Preparation of naphthyl functionalized magnetic nanoparticles for extraction of polycyclic aromatic hydrocarbons from river waters

Abstract

A novel core–shell structured magnetic sorbent, naphthyl functionalized magnetic nanoparticles (Fe₃O₄@SiO₂@Nap), was prepared and successfully applied for the magnetic solid-phase extraction (MSPE) of polycyclic aromatic hydrocarbons (PAHs) from river water samples. The analytes were finally determined by high performance liquid chromatography coupled with fluorescence detection (HPLC-FLD). Seven kinds of PAHs were selected as the model analytes, including fluorene (Flu), fluoranthene (Fla), anthracene (Ant), pyrene (Pyr), benz[a]anthracene (BaA), benzo[b]fluoranthene (BbF) and benzo[k]fluoranthene (BkF). Transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), Fourier transform infrared spectrometry (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the sorbent. The main influencing parameters including sorbent amount, desorption solvent, sample volume and extraction time were optimized to achieve the highest recovery rate. Under the optimal conditions, only 40 mg of the Fe₃O₄@SiO₂@Nap sorbent was used to extract the PAHs. The linear ranges of all the seven PAHs were 0.5–100 ng L⁻¹ with the limits of detection (S/N = 3) ranging from 0.04 to 0.12 ng L⁻¹. The repeatability was investigated by evaluating the intra- and inter-day precisions with relative standard deviations (RSDs) lower than 4.3%. Finally, the proposed method was successfully applied for the determination of PAHs in river water samples with recoveries in the range of 89.6–106.8%.

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1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) comprise a large group of compounds with two or more fused benzene rings. They are widespread environmental contaminants that result from incomplete combustion of organic materials during natural or anthropogenic processes. PAHs are toxic substances which are resistant to degradation. Numerous epidemiological studies indicate that exposed people have increased risks of cancer.¹ PAHs are therefore considered to be priority pollutants by both the US Environmental Protection Agency (EPA) and the European Environmental Agency.^2,3 Consequently, there are increasing interests in the detection of PAHs in environmental water sources for the protection of health and the environment. PAHs are hydrophobic in nature with low water solubility, being less soluble in water and less volatile with increasing molecular weight.⁴ To determine trace PAHs in aquatic environments by instrumental analysis, sample pre-concentration technique is usually required. Solid-phase extraction (SPE) is one of the most commonly used techniques for the preconcentration of PAHs.⁵ SPE has been widely exploited due to its strong separation capacity, high enrichment factor, minimal sample and solvent consumption, low cost and easy automation.⁶

Nowadays, a new mode of SPE, magnetic solid-phase extraction (MSPE) technology,⁷ has received increasing attention. Based on the use of magnetic nanoparticles, it has high extraction efficiency and rapid extraction kinetics. Magnetic nanoparticles (MNPs) are provided with many merits such as good stability, easy synthesis, high surface area, facile separation by magnetic forces, as well as low toxicity and low cost, and therefore have been widely applied in environmental and material science.⁸ The magnetic nanoparticles are always directly dispersed in the sample solutions to quickly extract analytes since they can be readily recovered by a magnet. Compared with traditional SPE, MSPE sorbents combine numerous advantages such as large surface area, unique magnetic property, convenient functional modification, high separation efficiency, high reusability and environmental friendliness.⁹ MSPE shows great potential applications in preconcentration and separation.^10–14 Typically, the magnetic nanoparticles were composed of a magnetic core (Fe₃O₄) and a functionalized shell which can selectively adsorb targets.¹⁵ Various functionalized magnetic nanoparticles were synthesized as sorbents to extract trace PAHs from various environmental matrices, including carbon coated Fe₃O₄ nanoparticles (Fe₃O₄/C),^16,17 alkyl (C₁₀–C₁₈) carboxylates,¹⁸ n-octadecylphosphonic acid modified mesoporous magnetic nanoparticles (OPA/MMNPs),¹⁴ C₁₈-functionalized magnetic nanoparticles,^19–22 diphenyl functionalization of Fe₃O₄ magnetic nanoparticles (Fe₃O₄-diphenyl),¹⁵ Fe₃O₄-doped poly(styrene-divinylbenzene-co-4-vinylbenzenesulfonic acid sodium salt) nanoparticles (Fe₃O₄-MPNP),³ phosphatidylcholine bilayer coated magnetic nanoparticles (Fe₃O₄/PC),²³ cholesterol-functionalized magnetic nanoparticles (Fe₃O₄@SiO₂@Chol),²⁴ polydopamine coated Fe₃O₄ nanoparticles (Fe₃O₄/PDA),²⁵ magnetic microsphere-confined graphene (Fe₃O₄@SiO₂-G),²⁶ metal–organic framework MIL-101,²⁷ triphenylamine-functionalized magnetic microparticles (Fe₃O₄/SiO₂/TPA),²⁸ ionic liquid coated magnetic nanoparticles (IL-MNPs)²⁹ and magnetic nanoparticles-nylon 6 composite.³⁰

