Translate this page into:
Ion imprinted polymer based potentiometric sensor for the trace determination of Cadmium (II) ions
⁎Corresponding author. Tel.: +98 511 8797022; fax: +98 511 8796416. abf_mashhad@yahoo.com (Abolfazl Darroudi)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of the King Saud University.
Abstract
A cadmium (II) ion selective electrode (ISE) based on an ion-imprinted polymer (IIP) as a novel ionophore has been prepared and studied. The ion-selective electrode (ISE) was prepared by dispersing cadmium (II) IIP particles in 2-nitrophenyloctyl ether as a plasticizer and then embedding them in a polyvinylchloride polymeric matrix. The Cd(II) ISE showed a nernstian response for cadmium (II) over the dynamic concentration range of 2.0 × 10−7–1.0 × 10−2 mol L−1, with a slope of 29.9 mV per decade. The limit of detection was 1.0 × 10−7 M. The proposed electrode revealed good selectivity over a wide variety of other cations including alkali, alkaline earth, heavy and transition metals. The accuracy of the proposed electrode was checked through the analysis of spiked water samples.
Keywords
Ion imprinted polymer particles
Ion-selective electrode
Cadmium (II)
Sensor
Potentiometry
1 Introduction
Cadmium is a toxic heavy metal that appears in the environment mainly due to industrial processes. Besides natural exposure from soil or the earth’s crust with high cadmium content, anthropogenic processes like the combustion of coal and mineral oils, smelting, mining, alloy processing, paint industries, etc. are the major sources of cadmium to the people in the vicinity of the industrial areas. Human uptake of cadmium takes place mainly through food. Foodstuffs that are rich in cadmium can greatly increase the cadmium concentration in human bodies. Examples are liver, mushrooms, shellfish, mussels, cocoa powder and dried seaweed. Another important source of cadmium emission is the production of artificial phosphate fertilizers. Cadmium strongly adsorbs to organic matter in soil (Rezaei et al., 2008). United States Environmental Protection Agency (EPA) has found that cadmium potentially causes the following health effects when people are exposed to it at levels above the maximum contaminant level (MCL) for relatively short periods of time: nausea, vomiting, diarrhea, muscle cramps, salivation, sensory disturbances, liver injury, convulsions, shock and renal failure. In addition, in long-term, cadmium has the potential to cause the following effects from a lifetime of exposure at levels above the MCL: kidney, liver, bone and blood damage. The MCL has also been set at 0.005 milligrams per liter (mg/L) or 5 parts per billion (ppb) by EPA (United States Environmental Protection Agency, http://www.epa.gov). Therefore, the determination of trace levels of cadmium in environmental samples is necessary and of great importance. Recently, several techniques including flame atomic absorption spectrometry (FAAS), electrothermal atomic absorption spectrometry (ETAAS), inductively coupled plasma mass spectrometry (ICP-MS) and atomic fluorescence spectrometry (AFS) have been used for the determination of trace amounts of cadmium in different samples (Li et al., 2002; Ye et al., 2003; Yaman, 2005; Xu et al., 2000; Tsalev et al., 2002; Dolan et al., 2003; Dias et al., 2005; Lu et al., 2002). However, most of these techniques are expensive, time consuming and/or inconvenient for in situ monitoring. These problems could be solved by the application of electrochemical methods, which are relatively simple, low cost and readily available techniques. Among the various electrochemical methods, potentiometry with ion selective electrodes is one of the most convenient and reliable electroanalytical tools for estimating metal-ion concentrations.
Imprinted polymers are biomimetic materials used for the sensitive and selective detection of analyte molecules. Through host guest interactions, imprinted polymers often display recognition capabilities comparable to those of antibody–antigen systems. An especially appealing feature of the imprinted polymers is their potential to have the affinity and selectivity analogous to antibodies, while retaining several benefits including greater stability, ease of production, lower cost and stability in harsh environments such as high temperatures and pressures (Prasada Rao et al., 2006). Mosbach and co-workers (Rosatzin et al., 1991) described the first imprinted polymer potentioselectrode for calcium and magnesium ions. Since that, until now, numerous studies on the ion imprinted polymers (IIP) and their use in ISE have been reported (Anuradha Baghel et al., 2007; Prasad et al., 2006). This paper reports a novel Cadmium (II) ion selective electrode based on Cadmium (II) IIP particles dispersed in 2-nitrophenyloctyl ether (NPOE) and embedded in a polyvinylchloride (PVC) matrix, for the monitoring of traces of Cadmium (II) ions.
