Organic Building Block Based Microporous Network SNW-1 Coating

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Organic Building Block Based Microporous Network SNW‑1 Coating Fabricated by Multilayer Interbridging Strategy for Efficient Enrichment of Trace Volatiles Jialiang Pan, Shu Jia, Gongke Li,* and Yuling Hu* School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275 Guangdong, China S Supporting Information *

ABSTRACT: Microporous organic polymers (MOPs) are an emerging class of functional porous materials for diverse potential applications. Typically, tailored microporous structures of MOPs are generated by linkages of organic polymerizable monomer building blocks, providing high permanent porosity and excellent stability. Herein, we reported the first example of the application of organic building block based MOPs (OBB-MOPs) as efficient enrichment media for sample preparation. A novel multilayer interbridging strategy was proposed to fabricate OBB-MOP coatings, and hereby SNW-1 (a kind of OBB-MOPs) was coated on silica substrate with well-controlled thickness. Strong covalent bonds throughout the network and interlayer bridging improved the durability of the coating significantly. Outstanding chemical stability was observed in diverse solvents as well as solutions with a wide range of pH or high ionic strength and even under extremely harsh conditions like boiling water. The SNW-1 coating possessed a microporous network structure constructed of conjugated and nitrogen-rich building blocks. Thus, the coating exhibited a superior enrichment performance of polycyclic aromatic hydrocarbons and volatile fatty acids (VFAs) over commercial coatings based on interactions including π−π affinity and acid−base interaction. For further application, this coating was combined with gas chromatography/mass spectrometry for the noninvasive analysis of VFAs from tea leaf and tobacco shred samples. The low detection limits of 0.014−0.026 μg/L were achieved with the relative standard deviations (RSDs) between 4.3 and 9.0%. Consequently, trace original VFAs from the samples were detected. Good recoveries were obtained in the range of 90−129% and 77−118% with the corresponding RSDs (n = 3) of 2.6−9.3% and 1.9−10%, respectively.

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hybrid microporous materials, like metal−organic frameworks (MOFs), has received great attention. MOFs are characterized by well-defined pore structures, small pore apertures, high adsorption capacity, and excellent “molecular sieving effect” for target analytes.6 Porous structure with well-defined cavities can be tailored by metal-containing inorganic cluster as nodes and organic ligands as linkers. Many MOFs possess excellent thermal stability, and some of them (like Zeolitic imidazolide frameworks7) are chemically stable. Though MOFs have shown exciting applications in separation in recent years,8−10 there are still some limitations. Since these coordination polymers are formed by linking organic ligands and metal moieties through coordination bonds, many MOFs have insufficient chemical

here has been an increasing trend toward the development of new efficient materials for sample preparation such as cleanup, enrichment, and separation, since the properties of materials are usually crucial to the sensitivity, selectivity, and accuracy of analytical methods. For example, adsorbents and coatings play key roles in the techniques of solid-phase extraction and solid-phase microextraction (SPME),1 respectively. Nowadays, microporous materials, featured by small pores and high surface areas, are a class of important adsorbents at the forefront of materials research, especially for their application in enrichment and separation.2 During the last few decades, inorganic microporous materials such as carbon molecular sieves, activated carbon, and zeolites have exhibited excellent enrichment capacity for low molecular weight compounds.3,4 However, carbon materials usually have wide pore size distribution and relatively limited scope for synthetic diversification,5 limiting their selectivity and applicability. To date, the development of novel inorganic−organic © XXXX American Chemical Society

Received: December 10, 2014 Accepted: February 23, 2015

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DOI: 10.1021/ac504594d Anal. Chem. XXXX, XXX, XXX−XXX

Analytical Chemistry

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sensing,31 and carbon dioxide capture32 due to its excellent properties. The fabrication of the SNW-1 coating was expected to extend application of this material to enrichment and separation. We found in this study the extraordinarily high adsorptive affinity of the SNW-1 coating toward both polycyclic aromatic hydrocarbons (PAHs) and volatile fatty acids (VFAs). VFAs were selected as the target analytes, since they are important malodorous compounds33 as the source of energy conversion (sewage and waste treatment plants). VFAs are also the important trace components of flavor from a wide range of foods, which can reflect the food quality. We applied the SNW1 enrichment coating which was combined with gas chromatography/mass spectometry (GC/MS) for the noninvasive sampling and analysis of trace VFAs emitted from tea and tobacco shred samples.

