112 Bull. Korean Chem. So. 21, Vol. 31, No. 5 Xing-Yuan Liu DOI 1.5/ks.21.31.5.112 A Novel Sensor Based on Eletropolymerization Poly(safranine) Film Eletrode for Voltammetri Determination of -Nitrophenol Xing-Yuan Liu Department of Chemistry, Teahers College of Simao, Yunnan 5, China. * E-mail: yunliuxy@sina.om.n Reeived Septemer, 29, Aepted Feruary 2, 21 A novel poly(safranine)-modified eletrode has een onstruted for the determination of -nitrophenol (-NP) in natural water sample. The eletrohemial ehavior of poly(safranine) film eletrode and its eletroatalyti ativity toward - NP were studied in detail y yli voltammetry (CV) and adsorptive linear stripping voltammetry (LSV). All experimental parameters were optimized and LSV was proposed for its determination. In optimal working onditions, the redution urrent of -NP at this poly(safranine)-modified eletrode exhiited a good linear relationship with -NP onentration in the range of. 1 to. 1 5 mol L 1. The detetion limit was 3. 1 mol L 1. The high sensitivity and seletivity of the sensor were demonstrated y its pratial appliation for the determination of trae amounts of -NP in natural water and fruit samples. Key Words: Sensor, Safranine, -Nitrophenol, Eletropolymerization Introdution Phenol and sustitute phenols have gained onsiderale attention in the past deade due to their toxi effets on human eings, animals and plants. Phenoli ompounds are produed y a numer of polluting proesses, inluding industry-related methods of plasti, paint, antioxidant, petroleum, paper, drug and pestiide prodution. 1 The ompound -NP is known for its toxiity, arinogenity and persistene in the environment. 2- It has eome a ommon pollutant in natural and wastewater. Moreover, -NP is involved in most of the degradation pathways of organophosphorous pestiides like fenitrothion, methyl-parathion, ethyl-parathion. These are deomposed in soil and water produing -NP as an intermediate or final produt of the reation. 5, As a onsequene, -NP is ited in the Environmental Protetion Ageny List of Priority Pollutants. 7 Hene, the determination of -NP in water and food samples is of paramount importane. Oviously there is a strong interest for the development of simple, sensitive and reliale determination method. The detetion of -NP is usually aomplished using hromatographi tehniques, suh as gas hromatography,,9 high performane liquid hromatography, 1,11 liquid hromatography assoiated with mass spetrosopy, and apillary eletrophoresis. 13 Although sensitive and seletive, these methods depend on multi-step sample lean-up proedures and are, therefore, generally time-onsuming and require skilled personnel. Eletrohemial method, whih an offer fast, simple and diret real time analysis is one of the most used tehniques in the determination of phenoli ompounds. Eletroanalytial methods have een proposed for -NP determination with a modified glassy aron eletrode(gce), 1-1 hanging merury drop eletrode (HMDE) 17,1 and oron-doped diamond (BDD) eletrode. The analytial signal is derived from the -eletron redution of the nitro group 19-2 or y the diret 2-eletron oxidation of phenol to the orresponding o-enzoquinone. 21-23 The amount of -NP in seawater has een measured with differential pulse voltammetry. This is potentially useful for on-line monitoring. 1 In reent years, water-solule dyes, suh as methylene lue, 2 methylene green, 25 armine, 2 and other dye derivatives, have een widely used as mediators to atalyze the redution or oxidation of organi moleules. Safranine is a water-solule red-olored phenazine-type dye used widely in paper and pharmaeutial industries. However, safranine polymer has rarely een used as a mediator to failitate the eletrohemial reation of ompounds. In this work, a novel sensor for the detetion of -NP was developed y the eletropolymerization of safranine on the surfae of a GCE. An eletrohemial measurement was proposed for the diret determination of -NP in water and apple samples. The results were onsistent with those of a standard hromatography proedure. H2N Experimental Setion Apparatus. All eletrohemial experiments were arried out on a CHI C eletrohemial workstation (CH Instrument Company, Shanghai, China) with a onventional three-eletrode system. The working eletrode was a GCE modified with eletropolymerization of safranine. The auxiliary and referene eletrodes were made of platinum wire and saturated alomel eletrode (SCE), respetively. Reagents. Analytial grade -NP was purhased from Shanghai Reagent Company (Shanghai, China). Stok standard solution was prepared y dissolving -NP in ethanol and then storing H N + N Cl NH2 Figure 1. The moleular struture of safranine ompounds.
