U.S. patent application number 13/476783 was filed with the patent office on 2013-03-21 for methods and apparatus for nanomembrane-based nucleic acid sensing platform for portable diagnostics.
This patent application is currently assigned to UNIVERSITY OF NOTRE DAME DU LAC. The applicant listed for this patent is Hsueh-Chia Chang, Li-Jing Cheng, Satyajyoti Senapati, Zdenek Slouka. Invention is credited to Hsueh-Chia Chang, Li-Jing Cheng, Satyajyoti Senapati, Zdenek Slouka.
Application Number | 20130068632 13/476783 |
Document ID | / |
Family ID | 47879602 |
Filed Date | 2013-03-21 |
United States Patent
Application |
20130068632 |
Kind Code |
A1 |
Chang; Hsueh-Chia ; et
al. |
March 21, 2013 |
METHODS AND APPARATUS FOR NANOMEMBRANE-BASED NUCLEIC ACID SENSING
PLATFORM FOR PORTABLE DIAGNOSTICS
Abstract
A DNA/RNA detection technology is provided. The open flow
detection technique includes a substrate defining a pair of
opposing microchannels, a pair of opposing electrodes in the
opposing microchannels, and at least one ion exchanging
nanomembrane coupled between the opposing microchannels such that
the opposing microchannels are connected to each other only through
the nanomembrane, wherein the nanomembrane is functionalized with a
probe complementary to the macromolecule. A voltammeter is provided
to measure the electrical current or potential across the
nanomembrane, and detect a change in the measured electrical
current or potential to quantify the presence of the
macromolecule
Inventors: |
Chang; Hsueh-Chia; (Granger,
IN) ; Slouka; Zdenek; (South Bend, IN) ;
Senapati; Satyajyoti; (Mishawaka, IN) ; Cheng;
Li-Jing; (Granger, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chang; Hsueh-Chia
Slouka; Zdenek
Senapati; Satyajyoti
Cheng; Li-Jing |
Granger
South Bend
Mishawaka
Granger |
IN
IN
IN
IN |
US
US
US
US |
|
|
Assignee: |
UNIVERSITY OF NOTRE DAME DU
LAC
Notre Dame
IN
|
Family ID: |
47879602 |
Appl. No.: |
13/476783 |
Filed: |
May 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61519331 |
May 20, 2011 |
|
|
|
Current U.S.
Class: |
205/780.5 ;
204/415; 205/787; 205/792; 205/793; 977/755; 977/904 |
Current CPC
Class: |
B82Y 30/00 20130101;
G01N 27/3278 20130101; G01N 27/44704 20130101; G01N 27/4146
20130101 |
Class at
Publication: |
205/780.5 ;
205/793; 205/792; 205/787; 204/415; 977/755; 977/904 |
International
Class: |
G01N 27/414 20060101
G01N027/414 |
Goverment Interests
GOVERNMENT LICENSE RIGHTS
[0003] This invention was made with government support under NSF
EFRI0937997 awarded by the National Science Foundation (NSF). The
government has certain rights in the invention.
Claims
1. A quantitative method for analyzing a macromolecule comprising:
making an assay mixture containing the macromolecule; providing a
microchamber electrochemical cell comprising: a substrate defining
a pair of opposing microchannels; a pair of opposing electrodes in
the opposing microchannels; at least one ion exchanging
nanomembrane coupled between the opposing microchannels such that
the opposing microchannels are connected to each other only through
the nanomembrane, wherein the nanomembrane is functionalized with a
probe complementary to the macromolecule; and a device for
measuring at least one of measuring electrical current or potential
across the nanomembrane; flowing the assay mixture through the
opposing microchannels of the microchamber electrochemical cell
such that assay mixture contacts the nanomembrane in a manner
suitable for hybridization of the assay mixture with the probe;
connecting the opposing electrodes to the device for measuring at
least one of measuring electrical current or potential across the
nanomembrane; and detecting a change in the measured electrical
current or potential across the nanomembrane to quantify the
presence of the macromolecule.
2. A method as defined in claim 1, wherein the nanomembrane is
provided in a nanoslot.
3. A method as defined in claim 2, wherein a depth of the nanoslot
is substantially the same as a depth of the microchannel.
4. A method as defined in claim 1, further comprising providing an
AC current across the pair of opposing electrodes.
5. A method as defined in claim 1, wherein the macromolecule is at
least one of a DNA macromolecule, an RNA macromolecule, a protein
macromolecule, or an organic polymer macromolecule.
6. A method as defined in claim 1, further comprising deionizing
the assay mixture proximate the nanomembrane.
7. A method as defined in claim 1, further comprising
bulk-to-membrane ion flux over the depletion region formed by the
membrane at over-potentials.
8. A method as defined in claim 1, further comprising forming a
non-uniform electroosmotic flow.
9. A method as defined in claim 8, wherein the non-uniform
electroosmotic flow leads to microvorticies formed in the fluid
flow.
10. A method as defined in claim 8, wherein the non-uniform
electroosmotic flow leads to a detectable enhanced ion current.
11. A method as defined in claim 1, further comprising eliminating
the overlimiting current when the surface is effectively
electroneutral.
12. A method as defined in claim 11, wherein eliminating the
overlimiting current comprises providing an oppositely charged
membrane from the hybridized or functionalized molecules, to invert
the charge on the membrane surface
13. A method as defined in claim 1, further comprising optimizing a
frequency for each electrolyte strength.
14. A method as defined in claim 13, wherein optimizing the
frequency scales as a function of D/.lamda..sup.2 where D is the
molecular diffusivity of the molecule and .lamda. is the Debye
length for the given electrolyte strength.
15. A microchamber electrochemical cell comprising: a substrate
defining a pair of opposing fluid microchannels for flowing a
solution of target macromolecules; a pair of opposing electrodes in
the opposing microchannels; at least one ion exchanging
nanomembrane connecting together the opposing fluid microchannels
such that the opposing fluid microchannels are connected to each
other only through each nanomembrane; a device for measuring at
least one of measuring electrical current or potential across the
nanomembrane; and a processor adapted to analyze the measured
electrical current or potential across the nanomembrane to
determine the presence of the target macromolecule.
