Universal Multilayer Assemblies of Graphene in Chemically Resistant

Jun 25, 2013 - ... Yong Qin , Juan Li , Fuqiang Chu , Yong Kong , Linhong Deng. Journal of Applied Polymer Science 2015 132 (10.1002/app.v132.37), n/a...
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Universal Multilayer Assemblies of Graphene in Chemically Resistant Microtubes for Microextraction Wenpeng Zhang, Juan Zhang, Tao Bao, Wei Zhou, Jiawei Meng, and Zilin Chen* Key Laboratory of Combinatorial Biosynthesis and Drug Discovery (Wuhan University), Ministry of Education, and Wuhan University School of Pharmaceutical Sciences, Wuhan 430071, China S Supporting Information *

ABSTRACT: Graphene is a new kind of two-dimensional carbon nanomaterial with excellent properties and is promising for solid-phase microextraction (SPME). Plastic microtubes such as poly(tetrafluoroethylene) (PTFE) and poly(ether ether ketone) are ideal substrates for in-tube SPME. However, immobilization of graphene layers onto these materials is still a problem due to their nature of chemical resistance. In order to solve the problem, we proposed a novel method based on universal mussel-inspired polydopamine (PD) and layer-bylayer assembly of graphene in this work. To make a graphene assembly layer inside PTFE, the strategy includes two major steps. First, a PD layer is made on the PTFE surface by noncovalent interaction. Second, multilayer graphene is assembled on the PD layer by covalent interaction. By repeating these two steps, a functional graphene oxide (FGO)-modified PTFE tube with a controllable number of layers can be obtained. Morphology of the multilayer structure of graphene has been confirmed by scanning electronic microscopy. Formation of the covalent layer has also been characterized by Foourier transform infrared and X-ray photoelectron spectroscopy. It is very interesting that (FGO-PD)3-PTFE shows exceptional efficiency for SPME. Enrichment from 1082- to 2331-fold was achieved for six polyaromatic hydrocarbons (PAHs). An online SPME-HPLCfluorescent detection method has been developed on the basis of (FGO-PD)3-PTFE. For qualitative analysis of PAHs, the method has low limits of detection of 0.05−0.1 pg/mL, which is significantly lower (up to 1000 times) than that reported in literature. The method shows wide linear range (0.3−200 pg/mL), good linearity (R2 ≥ 0.9968), and good reproducibility (relative standard deviation < 3.4%). The method has been applied to determine PAHs in environmental samples. Good recoveries were obtained, ranging from 85.1% to 96.7%.

G

suitability to most compounds.12−14 For coupling of SPME with HPLC, in-tube SPME is a great way to overcome problems often encountered by use of fiber-based SPME, such as swelling in mobile phase, stripping of the coating, and low capacities. Fused-silica capillaries are widely used for in-tube SPME, because the abundant active silanol groups in the inner wall can be easily functionalized to further immobilize sorbents. The problem with using silica capillaries is that they are fragile and easily broken down under the high pressure of HPLC, and the ends of the capillary are also easily crushed when connected to the HPLC flow system. In order to establish stable and highly efficient SPME-HPLC methods, it is necessary to develop new kinds of tube materials for in-tube SPME. Plastic microtubes such as poly(tetrafluoroethylene) (PTFE) and poly(ether ether ketone) (PEEK) are good choices for in-tube SPME substrates as they possess great properties such as high strength, high-pressure resistance, good flexibility, and easy

raphene is a new class of two-dimensional (2D) carbon nanomaterial that consists of single atomic layers of sp2hybridized carbon arranged in a hexagonal lattice.1−3 With their unique properties, graphene and its derivatives (such as graphene oxide, GO) have stimulated scientists’ intensive interest in applications of graphene in the fields of physics,4,5 chemistry,6 materials science,7 and biomedical science.8,9 Since the large delocalized π-electron system of graphene can form a strong π-stacking interaction with the benzene ring,6 graphene can strongly interact with aromatic compounds and their derivatives; along with the large surface area (theoretically about 2700 m2/g)4 and ease of preparation,2,10,11 graphene is also a promising material for chromatographic separation and sample pretreatment. Solid-phase microextraction (SPME) is a novel and simple sample pretreatment technique that is rapid, solvent-free, and economical for use as compared with conventional solid-phase extraction (SPE) and liquid−liquid extraction.12 SPME is usually coupled with gas chromatography (GC); SPME coupled with high-performance liquid chromatography (HPLC) has also been receiving more and more attraction because of the high separation efficiency of HPLC and its © XXXX American Chemical Society

