Valorization of Agricultural Waste as a Chemiresistor H2S-Gas Sensor: A Composite of Biodegradable-Electroactive Polyurethane-Urea and Activated-Carbon Composite Derived from Coconut-Shell Waste

In this study, a high-performance H2S sensor that operates at RT was successfully fabricated using biodegradable electroactive polymer-polyurethane-urea (PUU) and PUU-activated-carbon (AC) composites as sensitive material. The PUU was synthesized through the copolymerization of biodegradable polycaprolactone diol and an electroactive amine-capped aniline trimer. AC, with a large surface area of 1620 m2/g and a pore diameter of 2 nm, was derived from coconut-shell waste. The composites, labeled PUU-AC1 and PUU-AC3, were prepared using a physical mixing method. The H2S-gas-sensing performance of PUU-AC0, PUU-AC1, and PUU-AC3 was evaluated. It was found that the PUU sensor demonstrated good H2S-sensing performance, with a sensitivity of 0.1269 ppm−1 H2S. The H2S-gas-sensing results indicated that the PUU-AC composites showed a higher response, compared with PUU-AC0. The enhanced H2S-response of the PUU-AC composites was speculated to be due to the high surface-area and abounding reaction-sites, which accelerated gas diffusion and adsorption and electron transfer. When detecting trace levels of H2S gas at 20 ppm, the sensitivity of the sensors based on PUU-AC1 and PUU-AC3 increased significantly. An observed 1.66 and 2.42 times’ enhancement, respectively, in the sensors’ sensitivity was evident, compared with PUU-AC0 alone. Moreover, the as-prepared sensors exhibited significantly high selectivity toward H2S, with minimal to almost negligible responses toward other gases, such as SO2, NO2, NH3, CO, and CO2.


Introduction
H 2 S is a hazardous gas, even at the sub-ppm level, and may lead to poisoning or deaths at high concentrations [1]. People may be exposed to this gas through geothermal activities, organic decomposition, treatment plants, and as a by-product or intermediate of various industries. H 2 S is one of most dangerous gases in the workplace according to the Occupational Safety and Health Administration [2,3]. Hence, monitoring of this type of contaminant in the environment is of utmost significance [4].
Intrinsic conducting polymers (ICPs) exhibit the electronic properties of metals and semiconductors; they appeared as promising gas-sensing materials in the early 1980s [5]. Given its environmental stability, facile synthesis, unique redox behavior, low toxicity, and simple acid/base doping properties, polyaniline (PANI) is one of the most useful ICPs [6,7]. PANI has various applications, including as a corrosion inhibitor [8], supercapacitor [9], rethane pre-polymer. After 30 min, 3 mmol of ACT was dissolved in NMP, and the reaction continued to be refluxed further for 48 h. Thereafter, PUU films were prepared using sequential heating-steps in an oven. Scheme 1. Scheme for the preparation of polyurethane-urea (PUU-AC0). Figure 1 (top) showed the preparation of AC. The natural biomass coconut-shell was cleaned three times with distilled water, followed by overnight drying in an oven at 60 °C. Firstly, it was cut into small pieces and heated at 300 °C in an oven under N2 for 2 h, to obtain pre-carbonized biochar. Later, this was followed by washing (methanol, ethanol, distilled water) and drying. For the activation step, pre-carbonized biochar was mixed with ZnCl2 at a 1:10 weight ratio, and carbonized at 800 °C for 2 h under N2 flow in a tube furnace. The temperature ramp was set to 1.5 °C min −1 during carbonization. The as-synthesized activated carbon (AC) was then washed with H2O: Ispropanol (1:1) and vacuum dried at 80 °C for 6h [65].  Figure 1 (top) showed the preparation of AC. The natural biomass coconut-shell was cleaned three times with distilled water, followed by overnight drying in an oven at 60 • C. Firstly, it was cut into small pieces and heated at 300 • C in an oven under N 2 for 2 h, to obtain pre-carbonized biochar. Later, this was followed by washing (methanol, ethanol, distilled water) and drying. For the activation step, pre-carbonized biochar was mixed with ZnCl 2 at a 1:10 weight ratio, and carbonized at 800 • C for 2 h under N 2 flow in a tube furnace. The temperature ramp was set to 1.5 • C min −1 during carbonization. The as-synthesized activated carbon (AC) was then washed with H 2 O: Ispropanol (1:1) and vacuum dried at 80 • C for 6 h [65].

