Anal. Chem. 2001, 73, 5346-5351
Fabrication of Nanometer-Sized Electrodes and
Tips for Scanning Electrochemical Microscopy
Peng Sun, Zhiquan Zhang, Jidong Guo, and Yuanhua Shao*
Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences,
Changchun, Jilin 130022, China
A novel method for fabrication of nanometer-sized electrodes and tips suitable for scanning electrochemical
microscopy (SECM) is reported. A fine etched Pt wire is
coated with polyimide, which was produced by polymerization on the Pt surface initiated by heat. This method
can prepare electrodes with effective radii varying from a
few to hundreds of nanometers. Scanning electron microscopy, cyclic voltammetry, and SECM were used to
characterize these electrodes. Well-defined steady-state
voltammograms could be obtained in aqueous or in 1,2dichloroethane solutions. This method produced the
nanoelectrodes with exposed Pt on the apex, and they can
also be employed as the nanotips for SECM investigations.
Different sizes of Pt nanotips made by this method were
employed to evaluate the kinetics of the redox reaction of
Ru(NH3)63+ on the surface of a large Pt electrode by
SECM, and the standard rate constant K0 of this system
was calculated from the best fit of the SECM approach
curve. This result is similar to the values obtained by
analysis of the obtained voltammetric data.
During the last two decades, there has been growing interesting in using of nanoelectrodes in electrochemical investigations.1,2
The advantages arise from the fact that these nanoelectrodes can
reduce iR drop and double-layer charging effects.3,4 Additionally,
the rate of mass transport at these electrodes is so high that fast
heterogeneous electron-transfer kinetic measurements would be
carried out by steady-state measurements rather than by transient
techniques, which provided more reliable information and simpli* To whom the correspondence should be addressed. E-mail: yhshao@
ns.ciac.jl.cn.
(1) (a) Penner, R. M.; Heben, M. J.; Lewis, N. S. Anal. Chem. 1989, 61, 16301636. (b) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science
1990, 250, 1118-1121.
(2) (a) Morris, R. B.; Franta, D. J.; White, H. S. J. Phys. Chem. 1987, 91, 35593564. (b) Norton, J. D.; White, H. S.; Feldberg, S. W. J. Phys. Chem. 1990,
94, 6772-6780. (c) Conyers, J. L.; White, H. S. Anal. Chem. 2000, 72,
4441-4446.
(3) (a) Heinze, J. Angew. Chem., Int. Ed. Engl. 1993, 32, 1268-1288. (b)
Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J.,
Ed.; Marcel Dekker: New York, 1989; Vol. 15. (c) Fleischmann, M.; Pons,
S.; Rolison, D. R.; Schmidt, P. P. Ultromicroelectrodes; Datatech Systems,
Inc: Morganton, NC, 1987; Chapter 3.
(4) (a) Howell, J. O.; Wightman, R. M. Anal. Chem. 1984, 56, 524-529. (b)
Bond, A. M.; Fleischmann, M.; Robinson, J. J. Electroanal. Chem. 1984,
168, 299-312.
5346 Analytical Chemistry, Vol. 73, No. 21, November 1, 2001
fied data analysis.5 The nanoelectrodes also show promising
applications in the study of microenvironmental and biological
specimens.6,7
Scanning electrochemical microscopy (SECM) is one of the
major developments in electrochemistry in the past decade.8 It
combines the advantages of ultramicroelectrodes (UMEs) and
scanning probe microscopes (SPMs). Many electrochemical
techniques can be implemented in situ with additional high spatial
resolution with SECM. It has been proved to be a powerful tool
in investigation of fast heterogeneous kinetics on electrode
surfaces, microfabrication, and imaging of local electrochemical
reactivity. The attainable information by SECM strongly depends
on the type and size of the tips used, which is normally an UME
or a micro-ion-selective electrode (MISE). To obtain nanometer
resolution, and completely eliminate the iR effect in many lowconductivity solvents, fabrication of nanometer tips is desirable
and the trend of development.5 Crooks et al.9 recently reported
that single carbon nanotubes could be used for electrochemical
studies. The problem is that it is not easy to handle this tiny
creature. Nanopipets have also been used to evaluate the fast
kinetics of facilitated potassium ion transfer across a water/1,2dichloroethane (DCE) interface by dibenzo-18-crown-6 (DB18C6),
and it can be easily handled and fabricated.10a Nevertheless, it is
confined only to the study of charge transfer across liquid/liquid
interfaces.
