WO2012016296A1

WO2012016296A1 - Methods for preparing carbogenic nanoparticles and photoluminescent carbogenic nanoparticles

Info

Publication number: WO2012016296A1
Application number: PCT/AU2011/000998
Authority: WO
Inventors: Qin Li; Fu Wang
Original assignee: Curtin University Of Technology
Priority date: 2010-08-05
Filing date: 2011-08-05
Publication date: 2012-02-09

Abstract

A method for preparing carbogenic nanoparticles is provided. The method involves carbonizing a carbogenic precursor material, preferably by treating the carbogenic precursor material with an acid in a high boiling point solvent at elevated temperatures. The quantum yield of some of the carbogenic nanoparticules have a quantum yield greater than about 15%. The photoluminescence of the carbogenic nanoparticles may be further enhanced by treating the carbogenic precursor material with an acid in a high boiling point solvent in the presence of a surface passivating agent.

Description

METHODS FOR PREPARING CARBOGENIC NANOPARTICLES AND PHOTOLUMINESCENT CARBOGENIC NANOPARTICLES

FIELD

The present invention relates to methods for preparing carbogenic nanoparticles and photoluminescent carbogenic nanoparticles.

BACKGROUND

Luminescent semiconductor quantum dots (QDs) are nanoparticles with size dependent optical and electronic properties. They have been the subject of intensive research and development for a broad range of applications such as energy-efficient displays and lighting, photovoltaic devices, and biological markers. However, the intrinsic toxicity and potential environmental hazards associated with many of these nanomaterials represent considerable challenges to their practical usage. Light- emitting quantum-sized carbon dots (CDs) appears to be a promising alternative to semiconductor QDs in many of the applications owing to their low toxicity and cheaper cost.

Following the development of the laser ablation method for preparation of quantum- sized carbon nanoparticles, various methods have been developed to produce quantum-sized carbon nanoparticles. Some of these methods include electrochemical release or exfoliation from a graphitic source, separation of combusted carbon soot, carbonizing polymerized resols on silica spheres, and thermal oxidation of suitable molecular precursors.

Most of these synthetic methods require several steps to complete, including a surface passivation procedure in order to render resulting carbon nanomaterials photoluminescent. Moreover, the photoluminescent quantum yields (QY) of these resultant carbon dots are very low, usually below 6%. The highest quantum yield reported to date was 20%, where the laser ablated carbon dots were conjugated with PEG1500N in neat SOCI₂. In view of the significant potential of this zero-dimension carbon nanomaterial in various fields, a facile and scalable synthetic approach for luminescent carbon dots is highly desirable. The present invention seeks to overcome, at least in part, some of the problems described above. SUMMARY

Accordingly, in a first aspect the invention provides a method for preparing carbogenic nanoparticles comprising carbonizing a carbogenic precursor material in a high boiling point solvent.

In one embodiment of the invention, carbonizing the carbogenic precursor material comprises contacting the carbogenic precursor material with the high boiling point solvent at an elevated temperature. The elevated temperature may be in a range of about 120 °C to about 300 °C, preferably from about 140 °C to about 280 °C.

In some embodiments of the invention, the method comprises carbonizing the carbogenic precursor substance in the high boiling point solvent in the presence of an acid. In some embodiments of the invention, carbogenic nanoparticles prepared in accordance with the first aspect of the invention may be inherently photoluminescent. The quantum yield of such carbogenic nanoparticles may be enhanced by preparing such carbogenic nanoparticles in accordance with the first aspect of the invention in the presence of a surface passivating agent.

In other embodiments of the invention, the carbogenic nanoparticles of the present invention may be rendered photoluminescent by passivating the surface of the carbon nanoparticle. In one embodiment, passivating the carbogenic nanoparticles may be achieved by treating the carbogenic nanoparticles with a surface passivating agent after the carbogenic nanoparticles have been prepared according to the first aspect of the invention.

In an alternative embodiment, preparing photoluminescent carbogenic nanoparticles may be achieved by treating a carbogenic precursor material with an acid in a high boiling point solvent in the presence of a surface passivating agent. Surprisingly, the inventors have found that the method of the present invention is capable of preparing photoluminescent carbogenic nanoparticles in the absence of a surface passivating agent or without treating the surface of the carbogenic nanoparticles with a surface passivating agent.

Accordingly, a second aspect of the invention provides carbogenic nanoparticles comprising a core of carbogenic material, the carbogenic material comprising a plurality of fluorophores located in or on the core.

In one embodiment of the invention, the carbogenic nanoparticles are photoluminescent under excitation by near infrared frequencies. Photoluminescence may be induced by a two-photon absorption process. In one embodiment, the nanoparticle has a particle size in a range of about 0.1 nm to about 10 nm. The nanoparticles have a uniform particle size with a particle size distribution range of about 1 nm to about 2 nm.

In another aspect, the invention provides photoluminescent carbogenic nanoparticles comprising a core of nitrogen enriched-carbogenic material.

Brief Description of the Drawings

Figure 1 is a photograph of the toluene solution of Oil soluble' photoluminescent carbon nanoparticles prepared in accordance with one embodiment of the present invention after 180 min reaction excited by the fiber of the fluorescent spectrometer (A), and the absorption and photoluminescence emission spectra of the photoluminescent carbon nanoparticles reacted after (B) 5 min (C) 180min.

Figure 2 is an absorption and photoluminescence emission spectra of the 'water soluble' photoluminescent carbon nanoparticles prepared in accordance with one embodiment of the present invention when reacted after (A) 5 min (B) 180 min.

Figure 3 is a representation of the temporal evolution of the absorption and normalized photoluminescent spectra of Oil soluble' photoluminescent carbon nanoparticles shown in Figure 1 pyrolyzed in 1 -octadecylene (A), and the relationship between quantum yield and reaction time (B) excited at 360nm and 420nm using Quinine sulfate in 0.1 M H₂S0₄ and Coumarin 6 in ethanol as standard references, respectively. Figure 4 is a representation of the temporal evolution of the absorption and normalized photoluminescent spectra of 'water soluble' photoluminescent carbon nanoparticles shown in Figure 2 pyrolyzed in glycerin (A), and relationship between quantum yield and reaction time (B) excited at 360nm and 420nm using Quinine sulfate in 0.1 M H₂S0₄ and Coumarin 6 in ethanol as standard references, respectively.

