Vanesa Romero,
Isabel Costas-Mora,
Isela Lavilla and
Carlos Bendicho*
Departamento de Química Analítica y Alimentaria, Área de Química Analítica, Facultad de Química, Universidad de Vigo, Campus As Lagoas-Marcosende s/n, 36310 Vigo, Spain. E-mail: bendicho@uvigo.es; Fax: +34-986-812556; Tel: +34-986-812281
First published on 8th December 2015
Fabrication of unmodified graphene membranes for their application as selective sorptive platforms of hexavalent chromium [Cr(VI)] is described for the first time. Multilayer graphene membranes are synthesized by drop-casting of graphene oxide (GO) onto a glass substrate followed by mild thermal reduction. As-prepared membranes are formatted to fit the measurement area of total reflection X-ray fluorescence (TXRF). Structural and morphological characterization by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) shows that graphene membranes are 122 nm height and contain non-reduced functional groups resulting in lattice defects. Adsorption isotherm models and characterization by time-of-flight secondary ion mass spectrometry (TOF-SIMS) indicate that adsorption sites on graphene membranes are uniformly distributed and bind Cr(VI) as a monolayer, both by electrostatic interaction and chemisorption. Graphene membranes display high flexibility and become conical-shaped when immersed into stirred liquid samples. When combining graphene membrane preconcentration and TXRF, a detection limit of 0.08 μg L−1 Cr(VI) is obtained. Repeatability expressed as relative standard deviation is 3% (N = 5). Two certified reference materials, i.e. CASS-4 seawater and NWTM-27.2 lake water, are used for testing accuracy. The proposed method is simple, solvent-free and sensitive, being suitable for Cr speciation in water including high salinity samples.
In the last few years, GO and graphene have been reported as new sorptive materials for extraction and preconcentration.7,8 In contrast to other carbon-based nanomaterials, e.g. carbon nanotubes (CNTs), GO and graphene display sorptive properties in both surfaces of the planar sheet resulting in larger specific surface area and higher adsorption kinetics. Besides, unlike CNTs, chemical synthesis of GO and graphene from graphite does not require metal catalysts, which prevents sorbents from metal impurification. Both GO and graphene provide plenty of sites for functional modification.9 For sorption of trace metal ions, GO is usually preferable due to the abundance of functional groups that can interact with analytes by a variety of mechanisms (electrostatic interactions, cation–π interactions, etc.).10 GO can be used for in-column extraction, e.g. SiO2@GO composite can be loaded into a glass column or coated hollow fibers.11,12 However, low flow-rate is required in order to avoid overpressure problems and/or losses of the solid phase. Thus, most reported applications use dispersive micro-solid phase extraction (D-μ-SPE) involving GO nanosheets as nanocomposites (e.g., Fe3O4@SiO2@GO and Fe3O4@GO),13,14 which are further modified with specific functional groups (e.g. 3-mercaptopropyltrimethoxysilane-modified GO),15 or unmodified.16 After extraction, GO can be separated by filtration, aggregation by adding NaCl or by applying an external magnetic field. Graphene nanosheets have also been reported as a new solid phase for D-μ-SPE of trace metals. Given the extremely hydrophobic properties of graphene, complexation of metal ions with suitable chelating agents, e.g. ammonium pyrrolidinedithiocarbamate (APDC), is required for efficient sorption.17,18 It should be noted that before measurement, GO and graphene nanosheets have to be isolated from the sample solution. Losses during separation (filtration, centrifugation or decantation) can occur due to the small size of nanosheets.
New formats of carbon nanomaterials used as preconcentration platforms, such as easy-to-handle graphene membranes, would avoid centrifugation or filtration steps thus simplifying sorption procedures. In addition, graphene membranes should be well suited for direct measurement of metal species by total reflection X-ray fluorescence (TXRF) without the need for an elution step.
