Sheng Li

 system, the kinetic fluorescence method was used to determine formaldehyde. The experimental results show that in the sulfuric acid medium, the fading process of oxidized rhodamine B by potassium bromate is inhibited by the catalytic action of formaldehyde. The results showed that the formaldehyde concentration in the range of 0.4 to 4.4 μmol/L had a good linear relationship with lg (

) = 0.01385 c (μmol/L) + 0.02353, the correlation coefficient was 0.9974, and the method detection limit was 0.3027 μmol/L. In the determination of formaldehyde in the synthetical water sample and laundry water, the recoveries were 98.68% and 103.20%. The results were consistent with the certified value.

Formaldehyde can cause great harm to the human body. It can cause skin and mucous membrane irritation. When a high concentration of formaldehyde is inhaled, severe respiratory tract irritation and edema, eye irritation, headaches, and other symptoms can occur. Therefore, it is important to obtain a reliable and accurate formaldehyde result in environmental monitoring. The detection methods of formaldehyde mainly include spectrophotometry (1), kinetic spectrophotometry (2), fluorescence photometry (3), gas chromatography (GC) (4), liquid chromatography (LC) (5,6), and the electrochemical analysis method (7). Among them, kinetic spectrophotometry offers several key advantages as a technique, including high sensitivity, high selectivity, simple operation, and a large linear range. Compared with kinetic spectrophotometry, other methods have their own shortcomings (8–16). Rhodamine B is a red-purple or green crystalline powder. It is easily soluble in alcohol, water, and most organic solvents. In polar solvents, such as ethanol or water, the strong polarity of the solvent changes the molecular structure of rhodamine B, making it a zwitterion R

. These three cations can form various ion associations with halogen ions, such as Cl

. On the basis of the existing literature, (17,18), we found that in the sulfuric acid medium, the fading process of rhodamine B oxidation by potassium bromate can be inhibited by the catalysis of formaldehyde. The concentration of formaldehyde has a good linear relationship with lg (

) in the range of 0.4–4.4 μmol/L. Based on this, a new method for determining the content of formaldehyde has been established.

A F-7100 fluorescence spectrophotometer (Hitachi Ltd., Japan), a BS 110S electronic balance (Beijing Sartorius Instrument System Co., Ltd., China), a U3900H UV spectrophotometer (Hitachi Ltd., Japan), a HH-2 constant temperature water bath (Guohua Electric Co., Ltd.), and a KQ3200E ultrasonic cleaner (Kunshan Ultrasonic Instrument Co., Ltd.) was used in this study.

Potassium bromate was acquired from the Tianjin Quartz Clock Factory Baxian Chemical Plant; rhodamine B was acquired from the Tianjin Tianxin Fine Chemical Plant Development Center; the sulfuric acid originated from the Beijing Chemical Plant; formaldehyde came from the Tianjin Fuchen Chemical Reagent Plant; hydrochloric acid came from the Beijing Chemical Plant; nitric acid was acquired from the Baiyin Liangyou Chemical Reagent Co., Ltd.; and acetic acid came from the Tianjin Oboke Chemical Co., Ltd. All reagents were of analytical grade.

 mol/L rhodamine B solution, a 0.10 mol/L sulfuric acid solution, and a 0.10 mol/L potassium bromate solution was prepared with the above reagents and pure water.

The formaldehyde standard solution (1 mmol/L) was prepared with a 1.00 mmol/L formaldehyde standard solution with a 37% formaldehyde solution before being calibrated by an iodometric method and diluted with water to a 0.01 mmol/L standard working solution before use.

This experiment utilized four 25 mL colorimetric tubes (numbered 1–4). In each tube, the 2.0 mL 0.10 mol/L sulfuric acid solution and the 0.50 mL 1.00 × 10

 mol/L rhodamine B solution were added, respectively. Then, 5.0 mL of the 0.10 mol/L potassium bromate solution were added respectively to the colorimetric tubes numbered 2–4. Finally, the 2.00 mL 1.00 × 10

 mol/L formaldehyde standard solution was added to the tube labeled 3. The 4.000 mL 1.00 × 10

 mol/L formaldehyde standard solution was added to the tube labeled number 4, and the volume constant was made. All the colorimetric tubes were placed in a water bath at 90 °C for 15 min. Then, they were removed from the bath and cooled with flowing cold water for 3 min. Finally, the fluorescence intensity 

 of the catalytic solution and the fluorescence intensity 

The excitation emission matrix (EEM) spectra of the blank solution without formaldehyde is shown in Figure 1. It can be seen from Figure 1 that when the excitation wavelength is 572 nm and the emission wavelength is 584 nm, the sample has maximum fluorescence intensity.

