WO2006109059A1

WO2006109059A1 - Purification of nanoparticles by phase separation

Info

Publication number: WO2006109059A1
Application number: PCT/GB2006/001337
Authority: WO
Inventors: Shanju Zhang; Ian Anthony Kinloch; Alan H. Windle
Original assignee: Cambridge Enterprise Limited
Priority date: 2005-04-15
Filing date: 2006-04-12
Publication date: 2006-10-19

Abstract

A method of purifying nanoparticles comprises the steps of : preparing a first suspension of nanoparticles; causing or allowing phase separation of the first suspension into a liquid crystalline suspension of purified nanoparticles and a second suspension; and separating the liquid crystalline suspension of purified nanoparticles from the second suspension.

Description

PURIFICATION OF NANOPARTICLES BY PHASE SEPARATION

The present invention relates to a method of purifying nanoparticles and to nanoparticles purified by this method.

Nanoparticles are currently of great interest. In particular, carbon nanotubes (CNTs) have received interest since their discovery because of their unusual chemical structures and intriguing physical properties [1] •

It is desirable for a wide range of applications to produce macroscopically well-aligned fibres and films of CNTs. However, the current limited ability to process CNTs hinders the realization of their full potential. In many cases, the species of interest cannot be purified easily or dispersed at a significant concentration. The dispersion of CNTs in water is especially important because of their potential applications in biology and medicine.

The report [2] has shown that "cutting" CNTs by ultrasonication in a mixture of strong acids leads to a good dispersion of shortened CNTs in water or JV, JV- dimethylformamide (DMF) . However, the shortened CNTs show a wide range of size distribution. The separation of such polydisperse CNTs from the suspension has been reported by centrifugation, electrophoresis and applied external fields [3-5] . For all these methods, however, separation is not entirely satisfactory. Theoretically, it has been proposed that CNTs should form a lyotropic (multi-component) nematic (aligned) liquid crystalline phase when in suspension above the critical concentration [6], as do other rod-like entities (for example poly (p-phenylene terephthalamide) (PPTA) and Tobacco Mosaic Virus (TMV) ) . However, few experimental studies on lyotropic nematic liquid crystallinity of CNTs have been reported to date.

Recently, the inventors' research group reported a nematic phase of multiwall CNTs in aqueous suspension

[7] . To prepare such a suspension, the as-synthesised

CNTs were oxidized in a mixture of concentrated sulphuric acid and nitric acid under ultrasonication and extracted with deionised water. Different concentrations of the shortened CNTs were then dissolved in deionised water. A phase transition from isotropic phase to nematic phase was observed as the suspension concentration was increased above a critical value. It was found that the critical concentration value depended on the detailed characteristics of the starting material and the exact oxidation procedure used. As long as the same conditions are applied to a given batch of CNTs, the nematic suspension of CNTs forms above the same critical concentration . A phase change was observed on standing such a nematic liquid crystal aqueous suspension of multiwall carbon nanotubes for several weeks . When viewed under cross polar microscopy, the original nematic phase was found to separate into a strongly birefringent nematic phase and a dark isotropic phase. By studying the nanotube length and order distributions in the two phases, it was found that longer nanotubes dominate the nematic phase, leading to highly ordered arrangements whereas shorter nanotubes and carbon impurities occupy the isotropic phase, leading to completely disordered arrangements . This behaviour was analysed by Onsager rigid rod theory and interpreted in terms of a miscibility gap that is proportional to the degree of the nematic order, which is different for different lengths of nanotubes .

