WO2017065600A1

WO2017065600A1 - Stable iron oxide magnetic nanoparticle (nanomag) slurry and a method of producing the same

Info

Publication number: WO2017065600A1
Application number: PCT/MY2016/050067
Authority: WO
Inventors: Sharifah Bee Binti O.A. ABD HAMID; Hun Tiar ONG; Mun Foong TAI
Original assignee: Universiti Malaya
Priority date: 2015-10-15
Filing date: 2016-10-14
Publication date: 2017-04-20
Also published as: JP2018530513A

Abstract

Disclosed herein is an iron oxide nanoparticle comprising ≥95% magnetite (Fe₃O₄) and having a particle diameter size of from 7 to 27 nm when measured using transmission electron microscopy and a magnetisation strength (Ms) of from 60 to 80 emu/g. Also disclosed herein are coated iron oxide nanoparticles, methods to make said iron oxide nanoparticles and articles containing the same.

Description

STABLE IRON OXIDE MAGNETIC NANOPARTICLE (NANOMAG) SLURRY AND A METHOD OF PRODUCING THE SAME

FIELD OF THE INVENTION

This present invention relates to a method for producing an iron oxide nanoparticle that may be formed as a stable slurry, the nanoparticles and/or the slurry having a small particle size and having excellent dispersion properties in a polymer matrix when a coating layer is formed on said iron oxide nanoparticles. BACKGROUND OF THE INVENTION

Iron oxide nanoparticles possess unique features compared to equivalent larger-scale materials. Among iron oxide phases, such as magnetite (Fe₃0₄), maghemite (Y-Fe₂0₃) and hematite (a-Fe203), magnetite is frequently used because of its high saturation magnetization value. Therefore, iron oxide (Fe30₄) magnetic nanoparticles have received significant interest for biomedical applications, mineral separation, magneto- optic materials and microwave filters. However, iron oxide nanoparticles have extremely high adhesion properties, resulting in the tendency for iron oxide nanoparticles to aggregate together. This phenomena is because iron oxide nanoparticles have a strong magnetic dipole-dipole interaction between particles in conjunction with a large surface energy (>100 dyn cm^-1). This problem limits the use of iron oxide nanoparticles. Thus, for industrial applications, it is quite important to develop techniques to control the dispersion/agglomeration phenomena of the iron oxide nanoparticles to apply them into functional materials and products. To improve the dispersion stability of Fe₃0₄ nanoparticles in aqueous medium, it is necessary to modify the iron oxide surface by modifiers to generate an effective repulsive force. Besides that, the protecting shells not only stabilize the magnetic properties, but also can be used for further protection of the magnetic nanoparticles against oxidation by oxygen. The modifying agent should be able to prevent the agglomeration of iron oxide nanoparticles, not contain environmentally harmful elements such as sulfur and not impair the interaction between magnetite and surfactant. In US patent application publication No. 2013/0330280, there is disclosed a rod-shaped iron oxide nanoparticles coated with a crosslinked laminated biocompatible polymer. However, the magnetic strength was not stated and the size of the iron oxide produced is too big and the ranges were too broad (100 - 500 nm) to provide an effective iron oxide nano particle for use in many applications, for example for use in combination with natural and/or synthetic rubber in the manufacture of gloves. Said magnetic properties may be useful in helping to detect the presence of a ripped glove in a manufacturing process.

SUMMARY OF INVENTION

The present invention provides a method for the production of black iron oxide nanoparticle slurry having a magnetite structure which comprises precipitating the iron oxide with NH₄OH in a first stage at 60 °C and then followed by a washing step with deionized water. In a second stage, oleic acid was mixed with iron oxide slurry to homogenously disperse iron oxide slurry. In turn, the resulting iron oxide nanoparticles can be used as formed, or may be placed into a matrix or a medium (e.g. NBR (nitrile- butadiene rubber), titania, zinc oxides, silica, alumina etc). The resulting iron oxide nanoparticles have superior properties compared to other iron oxide nanoparticles.

Thus in a first aspect of the invention, there is provided an iron oxide nanoparticle comprising≥95% magnetite (Fe30₄) and the following properties: a particle diameter size of from 7 to 27 nm when measured using transmission electron microscopy (e.g. from 12 to 25 nm, such as from 20 to 23 nm); and

a magnetisation strength (Ms) of from 60 to 80 emu/g (e.g. from 65 to 75 emu/g, such as from 67 to 70 emu/g).

In certain embodiments of the invention, the iron oxide nanoparticle may have:

a percentage crystallinity of from 85 to 99% (e.g. from 90 to 95%);

a substantially spherical shape;

a polydispersity index of from 0.15 to 0.25 (e.g. from 0.16 to 0.25, such as from 0.17 to 0.20);

a remanence of from 0.19 to 1 .84 emu/g (e.g. from 0.25 to 1 .50, such as from 0.50 to 1 .00 emu/g);

a coercivity (He) of from 3.29 to 1 .71 kA/m (e.g. from 4.20 to 11 .00 kA/m, such as from 5.00 to 8.50 kA/m);

a Zeta potential of from -33 to -49 mV (e.g. from -45 to -48 mV, such as

-46.7 mV).

In further embodiments of the invention, the iron oxide nanoparticle may further comprise a coating agent that coats a surface of the iron oxide nanoparticle. For example, the coating agent may be selected from the group consisting of hexadecanoic acid, tetradecanoic acid, dodecanoic acid undecanoic acid, decanoic acid, stearic acid, hexanoic acid, nonaic acid, tridecanoic acid, pentadecanoic acid, heptadecanoic acid, mercapto silane, an amino silane, and, more particularly, oleic acid. In yet further embodiments of the invention, the coating agent may form a monolayer.bilayer or a multilayer coating on the surface of the iron oxide nanoparticle. Particular embodiments of the invention that may be mentioned herein, the coating may be in the form of a bilayer.

