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Jul 22, 2009 - Petroleum Industry (RIPI), Tehran, Iran, and Laboratoire de Chimie Analytique et ... 2-furaldehyde in several Iranian oil refinery wast...
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Anal. Chem. 2009, 81, 6789–6796

Highly Sensitive and Selective Poly(vinyl chloride)Membrane Potentiometric Sensors Based on a Calix[4]arene Derivative for 2-Furaldehyde Mojtaba Shamsipur,*,† Ali Akbar Miran Beigi,†,‡ Mohammad Teymouri,‡ Solmaz Rasoolipour,‡ and Zouhair Asfari§ Department of Chemistry, Razi University, Kermanshah, Iran, Oil Refinery Research Division, Research Institute of Petroleum Industry (RIPI), Tehran, Iran, and Laboratoire de Chimie Analytique et Mine´rale, UMR 778, ULP/CNRS/ IN2P3(LC4), ECPM, 25 Rue Becquerel, F-67087, Strasbourg Cedex, France A 2-furaldehyde-selective PVC-membrane electrode is designed based on the host-guest interaction between tetrabenzyl ether Calix[4]arene, as an ionophore, and a lipophilic hydrazone derivative generated in situ from reaction of 2-furaldehyde and Girard’s reagent T. At a pH of 9.2, the electrode exhibits a Nernstian response over the 2-furaldehyde concentration range of (5.0 × 10-5)(1.0 × 10-1) M. The electrode has found to be chemically inert and of adequate stability with a response time of 15 s with a good reproducibility ((0.2 mV) and can be used for a long working lifetime. In order to improve the minimum detectable concentration of 2-furaldehyde, further studies have been performed using a coated graphite electrode and coated platinum and gold disks. Some analytical aspects of adsorptive square wave voltammetry have also been presented in order to elucidate the adduct formation between 2-furaldehyde and Girard’s reagent T. The interfering effects of some Na+, K+, NH4+, formaldehyde, 5-hydroxymethyl 2-furaldehyde (HMF), excess of Girard’s reagent T and organic solvents such as isopropyl alcohol and N,N-dimethylformamide on the sensor’s response have been studied. The viability of using the electrode for the trace determination of 2-furaldehyde in several Iranian oil refinery wastewater samples is also demonstrated. The results obtained from the developed method for real samples are compared with those from UV-spectrophotometric and high-performance liquid chromatographic experiments. Because of the importance of furaldehydes in chemical, biological, and industrial areas, these compounds have been studied intensively for many years.1-5 Especially, the determina* To whom correspondence should be addressed. E-mail: mshamsipur@ yahoo.com and [email protected]. Fax: +98 21 66908030. † Razi University. ‡ Research Institute of Petroleum Industry (RIPI). § ECPM. (1) Nozal, M. J.; Bernal, J. L.; Toribio, L.; Jimenez, J. J.; Martin, M. T. J. Chromatogr., A 2001, 97, 95. (2) Leshkov, Y. R.; Chheda, J. N.; Dumesic, J. A. Science 2006, 312, 1933. (3) Moller, J. K. S.; Catharino, R. R.; Eberlin, M. N. Analyst 2005, 130, 890. (4) Chheda, J. N.; Leshkov, Y. R.; Dumesic, J. A. Green Chem. 2007, 9, 342. 10.1021/ac900920u CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

