Composite structured nanomaterials

Abstract

A composite structured nanomaterial comprises a plurality of discrete interconnected nanoparticles separated and bound within a molecular or atomic matrix material. The nanoparticles preferably compri

Description

Field of the Invention The invention concerns a composite structured nanomaterial comprising discrete, interconnected nanoparticles separated by and bound within a matrix material. These unique materials display optical properties that may be varied or tuned by altering their structural features. Due to a combination of their tuneable optical properties and ease of conjugation to organic / biological molecules, the nanomaterial of the invention finds multiple uses in, e.g., the diagnostic industry. Further, the unique chemical and physical properties of the structured nanomaterial suggest utility in a wide array of applications, including but not limited to catalysis, medicine and photonics. Also disclosed are simple, environmentally friendly methods to produce the structured nanomaterial, which do not require the use of harsh reducing agents or chemicals and hence provide a green production route to un-contaminated and pure nanomaterials. Background of the Invention Nanoparticles, also known as atomic clusters, are assemblies of atoms (or molecules) with a size of upto about 100 nm. The ability to exploit, and in some cases tailor, the unique chemical or physical properties of such nanoparticles has now enabled them to be used in a wide spectrum of applications, such as catalysis, medicine (both diagnostics and therapy) and photonics. Metal, in particular gold, nanoparticles, represent one particular subset of nanomaterials that has shown distinctive behaviour on the nanoscale, and thus has contributed to progress in nanomedicine and nanotechnology. Specifically, colloidal gold suspensions (in which a gold core is capped by an organic molecular, e.g. polymer, layer) are now commercially available, with the largest application area being biomedicine, with an estimated annual worldwide value of ~ $5 billion in 2022, which is expected to grow to ~$10 billion by 2027 with a compound annual growth rate in excess of 10%. The main conventional methods for the synthesis of colloidal gold nanoparticles, which enhances the commercial value of gold per gram by a factor of greater than 100 compared with gold bullion, can be divided into two types. The first type is based on the top-down processing of bulk gold under high-voltage electrical current or powerful irradiation (e.g., laser ablation) in a liquid, wherein the metal atoms are separated and then condense into nanoparticles in the liquid dispersion medium. However, such bulk gold processing methods tend to produce gold nanoparticles with rather non-uniform sizes, which can make the processes inefficient and / or the colloidal nanoparticles unsuitable for use in key market areas such as biomedicine as a result of, e.g., non-uniform optical properties. The second, more popular, type is based on the wet-chemical reduction of gold salts (e.g. halides), with subsequent centrifugation, and has been widely used for colloidal gold synthesis for a number of decades. Such wet-chemical reduction processes do offer a good degree of size control and, by extension, optical properties; greater power and concentration of reducing agent generally equates to smaller gold nanoparticles. However, the processes utilise harsh chemical reducing agents such as sodium borohydride and, typically, strong acids such as tetrachloroauric acid (HAuCk) in aqueous solution, which are reduced to produce approximately spherical, or non-spherical, gold nanoparticles. Such wet-chemical reduction processes, therefore, have the potential to result in the formation of impure nanoparticles that include some degree of contaminants (e.g. chlorine and / or boron from the residual salt and reducing agent, respectively), and may render the colloidal nanoparticles unsuitable for ideal use in key market areas such as biomedicine and cosmetics. The current state of the art in the synthesis of colloidal nanoparticles extends beyond the basic concept of a quasi-spherical core of, e.g., metal atoms capped by a layer of a molecular, e.g. organic, ligand layer to include non-spherical cores such as rods, core surfaces reconstructed by the ligands (e.g. thiolated gold clusters), and dimers of connected colloidal particles. Coresatellite structures are also known, in which a central colloidal core particle is fully or partially surrounded by a layer of smaller particles. Further it is known that some or all of the ligand material(s) can sometimes be partially or fully removed by selected processing methods such as heating, chemical reduction, chemical oxidation, plasma treatment or photochemical processes to leave, the colloidal particle partly denuded of ligands for, e.g. catalytic applications. The current invention extends beyond the current state of the art of colloidal nanoparticles, in that it provides a new composite nanomaterial comprising a quasi-two-dimensional raft type structure comprising multiple colloidal nanoparticles cross-connected to other colloidal nanoparticles (which may be of a single or multiple species) to form an extended network of finite extent. The raft type structure is robust, and can persist in or out of solution, e.g. on top of solid surfaces. In particular, raft type structures, either regular or irregular shaped, of about 10nm to about 100 microns in size are envisaged, comprising of nanoparticles in the diameter range of from about 1 to about 1000nm, and gaps between the closest points of proximity of adjacent nanoparticle cores in the range of from about 0.5 nm to less than about 1000nm. Further, the nanoparticles may be inorganic or organic and / or may be metallic, semiconducting or insulating in terms of their electronic structure (band gap). Their arrangement with respect to each other may be crystalline, disordered or partially ordered. Further, the cross-connecting species, or matrix materials, may be organic or inorganic, e.g. polymer or biological molecules, and are bonded (chemically or physically) to one or more nanoparticles, and possibly to other cross-connecting species. The structure of the quasi 2D nanomaterial or raft type structures of the invention is somewhat akin to structured three dimensional ‘metamaterials’, which are well known in the art of photonics. Therefore, these composite nanomaterial strictures have analogously been termed ‘metaparticles’, with the individual nanoparticles