Chiral metal–organic framework coated quartz crystal microbalance for chiral discrimination

Abstract

The development of novel methods for chiral discrimination remains a challenging and important area due to the structural similarity but significantly different roles of enantiomers in biological and environmental systems. Here we report a 3D chiral porous Zn–organic framework (Zn₂(bdc)(L-lac)(dmf)·DMF) coated quartz crystal microbalance (QCM) sensor for chiral discrimination of four pairs of enantiomers. The adsorption isotherms of the enantiomers on Zn₂(bdc)(L-lac)(dmf)·DMF follow the Dubinin–Astakhov equation. The QCM sensor shows excellent sensitivity and enantioselectivity. The chiral recognition ability of the Zn₂(bdc)(L-lac)(dmf)·DMF sensor was temperature and concentration dependent. The chiral selectivity factor ranged from 1.36 (S/R-1-(1-naphthyl)ethylamine) to 2.20 (S/R-1-phenylethylamine) at 25 °C. The good chiral recognition capacities of enantiomers provide Zn₂(bdc)(L-lac)(dmf)·DMF potential for the discrimination of enantiomers.

---

Introduction

The analysis of chiral enantiomers plays a significant role in the drug industry, environmental and life sciences. Although the physical and chemical properties of chiral enantiomers are almost the same, the biochemical and pharmacological effects are usually quite different.¹ In addition, chiral enantiomers also occupy an important position in the fields of food additives, chemical fertilizer and spices. Therefore, development of novel methods for the chiral discrimination of enantiomers is of great significance.

Chiral sensor systems allow rapid qualitative as well as quantitative determination or recognition of enantiomers in real time. Quartz crystal microbalance (QCM) is a rapid and cost effective technique,^2,3 which has been favorably adopted for chiral recognition and determination of enantiomers.^4–9 The key step for fabricating a QCM sensor is to build a chiral surface with recognition sites for enantiomers. Various materials such as cyclodextrins,^4,5 molecularly imprinted polymers,^6–8 and supramolecular compounds,⁹ have been applied to fabricate the QCM sensors for chiral recognition so far.

Metal–organic frameworks (MOFs) are a novel class of advanced materials with porous networks composed of organic ligands and metal ions or clusters. Their large surface areas, diverse structures and functions make them potential for diverse applications.^10–13 Recently, the utilization of MOFs as novel sorbents or coatings of QCM for probing the adsorption characteristics and sensing properties of MOFs has received increasing attention.^14,15 Uehara et al.¹⁴ presented the integration of size-controlled MOF crystals into the QCM device for investigating the effect of the crystal size on the sorption kinetics. Huang et al.¹⁵ reported the exploration of MIL-101(Cr) coated QCM device for rapid and sensitive probing of the adsorption of volatile organic solvents.

Chiral MOFs not only possess the intrinsic features of MOFs, but also offer chiral environment which is of vital importance to chiral recognition.¹⁶ However, the application of chiral MOFs to QCM for chiral adsorption and recognition has been rarely studied. Recently, Liu et al.¹⁷ showed the layer-by-layer growth of chiral MOF [{Zn₂(±cam)₂(dabco)}_n] for the direct QCM monitoring of chiral enantiomers.

Here we report a 3D chiral porous Zn–organic framework Zn₂(bdc)(L-lac)(dmf)·DMF (L-lactic acid = L-H₂lac, 4-benzenedicarboxylic acid = H₂bdc) coated QCM sensor for chiral discrimination. Zn₂(bdc)(L-lac) (dmf)·DMF is an open architecture, whose pores are interconnected in three directions. The L-lactate moieties offer the chiral centers within the voids and the pores in Zn₂(bdc)(L-lac)(dmf)·DMF have a homochiral environment.¹⁸ The fabrication of Zn₂(bdc)(L-lac)(dmf)·DMF coated QCM sensor was achieved via a simple drop-coating method.¹⁵ Four pairs of enantiomers (Fig. S1, ESI†), S/R-1-phenylethylamine (1 S/R), 1-phenylethanol (2 S/R), S/R-1-(4-methoxyphenyl)ethylamine (3 S/R), and S/R-1-(1-naphthyl)ethylamine (4 S/R), were used as target analytes. The developed QCM sensor allows the determination of the adsorption isotherms and monitoring of the adsorption process of chiral compounds with excellent enantioselectivity. The Zn₂(bdc)(L-lac)(dmf)·DMF based QCM sensor exhibited excellent sensitivity and chiral discriminating ability to the enantiomers.