tion of 2-furaldehyde (furfural) and its derivatives in polymer6,7 food,8,9 and petroleum products10,11 is of critical importance. Furfural is produced from renewable agricultural or forestry wastes (e.g., corncobs, oat and rice hulls, sugar cane bagasse, wood chips, etc.) through acid catalyzed dehydration of aldopentoses. It is used as a DNA degradation marker indicating the effects of some heavy metals such as Cr, Mn, and Cu on DNA oxidation and cleavage of deoxyribose by oxidative attacks on the C1′ and C5′ positions in DNA structure.12-14 It is also a marker for potential power transformer failures.15,16 Carbon adsorbents produced from furfural are suitable for the removal of heavy metal ions from drinking and waste waters.17 Furfural and its derivatives have been used as potential indicators of temperature abuse and inadequate time of storage in different foods and related materials such as orange juice, grape juice, honey, spirits, wine, milk, and infant milks.18-20 One of the steps in the production of many high grade lubricating oils is solvent extraction refining, and one of the most (5) Gerrard, J. A. Aust. J. Chem. 2002, 55, 299. (6) Rocha, S. M.; Coimbra, M. A.; Delgadillo, I. Carbohydr. Polym. 2004, 56, 287. (7) Davis, I. J.; Rawlings, R. D. Compos. Sci. Technol. 1999, 59, 97. (8) Ferrer, E.; Alegria, A.; Farre, R.; Abellan, P.; Romero, F. Food Chem. 2005, 89, 639. (9) Cha´vez-Servı´n, J. L.; Castellote, A. I.; Lo´pez-Sabater, M. C. J. Chromatogr., A 2005, 1076, 133. (10) ASTM D5837-99(2005). Standard Test Method for Furanic Compounds in Electrical Insulating Liquids by High- Performance Liquid Chromatography; ASTM: Philadelphia, PA, www.astm.org. (11) IEC 61198 (2003). Mineral Insulating Oils-Methods for the Determination of 2-Furfural and Related Compounds; International Electrochemical Commission: Geneva, Switzerland, www.iec.ch. (12) Bose, R. N.; Moghaddas, S.; Mazzer, P. A.; Dudones, L. P.; Joudah, L.; Stroup, D. Nucleic Acids Res. 1999, 27, 2219. (13) Pitie, M.; Burrows, C. J.; Meunier, B. Nucleic Acids Res. 2000, 28, 4856. (14) Barciszewski, J.; Siboska, G. E.; Pedersen, B. O.; Clark, B. F. C.; Rattan, S. I. S. Biochem. Biophys. Res. Commun. 1997, 238, 317. (15) Emsley, A. M.; Stevens, G. C. IEE Proc.: Sci., Meas. Technol. 1994, 141, 324. (16) Dilleen, J. W.; Lawrence, C. M.; Slater, J. M. Analyst 1996, 121, 755. (17) Kanto, T.; Ishizaki, K. Water-Absorbent Resin and Production Process Therefore. U.S. Patent 7,009,010, March 7, 2006. (18) Glordano, L.; Calabrese, R.; Davoll, E.; Rotilio, D. J. Chromatogr., A 2003, 1017, 141. (19) Granados, J. Q.; Mir, M. V.; Garcia-Serreda, L. M.; Lopez-Martinez, M. C. J. Ag. Food Chem. 1996, 44, 1378. (20) Shimizu, C.; Nakamura, Y.; Miyai, K.; Araki, S.; Takashio, M.; Shinotsuka, K. J. Am. Soc. Brewing Chem. 2001, 59, 51.

