WO2008039579A2 - Inorganic nanotubes and devices fabricated therefrom - Google Patents

Abstract

Nanofluidic devices are taught incorporating inorganic nanotubes fluidly coupled to channels or nanopores for supplying a fluid containing chemical or biochemical species. In one aspect, two channels are fluidly interconnected with a nanotube. Electrodes on opposing sides of the nanotube establish electrical contact with the fluid therein. A bias current is passed between the electrodes through the fluid, and current changes are detected to ascertain the passage of select molecules, such as DNA, through the nanotube. In another inventive aspect, a gate electrode is located proximal the nanotube between the two electrodes thus forming a nanofluidic transistor. The voltage applied to the gate controls the passage of ionic species through the nanotube selected as either or both ionic polarities. In either of these aspects the nanotube can be modified, or functionalized, to control the selectivity of detection or passage.

Description

INORGANIC NANOTUBES AND DEVICES FABRICATED THEREFROM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. provisional application serial number 60/814,264 filed on June 15, 2006, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

OR DEVELOPMENT [0002] This invention was made with Government support under Grant No.

CA103071 , awarded by the National Institutes of Health; and Grant No. DE- AC02-05CH11231 , awarded by the Department of Energy. The Government has certain rights in this invention.

INCORPORATION-BY-REFERENCE OF MATERIAL

SUBMITTED ON A COMPACT DISC [0003] Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION [0004] A portion of the material in this patent document is subject to copyright protection under the copyright laws of the United States and of other countries. The owner of the copyright rights has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office publicly available file or records, but otherwise reserves all copyright rights whatsoever. The copyright owner does not hereby waive any of its rights to have this patent document maintained in secrecy, including without limitation its rights pursuant to 37 C. F. R. § 1 .14. BACKGROUND OF THE INVENTION 1. Field of the Invention [0005] This invention pertains generally to nanofluidic devices, and more particularly to electronic devices for fluidic sensing and control. 2. Description of Related Art

[0006] In recent years microfluidics and nanofluidics have arisen as important technologies dealing with the behavior, precise detection, control and manipulation of microliter, nanoliter and even down to femtoliter volumes of fluids. Applications for microfluidics and nanofluidics are wide ranging and of increasing interest in the fields of chemistry, engineering, biotechnology (e.g.,

DNA, lab-on-a-chip), and so forth. Advances in microfluidics are revolutionizing molecular biology procedures for enzymatic analysis (e.g., glucose and lactate assays), DNA analysis (e.g., polymerase chain reaction and high-throughput sequencing), and proteomics. One of the important aspects of microfluidic biochip design is integrating assay operations such as detection, as well as sample pre-treatment and sample preparation on one chip.

[0007] Continued developments are arising in the area of DNA research with using microfluidics. Early biochips were largely based on the concept of a DNA microarray (e.g., the GeneChip DNAarray from Affymetrix) which is a piece of glass, plastic or silicon substrate on which pieces of DNA (probes) are affixed in a microscopic array. Protein arrays have been similarly configured with different capture agents used to determine the presence and/or amount of proteins contained in biological samples. [0008] Increasingly, interest is being directed toward the nanofluidic realm.

Nanofluidics is generally considered the study of the behavior, manipulation, and control of fluids that are confined to structures of nanometer (typically 1- 100 nm ) characteristic dimensions (1 nm = 10^"9 m). The design of these structures often diverges from the microfluidic realm in that fluids confined in these structures exhibit physical behaviors not observed in larger structures as a consequence of the characteristic changes which arise as the physical scaling lengths of the fluid, (e.g., hydrodynamic radius and Debye length) begin to converge on the nanostructural dimensions. [0009] A number of shortcomings exist with existing fluidic devices. Some of the drawbacks with existing devices relate to the transient nature of the testing (non-continuous) as well as the difficulty in registering results.

[0010] Accordingly, a need exists for nanofluidic device technology which can be run continuously while readily registering results. The nanofluidic devices according to the present invention fulfill those needs and others, while overcoming drawbacks of previous devices. BRIEF SUMMARY OF THE INVENTION(S)

[0011] Nanotubes are taught being successfully integrated with microfluidic systems to create nanofluidic devices for chemical and bio-chemical sensing and control. In at least one implementation a nanofluidic transistor is described which provides a fluidic analog of conventional electronic transistors, in that they allow for the electronic sensing and control of select chemical and bio-chemical constituents being fluidically communicated. [0012] One particularly well-suited application for this technology is in single

DNA molecule sensing. Inorganic nanotubes are utilized according to this aspect of the invention as they provide a high aspect ratio while exhibiting translocation characteristics in which the DNA is fully stretched. Transient changes of ionic current indicate DNA translocation events. A transition from current decrease to current enhancement during translocation was observed on changing the buffer concentration, suggesting an interplay between electrostatic charge and geometric blockage effects. These inorganic nanotube fluidic devices represent a category of devices for the study of single bio-molecule translocation with the potential for integration into nanofluidic circuits.

[0013] The integration of inorganic nanotubes into metal-oxide-solution field effect transistors (MOSo/FETs) is also described herein resulting in devices which exhibit rapid field effect modulation of ionic conductance. Surface functionalization, analogous to doping in semiconductors, can switch the nanofluidic transistors from p-type, to ambipolar and n-type field effect transistors. Transient study reveals the kinetics of field effect modulation is controlled by an ion-exchange step. Nanofluidic FETs have potential implications in sub-femtoliter analytical technology and large-scale nanofluidic integration.

[0014] The invention is amenable to being embodied in a number of ways, including but not limited to the following descriptions.

