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A palladium imprinted polymer for highly selective and sensitive electrochemical determination of ultra-trace of palladium ions
Electrochimica Acta 149 (2014) 108–116
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A palladium imprinted polymer for highly selective and sensitive electrochemical determination of ultra-trace of palladium ions Majid Kalate Bojdi a , Mohammad Behbahani b, *, Ali Sahragard b , Bahareh Golrokh Amin b , Alireza Fakhari b , Akbar Bagheri b a b
Faculty of Chemistry, Kharazmi (Tarbiat Moallem) University, Tehran, Iran Department of Chemistry, Shahid Beheshti University, G.C., Tehran 1983963113, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 August 2014 Received in revised form 16 October 2014 Accepted 21 October 2014 Available online 23 October 2014
We report on the design of a palladium(II)-selective electrode based on the use of palladium(II) imprinted polymer nanoparticles (IP-NPs), and its application for the anodic stripping deferential pulse voltammetry determination of palladium ions. The IP-NPs were obtained by precipitation polymerization of 4-vinylpyridine (the functional monomer), ethylene glycol dimethacrylate (the cross-linker), 2,2’azobisisobutyronitrile (the initiator), eriochrome cyanine R (the palladium-binding ligand), and palladium ions (the template ion) in methanol solution. After polymerization, the Pd2+ in the polymer nanoparticles were leached out with dilute hydrochloric acid to create cavities for hosting Pd2+. The new sensor showed high selectivity for palladium ions in the presence of common potential interferences according to the specific recognition nature of the synthesized imprinted material. A carbon paste electrode was modified with the IP-NPs, and differential pulse stripping voltammetry was applied as the detection technique after open-circuit sorption of Pd2+ ions and its reduction to the metallic form. An explicit difference in the response was observed between the electrodes modified with IP-NPs and electrodes modified with non-IP-NPs. The modified electrode responses to Pd2+ was linear in the 0.01 nmol dm 3 to 1.0 nmol dm 3 (with sensitivity of 92.25 nA/nmol dm 3) and in the 1.0 nmol dm 3 to 1.0 mmol dm 3 (with sensitivity of 16.12 mA/mmol dm 3) concentration ranges. The limit of detection (LOD) of the sensor was 3.0 pmol dm 3 (at S/N = 3). The sensor was successfully applied to the trace determination of Pd2+ in spiked environmental water and soil samples. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Carbon past electrode palladium imprinted polymer nanoparticles Differential pulse anodic stripping voltammetry Environmental water and soil samples
1. Introduction Palladium is an element with increasing demands in today’s industries. In 2007, an amount of 192 tons of palladium has been sold in the world market. The main demands are automotive catalyst (55%), electronics (17%), jewellery (11%) and dentistry (9%) [1], especially due to the catalytic properties it is widely used in the synthesis of many materials. Unfortunately, elevated level of palladium compared to geochemical background [2] has been found in airborne particulate matter [3], road dust [4–6], soil [7,8] and grass [9]. Anthropogenic palladium has been reported to be mobile and bioaccumulated by aquatic organisms and generally to a larger extent than other platinum group elements [10–13]. Moreover, metallic palladium has an allergenic potential on humans [14]. Therefore, monitoring of palladium in environmental
* Corresponding author. Tel.: +98 21 22431661; fax: +98 21 22431683. E-mail address: [email protected] (M. Behbahani). http://dx.doi.org/10.1016/j.electacta.2014.10.096 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
samples has great importance with respect to estimation of the future risk of the human health and the ecosystem. The toxicity of palladium demands for practical techniques to remove it from environment. In addition, because of the high economic value of palladium, its separation even in trace amounts is beneficial. Some of the valuable techniques, including ET-AAS [15,16], ICP-AES [17], FAAS [18], ICP-MS [19] and total reflection XRF-spectrometry [20] have been used for palladium determination. The above mentioned methods suffer from some sensitivity and selectivity limitations for real samples [15–20]. However, voltammetric methods are highly favorable techniques for the determination of metal ions because of its low cost, high sensitivity, easy operation and the ability for portability. In order to enhance the sensitivity and selectivity of the electrochemical determination of palladium(II), chemically modified electrodes have received increasing attentions in the past decades [21–23]. Anodic stripping deferential pulse voltammetry (ASDPV) is the most attractive electrochemical technique for the determination of trace heavy metals due to its high sensitivity and selectivity with modified electrodes.
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Advantage of ASVDPV over the other three methods (AAS, ICP-AES or ICP-MS) is the simplicity of the required instrumentation, which is relatively inexpensive, low electrical power consumption, portable as well as suitable for automation [24–26]. On the other hand, ion-imprinted polymers (IIPs) are recognizing metal ions after imprinting, while retaining all the virtues of MIPs. The production of polymers exhibiting selective binding of a specific cation, involves the formation of cavities equipped with complexing agents so arranged as to match the charge, coordination number, coordination geometry and size of the target cation. Although the bulk MIP prepared by conventional methods exhibits high selectivity [27,28], some disadvantages were suffered, such as the heterogeneous distribution of the binding sites, embedding of most binding sites, and poor site accessibility for template molecule [29]. Consequently, the consideration of researchers has moved toward achieving highly uniform spherical imprinted particles, particularly on the nanoscale [30]. In contrast to monoliths, MIPs possess cavities designed for a target analyte, providing a retention mechanism based on molecular recognition; by this, imprinted cavities are more easily available to templates and the binding kinetic is increased [31–35]. Different methods are used to obtain MIPs Nano particles, namely suspension, multistep swelling and precipitation polymerization. The different methods of preparation have been well documented in a recent review by Haginaka [36]. Among them, the precipitation technique is the most convenient one, because it is a homogeneous, one-step synthesis and doesnot require the use of a surfactant or stabilizer that can remain adsorbed to the surface of the polymer, interfering with the selective binding of the target to the imprinted materials. The first prepared MIPs, using precipitation polymerization technique, were reported by Ye et al. [37]. The polymer particles were obtained according to this procedure, have more uniform particles size and require no crushing and sieving steps in comparison to the polymers obtained by bulk polymerization. In the present work, combining the advantages of high selectivity from the IIP nanoparticles technique and high sensitivity from CPE detection, an IIP-CPE sensor has been developed for the determination of pallaium ions in complex matrices. Some advantages of this work are simplicity, rapidity, high selectivity, being operative and cheap, easy to usage and sensitive determination of low levels of palladium ions in aqueous solution. 2. Experimental 2.1. Materials 4-vinyl pyridine with high purity was purchased from Merck (Darmstadt, Germany, www.merck.de). Ethylene glycol dimethacrylate (EGDMA) and Eriochrome Cyanine R were obtained from Fluka (Buchs SG, Switzerland, www.sigmaaldrich.com). 2,2;Azobisisobutyronitrile (AIBN) was obtained from Acros Organics (New Jersey, USA). NaOH, HCl, HNO3, acetic acid (HOAC) and methanol were purchased from Merck (Darmstadt, Germany). All the other reagents used were of analytical grade and purchased from Merck (Darmstadt, Germany, www.merck.de). Ultrapure water was prepared using a Milli-Q system from Millipore (Bedford, MA, USA). 2.2. Apparatus All electrochemical measurements were performed with a Palm-Sens (EN 50,081–2) potentiostat. A personal computer was used for data storage and processing. The three electrode electrochemical cell used was equipped with a modified (Pd2+
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ion imprinted polymer) carbon paste electrode (MCPE) as a working electrode, a saturated calomel electrode (SCE) as a reference electrode and a platinum electrode as an auxiliary electrode (Azar Electrode Co, Iran). All potentials in the text were reported versus this reference electrode. The pH measurement was performed with a Metrohm model 691 pH/mV meter. Heidolph heater stirrer model MR 3001 (Germany) was employed for heating and stirring of the solutions. Scanning electron microscopy (SEM) was performed by gently distributing the powder sample on the stainless steel stubs, using SEM (KYKY, EM3200) instrument. The thermal properties of synthesized polymers were determined using a BAHR-Thermoanalyse GmbH (Germany) with employing, heating and cooling rates of 10 C min 1 and using a condenser as the coolant. The samples were weighed as a thin film and carefully packed into a clean aluminum pan (11.5–12.5 mg), and sealed by crimping an aluminum lid on the pan (Shimadzu universal crimper). An Al2O3 empty pan sealed with a cover pan was used as a reference sample. A scanning range of 10 to 800 C was used for samples at 10 C min 1 in nitrogen gas. The CHN analysis was performed on a Thermo Finnigan Flash EA112 elemental analyzer (Okehampton, UK). The Brunauer–Emmett–Teller (BET) of the synthesized imprinted and non-imprinted nanoparticles was analyzed by nitrogen adsorption in a nitrogen adsorption apparatus (BEL Japan, BELSORP-18 Plus). The samples were degassed at 150 C prior to the nitrogen adsorption measurements. 2.3. Synthesis of Pd2+ ion imprinted polymer (IIP) and non-imprinted polymer nanoparticles (NIP) The nanosized Pd2+-imprinted polymer was prepared by precipitation polymerization technique. In the first step, 2 mmol of 4-vinyl pyridine (functional monomer) and 2 mmol of Eriochrome Cyanine R (the palladium-binding ligand) were dissolved in 50 mL of methanol in a 100 mL glass flask. Subsequently, as the second step, 1 mmol of Pd (NO3)2 as an imprinted metal ion (template) was added slowly to a glass flask and the resulted mixture was stirred for 5 h at room temperature. In the third step, 14 mmol of EGDMA and 75 mg of AIBN were added as cross-linker and initiator. The oxygen of the sample solution was removed by bubbling nitrogen through the sample for 10.0 min. Polymerization was performed in an oil bath at 65 C for 24 h in the presence of nitrogen under magnetic stirring at 400 rpm. The prepared polymer was washed several times with 1:4 (v/v) methanol/water to remove the unreacted materials and then with HCl (1.0 mol L 1) for leaching the imprinted metal ions until the washing solution was free from palladium ions. Finally, it was washed with double distilled water until reached a neutral pH (Fig. 1 provides the images of synthesized nanosized IIP after each step). The resulting fine powder was dried under vacuum in a desiccator before sorption and desorption studies. In the same way, the nonimprinted polymer (NIP) was also prepared without palladium ions.
