The exploitation of SnO2/Polypyrrole interface for the  sensing of ammonia vapors using SPR technique: a theoretical and experimental analysis

doi:10.21203/rs.3.rs-1428599/v1

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The exploitation of SnO₂/Polypyrrole interface for the sensing of ammonia vapors using SPR technique: a theoretical and experimental analysis

https://doi.org/10.21203/rs.3.rs-1428599/v1

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We use a laboratory-assembled Surface Plasmon resonance (SPR) system for the application of gas sensors around the room temperature for our work, as described in this publication. The SPR system gas sensor is based on the variance in the refractive index at the nanocomposite sensing material of SnO₂/Polypyrrole (PPy) interface with gas adsorption (NH₃). The value of the best thickness of the nanocomposite of SnO₂as Metal Oxide Semiconductor (MOX ) and Polypyrrole(PPy), as Polymer, thin films were optimized using theoretical calculations based on the sharp SPR reflectance curve. In this work, we also include theoretical SPR curves by changing, one by one, the thickness of the SnO₂layer and PPy layer. The Surficial interaction of Ammonia (NH₃ ) gas with our system of prism/Au/ SnO₂/Polypyrrole nanocomposite gas sensing material was also studied experimentally for the validation of the theoretical findings. The gas sensing layer in the SnO₂/Polypyrrole nanocomposite multilayer structure system is used to design a sensitive and effective ammonia gas sensor based on the phenomenon of Surface Plasmon Resonance (SPR). The results obtained, highlighting the usefulness of the SPR setup for the study of the Ammonia vapors interaction of the metal-dielectric/polypyrrole nanocomposite material.

Gas Sensors

Polypyrrole

Nanocomposite

Surface Plasmon Resonance

The research community has long worked for the building of the very sensitive, stable, room temperature operating, low cost, and effective sensors to protect our living and workplaces getting the intoxicating effect of the deleterious and toxic gas leakage (1), (2). Various Nanostructures metal oxide semiconductors have been studied for a variety of applications such as photo-catalyst(3), Lithium-ion batteries(4), gas sensors(5), (6), and various other industrial applications. There are various gas sensing technologies available including conductometric and optical sensing technologies. Although the conductometric sensing technology is the simplest one, it has several drawbacks, including the requirement of operation at high temperatures, excessive power consumption, and poor selectivity of different analytes present simultaneously. As a result, the development of gas sensors having, selective detection ability and high sensitivity, which works on the principles of optical sensing phenomenon, are now considered as the best choice (7). Optical sensors have the advantage of being able to operate at room temperature and using less electricity as compared to other sensing technologies (8). For gas sensing applications, now a day, a variety of optical techniques are available, namely, IR spectroscopy techniques (9), UV spectroscopy techniques (10), ellipsometry techniques (11), and the SPR techniques (12). Surface Plasmon Resonance-based optical gas sensors are used for the sample preparation and they have advantages over other types of sensors like operating at room temperature, good selectivity, high response, low or room temperature operation, and lower detection limits. The Surface Plasmon Resonance (SPR) technique is very popular amongst researchers for the detection of harmful gases and bio-sensors.

The Kretschmann configuration is the most convenient SPR technique configuration among the other available configurations(13), (14), (15). However, the results were unsatisfactory due to the poor response, cumbersome and complex setup. So, the optimization of the thickness of the sensing layer is required for the development of a good optical gas sensor. In the present work, theoretical simulation is done for the estimation of the suitable thickness of the tin oxide nanolayer and polypyrrole nanolayer, one by one. SPR is a very sensitive technique for detecting even the smallest changes along with the metal-dielectric interface(16). So, the adsorption of a specific target gas molecule can lead to the change of the reflectance measurement of the sensor system.

