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Polyaniline-coated tilted fiber Bragg gratings for pH sensing
Accepted Manuscript Title: Polyaniline-coated tilted fiber Bragg gratings for pH sensing ´ Gonz´alez-Vila, M. Debliquy, Authors: A. Lopez Aldaba, A. M. Lopez-Amo, C. Caucheteur, D. Lahem PII: DOI: Reference:
S0925-4005(17)31381-3 http://dx.doi.org/doi:10.1016/j.snb.2017.07.167 SNB 22826
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
17-4-2017 21-7-2017 24-7-2017
´ Please cite this article as: A.Lopez Aldaba, A.Gonz´ alez-Vila, M.Debliquy, M.Lopez-Amo, C.Caucheteur, D.Lahem, Polyaniline-coated tilted fiber Bragg gratings for pH sensing, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.07.167 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyaniline-coated tilted fiber Bragg gratings for pH sensing A. Lopez Aldaba*a,b, Á. González-Vilab, M. Debliquyc, M. Lopez-Amoa, C. Caucheteurb and D. Lahemd a
Universidad Pública de Navarra, Dept. of Electrical and Electronic Engineering and ISC, Campus Arrosadia S/N, E-31006, Pamplona, Spain b Electromagnetism and Telecommunication Department, University of Mons, Boulevard Dolez 31, 7000 Mons, Belgium c Department of Materials Science, University of Mons, Rue de l’Epargne 56, 7000 Mons, Belgium d Materia Nova, Materials R&D Centre, Avenue N. Copernic 1, 7000 Mons, Belgium
* [email protected]; phone +34 948 16 9841; fax +34 948 16 9720
Highlights
The proposed pH sensor combines PAni chemical deposition and tilted fiber Bragg gratings. In situ chemical oxidative polymerization track of the coating thickness allows to optimize the sensor response. Determination of the sensor performance in terms of sensitivity and hysteresis between 212 pH. Fast response time and stabilization times comparable to commercial available devices. Sensors present biochemical compatibility, temperature independence, long-term stability and remote real-time multipoint sensing features.
ABSTRACT In this paper, we report an optical fiber pH sensor based on a polyaniline coating deposited on the surface of a tilted fiber Bragg grating. The pH-sensitive film was synthesized by in situ chemical oxidative polymerization keeping track of the deposition time in order to optimize the sensor response. As a result, the sensor reacts to pH changes in the range of 212 with a fast response and its sensitivity is directly related to the film thickness. The main advantages of this PAni-TFBG pH sensor are its biochemical compatibility, temperature independence, long-term stability and remote real-time multipoint sensing features. Keywords: optical fiber sensor, fiber Bragg grating, tilted fiber Bragg grating, polyaniline, pH sensor, chemical sensor
1. Introduction Optical and fiber-optic pH sensors complement the glass electrodes for pH monitoring and offer numerous advantages such as immunity from electrical interference, miniaturization, robustness, possibility of remote and real time sensing and in vivo measurements. Conducting polymers possess both the mechanical properties of polymers and the electrical properties of conductors, having a big potential in many technological applications. Polyaniline (PAni) is a well-known conducting polymer, which is easy to synthesize, shows excellent stability, biochemical compatibility [1] and has rapid and reversible adsorption or desorption kinetics [1]. It has attracted much attention in numerous sensing applications such as gas detection [3-5], ascorbic acid detection [6], multi-electrode sensor arrays [7], coatings for quartz crystal microbalance sensors [8], chemiresistors using single walled carbon nanotubes (SWNT) [9] and as modified cladding in optical fiber sensors [10]. Among chemical sensors, pH sensors are one of the most studied applications. Fiber optic sensors associated to conducting polymers have been developed to estimate the pH within the biofilms grown on stainless steel [11], to monitor small cluster of cells [12] or to develop pH sensors based on mode-filtered light [13]. Many non-fiber based typical pH sensors have also been studied too, such as carbon-dot-based for very weak acids detection [14], poly-dopamine films voltammetric sensors [15] and sol-gel sensors [16], among others. Tilted fiber Bragg grating (TFBG) sensors have proven to exhibit really interesting features when applied to chemical sensing [17]. Several sensors have indeed been proposed for physical and chemical parameter monitoring such as humidity [18], temperature [19], strain [20], gas [21], refractive index [22-24] or molecular binding [25]. Compared to uniform Bragg gratings (FBGs), the TFBG tilt angle causes a coupling of part of the guided light to the cladding of the optical fiber in the form of several cladding mode resonances [27]. Each of these modes propagates with a corresponding refractive index value and is confined in the cladding by reflection on the cladding-outer medium interface. Thus, when the refractive index value of the surrounding medium reaches a value close to the one of a mode, the total internal reflection condition is lost so that light is transmitted to this medium, producing a change on the grating transmitted spectrum. The monitoring of these alterations allows us to detect and quantify the variations of the refractive index of the outer medium, turning these devices into highly accurate refractometers. In this work, a novel pH sensor consisting on a polyaniline deposition on the surface of a TFBG is presented and characterized. PAni presents indeed a pH sensitive optical absorption spectrum in the NIR region. This effect is due to the partial protonation leading to a doping and a conformational change of the polymeric chains. The working principle of the sensor is based on the monitoring of the transmission spectrum of the TFBG, which varies according to the refractive index changes of the sensitive layer with pH. The pH measurements are thus based on tracking the wavelength shift of the TFBG cladding modes [26], achieving an overall sensitivity of 46 pm/pH unit in the range 2-12 pH. Due to the main properties of this combination, PAni-TFBG pH sensors could have applications in various domains such as medical smart textiles [28], bioelectrical and in vivo measurements [29-32], pipeline corrosion monitoring [33] or remote real-time multipoint for liquid solution measurements.