Received: April 13, 2013 Accepted: June 25, 2013

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Preparation of Multilayer FGO-PD-Modified PTFE Tube. A PTFE tube of 10 cm was cut, washed sufficiently with methanol, and dried in an oven (60 °C) before use. For polydopamine modification, dopamine basic solution (2 mg/ mL, pH 8.5 in Tris-HCl buffer) was agitated with a vortex mixer to facilitate the oxidation of dopamine. When the solution became slightly brown, 1 mL of solution was introduced into a syringe and then pushed through the PTFE tube by the syringe pump at a rate of 0.1 mL/h for 10 h. Then the two ends of the PTFE tube were sealed and held for 12 h; the residual solution was pushed out and the PTFE tube was washed with water. After drying in an oven (60 °C) for 3 h, PD-modified PTFE tube (PD-PTFE) was obtained. Graphene oxide was prepared from graphite powder according to the Hummers method,10,31 and for graphene modification, GO aqueous solution (1 mg/mL) was introduced into PD-PTFE, the two ends were sealed, and the tube was put in a water bath and heated at 65 °C for 10 h. After the tube was washed with water and dried in the oven (60 °C, 3 h), FGO-PD-PTFE was obtained. The above procedures were repeated three times to obtain multilayer graphene-modified PTFE tube, namely, (FGO-PD)3-PTFE. The modification of other kinds of tubes was the same as that of PTFE; the resultant modified tubes were named as (FGO-PD)3-PEEK and (FGO-PD)3-SS (stainless steel). The schematic is shown in Figure 1.

cutting. PTFE and PEEK tubes are also commonly used in HPLC pipelines, so it is convenient to realize the combination with HPLC because of the rich ready-made interfaces and connectors. However, due to high chemical resistance, it is difficult to immobilize sorbents, including graphene, onto plastic microtubes; to our knowledge, no work has been reported for immobilization of graphene or its derivatives onto chemically resistant plastic microtubes or fibers for sample pretreatment. Inspired by mussel’s unique binding capability to rocks, a polydopamine (PD)-based surface modification method has been developed by Messersmith’s group.15,16 In our previous work,17 we have explored the application of polydopamine method inside PEEK tubes and successfully realized a bond of monolithic sorbents18 with the surface of PEEK tube. The functionalized PD layer was found to be highly stable and the firm bonding between the PEEK surface and polymer monolith was established. Therefore, the exceptional results in our previous work encourage us to explore the possibility to bond graphene onto chemically resistant plastic microtubes by a PDbased method in this work. As graphene sheets are nanometer in size, graphene can be incorporated into monolithic materials such as organic polymer monolith.19 The disadvantages of incorporation are that the content of graphene in the sorbents is relatively small, and worse still, graphene is stuck in the dense polymer networks and the 2D structure of graphene could not be used to full advantage. Assembly of graphene on the surface of substrates seems to be a good way to make full use of graphene’s unique properties and to produce ultrathin films of large area surface.7,20 Recently, layer-by-layer (LBL) strategy has emerged as an excellent way to fabricate ultrathin materials.21,22 LBL has also been a powerful tool to fabricate graphene-based multilayer films and has greatly expanded the application of graphene in sensors,23 transistors,24 photodetectors,25 solar cells,26 supercapacitors,27 and so on. Noncovalent assemblies were effectively used in these works,24−26 where cationic molecules served as the “binders” of graphene layers. To overcome the low stability of the noncovalent interaction, a covalent LBL method27 has also been reported very recently for assembly of graphene layers. Herein, we describe a novel noncovalent and covalent combined LBL strategy for assembling graphene layers in chemically resistant plastic microtubes. First, a PD layer “grows” bioinspiredly on the surface of the microtubes; then GO would react with polydopamine layer covalently28−30 to form a functional graphene oxide (FGO) layer. Another PD layer would also grow on the FGO layer, which can bind a new FGO layer. Multilayer graphene can be formed and bond onto the chemically resistant microtubes and further used for SPME. The formation of multilayer FGO-PD was characterized by Xray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and scanning electron microscopy (SEM). Extraction efficiency of the multilayer graphenemodified microtubes was investigated and more than 1000fold enrichment was obtained for polyaromatic hydrocarbons (PAHs). To the best of our knowledge, this is the first time that graphene has been assembled in chemically resistant microtubes for sample pretreatment.

