Download Electroanalytical Properties of Band Electrodes of Submicrometer

1813
Anal. Chem. 1885, 57, 1913-1910
Electroanalytical Properties of Band Electrodes of
Submicrometer Width
Kenneth R. Wehmeyer, Mark R. Deakin, and R. Mark Wightman*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Platinum, gold, and carbon electrodes have been constructed
with a band geometry. Thin films of the electrode material
were obtained commercially or were formed on insulating
substrates by conventlonai sputtering techniques, and an insulating overlayer was placed on each of the films. The edge
of this assembly has been employed as a voltammetric
electrode which is characterized by a microscopic width
(5-2300 nm) and a macroscopic length (centimeters). These
electrodes exhibit sigmoidal, rather than peak-shaped, voitammehic curves because of nonlinear diffusiorl. The limiting
current at band eiedrodes is shown to depend on the eiectrode width in an inverse logarithmic manner. Therefore,
extremely large current densities are observed as the eiectrode radius is decreased. The large current densities lead
to a greater Sensnivlty of the vdtammetrlc curve to the effects
of slow heterogeneous charge transfer kinetics. The combination ot small area and high current denslty results in an
electrode that has a much larger faradalc to residual current
ratio than Is obtained at an electrode of conventional size.
This Is demonstrated by the determination of ruthenium hexaammine by conventional linear scan voltammetry with a
lowest detectable concentration of 7 X lo-' M.
In recent years increasing attention has been drawn to the
area of microvoltammetric electrodes. Electrodes of small
dimensions (10 pm or less) possess unique properties which
can make their use in many applications preferable to electrodes of conventional size. For example, in vivo measurements of neurotransmitters require electrodes of small size
to minimize damage to neuronal tissue (I,2). Due to their
small size, microvoltammetric electrodes generate extremely
small currents and, thus, i R drop is minimized. This allows
electrochemistryto be performed in highly resistive media (3,
4 ) arid cyclic voltammetry to be extended to scan rates of
greater than 10000 V s-l without need for instrumental corrections (3). The double layer capacitance on such electrodes
is also reduced due to the small area. This results in an
electrochemicalcell with a small RC time constant, which has
allowed potentiostatic experiments to be performed at submicrosecond time scales (5). The advantage of microvoltammetric electrodes has been demonstrated in nucleation studies
where, due to their small area, a single nucleus of metal atoms
can be grown (6, 7).
Another unique property of microvoltammetric electrodes
is the sigmoidal current response obtained in cyclic voltammetry. The sigmoidal response is a result of enhanced mass
transport due to nonlinear diffusion (8). The enhanced
transport obtained with disk electrodes of 0.6-25 pm diameter
allows the determination of the kinetics of homogeneous reactions coupled to electrode processes (9, 10). The enhanced
flux at microvoltammetric electrodes has also been demonstrated to be useful in the study of electrochemical kinetics
since reaction rates can be observed under steady-state conditions (6, 11).
The enhanced flux at microvoltammetric electrodes should
also lead to an increase in the sensitivity of electroanalytical
measurements (1). This is because the faradaic current density
0003-2700/85/0357-1Q13$01.50/0
increases with decreasing electrode dimensions, while many
contributors to the residual current are proportional to the
electrode area. However, the small magnitude of the currents
obtained with disk-shaped microvoltammetric electrodes can
pose instrumental difficulties in the measurement of dilute
concentrations. Therefore, some investigators have examined
the use of arrays of microvoltammetric electrodes. Digital
simulation has been employed to examine the optimum dimensions for designing ensembles of microvoltammetric
electrodes for high sensitivity (12,13). An array of 10-pm
carbon disks has been shown to have sufficient sensitivity for
trace analysis following liquid chromatographic separation
(14). An ensemble of randomly spaced microvoltammetric
electrodes formed from a combination of reticulated vitreous
carbon and epoxy also has been investigated (15). It has been
proposed that the favorable signal to noise properties of
composite electrodes are a result of this phenomena (16).
