Hydrogen peroxide-induced cathodic terbium(III) electroluminescence at a terbium-doped oxide-covered zirconium electrode

Analynca Chumca Acta, 266 W92) 51-66 Elsewer Science Pubhshers B V , Amsterdam

51

Hydrogen peroxide-induced cathodic terbium(II1) electroluminescence at a terbium-doped oxide-covered zirconium electrode K. Lplamen, S Kuhnala and K Haapakka Departmentof Chemmy, Umversztyof Turku, SF-20500 Turku

(Fuhnd)

(Recened 4th March 1992)

Abstract It has been found that the conventional low-voltage d c anodization of a zlrcomum electrode m an aqueous terbrum(IIIj-contammg sodmm acetate solution fumlsbes an electrode surface mth a thm onde layer which may contam terbmm at levels up to around 35 at % at the surface The cathodic polanxatlon of the mterface between the terbium-doped oxide layer and the aqueous hydrogen peroxlde-contammg electrolyte excites the oxrde-bound terbmm(III) to its lowest smglet state and thus 1s able to generate a terbmm(III)-charactenstlc electrolummescence This paper treats m detad the preparation and charactenzahon of highly e1ectrolummescent terbium-doped omde layers at a zIlw)nmm electrode surface and demonstrates that the cathodic terbmm(II1) electrohmunescence can be used to detect hydrogen peromde with Werent kmds of electrolummescence cells, e g , rotatmg workmg electrode cell, flow cell and static rmcrocell On the basu of the results from the expernnents anth the well known /3-D-glucose-glucose oxldase-oxygen system as a model system for enzymatIcally hberated hydrogen peroxtde, the use of this terbium-doped onde-covered zmzonmm electrode ISproposed as a versatde sensor for clmlcal analysis through Its response to hydrogen peromde Keywordr Chemdummescence, Electrolummescence, Glucose, Terbmm, Zmzomum electrode

In an oxldase-catalysed reaction, a clmlcal substrate IS oxldlzed and simultaneously oxygen 1s reduced to hydrogen peroxide substrate + 0, + H,O + product + H,O,

(1) Under appropriate condltlons, these oxldase reactions can be momtored electrochenucally on one of the followmg bases 0, + 2H,O + 4 e-+

40H-

(2a)

H,O,

+ 0, + 2H++ 2 e-

(2b)

H,O,

+ 2 e-+

@)

20H-

Comspondence to K Haapakka, Department of Chenustry, Umverslty of Turku, SF-20500 Turku (Finland)

1 e , detectmg the decrease m oxygen concentration by a four-electron reduction or detectmg the decrease m hydrogen peromde concentration elther by a two-electron oxldatlon or by a two-electron reduction Numerous studies have mdlcated that m these mstances, the electrochermcal response obtamed 1s also related to the substrate concentration m the sample solution On this basis, slgnlficant research and development have been devoted to the preparation of mstrumentally snnple, rapldly respondmg potentlometrrc and amperometnc detectors for the determmatlon of various substrates of clmxal Importance, where the appropnate oxldase has been nnmoblhzed either hrectly on the workmg electrode surface or on a separate oxldase reactor located

52

K L~tauzen et al /Anal

close to the workmg electrode Practical oxldase devices have been constructed m recent years for numerous chmcal analytes, e g , acetylcholme [1,2], cholesterol [3], cholme [1,2,41, P-D-glucose [5-121, glutamate [13-151, lactate [3,16-191, lactose [20] and oxalate [21] However, considerably less attention has been focused on the development of optical detectors for the determmatlon of these substrates, some chermlummescence detectors based on the well known hydrogen peroxldeinduced lummol and peroxyoxalate chemllumlnescent systems have been reported recently [22-

WI Hydrogen peroxide 1s reduced m two sequential steps as follows H,O, + e-+

OH + OH-

(3a)

OH+e-*OH-

(3b) where the one-electron mtermedlate, the hydroxyl radcal, 1s a powerful oxldmng agent (the standard electrode potential is ca 2 7 V vs Ag/AgCl [26]) Assummg that these electron transfer steps are comparatively sluggish and that the sample solution contams a chemical moiety capable of readily mteractmg with the highly energetic hydroxyl radical, this interaction may mltlate a reaction sequence which finally produces vlslble light enusslon, 1 e , chermlummescence (CL) rf hydrogen peroxide 1s reduced by chemical means, or electrogenerated lurmnescence (EL) if this reduction 1s carried out electrochenucally Previous studies have mdlcated that at an oxidecovered ahummum electrode m an aqueous electrolyte, the electrochermcally generated hydroxyl radical mteracts mth a hydrated terbmm(II1) cation adsorbed on the oxide-covered surface of the ahunmrum electrode, which results m a charactenstlc terbmm(II1) ermsaon [271 The followmg reaction scheme was proposed for the generation of this cathodic terbmm(II1) EL kAlOH+e-+H+

i-AlO-

(4a)

H,O,+H-+OH+OH-

(4b)

i- AlOTb’++

(4c)

OH + I- A101-b3+

I- A10Tb3++ e-* I- AloTb2+*

I- A10Tb2+*

-+ I- AloTb2++

hv

(4d) (4e)

