A Visual Color Response Test Paper for the Detection of Hydrogen Sulfide Gas in the Air

Hydrogen sulfide (H2S) is widely found in oil and natural gas wells and industrial wastewater tanks. Owing to its high toxicity, the monitoring and detection of H2S in the air is essential. However, recent techniques for the quantitative detection of H2S gas suffer from limitations such as high cost, complicated operation, and insufficient sensitivity, preventing their practical application in industry. Thus, we have developed a portable test paper for real-time and inexpensive monitoring of H2S gas by color changes. The test paper had a significantly low H2S detection limit of 200 ppb, which is considered safe for humans. Moreover, the color of the test paper did not change noticeably when exposed to CO2, N2, O2, and air environments, indicating that the test paper is selective for H2S gas and can be stored for a long time. In addition, we fitted a color difference linear model between the color difference values (ΔE) and the concentrations of H2S gas. The establishment of the linear model substantiates that the test paper can provide accurate intensity information when detecting H2S gas leakage.


Introduction
Hydrogen sulfide (H 2 S) is a suffocating, acidic, and highly toxic gas with a rotten egg odor that can be rapidly absorbed by the lungs [1]. H 2 S is mainly produced during the lowtemperature coking of coal and refining of sulfur-containing oil and natural gas [2][3][4]. The decomposition of sulfur-containing amino acids in meat can also release a small amount of H 2 S gas [5]. Furthermore, sulfate-reducing bacteria in industrial wastewater can reduce sulfates to H 2 S under the action of organic matter, which not only contaminates drinking water but also corrodes pipes and equipment [6,7]. The U.S. Occupational Safety and Health Administration (OSHA) considers it safe for workers to be exposed to 10 ppm H 2 S for a maximum of 8 h. When the human body inhales H 2 S at concentrations greater than 5 ppm, olfactory nerve fatigue may occur, and inhalation of H 2 S concentrations beyond 50 ppm can cause convulsions or shock. Sudden death occurs when the human body inhales H 2 S at concentrations higher than 1000 ppm [8]. Therefore, judging a H 2 S gas leakage in the air by smelling could be dangerous, and it is necessary to detect H 2 S gas leakage through instruments.

