Nitrogen-doped graphenic acid, its preparation method and use

Abstract

The present invention provides a nitrogen-doped graphene acid that is present in a sample and contains 3 at.% to 10 at.% (preferably 4 at.% to 6 at.%) of nitrogen and 25 at.% to 45 at.% (preferably 30 to 38 at.%) of oxygen for all atoms determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα X-ray source. In some embodiments, the nitrogen-doped graphene acid is in the form of dots. The nitrogen-doped graphene acid is produced by a method comprising the following steps: supplying nitrogen-doped graphene, oxidizing the nitrogen-doped graphene by reaction with an oxidizing inorganic acid, preferably nitric acid, and washing the resulting mixture with water. The nitrogen-doped graphene acid is for the isolation of Pb 2+ and / or Cd 2+ , or for the detection of Pb 2+ and / or Cd 2+ and is particularly useful.

Description

Detailed description of the invention

[0001] [Technical field] The present invention relates to nitrogen-doped graphenic acid, nitrogen-doped graphenic acid dots, methods for preparing the same, and their use in heavy metal sequestration and heavy metal sensing.

[0002] [Background technology] Access to clean water is one of the United Nations' primary Sustainable Development Goals and is essential for drinking water, sanitation, and food security. Therefore, technologies for water purification and water quality monitoring must be widely available and low-cost without sacrificing effectiveness, selectivity, sustainability, and environmentally friendly characteristics. To address this challenge, the inventors have developed a heterobifunctional nanographene fluorescent label (beacon) with a pocket that has high affinity for heavy metals, exhibiting high efficacy against cadmium and lead, respectively, at 870 mg / g. -1 and 450 mg.g -1 This achieved highly selective adsorption. This heterobifunctional, multidentate pocket also exhibits sub-nanomolecular sensitivity (Pb, respectively) due to its low binding affinity, comparable to that of antigen-antibody interactions. 2+ is 0.1nM, Cd 2+ It also functioned as a selective gate for controlling the fluorescence signal (0.2 nM; both 0.02 ppb). Importantly, acid-resistant nanographene is completely regenerative and reusable. Due to its broad visible light absorption, Pb 2+ and Cd 2+ A novel method of water quality monitoring based on reagent-free paper detection is provided, offering extremely low-cost, user-friendly, and naked-eye detection of ions at detection limits of 1 ppb and 10 ppb, respectively. This research demonstrates that photoactive nanomaterials densely functionalized with highly potent and selective ligands for targeted pollutants can successfully combine features such as excellent adsorption, reusability, and detection capabilities in a way that extends the applicability, lifecycle, and cost-effectiveness of the material.

[0003] The increasing global impact of anthropogenic activities on ecosystems is leading to the accumulation of toxic heavy metals in the upper crust, making it increasingly difficult to ensure the quality of drinking and sanitation water (a key UN Sustainable Development Goal). This has driven intensive research into the development of advanced adsorbents that integrate high-value-added functions, such as recyclability, catalytic activity after metal sorption, or the use of the material for water quality monitoring. The combination of water decontamination and user-friendly water quality monitoring is particularly attractive because these two applications are highly complementary both mechanically and practically. Powerful adsorbents require materials that can attract the target analyte to its highly affinity surface, thereby significantly increasing its concentration locally. This can have a significant impact on sensing if the adsorbent also provides an analyte-dependent signal. However, high selectivity for harmful metals is often a point of contention (Gogoi et al. ACS Appl. Mater. Interfaces 2015, 7, 3058; Wang et al. Sens. Actuators B Chem. 2015, 207, 25; Shi et al. ACS Appl. Mater. Interfaces 2014, 6, 2568), while combining sorption of the target analyte with sensing is particularly difficult (Pournara et al. J. Mater. Chem. A 2019, 7, 15432; Ding et al. J. Am. Chem. Soc. 2016, 138, 3031; Zhang et al. ACS Cent. Sci. 2018, 4, 1697).

[0004] [Overview of the prefecture] The present invention provides nitrogen-doped graphenic acid (NGA) containing 3 at.% to 10 at.% (preferably 4 at.% to 6 at.%) of nitrogen and 25 at.% to 45 at.% (preferably 30 at.% to 38 at.%) of oxygen for all atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα source.

[0005] Typically, at least a minimum residual amount of fluorine, from about 0.1 at.% to about 1.3 at.%, is present for all atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα source.

[0006] Nitrogen-doped graphene acid has infrared bands between 1690 cm -1 and 1750 cm -1 and infrared bands between 1180 cm -1 and 1250 cm -1 These bands are included among the five stronger bands in the infrared spectrum determined by the FT-IR spectroscopy method of total reflection measurement (ATR). Nitrogen-doped graphene acid further shows photoluminescence having peaks between 475 nm and 600 nm when excited at 470 nm, determined by a fluorescence spectrometer at room temperature using a sample dispersed in deionized water.

[0007] Nitrogen-doped graphene acid contains nitrogen atoms, carboxylic groups, carbonyl (C=O) groups, and C-O groups. In measurements by X-ray photoelectron spectroscopy (XPS) using an Al-Kα source, among all the carbon, about 10 at.% - 30 at.% corresponds to the carbon of carboxyl, about 5 at.% - 20 at.% corresponds to the carbon of carbonyl, and about 5 at.% - 20 at.% corresponds to C-O carbon.

[0008] Nitrogen-doped graphene acid is preferably in the form of particles with a maximum diameter of up to 500 nm, more preferably up to 200 nm, and even more preferably up to 100 nm. The particles are typically single-layered or few-layered sheets.

[0009] In some embodiments, the present invention provides nitrogen-doped graphenic acid in the form of nitrogen-doped graphenic acid dots (NGA-D). The nitrogen-doped graphenic acid dots are small particles having a diameter of 1 to 5 nm, preferably 2 to 3 nm (e.g., flocs of 1 to 5 nm in size and an average thickness of 3 nm).