Microvoltammetric electrodes with a band geometry can
provide larger currents than disk microvoltammetric electrodes, while maintaining the properties of nonlinear diffusion
(17). The band electrode can be designed to be microscopic
in one dimension (10 pm or less), but macroscopic in length,
thereby resulting in larger currents. Band electrodes with
thicknesees of 5-20 pm have been constructed from thin sheets
of conductors (In,or by vapor or chemical deposition techniques (18,19). An assembly of several band electrodes has
been fabricated with photolithography techniques, arid when
covered by a polypyrrole layer the assembIy was shown to have
properties similar to a solid-state field effect transistor (20).
At band electrodes of micrometer dimensions, and under
conditions of nonlinear diffusion, we have shown that the
faradaic current can be predicted by the equation for the
current at a hemicylinder of equivalent area (I 7)
i = 27rnFDCZ[l/(ln 40)]
(1)
In eq 1,i is the electrode length (cm), 8 = Dt/?, r is the radius
of the equivalent hemicylinder (cm), and the other terms have
their usual electrochemical meaning. We have previously
shown that this equation predicts the current obtained at
cylinders for 0 > 10 (17). This equation leads to the interesting
prediction that the faradaic current is relatively insensitive
to the radius of the electrode, while the residual current with
surface origins will decrease in a linear manner with a decrease
in radius. The detection capabilities of conventional solid
electrodes in voltammetric techniques are limited by residual
current due to double layer charging and to surface oxides
found on the electrodes (21). Thus, band-shaped microvoltammetric electrodes should provide a decrease in voltaminetric detection limits. This paper provides an experimental
realization of this prediction, and also provides a voltammetric
characterization of band electrodes of submicrometer dimensions.
EXPERIMENTAL SECTION
Reagents. All chemicals were of reagent grade, and solutions
were prepared in doubly distilled water (Mega-Pure System
MP-3A and D2,Corning G h s Works, Corning, NY).Ruthenium
hexaammine trichloride (ICN Pharmaceuticals, Plainview, NY)
was dissolved in in 0.1 M potassium phosphate buffer (pH 7.0)
and 0.1 M KNOs. Potassium ferricyanidesolutionswere prepared
0 lQS5 American Chemical Society
1914
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
daily in aqueous 0.1 or 1.0 M KCl.
Construction of Band Electrodes. Glass microscope slides
2.5 cm long, 1.9 cm wide, and 1mm thick (Corning Glass Works,
Corning, NY) were cleaned with detergent and water, dried in
an oven, cleaned with isopropyl alcohol, and redried. A portion
of each slide was masked with adhesive tape so that a strip of
glass 0.6 to 1.0 cm wide remained exposed in the center of the
slide. The slides were then covered with gold or platinum by a
sputtering process using a Polaron Model E 5100 Series I1 Cool
Sputter Coater (Polaron, Inc., Hatfield, PA). The tape was
carefully removed from the coated slides and the slides were
heated in an oven at approximately 200 OC for about 2 h. Metal
films with thicknesses ranging from 30 nm to 2300 nm could be
prepared in this manner. The thickness of these films was determined by scanning electron microscopy. The length of the
bands was measured by optical microscopy.
The films obtained from commerical sources and those prepared
locally were insulated from solution contact on the large, exposed
face by applying a drop of epoxy (Epon 828 with 14% mphenylenediamine, Miller Stephenson, Chicago, IL) to the conductor surface and placing a clean glass slide (1.9 cm x 1.9 cm)
over the epoxy. One edge of this assembly forms the band
electrode. Electrical contact was made at the other end with a
wire attached by silver epoxy (Epo-Tek H20E, Epoxy Technology
Inc, Billerica, MA) to the remaining exposed gold/glass surface.
The electrical contact and a portion of the wire were then completely covered with silicon rubber. The end of the assembled
electrodewas exposed by using 320 sandpaper. The exposed band
was then ground flat with 1000 grit carborundum and 30 pm
sandpaper. The electrode surface was subsequently polished with
6.0 and 1.0 pm diamond paste (Buehler, Ltd., Evanston, IL) and
0.05 pm alumina (Fisher Scientific, Cincinnati,OH). Before daily
use, each electrode was polished briefly with 0.05 pm alumina and
washed consecutivelywith distilled water, acetonitrile,and distilled
water. The limiting current normally was reproducible to within
10% after each polish. Polishing with diamond paste in the final
step led to irreproducible limiting currents, presumably because
the paste is difficult to remove from the electrode surface. Excessive polishing with 0.05 pm alumina was avoided since the
current response decreases and becomes peak-shaped, indicating
that the metal becomes recessed into the insulating material.