Figure 5 are TEM images (200 keV) and AFM topography images on mica substrates of 'oil soluble' photoluminescent carbon nanoparticles prepared in accordance with an embodiment of the present invention reacted after 5 min (A) (B) and after 180 min(C) (D). Figure 6 is a TEM and AFM image of 'oil soluble' photoluminescent carbon nanoparticles prepared in accordance with an embodiment of the present invention.

Figure 7 is a TEM and AFM image of carbon nanoparticles prepared in accordance with another embodiment of the present invention.

Figure 8 is the absorption and photoluminescence emission spectra of carbon nanoparticles prepared in accordance with another embodiment of the present invention. Figure 9 is a series of FT-IR spectra of citric acid (a); carbon nanoparticles (b);

'oil soluble' photoluminescent carbon nanoparticles (OCDs) (c); and 'water soluble' photoluminescent carbon nanoparticles (WCDs) (d).

Figure 10 are powder XRD patterns of 'oil soluble' photoluminescent carbon nanoparticles (OCDs) and 'water soluble' photoluminescent carbon nanoparticles (WCDs).

Figure 1 1 (a) is a UV-visible absorption spectrum of the three fractions nitrogen enriched carbogenic nano-particles referred to as <1 K, 1 K, and 3.5K in Example 7; and Figures 1 1 (b)-1 1 (d) are a series of photoluminescence emission spectra for the three fractions referred to as <1 K, 1 K, and 3.5K, respectively, where the excitation wavelength was step-wise increased from 330 nm to 450 nm. Figure 12 is a series of FTI spectra of several carbogenic precursor materials and carbogenic nanoparticles prepared in accordance with the present invention from said carbogenic precursor materials.

Figure 13 is an excitation-emission spectrum of nitrogen enriched-carbogenic nanoparticles prepared in accordance with the present invention, wherein excitation is by wavelength longer than 700 nm.

Detailed Description The present application relates to methods for preparing carbogenic nanoparticles and photoluminescent carbogenic nanoparticles, respectively.

The term 'carbogenic nanoparticle' is used to refer to nanoparticles substantially comprising a carbon-based material. Illustrative examples of carbon-based materials include, but are not limited to, amorphous carbon, semi-crystalline carbon, crystalline carbon, graphitic carbon, graphene-like carbon, carbogenic compounds, and carbogenic oligomers. It will be understood that the carbon-based material may be doped or enriched with heteroatoms, such as N, B, S, F, O, P, Si and so forth, by using a carbogenic precursor material which contains said heteroatoms. The carbon content of the carbogenic nanoparticle may be at least 40 wt%.

The carbogenic nanoparticles prepared in accordance with the methods of the present invention may have a particle size in a range of about 0.1 nm to about 10 nm. Generally, said nanoparticles have a substantially uniform particle size with a particle size distribution range of about 1 nm to about 2 nm.

In the prior art, photoluminescent carbon nanoparticles have been prepared. These photoluminescent carbon nanoparticles comprise a carbon core and a passivation agent coupled to the carbon core. The carbon core component may be produced via laser ablation of a graphite powder carbon target or by electric arc discharge from carbon powders. To attain the ability to exhibit photoluminescence, the passivation agent is then subsequently bound to the surface of the carbon core by known techniques. Excitation energy traps existing at the surface of the carbon core are stabilized by the passivation agent that is coupled to the carbon core, thereby rendering the carbon nanoparticle photoluminescent.

In contrast to the prior art, carbogenic nanoparticles prepared according to the present invention may be photoluminescent in the absence of a surface passivating agent. These photoluminescent carbogenic nanoparticles comprise a core of carbogenic material, wherein the carbogenic material comprises a plurality of fluorophores located in or on the core. The inventors have speculated that the fluorophores may be formed during carbonization of the carbogenic precursor material, and that the incidence of fluorophores located in or on the core may be enhanced (and therefore the photoluminescence of the carbongenic nanoparticles may be enhanced) by carbonizing the carbogenic precursor material in the presence of an acid and/or a surface passivating agent. The fluorophores include, but are not limited to, polyaromatic fluorophores or conjugated double bonds between carbon and oxygen atoms, carbon and nitrogen atoms, carbon and other heteroatoms, and/or carbon and other carbon atoms.

The term 'fluorophore' refers to a functional group in a molecule which will absorb energy of a specific wavelength and re-emit energy at a different (but equally specific) wavelength. The amount and wavelength of the emitted energy depend on both the fluorophore and the chemical environment of the fluorophore.

The photoluminescent carbogenic nanoparticles prepared in accordance with the present invention generally demonstrate higher quantum yields in comparison to photoluminescent surface passivated carbon nanoparticles prepared by prior art methods. The photoluminescence is in the UV-visible spectrum.

The quantum yields of the photoluminescent surface passivated carbon nanoparticles prepared in accordance with the present invention may be greater than 15%.

Some photoluminescent carbogenic nanoparticles, in particular nitrogen enriched- carbogenic nanoparticles, prepared in accordance with the present invention are photoluminescent under excitation by near infrared frequencies. Photoluminescence may be induced by a two-photon absorption process.

Early studies also indicate that the quantum yields of the photoluminescent carbogenic nanoparticles of the present invention demonstrate long term stability in PBS, phosphate buffer solutions.

Further, a dispersion of solid carbogenic nanoparticles in silane demonstrates photoluminescence with emissions in the range of 400-600 nm when excited at 360 nm. The inventors have therefore concluded that in some embodiments, the carbogenic nanoparticles of the present invention are photoluminescent in the solid state.

The carbogenic nanoparticles of the present invention may be prepared from a carbogenic precursor material.

The term 'carbogenic precursor material' refers to any suitable organic compound or an organic material which may be carbonised to a carbogenic material at elevated temperatures or with a carbonizing agent.

Illustrative examples of suitable organic materials include, but are not limited to, biomass. The biomass may be sourced from leaf vegetation.

