Journal of Chromatography A, 1362 (2014) 1–15
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Review
Graphene-based materials: Fabrication and application for adsorption in analytical chemistry夽 Xin Wang a , Bo Liu a , Qipeng Lu c , Qishu Qu b,∗ a b c
Department of Chemistry, School of Science, Beijing JiaoTong University, Beijing 100044, China School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, China Institute of Optoelectronic Technology, Beijing JiaoTong University, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 1 August 2014 Accepted 4 August 2014 Available online 15 August 2014 Keywords: Graphene Adsorption Organic compounds Metal ions Solid phase extraction Stationary phase
a b s t r a c t Graphene, a single layer of carbon atoms densely packed into a honeycomb crystal lattice with unique electronic, chemical, and mechanical properties, is the 2D allotrope of carbon. Owing to the remarkable properties, graphene and graphene-based materials are likely to find potential applications as a sorbent in analytical chemistry. The current review focuses predominantly on the recent development of graphenebased materials and demonstrates their enhanced performance in adsorption of organic compounds, metal ions, and solid phase extraction as well as in separation science since mostly 2012. © 2014 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Adsorption of organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Adsorption of metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Solid phase extraction (SPE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Solid-phase microextraction (SPME) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Stationary phase for chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction Trace analysis is always a subject of great interest especially in the field of analytical chemistry [1,2]. However, because of the strong matrix interferences, to determine the low concentrations of analytes directly from samples is very difficult. Therefore, it is necessary to increase the concentration of analytes and decrease the
夽 Paper originally submitted to the Advanced Materials for Separation Science special issue. ∗ Corresponding author. Tel.: +86 551 63828100. E-mail address: [email protected] (Q. Qu). http://dx.doi.org/10.1016/j.chroma.2014.08.023 0021-9673/© 2014 Elsevier B.V. All rights reserved.
background interference by enriching trace analytes before using preliminary separation techniques [3–6]. However, for the preconcentration technique, the high recovery and enrichment factor only can be achieved by using a suitable solid sorbent. Nowadays, with the advancements in nanomaterials synthesis, several complementary strategies have been developed for new analytical procedures including carbon-based materials, inorganic particles, and organicinorganic composites [7–10]. Among different types of materials, carbon nanomaterials are the research focus due to its chemical stability, durability, corrosion resistance and large surface area [11,12]. Carbon nanomaterials, which could be used as sorbent materials for preconcentration, comprise a wide range of allotropic forms
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of carbon, including nanodiamond, fullerenes, carbon nanotubes, graphite nanofibers, and graphene [7]. These nano-sized carbon materials could yield high preconcentration factor and good selectivity result from the large surface area. Since graphene was first described in the scientific literature in 2004 by Novoselov et al., the graphene-based materials have been studied with a view to developing several applications [13]. In the view of structure, graphene possesses one or a few layers thickness of sp2-hybridized carbon atoms arranged in a honeycomb pattern [14], which cause graphene with various superior electrical, electrochemical, optical and mechanical properties [15–18]. Therefore, versatile methods have been developed for fabrication, growth or synthesis of graphene and its derivatives [19–27]. RGO and GO have found potential applications like sensors [28–44], energy storage [45–51], solar cells [52], electrochemical devices, ultracapacitors [53–57], and so forth. In particular, very large specific surface area (theoretical value 2630 m2 /g) [58] and its electron-rich double-sided polycyclic aromatic scaffold makes it a promising analytical candidate as an wonderful adsorbent [59–65]. Recently, a series of new sorbents based on graphene (RGO) and graphene oxide (GO) represent a promising analytical technology with many advantages, including large specific surface area, high adsorption capacity and strong affinity. The most popular technique used for synthesizing graphene is chemical reduction of graphene oxide as shown in Fig. 1. GO was obtained by oxidation graphite with various oxidants in acidic media. The Hummer method of synthesizing GO was usually used and considered as one of the most efficient methods [66,67]. Subsequently GO can be chemically reduced to RGO by various inorganic and organic reducing agents (Fig. 1) [68]. Due to their huge surface area, spectacular physical and chemical properties, RGO and GO have attracted great interest in analytical chemistry. The present review attempts to summarize current progress in the preparation of graphene-based materials for the adsorption of various kinds of compounds, including organic compounds and metal ions. Also, the applications of graphene-based materials in solid-phase extraction and chromatography separation were introduced briefly.
2. Adsorption of organic compounds Graphene-based materials have been applied to adsorb various organic pollution via non-covalent interaction including Van der Waals type interactions, electrostatic interaction, hydrogen bonds, - stacking, dispersion forces, dative bonds and the hydrophobic effect [69]. Table 1 lists recent reports on the adsorption of organic compounds by graphene-based materials. The advantage of the RGO is selective adsorption. Various adsorption mechanisms play predominate roles in different kinds of organic chemical systems. Because of the large localized electron systems, the RGO could adsorb the aromatic rings from several organic compounds through strong – interaction. Different from the RGO, for GO, the rich functional groups play a key role in forming hydrogen bonding or electrostatic interaction with organic compounds containing oxygen- and nitrogen-functional groups. Xu et al. used RGO for the decontamination of bisphenol A (BPA) and the maximum adsorption capacity of RGO for BPA was 182 mg/g at 302.5 k [70]. The mechanism was schematically represented in Fig. 2. In this case, the - interactions between the benzene rings of BPA and RGO as well as the hydrogen bonds between the oxygen containing groups contained in BPA and RGO might be responsible for the adsorption of BPA on RGO. Pei et al. studied the adsorption of 1,2,4-trichlorobenzene (TCB), 2,4,6-trichlorophenol (TCP), 2-naphthol, and naphthalene (NAPH)
based on RGO and GO [71]. The structure of TCP and NAPH are similar with TCB and 2-naphthol. And the differences between them are TCP and 2-naphthol containing the polar hydroxyl group. From the experimental results, RGO had similar adsorption capacity for four aromatics at pH 5.0 in despite of their different chemical properties, which indicated that four aromatics were adsorbed on RGO mainly via – interaction than hydrophobic interaction. In the case of GO, the order of adsorption affinity for four aromatics was NAPH < TCB < TCP < 2-naphthol. The higher adsorption of TCP and 2-naphthol on GO could be attributed to H-bonding between hydroxyl groups of TCP or 2-naphthol and the oxygen-containing functional groups on GO. Maliyekka et al. studied the adsorption of chlorpyrifos (CP), endosulfan (ES), and malathion (ML) onto GO and RGO from water [72]. They determined the energies of adsorptive interactions between graphene and a pesticide (binary complex) as well as the interaction among graphene, pesticide and water complexes by the first-principles pseudopotential-based density functional theory (DFT) analysis. The results showed that both graphene and pesticide have an attractive interaction with water molecules and formed the Graphene–Water–Pesticide (G–W–P) complexes through electrostatic interaction. The adsorption of pesticide on graphene was feasible with the existence of water molecule, and the adsorption of pesticide on dry graphene was expected to be rather weak or unlikely. By using this water-assisted adsorption, the adsorption capacity of graphene observed up to 1200 mg/g which was much higher than any other materials. Wu et al. investigated the removal of hazardous chemicals from wastewater by using RGO as adsorbent [73]. It was found that the adsorption capacity and adsorption speed increased with the increasing of molecular size and number of the benzene rings possessed by these organic chemicals. The maximum adsorption capacity for p-toluenesulfonic acid, 1-naphthalenesulfonic acid, and methyl blue could reach to 1.43 g/g, 1.46 g/g, and 1.52 g/g, respectively. Although graphene-based materials have strong ability to adsorb the different kinds of compounds, it suffers the problem of difficult to be separated from the mother solution. Therefore, the introduction of magnetic properties into RGO and GO systems is an excellent strategy to solve the above problem. By combining the high adsorption capacity from graphene and the separation convenience from magnetic materials, the adsorbents can be easily separated and recycled by a simple magnetic process. With the advancements in nanomaterial’s synthesis, various kinds of magnetic nanoparticles (Ni, CoFe2 O4 , Fe3 O4 ) combined with functional GO and RGO were used successfully as adsorbents for removing dyes and organic contaminant. Geng et al. synthesized RGO/Fe3 O4 nanocomposites with tunable RGO/Fe3 O4 ratio and achieved an exceptionally high yield [74]. The RGO/Fe3 O4 hybrid was used for the decontamination of a serious dyes, such as rhodamine 6G (R6G), acid blue 92 (AB92), orange (II) (OII), malachite green (MG), and new cocaine (NC). Because of the large surface area of RGO and magnetic property of Fe3 O4 , the hybrid possessed quite a good adsorption capacity for these dyes. Moreover, the hybrid could be easily and rapidly extracted from water by magnetic attraction. Compared with the single dye adsorption, the hybrid could work well even exposed to a multi dye cocktail without suppressing the adsorption capacity for each of the dyes. This hybrid adsorbent could be easily and efficiently regenerated by simply annealing for reuse with hardly any compromise of the adsorption capacity. Zhang et al. decorated RGO with the superparamagnetic Fe3 O4 and used the RGO/Fe3 O4 composites as adsorbent for BPA [75]. The strong – interaction is the main drive force for the adsorption. Due to the superparamagnetism, RGO/Fe3 O4 could be easily
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Fig. 1. Preparation of reduced graphene oxide (RGO) by reduction of graphene oxide (GO) [68].
