In this research work we demonstrated negative axicon optical fiber tip filled with Polydimethylsiloxane (PDMS) as a sensor platform for volatile organic gases detection at room temperature. The response of the sensor was measured with various Volatile Organic Compounds (VOCs) such as Chloroform, Hexane, Isopropanol, Acetone, Toluene and Methanol in the concentration ranging from 5 to 200 ppm. The corresponding sensitivity and limit of detection (LOD) of the developed sensor for the measured VOCs were observed between the order of around 23.7 to 3.2 pm/ppm and 0.84 to 6.10 ppm, respectively. The response and recovery time of sensor were found between the order of 30 to 57 seconds and 8 to 25 seconds respectively for the measured VOCs. Thermal stability of the developed sensor was also studied at 30-70 °C with intervals of 10°C. The principle of sensing is based on change in the length of the Fabry-Perot Interferometric (FPI) cavity in the presence of varied concentrations of VOCs, which results in changes in the shift in wavelength of an interference pattern attributed to the change in PDMS filling the cavity length (swelling). The experimentally observed trends in the relative swelling of PDMS with VOCs are found in agreement with the theoretically calculated values obtained from the Hansen solubility parameter (HSP). The developed gas sensor has the potential to fulfill the demands of industrial applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Volatile organic compounds (VOCs) are a wide range of carbon based chemicals/compounds found in naturally objects and from various industries like oil and gas industry, solvent usage and transportation, food industry, paint industry, medical, cleaning supplies, furnishings, glues, permanent markers and printing equipment [1,2]. The VOCs have a high vapor pressure under normal atmospheric pressures and temperatures, which result they have a low boiling point and readily evaporate into the atmosphere, creating potentially toxic conditions. Some VOCs are harmful to animal or environmental health. Now a days VOCs detections are also exploited for the medical diagnostic and detection technologies [3,4]. Therefore, due to the volatile nature of these VOCs, their fast detection has become more and more essential for the environmental and our own safety .
Currently, electrochemical and optical based sensors are used for the VOC gases detection. The electrochemical (semiconducting oxide materials) gas sensors shows excellent performance in terms of sensitivity , however these sensors usually operated at high temperature (200 - 600°C) [7,8], also needs long measuring period, weak stability, and high manufacturing cost restrict its on-site gas monitoring and its real-time analysis applications .
Optical fiber based sensors could be the best option due to its major advantages, such as electrical neutrality, fast responses, light weight, high sensitivity and miniaturized setups [9–11]. In fiber optic sensor either certain portion of the optical fiber cladding is replaced with a chemically sensitive material composite [12,13] or the distal end of the optical fiber is coated with certain materials which interacts with gas [14–16]. Recent study reported tapered fiber-optic interferometer based gas sensors, which are realized by measuring the refractive index (RI) of the gaseous samples [17,18].
The fiber tip based gas sensors works on the FPI cavity [14,19]. The Polydimethylsiloxane (PDMS) polymer may be very attractive material for cavity coating due to their swelling properties in presence of gases moieties and ability to work at room temperature. This cavity physically interacts with the VOCs and by oxidation/reduction reaction the length of the cavity and refractive index of PDMS gets changed . With time and due to the swelling effect the coated PDMS or gas sensing material on the fiber tip could become unsterilized and may change its properties. However, negative axicon in general possesses large cavity volume which may benefit in making sensor working based on expansion or contraction of material within the cavity. Besides, the cavity in negative axicon fiber tip also possess better quality of optics which can also be exploited for various optical sensing applications. The optical fiber based negative axicon lens appears like a hollow cone which is fabricated at the distal end of special type of single mode optical fiber [21,22], which gives a possibility of filling a sensing reagent inside the cavity for chemical sensing application. This filling could be stable and protected in the conical shape. The depth of negative axicon inside the tip depends on the refractive index profile (RIP) of the fiber for a particular concentration of hydrofluoric acid (HF) and etching time.