2.4. Preparation of IIP-MCPE Based on optimized conditions, ion imprinted polymer modified carbon paste electrode (IIP-MCPE) was prepared. A mixture of IIP (15% w/w), graphite powder and paraffin oil (55:30% w/w) was blended by hand in a mortar with a pestle to construct the IIP-CPE. The body of the carbon paste working electrode was a Teflon rod with a hole (2 mm in diameter and 5 mm deep) bored at one end for paste filling. The electrical connection was made with a copper wire through the center of the rod.
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2.5. Real sample pretreatment The polyethylene bottles filled with the samples were cleaned with detergent, water, diluted nitric acid and water in sequence. The samples were immediately filtered through a cellulose filter membrane (pore size 0.45 mm), and were acidified to pH of 2.0 for storage. Tap water samples (60 mL) were taken from our research laboratory without pretreatment (pH adjusted to 6.0). Before analysis, the water samples (60 mL) which were taken from
Caspian Sea and river (Siahroodriver (Ghaemshahr, Iran)) were adjusted to pH of 6.0 according to optimized experimental conditions. To further study, the soil sample was collected from Mouteh gold mine (Iran). Soil samples (0.1 g) were digested using 8 mL mixture of 5% aqua regia with the assistance of a microwave digestion system. Digestion was carried out for 2 min at 250 W, 2 min at 0 W, 6 min at 250 W, 5 min at 400 W, and 8 min at 550 W, and the mixture was then vented for 8 min. The residue from the
Fig. 1. Schematic illustration of imprinting process for the preparation of palladium-imprinted polymer nanoparticles by precipitation polymerization technique.
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digestion, as well as a controlled digestion was then diluted with deionized water. Finally, the pH of each solution was adjusted to 6.0. 2.6. Determination procedure The prepared electrode was inserted into the solutions containing the Pd2+ at pH 6.0, being at a stirring state of 400 rpm for 7 min. Then, the electrode was rinsed and finally placed in the electrochemical cell containing 10 mL 0.5 mol L 1 HCl. At first, a negative pre-potential of -1.0 V was applied to the electrode for 35 s to reduce the recognized target ions (the stripping step was performed in a second solution to achieve the selectivity of the sensor for palladium ions because a reduction step which in the presence of other cations would cancel the selectivity of the method) and then the differential pulse voltammetry was performed in the potential range of 0.3 to 0.7 V. 3. Results and discussion 3.1. Characterization of IIP (SEM, TGA/DTA, CHN and N2 adsorption analysis) The resulting nanosized imprinted polymer were characterized by colorimetric studies, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential thermal analysis (DTA), N2 adsorption analysis and elemental analysis. After synthesis of un-leached and leached IIP, colorimetric studies were carried out to compare the changes in the color of prepared powders. An obvious change in the color from violet of un-leached IIPs to light-red after the leaching process clearly indicated the successful removal of palladium ions from the polymeric matrix. Meanwhile, adsorption process caused an instant color change in the leached sample from light-red to a violet color due to fast extraction of palladium ions into the imprinted polymeric matrix brown at the desired pH. Thermal stability of the leached (The imprinted polymer has been washed with optimized elution solvent and became free from target ions after washing) and unleached (The imprinted polymer which is not washed with optimized elution solvent and in this condition, the target ions is sorbed in the synthesized imprinted polymer) imprinted polymer was evaluated by TGA. Figure 1S (Electronic Supplementary Material (ESM)) and 2 S show DTA and TGA plots for leached and unleached imprinted polymer, respectively. In DTA plots of palladium imprinted polymer, exothermic peaks were observed at 371 and 512 C (for unleached) and 362 and 442 C (for leached). As can be seen in the DTA plots, the temperature for maximum of peaks in palladium-IIP was observed at higher temperature of 371 and 512 C; while this event for the leached IIP was happened in the lower temperature of 362 and 442 C. These events confirm the higher thermal stability of the un-leached relative to leached polymer, which is due to the presence of palladium ions in the un-leached polymer and also its strong complexation with Eriochrome Cyanine R in the polymeric network. As it is shown in TG plot for unleached ion imprinted polymer, weight loss for palladium-IIP was about 60%, and this amount of reduction in weight is related to the presence of palladium ions in polymer bead. While decrease in weight for leached imprinted polymer up to 100%, is due to the absence of palladium ions in polymer. These observations indicate that the formation of palladium-imprinted polymer and elution of palladium ions from the polymer was performed successfully. As evidence by the N2 sorption test, the imprinted sorbent showed a specific surface area of 39.2 m2 g 1 whereas the nonimprinted sorbent only had a specific surface area of 14.1 m2 g 1. The specific surface areas obtained for both sorbents are
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distinguishing indicating that the surface area was largely influenced by the imprinting technique. The results of elemental analysis (EA) of acid leached IIPs were found to be: (%found); C, (59.98%); H, (9.628%); S, (1.252%); N, (3.492%); O, (25.648%). These observations confirm the successful synthesize of palladium imprinted polymer nanoparticles. The morphology of the palladium-IIP was assessed by scanning electron microscopy, and the SEM micrograph is shown in Fig. 2. The SEM pattern showed particles with the size of about 25–45 nm. As a result, palladium-IIP can be used as a nano-size selective electrode for very fast extraction and trace detection of palladium ions in complex matrices. 3.2. Effect of nature and concentration of Leachant The influence of nature of mineral acids used as leachant on the desorption of Pd2+ ions from the IIP nanoparticles was examined by using 10 mL portions of 1 mol L 1 HCl, 1 mol L 1 HNO3 and 0.5 mol L 1 H2SO4, which resulted in respective percent desorptions of >99%, 93.4% and 84.5%. Thus, 1 mol L 1 HCl was selected as leachant because of its better leaching characteristics over the other two mineral acids. In order to study the optimum leachant concentration, for quantitative desorption of extracted Pd2+ ions from the prepared IIP, several 10 mL portions of hydrochloric acid solutions with different concentrations (i.e., 0.1, 0.2, 0.5 and 1.0 mol L 1) were used for leaching of Pd2+ ions from the imprinted sites in the polymer matrix. It was found that desorption of Pd2+ ions increases with increasing hydrochloric acid concentration (i.e., with respective percent desorptions of 48.2%, 58.1%, >99%, and >99% (for 0.1, 0.2, 0.5 and 1 mol L 1)). This is most possibly due to increased protonation of the heteroatoms of ligand in the polymer network. Thus, a 0.5 mol L 1 HCl concentration of hydrochloric acid was selected as electrolyte medium in the voltammetric determinations of palladium ions. 3.3. Evaluation of the effect of Pd2+ extraction conditions on the electrode response 3.3.1. Effect of IIP-CP composition on its response Since the amount of IIP in the modified CPE plays a key role in the response characteristics of the IIP-CPEs, the optimum composition of IIP was thoroughly studied. In a series of experiments, the amount of paraffin oil was kept constant at 30% level, and in the remaining 70% solid phase the relative amount of IIP/graphite was changed, and the voltammetric signals
Fig. 2. The SEM image of palladium-imprinted polymer nanoparticles.