Fig. 2. Schematic of – interaction and hydrogen bonding between BPA and graphene [70].
manipulated by an external magnetic field and exhibited excellent reproducibility and reusability. Li et al. found that addition of cyano radicals could improve the solubility of RGO (Fig. 3) [76]. The process of addition of cyano radicals could be carried out under mild and facile conditions. In addition, after the addition of the functional groups, the high surface area of RGO was kept. From the experimental results, magnetic Ni nanoparticles functionalized RGO nanocomposites showed great potential as an effective absorbent for removing aromatic compounds from wastewater. GO/Fe3 O4 and GO/multi-walled carbon nanotubes (GO/ MWCNTs) were prepared by Yang et al. and their adsorption capacities for 1-naphthylamine, 1-naphthol and naphthalene were
Fig. 3. Schematic diagram of synthesis of the Ni@GSs-C(CH3 )2 COONa nanocomposites [76].
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Table 1 Graphene-based materials for the removal of organic compounds. Adsorbate
Adsorbent
MACa (mg/g)
Ref.
RGO RGO RGO RGO RGO RGO RGO GO GO GO GO-SA GO/chitosan/Fe3 O4 GO/chitosan/Fe3 O4 GO/B-CD-chitosan/Fe3 O4 RGO/Fe3 O4 RGO/Fe3 O4 RGO/CoFeO4 RGO/CoFe2 O4 GO-SiO2 /Fe3 O4 RGO/Fe3 O4 RGO/Fe3 O4 RGO-SO3 H/Fe3 O4 RGO-SO3 H/Fe3 O4 RGO-SO3 H/Fe3 O4 GO/FeO·Fe2 O3 GO/FeO·Fe2 O3 GO/FeO·Fe2 O3 GO/Fe3 O4 RGO/sand RGO/sand GO/calcium alginate RGO/Poly (acrylamide) GO/brilliant blue GO/brilliant blue GO/poly-dopamine GO/poly-dopamine GO/poly-dopamine GO/poly-dopamine GO/poly-dopamine
Bisphenol A Chlorpyrifos (CP) Endosulfan (ES) Malathion (ML) Methyl blue Methyl blue Methyl blue Methyl blue Tetracycline Montmorillonite Methyl blue Methyl blue Methyl blue Methyl blue Fuchsine Congo red (CR) Methyl green Methyl orange Methylene Blue (MB) Ciprofloxacin (CIP) Norfloxacin (NOR) Safranine T (ST) Neutral red (NR) Victoria blue (VB) Naphthalene 1-Naphthol 1-Naphthylamine Bisphenol A Rhodamine 6G (R6G) Chloropyrifos (CP) Methyl blue Methyl blue Anthracenemethanol (AC) Fluoranthene (FL) Methyl violet (MV) Basicfuchsin (BF) Coomassie brilliant blue (CBB) Malachite green oxalate (MGO) Neutral red (NR)
182 1200 1100 800 1520 1449 153.85 244 313 212 833 181 95.16 50.12 89 33.66 49.83 72 111.1 18.22 22.2 199.3 216.8 200.6 283 389 408 123.2 55 48 182 1530 349 447.7 2100 1700 2100 2000 1400
[70] [72] [72] [72] [73] [144] [145] [146] [147] [147] [86] [148] [149] [150] [151] [152] [153] [154] [155] [156] [156] [157] [157] [157] [77] [77] [77] [75] [84] [84] [79] [82] [83] [83] [80] [80] [35] [35] [35]
a
Maximum adsorption concentration.
compared [77]. They found that the morphology of the adsorbent playing the key role in the adsorption. Although the surface area of GO/MWCNTs was higher than that of GO/Fe3 O4 , the net surface area used for adsorption was small, which may because interwall spacing between the coaxial tubes of MWCNTs was too small to allow the adsorbates to penetrate into. Therefore, the adsorption capacity of GO/Fe3 O4 was much higher than that of GO/MWCNTs. – Electron donor-acceptor (EDA) interaction between aromatic compounds and GO/Fe3 O4 surface was speculated as the primary mechanism for adsorption of aromatic compounds on GO/Fe3 O4 . As a result, the higher polarity of adsorbates leads to higher adsorption capacity. RGO or GO combined with the guest component (-cyclodextrin (-CD) [78], calcium alginate [79], brilliant blue [80], n-butyl lithium [81], polymer [82,83], etc.) could also bring many advantages such as easy to separate, excellent dispersity, improved adsorption capacity and biotoxicity to human cells. Therefore, graphene-based composites have attracted more and more attention. Yang et al. introduced poly(acrylamide) to the RGO sheets by in situ free-radical polymerization [82]. Thus obtained nanocomposite showed very high adsorption capacity for dye (methylene blue, 1530 mg/g) and metal ion (Pb, 1000 mg/g). Dong et al. fabricated a series of sub-nano thick poly-dopamine (PD) layer coated GO (GO/PD) composites and used them selectively adsorb the dyes containing an Eschenmoser structure (Fig. 4) [80]. Because of the high surface area and abundant functional groups exposed on the outer surface, the obtained GO/PD could
selectively adsorb the dyes containing an Eschenmoser structure and showed an extremely high adsorption capacity up to 2.1 g/g. The 1,4-Michael addition reaction between the ortho position of the catechol phenolic hydroxyl group from PD and Eschenmoser groups in the dyes promotes the adsorption process with the help of Eschenmoser salt. Zhang et al. developed a method to prepare GO and brilliant blue nanocomposites (GO/BB) [83]. The dispersity and stability of GO in solution was greatly enhanced by mixing BB with GO under sonicating. After adsorption, the products could be separated by simply changing the solution’s pH value or temperature. Flocculant was not needed making the second pollution avoided. Mondal et al. prepared a bifunctionalized graphitic system by using n-BuLi to simultaneously scavenge protons as a nucleophile [81]. The butyl groups could act as spacers to prevent the agglomeration of the graphene sheets while the carboxylic groups were used as sites for dye adsorption. To improve the adsorptive performance and overcome the limitations of graphene as a high-performance sorbent for pollutants, such as hydrophobic nature and difficulties to separate from treated water, graphene combine with sand or SiO2 were synthesized [84,85]. Liu et al. prepared graphene coated silica (RGO/SiO2 ), which acts as a highly efficient sorbent. And the ability of adsorption for eleven different organophosphorus pesticides (OPPs) was tested [85]. The adsorptions for nine of the eleven OPPs were above 95%, and the relative standard deviation (RSD) was under 5%, showing that the graphene-coated silica (GCS) was an efficient, stable, and consistent adsorbent. They demonstrated that the strong
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Fig. 4. (a-d) SEM images of PD-5%/GO (a), PD-15%/GO (b), PD-35%/GO (c), PD-70%/GO (d) after freeze drying (scale bars are 5 mm) [83].
