GB2464998A - Bacterial process for the preparation metal-doped magnetite nanoparticles - Google Patents

Abstract

A process for preparing magnetite nanoparticles incorporating one of more transition elements comprising exposing a ferric iron source to a culture of dissimilatory iron-reducing bacteria (DIRB) and/or a sulphate-reducing bacteria (SRB) in the presence of a redox mediator. The preferred ferric iron source is ferrihydrite, the preferred bacterium is G. sulfurreducens, the preferred redox mediator is an electron shuttle such as anthraquinone-2,6-disulfonate (AQDS). The preferred source of the metal dopant is sodium tetrachloropalladate (Na2PdCl4). The resultant metal-doped magnetite nanoparticles may be coated with a biomolecular coating derived from the bacterium. The metal-doped magnetite nanoparticles are useful catalytically, especially in the Heck reaction between iodobenzene and either styrene or ethyl acrylate. For this reaction, the metal-doped magnetite nanoparticles are more effective catylysts than commercially available colloidal palladium catalysts.

Description

MAGNETICALLY RECOVERABLE CATALYSTS

The present invention provides a process for producing iron minerals, and in particular, magnetite from a ferric iron source. More particularly, the present invention provides a process for producing such minerals via a biological means. The invention is therefore concerned with the production of biogenic magnetic minerals produced by microbial activity. More particularly, magnetite produced by the process of the present invention is in the form of crystalline particles. These biogenic particles may have a diameter of approximately 1 nm to approximately 50 nm. When the biogenic magnetite particles have a diameter in the nanometre range, i.e. approximately 1 to approximately 50 nm, the particles may be referred to as "biogenic nanomagnetite particles" or simply as "nanomagnetite particles".

The present invention thus relates to the preparation and use of iron-based magnetic nanoparticles and their use in the preparation of catalysts. In one preferred variant, the invention is concerned with the preparation of nanoscale magnetite particles which are produced by culturing bacteria with an iron based mineral. The bacteria used in the process of the invention are dissimilatory iron-reducing bacteria. The resulting nanoparticulate iron mineral is then reacted with a solution of a transition metal compound to produce a nanoparticulate iron-based catalytic material. The resulting catalytic material can be used in a number of chemical reactions where transition metals and compounds thereof are conventionally used as catalysts. One such example is in the Heck coupling reaction. The catalytic material of the present invention is magnetic and hence can easily be recovered after use.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Nanoparticles make highly desirable catalysts, often offering unique properties linked to their very high surface area. Magnetic nanoparticles are particularly useful support materials for catalysts as they can combine the advantages of a high dispersion through a liquid with ease of recovery. Palladium, and other platinum group metals, placed on a suitable support material, make exceptionally useful catalysts for mild reaction conditions that have excellent compatibility with many polar functional groups and a high degree of chemo-, regio-and even stereoselectivity.

Synthetic magnetite nanoparticles have been coated previously with precious metals such as palladium to give a catalyst which is potentially recoverable with a large reactive surface for use in the Heck Reaction and hydrogenation reactions.

Wang et al described the synthesis of palladium-coated magnetic nanoparticles and their application in the Heck Reaction in Colloids and Surfaces A: Physicochem. Eng.

Aspects 276, 116-121, 2006. Similarly, Rossi et al describe super paramagnetic nanoparticle-supported palladium as a highly stable magnetically recoverable and reusable catalyst which can be used in hydrogenation reactions in Green Chemistry, 9, 379-385, 2007.

Baruwati et al (Organic Letters, 2007, 9(26), 5377-5380 describes palladium on surface modified NiFe2O4 nanoparticles as a magnetically recoverable catalyst suitable for Suzuki and Heck reactions. The NiFe2O4 nanoparticles are synthesised by a hydrothermal route at 225°C. Anchoring of dopamine molecules on the surface of the as-synthesised NiFe2O4 nanoparticles was achieved by a ref luxing for 12 hours in aqueous medium. Pd(0) nanoparticles were then anchored on to the surface-modified NiFe2O4 nanoparticles by reducing Na2PdCI4 with hydrazine monohydrate. This process therefore requires extreme temperatures to produce the nanoparticles in the first place and also does not directly anchor the palladium metal to the magnetic substrate.

