CN116626136A - A method for the detection of different chromium ion species based on group interaction - Google Patents

Abstract

The invention discloses a method for realizing detection of different chromium ion forms based on radical interaction, which comprises the steps of carrying out electrochemical action on a functionalized material modified electrode and a chromium-containing solution under the induction of an electric field; the functional material is amino modified cobalt-based graphene and/or carboxylated modified cobalt-based graphene. The invention further fixes HCrO by electrostatic attraction and group interaction under the action of an electric field ₄ ^‑ And Cr (OH) ²⁺ The detection lower limit is 0.10 mug/L and 0.12 mug/L respectively, the detection sensitivity is 19.46 mug/L and 13.44 mug/L respectively, and the detection lower limit is improved by 2-3 orders of magnitude compared with the detection lower limit of the traditional spectrometry, the operation process is simple and mild, the cost is low, and the method has good application prospect.

Description

A method for the detection of different chromium ion species based on group interaction

technical field

The invention relates to the technical field of speciation analysis and detection of heavy metal chromium, in particular, the invention relates to a method for detecting the speciation of different chromium ions based on group interaction.

Background technique

In recent years, environmental pollution caused by heavy metal ions has received more and more attention. Different forms of heavy metal ions correspond to different bioavailability and physiological toxicity. Therefore, it is of great significance to focus on the rapid and accurate detection of different forms rather than comprehensive detection. At present, the sensitive and reliable detection of heavy metal pollutants still largely relies on traditional optical instrument methods, such as atomic absorption spectrometry, inductively coupled plasma mass spectrometry, etc. However, most of them have the characteristics of huge equipment, high operating costs, and high technical requirements for operators. In addition, electrochemical techniques and laser-induced breakdown spectroscopy (LIBS) have also been used for the detection of heavy metal ^ions1-3 , but there are characteristic problems that often rely on strong acid electrolytes or noble metal catalysis, and are often used for single-species analysis.

Contents of the invention

The invention proposes a mechanism based on the special interaction between oxygen-containing groups of target ions and functional materials under electric field induction, so as to achieve the purpose of ultra-low limit, accurate and rapid detection of chromium with different forms. To achieve this goal, the present invention adopts the following technical solutions:

A method for detecting different forms of chromium ions based on group interactions, which involves electrochemically interacting a functionalized material-modified electrode with a chromium-containing solution under the induction of an electric field; the functionalized material is cobalt-based graphite modified by amination ene and/or carboxylated cobalt-based graphene.

In the method described above, preferably, the cobalt-based graphene Co ₃ O ₄ -rGO can be aminated to obtain NH ₂ -Co ₃ O ₄ /rGO, and the cobalt-based graphene Co ₃ O ₄ rGO can be carboxylated COOH-Co ₃ O ₄ /rGO was obtained through modification. Wherein, the amination modification may be dissolving Co ₃ O ₄ rGO in ethanol and water, adding ammonia water and aminopropyltrimethoxysilane (APTMS), and ultrasonically vibrating. The amination modification method is a common and commonly used treatment process. The carboxylation modification comprises dissolving Co ₃ O ₄ rGO in ethanol and water, adding carboxylated graphene (COOHGO), and ultrasonically vibrating. The Co ₃ O ₄ -rGO can be obtained by using known and reported related literature methods such as RSC Adv., 2015, 5, 88567. The carboxylated graphene (COOHGO) can also be obtained by a common graphene carboxylation method or directly purchased from a professional manufacturer.