adsorption of RGO/SiO2 could attribute to the interactions between graphene’s large -system and electron-donating heteroatoms (N, S, and P) or the strong -bonding networks of benzene rings. Ma et al. developed a method to prepare ultra-light monolithic GO/sodium alginate (GO/SA) and RGO-SA gels [86]. Freezing, solvent exchange, and ethanol drying methods were used to fabricate these 3D network gels. The strong adsorption property of GO/SA for methylene blue was investigated. And the maximum adsorption capacity was 833.3 mg/g for GO/SA, which indicated that the gels obtained excellent adsorption properties for MB.
3. Adsorption of metal ions Graphene-based materials have shown the great potential to be efficient adsorbents for the adsorption of all kinds of metal ions, especially heavy metal ions. Table 2 lists recent results of the metal ions adsorption on graphene-based materials. The mechanisms of adsorption for metal ions on graphene-based materials are very complicated. Various kinds of interactions, such as physical adsorption, electrostatic attraction, and chemical interaction between the metal ions and the functional groups on the surface of those adsorbents, would contribute a lot to the adsorption. XPS measurements indicate that the adsorption of metal ions on GO nanosheets is mainly controlled by chemical adsorption involving the strong surface complexation of metal ions with the oxygen containing groups on the surface of GO [87]. Parameters such as treatment time, temperature, ionic strength, and pH value of the solution can affect the adsorption [88–91]. However, since the charge of both the adsorbents and the adsorbates can be changed by the pH value of the solution, pH value should be carefully adjusted during the adsorption process [92–95]. The pHpzc value of GO is ∼3.9. At pH < pHpzc , the surface charge of GO is positive. The positive metal ions are difficult to adsorb on the positively charged surface of GO nanosheets because of the electrostatic repulsion. At pH > pHpzc , the surface charge of GO nanosheets is negative because of the deprotonation reaction. With increasing pH, the surface charge was more negative and the electrostatic
interactions between the metal ions and GO nanosheets become stronger, and thereby resulted in the increase of metal ion sorption. Zhao et al. also studied the presence of effects of humic acid (HA) on the adsorption of Pb(II). HA prevent the adsorption of metal ions on GO at pH < 8, due to the strong surface complexation and high surface site density of graphene oxide [92,95]. Zhao et al. claimed that the metal ions could share an electron pair with the oxygen atom and form a metal complex for adsorption of metal ion [92]. The delocalized electron systems of graphene would act as Lewis base and form electron donor–acceptor complexes with Pb(II) ions (Fig. 5). The Lewis acid-base interaction between graphene and Pb(II) contributes to the ion adsorption on graphene. The adsorption capacity of GO is much higher than RGO and other carbon based materials due to the high oxygen content of GO. Li et al. studied the adsorption of U(VI) on GO and RGO, and found the maximum sorption capacity of GO was much higher than that of RGO, which indicated that the abundant oxygen-containing functional groups of GO playing important roles in the sorption [96].
Fig. 5. A schematic diagram of the formation of the surface complexes of Pb(II) ions on the surface of FGO [92].
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Table 2 Graphene based materials for the removal of metal ions. Adsorbate
Adsorbent
MAC (mg/g)
Ref.
Am(III) Au(III)
GO GO GO/chitosan RGO/Fe3 O4 /MnO2 GO RGO/Fe3 O4 /MnO2 GO GO/Fe3 O4 GO/-CD GO GO Tri-amino-RGO GO/Fe3 O4 /-CD RGO/CTAB GO GO GO GO/Fe3 O4 GO/Chitosan RGO/␦-MnO2 GO GO RGO RGO/CoFe2 O4 RGO/polypyrrole RGO/polyaniline RGO/␦-MnO2 GO GO GO EDTA-GO GO/poly(acrylamide) GO/chitosan GO/SiO2 GO/Fe3 O4 /chitozan RGO/␦-MnO2 Tri-amino-RGO GO GO/chitosan RGO/CoFe2 O4 GO GO GO GO GO RGO GO GO
8.5 108 1077 14 71 12 68 13 72 106 345 236 120 22 47 294 118 18 25.4 103 175 115 299 158 980 1000 47 842 81 1119 479 1000 100 114 77 162 461 98 217 299 71 24 28 98 299 47 411 345
[107] [158] [159] [90] [158] [90] [93] [98] [160] [93] [87] [161] [162] [91] [105] [87] [163] [95] [88] [104] [164] [107] [165] [100] [102] [103] [166] [92] [158] [87] [89] [82] [167] [168] [99] [104] [161] [159] [159] [100] [158] [107] [107] [97] [96] [96] [169] [87]
As(III) As(IV) Co(II)
Cd(II)
Cr(VI) Cu(II)
Eu(III) Fe(II) Hg(II)
Ni(II) Pb(II)
Pd(II)
Pt(IV) Sr(II) U(VI)
Th(IV) Zn(II)
Zhao et al. further confirmed that the oxygen-containing functional group is the key factor for the adsorption of U(VI) [97]. In order to solve the issue of hard to be separated of GO, magnetic GO/RGO composites were developed, such as GO/Fe3 O4 , RGO/CoFe2 O4 . On the other hand, the combination of GO/RGO with functional materials and groups could improve the adsorption performance efficiently. As a result, nanocomposites like GRO/polymer, GO/chitosan, RGO/functional groups have been synthesized. Owing to the fast magnetizable separation, graphene and magnet composite-based materials can overcome the shortcomings of many other methods, such as filtration, centrifugation, or gravitational separation. To date, the most used magnetic material is still Fe3 O4 [95,98,99]. Zhang et al. fabricated a magnet-GO composite by using CoFe2 O4 as magnetic core and showed excellent separation properties and adsorption capacity for Pb(II) and Hg (II) [100]. Li et al. studied the adsorption of Cu(II) and fulvic acid (FA) by GO/Fe3 O4 [95]. The results showed the FA led to a strong increase in Cu(II) sorption at low pH and a decrease at high pH, which attributed to the strong complexes between FA and Cu(II). On the other hand, the presence of Cu(II) led to an increase in FA sorption
