Nitrogen-doped graphene acid, method for preparing same, and use thereof

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 graphene acid, nitrogen-doped graphene acid dots, a method for preparing the same, and their use in heavy metal separation and heavy metal sensing.

[0002] [Background Art] Access to clean water is one of the major sustainable development goals of the United Nations and is essential for drinking, 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 developed a hetero-bifunctional nanographene fluorescent beacon with pockets having a high affinity for heavy metals, achieving selective adsorption ratios of 870 mg·g -1 and 450 mg·g -1 for cadmium and lead, respectively. This hetero-bifunctional and multi-dentate coordination pocket also functions as a selective gate for fluorescence signal control with a low binding affinity comparable to antigen-antibody interactions (0.1 nM for Pb 2+ and 0.2 nM for Cd 2+ , both 0.02 ppb). Importantly, the acid-resistant nanographene can be completely regenerated and reused. Due to its broad visible light absorption, a new method for water quality monitoring based on extremely low-cost and user-friendly reagent-free paper detection with detection limits of 1 ppb and 10 ppb for Pb 2+ and Cd 2+ ions by the naked eye is provided. This study shows that photoactive nanomaterials functionalized densely with strong and selective ligands for targeted contaminants can successfully combine features such as excellent adsorption, reusability, and detection ability in a way that extends the value commensurate with the applicability, life cycle, and cost of the material.

[0003] The increasing global impact of anthropogenic activities on ecosystems has led to the accumulation of toxic heavy metals in the upper crust, making the assurance of the quality of drinking and sanitation water (major United Nations sustainable development goals) an ever-greater challenge. As a result, intensive research has been promoted towards the development of advanced sorbents integrating high-value functions such as recyclability, catalytic activity after metal sorption, or use for water quality monitoring of the same material. The combination of water decontamination and user-friendly water quality monitoring is particularly attractive as these two applications are highly complementary both institutionally and operationally. A powerful sorbent requires a material that can attract the target analyte to its surface with high affinity and significantly increase the local concentration of that analyte. This can have a major impact on sensing if the sorbent simultaneously provides an analyte-dependent signal. However, high selectivity towards harmful metals is often an issue (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 the combination of sorption and sensing of the target analyte 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] [Summary of the Invention] The present invention provides nitrogen-doped graphene 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 a sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα X-ray source.

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

[0006] The 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 in the five stronger bands in the infrared spectrum determined by the FT-IR spectroscopy method of total reflection measurement (ATR). The 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 using a sample dispersed in deionized water at room temperature.

[0007] The 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, out of all the carbon, about 10 at.% to 30 at.% corresponds to the carbon of the carboxyl, about 5 at.% to 20 at.% corresponds to the carbon of the carbonyl, and about 5 at.% to 20 at.% corresponds to the C-O carbon.

[0008] The 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 graphene acid in the form of nitrogen-doped graphene acid dots (NGA-D). The nitrogen-doped graphene acid dots are small particles having a diameter of 1 to 5 nm, preferably 2 to 3 nm (for example, a floc size of 1 to 5 nm and an average thickness of 3 nm).

[0010] The particle diameter (including the maximum diameter of NGA) is determined by a transmission electron microscope (TEM), and its thickness is determined by an atomic force microscope (AFM).

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

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

[0013] Nitrogen-doped graphene is known, for example, from 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) Supplying a dispersion of fluorinated graphite; b) Subjecting the dispersion of fluorinated graphite to sonication and / or mechanical treatment and / or heat treatment; c) Contacting the product from step b) with an 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, dialyzing the product against water.

[0016] The term "fluorinated graphite" includes fluorographite, graphite fluoride, fluorinated graphite, and exfoliated 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 fluorinated graphite is typically at least 40 at.%, more preferably at least 45 at.% or at least 50 at.% with respect to 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" means graphene in which N atoms (nitrogen atoms) are incorporated into the graphene lattice. This term includes monolayer graphene, as well as materials containing monolayer graphene in a mixture with a portion (e.g., flake) or particles containing a plurality of graphene layers. However, this term also includes graphene in which a small proportion (e.g., up to 10% or up to 5% at most) 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. This term also includes graphene in which a small amount of fluorine is present (up to 16.6 at.% at most; preferably less than 5 at.%).

