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Abstract
In this study, a plasmonic sensor was designed based on the grating coupling to detect the low concentration of H2S as a toxic chemical. The polyaniline nanostructure was prepared using the laser ablation technique in a chitosan solution and the final products were tested using analytical methods. The chitosan-polyaniline nanocomposite layer was used as a sensing layer and coated on the surface of 1D polydimethylsiloxane grating. The variation of reflectivity with different concentrations of H2S was registered from the surface of the grating for evaluating the sensor’s response. Finally, it was explained using the Langmuir isotherm absorption model. The limit of detection and the sensitivity of chitosan-polyaniline-nanocomposite were about 1 ppm and 0.10767 for the detection of H2S, respectively.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Hydrogen sulfide (H2S) is an impurity and toxic chemical in some mineral products and environments. Plasmonic sensor is an accurate and sensitive tool based on nanocomposites to recognize toxic chemicals. Hydrogen sulfide (H2S) is corrosive, colorless, water-soluble, and flammable in ambient conditions and are hazardous substance at low concentrations. H2S can lead to poisoning deaths at higher concentrations [1–3]. H2S is usually found in nature by the decomposition of organic materials, such as oil and gas production chains [2,4,5]. H2S is in the gas phase [6] at normal temperature and pressure conditions. H2S can also be found in mineral environments such as coal deposits and salt. Physical, chemical, and biological agents are the main ones responsible for the formation of this compound. A common approach for monitoring H2S is through the gas chromatography (GC) method which has high detection capacity and accuracy [7]. The high potential technique including the electrochemical method, colorimetric gas detection tubes, chemical resistive, gas detectors based on lead acetate cassette, optical and optical fiber sensors based on laser beam modulation are the accurate methods for detection of H2S [8–12]. Gold film analyzers and SO2 conversion are the other methods to recognize the H2S [8,13,14].
The optical fiber sensor was used to detect the H2S in the range of 0.4 to 2.0 ppm using gold nanoparticles [15]. The fiber optic sensor based on surface plasmon resonance (SPR) was utilized to measure the low concentration (less than100 ppm) of H2S using ZnO layer [16–18]. SPR based on a D-shape optical fiber sensor with metal oxide layers was modelled and simulated using the finite element method for detection of the H2S at room temperature as a result, the limit of detection was 0.1 ppm [19].
Metal nanoparticles and metal oxide nanoparticles have been used to improve the selectivity and sensitivity of the H2S SPR sensor. For example, a thin layer of nickel oxide, indium tin oxide, and nickel oxide doped indium tin oxide were used to detect the H2S with the limit of detection less than 100 ppm, and the SPR spectra showed the shift when the concentration of H2S was increased [19,20].
The nanocrystal of WO3 has been applied to measure the low concentration of H2S in the range of 1 to 10 ppm using the electrochemical method [21–25].
Qin et al., [26] reported a molybdenum sulfide/citric acid composite for tracing the H2S gas. They coated the MoS/citric acid composite on the surface of fiber grating and the sensor limitation was about 0.5 ppm [26]. Au@Ag core shale nanoparticle was also used to detect in low concentration (0.04 nM) of H2S using the electrochemical method [27]. Shao et al., [28] used the p-CuO (particle)/n-SnO2 (nanowire) heterostructures complex nanocomponent to detect the H2S in the range of 1 to 10 ppm concentration [29]. Also, copper oxide (CuxO) [30] can use for the detection of H2S. Li et al., used fiber optics based on an SPR sensor using copper oxide for the detection of H2S, and the limit of detection was in the range of 10-60 ppm [30]. Magnetic nanoparticles have a high potential to detect toxic chemicals. α-Fe2O3 nanoparticles also were used to detect the H2S in concentration of about 0.05 ppm [31]. Zheng et al. [32,33] decorated the In2O3 nanofibers with Pt nanoparticles in the range of 5 to 10 nm size for the detection of H2S. The H2S was detected at low temperature, and they improved the sensitivity of the H2S sensor. SnO2 and ZnO are other metal oxide nanoparticles for the detection of H2S. The Organic field-effect transistor (OFET) integrated platform was fabricated with SnO2 and ZnO nanoparticles for detecting H2S gas [34,35]. This sensor can use in 25 temperatures and it is electronic based [12,36]. Consequently, the metal and metal oxide nanoparticles improved the sensitivity and selectivity of the H2S sensor based on the SPR sensor, optical sensor, electrochemical sensor, and electronic sensor.
