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Novel optical bulk membrane sensor and its application for determination of iron in plant and cereal samples
Journal of Food Composition and Analysis 29 (2013) 144–150
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Original Research Article
Novel optical bulk membrane sensor and its application for determination of iron in plant and cereal samples Mohammad Bagher-Gholivand a,*, Arash Babakhanian a,b,**, Moslem Mohammadi b, Payman Moradi b, Seyed Hossein Kiaie b a b
Department of Chemistry, Islamic Azad University, Kermanshah Branch, Iran Department of Chemical Biotechnology Engineering, kermanshah science and Research Branch, Islamic Azad University, Kermanshah, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 February 2012 Received in revised form 8 August 2012 Accepted 15 November 2012
A new ion selective optical bulk membrane sensor is fabricated for the highly sensitive and selective determination of iron(III) ion that is ion concentrations based on plasticized poly vinyl chloride containing 3% 1,10 -(iminobis(methan-1-yl-1-ylidene))dinaphthalen-2-ol as a chromo-ionophore, 3% sodium tetraphenylborate as an anionic additive and 64% tris(2-ethylhexyl)phosphate as a plasticizer. The proposed sensor displays a wide linear dynamic range from 1.0 109 to 1.0 103 mol L1 with a low limit of detection of 6.2 1010 mol L1 at pH 4.7. The sensor has a fast response time of less than 80 s. In addition to its high stability, reproducibility and stability over long life time period, the optical sensor shows a unique selectivity toward Fe(III) ions over a large number of organic and inorganic substances such as metal ions, sugars, amino acids and vitamins. The proposed optical sensor could readily be regenerated by exposure to 1.0 101 mol L1 sodium thiourea solution and is also applied successfully determination of the iron contents in plant and cereal samples with satisfactory results. ß 2012 Elsevier Inc. All rights reserved.
Keywords: Bulk optical sensor Plasticizer PVC membrane Food composition Food analysis 1,10 -(Iminobis(methan-1-yl-1ylidene))dinaphthalen-2-ol
1. Introduction Iron is a mineral that is necessary for producing red blood cells, and for reduction and oxidation (Redox) processes. Iron deficiency is considered to be the commonest worldwide nutritional deficiency, affecting approximately 20% of world population. Lack of iron may lead to unusual tiredness, shortness of breath, a decrease in physical performance, learning problems in children and adults, and consequently, increase the chance of getting an infection. This deficiency is partly induced by plant-based diets containing low levels of poorly bio-available iron. The most effective technological approaches to combat iron deficiency in developing countries include supplementation targeted to high-risk groups combined with a program of food fortification and dietary strategies designed to maximize the bioavailability of both the added and the intrinsic food iron (Navarretea et al., 2002). There is, thus, a large and rapidly growing interest on developing analytical methods, which could
* Corresponding author. Tel.: +98 8317242218/3182; fax: +98 8317242218. ** Corresponding author at: Department of Chemistry, Islamic Azad University, Kermanshah Branch, Iran. Tel.: +98 8317242218/3182; fax: +98 8317242218. E-mail addresses: [email protected] (M. Bagher-Gholivand), [email protected], [email protected] (A. Babakhanian). 0889-1575/$ – see front matter ß 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jfca.2012.11.003
determine iron contents at sub-micro-molar levels in different matrices, especially in food stuffs. Recently, a few studies have been published concerning iron determination in food stuffs such as flame atomic absorption spectrometry (Durukan et al., 2011), flow injection flame atomic absorption spectrometry (S¸ahin et al., 2010), energy dispersive X-ray fluorescence (Perring et al., 2005), plasma spectrometric techniques (Cubadda and Raggi, 2005), capillary electrophoresis (Xu et al., 1996), graphite furnace atomic absorption spectrometry (Poliana et al., 2003), spectrophotometric method (Riganakos and Veltsistas, 2003), inductively coupled plasma optical emission spectrometry (McKinstry et al., 1999), direct current plasma atomic emission spectrophotometry (Burnecka et al., 2004), electrothermal atomic absorption (Mierzwa et al., 1998) and cathodic stripping voltammetry (Lu et al., 2001). However, most of these methods are time consuming, costly and use complicated instruments and procedures. Compared to other analytical methods, sensors based on optical signal measurements and fabricated with ionophores, have been accepted as a fast and economical sample monitoring device with an excellent detection limit and sensitivity, which could minimize interferences from a real sample matrixes especially to heavymetal analysis in a real time (Chan et al., 2002). Schiff base ionophores has been extensively studied in coordination chemistry, mainly due to their facile synthesis, easily tunable steric feature, electronic properties and good solubility in common
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absorption measurements were carried out in an air–acetylene flame. The wavelength used to detect Fe(III) was 248.3 nm and the band width 0.2 nm. 2.3. Cereal and plant sample preparation
Fig. 1. Schematic structure of synthetic chromo-ionophore 1,10 -(iminobis(methan1-yl-1-ylidene))dinaphthalen-2-ol.
solvents (Garnovskii et al., 1993). They are stable under a variety of oxidative and reductive conditions. The Schiff bases or the imine ligands are border line between hard and soft Lewis’s bases (Garnovskii et al., 1993). Recent studies show the successful use of Schiff bases as neutral carriers to fabricate ion selective electrodes to determine some metal ions such as chromium(III) (Gupta et al., 2006a), cerium(III) (Gupta et al., 2006b), cobalt(II) (Gupta et al., 2006c) and iron(III) (Gupta et al., 2011). In our pervious work, we studied the prominent role of the synthetic Schiff base 1,10 -(iminobis (methan-1-yl-1-ylidene))dinaphthalen-2-ol (IBMYD) as an ionophore to fabricate the ion selective membrane sensor (ISE) for the direct and indirect determination of free iron species in some biological and nonbiological samples (Babakhanian et al., 2010). In the present work, we introduce a new Fe(III) optical PVC bulk membrane sensor base on the foregoing synthetic Schiff base (1,10 -(iminobis(methan-1yl-1-ylidene))dinaphthalen-2-ol (Fig. 1) as a chromo-ionophore. The proposed Fe(III) optical PVC membrane sensor has dynamic range, good limit of detection, and is used for successful sensitive and selective determination of iron contents in plant and cereal samples with satisfactory results. To the best our knowledge, this is the first report for the iron optical fabrication in type of PVC-based membrane sensors in the literature. 2. Experimental 2.1. Reagents and apparatus The 1,10 -(iminobis(methan-1-yl-1-ylidene))dinaphthalen-2-ol (IBMYD) chromo-ionophore (Fig. 1) was synthesized, and its structure was approved in accordance with a typical procedure, described before (Babakhanian et al., 2010). The following materials, namely 2-nitrophenyloctylether (o-NPOE), dibutylphthalate (DBP), dioctylphthalate (DOP), benzeylacetate (BA), acetophenone (AP), tris(2-ethylhexyl)phosphate (TEHP), dioctylsebasate (DOS), tetrahydrofuran (THF), sodium tetraphenylborate (NaTPB), oleic acid (OA), nitrates of metal ions, and high relative molecular weight poly vinyl chloride (PVC) were obtained from Merck or Fluka company, and used without any further purification. Besides, distilled deionized water was used throughout. Absorbance spectra were recorded using an HP spectrophotometer (Agilent 8453) equipped with a thermo stated bath (Huber poly stat cc1). The pH measurements were carried out with a digital pH-ion meter (Model HANA 302) using a combined glass electrode. 2.2. Flame atomic absorption spectrometry (FAAS) procedure The determinations of Fe(III) ion concentrations were performed on a Shimadzu AA-670 atomic absorption spectrophotometer (Kyoto, Japan) under the recommended conditions. Atomic
