Electroanalytical Applications Based on Carbon Nanotube/Prussian Blue Screen-printable Composite

Transcription

1 Electronlyticl Applictions Bsed on CNT/PB Electrode Bull. Koren Chem. Soc. 21, Vol. 31, No DOI 1.512/bkcs Electronlyticl Applictions Bsed on Crbon Nnotube/Prussin Blue Screen-printble Composite Jun Ho Shim, Je Seon Lee, Geun Sig Ch, nd Hkhyun Nm,* New nd Renewble Energy Reserch Center, Ewh Womns University, Seoul 12-75, Kore Chemicl Sensor Reserch Group, Deprtment of Chemistry, Kwngwoon University, Seoul , Kore * E-mil: nmh@kw.c.kr Received Februry 3, 21, Accepted Mrch 31, 21 A single step fbriction process of crbon nnotube/prussin Blue (CNT/PB) pste electrodes bsed on screen printing technology hs been studied s n mperometric sensor for the determintion of hydrogen peroxide nd free chlorine. Compred to the clssicl crbon pste (CP) electrode, the CNT pste electrode gretly enhnced the response in the presence of hydrogen peroxide due to the electroctlytic ctivity of the CNT. Bsed on the CNT/binder pste, PB ws lso incorported into network of CNT pste nd chrcterized. The best electronlyticl properties of PB-mixed sensors to hydrogen peroxide were obtined with PB rtio of 1 wt % composition, which showed fst response time (t 9 5 s;.2 - mm), low detection limit of µm, good liner response in the rnge from mol L 1 (r 2 =.9998), nd high sensitivity of 8.21 µa mm 1. In order to confirm the enhnced electrochemicl properties of CNT/PB electrode, the sensor ws further pplied for the determintion of chlorine in wter, which exhibited liner response behvior in the rnge of 5-2 ppb for chlorine with slope of 1.1 µa ppm 1 (r 2 = 9971). Key Words: Crbon nnotube, Prussin Blue, Screen printing technology, Hydrogen peroxide, Chlorine Introduction As the demnd for disposble biosensors increses in the field of point-of-cre testing (POCT), 1 the need for highly sensitive, relible nd economiclly mss producible electrodes increses. In this regrds, the thick film technology, fbricting films for electronic components bsed on screen printing process, is one of the most efficient mens of producing electrochemicl biosensors owing to their smll size, fst response time, high reproducibility, low cost, wide potentil window, nd low bckground currents. 2-4 The medited mperometric enzyme sensor cn be prepred on the crbon pste (CP) electrode which is one of widely studied the thick-film trnsducer for the fbriction nd ppliction of electronlyticl biosensors. The type of ech component nd their respective rtios determine the electrochemicl properties of resultnt electrodes. 4 Another criticl composition is the sensing lyer tightly dhered on the trnsducing lyer. The use of thick-film technology is promising for the mss production of sensors for biologicl nd environmentl monitoring. Since the first report in 1991, 5 crbon nnotubes (CNTs) hve received enormous ttention for the preprtion of electrochemicl sensors due to their unique properties. Some uthors 6,7 hve widely reviewed the bility of CNTs to promote electron trnsfer rections regrding different biomolecules nd their biosensing pplictions such s Pt nnoprticles nd CNT composite, 8 modifiction of glssy crbon electrodes with CNT, 9 nd binder-less composite CNT/grphite/enzyme mixture pcked in needle. 1 Severl methods hve lso been proposed for the immobiliztion of CNTs on electrochemicl trnsducer like the CNT/epoxy composites, 11 formultion of CNT/Teflon composite, 12,13 CNT/Nfion ssocition 14 nd CNT/chitosn system 15 becuse they re not redily dispersed in queous medium. In ddition, specil ttention hs been pid to the possibilities of crbon nnotube pste (CNTP)-bsed electrodes in electrochemicl investigtions nd in modern electronlysis. Rivs group hs reported on the dvntges of CNTP electrode on the electrochemicl behvior of different redox systems bsed on the use of CNTs dispersed in minerl oil: dopmine, scorbic cid, dopc nd uric cid s well s hydrogen peroxide; 16 gunine, denine nd nucleic cids; 17 phenol, ctechol, NADH nd hydroquinone; 18 epinephrine nd norepinephrine; 19 nd glucose. 