Preparation of PUU-AC Composites
A simple physical mixing-method was used to prepare the PUU-AC composites. First, 1 wt% solution of PUU was prepared in NMP via magnetic-stirring at room temperature. Subsequently, 1 wt% and 3 wt% AC was added to the previous solution, followed by overnight stirring. The as-prepared composites were labeled PUU-AC1 and PUU-AC3, respectively.

Sensor Preparation
An interdigitated ITO electrode was used as laser engraver to carve grooves into the ITO glass (12 pairs of electrodes, 0.3 mm spacing, and l × w of 20 × 20 mm) creating a nonconductive open-circuit on both sides of the glass. The spin-coating technique was used in the construction of ITO coated with PUU-AC0, PUU-AC1, and PUU-AC3. First, 1 wt% solution of the given samples PUU-AC0, PUU-AC1, and PUU-AC3 was prepared in NMP via stirring at room temperature, as shown in Figure 1 (bottom). Thereafter, thin films (0.005 mm) were prepared by spin-coating 200 µL of the respective solution on ITO, at a spin rate of 1600 rpm, followed by overnight drying at room temperature. The advantages of spin coating are to produce very fine, thin, and uniform coating. Using the spin-coating method, the desired thickness of the film can be achieved. On the other hand, the dropcasting method is used to prepare a film which is not uniform and also not thin. In addition, there will be the possibility of cracks after evaporation of the solvent. The dip-coating method involves the immersion of the substrate in the respective solution, which cannot be employed in the gas-sensing experiment.

Preparation of PUU-AC Composites
A simple physical mixing-method was used to prepare the PUU-AC composites. First, 1 wt% solution of PUU was prepared in NMP via magnetic-stirring at room temperature. Subsequently, 1 wt% and 3 wt% AC was added to the previous solution, followed by overnight stirring. The as-prepared composites were labeled PUU-AC1 and PUU-AC3, respectively.

Sensor Preparation
An interdigitated ITO electrode was used as laser engraver to carve grooves into the ITO glass (12 pairs of electrodes, 0.3 mm spacing, and l × w of 20 × 20 mm) creating a nonconductive open-circuit on both sides of the glass. The spin-coating technique was used in the construction of ITO coated with PUU-AC0, PUU-AC1, and PUU-AC3. First, 1 wt% solution of the given samples PUU-AC0, PUU-AC1, and PUU-AC3 was prepared in NMP via stirring at room temperature, as shown in Figure 1 (bottom). Thereafter, thin films (0.005 mm) were prepared by spin-coating 200 µL of the respective solution on ITO, at a spin rate of 1600 rpm, followed by overnight drying at room temperature. The advantages of spin coating are to produce very fine, thin, and uniform coating. Using the spin-coating method, the desired thickness of the film can be achieved. On the other hand, the drop-casting method is used to prepare a film which is not uniform and also not thin.

H 2 S Sensing Experiment
In the gas-sensing experiment, the films spin-coated onto ITO were exposed to H 2 S-gas concentration ranging from 1 ppm to 50 ppm [12] at room temperature. The ITO sensor fixed in a gas chamber was linked to the electrometer (Keithley 2450 SourceMeter, Keithlink Technology Co., Ltd.) to measure the response, as shown in Figure 2. All the measurements were performed at room temperature (25 ± 0.5 • C) after a steady state was achieved. The carrier gas (air) was blended with H 2 S gas from the cylinder at a fixed concentration (50 ppm in N 2 ) to determine the dependency of ITO responses on gas concentration. The gas chamber had a total-gas-flow value set to 1000 sccm. Each measurement was taken by flushing H 2 S gas through the measurement chamber for an interval of 150 s, followed by cleaning the sensor in which H 2 S gas was replaced with air, until the baseline was achieved. ITO responses were calculated as normalized resistance (R a −R g ), where R a and R g denote resistance under air and a given analyte, respectively. centration (50 ppm in N2) to determine the dependency of ITO responses on gas concen tration. The gas chamber had a total-gas-flow value set to 1000 sccm. Each measuremen was taken by flushing H2S gas through the measurement chamber for an interval of 150 s followed by cleaning the sensor in which H2S gas was replaced with air, until the baseline was achieved. ITO responses were calculated as normalized resistance (Ra−Rg), where R and Rg denote resistance under air and a given analyte, respectively.

