each other. The

domain size is determined by the relative counterbalance between both

energies [56]. With decreasing size of the MNPs, there is a critical value (rc)

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below which the magnetostatic energy no longer allows for the breaking of the

system into smaller domains and so the system is composed of a single

domain, as shown in Fig. 1.9.

Fig. 1.9 Schematic showing the transition from the multi-domain

configuration to the single-domain one upon size reduction [56].

Due to their decrease in size, MNPs usually present in SPM state,

meaning that the thermal energy may be enough to change spontaneously the

magnetisation within each MNP. It means that, the magnetic moment of each

MNP will be able to rotate randomly (in reference to the orientation of the

MNP) just due to the temperature effect. Numerous new fascinating

phenomena have been observed in MNPs that are not shared with their bulk

materials. In order to study the magnetic properties in bulk materials the

important parameters are coercivity (Hc) susceptibility (w) composition,

crystallographic structure, vacancies, defects and magnetic anisotropy. In bulk

material, the magnetization increases with respect the field via domain wall

nucleation and both the rotation of the magnetization vector away from the

easy axis of magnetization. In case of single domain particle, domain wall

movement is not possible and only coherent magnetization rotation can be

used to overcome the effective anisotropy (K) of the particle. Therefore, the

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maximum coercivity of a given material as a function of particle diameter

actually falls in the single domain region. The magnetization inside each

domain is uniform, but varies from domain to domain as they are separated by

an interface layer known as the domain wall.

The critical radius for a magnetic particle to reach the single domain

limit is calculated by the equation,

(1.1)

Where, A is the exchange constant, K is the effective anisotropy constant and

Ms is the saturation magnetization. The magnetic materials, having higher

magnetization, this diameter is in the range 10–100 nm, although for a few

high-anisotropy materials the single domain limit can be several hundred

nanometres. Also, for MNPs on the nanoscale range, the size and shape also

decide their magnetic behaviour. It is well known that for a single domain

element, the amount of energy essential to reverse the magnetization over the

energy barrier from one stable magnetic configuration to the other is

proportional to KV/kBT where, V is the particle volume, kB is Boltzmann’s

constant and T is temperature. If the thermal energy is larger enough to

overcome the anisotropy energy, the magnetization value is no longer constant

and the particle is said to be superparamagnetic. Stoner et al. studied simple

model for a noninteracting single domain spherical particle [57].

The magnetic anisotropy energy per particle which is responsible for

holding the magnetic moments along a certain direction can be expressed as

follows,

(1.2)

where, V is particle volume, Keff anisotropy constant and q is angle between

c 2

0

36r = AK

sM

2( ) sineffE K V

Chapter 1


magnetization and easy axis. The energy barrier KeffV splits the two

energetically equivalent easy directions of magnetization. With decreasing

size of the particle, the thermal energy (kBT), exceeds the energy barrier

(KeffV) and magnetization is easily flipped. The dependency of anisotropy

energy on θ is depicted schematically in Fig. 1.10. Clearly, either θ= 0 or π is

a direction of minimum energy and these directions are symmetrically

separated by an energy barrier as high as KV barrier for magnetization

reversal [58].

Fig. 1.10 Schematic representation of Stoner-Wohlfarth anisotropy

energy [58].

1.4.4.2 Superparamagnetic Properties of LSMO

The magnetic properties of LSMO NPs are studied by many authors for

their successful applications in the technical field, including magnetic

data storage and magnetic refrigeration [59-60]. Growing interest of the

researchers in LSMO NPs is due to the innovation of SPM nature. As the

LSMO material is a ferromagnetic in nature at room temperature when it is

Chapter 1


in bulk size, the magnetic material in bulk size is having multidomain nature

and LSMO exhibit the same. Daengsakul et al. [61] proposed that the LSMO

compound will be SPM in nature when its size is below critical size. Electron

paramagnetic resonance (EPR) study done by Krivoruchko et al. [62] and

studied the superparamagnetic nature of LSMO NPs (~12 nm) at room

temperature.