Voltammetri Determination of -Nitrophenol Bull. Korean Chem. So. 21, Vol. 31, No. 5 113 it in the refrigerator. An aqueous solution was prepared daily y simple dilution of the stok solution with.1 mol L 1 phosphate uffer solution (PBS, ph =.). Safranine (Fig. 1) was otained from Shanghai Reagent Company (Shanghai, China). All reagents are of analytial reagent grade unless stated otherwise. Doule-distilled water is used throughout the experiment. Preparation of the modified eletrode. The GCE (3. mm in diameter) was polished to a mirror finish with polish paper and alumina slurry and leaned onseutively and thoroughly in an ultrasoni leaner with 1:1 HNO 3, alohol, and redistilled water. Eletropolymerization of safranine on the GCE was aomplished y yli voltammetry(cv) in PBS (ph.) ontaining 1 5 mol L 1 safranine. A poly(safranine) film was formed on the eletrode surfae y sweeps etween 1. and V for 15 yles at a san rate of.1 V s 1. Sample preparation. A natural water sample was otained from a dam loated in Puer (Yunnan, China). The ph value was adjusted with KH 2PO -Na 2HPO. The apple sample was otained from the loal market. Its pretreatment followed the reommended proess in literature. 27 Briefly, the apple seedase sample was mashed with a lender. Then, 25. g of pulp was mixed with.5 L aetonitrile and 1.5 g NaCl, and the organi phase was olleted and evaporated on a steam ath. The final volume was adjusted to.5 L y the adding appropriate amount of dihloromethane. Part of the sample solution was transferred to PBS (ph.). It was then spiked with a known amount of -NP stok solution. This was used for the eletrohemial determination. Analytial proedure. An eletrohemial ell ontaining.1 L supporting eletrolyte (.1 mol L 1 PBS, ph.) and a speifi amount of standard -NP solution were used to perform eletrohemial measurements. The solution was de-aerated with nitrogen for 1min. After the potential was held at.7 V for 5 s in order to aumulate -NP on the modified eletrode, CV or adsorptive linear stripping voltammetry(lsv) was performed and voltammograms were reorded. The redution peak urrent, measured at approximately 1. V in LSV, was applied for the eletrohemial determination. After eah measurement, the poly(safranine) film eletrode was reativated y suessive yli potential sweeps etween 1.2 and +. V at.1 V s 1 in PBS (ph.). All experiments were arried out at room temperature. Results and Disussion Eletrohemial ehavior of -NP on the sensor. The redox ehavior of -NP was studied using the poly(safranine)-modified eletrode. Fig. 2 shows yli voltammograms of. 1 5 mol L 1 -NP on the sensor in PBS (ph.). An ovious redution peak (P) appeared at aout 1.5 V during the athodi sweep, and a ouple of not well-defined redox peaks (Pa1 and P1) were oserved in the potential range of 1.2 to. V. The anodi peak potential (Epa1) and the athodi potential (Ep1) were loated at.2 and.23 V, respetively. The peak-topeak potential separation was aout.5 V, indiating that the -NP eletroredution was a reversile proess. Its voltammetri ehavior is onsistent with the eletrohemial harateristis of -NP reported elsewhere. 1,15 Therefore, the same eletron 1.5 1..5.5 1. 1.5 P1 Pa2 Pa1 3 5 7 Figure 2. Voltammograms for a sensor in.1 mol L 1 ph. PBS with (urve a and ) and without (urve ). 