Results and Discussion
Characterization: ACAT was synthesized and fully characterized via FTIR. The iontrap mass spectrum is shown in Figure S1.
The nitrogen adsorption-desorption isotherms of AC are presented in Figure 3a. The profile of the isotherm curve is Type I, which is characteristic of a microporous material i.e., pores with a diameter of 2 nm (inset in Figure 3a). The surface area of AC was determined to be 1620 m 2 /g, with a pore volume of 0.91 cm 3 /g. The micropore area was 1392 m 2 /g, while single-point adsorption total-pore-volume was 1.05 cm 3 /g. Figure 3b shows the Raman spectrum of AC. A typical Raman spectrum of AC is characterized by two bands, as shown in Figure 3b. In the case of AC, the G-band observed at 1580 cm − 1 is related to the stretching vibration of sp 2 carbon in a hexagonal lattice, while the D-band that appeared at 1332 cm −1 is associated with the disordered or amorphous carbon atoms

Results and Discussion
Characterization: ACAT was synthesized and fully characterized via FTIR. The iontrap mass spectrum is shown in Figure S1.
The nitrogen adsorption-desorption isotherms of AC are presented in Figure 3a. The profile of the isotherm curve is Type I, which is characteristic of a microporous material, i.e., pores with a diameter of 2 nm (inset in Figure 3a). The surface area of AC was determined to be 1620 m 2 /g, with a pore volume of 0.91 cm 3 /g. The micropore area was 1392 m 2 /g, while single-point adsorption total-pore-volume was 1.05 cm 3 /g. Figure 3b shows the Raman spectrum of AC. A typical Raman spectrum of AC is characterized by two bands, as shown in Figure 3b. In the case of AC, the G-band observed at 1580 cm − 1 is related to the stretching vibration of sp 2 carbon in a hexagonal lattice, while the D-band that appeared at 1332 cm −1 is associated with the disordered or amorphous carbon atoms.
FTIR was used for the structural determination of PUU-AC0 and its composites. Figure 4a shows the FTIR spectra of PCL, PMDI, ACAT, and PUU-AC0. The absence of an absorption band at 2260 cm −1 of the -NCO group indicated that all of the -NCO groups were incorporated into the PUU-AC0 copolymer. The single absorption peak at 3422 cm −1 in PUU-AC0 demonstrated the formation of a urea group. Most of the absorption bands found in PUU-AC0 agreed with PCL diol except at 1539 cm −1 and 1500 cm −1 , exhibiting the presence of quinoid and benzenoid ring from ACAT. The carbonyl-group peak stretching from the urethane bond and urea linkage appeared at 1673 cm −1 and 1602 cm −1 , respectively. The molecular weight of PUU-AC0 was determined via gelpermeation-chromatography analysis with a value of Mw = 252,437, Mn = 160,550, and PDI = 1.5. FTIR was used for the structural determination of PUU-AC0 and its composites. Figure 4a shows the FTIR spectra of PCL, PMDI, ACAT, and PUU-AC0. The absence of an absorption band at 2260 cm −1 of the -NCO group indicated that all of the -NCO groups were incorporated into the PUU-AC0 copolymer. The single absorption peak at 3422 cm −1 in PUU-AC0 demonstrated the formation of a urea group. Most of the absorption bands found in PUU-AC0 agreed with PCL diol except at 1539 cm −1 and 1500 cm −1 , exhibiting the presence of quinoid and benzenoid ring from ACAT. The carbonyl-group peak stretching from the urethane bond and urea linkage appeared at 1673 cm −1 and 1602 cm −1 , respectively. The molecular weight of PUU-AC0 was determined via gel-permeation-chromatography analysis with a value of Mw = 252,437, Mn = 160,550, and PDI = 1.5.
