Method for manufacturing phosphorescent carbon dots and their application as article authentication markers

Abstract

Method for manufacturing phosphorescent carbon dots and their application as article authentication markers. The invention relates to a method for manufacturing phosphorescent carbon dots: a) ammonia or an ammonia solution is made available to which L-aspartic acid is added to obtain a mixture; b) the mixture is subjected to microwave heating to obtain a solid in gel form; c) said solid in gel form is dissolved in a basic solution containing sodium ions to obtain said phosphorescent carbon dots, the amounts of NH4OH and L-aspartic acid used in the mixture in step a) being chosen so that the molar ratio of NH4OH to L-aspartic acid is greater than or equal to 1.6. The invention also relates to a marking ink comprising said phosphorescent carbon dots, as well as a method for detecting them with a mobile phone.Figure for the summary: Fig. 2.

Description

Title of the invention: Method for manufacturing phosphorescent carbon dots and their application as article authentication markers

[0001] The invention relates to a method for manufacturing phosphorescent carbon dots and their application as article authentication markers.

[0002] In the context of the present invention, the term “carbon dots” means carbon nanomaterials, also referred to by their English name of “carbon dots”.

[0003] Counterfeiting today represents a real societal scourge that not only affects the global economy, but also creates risks for the health and safety of consumers. Among the sectors of activity most affected by counterfeiting are the luxury goods industry, pharmaceuticals, the production of certified documents (for example banknotes) and also the alcohol industry.

[0004] Indeed, the development and improvement of the techniques used by counterfeiters now allow them to copy items very effectively (for example everyday consumer items such as clothing, perfume bottles, but also medicines and even banknotes).

[0005] This is why, in order to be able to easily and with certainty distinguish counterfeit articles from authentic articles, it is known to incorporate or affix at least one authentication marker to said authentic articles. The embodiments of these authentication markers are more or less complex depending on the nature of their constituents and / or the technique used to manufacture them. The authentication markers are then more or less difficult for counterfeiters to reproduce. The more complex the authentication markers are to reproduce, the better their effectiveness and reliability.

[0006] The authentication markers may be present within or on the surface of the authenticatable article.

[0007] First of all, the authentication marker may be present in the “mass” of the authenticatable article. This is a “full material” marking. To do this, the authentication marker may be in the form of a micro- or nano-object having various physicochemical properties allowing its detection within the material or substance that the authenticatable article comprises (for example a polymer, a paper pulp, a metal, a paint or a varnish). However, authenticatable articles do not always have a manufacturing process compatible with the incorporation of such authentication markers in their mass. This is why, The incorporation of authentication markers within items to be authenticated has limited applications.

[0008] The authentication markers can also be present on the surface of a portion of the authenticatable article in the form of micro- or nano-prints, and this in two possible embodiments: - the authentication markers are affixed directly to the surface of the authenticatable item; - the authentication markers are integrated into an authentication label which is itself attached to the surface of the authenticatable item.

[0009] More specifically, these authentication markers may be incorporated into marking ink formulations that are affixed to the surface of a portion of the authenticatable article or an authentication label.

[0010] Among the very diverse and varied authentication markers that exist, phosphorescent markers are particularly preferred.

[0011] By "phosphorescent marker" is meant that the marker has the property of being able to absorb light and re-emit it at a different wavelength (the emission wavelength being able to be greater or less than the excitation wavelength of said phosphorescent marker). The emission of light persists for a certain time when the light excitation has ceased. Indeed, phosphorescence is a radiative transition between two different spin states having a lifetime of the order of a microsecond or more.

[0012] The color of the phosphorescence radiation of a phosphorescent marker constitutes a specific (in other words "unique") signature of said phosphorescent marker. This is why the use of phosphorescent markers is of great interest for authenticating articles.

[0013] In addition to their persistence of light emission which facilitates their detection, phosphorescent markers also have the advantage of being able to be easily detected by appropriate detection devices accessible to all as will be detailed below.

[0014] Among the phosphorescent markers, phosphorescent markers excitable at room temperature with visible light (namely light with a wavelength between approximately 400 nm and approximately 800 nm) are experiencing a certain enthusiasm, particularly for applications related to the detection of counterfeiting. Indeed, unlike phosphorescent markers excitable under ultraviolet light, these phosphorescent markers have lower phototoxicity and deeper penetrability.

[0015] Furthermore, the incorporation of these phosphorescent markers within or on an authenticatable article is perfectly discreet and does not modify the appearance of said article. fact that these phosphorescent markers are not visible under standard conditions of use of said article. It is indeed necessary to excite these phosphorescent markers with a visible light source so that they emit phosphorescence radiation for a certain duration. Also, this makes the development of counterfeit products more difficult, because the signature of these phosphorescent markers which, as explained above, is unique, is particularly difficult to imitate.

[0016] Among the phosphorescent markers excitable at room temperature with visible light, carbon dots have been particularly preferred since their discovery in 2004.

[0017] Carbon dots have the following advantages: a long emission life character, perfectly adjustable photoluminescence properties (phosphorescence emission can be obtained with adjustable excitation wavelengths which can be chosen in the ultraviolet or visible), non-toxic, biocompatible, highly photostable, chemically stable, as well as excellent dispersibility in aqueous media.

[0018] The chemical stability of phosphorescent markers is essential for their integration into or onto articles to be traced and / or authenticated through the detection of these phosphorescent markers.