Gold films on a Mylar support were purchased from Goodfellow
Metals, Cambridge,UK. The thickness of the film was given by
the manufacturer as 5 nm, and the nominal thickness was used
in all calculations. The Mylar supported gold film was sealed
between two glass slides by the same process described above.
Films of 1-nmnominal thickness were found to be nonconductive.
Glass microscope slides (2.5 X 7.5 cm) coated with a 30-nm carbon
film were purchased from a commerical source (Lebow Co., Goleta,
CA). A resistance of 7500 Q was reported by the manufacturer
for each slide. The slides were cut into smaller pieces and the
carbon removed from the edges of the slide so that a strip of
carbon 0.6-1.0 cm in length remained in the center of each piece.
Carbon band electrodes were prepared as described above.
An alternate method of preparing platinum band electrodes
involved in use of a soft glass tube (3.0 mm 0.d.) which was cleaned
with isopropyl alcohol and coated with a -30 nm film of platinum
by sputtering. The coated tube was then sealed inside a slightly
larger soft glass tube using an air/gas flame. Electrical contact
and polishing were carried out as described above.
Electrochemical Apparatus. A homebuilt potentiostat was
used to obtain voltammograms from 5 mV s-l to 200 V s-l (3).
A PARC 174A polarograph analyzer (Princeton Applied Research
Corp., Princeton, NJ) was used for all other electrochemical
experiments. Voltammograms were recorded on a flat bed X-Y
recorder (Houston Instruments, Austin, TX) or if scan rates
exceeded 500 mV s-l on a 10MHz storage oscilliscope (Tektronix,
Inc., Beaverton, OR). Conventionally sized gold and platinum
electrodes (2.0 mm diameter disks) in an insulating Teflon sheath
were purchased from BioanalyticalSystems, Inc., West Lafayette,
IN. The electrodes were polished with 0.05-pm alumina before
each run. The use of the same polishing techniques as used with
band electrodes facilitates comparisons between electrodes of
different sizes. The electrochemical cell was a 50-mL beaker which
was covered with Parafilm through which holes were made for
electrodes, A platinum wire auxiliary and a saturated calomel
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Figure 1. Cyclic voltammograms for the reduction of 1.3 mM (Ru(NH3)2+in 0.1 M phosphate buffer at a 5-nm gold band electrode at
various scan rates.
i (nA)
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1
1
1
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-2
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2
log v ( V S ” )
Figure 2. Dependence of the maximum current on scan rate for the
reduction of 1.3 mM Ru(NH3)2+ in 0.1 M phosphate buffer at two
different gold band electrodes.
electrode (SCE) were employed. For reductions, solutions were
purged with argon which was passed through an ammonium
vanadate scrubber and distilled water. Trace determination of
ruthenium(II1)hexaammine was performed inside a Faraday cage.
The diffusion coefficient for this compound in 0.1 M phosphate
buffer is 6.0 X lo4 cm2 s-l as determined from the limiting
steady-state current at a 10 pm diameter carbon disk (3).
RESULTS AND DISCUSSION
Voltammetric Behavior. Voltammetric electrodes with
a band geometry exhibit significant contributions from nonlinear diffusion ( I 7). Therefore, voltammograms at these
electrodes have a sigmoidal shape at slow scan rates. As the
scan rate is increased linear diffusion predominates and the
voltammograms become peak shaped (Figure 1). The contribution from linear diffusion is greatest for thick band
electrodes, as can be seen from a plot of the maximum current
as a function of scan rate (Figure 2). This is because transport
of molecules in a radial direction at a thin band electrode
provides a larger proportion of the total flux than at a larger
electrode.
Limiting Current. The enhanced diffusive mass transport
at band electrodes results in very large current densities. For
example, the steady-state current density obtained a t the 5
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
‘t
1
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0
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Flgure 4. Cyclic voltammograms for the reduction of 3.0 mM FHCN),”
in (a, c) 1.0 M KCI and (b, d) 0.1 M KCI at gold band electrodes: scan
rate, 50 mV/s.
log w(cm)
Flgwe 3. Maximum voltammetric cunents for the reductbn of 9.7 mh4
Ru(NH,)t+ in 0.1 M phosphate buffer at gold band electrodes (current
m l k e d by electrode length)as a function of the electrock bandwidth
(circled points). Voltammograms recorded at 20 mV s-’scan rate.
