2007 - BITZIOU - Microjet Ring Electrode (MJRE)- Development- Modelling and Characterization

8/12/2019 2007 - BITZIOU - Microjet Ring Electrode (MJRE)- Development- Modelling and Characterization

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Microjet ring electrode (MJRE): Development, modellingand experimental characterisation

Eleni Bitziou, Nicola C. Rudd, Patrick R. Unwin *

Electrochemistry and Interfaces Group, Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK

Received 7 August 2006; received in revised form 6 January 2007; accepted 16 January 2007Available online 1 February 2007

Abstract

A novel type of hydrodynamic ultramicroelectrode (UME) is described, which employs a ring UME coupled to a high speed perpen-dicular impinging microjet. Two types of ring UME have been fabricated, both based on an optical fibre, coated in a thin metal film,which is then sealed using either epoxy resin or glass. After polishing, a thin ring UME ( 3001000 nm) is obtained. When employedin the impinging microjet system, both UMEs show an increase in mass-transport-limited current with flow rate, for simple redox pro-cesses such as the reduction of RuNH3

36 or methyl viologen dication in aqueous solution. However, the mass-transport rates observed

are significantly lower than predicted by solving the NavierStokes and diffusion equations for an idealised coplanar UME. Character-isation of the UMEs with microscopy reveals imperfections on a 10 nm1 lm length scale which impact mass-transport significantly.When these imperfections are included in the simulations, it is possible to account for the transport-limiting currents observed experi-mentally. A general implication of the studies in this paper is that even small perturbations in electrode structure can dramatically influ-ence high-speed convective flow across small-scale UMEs, such that thorough geometric characterisation of UMEs employed in fast-flowsystems is important.2007 Elsevier B.V. All rights reserved.

Keywords: Microjet electrode; Ultramicroelectrodes; Fluid dynamics modelling; Ring electrode

1. Introduction

Hydrodynamic electrodes use forced convection ofsolution to provide well-defined and reproducible mass-transport rates under steady-state conditions [1]. Thedeployment of ultramicroelectrodes (UMEs) in convectivesystems has been shown to greatly enhance mass-transport

rates, compared to UMEs in quiescent solution, leading toadvantages for kinetic and analytical studies [2].Two classes of hydrodynamic UMEs are generally avail-

able. First, those in which solution moves with respect to astationary electrode, such as the microjet electrode (MJE)(miniaturised wall-tube) [213], the high speed channelelectrode [1419], and the radial flow microring electrode

(RFMRE) [20,21]. In the second type, the electrode ismoved mechanically in the solution, such as with vibratingmicroband electrodes [22,23]. Modulated versions of theMJE[24] and RFMRE[25] employ both forced flow andmechanical movement of the electrode.

In the MJE, solution containing the electroactive speciesof interest is fired at high velocities through a fine capillary

nozzle (typically 25120 lm diameter) usually onto a disc-shaped UME (25lm diameter or less) [35,7,8,10]. Mer-cury hemisphere UMEs, which are formed by depositingmercury onto conventional disc UMEs have also beenintroduced in the MJE arrangement [6]. The well-definedand variable mass transfer rates obtained by using themicrojet configuration led to the construction of theRFMRE, where a ring UME was introduced onto the finecapillary nozzle, insulated, and finely polished to produce ahydrodynamic system that yielded mass-transport ratescomparable with the high-speed channel electrode[2629].

0022-0728/$ - see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2007.01.014

* Corresponding author. Tel.: +44 24 7652 3264; fax: +44 24 7652 4112.E-mail address:[email protected](P.R. Unwin).

www.elsevier.com/locate/jelechem

Journal of Electroanalytical Chemistry 602 (2007) 263274

Journal of

ElectroanalyticalChemistry
mailto:[email protected]:[email protected]


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Ring UMEs have interesting mass-transport propertiescompared to other UME geometries. In particular, as thering thickness approaches the nanometer scale, highmass-transport rates are predicted[3038]. The inner andouter edge produce high current density improving analyt-ical detection limits and allowing the study of fast electrode

reactions[20,21,3941].Russell and co-workers [42] were among the first tointroduce a gold-ring UME to measure heterogeneous rateconstants for several fast one-electron-transfer reactionsunder steady-state diffusion conditions. A hydrodynamicthin-ring UME was used by Symanski and Bruckenstein[43], in which the ring electrode was rotated. However, neg-ative deviations of the experimental mass-transport behav-iour from theory were evident, due to difficulties inconstructing the electrode coplanar with the insulatingmaterial.

