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Engineering of metal–organic framework nanomaterials on long-period fiber grating for acetone vapor sensing

Metal–organic framework (MOF) material is one of the most promising porous nanomaterials for volatile organic compound (VOC) adsorption and sensing. The large surface area and the high porosity of MOF contribute to the high sensitivity of MOF-based VOC sensors. In this study, we engineer the coating of the zeolitic imidazolate framework material ZIF-8 grown on the surface of a long-period fiber grating (LPFG) for acetone vapor sensing. Being a periodic structure formed in a single-mode optical fiber, an LPFG is designed to couple light from the core to the cladding of the fiber at a specific resonance wavelength. Adsorption of acetone vapor molecules in the framework of the ZIF-8 coating can change the refractive index of the coating and cause a shift in the resonance wavelength of the LPFG. The sensitivity of the resonance shift of the LPFG to the acetone vapor concentration depends strongly on the thickness of the ZIF-8 coating. To create a dense ZIF-8 coating, at least five growth cycles of ZIF-8 (30 min growth for one cycle) are required, and nine growth cycles can create a 500 nm thick coating. The LPFG coated with nine growth cycles of ZIF-8 provides a high sensitivity of 21.9 nm ppm−1, a low detection limit of 1.4 ppm, and a wide detection range of about 1500 ppm. Our results can facilitate the development of high-performance optical fiber sensors based on MOF for VOC detection.


Introduction
Metal-organic frameworks (MOFs), which are formed with metal ions and organic ligands into a coordination pattern, have attracted a wide interest as a new class of porous crystalline materials. 1 Compared with porous organic polymers and porous carbon materials, MOFs have large surface area, highly ordered pore structure, and structural diversity and tunability. 2 These unique advantages allow strong host-guest interactions within the framework and, thus, make MOFs highly suitable as sensitive materials for the separation and the detection of gases or volatile organic compound (VOC) vapors. [3][4][5] Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs that are synthesized by coordination of zinc or cobalt with imidazole. ZIF-8, which consists of 2-methylimidazole and Zn 2+ ions, is one of the representative structures. 6 Owing to its large surface area (1946 m 2 g −1 ), high porosity, and excellent mechanical, thermal, and chemical stability, ZIF-8 is widely used for VOC adsorption and sensing. 7 ZIF-8 based uorescent sensors, microbalance sensors, electrochemical sensors, and optical ber/waveguide sensors have been developed for highly sensitive and rapid detection of various toxic and hazardous vapors, such as formaldehyde, explosives, aromatic hydrocarbons, etc. [8][9][10][11][12][13][14][15][16][17][18][19] Compared with traditional sensors, optical ber sensors (OFSs), which employ optical bers to achieve sensing with light, can offer a number of unique advantages, such as absolute electrical insulation, immunity to electromagnetic interference, non-invasiveness, corrosion resistance, and high sensitivity. OFSs also allow long-distance monitoring and interrogation of sensors. Among different types of structures, OFSs based on the structure of long-period ber grating (LPFGs) offer additional advantages, in particular, wavelength-encoded sensing and wavelength-multiplexing capability. 20 An LPFG is a structure formed in a single-mode ber by introducing a periodic modulation of the refractive index along the ber with a period of the order of hundreds of micrometers. 21 Such a structure allows the guided core mode to be coupled to the cladding modes of the ber at specic resonance wavelengths, which results in a series of rejection bands in the transmission spectrum of the ber. Perturbations in the refractive index of the surrounding medium can change the propagation characteristics of the cladding modes and shi the resonance wavelengths of the LPFG. Therefore, an LPFG coated with an effective adsorptive lm can measure the refractive index change of the lm caused by the adsorbed guest and hence function as a selective sensor. In fact, such LPFG sensors have been demonstrated for the detection of various chemical and biological substances. [22][23][24] The MOF nanotechnology provides more opportunities for the development of new OFSs, 25,26 in particular, for VOC sensing. 27 The interaction of MOF material with VOC molecules can lead to a signicant change in the refractive index of the material, [28][29][30] which allows VOC to be sensitively detected in OFSs. Optical ber Fabry-Perot cavities integrated with ZIF-8, HKUST-1, and Uio-66 for the detection of VOC, explosive, and carbon dioxide, as well as rhodamine-B in aqueous solutions, have been reported. [31][32][33][34] Mach-Zehnder interferometric OFSs covered with graphene-oxide nickel MOF and ZIF-8 have been demonstrated for sensitive detection of hydrogen and ethanol. 35 LPFGs coated with different MOFs have been proposed for VOC sensing, but their performances in terms of sensitivity and detection limit still need further optimization to achieve ultrasensitive detection. 36,37 In this paper, we report our study on engineering the ZIF-8 coating of an LPFG sensor for acetone vapor sensing. In particular, we study how an LPFG coated with different growth cycles of ZIF-8 affects its performance as an acetone vapor sensor. We show that nine growth cycles of ZIF-8 (30 min growth for one cycle) can form a 500 nm thick ZIF-8 coating and the coated LPFG can achieve a high detection sensitivity of 21.9 pm ppm −1 and a low detection limit of 1.4 ppm over a detection range of ∼1500 ppm. The use of a sufficiently thick ZIF-8 coating can greatly enhance the performance of an LPFG sensor for VOC detection.