2 Wng et l. reported the use of CNTP electrode for the oxidtion of hemocysteine. 22 Plleschi et l. 23 published CNT pste electrode using single-wll crbon nnotubes (SWCNTs) mixed with minerl oil for the detection of hydrogen peroxide, NADH nd dopmine. They lso reported for the first time the modifiction of SWCNT with Prussin Blue (PB) for hydrogen peroxide detection. 24 Wng et l. first proposed the screen printble CNT-bsed electrodes s n electrochemicl trnsducer by hnd printing on lumin cermic pltes for the detection of hydrogen peroxide, NADH nd glucose. 25 The hndprinted electrodes were fbricted using CNT-derived ink with isophorone solution contining severl different types of binders. Considering prcticl utiliztion nd mss production, these objectives of the invention in ll ppers cited here re often limited to be used s n efficient mens of producing electrochemicl biosensors becuse the conducting mtrix/binder system plys significnt role in the mss production technology like screen printing. PB hs ttrctive electroctlytic properties Mny reserchers reported tht PB exhibited high ctlytic ctivity for the determintion of hydrogen peroxide The PB complex cn exist in two different forms, i.e. insoluble (Fe III 4(Fe II

2 1584 Bull. Koren Chem. Soc. 21, Vol. 31, No. 6 Jun Ho Shim et l. (CN) 6) 3) nd soluble (KFe II Fe III (CN) 6). PB is electrochemiclly reduced to Prussin white (Everitt s slt), which is cpble of ctlyzing the reduction of hydrogen peroxide t low potentil, behving like peroxidse; for this reson, it is often referred s n rtificil peroxidse. 29 Hydrogen peroxide is one of the min products of the oxidse ctlyzed rections. Hence, biosensors re designed to mesure hydrogen peroxide to quntify the substrte. With this excellent bility s biosensing meditor, PB ws incorported with CNT-bsed electrodes by electrodeposition, 3,31 blend-nd-csting of PB/CNT lyer, nd modifiction of CNT with precipitted PB. 35 However, ll of them re multilyered structures formed on glssy crbon or grphite electrodes even onto the screen-printed electrodes nd requires multiple steps to fbricte desired electrodes, which re not relible nd my cuses incresing time nd cost. In the present work, the single step fbriction process of PB/CNT electrode is presented for the pplictions of screenprinted electrodes. The novel PB/CNT pste simplifies the production process since the screen-printing process directly yields highly sensitive PB/CNT electrode without further modifiction. Therefore, we believe our pproch will reduce cost nd time for sensor production. The screen-printed electrode bsed on PB/CNT composite ws evluted in terms of mperometric responses to hydrogen peroxide nd chlorine. The electrochemicl ctivity monitored by strip-type mperometric electrodes for hydrogen peroxide nd chlorine were compred to the homemde CP electrode. The gretly enhnced electrochemicl signl will ssure the possible ppliction in biosensor nd environmentl monitoring. The thick film technology studied in this work hs gret potentil in developing mss-producible, inexpensive, mechniclly robust, nd disposble sensing pltform. Experimentl Regents nd solutions. All regents were commercilly vilble nd were employed without further purifiction. Potssium hexcynoferrte (III), clcium hypochlorite nd ferric chloride were purchsed from Sigm-Aldrich. Terpineol nhydrous ws purchsed from Fluk. Multi-wll crbon nnotube (Nnolb; OD 15 ± 5 nm, length 1-5 µm), grphite powder (Fisher), silver pste LS-56 (Ashi Chemicl Reserch Lbortory), dielectric lyer (Seoul Chemicl Reserch Lbortory; SCR-55G solder resist ink nd SH-3 hrdner), polyester films (3M), nd high ethoxyl cellulose polymer (Dow Chemicl) were used in fbricting the screen-printed electrode. CNT pste ws prepred t working site in our lbortory. Throughout the