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Abstract
An all-fiber humidity sensor is proposed and fabricated by depositing three-dimensional graphene network (3DGN) around the surface of a freestanding microfiber (MF). The high specific surface area and porosity of 3DGN enhances its interaction with water molecules, allowing high performance of the humidity sensor. The sensor can operate in a wide relative humidity (RH) range of 11.6%RH-90.9%RH with a high sensitivity of -2.841 dB/%RH in the RH range (80.3%RH - 90.9%RH). The response and recovery times of this type of microfiber sensor are measured respectively to be 57 ms and 55 ms, which are one order magnitude faster than those of other fiber RH sensors activated by two-dimensional materials coating. Such an all-fiber RH sensor with high sensitivity and fast response property possesses great potential of application in widespread fields, such as biology, chemical processing and food processing.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Humidity measurement and control are essential in agriculture, food, and medical. For example, in the process of mass production of compounds, humidity control is a key point for quality assurance [1–4]. Therefore, it is necessary to develop a high-precision and fast-responding humidity sensor.
Compared with the traditional electric humidity sensors, the fiber optic humidity sensors possess some advantages such as low cost, immunity to electromagnetic radiation and corrosion, remote operation, and the capability to work in hazardous environments. Lots of efforts have been spent for improving the sensitivity and response time of the humidity sensor based on optical fiber. Various optical fiber structures were used for humidity sensing including side-polished fibers [5,6], fiber Bragg gratings [7–9], long period fiber gratings [10,11], and microfibers [12,13]. Since the bare optical fiber is insensitive to the water molecules, moisture sensitive materials are necessary to improve the humidity sensitivity of the fiber sensor. Various moisture sensitive materials such as gelatin [6], polyvinyl alcohol [14], tungsten disulfide [15], zinc oxide (ZnO) [16–18], titanium dioxide (TiO2) [19], carbon nanotubes, graphene oxide (GO) [20] and reduced graphene oxide (rGO) [21] were used to improve the performance of optical fiber humidity sensor and great success had been achieved [22].
The graphene family, including graphene oxide (GO) and reduced graphene oxide (rGO), is considered to be excellent humidity sensitizing material because of its unique photoelectric properties, high sensitivity to changes in the external environment, and strong adsorption capacity for water molecules. In 2014, Xiao et al. prepared a layer of reduced graphene oxide on the side polished fiber to prepare a humidity sensor with a response time of 5s and a recovery time of 29 s in the range of humidity 70%RH-95%RH and its sensitivity was 0.31 dB/%RH [23]. In 2016, Gao et al. deposited a reduced graphene oxide (rGO) on the outer surface of a hollow core fiber to prepare a novel humidity sensor based on power leakage at resonance wavelengths. The response time of the sensor was 5.2s, the recovery time was 8.1s, and its sensitivity was 0.22dB/%RH within the 60%RH-90%RH humidity range [24]. Beside graphene family, various two dimensional materials were also used as activated materials. For example, Luo et al. demonstrated a new all-fiber moisture sensor consisting of a WS2 film overlay on a side-polished fiber (SPF). In the range of relative humidity (RH) 35%RH-85%RH, the response time reached 1 s, the recovery time reached 5 s, and the sensitivity was 0.1213 dB/%RH [15]. In the above works, the response and recovery times were both on second scale. In 2017, Li et al. combined MoS2 with side-polished fiber to produce a humidity sensor with a response/recovery time of 0.85 s in the RH range of 40%RH – 85%RH. [25] In the same year, by applying MoS2 to an etched single mode fiber, Du et al. successfully demonstrated a humidity sensor with fast response time and recovery time (66ms response time, 2.395s recovery time) [26]. Although the response time or recovery time was less than a second by using MoS2 as activated material, the sum of the response and recovery time was still on second scale.