Illustrative examples of suitable organic compounds for use as the carbon precursor substance include, but are not limited to, carbohydrates such as monosaccharides, disaccharides, oligosaccharides, and polysaccharides; polyhydroxy-substituted aldehydes; polyhydroxy-substituted ketones; polyols; heterocyclic compounds including heterocyclic bases and heterocyclic acids; mono- and polyunsaturated hydrocarbons; and heteroatom-substituted oligomers or polymers of ethylene oxide such as PEG 1500N -

As used herein, the term 'carbohydrate^' generally refers to aldehyde or ketone compounds substituted with multiple hydroxy! groups, of the general formula (CH .0} ·. wherein n is 2-36, as well as their oligomers and polymers. The carbohydrates of the present invention can in addition, be substituted or deoxygenated at one or more positions. Carbohydrates, as used herein, encompass unmodified carbohydrates, carbohydrate derivatives, substituted carbohydrates, and modified carbohydrates. As used herein, the phrases "carbohydrate derivatives", ^"substituted carbohydrate", and "modified carbohydrates" are synonymous. Modified carbohydrate means any carbohydrate wherein at ieast one atom has been added, removed, substituted, or combinations thereof. Thus, carbohydrate derivatives or substituted carbohydrates include substituted and unsubstituted monosaccharides, disaccharides, oligosaccharides, and polysaccharides. The carbohydrate derivatives or substituted carbohydrates optionally can be deoxygenated at any corresponding C-position, and/or substituted with one or more moieties such as hydrogen, halogen, haloaikyL carboxyl, acyl, acyloxy, amino, amido, carboxyl derivatives, aikylamino, dia!kySamino, arylamino, aikoxy, aryioxy, nitres, cyano, suifo, mercapto. ί mines, suifonyl, suifenyl, sulfinyi, su!famoyi, carboa!koxy, carboxamido, phophonyl, phphinyi, phosphor/, phosphino, thioester, thioether, oximino, hydrazine, carbamyl, phospho, phosphonato, boro, si!yi, or any other viable functional group.

Non-limiting examples of suitable carbohydrates which may be used as the carbon precursor substance herein include glucose, fructose, galactose, xylose, ribose, sucrose, iaculose, lactose, maltose, trehalose, celiobiose, raffinose, melezitose, maitotriose, acarbose, sachyose, fructooligosaccharides. galactoo!igosaccharides, mannon-oligosaccharides, cyciodextrin. cellulose.

A heterocyclic compound is a cyclic compound which has atoms of at Ieast two different elements as members of its ring(s). The heterocyclic compounds used in the present invention contains at least one carbon atom, and one or more atoms of elements other than carbon with the ring structure, such as sulfuret, oxygen or nitrogen.

Heterocyclic bases are organic compounds comprising an aromatic ring in which a lone pair of electrons of a ring-heteroatom (e.g. N, B, S, F, O, P, Si and so forth) is not part of the aromatic system and extends in the plane of the ring. The heterocyclic bases of the present invention can in addition, be substituted at one or more positions or fused with one or more aromatic rings. The heterocyclic bases optionally can be substituted with one or more moieties such as hydrogen, halogen, haloaikyi, carboxyl, acyl, acyloxy, amino, amido, carboxyl derivatives, aikylamino, dialkyiamino, arylamino, aikoxy, aryioxy, nitro, cyano, su!fo, mercapto, imino, suifonyi, suifenyl, sulfinyl, sulfamoyi, carboaikoxy, carboxamido, phosphonyl, phosphinyl, phosphory, phosphino, thioester, thioether, oximino, hydrazine, carbamyl, phospho, phosphonato, boro, si!yi, or any other viable functional group.

Non-limiting examples of heterocyclic bases which may be used as the carbon precursor substance herein include pyridine, acricline, pyrazine, quinoxaiine, quinoiine, isoquinoiine, pyrazoie, indazoie, pyrimidine, quinazoiine, pyridazine, cirmoline, triazine, meiamine, and derivatives and combinations thereof.

Heterocyclic acids used in the present invention are organic compounds comprising an aromatic ring in which a ring heteroatom may be part of the aromatic ring system and which has an acidic functional group directly or indirectly coupled to the aromatic ring system. For example, hydroxyl groups directly coupled to the aromatic ring by virtue of substitution of the C-ring atoms have acidic functionality. The hetorocyclic acids of the present invention can in addition, be substituted at one or more positions or fused with one or more aromatic rings. The heterocyclic acids optionally can be substituted with one or more moieties such as hydrogen, halogen, haioalkyi, carboxyi, acy!, acyioxy, amino, amido, carboxyi derivatives, aikylarnino. diaiky!amino. arylamino. alkoxy, aryioxy, nitro, cyano, suifo, mercapto, imino, suifonyi, suifenyl, sulfinyl, sulfamoyi, carboaikoxy, carboxamido, phosphonyl, phosphinyl, phosphory, phosphino, thioester, thioether, oximino, hydrazine, carbamyl, phospho, phosphonato, boro, silyi, or any other viable functional group. Non-limiting examples of heterocyclic acids which may be used as the carbon precursor substance herein include cyanuric acid.

Mono- and unsaturated hydrocarbons used in the present invention are organic compounds comprising a C5-C36 backbone with one or more C=C bonds. The mono- and unsaturated hydrocarbons of the present invention can in addition, be substituted at one or more positions with one or more moieties such as alkyl, halogen, haioalkyi, carboxyi acyl, acyioxy, amino, amido, carboxyi derivatives, alky!amino, diaikylamirso, arylamino, alkoxy, aryioxy, nitro, cyano, suifo. mercapto, imino, suifonyi, suifenyl, su!finyi, sulfamoyi, carboaikoxy, carboxamido, phosphonyl, phosphinyl, phosphory, phosphino, thioester, thioeiher. oximino. hydrazine, carbamy!, phospho, phosphonato, boro, si!yi, or any other viable functional group.

In one specific embodiment of the invention the carbon precursor substance is glycerine. In another specific embodiment of the invention the carbon precursor substance is melamine. In another specific embodiment of the invention the carbon precursor substance is PEG 1500N - In another specific embodiment of the invention the carbon precursor substance is cyanuric acid. In another specific embodiment of the invention, the carbogenic precursor material is 1 -octadecene.