In yet still further embodiments of the invention, the coated iron oxide nanoparticle may have:

a particle diameter size of from 6 to 25 nm as measured using transmission electron microscopy (e.g. from 8 to 23 nm);

a magnetisation strength (Ms) of from 62 to 75 emu/g (e.g. 65 to 68 emu/g); a polydispersity index of from 0.13 to 0.25 (e.g. from 0.14 to 0.20); a remanence of from 0.80 to 1 .07 emu/g (e.g. from 0.85 to 1 .00 emu/g); and/or a coercivity (He) of from 6.47 to 8.57 G (e.g. from 6.95 to 7.75 G);

a Zeta potential of from -45 to -55 mV (e.g. from -50 to -51 mV);

a magnetic strength that only reduces by from 1 % to 6% (e.g. from 2% to 5.5%, such as 5.2%) when subjected to oxidation;

a water stability or a water/latex medium stability of from 20 days to 100 days (e.g. from 30 days to 90 days, such as 30 days or 90 days).

It will be appreciated that any technically feasible combination of the above features is contemplated as part of this invention.

In a second aspect of the invention, there is provided a method of producing iron oxide nanoparticles, the process comprising:

(i) providing a mixture of FeCI₂ (or a solvate thereof) and FeCI₃ (or a solvate thereof) in a solvent and reacting the mixture with N H4OH to form a first slurry comprising iron oxide nanoparticles;

(ii) separating the iron oxide nanoparticles from the first slurry; and

(iii) washing the uncoated iron oxide nanoparticles with a solvent to form a wet cake of uncoated iron oxide nanoparticles. In certain embodiments of the invention, the separating (ii) and washing (iii) steps may comprise:

(a) applying a magnetic force to the first slurry to settle the iron oxide nanoparticles and decanting the solvent; and

(b) adding a solvent to form a second slurry and then applying a magnetic force to the first slurry to settle the iron oxide nanoparticles and decanting the solvent to form a wet cake; and optionally

(c) repeating step (b) one or more times.

In yet further embodiments of the invention:

(a) the FeC may be present as the FeCl2»4H20 solvate and the FeCb is present as the FeCb»6H₂0 solvate; and/or

(b) the FeCI₂ (or a solvate thereof) and FeCI₃ (or a solvate thereof) may be present in a molar ratio of from 0.5:3 to 1 :1 (e.g. from 0.75:2 to 1 :1 .75, such as 1 :1 .5), provided that when FeCb and/or FeCb is present as a solvate, the molar ratios are calculated based upon the number of moles of the solvate(s) used.

In certain embodiments of the invention, in step (i)

(a) the solvent is water; and/or

(b) the N H4OH is an aqueous 12 M solution and is added to the mixture at a rate of 100 mL/minute; and/or

(c) the mixture and the first slurry are stirred at a rate of from 100 rpm to 1000 rpm (e.g. from 200 rpm to 700 rpm, such as 500 rpm) with a mechanical stirrer;

(d) the temperature of the reaction is from 50 to 70 °C (e.g. from 55 to 65 °C, such as 60 °C); and/or

(e) after the addition of the entire amount of N H4OH , the first slurry is stirred for from 20 min to 120 min, such as 90 min; and/or

(f) after the addition of the entire amount of NH4OH , the reaction is continued until the first slurry has a pH of not more than 9.5.

In certain embodiments of the invention, the resulting iron oxide nanoparticles of the method may:

(i) comprise≥95% magnetite (Fe30₄); and/or

(ii) have a magnetic strength of from 60 to 80 emu/g (e.g. from 65 to 66 emu/g); and/or

(iii) have a Zeta potential of from -33 to -49 mV (e.g. from -45 to -48 mV, such as -46.7 mV); and/or

(iv) the iron oxide nanoparticles may have a particle size of from 7 to 27 nm in diameter when measured by transmission electron microscopy (e.g. from 12 to 25 nm, such as from 20 to 23 nm, or from 10 to 15 nm).

In yet further embodiments of the invention, the process may further comprise forming coated iron oxide nanoparticles in a subsequent process comprising the steps of:

(a) providing iron oxide nanoparticles made by the method described above; and

(b) mixing the provided iron oxide nanoparticles with a coating agent to provide a mixture and producing coated iron oxide nanoparticles therefrom.

In embodiments of this further processing step: (i) in step (a), the iron oxide nanoparticles may be provided as a wet cake of iron oxide nanoparticles (e.g. the wet cake comprises the nanoparticles and water);

(ii) in step (b), the coating agent is added to the iron oxide nanoparticles in a weight/weight ratio of greater than 0.2:1 (e.g. from 2.5:1 to 2:1 , such as from 3:1 to 1 :1); and/or

(iii) in step (b), the mixture may be subjected to mechanical mixing for a first period of time before being subjected to sonication for a second period of time to provide the coated iron oxide nanoparticles (e.g. wherein the first period of time is from 1 minute to 1 hour and the sonication is from 5 minutes to 10 hours, such as from 15 minutes to 2 hours, for example 1 hour), optionally wherein the sonication power is from 85 to 90 W; and/or

(iv) in step (b) the coated iron oxide nanoparticles may be homogeneously dispersed.

In certain embodiments the coated iron oxide nanoparticles produced by the above method may:

(i) comprise≥95% magnetite (Fe30₄); and/or

(ii) have a magnetic strength of from 62 to 75 emu/g (e.g. 65 to 68 emu/g); and/or

(iii) have a Zeta potential of from -40 to -50 mV (e.g. from -45 to -48 mV, such as -46.7 mV); and/or

(iv) have a magnetic strength that only reduces by from 1 % to 6% (e.g. from 2% to 5.5%, such as 5.2%) when subjected to oxidation; and/or

(v) be stable in water or a water/latex medium for from 20 days to 100 days (e.g. from 30 days to 90 days, such as 30 days or 90 days); and/or

(vi) have a particle size of from 6 to 25 nm as measured using transmission electron microscopy (e.g. from 8 to 23 nm). In still further embodiments of the invention, the coated iron oxide nanoparticle may be subjected to one or more further processing steps, comprising:

(c) adding the coated iron oxide nanoparticles to a solution of a material or to a melted form of a material (e.g. a polymer solution, a melted polymer or a reaction mixture that can be reacted and/or cured to form a polymeric material, a paper precursor, a cardboard precursor or a fabric precursor, or, more particularly, a synthetic latex solution, such as nitrile rubber (NBR)) solution to form an to form an impregnated iron oxide nanoparticle mixture; and optionally

(d) forming an article from or that comprises the impregnated iron oxide nanoparticle mixture.

