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

Abstract

The invention relates to a method for manufacturing phosphorescent carbon dots, the method comprising: a) providing ammonia or an ammonia solution to which L-aspartic acid is added so as to obtain a mixture; b) microwave-heating the mixture so as to obtain a solid in gelled form; c) dissolving the solid in gelled form in a basic solution containing sodium ions so as to obtain the phosphorescent carbon dots, wherein the amounts of NH4OH and L-aspartic acid used in the mixture in step a) are selected 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 the phosphorescent carbon dots, and to a method for detecting them using a mobile telephone.

Description

DESCRIPTION TITLE: 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, “carbon dots” means carbon nanomaterials, also known by their English name “carbon dots”.

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

[0004] Indeed, the development and perfection 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] Therefore, in order to be able to easily and reliably distinguish counterfeit items from authentic items, it is known to incorporate or affix at least one authentication marker to said authentic items. 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] Authentication markers may be present within or on the surface of the authenticatable item.

[0007] First, the authentication marker may be present in the "mass" of the authenticatable article. This is a "full-material" marking. To achieve this, the authentication marker may be in the form of a micro- or nano-object with various physicochemical properties allowing its detection within the material or substance that the authenticatable article comprises (for example, a polymer, paper pulp, metal, paint or 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 articles to be authenticated has limited applications.

[0008] Authentication markers can also be present on the surface of a portion of the authenticatable article in the form of micro- or nano-prints, 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 higher or lower than the excitation wavelength of said phosphorescent marker). The light emission persists for a certain time after the light excitation has ceased. Indeed, phosphorescence is a radiative transition between two different spin states with 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 persistent light emission which facilitates their detection, phosphorescent markers also have the advantage of being easily detected by appropriate detection devices accessible to all as will be detailed below.

[0014] Among 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 gaining popularity, particularly for applications related to counterfeit detection. 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 because 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: long emission life, perfectly adjustable photoluminescence properties (phosphorescence emission can be obtained with adjustable excitation wavelengths that 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 permanently 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 moment of synthesis of the authentication markers and their use to authenticate articles (for example their incorporation into a marking ink). Preserving 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 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 carbon dots in a dried form do not absorb the surrounding moisture.

[0021] In addition, carbon dot synthesis routes have the advantage of being able to be implemented from a very wide variety of reagents that are common and inexpensive. For example, the so-called "bottom-up" synthesis route uses molecular precursors (e.g. glucose, citric acid) that 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 (e.g. 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: - L ère Category: Graphene quantum dots (also known as graphene quantum dots): These consist of a graphene disc 2 to 20 nm in diameter that contains only sp hybridized carbon atoms. 2 and which can present on their surface different functional groups (for example alcohol or carboxylic functions); - 2 ème 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 sp hybridized carbon atoms 2 and sp 3 ; - 3 ème Category: Carbon nanodots (also known as carbon nanodots): They have a quasi-spherical amorphous structure mainly made up of sp hybridized carbon atoms. 3 .

[0024] The publication entitled “Visible-light excited room temperature phosphorescent carbon dots” by Sizhe Hu et al., Nanomaterials, 2020, 10, 46 describes the fabrication of carbon dots of the 2 ème category according to the so-called bottom-up synthesis route from L-aspartic acid and an ammonia solution with a mass content of NH4OH between 25% and 28% (in other words ammonia diluted in water at a rate 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] For the purposes 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 preserving the carbon dots in a dried form. More precisely, it was found that in the presence of humidity, these carbon dots in a dried form quickly absorbed water in such a way that they agglomerated to each other to the point of causing the phenomenon of annihilation of their phosphorescence. As explained above. This annihilation of 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 absorb moisture when they are in a dry form (i.e., in the form of a powder).

[0031] The inventors surprisingly discovered that it was possible to improve the stability of the carbon dots obtained using the synthesis route described in the publication by Sizhe Hu et al. by appropriately selecting the molar ratio of NH4OH 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 in gelled form is dissolved in a basic aqueous solution containing at least sodium ions so as to obtain said carbon dots, said manufacturing method 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 surprisingly discovered that with a molar ratio of NH4OH to L-aspartic acid 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 quite 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 NH4OH 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), 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 rpm.

[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 treatment by ultrasound can be between 0.5 minutes and 30 minutes, for example 15 minutes. Ultrasonic treatment makes it possible to homogenize the mixture obtained at the end of step a).

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

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

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

[0042] Advantageously, before carrying out step c), the solid in gelled form 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 basic aqueous 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 sodium carbonate solution can 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 performed at a speed between 1,000 rpm and 20,000 rpm, for example 10,000 rpm. The duration of the centrifugation step can be between 1 minute and 30 minutes, for example 20 minutes.