Figure 1. Schematic of assemblies of multilayer graphene on chemically resistant plastic microtubes.

SPME-HPLC Procedures. FGO-PD-modified PTFE tube was directly connected with the HPLC system by a six-port valve (Figure S1, Supporting Information). SPME-HPLC consisted of two steps. In the sample loading step, the sixport valve was set at load mode; sample solution was introduced by a syringe pump at a rate of 0.8 mL/min and flowed through the PTFE tube for SPME. In the desorption step, the six-port valve was switched into injection mode. Mobile phase consisting of acetonitrile/H2O (65/15, v/v; 0.8 mL/min) was introduced and flowed through the PTFE tube, eluting the analytes into the HPLC column for separation and further detection by fluorescent detector. Sample Preparation. See Supporting Information.

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EXPERIMENTAL SECTION Chemicals and Materials. See Supporting Information. Instruments. See Supporting Information. B

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Evaluation of Extraction Efficiency. See Supporting Information.

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RESULTS AND DISCUSSION Layer-by-Layer-Assembled FGO-PD-Modified Microtubes. Commonly used microtubes such as PTFE, PEEK, and stainless steel tubes are highly chemically resistant, resulting in difficulty of fabricating graphene directly onto their surfaces. In this work, a mussel-inspired polydopamine method was developed for functionalization of chemically resistant surfaces, and graphene was assembled onto the surfaces by a LBL strategy. Taking PTFE as an example, dopamine basic solution was first introduced, and after reactions including oxidization, rearrangement, and intermolecular cross-linking,32 PD layers finally “grew” on the inner surface of PTFE tube. The interaction between PD and PTFE was noncovalent. Functional groups in the PD layer were active for binding with GO.28−30 As shown in Figure 1, amino groups of PD can react covalently with carboxyl and epoxy groups of GO; as a result, GO can be bonded with the inner surface of the PTFE tube, generating FGO-PD-PTFE. When dopamine basic solution was introduced again, dopamine could polymerize through intermolecular cross-linking and form another PD layer on the FGO layer. The interaction between FGO and PD was noncovalent; a new PD layer could “grow” on the modified FGO layer, and PDFGO-PD-PTFE could be obtained. The PD layer further reacted with another layer of GO. The procedures were repeated to obtain a multilayer graphene-assembled PTFE tube, which contained multilayer graphene and possibly possessed better adsorption capability. PD was assembled by noncovalent interaction onto PTFE and FGO, while GO was assembled onto the PD layer by covalent interaction. Therefore, the assemblies of multilayer graphene−PTFE were the result of a noncovalent and covalent combined LBL strategy, which combined advantages of the two strategies and possessed merits of simpleness and good stability.27 As the PTFE tube was transparent, the process of polydopamine and graphene immobilization could be observed directly. As can be seen in photographs of PTFE in Figure 2a−c, after polydopamine modification, the inner wall of PTFE became light brown (Figure 2b), and upon further reaction with GO, a black layer was observed in the PTFE (Figure 2c), which corresponded to FGO layer. Modification of graphene in silica surfaces has been reported;31,33 however, the methods were not suitable for chemical resistant surfaces, and toxic agents such as hydrazine were required for reduction of GO. The polydopamine-based method was performed under mild conditions and it was environmentally friendly, as almost no organic solvents or toxic reagents were required during the modification process. Stability of (FGO-PD)3-PTFE Tube. The stability of FGOPD layers in the surface of PTFE was investigated by treating the layers with harsh conditions, including organic solvent and strong acidic and basic solutions. Commonly used organic solvents such as methanol, ethanol, 2-propanol, dodecanol, acetonitrile, acetone, toluene, dimethyl sulfoxide, and chloroform were introduced into the (FGO-PD)3-PTFE tube. After ultrasonication for 5 min, no obvious stripping of the graphene layer was observed, indicating good endurance of the FGO-PD layer to organic solvents. The influence of acidic and basic conditions was also investigated by introducing HCl and NaOH solutions. The layer was stable even when treated with 1 M HCl and was stable under basic conditions where NaOH concentration was lower than 0.01 M. Under strong basic