Results and Discussion
The discussion in this chapter is based on three different types of test paper: SbCl 3 test paper, anthocyanin test paper, and mixed test paper. First, we found that anthocyanin test paper was responsive to hydrogen sulfide gas, but there was a problem with its high detection limit. Next, we used a more stable inorganic reagent SbCl 3 as a hydrogen sulfide indicator and found that although it lowered the detection limit, it was difficult to determine the leakage concentration of hydrogen sulfide. Finally, SbCl 3 and anthocyanin were used as mixed indicators to successfully prepare a composite test paper with a concentration gradient response to hydrogen sulfide gas. The effects of hydrogen sulfide concentration, temperature, and interfering gas on the response performance of different test strips were studied. The working principle of the test paper was inferred through SEM, XPS, and other surface verification methods. The changes in the surface color and ∆E values of different test papers (coated with different concentrations of SbCl 3 and/or anthocyanin) placed in 10 ppm H 2 S gas for 30 min are shown in Figure 1. As shown in Figure 1a, before the experiment, increasing anthocyanin concentration resulted in a gradual deepening of the blue on the test paper. However, the increase in SbCl 3 concentration led to the gradual deepening of purple. For example, when the SbCl 3 concentration was fixed at 0.05 wt.%, an increase in anthocyanin concentration led to a gradual change in the surface color of the test paper from light blue to dark blue, whereas at an anthocyanin concentration of 2 wt.%, the surface color of the test paper gradually turned from light blue to purple with the increase in SbCl 3 concentration.
blue, whereas at an anthocyanin concentration of 2 wt.%, the surface color of the test paper gradually turned from light blue to purple with the increase in SbCl3 concentration.
After exposure to 10 ppm H2S gas for 30 min, the surface of the test papers changed color to varying degrees. The surface of the anthocyanin test paper appeared red-dotted, while the SbCl3 test paper turned bright yellow due to an increase in indicator concentration. In the case of test papers coated with mixed solutions, an increase in anthocyanin concentration led to a gradual deepening of the brown on the test paper after the experiment. In contrast, an increase in SbCl3 concentration led to the gradual deepening of red. For example, after H2S exposure, the surface of the test paper coated with 1 wt.% SbCl3 and 2 wt.% anthocyanin turned brown while the surface of the test paper coated with 1 wt.% SbCl3 and 10 wt.% anthocyanin turned red. Figure 1b shows the ∆E value digitized using a colorimeter, where a higher ∆E indicates a higher sensitivity of the test paper to H2S gas. ∆E gradually increased as the concentration of the indicator increased, reaching the maximum (∆E = 48.5) when the indicator was 0.5 wt.% SbCl3 and 10 wt.% anthocyanin. However, when the indicator was 0.5 wt.% SbCl3 and 4 wt.% anthocyanin, the test paper demonstrated a clear detection of H2S, and the ∆E did not increase significantly with further increase in the indicator concentration. Thus, the response test paper coated with 0.5 wt.% SbCl3 and 4 wt.% anthocyanins was selected for further testing and analysis.  Figure 2a shows the color change in the test paper coated with 0.5 wt.% SbCl3 and 4 wt.% anthocyanin after exposure to 10 ppm H2S gas for 120 min at different temperatures. At 0 °C, the surface color of the test paper gradually changed from blue-violet to pinkpurple within 30 min, while at 25 °C the blue-violet color on the surface quickly disappeared and turned pink. At 60 °C, the surface was entirely covered by yellow-brown color within 1 min, and at 80 °C the surface color rapidly turned black-brown. Figure 2b shows the color change in the response test paper exposed to 10 ppm H2S and common gases (CO2, O2, N2, and air) for 120 min. The surface color of the test paper remained unchanged after exposure to a saturated CO2 environment for 120 min, indicating that CO2 did not interfere with the test paper. Similarly, the surface color did not change when exposed to O2, N2, and air, implying that test paper can be stored at room temperature for a long time without deterioration. The results indicate that the test paper has a characteristic response to H2S gas. Moreover, its low sensitivity to O2, N2, CO2, CH4,  After exposure to 10 ppm H 2 S gas for 30 min, the surface of the test papers changed color to varying degrees. The surface of the anthocyanin test paper appeared red-dotted, while the SbCl 3 test paper turned bright yellow due to an increase in indicator concentration. In the case of test papers coated with mixed solutions, an increase in anthocyanin concentration led to a gradual deepening of the brown on the test paper after the experiment. In contrast, an increase in SbCl 3 concentration led to the gradual deepening of red. For example, after H 2 S exposure, the surface of the test paper coated with 1 wt.% SbCl 3 and 2 wt.% anthocyanin turned brown while the surface of the test paper coated with 1 wt.% SbCl 3 and 10 wt.% anthocyanin turned red. Figure 1b shows the ∆E value digitized using a colorimeter, where a higher ∆E indicates a higher sensitivity of the test paper to H 2 S gas. ∆E gradually increased as the concentration of the indicator increased, reaching the maximum (∆E = 48.5) when the indicator was 0.5 wt.% SbCl 3 and 10 wt.% anthocyanin. However, when the indicator was 0.5 wt.% SbCl 3 and 4 wt.% anthocyanin, the test paper demonstrated a clear detection of H 2 S, and the ∆E did not increase significantly with further increase in the indicator concentration. Thus, the response test paper coated with 0.5 wt.% SbCl 3 and 4 wt.% anthocyanins was selected for further testing and analysis. Figure 2a shows the color change in the test paper coated with 0.5 wt.% SbCl 3 and 4 wt.% anthocyanin after exposure to 10 ppm H 2 S gas for 120 min at different temperatures. At 0 • C, the surface color of the test paper gradually changed from blue-violet to pinkpurple within 30 min, while at 25 • C the blue-violet color on the surface quickly disappeared and turned pink. At 60 • C, the surface was entirely covered by yellow-brown color within 1 min, and at 80 • C the surface color rapidly turned black-brown.  Figure 2a shows the color change in the test paper coated with 0.5 wt.% SbCl3 and 4 wt.% anthocyanin after exposure to 10 ppm H2S gas for 120 min at different temperatures. At 0 °C, the surface color of the test paper gradually changed from blue-violet to pinkpurple within 30 min, while at 25 °C the blue-violet color on the surface quickly disappeared and turned pink. At 60 °C, the surface was entirely covered by yellow-brown color within 1 min, and at 80 °C the surface color rapidly turned black-brown. Figure 2b shows the color change in the response test paper exposed to 10 ppm H2S and common gases (CO2, O2, N2, and air) for 120 min. The surface color of the test paper remained unchanged after exposure to a saturated CO2 environment for 120 min, indicating that CO2 did not interfere with the test paper. Similarly, the surface color did not change when exposed to O2, N2, and air, implying that test paper can be stored at room temperature for a long time without deterioration. The results indicate that the test paper has a characteristic response to H2S gas. Moreover, its low sensitivity to O2, N2, CO2, CH4, and air makes it a new method for the on-site detection of H2S gas.   Figure 2b shows the color change in the response test paper exposed to 10 ppm H 2 S and common gases (CO 2 , O 2 , N 2 , and air) for 120 min. The surface color of the test paper remained unchanged after exposure to a saturated CO 2 environment for 120 min, indicating that CO 2 did not interfere with the test paper. Similarly, the surface color did not change when exposed to O 2 , N 2 , and air, implying that test paper can be stored at room temperature for a long time without deterioration. The results indicate that the test paper has a characteristic response to H 2 S gas. Moreover, its low sensitivity to O 2 , N 2 , CO 2 , CH 4 , and air makes it a new method for the on-site detection of H 2 S gas. Figure 3a shows the color change in the test paper coated with 1 wt.% SbCl 3 after exposure to different H 2 S gas concentrations for different time intervals. The surface color of the test paper was white before the exposure, which turned into a uniform pale yellow within 10 min when exposed to H 2 S concentrations of 200 ppb and 500 ppb. However, at a higher H 2 S gas concentration of 1 ppm, the surface color changed quickly from white to pale yellow within 1 min, and the yellow gradually deepened with time. Further increase in the H 2 S gas concentration led to a faster response rate, with the surface color changing from white to yellow in 30 s at 4 ppm H 2 S. The above findings indicate that the test paper with 1 wt.% SbCl 3 coating can quickly and efficiently detect H 2 S gas in the air with a detection limit as low as 200 ppb. from white to yellow in 30 s at 4 ppm H2S. The above findings indicate that the test paper with 1 wt.% SbCl3 coating can quickly and efficiently detect H2S gas in the air with a detection limit as low as 200 ppb.