The FTIR spectra of PUU-AC0, PUU-AC1, and PUU-AC3 are shown in Figure 4b. In general, biomass-derived AC has surface oxygenated-functional groups. The surface functional groups present on AC from coconut shell were determined via FTIR, as shown in   In addition, the bending vibration at 828 cm −1 was ascribed to the C-H bond from a highly substituted aromatic ring. Overall, the AC may contain the functional groups of C-O, O-H, -CH2-or -CH3, C = C, and highly substituted aromatic rings [66,67,68,69]. The composites, i.e., PUU-AC1 and PUU-AC3 presented all the characteristic peaks of PUU-AC0 and AC, indicating the covering of the PUU network on the surface of AC. However, a slight shift occurred in the characteristic bands, due to the incorporation of AC into the polymer matrix. The interactions between PUU-AC0 and AC may be ascribed to the slight shift in the observed bands. Figure 5 shows the scanning-electron-microscopy (SEM) images of AC, PUU-AC0, PUU-AC1, and PUU-AC3. SEM imaging of the AC derived from coconut shell (Figure 5a) revealed that the production of rich porous structure was favored by the high ratio of ZnCl2/sample [70]. The SEM image shows cavities, pores, and more rough surfaces on AC. The holes and cave-type opening on the AC surface increased the surface available for adsorption of the given analyte. Figure 5b shows the SEM image of PUU with no evident surface morphology. As shown in Figure 5c,d, the morphology of PUU-AC0 changed after the addition of AC. During the blending process, PUU-AC0 diffused into the micropores of AC, as indicated by the absence of agglomeration. The absorption peaks at 1651 cm −1 and 1585 cm −1 were assigned to the C = C bond of the aromatic ring. The absorption peak at 1219 cm −1 was attributed to C-O in carboxylic acids, alcohols, phenols, and esters, or to the P = O bond in phosphate esters.
In addition, the bending vibration at 828 cm −1 was ascribed to the C-H bond from a highly substituted aromatic ring. Overall, the AC may contain the functional groups of C-O, O-H, -CH 2 -or -CH 3 , C = C, and highly substituted aromatic rings [66][67][68][69]. The composites, i.e., PUU-AC1 and PUU-AC3 presented all the characteristic peaks of PUU-AC0 and AC, indicating the covering of the PUU network on the surface of AC. However, a slight shift occurred in the characteristic bands, due to the incorporation of AC into the polymer matrix. The interactions between PUU-AC0 and AC may be ascribed to the slight shift in the observed bands. Figure 5 shows the scanning-electron-microscopy (SEM) images of AC, PUU-AC0, PUU-AC1, and PUU-AC3. SEM imaging of the AC derived from coconut shell (Figure 5a) revealed that the production of rich porous structure was favored by the high ratio of ZnCl 2 /sample [70]. The SEM image shows cavities, pores, and more rough surfaces on AC. The holes and cave-type opening on the AC surface increased the surface available for adsorption of the given analyte. Figure 5b shows the SEM image of PUU with no evident surface morphology. As shown in Figure 5c,d, the morphology of PUU-AC0 changed after the addition of AC. During the blending process, PUU-AC0 diffused into the micropores of AC, as indicated by the absence of agglomeration.

Biodegradability
The quantitative data of the weight loss [71] for PUU-AC0 films in PBS was given in Figure 6a. As can be seen, over the period of six months, the film show progressive massloss, with a degradation rate of 3.33 ± 0.0002%.