2. Experiments 2.1 Devices and setup The sensor centerpiece was a 1 cm long 6° TFBG photo-inscribed in the core of a hydrogen-loaded FiberCore PS-1250 single-mode photosensitive optical fiber using a frequency-doubled Ar laser emitting at 244 nm and a 1095 nm pitch uniform phase mask. After the photo-inscription process, the grating remained inside an oven at 85 °C for 12 hours to get rid of the residual hydrogen content and stabilize its behavior. Fig. 1 shows the experimental setup implemented to characterize the PAni-TFBG sensing heads. An FBG interrogator (NI PXIe-4844 + MicronOptics module) with a scanning frequency of 10Hz and a 5 pm resolution was employed to interrogate the TFBGs in the optical range 1510-1590 nm. To verify the correct performance of the sensing head in response to pH variations, the refractive index of the NaCl target solution was continuously monitored through a digital refractometer (Reichert AR200) with a resolution of 10-4 refractive index units (RIU). These modifications were always tracked in terms of refractive index to avoid crosstalk between refractive index and pH measurements, assuring that the refractive index of the NaCl solution remained always constant and verifying that the sensor response results from pH variations. Likewise, the pH value of the solution was tracked through a digital pH meter (Lab 850 Schott Instruments) with a resolution of 10-3 pH units.
2.2 PAni coating The PAni film was deposited by in situ oxidative polymerization starting from aniline and ammonium peroxodisulfate (oxidant) in an acid aqueous solution (Fig.2a) following the method described in [34]. The cleaned TFBG was immersed in a 1 M hydrochloric acid solution containing 0.1 M of aniline hydrochloride and 0.1 M ammonium persulfate. The polymerization reaction was carried out in several samples for 10, 20 and 30 min at room temperature. The formed film was then rinsed with deionized water and treated with 0.1 M hydrochloric acid before it was used for pH measurement. Polyaniline has many different oxidation states as shown in Fig. 2b. The basic form of polyaniline, commonly known as emeraldine base, can be reduced to the leucoemeraldine base or oxidized to the form of pernigraniline base. The acid–base interactions leading to changes in the spectral and electrical characteristics of PAni can be schematically represented by a sequence of reactions as shown in Fig.3 [35].
The TFBG transmitted amplitude spectrum was continuously tracked in order to monitor the peak-to-peak amplitude of the spectral comb, avoiding saturation effects. To avoid any disturbance due to unwanted polarization instabilities [36], all connecting fibers remained unaltered during the measurements. Fig. 4a displays the transmitted amplitude spectrum of an uncoated TFBG (blue curve) and after 20 minutes of PAni deposition (red curve). The PAni coating modifies (rises) the external refractive index of the TFBG leading to a higher number of resonant cladding modes coupled to the external medium and inducing a wavelength shift and an amplitude variation on the remaining cladding guided ones. These refractive index modifications, and therefore the sensitivity of the sensing head, directly depend on the deposition thickness. The exceptional features provided by the TFBG enable direct interaction between the cladding coupled light and the bounding medium, without the need for any additional processing treatment. A study of the saturation time in terms of wavelength shift was carried out and the obtained results are shown in Fig. 4b. In order to investigate the sensing performances, two TFBGs were immersed simultaneously in the PAni precursor solution. For each sensor, the wavelength shift of one of their most sensitive cladding mode resonances was monitored during the deposition process. In both cases, a saturation of the evolution was obtained for a deposition time of around 20 minutes. Above this deposition time, the PAni coating continued growing but due to the penetration depth of the TFBG cladding modes, no difference could be noticed. Attending to the morphology of the PAni layer, its thickness increases with the deposition time forming a homogeneous film around the optical fiber up to 30 minutes. Beyond this turning point the film is no longer homogeneous, generating “big spots” of PAni around the TFBG that peel off from the fiber when they become too big, creating “holes” on the thin film. It was observed that the sensor sensitivity increased with the deposition time but meanwhile undesirable effects arised dramatically such as hysteresis and response time.
For a 20 minutes deposition, at the limit of the saturation time, the PAni coating on the TFBG had a thickness of about 5 µm and exhibited a uniform distribution around the entire TFBG. Fig. 5 shows the SEM pictures of the coating surface at this stage with two magnifications and Fig. 6 presents the results of the surface roughness characterization with a by means of a profilometer. These figures show the deposition is quite homogenous although there are some bigger aggregates on the surface. At high magnification, a classical cauliflower structure is observed, being the typical size of the clusters about 200 nm. The roughness profile of the deposit shows a uniform layer (roughness of the order of 0.1 µm) with some aggregates confirming the SEM observations.
2.3 pH detection The pH sensing heads were immersed into a NaCl solution at a constant temperature of 20 ºC in order to perform controlled pH variations. HCl and NaOH were added to the NaCl solution to move towards acidic or basic aqueous solutions, respectively. During the experiments, the refractive index of the solution was monitored and could be considered as constant since the refractive index variations for the whole range corresponded to the resolution of the digital refractometer. pH variations induce a refractive index variation of the PAni coating and consequently a variation on the effective refractive index of the TFBG cladding mode resonances. This variation changes the penetration of the TFBG resonant cladding modes and are reflected on the TFBG spectrum, leading to a wavelength shift of these cladding mode resonances as described by Eq. (1). Where 𝜆cladding is the resonance wavelength, ΛPM is the period of the interference pattern that is used to create the grating, θ is the tilt angle of the grating planes relative to the plane of the fiber cross section, neff,core is the effective index of the core-guided mode at the wavelength where the resonance is observed and neff,clad,i is the effective refractive index of the mode i, that is the only parameter that changes as a function of the surrounding refractive index. 𝜆𝑐𝑙𝑎𝑑𝑑𝑖𝑛𝑔,𝑖 = (𝑛𝑒𝑓𝑓,𝑐𝑜𝑟𝑒 + 𝑛𝑒𝑓𝑓,𝑐𝑙𝑎𝑑,𝑖 )
Λ𝑃𝑀 2 cos 𝜗
(1)
Increasing pH is reflected by a decrease of the refractive index of the PAni coating and therefore in a wavelength shift of the cladding mode resonance towards lower wavelength values. Oppositely, decreasing pH leads to a wavelength shift towards higher wavelength values since the refractive index of the PAni coating increases. Fig. 7 illustrates the wavelength response of one of these cladding modes resonances with increasing pH for a sensor with a PAni thickness of 5 µm.