Therefore, due to the advantages of the negative axicon, the aim of this study was to explore new sensitive, stable negative axicon fiber optic sensor platform for volatile organic gases detection at room temperature. The effort was to improve present fiber optic gas detection techniques.
2. Materials and methods
2.1 Fabrication of sensor platform (negative axicon tip-based FPI cavity)
The refractive-reflective negative axicon cavity is fabricated inside a typical Cladding Mode Offset Photosensitive Single-Mode Fiber (CMOP-SMF), (highly Ge doped optical fiber with core diameter ∼3.5 μm and cladding diameter 125 µm, Nufern, USA) through chemical etching process as the optical fiber tip is superficially dipped in HF [21,22]. The high concentration of Ge accelerates etching rate of the core material in HF solution (48%) compared to silica cladding. The length of the negative axicon cavity is dependent on the etching time. Figure 1(a) shows schematic of the etching procedure, (b) fabricated axicon empty cavity, (c) schematic of filling of PDMS inside the fabricated cavity, and (d) cross-section image of developed PDMS filled negative axicon tip based sensor platform used for the experiments. The inner part of the axicon has length of ∼310 μm, inner cone angle ∼72°, opening diameter ∼71.7 μm, and depth filled with PDMS ~233 μm are measured from the microscopic image using Fiji software.
2.2 Experimental procedure
Figure 2 shows the negative axicon tip based fiber optic FPI cavity filled with PDMS gas detection experimental setup. It consists of a optical circulator (SMF-28, 60:40, Optolink Corp. Ltd., Hong Kong), broadband source (C-band, ASE light source, 1525 nm −1565 nm and maximum output power 28 mW, Optolink Corp. Ltd., Hong Kong) and OSA (model number AQ6370C, spectral range: 600 nm - 1700 nm, spectral resolution: ~ ± 0.02 nm, Yokogawa, Japan). The light source was connected to one end of the circulator through which 60% light will travel to the free end of the circulator tip where developed negative axicon tip fiber was spliced. The 40% light of the coupled light will travel to OSA through other end of the circulator. The reflected signal also coupled with OSA through the same circulator to form an interference pattern. The developed negative axicon tip filled with PDMS was placed at the middle of the three neck funnel (100 ml capacity, Borosil, India). Out of rest two necks, one neck was used for the gas pouring while rest was used for the degassing of the poured gas. Measurement were performed on Methanol, Isopropanol, Hexane, Chloroform and Acetone and Toluene gases for 0 to 200 ppm concentrations under different interval of time.
The signals were recorded for 30, 60, 90 and 120 seconds, respectively. In order to check the reproducibility of the experiment the measurements were repeated for three times for each concentration of gases. Furthermore, for each measurement, the sensor response and recovery were checked and recorded. During degassing of the gases, precaution of removal of measured gas was taken by pouring the air inside the funnel and by measuring the spectra on the OSA. The experiments were performed at room temperature (27°C) and at constant humidity environment (65%). In order to check the thermal stability of the developed sensor experiment were repeated at temperature ranging between 30 to 70°C with interval of 10°C.
2.3 Gas sensing theory
The PDMS filled axicon tip is as shown in Fig. 3. The FPI cavity filled with PDMS is formed between two reflection surfaces of R1 and R2. The majority of light propagates through the core of the fiber reflects back at R1 and remaining light propagates through the PDMS and reflects from R2. The interference is formed due to the change in the path difference created due to the cavity which is recorded on to the OSA.
