Application of CD and Eu3+ Dual Emission MOF Colorimetric Fluorescent Probe Based on Neural Network in Fe3+ Detection

A new highly fluorescent hybrid material (CD@Eu‐MOF) is synthesized by encapsulating carbon dots (CD) prepared from citric acid and ethylenediamine on the basis of a metal‐organic framework (MOF) prepared from Eu3+ and 1,2,4‐benzenetricarboxylic acid. The prepared composite not only maintains the excellent fluorescence properties of CD and Eu3+, respectively, but also forms a dual‐emission fluorescence system, and the system has good stability in an aqueous solution. It is further used as a novel fluorescent probe for the detection of Fe3+, which can effectively exclude the interference of other metal ions from the detection, and the intensity ratio of IEu/ICD of CD@Eu‐MOF material has a good linear relationship with Fe3+ in the range of 1–200 µm. In this study, computer vision and backpropagation (BP) neural networks are used to train and fit the sample data, and it is verified that the actual fluorescence color of CD has a good linear relationship with Fe3+ concentration. In addition, the BP neural network also verifies that the fluorescence spectrum data of CD@Eu‐MOF also have a good linear relationship with Fe3+ concentration. This study provides a new method for the fabrication of ratio and colorimetric Fe3+ fluorescence sensors.


Introduction
Fe 3+ plays a crucial role in all essential metabolic processes in nature, including the metabolism of oxygen and the catalysis of biological enzymes. [1] Fe 3+ is widely distributed in our environment, and its determination is therefore of great importance for human health. The existing detection schemes for small molecules, including Fe 3+ , include Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, liquid chromatography-mass spectrometry, and a series of methods with the help of modern instruments. [2][3][4] Although these methods have good upper and lower detection limits, they greatly increase the detection time due to the more complicated sampling and preparation process. Therefore, it is important to develop a fast and www.advancedsciencenews.com www.particle-journal.com Computer vision is the simulation of biological vision using computers and related devices. Its main task is to obtain 3D information about the corresponding scene by processing a captured picture or video. It is due to the limitations of the human's naked eye to discriminate colors that computer vision was introduced to distinguish colors that are difficult to distinguish with human eyes and to identify RGB values of colors. The detection of curcumin and hypochlorite, acetylcholinesterase, cephalexin, and parasites in drinking water was achieved by using computer vision for fluorescence acquisition. [27,28,41,42] The recognition and acquisition of fluorescence by computer vision not only improved the recognition speed but also greatly increased the upper and lower limits of recognition and the accuracy of recognition. Rapid and accurate detection of fluorescent probes became possible.
In this study, we encapsulated carbon dots in a controlled manner in a MOF material with Eu 3+ as the metal center, and the resulting CD@Eu-MOF complex contains not only the advantages of lanthanide luminescence of the MOF matrix but also the characteristic luminescence of CD. We found that this novel dual emission is selective for Fe 3+ . At the same time, we used computer vision to extract the RGB value of the actual fluorescence of CD and trained and predicted the dataset through the backpropagation (BP) neural network, which verified that the actual fluorescence RGB value of CD has a good linear relationship with the Fe 3+ concentration. In addition, the BP neural network also verified that the fluorescence spectrum data of CD@Eu-MOF also have a good linear relationship with Fe 3+ concentration. At present, there are few reports that the MOF-encapsulated CD and Eu 3+ are used as fluorescent substances, combined with ratio fluorescent probes, and the detection probe database is formed by computer color recognition, thereby, improving the detection accuracy. A feasible solution is provided to further simplify the detection of Fe 3+ in the environment.

Characterization of CD
The morphology and structure of the CD were determined by transmission electron microscopy (TEM) image analysis using the hydrothermal method. Figure 1a shows the TEM image of the synthesized CD, which shows that they are uniformly dispersed without aggregation, and their particle sizes are in the range of 5-10 nm. By randomly selecting 100 CDs and measuring their particle size, Figure 1b, the particle size distribution of the CD, shows that the particle size of the CD is about 2-5 nm, and the average particle size is about 3.12 nm. In Figure 1a, it can be observed that most of the CD are amorphous carbon particles and no lattice is observed, and only a very small number of CD can be observed. The X-ray diffraction (XRD) pattern of the CD in Figure 1c also confirms the amorphous state of the CD, showing that the CD has a broad peak centered at 25° (0.34 nm). The functional groups on the surface of the CD were characterized by FTIR spectroscopy. As shown in Figure 1d   From this infrared spectrogram, it can be analyzed that the surface of CD is successfully doped with nitrogen, and the presence of all these functional groups greatly promotes the solubility of CD in the water.