[0018] Mechanical treatment preferably includes at least one treatment selected from high-shear mixing, stirring, intense 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 treatment 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 non-polar solvent. The solvent is preferably selected from dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), glycols such as ethylene glycol, and mixtures thereof. In combination with a polar organic solvent (e.g., DMF, NMP, DMSO, DMA), a less polar or non-polar solvent such as acetonitrile, benzene, toluene, or chlorobenzene may be used.

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

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

[0023] The solvent is preferably a polar solvent. The solvent can preferably be selected from dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), glycols such as ethylene glycol, and mixtures thereof. A less polar or non-polar 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 as the solvent used to prepare the dispersion of fluorinated graphene prepared in step b).

[0024] The azide reagent can preferably be selected from metal azides, tri(C1-C4)alkylsilyl azides. More preferably, the azide reagent is NaN 3 , KN 3 , LiN 3 , Pb(N 3 ) 2 , trimethylsilyl azide.

[0025] After contacting the product containing the fluorinated graphene of 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, even more preferably 100°C to 140°C. The heating is preferably for at least 4 hours, preferably 4 hours to 20 days, even more preferably at least 8 hours, 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 step of isolating the product (nitrogen-doped graphene) can be carried out by known techniques such as centrifugation, sedimentation, or filtration.

[0027] Specifically, for the experiments and assays conducted by the present inventors, nitrogen-doped graphene was prepared as follows: 0.5 g of fluorographite was dispersed in 30 ml of DMF in a Teflon vial and sonicated for more than 24 hours. Next, 3 g of NaN 3 was added to the aforementioned mixture, transferred to a spherical flask, and stirred and heated at 130 °C for 3 days while using a condenser in a hood. After the reaction was completed, 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 is preferably carried out using concentrated nitric acid (i.e., 65% (v / v) - 98% (v / v) aqueous nitric acid), more preferably 65% (v / v) - 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, 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, 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 obtained nitrogen-doped graphene acid can be isolated by any of the known techniques such as centrifugation, sedimentation, or filtration.

[0031] The washing step is carried out to purify the product. Washing can be carried out using hot 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 graphene acid into dots, the nitrogen-doped graphene acid obtained from the oxidation step and the washing step 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, more preferably 90 °C to 100 °C. The heating is preferably carried out for at least 4 hours, preferably 4 hours to 5 days, even more preferably at least 8 hours, even more preferably at least 24 hours, and even more preferably at least 2 days (48 hours). This step is also referred to herein as "hydrothermal treatment".

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

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

[0035] The present invention further relates to the use of nitrogen-doped graphene acid or nitrogen-doped graphene acid dots for the sequestration of Pb 2+ and / or Cd 2+ from water. The water source can be, for example, river water, drinking water, wastewater. Within the framework of the present invention, the selectivity and affinity of nitrogen-doped graphene acid or nitrogen-doped graphene acid dots are excellent and have been found to be higher than those of any other sorbent except AuNP. However, nitrogen-doped graphene acid or nitrogen-doped graphene acid dots are superior to AuNP because they can be recycled and reused. Recycling is carried out 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 a method for the sequestration (i.e., adsorption removal, purification) of Pb 2+ and / or Cd 2+ from water, wherein Pb 2+and / or Cd 2+ including a method comprising a step of contacting water to be purified with the nitrogen-doped graphene acid of the present invention. In some embodiments, the method comprises washing the nitrogen-doped graphene acid after the contacting step with an inorganic acid, preferably hydrochloric acid or hydrobromic acid, and contacting the nitrogen-doped graphene acid after the contacting step with a new batch of water to be purified to recycle the nitrogen-doped graphene acid after the contacting step. 2+ and / or Cd 2+ The method further includes a step of recycling the nitrogen-doped graphene acid after the contacting step by contacting it with a new batch of water to be purified.