In this work, we developed a new kind of MNPs termed as naphthyl functionalized magnetic silica nanoparticles (Fe₃O₄@SiO₂@Nap) to extract PAHs from river water samples. To our knowledge, this is the first report on the introduction of naphthyl to functionalized magnetic nanoparticles for the extraction of PAHs. The condensed cyclic structure and hydrophobic property of naphthyl is expected to make it a good functional material to interact with PAHs through the π–π conjugative effect, which would increase the selectivity of the sorbent to PAHs. The proposed MNPs were applied successfully for the extraction of PAHs in three kinds of river water samples. Finally, according to the USA Environmental Protection Agency Method 610,³¹ PAH priority pollutants must be determined using HPLC in combination with ultraviolet absorption or fluorescence detectors. We used high performance liquid chromatography coupled with fluorescence detection (HPLC-FLD) to determine PAHs.

2. Experimental

2.1. Chemicals

Ferric chloride hexahydrate (FeCl₃·6H₂O, 99%), ferrous sulfate heptahydrate (FeSO₄·7H₂O, 99%), ammonia (26%), hydrazine hydrate (99%), isopropanol (99%), triethylamine (99%), toluene (99%) and naphthoyl chloride (99%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Methanol and acetonitrile of HPLC grade (99%) were bought from Merck (Darmstadt, Germany). Tetraethoxysilane (TEOS, 99%) and 3-aminopropyltriethoxysilane (APTES, 99%) were obtained from Adamas Reagent Ltd. (Switzerland). Other reagents including n-hexane, acetone, methanol, isopropanol and ethanol were of analytical grade. Ultrapure water was prepared using a Milli-Q system water purification system (Millipore Inc., USA). It was degassed using an ultrasonic bath for 5 min prior to use.

Certified reference standards of fluorene (Flu, 99%), fluoranthene (Fla, 99%), anthracene (Ant, 99%), pyrene (Pyr, 99%), benzo[a]anthracene (BaA, 99%), benzo[b]fluoranthene (BbF, 99%) and benzo[k]fluoranthene (BkF, 99%) were purchased from Acros Organics (NJ, USA).

Stock PAH solutions were prepared in HPLC grade methanol containing Flu, Ant, Pyr (1 mg mL⁻¹), Fla (1 mg mL⁻¹), BaA (0.05 mg mL⁻¹), BbF and BkF (0.5 mg mL⁻¹), and kept at 4 °C in darkness. Working solutions of PAHs composed of Flu, Ant, Pyr, BbF, BkF (100 ng mL⁻¹), Fla (200 ng mL⁻¹) and BaA (50 ng mL⁻¹) was prepared by diluting the stock solutions with methanol.

2.2. Apparatus

The size and morphological characterization of the particles were observed by transmission electron microscopy (TEM, JEM-2100F, JEOL Co., Tokyo, Japan). Fourier transform infrared spectra (FTIR) were recorded on Vertex 70 (Bruker Optics, Germany). X-ray photoelectron spectroscopy (XPS) was tested on a Thermo ESCALAB 250Xi X-ray photoelectron spectrometer (Thermo Scientific, USA). Samples were dried at 80 °C in a vacuum oven for 12 h, and mixed with KBr to fabricate a KBr pellet for FTIR analysis. PAHs were extracted from water samples with assistance of an ultrasonic instrument KQ-600KDE. The magnetic property was analyzed using a vibrating sample magnetometer (VSM, Model 7410, Lake Shore Cryotronics, Inc., Westerville, Ohio, USA).