Characterization of CD@Eu-MOF
CD was introduced into Eu-MOF by a one-step synthesis method. The generated product is denoted as CD@Eu-MOF. Figure 2a shows the TEM image of CD@Eu-MOF, which shows the tetrahedral lamellar structure, and also shows that there is no obvious aggregation of CD on the outer surface of MOF, indicating that the CD may have entered the framework system of MOF overwhelmingly. Figure 2b shows the XRD patterns of Eu-MOF and CD@Eu-MOF, from which it can be seen that the two spectra are roughly similar, which proves that the addition of carbon dot material to Eu-MOF does not change the crystalline shape of MOF. Figure 2c shows the IR spectra of CD, Eu-MOF, and CD@Eu-MOF, the peaks at 3428 cm −1 are the stretching vibration peaks of OH and NH bonds, and the peaks at 2975 cm −1 are the stretching vibration peak of CH, the peak at 1654 cm −1 is the stretching vibration peak of CN, the peak at 1556 cm −1 is the stretching vibration peak of CO, the peak at 1386 cm −1 is the stretching vibration peak by CC, and the peak at 1051 cm −1 belongs to the stretching vibration peak of CO(the vibration peak of carbon dioxide in the test environment appears around 2300 cm −1 ). It can be seen that CD@Eu-MOF contains both the characteristic absorption peaks of Eu-MOF and the characteristic peaks of CD, which proves that CD already exists in Eu-MOF. From Figure 2d, it can be seen that the electronic spectra of Eu-MOF and CD@ Eu-MOF contain the characteristic peaks of Eu, C, and O, but CD@Eu-MOF contains the characteristic peaks of N elements from CD, which can further indicate the existence and encapsulation of CD in Eu-MOF. At the same time, the actual content of CD in CD@Eu-MOF can be roughly estimated by the proportion of C, N, O, and Eu elements in the XPS spectrum. Calculated from the element content in Figure 2d, the proportion of CD in the CD@Eu-MOF is about 26.23%. This study also explored the effect of different CD loadings on the fluorescence properties and structural stability of CD@Eu-MOF. CD is the key factor for the detection of Fe 3+ in this fluorescent probe. Too low CD will affect the detection performance of CD@Eu-MOF for Fe 3+ , and make the ratio fluorescence transformation insignificant. If it is too high, it may have a certain impact on the structure of the MOF, thereby, affecting the fluorescence emission of lanthanide metal ions. [43] In addition, an excessively high loading amount of CD may lead to the aggregation of CD, resulting in the self-quenching of CD and reducing the detection performance of CD. As shown in Figure   www.advancedsciencenews.com www.particle-journal.com is 5 mg, CD@Eu-MOF has strong CD fluorescence emission, which can have good sensing performance for the detected substances. From the regularity of the crystal structure, it is also more excellent. More specific analysis content is in the Supporting Information.

Fluorescence Characterization
The fluorescence spectra of the prepared CD and CD@Eu-MOF were analyzed. As shown in Figure 3a, the CD solution has a clear emission peak at 443 under 340 nm excitation, which leads to a bright blue fluorescence. The prepared CD@Eu-MOF in the aqueous system showed almost unchanged fluorescence emission from the CD compared to the pure carbon dot solution emission, while a distinct emission peak of Eu-MOF appeared at around 617 nm. As shown in Figure 3b, the CD@ Eu-MOF solution on the left shows a purple color at 365 nm UV. Figure 3c shows that the Eu-MOF powder is white in visible light, while Figure 3d shows that the Eu-MOF powder is red in the 365 nm UV light, which is presumed to be the red fluorescence of Eu in the solid state. It should be noted that the luminescence mechanism of CD is still very complicated and not fully understood, however, there is increasing evidence that the fluorescence emission of CD is related to their surface state. [44] This result shows that the surface state of the CD does not change when they enter the Eu-MOF.