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of a 5.0 10 7 mol dm 3 Pd2+ ion were obtained under the same experimental conditions for the extraction and voltammetric determination steps. As shown in Fig. 3S (ESM) the IIP-CP voltammetric signal is increased with increasing amount of IIP in the paste up to 11%. However, a further increase in the IIP content resulted in some considerable decrease in the voltammetric signal, most possibly due to the diminished conductivity of the electrode. In addition, in the case of a paste with 13% IIP, it was found that the physical property of the electrode surface was not practically suitable for the renewal of the electrode surface. Thus, a graphite powder/IIP/paraffin oil electrode composition of 59%/11%/ 30% was selected as optimal condition for preparation of IIP-CPE in further studies. 3.3.2. The pH effect The pH of the extraction solution was also checked and its effect on the Pd2+ extraction in the electrode was studied. For this purpose, the prepared electrode was inserted into solutions with various pH values where they were incubated for 10 min at a constant stirring rate. After the mentioned time, the electrode was removed from the solution and immersed into the solution of the electrochemical cell. The results of this experiment are shown in Fig. 3. As can be seen, stabilizing the pH is crucial task in this work. The optimum pH for the proposed method was found to be 6.0. Decreasing of the pH, lower than 6.0, decreases the electrode signal due to the decrease in the amount of the target ion extracted into the electrode. This is probably because of the protonation of hetroatoms of the selective sites of the IIP, that weaken the interaction of Pd2+ with the hetroatoms, existing in the selective sites of the IIP. At higher pH values (above 7.0) the signal of the sensor toward palladium ions was constant. Therefore, pH of 6.0 was select as the optimum pH for next experiments. 3.3.3. The effect of extraction time The influence of extraction time on the sensitivity of the proposed palladium sensor was investigated over a time period of 1 to 13 min, while keeping the other experimental conditions constant, and the results are illustrated in Fig. 4S (ESM). As can be seen in Fig. 4S (ESM), an increase in extraction time resulted in an intensive increase in the extracted amount of Pd2+ ion and
consequently in the intensity of the corresponding voltammetric signals, until an extraction time of about 7 min is reached. While, upon a further increase in extraction time of palladium ions the intensity of the voltammetric signal remained more or less constant. In order to decrease analyzing time, as much as possible, the time of 7 min was selected for further studies. 3.3.4. The effect of stirring rate In order to optimize the stirring rate in the extraction period of 7 min, Pd2+ was extracted in the prepared IIP-CP electrodes at various stirring rates from 100 to 700 rpm, whereas the other extraction parameters were kept constant. The obtained results showed that an increase in stirring rate enhanced the electrode response for Pd2+ up to 400 rpm, indicating considerable effect of the stirring rate on the extraction of palladium ions in the IIP-CP electrode. However, further increase in stirring rate did not affect considerably the extraction amount. Therefore, a stirring rate of 400 rpm was chosen as optimal value for further experiments. 3.3.5. Optimization of the pre-reduction time In this work, a definite pre-reduction was applied to the electrode for a determined time and then the potential was scanned for recording the oxidation peak. It is clear that the prereduction magnitude and its applying time can influence the sensor performance. Thus, the effect of these parameters on the response magnitude of the developed sensor was investigated. Increasing of the time of the applied pre-reduction ( 1.0 V) in the time range of 5–35 s resulted increase in the electrode response. As is quite obvious from Fig. 5S (ESM), an increase in pre-reduction time resulted in an intensive increase in the signal and consequently in the intensity of the corresponding voltammetric signals, until a pre-reduction time of about 35 s is reached. While, upon a further increase in pre-reduction time of palladium ions the intensity of the voltammetric signal remained constant. In order to decrease analyzing time, as much as possible, the time of 35 s was selected for further studies. 3.3.6. Effect of sample volume In the analysis of real samples, the sample volume is one of the important parameters influencing the preconcentration factor of
Fig. 3. Differential pulse voltammetry of 1.0 10 9 mol dm 3 Pd2+ on IIP-CPE at different pH. Conditions: (i) extraction step: extraction time = 7 min, stirring rate = 400 rpm; (ii) stripping voltammetry step: E-conditioning = -1.0 V, conditioning time = 35 s, voltage step = 0.01 V, pulse amplitude = 0.1 V, pulse time = 0.01 s, scan rate = 0.1 V s 1 (inset: The obtained current peak vs. pH).
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Fig. 4. Differential pulse voltammetry of 1.0 10 9 mol dm 3 Pd2+ on IIP-CPE at different volume. Conditions: (i) extraction step: Solution pH = 6.0, extraction time = 7 min, stirring rate = 400 rpm; (ii) stripping voltammetry step: E-conditioning = -1.0 V, conditioning time = 35 s, voltage step = 0.01 V, pulseamplitude = 0.1 V, pulse time = 0.01 s, scan rate = 0.1 V s 1 (Inset:The obtained current peak vs. sample volume.)
target ion. Therefore, the effect of sample volume on quantitative adsorption of palladium ion was investigated. The extraction volume has a significant effect on the sensor performance, meaning that the extraction efficiency is influenced by the extraction volume. The first increase in the electrode signal (from 5–60 mL), as a result of extraction volume enhancement, is reasonable; because, as the solution volume increases, the preconcentration factor of the extraction step increases correspondingly. However, the finite adsorption sites exist on the electrode surface, limiting thus the signal augmentation manner to a limited extraction volume. After saturation of the adsorption sites of the electrode surface (nano IIP sites), further increase in the extraction solution volume can lead the solute-selective sites
interaction to be disturbed, decreasing thus the extraction efficiency (from 60–80 mL). It can be seen that when the extraction volume becomes 60 mL, the electrode reaches to an optima (Fig. 4). Thus, 60 mL was selected as an optimal for sample volume. 3.4. Comparison of the IIP-CP electrode with NIP-CP electrode and evaluation of the washing effect on the responses of the electrodes The carbon paste electrodes modified with IIP and NIP were incubated in the Pd2+ solution (1.0 10 9 mol dm 3) while the solution was stirred continuously. Then, the electrode was inserted into the electrochemical cell and a negative potential was applied to the electrode. Subsequently, differential pulse voltammetry
Fig. 5. Comparison of differential pulse voltammetry responses of IIP-CP electrode and NIP-CP electrodes with and without washing steps; [Pd2+] = 1.0 10 9 mol dm 3, Solution pH = 6.0, extraction time = 7 min, stirring rate = 400 rpm; stripping voltammetry conditions: E-conditioning = -1.0 V, conditioning time = 35 s, voltage step = 0.01 V, pulseamplitude = 0.1 V, pulse time = 0.01 s, scan rate = 0.1 V s 1. (a) IIP- before washing (b) IIP- after washing with water (c) NIP- before washing (d) NIP- after washing with water
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Fig. 6. Differential pulse voltammetric responses of 1.0 10 ratios of 25, 50, 75, 100, 125, 150, 175, 200.
