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Abstract
A novel fiber optic localized surface plasmon resonance (LSPR) hydrogen sensor has been developed based on the hetero-core structured with palladium nanoparticles (PdNPs) onto a cylindrical cladding surface. In a light-intensity-based experiment with an LED operating at 850 nm, it has been observed that a transmitted loss change of 0.23 dB was induced with response and recovery times of 1.5 and 3.2 s for 4% hydrogen which are the fastest response times among optical fiber hydrogen sensors. The proposed sensor resolved the inevitable trade-off issue between sensitivity and response time which existed in the previously reported SPR sensors, with keeping the response time below 2.0 s even in a high sensitivity region of interest.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, hydrogen gas has been used as an alternative to fossil fuel because it is a renewable and pollution-free energy source. Nevertheless, it requires strict control measures for preventing leakage because it is highly flammable and explosive in the presence of oxygen. In air, the lower explosive limit of hydrogen is known to be 4% by volume at room temperature and atmospheric pressure [1]. Because of its low density, hydrogen can easily migrate from one place to another, leading to a potentially destructive explosion via accidental ignition. Because even a small hydrogen gas leak can be hazardous, hydrogen gas sensors are required for real-time and online monitoring of its localized concentrations. Previous studies have so far reported various types of hydrogen sensors, such as semiconductor [2 –5] and thermal conductivity [6,7]. However, they pose an explosion hazard due to potential electrical leakage from electrical components within them.
On the other hand, optical fiber sensors are expected to operate at higher safety levels than electrical sensors, since they have no electrical contact at the sensing point and in the transmission line. In conventional optical hydrogen sensors, tungsten trioxide($\textrm {WO}_{\textrm {3}}$) and palladium (Pd) are mainly employed as sensitive layers [6,8]. When $\textrm {WO}_{\textrm {3}}$ is exposed to a hydrogen environment, it chemically interacts with hydrogen, modulating its optical properties [5,6,9]. However, $\textrm {WO}_{\textrm {3}}$ chemically interacts with other gases and exhibits a weak response to hydrogen gas; therefore, metal catalyst doping is necessary to improve the selectivity and response of $\textrm {WO}_{\textrm {3}}$ [9 –12]. However, Pd can also detect hydrogen via a change in its complex dielectric function caused by hydrogen absorption and selectively absorb and desorb hydrogen gas at room temperature and atmospheric pressure, unlike $\textrm {WO}_{\textrm {3}}$ [8]. Therefore, most current optical fiber hydrogen sensors employ Pd as a hydrogen-sensitive film because of its selectivity to hydrogen gas [13 –15].
Optical fiber hydrogen sensors using the Pd as the sensitive film have been proposed based on unclad [16,17] and tapered structures [18,19]; these approaches raise concerns regarding the reduced mechanical strength of optical fibers resulting from the need to remove the cladding layer to access transmitted light in the core. Other types of sensors based on fiber Bragg grating (FBG) structures with Pd and $\textrm {WO}_{\textrm {3}}$ as sensitive films were also proposed, which can detect strain changes based on increases in the Pd film volume and temperature changes in $\textrm {WO}_{\textrm {3}}$ [10,19 –21]. in hydrogen. However, the FBG sensor needs to be compensated due to its both of temperature and strain dependencies [22]. Optical fiber surface plasmon resonance (SPR) sensor has also been proposed [22]. Due to the phase transition of Pd in the presence of hydrogen, the SPR sensor utilized its intensity changes.
We have previously reported the hetero-core optical fiber hydrogen sensors with the Pd thin film as the sensitive layer, which comprises different core diameter fibers with no cladding removal, resulting in maintaining its mechanical strength [23 –26].The hetero-core optical fiber has higher mechanical strength than conventional optical fiber with optical leakage structure. Further, for practical usage, a short response time and durability are required. With respect to the deterioration of Pd, we have confirmed that the stability of sensitivity, which corresponds to durability, is improved by forming a Pt thin film on the Pd film [27]. In our previous work, we developed a dome-shaped Pd thin film by annealing a Pd thin film on a hetero-core optical fiber [28]. The response and recovery times of the dome-shaped Pd thin film hydrogen sensor improved to 5.5 and 19.5 s, compared to 15 s, 40 s for those of the merely Pd thin film.