2 Experimental
2.1 Reagent and materials
Cd (II) stock solution (0.10 mol L−1) was prepared by dissolving 2.013 g of CdCl2.H2O (Fluka) in acidified deionized water and making up the final volume to 100 mL. More dilute solutions were prepared daily by appropriate dilution of this stock solution with distilled water. 5,7-Dichloroquinoline-8-ol was purchased from Acros organics (New Jersey, USA). 4-vinyl pyridine, methyl methacrylate (MMA), ethyleneglycoldimethacrylate (EGDMA), triethylamine and 2, 2-azo bisisobutyronitrile (AIBN) were obtained from Merck (Germany). In addition, O-nitrophenyloctylether (o-NPOE), dioctylphthalate (DOP), dibutylphthalate (DBP), sodium tetraphenylborate (NaTPB) and high relative molecular weight polyvinylchloride (PVC) were obtained from Aldrich (Milwauke, USA). Other chemicals were of analytical grade and purchased from Merck (Germany). Deionized water has been used throughout all experiments.
2.2 Preparation of the ion-imprinted polymer based electrodes
The Cadmium (II) ion imprinted polymer (IIP) was synthesized via a single pot method by dissolving of the ternary complex of Cadmium with DCQ and VP in acetonitrile (porogen) and then, copolymerizing thermally after the addition of MMA (monomer) and EGDMA (crosslinking monomer) in the presence of AIBN (initiator). Then the obtained polymer was ground and the imprinted Cd(II) ions were leached from the polymer particles by stirring for 6 h with three portions of HNO3 (3 M, 25 mL). Finally, the particles were washed several times with double distilled water and dried at 45 °C. The non-imprinted polymer (NIP) was prepared using the same protocol except that Cd(II) did not exist in the polymerization mixture. A graphite disk electrode was fabricated from a graphite rod with 4 mm diameter housed and glued onto a Teflon holder. The exposed surface of the disk electrode was polished to a shiny finish with SiC paper. The electrode was rinsed with deionised water and allowed to dry. The PVC membrane electrodes were prepared by thorough mixing of PVC, NPOE, NaTPB and Cadmium (II) IIP particles (60–85 μm size) in proportions of 21%, 21%, 43% and 15% (w/w), respectively and dissolving in 2.5 mL of tetrahydrofuran (THF). The mixture was homogenized in a sonicator, poured into a watch glass until a viscose mixture was obtained. Then, the polished surface of a graphite electrode was dipped into this mixture and the solvent was allowed to evaporate completely. Thereafter, a thin PVC film formed on the graphite electrode surface.
2.3 Conditioning of the electrodes and emf measurement
The Cadmium (II) IIP based ISE was conditioned for 48 h in a 0.01 mM Cadmium (II) solution at the pH value of 6.0. The potentials were measured by varying the concentration in the range of 1 × 10−8–1.0 × 10−1 M. Potential measurements were made with a potentiometer, model pHM-632 (Metrohm, Swiss) having ±0.01 mv accuracy with an Ag/AgCl reference electrode. All measurements were carried out at 25 °C with cell type:
3 Results and discussion
In this paper, the suitability of the IIP particles dispersed in NPOE and embedded in PVC for selectively recognizing cadmium ions was examined.
3.1 Characterization of Cd (II) imprinted polymer
The Cadmium IIP particles were characterized by FT-IR and SEM techniques. The surface morphologies of the leached and unleached polymers were shown by SEM as depicted in Fig. 1. The SEM images confirm the Cd (II) removal as well as the enhanced surface area in the leached polymer. Thus, the ordered nano-pattern observed in the SEM image of the leached polymer is probably due to the metal ion imprinting of Cadmium (II) into the polymer. The FT-IR spectra of the unleached and leached IIP particles were recorded by the KBr pellet method using a Bomem spectrometer. The peak near 756 cm−1 corresponded to υC–Cl in the spectra of both the leached and unleached polymers, illustrating that DCQ was kept intact in the polymer even after leaching (Fig. 2).
SEM photographs of IIP (a) unleached and (b) leached.
FT-IR spectra of the unleached (a), leached (b) IIP particles.
3.2 Potential response
The potential responses of the electrodes prepared with Cadmium (II) IIP membrane has been given in Fig. 3 in which a slope of 29.9 mV per decade over a concentration range of 2.0 × 10−7–1.0 × 10−2 M of Cadmium (II) is observed. The limit of detection was 1 × 10−7 M Cadmium (II) calculated based on the IUPAC definition. The same experiment was carried out in the case of the electrode fabricated with NIP. This electrode showed almost a constatnt potential with respect to increase in cadmium (II) concentration from 2.0 × 10−7 to 1.0 × 10−2 M.
Potential response of ISE with Cadmium (II) IIP.