The model of superparamagnetism in LSMO is based on the Langevin

equation. The simplest variant of this model considers a system of N non-

interacting identical particles with magnetic moment (μef). If magnetic

moment of the particle is assumed to be large, the interaction of the particle

with the magnetic field H should be calculated without taking the quantum

effects into account. In case of isotropic particles, the equilibrium

magnetization (M) of the system is described by the Langevin equation,

(1.3)

Equation (1.3) has been derived with the assumption that single

particles are magnetically isotropic. In the isotropic system all directions for

their magnetic moments are energetically equivalent, but it is difficult to fulfil

the condition. Following are the some distinct features of the nanosize LSMO

that are responsible for its SPM nature.

Generally, if the particle size is smaller than the size of single domain,

each particle has a large magnetic moment (so-called super spin) which is

better for hyperthermia application, where MNPs of fairly uniform size,

having a TC above room temperature nearly 45˚C, are needed [63]. The

LSMO material is considered to be single domain when the dimensions of its

particles decrease to an extent where the ratio of the number of surface atoms

Ns to the total number N of atoms in the particle approaches 0.5. Actually,

cothef B

ef

B ef

H k TM N

k T H

Chapter 1


the name single domain' does not require a necessary uniform magnetization

throughout the whole particle bulk, but only involves the absence of domain

walls. The critical size or diameter is corresponding to particle transition from

the multi domain to the single domain state. The critical size differs for

different materials which can be calculated by equation 1.1. The typical values

reported for rc is about 15 nm for Fe, 35 nm for Co, 30 nm for γ-Fe2O3 [64]

and about 40 nm for LSMO [61].

One can find in the single domain structure, SPM materials can

fluctuate randomly by thermal fluctuation at high enough temperatures just as

an atom spin in paramagnetic materials. It is seen that at low temperatures, the

thermal energy decreased and the magnetic moments become blocked. This

temperature is called the blocking temperature (TB). Below TB, SPM material

loses its preferred direction of magnetization in zero magnetic fields [65]. The

TB for the LSMO is the temperature below which the magnetic moment of the

LSMO particle retains orientation in space, while the particle assembly

demonstrates a magnetic hysteresis. If the temperature of LSMO is higher

than TB the particle transfers from ferromagnetic to SPM state. In the TB< T

>TC region, the LSMO particle has a spontaneous magnetisation and a

nonzero total magnetic moment, which easily changes their orientation in the

external field. The TB is calculated by using following equation,

(1.4)

For LSMO NPs the value of K is 2.25×104 erg/cm3. It is remarkable

that the relation (1.4) indicates the TB for a zero magnetic field. As the

external or peripheral magnetic field is raised, TB reduces by power law [66],

(1.5)

( ) (0) 1

k

B B

c

HT H T

H

25B

B

KVT

k

Chapter 1


For low field (K=2) and for high field (K=2/3), HC =2K/MS.

In order to study the SPM nature of the system, one of the important

characteristics to find the SPM nature of the system containing NPs the

temperature dependence of the magnetic moment is i.e. field cooling (FC) and

zero field cooling (ZFC). For the ZFC measurements procedure, the sample is

cooled from room temperature to a particular low temperature in the absence

of magnetic field. Then a small magnetic field (about 100 Oe) is applied and

the magnetization is measured as the temperature is being increased. When

temperature goes on increasing, thermal energy will cause the moments to

align along the direction of the applied magnetic field (i.e., overcoming

anisotropy energy and freeing moments from being blocked at T < TB). The

number of these aligned moments will increase as the temperature increases

reaching a maximum at TB. As the temperature is increased above TB, the

thermal energy becomes large enough (larger than that of the aligning field) to

cause the magnetic moments to flip randomly which results in a suppression

of the magnetization of the particle. The ZFC measurement will result in a

peak in the magnetization versus temperature curve. This peak occurs at TB

[67].