stability in solvents and moisture due to the weak coordinative interaction,11 hampering their practical application. Recently, organic microporous materials, termed microporous organic polymers (MOPs), have been explored as a new generation of microporous materials which are comprised of light, nonmetallic elements such as C, H, O, N, and B.5 Micropores of MOPs are generated by linkage of organic polymerizable monomer building blocks or sometimes postpolymerization hyper-cross-linking.12 The MOPs synthesized by the former method can be considered as organic building block based MOPs (OBB-MOPs), while the latter method is specialized for the preparation of microporous hypercross-linked polymers (HCPs).13 Most of the HCPs are restricted to styrenic polymers, and control over the pore size is rather difficult.12 The OBB-MOPs, such as conjugated microporous polymers (CMPs),14 polymers of intrinsic microporosity (PIMs),15 and covalent organic frameworks (COFs),16 show great promise in addressing the limitation, which have received far more attention. These OBB-MOPs can be prepared using a plethora of organic reactions, which provide flexibility for the material design to achieve desirable pore properties. A great choice of monomers (organic building blocks) available makes it easy to introduce various functional groups in the pore walls, and hence the pore structure can be tailored by designing building blocks with targeted structure. Another impressive feature of these OBB-MOPs is their good chemical stability arising from the covalent linking of organic building blocks compared to MOFs containing coordinate bonds.17 Accordingly, these organic building block based microporous networks exhibited the permanent porosity with high surface area and remarkable stability. Due to this unique property, there is enormous scope for polymer postmodification to introduce specific chemical functionalities18 into OBBMOPs without problems such as framework degradation or loss of microporosity. 5 Compared with their inorganic or inorganic−organic hybrid counterparts, such as carbon nanometerials and MOFs, OBB-MOPs have possible advantages in terms of combining wide synthetic diversity with reasonable thermal and chemical stability.5,19 The two main advantages of OBB-MOPs bode well for their applications in catalysis,20 gas storage,21,22 and separation.23 Recent research also shows the excellent optical properties of OBB-MOPs and their potential application in sensing and related fields.17,24 In principle, excellent adsorptive properties of OBB-MOPs with inherent chemical stability can be tailored by selecting suitable building blocks and reaction strategies. Fine control over the chemical nature of the available surface area is possible. At this regard, OBB-MOPs show great promise as efficient adsorbents for enrichment of trace compounds. Since OBB-MOP materials are currently available in powder form as adsorbents, there is strong interest to develop other handy formats for practical applications such as thin films,25,26 magnetic microspheres,27 and nanoparticles with well-defined morphology.28 However, the immobilization of OBB-MOPs remains challenging especially for growth of OBB-MOPs on atomically smooth nonporous solid surfaces. Herein, we proposed a novel multilayer interbridging (MIB) strategy to fabricate OBB-MOP coatings on silica substrate with desirable thickness, stability, durability, and repeatability. Following this strategy, we successfully fabricated a novel OBB-MOP enrichment coating based on Schiff base network (SNW-1).29 In the previous studies, SNW-1 particles were applied to mercury ions removal,30 nitroaromatic explosives