[0010] The particle size (including the maximum diameter of the NGA) was determined by transmission electron microscopy (TEM), and its thickness was determined by atomic force microscopy (AFM).

[0011] The present invention further provides a method for preparing nitrogen-doped graphenic acid, the method comprising the following steps: - Supplying nitrogen-doped graphene, - The nitrogen-doped graphene is oxidized by reaction with an oxidizing inorganic acid, preferably nitric acid. - Wash the resulting mixture with water.

[0012] The preparation of nitrogen-doped graphenic acid dots further includes a step of heating nitrogen-doped graphenic acid at 80°C to 100°C for at least 24 hours, preferably at least 48 hours, and more preferably at least 70 hours (hydrothermal treatment).

[0013] Nitrogen-doped graphene is known, for example, from publication WO2021 / 223783, in which it is manufactured as a supercapacitor material.

[0014] Nitrogen-doped graphene can be prepared by the following procedure (according to WO2021 / 223783):

[0015] a) Supply a dispersion of fluorinated graphite; b) Subjecting a dispersion of fluorinated graphite to sonication and / or mechanical treatment and / or heat treatment; c) Contact the product from step b) with the azide reagent at a temperature of 40°C to 200°C; d) Separating the solid product (nitrogen-doped graphene) formed in step c) from the mixture; e) Optionally, dialyze the product against water.

[0016] The term "fluorinated graphite" includes fluorographite, fluorinated graphite, and flaked forms of these materials. Fluorinated graphite is also available under the names poly(carbon monofluoride), carbon monofluoride, or poly(carbon fluoride). The initial fluorine content in the starting material, fluorinated graphite, is typically at least 40 at.%, more preferably at least 45 at.%, or at least 50 at.%, for all atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα source.

[0017] The term "nitrogen-doped graphene" refers to graphene in which nitrogen atoms (N atoms) are incorporated into the graphene lattice. This term encompasses both monolayer graphene and materials containing monolayer graphene in a mixture with multiple graphene layers (e.g., flakes) or particles. However, the term also includes graphene in which only a small percentage (e.g., up to 10% or up to 5%) of nitrogen atoms are bonded to carbon atoms as out-of-plane substituents (e.g., amino groups), i.e., graphene in which nitrogen atoms are not incorporated into the graphene lattice. The term also includes graphene in which small amounts of fluorine are present (up to 16.6 at.%; preferably less than 5 at.%).

[0018] The mechanical treatment preferably includes at least one treatment selected from high-shear mixing, stirring, vigorous stirring, stirring with a magnetic bar, and stirring with a mechanical stirrer.

[0019] The heat treatment preferably includes heating the dispersion in step b) to a temperature in the range of 50°C to 250°C, or 80°C to 200°C, more preferably 100°C to 150°C. It may also include processing in a solvothermal reactor at a pressure higher than normal atmospheric pressure.

[0020] The dispersion prepared in step a) is a dispersion of fluorinated graphite in a solvent. The solvent is preferably a polar solvent or a mixture of a polar solvent and a nonpolar solvent. The solvent can preferably be selected from glycols such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), and ethylene glycol, and mixtures thereof. A less polar or nonpolar solvent such as acetonitrile, benzene, toluene, or chlorobenzene may be used in combination with a polar organic solvent (e.g., DMF, NMP, DMSO, DMA).

[0021] A mixture containing fluorinated graphene and / or exfoliated fluorinated graphite particles is obtained by ultrasonic treatment and / or mechanical treatment and / or heat treatment steps. Ultrasonic treatment is typically performed at a frequency range of 20 kHz to 100 kHz for at least 2 hours, more preferably at least 3 hours, and even more preferably at least 4 hours. Heat treatment is typically performed at a temperature range of 40°C to 200°C for at least 1 hour, more preferably at least 6 hours, more preferably at least 24 hours, and even more preferably at least 80 hours. Mechanical treatment is most typically performed by high-shear mixing or magnetic bar stirring.

[0022] The azide reagent is preferably added to the reaction solvent in the form of a powder or a suspension in the solvent.

[0023] The solvent is preferably a polar solvent. The solvent can preferably be selected from glycols such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), ethylene glycol, and mixtures thereof. A less polar or nonpolar solvent such as acetonitrile, benzene, toluene, or chlorobenzene may be used in combination with a polar organic solvent (e.g., DMF, NMP, DMSO, DMA). In a particularly preferred embodiment, the solvent is the same solvent used to prepare the dispersion of fluorinated graphene prepared in step b).

[0024] The azide reagent can preferably be selected from metal azides and tri(C1-C4) alkylsilyl azides. More preferably, the azide reagent can be selected from NaN3, KN3, LiN3, Pb(N3)2, and trimethylsilyl azides.

[0025] After contacting the product containing fluorinated graphene from step b) with the azide reagent, the mixture is typically heated to a temperature in the range of 40°C to 200°C, preferably 70°C to 170°C, and more preferably 100°C to 140°C. The heating is preferably carried out for at least 4 hours, preferably 4 hours to 20 days, more preferably at least 8 hours, even more preferably at least 24 hours, and even more preferably at least 2 days (48 hours) or at least 3 days (72 hours). The longer the heating period, the higher the nitrogen doping.

[0026] The process of isolating the product (nitrogen-doped graphene) can be carried out by known techniques such as centrifugation, sedimentation, or filtration.

[0027] Specifically, nitrogen-doped graphene was prepared for experiments and assays conducted by the inventors as follows: 0.5 g of fluorographite was dispersed in 30 ml of DMF in a Teflon vial and sonicated for 24 hours. Next, 3 g of NaN3 was added to the above mixture, transferred to a spherical flask, and stirred and heated at 130°C for 3 days using a condenser in a hood. After the reaction was complete, the sample was washed with DMF (3x), acetone (3x), ethanol (3x), distilled water (3x), and hot distilled water (2x) using centrifugation (14000 rcf) in a Falcon.

