Pe 22

A glass tube
with an outer diameter of 1.5 mm (G-1.5, Narishige Co., Ltd., Tokyo,
Japan) is ultrasonicated in 99.5% ethanol and pure water for 5 min
each and is baked on a hot plate at 473 K (200 °C) for 5 min.
After cleaning, it is pulled to form a glass capillary with desired
outer diameters of 0.6 or 1 μm using a glass tube puller (PE-22,
Narishige Co., Ltd., Tokyo, Japan) (Figure 2b). A Ag–AgCl wire with a diameter
of 0.2 mm (Nilaco Corp., Tokyo, Japan) is settled in the glass capillary
filled with a KCl solution, and this glass microelectrode is for the
RE. As a first step, the concentration of the inner solution is set
to be equal to the sample solution. To increase the impedance of the
glass microelectrode, an agarose gel is filled in a 1 μm tip
of the capillary. In this case, a 120 mg agar is stirred in a 100
mM KCl aqueous solution of 30 mL and is heated using an alcohol lamp
to harden the gel by decreasing the temperature. The quantity of agar
and the concentration of KCl solution are optimized to minimize errors
in the electrical measurements.

Cylindrical carbon-fiber microelectrodes
were made from 7-μm
in diameter T650 carbon-fibers (gift from Mitsubishi Chemical Carbon
Fiber and Composites Inc., Sacramento, CA). Carbon-fibers were vacuum
aspirated into a capillary glass tube (1.2 mm × 0.68 mm, A-M
Systems, Sequim, WA) and pulled into two using a vertical micropipet
puller (Narishige PE-22, Tokyo, Japan). To create a cylinder electrode,
fibers were trimmed to approximately 50–100 μm from the
glass seal using a microscope (Fisher Education). Electrodes were
pretested prior to modification. Electrodes were cleaned with isopropyl
alcohol (IPA) and water to remove salt and allowed to dry prior to
modification. To modify electrodes, either 0.5 mg/mL chloroauric acid
(HAuCl₄) or 0.5 mg/mL potassium hexachloroplatinate was
electrodeposited on the carbon-fiber surface and this electrodeposition
created formation of metal nanoparticles on the surface. Electrodeposition
was done by applying a potential sweep to the surface scanning from
−1.2 to 1.5 V at a rate of 5 V/s against a Ag/AgCl reference
electrode for 30 s. See Figure S-1 for
electrodeposition optimization data.