Tin Disulﬁde-Coated Microﬁber for Humidity Sensing with Fast Response and High Sensitivity

: Breath monitoring is signiﬁcant in assessing human body conditions, such as cardiac and pulmonary symptoms. Optical ﬁber-based sensors have attracted much attention since they are immune to electromagnetic radiation, thus are safe for patients. Here, a microﬁber (MF) humidity sensor is fabricated by coating tin disulﬁde (SnS 2 ) nanosheets onto the surface of MF. The small diameter (~8 µ m) and the long length (~5 mm) of the MF promise strong interaction between guiding light and SnS 2 . Thus, a small variation in the relative humidity (RH) will lead to a large change in optical transmitted power. A high RH sensitivity of 0.57 dB/%RH is therefore achieved. The response and recovery times are estimated to be 0.08 and 0.28 s, respectively. The high sensitivity and fast response speed enable our SnS 2 -MF sensor to monitor human breath in real time.


Introduction
Relative humidity (RH) measurement and monitoring are important for industry, food storage, human comfort, etc. [1][2][3]. In recent decades, various optical fiber devices have been developed for this purpose [1]. The optical fiber sensors show advantages over electronic ones, owing to their long lifetime, corrosion-free, light-weight, electromagnetic immunity, and remote sensing ability [4,5]. Microfiber and side-polished optical fiber are two kinds of optical fibers used frequently in the RH sensing [1]. To enhance the RH sensing performance, certain sensitive materials are deposited on the surface of microfiber/side-polished fiber [6,7]. In 2000, agarose gel was coated on a tapered fiber by Baríain et al. [8]. A transmitted optical power variation of 6.5 dB was obtained with a relative RH changing between 30% and 80% [8]. Recently, two dimensional (2D) materials have attracted increasing attention [9][10][11]. Luo et al. demonstrated an all-fiber-optic RH sensor comprised of a Tungsten disulfide (WS 2 ) film overlaid on a side polished fiber [11]. This sensor has a linear correlation coefficient of 99.39% [10]. Lately, Huang et al. demonstrated a high-sensitivity (0.145 nm/RH%) RH sensor by taking advantage of the swelling effect of graphene oxide film [12]. Du et al. fabricated a MoS 2 -based all-fiber RH sensor with fast response and recovery [13]. Monitoring human breath is significant for assessing human body conditions, such as cardiac and pulmonary symptoms [14,15]. Many fiber-optical RH sensors have been successfully used for monitoring human breathing [12,13]. However, ultrafast response and recovery times are required in some RH sensing application. As illustrated by [15], the infant pulmonary function testing requires a sampling rate of more than 200 Hz, which is still challenging for fiber-optical RH sensors.
Layered transition metal dichalcogenides (TMDs) have attracted much attention recently [16,17]. Compared with MoS 2 and WS 2 , Tin disulfide (SnS 2 ) has a larger bandgap of 2.18-2.44 eV and a higher carrier mobility of~50 cm 2 ·V −1 ·s −1 [18], which guarantee its potential applications in field effect transistors [19], fast photodetectors [20], lithium-ion batteries [21,22], and visible light sensitive photo-catalyst [23]. SnS 2 has been widely used in gas-sensing [24][25][26]. It was demonstrated that SnS 2 nanostructures exhibit a good response and reversibility to some organic gases, such as ethanol and n-butanol [25]. Moreover, the 2D SnS 2 was shown to have a selective and reversible response for nitrogen dioxide (NO 2 ) with a detection limit down to 30 ppb [26]. In 2016, Bharatula fabricated an electric RH sensor based on SnS 2 [27]. However, the optical RH sensing property of SnS 2 has still not been exploited.
Here, a high-performance RH sensor is fabricated by coating SnS 2 nanosheets onto the surface of MF. The RH sensor based on SnS 2 is demonstrated to have a high RH sensitivity of 0.57 dB/%, and fast response and recovery times of 0.08 s and 0.28 s, respectively.

Materials and Methods
The SnS 2 suspension is purchased from Mukenano co., and is made by liquid phase exfoliation method [28]. The Raman spectrum of the SnS 2 film is excited by a 488 nm laser and measured by LabRAM HR Evolution (HORIBA JY, France) at room temperature. The measured result is shown in Figure 1a. The peak at 314 cm −1 is the characteristic peak of SnS 2 [18]. Figure 1b shows the absorption spectra measured by UV-visible spectroscopy. There are two main peaks of 213 nm and 256 nm in the absorption spectra, as illustrated in [29]. The absorption decreases gradually when the wavelength increases from 252 nm to 600 nm.
