This document describes research on fabricating and testing an artificial ion pump using microfluidic and nanofluidic devices. The devices contain microchannels and nanochannels etched into glass slides. Electrolyte solutions containing different ions are flowed through the channels and voltages are applied to gates over the nanochannels to regulate ion transport. Variables tested include ion type, concentration, and alternating current versus direct current gate voltages. The experiments aim to better understand ion transport at the nanoscale and develop technologies for applications like water purification and drug delivery.

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Microsystems and Nanosystems Laboratory
David Horner
E-mail: horner.113@osu.edu
Introduction
The research fields of microfluidics and nanofluidics are widely regarded as some of the
most cutting edge with a broad array of practical applications. Microfluidic and nanofluidic
devices have many uses in medical, biological, and environmental applications. Micro and
nanochannel development can directly be used to fabricate artificial ion pumps, ion separators,
water desalination devices, biosensors, and more [14]. The biological ion pumps are inspired by
almost all cellular communication in order to maintain essential cell function [10]. Additional
applications include DNA sequencing and “smart” drug delivery, which could potentially
revolutionize the way medicine is delivered to patients [1-9]. This particular research experiment
explores the fabrication and testing of what will be a precursor to a full-fledged artificial ion pump.
In these devices, the flow is regulated within the channels of the pump using both AC and DC
voltages applied to the gate electrode over the bank of nanochannels.

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The device used for the laboratory experiments
were specially fabricated to test the effect of gate
voltage on transport of ions in a network of
nanochannels and are unique from other microfluidic
and nanofluidic devices. The device is fabricated from
three layers of material; a channel slide, a top cover
with evaporated gold electrodes, and a PDMS
(polydimethyl siliconaxe) dielectric layer. To begin
the fabrication process (Figure 1), a sheet of glass is
thoroughly cleaned and a chromium/gold (Cr/Au)
layer is deposited onto the surface. This Cr/Au layer
is patterned using a series of chemical baths in a
process referred to as photolithography or more
specifically in this case, UV lithography. This process
begins with a thin layer of a UV sensitive polymer
photoresist being spun over the surface of the device,
covering the top layer of gold. After the photoresist is
spun and baked onto the gold surface, the photoresist is exposed to a UV light through a mask with
the microchannels. This channel slide in the making is then soaked in a developer to remove any
polymer photoresist degraded by the UV leaving an imprint of the microchannels in the photoresist
layer. The microchannels are etched and then the photoresist is removed, followed by a similar
process in order to etch the nanochannels. The final step is the complete removal of the Cr/Au
mask resulting in a 1” x 3” borosilicate glass channel slide with micro and nanochannels.
Spin on a
Layer of
Photoresist
Develop and
Etch
Remove
Photoresist, Au,
and Cr
Start with
Clean Glass
Evaporate
Au/Cr Layer
Apply a
Mask and
Expose to
UV
Repeat for
Nanochannels
Figure 1: A visualization of the channel
slide process

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After the channel slide is complete, a variation of the photolithography process takes place
for the top cover. For the top cover, the glass is cleaned and the Cr/Au layer is evaporated onto
the surface. The photolithography process is used to pattern the gold into the shape of the
electrodes. Once all photolithography has taken place, the top covers are brought out of the clean
room and back to the lab for drilling. Four fluidic access ports are carefully patterned with a
sharpie and a stencil before they are drilled using a diamond tipped drill bit. Care must be taken
to keep a steady stream of nanopure deionized (DI) water over the glass for lubricant and to clear
away small particles of glass. The top covers are then cleaned in an Alconox solution before
returning to the clean room. Finally, the top cover has a layer of PDMS spun onto the surface
before the PDMS is allowed to cure overnight. The top cover with PDMS layer and the channel
slide are aligned and bonded after cleaning in an oxygen-plasma chamber, thus completing the
device fabrication [11].
While the process of photolithography is common when fabricating microfluidic and
nanofluidic devices, the combination of photolithography and the oxygen plasma process as well
as the device itself novel in its design [11]. Very few devices are embedded with gate electrodes
in order to directly apply a potential difference to the wall of the nanochannels, yet the device for
this experiment has been embedded with four individually addressable electrodes providing four
different connection points for gate tests. Beyond the added gate electrodes, the actual
nanochannels have been etched to a smaller depth than most devices, measuring in at only 16 nm
deep for the nanochannels.

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Figure 2: A model of a nanofluidic field effect device with four individually addressable electrodes used for
testing. Each device measures approximately 3” long and 2” wide. The gold electrodes are aligned on the
top and bottom of the device in rows of four with wire leading to the top wall or “roof” of each of the three
nanochannels. The microchannels are the visible lines configured in the }{ shape with the nanochannels
connecting the parallel section of the microchannels. The four holes or “reservoirs” in which the buffer
solutions are fed into are found at the end of the microchannels.