[0015] A first example of the invention is a nanofluidic device, comprising: (a) at least a first and second fluid supply structure configured for supplying a fluid containing chemical or bio-chemical species; (b) a nanotube of inorganic material which fluidly couples at least a first fluid supply structure to the second fluid supply structure; (c) at least a first and second electrode, on opposing ends of the nanotube (e.g., preferably in the nanodevice structure near where it joins to each end of the nanotube), configured for establishing electrical contact with the fluid in the nanotube; and (d) means for detecting or controlling the motion of the chemical or bio-chemical species flowing through the nanotube. Examples of the fluid supply structure include the use of channel structures, nanopore structures, and the like. [0016] In one mode of the invention the apparatus is configured for detecting molecular species, but not for controlling movement. In this mode the means for detecting or controlling is configured for detecting a change of current passing through the nanotube between the first and second electrode. One implementation of this means of detecting current changes comprises: (a) a voltage source configured for establishing a biasing current between the first and second electrode, the biasing current passing through the fluid which comprises an ionic solution containing molecules to be detected; and (b) a current detection circuit configured for registering transient changes in the biasing current in response to the translocation of the molecules through the nanotube. [0017] In a second mode of the invention the apparatus is configured for controlling the movement of ionic species. In this mode the means for detecting or controlling comprises: (a) a gate electrode configured for controlling the flow of ions between at least the first and second fluid supply structure in response to the voltage applied to the gate electrode; (b) wherein the gate electrode is retained proximal the inorganic nanotube (e.g., preferably fully or partially surrounding a portion of the nanotube); and (c) wherein the nanofluidic device operates as a field-effect transistor (FET). It will be appreciated that the gate structure can be configured to operate in either detection, control, or detection and control modes. [0018] Another example of the invention is a nanofluidic sensor, comprising: (a) a nanofluidic system configured for receiving DNA molecules, or other similarly sized molecular chains, retained within a fluid; (b) an inorganic nanotube coupled to the nanofluidic system through which the DNA molecules can be passed; and (c) means for detecting transient current changes through the fluid in response to translocation of the DNA molecules passing through the nanotube. The fluid is ionic and preferably has a known concentration. As a DNA sensor, the nanotube is configured to stretch the DNA molecule while it is passing through the nanotube, for example in response to the nanotube having a sufficiently high aspect ratio to confine the entire DNA molecule during translocation. [0019] This nanofluidic system is preferably configured with nanopores (e.g., membrane nanopores), channels (e.g., nanochannels), or a combination of nanopores and channels for supplying a fluid containing the DNA molecules, or other large molecules, to be passed through the nanotube. In one example, at least one access hole is provided in the microfluidic system through which the fluid is communicated to the nanotube. The nanotube can be modified or functionalized to change its translocation characteristics, such as to make it more specific to selected chemical or bio-chemical molecules. In one mode of the invention, the translocation of the molecules to be detected (e.g., DNA molecules) is electrophoretically driven. [0020] In one implementation of the nanofluidic sensor the means for detecting transient current changes comprises: (a) electrodes positioned toward opposing ends of the nanotube and configured for establishing contact with the fluid which comprises an ionic solution; (b) a voltage source configured for establishing a biasing current through the ionic solution; and (c) means for detecting transient changes in the biasing current in response to the translocation of the DNA molecules.

[0021] Another example of the invention is a nanofluidic transistor, comprising:

(a) a fluidic source; (b) a fluidic drain; (c) an inorganic nanotube coupled in fluidic communication between the fluidic source and fluidic drain; (d) a gate electrode retained proximal the inorganic nanotube for controlling the flow of ions (e.g., anions, cations, or both) between source and drain regions in response to the voltage applied to the gate electrode. In this aspect of the invention, the nanofluidic transistor operates as a field-effect transistor (FET) and, more preferably, a metal-oxide-solution field effect transistor (MOSoIFET). [0022] The fluidic transistor is configured to conduct either or both ionic polarities in response to voltage applied to the gate electrode. Depending on the configuration of the nanotube, the transistor comprises a p-type, n-type or amb/po/ar field effect transistor. The transistor exhibits rapid field effect modulation of ionic conductance. The voltage applied to the gate electrode of the transistor shifts the electrostatic potential distribution inside the nanotubes.

In at least one implementation, the nanotube comprises a silica material and is configured to have a diameter comparable with the diffuse layer of the electrical double layer (EDL) which forms in the nanotube to screen the surface potential which remains non-zero even at the center of the nanotube. [0023] Another example of the invention is a method of detecting molecular species, comprising: (a) establishing a flow path of ionic fluid through an inorganic nanotube configured with a diameter and length adapted for translocation of single molecules of desired molecular species; (b) conducting a current through the ionic fluid in the inorganic nanotube; and (c) detecting current transients in response to a translocation event of the desired molecular species. Additionally, the movement of molecules through the tube can be controlled in response to applying a voltage to a gate disposed adjacent the nanotube between its two ends.

[0024] Another example of the invention is a method of controlling molecular flow in a fluid, comprising: (a) communicating ionic fluid through an inorganic nanotube between a first end and second end; (b) establishing a bias current between the first end and second end which passes through the ionic fluid disposed in the inorganic nanotube; and (c) controlling ionic movement through the nanotube in response to a level of voltage applied to a gate electrode which is retained between the first and second ends of the nanotube.

[0025] It is preferable that the nanotube is configured with a sufficiently small diameter so that an electrical double layer (EDL) forms whose diffuse layer extends at least to the approximate center of the nanotube, thus rendering full control of movement across the whole cross-section of the nanotube. In addition, the nanotube can be functionalized to aid in selectively controlling the movement of molecules through the nanotube. For example, functionalizing can provide for selective control of the movement of anions, cations, or both anions and cations, through the nanotube.

[0026] Described within the teachings of the present invention are a number of inventive aspects, including but not necessarily limited to the following.

[0027] An aspect of the invention is a nanofluidic device technology providing for the electrical sensing and control of chemical and bio-chemical constituents.

[0028] Another aspect of the invention is to provide a nanofluidic device which provides reproducible detection of ionic and molecular species.

[0029] Another aspect of the invention is to provide a nanofluidic device which can be fabricated in a planar layout, and which for example may allow for both optical and electrical probing.

[0030] Another aspect of the invention is to provide a nanofluidic device which can benefit from the use of self-assembly techniques. [0031] Another aspect of the invention is to provide a nanofluidic device for sensing continuous flows or volumes down to sub-femtoliter regimes.

[0032] Another aspect of the invention is to provide a nanofluidic device for sensing fluids communicated through a fluid supply structure, such as channel structures and/or nanopores.

[0033] Another aspect of the invention is to provide a nanofluidic device which can be integrated with membrane nanopores.

[0034] Another aspect of the invention is to provide a nanofluidic device comprising chemically synthesized inorganic nanotubes within a nanofluidic system.

[0035] Another aspect of the invention is to provide a nanofluidic device incorporating inorganic nanotubes, which do not repel water, and within which a charged-oxide surface forms in response to filling with an ionic fluid under a bias potential. [0036] Another aspect of the invention is to provide a nanofluidic device comprising nanotubes having a high aspect ratio, such as having a length on the order of 10 μm , or more preferably of approximately 10 μm .

[0037] Another aspect of the invention is to provide a nanofluidic device using high aspect ratio nanotubes configured to confine entire bio-molecules toward registering translocation characteristics.

[0038] Another aspect of the invention is to provide a nanofluidic device configured for DNA molecular sensing. [0039] Another aspect of the invention is to provide a nanofluidic device configured to determine single molecule translocation in response to detecting current changes through the fluidic channel.

