In Situ Solvothermal Growth of Metal–Organic Framework-5 Supported

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In Situ Solvothermal Growth of Metal−Organic Framework‑5 Supported on Porous Copper Foam for Noninvasive Sampling of Plant Volatile Sulfides Yuling Hu, Haixian Lian, Langjun Zhou, and Gongke Li* School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275, China S Supporting Information *

ABSTRACT: The present study reported on an in situ solvothermal growth method for immobilization of metal−organic framework MOF-5 on porous copper foam support for enrichment of plant volatile sulfides. The porous copper support impregnated with mother liquor of MOF-5 anchors the nucleation and growth of MOF crystallites at its surface, and its architecture of the threedimensional channel enables accommodation of the MOF-5 crystallite seed. A continuous and well-intergrown MOF-5 layer, evidenced from scanning electron microscope imaging and X-ray diffraction, was successfully immobilized on the porous metal bar with good adhesion and high stability. Results show that the resultant MOF-5 coating was thermally stable up to 420 °C and robust enough for replicate extraction for at least 200 times. The MOF-5 bar was then applied to the headspace sorptive extraction of the volatile organic sulfur compounds in Chinese chive and garlic sprout in combination with thermal desorption-gas chromatography/mass spectrometry. It showed high extraction sensitivity and good selectivity to these plant volatile sulfides owing to the extraordinary porosity of the metal−organic framework as well as the interaction between the S-donor sites and the surface cations at the crystal edges. Several primary sulfur volatiles containing allyl methyl sulfide, dimethyl disulfide, diallyl sulfide, methyl allyl disulfide, and diallyl disulfide were quantified. Their limits of detection were found to be in the range of 0.2−1.7 μg/L. The organic sulfides were detected in the range of 6.0−23.8 μg/g with recoveries of 76.6−100.2% in Chinese chive and 11.4−54.6 μg/g with recoveries of 77.1−99.8% in garlic sprout. The results indicate the immobilization of MOF-5 on copper foam provides an efficient enrichment formats for noninvasive sampling of plant volatiles. fiber19,20,25,26 or magnetization of the material.21,22 However, coating the MOFs material directly on a smooth and nonporous silica fiber is not always easily realized. Weak adhesion of the MOFs coating with the substrate usually leads to poor coating and low stability. To find a substrate with compatible surface and porous structure to accommodate the nanosized MOFs would be helpful for the immobilization of this excellent material. For instance, assembly of ordered MOF membrane on porous alumina and silica were reported with good morphology and fascinating characteristics.27−29 Different methods of fabricating films of MOFs have been also explored. A step-bystep (layer-by-layer) method was proposed for the growth of the SURMOFs on a SAM-functionalized substrate. By using an appropriately functionalized organic surface as a nucleation site, highly ordered, oriented MOF structures could be obtained at mild conditions.30,31 De Vos and co-workers32−34 synthesized the metal−organic framework thin-film coatings grown by an electrochemical method, allowing fast deposition within short

M

etal−organic frameworks (MOFs) are an emerging class of crystalline porous hybrid materials consisting of inorganic building blocks and the organic linkers. The crystal lattice of these compounds is constructed by coordination bonds between nodes of metal ions and multidentate organic ligands.1 Their high specific surface areas and pore volumes together with tailored chemical functionality are interesting for a huge variety of applications, such as gas-storage,2,3 separation,4,5 heterogeneous catalysis,6,7 sensing,8−10 and drug delivery.11,12 In recent years, MOFs have found their application in diverse areas of analytical chemistry, for instance, as stationary phases for chromatography,13,14 sensors,15,16 sorbents for sampling,17,18 solid-phase microextraction,19,20 magnetic solidphase extraction,21,22 and enantioselective separation.23,24 Especially, MOFs have received more and more attention in sample preparation. However, while MOF materials are currently available in powder forms which are not very convenient to retrieve from a sample matrix, suitable fabrication formats will facilitate the convenience of enrichment procedures and simplify sampling devices. To address the problem, the immobilization of MOFs on a rigid substrate is essential, for example, the fabrication of solid-phase microextraction © XXXX American Chemical Society

