Electrospun polyethylene terephthalate (PET) nanofibers as a new adsorbent for micro-solid phase extraction of chromium(VI) in environmental water samples

Abstract

In this study, a polyethylene terephthalate (PET) nanofiber film was fabricated using a simple and low-cost electrospinning method and used as a novel adsorbent for solid phase microextraction (SPME) of chromium(VI) in water samples. 1,5-Diphenylcarbazide (DPC) as a selective complexing agent for Cr(VI) and sodium dodecyl sulfate (SDS) as a surfactant were added to sample solutions to improve the selectivity and sensitivity of the method. The extraction procedure was coupled with UV-Vis spectrophotometry for the determination of preconcentrated Cr(VI). The influence of the main parameters that affect the recovery of Cr(VI), including desorption conditions, pH, adsorbent dosage, adsorption time, and the concentration of DPC and SDS, were studied and optimized. The analytical figures of merit of the method were: the preconcentration factor, 125; the linear dynamic range, 1.8–60 ng mL⁻¹; the determination coefficient (R²), 0.9923; the limit of detection (LOD), 0.6 ng mL⁻¹, and the limit of quantification (LOQ), 1.8 ng mL⁻¹. The relative standard deviations (RSDs%) for intra-day and inter-day assays were 1.6% and 3.1% (n = 3), respectively. The proposed method was successfully applied for the determination of Cr(VI) in natural water samples, and the relative recoveries in the range of 96.9–99.1% were obtained.

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Introduction

Heavy metals are ranked as highly toxic elements in the environment. They can enter a water supply via industrial and consumer wastes, or even from acidic rain breaking down soils and releasing heavy metals into the water. These metals tend to bioaccumulate in the food chain and exert various health effects on humans.^1–5 Chromium (Cr) is found in the environment primarily in two oxidation states: Cr(III) which occurs naturally and is an essential nutrient, and Cr(VI) that is toxic and most commonly produced by industrial processes. The body can detoxify some amount of Cr(VI) to Cr(III).^6,7 However, trace levels of Cr(VI) in drinking water up to 1 ng mL⁻¹ is the target of strict official health norms. Accordingly, the trace determination of Cr(VI) in natural water is essential in environmental pollution monitoring.^8–10 A method for the selective measurement of this metal is based on the spectrophotometric determination of Cr(VI) after reaction with 1,5-diphenylcarbazide (DPC).^11,12 However, because the concentration of Cr(VI) in environmental water samples is usually below the detection limit of this technique, direct quantification of this element seems to be problematic. Therefore, a sample preparation step before the analysis is required.

In recent years, the use of electrospun polymeric nanofibers for the adsorption of heavy metals has increased.^13–30 Polymeric nanofibers can be fabricated using a number of techniques such as drawing, template synthesis, phase separation, self-assembly, and electrospinning. Electrospinning is a remarkably simple method for producing nanofibers of a wide variety of polymers. This is a process that creates nanofibers through an electrically charged jet of polymer solution or polymer melt.³¹ The basis of this technique is similar to electrospray ionization.³² Recently, electrospun nanofibers have been used as adsorbents in solid phase extraction (SPE)^31–35 and micro-SPE.^36,37 Since the surface area per unit volume is inversely proportional to the diameter of the nanofibers, the smaller the diameter, the greater the surface area per unit volume.³⁸ Moreover, the porous structure of the nanofibers increases the hydrophilicity of the surface and provides more paths for the diffusion of the analyte, and thus facilitates better interactions between the adsorbent and the analyte(s).

In the present work, an electrospun polyethylene terephthalate (PET) nanofiber film was fabricated and introduced as a new type of polymeric adsorbent for the selective extraction of Cr(VI) in the presence of DPC and SDS. To the best of our knowledge, this is the first time that PET has been used for the thin film microextraction of chromium(VI).

Experimental

Materials and reagents

All chemicals used were of analytical grade. Ethanol (EtOH), potassium dichromate (K₂Cr₂O₇), acetone, sulfuric acid (H₂SO₄), sodium chloride (NaCl), iron(III) chloride hexahydrate, copper(III) nitrate, ammonium molybdate tetrahydrate, 1,5-diphenylcarbazide (DPC), mercury(II) chloride, sodium dodecyl sulfate (SDS), and polystyrene were purchased from Merck Chemicals (Darmstadt, Germany). Polyethylene terephthalate (PET) and methanol (MeOH) were obtained from Sigma Aldrich Ltd (St Louis, USA). Trifluoroacetic acid (TFA) (99%) was obtained from Samchun Pure Chemical (Pyeongtaek, South Korea). The stock standard solution of Cr(VI) (1000 μg mL⁻¹) was prepared by dissolving a weighed amount of K₂Cr₂O₇ in distilled water and stored at 4 °C. The working standard solutions of Cr(VI) were prepared daily by appropriately diluting the stock solution with distilled water. The DPC solutions were prepared daily in MeOH at the concentration level of 10⁻² mol L⁻¹.