and cross-connecting linkers or matrix materials forming elements of the metaparticle. The structure of the metaparticles leads to a series of intriguing and potentially useful properties and behaviours, including: The optical properties of the metaparticle will depend on coupling between the optical properties of the component nanoparticles, which might be plasmonic or excitonic metal particles (depending on their size). This is expected to lead to a colour for the metaparticle that depends upon the spacing between the nanoparticles and their shape, and which may further depend on the viewing angle (i.e. iridescence). This colour may be structural in nature, i.e. dependent on light scattering, rather than absorptive like conventional dyes. The metaparticles may also be colourless in the visible region of the spectrum but show similar structural light scattering in other regions of the spectrum, to shorter or longer wavelength with respect to the visible range (~400-700nm). These colour effects may find a wide variety of applications, including but not limited to jewellery, make-up, paints, pigments and decorative arts. Moreover the concentration of field in the gaps between nanoparticles may give rise to optical hot spots of the kind that could be exploited in surface enhanced Raman spectroscopy (SERS) and kindred techniques. Since colloidal particles find application as visualisation agents and sensitisers in theranostics, it follows that the above tuneable colours of the metaparticles may also find utility in these applications, for example in multiplex diagnostic lateral flow assays (cf. the familiar home Covid diagnostic test in which the single red test line is formed from conventional colloidal gold); such a test could exploit the chemistry of gold or other nanoparticles in different metaparticles to generate a set of different coloured diagnostic tests lines for different diseases or conditions. Further, the extended quasi-two dimensional arrangement of the nanoparticles in the raft type structures presents a larger surface area than a sphere of equivalent volume. Moreover it provides the possibility of chemical or physical bonding of molecules across more than one individual nanoparticle in the metaparticle. For this reason the metaparticles may lead to improved conjugation behaviour, whether passive or covalent. This is relevant to diagnostic tests, wherein the visualisation particle is generally conjugated to a biological molecule, and to many other potential applications. The tunability of the metaparticles, in terms of the size of the nanoparticles, the interparticle spacing and the nature of the matrix materials linking them may control not only the optical properties but also other useful properties too, such as magnetic coupling, spin coupling and electronic conductivity (including neuromorphic properties) across the metaparticle structure. The metaparticles are tuneable, prefabricated and processable building blocks or products for applications in photonics, electronics, information processing / computing, magnetics, sensors as well as photovoltaics and other light-absorbing applications. A further potential application of the metaparticles is in catalysis, photocatalysis and electrocatalysis. The nanoparticles may be catalytically active, either with the linking molecules I matrix materials present or after limited removal thereof. This chemical activity will depend on the atomic arrangement and electronic structure of the nanoparticles. However the gaps (channels) between the nanoparticles present an additional class of potentially active sites for chemistry that could not exist in a single quasi-spherical particle. An especially positive prospect is the catalytic transformation of a molecule bonded to more than one of the interconnected nanoparticles within the raft type structure. In this case the channels in the metaparticles somewhat resemble natural enzyme structures. The channels may also impact the antimicrobial properties of the metaparticles. There is, therefore, a hitherto unmet need for an easy conjugatable and clean gold nanomaterial with varying optical properties, and for an efficient, environmentally friendly, route to producing same. These unmet needs are addressed by the unique composite nanomaterial disclosed herein, which comprises a stabilized interconnected network of nanoparticles, and the simple, cost-effective route to prepare said nanomaterial in a contaminant free suspension that reduces solvent and reagent usage and thus environmental and health risks. Statements of Invention The present invention is as set out in the accompanying claims. According to a first aspect, the invention provides a composite structured nanomaterial comprising a plurality of discrete, interconnected nanoparticles separated by and bound within a molecular or atomic matrix material. It has been surprisingly found that these structured nanomaterials can be produced using a cost-effective method that does not require the use of harsh chemicals or reducing agents and can be tailored to provide nanomaterials showing unique optical properties. As used herein, the term ‘structured nanomaterial’ refers to a material that a) has an average size (in at least one dimension) of from about 10 nm to about 100 pm, and b) comprises a plurality of interconnected nanoparticles. Throughout this application, the terms ’’nanomaterial” and / or “structured nanomaterial” is / are used interchangeably with the term ‘metaparticle’. In preferred embodiments, the structured nanomaterials of the present invention have an average unit diameter of at least about 50 nm, more preferably at least 100 nm, and more preferably at least about 200 nm. Alternatively or additionally, the nanomaterials preferably have an average unit diameter of no more than about 50 pm, more preferably no more than about 10 pm and still more preferably no more than about 1 pm. As used herein, the term ‘nanoparticles’ refers to organic or, preferably, inorganic particles that are between about 1 to about 1000 nm, preferably from about 2 to about 100 nm, and still more preferably from about 5 to about 50 nm in diameter, and comprise clusters or assemblies of atoms or molecules. As the skilled reader will readily appreciate, nanoparticle size is typically recorded as a circular equivalent diameter (ECD), i.e. the diameter of a circle having the same projected areas as the nanoparticle, using conventional image processing programs. In preferred embodiments, such nanoparticles comprise or consist of clusters or assemblies