Dry oxidation or ashing eliminates or minimizes the effect of organic materials in mineral element determination. It consists of ignition of organic compounds by air at atmospheric pressure and at relatively elevated temperatures in an electrical furnace. Resulting ash residues are dissolved in an appropriate acid. The proposed electrode was applied to determine iron contents in thyme ground, wheat bran, rosemary and cinnamon ground samples. To prepare the sample, 150 g of individual samples were heated for 4 h in an electrical furnace in accordance with commonly used dry ashing method at specific temperature 750 8C for Fe(III) analysis in all treated samples. The reminders were separately dissolved in nitric acid and diluted to 25 ml by distilled deionized water. The total iron content were determined by the proposed optical sensor with the standard addition method at pH 4.7 keeping the acetate buffer solution fixed. 2.4. Optode preparation Poly(vinylchloride)-based Fe(III) membrane sensor, was prepared by thoroughly mixing 3 mg of chromo-ionophore (IBMYD), 3 mg of anionic additive (NaTPB), 30 mg of powdered PVC and 64 mg of plasticizer TEHP in a glass dish of 2 cm diameter. An aliquot of 0.2 ml of this solution was poured and uniformly spread on a dust free activated glass plate (28 mm 13 mm) which was mounted on a spin device (rotating frequency 2600 rpm). After a spinning time of about 15 s, the glass support plate with sensing membrane was removed and allowed to stand in the ambient air for 1 h before use. The thickness of dry membrane was estimated to be approximately 4 mm. The polymer films were placed in diagonal position in the glass cell of spectrophotometer containing 2.5 ml of buffer solution (pH 4.7). A membrane (without chromo-ionophore), at the same condition, was used as a blank membrane. The sample cell was titrated with standardized metal ion solutions, and the absorbance value was measured. Standardized metal ion solutions were added to the cell with a 10–100 mL micro sampler. The titrations were done under a high concentration of acetate buffer solution which leads to the constant ionic strength. 3. Results and discussion 3.1. Response mechanism In our pervious study, the complex formation ability of IBMYD as a synthetic ionophore and some metal ions, were investigated. The results showed that the IBMYD has a good discriminating ability toward Fe(III) ions (log Kf = 6.05 0.02), over a wide variety of tested alkali, alkaline earth, transition, and heavy-metal ions. The theoretical Gaussian studies also shows that the IBMYD is a three dentate ligand and could form a 1:1 complex with Fe(III) which was more stable than the complexes of Fe(II) and other metal ions (Babakhanian et al., 2010). Thus, it is expected that the IBMYD will be a suitable neutral carrier to prepare the Fe(III) optical membrane sensor. The proposed membrane sensor was prepared by incorporation of the IBMYD lipophilic ionophore (3 mg) into a plasticized PVC (30 mg) containing NaTPB (3 mg) as a lipophilic anionic additive. The lipophilic anionic sites (TPB-127) provide the optode membrane with the necessary ion-exchange properties, because the IBMYD ionophore is a neutral ligand, and it
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could not function as an ion exchanger. Under the experimental conditions used, and in accordance with 1:1 complex formation between IBMYD and Fe(III) in the organic phase, the response mechanism of this optical system would be described by the following ion-exchange mechanism: Fe3þ ðaqÞ þ IBMYDðorgÞ þ 3NaTPBðorgÞ ! FeIBMYD3þ ðorgÞ þ 3TPB ðorgÞ þ 3Na þ ðaqÞ
(1)
Based on the response mechanism of the proposed optical sensor outlined above, a complexation equilibrium between ionophore L and Fe(III) would be concluded as MmLn complex formation with a mechanism as under: mMðaqÞ þ nLðorgÞ ¼ Mm Ln ðorgÞ
(2)
with K eq ¼
½Mm Ln org ; ½Mm aq½Ln org
(3)
where m and n designate the complexing ratio between iron ion and L, Keq is the equilibrium constant of the overall reaction. The relative absorbance, a, is defined as the ratio of un-complexed ionophore in the membrane phase [L]org, to the total amount of an ionophore present in the membrane [Lt]org. Concentration of the aqueous species can be used only if activity coefficients are constants, so that:
a ¼ ½Lorg=½Ltorg and ½Mm Ln org
(4.a)
or
a¼
½Ltorgð1 aÞ n
(4.b)
The a value is calculated by the absorbance measurements at the wavelength of the complexed chromo-ionophore at 377 nm as: A AL (5) AC AL where AC is the absorbance of the membrane for complete complexation (i.e., at a = 0), AL is the absorbance value for the free
a¼
chromo-ionophore (i.e., at a = 1) and A is the absorbance at any time during the titration procedure. Thus, the relationship between the a-value and the concentration of Fe(III) ion in aqueous sample solution [M]aq, could be obtained by combining Eqs. (3) and (4), as follows: 1a ¼ nK½Ltn 1 org½Mm aq an
(6)
Eq. (6) could be used as a basis for the quantitative determination of iron ions, using the proposed optical membrane.
3.2. Spectral characteristics The absorption spectra of the proposed membrane sensor in an acetate buffer (pH 4.7) and in the presence of different concentrations of Fe(III), are shown in Fig. 2. The spectral changes are due to the extraction of Fe(III) from the aqueous sample solution to the membrane phase and its complexation with the IBMYD. This illustrates that the optode membrane could be used for the assay of Fe(III) in aqueous sample solutions.
3.3. Dynamic range and detection limit The optical response of the proposed Fe(III)-selective sensor, at different Fe(III) ion concentrations, and under the optimal experimental conditions, is shown in Fig. 3. Three curves are calculated using Eq. (6) with different m and n ratios. As it is seen from Fig. 3, the curve with 1:1 complex ratio and an appropriate Keq value of 3.7 105 fits best to the experimental data. In fact, the mechanism of the response of optode is believed to be based on complexation of IBMYD and Fe(III). This curve could be suitably used as a calibration curve for Fe(III) determination over a concentration range of 1.0 109 to 1.0 103 mol L1. The limit of detection (LOD) based on that of the blank was 6.2 1010 mol L1.
Fig. 2. Absorption spectra for IBMYD immobilized in PVC matrix in the presence of increasing concentration of iron(III) ion: (1) buffer solution (2) 1.0 109 mol L1, (3) 3.15 109 mol L1, (4) 1.6 108 mol L1, (5) 1.0 107 mol L1, (6) 4.0 107 mol L1, (7) 1.0 106 mol L1, (8) 3.15 106 mol L1, (9) 2.5 105 mol L1, (10) 1.0 104 mol L1, (11) 3.15 104 mol L1, (12) 1.0 103 mol L1, (13) 3.15 103 mol L1.
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Fig. 3. Calibration curve plot of (1 a) versus logarithm of Fe(III) ion concentration (mol L1) for Fe(III)-optical membrane sensor at pH 4.7.
