Ni-MOF-74 as sensing material for resonant-gravimetric detection of ppb-level CO

Accepted Manuscript Title: Ni-MOF-74 as Sensing Material for Resonant-gravimetric Detection of ppb-level CO Authors: Yanqing Lv, Pengcheng Xu, Haitao Yu, Jiaqiang Xu, Xinxin Li PII: DOI: Reference:

S0925-4005(18)30334-4 https://doi.org/10.1016/j.snb.2018.02.058 SNB 24153

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

30-9-2017 5-2-2018 6-2-2018

Please cite this article as: Yanqing Lv, Pengcheng Xu, Haitao Yu, Jiaqiang Xu, Xinxin Li, Ni-MOF-74 as Sensing Material for Resonant-gravimetric Detection of ppb-level CO, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.02.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ni-MOF-74 as Sensing Material for Resonantgravimetric Detection of ppb-level CO Yanqing Lva,b, Pengcheng Xua, Haitao Yu*,a, Jiaqiang Xub and Xinxin

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Li*,a

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a. State Key Lab of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of

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Sciences, 865 Changning Road, Shanghai 200050, China.

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b. Department of Chemistry, College of Science, Shanghai University,

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99 Shangda Road, Shanghai 200444, China

*Corresponding authors: Tel.: +86 21 62131794; fax: +86 21 62131744

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E-mail: [email protected] (H.T. Yu); [email protected] (X.X. Li)

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Graphical abstract Ultrasensitive detection of 10 ppb CO has been realized using metal organic framework (MOF) crystals of

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Ni-MOF-74 as gravimetric-type sensing material with resonant microcantilever sensor.

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Abstract:

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For the first time, ultrasensitive detection of 10 ppb CO has been realized using metal organic framework (MOF) crystals of Ni-MOF-74 as gravimetric-type sensing material with

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resonant microcantilever sensors. Due to the specific interaction between CO molecules and the active transitional metal centers of Ni2+ in the Ni-MOF-74 crystal, the Ni-MOF-74

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material with ultra-high surface area and nanoporous structure possesses large absorption capacity to CO gas. By loading the Ni-MOF-74 material onto the resonant microcantilever,

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the formed gravimetric sensor exhibits high-performance sensing to CO at trace level. The experimentally obtained limit of detection (LOD) of the sensor to CO reaches finer than 10 ppb (parts per billion in volume), which indicates an extremely high CO-detection sensitivity

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of the sensor. Besides the excellent sensitivity, the sensor also shows remarkable repeatability and long-term stability. Eight kinds of interfering gases are tested and a fine selectivity of the sensor is verified. This work indicates that MOF materials such as Ni-MOF-74 are promising in gravimetric-type gas sensing applications.

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Keywords: Gas sensors; Carbon monoxide; Metal organic framework; Resonant

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microcantilever; Gravimetric-type sensor

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Introduction Carbon monoxide (CO) is one of the most dangerous toxic gases since it is colorless, odorless and tasteless that makes it undetectable to human sensing system [1-3]. Besides, CO is an indirect factor that influences on climate change and global warming [4, 5], and one of the major air pollutants produced by a variety of incomplete combustion activities [6, 7]. In

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medical domain, CO is found as a physiological tracer for diverse diseases including Parkinson’s, atherosclerosis and Alzheimer’s, etc [8]. Thus, CO sensors are highly demanded,

especially the ultra-sensitive ones which can detect CO in trace level. To date, trace-level

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detection of CO is mainly based on bulky instruments such as gas chromatography-mass spectrometer (GC-MS) or expensive optical devices such as quantum cascade laser (QCL) [9]. Compared with them, chemical sensors have the merits of small volume, low cost, easy to use

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and on-site detection [10, 11]. However, the sensitivity of chemical sensors is still unsatisfied

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and cannot achieve the requirements of the practical usages.