of a plurality of organic or, more preferably, inorganic atoms or molecules. In particularly preferred embodiments, such nanoparticles comprise or consist of clusters or assemblies of a plurality of metal atoms or metal containing molecules. In alternative embodiments, such nanoparticles may comprise or consist of a clusters or assemblies of a plurality of non-metallic semiconductor (e.g. silicon or germanium) or insulator (e.g. glass) particles. In further alternative embodiments, such nanoparticles may comprise or consist of a clusters or assemblies of a plurality of carbon atoms or carbon containing molecules. In particularly preferred embodiments, the nanoparticles comprise or consist of metal atoms, wherein said metal atoms are preferably selected from gold, silver, copper, palladium, platinum, iron, titanium or an alloy comprising one or more of said metals. In exemplary embodiments, the nanoparticles comprise or consist of gold. As the skilled person will readily appreciate, nanoparticles can adopt a wide variety of morphologies, and include but are not limited to spherical, elliptical, rod-like, star like, disc-like or worm-like particles. Therefore, nanoparticle morphology is not in any way limited in the broadest aspects of the invention. However, in preferred embodiments, the nanoparticles are of a spherical, elliptical, star-like, rod-like, disc-like or worm-like morphology. As used herein, the term ‘matrix material’ refers to any atomic or molecular material that can be co-deposited, either simultaneously or sequentially, together with a plurality of nanoparticles, onto the surface of a substrate in order to form a stabilized nanomaterial wherein said matrix material is disposed between and bound to adjacent nanoparticles. Further, these matrix materials can play a key role in altering the physico-chemical and biological characteristics of the formed nanomaterial, with the steric and chemical bonding (e.g. charge exchange) effects of said materials being responsible for modifying said characteristics. Suitable matrix materials include, but are not limited to, organic polymers, inorganic polymers and / or hybrid polymers, i.e. polymers formed from a mixture of organic and inorganic monomeric units. The matrix material may comprise a single material or a mixture of multiple different materials. Therefore, at least in the broadest aspects of the invention, the nanomaterial of the present invention is not limited to include any specific matrix material, which may be selected by the skilled reader based on need and / or availability without difficulty. However, in preferred embodiments, the matrix material comprises or consists of one or more organic agents and, in particularly preferred embodiments, the organic agent is selected from polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), bovine serum albumin (BSA), ethylene diamine tetra acetic acid (EDTA), Chitosan, a quaternary ammonium surfactant, an amino acid, a protein, citrate or any combination or polymerised variant thereof. In exemplary embodiments, the organic agent is PVP. The nanomaterial of the present invention comprises discrete, interconnected organic or, preferably, inorganic nanoparticles separated by and bound within a matrix material. It will be readily appreciated, therefore, that the nanomaterial can be further characterised by, e.g., the average interparticle spacing, the ratio of nanoparticles to matrix material, the average thickness of the nanomaterial, and / or the optical characteristics of the nanomaterial. In preferred embodiments, the structured nanomaterial has an average interparticle spacing, measured as an edge-to-edge distance between adjacent nanoparticles, of at least about 0.1 nm, more preferably at least about 0.5 nm and still more preferably at least about 1 nm. Further, it will be readily appreciated that the nanomaterial need not be restricted to any particular maximal interparticle spacing beyond that imparted by the physical dimensions of the structured nanomaterial. However, in preferred embodiments, the structured nanomaterial of the present invention has an average interparticle spacing of at no more than about 1 pm, more preferably no more than about 100 nm and still more preferably no more than about 10 nm. Alternatively or additionally, the structured nanomaterial of the invention preferably comprises said nanoparticles and said matrix material in a ratio of from about 0.5:99.5 to about 99:1, or from about 1:99 to about 50:50, or from about 5:95 to about 20:80 . As will be readily appreciated, this ratio can be 8 measured in a variety of ways, and can be based on, e.g., the deposited weight of nanoparticles and matrix materials, each of which may be estimated or measured in a variety of ways. For example, this ratio may be estimated using the deposition rates for PVD deposited nanoparticles and, if applicable, matrix material. Alternatively, if the matrix material is applied as a simple coating, this ratio can be estimated by comparing nanoparticle deposition rate with the concentration of the matrix material in the applied coating. Alternatively, it will be appreciated that this ratio may be measured more accurately using conventional spectroscopic and / or thermal analysis techniques. For example, inductively coupled plasma mass spectrometry (ICP-MS) may be utilised to measure the concentration of nanoparticles such as gold nanoparticles, and thermogravimetric analysis (TGA) may be used to measure the amount of matrix, particularly organic, matrix materials. However, for ease of measurement, in preferred embodiments in which the matrix material is visible using conventional microscopic techniques such as SEM and TEM, this ratio can be based on the total visible surface area of the nanoparticles and matrix materials within the nanomaterial, using conventional microscopy and image processing programs. The nanomaterial of the invention can be prepared as a single or a multiple layer structure. However, in preferred embodiments, the nanomaterial is a single layer structure. In such embodiments, the average thickness of the nanomaterial is preferably from about 1 nm to about 500 nm, and more preferably from about 2 nm to about 50 nm, and is primarily dependant on the size of the interconnected nanoparticles and ligand length. The nanomaterials of the invention have been shown to function as a photonic metamaterial, and can be tuned by varying structural features (e.g. interparticle spacing) to manipulate light at a range of optical frequencies. In preferred embodiments, the