Biodegradability
The quantitative data of the weight loss [71] for PUU-AC0 films in PBS was given in Figure 6a. As can be seen, over the period of six months, the film show progressive mass-loss, with a degradation rate of 3.33 ± 0.0002%.  Table 1 indicates that the electrical conductivity of the undoped and doped as-synthesized sensors was determined from four-point probe measurements. HCl is commonly used for the doping process. In the current study, H2S gas was also used for doping (because it is used as a dopant in gas-sensing properties). Thus, two types of dopants, 1 M HCl and 20 ppm of H2S(g) (50 ppm + N2), were used to dope thick and thin films, respectively. An electrical conductivity of 6.95 × 10 −7 , 8.46 × 10 −7 , and 9.94 × 10 −7 S/cm was exhibited by PUU-AC0, PUU-AC1, and PUU-AC3, respectively. After doping with 1 M HCl for 10 min, the electrical conductivity of PUU-AC0 increased up to 1.60 × 10 −6 S/cm. An increase of 3.5 and 3.7 times was observed for the PUU-AC1 and PUU-AC3 thick films, respectively. Moreover, the electrical conductivity determined for the thin films of PUU-AC0, PUU-AC1, and PUU-AC3 was 1.22 × 10 −5 , 7.92 × 10 −5 , and 3.6 × 10 −4 S/cm, respectively. After doping with 20 ppm of H2S gas for 10 min, a tremendous increase of ×5, ×9, and ×10.5  Table 1 indicates that the electrical conductivity of the undoped and doped as-synthesized sensors was determined from four-point probe measurements. HCl is commonly used for the doping process. In the current study, H 2 S gas was also used for doping (because it is used as a dopant in gas-sensing properties). Thus, two types of dopants, 1 M HCl and 20 ppm of H 2 S (g) (50 ppm + N 2 ), were used to dope thick and thin films, respectively. An electrical conductivity of 6.95 × 10 −7 , 8.46 × 10 −7 , and 9.94 × 10 −7 S/cm was exhibited by PUU-AC0, PUU-AC1, and PUU-AC3, respectively. After doping with 1 M HCl for 10 min, the electrical conductivity of PUU-AC0 increased up to 1.60 × 10 −6 S/cm. An increase of 3.5 and 3.7 times was observed for the PUU-AC1 and PUU-AC3 thick films, respectively. Moreover, the electrical conductivity determined for the thin films of PUU-AC0, PUU-AC1, and PUU-AC3 was 1.22 × 10 −5 , 7.92 × 10 −5 , and 3.6 × 10 −4 S/cm, respectively. After doping with 20 ppm of H 2 S gas for 10 min, a tremendous increase of ×5, ×9, and ×10.5 was observed for PUU-AC0, PUU-AC1, and PUU-AC3, respectively, as indicated in Table 1.   Figure 6c. The I-V curves for these films are linear, indicating an ohmic contact between the electrode and PUU-AC films. The σ of the films increased from PUU to PUU-AC3. The CV and I-V characteristics of the PUU and PUU-AC composite electrodes indicated that the AC-doped PUU resulted in an improved charge-transfer, due to the synergistic effect between the PUU matrix and AC, which, in turn, improved the sensor performance for H 2 S detection.

H 2 S-Gas-Sensing Properties
The H 2 S-sensing properties of all the sensors were studied at room temperature. The sensors exhibited an apparent difference in response with and without the filler, i.e., AC.