In embodiments of the invention, the coating agent may be selected from the group consisting of hexadecanoic acid, tetradecanoic acid, dodecanoic acid undecanoic acid, decanoic acid, stearic acid, hexanoic acid, nonaic acid, tridecanoic acid, pentadecanoic acid, heptadecanoic acid, mercapto silane, an amino silane, and, more particularly, oleic acid.

In a third aspect of the invention, there is provided an article comprising a material (e.g. a polymer, a paper, a cardboard or a fabric, carbon, a clay, a ceramic, a resin or, more particularly, a synthetic latex, such as nitrile rubber (NBR)) that comprises iron oxide nanoparticles according to the first or second aspects of the invention and embodiments thereof. In certain embodiments of this aspect, the coated iron oxide nanoparticles may be substantially homogeneously distributed throughout the material. BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention will be more readily understood and appreciated from the following detailed description when read in conjunction with the accompanying drawings of embodiments of the present invention, in which:

Figure 1 depicts uncoated (a) and oleic acid coated iron oxide nanoparticles according to the current invention.

Figure 2 depicts powder X-ray diffractograms of uncoated (FeOx) and oleic acid coated (FeOx(OA)) iron oxide (Fe₃0₄) nanoparticles.

Figure 3 depicts the Raman spectra of uncoated iron oxide (FeOx) and oleic acid coated iron oxide (FeOx(OA)). Figure 4 is the FT-IR spectra of KBr (control), uncoated iron oxide (FeOx), oleic acid coated iron oxide (FeOx(OA)), and oleic acid (OA).

Figure 5 is the hysteresis loop of uncoated iron oxide (FeOx) and oleic acid coated iron oxide (FeOx(OA)) measured at room temperature using a vibrating sample magnetometer (VSM).

Figure 6 is the magnetic strength of coated iron oxide slurry.

Figure 7 depicts the stability of the various ratio compositions of oleic acid functionalised iron oxide nanoparticles in aqueous solution.

Figure 8 depicts the stability of the oleic acid functionalised iron oxide nanoparticles in NBR latex and water over a period of up to 30 days.

DESCRIPTION OF INVENTION

It has been surprisingly found that iron oxide nanoparticles with particularly good properties can be made using the processes described hereinbelow. These iron oxide nanoparticles have particularly good magnetic properties, are capable of resisting oxidation (e.g when coated) and have the ability to form a suspension in a liquid medium (e.g. water, a polymer-precursor medium or a polymeric medium), allowing a more homogeneous dispersion of the iron oxide nanoparticles.

The resulting iron oxide nanoparticle comprises≥95% magnetite (Fe30₄) and:

a particle diameter size of from 7 to 27 nm when measured using transmission electron microscopy (e.g. from 12 to 25 nm, such as from 20 to 23 nm); and

The iron oxide nanoparticle may have a substantially spherical shape when examined using transmission electron microscopy. Additionally or alternatively:

(i) the iron oxide nanoparticle may have a percentage crystallinity of from 85 to 99% (e.g. from 90 to 95%); (ii) a polydispersity index of from 0.15 to 0.25 (e.g. from 0.16 to 0.25, such as from 0.17 to 0.20) measured as described in the experimental section below;

a remanence of from 0.19 to 1 .84 emu/g (e.g. from 0.25 to 1 .50, such as from 0.50 to 1 .00 emu/g) measured as described in the experimental section below;

a coercivity (He) of from 3.29 to 1 .71 kA m (e.g. from 4.20 to 11 .00 kA/m, such as from 5.00 to 8.50 kA/m) measured as described in the experimental section below; a Zeta potential of from -33 to -49 mV (e.g. from -45 to -48 mV, such as

-46.7 mV) measured as described in the experimental section below.

The percentage crystal I in inty of the iron oxide nanoparticles mentioned herein may be measured by any suitable method. For example, the percentage crystallinity may be measured using Mossbauer spectroscopy.

The above-mentioned iron oxide nanoparticles may be prepared using a method that comprises the following steps:

(i) providing a mixture of FeC (or a solvate thereof) and FeC (or a solvate thereof) in a solvent and reacting the mixture with NH₄OH to form a first slurry comprising iron oxide nanoparticles;

(ii) separating the iron oxide nanoparticles from the first slurry; and

(iii) washing the uncoated iron oxide nanoparticles with a solvent to form a wet cake of uncoated iron oxide nanoparticles. separating (ii) and washing (iii) steps may comprise the following process:

(c) repeating step (b) one or more times.