Chromatographic separations and analysis of PAHs were carried out on an Agilent 1260 Series HPLC system (Agilent, USA) including a G1311C quatpump, a G1322A degasser, a G1329B auto-sampler, a G1316A column oven and a G1321A fluorescence detector (FLD). Agilent Chem Station was used to control the system and the process of the chromatographic data. The chromatographic separation of PAHs was performed using an Ultimate XB-C₁₈ column (5 μm particle diameter, 3.0 mm i.d. × 250 mm length, Ultimate, Welch Materials, Inc.) with a column oven temperature maintained at 30 °C. The mobile phase consisted of methanol–water (v/v 85/15) at a flow rate of 1.0 mL min⁻¹. The excitation and emission wavelength programs used for the fluorescence detection were listed in Table S1.†

2.3. Preparation of Fe₃O₄@SiO₂@Nap MNPs

2.3.1. Preparation of Fe₃O₄ MNPs. The Fe₃O₄ magnetic nanoparticles (MNPs) were prepared by chemical co-precipitation. Briefly, FeCl₃·6H₂O (4.0 g) was dissolved in deionized water (30 mL) in a three-necked round bottom flask, followed by addition of hydrazine hydrate (2 mL) and FeSO₄·7H₂O (10.90 g) to prepare a stock solution. Afterwards, ammonia (35 mL 26.5% w/w) was added into the stock solution under vigorous stirring, followed by dropwise addition of ammonia until the solution pH reached 9. Then it was stirred at room temperature for 30 min, aged at 80 °C for 60 min, and then cooled to room temperature. The product was magnetically collected, and washed with water, finally vacuum-dried at 60 °C for 12 h.

2.3.2. Encapsulation of the MNPs with silica (Fe₃O₄@SiO₂). Nanoparticles (1 g) were dispersed in a mixture of 2-propanol (100 mL) and ultrapure water (8 mL), sonicated for 15 min, followed by addition of ammonia (10 mL) and TEOS (8 mL) sequentially. The mixture was then stirred for 12 h at 45 °C. The magnetic nanoparticles were collected by a magnet and washed with water and ethanol respectively and vacuum-dried at 60 °C for 12 h. In this step, the ferromagnetic nanoparticles were encapsulated with a mesoporous silica shell.

2.3.3. Preparation of naphthyl coated MNPs (Fe₃O₄@SiO₂@Nap). The preparation scheme of Fe₃O₄@SiO₂@Nap is depicted in Fig. 1. For this purpose, 40 mL of anhydrous toluene, 3.0 g of naphthoyl chloride, 1 mL of triethylamine and 2.5 mL of APTES were added to a 150 mL three-neck round-bottom flask under argon. The mixture was refluxed for 24 h at 80 °C under magnetic stirring, and cooled slowly to room temperature. Two grams of the dried silica gel-modified magnetic nanoparticles were added to the solution. The mixture was then refluxed for 12 h at 110 °C under mechanical stirring in argon atmosphere. The as-prepared product was magnetically collected and washed by n-hexane, acetone and ethanol successively. The resulting Fe₃O₄@SiO₂@Nap was dried under vacuum at 60 °C for 12 h.

Fig. 1 Preparation scheme of Fe₃O₄@SiO₂@Nap.

2.4. Sample collection

River water samples were collected from different districts of Nanchang city in September 2014. Yudai river water was collected from Yudai River (Nanchang, Jiangxi, China), Qingshan Lake water was collected from Qingshan Lake (Nanchang, Jiangxi, China), and the Ganjiang River water was collected from the Ganjiang River (Nanchang, Jiangxi, China). All samples were collected at 10 cm depth below the water surface, filtered through 0.45 μm cellulose membranes to remove suspended particles, and stored in an amber glass bottle at 4 °C. The filtered water samples were analyzed within 24 h.

2.5. MSPE procedure

Forty mg of Fe₃O₄@SiO₂@Nap was placed in a 250 mL vial and firstly activated with methanol, then dispersed into 150 mL of water sample spiked with the proper amounts of PAHs. After being sonicated for 30 s to form a homogeneous dispersion solution, the magnetic nanoparticles were isolated rapidly from the solution by applying an external magnetic field for 12 min. After decanting the supernatant solution, the captured PAHs were desorbed by 1.0 mL of acetonitrile. The desorption solution was collected and filtered through a 0.22 μm polytetrafluoroethylene (PTFE) membrane syringe filter, 20 μL of the filtrate was injected into HPLC for analysis.

