Method for producing selenium-doped carbon quantum dots and a solid lignocellulosic fraction, and selenium-doped carbon quantum dots and solid lignocellulosic fraction produced

Abstract

The present invention relates to a method that, starting with cocoa shell, produces selenium-doped carbon quantum dots, with an antioxidant capacity and luminescent nature, and a solid lignocellulosic fraction, which is stable at temperatures greater than 200°C. The method implemented requires grinding the cocoa shell, adding acid solvent and extracting with microwaves. The multifunctional supernatant produced is separated, a selenium precursor is added, and it is irradiated with microwaves, producing an intermediate solution to which a hydrothermal treatment is applied to produce selenium-doped carbon quantum dots. Optionally, a portion of the separated multifunctional supernatant is used to precipitate a pectin-rich insoluble fraction. Advantageously, the solid lignocellulosic fraction is used as a heavy metal adsorbent or a source of fibre.

Description

[0001] PROCEDURE FOR OBTAINING QUANTUM DOTS OF CARBON DOPED WITH SELENIUM AND A SOLID LIGNOCELLULOSIC FRACTION, AND QUANTUM DOTS OF CARBON DOPED WITH SELENIUM AND SOLID LIGNOCELLULOSIC FRACTION OBTAINED

[0002] OBJECT OF THE INVENTION

[0003] The present invention relates to a semi-pilot or industrial-scale process based on microwave radiation for obtaining selenium-doped carbon quantum dots and a lignocellulosic solid fraction from cocoa husks generated as a waste product in the chocolate industry of high economic value. Advantageously, the proposed process allows—in addition to the selenium-doped carbon quantum dots—for sequentially obtaining additional compounds with high added value, such as water-soluble proteins, pectin, antioxidants, pigments, and lignocellulosic material.

[0004] The object of the invention is to offer a process that enables the reduction of production costs of the products obtained, generating added value from the cocoa husk waste used as raw material in this process.

[0005] The present invention also relates to selenium-doped carbon quantum dots obtained using the developed method. These quantum dots can be used as biosensors for the controlled release of drugs, in imaging for the visualization of cells and tissues, and in general as photosensitizing agents, due to their advantageous properties in terms of photoluminescence, biocompatibility, and low toxicity. The invention also relates to the solid lignocellulosic fraction, which constitutes an adsorbent for heavy metals or as a source of fiber.

[0006] The compounds obtained through this process may be used in the chemical, cosmetic, pharmaceutical, food, medical, and electronic industries. BACKGROUND OF THE INVENTION

[0007] Cocoa husks are the main waste produced during chocolate manufacturing after roasting cocoa beans, constituting 12% of the total weight. The production of chocolate and chocolate derivatives represents a major economic and social impact in Spain, making it necessary to propose environmentally sustainable solutions to manage this type of high-production waste. Cocoa husks are composed of 70% carbohydrates, mainly cellulose and pectin, with low fat, ash, and moisture contents (2%, 4%, and 6%, respectively). Cocoa husks are also rich in antioxidant compounds (theobromine, caffeic acid, epicatechin, ferulic acid, protocatechuic acid, chlorogenic acid) and pigments, with potential applications in sectors such as food, cosmetics, medicine, and advanced materials (Mellinas et al., 2020; Sánchez et al., 2023).

[0008] Conventional extraction methods such as Soxhlet, maceration, or reflux have been widely used to obtain bioactive compounds from natural sources. These combinations of mixing, agitation, temperature, and / or maceration offer a simple and effective method for extracting these compounds. However, these processes require long extraction times, high solvent consumption, and can degrade the target compounds due to local overheating effects, yielding inaccurate results.

[0009] Sequential compound extraction has been applied in recent years (Aimone et al., 2023; Moreira et al., 2023; Pérez-Bassart et al., 2023) with the aim of increasing the efficiency and sustainability of extraction processes for different agro-food waste, minimizing their environmental impact. Patent ES2695874B2 proposes the sequential extraction of different high added value compounds (soluble proteins, lipids, pigments and carbohydrates) from wet biomass through successive extractions with pressurized subcritical water, ethanol expanded with sub- or supercritical CO2 and, finally, Supercritical CO2.

[0010] Patent US20060269633A1 develops a sequential extraction of cocoa husks to obtain the polyphenols and theobromine present in them. To achieve this, a conventional extraction process is proposed, repeated two to five times to obtain the maximum yield of these fractions, using a water-acetone mixture as a solvent and temperatures between 40 and 80 °C for four to six hours. Methanol, which is not recovered during the process, is used to separate the different fractions, increasing the use of organic solvents during extraction. The use of water as an extractant is contemplated, obtaining low yields in the process. Finally, the husks are subjected to a degreasing process prior to the extraction process, thus increasing the number of steps in the overall process.