In our work, the sensing of highly harmful and lethal ammonia gas is the prime focus. Ammonia is produced as a waste product from the animals as well as the by-product of Industries. A large amount of research work is still going on by the research community for the detection of ammonia gas(17), (18). The demand of time is the development of a gas sensor that can detect even a small amount of ammonia gas (less than 5 ppm) so that human life can be saved from the harmful presence of ammonia at an early stage of gas leakage(19). There are just a few publications of SPR gas sensors that are specifically designed to detect NH₃ gas(20) (21). But, the published work claims low sensitivity and methodology requires a higher ammonia concentration (more than 25 ppm) for detection. To name a few, Maciak et al. used the conducting polymer PANI (polyaniline) as the sensing material which utilizes the SPR technique with a detection limit of 32 ppm to monitor ammonia vapors (20). Dolbec et al.(21) used a different sensor layer, Nafion/WO₃, to detect larger ammonia concentrations.

Conducting polymers, which have good electrical and optical characteristics, is another type of material that has lately been investigated (21). Polypyrrole (PPy) is the most well-known conducting polymer due to its stability at high temperatures and ease of sample preparation (22)(23)(24)(25). PPy thin films with concentrations of 0.01–1% were electrochemically produced for NH₃ sensing (26)(27). As a result, the majority of the results reveal a small sensing response operating above the room temperatures. As known to the authors, to date no published research work on the construction of an SPR technique-based sensor for NH₃ gas detection, which employs a SnO₂/PPy multilayer structure, is there. So, this could open the way for the development of an effective SPR sensor for low-level ammonia gas detection.

The nanolayer thinness of gold (Au), Tin Oxide (SnO₂₎, and PPy thin films were simulated for a sharp reflectance curve, in this study. With various concentrations of harmful gas such as NH₃ gas, the change in optical characteristics of a gas-sensitive multilayer thin film structure produced at the optimal thickness was investigated using the SPR technique.

2.1.Details of Theoretical simulation:

When light is reflected from the metal/dielectric interfaces, the Intensity of reflected light is a function of the Incident angle (under angular interrogation mode). Here we utilize Fresnel's equations for the theoretical simulation (28). In the SPR technique for gas sensing, as pointed out previously, the optimization of the thickness plays an important and crucial role (29). Hence, the selection of the sensitive sensing material and having the appropriate thickness of that sensing material layer becomes a necessary step for the making of a good sensor. The interaction of the exposed target gas molecules with the sensing layer can lead to the variation of the refractive index of the sensing material as a whole. The thickness of the metal and dielectric layer plays an important role in minimizing the absorption losses. Hence, SPR reflectance curves have been theoretically simulated for optimizing the thickness.

2.2.Experimental Details:

The multilayer configuration was built by first depositing the metal layer directly on the prism by thermal evaporation as reported previously (29). Here, the dielectric layer used for sensing is the SnO₂ thin film. SnO₂ nanolayers were fabricated using the technique of sol-gel (30). Chemical polymerization was used to make PPy nanoparticles, and a spin coater was used to make films (31). For analyzing the gas sensing results based on SPR, a laboratory manufactured SPR measuring setup is being used which is indigenously developed and details are mentioned elsewhere (29). For gas sensing measurements, a separate cell has been designed so that the fixed concentration of the target ammonia gas may be introduced with the interaction of the sensing material coating (SnO₂/PPy). Any change in the optical properties of the interface (SnO₂/PPy) will be reflected in the developing gas sensor. To create a vacuum, a mechanical rotary pump is coupled with the gas cell unit. A calibrated needle linked to the inlet valve was used for the admission of ammonia vapors of desired concentration into the gas cell unit. The gas sensing results were analyzed in two modes of measurement i.e. static and dynamic. In the static mode, the rotating stage with the attached optical power meter was used to record the SPR reflectance curve while the ammonia gas was placed within the gas cell unit, for the whole range of incidence angle. The prism/gold/SnO₂/PPy/air sensing device was placed at a position where the minimal SPR reflectance (R_min) was recorded for the transient measurements. A CCD camera connected to a data acquisition system records the reflected light intensity as a function of time, for a given angle of incidence. The CCD camera was used to capture the variation of the intensity of the reflected light, at a given time, for the two types of measurements, namely,(1) when the target gas of a certain concentration was inserted into the gas cell and (2) when it was discharged out of the chamber.

3.1.Theoretical simulation results

Theoretically simulated reflectance curves of the SPR phenomenon are obtained by varying the counting of layers as depicted in Fig. 1, i.e. prism/Au/air (black curve), prism/Au/SnO₂/air (red curve), and prism/Au/SnO₂/PPy/air (blue curve). The angle of resonance moves towards higher values when the counting of layers increases from two (SnO₂/air interface) to three (SnO₂/PPy/air interface).