[0019] In this respect, in order to be able to use carbon dots as authentication markers, it is particularly advantageous, for reasons of logistics and handling in particular, to be able to store them in a durable manner without any alteration in a dried form (i.e. in the form of a powder) before their incorporation into the article to be authenticated or traced. Indeed, a certain time may elapse between the time of synthesis of the authentication markers and their use to authenticate articles (for example their incorporation into a marking ink). The storage of the authentication markers in a dried form (i.e. in the form of a powder) is the easiest to implement for reasons of logistics and space saving.

[0020] Therefore, it is particularly desirable that the carbon dots in a dried form are not hygroscopic. Indeed, if the carbon dots quickly absorb moisture so that they clump together, this can cause a phenomenon of annihilation of their phosphorescence, also known as "quenching". In other words, it is particularly desirable that the carbon dots in a dried form do not capture the surrounding moisture.

[0021] In addition, the carbon dot synthesis routes have the advantages of being able to be implemented from a very wide variety of reagents which are common and inexpensive. For example, the so-called "bottom-up" synthesis route uses molecular precursors (e.g. glucose, citric acid) which can be used in pure form or contained in more or less raw organic matter (cane sugar, fruit juice, coffee grounds, food waste) and which are pyrolyzed by solvothermal reactions, by high-temperature combustion or under microwave irradiation.

[0022] Once synthesized, the carbon dots can be subjected to different purification processes (for example centrifugation, dialysis, filtration, chromatography, electrophoresis) in order to obtain more or less pure and monodisperse nanoparticles.

[0023] According to their morphology and structure, carbon dots can be classified into the following three categories: - 1st category: graphene quantum dots (also known as “graphene quantum dots”): they consist of a graphene disc 2 to 20 nm in diameter which contains only sp2 hybridized carbon atoms and which can have different functional groups on their surface (for example alcohol or carboxylic functions); - 2nd category: carbon quantum dots (also known as "carbon quantum dots"): they consist of a stack of graphene discs and can have different functional groups on their surface. This stacking gives them a quasi-spherical crystalline structure made of a mixture of sp2 and sp3 hybridized carbon atoms; - 3rd category: carbon nanodots (also known as “carbon nano dots”): they have a quasi-spherical amorphous structure mainly made up of sp3 hybridized carbon atoms.

[0024] The publication entitled “Visible-light excited room temperature phosphorescent carbon dots” by Sizhe Hu et al., Nanomaterials, 2020, 10, 464 describes the manufacture of carbon dots of the 2nd category according to the so-called bottom-up synthesis route from L-aspartic acid and an ammonia solution with a mass content of NH 4OH between 25% and 28% (in other words ammonia diluted in water at a level of 25% to 28%).

[0025] In this regard, it is recalled that: - ammonia (NH3) is a gas that is very soluble in water; - ammonia (NH4OH) is a basic aqueous solution of ammonia. It is the result of dissolving gaseous ammonia in water.

[0026] In the context of the present invention, the term "ammonia solution" means a solution in which ammonia has been diluted in water, preferably deionized water.

[0027] In the above-mentioned publication, the synthesis was carried out with a molar ratio of NH4OH to L-aspartic acid of 1.41.

[0028] In the context of the present invention, the term "molar ratio of substance A to substance B" means the ratio between the number of moles of substance A and the number of moles of substance B.

[0029] However, the carbon dots obtained according to the parameters indicated in this publication do not prove to be stable over time. Indeed, these carbon dots, once dried, have proven to be particularly hygroscopic; which makes them unusable in all applications of authentication markers which must be spread over time and which require the carbon dots to be preserved in a dried form. More precisely, it has been found that in the presence of humidity, these carbon dots in a dried form rapidly absorb water so that they agglomerate with each other to the point of causing the phenomenon of annihilation of their phosphorescence. As explained above. This annihilation of the phosphorescence renders null and void all the interest of these carbon dots as phosphorescent authentication markers.

[0030] In view of these drawbacks, the inventors sought to improve the synthesis of carbon dots described in the publication by Sizhe Hu et al. in order to obtain carbon dots that remain perfectly stable over time, regardless of the surrounding conditions, particularly humidity. In other words, the inventors sought to obtain carbon dots with excellent phosphorescence properties that remain perfectly stable over time. They therefore sought to synthesize carbon dots that differ from those described in the publication in that they are not hygroscopic. Thus, the inventors sought to obtain carbon dots that do not capture moisture when they are in a dry form (i.e., in the form of a powder).

[0031] The inventors discovered, quite surprisingly, that it was possible to improve the stability of the carbon dots obtained according to the synthesis route described in the publication of Sizhe Hu et al. by appropriately selecting the molar ratio of NH40H to L-aspartic acid.

[0032] The invention thus relates to a method for manufacturing phosphorescent carbon dots which comprises at least the following steps: a) ammonia or an ammonia solution is provided to which L-aspartic acid is added so as to obtain a mixture, b) the mixture obtained at the end of step a) is subjected to microwave heating so as to obtain a solid in gelled form, c) said solid is dissolved in gelled form in a basic aqueous solution containing at least sodium ions so as to obtain said carbon points, said manufacturing process is characterized in that the quantities of NH4OH and L-aspartic acid used in the mixture in step a) are chosen such that the molar ratio of NH4OH to L-aspartic acid is greater than or equal to 1.6.

[0033] Indeed, the inventors have surprisingly discovered that with a molar ratio of NH40H to L-aspartic acid of a value of at least 1.6, when the carbon dots thus obtained with the manufacturing process are dried, they are not hygroscopic. More precisely, these carbon dots do not capture the surrounding humidity and remain perfectly stable over time. This is very different from the carbon dots obtained according to the synthesis described in the aforementioned publication which, once dried, prove to be hygroscopic.