Trace metal speciation in environmental and biological samples is required since metal bioavailability and toxicity depends on the different chemical forms. Cr occurs primarily in two oxidation states, namely, trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)]. Cr(III) is considered to be essential for carbohydrate and lipid metabolism, its toxicity being very low.19 On the contrary, Cr(VI) is highly toxic for humans and it is considered to be a strong carcinogen.20 Cr speciation can be performed by a variety of techniques. High performance liquid chromatography (HPLC) coupled to atomic spectrometry detection provides comprehensive information on the presence of Cr species in a variety of matrices.21 For speciation of Cr(III) and Cr(VI), derivatization with ethylenediaminetetraacetic acid followed by anion-exchange chromatography is commonly performed.22 Apart from hyphenated techniques, the development of non-chromatographic approaches for chemical speciation is attractive since they can provide simpler and less expensive ways to perform Cr speciation. Among the different strategies, solid phase extraction and microextraction are the most used techniques.23 Selective extraction can be achieved by ion-exchange resins,24 molecularly imprinted polymers (MIPs),25,26 or by complexing chromium species with specific organic groups before extraction, e.g., complexation with APDC before extraction onto CNTs,27 or complexation with ethylenediaminetetraacetic acid (EDTA) before extraction onto silver nanoparticles.28
In this work, a novel method for the preparation of multilayer graphene membranes by drop-casting of GO onto a glass substrate followed by mild thermal reduction is described. As-prepared graphene membranes are applied as preconcentration platforms for the detection of Cr(VI) and total Cr at trace level in waters by TXRF. Morphology and structure of the novel sorptive platforms are fully characterized and sorption mechanisms for Cr(VI) outlined.
A PowerPac™ basic power supply (programmable 10–300 V, 4–400 mA) equipped with a Sub-Cell® GT (Bio-Rad, California, USA) was used for gel electrophoresis. Graphene oxide aliquots (5 μL) are loaded in the agarose gel 0.8% (m/v) buffered at pH 1.5. Electric field with a voltage programmed at 90 V was applied for 30 min.
An Agimatic-N magnetic stirrer with heating (550 W) equipped with an electronic heater energy control from 0–100% (JP Selecta, Barcelona, Spain) was used for membrane preparation. Heater energy is programmed to 15% (T ≈ 100 °C) for thermal reduction of GO deposited onto glass substrates.
A JP Selecta model Ultrasons-1L ultrasonic bath (Barcelona, Spain) with a maximum output power of 50 W and an operating frequency of 20 kHz was employed for ultrasound-assisted exfoliation of graphite oxide.
A field emission transmission electron microscope (FETEM) model JEOL JEM-1010 (JEOL, Seoul, Korea) operating at an acceleration voltage of 100 kV was used for obtaining TEM images. A 10 μL aliquot of freshly prepared GO colloid was deposited onto a copper mesh cover (with a film of carbon/formvar) and left to dryness in a laminar flow hood.
A high resolution stylus profilometer (vertical resolution of 1 Å) model Dektak XT (Bruker GmbH, Berlin, Germany) and an AFM instrument model Multimode 8 (Veeco, Digital Instruments, Santa Barbara, CA) equipped with Nanoscope V controller were used for obtaining information about the membrane surface morphology, roughness and thickness. For XPS measurements, a Thermo Scientific K-alpha ESCA instrument (Thermo Scientific, UK) equipped with and aluminum K-alpha 1,2 monochromatized radiation X-ray source (1486.6 eV) was used. For XPS, AFM and profilometry measurement, samples were deposited onto pre-cleaned silicon wafers (orientation 〈111〉, type P, roughness 2 nm) and left to dryness in a laminar flow hood. Samples for XPS characterization were stored under vacuum to avoid contamination by atmospheric oxygen.
A time-of-flight secondary ion mass spectrometry (TOF-SIMS) instrument model TOF.SIMS4 (IonTof, Münster, Germany) was used for obtaining information about chromium distribution onto the graphene membrane. Bismuth liquid metal ion gum mode (BiHCBU) at 25 kev pulsed beam was used.
Graphite powder (grain size < 75 μm) 99% (Scharlau, Barcelona, Spain), KNO3 99% (Probus, Badalona, Spain), KMnO4 99.5% (Analyticals Carlo Erba, Milano, Italy), H2O2 30% w/w (Merck, Darmstadt, Germany) and H2SO4 95% w/v (Prolabo, Fontenay-sous-Bois, France) were used for synthesis of GO.
