Application of Fe3O4 quantum dots in Cr(vi) / Cr(iii) speciation analysis and photocatalytic reduction of Cr(vi) to Cr(iii)

Abstract

This invention provides an application of iron(III) oxide quantum dots in the analysis of Cr(VI) / Cr(III) speciation and the photocatalytic reduction of Cr(VI) to Cr(III), belonging to the field of heavy metal detection technology. This invention synthesizes iron(III) oxide quantum dots using iron(III) oxide nanoparticles. The fluorescence intensity of these iron(III) oxide quantum dots is enhanced in the presence of o-phenylenediamine, and further enhanced in the presence of Cr(VI), without interference from other metal ions. Therefore, the fluorescent probe composed of iron(III) oxide quantum dots and o-phenylenediamine can be used to detect the Cr(VI) content in samples and analyze the speciation of Cr(VI) / Cr(III), with a Cr(VI) detection limit of 2.44 μg / L. Simultaneously, these iron(III) oxide quantum dots possess nanozyme activity, capable of catalytically reducing Cr(VI) to Cr(III) under visible light conditions, with over 99% of Cr(VI) being degraded within 2 hours.

Description

Technical Field

[0001] This invention relates to the field of heavy metal detection technology, specifically to the application of iron tetroxide quantum dots in Cr(VI) / Cr(III) speciation analysis and photocatalytic reduction of Cr(VI) to Cr(III). Background Technology

[0002] The monitoring, removal, and degradation of heavy metal ions into less toxic or non-toxic forms have always been a research hotspot in analytical chemistry and environmental science. In actual samples, inorganic chromium ions mainly exist in two forms: Cr(III) and Cr(VI). Trace amounts of Cr(III) are an important nutrient element and a key component of glucose tolerance factor; conversely, Cr(VI), existing as an oxyanion, is a strong oxidizing agent and a highly toxic Group 1 carcinogen. It can cross cell membranes via non-specific anion carriers, leading to diseases such as skin ulcers, nasal perforation, and lung cancer. The World Health Organization specifies that the maximum permissible concentrations of Cr(VI) in drinking water and on surfaces are 50 μg / L and 100 μg / L, respectively. Due to the difference in toxicity between Cr(III) and Cr(VI), determining the total chromium content in a sample cannot accurately determine the individual contents of Cr(VI) and Cr(III). Therefore, establishing accurate analytical methods to distinguish and detect Cr(III) and Cr(VI), and developing methods to reduce and degrade highly toxic Cr(VI) into non-toxic Cr(III), are crucial.

[0003] Speciation analysis of Cr(III) and Cr(VI) typically requires the use of separation and enrichment techniques to separate them while preserving their morphologies. For example, when using atomic spectroscopy and mass spectrometry for Cr(III) and Cr(VI) speciation analysis, it is essential to combine them with some separation and enrichment techniques. The use of these separation and enrichment techniques inevitably leads to cumbersome, complex, time-consuming processes and is prone to contamination, which is not conducive to the rapid analysis of low-abundance components. In contrast, fluorescence spectroscopy, due to its excellent selectivity, can selectively identify Cr(III) and Cr(VI) based on the difference in response of fluorescent probes to Cr(III) and Cr(VI), without the need for separation and enrichment.

[0004] Due to its high toxicity, strong carcinogenicity, recalcitrant nature, and high migration capacity, Cr(VI) easily accumulates in aquatic ecosystems and organisms through the food chain, seriously endangering human health and ecosystem security. To address this, scientists have developed numerous methods for removing Cr(VI) from wastewater, such as ion exchange, chemical precipitation, coagulation / flocculation, adsorption, and photocatalytic degradation. Among these, photocatalytic degradation is a simple, efficient, and green technology. Under solar irradiation, the catalyst reduces Cr(VI) to the relatively non-toxic Cr(III) through electron transfer, achieving the goal of Cr(VI) removal. For example, Chinese patent application CN114180700A discloses a multilayer structure system for the synergistic conversion of Cr(VI) to Cr(III) through catalytic adsorption, including a catalytic layer A and an adsorption layer B. The catalytic layer A absorbs visible light to reduce Cr(VI) to the less toxic Cr(III), while the adsorption layer B effectively adsorbs the reduced Cr(III). A and B are combined using methods such as filtration and tableting to form a multi-layered system. In actual Cr(VI) wastewater treatment, this system is floated on the water surface. Catalytic layer A can absorb visible light significantly to achieve reduction, while adsorption layer B can effectively adsorb Cr(III), thus achieving efficient removal of Cr(VI) from the aqueous solution. This scheme only describes the reduction of Cr(VI) to Cr(III) and does not disclose how to analyze the speciation of Cr(III) and Cr(VI) without using separation and enrichment methods. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for detecting Cr(VI) and analyzing the Cr(VI) / Cr(III) morphology based on iron oxide quantum dots, as well as a method for catalytically reducing Cr(VI) to Cr(III) based on iron oxide quantum dots in the presence of visible light.

[0006] Specifically, the technical solution of the present invention is as follows:

[0007] A method for detecting Cr(VI) and analyzing the Cr(VI) / Cr(III) speciation based on iron oxide quantum dots includes the following steps:

[0008] S1. Mix Fe3O4 QDs solution and o-phenylenediamine, add buffer solution, then add different volumes of Cr(VI) standard solution, mix well, and dilute to volume with distilled water to obtain Cr(VI) solutions of different concentrations; detect the fluorescence intensity of Cr(VI) solutions of different concentrations at 355nm wavelength under an excitation wavelength of 330nm, establish the relationship between the fluorescence enhancement rate (F-F0) / F0 of Fe3O4 QDs / OPD fluorescent probe and the concentration of Cr(VI), and obtain a linear equation; where F0 is the initial fluorescence intensity of the fluorescent probe, F is the fluorescence intensity of the initial fluorescent probe after the addition of metal ions, and F and F0 have the same unit;

[0009] S2. Add Fe3O4 QDs solution, o-phenylenediamine and the buffer solution to a certain volume of sample, mix well and make up to volume with distilled water, detect the fluorescence intensity value of the sample at 355nm wavelength under 330nm excitation wavelength, and calculate the concentration C1 of Cr(VI) in the sample according to the linear equation in step S1.