[0011] Patent WO2020245293A1 applies a cascade process to obtain extracts from cocoa beans through hydrothermal extraction using mixtures of different organic solvents. To achieve this, it uses extraction times between 24 and 96 hours at temperatures below 70°C.

[0012] Patent CN110882289A performs a sequential extraction to extract the polyphenols present in cocoa beans, with the aim of reducing the costs of inactivating the polyphenol oxidase (PPO) enzyme. This process is necessary to increase the shelf life of cocoa beans. To achieve this, an initial extraction is carried out with hexane:acetone to defat the cocoa beans, and then the beans are extracted with inorganic acids at pH values ​​between 2 and 6 and a temperature range between 30 and 85 °C for 2 hours on a pilot scale, obtaining fractions with 54% polyphenols and 41% theobromine.

[0013] Patent W02020038905A1 describes a process for extracting cocoa pod husk components and utilizing the resulting soluble extract and its applications. This methodology proposes the use of unconventional techniques, such as microwaves and ultrasound, but involves 10-12 stages prior to obtaining the extract. In this case, the company also uses enzymes to maximize extraction yields, although increasing the cost of the process.

[0014] Patent W02007138118A1 uses unfermented cocoa beans as raw material in a pilot-scale sequential extraction to obtain lipid fractions I and polyphenols. To achieve this, a hydroalcoholic extraction (30-70%) is performed at a temperature range of 40-80 °C for 1-2 hours. Maximum polyphenol extraction yields of 50% are obtained, with extracts enriched by up to 90% through evaporation.

[0015] Grillo et al. propose the use of semi-pilot scale ultrasound to obtain essential oil and polyphenols derived from cocoa husks, using ternary mixtures of ethanol / hexane and water (25 L) and an energy of 6.82 kW.

[0016] There are different investigations published in recent years on the extraction of active compounds present in cocoa husks (Barbosa-Pereira et al., 2018; Bruna et al., 2009; Handojo et al., 2019; Mellinas et al., 2020; Nsor-Atindana et al., 2012; Okiyama et al., 2019; Patricia Isabel et al., 2017; Rojo-Poveda et al., 2020, 2019) where non-conventional extraction techniques such as supercritical fluids or microwaves are used, but their main disadvantage is that they are developed only on a laboratory scale and are not directly scalable for semi-pilot scale production in the production of larger quantities. Microwave-assisted extraction (MAE) preliminarily developed by Mellinas et al. (Mellinas et al.(2020) demonstrates the high potential of this technique for obtaining high-value compounds from cocoa husks by optimizing laboratory-scale processes and controlling the extraction temperature. It is important to note that the MAE scale-up process is not linear, so optimization of extraction parameters is necessary to maximize yields without affecting the quality of the products obtained.

[0017] Patents ES2615752T3 and WO2013067896A propose the use of microwave radiation on a pilot scale to extract polysaccharides from fungi and algae, respectively, through an extraction process with a pressure range of 20-760 mmHg, using organic and inorganic acids.

[0018] Patent WO2013150262A1 proposes a microwave-assisted citrus waste biorefinery for obtaining various value-added products such as essential oil, flavonoids, cellulose, and pectin. To achieve this, water-miscible organic solvents are used at an extraction temperature between 80 and 150°C and a power output between 100 W and 10 MW.

[0019] In recent years, alternative strategies for the synthesis of nanoparticles and nanomaterials with a more sustainable approach have been explored, employing natural agents such as plant extracts and microbial enzymes as reducing and / or stabilizing agents (Aswathi et al., 2023). On the other hand, the synthesis of nanoparticles using microwaves presents great advantages over conventional methods: improved microstructure, increased yield, energy savings, reduced manufacturing costs, and obtaining new materials with different properties. Several articles have been reported for the production of different types of nanoparticles at laboratory scale (Ahammed et al., 2020; Borowska et al., 2023; Kaur et al., 2023; Mellinas et al., 2019; Pauzi et al., 2019; Xiao et al., 2020).

[0020] The use of microwave radiation to obtain selenium nanoparticles using extracts derived from cocoa husks has been published, obtaining a mixture of small-sized amorphous nanoparticles (1-3 nm) and crystalline nanoparticles of an approximate size of 50 nm. For its development, the synthesis conditions have been optimized at laboratory scale, obtaining values ​​of 788.6 W of power and 15.6 minutes of synthesis for a sample volume of 50 mL. The main disadvantage of this synthesis method is related to the scalability of the obtained process, since these conditions would imply the use of high-power magnetrons and / or long synthesis times, which could lead to the degradation of the obtained products (Mellinas et al., 2019).