3.3 Effect of electrode composition
Literature shows that the sensitivity and selectivity of the ion selective electrodes depend upon various features of the membrane such as the nature and the amount of the plasticizers and the ion recognizing material used (Ganjali et al., 2003a, 2003b; Shamsipur et al., 2000). Thus, different aspects of electrode preparation using Cadmium (II) IIP particles were optimized and the results have been shown in Table 1. It is well known that the plasticizer has major effects on the electrochemical properties of conventional ISEs. These solvent mediators strongly influence the working concentration range of ISEs. Thus, the effect of different plasticizers, i.e. O-NPOE, DOP and DBP, on the potential response of the cadmium (II) IIP based ISE has been investigated. In addition, the addition of appropriate amounts of plasticizer leads to optimum physical properties and ensures the high mobility of ions in the membrane. As results show, the membrane with O-NPOE as plasticizer gives a nernstian slope over the concentration range of 2.0 × 10−7–1.0 × 10−2 M with a detection limit of 1 × 10−7 M. This is in accordance with the previously reported ISEs as in the case of ISE reported by Shamsipur et al. (2000). The ISE with plasticizer having higher dielectric constant viz., O-NPOE (ε = 24.0) gives better sensitivity compared to DOP (ε = 5.2) and DBP (ε = 6.4).
| No. | PVC (%) | Plasticizer (%) | IIP particle (%) | NaTPB (%) | Slope(mV decade−1) | Linear range |
|---|---|---|---|---|---|---|
| 1 | 21 | NPOE, 50 | 21 | 8 | 27.5 | 1 × 10−3 to 1 × 10−6 |
| 2 | 31 | NPOE, 40 | 21 | 8 | 23.1 | 1 × 10−2 to 1 × 10−6 |
| 3 | 21 | NPOE, 40 | 21 | 8 | 28.1 | 1 × 10−2 to 1 × 10−6 |
| 4 | 21 | NPOE, 43 | 21 | 15 | 29.9 | 1 × 10−2 to 2 × 10−7 |
| 5 | 21 | DOP, 43 | 21 | 15 | 39.1 | 1 × 10−2 to 1 × 10−6 |
| 6 | 21 | DBP, 43 | 21 | 15 | 19.2 | 1 × 10−3 to 1 × 10−6 |
Moreover, the effect of the addition of NaTPB to Cadmium (II) IIP particle based membrane on the potentiometric response of the electrode was examined. There are several reports on the effect of lipophilic salts or ionic additives upon the characteristics of ISEs as they reduce the anionic interference and lower the electrical resistance of the membrane in conventional ISEs (Gupta et al., 2007).
3.4 pH effect of the test solution
The pH dependence of the sensor number 4 (Table 1) has been tested over the range of 3–9 for 1.0 × 10−4 M Cd2+. The pH was adjusted with a dilute solution of nitric acid or sodium hydroxide. The potential of the sensor was plotted as a function of pH (Fig. 4). It is clear that the potential remains constant over the pH range of 4–7, which may be taken as the working pH range of the sensor assembly. The observed increase in the potential at low pH values indicates that the ionophore responds to the hydronium ions.
Effect of pH of the test solution (1 × 10−5 M of Cadmium (II)) on the potential response of the Cadmium (II) IIP based ISE.
3.5 Response time
Response time is the average time required for the electrodes to reach a potential response within ±1 mV of the final equilibrium value. The response time was recorded by changing the cadmium (II) concentration in solution over a concentration range of 1.0 × 10−6–1.0 × 10−4 M. The actual potential versus time traces has been shown in Fig. 5. As seen from the figure, the cadmium (II) IIP based ISE reaches the equilibrium response in a time of about 15 s. To evaluate the reversibility of the electrode, a similar procedure in the opposite direction was adopted. The measurements performed in the sequence of high to low concentration (from 1 × 10−4 to 1 × 10−6 M) indicate that the response of the cadmium (II) IIP based ISE was reversible.
Dynamic response of the Cadmium (II)IIP based ISE for stepwise concentration change of Cadmium (II) (A) 1.0 × 10−6 M to1.0 × 10−5 M, (B) 1.0 × 10−5 M to 1.0 × 10−4 M.
3.6 Interference studies
Potentiometric selectivity coefficients, describing the preference of the Cadmium (II) IIP based ISE for cadmium ion (A) relative to an interfering ion B were determined by the matched potential method, recommended by IUPAC (Umezawa et al., 1995). According to this method, a specified activity (concentration) of the primary ion (A = 1 × 10−5 M) is added to a reference solution (5.0 × 10−6 M of cadmium (II)) and the potential is measured. In a separate experiment, interfering ions are successively added to an identical reference solution, until the measured potential matched with that obtained on the addition of the primary ions. The selectivity coefficient has been given by the resulting primary ion to interfering ion activity (concentration) ratio. As shown in Table 2 the selectivity coefficients are in the order of ∼1.0 × 10−2 or smaller.
| Mn+ | |
|---|---|
| Na+ | 4.0 × 10−4 |
| K+ | 5.0 × 10−4 |
| Cu2+ | 1.0 × 10−2 |
| Ni2+ | 1.0 × 10−2 |
| Mg2+ | 6.0 × 10−4 |
| Ca2+ | 1.0 × 10−3 |
| Ag+ | 1.0 × 10−5 |
| Zn2+ | 4.0 × 10−3 |
3.7 Analytical application
The proposed cadmium (II)-selective electrode was applied to obtain recovery of cadmium (II) in a tap water sample with satisfactory results. The analysis was performed by using the standard addition technique. The results are summarized in Table 3. Good recovery was obtained in tap water.