[0037] In another aspect, the present invention relates to the use of nitrogen-doped graphene acid or nitrogen-doped graphene acid dots for detecting Pb 2+ and / or Cd 2+ The detection can be performed by a lateral flow paper sensor or by measuring the photoluminescence of nitrogen-doped graphene acid dots. The photoluminescence is excited at 400 nm to 500 nm, preferably 467 ± 2 nm, and the emission at 520 nm to 530 nm is recorded. The absorption / binding of Pb 2+ and / or Cd 2+ quenches this emission.

[0038] The prepared nitrogen-doped graphene acid shows top-class selective adsorption for cadmium and lead, reaching 870 mg g -1 and 450 mg g -1 respectively. Furthermore, the nitrogen-doped graphene acid maintains an adsorption capacity level of more than 90% for both Pb 2+ and Cd 2+ for at least 6 regeneration cycles. The same material also functions as a selective gate for fluorescence signal regulation with sub-nanomolar sensitivities (0.1 nM and 0.2 nM for Pb 2+ and Cd 2+ respectively; both 0.02 ppb).

[0039] The process enabling these properties is simple and effective, using economically viable starting compounds. In particular, the method of the present invention is the only wet process chemistry method capable of achieving such nitrogen doping and especially high oxidation. Furthermore, the method is the only way to achieve high nitrogen doping and high oxygen content at relatively low reaction temperatures.

[0040] Nitrogen-doped graphene acid has balanced mixture parameters that enable it to be used as a heavy metal sorbent and as a detection probe without the typical drawbacks of materials known in the art. In particular, the unprecedented detection ability of lead and cadmium by photoluminescence quenching, combined with the ability to absorb ions from solution, results in a material that exhibits these properties superior to any N- and O-containing graphene-based material to date. As described in the literature, the highest values obtained for materials showing both adsorption ability and heavy metal detection ability were an adsorption capacity (for Pb 2+ and Cd 2+ of about 522 mg g -1 ; and 220 mg g -1 ), respectively) and detection limits of 3.0 nM and 11.6 nM for Pb 2+ and Cd 2+ respectively.

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

[0042] [Example of the present invention] material and method Adsorption experiments. Apply the batch method, with a volume of 4 mL and 1-75 mg L of sorbent. -1 Therefore, the volume to mass ratio of the sorbent in this experiment, V:m, is approximately 13 L g -1 and the initial [Me n+ The feed rates ranged from 1 ppm to 100 ppm. The pH of the solutions was adjusted with 1% v / v HCl and 1% w / v NaOH solutions. The samples were shaken on a rotary shaker for 60 min and then filtered through a 200 nm syringe filter (Whatman, mixed cellulose esters). Residual metal concentrations were measured by the AAS method. All experiments were performed in triplicate. 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;NH 4 + <0.05;HCO 3 - 102;NO 3 - 6.3;SO 4 2- 13.2;NO 2 - <0.005;F - <0.2;Cl - 3.05;The composition of the river water is as follows;Mg 2+ 6.5; Ca2+ 37.3; Na + 9.7; K + 3.2。

[0043] The maximum adsorption capacity q m max (mg g -1 ) was calculated by the following equation:

Equation

[0044] where V is the volume of the sample (L), C 0 is the initial concentration of the metal (mg L -1 ), C e is the equilibrium concentration (mg L -1 ), and m is the mass of the sorbent used (g).

[0045] Another adsorption experiment was conducted to obtain information on the behavior of ions in solutions at different pH values without using the graphene material (NGA). Solutions of a given metal salt (Pb 2+ or Cd 2+ ) with an exact concentration (metal supply of 100 ppm) were intentionally adjusted to different pH values in the range of 3 - 8 (with 2% NaOH and 2% HCl). Subsequently, the effect of pH on the adsorption capacity of the thus - prepared metal salt solutions was investigated. The given solutions at different pH values were filtered using a syringe filter with a pore size of 200 nm, and then the filtrate was subjected to AAS measurement to obtain information on the metal concentration captured by the filter and the metal concentration passing through the filter, respectively.