Chun Acta 264 (1992) 51-66

Briefly, the reduction of an uncharged surface hydroxyl group of the oxide-covered alummmm electrode generates a hydrogen atom which, m turn, reduces hydrogen peroxide to the hydroxyl radical, this radical oxldlzes the adsorbed terbmm(II1) to adsorbed terbmm(IV), which 1s nnmediately reduced to adsorbed terbmm(II1) m Its lowest excited state and, Fmally, the deactivation of this excited state mltlates the terbmm(III) enusslon At a rotatmg oxide-covered alummmm electrode, this cathodic EL responded linearly to hydrogen peroxide and hydrated terbmm(III) concentrations down to ca 10e6 and lo-’ M, respectively In the presence of trace amounts of hydrated terbmm(II1) Ions m aqueous electrolytes, the low-amplitude anodlzatlon of a tantalum electrode with a symmetrlcal double-step potential, 1 e , sequential posltlve and negative potential pulses with mternuttent zero potential pulses, produces a terbium-doped oxide layer on the electrode surface wth a terbium content of ca 4 at % at the electrode/electrolyte mterface [28] The pulsed cathodic polamatlon of this terbmmdoped electrode m aqueous hydrogen peroxldecontammg electrolytes generates the cathodic terbmm(II1) EL analogously to that presented by reactions 4a-e Using the rotating workmg electrode, the terbnnn(II1) EL response was found to be linear with respect to hydrogen peroxide concentration m the range 10-6-10-3 M This terbmm-doped electrolummescent tantalum oxide layer generated a farly reproducible EL response during prolonged use, unhke the analogously formed terbium-doped oxide layer at an alummmm electrode surface, which proved to be unstable because of the nnmedlate dlssolutlon of terbium from the oxide layer after the connection of the EL excitation potential This paper discusses the preparation, characterlzatlon and cathodic terbmm(II1) EL of a terbmm-doped oxide-covered zlrcomum electrode These studies indicated that the low-voltage d c anodlzatlon of a zirconium electrode m terbmm(III)-contammg aqueous electrolytes furnishes an electrode surface with a thm oxide layer which may contam terbium up to ca 40 at % at the surface In the presence of hydrogen perox-

53

K Llpuunenet al /AnaL Chun.Acta 266 (1992)51-66 I&, the cathodic polarization of this electrode induces an intense terbmm(II1) EL whch can be used for the rehable measurement of hydrogen peroxide m aqueous solutions vvlth prolonged use Mamly on the basis of prehmmary experunents using an EL flow cell supphed with a glucose omdase reactor and a static EL nucrocell mth glucose oxldase m situ, this paper demonstrates that this hydrogen peroxide-Induced terbmm(II1) EL can be utilized for the determmatlon of B-Dglucose m the physlologxal concentration range through the detection of hydrogen peroxide hberated by the P-D-glucose-oxygen-glucose oxldase system The use of the terbium-doped oxidecovered ztrconrum electrode as an EL sensor for chmcal analytes on the basis of enzymatically liberated hydrogen peronde is proposed

EL Intenstty/au

-8

-6 -4 -2 log c(Terbum(ll0)

0

Fig 1 Effect of the terbmm(II1) concentration m the dopmg electrolyte on the cathodic terbmm(III) EL For anodlzatlon conditions, see text EL condltlons as m Fig 8

EXPERIMENTAL

Preparation of terbuma-doped ox&-covered zzrconturn electrodes Zlrcomum disc electrodes of 6 2- and 12 7-mm diameter were prepared by embeddmg z~rcoruum rods (Alfa, 99 99%) m FTFE and pohshmg the end, first mechamcally Hnth l-pm duunond paste and then chermcally for 10 s m concentrated mtrx acid-hydrofluonc acid-water (4 5 + 1 + 4 5) at 25°C Fmally, the electrodes were carefully rmsed m quartz-dlstdled water 111an ultrasomc bath The freshly polished zlrcomum electrodes were anodized m a vigorously stirred aqueous 0 10 M sodrum acetate (pH 7) which contamed an appropnate concentration of terbmm(II1) as acetate The d c anodlzatlon was conducted first galvanostatxally at 1 mA cmm2 to an appropnate end potential to estabhsh the oxide layer thickness and then potentlostatically until the current reached ca 10 FA cmv2, the anodrzatlon was generally completed m 2 h The optnnum dopmg pH and terbmm(II1) concentration of the electrolyte were evaluated mamly by utlhzmg terbmm(II1) EL measurements on the followmg basis a terbium-doped oxide layer was formed on the zlrcomum electrode surface under the appropnate condltlons and the mtenslty of the hydro-

gen peroxide-mduced cathodic terbmm(II1) EL was measured under the condltlons stated m Fig 8 It was assumed that the terbuun(III) EL mtenslty was related to the amount of terbium mcorporated m the oxide layer of the electrode On this basis, the optunum pH was found to be ca 7, m more acldlc electrolytes, the posltlvely charged electrode surface (see Fig 10) efficiently prevents the adsorptlon of terbmm(II1) cations on the electrode surface and, hence, the anodlzatlon-mduced mcorporatlon of terblum(II1) catlon mto the oxide layer becomes less probable, m more alkaline electrolytes, terbmm(II1) hydroxide begms to precipitate Figure 1 displays the cathodic terbuun(II1) EL mtenslty measured at zlrcoruum electrodes anodlzed with the optnnum potential of 8 4 V d c (a detailed treatment of the optunum anodtzatlon potential is presented below) m 0 10 M sodium acetate contammg vaflous concentrations of terbmm(III), on this basis, the optunum terbuun(II1) doping concentration seems to be ca 10e2 M, which was used throughout the work EL mstrumentatwn EL spectra were recorded on a photon counter constructed m our laboratory [29], where the workmg electrode was a 6 2-mm dmmeter ter-