[0021] Quantum dots are a special type of nanomaterial that exhibits significantly improved properties and can be defined as semiconductor nanomaterials that can convert an incoming light spectrum into a different energy output frequency. In this regard, carbon quantum dots (CQDs) have been the subject of numerous studies in recent years due to their exceptional optical and chemical properties, as well as their potential applications.

[0022] CQDs can present some drawbacks due to their low water solubility, low quantum yields and lack of functionalization. To solve these problems, several strategies have been proposed, highlighting the doping of CQDs with different heteroatoms to improve their properties (Wang et al., 2019). Selenium-doped carbon quantum dots (SeCQDs) have a high potential due to their high biocompatibility, which has allowed their use in the field of medicine, due to their high antioxidant capacity, to treat and detect different diseases (Bai et al., 2023; H. Huang et al., 2020; F. L¡ et al., 2017; Luo et al., 2020; Rosenkrans et al., 2020; Shi et al., 2020; Wang et al., 2022; X. Zhou et al., 2021). In addition, they have also been used as fertilizers (J. L¡ et al., 2023) in tomato plantations and as a selenium supplement in animal feed (Wang et al., 2022).Finally, we can find applications in the electrochemical area, being used for the development of batteries (Yang et al., 2021) and sensors (Chauhan & Chaudhary, 2021; G. Huang et al., 2020; D. Zhou et al., 2021).

[0023] Regarding the synthesis methods of SeCQDs, the most commonly used precursor is L-selenocysteine ​​through a procedure that consists of heating between 60-120 °C for at least 24 hours at pH 9, which favors the solubility of the precursor. Subsequently, the solution is purified by dialysis and brought to dryness until use (Bai et al., 2023; H. Huang et al., 2020; F. L¡ et al., 2017; Luo et al., 2020; Rosenkrans et al., 2020; Shi et al., 2020; Wang et al., 2022; X. Zhou et al., 2021).

[0024] L¡ et al. have recently reported the synthesis of SeCQDs from blue-green algae, composed mainly of cyanobacteria, and sodium selenite as a precursor. To do this, the algae powder is mixed with sodium selenite in a 2:1 ratio, with a volume of distilled water of 200 mL. This solution is treated at 200 °C for 6 hours in an autoclave, and the supernatant is dialyzed and lyophilized until use (J. L¡ et al., 2023). This process involves the use of high temperatures and pressures.

[0025] Another possible method for doping CQDs with selenium is to incorporate selenium through sodium selenite by heating at 90 °C for 15 minutes. This method, despite using shorter synthesis times, requires pre-production of the CQDs, increasing the number of steps and resources required to obtain the desired product (Chauhan & Chaudhary, 2021).

[0026] There are a limited number of patents related to the production of SeCQDs. Patent CN116554220A describes a biomedical application of selenium-doped carbon quantum dots in bone orientation, effectively improving the utilization rate of selenium in the human body by efficiently scavenging reactive oxygen species. SeCQDs can be used as medicines to treat diabetic bone diseases, osteoporosis, and the like.

[0027] Patent CN114479846A establishes a method for preparing and applying SeCQDs based on selenium-enriched yeast. To do this, a hydrothermal treatment followed by a purification process is carried out.

[0028] Patent CN104312588 refers to a method for preparing SeCQDs comprising the following steps: (1) preparing carbon oxide quantum dot powder, (2) dispersing the powder obtained above in a solvent to obtain a carbon oxide quantum dot solution, (3) adding a dopant containing selenium to the carbon oxide quantum dot solution by carrying out a solvothermal reaction process at a temperature of 50-500 °C for between 0.5 and 18 hours to obtain the selenium-doped carbon quantum dots.

[0029] To solve the problems posed by the use of conventional extraction methods and their scaling up, the present invention proposes the use of a microwave-assisted extraction method (MAE) on a semi-pilot or industrial scale, minimizing the consumption of organic solvents and the number of stages during the process, and improving the sustainability of the proposed processes through an overall zero-waste process, giving added value to all the products obtained.

[0030] Furthermore, the process proposed in the present invention allows the production of SeCQDs by microwave radiation on a semi-pilot scale, using an extract derived from cocoa husks as the carbon source, reducing agent, and stabilizer, which allows the production of a product with high photoluminescent potential and low toxicity. The use of cocoa husks as a carbon source, reducing agent, and stabilizer has not been previously reported to date, nor has the combination of microwave radiation with a low-pressure hydrothermal process for the synthesis of SeCQDs.

[0031] DESCRIPTION OF THE INVENTION The present invention proposes a method for obtaining selenium-doped carbon quantum dots (SeCQDs) and a lignocellulosic solid fraction on a semi-pilot or industrial scale from cocoa husks (a by-product of cocoa processing) by means of a sequential microwave-based process.