The carbogenic nanoparticles of the present invention may be prepared by carbonizing a carbogenic precursor material in a high boiling point solvent.

The term 'carbonizing' as used throughout the description refers to a process which reduces or converts a carbon-containing substance, such as any one of the suitable organic compounds referred to above, to a carbon-based material. The carbonizing process may include any one or more sequential depolymerisation, decomposition, dehydration including intramolecular dehydration, polymerisation and pyrolysis processes.

According to one embodiment of the invention, carbonizing the carbogenic precursor material in the high boiling point solvent comprises bringing the carbogenic precursor material in contact with the high boiling point solvent at an elevated temperature. Alternatively, carbonizing the carbogenic precursor material in the high boiling point solvent comprises heating a mixture or solution of the carbogenic precursor material in the high boiling point solvent to the elevated temperature and maintaining said mixture or solution at the elevated temperature. In general, the elevated temperature will be at or above a temperature at which the carbogenic precursor material will thermally decompose to a carbon-based material. Accordingly, in some embodiments the elevated temperature may be in a range of about 100 °C to about 300 °C. The high boiling point solvent is a solvent that has a boiling point temperature that is greater than the temperature at which the carbogenic precursor material may thermally decompose to the carbon-based material. Suitable high boiling point solvents include, but are not limited to, ionic liquids, glycerine, long chain unsaturated hydrocarbons such as 1-octadecene, and long chain saturated hydrocarbons. Preferably, the high boiling point solvent has a boiling point temperature greater than 250 °C.

In a further alternative embodiment, carbonizing the carbogenic precursor material may also comprise treating the carbogenic precursor material with a carbonizing agent.

The carbogenic nanoparticles of the present invention may also be prepared by carbonizing a carbogenic precursor material in a high boiling point solvent in the presence of an acid. The acid may be an inorganic acid or an organic acid.

Suitable inorganic acids include, but are not limited to, sulphuric acid, hydrochloric acid, nitric acid, perchloric acid, phosphoric acid, boric acid, hydrobromic acid, fluorosulphuric acid, and hexafluorophosphoric acid. Acid-enriched solid zeolite catalysts, such as ZSM-5 zeolite, are also suitable.

Suitable organic acids include but are not limited to organic acids such as monofunctional or polyfunctional carboxylic acids and/or anhydrides, in particular polyhydroxy-substituted carboxylic acids and/or anhydrides; sulphonic acids;

Monofunctional carboxylic acids as used herein are organic acids comprising a carboxylic acid group and optionally one or more functional groups, including functionalised and non-functionalised carboxylic acids. Monofunctional carboxylic acids useful herein, can be aliphatic, aromatic, saturated, linear and/or branched. The preferred monofunctiona! carboxylic acids have from about four to about twenty-four carbon atoms. The functionalisecl monofunctional carboxylic acids can be substituted with one or more moieties such as hydrogen, halogen, haloalkyl, carboxyl, acyl, acyloxy, amino, amiclo, carboxyl derivatives, alkylamino, dialkylamino, arylamino, a koxy, aryloxy, nitro, cyano, sulfo, mercapto, imino, sulfonyl, suifenyl, sulfinyl, sulfamoyi, carboalkoxy, carboxamido, phosphonyl, phosphinyl, phosphoryl, phosphino, thioester, thioether, oximino, hydrazine, carbamyi, phospho, phosphonato, boro, si!yi, or any other viable functional group.

Non-limiting examples of suitable monofunctiona! carboxy!ic acids which may be used as the carbon precursor substance herein include isohutyric acid, benzoic acid, 2-ethyl butyric acid, hexanoic acid, heptanoic acid, 2~ethyihexanoic acid, octanoic acid, nonanoic acid, 3,5,5-trimethyihexanoic acid, isononanoic acid, decanoic acid, isooctadecanoic acid, dodecanoic acid, 2-methyl butyric acid, isopentanoic acid, pentanoic acid, 2-methyl pentanoic acid, 2-methyl hexanoic acid, isooctanoic acid, undecyiinic acid, isolauric acid, isopaimitic acid, isosiearic acid, behenic acid, and derivatives and combinations thereof.

The polyfunctions! carboxy!ic acid is a oarboxyiic acid with at least two carboxyiic acid groups and optionally one or more additional functional groups, including functionaiized and non~functionaiized dicarboxyiic acids. Polyfu notional carboxyiic acids and/or anhydrides can be aliphatic, aromatic, saturated, linear and/or branched. Preferably, the polyfunctionai carboxyiic acids and/or anhydrides used herein have one to about thirty six carbon atoms. The functionalised polyfunctionai oarboxyiic acids can be substituted with one or more moieties such as hydrogen, halogen, ha!oaikyi, carboxyl, acyl, acy!oxy, amino, amido, carboxyl derivatives, alkyiamino, diaikyiamino, ary!amino, aikoxy, ary!oxy, nitro. cyano, sulfo, mercapto, imino, sulfony!, sulfenyl, sulfinyl, sulfamoyi, carboaikoxy, carboxamido, phosphonyl, phosphinyl, phosphoryi, phosphino, thioester, thioether, oximino, hydrazine, carbamyi, phospho, phosphonato, boro, silyi, or any other viable functional group.

Non-limiting examples of polyfunctionai carboxyiic acids and/or anhydrides which may be used as the organic acid herein include carbonic acid, hexanedioie acid, dimer acid, azelaic acid, sebacic acid, dodecanedioic acid, glutaric acid, succinic acid, citric acid, phtha!ic acid, isophthaiic acid, terephthalic acid, 2,6-naphthalene dicarboxyiic acid, and derivatives and combinations thereof.