[0050] A purification step can also consist of a filtration step. Filtration can be carried out with a filter membrane with a filtration threshold between 0.1 pm and 10 pm. This allows the removal of 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 method according to the invention after drying (in other words when they are in the form of clusters of carbon dots) were subjected to various analyses, namely: high-resolution X-ray induced photoelectron spectrometry, 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 de 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; - dried, they group together to form clusters of carbon dots, said clusters having a micrometric scale.

[0055] Furthermore, it was found that these carbon dot clusters appear as substantially spherical crystalline particles.

[0056] This is why the invention also relates to 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, said cluster of phosphorescent carbon dots being characterized in that: - the size of said substantially spherical crystalline particle may 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-layer distance can be explained by the intercalation of sodium ions between the layers. These sodium ions come from the basic aqueous solution containing sodium ions from step c) of the process for manufacturing phosphorescent carbon dots according to the invention.

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

[0060] Unlike the carbon dots of the aforementioned publication, the 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 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 item or an authentication label.

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

[0065] The solvent can 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 acts as a binder between the different constituents of the marking ink and which improves the adhesion of the phosphorescent carbon dots to 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 lower 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 higher 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 applicable, on the authentication label, the solvent it may contain evaporates. The other constituents of the marking ink remain adherent to each other and / or to the authenticatable article, where applicable, the authentication label.

[0073] The marking ink may include, 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 pm, preferably between 5 nm and 100 nm.

[0075] The authenticatable item may be an item chosen from everyday items, for example perfume bottles, bottles (including glass bottles), jewelry, watches, electronic devices, all types of packaging (including 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 authenticated item can be of a very varied nature. These are generally items that are likely to be counterfeit.

[0078] The invention also relates to a method of colorimetric detection with a device for authenticating a dispersion of phosphorescent carbon points of clusters of said phosphorescent carbon points according to the invention as described above or of phosphorescent carbon points obtained with the manufacturing method according to the invention as described above which are likely to be present in at least one 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) 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 the at least one determined portion of the article 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.;

[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 1 er color space and the so-called reference data is a 2 ème triplet E2 in said 1 er color space. The comparison step d) then consists of calculating a color difference AE between the 1 er triplet Ei and 2 ème triplet E2 or a color difference between the 1 er triplet Ei and 2 ème triplet E2 relating to 2 ème 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. 1] Figure 1 is a l ère photograph of phosphorescent carbon dots according to the invention which was obtained by scanning electron microscopy.

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

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

[0088] EXPERIMENTAL PART

[0089] Phosphorescent carbon dot samples according to the invention and comparative samples were synthesized in the manner as described below.

[0090] More specifically, 2 carbon dot samples according to the invention (hereinafter referred to as INV1 and INV2) and 5 comparative carbon dot samples (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 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 ammonia solution and L-aspartic acid mixture 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] Table 1

[0097] The COMP3 carbon dot sample with a molar ratio of NH4OH to L-aspartic acid of 1.41 corresponds to the carbon dots described in the aforementioned publication by Sizhe Hu et al.

[0098] All carbon dot syntheses were performed 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 under 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 ultrasonic treatment for a period of 15 minutes in order 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 gel form which was then cooled to room temperature.

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

[0103] The phosphorescent carbon dots thus obtained were purified as follows: - by centrifugation at a speed of 10,000 revolutions / minute, then - by filtration with a filter membrane whose filtration threshold was 0.lpm.

[0104] Finally, the purified phosphorescent carbon dots thus obtained were freeze-dried for a period of 12 hours in order 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 tests.

[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): h= arrtan in which a* and b* are the chromaticity coordinates in the CIELAB chromaticity space of the sample of phosphorescent carbon dots considered

[0108] A - Determination of the hue angle of all phosphorescent carbon dot samples during their synthesis:

[0109] First, on the day of their synthesis, the hue angle was determined for all synthesized phosphorescent carbon dot samples, in the absence of illumination, then after illumination (i.e. light excitation) with ultraviolet light at a wavelength of 365 nm for a duration of 1, 2, 3 and 4 seconds.

[0110] Table 2 below summarizes for all 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 illumination. Table 2

[0111] From the results detailed in Table 2, it is noted that all carbon dot samples have a hue angle of a different value from that determined in the absence of any illumination. This testifies that all synthesized phosphorescent carbon dot samples are able to emit phosphorescent light. This light emitted by the phosphorescent carbon dots varies depending on the duration of illumination.

[0112] After one week, all the comparative phosphorescent carbon dot samples were found to have formed agglomerates. They had absorbed the surrounding moisture. In contrast, the phosphorescent carbon dot samples according to the invention remained perfectly stable as in 1 erday of their synthesis. No agglomerates were formed. The phosphorescent carbon dot samples according to the invention were always in the form of a powder.

