Practical Problems in Voltammetry: 4. Preparation of Working Electrodes

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1 Prtil Prolems in Voltmmetry: 4. Preprtion of Working Eletrodes Adrin W. Bott, Ph.D. Bionlytil Systems, In. West Lfyette, IN E-mil: The ondition of the surfe of the working eletrode n hve signifint effet on the urrent response in oth voltmmetri nd mperometri experiments. The methods most ommonly used for the preprtion of working eletrodes (polishing, eletrohemil pretretment, nd het pretretment) re disussed in this rtile. The fundmentl proess in eletrohemil retions is the trnsfer of eletrons etween the eletrode surfe nd moleules in the interfil region (either in solution or immoilized t the eletrode surfe). The kinetis of this heterogeneous proess n e signifintly ffeted y the mirostruture nd roughness of the eletrode surfe, the loking of tive sites on the eletrode surfe y dsored mterils, nd the nture of the funtionl groups (e.g., oxides) present on the surfe (1, 2). Therefore, there hs een onsiderle effort devoted to finding methods tht remove dsored speies from the eletrode nd produe n eletrode surfe tht genertes reproduile results. Some of these methods hve lso resulted in the tivtion of the eletrode surfe (s judged y n inrese in the rte of eletron trnsfer). These methods re the sujet of this pper, nd inlude mehnil polishing, het pretretment, nd eletrohemil pretretment. The most ommon method for surfe preprtion is mehnil polishing. The protool used for polishing depends on the pplition for whih the eletrode is eing used nd the stte of the eletrode surfe. There re vriety of different mterils ville (e.g., dimond, lumin, silion ride), with different prtile sizes suspended in solution (BAS supplies.5 µm lumin polish nd 1, 3, 6, nd 15 µm dimond polishes). The pd used for polishing lso depends on the mteril eing used for polishing Texmet pds re used with lumin polish, nd nylon pds should e used with dimond polish. Working eletrodes supplied y BAS hve first een lpped to produe flt surfe, nd hve then een extensively polished to smooth, mirror-like finish t the ftory. Therefore, they typilly only require repolishing with.5 µm or 1 µm dimond polish y the user in etween experiments. Mterils tht hve rougher surfe (e.g., eletrodes whih hve een srthed) must first e polished using lrger-prtile polish in order to remove the surfe defets. After the defets hve een removed, the polishing should ontinue with suessively smller-prtile-size polish (e.g., 15 µm, then 6 µm, then 3 µm,ndthen1µm). One polishing hs een ompleted (this n require from 3 s to severl minutes, depending upon the stte of the eletrode), the eletrode surfe must e rinsed thoroughly with n pproprite solvent to remove ll tres of the polishing mteril (sine its presene n ffet the eletron trnsfer kinetis). Alumin polishes should e rinsed with distilled wter nd dimond polishes with methnol or ethnol. The rinsing solution should e spryed diretly onto the eletrode surfe. After the surfe hs een rinsed, eletrodes polished with lumin should lso e sonited in distilled wter for few minutes to ensure omplete removl of the lumin prtiles. If more thn one type of polish is used, then the eletrode surfe should e thoroughly rinsed etween the different polishes. As disussed ove, the effet of ny surfe pretretment n e determined y its effet on the rte of eletron trnsfer. This n e judged qulittively y exmining the seprtion of the pek potentils in yli voltmmogrm of moleule whose eletron trnsfer kinetis re known to e sensitive to the stte of the surfe; more quntittive determintion n e mde y lulting the vlue of k s from this pek potentil seprtion. For exmple, k s for potssium ferriynide t glssy ron surfe following simple polishing protool ws found to lie in the rnge m s -1 (3,4) (this should e ompred with the vlues mesured for k s for pltinum eletrode, whih re t lest one order of mgnitude lrger). The strong dependene of the eletron trnsfer kinetis of ferriynide on the stte of the eletrode surfe mens tht