nanomaterial of the present invention has, when suspended in a liquid solvent, preferably water, an absorption maximum (Amax) of from about 400 nm to about 800 nm, and more preferably from about 500 nm to about 700 nm. Highly pure, size-controlled nanomaterials according to the first aspect of the invention can be prepared via a cost-effective hybrid physical-chemical method that minimises I reduces, and preferably eliminates, usage of expensive and / or toxic solvents and reagents compared with conventional nanoparticle synthetic processes, and reduces environmental and health risks associated with such methods. Therefore, according to a second aspect, the invention provides, a method for preparing the structured nanomaterial according to the first aspect of the invention, said method comprising: i) providing a solid substrate; ii) depositing one or more molecular or atomic matrix materials and forming a plurality of nanoparticles on the surface of said substrate, wherein: (A) a film comprising or consisting of one or more matrix materials is deposited onto the surface of said substrate to form a coated substrate; and nanoparticles are formed on the surface of said coated substrate by physical vapour deposition under vacuum conditions to form a nanomaterial coated substrate; or (B) nanoparticles are formed on, and one or more matrix materials are deposited onto, the surface of said substrate by physical vapour deposition under vacuum conditions to form a nanomaterial coated substrate; iii) removing said nanomaterial coated substrate from vacuum conditions; and iv) washing said nanomaterial coated substrate in a liquid solvent under non vacuum conditions to dissolve the nanomaterial coating and form a suspension of said structured nanomaterial in said solvent. The process of the present invention comprises the creation of nanoparticles on the surface of said substrate by physical vapour deposition. As will be readily appreciated, such a process may comprise the direct deposition of preformed nanoparticles by, e.g., a cluster deposition process in which nanoparticle clusters are formed via condensation in the gas phase prior to deposition onto the surface of said substrate. Alternatively, the deposition process may comprise an atom / molecule deposition process, wherein individual atoms or molecules are deposited, and then diffuse and aggregate into nanoparticles, on the substrate surface. As would be readily appreciated by the skilled reader, the term ‘vacuum conditions’ refer in the context of the present specification to a closed environment having gas pressure of about 10-1 Pa or below, preferably about 10-3 Pa or below, and more preferably about 10-4 Pa or below. Conversely, in the context of the present specification, ‘non-vacuum conditions’ refers to an open or closed environment having a gas pressure of greater than about 102 Pa, preferably greater than about 103 Pa, and more preferably greater than about than about 104 Pa. In preferred embodiments, ‘non-vacuum conditions’ refer to a standard atmospheric pressure environment (at sea level, this equates to a pressure of about 101 kPa). According to the broadest embodiments of the second aspect, the matrix material may be deposited onto the surface of the substrate by a process of coating or by PVD. Where a coating process is utilized, the matrix material is preferably deposited by spin coating. Alternatively, where the matrix material is deposited by PVD, the nanoparticles and / or matrix materials are preferably independently formed / deposited / by an evaporation deposition or a pulsed injection deposition process. In one particularly preferred embodiment, the nanoparticles are formed by evaporation deposition, and the matrix materials are deposited by pulsed injection deposition. As will be readily appreciated, the term ‘pulsed injection deposition’ refers to a process in which a solution comprising molecules to be deposited are 11 transferred into a vacuum deposition chamber, where the solvent evaporates, and the molecules are subsequently deposited in the vapour phase onto a substrate surface. The use of such a deposition method thus avoids the need to couple thermal energy to sublimate the material prior to deposition, and so is particularly suitable for the deposition of fragile molecules, such as organic matrix materials used in embodiments of the present invention, without damage resulting from excitation of rotation I vibration modes. The PVD deposition I formation of nanoparticles or nanoparticle precursor materials and matrix material in step (ii) of the method of the invention can be carried out in separate independent deposition steps, or both the nanoparticles I nanoparticle precursors and matrix material may be co-deposited in a single deposition step. Therefore, in some embodiments, deposition step (ii) comprises or consists of the simultaneous co-deposition of nanoparticles or nanoparticle precursor material and matrix material on the surface of the substrate by physical vapour deposition under vacuum conditions to form a nanomaterial coated substrate. This co-deposition step may be repeated one or more times to build film thickness as required. In alternative embodiments, the deposition step (ii) comprises or consists of the sequential deposition of a matrix material layer followed by the formation of a nanoparticle layer on the surface of the substrate by physical vapour deposition under vacuum conditions to provide the nanomaterial coated substrate. As will be readily appreciated, these sequential deposition steps may be repeated one or more times to form two or more layers of the nanomaterial. The solid substrate on which the nanomaterial is formed is also not particularly limited and includes, but is not limited to, silicon or carbonbased materials (such as graphene, carbon nanotubes, fullerenes and amorphous-Carbon), an oxide, a nitride, a glass, or a MXene. However, in some embodiments, the substrate is a silicon material, which may optionally be doped with one or more heteroatoms (e.g., boron or phosphorus) which regulate the electrical and thermal properties. The presence of dopants or defects at the surface of the substrate may be advantageous in controlling surface diffusion of the deposited nanoparticles within the matrix material on the substrate surface. The extent of film thickness of the deposited nanomaterial on the surface of the substrate can be measured or calculated by a variety of methods. For example, film thickness is typically calculated by layer deposition rate and deposition