[0040] Another aspect of the invention is to provide a nanofluidic device filled with a fluid, preferably an ionic solution, through which a biasing current can be passed along the length of the passage within the nanotube. [0041] Another aspect of the invention is to provide a nanofluidic device in which ionic current drop events are attributable to the geometrical exclusion effect of conducting ions because of the finite size of λ -DNA which leads to transient ionic current blockage. [0042] Another aspect of the invention is to provide a nanofluidic device configured for detecting DNA whose translocation is electrophoretically driven. [0043] Another aspect of the invention is to provide a nanofluidic device configured for utilizing the critical ion concentration n_cr .

[0044] Another aspect of the invention is to provide a nanofluidic device having at least one nanotube through which fluid conveyance is controlled. [0045] Another aspect of the invention is to provide a nanofluidic device configured as a nanofluidic FET, and more particularly a metal-oxide-solution field effect transistor (MOSo/FET).

[0046] Another aspect of the invention is to provide a nanofluidic FET device which can be configured to selectively conduct either or both ionic polarities. [0047] Another aspect of the invention is to provide a MOSo/FET transistor device which exhibits rapid field effect modulation of ionic conductance. [0048] Another aspect of the invention is to provide a fluidic FET transistor wherein gate voltage changes shift the electrostatic potential distribution inside the nanotube. [0049] Another aspect of the invention is to provide a nanofluidic device in which the kinetics of field effect modulation are controlled by an ion-exchange step.

[0050] Another aspect of the invention is to provide a nanofluidic FET device incorporating a silica nanotube through which ionic species are passed. [0051] Another aspect of the invention is to provide a nanofluidic FET device incorporating a nanotube having an inner diameter on the order of 50 nm , or thinner, and a length on the order of 10-20//m .

[0052] Another aspect of the invention is to provide a nanofluidic FET device incorporating a nanotube having an inner diameter of approximately 40-50 nm , or thinner, and a length of approximately 10-20//m .

[0053] Another aspect of the invention is to provide a nanofluidic FET device having a transparent cover through which the interior of the device can be viewed. [0054] Another aspect of the invention is to provide a nanofluidic FET device having a cover of polydimethylsiloxane (PDMS). [0055] Another aspect of the invention is to provide a nanofluidic device subject to unipolar ionic transport under certain conditions of concentration and biasing.

[0056] Another aspect of the invention is to provide a nanofluidic FET device in which the inner surface, or portions thereof, of the nanotube is modified to control conductance.

[0057] Another aspect of the invention is to provide a nanofluidic FET device which utilizes surface functional ization in an analogous manner to which doping of semiconductors is performed, toward switching nanofluidic transistors from p-type, to ambipolar, and n-type field effect transistors. [0058] Another aspect of the invention is to provide a nanofluidic FET device which is functionalized with three-amino-propylthetheoxylsilane (APTES), or similar.

[0059] Another aspect of the invention is to provide a nanofluidic FET device in which an electrical double layer (EDL) forms in the nanotube to screen the surface potential.

[0060] Another aspect of the invention is to provide a nanofluidic FET device in which the diameter of the nanotube is comparable with the diffuse layer of the

EDL so that electrical potential remains non-zero even at the center of the nanotube. [0061] Another aspect of the invention is to provide a nanofluidic FET device having at least one nanotube whose inner surfaces are modified to change the surface potential, charge density and/or even switch the polarity of the channel. [0062] Another aspect of the invention is to provide a nanofluidic FET device in which the inner surfaces of the nanotube are modified by being functionalized, such as treated with three-amino-propyltrietheoxylsilane (APTES), or similar, according to a treatment regime and period. [0063] Another aspect of the invention is to provide a method of detecting the movement of chemical and bio-chemical species through a nanotube. [0064] Another aspect of the invention is to provide a method of controlling the movement of chemical and bio-chemical species through a nanotube. [0065] Another aspect of the invention is to provide a method of both detecting and controlling the movement of chemical and bio-chemical species through a nanotube. [0066] A still further aspect of the invention is to provide mechanisms for precisely detecting and controlling fluidic flow on a nanoscale level. [0067] Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0068] The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only: [0069] FIG. 1 is a schematic of an inorganic nanotube nanofluidic device according to an embodiment of the present invention, showing a single nanotube bridging two microfluidic channels to form the nanofluidic system.

[0070] FIG. 2 is an image rendition of a fully packaged nanotube device according to an embodiment of the present invention.

[0071] FIG. 3 is a rendition of a SEM image for a nanofluidic device according to an aspect of the present invention, shown prior to attachment of the cover. [0072] FIG. 4 is a graph of ionic signals during λ -DNA translocations with 2M

KCI buffer fluid in both microchannels according to an aspect of the present invention. [0073] FIG. 5 is a graph of a typical ionic current signal according to an aspect of the present invention, shown with magnification of the current axis. [0074] FIG. 6 is a graph of current drop and duration time for three measurements according to an aspect of the present invention. [0075] FIG. 7 is a graph of ionic current signals in response to λ -DNA translocations with 0.5M KCI buffer according to an aspect of the present invention.

[0076] FIG. 8 is a graph of typical ionic current signal recorded when 1 -DNAs (~6μg/mL ) test solution of 0.5M KCI buffer was loaded to microchannel according to an aspect of the present invention, shown with magnification of the current axis. [0077] FIG. 9 is a graph of current drop and duration time for four measurements according to an aspect of the present invention. [0078] FIG. 10 is a schematic of ionic distribution of counterions and co-ions in an inorganic nanotube when a DNA molecule is confined therein according to an aspect of the present invention. [0079] FIG. 11 is a graph of the interplay between charge effect and blockade effect according to an aspect of the present invention. [0080] FIG. 12 is a perspective view of a single nanotube nanofluidic transistor

(MOSo/FET) according to an embodiment of the present invention. [0081] FIG. 13 is a SEM image rendition of the device structure of FIG. 13.