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To enhance the sensitivity, sensing materials with enormous sensing sites are of critical

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importance. In this respect, the metal-organic frameworks (MOFs) material with ultra-high specific surface area is an ideal candidate [12]. Actually, MOFs materials have exhibited

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considerable capability in molecules capturing/storing and have been widely used as gas adsorbent [13-16]. However, to the best knowledge of the authors, only a few literatures with

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respect to gas sensing applications based on MOFs materials have been reported [17-23]. The possible causes may lie in that the insulated characteristic of MOFs materials made them

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difficult to construct the frequently-used chemiresistive-type gas sensors [22]. Fortunately, gravimetric-type transducers such as resonant cantilever or quartz crystal microbalance (QCM) are good at transforming adsorbing-induced mass-change into electrical sensing signals [18,

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24-27]. In this work, integrated resonant microcantilever is chosen as the gravimetric sensing device. The resonant microcantilever can convert the mass increase induced by the target analyte molecules adsorption into the decrease of the resonant frequency of the microcantilever. When the adsorbed mass, Δm, is much smaller than effective mass of the resonant microcantilever itself, meff, the frequency-shift signal can be considered being proportional to Δm, i.e. Δf ≈ −0.5 Δm (f0 / meff), where f0 is the initial resonant frequency 4

before mass adsorption. Thus, according to the designed sensitivity, the adsorbed mass of the target analyte can be calculated from the frequency-shift signal. Following the abovementioned working mechanism, nanoporous materials such as MOFs are advantageous as gravimetric-type sensing material owing to their ultra-high surface area, abundant specific adsorbing sites and large volume for molecules capturing. Herein, the MOF-74 crystals are developed as the gravimetric sensing material for trace-

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level CO detection. With regard to the use of MOFs as the sensing material, the CO capturing capability depends on the activity of the metal ions in the MOFs. Because CO molecule tends

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to be chelated with transitional metal ions to form coordinated complex [28], four kinds of MOF-74 crystals with different transitional metal ions are synthesized and compared in the CO sensing experiment. The Ni-MOF-74 material shows the best sensitivity and is chosen as

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the CO sensing material. Then the Ni-MOF-74 material is characterized and evaluated in a

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series of CO sensing experiments. The sensor has experimentally exhibited high sensitivity,

Experimental section

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good repeatability, long-term stability and satisfactory selectivity to CO gas.

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Chemicals. Nickel(II) acetate, 2,5-dihydroxyterephthalic acid and tetrahydrofuran (THF) were purchased from Sigma-Aldrich. Standard CO, CO2 and H2 was purchased from Shanghai

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SHENKAI Gases Technology Co., Ltd. Synthesis of Ni-MOF-74. Firstly, 1.306 g of Nickel(II) acetate is added into 35 mL of

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deionized water under stirring. After Nickel(II) acetate is completely dissolved in water, Stock solution (I) is formed. Meanwhile, 0.52 g of 2,5-dihydroxyterephthalic acid is dissolved completely into 35 mL of THF with stirring to form Stock solution (II). Then, Stock solution

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(II) is quickly poured into Stock solution (I). After mixing, the solution is transferred into a Teflon-lined autoclave and heated in an oven at 110°C for 3 days. After cooling down to room temperature, the precipitates are centrifugated and washed with deionized H2O for three times. Next, the products are immersed into 120 mL of ethanol for another 1 day. At last, the ethanol solvent is decanted and the solids are heated at 150°C under vacuum for 12 hours to obtain the Ni-MOF-74 crystal products. 5

Fabrication of the resonant microcantilever. The silicon resonant microcantilever is designed and top-down fabricated with the micromachining techniques in our lab. The microcantilever is integrated with a resonance-excitation heating resistor and a signal-readout piezoresistive full Wheatstone-bridge. The length, width and thickness of the microcantilever are designed as 200 μm, 100 μm and 3 μm, respectively. More technical details of the microcantilever can be found in our previously published literature [29].

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Characterization. The morphologies of the as-synthesized samples are characterized using a

field-emission scanning electron microscope (FE-SEM, Hitachi S4800) and a transmission

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electron microscope (TEM, FEI Tecnai G20). X-ray diffraction (XRD) patterns are obtained

using a Bruker model D8 focus diffractometer equipped with a copper anode to produce Xrays (40 kV, 40 mA). The produced X-ray beam contains Cu radiation with wavelengths K1

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= 0.15406 nm, K2 = 0.15444 nm and the intensity ratio of Cu K1/Cu K2 is 2:1. The data

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are collected in a 5° to 40° continuous scan mode, with a 0.02° sampling interval. Nitrogen

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sorption isotherm is measured at 77 K using a Micromeritics ASAP 2020M system. Specific surface area and pore size distribution are calculated using Brunauer-Emmett-Teller (BET)

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and density functional theory (DFT) methods.

