Hypersensitive and selective interferometric nose for ultra-trace

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Hypersensitive and selective interferometric nose for ultra-trace ammonia detection with fast response utilizing PANI@SnO2 nanocomposite Anand M Shrivastav, GAURAV SHARMA, Abhishek S. Rathore, and Rajan Jha ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00828 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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Hypersensitive and selective interferometric nose for ultra-trace ammonia detection with fast response utilizing PANI@SnO2 nanocomposite Anand M. Shrivastav, Gaurav Sharma, Abhishek S. Rathore, Rajan Jha* Nanophotonics & Plasmonics Laboratory, School of Basic Sciences, Indian Institute of Technology Bhubaneswar, India KEYWORDS: Nanocomposite, Ammonia, Photonic Crystal Fiber, Interferometer, Optical Sensor

ABSTRACT

Ammonia is one of the most toxic gases present in our environment and its presence in the atmosphere is very unpleasant since its higher concentration in blood causes the coma and convulsion. Hence, in light of advance photonics technology, we report a contemporary approach to design and develop hypersensitive ammonia gas sensor realizing Mach-Zehnder interferometer (MZI) by a Single mode fiber (SMF)-Photonic crystal fiber (PCF)-SMF fiber optic substrate for realizing the interference by immobilizing PANI@SnO2 nanocomposite to achieve sensing. In this case excitation of core and cladding modes of PCF is achieved using collapse region that is formed at the junction of SMF and PCF specialty fiber. The method

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claimed to have very fast response and recovery times of 7 sec and 2 sec respectively, which can detect as low as 8.09 ppt (47.59 fM). The reusable probe shows the potential for rapid detection of ultratrace ammonia with high selectivity and reproducible features thereby opening a new window for environmental monitoring and online measurements. Introduction Environmental safety is one of serious topic of concern including atmospheric surveillance and water quality control. Atmospheric parameter monitoring involves the detection of toxic gases and other pollutant exhausts from industries. Ammonia is one of toxic gases present in environment which originates naturally and as well as from industry. Naturally, ammonia is produced by the lightning during rainfall as it interacts with natural nitrogen present in air. Further, a large amount of ammonia is generated by bacteria in water and soil as a product of plant and animal waste decomposition. It is found in relatively low nontoxic concentrations in soil, air, and water and provides a source of nitrogen for plants.1,2 Also it is a highly reactive gas and has the vapor density about 0.59 of air and its exposure contributes significantly for the climate change and human health hazard3,4 in form of several dangerous diseases such as respiratory distress, bronchiolar, nasopharyngeal cancer, alveolar edema and tracheal burns etc. Further, ammonia also shows detrimental effects to humans and environmental species even at lower concentration by affecting the human respiratory system, eyes, skin etc. Numbers of studies have been reported for the detection of ammonia with higher concentrations5–7 but a state of art for ammonia detection is still lacking which requires less than 2 ppb ammonia concentration for environmental monitoring and need to be developed.8 Optical fiber sensors have shown a broad vitality in number of industries including biomedical diagnostics and environmental monitoring due to the advantageous factors such as simple and


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label-free monitoring, immunity towards external electromagnetic disturbances, applicability for online monitoring and remote sensing possibilities etc.9 Among the fiber optic sensors developed with various transducing mechanisms such as plasmonics, fluorescence, luminescence, interference etc, interference based sensor have been used in large proportion due to its high sensitivity and precise measurement10. In the present study, we have proposed ultra-sensitive optical fiber based MZI as the transducing mechanism for the ammonia gas detection with easy adjustment, low cost and high precision capabilities that is highly reproducible and reusable. The interferometer is obtained by an in-line splicing of SMF with specialty fiber also called PCF. Here PCF is spliced between two SMF and for simplicity we abbreviated this structure as SPS. When the light is launched into SMF, the fundamental mode diffracts thereby exciting core and cladding modes of PCF. Effective refractive index of a propagation mode in an optical fiber is governed by the weighted average of the local refractive index. Further, these modes coupled back at the second collapse region and act as the MZI, which is then collected at the core of single mode fiber.11 Obtained interference is highly sensitive towards the refractive index of the environment around the PCF cladding. Any change in this region produces shift in wavelength and also the depth of interference pattern. Hence by monitoring change in interference pattern, one can detect the ammonia concentration around the SPS platform by utilizing polyaniline (PANI) @ Tin Oxide (SnO2) nanocomposite as the recognition layer. Conducting polymers as recognition layer have shown great potential for the developments of the chemical and biological sensors due to their applicability to produce redox reactions with various analytes which result in the change in the electrical, optical and mechanical properties.12 Among the conducting polymers, polyaniline (PANI) manifested as one of the promising candidate for the gaseous sensing applications since it exhibits several advantages like high