A typical response time of approximately 15 s was reported for a multi-mode hydrogen fiber sensor coated with an Au/SiO2 multilayer film without cladding [8]. A hydrogen fiber sensor coated with a Pd film on a polarization-maintaining fiber interferometer exhibited a response time of approximately 10 s [8]. Therefore, the hetero-core fiber sensor with the dome-shaped Pd thin film exhibited a faster response than the above two sensors.
In this paper, we report the characteristics of a quick response hetero-core fiber optic hydrogen localized surface plasmon resonance (LSPR) sensor using spherical Palladium nanoparticles (PdNPs) can be expected to absorb and release hydrogen more quickly owing to their relatively large surface area per unit volume compared to a thin Pd layer and the dome-shaped Pd film layer. The PdNPs were immobilized on the cylindrical surface of the hetero-core fiber sensor that was positively charged by immersing the poly-L-lysine solution based on electrostatic interaction.
We compared calculated absorption spectra based on a Mie scattering model [29] with experimental ones with 4% $\textrm {H}_{\textrm {2}}$ gas in atmospheric $\textrm {N}_{\textrm {2}}$. At lower concentrations, since the adsorbed $\textrm {H}_{\textrm {2}}$ gas molecules are then dissociated into H atoms and they diffuse in the metal structure, the time of dissociation from molecules into atoms on the Pd surface and the rate of diffusion of hydrogen atoms into Pd nanoparticles determines the response time [30]. By clarifying the time response at 4% hydrogen gas, the response time is almost the same even at low concentrations. Additionally, it has been indicated in our previous study that lower concentration $\textrm {H}_{\textrm {2}}$ gas could be detected [25].Therefore, we evaluated the characteristics of PdNP hydrogen sensor using the explosion limit gas concentration of 4%. The hetero-core fiber optic hydrogen sensor successfully detected hydrogen via the difference between the light intensity spectra with and without $\textrm {H}_{\textrm {2}}$ in atmospheric $\textrm {N}_{\textrm {2}}$. In a light-intensity-based experiment at the infrared wavelength of 850 nm, changes in the optical loss of 0.23 dB with response and recovery times of 1.5 and 3.2 s, respectively, were observed on exposure to 4% $\textrm {H}_{\textrm {2}}$ in atmospheric $\textrm {N}_{\textrm {2}}$, which may be one of the fastest response times among optical fiber hydrogen sensors. According to the department of energy in USA [31], response time of less than 1 s. is required for the safety hydrogen sensors. It can be said that response time of 1.5 s is rapid enough to have good potential to be used as a safe hydrogen sensor in the practical application. In addition, we show how to control the amount of PdNPs immobilized by changing the immersion time in the PdNP suspension to enhance the sensitivity of the LSPR sensor. Therefore, employing the PdNPs for the sensitive material to the hydrogen gas, the hetero-core fiber hydrogen sensor resolved the inevitable trade-off issue between sensitivity and response time, which existed in the previously reported SPR sensor with Pd film as a sensitive layer.
2. Experimental arrangements
2.1 Sensor structure
Figure 1 depicts the structure of the hetero-core fiber optic hydrogen sensor with PdNPs. The hetero-core optical fiber comprised a multi-mode graded-index fiber, whose core diameter was 50 $\mu$m as the transmission line and an inserted single-mode step-index fiber by fusion splicing, whose core diameter and length were 3 $\mu$m and 15 mm [23 –26]. If the sensor is remotely interrogated, the bend-insensitive fibers should be used in intensity-based interrogation method to avoid the effect of loss on the optical path. PdNPs (4 nm in diameter, TANAKA Holdings Co., Ltd.) were immobilized on the hetero-core fiber sensor based on electrostatic interactions using an aqueous poly-L-lysine (M.W. 70000-150000, Sigma-Aldrich) solution with a concentration of 10 Mmol/L. The hetero-core fiber was immersed in the aqueous poly-L-lysine solution to form a positively charged surface with respect to the negatively charged silica fiber. The PdNPs were then immobilized on the surface of the sensor portion by immersing the hetero-core fiber in a suspension of PdNPs (0.04 wt%). Because of the difference in their core diameters, propagated light could leak into the cladding region of the inserted fiber. When light impinges on the PdNPs, LSPR is induced by polarization of the free electrons inside the nanoparticles. The metal nanoparticles optical extinction has a maximum at the plasma frequency, of which the PdNPs occurs at ultraviolet. Hydrogen gas absorption into the PdNPs shifted the transmitted light spectra due to a change in the dielectric complex function of the PdNPs. The reflected light was then recoupled into the core region of the multi-mode fiber. The leaked light excited LSPR in the PdNPs, which were immobilized on the fiber.