Stability Characterization
The fluorescence stability of CD@Eu-MOF materials is of great importance for their applications. The stability of the material at different pH values was first investigated. The test results are shown in Figure 4a. The fluorescence intensity of CD@Eu-MOF is relatively little affected by pH. The XRD patterns of CD@Eu-MOF after different pH treatments were then analyzed, as shown in Figure 4b; the XRD data were basically consistent with the original CD@Eu-MOF at pH 4, 7, and 9, so CD@Eu-MOF possessed better fluorescence stability at pH 4 to 9. Next, the effects of different storage times on the fluorescence of CD@Eu-MOF were analyzed. Figure 4c shows that the fluorescence emission was tested continuously at 1, 3, 5, 7, and 9 days, and its fluorescence emission remained stable at 9 days. As shown in Figure 4b, the XRD patterns of CD@Eu-MOF were tested after 5 days and 9 days of maintenance in an aqueous environment, and it was found that its crystal structure remained intact, which proved that CD@ Eu-MOF has good stability in the aqueous environment. For the thermal stability analysis as shown in Figure 4d, it can be obtained that the thermal stability of Eu-MOF is better than that of a single CD. For Eu-MOF, the mass change of Eu-MOF is not significant in the range of 40-200 °C with a decrease in the mass ratio of 9%, which corresponds to the loss of very little water, as well as dimethylformamide (DMF) in the MOF structure. In the 200-350 °C range, the mass of Eu-MOF decreases rapidly, with a mass loss of 11% occurring, a stage corresponding to the disintegration of some organic ligands.
In the interval of 350-600 °C, the mass of Eu-MOF decreases significantly, and the mass after warming is about 60% of the original mass; this stage may correspond to the removal of the remaining organic fraction. The final form after 600 °C corresponds to the amorphous europium oxide. The thermal stability of CD@Eu-MOF is similar to that of Eu-MOF when CD is loaded onto Eu-MOF, indicating that the loading of CD does not affect the thermal stability of MOF.

Selective Characterization
Since CD is an important component of colorimetric fluorescent probes, the ability of a single CD to detect metal ions such as Fe 3+ was investigated. According to Figure S1, Supporting Information, the fluorescence of CD has a good linear response relationship with Fe 3+ concentration. At the same time, in the detection of various metal ions, Fe 3+ has the best quenching effect on CD and good anti-interference performance. In order to further verify the relationship between the fluorescence of CD and Fe 3+ concentration, the RGB values corresponding to CD under different Fe 3+ concentrations were collected by computer vision, and the BP neural network was used to analyze and predict. As shown in Figure S2 and  www.advancedsciencenews.com www.particle-journal.com Information, the detection accuracy after training reaches 99.64%. All of these demonstrate the feasibility of constructing colorimetric fluorescent probes using the CD. More specific analysis content is in the Supporting Information. After completing the specificity and selectivity analysis of CD for Fe 3+ detection, we next analyzed the effect of CD@Eu-MOF on the specificity and selectivity of Fe 3+ detection. As shown in Figure 5a, the fluorescence of CD@Eu-MOF at 443 nm gradually decreased with the gradual increase of the concentration of Fe 3+ in the range of 0-200 µm, while the fluorescence intensity at 617 nm remained relatively constant, which proved that CD@Eu-MOF is an ideal ratio metric fluorescent material for the detection of Fe 3+ . The intensity ratio of I Eu /I CD of CD@ Eu-MOF material has a good linear relationship with Fe 3+ within 0-200 µm, as shown in Figure 5b, and the calculated LOD of the material is about 0.91 µm and R 2 is about 0.997 at a signal-to-noise ratio of 3, while the Environmental Protection Agency (EPA) allows the maximum concentration of Fe 3+ in drinking water to be 5000 µm, and the detection limit of the material is much lower than this requirement. BP neural network is used to train and analyze the spectral data of CD@ Eu-MOF. The input layer is the fluorescence intensity ratio of I Eu to I CD , the number of hidden layers is set to 10, and the output layer is the concentration of Fe 3+ . Training set: test set: verification set is 7:3:1. The prediction accuracy after training reached 99.38%. The model is used to predict and verify the Fe 3+ concentration, and the results are shown in Table 1.
In addition, the selectivity of CD@Eu-MOF for Fe 3+ detection was also analyzed, and a series of metal ions that may interfere with CD@Eu-MOF were selected as interference terms. As shown in Figure 5c, the I Eu /I CD fluorescence intensity ratio in CD@Eu-MOF basically did not change much when other metal ions were added. As shown in Figure 5d, the color of CD@Eu-MOF is purple when no iron ion is added, and when iron ions are added, as shown in Figure 5e, its blue fluorescence is obviously burst and the characteristic emission of Eu located at 617 nm remains unchanged, so it shows the red color of Eu. The color change is obvious and can be used for the visual detection of Fe 3+ by human eyes. In addition, this section analyzes and compares CD@Eu-MOF with Fe 3+ fluorescent probes reported in other literatures, and the results are shown in the following Table 2. By comparing the detection limit and detection range of different detection probes, it can be seen that compared with the existing research on Fe 3+ detection, such as pure CD and other fluorescent dyes, this study adopts the dual emission detection combined with CD and Eu-MOF. The method reduces the detection limit to 0.91 µm, and the detection range reaches 1-200 µm. CD@Eu-MOF achieves a low detection limit while taking into account a wide detection range.
It was pointed out in a related paper that Hg 2+ also has a certain burst effect on the citric acid CD, so it will cause some interference to the detection of CD on Fe 3+ , while CD@ Eu-MOF circumvents this drawback well. [21] As shown in Figure 6a, when CD@Eu-MOF is in an Hg 2+ solution, its red  www.advancedsciencenews.com www.particle-journal.com fluorescence completely disappears, leaving only the blue fluorescence of the CD. Analyzing the reason, it may be that Hg 2+ will destroy the structure of the MOF and collapse its structure, thus, Eu-MOF loses its red fluorescence. The XRD pattern of Figure 6b also illustrates the problem: the Fe 3+ treated CD@ Eu-MOF still has a good crystal structure, while the XRD spectrum after Hg 2+ treatment loses its characteristic peaks. To further verify this conclusion, SEM and XPS experiments were performed on CD@Eu-MOF before and after Fe 3+ and Hg 2+ detection, and the results are shown in Figure S4, Supporting Information. Thus, the encapsulation of CD within Eu-MOF not only effectively improves the detection ability of CD for Fe 3+ but also discriminates well for Hg 2+ , which can cause interference, with CD@Eu-MOF.