Results and discussion

Characterization of the synthesized Zn₂(bdc)(L-lac)(dmf)·DMF

The X-ray diffraction (XRD), thermal gravimetric analysis (TGA), N₂ adsorption–desorption, and scanning electron microscopy (SEM) experiments were performed to characterize the synthesized Zn₂(bdc)(L-lac)(dmf)·DMF. The XRD pattern of the synthesized Zn₂(bdc)(L-lac)(dmf)·DMF is in good agreement with the simulated one, revealing the successful preparation of Zn₂(bdc)(L-lac)(dmf)·DMF (Fig. 1A). The TGA curve shows that the Zn₂(bdc)(L-lac)(dmf)·DMF is stable up to 220 °C (Fig. 1B). N₂ adsorption–desorption isotherms reveal the synthesized Zn₂(bdc)(L-lac)(dmf)·DMF has a Brunauer–Emmett–Teller (BET) surface area of 377.5 m² g⁻¹ (Fig. 1C). SEM image shows the dense and homogeneous coating of Zn₂(bdc)(L-lac)(dmf)·DMF on the gold surface of quartz crystal (Fig. 1D).

Fig. 1 (A) Comparison of the synthesized and simulated XRD patterns of Zn₂(bdc)(L-lac)(dmf)·DMF; (B) TGA curve of the synthesized Zn₂(bdc)(L-lac)(dmf)·DMF; (C) N₂ adsorption–desorption isotherms of the synthesized Zn₂(bdc)(L-lac)(dmf)·DMF; (D) SEM image of the Zn₂(bdc)(L-lac)(dmf)·DMF coating on the gold surface of quartz crystal.

Frequency shifts

The resonance frequency (f) and the resonance frequency shifts (Δf) of Zn₂(bdc)(L-lac)(dmf)·DMF coated QCM sensor were measured to study the adsorption performance of Zn₂(bdc)(L-lac)(dmf)·DMF for the enantiomers. According to the Sauerbrey equation (eqn (1)):¹⁹

Δf = −(2nf₀²)Δm/[A(ρ_qμ_q)]^0.5 = −C_fΔm/A (1)

where f₀ is the fundamental resonant frequency, Δf is the frequency change due to mass loading, n is the harmonic number, ρ_q (2.648 g cm⁻³) and μ_q (2.947 × 1011 g cm⁻¹ s⁻²) are the density and the shear mode of the quartz material, respectively, Δm is the mass change at the crystal surface, and A is the crystal sensitive area. C_f is a constant, depending on physical parameters of the crystal only.

Δf is linearly proportional to the mass loaded at the exposed crystal surface (Δm). The Δf was measured against time for each single enantiomer (Fig. 2). After each single enantiomer was injected into the test chamber, the f of Zn₂(bdc)(L-lac)(dmf)·DMF coated QCM sensor began to decrease and reached equilibrium in a few minutes. The decreased frequency indicates the adsorption of enantiomer on Zn₂(bdc)(L-lac)(dmf)·DMF. The values of Δf were different when individual R-enantiomers or S-enantiomers were introduced into the test system, implying the enantioselectivity. After each adsorption experiment, the Zn₂(bdc)(L-lac)(dmf)·DMF could be regenerated by N₂ and reused in the next adsorption experiment without the loss of adsorption capability (Fig. S2, ESI†).

Fig. 2 The frequency shifts once each enantiomer was injected into the test chamber. For details, please see experimental section.

Selectivity factor

The chiral selectivity factor (α_QCM) of the QCM sensor for chiral enantiomers was calculated according to eqn (2).²⁰

α_QCM = Δf_S/Δf_R (2)

where Δf_S is the frequency shift of S-enantiomer while Δf_R is the frequency shift of R-enantiomer. The calculated α_QCM values for the studied four pairs of enantiomer are summarized in Table 1. In all cases, α_QCM > 1.0 (Table 1), indicating Zn₂(bdc)(L-lac)(dmf)·DMF gives higher affinity to S-enantiomer than R-enantiomer as the Δf_S is larger than Δf_R (Fig. 2). α_S/R is both temperature and initial concentration dependent. It increased with the initial concentration of the analytes, but decreased as temperature increased. The results reveal the enantioselectivity of Zn₂(bdc)(L-lac)(dmf)·DMF, and the favorableness of high concentration and low temperature for the chiral discrimination of Zn₂(bdc)(L-lac)(dmf)·DMF.