Fig. 1. Structure of a hetero-core fiber optic LSPR hydrogen sensor with 4 nm diameter Pd nanoparticles on a poly-L-lysine thin film coating.
2.2 Experimental setup
The experimental setup is shown in Fig. 2. This consisted of a halogen lamp (AQ4305, Yokogawa Co., Ltd) with a wavelength in the range of 400-1,800 nm and an optical spectrum analyzer (AQ6315A, Ando Electric Co., Ltd.). When measuring optical intensity changes, the halogen lamp and spectrum analyzer were replaced with an 850 nm LED light source and power meter (iLineBox8S, Core System Japan Co., Ltd), respectively. The hetero-core optical hydrogen sensor was placed straight and horizontally in a 15 mL acrylic gas chamber. Pure nitrogen (N2) and 4% hydrogen (H2) at atmospheric pressure were alternately introduced into the gas chamber with a maximum flow rate of 1,000 ml/min. Due to its low volume, the gas in the chamber was assumed to be replaced within less than 1 second when switching to another gas. The temperature environment was room temperature. The response was measured repeatedly by switching the gas between hydrogen and nitrogen, to ensure that the changes in the measured signal were due to hydrogen concentration variations.
Fig. 2. Experimental set-up for the detection of hydrogen gas using a hetero-core fiber LSPR hydrogen sensor.
3. Results
3.1 Spectra based on wavelength
In order to estimate the LSPR effect of PdNPs for 4% $\textrm {H}_{\textrm {2}}$ gas absorption, we calculated LSPR absorption spectra based on the Mie scattering theory and the Lambert-Beer law. The extinction, scattering, and absorption cross-sections, $\sigma _{ext}$, $\sigma _{sca}$, and $\sigma _{abs}$, respectively, are expressed as follows [29]:
Figure 3 shows calculated LSPR spectra with 4 nm diameter PdNPs, normalized with the spectra obtained without PdNPs, in 100% nitrogen and 4% hydrogen gas in atmospheric $\textrm {N}_{\textrm {2}}$. These spectra were normalized with an incident light spectrum. The normalized intensity in these spectrum monotonically attenuated as the wavelengths decreased in wide range of wavelength from visible to near infrared light 400 - 1000 nm, which was consistent with the resonance wavelength of Pd being around 200 nm [32]. The spectrum of normalized intensity was obviously shifted to the lower intensity level on exposure to 4% $\textrm {H}_{\textrm {2}}$ than the one on exposure to $\textrm {N}_{\textrm {2}}$.
Fig. 3. Calculated LSPR spectra with 4 nm diameter PdNPs, normalized with the spectra obtained without PdNPs, in 100% N2 (solid line) and 4% H2 (dash line) gas.