The inventors speculate that the role of the acid is to initiate, catalyze or facilitate conversion of the precursor carbogenic substance to carbogenic nanoparticles at elevated temperatures. In particular, the acid may be involved in several acid- catalysed reactions, such as amine-alkylization, which may increase the number of fluorophores in the core or surface of the carbogenic material of the nanoparticle, thereby enhancing the quantum yield of the carbon nanoparticles prepared by the methods of the present invention. Advantageously, the inventors have found that photoluminescent carbogenic nanoparticles may be prepared in a similar manner as described above by carbonizing a carbogenic precursor material, optionally with an acid, in a high boiling point solvent in the presence of a surface passivating agent. Additionally, while some carbon nanoparticles prepared in accordance with the present invention may display inherent photoluminescence properties, their quantum yield may be enhanced by preparing said carbon nanoparticles in the presence of a surface passivating agent.

The surface passivating agent may be any suitable organic compound capable of binding to a surface of the carbogenic nanoparticles by primary or secondary bonding interactions, such as for example, through amidation, hydrogen bonding, or electrostatic attraction forces or adsorption. Suitable examples of surface passivating agents include, but are not limited to, long chain amines, amphiphilic oligomeric polymers, long-chain surfactants, nucleotides, peptides, and so forth. In some embodiments, the passivating agent may be 1 -hexadecylamine, PEG1500, and organo-silanes such as triethylsilane.

In general, the surface passivating agent is soluble in the high boiling point solvent. The concentration of the surface passivating agent in the high boiling point solvent may be about 0.01 wt% to about 50 wt%.

In an alternative embodiment, the carbogenic nanoparticles of the present invention may be prepared in accordance with the present invention and subsequently treated with a surface passivating agent by methods well understood in the art to produce surface passivated carbogenic nanoparticles.

Subsequent treatment of carbogenic nanoparticles with a surface passivating agent may be particularly suitable where the surface passivating agent is an inorganic compound or a metal. Suitable inorganic compounds which may be surface passivating agents include, but are not limited to, CdS, ZnS. Suitable metals which may be surface passivating agents include, but are not limited to, Au, Ag, Zn. It will be appreciated that the carbogenic nanoparticles may be bonded to surface passivating agents which are metals by reacting the carbogenic nanoparticle with a metal precursor compound and subsequently converting the metal precursor compound to the metal. One such example includes reacting the carbogenic nanoparticles with silver nitride and subsequently reducing silver nitride to silver metal to prepare carbogenic nanoparticles surface passivated by silver metal.

The carbogenic precursor material may be in a solid or liquid phase or a gel form. It may be brought into contact with the high boiling point solvent by dispersing the carbogenic precursor material in the high boiling point solvent. In general, dispersing the carbogenic precursor material in the high boiling point solvent may be achieved by stirring a mixture of the carbogenic precursor material and the high boiling point solvent with a mixer or agitator. In a preferred form of the invention, dispersing the carbogenic precursor material in the high boiling point solvent may be achieved by rapidly stirring a mixture of the carbogenic precursor material and the high boiling point solvent with a mixer or agitator.

The acid may already be present in the high boiling point solvent, or the acid may be dispersed in the high boiling point solvent, as has been described above with respect to the carbogenic precursor material. The acid may be added to the high boiling point solvent in a liquid or a solid form.

In some embodiments, the carbogenic precursor material may be the high boiling point solvent itself and thus already be in the liquid phase. In these particular embodiments, the acid may be brought into contact with the high boiling point solvent (i.e. the carbogenic precursor material) by dispersing the acid in the high boiling point solvent, as has been described above with respect to the carbogenic precursor material. The acid may be added to the high boiling point solvent in a liquid or a solid form. The temperature of the high boiling point solvent may already be elevated to the desired elevated temperature when the carbogenic precursor material and/or the acid (if required) and/or surface passivating agent (if required) is added to the high boiling point solvent. Alternatively, a mixture or solution of the carbogenic precursor material and/or the acid (if required) and/or the surface passivating agent (if required) may be provided and then the mixture or solution may be heated to the desired elevated temperature and maintained at that temperature until the reaction is completed.

The invention may be readily adapted to a form of continuous processing. Suitable examples of continuous processing include, but are not limited to, spinning disc processing, tubular reactor processing, and rotating tubular reactor processing. In general, the method of the present invention may be performed in a period up to 24 hours, preferably in a period up to about 180 minutes. It will be appreciated that reactions performed at lower temperatures of under 150 °C are likely to proceed to completion in periods of 12 -24 hours, whereas reaction performed at higher temperatures are likely to proceed to completion in periods of 180 minutes or less.

Interestingly, the inventors have also found that it is possible to prepare carbogenic nanoparticles, including photoluminescent carbogenic nanoparticles, at room temperature by carbonizing a carbogenic precursor material in a high boiling point solvent in the presence of concentrated sulfuric acid (98%). Concentrated sulfuric acid (98%) is a known carbonizing agent.

The invention will be illustrated in greater detail with reference to the following examples. Example 1 Synthesis of oil soluble photoluminescent carbogenic nanoparticles ('carbon dots') (OCD)

A high boiling point solvent, 1-octadecene (15 ml), and a surface passivating agent, 1 - hexadecylamine (1 .5g), were placed into a 100 mL three-necked flask and degassed with Argon for 10 min. The temperature of the solution was raised to 300°C, and citric acid (1 g), was quickly added into the solution under vigorous stirring, and then kept at 300°C for 3 hours. The resulting solution was then allowed to cool to room temperature unassisted. Aliquots of the solution were removed at different time intervals and diluted with toluene for subsequent optical and morphology measurements.

The final products were purified by precipitating with acetone three times. The as- obtained photoluminescent carbon nanoparticles were highly soluble in common non- polar organic solvents, such as hexane, chloroform, and toluene.

For comparison, non-surface passivated carbon nanoparticles were also synthesized in the absence of a surface passivating agent (ie. 1 -hexadecylamine) with all other reaction conditions identical.

Example 2 Synthesis of water soluble photoluminescent carbogenic nanoparticles ('carbon dots') (WCD)

In a similar process to the process described in Example 1 , glycerine (15 ml) and PEG 1500 (1 g) were placed into a 100 mL three-necked flask and degassed with Argon for 10 min. The solution temperature was increased to 270°C, and citric acid (1 g) was quickly added into the hot solution. The temperature was maintained at 270°C for 3 hours and then allowed to cool to ambient temperature unassisted. Aliquots were taken at different time intervals and diluted with water for the optical and morphology measurements. The product was purified by dialyzing against Milli-Q water with a cellulose ester membrane bag (Mw =3500), during which excess PEG₁₅₀o and glycerin were removed.