For a lyotropic liquid crystal, the isotropic and nematic phases are separated by a two phase region (Flory chimney) extending over a fixed range of compositions [8] . The two phase region is a part of the overall thermodynamic equilibrium, and within it the co-existing isotropic and liquid crystalline phases each have a fixed composition but change in relevant amounts, as the overall concentration of the solute (i.e. CNTs) in suspension is increased. The two phase region covers a few % of the overall concentration range. Where there is a range of mesogenicity (or liquid crystalline forming ability) , which is enhanced by increasing in the nanotube aspect ratio and/or straightness , the CNTs of higher mesogenicity were found to diffuse preferentially to the liquid crystalline phase while those of lower mesogencinity or other non-rod like impurities diffuse preferentially to the isotropic region. Without being bound by this theory, the inventors believe that the physical origin of phase separation and length-selected fractionation in the lyotropic nematic suspension of polydisperse nanotubes is simple and general. The Onsager theory considers the competition between the orientational entropy and the translational entropy [11, 12]. The former favours mixing, whereas the latter reduces the excluded volume, increasing the alignment and resulting in elastic instabilities. The entropy which dominates will depend on the system density and the excluded volume of adjacent rigid rods. Monodisperse, rigid rod particles of different lengths have different critical densities for forming a nematic phase with the longer particles being more likely to undergo a transition at a lower density. Thus, longer and shorter particles have a different nematic order parameter at the same density. In the nanotube nematic suspension of polydisperse nanotubes, the nematic order parameter is a unique function measuring the mean field experienced by all nanotubes. Each individual nanotube is displaced from its optimal nematic configuration and this displacement generates a drive to separate from the other nanotubes with different lengths. As a result, the difference in the critical density for the nematic phase induces an effective repulsive interaction between longer and shorter nanotubes, leading to demixing of the nanotube sizes. Ultimately, the longer nanotubes preferentially enter the highly ordered nematic phase to enhance the nematic stability whereas the shorter nanotubes and carbon impurities tend to form the disordered, isotropic phase [12].

The ^' inventors have appreciated that this finding can be applied to provide a new method of purifying crude nanoparticles . The nematic and isotropic regions can be separately collected such that purified, higher mesogenicity CNTs are obtained. Furthermore, if the liquid crystalline phase is removed, it can be refractionated by the addition of further solvent and the process can be repeated successively to give further improvement. Successive fractionations lead to further enhancement of the purification.

Accordingly, in a first aspect the present invention relates to a method of purifying nanoparticles, comprising the steps of: preparing a first suspension of nanoparticles; causing phase separation of the first suspension into a liquid crystalline suspension of purified nanoparticles and a second suspension; and separating the liquid crystalline suspension of purified nanoparticles from the second suspension. The process may be a batch or continuous process . The term "phase separation" may be used to refer to macroscopic separation of two intimately mixed phases.

Preferably, the second suspension is an isotropic suspension. However, it may be a liquid crystalline suspension, for example in the separation of chiral nanoparticles discussed below. Preferably, the second suspension contains impurities. In this case, the first suspension may be a suspension of crude nanoparticles. However, the second suspension may contain nanoparticles which are also of interest. In this case, such particles may be recovered separately. The term "nanoparticles" includes particles which are between 0.5 (preferably 1.0) and 1000 run in at least one, preferably two, dimensions.

Preferred nanoparticles are those in which at least two dimensions are within the range set out above, with the third dimension being larger than the average of the other two by a factor of at least 5. These nanoparticles are variously referred to as "nanotubes", "nanorods", "nanofibrils" or "nanofibres" . The term "nanorods" will be used to include "nanofibrils" or "nanofibres" which have no clear hole along the centre of the particle as in the case of "nanotubes". The lengths of these nanotubes and/or nanorods can for example range from 10 nm to 10 mm.

Preferably, such particles will have a resistance to shape changes over time due to thermal energy as defined by a persistence length of at least 500 nm, preferably at least 1000 nm, more preferably at least 2000 nm. "Persistence length" is defined in "The Science of Polymer Molecules", page 190, Cambridge Solid State Series, R. H. Boyd and P. J. Phillips, Cambridge University Press 1993. Carbon nanotubes typically have a persistence length of around 10000 run.

Another sub set of "nanoparticles" of interest consists of sheet-like particles in which at least one dimension falls within the range set out above, with each of the other two dimensions being at least 5 times as large as the first dimension. Examples of such particles include platelets and discs. The method of the invention may be used for example to separate nanoparticles according to length, ratio of length to diameter or chirality.

Preferably, the purified nanotubes and/or nanorods are separated from nanotubes and/or nanorods of different length, curved nanotubes and/or nanorods and non-nanotube or nanorod nanoparticles. More preferably, the purified nanotubes and/or nanorods are longer nanotubes and/or nanorods and the impurities comprise shorter nanotubes and/or nanorods. More preferably, the longer nanotubes and/or nanorods have an average length of between 500 and 10,000 nm (for example between 500 and 1000 nm) , and the shorter nanotubes and/or nanorods have an average length below 600 nm. Preferably, the longer nanotubes and/or nanorods have an average aspect ratio of between 10 to IxIO⁶. Highly preferably, the longer nanotubes and/or nanorods have an average aspect ratio of between 16 and 70, and the shorter nanotubes and/or nanorods have an average aspect of below 20.