In the method described above, preferably, the electrode can be modified by using NH ₂ -Co ₃ O ₄ /rGO, and a positive voltage can be applied to detect the chromium-containing solution HCrO ₄ ^- ; COOH-Co ₃ O ₄ /rGO can be used to detect The electrode is modified, and a negative voltage is applied to detect Cr(OH) ²⁺ in chromium-containing solution. The electrode can be a common metal electrode such as an aluminum sheet electrode. Of course, other metals such as copper foil electrodes, titanium electrodes, silver electrodes or non-metal electrodes such as graphite electrodes can be used. It is found that compared with graphite electrodes, aluminum The sheet electrode has a superior electrochemical enrichment effect, which may be the reason for the better conductivity of the aluminum sheet. In addition, when modifying the electrode, the working electrode generally needs to be pre-treated, such as sanded to a bright state, and then 4-6mg/ml NH ₂ -Co ₃ O ₄ /rGO and 1-3mg/ml COOH-Co ₃ O ₄ /rGO was evenly spin-coated onto the surface of the electrode and dried for use. Preferably 5 mg/ml NH ₂ -Co ₃ O ₄ /rGO, 1.5 mg/ml COOH-Co ₃ O ₄ /rGO for spin coating. The electrode can adopt a rotating electrode that is easy to replace, so that it is convenient to adjust and replace the electrode. The inventors have found that too much concentration of the spin-coating solution will cause the dripping material to be too thick, which will cover the active sites on the electrode surface and affect the interaction with the target ions; while the concentration of the spin-coating solution is too low to provide enough functions. The interaction between the cation group and the target ion weakens the generation of spectral signal.

More preferably, the method for detecting and enriching different forms of chromium ions according to the present invention may adopt the following steps:

(1) Dissolve 8-12mg Co ₃ O ₄ rGO in 25-35mL ethanol and 1-3mL deionized water, add 2-6mL ammonia water and 200-300μL APTMS, sonicate for 2-4h and shake at room temperature at 200rpm for 6- 8h; after washing with ethanol and water several times, NH ₂ -Co ₃ O ₄ /rGO was obtained;

(2) When COOHGO is used to replace APTMS, COOH-Co ₃ O ₄ /rGO will be obtained; specifically: take 8-12mg Co ₃ O ₄ rGO and dissolve it in 25-35mL ethanol and 1-3mL deionized water, add 200-300μL COOHGO, sonicated for 2-4h, shaken at room temperature 200rpm for 6-8h; after washing with ethanol and water several times, COOH-Co ₃ O ₄ /rGO was obtained.

(3) The electrode is modified with NH ₂ -Co ₃ O ₄ /rGO, and a positive voltage is applied to detect the chromium-containing solution HCrO ₄ ^- ; the electrode is modified with COOH-Co ₃ O ₄ /rGO, and a negative voltage is applied to detect the Chromium solution Cr(OH) ²⁺ for detection.

Preferably, the pH of the chromium-containing solution is preferably 4-5. Within this pH range, hexavalent chromium and trivalent chromium exist in the form of HCrO4 ^- and Cr(OH) ²⁺ respectively. For hexavalent chromium, they generally exist in the form of oxygen-containing groups. For trivalent chromium, at pH In the case of 4-5, it exists in the form of oxygen-containing group Cr(OH) ²⁺ (Int.J.Environ.An.Ch.2019, 1051). Thus, the difference in charge of the two oxygen-containing groups can be used for electrochemical analysis. Of course, the chromium-containing solution can be adjusted to a suitable pH with a buffer solution. The applied positive voltage is preferably 0.5-2V, 0.8-1.5V is better, and 1.0V is the best; the negative voltage is -0.5--2V is better, -0.8--1.5V is better, and -0.9V is the best; If the voltage is too high, it will easily lead to hydrogen evolution on the surface of the electrode, and the generation of bubbles will affect the adsorption and interaction of nanomaterials with target ions, while if the voltage is too small, it will not be able to provide sufficient traction force under the electric field, which will affect the efficiency of electrical enrichment. The detection in step (3) is preferably combined with the LIBS quantitative analysis method. The invention fixes HCrO ₄ ^- or Cr(OH) ²⁺ on the surface of the electrode through the voltage regulation under the electric field and the chemical combination of the amino group or the carboxylation group, so as to perform quantitative analysis.