3. Results and discussion

3.1. Characterization of Fe₃O₄@SiO₂@Nap MNPs

The prepared MNPs were characterized with TEM, VSM, FTIR and XPS.

The TEM image of Fig. S1a† shows that the Fe₃O₄ nanoparticles exhibit spherical morphologies with an average diameter of 20 nm. The TEM image of Fig. S1b† shows that the obtained Fe₃O₄@SiO₂ MNPs at an average diameter of 120 nm are with a dark magnetite core and a uniform gray silica shell which provides abundant silanol groups for further chemical modification. The Fe₃O₄@SiO₂@Nap (Fig. S1c†) are larger than Fe₃O₄@SiO₂ in size, due to the modification of naphthyl.

The magnetic properties of the prepared MNPs were investigated with a vibrating sample magnetometer (VSM). Fig. 2a shows the magnetization curves of Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂@Nap at 300 K, giving the magnetic saturation values of 71.6, 30.73 and 27.56 emu g⁻¹, respectively. The coating results in decreases in the magnetic saturation values of Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂@Nap. It was reported that a saturation magnetization of 16.3 emu g⁻¹ is sufficient for a magnetic separation with a magnet.³² Thus, the Fe₃O₄@SiO₂@Nap sorbents loaded with analytes can be readily separated from solution with magnet due to their superparamagnetism and large saturation magnetization.

Fig. 2 Magnetization curves of Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂@Nap (a), FTIR spectroscopy of Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂@Nap (b), detail and deconvoluted XPS spectra of C 1s, N 1s, O 1s and Si 2p for Fe₃O₄@SiO₂@Nap (c).

FTIR analysis was employed to confirm the surface groups of the as-synthesized MNPs. FTIR spectra depicted in Fig. 2b are acquired for bare Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂@Nap MNPs between 4000 and 400 cm⁻¹. Two characteristic absorption peaks at 3400 and 590 cm⁻¹ are assigned to the stretching vibrations of hydroxyl groups of the hydrogen-bonded surface water molecules and the Fe–O–Fe transverse vibration of Fe₃O₄ NPs, respectively. Coating of SiO₂ onto Fe₃O₄ NPs was demonstrated by the appearance of characteristic peaks at 1090 and 466 cm⁻¹ (spectrum b and c), corresponding to the Si–O–Si stretching vibration and Si–O symmetric stretching vibration, respectively. The weak bands at 2850 and 2929 cm⁻¹ are assigned to the stretching vibrations of C–H bonds. After modification with naphthyl, there displays a prominent peak at 806 cm⁻¹, which is characteristic of –H on naphthalene ring. In addition, a band at 1540 cm⁻¹ corresponds to the bending vibration of skeleton of aromatic ring appears. These results confirmed the success surface modification of the magnetic nanoparticles.

To determine the chemical composition and to further affirm the modification on the surface of Fe₃O₄@SiO₂@Nap MNPs, the X-ray photoelectron spectroscopy analysis is employed. The C 1s, N 1s, O 1s and Si 2p deconvolution XPS spectra for Fe₃O₄@SiO₂@Nap are analyzed by curve fitting. As seen in Fig. 2c, the C 1s deconvolution spectra exhibit four components of the carbon bond at 284.4 eV (C=C/C–H), 284.6 eV (C–C/C–H), 285.4 eV (C–C aromatic) and 288.6 eV (C=O). The N 1s deconvolution spectra exhibit the nitrogen bond at 399.5 eV (N–C) and 399.8 eV (N–H). The O 1s deconvolution spectra exhibit three components of the oxygen bond at 532.4 eV (O=C) and 532.7 eV (–C=O/SiO₂), and Si 2p deconvolution spectra also exhibit SiO₂ (103.1 and 103.7 eV). In addition, the atomic ratio of the surface of Fe₃O₄@SiO₂@Nap is obtained from XPS data, and C/N/O/Si ratio is 20.53/3.3/51.1/24.67. The ratio of O–Si is approximately 2 : 1, which conforms to the construction of Fe₃O₄@SiO₂@Nap. But there may be some non-bonded SiO₂ on the surface of Fe₃O₄@SiO₂@Nap because of high ratio of Si/N. The number of carbon atoms may be associated with alkyl carbon and carbon atoms on naphthalene ring.