[0034] The molar ratio of NH40H to L-aspartic acid may be between 1.6 and 4, preferably between 2 and 3.

[0035] Preferably, in step a), an ammonia solution is used. This means that it is a solution in which ammonia has been diluted in water, preferably deionized water.

[0036] Preferably in step a), the L-aspartic acid is added gently into the ammonia or ammonia solution.

[0037] Advantageously, in step a), the L-aspartic acid is added while subjecting the ammonia or the ammonia solution to stirring, for example at a stirring speed of between 50 and 500 revolutions / minute.

[0038] Advantageously, before carrying out step b), the mixture obtained at the end of step a) is subjected to an ultrasound treatment. The duration of this ultrasound treatment may be between 0.5 minutes and 30 minutes, for example 15 minutes. The ultrasound treatment makes it possible to homogenize the mixture obtained at the end of step a).

[0039] Step b) of microwave heating can be carried out for a period of between 0.5 minutes and 30 minutes, for example 2 minutes.

[0040] The microwave heating power may be between 150 W and 1200 W, for example 750 W.

[0041] At the end of step b), the solid in gelled form has a yellow color.

[0042] Advantageously, before carrying out step c), the solid in the form The gelled mixture obtained at the end of step b) is cooled until it reaches room temperature (approximately 20°).

[0043] Preferably, the basic aqueous solution containing at least sodium ions has a pH greater than or equal to 7.5, more preferably greater than or equal to 9.

[0044] Preferably, said basic aqueous solution containing at least sodium ions further comprises an organic anion.

[0045] In preferred embodiments of the invention, said aqueous solution basic solution containing at least sodium ions is chosen from solutions of sodium carbonate, sodium oxalate, sodium borate, sodium hypochlorite, sodium aluminate, sodium sulfate, sodium silicate or sodium phosphate, taken alone or as a mixture thereof.

[0046] Most preferably, said basic aqueous solution containing at least sodium ions is a sodium carbonate solution.

[0047] The molar concentration of the sodium carbonate solution may be between 0.01 mol / L and 2 mol / L, for example 0.1 mol / L.

[0048] Advantageously, at the end of step c), the carbon points thus obtained can be subjected to at least one purification step.

[0049] A carbon dot purification step may consist of a centrifugation step. The centrifugation step may be carried out at a speed of between 1,000 rpm and 20,000 rpm, for example 10,000 rpm. The duration of the centrifugation step may be between 1 minute and 30 minutes, for example 20 minutes.

[0050] A purification step may also consist of a filtration step. Filtration may be carried out with a filter membrane whose filtration threshold is between 0.1 pm and 10 pm. This makes it possible to eliminate particles that are too large or that have agglomerated.

[0051] In an advantageous embodiment of the invention, the carbon dots thus obtained at the end of step c) are subjected to a centrifugation step, followed by a filtration step, and for example as described above. These two centrifugation and filtration steps correspond to purification steps of said phosphorescent carbon dots. In other words, the phosphorescent carbon dots obtained at the end of step c) are subjected to a centrifugation step carried out at a speed of between 1,000 rpm and 20,000 rpm for a duration of between 1 minute and 30 minutes, followed by a filtration step carried out with a filter membrane whose filtration threshold is between 0.1 μm and 10 μm.

[0052] Advantageously, at the end of step c), optionally at the end of at least one purification step if this purification step is implemented, the phosphorescent carbon dots are dried so as to obtain them in the form of a powder which is composed of a plurality of clusters of said carbon dots. The drying may for example consist of freeze-drying. The drying time may be between 10 minutes and several days, for example 24 hours.

[0053] The carbon dots obtained with the manufacturing process according to the invention after drying (in other words when they are in the form of clusters of carbon dots) were the subject of various analyses, namely: spectrometry of high-resolution X-ray induced photoelectrons, high-resolution transmission electron microscopy and scanning electron microscopy. This made it possible to characterize their morphology and to note that it was different from that of the carbon dots obtained with the synthesis method described in the aforementioned publication by Sizhe Hu et al.

[0054] In this regard, it should be specified that when the carbon points obtained according to the manufacturing method according to the invention are: - dispersed in a liquid (for example water), they have a nanometric size; - when dried, they group together to form clusters of carbon dots, said clusters having a micrometric scale.

[0055] Furthermore, it has been found that these clusters of carbon dots are in the form of substantially spherical crystalline particles.

[0056] This is why the invention also relates to a cluster of phosphorescent carbon dots having the form of a substantially spherical crystalline particle, each phosphorescent carbon dot comprising a stack of graphene sheets, said cluster of phosphorescent carbon dots being characterized in that: - the size of said substantially spherical crystalline particle can be between 0.1 pm and 2,000 pm, preferably between 0.15 pm and 1,000 pm; - the distance between two consecutive graphene sheets (in other words “the intersheet distance”) can be between 2.3 angstroms and 4.2 angstroms, preferably between 3.0 angstroms and 3.8 angstroms; - sodium ions are intercalated between the graphene sheets.

[0057] Thus, when the phosphorescent carbon dots according to the invention are dried so as to group together into a plurality of clusters of phosphorescent carbon dots, each cluster of phosphorescent carbon dots has the originality of presenting: - a micrometric particle size; - an interleaf distance which is higher than that of the carbon points of the aforementioned publication which is of the order of 2.1 angstroms; - sodium ions which are intercalated between the graphene sheets.

[0058] This higher inter-sheet distance can be explained by the intercalation of sodium ions between the sheets. These sodium ions come from the basic aqueous solution containing sodium ions from step c) of the method for manufacturing phosphorescent carbon dots according to the invention.