The total intensity (I()) from FPI cavity is given by the equation [19,23]:23,24]:23,24]:Eq. (4), the change in wavelength shift is directly proportional to the change in the length of the cavity and the corresponding change in the cavity length is also a correlated to the Hansen solubility parameters (HSP): dispersion interactions (), dipolar interactions ( ), hydrogen bonding interactions () and the vapor pressures () of the VOCs [25,26]. Therefore, the total change in length of the FPI cavity (due to swelling) can be expressed as follows :
Additionally, due to adsorption of gas molecules, the refractive index of the coated film gets changed, which further results in change in the Free spectral range (FSR) of the interference spectrum and is given by :19,28]:13]. The equation is given by:
2.4 Light propagation in cavity
Figure 4 shows the electric field distribution inside the axicon tip without and with PDMS filled cavity (of experimentally measured specifications) is simulated by using COMSOL Multiphysics 5.2. As shown in Fig. 4, there are two reflection surfaces R1 and R2 formed due to fiber to PDMS and PDMS to air, respectively. The reflection coefficient at the tip surface is given by Fresnels reflection coefficients. For air cavity the strong reflection is observed at surface R1 due to Fresnel’s reflection can be calculated from Eq. (2). The corresponding Fresnel’s reflections at R1 and R2 are obtained as 3.37% and 0. Thus, even the external refractive index will change it will not modify the reflection coefficient at R2 and hence the total reflection of an FPI cavity. In case of PDMS filled cavity (Fig. 4(b)), the Fresnel’s reflection coefficients are calculated 0.044% and 2.66%. Therefore, at R2 reflection coefficient in between PDMS and air is very strong and sensitive in nature to external ambient. Even small change in the external ambient refractive index at R2 which changes the light reflections from the R2 and modifies the total reflection coefficient of the cavity. Since, the measured reflection from the PDMS to air interface is more than 2.5% times larger than the reflection measured from air cavity, therefore any change in the PDMS to air cavity interface (cavity length) will drive the reflection and it will be proportional to the input changes accordingly.
3. Results and discussions
3.1 Light Interaction in negative axicon cavity filled with PDMS
Figure 5 shows the sensing output of PDMS coated FPI cavity sensor in terms of the power (dBm) vs wavelength (nm) measured for the methanol at concentrations of 0 ppm, 5 ppm, 10 ppm, 25 ppm, 50 ppm, 100 ppm and 200 ppm at 60 seconds. The 0 ppm (recorded spectrum in absence of gas) shows the spectrum with PDMS filled cavity only. The reflected interference spectrum of methanol of sensor shows shifts towards the longer wavelength of 0.11 nm, 0.13 nm, 0.21 nm, 0.25 nm, 0.32 nm and 0.75 nm with corresponding methanol ranging concentrations 5 ppm, 10 ppm, 25 ppm, 50 ppm, 100 ppm and 200 ppm respectively for 60 seconds (Fig. 5). Similar results of reflected interference spectrum shift towards the longer wavelength were observed for the Isopropanol, Acetone, Hexane, Toluene and Chloroform gases for the same concentrations are shown in Table 1. The observed shift in the reflectance spectra are due to the change in the FPI cavity length from the swelling effect resulted from the physical interaction between the measured VOCs and PDMS filled in the cavity [14,24]. Experimentally FSR of the interference spectrum found of the range of 3.85 nm.
Similarly, the measured changes in the intensity modulations also shows longer increase in extension ratio are measured for the Methanol, Isopropanol, Acetone, Hexane, Toluene and Chloroform gases concentrations of 5, 10, 25, 50, 100 and 200 ppm respectively at 60 seconds due to the change in the refractive index of the FP cavity resulted from the swelling effect (shown in Table 1). Relatively highest wavelength shift was reported from chloroform with increase in concentration from 5, 10, 25, 50, 100 and 200 ppm of 0.44, 0.57, 1.04, 2.83, 3.20 and 4.89 nm respectively at 60 seconds. Similar trend was observed from the extinction ratio for all concentrations.
In the concentration range of 0 - 200 ppm of Methanol, Isopropanol, Hexane, Chloroform, Acetone and Toluene gases, a linear dependence has been observed in wavelength shift with the linear coefficient of determination R2 = 0.94%, R2 = 0.92%, R2 = 0.90%, R2 = 0.89%, R2 = 0.94% and R2 = 0.85% (Fig. 6(a)). Nearly similar linear dependence has been observed in change in cavity length with respect to VOCs concentrations of linear coefficient of determination R2 = 0.94%, R2 = 0.92%, R2 = 0.89%, R2 = 0.89%, R2 = 0.93% and R2 = 0.85%, respectively (Fig. 6(b)) for 60 seconds.