Instrumentation

A double beam Rayleigh UV-2601 (Beijing, China) UV-Vis spectrophotometer, using a couple of 1 cm optical pathlength micro-cuvettes (Fisher Scientific, USA), was utilized for the spectrometric determination of the chromium(VI) complex. A Heidolph magnetic stirrer model MR 3001 K (Schwabach, Germany) was used for mixing the solutions. Eurosonic 4D (Euronda, Montecchio Precalcino (Vicenza) Italy) ultrasonic water was used for desorption process. The pH values were measured using a WTW Inolab 720 pH meter (Weilheim, Germany). The morphology of the electrospun nanofibers was characterized using scanning electron microscopy (SEM) (Zeiss DSM-960 Oberkochen, Germany) at an accelerating voltage of 15 kV. The infrared transmittance spectra were obtained using an Equinox 55 FT-IR spectrometer (Bruker, Bremen, Germany) in the 400–4000 cm⁻¹ region. The specific surface area and average pore diameter of the electrospun PET were measured using a Brunauer–Emmett–Teller (BET) surface area analyzer (BELCAT-A, Japan).

Nanofiber fabrication

The electrospinning set-up consisted of a direct current (DC) high voltage power supply, a syringe pump (SP 1000), and a collector fabricated by Fanavaran Nano-Meghyas (FNM, Tehran, Iran) was used to produce the nanofibers. The DC voltage supply had an electrical potential range of 0–25 kV. The feed rate of the polymer solution was regulated by using a programmable two channel syringe pump. The flow rate of the pump could be varied between 0.02 μL h⁻¹ and 0.064 mL h⁻¹. In this work, a 2 mL syringe (Soha, Alborz, Iran) with a needle diameter of 0.2 mm was used and the voltage (15 kV) was applied between the tip of the syringe and the ground collector. Electrospun fibers were collected using a metal collector, connected with aluminum foil placed at a distance of 10 cm from the tip of the syringe’s needle, at a flow rate of 0.3 mL h⁻¹. The polymer solution was prepared by dissolving appropriate amounts (0.18 g) of PET in 1 mL of TFA (18% w/v). The solutions were electrospun for two hours in all the experiments. After the electrospinning was completed, the aluminum foil was floated in MeOH to separate the adsorbent from the foil before use. The electrospinning set-up is illustrated in Fig. 1.

Fig. 1 Scheme of the electrospinning set-up.

The procedure

Firstly, 25 mL of a Cr(VI) solution (25 ng mL⁻¹) was placed in a beaker and 0.5 mL of sulfuric acid (0.5 mol L⁻¹) was added to it (pH 1.7). Then, 150 μL of a methanolic solution of DPC (0.01 mol L⁻¹) was added to the mixture and stirred for 5 min. After that, 5 mg of SDS (0.5 mL of aqueous solution, 1% w/v) and 5 mg of the adsorbent were added to the solution and stirred for 20 min. Thereafter, the adsorbent was removed from the solution and carefully transferred to a 0.5 mL safe-lock Eppendorf. Then, 200 μL of EtOH (eluent) was added to the adsorbent and ultrasonicated for 5 min. Finally, the absorbance of the solution was measured at 543 nm using a UV-Vis spectrophotometer.

Results and discussion

Characterization of the PET nanofibers

The SEM micrographs of the electrospun nanofibrous PET are shown in Fig. 2. The structure is composed of numerous, randomly oriented three-dimensional nonwoven nanofibers with uniform diameters in the range of 98–504 nm. Fig. 2 displays the FT-IR transmittance spectrum of the PET nanofibers. The strong band at 1727 cm⁻¹ is assigned to the stretching vibration of C=O. The peaks at 3068, 2960 and 730 cm⁻¹ are ascribed to the C–H aromatic and aliphatic vibrational stretching, and the out of plane C–H bending, respectively. Moreover, the bands at 1112 and 1260 cm⁻¹ are due to the C–O stretches. The specific surface area and average pore diameter of the electrospun PET were measured using a Brunauer–Emmett–Teller (BET) surface area analyzer and found to be equal to 1.67 m² g⁻¹ and 11.8 nm, respectively.

Fig. 2 The SEM micrographs of the electrospun polyethylene terephthalate (PET) (a) with magnification of 1000, (b) with magnification of 5000; and (c) the FT-IR spectrum.

