WO2009044146A1 - Metal nanoparticles - Google Patents

Abstract

There is described a material comprising a metal nanoparticle supported on a porous polysaccharide derived material wherein the nanoparticle has a diameter of from 1 to 30nm and uses thereof.

Description

Metal Nanoparticles

Field of the invention

The present invention relates to novel materials comprising metal nanoparticles, methods of their preparation and uses thereof.

More particularly the invention relates to metal nanoparticles supported on various porous polysaccharide derived materials (PPDM's).

Background to the invention

Metallic nanoparticles are receiving huge research literature interest, mainly due to their relatively higher chemical activity and specificity of interaction, compared to organic equivalents. This makes them particularly interesting in a variety of applications including sensors, non-linear optics, medical dressings, paints and catalysis, etc.¹ With particular regard to catalytic applications, palladium catalysts have several interesting features for organic synthesis, especially for C-C bond formation, due to their versatility among many transition metals employed in these reactions."

Control of nanoparticle size, shape and dispersity is the key to selective and enhanced activity. One mechanism to achieve this control, is to utilise another nano technology, that of nanoporous materials. Immobilisation and stabilisation of the nanoparticle form, allows exploitation of the special properties that occur at this size regime. The fusion between nanoporous and nanoparticle technology is potentially one of the most interesting and fruitful areas of this interdisciplinary research. It potentially allows the

l development of biomimetic devices with selectivity and sensitivity akin to enzymatic and biological sensory systems. Nanoparticles typically provide highly active centres; combining this with the nanopore environment, allows the generation of specific adsorption sites, creating a partition between the exterior and the interior pore structure, and inhibits nanoparticle aggregation during application. Potential for increased efficiency from nanoparticle catalysts, in combination with the advantages of such heterogeneous supports, increases the "green" credentials of the process, with higher selectivity, conversion, yield and catalyst recovery proposed advantages and targets.

The use of materials with pore sizes in the nanometre range has the advantage of inhibiting particle growth to a particular size regime as well as potentially reducing particle aggregation. By selection and manipulation of the textural properties of the heterogeneous support (in unison with the reduction step), it should be possible to determine the size and shape of the resulting nanoparticles. Coupling of size selective nanoparticle behaviour and that of pore size and geometry manipulation potentially allows the generation of "designer" materials for applications such as catalysis.

The support material on which nanoparticles are synthesised has received significant attention in the literature. Many different materials have been employed to support metals nanoparticles, including mesoporous silicas, activated carbons, polymers and even biomass."¹ Conventionally, the preparation of metal nanoparticles requires the use of a reducing agent, such as, NaBH₄, hydrazine, H₂, etc. and in most of the cases the reduction is not easily controllable, leading to non-uniform, 3-dimensional nanoparticle growth, also resulting in partially blocked pores, materials with a high content of inactive metal clusters or irregular nanoparticle size distribution. In spite of the exciting properties offered by materials at the nanoscale, conventional synthetic approaches for the fabrication of metallic nanoparticles present some environmental problems.^ιv Approaches predominantly apply organic solvents as the synthetic media and toxic reducing agents like sodium borohydride and N,N-dimethylformamide are commonly employed.

The use of polysaccharides as supports for stabilisation of the unstable metallic nanoparticle form, is an interesting option particularly if the reducing nature of polysaccharides can be exploited as support media and reducing agent in unison. The use of nanoporous polysaccharides by extension could potentially display the advantage of a surface with potential reducing behaviour, whilst limiting the nanoparticles to a specific size regime, as a product of the nanopore confinement, as the nanoparticle maturate. This would enable a "one-pot" synthetic route to nanoporous polysaccharide / nanoparticle hybrid materials. Furthermore, by utilising different porous polysaccharide forms (i.e. mesoporous starch or alginic acid), control of the nanoparticle size distribution may be inferred by the pore sizes available in different polysaccharide derived materials.

We have found a novel method of manufacturing metal nanoparticles which utilises the porous polysaccharide as a reducing agent, thus avoiding the need to use conventional reducing agents and thereby allowing the nanoparticles to be formed, essentially as thin films inside of pore network, thus, enabling the nanoparticles to be significantly smaller inside when compared to conventionally prepared nanoparticles. Statements of the invention

Thus, according to a first aspect of the invention we provide a material comprising a metal nanoparticle supported on a porous expanded polysaccharide derived material (PPDM) wherein the nanoparticle has a diameter of from 1 to 30nm.

In particular we provide a novel nanoporous material (a porous polysaccharides) which acts as a reducing agent, template, and support for metal nanoparticles.

In this aspect of the invention the nanoparticles are produced as a result of interaction of the metal precursor(s) and the surface of the polysaccharide. The nanoparticle has a diameter which is constrained by the porosity of support material.

The metal nanoparticles supported on a porous material, e.g. a porous polysaccharide is referred to as a nanocomposite. The pore size of the new nanocomposite materials varies depending, inter alia, upon the nature and pore size of the polysaccharide and the metal or metals present in the nanoparticles. The diameter of the nanoparticles may be from 1 to 30nm.

A variety of known polysaccharides may be used to host the nanoparticles of the invention. It is within the scope of the present invention for the polysaccharide to comprise more than one polysaccharide or more than one type of the same polysaccharide. The soluble, e.g. water soluble polysaccharides are preferred, examples of such polysaccharides include, but should not be limited to, amylopectin, amylose, alginic acid, pectin and xylan. The porous polysaccharide may be mesoporous or microporous. It should be understood that the polysaccharide may comprise a mixture of pores such that it may be mesoporous (diameter higher than 2nm) and microporous (diameter less than 2nm). The ratio between these two types of pore may be easily controlled by the method of preparation A further advantage of the porous polysaccharides used as supports in the present invention is that the size distribution of the pore size is substantially uniform. The pore size distribution in the polysaccharides of the present invention may be expressed by one of more of the graphs in figure 1 and 2 herein.

Preferably, the nanoparticle material retains a pore structure similar to that of the polysaccharide from which it is derived and preferably the degree of microporosity is not more than the degree of mesoporosity.

A variety of metals may be included in the nanoparticles of the invention. The metal may be selected from one or more of a noble and transition metals (e.g. Fe). By the term noble metal we mean a metal selected from the group consisting of copper, silver, gold, iridium, osmium, palladium, platinum, rhenium, rhodium, and ruthenium

. It is within the scope of the invention for the nanoparticles to contain a single metal or more than one metal. Similarly, a group of nanoparticles of the invention may comprise nanoparticles comprising different metal.

The use of a polysaccharide as a support for stabilisation of an otherwise unstable metallic nanoparticle form, is advantageous particularly if the reducing nature of polysaccharide can be exploited as support media and reducing agent in unison. The use of nanoporous polysaccharides displays the advantage of a surface with reducing behaviour, whilst limiting the nanoparticles to a specific size regime, as a product of the nanopore confinement, as the nanoparticle maturate. This enables a "one-pot" synthetic route to nanoporous polysaccharide/nanoparticle hybrid materials. Furthermore, by utilising different porous polysaccharide forms (i.e. mesoporous starch or alginic acid), control of the nanoparticle size distribution may be inferred by the pore sizes available in different polysaccharide derived materials. Furthermore, native polysaccharides are "green" materials, which are biodegradable and/or biocompatible. Thus, the nanoparticles structure may be controlled due to the combination of the mesopore confinement and the lack of added reducing agent, i.e. there is no need to have separate entities diffusing into the same space.

Gold nanoparticles (AuNPs) have been shown to demonstrate strong size selectivity, with the metal - oxygen interaction, important in oxidation reactions, altering as a product of the particle size. This leads to the possibility of size selective heterogeneous catalysis approach based on nanoparticle size, in tandem with conventional heterogeneous approaches utilising nanopore size.

In an aprotic solvent, the polysaccharide surface will possess abundant permanent dipoles, which means that the materials preferably adsorb charged, polar, and highly polarizable metal nanoparticle species (e.g. the complex precursor or nanoparticle nuclei), through dipole - charge, dipole-dipole, and dipole-induced dipole interactions. Furthermore, the introduction of more strongly charged functionality (i.e. carboxylic acid), should generate even greater adsorption potential. In this regard, the use of mesoporous forms of starch and alginic acid as novel support and reducing agent for the preparation of supported Au nanoparticle materials will now be discussed. Nanoparticle size values are reflective of the pore structure found in the mesoporous polysaccharide phase. Surface hydroxy! groups presumably facilitate the deposition of Au³⁺ or Au⁺ ions at the polysaccharide surface.

Heparin and chitosan have also been used to stabilise gold nanoparticles, with Yang et ai, reporting the preparation of gold nanoparticles in the 7 - 20 nm range, although TEM images demonstrated relatively poor dispersity and some agglomeration.^v By comparison here, materials demonstrate a much narrow distribution and very small nanoparticle size, with sub 3 nm diameters typical.