owing to Cu(II) that produced strong effects on the packing, spacing or alignment of adsorbed FA. Chen et al. compared the adsorption capacity of GO/activated carbon felt (GO/ACF) with activated carbon [101]. It was found that the maximum adsorption capacity for U(VI) of GO/ACF was much larger than that of ACF. This result confirmed that the carboxyl functional groups of GO playing important roles in the adsorption. A RGO/polypyrrole (RGO/PPy) composite was prepared by Chandra and Kim using environmentally friendly chemical polypyrrole as co-reactant in the process of the reduction of GO [102]. Thus prepared composites showed high selectivity for Hg(II) removal capacity which was as high as 980 mg/g. Yang et al. synthesized RGO/Polyacrylamide (RGO/PAM) nanocomposites by in situ freeradical polymerization [82]. In this case, the grafted PAM chains on RGO could enhance the dispersion property of RGO in aqueous solution and improve the adsorption capacity of RGO. As shown in Fig. 6, PAM have interaction with metal ions by chemical or physical adsorption, at the same time, interaction between PAM and aromatic structures can occur through hydrogen bond. Li et al. prepared a RGO/polyaniline (RGO/PANI) nanocomposite through in situ polymerization of aniline in the presence of GO and then reduction by hydrate hydrazine [103]. Compare to PANI, the maximum adsorption capacity of PANI/RGO for Hg(II) was increased from 515 mg/g to 1000 mg/g. The high adsorption capacity could be attributed to the enhanced adsorption sites caused by incorporating the high surface area of RGO into the PANI. Ren et al. fabricated RGO/␦-MnO2 to remove either Cu(II) or Pb(II) from simulated solution [104]. Graphene played dual roles in the adsorption process. Firstly, it could be used as a matrix for the growth of MnO2 nanoparticles. Secondly, it helped to prevent the aggregation of the nanoparticles. Oxygen-containing functional groups including hydroxyl (C-OH or Mn-OH) and carboxyl groups on the RGO surface could act as Lewis acids to adsorb the Cu(II) and Pb(II) by forming tetradentate monodentate complexes on the RGO/␦-MnO2 surface. Since the metal ions could insert into layered MnO2 , the adsorption capacities of RGO/␦-MnOn2 could be further increased compared to other adsorbents. Madadrang et al. linked N-(trimethoxysilylpropyl)ethylenediamine triacetic acid (EDTA-silane) with the hydroxyl groups on GO surface (Fig. 7) [89]. The high chelating ability of EDTA groups together with OH and COOH groups on the GO surface could greatly enhance the adsorption capacity of GO. The adsorption capacity of EDTA-GO reached 479 mg/g which was much higher than those of GO or carbon nanotubes. Furthermore, the adsorption rate was fast and the whole adsorption process could be completed in 20 min. Yang et al. found that GO was an efficient adsorbent for removal of Cu(II) [105]. The abundant oxygen-containing groups on the GO surface were believed to help much on forming GO-metal complexes. The neutralization of the negatively charged functional groups on the surface of GO would then induced the folding and aggregation of GO. Normally, GO is very stable in water [106]. However, these experimental results demonstrated that the GO is not stable enough in the aqueous solution. Later, a series of experimental results were further demonstrated by Sitko et al. [87] and Yu et al. [107]. From their results, they found that Zn(II), Cd(II), and Pb(II) and even most of the actinide metals and their fission products could also induce the aggregated of GO.
4. Solid phase extraction (SPE) Based on the results of organic compounds and metal ions adsorption, graphene shows exceptional adsorption properties. As a superior candidate as a good adsorbent for organic compounds and metal ions in various sample preparation methods,
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Fig. 6. Synthesis of PAM chains on RGO sheets by Free radical polymerization and adsorption of metal ions [82].
graphene-based materials have lot of applications in solid phase extraction and solid phase microextraction. SPE is a method of preconcentration and analyte/matrix separation by using the affinities of analytes dissolved in liquid for solid adsorbent. The core of SPE is sorbent which determines the efficiency and the selectivity of extraction. Recently, graphene and graphene based materials have attracted great of attentions as a new sorbent in SPE. Table 3 shows the published papers on applications of graphene based materials in SPE. By using graphene-packed SPE cartridges as adsorbent, Liu et al. extracted eight chlorophenols (CPs) from water samples [108]. From the experiment results, some advantages of graphene as SPE adsorbent were demonstrated, such as low consumption of adsorbent (20–30 mg) and organic solvent, good compatibility with various organic solvents, good reusability and no impact of sorbent drying. They also compared extraction of CPs with other adsorbents including C18 silica, graphitic carbon, single- and multi-walled
carbon nanotubes. Due to the improvement of water-wettability of RGO and the facilitating of retention and elution of polar compounds caused by residual polar groups after reduction, RGO gave the best results. Although RGO and GO have several advantages as adsorbents in SPE due to their adsorptive properties, according to many experimental results, there are several problems in using of RGO or GO as SPE adsorbent directly. Firstly, the aggregation of RGO or GO sheets may occur during isolation from a homogeneous solution, which may reduce efficiency and reusability. Secondly, very small RGO or GO sheets may cause high pressure and escape from the SPE cartridge/column. To avoid these problems and maintain the advantages of graphene, Liu et al. developed new SPE adsorbents by binding RGO or GO sheets with silica [109] (Fig. 8). Compared with commercially available adsorbents, superior and comparable performance was achieved which resulted from the large surface area and water wettabiltiy of RGO and GO. Notably, G@SiO2 could
Fig. 7. Chemical structure of EDTA-GO (left) and its interaction with heavy metal cations (right) [89].
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Table 3 Application of RGO, GO and their derivatives in SPE. Analyte
Matrix
Adsorbent
RSD (%)
Method
DL
Recovery (%)
Ref.