[0113] This means that the comparative phosphorescent carbon dot samples are hygroscopic and the phosphorescent carbon dot samples according to the invention are not hygroscopic.

[0114] Due to their instability, 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").

[0115] Finally, these experimental results demonstrate that the selection of a molar ratio of NH4OH to L-aspartic acid greater than or equal to 1.6 is decisive so that the synthesized phosphorescent carbon dots are not hygroscopic and therefore remain stable over time.

[0116] 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).

[0117] B- Determination of the hue angle of the phosphorescent carbon dot samples according to the invention during aging (1 to 25 days):

[0118] More specifically, aging experiments were carried out by measuring the hue angle of the phosphorescent carbon dot samples 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.

[0119] Table 3 below details the hue angles determined for the INV1 phosphorescent carbon dot sample. Table 3

[0120] Table 4 below details the hue angles determined for the INV2 phosphorescent carbon dot sample. Table 4

[0121] 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.

[0122] The experiments were continued with the INV2 phosphorescent carbon dot sample.

[0123] C- Determination of the hue angle of the INV2 phosphorescent carbon dot sample incorporated in a marking ink composition:

[0124] C- 1 Determination of the tint angle as a function of the surrounding brightness (morning / afternoon)

[0125] 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.

[0126] 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.

[0127] Ethylene glycol allowed for good dispersion of the INV2 phosphorescent carbon dots. It did not modify the colorimetry of the resulting marking ink.

[0128] 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.

[0129] The printer marketed by FUJIFILM under the trade name Dimatix Materials Printer DMP-2850 was used to produce prints on paper with the marking ink detailed above.

[0130] Table 5 below details the hue angle of the paper and the marking ink printed on the same paper as a function of the illumination duration (0, 1, 3 and 5 seconds) with ultraviolet light at a wavelength of 365 nm. The determination of the hue angle was carried out on the morning of the manufacturing of the marking ink. Table 5

[0131] Table 6 below details the hue angle of the paper and the marking ink printed on the same paper as a function of the illumination duration (0, 1, 3 and 5 seconds) with ultraviolet light at a wavelength of 365 nm. The determination of the hue angle was carried out the afternoon of the manufacturing of the marking ink. Table 6

[0132] Based on the detailed results in Tables 5 and 6, there is still a significant difference between the hue angle of the paper and that of the marking ink printed on the same paper. This difference is always approximately constant regardless of the illumination duration. This difference therefore does not depend on the illumination duration. This experiment shows that the INV2 phosphorescent carbon dot sample remains phosphorescent after its incorporation into a marking ink formulation. This confirms the interest in using such phosphorescent carbon dots as authentication markers in marking ink formulations.

[0133] 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 the same paper, regardless of the exposure conditions of the printed paper (i.e. morning or afternoon) or in other words independently of the surrounding brightness.

[0134] C- 2 Determination of the tint angle as a function of the aging of the marking ink

[0135] Then, the tint angle of the paper and that of the marking ink printed on the same paper were determined after aging the said marking ink for 1, 2 and 3 weeks.

[0136] Table 7 below details the tint angle of the paper and the marking ink printed on the 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. Table 7

[0137] Table 8 below details the tint angle of the paper and the marking ink printed on the 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. Table 8

[0138] Table 9 below details the tint angle of the paper and the marking ink printed on the 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. Table 9

[0139] In view of the detailed results in Tables 7 to 9, there is still a significant difference between the tint angle of the paper and that of the marking ink printed on it. 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.

[0140] After one year of synthesis, the INV1 and INV2 phosphorescent carbon dot samples are still in the form of a powder as in 1 er 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.

[0011] D- Additional analyses of the INV2 phosphorescent carbon dot sample:

[0142] The additional analyses detailed below were carried out with the INV2 phosphorescent carbon dot sample, 40 weeks after its synthesis.

[0143] Figures 1 and 2 are photographs of this INV2 phosphorescent carbon dot sample that were obtained by scanning electron microscopy.

[0144] Considering the added scale in each of the two photographs, the particle size of the phosphorescent carbon dots is approximately 300 pm.

[0145] Figure 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 Figure 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. Method for 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. Method 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 μm and 10 μm.

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), where appropriate 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 phosphorescent carbon dots of clusters of said phosphorescent carbon dots according to claim 7 or 8 or of 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. 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) illuminating the at least one determined portion of the article to be authenticated with a flash lamp with which the authentication device is equipped and which produces an excitation beam; b) acquiring at least one image, via a photographic apparatus with which the authentication device is equipped, of the at least one determined portion of the article 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.; 12. Detection method according to claim 11, characterized in that the authentication device is a mobile phone or a tablet.

13. 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.

14. 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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