54

bmm-doped oxide-covered zu-comum electrode, thts EL cell was also used to evaluate the effects of different experimental vanables on the cathodic terbmm(II0 EL intensity The analytical EL measurements were mainly done usmg the modlfied versions of the laboratory-made EL flow cell [30] and static EL cell [31] A schematic diagram of the prmclpal components of the EL flow cell system was presented prewously [30], Fig 2 shows the modified EL flow cell The workmg electrode was either a terbium-doped oxide-covered zlrconrum electrode (diameter 6 2 mm, prepared as described above, oxide layer thickness 215 nm, surfacebound terbium ca 35 at % at the surface), the optical filter was a 545-mn band-pass filter with a ca lo-nm band width and a 7-~1 cell volume correspondmg to a 0 15-mm thick 3 x 16 mm PTFE spacer frame was found to be most appropriate for the detection of hydrogen peroxide m mjected samples An unmoblllzed glucose oxldase reactor was fitted m the detector inlet ca 2 cm from the EL cell Stamless-steel tubmg [lo0 mm long x 6 5 mm thick (4 0 mm 1 d 11 was used to construct a reactor accordmg to Masoom and Townshend [32] Briefly, a 10-g amount of controlled-pore glass (CPG-240,120-200 mesh, mean pore diameter, 24 2 nm surface area 79 m2 g-‘, Sigma) was boded 1115% nitric acid, washed with quartz-dlstdled water and dried at 95°C The glass beads were alkylammated m an aqueous 10% 3-ammopropyltnethoxysdane (Pierce) solution adjusted to pH 3 5 A 1 O-g amount of alkylammo glass was cross-lmked m aqueous 0 10 M

Fig 2 EL flow cell (A) terbmm-doped oxide-covered wcomum workmg electrode, (B) platmum counter electrode, (C) interference filter, (D) PTFE spacer frame, (E) glucose OXIdase reactor See text for details

K Lpatnen

et al /Ad

Chun Acta 266 (1992) 51-66

I

PM

Fig 3 Static EL microcell (A) terbium-doped oxide-covered zirconium workmg electrode, (B) IT0 counter electrode, (C) interference filter, (D) FTFB spacer frame, (E) contact for the IT0 counter electrode

phosphate buffer (pH 7 0) containing 2 5% glutaraldehyde (Aldrich) and the activated glass was carefully washed with quartz-dlstdled water, 6 0 mg of glucose oxldase (from Aspetgdlus nrger, 1000 U ml-‘, Sigma) dissolved m 3 0 ml of 0 10 M phosphate buffer (pH 6 0) were added to the activated glass and the umnoblhzed enzyme derivative was washed with quartz-dlstdled water Fmally, the cohunn was packed with the unrnoblhzed glucose oxldase derivative A modified static EL mlcrocell 1s presented m Fig 3, a schematic diagram of the prmclpal components of this EL system was given m a previous paper [31] The workmg electrode was a terbnundoped oxide-covered zmxxmun electrode (duuneter 6 2 mm or 12 7 mm, prepared as described above, oxde layer thickness 215 nm, surfacebound terbium ca 35 at % at the surface) embed-

R Ltpmnen et al /Anal Chum Acta 266 (1992) 51-64

ded m PTFE and framed with an adhesive PTFE spacer The volume of the resultmg rmcrocell could be controlled by the sue of the workmg electrode and by the tluckness of the PTFE spacer frame, a 30-~1 volume, correspondmg to a 12 7mm diameter workmg electrode and a 0 25-mm frame thickness, proved to be feasible m this work The counter electrode was a 30 x 30 mm piece of mdmm tm oxl
55

ter (Perkm-Elmer) All these expernnents were done usmg freshly anodized oxide layers The EL measurements at the rotatmg electrode cell were conducted as follows 50 ml of sample solution contammg the appropnate amount of sodmm acetate was adjusted m the EL cell to the desired pH and the solution was deaerated wth nitrogen for 15 mm, an appropnate amount of hydrogen peromde was added to the cell and the sample solution was further deaerated for 2 mm Electrode rotation at a rate of 20 cps was started and the excltatlon potential (see Fig 8A) was apphed to the cell, the cathodic current density at this cell was found to be ca 45 mA cm-* The cathodrc terbmm(II1) EL was reglstered on the photon counter, the EL mtenslty was mtegrated for 2 nun over the whole cathodic potential pulse and 1s reported m arbltrary units The measurements at the EL flow cell were as follows a contmuous flow of aqueous oxygensaturated 0 10 M sodium acetate (pH 5 1, where 111the presence of oxygen 1 umt of glucose 0x1dase converts 10 pmol of @D-glucose to D-gluconic acid and hydrogen peroxide) at a rate of 3 ml nun-’ was provided by the Altex pump The excitation potential (see Fig 8A), which generated a cathodtc current density of ca 2 mA cm-*, was applied to the cell, a 20-~1 sample contammg the appropriate amount of glucose diluted m aqueous oxygen-saturated 0 10 M sodmm acetate (pH 5 1) was inJected into the carrier solution and the resulting terbmm(II1) EL was recorded The EL signal, 1 e , the average light output durmg the cathodic potential pulse, was measured as the peak height and 1s reported m arbitrary umts In the static 30-~1 EL nucrocell, the method of measurement was as follows a 15-~1 ahquot of aqueous oxygen-saturated 0 20 M sodnun acetate (pH 5 1) contammg appropriate amounts of B-Dglucose and a 15-~1 ahquot of glucose oxldase 0052 U ml-‘) saturated mth oxygen were plpetted mto the electrode cavity and the IT0 counter electrode was mstalled The murture was allowed to mcubate for 2 mm then the excltatlon potential (see Fig 8A), which generated a cathodic current density of ca 25 mA cm-*, was apphed to the cell The resultmg cathodic terbmm(II1)

56

K Lqxamen et al /Am!

emlsslon was reastered on the photon counter and the reported mtenslty values are the average numbers of emitted photons durmg 1000 sequential 2 0-ms cathodic excitation pulses All the experunental work was performed at ambient temperature