The liquid crystalline suspension may be nematic (exhibiting alignment between particles), smectic (exhibiting alignment between particles and layering) or cholesteric (chiral nematic) . Preferably, the liquid crystalline suspension is nematic. A nematic suspension can be used to separate nanoparticles by length.

A cholesteric suspension can be used to separate nanoparticles by chirality. Nanoparticles containing a mixture of chiralities or a mixture of chiral and non- chiral nanoparticles will partition in accordance with the relevant phase diagram.

Preferably, the nanoparticles are of inorganic material. The term "inorganic material" is used to include carbon-based nanoparticles such as optionally derivatised carbon nanotubes .

Preferably, the nanoparticles comprise carbon, boron nitride, titanium oxide, tungsten (IV) sulphide, or metal (in particular elemental metal such as gold or copper nanorods). More preferably, the nanoparticles consist essentially of one of these materials.

Carbon nanotubes are particularly preferred. The carbon nanotubes may be single wall or multiwall carbon nanotubes. The carbon nanotubes may be derivatised, for example with solubiUsing groups such as carboxylate groups .

In a preferred embodiment, the invention relates to a method of purifying crude carbon nanotubes, comprising the steps of: preparing a first suspension of crude carbon nanotubes ; causing phase separation of the first suspension into a nematic suspension of purified carbon nanotubes and an isotropic suspension of impurities; and separating the nematic suspension of purified carbon nanotubes from the isotropic suspension of impurities . Preferably, the first suspension is a liquid crystalline suspension, for example one of the types of liquid crystalline suspension discussed above.

Preferably, the concentration of the first suspension is within the Flory chimney of the phase diagram.

Preferably, the suspensions are in a solvent selected from water, polar organic solvents such as alcohol (for example ethanol) , acetone, DMSO, DMS, DMF, THF and NMP, halogenated solvents such as tetrachloroethylene, DCM and chloroform, acid (for example sulphuric acid or nitric acid) and oleum. A mixture of two or more of these solvents may be used.

Preferable the two phases are physically separated on the macroscopic scale such that they can be easily fractionated.

Preferably, the step of causing or allowing phase separation of the first suspension into a liquid crystalline suspension of purified nanoparticles and an second suspension is carried out in one or more of the following ways:

(a) adjusting the concentration of the first suspension such that two phases are present in the sample;

(b) applying a field to the first suspension; and (c) leaving the first suspension for a period of time .

Suitably, the concentration is adjusted such that it lies within the Flory chimney.

In a preferred embodiment, a simulated high gravity field is applied. Electrical or magnetic fields can also be used. The simulated high gravity field is preferably applied using a centrifuge. Suitably, more than one cycle of centrifugation is carried out on the first suspension. Preferably, the first suspension is left for a period of at least one week, and more preferably for a period of at least one month. This may be used as an alternative to centrifugation or in addition to this treatment. Preferably, the method further comprises an initial step of treating the nanoparticles to solubilise and/or to shorten them. This treatment may involve acids, surfactants and/or ultrasonication.

Acid can be used to shorten and to oxidise the nanoparticles.

Suitably, the surfaces of the nanoparticles are treated to aid their dispersion and/or separation. This treatment may use covalent and/or non-covalent approaches known to those skilled in the art. A treatment with saturated sodium hydroxide alcohol- water solutions is described in Q. Li, I.A. Kinloch and A. H. Wiήdle, Discrete Dispersion of Single-Walled Carbon Nanotubes, Chem. Comm. 2005 DOI 10.1039b419039d. Single wall carbon nanotubes treated in this way are easily dispersed in most common organic solvents.

Preferably, the purified nanoparticles are diluted with solvent and the purification process is repeated.

Preferably, the purified nanoparticles are recovered from the liquid crystalline suspension. Preferably, the purification process is successively applied. In this case one purification cycle typically comprises :

(a) Adjusting the concentration such that both a liquid crystalline suspension and second suspension are present;

(b) Allowing/encouraging the particles to segregate to their preferred phase, as discussed above; (c) Causing the two phases to physically segregate on a macroscopic scale; and

(d) Removing one phase from the other. This cycle is successively applied to one of the collected phases at the end of each cycle.

