Metal–Organic Framework Thin Film Coated Optical Fiber Sensors: A

Jan 5, 2018 - ... A. D. Gas sensing using porous materials for automotive applications Chem. Soc. Rev. 2015, 44, 4290– 4321 DOI: 10.1039/C5CS00040H...
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Metal-Organic Framework Thin Film Coated Optical Fiber Sensors: A Novel Waveguide-Based Chemical Sensing Platform Ki-Joong Kim, Ping Lu, Jeffrey T. Culp, and Paul R Ohodnicki ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00808 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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MetalMetal-Organic Framework Thin Film Coated Optical Fiber Sensors: A Novel WaveguideWaveguide-Based Chemical Sensing Platform Ki-Joong Kim,*,†,‡ Ping Lu,†,‡ Jeffrey T. Culp,†,‡ and Paul R. Ohodnicki†,§ †

National Energy Technology Laboratory, 626 Cochrans Mill Rd., Pittsburgh, Pennsylvania 15236, United States AECOM, 626 Cochrans Mill Rd., Pittsburgh, Pennsylvania 15236, United States § Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States KEYWORDS. optical fiber sensor, chemical sensor, small gases sensing, carbon dioxide sensing, metal organic framework thin film. ‡

ABSTRACT: Integration of optical fiber with sensitive thin films offers great potential for the realization of novel chemical sensing platforms. In this study, we present a simple design strategy and high performance of nanoporous metal organic framework (MOF) based optical gas sensors, which enables to detect a wide range of concentrations of small molecules based upon extremely small differences in refractive indices as a function of analyte adsorption within the MOF framework. Thin and compact MOF films can be uniformly formed and tightly bound on the surface of etched optical fiber through a simple solution method which is critical for manufacturability of MOF-based sensor devices. The resulting sensors show high sensitivity/selectivity to CO2 gas relative to other small gases (H2, N2, O2, and CO) with rapid (< tens of seconds) response time and excellent reversibility, which can be well correlated to the physisorption of gases into a nanoporous MOF. We propose a refractive index based sensing mechanism for the MOF-integrated optical fiber platform which results in an amplification of inherent optical absorption present within the MOFbased sensing layer with increasing values of effective refractive index associated with adsorption of gases.

Sensitive and selective sensing of hazardous and greenhouse gases is becoming increasingly important in environmental and industrial fields, and is also a fascinating subject for scientific study.1,2 Nanoporous materials such as metal organic frameworks (MOFs) have experienced a rapid increase in interest in recent years as potential materials for the fabrication of chemical sensors due to the excellent sorption kinetics, reversibility, and guest-induced changes in the MOF structure and/or properties.3-11 Integrating MOFs into optical devices will allow the measurement of optical property changes by adsorbing guest molecules. For example, the effective refractive index (RI) of a MOF should be sensitive to gas molecules due to pore filling. Although direct RI-based sensing of gases by Fabry-Perot based high-resolution spectroscopy was developed,12-14 reports of MOF-based optical devices for sensing based upon RI modifications in response to sorption of gases are relatively few and are only developed into relatively large RI such as hydrocarbons, alcohol, and water which are known to produce relatively large changes in RI upon adsorption.14-19 In contrast, application of this sensing technique for small molecules (e.q. H2, N2, O2, CO, and CO2) is significantly more challenging due to their extremely small differences in RIs (as small as ~5 x 10-4 RI units),20 and has not seen as significant progress and success as a result. To overcome the limitations of relatively weak RI responses for