Results and discussion
Design and fabrication of long-period ber grating (LPFG) The LPFG sensing system is illustrated in Fig. 1a. The surface of LPFG is coated with ZIF-8 nanomaterials. Light from an amplied spontaneous emission (ASE) source (B&A Technology, AS4600) is launched into the ber and transmitted as LP 01 core mode. The intensity distribution of the LP 01 mode in the ber is shown in Fig. 1b, which indicates that the LP 01 mode is well conned in the core of the ber. The LPFG in this study is designed to couple the LP 01 mode to the LP 08 cladding mode of the ber. The intensity distribution of the LP 08 mode in the ber is shown in Fig. 1c, which indicates that the mode is conned in the entire cladding of the ber. The transmission spectrum of the ber is measured with an optical spectrum analyzer (OSA) (Anritsu MS9740A).
The resonance wavelength l 0 at which the LP 01 mode is coupled most strongly to the LP 08 mode is governed by the phase-matching condition: 21 where N 01 and N 08 are the effective indices of the LP 01 and LP 08 modes, respectively, and L is the grating period. The transmission spectra of LPFGs with different periods are calculated and shown in Fig. 2a for a standard single-mode ber (SMF-28).  The index modulation in the ber core is assumed to be 1.3 × 10 −5 . As shown in Fig. 2a, the resonance wavelength calculated for L = 320 mm is 1557 nm and a longer period results in a longer resonance wavelength. The contrast of the grating is ∼3.5 dB for a 100-period grating and can be increased by increasing the index modulation and/or the grating length. Fig. 2b shows the transmission spectra of the grating with L = 320 mm and an index modulation of 2.1 × 10 −5 calculated for different grating lengths. In this case, the contrast is larger than 13 dB for a 100-period grating. In our study, the grating period is set at 320 mm, so that VOC sensing can be performed in the C band (1530-1565 nm), which is the spectral range of the ASE source. As the LP 08 mode is guided by the cladding of the ber, its effective index is very sensitive to the refractive-index change of the ZIF-8 coating. A shi in the resonance wavelength of the LPFG can thus provide a measure of the concentration of VOC.
The LPFG is fabricated by a CO 2 pulsed laser. The laser irradiation causes residual stress release in the ber and changes the density of the ber material and its refractive index. 38 The CO 2 laser is computer-controlled to periodically scan the ber along the transverse direction, resulting in a periodic change in the refractive index along the ber to form a grating structure. 38 Fig. 3 shows the measured transmission spectrum of the fabricated LPFG with a period of 320 mm and a length of 100 periods. The resonance wavelength of the LPFG is 1552.83 nm and the extinction ratio is 27 dB. Such high extinction ratio is attributed to careful tuning of the laser power and the exposure time to optimize the mode coupling. 38 In situ growth of ZIF-8 on optical ber A sensitive ZIF-8 coating needs to be grown on the fabricated LPFG to leverage the large surface area and the high porosity of ZIF-8 for the enrichment of VOC vapor on the ber. For this purpose, the thickness and the morphology of the ZIF-8 coating are critical. Uniform and densely ordered crystals can facilitate fast adsorption of VOC and thus fast detection. Meanwhile, the thickness of the ZIF-8 coating plays a key role in determining the sensitivity and the response speed of VOC sensing. In our study, one, two, three, ve, seven, and nine cycles of in situ growth of ZIF-8 on the ber, with 30 min growth for one cycle, are performed. The surface morphology of ZIF-8 on ber is characterized by SEM. The details of the ZIF-8 growth can be found in the Experimental section.
The SEM images show that, aer one ( Fig. 4a and b) and two ( Fig. 4c and d) growth cycles, the ber is not fully covered with ZiF-8 particles. Aer three growth cycles ( Fig. 4e and f), the ber is mostly covered with ZIF-8 particles, but the particle size is not uniform and most particles are crystal seeds, which suggests  that more growth cycles are required. Aer ve growth cycles ( Fig. 4g and h), the ZIF-8 particles on the ber is dense and uniform. Our results indicate that at least ve cycles are required to form a dense ZIF-8 coating. The thickness of the ZIF-8 coating aer ve cycles of in situ growth, measured by a surface step proler, is 300 ± 30 nm. The thickness of ZIF-8 coating can be further increased with more growth cycles. Aer seven and nine growth cycles, the thickness of the ZIF-8 coating is increased to 390 ± 20 nm and 500 ± 30 nm, respectively.
The thickness of the ZIF-8 coating affects the effective index of the LP 08 cladding mode and the resonance wavelength of the LPFG. Fig. 5 shows the measured transmission spectra of the LPFG aer different cycles of ZIF-8 growth, which are labelled as LPFG-0, LPFG-3, LPFG-5, LPFG-7, and LPFG-9 for zero, three, ve, seven, and nine growth cycles, respectively. As shown in Fig. 5, aer three growth cycles, the resonance wavelength shis from 1552.83 nm to 1550.88 nm. Aer ve growth cycles, a dense uniform ZIF-8 coating is formed and the resonance wavelength shis to 1546.50 nm. Aerwards, the resonance wavelength shis towards the shorter wavelength by about 2 nm for every two additional growth cycles. The resonance wavelength aer nine growth cycles is 1542.38 nm, with the 10.45 nm shi from the bare LPFG-0 without ZIF-8.