The Concentration of H 2 S Gas
A colorimeter was used to extract the ∆E values of the test papers, and the color change was digitized using Equation (4). When the test paper coated with 1 wt.% SbCl3 was exposed to 10 ppm H2S for 60 min, the ΔE reached a maximum value of 62.26, as shown in Figure 3b. Moreover, the ΔE was 6.31 when the test paper was placed in a 200 ppb H2S for 10 min. Typically, ΔE > 3.3 is considered a critical value for determining whether the difference between the two colors can be observed through the naked eye [22]. However, similar ΔE values and faster reaction speed result in no obvious gradient response to varying concentrations of H2S gas using the test paper coated only with SbCl3, which makes it difficult to determine the concentration of H2S gas. A colorimeter was used to extract the ∆E values of the test papers, and the color change was digitized using Equation (4). When the test paper coated with 1 wt.% SbCl 3 was exposed to 10 ppm H 2 S for 60 min, the ∆E reached a maximum value of 62.26, as shown in Figure 3b. Moreover, the ∆E was 6.31 when the test paper was placed in a 200 ppb H 2 S for 10 min. Typically, ∆E > 3.3 is considered a critical value for determining whether the difference between the two colors can be observed through the naked eye [22]. However, similar ∆E values and faster reaction speed result in no obvious gradient response to varying concentrations of H 2 S gas using the test paper coated only with SbCl 3 , which makes it difficult to determine the concentration of H 2 S gas. Figure 4a shows the color change in the test paper coated with 0.5 wt.% SbCl 3 and 4 wt.% anthocyanin after exposure to different H 2 S gas concentrations for different time intervals. The purple-blue surface color of the test paper turned into a uniform pale pink within 30-60 min when exposed to H 2 S concentrations of 200 ppb and 500 ppb. Furthermore, after exposure to 1 ppm H 2 S gas, the surface color of the test paper gradually turned purple to pink within 10-30 min. Further increase in the H 2 S gas concentration led to a faster response rate, with the test paper surface showing a uniform pink color in 5 min at 8 ppm and 10 ppm H 2 S concentrations. Thus, the H 2 S gas concentration significantly affects the color change rate of the test paper.
In the previous study, the detection limit of the response film containing only anthocyanin was 10 ppm [20]. In contrast, the composite test paper (coated with 0.5 wt.% SbCl3 and 4 wt.% anthocyanin) developed in this work demonstrated a significantly lower H2S detection limit of 200 ppb with a faster response rate. Compared to the response film, the surface color changes in the test paper were more uniform and stable. Moreover, the response film in the previous study was blue, and the composite test paper was blue-purple, suggesting the interaction of SbCl3 anthocyanin.
As shown in Figure 4b, after exposing the composite test paper to 1 ppm H2S for 60 min, the ΔE reached a maximum value of 55.55. However, the values of ΔE were 13.22 and 14.89 after 30 min of exposure to 200 ppb and 500 ppb H2S gas, respectively. Although the response time of the composite test paper was higher than the SbCl3 test paper, the surface color change in the composite test paper after 30 min was prominent to the naked eye. This indicated a lower H2S detection limit (200 ppb) of the composite test paper.  Although the SbCl3 and anthocyanin composite test paper had the disadvantages of a higher detection limit and slower detection speed than the 1 wt.% SbCl3 test paper, the ΔE and H2S concentration (1-10 ppm) showed a better gradient response (Figures 3 and  4). As shown in Figure 5, we fitted a color difference linear model of the test paper coated with different concentrations of SbCl3 and 4 wt.% anthocyanins as a function of H2S concentration. We found a linear relationship between the values of ΔE and the concentrations of H2S in the range of 1-10 ppm. The ΔE from two test papers increased as the H2S concentration increased. The test paper with 0.5 wt.% SbCl3 and 4 wt.% anthocyanin coating showed a significant ΔE value and conformed to the linear model. Establishing the linear model provides a judgment standard for the practical application of the response test paper combined with an intelligent device and provides accurate intensity information when detecting H2S gas leakage. It is expected that it will be possible to recognize and digitize the color change in test paper with the future development of AI robots, and implement an automatic alarm when the H2S concentration reaches the dangerous range.  In the previous study, the detection limit of the response film containing only anthocyanin was 10 ppm [20]. In contrast, the composite test paper (coated with 0.5 wt.% SbCl 3 and 4 wt.% anthocyanin) developed in this work demonstrated a significantly lower H 2 S detection limit of 200 ppb with a faster response rate. Compared to the response film, the surface color changes in the test paper were more uniform and stable. Moreover, the response film in the previous study was blue, and the composite test paper was blue-purple, suggesting the interaction of SbCl 3 anthocyanin.
As shown in Figure 4b, after exposing the composite test paper to 1 ppm H 2 S for 60 min, the ∆E reached a maximum value of 55.55. However, the values of ∆E were 13.22 and 14.89 after 30 min of exposure to 200 ppb and 500 ppb H 2 S gas, respectively. Although the response time of the composite test paper was higher than the SbCl 3 test paper, the surface color change in the composite test paper after 30 min was prominent to the naked eye. This indicated a lower H 2 S detection limit (200 ppb) of the composite test paper.
Although the SbCl 3 and anthocyanin composite test paper had the disadvantages of a higher detection limit and slower detection speed than the 1 wt.% SbCl 3 test paper, the ∆E and H 2 S concentration (1-10 ppm) showed a better gradient response (Figures 3  and 4). As shown in Figure 5, we fitted a color difference linear model of the test paper coated with different concentrations of SbCl 3 and 4 wt.% anthocyanins as a function of H 2 S concentration. We found a linear relationship between the values of ∆E and the concentrations of H 2 S in the range of 1-10 ppm. The ∆E from two test papers increased as the H 2 S concentration increased. The test paper with 0.5 wt.% SbCl 3 and 4 wt.% anthocyanin coating showed a significant ∆E value and conformed to the linear model. Establishing the linear model provides a judgment standard for the practical application of the response test paper combined with an intelligent device and provides accurate intensity information when detecting H 2 S gas leakage. It is expected that it will be possible to recognize and digitize the color change in test paper with the future development of AI robots, and implement an automatic alarm when the H 2 S concentration reaches the dangerous range. Although the SbCl3 and anthocyanin composite test paper had the disadvantages of a higher detection limit and slower detection speed than the 1 wt.% SbCl3 test paper, the ΔE and H2S concentration (1-10 ppm) showed a better gradient response (Figures 3 and  4). As shown in Figure 5, we fitted a color difference linear model of the test paper coated with different concentrations of SbCl3 and 4 wt.% anthocyanins as a function of H2S concentration. We found a linear relationship between the values of ΔE and the concentrations of H2S in the range of 1-10 ppm. The ΔE from two test papers increased as the H2S concentration increased. The test paper with 0.5 wt.% SbCl3 and 4 wt.% anthocyanin coating showed a significant ΔE value and conformed to the linear model. Establishing the linear model provides a judgment standard for the practical application of the response test paper combined with an intelligent device and provides accurate intensity information when detecting H2S gas leakage. It is expected that it will be possible to recognize and digitize the color change in test paper with the future development of AI robots, and implement an automatic alarm when the H2S concentration reaches the dangerous range.   Figure 5. Linear fitting of a* of the surface color change in 0.5 wt.% SbCl3 and 4 wt.% anthocyanin test paper exposed to H2S gas for 30 min. Figure 6 shows some excellent studies on the visual inspection of H2S. For example, the nanofibers prepared by Dong-Ha Kim [22] can detect H2S gas under dry conditions with a ppm-level detection limit (1 ppm). Compared with the gas phase, H2S detection in Figure 5. Linear fitting of a* of the surface color change in 0.5 wt.% SbCl 3 and 4 wt.% anthocyanin test paper exposed to H 2 S gas for 30 min. Figure 6 shows some excellent studies on the visual inspection of H 2 S. For example, the nanofibers prepared by Dong-Ha Kim [22] can detect H 2 S gas under dry conditions with a ppm-level detection limit (1 ppm). Compared with the gas phase, H 2 S detection in a solution is easier. For example, the fluorescent test paper prepared by Yan Feng [7] for H 2 S detection in wastewater had a low detection limit of 6.8 ppb. Similarly, the detection limit of the sensor designed by Yisheng Lin [23] for H 2 S detection in a solution was as low as 265 ppb. It is worth mentioning that the sensor prepared by Thomas S. [1] could detect 30 ppb of H 2 S gas, but it requires wet and alkaline conditions to achieve ppblevel detection. However, the H 2 S detection media developed by Xiaokun Yang [24], Xue Han [25], and Huanling Wu [26] had relatively higher H 2 S detection (in solutions) limits and the corresponding detection times were not clear. The response test paper developed in the current work can detect 200 ppb of H 2 S gas in a dry neutral environment without interference from other common gases and provide an accurate response time. Of course, the detection in wet conditions or detection methods based on nanomaterials may be our learnable methods for further lowering the H 2 S detection limit.
tion. However, the H2S detection media developed by Xiaokun Yang [24], Xue Han [25], and Huanling Wu [26] had relatively higher H2S detection (in solutions) limits and the corresponding detection times were not clear. The response test paper developed in the current work can detect 200 ppb of H2S gas in a dry neutral environment without interference from other common gases and provide an accurate response time. Of course, the detection in wet conditions or detection methods based on nanomaterials may be our learnable methods for further lowering the H2S detection limit.