Many literature examples show the suitability of silver nanoparticles in applications such as antimicrobial surfaces or wound dressings.™ Vigneshwaran et ah, have previously demonstrated the preparation of water soluble starch / silver nanoparticle systems, with the soluble starch component reportedly acting as reducing agent and stabiliser/" Literature shows the current interest in the use of starch™¹ and cellulose¹" for example for the synthesis of silver nanoparticles. However, this is not an efficient use of the polysaccharide, as in the native non porous form, hydroxyl accessibility is limited. The use of a nanoporous polysaccharide in this regard, presents a high surface area, pore structure and therefore accessible hydroxyl rich nanoporous network that could potentially stabilise and limit nanoparticle growth to a particular size regime. Furthermore, in many applications such as wound dressings or purifying agents, silver nanoparticles must be easy to handle and the applied material potentially recoverable. A solid support potentially represents a useful nanoparticle vehicle in this respect. The preparation, characteristics and application of mesoporous starch (MSl) supported silver nanoparticle materials is discussed in the examples herein.

Silver nanoparticles are well known to exhibit antimicrobial properties." At the nanometre level, high material density of states, permits the existence of Ag⁺, which can migrate from the nanoparticle structure to the organism of interest (i.e. bacteria). Here, the silver cation may interact with sulphur or phosphorous containing proteins or DNA, inhibiting cell function or replication.

The development and application of a wide range of heterogeneous supported Pd catalytic materials has been demonstrated. Palladium catalysts have several interesting features for organic synthesis, especially for C-C bond formation, due to their versatility among many transition metals employed in these reactions.¹" Therefore, according to a further aspect of the invention we provide a catalyst comprising a metal nanoparticle supported on a porous polysaccharide derived material.

Semiconductor quantum dots (QD) are of interest not only as an academic curiosity but also because these nanoparticles show exciting potential as biological labels, photocatalysts, and nanoelectronic devices.""'"¹¹¹''"^* Fabrication of nano inorganic- organic composite materials is of particular current interest, as the resulting materials may potentially possess the combined characteristics of the two original components/* Preparation of the quantum dot semiconductor nanoparticles either on the polysaccharide particle surfaces or enclosed within the porous structure, offers a potential route to materials for applications described above, as the resulting composite, should possess the biocompatibility properties of the polysaccharide support. Such biocompatibility may be useful in biomedical imaging applications.^xvι

Porous polysaccharides derived materials have been to shown to be useful novel support materials for the preparation of a wide range of nanoparticle species. Mesoporous forms of Starch and Alginic Acid have been employed as novel support for Au, Ag, Pd and CdS supported nanoparticle materials. The special features of the porous polysaccharide form have been exploited in the preparation of small (< 2 nm) gold nanoparticles, which potentially exist between the amylose double helical unit cell. This nanoconfinement was exemplified by high resolution XPS analysis of the Au 4(f) electrons, demonstrating a significant positive binding energy shift (~ 0.6 eV), potentially as a consequence of quantum confinement effects at this small size nanoparticle size. Utilisation of porous alginic acid, in the preparation of supported AuNP materials, produced materials with remarkably good size (~ 4nm) and dispersity. Employment of UV light (λ = 365 nm), enhanced the quality of this size dispersity, producing materials with excellent particle dispersity, as reflected by 0.6 eV shift in the binding energies for the Au 4(f) core electrons.

Ag nanoparticles materials, utilising mesoporous starch as support media, have been prepared in a simple approach, rendering materials antimicrobially active. These materials were tested and presented excellent growth inhibition behaviour against gram positive and gram negative bacteria, E.Coli and S. Aureus, at low Ag loadings.

The polysaccharide support is biocompatible and can be used directly in this form, as demonstrative by zone of inhibition testing. Palladium nanoparticles were successfully prepared using porous materials derived from starch as support media. The nanoparticle size distribution was controllable by selection of the preparation solvent. Well dispersed palladium nanoparticles were prepared utilising MSl mesoporous starch material as a heterogeneous support. These materials have been subsequently employed successfully in the Heck, Suzuki and Sonogashira C-C coupling reactions. The properties of the metal nanoparticles are reflective of the unique porous environment these novel polysaccharide derived porous materials provide, allowing access to a wide range of surface chemistries thus facilitating the preparation of metal nanoparticles of a controllable size and nature. Interesting differences in catalytic activity between materials- in which the Pd was reduced with EtOH and/or the starch surface were found. Supported CdS materials have also been prepared, providing a useful insight into the pore structure of mesoporous starch materials.

Finally, and perhaps most importantly, methods developed, are simple and reproducible, and may be applied to any porous polysaccharide form, providing small sized supported nanoparticles materials that can be readily integrated into systems for synthetic, pharmaceutical and biomedical applications. Porous polysaccharides perhaps demonstrate the advantages of colloidal approaches to nanoparticles synthesis whilst utilising the benefits of nanopore confinement to limit particle size growth offered by conventional porous solids.

Supported CdS materials have also found use as photocatalysts.^xv" Mesoporous starch supported Ag nanoparticles materials have been shown here to be useful antimicrobial materials. Thus, according to an aspect of the invention we provide a medical dressing comprising a metal nanoparticle supported on a porous polysaccharide derived material. The preparation of porous polysaccharide monoliths or films is possible which can, for example, be loaded with Ag nanoparticles, potentially allowing the generation of antimicrobial surfaces or sterilisation tablets. Polysaccharide based films may also be useful in wound dressing preparations.

A variety of porous polysaccharide derived materials can be employed as supports for the preparation of supported nanoparticle materials. For applications involving acidic or high temperature conditions (i.e. biorefinery streams oxidations or hydrogenations), the use of Starbon materials may be more applicable.

The metal nanoparticles as hereinbefore described may also be useful as a sensor material. Therefore, according to a further aspect of the invention we provide a sensor comprising a metal nanoparticle supported on a porous polysaccharide derived material.

According to a further aspect of the invention we provide a process for the preparation of a metal nanoparticle supported on a porous polysaccharide derived material (PPDM) which comprises the steps of;

(i) adding a porous polysaccharide to a solvent, e.g. making a slurry of a dried porous polysaccharide with a solvent; (ii) adding a metal containing compounds to the product of step (i);

(iii) agitating the mixture, e.g. at mild conditions (temperature 20-60⁰C); and

(iv) separating the supported nanoparticles from the mixture.

The process of the invention may also comprise a step of drying the supported nanoparticles material produced.

In the process of the invention the porous polysaccharide may be mesoporous or microporous. The process may comprise the use of one or more conventional energy sources, such as thermal energy. However, in one aspect of the invention, the process may comprise the preparation of a porous polysaccharide by providing microwave energy. The process may optionally include the use of solvent exchange techniques.

Microporosity is important in the development of catalytic centres on PPDM materials whereas the mesoporosity enable the efficient diffusion of reactants and/or products. Understanding of the nature and structural origin of porous polysaccharides has allowed the preparation of materials with selectable textural characteristics depending on processing conditions and polysaccharide selection. At one extreme it is possible to produce predominantly microporous polysaccharides, with tuneable pore size. Conversely, by altering processing conditions, it is possible to produce a predominantly mesoporous materials, again with tuneable textural properties. Manipulation of processing conditions and the choice of polysaccharide, allows for the synthesis of materials with textural properties intermediary between the two. The properties of the metal nanoparticles are reflective of the unique porous environment in these novel polysaccharide derived porous materials.

The preparation of PPDM's allows access to a wide range of surface chemistries, facilitating the preparation of metal nanoparticles of a controllable size and nature. The advantage of our PPDM's compared to previously reported systems, is the use of non toxic, natural support media, with easily tuneable properties, and a simple nanoparticle preparation route, eliminating the use of a reducing agents. Noble metal nanoparticles (e.g. Pd, Au and Ag; or combinations of) may be synthesised at the surface of the porous polysaccharide derived material, generating interesting materials in which the metal particle size can be easily tuned and optimised for a particular application.

Furthermore, polysaccharide derived porous materials may be thermally degraded under a wide range of conditions to give controllable surface carbonaceous materials, allowing manipulation of surface chemistry, enabling control of the metal nanoparticle microscale environment, and facilitating particular applications (e.g. antiseptic wound dressings).