[0046] Adsorption kinetics The batch method was applied, with a volume of 4 mL and a sorbent of 1 - 75 mg L -1 used. Thus, the volume - to - mass ratio of the sorbent in this experiment was approximately 13 L g -1 in the V:m approximation, and the initial [Me n+The supply amount was 1 ppm to 100 ppm. The pH of the solution was adjusted with 1% v / v HCl and 1% w / v NaOH solutions. The samples were shaken on a rotary shaker for the following different times: 0.5 minute, 1 minute, 5 minutes, 10 minutes, 15 minutes, 30 minutes, and 60 minutes. Then, they were filtered through a 200 nm syringe filter (Whatman, mixed cellulose ester). The residual metal concentration was measured using the AAS method.

[0047] Metal desorption. In the experiment on the regeneration and reusability of the sorbent, the weight ratio of NGA:Me (NGA to metal) was maintained at 10:1. The process included 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 the reusability test. All samples for characterization were prepared under the same conditions with a weight ratio of 75 mg L -1 of NGA and 1 mg L -1 of Me n+ by weight ratio.

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

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

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

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

Equation

[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 value and the standard deviation of the measured individual metal concentrations.

[0053] For the selectivity experiment of metal ions, mineral drinking water and spiked tap water were used. The bottled mineral drinking water had the following composition; (mg L -1 ): Mg 2+ 6.98; Ca 2+ 26.3; Na + 0.967; K + 2.08; Fe <0.002; NH 4+ <0.05;HCO 3 - 102;NO 3 - 6.3;SO 4 2- 13.2;NO 2 - <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;NH 4 + < 0.05;HCO 3 - 102;NO 3 - 16.3;NO 2 - <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) and the binding energies of Pb 2+ , Cd 2+ (at four binding sites) for relatively large models 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 a 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 structure optimization, frequency analysis, and evaluation of the binding energy. The calculations were performed using Gaussian16 (Revision B.01) [1]and D3 dispersion force correction function [3] with B3LYP [2] was performed. For main group elements (C, O, N, H), def2-SVP [4,5] basis set was used, and for cations, LANL2DZ [6] effective core potential was used. All calculations were carried out in an aqueous medium, and the density-based implicit solvation model (SMD) [7] water model was adopted. Natural bond orbital (NBO) analysis [8] was also performed with Gaussian16 (Gaussian NBO version 3.1) to examine all possible interactions between donor Lewis-type NBOs and acceptor non-Lewis-type NBOs, and the evaluation of E (2) energy by second-order perturbation theory [9] , and the calculation of Wiberg bond indices

[10] were carried out. All structures were relaxed to the minima of their structures as verified by frequency analysis.

[0055] Adsorption isotherm To determine the adsorption isotherm that describes the adsorption of Pb 2+ and Cd 2+ ions onto the NGA material, sorption experiments were conducted by dispersing 300 μg of the NGA material in 4 mL of a solution containing relevant heavy metal nitrates with metal weight concentrations in the range of 5 ppm to 150 ppm. After the samples were shaken on a rotary shaker at room temperature (25 °C) for 60 minutes, they were filtered through a 200 nm syringe filter (Whatman, mixed cellulose ester). The residual metal concentration was measured by the AAS method. Then, the equilibrium adsorption capacity of each system was calculated by the following equation.

Equation

[0056] where V is the volume of the sample (L), C 0 is the initial concentration of the metal (mg L -1 ), C eq is the equilibrium concentration of the non-adsorbed metal (mg L -1 ), and m is the mass of the sorbent used (g).

[0057] Machine The materials (NG, NGA, and NGA-D) were characterized by a transmission electron microscope (TEM) using a JEM 2010 TEM instrument (Jeol, Japan).

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

[0059] Steady-state and time-resolved PL measurements were performed using a FLS980 fluorescence spectrometer (Edinburgh Instruments) equipped with a 450 W xenon arc lamp and an EPL-375 ps pulsed diode laser (λ em = 372 nm, pulse width 66.5 ps, repetition rate 10 MHz, and average output 75 (μW)) as the excitation source. The PL decay curves were fitted using a stretched exponential function:

Equation

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

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

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

[0063]

Equation

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

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

[0066] The spiked tap water and river water were analyzed by a 7500ce inductively coupled plasma mass spectrometry (ICP-MS) instrument (Agilent).