[0010] S3. Add potassium permanganate and sulfuric acid to the sample from step S2 to completely oxidize Cr(III) to Cr(VI). Remove excess potassium permanganate with sodium nitrite. Detect the fluorescence intensity of the sample at a wavelength of 355 nm under an excitation wavelength of 330 nm. Calculate the concentration of Cr(VI) as C2 based on the fluorescence intensity and the linear equation from step S1. Obtain the Cr(III) content in the sample based on C2 and C1.

[0011] In a preferred embodiment, the concentration of the Cr(VI) solution in step S1 is 8–150 μg / L.

[0012] In the preferred embodiment, the final concentration of the Fe3O4 QDs solution in both steps S1 and S2 is 0.5–2.0 μg / L.

[0013] In the preferred embodiment, the final concentration of o-phenylenediamine in both steps S1 and S2 is 100–500 μM.

[0014] In a preferred embodiment, the buffer solutions in steps S1 and S2 are both Britton-Robinson buffer solutions with a pH of 2 to 12.

[0015] A method for catalytic reduction of Cr(VI) to Cr(III) based on Fe3O4 quantum dots in the presence of visible light includes the following steps: adding Britton-Robinson buffer solution with pH = 2 to 10 to the sample system in the presence of visible light, then adding Fe3O4 QDs solution and mixing evenly.

[0016] In the preferred embodiment, the pH of the Britton-Robinson buffer solution is 2 to 6.

[0017] In the preferred embodiment, the final concentration of the Fe3O4 QDs solution is 1.25–2.25 μg / mL.

[0018] The technical solution of this invention has the following beneficial effects: This invention successfully constructs a method for detecting Cr(VI), analyzing the speciation of Cr(VI) and Cr(III) based on iron(III) quantum dots, and catalyzing the reduction of Cr(VI) to Cr(III) under visible light. The detection limit for Cr(VI) using a fluorescent probe composed of iron(III) quantum dots and o-phenylenediamine is 2.44 μg / L, and the precision is 4.3%. The speciation analysis results of Cr(VI) and Cr(III) using the fluorescent probe composed of iron(III) quantum dots and o-phenylenediamine are in high agreement with the ICP-MS detection results. In the presence of visible light, iron(III) quantum dots can degrade more than 80.0% of Cr(VI) after 30 min of contact, and the degradation rate reaches 99.0% after 2 h of contact. Attached Figure Description

[0019] Figure 1 The graph shows the fluorescence intensity changes of Cr(VI) detected by Fe3O4 QDs with or without OPD.

[0020] Figure 2 The graph shows the change in fluorescence enhancement rate of Cr(VI) detected by Fe3O4 QDs under different OPD concentrations.

[0021] Figure 3 The graph shows the change in fluorescence enhancement rate of Cr(VI) detected by Fe3O4 QDs at different Fe3O4 QDs concentrations.

[0022] Figure 4 The figures show the fluorescence intensity changes of the Fe3O4 QDs-containing systems under different pH conditions. (a) The figures show the fluorescence intensity changes of the pure Fe3O4 QDs system and the pure Fe3O4 QDs / OPD system at 417 nm under different pH conditions. (b) The figures show the fluorescence intensity changes of the pure Fe3O4 QDs / OPD system and the Fe3O4 QDs / OPD+Cr(VI) system at 355 nm under different pH conditions.

[0023] Figure 5 The graph shows the fluorescence enhancement rate of the reaction system over time when the interaction time between the Fe3O4 QDs / OPD sensing system and Cr(VI) is different.

[0024] Figure 6 Bar graph showing the effect of different metal ions on the fluorescence intensity of the pure Fe3O4 QDs system;

[0025] Figure 7 Bar graph showing the effect of different metal ions on the fluorescence intensity of the Fe3O4 QDs / OPD system;

[0026] Figure 8 The graph shows the effect of different concentrations of Cr(VI) on the fluorescence enhancement rate of the Fe3O4 QDs / OPD system. Figure 8 (a) and the linear relationship between fluorescence enhancement rate and Cr(VI) concentration in the Fe3O4 QDs / OPD system. Figure 8 (Figure b)

[0027] Figure 9 The result of the colorimetric reaction of Fe3O4 QDs catalyzing the oxidation of TMB by H2O2 is shown in the figure. Figure 9 Figure (a) shows the degradation rate curves of different Cr(VI) catalyzed by Fe3O4QDs under visible light as a function of time. Figure 9 (Figure b)

[0028] Figure 10 Figure 1 shows the degradation efficiency of Cr(VI) to Cr(III) by photocatalytic degradation using Fe3O4 QDs. Figure 2 shows the degradation efficiency of Cr(VI) by photocatalytic degradation using Fe3O4 QDs at different concentrations over time. Figure 3 shows the fitting curves of Cr(VI) degradation and reduction using Fe3O4 QDs at different concentrations under photocatalytic conditions at different times. Figure 4 shows the degradation efficiency of Cr(VI) by photocatalytic degradation using Fe3O4 QDs at different pH conditions over time. Figure 5 shows the fitting curves of Cr(VI) degradation and reduction using Fe3O4 QDs at different pH conditions at different times.