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extensively used solvents for this purpose is furfural.21,22 It is unlikely that any finished product refined in this way would contain furfural, since finishing procedures which follow the furfural extraction eliminate all solvents. As furfural is recovered for reuse, analytical surveys and material balance studies are made to keep solvent losses to a practical minimum. However, significant amounts of furfural ranging from higher than 10 ppm up to 1 wt % are observed in related wastewaters unless in cases of biologically treated wastes. Aromatics, polar components, and mercaptans are removed from petroleum by means of furfural extraction. Furfural can also be used as a decolorizing agent to refine crude wood rosin. According to toxicity studies, it has been found that furfural leads to tumors, mutations, and liver and kidney damage in animals.23,24 When ingested or inhaled, furfural can cause symptoms similar to those of intoxication, including euphoria, headache, dizziness, nausea, and eventual unconsciousness and death due to respiratory failure. Because of its potential toxic effect, furfural regarded as a principal pollutant especially in lubricating oil refinery atmosphere and wastewater. Therefore, the development of simple and sensitive methods for determining trace amounts of furfural has been continued to be a topic of active research.25,26 Various spectrophotometric,27,28 HPLC,29,30 and voltammetric31 methods have been established for the determination of furfural. On the other hand, liquid membrane-type ion-selective electrodes (ISEs) exhibiting excellent selectivity for inorganic metal ions have long been commercially available.32,33 In contrast, the application of ISEs in organic analyses is less developed and have continued to be a fertile field for research. Most of the sensory elements designed for organic analyses are ion exchangers, which lack the differential power to discriminate analytes containing the same functional group.34,35 To improve the selectivity of the electrodes, a new generation of liquid-type ISEs based on (21) Zeitsch, K. J. The Chemistry and Technology of Furfural and Its Many By-Products; Elsevier Science Ltd.: Amsterdam, The Netherlands, 2000. (22) Gammie, I. I.; Douglas, J. Lubricating Oil Refining Process. U.S. Patent 5,328,596, July 12, 1994. (23) World Health Organization (WHO). Concise International Chemical Assessment Document 21, 2-Furaldehyde; World Health Organization (WHO): Geneva, Switzerland, 2000. (24) U.S. Dept. of Health and Human Services, Public Health Services, Center for Disease Control, NIOSH and U.S. Dept. of Labor, OSHA. NIOSH/OSHA Occupational Health Guidelines for Chemical Hazards, Furfural, DHHS (NIOSH) Publication No. 81; U.S. Dept. of Health and Human Services, Public Health Services, Center for Disease Control, NIOSH and U.S. Dept. of Labor, OSHA, U.S. Government Printing Office: Washington, DC, January 1981. (25) Slater, J. M.; Dilleen, J. W. Electroanalysis 1997, 9, 1353. (26) Gao, J.; Dai, H.; Yang, W.; Chen, H.; Lv, D.; Ren, J.; Wang, L. Anal. Bioanal. Chem. 2006, 384, 1438. (27) Khabarov, Y. G.; Kamakina, N. D.; Gusakov, L. V.; Veshnyakov, V. A. Russ. J. Appl. Chem. 2006, 79, 103. ¨ zdemir, Y.; Ekiz, H. L. Nahrung 2001, 45, 43. (28) Akkan, A. A.; O (29) Rodrı´guez, D. M.; Wrobel, K.; Wrobel, K. Chem. Anal. 2005, 50, 315. (30) Rodrı´guez, D. M.; Wrobel, K.; Wrobel, K. Eur. Food Res. Technol. 2005, 221, 798. (31) Shamsipur, M.; Miran Beigi, A. A.; Teymouri, M.; Farzinnejad, N.; Samimi, V. submitted to Food Chem. (32) Umezawa, Y., Ed. CRC Handbook of Ion-Selectice Electrodes: Selectivity Coefficients; CRC Press: Boca Raton, FL, 1990. (33) Bu ¨ hlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1998, 98, 1593. (34) Abdel-Ghani, N. T.; Rizk, M. S.; El-Nashar, R. M. Analyst 2000, 125, 1129. (35) Kharitonov, S. V.; Gorelov, I. P. Russ. J. Electrochem. 2001, 37, 823.

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Figure 1. Chemical structure of the ionophore tetrabenzyl ether Calix[4]arene.