This novel family of polysaccharide derived porous materials presents a very interesting, inexpensive, non-toxic, and abundant support material, for the preparation of metal nanoparticles, amenable to a wide range of applications. The invention relates to the novel preparation of tuneable well defined metal nanoparticles on polysaccharide derived porous materials. By the term mesoporous material used herein we mean a material with average pore diameter between 2 and 50 nm and by the term microporous material we mean a material containing pores with diameters ofless than 2 nm

The invention will now be described by way of example only and with reference to the accompanying figures

EXAMPLES

Example 1

The parent materials were prepared either thermally or by means of microwave irradiation. The preparation of mesoporous examples of PPDM's was performed as outlined below:

(a) Thermal preparation of Mesoporous Polysaccharide derived porous material lOOg of polysaccharide( e.g. starch, alginic acid, pectine) and 2L of deionised water was stirred at 700 rpm for 10 minutes in a modified domestic pressure cooker prior to heating (Volume = 3L; Operating conditions 120°C/80 KPa). The lid component of the device was modified with an aluminium enclosure facilitating insertion of a thermocouple. The system was heated to 120⁰C (30 minutes) and held at this temperature for a further forty five minutes. Upon returning to atmospheric pressure, the lid was detached, and the resulting solution / suspension decanted into powder drying jars. The vessels were then sealed and the gels retrograded at 5⁰C. (b) Microwave Assisted Preparation of Mesoporous Polysaccharide derived porous material

0.25g of polysaccharide was mixed with 5 mL of distilled water in a commercial microwave reactor vessel, placed in the reactor and the pressure sensor attached. The sample was then heated to the desired temperature (i.e. 9Q⁰C-180⁰C, typically 130⁰C) over 150 seconds in a CEM Discover microwave reactor with computer controlled operation. Upon returning to atmospheric pressure, the vessel remained sealed and was placed in a refrigerator and left to retrograde at 5⁰C for a desired time. The resulting gel/colloidal suspension was then solvent exchanged and dried.

(c) Solvent Exchange Procedure

Water was removed by a solvent exchange procedure. An initial volume of ethanol (10% v/v with water) was added and stirred for 2 hours. A second volume of ethanol (20% v/v) was then added followed by 2 hours stirring. This was followed by a further addition of ethanol (30% v/v) and another 2 hours stirring. A fourth volume of ethanol (50% v/v) was then added followed by another 2 hours of stirring. The resulting suspension was allowed to settle or was centrifuged. The excess solvent was decanted. A volume of ethanol was then added, equivalent to the volume of water used in the thermal hydration step stage and stirred for a time period of from 2 hours to overnight. The suspension was then allowed to settle and the excess solvent was decanted. This process of adding ethanol, stirring and removing was repeated twice. The product was then filtered until partially dry. The resulting solid at this point was then dried via rotary evaporation, and then held overnight at 40⁰C in a vacuum oven. Alternatively, the residual ethanol can be removed by supercritical CO₂ drying, (see Figure 1). Example 2

The parent materials were prepared either thermally or by means of microwave irradiation. The preparation of microporous examples of PPDM 's was performed as outlined below:

(a)Thermal preparation of Microporous Polysaccharide derived porous material

2g of polysaccharide and 2L of deionised water was stirred at 700 rpm for ten minutes in a modified domestic pressure cooker prior to heating (Volume = 3L; Operating conditions 120°C/80KPa). The lid component of the device was modified with an aluminium enclosure facilitating insertion of a thermocouple. The system was heated to 120⁰C (30 minutes) and held at this temperature for a further forty five minutes. Upon returning to atmospheric pressure, the lid was detached, and the resulting solution / suspension decanted into powder drying jars. The vessels were then sealed and the gels retrograded at 5⁰C.

(b) Microwave Assisted Preparation of Microporous Polysaccharide derived porous material

0.005g of polysaccharide was mixed with 5 mL of distilled water in a commercial microwave reactor vessel, placed in the reactor and the pressure sensor attached. The sample was then heated to the desired temperature (i.e. 90⁰C-180⁰C, typically 130⁰C) over 150 seconds in a CEM Discover microwave reactor with computer controlled operation. Upon returning to atmospheric pressure, the vessel remained sealed and was placed in a refrigerator and left to retrograde at 5⁰C for a desired time. The resulting gel/colloidal suspension was then solvent exchanged and dried. (c) Solvent Exchange Procedure

Water was removed by a solvent exchange procedure. An initial volume of ethanol (10% v/v with water) was added and stirred for 2 hours. A second volume of ethanol (20%v/v) was then added followed by 2 hours stirring. This was followed by a further addition of ethanol (30% v/v) and another 2 hours stirring. A fourth volume of ethanol (50%v/v) was then added followed by another 2 hours of stirring. The resulting suspension was allowed to settle or was centrifuged. The excess solvent was decanted. A volume of ethanol was then added, equivalent to the volume of water used in the thermal hydration step stage and stirred for a time period of from 2 hours to overnight. The suspension was then allowed to settle and the excess solvent was decanted. This process of adding ethanol, stirring and removing was repeated twice. The product was then filtered until partially dry. The resulting solid at this point was then dried via rotary evaporation, and then held overnight at 40⁰C in a vacuum oven. Alternatively, the residual ethanol can be removed by supercritical CO₂ drying. (See Figure 2).

Example 3

The preparation of Noble Metal Nanoparticles on PPDM's was performed in two different solvents and dried using rotary evaporation. A sample of dried polysaccharide derived porous material (e.g. mesoporous starch), was added to a desired volume of acetone (or ethanol) (1:25 /wt/mL) in a sealable vessel and left to stir for five minutes. The desired amount of noble metal or combination of noble metals (e.g. Pd, Ag or Au), was then added to system (i.e. 0.5 wt % and upwards). The vessel was then sealed and left to stir for 24 hours in an oil bath at 55⁰C. The resulting grey product was then vacuumed filtered, washed thoroughly with fresh acetone and dried in a vacuum oven at 45⁰C overnight.

Example 4 , The preparation of Noble Metal Nanoparticles on PPDM' s was performed in two different solvents and dried under ScCθ₂ conditions. A sample of dried polysaccharide derived porous material (e.g. mesoporous starch), was added to a desired volume of acetone (or ethanol) (1:25 /wt/mL) in a sealable vessel and left to stir for five minutes. The desired amount of noble metal or combination of noble metals (e.g. Pd, Ag or Au), was then added to system (i.e. 0.5 wt % and upwards). The vessel was then sealed and left to stir for 24 hours in an oil bath at 55⁰C The resulting grey product was then vacuumed filtered, and residual solvent was removed under ScCC>2 conditions (Figures 3-6).

Example s

The preparation of Noble Metal Nanoparticles on PPDM's was performed under ScCθ₂ conditions. A sample of dried mesoporous polysaccharides (e.g. starch, alginic acid, pectine) was added to the ScCθ₂ reactor. The system was then pressurised to supercritical conditions. A solution of known concentration of noble metal precursor (i.e. Palladium Triflate) was injected into the ScCθ₂ feed and the allowed to pass through the sample. The system was left at supercritical conditions for a desired time period (i.e. 2 hours). The system was then allowed to depressurise to STP overnight. (See Figures 7) Example 6

Noble nanoparticles supported on mesoporous examples of Alginic Acid or pectin PPDM's may be thermally degraded under a non reducing atmosphere (i.e. under N₂ or vacuum), to produce a palladium containing carbonaceous material. (See Figures 8 & 9).

Example 7

Palladium nanoparticles supported on mesoporous examples of PPDM's, as demonstrated here via a sample prepared on mesoporous high amylose corn starch, were tested in the Heck reaction of Iodobenzene and methyl acrylate. Iodobenzene (8 mmol, 0.85 mL), methyl acrylate (8 mmol, 0.70 mL), triethylamine (5 mmol) and the catalyst (0.1 g) were mixed together in a microwave tube and placed in a Microwave reactor under stirring at 90⁰C for 5 min. The microwave power employed in the reaction was 300 W. Dodecane was employed as internal standard and results were analysed by QC. Results are summarized in Table 1.

The reaction was complete for most of the palladium supported materials (except for the low loading ones) in ca. 5 min under microwave irradiation under the reaction conditions, with selectivities higher than 85% to the main product, methyl cinnamate. Table 1: Heck reaction of iodobenzene and methyl acrylate catalysed by Pd nanoparticles supported on mesoporous Starch catalyst.⁸

Iodobenzene (8 mmol), methyl acrylate (8 mmol), triethylamine (5 mmol), catalyst (0.1 g), 300W, 9CfC, 5 min.

Example 8

Palladium nanoparticles supported on mesoporous examples of PPDM's, as demonstrated here via a sample prepared on mesoporous high amylose corn starch, were tested in the Sonogashira reaction of Iodobenzene and phenyl acetylene. Iodobenzene (2 mmol), phenyl acetylene (2 mmol), 1, 4-diazabicyclo (2.2.2) octane (DABCO, 2 mmol) and the catalyst (0.025 g) were mixed together in a microwave tube and placed in a Microwave reactor under stirring at 13O⁰C for approx. 2 min. The microwave power employed in the reaction was 300 W. Results are summarized in Table 2.

The reaction was complete for all the palladium supported materials (no exceptions) in less than 2 minutes under microwave irradiation under the reaction conditions, with selectivities higher than 65% to the cross-coupled product (A). A fair quantity of the homocoupling product (typically 30-40%) was also obtained.

Table 2: Sonogashira reaction of iodobenzene and phenyl acetylene on catalysed by Pd nanoparticles supported on mesoporous Starch catalyst²

" Iodobenzene (2 mmol), phenyl acetylene (2 mmol), DABCO (2 mmol), catalyst (0.025 g), 300W, 13O⁰C, 2 min. Example 9

Preparation of supported metal nanoparticles on the Starbon surface.