In recent years, three-dimensional structures have been used in electrical sensors based on graphene family to increase their sensitivity [27,28]. In 2019, by coating with rGO/polystyrene microspheres, we have a kind of microfiber humidity sensor used a three-dimensional graphene network (3DGN) structure as a sensitizing layer. The sensor exhibits high humidity sensitivity with a sensitivity of -4.118 dB/% RH in the humidity range of 79.5% RH -85.0% RH, which were an order magnitude higher than that of the optical fiber humidity sensors activated by graphene family. Although this sensor possesses an ultrahigh sensitivity, the response and recovery times of this sensor were large (4s and 23.7s, respectively) [29]. It is due to the structure of 3DGN coating. Since the 3DGN coating was fabricated by dropping the suspension onto the microfiber with substrate, the microfiber sensor was adhered on the substrate. The substrate slowed down the adsorption and release speed of the water molecules.
In this paper, the suspended self-assembling method was introduced to fabricate 3DGN-covered microfiber RH sensor. The polystyrene (PS) microspheres coating with rGO nanosheets were self-assembling on the surface of a suspended microfiber. By using the suspended self-assembling method, the microfiber RH sensor was freestanding without substrate. Due to the high specific surface area and porosity of 3DGN around the freestanding microfiber, the interaction between water molecules and the 3DGN cladding is enhanced, allowing the sensor to achieve high RH sensitivity. Furthermore, the freestanding mode makes the absorption and release process of water molecules two orders of magnitude faster than the previous work [29]. By adjusting the thickness of the 3DGN cladding, an ultrafast RH response (on microsecond time scale) can be obtained. Such an all-fiber RH sensor with high sensitivity and fast response property possesses great potential of application in widespread fields, such as biology, chemical processing and food processing.
2. Sensor fabrication
The rGO nanosheet-coated PS microspheres (rGO/PS) were prepared by thermodynamically- driven heterocoagulation. The specific process is as follows: First, 15 mL of a 5 mg/mL GO nanosheet suspension is added to the emulsion of 0.69-µm diameter PS microspheres (50 mL, 70 mg/mL) that was synthesized by emulsifier-free emulsion polymerization. After ultra-sonication and stirring for 20 min, 0.3 g of NaCl was added to the mixture and ultra-sonication and stirring continued for a further 10 min. Thereafter, 3 mL of 80% hydrazine hydrate was added to the mixture and kept at 80 °C for 3 hours with mechanical stirring. The product was washed with deionized water several times and dispersed in deionized water. The scanning electron micrograph of the composite particles is shown in Fig. 1. The composite particles produced have an average diameter of 690 nm. The polymer dispersion index is 1.004. The left inset shows the close-up of one single particle, many fine wrinkles irregularly distributed on the surface of microsphere are observed. The Transmission Electron Microscope (TEM) image of one single particle (right inset) indicated that the rGO nanosheets were smoothly coated around the surfaces of the microsphere. As shown on the Fig. 2, the Raman spectrums of pure PS microspheres and rGO/PS microspheres (measured by Nanjing MKNANO Tech. Co., Ltd.) were studied to confirm the presence of the rGO coating on the microsphere. For the pure PS nanoparticles, a main feature peak was observed at 1001cm-1, which is due to the ν1 ring-breathing mode of PS [30]. In contrast, for the rGO/PS microspheres, two strong signal were observed at 1595cm-1 and 1355cm-1, which can be assigned to the G peak and D peak of rGO nanosheet respectively, while the featured peak of PS at 1001cm-1became weak. It implied that the rGO nanosheets were effectively covered around the surface of the PS microspheres. The weak feature peak of PS can be ascribed to the imperfect coverage of rGO to the PSs.
The diameter of the micro/nanofiber is a critical parameter of the micro/nanofiber device [31,32]. To decide the diameter of the microfiber, the optical field distribution of the fundamental mode for the microfiber with 3DGN cladding on transverse plane was calculated by COMSOL Multiphysics. Since the 3DGN cladding are similar to a close-packed structure consisting of rGO/PS microspheres, the effective refractive index of the 3DGN cladding is lower than that of the fiber core. Assume the refractive index of microfiber and 3DGN cladding are 1.463 and 1.458, respectively. In this simulation, the diameter of the microfiber with 3DGN cladding is 20 µm; the diameter of the bare microfiber is 9 µm in Fig. 3(a) and 6 µm in Fig. 3(b). As shown in Fig. 3, the less the diameter of microfiber is, the stronger evanescent wave is generated on the surface of the microfiber. Although the strong evanescent wave can contribute to the sensitivity of the microfiber sensor, decreasing the diameter of microfiber makes it more fragile. Since the surface tension of the rGO/PS suspension will break the microfiber the diameter of the microfiber should not be smaller than 8µm. In our experiment, if the diameter of the microfiber was less than 8 µm, two-thirds of microfiber was broken during the deposition process. Considering the production yield of the sensors the diameter of the fabricated microfiber is about 9 µm.