Figure 2. (A) Photographs and (B) scanning electron micrographs of PTFE tube: (a, d, g) bare PTFE; (b, e, h) PD-PTFE; (c, f, i) (FGOPD)3-PTFE.

conditions (1 M NaOH), part of the layer was observed to drop, which probably resulted from decomposition of the PD layer under strong basic conditions. As the conditions for SPME are usually mild, the influence of strong basic conditions could be avoided. Characterization by Transmission and Scanning Electron Microscopy and Infrared and X-ray Photoelectron Spectroscopy. The morphology of GO was investigated by transmission electron microscope (TEM). As shown in Figure S2 (Supporting Information), GO nanosheets exhibit a disordered paperlike structure. Figure 2B shows SEM images of the inner surface of bare PTFE tube. It can be seen that the bare inner surface of PTFE tube is smooth (Figure 2d,g). After PD modification (Figure 2e,h), small particles are observed, which are formed from polymerization of dopamine.18 After layer-by-layer assembly with FGO and PD (Figure 2f,i; an enlarged SEM image is shown in Figure S3, Supporting Information), thin wrinkled sheets of graphene are observed, indicating the immobilization of graphene onto the inner surface of PTFE tube.22 Chemical composition of the layers, including GO and (FGO-PD)3, was investigated by XPS. Peaks of C 1s and O 1s are observed in GO (Figure 3A). Peak fitting of the C 1s bands of GO (Figure 3B) yields several functional groups: nonoxygenated ring C (C−C), C−O bonds, carbonyl (C=O), and carboxylate C (HO−C=O).34 After reaction with PD, the N 1s peak was observed, which was attributed to the amino group from PD (Figure 3C). Peak fitting of the C 1s bands of (FGOPD)3 (Figure 3D) also yields functional groups including C−C, C−O, and C=O as well as C−N, which was from the bond of FGO-PD. Contents of O and C−O of PD are also found to decrease significantly compared with that of GO because of loss of O by reduction with PD. The layers were also analyzed by IR, shown in Figure S4 (Supporting Information). The characteristic peaks of GO are O−H stretching around at 3424 cm−1, C=O stretching at 1724 cm−1, O−H vibration of C−OH at 1390 cm−1, and C−O stretching of epoxy at 1104 cm−1.32 After reaction with PD, C− C

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Figure 3. XPS spectra of (A) GO and (C) (FGO-PD)3; peak fitting of C 1s XPS spectra of (B) GO and (D) (FGO-PD)3.

Figure 4. Effects of (A) sampling rate, (B) salt content, (C) acetonitrile content, and (D) sample volume on extraction efficiency of SPME. Standard PAH solution, 50 pg/mL.

O stretching at 1104 cm−1 decreased significantly, as well as O−H vibration of C−OH at 1390 cm−1; meanwhile, an

obvious red shift is observed from 1724 to 1710 cm−1, probably resulting from structure change of C(=O)−OH to C(=O)− D

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Figure 5. Chromatograms of PAHs after pretreatment by (A) PD-PTFE, (B) (FGO-PD)3-PTFE, (C) (FGO-PD)3-PEEK, and (D) (FGO-PD)3-SS: (1) fluoranthene (FLU), (2) pyrene (PYR), (3) benz[a]anthracene (B[a]AN), (4) benzo[a]fluorathene (B[a]FL), (5) benzo[a]pyrene (B[a]PY), and (6) dibenz[a,h]anthracene (D[a,h]AN). Sampling: 20 mL of PAH solution of 50 pg/mL, containing 5% acetonitrile, at a rate of 48 mL/h.

column pressure and stripping of the sorbents. Using opentubular in-tube SPME can avoid the problem of high column pressure, but the extraction efficiency may still be a problem at fast sampling rates. In this work, the sampling rate of (FGOPD)3-PTFE-based SPME was investigated from 10 to 60 mL/h. Sample solution of 5 mL was loaded, and the results are shown in Figure 4A. Peak areas of all six PAHs remain almost unchanged even at a sampling rate of 60 mL/h. It seems that extraction efficiencies of PAHs are not affected much by sampling rates, which is important to realize high-speed sampling and fast analysis. The graphene layer was also stable at this high flow rate after consecutive injections. A sampling rate of 48 mL/h (0.8 mL/min, same as the flow rate of mobile phase) was selected in consideration of high sampling speed and reproducibility of extraction. Salt Content. Salt is usually added to aqueous samples to decrease the solubility of some analytes in aqueous phase, which would increase analyte concentrations in the sorbents, especially in headspace sampling mode.33 NaCl was added to sample solutions to study the effect of salt content on (FGOPD)3-PTFE-based SPME. The content of NaCl from 0 to 10% (w/v) was investigated (5 mL sample solution, rate 0.8 mL/ min). As shown in Figure 4B, extraction efficiency of PAHs decreased with increasing NaCl content. This is the opposite of results obtained in headspace sampling mode.33 It was possibly due to the decreased content of analytes in sample medium along with the increased NaCl content. The amount (n) of PAHs extracted by (FGO-PD)3 layer can be determined by12