FIGURE 1: Excitation emission matrix spectra.

In Figure 2, curves 1 and 2 show that under acidic conditions, the fluorescence intensity of rhodamine B under the action of potassium bromate is significantly reduced, indicating that KBrO

 can oxidize rhodamine B in an acidic medium, with large peak intensity changes. From curve 3 and 4, it can be seen that the fluorescence intensity in the system increases as the amount of formaldehyde increases, indicating that formaldehyde can significantly inhibit the fading reaction, which can be used as a basis for measuring formaldehyde.

FIGURE 2: Fluorescence emission spectra of the rhodamine B system. 1: 2 mL 0.10 mol/L sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B; 2: 1 + 5 mL 0.10 mol/L potassium bromate solution; 3: 2 + 2.00 mL 1.00 × 10

 mol/L formaldehyde standard solution; 4: 2 + 4.00 mL 1.00 × 10

Different types of acids have a greater impact on the experimental results. The experiment investigated the effects of sulfuric acid, hydrochloric acid, nitric acid, and acetic acid on the fluorescence intensity of the reaction product. The results are shown in Figure 3. In the H

 medium, the inhibitory effect of formaldehyde was obvious, and the Δ

, HCl, and acetic acid (AcOH) mediums, the fluorescence intensity of the inhibitory reaction and the non-inhibitory reaction did not change much. Therefore, H

 was used as the reaction medium in the subsequent experiments.

FIGURE 3: The fluorescence intensity of the reaction system in different acid mediums. (a) a’: 2.0 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution; a: a’ + 2.00 mL formaldehyde standard solution. (b) b’: 2.0 mL hydrochloric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution; b: b’ + 2.00 mL formaldehyde standard solution. (c) c’: 2.0 mL nitric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution; c: c’ + 2.00 mL formaldehyde standard solution. (d) d’: 2.0 mL acetic acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution. d: d’ + 2.00 mL formaldehyde standard solution.

Effect of the Sulfuric Acid Concentration

The experiment investigated the effect of the different amounts of sulfuric acid on the fluorescence intensity of the reaction product. The results are shown in Figure 4. When the volume of H

 was 2.50 mL, the inhibitory effect of formaldehyde was obvious, and the Δ

 was the largest. Therefore, the optimum volume of the H

 solution in the subsequent experiments was 2.50 mL.

FIGURE 4: The fluorescence intensity of the reaction system in different volumes of sulfuric acid. (a) a’: 0.5 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution; a: a’ + 2.00 mL formaldehyde standard solution. (b) b’: 1.0 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution; b: b’ + 2.00 mL formaldehyde standard solution. (c) c’: 1.5 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution; c: c’ + 2.00 mL formaldehyde standard solution. (d) d’: 2.0 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution; d: d’ + 2.00 mL formaldehyde standard solution. (e) e’: 2.5 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution; e: e’ + 2.00 mL formaldehyde standard solution. (f) f’: 3.0 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B + 5 mL 0.10 mol/L potassium bromate solution; f: f’ + 2.00 mL formaldehyde standard solution.

Under the above selected conditions, the experiment investigated the influence of different volumes of the potassium bromate solution on the experimental results. The results are shown in Figure 5. When the volume of KBrO

 was 5.0 mL, the inhibitory effect of formaldehyde was obvious, and the Δ

 was the largest. Therefore, the volume of the KBrO

 solution in the subsequent experiments was 5.0 mL.

FIGURE 5: The fluorescence intensity of the reaction system in different volumes of the potassium bromate solution. (a) a’: 1 mL 0.10 mol/L potassium bromate solution + 2.5 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B; a: a’ + 2.00 mL formaldehyde standard solution. (b) b’: 3 mL 0.10 mol/L potassium bromate solution + 2.5 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B; b: b’ + 2.00 mL formaldehyde standard solution. (c) c’: 5 mL 0.10 mol/L potassium bromate solution + 2.5 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B; c: c’ + 2.00 mL formaldehyde standard solution. (d) d’: 7 mL 0.10 mol/L potassium bromate solution + 2.5 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L rhodamine B; d: d’ + 2.00 mL formaldehyde standard solution. (e) e’: 9 mL 0.10 mol/L potassium bromate solution + 2.5 mL sulfuric acid solution + 0.50 mL 1.00 × 10

 mol/L Rhodamine B; e: e’ + 2.00 mL formaldehyde standard solution.