Response and Sensitivity
The dynamic response and recovery-curves toward H 2 S with different concentrations (from 50 ppm to 1 ppm) of the PUU-AC0, PUU-AC1, and PUU-AC3 sensors are shown in Figure 7a. After the injection of H 2 S gas, resistance decreased sharply, returning to the original baseline as H 2 S gas was replaced with air [72]. As the concentration of H 2 S gas decreased, the response-value decreased correspondingly. For the PUU-AC0 sensor, the response value was 3 for detecting 20 ppm H 2 S gas. For the PUU-AC1 and PUU-AC3 sensors, the response values were as high as 4 and 7.27, respectively. For the 20 ppm H 2 S gas, the response value was 1.33, 2.42 times higher than that of PUU-AC0. The PUU-AC0, PUU-AC1, and PUU-AC3 sensors exhibited an evident response value of 0.11, 0.52, and 1, respectively, at a lower detection limit of 1 ppm. The large surface area of AC may be ascribed to the difference in sensor response. Considering the availability of a large number of active sites in the sensing layer, the relative response increased. Hence, the performance of the sensors increased in the following order: PUU-AC0 < PUU-AC1 < PUU-AC3.
sensor, the PUU-AC1 and PUU-AC3 sensors displayed better sensitivity. A sensitivity of 0.1269 ppm −1 was measured for PUU-AC0, which was increased up to 0.2047 ppm −1 for the PUU-AC1 sensor. The PUU-AC3 sensor exhibited the highest sensitivity, of 0.2706 ppm −1 , which was 2.1 times and 1.32 times higher than those of PUU-AC0 and PUU-AC1, respectively. The water contact-angle measurements of PUU-AC0, PUU-AC1, and PUU-AC3 were 79.7°, 73.3°, and 72.2°, respectively, indicating their hydrophilic nature. The hydrophilic groups may have developed intermolecular interactions with polar H2S gas, thus increasing the response of the sensor [27,73].  Figure 7b shows the response magnitudes of the PUU-AC0, PUU-AC1, and PUU-AC3 film sensors versus different H 2 S concentrations. The linear fitting equations of y = 0.1269x + 0.3249, y = 0.2047x + 0.4712, and 0.2706x + 1.8642 were determined for PUU-AC0, PUU-AC1, and PUU-AC3 respectively. The values of 0.9901, 0.9919, and 0.9847 were observed for correlation coefficients of the fitted data (R 2 ), respectively. Moreover, Figure 7c shows the bar plot for the sensor's sensitivity (S, [ppm −1 ]), which was calculated as the slope of the normalized sensor response, R a −R g . Evidently, compared with the PUU-AC0 sensor, the PUU-AC1 and PUU-AC3 sensors displayed better sensitivity. A sensitivity of 0.1269 ppm −1 was measured for PUU-AC0, which was increased up to 0.2047 ppm −1 for the PUU-AC1 sensor. The PUU-AC3 sensor exhibited the highest sensitivity, of 0.2706 ppm −1 , which was 2.1 times and 1.32 times higher than those of PUU-AC0 and PUU-AC1, respectively. The water contact-angle measurements of PUU-AC0, PUU-AC1, and PUU-AC3 were 79.7 • , 73.3 • , and 72.2 • , respectively, indicating their hydrophilic nature. The hydrophilic groups may have developed intermolecular interactions with polar H 2 S gas, thus increasing the response of the sensor [27,73].

Response/Recovery Time
For gas-sensing properties, an important parameter is response-recovery time. Upon exposure to gas, adsorption and desorption occur simultaneously. Thus, the adsorption/desorption rate is solely responsible for the response-recovery time. Figure 8 shows the response-recovery time of the as-prepared sensors as a function of the H 2 S gas at 20 ppm. Figure 8 shows that at 20 ppm, the quick response-time for PUU-AC0, PUU-AC1, and PUU-AC3 sensor was 55, 34, and 32 s (±0.05), respectively. The response-time window for PUU-AC0 toward H 2 S gas ranging from 1 ppm to 50 ppm was 120-15 s, as shown in Figure S2. However, this window was reduced to 57-16 s and 55-15 s for the PUU-AC1 and PUU-AC3 sensors, respectively. The short response-time window for the composites may be attributed to the higher absorption rate, due to a larger surface area compared with that of PUU-AC0. At 20 ppm, however, ITO coated with PUU-AC3 and PUU-AC1 required a longer recovery-time, of 315 s and 345 s (±0.05), respectively, compared with PUU-AC0, which had a recovery time of 200 s. The large surface area allowed the adsorption of a sufficient amount of H 2 S gas molecules to dope, probably leading to a higher adsorptionrate [74]. However, de-doping the sensing material took a longer time, i.e., the desorption rate was considerably slower, which may lead to the poor recovery time ( Figure S2) of the composites.

