An ultramicroelectrode assembly which generally comprises an electrically nonconducting host membrane having a plurality of micro-sized pores extending through the membrane, and a macro-sized substrate electrode in contact with this host membrane. The host membrane has its pores impregnated with an electrically conductive medium, and the substrate electrode is in electrical contact with the impregnated pores of the host membrane. The electrically conductive medium is a carbon paste, and the substrate electrode includes a confined volume of such carbon paste in contact with an interior surface of the impregnated membrane.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates generally to electrochemical electrode 
constructions, and particularly to ultramicroelectrode structures and 
methods of constructing such ultramicroelectrode structures. 
An "ultramicroelectrode" may be defined as an electrode which has at least 
one dimension which is 10 .mu.m or less. Thus, for example, an 
ultramicroelectrode may be a disk with a radius less than or equal to 10 
.mu.m, or a ring having its electrically conductive width less than or 
equal to this dimension. Alternatively, the ultramicroelectrode could be a 
band of undetermined length, which has a width less than or equal to 10 
.mu.m. In contrast, a "microelectrode" typically has its dimensions on the 
order of one to several millimeters. 
Ultramicroelectrodes are useful for several reasons. Firstly, 
electrochemical experiments can be conducted in highly electrically 
resistive media. Secondly, chemical or electrochemical processes which are 
too fast for larger electrodes can be studied using an 
ultramicroelectrode. Additionally, the mass transport rate to an 
ultramicroelectrode is higher than that can be achieved using larger 
electrodes. Furthermore, lower detection limits can be achieved using an 
ultramicroelectrode, due to its higher signal-to-background ratio. 
Finally, steady-state signals, as opposed to transient signals can often 
be obtained with ultramicroelectrodes. 
While ultramicroelectrodes provide enhanced analytical sensitivity in 
comparison with other electrode structures, one disadvantage of the 
ultramicroelectrode is that only very small currents can be obtained at 
such a minuscule electrode. However, this problem can be solved by 
constructing an ultramicroelectrode array or ensemble. In this regard, the 
term array is sometimes used to refer to assemblies where a plurality of 
ultramicroelectrodes are evenly spaced from each other, while the term 
ensemble is sometimes used to indicate that the ultramicroelectrode 
elements are not necessarily evenly spaced from each other. In any case, 
the total measured current from such ultramicroelectrode assemblies is the 
sum of the currents obtained at each of the ultramicroelectrode elements 
in the assembly. 
One pertinent example of an ultramicroelectrode ensemble is discussed in 
"Preparation and Electrochemical Characterization of Ultramicroelectrode 
Ensembles", by Reginald M. Penner and Charles R. Martin, Anal. Chem. 1987, 
59, 2625-2630. This article discloses a procedure for preparing 
ultramicrodisk electrode ensembles, and is hereby incorporated by 
reference. Specifically, this article discloses a method in which platinum 
is electrochemically deposited into the pores of a microporous 
polycarbonate host membrane until the platinum layer begins to overgrow 
the surface of the host membrane. The surface of this composite membrane 
is then impregnated with polyethylene by immersion, and the polyethylene 
and excess platinum are subsequently removed by polishing. This 
ultramicrodisk membrane is then stretched over a convex electrode, and 
held in place with a sleeve of heat shrinkable Teflon tubing. The host 
membrane used in this procedure is a Nuclepore .RTM. polycarbonate 
membrane from Nuclepore, Inc. 
It is a principal objective of the present invention to provide an 
ultramicroelectrode assembly and method of construction which does not 
require an electrochemical deposition step, nor requires the use of 
precious metals in the construction. 
It is also a principal objective of the present invention to demonstrate 
that an ultramicroelectrode assembly can yield lower electroanalytical 
detection limits. 
It is another objective of the present invention to provide a quick and 
inexpensive method of making an ultramicroelectrode assembly. 
It is an additional objective of the present invention to provide an 
ultramicroelectrode assembly which is capable of demonstrating all three 
of the theoretically predicted electrochemical response limiting cases. 
It is a further objective of the present invention to provide an 
ultramicroelectrode assembly which exhibits low capactive currents. 
To achieve the foregoing objectives, the present invention provides an 
ultramicroelectrode assembly which generally comprises an electrically 
nonconducting host membrane having a plurality of micro-sized pores 
extending through the membrane, and a macro-sized substrate electrode in 
contact with this host membrane. The host membrane has its pores 
impregnated with an electrically conductive medium, and the substrate 
electrode is in electrical contact with the impregnated pores of the host 
membrane. The electrically conductive medium is a carbon paste, and the 
substrate electrode includes a confined volume of such carbon paste in 
contact with an interior surface of the impregnated membrane. 