[0029] Figure 11 Figure 1 shows the solid UV-Vis diffuse reflectance spectrum of Fe3O4 QDs in the 200-800 nm range (Figure 1), the optical band gap energy of Fe3O4 QDs (Figure 2), and the Mott-Schottky curve of Fe3O4 QDs (Figure 2).

[0030] Figure 12 A schematic diagram of the electron transfer mechanism for the photocatalytic degradation of Cr(VI) to Cr(III) by Fe3O4 QDs;

[0031] Figure 13 This is a schematic diagram illustrating the principles of preparing Fe3O4 QDs in this invention, as well as the use of Fe3O4 QDs for detecting Cr(VI), performing speciation analysis on Cr(VI) / Cr(III), and photocatalytic degradation of Cr(VI) into Cr(III). Detailed Implementation

[0032] The following description, in conjunction with embodiments, clearly and completely describes the technical solutions of this application, so that those skilled in the art can fully understand this application. Obviously, the described embodiments are merely some preferred embodiments of this application, and not all embodiments. Any equivalent modifications or substitutions made by those skilled in the art to the following embodiments without creative effort are within the protection scope of this application.

[0033] The main experimental instruments used in the following examples are as follows: F-4600 fluorescence spectrophotometer (Hitachi, Japan); U-3010 UV-Vis spectrophotometer (Hitachi, Japan); PB-10 precision pH meter (Sartorius, Germany); transmission electron microscope (TEM, FEI Tecnai G2 F20, USA); X-ray photoelectron spectrometer (XPS, ThermoFischer, ESCALAB 250Xi, USA); X-ray powder diffractometer (XRD, Philips X'Pert, Netherlands); PCX-50C multichannel photocatalytic reaction system (Pofilai, Beijing).

[0034] The main reagents used were sourced as follows: 20nm iron tetroxide nanoparticles (Fe3O4 NPs) and ascorbic acid were purchased from Aladdin Chemical Co., Ltd. (Shanghai, China); o-phenylenediamine was purchased from Bodi Chemical Co., Ltd. (Tianjin, China); and various metal ion raw materials were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemical reagents were of analytical purity or higher and were not pretreated before use.

[0035] The lake water used in the following examples was taken from Qingshan Lake, Hubei Normal University; the tap water was taken from Hubei Normal University; and the red wine sample was purchased from the Hubei Normal University supermarket. All lake water, tap water, and red wine samples were centrifuged at 3500 rpm for 20 minutes. The resulting solutions were filtered through a 0.45 mm filter membrane, and the supernatant was collected for later use.

[0036] The synthesis method of Fe3O4 quantum dots (Fe3O4 QDs) used is as follows: 2.0 mg of Fe3O4 NPs and 220 mg of ascorbic acid (AA) were mixed in 20 mL of distilled water. The mixture was stirred vigorously for 30 min and placed in a 40 mL polytetrafluoroethylene-lined autoclave. The temperature was raised to 200 °C and heated for 2 h. After cooling to room temperature, the mixture was filtered through a 0.22 μm aqueous filter, retaining the brownish-yellow liquid to obtain a 100 μg / mL mother liquor, which was stored at 4 °C for later use. The Fe3O4 QDs synthesized from commercially available Fe3O4 NPs had a particle size of 3–4 nm and exhibited blue fluorescence under 365 nm UV light irradiation, with a quantum yield of 36.67%.

[0037] Example 1

[0038] This embodiment provides a method for detecting the speciation of Cr(VI) and Cr(III) in a sample using Fe3O4 QDs / OPD fluorescence sensing. The effects of parameters such as the presence or absence of OPD in the reaction system, the amount of OPD, the amount of Fe3O4 QDs, the pH value of the reaction system, and the reaction time of Fe3O4 QDs / OPD with Cr(VI) on the detection results were investigated. The selectivity of Fe3O4 QDs to different metal ions and the selectivity of Fe3O4 QDs / OPD to different metal ions were also studied.

[0039] Specifically, the method for detecting the speciation of Cr(VI) and Cr(III) in a sample using Fe3O4 QDs / OPD fluorescence sensing includes the following steps:

[0040] S1. In a test tube, mix 1.0 μg / L Fe3O4 QDs solution and 200 μM o-phenylenediamine (OPD), add BR (Britton-Robinson) buffer solution at pH=4, then add different volumes of Cr(VI) standard solution, mix well, and dilute to 5.0 mL with distilled water to obtain Cr(VI) solutions of different concentrations. Subsequently, test the fluorescence intensity of Cr(VI) solutions of different concentrations at a wavelength of 355 nm under an excitation wavelength of 330 nm to establish the relationship between the fluorescence enhancement rate (F-F0) / F0 of the Fe3O4 QDs / OPD fluorescence sensor and the Cr(VI) concentration, obtaining a standard curve equation. Where F0 is the initial fluorescence intensity of the fluorescent probe, and F is the fluorescence intensity of the initial fluorescent probe after the addition of metal ions; F and F0 have the same unit.

[0041] S2. Detection of Cr(VI) content in the sample: Add 50 μL (volume V = 5 × 10⁻⁶) to each test tube. -5 The pretreated sample (L) was then added with 1.0 μg / L Fe3O4 QDs solution, 200 μM o-phenylenediamine (OPD), and BR buffer solution at pH 4. After mixing, the volume was adjusted to 5.0 mL with distilled water and sonicated at room temperature for 30 s. Finally, the fluorescence intensity of the sample at 355 nm wavelength under an excitation wavelength of 330 nm was measured. The concentration of Cr(VI) in the sample was calculated as C1 according to the standard curve equation in step S1.