Attempts to make these materials using conventional chemical approaches have achieved varying degrees of success. In addition the protocols developed are relatively complicated, often including steps to functionalise the surface prior to the addition of precious metal coating. Additional problems have included loss of the precious metal during the recovery and recycling steps.

Thus, there are a number of problems with these prior art systems. The synthesis of such prior art catalysts is difficult, expensive, and may require the use of high temperatures. The resulting catalysts may not have a good turnover or lifetime.

There is therefore a need to provide a convenient route by which magnetic nanoparticulate catalysts can be produced. There is also a need for such catalysts to be able to be produced both economically and on a commercially viable scale if desired. In addition there is a need for a process which is robust, reliable and reproducible.

The present invention satisfies some or all of the above needs which are presently unmet by prior art processes. The present invention therefore provides a convenient route to magnetic nanoparticu late iron-based minerals which can then be converted to nanoparticulate catalytic materials following reaction with a transition metal compound.

We have found that biological routes can be harnessed to make nanoparticles of iron minerals, such as magnetite, efficiently and at low cost. This can be done with a degree of control regarding the rate of reduction, for example through the addition of redox mediators. We have also found that the magnetic properties of the end product are adequately retained despite substitution by transition metals. The resulting iron magnetic nanoparticles, otherwise referred to as the biogenic magnetic nanoparticles, bearing a catalyst have excellent activity.

The resulting biogenic material may also be crystalline. Microbes can be used in accordance with the invention to lay down a number of mineral phases under ambient conditions. The process of the invention is applicable to a number of catalytically active transition metals.

According to a first aspect of the present invention, there is provided a process for preparing magnetite nanoparticles incorporating one or more transition metal elements, the process comprising the steps: (a) exposing a compound or mixture of compounds as a source of ferric iron to a microbial system in a culture comprising a dissimilatory iron reducing bacteria (DIRB) and/or a sulphate reducing bacteria (SRB) in the presence of a redox mediator, (b) incubating the resulting mixture in a culture under anaerobic conditions to produce biogenic nanoparticles of an iron-based mineral, (c) separating the nanoparticles from the culture and optionally purifying the nanoparticles, (d) adding a solution containing one or more transition metal salts to a suspension of the nanoparticles to produce catalytic nanoparticles, and (e) recovering the catalytic nanoparticles.

The nanoparticles are preferably magnetic.

The source of iron can be an iron-based compound or a mixture of iron-based compounds which are capable of reacting with the bacteria to form magnetic nanoparticles.

In an embodiment, the source of iron is ferrihydrite.

In another embodiment, the source of iron is a waste iron material that would otherwise be land filled at large commercial and environmental cost. Examples of sources of Fe(III)-rich waste include acid mine drainage (AMD) sites and also water polishing steps in the water industry.

In an embodiment, step (c) does not include purifying the nanoparticles. In other words, in this embodiment, the nanoparticles are simply separated from the culture and the solution containing one or more transition metal salts is added immediately to the nanoparticles. In another embodiment, step (c) includes purifying the nanoparticles by washing.

In an embodiment, the redox mediator is an electron donor and an electron acceptor.

In an embodiment, the magnetic nanoparticles produced on exposure to the bacterial culture is one or more selected from the group comprising Fe304 (magnetite), Fe3S4 (greigite) and Fe7S8 (pyrrhotite). In a preferred embodiment, the magnetic material is magnetite.

In another embodiment, the bacterium is preferably a DIRB.

In an alternative embodiment, the bacterium is preferably a sulphate-reducing bacteria.

A number of suitable bacteria are described in the reference Microbiology Monograph (3), D Schüler: Magneto Reception and Magneto Somes in Bacteria DOl 10.