Table 1 QCM chiral selectivity factor α_S/R for four pairs of enantiomers

Adsorbate	Concentration (ppbV)	αS/R
		25 °C	30 °C	35 °C	40 °C
1	146	1.45	1.33	1.31	1.26
	292	1.48	1.43	1.38	1.29
	438	1.50	1.47	1.42	1.42
	584	1.59	1.54	1.45	1.36
	730	1.62	1.56	1.50	1.45
	1460	1.66	1.59	1.56	1.46
	2190	1.77	1.61	1.60	1.50
2	29	1.45	1.40	1.35	1.32
	88	1.52	1.41	1.41	1.35
	146	1.57	1.44	1.43	1.37
	292	1.58	1.51	1.46	1.38
	438	1.59	1.52	1.46	1.40
	584	1.60	1.53	1.47	1.41
	730	1.59	1.53	1.48	1.43
	1460	1.69	1.58	1.52	1.45
3	146	1.14	1.12	1.12	1.14
	438	1.28	1.21	1.18	1.15
	730	1.30	1.21	1.17	1.16
	1022	1.31	1.23	1.19	1.17
	1460	1.34	1.28	1.20	1.18
	1752	1.43	1.36	1.31	1.24
	2190	1.47	1.39	1.32	1.27
4	146	1.22	1.20	1.18	1.17
	292	1.23	1.22	1.20	1.22
	438	1.24	1.26	1.22	1.23
	584	1.26	1.28	1.23	1.24
	730	1.34	1.30	1.26	1.24
	1460	1.34	1.32	1.29	1.26

---

Adsorption isotherms on Zn₂(bdc)(L-lac)(dmf)·DMF

Adsorption isotherms of the four pairs of enantiomers on Zn₂(bdc)(L-lac)(dmf)·DMF were further measured at four different temperatures (25, 30, 35, and 40 °C) under atmospheric pressure (Fig. 3). The adsorption capacity of four pairs of enantiomers increased with temperature from 25 to 40 °C.

Fig. 3 Adsorption isotherms of the four pairs of enantiomers on Zn₂(bdc)(L-lac)(dmf)·DMF. Solid lines refer to the fitted adsorption isotherms with DA equation.

It is extremely important to predict the interaction between the sorbents and analytes in the adsorption and separation processes. So, we need to find an adequate isothermal equation for the examination and prediction of the adsorption experimental data. To describe the adsorption of gases on microporous materials, the Dubinin–Astakhov (DA) equation (eqn (3)) is often used.²¹

a = a₀ exp[−(A/E)ⁿ] (3)

where a is the adsorption capacity, a₀ is the limiting adsorption capacity, E is the characteristic energy of the adsorbent–adsorbate system, n is the heterogeneity parameter, and A is the Polanyi adsorption potential. The Polanyi adsorption potential A is related to the universal gas constant (R), the equilibrium temperature (T), the saturation vapor pressure (P₀) and equilibrium bulk vapor pressure (P) (eqn (4)):

A = RT ln(P₀/P) (4)