3.2. Optimization of extraction conditions

Several parameters that may affect the extraction efficiency were optimized, such as the sorbent amount, types of desorption solvent, solution volume and extraction time. The influence of all these parameters was evaluated in terms of recovery rate. The optimization experiments were conducted using pure water spiked with Flu, Ant, Pyr, BbF, BkF (10 ng L⁻¹), Fla (20 ng L⁻¹) and BaA (5 ng L⁻¹). Each experiment was performed in triplicate.

3.2.1. Effect of the sorbent amount. The sorbent amount is a key parameter affecting the extraction efficiency, which was investigated with various amounts of Fe₃O₄@SiO₂@Nap MNPs under the conditions of 150 mL sample volume, 1.0 mL acetonitrile as desorption solvent, and 12 min extraction time. Fig. 3a shows that the recovery rates of all the tested PAHs increase continuously with the increase of the sorbent amount from 10 to 40 mg. Further increasing the sorbent amount over 40 mg results in no obvious change in the recovery. These results indicate that 40 mg of sorbents are sufficient to extract PAHs. Excess sorbent may retain analytes resulting in the decrease in recovery. So, 40 mg of sorbent was used for the following experiments. Apparently, the required amount of Fe₃O₄@SiO₂@Nap in this work is far less than the amount of traditional SPE (C-18) sorbents.^33–35

Fig. 3 Effect of (a) amount of sorbent, (b) different desorption solvent, (c) the sample volume, (d) extraction time on the extraction recoveries of PAHs.

3.2.2. Effect of the desorption solvent. A complete desorption of analytes from the sorbent is highly related to the organic solvent. Five types of solvents were selected as desorption solvent, including acetonitrile, methanol, acetonitrile–isopropanol (v/v 1/1), acetone, and n-hexane. While other conditions were as follows: amount of sorbent, 40 mg; sample volume: 150 mL; and extraction time, 12 min. In order to achieve the best recoveries, Fe₃O₄@SiO₂@Nap nanoparticle sorbents were sonicated for 30 s in desorption solvents. As shown in Fig. 3b, acetonitrile yields the highest recovery for all of the tested PAHs. Hence, acetonitrile was used as the desorption solvent throughout the experiments.

3.2.3. Effect of sample volume. The sample volume from 10 to 250 mL was tested under conditions of 40 mg amount of sorbent, 1.0 mL acetonitrile as desorption solvent, and 12 min extraction time. As shown in Fig. 3c, the recovery rates of all the PAHs do not change significantly with increasing the sample volume from 10 to 150 mL. Further increasing the sample volume over 200 mL resulted in decreased in the recovery rates. The sample volume was therefore selected as 150 mL. The enrichment factors, defined as the ratio of the concentration of analytes in the final desorption solvent (1 mL acetonitrile) to the initial concentration of the analyte in the sample solution (150 mL), were determined to be 163, 141, 154, 139, 147, 145, and 149 for Flu, Ant, Fla, BaA, Pyr, BbF and BkF, respectively.

3.2.4. Effect of the extraction time. The recovery is also dependent on the extraction time which was evaluated within the range from 1 to 16 min under conditions of 40 mg sorbent, 150 mL sample volume, and 1.0 mL acetonitrile as the desorption solvent. As can be seen in Fig. 3d, 12 min was sufficient to achieve satisfactory extraction. Therefore, extraction time of 12 min was applied in the MSPE procedure.

Based on the above experimental results, the optimal conditions for MSPE of PAHs were as below: 40 mg of Fe₃O₄@SiO₂@Nap MNPs, 1.0 mL acetonitrile as the desorption solvent, 150 mL solution volume, and 12 min of the extraction time.

3.3. Investigation of the extraction mechanism

To prove that naphthyl played an important role on the extraction of PAHs, the extraction capacities of naked Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂@Nap were compared under the same conditions. The results are shown in Fig. 4. It can be seen that bare Fe₃O₄ has little enrichment ability towards PAHs, while Fe₃O₄@SiO₂ has better extraction capacity but recoveries of PAHs were all below 40%. Fe₃O₄@SiO₂@Nap shows the best extraction performance towards 7 PAHs, which are more hydrophobic. The condensed cyclic structure and hydrophobic property of naphthyl is expected to make it a good sorbent to interact with PAHs through the π–π conjugative effect and hydrophobic interaction, which would improve the enrichment abilities of the sorbent to PAHs. The results show that naphthyl on the surface of sorbent results in a significant improvement of extraction efficiency towards the PAHs.