Pt has been used widely in electrochemical investigations.
However, at present only a few attempts to make Pt electrodes
with the radii in nanometer ranges have been reported.1b,2,10b-12
Fabrication of a nanometer Pt electrode usually involves two major
steps. First, a Pt wire is electrochemically etched to an ultrafine
(5) (a) Oldham, K. B.; Myland, J. C.; Zoski, C. G.; Bond, A. M. J. Electroanal.
Chem. 1989, 270, 79-101. (b) Mirkin, M. V.; Fan, F.-R. F.; Bard, A. J. J.
Electroanal. Chem. 1992, 328, 47-62.
(6) (a) Pihel, K.; Schroeder, T. J.; Wightman, R. M. Anal. Chem. 1994, 66,
4532-4537. (b) Cahill, P. S.; Walker, Q. D.; Finnegan, J. M.; Mickelson, G.
E.; Travis, E. R.; Wightman, R. M. Anal. Chem. 1996, 68, 3180-3186. (c)
Giros, B.; Jaber, M.; Jones, S. R.; Wightman, R. M. Nature 1996, 379,
606-611.
(7) (a) Chien, J. B.; Wallingford, R. A.; Ewing, A. G. J. Neurochem. 1990, 54,
633-637. (b) Lau, Y. Y.; Chien, J. B.; Wong, D. K. Y.; Ewing, A. G.
Electroanalysis 1991, 3, 87-95.
(8) (a) Lee, C.; Miller, C. J.; Bard, A. J. Anal. Chem. 1991, 63, 78-83. (b)
Mirkin, M. V.; Arca, M.; Bard, A. J. J. Phys. Chem. 1993, 97, 10790-10795.
(9) Campbell, J. K.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 37793783.
(10) (a) Shao, Y.; Mirkin, M. V. J. Am. Chem. Soc. 1998, 119, 8103-8104. (b)
Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Plalanker, D.; Lewis, A. Anal.
Chem. 1997, 69, 1627-1634.
10.1021/ac010474w CCC: $20.00
© 2001 American Chemical Society
Published on Web 09/22/2001
point. This is easy to archive by using several environmentally
benign etching solutions.13 Second, the etched ultrafine Pt wire
is coated with an insulating material, except at the apex of the
wire. Although a number of coating materials have been employed,
such as Apiezon wax,14 varnish,15 molten paraffin,16 glass,1,10b,12
poly(R-methylstyrene),1a silica coatings,17 electrochemically polymerization of a layer of phenol,18 and electrophoretic paint,2c,11
these insulating procedures have other problems. Either the timeconsuming process19 or the easily cracking of this material14a
prevents these methods from wide use. Some of these materials
cannot be used in organic solvents. Recently, Conyers and White2c
fabricated submicroelectrodes following the method proposed by
Unwin et al.11a to cover the etched Pt wires by electrophoretic
paint. They demonstrated that well-defined cyclic voltammograms
could be obtained. Various sizes of Pt nanoelectrodes can also be
made using a laser puller. By this way, the fabricated nanoelectrodes could be employed as SECM tips.10b Another point needing
to be emphasized is that only a few methods mentioned previously
can fabricate nanoelectrodes that are suitable for SECM studies
because the nanometer-sized Pt wire must be on the apex of the
electrode for a tip of SECM.5b,10b,11a,c
In this paper, we describe a simple, inexpensive method to
produce Pt nanometer-sized electrodes by coating etched Pt wire
with polyimide produced by the polymerizing reaction. The
effective radii of the electrodes fabricated this way vary from a
few to hundreds of nanometers. The nanoelectrodes can be used
in aqueous or organic solvents and have a wide potential window
in both solutions. This method overcomes the defects in some of
the previous methods.2c,11,14-18 Scanning electron microscopy
(SEM) images and SECM experimental results indicated that the
fabricated nanoelectrodes can also be used as the SECM tips.