DA equation supports the micropore volume filling theory, and suggests the process of adsorption is the filling of the empty volume.²² The adsorption of the four pairs of enantiomers on Zn₂(bdc)(L-lac)(dmf)·DMF fits well with the DA equation with the regression coefficients (R²) from 0.970 to 0.990 (Fig. 3), indicating the suitability of DA equation for describing the adsorption behavior of the four pairs of enantiomers on Zn₂(bdc)(L-lac)(dmf)·DMF. The obtained values of E, n, and a₀ are summarized in Table 2. The relative standard deviations (RSDs) for E and n are 1.1–12.1% and 2.0–14.4%, respectively, showing temperature-independent nature of E and n, which satisfies the temperature invariance of the Polanyi adsorption potential.²¹ The above results show that the adsorption of the enantiomers on Zn₂(bdc)(L-lac)(dmf)·DMF follows the DA equation with the E value of 5.1–16.9 kJ mol⁻¹ for S-enantiomers, and 5.6–18.0 kJ mol⁻¹ for R-enantiomers, and the n value of 1.0–3.3 for S-enantiomers, and 1.0–3.6 for R-enantiomers, and the a₀ value of 0.06–0.97 mmol g⁻¹ (Table 2). The S-enantiomers always have larger a₀ values than the R-enantiomers. Otherwise, the a₀ at a specific temperature generally decreases in the order of 1 S/R > 2 S/R > 3 S/R > 4 S/R. Zn₂(bdc)(L-lac)(dmf)·DMF exhibited much stronger affinity to the 1 S/R, mainly due to proper size matching between the pore window of the framework and adsorbate molecules (the pore channel of Zn₂(bdc)(L-lac)(dmf)·DMF is roughly 5 Å, 1 S/R (4.3 Å *5.7 Å), 2 S/R (4.3 Å *6.1 Å), 3 S/R (4.3 Å *9.2 Å), 4 S/R (7.4 Å *6.7 Å)) (Fig. S3 and S4, ESI†).

Table 2 DA Parameters (E, n, a₀) obtained by fitting DA equation into the adsorption isotherms of four pairs of enantiomers on Zn₂(bdc)(L-lac) (dmf)·DMF at 25, 30, 35 and 40 °C

Adsorbate	E^a (kJ mol^−1)	n^a	a0 (mmol g^−1)
			25 °C	30 °C	35 °C	40 °C
a The value of E (or n) was calculated as mean ± standard deviation from four individual values at 25, 30, 35, and 40 °C.
1 S	16.9 ± 0.4	3.3 ± 0.3	0.34	0.73	0.75	0.97
1 R	18.0 ± 0.4	3.6 ± 0.5	0.16	0.39	0.41	0.56
2 S	15.7 ± 0.2	2.0 ± 0.1	0.29	0.30	0.35	0.43
2 R	17.2 ± 0.3	2.1 ± 0.1	0.15	0.16	0.21	0.26
3 S	9.1 ± 0.5	1.7 ± 0.1	0.15	0.21	0.23	0.34
3 R	11.1 ± 0.6	2.1 ± 0.2	0.08	0.11	0.14	0.22
4 S	5.1 ± 0.6	1.0 ± 0.0	0.08	0.13	0.14	0.15
4 R	5.6 ± 0.7	1.0 ± 0.0	0.06	0.09	0.11	0.12

---

The value of Astakhov exponent n is connected with the degree of surface heterogeneity, which is related to the width of the distribution curve of the adsorption potential.^23,24 The parameter n is also an indication of heterogeneity for Zn₂(bdc)(L-lac)(dmf)·DMF. In micropore system, the surface homogeneity increases with n from 1.5 to 3.²³ In this work, the n is related to the different interactions between the adsorbates and Zn₂(bdc)(L-lac)(dmf)·DMF. There are various adsorption sites such as phenyl group (Ph–), carbonyl group (–CO–) and carboxyl group (–COO–) in Zn₂(bdc)(L-lac)(dmf)·DMF. Thus, the adsorption behaviors of the four pairs of enantiomers on Zn₂(bdc)(L-lac)(dmf)·DMF are quite different. The n decreases in the order of 1 > 2 > 3 > 4 (Table 2), which means the MOF offers the most homogeneous adsorption for 1, but the most heterogeneous adsorption for 4. The results further show that the adsorption sites of Zn₂(bdc)(L-lac)(dmf)·DMF are diverse, and different pairs of enantiomers may interact with different adsorption sites. For 1, 2, and 3, the n values of S-enantiomers are a little smaller than those of R-enantiomers at the same temperature (Table 3), implying the interaction sites between S-enantiomers and Zn₂(bdc)(L-lac)(dmf)·DMF are more heterogeneous likely due to stronger interaction sites around the chiral carbon between S-enantiomers and Zn₂(bdc)(L-lac)(dmf)·DMF. For 4, the n values for its two enantiomers are almost the same probably because of their molecules are hardly to get into the channels of Zn₂(bdc)(L-lac)(dmf)·DMF. Four pairs of enantiomers were adsorbed by Zn₂(bdc)(L-lac)(dmf)·DMF, which may be mediated via hydrogen bonds between carboxyl groups and the OH, CH₃O or NH₂ groups, as well as π–π interaction between the phenyl rings. However, the carboxyl groups of Zn₂(bdc)(L-lac)(dmf)·DMF are located deep inside the cavities near the chiral carbon, which may affect the accessible R/S-analytes into Zn₂(bdc)(L-lac)(dmf)·DMF due to the steric effect. The S-enantiomers are easy to interact with carboxyl groups near the chiral carbon of Zn₂(bdc)(L-lac) (dmf)·DMF to form hydrogen bond as the CH₃– group of S-enantiomers meet the small H atom near the chiral carbon of Zn₂(bdc)(L-lac)(dmf)·DMF. However, the R-enantiomers are hard to interact with carboxyl groups near the chiral carbon of Zn₂(bdc)(L-lac)(dmf)·DMF to form hydrogen bond as the CH₃– group of R-enantiomers meet the CH₃– group near the chiral carbon of Zn₂(bdc)(L-lac)(dmf)·DMF (Fig. S5, ESI†). Therefore, the host MOF–QCM hybrid favors the S-enantiomer over the R-enantiomer. The carboxyl linkages also allow the aromatic moieties for maximizing π–π interactions.²⁵