[0028] The step of oxidizing nitrogen-doped graphene by reaction with an oxidizing inorganic acid can preferably be carried out using concentrated nitric acid (i.e., 65% (v / v) to 98% (v / v) aqueous nitric acid), more preferably 65% ​​(v / v) to 70% (v / v) aqueous nitric acid. The reaction is preferably carried out at a temperature in the range of 40°C to 200°C, preferably 70°C to 170°C, and even more preferably 100°C to 140°C. The heating is preferably carried out for at least 4 hours, preferably 4 hours to 20 days, more preferably at least 8 hours, even more preferably at least 24 hours, and even more preferably at least 2 days (48 hours).

[0029] Nitric acid is preferably added to nitrogen-doped graphene in the form of a liquid dispersion.

[0030] The resulting nitrogen-doped graphenic acid can be isolated by any known technique, such as centrifugation, sedimentation, or filtration.

[0031] The washing process is carried out to purify the product. Washing can be performed using high-temperature distilled water (at least 70°C) and distilled water. The purity can be further increased by dialysis of the product against water.

[0032] To convert nitrogen-doped graphenic acid into dots, the nitrogen-doped graphenic acid obtained from the oxidation and washing steps is placed in an autoclave and heated to a temperature typically in the range of 40°C to 150°C, preferably 70°C to 120°C, and more preferably 90°C to 100°C. The heating is preferably carried out for at least 4 hours, preferably 4 hours to 5 days, more preferably at least 8 hours, more preferably at least 24 hours, and more preferably at least 2 days (48 hours). This step is also referred to herein as “hydrothermal treatment”.

[0033] The process of isolating the product (nitrogen-doped graphenic acid dots) can preferably be carried out by dialysis with water or by filtration through an ultracentrifuge filter of at least 5 nm.

[0034] In a preferred embodiment, nitrogen-doped graphenic acid after the washing step and / or nitrogen-doped graphenic acid dots after hydrothermal treatment are subjected to a dialysis step with water.

[0035] The present invention further relates to Pb from water. 2+ and / or Cd 2+ This invention relates to the use of nitrogen-doped graphenate or nitrogen-doped graphenate dots for sequestration. The water source may be, for example, river water, drinking water, or wastewater. Within the framework of this invention, the selectivity and affinity of nitrogen-doped graphenate or nitrogen-doped graphenate dots have been found to be superior to any other sorbent except AuNP. However, nitrogen-doped graphenate or nitrogen-doped graphenate dots are superior to AuNP because they can be recycled and reused. Recycling is performed by desorbing the metal from the surface using a strong acid solution, such as hydrochloric acid, preferably a 1-5% v / v aqueous HCl solution.

[0036] The present invention further relates to Pb from water. 2+ and / or Cd 2+ A method of isolation (i.e., adsorption removal, purification) of Pb 2+and / or Cd 2+ The method includes the step of contacting water to be purified with the nitrogen-doped graphenic acid of the present invention. In some embodiments, the method includes washing the nitrogen-doped graphenic acid after the contact step with an inorganic acid, preferably hydrochloric acid or hydrobromic acid, and removing the nitrogen-doped graphenic acid after the contact step from Pb 2+ and / or Cd 2+ The process further includes a step of recycling the nitrogen-doped graphenic acid after the contact step by bringing it into contact with a new batch of water to be purified.

[0037] In another embodiment, the present invention relates to Pb 2+ and / or Cd 2+ This relates to the use of nitrogen-doped graphenate or nitrogen-doped graphenate dots for detection. Detection can be performed by a lateral flow paper sensor or by measuring the photoluminescence of nitrogen-doped graphenate dots. Photoluminescence is excited at 400 nm to 500 nm, preferably 467 ± 2 nm, and emission is recorded at 520 nm to 530 nm. 2+ and / or Cd 2+ Absorption / coupling quenches this luminescence.

[0038] The prepared nitrogen-doped graphenic acid exhibits top-class selective adsorption to cadmium and lead, at 870 mg g each. -1 and 450 mg g -1 It reaches [a certain level]. Furthermore, nitrogen-doped graphenic acid is Pb 2+ and Cd 2+ For both, it maintains an adsorption capacity of over 90% for at least six regeneration cycles. The same material has sub-nanomole sensitivity (Pb 2+ and Cd 2+ These also function as selective gates for adjusting the fluorescence signal (0.1 nM and 0.2 nM, respectively; both 0.02 ppb).

[0039] The process that makes these properties achievable is simple, effective, and uses economically viable starting compounds. In particular, the method of the present invention is the only wet process chemistry method that can achieve such nitrogen doping and especially high oxidation. Furthermore, this method is the only method that can achieve high nitrogen doping and high oxygen content at relatively low reaction temperatures.

[0040] Nitrogen-doped graphene acids possess well-balanced mixed parameters that enable their use as heavy metal adsorbents and detection probes without the typical drawbacks of materials known in the art. In particular, their unprecedented ability to detect lead and cadmium by photoluminescence quenching, combined with their ability to absorb ions from solution, results in a material exhibiting superior properties compared to any N and O-containing graphene-based material to date. As described in the literature, the highest values ​​obtained for materials exhibiting both adsorption capacity and heavy metal detection capacity are in adsorption capacity (Pb 2+ and Cd 2+ Each of these is approximately 522 mg g -1 ; and 220 mg g -1 ) and Pb 2+ and Cd 2+ The detection limits for each were 3.0 nM and 11.6 nM, respectively.