Fig. 4 Comparison of different sorbents on the extraction efficiencies of PAHs.

3.4. Reusability of Fe₃O₄@SiO₂@Nap

In order to investigate the recycling of the nanoparticle sorbents, the used Fe₃O₄@SiO₂@Nap (40 mg) was regenerated by rinsing it with 1 mL of acetonitrile twice to make sure that no PAHs was remained in the sorbent. Then the regenerated sorbent was applied in MSPE. The recoveries of PAHs are listed in Fig. S2.† After 10 times of regeneration, there are no obvious changes in the recoveries of analytes with the used Fe₃O₄@SiO₂@Nap as sorbent, indicating that the Fe₃O₄@SiO₂@Nap sorbents are stable and durable during MSPE procedure.

3.5. Analytical characteristics

Under the optimized conditions, a series of experiments with regard to the linearity, limit of detection (LOD), and precision were performed to validate the proposed method. In order to investigate the possible matrix effect on determination, the linearity of the proposed method was estimated by analyzing different standard solutions with various concentration of PAHs (0.5, 5, 10, 25, 50, 100 ng L⁻¹) in Ganjiang river water sample. Six-point calibration curve was constructed by plotting peak area vs. PAH concentrations. Intra-day and inter-day precision were calculated in terms of RSD% (five replicates) obtained with real water sample spiked with 5 ng L⁻¹ PAHs.

The achieved results of the validation procedure are listed in Table 1. The calibration curves were linear in the range of 0.5–100 ng L⁻¹ with coefficient of determination ranging from 0.9983 to 0.9997. The limit of detection (LOD) and the limit of quantification (LOQ) were calculated as the concentrations of the analytes at a signal-to-noise ratio (S/N) of 3 and 10, respectively. Our results show that the LOD and LOQ of the PAHs range from 0.04 to 0.12 ng L⁻¹ and 0.15 to 0.40 ng L⁻¹, respectively. The relative standard deviations (RSDs) for the PAHs were below 4.3%, illustrating a good repeatability. These results imply that the proposed method can be applied to the analysis of real samples containing PAHs at trace level.

Table 1 Figures and merit of the MSPE method for the determination of PAHs in water

PAHs	Calib. curve^a (n = 3)	Linear range (ng L^−1)	R^2	LOQ (ng L^−1)	LOD (ng L^−1)	RSD (n = 5) (%)
						Intra-day	Inter-day
a x is compound concentration (ng L^−1) and y is peak area.
Flu	y = 4100.62x + 276.88	0.5–100	0.9992	0.20	0.064	0.6	1.1
Ant	y = 8500.8x + 686.88	0.5–100	0.9983	0.15	0.044	0.7	2.0
FlA	y = 500.8959x + 73.842	0.5–100	0.9991	0.25	0.081	0.8	1.6
Pyr	y = 3900.216x + 206.45	0.5–100	0.9995	0.38	0.120	2.3	4.3
BaA	y = 4500.824x + 271.02	0.5–100	0.9997	0.40	0.095	1.1	2.0
BbF	y = 800.7977x + 86.625	0.5–100	0.9994	0.16	0.048	0.8	1.3
BKF	y = 6200.638x + 266.95	0.5–100	0.9993	0.26	0.085	1.2	3.0

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3.6. Analysis of river water samples

Three kinds of river water sampled from Yudai River, Qingshan Lake and Ganjiang River were analyzed under optimized conditions. Aliquots of 150 mL of each sample were filtered through a 0.45 μm cellulose membrane, and then spiked with the PAHs at three concentration levels (0.5, 5 and 10 ng L⁻¹). The spiked samples were stored in dark overnight, and then analyzed by the proposed method (n = 3). Fig. 5 shows typical chromatograms of MPSE of river water samples and river water samples spiked with PAHs. The results are listed in Table 2. Anthracene and fluoranthene were found in both Yudai river water samples and Qingshan Lake water samples. At the same time, anthracene and pyrene were found in Ganjiang water samples. The relative recoveries of PAHs at three concentration levels are in the range of 89.6–106.8%, with RSDs within 5.9%. These results imply that the established method can be applied to the analysis of PAHs at trace level in real samples.