small molecules to further develop a wide range of MOF based optical sensing applications, it is necessary to explore for new types of sensing and measurements systems that provides a highly sensitive, selective, and stable response for target gases. Waveguide based sensor applications, particularly optical fiber have unique characteristics; flexible design for in situ and in vivo analysis, online monitoring of various entities in hostile environments, and are insensitive to electromagnetic disturbances.21-25 The optical fiber transmission for evanescent wave absorption spectroscopy based sensors is typically attenuated through absorption and/or scattering losses within the sensing layer. If an uncladded optical fiber (or fiber tip) is covered with a porous material such as a MOF then interaction with analyte species can alter the magnitude of transmittance losses if engineered properly,26-29 which endows themselves to many optical applications. Here we report a new sensing technique that exploits small molecule induced RI changes of MOF thin films based upon characteristic absorption features associated with the MOF layer, which enables the identification of small molecules with extremely small differences in RIs by taking advantage of the optical fiber waveguide platform. To this end, an etched fiber optic was used as the optical sensing device, and the sensing layers were

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prepared by coating a MOF thin film via a simple solution method. For our initial study, we chose to work with the wellknown zeolitic imidazole framework (ZIF)-8 MOF since it is chemically/thermally stable and exhibits relative hydrophobicity as compared to other common MOFs,30,31 which are requirements for long term use and real application in the field.

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nanoparticles were obtained from the growth mixture solution by centrifuge. Determination of Optical Constants and Optical Mode Analysis. A multi-parameter curve-fitting method is used for determining optical constants (RI, n and extinction coefficient, k) of the ZIF-8 and Co/ZIF-8 films on the basis of dispersion models for RI and absorption which assumes a Tauc-Lorentz oscillator to describe the UV-range band edge absorption and a Lorentz oscillator for the absorption associated with Cobalt ions within a Co/ZIF-8. Mode analysis of the MOF-coated optical fiber was achieved by using COMSOL Multiphysics finite element analysis software to numerically simulate modal effective indices as well as intensity patterns. Sensing Measurement. In-situ optical transmission spectroscopy was recorded by connecting the one end of the fiber with a spectrometer (JAZ, Ocean Optics) operating over a wavelength range of ∼200-650 nm. The other end of the fiber was connected to a broad-band deuterium tungsten halogen light source (DH-2000-BAL, Ocean Optics). Gas sensing performance of MOF coated optical fiber sensor was evaluated at room temperature in a sealed quartz chamber of tube coupled with an automated gas delivery system and monitored the transmittance (%T) of the sensor in various gases. The total flow rate was maintained at 100 mL min-1 by mixing different concentrations of H2, O2, CO, and CO2 in a balance of N2. Different gas concentrations were obtained by controlling the flow rates of pure N2 and target gas. The chamber was first purged with N2 gas for a few hours and then the reference spectrum was recorded (100 %T). After this the target gas was inserted in the chamber and the transmitted spectrum was recorded. These two spectra were used to obtain the spectrum of the gas for a given concentration of the gas. Sensing measurements of planar substrate were also performed in a transmission geometry and the signal was monitored. To collect transmission spectra, light from the lamp was focused to a ~1 cm diameter spot on the sample using two lenses (Thorlabs); adjustable irises were used to produce a homogenous spot and control spot size. The sample was held in a home-built stainless-steel flow cell vertically. Light transmitted through the flow cell was refocused using a single lens into a spectrometer. All sensing tests were performed at room temperature and 1 bar without pretreatments such as heating and/or evacuating under vacuum, and pure N2 was used as the reference and/or carrier gas.