Figure 4 shows experimentally obtained LSPR spectra of a hetero-core fiber sensor immobilized 4 nm diameter PdNPs in 100% $\textrm {N}_{\textrm {2}}$ and 4% $\textrm {H}_{\textrm {2}}$ gas. These experimental spectra were normalized with the transmitted light spectrum in air after immobilizing the PdNPs. As shown in Fig. 4, the obtained spectrum was attenuated as the wavelength became shorter, which was in agreement with the calculated spectrum. In the calculation of Fig. 3, the absorption peak of PdNP was confirmed in the UV region. Since the calculated and the experimentally obtained spectra had the same shape, it was considered to be a spectrum derived from LSPR. In addition, the spectra obtained were attenuated over the measurement wavelength range from 400 to 1000 nm when changing from $\textrm {N}_{\textrm {2}}$ to 4% $\textrm {H}_{\textrm {2}}$ gas atmosphere. The overall agreement between the calculated and experimental spectra with and without hydrogen revealed that the spectral changes were caused by LSPR of PdNPs. The tendency of spectral change for 100% nitrogen and 4% hydrogen gas was different between the experimental and calculated LSPR spectra at the wavelength of 600 nm because the spectra was calculated without taking into account the volume expansion of PdNP due to hydrogen absorption. Therefore, the hetero-core fiber LSPR hydrogen sensor with PdNPs were successfully demonstrated for 4% $\textrm {H}_{\textrm {2}}$ gas detection.
Fig. 4. Experimental LSPR spectra of a hetero-core fiber sensor with 4 nm diameter Pd nanoparticles in 100% nitrogen (solid line) and 4% hydrogen (dash line) gas.
3.2 Hydrogen detection based on intensity
As shown in Fig. 4, the obtained broad LSPR spectra enabled light intensity mode operation, which means that the LSPR spectra change for 4% $\textrm {H}_{\textrm {2}}$ detection can be measured by an LED/PD system. The responsiveness and optical transmitted loss change were investigated by means of a series of experiments using the LED/PD system, whose wavelength was the near-infrared of 850 nm, based on light intensity mode operation. The experimental setup is indicated in Fig. 2 in which the white light source and spectrum analyzer are only replaced with the LED/PD system. The inflow of $\textrm {N}_{\textrm {2}}$ and $\textrm {H}_{\textrm {2}}$ gas at the flow rate of 1,000 mL/min was switched every two and a half minutes.
Figure 5 shows the real-time change in the optical loss of the sensor with immobilized PdNPs resulting from alternatively loading $\textrm {N}_{\textrm {2}}$ and 4% $\textrm {H}_{\textrm {2}}$ gases at operating wavelength of 850 nm. The sensor responded with a rapid optical loss changes with and without the $\textrm {H}_{\textrm {2}}$ gas as shown in the inset of Fig. 5. A certain drift was observed after some cycles, in which the hydrogen and nitrogen gas was switched alternatively. This drift was caused by the temporal fluctuation of the LED light source.
Fig. 5. Real-time responses in the optical loss changes of a hetero-core fiber hydrogen LSPR sensor with 4 nm Pd nanoparticles and expanded views of response and recovery for 0 and 4% $\textrm {H}_{\textrm {2}}$ contained $\textrm {N}_{\textrm {2}}$ at 850 nm wavelength.
The response time, which was defined as the period time for a loss change taken to reach from 10% above ground at the leading edge to a 90% maximum loss change after starting the 4% $\textrm {H}_{\textrm {2}}$ gas flow, was 1.5 s, and the recovery time, which was defined as the time taken for this to fall from 90% to 10% after switching the gas from $\textrm {H}_{\textrm {2}}$ to $\textrm {N}_{\textrm {2}}$, was 3.2 s, as shown in the expanded insets in Fig. 5. Consequently, it was clear that the temporal response of the proposed LSPR sensor with spherical PdNPs to 4% $\textrm {H}_{\textrm {2}}$ was superior to those of our previously reported dome-shaped sensor [25] and other relevant hydrogen sensors reported [26]. The surface area per unit volume of PdNP was 4.5 times larger than that of the 3 nm thick Pd thin films indicated in our previous research [24]. The larger surface area of the PdNPs per unit volume compared with those of the dome-shaped Pd and multilayer Pd films supported the superior temporal response. The loss change was observed to be 0.23 dB, which indicated that the sensor gave sufficient loss change for hydrogen detection using only PdNPs. Since the sensor characteristics show high reproducibility in terms of loss level and time response to $\textrm {N}_{\textrm {2}}$ and $\textrm {H}_{\textrm {2}}$ gas, 4 nm diameter PdNPs have proven to be practical hydrogen sensors that have a rapid time response and can be easily fabricated by a wet process.