RESULTS AND DISCUSSION

Charactentatwn of the terbzum-doped oxzdecovered zzrconzumelectrode An oxide layer at the zlrcomum electrode surface can be considered as the dlelectrlc of a parallel-plate capaator formed between the underlymg metal and the oxlde/solutlon Interface and, consequently, the followmg expression can be written for the oxide layer capacitance CC> [331 l/C = l/C,

+ l/C,

= d/q,er

= (d, + dl)/e,,er

(9

where C, 1s the oxide layer capacitance, C, 1s the Helmholtz layer capacitance, l0 1s the penttlvlty of free space, E 1s the dlelectnc constant of zlrcomum oxide, r IS the roughness factor of the electrode surface (for sunphclty assumed to be 1 m this work), d IS the total zlrcomum oxide thickness, d, IS the mltlal zlrconuun oxide thlckness and d, IS the thickness of zlrcomum oxide produced on the electrode surface by the anodlc polarlzatlon at the formation potential E, The Hehnholtz layer capacitance C, usually exceeds 20 FF cmV2 at oxide-covered metal electrodes and the term l/C, can be neglected m Eqn 5, hence the capacitance measurement of the zlrcomum electrode provides a sunple means of determmmg its oxide layer thickness In many cases concerning the characterlzatron of oxide-covered electrode/electrolyte interfaces, the Mott-Schottky equation [34] l/C&

= (141 x 1020/+)(

-4

- 0 0257)

(6)

where - 4 = E - &,, C,, 1s the space charge capacitance, E 1s the dlelectrlc constant of the oxide layer, ZVD1s the donor density and Eti IS the flat-band potential, has proved to be a useful

Chm Acta 264 (1992) 51-64

tool The slope of a plot of l/C& vs E IS proportional to the dopant level m the semconductmg oxide layer, and the intercept with the abscissa at l/C& = 0 yields the flat-band potent1al The surface of a zirconium metal electrode is not stable m aqueous electrolytes but IS always covered Hrlth an oxide layer havmg a thickness dependent to some extent on the formation conditions, e g , temperature, surface pretreatment and electrolyte, ml&-d oxide layer thicknesses ranging from 5 to 7 nm have generally been reported [33,35,36] The conventional anodlc d c polarlzatlon of this oxide-covered electrode surface can be used to thicken the oxtde layer and, accordmg to the literature, the polarmtlon-mduced change m the oxide layer thickness should be linearly dependent on the anodrc polarization potential, dependmg on the formation conditions, oxide layer growth rates m the range 16-3 0 mn V-’ have generally been reported [33,37-391 In this work, after chermcal pohshmg the zlrcomum electrode was mnnedlately anodized m aqueous 0 10 M sodium acetate (pH 7) m the absence and presence of 0 01 M terbmm(II1) acetate, fast galvanostatically at 1 mA cm-* to the required potential to establish the appropnate oxide layer thickness and then potentlostatltally until the anodlc current had reached ca 10 /LA cm-*, the anodlzatlon was generally completed m 2 h and furnished an electrode surface with an n-type oxide layer On the basis of capaatance measurements on the freshly formed oxide layers and assummg that the dielectric constant of undoped and terbium-doped zrrcomum oxide layers are equal, 1e , 22 obtamed m aqueous ammomum tartrate [40] (the literature reports values rangmg from 20 to 35 highly dependent on the anodlzatlon condltlons [33,39,41-4711, the calculated oxide layer thicknesses of the undoped and terbium-doped oxides at the zlrcomum electrode surfaces are presented 111Fig 4 as a function of the formation potential E, under the stated condltlons As expected at low formation potentials, the d vs E, plots are rectrlmear, with the oxide-covered zlrcomum electrode, the mltlal oxide layer thickness 1s ca 11 nm and the oxide growth rate ca 14 nm V-‘, whale these parame-

K Lt.ptawm et al /Anal

so3xide

0

57

Chm Acta 266 (1992) 51-66

Layer Thckness/nm

5

10

Anodlzatlon

&2/uFD2cm4

15

25

20

-197

7

Potential/V

.

PotentlaIN

I%g 4 Effect of the formation potentml on the oxide layer thickness at the zlrcomum electrode surface + = oxidecovered zxconnun electrode, * = terbmmdoped oxidecovered zircomum electrode For anodlzatlon combtlons, see text

F@ 5 Mott-Schottky plots of (+) the oxide-covered zlrcomum electrode and (*) the terbium-doped oxide-covered zlrconmm electrode Anodlzatlon conditions formation potential 84 V, 0 10 M soduun acetate, pH 7 0, 001 M tertuum(II1) acetate

ters for the terbium-doped oxide-covered zncomum electrode were found to be 10 nm and 14 IUIl v-1, respectively These values dtier to some extent from the aforementioned literature values

but, takmg mto account the uncertamty m the selected value of the dlelectnc constant (see above), this discrepancy can be still regarded as reasonable

,oo;~

loo Atomic % +-

0

20

40

60

00

Sputtermg Time/mm

100

-0

20

40

Sputtering

60

00

100

Time/min

Fig 6 Auger depth profilmg of the anodized zlrcomum electrodes (A) oxide-covered zmxxuum electrode, (B) terbmm-doped oxide-covered ixcomum electrode Anodization condltlons formation potentml 8 4 V, 0 10 M sodmm acetate, pH 7 0, 0 01 M terbmm(III) acetate