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conductivity, easy synthesis procedure, high stability, tunable properties along with a broad spectrum of applications.13 It is known that during the synthesis of PANI, its modification with hydrochloric acid (HCl) causes its protonation which results in the formation of N+-H chemical bond over the surface of PANI. Thus, the positive charge over nitrogen in PANI allows it to interact with ammonia (NH3) reversibly as the products of neutral PANI and positive ammonium ion (NH ). The reaction mechanism has been further illustrated in figure 1.

Figure 1: Ammonia sensing for polyaniline Although PANI possesses various advantages, it suffer from moderate sensitivity and selectivity towards the ammonia gas as compared to metal oxides. Further, n-type semiconducting metal oxides such tin oxide (SnO2), zinc oxide (ZnO), tungsten oxide (WO3), titanium oxide (TiO2) act as an efficient recognition element for the gas sensing.14 Among aforementioned metal oxides, SnO2 is the largely used semiconducting metal oxide for the ammonia sensing as it possesses high sensitivity and selectivity15 apart from being eco-friendly in nature, simple synthesis procedure, high band gap as well as dielectric constant. The SnO2 surface consists of a large number of grains and since the surface of SnO2 usually found in


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nonstoichiometric form due to its dual vacancy, it results in the variable oxygen composition which causes the SnO2 surface to have oxygen defects that facilitates ambient oxygen adsorption at the grain boundaries.16 When ammonia gas comes within the vicinity of absorbed oxygen over the SnO2 surface, it gets adsorbed resulting its oxidation as formation of NHx species on the SnO2 surface. These species move freely over the SnO2 surface resulting the production of nitrogen gas and H2O.17 Gaseous nitrogen then causes the change in the resistance and hence the conductivity of the surface that causes the change in the effective index of SnO2 layer. Further, the ammonia not only interacts with the absorbed oxygen over the surface but also reacts with the SnO2 by its reduction which causes the formation of tin nitride and tin hydroxide complexes. However these oxygen defects required higher temperature (150-200) for their activation which limits the solely application of SnO2 for ammonia sensing at room temperature.18–20

Figure 2: Pictorial representation of the SPS structure for obtaining interference and usage of PANI@SnO2 nanocomposite for ammonia sensing.


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To overcome the limitations of SnO2 and PANI, the nanocomposite of PANI and SnO2 have been introduced for the detection of ammonia in ppb concentration at room temperature with high sensitivity and selectivity. The conceptual diagram of the SPS structure for obtaining interference and usage of PANI@SnO2 nanocomposite for ammonia sensing has been shown in figure 2. In the present study, we report an ultra-sensitive optical fiber MZI based on SPS structure as an optical nose for ultratrace ammonia detection utilizing PANI and SnO2 (PANI@SnO2) nanocomposite thin film. A numerical and experimental analysis has been provided for the interference pattern and sensing mechanism. Further, the experiments have been performed on the fabricated probe to obtain the probe characteristics, dynamic response, reproducibility, reusability and the selectivity with ultralow detection limit of 47 fM. Ammonia Gas sensing mechanism using PANI@ SnO2 nanocomposite It is well known that SnO2 is a n-type semiconductor and PANI films act as the p-type semiconductor in general. Also PANI acts as a p-type semiconductor due to the presence of various acidic dopants (such as HCl) which bound with the central N atom of aniline as H-N+-Clas shown in figure 1.21 The presence of SnO2 nanoparticles affects the electrical properties of PANI@ SnO2 as shown in figure 3. The core shell nanostructure of p-type PANI and n-type SnO2 creates a p-n junction and leads to the formation of depletion layer at the interface, as ntype SnO2 eliminates the holes (central N-atom) of PANI resulting in the effective PANI@ SnO2 matrix electrically less conducting.22 When ammonia gas molecules enter at the p-n junction of porous PANI@SnO2 nanocomposite, these get polarized by the protonation (NH ions) and increases the mobility of central N atoms in PANI matrix towards depletion layer21,23 that causes the decrease in the width of depletion layer which in turn enhances the conductivity of the


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PANI@SnO2 nanocomposite film.24–27 The change in resistivity alters the conductivity (σ) of PANI@SnO2 nanocomposite film