The method of making an ultramicroelectrode assembly according to the 
present invention includes the steps of preparing the carbon paste, 
forcing the carbon paste into and through the pores of the host electrode, 
and joining the impregnated membrane to the substrate electrode. This 
assembly may also be heated in an oven to improve the adhesion between the 
impregnated membrane and the carbon paste substrate electrode. 
Additional advantages and features of the present invention will become 
apparent from a reading of the detailed description of the preferred 
embodiments which makes reference to the following set of drawings in 
which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, an ultramicroelectrode assembly 10 according to the 
present invention is shown. The ultramicroelectrode assembly 10 is 
generally comprised of a host membrane 12 which is supported by a 
macro-sized substrate electrode 14. As will be more fully discussed below, 
the host membrane is impregnated with a carbon paste. The substrate 
electrode 14 is referred to as being "macro-sized" in that its dimensions 
are several orders of magnitude greater than the dimensions of the 
"micro-sized" pores of the host membrane 12. Thus, for example, the 
substrate electrode 14 includes a glass tube housing 16 which may have a 
radius measured in centimeters; whereas, the pores of the host membrane 12 
will have a radius which is equal to or less than ten micrometers. 
The host membrane 12 is stretched over one end of the tube 16 and held in 
place by annular member 18. In one embodiment according to the present 
invention, the annular member 18 is an elastomeric band or o-ring. The 
substrate electrode 14 may also include another glass tube 20 which is 
joined to the opposite end of tube 16 via heat shrinkable Teflon tube 22. 
However, it should be appreciated that the principals of the present 
invention are not dependent upon any particular housing construction for 
the substrate electrode 14. 
A copper wire 24 is used to provide an electrical connection between the 
ultramicroelectrode assembly 10 and the conventional electrochemical 
instrumentation which employs the ultramicroelectrode assembly. Electrical 
contact between the copper wire 24 and the substrate electrode 14 may be 
provided by any suitable means, such as a drop 26 of mercury placed on top 
of the carbon paste 28. Electrical contact between the substrate electrode 
14 and the host membrane is provided by placing a supply of carbon paste 
28 in the end of the tube 16, such that this paste covers substantially 
the entire interior surface 30 of the host membrane 12. 
In this regard, FIG. 2 shows a greatly magnified cross-sectional view of a 
small portion of the interface between the host membrane 12 and the 
substrate electrode 14. As may be seen in FIG. 2, the host membrane 12 
includes an array or ensemble of micro-sized pores 32 having a generally 
cylindrical shape. The pores 32 extend completely through the host 
membrane 12 and are filled with a carbon paste 34, such that each 
impregnated pore creates an ultramicroelectrode. The pores 32 are 
separated by the electrically nonconductive regions 36 of the host 
membrane 12, such that the active surface area of the ultramicroelectrode 
assembly 10 is only a fraction of the geometric surface area of the host 
membrane. 
In one form of the present invention, it is preferred that the carbon paste 
28 and the carbon paste 34 be substantially identical in composition. FIG. 
2 also illustrates that the carbon paste ultramicroelectrodes 34 are in 
such substantial or intimate contact with the carbon paste 28 of the 
substrate electrode that there is essentially no boundary between them. 
Thus, the carbon paste 28 of the substrate electrode 14 will preferably be 
an extension of the ultramicroelectrodes 34, and this construction will 
provide a convenient way of collecting the currents obtained at each of 
the ultramicroelectrodes 34. 
With respect to the material used for the host membrane 12, it is preferred 
that the host membrane 12 be a Nuclepore .RTM. polycarbonate membrane, as 
these membranes provide linear, cylindrical pores of nearly uniform pore 
diameter. The specifications for one example of a suitable Nuclepore .RTM. 
membrane are set forth in the Table below: 
TABLE 
__________________________________________________________________________ 
Pore.sup.a 
Pore.sup.a Fractional.sup.a,b 
Thickness a 
Ave. dis..sup.c 
radius (.mu.m) 
density (pores cm-2) 
pore area 
(.mu.m) 
between pores (.mu.m) 
__________________________________________________________________________ 
4.0 1 .times. 105 
0.05 10 24 
__________________________________________________________________________ 
.sup.a From Nuclepore, Inc. product literature. Nominal precision of pore 
diameter is +0% to -20%. Nominal precision of pore density is +/- 15%. 
.sup.b Surface area of pores divided by total surface of the membrane. 