In a second aspect, the present invention relates to purified nanoparticles prepared by the method described above .

In a third aspect, the present invention relates to use of differing liquid crystalline suspension forming ability properties of two types of nanoparticles in performing a separation of the nanoparticles .

The invention will be described further with references to the following non-limiting examples, as illustrated by the Figures, in which:

Figure 1 (a) (Example 1) shows a polarised light micrograph of nematic structures in crude 8 wt% aqueous suspension of chopped CNTs between crossed polars . A Schlieren texture which can be confirmed on rotation of the crossed polars is observed, indicating a single-phase polydomain in the crude suspension.

Figure 1 (b) (Example 1) shows an electron micrograph of nematic structures in solid films after the solvent in 8 wt% aqueous suspension evaporated completely. SEM reveals individual nanotube orientation around ± ¹A disclinations . Different size CNTs and carbon impurities can be seen in the crude sample. Figure 2 (a) (Example 1) shows an optical micrograph of nematic structures in 8 wt% aqueous suspension chopped CNTs between crossed polars, after one month standing at room temperature. Two phases occur separated by a sharp interface: an isotropic drop of dark texture within a continuous nematic phase of Schlieren texture. Compared to the crude single- phase suspension, the Schlieren texture have stronger birefringence and larger domain size.

Figure 2 (b) (Example 1) shows an electron micrograph of the continuous nematic phase in Figure 2 (a) . Significantly, the longer CNTs dominate the nematic phase.

Figure 2 (c) (Example 1) shows an electron micrograph of the isotropic drop in Figure 2 (a) . Shorter CNTs and carbon impurities occupy the isotropic phase.

Figure 3 (Example 1) shows length distributions in the phase-separated sample: (a) nematic phase and (b) isotropic phase.

Figure 4 (Examples 2 and 3) shows the weight percentage of nanotubes found in the nematic and isotropic regions of a sample which has been through the repeated purification procedure. The lines are a guide to the eye only.

Figure 5 (Examples 2 and 3) shows electron micrographs of carbon nanotubes: (a) crude oxidized nanotubes in the biphasic suspension; (b) short nanotubes and carbon impurities in the upper layer (isotropic) after centrifligation,- (c) long nanotubes in the lower layer (nematic) after centrifugation. Significantly, the long nanotubes dominate the nematic phase and result in highly ordered patterns, whereas short nanotubes and carbon impurities occupy the isotropic phase and produce random arrangements.

Figure 6 (Examples 2 and 3) shows electron micrographs of carbon nanotubes in the nematic phase after cycles of centrifugation: (a) 1st centrifugation,- (b) 2nd centrifugation; (c) 3^rd centrifugation. After several cycles of centrifugation, the purity of nanotubes in the nematic phase increases greatly because short nanotubes and carbon impurities are removed.

Figure 7 (Example 3) shows optical micrographs of nanotube aqueous suspensions in reflected polarised light with the polars crossed at different concentrations (a) 1.2 wt%, no birefringence from isotropic phase; (b) 3.6 wt%, weak birefringence from nematic nuclei; (c) 6.0 wt%, strong birefringence from coexisting biphase; (d) 7.2 wt%.

Figure 8 (Example 3) shows a plot of the anisotropic phase volume fraction versus the total suspension concentration for nanotube aqueous suspensions .

Figure 9 (Example 3) shows a plot of individual concentrations of isotropic and anisotropic phases as a function of total nanotube suspension concentration. The lines are drawn to guide the eye only. Figure 10 (Example 3) shows a plot of the anisotropic phase volume fraction versus the total suspension concentration for nanotube aqueous suspension with various cycles of centrifugation as indicated in the Figure.

Figure 11 (Example 3) shows (a) electron micrograph of the disclination with s = +1 in a nematic suspension of nanotubes; (b) the molecular trajectory in a two dimensional ordered liquid crystal associated with the disclination of s = +1, predicted by Frank continuum theory (left) and separated into two disclinations of s = +1/2 (right) .

Example 1

In this example phase separation is carried out by leaving the first suspension to stand for several weeks. The aqueous suspensions of nanotubes were prepared by oxidising the as-synthesised nanotubes for 24 hours in a 3:1 mixture of concentrated sulphuric acid and concentrated nitric acid under ultrasonication and then extracting them with deionised water. The chopped nanotubes were then dissolved in deionised water at different concentrations.