3.3 Adjustment of optical loss change by immersion time in a PdNP suspension
Figure 6 shows the experimentally obtained LSPR spectra of 4 nm diameter PdNPs by immersing the hetero-core fiber sensor in PdNP suspension for immobilization every 60 seconds from 60 to 300 s. The experimental LSPR spectra were normalized with the spectrum obtained by the first poly-L-lysine layer coated the hetero-core fiber sensor, in which the LSPR was not excited. It is clear that the normalized intensity of the obtained spectra decreased with the immersion time in the PdNP suspension. The decrease in the normalized intensity resulted from the change in the dielectric function at the external surface of the hetero-core fiber. Therefore, it was considered that the number of PdNPs immobilized on the surface of the hetero-core fiber was changed with the immersion time.
Fig. 6. Experimental LSPR spectra obtained by immersing a hetero-core fiber sensor in PdNP suspension for immobilization of PdNP every 60 seconds from 60 to 300 seconds.
As shown in Fig. 6, the obtained broad LSPR spectra could be evaluated by light intensity observation at the specific wavelength of 850 nm. Figure 7 shows comparison between experimental results of hetero-core fiber sensors with PdNPs at normalized intensities, NI, of 0.879, 0.787, and 0.741 with PdNPs immobilized at wavelength 850 nm and calculated LSPR spectra with the number of Pd nanoparticles $\textrm {N}_{\textrm {PdNP}}$ of $1.8\times 10^{19}$, $3.5\times 10^{19}$ and $4.5\times 10^{19}$ for the length 15 mm in Lambert-Beer law [33]. The number of the PdNPs in the calculated spectra were adjusted to match the NI of the experimentally obtained spectrum at wavelength 850 nm. To reproduce the experimentally observed spectra in Fig. 7, the number of PdNPs was assumed to be the order of $10^{19}$.
Fig. 7. Comparison between experimental results of a herero-core fiber sensors with Pd nanoparticles at normalized intensities, NI, of 0.879, 0.787 and 0.741 with PdNP immobilized at wavelength 850 nm and calculated LSPR spectra with the amount of Pd nanoparticles $N_{PdNP}$ of $1.8\times 10^{19}$, $3.5\times 10^{19}$ and $4.5\times 10^{19}$ the length 15 mm in Lambert-Beer law.
Both of calculated and experimental spectra for $\textrm {N}_{\textrm {PdNP}}$, $1.8 \times 10^{19}$ and normalized intensity, 0.879, showed good agreement with their overall spectral shape. On the other hand, although the other spectra of normalized intensity, 0.787 and 0.741, and their calculated spectra also roughly matched in spectral shape but only at the point that normalized intensity monotonically decreased as wavelength decrease. This is probably because PdNPs on the sensor were immobilized nonuniformly in fabrication process, and that made a variation of the sensor characteristics due to the different broadness of the absorption spectra.
Focusing on the calculated spectra, the normalized intensity spectra was shifted to lower intensity level when the number of PdNPs was increased. Therefore, it was found that the number of PdNPs increases with the immersion time to the PdNPs suspension, because the experimentally obtained normalized intensity at wavelength of 850 nm was decreased with increase of immersion time in the PdNP suspension as shown in Fig. 6. As a result, the immersion time in the PdNP suspension can control the number of PdNPs immobilized on the surface of the fiber by monitoring the normalized intensity at a certain wavelength, such as 850 nm.
