Magnetic Fe3O4@C nanoparticles modified with 1-(2

Microchim Acta DOI 10.1007/s00604-014-1327-1

ORIGINAL PAPER

Magnetic Fe3O4@C nanoparticles modified with 1-(2-thiazolylazo)-2-naphthol as a novel solid-phase extraction sorbent for preconcentration of copper (II) Azam Samadi & Mohammad Amjadi

Received: 19 April 2014 / Accepted: 10 July 2014 # Springer-Verlag Wien 2014

Abstract We report on a new magnetic nanosorbent for solid phase extraction of Cu(II) before its determination by flame atomic absorption spectrometry. The magnetic sorbent is composed of carbon-coated magnetite nanoparticles (Fe3O4@C) synthesized by a single-step solvothermal reaction and then loaded with the chelator 1-(2-thiazolylazo)-2-naphthol. It was used for the preconcentration of Cu(II) ions from water and food samples. The effects of pH value and volume of sample, of type and volume of eluent, and of interfering ions were investigated. Under the optimum conditions, the calibration graph is linear in the 4.0–400 μg L−1 concentration range, with a detection limit of 1.5 μg L−1. The method was validated by using a certified reference material (NIST 1566b; oyster tissue) and applied to the determination of trace copper in spiked water and food samples. Keywords Fe3O4@C nanoparticles . Magnetic solid phase extraction . 1-(2-thiazolylazo)-2-naphthol . Copper

Introduction Copper is an essential element for human life at low concentrations, but, it can be toxic at higher levels [1]. Despite the sensitivity and selectivity of analytical techniques such as FAAS, there is a great necessity for separation and the preconcentration of trace metals from complex matrices,

Electronic supplementary material The online version of this article (doi:10.1007/s00604-014-1327-1) contains supplementary material, which is available to authorized users. A. Samadi : M. Amjadi (*) Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 5166616471, Iran e-mail: [email protected]

basically due to their low concentrations or matrix interferences. Among the preconcentration techniques, solid-phase extraction (SPE) have been extensively used for separation and determination of trace elements [2]. A lot of researches have been oriented towards the development of SPE to make it more efficient, economical and miniaturized sample preparation method. Nowadays, magnetic solid-phase extraction (MSPE) as a new mode of SPE, has gained considerable attention in trace analysis [3–5]. Compared with traditional sorbents, a distinct advantage of MSPE is that the magnetic sorbents can be readily isolated from sample solution by the application of an external magnetic field without the need for column passing operations and additional centrifugation or filtration procedures. The carbonaceous materials are effective sorbents for uptake of metal ions as well as their complexes from aqueous solutions [6–8]. Activated carbon as long been identified as an efficient sorbent for various species and has been commonly used for preconcentration of metal ions [9–12]. Metal retention on activated carbon could be improved by using complexing agents in the sorption process [13, 14]. Carbon nanotubes (CNTs) are relatively new carbon-based sorbents with a large specific surface area and small, hollow, and layered structures and have proved to possess great potential as sorbents for many divalent metal ions [15–19]. Magnetic Fe3O4@C nanoparticles (FCNPs) are a new class of carbonbased nanosorbents that have been synthesized by different methods and used as sorbents for some non-polar or slightly polar substances such as polycyclic aromatic hydrocarbons [20], organophosphorus pesticides [21], organic dyes [22], bisphenol A [23] and phenolic compounds [24]. FCNPs have been also examined for efficient removal and separation of some metal ions from aqueous solutions [25–28]. This paper reports the use of magnetic solid phase extractant, 1-(2-thiazolylazo)-2-naphthol modified Fe 3 O 4 @C

A. Samadi, M. Amjadi

nanoparticles (TAN-FCNPs), for the preconcentration of copper ions in water and food samples. FCNPs were synthesized by a facile one-step solvothermal method and then easily modified by a complexing agent. The resulting product was an efficient and selective nanosorbent for copper ion.

Experimental Instruments A Shimadzu Model AA-670G atomic absorption spectrometer (Kyoto, Japan; www.shimadzu.com) was used for the determination of copper in the following conditions: wavelength, 324.8 nm; lamp current, 3.0 mA; slit width, 0. 5 nm; burner height, 6.0 mm; acetylene flow, 1.8 L min−1; air flow,8.0 L min−1. A Metrohm model 654 pH meter was used for pH measurements. An ultrasonic bath (FALC, Italy; www. falcinstruments.it) was used for agitating the solutions in the modification and extraction processes. A strong magnet was applied for the magnetic separation. The size and structure of FCNPs were characterized by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) (Mira 3 FEG, Tescan Co., Czech Republic; www.tescan.com). The success of forming FCNPs and coating of TAN on them was proved by Fourier-transform infrared (FT-IR) spectra (Tensor-27, Bruker Co., Germany; www.bruker.com). The magnetic properties of nanoparticles were characterized by means of a vibrating sample magnetometer (Meghnatis Kavir Co., Kashan, Iran) at room temperature. Zeta potentials were measured by a Nanotrac zeta potential analyzer (Microtrac Inc., USA).