We can prepare a wide range of different noble metal supported nanoparticles (SMNP) on porous materials. The dispersion and particle size of the SMNP was found to be very dependent on the nature of the metal. Materials prepared under the same conditions followed the following trend in terms of increasing particle size and decreasing dispersion: Ru > Pd > Pt > Rh > Ag (TEM images of the actual samples Figure 10)

Example 10

Catalytic activity of supported metal nanoparticles in the hydrogenation of succinic acid.

An initial screening of the various SMNP showed all materials were active in the hydrogenation of succinic acid under the reaction conditions (10 mmol SA, 30 mmol EtOH, 50 mmol H2O, 10 bar H2, 80oC, 0.1 g catalyst, 24-48h reaction). The activity and selectivity of the SMNP was highly dependent on particle size and dispersion (see figure 11). Example 11

Catalytic activity of supported metal nanoparticles in hydrogenations of platform molecules

SMNP were found to be highly active and selective in the reduction of a range of platform molecules including fumaric, itaconic and levulinic acids. Under the reaction conditions (10 mmol acid, 30 mmol EtOH, 50 mmol H₂O, 10 bar H₂, 80⁰C, 0.1 g catalyst) and regardless of the metal, the exo-double bond in itaconic acid could be reduced in less than Ih of reaction, the trans double bond in fumaric acid could be reduced in less than 2h of reaction and the ketone functionality in levulinic acid was reduced to the alcohol within 2-4h reaction.

Example 12

The precursor porous polysaccharide materials (e.g. Amylopectin, Amylose, Alginic Acid, Pectin, and Xylan.) was prepared and dried before hand. Solutions Of AuBr₃ in acetone at known concentrations were prepared, to which the dried porous polysaccharide powder was added. The system was then left to stir at room temperature for 15 hours, after which the system was filtered and washed extensively to remove any unreacted complex or non weakly physisorbed Au nanoparticles. During this period the orange coloured solution of AuBr₃ in acetone, reduced in intensity and the suspended powder become salmon pink / red in appearance, indicating the formation of Au nanoparticles. In the absence of any polysaccharide support, the AuBr₃ acetone solution was stable and displayed no colour change. This system is summarised in Figure 1. Example 13

Mesoporous Starch (MS) as a support material for Gold nanoparticles (AuNPs /

MS).

S Transmission Electron Microscopy of AuNPs / MS.

Transmission electron microscopy (TEM) images revealed the presence of finely distributed sub 10 nm AuNPs, within the MS matrix (Figure 2). At low loadings (i.e. 0.08wt% Au), the resolution of the TEM was not capable of imaging the0 nanoparticles. Elemental analysis and XPS spectroscopy confirmed the presence of (metallic) Au in all samples. Importantly, the formation of AuNPs was observed without the addition of a conventional reducing agent (e.g. NaBI^).

The acetone solutions of AuB^ were stable, during the time frame of the preparation5 of these materials, exhibiting no colour change over a number of weeks. Au

Nanoparticles presented regular spherical shaped particulates, increasing in size as loading increases, potentially as a consequence of hierarchical pore filling (Figure 12

(A) to Figure 12 (B)). A nanoparticle size distribution from Figure 12 (D) was calculated (Figure 13). The particle size range of material prepared at 0.5 wt% Au,0 demonstrated excellent small sizes, with the majority of particle of a radius between 1

- 3 nm, remarkably good, and narrow in distribution. Figure 13 indicates two main maxima at 1.3 nm and 2.2 nm, with the lower value greater in intensity.

5 Example 14

The advantage of polysaccharide nanopore confinement was investigated via the preparation of gold nanoparticles on native starch, and as a comparison on native cellulose. Reduction of the Au¹¹¹ species progressed at the surface of these materials, with large irregular sized 100 run nano objects observed. Figure 14 (A) displays some unusual multifaceted rhomboidal and tube like nanoparticles. Interestingly both materials displayed the typical salmon pink/red colour of AuNP prepared on the porous starch form, and the surface plasmon was observed via diffuse reflectance UV- Vis absorption spectroscopy.

Example 15

Nitrogen Porosimetry Analysis

After the deposition and formation of the AuNP, the resulting materials continued to present promising surfaces areas and pore volumes. Supercritical CO2 dried mesoporous starch was used as the precursor support, and was prepared as a blank, without AuBr₃, dried from acetone, here labelled MS (Table 1). All materials presented type IV nitrogen isotherms typical of mesoporous starches.

Table 1: Textural Properties from Nitrogen Porosimetry Analysis of AuNP/MS materials prepared at increasing Au loading.

AuNPs growth occurred within the pore structure of the mesoporous starch support. As Au loading increases, surface area, mesopore and micropore volume all show near linear decreases in value, with significant reduction in value for each parameter, as Au loading approaches 0 1 Q wt%. Such trends may be reflective of pore filling by the AuBr₃ precursor. The particle size distribution is reflective of particle growth in small mesopores and large micropores at this Au loading. Data also suggests that the Au precursor first deposits at mesopore hydroxyl sites, followed by Au₂B^ dimer scission, and the migration of some molecular AuBr₃ or small nanoparticle nuclei to micropore site entrances. Consideration of the amylose helix model, hydroxyl groups will be involved in helix and nanopolysaccharide crystallite stabilisation at the micropore level. Therefore deposition and stabilisation of the nanoparticle is unlikely to occur within the helical interior, and is more likely to exist in between amylose double helices, that make up the unit cell.

Dubinin-Radushkevick surface energy (EQR) values are agreements with the idea of the hierarchical pore filling, as this value is derived from nitrogen adsorption at relative low pressures (0.2-0.35 P/P_o), reflective of AuNPs beginning to fill mesopore sites. Increases in nanoparticle size (from TEM) are demonstrative of this transition, and in good agreement with nitrogen porosimetry data.

The maintenance of the textural properties in these AuNP/MS materials is potentially useful, as this will provide accessibility to the Au nanoparticle surface, potentially allowing post fimctionalisation of the nanoparticles.™"

Example 16 Diffuse Reflectance UV- Vis Spectroscopy (DRUVs)

DRUVs analysis of AuNP/MS powders, displayed the characteristic surface plasmon for the Au NPs, ca λ = 500-600 nm.^xιx Increasing Au loading, resulted in a blue shift of the absorption λ_max for these materials, shifting from 532 ran to 509 nm, concurrently with an increase in the bandwidth of the absorption spectra.

This blue shift in the DRUVs absorption spectra (ca. Δλ ~ 23 nm), can be attributed to increased interparticle separation, as the deposition of Auⁿ⁺ transits from narrow confined pore sizes to more open expansive mesopores as hierarchical Au loading proceeds. Increased spectral bandwidth can be attributed to a bathochromic shift, consistent with a decreased interparticle distance, as seen from previous TEM images. The relationship between Au loading and the position of the absorption maximum is depicted graphically in Figure 14.

Example 17

X-ray Photoelectron Spectroscopy (XPS) Analysis

17.1 Survey Scan and Elemental Analysis

A representative XPS survey scan for AuNP/MS materials is displayed in Figure 16. The characteristics peaks for the core level electrons of elements present in the support (i.e. Q and C) dominate the spectra. The position of the Au 4(f) is indicated, but poorly resolved in the survey scan at these Au loadings. Quantification of the Au content of AuNP/MS materials prepared during this investigation was possible however, and results obtained for the different Au loadings are summarised with elemental analysis data for Au content, in Table 2. Elemental analysis values for Au wt% loading and those derived from the XPS survey scans, demonstrated and confirmed reduction of the Au precursor did not occur at the surface but within the bulk pore structure of the polysaccharide matrix; exemplified by discrepancy in elemental analysis and XPS results. This agrees well with previously discussed porosimetry data, indicating the formation of nanoparticles within the polysaccharide pore structure. Table 2: Comparison of Au loading, determined by elemental analysis (EA) and XPS survey scans (Atomic %).

17.2 High Resolution XPS spectra of the Au 4(1) Core electrons

To establish the formation of the metallic Au⁰ nanoparticle species, high resolution scans of the Au 4(f) core electrons were acquired. The emission of 4(f) photoelectrons from Au is identified by one pair of peaks in the high-resolution spectra of the 4f core level and can be characterized, due to spin-orbit coupling as the Au 4(f)_7/2 and Au 4(f)_5/2, present in the correct area ratios. Metallic gold is known to present twin peaks at 84.2 eV and 87.7 eV for the Au 4(Q₇₇₂ and Au 4(f)_5/2 respectively.™ All samples after spectral fittings presented one pair of peaks separated by 3.5 eV. Fittings did not allow for the introduction of further peaks, attributed to Au⁺ or Au³⁺ species, as a result of Auⁿ⁺ oxidation or physisorbed precursor complex. Analysis of the peak positions for the Au 4(f) electrons as a function of Au loading, indicated the positive binding energy shift for nanoparticle materials prepared at low wt% loadings (0.08 wt%). There is a shift from 84.2 eV and 87.7 eV positions to positively shifted binding energies, at 84.7 eV and 88.4 eV respectively. This difference of 0.50/0.70 eV is not trivial. Binding energy shifts of between 0.35-0.80 eV has been reported for different sized AuNPs prepared on different substrates (i.e. silica and titania).**¹ McFarland et al reported a maximum shift of 0.88 eV on a transition from 6.0 nm to 1.5 nm sized AuNPs. The size shift observed here upon decreasing Au loading to 0.08 wt% agrees well with the literature data. The origin of such sub 2 nm nanoparticles, as previously mentioned, may be the result of Au³⁺ reduction within interhelical spacing of amylose microchenal. Furthermore, the formation of these very small nanoparticles agrees well with sample TEM images, which demonstrated particles had diameters less than the machine resolution.

