CN113214828B

CN113214828B - Graphene quantum dot loaded fluorescent adsorption material and preparation method thereof

Info

Publication number: CN113214828B
Application number: CN202110476055.XA
Authority: CN
Inventors: 杨帆; 张良; 钟吕玲
Original assignee: Xian University of Architecture and Technology
Current assignee: Xian University of Architecture and Technology
Priority date: 2021-04-29
Filing date: 2021-04-29
Publication date: 2023-03-10
Anticipated expiration: 2041-04-29
Also published as: CN113214828A

Abstract

The invention also discloses a graphene quantum dot-loaded fluorescent adsorption material and a preparation method thereof. The material has the potential of being applied to the field of heavy metal adsorption. The NGQDs-PVA composite membrane has excellent fluorescence and heavy metal adsorption performance, and emits blue fluorescence under the excitation wavelength of 360 nm. Compared with the traditional adsorbent, the adsorbent has higher selective adsorption effect on Ag (I) (qe =317.35mg/g, T =40 ℃) and obvious quenching generation of fluorescence intensity after adsorbing Ag (I).

Description

Graphene quantum dot loaded fluorescent adsorption material and preparation method thereof

Technical Field

The invention belongs to the technical field of sewage treatment, and relates to a graphene quantum dot loaded fluorescent adsorption material and a preparation method thereof.

Background

Heavy metal water pollution is a major concern in developed and developing countries today. Common human sources include agricultural activities, atmospheric sedimentation, road runoff, emissions from industrial plants and sewage treatment plants, acid mine wastewater, and reservoir construction. The removal of heavy metals is imperative because of their toxicity, non-biodegradability and bioaccumulation potential risks to human health and the environment. Adsorption is one of the most efficient and economical methods for removing heavy metals from aqueous solutions. In recent years, many types of adsorbents including activated carbon, chitosan, fly ash, zeolite, perlite, kaolin, activated alumina, grape straw waste, rice hulls, tea waste, various resins and microorganisms have been used to remove various heavy metal ions from aqueous solutions. However, most of such materials are prepared under certain conditions, such as high temperature, controlled pressure and increased operating costs or inefficiencies, poor mechanical strength and other chemicals that are difficult to separate from the reaction system. Therefore, there is a need to develop low cost, readily available, effective and reusable adsorbents to remove heavy metal ions from aqueous environments.

Over the past decade, quantum Dots (QDs) have received increasing attention in recent years due to their good optical properties including tunable band gap, narrow emission bandwidth and high efficiency, good photochemical stability, long fluorescence lifetime and good water solubility. Because the functional groups on the quantum dots are related to their precursor functional groups, they are easily modified. Hg is treated by Nuengmatcha et al ²⁺ A strong quenching effect was observed with addition to the GQDs solution due to Hg ²⁺ Binding to GQDs. Fluorescence quenching is a process that reduces the intensity of fluorescence emission. In recent years, it has been used to detect many functionally upper-limited luminescence QDs of heavy metal ions, such as CdS, cdSe and CdTe QDs, and also exhibit reactions to copper, silver and mercury ions. It is therefore possible to adsorb heavy metals with GQDs and to react the progress of adsorption of the material by its fluorescent properties. However, due to the water solubility of quantum dots, a matrix material is required to support them.

Due to the water solubility of quantum dots, a matrix material is required to support them thereon. Many groups of topics at home and abroad attempt to solve the load problem of QDs. Xue et al intercalate carbon dots into zinc-ketone microspheres to detect and remove dichromate anions from water by luminescence. Truskewycz et al add quantum dots to PVP/ZnO hydrogels for an effective hexavalent chromium sensing platform. Wang et al supported carbon dots on mesoporous silica for rapid and selective U (VI) removal and in situ monitoring of adsorption behavior. However, most of the base materials used are expensive or metal base materials, so they are not good base materials and are troublesome to synthesize.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides a graphene quantum dot loaded fluorescent adsorbing material and a preparation method thereof, so as to solve the problem of high cost of a quantum dot matrix material in the prior art.

In order to achieve the purpose, the invention adopts the following technical scheme to realize the purpose:

a preparation method of a fluorescent adsorbing material loaded with graphene quantum dots,

step 1, adjusting the pH value of a nitrogen-doped quantum dot solution to 7.0 to obtain a quantum dot solution after the pH value is adjusted, namely an NGQDs solution;

and 2, mixing the PVA solution and the NGQDs solution to obtain a mixed solution, adding a citric acid solution into the mixed solution, performing crosslinking reaction to obtain a product solution, drying the product solution, and curing to obtain the fluorescent adsorption material loaded with the graphene quantum dots.