NH. The results demonstrated the covalent reaction between GO and PD. Application of the Multilayer Graphene-Modified PTFE Tube for SPME. The multilayer graphene-modified PTFE tube is convenient to directly connect with HPLC system by a six-port valve to realize online SPME-HPLC. The PTFE tube is of relatively high strength and high pressure resistance to facilitate use in the HPLC system. PTFE is also easy to cut and process; before connection with HPLC, the two ends of the tube were cut carefully to make the interface smooth, eliminate dead volume, and avoid residue of the sample in the valve between consecutive injections. Six PAH compounds were selected for their hydrophobic properties and demands of their low-content determination to demonstrate the enrichment efficiency of the multilayer graphene-modified PTFE tube.35 Several parameters that could affect SPME, including sample pH value, sampling rate, salt concentration, acetonitrile concentration, and sample volume, were systematically studied with a PAH standard solution of 50 pg/mL. Sample pH Value. Sample pH is usually an important parameter in the extraction process, especially where the sample solution and sorbents are in direct contact. Sample pH value was investigated from pH 3 to 9 with 10 mM phosphate buffers (5 mL, at a flow rate of 0.2 mL/min). The pH value would affect the charge of (FGO-PD)3 layer and further affect the extraction of PAHs. However, little difference between pH values was observed from experimental results, probably indicating that the charge of (FGO-PD)3 has insignificant interference on PAH extraction. To simplify sample preparation procedures, the pH of sample solution was maintained at about 7 in further studies. Sampling Rate. The duration of SPME procedures mainly depends on sampling rate, so a high sampling rate is desirable for rapid analysis. For in-tube SPME, the sampling rate is often limited by hindrance of sorbents and decrease of extraction efficiency along with increased sampling rate.17 With polymeric monolith as sorbent, sampling rates were mostly set slower than 12 mL/h,36 because a faster sampling rate will result in high

n=

KVgVs Vs + KVg

c0 (1)

where K is the distribution coefficient, c0 is the initial concentration of PAHs in the sample and Vs and Vg are the volume of sample and (FGO-PD)3 layer, respectively. As the sample volume is very large, Vs ≫ KVg, eq 1 can be simplified to E

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positively related mostly to log KOW, as well as to the molecular weight and carbon ring structure of PAHs. Other kinds of aromatic compounds, including toluene, 1methyl-4-nitrobenzene, biphenyl, bromobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene, phenol, 2,4dimethylphenol, and 1-naphthalenol were also loaded onto (FGO-PD)3-PTFE, eluted, and detected. UV detection was used because of low fluorescence quantum yield of these molecules. Enrichment of these compounds was determined (listed in Table S1 in Supporting Information), which was also found to be positive correlated with log KOW. The results show that the absorption mechanism of (FGO-PD)3-PTFE is mainly based on π−π stacking and hydrophobic effect; other functional groups in the layers such as amino and hydroxyl may also participate in the extraction process, but the effects were not significant. The relationship between enrichment (E) and log KOW of investigated compounds was fitted, as shown in Figure S5 (Supporting Information). Interestingly, the values were fitted well by an exponential curve with coefficient (r) 0.9878. The equation is

(2)