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

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

[0069] High-resolution X-ray photoelectron spectroscopy (HR-XPS) was performed using Al-K αPerformed using a line source (15 kV, 50 W) with a PHI VersaProbe II (Physical Electronics, Japan) spectrometer. The obtained data was evaluated using the MultiPak (Ulvac-PHI, Inc.) software package and deconvolution was performed. 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 the C–C bond at 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, droplets of an aqueous dispersion of the material were deposited on a carbon-coated copper grid under a test at a concentration of about 0.1 mg mL -1 and dried at room temperature for 24 h.

[0071] EPR spectra were collected using a JEOL JES-X-320 spectrometer operating at X-band frequency, equipped with an X-band Gunn oscillator bridge, a mode cylindrical cavity, and a temperature variable control device ES 13060DVT5 N 2 cryostat. In all experiments, the Q quality factor of the cavity was maintained above 6000. High-purity quartz tubes (Suprasil, Wilmad, ≤0.5 OD) were used and the accuracy of the g-value was Mn IIObtained relative to the / MgO standard (JEOL standard). The 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, a sweep time of 2 minutes, and 5 accumulations were used to improve the signal-to-noise ratio. In all experiments, the EPR tube was filled with 100 μl of a solution containing NGA-D or NGA (0.5 mg mL -1 ), and 100 μl of a solution of Pb 2+ , Cd 2+ or Ca 2+ (1 μm). In the CW-LEPR experiment, a HeCd laser (200 mW) line source operating at 325 nm was used and coupled to the EPR cavity resonator by an optical fiber.

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

[0073] Preparation of nitrogen-doped graphene: In a glass spherical 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 NaN 3 was dissolved in 20 ml of DMF and added to the graphite fluoride and / or few-layer fluorographene dispersion. The mixture was heated at 130 °C for 72 hours in a hood equipped with a condenser 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 centrifuging at 15000 rcf for about 10 minutes. The supernatant was discarded, and the tube was refilled with the following washing solvents. The sample was homogenized by shaking for at least 1 minute to redisperse the precipitate in the fresh solvent. Washing was performed with the following different solvents: DMF (3x), acetone (3x), ethanol (3x), hot ethanol (1x), distilled water (3x), and hot distilled water (1x), and then refilled with distilled water again.

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

[0075] Preparation of nitrogen-doped graphene acid dots: The nitrogen-doped graphene acid was filtered through a 200 nm syringe filter (Whatman, mixed cellulose ester), and then the filtrate was transferred to a Teflon-coated autoclave and heated in an oven at 90 °C for 72 h. After the reaction, the sample was filtered through a 5 nm ultracentrifugal filter (10 kDa cut-off membrane) and purified by dialysis (dialysis tubing, benzoyl-treated, 2 kDa cut-off) until the rise above about 10 μS / cm ceased and the conductivity inside the dialysis bag reached about 5 μS / cm.

[0076] X-ray photoelectron spectroscopy analysis of nitrogen-doped graphene (Figure 1a) showed that as a result of the reaction with NaN 3 N atoms were introduced into the product, reaching 11.6 at.% after 72 h of the reaction, and the fluorine atoms decreased significantly from 50.5 at.% to 2.2 at.% (Table 1).

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

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

[0079] By Fourier transform infrared spectroscopy (FT-IR), the spectra of nitrogen-doped graphene (Figure 2, NG) showed that two bands between 1560 cm -1 and 1000 - 1210 cm -1 were dominant. Both of these bands corresponded to the skeletal vibrations of the sp2 aromatic carbon network and the aromatic rings. The shape of 1395 cm -1 indicated the vibration of heteroatom substitution in the aromatic ring, such as the pyridine ring. Consistent with XPS, from the spectra of nitrogen-doped graphene acid NGA and nitrogen-doped graphene dots NGA-D, a characteristic stretching band of the carbonyl group was detected at 1720 cm -1 (Figure 2, NGA, NGA-D). The broad band at 1230 cm -1 had a changed pattern compared to NG and also included the C-O stretching mode derived from carboxylic acid groups. Ionization of the carboxylic acid groups to carboxylates resulted in additional vibrations at 1600 cm -1 and 1420 / 1350 cm -1 , corresponding to the asymmetric and symmetric stretching of -CO2-, respectively. The band at 1140 cm -1 in NGA reflected the presence of the C-OH group, which significantly decreased as oxidation progressed in NGA-D, corroborating the XPS results. The aromatic ring vibration at 1560 cm -1 was present in both NGA and NGA-D and was consistent with the sp2 carbon component observed by XPS. The new band at 1650 cm -1 in NGA-D was due to a small amount of organic nitrate groups (R-O-NO2).