Polyaniline is an intrinsically conductive polymer and can improve conductivity using dopants such as Sodium dodecyl sulfate (SDS). The polyaniline and polyaniline layer can be used in solar cells, drug delivery, plastic batteries, sensor, biosensor, optoelectronics devices, corrosion protection, and polymer light-emitting diode (PLED) displays [37]. The polyaniline was used to detect the H2S using the piezoelectric sensor at a low concentration of less than 100 ppm [38]. Raut et al. prepared the polyaniline - CdS nanocomposite to recognize the H2S and the limit of detection was about 100 ppm [39]. Zhang and his coworkers used the polyaniline/ZnO nano-heterostructure for the detection of H2S and the good sensitivity, good response time and good reproducibility were the advantages of that sensor [40]. Consequently, polyaniline has the potential to detect the H2S due to physically or chemically in one sentence.
On the other hand, the affinity of chitosan (poly(b-1-4)-2-amino-2-deoxy-D-glucopyranose) to transition electron is due to the abundant amino (–NH2) and hydroxy (–OH) groups on chitosan chains [41]. Therefore, the amino group of chitosan has the main role in binding the toxic chemical to the composite containing the chitosan. This mechanism was proved using infrared and Raman spectroscopies [42,43]. Hence, the toxic chemical can be immobilized on chitosan via four amino groups in a square-planar geometry [44,45]. The mentioned nanomaterials such as WO3, SnO2,CdS, CuO, In2O3 and etc. are not biocompatible. They should be prepared based on chemical methods which are difficult. Moreover, the results of mentioned analytical methods should be analyzed by experts. Therefore, a new biocompatible composite, simple and low-cost methods should be designed to measure the low concentration of H2S. Hence, laser ablation is a physical and green method to prepare the nanostructures and the plasmonic methods are user-friendly and low-cost techniques for the recognition and detection of toxic chemicals.
In this study, the chitosan-polyaniline nanocomposite (Chi-PANI-NC) layer was prepared using laser ablation of polyaniline in chitosan solution and it was characterized using UV-visible (UV-Vis), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopies, and field emission scanning electron microscopy (FE-SEM). The Chi-PANI-NC was coated on the surface of the gold-coated polydimethylsiloxane (PDMS) grating. The new grating/gold/Chi-PANI-NC system was used to detect the H2S gas based on the plasmonic response.
2. Materials and methods
2.1 Materials
Aniline, chitosan, H2SO4, and sodium dodecyl sulfate (SDS) were analytical grades purchased from Sigma Aldrich company. The polydimethylsiloxane (PDMS, (C2H6OSi)n) and silicon elastomer have been purchased from DOW CORNING company.
2.2 Preparation of 1D grating
In this study, the 1D grating was prepared using PDMS [46,47]. A polycarbonate one-dimensional grating pattern was a grating mold that was exploited from a commercial DVD (Digital Video Disc) 120 × 1.2 mm dimension. Each DVD has two polycarbonate layers that sandwich a reflective and colorful layer between themselves [48,49]. The grooves have height and periodicity of 162 nm and 741 nm, respectively. The colored layer on the surface of polycarbonate has been removed using ethanol and distilled water. The periodic polycarbonate layer should be utilized as a template to prepare the inexpensive soft diffraction grating with a soft lithography method [50]. Figure 1 demonstrates the polycarbonate grating as a template which was utilized for the preparation of PDMS grating. The mixture of degassing PDMS (21.8 g) and curing agent (2.2 g) 2.1 g of PDMS and 2.2 g curing agent were mixed to obtain the final elastomer. The polycarbonate patterns were transferred to the surface of the final elastomer to achieve the 1D grating. Finally, the PDMS was put in the oven at 80° temperature. Afterwards, the gold layer was coated on the surface of the prepared grating in the 40 nm thickness using a spattering coater device (DS3, Nano-Structured Coatings Co.).