HNO3 65% w/v (Prolabo, Fontenay-sous-Bois, France) was used to adjust the pH of the extraction medium to 1.0. Stock standard solutions of 1000 mg L−1 Cr(III) and Cr(VI)were prepared by dissolving 6.5 mg of Cr(NO3)3·9H2O 99% (Aldrich, Steinheim, Germany) and 0.14 g of K2Cr2O7 99.5% (Prolabo, Fontenay-sous-Bois, France), respectively, in 50 mL of ultrapure water. Diluted working standard solutions were prepared fresh daily from the stock solution.
The following reagents were employed for studying interferent effects: MgCl2·6H2O 98% (Prolabo, Fontenay-sous-Bois, France), Na2SO4 (Probus, Badalona, Spain), KH2PO4 99.5% (Panreac, Barcelona, Spain), Na2CO3 anhydrous 99.5% (Panreac, Barcelona, Spain), KNO3 99% (Probus, Badalona, Spain) and humic acid (Fluka, Buchs, Switzerland). Ammonium pyrrolidinedithiocarbamate (APDC) 98% (Fluka, Buchs, Switzerland) was used for sorption experiments of the Cr(VI)-PDC chelate onto graphene membranes.
Agarose standard low EEO (electroendosmosis) (Ecogen, Barcelona, Spain) was used for gel electrophoresis experiments.
Certified reference materials CASS-4 coastal Atlantic surface seawater (National Research Council of Canada) and NWTM-27.2 fortified lake water (National Water Research Institute of Canada) were used for testing accuracy of the method.
For membrane preparation, thermal reduction was followed. In order to obtain 10 mm diameter membranes, glass substrates (Ø 10 mm) are used as support for preparation. First, glass substrates were cleaned with propanol and acetone under ultrasonic irradiation and dried. After that, they were heated at 95 °C in a hot plate. Drop-casting method was used to prepare membranes. For this, three aliquots of 100 μL GO were added onto the pre-heated glass substrates, water was evaporated and color of the GO film on the glass substrate changed from yellow to black, thus indicating the formation of a multilayer graphene membrane. Since graphene has hydrophobic behavior in contrast to glass substrates, membranes can be easily separated by rinsing with ultrapure water.
For total Cr detection, samples are treated with 2.5% m/v potassium peroxydisulphate at 80 °C for 30 min in order to ensure the oxidation of Cr(III) to Cr(VI) before microextraction. The Cr(III) content can be calculated as the difference between the total Cr and Cr(VI) contents.
Taking into account this concentration, different mass of GO are tried by varying the volume of the aliquots during drop-casting operation. Three aliquots in the range of 20–200 μL are deposited onto the glass substrates for thermal reduction. As can be seen in Fig. 1a, the analytical signal for Cr(VI) increases on increasing GO mass. This behavior can be ascribed to the increased amount of functional groups that can interact with the analyte. However, it should be noted that the background signal in the TXRF spectra increases with increasing GO mass (Fig. 1a). Larger GO masses give rise to thicker graphene membranes, which results in higher background. For TXRF measurements, the limit of detection (LOD) directly depends on the background signal: the lower spectral background, the better LOD. A mass of 450 μg of GO (3 aliquots of 100 μL) was selected for the preparation of graphene membranes as a compromise between high analytical signal and low background.
Following thermal treatment, oxygen-containing functional groups, e.g. hydroxyl, carboxy and epoxy, tend to be reduced. However, incomplete reduction to pristine graphene may occur. The obtained graphene membrane may contain lattice defects originated from the formation of carbon oxides.32 The presence of non-reduced functional groups in the structure promotes the formation of holes along the layers of the membrane. Besides, oxygen functional groups attached to graphene keep the distance between layers33 and also can increase the membrane permeability for analyte retention. During the sorption process, Cr(VI) can pass through the graphene membrane and interact with non-oxidized groups. At pH of extraction (≈0.9), Cr(VI) exists as dichromate anion (Cr2O72−) and oxygen-containing functional groups in the membrane can have positive charge. Therefore, Cr(VI) may be retained by electrostatic interaction.