In certain embodiments of the process described above:

(a) the FeCb may be present as the FeCl2«4H20 solvate and the FeCb may be present as the FeCb^ei-bO solvate; and/or

(b) the FeCb (or a solvate thereof) and FeCb (or a solvate thereof) may be present in a molar ratio of from 0.5:3 to 1 :1 (e.g. from 0.75:2 to 1 :1 .75, such as 1 :1 .5), provided that when FeC and/or FeC is present as a solvate, the molar ratios are calculated based upon the number of moles of the solvate(s) used. In particular embodiments of the invention, the process described above may be subject to one or more of the following conditions:

(a) in step (i), the solvent may be water;

(b) in step (i), the N H4OH may be an aqueous 12 M solution and may be added to the mixture at a rate of 100 mL/minute;

(c) in step (i), the mixture and the first slurry may be stirred at a rate of from

100 rpm to 1000 rpm (e.g. from 200 rpm to 700 rpm, such as 500 rpm) with a mechanical stirrer;

(d) in step (i), the temperature of the reaction may be from 50 to 70 °C (e.g. from 55 to 65 °C, such as 60 °C);

(e) in step (i), after the addition of the entire amount of N H4OH , the first slurry may be stirred for from 20 min to 120 min, such as 90 min;

(f) in step (i), after the addition of the entire amount of N H4OH , the reaction may be continued until the first slurry has a pH of not more than 9.5. In addition, the iron oxide nanoparticles described above may be subjected to a further processing step to provide coated iron oxide nanoparticles. These coated iron oxide nanoparticle further comprise a coating agent that coats a surface of the iron oxide nanoparticle. For example, the coating agent may be selected from the group consisting of hexadecanoic acid, tetradecanoic acid, dodecanoic acid undecanoic acid, decanoic acid, stearic acid, hexanoic acid, nonaic acid, tridecanoic acid, pentadecanoic acid, heptadecanoic acid, mercapto silane, an amino silane, and oleic acid. Particular coating agents that may be described herein include oleic acid.

In certain embodiments of the invention, the coating agent may form a monolayer, abilayer or a multilayer coating on the surface of the iron oxide nanoparticle. That is, when the coating is a bilayer coating, the coating agent may form a first coating layer that is in interactive contact with the surface of the iron oxide nanoparticle and a second coating layer that interacts with the first coating layer. The terms monolayer and multilayer may be understood based upon the definition of bilayer provided herein. Particular embodiments of the invention that may be mentioned herein, the coating may be in the form of a bilayer. The coated iron oxide nanoparticle may have one or more of the following properties: a particle diameter size of from 6 to 25 nm as measured using transmission electron microscopy (e.g. from 8 to 23 nm);

a magnetisation strength (Ms) of from 62 to 75 emu/g (e.g. 65 to 68 emu/g); a polydispersity index of from 0.13 to 0.25 (e.g. from 0.14 to 0.20);

a remanence of from 0.80 to 1 .07 emu/g (e.g. from 0.85 to 1 .00 emu/g); and/or a coercivity (He) of from 6.47 to 8.57 kA/m (e.g. from 6.95 to 7.75 kA m);

a Zeta potential of from -45 to -55 mV (e.g. from -50 to -51 mV);

a water stability or a water/latex medium stability of from 20 days to 100 days

(e.g. from 30 days to 90 days, such as 30 days or 90 days).

It will be noted that resistance to oxidation, homogeneity in a medium and stability in a medium are particularly difficult properties to obtain for conventional iron oxide nanoparticles.

The coated iron oxide nanoparticles may be provided by:

(a) providing iron oxide nanoparticles made by the method described above; and

In particular embodiments of the invention that may be mentioned herein, the iron oxide nanoparticles may be provided as a wet cake of iron oxide nanoparticles. For example, the wet cake may comprise the nanoparticles and water.

In step (b) of the above process, the coating agent may be added to the uncoated iron oxide nanoparticles in a weight/weight ratio of greater than 0.2:1 (e.g. from 2.5:1 to 2:1 , such as from 3:1 to 1 :1). It will be understood that weight/weight ratio is based upon the dry weight of the materials used.

The coating agent may be selected from the group consisting of oleic acid; hexadecanoic acid, tetradecanoic acid, dodecanoic acid undecanoic acid, decanoic acid, stearic acid, hexanoic acid, nonaic acid, tridecanoic acid, pentadecanoic acid, heptadecanoic acid, mercapto silane and an amino silane. Particular coating agents that may be mentioned herein include oleic acid, such that the oleic acid is added to the uncoated iron oxide nanoparticles in a weight/weight ratio of greater than 0.2:1 (e.g. from 2.5:1 to 2:1 , such as from 3:1 to 1 :1).

In step (b), the mixture may be subjected to mechanical mixing for a first period of time before being subjected to sonication for a second period of time to provide the coated iron oxide nanoparticles (e.g. wherein the first period of time is from 1 minute to 1 hour and the sonication is from 5 minutes to 10 hours, such as from 15 minutes to 2 hours, for example 1 hour). For example, the sonication power is from 85 to 90 W.

It will be appreciated that the process above may provide coated iron oxide nanoparticles that may be homogeneously dispersed. The process above may further include an additional step of adding the coated iron oxide nanoparticles to a solution of a material or to a melted form of a material (e.g. a polymer, or a synthetic latex, such as a NBR latex) to form an impregnated iron oxide nanoparticle mixture. In addition, the impregnated iron oxide nanoparticle mixture may then be used to form an article from the impregnated iron oxide nanoparticle mixture (e.g. an NBR latex article).

The solution or melted form of a material may be a polymer solution, a melted polymer or a reaction mixture that can be reacted and/or cured to form a polymeric material. In alternative embodiments, the material may be a mixture of ingredients that are used to form a paper, a cardboard or a fabric. In particular embodiments of the invention, the material may be a latex solution (e.g. a synthetic latex solution, such as nitrile rubber (NBR)).

The process above may be used to make coated iron-oxide nanoparticles having one or more properties selected from:

(i) a magnetisation strength (Ms) of from 62 to 75 emu/g (e.g. 65 to 68 emu/g);

(ii) a Zeta potential of from -45 to -55 mV (e.g. from -50 to -51 mV);

(iii) a magnetic strength that only reduces by from 1 % to 6% (e.g. from 2% to 5.5%, such as 5.2%) when subjected to oxidation; and

(iv) stable in water or a water/latex medium for from 20 days to 100 days (e.g. from 30 days to 90 days, such as 30 days or 90 days),

A process for manufacturing the coated iron oxide nanoparticles that fall within the scope of the invention will now be described hereinbelow. It will be appreciated that the process described below may include additional components that are not essential to all aspects and embodiments of the invention. Different amounts of surfactant were added to the semi-dry nanomagnetic iron oxide nanoparticles and stirred manually (amounts listed in Table 1), followed by the addition of 12 M N H4OH . The resulting suspensions were treated by ultrasound using a sonicator for 1 hour and may be described herein as "nanoMAG".