Scheme 1

host-guest chemistry has emerged.36-40 However, to the best of our knowledge, there is no previous literature report on trace determination of furfural using ISEs. Calixarenes, which are cyclic oligomers of phenol-formaldehyde condensates, have attracted considerable attention in host-guest chemistry. A wide range of analytical methods employing the calixarene derivatives as active sensing materials have emerged.41-43 This paper describes our efforts to extend the use of Calix[4]arene ionophores as electrochemical sensors to 2-furaldehyde determination. Here, we have designed novel furfural-selective electrodes based on the host-guest interaction between tetrabenzyl ether Calix[4]arene (Figure 1), as a suitable ionophore, and a hydrazone derivative generated in situ from the reaction of 2-furaldehyde and Girard’s reagent T (FuGA, Scheme 1). In fact, the resulting ionic adduct is responsible for the observed potentiometric responses. EXPERIMENTAL SECTION Apparatus. Potentiometric measurements were made with a Mettler model DL40 digital potentiometer. Tetrabenzyl ether Calix[4]arene-poly(vinyl chloride) (PVC) matrix membrane electrode in conjunction with an internal Hg-Hg2Cl2 reference electrode and 1.0 × 1 0-3 M 2-furaldehyde-Girard’s reagent T adduct (FuGA) at a pH of 9.2 as the reference solution were used. A saturated calomel electrode (SCE, Azar Electrode Co., Urmieh, Iran) served as the external reference electrode. All (36) Li, X. W.; Yang, B. L.; Wu, Y. Y.; Lin, H. Y. Sensors 2005, 5, 604. (37) Suzuki, K.; Tohdak, K. Trends Anal. Chem. 1993, 12, 287. (38) Hassan, S. S.; Mahmoud, W. H.; Elmosallamy, M. A.; Almarzooqi, M. H. Anal. Sci. 2003, 19, 675. (39) Jennings, K.; Diamond, D. Analyst 2001, 26, 1063. (40) Diamond, D.; McKervey, M. A. Chem. Soc. Rev. 1996, 25, 15. (41) Liu, Y.; You, C. C.; Kang, S. Z.; Wang, C.; Chen, F.; He, X. W. Eur. J. Org. Chem. 2002, 4, 607. (42) Mahajan, R. K.; Kumar, M.; Sharma (nee Bhalla), V.; Kaur, I. Analyst 2001, 126, 505. (43) Chen, L.; Zeng, X.; Ju, H.; He, X.; Zhang, Z. Microchem. J. 2000, 65, 129.

measurements were made at a constant temperature in the range 24-27 °C. The representative electrochemical cell for emf measurements is as follows:

The pH of the solution was measured with a WTW model combined pH meter (Wissensehaftlich-Technische, supplied by Germany). A 384B polarograph analyzer supplied by EG&G Princeton Applied Research Company (PARC) equipped with a static mercury drop electrode (SMDE) was used for the voltammetric studies. The UV-vis spectra were recorded on a Cary 50 single detector double beam in-time spectrophotometer supplied by Varian Company (Australia). All membrane thickness studies were performed using a PHE-103 Microphotonics Co. variable angle spectroscopic ellipsometer (VASE). Reagents. All reagents were of analytical-reagent grade and used without any further purification. Deionized water with a conductivity of 0.7 µs was used throughout. High-relative molecular weight PVC, dioctyl phthalate (DOP), potassium tetrakis(pchlorophenyl)borate (KTpClPB), sodium tetraphenylborate (NaTPB), o-nitrophenyloctyl ether (o-NPOE), and tetrahydrofuran (THF) were purchased from Aldrich (Milwaukee, WI) and Merck (Germany). 2-Furaldehyde (assay; 99.0 wt %) was purchased from Riedel-de-Hae¨n (Sleeze, Germany) and Girard’s reagent T from Merck. Tetrabenzyl ether Calix[4]arene was synthesized and characterized as follows: In a 250 mL two-necked round-bottom flask equipped with a magnetic bar and a reflux condenser, a mixture of Calix[4]arene (3.0 g,7.1 mmol), K2CO3 (9.8. g,71.0 mmol), and dry acetone (100 mL) was stirred for 1 h at room temperature. Into this mixture, benzyl bromide (8.55 g, 50.0 mmol) in acetone (30 mL) was then added in a dropwise manner through an addition funnel. The mixture was refluxed under nitrogen atmosphere for 24 h. The reaction was allowed to cool to room temperature, and the solvent was evaporated under reduced pressure to give an orange-brown residue. The residue was dissolved in dichloromethane and to it was then added a 2 M hydrochloric acid solution until the pH of the aqueous layer became neutral. The organic phase was separated, and the aqueous layer was extracted again with dichloromethane. The combined organic layer was dried over anhydrous sodium sulfate. After filtration of sodium sulfate, the solvent was removed to give the pure product by precipitation in methanol. Upon slow evaporation of the solvent, the white solid of tetrabenzyl Calix[4]arene precipitated (4.2 g, 75%). The product was characterized by1H NMR (200 MHz, CDCl3), 7.65-7.61 (m, 8H, ArH), 7.35 (t, 12H, J ) 3.5 Hz, ArH), 6.89 (d, 8H, J ) 7.5 Hz, ArH), 6.61 (t, 4H, J ) 7.5 Hz, ArH), 4.95 (s, 8H, Ar-OCH2), 4.28 (d, 4H, J ) 13.0 Hz, ArCH2Ar), 3.36 (d, 4H, J ) 13.0 Hz, ArCH2Ar). Elemental analysis, calcd for C56H48O4: C, 85.68; H, 6.16. Found: C, 85.74; H, 6.32. Figure 1 shows the structure of tetrabenzyl ether Calix[4]arene ionophore. Electrode Preparation. The membrane solution was prepared by thoroughly dissolving milligram amounts of membrane ingre-