In addition, the reduction time may also influence the flexibility of graphene membranes. In this way, reduction times for thermal treatment at 70 °C and 100 °C are tried. Flexible graphene membranes that can be easily handled are obtained after 15 min of thermal treatment. On the other hand, when glass substrates are heated to 100 °C, shorter times are necessary (Fig. S1†). After drying aliquots of GO, color changes from yellow to black in 2 min, but membranes cannot be easily separated from the glass substrate. Membranes can break resulting in impaired precision for Cr(VI) extraction. For reduction times longer than 4 min, highly flexible graphene membranes are obtained, which can be readily separated from glass substrates and handled for sorption and measurement steps. Then, thermal reduction at 100 °C for 4 min is fixed for preparation of graphene membranes.
The synthesized GO was characterized by TEM. As can be see in Fig. S3,† GO is well-exfoliated giving rise to irregularly-shaped nanosheets (60 nm in length on average).
Besides, AFM studies provide information about morphology, thickness and roughness (Fig. S4†). Graphene membranes synthesized under optimal conditions are 122 nm height with a surface nanoroughness of 22.14 nm. Furthermore, XPS studies provide information about chemical composition of graphene membranes. Results show that complete reduction to pristine graphene is not achieved, so residual non-reduced functional groups (e.g. hydroxyl, carboxy and epoxy) are present in the as-prepared graphene membranes (Fig. S5†). Finally, TOF-SIMS studies provide information about the distribution of Cr(VI) onto the membrane. Results show that Cr(VI) is well-distributed onto the graphene membrane (Fig. S6†).
Moreover, the sorption of Cr(VI) in the presence of the aforementioned metal ions has also been attempted using APDC at pH 1. APDC–metal complexes are formed with all metal ions, which can interact with the graphene membrane by π–π interactions. This system displayed less selectivity as compared to the use of unmodified graphene membranes at acidic pH for Cr(VI) microextraction. Thus, direct interaction of the membrane graphene (i.e., without addition of chelating agents) with Cr(VI) is selected for further experiments.
Different HNO3 concentrations, i.e. 0.02, 0.1, 0.4, 1, 2.5 and 4.5 mol L−1, were tried for pH adjustment (Fig. 3b). For concentrations above 1 mol L−1 HNO3, the analytical signal reaches a plateau.
In addition, stirring rate of sample solution is a relevant parameter in order to allow fast mass transfer of the target analyte to the graphene membrane. The time required to reach equilibrium conditions can be reduced by using agitation, so the more effective the stirring is, the shorter the sorption times is required. For a fixed microextraction time (10 min), different stirring rates in the range of 0–900 rpm were tested (Fig. 3c). For stirring rates higher than 900 rpm with a 50 mL sample volume, it is difficult to keep the graphene membrane in the center of vortex. The graphene membrane can move to the bottom of the extraction vessel and hit the magnetic stirrer resulting in breakage.
In order to obtain good sensitivity and repeatability, a sorption time of 20 min is used for further experiments. Besides, it should be noted that if higher sample throughput is required, shorter sorption times can be applied by using pre-equilibrium conditions, but in this case, a precise control of sorption time is required to ensure good repeatability.
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Isotherms parameters indicate that the correlation coefficient for the adjustment of the experimental data are R = 0.9926 and R = 0.8706, for Langmuir and Freundlich model, respectively. Results clearly indicate that experimental data fit better using the Langmuir model. We can conclude that the adsorption sites on graphene are assumed to be uniformly distributed and bind Cr(VI) as a monolayer, in which positively charged oxygen-containing groups can interact with Cr(VI) as dichromate anion. Besides, localized adsorption is thus seen to be very plausible for chemisorption. Due to the strong oxidizing power of Cr(VI) in acid medium, hydroxyl groups present in the graphene membrane can be oxidized to the ketone form, with the subsequent reduction of Cr(VI) to Cr(III).36,37
Therefore, we can assume that Cr(VI) is retained both by electrostatic interaction with positively charged functional groups and by chemisorption upon reaction with hydroxyl groups.
