Quartz crystalmicrobalance (QCM) is a useful tool for detection themass change at the electrode surface in real time ^{[1, 2, 3]}. Usually, a QCM sensor is constructed with an AT-cut piezoelectric quartz crystal (PQC) disk sandwiched between two metal film electrodes, which induce the PQC resonator to oscillate at a frequency in the MHz. When an ultrathin,homogeneous and rigid mass loading is deposited on the electrode surface,a relationship between the frequency shift (△F) and mass change (△m) is expressed by the Sauerbrey equation ^[4].

Metal-organic frameworks (MOFs),a particular class of ordered porous solids,hold promise to solvemany key challenging societal needs (e.g.,hydrogen storage,carbon dioxide capture,renewable catalysts,controlled drug delivery) ^{[5, 6, 7, 8, 9, 10, 11]}. Recently,QCM is applied to characterize the adsorption of MOFs films ^{[12, 13, 14, 15, 16]}. In this work,a modified QCM sensor,separated-electrode piezoelec- tric sensor (SEPS),was used to monitor the adsorption process of iodine vapor on zeolitic-imidazolate framework-8 (ZIF-8) film. The motive of design SEPS is to eliminate the influence of the electrode corrosion by l₂ on the measurement of the resonant frequency. In the configuration of an SEPS (Fig. 1A),a naked PQC disk ismounted between two electrodes with a total air gap of several millimeters. The high-frequency excitation field is applied to the PQC resonator by the conductance of the air layer. The mass change on the separated-electrodes does not change the resonant frequency of the PQC resonator anymore. The influence of the electrode gap on the response of the SEPS was investigated by an impedance analysis method. The transfer of iodine between two ZIF-8 films was monitored by a series combination of two SEPS sensors.

Fig. 1

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Fig. 1.Schematic drawing of the configuration of the SEPS (A) and experimental setup for the measurement of iodine transfer (B) (not to scale) for ZIF-8.

A schematic representation of the experimental setup is illustrated in Fig. 1. AT-cut 6.03 MHz quartz crystal discs with diameter of 14 mm were purchased from Beijing Chenjing Electronics Co.,Ltd. (China). Two copper discs (with a diameter of 8 mm) were used as the excitation electrodes. A PQC disk was held by the bottom electrode with the help of a glass plate. The distance between the upper electrode and PQC was measure by a magnifier with scale. The electrodes were connected to a precision impedance analyzer (4294A,Agilent) by coaxial-cables. The resonant frequency and the intensity of the resonant peak were measured. In the experiment of iodine adsorption,ZIF-8 film was grown on one of the PQC surface by contact with the mixture of 25mmol/L zinc nitrate and 50mmol/L 2-methylimidazole in methanol. Then the SEPS with upwards ZIF-8 film was mounted in a sealed glass box. The electrode gap was ca. 2mm. With the frequency of the blank quartz resonator as the reference,the frequencydecrease of SEPSwithZIF-8filmwasmeasuredtoestimate themass offilmgrown. The thickness of the filmwas calculated from the mass per area by using the density of 0.95 g/cm^{3 [17]}.

The adsorption was started by adding 20 mg l₂ (in 1 g NaCl powder) in the box. In the experiment for iodine transfer,a series combination of two SEPS sensors was employed (Fig. 1B) and their resonant frequencies were measured in two independent scans. Prior to l₂ transfer,a PQC with ZIF-8 film was exposed to saturated l₂ vapor for 3 h and then mounted on the bottom of the cell. Immediately,the glass ring (with a thickness of 2 mm) coved by another PQC with a virgin ZIF-8 filmwere added. The amounts of l₂ adsorbed or desorbed were calculated from the frequency shifts of the SEPS sensors.

As shown in Fig. 2A,a symmetrical resonant peak occurs in the conductance-frequency curve of the SEPS. The frequency at maximum conductance (G_max) is corresponded to the resonant frequency of the SEPS (f_max). Because of the high impedance of the air layer,the value of G_max in the SEPS is much less than that in a conventional QCM in air (30-200 mS). But the width of the resonant peak at half of the peak height,f₂ - f₁,is only 28 Hz. Accordingly,the quality factor of the SEPS,calculated by Q = f_max/ (f₂ - f₁),is as high as 2.2 × 105 ,which is close to that of a conventional QCM with the same dimensions. With increasing electrode gap,the value of G_max in SEPS is decreased remarkably (Fig. 2B). However,that the quality factor of SEPS is reduced slightly (data were not shown). For example,Q = 7.9 × 104 was measured in the case with an electrode gap of 12 mm. The high quality factor in SEPS is reasonable because the air layer can be equivalent to a capacitor with little energy loss in conducting excitation field. On the other hand,the resonant frequency of the SEPS is independent of the mass change in the electrode. Because the resonant frequency of the SEPS increases with increasing electrode gap,it is essential tomaintain a constant electrode gap in its applications. The SEPS with a shorter electrode gap is favorable to improve the signal-to-noise ratio in the measurement of resonant frequency. In fact,a conventional QCMmay be considered as a special SEPS with zero electrode gap. By used an extrapolation, f_max = 6.046052 MHz and G_max = 87.2 mS were obtained at zero gap size. With the constant dimension and position of the electrodes used,the Sauerbrey equation in Eq. (1) is also valid for an SEPS. Hence,the SEPS is a useful mass sensor,especially in a corrosive gaseous phase.