The invention is further improved in that:

preferably, in step 1, the nitrogen-doped quantum dot solution is prepared by mixing citric acid and ammonia water, performing a hydrothermal reaction, and cooling to obtain the nitrogen-doped quantum dot solution.

Preferably, the mixing ratio of citric acid and ammonia water is 0.2g:0.3mL.

Preferably, the hydrothermal reaction temperature is 180 ℃ and the hydrothermal reaction time is 4h.

Preferably, in step 2, the PVA solution is prepared by adding 0.5-2g of PVA to 35mL of water to prepare a PVA solution.

Preferably, in step 2, the volume ratio of the PVA solution to the NGQDs solution is 35: (2-6.5).

Preferably, in step 2, the citric acid solution is prepared by dissolving 0.8-3g of citric acid in 35mL of water.

Preferably, in step 2, the crosslinking reaction time is 3h.

Preferably, in the step 2, the drying temperature of the product solution is 50 ℃, and the drying time is 12 hours; the curing temperature is 135 ℃, and the curing time is 5h.

The graphene quantum dot-loaded fluorescent adsorption material prepared by any one of the preparation methods comprises PVA colloid, wherein the graphene quantum dots are embedded in the PVA colloid.

Compared with the prior art, the invention has the following beneficial effects:

the invention discloses a preparation method of a graphene quantum dot-loaded fluorescent adsorption material, which is characterized in that after the pH value of a nitrogen-doped quantum dot solution is adjusted to be neutral, the nitrogen-doped quantum dot solution is suitable for heavy metal adsorption in the later period, and after the quantum dot solution and a PVA solution are mixed, quantum dots are embedded in the PVA material. Polyvinyl alcohol (PVA) is the most professional polymer due to the advantages of convenient preparation, atmospheric stability, low price, optical transparency and the like, and has unique performance and wide functional application prospect. All these valuable properties of PVA can be attributed to its skeletal structure, enabling it to form hydrogen bonds; thus, hydrophilicity predominates and the crosslinking ability increases with increasing dopant material.

Further, the use of GQDs made from citric acid as a precursor has more amino groups than amino-modified GQDs (NGQDs) made from citric acid and ammonia as precursors. Due to the small size and high surface area to volume ratio of nanoparticles, the photoluminescence of QDs is very sensitive to surface state modification. This is because changes in the QDs surface charge or ligand can affect the efficiency of electron-hole recombination, leading to significant changes in the amplitude of fluorescence emission in quenching or enhancement effects.

The invention also discloses a graphene quantum dot-loaded fluorescent adsorption material, and the cover material comprises PVA colloid, and the graphene quantum dots are embedded in the PVA colloid. The material has the potential of being applied to the field of heavy metal adsorption. The NGQDs-PVA composite membrane has excellent fluorescence and heavy metal adsorption performance, and emits blue fluorescence under the excitation wavelength of 360 nm. Compared with the traditional adsorbent, the adsorbent has higher selective adsorption effect on Ag (I) (qe =317.35mg/g, T =40 ℃) and obvious quenching generation of fluorescence intensity after adsorbing Ag (I). Researches find that the fluorescence response of the NGQDs-PVA composite membrane can be used for monitoring the adsorption behavior of the NGQDs-PVA composite membrane on metals, and the fluorescence quenching degree of the NGQDs-PVA composite membrane increases along with the increase of the adsorption quantity.

Drawings

FIG. 1 is a graph of the luminescence properties of NGQDs liquid prepared in example 1;

wherein, the picture (a) is a form picture of NGQDs under natural light; (b) the figure is a topographic map under an ultraviolet lamp with a wavelength of 365 nm; (c) is a form diagram of NGQDs-PVA under natural light; (d) The figure is a form diagram of NGQDs-PVA under an ultraviolet lamp with the wavelength of 365 nm; (e) The figure is a fluorescence emission spectrum of the NGQDs-PVA composite material under different excitation wavelengths.

FIG. 2 is a test image of the prepared composite film;

wherein (a) is FTIR images of PVA and NGQDs-PVA;

(b) The Raman spectrum of NGQDs-PVA is shown.

