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# Cysteine Functionalised Nanoparticles in the Removal of Arsenic Ions and Methylene Blue from Water

11550 words (46 pages) Dissertation

18th May 2020 Dissertation Reference this

Tags: Sciences

Abstract

In recent years the use of functionalised nanoparticles in number of applications has increased due to the ability to tailor the properties of these materials to have multifunctional capabilities. Subsequently nanoparticles can be used in a number of industries, such as catalytic converters in the automotive industry, and microbial resistant materials in the health sectors and, many other applications are being explored and accounts for another factor that influences the surge in popularity of researching hybrid nanoparticles. This is a paper which discusses the possibility of a cysteine functionalised nanoparticles in the removal of arsenic ions and methylene blue from water. We use two different synthesises to reach the cysteine functionalised nanoparticle and compare against the Fe3O4@SiO2 nanoparticle which we make close to the start of the reaction pathway. We use analytical methods such as Ultraviolet-Visible spectroscopy and inductively coupled plasma atomic emission spectroscopy to see whether the cysteine functionalised nanoparticle is effective.

1. Introduction

1.1  Brief history and introduction to nano-materials

The component ‘nano’ means one billionth part of a unit. In this instance a nano-material (or nano-particle) refers to a material in the nano-metre dimensions, which is one billionth of a metre or 1 x 10-9m. Nanoparticles often display starkly different electronic, optical and chemical properties when you compare them to their bulk material. These effects at the nano-scale can be a result of the restricted electronic environments in nano dimensions, the high specific surface area ( m2/kg) leading to high surface energies and enhanced adsorption ability, as well as diverse surface chemistry and functionalities. Furthermore nano-materials can exhibit a wide range of tailorable geometries, which can lead to highly diverse properties. [1] Despite the fact that the field of nanotechnology is relatively new, nano-materials and structures have been found naturally in the environment. There has also been evidence of civilisations making use of nanomaterials many centuries ago.

One of the earliest recorded examples of humans using nanotechnology was in 4th-century A.D., when glass made by Romans contained nano-sized metal particles of copper, gold or silver. It has been found that the Roman glass makers were able to finely tune the size and composition of the metal nanoparticles to result in glasses that had particular colours when light either reflected off them or when light passed through them.[2] An example of these types of coloured glass is ruby glass, which is silica glass consisting of c.a. 0.2 weight % of copper and/or gold nanoparticles. The nanoparticles undergo plasmonic effects which results in a high absorption band at visible wavelengths (c.a. 530 and 560 nm), therefore to the glass possess in this brilliant ruby colour and is found to be used in many ancient relics. [3]

Naturally occurring nanomaterials as well as nanocomposites can be found in many places, and they can have both beneficial and detrimental properties. For example, the ashes of erupting volcanoes, are mainly composed of graphitic nano-particles characteristically ranging from 50 to 200 nm in diameter and are freely buoyant in the air. Because of their suspension in the atmosphere, the inhalation of such nanoparticles can lead to severe respiratory disorders following the deposition of these particles in the upper, tracheobronchial and alveolar regions of the respiratory tract. [4] Furthermore, these nano-particles in volcanic ash can act as carriers of toxic elements such as Cd, Sn, Se, Hg, Te and Pb. [5] However quite amusingly, similar graphitic nano-powders, such as activated carbon nanoparticles with a nanoporous strucutre can be used in modern industrial processes to purify both gases and liquids. In such that they are used in ventilation systems or used to purify drinking water by removing hazardous chemicals. [6,7]

In addition to the nano-material alone, combining a nano-material with other (nano-) materials to form a multi-phasic system can lead to the formation of a material known as a nano-composite. Nanocomposites have been researched for over 50 years. Nanocomposites were first mentioned as early as the 1950’s, and plastic based polyamide nanocomposites were reported as early as 1976. [8] Though, it was not until Toyota researchers began a thorough examination of polymer/layered silicate clay mineral composites (known as organoclays) that nanocomposites became more broadly studied in both academic and industrial laboratories. [9]

By adding nanoparticles to a standard material, you can drastically change and improve its properties. It is possible to modify a materials strength, toughness, thermal and electrical conductivity, magnetic properties, charge capacity and even their colour without changing the actual chemical composition of the original material. Nanocomposited often require an addition of only ~5 wt% of nano material to the bulk material to observe these enhanced properties. [10,11]

Nano-composite can be further categorised and can consist of either three-dimensional metal matrix composites, two dimensional lamellar composites and nano-wires of single dimension to zero-dimensional core-shells all representing many variations of nano-mixed and layered material [10].