Received: June 10, 2014 Accepted: December 1, 2014

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was purchased from Shanghai Zhongwei New Materials Co., Ltd. (Shanghai, China). N, N′-Dimethylformamide (DMF) was from Tianjin Standard Chemical Reagent Co. Ltd. (Tianjin, China). Hexane was HPLC grade (Tianjin Fu Chen Chemical Reagents Factory, China). Other chemicals were of analytical grade. Instrumentals. An Agilent HP 6890 gas chromatography5973 mass detector system (GC/MS) (Palo Alto, CA) was employed for all experiments. A HP-VOC (Agilent Scientific) capillary column (60 m length × 0.32 mm i.d. × 0.18 μm film thickness) was used. The thermal desorption unit (TDU) (Gerstel, Germany) consists of a model Contrall C506 and a CIS 4 cool inlet system. Commercial PDMS twister (Gerstel, 20 mm length × 1.0 mm coating thickness) was used for comparison. A Hitachi S-4300 scanning electron microscope (Hitachi, Japan) was used to investigate the surface morphology of the MOF-5 crystal. A Nicolet Avatar 330 Fourier transform infrared (FT-IR) spectrometer (Nicolet, Waltham, MA) and a thermal gravity (TG) analyzer (Netzsch-209, Bavaria, Germany) were applied to study the composition and the thermal stability of the MOF-5 crystal, respectively. Powder X-ray diffraction (XRD) was performed on a D-MAX 2200 VPC (Rigaku, Japan), and X-ray photoelectron spectroscopy (XPS) was performed on a ESCALab 250 (Thermo Fisher Scientific). In Situ Solvothermal Growth of MOF-5 Bar. The MOF-5 was immobilized on the porous copper foam by in situ solvothermal growth of crystals on the support. The copper foam (20 mm × 2 mm × 2 mm) was ultrasonically pretreated by immersing in acetone, methanol, and distilled water for 30 min, respectively, and dried each time at 60 °C. A solution of Zn(NO3)2·6H2O (2.838 g) and terephthalic acid (0.536 g) was dissolved in DMF (20 mL). To this solution the pretreated foamed copper support was introduced and held for 30 min for solution impregnation. Then the solution was poured in a Teflon-lined autoclave and the support was placed vertically in the solution to allow crystal growth at 120 °C for 24 h. After crystallization, the MOF-5 bar was washed with DMF under 90 Hz shaking and dried at room temperature. The MOF-5 bar was activated under reduced pressure at 250 °C for 24 h and preserved in the vacuum desiccator. Headspace Microextraction Procedure and Thermal Desorption. The prepared MOF-5 bar was fixed with a homemade device at the head space of a vial. Then 1 μL of standard analytes dissolved in hexane was introduced in a 15 mL amber vial capped with a PTFE-coated septum. The device with MOF-5 bar was exposed to the headspace for 30 min. After enrichment, MOF-5 bar was immediately removed into a quartz tube and inserted into the thermal desorption (TD) inlet for thermal desorption. The TD conditions were set as follows: the TDU temperature was set at an initial temperature of 40 °C, delayed for 1 min, and then increased to 250 °C at a ramp rate of 150 °C/min and held for 5 min. The transfer line temperature between TDU and CIS was 300 °C. The desorbed analytes were reconcentrated on a focusing cryogenic quartz tube at −40 °C for 0.05 min and then rapid heating to 320 °C at a rate of 12 °C/s to release the concentrated analytes into the GC/MS system. GC/MS Conditions. The GC/MS conditions were as follows: injector temperature 250 °C; transfer line temperature 280 °C; energy of electron 70 eV; ion source temperature 230 °C; splitless mode. The mass spectrometer was operated in electron ionization (EI) mode with quadrupole temperature of