FIG. 3 is an SEM image of each substance;

wherein, the picture (a) is an SEM picture of PVA; (b) The figure is a SEM image of NGQDs-PVA, and the figure (c) is a SEM image of the NGQDs-PVA after adsorbing Ag ions; (d-f) SEM-Mapping image of NGQDs-PVA, wherein the image (d) is C element; (e) the figure is an N element; (f) the figure is O element; (g) the figure is Ag element; (g) The figure is an SEM-Mapping image of NGQDs-PVA after adsorbing Ag ions

FIG. 4 shows the adsorption capacity and fluorescence quenching degree of NGQDs-PVA at different pH values (T =303.15K, co =100mg/L, lambda. = _ex ＝360nm)。

FIG. 5 is a graph of adsorption performance; (T =313.15K, ph =4, co =100mg/L, λ _ex ＝360nm.)

Wherein, the graph (a) is an absorption kinetic curve chart of NGQDs-PVA; (b) the figure is a fitting graph of adsorption kinetics; (c) the figure is an intra-particle diffusion model; (d) The graph shows the change of adsorption capacity and fluorescence quenching degree with time.

FIG. 6 is a graph showing the adsorption performance test; (T =293.15K, ph =4, λ _ex ＝360nm.)

Wherein (a) is a graph showing the effect of initial concentration on the adsorption capacity of NGQDs-PVA adsorbents; (b) the figure is a fitting graph of NGQDs-PVA adsorption thermodynamics; (c) The graph is a graph of the change of the adsorption capacity and the fluorescence quenching degree of NGQDs-PVA under different initial concentrations; (d) The graph shows a fit graph of adsorption capacity and fluorescence quenching degree of NGQDs-PVA at different initial concentrations.

FIG. 7 is a diagram of a mechanism of NGQDs-PVA selective adsorption of silver;

FIG. 8 (a) is a graph showing adsorption capacity of NGQDs-PVA for different metals in a seven-membered ion solution and fluorescence quenching degree of NGQDs-PVA for different metals in seven-unit example solution (T =303.15K, co =100mg/L, λ =, λ) _ex ＝360nm.)；

Graph NGQDs-PVA in fig. 8 (b) adsorption capacity and fluorescence quenching degree in adsorption-desorption cycle (T =303.15k, co =100mg/L, λ = k) _ex ＝360nm)。

FIG. 9 shows the adsorption mechanism and fluorescence material diagram (lambda) of NGQDs-PVA in adsorption-desorption cycle _ex ＝360nm)。

Detailed Description

The invention is described in further detail below with reference to the accompanying drawings:

in the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention; the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance; furthermore, unless expressly stated or limited otherwise, the terms "mounted," "connected," and "connected" are to be construed broadly and encompass, for example, both fixed and removable connections; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.

The invention discloses an NGQDs-PVA composite membrane and a preparation method thereof, comprising the following steps:

(1) Preparation of NGQDs

2g of Citric Acid (CA) and 0.3mL of ammonia (28%) were weighed and placed in a 100mL Teflon lined stainless steel autoclave and heated at 180 ℃ for 4h, then allowed to cool naturally to room temperature to obtain an orange viscous solution as a nitrogen doped quantum dot solution. The orange viscous solution is stirred, simultaneously 30mL of 10mg/mL NaOH solution is added dropwise, and the concentrated orange solution is neutralized to pH 7.0, namely the nitrogen-doped quantum dot solution is the NGQDs solution after the pH value is adjusted.

(2) Preparation of NGQDs-PVA composite membrane

Adding 0.5-2g of PVA to 35mL of distilled water with stirring by a magnetic stirrer to prepare a PVA solution; uniformly mixing 2-6.5mL of NGQDs solution and PVA solution at room temperature to obtain a mixed solution, dissolving 0.8-3g of CA in 35mL of water, pouring the mixed solution, and performing crosslinking mixing for 3 hours to obtain the NGQDs-PVA mixed solution. The prepared NGQDs-PVA mixed solution was poured into a container. The composite film was then dried in an oven at 50 ℃ for 12 hours and cured in an oven at 135 ℃ for 5 hours.

Reference will now be made in detail to the present embodiments

Example 1

(1) Preparation of NGQDs

2g of Citric Acid (CA) and 0.3mL of aqueous ammonia (28%) were weighed and placed in a 100mL Teflon lined stainless steel autoclave and heated at 180 ℃ for 4h, then allowed to cool to room temperature to give an orange viscous solution. The orange viscous solution was stirred while 30mL of 10mg/mL NaOH solution was added dropwise to neutralize the concentrated orange solution to pH 7.0 to prepare NGQDs solution, i.e., nitrogen doped quantum dot solution.