The sdld line was calculated from eq 1 using r = w l r , t = 15 8, and
D = 8.0 X lo-’ cm2/s. The C denotes a carbon band electrode.
nm band electrode is 0.25 A cm-2 (Figure l),as calculated from
the manufacturer’s stated thickness and the measured length.
In contrast, the current density at a conventional rotating disk
electrode in the same solution would be 0.0005 A cm-2, at a
rotation speed of 20 000 rpm. It has been shown previously
that the limiting voltammetric current at band electrodes of
5 to 20 pm thickness could be predicted by eq 1 when the
radius of the hemicylinder is replaced with w / r , where w is
the thickness of the band (17). The voltammetric limiting
currents at band electrodes of thicknesses from 30 to 2000 nm
show good agreement with the currents calculated from eq
1(Figure 3). Due to the inverse logarithmic relationship the
current is largely independent of the electrode radius-a
decrease in the electrode radius by a factor of lo3 results in
only about a 3-fold decrease in the voltammetric current. For
the electrode of 5 nm nominal width less current is observed
than is expected. This may occur because eq 1is inappropriate
for an electrode of this small size or because a portion of the
electrode surface is blocked. Since an electrode of this size
approaches molecular dimensions, unusual phenomena may
occur.
Electrode Kinetics. As thG current density increases
because of increased mass transport, finite rates of electron
transfer become more apparent in the electrochemical response (22). This forms the basis for a variety of methods
used to measure heterogeneous rate constants (23). It has been
demonstrated that the increase in diffusional transport caused
by a decrease in the radius of a microdisk electrode can be
used for such measurements (24,25). Although exact equations are not available for band electrodes, it would be anticipated that the effect of electrode kinetics would be more
apparent for these electrodes than for electrodes of conventional size.
The effect of electron transfer rates on the voltammetric
shape at band electrodes was investigated with ferricyanide
solutions containing different amounts of KCI. The heterogenous rate constant for the ferrilferrocyanide couple has
been shown to increase linearly with KCl concentrations (26).
At a gold disk electrode of conventional size the standard
heterogeneous rate constant for the reduction of Fe(CN)63was measured to be 3.6 X
cm s-l in 0.1 M KCl and 2.8
X
cm s-l in 1.0 M KC1 (rate constants were determined
using the method of Nicholson, ref 27). At a scan rate of 50
mV s-l this corresponds to a separation of the anodic and
cathodic waves of 95 and 66 mV, respectively. Voltammograms in these solutions were also obtained with the 36 nm
and 2300 nm bold band electrodes (Figure 4). A dramatic
change in shape of the voltammogram is seen for the thinnest
electrode, indicative of its sensitivity to the rate of heterogenous electron transfer.
Electrical Properties. The resistivity of vacuum deposited thin films of gold or platinum is in the range of 10-25
D cm (28,29),and thus does not contribute significantly to
the overall cell resistance. The expression for the solution
resistance of a line electrode has been approximated by considering the case of two concentric hemicylindrical electrodes
(30)
P
R, = - In ( r 2 / r 1 )
Tl
where p is the specific resistance, r2 is the radius of the large
hemicylinder, and rl is the radius of the smaller hemicylinder.
Combination of eq 1 with eq 2 indicates that the iR drop at
hemicylindrical electrodes is relatively insensitive to the dimensions of the electrode under conditions where eq 1 is
appropriate. Thus is appears that a decrease in thickness of
the electrode should not increase the iR drop, in accord with
the experimental data.
The residual current (composed of capacitive current and
other contributions) was evaluated by comparing the current
density obtained at gold and platinum electrodes of conventional size in 0.1 M KN03 solution with that obtained at the
band electrodes. The residual current density was 2.2 X
and 1.8 X
A cmW2(at 0.4 V vs. SCE) at the large platinum
and gold electrodes, respectively. The gold and platinum films
prepared locally, as well as the commercial 30-nm carbon film,
showed an inverse relationship between thickness and residual
current density. The residual current density with the smallest
thickness (-40 nm) examined was 100 times greater than that
at the large electrode under the same conditions. Close inspection of the data indicated a resistive component in the
voltammograms, possibly because of an imperfect seal between
the electrode and the insulating material. As the electrode
thickness is decreased, the area to perimeter ratio is also
decreased, and such problems would be expected to be more
severe. Electron micrographs of the surface show regions
where an imperfect seal exists along the length of the band.