times. In addition, a simple dip-coating strategy allows the design of flexible thin films with tailored sorption properties.35,36 Macroporous metal materials are essential for many modern applications, due to their favorable properties including welldefined rigid macropores and good thermal and mechanical stability. The combination of large surface area of MOF material and the three-dimensional rigid skeleton of macroporous metal network would beneficial to create a desirable robust enrichment medium. The volatile plant constituents play important roles in the plant growth as well as can make significant impact on human health. For instance, volatile organic sulfur compounds play preventive roles in the development of human pathologies, including cancer, cardiovascular, and inflammatory diseases.37−39 As compared with main plant constituents, the concentrations of volatile compounds are very low in plants, and the plant matrix is very complex. Therefore, it is necessary to develop an efficient sampling method with high enrichment capability and the ability to remove interferences. Nowadays volatile enrichment techniques still remain an important challenge. Conventional sampling methods for volatiles such as steam-distillation and simultaneous distillation extraction always require long extraction times, consumption of large amounts of solvents, and multiple steps. Moreover, many unstable volatiles may be thermally decomposed or degraded during thermal extraction or distillation.40 Recently, the headspace microextraction, including solid-phase microextraction, liquid−liquid microextraction, and thin-film extraction, was developed for the analysis of volatile compounds in plants.40,41 Headspace microextraction has been proved to be a noninvasive sampling technique and may provide important information for volatile organic compounds. In this study, we proposed a facile method for immobilization of metal−organic framework MOF-5 on the porous copper support by in situ solvothermal growth of seed crystals confined within the macropores of support and then to form continuous MOF-5 coating. The resultant rigid MOF-5 bar was applied to the noninvasive sampling of the volatile organic sulfur compounds in Chinese chive and garlic sprout. The analytes are preconcentrated by the MOF-5 bar in the headspace mode and then subjected to thermal desorption-gas chromatography/ mass spectrometry (TD-GC/MS). The powerful adsorption ability and the microsized pores of MOF-5 makes it an especially excellent candidate for enrichment of volatile molecules, which would be very important for the improvement of sensitivity when trace analysis is needed. A significant enhancement of adsorption capacity and selectivity of the MOF-5 bar for the volatile organic sulfur compounds has been achieved.

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EXPERIMENTAL SECTION Materials and Chemicals. Terephthalic acid (H2BDC, 99%), Zn(NO3)2·6H2O (99%), allyl methyl sulfide (99%), methyl propyl disulfide (97%), dipropyl disulfide (99%), diallyl sulfide (98%), diallyl disulfide (85%), amyl butyrate (99%), isoamyl butyrate (99%), and butyl butyrate (99%) were bought from Jingchun Reagent Co. Ltd. (Shanghai, China). Dimethyl disulfide, methyl allyl disulfide (95%), and dimethyltrisulfide (98%) were obtained from J&K (Beijing, China). Hexyl butyrate (98%) and isoamyl isovalerate (98%) were obtained from TCI (Shanghai, China). Isoamyl acetate (99%) was purchased from Alfa Aesar (London, England). Ethylbenzene (99.8%) and o-xylene (98%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Copper foam B

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150 °C. The high-purity helium (99.999%, Guangzhou Xicheng Industrial Gas and Equipment Co., Ltd.) was used as the carrier gas at a constant flow rate of 1.0 mL/min. The temperature program for the analysis of VOCs in Chinese chive and garlic sprout was as follows: initial 50−120 °C at ramp rate of 3 °C/min, then increased to 180 °C at ramp rate of 5 °C/min, and finally from 180 to 250 °C at a ramp rate of 50 °C/min and held for 3 min. The mass spectra were acquired in both Scan and SIM mode. SIM mode ion conditions were as follows: 10−12 min, 41,73,88 m/z; 12−14 min, 91 m/z; 14−16 min, 45,79,94 m/z; 16−21 min, 91; 21−23 min, 45,73,99,114 m/z; 23−25 min, 91 m/z; 25−27 min, 41,73,120; 27−33 min, 91 m/z; 33−34 min, 41,81,146 m/z; 34−40 min, 91 m/z. Sample Analysis. Fresh and mature Chinese chive and garlic sprout samples were obtained from a local market (Guangzhou, China). For the determination of the organic sulfides, 0.3 g of Chinese chive or garlic sprout sample was weighed into a 15 mL septum-sealed amber vial and subjected to the headspace extraction using the MOF-5 bar for 30 min at room temperature. After that, the adsorbed volatiles were thermally desorbed by inserting the MOF-5 bar into the TD quartz tube for thermal desorption. For the analytical performance assessment, 1 μL of corresponding spiked mixed standard solution of sulfides in hexane was added to the sample in a 15 mL septum-sealed amber vial and extracted by the MOF-5 bar for 30 min.

