Philippine Journal of Science 134 (1): 51-56, June 2005 ISSN 0031 - 7683

A Feather-Trode Sensor For Detecting Lead Ions Elmer R.E. Mojica1, Arlene B. Tocino2, Jose R.L. Micor2, and Custer C. Deocaris2,3,4* 1

Institute of Chemistry, University of the Philippines Los Baños, College, Laguna, Philippines 2 Department of Chemistry and Biotechnology, School of Engineering, University of Tokyo 7-3-1, Hongo, Tokyo, Japan 3 National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba Science City, Ibaraki, Japan 4 Atomic Research Division, Philippine Nuclear Research Institute Commonwealth Avenue, Diliman, Quezon City, Philippines

Feathers have sulfur-containing proteins that could accumulate heavy metals. A method for the determination of trace amounts of lead ions using chicken feather-modified electrode is described. The modified electrode is prepared by mixing ground chicken feathers with graphite powder and mineral oil. The accumulation step performed by immersing the electrode in lead (II) solution under open circuit and its response is evaluated with respect to pH and accumulation time. The lead (II) adsorbed on the electrode surface was determined voltametrically using sodium hydroxide solution as supporting electrolyte. The response is evaluated in terms of deposition time and deposition potential. The electrode composition and regeneration methods were also taken into account. The best conditions for the preconcentration step were pH 6.0 and 3 min accumulation time while 90 sec deposition time and –120 mV deposition potential gave the optimum result. The modified electrode that contained 10% feathers gave the highest signal and multiple stripping allows the removal of the adsorbed lead ions in the electrode surface. Linear response was observed from 1 to 10 mg/L (r2 = 0.996). The detection limit was found to be 0.59 mg/L while relative standard deviation from a series of 5 readings using 5 and 10 mg/L were 2.43% and 3.75%, respectively. Key Words: chicken feather, electroanalysis, heavy metal sensor, lead

INTRODUCTION Lead (Pb) is one of the most common heavy metals in the environment. Lead determination is important and urgent because of its toxic effects on human health. There is an increasing attention on the determination of trace levels of lead in the environment. The most common technique used for lead determination is atomic absorption spectroscopy (AAS). AAS has the advantages of excellent sensitivity, good selectivity, and wide range of linearity. However, long waiting time, expensive instrumentation and sample pre-treatments are required for this technique. *Corresponding author. [email protected]

As a result, electrochemical methods using chemically modified electrodes (CMEs) have been introduced for the voltametric analyses of trace metals (Arrigan 1994; Gardea-Torresdey et al. 1988). A CME is advantageous because of its faster response, ease of fabrication, low cost and suitability for miniaturization (Wang 2000). This type of electrode utilizes chemical and biological-modifying moieties such as ligands, redox mediators, algae, enzymes, and tissues. Poultry feathers, which are considered farm wastes and of no economic value, are known to contain high amounts of water-insoluble fiber proteins known as keratin. Keratin was previously reported to be an 51

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Mojica et al.: Feather-Trode Sensor for Detecting Pb Ions

effective modifier in a carbon paste electrode format for the detection of silver ions (Sugawara et al. 1998). In this paper, we developed a carbon paste electrode using poultry feathers as modifier to bio-concentrate heavy metals, particularly lead ions (Pb2+). Optimization of parameters such as electrode composition, accumulation time, pH of the accumulating solution, deposition time, and deposition potential led to the voltametric determination of lead in actual aqueous samples.

MATERIALS AND METHODS Poultry feathers were collected from a market in Blumentritt, Manila. The samples were washed with tap water, sun-dried, and pulverized using a laboratory mill (Model 4). Ground samples were then thoroughly washed with deionized water and oven-dried at 95 °C to constant weight. The modified carbon paste electrodes were prepared by hand-mixing graphite powder (Aldrich) with the ground feathers at different proportions (5-25% feather by weight). Mineral oil (Nujol) was added to make a paste. A portion of the modified carbon paste was packed from the end of a 1.5 mm diam plastic tube where a copper rod (2.5 mm diam) was inserted to establish electrical contact (Figure 1).