3.4. Effect of membrane composition The membrane composition is well known to largely influence the response characteristics and 172 working concentration ranges of both potentiometric and optical sensors (Bakker et al., 1997; Eugster et al., 1993; Oter et al., 2007). Thus, the composition of membrane was optimized by studying the influences of plasticizer/PVC ratio, the nature of plasticizer, the amounts of chromoionophore and addition of NaTPB on the response behavior of the membrane sensor. The results are summarized in Table 1. For the preparation of a homogeneous membrane phase, the plasticizer should be physically compatible with the polymer. The nature of plasticizer is well known to influence largely the measuring concentration range of the solvent polymeric sensors as well as their selectivity coefficients (Ertas et al., 2000; Singh et al., 2007; Rosatzin et al., 1993). Thus, in this work, we studied the effect of seven kinds of the plasticizers (i.e., NPOE, DBP, DOP, DOS, TEHP, BA and AP) on the response of the Fe(III)-selective membrane optode, and the results are given in Table 1 (membrane Nos. 1–7). As it is obvious, among the seven different plasticizers used, TEHP was found to be the most effective solvent mediator to prepare the Fe(III)-selective optode, with the percent value of 64%. In bulk membrane optodes, a mass transfer of analyte from the sample solution into the membrane is required, to better establishment of
a thermodynamic ion-exchange equilibrium between the membrane and the sample (Singh et al., 2007; Rosatzin et al., 1993). Therefore, in the proposed iron-selective optical membrane sensor containing IBMYD as a neutral ionophore, the presence of a lipophilic anionic additive like NaTPB, was found to be necessary to facilitate the corresponding ion-exchange equilibrium. It has been reported that the absence of additive salts not only affects the working concentration range of the sensors, but also causes the prolonged response time and reduce selectivity (Bakker et al., 1997). Therefore, the membranes (Nos. 1–7), without additive used, result in narrower concentration ranges (Table 1) and the presence of 3 mg NaTPB is certainly required to reach the best optical response characteristic, compared with oleic acid used (OA). Thus, in this study, the membrane (No. 9, Table 1), with an optimized percent ratio of PVC:TEHP:NaTPB:IBMYD as 30:64:3:3, was chosen to fabricate further optical membrane sensors. 3.5. Effect of pH on the sensor response The effect of pH of test solution on the absorbance response of the proposed sensor was tested in the pH range of 1.5–7. The absorbance measurements were recorded for 1.0 106 mol L1 of Fe(III) ion concentration at different pH values at 377 nm. The blank membrane (membrane without IBMYD) was taken as a
Table 1 Optimization of membrane ingredients for the iron(III)-optical sensor (n = 5). Membrane number
1 2 3 4 5 6 7 8 9 10 11 12 13
Working concentration range (mol L1)
Composition (%) PVC (mg)
Plasticizer (mg)
Additive (mg)
Ionophore (mg)
30 30 30 30 30 30 30 30 30 30 30 30 30
67 67 67 67 67 67 67 64 64 64 64 64 64
0 0 0 0 0 0 0 3.0 3.0 4.0 5.0 5.5 0.5
3 3 3 3 3 3 3 3 3 2.0 1.0 0.5 5.5
(NPOE) (DOS) (AP) (DBP) (BA) (DOP) (TEHP) (TEHP) (TEHP) (TEHP) (TEHP) (TEHP) (TEHP)
(OA) (NaTPB) (NaTPB) (NaTPB) (NaTPB) (NaTPB)
5.0 104 4.0 105 4.0 105 7.0 105 8.0 105 7.0 106 2.0 106 3.0 108 1.0 109 7.0 109 9.0 106 7.0 104 3.0 103
to to to to to to to to to to to to to
7.0 102 7.0 103 3.0 103 3.0 103 2.0 103 5.0 103 3.0 103 5.0 103 1.0 103 2.0 101 3.0 102 3.0 102 7.0 101
148
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Fig. 4. Effect of pH on the response of the Fe(III)-optical membrane sensor.
reference. The absorbance measurements were expressed as absorbance difference, which was defined as the difference between the absorbance of the immobilized IBMYD alone and the absorbance of the Fe(III)–IBMYD complex at 377 nm. pH values were adjusted by the addition of dilute NaOH or HNO3. The results are shown in Fig. 4. As it is seen from Fig. 4, the absorbance difference intensity of the optical sensor increased by increasing the pH of test solution from 1.0 to 3.0, possess through a more or less plateau between pH 3.0–5.7. The diminished response at pH <3.0 could be due to the extraction of H+ from the aqueous sample solution into the PVC membrane via protonation of the nitrogen atoms in the structure of the ligand molecule On the other hand, the reduced optical response of the sensor at pH >5.7 could be also due to a possible slight swelling of the polymeric film under alkaline conditions as well as the hydroxide formation of Fe(III) ions in the sample solution. Therefore, the pH value at 4.7 was used for further studies. In the membrane fabricated with the IBMYD ionophore, this pH value is suitable for quantitative determination of Fe(III) in the mixture solution containing Fe(III)/Fe(II) or in the solution containing only Fe(III), because the Fe(III) is stable under this chosen pH value (Babakhanian et al., 2010; Ammann et al., 1985). 3.6. Solvent effect Sometimes in real situation, the samples may contain the nonaqueous contents and the functioning of the sensor system would be necessary to be investigated in partially non-aqueous media. Therefore, the sensor response was examined in the methanol– water and the acetone–water mixtures (Table 2). It was found that the membranes do not show any appreciable change in working concentration range in mixtures containing up to 5% (v/v) nonaqueous contents.
Fig. 5. Typical response curve of the film optode at 377 nm as a function of time when the film is exposed to 1.0 108 mol L1 iron ion.
containing 1.0 108 mol L1 of Fe(III) ion concentration. The response time of the present optode is controlled by the time required for the analyte to diffuse from the bulk of the solution to the membrane interface to associate with ligand. The response time was tested by recording the absorbance change from a pure buffer (pH 4.7) to a buffered 1.0 108 mol L1 Fe(III) solution. The response of the membrane was found to reach 90% of the total signal (<80 s) depending on the concentration of Fe(III). Generally, the response time is more rapid while proceeding from dilute to concentrated solution. 3.8. Reproducibility and reversibility Reproducibility and reversibility are two important characteristics in determining the suitability of an optical sensor for selective determination of the species of interest in different test solutions. The reversibility factor was examined by absorbance intensity measurements of the same membrane sensor when it was exposed to 1.0 106 mol L1 Fe(III) ion concentration in acetate buffer solution (pH 4.7) and 0.1 mol L1 of sodium thiourea. The results are shown in Fig. 6. The mean absorbance values (n = 5) were 0.987 0.007 and 0.475 0.005, in the presence of 1.0 106 mol L1 Fe(III) ion concentration and sodium thiourea
3.7. Response time Fig. 5 shows the time-dependent response characteristics of the optical membrane, which is immersed in a buffer solution Table 2 Performance of iron(III) optical PVC sensor (No. 9) in non-aqueous media. Non-aqueous content (%, v/v)
Working concentration range (mol L1)
0 Methanol 5 10 Acetone 5 10
1.0 109 to 1.0 103 1.3 109 to 1.5 103 1.5 109 to 1.7 103 1.9 109 to 1.5 103 1.7 109 to 1.5 103
Fig. 6. Variation of the absorbance of the membrane at 377 nm for repetitively exposing into 1.0 106 mol L1 of iron ion and a solution of 0.1 mol L1 of sodium thiourea.