According to the present invention, using tricobalt tetroxide and graphene as the medium, functionalized cobalt-based graphene composite material carrier is prepared by functionalizing amino group and carboxyl group, which can not only improve the electron transfer ability through the valence cycle of the transition metal cobalt, but also through the oxygen-containing The interaction between the group and the functionalized group realizes the selective and anti-interference detection of the target ion.

The invention proposes a new method for highly sensitive and selective detection of chromium in different forms at the ppb level under electric field regulation and the interaction of amino groups and carboxylation groups. Compared with traditional spectroscopic methods, this method can improve the detection limit and sensitivity by two orders of magnitude, and further explains the mechanism of selective and sensitive detection as the combined effect of "electric field induction" and "chemical binding". Amination and carboxylation groups are respectively modified on the surface of cobalt-based graphene. In a buffer solution with a pH of 4.0-5.0, especially 4.0, the amino group is protonated to form NH ₃ ⁺ and interact with HCrO ₄ ^- in a positive electric field of 1.0V Under the combined effect of electrostatic attraction and chemical combination, LIBS can identify the spectral signal of HCrO ₄ ^- with high sensitivity and high selectivity. The deprotonation of the carboxylation group to form COO ^- and Cr(OH) ²⁺ under the action of a negative electric field of 0.9V have electrostatic attraction and chemical combination, which promotes the highly sensitive and selective recognition of Cr(OH) ²⁺ by LIBS the spectral signal. The highly sensitive detection mechanism of the new method in this work is explained by TEM, XPS and XANES. It proposes a new selective detection method from the microscopic point of group interaction, and provides a new idea for the rapid and accurate analysis of heavy metal pollutants in the water environment.

Here, the inventors proposed a new method based on the interaction mechanism of different oxygen-containing groups and functional groups induced by an applied electric field, and assisted by laser-induced breakdown spectroscopy (LIBS) to detect different forms of chromium in water. Since the speciation distribution of heavy metal pollutants changes with the change of solution pH, by adjusting the electrochemical parameters, the charge difference of the two chromium ions is used to achieve selective enrichment, and then quantitative analysis is carried out by LIBS technology. Using Co ₃ O ₄ nanoparticles with excellent conductivity and rGO with a large surface area to construct an electrochemically sensitive interface, the composite material not only prevents the agglomeration of graphene sheets, but also improves the conductivity and electrochemical enrichment efficiency, reducing the LIBS lower limit of detection. Amino- and carboxyl-modified Co ₃ O ₄ -rGO were selectively enriched in HCrO ₄ ^- and Cr(OH) ²⁺ , respectively, and then quantitatively detected by LIBS. The results show that the method has good anti-interference and real water sample analysis effect. FTIR, XPS, and XANES indicated that the excellent detection performance was attributed to the strong adsorption capacity of thin-layer graphene, the selective adsorption and chemical interaction of functionalized groups (amino and carboxyl groups) for different morphological chromium ions, and the _{Co3O 4} synergy of Co(II)/(III) valence state cycle in the nanoparticle, in the present invention _{Co3O4 nanoparticle} size is not required, it mainly plays the role of enhancing electrical conductivity and valence state cycle to promote electron transfer effect.

The inventors found that the Co ₃ O ₄ nanoparticles enhanced the electrical conductivity of the composite material, prevented interlayer agglomeration of the graphene base material, and improved the enrichment efficiency. The DFT differential charge density map reveals that when COO ^- -Co ₃ O ₄ /rGO interacts with Cr(OH) ²⁺ , electrons flow from Co sites to Cr sites in Cr(OH) ²⁺ , and NH ₃ ⁺ -Co When ₃ O ₄ /rGO interacts with HCrO ₄ ^- , electrons flow from the Co site to the O site of HCrO ₄ ^- , as shown in Figure 3. XPS showed that the proportion of Co(II) decreased and the proportion of Co(III) increased after functionalized Co-based graphene adsorbed chromium ions. Electron transfer is facilitated through the Co(II)/(III) valence cycle. Figure 5.