3. Results and discussion 3.1 Influence of the deposition time To test the performance of the sensing head with respect to pH variations, several sensors were developed with different thicknesses and tested in pairs. The sensitive layers that were tested had thicknesses of 17 µm, 5 µm and 2 µm, each of them corresponding to a different stage of deposition: after the wavelength shift saturates, in the limit of saturation and when the wavelength shift is being produced, respectively. All the presented results were taken after 3 complete pH cycles (up and down between 2 and 12 pH) to ensure the stabilization of the sensitive PAni layer.
The aim of the first tests was to study the result of increasing the deposition time above the saturation point to check if the sensor features in terms of sensitivity and hysteresis change with regard to those achieved with a sensor whose deposition was stopped at the saturation point. As seen in Fig. 8, sensors have a sensitivity of 82 pm/pH and an error due to the hysteresis of ±2.91 pH what makes the sensor unsuitable for almost any practical application.
The response of the sensors with PAni thickness of 5 µm is shown in Fig. 9. In this case, the deposition was halted as soon as the resonances stop shifting during the film synthesis. As can be concluded from Fig. 9, these sensors present a sensitivity of 40 pm/pH and an error due to the hysteresis effect of ±1.14 pH. Compared to the previous sensors, they present lower sensitivity but a hysteresis that makes them suitable for some industrial applications.
Finally, on the lower deposition limit, the last sensors were deposited with a thickness of 2 µm. The results exhibited by these sensors can be found in Fig. 10. As can be noticed, they present a sensitivity of 30 pm/pH and a hysteresis of ±0.64 pH. As mentioned before, the sensitivity has slightly decreased but the hysteresis has been reduced almost in a factor of 2 with respect to the sensors with 5 µm coating thickness. The reduction of the hysteresis is explained by the faster diffusion in the thinner films [37]. Indeed, as shown in figure 5, the films are not very porous and the diffusion is slow. It is worth mentioning that the PAni coating was not uniform at all for thicknesses lower than 2 µm, leaving big areas of the TFBG uncovered. Therefore, sensors coated with thinner films result unsuitable for pH measurement.
As a summary, the results of the 3 types of thickness configurations are presented in Table 1. It can be noticed that a direct relation between the sensor sensitivity and its hysteresis exists, which leads to a compromise when choosing the proper film thickness during dip coating. According to the experiments, PAni-TFBG-based sensors are replicable (a high degree of similarity was indeed achieved between different sensors) and present a good long-term stability.
3.2 Influence of the temperature on the hysteresis A sensor with a PAni thickness of 5 µm was chosen to perform a temperature study in order to evaluate the performance of the sensitive layer, focusing on the hysteresis of the measurements. As previously reported [17], the optical spectrum of a TFBG is not temperature independent. A TFBG can be seen as a particular case of a FBG, so the main properties are similar, such as its typical temperature sensitivity of roughly 10 pm/ºC. All the TFBG spectrum shifts with temperature, and as seen, the cladding resonance modes also shift with pH variations, but 𝜆B (the core mode, also known as Bragg wavelength) does not. As the resonant mode reflected at 𝜆B is not coupled into the cladding (it is directly reflected confined in the fiber core), it is not affected by the variations of the effective refractive index of the surrounding medium and therefore it is insensitive to external perturbations. Therefore, by tracking 𝜆B it is possible to monitor continuously the temperature of the sensor and, if necessary, to compensate the induced wavelength shift. As a consequence of this particular property, a TFBG can be seen as a temperature independent sensing structure. However, the hysteresis effect is directly affected by temperature while sensitivity remains unaffected. Temperature tests were carried out for 20 ºC, 30 ºC, 40 ºC and 50 ºC, showing a comparison between just the results corresponding to 20 ºC and 50 ºC in Fig. 11 to ease the visualization of the differences. As can be seen, a reduction of the hysteresis effect is observed when temperature rises. This decrease of hysteresis is linear with temperature, giving results of 2.53 pH hysteresis for the sensor at 20 ºC, 2.38 pH at 30 ºC, 2.21 pH at 40 ºC and 2.04 pH at 50 ºC.
3.3 Response and stabilization time Another important feature for pH sensors is the response and stabilization times of measurements. The second can be defined as the time taken by the sensor to achieve the 90% of the final relative value. For a 5 µm thick PAni film, the stabilization results of the sensor are presented in Fig. 12. Measurements were performed between 4.8 and 6 pH giving a T1 of 29 seconds for the decreasing measurement (from 6 to 4.8 pH) and a Tt2 of 67 seconds for the increasing one (from 4.8 to 6 pH).
This difference on the increasing or decreasing measurement is due to the pH working range. Measurements performed in different regions are summarized in Table 2. Measurements for pH values approaching 7 take longer time to stabilize due to the sensor maximum sensitivity achieved in this region: very small pH variations cause high variations in the wavelength measured and thus high stabilization periods. All these results correspond to the theoretical performance behavior of the PAni sensitive layer to pH variations depending on the pH working region. The region between 4.8 and 6 was studied in order to have the stabilization times characterized in the pH region of the adult human skin.
With regard to the response time, the sensor was compared to the digital pHmeter used as a reference. The results of this study brought to light the fast response time of the sensor compared to the electronic device, the latter usually taking more than twice the PAni sensor´s time to obtain a valid measurement.
4. Conclusion To summarize, a new fiber optic pH sensor has been proposed and experimentally demonstrated. The sensing head is based on a PAni coating on the surface of a TFBG deposited through an in situ chemical oxidative method. The interrogation of the sensor was carried out by monitoring the wavelength shift of one of the resonant cladding modes of the TFBG. This platform presents different features depending on the thickness of the PAni film with a compromise between sensitivity and hysteresis effect, showing a maximum sensitivity of 82 pm/pH with and error of ±2.91 pH and a minimum of 30 pm/pH with an error of ±0.64 pH. The effect of the temperature on the hysteresis effect and the response and stabilization times have also been experimentally characterized. For a sensor with a 5 µm of PAni thickness hysteresis decreases with temperature from 2.53 pH at 20 ºC to 2.04 pH at 50 ºC. Furthermore, the stabilization times of pH measurements show different results depending on the pH region studied and the increasing or decreasing sense of the variation. Measurements around the neutral pH 7 take longer times as expected due to the high sensitivity of the sensor in this region. However, the sensors exhibit a faster response when being compared to a common digital pHmeter and a good degree of repeatability was achieved. Due to the characteristics of this device, it can be considered temperature independent and biochemical compatible, which make it suitable for new medical smart structures, pipeline corrosion monitoring or remote long-distance real-time multipoint sensing.