Fig. 1. a) peel off the DVD; b) after washing the DVD using ethanol and denoised distilled water; c) PDMS was poured into the DVD groove; d) PDMS and DVD were degassed in the chamber with a rotary vacuum pump and baked at 80°C; e) the PDMS was removed from DVD; f) the gold thin layer was coated on the surface of PDMS.
2.3 Preparation of chitosan solution
0.5003 g of chitosan powder has been solved in the 100 ml acetic acid (0.2 N). The chitosan powder was completely solved in the acetic acid solution after 48 hours at room temperature, and a clear solution was achieved to prepare the nanocomposite.
2.4 Preparation of polyaniline target
A 100 ml solution containing 0.1 M aniline in 1 M H2SO4 was prepared by dissolution of 20 microliters of vacuum distilled aniline. The equimolar amount of ammonium proxy disulfate was separately prepared as an oxidizing reagent. The prepared solutions were mixed using a stirrer for 1 h. The powder settled after 12 hours, and the solution was decantated. To purify powder, the precipitated polyaniline powder was washed, and it was separated by centrifuging at 4000 rpm for 3 minutes. The procedure was repeated three times to find the pure wet polyaniline power. The final tablet was achieved after drying in an oven (90°C) for 3 hours.
2.5 Preparation of the chitosan-polyaniline-nanocomposite layer
Figure 2(a) shows the laser ablation setup [51] for the synthesis of polyaniline nanostructures in the chitosan solution. The setup contains the Q-switch Nd: YAG laser in 1064 nm wavelength with 10 Hz repetition rate and 50 mJ energy, a convex lens, a mirror, a sample holder, and a sample tank. The ablation of the polyaniline target was carried out for 1, 2, and 3 mins in the 15 mL chitosan solution. The prepared samples were catheterized using Fourier transform infrared, UV-vis, and X-ray spectroscopies. The prepared sample was coated on the surface of the 1D grating using a spin coater device (backer, VCOAT4-HI). The chitosan-polyaniline-nanocomposite (Chi-PANI-NC) layer was formed on the surface of 1D grating during the 20s. The thickness of layer was about 40 nm. The grating/gold layer/ Chi-PANI-NC as a sensing layer was used to detect the H2S.
Fig. 2. a) The laser ablation setup contains Nd: YAG laser (1064 nm, 10 Hz, 50 mJ), a lens (f = 100 mm), a mirror (diameter = 25 mm), chitosan solution, and PANI target, b) plasmonic sensor setup based on grating coupling using a broadband Halogen light source.
2.6 Plasmonic sensor setup
The ellipsometry based on plasmonic properties of nanocomposite multilayer was used to detect the low concentration of H2S. The ellipsometry parameters were achieved from the optical setup [50] (Fig. 2(b)). The optical setup contains a broadband Halogen light source (Plasens-006, Inc.), a high-precision rotation mount (PR01, Thorlabs, Inc.), an iris diaphragm (ID25/M), a lens (f = 100 mm), a polarizer based on Glan–Taylor calcite prism (GT10-A, Thorlabs, Inc.), 3-Axis NanoMa Flexure Stages (MAX313D, Thorlabs, Inc.), XYZ Translation Stage (PT3/M, Thorlabs, Inc.), an optical fiber (QP600-2-VIS-NIR, Ocean Optics, Inc.), and a spectrometer (Maya2000 Pro, wavelength range from 498 nm to 941 nm, resolution of 0.32 nm, Ocean Optics, Inc.).