9
mol dm
3
Pd2+ at pH 6.0 in the absence and presence of excess amounts of Cu2+ ions with [Cu2+]/[Pd2+]molar
technique was applied for the determination of palladium ions. The obtained voltammetric signals of the two mentioned electrodes are shown in Fig. 5. As it can be seen, the obtained signal for IIP-CPE (a) is noticeably larger than that for NIP-CPE (c), indicating the existence and properly functioning of the selective cavities in the IIP, created in the polymerization step. It is interesting to note that, washing of both electrodes after removing them from the initial palladium sample solution, resulted in some decrease in intensity of the corresponding voltammetric signals, the extent of which in the NIP (d) signal being much larger than that of the IIP (b). This observation emphasizes the fact that the weakly surface adsorbed palladium ions on the NIP-CPE electrode are much larger than that on the IIP-CPE. 3.5. Evaluation of the selectivity of IIP-CPE The selectivity of Pd2+ imprinted polymer in the IIP-CPE was investigated by incubation of the electrode in different solutions containing a mixture of Pd2+ (1.0 10 9 mol dm 3) and some potential interfering cations at various concentrations and subsequent electrochemical analysis. Sample Pd2+ voltammograms obtained in the absence (a) and presence of Cu2+ ions (b, c, d, e, f and g) are shown in Figure 6 clearly shows that the presence of copper at a Cu2+/Pd2+ ratio of 150 possesses a negligible effect on the voltammetric signal of palladium ion. However, a 175-fold molar excess of Cu2+ ions significantly affects the voltammetric signal of Pd2+, indicating the presence of competition between these ions for occupying the selective cavities in the IIP. It is worth mentioning that when the electrode is washed, the weakly adsorbed species, which are most commonly the interfering ions, are removed from the electrode surface. Thus, such washing process can be used for improving the sensor selectivity by significantly decreasing the interference effects. The interference levels of several transition metal ions, including Co2+, Zn2+, Hg2+, Cu2+, Ni2+, Fe2+, Cd2+, Pb2+, Mn2+, Cr3+ and Ag+ ions, which are
usually present with palladium ion in complex matrices, are summarized in Table 1. As is obvious, these cations possess no significant effect on the determination of Pd2+ ion by the proposed IIP-based sensor. The tolerance limit was established as the maximum concentration of foreign species that caused a relative error of 5% in the analytical signal. 3.6. Stability and reproducibility of the sensor Reproducibility and stability of the sensor was also investigated. For 9 successive determinations of 1.0 10 9 mol dm 3 palladium ions at the same modified electrode, the relative standard deviation (R.S.D.) was 5.1%. Fabrication reproducibility was estimated with five different electrodes, which were constructed by the same procedure, R.S.D. was 6.8%. 3.7. Analytical performance of IIP-CPE Fig. 7 displays differential pulse voltammograms (DPVs) obtained upon increasing the Pd2+ concentrations for dynamic linear range under the optimized condition in water sample. Method detection limit (MDL) and limit of quantification (LOQ) of the sensor were 3.0 10 12 mol dm 3 (3 S/N) and 1.0 10 11 mol dm 3 (10 S/N), respectively. The calibration equations for detection of Pd2+ with proposed sensor were as following equation (the sensitivity of the sensor is different for low and high concentration, because the available cavities for palladium ions in low concentration is different to this value for high concentration.): I (mA) = 16.12 C (mmol dm 3) + 0.96 R2 = 0.995 I (nA) = 92.25 C (nmol dm 3) + 31.94 R2 = 0.995 The repeatability and intermediate precision for five replicate determinations of 1.0 10 9 mol dm 3 Pd2+ with the proposed sensor was 4.2 and 5.3%. 3.8. Real sample analysis
Table 1 Interference levels for some tested ions in the determination of Pd2+ by developed sensor. Species
Interference level
Cu2+, Cd2+, Zn2+, Hg2+, Pb2+ Ni2+, Co2+, Mn2+, Fe2+ Cr3+, Ag+NO3 , Cl PO43 CO32
>150 >125 >175 > 150 > 200 > 150
,
SO42
To demonstrate the potential application of our sensor in environment analysis, we applied the prepared sensor to evaluate the palladium content of different environmental samples (Table 2). The recoveries of palladium ions from the real and spiked samples varied in the range of 98.0–105.0%. The results clearly indicate the suitability of the new prepared IIP-CPE for determination of palladium ions at trace levels in real samples.
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Fig. 7. Linear calibration graphs of IIP-CPE at higher and lower Pd2+ ion concentrations under optimized experimental conditions.
Table 2 Determination of Pd2+ in different samples by the IIP-CP electrode (Number of replicate in each experiment). Sample
Pd2+ added (mol dm
Tap water
–– 1.00 10 9 1.00 10 7 –– 1.00 10 9 1.00 10 7 3 1.00 10 9 1.00 10 7 –– 1.00 10 7 5.00 10 7
Caspian sea water
River water
Soil sample
3
Pd2+ Found (mol dm
)
–– 0.99 10 9 0.98 10 7 –– 1.05 10 7 1.02 10 7 –– 1.02 10 7 1.04 10 7 1.50 10 7 2.51 10 7 6.48 10 7
3.9. Comparison of the figures of merit of the proposed sensor with those of some previous reported Pd2+ voltammetric sensors Table 3 compares the linear dynamic range (LDR) and limit of detection (LOD) of the proposed sensor for Pd2+ determination with those of other voltammetric sensors previously reported in Table 3 Comparison of the figures of merit of the proposed sensor with those of some previous reported Pd2+ sensors. Electrode
Detection limit
Linear range
Reference
Silver Amalgam Film Electrode Sodium humate-MCPE Thioridazine in situMCPE IIP-nanoparticles-CPE
1.4 nmol dm 3 –– 4.7 nmol dm 3 3.0 pmol dm 3
9.4–470 nmol dm
3
94 nM- 4700 nM 9.4–4220 nmol dm 0.01–1 nmol dm 1.0 nmol dm dm 3
3
21
3
3
- 1.0 mmol
22 23 This work
3
)
RSD (%)
Recovery (%)
–– 4.5 4.2 –– 4.1 4.7 –– 4.2 4.5 4.9 5.1 4.8
–– 99.0 98.0 –– 105.0 102.0 –– 102.0 104.0 –– 100.4 99.7
the literature. As it is obvious from these results, not only the LOD of the proposed IIP-CPE is superior to most of the other reported sensors, but also its LDR for Pd2+ ion is among the best previous reports in the literature. The results thus obtained emphasize the fact that the use of properly designed ion imprinted polymers, especially with nano particles; in modification of CPEs can considerably improve their limit of detection, linear range and selectivity over those modified with other modifying agents. 4. Conclusion In this paper a new electrochemical sensor for determination of palladium ions at trace levels was introduced. Application of IIP as a novel modifying agent in the carbon paste electrode made it very selective for palladium determination in the presence of common potential interfering agents according to the specific recognition nature of the synthesized material. The IIP, used in the carbon paste composition, acted as the selective chemical interface of the sensor as well as a pre-concentration agent.