In recent years, the use of hybrid organic/inorganic nanoparticles in a number of applications has increased due to their multifunctional properties. Subsequently, nanoparticles can be used in many industries, for instance catalytic converters in the car industry, microbial resistant materials in the health divisions, lithium ion batteries, windmill blades, and in the computer industry to make thin-film computer chips. [11]

One of the key advantages of using nanoparticles is that they have a very high surface to volume ratio which considerably changes their properties when equated to the normal sized equivalent. The properties of nanoparticles are dominated by its surface chemistry rather than bulk.  The high surface energy of a nanoparticle also changes the way in which nanoparticles bond with a bulk material. The outcome is that the composite can be improved dramatically with respect to the constituent parts. [12]

1.2 Introduction to magnetic nanoparticles

The root of magnetism comes from the orbital and spin motions of electrons and how the electrons interact with each other. The magnetic behaviour of a material is determined by the number of unpaired electrons in each atom. In atoms, the majority of electrons will exist in pairs with each electron spinning in opposite directions causing them to cancel out each other’s magnetic field, consequently resulting to no net magnetic field. Though, some materials have unpaired electrons which will produce a net magnetic field and therefore have a greater response to an external magnetic field. A magnetic field that is unified can be formed in materials where the magnetic moments of the electrons align with one another. [13] Arrangement of magnetic moments in various types of magnetic materials can be seen in Figure 1 below.

A more detailed explanation of these six different types of magnetism will be discussed below:

Ferromagnetism:

This type of magnetism is only possible when the atoms are aligned in a lattice and the atomic magnetic moments can interact to align parallel with one another. All the materials magnetic moments add to the net magnetisation. Once an external magnetic field is applied to the ferromagnetic material the individual domains are involuntary forced into alignment which they continue to uphold once the external field is withdrawn thus maintaining their magnetism, this is known as remanence. [14,15] Of all the elements in the periodic table only Fe, Co and Ni are ferromagnetic at room temperature and above. When ferromagnetic materials are heated thermal agitation of the atoms occurs, meaning that the degree of alignment of the atomic magnetic moments decreases and thus the saturation magnetisation also decreases. Ultimately the thermal agitation develops and builds so much that the material becomes paramagnetic; the transition at this temperature is referred to as the Curie temperature, TC (Fe =770°C, Co =1131°C and Ni =358°C). [15]

Ferrimagnetism:

Ferrimagnetism can only be found in compounds, they are known to have more complex crystal structures than pure elements. It is a permanent type of magnetism that occurs in solids in which the magnetic fields that are linked with individual atoms spontaneously arrange themselves, some are parallel, like in ferromagnetism, and the others are aligned in an antiparallel state, like in anti-ferromagnetism. The magnetic behaviour of single crystals of ferrimagnetic materials is likely connected to the parallel alignment; the weakening effect of the atoms in the antiparallel alignment keeps the magnetic strength of the materials are in general weaker than that of pure ferromagnetic solids such as metallic iron. [16] Comparable to ferromagnets, ferrimagnetic materials similarly show spontaneous magnetic moments and hysteresis below the materials Curie temperature Tc and acts paramagnetically above the Curie temperature. Ferrimagnetic materials are therefore distinguished from ferromagnetic and antiferromagnetic materials by the structure of their magnetic moments, and they are dependent on the resulting magnetic properties of temperature, which too depend on the varying types of elements in the material and crystal structure. [17]

Anti-ferromagnetism:

Anti-ferromagnetism, is a form of magnetism which is present in solids such as manganese oxide, where adjacent ions that work similar to small magnets that spontaneously align themselves at particularly low temperatures into antiparallel arrangements through the whole of the material so that it displays virtually no net external magnetism. The spontaneous antiparallel arrangement of antiferromagnetic material is disturbed by heating that disappears completely above a specific temperature, this is known as the Néel temperature, which is distinctive of each antiferromagnetic material. [18] When antiferromagnetic material is put through an external magnetic field, you can observe a slight amount of ferromagnetic behaviour. Another similarity to ferromagnetic materials is that antiferromagnetic materials also become paramagnetic above a transition temperature, known as the Néel temperature, TN. Chromium is the only element that demonstrates anti-ferromagnetism at room temperature (Cr: TN = 37 ºC). [19]

Paramagnetism:

Paramagnetism, is where the materials are faintly attracted by a strong magnet meaning they have almost zero net magnetic moment. They have randomly oriented magnetic moments and retain no magnetisation following the removal of the external magnetic field. The majority of elements are paramagnetic, but since their attractive force is many magnitudes weaker than ferromagnetic material they are mainly considered as ‘non-magnetic’. Strong paramagnetism (not to be mixed up with the ferromagnetic elements) is shown by compounds that contain iron, palladium, platinum, and all of the rare-earth elements. In compounds that contain these elements there are some inner electron shells that are incomplete, triggering their unpaired electrons to spin and orbit, consequently making the atoms in a permanent magnet that will align with and strengthen an applied magnetic field. [18,20]

Diamagnetism:

Diamagnetic materials do not have a permanent magnetic dipole moment, as all the electrons in the atoms of diamagnetic materials are all in pairs. When a diamagnetic element such as bismuth or silver is exposed to an external magnetic field, a weak magnetic dipole moment is induced in the opposite direction of the applied field. All materials are essentially diamagnetic, as a weak repulsive force is produced by a magnetic field by the current of the orbiting electron. However, some substances have stronger paramagnetic abilities that outweigh their natural diamagnetic abilities. This is because paramagnetic materials have unpaired electrons, such as iron and nickel. [21]