Response/Recovery Time
For gas-sensing properties, an important parameter is response-recovery time. Upon exposure to gas, adsorption and desorption occur simultaneously. Thus, the adsorption/desorption rate is solely responsible for the response-recovery time. Figure 8 shows the response-recovery time of the as-prepared sensors as a function of the H2S gas at 20 ppm. Figure 8 shows that at 20 ppm, the quick response-time for PUU-AC0, PUU-AC1, and PUU-AC3 sensor was 55, 34, and 32 s (±0.05), respectively. The response-time window for PUU-AC0 toward H2S gas ranging from 1 ppm to 50 ppm was 120-15 s, as shown in Figure S2. However, this window was reduced to 57-16 s and 55-15 s for the PUU-AC1 and PUU-AC3 sensors, respectively. The short response-time window for the composites may be attributed to the higher absorption rate, due to a larger surface area compared with that of PUU-AC0. At 20 ppm, however, ITO coated with PUU-AC3 and PUU-AC1 required a longer recovery-time, of 315 s and 345 s (±0.05), respectively, compared with PUU-AC0, which had a recovery time of 200 s. The large surface area allowed the adsorption of a sufficient amount of H2S gas molecules to dope, probably leading to a higher adsorption-rate [74]. However, de-doping the sensing material took a longer time, i.e., the desorption rate was considerably slower, which may lead to the poor recovery time (Figure S2) of the composites.
Based on the literature search, the performance of the H2S-gas sensor based on different conductor materials or composite materials is listed in Table 2. The PUU-AC3 synthesized in this work showed a good response for H2S at a room temperature, with a good sensitivity of 0.2724 ppm − of H2S gas.  Based on the literature search, the performance of the H 2 S-gas sensor based on different conductor materials or composite materials is listed in Table 2. The PUU-AC3 synthesized in this work showed a good response for H 2 S at a room temperature, with a good sensitivity of 0.2724 ppm − of H 2 S gas.

Selectivity
Another salient feature for gas-sensing execution is selectivity. This feature was studied upon exposure to various gases, including H 2 S, SO 2 , NO 2 , NH 3 , CO, and CO 2 at 20 ppm, at room temperature. Figure 9a illustrates the fact that all the sensors exhibited a similar trend, by showing the highest sensing selectivity toward H 2 S gas. Moreover, the PUU-AC3 sensor exhibited the highest response to H 2 S, the value of which was approximately 4.5 times higher than that for the other test gases.  Figure 9b shows the effect of humidity on the response of the as-prepared sensors to H2S gas at 20 ppm. The relative humidity (RH) range was 60% to 100%. The response value (of 3, 4.03, and 7.27) at 40% RH was set as the standard value (100%) for PUU-AC0, PUU-AC1, and PUU-AC3, respectively. As shown in the figure, moisture negatively affects the  Figure 9b shows the effect of humidity on the response of the as-prepared sensors to H 2 S gas at 20 ppm. The relative humidity (RH) range was 60% to 100%. The response value (of 3, 4.03, and 7.27) at 40% RH was set as the standard value (100%) for PUU-AC0, PUU-AC1, and PUU-AC3, respectively. As shown in the figure, moisture negatively affects the performance of the sensors, resulting in an attenuation of more than 95% of the initial value at 100% RH.

Stability
Stability (Figure 9c) was determined by the continuous measurements of the response values of the PUU-AC0, PUU-AC1, and PUU-AC3 sensors to 20 ppm of H 2 S gas at room temperature, for 30 days. The response value of all the sensors decreased with the passage of time, which coincided with the biodegradable nature of the as-prepared sensors.

Repeatability
Repeatability and stability are crucial parameters for the practical application of gas sensors, as shown in Figure 10. Figure 10 shows the response curves of PUU-AC0, PUU-AC1, and PUU-AC3 at 20 ppm of H 2 S gas, at room temperature. All the sensors demonstrated repeatability, without any attenuation in response.

Stability
Stability (Figure 9c) was determined by the continuous measurements of the response values of the PUU-AC0, PUU-AC1, and PUU-AC3 sensors to 20 ppm of H2S gas at room temperature, for 30 days. The response value of all the sensors decreased with the passage of time, which coincided with the biodegradable nature of the as-prepared sensors.