1007/71 71_047/published on line: 8 September 2006. The contents of this document in relation to dissimilatory iron-reducing bacteria and sulphate-reducing bacteria are intended to be incorporated herein as part of the present invention and form part of the 1 5 disclosure of the present invention in relation to the microbial process. Thus, preferred bacteria of the present invention are described on pages 278 to 282 of that reference.

Fig 2 of the review by Coker et al on page 279 shows a phylogenetic tree which presents a range of dissimilatory iron-reducing bacteria that can be used to synthesise magnetic nanoparticles. These bacteria specifically form part of the process of the present invention. Bacteria of particular interest in the present invention include the Geobacter species, Shewanella species and Geothrix species. In addition archaea, eg hyperthermophiles could also be used.

Preferred dissim ilatory iron-reducing bacteria include Geobacter, Shewanella and Geothrix.

Preferred sulphate-reducing bacteria include Desulfovibrio, Desulfococcus, Desulfobacter, Desulfobulbus and Desulfobotulus.

The nanoparticulate matter is produced from the iron source at around room temperature, typically 20°C using a range of different iron (III) reducing bacteria.

However, the process can be carried out at temperatures between 10° and 40°C. There is no need for elevated pressure, unless a barophilic organism is used.

The resulting magnetic nanoparticles may be recovered from the bacterial solution after incubation and washed. The material is then introduced into a solution containing one or more transition metals in the form of their salts. The magnetic iron nanoparticles are able to absorb and reduce the metals from the transition metal solution thereby forming particles in the range of ito 10 nm and preferably in the range of 5 to 7 nm of the transition metal on the magnetic nanoparticles. The magnetic nanoparticles have a size in the range of 15 to 50 nm and are preferably of 20 to 30 nm in size.

The culture may be incubated with the transition metal solution under anaerobic or aerobic conditions depending on whether an oxide or a zero-valent metal is required. In such a case, there is no separate step of recovering and purifying the particles. This is done for a period of between 8 and 72 hours. More preferably the incubation period is from 12 to 48 hours.

1 5 In a preferred embodiment, the final concentration of the transition metal or transition metals is between 0.5 and 40% by mass of the magnetic nanoparticles, and is preferably in the range of 1 and 20%. More preferably the final concentration of the transition metal or metals is in the range of 2 to 10% by mass, and most preferably it is in the range of 5 to 10% by mass.

In an embodiment, the source of the catalytic material is one or more transition metal salts. The transition metal salt is in aqueous solution or an aqueous-based solution.

The transition metal salts are reduced in solution by the magnetite to the transition metal itself. The transition metal solution may contain a single transition metal salt or a mixture of transition metal salts. A mixture of transition metal salts may be reacted using a single solution containing more than one transition metal salt, or by using more than one solution each containing a single transition metal salt.

The transition metal salt is a salt of one or more metals selected from the group comprising: palladium, platinum, rhodium, iridium, silver, gold, ruthenium, osmium, nickel and titanium. More preferably the transition metal is one or more selected from palladium, platinum or iridium.

The catalysts of the present invention have a number of advantages. Experimental tests show that the catalysts are more active than transition metal catalysts deposited by anaerobic bacteria in the absence of magnetic nanoparticles or are more active than transition metal colloids. For example, it can be seen from the figures that the palladium-magnetite complex is more active in Heck coupling than palladium which has been deposited using anaerobic bacteria or palladium colloids.

The use of more than one transition metal salt allows the production of nanoparticle alloys on the surface of the magnetic particles. X-ray diffraction provides evidence that alloys are being created on the surface of the magnetic particles rather than simply effecting deposition of two individual metals.

Two routes are possible for the biological synthesis of nano-scale magnetite. First, magnetotactic bacteria are able to synthesise intracellular crystals of single domain magnetite which are used to orientate the cell with the Earth's magnetic field, helping the organism guide itself to the sediment water interface. Although minor levels of substitution can be achieved using these organisms, growth yields and indeed the final yields of intracellular magnetite are generally low. In contrast, dissimilatory Fe(lll)-reducing bacteria such as Geobacter species make copious quantities of extracellular nano-scale magnetite through the respiration of poorly crystalline Fe(lll) oxides and oxyhydroxides. These specialist anaerobic bacteria live in environments depleted of oxygen and therefore conserve energy for growth by transferring electrons from the oxidation of simple carbon sources such as acetate to Fe(lll) and Mn(IV)-bearing minerals.