Superparamagnetism:

It is only found in ferri- or ferromagnetic nanoparticles. Superparamagnetism takes place in nanoparticles; due to their size, they consist of just a single magnetic domain, so the whole nanoparticle aligns with the applied field. Magnetic moments change direction at random, subject to the temperature. The time it takes is called the Néel relaxation time. Without an external magnetic field, the Néel relaxation time is a lot shorter than the time used to measure the magnetisation and the net magnetisation is detected as zero, this phenomenon is called superparamagnetism. The materials can be magnetised through an external field, more strongly than regular paramagnetic materials [16]. They do not hold magnetisation when the external magnetic field is removed. Magnetite nanoparticles are an example of a superparamagnetic material, having various magnetic properties to bulk magnetite [21,22]. Magnetite nanoparticles with particle sizes less than 20 nm are viewed as superparamagnetic as each individual particle is made out of a single magnetic domain. Such superparamagnetic particles display a distinct magnetization curve with no coercive and remnant responses when placed in a magnetic field, a required and important magnetic property in most biomedical applications of magnetic particles. [23,24]

Due the aforementioned reasons, superparamagnetic materials, such magnetite nanoparticles, are of great interest to a wide variety of the research and technological applications. One leading benefit to using magnetic particles overall is that they can effortlessly be removed from the reaction by using an external magnetic field. They have unique properties which allow them to have many different applications such as, the biomedical field, some uses for them are tissue repair by local heating, decontamination of biological fluids, magnetically controlled transport of drugs, also can be used as contrast agents in magnetic resonance imaging [25].  Another application where superparamagnetic nanoparticles could be useful is the magnetic removal of toxic substances from water, replacing complex filter systems that have complex maintenance. The role of superparamagnetic nanoparticles in water-cleaning is the adsorption of damaging substances to the nanoparticles’ surface followed by their consequential removal by using a magnetic field. [26]

1.3  Effect of organic pollutants in water

Organic pollution occurs when high amounts of organic compounds, which act as substrates for microorganisms, are released into water sources. Throughout the decomposition/digestion process the dissolved oxygen in the receiving water may be used up at a greater rate than it can be refilled, this then leads to oxygen depletion and has severe consequences for the stream. Organic sewages also regularly contain great quantities of suspended solids which reduces the light accessible to photosynthetic organisms and, in turn they alter the characteristics of the river bed, rendering it an unsuitable habitat for a lot of invertebrates. Organic pollutants originate from domestic sewage, urban run-off, industrial effluents and farm wastes. Sewage effluents is the greatest source of organic materials discharged to freshwaters. “In England and Wales there are nearly over 9000 discharges that release treated sewage waste to rivers and canals and several hundred more discharges of crude sewage”, the large majority of them tot the lower, tidal reaches of rivers or, by long outfalls, to the open sea. It has been assumed, certainly incorrectly, that the sea has an almost unlimited capacity for purifying biodegradable matter. [27]

The majority of organic pollutants comes from waste produced by tannery, paper, paint and textile industries, the waste contains mainly different types of dyes and are regularly released untreated into water bodies. The number one polluter of clean water is the textile industry, one of the most chemically rigorous industries on the planet. The World Bank estimates that “17 to 20 percent of industrial water pollution comes from textile dyeing and finishing treatment given to fabric” [28]. Some 72 toxic compounds have been recognised in water just from just textile dyeing alone, 30 of which cannot be removed. This causes a major environmental danger to aquatic and human life. Furthermore, water treatment plants are very vulnerable to fouling because of microorganisms that are present in the contaminated water, resulting in higher energy consumption and operational costs [28,29].

Organic pollution occurs when high amounts of organic complexes, which act as substrates for microorganisms, are released into water sources. Water systems with high concentrations of organic matter, can result in low oxygen concentrations in the water (known as aquatic hypoxia) this can result in the death of a wide range of organisms [29]. Organic sewages also regularly contain great quantities of suspended solids, which reduces the light accessible to photosynthetic organisms, and in turn, they alter the characteristics of the riverbed, rendering it an unsuitable habitat for many plant life and invertebrates. In addition, many dyes (such as para-phenylenediamine, lead acetate, and azo-functionalised molecules) and their breakdown products are carcinogenic, mutagenic and toxic to living organism [30]. Dyes, which are irritant to the eyes and skin, can also cause allergies for example; contact dermatitis and respiratory diseases. The presence of the smallest amount of dye in water, can severely affect the quality of water bodies and cause long term damage to aquatic life and the food chain. Organic pollutants originate from domestic sewage, urban run-off, industrial effluents and farm wastes [31].  Figure 2 shows the effect of organic pollutant to the ecosystem via sea water.