Reagents and solutions Doubly distilled de-ionized water obtained from Ghazi Serum Co. (Tabriz, Iran) was used for the preparation of all the solutions. FeCl3.6H2O, CO(NH2)2, glucose and ethylene glycol were prepared from Merck (Darmstadt, Germany;www. merck-chemicals.com). Stock standard solution of copper (1,000 mg L−1) was obtained from Merck. The working solutions were prepared daily from the stock solution by stepwise dilution with water. A 1.0 M nitric acid solution was prepared by appropriate dilution of concentrated HNO3 (Merck). Acetate buffer solution (0.1 M, pH=4.0) was prepared by dissolving appropriate amount of sodium acetate (Merck) in 100 mL of deionized water and adjusting the pH with HCl. A solution of TAN was prepared by dissolving appropriate amount of 1-(2thiazolylazo)-2-naphthol (Fluka, Tokyo, Japan; www. sigmaaldrich.com) in acetone.

Preparation of sorbent Fe3O4@C nanoparticles were synthesized according to the literature [29]. Briefly, 2.5 mmol of FeCl3.6H2O was dissolved in 30 mL of ethylene glycol to form a clear solution. Under vigorous stirring, 25 mmol of CO(NH2)2 and 0.5 mmol of glucose were added to the solution. After the mixture was vigorously stirred for 30 min, it was transferred into a Teflonlined stainless steel autoclave and heated at 200 °C for 12 h. Afterwards, the solid products were washed with deionized water and ethanol and dried at 60 °C for 6 h. Ligand loaded Fe3O4@C nanoparticles were prepared by addition of ligand solution (0.02 g TAN in 20 mL acetone) to the FCNPs (1.0 g). The resulting suspension was sonicated in a closed vessel for 1.0 h, then the vessel was opened and sonication continued until the solvent was completely evaporated. Modified FCNPs were washed with deionized water and finally dried in an oven at 60 °C. Characterization of the sorbent The morphology and particle size of the sorbent were studied using SEM image. SEM image in Fig. 1a demonstrates that the prepared Fe3O4@C nanoparticles are spherical with size distribution of 117±14 nm (Fig. 1b). The chemical composition of the nanoparticles was analyzed by EDX. The EDX spectrum (Fig. 1c) shows the iron, oxygen and carbon peaks. It reveals that the iron was from magnetic Fe3O4, and carbon was from glucose pyrolysis production. The synthesis of Fe3O4@C nanomaterial can also be proved by FT-IR spectrum in Fig. 2 (obtained by KBr pellet). It was reported that the characteristic absorption bands of the Fe-O vibrations of bulk Fe3O4 were at 570 and 375 cm−1 [30]. However, in Fig. 2 these bands have shifted to higher wavenumbers of about 600 and 440 cm−1 respectively. These changes can be explained by principal effect of finite size of nanoparticles. Nanoparticles due to the limited sizes, lead to the breaking of a large number of bonds for surface atoms and the enhancement of surface bond force constant so that the absorption bands of IR spectra shift to higher wavenumbers. Moreover, in Fig. 2, the band near 600 cm −1 has split into two peaks of 621.8 and 582.9 cm−1. This may be attributed to the split of the energy levels of the quantized Fe3O4 nanoparticles [31]. The peak at 1,620 cm−1 is attributed to C=C vibrations, which reflects the carbonization of glucose and formation of carbon on Fe3O4 surfaces. Finally, the broad band region at 3,100–3,700 cm−1 results from the stretching vibrations of O–H. FT-IR spectrum of TAN-FCNPs indicates the loading of TAN on Fe3O4@C nanoparticles (Fig. 2). Three bands at 1,452, 1,562 and 1,632 cm−1 in this spectrum are associated with the C=C stretching of aromatic rings of the ligand. Also, other differences between two spectra show up the presence of the ligand on FCNPs.