4 Conclusions
In conclusion, we have successfully demonstrated the design and development of Cadmium (II) ion sensitive ISE based on IIP particles which is highly selective for Cadmium (II) compared to several coexisting inorganic cations even with a nonselective DCQ and VP as chelating agents. By proper choice of chelating agents and employing the concept of ion imprinting, one can fabricate ISE, which can be specific for Cadmium (II) in the presence of other inorganics. Studies are in progress in choosing one such chelating agent.
Acknowledgements
The authors wish to thank the Ferdowsi University of Mashhad for the financial support of this work.
References
- Synthesis and characterization of metal ion imprinted nano-porous polymer for the selective recognition of copper. Biosens. Bioelectron.. 2007;22:3326-3334.
- [Google Scholar]
- Method development for the determination of cadmium, copper, lead, selenium and thallium in sediments by slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry and isotopic dilution calibration. Spectrochim. Acta B. 2005;60:117-124.
- [Google Scholar]
- Analysis of dietary supplements for arsenic, cadmium, mercury, and lead using inductively coupled plasma mass spectrometry. J. Agric. Food Chem.. 2003;51:1307-1312.
- [Google Scholar]
- Gliclazide as novel carrier in construction of PVC-based La(III)-selective membrane sensor. Talanta. 2003;59:613-619.
- [Google Scholar]
- Novel gadolinium PVC-based membrane sensor based on omeprazole as an antibiotic. Electroanalysis. 2003;18:1251-1255.
- [Google Scholar]
- Schiff bases as cadmium(II) selective ionophores in polymeric membrane electrodes. Anal. Chim. Acta. 2007;583:340-348.
- [Google Scholar]
- A flow injection on-line multiplexed sorption preconcentration procedure coupled with flame atomic absorption spectrometry for determination of trace lead in water, tea, and herb medicines. Anal. Chem.. 2002;74:1075-1080.
- [Google Scholar]
- Simultaneous determination of trace cadmium and arsenic in biological samples by hydride generationdouble channel atomic fluorescence spectrometry. Anal. Chem.. 2002;74:1525-1529.
- [Google Scholar]
- Ion imprinted polymer based ion- selective electrode for the trace determination of dysprosium(III) ions. Anal. Chim. Acta. 2006;566:69-74.
- [Google Scholar]
- Metal ion-imprinted polymers—Novel materials for selective recognition of inorganics. Anal. Chim. Acta. 2006;578:105-116.
- [Google Scholar]
- A fast response cadmium-selective polymeric membrane electrode based on N, N’-(4-methyl-1,2-phenylene)diquinoline-2-carboxamide as a new neutral carrier. Hazard. Mater.. 2008;153:179-186.
- [Google Scholar]
- Preparation of Ca2+ selective sorbents by molecular imprinting using polymer is able ionophores. J. Chem. Soc., Perkin Trans.. 1991;25:1261-1265.
- [Google Scholar]
- PVC-based 1,3,5-trithiane sensor for Cerium (III) ions. Anal. Chem.. 2000;72:2391-2394.
- [Google Scholar]
- Electrothermal atomic absorption spectrometric determination of cadmium and lead with stabilized phosphate deposited on permanently modified platforms. Talanta. 2002;58:331-340.
- [Google Scholar]
- Selectivity coefficients for ion-selective electrodes: recommended methods for reporting values (Technical Report) Pure Appl. Chem.. 1995;67:507-518.
- [Google Scholar]
- United States Environmental Protection Agency. <http://water.epa.gov/drink/contaminants/basicinformation/historical/upload/Archived-Consumer-Factsheet-on-Cadmium.pdf>.
- A sequential injection on-line column preconcentration system for determination of cadmium by electrothermal atomic absorption spectrometry. Spectrochim. Acta B. 2000;55:213-219.
- [Google Scholar]
- The improvement of sensitivity in lead and cadmium determinations using flame atomic absorption spectrometry. Anal. Biochem.. 2005;339:1-8.
- [Google Scholar]
- Determination of trace cadmium in rice by flow injection on-line filterless precipitation-dissolution preconcentration coupled with flame atomic absorption spectrometry. J. Agric. Food Chem.. 2003;51:2111-2114.
- [Google Scholar]