Effect of the desorption conditions

After the adsorption of the Cr(VI)–DPC complex onto the PET nanofibers was completed, an appropriate solvent was required for the elution of the complex from the surface of the nanofibers prior to spectrophotometric analysis. The recovery percentage of Cr(VI)–DPC, which is calculated using eqn (1), is highly dependent on the desorption solvent type and volume. Therefore, a suitable solvent should be selected that dissolves the adsorbed Cr(VI)–DPC well.where C_f is the concentration of the analyte in the desorption solvent, C_i is the concentration of the analyte in the initial sample solution, and V_i and V_f are the volume of the initial sample solution and the volume of the desorption solvent, respectively. Regarding the polarity of DPC,³⁹ solvents such as acetone, MeOH, and EtOH were tested as desorption solvents with the proposed procedure. According to the results, the following order of elution power was achieved: acetone > EtOH > MeOH. Although the highest extraction efficiency among these solvents was obtained using acetone, because it is highly evaporative, EtOH with a much lower volatility and toxicity was preferred to be used as the desorption solvent for subsequent experiments (Fig. 3a). In the next step, the effect of the eluent (EtOH) volume was investigated in the 100–350 μL range. The maximum efficiency was obtained using 200 μL of EtOH with the standard deviation of 0.0035 (n = 2) (Fig. 3b). Thus, 200 μL of EtOH was adopted for further experiments.

Fig. 3 Effect of: (a) the desorption solvent type; (b) volume, and time. Conditions: DPC, 6 × 10⁻⁵ mol L⁻¹; SDS, 5 mg; pH, 1.7; adsorbent, 5 mg; adsorption time, 20 min.

After adding the optimized volume of the desorption solvent to the adsorbent, it was subjected to ultrasound for an appropriate time to speed up the desorption of Cr(VI)–DPC. Since the recovery of the analyte is greatly dependent on the desorption time (ultrasonication time), the effect of this parameter was studied in the range of 1–15 min according to the proposed procedure. Fig. 3b shows that the maximum efficiency was obtained with 5 min of ultrasonication and thus it was chosen as the optimum desorption time.

Effect of pH

The complexation reaction between Cr(VI) and DPC occurs in strongly acidic media, and thus is a pH-dependent process. The influence of the pH of the sample solution on the performance of the method was studied in the range of 0.9–2.1. At higher pH values, the reaction is slow.⁴⁰ The pH was adjusted using H₂SO₄ (0.5 mol L⁻¹) and the experiments were carried out with the procedure in Section 2.4. Fig. 4 shows that the absorbance increased with increasing pH from 0.9 up to 1.7 and then decreased at higher values. This observation can be attributed to the fact that the rate of formation of the Cr(VI)–DPC complex is dependent on the pH of the sample solution. Since the maximum efficiency was obtained at pH 1.7, it was selected as the optimum value.

Fig. 4 Effect of the pH and DPC concentration. Conditions: desorption solvent, EtOH (200 μL); SDS, 5 mg; adsorbent, 4 mg; adsorption time, 20 min; desorption time, 5 min.

Effect of diphenylcarbazide concentration

Chromium(VI) can react with DPC in a strongly acidic solution (pH, 1.7) to form the reddish violet Cr–diphenylcarbazone complex, which is the carbazone inner salt of a chromous ion. This reaction is highly specific for Cr(VI) versus Cr(III), and the product can be detected using spectrophotometric analysis at λ_max of about 540 nm.⁴¹ Therefore, the influence of the ligand (DPC) concentration on the efficiency of the complexation process was studied in the range of 2 × 10⁻⁵ to 10 × 10⁻⁵ mol L⁻¹. As can be seen in Fig. 4, the absorbance increased with the increasing concentration of DPC and yielded a maximum at 6 × 10⁻⁵ mol L⁻¹, followed by a decline with its further increase. The decrease in the absorbance at concentrations higher than 6 × 10⁻⁵ mol L⁻¹ can be ascribed to the decrease in the complexation efficiency due to the dimerization of DPC in the sample solution. Thus, insufficient DPC was accessible for the formation of the complex with Cr(VI).⁴² Therefore, 6 × 10⁻⁵ mol L⁻¹ was chosen as the best concentration of DPC for complexation with Cr(VI).

Effect of sodium dodecyl sulfate concentration

Sodium dodecyl sulfate (SDS) is a widely used anionic surfactant that acts as a bridge between the PET nanofibers and the Cr(VI)–DPC complex, and thus plays an important role in the extraction of the analyte. The effect of the SDS concentration on the extraction efficiency was studied in the 2–6 mg (200 to 600 μL, 1% w/v) range using the procedure in Section 2.4. Fig. 5 shows that with increasing the amount of SDS up to 5 mg the absorbance increased, and then decreased after more SDS was added to the sample solution. The corresponding drop in the absorbance can be explained by the self-aggregation of the SDS molecules at higher concentrations in the sample solution. Accordingly, 5 mg was selected as the optimum amount of SDS in the proposed procedure.

Fig. 5 Effect of SDS and adsorbent dosage. Conditions: desorption solvent, EtOH (200 μL); DPC, 6 × 10⁻⁵ mol L⁻¹; pH, 1.7; adsorption time, 20 min; desorption time, 5 min.

