WO2011131979A2

WO2011131979A2 - Method of forming a one dimensional nanostructure and nanostructure formed by the method

Info

Publication number: WO2011131979A2
Application number: PCT/GB2011/050774
Authority: WO
Inventors: Amir Fahmi; Nabil Gindy
Original assignee: The University Of Nottingham
Priority date: 2010-04-19
Filing date: 2011-04-19
Publication date: 2011-10-27
Also published as: WO2011131979A3; GB201006520D0; EP2561009A2

Abstract

A method of forming a one dimensional nanostructure comprising the steps of; providing a solution of 4^th or 5^th generation dendrimers; attaining a pH of substantially 8.3 and adding a metal salt to the solution to cause cation induced self assembly of the dendrimers into one dimensional nanostructures. A metal species sensor, a pH sensor and a method of altering the spectra of a quantum dot is also disclosed.

Description

METHOD OF FORMING A ONE DIMENSIONAL NANOSTRUCTURE AND NANOSTRUCTURE FORMED BY THE METHOD

This invention relates to a method of forming a one dimensional nanostructure or one dimensional hybrid nanostructure. In particular, it relates to a method of forming an amine terminated dendrimers based one dimensional nanostructure and a method of forming a one dimensional nanostructure in aqueous solution. It also relates to such a one dimensional nanostructure. It also relates to a metallic species sensing substance. Further, it relates to a sensing substance for determining pH.

Nanofabrication is the construction of nanometre scale structures. Self assembly is one method of nanofabrication in which a system is manipulated such that the components therein form the desired structure. This invention is primarily related to a method of creating a series of conditions for the self assembly of one dimensional well-defined nano structures or nanofibres. However, two dimensional and three dimensional structures can be formed from the nanofibres. Dendrimers are globular macromolecules composed of a core, dendrons and surface groups. It has been found that dendrimers are useful for forming nano- objects.

One-dimensional nano structures based on multifunctional dendrimer-hybrid materials in aqueous media are particularly advantageous. Such nano structures are expected to show potential in wide range of applications from nanoelectronic, photoelectronic toward biotechnology and life-science. Nanofabrication via directed self-assembly of hybrid material into well-defined nano structures is a versatile and powerful tool for manufacturing the next generation of miniaturized devices. It is an object of the invention to provide a technique for mass fabrication of (multi)-functional nanostructure hybrid materials based on inorganic nanocomponents prepared in-situ within organic macromolecules. Embodiments of the invention comprise simple hybrid systems to fabricate high aspect ratio nanofibers. The molecular self-assembly of amino-terminated polypropylene imine (PPI) dendrimers into nanofibers is induced by complexing transition metal ions to the terminal amines groups of the dendrimers. The results disclosed herein show the creation of a shape and charge asymmetry in the dendritic scaffold together with the metal-ligand interactions to form uniform hybrid nanofibers of inorganic nanocomponents within the dendritic matrix. These hybrid nanofibers can be used as a scaffold to deposit different types of transition elements to form nanofiber alloys. The obtained one dimensional nanofibers possess unique physical properties useful for miniaturised electronic and photoelectronic devices.

According to a first aspect of the invention we provide a method of forming a one dimensional nanostructure comprising the steps of;

a) providing a solution of dendrimers

b) adding a cation, such as a metal salt, to the solution to direct the self assembly of the dendrimers into one dimensional nanostructures.

This is advantageous because the nanostructures fabricated by this method have been found to have well-defined structures. Further, the method provides a simple, cost-effective, manufacturing process that can be performed in-situ at room temperature in an aqueous medium. The method is environmentally friendly and flexible and yields multifunctional nanostructures that are light in weight, have a high surface area and can be modified further to form metallic, bimetallic, nano-alloy, magnetic material with semiconductor hybrid and metallic with semiconductor hybrid nanofibres. The method can be used for building hybrid nanofibers based alloys (CdSe/Au, CdS/Au etc) of inorganic ordered nanoparticles within the organic-dendritic scaffold; for fabricating different thickness of nanofibers with high aspect ratio. The resultant organic-inorganic nanofibres can be used as a scaffold to deposit different types of inorganic nanoparticles. The method can also be used in the fabrication of nanofibers based on alloys of different types of metals and semiconductors.

Preferably the solution of dendrimers is an aqueous solution. Alternatively, the solution may comprise dendrimers dissolved in an organic solvent. It will be appreciated that the solvent is chosen depending on the type of dendrimer and the dendrimer's generation. Preferably the dendrimers have two different families of functional groups, one functional group located in the core of each dendrimer molecule and the other functional group located at the terminus. Preferably the dendrimers comprise amine terminated dendrimers. Preferably the dendrimers comprise polypropylene imine (PPI) dendrimers or Polyamidoamine (PAMAM) dendrimers.

The dendrimers may have a generation between 0 and 7, preferably between 2 and 7. Most preferably, the dendrimers have a generation of 4 or 5.

Preferably the dendrimer solution has a concentration of between 0.1 mM and 1 mM and most preferably 0.3mM. However, it will be appreciated that the concentration may be dependent on the generation of the dendrimer. The method may include a step of attaining a pH of the solution of dendrimers of between 9 and 7. The method may include a step of reducing the pH of the solution of dendrimers to below pH 9 and most preferably to approximately pH 8.3. This is advantageous as the dendrimer solution typically has a pH of approximately 10.3 and with the reduction of pH the dendrimer surface becomes positively charged which advantageously affects the configuration and conformation of the dendrimers and promotes nanofiber formation.

Preferably the type of metal salt is selected such that the step of adding the metal salt attains a predetermined pH.

It has been found that tuning the solution properties, such as the pH and the ion concentration, provides control over the morphology of the nanostructure. Cationic metal ions are able to form multiple coordination bonds with the terminal primary amines of the PPI dendrimers, thereby acting as a linker between the dendrimer molecules in solution.