Time response curve of Negative axicon tip based fiber optic interferometer cavity sensor probe with 50 ppm concentrations of measured gases shown in Fig. 7. Time response curve shows that with increase in concentrations of measured VOCs shows increase in the corresponding wavelength shift.
The sensitivity of developed fiber optic sensor based on the wavelength shift and ER change were found of range 3.2, 8.8, 17.8, 9.1, 21.6 and 23.7 pm/ppm and 0.0004, 0.004, 0.016, 0.014, 0.019 and 0.024 dB/ppm for Methanol, Isopropanol, Hexane, Acetone, Toluene and Chloroform gases respectively (shown in Table 2). In wavelength shift measurement highest sensitivity of 23.7 pm/ppm was reported for the chloroform gas whereas lower sensitivity of 3.2 pm/ppm was reported for the methanol gas. The results from the repeated experiment show good reproducibility for the wavelength shifts and ER change measurement of ± 0.00033, ± 0.001, ± 0.002, ± 0.001, ± 0.004, ± 0.003 and ± 0.001, ± 0.007, ± 0.003, ± 0.004, ± 0.004 and ± 0.007 for the Methanol, Isopropanol, Hexane, Acetone, Toluene and Chloroform gases respectively. The sensitivity study confirms that the developed negative axicon tip based fiber optic interferometer cavity sensor is more sensitive for the wavelength shift measurement than the ER change based measurement. The detection limit of the developed sensor for Methanol, Isopropanol, Hexane, Acetone, Toluene and Chloroform gases detection was recorded of for 6.10, 2.26, 1.11, 2.19, 0.92 and 0.84 ppm respectively. Relatively high LOD of 0.84 ppm was recorded for the chloroform whereas low LOD of 6.10 ppm was recorded for the Methanol. From the wavelength shift, sensitivity and LOD study it was observed that the developed negative axicon tip based fiber optic interferometer cavity sensor shown better cross sensitivity with chloroform followed by toluene, hexane, acetone, isopropanol and methanol respectively.
The response and recovery time of the developed sensor for the Methanol, Isopropanol, Hexane, Acetone, Toluene and Chloroform gases are found 50, 55, 45, 30, 57, 35 seconds and 15, 20, 12, 8, 25 and 10 seconds respectively (shown in Table 2). Relatively high response time of 30 seconds was recorded for the Acetone gas, whereas low response time of 57 seconds was recorded for the Toluene gas. Similarly, high recovery time of 8 seconds was recorded for the acetone whereas low recovery time of 25 seconds was recorded for the Toluene gas.
Also, the vapor pressure of the VOCs is important parameter to analyze the sensor response and recovery time. The vapor pressure for Methanol, Isopropanol, Hexane, Acetone, Toluene and Chloroform are of 16.78 KPa, 5.76 KPa, 20.03 KPa, 30.39 KPa, 3.80 KPa and 26.05 KPa respectively at 25°C reported elsewhere [29–31]. Due to the lower vapor pressure of toluene, the response time of the developed sensor towards the toluene found low whereas the recovery time found to be high.
Figure 8 shows the cross-sensitivity study of PDMS coated FPI in terms of wavelength shift (Fig. 8(a)) and ER Change (Fig. 8(b)) with various VOCs at 50 ppm for 60 seconds. The sensor shows the highest sensitivity with chloroform followed by toluene, hexane, acetone, isopropanol, and least with the methanol. As per theory, the chloroform has highest swelling ratio with PDMS and least with methanol . The corresponding swelling ratios of these VOCs such as chloroform, toluene, hexane, acetone, isopropanol, and methanol with PDMS are given by: 1.39, 1.35, 1.31, 1.06, 1.09 and 1.02 respectively .