Effect of adsorbent type and dosage

The adsorbent is a crucial factor in micro-solid phase extraction that has a very significant effect on the extraction efficiency. Different polymers such as PET, polystyrene, and nylon 6,6 were electrospun and their fabricated nanofibers were tested as adsorbents in accordance with the proposed procedure. The results showed that among these polymers, PET was the most effective adsorbent toward the Cr(VI)–DPC complex. After the selection of the nanofibrous PET as an appropriate adsorbent in the solid phase microextraction of Cr(VI), the influence of PET dosage was also investigated in the 2 to 7 mg range. Fig. 5 shows that with increasing the amount of PET in the sample solution up to 5 mg, the absorbance increases and reduces after that. The decline of the absorbance at amounts higher than 5 mg may be attributed to the fact that the volume of the added eluent in the desorption step was insufficient for removing all of the Cr(VI)–DPC complex. Therefore, 5 mg of the nanofibrous PET was recognized as the optimum amount in the extraction of the analyte.

Effect of extraction time (stirring time)

The extraction efficiency strongly depends on the amount of mass transfer of the analyte from the sample solution onto the adsorbent. Stirring the sample solution can speed up this process when the adsorbent is in contact with the sample solution. Therefore, after adding an appropriate amount of DPC (6 × 10⁻⁵ mol L⁻¹), SDS (5 mg), and PET (5 mg) into 25 mL of the sample solution (Cr(VI), 25 ng mL⁻¹), the effect of the contact time (stirring time) was studied in the range of 5–30 min while stirring the mixture at 400 rpm. The results recorded in Fig. 6, reveal that the highest efficiency was obtained in 20 min, thus this time was chosen as the optimum stirring time.

Fig. 6 Effect of the salt and adsorption time (stirring time). Conditions: desorption solvent, EtOH (200 μL); DPC, 6 × 10⁻⁵ mol L⁻¹; SDS, 5 mg; pH, 1.7; adsorbent dosage, 5 mg; desorption time, 5 min.

Salt effect

The influence of the ionic strength of the sample solution on the extraction efficiency was investigated by adding NaCl in the concentration range of 0–10%, w/v. The results in Fig. 6 show that with the increasing concentration of NaCl in the sample solution, the efficiency of the method was continuously decreased. This decrease can be due to the increasing solubility of the Cr(VI)–DPC complex with the addition of NaCl to the sample solution (salting-in effect). Therefore, the method was adopted without adding salt to the sample solution.

Influence of coexisting ions

The species that can change the valence state of Cr(VI) and/or react with DPC to form complexes may interfere with the spectrophotometric determination of the Cr(VI)–DPC complex. Although the reaction of Cr(VI) with DPC is highly specific versus Cr(III), it has been reported that several other cationic species i.e. Cu(II), Fe(III), Hg(II), Mo(VI), and V(V) may react with DPC and compete with Cr(VI).⁴² Therefore, the effect of these cations and some other common ions such as Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄²⁻, NO₃⁻, PO₄³⁻, CO₃²⁻ and Cl⁻, that may be present in environmental water samples, on the detection of Cr(VI)–DPC was studied. A coexisting ion was considered as an interference ion if its presence resulted in a variation of more than 5% in the recovery. According to the results, no significant interference in the detection of Cr(VI) due to the presence of the above-mentioned ions was observed (Table 1).

Table 1 Effect of potential interfering ions on the determination of a 25 ng mL⁻¹ chromium(VI) solution

Interfering ion	Interfering ion to Cr(VI) ratio (w/w)	RR^a (%) ± SD
a Relative recovery.
Hg^2+	50	97.8 ± 1
VO3^−	50	98.3 ± 2
MoO4^2−	100	99.2 ± 1
Cu^2+	10 000	98.4 ± 1
NO3^−	10 000	99.1 ± 2
PO4^3−	10 000	98.7 ± 3
SO4^2−	10 000	99.5 ± 1
Fe^3+	10 000	98.4 ± 2

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Method evaluation

Under the optimized conditions (desorption solvent, EtOH, 200 μL; DPC, 6 × 10⁻⁵ mol L⁻¹; SDS, 5 mg; pH, 1.7; adsorbent, 5 mg; adsorption time, 20 min; desorption time, 5 min), the analytical figures of merit including LDR, R², LOD, LOQ, and RSDs% were evaluated. The linearity of the method was investigated using the standard solutions of Cr(VI) at fourteen concentration levels with two replicates at each level. The calibration curve was constructed by plotting the absorbance against the concentration (1.8–60 ng mL⁻¹) of Cr(VI) with the satisfactory determination coefficient (R²) of 0.9923 (Fig. 1S of ESI†). The limit of detection (LOD) based on a signal to noise ratio (S/N) of 3 and the limit of quantification (LOQ) with a S/N of 10 were obtained equal to 0.6 and 1.8 ng mL⁻¹, respectively. The precision of the method was studied with three replicate measurements using 25 mL of Cr(VI) solution (25 ng mL⁻¹). The relative standard deviations (RSDs%) for intra-day (n = 3) and inter-day (three consecutive days, n = 3) were 1.6% and 3.1%, respectively. These RSD% values indicate that the proposed method is reproducible and suitable for the quantitative determination of Cr(VI) in water samples.