The magnetic moment of the SPM LSMO in the FC mode at T < TB

and in ZFC mode at T = TB given by,

(1.6)

The magnetic moment is approximate in case of FC and it is exact in

ZFC case. An equation (1.6) is common for both. In FC measurements

procedure, the sample is cooled from room temperature to a particular low

temperature in the existence of magnetic field. The magnetization is calculated

as the temperature is being cooled. At T > TB, thermal energy is large enough

to randomize the magnetic moments in the particle leading to very small net

2

3

s

B

M VHm

k T

Chapter 1


magnetization. When temperature is decreased for some extent, thermal

energy will reduce it becomes smaller than that produced by the aligning field.

This is the reason why some moments align along the field direction leading

to an increase in magnetization. Further decrease in temperature causes, most

of moments to froz along the direction of the applied field. The magnetization

of the material is expected to keep increasing down to the lowest temperature

of the experiment. If the NPs are not single-domain particles or/and if they are

not seprated, then other interactions will be involved and the results of the

ZFC and FC measurements will be complicated. Thus, the shape of the

magnetization versus temperature plots in the ZFC and FC measurements can

provide qualitative information about the size distribution and the strength of

interaction among the particles making the sample [67].

1.5 Statement of the Problem

Recent advances in nanomedicine have led us towards the development

of MNPs probes for molecular and cellular imaging, NPs drugs for targeted

therapy, and integrated nanocarriers for early cancer detection and screening.

Cancer is a disease, which is one of the leading cause of death in the present

world. Even five years into the 21st millennium, cancer continues to torment

humanity as the second leading cause of death with 12.7 million newly

diagnosed cases worldwide in the year 2008 alone, which equates to around

188 cases for every 1,00,000 people. Out of the 12.7 million cases of cancer,

6.6 million cases were in men and in women it is about 6.0 million. This

number is expected to increase upto 21 million by 2030. Improper diagnosis

and detection at later stages are the some of the reasons behind it.

MNP is one of the subclass of NPs that can be manipulated under the

influence of an external magnetic field. The ability of the MNPs that they

resonantly respond to an external magnetic field has been used in biomedical

applications such as MRI, targeted drug and MFH. Several methods have

Chapter 1


been developed in order to improve the diagnosis and treatment of cancer. In

comparison to the traditional therapies of cancer like chemotherapy and

radiotherapy, hyperthermia in general and MFH in particular can reduce

severe side effects caused to normal tissue. Therefore in present work heat

induction study of MNPs for cancer hyperthermia therapy has been focused.

Rare-earth based group of half-metallic ferromagnetic materials, such

as manganites with a typical composition LSMO, are of interest in

biomedicine due to its high TC of 380 K and a large magnetic moment at room

temperature [68]. The half-metallic manganites are fairly metallic and can

have large microwave absorption with the possibility of its use in

hyperthermia applications and the large moment can also allow its use in

marker experiments in biodetection [69]. The thesis describes the

experimental investigation of SPM perovskite LSMO manganite NPs

synthesized by facile combustion method. Thesis mainly focuses on following

main objectives,

1. The development of high quality SPM LSMO NPs. Special attention

was paid to the surface functionalization of MNPs and to control the size of

NPs.

2. Optimisation of surface functionalization and as a result optimized process

parameter such as concentration of surfactant, reaction temperature can be

identified and used to fabricate high quality monodisperse MNPs. MNPs are

planned to functionalize with different material such as Polyvinylpyrrolidone

(PVP), acrypol, PEG and glycine in order to improve biocompatibility and to

study the effect of surface functionalization on structural, morphological and

magnetic properties of NPs.

3. To investigate the colloidal behaviour of bare and functionalized LSMO

NPs in different physiological conditions such as Phosphate buffered solution,

(PBS), NaCl, Glucose and in protein Bovine serum albumin (BSA).

Chapter 1


4. To check in vitro biocompatibility study of LSMO NPs with the Trypan

blue assay and MTT assay on L929 cell lines, MCF-7 and HeLa cell lines and

to investigate the influence of MNPs concentration and surface

functionalization on cell death.

5. To study the induction heating ability of bare and functionalized LSMO

NPs for cancer hyperthermia therapy application in different physiological

conditions.

Chapter 1


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