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solution, respectively. The mean difference in absorbance was about 0.498 0.004. Thus, it could be concluded that the sensor performance is reversible and shows good reproducibility. From Fig. 6, it could also be found that the response time of the sensor is less than 3 min. The reproducibility was also examined by preparing five different membranes with the same composition, and measuring the absorbance intensity of each membrane in a 1.0 108 mol L1 of Fe(III) ion concentration (five repeated determinations) in acetate buffer solutions (pH 4.7). The resulting coefficient of variation was found to be 2.5%. Some regenerating compounds such as ethylenediamine tetraacetic acid, sodium florid, citric acid, lactic acid and sodium thiourea solutions were tested to fully recover the proposed optical membrane sensor. Among the reagents used, sodium thiourea was capable of fully recovering of the membrane at a short membrane regeneration time (<3 min). To study the influence of sodium thiourea concentration on the efficiency and time of regeneration process, different concentrations (0.01–0.1 mol L1) of sodium thiourea were tested. The best result was obtained when 0.1 mol L1 of sodium thiourea were used. 3.9. Recovery, short-term stability, lifetime and regeneration of the optode To evaluate the recovery of the proposed optical sensor, two solutions of Fe(III) with different concentrations were examined. The found values of (1.04 0.04) 106 for 1.0 106 mol L1 with n = 5 and (1.02 0.03) 105 for 1.0 105 mol L1 with n = 5 were obtained, respectively. The short term stability of the proposed optical sensor was investigated by its absorbance intensity measurements in contact with a 1.0 105 mol L1 of Fe(III) ion concentration and buffered at pH 4.7 over a period of 6 h. From the absorbance intensities, taken every 30 min (n = 12), it was found that the response was almost complete with only 1.9% increase in intensity after 6 h monitoring. The membrane sensor could also be stored in wet conditions (acetate buffer solution pH at 4.7) without any measurable change in its intensity for at least 9 weeks. To over come detaching the optical PVC membranes from the glass surfaces. The glass plates were activated according to the following procedure before use: the glass plates were activated by treatment with concentrated HNO3 for 12 h, followed by 3% HF, and 10% H2O2 each for 30 min, then washed with distilled water and ethanol (Luo et al., 2007). 3.10. Selectivity The optical membrane selectivity, which reflects the relative response of the optode for primary ion over diverse ions present in solution, might be the most important characteristic of an ionselective optode. The experiment was carried out with a fixed concentration of Fe(III) at 1.0 106 mol L1 and then measuring the changes in absorbance intensity before (A0) and after adding different foreign interfere (A) in the iron solution buffered at pH 4.7. The resulting relative error is defined as %RE = [(A A0)/ A0] 100. The tolerance limit was taken as the concentration causing an error of 5% in the analytical signal for determination of Fe(III). The results are shown in Table 3. As it can be seen, the proposed optical sensor has a good discriminating ability to Fe(III) ions over a wide variety of organic and inorganic substances. 3.11. Analytical applications The proposed sensor was examined successfully for the direct determination of iron contents in plant and cereal samples (Table 4). The samples were pre-treated by the suitable method described in this paper and analyzed by the optical sensor at pH 4.7. The results obtained by the proposed sensor were quite precise
149
Table 3 Effect of interfering ions on the determination of 1.0 106 mol L1 of Fe(III). Interfering ion
Tolerated ratio [interference]/[Fe(III)]
Interfering ion
Tolerated ratio [interference]/[Fe(III)]
Cu2+ Cd2+ Pb2+ Ag+ Zn2+ Cr3+ K+ Vitamin B6 Vitamin E D-Arginine Triton X-100
5 293 502 314 132 9 983 1084 1372 1055 962
Ni2+ Mn2+ Sr2+ Hg2+ Co2+ Al3+ Fe2+ Fructose CTAB Glucose Vitamin A
85 219 922 437 251 11 213 1147 998 1263 1429
Table 4 Determination of iron contents in plant and cereal samples with the proposed optical sensor (n = 5). Sample
Found by optode (mg per 100 g)a
Found by FAAS (mg per 100 g)a
Thyme ground Wheat bran Rosemary Cinnamon ground
126.5 2.5 46.2 1.2 31.05 1.04 39.9 1.2
126.3 2.3 46.3 1.3 31.07 1.02 39.3 1.1
a
Number of analysis; n = 5.
and accurate and in accordance with the data taken by flame atomic absorption spectrometry (FAAS). 4. Conclusion In this study, 1,10 -(iminobis(methan-1-yl-1-ylidene))dinaphthalen-2-ol (IBMYD) could be considered as a suitable chromo-ionophore agent to fabricate a new Fe(III) optical membrane sensor. The proposed sensor shows inherent advantages such as simple operation, stability, precise results, low cost, wide dynamic range, low detection limit and relatively fast response time. The sensor is also recommended for iron ion determination in some real samples such as plant and cereal samples with satisfactory results. References Ammann, D., Pretsch, E., Simon, W., Lindner, E., Bezegh, A., Pungor, E., 1985. Lipophilic salts as membrane additives and their influence on the properties of macro- and micro-electrodes based on neutral carriers. Analytica Chimica Acta 171, 119–129. Burnecka, J.B., Jankowiak, U., Zyrnicki, W., Wilk, K.A., 2004. Heavy metal content of canned orange juice as determined by direct current plasma atomic emission spectrophotometry (DCPAES). Spectrochimica Acta Part B: Atomic Spectroscopy 59, 585–590. Babakhanian, A., Gholivand, M.B., Mohammadi, M., Khodadadian, M., Shockravi, A., Abbaszadeh, M., Ghanbary, A., 2010. Fabrication of a novel iron(III)-PVC membrane sensor based on a new 1,10 -(iminobis(methan-1-yl-1-ylidene))dinaphthalen-2-ol synthetic ionophore for direct and indirect determination of free iron species in some biological and non-biological samples. Journal of Hazardous Materials 177, 159–166. Bakker, E., Bulmann, P., Pretsch, E., 1997. Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics. Chemical Review 97, 3083–3132. Chan, W.H., Yang, R.H., Mo, T., Wang, K.M., 2002. Lead-selective fluorescent optode membrane based on 3,30 , 5,50 -tetramethyl-N-(9-anthrylmethyl)benzidine. Analytica Chimica Acta 460, 123–132. Cubadda, F., Raggi, A., 2005. Determination of cadmium, lead, iron, nickel and chromium in selected food matrices by plasma spectrometric techniques. Microchemical Journal 79, 91–96. Durukan, I., S¸ahin, C.A., S¸atiroglu, N., Bektas, S., 2011. Selective determination of copper and iron in various food samples by the solid phase extraction. Microchemical Journal 99, 159–163.