In order to examine the reusability of the PET nanofibers, the adsorbent was washed twice with 1 mL of methanol, then with 1 mL of water, and was reused for the next analysis run. In each run, 5 mg of the adsorbent was used for the extraction of Cr(VI) in a 25 ng mL⁻¹ solution under the optimal conditions. The results showed that the recoveries were higher than 90% after 5 uses.

Analysis of real samples

In order to evaluate the applicability of the developed method, real water samples such as tap water, river water (Zayandeh-rud river), ground water, and sewage water samples were prepared. All water samples were filtered using a paper filter, the pH values were adjusted at 1.7 and spiked with Cr(VI) at two concentration levels of 10 ng mL⁻¹ and 40 ng mL⁻¹. Then, the water samples were extracted and analyzed in accordance with the proposed procedure. The relative recoveries (RR%) were calculated based on the following equation:where C_found is the concentration of the analyte after adding a known amount of standard in the real sample, C_real is the concentration of the analyte in the real sample, and C_added is the concentration of the standard solution that was spiked into the real sample. The obtained relative recoveries, given in Table 2 are in the acceptable range of 96.9–99.1%.

Table 2 Analytical results for chromium(VI) in real samples

Sample	Added (ng mL^−1)	Found (ng mL^−1)	RSD^a (%)	RR^b (%)
a Relative standard deviation (n = 3).b Relative recovery.c Isfahan city, Iran.d Tap water was taken from our laboratory (Tehran, Iran).e Karaj city, Alborz province, North of Iran.f Karaj city, Alborz province, North of Iran.
Zayandeh-rud river^c	0	—	—	—
	10	9.9	1.2	99
	40	38.8	1.3	97
Tap water^d	0	—	—	—
	10	9.76	1.1	97.6
	40	38.9	1.4	97.2
Ground water^e	0	—	—	—
	10	9.91	1.3	99.1
	40	39.1	1.6	97.8
Sewage^f	0	—	—	—
	10	9.85	1.2	98.5
	40	38.76	1.5	96.9

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A search in the literature databases was carried out to find the previously developed methods^42–51 for the extraction and determination of Cr(VI) and the results are summarized in Table 3. In comparison, the main advantage is that the proposed method benefits from a remarkably simple electrospinning method for producing the adsorbent as PET nanofibers. In addition, the method precision (RSD%) is better than those reported in other studies. High relative recoveries (∼97–99%) were obtained for the extraction in 25 mL of several complex matrix samples such as tap, river, sewage and ground water samples. The high relative recoveries improve the detection limits, simplify quantification, and decrease disturbance to the system being studied.⁵² The method’s limit of detection (0.6 ng mL⁻¹) and the enhancement factor (125) are better than for most other existing techniques. The linear dynamic range is comparable to that of other methods. The proposed procedure is faster than most of the others previously developed.

Table 3 A comparison of the proposed method with other reported studies for the determination of Cr(VI)

Adsorbent	Detection method	Desorption solvent (mL)	Application	Sample (mL)	EF^a	Time (min)	LOD^b	LDR^c	RR (%)	RSD (%)	Ref.^d
a Enrichment factor.b Limit of detection (ng mL^−1).c Linear dynamic range (ng mL^−1).d Reference.e Thermally reduced graphene.f Flame atomic absorption spectroscopy.g Acid activated montmorillonite.h Polyethylene glycol.i Dimethyl formamide.
TRG^e modified SiO2 composite	UV-Vis	EtOH, 0.15	Tap, sewage, ground and river water	25	166.6	17	0.4	1.3–40	92.6–109.9	2.3	42
Amberlite XAD-16	FAAS^f	MeOH/H2SO4, 10	Tap water	250	25	>72	45	45–2000	99.4	1	43
Chitin	UV-Vis	MeOH/CH3COOH, 1	Natural water	100	100	15	50	50–600	98–105	1.7	44
C18	UV-Vis	MeOH, 2	Drinking water	50	25	10	3	0–500	96.1–102	3.4	45
Alumina	UV-Vis	MeOH, 10	Electroplating wastewater	250	25	>55	5	0–500	98–98.7	5	46
Ambersorb 563	UV-Vis	Acetone, 5	Wastewater	150	30	>75	3.4	—	100–104	<6	47
AAM^g-silica gel	UV-Vis	PEG^h/H2SO4, 10	Electroplating wastewater	250	25	>62	6	0–1000	97–99	3.5	48
Naphthalene	FAAS	DMF^i, 10	Tannery effluents	1000	100	>15	0.5	0–200	—	3.1	49
SDS-coated alumina	UV-Vis	Acetone/MeOH/HCl, 8	Tap water	800	100	>30	0.033	—	98–102	3.4	50
Amberlite XAD-1180	FAAS	HNO3/acetone, 2	Wastewater	150	75	>40	7.7	—	96–104	5.1	51
Electrospun PET	UV-Vis	EtOH, 0.2	Tap, sewage, ground and river water	25	125	25	0.6	1.8–60	96.9 99.1	1.6	This work