[0032] Obtaining SeCQDs using microwaves offers higher extraction yields, fewer steps, and lower energy requirements than previously reported.

[0033] Advantageously, the proposed process also allows for the production of other high-value compounds, such as a lignocellulosic solid fraction, water-soluble proteins, pectins, antioxidants, and pigments, resulting in the complete valorization of the cocoa husk, with all the raw material entering the process ultimately becoming a product.

[0034] Furthermore, the proposed extraction processes utilize an aqueous medium, avoiding the use of organic solvents, improving the sustainability of the processes compared to the reported methodologies.

[0035] Since this is a non-selective microwave-assisted extraction in an aqueous medium and on a semi-pilot or industrial scale, the present invention allows for the production of multifunctional products with great potential for use in multiple sectors such as food, food packaging, cosmetics, electronics, and biomedicine.

[0036] The SeCQD synthesis process involves the use of a salt as a precursor, preferably in the form of an aqueous solution, together with a natural extract rich in natural antioxidants obtained after the extraction of cocoa husks in an acidic medium (extract P1), used as a carbon source, reducing agent, and stabilizer. Thus, this invention results in the production of a non-toxic, economically viable, and easily accessible reducing agent for the production of SeCQDs with great potential for use in various fields such as electronics, agriculture, food packaging, and medicine.

[0037] This approach not only significantly reduces production costs, but also represents a "green" synthesis method, incorporating the principles of the "circular economy" by reusing materials previously considered waste.

[0038] The SeCQDs obtained by this procedure present exceptional results in terms of photoluminescence, polydispersity, stability, antioxidant capacity and low toxicity compared to those reported so far, thus demonstrating the viability and efficacy of the present invention.

[0039] Thus, the present invention allows the production of SeCQDs and, optionally, other high added value compounds from cocoa husks according to the following steps:

[0040] - Grinding of the cocoa husk, where it is preferably ground to a size between 0.5 and 2 mm.

[0041] - Addition of an acid solvent to the ground cocoa husk until pH equal to or less than 5.5 and obtaining a solution with a ratio of 10-70 g husk / L acid solvent.

[0042] - Microwave-assisted extraction of the solution obtained in the previous step, carried out at an energy density of between 0.3 and 0.7 kW s / mL, generating a solid lignocellulosic fraction (SLC) and a multifunctional supernatant (P1). Optionally, the microwave-assisted extraction is carried out for at least 20 minutes (preferably for a time interval between 35-100 minutes) working under stirring and atmospheric pressure. Advantageously, in microwave-assisted extraction, it is not necessary to perform any pretreatment on the ground cocoa husk to obtain a first extract rich in pectins, water-soluble proteins and other value-added compounds, as well as the SLC. In fact, the SLC can be used as a metal adsorbent or as a dietary fiber without the need for pretreatment, as it is rich in lignocellulosic material.

[0043] - Solid-liquid separation obtaining a solid lignocellulosic fraction (SLC) and a multifunctional supernatant (P1), where the multifunctional supernatant (P1) is composed of, at least, a minimum concentration of 235 mg of Galacturonic Acid / g dry extract and by antioxidant compounds with a minimum antioxidant capacity of 18 mg eq. Gallic Acid / g extract and 47 mg eq. Gallic Acid / g extract, measured by FRAP (acronym for the iron-reducing antioxidant power measurement method) and ABTS (acronym for the ABTS+ cation radical reduction measurement method), respectively.

[0044] Synthesis of selenium-doped carbon quantum dots (SeCQDs) by microwave irradiation at an energy density between 0.072 - 0.720 kW s / mL of a solution formed by the multifunctional supernatant (P1) and a selenium precursor with a concentration between 1 and 4 g / L, obtaining an intermediate solution. Thus, P1 is used as a carbon source, reducing agent and stabilizer for the synthesis of selenium-doped carbon quantum dots; while the selenium precursor is a compound such as: selenium oxide, trioxoselenic acid and / or sodium selenite. Likewise, it should be noted that, preferably, the synthesis of selenium-doped carbon quantum dots (SeCQDs) by microwave irradiation is carried out for a period of between 5 and 50 minutes.

[0045] - Hydrothermal treatment of the intermediate solution at a temperature below 60 °C, and a pressure between 50 and 120 mbar, obtaining SeCQDs with a size between 10 and 50 nm, presenting a GAP band of 3.1 eV, a stability between -3 and -52 mV, a minimum antioxidant capacity, measured by ABTS and FRAP, of 22.6 ± 0.2 and 40.2 ± 0.2 mg eq. Trolox / g SeCQDs, respectively, and luminescent character.