Under the above selected conditions, the experiment investigated the influence of different volumes of rhodamine B on the experimental results. The results are shown in Figure 6. The fluorescence intensity value of the non-inhibitory reaction gradually increased with the increase of the volume of rhodamine B, but when the volume of rhodamine B was 0.50 mL, the reaction had a higher fluorescence intensity and the inhibitory effect of formaldehyde was obvious, so the volume of the rhodamine B solution in the subsequent experiments was 0.50 mL.

FIGURE 6: The fluorescence intensity of the reaction system in a rhodamine B solution with different volumes. (a) a’: 0.10 mL 1.00 × 10

 mol/L rhodamine B + 2.5 mL sulfuric acid solution + 5 mL 0.10 mol/L potassium bromate solution; a: a’ + 2.00 mL formaldehyde standard solution. (b) b’: 0.30 mL 1.00 × 10

 mol/L rhodamine B + 2.5 mL sulfuric acid solution + 5 mL 0.10 mol/L potassium bromate solution; b: b’ + 2.00 mL formaldehyde standard solution. (c) c’: 0.50 mL 1.00 × 10

 mol/L rhodamine B + 2.5 mL sulfuric acid solution + 5 mL 0.10 mol/L potassium bromate solution; c: c’ + 2.0 mL formaldehyde standard solution. (d) d’: 0.70 mL 1.00 × 10

 mol/L rhodamine B + 2.5 mL sulfuric acid solution + 5 mL 0.10 mol/L potassium bromate solution; d: d’ + 2.00 mL formaldehyde standard solution. (e) e’: 0.90 mL 1.00 × 10

 mol/L rhodamine B + 2.5 mL sulfuric acid solution + 5 mL 0.10 mol/L potassium bromate solution; e: e’ + 2.00 mL formaldehyde standard solution.

Under the above selected conditions, the experiment investigated the influence of the water bath temperature on the experimental results. The results are shown in Figure 7. When the temperature was 90 °C, Δ

 had a maximum value, so in the subsequent experiments, the reaction temperature was selected to be heated in a water bath at 90 °C.

FIGURE 7: The fluorescence intensity of the reaction system at different water bath temperatures: (a) 30 °C, (b) 60 °C, (c) 90 °C, and (d) 100 °C.

Under the above selected conditions, the experiment investigated the influence of heating temperature on the experimental results. The results are shown in Figure 8. As the reaction time increased, Δ

 first increased and then decreased. When the reaction time was 15 min, Δ

 was relatively stable and had a maximum value, so the reaction time was 15 min in the subsequent experiments.

FIGURE 8: The fluorescence intensity of the reaction system under different reaction times: (a) 6 min; (b) 9 min; (c) 12 min; (d) 15 min; and (e) 18 min.

As a result, the optimum conditions were as follows: 2.50 mL of the dilute H

 solution was added, and 0.50 mL of the rhodamine B solution was placed in a 25-mL colorimetric tube and put in a water bath at 90 °C for 15 min before being cooled with cold water for 3 min.

For this part of the experiment, 12 25 mL colorimetric tubes were used and 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11 mL of formaldehyde standard solution was added to them before reacting under the optimal experimental conditions described above. Then, the fluorescence spectrum was measured in the range of 500~700 nm. The experimental results are shown in Figure 9. The results show that when the formaldehyde concentration is 0.4~4.4 μmol/L, the lg (

) of the system has a linear relationship with the concentration of formaldehyde, as shown in Figure 10. The linear equation was lg (

) = 0.01385 c (μmol/L) + 0.02353, and the maximum linear correlation coefficient was 0.9974.

FIGURE 9: The fluorescence diagram of the standard curve experiment. The image to the right shows a close-up of the peak area.

FIGURE 10: The standard working curve with the correlation results.