NAAS CPA PAHs CPs Sulfonamide SAs Malachite green Organophosphorus pesticides OPPs PAHs PAHs PAEs PAEs Pesticide residues Pesticides Carbamates pesticides Phthalate acid esters Pesticide Pb(II) Cr(III) Cd(II) Cd(II) Pd(II) Aflatoxins
Rat brain Aquatic products Water Water Environmental water Meat Fish tissues Environmental water Apple juice Environmental water Environmental water Environmental water Water and soybean milk Oil crops Tomato and rape Environmental water Environmental water Tea Environmental water Environmental water Water and vegetable Water and vegetable Water and Vegetable Peanut
RGO RGO Sulfonated RGO RGO@SiO2 RGO RGO RGO RGO RGO RGO Fe3 O4 @SiO2 -RGO RGO Fe3 O4 @SiO2 -RGO RGO/CH3 NH Fe3 O4 @SiO2 -RGO RGO/Fe3 O4 RGO/Fe3 O4 RGO/PSA RGO RGO RGO/amino RGO/Fe3 O4 RGO/amino GO
6.0 1.5–4.3 1.0–9.0 2.2–7.7
Contents lists available at ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Review
Graphene-based materials: Fabrication and application for adsorption in analytical chemistry夽 Xin Wang a , Bo Liu a , Qipeng Lu c , Qishu Qu b,∗ a b c
Department of Chemistry, School of Science, Beijing JiaoTong University, Beijing 100044, China School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, China Institute of Optoelectronic Technology, Beijing JiaoTong University, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 1 August 2014 Accepted 4 August 2014 Available online 15 August 2014 Keywords: Graphene Adsorption Organic compounds Metal ions Solid phase extraction Stationary phase
a b s t r a c t Graphene, a single layer of carbon atoms densely packed into a honeycomb crystal lattice with unique electronic, chemical, and mechanical properties, is the 2D allotrope of carbon. Owing to the remarkable properties, graphene and graphene-based materials are likely to find potential applications as a sorbent in analytical chemistry. The current review focuses predominantly on the recent development of graphenebased materials and demonstrates their enhanced performance in adsorption of organic compounds, metal ions, and solid phase extraction as well as in separation science since mostly 2012. © 2014 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Adsorption of organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Adsorption of metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Solid phase extraction (SPE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Solid-phase microextraction (SPME) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Stationary phase for chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction Trace analysis is always a subject of great interest especially in the field of analytical chemistry [1,2]. However, because of the strong matrix interferences, to determine the low concentrations of analytes directly from samples is very difficult. Therefore, it is necessary to increase the concentration of analytes and decrease the
夽 Paper originally submitted to the Advanced Materials for Separation Science special issue. ∗ Corresponding author. Tel.: +86 551 63828100. E-mail address: [email protected] (Q. Qu). http://dx.doi.org/10.1016/j.chroma.2014.08.023 0021-9673/© 2014 Elsevier B.V. All rights reserved.
background interference by enriching trace analytes before using preliminary separation techniques [3–6]. However, for the preconcentration technique, the high recovery and enrichment factor only can be achieved by using a suitable solid sorbent. Nowadays, with the advancements in nanomaterials synthesis, several complementary strategies have been developed for new analytical procedures including carbon-based materials, inorganic particles, and organicinorganic composites [7–10]. Among different types of materials, carbon nanomaterials are the research focus due to its chemical stability, durability, corrosion resistance and large surface area [11,12]. Carbon nanomaterials, which could be used as sorbent materials for preconcentration, comprise a wide range of allotropic forms
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of carbon, including nanodiamond, fullerenes, carbon nanotubes, graphite nanofibers, and graphene [7]. These nano-sized carbon materials could yield high preconcentration factor and good selectivity result from the large surface area. Since graphene was first described in the scientific literature in 2004 by Novoselov et al., the graphene-based materials have been studied with a view to developing several applications [13]. In the view of structure, graphene possesses one or a few layers thickness of sp2-hybridized carbon atoms arranged in a honeycomb pattern [14], which cause graphene with various superior electrical, electrochemical, optical and mechanical properties [15–18]. Therefore, versatile methods have been developed for fabrication, growth or synthesis of graphene and its derivatives [19–27]. RGO and GO have found potential applications like sensors [28–44], energy storage [45–51], solar cells [52], electrochemical devices, ultracapacitors [53–57], and so forth. In particular, very large specific surface area (theoretical value 2630 m2 /g) [58] and its electron-rich double-sided polycyclic aromatic scaffold makes it a promising analytical candidate as an wonderful adsorbent [59–65]. Recently, a series of new sorbents based on graphene (RGO) and graphene oxide (GO) represent a promising analytical technology with many advantages, including large specific surface area, high adsorption capacity and strong affinity. The most popular technique used for synthesizing graphene is chemical reduction of graphene oxide as shown in Fig. 1. GO was obtained by oxidation graphite with various oxidants in acidic media. The Hummer method of synthesizing GO was usually used and considered as one of the most efficient methods [66,67]. Subsequently GO can be chemically reduced to RGO by various inorganic and organic reducing agents (Fig. 1) [68]. Due to their huge surface area, spectacular physical and chemical properties, RGO and GO have attracted great interest in analytical chemistry. The present review attempts to summarize current progress in the preparation of graphene-based materials for the adsorption of various kinds of compounds, including organic compounds and metal ions. Also, the applications of graphene-based materials in solid-phase extraction and chromatography separation were introduced briefly.
2. Adsorption of organic compounds Graphene-based materials have been applied to adsorb various organic pollution via non-covalent interaction including Van der Waals type interactions, electrostatic interaction, hydrogen bonds, - stacking, dispersion forces, dative bonds and the hydrophobic effect [69]. Table 1 lists recent reports on the adsorption of organic compounds by graphene-based materials. The advantage of the RGO is selective adsorption. Various adsorption mechanisms play predominate roles in different kinds of organic chemical systems. Because of the large localized electron systems, the RGO could adsorb the aromatic rings from several organic compounds through strong – interaction. Different from the RGO, for GO, the rich functional groups play a key role in forming hydrogen bonding or electrostatic interaction with organic compounds containing oxygen- and nitrogen-functional groups. Xu et al. used RGO for the decontamination of bisphenol A (BPA) and the maximum adsorption capacity of RGO for BPA was 182 mg/g at 302.5 k [70]. The mechanism was schematically represented in Fig. 2. In this case, the - interactions between the benzene rings of BPA and RGO as well as the hydrogen bonds between the oxygen containing groups contained in BPA and RGO might be responsible for the adsorption of BPA on RGO. Pei et al. studied the adsorption of 1,2,4-trichlorobenzene (TCB), 2,4,6-trichlorophenol (TCP), 2-naphthol, and naphthalene (NAPH)
based on RGO and GO [71]. The structure of TCP and NAPH are similar with TCB and 2-naphthol. And the differences between them are TCP and 2-naphthol containing the polar hydroxyl group. From the experimental results, RGO had similar adsorption capacity for four aromatics at pH 5.0 in despite of their different chemical properties, which indicated that four aromatics were adsorbed on RGO mainly via – interaction than hydrophobic interaction. In the case of GO, the order of adsorption affinity for four aromatics was NAPH < TCB < TCP < 2-naphthol. The higher adsorption of TCP and 2-naphthol on GO could be attributed to H-bonding between hydroxyl groups of TCP or 2-naphthol and the oxygen-containing functional groups on GO. Maliyekka et al. studied the adsorption of chlorpyrifos (CP), endosulfan (ES), and malathion (ML) onto GO and RGO from water [72]. They determined the energies of adsorptive interactions between graphene and a pesticide (binary complex) as well as the interaction among graphene, pesticide and water complexes by the first-principles pseudopotential-based density functional theory (DFT) analysis. The results showed that both graphene and pesticide have an attractive interaction with water molecules and formed the Graphene–Water–Pesticide (G–W–P) complexes through electrostatic interaction. The adsorption of pesticide on graphene was feasible with the existence of water molecule, and the adsorption of pesticide on dry graphene was expected to be rather weak or unlikely. By using this water-assisted adsorption, the adsorption capacity of graphene observed up to 1200 mg/g which was much higher than any other materials. Wu et al. investigated the removal of hazardous chemicals from wastewater by using RGO as adsorbent [73]. It was found that the adsorption capacity and adsorption speed increased with the increasing of molecular size and number of the benzene rings possessed by these organic chemicals. The maximum adsorption capacity for p-toluenesulfonic acid, 1-naphthalenesulfonic acid, and methyl blue could reach to 1.43 g/g, 1.46 g/g, and 1.52 g/g, respectively. Although graphene-based materials have strong ability to adsorb the different kinds of compounds, it suffers the problem of difficult to be separated from the mother solution. Therefore, the introduction of magnetic properties into RGO and GO systems is an excellent strategy to solve the above problem. By combining the high adsorption capacity from graphene and the separation convenience from magnetic materials, the adsorbents can be easily separated and recycled by a simple magnetic process. With the advancements in nanomaterial’s synthesis, various kinds of magnetic nanoparticles (Ni, CoFe2 O4 , Fe3 O4 ) combined with functional GO and RGO were used successfully as adsorbents for removing dyes and organic contaminant. Geng et al. synthesized RGO/Fe3 O4 nanocomposites with tunable RGO/Fe3 O4 ratio and achieved an exceptionally high yield [74]. The RGO/Fe3 O4 hybrid was used for the decontamination of a serious dyes, such as rhodamine 6G (R6G), acid blue 92 (AB92), orange (II) (OII), malachite green (MG), and new cocaine (NC). Because of the large surface area of RGO and magnetic property of Fe3 O4 , the hybrid possessed quite a good adsorption capacity for these dyes. Moreover, the hybrid could be easily and rapidly extracted from water by magnetic attraction. Compared with the single dye adsorption, the hybrid could work well even exposed to a multi dye cocktail without suppressing the adsorption capacity for each of the dyes. This hybrid adsorbent could be easily and efficiently regenerated by simply annealing for reuse with hardly any compromise of the adsorption capacity. Zhang et al. decorated RGO with the superparamagnetic Fe3 O4 and used the RGO/Fe3 O4 composites as adsorbent for BPA [75]. The strong – interaction is the main drive force for the adsorption. Due to the superparamagnetism, RGO/Fe3 O4 could be easily
X. Wang et al. / J. Chromatogr. A 1362 (2014) 1–15
3
Fig. 1. Preparation of reduced graphene oxide (RGO) by reduction of graphene oxide (GO) [68].