[0059] It was observed during analyses with the techniques detailed above that the carbon dots of the aforementioned publication in a dried form do not contain sodium ions intercalated between the graphene sheets.

[0060] Unlike the carbon dots of the aforementioned publication, the carbon dots phosphorescent carbon dots according to the invention in a dried form are not hygroscopic, because they comprise sodium ions intercalated between the graphene sheets which prevent the possible intercalation of water molecules which could cause the hygrometry of said phosphorescent carbon dots.

[0061] The cluster of phosphorescent carbon dots according to the invention comprises carbon atoms and it further comprises sodium atoms, the number of sodium atoms may be between 1% and 25%, preferably between 3% and 15%, of the number of carbon atoms that said cluster of phosphorescent carbon dots comprises.

[0062] The invention also relates to a marking ink which is characterized in that it comprises: - a dispersion of phosphorescent carbon dots of the clusters of said phosphorescent carbon dots according to the invention as described above Or - phosphorescent carbon dots obtained according to the manufacturing method according to the invention as described above.

[0063] The marking ink is intended to be deposited on the surface of an authenticatable article or an authentication label.

[0064] The marking ink may further comprise a solvent.

[0065] The solvent may be chosen from the group consisting of: - ketones chosen from acetone, butanone, diethyl ketone, methyl isobutyl ketone, cyclohexanone and acetophenone; - esters chosen from methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, t-butyl acetate, amyl acetate, methyl lactate, ethyl lactate, n-propyl lactate, isopropyl lactate, n-butyl lactate and methoxy propanol acetate; - alcohols chosen from methanol, ethanol, n-propanol, isopropanol, n-butanol, n-pentanol, n-hexanol and benzyl alcohol; - diacetone alcohol; - anisole; - glycol ethers or glycol ether acetates selected from propylene glycol methyl ether, dipropylene glycol methyl ether, propylene glycol propyl ether, n-butyl propyl ether, tripropylene glycol methyl ether, butylene glycol methyl ether, dibutylene glycol methyl ether, dipropylene glycol methyl ether acetate, propylene glycol propyl ether acetate and propylene glycol butyl ether acetate.

[0066] Preferably, the solvent is chosen from ethyl acetate, butanone, acetophenone and anisole.

[0067] The marking ink may further comprise a binder resin.

[0068] By "binding resin" is meant a resin which has a binding function between the different constituents of the marking ink and which improves the adhesion of the phosphorescent carbon dots on the authenticatable article.

[0069] The binder resin may be selected from the group consisting of poly(methyl methacrylate) (hereinafter abbreviated as "PMMA"), vinyl chloride / vinyl acetate copolymers, polyester resins, polyvinyl butyral resins, ethylcellulose resins, polyurethane resins, rosin resins, phenolic resins, polyamide resins, cellulose ether resins, cellulose nitrate-based resins, polymaleic anhydride resins, acetal polymers, styrene / methacrylate copolymers, aldehyde resins, styrene and allyl alcohol copolymers, epoxies, polyhydroxystyrenes and polyketone resins.

[0070] Preferably, the binding resin is PMMA.

[0071] The viscosity of the marking ink may be between 8 cP and 16 cP, preferably between 10 cP and 11 cP. If the viscosity is less than 8 cP, the marking ink is very liquid and may be difficult to deposit on the surface of the authenticatable article. If the viscosity is greater than 16 cP, the deposition of the marking ink on the surface of the authenticatable article may be of poor quality.

[0072] When the marking ink has been deposited on the authenticatable article or, where appropriate, on the authentication label, the solvent that it may contain evaporates. The other constituents of the marking ink remain adherent to each other and / or to the authenticatable article, where appropriate, the authentication label.

[0073] The marking ink may comprise, in mass percentages: - between 0.01% and 10%, preferably between 0.05% and 1%, of a dispersion of phosphorescent carbon dots of the clusters of said phosphorescent carbon dots according to the invention as described above or of carbon dots obtained according to the manufacturing method according to the invention as described above, - between 90% and 98%, preferably between 94% and 98%, of solvent, - optionally between 0.5% and 10%, preferably between 1% and 3%, of binder resin.

[0074] The marking ink may be present on the surface of a portion of the authenticatable article over a thickness which may be between 1 nm and 10 μm, preferably between 5 nm and 100 nm.

[0075] The authenticatable article may be an article chosen from everyday articles, for example from perfume bottles, bottles (in particular glass bottles), jewelry, watches, electronic devices, all types of packaging (in particular packaging for medicines, cigarette cartons and spirits) and banknotes.

[0076] Furthermore, since the phosphorescent carbon dots according to the invention are biocompatible, the authenticatable article can also be any cosmetic composition (for example in the form of a cream, a gel, a stick or an oil), any pharmaceutical composition or any food supplement composition (for example in the form of capsules or powder).

[0077] In other words, the authenticatable article can be of a very varied nature. These are generally articles likely to be counterfeited.

[0078] The invention also relates to a method of colorimetric detection with a device for authenticating a dispersion of phosphorescent carbon dots of clusters of said phosphorescent carbon dots according to the invention as described above or of phosphorescent carbon dots obtained with the manufacturing method according to the invention as described above which are likely to be present in at least a determined portion of an article to be authenticated of a determined color, said detection method is characterized in that it comprises at least the following steps: a) at least one determined portion of the article to be authenticated is illuminated with a flash lamp with which the authentication device is equipped and which produces an excitation beam; (b) at least one image is acquired, via a photographic device with which the authentication device is equipped, of at least one specific portion of the item to be authenticated; c) a colorimetric analysis of the at least one image is carried out via a processing unit with which the authentication device is equipped, so as to determine at least one measurement of a color of phosphorescence radiation which is likely to have been induced by the excitation beam illuminating the carbon points likely to be present in the determined portion of the article to be authenticated; d) comparing, via said processing unit, the at least one measurement of the color of the phosphorescence radiation with a so-called reference datum which was determined prior to the implementation of the detection method as a function of the phosphorescent carbon dots and the determined color of the article to be authenticated in order to determine whether in the at least one determined portion of the article to be authenticated said phosphorescent carbon dots are present.