Fig. 3. The optical field distribution of the fundamental mode for the microfiber with 3DGN cladding on transverse plane. The diameters of the bare microfibers are 9 µm (a) and 6 µm (b), respectively.
The microfiber was made by the flame heating and tapering of standard single mode fiber. As shown in Fig. 4(a), the microfiber was composed of transition region and flat region. The diameter of the flat region of the fabricated microfiber was about 9.4 µm and the lengths of the flat and transition regions of microfiber were about 3.6 mm and 5.5 mm, respectively. The Fig. 4(b) shows the microscope image of the flat region of the fabricated microfiber.
Fig. 4. (a) The schematic of the microfiber structure; (b) The microscope image of the flat region of the microfiber.
The fabricated microfiber was tightened and placed in a 12 mm × 1 mm × 1.2 mm square channel for the suspended self-assemble. The distance between the microfiber and the bottom of channel was about 600 µm. Before the suspended self-assembling process, the rGO/PS composite microspheres suspension was treated by ultra-sonication in a 30°C thermostatic water bath for 5 min to make the suspension uniform and avoid agglomeration. 40 µl of the prepared 2 wt% rGO/PS suspension was dispensed into the channel using a micropipette, as shown by Fig. 5(a). Then the water of the suspension was naturally volatilized in air at room temperature for 3 hours. During this period, the rGO/PS composite microspheres were self-assembled by gravity deposition onto the surface of the microfiber and the microfiber was separated from the channel. After the deposition process, the microfiber fiber with 3DGN coating was lift-off from the substrate and fixed on a glass slide. Since the uneven of the cladding could strongly scatter the propagation mode of the microfiber, leading to a dramatically decreasing of output power, the fabricated cladding should be quite uniform. Figure 5(b) shows a SEM image of a 3DGN coated microfiber, from which one can see that the surface of the microfiber fiber is completely and uniformly covered by the 3DGN cladding. Thus, the output power of the microfiber was decreased only 6 dBm after the deposition of the 3DGN cladding. The uniform coating of 3DGN is due to the uniform diameter of the rGO/PS microspheres whose polymer dispersity index (PDI) is 1.004. The inset is the close-up of the red square region, indicating the diameter of the 3DGN coated microfiber is ∼18 µm.
Fig. 5. (a) The suspended self-assembly process of the 3DNG coating around microfiber; (b) The SEM image of the freestanding microfiber covered by 3DGN; The inset is the close-up of the red square region.
3. Humidity sensing experiments and discussions
The experimental setup for measuring humidity sensing performance is shown in Fig. 6. The optical system comprised a distributed feedback laser at 1550 nm and an optical power meter. The sensor was placed in a controlled humidity chamber. Wet and dry airs were mixed by an air mixer to produce the humid air flow. Humid air, whose humidity was controllable by two flow controller, flowed into the chamber to control the humidity of the chamber. The humidity and temperature in the gas chamber are measured by the commercial humidity/thermometer (175H1, Testo SE & Co. KGaA). The transmission power of the 1550 nm light source passing through the sensor is monitored by an optical power meter. The temperature, humidity, and transmission power of the microfiber sensor during the experiment were recorded by the computer in real time.
During the humidity measurement, the temperature of the chamber was fixed at 20 ℃. As shown in Fig. 3(a), it is clearly that almost all the light was still confined in the microfiber. Considering the low laser power launching into the fiber (-9 dBm / 0.13mW) and and the good thermal conductivity of rGO (∼600W m-1 K-1), the photothermal effect of rGO could be ignored [33].