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[19] M.V. Krishna, M. Ranjit, K. Chandrasekaran, G. Venkateswarlu, D. Karunasagar, On-line preconcentration and recovery of palladium from waters using polyaniline (PANI) loaded in mini-column and determination by ICP-MS; elimination of spectral interferences, Talanta 79 (2009) 1454. [20] K. Van Meel, A. Smekens, M. Behets, P. Kazandjian, R. Van Grieken, Determination of Platinum Palladium, and Rhodium in Automotive Catalysts Using High-Energy Secondary Target X-ray Fluorescence Spectrometry, Anal Chem 79 (2007) 6383. [21] A. Bobrowski, M. Gawlicki, P. Kapturski, V. Mirceski, F. Spasovski, J. Zare bskia, The Silver Amalgam Film Electrode in Adsorptive Stripping Voltammetric Determination of Palladium(II) as Its Dimethyldioxime Complex, Electroanalysis 21 (2009) 36. [22] Q. Sun, C. Wang, L. Li, H. Li, Preconcentration and voltammetric determination of palladium (II) at sodium humate modified carbon paste electrodes, Fresenius J Anal Chem 363 (1999) 114. [23] G. Raber, K. Kalcher, C.G. Neuhold, C. Talaber, G. Kolbl, Adsorptive Stripping Voltammetry of Palladium(I1) with Thioridazine In Situ Modified Carbon Paste Electrodes, Electroanalysis 7 (1995) 138. [24] B.S. Sherigara, Y. Shivaraj, R.J. Mascarenhas, A.K. Satpati, Simultaneous determination of lead, copper and cadmium onto mercury film supported on wax impregnated carbon paste electrode: Assessment of quantification procedures by anodic stripping voltammetry, Electrochim Acta 52 (2007) 3137. [25] E.A. McGaw, G.M. Swain, A comparison of boron-doped diamond thin-film and Hg-coated glassy carbon electrodes for anodic stripping voltammetric determination of heavy metal ions in aqueous media, Anal. Chim. Acta 575 (2006) 180. [26] P. Sonthalia, E.A. McGaw, Y. Show, G.M. Swain, Metal ion analysis in contaminated water samples using anodic stripping voltammetry and a nanocrystalline diamond thin-film electrode, Anal. Chim. Acta 522 (2004) 35. [27] T. Akiyama, H. Hishiya, Molecular imprinting of cyclodextrin on silica-gel support for the stationary phase of high-performance-liquid-chromatography, J. Inclusion Phenom Macrocyclic Chem 41 (1–4) (2001) 149. [28] H. Zhang, P. Dramou, H. He, S. Tan, C. Pham-Huy, H. Pan, Molecularly Imprinted Stationary Phase Prepared by Reverse Micro-Emulsion Polymerization for Selective Recognition of Gatifloxacin in Aqueous Media, Journal of Chromatography Sci. 50 (6) (2012) 499. [29] X. Kan, Y. Zhao, Z. Geng, Z. Wang, J. Zhu, Composites of multiwalled carbon nanotubes and molecularly imprinted polymers for dopamine recognition, J. Phys. Chem. C. 112 (13) (2008) 4849. [30] L. Ye, K. Mosbach, editors. Molecularly imprinted materials: towards the next generation. Materials Research Society Symposium Proceedings; 2002: Cambridge Univ Press. [31] S. Tokonami, H. Shiigi, T. Nagaoka, Review: Micro- and nanosized molecularly imprinted polymers for high-throughput analytical applications, Analytica Chimica Acta 641 (1–2) (2009) 7. [32] M. Kalate Bojdi, M.H. Mashhadi Zadeh, M. Behbahani, A. Farahani, S.S. Hosseini Davarani, A. Bagheri, Synthesis, characterization and application of novel lead imprinted polymer nanoparticles as a high selective electrochemical sensor for ultra-trace determination of lead ions in complex matrixes, Electrochimica Acta 136 (2014) 59. [33] M. Behbahani, M.M. Ghareh Hassanlou, H.R. Amini, A. Sadeghi Abandansari, S. Hassan Zadeh, Selective Solid-Phase Extraction and Trace Monitoring of Lead Ions in Food and Water Samples Using New Lead-Imprinted Polymer Nanoparticles, Food Anal. Methods (2014), doi:http://dx.doi.org/10.1007/ s12161-014-9924-5. [34] M. Behbahani, M. Barati, M.K. Bojdi, A.R. Pourali, A. Bagheri, N.A. Ghareh Tapeh, A nanosized cadmium (II)-imprinted polymer for use in selective trace determination of cadmium in complex matrices, Microchim. Acta 180 (2013) 1117. [35] H. Ebrahimzadeh, M. Behbahani, Y. Yamini, L. Adlnasab, A.A. Asgharinezhad, Optimization of Cu (II)-ion imprinted nanoparticles for trace monitoring of copper in water and fish samples using a Box–Behnken design, React. Funct. Polym. 73 (2013) 23. [36] J. Haginaka, Monodispersed, molecularly imprinted polymers as affinity-based chromatography media, J. Chromatogr. B 866 (1–2) (2008) 3. [37] L. Ye, P.A. Cormack, K. Mosbach, Molecularly imprinted monodisperse microspheres for competitive radioassay, Anal. Commun. 36 (2) (1999) 35.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A palladium imprinted polymer for highly selective and sensitive electrochemical determination of ultra-trace of palladium ions Majid Kalate Bojdi a , Mohammad Behbahani b, *, Ali Sahragard b , Bahareh Golrokh Amin b , Alireza Fakhari b , Akbar Bagheri b a b
Faculty of Chemistry, Kharazmi (Tarbiat Moallem) University, Tehran, Iran Department of Chemistry, Shahid Beheshti University, G.C., Tehran 1983963113, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 August 2014 Received in revised form 16 October 2014 Accepted 21 October 2014 Available online 23 October 2014
We report on the design of a palladium(II)-selective electrode based on the use of palladium(II) imprinted polymer nanoparticles (IP-NPs), and its application for the anodic stripping deferential pulse voltammetry determination of palladium ions. The IP-NPs were obtained by precipitation polymerization of 4-vinylpyridine (the functional monomer), ethylene glycol dimethacrylate (the cross-linker), 2,2’azobisisobutyronitrile (the initiator), eriochrome cyanine R (the palladium-binding ligand), and palladium ions (the template ion) in methanol solution. After polymerization, the Pd2+ in the polymer nanoparticles were leached out with dilute hydrochloric acid to create cavities for hosting Pd2+. The new sensor showed high selectivity for palladium ions in the presence of common potential interferences according to the specific recognition nature of the synthesized imprinted material. A carbon paste electrode was modified with the IP-NPs, and differential pulse stripping voltammetry was applied as the detection technique after open-circuit sorption of Pd2+ ions and its reduction to the metallic form. An explicit difference in the response was observed between the electrodes modified with IP-NPs and electrodes modified with non-IP-NPs. The modified electrode responses to Pd2+ was linear in the 0.01 nmol dm 3 to 1.0 nmol dm 3 (with sensitivity of 92.25 nA/nmol dm 3) and in the 1.0 nmol dm 3 to 1.0 mmol dm 3 (with sensitivity of 16.12 mA/mmol dm 3) concentration ranges. The limit of detection (LOD) of the sensor was 3.0 pmol dm 3 (at S/N = 3). The sensor was successfully applied to the trace determination of Pd2+ in spiked environmental water and soil samples. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Carbon past electrode palladium imprinted polymer nanoparticles Differential pulse anodic stripping voltammetry Environmental water and soil samples
1. Introduction Palladium is an element with increasing demands in today’s industries. In 2007, an amount of 192 tons of palladium has been sold in the world market. The main demands are automotive catalyst (55%), electronics (17%), jewellery (11%) and dentistry (9%) [1], especially due to the catalytic properties it is widely used in the synthesis of many materials. Unfortunately, elevated level of palladium compared to geochemical background [2] has been found in airborne particulate matter [3], road dust [4–6], soil [7,8] and grass [9]. Anthropogenic palladium has been reported to be mobile and bioaccumulated by aquatic organisms and generally to a larger extent than other platinum group elements [10–13]. Moreover, metallic palladium has an allergenic potential on humans [14]. Therefore, monitoring of palladium in environmental
* Corresponding author. Tel.: +98 21 22431661; fax: +98 21 22431683. E-mail address: [email protected] (M. Behbahani). http://dx.doi.org/10.1016/j.electacta.2014.10.096 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
samples has great importance with respect to estimation of the future risk of the human health and the ecosystem. The toxicity of palladium demands for practical techniques to remove it from environment. In addition, because of the high economic value of palladium, its separation even in trace amounts is beneficial. Some of the valuable techniques, including ET-AAS [15,16], ICP-AES [17], FAAS [18], ICP-MS [19] and total reflection XRF-spectrometry [20] have been used for palladium determination. The above mentioned methods suffer from some sensitivity and selectivity limitations for real samples [15–20]. However, voltammetric methods are highly favorable techniques for the determination of metal ions because of its low cost, high sensitivity, easy operation and the ability for portability. In order to enhance the sensitivity and selectivity of the electrochemical determination of palladium(II), chemically modified electrodes have received increasing attentions in the past decades [21–23]. Anodic stripping deferential pulse voltammetry (ASDPV) is the most attractive electrochemical technique for the determination of trace heavy metals due to its high sensitivity and selectivity with modified electrodes.