The invention achieves selective electrochemical enrichment of Cr(OH) ²⁺ and HCrO ₄ ^- by modifying electrodes with functionalized materials under different electric fields, and produces specific chemical interactions, which is the key to improving detection sensitivity and selectivity Factors, modify specific functional groups (amino and carboxyl) on the surface of the material, under the action of an electric field, further immobilize HCrO ₄ ^- and Cr(OH) ²⁺ through electrostatic attraction and group interaction, and the lower detection limits obtained are 0.10 μg/ L and 0.12μg/L, the detection sensitivities are 19.46μg/L and 13.44μg/L, which is 2-3 orders of magnitude higher than the traditional laser-induced breakdown spectroscopy ^4-8 (lower detection limit 100μg/L-1000μg/L) . The operation process is simple and gentle, and the cost is low, showing good application prospects. The mechanism of high-sensitivity detection is the electric field induction during the pre-enrichment process and the interaction between functionalized groups and target ions. The results show that the amino group protonated NH ₃ ⁺ and HCrO ₄ ^- have electrostatic attraction and chemical combination, and at the same time avoid the interference of cations (electrostatic repulsion) and other simple forms (without oxygen-containing group) the interference of anions present. The carboxylation group is deprotonated to form COO ^- and Cr(OH) ²⁺ has electrostatic attraction and chemical combination, and at the same time, it avoids the interference of anions (electrostatic repulsion) and other simple forms (without containing Oxygen group) the presence of cation interference. And the Co ₃ O ₄ nanoparticles with excellent conductivity synergistically improved the efficiency of electrical enrichment and electron transfer, which promoted the detection of different forms of chromium with high sensitivity and selectivity by LIBS. The invention provides a new thought for the electrochemical method of heavy metal ion speciation analysis.

Description of drawings

Figure 1 is a TEM image of the prepared amino and carboxylated cobalt-based graphene.

Figure 2 is a schematic diagram of the detection. The detection of different forms of chromium is divided into two parts: electrochemical separation and enrichment and LIBS quantitative analysis. Electrocatalytic adsorption was performed on a replaceable rotating electrode, after which both electrodes were used for in situ LIBS quantification. The linear equation was established through the spectral signal results to obtain the analysis results of different forms of chromium.

Figure 3 is the DFT adsorption configuration diagram (adsorption energy).

(a) Co ₃ O ₄ /rGO, (b) NH ₃ ⁺ -Co ₃ O ₄ /rGO adsorption configuration and adsorption energy of HCrO ₄ ^- at 1V; (c) Co ₃ O ₄ /rGO, (d )COO--Co ₃ O ₄ /rGO adsorption configuration and adsorption energy for Cr(OH) ²⁺ at -0.9V.

Figure 4 is a) the LIBS response diagram of NH ₂ -Co ₃ O ₄ /rGO to HCrO ₄ ^- in the example, the inset is the corresponding linear fitting diagram, b) COOH-Co ₃ O ₄ /rGO to Cr(OH) ²⁺ LIBS response plot, inset is the corresponding linear fitting plot. (50-500ppb)

Figure 5 is a comparison of XPS high-resolution spectra of each element before and after adsorption of Cr on a)-e) NH ₂ -Co ₃ O ₄ /rGO and COOH-Co ₃ O ₄ /rGO prepared in the example, f) Cr foil, CrO ₃ , Comparison of Cr(OH) ²⁺ /COOH-Co ₃ O ₄ /rGO, Cr ₂ O ₃ , HCrO ₄ ^- /NH ₂ -Co ₃ O ₄ /rGO Cr K-edge and near-edge spectra. The inset is a partially enlarged view.

Fig. 6 Spectral signals for 100ppb (a) HCrO ₄ ^- and (b) Cr(OH) ²⁺ induced by pure electric field.