[0046] It should be noted that the solid lignocellulosic fraction (SLC) is stable at temperatures greater than 200 °C and the intermediate solution obtained after the synthesis of SeCQDs has an antioxidant capacity between 10-30 mg Trolox / g extract, with the yield of the synthesis reaction being greater than 90%.

[0047] The present extraction process using cocoa hulls presents higher yields for the main products obtained (pectin, proteins, and antioxidants) than conventional methods reported to date, given that the extraction occurs as a result of changes in the cellular structure of the agro-food residue upon interaction with microwave waves, under the conditions described above. In this regard, it is worth highlighting that the microwave extraction yield is between 30 and 40% compared to the initial raw material.

[0048] Thus, the acidic solvent added to the ground cocoa husk is an organic or inorganic acid, such as HCl, H2SO4, citric acid, acetic acid, and / or malic acid. In this regard, the efficiency and effectiveness of organic acids, such as citric acid, have been proven, thus improving the sustainability of the overall process of the invention.

[0049] Optionally, a portion of the multifunctional supernatant (P1) obtained from the solid-liquid separation is contacted with alcohol at a temperature lower than 25 °C, generating the precipitation of an insoluble fraction (P2) - rich in pectins -, and a fraction (P3); where the precipitated insoluble fraction (P2) is stable against temperatures greater than 200 °C and is recovered by means of a second solid-liquid separation at room temperature, whose extraction yield is greater than 45% with respect to the multifunctional supernatant (P1). Thus, this second solid-liquid separation is preferably carried out by centrifugation or filtration.

[0050] The different products obtained during the extractions (P1 and P2) are stable at temperatures above 200 °C, presenting high thermal stability for processing in different industrial applications.

[0051] Fractionation with alcohol (second solid-liquid separation) after extraction allows the production of concentrated pectin (P2) and antioxidant fractions (P3 fraction), allowing the recovery of the used alcohol for reuse in future fractionation processes, significantly increasing the sustainability of the process. That is, fraction (P3) - rich in antioxidant compounds and pigments - is concentrated by evaporation to recover the alcohol, preferably, subsequently the fraction (P3) is subjected to a drying process, by atomization, nebulization, lyophilization or oven. Meanwhile, the insoluble fraction (P2) is subjected to a drying process, by atomization, nebulization, lyophilization or oven.

[0052] Advantageously, the present invention develops a zero-waste process, where all raw materials are transformed into products using microwave radiation.

[0053] The use of water to obtain polyphenol-rich fractions with high yields improves the sustainability of the processes proposed to date.

[0054] The present invention allows to obtain selenium-doped carbon quantum dots (SeCQDs) that have a size between 10 and 50 nm, a GAP band of 3.1 eV, a stability between -3 and -52 mV, a minimum antioxidant capacity, measured by ABTS and FRAP, of 22.6 and 40.2 mg eq. Trolox / g SeCQDs, respectively, and luminescent character. In this sense, the luminescent character of SeCQDs offers an advantageous behavior for their application in biosensors, for the controlled release of drugs, in imaging for the visualization of cells and tissues, and in general as a photosensitizing agent.

[0055] Likewise, the process of the present invention allows obtaining a solid lignocellulosic fraction (SLC), with application as an adsorbent of metals, preferably heavy metals such as lead or cadmium, present in wastewater due to its porous structure, with a minimum efficiency between 26-50%, under the conditions studied, without the need to carry out any subsequent treatment after the solid-liquid separation process.

[0056] Likewise, said solid lignocellulosic fraction (SLC) obtained by the process of the invention can be used as a source of soluble and insoluble fiber with a concentration greater than 40%.

[0057] Table 1 summarizes the functional properties of the supernatant or multifunctional extract (P1) obtained after the first microwave-assisted extraction, which is not intended to limit its scope, but rather to be illustrative, since the final composition of the extracts will depend on the conditions used during the extraction.

[0058] Table 1. Characterization of the extracts obtained after the first extraction (P1): 0.61 kW s / mL, 30 min, 0.04 g / L, pH 3, 7500 mL. In short, the "in situ" production of SeCQDs from the liquid extract (P1) as a synthesis medium, which reacts with the precursor by microwave irradiation, allows obtaining interesting active properties in the fields of electronics, agriculture and medicine.

[0059] Furthermore, the synthesis times previously proposed in other works are reduced, also considering a semi-pilot or industrial production scale.

[0060] Scaling up the synthesis conditions for SeCQDs using microwaves represents an improvement for their implementation in industrial processes. To date, no microwave synthesis conditions have been reported for the production of this type of nanomaterial.

