Supporting Information
Facile Fabrication of Carbon Ultramicro- to Nanoelectrode Arrays with Tunable Cyclic Voltammetry
Response
Jonathon Duay, Jacob M. Goran, and Keith J. Stevenson∗
Department of Chemistry, Center for Nano- and Molecular Science and Technology
The University of Texas at Austin, Austin, TX 78712, USA
Detailed Experimental Protocols
∗
Corresponding authors: Keith J. Stevenson (stevenson@mail.utexas.edu) (T) +1-512- 232-9160; (F) +1-512-4718696
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Reagents and Materials. All chemicals were used as received. Photoresist AZ 1518 was
purchased from Microchemicals. Polystyrenes spheres (Polybead®) with stated diameters of
1.54, 11, and 90 µm were purchase from Polysciences, Inc. Ferrocenemethanol and KCl was
acquired from Sigma-Aldrich, while NaH2PO4 and Na2HPO4 used for the sodium phosphate
buffer were purchased from Fisher.
PPF Fabrication. Briefly, quartz microscopic slides (6.45 cm2 and 1 mm thick, Technical Glass
Products) were heated at 800 °C in air to remove organic contaminations and further placed in
piranha (3:1 H2SO4: 30% H2O2) to remove any additional residual organics. (Caution: Piranha is
a strong oxidizing solution and must be prepared in a fume hood with proper protection. Always
add the H2O2 to H2SO4.) Undiluted AZ1518 photoresist was spun onto the piranha cleaned
quartz slides at 6000 rpm for 60 s.
After spincoating, the photoresist slides were soft baked for 10 min at 90°C on a hotplate, and
then transferred to a tube furnace. After purging with 5% H2: 95% N2 (~100 mL/min) for 15
minutes, the photoresist slides were pyrolized by heating to 1000 °C at 5 °C/min and holding at
that temp for 1 hour before allowing them to cool slowly back to room temperature at 5°C/min.
The PPFs were then removed from the furnace and stored for 3 days prior to use to allow for the
oxide layer to stabilize.34 As detailed elsewhere,35 PPFs prepared by this method are 250±20 nm
thick, display a rms roughness of 0.39±0.07 nm, and have a sheet resistance of 97±3 Ω/□,
respectively.
Finally the PPFs were O2 plasma cleaned prior to microelectrode array fabrication.
Al2O3 ALD Recipe. Electrodes were placed in an 80°C ALD chamber. For each cycle,
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Trimethylaluminum (TMA) was pulsed for 0.015 seconds followed by a 30 second wait time
followed by a water pulse of 0.015 seconds which is then followed by an additional 60 s wait
time.
This process was previously calibrated by the manufacturer to deposit a 0.089 nm
conformal Al2O3 layer for each cycle on silicon. Therefore, the process was cycled 112 times in
order to form a 10nm thick Al2O3 film. The thickness of this film is supported by atomic force
microscopy measurements.
Atomic Force Microscopy. All measurements were obtained in tapping mode with aluminum
coated monolithic SPM tips with alignment grooves (BudgetSensors, cantilever length 125 µm,
resonance frequency ~300 kHz, tip radius 10 nm).
Electrochemical Analysis. All electrochemical measurements were performed in a 1 mM
ferrocenemethanol aqueous solution utilizing 0.1 M KCl as a supporting electrolyte with 0.1 M
sodium phosphate buffer to maintain a neutral pH of 7. The ultramicroelectrode arrays, a nonpatterned macroelectrode PPF control, and a completely ALD Al2O3 coated PPF negative control
were used as the working electrode with a total 0.495 cm2 electrode area. A saturated calomel
electrode (SCE) was used as the reference electrode, and a platinum mesh as the counter
electrode. All experiments were performed in a Faraday cage. Voltammograms are displayed
with a positive cathodic current and a negative anodic current.
Electrochemical impedance spectroscopy was done between 10 mHz and 100 kHz with a 10 mV
perturbation voltage around the potential of 0.185 V vs. SCE.
Cyclic voltammetry (CV)
experiments were performed at scan rates of 0.001, 0.002, 0.005, 0.01, 0.02, 0.04, 0.05, 0.1, 0.2,
0.3, 0.4, 0.5, 1, and 2 V/s between the potentials of -0.05 and 0.45 V vs. SCE.
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Johnson-Kendall-Roberts Contact Mechanics
The critical contact radius, which governs the size of the electrodes, is directed by the JohnsonKendall-Roberts model for elastic contact of a sphere on a substrate at zero applied load:
=
9 4 ∗
(1)
where a is the critical contact radius, R is the radius of the sphere, ∆γ is the work of adhesion,
and E* is the effective elastic modulus. Simplification of equation 1 results in the following
proportionality:
∝ ⁄
(2)
Furthermore, since d is linearly proportional to R, the following proportionality relates d/a to the
PSS radius:
⁄ ∝ ⁄ ⁄ ∝ ⁄
(3)
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Figure S1. d/a, ratio of the center-to-center UME distance (d) over the radius (a) of the UME
versus the cube root of the PSS radius used to fabricate the electrode array. Horizontal error bars
represent the standard deviation from the manufacturer of the radius of the spheres while the
vertical error bars are calculated utilizing the standard deviation for d/a ratio for three electrodes.
riate size of PSS (commercially available with diameters between 0.05 and 90 µm).
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Figure S2. EIS of uncoated PPF macroelectrode
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Figure S3. (a) Cyclic voltammogram of a 1.54CUA microelectrode array and a carbon PPF
macroelectrode at a 0.001 V/s scan rate using 1 mM FcMeOH as an analyte. (b) Cyclic
voltammetry without any analyte demonstrating the reduced noise and/or background double
layer charging current for the microelectrode array. (c) Cyclic voltammograms of a 1.54CUA
microelectrode array between 2 and 15 V/s scan rates.
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Figure S4. I-V curve of a completely ALD Al2O3 coated PPF compared to the 90CUA (the
smallest current response reported here) at the same scan rate of 1mV/sec. The inset is the
expanded view of the ALD coated PPF showing some defects in the ALD layer.
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