VOC sensing of LPFG
The VOC sensing mechanism is based on the refractive index change of ZIF-8 coating on the surface of optical ber. Typically, due to the strong during the adsorption. Due to the strong electrostatic interactions, the guest VOC molecules can be easily adsorbed inside the pore of ZIF-8, causing a change in the refractive index of ZIF-8 on the ber surface; and the light of the LP 08 mode excited by the long-period grating senses this change in refractive index in ZIF-8, resulting in a change in the effective refractive index of the LP 08 mode, but the effective refractive index of the fundamental mode LP 01 in the ber core layer refractive index does not change. Therefore, according to the phase-matching condition of eqn (1), it is known that the change in refractive index of ZIF-8 corresponds to the change in the resonant center wavelength of the LP 08 and LP 01 mode coupling of the long-period grating. Different concentrations of VOC adsorption will produce different ZIF-8 refractive index changes, leading to the different resonance center wavelength changes. Therefore, it can correspond to the VOC concentration, so as to quantitatively obtain the gas concentration.
To evaluate the effect of the ZIF-8 thickness on the performance of VOC sensing, the VOC adsorption dynamics is rstly monitored. The ZIF-8 coated LPFG is placed in a sealed chamber lled with saturated acetone vapor and the resonance wavelength of the LPFG is measured with the OSA. As the ZIF-8 coating adsorbs acetone molecules, the refractive index of the coating should be increased, which should result in an increase in the effective index of the cladding mode and a blue-shi in the resonance wavelength of the LPFG. The adsorption kinetic process is obtained by recording the variation of the resonance wavelength of the LPFG with time during the adsorption process. Fig. 6a-d show the transmission spectra of LPFG-3, LPFG-5, LPFG-7, and LPFG-9, respectively, measured for different acetone vapor exposure times. Fig. 6e shows the variation of the resonance shi with the exposure time for the LPFG with different cycles of ZIF-8 growth. As shown in Fig. 6e, the ZIF-8 coating responds quickly. For a relatively thin ZIF-8 coating, as in LPFG-3 and LPFG-5, it takes only one to two minutes for the resonance shi to settle. For a thicker ZIF-8, as in LPFG-7 and LPFG-9, most of the resonance shi still occurs in the rst two minutes and it takes another ve to ten minutes to reach the nal value. The fast response can be attributed to the highly ordered porosity of the ZIF-8 coating, which is consistent with previous conclusion on different sensing device with ZIF-8. 32,36 Fig. 6f shows the nal resonance shis of the LPFG with different cycles of ZIF-8 growth. Aer three growth cycles, the distribution of the ZIF-8 particles on the LPFG remains sparse and the adsorption effect causes only a small resonance shi of about 2 nm. Aer ve growth cycles, the ZIF-8 coating becomes denser and the nal resonance shi is about 7 nm. Aer seven growth cycles, the ZIF-8 coating becomes thicker and the nal resonance shi is 14.2 nm. Aer nine growth cycles, the nal resonance shi is as large as 28.8 nm. Clearly, a thicker ZIF-8 coating results in a larger wavelength shi and, hence, a higher sensitivity for acetone vapor detection. For a bare LPFG without any ZIF-8 coating, however, the resonance shi is smaller than 0.1 nm in saturated acetone vapor. Our experimental results conrm that the strong adsorption effect in the porous ZIF-8 coating can signicantly increase the refractive index of the coating and thus lead to a large blue-shi of the resonance wavelength of the LPFG. Fig. 7 shows the resonance shis of LPFG-7 and LPFG-9 measured at different concentrations of acetone vapor in air. The sensitivities for LPFG-7 and LPFG-9, which are the slopes of the tted straight lines in Fig. 7, are 11.0 pm ppm −1 and 21.9 pm ppm −1 , respectively, and their detection ranges are approximately 1200 ppm and 1500 ppm, respectively. The detection range is limited by the saturation of the adsorption effect in the ZIF-8 coating. The detection limit D of the sensor is given by, where R is the resolution of OSA and S is the sensitivity. Given an OSA resolution of 0.03 nm, the detection limits calculated for LPFG-7 and LPFG-9 are 2.7 ppm and 1.4 ppm, respectively. For most sensors, there is a trade-off between the detection range and the sensitivity, i.e., a higher sensitivity results in a narrower detection range. In our case, however, LPFG-9 offers not only a higher sensitivity, but also a wider detection range, which is due to the fact that the thicker ZIF-8 coating in LPFG-9 provides more porosity and a larger surface area to adsorb more acetone vapor molecules. Our study proves the effectiveness and the exibility in tuning the sensitivity and the detection range of the LPFG sensor for VOC sensing by controlling the thickness of the ZIF-8 coating.