The RH in the chamber was gradually increased from 11.6%RH to 90.9%RH and then decreased back to 11.6% RH with a step of about 10% RH. Each RH step was kept for 4 minutes. The variations of RH in the chamber and transmission power of sensor over time are depicted in Fig. 7. The transmission power of sensor showed significant sensitivity to humidity. As shown in Fig. 7, the humidity of the chamber was fluctuated in the beginning of some steps, which was caused by the feedback control of the humidity. The transmission power of the sensor closely follows the fluctuation of RH.
The experiment above was repeated to measure the average transmitted power at different humidity levels. The relationship between the transmitted power and the humidity is shown in Fig. 8. When the RH ascending from 11.6% RH to 90.9% RH, the transmitted light power decreased by 41.39 dBm (range from -28.86 dBm to -70.25 dBm). When the RH descending from 90.9%RH to 12.4%RH, the transmitted light power increased by 41.26 dBm. It can be seen from Fig. 8, the black squares (RH ascending) and red circles (RH descending) almost overlap, indicating that this microfiber sensor activated by the 3DGN cladding possess good repeatability. Although the optical transmission and the RH exhibited a nonlinear relationship as a whole, two linear regions can be distinguished. The linear fitting shows that the RH sensitivity of the microfiber sensor is -0.172 dB/%RH in the RH range 11.6%RH -71.3%RH, and the linear correlation coefficient of the fitting curve is 98.9%. In the higher RH range of 80.3% RH - 90.9% RH, the RH sensitivity is as high as -2.841 dB/%RH, and the linear correlation coefficient is 96.9%. This sensitivity is several times higher than those RH sensors (0.427dB/%RH, 0.31dB/%RH) that directly combined GO or rGO nanosheets with fiber [2,8].
Since the PS has low sensitivity to RH, the rGO nanosheets coating on the surface of the PS microspheres is considered as the activated materials. The chemically active defect sites and hydrophilic functional groups of the chemically reduced GO nanosheets make it is liable to absorb water molecules from the external environment. When RH increases, the carrier density of rGO, with holes as the dominant charge carriers, will increase as water molecules are adsorbed, since water molecules act as electron accepters on the rGO surface.
Because the thickness of the rGO covered on the PS microspheres is only a few nanometers, the refractive index of rGO can be calculated by the following two formulas similar to the monolayer graphene [34,35]:
When ${\; \Gamma } = {10^{12}}{\; Hz}$, λ = 1550 nm, T = 298K, and tg = 0.34 nm (the thickness of graphene), the refractive index for the rGO with different chemical potentials (0 ħω to 0.6 ħω) was calculated and shown in the Fig. 9(a).
Fig. 9. (a) The refractive index for the rGO with different chemical potentials (b) The effective refractive index of 3DGN cladding with different chemical potentials (range from 0 ħω to 0.5 ħω)
The effective refractive index can be evaluated by the volume average of refractive indices [36]:
In order to measure the dynamic response of the sensor, the experimental setup of Fig. 6 had been changed a little. The air mixer was substituted for a solenoid valve. Either dry air (humidity: 10%RH) or wet air (humidity: 90%RH) was selected to flow into the gas chamber through the solenoid valve. The humidity of the chamber was changed between high humidity and low humidity periodically controlled by the solenoid valve. A photodetector and an oscilloscope replaced the optical power meter to monitor the dynamic response of the sensor. The response of each sensor was normalized with respect to the values measure at low humidity (VH, dry air flowed into the chamber) and high humidity (VL, wet air flowed into the chamber): VN = (V - VL)/(VH - VL).
Figure 10 shows the repeatability, stability and dynamic response characteristics of the upper 3DGN covered-microfiber sensor, defined as sample S1. The dynamic response of the sensor to a modulated humid air flow at 0.5 Hz is shown in Figs. 10(a) and (b). As shown in Fig. 10(a), the transmitted optical power recovered each cycle to the same level (the red dashed line) each time that the RH changed between high and low humidity, which indicated that the sensor has excellent repeatability and stability. The dynamic response of the sensor to a modulated humid air flow at 0.5 Hz is shown in Fig. 10(b), the response and recovery times of the sensor S1 were 109 ms and 193 ms, respectively.