Figure 7 Selectivity and anti-interference test.

Fig.8 Adsorption configuration and adsorption energy of (a) HCrO _4- and (b) Cr(OH) ²⁺ by _{NH 3} ⁺ _-Co 3 O 4 /rGO at ^1V ; COO-Co ₃ O ₄ /rGO at -0.9 The adsorption configuration and adsorption energy of (c) Cr(OH) ²⁺ and (d) HCrO ₄ ^- at V.

Detailed ways

The following examples are a further description of the content of the present invention as an explanation of the technical content of the present invention, but the essential content of the present invention is not limited to the following examples, those of ordinary skill in the art can and should know any Simple changes or replacements of the essential spirit of the invention shall fall within the scope of protection required by the present invention.

Example 1

(1) The preparation process of functional materials is as follows: First, Co ₃ O ₄ rGO (10 mg) was dissolved in ethanol (30 mL) and deionized water (2 mL), ammonia (5 mL) and APTMS (200 μL) were added, and after ultrasonic treatment for 2 h, the Shake at room temperature (200rpm) for 6h. After multiple washes with ethanol and water, NH ₂ -Co ₃ O ₄ /rGO was obtained;

When APTMS is replaced by COOHGO in the above steps, COOH-Co ₃ O ₄ /rGO can be obtained. Specifically, Co ₃ O ₄ rGO (10 mg) was dissolved in ethanol (30 mL) and deionized water (2 mL), COOHGO (300 μL) was added, sonicated for 3 h and then shaken at room temperature (200 rpm) for 8 h. After multiple washes with ethanol and water, COOH-Co ₃ O ₄ /rGO was obtained. Fig. 1 shows the TEM figure of amination and carboxylation cobalt-based graphene prepared in the embodiment.

(2) When the pH of the solution is 4, Cr(VI) exists in the form of HCrO ₄ ^- , and Cr(III) exists in the form of Cr(OH) ²⁺ . Depending on the charge carried by the two ions, different voltages are applied. Electrochemical separation and enrichment can be described as the following steps: using an electrochemical workstation, when a positive voltage is applied, the electrode modified by NH ₂ -Co ₃ O ₄ /rGO is used as the working electrode, and the electrode modified by COOH-Co ₃ O ₄ /rGO is used as the Reference electrode/counter electrode; when a negative voltage is applied, the electrode modified by COOH-Co ₃ O ₄ /rGO is used as the working electrode, and the electrode modified by NH ₂ -Co ₃ O ₄ /rGO is used as the reference electrode/counter electrode. Specifically, the NH ₂ -Co ₃ O ₄ rGO modified aluminum sheet is used as the working electrode 1 to apply a positive voltage to form a positive charge area, and the chemical combination of hexavalent chromate ions by electric field induction and protonation NH ₃ ⁺ is used for HCrO ₄ The transfer of ^- is immobilized on the NH ₂ -Co ₃ O ₄ rGO electrode; when a negative voltage is applied to the COOH-Co ₃ O ₄ rGO modified aluminum electrode, the migration of the negative electric field assists Cr(OH) ²⁺ to focus on the work Around the electrode 2, Cr(OH) ²⁺ was immobilized on the COOH-Co ₃ O ₄ rGO electrode under the action of negative electric field and deprotonated COO-, and the LIBS laser performed in-situ analysis on the rotatable electrochemical electrode sheet, Obtain spectral signals.

(3) DFT calculations verified that the introduction of functional groups enhanced the interaction between the substrate material and the target ions, resulting in the mechanism of high selectivity and high sensitivity detection of different chromium ions. Figure 3 shows the comparison of the adsorption configuration and adsorption energy value of the substrate material for the target ion in the presence and absence of functional groups.