16. A microchamber as defined in claim 15, further comprising a
nanoslot housing the nanomembrane.
17. A microchamber as defined in claim 16, wherein a depth of the
nanoslot is substantially the same as a depth of the
microchannel.
18. A microchamber as defined in claim 17, further comprising an AC
current supply for supplying an AC current across the pair of
opposing electrodes.
19. A real-time quantitative method comprising: preparing an assay
mixture comprising a target template; providing a microchamber
electrochemical cell comprising: a substrate defining a pair of
opposing fluid microchannels; a pair of opposing electrodes in the
opposing fluid microchannels; and at least one ion exchanging
nanomembrane housed in a nanoslot fluidly connecting together the
opposing fluid microchannels such that the opposing fluid
microchannels are connected to each other only through the
nanomembrane, the height porosity of the nanomembrane preventing
the passage of the target template therthrough, wherein the
nenomembrane is functionalized with a probe complementary to the
target template; and a device for measuring at least one of
measuring electrical current or potential across the nanomembrane;
flowing the assay mixture through the opposing fluid microchannels
of the microchamber electrochemical cell such that the assay
mixture is in fluid communications with the nanomembrane; coupling
the opposing electrodes to the a device for measuring at least one
of measuring electrical current or potential across the
nanomembrane; measuring an AC electrical current or potential
across the opposing electrodes while providing an AC sinusoidal
electromagnetic perturbation; and detecting a change in the
measured electrical current or potential across the nanomembrane to
quantify the presence of the macomolecule.
20. A method as defined in claim 19, wherein the nanomembrane is
provided in a nanoslot.
21. A method as defined in claim 20, wherein a depth of the
nanoslot is substantially the same as a depth of the microchannel.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a non-provisional application claiming
priority from U.S. Provisional Application Ser. No. 61/519,331,
filed May 20, 2011, entitled "Electrokinetic Membrane Biosensor"
and incorporated herein by reference in its entirety.
[0002] This application is also related to International Patent
Application No. PCT/US10/55679 entitled "Microchamber
Electrochemical Cell Having a Nanoslot," filed Nov. 5, 2010, the
contents of which are incorporated herein by reference in their
entirety.
FIELD OF THE DISCLOSURE
[0004] The present description relates generally to microfluidic
sensing technology and more particularly to methods and apparatus
for nanomembrane-based nucleic acid sensing platform for portable
diagnostics.
BACKGROUND OF RELATED ART
[0005] The development of rapid, portable, cheap, and/or
easy-to-use detection devices for point-of-care application is
oftentimes a challenge for the modern medical diagnostic industry
to effectively diagnosis any number of diseases, including diseases
that result in deaths of millions each year in developing
countries. For example, conventional laboratory based technology,
such as microarray, reverse transcription polymerase chain reaction
(RT-PCR), etc. is relatively slow, employs multistep procedures,
and/or uses bulky, expensive fluorescent detection units operated
by trained technician. The cumbersome equipment requirements
typically restrict the usage of such systems to the laboratory
settings. The present disclosure provides for a nanomembrane based
electrochemical nucleic acid detection platform that can be turned
into a hand-held, portable device operated by workers with minimal
instruction.
[0006] Rapid and portable devices for point-of-care application
would allow for recognition of contamination and effective
diagnosis of diseases that result in the deaths of millions each
year in developing countries. The main challenges for the platform
have been the elimination of sophisticated instruments and
reagents, reduction in size to allow portability, acceptable
detection sensitivity and robustness towards field sample
variability, and sufficiently high assay rapidity to be compatible
with portability. Presently, the genetic identification is mostly
achieved by Enzyme-linked immunosorbent assay (ELISA), microarrays,
and/or by real-time polymerase chain reaction (PCR). As previously
noted, however, these conventional laboratory based technologies
are relatively slow, employ multistep procedures, and use bulky and
expensive fluorescent detection units operated by trained
technicians. The cumbersome equipment requirements restrict the
usage of such systems to the laboratory settings. Recent progress
in dip-stick ELISA type assay is intended to circumvent the
instrumentation and personnel demands, but its sensitivity remains
unacceptable for field employment.
[0007] Electrochemical sensing with molecular probes functionalized
onto the electrode sensor has also been developed as a candidate
for label-free detection, particularly those that link the probe to
the electrode with a linker that can enhance the electron transfer
rate to the electrode once the target DNA has hybridized onto the
probe. However, such self-assembled layer sensors remain unstable
and hence not currently robust to the sample variability. Another
label-free sensor technology that has been developed is the DNA
chip technology which uses capacitance and field-effect transistor
(FET) structures. Both techniques rely on the detection of the
charges brought to the sensor surface by the hybridized target DNA.
However, a recent survey has found that only DNA charges within the
Debye electric double layer on the sensor can produce a capacitance
or field-effect transistor signal. As the Debye layer is only a few
nm thick under most practical conditions, there is a limit to the
sensitivity of such capacitance and FET sensors, typically
nano-molar. There is also a relatively significant fabrication cost
typically associated with the capacitance and FET sensors.
Electrochemical sensing and FET sensors all commonly suffer from
long assay time as the hybridization reaction rate is limited by
the diffusion of the molecules towards the probe, which can usually
take hours for typical sample volumes.
[0008] As such, it is apparent that there is a need for an improved
DNA/RNA detection technology. The present disclosure represents a
new microfluidic technology that fully exploits the small spatial
dimensions of a biochip and some new phenomena unique to the micro-
and nanoscales. More specifically, the present disclosure addresses
all the typical requisites for portable on-field applications:
fast, small, sensitive, selective, robust, label- and reagent-free,
economical to produce, and possibly PCR-free.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is an illustration of an example open-flow DEP
chip.
[0010] FIG. 2A is an illustration of an example enrichment and
depletion across a nanoporous silica granule.
[0011] FIG. 2B is an example plot showing the depletion of charged
fluorescent dye between two microreservoirs.
[0012] FIGS. 3A-3D together are diagrams showing an example effect
of entrance surface charge density and polarity on ion transport in
a nanochannel.
[0013] FIG. 4A is graph showing a current voltage (I-V) curve for
an example device of FIG. 4B after hybridization.