In order to analyze this swelling mechanism, the HSP including solubility parameters ( ), swelling ratio , dipole moments and the vapor pressure of VOCs are very important parameter in such interaction. The corresponding parameters of these VOCs are reported elsewhere . The sensing mechanism of this sensor is based on swelling of PDMS polymer due to the interaction of VOCs molecules with PDMS polymer. The PDMS contains cross-linked structure of the repeating units of -OSi(CH3)2- groups. Depending upon the surface chemistry of PDMS polymer, the VOCs may interact with the -OSi(CH3)2- group of PDMS, which will induce the swelling in the PDMS polymer [8,14,19,32] (shown in Fig. 9). From Eq. (5), it is confirmed that the swelling of PDMS is dependent upon the HSP and the vapor pressure of the VOCs. In general, theory suggests that when the HSP of VOCs are similar to that of PDMS will result in to the greatest swelling of PDMS upon the exposure with VOCs. The swelling is based on the HSP, where the relative energy difference (RED) used for the possible interaction of VOCs with PDMS. The RED can be defined as .Table 3, the RED values of each VOCs and PDMS are shown with their HSPs  calculated from the Eq. (9).
In Table 3 high RED value of 3.74 were observed for Methanol followed by 2.43, 1.31, 1.12, 1.06 and 0.93 for Isopropanol, Acetone, Toluene, Chloroform and Hexane respectively. If RED value less than one, then it shows that a high miscibility of a VOC in a polymer results into the more swelling of the polymer and it corresponds to the increase in sensitivity. Thus, swelling is directly related to the change in the cavity length of the sensor. We measured highest sensitivity for chloroform (RED = 1.06) followed by toluene, hexane, acetone, isopropanol and methanol respectively. In our experimentally reported values, we have observed similar trend as per the theory suggested and observed values are found nearly in agreement with the HSP values reported elsewhere [26,34] except Hexane as shown in Table 3. The swelling results in change in cavity length of the FPI cavity (ΔL), and which changes the optical path length of the cavity and induces the wavelength shift in the interference pattern.
The thermal stability of the developed sensor in terms of the wavelength shift and ER change for measured gas are shown in Fig. 10 for temperature change from 30 to 70 °C with interval of 10 °C. Overall, the wavelength shifts and ER change with temperature shows quite linear response with R2 = 0.94% each. The repeated experiment shows good reproducibility for the wavelength shifts of ± 0, ± 0.009, ± 0.017, ± 0.007, ± 0.009 measured for 30 °C, 40 °C, 50 °C, 60 °C and 70 °C respectively.
3.2 Concentration and temperature detections algorithm
As discussed wavelength shift (Fig. 10(a)) and ER (Fig. 10(b)) change also depends on the concentration of the gases. Therefore, concentration and temperature affects correspondingly change in the ER and wavelength shift of interference signal. Thus, to evaluate the concentration and temperature separately a second order of matrix algorithm can be employed as follows [14,35]:Eq. (11), when ER change and wavelength shift were detected then concentration of gas and temperature can be determined by matrix algorithm as follows:
Therefore, Eq. (12) can be expressed as, follows,Equation (13) can be the performance equation of the developed sensor for the chloroform gas under variable temperature and concentration.