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coupled plasma-optical emission spectrometry (ICP-OES). Food Chemistry 65, 245–252. Mierzwa, J., Sun, Y.C., Chung, Y.T., Yang, M.H., 1998. Comparative determination of Ba, Cu, Fe, Pb and Zn in tea leaves by slurry sampling electrothermal atomic absorption and liquid sampling inductively coupled plasma atomic emission spectrometry. Talanta 1263–1270. Navarretea, N.M., Camachoa, M.M., Lahuertab, J.M., Martinez, M.J., Fitoa, A.P., 2002. Iron deficiency and iron fortified foods—a review. Food Research International 35, 225–231. Oter, O., Ertekin, K., Kirimi, C., Koca, H., Ahmedzade, M., 2007. Characterization of a newly synthesized fluorescent benzofuran derivative and usage as a selective fiber optic sensor for Fe(III). Sensors and Actuators B 122, 450–456. Perring, L., Andrey, D., Dvorzak, M.B., Hammer, D., 2005. Rapid quantification of iron, copper and zinc in food premixes using energy dispersive X-ray fluorescence. Journal of Food Composition and Analysis 18, 655–663. Riganakos, K.A., Veltsistas, P.G., 2003. Comparative spectrophotometric determination of the total iron content in various white and red Greek wines. Food Chemistry 82, 637–641 643. Rosatzin, T., Bakker, E., Suzuki, K., Suzuki, K., Simon, W., 1993. Lipophilic and immobilized anionic additives in solvent polymeric membranes of cationselective chemical sensors. Analytica Chimica Acta 280, 197–208. S¸ahin, C¸.A., Tokgoz, I., Bektas¸, S., 2010. Preconcentration and determination of iron and copper in spice samples by cloud point extraction and flow injection flame atomic absorption spectrometry. Journal of Hazardous Materials 181, 359–365. Singh, A.K., Gupta, V.K., Gupta, B., 2007. Chromium(III) selective membrane sensors based on Schiff bases as chelating ionophores. Analytica Chimica Acta 585, 171–178. Xu, J., Che, P., Ma, Y., 1996. More sensitive way to determine iron using an iron(II)– 1,10-phenanthroline complex and capillary electrophoresis. Journal of Chromatography A 749, 287–294.
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Journal of Food Composition and Analysis journal homepage: www.elsevier.com/locate/jfca
Original Research Article
Novel optical bulk membrane sensor and its application for determination of iron in plant and cereal samples Mohammad Bagher-Gholivand a,*, Arash Babakhanian a,b,**, Moslem Mohammadi b, Payman Moradi b, Seyed Hossein Kiaie b a b
Department of Chemistry, Islamic Azad University, Kermanshah Branch, Iran Department of Chemical Biotechnology Engineering, kermanshah science and Research Branch, Islamic Azad University, Kermanshah, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 February 2012 Received in revised form 8 August 2012 Accepted 15 November 2012
A new ion selective optical bulk membrane sensor is fabricated for the highly sensitive and selective determination of iron(III) ion that is ion concentrations based on plasticized poly vinyl chloride containing 3% 1,10 -(iminobis(methan-1-yl-1-ylidene))dinaphthalen-2-ol as a chromo-ionophore, 3% sodium tetraphenylborate as an anionic additive and 64% tris(2-ethylhexyl)phosphate as a plasticizer. The proposed sensor displays a wide linear dynamic range from 1.0 109 to 1.0 103 mol L1 with a low limit of detection of 6.2 1010 mol L1 at pH 4.7. The sensor has a fast response time of less than 80 s. In addition to its high stability, reproducibility and stability over long life time period, the optical sensor shows a unique selectivity toward Fe(III) ions over a large number of organic and inorganic substances such as metal ions, sugars, amino acids and vitamins. The proposed optical sensor could readily be regenerated by exposure to 1.0 101 mol L1 sodium thiourea solution and is also applied successfully determination of the iron contents in plant and cereal samples with satisfactory results. ß 2012 Elsevier Inc. All rights reserved.
Keywords: Bulk optical sensor Plasticizer PVC membrane Food composition Food analysis 1,10 -(Iminobis(methan-1-yl-1ylidene))dinaphthalen-2-ol
1. Introduction Iron is a mineral that is necessary for producing red blood cells, and for reduction and oxidation (Redox) processes. Iron deficiency is considered to be the commonest worldwide nutritional deficiency, affecting approximately 20% of world population. Lack of iron may lead to unusual tiredness, shortness of breath, a decrease in physical performance, learning problems in children and adults, and consequently, increase the chance of getting an infection. This deficiency is partly induced by plant-based diets containing low levels of poorly bio-available iron. The most effective technological approaches to combat iron deficiency in developing countries include supplementation targeted to high-risk groups combined with a program of food fortification and dietary strategies designed to maximize the bioavailability of both the added and the intrinsic food iron (Navarretea et al., 2002). There is, thus, a large and rapidly growing interest on developing analytical methods, which could
* Corresponding author. Tel.: +98 8317242218/3182; fax: +98 8317242218. ** Corresponding author at: Department of Chemistry, Islamic Azad University, Kermanshah Branch, Iran. Tel.: +98 8317242218/3182; fax: +98 8317242218. E-mail addresses: [email protected] (M. Bagher-Gholivand), [email protected], [email protected] (A. Babakhanian). 0889-1575/$ – see front matter ß 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jfca.2012.11.003
determine iron contents at sub-micro-molar levels in different matrices, especially in food stuffs. Recently, a few studies have been published concerning iron determination in food stuffs such as flame atomic absorption spectrometry (Durukan et al., 2011), flow injection flame atomic absorption spectrometry (S¸ahin et al., 2010), energy dispersive X-ray fluorescence (Perring et al., 2005), plasma spectrometric techniques (Cubadda and Raggi, 2005), capillary electrophoresis (Xu et al., 1996), graphite furnace atomic absorption spectrometry (Poliana et al., 2003), spectrophotometric method (Riganakos and Veltsistas, 2003), inductively coupled plasma optical emission spectrometry (McKinstry et al., 1999), direct current plasma atomic emission spectrophotometry (Burnecka et al., 2004), electrothermal atomic absorption (Mierzwa et al., 1998) and cathodic stripping voltammetry (Lu et al., 2001). However, most of these methods are time consuming, costly and use complicated instruments and procedures. Compared to other analytical methods, sensors based on optical signal measurements and fabricated with ionophores, have been accepted as a fast and economical sample monitoring device with an excellent detection limit and sensitivity, which could minimize interferences from a real sample matrixes especially to heavymetal analysis in a real time (Chan et al., 2002). Schiff base ionophores has been extensively studied in coordination chemistry, mainly due to their facile synthesis, easily tunable steric feature, electronic properties and good solubility in common
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absorption measurements were carried out in an air–acetylene flame. The wavelength used to detect Fe(III) was 248.3 nm and the band width 0.2 nm. 2.3. Cereal and plant sample preparation
Fig. 1. Schematic structure of synthetic chromo-ionophore 1,10 -(iminobis(methan1-yl-1-ylidene))dinaphthalen-2-ol.