Crystals 2021, 11, x FOR PEER REVIEW 2 of 10 of graphene oxide film [12]. Du et al. fabricated a MoS2-based all-fiber RH sensor with fast response and recovery [13]. Monitoring human breath is significant for assessing human body conditions, such as cardiac and pulmonary symptoms [14,15]. Many fiber-optical RH sensors have been successfully used for monitoring human breathing [12,13]. However, ultrafast response and recovery times are required in some RH sensing application. As illustrated by [15], the infant pulmonary function testing requires a sampling rate of more than 200 Hz, which is still challenging for fiber-optical RH sensors. Layered transition metal dichalcogenides (TMDs) have attracted much attention recently [16,17]. Compared with MoS2 and WS2, Tin disulfide (SnS2) has a larger bandgap of 2.18-2.44 eV and a higher carrier mobility of ~50 cm 2 ·V −1 ·s −1 [18], which guarantee its potential applications in field effect transistors [19], fast photodetectors [20], lithium-ion batteries [21,22], and visible light sensitive photo-catalyst [23]. SnS2 has been widely used in gas-sensing [24][25][26]. It was demonstrated that SnS2 nanostructures exhibit a good response and reversibility to some organic gases, such as ethanol and n-butanol [25]. Moreover, the 2D SnS2 was shown to have a selective and reversible response for nitrogen dioxide (NO2) with a detection limit down to 30 ppb [26]. In 2016, Bharatula fabricated an electric RH sensor based on SnS2 [27]. However, the optical RH sensing property of SnS2 has still not been exploited.
Here, a high-performance RH sensor is fabricated by coating SnS2 nanosheets onto the surface of MF. The RH sensor based on SnS2 is demonstrated to have a high RH sensitivity of 0.57 dB/%, and fast response and recovery times of 0.08 s and 0.28 s, respectively.

Materials and Methods
The SnS2 suspension is purchased from Mukenano co., and is made by liquid phase exfoliation method [28]. The Raman spectrum of the SnS2 film is excited by a 488 nm laser and measured by LabRAM HR Evolution (HORIBA JY, France) at room temperature. The measured result is shown in Figure 1a. The peak at 314 cm −1 is the characteristic peak of SnS2 [18]. Figure 1b shows the absorption spectra measured by UV-visible spectroscopy. There are two main peaks of 213 nm and 256 nm in the absorption spectra, as illustrated in [29]. The absorption decreases gradually when the wavelength increases from 252 nm to 600 nm. The MF shown in Figure 2a is manufactured with "flame-brushing" technique [30] from a single-mode fiber (Corning SMF-28e), and has a core diameter of 8.2 μm and a cladding diameter of 125 μm. With a drawing speed of 0.2 mm/s, a MF with a diameter of ~8.18 μm in the uniform waist region is fabricated, as shown in Figure 2b, where an image of the central MF region by optical microscope is given. The dimeter changing along the MF is obtained by a series of microscopy images captured when moving the MF step by step, and the result is shown by Figure 2c. The length of uniform waist region is measured to be ~5 mm. The MF is then fixed on a glass slide by ultraviolet curing adhesive (Loctite The MF shown in Figure 2a is manufactured with "flame-brushing" technique [30] from a single-mode fiber (Corning SMF-28e), and has a core diameter of 8.2 µm and a cladding diameter of 125 µm. With a drawing speed of 0.2 mm/s, a MF with a diameter of~8.18 µm in the uniform waist region is fabricated, as shown in Figure 2b, where an image of the central MF region by optical microscope is given. The dimeter changing along the MF is obtained by a series of microscopy images captured when moving the MF step by step, and the result is shown by Figure 2c. The length of uniform waist region is measured to be~5 mm. The MF is then fixed on a glass slide by ultraviolet curing adhesive (Loctite 352, Henkel Loctite Asia Pacific). In addition, a basin (15 mm × 5 mm × 1 mm) is constituted by using the UV adhesive to contain SnS 2 solution.