(4) Using aminated and carboxylated cobalt-based graphene materials as substrates, the quantitative analysis of different forms of chromium was realized based on LIBS method through electrochemical enrichment and interaction between groups. The results show that under the action of an electric field, HCrO ₄ ^- and Cr(OH) ²⁺ are immobilized by electrostatic attraction, the lower detection limits are 0.10μg/L and 0.12μg/L, and the detection sensitivities are 19.46μg/L and 13.44μg, respectively /L, which is 2-3 orders of magnitude higher than the detection limit of traditional spectroscopic methods. Figure 4 shows the spectral detection signal diagrams of the prepared composite material for (a) HCrO ₄ ^- and (b) Cr(OH) ²⁺ .

(5) The XPS results revealed the mechanism of the chemical shift produced by the interaction between the oxygen-containing groups of the target ions and the functionalized groups, which proved the anti-interference and selection of the functionalized materials (composite materials) we prepared for the detection of different forms of chromium ions sex. Figure 5 shows the XPS high-resolution spectrum of the interaction between the composite material and the target ion.

In addition, the inventors found that, as shown in FIG. 6 , the spectral signals of HCrO ₄ ^- and Cr(OH) ²⁺ induced by a simple electric field are very weak in the absence of modified functionalized materials. It shows that the interaction between materials and target ions is the key factor to enhance the spectral signal.

At the same time, the inventor has also carried out related interference experiments, and the results are shown in Figure 7, the interference ions within a certain concentration range have a smaller spectral signal to the target ion, which is due to the positive electric field induction of 1.0V avoiding the interference of other cations , the joint effect of the chemical combination and electric field attraction of oxygen-containing group HCrO ₄ ^- and protonated NH ₃ ⁺ is greater than the electric field adsorption of other simple (oxygen-free) anions, so the interference of other anions on the target ion HCrO ₄ ^- is less Small. Negative electric field induction at minus 0.9 V circumvents the interference of anions, and the combination of chemical combination and electric field attraction of oxygen-containing group Cr(OH) ²⁺ with deprotonated ^COO- is greater than that of other simple (oxygen-free) cations Adsorption, so other cations have less interference on the target ion Cr(OH) ²⁺ .

Figure 8 shows the optimization process of DFT configuration and adsorption energy. It can be seen that the adsorption energy of NH ₃ ⁺ -Co ₃ O ₄ /rGO on HCrO ₄ ^- is greater than that on Cr(OH) ²⁺ when the applied electric field is 1V energy; when the applied electric field is minus 0.9V, the adsorption energy of COO ^- -Co ₃ O ₄ /rGO on Cr(OH) ²⁺ is greater than that on HCrO ₄ ^- . In the presence of two different forms of chromium ions, the protonated NH ₃ ⁺ -Co ₃ O ₄ /rGO tends to adsorb and interact with HCrO ₄ ^- under positive electric field, and the deprotonated COO ^- -Co ₃ O ₄ /rGO tends to adsorb and interact with Cr(OH) ²⁺ under negative electric field. The above results illustrate the selectivity of the specific groups for the two target ions.

The present invention reveals a mechanism of interaction between different functionalized groups and different oxygen-containing groups of target ions, and finds that the existence of "oxygen-containing groups" is an important factor for the specific combination of groups and the realization of anti-interference detection, including Oxygen group HCrO ₄ ^- chemical combination with protonated NH ₃ ⁺ and positive electric field induction of 1.0V avoids the interference of cations and other anion ions, oxygen-containing group Cr(OH) ²⁺ and deprotonated COO The chemical combination of ^- and the negative electric field induction of -0.9V avoid the interference of anions and other cations. In addition, the valence cycle of Co(II)/(III) in the substrate material promotes the transfer of electrons to Cr, promotes the reduction and immobilization of Cr on the electrode, and is beneficial to obtain enhanced spectral signals. Combined with DFT theoretical calculation and XPS characterization, the coordination environment of target ions was further analyzed, and a synergistic mechanism of "electric field induction" and "chemical binding" was proposed to realize the highly sensitive and selective detection of different forms of chromium ions.