Figure 3: Interaction behaviour of ammonia with PANI@SnO2 nanocomposite for interferometric sensing application affecting the dielectric constant ε of the film as per the relation28,29 ε = ε r + i σ ωε 0 . Further, the change in dielectric constant (or effective index) of the PANI@SnO2 nanocomposite film alters the modal characteristics of higher order cladding modes of the PCF. When light from the broadband source is coupled, the fundamental mode of the SMF diffracts at the first collapsed region. This leads to the broadening of the modes which, in turn, excites the core and cladding modes in the PCF. The propagation constants of PCF core and cladding modes are different. Thus, the modes accumulate a phase difference as they propagate along the PCF section. The phase difference depends on the wavelength of the guided light and also on the distance over


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which the modes travel or length of PCF. After the PCF, the modes reach another solid piece of glass, i.e., the other collapsed end of PCF. They will thus further diffract and will be recombined through the filtering of the subsequent SMF. Therefore, the transmission of our interferometer can be expressed as that of a two-mode interferometer.11 As the effective index of the PANI@SnO2 nanocomposite film changes with the increase in ammonia concentration, the condition for interference maxima and minima is satisfied for other value of wavelength that causes the shift in the interference pattern. Hence, by monitoring the shift in the interference pattern one can detect the very small quantity of NH3 along with the high response at room temperature. Theory and numerical simulations: Simulated core mode for LMA8 PCF structure under consideration is shown in figure 4(a). When a section of PCF is spliced between SMFs, collapsed regime is formed at the interface of SMF and PCF that excites higher order cladding modes of PCF. One such mode is shown in figure 4(b). The mode intensity variation can be expressed as: = + + 2 cos

(1)

Where and are the light intensities of core mode and cladding modes propagating through PCF and corresponds to the phase difference between the core mode and cladding mode at the point of interference, which can be given as follows: =

Where

and

(2)

represents the effective refractive index of PCF core and cladding

respectively, L is the length of PCF and is the wavelength of light in free space. Since

and

depends upon the medium surrounding the PCF. Figure 4(c) shows the simulated light


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transmission in SMF-PCF-SMF structure which affirms the transmitted light is partially diffused into PCF cladding and recoupled back to SMF core to produces interference. Further, the typical interference pattern corresponding to different coating over PCF region using MATLAB is shown in figure 4(d).

Figure 4: (a) Core mode of LMA 8 PCF, (b) Excitation of higher order cladding mode of LMA 8 PCF (c) Electric field energy distribution for fabricated SPS structure and (d) Typical shift in interference pattern achieved for SPS, SPS/PANI@SnO2 and SPS/PANI@SnO2/ammonia. Results and discussion Probe Characterization: The fabricated probe was fixed in the gas sensing setup as discussed later in the methods section. Light was launched from SLED (Thorlabs:S5FC10005S) source and its corresponding transmitted spectrum was collected by the optical spectrum analyzer. The ammonia gas with


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varying concentration from 0 to 8 ppb was purged within the gaseous chamber and the output interference spectra with respect to varying ammonia concentration were recorded. The vacuum within the chamber was maintained within two consecutive readings to ensure that there should not be any essence of previous gas concentration in the spectrum. Figure 5(a) shows the observed interference pattern with varying probe configurations of bare SPS, SPS/PANI nanocomposite and SPS/PANI@SnO2 nanocomposite after exposing ammonia gas.

Figure 5: (a) Interference pattern for SPS probe with and without coating of PANI@SnO2 (10 wt%) nanocomposite and, then after its interaction with ammonia gas, variation in (b) interference dip wavelength and (c) sensitivity for SPS/PANI@SnO2 nanocomposite (10 wt%) probe with ammonia concentration ranging from 0 to 8 ppb. For bare SPS probe, the interference pattern represented by solid line is obtained due to the interference of core mode and diffracted cladding modes of PCF. The dashed spectrum in the