Preferably the metal salt is inorganic and may comprises a Cadmium salt and, in particular, Cadmium Acetate or Cadmium Nitrate. Alternative the metal salt may be a Zinc salt, a Palladium salt, a Silver Salt or other transition metal salt. This is advantageous as the method can be used to control the assembly of many types of functional inorganic nanoparticles such as Cadmium or Zinc into well-defined I D and 2D-nanostructures via directed self-assembly of the dendrimers units. The unidirectional self-assembly through the Cd(II)-bridging is believed to be a consequence of the relative flexibility of the dendritic scaffold which controls the shape and charge asymmetry.

Preferably the molar ratio of metal salt and dendrimers is greater than 5 : 1. Most preferably the molar ratio of metal salt and dendrimers is substantially 10: 1.

The method may include a further step of metallisation of the I D nanostructure; or a further step of introducing a semiconductor into the I D nanostructure to form a nanostructure incorporating a quantum dot; or a further step of introducing a semiconductor into the I D nanostructure and metallisation.

To achieve metallisation of the nanostructure, the method may include the steps of;

c) adding a metal precursor to the solution of I D nanostructures; and d) reducing the metal to obtain a metallised I D nanostructure.

Preferably the metal precursor comprises a Gold precursor, which may comprise Chloroauric acid. The molar ratio of dendrimer to metal precursor may be between 1 : 1 and 1 :7 and preferably is substantially 1 :3.

Preferably the step of reducing the metal of the metal precursor comprises the addition of a reducing agent which may comprise Sodium borohydride (NaBH₄).

To achieve the incorporation of a semiconductor into the I D nanostructure the method may include the steps of; c) adding a semiconductor precursor such that the metal linked nanostructure and semiconductor precursor react together to form semiconductor nanoparticles. Preferably the semiconductor precursor comprises one of Sodium hydrogen Selenide (NaHSe), Sulphur dichloride (SCI₂), Sodium sulfide (Na₂S) and Sodium Telluride (Na₂Te). This is advantageous as when the metal salt comprises a Cadmium salt, these precursors can be used to prepare fluorescent CdSe or CdS or CdTe quantum dots within the nanostructure. Thus, using Cadmium acetate as the metal salt precursor, the Cd(II) precursor is further modified to incorporate fluorescent CdSe, CdS, CdTe quantum dots. The optical properties of these nanofibers are not only sensitive to the particle size and shape, but also to their spatial distribution and the chemical environment.

Once the semiconductor has been incorporated into the nanostructure, the method may include the step of metallisation, comprising;

e) adding a metal precursor to the solution of I D nanostructures incorporating the semiconductor; and

f) reducing the metal to obtain a hybrid semiconductor-metallised I D nanostructure.

Self-assembled nanofibers of dendrimer stabilised metal and semiconductor quantum dots in aqueous media could be used as labels or markers in biotechnological applications. Modification of the fibers' surface, e.g. via metallisation, yields multifunctional I D-nanostructures that extend the range of accessible applications. The ability of dendrimers to act as a unimolecular micelles for the controlled synthesis and stabilisation of inorganic nanoparticles is not the only one of their outstanding features. Their large number of functional groups and the unique molecular architecture also give rise to a great diversity of self-assembled structures in both thin films and solutions. One-dimensional nanostructures are anisotropic and often possessing unique physical properties. The invention demonstrates the ability of amine-terminated dendrimers (e.g. PPI) to direct the self-assembly of inorganic nanoparticles into one-dimensional aggregates in solution. It has been found that the decoration of the nanofibers with metallic gold nanoparticles leads to complete quenching of the fluorescence of a nanostructure having CdSe quantum dots incorporated therein. Thus, the nano structures can be used as sensors to determine the presence of metallic species.

Preferably the method includes the step of forming 2D-nanostructures by applying an external field, such as electric or magnetic field, to direct the assembly of the one dimensional nanostructures into a two dimensional structure. This is advantageous as electrically charged nanoparticles or magnetic nanoparticles within the nanofibers can interact with an electric field or a magnetic field respectively and can therefore be aligned according to the field direction and shape. According to a second aspect of the invention we provide a one dimensional nanostructure formed in solution by the method of the first aspect of the invention.

According to a third aspect of the invention we provide a sensor for detecting the presence of metallic species in solution comprising a one dimensional nanostructure, said nanostructure having semiconductor quantum dots incorporated into its structure.

This is advantageous as the introduction of a metal species has been found to significantly alter the absorption/transmission spectra of the nanostructure solution, which can be used to determine the presence of the metallic species. Further, using spectroscopy, the type of metallic species can be identified using the detected surface Plasmon band. It will be appreciated that the absorption/transmission spectra is dependent on the structure of the nanofibres. Thus, any change in the medium in which the fibres are contained, which results in a change in the fibre structure, can be detected by looking for changes in the absorption/transmission spectra. Thus, the nanofibres can also be used to detect temperatures changes, pH changes and ionic strength changes. Preferably the metal species detector comprises a one dimensional nanostructure in aqueous solution having CdSe, CdS or CdTe quantum dots integrated into its structure. In particular, the one dimensional nanostructure may comprise a plurality of dendrimers linked together with the quantum dots to form the one dimensional nanostructure. The one dimensional nanostructure may be assembled into a 2D structure.

Preferably the sensor comprises a nanostructure having semiconductor quantum dots formed by the method of the first aspect of the invention.

According to a fourth aspect of the invention we provide a method of altering the absorption/transmission spectra of a quantum dot, said quantum dot incorporated into the structure of a one dimensional nanostructure, the method comprising the steps of;

providing a one dimensional nanostructure incorporating at least one quantum dot in its structure in solution;

adding a metal containing substance, such as a metal salt, to the solution.

This is advantageous as the quantum dot nanofibers can be modified such that the spectra changes and these changes utilised to produce labels or markers in biotechnology applications or detectors or other electro-chemical devices.

Preferably the one dimensional nanostructure comprises a structure described in relation to the first aspect of the invention.

According to a fifth aspect of the invention we provide a method determining the presence of a metal species in a test solution comprising the steps of;

providing a quantum dot incorporated into the structure of a one dimensional nanostructure;

introducing the nanostructure to the test solution;

determining if a change in the spectra of the quantum dot is observed to determine the presence of a metal species. Preferably the method includes the step of determining the type of metal species by analysing the spectra of the quantum dot.