Fig. 2

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Fig. 2. Conductance-frequency curve of SEPS with electrode gap of 2 mm (A) and dependence of the resonant frequency and intensity of the resonant peak of SEPS on the electrode gap (B).

ZIF-8 is an excellent sorbent for molecular l₂ because of its suitable pore aperture size,large specific surface area,and high chemical and thermal stability ^{[18, 19]}. With ZIF-8 film grown on PQC surface,the kinetic process of the adsorption l₂ vapor on the filmwasmonitored by the SEPS. In this experiment,the gap size of 2 mmwas chosenwith the compromise of resonance intensity and diffusion rate of l₂ to ZIF-8 film. As can be seen in Fig. 3A,the amount of l₂ adsorbed increases rapidly in the initial 30 min and approaches to a plateau after 4 h. For a ZIF-8 filmwith thickness of 0.38 μm,the amount of l₂ adsorbed is about 87.9 μg/cm² , corresponding to a high adsorption capacity of 2.4 g/g. In a control experiment,the adsorption equilibrium of l₂ on quartz surface is achieved within 10 min with a mass adsorbed of 0.27 μg/cm² , indicating the condensation of l₂ vapor on the cell wall is slight. It should be pointed out that l₂ shows strong chemical adsorption on metal surface. When a conventional QCM with Au film electrodes was employed,the mass of l₂ adsorbed in ZIF-8 film on Au surface is higher than that in ZIF-8 film on quartz surface. The adsorption did not reach equilibrium even after a contact time of 8 h. On the other hand,the intensity of the resonant peak of the QCM is decreased obviously in the later stage of adsorption. This result indicates that part of l₂ passes through ZIF-8 film and adsorbed on the Au surface,resulting in the corrosion of the Au electrodes. In the case of SEPS,the corrosion of the Cu electrodes was observed fromthe change in color of electrode surface. But the Cu electrodes are separated fromthe PQC resonator. As a result,the adsorption of l₂ on Cu electrodes does not change the resonant frequency of SEPS. Hence,a SEPS offers the advantage over a conventional QCMin the situation of corrosive gases. On the other hand,the naked quartz disk used in SEPS offers the convenience in the combination of mass and optical measurements.

Fig. 3

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Fig. 3.Adsorption of l2 at saturated vapor on ZIF-8 films and quartz surface (A) and transfer of l2 between two ZIF-8 film (B). The thickness of ZIF-8 films is 0.38 μm.

When the ZIF-8 film with pre-adsorbed l₂ was coexisted with a virgin ZIF-8 film in a sealed cell,as can be seen in Fig. 3B,the transfer of l₂ took place. The transfer process is related to the volatility of l₂ and the adsorption reversibility of l₂ in ZIF-8 film.Part of l₂ was escaped from the ZIF-8 film with higher adsorption and was adsorbed in the film with lower adsorption. Under the experimental conditions used,the amount of l₂ adsorbed in the virgin ZIF-8 film is close to the amount of l₂ desorbed from pre- adsorbed ZIF-8 film,except in the initial 3 min,indicating the transfer of l₂ between the two films. After 30 min,the total l₂ adsorbed in the two films approached to a plateau of 1.97 g/g, which is less slightly than the initial of 2.08 g/g. The loss of l₂ is ascribed to the presence of l₂ in the gaseous phase in equilibrium with the films and adsorption by the wall in the initial stage. The amounts of l₂ adsorbed on the two ZIF-8 films approached to plateaus after 100 min. Noted that the plateau in ZIF-8 film with l₂ pre-adsorbed is higher than that in the virgin film. The reason is that the adsorption of l₂ in ZIF-8 film is partly irreversible with respect to the concentration dilution of the adsorbate due to the chemisorption mechanism ^[18]. The kinetics of the transfer of l₂ between different MOFs films is under investigation.

This work demonstrates that the SEPS in gaseous phase is oscillated with excellent frequency stability. With increasing electrode gap,the resonant frequency increaseswhile the intensity of the resonant peak reducesmarkedly,the quality factor decreases slightly. In the design of SEPS,the mass change on the electrode surface does not shift the resonant frequency,eliminating the influence of electrode corrosion on the resonant frequency measurement. It was shown that the adsorption of l₂ at its saturated vapor on quartz surface and ZIF-8 film with thickness of 0.38 mm are 0.27 μg/cm² and 110 μg/cm² ,respectively. The transfer of l₂ between two ZIF-8 films approaches to equilibrium after a contact time of 100 min. The SEPS offers the advantage over a conventional QCM in the situation of corrosive gaseous phase.

The authors gratefully acknowledge financial support by National Natural Science Foundation of China (Nos. 21175084, 21275091),theOpening Fund ofKey Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Hunan Normal University),Ministry of Education (No. KLCBTCMR2001-01) and Research Fund for the Doctoral Program of Higher Education of China (No. 20113704110003).