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figure 5(a) corresponds to the interference pattern corresponding to the probe having PANI@SnO2 nanocomposite (10 wt%), coated around the PCF regime. The pattern shows a change in the depth of the interference pattern with respect to bare SPS probe (solid plot) caused by the dependency of the modal characteristics on refractive index of the medium surrounding it. Further, when ammonia gas is injected within the chamber, a change in the intensity and as well as in the wavelength of interference dips is observed as dotted line in the figure due to the change in the refractive index of the sensing layer (PANI@SnO2 nanocomposite) after its interaction with the ammonia gas. Further figure 5(b) represents the change in the interference curves with varying concentration of ammonia. A red shift in the interference pattern has been observed with increase in the ammonia gas concentration due to the increase in effective index around the probe, which is a result of the interaction of ammonia with the PANI@SnO2 nanocomposite. While interacting with PANI, ammonia causes the reversible chemical interaction with protonic PANI in the presence of HCl. Further, during the reaction with SnO2, ammonia gets oxidized with oxygen defects, caused by variable oxygen vacancies over the SnO2 surface. These interactions result in the effective index increase of PANI@SnO2 nanocomposite around the sensing probe thereby shifting the pattern. Figure 5(c) represents the variation in dip wavelength with ammonia gas concentration within the chamber. The calibration plot shows the saturation behavior of the sensor probe at higher concentration of ammonia. This is because of the limited number of active surface sites over the sensing layer which can interact with ammonia, as the PCF regime of SPS structure has finite surface area. Also for higher ammonia concentration, the number of active sites decreases, hence there is less change in effective index of sensing layer, thus a lesser shift in dip wavelength as evident by the saturation behavior of calibration curve at higher concentrations. The error bars in the calibration curve shows the standard deviation in the


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dip wavelength while performing the experiments multiple times, which shows the good repeatability of the fabricated sensing probe. The calibration curve of proposed sensing probe i.e the change in dip wavelength and ammonia concentration as follows: = −54.13& + 1252.18&

(3)

Additionally, the inset of figure 5(c) shows the sensitivity response of the probe, estimated by taking the slope of calibration curve equation (3) gives linear relation between the sensitivity and the ammonia concentration: (=

) )+

= −108.26& + 125.18

(4)

The maximum sensitivity of the probe is obtained as 1236 pm/ppb near zero concentration of ammonia gas. Effect of SnO2 concentration for ammonia sensing: To analyze the effect of SnO2 for the ammonia sensing, three type of sensing materials were synthesized: PANI, PANI@SnO2 nanocomposite (10 wt%) and PANI@SnO2 nanocomposite (20 wt%). The SPS structured were coated with these materials. Figure 6(a) shows the SPS/PANI probe characteristics with varying ammonia concentration from 0 to 8 ppb, while figure 6(b) corresponds to the characteristic plot for the probe having configuration SPS/PANI@SnO2 nanocomposite (20 wt%). The response for the probe with SPS/PANI@SnO2 nanocomposite (10 wt%) was already shown in figure 5(b). From the characteristic curves of all the probes, the comparative calibrations curves have been plotted as figure 6(c), where the maximum change in dip wavelength was observed for the probe having configuration SPS/PANI@SnO2 nanocomposite (10 wt%) due to difference in the surface morphology of the sensing materials namely PANI, PANI@SnO2 nanocomposite (10 wt%) and PANI@SnO2 nanocomposite (20 wt%) as evident from the Field emission scanning electron microscopy (FESEM) images. Figure


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7(a) corresponds to the magnified surface of PANI matrix coated SPS structures. The inset of the figure

Figure 6: Shift in interference dip for the (a) SPS/PANI and (b) SPS/PANI@SnO2 nanocomposite (10 wt%) probe. (c) Shift in dip wavelength for ammonia concentration change from 0 to 8 ppb for three probes. refers to the surface morphology of the PANI matrix at 200 nm scale. The circled area in the figure 7(a) shows that PANI has the porous structure which is responsible for its gas sensing capability and the interaction with ammonia. Further, the inset of figure 7(b) reveals that the PANI@SnO2 nanocomposite (10 wt%) has the surface with netlike structure where SnO2 nanoparticles are encapsulated within the PANI matrix. Figure 7(b) also depicts the increased porosity of the surface, which lead the increased sensing behavior for ammonia as compared to bare PANI film. Figure 7(c) shows the surface structure of PANI@SnO2 nanocomposite (20 wt


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%), with increased loading of SnO2 compared to figure 7(b). The clusters (circled) in the figure shows that the SnO2 particles get agglomerated over the surface that may reduce the ammonia sensing capability as the gas be able to interact within the PANI surface. From the figures, one can find that SPS/PANI@SnO2 nanocomposite (10 wt %) surface contains the maximum porosity with the respect to rest two. Hence, we have used the probe SPS/PANI@SnO2 nanocomposite (10 wt %) for further experiments.