power is monitored. The SnS2 alcohol suspension is dropped into the basin and evaporated naturally in ambient surrounding. During the alcohol evaporation, the SnS2 nanosheets are self-assembling onto the MF owing to the physisorption effect, which results in the change of transmitted power, as shown by Figure 3b. After ~115 min, the power is stable at −35 dBm, indicating the complement of the self-assembly process. Recently, Zhong et al. have developed a suspended self-assembling process [31], which can improve the RH response time.  For the RH sensing, we combine a MF with SnS 2 nanosheets. This is because the controllable diameter of MF can result in the strong interaction between light field and SnS 2 nanosheets. In addition, the large specific surface area of SnS 2 nanosheets enhances the absorption of water molecules. The method of depositing SnS 2 nanosheets on the MF is based on a self-assembly method. The concentration of SnS 2 alcohol suspension is 1 mg/mL. To avoid agglomeration, the SnS 2 suspension is treated by ultrasonication for 60 min. The MF is fixed in a basin, as shown by Figure 2a. As shown by Figure 3a, a 1550 nm distributed feedback (DFB) laser is launched into the MF, and the optical transmitted power is monitored. The SnS 2 alcohol suspension is dropped into the basin and evaporated naturally in ambient surrounding. During the alcohol evaporation, the SnS 2 nanosheets are self-assembling onto the MF owing to the physisorption effect, which results in the change of transmitted power, as shown by Figure 3b. After~115 min, the power is stable at −35 dBm, indicating the complement of the self-assembly process. Recently, Zhong et al. have developed a suspended self-assembling process [31], which can improve the RH response time.  Figure 4c shows the SEM image of the cross section of MF, from which the thickness of the SnS2 film is estimated to be ~161 nm.   Figure 4c shows the SEM image of the cross section of MF, from which the thickness of the SnS 2 film is estimated to be~161 nm.  Figure 4c shows the SEM image of the cross section of MF, from which the thickness of the SnS2 film is estimated to be ~161 nm.

Results
The RH sensing schematic is shown in Figure 5. A light from 1550 nm DFB laser source (SOF-155-D DFB LASER) is sent into MF coated with SnS 2 , which is placed in a temperaturehumidity chamber (BPS-100CL). The MF transmitted light is measured by optical power meter. The RH in the chamber is monitored by a commercial humidity sensor (Testo 175H1) in real time. During the RH sensing experiments, the chamber temperature is fixed at 27 • C, while the RH in the chamber ranges from~55 %RH to~100 %RH. Both the optical transmitted power and the RH were recorded during the whole experimental process.

Results
The RH sensing schematic is shown in Figure 5. A light from 1550 nm DFB laser source (SOF-155-D DFB LASER) is sent into MF coated with SnS2, which is placed in a temperature-humidity chamber (BPS-100CL). The MF transmitted light is measured by optical power meter. The RH in the chamber is monitored by a commercial humidity sensor (Testo 175H1) in real time. During the RH sensing experiments, the chamber temperature is fixed at 27 °C, while the RH in the chamber ranges from ~55 %RH to ~100 %RH. Both the optical transmitted power and the RH were recorded during the whole experimental process. To test the RH sensing property, the RH in the chamber is increased and then decreased with by a step of 13 %RH in the RH range of 55% to 95%. With the change of the chamber RH, the transmitted optical power of MF coated with SnS2 varies accordingly, as shown by Figure 6a. The variation is ~22.5 dB, ranging from ~−35 dBm to ~−13 dBm. The relationship between the transmitted power and the humidity is depicted in Figure 6b. The transmitted power in humidity ascending and descending processes are well overlapped. The transmitted power changes linearly with the chamber humidity. The sensitivity is ~0.57 dB/%RH with the R-square of ~0.98. The high sensitivity and good linearity of the SnS2 nanosheets is appealing in the humidity sensing.  To test the RH sensing property, the RH in the chamber is increased and then decreased with by a step of 13 %RH in the RH range of 55% to 95%. With the change of the chamber RH, the transmitted optical power of MF coated with SnS 2 varies accordingly, as shown by Figure 6a. The variation is~22.5 dB, ranging from~−35 dBm to~−13 dBm. The relationship between the transmitted power and the humidity is depicted in Figure 6b. The transmitted power in humidity ascending and descending processes are well overlapped. The transmitted power changes linearly with the chamber humidity. The sensitivity is 0.57 dB/%RH with the R-square of~0.98. The high sensitivity and good linearity of the SnS 2 nanosheets is appealing in the humidity sensing.