the scanning electron microscopy (SEM) images of a MOF-5 coating synthesized for 12, 18, 24, and 36 h. As shown in Figure 2A, the bare porous copper foam has a three-dimensional porous skeleton. The MOF-5 crystals become more intact with the increase of growing time, and finally a continuous and dense coating layer is obtained. Intergrowth between grains can be observed. From the cross section view, one can see the coating thickness of the MOF-5 is around 500 μm. The cubic crystals of MOF-5 were partially entrapped within the skeleton of the support, and the intergrowth characteristics of the film guarantees the high mechanical stability of the MOF-5 coating. Results showed the resultant MOF-5 bar was robust enough for replicate extraction for at least 200 times. Temperature would affect the rate of heterogeneous nucleation and growth of MOF crystals. With the increase of temperature, the crystalline became more intact, because high temperature caused substantial increase in the rate of heterogeneous nucleation and growth. A continuous MOF-5 coating was well fabricated on the support with the solvothermal treatment at 120 °C for 24 h. Chemical Structure and Thermal Stability. In order to demonstrate the chemical structure of the MOF-5 bar, XRD, XPS, and IR characteristics were investigated. The XRD patterns of the MOF-5 bar and the corresponding crystal powder are presented in Figure 3A. The main diffraction peaks at low angles, 2θ = 6.9°(⟨200⟩ plane, d = 12.8 Å) and 9.7° (⟨220⟩ plane, d = 9.1 Å) are indicative of the modular arrangement of the MOF-5 cubic lattice.42 Most peaks from MOF-5 bar are in accordance with the XRD patterns of the as-synthesized MOF-5 crystal powder and are also in good agreement with that reported by Yaghi,43 indicating the successful immobilization of the MOF-5. After removal of the entrapped solvent by thermal treatment at 250 °C, the activated MOF-bar exhibited improved crystal structure, which is indicated from the reversion of the relative intensities of the 6.9° and 9.7° diffraction signals. The chemical composition of the MOF-5 bar was analyzed by X-ray photoelectron spectroscopy (XPS, Figure 3B) and infrared spectroscopy (IR, Figure 3C). The zinc, oxygen- and carbon-related peaks corresponding to Zn2P1, Zn2P3, O1S, and C1S were detected in the XPS survey spectra, while no observable signals for copper was detected, indicating that the metal copper did not participate in the crystallization and only served as the support. The FT-IR spectrum of the assynthesized MOF-5 bar is consistent with the reported characteristics of MOF-5.44 Two absorption bands located at 1572 and 1507 cm−1 can be assigned to the vas (COO) asymmetric stretching, whereas the band at 1391 cm−1 can be assigned to the corresponding symmetric stretching vibration. The thermal stability of MOF-5 bar was studied by thermal gravimetric analysis (TGA) and shown in Figure 3D. Two weight-loss steps were observed. The first occurred in the range of 100−250 °C relates to the loss of solvent molecules. The second between 420 and 550 °C is due to the decomposition of MOF-5. The result indicates that the structure of MOF-5 is stable up to 420 °C, which meets the requirements of the operation temperature of TD-GC/MS detection. Extraction Performance of the MOF-5 bar. The research about the extraction characteristics of the MOF-5 bar for organic analytes is critical to explore their potential applications. The general extraction performance of the MOF-5 bar was evaluated by headspace extraction of a series of organic compounds, including the volatile aliphatic esters, benzene series, and organic sulfides. Commercial polydimethylsiloxane (PDMS) twister were selected for comparison. The comparison

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RESULTS AND DISCUSSION In Situ Solvothermal Growth of the MOF-5 on the Porous Copper Support. Despite the current immense investigations into the chemistry of MOF materials, nowadays it is still the challenge to direct and control the crystal growth of metal−organic framework at support surfaces to facilitate the usability of the advanced material for adsorption and separation. In this work, the MOF-5 bar was fabricated by in situ solvothermal growth of the crystals on the porous copper supports. The scheme of preparation was illustrated in Figure 1.