According to a sixth aspect of the invention we provide a pH sensor for detecting the pH of a solution comprising a one dimensional nanostructure, said nanostructure having semiconductor quantum dots incorporated into its structure.

This pH sensor is advantageous as the spectra of the quantum dots can be monitored to determine the integrity of the nanostructure that incorporates the quantum dots. As the pH increases and decreases from around 8 to 10, the nanostructure disassembles with an associated reduction in the intensity of the spectra obtained from the quantum dots.

According to a seventh aspect of the invention we provide a method of determining changes in the pH of a test solution comprising the steps of;

providing a one dimensional nanostructure, said nanostructure having at least one semiconductor quantum dot incorporated into its structure;

introducing the nano structures to the test solution;

determining if a change in the spectra of the quantum dot is observed to determine a change in pH.

According to a further aspect we provide a simple method to fabricate hybrid one dimensional nano structures (I D) such as Morphology of nanorods, nanofibers, nanofibres, nano-objects. According to a further aspect we provide a route to generate two dimensional (2D) such as nanoporous, nanosheets, worm-like structures, of ordered I D structures via an external field (electric field and/ or magnetic field, mechanical shear) and/or at water-air interface by the Langmuir- Blodgett technique. According to a further aspect we provide a method of directed self-assembly of organic dendrimers macromolecules via inorganic components to form well-defined hybrid of I D and 2D. According to a further aspect we provide a well-defined one dimensional and two dimensional nano structures based on inorganic nano-particles prepared in-situ within the organic polymeric matrix. According to a further aspect we provide a method for gaining precise control over the assembly of the inorganic moieties within the well-defined polymeric metrics. According to a further aspect we provide a method of precise control over the size and the size distribution of the inorganic moieties inside the dendritic matrices. According to a further aspect we provide a method of directing the self-assembly of hybrid materials based on in-situ prepared inorganic moieties inside the dendritic macromolecules to obtain well- defined architectures. According to a further aspect we provide a simple synthesis method of nanofibers comprising a one pot reaction at room temperature. According to a further aspect we provide accessibility to self assembly tool box consisting of a range of hybrid nano-objects to fabricate functional 2D-nanostructures. According to a further aspect we provide a method for accessibility of a vast many different types of metallic l D-nanostructures. According to a further aspect we provide a method of nanofabrication of hybrid I D and 2D architectures based on bimetallic moieties within the dendritic matrices. According to a further aspect we provide use of self-assembly as a nanofabrication tool for wide range of functional hybrid nano-objects. According to a further aspect we provide a method of fabricating high aspect ratio nanofibers comprising both metallic, magnetic and semiconductors for unique electronic, optoelectronic and photoelectronic properties. According to a further aspect we provide a method of preparing the hybrid materials in a liquid medium and on a solid substrate (wet chemistry, non-electro methods and successive ionic layer deposition). According to a further aspect we provide accessibility to hybrid building blocks for nano-engineering 2D and 3D structures. According to a further aspect we provide a method of nano-alloy fabrication including metallic, bimetallic, semiconductors, magnetic, semiconductors-metallic, semiconductors coating with metallic layers, and semiconductors coated with magnetic layers. According to a further aspect we provide a hybrid material based on different generation of dendrimers possessing key characteristic of donor-acceptor, gluco- dendrimers, Azo-dendrimers, hyper-branch dendrimers for many applications such as solar cell, nano-electronic, catalyst, biotechnology, optoelectronic, optics nanofibers, Photovoltaics, optics, Flexible electronics, MEMs, NEMs, Nanotechnology and High tech-devices.

There now follows by way of example only a detailed description of the present invention with reference to the accompanying drawings, in which; Figure 1 shows a schematic illustration of dendrimer conformation and self-assembly, (a) molecular structure of 4^th generation polympropylene imine (PPI) dendrimers, (b) dendrimer conformation in aqueous media as a function of pH and the addition of a metal salt, (c) cation-induced, unidirectional self- assembly of dendrimer molecules into nanofibers, (d) metallization of Cd- complexed PPI nanofibers with gold;

Figure 2 shows an Atomic Force Microscope (AFM) topogram (a) and Transmission Electron Microscopy (TEM) micrographs (b, c) of self- assembled nanofibers of polypropylene imine (PPI) dendrimers in the presence of Cd(AcO)₂. The micrograph in (d) is a magnified view showing the enrichment of Cd(II) in the nanofibers; Figure 3 shows an AFM topogram of nanofibers of PPI[G4]-Cd(AcO)2.

The nanofibers are decorated with gold nanoparticles, the molar ratio of PPI[G4] :Au is (a) 1 : 1 , (b) 1 :3. The TEM micrograph (c) and HR-TEM micrograph (d) show a regular distribution of Au nanoparticles along the nanofiber contour; Figure 4 shows an AFM topogram (a) and EFM phase image (b) of self-assembled nanofibers of PPI[G4]-CdSe. The CdSe particles appear as dark spots in the TEM micrographs (c). The presence o fringes in the HR- TEM micrographs (d) also suggest the existence of crystalline nanoparticles;

Figure 5 shows AFM topograms of nanofibers of (a, b) PPI[G4]- Cd(AcO)₂ and (c, d) PPI[G4]-CdSe. aThe nanofibers are decorated with gold nanoparticles, the molar ratio of PPI[G4] :Au is (a) 1 : 1 , (b) 1 :3 , (c) 1 : 1 and (d) 1 :7 respectively;

Figure 6 shows TEM micrographs of nanofibers of PPI[G4]-CdSe. The nanofibers are metallised leading to necklace-like structures of metal (Au) nanoparticles following the nanofibers contour, (a) low ratio PPI[G4] :Au= 1 : 1 , (b) low ratio PPI[G4] :Au= 1 :7 and (c) PPI[G4] :Au= 1 :3 after chemical reduction with NaBH₄. The insets in (a) and (c) are high resolution micrographs, (d) Histograms of the particle size distribution for both the CdSe and the Au nanoparticles.;

Figure 7 shows UV-Vis and photoluminescence (PL) spectra of (a) nanofibers of G4 dendrimer-stabilised CdSe quantum dots and (b) the same nanofibers after metallisation with Au. The insets are optical photographs of aqueous solutions of nanofibers taken under UV-illumination;

Figure 8 shows AFM topograms of self-assembled nanofibers of (a) G2 and (b) G3 dendrimers with (- 1) Cd(AcO)₂ and (-2) CdSe particles;

Figure 9 shows AFM topograms of self-assembled nanofibres of (a) G4 and (b) G5 dendrimers with (- 1) Cd(AcO)₂ and (-2) CdSe particles;

Figure 10 shows a flow chart illustrating the method of producing the nanostructures; and Figure 11 shows a flow chart illustrating the use of the quantum dot nanostructure as a pH detector and a metal species detector.