[0079] Preferably, the authentication device is a mobile phone or a tablet. The invention thus has the advantage of providing a method for detecting phosphorescent carbon dots according to the invention for the purpose of authenticating an item to be authenticated which can be easily implemented with an authentication device that is perfectly within everyone's reach (namely a mobile phone or a tablet).

[0080] Preferably, in step c), the measurement of a color of the phosphorescence radiation is a first triplet Ei in a 1st color space and the so-called reference data is a 2nd triplet E2 in said 1st color space. The comparison step d) then consists of calculating a color difference AE between the 1st triplet Ei and the 2nd triplet E2 or a color difference between the 1st triplet Ei and the 2nd triplet E2 relative to the 2nd triplet E2 (in other words “AE / E2”). Most preferably, step d) consists of determining that the article to be authenticated is authentic when AE / E2 is less than or equal to 5%.

[0081] Preferably, the excitation beam may have wavelengths in a spectral range from 200 nm to 900 nm.

[0082] Advantageously, step b) of acquiring the image can be carried out between 0.1 and 10 seconds after illumination by the excitation beam of at least one determined portion of the article to be authenticated in which said phosphorescent carbon dots are likely to be present.

[0083] Advantageously, the processing unit is configured to display on a screen of the authentication device a message indicating whether the item to be authenticated is authentic or not.

[0084] The invention will be better understood with the aid of the detailed description which is set out below with reference to the appended drawing representing, by way of non-limiting example, analyses of phosphorescent carbon points according to the invention.

[0085] [Fig.l] [Fig.l] is a first photograph of phosphorescent carbon dots according to the invention which was obtained by scanning electron microscopy.

[0086] [Fig.2] The figure is a 2nd photograph of the phosphorescent carbon dots according to the invention which was obtained by scanning transmission electron microscopy.

[0087] [Fig.3] [Fig.3] is a photograph of the phos carbon dot sample phorescent according to the invention which was obtained by high-resolution transmission electron microscopy.

[0088] EXPERIMENTAL PART

[0089] Samples of phosphorescent carbon dots according to the invention and comparative samples were synthesized in the manner as described below.

[0090] More specifically, 2 samples of carbon dots according to the invention (hereinafter referred to as INV1 and INV2) and 5 samples of comparative carbon dots (hereinafter referred to as COMP1, COMP2, COMP3, COMP4 and COMP5) were synthesized.

[0091] The carbon dot samples according to the invention were synthesized by implementing the steps of the manufacturing method according to the invention as described above.

[0092] The comparative carbon dot samples were synthesized in the same manner as the carbon dot samples according to the invention, with the sole exception that the quantities of ammonia solution and L-aspartic acid used were such that the molar ratio of NH4OH to L-aspartic acid was less than 1.6.

[0093] We had: - an ammonia solution with a mass content of NH4OH of 28% (in other words an ammonia solution diluted in water to a level of 28%); - L-aspartic acid.

[0094] Different volumes of the ammonia solution and different masses of L-aspartic acid were used to obtain the different samples of phosphorescent carbon dots. In addition, for each synthesis of phosphorescent carbon dots, deionized water was added so that the volume of the mixture of ammonia solution and L-aspartic acid was always 10 mL.

[0095] Table 1 below details for each sample of phosphorescent carbon dots (i.e. according to the invention or comparative): - the volume (in mL) of the ammonia solution, as well as the mass (in g) and the quantity of matter (in mol) corresponding to NH4OH; - the mass (g) of L-aspartic acid and the corresponding quantity of matter (in mol) - the volume of deionized water added; - the molar ratio of NH4OH to L-aspartic acid.

[0096] [Tables 1] COMP1 COMP2 INV1 COMP3 COMP4 INV2 COMP5 volume of ammonia solution (mL) 1 1.5 2 2 2.5 4 4 mass of NH40H (g) 0.28 0.42 0.56 0.56 0.70 1.12 1.12 amount of substance of NH40H (mol) 0.008 0.012 0.016 0.016 0.020 0.032 0.032 mass of L-aspartic acid (g) 0.75 1.5 0.75 1.5 3 1.5 3 amount of substance of L-aspartic acid (mol) 0.0056 0.0113 0.0056 0.0113 0.0225 0.0113 0.0225 molar ratio NH4OH / L-aspartic acid 1.43 1.06 2.85 1.41 0.88 2.83 1.42 volume of water added (mL) 9 8.5 8 8 7.5 6 6

[0097] The carbon dot sample C0MP3 whose molar ratio of NH40H to L-aspartic acid is 1.41 corresponds to the carbon dots described in the aforementioned publication by Sizhe Hu et al.

[0098] All carbon dot syntheses were carried out in the manner as described below with the amounts of ammonia solution, L-aspartic acid and deionized water as detailed in Table 1 above.

[0099] The ammonia solution was first diluted in deionized water. Then, L-aspartic acid was gently added with stirring at a speed of 100 rpm to this mixture of ammonia solution and deionized water.

[0100] Then, the mixture thus obtained was subjected to an ultrasonic treatment for a period of 15 minutes so as to obtain a transparent homogeneous solution.