solvents (Garnovskii et al., 1993). They are stable under a variety of oxidative and reductive conditions. The Schiff bases or the imine ligands are border line between hard and soft Lewis’s bases (Garnovskii et al., 1993). Recent studies show the successful use of Schiff bases as neutral carriers to fabricate ion selective electrodes to determine some metal ions such as chromium(III) (Gupta et al., 2006a), cerium(III) (Gupta et al., 2006b), cobalt(II) (Gupta et al., 2006c) and iron(III) (Gupta et al., 2011). In our pervious work, we studied the prominent role of the synthetic Schiff base 1,10 -(iminobis (methan-1-yl-1-ylidene))dinaphthalen-2-ol (IBMYD) as an ionophore to fabricate the ion selective membrane sensor (ISE) for the direct and indirect determination of free iron species in some biological and nonbiological samples (Babakhanian et al., 2010). In the present work, we introduce a new Fe(III) optical PVC bulk membrane sensor base on the foregoing synthetic Schiff base (1,10 -(iminobis(methan-1yl-1-ylidene))dinaphthalen-2-ol (Fig. 1) as a chromo-ionophore. The proposed Fe(III) optical PVC membrane sensor has dynamic range, good limit of detection, and is used for successful sensitive and selective determination of iron contents in plant and cereal samples with satisfactory results. To the best our knowledge, this is the first report for the iron optical fabrication in type of PVC-based membrane sensors in the literature. 2. Experimental 2.1. Reagents and apparatus The 1,10 -(iminobis(methan-1-yl-1-ylidene))dinaphthalen-2-ol (IBMYD) chromo-ionophore (Fig. 1) was synthesized, and its structure was approved in accordance with a typical procedure, described before (Babakhanian et al., 2010). The following materials, namely 2-nitrophenyloctylether (o-NPOE), dibutylphthalate (DBP), dioctylphthalate (DOP), benzeylacetate (BA), acetophenone (AP), tris(2-ethylhexyl)phosphate (TEHP), dioctylsebasate (DOS), tetrahydrofuran (THF), sodium tetraphenylborate (NaTPB), oleic acid (OA), nitrates of metal ions, and high relative molecular weight poly vinyl chloride (PVC) were obtained from Merck or Fluka company, and used without any further purification. Besides, distilled deionized water was used throughout. Absorbance spectra were recorded using an HP spectrophotometer (Agilent 8453) equipped with a thermo stated bath (Huber poly stat cc1). The pH measurements were carried out with a digital pH-ion meter (Model HANA 302) using a combined glass electrode. 2.2. Flame atomic absorption spectrometry (FAAS) procedure The determinations of Fe(III) ion concentrations were performed on a Shimadzu AA-670 atomic absorption spectrophotometer (Kyoto, Japan) under the recommended conditions. Atomic
Dry oxidation or ashing eliminates or minimizes the effect of organic materials in mineral element determination. It consists of ignition of organic compounds by air at atmospheric pressure and at relatively elevated temperatures in an electrical furnace. Resulting ash residues are dissolved in an appropriate acid. The proposed electrode was applied to determine iron contents in thyme ground, wheat bran, rosemary and cinnamon ground samples. To prepare the sample, 150 g of individual samples were heated for 4 h in an electrical furnace in accordance with commonly used dry ashing method at specific temperature 750 8C for Fe(III) analysis in all treated samples. The reminders were separately dissolved in nitric acid and diluted to 25 ml by distilled deionized water. The total iron content were determined by the proposed optical sensor with the standard addition method at pH 4.7 keeping the acetate buffer solution fixed. 2.4. Optode preparation Poly(vinylchloride)-based Fe(III) membrane sensor, was prepared by thoroughly mixing 3 mg of chromo-ionophore (IBMYD), 3 mg of anionic additive (NaTPB), 30 mg of powdered PVC and 64 mg of plasticizer TEHP in a glass dish of 2 cm diameter. An aliquot of 0.2 ml of this solution was poured and uniformly spread on a dust free activated glass plate (28 mm 13 mm) which was mounted on a spin device (rotating frequency 2600 rpm). After a spinning time of about 15 s, the glass support plate with sensing membrane was removed and allowed to stand in the ambient air for 1 h before use. The thickness of dry membrane was estimated to be approximately 4 mm. The polymer films were placed in diagonal position in the glass cell of spectrophotometer containing 2.5 ml of buffer solution (pH 4.7). A membrane (without chromo-ionophore), at the same condition, was used as a blank membrane. The sample cell was titrated with standardized metal ion solutions, and the absorbance value was measured. Standardized metal ion solutions were added to the cell with a 10–100 mL micro sampler. The titrations were done under a high concentration of acetate buffer solution which leads to the constant ionic strength. 3. Results and discussion 3.1. Response mechanism In our pervious study, the complex formation ability of IBMYD as a synthetic ionophore and some metal ions, were investigated. The results showed that the IBMYD has a good discriminating ability toward Fe(III) ions (log Kf = 6.05 0.02), over a wide variety of tested alkali, alkaline earth, transition, and heavy-metal ions. The theoretical Gaussian studies also shows that the IBMYD is a three dentate ligand and could form a 1:1 complex with Fe(III) which was more stable than the complexes of Fe(II) and other metal ions (Babakhanian et al., 2010). Thus, it is expected that the IBMYD will be a suitable neutral carrier to prepare the Fe(III) optical membrane sensor. The proposed membrane sensor was prepared by incorporation of the IBMYD lipophilic ionophore (3 mg) into a plasticized PVC (30 mg) containing NaTPB (3 mg) as a lipophilic anionic additive. The lipophilic anionic sites (TPB-127) provide the optode membrane with the necessary ion-exchange properties, because the IBMYD ionophore is a neutral ligand, and it
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could not function as an ion exchanger. Under the experimental conditions used, and in accordance with 1:1 complex formation between IBMYD and Fe(III) in the organic phase, the response mechanism of this optical system would be described by the following ion-exchange mechanism: Fe3þ ðaqÞ þ IBMYDðorgÞ þ 3NaTPBðorgÞ ! FeIBMYD3þ ðorgÞ þ 3TPB ðorgÞ þ 3Na þ ðaqÞ
(1)
Based on the response mechanism of the proposed optical sensor outlined above, a complexation equilibrium between ionophore L and Fe(III) would be concluded as MmLn complex formation with a mechanism as under: mMðaqÞ þ nLðorgÞ ¼ Mm Ln ðorgÞ
(2)
with K eq ¼
½Mm Ln org ; ½Mm aq½Ln org
(3)
where m and n designate the complexing ratio between iron ion and L, Keq is the equilibrium constant of the overall reaction. The relative absorbance, a, is defined as the ratio of un-complexed ionophore in the membrane phase [L]org, to the total amount of an ionophore present in the membrane [Lt]org. Concentration of the aqueous species can be used only if activity coefficients are constants, so that:
a ¼ ½Lorg=½Ltorg and ½Mm Ln org
(4.a)
or
a¼
½Ltorgð1 aÞ n
(4.b)
The a value is calculated by the absorbance measurements at the wavelength of the complexed chromo-ionophore at 377 nm as: A AL (5) AC AL where AC is the absorbance of the membrane for complete complexation (i.e., at a = 0), AL is the absorbance value for the free
a¼
chromo-ionophore (i.e., at a = 1) and A is the absorbance at any time during the titration procedure. Thus, the relationship between the a-value and the concentration of Fe(III) ion in aqueous sample solution [M]aq, could be obtained by combining Eqs. (3) and (4), as follows: 1a ¼ nK½Ltn 1 org½Mm aq an
(6)
Eq. (6) could be used as a basis for the quantitative determination of iron ions, using the proposed optical membrane.
3.2. Spectral characteristics The absorption spectra of the proposed membrane sensor in an acetate buffer (pH 4.7) and in the presence of different concentrations of Fe(III), are shown in Fig. 2. The spectral changes are due to the extraction of Fe(III) from the aqueous sample solution to the membrane phase and its complexation with the IBMYD. This illustrates that the optode membrane could be used for the assay of Fe(III) in aqueous sample solutions.
3.3. Dynamic range and detection limit The optical response of the proposed Fe(III)-selective sensor, at different Fe(III) ion concentrations, and under the optimal experimental conditions, is shown in Fig. 3. Three curves are calculated using Eq. (6) with different m and n ratios. As it is seen from Fig. 3, the curve with 1:1 complex ratio and an appropriate Keq value of 3.7 105 fits best to the experimental data. In fact, the mechanism of the response of optode is believed to be based on complexation of IBMYD and Fe(III). This curve could be suitably used as a calibration curve for Fe(III) determination over a concentration range of 1.0 109 to 1.0 103 mol L1. The limit of detection (LOD) based on that of the blank was 6.2 1010 mol L1.