EXPERIMENTAL SECTION SECTION Materials. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), and methanol of analytical grade were purchased from Sigma-Aldrich. 2methylimidazole (2-mIm) and quartz substrate were obtained from Alfa Aesar. Fiber optic was purchased from ThorLabs (FG105UCA, Multimode fiber, 0.22 NA, high-OH, 105 um). Characterization. X-ray diffraction (XRD) pattern was obtained using a Rigaku Ultima IV Diffractometer, operating at 40 kV and 40 mA with Cu kα radiation (0.154 nm) in the range from 5 to 20 degrees with a step size of 0.01. Field emission-scanning electron microscope (FE-SEM) analysis was conducted with an FEI Quanta 600 using 15 kV accelerating voltage. UV-Vis-NIR spectra were measured by using a Lambda 1050 spectrophotometer equipped with an integrating sphere to include non-specular light associated with diffuse scattering. Gravimetric gas adsorption measurements were obtained using a Hiden IGA microbalance at room temperature. The sample was dried under vacuum at 90 °C until the mass loss stabilized prior to measurement. Preparation of ZIF-8 and Cobalt-doped ZIF-8 (Co/ZIF8) Thin Films on Optical Fiber and Quartz Substrates. Fiber optic was first heated at 550 °C in air for 60 minutes to remove the polymer jacket and then etched (~5 cm long) in a buffered oxide hydrofluoric acid (HF) etchant solution for 50 minutes. Then it is washed thoroughly by DI water and dried naturally. The etched fiber optic (or quartz substrate) was also cleaned in Piranha solution (H2SO4/H2O2, 70/30 v/v%) at 70 °C for 30 minutes. Then it is washed thoroughly by DI water and dried under N2 flow. (Caution: HF and Piranha solutions are an acutely toxic chemical, and are dangerous to handle without personal protective equipment; splash goggles or a face shield with safety glasses, lab coat, 18 mil neoprene gloves, chemically-resistant sleeves and apron, and close toed shoes. Working alone with this chemical is not allowed). To grow MOF thin film, the etched/cleaned fiber optic (or quartz substrate) was immersed in a freshly mixed methanolic solution of 2-mIm (2.5 mmol, 50 mL) and Zn(NO3)2·6H2O (1.25 mmol, 50 mL) for 30 min at room temperature, followed by washing using methanol several times and drying in air. This process was repeated in order to obtain a desired MOF thickness. For the study of the sensing mechanism, the Co/ZIF-8 thin film was also prepared by a similar manner. The HF and Piranha solutions cleaned fiber optic (or quartz substrate) was immersed in a freshly mixed methanolic solution of 2-mIm (5 mmol, 50 mL) and mixture solution of Zn(NO3)2·9H2O (1.25 mmol, 25 mL) and Co(NO3)·6H2O (1.25 mmol, 25 mL) for 60 min at room temperature, followed by washing using methanol and drying under N2 flow. Each ZIF-8 and Co/ZIF-8

RESULTS AND DISCUSSION Schematic diagram of gas sensing system designed in this study and MOF-coated optical fiber seosor is illustrated in Figure 1a. The morphological features and growth kinetics of ZIF-8 thin films on optic fibers with increasing the number of deposition cycles were investigated by FE-SEM images. Top (Figure 1b) and cross-sectional (Figure 1c) FE-SEM images of ZIF-8 coated optical fiber show the successful coating of MOF layer with very uniform, dense, and continuous ZIF-8 film, which is essential for high performance sensing devices. The film thickness of the MOF film on the optical fiber was proportionally increased at a rate of ~75 nm per cycle when the deposition cycle increases (Figure S1 and S2a). XRD

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pattern confirms crystalline ZIF-8 MOF on the optical fiber without the presence of any peaks assigned to other impurities (Figure S2b).32