The mechanism of this transformation involves the formation of magnetite through the extracellular reduction of the Fe(lll) mineral phase causing the release of soluble Fe(ll), and ultimately resulting in complete recrystallization of the amorphous mineral into a new potentially high-value magnetic phase. Most importantly these enzyme-driven reactions take place rapidly at ambient pressures and temperatures, using inexpensive feedstocks.

Previously Fe(lll)-oxyhydroxides converted to magnetite by Geobacter sulfurreducens have also been altered to include significant quantities of additional transition metals such as Co and Ni forming nano-ferrites thus optimising magnetic properties for different commercial applications and also for bioremediation through the reduction of toxic metals and radionuclides such as Cr(Vl) and Tc(Vll), and reductive dehalogenations of xenobiotic organics contaminants. These nano-scale biomagnetite crystals could also be very useful as support materials in accordance with the present invention for industrial catalysts.

The present invention enables biomagnetic nanoparticles based on iron minerals to be prepared by microbial reduction which can then be functionalised with one or more transition metals to produce a nanoscale magnetic catalytic particle. For example, using palladium and magnetite we are able to produce bio-magnetite functionalised with palladium nanoparticles with a minimal level of downstream processing.

The catalyst can then be used in the Heck coupling of a range of materials such as, for example, iodobenzene with styrene or ethyl acrylate. Heck chemistry is of wide-ranging industrial importance as it provides one of the simplest routes to substituted olefins, more traditionally via a palladium-phosphine catalyst although a large amount of literature is devoted to the study of a variety of different catalysts for these reactions.

We also believe that this simple, energy efficient route to novel biologically-derived heterostructures can be harnessed to make a wide range of functionalised nanocatalysts for contrasting applications.

The following Figures are illustrative of the invention. Fig 1 shows images of palladium nanoparticles on biomagnetite. Fig 2 is a XPS trace that shows that the palladium is metallic and that any oxide component is negligible. Fig 3 shows the results for Heck coupling of iodobenzene and styrene using palladium-magnetite. Fig 4 shows the Heck coupling of iodobenzene and ethyl acrylate using palladium magnetite. Fig 5 shows the size and shape of gold nanoparticles supported on biogenic magnetite. Fig 6 shows XRD results for nanoparticle alloys on the surface of magnetite. The alloy is a palladium gold alloy. Fig 7 shows the X-ray diffraction results which are indicative of the formation of a palladium gold alloy rather than the deposition of palladium and gold as separate entities. Figure 8. TEM images of Pd-coated biogenic magnetite. Image b contains annotation indicating where EDX spectra were taken. Figure 9. (a) X-ray diffraction (XRD) and (b) selected area diffraction (SAD) pattern for Pd-coated biogenic magnetite with reflections labelled in black (magnetite) and grey (palladium). Figure 10. X-ray magnetic circular dichroism spectra for (a) biogenic magnetite and (b) Pd-coated biogenic magnetite. Figure 11. XPS of Pd 3d of biogenic Pd-magnetite before (a) and after (b) use as a catalyst in the Heck coupling of iodobenzene and styrene.

The basic ingredients required for operation of the process of the present invention include a suitable buffer. A redox system is also required to biosynthesise the magnetite and this includes an electron donor (acetate or hydrogen for Geobacter, lactate/f ormate or hydrogen for She wane/Ia and an electron shuttle such as AQDS, humic materials or flavins such as riboflavin. It is also preferable for the microbial cells to be pre-grown to a suitable density. This can be done using conventional procedures and usually takes place on fumarate-containing media (fumarate is an electron acceptor).