Table 3 n values of four pairs of enantiomers at 25, 30, 35, and 40 °C

Adsorbate	25 °C		30 °C		35 °C		40 °C
	nS	nR	nS	nR	nS	nR	nS	nR
1	3.4	4.1	3.1	3.3	3.8	4.2	2.7	2.9
2	2.0	2.3	1.9	2.1	2.0	2.1	1.9	2.1
3	1.5	1.8	1.7	2.1	1.9	2.6	1.8	2.1
4	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0

---

The different fittings of enantiomers onto the chiral sensor due to the different stereo-configurations of the enantiomers result in different binding energies with R/S-enantiomer. Moreover, R/S-analytes may change their conformation upon binding to Zn₂(bdc)(L-lac)(dmf)·DMF, leading to a tight fitting.²⁶ The transient complexes stabilized via interactions such as hydrogen bonding, coordination bonding, electrostatic attraction, π–π interactions, van der Waals forces, steric affects, dipole-induced dipole attraction, and dispersive forces with low energy are in favor of adsorption. For all cases, the characteristic energy E of S-enantiomer is smaller than that of R-enantiomer at the same temperature (Table 4), which means S-enantiomer is more favorable conformation to interact with Zn₂(bdc)(L-lac)(dmf)·DMF.

Table 4 The E of the four pairs of enantiomers at 25, 30, 35, and 40 °C

	25 °C		30 °C		35 °C		40 °C
	ES	ER	ES	ER	ES	ER	ES	ER
1	16.3	17.8	16.5	17.5	17.8	18.8	17.0	18.2
2	15.5	17.0	16.0	17.8	15.8	17.0	15.6	17.0
3	8.7	11.3	8.7	10.7	10.7	12.0	8.9	10.3
4	5.8	6.4	4.0	4.6	5.0	5.2	5.7	6.1

---

The α_S/R of 1 is the largest, and the α_S/R of 4 is the smallest (Table 5), mainly because the smallest molecule of 1 can get into the chiral channels easily, but hardly for the biggest molecule of 4. This explains why a proper matching of the size and shape between guest and host molecules is so important for enantioselectivity. In all cases, the value of α_S/R decreases with the increase of temperature likely because temperature affects the stability of diastereoisomeric complexes between hosts and guests.

Table 5 Chiral selectivity factors (α_S/R) under limiting adsorption capacity at 25, 30, 35, and 40 °C

Adsorbate	αS/R
	25 °C	30 °C	35 °C	40 °C
1	2.20	1.84	1.81	1.74
2	1.92	1.83	1.69	1.66
3	1.98	1.86	1.71	1.55
4	1.36	1.34	1.29	1.28

---

The determined DA parameters reveal that Zn₂(bdc)(L-lac)(dmf)·DMF has higher affinity to the S-enantiomer. On one hand, the steric hindrance of the analytes plays an important role on the chiral recognition. On the other hand, the size matching between the guest analyte and the framework also plays a significant role in the host–guest interactions which leads to chiral discrimination.