work, deionized wter ( 18 MΩ cm) further purified vi Millipore Direct-Q unit (Billeric, USA) ws used. The supporting electrolyte ws n queous solution buffered t ph 7.4 with 5 M phosphte nd.14 M NCl. Stock solutions of hydrogen peroxide were prepred dily in buffer solution nd stored in the drk t 4 o C before use. Residul chlorine nlysis ws crried out in 5 M phosphte contining.14 M NCl solution t ph 6.9. Instrumenttion. Cyclic voltmmogrm (CV) mesurements were performed with n EG&G PAR (Princeton, USA) Model 273A potentiostt/glvnostt controlled by n IBM PC with n EG&G M27 softwre progrm. Eight chnnel multipotentiostt (ELBIO cdaq-164, Kore) ws used to run currenttime responses. All mesurements were conducted using three-electrode configurtion except for the detection of residul chlorine. Scnning electron microgrphs (SEMs) were obtined with Hitchi Model S-27 microscope (Tokyo, Jpn). A semi-utomted screen-printer (LS-15 NEW LONG, Tokyo, Jpn) nd set of stencils (Deshin Co., Kore) ptterned for the electrodes were used for constructed plnr rrys of thirty electrodes. The resultnt plnr rrys were used to produce disposble strip electrodes. A 3-roll mill (EXAKT 5, EXAKT Co., Germny) ws used for mixing nd milling of CNT with polymer binder. Preprtion of CNTP. All printing composites were prepred t room temperture. Polymer binder ws diluted by thoroughly mixing 25 wt % of high ethoxyl cellulose with 75 wt % of terpineol in vil (3 cm 5 cm) for 3 min, nd then CNT ws crefully dded to the vil with mechnicl stirrer (IKA, USA). The preliminry respective inks were then homogeneously dmixed in 3-roll mill (below 1-µm intervl between rolls) for 3 min. Three different mixed proportions were prepred by vrying the mount of CNT; 1, 33, nd 5 (wt %). Other pstes with different sensor mterils (grphite or PB) were prepred in similr fshion. The viscosity of homogenized composites ws djusted with terpineol. The chieved pstes were stored in refrigertor t 5-1 o C before use. Construction of the mperometric trnsducer. The thick-film mperometric electrodes were fbricted using screen-printing technology. The substrte of the screen-printed CNTP electrode ws.5 mm thick flexible PE films which were cut into 1 cm 15 cm nd thermlly treted t 13 o C for 3 hours before the printing to prevent further therml shrinkge in the following process. Three silver lines (electricl connection nd reference electrode), CNT pttern (for working nd counter electrode) nd dielectric lyer were sequentilly screen-printed onto the thermlly pretreted PE film using the stencils corresponding to ech lyer pttern. Ech pttern ws dried t 13 o C for 13 min fter the printing. The overll dimension of individul screenprinted CNTP electrode ws 8 mm 34 mm. The projected re of the round-shped working electrode ws 7 cm 2. Thirty strips were printed on one film. The Ag/AgCl reference electrode ws prepred by oxidtive tretment using 3. M FeCl 3 solution on the dried silver pste for 1 minutes. The surfce of the printed electrodes ws rinsed thoroughly with doubledistilled wter prior to use. Results nd Discussion Structure chrcters of thick-film electrodes. The CNTP electrodes in this study were prepred by mixing different weight percentges of CNT with polymer binder, i.e., 1, 33, nd 5 (wt %) rtios of CNT/binder. Beside these rtios in pste blending, the ink contining over 5 (wt %) of CNT in the sme ethoxyl cellulose level cn not be used for the screen printing purpose, becuse the lrge porous CNT requires extremely high mount of polymer/solvent mixture, which llows surfce blocking for the electron trnsfer. It is importnt to