Mineral phases that have been successfully used in the process of the present invention can be referred to as iron (III) oxides, oxyhydroxides and oxyhydroxy sulphates, including poorly crystalline phases such as ferrihydrite. Other substrates that have also been used include schwertmannite lepidocrocite akaganeite and feroxyhyte.

Separation techniques used to isolate the magnetic nanoparticles include the use of magnetic separating devices, centrifugation or filtration.

In accordance with another aspect of the present invention, the invention provides magnetic nanoparticles including a conditioning film. The conditioning film helps support the transition metal functionalisation. Thus, the magnetic nanoparticles of the present invention also include a biomolecular coating on the particle which provide the particles with a number of advantageous properties. We have conducted studies using XPS that demonstrate the presence of the biomolecular film. It is possible to show, for example, the presence of nitrogen in the biomolecular film using this technique.

Magnetic nanoparticles that include a conditioning film may be manufactured by a method according to a first aspect of the present invention wherein step (c) does not include purifying the nanoparticles or wherein step (c) does not include extensive purification of the nanoparticles. In other words, the nanoparticles are separated from the culture and the solution containing one or more transition metal salts is added immediately to the nanoparticles.

Alternatively, magnetic nanoparticles that include a conditioning film may be manufactured by a method according to a first aspect of the present invention which further includes an additional step (ci) in between steps (c) and (d). Step (ci) may include the addition of a conditioning film to purified or non-purified nanoparticles.

The principle described below specifically in relation to magnetite can be applied to a number of different sources of iron, including iron waste. In the case of the exemplary preparation of magnetite, the iron source is iron ferrihydrite (amorphous iron (Ill) oxyhdroxide).

Biogenic nano-magnetite was produced by anoxic washed cell suspensions of G. sulfurreducens challenged with Fe(lll)-oxyhydroxide, and electron donor (acetate) and a redox mediator (AQDS). After approximately 24 hours, the Fe(lll)-oxyhydroxide had converted completely to magnetite. Production of a Pd-coated magnetite was completed by adding a solution of NaPdCI4 to the magnetite suspension. Optimization studies suggested that removal of soluble palladium, presumably by Fe(ll)-mediated reduction to Pd(0) on the surface of the magnetite nanoparticles, was rapid and efficient at a range of Pd(ll) concentrations. For further detailed analyses of the material, a bionanomagnetite suspension was prepared to give a 5% Pd loading, by mixing the Pd(ll) solution and magnetite for 12 hours prior to washing in deionised water. No additional ligands were used to functionalise the surface of the magnetite prior to the addition of the solution of Pd(ll). For example, the common and costly practise of first modifying the surface of the carrier with a ligand such as 3-aminopropyl triethoxysilane (APTS) to aid attachment before adding a reducing agent such as hydrogen to convert the Pd to the metallic form was not required in accordance with the process of the invention.

Transmission electron microscope (TEM) images of the material produced by precipitating Pd onto the surface of biogenic magnetite are shown in Fig. 8(a-b). Fig. 8a showed the sample to contain particles of two contrasting sizes while Energy Dispersive X-ray (EDX) analysis of the general area showed the sample to contain approx 3.3 at% Pd. The high-resolution image in Fig. 8b shows a representative sample of two different size particles with the smaller ( 5 nm) attached to the larger (20 nm).

Using spot EDX elemental analysis (labelled on Fig. 8b) on both these particles, and other similar clusters, the larger particles (point 1) were found to contain less than 1 at% Pd whereas the smaller contained 9-10 at% Pd (point 2). Since the Pd-rich particles were smaller than the spot size used for EDX it is thought that the small particles were pure Pd and the larger were the magnetite crystals, indicating that Pd formed smaller nanoparticles on the larger nanoparticles of magnetite. Both small and large particles showed continuous lattice fringes, indicative of well-crystalline single crystals of both materials.