Figure 8 show the optical loss changes in the hetero-core fiber optic LSPR sensors, which were fabricated by adjusting the amount of PdNP immobilized, with 0% and 4% $\textrm {H}_{\textrm {2}}$ gas in atmospheric $\textrm {N}_{\textrm {2}}$. The amount of PdNP immobilized was controlled by the NI at 850nm wavelength, and the NI was defined as $\textrm {NI}_{\textrm {850nm}}$. Figure 8(a) shows a comparison of sensor responses fabricated by $\textrm {NI}_{\textrm {850nm}}$ as 0.887, 0.782 and 0.775. Figure 8(a) indicates that optical loss changes, when exposed gas to sensors switched $\textrm {N}_{\textrm {2}}$ to $\textrm {H}_{\textrm {2}}$, increased as $\textrm {NI}_{\textrm {850nm}}$ decreased. It can be said that this was caused by increase of the amount of PdNPs immobilized on a sensor. Optical loss changes were 0.118, 0.227, and 0.295 dB of the normalized intensities, 0.887, 0.782, and 0.755 respectively. On the other hand, Fig. 8(b) indicates the characteristics of three sensors fabricated with $\textrm {NI}_{\textrm {850nm}}$ as the almost same 0.782. They showed almost nearly same optical losses, 0.225, 0.227, and 0.227 dB. Therefore, it was revealed that the sensor sensitivity can be controlled by adjusting the amount of PdNPs immobilized on the sensor as mentioned above in this section.
Fig. 8. Real-time responses in the optical loss changes of hetero- core hydrogen LSPR sensors with 4nm diameter Pd nanoparticles at normalized light intensities of (a) 0.887, 0.782 and 0.775, and (b) about 0.782 for all three sensors with PdNP immobilized at wavelength 850 nm.
Figure 9 shows the optical loss and the response time of hetero-core optical fiber LSPR hydrogen sensors as a function of the NI. Focusing on the optical loss, it was found that the optical loss increased with the NI decrease, which means that the amount of PdNPs immobilized on the fiber was increased. On the other hand, the response time of the hetero-core optical fiber LSPR hydrogen sensors kept their rapid response time less than 2.0 s. This means that it is possible for the hetero-core optical fiber LSPR hydrogen sensor to simultaneously achieve high sensitivity and fast response time. The conventional hetero-core hydrogen SPR sensor with a Pd thin film indicated the inevitable trade-off between sensitivity and response time. Therefore, employing the PdNPs for the sensitive material to the hydrogen gas, the hetero-core fiber hydrogen sensor resolved the trade-off issue which inevitably existed in the previously reported SPR sensors.
Fig. 9. Optical losses (circles) and response times (squares) of hetero-core fiber LSPR hydrogen sensors as a function of normalized light intensity with PdNP immobilized at wavelength 850 nm.
4. Conclusion
A hetero-core optical fiber LSPR hydrogen sensor has been developed using PdNPs as the sensitive layer. The PdNPs were immobilized on the hetero-core fiber sensor by electrostatic interaction as a simple wet process. As a result, the hetero-core fiber optic LSPR hydrogen sensor successfully detected hydrogen based on the difference between the light intensity spectra with and without $\textrm {H}_{\textrm {2}}$ in atmospheric $\textrm {N}_{\textrm {2}}$. In a light-intensity-based experiment at the near infrared wavelength of 850 nm, the typical optical loss change, the response and recovery times of 0.23 dB and 1.5 and 3.2 s, respectively, were observed for 4% hydrogen, which may be one of the fastest response times among optical fiber hydrogen sensors. In addition, we showed how to control sensor characteristics by monitoring spectra of PdNPs immobilizations to the hetero-core fiber optic. In the experiment based on the light intensity mode using an 850 nm LED light source , LSPR hydrogen sensors with different PdNPs immobilized on the fiber were fabricated and the sensitivity increased with the amount of PdNPs. The response time could be kept to be less than 2.0 s with increasing the sensitivity. It can be said that response time less than 2.0 s is sufficiently rapid enough to realize a hydrogen sensor in actual field.
Disclosures
The authors declare no conflicts of interest.