[0082] FIG. 14 is a schematic of field effect modulation of electrical potential diagram in MOSo/FETs according to an aspect of the present invention. [0083] FIG. 15 is a graph of ionic conductance with respect to gate voltage for the device of FIG. 12, with an inset showing selected IA/ curves. [0084] FIG. 16 is a schematic of "doping" a nanotube inner surface with

APTES according to an aspect of the present invention. [0085] FIG. 17 is a graph of selected current/voltage (IA/) curves for the nanofluidic transistor after 1 day of APTES treatment according to an aspect of the present invention. [0086] FIGS. 18A-18B are graphs of measured "as-made" ionic conductances

(S) and the effective conductance at gate controlled regions (S_Gc) according to an aspect of the present invention. [0087] FIGS. 19A-19B are graphs of measured "APTES 1 day" ionic conductances (S) and the effective conductance at gate controlled regions (S_GC) according to an aspect of the present invention. [0088] FIGS. 20A-20B are graphs of measured "APTES 2 day" ionic conductances (S) and the effective conductance at gate controlled regions

(S_GC) according to an aspect of the present invention. [0089] FIGS. 21 A-21 B are graphs of measured "APTES 4 day" ionic conductances (S) and the effective conductance at gate controlled regions

(S_GC) according to an aspect of the present invention. [0090] FIG. 22 is a graph of field effect modulation of ζ potentials for as-made and all functionalized devices according to an aspect of the present invention, with inset showing the three-capacitor model.

[0091] FIG. 23 is a graph of surface charge densities for as-made and all functionalized devices according to an embodiment/aspect of the present invention.

[0092] FIG. 24 is a schematic of surface chemical reactions and electrokinetic effect involved in field effect modulation according to an aspect of the present invention. [0093] FIG. 25 is a graph of transient responses of ionic conductance when turning on the gate voltages according to an aspect of the present invention. [0094] FIG. 26 is a cross-section of an inorganic nanotube nanofluidic transistor according to an aspect of the present invention.

[0095] FIG. 27 is a rendering of a FESEM image of the nanofluidic transistor device shown in FIG. 27. [0096] FIG. 28 is a rendering of a FESEM image showing magnified details of the nanofluidic transistor device shown in FIG. 26. [0097] FIG. 29 is a schematic of ion distribution for a silica microfluidic channel according to an aspect of the present invention. [0098] FIG. 30 is a graph of electric potential for the silica microfluidic channel of FIG. 29.

[0099] FIG. 31 is a schematic of ion distribution for a silica nanofluidic channel according to an aspect of the present invention. [00100] FIG. 32 is a graph of electric potential for the silica nanofluidic channel of FIG. 31 . [00101] FIG. 33 is a graph of a theoretical calculation of total ionic density at the nanotube size according to an aspect of the present invention. [00102] FIG. 34 is a graph of experimental data of KCI salt concentration dependence of the ionic conductance in a single silica nanotube nanofluidic transistor according to an aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION [00103] Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus generally shown in FIG. 1 through FIG. 34. It will be appreciated that the apparatus may vary as to configuration and as to details of the parts, and that the method may vary as to the specific steps and sequence, without departing from the basic concepts as disclosed herein. [00104] 1 . DNA Translocation in Inorganic Nanotubes.

[00105] The detection of individual bio-molecules has been realized in nanofluidic devices according to the present invention for which potential applications exist ranging from single molecule study of biological activity to rapid diagnosis of diseases. Biological nanochannels/pores, e.g., a - hemolysin, have been used for detecting single-stranded polynucleotides, and show substantial promise for ultrafast DNA sequencing. Recently, artificial inorganic nanopores are attracting increasing attention due to the robustness of solid-state nanopore membranes, the flexibility of surface modification, and the precise control of nanopore sizes. The artificial nanopores have been used to study analytes ranging from small molecules, single-stranded polynucleotides to double-stranded DNAs (dsDNAs). The molecular translocation can be probed from ionic current signals. In addition, nanotubule membranes have been used to sense DNAs with single base mismatch selectivity. [00106] Inorganic nanotubes, which represent a new class of one-dimensional nanostructures (e.g., elongate high-aspect ratio structures), are attracting increasing attention. Inspired by nanopore technology, chemically synthesized inorganic nanotubes are utilized as the core elements and integrated with nanofluidic systems for single-molecule sensing. Compared with traditional nanopore devices, these nanotube devices feature three distinct differences. First, these nanotubes have a length on the order of approximately 10 μm , thus providing a very high aspect ratio, which for example can confine the entire bio-molecule, which is likely to result in new translocation characteristics. Second, the nanotube devices taught herein provide a planar layout, which could enable simultaneous optical and electrical probing. Third, the current device geometry of these nanotube devices is compatible and amenable to integration with lab-on-a-chip micro-total-analysis systems (// TAS), and microelectronics. Moreover, advancing self-assembly techniques, such as Langmuir-Blodgett assembly, provide convenient routes for fabricating large-scale arrays of nanofluidic devices for parallel processing. [00107] 1 .1 Example Nanofluidic Device 1 .

[00108] FIG. 1 illustrates an example embodiment 10 of a nanotube nanofluidic device that features a single inorganic nanotube 12 bridging two microfluidic fluid supply structures, depicted here as channels 14, 16, mounted to a base 20, such as a silica substrate. The channels are shown with access holes 18a, 18b, 18c and 18d. It should be appreciated that although the fluid supply structure is shown comprising a channel structure (e.g., nanochannel), it can also be configured with a nanopore structure (e.g., nanoporous membrane) or combination thereof. [00109] Uniform silicon nanowires were utilized during testing which had a controlled wall thickness and a pore size down to about 10 nm . By way of example, nanowires for use in the nanofluidic device can be chemically synthesized, such as by SiCI₄ chemical vapor deposition, and translated into silica nanotubes through an oxidation/etching process. Nanotubes utilized in testing had an inner diameter of typically 50 nm or less. [00110] FIG. 2 is a representation (rendition) of an actual SEM image taken of a fully packaged nanofluidic device with microfluidic channels and inlet/outlet ports. [00111] FIG. 3 is another representation (rendition) of an actual SEM image showing the integration of a single nanotube with microfl iridic channels. Scale bar 10 μm . The cross-sectional view (inset) clearly shows the opening of the nanotube embedded between two silicon dioxide layers. It should be appreciated that the open end of the nanotube section will be fluidically available on either side of the wall separating the nanofluidic channels. [00112] FIGS. 4 through FIG. 6 illustrate electrical characteristics of the nanofluidic device of FIG. 2 during testing of /t -DNA translocations in an aqueous solution of a first concentration of buffer solution in both microchannels. In these tests both microfluidic channels were filled with 2M potassium chloride (KCI) buffer solution, and ionic current was recorded in FIG. 4 in response to applied voltage bias. All fluid solutions in these tests were prepared with DNA buffer solution, which consists of 1 OmM NaCI, 1 OmM Tris-HCI and 1 mM EDTA in aqueous solution with pH=7.6. No transient current change was observed although the baseline shifted slightly over time. When 1 -DNA molecules in 2 M KCI buffer were introduced to the negatively biased microchannel while the other microchannel was filled only with buffer solution, the ionic current exhibited frequent drops in current as seen in FIG. 5, which corresponded with the passage of 1 -DNAs through the nanotube. The graph shows typical ionic current signals recorded when a λ - DNA (-30 μg/mL ) test solution (prepared with 2M KCI buffer) was loaded to the negatively biased (0.4V) microchannel, in response to which extensive current drop spikes were seen. Current drop signals were not observed when the bias polarity was reversed.