Example 18

Mesoporous Alginic Acid (AS) as a support material for Au nanoparticles.

Porous Alginic Acid (AS) material presents higher surface areas and pore volumes compared to MS materials, with the addition of carboxylic acid surface functionality. Supercritical CO₂ dried porous AS was used as support material for this investigation. The use of AS aerogel material offers a surface with high porosity, allowing rapid analyte flux to and from the metal particle centres, in any future application (e.g. catalysis or sensors). The Bronsted acidity of the AS surface was discussed earlier in this thesis, and it was envisaged that surface carboxylic acids, as well as hydroxyl groups may participate in the reduction of the gold bromide precursors. The higher activity of the carboxylic acid surface groups, coupled with the differing textural properties of AS material, may produce Au naπoparticles presenting different size distributions. Therefore, AS aerogels presented excellent candidate materials to investigate the effect of increased Au loading (i.e. higher than that used for AuNP/MS materials).

18.1 Transmission Electron Microscopy Analysis (TEM)

Materials were prepared in the same manner as for MS supported Au nanoparticles. And the transition from the orange AuBr₃ acetone solution to purple / red solid was observed.

Figure 17 demonstrates the excellent dispersion and size characteristics of AuNPs prepared on porous AS material. As for MS supported materials, increasing Au loading, resulted in an overall increase in the nanoparticle size. The particle size distributions of AuNPs prepared at lowest and highest Au loadings are displayed in Figure 18 (A).

18.2 Effect of System Expose to UV Light at 365 nm.

Pal et al., have previously demonstrated the preparation of AuNPs via the photo excitation of the hydrogen tetrachloroaurate(III) trihydrate, in the presence of poly(oxyethylene) isooctyl phenyl ether surfactant which acted as both reducing agent and stabiliser, using a wavelength of 365 nm.^xx" Interestingly, Ahmadi et al, proposed that acetone may play a major role in the reductive step for the preparation of Au nanorods.**"¹ As a comparative investigation, porous AS supported AuNPs were prepared under a UV lamp (λ =365 nm), under the same experimental conditions used in the preparation of RW-AAu materials (see previous section).

AuNPs were clearly visible, well dispersed and presented spherical shaped nanoparticles. Figure 18a shows a comparison of the nanoparticle size distribution of the highest and lowest Au loaded materials for material conventionally prepared (Figure 18(A)), and as prepared under the UV lamp (Figure 18 (B)).

Nanoparticle size analysis, indicated a transition in size upon increasing Au loading, with the predominant nanoparticle at the low loadings (0.16wt% Au) ~ 2.5 nm, whereas at the higher loadings investigated (0.80wt% Au), the predominant size for both preparation routes shifted to around ~ 4.0 nm. Regardless of loadings or preparation route, the small size of these nanoparticles was still impressive, and comparative, if not better than literature examples for supported Au nanoparticles.^xxιv The route is made more impressive if the simplicity of the preparation route is also considered. Figure 17(B) and (D) demonstrate well the dispersity and uniformity of the AuNPs prepared in this manner.

Comparison of TEM images of AuNPs/ AS materials indicated increased uniformity, size and dispersity for nanoparticles prepared under the UV lamp, although AuNPs prepared without the lamp, produced smaller particle diameters, but with a greater size range.

Strong electrostatic interaction is expected between surface carboxylic / carboxylate surface groups with the positively charged AuNPs growth nuclei, providing a strong adsorption, inhibiting particle agglomeration, and generating the excellent dispersity observed from TEM.

Exposure of the AuBr3 acetone solution to the UV lamp, did not result in any red colouring, in the time frame of the experiment undertaken here (-15 hours), and no characteristic peak for the formation of solution dispersed AuNPs at 500-600 run was observed by liquid UV- Vis absorption spectroscopy of the reaction solution. ^xxv The role of UV light and differences in material properties (i.e. nanoparticle size and dispersion) is not certain. Literature indicates that the acetone triplet state may be generated by excitation using pulsed laser exposure, and that the excited state is an extremely mobile probe in micellar solution, Scaiano et al. commenting that photoreduction by a surfactant plays an important role in the absence of added quenchers."™ Yamada et al. have also commented on the necessity for acetone in the synthesis of Au nanorods structure synthesis. ^xxv11 The fact that Au nanoparticlεs were not obtained in the absence of the polysaccharide support, but different particle size distributions were produced as a consequence of exposure to the UV light, is an interesting observation. Results suggest a synergistic relationship exist between polysaccharide surface and the Auⁿ⁺ reductive cycle involved in nanoparticle synthesis. Proton donation from the dense hydrogen bond network of the porous polysaccharide may participate in quenching of any excited acetone states, the liberated energy from such processes aiding faster reduction at surface sites. The formation of the acyl radical via such mechanisms may also be important. More research is still needed to clarify the role of acetone in these UV irradiated systems. However, acetone is a convenient and potentially renewable solvent, and results are very promising, with small, well dispersed AuNP prepared in a simple and efficient manner.

18.3 Nitrogen Porosimetry Analysis of AS supported AuNP materials. Samples prepared under conventional conditions (AAu series), presented typical type FV nitrogen isotherms, high porosity (Vm₆₅₀ > 0.60 cm³g '), and surfaces areas (S_BE_T > 160 m²g^'') with nitrogen adsorption capacity decreasing as the loading of Au increased, as evidenced by reducing pore and mesopore volumes (Table 3). The reduction in value for these textural characteristics presented a negative linear relationship. Micropore volumes significantly also reduced in size as a function of Au loading.

Table 3: Nitrogen Porosimetry Data for AS supported Au Nanoparticle Materials prepared under conventional conditions as compared to the AS control sample.

By comparison, samples prepared under UV exposure, presented significantly different behaviour. Textural properties decreased more rapidly up to loading of 0.34 wt% Au. After which, textural properties actually started to increase in value (Table 4). Reduction in the size of the pore diameters value was also far greater than for the sample not exposed to the UV source. This data coupled with TEM images, XPS and I⁾RUVs, suggests that Auⁿ⁺ reduction occurs at a much faster rate than for samples not exposed to UV.

Table 4: Nitrogen Porosimetry Data for AS supported Au Nanoparticle Materials prepared under conventional conditions as compared to the AS control sample.

For conventionally prepared sample the gold nanoparticle precursor (i.e. precursor complex or more cluster nuclei), migrated slowly to the micropore region and was subsequently reduced. As system Au content increased, a hierarchical nanoparticle loading commenced. Nanoparticle formation within the micropore region was followed by deposition in the small and then medium mesopore region. This behaviour, generating the broad particle size range observed from particle size distribution measurements.

By contrast, for samples prepared under UV light, data in Table 4 suggests that the nanoparticle precursor nuclei, reached a critical size (i.e. larger than the micropore entrance), and deposited in the smaller mesopores. Consequently as Au loading increased, the mesopore walls became coated with the nanoparticles of this critical size, which potentially was stabilised by the strong electrostatic interactions with carboxylic acid functions. Increasing loading to values higher than 0.34 wt% Au, resulted in a narrowing of the mesopore diameters throughout the 2 -50 nm pore size range, resulting in the observed increase in surface and pore volumes (Table 4).

A comparison of porosimetry data for AAu5 and LAuIO samples agrees with observed changes in particle size and distribution. A higher surface area of the material is obtained in LAuIO material as compared to the AAu5 equivalent (Figure 12).

18.4 Diffuse Reflectance UV- Vis Absorption Spectroscopy (DRUVs) Analysis of Porous Alginic Acid supported Au nanoparticle materials

DRUVs analysis of AuNP/ AS powders, displayed the characteristic surface plasmon for the Au NPs, ca λ = 500 - 600 nm, as previously discussed for MS materials.

Samples prepared under normal conditions (i.e. without UV light), present variable λ_max values as a function of Au loading (Figure 19(A)). Contrastingly, AuNP/AS materials prepared under the UV light, show a far regular blue shift in λ_max as Au loading increases, shifting from 523 nm to 504 nm, concurrently with an increase in the absorption spectra bandwidth.