dients under optimal conditions (i.e., 30 mg of PVC, 61 mg of o-NPOE, 4 mg of KTpClPB, and 5.0 mg of the ionophore) in 5 mL of THF and mixed well. The resulting mixture was transferred into a 10 mL beaker. The THF content of the mixture was slowly evaporated, until an oily concentrated mixture was obtained. A polyethylene micropipet tip was dipped into the mixture for about 5 s, so that a transparent membrane of about 0.3 mm thickness was formed. The tube was then filled with the internal filling solution (1.0 × 10-3 M FuGA at pH 9.2). The electrode was finally conditioned by soaking in a 1.0 × 10-3 M FuGA solution for several hours before use. The coated graphite and coated Pt and Au disk electrodes were prepared by coating and conditioning the disks (1 mm radius) in a similar manner. Derivatization of 2-Furaldehyde and Calibration. A stock standard solution of 0.10 M FuGA was prepared by reaction of excess of Girard’s reagent T with 2-furaldehyde (5 + 1 by mole ratio) in a buffer solution of 0.10 M NH3-NH4Cl (Scheme 1). Derivatization reaction was carried out at room temperature for 30 min. The ionic adduct FuGA thus prepared is responsible for the observed potentiometric responses. Spectrophotometric and voltammetric evidence are also given in the following sections in order to elucidate the produced FuGA. Subsequently, after appropriate dilution with an ammonium buffer solution, a series of standard solutions containing 0.10 M NH3-NH4Cl and different concentrations of FuGA in the range of (1.0 × 10-1)-(1.0 × 10-7) M were prepared. Aliquots of 25 mL of these standard solutions were used for ion-selective electrode emf measurements. Calibration graphs were constructed by plotting the potential readings against the logarithm of the concentration of the adduct solution. RESULTS AND DISCUSSION Effect of Membrane Composition. The sensitivities and selectivities obtained for a given ion-carrier significantly depend on the membrane ingredients and the nature of the solvent mediator and additive used.44-47 Thus, the influences of membrane compositions on the potential responses of the furfural sensor were studied, and the results are given in Table 1. The potentials of the cells setup with the sensors having membranes with and without additives were measured in the concentration range of (1.0 × 10-1)-(1.0 × 10-7) M of FuGA solutions and plotted as depicted in Figure 2. The working concentration range and slope for different sensors have been determined from the plots and are also summarized in Table 1. Sensor no.1, a membrane without any additive, exhibits a narrow working concentration range ((1.0 × 10-2)-(1.0 × 10-3) M) with a slope of 65.4 mV decade-1. The response time, defined as the time taken by the sensor to generate a stable potential with a ±0.2 mV fluctuation in potential, of this sensor was found to be 65 s. In order to improve the performance of the membrane, different plasticizers (i.e., DBP, DOP, and o-NPOE) and additives (i.e., NaTPB, KTpClPB, and an ionic liquid 1-ethyl-3-methyl (44) Shamsipur, M.; Yousefi, M.; Hosseini, M.; Ganjali, M. R. Anal. Chem. 2002, 74, 5538. (45) Gupta, V. K.; Jain, S.; Chandra, S. Anal. Chim. Acta 2003, 486, 199. (46) Shamsipur, M.; Hosseini, M.; Alizadeh, K.; Talebpour, Z.; Mousavi, M. F.; Ganjali, M. R.; Arca, M.; Lippolis, V. Anal. Chem. 2003, 75, 5680. (47) Shamsipur, M.; Hosseini, M.; Alizadeh, K.; Mousavi, M. F.; Garau, A.; Lippolis, V.; Yari, A. Anal. Chem. 2005, 77, 276.