wavelengths above 250 nm associated with the 2-mIm absorption band tail that is not very intense. Furthermore, a linear correlation between the transmittance and CO2 concentration is observed (Insert of Figure 2a). To interpret the results, a sensing mechanism is proposed which involves a change of RI in the ZIF-8 coating concurrent with the adsorption of CO2 into the MOF pores. First, the measured adsorption isotherm of ZIF-8 powders (Figure S3) shows the gas uptake of ZIF-8 for CO2 is ~0.73 mmol g-1 and that for N2 is ~0.09 mmol g-1 at room temperature and 1 bar. Therefore, an increase in the density and the consequent RI change of ZIF-8 film will occur according to the LorenzLorentz law when CO2 replaces N2.39 A real RI change of ~0.005 in the CO2 adsorbed ZIF-8 MOF layer is estimated when the ambient CO2 gas concentration is 100% as compared to the corresponding value in a pure N2 atmosphere.40 The change in the corresponding imaginary optical constant is not easily estimated as it will depend upon the detailed interaction between the MOF and the CO2 which is expected to be weak and negligible due to the nature of the physisorption process. Since the RI change in the MOF layer is dependent on the amount of CO2 adsorbed, the transmitted light intensity changes in direct correlation with the partial pressure of CO2. As the RI of the MOF layer increases closer to the RI of the fiber, more light propagates from the optical fiber into the MOF layer. As discussed in prior work in the context of solution phase pH sensing using metal nanoparticle incorporated sol-gel sensing layers, a modified effective real index (n) of the sensing layer can proportionally amplify the effective absorption inherent within the sensing layer by modifying the fraction of light which propagates within the sensing layer.41 To further prove the consistency of the proposed sensing mechanism, a new optical fiber sensor was prepared using Co/ZIF-8. Incorporation of Cobalt ions into the ZIF-8 provides an additional spectral band in the visible range due to the d-d band transition of the cobalt ion.42 A RI change induced by gas adsorption in the Co/ZIF-8 should lead to a correlated change in absorption intensities of both the d-d band transition of the cobalt ion in the visible range and the absorption band of the organic linker in the UV range. It can be clearly seen that transmittances of the Co/ZIF-8 at ~265 nm and ~597 nm decreased with increased CO2 concentration proportionally (Figure 2b), consistent with a mechanism that results from a larger fraction of incident light propagating within the MOF layer. In comparison to the optical fiber based MOF gas sensor, we grew the ZIF-8 and Co/ZIF-8 layers on quartz substrates (e.q. Fabry-Perot devices), and tested CO2 sensing performance through direct transmission spectroscopy measurements (for details see the experimental senction). No changes and/or shifts in %T to different CO2 concentration could be resolved as the relatively small RI changes of the sensing layers do not have a significant impact on the film optical properties in this type of optical device configuration (Figure S4). This result clearly indicates that MOF-coated optical fiber sensor exhibits highly sensitive detection of small gases as compared to corresponding sensing layers interrogated through normal incidence transmission

Figure 1. (a) Schematic diagram of gas sensing system and optical fiber sensor integrated with MOF thin film. (b) Top and (c) crosssectional FE-SEM images of 200 nm ZIF-8 coated optical fiber illustrating the uniform and continues thin film.

The integration of nanoporous MOF thin film as coatings on optical fibers is anticipated to lead to new sensing materials with novel functionality. Particularly, their selective gas adsorption behavior makes MOF very attractive for overcoming the selectivity problem in gas sensors.33,34 For example, ZIF series MOF has a CO2 adsorption selectivity of approximately ≥ 10 times over competitive gases such as H2, CO, O2 and N2 near ambient conditions.35-38 Nevertheless, we have not identified any reports that discuss the successful discrimination of sensing responses between these species for an optical waveguide based gas sensor based upon the ZIF-8 sensing layers. Sensing tests of MOF coated optical fiber sensors were performed at room temperature and 1 bar without pretreatments such as heating and/or evacuating under vacuum, and N2 was used as the reference and/or carrier gas. The gas chamber was first purged with N2 gas for a half hour and then the reference spectrum was recorded (100 %T). After this the target gas was inserted in the chamber and the transmitted spectrum was recorded. The transmittances of the 350 nm ZIF-8 coated optical fiber decrease as the CO2 concentration increases (Figure 2a), indicative of a guest molecule-induced optical response modification. The overall drop in transmitted light intensity is predominantly due to a strong absorption band at ~242 nm caused by the 2-mIm linker within the ZIF-8 as demonstrated by the similarity with the absorption peak wavelength observed for the free 2-mIm linker in dilute solution. The changes in transmittances as a function of CO2 concentration are relatively small at longer