The fluorescence intensity of the 11 blank samples was measured to obtain the standard deviation σ. Then, we divided 3σ by the slope of the standard working curve to obtain a detection limit of 0.3027 μmol/L.

The experiment investigated the influence of common ions and compounds on the determination of formaldehyde. The allowable relative error was controlled within ±5%. The allowable multiples of the following interfering ions are shown in Table I.

In polar solvents such as ethanol or water, the structure of rhodamine B would be changed by the strong polarity of the solvent, making it a zwitterion R

 can combine with the negatively charged BrO

 and be oxidized by it. The conjugated structure of rhodamine B was destroyed as the fluorescence intensity of the solution decreases (19–21); its fading process is shown in Figure 11.

FIGURE 11: Schematic illustrating the decreased intensity of rhodamine b.

Formaldehyde has weak reducibility and can be oxidized by potassium bromate to formic acid, which can oxidize rhodamine B and generate formaldehyde. Therefore, formaldehyde competes with rhodamine B for the opportunity to combine with potassium bromate, so it can inhibit the fading process of rhodamine B. The fluorescence intensity of the reaction system increases with the increase of the formaldehyde concentration. Within a certain range, lg (

) shows a good linear relationship with the concentration of formaldehyde.

A water sample was prepared with the following components: 0.8 μmol/L formaldehyde; 0.1 μmol/L Na

. Next, an appropriate amount of the laboratory synthetical water sample was taken. Then, 10 mL was measured after filtration and transferred to a 100 mL volumetric flask and diluted with water to the volume and mixed. After that step was completed, 3.0 mL of the sample was added to the 25-mL colorimetric tube before the test was carried out according to the best experimental conditions while the addition experiment was performed at the same time. After the reaction, the fluorescence spectrum was measured in the range of 500~700 nm on a fluorescence spectrophotometer, and the fluorescence intensity value corresponding to 584 nm was taken. Then, the fluorescence intensity value obtained was brought into the standard working curve equation, where the formaldehyde concentration and standard recovery rate was calculated. The results are shown in Table II.

To analyze the laundry water samples, a new piece of clothing was soaked in tap water for 5 h, and 10 mL of washing water was measured with a graduated cylinder. The liquid was then transferred to a 100 mL volumetric flask, and the volume and mix was diluted with water. Then, 3.0 mL of the sample was added to a 25 mL colorimetric tube, and then the procedural steps outlined in “Analysis of Synthetical Water Sample” were repeated. The results are shown in Table II.

In this paper, a kinetic fluorimetric method for the determination of formaldehyde by potassium bromate oxidation of rhodamine B was established. In sulfuric acid medium, the fading process of potassium bromate oxidation of rhodamine B would be inhibited by the catalysis of formaldehyde. As the concentration of formaldehyde increased, the fluorescence intensity gradually increased. In a certain range, lg (

) had a good linear relationship with the concentration of formaldehyde, so it could be used as a basis for determining formaldehyde. After defining the optimal experimental conditions, further experiments were carried out on this basis. The experimental results showed that the concentration of formaldehyde had a good linear relationship with lg (

) in the range of 0.4 μmol/L to 4.4 μmol/L. The standard working curve was lg (

) = 0.01385 c (μmol/L) + 0.02353, the correlation coefficient was 0.9974, and the detection limit of the method was 0.3027 μmol/L. In the determination of formaldehyde in the synthetical water sample and laundry water, the spike-and-recovery rates were 98.68% and 103.20%, respectively, and the results were good.

The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 62075007).

(1) G.F. Pinto, D.L. Rocha, E.M. Richter, R.A.A. Muñoz, and S.G. da Silva, 

(3) G. Nájera-Estebanjuan, E.G. Carrillo-Cedillo, and M.P. Cañizares-Macías, 

(5) A.S. Sebaei, A.M. Gomaa, A.A. El-Zwahry, E.A. Emara, and C.L. Wilkins, 

(6) B.K. Ma, F.J. Xu, M. He, Y.Q. Lin, et al, 

, 105222 (2020). DOI: 10.1016/j.microc.2020.105222

(7) D. Pavel, R.R. Miguel, V. Vlastimil, and R.J. António, 

, 121068 (2020). DOI: 10.1016/j.talanta.2020.121068

(9) N.G. Yasri, H. Seddik, and M.A. Mosallb, 

(10) T.T. Ma, D.Z. Wang, Y.H. Zhang, S. Zhang, S.H. Sun, X. Li, and J.X. Zhang, 

(12) F.B.de.F. Rezende, A.M.de.S.S. Cheibub, A.D.P. Netto, and F.F.de.C. Marques, 

(13) M.Y. Wang, Z.B. Guo, H.Y. Guan, and M. Li, 

(14) F.A. Lobo, T.M.O. Santos, K.M. Vieira, V.M. Osório, and J.G. Taylor, 

(15) M. Hladová, J. Martinka, and P. Rantuch, 

Res. Pap. Fac. Mater. Sci. Technol. Slovak. U. Technol.