Figure 1.2. Effect of the accumulation of one particular organic pollutant known as polychlorinated biphenyl (PCB) in water (world ocean review) [32]

1.4.1. Arsenic ions in water

In the environment, volcanic activity, weathering and erosion of sulphide minerals (such as pyrite, FeS2) are the major emitters of naturally occurring water soluble arsenic.[33,34] Arsenic containing minerals can interact with oxygen or other molecules present in air, water or soil, as well as with bacteria can react with these mineral deposits and cause the release of Arsenic compounds into ground water systems.[35] Many common arsenic compounds are soluble in water, so arsenic can contaminate lakes, rivers, or underground water by dissolving in rain, snow and through waste produced by industry. Therefore, arsenic contamination in ground water is a serious public health threat worldwide.[36] Ground water is a major source of drinking water, and elevated concentration of arsenic in ground water has been associated with various negative health effects in humans [37]. Arsenic in drinking water is considered to be one of the most significant environmental causes of cancer in the world [38].

In drinking water, arsenic is frequently found to be in two different oxidation states such as arsenic (III) (arsenite) or arsenic (V) (arsenate) as shown in figure 3. Common arsenate and arsenite substances are soluble in water.[39].The discharge of arsenic from industrial wastewater into groundwater causes significant contamination, require appropriate treatment before it is to be used as safe drinking water.[40]

Inorganic arsenic is a well-known carcinogen and is the most abundant toxin that is found in drinking water globally. Inorganic arsenic compounds are extremely toxic while organic arsenic compounds are moderately harmful to health. The first symptoms of being exposed to unnatural high levels of inorganic arsenic is commonly first observed in the skin such as pigmentation changes, skin lesions and rough patches on the surface of palms and the soles of the feet [41]. This usually only happens only after a minimum exposure of approximately five years and might eventually lead to skin cancer. Long-term exposure to arsenic might cause cancer to many other organs such as the lungs and bladder [42]. Figure 4 shows the effect of arsenic pollution to public health.

Figure 1.4. Effects of Arsenic on the human body (Humans right watch 2017). [43]

1.4.2. Methylene blue in water

Methylene blue (methylthioninium chloride) is a compound that forms dark green crystals or crystalline powders. Methylene blue is commonly used in the laboratory as a bacteriologic stain, or as a dye for its distinct light absorption properties. It also has a range of medicinal and biological functions, for example it can be used as a treatment to cyanide poisoning and it also assists in lowering the levels of Methemoglin by reducing the ferric (Fe3+) ion back to the ferrous (Fe2+) state and therefore re-establishing levels of functional haemoglobin.[44,44] Methylene blue is a toxic dye that is regularly used for dyeing cotton, wood and silk [45]. It has negative effects on the human body as it can cause vomiting, nausea, diarrhoea.[46] Therefore, controlling the level of methylene blue from wastewater is very important environmentally.

Due to the reasons listed above, for experimental purposes, Methylene blue acts as an ideal molecular standard for screening and testing water purification processes, particularly designed to remove organic pollutants.

Many physicochemical approaches have been used to remove dyes from textile effluents, such as chemical precipitation, nano-filtration, and coagulation-flocculation.[47,48] Since there is a growing demand for it in many industries such as the pharmaceutical industry, chemical industry and biological industry, the removal techniques for methylene blue (and other common organic compounds found in aqueous water) is imperative.

To be able to capture pollutants before they accumulate in a food chain is crucial, as you prevent the build-up of many different toxic chemicals such as organic dyes from entering the food we eat every day and to reduce the amount of naturally occurring arsenic in water is essential to people who live in these areas. Therefore, by constructing the proposed SPION it can be effective in dealing with these worldwide problems.[49]

1.5. Magnetites in water purification

Magnetite is the name given to a highly earth abundant, low cost and non-toxic iron oxide mineral with the formula Fe3O4. It is composed of Fe2+ and Fe3+ cations, subsequently magnetite is known for being ferrimagnetic (hence the name) and exhibits supermagnatism at the nanoscale, such that magnetite bulk and nano-particles exhibits a strong attraction to a magnetic field. Hence, due to the aforementioned reasons, magnetite is an ideal inorganic material candidate to use as a superparamagnetic nanoparticle for water remediation. Due to the inherent mixed oxidation states of iron (Fe2+/Fe3+) in magnetite it provides useful chemistry for the functionalisation of the surface. The Fe2+ cations can act as reductive species for the chemisorption (adsorption) of functionalised molecules, but also can be used for the reduction/ degradation of chemical pollutants. [50]

The use of magnetites for removal of water contaminants has already been recognised from both the research level, [52] and has also become to be established at the industrial scale. [53] SIROFLOCTM is one example of an established sewage treatment process which utilises magnetite nanoparticles for the removal of suspended solids and organic materials at a scale of processing >5ML/day of waste water. [54]

A particular property of magnetite which has already been eluded to is its stability and reactivity, particularly in aqueous environments. Although the high surface reactivity of iron oxide nanoparticles, such as magnetite, is favourable for functionalisation, it can also lead to erosion and complete solvation or decomposition of the magnetite nanoparticles.