EXPERIMENTAL SECTION
Chemicals and Apparatus. Hexaammineruthenium(III) chloride ((Ru(NH3)63+, 98%, Aldrich), 7,7,8,8-tetracyanoquinodimethane
(TCNQ, 98%, ACROS), and potassium ferricyanide (K3Fe(CN)6,
A. R. Beijing Chemicals, Co., China) were used. 1,2,4,5-Benzenetetracarboxylic anhydride, 4,4′-diaminodiphenyl ether, 1,2-dichloroethane (DCE), N,N-dimethylacetamide, potassium chloride,
cadmium chloride, and hydrochloric acid were supplied by
Shanghai Chemicals Co., and they are all analytical grade or better.
1,2,4,5-Benzenetetracarboxylic anhydride and 4,4′-diaminodiphenyl ether were sublimated at 260 and 160 °C in a vacuum,
(11) (a) Slevin, C. J.; Gray, N. J.; Macpherson, J. V.; Webb, M. A.; Unwin, P. R.
Electrochem. Commun. 1999, 1, 282-288. (b) Bach, C. E.; Nichols, R. J.;
Beckmann, W.; Meyer, H. J. Electrochem. Soc. 1993, 40, 1281-1284. (c)
Zu, Y.; Ding, Z.; Zhou, J.; Lee, Y.; Bard, A. J. Anal. Chem. 2001, 73, 21532156.
(12) Lee, Y.H.; Tsao, G. T.; Wankant, P. C. Ind. Eng. Chem. Fundam. 1978,
17, 59-81.
(13) Nam, A. J.; Teren, A.; Lusby, T. A.; Melmed, A. J. J. Vac. Sci. Technol. B
1995, 13, 1556-1559.
(14) (a) Nagahara, L. A.; Tundat, T.; Lindsay, S. M. Rev. Sci. Instrum. 1989,
60, 3128-3130. (b) Wiechers, J.; Twomey, T.; Kolb, D. M.; Behm, R. J. J.
Electroanal. Chem. 1988, 248, 451-457.
(15) (a) Gewirth, A. A.; Craston, D. H.; Bard, A. J. J. Electroanal. Chem. 1989,
261, 477-482. (b) Vitus, C. M.; Chang, S. C.; Weaver, M. J. J. Phys. Chem.
1991, 95, 7559-7563.
(16) Zhang, B.; Wang, E. Electrochim. Acta 1994, 39, 103-107.
(17) Zhao, G.; Giolando, D. M.; Kirchhoff, J. R. Anal. Chem. 1995, 67, 25922598.
(18) Potje-Kamloth, K.; Janata, J.; Jossowicz, M. Ber. Bunsen-Ges. Phys. Chem.
1990, 93, 1480-1485.
(19) Schulte, A.; Chow, R. H. Anal. Chem. 1996, 68, 3054-3058.
respectively. N,N-Dimethylacetamide was newly distilled, and
some molecular sieves were added in it to ensure that it was very
dry.
Etching solution contained 50% (by volume) saturated CaCl2,
25% HCl, and 25% deionized water. Coating solution was prepared
by dissolution of 0.5000 g of 1,2,4,5-benzenetetracarboxylic anhydride and 0.4590 g of 4,4′-diaminodiphenyl ether in 7 mL of N,Ndimethylacetamide; this solution was mixed thoroughly before use.
The aqueous solution used to characterize the fabricated
nanoelectrodes was 1 × 10-2 mol dm-3 Ru(NH3)63+ with 0.2 mol
dm-3 KCl as supporting electrolyte, and the DCE solution
contained 1 × 10-3 mol dm-3 TCNQ without supporting electrolyte. All aqueous solutions were prepared using deionized water
(Millipore Corp.).
A CHI 900 setup (CH Instrument) was employed to perform
cyclic voltammetry and SECM experiments. An optical microscope
(BX-60, Olympus) and the JXA-840 scanning electronic microscope
(JEOL) were used to check the shapes and insulating coverage
of the electrodes. A 0.2-cm-diameter Pt disk electrode acted as
the SECM substrate. A KCl-saturated Ag/AgCl electrode and a
0.125-cm-diamter Pt wire were used as reference and counter
electrodes, respectively.