Acknowledgments Á. González-Vila is supported by the F.R.S.-FNRS through a FRIA grant. C. Caucheteur is Associate Researcher of the F.R.S.-FNRS. The authors would also like to thank the financial support from the ERC (European Research Council) Starting Independent Grant PROSPER (grant agreement No. 280161 – http://www.umons.ac.be/erc-prosper), from the Spanish Comisión Interministerial de Ciencia y Tecnología within projects TEC2016-76021-C2-1-R and TEC2013-47264C2-2-R and from SUDOE ECOAL-MGT and FEDER funds from the European Union. M. Debliquy and D. Lahem also acknowledge the financial support of CAPTINDOOR WBGREEN programs (Walloon Region of Belgium).
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Author biographies Aitor López Aldaba was born in Navarra, Spain, in July 1988. He received the, Telecommunication Engineering degree from the Universidad Pública de Navarra, Navarra, in 2014. In 2013, he joined the Optical Communications Group, Department of Electrical and Electronic Engineering, Universidad Pública de Navarra. His research interests include fiber optic lasers, optical amplifiers, optical fiber sensor networks, photonic crystal fibers and chemical fiber optic sensors. Álvaro González-Vila received his Master degree in Telecommunications Engineering in 2014 from the University of Cantabria, Spain. Since January 2015, he is a full-time Ph. D. student at the Electromagnetism and Telecommunications Department in the University of Mons, Belgium. He is the holder of a FRIA grant from the Belgian F.R.S.-FNRS and a student member of the international society for optics and photonics (SPIE). Marc Debliquy received his Ph.D. at Faculty of Engineering in Mons (Belgium) in 1999 in the field of organic semiconductors for fire detection. He joined the Sochinor Company in 2000. He left in 2003 for joining Materia Nova. He was responsible for the research activities in the field of gas sensors. Since October 2008, he joined the Material Science Department of Faculty of Engineering of University of Mons and worked as a team leader of semiconductor and sensor group. He was promoted as associate professor in September 2013. His main research interest is smart coatings for chemical detection. Manuel López-Amo (M’91–SM’98) was born in Madrid, Spain, in 1960. He received the Telecommunications Engineering and Ph.D. degrees from the Universidad Politécnica de Madrid, Madrid, in 1985 and 1989, respectively. From 1985 to 1996, he belonged to the Photonic Technology Department, Universidad Politécnica de Madrid, where, in 1990, he became an Associate Professor. In 1996, he moved to the Electrical and Electronic Engineering Department, Public University of Navarra, Pamplona, Spain, where he became a Full Professor and is currently the Head of the Optical Communications Group. He has been the Chairman of the Optoelectronic Committee of Spain. He has been the leader of more than 40 research projects and he has coauthored more than 250 works in international refereed journals and conferences related with fiber-optic networks, optical amplifiers, fiber-optic sensors, and integrated optics. He is a member of the technical committees of the International Conference on Fiber Optic Sensors and the European Workshop on Optical Fiber Sensors, among others. He is a senior member of the IEEE and member of the Optical Society of America. Christophe Caucheteur received his Master degree in Electrical Engineering in 2003 and PhD degree in Applied Sciences in 2007 from the Faculté Polytechnique de Mons, Belgium. He is now Research Associate of the F.R.S.-FNRS at the University of Mons and team leader of the fiber Bragg grating sensing group. He is co-author of about 250 papers in journals and conference proceedings and 5 patent applications regarding the development of innovative fiber Bragg grating sensors. He is recipient of an ERC (European Research Council) Starting Grant for a research project focused on the development of innovative plasmonic biochemical optical fiber sensors. Driss Lahem received his Ph.D. at the University of Mons - Hainaut in Mons (Belgium) in 1999 in the field of organic chemistry. Since 2000, he has worked at Materia Nova, a Research Center in the field of Materials in Mons (Belgium). He is Scientific Leader for the research activities in the field of chemical sensors. His main research areas are the development and the validation of active materials specifically for chemical sensing and pollutants degradation.
Figure captions Fig. 1. Experimental setup used to interrogate the TFBGs in transmission.
Fig. 2a. Scheme of the synthesis of emeraldine base polyaniline.
Fig. 2b. Structure of polyaniline.
Fig.3. Scheme of PAni changes during acid–base reactions.
Fig. 4. (a) Transmitted amplitude spectrum of the TFBG before and after the PAni deposition and (b) wavelength shift of the most sensitive TFBG cladding mode resonance during the PAni deposition
Fig. 5. SEM pictures of the PAni: a) 50 µm scale and 500nm scale.
Fig. 6. Roughness profile of the surface of the PAni layer.
Fig. 7. Wavelength shift with increasing pH of a sensor with 5 µm of PAni thickness.
Fig. 8. Response to pH evolution of the sensor coated with 17 µm of PAni.
Fig. 9. Response to pH evolution of the sensor coated with 5 µm of PAni.
Fig. 10. Response to pH evolution of the sensor coated with 2 µm of PAni.
Fig. 11. Temperature effect on pH-sensing with 5 µm PAni-coated TFBGs.
Fig. 12. Stabilization time of the sensor response (5 µm of PAni thickness) for increasing and decreasing pH measurements.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Tables Table 1. Summary of pH sensing features for PAni-coated TFBGs. PAni thickness (µm)
Sensitivity (pm/pH)
Error due to hysteresis
17
82
±2.91
5
40
±1.14
2
30
±0.64
Table 2. Response time summary.