Modified magnetic Fe3O4@C nanoparticles as a sorbent for copper

Frequency

Fig. 1 a SEM image b particle size distribution and c EDX spectrum of the prepared FCNPs 8 7 6 5 4 3 2 1 0

90

100

110

120

130

140

150

Particles diameter (nm)

a

b Element

Wt%

At%

CK

41.66

60.28

OK

27.83

30.23

Fe K

30.51

9.49

c

SPE procedure First, an aliquot of 100 mL of aqueous sample or standard solution was transferred to a beaker and its pH was adjusted to 4.0 with 2 mL acetate buffer (0.1 M). Then, 0.05 g of TAN-

Fig. 2 IR spectra of FCNPs and TAN-FCNPs

FCNPs was added to the solution and placed in an ultrasonic bath and sonicated for 15 min at 25 °C. Afterwards, a strong magnet (with strength of~0.4 Tesla) was positioned at the bottom of the beaker, and the Fe3O4@C nanoparticles were isolated from the suspension (which takes about 7 min). The


preconcentrated target analyte was desorbed with 2 mL of HNO3 (1.0 M) in two 1-mL portions. The sorbent was sonicated for 1 min during each desorption process and nanoparticles were isolated by the magnet in a very short time (about 1 min). The final combined solution was analyzed by FAAS. Real sample preparation Oyster tissue 0.25 g of oyster tissue (NIST 1566b) was decomposed by heating with 30 mL of concentrated nitric acid (Suprapure, Merck) and 8 mL of 60 % perchloric acid (Merck) in a beaker. The solution was heated on a hot plate until dryness. Distilled water was added to the residue and the solution was filtered and diluted to 500 mL in a calibrated flask. 100 mL of this solution was taken through the general procedure. Water samples Two water samples including tap water and spring water were selected and the developed method was applied to determine their copper contents. Tap water was collected from our laboratory (University of Tabriz, Tabriz, Iran) and spring water was collected from environs of Tabriz, Iran. The water samples were filtered through a Millipore 0.45 μm pore-size membrane into polyethylene bottles. Their pH values were adjusted to 4.0 by addition of acetate buffer and analyzed according to the general procedure. Food samples Food samples including black tea and mushroom were purchased from local supermarkets.1.0 g of ground tea sample was accurately weighed and digested on a hot plate with 20 mL of HNO3:HClO4 (1:4) mixture at 200 °C until dryness. The residue was dissolved in 5 mL of HNO3 (5 %v/v) and the clear sample solution was diluted to 250 mL in a calibrated flask. 100 mL of this solution was taken through the general procedure. The mushroom sample was dried at 105 °C for 24 h. 1.0 g of the resulting sample was placed into a porcelain crucible and ashed at 500 °C in a furnace for approximately 4 h until a white or grey ash residue was obtained. The residue was dissolved in 4 mL of HNO3 and 8 mL of HClO4 and heated on a hot plate at 190 °C for 1 h. The solution was then heated to dryness on a hot plate at 200 °C. The residue was dissolved in 5 mL of HNO3 (5 %v/v) and the clear sample solution was transferred to 250 ml volumetric flask and made up to volume. 100 mL of this solution was taken through the general procedure.

Results and discussion Carbon materials have been used as powerful sorbents for solid phase extraction of non-polar compounds and also metal ions for many years. Nature of a carbon material surface determines the mechanism of adsorption. The non-polar or slightly polar substances in aqueous solution can be simply adsorbed on the hydrophobic carbon materials but metal ions usually need to be transformed to the corresponding chelates. Metal chelates could provide higher selectivity and high enrichment factor for such separation and preconcentration techniques. The adsorption of chelating agents on the carbonbased sorbents may be justified by two mechanism; π-π bonding [32] and the electron donor process [33]. The electron donor theory suggests that an exchange of electrons takes place during the adsorption process, whereas in π-π bonding dispersive interactions are dominant. In this work, magnetic Fe3O4@C nanoparticles were modified by 1-(2-thiazolylazo)-2-naphthol, which is an azo dye. Azo dyes are important class of organic colorants consist of at least one conjugated azo group (−N=N-). TAN is associated with two aromatic and one heterocyclic ring [34]. Therefore, the aromatic moiety of TAN can be adsorbed on FCNPs via two above-mentioned mechanisms. It should be mentioned that the prepared nanoparticles are stable for at least 6 months and the preparation procedure is quite reproducible and yields the particles with same sorption characteristics. The magnetic properties of Fe3O4@C nanoparticles were characterized by room-temperature magnetization hysteresis loops. As shown in Fig. 3, the maximum saturation magnetization value was 65.0 emu·g−1. Furthermore, the Fe3O4@C nanoparticles exhibited typical superparamagnetic behaviors with no coercivity and remanence. Therefore, they are suitable for the use as sorbent materials in MSPE. As mentioned in experimental section, after being exposed to a magnet with strength of about 0.4 Tesla, all nanoparticles in 100 mL solution can be isolated from the dispersion in about 7 min. The zeta potential of Fe3O4@C nanoparticles was also studied in various conditions. It was found that in acidic solution the zeta potential is positive while in neutral solution the potential becomes negative. The isoelectric point of the nanoparticles was found to be ~4.5. Optimization of preconcentration conditions Effect of pH The effect of pH on the recovery of 5 μg of Cu(II) present in 100 mL solution was studied in the range of 3.0–9.0. The results, shown in Table S1 (Electronic Supplementary Material, ESM), revealed that a quantitative recovery occurs at the pH range of 4.0–7.0. The low recovery of Cu(II) at pHs >7.0 is due to desorption of TAN from sorbent surface. Since