X-ray Diffraction (XRD) and Selected Area Diffraction (SAD) in Fig. 9(a-b) confirmed the presence of magnetite alongside faint reflections due to Pd metal, with no reflections from any other phase. The reflections in the XRD due to Pd are broader than those due to magnetite most likely as the particle size is smaller, as confirmed by TEM analyses (e.g. Fig. 8b). Crystallite size was estimated by applying the Scherrer equation to the (311) peak of magnetite and the (111) peak of Pd in Fig. 9 and the results gave a mean crystallite size of 27.2 nm for magnetite and 5.4 nm for Pd, again in good agreement with the TEM images.

The Fe L-edge and K-edges of magnetite were collected to give the X-ray Magnetic Circular Dichroism (XMCD) and Extended X-ray Absorption Fine Structure (EXAFS), respectively, before and after addition of Pd. It is speculated that either the amount of Pd bonded directly to Fe is too low for detection or the Pd is bonded to magnetite via an 0 bridging atom rather than directly to Fe. XMCD, a technique that can differentiate between Fe2 and Fe3 as well as between Td and Oh coordinated Fe in magnetite, was used to analyse the surface of the magnetite (top 4.5 nm). Fig. lOa shows the Fe L-edge difference spectrum for biogenic magnetite with and without Pd that after fitting gives Fe2/Fe3 ratio of 0.55, indicating an excess of Fe2 compared to stoichiometric magnetite which would give a ratio of 0.50. After the addition of Pd it was found that there was a small reduction in the amount of Fe2 at the surface, forming a spinel that was closer to stoichiometric magnetite with an Fe2/Fe3 ratio of 0.53. This indicates that the magnetite was partially oxidised during Pd(0) nanoparticle formation, possibly during reduction of the Pd(ll) by Fe(ll) at the surface in agreement with the EXAFS data. Fe 2p spectra (Fig. 11 b) collected using X-ray Photoelectron spectroscopy (XPS) before and after addition of Pd to the magnetite also indicates that the surface Fe2:Fe3 ratio decreases as Pd(ll) is introduced, indicating that the sample was oxidised at the surface.

In addition an XPS wide scan indicates that the surface of these nanoparticles has an Fe:Pd ratio of 1:0.22 (Fig. 11 a).

EXAFS and XPS were also used to investigate the structure of the Pd particles deposited on the surface of the magnetite. These show the main peak to be Pd(0) and the weak peak to indicate a second phase that is possibly PdO or Pd02. The Pd-oxide could be due to either partial oxidation of the surface of the metallic Pd clusters or could be from the bonding of the Pd nanoparticles to the magnetite via oxygen, which supports the hypothesis that the Pd is attached to the magnetite by a bond to the nearest 0 and not Fe ion.

This sample was then tested in the Heck Reaction for coupling iodobenzene to styrene and ethyl acrylate and found to be better than commercially available colloidal palladium catalyst. ICP-AES analysis of the supernatant from the Heck reaction found that there was negligible loss of Pd or Fe to the solution.

The experimental results show a simple method for producing highly catalytically active 1 0 nanoparticles using a low-cost synthesis route.

G. sulfurreducens was obtained from our laboratory culture collection and grown under strictly anaerobic conditions at 30°C in modified fresh water medium as described previously in [quote ref 14]. Sodium acetate (20 mM) and fumarate (40 mM) were provided as the electron donor and acceptor, respectively. However, a range of electron donors and acceptors could be used. All manipulations were done under an atmosphere of N2-C02 (80:20). Late log-phase cultures of G. sulfurreducens was harvested by centrifugation at 4920 g for 20 minutes and washed twice in carbonate buffer (NaHCO3; mM, pH 7.1) under N2-C02 (80:20) gas prior to use. Aliquots of the washed cell suspension (1.5 ml) were added to sealed anaerobic bottles containing 28.5 ml bicarbonate buffer (30 mM), poorly crystalline Fe(lll) oxide (50 mM) (prepared according to [quote ref 15]), sodium acetate (20 mM) and an electron shuttling compound anthraquinone-2,6-disulfonate (10 tiM) to increase the rate of Fe(lll) reduction. The final concentration of bacteria corresponded to 0.2 mg protein per mL. Bottles were incubated in the dark at 20°C for two days after which magnetite had been produced. The resultant magnetite was washed twice in degassed deionised water and then resuspended in water using magnetic separation to remove the supernatant. An aliquot of a solution of sodium tetrachloropalladate (Na2PdCI4, Sigma-Aldrich CAS no. 13820-53-6) was then added so that the final concentration of Pd was 5% by mass of the magnetite. The magnetite suspension was left overnight in a shaking incubator at 150 rpm and 20°C.

