A novel modified fiber Bragg grating (FBG) based ammonia sensor coated with polyaniline/graphite nanofibers nanocomposites

Highlights

• Deposition of PANI/GNF nanocomposite on the etched-tapered FBG sensors enhances sensor sensitivity towards ammonia.

• FBG sensors coated with PANI/GNF nanocomposite exhibited good sensitivity and selectivity towards ammonia at room temperature in the C-band wavelength band.

• Etched-tapered FBGs produced highly sensitive ammonia sensors with good selectivity.

Abstract

The sensing of ammonia leakage at room temperature has potential applications in many areas such as industry. A sensitive etched-tapered fiber Bragg grating (FBG) based ammonia sensor is developed and investigated towards different concentrations of ammonia. The investigations are performed at room temperature. FBGs are etched with 50 µm diameter and tapered with 15 and 30 µm waist diameters. These FBGs are coated with polyaniline/graphite nanofibers (PANI/GNF) nanostructures as a sensing layer using spray method. The developed FBG sensors are based on Bragg wavelength shift on exposure to different ammonia concentrations due to change in refractive index of the sensing layers as interact with ammonia molecules. FBG sensors proved a linear Bragg wavelength shift with ammonia concentrations. The developed sensors exhibited linear, stable response, high sensitivity and good repeatability. Furthermore, FBGs sensors proved strong response towards NH₃ by exhibiting high Bragg wavelength shift. This is because of porous structures of PANI\GNF nanostructures which improves the capability to adsorb additional NH₃ molecules. This is in turn eventually enhances the interaction between NH₃ and PANI\GNF nanocomposites. PANI\GNF coated FBG sensor with 15 µm waist diameter shows the highest sensitivity and good selectivity towards NH₃.

Introduction

Ammonia (NH₃) is widely used for different applications such as industry, agricultures and medicines [1], [2], [3]. Ammonia has also dangerously impacted human health even at low concentrations [1], [2], [4]. Ammonia can be characterized by its colorless, pungent smell, and explosive, toxic at a high-concentration ammonia atmosphere [3], [5], [6], [7], [8]. Hence, the development of highly-sensitive, fast and reliable sensors to continuously detect leakages of ammonia is a key issue for the safety of environments for both indoor and outdoor applications. [4], [9]. Most of developed ammonia sensors are performed electrically [2], [5], [10], [11], [12], [13]. Even though it is simple and low cost; the electrical based sensor has poor selectivity by responding to other gases and susceptible towards electrical noise such as electromagnetic interference (EMI). The optical fiber sensor is an excellent candidate to avoid these drawbacks introduced by the conventional sensors as it has immunity towards electromagnetic interference [14]. Several schemes are used for optical fiber ammonia sensors including evanescent sensor [15], [16], [17], surface plasmon resonance sensor [18] and fiber Bragg grating sensor [19], [20], [21]. The cladding modified optical fiber platforms are the most used for optical fiber sensors. This is achieved using techniques such as removing the cladding using methods such as chemical etching [22], [23], [24], tapering [25], [26], [27] or combination of etching and tapering [16], [17]. FBG has the unique property to reflect a specific wavelength of light which is not own by normal optical fibers. FBG ammonia sensor is a most suitable candidate for multiple points or distributed sensors because it based on Bragg wavelength shift. Besides, FBG sensor can have impact in intensity fluctuations but could not be significant if you measure by wavelength results in a measurement with higher accuracy [28]. Most of recommended FBG based sensors use etched FBGs [21], [29], [30]. Combining FBG sensors with the nanostructured thin films such as polyaniline/graphite nanofiber (PANI/GNF) nanocomposites, leads to great enhancement in sensitivity and selectivity towards NH₃.

Polyaniline (PANI) as a conductive polymer (CP) has been attained great attention by researchers as a sensing layer in gas sensing applications. PANI is capable to perform at room temperature as a sensing layer and exhibits high sensitivity with fast response [9], [16], [31], [32], [33], [34], [35]. The electrical and optical characteristics of PANI sensing layer shift when exposed to NH₃ accompanied with change in state of oxidation and protonation of the polymer. Exposing the acid or doped form emeraldine salt (ES) PANI to NH₃ will be deprotonated or dedoped and becomes non-conducting emeraldine base (EB) PANI [8], [16]. This is observed to be a reversible switching. PANI coated optical fiber sensors are based on this unique switching action. PANI/GNF nanocomposites have generated increasing interest in the sensor field, because of the unique electrical, mechanical, optical, chemical, and structural properties of CP and graphite [10]. The dispersion of graphite in the polymer matrix offers proficient use for polymer composite. Graphite-based polymer nanocomposite systems have been explored for superior properties to those of neat polymers, including electrical conductivity and mechanical and barrier properties. These improved properties are attributed to the confinement of graphite with the polymer matrix and the high surface area offered by the resulting nanocomposite [12]. The combination of PANI and graphite to form a nanocomposite is expected to produce a highly sensitive gas sensing layer.

By integrating PANI with graphite nanofiber, its propertied, such as conductivity and optical properties, can be enhanced [17]. PANI/GNF nanocomposites is introduced recently by authors for ammonia sensing applications deposited on a single mode optical fiber [17]. PANI/GNF nanocomposites produces a highly sensitive ammonia sensing layer. Graphite exists naturally and thus it is a good candidate material to shift polymer properties such as electrical conductivity, thermal and optical features. GNFs is a unique and new material that belong to the family of carbon nanostructures [17]. GNF has a unique configuration of only edges exposed platelets. Therefore, GNF is highly preferable for gas sorption. The performance of PANI/GNF nanocomposite based ammonia sensors can be greatly enhanced by integrating GNFs as inorganic nanofiller and PANI [17]. There are many articles reported electrical sensors based on nanocomposites of PANI and carbon-based structures such as graphene (G) and carbon nanotube (CNT). PANI/GNF nanocomposites is investigated for NH₃ sensing applications only with single mode optical fiber and is not explored for FBG based sensors.

Herein, a novel etched-tapered FBG sensor is developed. FBG is etched using chemical agent to remove some of the cladding layer and followed by tapering the etched area. PANI/GNF nanocomposites is prepared and sprayed on the modified area of FBG sensor as a sensing layer. By this, the cladding layer of FBG is replaced with PANI/GNF sensing layers to enhance the sensitivity of FBG towards change in its surroundings. FBG sensors are etched with 50 µm diameters after etching and later tapered with 15 and 30 µm waist diameters. The developed sensors are exposed to ammonia with concentrations in the range of 0.004–0.1% at room temperature. Bragg wavelength is modulated as FBGs are exposed to ammonia with different concentrations due to the change in refractive index (RI) of the sensing layer changes with the concentration change of ammonia. PANI/GNF nanocomposites coated FBGs have not been explored towards ammonia at room temperature.