[0061] Microwave pretreatment significantly improves the stability of selenium-doped carbon quantum dots.

[0062] Low-pressure hydrothermal treatment following microwave synthesis induces the formation of selenium-doped carbon quantum dots. Furthermore, up to two-thirds of the water (solvent) volume evaporates during this process, resulting in a more concentrated product than the original.

[0063] The combination of a microwave process followed by a low-pressure treatment has not been reported to date for the development of these products, allowing for improved energy consumption and process sustainability.

[0064] The method proposed in this invention works on a semi-pilot scale in an aqueous medium, adjusting the necessary energy density based on the amount of raw material entering the system.

[0065] Cascade steps and the circularity of the process allow for a zero-waste procedure by eliminating sample pretreatment steps proposed by other authors, such as concentrating or drying, between the extraction and synthesis processes, which involve the use of organic solvents and high time and energy consumption. This invention reduces the carbon footprint along with a significant reduction in total process costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] To complement the description that follows and in order to help better understand the characteristics of the invention, according to preferred examples of practical implementation thereof, a set of figures is attached as an integral part of said description, in which the following has been represented for illustrative and non-limiting purposes:

[0067] Figure 1.- Shows a flow diagram of the method for obtaining the different compounds mentioned above, object of the invention, where the sequential microwave processes and the products obtained in the different stages have been represented.

[0068] Figure 2.- TEM images showing the size of the SeQDs.

[0069] Figure 3.- Images of SeQDs showing photoluminescence or fluorescence after being irradiated with a lamp at 365 nm under synthesis conditions included in the preferred embodiments.

[0070] PREFERRED REALIZATION

[0071] The preferred embodiments detailed in this section constitute non-limiting examples of the invention.

[0072] Thus, the experimental procedure in which up to five types of products can be obtained is detailed first (as detailed in Figure 1), which consists of the following stages:

[0073] 1. Obtaining multifunctional extract (P1), pectin (P2), antioxidant compounds (P3), liqnocellulosic fraction (SLC):

[0074] 1.1. The previously ground residue with a preferred particle size of 1000 microns is introduced into an aqueous solution and subjected to a microwave extraction process under a wide range of conditions: pH less than or equal to 5.5 by the addition of an organic or inorganic acid (HCl, H2SO4, citric acid, acetic acid, malic acid,...), an energy density preferably between 0.3 and 0.7 kW s / mL and a ratio of 10-70 g husk / L acid solvent, for at least 20 minutes working in a static tank under stirring and atmospheric pressure, preferably between 35-100 minutes. After a solid-liquid separation, a solid lignocellulosic fraction (SLC) and a multifunctional supernatant (P1) are obtained with a microwave extraction yield of between 30 - 40% with respect to the initial raw material. The multifunctional product (P1) is mainly composed of pectins with a minimum concentration of 235 mg of Ac.Galacturonic acid / g dry extract (determined using the uronic acid method) and antioxidant compounds with a minimum antioxidant capacity of 18 mg eq. Gallic acid / g extract and 47 mg eq. Gallic acid / g extract, determined using the FRAP and ABTS spectrophotometric methods, respectively.

[0075] The SLC product is a lignocellulosic fraction that presents a highly porous structure with a thermal stability greater than 250 °C. This type of material can be used to adsorb heavy metals (cadmium, lead or arsenic) in wastewater purification, in addition to other possible applications in sectors such as cosmetics, food or the development of new advanced materials. Optionally, from the supernatant obtained in the previous stage (P1), products can be obtained by precipitation with alcohol at controlled temperatures, preferably below 25 °C, until complete precipitation of the insoluble fraction (P2), mainly pectins, being recovered by a solid-liquid separation method at room temperature, such as centrifugation or filtration, whose extraction yield is above 45% compared to P1.

[0076] The P3 fraction, rich in antioxidant compounds and pigments, is concentrated to recover the alcohol, which will be reused in future precipitation processes through evaporation. P2 and P3 can be dried using atomization (nebulization), freeze-drying, or oven drying, depending on the required moisture content, and then stored.

[0077] 2. Obtaining selenium-doped carbon quantum dots:

[0078] 2.1. The multifunctional supernatant (P1) obtained in the previous extraction is used as a reducing and stabilizing agent, without the need for pretreatment, for the "in situ" synthesis of SeCQDs, from a selenium precursor, such as: selenium oxide, trioxoselenic acid or sodium selenite.

[0079] This solution is irradiated with microwaves at an energy density of 0.072–0.720 kW s / mL for 5–50 minutes. The concentration of selenium precursor in the solution is between 1 and 4 g / L, preferably between 2.5 and 3.5 g / L. The intermediate solution obtained after microwave (MW) synthesis at this stage has an antioxidant capacity of 10–30 mg Trolox / g extract, and the synthesis reaction yield is greater than 90%.