Repeatability
Repeatability and stability are crucial parameters for the practical application of gas sensors, as shown in Figure 10. Figure 10 shows the response curves of PUU-AC0, PUU-AC1, and PUU-AC3 at 20 ppm of H2S gas, at room temperature. All the sensors demonstrated repeatability, without any attenuation in response.

Gas-Sensing Mechanism
Doping/de-doping plays a crucial role in the gas-sensing mechanism of ICP-based [79] sensors. Similar to PANI, aniline oligomer-based PUU may also be doped via redox reaction or protonation. H2S is a reducing gas. It may undergo ionization in the presence of water (Equation (1)), as shown in Figure 11. The resulting proton ions subsequently dope the PUU polymer reversibly [80], as shown in Equation (2).

Gas-Sensing Mechanism
Doping/de-doping plays a crucial role in the gas-sensing mechanism of ICP-based [79] sensors. Similar to PANI, aniline oligomer-based PUU may also be doped via redox reaction or protonation. H 2 S is a reducing gas. It may undergo ionization in the presence of water (Equation (1)), as shown in Figure 11. The resulting proton ions subsequently dope the PUU polymer reversibly [80], as shown in Equation (2).
The equilibrium shifted toward right and left under and after H2S-gas exposure, respectively. In addition to the doping effect mentioned above, the functional groups present in PUU may also exhibit weak intermolecular interactions with H2S gas [27].

Conclusions
The H2S-gas-sensing performance of the biodegradable electroactive polymer (PUU-AC0) and composites (PUU-AC1 and PUU-AC3) was investigated at room temperature. A sensor was fabricated using the PUU-AC0, PUU-AC1, and PUU-AC3. This sensor was able to detect H2S with a concentration ranging from 1 ppm to 50 ppm. All the given sensors demonstrated good selectivity, rapid response, and good reproducibility against H2S gas. For 20 ppm H2S, the PUU-AC0, PUU-AC1, and PUU-AC3 sensors exhibited a high gas-sensing response of 3, 5, and 7.3, respectively, with a response-time window of 55-32 s. Among all of these, the PUU-AC3 sensor exhibited the smallest response-time, while PUU-AC0 showed the smallest recovery-time. Given the synergistic effect between PUU and AC, the PUU-AC1 and PUU-AC3 gas-sensors demonstrated a highly sensitive and selective gas-sensing performance toward H2S gas relative to PUU-AC0, with a humidity of 60%. All sensors showed stability of up to 30 days, and thus may be employed for practical application. Our experimental results indicate that porous PUU-AC is a potential candidate for high-performance H2S-sensing materials. The equilibrium shifted toward right and left under and after H 2 S-gas exposure, respectively. In addition to the doping effect mentioned above, the functional groups present in PUU may also exhibit weak intermolecular interactions with H 2 S gas [27].

Conclusions
The H 2 S-gas-sensing performance of the biodegradable electroactive polymer (PUU-AC0) and composites (PUU-AC1 and PUU-AC3) was investigated at room temperature. A sensor was fabricated using the PUU-AC0, PUU-AC1, and PUU-AC3. This sensor was able to detect H 2 S with a concentration ranging from 1 ppm to 50 ppm. All the given sensors demonstrated good selectivity, rapid response, and good reproducibility against H 2 S gas. For 20 ppm H 2 S, the PUU-AC0, PUU-AC1, and PUU-AC3 sensors exhibited a high gas-sensing response of 3, 5, and 7.3, respectively, with a response-time window of 55-32 s. Among all of these, the PUU-AC3 sensor exhibited the smallest response-time, while PUU-AC0 showed the smallest recovery-time. Given the synergistic effect between PUU and AC, the PUU-AC1 and PUU-AC3 gas-sensors demonstrated a highly sensitive and selective gas-sensing performance toward H 2 S gas relative to PUU-AC0, with a humidity of 60%. All sensors showed stability of up to 30 days, and thus may be employed for practical application. Our experimental results indicate that porous PUU-AC is a potential candidate for high-performance H 2 S-sensing materials.