[0041] [Brief description of the drawing] Figure 1. a) Starting nitrogen-doped graphene (NG); b) Nitrogen-doped graphenic acid (NGA); c) X-ray photoelectron spectra of nitrogen-doped graphenic acid after hydrothermal treatment (dot, NGA-D). Figure 2. Infrared spectra of a) starting nitrogen-doped graphene, b) nitrogen-doped graphenic acid, and c) nitrogen-doped graphenic acid after hydrothermal treatment. Figure 3.a) Comparison of PL spectra of NG, NGA, and NGA-D (λex=470nm, 0.5mg mL) -1b) UV-Vis absorption spectra of NG, NGA, and NGA-D, and excitation (λex=467nm) and PL (λem=527nm) spectra of NGA-D. c) Time-resolved PL decay (λem=527nm) and corresponding extensional exponential function fit for derivation of PL lifetime. Figure 4. Transmission electron microscope (TEM) images of a) nitrogen-doped graphene, b) nitrogen-doped graphenic acid, and c) nitrogen-doped graphenic acid dots. d) Histogram of particle size distribution.

[0042] [Examples of the present invention] material and method Adsorption experiment. Apply the batch method, distributing 1-75 mg of sorbent in a volume of 4 mL. -1 We used [a specific method]. Therefore, the ratio of volume to mass of the sorbent in this experiment is 13L g using the V:m approximation. -1 And initially [Me n+ The supply concentration was 1 ppm to 100 ppm. The pH of the solution was adjusted with 1% v / v HCl and 1% w / v NaOH solution. The sample was shaken in a rotary shaker for 60 minutes and then filtered through a 200 nm syringe filter (Whatman, mixed cellulose ester). The residual metal concentration was measured by the AAS method. All experiments were performed three times. Mineral drinking water and river water were used for the metal ion selectivity experiments, and the composition of the mineral drinking water was as follows (mg L). -1 ): Mg 2+ 6.98;Ca 2+ 26.3; Na + 0.967;K + 2.08; Fe <0.002; NH4 + <0.05; HCO3 - 102; NO3 - 6.3; SO4 2- 13.2; NO2 - <0.005;F - <0.2;Cl - 3.05; Furthermore, the composition of river water is as follows: Mg 2+ 6.5; Ca 2+ 37.3; Na + 9.7;K + 3.2.

[0043] Maximum adsorption capacity q m max(mg g -1 ) was calculated using the following formula:

number

[0044] Here, V is the volume of the sample (L), and C0 is the initial concentration of the metal (mg L). -1 ), C e This is the equilibrium concentration (mg L) -1 ), and m are the mass (g) of the sorbent used.

[0045] Another adsorption experiment was conducted to obtain information on the behavior of ions in solution at different pH levels, without using graphene material (NGA). A specific metal salt (Pb) was used at precise concentrations (metal supply amount: 100 ppm). 2+ or Cd 2+ Solutions of ) were intentionally adjusted to have different pH values ​​in the range of 3 to 8 (2% NaOH and 2% HCl). Subsequently, the effect of pH on the sorption capacity of the metal salt solutions thus prepared was investigated. The predetermined solutions with different pH values ​​were filtered using a syringe filter with a pore size of 200 nm, and the filtrates were then subjected to AAS measurement to obtain information on the metal concentration captured by the filter and the metal concentration that passed through the filter, respectively.

[0046] Sorption kinetics Apply the batch method, distributing 1-75 mg of sorbent in a volume of 4 mL. -1 We used [a specific method]. Therefore, the ratio of volume to mass of the sorbent in this experiment is approximately V:m, which is 13L g -1 And initially [Me n+The supply concentration ranged from 1 ppm to 100 ppm. The pH of the solution was adjusted with 1% v / v HCl and 1% w / v NaOH solution. The sample was shaken in a rotary shaker for the following different times: 0.5 min, 1 min, 5 min, 10 min, 15 min, 30 min, and 60 min. It was then filtered through a 200 nm syringe filter (Whatman, mixed cellulose ester). The residual metal concentration was measured using the AAS method.

[0047] Metal removal. In experiments on the regeneration and reusability of the sorbent, the weight ratio NGA:Me (NGA to metal) was maintained at 10:1. The process involved three steps: (i) washing the sorbent with 4 mL of water (3 times) to examine the elution of metal from the NGA surface; (ii) desorbing the metal from the NGA surface with 4 mL of 2% v / v HCl solution (3 times); and (iii) washing with distilled water or 1% w / v NaOH and neutralizing the NGA with 4 mL of water (3 times) for reusability testing. All samples for characterization were 75 mg L -1 NGA, and 1 mg L -1 Me n+ It was prepared under the same conditions based on the weight ratio of the components.

[0048] Paper-based detection. Sensor preparation: Chromatography paper (Whatman) was used as the substrate for lateral flow of NGA material (NGA filtered through a 200 nm cellulose filter). The paper was cut to a size of 8.5 cm x 2 cm, and in the next step, a barrier for NGA flow was applied using a wax printer (Xerox, ColorQube 8580). After the experiment, the channel width for proper NGA flow was set to 2 mm. After applying the wax ink, the sensor was placed in a preheated oven at 90°C for 2 minutes to allow the wax to penetrate the paper.

[0049] Preparation for detection: This process involved four steps: (I) Paper sensor, metal (Pb 2+ or Cd 2+(1) The sample was immersed in a solution containing ) and then dried in the air. This process was repeated three times to increase the detection limit. (2) A total volume of 1.5 μL of NGA (13.5 μg) was placed on the paper sensor by drop casting in 0.5 μL volumes three times. (3) A total volume of 1.5 μL of this metal solution sample was applied to the NGA spot by drop casting in 0.5 μL volumes three times and left for 1 hour. (4) The prepared sensor was placed in a container of metal solution (0.5 mL) for 30 minutes, then dried in the air, and the path distance of the NGA was measured. To increase the detection limit, 50 mL of metal solution was evaporated on a 60°C water bath for 2 hours to a final volume of 0.5 mL, and then the same procedure (I-IV) was applied.

[0050] PL detection: For this detection purpose, 0.5 mg mL -1 NGA-D material of the specified concentration was used. 1.8 mL of NGA-D was placed in a quartz cuvette, and then 0.2 mL of a metal solution of a concentration predetermined by AAS was added. The solution thus prepared was reacted for 5 minutes, and then the PL was measured to determine the contamination of the solution by PL quenching.