[0042] S3. Detection of total Cr(VI) and Cr(III) in the sample: Because the Fe3O4 QDs / OPD fluorescence sensing platform is only sensitive to Cr(VI), to achieve the detection of total Cr(VI) and Cr(III), Cr(III) in the sample is first oxidized to Cr(VI). Therefore, 0.20 mL of potassium permanganate (0.05 M) and 1 mL of sulfuric acid (0.1 M) are added to the sample solution sequentially, and the solution is heated at 50 °C for 30 min to ensure that Cr(III) is completely oxidized to Cr(VI); then 0.1 g of sodium nitrite is added to reduce the excess potassium permanganate to eliminate its influence on the detection of Cr(VI) by the Fe3O4 QDs / OPD fluorescence sensing platform. The fluorescence intensity of the treated sample at a wavelength of 355 nm under an excitation wavelength of 330 nm was detected. The concentration of Cr(VI) was calculated as C2 based on the fluorescence intensity and the standard curve from step S1. The content of Cr(III) in the sample was obtained based on the difference in Cr(VI) before and after oxidation, thus achieving speciation analysis of Cr(VI) / Cr(III). The concentration of Cr(III) in the sample was calculated as C3 = (VC2 - VC1) / V = C2 - C1; where C1 and C2 are in μg / L, and V is in L.

[0043] 1.1 Effect of o-phenylenediamine (OPD) on the detection of Cr(VI) by Fe3O4 QDs

[0044] 1.1.1 Effect of OPD Presence on Fe3O4 QDs Detection of Cr(VI)

[0045] Following steps S1 to S3 above, the effect of Fe3O4 QDs on the response of Cr(VI) was investigated with and without the addition of OPD at pH 4 (using Britton-Robinson buffer solution). The results are listed below. Figure 1 .from Figure 1 As can be seen, without OPD, Fe3O4 QDs showed no response to Cr(VI). With the addition of OPD, the fluorescence intensity of the Fe3O4 QDs / OPD system increased, the emission peak at 417 nm gradually blue-shifted, and the fluorescence emission peak broadened, with two new peaks appearing at 355 and 370 nm. When Cr(VI) was added to the Fe3O4 QDs / OPD system, the fluorescence intensity further increased, the fluorescence emission peak narrowed, and the blue shift continued, with the two peaks at 355 and 370 nm becoming more prominent, and the peak at 355 nm being higher and more sensitive. Therefore, the addition of OPD provides a possibility for establishing a new method for the quantitative detection of Cr(VI) using Fe3O4 QDs.

[0046] 1.1.2 Optimization of OPD Usage

[0047] To further investigate the effect of OPD dosage on the quantitative detection of Cr(VI) in the Fe3O4 QDs / OPD system, following the methods described in steps S1-S3 above, and under conditions of pH 4 (using Britton-Robinson buffer solution to provide the pH environment), the concentration of Fe3O4 QDs was fixed at 1.0 μg / mL. The OPD concentration in the reaction system was varied (i.e., the OPD concentrations in step S1 were 0 μM, 50 μM, 100 μM, 150 μM, 200 μM, 250 μM, 300 μM, 350 μM, 400 μM, and 500 μM, respectively), and its effect on the fluorescence intensity of Fe3O4 QDs was examined. The results are as follows: Figure 2 As shown. From Figure 2 As can be seen, the fluorescence intensity of Fe3O4 QDs gradually increases with increasing OPD concentration, reaching a plateau when the OPD concentration is 200 μM. At this point, OPD increases the fluorescence intensity of Fe3O4 QDs by 1.47 times compared to Fe3O4 QDs. Therefore, the Fe3O4 QDs / OPD system can be used to detect Cr(VI) when the OPD concentration in the reaction system is between 50 and 500 μM. Furthermore, the preferred concentration ranges of OPD in the reaction system are 100–500 μM, 150–500 μM, 150–400 μM, 150–350 μM, 150–300 μM, and 200–300 μM, with the optimal concentration being 200 μM.

[0048] 1.2 Optimization of Fe3O4 QDs Dosage

[0049] To obtain optimal fluorescence detection performance, the dosage of Fe3O4 QDs was further optimized. Following the methods described in steps S1-S3 above, under conditions of pH 4 (using Britton-Robinson buffer solution to provide the pH environment), the concentration of OPD was kept constant at 200 μM. The concentration of Fe3O4 QDs was varied (i.e., the Fe3O4 QDs concentrations in step S1 were 0.50 μg / mL, 0.75 μg / mL, 1.00 μg / mL, 1.25 μg / mL, 1.50 μg / mL, 1.75 μg / mL, and 2.00 μg / mL, respectively) to investigate its effect on the fluorescence enhancement rate of the Fe3O4 QDs / OPD system. The results are as follows: Figure 3 As shown. From Figure 3As can be seen, when the Fe3O4 QDs concentration in the reaction system is 0.5–2.0 μg / mL, the fluorescence enhancement first increases and then decreases with increasing Fe3O4 QDs concentration, which may be due to the concentration effect. Furthermore, the preferred concentration ranges for Fe3O4 QDs are 0.5–1.5 μg / mL, 0.5–1.25 μg / mL, and 0.75–1.25 μg / mL, with the optimal concentration range being 1.0 μg / mL.