[0080] 2.2. The above intermediate solution is subjected to hydrothermal treatment at a temperature not exceeding 60 °C and a pressure between 50 and 120 mbar, the time required to reduce the solvent volume (water) to two-thirds of the initial volume. The SeCQDs obtained have a size between 10 and 50 nm (as shown in Figure 2) and an amorphous structure, presenting a GAP band of 3.1 eV, a stability between -3 and -52 mV (determined by dynamic light scattering, DLS) and a minimum antioxidant capacity, measured by ABTS and FRAP, of 22.6 ± 0.2 and 40.2 ± 0.2 mg eq. Trolox / g SeCQDs, respectively.

[0081] Two examples of preferred embodiments for obtaining selenium-doped carbon quantum dots (SeCQDs) according to the process proposed in the present invention are detailed below.

[0082] Example A

[0083] 300 g of cocoa husk, previously ground to a size between 0.5-1 mm, are deposited, a sufficient quantity of water is added to achieve a 40 g / L ratio and the pH is adjusted to a value of 2 with hydrochloric acid.

[0084] The mixture is subjected to open microwave treatment under the following conditions: 0.5 kW s / mL for 40 minutes. The solid lignocellulosic fraction (SLC) is then separated by centrifugation for the time required to separate the two phases.

[0085] To the obtained supernatant (P1) rich in pectins and antioxidants, alcohol is added in a volume of ethanol equivalent to 80% of the solution to be precipitated in order to obtain P2 and P3, which are separated by centrifugation at room temperature at 10,000 rpm for the time necessary to separate both phases.

[0086] P2 is a solid extract rich in pectin, and P3 is a liquid extract rich in polyphenols. P3 is evaporated and concentrated in a rotary evaporator, recovering the ethanol, at 60°C and at least 100 mbar of pressure.

[0087] The recovered solid lignocellulosic fraction (SLC) is oven-dried and stored under vacuum for use as a metal adsorbent.

[0088] Following the circularity of the process, the P1 extract is used without the need for pretreatment in the "in-situ" synthesis of SeCQDs. 3 g of sodium selenite are added per liter of solution and an energy density of 0.07 kW s / mL is used for 20 minutes in a microwave system working at atmospheric pressure. The liquid fraction is subjected to a hydrothermal process at 60 °C and 50 mbar of pressure, removing part of the solvent up to a third of the initial volume for 4 hours and ultracentrifuged to obtain an isolated selenium-doped carbon quantum dots (SeCQDs).

[0089] Example B

[0090] 600 g of cocoa husks, previously ground to a size between 0.5 and 1 mm, are deposited, sufficient water is added to achieve a ratio of 25 g / L, and the pH is adjusted to 2 with citric acid. The mixture is subjected to open microwave treatment under the following conditions: 0.3 kW s / mL for 50 minutes. The solid lignocellulosic fraction (SLC) is then separated by centrifugation for the time necessary to separate both phases.

[0091] To the obtained supernatant (P1) rich in pectins and antioxidants, alcohol is added in a volume of ethanol equivalent to 80% of the solution to be precipitated in order to obtain P2 and P3, which are separated by centrifugation at room temperature for the time necessary to separate both phases.

[0092] P2 is a solid extract rich in pectin, and P3 is a liquid extract rich in polyphenols. P3 is evaporated and concentrated in a rotary evaporator, recovering the ethanol, at 60°C and at least 100 mbar of pressure.

[0093] The solid lignocellulosic fraction (SLC) is dried to a water content of less than 10% and stored under vacuum until use.

[0094] Following the circularity of the process, the P1 extract is used without the need for pretreatment in the "in-situ" synthesis of SeCQDs. 40 g of sodium selenite are added to 10 L of P1 and an energy density of 0.095 kW s / mL is used for 10 minutes in a microwave system operating at atmospheric pressure.

[0095] The liquid fraction is subjected to a hydrothermal process at 50 °C and 80 mbar pressure, removing part of the solvent up to a third of the initial volume for 4 hours and ultracentrifuged to obtain an isolated SeCQDs.

[0096] Finally, it should be noted that for the SeQDs obtained in example A and example B detailed above, they have been irradiated with a lamp at 365 nm, showing said SeQDs their luminescent character, as observed in Figure 3. Thus, it should be noted that in order to show the photoluminescence or fluorescence of the SeQDs obtained in example A and example B, Figure 3 is included in color.