[0101] This transparent homogeneous solution was subjected to microwave heating for a period of 2 minutes at a power of 750 W so as to obtain a yellow solid in gelled form which was then cooled to room temperature.

[0102] The yellow solid in gel form was completely dissolved in a sodium carbonate solution whose molar concentration was 0.1 mol / L so as to obtain phosphorescent carbon dots.

[0103] The phosphorescent carbon dots thus obtained were purified in the following manner: - by centrifugation at a speed of 10,000 revolutions / minute, then - by filtration with a filter membrane whose filtration threshold was 0.1 pm.

[0104] Finally, the purified phosphorescent carbon dots thus obtained were freeze-dried for a period of 12 hours so as to recover them in the form of a powder. Thus, at the end of this drying, the powder obtained consisted of clusters of phosphorescent carbon dots.

[0105] Then, experiments as described below were carried out on these different phosphorescent carbon dot powders according to the invention and comparative ones.

[0106] During all these experiments, the parameter of the hue angle (hereinafter abbreviated as "h", expressed in degrees and between 0° and 360°) of the sample of phosphorescent carbon dots considered was determined.

[0107] The hue angle is defined according to the following mathematical equation (1):

[0108] [Math.l] b= arctan ) U)

[0109] in which a* and b* are the chromaticity coordinates in the CIELAB chromatic space of the sample of phosphorescent carbon dots considered

[0110] A - Determination of the hue angle of all the samples of phosphorescent carbon dots on the day of their synthesis:

[0111] First, on the day of their synthesis, the hue angle was determined for all the samples of synthesized phosphorescent carbon dots, in the absence of illumination, then after illumination (in other words light excitation) with ultraviolet light at a wavelength of 365 nm for a duration of 1, 2, 3 and 4 seconds.

[0112] Table 2 below summarizes for all the samples of phosphorescent carbon dots the tint angle thus measured in the absence of illumination (in other words “0 s”) and as a function of the duration of the illumination.

[0113] [T ables 2] carbon dot sample illumination time (in s) 0 1 2 3 4 hue angle h (in °) COMP1 80 126 160 165 197 COMP2 80 198 198 202 227 IN VI 80 152 177 173 238 COMP3 80 185 205 207 227 COMP4 80 126 165 162 193 INV2 80 123 118 113 185 COMP5 80 107 160 122 182

[0114] In view of the results detailed in Table 2, it is noted that all the carbon dot samples have a hue angle of a value different from that determined in the absence of any illumination. This testifies that all the synthesized phosphorescent carbon dot samples are capable of emitting phosphorescent light. This light emitted by the phosphorescent carbon dots varies depending on the duration of illumination.

[0115] After one week, it was found that all the comparative phosphorescent carbon dot samples formed agglomerates. They had absorbed the surrounding moisture. In contrast, the phosphorescent carbon dot samples according to the invention remained perfectly stable as on the first day of their synthesis. No agglomerates were formed. The phosphorescent carbon dot samples according to the invention were always in the form of a powder.

[0116] This means that the comparative phosphorescent carbon dot samples are hygroscopic and the phosphorescent carbon dot samples according to the invention are not hygroscopic.

[0117] Due to their instability, the comparative phosphorescent carbon dot samples cannot be used as phosphorescent authentication markers. Indeed, as explained above, the formation of phosphorescent carbon dot agglomerates is accompanied by the loss of the phosphorescence property due to the phenomenon of phosphorescence annihilation (“quenching”).

[0118] Finally, these experimental results show that the selection of a molar ratio of NH40H to L-aspartic acid greater than or equal to 1.6 is decisive for the synthesized phosphorescent carbon dots are not hygroscopic and therefore remain stable over time.

[0119] Given this instability observed with the comparative phosphorescent carbon dot samples, the experiments were continued only with the 2 phosphorescent carbon dot samples according to the invention (INV1 and INV2).

[0120] B- Determination of the hue angle of the samples of phosphorescent carbon dots according to the invention during aging (1 to 25 days):

[0121] More precisely, aging experiments were carried out by measuring the hue angle of the samples of phosphorescent carbon dots according to the invention (INV1 and INV 2) 1, 8, 18 and 25 days after their synthesis, and this as a function of the duration of illumination with ultraviolet light at a wavelength of 365 nm.

[0122] Table 3 below details the hue angles determined for the INV1 phosphorescent carbon dot sample.

[0123] [Tables3] illumination duration (in s) number of days of aging 0 1 2 3 4 tint angle h (in °) of the sample INV1 1 80 152 177 173 238 8 80 115 127 127 150 18 80 135 172 177 218 25 80 187 207 207 223

[0124] Table 4 below details the hue angles determined for the INV2 phosphorescent carbon dot sample.

[0125] [Tables4] illumination duration (in s) number of days of aging 0 1 2 3 4 tint angle h (in °) of the sample INV2 1 80 123 118 113 185 8 80 118 123 98 110 18 80 143 182 173 227 25 80 190 223 223 235

[0126] In view of the results detailed in Tables 3 and 4, it is noted that the samples of phosphorescent carbon dots according to the invention always have a hue angle of a value different from that determined in the absence of any illumination, and this throughout the aging period. This shows that the samples of phosphorescent carbon dots according to the invention continue to emit phosphorescent light, and this throughout the aging period. This light emitted by the phosphorescent carbon dots varies depending on the illumination time. In conclusion, after one month of aging, the samples of phosphorescent carbon dots according to the invention continue to emit phosphorescent light. This shows their perfect stability over time.

[0127] The experiments were continued with the INV2 phosphorescent carbon dot sample.