An initial approach to understanding the stability of a material in aqueous medium is to refer to the Pourbaix diagram of that particular system. The Pourbaix diagram, can be interpreted, particularly by electrochemist like a standard phase diagram, where is shows the stability of a particular metal oxide, metal hydrate or metal cation species in aqueous electrolytes under different applied potentials. The Pourbaix diagram shows at which pH (x-axis) and under which applied potential (y-axis) referenced to a reversible hydrogen electrode (RHE), species is thermodynamically stable, but does not reveal the effects of kinetic stability. Figure 7 shows the Pourbaix diagram for iron in aqueous medium, and its clear that magnetite is relatively stable in alkaline to near neutral pH, but likely to form soluble species in acid electrolytes under positively applied potentials.[55]

Another important value particularly when addressing the stability of nanoparticles, is the point of zero charge (PZC) or isoelectronic point (IEP). With most metal oxides, such as iron oxide, in aqueous systems the surfaces are hydrated to form Fe-OH groups. These hydrate terminations can react with H+ or OH− ions. Depending on the pH, positive (Fe-OH2+) or negative (Fe-O) charges can develop on the surface of the nanoparticle. In the absence of specific adsorbing ions, such as a functionalised molecule, amphoteric metal oxides have a characteristic pH, where there is a point of zero charge (PZC) at the surface such that the concentration of positive and the negative sites are equal. Therefore, a pH above pHPZC will result in negatively charged particle surfaces and below pHPZCthe surface is positively charge. This value is significant and helpful when determining how stable the colloidal dispersion of a nano particle is as well it will determine how well a particular adsorbate ion will interact with the surface of the metal oxide nanoparticle. For ground magnetite the pHPZC is ~6.5. [56]

Magnetite’s have high potential in distinguishing and targeting bacteria for water purification, it depends on whether you can stabilise the magnetite and enhance its coating efficiency on the bacterial cell surface.[57].They have a high capability and efficiency in the capture of high concentrations of organic substances and multiple metal ions, because of their very large surface area. [57] Furthermore magnetite nanoparticles can be inserted directly into polluted ground and can be used to capture biological pollutants (such as bacteriophage), which can be easily removed by use of a magnetic field.[59]

1.6. Effect of surface coating and attaching ligands to magnetite nanoparticles

In order to improve the stability, as well as adjust the surface chemistry, core shell nanoparticles can be synthesised, which consist of a magnetite core coated with a more stable metal oxide material, such as zirconia (ZrO2),[REF] titania (TiO2), or silica (SiO2).[60,61]

Attachment of ligands is used to increase the sorption properties of the silica-coated magnetite nanoparticles in solid phase removal processes and is typically attained by the surface silanization, using silane-coupling agents, with a thiol, amine, isocyanate or carboxylic acid moiety.[62] For coated nanoparticles, the surface functionality plays a key role in the interaction between nanoparticles and biological bodies.[63] It is known that silica coating is one of the most prevalent techniques for nanoparticle surface alteration; it improves colloidal stability, biocompatibility and surface bio-conjugation.[64,65]

Particular functional groups attached to the surface of the nanoparticle can be selective for different metal cations (Cd2+, Cr3+/6+, Cu+/2+, Hg2+, Ni2+, Pb2+, etc) or anions (AsO33-, AsO43-, etc), organic molecules, and biological pollutants (bacteria, bacteriophage, etc). It has been shown that Fe3O4/SiO2 magnetic nanoparticles functionalised by silanisation and attachement of aminopropyl groups can efficiently adsorb As5+ and As3+ from polluted ground water. Initial studies by J. Saiz et al have shown that maximum adorption capacities of 127 mgAs5+g-1 and 14.7 mgAs3+g-1 were achievable at a pH of ~8. [67]

Clearly these nano-compositie systems are highly promising and aim to deliver novel,  more efficient, and practical purification processes for the elimination of organic and inorganic pollutants, such as heavy metals and organic dyes from ground water. However, further understand of how these nanoparticles can be optimised functionalised and stabilised in a range of aqueous electrolytes under different conditions is imperative. [68

1. Materials and Methods

2.1 Materials

2.1.1 List of chemicals used

1. Methylene blue
2. Fe3O4 (Iron magnetic Nanoparticles)
3. Iron(II) chloride tetrahydrate
4. Iron(III) chloride hexahydrate
5. Ammonium hydroxide
6. Iron Sulphate
7. Potassium hydroxide
8. Potassium nitrate
9. (3-Aminopropyl)-triethoxysilane
10. Glutaraldehyde
11. THF (Tetrahydrofuran)
12. Toluene
13. Methanol
14. L-Cysteine ((2R)-2-amino-3-sulfanylpropanoic acid)
15. L-Glutathione
16. ICPTES (3-(Triethoxysilyl)propyl isocyanate)
17. Sodium meta arsenite (NaAsO2)
18. Sodium arsenate dibasic heptahydrate (Na2HAsO4 · 7H2O)
19. Distilled, deionised water
20. 4-Nitrobenzaldehyde