In addition, the slope of linear adjustment to Langmuir isotherm (1/qmax) provides information about the maximum adsorption capacity of the graphene membrane (qmax), obtaining a result of 22 μg Cr(VI) per mg of graphene membrane. The adsorption capacity for Cr(VI) can vary in a wide range depending on the sorbent.38–40 Unmodified CNTs (used as dispersions for batch extraction or packed into a column) provide an adsorption capacity in the range 1–11 μg Cr(VI) per mg of CNTs.27,41–43 Besides, non-nanostructured membranes, e.g. modified silica membrane, shows an adsorption capacity around 3 μg Cr(VI) per mg of adsorbent.44 Moreover, using CNTs large extraction times are usually needed to reach equilibrium (several hours), and also poor selectivity is achieved. Using graphene membranes, highly selective extraction of Cr(VI) can be achieved with high adsorption capacity and short extraction time. Moreover, operations for isolating the sorbent by centrifugation or filtration are avoided.
Interferent | Studied level (mg L−1) | Cr(VI) recoveries (%) | Effect (%) |
---|---|---|---|
NaCl | 100 | 99 | +1.1 |
1000 | 100 | +2 | |
20![]() |
98 | +4.8 | |
30![]() |
101 | +7.5 | |
MgCl2 | 10 | 101 | +0.6 |
100 | 99 | −0.4 | |
200 | 100 | −0.9 | |
KH2PO4 | 10 | 100 | +5.6 |
100 | 101 | +7.1 | |
200 | 99 | +9.5 | |
NaSO4 | 10 | 100 | +0.5 |
100 | 99 | +1.5 | |
200 | 99 | +3.5 | |
KNO3 | 10 | 95 | +0.6 |
100 | 100 | −0.9 | |
200 | 100 | −2.3 | |
Na2CO3 | 50 | 100 | +0.6 |
100 | 97 | +0.8 | |
500 | 101 | +1.3 | |
Humic acid | 1 | 99 | +2.3 |
50 | 101 | +4 | |
100 | 98 | +5 |
In addition, the accuracy of the proposed method for detection of Cr(VI) was evaluated by analyzing two certified reference materials (CRMs), i.e. CASS-4 seawater and NWTM-27.2 lake water (Table 2). Prior to preconcentration, samples are treated with 2.5% m/v potassium peroxydisulphate at 80 °C for 30 min in order to ensure the oxidation of Cr(III) to Cr(VI). The found content for NWTM-27.2 is in good agreement with the certified value for Cr, a recovery of 101% being achieved. The condition texp < tcrit (tcrit = 4.303 for N − 1 = 2 freedom degrees) was fulfilled for the studied certified material and consequently, non-significant differences (p = 0.05) occurred between the certified and the found concentration. For the certified material CASS-4, the Cr content was below the limit of quantification (LOQ). In addition, CRMs were spiked with Cr(VI) and Cr(III) at different levels, recoveries for Cr(VI) and total Cr being in the range of 99–102% and 99–101%, respectively.
CRMb | Spiked content (μg L−1) | Found content (μg L−1) ± standard deviation (N = 3) | Recovery (%) | |||
---|---|---|---|---|---|---|
Cr(VI) | Cr(III) | Cr(VI) | Total Cr | Cr(VI) | Total Cr | |
a [Total Cr] = [Cr(VI)] + [Cr(III)].b Certified values for total Cr content: NWTM-27.2 (1.9 μg L−1); CASS-4 (0.144 μg L−1).c The limit of quantification (LOQ) was 0.25 μg L−1 of Cr(VI). | ||||||
NWTM-27.2 | — | — | — | 1.9 ± 0.1 | — | 101 |
10 | 10 | 9.9 ± 0.1 | 19.8 ± 0.5 | 99.4 | 99.3 | |
50 | 50 | 50.7 ± 0.4 | 101 ± 0.7 | 99.5 | 101 | |
CASS-4 | — | — | — | <LOQc | — | — |
10 | 10 | 10.1 ± 0.2 | 19.9 ± 0.2 | 100 | 99.5 | |
50 | 50 | 50.6 ± 0.4 | 101 ± 1 | 102 | 101 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23116g |
This journal is © The Royal Society of Chemistry 2016 |