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Loading Ni-MOF-74 onto microcantilever. The Ni-MOF-74 material (10 mg) is added into 1 mL of deionized water (under ultrasonic) to form a suspension, which is used as ink in the following material deposition. Then, several drops of the ink are printed onto the top surface

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near the free end of the microcantilever using a commercial GIX II Microplotter (Sonoplot Inc.). Thereafter, the microcantilever is dried in an oven at 60°C for 2 hours.

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CO sensing setup. The CO sensing setup is schematically shown in Fig. 1. There are three independent gas inlets: a standard CO gas, a diluting N2 gas and a balance N2 gas. The flow rate of the gases flowing into them is controlled by a mass flow controller (MFC), respectively.

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The CO gas and the diluting N2 gas are introduced into a glass gas mixing chamber to make the measure gas with the desired CO concentration (10 ppb-2.6 ppm). It should be pointed out that two types of standard CO gas are used: one is with concentration of 200 ppb, and the other one is 5.2 ppm (diluted by N2 for both standard gases). The measure gases with CO concentration lower than 200 ppb use the former standard CO gas, while the measure gases higher than 200 ppb use the latter one. The fabricated sensor is sealed in a glass testing 6

chamber with a gas inlet and outlet. A four-way valve is used to switch the gas into the testing chamber between the measure CO gas and the balance N2 gas. The testing chamber is set into a heating oven to keep the temperature stable. A lab-made interface circuit with software PLL (phase-locked loop) is used to record the resonant frequencies of the microcantilever in real time. Firstly, the balance N2 gas with a fixed flow rate of 200 mL/min is introduced into the testing chamber and the microcantilever sensor vibrates in N2 atmosphere for more than 30

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minutes to warm up. Then the resonant frequencies of the sensor begin to be recorded as the

base line. After a few minutes, the measure CO gas with the desired concentration is

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introduced into the testing chamber with the help of the four-way valve. It must be noted that the flow rate of the measure CO gas (i.e., sum of the standard CO gas and the diluting N2 gas) should be as same as the balance N2 gas to eliminate the interference of the flow rate

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fluctuation. The resonant frequency of the sensor will decrease due to the specific adsorption

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of CO molecules. When the changes of the resonant frequency become very slowly, which

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indicates that the molecule adsorption is close to the saturation state, the gas passing through the testing chamber is switched back to the balance N2 gas. The resonant frequency of the

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sensor will gradually increase to the base line. These steps can be repeated for several times.

Results and discussion

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Due to the reversible adsorption/desorption capability to CO, MOF-74 materials exhibit potential applications in CO separation from gas mixtures [30]. Inspired from the CO

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separations, MOF-74 materials have been synthesized for gravimetric-type CO sensing applications. Software of Materials Studio with Sorption module is used to explore the sensing mechanism of the MOF-74 crystal to CO molecules. The simulation result in Fig. 2(a)

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indicates that CO molecules tend to be adsorbed onto the metal site of the Ni-MOF-74 material. It should be noted that MOF-74 material can be obtained with various kinds of metal sites such as Mg, Mn, Fe, and Ni. Obviously, different metal sites bring different adsorption capabilities of the MOF-74 material. Thus, it is critical to quantitatively compare the strength of the adsorption capabilities of the MOF-74 material synthesized with different metal sites. Adsorption enthalpy -ΔH (i.e., isosteric heat of adsorption Qst) can be used to quantitatively 7

indicate the interaction strength between the adsorbent material and the target molecules. According to the data reported in literature, the four kinds of M-MOF-74 (M= Ni, Fe, Mn, Mg) materials feature moderate adsorption enthalpy -ΔH in the range of 29.4 kJ/mol to 52.7 kJ/mol [31]. The moderate value of -ΔH implies a reversible but specific interaction between the MOF materials and the CO molecules, which is suitable for CO sensing. Among the four kinds of M-MOF-74 (M= Ni, Fe, Mn, Mg), Ni-MOF-74 has the highest value of -ΔH which

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is anticipated to display a good sensing performance to CO. The above-mentioned deduction is verified using resonant microcantilever as gravimetric-type sensing platform. As shown in

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Fig. 2(b), Ni-MOF-74 shows the highest response to CO with identical concentration.