References
1. U. Maas and J. Warnatz, “Ignition processes in hydrogen oxygen mixtures,” Combust. Flame 74(1), 53–69 (1988). [CrossRef]
2. H. Gu, Z. Wang, and Y. Hu, “Hydrogen gas sensors based on semiconductor oxide nanostructures,” Sensors 12(5), 5517–5550 (2012). [CrossRef]
3. D. V. Esposito, I. Levin, T. P. Moffat, and A. A. Talin, “H2 evolution at Si-based metal-insulator-semiconductor photoelectrodes enhanced by inversion channel charge collection and H spillover,” Nat. Mater. 12(6), 562–568 (2013). [CrossRef]
4. J.-K. Park, S. Takagi, and M. Takenaka, “InGaAsP Mach–Zehnder interferometer optical modulator monolithically integrated with InGaAs driver MOSFET on a III-V CMOS photonics platform,” Opt. Express 26(4), 4842–4852 (2018). [CrossRef]
5. Y. Luo, C. Zhang, B. Zheng, X. Geng, and M. Debliquy, “Hydrogen sensors based on noble metal doped metal-oxide semiconductor: A review,” Int. J. Hydrogen Energy 42(31), 20386–20397 (2017). [CrossRef]
6. T. Hübert, L. Boon-Brett, G. Black, and U. Banach, “Hydrogen sensors - A review,” Sens. Actuators, B 157(2), 329–352 (2011). [CrossRef]
7. L. Boon-Brett, G. Black, P. Moretto, and J. Bousek, “A comparison of test methods for the measurement of hydrogen sensor response and recovery times,” Int. J. Hydrogen Energy 35(14), 7652–7663 (2010). [CrossRef]
8. Y. nan Zhang, H. Peng, X. Qian, Y. Zhang, G. An, and Y. Zhao, “Recent advancements in optical fiber hydrogen sensors,” Sens. Actuators, B 244, 393–416 (2017). [CrossRef]
9. W. C. Hsu, C. C. Chan, C. H. Peng, and C. C. Chang, “Hydrogen sensing characteristics of an electrodeposited WO3 thin film gasochromic sensor activated by Pt catalyst,” Thin Solid Films 516(2-4), 407–411 (2007). [CrossRef]
10. X. Zhou, Y. Dai, J. M. Karanja, F. Liu, and M. Yang, “Microstructured FBG hydrogen sensor based on Pt-loaded WO3,” Opt. Express 25(8), 8777–8786 (2017). [CrossRef]
11. S. Ippolito, S. Kandasamy, K. Kalantar-zadeh, and W. Wlodarski, “Hydrogen sensing characteristics of WO3 thin film conductometric sensors activated by Pt and Au catalysts,” Sens. Actuators, B 108(1-2), 154–158 (2005). [CrossRef]
12. M. Ando, R. Chabicovsky, and M. Haruta, “Optical hydrogen sensitivity of noble metal-tungsten oxide composite films prepared by sputtering deposition,” Sens. Actuators, B 76(1-3), 13–17 (2001). [CrossRef]
13. Z. Zhao and M. A. Carpenter, “Annealing enhanced hydrogen absorption in nanocrystalline Pd/Au sensing films,” J. Appl. Phys. 97(12), 124301 (2005). [CrossRef]
14. R. R. J. Maier, B. J. S. Jones, J. S. Barton, S. McCulloch, T. Allsop, J. D. C. Jones, and I. Bennion, “Fibre optics in palladium-based hydrogen sensing,” J. Opt. A: Pure Appl. Opt. 9(6), S45–S59 (2007). [CrossRef]
15. F. Lewis, “Hydrogen in palladium and palladium alloys,” Int. J. Hydrogen Energy 21(6), 461–464 (1996). [CrossRef]
16. R. Tabassum and B. D. Gupta, “Surface plasmon resonance-based fiber-optic hydrogen gas sensor utilizing palladium supported zinc oxide multilayers and their nanocomposite,” Appl. Opt. 54(5), 1032–1040 (2015). [CrossRef]
17. S. P. Usha, S. K. Mishra, and B. D. Gupta, “Fiber optic hydrogen sulfide gas sensors utilizing ZnO thin film/ZnO nanoparticles: A comparison of surface plasmon resonance and lossy mode resonance,” Sens. Actuators, B 218, 196–204 (2015). [CrossRef]