3 Electronlyticl Applictions Bsed on CNT/PB Electrode Bull. Koren Chem. Soc. 21, Vol. 31, No. 6 () 4 (b) b Current (µa A) c (c) d (d) Potentil (V) Figure 1. SEM imges of the () grphite nd (b) CNTP electrodes. Insets re the powders of respective grphite nd CNT. describe tht the electrochemicl sensor properties re corresponded to bckground current nd cpcitnce. Figure 1 shows the typicl morphologicl fetures of grphite nd CNTP electrodes prior to the electrochemicl mesurements. SEM imge (Figure 1) of screen-printed electrode displys surfce of the grphite electrode obtined by dispersion of grphite within the ethoxyl cellulose binder (5 wt %). The electrode surfce is formed by regulrly shped micrometer sized grphite flke corresponding to the grphite powder in inset (Figure 1). A homogeneous distribution of CNT within the polymer binder (5 wt %) fter printing on the substrte cn be observed in Figure 1b. The CNT-bsed electrode shows more fine surfce morphology thn tht of grphite in micrometer scle. The inset (Figure 1b) shows the nnotube powder used in this study. The qulity of printed electrode ws not dmged by the use of CNT-bsed ink. The screen-printed CNTP electrode remined like hrd shell nd stble fter severl wshings nd electrochemicl mesurements. Electrochemicl behvior of trnsducers. The electrolytes t CNTP electrode re likely to ccess the interior surfce leding to high differentil cpcitnce compred with grphite electrode since CNT is highly porous mteril. In this respect, we first explored the electrochemicl response of electrodes modified with CNT nd grphite in phosphte buffer solution 3/4 contining Fe(CN)6 couple between nd V t scn 1 rte of 5 mv s. The cyclic voltmmogrms of the electrodes printed with the pste formulted with vrying CNT/polymer binder rtio (1, 33, nd 5 (wt %)) nd grphite/polymer binder (5 wt %) re shown in Figure 2. When the CNT ws used insted of the grphite, we observed gretly incresed redox ctivities. It is importnt to remrk tht the excellent electroctlytic properties of the crbon composite mteril were observed with thick-film electrode contining CNT. As presented Figure 2. Cyclic voltmmogrms obtined t CNTP nd grphite 1 electrode in.5 mm potssium ferricynide with scn rte, 5 mv s : () CNT rtio (wt %) of 1; (b) 33; (c) 5; nd (d) grphite rtio (wt %) of 5. Tble 1. Pek potentil seprtion ( Ep) nd pek currents (Ip; Ipc) t the CNTP nd grphite electrode in.5 mm K3Fe(CN)6 contining.1 M KCl % CNT in the ink formultion Ep (mv) Ip (µa) Ipc (µa) Ip/Ipc % Grphite Potentil rnge from V to V (vs. Ag/AgCl). Scn rte is 5 mv s 1. in Tble 1, the cyclic voltmmogrm obtined with the CNTP electrode contining the lest mount of binder (5 wt % of CNT) exhibited improved reversibility in terms of Ep nd pek 3/4 current (Ip) for Fe(CN)6 couple. It cn be seen tht the elec3/4 tron trnsfer of Fe(CN)6 redox couple t CNTP is much fster ( Ep = 85.1 mv of Figure 2c) thn t grphite ( Ep = 782 mv of Figure 2d). Apprently, due to the CNT dimensions, these results demonstrte tht the improved electroctlytic ctivity of CNT presents on its surfce; the redox couples in the proximity of the electrode my esily trnsfer the electrons to the needle-like CNT exposed on the surfce of the electrode, resulting in enhnced electroctlytic property. Since hydrogen peroxide is n importnt molecule involved in numerous enzymtic processes, the development of new methodologies for the sensitive nd selective hydrogen peroxide determintion is crucil. Figure 3 shows typicl current response to hydrogen peroxide s the compositions of the CNTnd grphite-bsed thick-film electrodes. The mperometric responses were operted t potentil of +.65 V. The liner 5 3 rnges of ech electrode were between 5 1 M nd 1 1 M