Fig. 5 Magnetic solid-phase extraction HPLC-FLD chromatograms of river water samples. (a) Qingshan Lake river water sample spiked with 5 ng L⁻¹ of each analyte; (b) Ganjiang river water sample; (c) Qingshan Lake river water sample and (d) Yudai river water sample. Peak assignment: (1) Flu, (2) Ant, (3) Fla, (4) Pyr, (5) BaA, (6) BbF, (7) BkF.

Table 2 Results of determination and recoveries of river water samples by MSPE

Analytes	Yudai river water samples				Qingshan Lake water samples				Ganjiang water samples
	Found (ng L^−1)	Added (ng L^−1)	Recovery (%)	RSDs (%, n = 3)	Found (ng L^−1)	Added (ng L^−1)	Recovery (%)	RSD (%, n = 3)	Found (ng L^−1)	Added (ng L^−1)	Recovery (%)	RSDs (%, n = 3)
a N.D.: not detected.b N.Q.: found but can't be quantified.
Flu	N.D.^a	0.5	95.8	2.3	N.D.	0.5	98.1	3.6	N.D.	0.5	92.6	3.8
		5	104.0	1.8		5	104.4	4.7		5	104.4	2.6
		10	92.7	2.9		10	89.9	3.9		10	93.6	4.5
Ant	0.60 ± 0.05	0.5	104.3	4.9	0.73 ± 0.05	0.5	94.8	4.7	0.77 ± 0.05	0.5	105.8	4.2
		5	101.2	3.0		5	101.4	3.3		5	95.2	3.4
		10	95.0	3.4		10	90.5	5.3		10	92.6	2.7
Fla	1.49 ± 0.05	0.5	101.3	4.5	0.86 ± 0.05	0.5	102.4	3.2	N.D.	0.5	103.6	3.8
		5	106.5	5.2		5	103.4	4.2		5	106.6	2.2
		10	94.7	2.8		10	96.8	2.5		10	95.5	3.2
Pyr	N.D.	0.5	106.7	4.8	N.D.	0.5	106.6	2.5	N.Q.^b	0.5	103.9	4.4
		5	103.9	3.6		5	104.7	5.3		5	105.6	2.9
		10	93.5	3.9		10	95.6	2.7		10	101.5	3.5
BaA	N.D.	0.5	105.7	5.8	N.D.	0.5	104.6	3.2	N.D.	0.5	104.8	5.0
		5	102.5	2.6		5	106.8	4.4		5	102.7	3.2
		10	93.8	4.9		10	94.0	2.9		10	90.4	5.2
BbF	N.D.	0.5	105.8	5.9	N.D.	0.5	106.3	4.1	N.D.	0.5	105.2	3.5
		5	94.8	3.8		5	102.8	5.2		5	94.4	4.3
		10	96.7	3.2		10	95.6	4.9		10	91.4	2.6
BkF	N.D.	0.5	93.8	4.5	N.D.	0.5	106.7	4.1	N.D.	0.5	95.3	3.6
		5	102.7	3.8		5	95.6	3.3		5	93.5	2.8
		10	96.1	4.0		10	89.6	4.7		10	104.6	4.9

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3.7. Comparison of Fe₃O₄@SiO₂@Nap with other sorbents

The extraction efficiencies of Fe₃O₄@SiO₂@Nap to target PAHs were compared with other magnetic materials including Fe₃O₄/C,^16,17 C₁₀–C₁₈ carboxylates,¹⁸ OPA/MMNPs,¹⁴ Fe₃O₄–C₁₈,^19–21 Fe₃O₄-MPNP,³ Fe₃O₄/PC,²³ Fe₃O₄/PDA,²⁵ Fe₃O₄@SiO₂-G,²⁶ MIL-101,²⁷ Fe₃O₄/SiO₂/TPA,²⁸ IL-MNPs²⁹ and nylon 6 (ref. 30) reported in literatures. The sorbent amount, loading volume, LODs, RSDs and recoveries obtained with different materials are listed in Table 3. The proposed sorbent shows similar extraction efficiency to other reported sorbents.^{23,26–28,30} The LODs of the proposed method were comparable with those methods^{16,18,20,23,25,26,28} that HPLC-FLD was also used. Besides the sample volume, less amount of Fe₃O₄@SiO₂@Nap sorbent was needed in comparison with some other MNPs.^{3,14,16,18–20,23,28} But in comparison with these sorbents: Fe₃O₄/C,¹⁷ Fe₃O₄/SiO₂/SiO₂–C₁₈,²¹ Fe₃O₄/PDA,²⁵ Fe₃O₄@SiO₂-G,²⁶ MIL-101 (ref. 27) and IL-MNPs,²⁹ more amount of Fe₃O₄@SiO₂@Nap was needed. In addition, the abundant π electrons of naphthyl provide potent π–π stacking interactions with PAHs, contributing to the selectivity to PAHs. The adsorption equilibrium in the process of extraction can be quickly achieved due to the good hydrophilicity of Fe₃O₄@SiO₂@Nap. Shorter extraction time was needed than several MSPE methods.^{5,17,18,20,21,27,28,30} Considering these results, the proposed sorbent is a sensitive, efficient, convenient and reliable material for the pre-concentration of trace PAHs.