[0050] 1.3 Effect of pH of the reaction system on the interaction between Fe3O4 QDs / OPD and Cr(VI)

[0051] Following the methods described in steps S1 to S3 above, the effects of fluorescence intensity on Fe3O4 QDs, Fe3O4 QDs / OPD, and Fe3O4 QDs / OPD+Cr(VI) systems under BR buffer conditions with pH values ​​ranging from 2 to 12 (i.e., pH values ​​of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12 in step S1, respectively) were investigated. The results are as follows: Figure 4 As shown. From Figure 4 As can be seen in Figure (a), the fluorescence intensity (λ) of Fe3O4 QDs em The fluorescence intensity (417 nm) gradually increases with increasing pH in the pH range of 2–3, and gradually decreases with increasing pH in the pH range of 3–12. This may be because as the pH value gradually increases, Fe... 3+ Gradual hydrolysis leads to a gradual decrease in fluorescence intensity. However, in the Fe3O4 QDs / OPD system, at pH 2–4, λ… em The fluorescence intensity at 417 nm also increased with increasing pH, then decreased with increasing pH in the pH range of 4–12; the highest fluorescence intensity of Fe3O4 QDs / OPD was observed at pH 4. Figure 4 As shown in Figure (b), under the same pH conditions, the fluorescence intensity of the Fe3O4 QDs / OPD+Cr(VI) system is further enhanced compared to the Fe3O4 QDs / OPD system, at λ em The fluorescence intensity at 355 nm also increases with increasing pH in the range of 2–4, and decreases with increasing pH in the range of 4–12. The highest fluorescence intensity is observed in the Fe3O4 QDs / OPD+Cr(VI) system at pH 4. Therefore, when using the Fe3O4 QDs / OPD system to detect Cr(VI), the optimal pH range is 2–12, with the preferred pH ranges being 2–9, 2–8, 2–7, 2–6, 2–5, 3–5, and 3–4, respectively. Considering the results of both fluorescence enhancement steps, the optimal pH is 4.

[0052] 1.4 Optimization of the interaction time between Fe3O4 QDs / OPD and Cr(VI)

[0053] Under pH 4 (using Britton-Robinson buffer solution to provide the pH environment), with the concentrations of Fe3O4 QDs, OPD, and Cr(VI) fixed at 1.5 μg / mL, 200 μM, and 100 μg / L, respectively, the effect of the interaction time (0 min, 0.5 min, 2.5 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min) between the Fe3O4 QDs / OPD sensing system and Cr(VI) on the fluorescence enhancement rate was investigated according to steps S1-S3 above. The results are as follows: Figure 5 As shown. From Figure 5 As can be seen, in the presence of Cr(VI), the fluorescence enhancement rate of the Fe3O4 QDs / OPD sensing system remains almost unchanged within 0.5–30 min, indicating that the fluorescence sensor responds very quickly to Cr(VI) and has the advantage of rapid detection. Finally, 0.5 min was selected as the optimal response time.

[0054] 1.5 Selectivity of Fe3O4 QDs and Fe3O4 QDs / OPD

[0055] Under pH 4 (using Britton-Robinson buffer solution to provide the pH environment), with the concentrations of Fe3O4 QDs and OPD fixed at 1.5 μg / mL and 200 μM respectively, the responses of the Fe3O4 QDs system and the Fe3O4 QDs / OPD sensing system to Cr(VI), Mn(VII), K(I), Ca(II), Na(I), Mg(II), Al(III), Cu(II), Zn(II), Ba(II), Cd(II), Mn(II), Ag(I), Zr(IV), Cr(III), Pb(II), Hg(II), Fe(III), Fe(II), Co(II), and Ni(II) ions at a concentration of 50 μg / L were studied according to the methods described in steps S1 to S3 above. The results are as follows: Figure 6 and Figure 7 As shown. Figure 6 and Figure 7 In this context, F0 represents the initial fluorescence intensity of the fluorescent probe, and F represents the fluorescence intensity of the initial fluorescent probe after the addition of metal ions. Figure 6 The fluorescent probe on it is Fe3O4 QDs. Figure 7 The fluorescent probe on it is Fe3O4 QDs / OPD. From Figure 6 As can be seen, the added metal ions have almost no effect on the fluorescence intensity of Fe3O4 QDs. From... Figure 7 As can be seen above, only Cr(VI) enhances the fluorescence intensity of the Fe3O4 QDs / OPD fluorescence sensing system, while other metal ions, including Cr(III), have almost no effect on the fluorescence intensity of Fe3O4 QDs / OPD. This indicates that the Fe3O4 QDs / OPD fluorescence sensing system exhibits excellent selectivity for Cr(VI). Furthermore, based on the difference in response of the Fe3O4 QDs / OPD fluorescence sensing system to Cr(VI) and Cr(III), speciation analysis of Cr(VI) and Cr(III) can be performed conveniently and simply without any complex sample pretreatment such as extraction or chromatographic separation, demonstrating significant advantages.

[0056] 1.6 Performance Analysis of Fe3O4 QDs / OPD Fluorescence Sensor for Detecting Cr(VI)

[0057] Under pH 4 (using Britton-Robinson buffer to provide the pH environment), with the concentrations of Fe3O4 QDs and OPD fixed at 1.5 μg / mL and 200 μM, respectively, the effects of different concentrations of Cr(VI) (8 μg / L, 10 μg / L, 15 μg / L, 20 μg / L, 35 μg / L, 50 μg / L, 75 μg / L, 100 μg / L, 125 μg / L, and 150 μg / L) on the fluorescence enhancement rate of the Fe3O4 QDs / OPD system were studied according to steps S1-S3 above. The results are as follows: Figure 8 As shown. From Figure 8 As can be seen, when the Cr(VI) concentration is 8–150 μg / L, the emission peak at 355 nm gradually increases with the increase of Cr(VI) concentration, and there is a good linear relationship between its fluorescence enhancement rate (F-F0) / F0 and the Cr(VI) concentration. The linear equation is: (F-F0) / F0=0.0269C-0.1890, and the correlation coefficient (R) is 0.0269C-0.1890. 2 The fluorescence intensity was 0.9936, the limit of detection (LOD) and precision (RSD) were 2.44 μg / L and 4.3%, respectively. Here, F0 is the initial fluorescence intensity of the fluorescent probe, F is the fluorescence intensity of the initial fluorescent probe after the addition of metal ions, and F and F0 have the same unit. The fluorescent probe is Fe3O4 QDs / OPD.