[0051] The limit of detection (LoD) was calculated using the following formula:

number

[0052] Here, SD is the standard deviation of the blank (NGA-D), and S is the slope of the linear fit from the F0-F values ​​and the standard deviation of the individual metal concentrations measured.

[0053] For the metal ion selectivity experiment, mineral water and spiked tap water were used, and the composition of the bottled mineral water was as follows: (mg L -1 ): Mg 2+ 6.98;Ca 2+ 26.3; Na + 0.967;K + 2.08; Fe <0.002; NH4 +< 0.05; HCO3 - 102; NO3 - 6.3; SO4 2- 13.2; NO2 - <0.005; F - <0.2; Cl - 3.05. The tap water in the region had the following composition; Mg 2+ 10.3; Ca 2+ 91.7; Na + 0.767; K + 1.9; Fe <0.1; NH4 + < 0.05; HCO3 - 102; NO3 - 16.3; NO2 - <0.007; F - <0.19; Cl - 0.09.

[0054] Details of the calculation The strength of the interaction between divalent cations and NGA-D was also evaluated by theoretical calculations. In this regard, the binding energies of Pb 2+ , Cd 2+ , and Ca 2+ (at three binding sites) with a relatively small model of NGA-D and the binding energies of Pb 2+ , Cd 2+ (at four binding sites) with a relatively large model of NGA-D were calculated. The binding energy corresponds to the energy difference between the complex and the fragments (NGA-D and the ion), where the fragments have the shape of the complex. All models were modeled in the protonated state representing the low-pH condition, and one binding site was also modeled with a relatively small NGA-D model having all deprotonated carboxylic acid groups representing the behavior in a basic environment. Density functional theory (DFT) was used for all calculations, including structural optimization, frequency analysis, and evaluation of the binding energy. The calculations were performed with Gaussian16 (revision B.01) [1] and the D3 dispersion force correction function [3] with B3LYP [2] for main group elements (C, O, N, H), the def2-SVP [4,5] basis set was used, and for cations, LANL2DZ[6] The effective core potential was used. All calculations were performed in an aqueous medium, and a density-based ansolvation model (SMD) was employed. [7] A water model was adopted. Natural bond orbital (NBO) analysis. [8] Furthermore, Gaussian16 (Gaussian NBO version 3.1) examines all expected interactions between the donor Lewis-type NBO and the acceptor non-Lewis-type NBO, and E is analyzed using second-order perturbation theory. (2) Energy evaluation [9] , and calculation of Wiberg combined exponents

[10] This was done as follows: All structures were relaxed to their minimum values, as verified by frequency analysis.

[0055] Adsorption isotherm Pb in NGA material 2+ and Cd 2+ To determine the adsorption isotherms that explain ion adsorption, sorbation experiments were performed by dispersing 300 μg of NGA material in 4 mL of a solution containing relevant heavy metal nitrates with metal weight concentrations ranging from 5 ppm to 150 ppm. The samples were shaken in a rotary shaker at room temperature (25°C) for 60 minutes and then filtered through a 200 nm syringe filter (Whatman, mixed cellulose ester). The residual metal concentration was measured by the AAS method. Subsequently, the equilibrium adsorption capacity of each system was calculated using the following formula.

number

[0056] Here, V is the volume of the sample (L), and C0 is the initial concentration of the metal (mg L). -1 ), C eq This is the equilibrium concentration of the non-adsorbed metal (mg L). -1 ) where m is the mass (g) of the sorbent used.

[0057] device. The materials (NG, NGA, and NGA-D) were characterized by transmission electron microscopy (TEM) using the JEM 2010 TEM apparatus (Jeol, Japan).

[0058] The UV-Vis absorption spectra of all materials were obtained using a Specord S600 spectrometer in dilute colloidal suspensions (0.5 mg mL). -1 Measurements were taken inside (Analytik Jena, Germany).

[0059] Steady-state and time-resolved PL measurements were performed using a 450W xenon arc lamp and an EPL-375ps pulsed diode laser (λ) as excitation sources. em The analysis was performed using an FLS980 fluorescence spectrometer (Edinburgh Instruments) equipped with a pulse width of 66.5 ps, a repetition rate of 10 MHz, and an average output of 75 μW. The PL decay curve was fitted using an exponential extension function:

number

[0060] Here, the fit parameters τ and β are the PL decay time and extension parameter, respectively.

[0061] The band gap of NGA-D was calculated using Tauc's formula. A Lambda 1050 UV / Vis / NIR spectrophotometer (PerkinElmer) was used.

[0062] The kinetics of PL extinction were evaluated using a function based on bimolecular theory, with a Stern-Volmer plot of F0 / F versus metal concentration:

[0063]

number

[0064] Here, K sv And Q represent the Stern-Volmer extinction constant and metal concentration, respectively. qis the bimolecular extinction rate constant, and τ is the average lifetime of NGA-D without any quenching agent. F and F0 are the fluorescence intensities of NGA-D under 467 nm excitation, with and without metal, respectively.

[0065] The concentrations of heavy metal dispersions were measured using atomic absorption spectroscopy (AAS) on a ContrAA600 (Analytik Jena AG, Germany) equipped with a graphite furnace, a high-resolution Eschelle double monochromator (spectral bandwidth, 2 pm at 200 nm), and a xenon lamp as a continuous radiation source. For AAS measurements, the heavy metal dispersions were added to a nitric acid (2% w / w) solution and sonicated for 10 minutes to quantitatively dissolve all ions.

[0066] Spiked tap water and river water were analyzed using a 7500ce inductively coupled plasma mass spectrometer (ICP-MS) (Agilent).

[0067] FTIR spectra were recorded using an iS5 FTIR spectrometer (Thermo Nicolet) with the Smart Orbit ZnSe ATR accessory. Briefly, droplets of an ethanol dispersion of the relevant material were placed on a ZnSe crystal and dried. The spectrum was then acquired by summing 52 scans using a nitrogen gas stream through the ATR accessory. ATR and baseline corrections were applied to the recovered spectra.