As the salt content increased, the solubility of the analytes decreased (c0 decreased), and as a result, PAHs extracted by the (FGO-PD)3 layer decreased. Acetonitrile Content. Polycyclic aromatic hydrocarbons are highly hydrophobic, so acetonitrile was added to aqueous solution to facilitate their solubility. Acetonitrile contents from 0.5% to 20% (v/v) were investigated (5 mL sample solution, rate 0.8 mL/min). As shown in Figure 4C, along with the increase in acetonitrile, peak areas of PAHs were observed to increase within certain acetonitrile contents and then decrease after the high points. The reason is possibly related to the change of the distribution coefficient and PAH concentrations in sample medium. According to eq 2, K decreased along with increased acetonitrile content, but when acetonitrile was at a low concentration, c0 was also influenced and increased along with increased acetonitrile concentration. The high points of the curves in Figure 4C are the result of an inversely proportional relationship between K and c0 at low acetonitrile concentration. The difference between PAHs probably resulted from their different polarities. The relationship of extraction efficiency to acetonitrile content has also been studied in previous reports, which showed that it is also possibly due to different swelling properties of the solvent passing through the extraction capillary.37,38 In consideration of extraction efficiency and solubility of PAHs at a higher concentration, acetonitrile concentration was chosen as 5%. Sampling Volume. Unlike traditional fiber-based SPME, (FGO-PD)3-based in-tube SPME is a nonequilibrium absorption process; therefore, the extraction efficiency (FGO-PD)3PTFE is related to sample volume. Volumes of PAHs ranging from 5 to 25 mL were loaded onto (FGO-PD)3-PTFE, and the extraction efficiencies were studied. As can be seen in Figure 4D, the peak areas of six PAHs increase rapidly along with the increase of the sample volume from 5 to 20 mL and then increase slowly above 20 mL. Sample volume of 20 mL was selected for high extraction efficiency and rapid analysis. Enrichment Performance of (FGO-PD)3-PTFE. Under optimum experimental conditions, the enrichment performance of prepared (FGO-PD)3-PTFE was evaluated. Figure 5A shows the chromatogram of PAHs after pretreatment with PD-PTFE; Figure 5B shows that with (FGO-PD)3-PTFE. Little PAH was extracted by PD-PTFE, while significant enrichment efficiency was observed for (FGO-PD)3-PTFE, indicating that the extraction capability mainly came from FGO layers. For the study of enrichment efficiency of (FGO-PD)3-PTFE, a standard sample of 5 μL was directly injected into the HPLC system for comparison because the volume of the bare PTFE tube was calculated to be about 5 μL, and the injection volume of SPMEHPLC was also counted as 5 μL. Enrichment was determined by comparing the analyte concentration with and without extraction by (FGO-PD)3-PTFE. As listed in Table S1 (Supporting Information), (FGO-PD)3-PTFE showed very high extraction efficiency toward PAHs, with enrichment larger than 1000-fold: 1082 for fluoranthene (FLU), 1243 for pyrene (PYR), 1533 for benz[a]anthracene (B[a]AN), 1772 for benzo[a]fluorathene (B[a]FL), 2065 for benzo[a]pyrene (B[a]PY), and 2331 for dibenz[a,h]anthracene (D[a,h]AN). Log KOW exhibits the potential of molecules to partition into organic solvent rather than water; it is also an indication of hydrophobicity of molecules. Enrichment was observed to be

E = 133.9e0.44 log KOW − 94.9 mg /Vp c0

= E = 133.9e0.44 log KOW − 94.9

mg = c0Vp(133.9e 0.44 log KOW − 94.9)

(3)

(4) (5)

where mg is the amount of analyte finally adsorbed on (GOPD)3-PTFE, VP is the volume of the PTFE tube (in this work, the value is 5 μL), and c0 is the initial concentration of analytes in sample solutions. Equation 5 is useful to estimate how much the (FGO-PD)3-PTFE could extract a certain kind of compound based on its log KOW value and its initial concentration (c0). For example, for a solution containing 50 pg/mL of six PAHs, the amounts extracted by (FGO-PD)3PTFE were 259.1 pg of FLU, 285.1 pg of PYR, 427.2 pg of B[a]AN), 457.9 pg of B[a]FL, 523.5 pg of B[a]PY, and 579.1 pg of D[a,h]AN. The performance of (FGO-PD)3-PTFE after 2 months of use has also been studied to further evaluate the stability of (FGOPD)3-PTFE. As shown in Figure S6 (Supporting Information), only a small decrease of peak areas (less than 10%) was observed after 2 months of use, which suggests that (FGOPD)3-PTFE has good stability and reusability. Universally Assembling (FGO-PD)3 in Different Kinds of Microtubes. The proposed FGO-PD method is a universal method that can be used for fabricating multilayer graphene on different kinds of surfaces. In this work, three kinds of commonly used materials, PTFE, PEEK, and SS, were used to demonstrate the efficiency of the assembly method. The assembly method was the same as described for PTFE. The confirmation of assembly of (FGO-PD)3 was performed by connecting PEEK and SS tubes into the HPLC system in the same way as the PTFE tube and investigating the extraction efficiency. Figure 5B−D shows chromatograms of PAHs enriched by (GO-PD)3-modified PTFE, PEEK, and SS tubes, respectively. All of the tubes exhibited excellent enrichment efficiency, the average enrichment for six PAHs was 1789-fold for (FGO-PD)3-PTFE, 1622-fold for (FGO-PD)3-PEEK, and 1991-fold for (FGO-PD)3-SS. (FGO-PD)3-SS has a slightly higher enrichment efficiency, possibly due to the better F