Figure 1. Schematic illustration of the in situ solvothermal growth of MOF-5 on the porous copper support.

In this procedure, the pretreated porous copper foam was immersed into the solution of Zn(NO3)2·6H2O and terephthalic acid and then subjected to solvothermal treatment. During the crystallization, the porous structure of the copper support could be impregnated with the mother liquor. Then, nucleation took place to form seed crystals. The threedimensional metal skeleton provided suitable sites to accommodate the seed crystals and subsequently to promote growth of MOF-5 microcrystallines, such that MOF-5 grew preferentially on the support instead of self-nucleating in the bulk solution. The desired growing of the MOF-5 crystals into a dense layer on the copper support could be realized by controlling the reaction time and temperature. Figure 2 shows C

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Figure 2. SEM images of the MOF-5 bar with different growth time: (A) bare porous copper foam; (B−E) MOF-5 grown on the support with 12, 18, 24, 36 h; and (F) cross-section of the MOF-5 bar.

Figure 3. Characterization of the MOF-5 bar. (A) XRD patterns of the MOF-5 crystal powder and the as-synthesized MOF-5 bar; (B) XPS spectra of the MOF-5 bar; (C) FT-IR spectra of the precursor terephthalic acid, MOF-5 crystal powder and the as-synthesized MOF-5 bar; and (D) thermogravimetric curves of the as-synthesized MOF-5 bar. D

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was accomplished by calculating the adsorption amounts of analytes per coating volume, and the results are shown in Figure 4. Commercial PDMS twister is nonpolar polymeric structure. It discriminated analytes mainly according to their polarity and exhibited higher affinity to nonpolar compounds than polar compounds. Therefore, the extraction amounts of compounds on commercial PDMS twister decreased in the order of aliphatic esters, benzene series, and organic sulfides. Generally the commercial PDMS twister showed little selectivity for compounds with different functional groups. On the other hand, the MOF-5 bar exhibited higher extraction amounts for all the investigated analytes than the commercial PDMS twister. To explain the adsorption mechanism of analytes on MOF-5, several factors can be involved: (1) highly porous structure of the metal−organic framework with appropriate pore structure and suitable pore size; (2) electrostatic interactions between the electric field generated by the framework and the dipole moment of the aromatic molecules; (3) interactions of the metal centers in the MOF frameworks with the delocalized π electrons of the aromatic ring. It can be observed in Figure 4 that enhanced selectivity was obtained for benzene series than aliphatic esters, owing to the π−π affinity between MOF-5 and phenyl ring in these compounds. Specifically, the highest selectivity was obtained for the organic sulfides. The extraction amounts of sulfides with the MOF-5 bar were 2.3−7.7 times of that with the PDMS twister. We speculate that the high extraction sensitivity and good selectivity of MOF-5 to volatile sulfides was mainly due to the highly porous structure of the metal−organic framework. On the other hand, the interaction between the S-donor sites and the surface cations at the crystal edges of MOF-5 may be another contribution. According to Pearson’ s hard−soft acid−base principle, S-compounds tend to be intermediate to soft bases, and the soft S-compounds prefer to interact with intermediate or soft Lewis acid sites, such as Cu2+, Zn2+, or Ag+.45 This specific adsorptive property makes the MOF-5 bar a good candidate for the practical application to enrichment of volatile organic sulfur in plants. The adsorption isotherms of the MOF5 bar for diallyl sulfide was then accessed (Figure S1 in the Supporting Information). The adsorption capacity is estimated about 3.0 μg mg−1. The high adsorption affinity to organic sulfides makes the hybrid material to be an excellent adsorbent in the following study of real sample analysis. Effect of Sampling and Desorption Time. The assynthesized MOF-5 bar was used for noninvasive sampling of the volatile organic sulfur in Chinese chive and garlic sprout samples. The analytes were preconcentrated by the MOF-5 bar in the headspace mode. In order to obtain the best extraction performance for volatile organic sulfides present in the two Allium plants, sampling and desorption time was investigated. Sensitivity increased with the increase of sampling time for all compounds. The sampling time of 30 min was selected because no significant increase of sensitivity was obtained with longer sampling time. Desorption is carried out in two steps. First, the MOF-5 bar is heated inside the desorption unit (TDU) and purged with a high flow of helium. Then the analytes are cryofocused and concentrated in the glass line at extremely low temperature. Second, the compounds retained are introduced into the chromatographic column by heating the inlet while the carrier gas flows through the inlet to the column and carries the analytes. In this study, desorption temperature and time for the TDU, which are compound-dependent, were investigated to allow the maximum desorption efficiency of the analytes,