Figure 7: Surface morphology for the PCF coated with (a) PANI, (b) PANI@SnO2 nanocomposite (10 wt %) and (c) PANI@SnO2 nanocomposite (20 wt %). Probe performance parameters The various performance parameters of the probes such as selectivity, stability, repeatability were evaluated. The selectivity of finalized sensing probe, SPS/PANI@SnO2 nanocomposite (10 wt%) was checked by various other gases like Hydrogen (H2), Nitrogen (N2), Oxygen (N2), Methane (CH4) and Hydrogen sulfide (H2S), where CH4 and H2S are the reducing gases and H2


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and N2 belongs to oxidizing gas family. Figure 8(a) shows the observed change in dip wavelength in interference pattern of the probe corresponding to each with concentration change from 0 to 8 ppb. Figure confirms that the probe possessed maximum selectivity for ammonia gas detection. For the other gases, insignificant shifts in dip wavelength are observed due to the adsorption of the gases over the nanocomposite matrix. However, CH4 and H2S reacts a little with SnO2 film, causes a small shift in dip wavelength compared to other gases. The reason for high selectivity of the proposed probe towards NH3 detection is based on the its sensing mechanism as, when NH3 gas interacts with the PANI, resulting the NH ions via its protonation. This in turns the increased conductivity of PANI@SnO2 nanocomposite film (Figure 3) which transmutes the phase shifts in


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Figure 8: The performance of the SPS/PANI@SnO2 nanocomposite (10 wt%) probe in terms of (a) Selectivity, (b) stability, (c) reproducibility and (d) response and recovery time. interference pattern. Hence, it can be concluded that the protonation of ammonia plays a very crucial role in the sensing and the ability of protonation of a molecule is evaluated by its proton affinity (PA) value.23,30,31 In our case, the PA value of ammonia is maximum (853.6 kJ/mol) as compared to that of other used gases. The PA values of N2, O2, CH4, H2 and H2S are 393 kJ/mol, 421 kJ/mol, 543.5 kJ/mol, 422 kJ/mol and 705 kJ/mol respectively.32,33 This is one of the reasons behind PANI@SnO2 film being highly sensitive and selective towards ammonia detection compared to various other gases. Further, one can enhance the performance of the probe in the presence of water vapor (or humid environment) as reported27. To observe the stability and shelf life of the probe, the finalized probe was experimented several numbers of times over number of days, by exposing the ammonia gas concentration from 0 to 8 ppb. Figure 8(b) represents the variation in dip wavelength of the interference pattern of the probe, observed over number of days. A variation in dip wavelengths corresponding to each concentration of ammonia gas were observed, which is within the error limit of the probe calibration curve as shown in figure 5(c). Thus, the results showed that probe is highly stable and have significantly high shelf life. Further, the response and recovery times of the sensing probe was obtained from the transient response of the sensing probe. For this, the experiment was conducted in the following manner. Initially, the probe was fixed in the gas chamber as shown in the experimental setup (as shown in Figure 12) except that output spectrum was recorded with the integrated monitoring (I-MON 512 USB) system (instead of OSA) which track the dip wavelength as a function of time. Further, the vacuum was created in the gas chamber using rotary pump. The light from SLED (Thorlabs: S5FC10005S) source was launched in the proposed sensing probe and corresponding


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interference pattern was recorded. In the interference pattern, a specific dip wavelength (1572.2. nm) was selected, and its dynamic behavior was recorded. Now, the ammonia gas with 1 ppb concentration, was purged in the sensing chamber which interacts with the sensing medium and shifts the interference dip to the higher wavelength. The dynamic behavior of the dip wavelength was recorded until this saturates at λ=1573.5 nm. At this point the chamber was evacuated with the rotary pump to achieve vacuum that causes the decreased in dip wavelength so that the probe attains its initial value (1572.2 nm). This completed the first cycle of the experiment. During the cycle, the wavelength was traced and plotted as a function of time. Further, these cycles were repeated with varying ammonia concentrations of 2, 3, 6 and 8 ppb in the gas chamber and their dynamic behavior with wavelength change was plotted as figure 8(c) showing the transient response of the sensing probe. Further, to find the response and recovery times of the sensing probe, first cycle of above experiment was zoomed as figure 8(d). The response time and recovery time are defined as the time required to achieve the 90% of maximum response (when gas is inserted) and 10% of maximum response (when gas is evacuated) respectively.34 From figure 8(d), these values lie within the 7 secs and 2 secs during the insertion and evacuation of the ammonia gas respectively. This confirms that response and recovery times of the proposed sensing probe were 7 secs and 2 secs respectively which shows the possibility to fabricate

an ammonia sensing device with fast response time and good reusability. To validate the novelty and advantages of the proposed sensing method, we have calculated the detection limits of our detection method and compared with previous studies reported for ammonia sensing. The detection limit of our proposed method is calculated by taking the ratio of resolution of OSA and the sensitivity of the probe near 0 ppb concentration of ammonia gas35 and the observed value has been found as 8.09 ppt or 47.59 fM of ammonia concentration. A comparative table of the detection limits and operating range of several ammonia sensors reported in literature is shown