Table 1

Materials Portion

Iron oxide 1 g

Surfactant 0.4 - 1 .0 g

The iron oxide used herein is maghemite (Y-Fe203); or magnetite (Fe304), or a combination thereof, unless stated otherwise. The surfactant used herein is Fatty acid (Oleic acid; Hexadecanoic acid (Palmitic acid); Tetradecanoic acid (Myristic acid); Dodecanoic acid (Luaric acid); Undecanoic acid; Decanoic acid (Capric acid); Stearic acid: Hexanoic acid: Nonaic acid; Tridecanoic acid (C13); Pentadecanoic acid (C15); or Heptadecanoic acid (C17)); or Coupling agent (3-(Trimethoxysilyl)-1 -propanediol or N- (2-Aminoethyl)-3-amino propyltrimethoxysilane) or a combination thereof, unless stated otherwise. There is also provided an article formed from a material (e.g. a polymer, a paper, a cardboard or a fabric, or, more particularly, a synthetic latex, such as nitrile rubber (NBR)) comprising the iron oxide nanoparticles described herein.

Examples

Methods

Zetasizer

Malvern Zetasizer Nano ZS was selected to investigate zeta potential, particle size distribution and polydispersity of ION P (iron oxide nanoparticles). It was a high performance two angle particle and molecule size analyser for the enhanced detection of aggregates and measurement of small or dilute samples as well as samples at very low or very high concentration using dynamic light scattering. This technique measured the diffusion of particles moving under Brownian motion, subsequently converted it into size and size distribution using Stokes- Einstein relationship. Approximately 0.001 g of IONP was prepared and dispersed in 5 ml of Dl water. Besides 0.2 ml of 3M ammonium hydroxide was dropped and the solution is around pH 10. The solution was ultra-sonicated for 1 hour in order to ensure IONP was well dispersed in deionized water. Zetasizer instrument was turned on for 30 mins in order to stabilise the laser. Next, the dispersed solution was injected into disposable polystyrene (DTS0012) and folded capillary cell (DTS1060), measuring hydrodynamic size and zeta potential, respectively. Then, the required cell was inserted into the instrument and allowed for the temperature to stabilise. Finally, slurry charge and particle size distribution measurement were analysed, and results were collected. High Resolution Transmission Electron Microscope (HRTEM)

HRTEM analysis was performed by JEM-2100F instrument with accelerating voltage 200 kV. It has become a major support in the list of characterization techniques for materials scientists attributed to its capability of producing both image and diffraction information from a single sample. Furthermore, materials characterization could be determined via radiation produced by beam electrons accelerated on the sample. It offers high magnification up to 1 .6 times magnification which is able to observe extremely small and tiny lattice-fringe spacing. Prior to HRTEM characterization, samples were prepared by dropping dispersed IONP on copper grids of 300 mesh. The sample prepared was left overnight. After that, sample prepared on copper grid was placed into HRTEM sample holder before inserting into the HRTEM. Image of IONP was selected and taken at 50000, 100000 and 500000 times magnification. Size and lattice-fringe spacing was measured by image-J. Particle size distribution can be plotted after 100 IONP particles were measured and lattice-fringe spacing was used to support XRD information.

Vibrating Sample Magnetometer (VSM)

VSM was invented by Simon Foner (a scientist of the MIT) in 1956, and it has been widely used to determine magnetic properties of large variety materials: diamagmetic, paramagnetic, ferromagnetic and antiferromagnetic. Vibrating Sample Magnetometer (Lakeshore - VSM 7407) was used to study magnetic properties of IONP. Roughly 0.03g of IONP was prepared in the sample holder and placed at centre in a pair of pickup coils between the poles of an electromagnet. The sample was mounted with a sample rod in a transducer assembly and it passes via the centre of driving coil. The transducer was driven by power amplifier which itself is driven by an oscillator at frequency of 71 Hz. The sample vibrated along the z-axis perpendicular to the magnetizing field and it induced signal that indicated the magnetic properties itself. Continuously hysteresis and magnetic field from 10000 kA m to -10000 kA/m and from -10000 kA/m to 10000 kA/m was applied to identify magnetization saturation (Ms), coercivity (He) and remanence (Mr) of ION P.

Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR spectrum showed the molecular absorption and transmission (specific frequency of energy) which is useful to determine lONP's functional group, other attached molecule's functional group and the functional group linked in between IONP with the molecules. Before putting the sample in sample holder, the sample holder was cleaned by acetone. About 0.3 mg of ION P was mixed with 4 mg of KBr and moulded to form pellet. Pellet was placed in the FTIR and IR radiation was passed through it. Some of the IR radiation was absorbed by sample and some of it was passed via transmission. Resolution of FTIR was set at 4 cm^'1 with 16 scans in the wavelength range 400 cm-1 to 4000 cm-1 . FTIR spectrum was collected with labelled peaks.

Raman spectroscopy

Raman Spectroscopy (Renishaw in Via Reflux with high performance CCD camera and LEILA microscope is used in this study. It is one of the vibrational spectroscopy techniques widely used to provide information on chemical structures and physical forms. It was used to determine substance (phase) from the characteristic spectral patterns and quantitatively or semiquantitatively amount of substance in IONP with high lateral resolution. Approximately 0.05g of sample was placed into the sample holder, subsequently inserted into Raman spectrometer. Sample degradation and fluorescence might be happened in the process of testing. It will give a big impact on highly sensitive material like IONP. Hence, 1 percent of 0.2 mV laser power with 180 seconds exposure time was chosen to analyse ION P. Argon gas laser (514 nm) was selected in this research as 1800 mm-1 spectral due to its resolution is sufficient to plot a good spectra. Scattered radiation was collected by focusing laser beam via *50 objective and the laser spot on sample was approximately 0.836 μηη with 514 nm excitation. The obtained Raman spectra was analysed and different phases of IONP was determined.