Figure 19(A) shows variability in the λ_maX position for samples prepared under normal conditions, whereas contrastingly for samples prepared under the UV lamp, shown a consistent bathochromic blue shift as Au loading increases. The difference in the two spectra sets may be reflective of the more uniform particle size dispersity observed in materials prepared under the UV lamp.

18.5 High Resolution XPS spectra of the Au 4(f) Core electrons The synthesis of the Au⁰ metallic nanoparticle phase was confirmed by high resolution XPS analysis of the Au 4(f) core electron level. The emission of 4(f) photoelectrons was identified for the AAu5 sample, by the spin-orbit coupling peak pair at 84.2 eV and 87.7 eV assigned as Au 4(f)_7/2 and Au 4(f)₅/₂ respectively, and were found in correct peak area ratio. The two peaks were separated by 3.5 eV. Spectral fittings did not allow for the introduction of further peaks, which may have identified partially reduced Au⁺ or unreduced Au³⁺ species. This was not case, and Au⁰ was only found in the metallic state. LAuIO material presented the same peak pair, binding energy separation and peak area ratio, as for AAu5, but binding energies were positively shifted by ~ 0.6 eV for Au 4(f)s/₂ peak.

As previously mentioned, a similar shift was observed for AuNPs prepared on mesoporous starch at low Au loadings (0.08 wt% Au). In this case, the shift was attributed to the synthesis of very small nanoparticles, potentially occupying interhelical amylose spacings, which restricted the maturation Qf the nanoparticle to the nanoconfinement of this porous region. However, here the Au wt% loading is much higher (0.80 wt% Au). Comparison of the TEM images, indicate that LAuIO materials present narrowly distributed and well dispersed AuNPs. Contrastingly, AAu5 materials presents materials with a much wide spread of sizes and marginally poorer distribution and dispersity. The quality of dispersion, much lower average particle size and range (< 5 nm; 2-8 nm) impacting may be the cause for 4(f) electron binding energy shift.

Example 19

Supported Silver Nanoparticle Materials

For the intended study here, the loading of silver was expressed in terms of mmol/g, in order to directly compare with antimicrobial loading of active pharmaceuticals (Table 5).

Synthesis was performed during the solvent exchange step of the preparation of mesoporous starch. Together AgNO₃ and starch were mixed in the desired quantity, and stirred vigorously for 15 hours, before exchange for ethanol was started. The colour of the gel system at this point had turned an off yellow; indicative of the formation of silver nanoparticles, yielding upon completion of the solvent exchange for ethanol, and drying of the solid under vacuum, a yellowy/brown solid. 19.1 Transmission Electron Microscopy (TEM) Analysis.

Figure 3 shows TEM micrographs of porous starch (MSl) supported Ag nanoparticle materials prepared at different Ag loadings.

MSl supported AgNP materials presented reasonably good nanoparticles sizes (ca 10 - 20 run) and distributions. Lianos et al, reported the synthesis of photocatalytically deposited silver nanoparticles on mesoporous titania films with nanoparticles possessing diameters of between 35-60 nm.^xxv"¹ Contrastingly, Homebecq et al, successfully prepared Ag nanoparticles with diameters of 2 and 3 - 4 nm, confined in mesoporous silica, prepared via γ-irradiation reduction approach.'""'

The large and relatively broad particle size and distribution of the AgNPs reported here, may be attributed to the flexibility of the porous polysaccharide structure. Starch hydroxyl groups presumably complex with Ag⁺ ions in aqueous solution. Reduction proceeds at these adsorption sites, and is accelerated by the slow addition of ethanol during the solvent exchange. Therefore, polysaccharide configuration may therefore, adapt to increases in nanoparticles size, in contrast to example of supported Au materials previously discussed, prepared in acetone, where the solvent polarity is much lower, and the structure can be thought of as essentially "fixed". Conducting the reduction in water, allows conformational change to assume the lowest energy configuration in the presence of maturing nanoparticles.

19.2 Nitrogen Porosimetry Analysis

Data collected from the nitrogen porosimetry analysis of MSl supported Ag nanoparticles, along with data of the MSl parent material, is presented in Table 1. All samples presented promising surface areas and mesopore volumes. AU samples presented nitrogen isotherms similar to those discussed previously for mesoporous (MSl) starches (i.e. type FV isotherm). Upon increased loading of Ag, textural properties as compared to the original MSl control samples, for supported silver (SAg) samples showed no real trend in terms of a reduction in surface area or mesopore volume for example, and in fact remained relative constant regardless of loading. However, the maximum pore diameter in the mesopore region seemed more sensitive to the increase in silver loading.

Table 5: Textural properties of SAg mesoporous starch (MSl) supported Ag nanoparticle samples investigated at different loadings.

Pore size distributions for these supported silver nanoparticle materials as compared against the mesoporous starch prepared as a control, show that with increasing loading, the distribution appears to be compressing, and becoming narrower and potentially more Gaussian like rather than the pore size shrinking and reducing consistently in terms of volume, Furthermore, the maximum in the mesopore region (NB: peak at 3.7 nm represent analysis artifact), gradually decrease in size as a function of Ag loading..

This behaviour is contrasting to previously discussed Au nanoparticles/MS materials, which showed a gradual decrease in all textural properties as a consequence of Au nanoparticle formation. Due to the limited solubility of the AgNθ₃ precursor in any solvent except water, the nanoparticle preparation step commenced in the aqueous phase. This allowed for deposition of the silver at the hydroxyl rich polysaccharide surface, where reduction was initiated. At this point, the porous polysaccharide network was quite flexible in the presence of water, and therefore any growth of nanoparticles in the mesopores of the gel state, may have induced alternative polymer network configuration, altering the pore size distribution. Wallen et al. have speculated that silver ions in aqueous solution may play a role in guiding the starch polysaccharide supramolecular organization.'⁰⁰' Whilst Vigneshwaran et al. have previously proposed that silver nanoparticles may be stabilised within the helical structure of soluble starch (amylose).^xxxι Ethanol was added slowly during the solvent exchange step, and presumably participated in the chemical reduction of any non adsorbed silver nitrate, and a competition between reduction at the surface or in solution was set up. This competitive reduction step plus the potential for pore size flexibility during initial reduction, may be the cause of the rather broad size of distribution of the Ag nanoparticles. 19.3 X-Ray Photoelectron Spectroscopy Analysis of Porous Starch Supported Ag Nanoparticle Materials.

MS supported silver nanoparticle materials (RW-SAg series) were characterised by X-ray Photoelectron Spectroscopy (XPS)..

The survey depicts the main chemical signatures for the polysaccharide support (i.e. C l(s) and O l(s) electrons). A small peak corresponding to the Ag 3(d) electrons can just be made out from the background. Unfortunately the loading of Ag in the RW- SAg series, was not high enough to enable determination of the Ag loading from the survey scan. High resolution XPS spectra of Ag 3d region for RW-SAg materials, shows single peaks at 368.4 and 374.4 eV which are assigned to the Ag 3(d)s_/2 and Ag 3(d)3/2 peaks, with relative intensities of 2:3, typical from electron orbitals such as the d orbitals, possessing an angular momentum quantum number greater than . Binding energy values reported here are similar to those for zero oxidation state silver (i.e. metallic), reported previously by Shah et al. for antibacterial silver doped titania materials.⁵⁰""

As loading of Ag nanoparticles increased, the relative intensity of the Ag 3(d) peaks increases accordingly, and become more resolved from the baseline. Previously, Ley et al. assigned the Ag 3d_S/2 peak for metallic Ag⁰ as ~ 368.5 eV; Ag²⁺ 367.2 and Ag⁺ ~- 367.6 eV. Silver nanoparticles prepared in this investigation, presented a Ag 3dsn peak at 368.9 (± ) eV, which is close to the literature value reported for Ag⁰. There was no evidence from this XPS analysis to indicate the presence OfAg⁺ or Ag²⁺ from physisorbed silver nitrate. Small variations in the binding energy may be attributed to silver interacting with electronegative oxygen rich polysaccharide surface, resulting in a small distortion of the silver atom electron density toward these electronegative atoms, leading to the small shift in binding energy, as compared to the literature values. Unfortunately, the loading of Ag in these materials was not significant enough to allow detection of the Auger core level of the Ag 3(d) orbital, which would have given the exact oxidation state value.

19.4 Diffuse Reflectance Solid State UV-Vis Spectroscopy (DRUVS)

Support for the finding of the metallic state of Ag via comes from the characteristic nanoparticle surface plasmon of metallic silver at about 447.5 (± 2.8) nm in DRUVS spectra for these supported silver materials. Some variation is seen with regard to the maximum of this surface plasmon and may be reflective of the transition in nanoparticle size, as the loading of silver increases.