XPS Analysis of Test Paper
To confirm the reaction mechanism between H2S and the response test paper, XPS was used to analyze the changes in the valence states of surface elements before and after exposure to 10 ppm H2S gas for 30 min. Figure 7a shows the binding energy peaks of 288.0 eV, 286.3 eV, and 284.8 eV corresponding to C=O, C-O-C, and C-C functional groups. The newly added binding energy peak of 289.4 eV after exposure to H2S corresponds to the C-S bond [27]. In the exposed S2p spectra (Figure 7b), binding energy peaks of 168.0 eV and 163.3 eV correspond to the S-O and C-S functional groups. This demonstrates a chemical reaction between anthocyanin test paper and H2S, which is consistent with the previous research findings [20].
As shown in Figure 7c, the binding energy peaks of the Sb element before and after the exposure of the 0.5 wt.% SbCl3-coated test papers were 539.3 eV and 529.9 eV, respectively, indicating that the Sb element always existed in the form of Sb (III) [28]. The exposed test paper showed binding energy peaks of 161.6 eV and 162.8 eV, indicating that SbCl3 and H2S reacted to form Sb2S3 [29]. Figure 7e shows that the binding energy peak shifts of the Sb element before the exposure of the composite (0.5 wt.% SbCl3 and 4 wt.% anthocyanin-coated) test paper were 538.6 eV and 529.1 eV, which may be attributed to the interaction of the Sb element with some organic substances during the preparation of the test paper. It is speculated that the O atom of the C=O bond in the PC extract had a lone pair of electrons that coordinated with the empty orbital of Sb (III). Therefore, the binding energy peak of 538.6 eV corresponds to the Sb3d3/2 orbital spin splitting of C=O-Sb [30]. After exposure to H2S gas, the binding energy peaks of 530.0 eV and 539.4 eV correspond to Sb (III). Combined with the S2p spectra, the binding energy peaks of 164.