Example 1

Preparation of Iron Oxide Nanoparticles To deionised (Dl) water (680 mL) at 60 °C was sequentially added fully dissolved aqueous solutions of FeCI₂«4H₂0 (28.16 g, 141 .64 mmol, in 100 mL H₂0), and FeCI₃»6H₂0 (57.42 g, 212.43 mmol in 100 mL H₂0; molar ratio of FeCI₂»4H₂0: FeCl3«6H₂0 is 1 :1 .5) with vigorous stirring (500 rpm). Ammonium hydroxide (12M, 1000 mL) was then added to the solution at a rate of 100 mL/min. The reaction mixture was stirred at 60 °C for a further 90 minutes. The precipitate was separated from the reaction mixture by use of a magnet to form a wet cake and Dl waterwas then added and the cycle of removal of water and removal repeated a number of times. Excess water was then removed and the nanomagentic particles were kept in a semi-dry state to avoid agglomeration.

Figure 1 a depicts a TEM image of the resulting uncoated nanomagnetic iron oxide. Example 2

Preparation of functionalised Iron Oxide Nanoparticles

To 1 g of the semi-dry iron oxide nanomagnetic particles of preparation 1 was added oleic acid (5 ml_, 0.45 g) with stirring, followed by the addition of NH₄OH (12M, 5 ml_). The resulting mixture was sonicated for 1 hour.

Figure 1 b depicts the coated iron oxide particle of the current invention.

Figure 2 depicts the X-ray powder diffraction results obtained for the uncoated iron oxide (FeOx) obtained in preparation 1 against the oleic acid coated (FeOx(OA)) iron oxide nanoparticle of Example 1. As shown, the powder diffraction results indicated that both materials contain iron oxide with characteristic peaks. Based upon the results obtained in the X-ray powder diffraction experiments, the crystal sizes of FeOx and FeOx(OA) appear to be of comparable size. It will be appreciated however, that this does not consider the potential for the uncoated iron oxide nanoparticles to agglomerate.

When considered using hydrodynamic measurement techniques (e.g. zeta sizer), the uncoated particles are slightly larger in size (see Table 2). This difference is most probably due to the agglomeration of uncoated iron oxide nanoparticles in the solution, as suggested by the lower zeta potential, Zp (Table 2). Zp is an indicator of stability of the particle in solution. Thus, while the hydrodynamic particles have a slightly larger size compared to that measured using TEM analysis, the particle size of FeOx and FeOx (OA) are still nanosized. In addition, it should be noted that the actual level of agglomeration in the uncoated iron oxide nanoparticles may be somewhat higher than displayed here. Sample Crystalline size Particle size (nm) Zp (mV)"

(nm) PSD TEM

FeOx 11.8 38.0 10-15 -38.4

FeOx(OA) 11.6 33.6 11 -18 -46.7

"characterized in pH10

TABLE 2 Figure 3 shows the Raman spectra of uncoated (FeO_x) and oleic acid coated iron oxide (FeO_x (OA)). It was found that both samples contain the magnetite and maghemite magnetic phases due to the presence of strong peaks at 668 and 700 cm ¹ , respectively (Slavov, L, M. V. Abrashev, et al. (2010). Journal of Magnetism and Magnetic Materials 322(14): 1904-1911 .). However, neither composition showed the presence of hematite or geotite phases.

Figure 4 show the FT-IR spectra of oleic acid coated iron oxide (FeO_x(OA)), uncoated iron (FeO_x), KBr(control) and oleic acid (OA). As shown, it can be seen that FeO_x and FeOx(OA) show the characteristic absorption bands of the (Fe-O) (A) bond at 580 cm^"1 (Ma, M., Y. Zhang, et a I. (2003). Colloids and Surfaces A: Physicochemical and Engineering Aspects 212(2-3): 219-226; Slavov ibid) which indicates that the samples are iron oxide, confirming that both samples contain iron oxide particles. OA can be detected by the presence of C-H stretching band (D) and bending band (B) at 2800- 3000 cm^-1 and 1390-1420 cm^"1, respectively. These bands also were shown in FeO_x (OA), with the small peak size for the bands due to the small amount of OA contained in the sample. OA also shows a band for C=0 at (C) (1690-1760 cm ¹). However, this band could not be detected in FeO_x(OA) due to the strong interaction between the carboxyl group and iron oxide, which obstructs the C=0 vibration. The bands at 3400 cm^"1 are attributed to the bending (E) vibration of O-H. These results show that the FeOx(OA) contains vibrations corresponding to Fe-O and C-H, confirming presence of both iron oxide and OA, respectively, in the composition.

The hysteresis loops for FeO_x and FeO_x(OA) are depicted in Figure 5, which was measured at room temperature using a vibrating sample magnetometer (VSM). The synthesized magnetite NPs show a superparamagnetic behaviour with very small coercivity and magnetic remanence observed. The saturation magnetization values of FeOx and FeO_x(OA) are ~ 80.6 and ~ 66.2, respectively. It is speculated that the coated nanoparticle gave the lower value due to the coating material which behaves as a barrier to the magnetic properties of the composition.

The use of oleic acid as a coating agent for iron oxide nanoparticles not only prevents the agglomeration of the nanoparticles, but also oxidation by oxygen as shown in Figure 6. The samples were compared using an oxidation retarder, where the samples were heated to a temperature of 125 °C for 25 min and then the magnetic properties were characterised. It was found that the iron oxide nanoparticles coated by oleic acid (FeO_x(OA)) could retard the oxidation as the magnetic strength only reduced 5.2 % compared to a sample that had not been heated compared to a reduction of 18.9 % for the uncoated iron oxide (FeOx) compared to an untreated FeOx sample.