Initial pH
Final pH
Sensor stabilization time (seconds)
pHmeter stabilization time (seconds)
2.5
3.5
15
31
3.5
2.5
8
17
4.8
6
67
84
6
4.8
29
41
9.2
10.2
16
33
10.2
9.2
26
57
S0925-4005(17)31381-3 http://dx.doi.org/doi:10.1016/j.snb.2017.07.167 SNB 22826
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Sensors and Actuators B
Received date: Revised date: Accepted date:
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´ Please cite this article as: A.Lopez Aldaba, A.Gonz´ alez-Vila, M.Debliquy, M.Lopez-Amo, C.Caucheteur, D.Lahem, Polyaniline-coated tilted fiber Bragg gratings for pH sensing, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.07.167 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyaniline-coated tilted fiber Bragg gratings for pH sensing A. Lopez Aldaba*a,b, Á. González-Vilab, M. Debliquyc, M. Lopez-Amoa, C. Caucheteurb and D. Lahemd a
Universidad Pública de Navarra, Dept. of Electrical and Electronic Engineering and ISC, Campus Arrosadia S/N, E-31006, Pamplona, Spain b Electromagnetism and Telecommunication Department, University of Mons, Boulevard Dolez 31, 7000 Mons, Belgium c Department of Materials Science, University of Mons, Rue de l’Epargne 56, 7000 Mons, Belgium d Materia Nova, Materials R&D Centre, Avenue N. Copernic 1, 7000 Mons, Belgium
* [email protected]; phone +34 948 16 9841; fax +34 948 16 9720
Highlights
The proposed pH sensor combines PAni chemical deposition and tilted fiber Bragg gratings. In situ chemical oxidative polymerization track of the coating thickness allows to optimize the sensor response. Determination of the sensor performance in terms of sensitivity and hysteresis between 212 pH. Fast response time and stabilization times comparable to commercial available devices. Sensors present biochemical compatibility, temperature independence, long-term stability and remote real-time multipoint sensing features.
ABSTRACT In this paper, we report an optical fiber pH sensor based on a polyaniline coating deposited on the surface of a tilted fiber Bragg grating. The pH-sensitive film was synthesized by in situ chemical oxidative polymerization keeping track of the deposition time in order to optimize the sensor response. As a result, the sensor reacts to pH changes in the range of 212 with a fast response and its sensitivity is directly related to the film thickness. The main advantages of this PAni-TFBG pH sensor are its biochemical compatibility, temperature independence, long-term stability and remote real-time multipoint sensing features. Keywords: optical fiber sensor, fiber Bragg grating, tilted fiber Bragg grating, polyaniline, pH sensor, chemical sensor
1. Introduction Optical and fiber-optic pH sensors complement the glass electrodes for pH monitoring and offer numerous advantages such as immunity from electrical interference, miniaturization, robustness, possibility of remote and real time sensing and in vivo measurements. Conducting polymers possess both the mechanical properties of polymers and the electrical properties of conductors, having a big potential in many technological applications. Polyaniline (PAni) is a well-known conducting polymer, which is easy to synthesize, shows excellent stability, biochemical compatibility [1] and has rapid and reversible adsorption or desorption kinetics [1]. It has attracted much attention in numerous sensing applications such as gas detection [3-5], ascorbic acid detection [6], multi-electrode sensor arrays [7], coatings for quartz crystal microbalance sensors [8], chemiresistors using single walled carbon nanotubes (SWNT) [9] and as modified cladding in optical fiber sensors [10]. Among chemical sensors, pH sensors are one of the most studied applications. Fiber optic sensors associated to conducting polymers have been developed to estimate the pH within the biofilms grown on stainless steel [11], to monitor small cluster of cells [12] or to develop pH sensors based on mode-filtered light [13]. Many non-fiber based typical pH sensors have also been studied too, such as carbon-dot-based for very weak acids detection [14], poly-dopamine films voltammetric sensors [15] and sol-gel sensors [16], among others. Tilted fiber Bragg grating (TFBG) sensors have proven to exhibit really interesting features when applied to chemical sensing [17]. Several sensors have indeed been proposed for physical and chemical parameter monitoring such as humidity [18], temperature [19], strain [20], gas [21], refractive index [22-24] or molecular binding [25]. Compared to uniform Bragg gratings (FBGs), the TFBG tilt angle causes a coupling of part of the guided light to the cladding of the optical fiber in the form of several cladding mode resonances [27]. Each of these modes propagates with a corresponding refractive index value and is confined in the cladding by reflection on the cladding-outer medium interface. Thus, when the refractive index value of the surrounding medium reaches a value close to the one of a mode, the total internal reflection condition is lost so that light is transmitted to this medium, producing a change on the grating transmitted spectrum. The monitoring of these alterations allows us to detect and quantify the variations of the refractive index of the outer medium, turning these devices into highly accurate refractometers. In this work, a novel pH sensor consisting on a polyaniline deposition on the surface of a TFBG is presented and characterized. PAni presents indeed a pH sensitive optical absorption spectrum in the NIR region. This effect is due to the partial protonation leading to a doping and a conformational change of the polymeric chains. The working principle of the sensor is based on the monitoring of the transmission spectrum of the TFBG, which varies according to the refractive index changes of the sensitive layer with pH. The pH measurements are thus based on tracking the wavelength shift of the TFBG cladding modes [26], achieving an overall sensitivity of 46 pm/pH unit in the range 2-12 pH. Due to the main properties of this combination, PAni-TFBG pH sensors could have applications in various domains such as medical smart textiles [28], bioelectrical and in vivo measurements [29-32], pipeline corrosion monitoring [33] or remote real-time multipoint for liquid solution measurements.