Fig. 2. Schematic of – interaction and hydrogen bonding between BPA and graphene [70].
manipulated by an external magnetic field and exhibited excellent reproducibility and reusability. Li et al. found that addition of cyano radicals could improve the solubility of RGO (Fig. 3) [76]. The process of addition of cyano radicals could be carried out under mild and facile conditions. In addition, after the addition of the functional groups, the high surface area of RGO was kept. From the experimental results, magnetic Ni nanoparticles functionalized RGO nanocomposites showed great potential as an effective absorbent for removing aromatic compounds from wastewater. GO/Fe3 O4 and GO/multi-walled carbon nanotubes (GO/ MWCNTs) were prepared by Yang et al. and their adsorption capacities for 1-naphthylamine, 1-naphthol and naphthalene were
Fig. 3. Schematic diagram of synthesis of the Ni@GSs-C(CH3 )2 COONa nanocomposites [76].
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Table 1 Graphene-based materials for the removal of organic compounds. Adsorbate
Adsorbent
MACa (mg/g)
Ref.
RGO RGO RGO RGO RGO RGO RGO GO GO GO GO-SA GO/chitosan/Fe3 O4 GO/chitosan/Fe3 O4 GO/B-CD-chitosan/Fe3 O4 RGO/Fe3 O4 RGO/Fe3 O4 RGO/CoFeO4 RGO/CoFe2 O4 GO-SiO2 /Fe3 O4 RGO/Fe3 O4 RGO/Fe3 O4 RGO-SO3 H/Fe3 O4 RGO-SO3 H/Fe3 O4 RGO-SO3 H/Fe3 O4 GO/FeO·Fe2 O3 GO/FeO·Fe2 O3 GO/FeO·Fe2 O3 GO/Fe3 O4 RGO/sand RGO/sand GO/calcium alginate RGO/Poly (acrylamide) GO/brilliant blue GO/brilliant blue GO/poly-dopamine GO/poly-dopamine GO/poly-dopamine GO/poly-dopamine GO/poly-dopamine
Bisphenol A Chlorpyrifos (CP) Endosulfan (ES) Malathion (ML) Methyl blue Methyl blue Methyl blue Methyl blue Tetracycline Montmorillonite Methyl blue Methyl blue Methyl blue Methyl blue Fuchsine Congo red (CR) Methyl green Methyl orange Methylene Blue (MB) Ciprofloxacin (CIP) Norfloxacin (NOR) Safranine T (ST) Neutral red (NR) Victoria blue (VB) Naphthalene 1-Naphthol 1-Naphthylamine Bisphenol A Rhodamine 6G (R6G) Chloropyrifos (CP) Methyl blue Methyl blue Anthracenemethanol (AC) Fluoranthene (FL) Methyl violet (MV) Basicfuchsin (BF) Coomassie brilliant blue (CBB) Malachite green oxalate (MGO) Neutral red (NR)
182 1200 1100 800 1520 1449 153.85 244 313 212 833 181 95.16 50.12 89 33.66 49.83 72 111.1 18.22 22.2 199.3 216.8 200.6 283 389 408 123.2 55 48 182 1530 349 447.7 2100 1700 2100 2000 1400
[70] [72] [72] [72] [73] [144] [145] [146] [147] [147] [86] [148] [149] [150] [151] [152] [153] [154] [155] [156] [156] [157] [157] [157] [77] [77] [77] [75] [84] [84] [79] [82] [83] [83] [80] [80] [35] [35] [35]
a
Maximum adsorption concentration.
compared [77]. They found that the morphology of the adsorbent playing the key role in the adsorption. Although the surface area of GO/MWCNTs was higher than that of GO/Fe3 O4 , the net surface area used for adsorption was small, which may because interwall spacing between the coaxial tubes of MWCNTs was too small to allow the adsorbates to penetrate into. Therefore, the adsorption capacity of GO/Fe3 O4 was much higher than that of GO/MWCNTs. – Electron donor-acceptor (EDA) interaction between aromatic compounds and GO/Fe3 O4 surface was speculated as the primary mechanism for adsorption of aromatic compounds on GO/Fe3 O4 . As a result, the higher polarity of adsorbates leads to higher adsorption capacity. RGO or GO combined with the guest component (-cyclodextrin (-CD) [78], calcium alginate [79], brilliant blue [80], n-butyl lithium [81], polymer [82,83], etc.) could also bring many advantages such as easy to separate, excellent dispersity, improved adsorption capacity and biotoxicity to human cells. Therefore, graphene-based composites have attracted more and more attention. Yang et al. introduced poly(acrylamide) to the RGO sheets by in situ free-radical polymerization [82]. Thus obtained nanocomposite showed very high adsorption capacity for dye (methylene blue, 1530 mg/g) and metal ion (Pb, 1000 mg/g). Dong et al. fabricated a series of sub-nano thick poly-dopamine (PD) layer coated GO (GO/PD) composites and used them selectively adsorb the dyes containing an Eschenmoser structure (Fig. 4) [80]. Because of the high surface area and abundant functional groups exposed on the outer surface, the obtained GO/PD could
selectively adsorb the dyes containing an Eschenmoser structure and showed an extremely high adsorption capacity up to 2.1 g/g. The 1,4-Michael addition reaction between the ortho position of the catechol phenolic hydroxyl group from PD and Eschenmoser groups in the dyes promotes the adsorption process with the help of Eschenmoser salt. Zhang et al. developed a method to prepare GO and brilliant blue nanocomposites (GO/BB) [83]. The dispersity and stability of GO in solution was greatly enhanced by mixing BB with GO under sonicating. After adsorption, the products could be separated by simply changing the solution’s pH value or temperature. Flocculant was not needed making the second pollution avoided. Mondal et al. prepared a bifunctionalized graphitic system by using n-BuLi to simultaneously scavenge protons as a nucleophile [81]. The butyl groups could act as spacers to prevent the agglomeration of the graphene sheets while the carboxylic groups were used as sites for dye adsorption. To improve the adsorptive performance and overcome the limitations of graphene as a high-performance sorbent for pollutants, such as hydrophobic nature and difficulties to separate from treated water, graphene combine with sand or SiO2 were synthesized [84,85]. Liu et al. prepared graphene coated silica (RGO/SiO2 ), which acts as a highly efficient sorbent. And the ability of adsorption for eleven different organophosphorus pesticides (OPPs) was tested [85]. The adsorptions for nine of the eleven OPPs were above 95%, and the relative standard deviation (RSD) was under 5%, showing that the graphene-coated silica (GCS) was an efficient, stable, and consistent adsorbent. They demonstrated that the strong
X. Wang et al. / J. Chromatogr. A 1362 (2014) 1–15
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Fig. 4. (a-d) SEM images of PD-5%/GO (a), PD-15%/GO (b), PD-35%/GO (c), PD-70%/GO (d) after freeze drying (scale bars are 5 mm) [83].