(2) Preparation of NGQDs-PVA composite membrane

0.8g of PVA was added to 35mL of distilled water with stirring by a magnetic stirrer to prepare a PVA solution; 5.5mL of the NGQDs solution and the PVA solution were uniformly mixed at room temperature to obtain a mixed solution, 0.8g of CA was dissolved in water and then poured into the mixed solution, and cross-linked and mixed for 3 hours to obtain the NGQDs-PVA mixed solution. The prepared NGQDs-PVA mixed solution was poured into a container. Then drying the composite film in an oven at 50 ℃ for 12 hours, and curing in the oven at 135 ℃ for 5 hours to obtain the NGQDs-PVA fluorescent composite film.

As can be seen from the graphs (a) to (d) in FIG. 1, the NGQDs-PVA composite membrane shows blue fluorescence under an ultraviolet lamp with an excitation wavelength of 365nm due to the addition of the NGQDs. This demonstrates the successful preparation of NGQDs-PVA composite membranes, since PVA does not have fluorescent properties. FIG. 1 (e) is a fluorescence emission spectrum of the NGQDs-PVA composite membrane at different excitation wavelengths, and it can be seen that 360nm is the optimal excitation wavelength.

Detailed changes in Fourier transform Infrared Spectroscopy (FTIR) of PVA and NGQDs-PVA are depicted in the graph (a) in FIG. 2. 3362.22cm ^-1 The stretching vibration of carbonyl O-H and the N-H stretching vibration of secondary amine are overlapped, and the success of N doping in GQDs is proved. 2923.15cm ^-1 Is of the formula-CH ₂ -stretching vibration peak. 1709.37cm ^-1 Here is a stretching vibration of C = O, but the absorption peak of C = O is shifted to a low wavenumber direction due to bimolecular association in the carboxylic acid molecule. 1179.74cm ^-1 And 1075.08cm ^-1 There is a C-O stretching vibration region, and the double peak indicates the production of ester. 824.36cm ^-1 In the form of primary amine-NH ₂ The superposition of the out-of-plane bending vibration of the medium N-H plane and the out-of-plane swinging vibration absorption peak of the C-H bond on the double bond of the olefin, and the existence of the olefin can also be verified in a Raman spectrum. This demonstrates the successful loading of NGQDs. And 1464.92cm in FIG. 2 (b) ^-1 And 1607.47cm ^-1 The D band and the G band can also indicate that graphene structures exist in the composite membrane, and the successful preparation of NGQDs and the successful doping in PVA are proved. And 1607.47cm ^-1 Is also-NH ₂ The deformation vibration peak proves the successful doping of N in the graphene quantum dots. By scanning electricityThe surface appearance and element distribution of the PVA, the NGQDs-PVA and the NGQDs-PVA-Ag nano composite material are observed by a mirror.

FIG. 3 (a) is a SEM micrograph showing a PVA, NGQDs-PVA and NGQDs-PVA-Ag nanocomposite film. In FIG. 3 (a), the PVA/CA blend composite exhibits a uniform, smooth surface topography. FIG. 3 (d) Panel- (f) shows SEM micrograph of surface of NGQDs-PVA nanocomposite film, which shows that the dispersion of GQDs in polymer matrix is uniform without any significant aggregation. FIG. 3 (g) is SEM micrograph of surface of NGQDs-PVA nano composite membrane after adsorbing Ag (I), and the surface is uniformly distributed with complexes generated after adsorbing Ag (I), which further shows that GQDs are uniformly dispersed in PVA matrix. FIGS. 3 (d) - (g) illustrate the participation of the three elements N, C and O in the adsorption of Ag (I) by the change in the amounts of the elements before and after the adsorption of the different elements. The fluorescent composite membrane NGQDs-PVA is used for statically adsorbing the Ag (I) solution to examine the adsorption performance of the Ag (I) solution.