In 1990, Kuhn et al. [44] introduced a combination ofoptical and electrochemical methods, in which an optical

fibre was coated with a thin metal and then an insulatorto produce a micro-optical ring electrode. This was devel-oped further by both Smyrl et al. [45,46] for imagingpurposes and Boxall and OHare [47,48] for kineticstudies of photochemical processes. More recently, Leeand Bard[49] combined scanning electrochemical micros-copy (SECM) and optical microscopy (OM) using a ringUME that acted as both an optical and electrochem-ical probe for imaging microstructures. When a con-stant shear-force (tuning fork) mode was introduced,simultaneous topographical, electrochemical, and opticalimages of an interdigitated array electrode were obtained

[49].To support the practical development of hydrodynamic

UMEs, recent theoretical studies[50]have used numericalmethods to provide detailed information on mass trans-port, for realistic cell geometries. These approaches haveimproved upon analytical expressions [51,52] which makeassumptions that have been shown to unrealistic in practi-cal experimental systems of interest[8,50].

In this paper we describe simple procedures for makingring UMEs using metal-coated optical fibres, insulatedwith two different methods. Characterisation of theseprobes reveals small geometric imperfections which donot significantly affect studies in quiescent solutions, buthave a major impact when the electrodes are introducedinto a hydrodynamic system such as the impinging micro-jet. Nonetheless, by characterising these electrodes anddeveloping simulations that reflect the true electrode geom-etry, it is possible to fully account for the experimentalmass-transport behaviour.

2. Experimental

2.1. Ring electrode fabrication

The methods for fabricating the ring UMEs involved

sputter-coating gold onto an optical fibre, and introducing

a surrounding insulating sheath, fabricated from epoxyresin or glass. First, an all-silica optical fibre (F-MCC-T,core diameter 200 5lm, Newport Corp., US) wasstripped from its polyimide coating and was sputter-coatedusing an Edwards E306 vacuum evaporator (Moorfield,UK) fitted with a minibox conversion to configure the sys-

tem into a true multitechnique vacuum deposition system.Optical fibres were metal-coated when placed vertically ontop of the gold metal target (99.99% pure, Kurt J. LeskerComp., USA). A thin (few A) titanium under-layer(99.97% pure, Kurt J. Lesker Comp., USA) was sometimesused to enhance the adhesion of the gold coating with thequartz body of the fibre (vide infra). Electrical contact tothe gold-coated fibre was made with tincopper wire andsilver loaded epoxy (RS, UK).

Two different insulation materials were used to coat themetal films and so produce electrically isolated ringelectrodes (after subsequent polishing). Initially, the tipof the metal-coated fibre was placed firmly in the centre

of a cylindrical Teflon mould (i.d. 2 mm, height 6 mm)which was filled with epoxy resin (Delta Resins, UK)and left to cure for 48 h. However, the epoxy resin sealingprocedure would ultimately restrict the choice of solventsamenable to study. Consequently, glass insulation wasconsidered which, as highlighted herein, appeared toprovide a more coplanar electrode surface. In this case,the optical fibre and connecting wire were inserted intoa pulled and sealed borosilicate glass capillary (o.d.2.0 mm and i.d. 1.16 mm, Havard Apparatus), whichwas left for 30 min under vacuum. A vertical pipettepuller (Narishige, Japan) was then used to seal the end

of the capillary containing the optical fibre. The tempera-ture of the heating coil was in the range of 700800 Cto melt the borosilicate glass (without the formation ofair bubbles), while ensuring that the metal did not formsmall clusters on the fibre surface, which was observedat higher temperature. The glass sealed thin-ring UMEwas exposed by polishing the sealed end on a coarse Car-bimet paper (Buehler, UK) by hand. To finally ensure aflat surface, the electrodes were polished using diamondimpregnated pads fixed to a home-build polishing wheel,starting with a grain of 15 lm and progressing to 0.1 lm(Buehler).

Optical inspection of the exposed electrode surface wasachieved using an Olympus BH2 light microscope (overallmagnifications 50 to 1000) equipped with a 3-CCD col-our video system. High resolution field-emission scanningelectron microscopy (Zeiss Supra55-VP FESEM) and amultimode atomic force microscope (Veeco multimodeAFM, UK) were employed to examine the topography ofthe ring electrodes in detail and quantitatively measurethe ring thickness. Furthermore, fluorescence confocalmicroscopy imaging of pH gradients during water electrol-ysis (see below) proved to be a very useful tool to monitorconcentration gradients at the ring electrodes[12,53]givinginformation on ring uniformity and sometimes revealing

faults in the sealing process.

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2.2. Apparatus and instrumentation

All potentiostatic measurements were made using a two-electrode set-up with the thin-ring UME operating as theworking electrode and a coated silversilver chloride wire(Ag/AgCl, saturated AgCl) or a saturated calomel elec-

trode (SCE) acting as a reference electrode. Most electro-chemical measurements, including voltammetry andchronoamperometry, were carried out with a model 730Aelectrochemical workstation (CH Instruments, USA). Ahome-built galvanostat, described elsewhere[12], was usedto control the current (in the range 1 nA to 1lA) forgalvanostatic studies of water reduction on the fabricatedring electrodes with simultaneous monitoring by CLSM[12].