4 1586 Bull. Koren Chem. Soc. 21, Vol. 31, No. 6 Jun Ho Shim et l. Current (µa) ( A) Current ( (µa) A) 5 mm mm H2O2 (mm).2.6 H 2O 2 O 2 (mm) ( µa A) ( µa A) 1 s 1 s () (c) c 5 mm Current (µa) ( A) mm Current (µa) ( A) µa ( µ A) H 2O 2 (mm) H 2OO 2 (mm) 1 s ( µa µ A) 1 s (b) b (d) d Figure 3. Current-time response obtined on incresing the hydrogen peroxide concentrtion stepwise from 5 to 1 µm:, CNT rtio (wt %) of 1; b, 33; c, 5; nd d, grphite rtio (wt %) of 5. Applied potentil ws.65 V (vs. Ag/AgCl) in 5 M phosphte buffer (ph 7.4). of hydrogen peroxide, nd the slopes of the clibrtion: 2 µa/mm (r 2 =.9986) for 1 wt % of CNT; 1.2 µa/mm (r 2 =.9982) for 33 wt % of CNT; 1.69 µa/mm (r 2 =.997) for 5 wt % of CNT; nd.63 µa/mm (r 2 =.9979) for 5 wt % of grphite. The obtined results shown in Figure 2 prove tht the CNTP electrode bsed on CNT rtio of the 5 wt % exhibited the most effective ctlytic ctivity on the electrode surfce. Chrcteriztion of PB-derived CNTP electrode. CNTP electrode incorported with vrying weight percentge of PB were prepred s described in experimentl section, nd their electrochemicl chrcteristic ws evluted by recording cyclic voltmmogrm (scn rte of 6 mv s 1 ) of the working electrode in M KCl solution. Figure 4 shows typicl qusi reversible redox peks which correspond to the redox rection between Fe III [Fe +2 (CN) 6] 1 nd Fe II [Fe +2 (CN 6)] 2, indicting tht the redox rection of PB occurs reversibly within the CNTP electrode. The PB/CNT thick-film electrode hs lso been investigted further for the determintion of hydrogen peroxide in 5 M phosphte buffer contining.1 M KCl s supporting electrolyte. As displyed in Figure 5, the increse in PB rtio t the CNT/binder composite substntilly enhnces the mperometric responses of hydrogen peroxide indicting tht the PB ctlyzes the electrochemicl process t the surfce of the electrode. The sensitivity to the chnge in the concentrtion of hydrogen peroxide (between M nd M):.62 µa/mm for grphite (5 wt %); 1.49 µa/mm for CNT (5 wt %); 1.68 µa/mm for PB rtio (R, wt %) of R =.1; 4.53 µa/mm for R = 1; nd 8.21 µa/mm for R = 1. The optimized composition of CNT (5 wt %) ws used to exmine the comprtive performnce of hydrogen peroxide t the PB-derived CNTP electrode. Current ( µa A) Reduced form Oxidized form Potentil (V) Figure 4. Cyclic voltmmogrm obtined t 1 wt % PB-mixed CNTP electrode in M KCl (ph 4.). Scn rte is 6 mv s 1. Current (µa) ( µa Concentrtion of of H 2O 2 O 2 2 (mm) (mm) b c d e e mm Time (s) Figure 5. Current-time response obtined on incresing hydrogen peroxide concentrtion stepwise from 5 to 1 µm:, grphite; b, PB rtio (wt %) of ; c,.1; d, 1; nd e, 1. Applied potentil ws.65 V (vs. Ag/AgCl) in 5 M phosphte buffer (ph 7.4). Chrcteriztion of PB-CNTP electrode for chlorine detection. In order to confirm the enhnced electrochemicl properties of PB mixed thick-film electrode, we lso pplied the electrode for the determintion of residul chlorine. Chlorine is extensively used s disinfectnt for drinking wter nd swimming pool s well s strong oxidizing gent in vrious mnufcturing processes. Its concentrtion must be strictly regulted t the purifiction plnt of public wter systems nd swimming pools since chlorine is highly toxic nd rects with other contminnts to produce crcinogens. Conventionl electrochemicl nlysis for residul chlorine hs been crried out on the noble metls like Pt or Au electrodes The screen-printed CNTP electrode would be more dvntgeous in the detection of chlorine over those conventionl electrodes; it is much less expensive thn noble metls nd suitble for mss production.

5 Electronlyticl Applictions Bsed on CNT/PB Electrode Bull. Koren Chem. Soc. 21, Vol. 31, No Current ( µa A) Current (µa) Current ( µa A) Chlorine (ppm) Chlorine (ppm) ppm Time (s) Figure 6. Current-time response obtined on incresing residul chlorine concentrtion:, grphite; b, CNT only; nd c, CNT with PB rtio of 1 wt %. Applied potentil ws -.1 V (vs. Ag/AgCl) in 5 M phosphte buffer (ph 6.9) contining.1 M KCl. The proposed mechnism of hypochlorous cid to hydroxide ions is s follows: 39 c b 2Ag + + Cl + HOCl 2AgCl + OH (1) The screen-printed CNTP electrodes in both the presence nd bsence of PB were investigted s possible new trnsducer for the determintion of free chlorine. Sequentilly ddition of chlorine stock solution ws mde to the 5 M PBS buffer (ph 6.9) since pk of HOCl ws ssumed to be 7.48 t 25 o C. Figure 6 shows the dynmic nd clibrtion curves (inset) of the respective three different electrodes