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Table 1. Composition of PVC Membranes of Tetrabenzyl Ether Calix[4]arene and Performance Characteristics of the 2-Furaldehyde-Selective Electrode working composition of the membrane (mass %) concentration slope correlation response no. PVC ionophore plasticizer additive range (M) (mV decade-1) coefficient time (s) 1 2 3 4 5 6 7 8 9 a

30 30 30 30 30 30 30 30 30

5 5 5 5 5 5 5 4

65 DBP 61 DBP 61 DBP 61 DOP 61 DOP 61 o-NPOE 61 o-NPOE 60 o-NPOE 65 o-NPOE

4 ILa 4 NaTPB 4 NaTPB 4 KTpClPB 4 NaTPB 4 KTpClPB 6 KTpClPB 5 KTpClPB

(1.0 × 10-2)-(1.0 × 10-3) (1.0 × 10-1)-(1.0 × 10-3) (1.0 × 10-1)-(1.0 × 10-3) (1.0 × 10-2)-(1.0 × 10-4) (1.0 × 10-1)-(1.0 × 10-4) (1.0 × 10-1)-(1.0 × 10-4) (1.0 × 10-1)-(5.0 × 10-5) (1.0 × 10-1)-(5.0 × 10-5)

65.4 52.5 32.8 31.2 52.8 55.9 59.5 56.9

0.9851 0.9855 0.9998 0.9916 0.9945 0.9905 0.9976 0.9931

65 40 25 45 30 25 15 20

IL, ionic liquid (1-ethyl-3-methylimidazolium hexafluorophosphate).

Figure 2. Potential response of different synthetic sensors (Table 1) based on tetrabenzyl ether Calix[4]arene.

imidazolium hexafluorophosphate) were studied. Table 1 revealed that the addition of 4% of each additive will increase the dynamic range and results in a shorter response time. In fact, the use of 5% of the ionophore and 4% KTpClPB in the membrane resulted in the Nernstian slope of the electrode toward FuGA concentration (membrane nos. 5, 7, and 8). As it is obvious from Table 1, o-NPOE is a more effective solvent mediator than DBP and DOP in preparing the furfural-selective electrode. It should be noted that the nature of the plasticizer influences both the dielectric constant of the membrane and the mobility of the ionophore and its complex. It is also well-known that the presence of lipophilic anions in cation-selective membranes based on neutral ionophores not only diminishes the ohmic resistance and enhances the response behavior and selectivity but also, in cases where the extraction capability is poor, increases the sensitivity of the membrane electrodes.48,49 A comparison of the performance characteristics of all the sensors clearly revealed that the sensor no.7 with PVC/o-NPOE/ KTpClPB/ionophore ratio of 30:61:4:5 is the best electrode exhibiting the widest concentration range of (5.0 × 10-5)-(1.0 × 10-1) M with an almost Nernstian slope of 59.5 mV decade-1 and a short response time of 15 ± 1 s. This optimal membrane ingredient composition was also used for the preparation of (48) Teixeria, M. F. S.; Pinto, A. Z.; Fatibello-Filho, O. Talanta 1997, 45, 249. (49) Ammann, E.; Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A.; Pungor, E. Anal. Chim. Acta 1985, 171, 119.