XPS Analysis of Test Paper
To confirm the reaction mechanism between H 2 S and the response test paper, XPS was used to analyze the changes in the valence states of surface elements before and after exposure to 10 ppm H 2 S gas for 30 min. Figure 7a shows the binding energy peaks of 288.0 eV, 286.3 eV, and 284.8 eV corresponding to C=O, C-O-C, and C-C functional groups. The newly added binding energy peak of 289.4 eV after exposure to H 2 S corresponds to the C-S bond [27]. In the exposed S2p spectra (Figure 7b), binding energy peaks of 168.0 eV and 163.3 eV correspond to the S-O and C-S functional groups. This demonstrates a chemical reaction between anthocyanin test paper and H 2 S, which is consistent with the previous research findings [20].
As shown in Figure 7c, the binding energy peaks of the Sb element before and after the exposure of the 0.5 wt.% SbCl 3 -coated test papers were 539.3 eV and 529.9 eV, respectively, indicating that the Sb element always existed in the form of Sb (III) [28]. The exposed test paper showed binding energy peaks of 161.6 eV and 162.8 eV, indicating that SbCl 3 and H 2 S reacted to form Sb 2 S 3 [29]. Figure 7e shows that the binding energy peak shifts of the Sb element before the exposure of the composite (0.5 wt.% SbCl 3 and 4 wt.% anthocyanin-coated) test paper were 538.6 eV and 529.1 eV, which may be attributed to the interaction of the Sb element with some organic substances during the preparation of the test paper. It is speculated that the O atom of the C=O bond in the PC extract had a lone pair of electrons that coordinated with the empty orbital of Sb (III). Therefore, the binding energy peak of 538.6 eV corresponds to the Sb3d 3/2 orbital spin splitting of C=O-Sb [30]. After exposure to H 2 S gas, the binding energy peaks of 530.0 eV and 539.4 eV correspond to Sb (III). Combined with the S2p spectra, the binding energy peaks of 164.5 eV and 163.3 eV can be attributed to the S2p 1/2 and S2p 3/2 orbital spin splits of C-S, while the binding energy peaks of 162.2 eV and 161.0 eV correspond to Sb 2 S 3 [31].   Figure 8 shows the SEM images and EDX map of the different test papers before and after exposure to 10 ppm H2S for 30 min. Figure 8(a1,b1,c1) shows a large number of fibrous holes on the surface of the three test papers before exposure. These fibrous holes increased the contact area between the test paper and the H2S gas, making H2S adsorption easier. As shown in Figure 8(a1,a2), the test paper coated with 4 wt.% anthocyanin did not change significantly before and after exposure. However, the EDX analysis of the exposed surface showed that the atomic ratio of the S element was 0.33% (Figure 8(a3,a4)), indicating that the test paper coated with 4 wt.% anthocyanin could detect H2S gas, which is consistent with our previous research [20].   Figure 8 shows the SEM images and EDX map of the different test papers before and after exposure to 10 ppm H 2 S for 30 min. Figure 8(a1,b1,c1) shows a large number of fibrous holes on the surface of the three test papers before exposure. These fibrous holes increased the contact area between the test paper and the H 2 S gas, making H 2 S adsorption easier. As shown in Figure 8(a1,a2), the test paper coated with 4 wt.% anthocyanin did not change significantly before and after exposure. However, the EDX analysis of the exposed surface showed that the atomic ratio of the S element was 0.33% (Figure 8(a3,a4)), indicating that the test paper coated with 4 wt.% anthocyanin could detect H 2 S gas, which is consistent with our previous research [20].

SEM Images of Test Paper
As shown in Figure 8(b1,b2), the test paper coated with 0.5 wt.% SbCl 3 presented a uniform flocculent structure before exposure, which diminished after exposure. Combined with XPS, it was found that SbCl 3 reacts with H 2 S gas to generate Sb 2 S 3 , but Sb 2 S 3 has a poor binding force with the test paper and falls off slowly during the experiment. Similarly, the EDX analysis of the exposed test paper showed that the atomic ratio of the S element was 0.16% (Figure 8(b3-b5)), indicating that SbCl 3 can effectively capture H 2 S gas. Figure 8(c1,c2) show that the microstructure of the composite (0.5 wt.% SbCl 3 and 4 wt.% anthocyanin-coated) test paper before exposure was similar to the 4 wt.% anthocyanincoated test paper and after exposure, and it was identical to the 0.5 wt.%SbCl 3 -coated test paper. The S element was found in the EDX analysis of the exposed composite test paper. The contact with XPS indicated that SbCl 3 and anthocyanin were bound in the process of test paper preparation. The combined SbCl 3 was adsorbed on the test paper in the form of organic Sb. When exposed to H 2 S gas, some Sb elements reacted with H 2 S to form orange-yellow Sb 2 S 3 , which covered the surface of the test paper with a flocculent structure.
As shown in Figure 8(b1,b2), the test paper coated with 0.5 wt.% SbCl3 presented a uniform flocculent structure before exposure, which diminished after exposure. Combined with XPS, it was found that SbCl3 reacts with H2S gas to generate Sb2S3, but Sb2S3 has a poor binding force with the test paper and falls off slowly during the experiment. Similarly, the EDX analysis of the exposed test paper showed that the atomic ratio of the S element was 0.16% (Figure 8(b3,b4,b5)), indicating that SbCl3 can effectively capture H2S gas. Figure 8(c1,c2) show that the microstructure of the composite (0.5 wt.% SbCl3 and 4 wt.% anthocyanin-coated) test paper before exposure was similar to the 4 wt.% anthocyanin-coated test paper and after exposure, and it was identical to the 0.5 wt.%SbCl3-coated test paper. The S element was found in the EDX analysis of the exposed composite test paper. The contact with XPS indicated that SbCl3 and anthocyanin were bound in the process of test paper preparation. The combined SbCl3 was adsorbed on the test paper in the form of organic Sb. When exposed to H2S gas, some Sb elements reacted with H2S to form orange-yellow Sb2S3, which covered the surface of the test paper with a flocculent structure.