(16) R. Giesen, T. Schripp, D. Markewitz, B. Meyer, H. Schwab, E. Uhde, and T. Salthammer, 

(17) T. Zhang, Q.Q. Xu, L.Y. Chen, and Y.F. Zheng, 

(20) X.K. Wang, J.G. Wang, P.Q. Guo, W.L. Guo and G.L. Li, 

 is with the College of Chemistry and Chemical Engineering at Lanzhou Jiaotong University, in Lanzhou, China. 

 is with the Key Institute of Lightwave Technology at Beijing Jiaotong University, in Beijing, China. Direct correspondence to 

Articles

A kinetic fluorimetric method was established for the determination of formaldehyde in synthetic water samples and laundry water by potassium bromate oxidation of rhodamine B.

Kinetic Fluorimetric Determination of Formaldehyde by Oxidation of Rhodamine B with Potassium Bromate

A catalytic kinetic fluorescence method for the detection of trace bismuth (III) was developed, which focuses on the fading reaction of hydrogen peroxide that oxidizes eosin Y under the acidity of pH 4.50 and 90 

C. Experiments showed that bismuth could catalyze the oxidation of eosin Y by hydrogen peroxide; simultaneously, the fluorescence in- tensity was weakened or lost. The linear relationship between bismuth and eosin Y can be used to determine trace amounts of bismuth. The results showed that the concentration of bismuth in the range of 4.0 × 10

 μg/L showed a good linear relationship with lg(

), the correlation coefficient was 0.9971, and the detection limit of the method was 1.9 × 10

 μg/L. The method could be used for the determination of bismuth in human hair and water samples.

Bismuth is known as the “green metal” and is widely used in the medical and health industry. Bismuth has a significant role in the treatment of gastric diseases such as gastric ulcers. In addition, bismuth is also used in semiconductor materials, cosmetics, dyes, biomedical science, combustion aids, and other industries (1–3). Therefore, it is significant to study methods for the determination of bismuth in the environment. Currently, measurement methods for bismuth mainly include resonance light scattering spectrometry (RLS), atomic absorption spectrometry (AAS), ethylenediaminetetraacetic acid (EDTA) titration, extraction spectrophotometry, flow injection analysis (FIA), inhibition kinetic spectrophotometry, inductively coupled plasma–atomic emission spectrometry (ICP-AES), adsorptive stripping voltammetry, and fluorescence spectrometry (4–13). Kinetic spectrophotometry has the advantages of high sensitivity and simple operation and has been extensively discussed in reports of determination of trace elements. Therefore, kinetic spectrophotometry has become a very important method for the analysis and detection of microtrace elements (14–20). Three-dimensional fluorescence spectroscopy is a new fluorescence analysis technology that was developed within the past 10 years. It is a function of two wavelength parameters, of excitation and emission. The three-dimensional fluorescence spectrum can be displayed by a three-dimensional stereogram or a planar contour map, which has the advantages of simplicity and intuitiveness (21–23).

It was found that trace amounts of bismuth had a significant catalytic effect on the reaction of eosin Y with hydrogen peroxide under pH 4.50 and 90 

C. A novel catalytic kinetic fluorescence method was established for the determination of trace bismuth. The method has a low detection limit, high sensitivity, and simple operation. It also has the ability to determine trace bismuth in human hair and sewage samples.

The fluorescence measurements were carried out with an F-7100 fluorescence spectrophotometer (Hitachi). Absorption spectra were recorded on a U-3900H UV-Vis spectrophotometer (Hitachi). The pH was measured by a pHS-3C meter (Shanghai Leici Instruments). The solution was heated in a HH-2 thermostatic water bath (Shanghai Lichen Instrument Technology).