[0058] 1.7 Detection of Cr(VI) / Cr(III) speciation in actual samples using Fe3O4 QDs / OPD fluorescence sensor

[0059] Under pH 4 (using Britton-Robinson buffer solution to provide the pH environment), with the concentrations of Fe3O4 QDs and OPD fixed at 1.5 μg / mL and 200 μM respectively, the Fe3O4 QDs / OPD fluorescence sensor was used for Cr(VI) / Cr(III) analysis in different matrix samples according to steps S1-S3 above to verify its applicability and feasibility. Table 1 shows the detection results and spiked recoveries of Cr in Qingshan Lake water, tap water, and red wine. Each sample was measured in parallel five times.

[0060] Table 1. Speciation analysis results of Cr(VI) / Cr(III) in different samples using Fe3O4 QDs / OPD fluorescence sensor.

[0061]

[0062]

[0063] As shown in Table 1, the recoveries ranged from 89.2% to 102.6%. The total Cr and Cr(VI) concentrations detected in the red wine were 4.89 μg / L and 2.47 μg / L, respectively, with a calculated Cr(III) concentration of 2.42 μg / L. The normal range for Cr(VI) concentration in red wine is 2–90 μg / L (refer to "Graphene-based materials as solid phase extraction sorbent for chromium(VI) determination in red wine", https: / / doi.org / 10.1016 / j.microc.2018.05.043). Although Cr(VI) was detected in the red wine, this is likely due to contamination of raw materials and equipment during the winemaking process, and its concentration is within the prescribed safety range.

[0064] To further verify the accuracy of the results, the same samples were analyzed using ICP-MS (inductively coupled plasma mass spectrometry), with each sample measured in parallel three times. The results are shown in Table 2.

[0065] Table 2. ICP-MS results of Cr(VI) / Cr(III) speciation in different samples.

[0066]

[0067]

[0068] Comparing Tables 2 and 1, it can be seen that the results from the two methods show a high degree of agreement. SPSS Statistics 27 software analysis indicates a strong correlation between the two methods (see Table 3, where "**" indicates a P-value ≤ 0.01). Therefore, the established Fe3O4 QDs / OPD fluorescence sensor is suitable for speciation analysis of Cr(VI) / Cr(III) in liquid samples (e.g., speciation analysis of Cr(VI) / Cr(III) in environmental water samples and red wine), possessing advantages of simplicity, convenience, speed, high sensitivity, and good selectivity.

[0069] Table 3. Correlation analysis between Fe3O4 QDs / OPD fluorescence sensor and ICP-MS detection results.

[0070]

[0071] Example 2

[0072] Because Fe3O4 QDs possess peroxidase-like catalytic activity, the inventors investigated this activity through the redox reaction between H2O2 and 3,3,5,5-tetramethylbenzylamine (TMB). The results are as follows: Figure 9 As shown in Figure (a), in the presence of both H2O2 and TMB, Fe3O4 QDs can catalyze the generation of hydroxyl radicals (·OH) from H2O2. The ·OH radicals then undergo a redox reaction with TMB, turning blue, indicating that Fe3O4 QDs have high peroxidase-like catalytic activity. This provides a feasibility for utilizing the enzymatic catalytic activity of Fe3O4 QDs to photocatalytically reduce and degrade Cr(VI) to Cr(III).

[0073] This embodiment provides a method for photocatalytic reduction and degradation of Cr(VI) to Cr(III) using Fe3O4 QDs, comprising the following steps:

[0074] Under conditions of pH 4 (using Britton-Robinson buffer solution to provide the pH environment), with the concentration of Fe3O4 QDs fixed at 1.0 μg / mL, different concentrations of Cr(VI) (5 μg / mL, 7.5 μg / mL, and 10 μg / mL) were added, and the samples were irradiated with a xenon lamp (>420 nm) for 0–2 h (0 min, 5 min, 10 min, 25 min, 60 min, 90 min, and 120 min). To avoid absorption interference from Fe3O4 QDs, a wavelength of 350 nm was selected as the absorption peak for detecting the residual Cr(VI) content in the samples. The detection results are as follows: Figure 9As shown in Figure (b), the degradation rate of Cr(VI) gradually increases with increasing irradiation time, while it gradually decreases with increasing Cr(VI) concentration. This indicates that Fe3O4 QDs have the potential for photocatalytic degradation of Cr(VI), and its efficiency is related to the Cr(VI) concentration, which is also related to the amount of Fe3O4 QDs used (i.e., the concentration of Fe3O4 QDs in the reaction system).