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works.12-14

spectroscopy such as Fabry-Perot devices reported in previous

Figure 2. Transmission spectra of (a) 350 nm ZIF-8 and (b) 160 nm Co/ZIF-8 coated optical fibers after exposure to CO2 of different concentrations. Absorption spectra of 2-mIm, ZIF-8 and Co/ZIF-8 were obtained from diluted in methanol, respectably. (c) Optical response of 350 nm ZIF-8 coated optical fiber sensor to different pure gases. (d) Dynamic response at ~242 nm of 350 nm ZIF-8 coated optical fiber after exposure to various gases or gas mixtures as identified. All sensing tests were performed at room temperature and 1 bar without pretreatments, and N2 was used as the reference and/or carrier gas.

The sensing performance of the 350 nm ZIF-8 coated fiber sensor to various gases with extremely similar RIs was investigated. Single gases (100 v/v% of H2, CO, O2, and CO2) or a 1:1 mixture gases with CO2 (50 v/v% of H2, CO, O2 and N2) were sequentially switched to N2, which represents the base signal, %T = 100. For reference, no responses to any gases were observed for a bare etched fiber. This observation is due to very small difference in RIs between N2 as a reference gas and other gases (H2, O2, CO, and CO2) combined with the lack of an inherent optical absorption of small molecule gases in the UV or visible range. Impressively, the ZIF-8 coated optical fiber sensor shows highly sensitive/selective sensing toward only CO2 gas, whereas negligible responses for H2, CO, and O2 gases are observed (Figure 2c and Figure S5). The ratio of the responses, between %T (100% CO2) and %T (50% CO2/N2),

is close to 2 (Figure 2d), suggesting that the sensing response shows a proportionality with the amount of gases physically adsorbed and the effective real index of the sensing layer. Furthermore, the mixed gases revealed almost the same sensing response compared to the CO2/N2 sensing for the same CO2 concentration, indicating that the other gases do not influences the selectivity of the CO2 sensing. This result on CO2 sensing in the gas mixture provides that the MOFbased optical sensors significantly expand the flexibility to be useful in many industrial processes and environmental monitoring applications. For example, one primary application driver for CO2 based chemical sensing is carbon sequestration in which it is highly desirable to have a probe which can detect and quantify CO2 throughout a geological formation.43,44

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Figure 3. (a) Response time of 200 nm and 530 nm ZIF-8 coated optical fiber sensors. (b) Simulated percentage of maximum response as a function of time. (c) Optical responses of optical fiber sensors with different ZIF-8 thickness at numbers of growth cycles. (d) %T as a function of CO2 concentration in N2 (The inset shows a zoomed-in plot for low concentration). (e) Dynamic responses for different CO2 concentration on the 200 nm ZIF-8 coated optical fiber sensor.

Estimates of response time can be of great utility in elucidating the kinetics of gas diffusion (adsorption/desorption) into the MOF layer. The response time of the MOF coated optical fiber gas sensor is defined as the time period from the response onset to 90% of maximum response in a steady state, and were estimated by instantaneously supplying CO2 and N2, respectively. The 200 nm ZIF-8 coated fiber sensor shows the CO2 adsorption and desorption times of ~9 and ~14 seconds, respectively, which are faster than those for a 530 nm ZIF-8 sensor with the CO2

adsorption and desorption times of ~24 and ~84 seconds (Figure 3a). The kinetics of gases diffusion from the surrounding media into MOF pores are relatively rapid and it dominates the initial dynamic gas diffusion/adsorption process in the ZIF-8 film outer surface for which local equilibrium can be assumed. Once the local equilibrium is established at the surface, gas phase mass transfer through the assemblage of ZIF-8 crystallites occurs towards interior of the film with a certain long-range diffusion rate.