1 5 mol L 1 -NP. Curve a and represent the first and the seond yle respetively. San rate: 1 mv s 1. NO 2 +e + NHOH P2 OH OH OH +e 2e, 2H + +2e, +2H + NO P a Sheme 1. Moleular strutures and eletrohemial reation mehanism of -nitrophenol on the sensor. transfer mehanism of -NP at the poly(safranine) film eletrode an e applied in this study (Sheme 1). The P peak was the result of an initial e /H + redution of the nitro group to the hydroxylamine speies. The ouple Pa1/ P1 represented for the oxidation of hydroxylamine speies and the redution of the NO group, eah transferring 2e and 2H +. Moreover, the signifiant derease of the urrent peak of the P proess in the seond san (Fig. 2) ould e related to the adsorption of -NP on the eletrode surfae. During the first yle, most of the -NP adsored was redued, and the seond redution peak appeared smaller than it was previously. With the voltammogram of the sensor in.1 mol L 1 PBS (ph.) and in the asene of -NP (urve ) as asis, another ouple of redox peaks (Pa2 and P2) in urve was asried to the redox reation of the safranine polymer film. However, the urrent of urve was different from that of urve around. V region. The urrent differene ould e aused y the adsorption of an unstale intermediate and/or the adsorption of the final produt of the eletrode reation (Sheme 1). Furthermore, adsorptive linear stripping voltammetry was applied to study the eletrohemial ehavior of -NP on the sensor. Fig. 3 shows the adsorptive linear stripping voltammograms of are GCE (urve ) and the sensor with (urve a) and without (urve ). 1 mol L 1 -NP in PBS (ph.). One redution peak of safranine polymer film was also oserved at around.5 V on the sensor (urve a and ). The voltammetri ehavior of -NP on a are GCE was also investigated in
11 Bull. Korean Chem. So. 21, Vol. 31, No. 5 Xing-Yuan Liu 1 1. a 1.2 1...2..2..2... 1. 1.2 Figure 3. Adsorptive linear stripping voltammograms for. 1 mol L 1 -NP on the sensor (a) and a are glassy aron eletrode (). Curve represents linear sweep voltammogram of the sensor in.1 mol L 1 ph. PBS in the asene of -NP. San rate: 1 mv s 1. 2 5 1 15 2 25 t (s) Figure 5. Influene of aumulation time on the redution peak urrent of. 1 mol L 1 -NP in.1mol L 1 ph. PBS. Aumulation potential:.7 V..95 11 1. 1 1 1.5 1.1 9 7 1.15 1.2 5 7 9 1 ph Figure. Effet of ph on the peak potential (full irle) and urrent (open irle) of. 1 mol L 1 -NP on the sensor. Supporting eletrolyte: 1/15 mol L 1 phosphate uffer..1 mol L 1 PBS. Only a small peak was oserved at 1. V when the -NP onentration was ontrolled at. 1 mol L 1. However, the redution peak urrent of -NP inreased signifiantly, and the peak potential shifted positively to 1. V when the poly (safranine) film-modified GCE was employed. The remarkale peak urrent enhanement was undoutedly attriuted to the extraordinary properties of the polymer film, suh as its three-dimensional distriution mediators and strong adsorption aility. In Fig. 1, phenazine groups are present in the hemial struture of safranine, Thus, these are also present in the struture of polymer (safranine). Moreover, a phenyl group is present in the struture of -NP. Therefore, during the proess of pre-onentration, -NP an e aumulated in the surfae of the eletrode via π-π hydrophoi interation and/or hydrogen onding with the poly (safranine) film to enhane the urrent response. These results suggest that the sensor exhiits 1.....2..2. Figure. Effet of the aumulation potential on the redution peak urrent of. 1 mol L 1 -NP in.1 mol L 1 ph. PBS. Aumulation time: 1 s. strong adsorptive aility and sutle eletroni properties for -NP. Influene of solution ph. The effet of ph on the peak urrent and peak potential of -NP is displayed in Fig.. The redution peak potential shifted negatively with inreasing ph values. In the ph range of 5. - 9., a linear relationship existed etween them. The regression equation was: Ep =. ph.729 (r =.99). A slope of approximately.5 V/pH suggests that the overall proess was proton-dependent and that the eletron transfer step was preeded y protonation, with an equal numer of protons and eletrons involved in the -NP redution. On the other hand, the redution peak urrent vs. ph dependene revealed that the peak urrent had a maximum value at ph.. The urrent dereased sharply in oth diretions for higher and lower ph values. For this reason, all susequent experiments using the sensor were arried out in phosphate uffer at ph..