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Figure 3. Calibration graphs for 2-furaldehyde sensors based on tetrabenzyl ether Calix[4]arene: (A) PME and CGE and (B) CPtE and CAuE.

coated graphite (CGE), coated gold (CAuE), and platinum disk (CPtE) electrodes. Calibration Curves and Statistical Data. The plot of EMF vs -log [2-furaldehyde] shown in Figure 3A indicates that the PME and CGE sensors have a Nernstian behavior over a wide concentration ranges of (5.0 × 10-5)-(1.0 × 10-1) M and (5.0 × 10-6)-(1.0 × 10-1) M of 2-furaldehyde, respectively. The respective slopes of the resulting calibration graphs for PME and CGE are 59.5 ± 0.3 and 56.4 ± 0.5 mV decade-1. At a 10 µg mL-1 of 2-furaldehyde, the standard deviation of four replicate potential measurements for both PME and CGE sensors was at the most ±0.2 mV. The respective limits of detection (LOD), defined as the concentration of furfural obtained when the

Figure 4. SEM images of a synthetic polymer membrane coated on gold disk electrode: magnification ) ×2500.

linear regions of the calibration graphs extrapolated to the baseline potential, for PME and CGE found to be 2.0 × 10-5 M (∼2.0 µg mL-1) and 3.0 × 10-6M (∼0.3 µg mL-1). In order to improve the minimum detectable concentration of 2-furaldehyde, further studies were performed using the coated gold (CAuE) and coated platinum (CPtE) disk electrodes. In a preliminary survey on the characteristics of the proposed electrodes, it was found that the dynamic ranges and the detection limits were surprisingly improved when the films were prepared with a thickness of less than 70 nm (Figure 4) and resistance of 0.5 MΩ. Membrane thickness was determined from Y and ∆ functions measured in a spectral range of 250-1700 nm with a wavelength resolution of 10 nm and at an angle of incident of 70°. Apparently, the film thickness, membrane resistance and adsorptive properties of support material (Pt or Au) were responsible for this phenomenon. As shown in Figure 3B, sub-Nernstian responses were obtained for the CAuE and CPtE over very wide ranges of the analyte. Under the optimal conditions, the CAuE and CPtE exhibited linear response in the range of (1.0 × 10-8)-(1.0 × 10-1) M (slope ) 43.1 mV decade-1) and (1.0 × 10-8)-(1.0 × 10-2) M (slope ) 44.8 mV decade-1), respectively. On the other hand, although sub-Nernstian behaviors with good linearity were observed for both electrodes, a major drift in potential readings was also recorded for each experimental point of the curves. Thus, in the case of both CAuE and CPtE, it was necessary to rapidly measure the most stable potential readings for all standards before their decay. Some UV-vis experiments were performed in order to elucidate the produced adduct between Girard’s reagent T and 2-furaldehyde, FuGA. The spectra were recorded on a double beam spectrophotometer, while a 0.10 M ammonium-ammonia buffer solution was used as a reference. Figure 5A shows the absorption bands of 10 µg g-1 of each one of test solutions including Girard’s reagent T, 2-furaldehyde, and the FuGA adduct. By comparison of spectra 2 and 3, a bathochromic effect at the maximum wavelength of 2-furaldehyde, from 279 nm toward 315 nm, appeared which was allocated to the increasing π-bonding system in the formed adduct. Besides, the broadening in peaks was noticeable. As shown in Figure 5B, adsorptive voltammetry of 2-furaldehyde via the in situ derivatization of 2-furaldehyde using Girard’s reagent T was also performed. Here, two interesting points were worth commenting. The peak potential of the FuGA adduct under

Figure 5. (A) UV-vis spectrum of 10 µg g-1 of Girard’s reagent T (1), 2-furaldehyde (2), and FuGA (3) in 0.10 M NH3-NH4Cl solution as the reference solvent. (B) Typical SWV voltammograms of blank solution of 0.10 M NH3-NH4Cl(1), 5 µg g-1 of 2-furaldehyde (2), and Ad-SWSV voltammogram of 0.6 µg g-1 2-furaldehyde as FuGA (3).