[00113] These current drop events may be attributed to the geometrical exclusion effect of conducting ions because of the finite size of λ -DNA, leading to transient ionic current blockage. Although, the noise level is relatively high, these ionic current drops can still be resolved and were highly reproducible. Statistics of current drop and duration for three measurements are shown in FIG. 6. The main plot shows the pattern of all events, with a tight distribution indicating a single molecule translocation scheme. The top and right insets are the event frequency as a function of duration time and current drop, respectively. The typical current change was 10 to 4OpA ; the typical duration was 4-1 O mS with a small fraction of events up to 2OmS . The overall distribution was quite narrow and centered around 23 pA and 7.5 mS , suggesting that most of the events were identical, corresponding to single molecule translocation. [00114] Furthermore, once the polarity of the applied bias was reversed no signal peaks were observed, indicative of the DNA translocation being electrophoretically driven. For electrically charged particles (spherical particles) in aqueous solution, the electrophoretic mobility is roughly approximated by μ_EP = q/6πηr (wherein q is the charge of particles, η is the viscosity, and r is the particle size). [00115] Since the persistence length of double-stranded DNA is about 50 nm it is likely that the DNA molecules are fully stretched in the nanotube with a diameter of 50 nm . The particle size r can be approximated by the end-to- end distance of the stretched DNA molecule. The end-to-end distance L_z, estimated using de Gennes dynamics model, is about 5.8 μm . It is of interest that an experimental study of stretching /t -DNA in nanochannels gave an average L_z about 8 μm . This means that the entire λ -DNA molecule could stay inside the nanotube during translocation. The electrophoretic mobility of /t -DNA in the nanotube is calculated to be approximately 1x10^~8 m²A/sec and the resulting DNA transport velocity is about 2 μm/mS under a bias potential of 1 volt. [00116] Consequently, the predicted translocation duration is 2.5 mS , which is in agreement with the shortest limit of observed events. Due to the interactions of biomolecules with the surface electric double layer, it is reasonable to expect a longer translocation time. Moreover, electro-osmotic flow could add an opposite force to slow down the translocation of DNA molecules. [00117] 1 .2 Example Nanofluidic Device 2.

[00118] FIG. 7 through FIG. 9 illustrate electrical characteristics of the nanofluidic device during the same testing, as shown in FIG. 4 through FIG. 6, in a second aqueous solution having a second, lower, concentration of KCI (0.5 M) buffer solution, wherein a distinctly different phenomenon was observed. In FIG. 8 a typical ionic current signal is shown when 1 -DNAs (~6 μg/mL ) test solution (prepared with 0.5M KCI buffer) was loaded to the negatively biased (1V) microchannel. Instead of a current decrease in response to translocation, as seen in FIG. 4 through FIG. 6, frequent current increases were observed corresponding to the passage of λ -DNA molecules.

The control experiment exhibited some baseline shifts as seen in FIG. 7, although no abrupt current changes were observed. FIG. 9 depicts current drop and duration time for four measurements; the main plot shows the pattern of all events showing a relatively broader distribution. The top and right insets depict the event frequency as a function of duration time and current drop, respectively.

[00119] It should be appreciated that translocation characteristics were different compared with the 2M KCI case, wherein the statistics of current change and duration time showed a much broader distribution. This appears to be the first observation of ionic current crossover, for example a transition from current blockage to current enhancement during DNA translocation through nanotubes as a result of their unique dimensions. Ionic current increase during DNA translocation through 50 nm to 60 nm long nanotubes was previously observed by Chang et al. The current enhancement was attributed to the introduction of counterions in the nanotube due to the charge on DNA molecules dominating over the steric blockage effect observed in other studies. Tests on the nanofluidic devices confirm their observations and further elucidate that there is a crossover from current enhancement to current blockage that depends on ionic concentration. [00120] When DNA molecules enter the nanotube, the number of cations and anions in the nanotubes would decrease due to the volume exclusion effect. At the same time, DNA molecules also carry counterions (cations) into the nanotube, which may transiently increase the cation concentration. A simple model is developed here which build on the arguments presented by Chang et al., in the article: Chang, H.; Kosari, F.; Andreadakis, G.; Alam, M. A.; Vasmatzis, G.; Bashir, R. Nano. Lett. 2004, 4, 1551. Assuming that Manning condensation (refer to Manning, G. Biophys. Chem. 2002, 101, 461.) of counter-ions neutralizes a fraction (1 -^ ) of the negative phosphate groups of DNA leaving fraction φ for mobile counter-ions to interact with, the change An in the mobile counter-ion density inside the nanotube is:

A_H (1 )

where b is the number of base pairs (48500 for λ -DNA), V_nt is the volume of the nanotube, V_mol is the volume occupied by the DNA molecule, n₊ and n_ are the cation and anion densities, respectively, within the nanotube in the absence of DNA, and N_A is Avagadro's number. The first term (An_CHARGE ) corresponds to the increase in mobile counter-ion concentration due to the presence of DNA (e.g., the molecular gating effect), whereas the second term (An_BLOCK ) is the steric exclusion of both cations and anions. It should be recognized that the expression for An_CHARGE is only approximately given by

Eq. (1 ). In these tests KCI was used as the electrolyte, although it will be appreciated that various other electrolytes may be utilized according to application and characteristics without departing from the teachings of the invention. The ionic mobilities for K⁺ and C/^~ are μ_κ+ at approximately 7.62 x 10^~8 m²A/s and μ_cι_ at approximately 7.91 x 10^~8 m²A/s, respectively. So the current change during DNA molecule translocation is given by:

]|e ^• N_A ^■ A ^• E (2)

where A is the cross sectional area of the nanotube, E is the magnitude of the electric field across the nanotube. The values An_CHARGE and An_BLOCK denote the current changes corresponding to the charge effect and the steric blockade effect, respectively. Current carried by a translocating biomolecule itself is negligible compared to ionic current due to low bio-molecular mobility. [00121] FIG. 10 and FIG. 11 illustrate ion distribution for the nanotube in the nanofluidic device. FIG. 10 depicts ionic distribution of countehons and co- ions in an inorganic nanotube when DNA molecules are confined inside. FIG. 11 depicts the interplay of charge effect and blockade effect of Eqs. (1 -2) which shows that a critical ion concentration n_cr exists such that AI > 0 when