(1) Intermittent adsorption experiment

The pH influence determination is carried out in sequence: weighing 5 parts of NGQDs-PVA with the mass of 0.1g, adding the NGQDs-PVA with the Ag (I) concentration of 100.0 mg.L ^-1 AgNO of ₃ 50mL of the solution, and respectively adjusting the pH value of the solution to 2.0-6.0. Therefore, the influence of different pH values on the Ag (I) adsorption capacity is examined. Study of adsorption kinetics: respectively weighing 10 parts of 0.1g NGQDs-PVA, adding 100.0 mg.L Ag (I) ^-1 AgNO of ₃ 50mL of the solution was added, and the pH was adjusted to 4.0. Sequentially sampling at three temperatures of 20 ℃, 30 ℃ and 40 ℃ for 15, 30, 60, 90, 120, 180, 240, 360, 540 and 720min at different time points to determine the heavy metal content. Study of adsorption thermodynamics: respectively weighing 6 parts of 0.05g NGQDs-PVA, respectively adding Ag (I) with the concentration of 50, 100, 150, 200, 250, 300, 400 and 500 mg.L ^-1 AgNO of ₃ 50mL of the solution was adjusted to pH 4.0. Reacting at three temperatures of 20 ℃, 30 ℃ and 40 ℃ for 12 hours. The above experiments were repeated three times each. FIG. 4 shows a graph of pH-dependent adsorption capacity, and it can be seen that the optimum pH was 4.0. FIG. 5 shows the effect of adsorption time on the adsorption capacity of NGQDs-PVA adsorbents at 20 deg.C, 30 deg.C and 40 deg.C. The adsorbent shows a high level in 0-60minThe adsorption rate, then gradually slowed down until adsorption equilibrium was substantially reached. When the temperature is increased from 20 ℃ to 30 ℃, the adsorption capacity is not obviously changed; but when the temperature is increased from 30 ℃ to 40 ℃, the adsorption capacity is greatly increased. The results of fitting the pseudo-first order kinetic model, the pseudo-second order kinetic model and the intra-particle diffusion model to the adsorption kinetic data of NGQDs-PVA adsorbent are shown in FIGS. 5 (b) and 5 (c), and the relevant parameters of the kinetic models are shown in tables 1 and 2. As can be seen from Table 2, the quasi-first order kinetic model can better represent the adsorption kinetics of GQDs-PVA. FIG. 6 shows the effect of different initial concentrations on the adsorption capacity of NGQDs-PVA at 20 deg.C, 30 deg.C and 40 deg.C. As with the kinetics, there was no significant change in adsorption capacity from 20 ℃ to 30 ℃; but when the temperature is increased from 30 ℃ to 40 ℃, the adsorption capacity is greatly increased. Langmuir adsorption isothermal model and Freundlich isothermal adsorption model fitting were performed on the NGQDs-PVA adsorbent adsorption thermodynamic data, and the results are shown in FIG. 6 (b). As can be seen from the correlation coefficients in Table 3, the Langmuir isothermal adsorption model can better represent the adsorption kinetics of NGQDs-PVA. The adsorption capacity of the NGQDs-loaded PVA on Ag (I) ions reaches 317.35mg/g through thermodynamic fitting. This is due to COOH, -NH ₃ The presence of-NH-and-OH results in more active centers on the surface of NGQDs-PVA.

FIG. 6 (c) shows that the amount of NGQDs-PVA adsorbed increases and the degree of fluorescence quenching increases with increasing initial concentration of Ag (I). And the material fluorescence becomes significantly darker with increasing concentration under the uv lamp. Adsorption capacity (q) _e ) And degree of fluorescence quenching ((F) ₀ -F)/F ₀ ) A polynomial fit can be well established: (F) ₀ -F)/F ₀ ＝-6.70×10 ^-5 q ² _e +0.016q _e -0.0517，R ² =0.999 (fig. 6 (d)). The above results indicate that it is feasible to monitor the progress of adsorption of NGQDs-PVA by the degree of fluorescence quenching of NGQDs-PVA.

TABLE 1 parameter Table of NGQDs-PVA model for in-particle diffusion

Table 2 quasi-first-level model, quasi-second-level model, elovich model parameter table of NGQDs-PVA.