3. Results and discussion
Figure 3(a) shows the UV-vis spectrum from 250 to 800 nm for PANI nanostructure with the different ablation times in the chitosan solution. The main peaks appeared at 277, 282 and 285 nm. They related to the π–π* transition of nitrogen excitation [52] in benzenoid segments of PANI nanostructure. The peaks at 445, 451, and 453 nm assigned the shift of polariton to π* band of PANI [52–54]. The red shift in the UV-Vis spectra shows the main chain of PANI increased.
Fig. 3. a) UV-vis spectrum for different ablation times of PANI nanostructure including 1 min, 2 mins and 3 mins, b) XRD spectrum, FITR spectrum for c) pure chitosan, and d) PANI nanostructure in the chitosan solution for 2 mins ablation time before contacted with H2S.
Figure 3(b) depicts the X-ray spectrum result for Chi-PANI-NC. The main peaks appeared at 8.23°, 17.74°, 26.77°, and 36.07°. The board peak appeared in the range of 18° to 35° and 36.07° related to amorph and chitosan. The sharp peak at 8.50° is related to the acetic acid molecules [55] as a solution for chitosan in the tunnels between the PANI chains. The peak at 17.74° and 26.77° depicted the inter-chain distance between adjoining benzene rings in PANI, and the dispersion of PANI chains at an interplanar distance [56]. The X-ray wavelength ($\lambda $) was 1.54 A° from the Cu target and the d-spacing was obtained from Debye–Scherrer technique using the Bragg relation (Eq. (1)) [57] as follows:
where n and d are an integer and d-spacing which is the distance among the planes. θ is the angle between the diffraction plans and path of X-ray. The crystallite size was calculated from the Scherrer equation, $l = K\lambda /B \cos\theta $ [58]. Where l is the crystallite size, K is the ordinary crystallite (∼0.9) the shape factor, B is the entire full width half maximum (FWHM) in radians [36] for each peaks. The d spacing, FWHM and crystalline size have been presented in the Table 1 for mentioned peaks. Consequently, the inter-chain distance between adjoining benzene rings in PANI and PANI chains at an interplanar distance have a crystalline size of about 58.02 and 34.17 nm, respectively. As a result, the acetic acid molecules contain chitosan, and they capped the PANI using tunneling the molecules between the PANI chain [57].
Table 1. The pertinent parameters from the analysis of the XRD spectrum for Chi-PANI-NC.
Figures 3(c) and 3(d) show the FT-IR spectrum in the range of 650 to 4000 cm−1 for pure chitosan and Chi-PANI-NC. The main peaks appeared at 3446.59, 3280.53, 3256.23, 2935.49, 2911.57, 2891.13, 2850.25, 2028.75, 1641.32, 1635.36, 1544.89, 1374.99, 1325.02,1163.01, 1091.64, 1058.73, 885.27, and 750.26 cm−1. The peak at 3446.59, 3280.53 and 3256.12 cm−1 assigned the O-H, symmetric and asymmetric NH2, and N-H stretching of PANI and chitosan that overlapped together in the chitosan solution. The peaks at 2935.49, 2911.57, 2891.13, and 2850.25 cm−1 corresponded to C-H in the chitosan, [59] aromatic aniline ring of PANI and C = O stretching in the chitosan. Moreover, the peaks at 1641.32, and 1635.36 cm−1 corresponded to CO amid and NH primary amino bends [60]. The chitosan peak at 2028.75 cm−1 attributed to the weak absorption of a small amount of stretching vibration of the -N≡ group C- due to presence of acetic acid. The peak at 1544.89 and 1325.02 cm−1 related to C = C quinoid ring stretching of PANI, H-N bending of chitosan skeleton and the C-N stretching of the benzenoid ring, and CH2-OH binding in the chitosan structure [59,61], which overlapped together. In addition, the peaks at 1374.99, and 1058.73 cm−1 are related to C-O starching and C-O bending of pure chitosan [60]. The peaks at 1091.64, 885.27, and 750.25 cm−1 related to C-O-C stretching, ortho substitutions, 1,2 di-substitution in the benzene ring of chitosan and PANI structures, respectively. The peaks in the range of 1163.01 to 885.27cm−1 are related to symmetric and asymmetric stretching vibrations of the C-O and C-O-C polysaccharide skeleton [62]. As a result, the FTIR spectra depict the shift in the peaks at 3280.53, 1635.36, 2850.25, 1374.99, and 1058.73 cm−1 to 3256.12, 2891.13, 1641.32, 1325.02, and 1091.64 cm−1, respectively. In addition, the peak intensity of the FTIR spectrum for Chi-PANI-NC was decreased. The shift in the FTIR absorption and intensity of peaks are due to interaction between the PANI and chitosan. Consequently, the chitosan capped the PANI in the solution.