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EXPERIMENTAL SECTION Reagents and Materials. Melamine, terephthalaldehyde, and the VFAs standards such as acetic acid (ACA), propionic acid (PPA), isobutyric acid (IBA), n-butyric acid (BTA), isovaleric acid (IVA), valeric acid (VLA), and n-hexylic acid (HLA) were obtained from Aladdin (Shanghai, China). The PAHs standards such as naphthalene (NAP), acenaphthene (AcPy), fluorene (FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLA), and pyrene (PYR) were purchased from Chem Service (America). The silane coupling agent γ-(2,3-epoxypropoxy)propytrimethoxysilane (GPTMS) was obtained from Juzhaoyuecheng organic silicon raw material Co., Ltd. (Guangzhou, China). Methanol, ethanol, dimethyl sulfoxide, nitrobenzene, benzaldehyde, benzene, toluene, styrene, chlorobenzene, and sodium hydroxide were all analytical reagent-grade and purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). HPLC grade methanol was obtained from Dikma (Beijing, China). Water used in this work was doubly distilled. The fused-silica fibers (125 μm i.d.) were taken from Yongnian Ruifeng Optical Fiber Factory (Hebei, China). Other chemical reagents were all of analytical grade. Stock solutions of PAHs and VFAs at 50000 mg/L were prepared, and all of them were refrigerated for storage. Working solutions were prepared by step-by-step dilution with methanol just before use. The commercially available fibers including polydimethylsiloxane (PDMS, 100 μm), polydimethylsiloxane/divinylbenzene (PDMS/DVB, 65 μm), carboxen/polydimethylsiloxane (CAR/ PDMS, 85 μm), and polyacrylate (PA, 85 μm) were purchased from Supelco (Bellefonte, PA, USA), which were used for the comparison experiment. Tea leaf was purchased from a supermarket in Guangzhou, and tobacco shred was kindly provided by China Tobacco Guangdong Industrial Co., Ltd. Instruments. MAS-I microwave oven (Sineo, China) was employed for synthesis of the SNW-1 powders. A 200 W ultrasonicator (KQ 5200, Kunshan, China) was used for the preparation of the SNW-1 dispersion in methanol. Scanning electron microscopy (SEM) images of the enrichment coatings were acquired by XL-30 SEM instrument (Philips, Netherlands). Transmission electron microscopic (TEM) characterization was performed on a PHILIPS TECNAI 10 TEM instrument (Philips, Netherlands). IR spectra were obtained on a NICOLET AVATAR 330 Fourier transform infrared (FT-IR) spectrometer. Thermal stability evaluation was conducted on a thermogravimetric analyzer (TGA) (Netzsch-209, Bavaria, Germany) from room temperature to 900 °C in flowing N2 at a heating rate of 10 °C/min. N2 and CO2 adsorption as well B



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Figure 1. Schematic diagrams of preparation of SNW-1 (A) and fabrication of the SNW-1 enrichment coating (B).

precipitate was observed, the reaction lasted for 24 h. The product was washed extensively with acetone, dichloromethane, and methanol in sequence and then was dried at 80 °C. The fabrication of the SNW-1 enrichment coating by MIB strategy involved the following processes (Figure 1). (a) The silica fiber was cut into 17 cm in length. The protecting polyimide layer of its one tip (3.0 cm) was removed by a pair of special decortication pliers. Subsequently, the fiber was dipped into 1 mol/L NaOH solution for 1 h and 1 mol/L HCl solution to expose the maximum number of silanol groups, which was finally washed with water thoroughly and dried. (b) The fiber was immersed into the GPTMS solution for 5 min at room temperature, leading to the formation of Si−O−Si bonding by the reaction between GPTMS and hydroxyl groups on the silica fiber surface. Then, the fiber was pulled out and placed into an oven at 80 °C for 30 min to complete this silanization reaction. (c) The 6.7 wt % SNW-1 dispersion in methanol with particle size in the range of 20−30 nm was prepared under an ultrasonic bath for 30 min. The dispersion was stable and uniform for at least 8 h. Before each coating, the dispersion was freshly made. (d) The silylated fiber was inserted into the 6.7 wt % SNW-1 dispersion in methanol and pulled out immediately to give a thin layer of the SNW-1 coating. Then, it was immersed in GPTMS solution for 1 min and subsequently taken out and placed into an oven at 80 °C for 30 min to complete the chemical linking between SNW-1 and GPTMS. The procedures of (d) were repeated to get a multilayer SNW-1 coating until the required thickness was achieved. The obtained fiber was assembled into a homemade SPME fiber holder modified from a 5-μL GC microsyringe and then conditioned in the GC injector at 250 °C under argon for 1 h prior to use. SNW-1 coatings were carefully scraped off the fibers using a knife for the FT-IR and TGA characterization. Enrichment Procedure. The enrichment of PAHs was carried out in a 15 mL working solution containing 20% of NaCl in water, which was introduced to a 40 mL vial capped with PTFE-coated septa. Magnetic stirring with a Teflon-coated stir bar was used to agitate the solution at 1500 rpm. The enrichment temperature was set at 60 °C controlled by a