Example 3 Characterization of CDs, photoluminescent OCDs, and photoluminescent WCDs.

TEM measurements were performed on Philips Tecnai F20 at operating voltage of 200 kV. AFM Imaging were obtained by dropping diluted solutions of bare CDs, OCDs and WCDs onto freshly cleaved mica surface and dried in air. All samples were imaged in air by tapping mode AFM on a Dimension 3100 with OMCLAC 160 tip (TS-W2, silicon). Fourier-transform infrared spectra of the purified samples were recorded on a Nicolet 730 FT-IR spectrometer. UV Vis spectra were recorded at room temperature on a Perkin-Elmer Lambda 9 spectrophotometer. PL spectra were measured on a J&M Fluoreszenzspektrometer 3095SL. XRD patterns were recorded using a D/MAX-2550 diffract meter (Rigaku, Japan), with a Cu Ka (λ=1.54178Α) radiation source. The Elemental Analysis was measured in Institute fur Organische Chemie der Johannes Gutenberg-Universitat Mainz for: C, H, N Foss Heraeus vario EL.

As shown in Figure 1A, bright and colorful photoluminescence (PL) emissions were observed from the as-made OCDs, and the emission maxima shifted bathochromically as the excitation wavelength increased (Figure 1 B and 1 C). In addition, the full-width at half-maximum intensity of the photoluminescent (PL) peaks were considerably large (>100nm). For the WCDs, as shown in Figure 2, similar optical behavior was also observed. When the reactions proceeded to 5 min, both the OCDs and WCDs can only be excited in a short range of excitation wavelengths (Figure 1 B and Figure 2A), however, when the reactions were left to proceed to 3h, the excitable range broadened significantly into the longer wavelength with much enhanced emission intensity in the longer wavelength spectrum (Figure 1 C and Figure 2B). This phenomenon is particularly pronounced in the case of the OCDs, where the upper end of the excitable wavelength increased from 480nm to 560 nm.

Several mechanisms have been proposed to explain these unique optical characteristics, such as the size distribution of the CD particles within the sample, a distribution of different emissive trap sites, and the pyrolytic formation of several different polyaromatic fluorophores within the carbon core. However, lacking clear experimental evidence, the relative contribution of these mechanisms and the possible existence of additional effects is still an open question. In order to understand the temporal evolution during the reaction time, absorption and PL spectra of the OCDs and WCDs solutions were measured at different reaction time by taking the aliquots. As shown in Figure 3A, neither the absorption nor the PL maxima of OCDs showed obvious shift along the reaction time. However, the distinct absorption peak around 360nm gradually broadened and eventually disappeared as the time increases. Absorption below 340nm also increased as the reaction proceeded. The PL spectra reveal that emissions at the longer wavelength were enhanced as the reaction time extended (Figure 1 ). Further evidence can be drawn by calculating the QY at different reaction time under different excitation wavelengths, as shown in Figure 3B, the QY excited at 360nm reached the maximum in the beginning of the reaction (about 5 minutes) and then decreased, reaching a plateau around 30 minutes. Conversely, the QY excited at 420nm increased at a much slower pace, the steady increase of QY appeared to reach the maximum around 120 minutes, followed by a slight decrease thereafter. Figure 4A shows the absorption spectra of the WCDs colloidal solutions taken from the reaction flasks at different stages. It is noticeable that a broad absorption peak gradually appeared with the increase of reaction time, and correspondingly, the emissions were enhanced at the longer wavelength. As depicted in the Figure 4B, the highest quantum yield (17%) excited at 360nm appeared after 90min reaction, however, the highest quantum yield (9%) excited at 420nm appeared when the reaction preceded about 150min.

The morphology of the OCDs extracted at 5min and 180min (Figure 5) were characterized by high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). The results revealed that the OCDs are highly mono- disperse and are mostly spherical dots with diameters in the range of 4-7 nm without noticeable difference. Correspondingly, as shown in Figure 6, the WCDs have similar size and morphology in comparison with the OCDs. This leads to our speculation that it was not the size change that has altered the QY of the OCDs over the reaction time, whereas it may be the OCD particle surface that has continuously evolved, which may have resulted in the changes in emissive sites, hence the QYs. This outcome also confirms that the CD formation by 'hot injection' method in a high boiling point solvent undergoes a depolymerization, decomposition and pyrolysis process accompanied by formation of gas and condensable vapours, which is a starkly different reaction compared to the processes used to prepare semiconductor QDs. The addition of surface passivating agents in the one-step synthesis for both the OCDs and WCDs enhances their photoluminescent emission. Here, a parallel experiment was conducted without adding HDA. Although the as-made bare CDs showed similar morphology (Figure 7), the QY is quite low (less than 10% excited at 360nm, emission spectra in Figure 8). Elemental analysis reveals that the bare CDs are composed of C 57.33wt%, H 6.03wt% and O 36.64% (calculated). The surface state of the bare CDs was characterized by means of Fourier-transform infrared spectroscopy: carboxyl groups were clearly identified, through both the broad 3100 cm^"1 O-H stretching vibration and the sharp 1706 cm^"1 C=0 stretching vibration (Figure 9d). In the case of HDA modified OCDs, two sharp peaks at 1707 cm^"1 and 1658 cm^"1 indicate the existence of v_(C=₀₎ and_D V_(CONR) vibrations, suggesting that carbonyl groups have been converted into amide groups during the passivation process. The other characteristic peaks provide further evidence for the formation of amide groups: a small peak at 1537 cm^"1 vDassigned to_(C-_N) and a broad peak at 3300 cm^"1 assigned to v_(N._H). This result is consistent with the decreased oxygen content and the appearance of nitrogen, as observed by the elemental analysis: C 80.79wt%, H 12.79 wt%, N 1.65% and O 5.77 wt% (calculated). This set of comparative experiments suggests that the introduction of N atoms on the surface may be responsible for the significantly enhanced PL, however, it is not the prerequisite for the light emission, which is also evidenced by the WCDs that does not have N atoms in its chemical composition characterized by the elemental analysis: C 42.70wt%, H 8.29wt% and O 49.01 (calculated). The incrassation of oxygen content in the WCDs and the surface modification can be further verified by the appearance of ester groups and ether linkages in the FT-IR spectra (Figure 9d), which shown the vibration of C=0 and C-0 around 1735 cm^"1 and 1 100 cm^"1 , respectively. Attempts to obtain Raman spectra of OCDs and WCDs were unsuccessful due to the fluorescence of the nanoparticles, however, a typical X-ray diffraction (XRD) pattern expressed a broad peak located at 20=25° (Figure 10), suggesting an amorphous nature.