time, and / or measured with spectroscopic ellipsometry or a surface profiler. Preferably, the nanomaterial film thickness varies from a single monolayer to about 100 pm, more preferably from about 2 nm to about 50 pm. Where a single monolayer is formed, the film thickness is preferably from about 2 nm to about 50 nm, and is primarily dependent on the size of the deposited nanoparticles and ligand length. Additionally or alternatively, the weight ratio of deposited nanoparticles to matrix material within the nanomaterial layer, which can be controlled and calculated by the values from quartz crystal microbalance (QCM) measurements, preferably ranges from about 0.5:99.5 to about 99:1 or from about 1:99 to about 50:50, or from about 5:95 to about 20:80. As will be readily appreciated, the choice of solvent used in wash I dissolution step (iv) is not particularly limited, provided the chosen solvent is compatible with the matrix component of the nanomaterial coating. For example, in preferred embodiments the solvent is water (for environmental and biocompatibility reasons), and the matrix material is an organic water-soluble ligand such as PVP. Wash step (iv) may immediately follow (e.g. within 1 day) the removal of the nanomaterial coated substrate from vacuum conditions. However, it will also be readily appreciated that said wash step may be delayed for a period of time (e.g. from 2 days to 6 months or more), thereby permitting storage and / or transport of the nanomaterial coated substrate in a preferred solid form, before forming the colloidal structured nanomaterial at a later time point and / or location as desired. The specific method by which the coated substrate is washed in dissolution step (iv) is not limited, provided that the substrate is brought into contact with the solvent to dissolve the nanomaterial coating, thereby forming a suspension of nanomaterial in said solvent. Therefore, in some embodiments, the coated substrate is washed by exposure to a flow of solvent, e.g., by spraying. However, in preferred embodiments, the coated substrate is washed by immersion of the coated substrate in a fixed volume of solvent, thereby forming a high concentration of nanomaterial in said solvent. In such embodiments, the immersed coated substrate is preferably subjected to sonication (i.e. the application of ultrasonic (> 20 kHz) sound energy) to agitate the substrate and enhance the release of the structured nanomaterial from the substrate. In an alternative, also preferred, embodiment dissolution step (iv) comprises contacting the surface of said coated substrate with an absorbent pad or swab to at least partially, and preferably fully, transfer the nanomaterial to said absorbent pad or swab, and then submerging said pad or swab in a fixed volume of said solvent thereby forming a suspension of said nanoparticles. Such a dissolution process enables the production of a highly concentrated suspension of nanomaterial in said solvent. The skilled reader will also appreciate that the method of the invention may advantageously comprise one or more additional process steps. However, in preferred embodiments, if any additional processing step(s) are included, said steps do not include the deposition of one or more additional solvent (e.g. water) soluble layer(s), and more preferably does not include the deposition of any other layer(s) to the nanomaterial coated substrate formed in step (ii). In one such preferred example, the nanomaterial coated substrate is subjected to a processing step, after step (ii) but prior to step (iv), to modify nanoparticle size and / or interparticle spacing. Suitable processing methods include, but are not limited to, heating, ion-beam irradiation, laser irradiation, and electron beam irradiation. Where ion-beam or electron beam irradiation is employed, the substrate upon which the nanomaterial is formed is preferably doped with one or more dopant materials to provide a charge conducting substrate or enhance the conducting property of an inherently conducting substrate. In such embodiments, said processing step may be conducted under vacuum conditions, prior to step (iii). In alternative embodiments, the processing step is conducted ex vacuo, after step (iii). In particularly preferred embodiments, said processing step is a heating step, wherein said heating step preferably comprises exposing said nanomaterial coated substrate to an elevated temperature of from about 100 °C to about 250 °C for a period of at least about 2 hours. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises” mean “including but not limited to” and do 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. Throughout the description and claims of this specification, the term “about” includes a variation of ± 10%, preferably ± 5%, more preferably ± 4%, still more preferably ± 3%, still more preferably ± 2% and most preferably ± 1% unless the context otherwise requires. Any references, including any patent or patent application, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. Further, no admission is made that any of the prior art constitutes part of the common general knowledge in the art. Other features of the present invention will become apparent from the following examples. Generally speaking, the invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including the accompanying claims and drawings). Thus, features, integers, characteristics, compounds or chemical moieties 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. Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose. The Invention will now be described by way of example only with reference to the Examples below and to the following Figures, wherein: Figure 1.Image of the vacuum system used for one of the methods described here. (A) Main Chamber; (B) Load Lock; (C); Sample Stage; (D) Manipulator arm; (E) Gate Valve; (F) QCM; (G) High Temperature (gold) evaporator; (H) HTC Evaporator Controller; (I) OLED (ligand) Evaporator; (J) OLED Evaporator Controller; (K) Pulse Valve; (L) Pulse Valve Controller; Figure 2. Composite Structured Nanomaterials formed by Co-Evaporation of Gold and PVP; Figure 3. Composite Structured Nanomaterials formed by Co-Deposition of Gold (Evaporation) and PVP (Vacuum Spray); Figure 4. Composite Structured Nanomaterials formed by Co-Deposition of Gold (Evaporation) and PVP (Vacuum Spray), followed by ex vacuo processing (heating); Figure 5. Composite Structured Nanomaterials formed by Co-Evaporation of Gold and Sodium Citrate; Figure 6. Composite Structured Nanomaterials by Co-Deposition