TABLE 3 parameter Table of the thermodynamic model of NGQDs-PVA

(2) Selective adsorption experiment

Respectively weighing 8 parts of 0.05g of adsorbent NGQDs-PVA, and respectively adding 50ml of adsorbent NGQDs-PVA with the concentration of 100 mg.L ^-1 The solutions of Cu (II), pb (II), zn (II), ni (II), ag (I), cd (II), na (I) and Mg (II) were subjected to fluorescence selectivity study at 30 ℃ and the pH was adjusted to 4.0. Then 1 part of 0.05g of adsorbent NGQDs-PVA is weighed and added with 50ml of adsorbent with the concentration of 100 mg.L ^-1 The solutions of Cu (II), pb (II), zn (II), ni (II), ag (I), cd (II), na (I) and Mg (II) were studied for adsorption selectivity at 30 ℃ and the pH was adjusted to 4.0. The above experiments were repeated three times each. As can be seen from fig. 8 (a), the material has both adsorption selectivity and fluorescence selectivity for Ag (I). The results in FIG. 8 (a) show that NGQDs-PVA have different adsorption capacities for different metal ions, where the adsorption amount for Ag (I) is optimal. FIG. 8 (a) shows the fluorescence quenching effect of NGQDs solution on seven metal ions, cu (II), pb (II), zn (II), ni (II), ag (I), cd (II) and Mg (II), wherein the quenching effect on Ag (I) is most obvious. However, the material compounded by NGQDs and PVA has obvious quenching effect except for Ag (I) ((F) ₀ -F)/F ₀ And the adsorption capacity of the quantum dot is certain for Pb (II) and Cu (II) except for 62 percent, which indicates that the high molecular structure has influence on the fluorescent metal selectivity of the quantum dot. The occurrence of this phenomenonCan be explained by steric hindrance. Firstly, the method comprises the following steps: the number of stabilizing ligands for Ag (I) is 2 and the number of stabilizing ligands for the other ions is 3, 4, 5 and 6. The three-coordination of Pb (II) results in a coordination which is easily achieved without Ag (I) but which has a higher adsorption capacity than other metals. The graphene molecules in NGQDs are not arranged in layers, resulting in difficulties in coordinating metals with ligand numbers of 4, 5, 6 and 8. Secondly, the method comprises the following steps: for the solution of NGQDs, the NGQDs in the solution can freely move the NGQDs according to coordination conditions when the NGQDs are coordinated with metals. However, after the NGQDs are combined with PVA and solidified, the molecular positions of the NGQDs in the NGQDs-PVA are fixed, so that the linear 2-coordinated Ag (I) can also keep higher adsorption quantity. It is the selectivity of this structure in (1) that leads to the selectivity of adsorption and ultimately to the selective quenching of fluorescence.

And the fluorescence quenching degree of different metals under the irradiation of an ultraviolet lamp is increased with the increase of the absorption amount of NGQDs-PVA. The result shows that the adsorption selectivity can be more intuitively and simply predicted by observing the fluorescence intensity of the NGQDs-PVA after adsorbing the metal ions.

Referring to FIG. 7, the adsorption of Ag (I) by NGQDs-PVA adsorbents is a result of the combination of complexation and reduction. GQDs modified by ammonia water are rich in-NH 2 and loaded on PVA, so that the surface of the NGQDs-PVA is rich in C-O, -COOH, -NH ₂ -NH-and-OH. Therefore, the NGQDs-PVA adsorbent has good adsorbability to metal ions. When adsorbing Ag (I) having oxidizing property, the imino group will adsorb Ag ⁺ Reduction to Ag ⁰ This conclusion can also be confirmed by XPS analysis. The resulting C-O, -COOH, -NH ₂ and-OH coordinates to Ag (I) and forms a complex. Resulting in positively charged Ag ⁺ Ions tend to adsorb on the surface of negatively charged NGQDs-PVA. As shown in fig. 9, NGQDs-PVA undergoes quenching of fluorescence after Ag (I) adsorption because efficient electron transfer processes promote recombination and annihilation of non-radiative electron holes. While in the case of eluting NGQDs-PVA-Ag with sodium thiosulfate, ag ⁺ And S ₂ O ₃ ^- The reaction takes the Ag (I) in the material into the eluent, resulting in the recovery of the fluorescence of the material (fig. 9). By the aboveThe explanation proves that the NGQDs-PVA material has monitoring effect on the adsorption and desorption of Ag (I) through the change of the fluorescence characteristics.

(3) Regeneration of adsorbents

Respectively weighing 6 parts of 0.05g NGQDs-PVA, adding 100.0 mg.L of Ag (I) ^-1 AgNO of ₃ 50mL of the solution was added, and the pH was adjusted to 4.0. The reaction was carried out at 30 ℃ for 12h. And averaging the measured solubility of the adsorbed solution to obtain a final adsorption value.