Mechanism Analysis
The reaction mechanisms of SbCl3-coated test paper and the SbCl3-and-anthocyanincoated composite test paper were slightly different. Sb2S3 was generated by an inorganic reaction between the SbCl3-coated test paper and H2S, shown in Equation (1). Equation (2) shows the reaction mechanism of anthocyanins with H2S, which has been discussed in previous studies [20]. The mechanism of the composite test paper is that SbCl3 and anthocyanin are combined during its preparation. As shown in Equation (3), Sb (III) provides an empty orbital, and the O atom of the C=O bond in the PC extract feeds a lone pair of electrons that coordinate with the empty orbital of Sb (III). After exposure to H2S gas, SbCl3 and anthocyanin react with H2S to form Sb2S3 and C-S bonds, respectively.

The Simulation of Pipeline Leakage
To validate the practical utility of our H2S test paper, we used it to test the discoloration of trace amounts of H2S gas after leakage from the pipeline (Figure 9 and Video S1, the video plays at triple speed). The transparent hose and the black plastic tube have small Figure 8. SEM images and EDX map of three test papers before and after being placed in 10 ppm H 2 S at 25 • C for 30 min. SEM images of 4 wt.% anthocyanin test paper (a1) before and (a2) after response and (a3-a6) EDS map after response. SEM images of 0.5 wt.% SbCl 3 test paper (b1) before and (b2) after response and (b3-b7) EDS map after response. SEM images of 0.5 wt.% SbCl 3 and 4 wt.% anthocyanin test paper (c1) before and (c2) after response and (c3-c7) EDS map after response.

Mechanism Analysis
The reaction mechanisms of SbCl 3 -coated test paper and the SbCl 3 -and-anthocyanincoated composite test paper were slightly different. Sb 2 S 3 was generated by an inorganic reaction between the SbCl 3 -coated test paper and H 2 S, shown in Equation (1). Equation (2) shows the reaction mechanism of anthocyanins with H 2 S, which has been discussed in previous studies [20]. The mechanism of the composite test paper is that SbCl 3 and anthocyanin are combined during its preparation. As shown in Equation (3), Sb (III) provides an empty orbital, and the O atom of the C=O bond in the PC extract feeds a lone pair of electrons that coordinate with the empty orbital of Sb (III). After exposure to H 2 S gas, SbCl 3 and anthocyanin react with H 2 S to form Sb 2 S 3 and C-S bonds, respectively.

Mechanism Analysis
The reaction mechanisms of SbCl3-coated test paper and the SbCl3-and-anthocyanincoated composite test paper were slightly different. Sb2S3 was generated by an inorganic reaction between the SbCl3-coated test paper and H2S, shown in Equation (1). Equation (2) shows the reaction mechanism of anthocyanins with H2S, which has been discussed in previous studies [20]. The mechanism of the composite test paper is that SbCl3 and anthocyanin are combined during its preparation. As shown in Equation (3), Sb (III) provides an empty orbital, and the O atom of the C=O bond in the PC extract feeds a lone pair of electrons that coordinate with the empty orbital of Sb (III). After exposure to H2S gas, SbCl3 and anthocyanin react with H2S to form Sb2S3 and C-S bonds, respectively.

The Simulation of Pipeline Leakage
To validate the practical utility of our H2S test paper, we used it to test the discoloration of trace amounts of H2S gas after leakage from the pipeline (Figure 9 and Video S1, the video plays at triple speed). The transparent hose and the black plastic tube have small

Mechanism Analysis
The reaction mechanisms of SbCl3-coated test paper and the SbCl3-and-anthocyanincoated composite test paper were slightly different. Sb2S3 was generated by an inorganic reaction between the SbCl3-coated test paper and H2S, shown in Equation (1). Equation (2) shows the reaction mechanism of anthocyanins with H2S, which has been discussed in previous studies [20]. The mechanism of the composite test paper is that SbCl3 and anthocyanin are combined during its preparation. As shown in Equation (3), Sb (III) provides an empty orbital, and the O atom of the C=O bond in the PC extract feeds a lone pair of electrons that coordinate with the empty orbital of Sb (III). After exposure to H2S gas, SbCl3 and anthocyanin react with H2S to form Sb2S3 and C-S bonds, respectively.