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Advantage of ASVDPV over the other three methods (AAS, ICP-AES or ICP-MS) is the simplicity of the required instrumentation, which is relatively inexpensive, low electrical power consumption, portable as well as suitable for automation [24–26]. On the other hand, ion-imprinted polymers (IIPs) are recognizing metal ions after imprinting, while retaining all the virtues of MIPs. The production of polymers exhibiting selective binding of a specific cation, involves the formation of cavities equipped with complexing agents so arranged as to match the charge, coordination number, coordination geometry and size of the target cation. Although the bulk MIP prepared by conventional methods exhibits high selectivity [27,28], some disadvantages were suffered, such as the heterogeneous distribution of the binding sites, embedding of most binding sites, and poor site accessibility for template molecule [29]. Consequently, the consideration of researchers has moved toward achieving highly uniform spherical imprinted particles, particularly on the nanoscale [30]. In contrast to monoliths, MIPs possess cavities designed for a target analyte, providing a retention mechanism based on molecular recognition; by this, imprinted cavities are more easily available to templates and the binding kinetic is increased [31–35]. Different methods are used to obtain MIPs Nano particles, namely suspension, multistep swelling and precipitation polymerization. The different methods of preparation have been well documented in a recent review by Haginaka [36]. Among them, the precipitation technique is the most convenient one, because it is a homogeneous, one-step synthesis and doesnot require the use of a surfactant or stabilizer that can remain adsorbed to the surface of the polymer, interfering with the selective binding of the target to the imprinted materials. The first prepared MIPs, using precipitation polymerization technique, were reported by Ye et al. [37]. The polymer particles were obtained according to this procedure, have more uniform particles size and require no crushing and sieving steps in comparison to the polymers obtained by bulk polymerization. In the present work, combining the advantages of high selectivity from the IIP nanoparticles technique and high sensitivity from CPE detection, an IIP-CPE sensor has been developed for the determination of pallaium ions in complex matrices. Some advantages of this work are simplicity, rapidity, high selectivity, being operative and cheap, easy to usage and sensitive determination of low levels of palladium ions in aqueous solution. 2. Experimental 2.1. Materials 4-vinyl pyridine with high purity was purchased from Merck (Darmstadt, Germany, www.merck.de). Ethylene glycol dimethacrylate (EGDMA) and Eriochrome Cyanine R were obtained from Fluka (Buchs SG, Switzerland, www.sigmaaldrich.com). 2,2;Azobisisobutyronitrile (AIBN) was obtained from Acros Organics (New Jersey, USA). NaOH, HCl, HNO3, acetic acid (HOAC) and methanol were purchased from Merck (Darmstadt, Germany). All the other reagents used were of analytical grade and purchased from Merck (Darmstadt, Germany, www.merck.de). Ultrapure water was prepared using a Milli-Q system from Millipore (Bedford, MA, USA). 2.2. Apparatus All electrochemical measurements were performed with a Palm-Sens (EN 50,081–2) potentiostat. A personal computer was used for data storage and processing. The three electrode electrochemical cell used was equipped with a modified (Pd2+
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ion imprinted polymer) carbon paste electrode (MCPE) as a working electrode, a saturated calomel electrode (SCE) as a reference electrode and a platinum electrode as an auxiliary electrode (Azar Electrode Co, Iran). All potentials in the text were reported versus this reference electrode. The pH measurement was performed with a Metrohm model 691 pH/mV meter. Heidolph heater stirrer model MR 3001 (Germany) was employed for heating and stirring of the solutions. Scanning electron microscopy (SEM) was performed by gently distributing the powder sample on the stainless steel stubs, using SEM (KYKY, EM3200) instrument. The thermal properties of synthesized polymers were determined using a BAHR-Thermoanalyse GmbH (Germany) with employing, heating and cooling rates of 10 C min 1 and using a condenser as the coolant. The samples were weighed as a thin film and carefully packed into a clean aluminum pan (11.5–12.5 mg), and sealed by crimping an aluminum lid on the pan (Shimadzu universal crimper). An Al2O3 empty pan sealed with a cover pan was used as a reference sample. A scanning range of 10 to 800 C was used for samples at 10 C min 1 in nitrogen gas. The CHN analysis was performed on a Thermo Finnigan Flash EA112 elemental analyzer (Okehampton, UK). The Brunauer–Emmett–Teller (BET) of the synthesized imprinted and non-imprinted nanoparticles was analyzed by nitrogen adsorption in a nitrogen adsorption apparatus (BEL Japan, BELSORP-18 Plus). The samples were degassed at 150 C prior to the nitrogen adsorption measurements. 2.3. Synthesis of Pd2+ ion imprinted polymer (IIP) and non-imprinted polymer nanoparticles (NIP) The nanosized Pd2+-imprinted polymer was prepared by precipitation polymerization technique. In the first step, 2 mmol of 4-vinyl pyridine (functional monomer) and 2 mmol of Eriochrome Cyanine R (the palladium-binding ligand) were dissolved in 50 mL of methanol in a 100 mL glass flask. Subsequently, as the second step, 1 mmol of Pd (NO3)2 as an imprinted metal ion (template) was added slowly to a glass flask and the resulted mixture was stirred for 5 h at room temperature. In the third step, 14 mmol of EGDMA and 75 mg of AIBN were added as cross-linker and initiator. The oxygen of the sample solution was removed by bubbling nitrogen through the sample for 10.0 min. Polymerization was performed in an oil bath at 65 C for 24 h in the presence of nitrogen under magnetic stirring at 400 rpm. The prepared polymer was washed several times with 1:4 (v/v) methanol/water to remove the unreacted materials and then with HCl (1.0 mol L 1) for leaching the imprinted metal ions until the washing solution was free from palladium ions. Finally, it was washed with double distilled water until reached a neutral pH (Fig. 1 provides the images of synthesized nanosized IIP after each step). The resulting fine powder was dried under vacuum in a desiccator before sorption and desorption studies. In the same way, the nonimprinted polymer (NIP) was also prepared without palladium ions.
2.4. Preparation of IIP-MCPE Based on optimized conditions, ion imprinted polymer modified carbon paste electrode (IIP-MCPE) was prepared. A mixture of IIP (15% w/w), graphite powder and paraffin oil (55:30% w/w) was blended by hand in a mortar with a pestle to construct the IIP-CPE. The body of the carbon paste working electrode was a Teflon rod with a hole (2 mm in diameter and 5 mm deep) bored at one end for paste filling. The electrical connection was made with a copper wire through the center of the rod.
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2.5. Real sample pretreatment The polyethylene bottles filled with the samples were cleaned with detergent, water, diluted nitric acid and water in sequence. The samples were immediately filtered through a cellulose filter membrane (pore size 0.45 mm), and were acidified to pH of 2.0 for storage. Tap water samples (60 mL) were taken from our research laboratory without pretreatment (pH adjusted to 6.0). Before analysis, the water samples (60 mL) which were taken from
Caspian Sea and river (Siahroodriver (Ghaemshahr, Iran)) were adjusted to pH of 6.0 according to optimized experimental conditions. To further study, the soil sample was collected from Mouteh gold mine (Iran). Soil samples (0.1 g) were digested using 8 mL mixture of 5% aqua regia with the assistance of a microwave digestion system. Digestion was carried out for 2 min at 250 W, 2 min at 0 W, 6 min at 250 W, 5 min at 400 W, and 8 min at 550 W, and the mixture was then vented for 8 min. The residue from the
Fig. 1. Schematic illustration of imprinting process for the preparation of palladium-imprinted polymer nanoparticles by precipitation polymerization technique.