Figure 4. Adsorption amounts obtained with the MOF-5 bar and PDMS twister for aliphatic esters (A), benzene series (B), and organic sulfur compounds (C).

thus avoiding their thermal degradation and memory effects. The desorption temperature of 250 °C was selected because it provided the best sensitivity for all the compounds. In regards to desorption time, 5 min was observed to be sufficient to desorb the analytes from the MOF-5 bar heated at 250 °C. E

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Table 1. Analytical Performance for TD-GC/MS Determination of the Five Sulfide Compounds RSD analytes allyl methyl sulfide dimethyl disulfide diallyl sulfide methyl allyl disulfide diallyl disulfide

R2

linear range (μg/L)

detection limits (μg/L)

one bar (%, n = 5)

bar-to-bar (%, n = 3)

0.9928 0.9904 0.9942 0.9974 0.9986

1.0−200.0 5.0−300.0 5.0−300.0 1.0−200.0 5.0−150.0

0.2 1.4 1.2 0.3 1.7

7.8 2.7 4.9 5.1 1.8

10.6 7.4 10.3 6.8 5.4

regression equation y y y y y

= = = = =

1.7 3.1 1.3 4.8 5.1

× × × × ×

6

10 x 105x 106x 105x 105x

− 3.0 × 10 + 3.0 × 105 + 9.1 × 106 − 2.4 × 106 + 8.5 × 105 6

Figure 5. Chromatograms of extracted volatile sulfides from (a) standard solution of the five sulfur compounds, (b) Chinese chive or garlic sprout samples, and (c) spiked Chinese chive or garlic sprout samples using the MOF-5 bar. The spiked concentrations are indicated in Table 2. Peaks: 1, allyl methyl sulfide; 2, dimethyl disulfide; 3, diallyl sulfide; 4, methyl allyl disulfide; 5, diallyl disulfide.

Table 2. Analytical Results for the Determination of Five Sulfide Compounds in Chinese Chive and Garlic Sprout (n = 3) Chinese chive

a

garlic sprout

analytes

content (μg/g)

spiked (μg/g)

recovery (%)

RSD (%)

content (μg/g)

spiked (μg/g)

recovery (%)

RSD (%)

allyl methyl sulfide dimethyl disulfide diallyl sulfide methyl allyl disulfide diallyl disulfide

23.8 18.4 22.9 22.9 6.0

23.4 23.4 23.4 23.4 6.7

82.6 83.2 83.6 100.2 76.6

7.8 6.4 8.9 10.6 4.5

16.7 N.D.a 54.6 11.4 26.7

16.7 8.4 50.0 13.4 33.4

77.1 80.5 99.8 78.3 88.6

2.6 4.1 8.1 10.8 10.2

N.D., not detected.