[0014] FIG. 4B is an example nanomembrane device constructed in
accordance with the teachings of the present invention.
[0015] FIG. 4C is a schematic illustration of an example nucleic
acid hybridization onto immobilized oligoprobes.
[0016] FIG. 5 is an illustration of an example device suitable for
FCS detection of DNS hybridization.
[0017] FIGS. 6A-6C together illustrate images of an example
fluorescence of an example trapping electrode tip.
[0018] FIG. 6D is a graph showing an example plot of detection time
with decreasing concentration.
[0019] FIG. 6E is a graph showing an optimum frequency for a given
electrolyte strength.
[0020] FIG. 6F is a graph showing fluorescence intensity from
different example flow rates.
[0021] FIG. 7A is optical microscope image of an example
pre-concentrator based on a charge-selective membrane.
[0022] FIG. 7B is an illustration of an example electroosmotic flow
of fluorescently labeled molecules in the device of FIG. 7A.
[0023] FIG. 8A is a graph showing the Warburg impedance spectrum of
an example nanoslot.
[0024] FIG. 8B is a graph showing a shift in the intercept allowing
quantification of the number of target macromolecules in the
microchannel.
[0025] FIGS. 9A-9C together illustrate an example field-enhanced
water disassociation.
[0026] FIG. 10 is a graph showing the Warburg signal measured
across the example membrane sensor assembly.
[0027] FIG. 11 is an example integrated hybridization sensor in
accordance with the teachings of the present disclosure.
[0028] FIGS. 12A-12B together are illustrations of another example
device constructed in accordance with the teachings of the present
disclosure and showing a graph of the change in characteristics of
an I-V plot.
DETAILED DESCRIPTION
[0029] The following description of example methods and apparatus
is not intended to limit the scope of the description to the
precise form or forms detailed herein. Instead the following
description is intended to be illustrative so that others may
follow its teachings.
[0030] As noted above in the background, development of rapid and
portable detection devices for point-of-care application is
believed to be an important aspect of the modern diagnostics
industry for effective detection of diseases in developing
countries, from anti-terrorism and biowarfare applications to
environmental monitoring, including the detection of harmful
organisms on beaches. The most specific known sensing platform is
currently the genetic detection platform, which identifies a
particular sequence of the target pathogen's genome. As a result of
active research in this area, small pretreatment units are now
available that can concentrate the pathogens with membranes and
beads, lyse cells, and remove chromosomal DNA for amplification in
an integrated PCR chip. However, the key technological bottleneck
remains the detection and quantification of the amplified DNAs.
[0031] Two known standards for genetic detection have appeared in
the last decade, both involving labeling of fluorophores or
quenchers onto the target molecule during PCR amplification: DNA
microarray and real-time PCR. DNA microarrays offer sensitivity and
large library volume. However, the assay time is long due to
diffusion limitations. It also requires periodic rinsing to avoid
nonspecific binding. Finally, the fluorescent confocal detection
instrumentation is still too bulky and costly for portable
applications. Quantification of the number of target DNAs is also
impossible. Real-time PCR sacrifices large library volume for rapid
and quantifiable detection, higher sensitivity, and good
selectivity. However, it still requires bulky fluorescent detection
instrumentation as well as expensive reagents. For instance, Model
ViiATM7 available from Applied Biosystems is the size of a small
refrigerator and can cost thousands of dollars. Thus, one challenge
for portable diagnostics is then a miniature label-free nucleic
acid sensing platform without any sophisticated instruments and
reagents. The elimination of the PCR step would also be
advantageous, as it would remove the 30-min thermal cycling time
and the need for a PCR unit. In many medical applications, over a
million DNA and RNA copies are available in a typical sample volume
of 100 .mu.L. Consequently, a detection platform capable of sensing
one million copies of DNA/RNA can be PCR-free. For bacterial
pathogens, each cell produces a million copies of mRNA and only one
copy of DNA. However, the tradeoff for this relative abundance of
RNA is its short life-time (typically less than an hour) due to
rapid degradation. Hence, an RNA detection platform with an assay
time of less than one hour (and without reverse-transcription PCR)
would be the first known RNA detection platform of its kind
[0032] Several label-free field-use DNA/RNA sensing technologies
have been studied in the last decade. The most viable field-use
sensing technology to date appears to be, electrochemical sensing.
Electrochemical sensing with molecular probe functionalized
electrode sensors can measure the change in electron-transfer rate
upon docking of the target DNA/RNA molecules and redox reporter
agents that can magnify this electrochemical current. Because many
current carriers and inhibitors in the buffer can affect this
electrochemical signal, even in the presence of surface-assembled
monolayers, this sensing technology illustrates a lack of
robustness and is oftentimes difficult to calibrate.
[0033] Capacitance, conductance, and FET electrode sensors have
also attracted considerable interest recently. For such
non-Faradaic sensors, excess charges brought to the surface by the
docked DNA/RNA molecules and their associated potential can produce
a local change in Debye double-layer conductance/capacitance and
sub-surface current of the sensor. Conductance measurements are
typically insensitive at practical ionic strengths because the
presence of the DNA/RNA molecules in the high-conductivity Debye
layer would not significantly affect the local conductance.
Moreover, the same Debye layer is only a few nanometers thick for
practical RNA samples, and only the lower fraction of the charges
on the long (>10 kb) linear DNA/RNA is responsible for the
capacitance signal, again resulting in low sensitivity. At its
current state, conductance/capacitance/FET sensors have a detection
limit higher than nanomolar, which translates into approximately
10.sup.8 copies of nucleic acid molecules for practical sample
volumes, which is too high for field-based detection. Additionally,
another drawback of all electrode sensors is their long assay time.
For instance, at the low target molecule concentrations (picomolar)
of practical samples, the diffusion time of long (more than
kilobase) nucleic acids to the electrode sensor often exceeds
hours, thus rendering such a platform ineffective for rapidly
degrading RNA.