The nematic aqueous suspensions of crude, as- treated, multiwall CNTs were held in a flat optical cell. These crude CNTs had an average diameter of about 30 run and an average length of about 800 nm.

The threshold concentration for a single nematic phase was 7 wt%, with the suspension displaying a typical single polydomain nematic phase. Observed under polarized light with a reflection mode, the single phase showed strong birefringence.

A suspension of 8 wt% nanotubes (corresponding to the middle of the two phase region in the phase diagram) , was prepared.

A thin liquid film was prepared by placing a drop of nanotube suspensions between two glass slides to form a sandwich cell, which was subsequently sealed using epoxy glue. The wet samples were left standing at room temperature for several days to develop the texture. The phases were identified by characteristic textures observed under the reflected polarised optical microscopy (Olympus BX-50) . The morphologies and microstructures of nanotubes in isotropic and anisotropic phases were examined using a JEOL 6340F field emission gun scanning electron microscopy at an accelerating voltage of 5 kV. This analysis method was also used as appropriate in Examples 2 and 3.

Figure Ia shows a typical optical micrograph in 8 wt% aqueous suspension of CNTs . One can immediately observe the dark brushes of a Schlieren texture emanating from singularities typical of the nematic phase [8] . It has been shown that this texture can be preserved upon evaporating the solvent to form a film and allowing the microstructure to be studied [7] . Scanning electron microscope (SEM) on these films revealed that individual nanotubes organized into ± % disclinations (Figure Ib) [9] . Also, despite the extensive oxidative process and microfiltration processing steps applied in treating the nanotubes [7] , short nanotubes and carbon impurities were found in the crude sample. (It should be noted that carbon impurities also refer to the curved tubes present within the samples since the curvature would prevent alignment and liquid crystallinity. ) After the crude suspension had been held at room temperature for more than one week, the scale of the phase separation had increased to the extent it could easily be observed by cross polar light microscopy. Figure 2a shows a typical optical micrograph of the texture obtained when the sample of 8 wt % concentration has been annealed for one month at room temperature. A region of an isotropic phase (dark) could be seen within a matrix of liquid crystalline phase (speckled) . The regions are of macroscopic size, indicating that the characteristic scale of phase separation is very large. The interface between two phases is sharp as indicated by the significant contrast; the nematic phase with Schlieren texture has strong birefringence compared to the isotropic phase. The dark and light regions of the liquid crystalline phase changed contrast as the polars were rotated.

This phase separation was studied further by drying the films and then observing in the SEM the arrangement of the individual tubes within the films. Figures 2b and 2c show typical electron micrographs for the nanotube distributions and alignments in the two phases. In the region of the Schlieren texture, the nematic consists of highly ordered, longer nanotubes (Figure 2b) . The liquid crystalline region, which is classified as nematic, contains topological defects or disclinations, two of which are present in Figure 2b. The disclination density and the lateral separation between individual nanotubes decreased considerably compared to the crude nematic sample (Figure Ib) . By carefully controlling the conditions, large solid films consisting of purified longer nanotubes packed parallel to each other can be obtained. In contrast to the nematic regions, short nanotubes and impurities were found in the isotropic regions (Figure 2c) . These nanotubes were not aligned and were randomly organized. The impurities found included round carbon particles and highly curved nanotubes .

Figure 3 shows length distributions of CNTs in the two phases obtained by measuring over 100 nanotubes, taken from different areas of the sample. ~ 94% nanotubes in the nematic have maximal lengths between 500 and 1000 run, while about 90% nanotubes in the isotropic have maximal lengths below 600 nm. The mean length of the nanotubes in the nematic phase was 750 nm and the mean length of the nanotubes in the isotropic phase was 300 nm.

In addition, there were significant carbon impurities in the isotropic phase [10]. These statistics show that the longer the nanotubes, the more stable the nematic phase, with the system naturally separating out the shorter nanotubes and impurities.

Example 2 In this example phase separation is carried out by centrifugation.

Multi-wall carbon nanotubes were synthesised in the inventors' laboratory by pyrolysis of a solution of ferrocene in toluene via an injection CVD method. The nanotubes produced are fairly straight with a diameter in a range of 10 to 100 nm and a length from 20 to 200 microns. To prepare nematic aqueous suspensions of nanotubes, the as-synthesised nanotubes were oxidised for 24 hours in a 3:1 mixture of concentrated sulphuric acid and concentrated nitric acid under ultrasonication and then extracted with deionised water.