Conclusions

In summary, Zn₂(bdc)(L-lac)(dmf)·DMF coated QCM sensor was fabricated and applied as a novel chiral sensor for the discrimination of four pairs of enantiomers. The developed Zn₂(bdc)(L-lac)(dmf)·DMF coated QCM sensor not only allows the determination of the adsorption isotherms and monitoring of the adsorption process, and also provides a useful tool for characterization of chiral MOFs. The adsorption of four pairs of enantiomers on Zn₂(bdc)(L-lac)(dmf)·DMF follows the DA equation. The excellent sensitivity and selectivity of Zn₂(bdc)(L-lac)(dmf)·DMF for chiral enantiomers also reveal the potential of chiral MOFs as novel sorbents for QCM.

Experimental

Chemicals and reagents

All chemicals and reagents used were at least of analytical grade. R/S-1-phenylethylamine, R/S-1-phenylethanol, R/S-1-(4-methoxyphenyl)ethylamine, R/S-1-(1-naphthyl)ethylamine, 1,4-benzenedicarboxylic acid (H₂bdc), Zn(NO₃)₂·6H₂O were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). Ethanol and N,N-dimethylformamide (DMF) were obtained from Tianjin Concord Technology Co. Ltd. (Tianjin, China). L-lactic acid (L-H₂lac) was obtained from Hefei Bomei Biotechnology Co. Ltd. (Hefei, China). H₂SO₄ (98%) and H₂O₂ (30%) were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). High purity nitrogen (99.99%) was obtained from BOC Gases Co. Ltd. (Tianjin, China). Ultrapure water (Wahaha Foods Co. Ltd., Tianjin, China) was used throughout this work.

Instrumentation

QCM 200 fitted with 5 MHz AT-cut quartz crystals (the surface area of the gold is 1.33 cm²) (Stanford Research Systems, Sunnyvale, CA) was used throughout the experiments. The temperature of the test chamber was controlled by a M312774 thermostat water bath (Zhongxihua, Beijing, China). A glass flask (685 mL) was used as the test chamber.

Fabrication of Zn₂(bdc)(L-lac)(dmf)·DMF coated QCM sensor

Zn₂(bdc)(L-lac)(dmf)·DMF was prepared according to Dybtsev et al.¹⁸ A homemade polytetrafluoroethylene (PTFE) crystal-holder (Fig. S6, ESI†) was used to fabricate the Zn₂(bdc)(L-lac)(dmf)·DMF coating on the gold surface of the quartz crystal. Before coating, the quartz crystal was immersed into piranha solution (98% H₂SO₄ : 30% H₂O₂, 3 : 1 v/v) for 5 min to remove organic residues (caution: piranha solution is a strong oxidizing agent which is very corrosive, reactive, and potentially explosive, and should be handled with extreme care), followed by rinsing with ultrapure water, ethanol, ultrapure water in sequence. The quartz crystal was dried under nitrogen gas. Zn₂(bdc)(L-lac) (dmf)·DMF (8 mg) was suspended in ethanol (1 mL), and sonicated (80 W) for 3 h. The obtained suspension (20 μL) was dropped onto the cleaned gold surface of the quartz crystal to obtain the Zn₂(bdc)(L-lac)(dmf)·DMF coating. The Zn₂(bdc)(L-lac)(dmf)·DMF coated quartz crystal was then dried in air. The amount of Zn₂(bdc)(L-lac)(dmf)·DMF deposited on the quartz crystal was calculated by the Sauerbrey equation from the decreased frequency. The Zn₂(bdc)(L-lac)(dmf)·DMF coated quartz crystals were heated at 75 °C for 3 h before detection.

Adsorption experiments

A QCM 200 fitted with 5 MHz AT-cut quartz crystals (gold-coated) was used to study the adsorption and recognition performance of Zn₂(bdc)(L-lac)(dmf)·DMF for chiral compounds (Fig. 4).

Fig. 4 Schematic illustration of the QCM measurement system for chiral recognition. The measurement cell was thermostatted in a water-bath throughout the experiments.

Before measurement, the QCM system was conditioned with nitrogen to get a stable oscillating frequency. Then, different amounts of enantiomers were injected into the test chamber respectively using a microsyringe. When the system reached to the adsorption equilibrium, the frequency was recorded. After each adsorption, the QCM system was conditioned with nitrogen to return to the starting frequency. After desorption with nitrogen gas, the frequency would deviate from the starting frequency within a few Hertz in a reasonable range, so sometimes Δf would achieve the value higher than zero (Fig. 2).