[0080]

Table 1

[0081]

Table 2

[0082]

Table 3

[0083]

Table 4

[0084]

Table 5

[0085] Properties of NGA and NGA-D: Nitrogen-doped graphene acid is virtually carboxyl-functionalized nitrogen-doped graphene. Oxidation by oxidative acids and size confinement lead to the synthesis of fluorescent and carboxyl-functionalized nitrogen-doped graphene acid (NGA). NGA is a very efficient and selective trap for Pb 2+ ions and Cd 2+ ions, and has been proven to promote excellent water purification in both spiked and real samples in the presence of competing ions (Table 6). After hydrothermal treatment of NGA to form 2 - 3 nm NGA dots (NGA-D) with further restricted lateral dimension to improve photoluminescence (PL) properties, fast, reagent-free, selective detection of Pb 2+ and Cd 2+ was achieved by PL quenching. The obtained LoD (Pb 2+ and Cd 2+(0.1 nM and 0.2 nM, respectively) were unprecedented and exceeded even many sensors using state-of-the-art schemes based on fluorescence and potentiometry, and aptamers and DNA strands for sensitivity improvement (Table 7). The high sensitivity of NGA-D is due to the very strong binding of Pb 2+ and Cd 2+ within the multivalent coordination pockets formed by nitrogen-doped vacancies and carboxyl groups, and a direct binding mechanism involving the association constant (10 -6 ~ 10 -9 L M -1 ) of the order of antigen-antibody interactions, and the formation of dark complexes with the ground state of NGA dots is involved. Importantly, the acid-resistant structure of NGA enables its complete regeneration and reuse as an adsorbent. On the other hand, its broad visible light absorption provides two additional sensing modes based on naked-eye detection on paper without reagents, offering a very 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 had several layers of graphene flakes with a size of about 600 nm. The NGA flakes were reduced to about 50 nm by oxidative cleavage due to the action of concentrated nitric acid and tended 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, and the lattice spacing was 0.21 nm corresponding to the lattice fringes of graphene {1100}. After measuring the lattice spacing from more dots, only these fringes were observed, and it was confirmed that the presence of single-layer or few-layer graphene dots lying flat on the lattice was dominant rather than carbon dots with a multilayer structure having an interlayer spacing of about 3.4 nm. The abundance of single-layer graphene dots was further confirmed by selected area diffraction obtained from many dots, and the {1100} diffraction peak was much brighter than the {2110} diffraction peak. Considering that the AFM thickness of non-functionalized single-layer graphene is up to 2.6 nm in atomic force microscopy (AFM), the average thickness corresponding to a single-layer sheet was 3 nm.