The sample was then washed again using degassed, distilled deionised water twice before drying under anoxic conditions.

X-ray absorption spectra (XAS) were collected for the Fe and Pd Kedges on beamline 9.3 at the Synchrotron Radiation Source (SRS), Daresbury Laboratory.

XAS for X-ray magnetic circular dichroism (XMCD) were collected on beamline 4.0.2 at the Advanced Light Source (ALS), Berkeley, CA, using the octopole magnet endstation.

Transmission electron microscopy (TEM) was conducted using a commercial instrument (Phillips/FEI CM200) equipped with a Field Emission Gun (FEG), EDX system (Oxford Instruments UTW ISIS) and a Gatan Imaging Filter (GIF). All TEM images presented here are bright-field images obtained using an operating beam voltage of 200 keV.

Selected Area Electron Diffraction (SAED) patterns were acquired using an appropriate diffraction aperture.

X-ray photoelectron spectroscopy (XPS) data were recorded using a commercial system (Kratos Axis Ultra) employing a monochromated Al Ka X-ray source and an analyser pass energy of 20eV, resulting in a total energy resolution of ca. 0.9eV.

Claims (13)

1. CLAIMS1. A process for preparing magnetite nanoparticles incorporating one or more transition metal elements, the process comprising the steps: (a) exposing a compound or mixture of compounds as a source of ferric iron to a microbial system in a culture comprising a dissimilatory iron reducing bacteria (DIRB) and/or a sulphate reducing bacteria (SRB) in the presence of a redox mediator, (b) incubating the resulting mixture in a culture under anaerobic conditions to produce biogenic nanoparticles of an iron-based mineral, (c) separating the nanoparticles from the culture and optionally purifying the nanoparticles, (d) adding a solution containing one or more transition metal salts to a suspension of the nanoparticles to produce catalytic nanoparticles, and (e) recovering the catalytic nanoparticles.
2. 2. A process as claimed in claim 1, wherein the nanoparticles are magnetic.
3. 3. A process as claimed in claim 1 or 2, wherein the source of iron can be an iron-based compound or a mixture of iron-based compounds which are capable of reacting with the bacteria to form magnetic nanoparticles.
4. 4. A process as claimed in any of claims I to 3, wherein the source of iron is ferrihydrite.
5. 5. A process as claimed in any preceding claim, wherein step (c) does not include purifying the nanoparticles.
6. 6. A process as claimed in any preceding claim, wherein the redox mediator is an electron donor and an electron acceptor.
7. 7. A process as claimed in any preceding claim, wherein the bacterium is a DIRB, or wherein the bacterium is preferably a sulphate-reducing bacteria.
8. 8. A process as claimed in claim 7, wherein the dissimilatory iron-reducing bacteria is Geobacter, Shewanella or Geothrix, or wherein the sulphate-reducing bacteria is Desulfovibrio, Desulfococcus, Desulfobacter, Desulfobulbus or Desulfobotulus.
9. 9. A process as claimed in any preceding claim, wherein the transition metal solution may contain a single transition metal salt or a mixture of transition metal salts.
10. 10. A process as claimed in claim 9, wherein the transition metal salt is a salt of one or more metals selected from the group comprising: palladium, platinum, rhodium, iridium, silver, gold, ruthenium, osmium, nickel and titanium.
11. 11. A process substantially as hereinbefore described.
12. 12. A magnetic nanoparticle including a conditioning film, which is a biomolecular coating on the particle derived from the bacterium. a)Q
13. 13. A magnetic nanoparticle substantially as hereinbefore described. a)