Conclusions
We have studied in detail how the number of growth cycles of ZIF-8 on an LPFG affects the coating quality and the response of the LPFG to acetone vapor. We nd that at least ve growth cycles are needed for producing a dense ZIF-8 coating and the thickness of the ZIF-8 coating increases further by applying more growth cycles. Our experiments show that the shi in the resonance wavelength of an LPFG properly coated with ZIF-8 can serve as a sensitive measure of the acetone vapor concentration. In particular, our LPFG sensor coated with nine growth cycles of ZIF-8 can provide a high sensitivity (21.9 pm ppm −1 ) and a wide detection range (∼1500 ppm) for acetone vapor detection. The response time of the LPFG sensor is also short (less than 2 minutes). Our results can facilitate the development of MOF-based optical ber sensors for VOC detection.

Materials and instruments
All chemicals were purchased from Sigma-Aldrich and used as received. Optical ber simulation was performed with a commercial mode solver (COMSOL Multiphysics) based on the full-vector nite-element method and Matlab. Scanning electronic microscopy (SEM) images were collected by Inspect F (FEI). The LPFG was fabricated with a CO 2 laser using a wellestablished procedure. 38 The thickness of ZIF-8 on the surface of optical ber was measured by Dektak 150 step proler. The  In situ growth of ZIF-8 on LPFG Zn(NO 3 ) 2 $6H 2 O and 2-methyl-imidazole were dissolved in methanol with respective concentration of 12.5 mM and 25 mM. The LPFG was immersed in the mixed solution with a 1 : 1 volume ratio for 30 min at 25°C and then washed with methanol for three times. ZIF-8 was grown on the surface of the LPFG and a growth time of 30 min is designated as one growth cycle. Aer completing a desired number of growth cycles, the LPFG was dried in a vacuum oven at 80°C overnight to remove any residual solvent. The transmission spectrum of the LPFG was monitored with an OSA to determine its resonance wavelength. The procedure was repeated when the LPFG was coated with additional growth cycles of ZIF-8. Meanwhile, the crystal particles were collected by centrifugation and washed in methanol for three times. The XRD pattern of ZIF-8, as shown in Fig. 8, illustrated the correct diffraction peaks of ZIF-8. Meanwhile, the BET measurement was performed and the measured surface area of ZIF-8 is 1587 m 2 g −1 , showing the ultra large surface to absorb guest VOC gas.

Conflicts of interest
There are no conicts to declare.