58

Figure 5 presents the lmear parts of the Mott-Schottky plots measured for the 215-nm thrck oxide and terbium-doped oxide layers at the zmxxmun electrode surface On the basis of the extrapolation of these plots to l/C& = 0, the flat-band potential of both the electrodes 1s ca -2 06 V vs Ag/AgCl, but m both mstances it was found to be frequency dependent which makes the flat band potential values obtained unrehable, accordmg to the literature, the flatband potential of the oxide-covered zlrcomum electrode at pH 7 should be ca - 15 V vs Ag/AgCl [33,46,47] In spite of these frequency dependences, the correct dopant levels can still be deduced from the slopes of the l/C& vs E plots and, assummg the dlelectrlc contants of z11%omum oxide and terbmm--doped zmxnmm oxide to be 22 as above, the slopes gwe frequency-mdependent dopant levels of 9 8 X 101’ and 1 1 x 101* cmm3 for the oxide-covered and terbium-doped oxide-covered electrodes, respectively which can be regarded as charactenstlc for the oxide-covered semiconductor electrode These dopant levels are lower than that obtamed by MelsterJahn et al [33], I e , ca lo”, but takmg mto account the high senatWy of the dopant level towards the anodlzatlon condltlons (I e , surface pretreatment, electrolyte, unpuntles m the electrolyte, temperature, etc 1, this discrepancy 1s reasonable Figure 6 displays Auger profiles from the surfaces of the electrodes anodized at E, = 8 4 V m 0 10 M sodmm acetate (pH 7) m the presence of 0 01 M terbmm(II1) acetate and, for companson, m the absence of terbnun(II1) acetate, addltlonally, Fig 7 shows the Auger profiles from the terbium-doped oxide-covered zucomum electrode anodized at a formatlon potential of 17 8 V under the same condltlons Although this terbmm-doping method 1s not very reproduable, these experiments make it possible to conclude that terbium 1s not homogeneously dlstnbuted m the anodlcally formed oxide layer but IS strongly accumulated at the electrolyte/oxide layer mterface, the terbium content at the surface seems to be mdependent of the formation potential, at least m the range used m this work, I e , the 4 9, 8 4 and 17 8 V anodlzatlons produced oxide lay-

K Lqmwen et a~!/Ad

1001

Chm Acta 266 (1992) 51-64

Atomic %

-i 0

20

40

60

00

100

Sputtermg Time/mm Fig 7 Auger depth profihng of the terbmm-doped mdecovered zrcomum electrode An&bon condlbons as m Fu 5 except formation potentud 17 8 V

ers with terbium contents of ca 30, 35 and 20 at %, respectrvely, and, further, terbmm 1s capable of bemg mcorporated mto the formmg oxide layer m an appreciable amount only to a depth of ca 10 nm (I e , the sputtermg tune m the Auger profiles 1s ca 10 mm) from the electrolyte/omde layer interface At electrodes furmshed vvlth thick oxide layers, the terbium-free oxide layer zone becomes much mder, as can be concluded from the results m Figs 6A and 7 (e g ,111 this mstance ca 12 nm), which may be the reason for the thxkness-Induced quenchmg of terbmm011) EL shown m Fig 13 Hydrogen peroxuie-wuiuced cathodz EL at the terbuun-doped o&e-covered zwconzum electrode As shown m Figs 8 and 9, the terbium-doped, 215mu thxk oxide layer at the zlrcomum electrode surface 1s cathodically conductive when polarlzed with the pulse potentml presented m Fig 8A, the breakdown amphtude 1s ca 2 V and the cathodtc current dens@ mcreases linearly Hrlth the pulse amphtude and reaches around 75 mA cme2 at a 10-V pulse amphtude The cathodic current mduces mtense EL Hnth the spectrum shown m Fig 10 but only when the aqueous

R Lqnamm

et d/Anal

Chm

Acta 266 (1992) 51-66

59

EL Intensity/au

l-l

_/A.,

-8 V

,

400

I

500

800

700

Wavelength/nm Ftg 10 Spectrum of the cathodrc electrolummescence at the terbmm-doped onde-covered xnconmm electrode Condlttons as m Ftg 8

Ftg 8 (A) Excrtatton potenttal and (B) excrtatton current Gmdrtrons 0 10 M sodnun actetate, pH 6 0, 1 OX 10m3 M hydrogen peroxtde, electrode rotatton rate 20 s-l, solutton deaerated wrth mtrogen

electrolyte also contams addmonally hydrogen peroxrde, a cathodic current density of ca 50 mA cm-’ 1s required for the maxmuun EL output at the rotating workmg electrode EL cell under these cncumstances This cathodic EL consists of four sharp peak ermssrons at 488, 546, 585 and

80.

JC

EL Intensity/al

,,JmAcm-*

80

624 nm, whrch can be assigned to the terbmm(111) tranatrons from the excited smglet state ‘Da to the ground states 7F6JA3, respectively [48] This cathodtc EL 1s subsequently called the hydrogen peroxrde-mduced terbmm(II1) EL For comparison,, Fig 11 displays the spectrum of the cathodic EL obtained at the zucomum electrode furnished with an undoped, 215mn thick oxrde layer, the spectrum was measured using the 50 mA cm-* cathodic current density under the condmons stated for Fig 10 The assrgnment of this cathodrc EL 1s not as unambrguous as above but, taking rnto account that the band-gap energy of z~rconuun oxrde 1s ca 5 eV [49] and corresponds to the band-gap enussion at the peak wavelength of ca 250 nm, the observed 510~nm EL can be regarded as a low-energy sub-band-gap EL of the electrode and is obvr-

EL Intenstty/au

0

I

0

2

4

8

8

10

Pulse AmphtudeN Ftg 9 Effect of the pulse ampbtude of the excttatton potenhal on (+) the cathodrc current densrty and (*I the cathodrc terbmm(II1) EL Ccmdrttonsas m Frg 8

400

500

800

700

Wavelength/nm Ftg 11 Spectrum of the cathodic electrolummescence at the oxrde-covered xnconmm electrode Condrtrons as m Fig 8