^_CHARGE > ^¹ _BLOCK . ana AI < 0 when Al ^' _CHARGE < AI_BLOCK . The critical total ion concentration is given as:

with the critical KCI concentration at c_cr = n_cr/2 . Note that for a linear molecule such as DNA, V_mol = πr²pb , where r is the radius of the double helix and p is the length per base pair (e.g., 0.34 nm ). For dsDNA molecules, φ ranges from 0.17-0.5 based on previous reports and simulation. Here it is assumed that φ = 0.5 for Manning condensation, and r = \ nm . Therefore, the critical KCI concentration is estimated as n_cr , which is approximately

0.79M. [00122] This simple scheme indicates there is an ionic current crossover (from current blockade to current enhancement) for DNA translocation through a nanotube as the KCI concentration is decreased below the critical concentration (-0.79 M). This unexpected result has indeed been observed in our experiments. Eq. (4) predicts a net current blockade of 2OpA for 2M KCI, which again agrees very well with the experimental data (-20-30 pA ). The predicted current increase for 0.5M KCI is 6 pA , which corresponds to the lowest limit in our observation. While this scheme qualitatively explains the experimental observations, it should be pointed out that this may be an oversimplified account of a highly complex DNA transport process within nanotubes filled with electrolytes.

[00123] The inorganic nanotube nanofluidic device of the present invention significantly extends the time scale of single molecule transport events compared to the use of nanopore devices. In addition, useful information on bio-molecules within a confined geometry can be obtained from duration, current change, and current decay characteristics measured at different ionic concentrations and bias currents. Therefore, the nanotube devices of the present invention represent a new platform for studying single molecule behavior. Due to their planar design and compatibility with standard microfabhcation technology, this basic module of inorganic nanotube nanofluidics could enable simultaneous electrical and optical probing. Moreover, nanotube devices could be further integrated into nanofluidic circuits for high throughput and parallel analysis of biological species at the single molecule level. [00124] 1 .3 Method of Detecting Molecular Species.

[00125] It can be seen from the above that a number of steps are involved toward providing an ability to detect molecular species, such as DNA, from within an ionic fluid. In general, the steps of molecular detection comprise the following. A flow path is established for the ionic fluid through a nanotube. The diameter of the nanotube is adapted with a diameter and length for translocation of single molecules of the desired molecular species. Electrodes are positioned proximal each end of the nanotube to establish electrical connection with the fluid within the nanotube. A current is conducted between the electrodes and thus through the ionic fluid in the nanotube. Current transients are detected in the bias current, which indicate the occurrence of translocation events of the desired molecular species. It will be appreciated that other methods and variations can be implemented by one of ordinary skill in the art without departing from the teachings of the present invention. [00126] 2. Polarity Switching and Transient Responses. [00127] The ability to manipulate charge carriers (electrons and holes) in metal- oxide-semiconductor field effect transistors (MOSFETs) has revolutionized how information is processed and stored, and created the modern digital age. Analogous to MOSFETs, introducing field effect modulation in micro/nanofluidic systems in a three-terminal device enables the manipulation of ionic and molecular species at a similar level and even logic operations. Inorganic nanotubes are preferably utilized in these nanofluidic transistors, such as silicon, because a charged oxide surface forms within the nanotube which facilitates attracting ions in solution. In contrast to inorganic nanotube use would be trying to utilize organic nanotubes, such as carbon nanotubes. However, organic nanotubes repel water and thus do not form the charge layers which are relied upon in these fluidic transistors. It should also be noted that due to strong Debye screening in aqueous solution, field effect modulation of ion transport arises only in systems whose dimensions are comparable to the critical Debye Length, such as in nanofluidic channels. [00128] A nanofluidic transistor is fabricated which incorporates an inorganic nanotube configured to conduct either positively or negatively charged ions dissolved in a fluid. Charge flows through the tube in response to ions which flow through the fluidic channel nanotube as controlled by the voltage applied to the gate electrode. Modification of the nanotube, such as the degree of chemical modification or functionalization, determines whether one or both ion polarities are conducted through the nanotube depending on applied gate voltage.

[00129] Numerous applications for the technology can be considered, such as highly sensitive biological sensors, selective pumps, fluid-based "computer chips", and so forth. Manipulating individual charged molecule flow can be highly beneficial toward creating these and many other advanced chemical and bio-chemical systems. These fluidic transistors are capable of operating with very small quantities of material, such as femtoliter amounts of blood or saliva. [00130] Previously, mechanical means were required in transporting small quantities of conductive fluids, such as blood, saliva or urine. However, the present invention makes use of the conductivity of these fluids toward inducing movement of the fluids. Nanofluidics has already attracted attention for ultrasensitive or even single molecule level detection and biological activity study. For instance, membrane channel proteins and artificial solid state nanopores were utilized for single molecule sensors, configuration study, and DNA sequencing. These nanopore devices usually passively transport ionic species, similar to an electrical resistor. Analogous to unipolar MOSFETs, introducing an external electrical field to modulate ionic conductivity according to the present invention would elevate nanofluidics to a higher level of controllability or even logic. It is also notable that a single conical nanopore has been reported to exhibit active rectified ion transport in a two-terminal device configuration. Single nanochannel studies have shown that the surface charge governs the ionic transport and induces the formation of unipolar solutions as in unipolar MOSFETs. Metal nanotubule membranes exhibited selective ion flux upon electrochemically tuning surface charges. These results are indicative of feasibility toward fabricating single nanotube ionic field effect transistors. In the following tests a rapid field effect control of electrical conductance in single nanotube nanofluidic transistors was provided. Furthermore, the polarity switching of the devices could be controlled in response to the level of surface functionalization and the transient responses upon field effect control.

[00131] 2.1 Example Nanotube Ionic FET 1.