2. Experiments 2.1 Devices and setup The sensor centerpiece was a 1 cm long 6° TFBG photo-inscribed in the core of a hydrogen-loaded FiberCore PS-1250 single-mode photosensitive optical fiber using a frequency-doubled Ar laser emitting at 244 nm and a 1095 nm pitch uniform phase mask. After the photo-inscription process, the grating remained inside an oven at 85 °C for 12 hours to get rid of the residual hydrogen content and stabilize its behavior. Fig. 1 shows the experimental setup implemented to characterize the PAni-TFBG sensing heads. An FBG interrogator (NI PXIe-4844 + MicronOptics module) with a scanning frequency of 10Hz and a 5 pm resolution was employed to interrogate the TFBGs in the optical range 1510-1590 nm. To verify the correct performance of the sensing head in response to pH variations, the refractive index of the NaCl target solution was continuously monitored through a digital refractometer (Reichert AR200) with a resolution of 10-4 refractive index units (RIU). These modifications were always tracked in terms of refractive index to avoid crosstalk between refractive index and pH measurements, assuring that the refractive index of the NaCl solution remained always constant and verifying that the sensor response results from pH variations. Likewise, the pH value of the solution was tracked through a digital pH meter (Lab 850 Schott Instruments) with a resolution of 10-3 pH units.
2.2 PAni coating The PAni film was deposited by in situ oxidative polymerization starting from aniline and ammonium peroxodisulfate (oxidant) in an acid aqueous solution (Fig.2a) following the method described in [34]. The cleaned TFBG was immersed in a 1 M hydrochloric acid solution containing 0.1 M of aniline hydrochloride and 0.1 M ammonium persulfate. The polymerization reaction was carried out in several samples for 10, 20 and 30 min at room temperature. The formed film was then rinsed with deionized water and treated with 0.1 M hydrochloric acid before it was used for pH measurement. Polyaniline has many different oxidation states as shown in Fig. 2b. The basic form of polyaniline, commonly known as emeraldine base, can be reduced to the leucoemeraldine base or oxidized to the form of pernigraniline base. The acid–base interactions leading to changes in the spectral and electrical characteristics of PAni can be schematically represented by a sequence of reactions as shown in Fig.3 [35].
The TFBG transmitted amplitude spectrum was continuously tracked in order to monitor the peak-to-peak amplitude of the spectral comb, avoiding saturation effects. To avoid any disturbance due to unwanted polarization instabilities [36], all connecting fibers remained unaltered during the measurements. Fig. 4a displays the transmitted amplitude spectrum of an uncoated TFBG (blue curve) and after 20 minutes of PAni deposition (red curve). The PAni coating modifies (rises) the external refractive index of the TFBG leading to a higher number of resonant cladding modes coupled to the external medium and inducing a wavelength shift and an amplitude variation on the remaining cladding guided ones. These refractive index modifications, and therefore the sensitivity of the sensing head, directly depend on the deposition thickness. The exceptional features provided by the TFBG enable direct interaction between the cladding coupled light and the bounding medium, without the need for any additional processing treatment. A study of the saturation time in terms of wavelength shift was carried out and the obtained results are shown in Fig. 4b. In order to investigate the sensing performances, two TFBGs were immersed simultaneously in the PAni precursor solution. For each sensor, the wavelength shift of one of their most sensitive cladding mode resonances was monitored during the deposition process. In both cases, a saturation of the evolution was obtained for a deposition time of around 20 minutes. Above this deposition time, the PAni coating continued growing but due to the penetration depth of the TFBG cladding modes, no difference could be noticed. Attending to the morphology of the PAni layer, its thickness increases with the deposition time forming a homogeneous film around the optical fiber up to 30 minutes. Beyond this turning point the film is no longer homogeneous, generating “big spots” of PAni around the TFBG that peel off from the fiber when they become too big, creating “holes” on the thin film. It was observed that the sensor sensitivity increased with the deposition time but meanwhile undesirable effects arised dramatically such as hysteresis and response time.
For a 20 minutes deposition, at the limit of the saturation time, the PAni coating on the TFBG had a thickness of about 5 µm and exhibited a uniform distribution around the entire TFBG. Fig. 5 shows the SEM pictures of the coating surface at this stage with two magnifications and Fig. 6 presents the results of the surface roughness characterization with a by means of a profilometer. These figures show the deposition is quite homogenous although there are some bigger aggregates on the surface. At high magnification, a classical cauliflower structure is observed, being the typical size of the clusters about 200 nm. The roughness profile of the deposit shows a uniform layer (roughness of the order of 0.1 µm) with some aggregates confirming the SEM observations.
2.3 pH detection The pH sensing heads were immersed into a NaCl solution at a constant temperature of 20 ºC in order to perform controlled pH variations. HCl and NaOH were added to the NaCl solution to move towards acidic or basic aqueous solutions, respectively. During the experiments, the refractive index of the solution was monitored and could be considered as constant since the refractive index variations for the whole range corresponded to the resolution of the digital refractometer. pH variations induce a refractive index variation of the PAni coating and consequently a variation on the effective refractive index of the TFBG cladding mode resonances. This variation changes the penetration of the TFBG resonant cladding modes and are reflected on the TFBG spectrum, leading to a wavelength shift of these cladding mode resonances as described by Eq. (1). Where 𝜆cladding is the resonance wavelength, ΛPM is the period of the interference pattern that is used to create the grating, θ is the tilt angle of the grating planes relative to the plane of the fiber cross section, neff,core is the effective index of the core-guided mode at the wavelength where the resonance is observed and neff,clad,i is the effective refractive index of the mode i, that is the only parameter that changes as a function of the surrounding refractive index. 𝜆𝑐𝑙𝑎𝑑𝑑𝑖𝑛𝑔,𝑖 = (𝑛𝑒𝑓𝑓,𝑐𝑜𝑟𝑒 + 𝑛𝑒𝑓𝑓,𝑐𝑙𝑎𝑑,𝑖 )
Λ𝑃𝑀 2 cos 𝜗
(1)
Increasing pH is reflected by a decrease of the refractive index of the PAni coating and therefore in a wavelength shift of the cladding mode resonance towards lower wavelength values. Oppositely, decreasing pH leads to a wavelength shift towards higher wavelength values since the refractive index of the PAni coating increases. Fig. 7 illustrates the wavelength response of one of these cladding modes resonances with increasing pH for a sensor with a PAni thickness of 5 µm.