Since the pores of the membrane become the ultramicroelectrode elements, 
this fraction is the electrochemically active area divided by the 
geometric area. 
.sup.c Average distances are calculated from pore radii and densities, 
assuming regularly spaced pores. 
While Nuclepore .RTM. membranes are preferred, it should be understood that 
other suitable electrically nonconductive membrane materials may be used, 
providing they are inert to the chemistry of the application and have the 
pore structures necessary to create the ultramicroelectrode elements. 
Thus, for example other polymer materials may be used after they have been 
bombarded with fission fragments and chemically etched to enlarge the 
tracks created into micro-sized pores. Inorganic materials such as mica 
may also be used. 
With respect to the method of making the ultramicroelectrode assembly 10 
according to the present invention, the carbon paste was first prepared by 
intimately mixing carbon powder (Carbopack C, 80/100 mesh, Supelco, 
Belletonte, PA) with silicon-based high vacuum grease (Dow Corning) using 
a glass mortar and pestle. The Supelco carbon was selected for its small 
uniform size spherical particles and high electrical conductivity. In one 
example according to the present invention, 1.0 grams of the carbon powder 
was mixed with 1.0 grams of the vacuum grease. While in another example, 
0.42 grams of the carbon powder was mixed with 0.20 grams of the vacuum 
grease. In general, the ratio of carbon power to vacuum grease should be 
such as to provide sufficient electrical conductivity, while still 
providing adequate binding of the carbon particles. 
The Nuclepore .RTM. membrane 12 was stripped of its wetting agent, 
polyvinylpyrrolidone, by sonicating in glacial acetic acid and followed by 
washing with purified water. The water was purified by passing distilled 
water through a Milli-Q water purification system. This stripping step was 
used to insure that that any aqueous solution did not leak into the 
interface between the host membrane 12 and the substrate electrode 14. 
Nuclepore .RTM. membranes generally have a smooth (shiny) face and a rough 
(dull) face. The carbon paste was applied to the dull face of the membrane 
12, and the pestle was used to rub the paste into and through the pores 32 
of the membrane. This procedure was continued until the carbon paste began 
to leak out from the opposite (shiny) side of the membrane 12. The carbon 
paste-impregnated membrane 12 was then applied to the surface of the 
carbon paste substrate electrode 14, with the dull surface of the membrane 
facing the carbon paste 28 of the substrate electrode. Excess carbon paste 
was removed from the exposed surface of the impregnated membrane 12 by 
polishing this surface on a sheet of glycine paper. 
The assembly 10 was then heated in an oven at 100.degree. C. for 20 minutes 
to improve the adhesion between the membrane 12 and the carbon paste 
substrate electrode 14. The surface of the impregnated membrane 12 was 
then polished once more while it was still warm. It should be noted that 
if any defects are observed in the seal between the interior surface 30 of 
the impregnated membrane 12 and the carbon paste substrate electrode 14, 
more carbon paste may be applied to the exposed surface of the membrane to 
fill in such defects. The membrane 12 may then be repolished. 
In this regard, it should be noted that one of the advantages of this 
carbon paste construction is that it permits the active surface area of 
the ultramicroelectrode assembly to be renewable by polishing off old 
paste from the surface and forcing new paste into the pores 32 of the 
membrane 12. It should also be appreciated that this carbon paste 
construction is very inexpensive, and it permits the ultramicroelectrode 
assembly to be molded or formed into a variety of desirable shapes. 
While carbon powder has been found to be quite effective, it may be 
possible to construct ultramicroelectrode assemblies according to the 
present invention using other electrically conductive, micro-sized 
particles. Thus, for example, it may be possible to use carbon black or 
furnace black. Similarly, it may be possible in the appropriate 
application to use other suitable electrically nonconducting materials as 
a binder, besides high vacuum grease. For example, polymeric materials 
such as polyethylene or polypropylene may serve as the binder. However, 
the binding material chosen must be capable of adhering to the surface of 
the membrane 12. Additionally, it should be appreciated that the use of 
highly viscous fluids, such as high vacuum grease, provide a particularly 
advantageous non-conductive material for suspending the carbon particles. 
Referring generally to FIGS. 3 and 4, the ability of the 
ultramicroelectrode assembly 10 to yield enhanced sensitivity in 
electroanalysis will now be discussed. First, with respect to the examples 
of FIG. 3A and FIG. 3B, the ultramicroelectrode assembly 10 and a carbon 
paste macro-sized electrode were alternatively placed into an analyte 
solution containing 0.5M H.sub.2 SO.sub.4 and various concentrations of 
Fe(CN)6.sup.-4. A suitable reference electrode (e.g., a saturated calomel 
electrode) and a counter electrode (e.g., a Pt-Flag electrode) were also 
placed into the solution. Linear sweep voltammetry was used to effect the 
oxidation of this ion to Fe(CN)6.sup.3- for various concentrations of this 
ferrocyanide analyte. 