[0034] Several techniques have been previously suggested for
removing the slow transport of long nucleic acid molecules to the
electrode sensor. One known technique involves the activation of a
high voltage at the electrode sensor to electrophoretically attract
nearby DNAs. However, this electrophoretic concentration technique
is highly nonspecific and other like-charge molecules can also be
attracted to the sensor. Moreover, for buffers of high ionic
strength, the elevated voltage can produce undesirable Faradaic
reactions that can produce false current or voltage signals.
Internal vortices, generated on microelectrodes by various
ingenious but unreliable mechanisms, have also been suggested as a
means of concentrating the target molecules towards the sensor.
Generation of internal vortices remains, however, an imperfect
science. It would be more desirable for the sensor to generate such
vortices automatically at a precise location and for the vortices
to exhibit a strong electric signal such that they can be detected
and automatically controlled, this new technology will be described
hereinbelow.
[0035] The present disclosure for portable DNA/RNA diagnostics
includes a label-free electrode sensor that at least reduces
diffusion limitation (i.e., short assay time), is relatively highly
selective and sensitive, and yet is relatively insensitive to
buffer ionic strength and chemical composition. The disclosed
example ion-selective membrane sensor technologies, with properly
tuned electrokinetic features and dynamic feedback actuation,
address these specifications.
[0036] The applicants have recently developed an on-chip sol-gel
silica fabrication technique and a nanocolloid assembly technique
for on-chip membrane synthesis. Additionally, the applicants have
also applied several photocuring polystyrene sulfonate or
polyallylamine synthesis techniques to fabricate on-chip membranes.
Still further, as described in International Patent Application No.
PCT/US10/55679, the applicants have developed the technology to
fabricate nanoslots on chips, which behave like single-pore
membranes, for application in diagnostic chips. The membranes are
used for molecular detection and involve continuous pumping of the
sample solution in a cross-flow (tangential to the membrane
surface) format to minimize hydrodynamic resistance. On-chip
electrodes control the ionic current and voltage drop across these
membrane components to produce the desired phenomena for rapid
molecular concentration, transport, and detection.
[0037] Turning now to FIG. 1, an example integrated chip 100 for
rapid detection of kilobase DNA with probe-functionalized
nanocolloid assemblies is shown. The example integrated chip 100 is
passive, not automated, and does not involve feedback control
because the example chip 100 does not include several sensors and
activation components that the present applicants have developed.
In this example, the chip 100 is an open-flow disposable
electrochemical printed (DEP) chip through which nanocolloids
functionalized with complementary oligonucleotides are pumped. The
example chip 100 includes an inlet, a filter, a focuser, a sorter,
a trapper, and an outlet. In one example, a larger colloid (500 nm)
with a long oligonucleotide is focused, sorted, and assembled
passively at a microelectrode gate with symmetric and aligned
top-down electrode pairs. The trapping of nanocolloids occur within
a micrometer-sized region, and DNA solutions, ranging from
picomolar to nanomolar concentrations, are then pumped over the
nanocolloid assembly (membrane). A fluorescent imaging method may
be used to quantify the specificity and concentration factor,
whereas label-free detection yields quantifiable electrical
signals.
[0038] One example solution to the robustness issue is to deplete
the inhibitors and chemicals around the sensor such that close to
deionized water conditions are always produced near the sensor,
regardless of the buffer ionic strength and composition. In
laboratory testing, several of these depletion technologies have
been developed based on fabricated ion-selective nanoslots and
on-chip nanoporous membranes. Significant counterion transport can
rapidly deplete the counterions on one side of the membrane. To
sustain electroneutrality, the co-ions also deplete rapidly to
produce an ion-depleted zone. Sufficiently high DC fields (>100
V/cm) can deionize a 100 mm neighborhood (the depletion zone) near
the membrane. The depletion layer with low interfacial ionic
strength produces the maximum possible ion current without
convection and exhibits a distinct limiting-current plateau in the
polarization I-V or cyclic voltammetry spectrum as illustrated in
the plot 200 shown in FIG. 2A. This nonlinear I-V polarization plot
200 is not due to electron-transfer reactions but bulk-to-membrane
ion flux across the extended and depleted interfacial double layer.
Its sensitivity to the interfacial charge in the depleted double
layer allows sensitive conduction/capacitance detection of
hybridization with the same actuation on-chip electrodes that drive
the ion current.
[0039] At another critical voltage region shown in the plot 200,
the limiting current gives way to a sharp increase in the current,
the overlimiting current, which is a very sensitive signature of
vortices driven by an extended polarized (Debye) layer at the
membrane interface. Nonequilibrium (counter)ion transport across
the ion-selective membrane produces an extended polarized layer and
nonequilibrium over-potential that is orders of magnitude
thicker/higher than the Debye screening length and the equilibrium
zeta-potential. As such field-induced polarization is curvature-
and perturbation-sensitive, the induced electro-osmotic flow is not
uniform and the resulting backpressure can drive microvortices of
specific dimension, and linear velocity at precise voltage windows.
Such microvortices enhance the ion current through the membrane or
nanoslot (hence the overlimiting current) and thus exhibit a
sensitive polarization or single-sweep cyclic-voltammetry
overlimiting signal as shown in the polarization curve (plot 200)
in FIG. 2A. This strong conductance signature allows us to develop
a smart platform that can generate such vortices on demand.
Concentration of the charged dye by five orders of magnitude, as
illustrated in FIG. 2B by the plot 210, is mostly due to convective
concentration of the molecules at the stagnation points of the
vortices. Other than the distinctive conductance signals of the
membrane depletion/vortex phenomena, their actuation and sensing
time is also very rapid. With thin membranes and short nanoslots as
shown in FIG. 2A, the ion depletion and hydrodynamic timescales
range from microseconds to seconds, allowing for rapid
automation.
[0040] Specifically, FIG. 2A illustrates the depletion of charged
fluorescent dye 212 at one entrance of a 50 nm nanoslot between two
circular microreservoirs 214. The depletion has a very distinct
polarization signature: the current plateaus at a limiting current
value when depletion occurs. When vortices are observed in both the
silica granule and the nanoslot beyond a critical voltage, the
polarization (single-sweep cyclic voltammetry) curve shows a large
overlimiting current beyond the limiting current plateau. The plot
200 shows the polarization characteristics of the nanoslot for
different ionic concentrations of solution. Linear polarization
curves missing the limiting region can be observed for
concentrations above 0.1 mM. The disappearance of the limiting
region is given by the loss of the ion-selective properties of the
nanochannel as a result of decreasing Debye layer thickness inside
the nanochannel.