Fluorescent Quenching Mechanism
Finally, the fluorescence quenching mechanism of Fe 3+ on CD@Eu-MOF was analyzed by UV absorption spectroscopy, and the results are shown in Figure 3c. There is a strong absorption peak at 340 nm, which may be caused by the energy jump captured from the ground state to the excited state on the surface of CD, and this is the reason for the strong fluorescence phenomenon of CD. When CD is loaded into Eu-MOF, a significant blue shift (325 nm) of this peak can be found.  the solvent used is DMF, and CD is dissolved in DMF to participate in the hydrothermal reaction, while CD in DMF, its UV absorption and fluorescence emission are significantly blue-shifted, as can be seen from the UV absorption of CD@ Eu-MOF and CD in DMF. Both of them have obvious absorption peaks at 325 nm. When CD@Eu-MOF is in contact with Fe 3+ , it can be seen that the absorption peak at 325 nm disappears. According to the UV absorption of Fe 3+ , it has no absorption peak in the 200-500 nm region, and the reason for competitive absorption can be excluded. Combined with the XRD plots of CD@Eu-MOF after reaction with Fe 3+ , etc., it is speculated that the reason is that Fe 3+ complexes with groups such as COOH on the surface of CD, and the resulting complexes burst the blue fluorescence of CD without interfering with the red fluorescence of Eu-MOF, which is the mechanism of the ratio metric fluorescence detection of Fe 3+ by CD@ Eu-MOF.

Actual Sample Analysis
In this section, in order to verify the detection performance of CD@Eu-MOF for Fe 3+ in real water samples, a series of experiments were carried out using Xinghu water from Wuhan University and tap water from our university. The results are shown in Table 3. The recoveries for tap water and Xinghu water are between 96.5-105.03% and 98.63-103.83%, respectively, which proves that the CD@Eu-MOF material can actually detect the Fe 3+ content in the samples.

CD@Eu-MOF as Fluorescent Patterning Agent
In order to explore the practical application scenarios of the prepared CD@Eu-MOF, three fluorescent substances, CD, CD@ Eu-MOF, and Eu-MOF, were prepared into inks according to the methods shown in the literature. [50] The three prepared inks were printed into a dot matrix by screen printing, and the results are shown in Figure 6d. It can be seen that it can still maintain a good fluorescence effect after printing, which