Fig. 2. Absorption spectra for IBMYD immobilized in PVC matrix in the presence of increasing concentration of iron(III) ion: (1) buffer solution (2) 1.0 109 mol L1, (3) 3.15 109 mol L1, (4) 1.6 108 mol L1, (5) 1.0 107 mol L1, (6) 4.0 107 mol L1, (7) 1.0 106 mol L1, (8) 3.15 106 mol L1, (9) 2.5 105 mol L1, (10) 1.0 104 mol L1, (11) 3.15 104 mol L1, (12) 1.0 103 mol L1, (13) 3.15 103 mol L1.
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Fig. 3. Calibration curve plot of (1 a) versus logarithm of Fe(III) ion concentration (mol L1) for Fe(III)-optical membrane sensor at pH 4.7.
3.4. Effect of membrane composition The membrane composition is well known to largely influence the response characteristics and 172 working concentration ranges of both potentiometric and optical sensors (Bakker et al., 1997; Eugster et al., 1993; Oter et al., 2007). Thus, the composition of membrane was optimized by studying the influences of plasticizer/PVC ratio, the nature of plasticizer, the amounts of chromoionophore and addition of NaTPB on the response behavior of the membrane sensor. The results are summarized in Table 1. For the preparation of a homogeneous membrane phase, the plasticizer should be physically compatible with the polymer. The nature of plasticizer is well known to influence largely the measuring concentration range of the solvent polymeric sensors as well as their selectivity coefficients (Ertas et al., 2000; Singh et al., 2007; Rosatzin et al., 1993). Thus, in this work, we studied the effect of seven kinds of the plasticizers (i.e., NPOE, DBP, DOP, DOS, TEHP, BA and AP) on the response of the Fe(III)-selective membrane optode, and the results are given in Table 1 (membrane Nos. 1–7). As it is obvious, among the seven different plasticizers used, TEHP was found to be the most effective solvent mediator to prepare the Fe(III)-selective optode, with the percent value of 64%. In bulk membrane optodes, a mass transfer of analyte from the sample solution into the membrane is required, to better establishment of
a thermodynamic ion-exchange equilibrium between the membrane and the sample (Singh et al., 2007; Rosatzin et al., 1993). Therefore, in the proposed iron-selective optical membrane sensor containing IBMYD as a neutral ionophore, the presence of a lipophilic anionic additive like NaTPB, was found to be necessary to facilitate the corresponding ion-exchange equilibrium. It has been reported that the absence of additive salts not only affects the working concentration range of the sensors, but also causes the prolonged response time and reduce selectivity (Bakker et al., 1997). Therefore, the membranes (Nos. 1–7), without additive used, result in narrower concentration ranges (Table 1) and the presence of 3 mg NaTPB is certainly required to reach the best optical response characteristic, compared with oleic acid used (OA). Thus, in this study, the membrane (No. 9, Table 1), with an optimized percent ratio of PVC:TEHP:NaTPB:IBMYD as 30:64:3:3, was chosen to fabricate further optical membrane sensors. 3.5. Effect of pH on the sensor response The effect of pH of test solution on the absorbance response of the proposed sensor was tested in the pH range of 1.5–7. The absorbance measurements were recorded for 1.0 106 mol L1 of Fe(III) ion concentration at different pH values at 377 nm. The blank membrane (membrane without IBMYD) was taken as a
Table 1 Optimization of membrane ingredients for the iron(III)-optical sensor (n = 5). Membrane number
1 2 3 4 5 6 7 8 9 10 11 12 13
Working concentration range (mol L1)
Composition (%) PVC (mg)
Plasticizer (mg)
Additive (mg)
Ionophore (mg)
30 30 30 30 30 30 30 30 30 30 30 30 30
67 67 67 67 67 67 67 64 64 64 64 64 64
0 0 0 0 0 0 0 3.0 3.0 4.0 5.0 5.5 0.5
3 3 3 3 3 3 3 3 3 2.0 1.0 0.5 5.5
(NPOE) (DOS) (AP) (DBP) (BA) (DOP) (TEHP) (TEHP) (TEHP) (TEHP) (TEHP) (TEHP) (TEHP)
(OA) (NaTPB) (NaTPB) (NaTPB) (NaTPB) (NaTPB)
5.0 104 4.0 105 4.0 105 7.0 105 8.0 105 7.0 106 2.0 106 3.0 108 1.0 109 7.0 109 9.0 106 7.0 104 3.0 103
to to to to to to to to to to to to to
7.0 102 7.0 103 3.0 103 3.0 103 2.0 103 5.0 103 3.0 103 5.0 103 1.0 103 2.0 101 3.0 102 3.0 102 7.0 101
148
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Fig. 4. Effect of pH on the response of the Fe(III)-optical membrane sensor.
reference. The absorbance measurements were expressed as absorbance difference, which was defined as the difference between the absorbance of the immobilized IBMYD alone and the absorbance of the Fe(III)–IBMYD complex at 377 nm. pH values were adjusted by the addition of dilute NaOH or HNO3. The results are shown in Fig. 4. As it is seen from Fig. 4, the absorbance difference intensity of the optical sensor increased by increasing the pH of test solution from 1.0 to 3.0, possess through a more or less plateau between pH 3.0–5.7. The diminished response at pH <3.0 could be due to the extraction of H+ from the aqueous sample solution into the PVC membrane via protonation of the nitrogen atoms in the structure of the ligand molecule On the other hand, the reduced optical response of the sensor at pH >5.7 could be also due to a possible slight swelling of the polymeric film under alkaline conditions as well as the hydroxide formation of Fe(III) ions in the sample solution. Therefore, the pH value at 4.7 was used for further studies. In the membrane fabricated with the IBMYD ionophore, this pH value is suitable for quantitative determination of Fe(III) in the mixture solution containing Fe(III)/Fe(II) or in the solution containing only Fe(III), because the Fe(III) is stable under this chosen pH value (Babakhanian et al., 2010; Ammann et al., 1985). 3.6. Solvent effect Sometimes in real situation, the samples may contain the nonaqueous contents and the functioning of the sensor system would be necessary to be investigated in partially non-aqueous media. Therefore, the sensor response was examined in the methanol– water and the acetone–water mixtures (Table 2). It was found that the membranes do not show any appreciable change in working concentration range in mixtures containing up to 5% (v/v) nonaqueous contents.
Fig. 5. Typical response curve of the film optode at 377 nm as a function of time when the film is exposed to 1.0 108 mol L1 iron ion.
containing 1.0 108 mol L1 of Fe(III) ion concentration. The response time of the present optode is controlled by the time required for the analyte to diffuse from the bulk of the solution to the membrane interface to associate with ligand. The response time was tested by recording the absorbance change from a pure buffer (pH 4.7) to a buffered 1.0 108 mol L1 Fe(III) solution. The response of the membrane was found to reach 90% of the total signal (<80 s) depending on the concentration of Fe(III). Generally, the response time is more rapid while proceeding from dilute to concentrated solution. 3.8. Reproducibility and reversibility Reproducibility and reversibility are two important characteristics in determining the suitability of an optical sensor for selective determination of the species of interest in different test solutions. The reversibility factor was examined by absorbance intensity measurements of the same membrane sensor when it was exposed to 1.0 106 mol L1 Fe(III) ion concentration in acetate buffer solution (pH 4.7) and 0.1 mol L1 of sodium thiourea. The results are shown in Fig. 6. The mean absorbance values (n = 5) were 0.987 0.007 and 0.475 0.005, in the presence of 1.0 106 mol L1 Fe(III) ion concentration and sodium thiourea
3.7. Response time Fig. 5 shows the time-dependent response characteristics of the optical membrane, which is immersed in a buffer solution Table 2 Performance of iron(III) optical PVC sensor (No. 9) in non-aqueous media. Non-aqueous content (%, v/v)
Working concentration range (mol L1)
0 Methanol 5 10 Acetone 5 10
1.0 109 to 1.0 103 1.3 109 to 1.5 103 1.5 109 to 1.7 103 1.9 109 to 1.5 103 1.7 109 to 1.5 103
Fig. 6. Variation of the absorbance of the membrane at 377 nm for repetitively exposing into 1.0 106 mol L1 of iron ion and a solution of 0.1 mol L1 of sodium thiourea.