The regeneration performance of the adsorbents plays an important role for the commercial application of the adsorbents, and the regenerability and the fluorescence recovery rate of the NGQDs-PVA adsorbents are shown in FIG. 8 (b). After the first adsorption-desorption cycle, the adsorption capacity of the adsorbent is reduced to a certain degree, and the adsorption amount of the Ag (I) by the NGQDs-PVA from the second time to the fifth time is maintained to be about 74 percent of the first adsorption. This is because a small amount of the reducing imino group present in NGQDs-PVA is consumed in the first adsorption, resulting in a decrease in the next adsorption (FIG. 9). The adsorption amounts of the last four times are maintained at a certain level because of C-O, -COOH, -NH ₃ and-OH can be almost completely eluted by sodium thiosulfate after complexing with Ag (I). By measuring the fluorescence loss rate of the NGQDs-PVA adsorbent, it was found that the degree of fluorescence loss was large after the first adsorption. However, quenched NGQDs-PVA-Ag still has the potential to weakly emit FL, which is fully recovered in fluorescence after elution of Ag (I) by sodium thiosulfate (fig. 9). The degree of fluorescence loss becomes smaller in the latter several times of adsorption. The results indicate that the fluorescence loss rate of NGQDs-PVA adsorbents can be used as a measure of their metal adsorption capacity.

The data and adsorption effects of examples 2 to 18 are shown in Table 4 below, in which the amounts of the substances used are the (2) th step in example 1, and the portions not shown in the table are the same as in example 1.

Table 4 process data for examples 2-18

Serial number	pva(g)	Water (ml)	NGQD(ml)	CA(g)	Adsorption capacity (mg/g)
						1	1	35	2	1	25.24
2	1	35	2	3	24.63
						3	1	35	2	2	13.86
4	1	35	4	1	16.54
						5	1	35	6	1	17.77
6	0.5	35	2	1	11.47
						7	2	35	2	1	10.92
8	0.8	35	5.5	0.8	25.20
						9	0.8	35	6	1	11.23
10	0.8	35	6.5	1.2	3.51
						11	1	35	5.5	1	10.65
12	1	35	6	1.2	3.28
						13	1	35	6.5	0.8	21.75
14	1.2	35	5.5	1.2	18.64
						15	1.2	35	6	0.8	9.50
16	1.2	35	6.5	1	21.18

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims

1. A preparation method of a fluorescent adsorption material loaded with graphene quantum dots is characterized in that,

2. The method for preparing a graphene quantum dot-loaded fluorescent adsorbing material according to claim 1, wherein in the step 1, the nitrogen-doped quantum dot solution is prepared by mixing citric acid and ammonia water, performing a hydrothermal reaction, and cooling to obtain the nitrogen-doped quantum dot solution.

3. The preparation method of the graphene quantum dot-loaded fluorescent adsorbing material according to claim 2, wherein the mixing ratio of citric acid to ammonia water is 0.2g:0.3mL.

4. The method for preparing the graphene quantum dot-loaded fluorescent adsorbing material according to claim 2, wherein the hydrothermal reaction temperature is 180 ℃ and the hydrothermal reaction time is 4 hours.

5. The method for preparing a graphene quantum dot-loaded fluorescent adsorbent material according to claim 1, wherein in step 2, the PVA solution is prepared by adding 0.5-2g of PVA to 35mL of water.

6. The method for preparing a graphene quantum dot-loaded fluorescent adsorbent material according to claim 1, wherein in the step 2, the volume ratio of the PVA solution to the NGQDs solution is 35: (2-6.5).

7. The method for preparing a graphene quantum dot-loaded fluorescent adsorbent according to claim 1, wherein in step 2, 0.8-3g of citric acid is dissolved in 35mL of water.

8. The method for preparing a fluorescent adsorbing material loaded with graphene quantum dots according to claim 1, wherein in the step 2, the crosslinking reaction time is 3 hours.

9. The method for preparing the graphene quantum dot-loaded fluorescent adsorbing material according to claim 1, wherein in the step 2, the drying temperature of the product solution is 50 ℃, and the drying time is 12 hours; the curing temperature is 135 ℃, and the curing time is 5h.

10. The graphene quantum dot-loaded fluorescent adsorbing material prepared by the preparation method of any one of claims 1 to 9, characterized by comprising PVA colloid in which the graphene quantum dots are embedded.