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To demonstrate this concept mathematically, as the CO2 concentration in a homogeneous ZIF-8 film evolves, the diffusion of CO2 molecules is a non-steady state diffusion process that can be described by Fick’s second law. There are two boundary conditions: (1) the surface CO2 concentration at the MOF-External interface is a constant CS determined by the partial pressure of CO2 in the surrounding media and the local concentrations of any existing species in the MOF, (2) the pre-adsorbed CO2 concentration at the MOF-FIBER interface is a constant CE, which depends upon the gas stream chemistry for the prior gas phase sensing step assuming that steady state has been reached. Then the CO2 molecules concentration C(r,t) in the ZIF-8 thin film can be expressed as (Equation 1):

 r  C (r , t ) = (CS − CE )erfc  + CE  2 Dt 

layer (Figure S6). This can be associated with the molecular sieving properties of ZIF-8 with small pore apertures (0.34 nm); such that N2 molecules with 0.36 nm larger than 0.34 nm could not efficiently replace the pre-adsorbed CO2 molecule (kinetic diameter of 0.33 nm).45,46 Optimum thicknesses of ZIF-8 thin film for the maximum CO2 sensing response was observed for 200 nm ZIF-8 coated on optical fiber (Figure 3c). Increasing the thickness of ZIF8 to more than 350 nm resulted in lower absolute responses. Some inhomogeneities such as increased surface roughness and more coarse grains are induced during the multiple-cycle MOF film deposition process as seen from FE-SEM images (Figure S1), showing the evolution of film morphology with increasing film thickness. Such details are expected to modify the corresponding optical response including the potential for introducing a significant degree of optical scattering to the MOF integrated waveguide structure, thus a lower sensitivity is observed. Optical responses of the 200 nm ZIF-8 coated optical fiber sensor clearly show the linear relationship between %T and the CO2 concentration (Figure 3d and Figure S7). The reversibility of the 200 nm ZIF-8 coated sensor was also examined for the structural stability. When the sensing experiments were repeated 10 cycles upon alternative exposure to N2 and CO2, nearly identical responses without showing significant variations in %T (±0.5%) were obtained (Figure 3e). Cycles of 350 nm ZIF-8 coated optical fiber sensor was also fully recovered and maintained during the entire set of cycling experiments (Figure S8). These observations indicate that the MOF material coated on optical fiber can retain the robust framework structure, leading to fast response, fatigueless reversibility and high photophysical stability. It is worth noting that the presented results demonstrate that pretreatments such as heating and/or evacuating under vacuum are not required for the sake of sensor regeneration, a critical observation for practical sensor applications. The measurement of normal incidence transmittance (reflectance) of thin films on planar substrates (Figure S9) can be exploited to determine the real and imaginary parts of the complex RI in ambient air conditions. As shown in Figure 4, the real RI of ZIF-8 and/or Co/ZIF-8 is less than that of the silica fiber over the visible and near-IR range and thus the fiber supports confined modes within the waveguide core by which light propagates along the fiber. The intensity pattern of the fundamental mode in the Co/ZIF-8 coated optical fiber at 597 nm is shown in the inset of Figure 4, and the evanescent wave power fraction in ambient gas environments in the visible range is more susceptible to attenuation due to its non-zero extinction coefficient compared to that of the ZIF-8 coated fiber, making it attractive for evanescent wave sensing.

(1)

where C(r,t) is the gas concentration at the position r along the radial direction in the cross-section of waveguide at the time t, and D is the long-range diffusivity of the gas molecules in the thin film. The overall average concentration of CO2 in the entire ZIF-8 thin film is assumed to contribute equally to the linear transmittance sensing response according to the Beer-Lambert law. The percentage of maximum response η is defined to be the average composition of CO2 throughout the sensing layer which represents the completion level of the gas sorption process (Equation 2),