The stock solution of bismuth (0.1000 g/L) was prepared as follows: 0.2320 g of Bi(NO

O (Tianjin Comeo Chemical Reagent Development Center, China) dis- solved in 10 mL of nitric acid solution (φ = 25%) and diluted to 1000 mL. The stock solution was diluted to 0.1000 μg/L as a standard working solution with water, 25% nitric acid solution, 0.20 mg/mL (w/v) eosin Y solution, 3% (v/v) hydrogen peroxide solution, and 1.00% (w/v) EDTA solution. The reagents were of analytical regent (AR) grade, and doubly distilled water was used throughout this work.

At a pH of 4.50, 1.50 mL of 0.20 mg/ mL eosin Y solution, 5.00 mL of 3% H

 solution, and 0.1000 μg/L in different volumes of the standard solution of bismuth were poured into a 25-mL colorimetric tube with stopper. The mixture was diluted to the mark with water and mixed thoroughly. The solution was heated in a 90 

C water bath for 15 min before it was cooled to room temperature. The solution was then cooled with cold running water for 5 min before 2 mL of 1% EDTA was added to terminate the reaction. The fluorescence spectra of the system were recorded at λex = 515 nm and λem = 542 nm, and the absorbance was measured in a 1-cm cell at the wavelength of the maximum absorbance against a reagent blank. The variation of the fluorescence intensities was denoted as the relative fluorescence intensity lg(

. When the relative intensity was calculated, a standard curve within a specific concentration range was made.

 were the fluorescence intensities of system and reagent blank, respectively.

The verification test examined whether bismuth catalyzed with eosin Y and caused the discoloration in the presence of hydrogen peroxide. If it does, the eosin Y system should see a reduction in its fluorescence intensity and show a linear relationship. To determine the accuracy of the verification test, a reference was selected, according to the principles of the experiment.

There were two prepared sets of comparative experiments, according to the experimental conditions. In one of them, 0.50 mL of bismuth was added, and in the other, no bismuth was added. The fluorescence intensity spectrum was measured after the reaction concluded. As shown in Figure 1, the fluorescence intensity was excellent when bismuth was not added, and the fluorescence intensity decreased after adding bismuth. Therefore, bismuth can reduce the fluorescence intensity of eosin Y.

According to the experimental method, different component solutions were prepared to obtain the fluorescence intensity spectrum curve, as shown in Figure 2. From Figure 2, it is verified again that bismuth has a greater catalytic effect on the fading reaction of hydrogen peroxide–oxidized eosin Y, which meets the experimental principle. The fluorescence intensity shown in Figures 2a and 2b without bismuth is strong and the difference is very small, which is quite different from the weakened fluorescence intensity shown in Figure 2c, 2d, 2e, and 2f. At the same time, comparing Figure 2c and 2d, the fluorescence intensity of 2c without hydrogen peroxide is stronger, indicating that hydrogen peroxide has reacted with eosin Y and has a strong and measurable oxidation effect.

Figure 3 displays a three-dimensional fluorescence plane contour plot of a blank solution without bismuth. As shown in Figure 3, there are three spectral lines from the lower left corner to the upper right corner that are generated by Rayleigh scattering of the solution. These three lines are stronger in the shorter wavelength portion (bottom left corner), but weaken and disappear in the longer wavelength portion (upper right corner). Rayleigh scattering lines and fluorescence contour lines cross between 500 and 580 nm. As can be seen from the fluorescence 3-D graph of Figure 3, the fluorescence intensity is the largest at λ

Acidity has an important effect on the progress of the catalytic reaction. Under acidic conditions, bismuth can catalyze the oxidation of eosin Y by H

. The experiment was chosen to investigate the effect of H2O2 on the fluorescence intensity of Eosin Y when the pH was be- tween 2 and 6. The experimental results showed that when the pH value was between 4 and 5, the relative fluorescence intensity Δ

) was large and remained constant. The relative fluorescence intensity of the system was the largest at pH = 4.50, so that pH level was used in the experiment.