Fabrication of Nanoelectrodes. A micropipet with a ∼30µm (i.d) sharp tip was pulled from a piece of borosilicate capillary
(1-mm o.d, 0.58-mm i.d) by use of a P-2000 laser puller (Sutter
Instrument Co.). A 1-cm length of 20-µm Pt wire was transferred
carefully inside the capillary from the back and left ∼0.5 cm of
the Pt wire outside of the sharp tip. The Pt wire was secured in
position by melting the sharp tip around the Pt wire using a gas
flame. Electrical connection to the inside end of the Pt wire was
made with silver epoxy to a copper wire. The open end of the
capillary was sealed with epoxy resin. This provided strain relief
for the conductive wire.
Etching of a Pt wire to form an ultrafine point was accomplished using the procedure that is outlined in Figure 1a. A
Pt (radius 0.125 mm) ring was covered with a film of etching
solution, and this film can exist stably because of the surface
tension. An ac voltage of 2 V was applied between the Pt wire
and the Pt ring. The etching procedure was completed when the
current dropped to zero. The image of an etched Pt wire is shown
in Figure 2a.
The etched Pt wire was cleaned by using 0.01 mol dm-3 NaOH,
1 mol dm-3 HNO3, and amounts of deionized water, respectively.
Then, the tip was dipped in the coating solution, and heat was
added to the part of the capillary that was very near to the tip
(See Figure 1b). The heat can be transferred through the glass
tube to the tip, so that the solution temperature around the tip is
much higher than the solution far from the tip. The high
temperature can induce the formation of a layer of polyimide
around the tip. The polyimide is a tightly adherent and highly
resistive insulator. Finally, the tip was taken out of the solution
and was put in an oven at 180-220 °C for ∼1 h (See Figure 1c.).
The major problem is that sometimes the electrodes are thoroughly insulated. Since the polyimide can hydrolyze in 0.01 M
NaOH solution very slowly, those very small electrodes (only a
few nanometers in diameter) can be obtained by dipping the
electrodes in 0.01 M NaOH for a few minutes.
Analytical Chemistry, Vol. 73, No. 21, November 1, 2001
5347
Figure 1. Schematic diagram of the fabrication of nanometer-sized
electrodes. (a) The setup was used to etch the Pt wires; (b) and (c)
the setups were employed to insulate the etched Pt wires.
RESULTS AND DISCUSSION
Characterization of the Fabricated Nanoelectrodes by
SEM and Cyclic Voltammetry. The polymerization is as follows:
The polyimide is a very good insulating material. It is hard
and oil insoluble and has an expansion coefficient similar to that
of Pt.20 Figure 2 shows the SEM images of an electrode before
5348 Analytical Chemistry, Vol. 73, No. 21, November 1, 2001
Figure 2. SEM images of the nanoelectrodes: (a) an etched Pt
wire, (b) the side view of a nanoelectrode, and (c) the top view of a
nanoelectrode.
and after insulation; its radius could be estimated from Figure 2c
and is equal to ∼120 nm. The surface is smooth with no cracks.
There are no other defects in the polymer/metal seal. From the
top view and the side view of the insulated tip, we could assume
that the exposed area is almost a disk shape.
The stability of the polyimide coated on the surface of Pt
nanoelectrodes in various solvents can be tested experimentally
by cyclic voltammetry. Ru(NH3)63+ and TCNQ were chosen as
the mediators in the aqueous and the DCE solutions, respectively.
For both cases, it is profitable to perform the experiments in dim
light. Figure 3a shows the voltammograms of reduction of Ru(20) Ying, L.; Xu, X. Fundmental of Adhension and Adhensive; Aviation Industrual
Press Inc.: Beijing, 1988; Chapter 5.
Figure 3. Cyclic voltammograms of the mediators at Pt nanoelectrodes obtained both in aqueous and in DCE solutions. The numbers
refer to the cycling numbers: (a) A radius of 46-nm nanoelectrode in
1 × 10-2 mol dm-3 Ru(NH3)63+ and 0.2 M KCl aqueous solution and
its potential window (see the inset). (b) A radius of 52-nm nanoelectrode in 1 × 10-3 mol dm-3 TCNQ in the DCE phase and its potential
window (see the inset).