adsorption of RGO/SiO2 could attribute to the interactions between graphene’s large -system and electron-donating heteroatoms (N, S, and P) or the strong -bonding networks of benzene rings. Ma et al. developed a method to prepare ultra-light monolithic GO/sodium alginate (GO/SA) and RGO-SA gels [86]. Freezing, solvent exchange, and ethanol drying methods were used to fabricate these 3D network gels. The strong adsorption property of GO/SA for methylene blue was investigated. And the maximum adsorption capacity was 833.3 mg/g for GO/SA, which indicated that the gels obtained excellent adsorption properties for MB.
3. Adsorption of metal ions Graphene-based materials have shown the great potential to be efficient adsorbents for the adsorption of all kinds of metal ions, especially heavy metal ions. Table 2 lists recent results of the metal ions adsorption on graphene-based materials. The mechanisms of adsorption for metal ions on graphene-based materials are very complicated. Various kinds of interactions, such as physical adsorption, electrostatic attraction, and chemical interaction between the metal ions and the functional groups on the surface of those adsorbents, would contribute a lot to the adsorption. XPS measurements indicate that the adsorption of metal ions on GO nanosheets is mainly controlled by chemical adsorption involving the strong surface complexation of metal ions with the oxygen containing groups on the surface of GO [87]. Parameters such as treatment time, temperature, ionic strength, and pH value of the solution can affect the adsorption [88–91]. However, since the charge of both the adsorbents and the adsorbates can be changed by the pH value of the solution, pH value should be carefully adjusted during the adsorption process [92–95]. The pHpzc value of GO is ∼3.9. At pH < pHpzc , the surface charge of GO is positive. The positive metal ions are difficult to adsorb on the positively charged surface of GO nanosheets because of the electrostatic repulsion. At pH > pHpzc , the surface charge of GO nanosheets is negative because of the deprotonation reaction. With increasing pH, the surface charge was more negative and the electrostatic
interactions between the metal ions and GO nanosheets become stronger, and thereby resulted in the increase of metal ion sorption. Zhao et al. also studied the presence of effects of humic acid (HA) on the adsorption of Pb(II). HA prevent the adsorption of metal ions on GO at pH < 8, due to the strong surface complexation and high surface site density of graphene oxide [92,95]. Zhao et al. claimed that the metal ions could share an electron pair with the oxygen atom and form a metal complex for adsorption of metal ion [92]. The delocalized electron systems of graphene would act as Lewis base and form electron donor–acceptor complexes with Pb(II) ions (Fig. 5). The Lewis acid-base interaction between graphene and Pb(II) contributes to the ion adsorption on graphene. The adsorption capacity of GO is much higher than RGO and other carbon based materials due to the high oxygen content of GO. Li et al. studied the adsorption of U(VI) on GO and RGO, and found the maximum sorption capacity of GO was much higher than that of RGO, which indicated that the abundant oxygen-containing functional groups of GO playing important roles in the sorption [96].
Fig. 5. A schematic diagram of the formation of the surface complexes of Pb(II) ions on the surface of FGO [92].
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Table 2 Graphene based materials for the removal of metal ions. Adsorbate
Adsorbent
MAC (mg/g)
Ref.
Am(III) Au(III)
GO GO GO/chitosan RGO/Fe3 O4 /MnO2 GO RGO/Fe3 O4 /MnO2 GO GO/Fe3 O4 GO/-CD GO GO Tri-amino-RGO GO/Fe3 O4 /-CD RGO/CTAB GO GO GO GO/Fe3 O4 GO/Chitosan RGO/␦-MnO2 GO GO RGO RGO/CoFe2 O4 RGO/polypyrrole RGO/polyaniline RGO/␦-MnO2 GO GO GO EDTA-GO GO/poly(acrylamide) GO/chitosan GO/SiO2 GO/Fe3 O4 /chitozan RGO/␦-MnO2 Tri-amino-RGO GO GO/chitosan RGO/CoFe2 O4 GO GO GO GO GO RGO GO GO
8.5 108 1077 14 71 12 68 13 72 106 345 236 120 22 47 294 118 18 25.4 103 175 115 299 158 980 1000 47 842 81 1119 479 1000 100 114 77 162 461 98 217 299 71 24 28 98 299 47 411 345
[107] [158] [159] [90] [158] [90] [93] [98] [160] [93] [87] [161] [162] [91] [105] [87] [163] [95] [88] [104] [164] [107] [165] [100] [102] [103] [166] [92] [158] [87] [89] [82] [167] [168] [99] [104] [161] [159] [159] [100] [158] [107] [107] [97] [96] [96] [169] [87]
As(III) As(IV) Co(II)
Cd(II)
Cr(VI) Cu(II)
Eu(III) Fe(II) Hg(II)
Ni(II) Pb(II)
Pd(II)
Pt(IV) Sr(II) U(VI)
Th(IV) Zn(II)
Zhao et al. further confirmed that the oxygen-containing functional group is the key factor for the adsorption of U(VI) [97]. In order to solve the issue of hard to be separated of GO, magnetic GO/RGO composites were developed, such as GO/Fe3 O4 , RGO/CoFe2 O4 . On the other hand, the combination of GO/RGO with functional materials and groups could improve the adsorption performance efficiently. As a result, nanocomposites like GRO/polymer, GO/chitosan, RGO/functional groups have been synthesized. Owing to the fast magnetizable separation, graphene and magnet composite-based materials can overcome the shortcomings of many other methods, such as filtration, centrifugation, or gravitational separation. To date, the most used magnetic material is still Fe3 O4 [95,98,99]. Zhang et al. fabricated a magnet-GO composite by using CoFe2 O4 as magnetic core and showed excellent separation properties and adsorption capacity for Pb(II) and Hg (II) [100]. Li et al. studied the adsorption of Cu(II) and fulvic acid (FA) by GO/Fe3 O4 [95]. The results showed the FA led to a strong increase in Cu(II) sorption at low pH and a decrease at high pH, which attributed to the strong complexes between FA and Cu(II). On the other hand, the presence of Cu(II) led to an increase in FA sorption