It should be noted that the above-mentioned technical content of the present invention is only an explanation and clarification to enable those skilled in the art to understand the technical essence of the present invention, so the described technical content is not intended to limit the scope of the present invention. . The substantive protection scope of the present invention should be defined by the claims. Those skilled in the art should know that any modifications, equivalent replacements and improvements made based on the essential spirit of the present invention shall fall within the scope of the essential protection of the present invention.

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Claims (10)

1. the method for detecting different chromium ion forms based on group interaction comprises the steps of carrying out electrochemical action on a functionalized material modified electrode and a chromium-containing solution under the induction of an electric field; the functional material is amino modified cobalt-based graphene and/or carboxylated modified cobalt-based graphene.

2. The method according to claim 1, wherein cobalt-based graphene Co ₃ O ₄ Amination modification of rGO to NH ₂ -Co ₃ O ₄ rGO, cobalt-based graphene Co ₃ O ₄ Carboxylation modification of rGO to give COOH-Co ₃ O ₄ /rGO。

3. The method of claim 2, wherein the amination modification is the modification of Co ₃ O ₄ -rGO was dissolved in ethanol and water, ammonia and APTMS were added and sonicated.

4. The method of claim 2, wherein the carboxylation repairThe decoration is Co ₃ O ₄ -rGO was dissolved in ethanol and water, COOHGO was added and sonicated.

5. The method of claim 2, wherein NH is employed ₂ -Co ₃ O ₄ Modification of electrode by rGO, application of positive voltage to HCrO in chromium-containing solution ₄ ^- Detecting; by COOH-Co ₃ O ₄ Modification of electrode/rGO, application of negative voltage to Cr (OH) in chromium-containing solution ²⁺ And (5) detecting.

6. The method of claim 1, wherein the chromium-containing solution has a pH = 4-5.

7. The method of claim 2, comprising the steps of:

(1) Taking 8-12mg Co ₃ O ₄ -rGO is dissolved in 25-35mL ethanol and 1-3mL deionized water, 2-6mL ammonia water and 200-300 mu L APTMS are added, and after ultrasonic treatment for 2-4h, the solution is oscillated at 200rpm at room temperature for 6-8h; washing with ethanol and water for several times to obtain NH ₂ -Co ₃ O ₄ /rGO；

(2) When COOHGO is used for replacing APTMS, COOH-Co is obtained ₃ O ₄ rGO; specific: taking 8-12mg Co ₃ O ₄ rGO is dissolved in 25-35mL of ethanol and 1-3mL of deionized water, 200-300 mu L of COOHGO is added, and after ultrasonic treatment for 2-4 hours, the solution is oscillated for 6-8 hours at room temperature and 200 rpm; washing with ethanol and water for several times to obtain COOH-Co ₃ O ₄ /rGO；

(3) By NH ₂ -Co ₃ O ₄ Modification of electrode/rGO, application of positive voltage to chromium-containing solution HCrO with ph=4-5 ₄ ^- Detecting; by COOH-Co ₃ O ₄ Modification of electrode/rGO, application of negative voltage to chromium-containing solution Cr (OH) at ph=4-5 ²⁺ And (5) detecting.

8. The method of claim 6, wherein the positive voltage is applied at 0.5-2V and the negative voltage is applied at-0.5 to-2V.

9. The method of claim 6, wherein the electrode is a rotating electrode.

10. The method of claim 6, wherein step (3) employs NH ₂ -Co ₃ O ₄ Modification of electrode/rGO, application of positive voltage 1V to chromium-containing solution HCrO with ph=4-5 ₄ ^- Detecting; by COOH-Co ₃ O ₄ Modification of electrode/rGO, application of negative voltage 0.9V to chromium-containing solution Cr (OH) with pH=4-5 ²⁺ And (5) detecting.