CLSM imaging employed a Zeiss LSM 510, Axioplan 2,microscope with a water immersion objective lens (Zeiss,Achroplan 20/0.50W Ph2). An argon laser (k= 488 nm)was used for excitation in conjunction with a long-pass fil-

ter (k= 505 nm) to measure the fluorescence from fluores-cein. All images in this paper were obtained over an area of612 612 lm in thexyplane (parallel to the electrode sur-face).Z-stack images were obtained by taking serial opticalslices in this plane over a range of distances.

The microjet electrochemical cell consisted of a fullydetachable Teflon base and a cylindrical glass body (vol-ume 25 cm3) containing an optical window to aid moni-toring the position of the ring electrode relative to thenozzle. The cell base contained a 2 mm hole to accommo-date a pulled borosilicate glass capillary in an invertedconfiguration, which served as the jet nozzle. The tip of

the nozzle was polished flat using the polishing wheeldescribed earlier. Flow of electrolyte solution throughthe nozzle was achieved using a Gilson HPLC pump(Model 305, Villiers-le-Bel, France) with a 25WTi pumphead and model 806 manometric module. The positionof the ring UME relative to the nozzle was monitoredby video microscopy with a CCD camera attached, whichoffered maximum on-screen resolution of 2.2lm/pixel.The video camera and the positioning system were placedon a home-built granite bench and shielded in a home-made Faraday cage. An airtight Teflon lid with small holesto accommodate tubes was fitted to the glass cell to deaer-ate the solution with argon whenever necessary. The posi-tion of the ring UME was controlled and manipulated inthe x and y directions (parallel to the face of the nozzle)with sub-micrometer resolution using a set ofxy transla-tion stages (M015.00, Physik Instrumente, Germany). Theelectrode was mounted on a piezoelectric positioner (pie-zoelectric element P-173.07, and controller P-267/P277,Physik Instrumente), attached to a one-axis stage (New-port), which was used to control the movement of theelectrode in the z-direction perpendicular to the nozzle,over a maximum expansion of 45 lm. A larger range ofmovement in the z-axis (25 mm) was afforded throughthe use of a differential micrometer (model DM-13,

Newport).

The microjet-CLSM set-up consisted of a rectangularPTFE cell with an optical window, described in detail else-where[50]. A finely polished borosilicate nozzle (typicallyof internal diameter, d, 100lm) was fixed in a 2 mm holeon one side of the cell. On the opposite side of the cell, thering electrode was passed though a 12 mm hole fitted with a

latex finger cot, PTFE collar and o-ring to prevent leakageof solution from the cell. The substrate was mounted on anxyzminiature positioner (Physik Instrumente) to manu-ally align the UME. Solution was delivered through theglass nozzle onto the substrate surface at a constant rateusing a dual-syringe pump (U-74900-15, Cole Palmer Instr.Comp.) via a 100 cm3 glass syringe (Hamilton). The systemwas capable of delivering flow rates in the range of 0.014.0 cm3 min1.

2.3. Chemicals

Hexaammineruthenium (III) chloride (99%, Strem),potassium nitrate (min. 98%, Fisher), methyl viologendichloride hydrate (Aldrich), and fluorescein (98%, Sigma)were used as received. The composition of the solutionsused for different experiments is outlined in the text. Solu-tions were prepared fresh before each experiment, and weredeaerated with Ar. All aqueous solutions were preparedusing Milli-Q-reagent water (Millipore Corp., resistiv-ityP 18 MXcm). The solutions were prepared in ambientconditions and measurements were made in air-condi-tioned laboratories (23 1 C).

2.4. Simulations

Modelling utilised a commercial finite element methodmodelling package (COMSOL Multiphysics, version3.2a), used in conjunction with MATLAB (version 7.0,release 14). Simulations were run (under Windows XP)on a Dell PC with an Intel Pentium 4 processor(2.50 GHz) and 1.5 GB of RAM.

3. Simulations

The finite element method [54] was used to simulatevelocity profiles in real space as a consequence of flow from

a microcapillary nozzle impinging on a finite solid ring elec-trode. The incompressible NavierStokes equations[55]formomentum balance (Eq. 1) and continuity (Eq. 2) weresolved in axisymmetric cylindrical coordinates (understeady-state conditions).

qV rV rp gr2V 1

r V 0 2

where q is the density of water (1.00 g cm3 [56]), V is thevelocity vector (with components u and v in the r and zdirections, respectively), p is pressure and g is the dynamicviscosity of water (1.002 mPa s, for the conditions of our

experiments[57]). The boundary conditions used were

E. Bitziou et al. / Journal of Electroanalytical Chemistry 602 (2007) 263274 265
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r 0; 0< z< zmax n V 0 3