of Gold (Evaporation) and PVP (Spin Coating); Figure 7. Composite Structured Nanomaterials by Co-Deposition of Gold (Evaporation) and PVP (Spin Coating) (no post-processing); Figure 8. Composite Structured Nanomaterials by Co-Deposition of Gold (Evaporation) and PVP (Spin Coating), followed by ex vacuo processing (heating); and Figure 9. Composite Structured Nanomaterial formed by Evaporation of Gold (180 minutes) and Spin Coating of PVP (10 kDa). MATERIALS AND METHODS The production of composite structured nanomaterials comprising discrete, interconnected nanoparticles separated by and bound within a molecular or atomic matrix material was explored using two-step processes wherein, in a first (deposition) step, a nanomaterial coated substrate was produced, either by physical vapour deposition (PVD) of both gold and an organic matrix material under vacuum conditions, or by a combination of spin coating deposition of an organic matrix material and PVD of gold. In a specific example of this approach, a silicon wafer, either uncoated or pre (spin) coated with matrix material, was mounted onto a stage at the end of a manipulator arm inside the load lock of the vacuum system shown in Figure 1. Once the load lock chamber had been evacuated to a vacuum in the order of 10-5 to 10-8 mbar, a gate valve was opened, and the coated or uncoated wafer transferred into the main chamber for deposition. To produce the nanomaterial coated substrate, the organic matrix material (PVP or sodium citrate) was, if required, first deposited either by thermal evaporation of solid matrix material or by pulsed injection spraying into the vacuum chamber of a solution comprising matrix material dissolved in isopropyl alcohol (via a pulsed valve). This step was subsequently followed by the simultaneous deposition of both gold and organic matrix as a layer onto the support, either by metal and matrix evaporation (co-evaporation) or by metal evaporation and ligand pulse valve spray (co-deposition), or by sequential deposition of layers of the two materials. Detailed descriptions of each method step are given below. Thermal evaporation of the organic matrix material was conducted using an OLED Effusion Cell, from Createc, and the evaporator was fitted with a quartz crucible which was controlled by a PID control box. QCM measurements were also used to determine the deposition rate of organic matrix material. For vacuum spray deposition of the organic matrix material, a solution of said material was produced and placed into a Swagelok 125 cm3 cylinder which was attached, via a two-way ball valve, to a pulsed valve. The pulsed valve (Parker Series 9 High-Speed Pulsed Solenoid Valve, 0.031 in. Orifice, 28 V, Stainless Steel Body) was attached to the main chamber via a custom DN40 flange containing a 3 mm hole for the liquid to enter the vacuum chamber. The valve was controlled via a custom control box fitted with an Arduino controller, the open and closed times of the pulse valve could be changed to control the vacuum spray deposition rate. Prior to vacuum spray deposition the pulsed valve set up was purged with Argon, the two-way valve connecting the sample reservoir to the valve was then opened, and the control box used to commence deposition. QCM measurements were also used to determine the deposition rate of matrix material. Gold evaporation was conducted at varying evaporator temperatures, with deposition rate being exponentially related to the temperature. Gold was placed into a tungsten crucible which was placed inside the high temperature evaporator (High Temperature Cell, HTC, Createc). The temperature was set and controlled via a PID control box. Once the gold evaporator was at temperature, with the sample stage removed, the deposition rate was measured using the QCM by opening the shutter lid of the evaporator. Gold deposition along with matrix deposition was then conducted once the sample stage was in the centre of the main chamber with the stage, and silicon wafer, facing the bottom of the chamber. Upon completion of the deposition steps, the sample stage was rotated and removed from the main chamber and into the load lock. The gate valve could then be closed, and the load lock vented which enabled the vacuum in the main chamber to be preserved. Once the load lock was vented to atmospheric pressure the sample stage could be removed. The wafer supporting the nanomaterial film was then transferred into deionised water (3 mL), and in some embodiments was sonicated in a water bath, for typically 60 minutes, to form a suspension of nanomaterial in water. Alternatively, to ensure complete removal of the nanomaterial, the surface of the wafer could be swabbed using an absorbent pad or swab such as, e.g., a cotton bud. The absorbent pad or swab can then be placed into water to remove the sample and produce a concentrated nanomaterial suspension. The suspension can then be sonicated to ensure full dissolution of the sample in the solvent. Sonication was carried out with a Bransonic CPX2800-E Ultrasonic bath, 110W, 40kHz, operating in high power Sonics mode. For TEM imaging of the colloidal particles, solutions were drop cast onto TEM grids. 3 uL of colloid was placed onto a Carbon Film supported on a Cu TEM support grid and left to dry at room temperature. STEM images were taken using an FEI TALOS F200X TEM using a STEM High Angle Annular Dark Field (HAADF) detector at 200 kV acceleration voltage. UV-Visible spectra were measured on a Jenway 7135 Spectrophotometer fitted with a Xenon lamp light source. Spectra were collected between wavelengths of 300 to 900 nm at 1 nm intervals. Baseline spectra were determined prior to analysis using deionised water. UV-Visible measurements were taken of the colloids as produced after sonication, unless stated otherwise. Example 1: Formation of Composite Nanomaterials by Co-Evaporation of Gold and PVP A structured nanomaterial comprising gold nanoparticles separated by and bound within a PVP matrix was built on the surface of a silicon wafer, precoated with a layer of PVP, by simultaneous direct evaporation; PVP was evaporated at 220 °C (0.05 A / s) and gold nanoparticles were evaporated at 1200 °C (0.06 A / s). Figure 2 displays the results of such a deposition sequence which resulted in the formation of composite structured nanomaterials I metaparticles. Notably, Figure 2 shows a Scanning Transmission Electron Microscope (STEM) image of the composite nanomaterial which was generated by dissolution in water of the coated wafer, and the nanomaterial was subsequently deposited and