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digestion, as well as a controlled digestion was then diluted with deionized water. Finally, the pH of each solution was adjusted to 6.0. 2.6. Determination procedure The prepared electrode was inserted into the solutions containing the Pd2+ at pH 6.0, being at a stirring state of 400 rpm for 7 min. Then, the electrode was rinsed and finally placed in the electrochemical cell containing 10 mL 0.5 mol L 1 HCl. At first, a negative pre-potential of -1.0 V was applied to the electrode for 35 s to reduce the recognized target ions (the stripping step was performed in a second solution to achieve the selectivity of the sensor for palladium ions because a reduction step which in the presence of other cations would cancel the selectivity of the method) and then the differential pulse voltammetry was performed in the potential range of 0.3 to 0.7 V. 3. Results and discussion 3.1. Characterization of IIP (SEM, TGA/DTA, CHN and N2 adsorption analysis) The resulting nanosized imprinted polymer were characterized by colorimetric studies, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential thermal analysis (DTA), N2 adsorption analysis and elemental analysis. After synthesis of un-leached and leached IIP, colorimetric studies were carried out to compare the changes in the color of prepared powders. An obvious change in the color from violet of un-leached IIPs to light-red after the leaching process clearly indicated the successful removal of palladium ions from the polymeric matrix. Meanwhile, adsorption process caused an instant color change in the leached sample from light-red to a violet color due to fast extraction of palladium ions into the imprinted polymeric matrix brown at the desired pH. Thermal stability of the leached (The imprinted polymer has been washed with optimized elution solvent and became free from target ions after washing) and unleached (The imprinted polymer which is not washed with optimized elution solvent and in this condition, the target ions is sorbed in the synthesized imprinted polymer) imprinted polymer was evaluated by TGA. Figure 1S (Electronic Supplementary Material (ESM)) and 2 S show DTA and TGA plots for leached and unleached imprinted polymer, respectively. In DTA plots of palladium imprinted polymer, exothermic peaks were observed at 371 and 512 C (for unleached) and 362 and 442 C (for leached). As can be seen in the DTA plots, the temperature for maximum of peaks in palladium-IIP was observed at higher temperature of 371 and 512 C; while this event for the leached IIP was happened in the lower temperature of 362 and 442 C. These events confirm the higher thermal stability of the un-leached relative to leached polymer, which is due to the presence of palladium ions in the un-leached polymer and also its strong complexation with Eriochrome Cyanine R in the polymeric network. As it is shown in TG plot for unleached ion imprinted polymer, weight loss for palladium-IIP was about 60%, and this amount of reduction in weight is related to the presence of palladium ions in polymer bead. While decrease in weight for leached imprinted polymer up to 100%, is due to the absence of palladium ions in polymer. These observations indicate that the formation of palladium-imprinted polymer and elution of palladium ions from the polymer was performed successfully. As evidence by the N2 sorption test, the imprinted sorbent showed a specific surface area of 39.2 m2 g 1 whereas the nonimprinted sorbent only had a specific surface area of 14.1 m2 g 1. The specific surface areas obtained for both sorbents are
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distinguishing indicating that the surface area was largely influenced by the imprinting technique. The results of elemental analysis (EA) of acid leached IIPs were found to be: (%found); C, (59.98%); H, (9.628%); S, (1.252%); N, (3.492%); O, (25.648%). These observations confirm the successful synthesize of palladium imprinted polymer nanoparticles. The morphology of the palladium-IIP was assessed by scanning electron microscopy, and the SEM micrograph is shown in Fig. 2. The SEM pattern showed particles with the size of about 25–45 nm. As a result, palladium-IIP can be used as a nano-size selective electrode for very fast extraction and trace detection of palladium ions in complex matrices. 3.2. Effect of nature and concentration of Leachant The influence of nature of mineral acids used as leachant on the desorption of Pd2+ ions from the IIP nanoparticles was examined by using 10 mL portions of 1 mol L 1 HCl, 1 mol L 1 HNO3 and 0.5 mol L 1 H2SO4, which resulted in respective percent desorptions of >99%, 93.4% and 84.5%. Thus, 1 mol L 1 HCl was selected as leachant because of its better leaching characteristics over the other two mineral acids. In order to study the optimum leachant concentration, for quantitative desorption of extracted Pd2+ ions from the prepared IIP, several 10 mL portions of hydrochloric acid solutions with different concentrations (i.e., 0.1, 0.2, 0.5 and 1.0 mol L 1) were used for leaching of Pd2+ ions from the imprinted sites in the polymer matrix. It was found that desorption of Pd2+ ions increases with increasing hydrochloric acid concentration (i.e., with respective percent desorptions of 48.2%, 58.1%, >99%, and >99% (for 0.1, 0.2, 0.5 and 1 mol L 1)). This is most possibly due to increased protonation of the heteroatoms of ligand in the polymer network. Thus, a 0.5 mol L 1 HCl concentration of hydrochloric acid was selected as electrolyte medium in the voltammetric determinations of palladium ions. 3.3. Evaluation of the effect of Pd2+ extraction conditions on the electrode response 3.3.1. Effect of IIP-CP composition on its response Since the amount of IIP in the modified CPE plays a key role in the response characteristics of the IIP-CPEs, the optimum composition of IIP was thoroughly studied. In a series of experiments, the amount of paraffin oil was kept constant at 30% level, and in the remaining 70% solid phase the relative amount of IIP/graphite was changed, and the voltammetric signals
Fig. 2. The SEM image of palladium-imprinted polymer nanoparticles.
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of a 5.0 10 7 mol dm 3 Pd2+ ion were obtained under the same experimental conditions for the extraction and voltammetric determination steps. As shown in Fig. 3S (ESM) the IIP-CP voltammetric signal is increased with increasing amount of IIP in the paste up to 11%. However, a further increase in the IIP content resulted in some considerable decrease in the voltammetric signal, most possibly due to the diminished conductivity of the electrode. In addition, in the case of a paste with 13% IIP, it was found that the physical property of the electrode surface was not practically suitable for the renewal of the electrode surface. Thus, a graphite powder/IIP/paraffin oil electrode composition of 59%/11%/ 30% was selected as optimal condition for preparation of IIP-CPE in further studies. 3.3.2. The pH effect The pH of the extraction solution was also checked and its effect on the Pd2+ extraction in the electrode was studied. For this purpose, the prepared electrode was inserted into solutions with various pH values where they were incubated for 10 min at a constant stirring rate. After the mentioned time, the electrode was removed from the solution and immersed into the solution of the electrochemical cell. The results of this experiment are shown in Fig. 3. As can be seen, stabilizing the pH is crucial task in this work. The optimum pH for the proposed method was found to be 6.0. Decreasing of the pH, lower than 6.0, decreases the electrode signal due to the decrease in the amount of the target ion extracted into the electrode. This is probably because of the protonation of hetroatoms of the selective sites of the IIP, that weaken the interaction of Pd2+ with the hetroatoms, existing in the selective sites of the IIP. At higher pH values (above 7.0) the signal of the sensor toward palladium ions was constant. Therefore, pH of 6.0 was select as the optimum pH for next experiments. 3.3.3. The effect of extraction time The influence of extraction time on the sensitivity of the proposed palladium sensor was investigated over a time period of 1 to 13 min, while keeping the other experimental conditions constant, and the results are illustrated in Fig. 4S (ESM). As can be seen in Fig. 4S (ESM), an increase in extraction time resulted in an intensive increase in the extracted amount of Pd2+ ion and
consequently in the intensity of the corresponding voltammetric signals, until an extraction time of about 7 min is reached. While, upon a further increase in extraction time of palladium ions the intensity of the voltammetric signal remained more or less constant. In order to decrease analyzing time, as much as possible, the time of 7 min was selected for further studies. 3.3.4. The effect of stirring rate In order to optimize the stirring rate in the extraction period of 7 min, Pd2+ was extracted in the prepared IIP-CP electrodes at various stirring rates from 100 to 700 rpm, whereas the other extraction parameters were kept constant. The obtained results showed that an increase in stirring rate enhanced the electrode response for Pd2+ up to 400 rpm, indicating considerable effect of the stirring rate on the extraction of palladium ions in the IIP-CP electrode. However, further increase in stirring rate did not affect considerably the extraction amount. Therefore, a stirring rate of 400 rpm was chosen as optimal value for further experiments. 3.3.5. Optimization of the pre-reduction time In this work, a definite pre-reduction was applied to the electrode for a determined time and then the potential was scanned for recording the oxidation peak. It is clear that the prereduction magnitude and its applying time can influence the sensor performance. Thus, the effect of these parameters on the response magnitude of the developed sensor was investigated. Increasing of the time of the applied pre-reduction ( 1.0 V) in the time range of 5–35 s resulted increase in the electrode response. As is quite obvious from Fig. 5S (ESM), an increase in pre-reduction time resulted in an intensive increase in the signal and consequently in the intensity of the corresponding voltammetric signals, until a pre-reduction time of about 35 s is reached. While, upon a further increase in pre-reduction time of palladium ions the intensity of the voltammetric signal remained constant. In order to decrease analyzing time, as much as possible, the time of 35 s was selected for further studies. 3.3.6. Effect of sample volume In the analysis of real samples, the sample volume is one of the important parameters influencing the preconcentration factor of
Fig. 3. Differential pulse voltammetry of 1.0 10 9 mol dm 3 Pd2+ on IIP-CPE at different pH. Conditions: (i) extraction step: extraction time = 7 min, stirring rate = 400 rpm; (ii) stripping voltammetry step: E-conditioning = -1.0 V, conditioning time = 35 s, voltage step = 0.01 V, pulse amplitude = 0.1 V, pulse time = 0.01 s, scan rate = 0.1 V s 1 (inset: The obtained current peak vs. pH).