Results
The RH sensing schematic is shown in Figure 5. A light from 1550 nm DFB laser source (SOF-155-D DFB LASER) is sent into MF coated with SnS2, which is placed in a temperature-humidity chamber (BPS-100CL). The MF transmitted light is measured by optical power meter. The RH in the chamber is monitored by a commercial humidity sensor (Testo 175H1) in real time. During the RH sensing experiments, the chamber temperature is fixed at 27 °C, while the RH in the chamber ranges from ~55 %RH to ~100 %RH. Both the optical transmitted power and the RH were recorded during the whole experimental process. To test the RH sensing property, the RH in the chamber is increased and then decreased with by a step of 13 %RH in the RH range of 55% to 95%. With the change of the chamber RH, the transmitted optical power of MF coated with SnS2 varies accordingly, as shown by Figure 6a. The variation is ~22.5 dB, ranging from ~−35 dBm to ~−13 dBm. The relationship between the transmitted power and the humidity is depicted in Figure 6b. The transmitted power in humidity ascending and descending processes are well overlapped. The transmitted power changes linearly with the chamber humidity. The sensitivity is ~0.57 dB/%RH with the R-square of ~0.98. The high sensitivity and good linearity of the SnS2 nanosheets is appealing in the humidity sensing.   Figure 7 shows the repeatability of the RH sensing of SnS 2 -based sensor, where the chamber was switched between two RH of 45% and 100%. From the three cycles of the RH switching in Figure 7, one finds that the optical transmitted power holds the same variation  Figure 7 shows the repeatability of the RH sensing of SnS2-based sensor, where the chamber was switched between two RH of 45% and 100%. From the three cycles of the RH switching in Figure 7, one finds that the optical transmitted power holds the same variation as the RH, which confirms the good repeatability and reversibility of the SnS2based RH sensor. The transmitted spectrum responses of SnS2-based sensor for the humidity were further investigated. A tunable laser (TLS, AQ4321D, ANDO) and an optical spectrum analyzer (OSA, AQ6370D, Yokogawa) were used in the experiment. Figure 8a,b show the variations of the transmitted spectrum (1520-1620 nm) for the RH ascending (a) and descending (b) processes, respectively. The transmitted spectrum of the MF with SnS2 undergoes a relative change when the RH changes in the range of 50% to 95%. As shown by Figures 6 and 8, the transmitted power increases with the relative RH, which should be a result of the increase of the real part of the refractive index of SnS2. In contrast to the RH sensor-based graphene oxide [12,31], the swelling effect of TMDs is not obvious [32]. When water molecules are absorbed by SnS2 film, some air is replaced by the water molecules. Thus, the average refractive index should be n = fSnS2nSnS2+fair-nair+fwaternwater, where nx and fx (x = SnS2, air, water) are the refractive index and filling factor, respectively. As fSnS2+fair+fwater = 1 and nwater > nair, the absorption of water molecules increases the average refractive index. The transmitted spectrum responses of SnS 2 -based sensor for the humidity were further investigated. A tunable laser (TLS, AQ4321D, ANDO) and an optical spectrum analyzer (OSA, AQ6370D, Yokogawa) were used in the experiment. Figure 8a,b show the variations of the transmitted spectrum (1520-1620 nm) for the RH ascending (a) and descending (b) processes, respectively. The transmitted spectrum of the MF with SnS 2 undergoes a relative change when the RH changes in the range of 50% to 95%.
Crystals 2021, 11, x FOR PEER REVIEW 6 of 10 Figure 7 shows the repeatability of the RH sensing of SnS2-based sensor, where the chamber was switched between two RH of 45% and 100%. From the three cycles of the RH switching in Figure 7, one finds that the optical transmitted power holds the same variation as the RH, which confirms the good repeatability and reversibility of the SnS2based RH sensor. The transmitted spectrum responses of SnS2-based sensor for the humidity were further investigated. A tunable laser (TLS, AQ4321D, ANDO) and an optical spectrum analyzer (OSA, AQ6370D, Yokogawa) were used in the experiment. Figure 8a,b show the variations of the transmitted spectrum (1520-1620 nm) for the RH ascending (a) and descending (b) processes, respectively. The transmitted spectrum of the MF with SnS2 undergoes a relative change when the RH changes in the range of 50% to 95%. As shown by Figures 6 and 8, the transmitted power increases with the relative RH, which should be a result of the increase of the real part of the refractive index of SnS2. In contrast to the RH sensor-based graphene oxide [12,31], the swelling effect of TMDs is not obvious [32]. When water molecules are absorbed by SnS2 film, some air is replaced by the water molecules. Thus, the average refractive index should be n = fSnS2nSnS2+fair-nair+fwaternwater, where nx and fx (x = SnS2, air, water) are the refractive index and filling factor, respectively. As fSnS2+fair+fwater = 1 and nwater > nair, the absorption of water molecules increases the average refractive index. As shown by Figures 6 and 8, the transmitted power increases with the relative RH, which should be a result of the increase of the real part of the refractive index of SnS 2 . In contrast to the RH sensor-based graphene oxide [12,31], the swelling effect of TMDs is not obvious [32]. When water molecules are absorbed by SnS 2 film, some air is replaced by the water molecules. Thus, the average refractive index should be n = f SnS 2 n SnS 2 +f air n air +f water n water , where n x and f x (x = SnS 2 , air, water) are the refractive index and filling factor, respectively. As f SnS 2 +f air +f water = 1 and n water > n air , the absorption of water molecules increases the average refractive index.