as surface area measurement were performed on an ASAP-2020 M gas adsorption instrument (Micromeritics, Atlanta, USA). Elemental analysis was performed on a vario EL cube analyzer (Elementar, Germany). The 13C NMR spectrum was recorded with a BRUKER AVANCE 400 NMR spectrometer ((Bruker, Switzerland). All chromatographic measurements were performed using a Shimadzu QP-2010 gas chromatograph equipped with mass spectrometry detector (GC/MS; Japan). An HP-VOC (Agilent Scientific, USA) capillary column (60 m length × 0.32 mm i.d. × 1.8 μm film thickness) was used for the chromatographic separation of PAHs. The instrument conditions were as follows: splitless mode; injector temperature, 250 °C; ion source temperature, 230 °C; interface temperature, 260 °C; argon flow, 1.96 mL/min. The GC oven temperature program was as follows: 80 °C for 1 min; 26 °C/min to 270 °C for 25 min. A VF-WAX (Agilent Scientific, USA) capillary column (30 m length × 0.25 mm i.d. × 0.25 μm film thickness) was used for the chromatographic separation of VFAs. The instrument conditions were as follows: splitless mode; injector temperature, 200 °C; ion source temperature, 230 °C; interface temperature, 240 °C; argon flow, 0.99 mL/min. The GC oven temperature program was as follows: 50 °C for 1 min; 2 °C/ min to 178 °C; 60 °C/min to 240 °C for 7 min. Preparation of the SNW-1 Enrichment Coating. The SNW-1 nanoparticles were prepared by a microwave-assisted synthesis method modified from the previous study.31 Melamine (0.313 g) and terephthalaldehyde (0.500 g) were dissolved in 15.5 mL of dimethyl sulfoxide and transferred into a 50 mL single-necked flask. After ultrasonic degassing, the solution was sparged with nitrogen and put in a microwave oven. Then the solution was dispersed by vigorous agitation (800 rpm) with a magnetic stir bar throughout the whole reaction under reflux. Microwave irradiation was carried out with a programmed temperature control as follows. The temperature increased from room temperature to 50 °C within 2 min. Then the temperature increased from 50 to 90 °C within 20 min and was finally held at 90 °C. After resultant white C



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between the silica fiber and SNW-1 particles as well as the interparticle bridging by self-polymerization. The pretreated fiber was immersed into GPTMS so that the exposed silanol groups would form the covalent bonding with GPTMS. The silane-modified fiber was in sequence inserted and pulled out from methanol solution of well-dispersed SNW-1 particles, forming a homogeneous coating on the substrate. Then vital linkages were formed by heat treatment due to a series of chemical reactions discussed as follows. The SNW-1 particles processed the amine-rich structure with a nitrogen content of up to 44 wt %, as confirmed by elemental analysis. Thus, epoxy groups of GPTMS could easily react with the amino groups of SNW-1, forming C−N bonds via the ring-opening reaction. The resultant intralayer and layer-to-layer bridging were attributed to the self-polymerization of GPTMS. This entirely covalent-linking network played a crucial role in the enhancement of the durability and stability of the coating. There was no cracking and desquamation of the coating, as well as no significant change of the enrichment performance after 100 uses (with RSDs ranged from 8.3% to 12% for enrichment of VFAs). The result indicated that the SNW-1 enrichment coating was able to be used repeatedly at least for 100 enrichment cycles. The stability was significantly superior to that of the physically deposited coating which was prepared following the identical procedure without GPTMS treatment. The result suggested that the interparticle bridging of the coating, and the covalent bridging between the SNW-1 and the silica substrate indeed promoted the formation of robust SNW1 multlayer on the GPTMS modified substrate. Fourier transform infrared spectra confirmed the presence of a ring-opening reaction between the amine groups of SNW-1 and epoxy groups on the GPTMS-treated silica substrate, as could be seen from Figure 2. The characteristic peak of the

thermostatic water bath. The SNW-1 coated fiber was exposed to the headspace above water for a certain time to perform the enrichment. Finally, the fiber was removed from the vial and immediately inserted into a GC inlet for thermal desorption followed by GC/MS analysis. Before each use, the fiber was conditioned at 250 °C for 2 min. For VFAs analysis, a 4.0 μL aliquot of standard solution was introduced to a 40 mL gastight sealed glass vial and then gasified and homogenized in the vial at 35 °C for 5 min. Then, the needle of homemade SPME device passed through the septum, and the SNW-1-coated fiber was pushed in and exposed to the standard vapor for enrichment. Finally, the fiber was immediately transferred to the GC inlet for thermal desorption followed by GC/MS analysis. Before each use, the fiber was conditioned at 200 °C for 2 min. The enhancement factor (EF) was defined as the ratio of the sensitivity of an analyte after extraction to that before extraction (i.e., by direct injection of 1 μL of standard solution) using the chromatographic peak area for quantification.34 Since SNW-1 and commercial coatings have different thickness and length, EFs of these coatings for various target compounds were finally calculated and compared based on per cubic millimeter of fiber coatings in this study. For sampling, the accurately weighed samples (120 mg of tea and 100 mg of tobacco shred) were put in a 40 mL sealed vial. After the odors emitted from tea leaf and tobacco shred were collected in the sealed headspace vial (see the Experimental Section of the Supporting Information), the SNW-1 coated fiber was inserted into the headspace vial for capturing the VFAs. Subsequently, the SNW-1 enrichment coating was exposed to the vial headspace for 45 min at room temperature and then transferred to a GC inlet at 200 °C for thermal desorption followed by GC/MS analysis.