2 F1 Voltmmetry t ylindril ron fier eletrode (1) efore nd (2) fter eletrohemil pretretment: (A).1 mm dopmine nd (B) 1. mm sori id, ph 7 solutions, sn rte =.1 v/s. (Adpted from referene 11.) 1 2 na NH 2 (A) there n e signifint vritions in the pek potentil seprtion fter eh polishing. Polishing lters the mirostruture, roughness, nd funtionl groups of the eletrode surfe in ddition to removing dsored speies (for exmple, it hs een shown tht the oxygen-toron rtio is inresed y polishing (5)). It hs lso een reported tht mterils used for the polishing n ffet the vlue of k s (1,4,6). For exmple, the eletrode surfe n e ontminted y the gglomerting gents required to keep the lumin prtiles suspended in solution nd y the omponents of the polishing pd. The presene of these speies n hve deleterious effet on the eletron trnsfer kinetis y loking the tive sites for the eletron trnsfer retion. For the most exting studies, it ws suggested tht the lumin suspension e freshly mde with ultrpure wter nd tht the eletrode should e polished on glss ( k s vlue of.14 m s -1 for ferriynide ws reported following polishing under these stringent onditions (4)). However, it should e noted tht suh pronouned dependene on the stte of the eletrode surfe is only oserved for ertin systems (the most well hrterized exmples re the redution of ferriynide, the oxidtion of sorte, nd the dsorption of dopmine). For suh systems, polishing is often used in omintion with nother pretretment (e.g., het or eletrohemil). However, for mny other systems, the simple polishing 2 OH O E (V vs. SCE) 1 O OH 5 na (B) desried ove is dequte (for exmple, when using non-queous eletrolytes, sine loking of tive sites y dsored speies is less ommon in suh eletrolytes thn in queous solutions). Another method for preprtion of the eletrode surfe tht is eoming more widely used is eletrohemil pretretment (ECP), prtiulrly for eletrodes whih nnot redily e polished (e.g., ron fier ylinder eletrodes). ECP onsists of pplying onditioning potentils to the eletrode surfe efore the experiment. As for polishing, this hs the effet of removing dsored speies nd ltering the mirostruture, roughness, nd funtionl groups of the eletrode surfe. The preise ECP protool depends upon the pplition nd vries onsiderly. The potentil wveforms typilly re held t, or yle to, lrge positive or negtive potentil, either using steps or sweeps (onstnt potentil (6), potentil sn (7,8), tringulr wve (9-15) nd squre wve (16, 17)). Although the development of the preonditioning protools hs een lrgely empiril, the pretreted eletrode surfe hs een hrteriztized in order to eluidte the resons for the tivtion of the eletrode surfe (6,7,17,18). For glssy ron eletrodes, in ddition to the removl of dsored speies, the preonditioning potentil leds to the formtion of n oxygen-rih lyer on the ron surfe. This lyer ontins oxides s well s other oxygen-ontining 2 i OX funtionl groups whih my tlyze eletron trnsfer retions (the omposition of the funtionl groups in this lyer is sensitive to the pretretment onditions nd depends on the solution ph s well s the potentils used for the pretretment (19)). The oxide lyer n lso dsor nd/or exhnge ions from the solution, whih leds to improved detetion limits. However, eletrohemil pretretment of eletrodes inreses the kground urrent of the eletrode reltive to tht of polished eletrode, whih my e disdvntgeous for some pplitions. Some of the speifi effets nd pplitions of eletrohemil pretretment n est e illustrted y numer of exmples. Disrimintion Between Asorte nd Dopmine Using Pretreted Cron Fier Eletrodes In vivo determintion of neurotrnsmitters suh s dopmine is hindered y the uiquitous presene of sorte, sine dopmine nd sorte re oxidized t similr potentils. However, it hs een found tht pretretment of ron fier eletrodes using tringulr wveform (yling etween out V nd +3 V t frequeny of 7 Hz for 2 s, followed y holding t onstnt potentil of +1.5 V for 2 s) inreses the rte of eletron trnsfer for oth sorte nd dopmine (F1) nd hnges the sensitivity of the eletrode to these two nlytes (9-12). The shift in the pek potentils llowed resolution of the peks due to sorte nd dopmine mesured using differentil pulse voltmmetry. The reltive sensitivities for dopmine nd sorte for pretreted eletrodes were out 1:1, whih provides further disrimintion ginst interferene y sorte. The effet of this pretretment on ron fier eletrodes ws further exmined y studying the ehvior of rnge of moleules t