In summary, in this research work the refractive-reflective negative axicon cavity of 310 µm is fabricated inside a typical CMOP-SMF through chemical etching process. The fabricated cavity is filled with PDMS of 233 µm length and is demonstrated for the Methanol, Isopropanol, Hexane, Acetone, Toluene and Chloroform VOCs sensing based on principle of fiber optic FPI. The simulation results confirm that the reflection coefficient measured from the cavity filled with PDMS (between the and air) is very strong and sensitive as compare to the reflection coefficient measured from the empty cavity. Furthermore, the experimentally calculated FSR of the interference spectrum of 3.85 nm found nearly matching with theoretically calculated FSR of 3.71 nm. During interaction of different VOCs of varied concentrations with PDMS in the cavity resulted in the respective change in the FPI cavity length observed due to swelling of the PDMS. During these experiments Chloroform shown relatively highest wavelength shift which indicate highest swelling length of PDMS due to the interaction with the chloroform VOC molecules followed by Toluene, Hexane, Acetone, Isopropanol and Methanol. The change in cavity length of the FPI cavity (ΔL) changes the wavelength shift in the interference pattern as well as changes in the optical intensity modulation of the transmitted optical signal resulted from the change in refractive index of the PDMS polymer. According to the HSP Hexane, Toluene and Chloroform shows nearly RED value close to 1, which implies that these VOCs are relatively sensitive. The measured trend of the relative swelling of the PDMS cavity due to the VOCs interaction are nearly matches with the obtained from the HSP .
Highest limit of detection (LOD) for the developed sensor of 0.84 ppm was found for the Chloroform followed by 0.92 ppm, 1.11 ppm, 2.19 ppm, 2.26 ppm and 6.10 ppm for Toluene, Hexane, Acetone, Isopropanol and Methanol respectively. The sensitivity of the developed sensor measured from the changes in the wavelength shift shows much better results than the sensitivity measured from the changes in intensity modulation. Therefore, for the real time and high sensitivity gas detection application, the wavelength shift based analysis can be considered.
Although the developed negative axicon tip based FPI cavity sensor filled with PDMS has provided interesting preliminary results of Methanol, Isopropanol, Hexane, Chloroform and Acetone and Toluene detection, however some limitations of the presented study need to be noted. The measured sensitivity is limited by thickness of PDMS filling inside the negative axicon tip. Similarly, the limit of detection for various gas is also limited by the resolution of the used OSA (spectral resolution ~ ± 0.02 nm). Therefore, the sensitivity of the sensor can be increases by decreasing the film thickness whereas detection limit of the sensor can be increased by using high spectral resolution OSA or spectrometer. Furthermore, these measurements were performed on the VOCs in a closed environment individually. Therefore, performance of the developed sensor in terms of sensitivity and selectivity need to the evaluated for their mixtures. The developed sensor shows linear thermal stability of wavelength shift and with increase in temperature. However, for real time gas sensing applications at different temperature the sensor needs to calibrated accordingly.
We propose and demonstrate a negative axicon tip based fiber optic interferometer cavity sensor filled with PDMS for volatile Gas Sensing for the first time to our knowledge. The Methanol, Isopropanol, Hexane, Chloroform, Acetone and Toluene gases concentration ranging from 5 ppm to 200 ppm were effectively measured with novel FPI approach. Reflected interference spectrum shift towards the longer wavelength were observed for the measured VOCs for the concentrations ranging from 5 ppm, 10 ppm, 25 ppm, 50 ppm, 100 ppm and 200 ppm respectively. From the wavelength shift, sensitivity and LOD study it was observed that the developed novel sensor shown high sensitivity and good selectivity with chloroform followed by toluene, hexane, acetone, isopropanol and methanol respectively. The measured VOCs gas sensing results were shown as the specific of optical wavelength shift. By using this proposed method, we can monitor and quantify various VOCs in real-time. The developed sensor exhibits many desirable characteristics, including sensing range (VOCs concentration), high sensitivity, high recovery time, good linearity, a linear response over a dynamic range, immunity to electromagnetic interference high resolution, easy operation, linear thermal sensitivity, low fabrication cost robust, capacity for online monitoring, remote sensing, fast and efficient approach for various VOCs sensing at room temperature. This work provides important guidance for the design of high-response gas sensors and breath analyzers.
Council of Scientific & Industrial Research (CSIR), Mission Mode Program on Food and Consumer Safety Solutions (FOCUS), Multi Analyte Detection Methods (HCP0016, WP5.4).
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