[0041] As illustrated in FIG. 2B, enrichment and depletion across a
nanoporous silica granule synthesized within a glass chip by
sol-gel chemistry, produces a five orders of magnitude
concentration of ions on one side of the granule and a comparable
degree of ion depletion on the other side. More particularly, an
example high magnification SEM image 220 of a silica granule with
superimposed plot showing ion concentration is shown. Meanwhile, a
schematic illustration of counterion movement 222 is also shown.
Examination of the plot 210 illustrates that the concentration
factor c/c1 as a function of the ionic concentration (c1) of the
fluorescent solution for different sizes of silica beads.
[0042] Turning now to FIGS. 3A-3D, as can be seen, the ion current
across an ion-selective medium can be very sensitive to the charge
polarity and density on the surface outside the medium. For
example, as illustrated in FIG. 3A, alumina nanochannels
demonstrate that with negatively charged SiO.sub.2 entrance
side-walls, the ion conductance across the positive-charged
Al.sub.2O.sub.3 nanochannel is suppressed and shows a nonlinear I-V
characteristic at 300, 302. The ion charge inversion induced by the
heterogeneous entrance charge enhances ion depletion as shown in
FIG. 3B and hence creates a large voltage drop at the channel
entrance as seen in FIG. 3C. The heterogeneous entrance charge
efficiently suppresses the flow of counterions through the nanoslot
(anions in the case of the positively charged Al.sub.2O.sub.3
nanochannel). This effect is clearly seen in FIG. 3D and is
reflected in the measured I-V curves 300, 302 depicted in FIG. 3A.
The ion conductance is found to change significantly when the
surface charge of the entrance side-walls converts its polarity and
density. The shift of ion conductance induced by surface charge
conversion will be utilized as a basis of DNA/RNA sensing.
Hybridization of DNA or RNA on a positively charged anion-selective
medium can be detected by measuring the nonlinear I-V
characteristics.
[0043] More particularly, FIG. 3A illustrates the effect of
entrance surface charge density and polarity on the ion transport
in a 20 nm thick, 60 mm long, positively charged Al.sub.2O.sub.3
nanochannel. In FIG. 3A, a heterogeneous nanochannel entrance 304
induces ion charge inversion at the channel access. A homogenous
nanochannel entrance 306 is also shown. The charge of entrance
side-walls, .sigma..sub.w and the charge of the nanochannel
.sigma..sub.ch appear in opposite polarities, with
.sigma..sub.w/.sigma..sub.ch<0. The figure also illustrates the
experimental I-V characteristics of an Al.sub.2O.sub.3 nanochannel
device with negatively charged silica entrance side-walls 300 and
Al.sub.2O.sub.3 entrance side-walls 302 measured with 0.1 mM KCl.
FIG. 3B illustrates the calculated Cl ion 310 and K ion 312
distributions near left channel entrance with values of
.sigma..sub.w/.sigma..sub.ch varying from 2 to -2. FIG. 3C
illustrates the calculated potential profile 320 along the
nanochannels with varied .sigma..sub.w/.sigma..sub.ch
(.sigma..sub.ch=4.5 mC/m.sup.2) under V.sub.d (Voltage applied
across the nanochannel)=5 V. FIG. 3D, meanwhile illustrates a
summarized theoretical ion current density 322 (current per channel
width) of the nanochannels with varied
.sigma..sub.w/.sigma..sub.ch.
[0044] Another step is the proper development of surface chemistry
to attach addressable probes onto different membrane sensors. In
the disclosed example, this can be achieved by patterning
UV-curable acrylic-based polymers inside the microfluidic channel
doped with different monomers containing charged or functional
groups. Such polymers may be ion-selective and provide reactive
chemical groups on their surfaces for the attachment of DNA/RNA
probes. The functionality of at least some of the example devices
disclosed herein relies on the ion-selectivity of the polymeric
material, which is less dependent on ionic strength than the
nanofluidic counterparts. Briefly, using photolithographic
techniques, cation- and anion-exchange membranes are defined in
glass microfluidic channels by crosslinking positively charged
diallyldimethylammonium (DADMA) and negatively charged
2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) using a
crosslinker (N,N'-methylene bisacrylamide) and photo-initiator.
Each example membrane has a defined width and length of few tens to
hundreds of micrometers, bridging two microfluidic channels that
are about 20 mm deep and 20-100 mm wide. In one instance, the pore
size of the nanoporous membrane can be controlled by varying the
concentration of the monomers and crosslinker. To achieve surface
functionalization of the oligo probes, the surface of an
anion-exchange membrane is modified with amino groups by using
allylamine as an additive in the prepolymer solution. The DNA or
RNA probe (.about.27 bases) pre-attached with functional groups of
choice can then be used to functionalize the probes onto membrane
surface. Through examination by microscope and measurement of the
ability to deplete ions, the polymerization time and the
concentrations of crosslinker and photo-initiator have been
optimized to produce reproducible, well-defined ion-selective
membranes with functional chemical groups inside microchannels.
[0045] For example, turning to FIGS. 4A-4C, there is shown
experimental evidence that the onset voltage and the onset of
overlimiting current, features of the nonlinear I-V curve of our
sensor, are sensitive to nucleic acid hybridization onto oligo
probes functionalized onto the surface of the ion-selective medium,
as the resulting change in the surface charge can enhance or
eliminate the extended Debye layer. Specifically, FIG. 4A
illustrates a plot 400 of a I-V curve of a bare membrane 402, an
oligo probe attachment 404, and a hybridization 406. The voltage
differential between the plots 402, 404, 406, is particularly large
because of the nearly infinite differential resistance at the
limiting current conditions. More particularly, significant change
of I-V characteristics in the overlimiting current regime is
observed after RNA hybridization from a picomolar sample. The 50%
change in conductance is compared to typical 5% changes of
electrochemical electrode sensors at the same concentration
(low-voltage region). In contrast, the low-voltage linear ohmic
region, where classical electrochemical sensors operate, registers
an insignificant shift. Conveniently, the depleted and extended
double layer, which can be three orders of magnitude thicker than
the Debye layer, also allows more charges on the target RNA to
contribute to the effective surface charge. If the membrane is
oppositely charged from the hybridized or functionalized molecules,
the latter can even invert the charge on the membrane surface,
eliminating the overlimiting current sustantially when the surface
is effective electroneutral with exact compensation. The result is
a very sensitive RNA sensor with picomolar sensitivity, compared to
the nanomolar sensitivity of most electrode electrochemical
sensors.