Ions-induced nanofiber formation

Figures 1 and 10 show a first embodiment of a method of forming a one dimensional nanostructure comprising a nanofiber. One-dimensional nanostructures, such as nanofibers are anisotropic and often possess unique physical properties. Self-assembled nanofibers of dendrimer stabilised metal and semiconductor quantum dots in aqueous media could be used as labels or markers in biotechnological applications. Modification of the fiber surface, e.g. via metallisation, yields multifunctional one dimensional (ID) nanostructures that extend the range of accessible applications. It has been found that the number of repeating branches of a dendrimer (i.e. its generation) as well as the pH of the dendrimer solution and the metal precursors introduced into the solution are particularly important for the formation of dendrimer based nanofibers in solution. The present embodiment utilises polypropylene imine (PPI) dendrimers in aqueous solution at a concentration of approximately 0.3 millimolar. It will be appreciated that other types of amine terminated dendrimers could be used. The pH of the solution and the type of metal precursor has been found to affect the conformation and self-assembly of polypropylene imine (PPI) dendrimers, particularly of 2^nd to 5^th generation dendrimers.

The provision of an aqueous dendrimer solution is shown in step 101. The molecular structure of a 4^th generation amino-terminated PPI dendrimer is shown in Figure l a. These dendrimers consist of an interior based on 30 tertiary amines and a periphery of 32 primary amines. The primary amines at the dendrimer' s surface are more alkaline (p _a= 9.23) than the interior tertiary amines (p _a= 6.3). It has therefore been realised that the selective protonation of the dendrimer's surface is possible while the interior amines remain uncharged. As a consequence, the conformation of amino-terminated PPI dendrimers has been found to be strongly dependent on factors such as the type of surrounding medium, its acidity and ionic strength.

Step 102 of Figure 10 comprises attaining a pH that promotes assembly of the dendrimers into nanofibers. Figure lb shows, schematically, typical conformations of a 4^th generation amine terminated PPI dendrimer (G4) as the pH conditions are altered. It has been shown that PPI and Polyamidoamine (PAMAM) dendrimers at intermediate generations (i. e. 4^th and 5^th generations) are susceptible to conformational changes because the flexible dendrimer architecture allows back-folding of peripheral groups into the dendrimer's interior leading to a more globular shape.

Figure l c and step 103 of Figure 10 show the addition of a metal salt to initiate self assembly of the dendrimers into fibers. With an appropriate metal salt, such as Cadmium Acetate, the step 102 may be achieved on addition of the metal salt to the solution thereby obviating the need for a separate pH adjustment step. If the metal salt is not introduced the conformational changes as a function of pH result in a topology of self-assembled dendrimer structures in thin film. In this embodiment, the metal salt cadmium acetate (Cd(CH₃COO)₂) is added to the aqueous solution of the 4^th generation PPI dendrimer in a molar ratio of PPI:Cd = 1 : 10. This step results in the formation of tens of micrometer long fibers with a diameter of just 4-6nm as shown in Figure 2a and step 104 of Figure 10. A first indication of the fiber formation is that the dendrimer solution turns slightly opaque after the addition of the cadmium acetate. Dynamic light scattering experiments confirmed increase in hydrodynamic radius from 1.6nm for the unmodified PPI dendrimer to >500nm for the Cd(II)-complexed dendrimer nanofibers. This confirms that the fibers are already formed in aqueous solutions and not during film deposition.

The mechanism of fiber formation is demonstrated schematically in Figure l c. As the cadmium acetate (Cd(AcO)₂) is added to the aqueous dendrimer solutions, the Cd(II) cations coordinate to the terminal primary amines. These Cd(II) cations can form multiple coordination bonds with the monodentate peripheral primary amines of the dendrimers. As results of this process bridging between the dendrimer molecules is taking place. Hence, the Cd(II) cations can act as a linker between dendrimers in aqueous media and promote the unidirectional self- assembly of dendrimers with asymmetric shape and negative charge distribution. As a result, nanofibers with diameters of 4-6nm are formed.

It has been found that this growth mechanisms is, however, not effective for growth perpendicular to the fiber axis and repulsive interaction among the remaining charges of protonated peripheral amines reduces the tendency of nanofibers to aggregate into bundles. In fact, the TEM micrographs in Figures 2b and 2c show that micrometer long nanofibers are randomly distributed on the substrate and only few bundles with larger diameters can be found. The TEM micrograph in Figure 2d also shows that within these bundles, the nanofibers are aligned parallel, but do not aggregate to form a single fiber. The Cd(II) precursor, which is reduced to Cd° upon exposure to the electron beam, appears as dark spots in the TEM micrographs due to its higher electron density compared to the organic dendrimer matrix. A regular distribution of small Cd clusters along the dendrimer nanofibers is observed, which supports the inventor's hypothesis of the growth mechanism shown in Figure 1.

The formation of nanofibers based on the mechanism proposed above relies on the presence of cations, which can act as a linker between the dendrimers. It has been found that the concentration of cations (Cd² ) is an important parameter that significantly affects the stability and size of the nanofibers. Several control experiments indicate that nanofibers are formed only above a certain concentration-threshold of Cd(II) in solution. The concentration threshold of Cd(II) is dependent on the dendrimers' generation but in general a dendrimer to Cadmium(II) ratio of 1 : 10 is preferred for reliable nanofiber formation. If the ratio is 1 :20 then Cadmium ions have been found to be present in solution after the nanofibers have formed. Moreover, the growth of I D dendrimer aggregates relies on the presence of a charge and shape asymmetry in the Cd(II)-complexed dendrimer molecules. To verify the proposed mechanism, the inventors studied the fiber formation as a function of dendrimer generation. Their results demonstrated that due to the higher flexibility of the dendritic scaffold at intermediate generations (G4 - G5) nanofibers of up to a micrometer long can be formed, while at lower generations fewer and shorter fibers are achieved.