[00132] FIG. 12 illustrates an example embodiment 30 of a nanotube ionic field effect transistor. A nanotube 32 is shown coupled between a source 34 and drain 36 which are configured for passing a fluid therebetween. By way of example, the source and drain each have at least one fluid coupling means, such as access holes 38a, 38b. Source 34 and drain 36 are mounted to a base, such as substrate 40. A gate 42 is retained proximal to (e.g., preferably fully or partially surrounding) nanotube 32, with conductors 44, 46 in contact with opposing sides of the nanotube for making electrical contact with the fluid being communicated within the nanotube. For the example shown, conductors 44, 46 are contained within source 34 and drain 36 configured for establishing electrical contact with the fluid contained within nanotube 32. [00133] FIG. 13 illustrates an example implementation 50 (shown as a rendition of an actual SEM image) having nanotube 52, source channel 54, drain channel 56, and gate 58. It this embodiment the nanotube is a chemically synthesized silica nanotube with high aspect ratio, and having both excellent uniformity and surface smoothness. The nanotubes for use in this embodiment should have an inner diameter of about 40-50 nm and a length of about 15//m . The nanotube is integrated into this single nanotube nanofluidic transistor by interfacing with the two microfluidic channels. The device includes a lithographically defined gate electrode 58, and deep etched source/drain microfluidic channels 54, 56, and a polydimethylsiloxane (PDMS) cover (not visible). [00134] Concentration dependence of ionic conductance deviates from bulk behavior when [KCI]<10 mM, which indeed confirms the formation of unipolar ion transport, and lays down the foundation for further field effect modulation.

When applying gate voltages (Vg), the electrical conductance of KCI solution (< 1 mM) decreases with changing Vg from negative to positive, a characteristic of p-type transistors. [00135] FIG. 14 and FIG. 15 illustrate shifting of electrostatic potential within the nanotube transistor. Similar to field modulation in a metal-oxide- semiconductor (MOS) system, gate voltages in the nanofluidic transistor shift the electrostatic potential distribution inside the nanotube. FIG. 14 illustrates three cases of gate voltage, specifically negative, zero and positive. In the solution an electrical double layer (EDL) forms on the interior of the nanotube to screen the surface potential. The EDL consists of an inner compact layer

(Stern layer) and an outer diffuse layer. In a fluid medium sufficiently larger than the thickness of the diffuse layer, the electrostatic potential decays from the effective surface potential ( ζ potential) to zero. When the solution is confined in nanotubes, whose dimension is comparable to or smaller than the diffuse layer, the electrical potential remains non-zero even in the middle of nanotubes. In the case of a silica nanotube having negative surface charges, cations are majority carriers and the resulting transistors are p-FETs. Negative Vg enhances cation concentration while positive Vg depletes cations. This simple scheme explains qualitatively how field effect control works in nanofluidic systems. [00136] A number of benefits can be derived by tuning the "doping level", in a similar manner as doping levels are tuned in a semiconductor, to change inherent carrier concentrations or type and systematically study field effect operation in nanofluidic transistors. It is clear that inherent carrier concentrations in nanofluidic transistors are controlled by inner surface potential and charge density. In this regard, surface modification is expected to have a similar consequence for nanofluidic transistors as semiconductor doping has for MOSFETs. Reduced doping level in semiconductors is generally associated with pronounced field effect modulation. The following describes the impact of surface modification on the field effect for our metal- oxide-solution field effect transistors (MOSo/FETs) according to the present invention.

[00137] 2.2 Example Nanotube Ionic FET 2.

[00138] FIG. 16 illustrates surface modification within the nanotube of a nanofluidic transistor. By way of example and not limitation, aminosilane chemistry was used to modify inner surfaces of silica nanotubes in order to change the surface potential and charge density or even switch channel polarity. As represented by the figure, right before PDMS cover bonding, the nanotube was treated with three-amino-propylthetheoxylsilane (APTES) while the transistor characteristics were monitored over the surface functional ization duration.

[00139] FIG. 17 is a graph indicating that one day of APTES functionalization did not change polarity (still p-type behavior) of the material, but led to greatly reduced ionic conductance and more pronounced gating effect. [00140] FIG. 18A through FIG. 21 B illustrate ionic conductance aspects with respect to functionalization time. FIG. 18A and FIG. 19A illustrate that ionic conductance is more profound and provides a fairly stable field effect modulation, while it is subject to lower noise levels compared to as-made devices. [00141] FIG. 2OA is a graph which illustrates that two days of APTES functionalization resulted in ambipolar transport behavior. Negative gate voltage increased conductance significantly due to enhancement of cation as in p-FETs. A positive Vg of 5V slightly decreased conductance due to the depletion of cations, but when Vg was above 5V, ionic conductance again increased as one would expect for n-FETs. This observation is attributed to surface charge reversal after passing the ambipolar point. Under large positive gate voltage (e.g., Vg > 5V), anions became the majority carrier resulting in n-FET characteristics. It will be appreciated that within semiconductor systems, small band gap materials tend to exhibit more profound ambipolar behavior. Nanofluidic transistors, in which both cation and anion densities are associated with the same electrical potential level, are essentially gapless transport systems. Consequently, ambipolar behavior would be expected once the inherent carrier concentration falls into suitable low concentration regime.

[00142] FIG. 21 A is a graph indicating that the polarity of the nanotube ionic transistors can be completely reversed after a long period of surface modification. For example, in the graph of FIG. 21 A four days of APTES treatment converted as-made p-FETs into n-FETs. Within the experimental range of Vg, conductance increased monotonically with increasing Vg. For all n-FET devices, the conductance at zero Vg is always lower than that for as- made nanotube devices, yet greater than that for the devices after one or two days of APTES treatment. This polarity switching is highly reproducible in various devices and by using different APTES concentrations. [00143] FIGS. 18B, 19B, 2OB and 21 B depict gate electrode control (S_GR) in a nanofluidic transistor according to the invention, wherein regions that were not covered by gate electrodes, as shown in FIG. 13, resulted in series resistance. The effective conductance under gate electrode control (S_GR) was obtained for the as-made transistors and all functionalized transistors after abstracting series resistances. Gate electrode control, SGR_, was found to exhibit up to a ten-fold field effect modulation according to these tests. The region of a nanotube underneath a single gate electrode, such as approximately 4 μm wide, is as small as about 8 attoliter. The rapid and reversible modulation of ionic concentration or even local switching of the carrier polarity in such a small volume might represent the finest control of ion distribution which is available at this time within fluidic systems. [00144] Field effect modulation in metal-oxide-semiconductor (MOS) systems relies on capacitive coupling between metallic gates and semiconductors. Capacitive coupling also plays a key role in our metal-oxide-solution (MOSo/) systems. A three capacitor model was proposed by van den Berg et al. (refer to R. B. Schasfoort, S. Schlautmann, J. Hendrikse, and A. van den Berg, Science 286, 942 (1999)) to semi-quantitatively explain the relationship between gate voltage and ζ potential change in flow FETs. [00145] FIG. 22 illustrates ζ potential change with respect to gate voltage for the present invention. The inset schematic depicts C_0x, C_Sτ and C_DL representing series capacitances of the silica nanotube wall, the compact layer and diffuse layer, respectively. Since the compact layer is very thin (< 1 nm ) and has a large dielectric constant (e.g., -80) as in aqueous solutions, C_Sτ is thus very large and therefore negligible when connected in series with C_0x and