Fig. 10. (a) Characterization of repeatability and stability of the microfiber sensor; (b) Response of the microfiber sensor to a modulated humid air flow at 0.5 Hz.
The other two samples (S2 and S3) with different humidity sensitivity had been fabricated to investigate the sensor response and recovery times. In the range of 81%RH-90.3%RH, the humidity sensitivities of S2 and S3 were -0.536 dB/%RH and -0.12 dB/%RH, respectively. Figures 11(a) and (b) show the response characteristics of these tow samples, respectively, while the air flowed into the chamber was switched between dry air (10%RH) and wet air (90%RH) at 0.33 Hz. The test results show that they have different response and recovery times. The response times of samples S2 and S3 are 80 ms and 57 ms, respectively. The recovery times of samples S2 and S3 are 139 ms and 55 ms, respectively.
Fig. 11. (a) the response of the sensor S2 to a modulated humid air flow at 0.33 Hz; (b) the response of the sensor S3 to a modulated humid air flow at 0.33 Hz.
Table 1 summaries up the response time and humidity sensitivity for the three samples. It can be seen that the sample with lower humidity sensitivity has faster humidity response and recovery speeds. The main reason is that the absorption and release process of water molecule are faster when the 3DGN cladding is thinner. Furthermore, the response and recovery times of the freestanding microfiber humidity sensor (S3) are two orders of magnitude lower than that of the previous work, in which the microfiber sensor is adhered on the substrate. [29] The freestanding mode makes the absorption and release processes of water molecules faster.
Table 1. Response time and RH sensitivity for four different samples
Table 2 compares the performance of fiber RH fiber sensors with different coated material. It can be seen that the 3DGN coated-microfiber sensor possesses not only highest humidity sensitivity but also fastest recovery time. Furthermore, the sum of response and recovery time (total time) of our sensor is the smallest (0.302 s for the higher sensitivity sample and 0.112 s for the lower sensitivity sample), which is at least more than 8 times smaller than other fiber RH sensors activated with two dimensional materials. The simultaneously fast response and recovery speed promises our 3DGN coated-microfiber sensor having best performance in the RH dynamic response. Since the rGO covering the PS microsphere is quite thin (about 3 nm), the three dimensional graphene network not only enlarge the specific surface area dramatically, but also keep the good properties of monolayer graphene. Due to the high specific surface area the interaction between water molecules and the 3DGN cladding is enhanced, allowing the sensor to achieve high RH sensitivity. Furthermore, by using the suspended self-assembling method the microfiber RH sensor was freestanding without substrate. The freestanding mode makes the absorption and release process of water molecules two orders of magnitude faster than the previous work [29].
Table 2. Comparison of Response and Recovery Time Obtained by Different Methods
4. Conclusions
An ultrafast humidity sensor based on freestanding microfiber with 3DGN cladding had been fabricated. The 3DGN was fabricated by suspended self-assembling method which made it is possible to separate the microfiber sensor from the substrate. Since the 3D graphene network has a larger specific surface area than pure rGO, this type of microfiber sensor can be interacted better with water molecules. The humidity sensing characteristics were demonstrated over the RH range of 11.6%RH to 90.9%RH. The linear fitting shows that the RH sensitivity of the microfiber sensor is -0.172 dB/%RH in the RH range 11.6%RH -71.3%RH with linear correlation coefficient of 98.9%. In the higher RH range of 80.3% RH - 90.9% RH, the RH sensitivity is as high as -2.841 dB/%RH, and the linear correlation coefficient is 96.9%. This sensitivity is several times higher than those RH sensors (0.427dB/%RH, 0.31dB/%RH) that directly combined GO or rGO nanosheets with fiber [2,8]. The freestanding mode of the microfiber sensor accelerates the absorption and release process of water molecules. The sum of response and recovery time is 0.302 s for the higher sensitivity sample, which is at least more than 8 times smaller than other fiber RH sensors activated with two dimensional materials. [15,23–26,37,38] Furthermore, the sample 3 (S3) with lower sensitivity (-0.228 dB/%RH in the range from 81.2%RH to 90.8%RH) possesses ultrafast response and recovery times of 57 ms and 55 ms, respectively. The high sensitivity and fast response properties promise the proposed 3DGN covered freestanding microfiber sensor possesses great potential of application in widespread fields, such as biology, chemical processing and food processing.