Too much or too little eosin Y will affect the results of the experiment. On the premise of ensuring that eosin Y has no self-extinguishing fluorescence, 0.50 to 3.00 mL of eosin Y was added, according to the experimental method. The results are shown in Figure 4. When 1.50 mL of eosin Y was added, the fluorescence intensity was the strongest, and it was stable. The fluorescence intensity was the worst when 0.50 mL of eosin Y was added. The fluorescence intensity was similar when 2.00 mL and 2.50 mL of eosin Y was added. Because the concentration of the developer is too high, self-absorption will occur, and the fluorescence intensity will decrease. The optimal amount of eosin Y was determined to be 1.50 mL. Under the selected test conditions, the experiment was carried out by changing the amount of H

 to investigate its effect on the catalytic reaction. As shown in Figure 4, increasing amounts of oxidant H O can speed up the catalytic reaction. When the amount of H

 is 5.00 mL, the fluorescence intensity is the strongest and stable. When the amount of H

 is too large, the excess oxidant greatly reduces the absorbance of noncatalytic reactions. Thus, the optimal amount of H

Under the selected experimental conditions, the effect of the reaction temperature and time on the experimental reaction was investigated. The results showed that when the temperature was above 25 

C, the catalytic reaction sped up and the absorbance increased with increasing temperature. When the temperature was 90 

C, the relative fluorescence intensity of the system was the largest and stable. When the temperature was higher than 90 

C, the absorbance decreased because the temperature was too high. Thus, the optimal reaction temperature was determined to be 90 

C. Subsequently, the fluorescence emission intensity of the system at different reaction times (0–25 min) was measured at 90 

C. The results showed that the fluorescence intensity increased rapidly when the heating time was within 0–15 min, the fluorescence intensity decreased when the heating time was between 15–25 min, and the relative fluorescence emission intensity reached the maximum when the heating time was 15 min. Therefore, the optimal response time was determined to be 15 min. When the reaction was terminated with EDTA, the amount of 1.00% EDTA was 2.00 mL.

According to the above experimental method, the calibration curve was established by conducting a series of similar experiments at various concentrations of bismuth within a certain concentration range. The results of these are shown in Figures 5a and 5b. The results show that the fluorescence intensity of the solution decreased with increasing concentrations of bismuth and when the volume of bismuth solution was 0.10–1.00 mL, that is, the actual concentration was 4.0 × 10

 μg/L, there was a good linear relationship between the lg(

) and the concentrations of bismuth. The linear regression equation has the form lg (

) = 0.00277 + 1.44689ρBi3+(μg/L), and the correlation coefficient was 0.9971. The bismuth solutions of 0.20 mL and 1.00 mL were measured 21 times, and the relative standard deviations (RSD) were 3.20% and 1.97%, respectively. The sensitivity of this experimental method is based on the minimum amount of the test substance that can be detected, the detection limit was 1.9 × 10

According to the procedure, if the coexisting ions cause a relative error of less than or equal to ±5% in the fluorescence intensity change of system, the common ions were considered to have few interferences for the determination of bismuth. The measurement results are shown in Table I.

A 100 mL sample of local sewage was poured into a volumetric flask for subsequent use and filtered with quick filter paper to remove insoluble impurities. Then, 50 mL of the filtrate was drawn into a beaker, then 1 mL of concentrated hydro- chloric acid, 2 mL of concentrated nitric acid, and the solution were added and heated on an electric furnace until the sewage evaporated to near dryness. When the sample reached room temperature, 1 mL of concentrated hydrochloric acid and a small amount of water was poured into the beaker. The beaker was then heated to boiling for several minutes and cooled to room temperature. The pH level was adjusted to 4.50, before the solution was transferred to a 100-mL volumetric flask. The mixture was diluted to the mark with water and mixed thoroughly. The measurement was performed according to the selected experimental method, and the spiked recovery experiment was performed at the same time. Table II shows these results.

Female human hair samples were digested by the wet nitrification method, in which a 3.006 g hair sample was washed with acetone and ultrapure water and dried, then added to a beaker, nitrated with 60 mL of a concentrated HNO

 solution [(5:1) v:v] at room temperature for 3 h, and heated on a hot plate until the brown smoke was gone. After it was cooled to room temperature, following the same procedure as above, 1.50 mL of eosin Y, 5.00 mL of hydrogen peroxide, 2.00 mL of EDTA solution, and 0.50 mL of bismuth solution were poured into the beaker before adding 0.00 mL, 0.50 mL, 1.00 mL, and 1.50 mL of human hair for reaction, respectively. The reaction was terminated, the residue was re-nitrated with 30 mL of a mixture of HNO

 [(2:1) v:v]. The final residue was dissolved in distilled water and transferred to a 50–mL volumetric flask to obtain a transparent solution for experiments. The measurement was performed according to the experimental method, and the results were shown in Table II.