0 6 r6 rglass; z 0 u 0; v 0 4

0 6 r6 rin; z zmax u 0; v Vf

pr2in5

r rin; H


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108.5lm, H= 300lm, and rin= 63.5lm. The ring thick-ness, d, was varied from 100 to 400 nm. For a stationarysolution (Vf= 0), the simulated limiting currents rangefrom 17 nA (d= 100 nm) to 20 nA (d= 400 nm). Thissmall margin by which the current varies with ring thick-ness suggests that caution should be exercised in the mea-

surement of ring thicknesses from limiting currents underquiescent conditions alone. However, upon the introduc-tion of convective flow, much larger changes in limitingcurrent with ring thickness are apparent, as shown inFig. 2; the effect becomes more significant with increasingmass-transport rate. Even at a relatively low flow rate of0.5 cm3 min1, the limiting current range is from 62 nA(d= 100 nm) up to 112 nA (d= 400 nm), which would cer-tainly allow these ring thicknesses to be distinguished.

One of the main outcomes of the studies described inthis paper is that ring UMEs are rarely coplanar. After pol-ishing, rings sealed in epoxy resin appeared to be of thestepped geometry, shown in Fig. 3b, due to the hard-

wearing nature of the quartz optical fibre in the centre ofthe ring compared to the softer outer epoxy resin. Glass-sealed ring UMEs were found to be recessed since the sput-tered gold is much less hard-wearing than either quartz orglass when subjected to polishing. Mass transport to thesenon-ideal geometries was thus investigated by simulation.

The transport limiting current-flow rate characteristicsfor a stepped ring electrode geometry (Fig. 3b) areshown inFig. 4. These simulations were for a ring electrodecharacterised by d= 1000 nm and a= 108.5lm. The noz-zle (rin= 60lm) was positioned at H= 300lm. The solu-tion contained 10 mM MV2+ and the effect of step height

from 0 to 1000 nm was considered. When there is no flow,the step has a relatively minor effect on the limiting current,varying from 239 nA when L= 0 to 216.5 nA when

L= 1lm. Nonetheless, this variation would introduce sig-nificant error into ring thickness determinations thatassumed a coplanar geometry. It can be seen that withflow, the presence of a step has a significant effect, notablyat the highest velocities. For example, atVf= 0.5 cm

3 s1 acurrent of 2017 nA results while a step of 1 lm reduces thecurrent to 994 nA. At Vf= 2.0 cm

3 s1 the effect is morepronounced with a limiting current of 4277 nA withoutthe step, and 1360 nA with a 1lm step.

The consequences of the step on mass transport to thering electrode, with and without flow, are readily visualisedvia calculated concentration profiles presented in theaxisymmetric r, z geometry. Fig. 5 shows the profiles for

MV2+ at a coplanar electrode (a) and stepped electrode

0.0 0.5 1.0 1.5 2.0

0

50

100

150

200

250

i

/nA

Vf/ mL min

-1

Fig. 2. Effect of ring thickness (from top to bottomd= 400, 350, 300, 250,200, 150, and 100 nm) on the limiting current for the reduction of 1 mMsolution of RuNH3

36 at various flow rates. The following parameters

were used: a= 108.5lm, H= 300lm and rin= 63.5lm, for a coplanar

ring geometry.

Optical fibre Epoxy

or glasssheath

Active ring

electrode

L

L

Optical fibre Epoxy

or glasssheath

Active ring

electrode

Optical fibre Epoxy

or glasssheath

Active ring

electrode

LL

L

Fig. 3. The three ring geometries considered: (a) coplanar, (b) stepped,

and (c) recessed.

0.0 0.5 1.0 1.5 2.0

0

1000

2000

3000

4000

5000

i

/nA

Vf/ mL min

-1

Fig. 4. Effect of step height (from top to bottomL= 0, 25, 50, 100, 250,500, and 1000 nm) on the limiting current for the reduction of a 10 mMsolution of MV2+ at various flow rates with d= 1000 nm, a = 108.5lm,H= 300lm and r in= 60 lm.



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(L= 1000 nm) (b), with no flow (i) and at Vf= 2.0 cm3 s1

(ii). Because the diffusion field at the UME, without flow,extends over a large range (partially due to the large sizeof the ring considered), the step diminishes mass transportto the upstream edge of the electrode only, to a relativelysmall extent. However, with flow the concentration bound-

ary shrinks in size dramatically (Fig. 5a(ii)) at a coplanarring electrode, so that the introduction of the step dis-rupts flow and mass transport significantly. This resultsin the characteristic concentration profile shown inFig. 5b(ii) which extends over a larger distance then forthe coplanar electrode, resulting in the much lower currentsobserved.

The effect of recess depth on the limiting current-flowrate characteristics, for the geometry shown in Fig. 3c, isillustrated in Fig. 6. Based on electrode characterisationresults presented later, the recess depth was varied from 0to 80 nm, and the following ring UME parameters wereemployed, which were characteristic of the glass-sealed

UMEs discussed later: d= 350 nm, a= 108.5lm, H=300lm, and rin= 63.5lm, with [RuNH3

36 1 mM.

