IntroductionWith the establishment of microelectrodes as powerful electroanalytical tools, the demand for reliable, commercially available microelectrode devices is ever increasing. Microdisc or microhole array electrodes are particularly promising since they combine microelectrode behaviour with increased signal while also offering the possibility of mass-production via screen-printing or lithographic techniques.

In this work, planar electrode arrays of 29x29 Au microdiscs (30 μm in diameter), lying 300 nm apart and separated by a 100 nm thick SiO2 insulating layer, were fabricated on a SiO2/Au/Cr/Si layered

substrate using a combination of photolithographic and silicon technology techniques. They were characterised for their electroactive surface area by Au surface electrochemistry in acid and for their nominal geometric area by the reversible reduction of ferricyanide. Finally, these electrodes were used for the electrochemical determination of cystine, a common aminoacid.

Fabrication and characterisation of Au microdisc array electrodes

-application to the electroanalysis of cystine

S.Papadimitriou1, D.P.Casey2, J.F.Rohan2 and .Sotiropoulos1*

1Chemistry Department, Aristotle University, Thessaloniki 54124, Greece2National Microelectronics Research Centre, Cork, Ireland

Aristotle UniversityAristotle University Aristotle UniversityAristotle University

Fabrication and microscopic characterisationPhotolithography was used to pattern (through an appropriate glass mask) a ca. 8 μm thick positive photoresist film span over the SiO2 topmost layer of Si wafers and this was followed by resist

development and resist removal over the designed microdisc areas. Selective etching of the insulating oxide layer through the patterned resist and down to the Au layer resulted in an array of 30 μm Au microholes (Figure 1 (A)), whereas further, complete, resist removal resulted in an array of Au microdiscs (strictly speaking, 100 nm shallow microholes, Figure 1(B)). The wafers were diced in individual electrode arrays of 2 x 1 cm, having 29x29=841 Au microholes. The arrays were characterised by profilometry, optical microscopy and SEM microscopy and conformed to the designed dimensions.

Electrochemical oxidation of cystine on Au microdisc array electrodesThe thus produced Au array electrodes were tested for the oxidation of cystine in the 10-5 – 10-4 M range, from acid solutions. Cystine is one of the few aminoacids that is electroactive in its uderivatised form on C, Pt and Au electrodes. Its oxidation proceeds via either a 5 or 6 or 10e- loss resulting in sulfone, sylfinyl sulfone or cysteic acid respectively. The voltammetric picture on Au is known to depend on the less positive potential limit and, recently on the roughness factor of the electrode (for high rougness factors, cystine is oxidised to cysteic acid).

Figure 4 shows voltamograms for cystine oxidation on a rough Au microdisc array with a starting potential of –0.5 V vs. Ag/AgCl (not shown). It can be seen that the typical picture reported in the literature has been obtained: a pre-wave of pre-adsorbed cystine oxidation on an oxide-free surface (i.e. at potentials less positive than 1.050 V vs. Ag/AgCl) and a main wave showing a maximum (due to competition between the good catalytic activity and site blocking of the Au oxide formed) beyond which mass transfer control prevails for electrodes of high catalytic activity. Based on the microdisc limiting current formula (given above) and cystine concentration and diffusion coefficient, it is estimated that at high positive potentials cystine is fully oxidised to cysteic acid via a 10 e - loss, on rough Au microelectrodes.

Our choice of a negative starting potential and of rough electrodes, despite the complications of an additional pre-wave in the voltammogam and an increased background current in the Au oxide formation region respectively, is justified as follows: First, although voltammograms starting at +1.00 V vs. Ag/AgCl did indeed show a single wave ( Figure 5), the current deteriorated fast in subsequent runs indicating that the activation of the Au surface by oxide formation-stripping is a prerequisite for reproducible cystine oxidation. Second, although Au arrays of a smaller roughness factor, e.g. as received, did show

suppressed surface electrochemistry-background currents (Figure 6), the change of the cystine oxidation mechanism (from 5 to 10 e -) and current as the rougnhess factor of electrodes changed upon experimentation, made it necessary to scan them between the oxide formation and stripping regions continuously until a stable, high roughness factor and a stable, maximum oxidation current (corresponding to 10e-) was atained.

The background-corrected current at +1.15V vs. Ag/AgCl of the voltammograms of Figure 4, varies in a reasonably linear fashion with cystine concentration ( Figure 7), allowing its determination for concentrations below 10-5 M (a detection limit of 4 x 10-6 M has been estimated, corresponding to a signal equal to three times the intercept of the line of Figure 7).