The functions of the obtained nanofiber' s surface are dependant not only on the choice of the metallic types but also on the assembly of the nanoparticles along the nanofibers. Nanofibres having metallic Cadmium can be formed by a reduction step, however, alternative nanofibres can be formed as described below. It has been found that metal or semiconductor nanoparticles can be easily deposited through an extra wet-chemistry step to form bimetallic nanofibers (nanofibers based on nano-alloys) and nanofibres including semiconductors that are well aligned with the nanofibers, as will be described in more detail below.

Fabrication of Bimetallic Nanofibers Figure l e and step 105 of Figure 10 shows the metallisation of Cd(II)-complexed PPI dendrimer nanofibers. In particular, Figure l e shows a method step of populating the nanofiber with nanoparticles of gold (Au). This method of creating bimetal nanofibres bypasses step I d shown in Figure 1 and proceeds directly from the step shown in Figure l c to the step shown in Figure l e. Firstly, a gold precursor of Chloroauric acid (HAuC ) is added to an aqueous solution of the Cadmium complexed nanofibers. Preferably, the aqueous solution is dilute to a dendrimer water ratio of 1 : 10. The ratio of 4^th generation PPI to Chloroauric acid (PPI [G4] :HAuCLi) is preferably of a molar ratio between 1 : 1 and 1 : 3. The gold precursor coordinates selectively to the terminal primary amines of the PPI dendrimers. The gold is then reduced as shown in step 106. Following chemical reduction, gold nanoparticles are obtained which follow the contour of the nanofibers. Hence, the Cd(II)-complexed PPI nanofibers can be used as scaffold for Au nanoparticles to obtain one-dimensional particle assemblies. The reducing agent used in this embodiment is Sodium borohydride (NaBH₄) in solution.

The AFM topograms in Figure 3 a and 3b show metallised nanofibers of PP1[G4] - Cd(AcO)2, where necklace-like structures of Au nanoparticles are obtained, which mimic the contour of the nanofibers. The gold nanoparticles are discrete and spaced from one another along the nano-fibers. The gold precursor (HAuCLi) coordinates to the available remaining primary amine groups at the dendrimer periphery to obtain Au-nanoparticles after the chemical reduction with NaBH₄ in solution or after spin-coating on the substrate. The nucleation and growth of the Au particles has been found to be governed by the kinetics of both the coordination of HAuCLi to the primary amines of PPI[G4] and the reduction of the gold precursor to metallic gold.

In particular, the degree of metallisation can be adjusted through the molar ratio of PPI [G4] :HAuCLi. A higher density of Au particles decorating the nanofibers is achieved at higher concentrations of gold precursor. For example, only few Au nanoparticles are formed at molar ratios of PPI [G4]

1 : 1 as shown in Figure3a. However, regular necklace-structures of nanofibers decorated with Au particles can be obtained at PPI[G4] 1 :3 as shown in Figure3b. Markedly, the spacing of the Au nanoparticles that is achieved by the above method appears to be relatively constant along the nanofibers with a narrow size distribution. No additional stabiliser was employed during the preparation. The average particle size, as determined from TEM measurements (Figure3-c), is 5.4nm with a standard deviation of 0.9nm. Moreover, the presence of fringes in the HR-TEM micrograph (Figure3-d) suggests that the particles are crystalline. The inventors believe that the low polydispersity and constant spacing of the Au nanoparticles can be understood considering their formation. As the gold precursor coordinates to available primary amines at the dendrimer surface a large number of gold particles are nucleated upon chemical reduction. Subsequently, the particles grow via coalescence of gold clusters, presumably in the fashion of an Ostwald ripening mechanism. A narrow particle size distribution can be obtained, since the mobility of the initial Au clusters along the nanofibers is limited by the adsorption of Au clusters to the primary amines of the dendrimers. Hence, the particle growth is controlled and limited by the depletion of Au clusters in the vicinity of a growing particle. Assuming that the concentration of gold precursor at the nanofibers surface is consistent through a uniform charge distribution along the nanofibers, this leads to guide the nucleation mechanism to grow the Au-nanoparticles in a controlled manner toward low polydispersity and regular spacing.

Figure 3 also suggests that there are no free dendrimers present in the solutions because the Au precursor would also coordinate to available primary amines of these dendrimers instead of the nanofibers. This in return would lead to spherical molecules comprising single dendrimers associated with the gold particles that do not follow the contour of the nanofibers. Hence metallisation of Cd(II)- complexed nanofibers at intermediate Cd-concentrations (PPI[G4] :Cd(II) > 3 has been found to lead to uniform metallised nanofibers, which exceed several micrometers in length. The above described self-assembly process is simple and flexible and the specific organic-inorganic interactions have been found to facilitate the formation of defect free well-defined I D-structures. These I D structures have potential uses in electronic applications and molecular biology.

Fabrication of Semiconductor Nanofibers It has also been found possible to introduce semiconductor nanoparticles to the nanofibres formed be the above described method. Following the formation of nanofibers in the presence of Cd(II) ions, a semiconductor precursor can be added to the solution to prepare a semiconductor nanofiber as shown in step 107 of Figure 10. The semiconductor precursors may be Selenium based, Sulphur based or Tellurium based. In particular, it has been found that semiconductor precursors such as Sodium Selenide (NaHSe), Sulphur dichloride (SCI₂) and Sodium Telluride (Na₂Te) can be used to prepare fluorescent CdSe/CdS/CdTe quantum dots within the dendrimers aggregates. The resultant I D-structures have been found to be stable under the altered solution conditions, such that nanofibers with unique optical properties can be obtained. The physical properties and micro structure of the nanofibers are characterised by means of Ultraviolet- Visible light (UV-Vis) and photoluminescence (PL) spectroscopy as well as Scanning Force Microscopy (SFM) and Transmission Electron Microscopy (TEM).