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Figure 2: A cross-section of a standard device used for testing. The width of the nanochannels can be
seen in the cross-section separated by the gold electrode wires. Each microchannel measures 3.2 cm in
length, 50 µm in width and 10 µm deep while each nanochannel measures 5 mm long, 30µm wide, and 16
nm deep.
Experimental Setup
After fabrication of the devices is complete, several different electrolyte solutions are
manipulated in order to vary factors such as cation, electrolyte concentrations, axial voltage, gate
voltage, and electrolyte pH to gain insight into ionic transport. All the solutions created throughout
the laboratory experiment were composed of a mixture of DI water, as well as a single type of salt
made with a monovalent (KCl and NaCl) or divalent cation (MgCl2, CaCl2) in each solution. All
electrolyte solutions are pH controlled to regulate surface charge of the walls of the nanochannel.
Before the electrolyte solutions are inserted into the reservoirs, the devices are cleaned in an O2-

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plasma chamber in order to ensure the surface of the glass is hydrophilic. This facilitates filling of
the microchannels and nanochannels. When the plasma treatment is complete, an electrolyte
solution is inserted into the device using a standard dropper and then drawn through the channels
using an Edward’s pump.
Once the devices are filled with their respective electrolyte solutions, electrodes are
connected in three separate configurations for axial, gate and leakage tests. The first test, an axial
sweep, has the nodes from the positive terminal of the function generator serving as a power supply
are placed in two of the reservoirs aligned vertically and then excited with a DC voltage ranging
from 0 – 9500 mV. Leads from the picoammeter were placed in the other two reservoirs aligned
vertically on the right side of the device in order to measure the resultant current across each
channel. In order to decrease any residual charge and ensure the top of the device is completely
dry after filling the channel, a 0 V voltage is applied before and after each trial for the axial sweeps
until a steady current near 0 pA is measured. The axial test is used to find the conductance of the
device without the gate electrode attached. Looking at figure 4, this would be the setup with the
red wires removed.
For gate sweeps, the two leads connected to the power supply as well as the picoammeter
leads remain in place and an additional two wires are hooked to the embedded gold gate electrodes.
This is illustrated in figure 4. These axial wires (green wires in figure 4) are excited with a constant
DC voltage of either 3 V or 5 V applied directly across the nanochannels. The gate (red wire in
figure 4) voltage ranges from 0 – 5000 mV. Like the axial sweeps, a 0 V voltage is applied before
and after each trial yet for gate sweeps each initial zero point consists of two parts. First, a zero
point is taken where the variable voltage is set at zero and the axial voltage is turned completely
off. The second zero point consists of the variable voltage remaining zero, yet the axial voltage is

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turned on and set to 0 V as well. Both zeros are required to make sure there is a stable nearly zero
current reading before starting the gate sweep. The first is effectively an open circuit with a break
in the axial voltage loop while the second zero gives a complete loop with no applied currents.
Finally, for determining any electrical noise within the system, a leakage test is conducted.
In two separate trials, a single wire connects the two reservoirs vertically on each side of the device
and then is connected to the picoammeter. A voltage is applied on the opposite side of the device
ranging from 0 – 5000 mV. The current reading is the opposite channel should remain low to
signify there are no low resistance paths between the microchannels that circumvent the
nanochannels within the system. In order to solidify the validity of the measured data, both positive
and negative variable DC voltages are applied during separate trials.

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Figure 4: A pictorial representation of the testing setup throughout the course of the laboratory experiment.
A power supply and a picoammeter were connected to the device for all testing runs where the power
supply provided a variable voltage and the picoammeter measured the current within the device. The
copper colored box represents an Earth-grounded Faraday cage used in order to reduce electrical noise
within the system, inside the cage resides a second power supply which is connected to the imbedded
electrodes and a constant voltage is for the gate tests.
The purpose of these tests is to determine the ability of each solution to be manipulated
within the nanochannels of the device. By applying a variable voltage across the nanochannels of
each device, a potential difference is created thus pulling the solution through the bank of
nanochannels. The picoammeter is used to measure the resultant current caused by the potential
difference across the channels which determines the direction and magnitude of the flow [12]. The
electrolyte in the channel causes what is called an electric double layer (EDL) on the walls of each
nanochannel. The EDL is composed of the stern layer which is held tightly to the walls of the
channel and the diffuse layer which is between the stern layer and the bulk region of the channel.
Together, these two layers form the EDL and are defined by what is called to Debye length (λD)
which is the sum of the lengths of each layer. The Debye length defined by the equation
𝜆 𝐷 = √
𝜀0 𝜀 𝑟 𝑅𝑇
2𝐹2 𝐼
where ε0 is the permittivity of free space constant, εr is the dielectric constant, R is the gas constant,
T is temperature, F is Faraday’s constant, and I is the current within the channel. The EDL is in
direct contact with the walls of the channel and acts as an opposing charge in order to create a zero
net charge within the channel [14]. This is to counteract the naturally negative charge of the
nanochannel walls and maintain electroneutrality. While there is a zero net charge within the