Defects or porosity in the locally constructed films could also
lead to the observed results (31,32). The effect of the insulating material or surface roughness caused by polishing on
the double layer capacity at very small thicknesses may also
play a role.
Voltammograms obtained at the electrode fabricated from
the commercial 5-nm gold film and the platinum film sealed
in glass (-30 nm thickness) did not exhibit as large of a
resistive behavior. For these electrodes the residual current
density was -30 times greater than that at the electrodes of
conventional size.
Trace Analysis. Cyclic voltammetry is regarded as a
qualitative rather than a quantitative tool because of ita poor
detection limits. The detection limit for all voltammetric
methods is ultimately limited by the magnitude of the residual
1916
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
0
CONCLUSIONS
Band electrodes with geometries that are microscopic in one
dimension and macroscopic in another dimension have been
shown to have many of the properties of disk shaped microelectrodes due to a significant amount of nonlinear diffusion. However, due to their macroscopic length, measurable
currents are obtained at ultrasmall bandwidths. These
electrodes provide a useful tool to study the effect of extremely
high current density. Furthermore, they show properties of
great utility for electroanalysis because of their small area and
high current density.
b
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ACKNOWLEDGMENT
The technical assistance of Susan Richardson and Frank
Qualls is gratefully acknowledged.
E ( V v s SCE)
Flgure 5. Cyclic voltammograms for the reduction of (a) 45 pM Ru(NH3)t+ in 0.1 M KNO, at a 2 mm diameter platinum disk (scan rate,
5 mV s-‘) and (b) 0.37 p M Ru(NH,):’
in 0.1 M KNO, at a -30-nm
platinurn band electrode sealed in soft glass (scan rate, 2 mV s-’),
current. Residual or background currenta have several sources:
electrolysis of the electrolyte or solvent, double layer charging,
and redox processes of the electrode surface. Pulse techniques
employing various potential wave forms have been used to
provide a discriminationagainst double layer charging current
with a resulting decrease in the detection limit to the lo-’ to
M range (33). However, Evans (34) has pointed out that
the detection limit of pulse techniques is determined by the
oxidation and reduction of surface groups.
The data in this paper show that the faradaic current at
a band electrode is only slightly dependent on the electrode
thickness. In contrast, the current arising from the double
layer charging and the electrolysis of surface groups should
be directly proportional to the electrode area. Thus,a decrease
in the width of the band electrode should result in a decrease
in the residual current with a concurrent improvement in the
detection limits. As has been shown, some of the band
electrodes have approached the values given by these predictions. Platinum films sealed in glass exhibited the best
performance and thus these were used for trace determinations. In Figure 5 voltammograms for 0.37 pM and 43 pM
solutions of ruthenium hexaammine at the platinum line and
a conventional 2 mm diameter platinum disk electrode are
compared. The platinum line electrode gave a well-defined
voltammogram whereas the voltammogram from the conventional platinum electrode is largely composed of background current. Limiting currents from the band electrode
were linear with the concentration of ruthenium hexaammine
up to 12 p M , the highest concentration examined (r = 0.9999,
n = 8). The lowest concentration examined was 7.1 X
M ruthenium hexaammine, and at this concentration the
residual current was twice the value of the faradaic current
at a scan rate of 2 mV s-l. The range of concentration detectable by cyclic voltammetry with the platinum band
electrode compares favorably with the detection limits obtained with conventionally sized electrodes using the more
exotic pulse potential wave forms. It should be noted that
these results are only an indication of the potential capabilities
of line electrodes. If a means of fabricating band electrodes
which give the expected values for the residual current is
achieved, it can be easily seen that detection limits in the IO”
to
M range should be possible by linear scan voltammetry.
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RECEIVED
for review February 21, 1985. Accepted April 9,
1985. This research was supported by USARO and NSF.
R.M.W. is an Alfred P. Sloan Fellow.