Figures 4(a), 4(b1), 4(c1), and 4(d1) show the morphology of the chitosan before ablation of the PANI target and Chi-PANI-NC layer with the magnification and scale bar equivalent to 10 K and 1 µm, respectively. Figure 4(a) demonstrates the structure of chitosan in the acetic acid solution and the image depicts the chitosan in flower form. Figures 4(b2), 4(c2), and 4(d2) are the particular parts which were defined with the black circle. The magnification and scale bars were 50 k and 500 nm, respectively. The images show the PANI growth in the chitosan solution and the concentration of PANI nanostructure increased with an increase in the ablation time from 1 to 3 mins. The aggregation and low specific surface area were obtained due to drastic surface tension [52,63,64]. The formation and shape of PANI nanostructure were authenticated the polymer chain grow up by increasing the ablation time from 1 to 3 mins in the chitosan matric using an FE-SEM image. The FESEM picture was achieved. Therefore, the large agglomeration of PANI nanostructure was generated in the chitosan solution.
Fig. 4. The FE-SEM images show a) chitosan before ablation the target of PANI, the PANI nanostructure in the chitosan solution for b1&b2) 1 min, c1&c2) 2 mins, and d1&d2) 3 mins.
The reflectivity of the broadband light beam from the surface of Chi-PANI-NC was registered using a plasmonic sensor setup (Fig. 2(b)). The experiment was carried out using p-polarized light (Transverse magnetic (TM) mode) and s-polarized light (Transverse electric (TE) mode). Figures 5(a), 5(b), 5(c) and 5(d) depict the electrical field distribution when the p- and s- polarized reflected from the sensor chip at different wavelength. The distribution and the intensity of plasmonic waves are different for s- and p- polarized light at the interface of the medium. Figures 5(a) and 5(b) show the plasmonic waves are strongly excited with p-polarized light and the plasmonic wave is stronger than the plasmonic wave when the s-polarized light (Figs. 5(c) and 5(d)) is used. The reflectivity was registered in the presence of air and H2S gas when the p-polarized light reflected from the surface of Chi-PANI-NC thin layers. Figure 5(e) shows the reflectivity of the light beam from the surface of the Chi-PANI -NC thin layer at different angles in the range of 44° to 52°. As a result, the reflectivity decreased by increasing the incident angle from 0.70 to 0.30 a.u. The variation of reflectivity and the curve slope are a minimum of 52° in the presence of air. Therefore, the experiments were carried out at 52°.
Fig. 5. The electrical field distribution for different wavelength a & b) p-polarization, c & d) for s-polarization of light beam, e) variation of reflectance for Chi-PANI-NC in the presence of air.
The experiment was continued when the H2S gas was injected with the different concentrations in the sample tank. Because the detection of H2S is important in the low concentration, Fig. 6 shows the variation of reflectivity for 1, 10, and 20 ppm. The reflectivity shift was compared with the reflectivity shift of broadband light beam in the presence of air.
Fig. 6. The intensity and wavelength shift for different concentrations of H2S gas including a1 and b1) 0 and 1 ppm, a2, and b2) 0 and 10 ppm, and a3 and b3) 0 and 20 ppm.