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channel, the EDL creates an electro-kinetic or surface potential within the channel and can be
controlled directly beneath the gate electrode in order to direct the flow of the ions in the
nanochannels.
Variables
As referenced in the previous section, the electrolyte solutions placed within the channels
of the devices for testing consisted of four types; potassium chloride (KCl), sodium chloride
(NaCl), magnesium chloride (MgCl2), and calcium chloride (CaCl2). Four different types of
electrolyte were tested in the devices in order to determine factors affecting ionic transport or the
ability to selectively transport ions within a solution. Beyond the different types of electrolytes,
different concentrations of each electrolyte solution were tested in the devices and AC versus DC
stimulation on the gate electrode is currently being tested.
The concentrations of the electrolyte solutions ranged from 10-7 M to 10-1 M and consisted
of a combination of each salt along with deionized water to form an electrolyte. The concentration
of each solution directly affects the conductance of the system. For solutions greater than 1 mM
where EDL does overlap, as the concentration increases the conductance increases linearly. For
solutions under 1 mM where the EDL overlaps, the conductance is constant [12]. For lower
concentrations the EDL will span the whole width of the channel forcing there to be more
positively charges ions within the channel to counteract the negatively charged channel walls. The
relationship of current through the channel to concentration is given by the equation
𝐼 = ∫ 𝐹𝑤 ∑ 𝑧𝑖
𝑖
(−𝐷𝑖
𝜕𝑐𝑖
𝜕𝑥
− Ω𝑖 𝑐𝑖∇𝜙 + 𝑐𝑖 𝑢
→
ℎ
0
)𝑑𝑥

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where F is Faraday’s constant, w is the channel width, zi is the species’ valence, Di is the diffusion
coefficient, Ωi is the ionic mobility, ci is to concentration in the nanofluidic channel, -∇𝜙 is the
electric field,
𝑢
→ is the flow rate, and finally h is the height of the channel. While the trends for
conductance’s dependence on concentration are easily observable, its dependence on each cation
is not [13].
The dependence of current on each cation is a variable that has very little testing to support
it so far outside of this group. Currently, research is being conducted on both monovalent and
divalent cations in order to determine how valence affects the ionic transport seen through the
current measured through each device. Recently, it has been shown that monovalent cations behave
very similarly to one another while the trends of divalent cations are distinctly different indicating
differences in their transport through the channel.
Another variable that is currently being tested is the effect of AC voltage on the device
when applied at the gate electrode. AC current’s effect on the conductance of nanochannels has
only recently been examined as a potentially more effective way to encourage ionic transport
through nanochannels. Preliminary tests show that the AC voltage does indeed cause the device
to behave differently than DC in that the current modulation is much greater within the channels
for RMS matched signals. More testing is being done in order to solidify the effects of AC voltage
on the conductance as well as the device as a whole.
Knowledge Gained

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Over the course of this laboratory experiment, and abundance of information has been
presented regarded the field of microfluidics and nanofluidics as well as safe lab practices.
Programs such as LabView are an integral part of device testing for microfluidics and nanofluidics
as well as many other fields of research. In addition to the software knowledge, information
regarding machinery (drill press operation) was provided as well.
In order to conduct safe and credible research, the laboratory environment needs to be safe
as well as clean. Due to the strong acids and bases used to form the electrolyte solutions, an
important safety aspect is to wear the appropriate protective equipment. For all lab practices
involving the electrolyte solutions, nitrile plastic gloves were worn as well as a pair of safety
goggles in order to prevent chemical burns. These items were also worn for drill press operation
in order to protect from glass particles and potential shrapnel caused by a broken device. While
safety was the primary focus of the laboratory practices, cleanliness was a close second. Nitrile
gloves are required to be worn when touching any of the devices in order to prevent contamination.
After each solution was done being used, beakers are triple rinsed in order to assure following
solutions are not contaminated. Many of the beakers are also used exclusively for a certain type of
solution to prevent the mixture of the separate cations.
Many devices are used throughout each lab practice that requires training in order to
operate. When drilling holes in the top covers of the devices, a drill press has to be operated
efficiently in order to prevent breakage. Before each hole is drilled, the top covers are placed in a
vice and sprayed with DI water. Throughout the drilling process the holes are periodically sprayed
with DI water in order to cool down the drill bit and remove debris. The handle of the drill press
is raised up and down in minute increments in order to prevent the device from breaking as well
as to create the best reservoirs. In addition to the drill press, both a Keithley picoammeter and

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arbitrary waveform generator had to be operated. While LabView controls the picoammeter
through the designated program, the waveform generator used during gate sweeps has to be
operated manually. A DC voltage of either 3 V or 5 V is selected on the generator and then
outputted to the device using to output setting. Care had to be taken while operating the voltage
generators to ensure electrocution was avoided.

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