Methodology to obtain nanofibers incorporating CdSe Quantum Dots

Following the formation of Cd(II) containing dendrimer nanofibers in aqueous solution, it has been found that fluorescent CdSe nanoparticles can be fabricated at room temperature. Under an inert atmosphere, such as Nitrogen (N₂) or Argon, and with vigorous agitation or stirring, a freshly prepared solution of NaHSe is added to the dendrimer-complexed Cd(II) solution as shown at step 107. The Sodium hydrogen Selenide solution provides a source of Selenium. In the present embodiment 4 mg of Selenium powder was suspended in 10 millilitres of ethanol and NaBH₄ was added slowly to the Selenium suspension under an Nitrogen atmosphere. This step is shown in Figure I d. Upon addition of NaHSe, the initially colourless Cd(II) solution turns bright yellow signifying the formation of small CdSe nanoparticles with low Se content, as shown in Figure 4. The optical properties of the CdSe quantum dots (QDs) were evaluated by means of UV-Vis and photo-luminescence (PL) spectroscopy; the results are presented in Figure 7. The PPI[Cd(II)] nanofibers have been found to be stable during the synthesis of CdSe nanoparticles. Accordingly, this method achieves micrometer long nanofibers of CdSe nanofibers based on amine-terminated PPI dendrimers and CdSe nanoparticles. The CdSe particles act as a linkage between adjacent dendrimer molecules. The morphology of nanofibers containing CdSe nanoparticles (Figure 4a) are similar to the ones obtained in the presence of Cd(AcO)2 precursor. It has been found that in the case of a 4^th generation dendrimer based nanofiber including CdSe, the nanoparticles in the nanofibers have an average diameter of 6.4 nm and a length of up to 8 μιη (Figure 4a). The presence of the CdSe nanoparticles in the nanofibers is apparent in the electric force microscopy measurements, shown in Figure 4b, where the nanofibers containing nanoparticles yield a strong phase contrast. The cross section, shown in Figure 4b, also suggests a relatively constant spacing ( 1 lnm) between the nanoparticles. Indeed the TEM micrographs shown in Figures 4c and 4d reveal a large number of nanoparticles following the contour of the nanofibers with a constant spacing. The spatial arrangement of the CdSe Quantum Dots in the nanofibers, and in particular their spacing, supports the inventor's hypothesis that the particles act as a linkage between the amine- terminated PPI dendrimers (Figure 4b). Moreover, the results indicate that the nanofibers are stable under the experimental conditions during the synthesis of CdSe Quantum Dots, where the pH increases from 8.3 to 9.2. The presence of fringes in the HR-TEM micrographs shown in Figure 4d further suggests that the CdSe nanoparticles are crystalline. Analysis of the TEM micrographs yields a mean particle diameter of 2.9 nm with a standard deviation of 0.6 nm as shown in Figure 6d. Hence, it has been found that the PPI dendrimers not only stabilise the nanoparticles and promote the formation of nanofibers in aqueous solution but they also control the particles size and size distribution for advantages of low polydispersity.

Metallisation of Semiconductor Nanofibers (Hybrid Metallised/Semiconductor nanofiber) Figures l c to I d to l e show the steps performed to create a hybrid metal/semiconductor one-dimensional nanofiber. Nanofibers are anisotropic and often possess unique physical properties such as ballistic electron transport and unusual optical characteristics. It is expected that modification of the surface of a semiconductor complexed nanofiber by metallisation with different types of metals (such as Au, Ni, Pd, Co, ) may lead to unusual physical properties that increase the range of accessible applications. In the present embodiment, dendrimer nanofibres linked with Cadmium and Selenium (CdSe) are metallised with metallic gold (Au) nanoparticles. Such fibres can be used as optoelectronic components, and also as a diagnostic agent using the strong fluorescence in the fibers which could hold Antigens or antibodies for detection.

Firstly, the semiconductor nanofibres are created using the method described above to achieve the CdSe joined dendrimer fibres shown in Figure I d. A gold precursor of Chloroauric acid (HAuCL_t) is added (step 109) to dilute aqueous solutions of the nanofibers (PPI[G4]-Cd(AcO)₂ and PPI[G4] -CdSe) in a certain molar ratio. It has been found that a molar ratio of PPI[G4] :HAuCLi of 1 : 1 to 1 :3 is preferable. The gold precursor coordinates selectively to the terminal primary amines of the PPI dendrimers. Gold nanoparticles that follow the contour of the nanofibers are obtained after chemical reduction with NaBH₄. Hence, the nanofibers incorporating CdSe Quantum Dots can be used to template Au nanoparticles and direct their spatial order into one-dimensional assemblies.

The AFM topograms in Figure 5 show metallised nanofibers of PPI[G4]- Cd(AcO)₂ and PPI[G4] -CdSe respectively. It has been found that the metallisation of nanofibers can be successfully achieved with Cd(II) or CdSe containing nanofibers. In either case, necklace-like structures of Au nanoparticles are obtained, which mimic the contour of the nanofibers as shown in Figure 3. It has been found that the addition of a reducing agent, such as NaBH₄, is not always necessary although it is shown as step 1 10 in Figure 10. In particular, the gold precursor is reduced practically instantaneously when added to the PPI[G4]- CdSe nanofibers (Figure 5c, d), due to residual NaBH₄, which had been added to the solution during the synthesis of CdSe nanoparticles.

It is believed that the nucleation and growth of the Au-nanoparticles are governed by the kinetics rate of both process (i) the coordination of HAuCLi to the primary amines of PPI[G4] and (ii) the reduction of the gold precursor to metallic gold, followed by uncontrolled growth in solution. This may explain the larger Au particles in the case PPI[G4]-CdSe nanofibers.