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Conclusions

The simple and low-cost prepared electrospun polyethylene terephthalate (PET) nanofiber film, with a large specific surface area and a fine porous structure, was used as a new and excellent adsorbent for micro-solid phase extraction of water samples. The nanofibrous structure of the electrospun PET increased the hydrophilicity of the surface area and provided more paths for the diffusion of the analyte through the fibrous polymeric matrix, and thus facilitated better interactions between the adsorbent and Cr(VI)–diphenylcarbazide. The selectivity and sensitivity of the method were improved by adding 1,5-diphenylcarbazide as the complexing agent for Cr(VI), and sodium dodecyl sulfate (SDS) as the surfactant. The adsorption and desorption conditions were studied and optimized using a one-at-a-time methodology. In comparison to the previously reported methods, the proposed method benefits from the advantages of a lower LOD, a relatively wide LDR, a high enrichment factor, a low volume of the sample solution, and a low volume of the nontoxic desorption solvent (EtOH).

References

1. V. K. Gupta, A. K. Singh and B. Gupta, Schiff bases as cadmium(II) selective ionophores in polymeric membrane electrodes, Anal. Chim. Acta, 2007, 583, 340.
2. Y. M. Scindia, A. K. Pandey, A. V. R. Reddy and S. B. Manohar, Chemically selective membrane optode for Cr(VI) determination in aqueous samples, Anal. Chim. Acta, 2004, 515, 311.
3. W. Ngeontae, W. Aeungmaitrepirom and T. Tuntulani, Chemically modified silica gel with aminothioamidoanthraquinone for solid phase extraction and preconcentration of Pb(II), Cu(II), Ni(II), Co(II) and Cd(II), Talanta, 2007, 71, 1075.
4. F. Xiao and J. C. Howard Huang, Comparison of biosorbents with inorganic sorbents for removing copper(II) from aqueous solutions, J. Environ. Manage., 2009, 90, 3105.
5. A. Jang, Z. Zou, K. K. Lee, C. H. Ahn and P. L. Bishop, Potentiometric and voltammetric polymer lab chip sensors for determination of nitrate, pH and Cd(II) in water, Talanta, 2010, 83, 1.
6. Agency for Toxic Substances and Disease Registry (ATSDR), Toxicological Profile for Chromium U.S. Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA, 1998.
7. U.S. Environmental Protection Agency, Toxicological Review of Trivalent Chromium. National Center for Environmental Assessment, Office of Research and Development, Washington, DC, 1998.
8. M. J. Marques, A. Salvador, A. Morales-Rubio and M. de la Guardia, Chromium speciation in liquid matrices: a survey of the literature, Fresenius. J. Anal. Chem., 2000, 367, 601.
9. M. A. Taher, Flame atomic absorption spectrometric determination of trace chromium in various standard samples after preconcentration with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol, Fresenius. J. Anal. Chem., 2000, 368, 421.
10. R. Milaci and J. Scancar, Determination of hexavalent chromium in lime-treated sewage sludge by anion-exchange fast protein liquid chromatography with electrothermal atomic absorption spectrometry detection, Analyst, 2000, 125, 1938.
11. J. Mocfik and J. Labuda, Determination of extractable chromium(VI) in the presence of large excess of chromium(III) in solid materials, Microchim. Acta, 1985, 231.
12. M. Sikovec, M. Franko, F. G. Cruzc and S. A. Katz, Thermal lens spectrometric determination of hexavalent chromium, Anal. Chim. Acta, 1996, 330, 245.
13. C. S. Ki, E. H. Gang, I. C. Um and Y. H. Park, Nanofibrous membrane of wool keratose/silk fibroin blend for heavy metal ion adsorption, J. Membr. Sci., 2007, 302, 20.
14. W. Wang, Q. Yang, L. Sun, H. Wang, C. Zhang, X. Fei, M. Sun and Y. Li, Preparation of fluorescent nanofibrous film as a sensing material and adsorbent for Cu²⁺in aqueous solution via copolymerization and electrospinning, J. Hazard. Mater., 2011, 194, 185.