Example 4 Synthesis of water soluble photoluminescent nitrogen-containing carbogenic nanoparticles ('carbon dots') (N-WCD)

In a similar process to the process described in Example 2, glycerine (15 ml) and PEG₁₅₀oN (1 g) (amine-terminated PEG) were placed into a 100 mL three-necked flask and degassed with Argon for 10 min. The solution temperature was increased to 270°C. The temperature was maintained at 270°C for minutes to 1 hour and then allowed to cool to ambient temperature unassisted. The product was purified by dialyzing against Milli-Q water with a cellulose ester membrane bag (Mw =3500), during which excess PEG_150ON and glycerine were removed. Quantum yield of the water-suspended fluorescent species at excitation 360nm was 7.8%.

Example 5 Synthesis of water soluble photoluminescent nitrogen-enriched carbogenic nanoparticles (N-WCD)

In a similar process to the process described in Example 3, glycerine (15 ml) was placed into a 100 mL three-necked flask and degassed with Argon for 10 min. The solution temperature was increased to 270°C, and melamine (1 g) was quickly added into the hot solution. The temperature was maintained at 270°C for up to 3 hours and then allowed to cool to ambient temperature unassisted. The product was purified by dialyzing against Milli-Q water with a cellulose ester membrane bag (Mw =3500), during which excess melamine and glycerine were removed.

Example 6 Synthesis of photoluminescent carbogenic nanoparticles from biomass In a similar process to the process described in Example 1 , 1 -octadecene (15 ml) was placed into a 100 mL three-necked flask and degassed with Argon for 10 min. The solution temperature was increased to 300°C. Dried tree leaves (no specification) ground into small pieces was added into the flask. The temperature was maintained at 300°C for up to 3 hours and then allowed to cool to ambient temperature unassisted. The product was photoluminescent with a quantum yield of the water-suspended fluorescent species at excitation 360 nm 6 - 7 %.

Example 7 Synthesis of photolumuniscent nitrogen-enriched carbogenic nanoparticles

Melamine (0.5 g) was dissolved in glycerine (18 ml) in a 100 mL three-necked flask and degassed with Argon for 10 min. The solution was heated to 280 °C and then citric acid (0.5 g) was added to the hot solution. Upon addition, vigorous gas release was observed in the flask. The solution was maintained at 280°C for 15 minutes and then allowed to cool to ambient temperature. The resulting product was a dark brown viscous mixture which was found to be highly soluble in water and emitted strong fluorescence under a UV lamp.

The mixture was purified by dialyzing against Milli-Q water with MWCO (molecular weight cut-off) 3500 dialysis tubing (SpectroPor 6). It was found that part of the sample could easily penetrate the tubing of MWCO3500. The fraction passing through the MSCO 3500 dialysis tube was further separated by MWCO 1000 dialysis tubing and column chromatograph. The three fractions obtained, which were dialyzed by MWCO 3500, MWCO 1000 dialysis tubing, and column chromatography, are labeled as 3.5K, 1 K, and <1 K, respectively. The particle size of each fraction was determined by fluorescence correlation spectra as indicated in the table below.

The spectra of UV-visible absorption, photoluminescence, and excitation the three fractions are presented in Figure 1 1 . Figures 1 1 (b)-(d) shows the photoluminescence spectra of <1 K, 1 K, and 3.5K fractions under excitation from 330 nm to 450 nm, respectively. They are all multicolored and the emission maxima shift bathochromically as the excitation wavelength increases from 32 nm to 600 nm. The emission intensity of the <1 K fraction is higher than the 1 K fraction, which is higher again than the 3.5K fraction. The quantum yields, which were calibrated against quinine sulphate, were 22.4 %, 10.2 % and 5.4% for the <1 K, 1 K, and 3.5K fractions, respectively.

A parallel experiment where the carbogenic nanoparticles were produced in the absence of citric acid also resulted in fluorescent carbongenic nanoparticles but with a light lower quantum yield. Moreover, when citric acid is replaced by concentrated H2S04 or a solid acid ZSM-5 zeolite, carbogenic nanoparticles with similar quantum yields were also obtained.

Fourier transform infrared spectra (FTIR) of the starting materials and the resulting carbogenic nanoparticles are presented in Figure 12.

Two photon absorption of these nitrogen-enriched carbogenic nanoparticles has also been observed. As shown in Figure 13, when the nitrogen-enriched carbogenic nanoparticles were excited by wavelength longer than 700 nm, emissions range from 350 nm to 650 nm were observed. The two-photon emission process involves the adsorption of two or more near infra red photons by the carbogenic nanoparticles, followed by excitation of electrons from ground level to excited state and finally relaxation to ground level with emission.

Example 8 Synthesis of photoluminescent carbogenic nanoparticles

A mixture of melamine (0.2 g), citric acid (0.2 g), water ( 5 ml) and ethanol (5 ml) was stirred until all solids had dissolved and the resultant solution was placed into a hydrothermal bomb. The bomb was sealed and placed in an oven with a reaction temperature of 180 °C and allowed to react for 24 hours.

After completion of the reaction, black solids had formed at the bottom of the bomb. The supernatant was brown and appeared yellow after dilution. The sample was purified with a 3500 MWCO membrane and the resulting products exhibited photoluminescence when excited by UV light.