COMPUTER

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Figure 2. Electroanalytical system used to test the capacity of fether - modified CPE (working electrode) in measuring ionic voltage

RS232 connection, which converts the generated data into an ASCII format. The data obtained can be exported into the Microcal Origin version 5. Copper wire

Carbon paste with feathers Figure 1. Feather modified carbon paste electrode

Voltametric measurements were carried out in a threeelectrode cell connected to Metrohm 693VA processor (Figure 2). The three-electrode system is made up of Ag/ AgCl as reference electrode, feather-modified Carbon Paste Electrode (CPE) as working electrode, and platinum wire as auxiliary electrode and contained on an electrochemical glass cell where the supporting electrolyte was also placed. The processor was interfaced to a personal computer via a 52

The electrochemical nature of the feather-modified CPE was evaluated using cyclic voltammetry (CV) and differential pulse anodic stripping voltametry (DPASV). Cyclic voltammetry was performed with both bare CPE and feather-modified CPE in a 0.1 M NaOH solution. A potential range of –1000 to 1000mV was applied and reversed. A scan rate of 100 mV/s was used in the analysis. Each DPASV run was made up of three steps: (1) accumulation under open circuit where the modified electrode is immersed in lead (II) solution then rinsed with deionized distilled water and then connected to Metrohm 693VA processor for (2) deposition and (3) stripping. The scan rate for DPASV was set at 40 mV/s. Optimization The following conditions were optimized to facilitate the determination of lead ions using the modified CPE: electrode composition, accumulation time, pH of accumulating solution and deposition time, deposition potential, and regeneration method. For the effect of pH on accumulating solution, different buffers were used and, adjusted by HCl. For supporting electrolyte, 0.1

Philippine Journal of Science Vol. 134 No. 1, June 2005

Mojica et al.: Feather-Trode Sensor for Detecting Pb Ions

M NaOH was used throughout the analysis. Multiple stripping was used as a possible way of regenerating or reusing the modified electrodes. The detection limit and other figure of merits of the modified electrode were also determined.

the -800- -600 mV region when the electrode was placed in a solution containing lead ions (Figure 4). This was not observed when unmodified electrode was used.

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Cyclic voltmeter was used to initially characterize the electrochemical properties of unmodified CPE and the feather-modified CPE. Figure 3 shows the cyclic voltammograms of unmodified CPE and the feathermodified CPE. Broadening of the cyclic voltammogram in the modified carbon paste electrode was observed. In addition, small anodic peak currents at Epa ~ -250 mV and small cathodic peak currents at Epc~ -550 mV can be seen. This suggests the presence of inherent redox reactions in the modifier albeit to a minimum extent. It is possible that some of the functional groups present in the feather undergo redox reactions. However potential region where lead (II) ions is usually detected did not show any current signal. From this, we can confidently say that any signal during our voltametric analysis at the given potential range can be attributed to the analyte itself. DPASV analysis confirmed this after a distinct peak was obtained within 150

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Figure 4. Differential voltammogram of the feather modified electrode and unmodified electrode on lead (II) solution

RESULTS AND DISCUSSION

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Determination of Lead Ions in Waste Samples Using the optimized conditions for accumulation and deposition steps, the amount of lead ions in laboratory waste samples (n=5) was analyzed. The samples were adjusted to pH 6 using HCl. The concentration of lead ions in the waste samples was determined from the calibration curve by interpolation. Spiking known amount of lead was also done to determine the lead ion content. The performance of the modified CPE was compared with the results obtained using AAS.

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Figure 3. Cyclic voltammogram of unmodified and feather modified carbon paste electrode in 0.1 M NaOH

The surface active material present in the electrode surface allows for the accumulation of the lead ion. The process of such metal accumulation by the ground feather is not yet fully understood. Feathers are known to be an abundant source of keratin, a protein rich in sulfur-containing amino acids known to favor binding to metal ions by means of ion-exchange or complexation. Amino acids in proteins can serve as a very effective and specific ligand for a variety of metal ions since they contain a great number of potential donor atoms through the peptide backbone and amino acid side chains (Gooding et al. 2001). The complexes formed exist in a variety of conformation that is sensitive to the pH environment of the complex (Sigel & Martin 1982; Kozlowski et al. 1999). Electrode Development Amount of feather modifier. Optimization of the different parameters was done using DPASV. The dependence of electrochemical response on the concentration of the modifier was made with concentrations ranging from 5% to 25% (by weight) modifier. The peak current was observed to increase from 5 to 10% and afterward started to decrease and almost reach zero level at 25% (Figure 5). This observation could be due to the effect of the volume of feathers used. Increasing the amount of feather in the electrode means an increase in volume of the feathers mixed with carbon powder. A much higher amount of feathers would affect electrode conductivity since less carbon powder are present, and such conditions result to a low peak current. 53

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Mojica et al.: Feather-Trode Sensor for Detecting Pb Ions

gradually with increasing accumulation time (Figure 7), demonstrating that the transport of lead ions toward the surface of the electrode increases with time. Highest peak current was observed within 3 min. Afterwhich, the signal levels off possibly due to the saturation of binding sites on the electrode surface.