η=

C (r , t ) − C E CS − C E

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(2)

where C (r , t ) represents the average concentration with respect to the ZIF-8 film thickness. It is noted that the response time is proportional to the term l2/D. Thus, D can be found by fitting the percentage of maximum response curves to be well suited to the corresponding adsorption and desorption time η = 90% and film thickness l, giving Dadsorption (CO2 adsorption) and Ddesorption (CO2 desorption) on the order 10-14 m2 s-1 (Figure 3b). For instance, Dadsorption = 3.50 x 10-14 m2 s-1, Ddesorption = 2.24 x 10-14 m2 s-1 for 200 nm ZIF-8, and Dadsorption = 9.20 x 10-14 m2 s-1, Ddesorption = 2.63 x 10-14 m2 s-1 for 530 nm ZIF-8, respectively. An ideal homogeneous structure in the absence of significant barriers such as polycrystals and grain boundaries would be expected to give the same long-range diffusivity with negligible film thickness dependence, whereas the experimental results of real ZIF-8 films show that the thicker film exhibits increased diffusivities (Dadsorption or Ddesorption) than those of the thinner film, implying the presence of more rapid grain boundary diffusion. Such differences are likely due to the detailed microstructure of thicker films as compared to thinner films with much larger average grain sizes and significantly increased grain boundary regions acting as shortcuts for gas molecule transport (Figure S1). In addition, a relative longer CO2 desorption time compared to CO2 adsorption time (Ddesorption < Dadsorption) for both film thicknesses are presumably due to a slower diffusion rate of N2 molecule into the MOF sensing

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in the relative transmission intensity is estimated to be ~60% as the sensor is transfered from 100% ambient N2 gas to 100% ambient CO2 gas. It remains an open question the extent to which such sensing layer guided modes are excited in the current set-up due to the details of the sensor fabrication process and the potential for mode conversion and loss at the intersection between the cladded and uncladded portions of the fiber sensor.

Figure 4. Refractive indices (n) and extinction coefficients (k) of ZIF-8, Co/ZIF-8, and silica fiber (SiO2) in air. These were determined from transmittance and reflectance of ZIF-8 and Co/ZIF-8 films on a planar quartz substrate by using high resolution UV-Vis-NIR spectrometry.

A subtlety in the observed sensing responses should be pointed out such that at short wavelengths below ~250 nm the RIs of both ZIF-8 and Co/ZIF-8 with a thickness of 200 nm with ambient gas exceeds that of the etched fiber core with a diameter of 90 µm. Because of the relatively short wavelength relative to the film thickness and the high realrefractive index, it is expected that the sensing layer can become a waveguide under certain modal excitation conditions. This waveguide supports not only optical modes still confined in the core by Fresnel reflection at the boundary between the core and the ZIF-8 (or Co/ZIF-8) film but also some optical modes guided by the ZIF-8 film with the high index of refraction due to total internal reflection. The intensity pattern of the fundamental mode in the MOF coated optical fiber at 220 nm is shown in the inset of Figure 4, and these optical modes supported in the ZIF-8 (or Co/ZIF-8) film with very large extinction coefficient (k = 0.11 at wavelength of 220 nm) are high loss modes, leading to a very strong enhancement of light-matter interaction compared to the evanescent wave sensing regime. Figure 5 shows the simulated evolution of light intensity along the ZIF-8 integrated sensor device with ambient N2 and CO2 gases. The additional transmission loss with 100% ambient CO2 gas compared to that with 100% ambient N2 gas is created by an increase in the RI of the ZIF-8 film estimated to be ~0.005 according to Ref. 40. The intensity oscillations at the front sections of the unetched/etched and etched/unetched fiber transition regions indicate that the mode coupling and interference would occur. The intensity decreases exponentially with the increasing of the etched fiber length due to the high extinction coefficient of ZIF-8 while the intensity flattens in the multimode fiber sections considering the low attenuation coefficient in the pristine optical fiber. The simulation result shows that the reduction

Figure 5. Simulated evolution of light intensities along the sensor device with the ambient (black) N2 gas and (red) CO2 gas at 220 nm wavelength.