60

K. Lqnamen et al /Anal

ously caused by flaws and/or lmpurlttes m the electrode oxide layer This weak cathodic subband-gap EL depends on the excltatlon condotions, I e , excitation potential, electrolyte, etc , but does not seem to depend on the presence of hydrogen peroxide m the sample solution At the terbmm-doped oxide-covered wrcomum electrode, this 510-nm sub-band-gap EL is generated smultaneously unth the hydrogen peromdemduced terbmm(II1) EL and hence provides the cathodic background EL Although Its mtenslty is low, this sub-band-gap EL must be efficiently separated from the hydrogen peroxide-induced terbmm(II1) EL m order to a&eve a suitable signal-to-noise ratio for the EL detectlon of hydrogen peromde In the present work, the lo-mn mterference filter selected for the 546-nm terbmm(II1) emlsslon hne was found to be appropnate for thts purpose and, as shown m Fig 12, under the optlmlzed condltlons the cathodic terbmm(II1) EL at the rotatmg workmg electrode responds linearly to hydrogen peroxide concentration over the range 10-6-10-3 M

)g EL Intensity

-6

-5 log

-4

-3

-2

c(Hydrogen PeroxIde)

Fig 12 Effect of the hydrogen peroxide concentration on the cathodic terbmm(III) EL at the rotating workmg electrode cell Condltlons as m Fig 8

Chm Acta 264 (1992) 51-46

EL Intensttyiau

16

24

30

is”

Oxrde Layer Thckness/nm Fig 13 Effect of the oxide layer thickness of the zuxxmlum electrode on (*) the cathodic terbmm(II1) EL and (+) the cathodic current density Condltlons as m Fig 8

Effect of other expenmental vanables on the hydrogen perox&-mduced terbtum(III) EL The results m Fig 13 demonstrate clearly that the oxide layer at the zlrcomum electrode surface 1s cathodlcally more conductive the thicker IS the oxide layer, at least m the thickness range used m this work However, this thickness-mduced mcreasing cathodic current dens@ does not result m a linearly mcreasmg terbmm(II1) EL intensity, instead, strong EL quenchmg is observed for oxide layer thicknesses 2 215 nm, so that virtually no terbmm(II1) EL could be observed with oxide layers thicker than ca 40 nm This terbnun(II1) EL quenchmg seems to be connected with the wldenmg of the terbium-free oxide layer zone as a function of increasing oxide layer thickness (see the results m Frgs 6A and 71, which suggests that the oxide-bound terbium may have an electrolummescently essential role m the electron transfer across the oxide layer at the zlrcomum electrode surface Mainly because of the most mtense terbmm(II1) EL response, further experiments were conducted at an electrode with a 215-nm thtck oxide layer As m numerous cathodically induced electrolummescent systems at oxide-covered alummnun

61

K Lpmnen et al /AnaL Chm Acta 266 (1992) 51-46

and tantalum electrodes [27,28,50-531, a short anodlc pulse m the excltatlon potential (see Fig SA) was found to be of crucial importance also for the generation of an mtense and reproducible cathodic terbmm(II1) EL at the terbium-doped oade-covered zlrcomum electrode Three alternatrves can be proposed for the role of this anodlc mternuttent polanzatlon first, the resulting small anodlc current repairs the cathodically mduced damage m the oxide layer and thus mamtams a constant oxide layer thickness at the electrode surface, which seems to be essential for a reproducible EL response, second, it generates intermediate oxide-bound terbmm(II1) by oxldatlon of the oxide-bound terbmm(III), finally, it keeps the terbium-contammg zone at the certain distance from the electrode/electrolyte mterface necessary to generate the mtense EL The present expemnental results do not allow any detailed comparison between these alternatives, but studies to shed light on this uncertainty are m progress The effect of pH on the mtenslty of the cathodic terbmm(II1) EL is shown m Fig 14 The results demonstrate that an mtense terbmm(II1) EL can be obtamed m the pH range 3-10 with the maxnnum response close to pH 6 According EL Intenstty/au

3

6

5

6

7

6

PH Fig 14 Effect of pH on the cathodic terbmm(III) EL Condotlons as m F@ 8

11

PH species

on the

to our studies, this pH dependence of terbmm(III) cannot be explained by any pH-mduced changes m the current densities, 1 e , the anodlc and cathodic current denntles were found to be mdependent of pH m thrs range, but rather by an effect of pH on the hydroxylated surface of the oxide-covered zlrconmm electrode The hydroxylated z1rcomum oxide surface can be regarded as a dlprotlc acid +I-ZrOH+H+

I-ZrOH+kZrO-+H+

4

9

FIN 15 Dlstnbufion of protolyt~c surface onde-covered zmzomum electrode

I- ZrOH;

3

7

(7)

(8) where l- designates a surface species For powdered zlrconmm oxide, the surface acidity constants are pKc = 5 7 and pK,*2 = 7 9 [54], and the calculated mole fractions of the dtierent hydroxyl groups at the zlrconmm electrode surface are presented m Fig 15 The slmllar profiles of the terbuunCII1) EL mtenslty vs pH plot and the mole fraction curve of uncharged surface species make possible to infer at least on a qualitative basis that the cathodic terbmm(II1) EL 1s generated at the electrode surface covered vvlth the uncharged surface species Previous studies wrth a variety of electrohunmescent systems [28,30,31, 50-531 indicate that uncharged surface hydroxyl

62

K Lgmnen et al /Ad

species seem to play an essential role m EL generation also at oxide-covered alummmm and tantalum electrodes This suggests a common mtermedlate for the cathodlcally induced EL at these oxide-covered valve metal electrodes, which may well be atormc hydrogen produced by a one-electron reduction of the uncharged surface species as shown by reaction 9 and 10 m the case of the oxide-covered zlrcomum electrode