Example 3

To optimize the ratio of iron oxide and oleic acid, semi-dry iron oxide was prepared as described in preparation 1 . Different amounts of Oleic acid were added to the semi-dry nanomagnetic iron oxide nanoparticles and stirred manually (amounts listed in Table 3), followed by the addition of 12 M NH₄OH. The resulting suspensions were treated by ultrasound using a sonicator for 1 hour.

Chemicals A1 A2 A3 A4 A5 A6 A7

Iron oxide (g) 1 1 1 1 1 1 1

Oleic acid (g) 0 0.2 0.4 0.5 0.6 0.8 1

12M NH₄OH 1 1 1 1 1 1 1

(ml)

TABLE 3

As shown in Figure 7, it appears that a ratio of greater than 0.2:1 of oleic acid: iron oxide nanoparticle is necessary to obtain a coated nanoparticle that is stable in solution. It will be appreciated that the ammonium hydroxide does not appear to affect the stability of the coated iron oxide nanoparticles in solution. Example 4 The stability of oleic acid coated iron oxide nanoparticles in water and NBR latex were subsequently examined.

Firstly, mixing tank was cleaned thoroughly by using brushes and detergent to ensure that no contamination during compounding process. 700 g of 43% total solid content (TSC) of NBR latex was filtered and transferred to the mixing tank. It was stirred for 30 minutes with 50 rpm stirring speed. After that, pH of the latex was adjusted at pH 9.6 to 9.7. The TSC of latex was continuously checked to be maintaining maintained at 43 %.. Next, 18.06 g of Sodium Dodecyl Benzene Sulphonate (SDBS) was slowly added into the latex and continued by stirring process for an hour. Once again, pH of the compound latex was measured and it was adjusted to pH 9.6 to 9.7. In the meantime, 3.61 g of Dibutyldithiocarbamate (ZDBC), 7.02 g of sulphur, 6.52 g of Zinc oxide (ZnO) and 9.03 g of Titanium dioxide (Ti02) were weighed. All of these chemicals were slowly added into the latex compound and stirred for another 1 hour. Then, stirring speed was slow down to 30 rpm and left overnight. TSC of compounded latex was checked and diluted by Dl water to 23%. At this stage, 5, 10, 15 and 20 phr of coated IONP as well as 12.04 g of aqua wax was added slowly into the compounded latex. It was stirred for 1 hour with the controlled and recorded of TSC and pH .

Figure 8 shows the stability of FeOx(OA) in water and in NBR latex over a period of time. The loading amount of Oleic acid was 0.4 g per 1 g iron oxide. As depicted by Figure 8, the FeOx(OA) nanoparticles are very stable in both water and NBR latex for at least 30 days. This observed stability is in agreement with the Zp values (see Example 1).

Claims

1 . An iron oxide nanopartide comprising >95% magnetite (Fe₃0₄) and the following properties:

2. The iron oxide nanopartide of Claim 1 , wherein the iron oxide nanopartide has: a percentage cr stallinity of from 85 to 99% (e.g. from 90 to 95%); and/or a substantially spherical shape.

3. The iron oxide nanopartide of Claim 1 or Claim 2, wherein the iron oxide nanopartide has a polydispersity index of fromO.15 to 0.25 (e.g. from 0.16 to 0.25, such as from 0.17 to 0.20).

4. The iron oxide nanopartide of any one of the preceding claims, wherein the iron oxide nanopartide has:

a remanence of from 0.19 to 1 .84 emu/g (e.g. from 0.25 to 1 .50, such as from 0.50 to 1 .00 emu/g); and

a coercivity (He) of from 3.29 to 14.71 kA/m (e.g. from 4.20 to 11 .00 kA/m, such as from 5.00 to 8.50 kA/m).

5. The iron oxide nanopartide of any one of the preceding claims, wherein the iron oxide nanopartide has a Zeta potential of from -33 to -49 mV (e.g. from -45 to -48 mV, such as -46.7 mV).

6. The iron oxide nanopartide of any one of the preceding claims, wherein the iron oxide nanopartide further comprises a coating agent that coats a surface of the iron oxide nanopartide.

7. The iron oxide nanopartide of Claim 6, wherein the coated iron oxide nanopartide has:

a particle diameter size of from 6 to 25 nm as measured using transmission electron microscopy (e.g. from 8 to 23 nm); and/or

a magnetisation strength (Ms) of from 62 to 75 emu/g (e.g. 65 to 68 emu/g).

8. The iron oxide nanoparticle of Claim 6 or Claim 7, wherein the coating agent is selected from the group consisting of oleic acid; hexadecanoic acid, tetradecanoic acid, dodecanoic acid undecanoic acid, decanoic acid, stearic acid, hexanoic acid, nonaic acid, tridecanoic acid, pentadecanoic acid, heptadecanoic acid, mercapto silane and an amino silane.

9. The iron oxide nanoparticle of Claim 8, wherein the coating agent is oleic acid.

10. The iron oxide nanoparticle of any one of Claims 6 to 9, wherein the coating agent forms a monolayer, a bilayer or a multilayer coating on the surface of the iron oxide nanoparticle.

11 . The iron oxide nanoparticle of any one of Claims 6 to 10, wherein the iron oxide nanoparticle has a polydispersity index of from 0.13 to 0.25 (e.g. from 0.14 to 0.20).

12. The iron oxide nanoparticle of any one of Claims 6 to 11 , wherein the iron oxide nanoparticle has:

a remanence of from 0.80 to 1 .07 emu/g (e.g. from 0.85 to 1 .00 emu/g); and/or a coercivity (He) of from 6.47 to 8.57 kA/m (e.g. from 6.95 to 7.75 kA/m).