Figure 6 depicts the variation of reflectivity from the surface of the Chi-PANI-NC layer when the different concentrations of H2S flowed in the gas tank and the H2S interacted with the sensing layer. The broad peak appeared in the range of 500 to 700 nm wavelength. The maximum peak was observed at 585.68 and 451.6 nm which is related to the π* band of PANI and minimum absorption of Chi-PANI-NC (Fig. 3(a)). As a result, the reflectivity increased with an increase in the concentration of H2S. The intensity of the reflected beam increased from 2648.75 to 62260.35 when the concentration of H2S increased from 1 to 20 ppm. As a result, the reflectivity increased 23.5 times when the Chi-PANI-NC interacted and absorbed the H2S. The absorption mechanism of Chi-PANI-NC was based on the electrostatic interaction of oxygen and nitrogen in the PANI and chitosan chain with H2S. The π-conjugation along the PANI authorized the organization of delocalized charge carrier such as electron at the PANI and the electron were free in the π-conjugate (double bonds) and it moved along the carbon bonds which overlaps the p orbitals. This procedure managed the conductivity of PANI. When the nitrogen and the oxygen of the PANI chain had indeed with the H2S, the conductivity decreased. Chitosan is poly(b-1-4)-2-amino-2-deoxy-D-glucopyranose, and it has amino group. Chitosan capped the PANI and H2S via amino (-NH2) and (-OH) groups, they were the main group implicated in the interaction between chitosan and impurity. Therefore, PANI immobilized on the chitosan with the amino group, and the H2S interacts with the NH+ in the PANI chain and amino group in the chitosan. Moreover, the degree of deacetylation of chitosan was a major factor in capping the PANI and interacting with the H2S. Therefore, Chi-PANI-NC absorbed the H2S and the delocalized electron in the nanocomposite chain involved the interaction between H2S and PANI, and the concentration of free electron decreased. As a Mie theory, absorption cross-section of $({C_{abs}} = k{\; \textrm{Im}}[\alpha ]$ ) absorption coefficient ($\alpha $) which depends on refractive index ($n$) and conductivity ($\sigma )$ as a function of the speed and frequency of light, c and ω, as follows:
Therefore, when the concentration of free electrons decreased, the absorption coefficient also decreased, and based on Fresnel’s theory, the reflectance increased.
Figure 7(a) demonstrates the variation of reflectance in association and dissociation process. The results show that the response of the sensor and reflectance increased by increasing the concentration of H2S. Figure 7(b) shows the variation of reflectance for three times that confirms the response of the sensor is stable to detect the low concentration of H2S.
Fig. 7. Variation of reflectance with time a) association process and dissociation process for 1,10, 20 ppm concentrations of H2S b) for association process and dissociation process in three times when the concentration of H2S increased; the normalized curved for c) the association process and fitted curve, d) the dissociation process.
Figures 7(c) and 7(d) exhibit the variation of reflectance with time. Figure 7(c) shows the association process and the association time to find the terminal value is about 100 s. Figure 7(d) demonstrates the dissociation process and the release time is about 200 s. The doted points are the experimental value, and the solid line depicts the theory based on Langmuir’s first order adsorption model as follows:
Where ka is the rate constant [65,66]. As a result, the experimental value fitted well with the theoretical formula. The rate constant for 1, 10, and 20 ppm concentration of H2S were achieved at 0.05, 0.56, and 0.061, respectively. As a result, the terminal (Rter) value increased when the concentration of H2S also increased. At the terminal value, the reflectance increased from 0.93 to 1.024 with an increase in the concentration of H2S from 1 to 20 ppm (see Fig. 7(c)). Consequently, the experimental values fitted well with the theoretical model.Figures 8(a) and 8(b) exhibit the variation of reflectivity with the different concentrations of H2S in the range of 1 to 100 ppm. The experimental value fitted to Langmuir’s equation as follows [66]:
Where C, $\Delta R$, $\Delta {R_{sat}}$, and K are the concentration of H2S, the integrated intensity shift with the concentration of H2S, the integrated intensity shift at the saturation value, and the equilibrium constant, respectively. Langmuir’s equation [66] can utilize to explain the surface absorption of H2S on the surface of Chi-PANI-NC. The value of the integrated intensity shift at the saturation value ($\Delta {R_{sat}}$) and the equilibrium constant ($K$) are 3.9818 and 0.003331, respectively. Consequently, the saturation value occurs after 3.9818 and it is related to a concentration over 100 ppm. Figure 8(c) shows the variation of reflectivity in low concentrations of H2S in the range of 1 to 20 ppm. It has a linear form with the slope of 0.10767. The sensitivity of the sensor is as follows:Fig. 8. Variation of reflectivity with different concentrations of H2S in the range of a) 1-100 ppm, b) 1-100 ppm using Langmuir equation fitted, c) 1-20 ppm as a linear part.