With no flow, the recess has only a small effect on the lim-iting current, which varies from 17.7 nA (coplanar elec-trode) to 16.8 nA (L= 80 nm). However, at Vf=2.0 cm3 s1, the effect on the current is significant, decreas-

ing from 189 nA (coplanar electrode) to 116.4 nA(L= 80 nA). The reason for the significant decrease canbe seen in Fig. 7, which shows concentration profiles for

Fig. 5. Simulated concentration profiles (in the axisymmetric cylindrical r, z co-ordinate system) for the one-electron reduction of 10 mM MV 2+ at a

coplanar (a) and a stepped (b) ring electrode geometry when Vf= 0 (i) and Vf= 2 cm3 s

1 (ii).

0.0 0.5 1.0 1.5 2.0

0

50

100

150

200

i

/nA

Vf/ mL min

-1

Fig. 6. Effect of recess depth (from top to bottomL = 0, 20, 40, 60, and80 nm) on the limiting current for the reduction of 1 mM RuNH3

36 at

various flow rates with d= 350 nm, a= 108.5lm, H= 300lm and

rin= 63.5lm.



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Vf= 0 cm3 s1 (i) and Vf= 2.0 cm

3 s1 (ii) at the recessedelectrode. The profile for Vf= 0 is similar to that showninFig. 5a, indicating that the small recess has only a smalleffect in diminishing mass transport to the ring UME.However, with flow the effect of recess depth in restrictingmass transport becomes apparent due to the much smallerconcentration boundary layer.

4. Experimental results and discussion

4.1. Voltammetry at ring UMEs in quiescent solution

Fig. 8 shows typical cyclic voltammograms for thereduction of 1 mM RuNH3

36 at (a) an epoxy-sealed elec-

trode; and (b) a glass-sealed Au sputtered fibre. The elec-trode comprised the same fibre dimensions but differentring thicknesses, as a longer sputtering time and higherpower was used for the epoxy-coated electrode. Further-more, the epoxy-sealed electrode included a Cr underlayer(50 A) used to promote better adhesion between the gold

and the plasma cleaned quartz fibre.

The voltammetry of both ring electrodes exhibited a typ-ical sigmoidal waveform as expected for the reversibleRuNH3

3=26 system for thin-ring UMEs at low sweep

rates[36]. However, the glass-sealed ring electrode clearlyproduces a much better voltammetric response, which weattribute to a more faithful seal with the glass coating pro-cedure and the fact that a Cr underlayer was not used.Nonetheless, the epoxy-sealed electrode shows a reasonableresponse, so that both electrodes could be used for furtherinvestigations of mass transport.

A 25lm diameter Au-disc UME was used to measurethe diffusion coefficient of 1 mM RuNH3

36 in 0.1 M

KNO3 via linear sweep voltammetry (LSV) at 10 mV s1

.A value of D= 7.7 106 cm2 s1 was obtained, whichcompares reasonably with literature values[60,61]. Armedwith a knowledge of the diffusion coefficient and the innerradius of the rings (a= 108.5lm) from optical microscopy,the current measured by LSV could be compared to thatexpected, given d= 1lm for the epoxy-sealed electrode(estimated by optical microscopy) and d= 0.35lm forthe glass-sealed electrode (electron microscopy). For athin-ring geometry, i.e. where a/b > 0.91, the followingapplies for a coplanar ring[30,32,33,38,43,47,62]

iT;Ring nFDAC1

A

p2 a b

ln16b a=b a 22

wheren is the number of electrons per redox event,Fis theFaraday constant,DAthe diffusion coefficient of electro-ac-tive species, A, and C1A is the bulk concentration ofA.

From steady-state voltammetry shown inFig. 8, for theepoxy-sealed electrode, the experimental limiting currentvalue is approximately 20.5 0.4 nA, whereas for theglass-sealed electrode the steady-state current value is closeto 17.7 0.2 nA. The calculated current values, using Eq.(22), for the epoxy-sealed and the glass-insulated ringsyields values of 19.3 and 17.5 nA, respectively, which bothcompare well with experimental observation, especially for

the glass-sealed ring. Notice, however, that if one was to

Fig. 7. Simulated concentration profiles for the one-electron reduction of1 mM RuNH3

36 at a ring UME recessed byL = 90 nm when Vf= 0 (i)

and Vf= 2 cm3 s1 (ii).

-30

-25

-20

-15

-10

-5

0

5

-0.4 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0

Potential / Volts (vs. SCE)

Curren

t/nA

(a)

(b)

Fig. 8. Steady-state voltammograms for the reduction of 1 mMRuNH3

36 in 0.1 M KNO3 at (a) 10 mV s

1 for an epoxy-sealed ringUME; and (b) 5 mV s1 for a glass-sealed UME under quiescentconditions.