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RESULTS AND DISCUSSION Multilayer Interbridging Strategy. Covalent Bonding for Durability. Despite much progress in the synthesis of OBBMOPs, there is still a lack of research for the fabrication of the thin coating on substrate acting as reliable and robust formats. Initially, we attempted to immobilize the SNW-1 particles on a silica fiber simply by physical deposition. Unfortunately, cracking and exfoliation of the SNW-1 coating was observed after several enrichment cycles. The difficulties in immobilization may be attributed to the weak force between the SNW-1 particles and the silica substrate as well as the weak interparticle cohesion. Thus, a new multilayer interbridging strategy was developed here for the fabrication of the SNW-1 enrichment coating. Following this strategy, strong linkages including coating-to-substrate and interparticle bridging were formed, generating a robust SNW-1 coating with excellent durability and long lifetime. The schematic of the coating procedure is shown in Figure 1. SNW-1 nanoparticles were synthesized as shown in Figure 1A, and the given structure was confirmed by 13C solid-state NMR spectroscopy (Figure S-1). The resonances of 167 ppm can be assigned to the carbon atoms present in the triazine ring of the melamine, whereas the signal at 128 ppm originates from CH aromatic carbons of the benzene. The resonance at 55 ppm can be correlated to the tertiary carbon atoms formed upon the addition of the primary amine groups of melamine to the newly formed carbon−nitrogen double bond leading to the aminal structure.29 As shown in Figure 1B, the silane coupling agent GPTMS was used for the formation of covalent attachment

Figure 2. Infrared (KBr pellet) spectra for SNW-1 particles (a), the SNW-1 coating (b), and GPTMS (c).

epoxy group at 910 cm−1 in the spectrum of GPTMS (see Figure 2c) was absent in the spectrum of the SNW-1 coating (see Figure 2b), which provided possible evidence that epoxy groups reacted with amine groups via a ring-opening reaction. A new absorption band at 1383 cm−1 in the spectrum of the SNW-1 coating might be attributed to the −OH groups formed via ring-opening of epoxy groups. Generally, the SNW-1 coating had a similar absorption peak with the SNW-1 powders (Figure 2a). Typical bands of benzene and triazine rings were D



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Figure 3. Scanning electron micrographs at a magnification of 500 (A) and 50000 (B) and transmission electron microscopy image (C).

clearly observed at 814 cm−1 and in the range from 1300 to 1650 cm−1, respectively. As shown in Figure 2a, the spectrum of SNW-1 powders was almost identical to that of coatings except for the typical absorption band of GPTMS (2929 cm−1). These results reflected that the immobilization process by MIB strategy may not deform or destroy the structure of the SNW-1. Thickness Control. Following MIB strategy, the thickness of the multilayer SNW-1 coating could be well-controlled by the coating times. The coating thickness was observed to increase linearly with the number of coating cycles with the good correlation coefficient (R) of 0.9980 as illustrated in Figure S-2. Considering the time consumption and enrichment efficiency (generally increased with the thickness), the optimal coating cycles were selected as 11 to fabricate the SNW-1 enrichment coating (14 μm) for further application. The relative standard deviation (RSD) of thickness (9 fibers) was 6.0%, which suggested the good preparation reproducibility of this MIB method. The SEM (Figure 3A and 3B) and TEM images (Figure 3C) of the obtained coatings indicated that a homogeneous, dense, and porous SNW-1 coating could be successfully fabricated by the proposed MIB method. Thermal and Chemical Stability of the SNW-1 Enrichment Coating. The robustness of the SNW-1 coating was assessed by performing a series of thermal assays as well as chemical stability tests. The good thermal stability of the SNW1 coating was evidenced by the TGA data as shown in Figure S3. The result suggested that the SNW-1 coating was thermally stable up to 250 °C, which satisfied the requirement for the consequent GC applications. Outstanding chemical stability of the SNW-1 enrichment coating was observed under diverse chemical conditions. The test was performed by successively immersing the fiber in ethanol, hexane, boiling water, and solutions of 0.1 mol/L NaOH (pH = 13.0), 0.005 mol/L H2SO4 (pH = 2.0), and saturated NaCl for 5 h. The enrichment performances of the treated and untreated coatings were compared in Figure 4. The RSDs for extractions under different treatments were below 10% for VFAs. The results indicated that the SNW-1 enrichment coating showed remarkable chemical resistance to polar or nonpolar organic solvents, aqueous solutions with a wide range of pH (2.0−13.0) or high ionic strength, and even an extremely harsh condition like boiling water. It is worth noting that the main drawback of silica-based sorbents (commonly used coating materials) is the narrow range of pH stability.35 Currently, the high pH-resistant coatings are often made from organic polymers and copolymers, which are easy to swell in organic solvents and slip off the fiber core.36 In addition, many MOFs have insufficient stability toward harsh chemical conditions except Zeolitic imidazolide frameworks.7 Some MOFs coatings are even water-sensitive, showing a sharp reduction in adsorption amounts after prolonged exposure to humidity.5,34 The good