So, the sensitivity is equivalent to the slope of the linear curve. Therefore, the sensitivity of the plasmonic sensor based on Chi-PANI-NC is about 0.10767 for the detection of H2S in low concentration in the range of 1 to 20 ppm.
Figure 9(a) shows the FT-IR spectrum for sensing later after the interaction with H2S in the range of 650 to 4000 cm−1. The main peaks appeared at 3360.31, 2890.20, 1740.51, 1620.32, 1530.02, 1380.43, 1150.42, 1030.05, 874 and 766 cm−1. The shift in wavenumber and intensity observed in the peak at 1740.51, 1620.02, 1380.43, 1150.42 and 1030.05 cm−1 from 1641.32, 1325.02, 1163.01, and 1090.64 cm−1. The peaks correspond to the aromatic aniline ring of PANI and C = O stretching in the chitosan, C-N stretching of the benzenoid ring, and to C-O-C stretching of PANI. Therefore, PANI and chitosan interacted with H2S and they contributed to detecting the H2S. As a result, the FTIR spectrum authenticated Chi-PANI-NC can adsorb the H2S. Figure 9(b) shows the molecule structure [67] of Chi-PANI-NC and it demonstrates the interaction of H2S with Chi-PANI-NC to clarify the absorption mechanism of hydrogen sulfide.
Fig. 9. The FT-IR spectra for a) PANI nanostructure and b) PANI nanostructure after contacted with H2S.
4. Conclusion
The Chi-PANI-nanocomposite was fabricated using laser ablation technique. The Chi-PANI-NC layer (3 mins ablation time) was coated on the surface of the gold-coated layer side of PDMS grating. The UV-vis, XRD, and FTIR spectra confirmed the formation of PANI-nanostructure in chitosan. The FESEM images demonstrated the distribution of PANI nanostructure increased on the surface of chitosan and they authenticated that the concentration of PANI increased when the ablation time was increased. The reflectivity of broadband light increased with an increase in the concentration of H2S and the Freundlich isotherm model explained the interaction of Chi-PANI-NC with H2S. Moreover, the main peaks of the FTIR spectrum were investigated after the experiment and the shift in the main peaks confirmed the surface absorption of H2S by Chi-PANI-NC. Consequently, the Chi-PANI-NC can adsorb the H2S gas with the limit in the concentration and sensitivity of about 1 ppm and 0.10767, respectively.
Acknowledgments
The authors acknowledge Magneto-plasmonic Lab, Laser and Plasma Research Institute, Shahid Beheshti University.
Author Contribution. S. M. Hamidi supervised the work and checked all of the measurements and the writing process, Farnaz Amouyan measured, analyzed, and writing some part of the manuscript, A. R. Sadrolhosseini, project designed, sample preparation, measured, writing the manuscript, R. Taheri Ghahrizjani, measured, data analyzed, virtualization and M. Kazemzad fabricated the chitosan sample.
Disclosures
The authors declare that they have no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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