3 F2 Pek urrent vrition with inresing onentrtion, (A) without eletrohemil pretretment nd (B) with eletrohemil pretretment. Plting time = 3 se. Eh dt point is the verge of three repetitions. (Reprinted from referene 21.) F3 Vrition of pek potentils with inresing onentrtion, (A) without eletrohemil pretretment nd (B) with eletrohemil pretretment. Plting time = 3 se. Eh dt point is the verge of three repetitions. (Reprinted from referene 21.) Pek Current (na) Pek Potentil (mv) y = x R =.99 these eletrodes (12). Speifilly, the urrent responses mesured using yli voltmmetry were ompred with those lulted from theory. Although the eletron trnsfer kinetis for ll the systems exmined were inresed y the pretretment, the urrents mesured for tions suh s dopmine nd trnsition metl mine omplexes were lrger thn those lulted, wheres urrents for nions suh s sorte nd ferriynide were smller. The shpe of the yli voltmmogrms for the tions were onsistent with dsorption, nd this ws onfirmed using hronooulometry. The model proposed on the sis of these results involved the formtion of multilyer insulting oxide film on the ron surfe, together with frturing of the surfe. The oxide () () Added P (II) Conentrtion (pp) Added P(ll) Conetrtion (pp) () () lyer n preferentilly tke up positively hrged speies, whih leds to the lrger urrents oserved for dopmine nd the other tions. Anions, suh s sorte nd ferriynide, n only ret t the tive sites exposed y the frturing; tht is, they n only ret t smll frtion of the eletrode surfe, whih is onsistent with the smll urrent response. Although the ove method does led to high sensitivity for dopmine, the preonentrtion required mens tht there is dely in the response time. The response time n e improved y using less positive potentils in the pretretment wveform (out V) (13-15). A surfe oxide lyer is still formed t these potentils, ut it is thinner, nd hene it provides some inrese in its sensitivity for dopmine, while mintining good response time. Anlysis of Led(II) y Squre Wve Voltmmetry Using Gold Disk Eletrode Merury eletrodes re more ommonly used thn solid eletrodes for the detetion of led y nodi stripping voltmmetry, due to the more omplex intertions of led with the surfes of solid metls. This leds to non-liner reltionship etween the urrent nd the led onentrtion (F2A) nd signifint vrition in the pek potentil with inresing led onentrtion (F3A). These poor dt were ttriuted to umultion of the plted metl on the gold surfe, whih leds to vritions in the surfe ondition from one experiment to the next. These vritions n e eliminted y holding the eletrode t potentil of +.8 V vs. Ag/AgCl for five minutes efore strting the stripping experiments, nd then holding the eletrode t this potentil for 5 s etween experiments. The improvements in the linerity of the reltionship etween the urrent nd the onentrtion, nd in the onsisteny of the pek potentil, re redily pprent from F2B nd F3B, respetively. These dt show tht this pretretment restores the eletrode surfe to well-defined ondition. Detetion of Sugrs, Amines, nd Sulfur Compounds Using Pulsed Eletrohemil Detetion In the two exmples ove, the pretretment potentil ws pplied efore eh experiment. However, the preonditioning potentil n lso e inorported into the experimentl wveform to provide eletrode lening nd tivtion t regulr intervls during the experiment. One exmple of suh n experiment is the Pulsed Eletrohemil Detetion (PED) used for

4 F4 Potentil wveform for PED. F5 Comprison of urrent vs. time plots for (A) PED nd (B) onstnt potentil mperometry. Solutions: () lysine, 3 ppm; () gluose, 1 ppm; () surose, 4 ppm. (Reprinted with permission from referene 23.) (A) Potentil (V) (B) 2 nc the detetion of, for exmple, sugrs, mines, nd sulfur ompounds t gold nd pltinum eletrodes (22, 23). These moleules n e oxidized t the surfe of pltinum or gold metl vi n eletrotlyti retion whih is thought to involve dsorption of the nlyte nd retion with dsored hydroxyl groups ( detiled mehnism hs not yet een eluidted). Sine the retion involves dsorption, the eletrotlyti tivity of the eletrode (nd hene the urrent response) dereses with time. This prolem n e solved through the pplition of triple potentil pulse Time (s) Ι Ι Ι Ι Ι Ι Ι 2 nc Ι Ι Ι Time (min) wveform (F4). The first pulse is t potentil t whih the eletrotlyti retion ours (detetion step). Sine this step pssivtes the eletrode surfe, the next pulse is t more positive potentil. This results in the desorption of the pssivting speies, onomitnt with the formtion of n inert oxide lyer (lening step). The eletrode is now retivted y the removl of the oxide lyer using negtive potentil (retivtion step). The eletrode is now redy for the next detetion step. The triple pulse sequene n e used either mperometrilly for EC detetion following seprtion y LC, or voltmmetrilly, y omining the triple pulse with, for exmple, stirse potentil wveform (oth options re ville on the BAS 1B/W). The effet of the lening/detivtion potentil pulses is illustrted in F5. F5A shows the urrent response using fixed potentil for the detetion of sugrs following hromtogrphi seprtion. The derese of the urrent response with time is due to the progressive pssivtion of the eletrode surfe. In ontrst, the urrent response for the triple pulse sequene does not diminish with time (F5B)(23). Ativtion of the eletrode surfe n lso e hieved y therml pretretment. The eletrodes n either y heted under vuum (24,25) or n e exposed to lser (26-28). Although suh tretments do give rise to enhned rtes of eletron trnsfer, s well s reproduile surfes, they re not prtil for routine use. The tivtion for these pretretments ws ttriuted to the removl of dsored speies from the eletrode surfe (29,3). The development of pretretment methods hs een ompnied y the hrteriztion of eletrode surfes efore nd fter suh pretretments in order to eluidte the hnges in the surfe tht led to tivtion (4-7,17,18,24-26,29-34). However, for most pretretments, it is not possile to identify unmiguously ny one hnge in the surfe tht n e orrelted with the tivtion (for exmple, the tivtion in some instnes my e due simply to the removl of dsored speies from tive sites, wheres in other instnes the hnges in the surfe funtionl groups my lso e importnt). The effetiveness of given pretretment lso depends on the nlyte under investigtion. Therefore, the optiml pretretment for given pplition n only e found experimentlly.

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