[0091] In the PL measurement at room temperature of NG, NGA, and NGA-D dispersed in water, PL of NG was not observed, but oxidized NGA showed a PL maximum at 523 nm under excitation at 470 nm (Figure 3). The appearance of PL was due to sp 3 small sp isolated by carbon 2It is considered to be due to the quantum confinement effect caused by the formation of islands and the formation of a PL active surface state as a whole. As a result of heat-treating NGA, in the NGA-D product, PL was enhanced 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, and the amount of the quantum confinement region further increased. The increase in PL intensity is also thought to be due to the improved quality of the dispersion of the NGA-D product compared to the NGA system because interparticle interactions can cause PL quenching. The excitation-emission map of NGA-D shows excitation-independent luminescence, suggesting that PL originates from dots with very similar bandgap states due to its monodispersity. Together with a small Stokes shift of about 50 nm, these observations suggest that there is no interlayer interaction that usually leads to excimer formation and energy transfer (causing a large Stokes shift and excitation-dependent PL, respectively) in carbon dots. The absorption spectrum of NGA-D shows a perturbation between 400 and 500 nm, with a maximum at 460 nm as verified by the differential curve, which is in good agreement with the excitation maximum of 467 ± 2 nm. It was also found that the bandgap value of NGA-D calculated from the Tauc plot, 2.8 eV (i.e., 440 nm), is consistent with the characteristics of the spectrum. The PL emission peak of NGA-D under excitation at 467 nm was centered at 527 nm (FWHM 119 nm), and the PL lifetime was 1.1 ns as obtained from the fitting of the PL decay by the stretched exponential function.

Brief Description of the Drawings

[0092]

Figure 1

Figure 2

Figure 3

Figure 4

Claims

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

2. The nitrogen-doped graphene acid according to claim 1, present in a sample and containing, for all atoms determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα line source, 4 at.% to 6 at.% nitrogen and 30 at.% to 38 at.% oxygen.

3. 1690 cm －１ and 1750 cm －１ infrared bands between, and 1180 cm －１ and 1250 cm －１ show infrared bands between, and these bands are included among the five strongest bands in the infrared spectrum determined by the FT-IR spectral method of the total reflection measurement method (ATR); using a sample dispersed in deionized water, showing photoluminescence having a peak between 475 nm and 600 nm when excited at 470 nm, determined by a fluorescence spectrometer at room temperature, the nitrogen-doped graphene acid according to claim 1 or 2.

4. The nitrogen-doped graphene acid according to any one of claims 1 to 3, in the form of particles with a maximum diameter of at most 500 nm, more preferably at most 200 nm, and even more preferably at most 100 nm.

5. The nitrogen-doped graphene acid according to any one of claims 1 to 3, in the form of nitrogen-doped graphene acid dots with a diameter of 1 to 5 nm, preferably 2 to 3 nm.

6. A method for preparing the nitrogen-doped graphene acid according to any one of claims 1 to 5, the method comprising the following steps: - Supplying nitrogen-doped graphene, - Oxidizing the nitrogen-doped graphene by reaction with an oxidizing inorganic acid, preferably nitric acid, - Washing the resulting mixture with water.

7. The method according to claim 6, 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, preferably 70 °C to 170 °C, and the heating is carried out for at least 4 hours, preferably at least 24 hours.

8. The method according to claim 6 or 7, further comprising a subsequent step of hydrothermally treating the nitrogen-doped graphene acid in an autoclave by heating at a temperature in the range of 40 °C to 150 °C, preferably 70 °C to 120 °C, for at least 4 hours, preferably at least 24 hours.

9. The method according to any one of claims 6 to 8, subjecting the nitrogen-doped graphene acid after the washing step and / or the nitrogen-doped graphene acid dots after the hydrothermal treatment step to a step of dialysis against water.

10. Pb from water ２＋ and / or Cd ２＋ Use of the nitrogen-doped graphene acid according to any one of claims 1 to 5 for the isolation of

11. The use according to claim 10, wherein the water is selected from river water, drinking water, and wastewater.

12. Pb from water ２＋ and / or Cd ２＋ is a method for separating, Pb ２＋ and / or Cd ２＋ A method comprising the step of contacting water to be purified of with the nitrogen-doped graphene acid according to any one of claims 1 to 5.

13. Washing the nitrogen-doped graphene acid after the contacting step with an inorganic acid, preferably hydrochloric acid or hydrobromic acid, and recycling the nitrogen-doped graphene acid after the contacting step by contacting it with a fresh batch of water to be purified of Pb ２＋ and / or Cd ２＋ The method according to claim 12, further comprising the step of recycling the nitrogen-doped graphene acid after the contacting step by doing so.

14. Use of the nitrogen-doped graphene acid dots according to claim 5 for detecting Pb ２＋ and / or Cd ２＋ by photoluminescence.

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