The chopped nanotubes were then dissolved in deionised water with different concentrations. As explained previously, the phase transition from isotropic phase to nematic phase was observed with increase of the suspension concentration above a critical value. Biphasic test suspensions of different concentrations were prepared by mixing the nematic suspension with deionised water in a test tube and were briefly ultrasonicated at room temperature .

Each suspension was centrifuged at 3500 rpm for 90 min in a MISTRAL 1000 centrifuge to achieve layering of the sample, with the isotropic phase above the denser nematic phase. The relative volumes of the two phases were measured and a sample from each was removed by pipette and weighed before and after drying in an oven to determine the concentration of nanotubes. These measurements are shown in Fig. 4 which shows the concentrations in both the nematic and isotropic phases for the first and two subsequent re-centrifugations .

For each re-centrifugation the nematic material only was rediluted to give the same overall concentration as the original crude sample and the centrifugation process repeated. The fact that the upper concentration limit of the biphasic region reduces compared with the original crude sample (Fig. 5a) on successive treatments indicates that the mesogenicity of the CNTs in the sample is increased. Samples were also taken for microscopy. Figs. 5b and 5c show typical images of the dried out samples of isotropic and nematic phases .

Example 3 This example relates to a more detailed study of the method and product of Example 2.

Isotropic-nematic phase transition was studied as a function of suspension concentrations. Optical micrographs of nanotube aqueous suspensions in reflected polarized light with the polars crossed showed that there existed a biphasic range where isotropic and anisotropic phases coexisted (Fig. 7) . The wide biphasic range (so-called Flory chimney) is due to the polydispersity of chopped nanotubes in aqueous suspensions. Phase separation in biphasic suspensions was studied (Figs. 8-9) . Fig. 8 shows that the volume fraction of anisotropic phase increases linearly with the total concentration. The approximate critical concentration for the onset limit of biphasic behaviour was estimated from the experimental data in Fig. 8 by extrapolating the linear curve to the zero value of volume fraction of anisotropic phase. This critical concentration defined as ci* is ~ 1.6 wt%. According to the rigid rod theory, an anisotropic phase starts to form at a weight fraction of about 3.3pd/l, where p, d and 1 are, respectively, rod density, rod diameter and rod length. For chopped nanotubes in this work, p ~ 1.75, d ~ 30 nm, 1 ~ 900 nm,- thereof, the critical concentration at the start of phase transition is ~ 1.9 wt% . The experimental data in this work are in agreement with the theoretical prediction.

The weight concentrations for both isotropic and nematic phases against the total concentration were determined (Fig. 9) and no significant trend was observed with the total concentration. This result indicates that the concentrations of the coexisting phases are insensitive to the suspension concentration. This is expected since the effect of electrostatic interaction on the phase separation will remain constant as the suspension concentration is increased, predicted by the theory . By studying nanotube length and order distributions in the two phases (Fig/ 5), we found that longer nanotubes dominated the nematic phase showing highly ordered alignments whereas shorter nanotubes and carbon impurities occupied the isotropic phase leading to completely disordered arrangements . This behaviour is analysed by the rigid rod theory and interpreted in terms of a miscibility gap that proportional to the degree of nematic order, which is different for different lengths of nematic nanotubes .

Several cycles of centrifugation were carried out and the different phase behaviours found. Fig. 10 shows the volume fraction of anisotropic phase against the total concentration for different cycles of centrifugation. For each cycle of centrifugation, increasing the total concentration resulted in an increase of volume fraction of anisotropic phase, following a simple linear function. The slope of each linear curve increases with increase of cycle number. It should be noted that the value of critical concentration (ci*) of starting anisotropic is roughly same for each cycle, e.g. Ci* ~ 1.6 wt% . If the critical concentration of the upper limit of biphasic behaviour is defined as C₂* where the isotropic phase disappears completely, we can estimate the biphasic width (so-called Flory chimney) from the range of C₂* - Ci* . The data from Figure 10 indicate that the Flory chimney decreases with increase of cycle number. Therefore oligo-dispersed nanotubes will be obtained after several cycles of centrifugation. Fig. 6 shows the length distribution of nanotubes with different cycles of centrifugation. We can see that the short nanotubes and carbon impurities have been removed after cycles of centrifugation. Fig. 4 shows the concentrations for isotropic and nematic phases against the cycle number of centrifugation. The concentration for the nematic phase decreases greatly whereas the concentration for the isotropic phase changes little with increasing number of cycles of centrifugation. This result is expected as the concentration for the nematic phase is strongly dependent on the shorter nanotubes whereas the concentration for isotropic is determined by the longer nanotubes . After centrifugation, the short nanotubes have been removed and only long nanotubes are maintained.