3. Results and discussion 3.1 Influence of the deposition time To test the performance of the sensing head with respect to pH variations, several sensors were developed with different thicknesses and tested in pairs. The sensitive layers that were tested had thicknesses of 17 µm, 5 µm and 2 µm, each of them corresponding to a different stage of deposition: after the wavelength shift saturates, in the limit of saturation and when the wavelength shift is being produced, respectively. All the presented results were taken after 3 complete pH cycles (up and down between 2 and 12 pH) to ensure the stabilization of the sensitive PAni layer.
The aim of the first tests was to study the result of increasing the deposition time above the saturation point to check if the sensor features in terms of sensitivity and hysteresis change with regard to those achieved with a sensor whose deposition was stopped at the saturation point. As seen in Fig. 8, sensors have a sensitivity of 82 pm/pH and an error due to the hysteresis of ±2.91 pH what makes the sensor unsuitable for almost any practical application.
The response of the sensors with PAni thickness of 5 µm is shown in Fig. 9. In this case, the deposition was halted as soon as the resonances stop shifting during the film synthesis. As can be concluded from Fig. 9, these sensors present a sensitivity of 40 pm/pH and an error due to the hysteresis effect of ±1.14 pH. Compared to the previous sensors, they present lower sensitivity but a hysteresis that makes them suitable for some industrial applications.
Finally, on the lower deposition limit, the last sensors were deposited with a thickness of 2 µm. The results exhibited by these sensors can be found in Fig. 10. As can be noticed, they present a sensitivity of 30 pm/pH and a hysteresis of ±0.64 pH. As mentioned before, the sensitivity has slightly decreased but the hysteresis has been reduced almost in a factor of 2 with respect to the sensors with 5 µm coating thickness. The reduction of the hysteresis is explained by the faster diffusion in the thinner films [37]. Indeed, as shown in figure 5, the films are not very porous and the diffusion is slow. It is worth mentioning that the PAni coating was not uniform at all for thicknesses lower than 2 µm, leaving big areas of the TFBG uncovered. Therefore, sensors coated with thinner films result unsuitable for pH measurement.
As a summary, the results of the 3 types of thickness configurations are presented in Table 1. It can be noticed that a direct relation between the sensor sensitivity and its hysteresis exists, which leads to a compromise when choosing the proper film thickness during dip coating. According to the experiments, PAni-TFBG-based sensors are replicable (a high degree of similarity was indeed achieved between different sensors) and present a good long-term stability.
3.2 Influence of the temperature on the hysteresis A sensor with a PAni thickness of 5 µm was chosen to perform a temperature study in order to evaluate the performance of the sensitive layer, focusing on the hysteresis of the measurements. As previously reported [17], the optical spectrum of a TFBG is not temperature independent. A TFBG can be seen as a particular case of a FBG, so the main properties are similar, such as its typical temperature sensitivity of roughly 10 pm/ºC. All the TFBG spectrum shifts with temperature, and as seen, the cladding resonance modes also shift with pH variations, but 𝜆B (the core mode, also known as Bragg wavelength) does not. As the resonant mode reflected at 𝜆B is not coupled into the cladding (it is directly reflected confined in the fiber core), it is not affected by the variations of the effective refractive index of the surrounding medium and therefore it is insensitive to external perturbations. Therefore, by tracking 𝜆B it is possible to monitor continuously the temperature of the sensor and, if necessary, to compensate the induced wavelength shift. As a consequence of this particular property, a TFBG can be seen as a temperature independent sensing structure. However, the hysteresis effect is directly affected by temperature while sensitivity remains unaffected. Temperature tests were carried out for 20 ºC, 30 ºC, 40 ºC and 50 ºC, showing a comparison between just the results corresponding to 20 ºC and 50 ºC in Fig. 11 to ease the visualization of the differences. As can be seen, a reduction of the hysteresis effect is observed when temperature rises. This decrease of hysteresis is linear with temperature, giving results of 2.53 pH hysteresis for the sensor at 20 ºC, 2.38 pH at 30 ºC, 2.21 pH at 40 ºC and 2.04 pH at 50 ºC.
3.3 Response and stabilization time Another important feature for pH sensors is the response and stabilization times of measurements. The second can be defined as the time taken by the sensor to achieve the 90% of the final relative value. For a 5 µm thick PAni film, the stabilization results of the sensor are presented in Fig. 12. Measurements were performed between 4.8 and 6 pH giving a T1 of 29 seconds for the decreasing measurement (from 6 to 4.8 pH) and a Tt2 of 67 seconds for the increasing one (from 4.8 to 6 pH).
This difference on the increasing or decreasing measurement is due to the pH working range. Measurements performed in different regions are summarized in Table 2. Measurements for pH values approaching 7 take longer time to stabilize due to the sensor maximum sensitivity achieved in this region: very small pH variations cause high variations in the wavelength measured and thus high stabilization periods. All these results correspond to the theoretical performance behavior of the PAni sensitive layer to pH variations depending on the pH working region. The region between 4.8 and 6 was studied in order to have the stabilization times characterized in the pH region of the adult human skin.
With regard to the response time, the sensor was compared to the digital pHmeter used as a reference. The results of this study brought to light the fast response time of the sensor compared to the electronic device, the latter usually taking more than twice the PAni sensor´s time to obtain a valid measurement.
4. Conclusion To summarize, a new fiber optic pH sensor has been proposed and experimentally demonstrated. The sensing head is based on a PAni coating on the surface of a TFBG deposited through an in situ chemical oxidative method. The interrogation of the sensor was carried out by monitoring the wavelength shift of one of the resonant cladding modes of the TFBG. This platform presents different features depending on the thickness of the PAni film with a compromise between sensitivity and hysteresis effect, showing a maximum sensitivity of 82 pm/pH with and error of ±2.91 pH and a minimum of 30 pm/pH with an error of ±0.64 pH. The effect of the temperature on the hysteresis effect and the response and stabilization times have also been experimentally characterized. For a sensor with a 5 µm of PAni thickness hysteresis decreases with temperature from 2.53 pH at 20 ºC to 2.04 pH at 50 ºC. Furthermore, the stabilization times of pH measurements show different results depending on the pH region studied and the increasing or decreasing sense of the variation. Measurements around the neutral pH 7 take longer times as expected due to the high sensitivity of the sensor in this region. However, the sensors exhibit a faster response when being compared to a common digital pHmeter and a good degree of repeatability was achieved. Due to the characteristics of this device, it can be considered temperature independent and biochemical compatible, which make it suitable for new medical smart structures, pipeline corrosion monitoring or remote long-distance real-time multipoint sensing.