[0046] Referring to FIG. 4B, an example sensor 410 includes a
plurality of microchannels 412, 414, and an oligo probe
functionalized membrane 416. A current source 418 and a voltammeter
420 are also included, the operation of which is described in
detail herein. As will be detailed further, the sensor 410 may
include any number of microchannels including multiple pairs of
microchannels. Additionally, as will be appreciated, the
microchannels can each function as an inlet and/or outlet channel
as desired. FIG. 4C, meanwhile illustrates the membrane 416 prior
to hybridization 440 and post hybridization 442.
[0047] Dielectrophoresis (DEP), a molecular force due to induced
molecular dipoles, has been shown to be an effective means of
concentrating large DNA/RNA molecules into the depleted region near
the membrane surface (see FIG. 4B) where the probes are
functionalized. The electric field is focused by the nanopores in
the membrane to produce a high field gradient at the membrane
interface. A polarizable molecule in the bulk, with a large induced
dipole, would then experience a net force towards the high-field
region (the membrane surface). With the intense field amplification
of nanopores, this DEP force on the molecules can overcome
molecule--membrane repulsive interaction.
[0048] In one example fluorescent correlation spectroscopy (FCS)
experiment this domination of dielectrophoretic attraction over
like-charge repulsion with floating probe-functionalized carbon
nanotubes (CNTs) and the fluorescently labeled kilobase target
single-stranded DNA (ssDNA) was confirmed. Because CNTs quench the
fluorophores on hybridization of target DNA, reduction in the
fluorescent intensity can be used to quantify the hybridization
degree and the attraction of the molecules to the nanoelectrode. As
seen in the experimental setup of FIG. 5A, and the results of the
experiment of FIG. 5B, an FCS detection experiment 500 of DNA
hybridization from a picomolar solution is shown. The setup 500
includes a Detector 502, a Flourescence 504, a laser 506, an
objective 508, and a flourescent quench of ssDNA 510.
[0049] As a result of the experimental setup 500, as shown in FIG.
5, dielectrophoretic attraction due to the field-focusing CNTs
allows hybridization in less than 2 min at picomolar concentrations
520. In contrast, the diffusion time for the long ssDNA at this
concentration is hours 522.
[0050] Instead of floating CNTs, one example system is able to
utilize 50 nm nanoslots on glass (see FIG. 3B) and is able to show
concentration of ssDNA to the nanoslot. This same DNA concentration
is shown with the nanoporous membrane 416 in FIG. 4B, with a
concentration factor of up to five orders of magnitude.
Alternatively, 100 nm nanocolloids can be assembled into a
nanocolloid crystal (e.g., a membrane) at a top-down electrode pair
by nanocolloid DEP, such as shown in FIGS. 6A-6F. The 10 nm spacing
between the nanocolloids focuses the electric field of the
electrode gate and can rapidly (order of seconds) trap and
concentrate ssDNA molecules of a Green Crab species from a
picomolar solution onto the on-chip nanocolloid membrane by
molecular DEP.
[0051] Specifically, as illustrated in FIG. 6A-6C, a properly tuned
DEP force can drive the DNAs towards the nanostructure against
electrostatic repulsion from the likecharged structures, but they
will not deposit onto the surface until they are convected to a
sharp tip (.about.10 .mu.m) at the nanostructure, see FIG. 6A. More
particularly, FIGS. 6A-6C illustrate a schemetic of fluorescence
images of the trapping electrode tip in FIG. 2A, showing the 100 nm
nanocolloid assembly at fixed times after the Green Crab DNA
solution had been injected but at different AC frequencies 600,
602, 604. For instance in the schematic 600, the chosen AC
frequency causes the dielectrophretic force (F.sub.DEP) to be
approximately the same as the repulsive force (F.sub.REP). In the
schematic 602, the chosen AC frequency causes the dielectrophretic
force (F.sub.DEP) to be greater than the repulsive force
(F.sub.REP), and in the schematic 604, the chosen AC frequency
causes the dielectrophretic force (F.sub.DEP) to be less than the
repulsive force (F.sub.REP). In these examples, the fluorescence is
detectable only when the ssDNA is concentrated beyond the
micromolar level from the undetectable concentrations (nanomolar to
picomolar) of the injected solution. Trapping at the assembly is
achieved at low frequencies, whereas none occurs at high
frequencies.
[0052] As can be seen from the plot in FIG. 6D, a detection time
increases with decreasing concentration. Specifically, in this
example, a 100 pM concentration 610 compared to a 1 pM
concentration 612 demonstrates a dramatic time difference.
Additionally, as can be seen in FIG. 6E, there is an optimum
frequency for each electrolyte strength 614, 616, 618 with a sharp
minimum in detection time, which scales as D/.lamda..sup.2 where D
is the molecular diffusivity of the molecule and .lamda. is the
Debye length for the given electrolyte strength. Referring to FIG.
6F, fluorescence intensity at 2 min from different flow rates of
100 pM of a 1 kb ssDNA target from a Green Crab species with a 26
base docking segment in the middle (620) and with a complementary
26 base oligo on the nanocolloid (622) or with a single end
mismatch (624). The flow rate window with single-mismatch (624)
discrimination is indicated by a vertical dashed line. The scheme
above the plot of FIG. 6C shows the actual 26 base ssDNA docking
sequence and the location of mismatched bases
[0053] It will be appreciated by one of ordinary skill in the art
that intermolecular interaction can be adjusted to minimize
nonspecific binding. A concentration factor exceeding 10.sup.5
within minutes is observed from the fluorescent imaging in FIGS.