Voltammetri Determination of -Nitrophenol Bull. Korean Chem. So. 21, Vol. 31, No. 5 115 1 2 log I/C... 5. 5.2 5. 5..5..5 1. 1.5 log C 1 2 3 C/µmol L 1 Tale 1. Analytial results of water samples and reovery test Sample Spiked Found Reovery (%) 1 2. 3 1 2.39 99.5.3 97..37 9..93 9.3.995 99.5 1.1 1.1 2.1 1. 2.11 15. Figure 7. Caliration urve for the determination of -NP on the sensor. Inset: The log i/c vs. log C plot. Influene of aumulation time and aumulation potential. Effets of aumulation time on the redution peak urrent of. 1 mol L 1 -NP were investigated y LSV, and the results are illustrated in Fig. 5. The peak urrent inreased with the aumulation time in the range of 2-1 s. This indiated the ourrene of adsorptive aumulation of -NP on the surfae of the modified eletrode. However, further inrease in aumulation time did not inrease the amount of -NP at the eletrode surfae owing to surfae saturation. Therefore, 1 s was seleted as optimum aumulation time for the determination of -NP on the sensor. Fig. shows the effet of aumulation potential on the response of -NP. A maximum urrent response was reahed at the aumulation potential value of.7 V, and a sharp derease in urrent was oserved in oth sides of this potential. Thus, an aumulation potential of.7 V was used in susequent studies to ahieve optimum results for -NP determination. Influene of san rate. The effet of san rate on the redution of -NP was investigated y adsorptive linear stripping voltammetry in the range of.25 -.2 V s 1. A good linear relationship etween the peak urrent and san rate was oserved in the range studied. The regression equation was: Ip (1µA) =.55v (mv s 1 ) + 1.717 (r =.997). The linear urve indiated that the eletrohemial reation was predominantly adsorption-ontrolled in a onfined thin film. Analytial harateristis. A series of -NP solutions with different onentrations was measured using the optimized onditions. As a result, the peak urrent was linear to the -NP onentration over the range of. 1 to. 1 5 mol L 1. Fig. 7 shows the aliration urve of the sensor for the determination of -NP. The linear regression equation is Ip (1 µa) =.239 C +.9 (r =.999). The detetion limit was estimated to e 3. 1 mol L 1. To explore the reproduiility of the sensor, a. 1 mol L 1 -NP solution was measured ten times, and the relative standard deviation of the peak urrent was 3%. The -NP solution was also determined with six eletrodes that were fariated independently, and the relative standard deviation of Tale 2. Analytial results of apple samples and reovery test Sample Spiked Found Reovery (%) 1 2. 3 1 2.15 13.7. 1.397 99.2 1.3 1.3 1.15 11.5.97 97. 2.12 17.1 1 1.9 1.91 9.5 the peak urrent was 3.1%. Hene, the reproduiility was aeptale. The staility of the sensor was examined in PBS ontaining. 1 mol L 1 -NP y means of voltammetry. The sensor was used and stored in redistilled water at room temperature for at least 2 weeks. The test results showed that the urrent responses of the sensor only had a deviation of 3.5% from its original urrent. The sensor therefore exhiited longtime staility. The interferene of some oexistent ions and moleules was examined. The results showed that for. 1 mol L 1 -NP, at least 1-fold onentration of Ca 2+, Mg 2+, Cu 2+, Mn 2+, P 2+, Zn 2+, Fe 2+, Fe 3+, and Al 3+ and 1-fold onentration of dopamine, asori aid, vitamin B, gluose, hypoxanthine, and holesterol did not interfere with the determination of -NP. However, o-nitrophenol, m-nitrophenol, p-nitroenzoi