[0128] C- Determination of the hue angle of the INV2 phosphorescent carbon dot sample incorporated in a marking ink composition:

[0129] C- 1 Determination of the tint angle as a function of the surrounding brightness (morning / afternoon)

[0130] In order to evaluate their properties as authentication markers in a marking ink composition, the INV2 phosphorescent carbon dot sample was incorporated into the formulation of a marking ink.

[0131] Specifically, 500 mg of the INV2 phosphorescent carbon dot sample (aged for 1 month) was dispersed in 1 mL of water. Then, 4 mL of ethylene glycol was added to this mixture to obtain a marking ink.

[0132] Ethylene glycol allowed the good dispersion of the INV2 phosphorescent carbon dots. It did not modify the colorimetry of the marking ink thus obtained.

[0133] Furthermore, in order to correctly determine the phosphorescence effect provided by the INV2 phosphorescent carbon dots, it should be noted that this marking ink was a so-called “transparent” marking ink, namely that it did not contain any other constituent (for example pigments) likely to influence the colorimetry of said marking ink.

[0134] The printer marketed by the company FUJIFILM under the trade name Dimatix Materials Printer DMP-2850 was used to produce prints on paper with the marking ink detailed above.

[0135] Table 5 below details the tint angle of the paper and the marking ink printed on the same paper as a function of the illumination time (0, 1, 3 and 5 seconds) with ultraviolet light at a wavelength of 365 nm. The determination of the tint angle was carried out on the morning of the manufacturing of the marking ink.

[0136] [Tables5] illumination time (in s) 0 1 3 5 tint angle h (in °) of the sample INV2 paper 82 88 81 82 ink 87 90 90 90

[0137] Table 6 below details the tint angle of the paper and the marking ink printed on the same paper as a function of the illumination time (0, 1, 3 and 5 seconds) with ultraviolet light at a wavelength of 365 nm. The determination of the tint angle was carried out the afternoon of the manufacture of the marking ink.

[0138] [Tableauxô] illumination time (in s) 0 1 3 5 tint angle h (in °) of the sample INV2 paper 85 85 87 81 ink 89 95 94 94

[0139] In view of the results detailed in Tables 5 and 6, there is always a significant difference between the tint angle of the paper and that of the marking ink printed on the same paper. This difference is always more or less constant regardless of the duration of illumination. This difference therefore does not depend on the duration of illumination. This experiment mentation testifies that the INV2 phosphorescent carbon dot sample remains phosphorescent after its incorporation into a marking ink formulation. This confirms the interest of using such phosphorescent carbon dots as authentication markers in marking ink formulations.

[0140] Furthermore, the results of Tables 5 and 6 show that there is always a difference between the hue angle of the paper and that of the marking ink printed on this same paper, regardless of the exposure conditions of the printed paper (namely morning or afternoon) or in other words independently of the surrounding brightness.

[0141] C- 2 Determination of the tint angle as a function of the aging of the marking ink

[0142] Then, the tint angle of the paper and that of the marking ink printed on this same paper were determined after aging said marking ink for 1, 2 and 3 weeks.

[0143] Table 7 below details the tint angle of the paper and the marking ink printed on this same paper after 1 week of aging of said marking ink as a function of the duration of illumination (0, 1, 3 and 5 seconds) with ultraviolet light at a wavelength of 365 nm.

[0144] [Tables?] illumination time (in s) 0 1 3 5 tint angle h (in °) of the sample INV2 paper 82 87 86 87 ink 89 93 94 94

[0145] Table 8 below details the tint angle of the paper and the marking ink printed on this same paper after 2 weeks of aging of said marking ink as a function of the duration of illumination (0, 1, 3 and 5 seconds) with ultraviolet light at a wavelength of 365 nm.

[0146] [Tables8] illumination time (in s) 0 1 3 5 tint angle h (in °) of the sample INV2 paper 86 99 94 97 ink 89 101 102 99

[0147] Table 9 below details the tint angle of the paper and the marking ink printed on this same paper after 3 weeks of aging of said marking ink as a function of the duration of illumination (0, 1, 3 and 5 seconds) with ultraviolet light at a wavelength of 365 nm.

[0148] [Tables9] illumination time (in s) 0 1 3 5 tint angle h (in °) of the sample INV2 paper 82 92 95 94 ink 86 99 98 101

[0149] In view of the results detailed in Tables 7 to 9, there is always a significant difference between the hue angle of the paper and that of the marking ink printed on the same paper, regardless of the aging of the marking ink. This experiment shows that the INV2 phosphorescent carbon dot sample remains phosphorescent after its incorporation into a marking ink formulation, regardless of the aging of the marking ink. This confirms the interest in using such phosphorescent carbon dots as authentication markers in marking ink formulations.

[0150] After one year of synthesis, the INV1 and INV2 phosphorescent carbon dot samples are still in the form of a powder as on the first day of their synthesis. They remain perfectly stable over time and are not at all hygroscopic. This maintenance of their stability confirms the full interest of the phosphorescent carbon dots according to the invention as phosphorescent authentication markers.

[0151] D- Additional analyses of the INV2 phosphorescent carbon dot sample:

[0152] The additional analyses detailed below were carried out with the INV2 phosphorescent carbon dot sample, 40 weeks after its synthesis.

[0153] Figures 1 and 2 are photographs of this INV2 phosphorescent carbon dot sample that were obtained by scanning electron microscopy.

[0154] Considering the scale added to each of the two photographs, the particle size of the phosphorescent carbon dots is approximately 300 pm.