Figure 1 SEM micrographs of (A) a segment of patterned resist –covered Au microhole array and

(B) a single “microdisc” (shallow microhole) Au electrode following complete resist removal

Figure 2 Cyclic voltammogram at 50 mV/s of a Au microdisc array in (non-dearated ) 0.1 M H2SO4,

500 μm

(A)

12 μm

(B)

Electrochemical characterisation of Au electrode array Electrochemical characterisation was first carried out by continuous cyclic voltammetry in sulfuric acid (serving also as a cleaning-activation procedure) which allowed calculation of microelectrode roughness (true to nominal electrode area), based on Au oxide surface electrochemistry. The charge associated with the oxide stripping peak (cathodic peak in Figure 2) gives an estimate of the true electrode surface area, taking into account that the charge corresponding to the formation/stripping of a Au oxide monolayer from a true electrode area of 1 cm2 is 420 μC . As received electrodes exhibited a roughness factor in the 2-3 range which, upon continuous cycling up to a stable voltammetric picture, reached higher values in the 7-10 range (Figure 2).

Microelectrode behaviour as well as conformity to expected geometric area-number of electrodes per array were asserted through the reduction of the reversible electroactive probe of ferricyanide. Figure 3 shows the corresponding S-shaped voltammogram, typical of steady-state mass transfer conditions and attainable for stationary solutions in cases of non-linear diffusion. The total array geometric area or, conversely, the number of active microdiscs, can be estimated from the limiting current equation for microdisc electrodes:

where N is the number of active-operating microdiscs, r their radius (30 μm), n the number of electrons exhanged in the electrochemical transformation of a molecule of the electroactive species (n=1 for ferricyanide reduction) and D, C its diffusion coefficient and concentration. The number of estimated active microelectrodes varied by 10% around the nominal value of 841 fabricated microdiscs for different devices.

(4nFDCr)NiNI microdisc

Voltammogram (50 mV/s)of a Au microdisc array

in 5 mM Ferri + 0.1 M KCl

-1600

-1200

-800

-400

0

400

-0.3 -0.1 0.1 0.3 0.5E / (V vs. Ag/AgCl)

I / n

A

Voltammogram (50 mV/s) of Au microdisc array in 0.1 M H2SO4

-15

-10

-5

0

5

10

-0.5 0 0.5 1

E / V vs. Ag/AgCl

I / μ

Α

Figure 3 Voltammogram at 50 mV/s of a Au microdisc array in 5mM

ferricyanide + 0.1 M KCl

Cystine + 0.1 M H2 SO 4 (10 mV/s); starting potential -0.5 V

-4

0

4

8

0.5 0.7 0.9 1.1 1.3E / (V vs. Ag/AgCl)

I / μA

10-4 M5x10-5 M

0 M

Voltammogram (10 mV/s) at Au array electrodes

in 5x10-4 M cystine + 0.1 M H2SO4;

starting potential of +0.4 V

-10

0

10

20

30

40

50

0.35 0.85 1.35 1.85E / (V vs. Ag/AgCl)

I / μA

Variation of background-corrected current at +1.15 V, on Au microdisc arrays,

with cystine concentration

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.00002 0.00004 0.00006 0.00008 0.0001

C / M

I / μ

Α

Figure 4 Cyclic voltammograms at 10 mV/s on a Au microdisc array in cystine solutions of 0.1 M H2SO4,;

starting potential of –0.5 V and electrode roughness of 8.5

Figure 7 Background –corrected current vs. concentration plot based on limiting currents at +1.15 V of

cyclic voltammograms at Au microdisc arrays

in 0.1 M H2SO4, cystine solutions.

Voltammetry (10 mV/s) at moderately rough Au microdisc arrays

in a 10-4 M cystine + 0.1 M H2SO4

-4-3-2-10123456

-0.25 0.25 0.75 1.25E / (V vs. Ag/AgCl)

I / μ

Α

10-4 M cystine + 0.1 M H2SO4

0.1 M H2SO4

Roughness factor of 3Roughness factor of 8.5

Acknowledgements Acknowledgements to:to: EU EU for a RIMDAC II for a RIMDAC II (Project 6) grant to NMRC and S.Sotiropoulos;(Project 6) grant to NMRC and S.Sotiropoulos;Staff at National Microelectronics Research CentreStaff at National Microelectronics Research Centre, Cork, Ireland, Cork, Ireland

Figure 5 Cyclic voltammogram at 10 mV/s on a Au microdisc array in a 5 x 10-4 M cystine + 0.1 M H2SO4, solution; starting potential of +0.4 V

and electrode roughness of 8.

Figure 6 Cyclic voltammograms at 10 mV/s on a Au microdisc array in a 10-4 M cystine + 0.1 M H2SO4, solution; starting potential of -0.5V

and electrode roughness of 3.