The Simulation of Pipeline Leakage
To validate the practical utility of our H2S test paper, we used it to test the discoloration of trace amounts of H2S gas after leakage from the pipeline (Figure 9 and Video S1, the video plays at triple speed). The transparent hose and the black plastic tube have small

The Simulation of Pipeline Leakage
To validate the practical utility of our H 2 S test paper, we used it to test the discoloration of trace amounts of H 2 S gas after leakage from the pipeline (Figure 9 and Video S1, the video plays at triple speed). The transparent hose and the black plastic tube have small holes of 1 mm and the hose was placed inside the plastic tube, which was sealed with red glue at both ends. A known concentration of H 2 S gas (1000 ppm) generated by Na 2 S was pumped into the transparent hose through a circulation pump while the H 2 S sensor and video shooting software (HIKVISION) were turned on. The H 2 S sensor did not respond in any way during the experiment. After the experiment was completed, the time data of the color change were obtained by video capture. This result demonstrates that the H 2 S test paper prepared by us can detect microleakages that cannot be detected by ordinary instruments. At the same time, it has the characteristics of being cheap, convenient, and suitable for large-scale application, and is expected to provide a new H 2 S gas monitoring method for the field. holes of 1 mm and the hose was placed inside the plastic tube, which was sealed with red glue at both ends. A known concentration of H2S gas (1000 ppm) generated by Na2S was pumped into the transparent hose through a circulation pump while the H2S sensor and video shooting software (HIKVISION) were turned on. The H2S sensor did not respond in any way during the experiment. After the experiment was completed, the time data of the color change were obtained by video capture. This result demonstrates that the H2S test paper prepared by us can detect microleakages that cannot be detected by ordinary instruments. At the same time, it has the characteristics of being cheap, convenient, and suitable for large-scale application, and is expected to provide a new H2S gas monitoring method for the field.

Experimental Material
The SbCl3, ethanol, and filter paper were purchased from Chengdu Kelon Chemical Reagent Factory (Chengdu, China). The anthocyanins were extracted from purple cabbage (PC), purchased from the local Carrefour.

Preparation of Response Test Paper
Different SbCl3 solutions were obtained by dissolving 0.005 g, 0.01 g, 0.02 g, 0.05 g, and 0.10 g of SbCl3 powder in 10 g of the ethanol-water mixture (1:1, wt/wt), respectively. The SbCl3 solutions (0.5 mL) were coated on different filter papers of the same size (2 cm × 5 cm). The filter papers were dried at room temperature (25 °C) and again coated with respective SbCl3 solution (0.5 mL). After drying, the response test papers coated with different concentrations of SbCl3 were obtained.
PC was dried in a vacuum oven (60 °C) for 36 h, then ground into powder for later use. Different weights of PC powder (1 g, 2 g, 3 g, 4 g, 5 g) were separately immersed in 50 mL ethanol-water (1:1, wt/wt) mixture, soaked for 2 h at room temperature, and then extracted using ultrasonic extractor for 30 min at 30 °C and 200 W. The extract was centrifuged at 3500 r/min and 10 °C for 15 min. The supernatant obtained was an anthocyanin solution. Mixed solutions of anthocyanin and SbCl3 were prepared by adding SbCl3 powder (0.05 wt.%, 0.1 wt.%, 0.2 wt.%, 0.5 wt.%, 1 wt.%, based on the weight of the Figure 9. Schematic diagram of the device using test paper to monitor microleakage of pipeline and color change in test paper.

Experimental Material
The SbCl 3 , ethanol, and filter paper were purchased from Chengdu Kelon Chemical Reagent Factory (Chengdu, China). The anthocyanins were extracted from purple cabbage (PC), purchased from the local Carrefour.

Preparation of Response Test Paper
Different SbCl 3 solutions were obtained by dissolving 0.005 g, 0.01 g, 0.02 g, 0.05 g, and 0.10 g of SbCl 3 powder in 10 g of the ethanol-water mixture (1:1, wt/wt), respectively. The SbCl 3 solutions (0.5 mL) were coated on different filter papers of the same size (2 cm × 5 cm). The filter papers were dried at room temperature (25 • C) and again coated with respective SbCl 3 solution (0.5 mL). After drying, the response test papers coated with different concentrations of SbCl 3 were obtained.
PC was dried in a vacuum oven (60 • C) for 36 h, then ground into powder for later use. Different weights of PC powder (1 g, 2 g, 3 g, 4 g, 5 g) were separately immersed in 50 mL ethanol-water (1:1, wt/wt) mixture, soaked for 2 h at room temperature, and then extracted using ultrasonic extractor for 30 min at 30 • C and 200 W. The extract was centrifuged at 3500 r/min and 10 • C for 15 min. The supernatant obtained was an anthocyanin solution. Mixed solutions of anthocyanin and SbCl 3 were prepared by adding SbCl 3 powder (0.05 wt.%, 0.1 wt.%, 0.2 wt.%, 0.5 wt.%, 1 wt.%, based on the weight of the anthocyanin solution) to different concentrations of anthocyanin solution and stirring for 10 min. Furthermore, the response test papers coated with mixed solutions were prepared similarly to SbCl 3 -coated test papers.