dried onto a TEM grid for imaging and size analysis. The STEM image shows one such metaparticle and part of another from the set of structured nanoparticles which were obtained. Example 2: Formation of Composite Nanomaterials by Co-Deposition of Gold (Evaporation) and PVP (Vacuum Spray) In an alternative method to that disclosed in Example 1, a composite Gold-PVP nanomaterial was prepared using a combination of evaporation and vacuum spray methods to deposit gold nanoparticles and PVP matrix material respectively, upon the surface of the solid support (the silicon wafer) mounted on the stage of the manipulator arm. Here a 1% solution of PVP in isopropyl alcohol (IPA) solution was gravity fed into a high-speed pulsed valve mounted directly onto a flange of the main vacuum chamber. The solution of PVP was pulsed into the main chamber for 5 minutes (5ms open, 10 s closed), prior to deposition of gold nanoparticles by direct evaporation (1400 °C for 10 s). Figure 3 shows a STEM image of the composite nanomaterial which was generated by dissolution in water of half the coated wafer, and the nanomaterial was subsequently deposited and dried onto a TEM grid for imaging and size analysis. The STEM image showed isolated rafts of particles, and within the rafts, a mixture of round particles and worms / extended structures was observed. Some more spherical isolated particles were also obtained. Average nanoparticle diameter and metaparticle size were not recorded, but UV-Vis spectroscopy showed a Amax of absorption of around 612 nm. In an additional method step, approximately half of this Gold-PVP nanomaterial was subjected to an ex vacuo processing step in which the structured nanomaterial was heated, prior to dissolution, in a static air furnace (200 °C, 6 hours). The effects of this ex vacuo processing step are shown in Figure 4, wherein the Scanning STEM image of the composite nanomaterial shows more discrete and circular particles inside, with an average nanoparticle diameter of 7.19 ± 1.91 nm and an interparticle spacing of 6.3 ± 2.4 nm. Further, UV-Vis spectroscopy showed that the processing step resulted in colour shift from dark blue to purple / pink, with a Amax of absorption red shift to around 543 nm. Example 3: Formation of Composite Nanomaterials by Co-Evaporation of Gold and Sodium Citrate In an analogous method to that disclosed in Example 1, a composite Gold-Citrate structured nanomaterial was prepared on the surface of a silicon wafer by simultaneous direct evaporation; Sodium citrate was first evaporated at 300 °C for 30 minutes, followed by co-evaporation of sodium citrate (300°C) and gold nanoparticles (1400°C) for 15 seconds. Alternating layers were prepared by repeating these evaporation steps 14 times. Figure 5 shows a STEM image of the composite structured nanomaterial which was generated by dissolution in water of the coated wafer, the nanomaterial being subsequently deposited and dried onto a TEM grid for imaging. The STEM images again showed isolated rafts of particles. Average size of the structured nanomaterials is also shown in Figure 5. Further, UV-Vis spectroscopy, following 10 x dilution due to colour depth, revealed a Amax of absorption of around 685 nm. Example 4: Formation of Composite Nanomaterials by Co-Deposition of Gold (Evaporation) and PVP (Spin Coating) In an additional method, composite Gold-PVP structured nanomaterials were prepared using a combination of evaporation and spin coating methods to create gold nanoparticles and PVP matrix material respectively upon the surface of the solid support (the silicon wafer), mounted on the stage of the manipulator arm. Here, gold nanoparticles were formed by evaporation of gold (1100 °C; 120 min) onto a PVP spun coated silicon wafer. Figure 6 shows STEM images of the composite structured nanomaterial which was generated by dissolution in water of the coated wafer, with the nanomaterial being subsequently deposited and dried onto a TEM grid for imaging. The STEM image showed isolated rafts formed from irregularly shaped nanoparticles, with an average nanoparticle diameter of 13.17 ± 7.36 nm, an interparticle spacing of 2.6 + 0.8 nm and an average size of the structured nanomaterial of 119 ± 137 nm. Further, UV-Vis spectroscopy revealed a teal coloured solution, with a Amax of absorption of 678 nm. In addition, composite gold-PVP structured nanomaterials were prepared using a combination of evaporation and spin coating methods to deposit gold nanoparticles and PVP matrix material respectively, upon the surface of the solid support (the silicon wafer) mounted on the stage of the manipulator arm. Here, gold nanoparticles were evaporated (1100 °C; 30 min) onto a PVP spun coated silicon wafer, with approximately half of this Gold-PVP nanomaterial, still on the support wafer, being subjected to an ex vacuo processing step in which the nanomaterial was heated in a static air furnace (200 °C, 6 hours). The effects of this ex vacuo processing step are apparent upon comparing Figure 7 and Figure 8. In particular, as shown in Figure 7, the non-processed structured nanomaterial was found to have an average nanoparticle diameter of 4.28 ± 1.06 nm and an interparticle spacing of 2.0 ± 0.8 nm. In contrast, Figure 8 shows that the processed material had an average nanoparticle diameter of 6.55 ± 1.82 nm and an interparticle spacing of 3.7 ± 1.4 nm. Further, UV-Vis spectroscopy showed that the processing step resulted in a colour shift, with a Amax of absorption red shift from around 579 nm to around 544 nm. Example 5: Formation of Composite Nanomaterials by Co-Deposition of Gold by Evaporation (1100 °C, 180 mins) and PVP (10 kDa) by Spin Coating Using an analogous method to that disclosed in Example 4, composite Gold-PVP nanomaterials were prepared using a combination of evaporation and spin coating methods to deposit gold nanoparticles and PVP matrix materials upon the surface of the silicon wafer mounted on the stage of the manipulator arm. Here, gold particles were evaporated at a temperature of 1100 °C onto a silicon wafer that has been spun coated with a 10 kDa PVP for a period of 180 minutes. Figure 9 shows a representative STEM image of the composite structured nanomaterial that was generated by dissolution in water of the coated wafer 22 (10 kDa PVP, 180 minutes gold deposition). The STEM image clearly showed that isolated rafts, i.e. structured composite nanomaterials, were formed containing irregular shaped nanoparticles, with an average nanoparticle diameter of 20.0 nm and an average interparticle spacing of 4.0 nm.