[0068] Raman spectra were recorded using a DXR Raman microscope with a 633 nm excitation line from a diode laser. For measurement, 1.5 mg of the material was diluted in 3 mL of distilled water.

[0069] High-resolution X-ray photoelectron spectroscopy (HR-XPS) αSpectroscopic analysis was performed using a PHI VersaProbe II (Physical Electronics, Japan) spectrometer with a radiation source (15kV, 50W). The obtained data were evaluated and deconvolution was performed using the MultiPak (Ulvac-PHI, Inc.) software package. In the spectral analysis process, Shirley background subtraction and peak deconvolution using a mixed Gaussian-Lorentzian function were performed. All binding energies are referenced to a CC bond of 284.8 eV.

[0070] HR-TEM images were acquired using an HR-TEM TITAN 60-300 microscope equipped with an X-FEG type emission gun operating at 300 kV. Scanning transmission electron microscope high-angle annular dark-field imaging (STEM-HAADF) analysis for EDS (energy-dispersive X-ray spectroscopy) elemental mapping of the product was performed using an FEI Titan HR-TEM microscope operating at 80 kV. For this analysis, approximately 0.1 mg mL -1 Under testing of the concentration, droplets of an aqueous dispersion of the material were deposited onto a carbon-coated copper grid and dried at room temperature for 24 hours.

[0071] EPR spectra were collected using a JEOL JES-X-320 spectrometer operating at X-band frequencies, equipped with an X-band Gunn oscillator bridge, a mode cylindrical cavity, and a temperature-variable control device ES 13060DVT5 N2 cryostat. In all experiments, the cavity's Q quality factor was maintained above 6000. High-purity quartz tubes (Suprasil, Wilmad, ≤0.5 OD) were used, and the g-value accuracy was Mn IIEPR spectra were obtained relative to the MgO standard (JEOL standard). EPR spectra were measured with the following parameters: microwave frequency = 9.088 GHz, microwave power = 1.0 mW, modulation width = 0.35 mT, modulation frequency = 100 Hz, and temperature T = 80 K. All spectra were recorded with a time constant of 30 ms and a sweep time of 2 minutes, and five accumulations were used to improve the signal-to-noise ratio. In all experiments, the EPR tube was filled with NGA-D or NGA (0.5 mg mL). -1 100 μl of solution containing ) and Pb 2+ , Cd 2+ or Ca 2+ The solution (1 μm) was loaded. In the CW-LEPR experiment, a HeCd laser (200 mW) source operating at 325 nm was used and coupled to the EPR cavity resonator via an optical fiber.

[0072] Synthesis of nitrogen-doped graphene and nitrogen-doped graphene dots

[0073] Preparation of nitrogen-doped graphene: In a spherical glass flask, 1 g of graphite fluoride was dispersed in 40 ml of DMF. The flask was covered and stirred for 2 days. Then, it was sonicated for 4 hours and stirred overnight. In a glass beaker, 2 g of NaN3 was dissolved in 20 ml of DMF and added to the graphite fluoride and / or fluorographene dispersion with fewer layers. The mixture was heated at 130°C for 72 hours in a condenser-equipped hood while stirring with a Teflon-coated magnetic bar. After heating, the reaction mixture was allowed to cool and transferred to a 50 ml Falcon centrifuge tube. The solid particles (product) were separated from the solvent and by-products by centrifugation at 15000 rcf for approximately 10 minutes. The supernatant was discarded and the tube was replenished with the next washing solvent. The sample was homogenized by shaking for at least 1 minute to redisperse the precipitate in the new solvent. The following washings were performed with different solvents: DMF (3x), acetone (3x), ethanol (3x), hot ethanol (1x), distilled water (3x), and hot distilled water (1x), after which the container was refilled with distilled water.

[0074] Preparation of nitrogen-doped graphenic acid: The previously prepared N-doped graphene derivative was treated with 65% v / v nitric acid in a glass flask with a condenser at 100°C for 24 hours. After the reaction was complete, the product was purified by washing with 3x hot distilled water and 5x distilled water in 15 ml of Falcon. Finally, the dispersed solid was placed in a dialysis bag (molecular weight cutoff 10 kDa) until the conductivity of the surrounding water stopped rising above approximately 10 μS / cm and the conductivity in the dialysis bag became approximately 5 μS / cm. This dispersion was finally removed from the dialysis bag and stored or dried for further use.

[0075] Preparation of nitrogen-doped graphenic acid dots: Nitrogen-doped graphenic acid was filtered through a 200 nm syringe filter (Whatman, mixed cellulose ester), and the filtrate was then transferred to a Teflon-coated autoclave and heated in an oven at 90°C for 72 hours. After the reaction was complete, the sample was filtered through a 5 nm ultracentrifugal filter (10 kDa cutoff membrane) and purified by dialysis (dialysis tube, benzoyl treatment, 2 kDa cutoff) until the conductivity in the dialysis bag stopped rising above approximately 10 μS / cm and the conductivity was approximately 5 μS / cm.

[0076] X-ray photoelectron spectroscopy analysis of nitrogen-doped graphene (Figure 1a) showed that the reaction with NaN3 resulted in the introduction of nitrogen atoms into the product, reaching 11.6 at.% after 72 hours of reaction, while the fluorine content decreased significantly from 50.5 at.% to 2.2 at.% (Table 1).

[0077] X-ray photoelectron spectroscopy analysis of nitrogen-doped graphenic acid (Figure 1b) showed that the proportion of nitrogen atoms in the product decreased as a result of the reaction with HNO3, reaching 5.2 at.% after 24 hours of reaction, while the oxygen atoms increased significantly from 3 at.% to 32.7 at.% (Table 2).