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as table 1. From table, it can be found that our proposed method possessed an excellent detection limit as compared to other studies. The studies which possessed the detection limits lower than our methods are based on electrochemical detection and limited by the response and recovery times.6,7 Table 1: A comparison of the various ammonia gas sensing methods reported in literature. Method

Sensing Layer

Op. Range

LOD

Electrical resistance36

Pt activated SnO2 nanoparticles

0-1000 ppm

50 ppm

Impedometric37

MnFe2O4 Nanocomposite 0-1000 ppm

10 ppm


CeO2@PANI Nanocomposite

0-400 ppm

6.5 ppm


PANI@Fe3O4 nanocomposite

0-100 ppm

5 ppm

Optical absorption40

Dye doped Polypyrrole

15-260 ppm

5 ppm


SnO2 Nanowires

50-200 ppm

-


SnO2 Nanowires

25-200 ppm

-


PANI@SnO2 Nanocomposite

100-500 ppm

-


CNT modified PANI

30-60 ppm

4 ppm

Interferrometric5

Graphene-microfiber

0 -360 ppm

0.3 ppm

Interferrometric10

PAA-PAH modified CNT

0-960 ppm

-

Fluoroscence42

Fluorographene

1 pM- 0.1 µM

0.44 pM (7.48 ppb)

Plasmonics17

SnO2 thin films

0-100 ppm

0.154 ppm

Interferometric

PANI

0-8 ppb

32 ppt

(Present study) Interferometric (Present study)

(196 fM) PANI@SnO2 Nanocomposite

0-8 ppb

8.09 ppt (47 fM)


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Amperometric6

Chemiresistive7

Modified bilayer graphene (MBLG) integrated with human olfactory receptor 2AG1

0-4 nM

0.04 fM

(0-68 ppb)

(0.68 ppt)

Au Nanoparticlepolysaccharide nanocomposite

0.1 ppq-75000 0.1 ppq ppm

Further for the sensors applicability we would like to mention that there are several fields where the ppb level monitoring of ammonia gas is required. It may also be noted that the operating range of our sensor basically depends upon the number of interaction sites available for NH3 in the PANI@SnO2 nanocomposite film. This can be increased by the enhancing the interaction volume of sensing region by increasing the length of PCF. The increased PCF length will facilitates in loading more amount of PANI@SnO2 nanocomposite over the PCF region so that ammonia with higher concentration can interact thereby increasing the operating range of the probe for applications such as in agricultural industry, where the various plant species like likens are sensitive to a few ppb concentrations of ammonia gas.43 Further, in semiconductor industry where extremely low concentration of ammonia in the clean room may drastically deteriorate the lithographic film performance.44 Additionally, ppb level detection of ammonia is required in DeNOx process and human breath analysis etc.45 In summary, we have successfully demonstrated a highly selective, stable, reproducible and reusable contemporary approach for the detection of the ultra-trace ammonia by interferometric technique. A thin film of PANI@SnO2 nanocomposite was coated over the PCF region of the SPS probe which acts as the sensing layer towards the ammonia gas detection. The modal analysis and simulations have been executed to analyze the fundamental of interference phenomenon and its application to the sensing. Further, the experiments have been performed to obtain the probe characteristics and various performance