Claims

1) A composite structured nanomaterial comprising a plurality of discrete, interconnected nanoparticles separated by and bound within a molecular or atomic matrix material.2) The structured nanomaterial according to claim 1, wherein said nanoparticles are inorganic nanoparticles, and preferably comprise or consist of metal atoms, and wherein said metal atoms are optionally selected from gold, silver, copper, palladium, platinum, iron, titanium or an alloy comprising one or more of said metals.3) The structured nanomaterial according to claim 2, wherein said nanoparticles comprise or consist of gold.4) The structured nanomaterial according to any one of the preceding claims, wherein said nanoparticles have an average particle size of from about 1 to about 1000 nm.5) The structured nanomaterial according to any one of the preceding claims, wherein said nanoparticles are of a spherical, elliptical, rod-like, star-like, disc-like or worm-like morphology.6) The structured nanomaterial according to any one of the preceding claims, wherein said matrix material comprises or consists of one or more organic agents.7) The structured nanomaterial according to any of the preceding claims, wherein said one or more organic agents are selected from: polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), polyvinyl alcohol(PVA), bovine serum albumin (BSA), ethylene diamine tetra acetic acid (EDTA), Chitosan, a quaternary ammonium surfactant, an amino acid, citrate or any combination or polymerised variant thereof.8) The structured nanomaterial according to claim 7, wherein said one or more organic agents are selected from PVP and citrate.9) The structured nanomaterial according to any of the preceding claims, wherein the average interparticle spacing, measured as an edge-to-edge distance between two adjacent nanoparticles, is at least about 0.1 nm.10)The structured nanomaterial according to any of the preceding claims, wherein the ratio of said nanoparticles to said matrix material is from about 0.5:99.5 to about 99:1.11)The structured nanomaterial according to any of the preceding claims, having an average diameter of from about 10 nm to about 100 pm, and optionally from about 50 nm to about 50 pm.12)The structured nanomaterial according to any one of the preceding claims, having an average thickness of from about 1 nm to about 500 nm.13)The structured nanomaterial according to any one of the preceding claims, which, when suspended in a liquid solvent, has an absorption maximum (Amax) of from about 400 nm to about 800 nm.14)A method for preparing the structured nanomaterial according to any of the preceding claims, said method comprising:i) providing a solid substrate;ii) depositing one or more molecular or atomic matrix materials and forming a plurality of nanoparticles on the surface of said substrate, wherein:A. a film comprising or consisting of one or more matrix materials is deposited onto the surface of said substrate to form a coated substrate; and nanoparticles are formed on the surface of said coated substrate by physical vapour deposition under vacuum conditions to form a nanomaterial coated substrate;orB. nanoparticles are formed on, and one or more matrix materials are deposited onto, the surface of said substrate by physical vapour deposition under vacuum conditions to form a nanomaterial coated substrate;iii) removing said nanomaterial coated substrate from vacuum conditions; andiv) washing said nanomaterial coated substrate in a liquid solvent under non vacuum conditions to dissolve the nanomaterial coating and form a suspension of said structured nanomaterial in said solvent.15)The method according to claim 14, wherein a film comprising or consisting of one or more matrix materials is deposited onto the surface in step (ii-A) by spin coating.16)The method according to claim 14, wherein said nanoparticles and said matrix material are formed / deposited by physical vapour deposition, and wherein said nanoparticles and / or said matrix materials are independently formed / deposited by evaporation deposition or by pulsed injection deposition.17)The method according to claim 16, wherein said nanoparticles are formed by evaporation deposition, and said matrix material is deposited by pulsed injection deposition.18)The method according to any one of claims 14 to 17, wherein said substrate is selected from silicon; a carbon-based material such as graphene, carbon nanotubes, fullerenes or amorphous-carbon; an oxide, a nitride, a glass, or a MXene.19)The method according to claim 18, wherein said substrate is a silicon material, optionally doped with one or more heteroatoms (e.g. boron or phosphorus).20)The method according to any one of claims 14 to 19, wherein step (ii-B) comprises or consists of the simultaneous co-deposition of said nanoparticles or a nanoparticle precursor and said matrix material on the surface of the substrate by physical vapour deposition.21)The method according any one of claims 14 to 20, wherein said liquid solvent is water.22)The method according to any one of the preceding claims, wherein said nanocomposite coated substrate is subjected to a processing step, after step (ii) but prior to step (iv).23)The method according to claim 22, wherein said processing step is conducted in vacuum conditions, prior to step (iii).24)The method according to claim 22, wherein said processing step isconducted ex vacuo, after step (iii).25)The method according to any one of claims 22 to 24, wherein said processing step is a heating step, wherein said heating preferably comprises exposing said nanomaterial coated substrate to an elevated temperature of from about 100 °C to about 250 °C for a period of at least about 2 hours.

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