2.1.2 Consumables

a)     1.5 mL Eppendorf tubes

b)     15 mL Falcon tubes

c)     50ml Falcon tubes

d)     Nitrile free gloves

e)     Glass pipettes

f)       1ml pipette tips

2.1.3 Equipment

1. Adjustable 10-1000μL pipette
2. Magnetic stand
3. Fridge
4. Incubator
5. Oven
6. Rotator
7. UV Visible Spectrometer
8. FT-IR (Fourier-transform infrared spectroscopy)
9. VSM (Vibrating sample magnetometer)
10. SEM (Scanning electron microscope)
11. CHNS elemental analyser
12. ICP-OES (Inductively coupled plasma – optical emission spectrometry)

2.2 Methods

2.2.1 Synthesis of Iron oxide magnetic nanoparticles

2.2.1.1 Iron Chloride method

Fe3O4 magnetite was prepared using ferrous and ferric chloride in the presence of an alkaline solution. Iron(III) chloride hexahydrate (0.4070g) was dissolved in 100mL distilled, deionised water in a 250ml litre round-bottomed flask with a magnetic stirrer. [69] To this, iron(II) chloride tetrahydrate (0.2080g) was added whilst stirring. The solution appears to be brown/orange, this was left to stir for 30 minutes. Afterwards at room temperature while stirring 25 mL of ammonium hydroxide solution was added gradually over the course of 30 minutes with continuous stirring, the solution turns pitch black. The reaction continued with stirring at room temperature for another 30 minutes. Subsequently, the solution was transferred to a 250 mL conical flask and the black precipitate was washed using distilled, deionised water, so the pH can reach neutral, it was done by using magnetic sedimentation and decantation through the use of a slab magnet. The product formed was a dark red colour, and it showed a weak response to the slab magnet. The reaction process is shown below.

Reaction scheme 1 showing the co-precipitation of Iron(II) and Iron(III) chloride. [70]

2.2.1.2 Iron Sulphate method

This alternate method involves the oxidative hydrolysis of iron(II) sulphate heptahydrate under nitrogen at 90ºC. Iron(II) sulphate heptahydrate (2.003g) was dissolved in 100 mL deionised, distilled water in a 250 mL round-bottomed three-necked flask with a magnetic stirrer, condenser and thermometer while under a nitrogen environment. [69] This solution was heated to 90oC with continuous stirring under nitrogen. To this reaction mixture, potassium nitrate (0.539g) and potassium hydroxide (1.242g) were added to the pale green solution whilst stirring. The reaction mixture turns jet-black and was kept at 90ºC and was still kept in a nitrogen environment for 4 hours with stirring. Afterwards, the mixture was left to cool for 1 hour and then was moved to a 250mL litre conical flask. The resulting black precipitate was washed to neutral pH with distilled, deionised so the pH can reach neutral, it was done by using magnetic sedimentation and decantation through the use of a slab magnet. The product formed was black in colour and had a strong attraction for the slab magnet when compared to the other method. This process is shown below.

Reaction scheme 2 showing the oxidative hydrolysis of Iron(II) sulphate. [71]

2.2.2      Addition of a silicon layer to the magnetite nanoparticle

Silica-coated magnetite nanoparticles were prepared via the small-scale deposition of silica onto magnetite nanoparticles, from silicic acid solution at pH 10: [72]

Aqueous sodium silicate solution, 4.15g was dissolved in water to a total volume of 100 mL with distilled, deionised water. A column containing 22 g of Amberlite IR-120 ion-exchange resin was regenerated with 200ml each of the following: Hot water (70ºC), 3 M HCl and cold water. Sodium silicate solution was passed down the column, allowing the first 10 mL to pass uncollected. The subsequent 90 mL of the eluate (now in the form of silicic acid) was taken and it’s pH was immediately raised to pH 12 with aqueous TMAOH (25%) to prevent homogeneous silica nucleation.

In addition to this, 1g of magnetite suspension was mixed with 45 mL fresh distilled, deionised water and titrated to pH 12 with aqueous TMAOH (25%). With continued stirring, the silicic acid eluate (at pH 12) was added to the magnetite suspension. The mixture was then slowly titrated to pH 10.0 using 0.5 M HCl over approximately 1 hour. The mixture at pH 10.0 was stirred for another 2 hours at room temperature before transferring to a 1 litre conical flask. The silica-magnetite nanoparticles were washed once with 200ml of TMAOH solution at pH 10.0 and then many times with 200ml litre distilled, deionised water via magnetic sedimentation until the supernatant reached neutral pH.

2.2.3 Amine functionalisation of magnetite nanoparticles

Amine functionalisation was carried out using (3-Aminopropyl)-triethoxysilane. Firstly, 8.0ml of silica coated nanoparticles was added to a 50 mL falcon tube. Excess water was removed by using magnetic separation through the means of a slab magnet. After 40 mL of THF was added and 120 µl of APTS was added to the reaction mixture in the falcon tube then the tube was sealed. Then it was subsequently placed in an incubator with end-to-end rotation at 25

$°C$

at an RPM of 25 for 20 hours. Afterwards, the nanoparticles were magnetically immobilised, and the solvent was separated from the solid. They were washed with 20 mL THF and again magnetically immobilised to remove the solvent. The nanoparticles are then washed 3 times with 20 mL of methanol, and then stored in 20 mL of the same. This product is then stored in a fridge until further use.