Fluorescence quenching usually includes static quenching and dynamic quenching. It can be distinguished by measuring the relationship between the quenching constant and temperature. Static quenching is due to the combination of the quencher and the fluorophore to generate non-fluorescent substances. Therefore, when the temperature increases, the degree of turbulence in the system increases, resulting in a decrease in the quenching constant. While dynamic quenching is due to quenching, the collision between the extinguishing agent and the fluorophore causes the fluorescence intensity to decrease. Therefore, when the temperature increases and the turbulence of the system increases, the collision is intensified, and the quenching constant increases.

Dynamic quenching is related to diffusion. When the temperature increases, the viscosity of the solution decreases and the molecular movement accelerates. As a result, the diffusion coefficient of the molecule will increase, which will increase the bimolecular quenching constant. Conversely, an increase in temperature may cause the stability of the complex to decrease, thereby reducing the extent of static quenching. According to the Stern-Volmer equation, if the slope at high temperature is greater than the slope at low temperature where dynamic quenching is occurring; otherwise, it is static quenching (24).

According to the experimental scheme, the ultraviolet absorption intensity of the eosin Y system with different amounts of bismuth standard solution was measured, and the quenching type was analyzed at different temperatures. The results are shown in Figures 6 and 7. As the temperature increases, the quenching type observed in this experiment is presumed to be dynamic quenching.

R is a kind of unsaturated lactone containing two phenol light groups to produce acidity. Even a weak oxidant can convert it into dehydrogenated R. Under its own acidity pH = 4.50 and heating in a boiling water bath, the reaction as to what this looks like can be seen in Figure 8.

 to produce a ·OH radical based on a bimolecular reaction:

Because ·OH free radicals are small, uncharged, and have a high oxidation potential, they can further oxidize R and accelerate the fading reaction of R, as seen in Figure 9.

 to generate a ·OH radical, which catalyzes the fading reaction of H

A novel method for the detection of trace bismuth by catalytic kinetic fluorescence was established. Experiments showed that bismuth could catalyze the discoloration of eosin Y in the presence of peroxy hydroxide, weaken the fluorescence intensity, and even lose fluorescence. lg(

) and the concentration of bismuth show a good linear relationship, which can be used as a basis for the detection of trace bismuth. The concentration of bismuth in the range of 4.0 × 10

 μg/L has a good linear relationship with lg(

), the correlation coefficient is 0.9971, and the detection limit of the method is 1.9 × 10

 μg/L. The method used for the detection of bismuth in human hair samples and water samples, with simple operation and high accuracy, and the results were consistent with the certified values.

The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 61805010).

(3) H. T. Sun, J. J. Zhou, and J. R. Qiu, 

(7) A. Afkhami, T. Madrakian, and H. Siampour, 

(8) G. Gumus, H. Filik, and B. Demirata, 

(9) S. H. Xu, F. J. Jiao, and G. J. Dang, 

(11) N. Velitchkova, E. N. Pentcheva, and N. Daskalova,

(13) F. W. Wan, M. Zhang, and S. W. Wang, 

(14) D. Meneses, Júnior, F. José Guimarães F, and P. C. C. De Oliveira, 

(15) A. Masek, E. Chrzescijanska, and M. Latos-Brozio, 

(16) B. Moavenian, M. H. Hosseini, and M. Arabieh, 

(18) T. N. Al-Sabha, M. Y. Dhamra, and T. S. Al-Ghabsha, 

(19) O. Jelena, H. Safija, and B. Zerina, 

(21) N. V. D. Santos, C. F. Saponi, and T. L. Greaves, 

(22) R. R. Wang, X. G. Lin, and S. Zhou, 

(25) W. Lin, J. Zhangzhou Teachers College 

 are with the College of Chemical and Biological Engineering at Lanzhou Jiaotong University in Lanzhou, China. 

 is with the Key Institute of Lightwave Technology at Beijing Jiaotong University in Beijing, China. 

 is with the Zhangye Food and Drug Inspection Testing Center in Zhangye, China. Direct correspondence to: shengl@mail.lzjtu.cn

Sheng Li

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