6A-6C thus rapidly and significantly enhancing the sensitivity of
any sensor at the trapping location. The shear rate and AC
frequency can be optimized so that the sensor can selectively
discriminate against kilobase target molecules with a single
mismatch in the 26 base pairing segment in the middle (FIG. 6F).
This shear-enhanced selectivity eliminates the need for rinsing and
washing steps.
[0054] Apart from dielectrophoretic concentration, which is not
effective for small nucleic acids because the DEP force scales as
the cubed power of the hydrodynamic radius of the molecule, the
present disclosure successfully demonstrates rapid analyte
preconcentration based on ion depletion at an ion-selective
membrane in microfluidic chips. Ion depletion at the surface of a
membrane establishes a conductivity gradient across the boundary of
the ion-depleted region. When flux of charged analyte molecules
(due to flow, electrophoresis or other particle forces) occurs
across this boundary, the concentration gradient at the boundary
produces an accumulation of the analyte molecule at the boundary of
the depletion region.
[0055] Referring now to FIG. 7A, a cation-exchange membrane 710
UV-polymerized in a microslot 711 bridging two microfluidic
channels 712, 714 can induce deionization under voltage biases. The
ion-depletion region functioning as an energy barrier traps the
molecule passing across it in an electroosmotic flow tangential to
the membrane 710 on the side. The example UV-curable ion-selective
membrane 710 disclosed herein offers concentration efficiency and
proccessability compared to the microfabricated nanochannels of the
prior devices or Nafion resins. Unlike the 100 nm thick
nanochannels and surface-patterned Nafion thin films, the example
membrane slot 711 has the same depth as the microfluidic channels
712, 714, yielding a large junction area. The large cross-section
area provides greater ion current and better control of
iondepletion in the microchannels 712, 714. Therefore,
preconcentration can be achieved in few seconds. The fluorescence
image 720 in FIG. 7B shows the concentration of labeled molecules
by several orders of magnitude in 10 s from a solution being pumped
by electro-osmosis from the left to right in the top microfluidic
channel 714, after 10 V is applied across the membrane 710. In
particular, the image 720 shows a concentration of fluorescently
labeled molecules taking place 10 s after applying a voltage bias
of 10 V with a scale bar of 50 mm, wherein the microchannel 714
experiences electroosmotic flow (EOF). Moreover, the example
membrane 710 adheres to acrylfunctionalized glass surfaces well.
Although the current membranes are synthesized on glass chips to
allow easy inspection and testing, the same technology can be
transferred to hard polymer chips, which should be cheaper to
produce and easier to bond.
[0056] The presence of the docked RNA/DNA and their mobile
counterions produce a large conductivity change at the depleted
region, which is where most of the voltage drop occurs. Moreover,
the extended Debye (polarization) layer allows more of the charges
on a long (>2 nm) DNA/RNA molecule to contribute to the charging
capacitance and surface-charge compensation on the surface. As
described earlier, the surface charge can sensitively alter the
onset voltage for microvortices and the overlimiting currents that
the vortices contribute to. These effects greatly enhance the
capacitance, conductance, and polarization signatures of the docked
nucleic acids, resulting in sensitive nonlinear I-V polarization
signatures, such as those due to the charge-inversion after
hybridization shown in FIG. 4A.
[0057] The dynamics of depletion layer formation with strong
charging also exhibits a distinct capacitance signature in the AC
impedance spectrum, such as shown in FIG. 8B. A plot 800 of a
Warburg spectrum has a constant phase angle of .pi./4, whose
modulus increases with decreasing frequency and is classically
associated with diffusion controlled ion transport. For instance,
as shown in FIG. 8B, the warburg impedance spectrum 800 of the
nanoslot illustrated in FIG. 2B, shows a shift to lower resistance
with a 2.7 nM 1 kb E. coli ssDNA solution relative to the control
without DNA. The identified shift in the intercept with the real
axis between a 2.7 nM 1 kb E. coli ssDNA solution 804 relative to
the control without DNA 806 allows precise quantification of the
number of ssDNA molecules 808 in the microreservoir down to
10.sup.7 copies (FIG. 8A).
[0058] It will be appreciate that under an AC field, the depletion
region next to a membrane sensor is created periodically during the
half-cycle when the mobile counterions are driven into the nanoslot
or on-chip membrane 711. The depletion layer dynamics was verified
by high-speed confocal imaging to be a diffusive one such that its
thickness grows in a self-similar manner as {square root over (Dt)}
and was shown to exhibit the Warburg spectrum, with a constant
phase of .pi./4 (FIG. 8B). The intercept of the Warburg spectrum
with the real axis represents the limiting ion flux when the
depletion layer is smallest in dimension--just above the critical
voltage where the depletion phenomenon can be sustained. It hence
offers an accurate estimate of the low conductivity in this small
region, as most of the voltage drop occurs there. As mentioned
earlier, the presence of a few macroions attracted to this small
depleted region by DEP can significantly change its local
conductance. In FIG. 8B, there is illustrated sensitive detection
of E. coli ssDNA below nanomolar concentrations or 10.sup.7
molecules. With a reduction of the nanoslot width, down to the
micrometer size of the nanocolloid assembly in FIG. 6A, the
detection limit can be expected to reach below picomolar
concentrations or 10.sup.5 copies of nucleic acid.
[0059] The same Warburg signal can be captured with the field
across the nanocolloid assembly of FIG. 6A to allow label-free
quantification of the docked DNA/RNAs. This large-voltage AC
impedance technique is quite distinct from the classical
low-amplitude impedance spectroscopy for electron transfer rates
because the current example induces nonequilibrium ion transport
through the ion-selective nanoslot or membrane to produce extended
polarized Debye layer and concentration depletion layers.