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Figure 5. Effect of increasing amount of feather in the modified carbon paste electrode on peak current

pH effects. The effect of pH of the accumulating solution on the current response of the electrode is shown in Figure 6. Peak current is shown to increase within the pH range of 3-6. Dissociation of the proton from the feather’s thiol group hardly occurs and the complexation between Pb (II) and sulfur is difficult below pH 6. The result obtained is related to the study done by GardeaTorresdey et al. (1988), which demonstrated that lead belongs to the first class of metals where metal ions can tightly and rapidly bind at pH > 5.0. Arrigan (1994) also classified lead (II) as among those that are tightly bound at pH greater than or equal to 5, and not bound at pH values of 2 and below. The decrease in peak current after pH 6 could be due to the formation of hydroxylic lead complexes that prevent lead incorporation to the electrode (Ramos et al. 1993). Accumulation time. In conventional stripping voltammetry, the accumulation step serves as a pre-concentration step. The peak current increases

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Figure 7. Effect of increasing accumulation time on peak current

Deposition time and deposition potential. Deposition time refers to the time required for the reduction of lead (II) to its metallic state, which takes place on the electrode surface. Peak current increases up to 90 sec and then begins to decrease afterward (Figure 8). Deposition potential is the potential at which the accumulated metal on the electrode surface is deposited or reduced. A more negative potential was expected to give a higher current response because during deposition, more ions are converted to their reduced form. This was observed as shown by the different voltammograms in Figure 9. The highest peak current was observed at –1200 mV, the most negative potential used in the study. 18

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Figure 6. Effect of pH on peak current

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Figure 8. Effect of increasing deposition time on peak current

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and concentration due possibly to saturation of the binding sites. Reproducibility was illustrated by the precision obtained from a series of 5 readings obtained using 5 and 10 mg/L. For 5 mg/L of lead solution. The mean peak current obtained was 4.52 uA with a range of 4.39-4.67 uA and a relative standard deviation of 2.43% while for 10 mg/L lead solution, a mean peak current of 8.64 uA was obtained with a range of 8.198.97 uA and a relative standard deviation of 3.75%. The detection limit (signal-to-noise ratio =3) was found to be 0.59 mg/L.

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Figure 9. Voltammograms at different deposition potential

Regeneration. After analysis, it is important to determine the extent by which electrodes can be re-used. Regeneration of the modified electrodes are typically done by multiple stripping, or by regeneration with a metal chelator, e.g. EDTA, to remove bound metal ions. Results in Figure 10 shows that concentration of lead ions or current peak decreased to a certain extent with more stripping steps. After five strippings, no peak was observed, which means that no more lead ions are attached to the electrode. This would indicate that multiple stripping would be an appropriate regeneration method to allow the re-use of electrodes.

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Actual laboratory waste samples were analyzed for lead ion content using differential pulse anodic stripping voltametry, with our novel feather-modified carbon paste electrode. DPASV analysis done using the optimized parameters showed an average lead content of 7.48 + 0.44 mg/L on the laboratory waste samples. The value obtained by the electrochemical procedure is a little lower than the values obtained by atomic absorption spectroscopy (AAS), a standard method for lead analysis, which is 7.74 + 0.23. The discrepancy could be due to the presence of other metal ions in the samples. These metal ions could also bind with the feather in the electrode, and thus compete with the lead ions for the binding sites in the electrode. This limitation can be surmounted by performing a series of quality control methods with various interfering metal ions to statistically determine correction factors for our electrode. To further verify the correctness of the method, the laboratory waste samples were spiked with 5 mg/L lead solution standard. The DPASV method using the feather CPE gave 12.64 + 0.35 mg/L while AAS has 12.90 + 0.03 mg/L. The agreement between the above results confirms the validity of the method for lead analysis in water samples.

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REFERENCES 0 -1.0

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Figure 10. Regeneration of feather-modified carbon paste electrode surface using multiple stripping

Calibration Curve and Electrode Testing. The calibration graph of peak current and lead concentration in the range of 1 to 10 mg/L was linear (r2 = 0.996). Concentration significantly higher than 10 mg/L show deviations from the linear relationship between current

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KOZLOWSKI H, BAL W, DYBA M and KOWALIKJANKOWSKA T. 1999. ����������������������������� Specific structure-stability relations in metallopeptides. Coord ��������������� Chem Rev 184:319-346 RAMOS JA, BERMEJO E, ZAPARDIEL A, PEREZ JA and HERNANDEZ L. 1993. Direct determination of lead by accumulation at a moss modified carbon paste electrode. Anal Chem Acta 273:219-227. SIGEL H and MARTIN RB. 1982. Coordinating properties of the amide bond. Stability and structure of metal ion complexes of peptides and related ligands. Chem Rev 82:385-426. SUGAWARA K, MATSUI H, HOSHI S and AKATSUKA K. 1998. Voltammetric detection of silver (I) using a carbon paste electrode modified with keratin. Analyst 123: 2013-2016. WANG J. 2000. Analytical Electrochemistry. 2nd ed. USA: VCH Publishers.

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