2.2.4. Surface Amine Density Assay

Nanoparticles (5 mg) were placed in a 1.5 mL Eppendorf tube and washed (x4) with 1 mL of coupling solution [0.8% (v/v) glacial acetic acid in dry methanol]. Subsequently, 1 mL of 4-nitrobenzaldehyde solution (7 mg in 10 mL of coupling solution) was added to the particles and the suspension was allowed to react for 3 h with gentle end-over-end rotation.

After removal of the supernatant and washing (x4 in 1mLof coupling solution), 1mLof hydrolysis solution (75 mL of H2O, 75 mL of methanol, and 0.2 mL of glacial acetic acid) was added to the particles and the tube was shaken for a further hour.

The supernatant was then removed from the particles with a magnetic separator and its absorbance was measured at 282 nm. The amount of 4-nitrobenzaldehyde in the hydrolysis solution was calculated by interpolation, by use of a calibration curve constructed from a range of standard solutions of 4-nitrobenzaldehyde prepared separately.

2.2.5. Modifying the silica layer to add ligands to its surface

2.2.5.1. Glutaraldehyde method

SSC buffers (1x and 13x) were prepared by diluting a stock solution of SSC buffer (20x) (175.3 g of NaCl, 88.2 g of sodium citrate, and 1 L of H2O, pH 7.4) with distilled, deionized water, adjusted to pH 7.4, and autoclaved before use. Glutaraldehyde solutions were prepared immediately before use. Modified nanoparticles (2 mg) were washed (x3) with 1 Ml of coupling buffer (1xSSC buffer, pH 7.3) for 2 min at 18 °C.

After removal of the supernatant, 0.5 mL of a 5% (w/v) glutaraldehyde solution in coupling buffer was added and the suspension was incubated for 3 h with end-over-end rotation at 18°C. The material was subsequently washed 3 times with 1 ml of coupling buffer to remove excess glutaraldehyde. A 3.3 uM solution (0.5 mL) of 5¢-amine-modified dT25 was added and the mixture was left incubating overnight with shaking.

The oligomodified nanoparticles were then washed once with coupling buffer and placed in 0.8 mL of NaBH4 solution [0.03% (w/v) in coupling buffer] for 30 min at 18 °C. The material was then washed 3 times with 0.8 mL of coupling buffer and finally resuspended in 200 íL of the same.

2.2.5.2 ICPTES method

For the surface modification of Fe3O4@SiO2, the as-prepared coated nanoparticles (0.2 g Fe3O4@SiO2), this is washed 3 times with THF and then once with toluene, then they are dispersed in dry toluene (40 mL) and, to this dispersion, 0.1 mL 3-(triethoxysilyl) propyl isocyanate (ICPTES) was added. The reaction mixture was refluxed for 24 h, at 110 °C. Subsequently, the solvent was evaporated and the precipitate was washed several times with warm methanol and then collected with a Nd–Fe–B magnet.

Chemical grafting of cysteine onto functionalized silica surface of magnetite nanoparticles was carried out in tetrahydrofuran (THF) as follows: 0.5 g of cysteine was dissolved in 50 mL THF and then, 0.2 g of Fe3O4@SiO2@ICPTES were added to the solution. Under reflux, for 5 h at 67 °C, the isocyanate group in the alkoxysilane precursor ICPTES reacted with the amine group in cysteine to form an urea derivative anchored onto silica surface. The resulted Fe3O4@SiO2@ICPTES-cysteine nanoparticles have been magnetically separated and further washed with methanol and deionized water.

1. Results

3.1 VSM analysis of magnetite

Figure 3.1: Shows VSM analysis of the Iron sulphate method magnetite (), and the Iron chloride method magnetite (). These curves displays the superparamagnetic properties of the magnetite, as the magnetic remanence is almost zero and there is high magnetisation saturation.

The Iron sulphate method magnetite is the preferred core of the functionalised nanoparticle, as it displayed a stronger magnetic affinity than the iron chloride method magnetite after it was synthesised and its curve is closer to the ideal curve which represents superparamagnetic behaviour. This is an important in step in the synthesis, as without superparamagnetic properties we would not be able to remove the magnetite from the water using an external magnetic field, after the magnetite has adsorbed its target pollutants.

3.2 FT-IR analysis and reaction pathway

Figure 3.2 FT-IR spectra of a) silica coated NPs, b) cysteine modified amine functionalized NPs, c) cysteine modified glutaraldehyde functionalised NPs, d) cysteine modified NPs and e) cysteine + 3-Isocyanatopropyltriethoxysilane (ICPTES) modified nanoparticles. Note the reaction pathways to attach cysteine to the nanoparticles are different. The glutaraldehyde functionalized NPs involves a two-step method to attach cysteine (b < c < d), whereas the ICPTES modified NPs involves a single step to attach cysteine (b < e). The FTIR could not be taken for the intermediate step of the ICPTES method as the process is a series of continuous reactions.