Modified magnetic Fe3O4@C nanoparticles as a sorbent for copper 80

Desorption conditions

60

Moment (emu/g)

40 20 0 -20 -40 -60 -80 -10

-8

-6

-4

-2

0

2

4

6

8

10

Applied field (KOe)

Fig. 3 Magnetization curve of Fe3O4@C nanoparticles at roomtemperature

most of the digested real samples are acidic and also, since at lower pH values there is probably less interference, in all subsequent works, the pH was adjusted at 4.0 by addition of 2 mL of 0.1 M of acetate buffer. Effect of concentration of TAN solution and amount of sorbent The percentage of TAN in TAN-acetone solution used for modification of FCNP was varied from 0.1 to 5.0 % (w/v). The recovery of 5.0 μg of Cu(II) in 100 mL of solution was quantitative when the percentage of TAN was greater than 2.0 % (Table S1, ESM). Hence, 2 % TAN solution was used for modification of FCNPs. Furthermore, a minimum of 50 mg of modified Fe3O4@C nanoparticles is required for quantitative preconcentration of copper. Considering the fact that sorption capacity of modified Fe3O4@C was about 1,500 μg g−1, 50 mg of sorbent would be sufficient for entire range of calibration (up to 0.4 mg L−1, as discussed below). Effect of sonication time The time of sonication was varied from 5 to 20 min during recovery of 5 μg of Cu(II) in 100 mL of solution by using modified Fe3O4@C nanoparticles. The obtained results have been shown in Table S1 (ESM) from which it is clear that a 15min stirring time was sufficient for quantitative recovery of Cu(II) by TAN-FCNPs.

Elution of Cu(II) from TAN-FCNPs was performed by using HCl and HNO3 as eluent. The obtained results showed that HNO3 is better eluent because of better reproducibility and higher recovery. Hence, the effect of HNO3 volume on the recovery of analyte was studied. It was found that satisfactory recoveries were obtained by 2× 1 mL of 1.0 M HNO3 (two times washing, each with 1 mL of HNO3). In order to achieve complete desorption of analyte, sorbent were sonicated for 60 s during each desorption process. It should be mentioned that coating the Fe3O4 nanoparticles with carbon can protect them from being dissolved in acid solution [35]. So in our experiments, FCNPs were not dissolved and can be reused for further two times after loading with TAN. Selectivity study The solid phase extraction of Cu(II) in presence of different amounts of potentially interfering ions was investigated and the results are shown in Table 1. An ion was considered to interfere when its presence produced a variation of more than 5 % in the absorbance of the sample. It is clear that none of the tested species interfere with the determination of Cu(II). Thus, TAN-FCNPs selectively separates Cu(II) from several alkali, alkaline earth, transition and heavy metal ions. Analytical figures of merit Calibration graphs were obtained both with and without preconcentration. While the linear range without preconcentration was 0.5–30 mg L−1, the calibration graph after preconcentration by using the developed method was linear in the range of 4.0–400 μg L−1 with a correlation coefficient of 0.9994 (n=8). Considering the volumes of sample and eluent, the maximum pre-concentration factor for this method is 50. The detection limit according to the definition of IUPAC (3Sb/b, where Sb is the standard deviation of blank and b is the slope of calibration graph) is 1.5 μg L−1. A study of precision was

Table 1 Effect of potentially interfering ions on the recovery of copper (50 ng mL−1) Interfering ion

Tolerance limit (interfering to analyte mass ratio)

Na+, K+, Ni2+, Pb2+, Al3+, As(V), Cr(VI), NO3− Ca2+, Mg2+ − − Cl ,F ,SO42−, Mo(VI), Co2+, Mn2+ Cr3+, Fe2+, Ba2+ Fe3+, Zn2+, Cd2+, V(V)