--2

&

--1 -

0

-

1

H+e,+H+

H

OH +,02

-

OHYOH

(9) (10)

The acid dlssoclatlon constant and standard electrode potential of atomic hydrogen are pKaI = 9 6 [55] and 2 1 V [56], respectively Possibly the role of this powerful reductant m the terbmm(II1) EL reaction scheme 1s to generate the hydroxyl radlcal as presented below (reactIon 13) Pombie reactwn pathways for the cathodzc terbum (III) EL Supposing that terbium nmlally penetrates as terbmm(II1) mto the anodlcally formed oxide layer at the zlrcomum electrode surface, the energetic hydroxyl radical partrapates m the electrohunmescent pathway, the oxide-bound terbmm(III)-terbmm(IV) redox process 1s an essential process for the cathodic terbmm(II1) EL and finally the cathodic polarlzatlon of the electrode/ electrolyte does not essentmlly change the mltlal solution pH of 6 m the vlcmlty of the electrode surface m spite of the high cathodic current density, then the mechamsm of the hydrogen peroxide-induced cathodic terbmm(II1) EL (condltlons as stated for Fig 8) can be briefly dlscussed on a quahtatlve basis as follows Hydrogen peroxtde is dlssoclated in aqueous solutions H,O, +HOO-+H+

Eg=5 OeV

Tb~lll)/TbWl

I- ZrOH + e--+ I- ZrO-+

Chun. Acta 266 (1992) 51-66

(11)

with an acid dlssoclatlon constant pK,, = 117 [57] Two alternative ways to produce the highly oxldrzmg hydroxyl radical by the reduction of undlssoclated hydrogen peroxide can be proposed hydrogen peroxide takes the electron elther directly from the conductlon band of the oxide-covered zlrcomum electrode (reactIon 12)

Fig 16 Schematic energy-level dmgram of the mterface between the terbmm-doped oxide-covered zncomum electrode and the hydrogen peroxide-contammg electrolyte

or from the highly reducmg atormc hydrogen (reactlon 13) which 1s generated by the reduction of the uncharged surface hydroxyl group as presented m reactron 9 H,O,

+ e-+

H202+H+ HO --* 0-+

OH + OH-

(12)

OH+H,O

(13)

H+

(14) It IS tempting to propose that the latter altematlve apphes to the cathodic terbrum(II1) EL because It addltlonally gives a reasonable explanatlon of the expernnentally observed effect of pH on the mtenslty of this EL The acid dlssoclatlon constant of the hydroxyl radical is pK,, = 116 [551 and, consequently, the undlssoclated radtcal 1s the prevailing species under these condltlons and obviously participates m the light-emlttmg reaction sequence Figure 16 shows a schematic energy-level dlagram of the Interface between the terbmm(III)doped oxide-covered zirconnun electrode and the hydrogen peroxide-contammg electrolyte on the basis of the currently avallable data, I e , the band-gap energy of zlrcomum oxide 1s 5 0 eV [49], the flat-band potential of the oxide-covered electrode is - 1 4 V vs Ag/AgCl at pH 6 0 [33,46,47], the conduction band edge of the oxide-covered electrode 1s 0 25 eV above the Fenrzl level of the metal at the flat-band potential (zlrcomum oxide is a typlcal n-type senuconductmg oxide [33]), the conductlon band (CB) and valence band (VB)

K Lqmnen et aL/AnaL Chm Acta 266 (1992) 51-64

edges of the oxide-covered electrode are located at 19 V and - 3 1 V vs Ag/AgCl, respectively, the standard electrode potential of the oxidebound terbmm(III)-terbmmW) couple 1s ca 2 7 V vs Ag/AgCl [58] and the standard electrode potentials of the two-step reduction of hydrogen peroxide are 0 4 and 2 7 V vs Ag/AgCl [26] Hence, the energy levels of the valence band of the oxide-covered electrode and the oxide layerincorporated terbmm(III)-terbmm(IV) redox couple are located close to each other, which makes it possible that the hydroxyl radical can generate the terbmm(II1) EL by two different routes m the first alternatnre, the hydroxyl radlcal captures an electron from the oxide-bound terbnnn(II1) and the resultmg oxide-bound terbmm(IV) 1s mnnedlately reduced to the oxidebound terbmm(II1) m its excited ‘D4 level by takmg the electron from the valence band of the oxide-covered electrode, m the second altematlve, the hydroxyl radical takes an electron from the valence band of the oxide-covered zlrconmm electrode followed by an nnmedlate electron transfer from the oxide-bound terblum(II1) to this valence band and the resultmg oxide-bound terbnun(IV) captures an electron from the conduc-

710g EL lntenslty I 6-

5-

4-

3-

2’

t

’

-6

’

I

-5

’

’

-4

log c(Hydrogen

I

’

-3

I

’

-2

8

1

PeroxIde)

F@ 17 Callbratioon graph for hydrogenperomde at the static EL mmocell For conddlons, see text

63

tlon band of the onde-covered electrode which then produces the oxide-bound terbmm(II1) m its excited 5D4 level The relaxation of this excited singlet state of the oxide-bound terbnun(II1) to the ground-states ‘F,_, of the oMde-bound terbmm(II1) mduces the charactenstlc terbmm(II1) peak enusslons shown m Fig 6 Present knowledge does not allow any profound mechamstlc discussion and the aforementioned models treat the cathodic terbmm(II1) EL only on a prehmmary basis, however, they give reasonable explanations for the expemnentally observed effects of pH and hydrogen peroxide on this EL Substantial further work 1s m progress to elucidate the mtrlgumg characterlstlcs of the cathodic terbmm(II1) EL and especially to shed light on cathodlcally mduced energetic mtermedlates and on their roles m the electrolummescent processes The results from these expenments wdl be presented later Analyttcal feaszbtity of the hydrogen peroxzdemduced terbium (III) EL