15. M. Stephen, N. Catherine, M. Brenda, K. Andrew, P. Leslie and G. Corrine, Oxolane-2,5-dione modified electrospun cellulose nanofibers for heavy metals adsorption, J. Hazard. Mater., 2011, 192, 922.
16. S. Li, X. Yue, Y. Jing, S. Bai and Z. Dai, Fabrication of zonal thiol-functionalized silica nanofibers for removal of heavy metal ions from wastewater, Colloids Surf., A, 2011, 380, 229.
17. Y. Tian, M. Wu, R. Liu, Y. Li, D. Wang, J. Tan, R. Wu and Y. Huang, Electrospun membrane of cellulose acetate for heavy metal ion adsorption in water treatment, Carbohydr. Polym., 2011, 83, 743.
18. S. Wu, F. Li, H. Wang, L. Fu, B. Zhang and G. Li, Effects of poly(vinyl alcohol) (PVA) content on preparation of novel thiol-functionalized mesoporous PVA/SiO₂ composite nanofibers membranes and their application for adsorption of heavy metal ions from aqueous solution, Polymer, 2010, 51, 6203.
19. Y. Sang, F. Li, Q. Gu, C. Liang and J. Chen, Heavy metal-contaminated groundwater treatment by a novel nanofiber membrane, Desalination, 2008, 223, 349.
20. S. Wu, F. Li, H. Wang, L. Fu, B. Zhang and G. Li, Effects of poly(vinyl alcohol) (PVA) content on preparation of novel thiol-functionalized mesoporous PVA/SiO₂ composite nanofiber membranes and their application for adsorption of heavy metal ions from aqueous solution, Polymer, 2010, 51, 6203.
21. S. Li, X. Yue, Y. Jing, S. Bai and Z. Dai, Fabrication of zonal thiol-functionalized silica nanofibers for removal of heavy metal ions from wastewater, Colloids Surf., A, 2011, 380, 229.
22. S. Abbasizadeh, A. R. Keshtkar and M. A. Mousavian, Preparation of a novel electrospun polyvinyl alcohol/titanium oxide nanofiber adsorbent modified with mercapto groups for uranium(VI) and thorium(IV) removal from aqueous solution, Chem. Eng. J., 2013, 220, 161.
23. D. K. Sharma, F. Li and Y. N. Wu, Electrospinning of Nafion and polyvinyl alcohol into nanofiber membranes: a facile approach to fabricate functional adsorbent for heavy metals, Colloids Surf., A, 2014, 457, 236.
24. J. Wang, C. Luo, G. Qi, K. Pan and B. Cao, Mechanism study of selective heavy metal ion removal with polypyrrole-functionalized polyacrylonitrile nanofiber mats, Appl. Surf. Sci., 2014, 316, 245.
25. Y. Lu, Z. Wu, M. Li, Q. Liu and D. Wang, Hydrophilic PVA-co-PE nanofiber membrane functionalized with iminodiacetic acid by solid-phase synthesis for heavy metal ions removal, React. Funct. Polym., 2014, 82, 98.
26. M. Aliabadi, M. Irani, J. Ismaeili and S. Najafzadeh, Design and evaluation of chitosan/hydroxyapatite composite nanofiber membrane for the removal of heavy metal ions from aqueous solution, J. Taiwan Inst. Chem. Eng., 2014, 45, 518.
27. P. Kampalanonwat and P. Supaphol, The study of competitive adsorption of heavy metal ions from aqueous solution by aminated polyacrylonitrile nanofiber mats, Energy Procedia, 2014, 56, 142.
28. A. Razzaz, S. Ghorban, L. Hosayni, M. Irani and M. Aliabadi, Chitosan nanofibers functionalized by TiO₂ nanoparticles for the removal of heavy metal ions, J. Taiwan Inst. Chem. Eng., 2015, 1 DOI:10.1016/j.jtice.2015.06.003.
29. H. M. Marwani, M. U. Lodhi, S. B. Khan and A. M. Asiri, Selective extraction and determination of toxic lead based on doped metal oxide nanofiber, J. Taiwan Inst. Chem. Eng., 2015, 51, 34.
30. R. Zhao, X. Li, B. Sun, M. Shen, X. Tan, Y. Ding, Z. Jiang and C. Wang, Preparation of phosphorylated polyacrylonitrile-based nanofiber mat and its application for heavy metal ion removal, Chem. Eng. J., 2015, 268, 290.
31. S. Ramakrishna, K. Fujihara, W. E. Teo, T. C. Lim and Z. Ma, An introduction to electrospinning and nanofibers, World Scientific Publishing Co. Pte. Ltd, 2005, ch. 1 and 2.
32. G. C. Rutledge and S. V. Fridrikh, Formation of fibers by electrospinning, Adv. Drug Delivery Rev., 1384, 2007, 59.
33. Y. Zhang, X. U. Kang, L. Chen, C. Pan, Y. Yao and Z. Z. Gu, Fiber packed SPE tips based on electrospun fibers, Anal. Bioanal. Chem., 2008, 391, 2189.