Funding
National Natural Science Foundation of China (61401176, 61405075, 61475066, 1505069, 61575084, 61705086); Natural Science Foundation of Guangdong Province (2017A030313359, 2017A030313375); Science & Technology Project of Guangzhou (201803020023); Natural Science for Youth Foundation (61401432); Changjiang Scholar Program of Chinese Ministry of Education (IRT1252).
Disclosures
The authors declare no conflicts of interest.
References
1. G. Y. Chen, X. Wu, Y. Q. Y. Kang, L. Yu, T. M. Y. Monro, D. G. Lancaster, X. k. Liu, and H. L. Xu, “Ultra-fast hygrometer based on U-shaped optical microfiber with nanoporous polyelectrolyte coating,” Sci. Rep. 7(1), 7943 (2017). [CrossRef]
2. S. Borini, R. White, D. Wei, M. Astley, S. Haque, E. Spigone, N. Harris, J. Kivioja, and T. Ryhänen, “Ultrafast Graphene Oxide Humidity Sensors,” ACS Nano 7(12), 11166–11173 (2013). [CrossRef]
3. D. Z. Zhang, J. Tong, and B. K. Xia, “Humidity-sensing properties of chemically reduced graphene oxide/polymer nanocomposite film sensor based on layer-by-layer nano self-assembly,” Sens. Actuators, B 197, 66–72 (2014). [CrossRef]
4. U. Mogera, A. A. Sagade, S. J. George, and G. U. Kulkarni, “Ultrafast response humidity sensor using supramolecular nanofibre and its application in monitoring breath humidity and flow,” Sci. Rep. 4, 4103 (2015). [CrossRef]
5. Y. M. Huang, W. G. Zhu, Z. B. Li, G. L. Chen, L. H. Chen, J. J. Zhou, H. Lin, J. W. Guan, W. X. Fang, X. Liu, H. Z. Dong, J. Y. Tang, H. Y. Guan, H. H. Lu, Y. Xiao, J. Zhang, H. C. Wang, Z. Chen, and J. H. Yu, “High-performance fiberoptic humidity sensor based on a side-polished fibre wavelength selectively coupled with graphene oxide film,” Sens. Actuators, B 255, 57–69 (2018). [CrossRef]
6. L. Tang, Y. M. Feng, Z. S. Xing, Z. Chen, J. H. Yu, H. Y. Guan, H. H. Lu, J. B. Fan, and Y. C. Zhong, “High-sensitivity humidity sensing of side-polished optical fiber with polymer nanostructure cladding,” Appl. Opt. 57(10), 2539 (2018). [CrossRef]
7. B. N. Shivananju, S. Yamdagni, R. Fazuldeen, A. K. S. Kumar, S. P. Nithin, M. M. Varma, and S. Asokan, “Highly sensitive carbon nanotubes coated etched fiber bragg grating sensor for humidity sensing,” IEEE Sens. J. 14(8), 2615–2619 (2014). [CrossRef]
8. P. Kronenberg, P. K. Rastogi, P. Giaccari, and H. G. Limberger, “Relative humidity sensor with optical fiber Bragg gratings,” Opt. Lett. 27(16), 1385 (2002). [CrossRef]
9. T. L. Yeo, T. Sun, K. T. V. Grattan, D. Parry, R. Lade, and B. D. Powell, “Characterisation of a polymer-coated fibre Bragg grating sensor for relative humidity sensing,” Sens. Actuators, B 110(1), 148–156 (2005). [CrossRef]
10. M. Konstantaki, S. Pissadakis, S. Pispas, N. Madamopoulos, and N. A. Vainos, “Optical fiber long-period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating,” Appl. Opt. 45(19), 4567–4571 (2006). [CrossRef]
11. T. Venugopalan, T. Sun, and K. T. V. Grattan, “Long period grating-based humidity sensor for potentialstructural health monitoring,” Sens. Actuators, A 148(1), 57–62 (2008). [CrossRef]