13. The iron oxide nanoparticle of any one of Claims 6 to 12, wherein the coated iron oxide nanoparticles:

(i) have a Zeta potential of from -45 to -55 mV (e.g. from -50 to -51 mV); and/or

(ii) have a magnetic strength that only reduces by from 1 % to 6% (e.g. from 2% to 5.5%, such as 5.2%) when subjected to oxidation; and/or

(iii) are stable in water or a water/latex medium for from 20 days to 100 days (e.g. from 30 days to 90 days, such as 30 days or 90 days).

14. A method of producing iron oxide nanoparticles (e.g. the iron oxide nanoparticles of Claims 1 to 5), the process comprising:

(i) providing a mixture of FeCb (or a solvate thereof) and FeCb (or a solvate thereof) in a solvent and reacting the mixture with NH₄OH to form a first slurry comprising iron oxide nanoparticles;

(ii) separating the iron oxide nanoparticles from the first slurry; and

(iii) washing the uncoated iron oxide nanoparticles with a solvent to form a wet cake of uncoated iron oxide nanoparticles.

15. The method of Claim 14, wherein the separating (ii) and washing (iii) steps comprise:

(c) repeating step (b) one or more times.

16. The method of Claim 14 or Claim 15, wherein:

(a) the FeCb is present as the FeCb»4H₂0 solvate and the FeCI₃ is present as the FeCI₃»6H₂0 solvate; and/or

(b) the FeCb (or a solvate thereof) and FeCI₃ (or a solvate thereof) are present in a molar ratio of from 0.5:3 to 1 :1 (e.g. from 0.75:2 to 1 :1 .75, such as 1 :1 .5), provided that when FeCb and/or FeCb is present as a solvate, the molar ratios are calculated based upon the number of moles of the solvate(s) used.

17. The method of any one of Claims 14 to 16, wherein in step (i):

(a) the solvent is water; and/or

(b) the NH OH is an aqueous 12 M solution and is added to the mixture at a rate of 100 mL/minute; and/or

(e) after the addition of the entire amount of NH₄OH, the first slurry is stirred for from 20 min to 120 min, such as 90 min; and/or

(f) after the addition of the entire amount of NH₄OH , the reaction is continued until the first slurry has a pH of not more than 9.5.

18. The method of any one of Claims 14 to 17, wherein the iron oxide nanoparticles:

(i) comprise >95% magnetite (Fe₃0₄); and/or

(ii) have a magnetisation strength (Ms) of from 60 to 80 emu/g (e.g. from 65 to 75 emu/g, such as from 67 to 70 emu/g); and/or

(iv) the iron oxide nanoparticles have a particle size of from 7 to 27 nm when measured using transmission electron microscopy (e.g. from 12 to 25 nm, such as from 20 to 23 nm).

19. The method of any one of Claims 14 to 18, wherein the process further comprises forming coated iron oxide nanoparticles in a subsequent process comprising the steps of:

(a) providing iron oxide nanoparticles made in any one of Claims 14 to 18; and

(b) mixing the uncoated iron oxide nanoparticles with a coating agent to provide a mixture and producing coated iron oxide nanoparticles therefrom.

20. The method of Claim 19, wherein in step (a), the iron oxide nanoparticles are provided as a wet cake of iron oxide nanoparticles (e.g. the wet cake comprises the nanoparticles and water).

21 . The method of Claim 19 or Claim 20, wherein in step (b), the coating agent is added to the iron oxide nanoparticles in a weight/weight ratio of greater than 0.2:1 (e.g. from 2.5:1 to 2:1 , such as from 3:1 to 1 :1).

22. The method of any one of Claims 19 to 21 , wherein in step (b), the mixture is subjected to mechanical mixing for a first period of time before being subjected to sonication for a second period of time to provide the coated iron oxide nanoparticles (e.g. wherein the first period of time is from 1 minute to 1 hour and the sonication is from 5 minutes to 10 hours, such as from 15 minutes to 2 hours, for example 1 hour).

23. The method of Claim 22, wherein the sonication power is from 85 to 90 W.

24. The method of any one Claims 19 to 24, wherein the coated iron oxide nanoparticles:

(i) comprise >95% magnetite (Fe30₄); and/or

(ii) have a magnetisation strength (Ms) of from 62 to 75 emu/g (e.g. 65 to 68 emu/g); and/or

(iii) have a Zeta potential of from -45 to -55 mV (e.g. from -50 to -51 mV); and/or

(v) are stable in water or a water/latex medium for from 20 days to 100 days (e.g. from 30 days to 90 days, such as 30 days or 90 days); and/or

(vi) have a particle size of from 6 to 25 nm as measured using transmission electron microscopy (e.g. from 8 to 23 nm).

25. The method of any one of Claims 19 to 24, wherein in step (b) the coated iron oxide nanoparticles are homogeneously dispersed.

26. The method of any one of Claims 19 to 25, wherein the process further comprises:

(c) adding the coated iron oxide nanoparticles to a solution of a material or to a melted form of a material (e.g. a polymer solution, a melted polymer or a reaction mixture that can be reacted and/or cured to form a polymeric material, a paper precursor, a cardboard precursor or a fabric precursor, or, more particularly, a synthetic latex solution, such as nitrile rubber (NBR)) solution to form an to form an impregnated iron oxide nanoparticle mixture.

27. The method of Claim 26, wherein the process further comprises:

28. The method of any one of Claims 19 to 27, wherein the coating agent is selected from the group consisting of oleic acid; hexadecanoic acid, tetradecanoic acid, dodecanoic acid undecanoic acid, decanoic acid, stearic acid, hexanoic acid, nonaic acid, tridecanoic acid, pentadecanoic acid, heptadecanoic acid, mercapto silane and an amino silane.

29. The method of Claim 28, wherein the coating agent is oleic acid.

30. An article comprising a material that comprises iron oxide nanoparticles according to any one of Claims 1 to 18.

31 . The article of Claim 30, wherein the iron oxide nanoparticles are substantially homogeneously distributed throughout the material.