Considering that the four kinds of MOF-74 materials have similar surface area and pore volume as shown in Table 1 (measuring method will be described later), therefore, Ni-MOF-

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74 is selected as the optimized sensing material for the following characterization and

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gravimetric-type sensing evaluations.

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Figure 3 shows the SEM images of the Ni-MOF-74 sample prepared in this work. The crystals of Ni-MOF-74 are small polyhedral aggregates, just like flowers. The particle size is

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in the range of 1-10 μm. The sample is further characterized by TEM. In low magnitude TEM

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images shown in Fig. 4(a), the flower-like structure can be clearly observed. The high resolution TEM images in Figs. 4(b)-4(c) clearly show that the Ni-MOF-74 crystals are with numerous nanopores.

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The powder XRD is used to confirm the phase structure of the Ni-MOF-74 samples. The XRD pattern shown in Fig. 5(a) well matches the already published XRD patterns of Ni-MOF-

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74 material [32]. The two main diffraction peaks (6.8° and 11.8°, 2h) indicate that the assynthesized sample has the correct structure with good crystallinity. In order to determine the pore textural properties, N2 adsorption/desorption isotherm on the Ni-MOF-74 sample at 77

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K is measured and depicted in Fig. 5(b). The nitrogen isotherm is of a typical type I isotherm by IUPAC definition, with a very sharp uptake at P/P0 from 10-5 to 10-1, showing a signature characteristic of microporous (pore sizes in the range of 0.5 to 1.8 nm) materials. The poresize distribution in the inset of Fig. 5(b) indicates that the average pore size is about 0.9 nm. According to the standard N2 sorption measurements, the BET surface area of the material is as high as 1152 m2/g and the total pore volume reaches 0.56 cm3/g. The porous structure of 8

the obtained Ni-MOF-74 material is potential for gravimetric-type sensing applications. With the help of inkjet printer, the as-prepared Ni-MOF-74 material is precisely deposited onto the designated micro-region of a lab-made resonant microcantilever. Figure 6 shows the SEM image of the microcantilever sensor with the Ni-MOF-74 material loaded uniformly on its free-end surface. Then the sensor is put into the testing chamber for CO detecting experiments. When the sensor is exposed to CO-containing atmosphere, the CO molecules

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will be selectively captured by the MOF material. Figure 7(a) shows the real-time sensing

curve of the sensor to CO with various concentrations ranging from 10 ppb to 2.6 ppm at 25C.

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The sensing experiments for different concentrations of CO are performed continuously. The

obtained data sets show that the sensor response to as low as 10 ppb concentration CO can be well recognized and the sensing signal increases with the raising CO concentration.

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Considering that the noise of the base-line of the output signals is about ±0.1 Hz, the LOD

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(limit of detection) of the sensor to CO is far finer than 10 ppb based on the results of the inset

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of Fig. 7(a). Compared with the previous reported nano-materials coated gravimetric CO sensors, the Ni-MOF-74 based microcantilever sensor shows a superior sensitivity to CO [33-

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35]. Both the microcantilever transducer and the MOF sensing material contribute to the good

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sensing performance. Considering that sub-10-ppb LOD is better than most of the microcantilever gas sensors (although the target gas may be different) [24-26, 29], the NiMOF-74 is an excellent CO sensing material with no doubt. Figure 7(b) shows the relationship

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between the response signals and the CO concentrations, which well fits the function of Langmuir equation and Freundlich isotherm.

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Based on the designed mass sensitivity of the microcantilever (i.e. 1.5 Hz/pg), it is facile

to calculate the adsorbed CO molecule number under a given concentration. For example, the response value, f, is obtained as 8.7 Hz for 40 ppb CO shown in Fig. 7(a). Thus, the adsorbed

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mass of CO molecules can be calculated as 5.8 pg. According to the known CO molecular weight of 28 g/mol, the number of adsorbed CO molecules, n, can be obtained as 0.21 pmol. As for the mass-type chemical sensor, the total adsorbing sites is more important than the specific surface area. Obviously, the sensing performance is directly determined by the number of adsorbing sites: more adsorbing sites of the sensing material will bring higher sensing response of the microcantilever sensor. Thus, it is meaningful to calculate the total 9

adsorbing sites of the sensing material. Here, the number of total adsorbing sites is represented as N, and the fractional coverage, , can be defined as

=n/N

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Substituting the definition of =n/N into Langmuir equation of =Kp/1+Kp, the following relationship can be obtained: p/n=1/KN+p/N

(2)

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where K is the equilibrium constant, and p is the partial pressure. Fitting the curve in Fig. 7b with Equation (2), the value of N can be calculated as 1.3 pmol. Hence, for 40 ppb CO, the

fractional coverage  can be obtained as =0.21/1.3=16%, which is far smaller than 1. Thus,

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there are still abundant adsorbing sites for further CO molecule sensing and the sensor exhibits potential for CO detection in a higher concentration.