3.1.1.Variation of the thickness of SnO₂:

The theoretical simulation results by varying the thickness of SnO₂ are presented in Fig. 2.

Resonance angle and minimum reflectance are the coordinates on the x and y-axis respectively for minima in the SPR reflectance curve. There is a continuous increase in resonance angle observed for an increase in thickness of SnO₂ where minimum reflectance decreases.

Figure 3 confirms that for a width of 100 nm of SnO₂, the reflectance curves of SPR, have the lowest value of Full Wave Half Maximum (FWHM), hence, this is the required value of optimized width of SnO₂ layer for optimum functioning.

3.1.2.Variation of the width of the Polypyrrole conducting polymer layer :

The simulation results of the variation in the width of the polypyrrole layer of the prism/Au/SnO₂/PPy/air interface are depicted in Fig. 4.

As seen from Fig. 4, The angle of resonance increases with the increase in the thickness of the polypyrrole layer, while the minimum reflectance decrease with the thickness of the polymer polypyrrole layer.

Figure 5, shows the full-wave half maximum (FWHM). It is observed that the FWHM, first increases with the thickness of the polypyrrole layer, reaches a maximum then decreases to a minimum, and again increases with the thickness. The minimum value of FWHM is obtained for 300 nm polypyrrole layer thickness.

3.2 Experimental studies

Figure 6 represents the static mode of measurement of the reflectance curves as recorded for prism/gold/SnO₂/PPy multi-layer interfaces when exposed with the varying concentration of NH₃ molecules (from 0 to 200 ppm). From Fig. 6, there is a clear indication of the fact that a continuous shift in the minimum resonance angle, θ_SPR, to the higher angle values, varying from 40.7° to 51.1°, which is due to the variation of the refractive index of SnO₂ /PPy nanolayer interface having adsorbed NH₃ molecules, when the variation of concentration of inserted gas occurs. This observation can be observed as shown in Fig. 7, when the corresponding graphs of the resonance angle, minimum reflectance, and FWHM, respectively, versus varying concentration of the ammonia gas, is plotted separately.

From Fig. 7, it is observed that there is a sharp increase of values of resonance angle (θ_SPR) and Minimum Reflectance value (R_min) for the prism/gold/Tin oxide/Polypyrrole/NH₃ adsorbed nanolayer sensing device with the varying concentration of ammonia(NH₃) molecules from 0 to 200 ppm. The obtained data of Reflectance values concerning change in the angle of resonance may be the effect of the variation of the effective refractive index of Tin oxide/Polypyrrole interface due to the adsorption of ammonia (NH₃)molecules. As evident from Figs. 7 (a) and (b), there is a linear increase in the angle of resonance (θ_SPR) and the minimum value of reflectance (R_min) as the concentration reaches 10 ppm. With a further increase in concentration, there is saturation observed for the curves of the angle of resonance and the minimum value of reflectance, respectively. The reason for this observation may be the fact that with a higher concentration of NH₃ gas, the absorption of more ammonia molecules does not produce any appreciable variation in the value of the refractive index, hence there is non-linear variation (saturation) of the values of, θ_SPR and R_min. Further, the FWHM of the reflectance curves of the sensing system also behaves similar to the behavior of,θ_SPR and R_min (Fig. 7 (c)), till 10 ppm concentration.

3.3.Sensing mechanism

Tin oxide is an n-type semiconductor, so it gets reduced when a reducing gas, like ammonia, gets adsorbed on its surfaces. Now there is a transfer of an electron from the adsorbed oxygen molecules, hence releasing the trapped electron to the tin oxide semiconductor layer. This, in turn, increases the conductivity of the material which corresponding changes the refractive index of the tin oxide nanolayer.

This change in the effective refractive index of the ammonia adsorbed tin oxide/polypyrrole nanocomposite layer may occur due to two reasons:

(i) Variation of the refractive index of the bulk media from air to the higher refractive index of the ammonia gaseous molecules, and

(ii) Variation of the refractive index of the gas sensing layer of Tin oxide (SnO₂)/Polypyrrole nanolayer having adsorbed ammonia molecules on its interface.