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indicate the superior sensitivity of the (FGO-PD)3-PTFE-based online SPME-HPLC method. Absolute recovery of (FGO-PD)3-PTFE has also been studied and compared with that of a commercial GC capillary coated with a hydrophobic poly(dimethylsiloxane) layer. PAHs could also be extracted by the GC capillary (Figure S7, Supporting Information) but much less than that by (FGOPD)3-PTFE. As listed in Table 3, absolute recoveries of the GC

polydopamine modification of SS because of relatively strong interaction of dopamine with metal surfaces.39 Analytical Performance of Online SPME-HPLC. A (FGO-PD)3-PTFE-based online SPME-HPLC method was developed for quantification of PAHs in aqueous samples. Parameters for analytical performance were investigated and are listed in Table 1. All six PAHs possess good linearity in the Table 1. Analytical Performance of (FGO-PD)3-PTFE-Based Online SPME-HPLC for Determination of PAHs linearitya

compd FLU PYR B[a]AN B[a]FL B[a]PY D[a,h] AN a

regression eq

RSD, % (n = 5) LOD (pg/ mL)

R2

y = 2007x + 1053.1 y = 1841.1x + 2017.7 y = 5053.3x + 1614.5 y = 15273x + 3106.0 y = 25477x + 3125.0 y = 13265x + 2913.3

Table 3. Comparison of Absolute Recoveries of (FGO-PD)3PTFE and Commercial GC Capillary

LOQ (pg/ mL)

absolute recovery (%) between runs

between tubes

0.9968

0.1

0.3

1.5

3.9

0.9977

0.1

0.3

2.3

3.2

0.9993

0.1

0.3

1.8

4.7

0.9992

0.05

0.3

1.3

4.1

0.9985

0.05

0.3

2.7

3.8

0.9995

0.1

0.3

3.4

4.4

range 0.3−200 pg/mL, with R2 ≥ 0.9968. Low limits of detection (LODs) of the method can reach 0.1 pg/mL for FLU, PYR, B[a]AN, and D[a,h]AN and 0.05 pg/mL for B[a]FL and B[a]PY. The limit of quantification (LOQ) of six PAHs is 0.3 pg/mL, which is sufficient to determine trace amounts of PAH in aqueous samples. The reproducibility between five duplicate runs and between batches of tubes was determined by loading PAH solution of 50 pg/mL, and relative standard deviation (RSD) for each peak was calculated. RSD values less than 3.4% and 4.7% were found (Table 1), which indicates good reproducibility of the method. Comparison of (FGO-PD)3-PTFE-Based Online SPMEHPLC with Other Methods. Sensitivity is an important factor in a method for determining trace amount of pollutants in water or environmental samples. Therefore, the proposed method was also compared with other methods31,40−43 recently reported for PAH enrichment and determination, mainly concerning sensitivity. As shown in Table 2, the LOD of (FGO-PD)3-PTFE-based online SPME-HPLC method was significantly lower than those of reported carbon nanostructurebased SPE/SPME method, even by GC/MS. In comparison with SPE/SPME-HPLC method with C18-PAH silica, polymer monolith, and sol−gel as sorbents, a much lower LOD of the (GO-PD)3-PTFE based method was also observed. The results

limit of detection (pg/ mL)

this method SPME-GC/MS31 SPE-GC/MS40 SPE-HPLC-FD41 SPME-HPLC-UV42 SPME-HPLC-FD43

(FGO-PD)3-PTFE RGO-silica fiber multiwall carbon nanotubes C18-PAH silica particles polymer monolith sol−gel capillary