Acknowledgments Á. González-Vila is supported by the F.R.S.-FNRS through a FRIA grant. C. Caucheteur is Associate Researcher of the F.R.S.-FNRS. The authors would also like to thank the financial support from the ERC (European Research Council) Starting Independent Grant PROSPER (grant agreement No. 280161 – http://www.umons.ac.be/erc-prosper), from the Spanish Comisión Interministerial de Ciencia y Tecnología within projects TEC2016-76021-C2-1-R and TEC2013-47264C2-2-R and from SUDOE ECOAL-MGT and FEDER funds from the European Union. M. Debliquy and D. Lahem also acknowledge the financial support of CAPTINDOOR WBGREEN programs (Walloon Region of Belgium).
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Author biographies Aitor López Aldaba was born in Navarra, Spain, in July 1988. He received the, Telecommunication Engineering degree from the Universidad Pública de Navarra, Navarra, in 2014. In 2013, he joined the Optical Communications Group, Department of Electrical and Electronic Engineering, Universidad Pública de Navarra. His research interests include fiber optic lasers, optical amplifiers, optical fiber sensor networks, photonic crystal fibers and chemical fiber optic sensors. Álvaro González-Vila received his Master degree in Telecommunications Engineering in 2014 from the University of Cantabria, Spain. Since January 2015, he is a full-time Ph. D. student at the Electromagnetism and Telecommunications Department in the University of Mons, Belgium. He is the holder of a FRIA grant from the Belgian F.R.S.-FNRS and a student member of the international society for optics and photonics (SPIE). Marc Debliquy received his Ph.D. at Faculty of Engineering in Mons (Belgium) in 1999 in the field of organic semiconductors for fire detection. He joined the Sochinor Company in 2000. He left in 2003 for joining Materia Nova. He was responsible for the research activities in the field of gas sensors. Since October 2008, he joined the Material Science Department of Faculty of Engineering of University of Mons and worked as a team leader of semiconductor and sensor group. He was promoted as associate professor in September 2013. His main research interest is smart coatings for chemical detection. Manuel López-Amo (M’91–SM’98) was born in Madrid, Spain, in 1960. He received the Telecommunications Engineering and Ph.D. degrees from the Universidad Politécnica de Madrid, Madrid, in 1985 and 1989, respectively. From 1985 to 1996, he belonged to the Photonic Technology Department, Universidad Politécnica de Madrid, where, in 1990, he became an Associate Professor. In 1996, he moved to the Electrical and Electronic Engineering Department, Public University of Navarra, Pamplona, Spain, where he became a Full Professor and is currently the Head of the Optical Communications Group. He has been the Chairman of the Optoelectronic Committee of Spain. He has been the leader of more than 40 research projects and he has coauthored more than 250 works in international refereed journals and conferences related with fiber-optic networks, optical amplifiers, fiber-optic sensors, and integrated optics. He is a member of the technical committees of the International Conference on Fiber Optic Sensors and the European Workshop on Optical Fiber Sensors, among others. He is a senior member of the IEEE and member of the Optical Society of America. Christophe Caucheteur received his Master degree in Electrical Engineering in 2003 and PhD degree in Applied Sciences in 2007 from the Faculté Polytechnique de Mons, Belgium. He is now Research Associate of the F.R.S.-FNRS at the University of Mons and team leader of the fiber Bragg grating sensing group. He is co-author of about 250 papers in journals and conference proceedings and 5 patent applications regarding the development of innovative fiber Bragg grating sensors. He is recipient of an ERC (European Research Council) Starting Grant for a research project focused on the development of innovative plasmonic biochemical optical fiber sensors. Driss Lahem received his Ph.D. at the University of Mons - Hainaut in Mons (Belgium) in 1999 in the field of organic chemistry. Since 2000, he has worked at Materia Nova, a Research Center in the field of Materials in Mons (Belgium). He is Scientific Leader for the research activities in the field of chemical sensors. His main research areas are the development and the validation of active materials specifically for chemical sensing and pollutants degradation.
Figure captions Fig. 1. Experimental setup used to interrogate the TFBGs in transmission.
Fig. 2a. Scheme of the synthesis of emeraldine base polyaniline.
Fig. 2b. Structure of polyaniline.
Fig.3. Scheme of PAni changes during acid–base reactions.
Fig. 4. (a) Transmitted amplitude spectrum of the TFBG before and after the PAni deposition and (b) wavelength shift of the most sensitive TFBG cladding mode resonance during the PAni deposition
Fig. 5. SEM pictures of the PAni: a) 50 µm scale and 500nm scale.
Fig. 6. Roughness profile of the surface of the PAni layer.
Fig. 7. Wavelength shift with increasing pH of a sensor with 5 µm of PAni thickness.
Fig. 8. Response to pH evolution of the sensor coated with 17 µm of PAni.
Fig. 9. Response to pH evolution of the sensor coated with 5 µm of PAni.
Fig. 10. Response to pH evolution of the sensor coated with 2 µm of PAni.
Fig. 11. Temperature effect on pH-sensing with 5 µm PAni-coated TFBGs.
Fig. 12. Stabilization time of the sensor response (5 µm of PAni thickness) for increasing and decreasing pH measurements.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Tables Table 1. Summary of pH sensing features for PAni-coated TFBGs. PAni thickness (µm)
Sensitivity (pm/pH)
Error due to hysteresis
17
82
±2.91
5
40
±1.14
2
30
±0.64
Table 2. Response time summary.
Initial pH
Final pH
Sensor stabilization time (seconds)
pHmeter stabilization time (seconds)
2.5
3.5
15
31
3.5
2.5
8
17
4.8
6
67
84
6
4.8
29
41
9.2
10.2
16
33
10.2
9.2
26
57