The curve fitting of the theoretically calculated values of the refractive indices from the Fresnel coefficients with the experimentally observed reflectance curves data values is done. The variation of the effective refractive index of the gas sensing device of tin oxide/polypyrrole interface having adsorbed ammonia molecules with the changing concentration value is depicted in Fig. 8 (a) (the calibration curve). As observed earlier, The value of the refractive index increases, almost in a linear fashion, to 10 ppm of the concentration, as clearly indicated by Fig. 8(b). A linear fit of the curve is done for the calculation of the Sensitivity by finding the slope of the line from the calibration curve of Fig. 8(b) and the value obtained is 4.5x10^− 3 RIU/ppm.

The transient mode of measurement curves are shown in Fig. 9 (a) for the prism/gold/Tin oxide/Polypyrrole/ammonia adsorbed sensing device. As depicted in the graph, at a fixed angle of incidence, θ = 40.7°, the gas sensing device show step like variation of the effective index of refraction with the variation of the concentration of ammonia gas from 0 to 200 ppm. The variation in the reflectance of the sensing system is measured using a CCD. The sensor shows a continuous increase in the response (change in reflectance) with increasing concentration of NH₃ gas from 1 to 200 ppm at room temperature (Fig. 9 (a)). The calibration curve of the SPR sensor is also plotted as shown in Fig. 9 (b). Figure 9 (b) shows a linear increase in the response i.e. change in reflectance with an increase in the concentration of NH₃ gas over the range of 1 to 200 ppm. The calculated value of the sensitivity of the ammonia gas sensing device is 0.202 /ppm with varying concentrations from 1 ppm to 200 ppm (Fig. 9 (b)). So, the obtained values of sensitivity pave the way to develop an efficient ammonia gas sensor based on the surface plasmon resonance technique.

Surface Plasmon Resonance (SPR) laboratory setup is utilized for the fabrication of a gas sensing device for the detection of deleterious and hazardous ammonia gas. The indigenous lab setup SPR system uses the metal oxide semiconductor, tin oxide(SnO2), and conducting polymer, polypyrrole (PPy) nanocomposite interface for efficient detection of the ammonia gas. The best thicknesses of the nanolayers of tin oxide and polypyrrole are obtained by the theoretical simulation method, which constitutes part of the multi-layer system of the gold-coated prism with the tin oxide and polypyrrole nanocomposite. The deposited thin films of 100 nm width layer of the tin oxide and 300 nm width layer of Polypyrrole were obtained theoretically for the sharpest reflectance curve with the minimum reflectance values, which work as the efficient gas sensor for the ammonia gas detection.

The lab-made SPR ammonia gas sensor has a fast response time of 1s with a good sensitivity of (4.5x10^− 3 °/ppm) for the ammonia gas over a concentration range of 1 to 10 ppm. The dynamic response has also been obtained with a good sensitivity of 0.202 /ppm.

Hence good and high sensitive Surface Plasmon Resonance-based optical gas sensor is realized in the work.

Ethics approval and consent to participate:

The Authors gives their consent to participate.

Consent for publication:

The Authors gives their consent to publication.

Availability of data and material:

All data will be available from authors on reasonable demand.

Competing interests:

The authors declare that they have no competing interests in this section.

Funding:

No funding was received from any agency for the present work.

Authors' contributions:

APSG has devised the problem for the present work. All the literature survey and work plan of the present work has been done. Analysis of the performed work.

AP has done all the experimental work. Some part of the analysis of data has also been done

AK has done the final proofing of the article and reviewed the article for publication.

Scientific inputs in the analysis of the data have also been given.

Acknowledgments:

The authors are highly obliged to late Prof. Vinay Gupta, Department of Physics and Astrophysics, University of Delhi for his kind help and suggestions.

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The exploitation of SnO₂/Polypyrrole interface for the sensing of ammonia vapors using SPR technique: a theoretical and experimental analysis

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1. Introduction

2. Details Of Theoretical Simulation And Experiment

3. Results And Discussion

3.2 Experimental studies

4. Conclusion

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