aid, and nitroenzene severely interfered with the determination. Determination of -NP in water and fruit samples. The sensor was used to determine -NP in natural water and fruit samples. A standard addition method was adopted to estimate the auray. The measurement results are shown in Tales 1 and 2. In addition, Fig. shows the voltammograms from typial analysis of -NP in apple samples. No voltammetri peaks orresponding to -NP were oserved when the samples were analyzed. This means that their onentrations were elow the de-
11 Bull. Korean Chem. So. 21, Vol. 31, No. 5 Xing-Yuan Liu 1.5 1..5..2..2... 1. 1.2 Figure. Voltammograms from typial analysis of -NP in apple samples. Curve a represents a apple sample without -NP spiked. Curve, and d represent the voltammograms of apple samples spiked with. 1 7, 1. 1, 1 mol L 1 -NP respetively. tetion limit. The reoveries of the standards added were lose to 1%, indiating that the method was reliale. Conlusions An eletrohemial sensor for the determination of -NP has een developed ased on the eletropolymerization of safranine on GCE. The results suggest that the urrent response of the sensor to -NP is highly sensitive, seletive and stale. The suessful determination of -NP spiked into natural water and apple samples suggests that it is a promising eletrohemial sensor for the detetion of -NP in real samples. Referenes 1. Frenzel, W.; Frenzel, J. O.; Moeler, J. Anal. Chim. Ata 2, 21, d a 253. 2. Penalver, A.; Pourull, E.; Borrull, F.; Mare, R. M. J. Chromatogr. A 22, 953, 79. 3. Nasr, B.; Aedllatif, G. J. Eletrohem. So. 25, 152(), D113.. Cong, Y.; Wu, Z.; Ye, Q.; Tan, T. J. Zhejiang. SCI 25, 2, 1. 5. Castillo, M.; Domingues, R.; Alpendurada, M. F.; Barelo, D. Anal. Chim. Ata 1997, 353, 133.. Dzyadevyh, S. V.; Chovelon J. M. Mater. Si. Eng. C 22, 21, 55. 7. ATSDR. Tox FAQs nitrophenols; Ageny for Toxi Sustanes and Disease Registry, 21.. Wang, S. P.; Chen, H. J. J. Chromatogr. A 22, 979, 39. 9. Kontsas, H.; Rosenerg, C.; Pfaeffli, P.; Jappinen, P. Analysy. 1995,, 175. 1. Brega, A.; Prandini, P.; Amaglio, C.; Pafuni, E. J. Chromatogr. A 199, 535, 313. 11. Makuh, B.; Gazda, K. J. Chromatogr. A 199, 733, 171.. Wissiak, R.; Rosenerg, E. J. Chromatogr. A 22, 93, 19. 13. Kaniansky, D.; Krmova, E.; Madajova, V.; Masar, M.; Marek, J. J. Chromatogr. A 1997, 772, 327. 1. Luz, R. D. S.; Damos, F. S.; De Oliveira, A. B.; Beek, J. Talanta 2,, 935. 15. Hu, S. S.; Xu, C. L.; Wang, G. P.; Cui, D. F. Talanta 21, 5, 115. 1. Cordero-Rando, M. D.; Barea-Zamora, M.; Barer-Salvador, J. M.; Naranjo-Salvador, I. Mirohim. Ata 1999, 132, 7. 17. Barek, J.; Eertova, H.; Mejstrik, V.; Zima, J. Collet. Czeh. Chem. C 199, 59, 171. 1. Ni, Y.; Wang, L.; Kokot, S. Anal. Chim. Ata 21, 31, 11. 19. Lawrene, N. S.; Pagels, M.; Meredith, A.; Jones, T. G.. J.; Hall, C. E.; Pikles, C. S. J.; Godfried, H. P. Talanta 2, 9, 29. 2. Lund, H., Baizer, M. M., Eds., Organi Eletrohemistry: An Introdution and a Guide; Marel Dekker: New York, 1991; p 11. 21. Pedrosa, V. A.; Codognoto, L.; Avaa, L. A. J. Braz. Chem. So. 23, 1, 53. 22. Pedrosa, V. A.; Codognoto, L.; Mahado, S. A. S.; Avaa, L. A. J. Eletroanal. Chem. 2, 573, 11. 23. Garellini, G. S.; Salazar-Banda, G. R.; Avaa, L. A. J. Braz. Chem. So. 27, 1, 195. 2. Ensafi, A. A. Anal. Lett. 23, 3, 591. 25. Wang, B.; Dong, S. Talanta 2, 51, 55. 2. Liu, X. Y.; Li, C. Y.; Hu, S. S. Mirohim. Ata 2, 15, 275. 27. Zhou, S.; Zhao, L. W.; Luo, L. N. Environ. Chem. 2, 25, 3.