Patternable Nanowire Sensors for Electrochemical Recording of Dopamine

Article (PDF Available)inAnalytical Chemistry 81(24):9979-84 · November 2009with44 Reads
DOI: 10.1021/ac901744s · Source: PubMed
Abstract
Spatially resolved electrochemical recording of neurochemicals is difficult due to the challenges associated with producing nanometer-scale patternable and integrated sensors. We describe the lithographic fabrication and characterization of patternable gold (Au) nanowire (NW) based sensors for the electrochemical recording of dopamine (DA). We demonstrate a straightforward NW-size-independent approach to align contact pads to NWs. Sensors, with NW widths as small as 30 nm, exhibited considerable insensitivity to scan rates during cyclic voltammetry, a nonlinear increase in oxidation current with increasing NW width, and the selectivity to measure submaximal synaptic concentrations of DA in the presence of interfering ascorbic acid. The electrochemical sensitivity of Au NW electrode sensors was much larger than that of Au thin-film electrodes. In chronoamperometric measurements, the NW sensors were found to be sensitive for submicromolar concentration of DA. Hence, the patternable NW sensors represent an attractive platform for electrochemical sensing and recording.
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Nanowiring for neurons
If you want to know what’s going on in
a single synapse of the brain, you’ve got
to get down to the level of that synapse
with your measurement tools. That’s
how David Gracias and colleagues at
Johns Hopkins University have tackled
the problem of measuring local concen-
trations of an important neurotransmit-
ter, dopamine. In their recent AC paper
(DOI 10.1021/ac901744s), the investi-
gators describe long, thin, gold nanow-
ires that can be mass-produced, ar-
ranged in any pattern, and integrated
with the outside world.
“There is much interest in developing
smaller electrodes because the normal
electrodes used for dopamine detection
in the brain are on the micron scale,
larger than a synapse,” explains B. Jill
Venton at the University of Virginia.
She says that, at 30 nm in diameter, the
smallest nanowires “in this report are on
the scale that could fit in a synapse.”
Electrochemical methods for measur-
ing neurotransmitters go back to the
1970s when Ralph Adams at the Univer-
sity of Kansas and his team were demon-
strating the ability of small electrodes to
monitor neurochemistry. “We’re not rein-
venting the wheel,” states Gracias. “We’re
just miniaturizing [the system] and show-
ing that we can have electrochemical
methods at the 30-nm scaleOat this scale,
we can measure dopamine at relevant syn-
aptic concentrations.”
For instance, the investigators showed
that when used for chronoamperometry,
the sensors detected submicromolar con-
centrations of dopamine (the synaptic
concentration of dopamine is estimated to
be 1.6 mM). Gracias says that, to his
knowledge, only one other demonstration
has electrochemically measured dopamine
at the nanoscale, with carbon nanotubes.
But carbon nanotubes can have fabrication
issuesOthey have to be grown at high
temperatures and they are difficult to inte-
grate with existing systems.
In contrast, the gold nanowires that
Gracias and colleagues produced are
easy to fabricate by lithography and can
be readily insulated and connected to
the outside world with a macroscale
contact pad. The investigators adapted
the fabrication method called litho-
graphically patterned nanowire elec-
trodeposition (LPNE), which was devel-
oped by Reginald Penner’s group at the
University of California Irvine. The
method involves cycles of deposition of
various materials and selective etching to
let researchers have exquisite control
over the dimensions of an object’s fea-
tures. Using LPNE, Gracias and col-
leagues could fabricate nanowires with
diameters ranging from 30 to 1000 nm,
and lengths from 1 to 20 mm. The
nanowires were easily integrated and
attached to macroscale connectors with
alligator clips.
As Penner explains, small electrodes
demand high, mass-transport-based
fluxes to generate currents. “The nice
thing about nanowires is you get these
high fluxes by shrinking the electrode in
just two dimensions. But you leave one
macroscopic dimension, and the result is
the currents aren’t in the nano-amp
rangeOthey are in the micro-amp
range,” he says. “That’s the conceptual
leap these investigators have made. They
are making micro- and nanoelectrodes
out of these wires.”
Gracias says it’s the first time gold has
been used to make nanowires for the
electrochemical detection of dopamine.
The metal takes care of a nagging prob-
lem in dopamine detection: ascorbic
acid. Ascorbic acid is present in high
concentrations in synapses and has a
similar oxidative potential to dopamine.
“A gold electrode is more selective to
dopamine than to ascorbic acid, which,
in the brain, is a common interfering
agent. When you do electrochemical
recording, you need to have your elec-
trode more sensitive to dopamine,” ex-
plains Gracias. “Otherwise you just mea-
sure the background of ascorbic acid.”
The investigators discovered an-
other surprising attribute of the
nanowires: their sensitivity increased
nonlinearly with decreasing diameter.
“You would not intuitively think sensi-
tivity necessarily increases with smaller
size,” Gracias says, adding that he
thinks some interesting diffusion be-
havior may explain the observation.
He’s also encouraged by the phenom-
enon because it means that when the
investigators move to an in vivo ex-
perimental system, they will get high
sensitivity with the narrowest nanow-
ires at single synaptic junctions.
But in gearing up to apply these
long, skinny, nanowire-based sensors to
an in vivo setting, Gracias and colleagues
are anticipating hurdles. For instance,
“we have to position our nanowire rela-
tive to a single synapse. That is challeng-
ing,” Gracias says. “We would need to
actually grow the synapse to make it
come towards the wire.” The investiga-
tors plan on creating neuronal-growth-
factor gradients to get the neurons to
migrate toward the wire.
—Rajendrani Mukhopadhyay
Optical images of insulated, gold-nanowire-
based electrochemical sensors for dopamine, a
neurotransmitter implicated in several neurologi-
cal disorders, including Parkinson’s disease.
n ews
10.1021/AC9025913 2010 AMERICAN CHEMICAL SOCIETY2 ANALYTICAL CHEMISTRY / JANUARY 1, 2010
Published on Web 11/20/2009
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