18. J. Villatoro and D. Monzón-Hernández, “Fast detection of hydrogen with nano fiber tapers coated with ultra thin palladium layers,” Opt. Express 13(13), 5087–5092 (2005). [CrossRef]
19. R. Chen, W. Liu, G. Huang, D. Wang, X. Qin, and W. Feng, “Hydrogen sulfide sensor based on tapered fiber sandwiched between twomolybdenum disulfide/citric acid composite membrane coated long-periodfiber gratings,” Appl. Opt. 57(33), 9755–9759 (2018). [CrossRef]
20. J. Dai, M. Yang, X. Yu, and H. Lu, “Optical hydrogen sensor based on etched fiber Bragg grating sputtered with Pd/Ag composite film,” Opt. Fiber Technol. 19(1), 26–30 (2013). [CrossRef]
21. J. Dai, M. Yang, Z. Yang, Z. Li, Y. Wang, G. Wang, Y. Zhang, and Z. Zhuang, “Performance of fiber bragg grating hydrogen sensor coated with Pt-loaded WO3 coating,” Sens. Actuators, B 190, 657–663 (2014). [CrossRef]
22. S. Cai, A. González-Vila, X. Zhang, T. Guo, and C. Caucheteur, “Palladium-coated plasmonic optical fiber gratings for hydrogen detection,” Opt. Lett. 44(18), 4483–4486 (2019). [CrossRef]
23. A. Hosoki, M. Nishiyama, H. Igawa, Y. Choi, and K. Watanbea, “Surface plasmon resonance hydrogen sensor based on hetero-core optical fiber structure,” Procedia Eng. 47, 128–131 (2012). [CrossRef]
24. A. Hosoki, M. Nishiyama, H. Igawa, A. Seki, Y. Choi, and K. Watanabe, “A surface plasmon resonance hydrogen sensor using Au/Ta2O5/Pd multi-layers on hetero-core optical fiber structures,” Sens. Actuators, B 185, 53–58 (2013). [CrossRef]
25. A. Hosoki, M. Nishiyama, H. Igawa, A. Seki, and K. Watanabe, “A hydrogen curing effect on surface plasmon resonance fiber optic hydrogen sensors using an annealed Au/Ta2O5/Pd multi-layers film,” Opt. Express 22(15), 18556–18563 (2014). [CrossRef]
26. D. Komatsu, A. Hosoki, M. Nishiyama, H. Igawa, and K. Watanabe, “Response characteristics of annealed Pd film on hetero-core optical fiber structure,” Photonics and Fiber Technology 2016 (ACOFT, BGPP, NP) (Optical Society of America, 2016), p. JM6A.8.
27. A. Hosoki, M. Nishiyama, N. Sakurai, H. Igawa, and K. Watanabe, “Long-term hydrogen detection using a hetero-core optical fiber structure featuring Au/Ta2O5/Pd/Pt multilayer films,” IEEE Sens. J. 20(1), 227–233 (2020). [CrossRef]
28. M. Yamauchi and H. Kitagawa, “Hydrogen absorption of the polymer-coated Pd nanoparticle,” Synth. Met. 153(1-3), 353–356 (2005). [CrossRef]
29. K. M. Mayer and J. H. Hafner, “Localized surface plasmon resonance sensors,” Chem. Rev. 111(6), 3828–3857 (2011). [CrossRef]
30. C. C. Ndaya, N. Javahiraly, and A. Brioude, “Recent advances in palladium nanoparticles-based hydrogen sensors for leak detection,” Sensors 19(20), 4478 (2019). [CrossRef]
31. W. J. Buttner, M. B. Post, R. Burgess, and C. Rivkin, “An overview of hydrogen safety sensors and requirements,” Int. J. Hydrogen Energy 36(3), 2462–2470 (2011). [CrossRef]
32. M. Kracker, C. Worsch, and C. Rüssel, “Optical properties of palladium nanoparticles under exposure of hydrogen and inert gas prepared by dewetting synthesis of thin-sputtered layers,” J. Nanopart. Res. 15(4), 1594 (2013). [CrossRef]
33. M. Fox, Optical Properties of Solids, 2nd edition, (Oxford university press, 2010), chap. 1, pp. 1–23.