Figure 4. Extraction amounts of VFAs by the SNW-1 enrichment coating as originally made (untreated) and treated with various conditions including immersion into ethanol, hexane, boiling water, and solutions of 0.1 mol/L NaOH, 0.05 mol/L H2SO4, and saturated NaCl. Concentrations of VFAs: 4 μg/L; extraction time: 45 min; desorption time: 2 min; extraction temperature: 35 °C. VFAs: acetic acid (ACA), propionic acid (PPA), isobutyric acid (IBA), n-butyric acid (BTA), isovaleric acid (IVA), valeric acid (VLA), and n-hexylic acid (HLA).

chemical stability of the SNW-1 enrichment coating might be attributed to the highly cross-linking network based on robust covalent bonds. This feature facilitated wide application of the SNW-1 coating to enrichment and separation under various conditions. Adsorption Characteristics of the SNW-1 Enrichment Coating. Gas Adsorption. The research about the adsorption characteristics of the SNW-1 enrichment coatings for gases and organic analytes is critical to explore their potential applications. According to the nitrogen adsorption−desorption isotherms (see Figure 5A) of the SNW-1 powders, the specific surface area was 231 m2/g (determined by the Brunauer− Emmett−Teller N2 adsorption method). Pores size distribution was further estimated by the density functional theory (DFT) method according to the nitrogen adsorption−desorption isotherms. As shown in Figure S-4, the SNW-1 network had a narrow pore size distribution from 1.2 to 1.6 nm with an average pore size around 1.4 nm, indicating the micropore structure of this material. In the previous studies,28−32 the reported specific surface areas of SNW-1 vary from 133 to 1377 m2/g. It probably resulted from differences of the particle sizes, degree of cross-linking, and open channels of MOPs produced under various synthetic conditions. In our work, a relative simple, mild, and rapid synthesis method using microwave irradiation was adopted to prepare SNW-1. In accordance with E



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commercial PDMS and PDMS/DVB coatings as shown in Figure 6A. The superiority was more obvious for the PAHs with more conjugated double bonds and condensed rings. It could be explained that the adsorption affinity was controlled primarily by π−π stacking interaction. There occurred a π−π stacking interaction between the condensed rings of PAHs and π-conjugated groups of the SNW-1 coating such as aromatic and triazine rings, facilitating the adsorption of PAHs. The πconjugation of the coating can be confirmed by its remarkable fluorescence emission feature in solid-state as shown in Figure S-5. Additionally, the π−π interaction can be confirmed by the variation in the fluorescence intensity after the exposure of the SNW-1 to arene vapors.24,31 As shown in Figure S-6, drastic fluorescence quenching was observed in the presence of electron-deficient arene vapors such as nitrobenzene and benzaldehyde. In contrast, fluorescence enhancement was found after exposure to electron-rich arene vapors such as benzene, toluene, styrene, and chlorobenzene. Benefited by π−π interaction, the SNW-1 coating had great potential to be applied to the enrichment of analytes with conjugated structure. As shown in Figure 6B, the enrichment factors for VFAs of the SNW-1 coating also exhibited obvious superiority over those of its commercial counterparts. The high adsorption affinity might be attributed to the acid−base interaction between the amine groups of the SNW-1 coating and the carboxyl groups of VFAs.37 The SNW-1 coating offered abundant electron donors (basic groups) as binding sites for the acceptors (acidic targets). The acid−base interaction had proved to be useful for selective binding and vapor sensing38 as well as binary CO2 and H2O capture.39 Thus, based on the interaction, the nitrogen-rich feature of the SNW-1 coating was expected to facilitate the highly efficient enrichment of acidic volatiles like VFAs. Interest in the analysis of VFAs has grown steadily, since VFAs are one of the vital volatile organic compounds that present a challenge to sampling.40 It is therefore of highly desirable importance to develop innovative efficient sorbents for capturing VFAs. Hence, they were selected as target analytes for further application of the obtained SNW-1 enrichment coating. Application to Analysis of Trace VFAs in Food Odor. A new analytical method incorporating the SNW-1 coating and