The repeatability of the sensor is also investigated. The six-time repeated response signals

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to 240 ppb CO are recorded in Fig. 8(a). The sensor responses are always 22.6±0.3 Hz and

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the relative standard deviation (RSD) is calculated as 1%, which indicates satisfactory

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detection repeatability. Long-term stability of the sensor is also very important. The responses

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of the same sensor to 240 ppb CO which are obtained with a time interval of six months are recorded and compared in Fig. 8(b). The sensor is preserved in ambient atmosphere, and after

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six months only 10.5% sensitivity attenuation of the sensor can be observed, which indicates a good long-term stability. To validate the uniformity between different sensors in a batch, we

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choose four CO sensors which possesses similar resonant frequency shift before and after NiMOF-74 material coating (i.e., similar coated mass of the MOF material). The responses of

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the four sensors to CO with 640 ppb concentration is drawn in Fig. 8(c). The maximum relative sensitivity error is less than 15%, which indicates that the fabrication repeatability can be ensured through device selection.

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In order to evaluate the selectivity of the sensor, eight kinds of common gases/vapors

including NH3, CO2, H2, C2H5OH, toluene, acetone, CH2Cl2 and hexane are selected as interfering gases. Among these gases, CO2 and H2 are from cylinder gases, and the others are generated by a Standard Gas Generator (model: 8000S, Jusun Instrument Co., Ltd.). The sensing results in Fig. 9(a) indicates that the sensor outputs smaller than 8.2 Hz frequencyshift to all of the interfering gases with concentration of 1 ppm, while the response of the 10

sensor to CO with the same concentration is as high as 40.2 Hz, which demonstrates that the Ni-MOF-74 based microcantilever sensor has a good selectivity to CO. Further experiments have been carried out to examine the sensor response to CO in the presence of interfering gas. To achieve CO detection in mixed environment, a different sensing setup is used. The CO sensor is sealed into a glass testing chamber with volume of 20 L, certain volume of acetone, CO2 and CO is one by one injected into the testing chamber through a small hole, and the

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concentrations of the mixed gases are controlled as 1 ppm that can be calculated through ideal gas equation of state. It should be pointed out that the liquid acetone is injected onto a heater

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in the testing chamber to form vaporized acetone. Finally, the lid of the testing chamber is

removed and the sensor is exposed in ambient air. The sensing results are shown in Fig. 9(b). In the presence of interfering gases of acetone and CO2, the sensor still can detect CO gas

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without obvious attenuation in sensitivity.

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Conclusion

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In summary, MOF materials of Ni-MOF-74 crystals with nanoporous structure is prepared and systematically characterized. CO sensing performances of the MOF materials have been

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investigated by using the gravimetric sensing platform of resonant microcantilevers. Due to the ultra-high specific surface area and specifically host-guest interaction, the Ni-MOF-74

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material shows satisfactory sensing performance to trace CO. The sensor shows a Langmuirtype response to CO within the concentration range of 10 ppb-2.6 ppm. The LOD of the MOF

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based sensor is better than 10 ppb. The sensor also shows good selectivity, repeatability and long-term stability. Such an ultrasensitive sensor is promising in CO detection and monitoring

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applications.

Acknowledgement This research is supported by MOST of China (2016YFA0200800), NSF of China

(61571430, 61527818, 61604163), Key Research Program of Frontier Sciences of Chinese Academy of Sciences (QYZDJ-SSW-JSC001). H. T. Yu and P. C. Xu appreciate the financial support of the Youth Innovation Promotion Association CAS (2017278 and 2016213). 11

Conflict of Interests

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The authors declare no conflict of interests.