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solution, respectively. The mean difference in absorbance was about 0.498 0.004. Thus, it could be concluded that the sensor performance is reversible and shows good reproducibility. From Fig. 6, it could also be found that the response time of the sensor is less than 3 min. The reproducibility was also examined by preparing five different membranes with the same composition, and measuring the absorbance intensity of each membrane in a 1.0 108 mol L1 of Fe(III) ion concentration (five repeated determinations) in acetate buffer solutions (pH 4.7). The resulting coefficient of variation was found to be 2.5%. Some regenerating compounds such as ethylenediamine tetraacetic acid, sodium florid, citric acid, lactic acid and sodium thiourea solutions were tested to fully recover the proposed optical membrane sensor. Among the reagents used, sodium thiourea was capable of fully recovering of the membrane at a short membrane regeneration time (<3 min). To study the influence of sodium thiourea concentration on the efficiency and time of regeneration process, different concentrations (0.01–0.1 mol L1) of sodium thiourea were tested. The best result was obtained when 0.1 mol L1 of sodium thiourea were used. 3.9. Recovery, short-term stability, lifetime and regeneration of the optode To evaluate the recovery of the proposed optical sensor, two solutions of Fe(III) with different concentrations were examined. The found values of (1.04 0.04) 106 for 1.0 106 mol L1 with n = 5 and (1.02 0.03) 105 for 1.0 105 mol L1 with n = 5 were obtained, respectively. The short term stability of the proposed optical sensor was investigated by its absorbance intensity measurements in contact with a 1.0 105 mol L1 of Fe(III) ion concentration and buffered at pH 4.7 over a period of 6 h. From the absorbance intensities, taken every 30 min (n = 12), it was found that the response was almost complete with only 1.9% increase in intensity after 6 h monitoring. The membrane sensor could also be stored in wet conditions (acetate buffer solution pH at 4.7) without any measurable change in its intensity for at least 9 weeks. To over come detaching the optical PVC membranes from the glass surfaces. The glass plates were activated according to the following procedure before use: the glass plates were activated by treatment with concentrated HNO3 for 12 h, followed by 3% HF, and 10% H2O2 each for 30 min, then washed with distilled water and ethanol (Luo et al., 2007). 3.10. Selectivity The optical membrane selectivity, which reflects the relative response of the optode for primary ion over diverse ions present in solution, might be the most important characteristic of an ionselective optode. The experiment was carried out with a fixed concentration of Fe(III) at 1.0 106 mol L1 and then measuring the changes in absorbance intensity before (A0) and after adding different foreign interfere (A) in the iron solution buffered at pH 4.7. The resulting relative error is defined as %RE = [(A A0)/ A0] 100. The tolerance limit was taken as the concentration causing an error of 5% in the analytical signal for determination of Fe(III). The results are shown in Table 3. As it can be seen, the proposed optical sensor has a good discriminating ability to Fe(III) ions over a wide variety of organic and inorganic substances. 3.11. Analytical applications The proposed sensor was examined successfully for the direct determination of iron contents in plant and cereal samples (Table 4). The samples were pre-treated by the suitable method described in this paper and analyzed by the optical sensor at pH 4.7. The results obtained by the proposed sensor were quite precise
149
Table 3 Effect of interfering ions on the determination of 1.0 106 mol L1 of Fe(III). Interfering ion
Tolerated ratio [interference]/[Fe(III)]
Interfering ion
Tolerated ratio [interference]/[Fe(III)]
Cu2+ Cd2+ Pb2+ Ag+ Zn2+ Cr3+ K+ Vitamin B6 Vitamin E D-Arginine Triton X-100
5 293 502 314 132 9 983 1084 1372 1055 962
Ni2+ Mn2+ Sr2+ Hg2+ Co2+ Al3+ Fe2+ Fructose CTAB Glucose Vitamin A
85 219 922 437 251 11 213 1147 998 1263 1429
Table 4 Determination of iron contents in plant and cereal samples with the proposed optical sensor (n = 5). Sample
Found by optode (mg per 100 g)a
Found by FAAS (mg per 100 g)a
Thyme ground Wheat bran Rosemary Cinnamon ground
126.5 2.5 46.2 1.2 31.05 1.04 39.9 1.2
126.3 2.3 46.3 1.3 31.07 1.02 39.3 1.1
a
Number of analysis; n = 5.
and accurate and in accordance with the data taken by flame atomic absorption spectrometry (FAAS). 4. Conclusion In this study, 1,10 -(iminobis(methan-1-yl-1-ylidene))dinaphthalen-2-ol (IBMYD) could be considered as a suitable chromo-ionophore agent to fabricate a new Fe(III) optical membrane sensor. The proposed sensor shows inherent advantages such as simple operation, stability, precise results, low cost, wide dynamic range, low detection limit and relatively fast response time. The sensor is also recommended for iron ion determination in some real samples such as plant and cereal samples with satisfactory results. References Ammann, D., Pretsch, E., Simon, W., Lindner, E., Bezegh, A., Pungor, E., 1985. Lipophilic salts as membrane additives and their influence on the properties of macro- and micro-electrodes based on neutral carriers. Analytica Chimica Acta 171, 119–129. Burnecka, J.B., Jankowiak, U., Zyrnicki, W., Wilk, K.A., 2004. Heavy metal content of canned orange juice as determined by direct current plasma atomic emission spectrophotometry (DCPAES). Spectrochimica Acta Part B: Atomic Spectroscopy 59, 585–590. Babakhanian, A., Gholivand, M.B., Mohammadi, M., Khodadadian, M., Shockravi, A., Abbaszadeh, M., Ghanbary, A., 2010. Fabrication of a novel iron(III)-PVC membrane sensor based on a new 1,10 -(iminobis(methan-1-yl-1-ylidene))dinaphthalen-2-ol synthetic ionophore for direct and indirect determination of free iron species in some biological and non-biological samples. Journal of Hazardous Materials 177, 159–166. Bakker, E., Bulmann, P., Pretsch, E., 1997. Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics. Chemical Review 97, 3083–3132. Chan, W.H., Yang, R.H., Mo, T., Wang, K.M., 2002. Lead-selective fluorescent optode membrane based on 3,30 , 5,50 -tetramethyl-N-(9-anthrylmethyl)benzidine. Analytica Chimica Acta 460, 123–132. Cubadda, F., Raggi, A., 2005. Determination of cadmium, lead, iron, nickel and chromium in selected food matrices by plasma spectrometric techniques. Microchemical Journal 79, 91–96. Durukan, I., S¸ahin, C.A., S¸atiroglu, N., Bektas, S., 2011. Selective determination of copper and iron in various food samples by the solid phase extraction. Microchemical Journal 99, 159–163.
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