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parameters. The sensor possesses the ammonia operating range from 0 to 8 ppb with linear range of 0 to 6 ppb that can be tuned for higher concentration. The detection limit of the proposed method is calculated as 8.09 ppt (47.59 fM). The sensor has an ultrafast response and recovery time of 7 seconds and 2 seconds respectively. This unprecedented performance of the proposed method for ammonia detection with high sensitivity and selectivity makes this inline sensor advantageous for next generation online and remote monitoring of ammonia in harsh environmental condition. Methods Fabrication of Probe: The probe fabrication process consists of three steps. In the first step, the SPS structure is fabricated as a transducing support for the sensor fabrication, while the second step involves the synthesis of sensing medium which includes the fabrication of PANI and PANI@SnO2 nanocomposites. Further, in the final step, the immobilization of thin nanocomposite sensing film over the SPS structure has been performed. We shall discuss these briefly in the following sections. Fabrication of SPS structure: As discussed in the introduction section, SPS structure in an in-line splicing of SMF-PCF-SMF fiber to form the Mach-Zehnder interferometer. For this standard LMA-8 PCF and standard SMF-28 optical fiber has been taken. Further, both the ends of about 2 cm long PCF is spliced between two SMF fibers using a fiber fusion splicer. A schematic of the spliced SPS structure is shown in figure 2. The splicing points of SPS structure results in the formation of Mach-Zehnder interferometer. In this case, the first collapsed region acts as the input coupler, that couples the light from SMF to PCF while the second collapse works as the output coupling regime which is


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used to couple the light from PCF to the core of SMF, which in turns the form of Mach-Zehnder interferometer. Synthesis of recognition mediums: In this step, PANI and PANI@SnO2 nanocomposites were prepared in following manners: Synthesis of PANI: The oxidative polymerization process was adopted for the PANI synthesis.46 For this, 1 ml aniline monomer was dissolved in 100 ml HCl and named as solution 1. Further, another separated solution (solution 2) was prepared by mixing 2.2820 gm APS in 100 ml HCl. APS was used as the oxidative agent during polymerization. Now, solution 1 was kept at 5 ºC temperature using an ice bath and solution 2 was added dropwise in solution 1. During reaction, the colour of solution 1 was turned dark green from milky white, which confirmed the PANI synthesis. Further, PANI was extracted by centrifuging the resulted solution at 8000 rpm for 30 mins. The reaction mechanism for the synthesis of PANI has been shown in figure 9(a). Synthesis of in-situ polymerized PANI@SnO2 nanocomposite: PANI@SnO2 nanocomposite was synthesized with varying SnO2 weight percentages via inorganic/organic interface reaction. Initially, PANI@SnO2 nanocomposite having 10 wt% of SnO2 was prepared by mixing of 0.1 g tin chloride (SnCl4.5H2O) in 100 ml de-ionized water, maintaining the pH of the solution at 4 using 0.1 M HCl. Further, 20 ml H2O2 solution was then added to the solution, for oxidation of tin ions to tin oxide resulting the white coloured suspension solution. This is then followed by mixing 0.9 g aniline and 0.1 M Ammonium per sulfate (APS) in the resultant and keeping the product at 5 ºC within an ice bath for polymerization. The colour of final solution turned green from blue within few minutes. The


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precipitation of the solution was collected by centrifugation and washing repeatedly with 1 M HCl. Additionally, one more nanocomposite with 20 wt% of SnO2 with respect to PANI was also prepared to observe the effect of increased SnO2 for ammonia sensing. Figure 9(b) represents the reaction for the synthesis of PANI@SnO2 nanocomposite. The syntheses of these materials have been further confirmed via XRD response of synthesized materials.

Figure 9: Synthesis of (a) PANI and (b) PANI@SnO2 nanocomposite. Immobilization of sensing layers: For coating the sensing films over the SPS structure, chemical deposition method was opted. In this method, PCF region of the SPS fiber was treated with sulfochromic solution (the mixed solution of 1 mg of potassium dichromate, 0.1 ml de-ionized water and 10 ml of sulfuric acid) for 10 mins. The probe was then washed thrice by de-ionized water and dried at 115 ºC for 2 h. This results in the functionalized of PCF by hydroxyl bond (-OH). The functionalized PCF surface was then modified by dipping in the silane solution (a sonicated solution of 1 ml APTMS, 4 ml acetic acid and 10 ml ethanol), followed by drying at 100 ⁰C in nitrogen ambient for 1 hr. The silane group makes the bonding with corresponding synthesized sensing materials. Three different probes were prepared by dipping the modified regime of SPS structure in PANI,


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PANI@SnO2 nanocomposite (10 wt%) and PANI@SnO2 nanocomposite (20 wt%) for 3 h. All the three probes were then left overnight for drying. A FESEM image of uniformly coated PANI@SnO2 nanocomposite (10 wt%) film over the PCF surface has been shown in figure 10(a) whereas figures 10(b) and (c) represents the cross-sectional image of PCF coated with 700 nm film thickness approximately.