the Ad-SWSV scan is substantially more positive than that of 2-furaldehyde (∆Ep ∼ 250 mV), so that the detection sensitivity of the method has been significantly improved. Both observations indicated that a selective reaction of 2-furaldehyde (F) with Girard’s reagent T occurred at room temperature for 10 min. Furthermore, FuGA was indeed adsorbed at the surface of the electrode prior to the electroreduction. In our recent work,31 we studied the adsorptive voltammetry of furaldehydes via in situ derivatization using Girard’s reagent T in order to separate of the cathodic peaks for the sake of simultaneous determination of F and 5-hydroxymethyl-2-furaldehyde (HMF). Effect of pH. In order to study the effect of pH on the performance of the sensor, the potentials were determined at two concentrations (1.0 × 10-3 and 1.0 × 10-2 M) of FuGA as a function of pH. The pH of solutions was adjusted by the addition of NaOH and HNO3. The obtained results shown in Figure 6 indicate that the potential remains approximately constant over pH values of 2.0-9.5. In strongly acidic conditions (pH < 2.0), the produced adduct would be rapidly hydrolyzed, especially in cases of dilute solutions. Apparently, under such circumstances, the basic nitrogen of the hydrazone moiety of the resulting adduct, that may be exposed outside the calix sheath, will be susceptible to protonation at low pH values. Keto-enol tautomerization can Analytical Chemistry, Vol. 81, No. 16, August 15, 2009

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Figure 6. Effect of pH of 1.0 × 10-2 M (A) and 1.0 × 10-3 M (B) of test solutions on the potential response of the 2-furaldehyde membrane sensor.

Figure 7. Potential response of PME in 0.1 M of three different electrolytes: (A) NH4CH3CO2, (B) KNO3, and (C) NH3-NH4Cl.

also occur as the pH of the solution changes. With an increase in the pH, enol to keto conversion takes place and the predominant species will be the keto form. On the other hand, the main reason for the observed potential drift at the higher pH values than 10 could be due to the response of the membrane sensor to hydroxyl ions by the deprotonation of the ether group of the ionophore. Figure 7 shows the response characteristics of the PME in 0.10 M of different buffer solution of pH 7.3 (potassium nitrate), pH 5.5 (ammonium acetate), and pH 9.2 (ammonium-ammonia). As seen, the electrode exhibits Nernstian response over a wide concentration range in both pH 9.2 (slope ) 59.5 mV decade-1, r2 ) 0.9916) and pH 7.3 (slope ) 63.9 mV decade-1, r2 ) 0.9853) while, at pH 5.5, a sub-Nernstian response (slope ) 40.9 mV decade-1, r2 ) 0.9833) in a narrower working concentration range ((1.0 × 10-4)-(1.0 × 10-1) M) was observed. It is worth mentioning that the adduct formation was more convenient over a period of less than 15 min in a 0.10 M ammoniumammonia solution, and thus, it was used in further studies. Static and Dynamic Response Times and Reversibility of the Electrode. Response time is one of the most important factors for analytical applications of selective electrodes. In order to evaluate the practical static response time of the electrode, the average time required to achieve a potential within ±1 mV of the final steady-state potential was measured by recording the potential-time plots of three different furaldehyde concentrations of FuGA in 0.10 M NH3-NH4Cl buffer of pH 9.2 and the results 6794

Analytical Chemistry, Vol. 81, No. 16, August 15, 2009

Figure 8. (A) Static potential-time plots of three different furaldehyde concentrations: (1) 1.0 × 10-2, (2) 1.0 × 10-3, and (3) 1.0 × 10-4 M. Dynamic response time of the 2-furaldehyde sensor PME with step changes in the analyte concentration: (A) 5.0 × 10-5, (B) 1.0 × 10-4, (C) 5.0 × 10-4, (D) 1.0 × 10-3, (E) 5.0 × 10-3, (F) 1.0 × 10-2, (G) 5.0 × 10-2, and (H) 1.0 × 10-1 M. (C) Typical high to low curve of the sensor (PME type) for five cycles.

are shown in Figure 8A. The results clearly indicate that, in all cases, the electrode exhibits a constant and stable potential within 15 s. Moreover, the practical dynamic response time of the electrode was recorded by immediate changing of furfural concentration from low-to-high over a concentration range of (5.0 × 10-5)-(1.0 × 10-1) M and the results are shown in Figure 8B. As it can be seen, by an increase in the concentration of the analyte, the potential changes very rapidly (