FIG. 3A illustrates a calibration curve of the peak current obtained as a 
function of the concentration of ferrocyanide, using 3.0 .mu.m 
ultramicroelectrode elements. In this regard, it should be observed that 
this calibration curve is linear down to at least 10.sup.-7 M. This 
detection limit is on the order of seven times lower than the detection 
limit obtained using the macro-sized electrode (0.071 cm2 electrode area), 
as shown in FIG. 3B. 
The capacitive (background) current arises from double layer charging at 
the electrode surface. This background can limit the detection of 
electroactive species by masking the faradaic current. This is 
demonstrated in FIGS. 4A and 4B, where the solution is 0.12 .mu.m 
ferrocyanide, and the faradaic signal results from the oxidation of 
ferrocyanide. As shown in FIG. 4B, the faradaic signal is barely 
perceptible over the background using the macro-sized electrode. However, 
with the ultramicroelectrode assembly 10, FIG. 4A shows that the faradaic 
signal is considerably more apparent over the background. By decreasing 
the active area of the electrode it is possible to decrease the capacitive 
current and allow the faradaic signal to remain the same. This is because 
capacitive currents are directly proportional only to the 
electrochemically active area of the electrode, whereas the faradaic 
signal is proportional to the total (active plus inactive) area of the 
electrode at slow scan rates (e.g., 5 mV/S). 
Referring now to FIG. 5A-5C, three cyclic voltammograms are shown to 
demonstrate that the ultramicroelectrode assembly 10 is capable of 
verifying all three theoretical limiting cases. In this regard, these 
cyclic voltammograms were obtained using a C model 175 programmer in 
conjunction with a C 173 potentiostat, and either a Soltec VP-64245 X-Y 
recorder or a Nicolet 206 digital oscilloscope. Again, a three electrode 
cell comprising a carbon paste working electrode (either assembly 10 or 
the macro-sized electrode), a Ag/AgCL reference electrode and a Pt-Flag 
counter electrode was used. 0.1M KCL served as the supporting electrolyte. 
The shape of the cyclic voltammogram using an ultramicroelectrode assembly 
depends on the time scale (scan rate) of the experiment. The first 
limiting case is obtained at very high scan rates (e.g., greater than 100 
V/s), where the diffusion layers at the ensemble elements are thin, linear 
and completely isolated. This linear diffusion situation produces a 
conventional, peak-shaped voltammogram, where currents are proportional to 
the electrochemically active surface area. 
The second limiting case occurs at lower scan rates (e.g., 0.5 V/s), where 
the diffusion layers take on a radial character. This radial diffusion 
field yields sigmoidal voltammograms. The final limiting case occurs at 
very low scan rates (e.g., less than 10 mV/s) where the radial diffusion 
layers merge, yielding a net linear diffusion field. Because linear 
diffusion is obtained, a peak-shaped voltammogram is again observed. 
However, in this case, currents are proportional to the entire geometric 
area of the membrane for the ultramicroelectrode assembly. 
In this regard, FIG. 5A shows slow scan voltammogram for ferrocenylmethyl 
trimethyl ammonium at the ultramicroelectrode assembly 10 and at the 
carbon paste macro-sized electrode of equivalent geometric area. A 
conventional, peak-shaped voltammogram is obtained at the assembly 10, and 
peak currents at the assembly 10 and macro-sized electrode are almost 
identical. This demonstrates that the assembly 10 can achieve the third 
limiting. 
At the higher scan rates of FIG. 5B, the ferrocenylmethyl trimethyl 
ammonium voltammogram takes on a sigmoidal shape, indicating approach to 
the second limiting case. FIG. 5C shows voltammograms at very high scan 
rates. In agreement with the predictions of the third limiting case, the 
assembly 10 voltammogram again becomes peak-shaped and current is 
proportional to only the electrochemically active area. 
It will be appreciated that the above disclosed embodiment is well 
calculated to achieve the aforementioned objects of the present invention. 
In addition, it is evident that those skilled in the art, once given the 
benefit of the foregoing disclosure, may now make modifications of the 
specific embodiment described herein without departing from the spirit of 
the present invention. Such modifications are to be considered within the 
scope of the present invention which is limited solely by the scope and 
spirit of the appended claims.