Example 9 Synthesis of photoluminescent carbogenic nanoparticles

Glycerine (10 ml) was added to a 250 ml three neck round bottom flask and heated to 280 °C under nitrogen. One drop of concentrated sulfuric acid (98%) was added into the flask and the mixture was refluxed for 15 minutes, followed by cooling to ambient temperature.

Example 10 Synthesis of photoluminescent carbogenic nanoparticles at room temperature a) Cyanuric acid (0.5 g) was mixed with glycerine (10 ml) with stirring at room temperature. Concentrated sulfuric acid (10 ml, 98%) was added to the mixture. b) Melamine (0.5 g) was mixed with glycerine (10 ml) with stirring at room temperature. Concentrated sulfuric acid (10 ml, 98%) was added to the mixture.

c) Citric acid (0.5 g) was mixed with glycerine (10 ml) with stirring at room temperature. Concentrated sulfuric acid (10 ml, 98%) was added to the mixtures.

d) Concentrated sulfuric acid (10 ml, 98%) was added to glycerine (10 ml) with stirring at room temperature. After completion of each of the reactions, black solids had formed. The resulting solids and supernatants exhibited photoluminescence when excited by UV light.

In the description of the invention, except where the context requires otherwise due to express language or necessary implication, the words "comprise" or variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features, but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that, although prior art use and publications may be referred to herein, such reference does not constitute an admission that any of these form a part of the common general knowledge in the art, in Australia or any other country.

Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from the basic inventive concepts. All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description.

Claims

CLAIMS:

1. A method for preparing carbogenic nanoparticles comprising carbonizing a carbogenic precursor material in a high boiling point solvent.

2. The method according to claim 1 comprising carbonizing the carbogenic precursor material in the presence of an acid.

3. The method according to claim 1 or claim 2 comprising carbonizing the carbogenic precursor material in the presence of a surface passivating agent.

4. The method according to any one of claims 1 to 3, wherein carbonizing the carbogenic precursor material comprises bringing the carbogenic precursor material in contact with the high boiling point solvent at an elevated temperature.

5. The method according to any one of claims 1 to 3, wherein carbonizing the carbogenic precursor material comprises heating a mixture or solution of the carbogenic precursor material in the high boiling point solvent to the elevated temperature and maintaining said mixture or solution at an elevated temperature.

6. The method according to claim 4 or claim 5, wherein the elevated temperature is at or above a temperature at which the carbogenic precursor material will thermally decompose.

7. The method according to any one of claims 4 to 6, wherein the elevated temperature may be in a range of about 100 °C to about 300 °C.

8. The method according to any one of claims 1 to 3, wherein carbonizing the carbogenic precursor material comprises reacting the carbogenic precursor material with concentrated sulfuric acid (98%).

9. The method according to any one of the preceding claims, wherein the carbonizing is performed in a period of up to 24 hours.

10. The method according to claim 9, wherein the period is up to about 180 minutes.

1 1. The method according to any one of the preceding claims, wherein the carbogenic precursor material is an organic compound selected from a group comprising carbohydrates, polyhydroxy-substituted aldehydes; polyhydroxy- substituted ketones; any one of the aforesaid organic compounds substituted with heteroatoms; heterocyclic compounds including heterocyclic bases and heterocyclic acids; polyols; mono- and polyunsaturated hydrocarbons; and heteroatom-substituted oligomers or polymers of ethylene oxide.

12. The method according to any one of the preceding claims, wherein the carbogenic precursor material is a solid, liquid or a gel.

13. The method according to any one of the preceding claims, wherein the carbogenic precursor material is the high boiling point solvent.

14. The method according to claim 1 1 , wherein the carbogenic precursor material is glycerine, melamine, or cyanuric acid.

15. The method according to any one of claims 2 to 14, wherein the acid comprises an inorganic acid or an organic acid.

16. The method according to claim 15, wherein the inorganic acid comprises concentrated H₂S0₄ (98%) or a solid acid-enriched ZSM-5 zeolite.

17. The method according to claim 15, wherein the organic acid is selected from a group comprising monofunctional or polyfunctional carboxylic acids and/or anhydrides, polyhydroxy-substituted carboxylic acids and/or anhydrides or sulphonic acids.

18. The method according to claim 17, wherein the organic acid comprises citric acid.

19. The method according to any one of claims 3 to 18, wherein the surface passivating agent is selected from a group comprising long chain amines, amphiphilic oligomeric polymers, long-chained surfactants, nucleotides, peptides.

20. The method according to any one of claims 3 to 19, wherein a concentration of the surface passivating agent in the high boiling point solvent is about 0.1 wt% to about 50 wt%.

21. A method of preparing heteroatom-enriched carbogenic nanoparticles according to the method as defined by any one of claims 1 to 20, wherein the carbogenic precursor material is a heteroatom-containing carbogenic precursor material.

22. Carbogenic nanoparticles comprising a core of carbogenic material, the carbogenic material comprising a plurality of fluorophores located in or on the core.

23. The nanoparticles according to claim 22, wherein the core is not surface passivated.

24. The nanoparticles according to claim 22, wherein the core is coupled with or adsorbed with a surface passivating agent.

25. The nanoparticles according to any one of claims 22 to 24, wherein the nanoparticles have a particle size in a range of about 0.1 nm to about 10 nm.

26. The nanoparticles according to claim 25, wherein the nanoparticles have a substantially uniform particle size.

27. The nanoparticles according to claim 26, wherein the nanoparticles have a particle size distribution range of about 1 nm to about 2 nm.

28. The nanoparticles according to any one of claims 22 to 27, wherein the nanoparticles are photoluminescent.

29. The nanoparticles according to claim 28, wherein the photoluminescence is in the UV-visible spectrum.

30. The nanoparticles according to any one of claims 22 to 29, wherein the nanoparticles have a quantum yield greater than about 15%.

31. The nanoparticles according to any one of claims 22 to 30, wherein the carbon content of the nanoparticles is at least 40 wt%.

32. Photoluminescent carbogenic nanoparticles comprising a core of nitrogen enriched-carbogenic material.

33. The nanoparticles according to claim 32, wherein the nanoparticles are photoluminescent under excitation by near infrared frequencies.

34. The nanoparticles according to claim 33, wherein the photoluminescence is induced by a two-photon absorption process.