[0075] Generally, the main factors affecting photocatalytic efficiency are the amount of photocatalytic material and pH. Therefore, to obtain the best photocatalytic effect, the concentration of Fe3O4 QDs and pH were optimized. Under the condition of pH 4 (using Britton-Robinson buffer solution to provide the pH environment), with a fixed Cr(VI) content of 5 μg / mL, the effect of Fe3O4 QDs concentrations (1.00 μg / mL, 1.25 μg / mL, 1.50 μg / mL, 1.75 μg / mL, 2.00 μg / mL, 2.25 μg / mL) was optimized. The results are as follows: Figure 10 As shown in Figure (a), the photocatalytic degradation effect of Fe3O4 QDs on Cr(VI) increases with the increase of Fe3O4 QDs concentration. When the concentration of Fe3O4 QDs is 1.00 μg / mL, the degradation efficiency of Cr(VI) reaches 48.9% after 2 hours of irradiation; when the concentration is 1.25 μg / mL, the degradation efficiency reaches 72.4% after 2 hours; when the concentration is 1.75 μg / mL, the degradation efficiency reaches 79.7% after 2 hours; and when the concentration is 2.00 μg / mL, the degradation efficiency reaches 90.0% after 2 hours. When the concentration of Fe3O4 QDs is 2.25 μg / mL, the degradation rate of Cr(VI) reaches 80.4% after 10 minutes of xenon lamp irradiation, and reaches as high as 99.0% after 2 hours of irradiation. Therefore, under visible light irradiation for 2 hours, the final concentration of Fe3O4 QDs at 1.25–2.25 μg / mL showed better photocatalytic effect on Cr(VI); the final concentration of Fe3O4 QDs at 1.75–2.25 μg / mL further improved the photocatalytic effect on Cr(VI); the final concentration of Fe3O4 QDs at 2.00–2.25 μg / mL further improved the photocatalytic effect on Cr(VI); and the final concentration of Fe3O4 QDs at 2.25 μg / mL showed the best photocatalytic effect on Cr(VI). The pseudo-first-order kinetic equation (1) was used to fit the relationship between the degradation and reduction of Cr(VI) under different photocatalytic conditions at different times using different amounts of Fe3O4 QDs. Figure 10Figure (b) shows that the results conform to pseudo-first-order kinetics. With increasing Fe3O4 QDs dosage, the steeper the slope, the higher the photocatalytic degradation efficiency. The efficiency is highest when the Fe3O4 QDs dosage is 2.25 μg / mL. Figure 10 The results in Figure (a) are very consistent; the degradation rate can reach more than 90% after 1 hour of illumination, and as high as 99% after 2 hours, indicating that Fe3O4 QDs have a strong ability to photocatalytically degrade Cr(VI). The pseudo-first-order kinetic equation (1) is as follows: -ln(C / C0)=kt.

[0076] The concentrations of Cr(VI) and Fe3O4 QDs were fixed at 5 μg / mL and 2.25 μg / mL, respectively, and the effects of different pH values ​​(2, 4, 6, 7, 8, 10) were optimized. The results are as follows: Figure 10 As shown in Figure (c), the efficiency of Fe3O4 QDs in photocatalytic degradation of Cr(VI) gradually increases with increasing pH from pH 2 to 4, reaching its highest value at pH 4, and then gradually decreases with increasing pH from pH 4 to 10. Clearly, in acidic solutions (pH 2, 4, or 6), especially at pH 4, the efficiency of Fe3O4 QDs in photocatalytic reduction of Cr(VI) is higher than in neutral and alkaline solutions. This is mainly because in acidic solutions, Cr(VI) ions are primarily HCrO4 ions. - and Cr2O7 2- Cr(III) ions are mainly Cr 3+ , and CrO4 2- In neutral and alkaline solutions, Cr(III) ions predominate, existing mainly in the form of Cr(OH)3. Based on the Nernst reduction potential, in neutral and alkaline solutions, CrO4... 2- The reduction potential of Cr(OH)3 (-0.13 eV relative to NHE) (NHE: general hydrogen electrode) is much lower than that of HCrO4 in acidic solution. - / Cr 3+ The reduction potential (1.35 eV relative to NHE) and Cr2O7 2- / Cr 3+ The reduction potential (1.33 eV relative to NHE). Therefore, HCrO4 - and Cr2O7 2- The photogenerated electrons provide a greater thermodynamic driving force for the reduction of Cr(VI) ions to Cr(III) ions, so HCrO4 - and Cr2O7 2- Compared to CrO4 2-It is more easily reduced. Furthermore, during the photocatalytic reduction of Cr(VI) by Fe3O4 QDs, the product Cr(OH)3 precipitates in high pH solutions and can be adsorbed onto the surface of Fe3O4 QDs, leading to a decrease in photoreduction efficiency. Similarly, the relationship between the photocatalytic degradation and reduction of Cr(VI) by Fe3O4 QDs at different pH conditions and at different times (…) Figure 10 The middle (d) figure also conforms to pseudo-first-order kinetics; the slope is largest at pH 4, indicating the highest photocatalytic degradation efficiency. Figure 10 The results in Figure (c) are very consistent.

[0077] The inventors explored the mechanism of Fe3O4 QDs photocatalytic reduction and degradation of Cr(VI) to Cr(III). Figure 11 Figure (a) shows the solid-state UV-Vis diffuse reflectance spectrum of Fe3O4 QDs in the 200-800 nm range. The figure reveals strong light absorption around 330 nm, verifying its photoresponse characteristics. The optical band gap was further calculated using the Kubelka-Munk method, as shown below. Figure 11 As shown in Figure (b), via (ahν) 2 The relationship between the voltage and hν yielded an optical bandgap energy (Eg) of 1.75 eV for Fe3O4 QDs. Mott-Schottky curves were used to determine the positions of the valence band (VB) and conduction band (CB) of the Fe3O4 QDs. n-type and p-type semiconductors exhibit positive and negative slopes, respectively. Figure 11 As shown in Figure (c), its slope is negative, therefore Fe3O4 QDs are p-type semiconductors. The E0 of Fe3O4 QDs is calculated according to formulas (2) and (3). VB =0.08eV, E CB = -1.67eV. Formulas (2) and (3) are as follows:

[0078] Formula (2): E VB =0.28eV-E Ag / AgCl ;

[0079] Formula (3): E VB =E CB +Eg.

[0080] E based on Fe3O4 QDs VB and E CB Based on the calculation results and the Cr(VI) / Cr(III) redox potential (1.33 eV), a possible electron transfer mechanism for the photocatalytic degradation of Cr(VI) to Cr(III) by Fe3O4 QDs was derived. The results are as follows: Figure 12As shown. Under xenon lamp irradiation, Fe3O4 QDs are activated, and photogenerated electrons (e... - The signal is excited to the conduction band, leaving a hole in the valence band. + ), e - Cr(VI) can be directly reduced to Cr(III). The mechanism of photocatalytic reduction of Cr(VI) to Cr(III) is shown in equations (4) and (5).