owing to Cu(II) that produced strong effects on the packing, spacing or alignment of adsorbed FA. Chen et al. compared the adsorption capacity of GO/activated carbon felt (GO/ACF) with activated carbon [101]. It was found that the maximum adsorption capacity for U(VI) of GO/ACF was much larger than that of ACF. This result confirmed that the carboxyl functional groups of GO playing important roles in the adsorption. A RGO/polypyrrole (RGO/PPy) composite was prepared by Chandra and Kim using environmentally friendly chemical polypyrrole as co-reactant in the process of the reduction of GO [102]. Thus prepared composites showed high selectivity for Hg(II) removal capacity which was as high as 980 mg/g. Yang et al. synthesized RGO/Polyacrylamide (RGO/PAM) nanocomposites by in situ freeradical polymerization [82]. In this case, the grafted PAM chains on RGO could enhance the dispersion property of RGO in aqueous solution and improve the adsorption capacity of RGO. As shown in Fig. 6, PAM have interaction with metal ions by chemical or physical adsorption, at the same time, interaction between PAM and aromatic structures can occur through hydrogen bond. Li et al. prepared a RGO/polyaniline (RGO/PANI) nanocomposite through in situ polymerization of aniline in the presence of GO and then reduction by hydrate hydrazine [103]. Compare to PANI, the maximum adsorption capacity of PANI/RGO for Hg(II) was increased from 515 mg/g to 1000 mg/g. The high adsorption capacity could be attributed to the enhanced adsorption sites caused by incorporating the high surface area of RGO into the PANI. Ren et al. fabricated RGO/␦-MnO2 to remove either Cu(II) or Pb(II) from simulated solution [104]. Graphene played dual roles in the adsorption process. Firstly, it could be used as a matrix for the growth of MnO2 nanoparticles. Secondly, it helped to prevent the aggregation of the nanoparticles. Oxygen-containing functional groups including hydroxyl (C-OH or Mn-OH) and carboxyl groups on the RGO surface could act as Lewis acids to adsorb the Cu(II) and Pb(II) by forming tetradentate monodentate complexes on the RGO/␦-MnO2 surface. Since the metal ions could insert into layered MnO2 , the adsorption capacities of RGO/␦-MnOn2 could be further increased compared to other adsorbents. Madadrang et al. linked N-(trimethoxysilylpropyl)ethylenediamine triacetic acid (EDTA-silane) with the hydroxyl groups on GO surface (Fig. 7) [89]. The high chelating ability of EDTA groups together with OH and COOH groups on the GO surface could greatly enhance the adsorption capacity of GO. The adsorption capacity of EDTA-GO reached 479 mg/g which was much higher than those of GO or carbon nanotubes. Furthermore, the adsorption rate was fast and the whole adsorption process could be completed in 20 min. Yang et al. found that GO was an efficient adsorbent for removal of Cu(II) [105]. The abundant oxygen-containing groups on the GO surface were believed to help much on forming GO-metal complexes. The neutralization of the negatively charged functional groups on the surface of GO would then induced the folding and aggregation of GO. Normally, GO is very stable in water [106]. However, these experimental results demonstrated that the GO is not stable enough in the aqueous solution. Later, a series of experimental results were further demonstrated by Sitko et al. [87] and Yu et al. [107]. From their results, they found that Zn(II), Cd(II), and Pb(II) and even most of the actinide metals and their fission products could also induce the aggregated of GO.
4. Solid phase extraction (SPE) Based on the results of organic compounds and metal ions adsorption, graphene shows exceptional adsorption properties. As a superior candidate as a good adsorbent for organic compounds and metal ions in various sample preparation methods,
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Fig. 6. Synthesis of PAM chains on RGO sheets by Free radical polymerization and adsorption of metal ions [82].
graphene-based materials have lot of applications in solid phase extraction and solid phase microextraction. SPE is a method of preconcentration and analyte/matrix separation by using the affinities of analytes dissolved in liquid for solid adsorbent. The core of SPE is sorbent which determines the efficiency and the selectivity of extraction. Recently, graphene and graphene based materials have attracted great of attentions as a new sorbent in SPE. Table 3 shows the published papers on applications of graphene based materials in SPE. By using graphene-packed SPE cartridges as adsorbent, Liu et al. extracted eight chlorophenols (CPs) from water samples [108]. From the experiment results, some advantages of graphene as SPE adsorbent were demonstrated, such as low consumption of adsorbent (20–30 mg) and organic solvent, good compatibility with various organic solvents, good reusability and no impact of sorbent drying. They also compared extraction of CPs with other adsorbents including C18 silica, graphitic carbon, single- and multi-walled
carbon nanotubes. Due to the improvement of water-wettability of RGO and the facilitating of retention and elution of polar compounds caused by residual polar groups after reduction, RGO gave the best results. Although RGO and GO have several advantages as adsorbents in SPE due to their adsorptive properties, according to many experimental results, there are several problems in using of RGO or GO as SPE adsorbent directly. Firstly, the aggregation of RGO or GO sheets may occur during isolation from a homogeneous solution, which may reduce efficiency and reusability. Secondly, very small RGO or GO sheets may cause high pressure and escape from the SPE cartridge/column. To avoid these problems and maintain the advantages of graphene, Liu et al. developed new SPE adsorbents by binding RGO or GO sheets with silica [109] (Fig. 8). Compared with commercially available adsorbents, superior and comparable performance was achieved which resulted from the large surface area and water wettabiltiy of RGO and GO. Notably, G@SiO2 could
Fig. 7. Chemical structure of EDTA-GO (left) and its interaction with heavy metal cations (right) [89].
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Table 3 Application of RGO, GO and their derivatives in SPE. Analyte
Matrix
Adsorbent
RSD (%)
Method
DL
Recovery (%)
Ref.
NAAS CPA PAHs CPs Sulfonamide SAs Malachite green Organophosphorus pesticides OPPs PAHs PAHs PAEs PAEs Pesticide residues Pesticides Carbamates pesticides Phthalate acid esters Pesticide Pb(II) Cr(III) Cd(II) Cd(II) Pd(II) Aflatoxins
Rat brain Aquatic products Water Water Environmental water Meat Fish tissues Environmental water Apple juice Environmental water Environmental water Environmental water Water and soybean milk Oil crops Tomato and rape Environmental water Environmental water Tea Environmental water Environmental water Water and vegetable Water and vegetable Water and Vegetable Peanut
RGO RGO Sulfonated RGO RGO@SiO2 RGO RGO RGO RGO RGO RGO Fe3 O4 @SiO2 -RGO RGO Fe3 O4 @SiO2 -RGO RGO/CH3 NH Fe3 O4 @SiO2 -RGO RGO/Fe3 O4 RGO/Fe3 O4 RGO/PSA RGO RGO RGO/amino RGO/Fe3 O4 RGO/amino GO
6.0 1.5–4.3 1.0–9.0 2.2–7.7