Figure 5. Nitrogen adsorption/desorption at 77 K of the SNW-1 powders.

these studies,28−32 the porosity and surface area of SNW-1 were lower than those achieved by the conventional heating methods; however, the microwave methods can boost the yield and speed of the reaction. As the previous report pointed out,30 the SNW-1 prepared by microwave methods possessed a lower degree of cross-linking and thus afforded more amineterminated groups. These amine groups are easy to form hydrogen bonds between the melamine-based molecules, leading to the decrease in porosity as well as specific surface area.30 However, from another perspective, abundant amineterminated groups might benefit the adsorption of organic volatiles, since the SNW-1 network afforded more binding sites on the surface. Organic Volatiles Adsorption. In view of the abundant conjugated rings and amine groups throughout the networks of SNW-1, conjugated compounds (PAHs) and acidic compounds (VFAs) were selected as model analytes. Their enrichment factors (EFs) were evaluated and compared with those of the most commonly used commercial coatings as shown in Figure 6. The SNW-1 enrichment coating displayed significantly higher enrichment efficiency of PAHs compared to the

Figure 6. Enrichment factors (EFs) obtained with the SNW-1 enrichment coating and commercially available coatings for PAHs (A) and VFAs (B). VFAs: acetic acid (ACA), propionic acid (PPA), isobutyric acid (IBA), n-butyric acid (BTA), isovaleric acid (IVA), valeric acid (VLA), and n-hexylic acid (HLA). PAHs: naphthalene (NAP), acenaphthene (AcPy), fluorene (FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLA), and pyrene (PYR). F



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Figure 7. Adsorption (A) and desorption (B) time curves of the SNW-1 coating. The corresponding desorption time and the adsorption time were 2 and 45 min, respectively. Compounds: 1, acetic acid; 2, propionic acid; 3, isobutyric acid; 4, n-butyric acid; 5, isovaleric acid; 6, valeric acid; 7, nhexylic acid.

are shown in Figure 8, and the quantification results were summarized in Table 2. It was satisfactory that most of the VFAs could be detected from the two samples by this method and quantified to be in the range of 0.13−4.9 μg/L. The spiked experiment was conducted for the method validation. The recoveries of tea leaf and tobacco shred odor samples were 90− 129% and 77−118%, and the corresponding RSDs were 2.6− 9.3% and 1.9−10%, respectively. The results demonstrated that this sensitive method was reliable and practical for the noninvasive analysis of trace VFAs from real samples. The method was expected to be used for noninvasive monitoring the VFAs in the odor emitted from various foods.

GC/MS was developed for the determination of VFAs emitted from food samples. Important conditions including adsorption time and desorption time were optimized as shown in Figure 7, and the results suggested that the SNW-1 coating processed fast adsorption and desorption kinetics. As summarized in Table 1, the new method exhibited satisfactory linearity in the Table 1. Linear Range, Detection Limit, and Precision of SNW-1 Enrichment Based GC/MS Method for the Determination of VFAs compound acetic acid propionic acid isobutyric acid n-butyric acid isovaleric acid valeric acid n-hexylic acid a

linear range (μg/L)

correlation coefficient

LOD (μg/L)

RSDa (%, n = 5)