12. H. F. Liu, Y. P. Miao, B. Liu, W. Lin, H. Zhang, B. B. Song, M. D. Huang, and L. Lin, “Relative Humidity Sensor Based on S-Taper Fiber Coated with SiO2 Nanoparticles,” IEEE Sens. J. 15(6), 3424–3428 (2015). [CrossRef]
13. Y. Wu, T. H. Zhang, Y. J. Rao, and Y. Gong, “Miniature interferometric humidity sensors based on silica/polymer microfiber knot resonators,” Sens. Actuators, B 155(1), 258–263 (2011). [CrossRef]
14. A. Gastón, F. Pérez, and J. Sevilla, “Optical fiber relative-humidity sensorwith polyvinyl alcohol film,” Appl. Opt. 43(21), 4127–4132 (2004). [CrossRef]
15. Y. H. Luo, C. Y. Chen, K. Xia, S. H. Peng, H. Y. Guan, J. Y. Tang, H. H. Lu, J. H. Yu, J. Zhang, Y. Xiao, and Z. Chen, “Tungsten disulfide (WS2) based all-fiber-optic humidity sensor,” Opt. Express 24(8), 8956–8966 (2016). [CrossRef]
16. Z. Harith, N. Irawati, M. Batumalay, H. A. Rafaie, G. Yun II, S. W. Harun, R. M. Nor, and H. Ahmad, “Relative Humidity Sensor Employing Optical FibersCoated with ZnO Nanostructures,” Indian J. Sci. Technol. 8(35), 1–5 (2015). [CrossRef]
17. H. A. Rahman, N. Irawati, T. N. R. Abdullah, and S. W. Harun, “PMMA microfiber coated with ZnO nanostructure for the measurement of relative humidity,” IOP Conf. Ser.: Mater. Sci. Eng. 99, 012025 (2015). [CrossRef]
18. N. Irawati, H. A. Rahman, H. Ahmad, and S. W. Harun, “A PMMA microfiber loop resonator based humidity sensor with ZnO nanorods coating,” Measurement 99, 128–133 (2017). [CrossRef]
19. R. Aneesh and S. K. Khijwania, “Titanium dioxide nanoparticle based optical fiberhumidity sensor with linear responseand enhanced sensitivity,” Appl. Opt. 51(12), 2164–2171 (2012). [CrossRef]
20. W. H. Lim, Y. K. Yap, W. Y. Chong, and H. Ahmad, “All-Optical Graphene Oxide Humidity Sensors,” Sensors 14(12), 24329–24337 (2014). [CrossRef]
21. H. Ahmad, M. T. Rahman, S. N. A. Sakeh, M. Z. A. Razak, and M. Z. Zulkifli, “Humidity sensor based on microfiber resonator with reduced graphene oxide,” Optik 127(5), 3158–3161 (2016). [CrossRef]
22. Y. Peng, Y. Zhao, M.-Q. Chen, and F. Xia, “Research advances in microfiber humidity sensors,” Small 14(29), 1800524 (2018). [CrossRef]
23. Y. Xiao, J. Zhang, X. Cai, S. Z. Tan, J. H. Yu, H. H. Lu, Y. H. Luo, G. Z. Liao, S. P. Li, J. Y. Tang, and Z. Chen, “Reduced graphene oxide for fiber-optic humidity sensing,” Opt. Express 22(25), 31555–31567 (2014). [CrossRef]
24. R. Gao, D.-F. Lu, J. Cheng, Y. Jiang, L. Jiang, and Z.-M. Qi, “Humidity sensor based on power leakage at resonance wavelengths of a hollow core fiber coated with reduced graphene oxide,” Sens. Actuators, B 222, 618–624 (2016). [CrossRef]
25. D. Q. Li, H. H. Lu, W. T. Qiu, J. L. Dong, H. Y. Guan, W. G. Zhu, J. H. Yu, Y. H. Luo, J. Zhang, and Z. Chen, “Molybdenum disulfide nanosheets deposited on polished optical fiber for humidity sensing and human breath monitoring,” Opt. Express 25(23), 28407–28416 (2017). [CrossRef]