[0081] Formula (4): Fe3O4 QDs+hν→e - +h + ;

[0082] Equation (5): Cr 6+ +e - →Cr 5+ +e - →Cr 4+ +e - →Cr 3+ .

[0083] In summary, such as Figure 13 As shown, in this invention, Fe3O4 QDs (λ) were successfully synthesized via a one-step hydrothermal method using Fe3O4 NPs as raw materials and ascorbic acid (AA) as an etching agent. ex =330nm, λ em =417nm), with a quantum yield of 36.67%. After adding OPD to Fe3O4 QDs, the two form a chelate through Fe-N linkage, enhancing fluorescence. This results in a blue shift of the fluorescence peak at 417nm, accompanied by the appearance of two new peaks (λ). ex =330nm, λ em=355nm). After adding Cr(VI) to the Fe3O4 QDs / OPD system, the fluorescence of the system is further enhanced based on electrostatic interactions and fluorescence resonance energy transfer (FRET). The fluorescence intensity of Fe3O4 QDs / OPD increases linearly with increasing Cr(VI) concentration, but shows no response to Cr(III). Based on the difference in the response of Fe3O4 QDs / OPD to Cr(VI) and Cr(III), a novel, simple, sensitive, and rapid two-step fluorescence enhancement method for detecting Cr(VI) / Cr(III) speciation is established. This two-step fluorescence enhancement mechanism has strong resistance to background interference and selectivity. The linear range of this method is 8–150 μg / L; the limit of detection is 2.44 μg / L, which is far below the World Health Organization's recommended upper limit for Cr(VI) concentration in drinking water (50 μg / L). Meanwhile, leveraging the nanozyme properties of Fe3O4 QDs, and utilizing their catalytic activity, a new method for the photocatalytic reduction of Cr(VI) to Cr(III) was developed, providing a novel approach for the removal of toxic Cr(VI) from the aquatic environment. Fe3O4 QDs exhibited excellent photocatalytic activity for the reduction of Cr(VI) to Cr(III) under mild, weakly acidic conditions. During the photoreduction degradation process, Fe3O4 QDs were activated, and photogenerated electrons were excited to the conduction band, leaving holes in the valence band. These electrons could directly reduce Cr(VI) to Cr(III). Fe3O4 QDs achieved a degradation rate of over 80.0% after 30 minutes of xenon lamp irradiation, and a degradation rate as high as 99.0% after 2 hours of irradiation. Fe3O4 QDs serve both as a fluorescent probe for Cr(VI) / Cr(III) speciation analysis and as a nanozyme for the photocatalytic reduction of Cr(VI) to Cr(III), greatly expanding the application scope of Fe3O4 QDs.

[0084] The embodiments described above are merely preferred embodiments of this application and are not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by anyone skilled in the art. Any simple equivalent changes and modifications made based on the scope of protection of this application and the content of the specification should be included within the scope of protection of this application.

Claims

1. The application of iron(III) oxide quantum dots, characterized in that, The application is any one or more of the following: (1) The use of iron(III) oxide quantum dots in the presence of o-phenylenediamine to detect the Cr(VI) content in liquid samples includes the following steps: S1. Mix Fe3O4QDs solution and o-phenylenediamine, add buffer solution, then add different volumes of Cr(VI) standard solution, mix well, and dilute to volume with distilled water to obtain Cr(VI) solutions of different concentrations; detect the fluorescence intensity of different concentrations of Cr(VI) solutions at 355 nm wavelength under an excitation wavelength of 330 nm to establish the fluorescence enhancement rate of the Fe3O4QDs / OPD fluorescent probe. F F 0) / F The relationship between F0 and Cr(VI) concentration was obtained as a linear equation; where F0 is the initial fluorescence intensity of the fluorescent probe, and F is the fluorescence intensity of the initial fluorescent probe after the addition of metal ions, and the units of F and F0 are the same. S2. Add Fe3O4QDs solution, o-phenylenediamine and the buffer solution to a certain volume of sample, mix well and make up to volume with distilled water, detect the fluorescence intensity value of the sample at 355nm wavelength under 330nm excitation wavelength, and calculate the concentration C1 of Cr(VI) in the sample according to the linear equation in step S1. (2) In the presence of o-phenylenediamine, iron(III) quantum dots are used to analyze the speciation of Cr(VI) and Cr(III) in liquid samples, including steps S1 and S2 in item (1) and step S3. Step S3 includes the following steps: S3, add potassium permanganate and sulfuric acid to the sample in step S2 to completely oxidize Cr(III) to Cr(VI), and remove excess potassium permanganate with sodium nitrite; detect the fluorescence intensity value of the sample at a wavelength of 355 nm under an excitation wavelength of 330 nm, calculate the concentration of Cr(VI) as C2 according to the fluorescence intensity value and the linear equation in step S1, and obtain the content of Cr(III) in the sample according to C2 and C1.

2. The application according to claim 1, characterized in that, In step S1, the concentration of the Cr(VI) solution is 8~150 μg / L.

3. The application according to claim 1, characterized in that, The final concentration of the Fe3O4QDs solution in both steps S1 and S2 is 0.5~2.0 μg / L.

4. The application according to claim 1, characterized in that, The final concentration of o-phenylenediamine in both steps S1 and S2 is 100~500 μM.

5. The application according to claim 1, characterized in that, The buffer solutions used in steps S1 and S2 are Britton-Robinson buffer solutions with a pH of 2 to 12.

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