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conductive 2D metal-organic frameworks, J. Am. Chem. Soc. 137 (2015) 13780-13783. [18] T. Xu, P.C. Xu, D. Zheng, H.T. Yu, X.X. Li, Metal-Organic frameworks for resonantgravimetric detection of trace-level xylene molecules, Anal. Chem. 88 (2016) 1223412240. [19] Y. Hwang, H. Sohn, P. Anh, O.M. Yaghi, R.N. Candler, Dielectrophoresis-assembled zeolitic imidazolate framework nanoparticle-coupled resonators for highly sensitive and selective gas detection, Nano Lett. 13 (2013) 5271-5276. [20] G. Lu, J.T. Hupp, Metal-organic frameworks as sensors: a ZIF-8 based Fabry-Pérot device as a selective sensor for chemical vapors and gases, J. Am. Chem. Soc. 132 (2010) 7832-7833. [21] E.-X. Chen, H. Yang, J. Zhang, Zeolitic imidazolate framework as formaldehyde gas sensor, Inorg. Chem. 53 (2014) 5411-5413. [22] M.G. Campbell, D. Sheberla, S.F. Liu, T.M. Swager, M. Dincă, Cu3(hexaiminotriphenylene)2: an electrically conductive 2D metal-organic framework for chemiresistive sensing, Angew. Chem. Int. Edit. 54 (2015) 4349-4352. [23] N.L. Torad, M. Hu, Y. Kamachi, K. Takai, M. Imura, M. Naito, Y. Yamauchi, Facile synthesis of nanoporous carbons with controlled particle sizes by direct carbonization of monodispersed ZIF-8 crystals, Chem. Commun. 49 (2013) 2521-2523. [24] H.T. Yu, P.C. Xu, X.Y. Xia, D.-W. Lee, X.X. Li, Micro-/nanocombined gas sensors with functionalized mesoporous thin film self-assembled in batches onto resonant cantilevers, IEEE Trans. Ind. Electron. 59 (2012) 4881-4887. [25] H.T. Yu, P.C. Xu, D.-W. Lee, X.X. Li, Porous-layered stack of functionalized AuNP– rGO (gold nanoparticles–reduced graphene oxide) nanosheets as a sensing material for the micro-gravimetric detection of chemical vapor, J. Mater. Chem. A 1 (2013) 44444450. [26] H.T. Yu, T.T. Yang, Y. Chen, P.C. Xu, D.-W Lee, X.X. Li, Chemo-mechanical joint detection with both dynamic and static microcantilevers for interhomologue molecular identification, Anal. Chem. 84 (2012) 6679-6685. [27] P.C. Xu, H.T. Yu, X.X. Li, Functionalized mesoporous silica for microgravimetric sensing of trace chemical vapors, Anal. Chem. 83 (2011) 3448-3454. [28] G. Sethia, R.S. Somani, H.C. Bajaj, Adsorption of carbon monoxide, methane and nitrogen on alkaline earth metal ion exchanged zeolite-X: structure, cation position and adsorption relationship, RSC Adv. 5 (2015) 12773-12781. [29] H.T. Yu, X.X. Li, X.H. Gan, Y.J. Liu, X. Liu, P.C. Xu, J.G. Li, M. Liu, Resonantcantilever bio/chemical sensors with an integrated heater for both resonance exciting optimization and sensing repeatability enhancement, J. Micromech. Microeng. 19 (2009) 045023. [30] L. Valenzano, B. Civalleri, S. Chavan, G.T. Palomino, C.O. Areán, S. Bordiga, Computational and experimental studies on the adsorption of CO, N2, and CO2 on MgMOF-74, J. Phys. Chem. C 114 (2010) 11185-11191. [31] E.D. Bloch, M.R. Hudson, J.A. Mason, S. Chavan, et al., Reversible CO binding enables tunable CO/H2 and CO/N2 separations in metal-organic frameworks with exposed divalent metal cations, J. Am. Chem. Soc. 136 (2014) 10752-10761. [32] X.F. Wu, Z.B. Bao, B. Yuan, J. Wang, Y.Q. Sun, H.M. Luo, S.G. Deng, Microwave 14

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Biographies Yanqing Lv received her B.S. degree (2015) in Chemistry from Lanzhou University. Presently she is pursuing her M.S. degree in Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. Her research is focused on masstype gas sensing materials.