[0078] X-ray photoelectron spectroscopy analysis of nitrogen-doped graphenic acid dots (Figure 1c) showed that the proportion of nitrogen atoms in the product further decreased as a result of the reaction in the autoclave, reaching 4.9 at.% after 72 hours of reaction, while the oxygen atoms increased from 32.7 at.% to 36.6 at.% (Table 3).

[0079] Fourier transform infrared spectroscopy (FT-IR) revealed that the spectrum of nitrogen-doped graphene (Figure 2, NG) is at 1560 cm⁻¹. -1 and 1000~1210cm -1 Two bands between 1395 cm² were shown to be dominant. These two bands correspond to the skeletal vibrations of the sp2 aromatic carbon network and the aromatic ring. -1 The shape indicates heteroatom substitution of the aromatic ring, e.g., vibration of the pyridine ring. To match the XPS, the spectra of nitrogen-doped graphenate NGA and nitrogen-doped graphenate dot NGA-D show a range from carboxyl to 1720 cm⁻¹. -1 (Figure 2, NGA, NGA-D) shows characteristic stretching bands of the carbonyl group. 1230 cm -1 The broadband pattern is different compared to NG, and includes CO stretching modes derived from carboxylic acid groups. Ionization of carboxylic acid groups to carboxylates occurs at 1600 cm². -1 , and 1420 / 1350cm -1 As a result, additional vibrations occurred, which are considered asymmetric and symmetric stretching of -CO2-. 1140cm at NGA -1 The band reflects the presence of a C-OH group, which decreases significantly as oxidation progresses with NGA-D, supporting the XPS results. 1560cm² -1 Aromatic ring oscillations were present in both NGA and NGA-D and were consistent with the sp2 carbon component observed by XPS. -1 The new band is due to a small amount of organic nitrate group (RO-NO2).

[0080] [Table 1]

[0081] [Table 2]

[0082] [Table 3]

[0083] [Table 4]

[0084] [Table 5]

[0085] Characteristics of NGA and NGA-D: Nitrogen-doped graphenate is, in effect, carboxylated nitrogen-doped graphene. It is synthesized by oxidation with oxidizing acids and size confinement, leading to fluorescent, carboxylated nitrogen-doped graphenate (NGA). NGA is Pb 2+ Ions and Cd 2+ It was proven to be a highly efficient and selective trap for ions, promoting outstanding water purification in both simulated and real samples in the presence of competing ions (Table 6). NGA was hydrothermally treated to form 2-3 nm NGA dots (NGA-D), further restricting the lateral dimension to improve photoluminescence (PL) properties, and then Pb 2+ and Cd 2+ High-speed, re-reagent-free, and selective detection was achieved by PL quenching. The obtained LoD(Pb 2+ and Cd 2+The high sensitivity of NGA-D (0.1 nM and 0.2 nM respectively) was unprecedented and surpassed even many sensors using state-of-the-art schemes based on fluorescence and potentiometric methods, as well as aptamers and DNA strands for improved sensitivity (Table 7). The high sensitivity of NGA-D is due to the presence of Pb within the multivalent coordination pocket formed by nitrogen-doped vacancies and carboxyl groups. 2+ and Cd 2+ This is because they are bound very strongly, and the association constant (10) is on the order of the antigen-antibody interaction. -6 ~10 -9 LM -1 ), and a direct binding mechanism is involved, including the formation of a dark complex with the ground state of the NGA dot. Importantly, the acid-resistant structure of NGA allows for its complete regeneration and reuse as an sorbent. On the other hand, its broad visible light absorption provides an additional sensing mode for two types of hazardous metals, based on naked-eye, reagent-free detection on paper, offering an extremely low-cost and user-friendly solution for water quality monitoring.

[0086] [Table 6]

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[0090] The morphology and microstructure of NG, NGA, and NGA-D were analyzed by transmission electron microscopy (TEM, Figure 4). NG contained several layers of graphene flakes approximately 600 nm in size. The NGA flakes were reduced to about 50 nm by oxidative sclerature with concentrated nitric acid and showed a tendency to interact with each other. After hydrothermal treatment at 90°C, very small graphene dots (NGA-D) with a diameter in the range of 2-3 nm were formed, with a lattice spacing of 0.21 nm, corresponding to the lattice fringes of graphene {1100}. After measuring the lattice spacing from many more dots, only these fringes were observed, confirming that the presence of single-layer or multi-layer graphene dots placed flat on the lattice was dominant, rather than multilayer carbon dots with an interlayer spacing of approximately 3.4 nm. The abundance of single-layer graphene dots was further confirmed by selective region diffraction obtained from many dots, with the {1100} diffraction peak being much brighter than the {2110} diffraction peak. Considering that the AFM thickness of unfunctionalized monolayer graphene is up to 2.6 nm, the average thickness equivalent to a monolayer sheet was 3 nm.

[0091] PL measurements of NG, NGA, and NGA-D dispersed in water at room temperature showed no PL for NG, but oxidized NGA showed a PL maximum at 523 nm under excitation at 470 nm (Figure 3). The appearance of PL is due to sp groups bonded to the oxygen functional group. 3 Tiny sp. isolated by carbon 2The formation of islands and the overall formation of a PL-active surface state are thought to be due to a quantum confinement effect. Heat treatment of NGA resulted in enhanced PL in the NGA-D product while maintaining the same photophysical properties. This is thought to be because the lateral diameter of the NGA sheet decreased significantly in the form of discrete dots, further increasing the amount of quantum confinement regions. The increase in PL intensity is also thought to be due to improved dispersion quality in the NGA-D product compared to the NGA system, as interparticle interactions may cause PL quenching. The excitation-emission map of NGA-D shows excitation-independent emission, suggesting that the PL, due to its monodispersity, originates from dots with very similar bandgap states. Along with a small Stokes shift of approximately 50 nm, these observations suggest the absence of interlayer interactions in carbon dots that would normally lead to excimer formation and energy transfer (causing a large Stokes shift and excitation-dependent PL, respectively). The absorption spectrum of NGA-D showed perturbation between 400 and 500 nm, with a maximum at 460 nm, as verified by the differential curve, which closely matched the excitation maximum of 467 ± 2 nm. The band gap value of NGA-D, calculated from the Tauc plot, of 2.8 eV (i.e., 440 nm), also matched the spectral characteristics. Under excitation at 467 nm, the PL emission peak of NGA-D was centered at 527 nm (FWHM 119 nm), and the PL lifetime was 1.1 ns, as obtained from fitting the PL decay using the extensional exponential function. [Brief explanation of the drawing]