34. X. J. Kang, L. Q. Chen, Y. Zhang, Y. W. Liu and Z. Z. Gu, Performance of electrospun nanofibers for SPE of drugs from aqueous solutions, J. Sep. Sci., 2008, 31, 3272.
35. X. Kang, C. Pan, Q. Xu, Y. Yao, Y. Wang, D. Qi and Z. Gu, The investigation of electrospun polymer nanofibers as a solid-phase extraction sorbent for the determination of trazodone in human plasma, Anal. Chim. Acta, 2007, 587, 75.
36. A. Aghakhani and M. Asadollahi-Baboli, Rapid analysis of Origanum majorana L. fragrance using a nanofiber sheet, gas chromatography with mass spectrometry, and chemometrics, J. Sep. Sci., 2014, 37, 990.
37. H. Bagheri, A. Aghakhani and M. Akbari, Electrospun composite of polypyrrole-polyamide as a micro-solid phase extraction sorbent, Anal. Bioanal. Chem., 2011, 400, 3607.
38. A. Bertsch, H. Lorenz and P. Renaud, Combining microstereolithography and thick resist UV lithography for 3D microfabrication, in Micro Electro Mechanical Systems, MEMS 98, Proceedings, The Eleventh Annual International Workshop on, 1998, pp. 18–23.
39. http://www.chemspider.com/Chemical-Structure.8459.html.
40. T. A. Thomas, Microdetermination of chromium with 1,5-diphenylcarbohydrazide, Anal. Chem., 1958, 30, 447.
41. K. Ashley, A. M. Howe, M. Demange and O. Nygren, Sampling and analysis considerations for the determination of hexavalent chromium in workplace air, J. Environ. Monit., 2003, 5, 707.
42. H. Sereshti, M. Vasheghani Farahani and M. Baghdadi, Trace determination of chromium(VI) in environmental water samples using innovative thermally reduced graphene (TRG) modified SiO₂ adsorbent for solid phase extraction and UV-Vis spectrophotometry, Talanta, 2015 DOI:10.1016/j.talanta.2015.06.051.
43. A. T. Eli and A. R. Turker, Speciation of Cr(III) and Cr(VI) in water after preconcentration of its 1,5-diphenylcarbazone complex on amberlite XAD-16 resin and determination by FAAS, Talanta, 2002, 57, 1199.
44. S. Hoshi, K. Konuma, K. Sugawara, M. Uto and K. Akatsuka, The simple and rapid spectrophotometric determination of trace chromium(VI) after preconcentration as its colored complex on chitin, Talanta, 1998, 47, 659.
45. J. Ma, B. Yang and R. H. Byrne, Determination of nano-molar chromate in drinking water with solid phase extraction and a portable spectrophotometer, J. Hazard. Mater., 2012, 219–220, 247.
46. I. Narin, Y. Surme, M. Soylak and M. Dogan, Speciation of Cr(III) and Cr(VI) in environmental samples by solid phase extraction on Ambersorb 563 resin, J. Hazard. Mater., 2006, B136, 579.
47. N. Rajesh, B. Deepthi and A. Subramaniam, Solid phase extraction of chromium(VI) from aqueous solutions by adsorption of its ion-association complex with cetyltrimethylammoniumbromide on an alumina column, J. Hazard. Mater., 2007, 144, 464.
48. N. Rajesh, B. G. Mishra and P. K. Pareek, Solid phase extraction of chromium(VI) from aqueous solutions by adsorption of its diphenylcarbazide complex on a mixed bed adsorbent (acid activated montmorillonite–silica gel) column, Spectrochim. Acta, Part A, 2008, 69, 612.
49. P. G. Krishna, J. M. Gladis, U. Rambabu, T. Prasada Rao and G. R. K. Naidu, Preconcentrative separation of chromium(VI) species from chromium(III) by coprecipitation of its ethyl xanthate complex onto naphthalene, Talanta, 2004, 63, 541.
50. J. L. Manzoori, M. M. Sorouraddin and F. Shemiran, Preconcentration and spectrophotometric determination of chromium(VI) and total chromium in drinking water by the sorption of chromium diphenylcarbazone with surfactant coated alumina, Anal. Lett., 1996, 29(11), 2007.
51. I. Narin, A. Kars and M. Soylak, A novel solid phase extraction procedure on Amberlite XAD-1180 for speciation of Cr(III), Cr(VI) and total chromium in environmental and pharmaceutical samples, J. Hazard. Mater., 2008, 150, 453.
52. J. P. Landers, Handbook of capillary electrophoresis, CRC Press LLC, USA, 2nd edn, 1997.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14788c

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