0.05−0.1 1.52−2.72 4.2−46.5 1−2.3 400−2000 5−500

InertCap 1 capillary

25.9 28.5 42.7 45.5 52.3 57.9

3.8 4.3 8.8 13.7 16.7 18.9

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CONCLUSIONS In this work, we reported the first example of universally assembling multilayer graphene in chemically resistant microtubes for microextraction. FGO and PD were assembled by a novel noncovalent and covalent combined layer-by-layer method on the inner surface of microtubes. The assemblies were performed under mild and environmentally friendly conditions, free of organic solvents and toxic reduction reagents. The (FGO-PD)3-PTFE tube was used for in-tube solid-phase microextraction and exhibited good stability, high sampling speed, and excellent extraction efficiency. An online SPME-HPLC method was developed to enrich and determine

Table 2. Comparison of Sensitivity of (FGO-PD)3-PTFEBased Online SPME-HPLC Method with Reported Methods sorbents

(FGO-PD)3-PTFE

capillary were 3.8−18.9%; for (FGO-PD)3-PTFE, better absolute recovery from 25.9% to 57.9% were obtained, probably resulting from its better extraction capability. Application in Environmental Samples. The (FGOPD)3-PTFE-based online SPME-HPLC method was also applied to analysis of PAHs in environmental samples. Four typical samples were collected, two water samples and two soil samples. The chromatograms are shown in Figure S8 (Supporting Information), and the results are listed in Table 4. No PAHs were found in tap water, while in lake water B[a]AN, B[a]FL, and B[a]PY were detected, in which B[a]FL and B[a]PY could be quantified and calculated to be 0.39 and 0.72 pg/mL. All six PAHs were detected in the soil from lakeshore of the East Lake, Wuhan (soil A), calculated to be 0.10−1.37 ng/g; FLU, PYR, B[a]AN, B[a]FL, and B[a]PY were detected in the soil from grassland beside a local road, and calculated to be 0.59−7.67 ng/g. Although the contents of six PAHs in these environmental samples are lower than limit values,44 more attention should still be paid to the risk of accumulation of PAHs. Recoveries were also tested in water and soil samples, in which six PAH standards were added at 10 pg/mL and 0.8 ng/ g, respectively. By calculating the mean value of three duplicates, recoveries in water samples were in the range 89.8−96.7%, and recoveries in soil samples were in the range 84.6−90.5% (Table 4). Although recovery in soil samples was a little lower, possibly due to the complexity of soil matrix,31 the results also showed good accuracy of the (FGO-PD)3-PTFEbased online SPME-HPLC method.

The method had a range of 0.3−200 pg/mL for all compounds.

method

compd FLU PYR B[a]AN B[a]FL B[a]PY D[a,h]AN

G

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Table 4. Results and Recoveries for Determination of PAHs in Water and Soil Samples water compd FLU PYR B[a]AN B[a]FL B[a]PY D[a,h]AN a

tap (pg/mL)

soil

recoverya (%)

lake (pg/mL)

recoverya (%)

roadside (ng/g)

recoveryb (%)

lakeshore (ng/g)

recoveryb (%)

92.1 93.5 96.7 93.1 95.0 91.9

nd nd nqd 0.39 0.72 nd

91.2 93.3 95.5 94.5 90.8 89.8

1.37 1.26 0.30 0.55 0.10 nd

87.7 89.1 88.9 87.2 89.8 87.2

5.13 3.31 3.19 7.67 1.31 0.59

84.6 87.7 86.5 90.5 90.1 85.1

c

nd nd nd nd nd nd

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pollutant PAHs in water samples; the method showed high sensitivity, wide linear range, good linearity, and good reproducibility. The bioinspired polydopamine assembly method is universal for fabricating multilayer graphene onto different various microtubes, and the FGO-PD-based online SPME-HPLC method is promising for sample pretreatment and quantitative analysis in different kinds of matrix.

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ASSOCIATED CONTENT

S Supporting Information *

Additional text, six figures, and one table as described in the main 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 86-27-68759893; fax 86-27-68759850; e-mail chenzl@ whu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Scientific Foundation of China (Grants 90817103, 30973672, and 20775055), Doctoral Fund of Ministry of Education of China (20110141110024), Hubei Provincial Scientific Foundation (2011CDB475), and the Fundamental Research Funds for the Central Universities.

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