It will be appreciated that while the invention has been described with reference to the illustrated examples, many modifications are possible within the scope of the invention.

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Claims

1. A method of purifying nanopartides, comprising the steps of: preparing a first suspension of nanoparticles; causing or allowing phase separation of the first suspension into a liquid crystalline suspension of purified nanoparticles and a second suspension; and ^' separating the liquid crystalline suspension of purified nanoparticles from the second suspension.

2. A method as claimed in Claim 1, wherein the second suspension contains impurities.

3. A method as claimed in Claim 1 or Claim 2 , wherein the second suspension is an isotropic suspension.

4. A method as claimed in any one of the preceding claims, wherein the liquid crystalline suspension is nematic, smectic or cholesteric.

5. A method as claimed in any one of the preceding claims, wherein the purified nanoparticles comprise nanotubes and/or nanorods .

6. A method as claimed in Claim 5, wherein the purified nanotubes and/or nanorods are separated from nanotubes and/or nanorods of different length, curved nanotubes and/or nanorods and non-nanotube or nanorod nanoparticles .

7. A method as claimed in Claim 6, wherein the purified nanotubes and/or nanorods are longer nanotubes and/or nanorods and the impurities comprise shorter nanotubes and/or nanorods.

8. A method as claimed in Claim 7 , wherein the longer nanotubes and/or nanorods have an average length of between 500 and 10000 run, and the shorter nanotubes and/or nanorods have an average length below 600 run.

9. A method as claimed in either one of Claims 7 or 8 , wherein the longer nanotubes and/or nanorods have an average aspect ratio of between 16 and 70, and the shorter nanotubes and/or nanorods have an average aspect of below 20.

10. A method as claimed in any one of the preceding claims, wherein the nanoparticles comprise carbon, boron nitride, titanium oxide, tungsten (IV) sulphide, or metal .

11. A method as claimed in Claim 10, wherein the nanoparticles are gold or copper nanorods .

12. A method as claimed in Claim 10, wherein the nanoparticles are optionally derivatised carbon nanotubes .

13. A method as claimed in Claim 12, wherein the carbon nanotubes are optionally derivatised single wall or multiwall carbon nanotubes.

14. A method as claimed in any one of the preceding claims, wherein the first suspension is a liquid crystalline suspension.

15. A method as claimed in any one of the preceding claims, wherein the concentration of the first suspension is within the Flory chimney.

16. A method as claimed in any one of the preceding claims, wherein the suspensions are in a solvent selected from water, polar organic solvents, halogenated solvents, acid and oleum.

17. A method as claimed in any one of the preceding claims, wherein the step of causing or allowing phase separation of the first suspension into a liquid crystalline suspension of purified nanoparticles and a second suspension is carried out in one or more of the following ways :

(a) adjusting the concentration of the first suspension;

(b) applying a field to the first suspension,- and

(c) leaving the first suspension for a period of time.

18. A method as claimed in Claim 17, where a simulated high gravity field is applied.

19. A method as claimed in Claim 18, wherein the field is applied using a centrifuge.

20. A method as claimed in Claim 19, wherein more than one cycle of centrifugation is carried out on the first suspension.

21. A method as claimed in Claim 17, wherein the first suspension is left for a period of at least one week.

22. A method as claimed in Claim 21, wherein the first suspension is left for a period of at least one month.

23. A method as claimed in any one of the preceding claims, further comprising an initial step of treating the nanoparticles to solubilise and/or to shorten them.

24. A method as claimed in Claim 23, wherein the nanoparticles are treated with acid, surfactant and/or ultrasonication.

25. A method as claimed in any one of the preceding claims, where the purified nanoparticles are diluted with solvent and the purification process is repeated.

26. Purified nanoparticles prepared by the method of any of the preceding claims .

27. Use of differing liquid crystalline suspension forming ability properties of two types of nanoparticles in performing a separation of the nanoparticles .