Acknowledgements

This work was supported by National Basic Research Program of China (Grant 2015CB932001), National Natural Science Foundation of China (Grant 21305071), Tianjin Natural Science Foundation (Grants 14JCZDJC37600 and 14JCQNJC06600), and the Fundamental Research Funds for the Central Universities.

Notes and references

1. I. Agranat, H. Caner and A. Caldwell, Nat. Rev. Drug Discovery, 2002, 1, 753.
2. M. D. Ward and D. A. Buttry, Science, 1990, 249, 1000.
3. J. J. McCallum, Analyst, 1989, 114, 1173.
4. G. S. Ding, X. J. Huang, X. L. Liu and J. D. Wang, Chin. J. Pharm. Anal., 2004, 24, 68.
5. C. Fietzek, T. Hermle, W. Rosenstiel and V. Schurig, Fresenius. J. Anal. Chem., 2001, 371, 58.
6. N. M. Maier and W. Linder, Anal. Bioanal. Chem., 2007, 389, 377.
7. K. Haupt, K. Noworyta and W. Kutner, Anal. Commun., 1999, 36, 391.
8. C. J. Percival, S. Stanley, M. Galle, A. Braithwaite, M. I. Newton, G. Mchale and W. Hayes, Anal. Chem., 2001, 73, 4225.
9. M. Miyauchi, Y. Takashima, H. Yamaguchi and A. Harada, J. Am. Chem. Soc., 2005, 127, 2984.
10. D. J. Collins and H. C. Zhou, J. Mater. Chem., 2007, 17, 3154.
11. J. R. Li, Y. Ma, M. C. McCarthy, J. Sculley, J. Yu, H. K. Jeong, P. B. Balbuena and H. C. Zhou, Coord. Chem. Rev., 2011, 255, 1791.
12. J. R. Li, R. J. Kuppler and H.-C. Zhou, Chem. Soc. Rev., 2009, 38, 1477.
13. J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450.
14. H. Uehara, S. Diring, S. Furukawa, Z. Kalay, M. Tsotsalas, M. Nakahama, K. Hirai, M. Kondo, O. Sakata and S. Kitagawa, J. Am. Chem. Soc., 2011, 133, 11932.
15. C. Y. Huang, M. Song, Z. Y. Gu, H. F. Wang and X. P. Yan, Environ. Sci. Technol., 2011, 45, 4490.
16. G. Nickerl, A. Henschel, R. Grünker, K. Gedrich and S. Kaskel, Chem. Ing. Tech., 2011, 83, 90.
17. B. Liu, O. Shekhah, H. K. Arslan, J. Liu, C. Wöll and R. A. Fischer, Angew. Chem., Int. Ed., 2012, 51, 807.
18. D. N. Dybtsev, A. L. Nuzhdin, H. K. Chun, P. Bryliakov, E. P. Talsi, V. P. Fedin and K. Kim, Angew. Chem., Int. Ed., 2006, 45, 916.
19. G. Sauerbrey, Z. Phys., 1959, 155, 206.
20. K. Bodenhofer, A. Hierlemann, J. Seemann, G. Gauglitz, B. Christian, B. Koppenhoefer and W. Gopel, Anal. Chem., 1997, 69, 3058.
21. D. D. Do, Adsorption Analysis: Equilibria and Kinetics, Series on Chemical Engineering 2, Imperial College Press, London, 1998.
22. F. Stoeckli, M. V. Lopez-Ramon, D. Hugi-Cleary and A. Guillot, Carbon, 2001, 39, 1115.
23. J. Pires, A. Carvalho and M. B. De Carvalho, Microporous Mesoporous Mater., 2001, 43, 277.
24. A. M. Puziy, Langmuir, 1995, 11, 543.
25. G. K. E. Scriba, Chromatographia, 2012, 75, 815.
26. Y. K. Ye, S. Bai, S. Vyas and M. J. Wirth, J. Phys. Chem. B, 2007, 111, 1189.

---

Footnote

† Electronic supplementary information (ESI) available: Supplementary information Fig. S1–S3. See DOI: 10.1039/c5ra01204j

---