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use the limiting current to estimate the ring thickness, onewould obtain d= 1.58 0.23 lm (epoxy-sealed electrode)and d= 0.47 0.05 lm (glass-sealed electrode). This isbecause the limiting current is relatively insensitive to ringthickness, in the range of interest, but requires a very accu-rate value of DA. Consequently, the independent physical

characterisation of ring UMEs is particularly important,as we highlight further, later in this paper.

4.2. Mass-transfer rate imaging under flow conditions

To investigate mass transport in the microring geometrywith flow, mass-transfer imaging experiments were carriedout by recording the transport-limited current for thereduction of methyl viologen dication as a function of theimpinging jet position, which was physically scanned overthe electrode, usually to form a square image (Fig. 9).The application of this methodology to disc UMEs

revealed considerable information on local mass transport[4] and it was anticipated that useful insights into localmass transport could also be obtained for ring UMEs.The nozzle was scanned in the xy plane above the elec-trode, holding the electrode potential at a constant valuefor the transport-limited reduction of 10 mM MV2+

(0.85 Vvs.SCE). The data were obtained using a nozzleof internal diameter, rin= 60lm, which was placed at adistance, H= 300lm, from the surface of the electrodeusing the z-axis micropositioner. The solution flow ratewas moderate (Vf= 0.5 cm

3 min1) to minimise solutionconsumption, and the current response was normalisedwith respect to the diffusion-limited current (ilim= 208 nA)

measured under quiescent conditions.Fig. 9shows a map of the normalised current vs. nozzle

position. The form of the image, which shows a positionwith a single central maximum and a fairly symmetricaldecrease in current with radial distance from this point,implies that the maximum enhancement occurs when the

nozzle axis is coaligned with that of the ring UME. Radialmotion of the nozzle away from this position causes thecurrent to decrease. This behaviour contrasts with thatobserved with a 25lm diameter Pt-disc and a Hg-hemi-sphere UME positioned in the impinging jet arrangement[4,6], which both produced mass-transport-limited current

images that comprised a ring of maximum current thatsurrounded a central circular area where the transport-lim-iting current was slightly smaller. These responses wereobtained because mass transport to the impinging jetreaches a maximum at small radial displacements fromthe area where the jet impinges on the surface[4,6,12]. Withthe ring electrode geometry, the electrode is naturally inthis position when the nozzle and electrode are alignedcoaxially. It should be noted, however, that the currentenhancements observed with the ring UME are consider-ably lower then for the disc and hemisphere UMEs, studiedearlier [2,46]. This is partly due to imperfections in theelectrode geometry, alluded to above and considered fur-

ther bellow.

4.3. Hydrodynamic voltammetry with an epoxy-sealed UME

A series of steady-state LSVs were recorded when thenozzle was positioned coaxially with the ring electrodeusing mass transfer imaging, as discussed in relation toFig. 9. Solution was flowed directly at the surface of theelectrode at various flow rates. The resulting voltammetryobtained by scanning the electrode potential between 0 Vand 0.9 V vs. SCE at a scan rate of 20 mV s1 for theone-electron reduction of MV2+ to MV+ is shown in

Fig. 10. This plot demonstrates the enhanced rate of masstransport to the epoxy-sealed ring (d= 1lm) when a well-established flow is present, with a current increase of almostone order of magnitude at a flow rate of 7 cm3 min1, com-pared to quiescent solution. However, the enhancement ismuch less than expected for a coplanar ring electrode (see,for example,Fig. 4in the theory section).

0 100 200 300 400 5000

100

200

300

400

500

Normalised Current:3.71+3.54 to 3.713.36 to 3.543.19 to 3.36

3.02 to 3.192.84 to 3.022.67 to 2.842.49 to 2.672.32 to 2.492.15 to 2.321.97 to 2.151.80 to 1.97

x-axis / m

y-axis/m

0 100 200 300 400 5000

100

200

300

400

500

Normalised Current:3.71+3.54 to 3.713.36 to 3.543.19 to 3.36

3.02 to 3.192.84 to 3.022.67 to 2.842.49 to 2.672.32 to 2.492.15 to 2.321.97 to 2.151.80 to 1.97

x-axis / m

y-axis/m

Fig. 9. Variation of the transport-limited current with nozzle position inthe xyplane for the reduction of 10 mM MV 2+ at an epoxy-sealed ringUME under an impinging microjet. The current maximum was obtainedwhen the nozzle and the ring were in coaxial alignment (x= 270lm,

y= 160lm), Vf= 0.5 cm3 min

1, H= 300lm, and r in= 55 lm.