Table 3 Comparison of the analytical performance of the proposed MNPs with other magnetic nanomaterials^a

Sorbent	Amount of sorbent (mg)	Extraction time (min)	Loading volume (mL)	Method	LODs (ng L^−1)	RSDs (%)	Recoveries (%)	Enrichment factor	Ref.
a n.r.: not reported.
Fe3O4/C	50	Very short time	1000	HPLC-FLD	0.2–0.6	0.8–9.7	76–110	n.r.	16
Fe3O4/C	10	30	20	GC-MS	15–335	3.6–9.3	n.r.	35–133	17
C10–C18 carboxylates	200	18	350	HPLC-FLD	0.1–0.25	1–7	85–94	n.r.	18
OPA/MMNPs	50	1	10	GC-MS	14.1–70.0 × 10^3	1.2–11.7	61.9–119.1	n.r.	14
Magnetic C18	50	6	20	GC-MS	0.8–36 × 10^3	2.0–10	35–99	n.r.	19
Fe3O4@C18	100	30	500	HPLC-FLD	2–5	1–8	72–108	n.r.	20
Fe3O4/SiO2/SiO2–C18	30	20	500	HPLC-FLD	n.r.	n.r.	>60%	n.r.	21
Fe3O4/MPNP	200	15	200	UHPLC-DAD	10.83–18.53 nM	0.3–8.2	75.7–106.4	157–186	3
Fe3O4/PC	100	10	500	HPLC-FLD	0.2–0.6	1–8	89–115	n.r.	23
Fe3O4/PDA	20	5	500	HPLC-FLD	0.5–0.9	1–9.7	76.4–107	n.r.	25
Fe3O4@SiO2-G	15	5	250	HPLC-FLD	0.5–5	2.8–5.6	83.2–108.2	137–173	26
MIL-101	1.6	20	20	HPLC-PDA	2.8–27.2	3.1–8.7	81.3–105	101–180	27
Fe3O4/SiO2/TPA	50	15	200	HPLC-FLD	0.04–37.5	<10	80–108.33	n.r.	28
IL-MNPs	30	8	100	GC-MS	0.04–1.11 × 10^3	4.0–8.9	75–102	49–158	29
Nylon 6	40	30	25	HPLC-PDA	0.05–0.58 × 10^3	3.8–6.8	80–110	18.1–43.5	30
Fe3O4@SiO2@Nap	40	12	150	HPLC-FLD	0.04–0.12	0.6–3.0	89.6–106.8	139–163	This work

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4. Conclusions

Naphthyl functionalized magnetic nanoparticles were successfully synthesized as a novel sorbent for the enrichment of PAHs from river water samples. Due to the condensed cyclic structure and hydrophobic property of naphthyl, the Fe₃O₄@SiO₂@Nap magnetic nanoparticles display satisfying extraction efficiency. Compared to other magnetic materials reported in recent years, Fe₃O₄@SiO₂@Nap magnetic nanoparticles have some advantages. In the analysis of seven kinds of PAHs in river water samples, the Fe₃O₄@SiO₂@Nap magnetic sorbents showed reliable analytical performance.

Acknowledgements

We gratefully acknowledge the National Science Foundation of China (grant 21175038, 21235002), and the National Basic Research Program of China (Grants no. 2009CB421601) for financial support.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra10054b

‡ These authors contributed equally to this study and share first authorship.

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