CONCLUSION Here we show that integration of nanoporous MOF thin film on optical fibers provides a way for designing high performance of chemical sensors towards outperforming sensitivity/selectivity of target molecules. In comparison with the normal incidence transmission spectroscopy (e.q. Fabry-Perot devices) based MOF sensors,12-14 sensing gases can be developed based upon extremely small differences in RIs. This distinction is due to the waveguide-based device platform which results in an increased propagation of electromagnetic radiation within the sensing layer and hence an effective amplification of inherent optical absorption present within the MOF sensing layer with increasing values of effective RI associated with adsorption of gases. Co/ZIF-8 with d-d band transition was used as a separate observation with a similar CO2-dependence in order to verify a RI change induced by gas adsorption. The results show that the intensities of incident light for both the d-d band transition of Cobalt ion and the absorption band of the organic linker decreased with increased gas concentration proportionally. This is consistent with a sensing mechanism that results from a larger fraction of incident light propagating within the MOF layer due to a modification of the real part of the RI, leading to a stronger absorption by the organic linker of the MOF materials. In summary, we report a novel waveguide-based chemical sensing platform with MOF thin film on optical fiber. The ZIF-8 MOF coated optical fiber sensor studied in this paper

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shows a high selectivity to CO2 gas relative to other small gases (H2, N2, O2, and CO), which is originated from the specific adsorption capability of the ZIF-8 MOF. In addition, the sensor exhibits an excellent reversibility with rapid (< tens of seconds) response time at atmospheric conditions (room temperature and 1 bar) without pretreatments such as heating and/or evacuating under vacuum for the sake of sensor regeneration. Although non-dispersive infrared absorption (NDIR) sensor is widely used as CO2 sensor,47 it makes them unsuitable for mobile sensing applications due to the necessity of a bulky optical system. On the other hand, chemiresistive sensors, based on metal oxides, have been proposed as miniature CO2 sensors, feasibly integrated into a chip and is most suitable for mobile sensors in terms of simplicity; however they are typically required high temeprature to promote reaction of surface-bound chemical species, and they exhibit cross-sensitivity and significant baseline drift over the life of the sensors.48,49 Considering that this CO2 sensor approach outperforms other approaches reported to date in detecting CO2 selectively and reversibly, we anticipate that this new type of gas sensing system could develop a wide range of optical sensing applications such as monitoring of CO2 in geological formations relevant for carbon capture and sequestration as well as monitoring in the flue gas of a power plant amongst a range of other industrial process applications.50-53 Moreover, the sensing response of the MOF-coated optical fiber depends on the amount of gases adsorbed, which also makes it possible to sense a range of gas molecules by designing or discovering MOF materials that respond very selectively to specific gas molecules. This sensing system could also be miniatured for mobile sensing system, and will significantly accelerated the progress of gas sensors in the field application. However, the sensors are often affected by humidity and temperature, therefore it is necessary to investigate the effect of water and temperature on the sensing response, which are under way.

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ACKNOWLEDGMENT This technical effort was performed in support of the National Energy Technology Laboratory's ongoing research under the RES contract DE-FE0004000. This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with AECOM. Neither the United States Government nor any agency thereof, nor any of their employees, nor AECOM, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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ASSOCIATED CONTENT Supporting Information. FE-SEM images, XRD pattern, optical responses to various gases with different concentrations, transmission spectra, reflectance and stability test of MOF coated optical fiber sensor and planar substrates. Adsorption isotherms of ZIF-8 nanoparticles. Simulated total diffusion flux of MOF coated optical fiber sensor. “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

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capture in metal-organic frameworks. Chem. Rev. 2012, 112, 724781. (53) McDonald, T. M.; Lee, W. R.; Mason, J. A.; Wiers, B. M.; Hong, C. S.; Long, J. R. Capture of carbon dioxide from air and flue

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gas in the alkylamine-appended metal-organic framework mmenMg2(dobpdc). J. Am. Chem. Soc. 2012, 134, 7056-7065.

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SYNOPSIS TOC. “for TOC only”

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