It has been found that the degree of metallisation can be adjusted through the molar ratio of PPI[G4] :HAuCLi; a higher density of Au particles decorating the nanofibers can be achieved if more gold precursor is added to the nanofiber solutions. For example, only few Au-nanoparticle are formed at molar ratios of PPI[G4] :HAuCL_t= 1 : 1 (Figure 5a, c), while regular necklace-structures of nanofibers decorated with Au particles can be obtained at PPI[G4] :HAuCl₄= 1 :3 (Figure 5b) and PPI[G4] :HAuCL_t= 1 :7 (Figure5d). However, the inventors found that the nanofiber morphology is perturbed at high concentrations of gold precursor (PPI[G4] :HAuCl₄< 1 :7), especially for PPI[G4]-Cd(AcO)₂ nanofibers. It is suspected that the addition of high concentrations of Chloroauric acid (HAuCLi) decreases the solution pH and shifts the balance between attractive and repulsive interactions among the dendrimers. In particular, the higher degree of protonation of primary amines (NH₃ ⁺) at the dendrimer surface and tertiary interior amines enhances the coulomb repulsion and may lead to the disassembly of nanofibers and the formation of cadmium-complexed PPI[G4] dendrimer structural units.

Due to their larger size and high electron density, the Au particles provide a much higher contrast in the TEM micrographs (Figure 6), while the PPI[G4]-CdSe nanofibers are hardly visible. At low concentrations of Au, e.g. PPI[G4] :Au= 1 : 1 , small particles can be observed (Figure 6a), which follow the contour of the nanofibers. However, it appears that the distribution of Au particles is inhomogeneous in the sense that few nanofibers contain more Au particles than others. In contest, adding high concentrations of Au-precursor (PPI[G4] :Au= 1 :7) larger particles are formed along the nanofibers (Figure 5b). However, as a side effect, the nanofibers become shorter and their morphology is partially disrupted possibly due to the strong changes in solution conditions (by reduction in pH) upon adding the acidic Au precursor (HAuC ). Hence, balancing the solution conditions and the degree of metallisation is important to fabricate homogeneously metallised nanofibers, whose morphology remains intact. It has been found that small Au particles (substantially l Onm across) can be formed on the surface of the PPI[G4] -CdSe nanofibers (Figure 6c) at intermediate Au concentrations ((PPI[G4] :Au= 1 :3) and low solution concentrations (O. lmM). The spacing of the Au nanoparticles appears to be relatively constant along the nanofibers. Moreover, the Au particles show a narrow size distribution, when no additional stabiliser was added during their preparation process. The average particle size determined from TEM measurements is found to be around l Onm with a standard deviation of 1.8nm.

The adsorption of Au nanoparticles to the surface of the nanofibers has been found to have a profound effect on their optical properties. Nanofibers of PPI[G4]-CdSe show fluorescent properties with a strong emission band at as shown in Figure 7a. The position of the emission band of semiconductor quantum dots depends not only on the particle size and composition but also the state of their surface and the surrounding medium. Hence, upon the formation of Au nanoparticles the characteristic fluorescence band of the CdSe Quantum Dots is completely quenched as shown in Figure 7b, in which only a very weak emission at λ= 553 nm can be observed. Additionally, the presence of gold nanoparticles in the solution is signified by the appearance of the typical surface plasmon band at

530 nm in the UV-Vis absorption spectra. Thus, a method of altering the optical properties of a semiconductor linked nanofiber is provided. Figure 1 1 shows a method of using this effect to detect the presence of a metal species. Step 1 1 1 shows the provision of a nanofibre having quantum dots in its structure. Step 1 12 shows the introduction of a solution to be tested. Step 1 13 shows the step of observing spectra changes of the quantum dots to determine the presence of metal species.

These results indicate that self-assembled nanofibers containing fluorescent CdSe quantum dots in aqueous solutions may be used as labels or markers in biotechnological applications. Another possibility could be to utilise the fluorescence quenching or a shift in the emission/ absorption band of the nanofibers as a sensor to detect other (metallic) species in solution. Moreover, it could be imagined that the controlled assembly and disassembly of the nanofibers as a function of pH is detectable as a change in the characteristic fluorescence/ absorption band. Hence, self-assembled PPI[G4] -CdSe nanofibers may also serve as a pH sensor in biological media. Figure 1 1 is also illustrative of a method of using this effect to determine changes in pH. Step 1 1 1 shows the provision of a nanofibre having quantum dots in its structure. Step 1 12 shows the introduction of a solution to be tested. Step 1 13 shows the step of observing spectra changes of the quantum dots to determine the changes in the pH, which can cause disassembly of the nanofibers.

Effect of Dendrimer Generation on Nanofiber Formation In the embodiment described above, 4^th generation dendrimers have been used. The nanofiber formation relies on balancing attractive and repulsive interaction among the dendrimer molecules as well as creating a shape and charge asymmetry, which promotes unidirectional growth of the nanofibers. The latter, in particular, requires dendrimer molecules with a flexible molecular structure, which is susceptive to changes of external factors, such as the solution pH and ionic strength. It has been found that the conformation of low generation dendrimers (G2 and G3) is less sensitive to external changes due to their small size compared to G4 and G5. However, it is believed that Steric hindrance within the densely packed surface of high generation dendrimers (i. e. >G7) also limits the extent of conformational rearrangements of the molecular structure. Hence, it is expected that the formation of nanofibers following the mechanism proposed in the present application is most effective for dendrimers of 4^th to 6th generation (G4 - G6). AFM topograms of PPI nanofibers with Cd(AcO)2 (- 1 ) and CdSe particles (-2) are shown in Figure 8 for low generation dendrimers (G2, G3) and in Figure 9 for higher generations (G4, G5). It has been found that nanofibers with similar morphologies can be obtained regardless the generation of the dendrimers. However, the nanofibers are relatively short in the case of low dendrimer generations (G2 and G3) compared to higher generations. Moreover, it has been observed that the nanofiber diameter is lower for nanofibers with Cd(AcO)2 and increases upon Selenisation of the Cd(II) precursor to form CdSe nanoparticles. This effect is pronounced again for low dendrimer generations. For example, in the case of PPI[G2]-Cd(AcO)2 the nanofiber diameter is below 1.5nm (Figure 8a) and increases to 4nm upon the formation of CdSe nanoparticles (Figure 8a). Similarly, for PPI[G3] the nanofiber diameter increases from 1.7nm with Cd(AcO)₂ precursor to 5.4nm with CdSe nanoparticles. The diameter gain is due to the growth of CdSe nanoparticles, which are larger than the radius of gyration of the pristine dendrimers. Since the particle diameter is only weakly dependent on the dendrimer generation, the final thickness of PPI-CdSe nanofibers is relatively constant around 6nm (Figure 9). For the high generation dendrimers (G4 and G5) we obtain nanofibers exceeding 4μιη in length (Figure 9). Again their diameter increases upon the formation of CdSe nanoparticles; however the diameter again is much lower compared to G2 and G3.