There is a broad absorption band in the range 320 - 600 nm, with X^_x in the region of 444 - 450 nm depending on Ag loading. This absorption band arises from the excitation of surface plasmon vibrations of silver nanoparticles in the MSl supported media. As seen from TEM images of the silver nanoparticles, AgNP prepared here, are relatively well-dispersed within the porous polysaccharide matrix, presenting spherical nanoparticles of diameters between 10-20 nm. Sanchez-Cortes et a\. have previously reported that 10 nm silver nanoparticles in solution, present a well resolved surface plasmon resonance at ~ 400 nm.^mι" Here, the surface plasmon band for MSl supported silver nanoparticles was red-shifted and broadened, compared to spectra results reported previously by Sanchez-Cortes et al and Kapoor.^xxxιv Interestingly

Kapoor showed the ability to red-shift this surface plasmon feature by the addition of polyvinylpyrrolidone), gelatin and carboxymethyl cellulose. Here a red-shift is observed as function of increasing Ag loading. This shift (- 6 ran) is relatively small but could be related to firstly the change in electronic environment as a consequence of the polysaccharide matrix and / or the consequences of interactions between silver nanoparticles as their concentration increases and the resulting changes in the dielectric properties of the system.

Example 20

Investigation of the Antimicrobial Activity of mesoporous starch supported Ag nanoparticles.

The inhibitory effect of these mesoporous starch supported Ag nanoparticles was measured during the growth of E. coli and S. Aureus in LB broth at 37⁰C, by optical density at 600 nm. Both the gram-negative (E.Coli) and gram positive bacteria (S.Aureus) grew well and positively in the presence of the control experiment of mesoporous starch. The addition of mesoporous starch supported Ag nanoparticles on the other hand had a profound effect on the reproductive behaviour of the two bacteria. In both cases, materials appeared extremely inhibitory, even at these very low Ag loadings. The growth of both bacteria seemed to be more or less completely inhibited in the presence of these starch based nanoparticles. Marginal variation was observed for the inhibitory behaviour of different loadings. Results were almost identical for all Ag NP materials against E.Coli, but at sample RW-SAg2 (0.029 mmol/g Ag loading) appeared to be marginally less efficient for the inhibition of S.Aureus. MS supported Ag nanoparticles materials were also tested via Growth Inhibition studies on agar plates. Representative images of these studies are shown in Figure X. Approximately, 50 mg of material was loaded into a void cut into the growth plate which had previously been inoculated with either E. CoIi or S. Aureus. Plates were then left to incubate overnight at 37⁰C.

Materials presented good growth inhibition almost independent of Ag loading, and appeared more efficient against E.Coli, as demonstrated by slightly increased inhibition ring diameter (Figure 20).

Example 21

Supported Palladium Nanoparticle Materials

Herein the use of mesoporous starch as a novel promising support media for nanoparticle catalyst preparation, rendering materials active in a wide range of C-C coupling reactions. Materials were prepared in either acetone or ethanol solution, and the impact of such is discussed, with particular regard to nanoparticle size distribution.

21.1 Porosimetry Analysis

The starch-derived MSl support exhibited the typical characteristics of a mesoporous solid (i.e. a Type IV isotherm), as discussed earlier in this thesis. The BET surface area of the parent material prior to palladium addition was measured as 190 m² g^~l with an average pore size of 8.2 run. Table 6 summarises the textural properties and the palladium content of a series of mesoporous starch-supported materials. The surface area and the mesoporous structure of the material were preserved at lower Pd content. An increase of Pd content from 2.5 to 5 wt% resulted in a significant decrease in the surface area of the materials (Table 6; entries 3 and 6), as expected due to Pd deposition within the pores and/or on the surface that may lead to pore blocking. This is in good agreement with porosimetry results, in which a reduction of the average pore diameter was initially observed, followed by an increase as the smaller mesopores became filled at higher Pd loadings.

Table 6: Textural properties of MS 1 Supported Pd nanoparticle materials.

Upon incorporation of the Pd into the mesoporous starch, the pore size was reduced to ~7 nm for the materials prepared under reduction in acetone and to ca. 6 nm for the catalysts reduced in EtOH.

21.2 Powder XRD Characterisation

Powder XRP diffraction patterns obtained for the Pd-starch materials prepared at high Pd loadings, exhibited a broad reflection corresponding to the amorphous starch support. Four additional reflections were found in the XRD pattern that could be attributed to elemental palladium, in good agreement with JCPDS file: 46-1043.^xxxv Of note was the absence of any reflections in the diffractogram due to the presence of significant quantities of the Pd precursor on the support.

21.3 TEM Analysis

TEM micrographs showed well-dispersed Pd nanoparticles homogeneously distributed with a reasonably narrow nanoparticle diameter (ca. ~ 5 nm) (Figure 21).

The nanoparticle morphology was spherical with the average particle diameter at 4.0 and 2.5 nm, respectively, for materials prepared using ethanol and acetone as solvents.

Importantly, no significant Pd cluster formation was observed in the materials, even at high Pd content (i.e. 5 wt%). The average nanoparticle size distribution was further investigated from the TEM micrographs. There was a significant different in particle size and nanoparticle size distribution, as shown on Figure 21, for the two different solvents (ethanol and acetone) employed in the synthesis procedure. Both materials, irrespective of the palladium loading, exhibited two major peaks in the nanoparticle size distribution at ca. 1.5-2 and 3.5 - 4.5 nm. The smaller nanoparticles are believed to be formed within the starch micropores, as suggested earlier with regard to Au nanoparticles, whereas the bigger palladium nanoparticles grew within the starch mesopores.

The nanoparticle size distribution resembles the pore size distribution of the parent mesoporous starch. This difference in nanoparticle size can be related to the way the palladium nanoparticles are either reduced mainly by the solvent (ethanol) or by the functionalities present on the starch surface (acetone). Differences in nanoparticle sizes were found to have a notable effect on the catalytic activity of the materials, when employed in Suzuki, Sonogashira and Heck reactions

Example 22

Supported Cadmium Sulphide (CdS) Quantum Dot Materials.

Preparation of CdS QD/porous starch hybrid materials was conducted, adapting a method developed by from Hullavarad et al., which conveniently allowed the introduction of the cadmium acetate and sodium sulphide precursors, during the solvent exchange step in the preparation of mesoporous starch.^xxxvι Cd²⁺ was adsorbed within the pores / on the surface of the nanoporous polysaccharide by means of a ethanol solution of cadmium acetate. The formation of the CdS nanoparticles was evident from the immediate change of colour to yellow, upon addition of an ethanol solution of sodium sulphide. The system was solvent exchanged to 100% ethanol, and then extensively washed with ethanol to remove any unreacted reagents. Acetone was not used for solvent exchange in this experiment. Porous starch sample (MS) and CdS materials were also prepared in the absence of each other, as control experiments. Nitrogen Porosimetry Analysis

Materials were prepared at varying cadmium to starch ratio (w/w). Promisingly all samples presented good textural properties, similar to conventional prepared mesoporous starch (MSl). A sample pf CdS prepared in the absence of starch, presented a low surface area material (~ 5 m²g^~l). Results from the nitrogen porosimetry analysis of these materials are displayed in Table 3.

Table 7: Nitrogen Porosimetry Data for CdS nanoparticle / mesoporous starch hybrid materials.

All materials presented excellent textural characteristics independent of CdS loading, with high surfaces areas and pore volumes typical. Surprisingly, and contrary to results observed for Pd and Au nanoparticle supported on MS, the textural properties of these materials actually increased as a consequence of a CdS loading increase. This is seen as an increase in the average pore diameter values for these materials, and a transition in the pore size distributions. Increasing CdS loading, results in a shift to higher pore sizes and broader pore size distribution sizes relative to the control MS samples. This is reflected in a corresponding increase in the total pore and mesopore volume of the materials as the number of large mesopores (PD > 20 nm) increases. These observed changes in textural properties as a result of increased CdS nanoparticle loading, as opposed to opposite trends for Au nanoparticle preparation for example, may be in part due to the preparation methodology, in terms of the sensitivity of the polysaccharide gel system to alterations in preparation and retrogradation temperature. It therefore can be envisaged that alterations in the ionic potential of the aquagel system, for example by salts (e.g. sodium acetate by-product), may act to reorganises the polysaccharide network (e.g. starch amylose), during the solvent exchange process. The polysaccharide is in a metastable state and will seek to order itself to derive the most entropy from the system; the pore system adapts to accommodate the growing nanoparticles.

During the preparation of Au nanoparticles, by contrast the polysaccharide network is essentially stable in acetone solvent, with surface area and pore volume lost as the reduction of the metal precursor occurs at the hydroxyl adsorption sites within the pore structure. Here, as CdS nanoparticles grow, the polysaccharide structure is still flexible, particular in the presence of water, and may rearrange the hydrogen bond network accordingly, whilst in acetone, the material structure is stable. REFERENCES

' (a) X.L. Luo, A. Morrin, A.J. Killard, M.R. Smyth, Electroanalysis, 2006, 18, 319; (b) Y.Sun, W.

Yugang; H. Hau, Appl. Phys. Lett. 2007, 90, 213107/l;(c) Y.C. Chen, Z.M. Tang, M.Z. Gui, J.Q.