prepred with () grphite, (b) CNT nd (c) PB (1 wt %) mixed CNT from 5-2 ppb. The nlyticl performnce of the PB/CNT electrode (curve c) ws evluted: sensitivity of 1.1 µa/ppm; detection limit of 1 ppb; response time, t 9% = c. 7 s; nd relible liner dynmic rnge (5-2 ppb, r 2 = 9971). It is noted tht the obtined result is even three times bigger sensitivity thn tht of CNTP electrode (3 µa/ppm, curve b). Compred with CNT-derived electrodes, however, the sensitivity (.15 µa/ppm) of grphite electrode (Figure 6) ws much inferior with slow response (t 9% = c. 15 s) nd high detection limits (> 1 ppb). As result, considering tht the environmentlly meningful rnge of residul chlorine is in M, the sensitivity nd the detection limits of PB mixed CNT-bsed electrode shll find useful pplictions in environmentl residul chlorine monitoring. Conclusions In this work, CNT-derived pste electrodes hve been developed nd used to prepre screen printing type electrode system. CNT ws blended with suitble polymer binder in vrying rtios, nd their electrochemicl properties in the printed electrode formt hve been exmined. Cyclic voltmmogrms of the CNT-derived electrodes in Fe(CN) 6 3/4 redox couple in queous solution exhibited qusi-reversible redox peks while the conventionl grphite thick-film ones did not, implying tht the electron trnsfer kinetics t the CNT-bsed electrode surfce ws more fvorble thn the grphite-bsed. To further enhnce the performnce of CNT-bsed electrodes, PB ws mixed with CNT in different rtios, nd their response to hydrogen peroxide ws exmined. In ddition, the screen printed PB/CNT pste electrode hs been utilized to determine residul chlorine in the 5-2 ppb rnge. The overll results clerly demonstrted the dvntge of CNT-derived electrodes over conventionl grphite-derived ones in the detection of redox species. Acknowledgments. H. Nm grtefully cknowledges the finncil support from Kwngwoon University in 27. J. H. Shim ws supported by RP-Grnt 29 of Ewh Womns University. References 1. Wng, J. Biosens. Bioelectron. 26, 21, Alvrez-Icz, M.; Bilitewski, U. Anl. Chem. 1993, 65, 525A. 3. Glán-Vidl, C. A. ; Muñoz, J.; Domínguez, C.; Alegret, S. Trends. Anl. Chem. 1995, 14, Švncr, I.; Vytřs, K.; Brek, J.; Zim, J. Crit. Rev. Anl. Chem. 21, 31, Iijim, S. Nture 1991, 354, Wng, J. Electronlysis 25, 17, Fernández-Abedul, M. T.; Cost-Grcí, A. Anl. Bionl. Chem. 28, 39, Hrpovic, S.; Liu, Y.; Mle, K. B.; Luong, J. H. T. Anl. Chem. 24, 76, Di, Y.; Shiu, K. Electronlysis 24, 16, Wng, J.; Musmeh, M. Anlyst 23, 128, Gojny, F. H.; Nstlczyk, J.; Roslniec, Z.; Schulte, K. Chem. Phys. Lett. 23, 37, Wng, J.; Musmeh, M. Anl. Chem. 23, 75, Wng, J.; Musmeh, M. Anl. Lett. 23, 36, Wng, J.; Musmeh, M.; Lin, Y. J. Am. Chem. Soc. 23, 125, Zhng, M.; Smith, A.; Gorski, W. Anl. Chem. 24, 76, Rubines, M. D.; Rivs, G. A. Electrochem. Commun. 23, 5, Pedno, M. L.; Rivs, G. A. Electrochem. Commun. 24, 6, Rubines, M. D.; Rivs, G. A. Electronlysis 25, 17, Chichrro, M.; Sánchez, A.; Bermejo, E.; Zprdiel, A.; Rubines, M. D.; Rivs, G. A. Anl. Chim. Act 25, 543, Luque, G. L.; Ferreyr, N. F.; Rivs, G. A. Microchim. Act 26, 152, Rivs, G. A.; Rubines, M. D.; Pedno, M. L.; Ferreyr, N. F.; Luque, G. L.; Rodríguez, M. C.; Miscori, S. A. Electronlysis 27, 19, Lwrence, N. S.; Deo, R. P.; Wng, J. Tlnt 24, 63, Vlentini, F.; Amine, A.; Orlnducci, S.; Terrnov, M. L.; Plleschi, G. Anl. Chem. 23, 75, Ricci, F.; Amine, A.; Moscone, D.; Plleschi, G. Anl. Lett. 23, 36, Wng, J.; Musmeh, M. Anlyst 24, 129, Neff, V. D. J. Electrochem. Soc. 1978, 125, Krykin, A. A.; Pugnov, E. A.; Budshov, I. A.; Kurochkin, I. N.; Krykin, E. E.; Levchenko, V. A.; Mtveyenko, V. N.; Vrfolomeyev, S. D. Anl. Chem. 24, 76, Krykin, A. A.; Gitelmcher, O. V.; Krykin, E. E. Anl. Chem. 1995, 67, Krykin, A. A.; Krykin, E. E. Sens. Actutors B 1999, 57, Li, Z.; Chen, J.; Li, W.; Chen, K.; Nie, L.; Yo, S. J. Electronl. Chem. 27, 63, 59.

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