(RSDs) were calculated to be 1.8−7.8%. Also, the bar-to-bar reproducibility of three different MOF-5 bar prepared in the same way was less than 10.6%. Furthermore, the extraction performance of the MOF-5 bar was monitored during its use, and the result showed that no measurable loss was observed after it had been used for more than 200 extractions. Application to Allium Species. After the methodology was successfully validated, it was applied to the determination of volatile organic sulfurs in Chinese chive and garlic sprout. The results are shown in Figure 5 and Table 2. From Table 2, it can be seen that the contents of the five sulfides in Chinese chive were 23.8, 18.4, 22.9, 22.9, and 6.0 μg/g for allyl methyl sulfide, dimethyl disulfide, diallyl sulfide, methyl allyl disulfide, and diallyl disulfide, respectively. The concentration of sulfides in garlic sprout was measured to be 16.7, 54.6, 11.4, and 26.7 μg/g for allyl methyl sulfide, diallyl sulfide, methyl allyl disulfide, and diallyl disulfide, respectively. Recovery studies were carried out in order to check the accuracy of the proposed method. The contents of the five sulfides in spiked mixed standard solution were set at the corresponding levels according to the contents in the real samples that present in Table 2. The recoveries were found to be in the range of 77.1−99.8% with RSDs less than 10.8% for all the samples. The results indicate the proposed method is

To study the carryover effect, blank tests were run with the MOF5 bar after desorption of the extracted compounds. No signal of analytes was detected after desorption at 250 °C for 5 min. Development of Quantitative Method. The composition of volatiles in Chinese chive and garlic sprout was obtained by headspace sorptive extraction using the MOF-5 bar coupling with TD-GC/MS analysis. The abundant volatiles identified were mostly organic sulfides. The five sulfides, including allyl methyl sulfide, dimethyl disulfide, diallyl sulfide, methyl allyl disulfide, and diallyl disulfide were further quantified for both kinds of Allium plants. To validate the proposed method, the calibration curves, limits of detection, linearity coefficients (R2), and the repeatability were evaluated (Table 1). The linear ranges were tested by varying the concentration of the standard solution and were found to be in the range of 1.0−200.0 μg/L for allyl methyl sulfide and methyl allyl disulfide, 5.0−300.0 μg/L for dimethyl disulfide, diallyl sulfide, and 5.0−150.0 μg/L for diallyl disulfide with correlation coefficients higher than 0.9904. The detection limits (LODs), calculated as 3 times the standard deviation of the obtained peak area at the lowest sample concentration divided by the slope of the calibration curve, ranged from 0.2 to 1.7 μg/L. The repeatability of the method was tested with five replicate analyses of sulfides at a concentration of 25.0 μg/L and the relative standard deviations F

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efficient for noninvasive sampling and quantification of volatile organic sulfurs in different plant samples.

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CONCLUSION We have demonstrated the fabrication of continuous, wellintergrown MOF-5 on three-dimensional porous copper foam by an in situ solvothermal growth technique. The porous metal support with a pore size appropriate to the MOF-5 seed is suitable for growing a compact MOF-5 layer thus improving the integrity of the whole coating. The resultant robust MOF-5 bar was recognized as an excellent adsorbent for trapping volatile compounds and thus was applied to the headspace sorptive extraction of the volatile organic sulfur compounds in Chinese chive and garlic sprout in combination with TD-GC/MS. The several primary sulfur volatiles containing allyl methyl sulfide, dimethyl disulfide, diallyl sulfide, methyl allyl disulfide, and diallyl disulfide were quantified. It was satisfactory that trace sulfides could actually be detected from the plant samples by this noninvasive method. It is expected that this novel MOF-5 bar may also be potential for preconcentration of other plant volatiles due to the unique properties of the metal−organic framework, which can be judiciously tailored by choosing different framework building blocks to create diverse MOF material.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-20-84110922. Fax: +86-20-84115107. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Grant Nos. 21127008 and 21277176), the National Key Scientific Instrument and Equipment Development Project (Grant No. 2011YQ03012409), and the Specialized Research Fund for the Doctoral Program of Higher Education (Grant 20120171110001), respectively.

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dx.doi.org/10.1021/ac502146c | Anal. Chem. XXXX, XXX, XXX−XXX