Figure 10: FESEM image of PANI@SnO2 (10 wt%) coated PCF probe: (a) Side view and (b) cross-sectional view. Characterizations XRD analysis The XRD measurements of PANI and PANI@SnO2 nanocomposites have been recorded using Burker AXSD 8 (Germany) with the Cu-Kα rays with wavelength of 0.15405 nm within the 2θ range of 15 to 80º, as shown in figure 11. A single broad diffraction peak, centered about 2θ value of 25.9º suggests the amorphous nature of PANI. This observed peak is little misalinged with that of standard spectrum of PANI which attributed the presence of the HCl within the matrix.47 The peak at 25.9º corresponds to the (200) of emeraldine of the matrix. However, the XRD spectrum corresponding to PANI@SnO2 nanocomposite shows the prominent peaks


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attributed to (110), (211), (310) and (301) at the corresponding 2θ values of 26.51º,52.02º, 64.59º and 65.80º respectively which depicts the tretragonal structure of SnO2 (as per JCPDS data card 41-1445). In addition, the observed peaks are little shifted from the standard SnO2 XRD spectrum, that may be because of the presence of PANI matrix. futher, the reduced intensity of SnO2 peaks indicates the relativelly larger size of nanocomposite with respect to that of pure SnO2, it suggests that presence of PANI influence the orientation of SnO2 grains significantly.

Figure 11: XRD response for PANI and PANI@SnO2 nanocomposite Experimental Setup: Figure 12 shows the schematic of experimental setup used for the characterization of the fabricated probes. The in-house customized setup consisted of a box shaped metallic chamber with the facilities of inserting the optical fiber probe along with gaseous facilities. The chamber was connected via a rotary pump to remove gas and maintain pressure inside the chamber, whenever required. An ammonia gas cylinder was also attached with the chamber via a précised Mass Flow controller (MFC). The fiber optic probe with SPS structure was fixed inside the chamber with one end was connected to SLED (Super Luminescent Diode) for light input and other end to OSA (Optical Spectrum Analyzer). The experiments were performed by inserting


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the ammonia gas with varying concentration from 0 to 8 ppb using MFC. The gas concentration was varied with the range and for each concentration, the output interference pattern was recorded. The ammonia concentration and flow rate of the gas was controlled by computer operated mass flow controller (Alicat-MCE) which controls the flow rate up as 0.0025 sccm (cm3/min) in the volumetric flow control mode. The volumetric flow rate is controlled by creating a pressure drop across a unique internal restriction, known as a Laminar Flow Element (LFE), and measuring differential pressure across it. Further, the relation between the gaseous concentration and flow rate is governed by . = /. 01⁄23. Where c, f, t and v represents the gaseous concentration, volumetric flow rate, loading time and volume of the gas chamber respectively. In our case, for 1 ppb concentration of ammonia gas the values of f, t and V were kept as 0.006 sccm (0.1 µl/sec), 20 mili-secs and 5 l respectively. The additional required parameters for ammonia gas such as absolute viscosity, density and compressibility were kept as 100.92 kg/m-sec, 0.70352 kg/m3 and 0.98945 at 1 atmospheric pressure and 25ºC temperature.

Figure 12: Schematic of the gas sensing setup.


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AUTHOR INFORMATION Corresponding Author *Electronic mail: [email protected]; [email protected] Author Contributions: RJ conceive the experimental idea. ASR, AMS and GS executed the experiments. AMS and GS and RJ discussed and analyzed the results. AMS, GS and RJ wrote and revised the manuscript. AMS and GS contributed equally for this work. Funding Sources This work is partially supported by MHRD funded project CENEMA, DST funded project EMR/2016/003446 and also ISRO funded project ISRO/RES/3/757/17-18. GS thanks SERB for NPDF-2017/0529. ACKNOWLEDGEMENT We are thankful to Dr. Shantanu Pal (School of Basic Sciences, IIT Bhubaneswar) for providing ammonia gas. Mr. Dhrubraj Dhora help with the schematic drawing of the gas sensing setup. ABBREVIATIONS MZI, Mach-Zehnder Interferometer; SMF, Single Mode Fiber; PCF, Photonic Crystal Fiber; SPS, SMF-PCF-SMF; PANI, Polyaniline.


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Table of content

Pictorial representation of the Single Mode Fiber (SMF)- Photonic Crystal Fiber (PCF)- Single Mode Fiber structure for obtaining highly stable interference pattern and usage of PANI@SnO2 nanocomposite for ultra-trace online ammonia sensing.


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Hypersensitive and selective interferometric nose for ultra-trace

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