[0155] [Fig.3] is a photograph of the INV2 phosphorescent carbon dot sample obtained by high-resolution transmission electron microscopy. The two white dotted segments parallel to each other and indicated by the white arrow correspond to two consecutive graphene sheets. Given the scale indicated in this [Fig.3], an intersheet distance of 3.42 angstroms was measured.

Claims

Claims

1. A method for manufacturing phosphorescent carbon dots which comprises at least the following steps: a) ammonia or an ammonia solution is provided to which L-aspartic acid is added so as to obtain a mixture, b) the mixture obtained at the end of step a) is subjected to microwave heating so as to obtain a solid in gelled form, c) said solid in gelled form is dissolved in a basic aqueous solution containing at least sodium ions so as to obtain said carbon dots, characterized in that the quantities of NH4OH and L-aspartic acid used in the mixture in step a) are chosen such that the molar ratio of NH4OH to L-aspartic acid is greater than or equal to 1.

6.

2. Method for manufacturing phosphorescent carbon dots according to claim 1, characterized in that before carrying out step b), the mixture obtained at the end of step a) is subjected to an ultrasound treatment lasting between 0.5 minutes and 30 minutes.

3. A method of manufacturing phosphorescent carbon dots according to claim 1 or 2, characterized in that step b) of microwave heating is carried out for a duration of between 0.5 minutes and 30 minutes and the power of the microwave heating is between 150 W and 1200 W.

4. Process for manufacturing phosphorescent carbon dots according to any one of claims 1 to 3, characterized in that in step c), the basic aqueous solution containing at least sodium ions is a sodium carbonate solution whose molar concentration is between 0.01 mol / L and 2 mol / L.

5. Method for manufacturing phosphorescent carbon dots according to any one of claims 1 to 4, characterized in that the phosphorescent carbon dots obtained at the end of step c) are subjected to a centrifugation step carried out at a speed of between 1,000 rpm and 20,000 rpm for a duration of between 1 minute and 30 minutes, followed by a filtration step carried out with a filter membrane whose filtration threshold is between 0.1 pm and 10 pm.

6. Method for manufacturing phosphorescent carbon dots according to any one of claims 1 to 5, characterized in that at the end of step c), optionally at the end of the centrifugation step followed by the filtration step, the phosphorescent carbon dots are dried so as to obtain them in the form of a powder which is composed of a plurality of clusters of said phosphorescent carbon dots.

7. A cluster of phosphorescent carbon dots in the form of a substantially spherical crystalline particle, each phosphorescent carbon dot comprising a stack of graphene sheets, characterized in that: - the size of said substantially spherical crystalline particle is between 0.1 pm and 2,000 pm, preferably between 0.15 pm and 1,000 pm; - the distance between two consecutive graphene sheets is between 2.3 angstroms and 4.2 angstroms, preferably between 3 angstroms and 3.8 angstroms; - sodium ions are intercalated between the graphene sheets.

8. A cluster of phosphorescent carbon dots according to claim 7 comprising carbon atoms, characterized in that it further comprises sodium atoms, the number of sodium atoms being between 1% and 25% of the number of carbon atoms that said cluster of phosphorescent carbon dots comprises.

9. Marking ink, characterized in that it comprises: - a dispersion of phosphorescent carbon dots of clusters of said phosphorescent carbon dots according to claim 7 or 8 or - phosphorescent carbon dots obtained according to the manufacturing method according to any one of claims 1 to 6.

10. Marking ink according to claim 9, characterized in that it comprises, in mass percentages: - between 0.01% and 10%, preferably between 0.05% and 1%, of a dispersion of the phosphorescent carbon dots of clusters of said phosphorescent carbon dots according to claim 7 or 8 or of the phosphorescent carbon dots obtained according to the manufacturing method according to any one of claims 1 to 6, - between 90% and 98%, preferably between 94% and 98%, of solvent, - optionally between 0.5% and 10%, preferably between 1% and 3%, of binder resin.

11.

12.

13.

14. Method for colorimetric detection with an authentication device of a dispersion of phosphorescent carbon dots of clusters of said phosphorescent carbon dots according to claim 7 or 8 or phosphorescent carbon dots obtained according to the manufacturing method according to any one of claims 1 to 6 which are likely to be present in at least one determined portion of an article to be authenticated of a determined color, characterized in that it comprises at least the following steps: a) the at least one determined portion of the article to be authenticated is illuminated with a flash lamp with which the authentication device is equipped and which produces an excitation beam; (b) at least one image is acquired, via a photographic device with which the authentication device is equipped, of at least one specific portion of the item to be authenticated; c) a colorimetric analysis of the at least one image is carried out via a processing unit with which the authentication device is equipped, so as to determine at least one measurement of a color of phosphorescence radiation which is likely to have been induced by the excitation beam illuminating the carbon dots likely to be present in the determined portion of the article to be authenticated; d) the at least one measurement of the color of the phosphorescence radiation is compared via said processing unit with a so-called reference datum which was determined prior to the implementation of the detection method as a function of the phosphorescent carbon dots and the determined color of the article to be authenticated in order to determine whether said phosphorescent carbon dots are present in the at least one determined portion of the article to be authenticated. Detection method according to claim 11, characterized in that the authentication device is a mobile phone or a tablet. Detection method according to claim 11 or 12, characterized in that the excitation beam has wavelengths in a spectral range from 200 nm to 900 nm. Detection method according to any one of claims 11 to 13, characterized in that step b) of acquiring the image is carried out between 0.1 and 10 seconds after illumination by the excitation beam of at least one determined portion of the article to be authenticated in which said phosphorescent carbon dots are likely to be present.

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