[0097] Bibliographic references:

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Claims

1. Procedure for obtaining selenium-doped carbon quantum dots and a lignocellulosic solid fraction from cocoa husks, comprising the following steps: - Grinding of cocoa husks. - Addition of an acid solvent to the ground cocoa husk until pH equal to or less than 5.5 and obtaining a solution with a ratio of 10-70 g husk / L acid solvent. - Microwave-assisted extraction of the solution obtained in the previous stage, which is carried out at an energy density of between 0.3 and 0.7 kW s / mL, generating a solid lignocellulosic fraction (SLC) and a multifunctional supernatant (P1). - Solid-liquid separation obtaining a solid lignocellulosic fraction (SLC) and a multifunctional supernatant (P1), where the multifunctional supernatant (P1) is composed of, at least, a minimum concentration of 235 mg of Galacturonic Acid / g dry extract and antioxidant compounds with a minimum antioxidant capacity of 18 mg eq. Gallic Acid / g extract and 47 mg eq. Gallic Acid / g extract, measured by FRAP and ABTS, respectively. - Synthesis of selenium-doped carbon quantum dots (SeCQDs) by microwave irradiation at an energy density between 0.072 - 0.720 kW s / mL of a solution formed by the multifunctional supernatant (P1) and a selenium precursor with a concentration between 1 and 4 g / L, obtaining an intermediate solution. - Hydrothermal treatment of the intermediate solution at a temperature lower than 60 °C, and a pressure between 50 and 120 mbar, obtaining SeCQDs with a size between 10 and 50 nm presenting a GAP band of 3.1 eV, a stability between -3 and -52 mV, a minimum antioxidant capacity, measured by ABTS and FRAP, of 22.6 and 40.2 mg eq. Trolox / g SeCQDs, respectively, and luminescent character, where the solid lignocellulosic fraction (SLC) is stable against temperatures greater than 200 °C and where the intermediate solution obtained after the synthesis of SeCQDs has an antioxidant capacity between 10-30 mg Trolox / g extract, the yield of the synthesis reaction being greater than 90%.

2. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized in that in the grinding stage the cocoa husk is ground to a size between 0.5 and 2 mm.

3. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized in that the acid solvent is an organic or inorganic acid, such as HCl, H2SO4, citric acid, acetic acid and / or malic acid.

4. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized in that the microwave-assisted extraction is carried out for at least 20 minutes working under agitation and atmospheric pressure.

5. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized in that microwave extraction takes place over a time interval of between 35-100 minutes.

6. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized by the fact that the microwave extraction yield is between 30 and 40% compared to the initial raw material.

7. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a, characterized in that a portion of the multifunctional supernatant (P1) obtained from the solid-liquid separation is contacted with alcohol at a temperature lower than 25 °C, generating the precipitation of an insoluble fraction (P2) - rich in pectins -, and a fraction (P3); where the precipitated insoluble fraction (P2) is stable against temperatures greater than 200 °C and is recovered by means of a second solid-liquid separation at room temperature, whose extraction yield is greater than 45% with respect to the multifunctional supernatant (P1).

8. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 7 a , characterized in that the second solid-liquid separation is carried out by centrifugation or filtration.

9. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 7 a , characterized in that the fraction (P3) - rich in antioxidant compounds and pigments - is concentrated by evaporation to recover the alcohol.

10. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 7 a , characterized in that the insoluble fraction (P2) is subjected to a drying process, by atomization, nebulization, lyophilization or oven.

11. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 9 a, characterized in that the fraction (P3) is subjected to a drying process, by atomization, nebulization, lyophilization or oven.

12. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized in that the selenium precursor is a compound such as selenium oxide, trioxoselenic acid and / or sodium selenite.

13. Procedure for obtaining carbon quantum dots doped with selenium and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized in that the synthesis of selenium-doped carbon quantum dots (SeCQDs) by microwave irradiation is carried out over a period of between 5 and 50 minutes.

14. Selenium-doped carbon quantum dots obtained according to any of the preceding claims, characterized in that they have a size between 10 and 50 nm, a GAP band of 3.1 eV, a stability between -3 and -52 mV, a minimum antioxidant capacity, measured by ABTS and FRAP, of 22.6 and 40.2 mg eq. Trolox / g SeCQDs, respectively, and luminescent character.

15. Selenium-doped carbon quantum dots, according to claim 14 a , characterized by having a luminescent character after being irradiated with a wavelength of 365 nm.

16. Lignocellulosic solid fraction obtained in the solid-liquid separation of the process, according to claim 1 a , for use as a heavy metal adsorbent when brought into contact with compounds containing heavy metals, such as wastewater.

17. Lignocellulosic solid fraction obtained in the solid-liquid separation of the process, according to claim 1 a , for use as a source of soluble and insoluble fiber with a concentration greater than 40%.

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