[0092] [Figure 1] a) Starting nitrogen-doped graphene (NG); b) Nitrogen-doped graphenic acid (NGA); c) X-ray photoelectron spectra of nitrogen-doped graphenic acid (dot, NGA-D) after hydrothermal treatment. [Figure 2] a) Infrared spectra of starting nitrogen-doped graphene, b) nitrogen-doped graphenic acid, and c) nitrogen-doped graphenic acid after hydrothermal treatment. [Figure 3]a) Comparison of PL spectra of NG, NGA, and NGA-D (λex=470nm, 0.5mg mL-1). b) UV-Vis absorption spectra of NG, NGA, and NGA-D, and excitation (λex=467nm) and PL (λem=527nm) spectra of NGA-D. c) Time-resolved PL decay (λem=527nm) and corresponding extensional exponential function fit for derivation of PL lifetime. [Figure 4] a) Transmission electron microscope (TEM) images of nitrogen-doped graphene, b) nitrogen-doped graphenic acid, and c) nitrogen-doped graphenic acid dots. d) Histogram of particle size distribution.

Claims

1. Nitrogen-doped graphenic acid containing 3 at.% to 10 at.% nitrogen, 25 at.% to 45 at.% oxygen, and 0.1 at.% to 1.3 at.% fluorine for all atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα source.

2. The nitrogen-doped graphenic acid according to claim 1, wherein the sample contains 4 at.% to 6 at.% nitrogen and 30 at.% to 38 at.% oxygen relative to all atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα source.

3. 1690cm －１ and 1750cm －１ Infrared band between, and 1180 cm －１ and 1250 cm －１ The nitrogen-doped graphenic acid according to claim 1 or 2, exhibiting infrared bands between 1690 cm⁻¹ and 1750 cm⁻¹, and infrared bands between 1180 cm⁻¹ and 1250 cm⁻¹, which are among the five strongest bands in the infrared spectrum determined by the FT-IR spectroscopy method of total internal reflection measurement; and exhibiting photoluminescence having peaks between 475 nm and 600 nm when excited at 470 nm, as determined by a fluorescence spectrometer at room temperature using a sample dispersed in deionized water.

4. The nitrogen-doped graphene acid according to claim 1, wherein the particle morphology is such that the maximum diameter is up to 500 nm, as determined by transmission electron microscopy.

5. The nitrogen-doped graphenic acid according to claim 1, wherein the particle morphology is such that the maximum diameter is up to 200 nm, as determined by a transmission electron microscope.

6. The nitrogen-doped graphenic acid according to claim 1, wherein the particle morphology is such that the maximum diameter is up to 100 nm, as determined by a transmission electron microscope.

7. The nitrogen-doped graphenic acid according to claim 1, wherein the morphology is nitrogen-doped graphenic acid dots of 1 to 5 nm in size, as determined by transmission electron microscopy.

8. A method for preparing nitrogen-doped graphenic acid according to claim 1, comprising the following steps: - Supply nitrogen-doped graphene, which is prepared using the following procedure: a) Supplying a dispersion of fluorinated graphite; b) Subjecting a dispersion of fluorinated graphite to ultrasonic treatment and / or mechanical treatment and / or heat treatment; c) Contact the product from step b) with an azide reagent at a temperature of 40°C to 200°C; d) Separating the solid nitrogen-doped graphene formed in step c) from the mixture. - Oxidizing the nitrogen-doped graphene by reaction with an oxidizing inorganic acid, - Wash the resulting mixture with water.

9. The method according to claim 8, wherein the oxidizing inorganic acid is nitric acid.

10. The method according to claim 8, wherein step d) is followed by step e): dialyzing the nitrogen-doped graphene with water.

11. The method according to claim 8, wherein the step of oxidizing the nitrogen-doped graphene by reaction with an oxidizing inorganic acid is carried out using concentrated nitric acid at a temperature in the range of 40°C to 200°C for at least 4 hours.

12. The method according to claim 8, further comprising a subsequent step of hydrothermally treating the nitrogen-doped graphenic acid in an autoclave by heating it to a temperature in the range of 40°C to 150°C for at least 4 hours.

13. The method according to claim 8, wherein the nitrogen-doped graphenic acid produced is subjected to a step of dialyzing with water.

14. Pb from water ２＋ and / or Cd ２＋ Use of nitrogen-doped graphenic acid according to claim 1 for isolation.

15. The use according to claim 14, wherein the water is selected from river water, drinking water, and wastewater.

16. Pb from water ２＋ and / or Cd ２＋ is a method for separating, Pb ２＋ and / or Cd ２＋ A method comprising a contacting step of contacting water to be purified of with the nitrogen-doped graphene acid according to claim 1.

17. Wash the nitrogen-doped graphenic acid after the contact step described in Claim 16 with an inorganic acid, and remove the nitrogen-doped graphenic acid from Pb ２＋ and / or Cd ２＋ The method according to claim 16, further comprising the step of recycling the nitrogen-doped graphenic acid by reusing it by bringing it into contact with a new batch of water to be purified.

18. The method according to claim 17, wherein the inorganic acid is hydrochloric acid or hydrobromic acid.

19. Photoluminescence of Pb ２＋ and / or Cd ２＋ Use of nitrogen-doped graphenic acid dots according to claim 7 for detection.

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