Visual Detection of H 2 S Gas
As shown in Figure 10, the response test paper was placed in a glass container (on the right) to evaluate its responsiveness. A rubber hose and an air pump were used to connect the glass container to a 500 mL wide-mouth bottle. The glass container was filled with 100 mL of 5 wt.% H 2 SO 4 solution and sealed with a rubber plug. Before the test, N 2 was continuously introduced into the device to check the sealing performance. After the supply of N 2 was stopped, solid Na 2 S·9H 2 O (mass of Na 2 S·9H 2 O corresponds to H 2 S gas concentration) was added to the H 2 SO 4 solution to obtain the H 2 S gas. Simultaneously, the air pump was turned on to quickly fill the device with H 2 S gas. The H 2 S sensor in the glass container accurately indicated the H 2 S gas concentration. The change in color of the test paper was recorded using a colorimeter (YS4510 plus, 3nh, Shenzhen Sanenshi Technology Co., Ltd., Shenzhen, China) and a smartphone camera. The color difference value (∆E) was calculated using Equation (4) to digitize the color change in the test paper [17]: where L * 0 (luminance), a * 0 (red-green value), and b * 0 (yellow-blue value) were the initial parameters, and L*, a*, and b* were the parameters obtained after H 2 S gas exposure.
Additionally, the environmental influences on the response test paper were investigated by varying the experimental temperature (0 • C, 25 • C, 60 • C, 80 • C) and introducing other gases (N 2 , CO 2 , O 2 , air) into the device. The H 2 S detection was investigated using different test papers coated with different concentrations of SbCl 3 and/or anthocyanin. anthocyanin solution) to different concentrations of anthocyanin solution and stirring for 10 min. Furthermore, the response test papers coated with mixed solutions were prepared similarly to SbCl3-coated test papers.

Visual Detection of H2S Gas
As shown in Figure 10, the response test paper was placed in a glass container (on the right) to evaluate its responsiveness. A rubber hose and an air pump were used to connect the glass container to a 500 mL wide-mouth bottle. The glass container was filled with 100 mL of 5 wt.% H2SO4 solution and sealed with a rubber plug. Before the test, N2 was continuously introduced into the device to check the sealing performance. After the supply of N2 was stopped, solid Na2S·9H2O (mass of Na2S·9H2O corresponds to H2S gas concentration) was added to the H2SO4 solution to obtain the H2S gas. Simultaneously, the air pump was turned on to quickly fill the device with H2S gas. The H2S sensor in the glass container accurately indicated the H2S gas concentration. The change in color of the test paper was recorded using a colorimeter (YS4510 plus, 3nh, Shenzhen Sanenshi Technology Co., Ltd., Shenzhen, China) and a smartphone camera. The color difference value (∆E) was calculated using Equation (4) to digitize the color change in the test paper [17]: where 0 * (luminance), 0 * (red-green value), and 0 * (yellow-blue value) were the initial parameters, and L*, a*, and b* were the parameters obtained after H2S gas exposure.
Additionally, the environmental influences on the response test paper were investigated by varying the experimental temperature (0 °C, 25 °C, 60 °C, 80 °C) and introducing other gases (N2, CO2, O2, air) into the device. The H2S detection was investigated using different test papers coated with different concentrations of SbCl3 and/or anthocyanin.

Characterization Methods
A scanning electron microscope (SEM, EVOMA15, ZEISS, Aalen) was used to examine the microscopic morphology of the test papers before and after the introduction of H2S gas. Before observing under SEM, the surface of the test papers was coated with gold nanoparticles. The surface element content was analyzed using energy-dispersive X-ray spectroscopy (EDX). Furthermore, the surface composition and chemical valences were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA).

Conclusions
In summary, a portable and efficient colorimetric test paper was developed using SbCl3 and anthocyanin to detect H2S gas quantitatively. It can be used to monitor the leakage of H2S gas by color change that is visual to the naked eye and can determine the leakage concentration of H2S gas when used in conjunction with a colorimeter. The H2S Figure 10. Schematic illustration of H 2 S response performance test process of the test paper.

Characterization Methods
A scanning electron microscope (SEM, EVOMA15, ZEISS, Aalen) was used to examine the microscopic morphology of the test papers before and after the introduction of H 2 S gas. Before observing under SEM, the surface of the test papers was coated with gold nanoparticles. The surface element content was analyzed using energy-dispersive X-ray spectroscopy (EDX). Furthermore, the surface composition and chemical valences were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA).

Conclusions
In summary, a portable and efficient colorimetric test paper was developed using SbCl 3 and anthocyanin to detect H 2 S gas quantitatively. It can be used to monitor the leakage of H 2 S gas by color change that is visual to the naked eye and can determine the leakage concentration of H 2 S gas when used in conjunction with a colorimeter. The H 2 S detection limit of the test paper prepared in this work is 200 ppb, which is significantly lower than the existing test papers for H 2 S detection. Moreover, the test paper is not sensitive to common gases in the air such as O 2 , N 2 , and CO 2 , demonstrating that it is not affected by other gases and can be stored for a long time. Overall, the selectivity of this test paper to H 2 S provides a new strategy for the rapid detection and quantification of H 2 S gas leakage in industries.
Under the background of the gradual development of artificial intelligence, the robot is expected to recognize and digitize the color change during the patrol process and implement an automatic alarm when the H 2 S concentration reaches the dangerous range.
Supplementary Materials: The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/molecules28135044/s1: Video S1: Three times the speed of the simulated pipeline leak experiment.  Data Availability Statement: All raw/processed data necessary for reproducing the results in this study can be accessed upon reasonable request.