(NH3)63+ on one Pt nanoelectrode in the 0.2 mol dm-3 KCl
solution. Figure 3b shows the voltammograms of reduction of
TCNQ obtained on another Pt nanoelectrode in pure DCE. The
voltammograms in both cases were recorded after more than 30
times cycling. For the first cycling and the one after 30 times, the
steady-state current is almost the same, suggesting that the
insulation layer does not crack or deteriorate with time in both
solutions. Moreover, the fabricated nanoelectrodes can work in a
rather wide potential window for both systems (see the insets in
Figure 3), and they can be also used to study many other systems.
The following eq 1, where c° is the buck concentration of mediator
iT,∞ ) 4nFDc°r
(1)
species, F is the Faraday constant, n is the number of electrons
transferred, D is the diffusion coefficient of the mediator, and r is
the effective radius of the electrode, can be employed to quantitatively analyze the voltammetric data and evaluate the effective
radii of these nanoelectrodes because the SEM images suggested
that they are nanodisk electrodes. By use of this equation, and
the diffusion coefficients of Ru(NH3)63+ and TCNQ, 7 × 10-6 8b
and 1.5 × 10-5cm2 s-1,21 the radii of the nanoelectrodes used in
parts a and b of Figure 3 can be calculated and are equal to 46
and 52 nm, respectively.
(21) Cheng, Y. F.; Schiffrin, D. J. J. Chem. Soc., Farady Trans. 1994, 90, 25172523.
Figure 4. Cyclic voltammograms obtained with different sizes of
Pt electrodes. System: 1 × 10-2 mol dm-3 Ru(NH3)63+ and 0.2 mol
dm-3 KCl for (a-c), The effective radii calculated by use eq 1 are (a)
91, (b) 17, and (c) 1.8 nm.
Voltammetric responses of different sizes of electrodes obtained in the aqueous solution containing 1 × 10-2 mol dm-3
Ru(NH3)63+ and 0.2 mol dm-3 KCl are shown in Figure 4. All of
them are well-defined steady-state voltammograms. The effective
radii of these nanoelectrodes can be calculated from the diffusionlimiting steady-state currents using eq 1. All of them show a
relative small charging current except for the very small electrodes. This indicates that these electrodes are not “lagooned”
electrodes.10b We even can obtain a reasonable steady-state
voltammogram for a radius of a 1.8-nm electrode in this solution
(See Figure 4c). Figure 4 also shows that the voltammograms of
the reduction of Ru(NH3)63+ become worse with decreasing sizes
of the electrodes. It means that the reversibility of this process
becomes worse with increase of the mass transport rate with
smaller electrodes. Using the approach developed by Mirkin and
Bard,22 the kinetic parameters can be obtained and are listed in
Table 1. A number of voltammograms obtained at radii of 1.8-17
Analytical Chemistry, Vol. 73, No. 21, November 1, 2001
5349
Table 1. Kinetic Parameters for the Reduction of
Ru(NH3)63+
no.
radii
(nm)
∆E1/4
(mV)
∆E3/4
(mV)
κ0
(cm/s)
1
2
3
4
5
6
91
46
17
12
8
1.8
28.6
28.6
30.5
31.0
35.1
36.2
28.6
28.6
31.0
32.3
37.4
38.7
a
a
2.0
5.3
4.5
4.9
a
Reversible reaction (no kinetic parameter can be extracted).
nm electrodes yield a value of the standard rate constant of κ0 )
4.2(0.9 cm s-1.
Characterization of the Fabricated Nanoelectrodes by
Scanning Electrochemical Microscopy. SECM has been proved
to be a reliable method to study fast heterogeneous electrontransfer kinetics.23 The resolution of SECM largely depends on
the size of the tip and its geometry. To determine very fast
heterogeneous electron-transfer kinetics and to obtain high
resolution of the specimen, it is necessary to use a nanometer
electrode. Practically, it is not easy to obtain good approach curves
using a very small tip,24 for example, <50 nm. Many types of
nanoelectrodes made by various procedures could not be used
for SECM tips. This is because the nanometer-sized Pt wire must
be on the apex of the electrode to be used as a tip of SECM.