To investigate the influence of the SnS 2 refractive index on the absorption of SnS 2coated MF, we perform a simulation with COMSOL Multiphysics. The simulated structure is shown by Figure 9a, which is the same as the experimental one. The refractive index of MF and SnS 2 are 1.46 and 2.29 + 0.3i [33], respectively. The intensity distributions of fundamental modes with and without SnS 2 are shown by Figure 9b. The light field is To investigate the influence of the SnS2 refractive index on the absorption of SnS2coated MF, we perform a simulation with COMSOL Multiphysics. The simulated structure is shown by Figure 9a, which is the same as the experimental one. The refractive index of MF and SnS2 are 1.46 and 2.29 + 0.3i [33], respectively. The intensity distributions of fundamental modes with and without SnS2 are shown by Figure 9b. The light field is enhanced and absorbed by the SnS2 film, as shown by Figure 9c. With the increase of the real part of the refractive index of SnS2 film, the imaginary part of effective index of SnS2coated MF decreases gradually, resulting in the increase of the transmitted power as observed experimentally. It is interesting to apply the SnS2-based MF sensor for monitoring human breath. The experimental setup to monitor human breath is shown in Figure 10a. A light from 1550 nm DFB laser is launched into SnS2-coated MF. The transmitted optical light is detected by a photodetector (1811, New Focus), and the transformed electric signal is analyzed by an oscilloscope (DS1052E, Rigol). Figure 10b shows the five cycles of human breathing. The photodetector voltage varies in accordance with the evolution of the exhale/inhale cycles with a maximum voltage variation of ~0.2 V. According to the enlarged view of the response from the fourth cycle, the best response and recovery times are 0.08 s and 0.28 s, respectively. The average response and recovery times over five human breathing cycles are 0.10 s and 0.31 s, respectively. It is interesting to apply the SnS 2 -based MF sensor for monitoring human breath. The experimental setup to monitor human breath is shown in Figure 10a. A light from 1550 nm DFB laser is launched into SnS 2 -coated MF. The transmitted optical light is detected by a photodetector (1811, New Focus), and the transformed electric signal is analyzed by an oscilloscope (DS1052E, Rigol). Figure 10b shows the five cycles of human breathing. The photodetector voltage varies in accordance with the evolution of the exhale/inhale cycles with a maximum voltage variation of~0.2 V. According to the enlarged view of the response from the fourth cycle, the best response and recovery times are 0.08 s and 0.28 s, respectively. The average response and recovery times over five human breathing cycles are 0.10 s and 0.31 s, respectively.  Table 1 shows the main performances of our proposed device and other types of recently developed fiber-optical humidity sensors in literature. Compared to the other fiberoptical RH sensors, the SnS2-based MF device possesses a much higher sensitivity with a  Table 1 shows the main performances of our proposed device and other types of recently developed fiber-optical humidity sensors in literature. Compared to the other fiber-optical RH sensors, the SnS 2 -based MF device possesses a much higher sensitivity with a high linearity in the RH range of 55 %RH-95 %RH. Moreover, the sum of response and recovery time (total time) of our sensor is the smallest (0.36 s), which is at least more than 4.7 times smaller than other fiber RH sensors activated with layered TMDs. Compared to the MoS 2 -coated etched single-mode fiber, our sensor possesses a little slower response time of 0.08 s, but a faster recovery time of 0.28 s [13]. Therefore, the SnS 2 -based MF device is more suitable for the human breath monitoring.

Conclusions
A high-performance RH sensor has been proposed by coating SnS 2 nanosheets onto the surface of an MF. Due to the strong interaction between evanescent wave of MF and SnS 2 nanosheets, the SnS 2 -based sensor possesses a high sensitivity of 0.57 dB/%RH in the RH range of 55 %RH-95 %RH. The response and recovery times respectively are 0.08 s and 0.28 s, allowing us to monitor human breath in real time. This optical RH sensor may find applications in medical diagnosis.