26. B. B. Du, D. X. Yang, X. Y. She, Y. Yuan, D. Mao, Y. J. Jiang, and F. F. Lu, “MoS2-based all-fiber humidity sensor for monitoring human breath with fast response and recovery,” Sens. Actuators, B 251, 180–184 (2017). [CrossRef]
27. J. J. Song, Y. Y. Wang, F. Zhang, Y. Ye, Y. H. Liu, X. H. Zhou, L. S. Chen, and C. S. Peng, “Polymer foam-supported chemically reduced graphene oxide conductive networks for gas sensing,” J. Nanosci. Nanotechnol. 18(4), 2965–2970 (2018). [CrossRef]
28. C. F. Cheng, C. M. Zhang, X. H. Gao, Z. H. Zhuang, C. Du, and W. Chen, “3d network and 2d paper of reduced graphene oxide/cu2o composite for electrochemical sensing of hydrogen peroxide,” Anal. Chem. 90(3), 1983–1991 (2018). [CrossRef]
29. Z. S. Xing, Y. Zheng, Z. F. Yan, Y. M. Feng, Y. Xiao, J. H. Yu, H. Y. Guan, Y. H. Luo, Z. Q. Wang, Y. C. Zhong, and Z. Chen, “High-sensitivity humidity sensing of microfiber coated with three-dimensional graphene network,” Sens. Actuators, B 281, 953–959 (2019). [CrossRef]
30. R. Venkatachalam, F. Boerio, P. Roth, and W. Tsai, “Surface-enhanced Raman scattering from bilayers of polystyrene, diglycidyl ether of bisphenol-A, poly (4-vinyl pyridine), and poly (4-styrene sulfonate),” J. Polym. Sci., Part B: Polym. Phys. 26(12), 2447–2461 (1988). [CrossRef]
31. S. Liu, Z. Li, Z. Weng, Y. Li, L. Shui, Z. Jiao, Y. Chen, A. Luo, X. Xing, and S. He, “Miniaturized optical fiber tweezers for cell separation by optical force,” Opt. Lett. 44(7), 1868–1871 (2019). [CrossRef]
32. H. Xin, C. Cheng, and B. Li, “Trapping and delivery of Escherichia coli in a microfluidic channel using an optical nanofiber,” Nanoscale 5(15), 6720–6724 (2013). [CrossRef]
33. J. Zheng, X. Xing, J. Evans, and S. He, “Optofluidic vortex arrays generated by graphene oxide for tweezers, motors and self-assembly,” NPG Asia Mater. 8(4), e257 (2016). [CrossRef]
34. G. W. Hanson, “Dyadic Green’s functions and guided surface waves for a surface conductivity model of graphene,” J. Appl. Phys. 103(6), 064302 (2008). [CrossRef]
35. M. Mohsin, D. Neumaier, D. Schall, M. Otto, C. Matheisen, A. L. Giesecke, A. A. Sagade, and H. Kurz, “Experimental verification of electro-refractive phase modulation in graphene,” Sci. Rep. 5(1), 10967 (2015). [CrossRef]
36. P. Chýlek, V. Srivastava, R. G. Pinnick, and R. T. Wang, “Scattering of electromagnetic waves by composite spherical particles: experiment and effective medium approximations,” Appl. Opt. 27(12), 2396–2404 (1988). [CrossRef]
37. T. H. Ouyang, L. M. Lin, K. Xia, M. J. Jiang, Y. W. Lang, H. Y. Guan, J. H. Yu, D. Q. Li, G. L. Che, W. G. Zhu, Y. C. Zhong, J. Y. Tang, J. L. Dong, H. H. Lu, Y. H. Luo, J. Zhang, and Z. Chen, “Enhanced optical sensitivity of molybdenum diselenide (MoSe2) coated side polished fiber for humidity sensing,” Opt. Express 25(9), 9823–9833 (2017). [CrossRef]
38. H. Y. Guan, K. Xia, C. Y. Chen, Y. H. Luo, J. Y. Tang, H. H. Lu, J. H. Yu, J. Zhang, Y. C. Zhong, and Z. Chen, “Tungsten disulfide wrapped on micro fiber for enhanced humidity sensing,” Opt. Mater. Express 7(5), 1686–1696 (2017). [CrossRef]