Generally, this is not easy to achieve. From the SEM images, the
nanometer-sized Pt wire seems to be on the apex of the electrode
fabricated by this method. It is maybe more easy and practical to
test whether these wires can be used as SECM tips by doing
approach curves.
The following system was chosen to characterize the fabricated
nanotips: 1 × 10-2 mol dm-3 Ru(NH3)63+ and 0.2 mol dm-3 KCl
in aqueous solution. A saturated Ag/AgCl electrode was immersed
in the solution as reference electrode. A CHI 900 SECM setup
was employed to record the approach curves. Figure 5 shows the
experimental normalized I-L curves fitted with the theoretical
values obtained with radii of 151 and 46 nm, respectively. L )
d/r is the normalized distance and d is the distance between the
tip and the substrate. In this case, the positive feedback is due to
the following reactions:
κf
Ru(NH3)63+ + e 98 Ru(NH3)62+
(tip potential ET ) -0.3 V vs Ag/AgCl)
κb
Ru(NH3)62+ - e 98 Ru(NH3)63+
(substrate potential ES ) 0.0 V vs Ag/AgCl)
The tip was held at a potential (ET) where the reduction of Ru(NH3)63+ on the nanoelectrode is diffusion-controlled. The substrate was kept at a potential (ES) where the Ru(NH3)62+ could
be completely reoxidized back to Ru(NH3)63+.
(22) Mirkin, M. V.; Bard, A. J. Anal. Chem. 1992, 64, 2293-2302.
5350
Analytical Chemistry, Vol. 73, No. 21, November 1, 2001
Figure 5. Normalized experimental approach curves (square) fitted
with the theoretical values (line). System: 1 × 10-2 mol dm-3
Ru(NH3)63+ and 0.2 mol dm-3 KCl in aqueous solution. Tip radius
(a) 151,and (b) 46 nm. Approach rate, 10 nm s-1.
The equations used to fit the experimental approach curves
with theoretical values can be found in ref 24b. The d ) 0 point
was precisely determined when the tip came in contact with the
substrate, which causes a sharp increase of the tip current. The
parameter κf can be extracted from these fittings. The values of
κ0 can be obtained from the Butler-Volmer equation if R is
assumed to be equal to 0.5 for 46- and 151-nm electrodes as the
SECM tips, and they are equal to 2.1 and 1.0 cm s-1, respectively.
These values are similar to the value calculated from the
nanoelectrodes and previous reports,25 but 2 orders smaller than
the data in ref 1b. It is hard to reuse the nanotip after one approach
curve is obtained because it is rather fragile, and it will break
after it touches the substrate. Nevertheless, these results indicate
that the nanoelectrodes produced by this method could also be
used as SECM tips.
CONCLUSIONS
A novel method of fabrication of nanometer electrodes based
on polymerization on Pt wire surface has been presented. The
SEM images and cyclic voltammetric responses indicate that the
nanoelectrodes produced this way can work well either in aqueous
or organic solutions. This method can also be used to make useful
nanometer SECM tips.
(23) (a) Mirkin, M. V.; Richard, T. C.; Bard, A. J. J. Phys. Chem. 1993, 97, 76727681. (b) Horrocks, B. R.; Mirkin, M. V.; Bard, A. J. J. Phys. Chem. 1994,
98, 9106-9111.
(24) (a) Mirkin, M. V.; Horrocks, B. R. Anal. Chim. Acta 2000, 406, 119-146.
(b) Wei, C.; Bard, A. J.; Mirkin, M. V. J. Phys. Chem. 1995, 99, 1603316042.
(25) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928.
ACKNOWLEDGMENT
The authors thank the Chinese Academy of Sciences (CAS),
the National Natural Science Foundation of China (NSFC,
29825111), the Third World Academy of Sciences (TWAS), and
the Laboratory of Electroanalytical Chemistry of the CAS for the
financial support for this work. The authors also appreciate Mr.
Xingzhong Fang for his help in this work.
Received for review April 27, 2001. Accepted July 18,
2001.
AC010474W
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5351