[0060] The single mismatch (SNP) discrimination capability of the
device shown in FIG. 6F is due to hydrodynamic shear. It will be
understood that shear is most discriminating because it can meter
small thermal-energy-level hydrogen bond energies to dehybridize
the target molecules. Microscale bipolar membrane technologies may
be used to control the local pH in microfluidic chips to improve
both the specificity and selectivity of the membrane sensor. These
bipolar membranes/nanopores exhibit distinct hysteretic I-V
polarization and cyclic voltammetry signatures due to local
field-induced water-breaking reactions that generate more ions. An
image 900 of the pH fronts generated by a UV-polymerized bipolar
membrane composed of positively charged dimethylammonium and
negatively charged sulfonic groups are shown in FIG. 9C. The ion
currents can drastically increase when reversely biased at a high
voltage, forming a breakdown regime. In accordance with the second
Wien effect, the ionic current breakdown results from the enhanced
water dissociation into cation (H.sup.+) and anion (OH.sup.-) at
the bipolar junction, in which a strong electric field greater than
10 MV/cm can build up at a reverse bias. These membrane actuation
components can be used to control the local pH for the example
integrated devices disclosed herein, with feedback control based on
the distinctive hysteretic polarization signals and I-V
characteristics seen in FIGS. 9A and 9B, respectively.
[0061] More particularly, FIGS. 9A-9C illustrate that
field-enhanced water dissociation increases ionic currents in a
reverse-biased 20 nm thick bipolar-junction nanofluidic channel
containing positive and negative surface charges (|Vm|>0.6 V)
(plot 902 at FIG. 9A), and a UV-polymerized bipolar membrane
(Vm<-10 V) (plot 904 at FIG. 9B). Still further, Hydroxide ions
and protons are produced at the bipolar membrane junction and
transport to opposite sides of the membrane as shown at FIG. 9C
(906). The pH change of the solution in the microchannels can be
observed with a mixture of universal pH indicator, wherein in the
example illustrated, the left half of the bipolar membrane is
positively charged whereas the right half is negatively
charged.
[0062] Integration of the example chips disclosed herein into the
passive sensor chip 1000, may lead to an interrogated Warburg
signal 1030 which may be seen illustrated in FIG. 10. The detection
limit of the Warburg impedance signal is picomolar concentrations
or about the desired 10.sup.5 copies, the detection time is about
15 min, and the selectivity is three mismatches in the 27 base
pairing segments. The example device 1000 shows that long kilobase
target ssDNA produces a larger signal, consistent with the extended
Debye layer allowing more of the charges of a long molecule to
contribute to the local charge capacitance and conductance. A
sensor of the present invention is capable to differentiate a Green
Crab species from a congener species with three mismatches over the
26 pair docking segment due to the hydrodynamic shear offered by
the high through flow.
[0063] An example multi-target unit 1100 is shown in FIG. 11 as a
schematic view. The example multi-target unit 1100 generally
comprises an integrated smart RNA hybridization sensor composed of
a plurality of nanomembrane sensing units, 400, a preconcentration
unit 1110, and dehybridization unit 1120. In operation, the unit
1100 offers sequential detection of different targets by moving the
sample from one sensor 400 location to the next with the depletion
technique. A processor 1140 receives data regarding the sensed
hybridization and is able to make a determination regarding the
presence and/or lack of target material based upon the sensed data.
While the present unit 1100 may be well suited for small sample
volumes, for larger volumes, the unit 1100 may be implemented in a
parallel design that allows for multiple sample movements.
[0064] The Nanoporous membranes disclosed herein can greatly enable
and sensitize on-chip molecular sensing. The example disclosed can
deplete inhibitors near their surface where the probes are
functionalized, such that the platform is robust to a large variety
of sample ionic strengths and pH. Still further, the same
ion-depletion dynamics extends the Debye layer and hence allows
more sensitive conductance and capacitance detection of the
hybridized molecules. The high field in the same depletion region
can produce fast dielectrophoretic trapping of the larger target
molecules. If the depletion region extends across the entire flow
channel, it can also trap smaller molecules. Hence, by activating
different membrane components on a chip, the molecules can be
concentrated and transported to different sensors. The membrane's
ability to invert its surface charge upon hybridization produces a
large conductance signal for hybridization. A large capacitance
signal is also produced, corresponding to the intercept of the
Warburg spectrum with the real axis, when the depletion layer is
formed periodically under an AC field such that the hybridized
target molecules and their counterions are responsible for this
asymptotic conductance when all other ions are depleted within the
small depletion layer. These nanoporous membranes are fabricated on
the chip and are situated on the side of the flowing channel
without blocking the flow, such that a high throughput (>1
mL/min) can be achieved. Bipolar nanoporousmembranes can also be
used to split water and to exercise precise control of pH near the
sensor, to enhance selectivity. This rapid and precise pH control
can also allow multitarget sensing with the same probe if the
probes are designed to be pH-sensitive.
[0065] Referring now to FIG. 12A, there is illustrated another
schematic representation of a nanomembrane sensing platform 1200
including a single-sensor device 400 in Polydimethylsiloxane (PDMS)
with a sensing region 1210 having a dimension of approximately 5
mm.times.5 mm. In operation, the example sensing platform 1200
utilizes a positively charged anion-exchange membrane 1220 placed
to bridge two microchannels 1230, 1232. The example membrane 1220
includes DNA probes (27 nt.) functionalized on one of its openings.
As the fluid flows through the microchannels 1230, 1232, the I-V
characteristics were measured via Pt electrodes using the
previously described circuit, and the processor 1140.
[0066] As illustrated in FIG. 12B, a significant change of I-V
characteristics in the over limiting-current regime was observed
after complementary 10 nM RNA (1250) or 1 nM DNA (1252)
hybridization. The 30% to 50% change in conductance is observed
compared to typical 5% changes of electrochemical electrode sensors
at the same concentration (low-voltage region). With almost no
voltage shift for the negative control sample, the sensor exhibits
excellent selectivity.
[0067] It will be further appreciated by one of ordinary skill in
the art that the membrane disclosed herein may be any suitably
formed membrane, including a membrane specifically pre-formed and
or formed during operation of the device illustrated. For example,
the membrane may be formed by the interaction of a nanocolloid,
nanostruicture, etc, in the microchamber as desired.
[0068] Although certain example methods and apparatus have been
described herein, the scope of coverage of this patent is not
limited thereto. On the contrary, this patent covers all methods,
apparatus, and articles of manufacture fairly falling within the
scope of the appended claims either literally or under the doctrine
of equivalents.
* * * * *