Thus, the present invention provides a simple bottom-up tool and versatile method to fabricate organic-inorganic nanofibers via directed self-assembly in aqueous solution. The resulting hybrid nanofibers, especially using 4^th and 5^th generation dendrimers, have been found to be stable for several weeks in aqueous solution at ambient temperature. It will be appreciated that while the above embodiments describe the metallisation of the nanofibres with gold, the method could be applied to other metals using an appropriate metal precursor. For example, Silver, Platinum and Palladium metal precursors may be used. In particular, the CdSe nanofibers have a positive charge and therefore negatively charged metal precursors such as AuCLf electrostatically interact with the CdSe nanofibers.

It will be appreciated that while the above embodiments describe the introduction of Selenium into the nanofibers to form a nanofiber having CdSe quantum dots, the method could be applied to other semiconductors using an appropriate semiconductor precursor. For example, Na₂S, SCI₂ or Na₂Te. Alternatively thiol based dendrimers could be used. When using thiol based dendrimers, the metal salt is preferably a gold based metal salt.

Claims

1. A method of forming a one dimensional nanostructure comprising the steps of; a) providing a solution of dendrimers

b) adding a metal salt to the solution to cause cation induced self assembly of the dendrimers into one dimensional nanostructures.

2. A method according to claim 1 , in which the solution of dendrimers is an aqueous solution.

3. A method according to claim 1 or claim 2, in which the dendrimers comprise amine terminated dendrimers.

4. A method according to any preceding claim, in which the dendrimers comprise polypropylene imine (PPI) dendrimers or Polyamidoamine (PAMAM) dendrimers.

5. A method according to any preceding claim, in which the dendrimers have a generation between 2 and 7.

6. A method according to any preceding claim, in which the dendrimers have a generation of 4 or 5.

7. A method according to any preceding claim 1 , in which the dendrimer solution has a concentration of between 0.1 mM and 1 mM and most preferably 0.3mM.

8. A method according to any preceding claim, in which the method includes a step of attaining a pH of the solution of dendrimers of between pH 9 and pH 7.

9. A method according to any preceding claim, in which the method includes a step of attaining a pH of the solution of dendrimers of below pH 9.

10. A method according to any preceding claim, in which the method includes a step of attaining a pH of the solution of dendrimers of substantially pH 8.3.

1 1. A method according to any one of claims 8 to 10, in which the step of attaining the pH comprises selecting a metal salt that achieves the desired solution pH when added thereto.

12. A method according to any preceding claim, in which the metal salt is inorganic and cationic.

13. A method according to any preceding claim, in which the metal salt comprises a Cadmium salt and, in particular, Cadmium Acetate or Cadmium Nitrate.

14. A method according to any preceding claim, in which the molar ratio of dendrimer to metal salt is greater than 5 : 1.

15. A method according to any preceding claim, in which the molar ratio of dendrimer to metal salt is substantially 1 : 10.

16. A method according to any preceding claim, in which the method includes steps to achieve metallisation of the nanostructure, comprising the steps of;

17. A method according to claim 16, in which the metal precursor comprises a Gold precursor, which may comprise Chloroauric acid.

18. A method according to claim 16 or claim 17, in which the molar ratio of dendrimer to metal precursor may be between 1 : 1 and 1 :7 and preferably is substantially 1 :3.

19. A method according to any one of claims 16 to 18, in which the step of reducing the metal of the metal precursor comprises the addition of a reducing agent which may comprise Sodium borohydride (NaBH₄).

20. A method according to any one of claims 1 to 15, in which the method includes steps to achieve the incorporation of a semiconductor into the I D nanostructure comprising the steps of; c) adding a semiconductor precursor.

21. A method according to claim 20, in which the semiconductor precursor comprises one of Sodium Selenide (NaHSe), Sodium Sulfide Na₂S, Sulphur dichloride (SC1₂) and Sodium Telluride (Na₂Te).

22. A method according to claim 20 or claim 21 , in which the method includes the step of metallisation, comprising;

23. A method according to any preceding claim, in which the method includes the step of forming 2D-nanostructures by applying an external electric or magnetic field to direct the assembly of the one dimensional nanostructures into a two dimensional structure.

24. A one dimensional nanostructure formed in solution by the method of any one of claims 1 to 22.

25. A sensor for detecting the presence of metallic species in solution comprising a one dimensional nanostructure, said nanostructure having semiconductor quantum dots incorporated into its structure.

26. A sensor as defined in claim 25, in which the metal species detector comprises a one dimensional nanostructure in aqueous solution having CdSe, CdS or CdTe quantum dots integrated into its structure.

27. A sensor as defined in claim 25, in which the sensor comprises a nanostructure having semiconductor quantum dots formed by the method of any one of claims 1 to 22.

28. A method of altering the absorption/transmission spectra of a quantum dot, said quantum dot incorporated into the structure of a one dimensional nanostructure, the method comprising the steps of;

adding a metal containing substance, such as a metal salt, to the solution.

29. A method determining the presence of a metal species in a test solution comprising the steps of;

introducing the nanostructure to the test solution;

determining if a change in the spectra of the quantum dot is observed to determine the presence of a metal species.

30. A method according to claim 28, in which the method includes the step of determining the type of metal species by analysing the spectra of the quantum dot.

31. A pH sensor for detecting the pH of a solution comprising a one dimensional nanostructure, said nanostructure having at least one semiconductor quantum dot incorporated into its structure.

32. A method of detecting changes in the pH of a test solution comprising the steps of;

introducing the nanostructure to the test solution;