0.07−10 0.08−10

0.9993 0.9994

0.019 0.022

4.3 8.4

0.06−10

0.9984

0.017

8.5

0.07−10

0.9986

0.020

8.0

0.07−10

0.9973

0.021

9.2

0.05−10 0.09−8.2

0.9974 0.9935

0.014 0.026

8.0 9.0

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CONCLUSION

We have reported the first example of the application of OBBMOPs as efficient enrichment media for sample preparation. A novel MIB strategy was proposed for the fabrication of OBBMOP enrichment coating with well-controlled thickness and stable covalent bridging. Excellent chemical stability under diverse chemical conditions was observed due to its highly cross-linked networks formed by strong covalent bonds. The resultant SNW-1 coating provided the significant enrichment superiority for conjugated compounds (like PAHs) and acidic compounds (like VFAs) over commonly used commercial coatings. We propose that the driving force of the recognition process is mainly dictated by π−π affinity and acid−base interactions, which were attributed to the conjugated rings and abundant amine groups of SNW-1. The SNW-1 coating was successfully applied for the noninvasive enrichment and analysis of trace VFAs emitted from tea and tobacco shred samples. It is expected that OBB-MOPs are a kind of promising materials for sampling, enrichment, and separation. They process high enrichment performance with exceptional chemical stability, and their adsorption properties can be tailored by using suitable building blocks and diverse synthetic strategies. The results of our study signify the prospect of OBB-MOPs for wide application in analytical chemistry, and MIB strategy would provide a useful way to immobilize OBB-MOPs for practical use.

RSD: Calculated at a concentration of 0.5 μg/L.

range of 0.07−10 μg/L for acetic acid, n-butyric acid and isovaleric acid, 0.08−10 μg/L for propionic acid, 0.06−10 μg/L for isobutyric acid, 0.05−10 μg/L for valeric acid, and 0.09−8 μg/L for n-hexylic acid. Their correlation coefficients (R) ranged from 0.9935 to 0.9993. The detection limits of the VFAs were in the range of 0.014−0.026 μg/L based on a signal-tonoise ratio of 3 (S/N = 3). The RSDs (n = 5) ranged from 4.3% to 9.0% at a level of 0.5 μg/L. The results proved that the novel SNW-1 coating was reproducible and stable, and the method was reliable and sensitive for the trace analysis of VFAs. VFAs are important trace components of flavor emitted from tea and tobacco shred, which required a reliable and sensitive quantitative method to study the relationship between their concentrations and human smell and taste. Thus, the proposed method was applied to the analysis of trace VFAs in the odor of the two samples in a noninvasive operation mode. Typical chromatograms of trace VFAs from tea leaf and tobacco shred G



Article

Figure 8. Chromatograms of tea leaf (A) and tobacco shred (B) samples. The standard at 0.41 μg/L(a), obtained by the proposed SNW-1 enrichment(b) based GC/MS method. Peak identity: 1, acetic acid; 2, propionic acid; 3, isobutyric acid; 4, n-butyric acid; 5, isovaleric acid; 6, valeric acid; 7, n-hexylic acid.

Table 2. Original Amounts and Recoveries of VFAs for Spiked Tea Leaf and Tobacco Shred Samples (n = 3) tea compound

a

original amount (μg/L)

acetic acid

4.9

propionic acid

0.63

isobutyric acid

nda

n-butyric acid

nda

isovaleric acid

nda

valeric acid

0.13

n-hexylic acid

0.17

added (μg/L)

tobacco shred recovery (%)

4.31 2.15 1.23 0.615 1.23 0.615 1.23 0.615 1.23 0.615 0.205 0.102 0.205 0.102

RSD (%)

112 90 102 95 108 109 120 120 118 117 129 129 115 107

6.4 2.6 5.8 3.6 4.0 5.8 2.9 3.8 3.6 5.1 7.5 3.3 9.3 5.2

nd: the VFAs was not detected. bnq: the signal was saturated.

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ASSOCIATED CONTENT

S Supporting Information *

nq

added (μg/L)

recovery (%)

RSD (%)

1.44 0.718 1.23 0.615 0.205 0.102 0.390 0.195 0.513 0.256 0.369 0.185

105 108 102 109 118 117 115 89 113 105 96 77

7.2 3.4 5.6 10 1.9 5.4 4.0 4.9 7.0 1.0 9.6 10

b

0.76 nda 0.10 0.20 0.26 0.18

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Experimental Section and Figures S-1−S-6. This material is available free of charge via the Internet at http://pubs.acs.org.

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original amount (μg/L)

AUTHOR INFORMATION

Corresponding Authors

*Phone: 86-20-84110922. Fax: 86-20-84115107. E-mail: [email protected] (G.L.). *E-mail: [email protected] (Y.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Special Funds of the National Natural Science Foundation of China (Grant No. 21127008), the National Natural Science Foundation of China (Grant Nos. 91232703 and 21475153), and the Specialized Research Fund for the Doctoral Program of Higher Education (20120171110001), respectively. H



Article

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Organic Building Block Based Microporous Network SNW-1 Coating

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