Summary of FT-IR spectra for the different functionalised NPs:

Table 1: Assigning the FT-IR peaks from Figure 3.2, to show the functional groups that are present at the start and the ones that are added during the course of the synthesis. Identifying the peaks show us if the method to reach the final product was successful. Furthermore, it shows the slightly different reaction pathways the two methods took in terms of functional groups found by the FT-IR spectra.

 Functional groups found in FT-IR spectra Silica coated NP’s a) Amine functionalised NP’s b) Glutaraldehyde functionalised NP’s c) Cysteine + ICPTES functionalised NP’s d,e) Fe-O bond: 450-550 cm-1 ✔ ✔ ✔ ✔ Si-O bond: 1000~ cm-1 ✔ ✔ ✔ ✔ Fe-O-Si: 1100 cm-1 ✔ ✔ ✔ ✔ C-C bond: 1130 cm-1 ✔ ✔ ✔ ✔ N-H bend: 1600~cm-1 ✔ ✔ ✔ C-N: 1000-1300 cm-1 ✔ ✔ ✔ N-H stretch: 2900~cm-1 ✔ ✔ ✔ CHO bond C=O stretch: 1690-1740 cm-1 ✔ COOH bond O-H stretch: 2500-3000 cm-1 ✔ COOH bond C=O stretch: 1710-1780 cm-1 ✔ C-S bond: 570-710 cm-1 ✔

Figure 3.2.1:  Showing the reaction pathway of the glutaraldehyde method

Figure 3.2.2:  Showing the reaction pathway of the ICPTES method

3.3. Detecting separation of arsenic ions from water using inductively coupled plasma atomic emission spectroscopy (ICP-OES)

Figure 3.3.1:  Showing the reduction of arsenic concentration at 189nm

Figure 3.3.2: Showing the reduction of arsenic concentration at 193nm

Figure 3.3.1 and 3.3.2 shows the decrease in Arsenic concentration over a period of one hour, all the magnetites have the same starting concentration. Arsenic is detected at 189nm and 193nm, you can see that all the magnetites effectively reduce the concentration of both arsenic 189nm and 193 nm in water. From the graph you can see that the glutaraldehyde method magnetite is the most effective in reducing the amount of arsenic at 189nm present in water, closely followed by the ICPTES method magnetite. However, surprisingly Fe3O4@SiO2 magnetite was also effective in reducing arsenic concentration in water but not as effective as the other methods. For arsenic at 193m, all the magnetites perform very similarly this shows the attachment of L-cysteine ligand may not help that increase performance in removing arsenic at 193 nm. As the Fe3O4@SiO2 has all almost identical performance in reducing arsenic concentration at 193nm.

3.4. Detecting separation / decomposition of methylene blue in water using Ultraviolet–visible spectroscopy (UV-Vis)

Figure 3.4 shows the absorbance of methylene blue at 668nm. Similarly like in ICP-OES results you can see that the Fe3O4@SiO2 magnetite performs well in reducing the concentration of methylene blue in water. For the glutaraldehyde method and ICPTES method magnetites you can see that there performance is almost identical, with the ICPTES method magnetite preforming just slightly better at removing the amount of methylene blue present in water.

1. Conclusion

This study has showed the development of cysteine functionalised superparamagnetic nanoparticles (CFSNP) to remove arsenic ions and methylene blue is successful. As both methods of producing CFSNP have shown to be effective at both reducing the concentration and removing these pollutants from water. The results from the ICP-OES and UV-Vis both confirm this outcome. It is important to look at the results produced by the Fe3O4@SiO2 magnetite as it performed considerably well at reducing arsenic concentration in water, and the separation and decomposition of methylene blue. These results could mean one of many things.

It might be that the attachment of the L-cysteine ligand to the Fe3O4@SiO2 magnetite does not increase its effective at reducing concentrations of both reducing arsenic and methylene blue concentrations in water. Firstly, it could be that the property of the CFSNP increased surface area is negligible as the ligand concentration on the surface might not be high, to be able to significantly impact the binding of arsenic ions to it and the separation and decomposition of methylene blue. To add to, if the concentration of ligands is indeed negligible the results from the ICP-OES can show that there is little difference between the Fe3O4@SiO2 magnetite and CFSNP, you could conclude that there might be a difference made if we are able to increase that concentration of the ligand on the surface of the nanoparticle. However, the results from the methylene blue separation and decomposition are not as close the results from the separation of arsenic ions from water.

Moreover, we can conclude that the ability of the CFSNP is successful at removing our target pollutants from water, but not that much more effective than the Fe3O4@SiO2 magnetite. Considering the extra cost involved in terms of time and money to reach the cysteine functionalised superparamagnetic nanoparticle, it can be seen as not efficient of a method at removing the target pollutants from water. Though further investigation would be required to make that final conclusion, to get a more accurate conclusion of this study we would have to see if we can see if the ligand concentration on the surface can make a significant impact at reducing concentration levels of the pollutant, this would also help out in finding if cysteine is really an effective enough ligand. A comparison with another similar ligand would also be an effective way to see if cysteine is the most appropriate ligand to attach to the surface the superparamagnetic nanoparticle.

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