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Fig. 4. Differential pulse voltammetry of 1.0 10 9 mol dm 3 Pd2+ on IIP-CPE at different volume. Conditions: (i) extraction step: Solution pH = 6.0, extraction time = 7 min, stirring rate = 400 rpm; (ii) stripping voltammetry step: E-conditioning = -1.0 V, conditioning time = 35 s, voltage step = 0.01 V, pulseamplitude = 0.1 V, pulse time = 0.01 s, scan rate = 0.1 V s 1 (Inset:The obtained current peak vs. sample volume.)
target ion. Therefore, the effect of sample volume on quantitative adsorption of palladium ion was investigated. The extraction volume has a significant effect on the sensor performance, meaning that the extraction efficiency is influenced by the extraction volume. The first increase in the electrode signal (from 5–60 mL), as a result of extraction volume enhancement, is reasonable; because, as the solution volume increases, the preconcentration factor of the extraction step increases correspondingly. However, the finite adsorption sites exist on the electrode surface, limiting thus the signal augmentation manner to a limited extraction volume. After saturation of the adsorption sites of the electrode surface (nano IIP sites), further increase in the extraction solution volume can lead the solute-selective sites
interaction to be disturbed, decreasing thus the extraction efficiency (from 60–80 mL). It can be seen that when the extraction volume becomes 60 mL, the electrode reaches to an optima (Fig. 4). Thus, 60 mL was selected as an optimal for sample volume. 3.4. Comparison of the IIP-CP electrode with NIP-CP electrode and evaluation of the washing effect on the responses of the electrodes The carbon paste electrodes modified with IIP and NIP were incubated in the Pd2+ solution (1.0 10 9 mol dm 3) while the solution was stirred continuously. Then, the electrode was inserted into the electrochemical cell and a negative potential was applied to the electrode. Subsequently, differential pulse voltammetry
Fig. 5. Comparison of differential pulse voltammetry responses of IIP-CP electrode and NIP-CP electrodes with and without washing steps; [Pd2+] = 1.0 10 9 mol dm 3, Solution pH = 6.0, extraction time = 7 min, stirring rate = 400 rpm; stripping voltammetry conditions: E-conditioning = -1.0 V, conditioning time = 35 s, voltage step = 0.01 V, pulseamplitude = 0.1 V, pulse time = 0.01 s, scan rate = 0.1 V s 1. (a) IIP- before washing (b) IIP- after washing with water (c) NIP- before washing (d) NIP- after washing with water
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Fig. 6. Differential pulse voltammetric responses of 1.0 10 ratios of 25, 50, 75, 100, 125, 150, 175, 200.
9
mol dm
3
Pd2+ at pH 6.0 in the absence and presence of excess amounts of Cu2+ ions with [Cu2+]/[Pd2+]molar
technique was applied for the determination of palladium ions. The obtained voltammetric signals of the two mentioned electrodes are shown in Fig. 5. As it can be seen, the obtained signal for IIP-CPE (a) is noticeably larger than that for NIP-CPE (c), indicating the existence and properly functioning of the selective cavities in the IIP, created in the polymerization step. It is interesting to note that, washing of both electrodes after removing them from the initial palladium sample solution, resulted in some decrease in intensity of the corresponding voltammetric signals, the extent of which in the NIP (d) signal being much larger than that of the IIP (b). This observation emphasizes the fact that the weakly surface adsorbed palladium ions on the NIP-CPE electrode are much larger than that on the IIP-CPE. 3.5. Evaluation of the selectivity of IIP-CPE The selectivity of Pd2+ imprinted polymer in the IIP-CPE was investigated by incubation of the electrode in different solutions containing a mixture of Pd2+ (1.0 10 9 mol dm 3) and some potential interfering cations at various concentrations and subsequent electrochemical analysis. Sample Pd2+ voltammograms obtained in the absence (a) and presence of Cu2+ ions (b, c, d, e, f and g) are shown in Figure 6 clearly shows that the presence of copper at a Cu2+/Pd2+ ratio of 150 possesses a negligible effect on the voltammetric signal of palladium ion. However, a 175-fold molar excess of Cu2+ ions significantly affects the voltammetric signal of Pd2+, indicating the presence of competition between these ions for occupying the selective cavities in the IIP. It is worth mentioning that when the electrode is washed, the weakly adsorbed species, which are most commonly the interfering ions, are removed from the electrode surface. Thus, such washing process can be used for improving the sensor selectivity by significantly decreasing the interference effects. The interference levels of several transition metal ions, including Co2+, Zn2+, Hg2+, Cu2+, Ni2+, Fe2+, Cd2+, Pb2+, Mn2+, Cr3+ and Ag+ ions, which are
usually present with palladium ion in complex matrices, are summarized in Table 1. As is obvious, these cations possess no significant effect on the determination of Pd2+ ion by the proposed IIP-based sensor. The tolerance limit was established as the maximum concentration of foreign species that caused a relative error of 5% in the analytical signal. 3.6. Stability and reproducibility of the sensor Reproducibility and stability of the sensor was also investigated. For 9 successive determinations of 1.0 10 9 mol dm 3 palladium ions at the same modified electrode, the relative standard deviation (R.S.D.) was 5.1%. Fabrication reproducibility was estimated with five different electrodes, which were constructed by the same procedure, R.S.D. was 6.8%. 3.7. Analytical performance of IIP-CPE Fig. 7 displays differential pulse voltammograms (DPVs) obtained upon increasing the Pd2+ concentrations for dynamic linear range under the optimized condition in water sample. Method detection limit (MDL) and limit of quantification (LOQ) of the sensor were 3.0 10 12 mol dm 3 (3 S/N) and 1.0 10 11 mol dm 3 (10 S/N), respectively. The calibration equations for detection of Pd2+ with proposed sensor were as following equation (the sensitivity of the sensor is different for low and high concentration, because the available cavities for palladium ions in low concentration is different to this value for high concentration.): I (mA) = 16.12 C (mmol dm 3) + 0.96 R2 = 0.995 I (nA) = 92.25 C (nmol dm 3) + 31.94 R2 = 0.995 The repeatability and intermediate precision for five replicate determinations of 1.0 10 9 mol dm 3 Pd2+ with the proposed sensor was 4.2 and 5.3%. 3.8. Real sample analysis
Table 1 Interference levels for some tested ions in the determination of Pd2+ by developed sensor. Species
Interference level
Cu2+, Cd2+, Zn2+, Hg2+, Pb2+ Ni2+, Co2+, Mn2+, Fe2+ Cr3+, Ag+NO3 , Cl PO43 CO32
>150 >125 >175 > 150 > 200 > 150
,
SO42
To demonstrate the potential application of our sensor in environment analysis, we applied the prepared sensor to evaluate the palladium content of different environmental samples (Table 2). The recoveries of palladium ions from the real and spiked samples varied in the range of 98.0–105.0%. The results clearly indicate the suitability of the new prepared IIP-CPE for determination of palladium ions at trace levels in real samples.
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Fig. 7. Linear calibration graphs of IIP-CPE at higher and lower Pd2+ ion concentrations under optimized experimental conditions.
Table 2 Determination of Pd2+ in different samples by the IIP-CP electrode (Number of replicate in each experiment). Sample
Pd2+ added (mol dm
Tap water
–– 1.00 10 9 1.00 10 7 –– 1.00 10 9 1.00 10 7 3 1.00 10 9 1.00 10 7 –– 1.00 10 7 5.00 10 7
Caspian sea water
River water
Soil sample
3
Pd2+ Found (mol dm
)
–– 0.99 10 9 0.98 10 7 –– 1.05 10 7 1.02 10 7 –– 1.02 10 7 1.04 10 7 1.50 10 7 2.51 10 7 6.48 10 7
3.9. Comparison of the figures of merit of the proposed sensor with those of some previous reported Pd2+ voltammetric sensors Table 3 compares the linear dynamic range (LDR) and limit of detection (LOD) of the proposed sensor for Pd2+ determination with those of other voltammetric sensors previously reported in Table 3 Comparison of the figures of merit of the proposed sensor with those of some previous reported Pd2+ sensors. Electrode
Detection limit
Linear range
Reference
Silver Amalgam Film Electrode Sodium humate-MCPE Thioridazine in situMCPE IIP-nanoparticles-CPE
1.4 nmol dm 3 –– 4.7 nmol dm 3 3.0 pmol dm 3
9.4–470 nmol dm
3
94 nM- 4700 nM 9.4–4220 nmol dm 0.01–1 nmol dm 1.0 nmol dm dm 3
3
21
3
3
- 1.0 mmol
22 23 This work
3
)
RSD (%)
Recovery (%)
–– 4.5 4.2 –– 4.1 4.7 –– 4.2 4.5 4.9 5.1 4.8
–– 99.0 98.0 –– 105.0 102.0 –– 102.0 104.0 –– 100.4 99.7
the literature. As it is obvious from these results, not only the LOD of the proposed IIP-CPE is superior to most of the other reported sensors, but also its LDR for Pd2+ ion is among the best previous reports in the literature. The results thus obtained emphasize the fact that the use of properly designed ion imprinted polymers, especially with nano particles; in modification of CPEs can considerably improve their limit of detection, linear range and selectivity over those modified with other modifying agents. 4. Conclusion In this paper a new electrochemical sensor for determination of palladium ions at trace levels was introduced. Application of IIP as a novel modifying agent in the carbon paste electrode made it very selective for palladium determination in the presence of common potential interfering agents according to the specific recognition nature of the synthesized material. The IIP, used in the carbon paste composition, acted as the selective chemical interface of the sensor as well as a pre-concentration agent.
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