Cheng, X.Wang, Z H. Lu, Chem. Lett. 2000, 1140; (d) M.J. Gronnow, RJ. White, J.H. Clark and D.J.

Macquarrie, Org. Proc. Res. Dev., 2005, 9, 516; (ε) M. Harada, K. Asakura, N. Toshima, J. Phys.

Chem. 1993, 97, 5103

" J. Tsuji, "Palladium Reagents and catalysts", Wiley, 2004

'" (a) A. Barau, R. Luque, V. Budarin, A. Prelle, V.S. Teodorescu, A. Caragheorgheopol, D.J.

Macquairie, M. Zaharescu, Proceedings of the 14* International Sol-Gel Conference, September 2007,

Montpellier (France); (b) A. Safavi, M. Afsaneh; N. Maleki, F. Tajabadi, E.Farjami, Electrochem.

Commun. 2007, 9, 1963; (c) M.F. Lengke, M.E. Fleet, G. Southam, Langmuir, 2007, 23, 8982 ^iv L. N. Lewis, Chemical Reviews, (1993), 93, 2693 - 2730

^¥ H. Huang and X. Yang, Carbohydrate Research, (2004), 339, 2627 - 2631

" C- Aymonier, U. Schlotterbeck, L. Antonietti, P. Zacharias, R. Thomann, J. C. Tiller and S. Mecking,

Chemical Communications^ 2002), 3018 - 3019 m N. Vigneshwaran, R. P. Nachane, R. H. Balasubramanya and P. V. Varadarajan, Carbohydrate

Research, (2006), 2012 - 2018

™' P. Raveendran, J. Fu, and S. L. Wallen, Journal of the American Chemical Society, (2003), 125,

13940 - 13941

" W. K. Son, J. H. Youk, T. S. Lee, and W. H. Park, Macromolecular Rapid Communications, (2004),

25, 1632 - 1637

¹ 1. Sondi, and B. Salopek-Sondi, Journal of Colloid and Interface Science, (2004), 275, 177-182 ^xi J. Tsuji, Palladium Reagents and Catalysts, Wiley International Ltd, Chichester, 2004. xⁱⁱ W. C. W. Chan, and S. Nie, Science, (1998), 281, 2016 - 2018

"^H K. J. Green and R. Rudham, Journal of the Chemical Society, Faraday Transactions, (1992), 88,

3599 - 3603 x^iv D. L. Klein, R. Roth, A. K. L. Lim, A. P. Alivisatos, and P. L. McEuen, Nature, (1997), 389, 699 -

701. x^v (a) F. Caruso, Advanced Materials, (2001), 13, 1, 11 - 22; (b) C. Sanchez, G. J. de A. A. Soler-Illia,

F. Ribot, T. Lalot, C. R. Mayer, and V. Cabuil, Chemistry of Materials, (2001), 13, 3061 - 3083. ^xvi W. Jiang, A. Singhal, J. Zheng, C. Wang, and W. C. W. Chan, Chemistry of Materials, (2006), 18,

4845 - 4854 x^vii J. C. Yu, L. Wu, J. Lin, P. Li, and Q. Li, Chemical Communications, (2003), 1552 - 1553 x^viii M. J. Hosteller, A. C. Templeton, and R. W. Murray, Langmuir, (1999), 15, 3782 - 3789 ^Λ S. Link and M. A. El-Sayed, Journal of Physical Chemistry B, (1999), 103, 8410 - 8426

^Xlt (a) M. Brust, M. Walker, D. Bethell, D. J. Schifirin and R. Whyman, Chemical Communications,

(1994), 801 - 802; (b) H-G. Boyen, G. Kastle, F. Weigl, B. Koslowski, C. Dietrich, P. Ziemann, J. P.

Spate, S. Riethmuller, C. Hartmann, M. Moller, G. Schmid, M. G. Gamier, and P. Oelhafen, Science,

(2002), 297, 1533

**' (a) B. R. Cuenya, S-H. Baeck, T. F. Jaramillo, and E. W. McFarland, Journal of the American

Chemical Society, (2003), 125, 12928 - 12934; (b) A. Howard, D. N. S. Clark, C. E. J. Mitchell, R. G.

Egdell, and V. Dhanak, Surface Science, (2002), 518, 210 - 224.

*™ M. Mandal, S. K. Ghosh, S. Kundu, K. Esumi, and T. Pal, Langmuir, (2002), 18, 7792 - 7797 O. R. Miranda and T. S. Ahmadi, Journal of Physical Chemistry B, (2005), 109, 15724 - 15734 xxiv A. K. Sinha, S. Seelan, S. Tsubota, and M. Haruta, Angewandte Chemie International Edition,

(2004), 43, 1546 - 1548 x^xv M. L. Anderson, C. A. Morris, R. M. Stroud, C. I. Merzbacher, and D. R. Rolison, Langmuir,

(1999), 15, 674 - 681

"" W. J. Leigh and J. C. Scaiano, Journal of American Chemical Society, (1983), 105, 5652 - 5657

*^Xϊii K. Nishioka, Y. Niidome, and S. Yamada, Langmuir, (2007), 23, 10353 - 10356

**** E. Stathatos and P. Lianos, Langmuir, (2000), 16, 2398 - 2400

"* V. Homebecq, M. Antonietti, T. Cardinal, and M. Treguer-Delapierre, Chemistry of Materials,

(2003), 15, 1993 - 1999

*" P. Raveendran, J. Fu, and S. L. Wallen, Journal of the American Chemical Society, (2003), 125,

13940 - 13941

"" N. Vigneshwaran, R. P. Nachane, R. H. Balasubramanya and P. V. Varadarajaπ, Carbohydrate

Research, (2006), 341, 2012 - 2018

*"" J. Thiel, L. Pakstis, S. Buzby, M. Raffi, C. Ni, D. J. Pochan, and S. I. Shah, Small, (2007), 3, 799 -

803

*^»>"" M V. Canamares, J. V. Garcia-Ramos, J. D. Gomez-Varga, C. Domingo, and S. Sanchez-Cortes,

Langmuir, (2005), 21, 8546 - 8553.

™*^r S. Kapoor, Langmuir, (1998), 14, 1021 - 1025

""" Palladium, JCPDS 46-1043, 1969.

^*"¹ N. V. Hullavarad, and S S. Hullavarad, Photonics and Nanostructures - Fundamentals and

Applications, (2007), 5, 156- 163.

Claims

Claims

1. A material comprising a metal nanoparticle supported on a porous expanded polysaccharide derived material (PPDM) wherein the nanoparticle has a diameter of from 1 to 30nm.

2. A material which is nanoporous and which acts as a reducing agent, template, and support for metal nanoparticles.

3. A material according to claim 2 wherein the nanoparticles are produced as a result of interaction of the metal precursor(s) and the surface of the polysaccharide.

4. A material according to claim 3 wherein the nanoparticle has a diameter which is constrained by the porosity of support material.

5. A material according to claims 1 or 2 wherein the polysaccharide is selected from one or more of amylopectin, amylose, alginic acid, pectin and xylan.

6. A material according to claim 1 wherein the polysaccharide is microporous.

7. A material according to claims 1 or 2 wherein the polysaccharide is mesoporous.

8. A material according to claims 1 or 2 wherein the distribution of the pore size is substantially uniform.

9. A material according to claim 1 or 2 wherein a reducing agent is absent.

10. A material according to claim 5 wherein the polysaccharide is microporous and mesoporous.

11. A material according to claims 1 or 2 wherein the metal is selected from a noble metal and Fe.

12. A material according to claim 11 wherein the noble metal is selected from the group consisting of copper, gold, iridium, , osmium, palladium, platinum, rhenium, rhodium, ruthenium and silver.

13. A process for the preparation of a material comprising a metal nanoparticle supported on a porous expanded polysaccharide derived material (PPDM) which comprises the steps of;

(i) adding a porous polysaccharide to a solvent;

(ii) adding a metal salt to the product of step (i); (iii) agitating the mixture at elevated temperature; and

(iv) separating the supported nanoparticles from the mixture.

14. A process according to claim 13 wherein the porous polysaccharide is mesoporous.

15. A process according to claim 13 wherein the porous polysaccharide is microporous.

16. A process according to claim 13 wherein the porous polysaccharide is prepared by providing microwave energy.

17. A process according to claim 13 wherein the porous polysaccharide is prepared by providing thermal energy.

18. A process according to claim 13 wherein the porous polysaccharide is dried by solvent exchange.

19. A process according to claim 13 wherein the porous polysaccharide is dried by supercritical carbon dioxide.

20. A catalyst comprising a metal nanoparticle supported on a porous polysaccharide derived material.

21. A medical dressing comprising a metal nanoparticle supported on a porous polysaccharide derived material.

22. A sensor comprising a metal nanoparticle supported on a porous polysaccharide derived material.

23. A metal nanoparticle or a process or the use substantially as hereinbefore described with reference to the accompanying drawings.