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Pengcheng Xu received his B.S. degree in Applied Chemistry from Zhenzhou Institute of

Light Industry (2005), and his M.S. degree in Inorganic Chemistry from Shanghai University

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(2008). Now he is an assistant professor at the State Key Laboratory of Transducer

Technology, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences. His present research covers advanced sensing materials (like

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nanowires, hyperbranched polymers and nanoporous materials), micro/nanofluidic chips and

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MEMS-based chemical sensors. Since 2016, he has been appointed as a member of Youth

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Innovation Promotion Association CAS.

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Haitao Yu received his B.S. degree (2004) in Physics from Peking University, and his Ph.D.

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degree (2009) in Microelectronics and Solid State Electronics from Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. He is currently an Associate Professor at State Key Laboratory of Transducer Technology, Shanghai Institute of

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Microsystem and Information Technology, Chinese Academy of Sciences. His present

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research interests include microcantilever chemical sensors and lab-on-chip.

Jiaqiang Xu obtained his B.S. degree from Zhengzhou University, M.S. degree from University of Science and Technology of China and Ph.D. degree from Shanghai University.

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From 2001 to now, he has been promoted as a professor. Presently, he served as the director of Chinese Professional Committee on Gas and Humidity Sensing Technology. His research interests include synthesis of nanostructured materials and gas sensors. Xinxin Li received B.S. degree from Tsinghua University, Beijing and Ph.D. degree from Fudan University, Shanghai. Thereafter, he sequentially worked in Hong Kong University of Science and Technology as a Research Associate, in Nanyang Technological University, 16

Singapore as a Research Fellow and, then, joined Tohoku University, Japan, as a Lecturer (COE fellowship). From 2001 to now, he has been a professor and now serves as the Director of the State Key Lab of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. He has also served as Adjunct Professor in Fudan University, Shanghai Jiaotong University, Dalian University of Technology, ShanghaiTech University and Suzhou University. From 2009 to 2013, he had

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served as Consultant Professor for World Class University Program of Korean in Chonnam National University, Korea. He was granted the National Science Fund for Distinguished

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Young Scholar in 2007. His PhD student was awarded National Excellent 100 PhD Dissertation in 2009. Prof. Li’s research interest includes micro/nano sensors and MEMS/NEMS. He has invented about 80 patents and published numerous papers in referred

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journals and conferences (including about 150 SCI journal papers). He ever served as TPC

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member for the conferences of IEEE MEMS, Transducers and IEEE Sensors. He is now the

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editorial member for Journal of Micromechanics and Microengineering and the International

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Steering Committee member for Transducers.

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Figure Captions Fig. 1. Schematic of the CO sensing setup. Fig. 2. (a) Materials Studio simulation results that show CO adsorbing sites in the Ni-MOF74 crystals. (b) Comparison of responses among the resonant microcantilever sensor loaded with four kinds of M-MOF-74 (M=Fe, Mn, Mg, Ni) to 240 ppb CO.

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Fig. 3. SEM images of the prepared Ni-MOF-74 sample. Fig. 4. TEM images of the prepared Ni-MOF-74 sample.

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Fig. 5. XRD pattern (a) and N2 sorption isotherms (b) of the prepared Ni-MOF-74 sample.

The inset of (a) plots the molecular model of a fragment of Ni-MOF-74 crystal and the inset of (b) shows the pore-size distribution of Ni-MOF-74 material.

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Fig. 6. The MOF-loaded resonant microcantilever sensor where the Ni-MOF-74 material is

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regio-selectively deposited onto the free-end of the microcantilever.

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Fig. 7. (a) Responses of the sensor to various concentrations of CO gas ranging from 10 ppb to 2.6 ppm. The inset shows the enlarged view of the sensor response to CO with ultra-low

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concentration of 10 ppb. (b) The relationship between the response signals and the CO

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concentrations, which well fits the function of Langmuir equation and Freundlich isotherm. Fig. 8. (a) Rapidly reproducible and repeatable response of the MOF-loaded resonant microcantilever sensor to CO with identical concentration of 240 ppb; (b) Lone-term stability

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testing results of the sensor to CO. The two sensing responses of the same sensor to 240 ppb CO are obtained with an interval of two weeks; (c) Responses comparation of four different

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sensors to 640 ppb CO.

Fig. 9. (a) Responses of the MOF-loaded resonant microcantilever sensor to various interfering gases and CO with the identical concentration of 1 ppm; (b) Sensor response to

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CO in the presence of interfering gases of acetone and CO2.

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