1000 a

The effect of sample volume The effect of sample volume on the recovery of 5 μg of Cu(II) with TAN-FCNPs was studied in the range 50–500 mL. As seen from Table S1 (ESM), the recovery decreases when the volume was higher than 100 mL. Therefore, in order to obtain high recovery, 100 mL was considered to be the maximum sample volume.

a

Highest mass ratio tested

750 500 250

A. Samadi, M. Amjadi Table 2 Comparison of the performances of TAN-FCNPs with some other sorbents for copper Solid support

Ligand

DL1 (μg L−1)

LDR (μg L−1)

PF2

Sample Volume (mL)

Approximate Extraction time (min)

Ref

Alumina nanoparticles Silica gel Activated carbon MCNTs MCNTs MCNTs MCNTs Fe3O4@C nanoparticles

APDC3 gallic acid BSPDI4 CPHC5 PIDA6 D2EHPA-TOPO7 TAN

2.5 0.86 0.27 1.64 0.15 1.46 50 1.5

400–2600 20–800 4.0–400

25 200 175 40 100 60 25 50

50 2000 1750 400 300 600 250 100

104 16 h 14 h 103 75 305 130 26

[36] [37] [38] [17] [39] [18] [40] This work

1 2

detection limit preconcentration factor

3

ammonium pyrrolidindithiocarbamate

4

bissalicyl aldehyde, 1,3propandiimine

5

o-cresolphthaleincomplexone

6

pheyliminodiacetic acid

7

di-(2-ethyl hexyl phosphoric acid) - tri-noctyl phosphine oxide

performed by carrying out five independent measurements of solutions of Cu(II) at 50 μg L−1 level and gave a relative standard deviation of 2.6 %. The characteristics of the developed SPE procedure compared with some reported methods are shown in Table 2. As can be seen, our method is comparable with most of other methods in terms of detection limit and linear range. However, due to the magnetic feature of TAN-FCNPs, SPE procedure is much faster and simpler than other methods since there is no need for column preparation, centrifugation or filtration process. The whole preconcentration process including sonication, isolation of FCNPs and two desorption steps is taken about 26 min, which is at least 3 times shorter than other procedures shown in Table 2. Table 3 Results obtained for copper determination in various water and food samples

Added a

Found b

Recovery (%)

Absolute value of t-statistics c

Black tea

0 10 100 0 10 100 0 10

17.8±0.9 27.8±0.15 116.6±1.2 38.2±1.0 48.3±0.6 136.7±1.0 6.3±0.8 16.1±1.1

100 98.8 101 98.3 98

0 1.73 0.28 2.6 0.31

100 0 10 100

106.2±0.9 2.8±0.5 12.6±1.0 103±2.3

99.9 98 102

0.19 0.34 0.15

a

b

Averages of three determinations±standard deviation

c Critical t-value at 95 %confidence level is 4.30

In order to verify the accuracy of the developed procedure, the method was first applied to the determination of copper in a standard reference material, NIST SRM 1566be Oyster Tissue with certified Cu content of 71.6±1.6 μg g−1. The obtained value for Cu by using our method was 72.5±2.2 μg g−1 (mean of three determinations±standard deviation) which is in good agreement with the certified concentration (t-statistic=0.71). It can be concluded that the method is accurate and free from systematic errors. The method was then applied to the determination of copper in water, black tea and mushroom samples. Table 3

Sample

Mushroom

The added and found values have μg L−1 unit in the case of water samples and μg g−1 unit in the case of food samples

Analytical applications

Tap water

spring water

Modified magnetic Fe3O4@C nanoparticles as a sorbent for copper

shows the obtained results. The recovery tests were also performed by spiking the samples with a known amount of copper before any pretreatment that proved that the developed procedure is not affected by matrix interferences and can be applied satisfactorily for natural food and water analysis.

Conclusions In this work magnetic Fe3O4@C nanoparticles, a new member of carbon nanomaterials, was applied for preconcentration of trace Cu(II) ions from water and food samples. FCNPs were synthesized by a one-step solvothermal reaction and loaded with TAN by a fast and simple procedure. Compared to other sorbents, TAN-FCNPs have some unique advantages such as good selectivity (see Table 2) and simplicity of modification. Furthermore, due to the magnetic feature of the sorbent, SPE procedure is much simpler and faster. The prepared nanosorbent may have general applicability for separation and/or preconcentration of traces of some other elements which form complexes with TAN. However, for each element, the corresponding optimized conditions will be needed.

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