-2500

-2000

-1500

-1000

-500

0

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2

E / V (vs. SCE)

i/nA

(a)

(m)

-2500

-2000

-1500

-1000

-500

0

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2

E / V (vs. SCE)

(a)

(m)

Fig. 10. Hydrodynamic LSVs (20 mV s1) for the reduction of 10 mMMV2+ at an epoxy-sealed ring UME at solution flow rates of: (a) 0 , (b)0.25, (c) 0.5, (d) 0.75, (e) 1.0, (f) 1.5, (g) 2.0, (h) 2.5, (i) 3.0, (j) 4.0, (k) 5.0,(l) 6.0 and (m) 7.0 cm3 min1 (d= 1 lm, a = 108.5lm, H= 300lm, and

rin= 55 lm).



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Fig. 11 summarises the experimental limiting-currentwith respect to the flow rate, together with the theoreticalpredictions when the ring electrode is considered to havea stepped geometry (Fig. 3b) caused from the polishingprocedure, rather then being coplanar. The theory fits wellwith the experimental values if a step of 1200 nm isassumed.

Evidence for this type of geometry became apparentwhen CLSM was used to visualise the reactivity of the ringelectrode [12,53]. Fig. 12 shows fluorescence images: (i)parallel at the electrode surface; and (ii) perpendicular tothe electrode surface through the central plane of symme-try, when the reduction of oxygen (E= 0.7 V vs. Ag/AgCl) from aerated solution was carried out at a diffu-sion-limited rate. The experiment was carried out in a solu-tion containing 5lM fluorescein (initial pH 4.80) so thatthe generation of hydroxide ions at the surface of the elec-trode caused the pH to increase significantly, resulting in afluorescent zone due to the fluorescence of fluorescein (fordetails see Ref.[12]). The top xyimage (Fig. 12(i)) of theepoxy-sealed ring electrode clearly shows the fluorescentregion around the electrode but the centre (optical fibre)

appears darker in the plane just above the ring surface.From the bottomxzimage inFig. 12(ii), constructed fromcross-sectioning a z-stack consisting of 51 slides (interval4.54lm), the quartz fibre appears slightly above the epoxylevel as a small perturbation.

4.4. Characterisation of glass-sealed UMEs

The glass insulation of the metal ring surrounding anoptical fibre appeared to prevent the stepped geometrydiscussed in the preceding section, due to the insulatingsheath and optical fibre having similar abrasion character-

istics. Further advantages of the glass insulation method

include the straight forward fabrication method, whichresembles that of glass-sealed disc UMEs, as well as thepossibility of application to a broader choice of solvents.

As the glass-sealed ring emerged as the more appealingof the two different insulated electrodes, further character-isation was introduced through the use of field emissionscanning electron microscopy (FE-SEM). Typical imagesare shown inFig. 13a and b. FE-SEM was used to obtaindetailed information on the uniformity of the ring elec-trodes and to quantitatively measure the ring thickness.The ring thickness is seen to vary a little (30 nm) aboutthe average value of 350 nm, which was used in all simula-tions which follow. Even though extremely useful, electronmicroscopy does not provide detailed information on anyheight variations of the sample. Thus, for accurate determi-nation of the topography, and to identify height-variationsin the ring electrode, tapping mode AFM was used. Elec-tric force microscopy (EFM) was used prior to tappingmode to ensure the image contained an electrode and notjust a surface abnormality. Cross-sectioning various pointsacross the electrode perimeter produce height-variationplots, like the one shown in Fig. 11d, which indicates a

recess of20 nm. An average recess of the metal ring elec-

0.0 0.2 0.4 0.6 0.8 1.0

0

200

400

600

800

1000

1200

i

/nA

Vf/ mL min

-1

Fig. 11. Experimental limiting current (diamonds) and theoretical fit (dotsand line) for a stepped ring UME geometry (L= 1200 nm) for thereduction of 10 mM MV2+ at various flow rates. The epoxy-sealed ring

UME was characterised by: d= 1 lm, a= 108.5lm, H= 300lm andrin= 63.5lm.

Fig. 12. CLSM image of the region close to a 1000 nm thick epoxy-sealedring electrode during diffusion-limited oxygen reduction in 5 lM fluores-cein solution (0.1 M KNO3). A potential of 0.7 V (vs. Ag/AgCl) wasapplied to the electrode, with an initial bulk solution of pH 4.8. The topxy plane image (i) measures 512 512 lm and is parallel to the electrode,just above the surface. The bottom xz plane image (ii) (measuring512 231.5lm) was constructed taking the cross-section of a z-stack

consisting of 51 slices (interval 4.54lm).



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trode of 90 50 nm was determined, with the recessionvarying from 0 to 180 nm.

4.5. Hydrodynamic voltammetry with a glass-sealed ring

UME

As described above for the epoxy-sealed ring electrode,hydrodynamic voltammetry was carried out by positioningthe nozzle coaxially with respect to the glass-sealed ring elec-trode at a distance ofH= 300lm from the electrode sur-face. Potentiostatic reduction of 1 mM RuNH3

36 in

0.1 M KNO3was carried out as a function of volume flowrate and the steady-state limiting-current response recorded.

Fig. 14 shows the limiting-current values recorded atflow rates between 0 and 2.0 cm3 min1 alongside the sim-ulations which took into consideration the characteristic

90 nm recess of the ring electrode. This plot clearly shows

reasonable agreement of the experimental data and the the-oretical model once the electrode has been subjected toextensive and detailed characterisation.