Prehmmary expenments to test the feaslblhty of the terblum(III)_doped oxide-covered zlrcomum electrode as an EL sensor for hydrogen peroxide detectlon were carried out usmg three kmds of EL cells, wz , rotatmg workmg electrode EL cell, EL flow cell and static EL mlcrocell, m each cell, the workmg electrode surface was furnished with a 215mn thick oxide layer contammg ca 35 at % terbmm(II1) at the surface The cahbratlon graphs for the rotatmg workmg electrode EL cell (Fig 12) and at the static EL rmcrocell (Fig 17) were determmed m 0 10 M sodium acetate (pH 6 0) usmg the optnmzed excltatlon potential shown m Fig 8A On this basis, these EL cells respond lmearly to hydrogen peroxide concentration over three order of magmtude with the detectlon lout well below 10m6 M The detectlon lumt at the EL flow cell was found to be a decade higher, which was a consequence of the lowermg of the cathodic current density to ca 2 mA cm-* (accordmg to the results m Fig 9, the optimum cathodic current density 1s ca 45 mA cm-*) which had to be done to avold a decrease m reproduclblllty durmg long-term use owing to the high cathodic current-mduced gas

K Ltpuunen et al /Ad

64

formatlon m the cell compartment The electrolummescent terbmm(III)-doped oxide layer at the electrode surface proved to be stable m long-term use, 1 e , neither essential changes 111the &h&c current density nor promment damage to the oxide layer were observed, and also no detectable amount of terbrum(II1) was leached from the electrode surface durmg prolonged polamatlon Hnth the appropriate excitation potential or when the electrode was soaked overnight m aqueous alkaline electrolytes contammg a terbnun(III)-brndmg chelatmg agent (EDTA) Finally, no essential dtit m the terbmm011) EL response was observed with the aforementioned EL cells Encouraged by these prormsmg results, the posslblhty of momtormg hydrogen peroxldeliberating enzyme reactions with the terblum(III)-doped oxide-covered zirconium electrode was studied The well known glucose oxldase (GOD)-catalysed oxtdatlon of P-D-glucose m the presence of oxygen

3log EL Intensity

2-

l-

0’

-5

I

-45

-4

-35 log

-3

-25

-2

-15

-1

c(Glucose)

Fig 18 Cahbratlon graph for glucose at the static EL msrocell For conditions, see text

the concentration range usually encountered conventional clinical analysis

P-D-glucose + H,O + 0, i= D-glucomc acid + H,O,

Chum Acta 264 (1992) 51-66

(15)

was selected for the model system These expenments were carried out using the EL flow cell equipped vvlth the lmmoblllzed GOD reactor and the static EL microcell according to the procedures described m detail under Experimental, analogous expernnents with the rotatmg workmg electrode EL cell were not carried out because of the inherent mconvemence of using the rotating electrode m routme analysis Figure 18 shows the cahbratlon graph for glucose determmatlon mth the static EL microcell, the detection limit for glucose 1s 3 x lo-’ M and the log-log plot shows a linear range of ca three decades with a slope of 0 8 In the EL flow cell, glucose determination proved to be possible for concentrations from 1 x 1O-4 to 1 x lo-* M Takmg mto account that these EL cells, especially the static EL microcell, are mstrumentally and operationally sunple, it can be concluded from the aforementioned results that the terbmm(III)_doped oxide-covered zirconium electrode 1s feasible as a basis for the construction of versatile EL sensors for glucose determmatton m

m

Conclusions It has been found that a heavdy terbium-doped oxide layer at the zlrcomum electrode surface 1s cathodically electrolummescent m aqueous electrolytes The electroluminescent performance of the terbium-doped oxide-covered zlrcomum electrode m aqueous hydrogen peroxide-contammg electrolytes has been established, the reduction of hydrogen peroxide mlttates the process which generates the terbmm(II1) EL with the well known emitting transitions ‘D4 + 7F6,5,4,3 This cathodic terblum(III) EL can be regarded as a potential alternative for hydrogen peroxide detection m aqueous solutions v&h dtierent kmds of cells on the followmg reasons the oxide layer at the zlrcomum electrode surface 1s stable even under extreme conditions, terbium 1s tightly mcorporated mto the oxide layer of the zlrcoruum electrode, the sub-band-gap background EL of the oxide-covered zu-comum electrode 1s of low intensity and the terbnun(II1) EL consist of relatively narrow emrsslon bands and its mam emrsslon peak at 546 nm can be easily dlscrlmmated against the background EL usmg the appropriate

K Lqmmen et aL/AnaL Chm Acta 266 (1992) 51-64

interference filter Especially with the static nucrocell, this terbmmUI1) EL system 1s of promrsmg analytical value m terms of mstrumental and procedure snnphclty Mamly on the basis of the results from the above expernnents vvlth glucose oxldase-hberated hydrogen peroxtde and from analogous expenments wth cholesterol-cholesterol oxldase and phenylalanme-ammo acid omdase. systems, the terbium-doped oxide-covered zmxxmun electrode 1s proposed as a versatile tool for momtormg enzymatrcally liberated hydrogen peroxide and thus for the determination of clmlcally nnportant compounds The authors thank Tama Leppamerm and Mmna Luoma for assrstance urlth the experunental work The assistance provided by M Hemonen with the measurement of the Auger spectra 1s gratefully acknowledged

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Chm Acta 246 (1992) 51-66

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