4.6. CLSM imaging during hydrodynamic experiments

Fluorescence CLSM has recently been shown to be apowerful method of flow visualisation of an impingingmicrojet, yielding images that map, with high precision,the path of solution flow from the nozzle as it impingesonto a substrate[12].

In the present studies, CLSM was used to probe the dif-fusion layer established at the ring electrode under flowfrom the impinging microjet.Fig. 15shows a CLSM imageof a 5lM fluorescein unbuffered solution of pH 5.0 flowingtowards a glass sealed UME (d= 350 nm). The flow rate

was 0.2 cm3 min

1 and a current of150 nA was applied

0.0 0.5 1.0 1.5 2.0

-20

-15

-10

-5

0

5

10

Height/nm

Length /m

~20 nm

0.0 0.5 1.0 1.5 2.0

-20

-15

-10

-5

0

5

10

Height/nm

Length /m

~20 nm

200 nm

CrossSection

Fig. 13. FE-SEM images (a and b) and AFM image (c) of a 350 nm thick glass-sealed ring UME. A cross-section of the ring electrode from the AFMimage (c) reveals a plot of heightvs.cross-section length (d) and a ring recess depth of approximately 20 nm with values varying from 0 to 180 nm acrossthe perimeter of the ring (mean value of 90 50 nm).



11/12

to the UME galvanostatically. Note that the similar unbuf-fered solution conditions in the nozzle and the cell meantthere was little contrast between the impinging jet andthe surrounding solution. This was intentional as, underthese conditions, water reduction at the electrode, causesan increase of the pH (production of hydroxide ions) [12]which is readily visualised as a fluorescent zone adjacentto the electrode. The fluorescence image clearly shows aconcentration boundary which extends symmetrically a sig-

nificant distance radially beyond the downstream ring elec-trode edge, which is consistent with the predictions fromthe simulations (e.g.Fig. 7(ii)).

5. Conclusions

This paper has considered the deployment of a thin ringUME in an impinging jet system for the first time. The innerdiameter of the ring electrode is about twice that of the noz-zle from which solution flows, so as to optimise mass trans-

port [4,8]. Simulations of mass transport, by solving theNavierStokes and diffusion equations, have revealed thatmass transport to coplanar electrodes should be greatlyenhanced in this configuration. However, smaller enhance-ments were observed experimentally and this was rationa-lised in terms of deviations of the ring UME geometriesfrom the ideal coplanar case. Simulations indicate thatsmall imperfections, which have length scales of the orderof the concentration boundary layer (under flow condi-tions), have a dramatic effect in diminishing mass transportto ring UMEs. Clearly, a major challenge in the furtherdevelopment of ring UMEs in high speed convective sys-tems would be to develop fabrication methods which mini-

mise such imperfections. With current fabrication protocolsconsiderable effort is required to characterise electrodesbefore use and the studies in this paper indicate that for ringelectrodes formed on optical-fibres it is difficult to eliminategeometric imperfections. We note, in passing that othermethods for forming ring UMEs yield better-defined elec-trodes[20,21]. More generally, other convective systems inwhich solution flows across an UME at high speed may alsobe influenced by small geometric imperfections and it wouldbe interesting to examine the extent to which such effects areimportant using simulation methods.

Two types of ring UMEs have been described in this

paper. Both employ an optical fibre core, which will allowfuture photoelectrochemical applications. The novel glass-sealed electrode, fabricated in an analogous manner to con-vectional disc-shaped UMEs, appears to be particularlyattractive for studies in a wide range of media, includingorganic solvents for which the epoxy-resin electrode wouldbe unsuitable.

Acknowledgements

This research was supported by the EPSRC (E.B.) andUniversity of Warwick Postgraduate Research Fellowship

Scheme (N.C.R.). We thank Dr. Neil R. Wilson and So-phie Martin for producing the AFM images. We wouldalso like to express our appreciation to Dr. Julie Macpher-son (University of Warwick), Mr. Martin Edwards (Uni-versity of Warwick), and Dr. Sabine Szunerits (DomaineUniversitaire, Grenoble) for helpful discussions and advice.

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Fig. 15. Fluorescence CLSM image of a 5lM fluorescein (in 0.1 MKNO3) unbuffered solution (pH 5) flowing towards a glass-sealed ringelectrode (d= 350 nm) with 1.08 mm total diameter substrate, immersedin an unbuffered solution of similar pH. The CLSM image measures651 651 lm, H= 400lm, rin= 60 lm, and Vf= 0.2 cm

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12/12

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2007 - BITZIOU - Microjet Ring Electrode (MJRE)- Development- Modelling and Characterization