CDL- Accordingly, the ζ potential change {Aζ ) can be calculated by Aζ =

(^coχ/ ^CDLW9. This model predicts a linear relation between ζ potential and gate voltages. The value for Aζ could be as large as 200«^ at Vg = 20 V. In order to find out the experimental values of ζ potential and surface charge density, Possion-Boltzmann equations were used to numerically solve for the potential and ion distributions across the nanotube as a function of ζ potential. Integration of the anion and cation densities in the whole nanotube yields conductance enhancement factor as a function of ζ potential and surface charge density. [00146] FIG. 22 and FIG. 23 depict estimated ζ potential and surface charge density. This estimate was based on comparing theoretical and experimental conductance enhancement factor, effective conductance (S_GR), over the conductance of bulk solution if confined in the same volume to arrive at corresponding ζ potential and surface charge density. It turns out that ζ potential changes with Vg almost linearly, in agreement with the prediction of the three capacitor model. In contrast to its prediction that field effect modulation of ζ potential is concentration independent, the ζ potential modulation is greater in low inherent ζ potential devices (low "doping" level). The observed ζ potential changes between Vg = -20V and +20V range from

~1 k_BTle to 5.5 k_BTle, which is smaller than the theoretical value 20OmV (7.8 k_BTle). This discrepancy may be attributable to the effective distance of the EDL diffuse layer which varies with surface potential and is not simply equal to Debye length. [00147] Effective surface charge densities with respect to Vg are shown in FIG. 23. As-made p-type nanofluidic transistors exhibit four-fold field effect modulation of surface charge density. The "APTES 1 day" device has greatly reduced inherent surface charge density, and exhibits increased field effect modulation, such as about ten-fold. The n-type FET exhibits five-fold field effect modulation and lower inherent surface charge density (at Vg=O V) compared to as-made p-type FETs. This difference might arise from the different deprotonation/protonation constants of the hydroxyl group as compared with the amino group. The ambipolar device exhibits a surface charge density switch from negative to positive when Vg exceeds approximately 5V. Theoretically, the surface charge density must be reduced down to zero to completely turn off the fluidic transistor. The experimental data shows large OFF state conductance and charge density, possibly due to the existence of parasitic conductance at the bottom side of nanotubes, which is not wrapped by metallic gate electrode and does not necessarily respond to applied gate voltage. [00148] FIG. 24 illustrates basic kinetic processes at work within the nanotube of the nanofluidic device. In testing the present invention, the kinetic process of field effect control in a nanofluidic transistor has been examined. By way of example, consider a hydroxyl group terminated silica nanotube filled with KCI solution. Upon application of a gate voltage the three basic kinetic processes of FIG. 24 are shown comprising: (i) deprotonation or protonation in response to the external electrical field, (ii) adsorption or desorption of counter ions in compact layers; and (iii) ion exchanges between the transiently generated counter ions and bulk solution in microfluidic channels leading to a steady state of ion distribution. These tests illustrate that ionic conductance jumps in response to turning on the gate voltage, because of transient accumulation of the ions induced by surface reaction (i) and/or (ii). This level of conductance then decays gradually down to steady states (dashed line). At the same gate voltages, high bias sweeping leads to rapid decay (relaxation time less than 60 seconds at 1V) while low bias gives slow decay (relaxation time of about 250 seconds at 0.1V). Such strong bias dependence was observed for both a negative and positive Vg. When the bias was swept between +5V, conductance reached steady state immediately in a single IA/ scan (10-20 sec). However, it was found that at the same bias, different gate voltages did not result in substantially different transient responses. Hence, the ion exchange step is believed to control the kinetics of field effect modulation in nanofluidic transistors. Faster field effect operation is expected in response to further reducing the nanotube dimension and increasing bias currents. [00149] 2.3 Method of Controlling Molecular Flow. [00150] The above examples discuss apparatus for controlling molecular flow in a fluid. It will be appreciated that these aspects of the invention also describe one or more methods for controlling molecular flow. Generally, the method involves the following principle steps. First, an inorganic nanotube is configured for communicating ionic fluid between a first end (e.g., source) and a second end (e.g., drain). Electrodes are coupled to the first and second ends of the nanotube, with a gate electrode positioned proximal the nanotube (preferably fully or partially surrounding the nanotube). A bias current is established between the first end and second end which passes through the ionic fluid in the inorganic nanotube. Ionic movement through the nanotube is then controlled in response to the level of applied gate voltage, thus a fluidic transistor structure is formed.

[00151] The nanotube is preferably adapted with a small diameter to assure that an electrical double layer (EDL) forms whose diffuse layer extends sufficiently into the interior of the nanotube to control movement. Preferably, the diffuse layer extends to the center (or overlaps the center) which results in full control of movement across the whole cross-section of the nanotube. It will be appreciated that in tubes of larger diameter (e.g., above about 50-100 nm ) the diffuse layer will be unable to direct molecular movement at the center of the tube. Molecular control can be increased or altered by functionalizing the nanotube. An example above was the exposure to APTES over a desired preparation time, though other forms of functional ization can be utilized without departing from the teachings of the present invention. In its most basic form, functionalizing provides for the selective control of the movement of anions, cations, or both anions and cations through the nanotube thus making n-type, p-type or ambipolar fluidic nanotransistors. [00152] 3. Nanotube Device Fabrication.

[00153] FIG. 26 illustrates an example embodiment 70 of an inorganic nanotube nanofluidic transistor. A nanotube 72 (e.g., single silica nanotube) is shown fluidly coupled between a source volume 74, incorporating electrode 76, and a drain volume 78, incorporating electrode 80. It should be appreciated that the terms "source" and "drain" are adopted above in response to the electrical characteristics which are an analog of a MOSFET device, and not in response to a limitation on the direction of fluid flow, such as indicative of flow originating from source and moving into the drain. A conductive gate 82 (e.g., chromium) is shown proximal (e.g., adjacent, partially surrounding, or fully surrounding) at least a portion of nanotube 72. Access holes 84a, 84b are shown for communicating fluid to the source and drain regions. By way of example and not limitation, the base portion of the nanofluidic transistor comprises a quartz substrate 86, while the upper portion 88 comprises one or more layers of SiO₂, such as deposited by chemical vapor deposition (CVD). [00154] The transistor shown has been fabricated by two separate steps, specifically chemical synthesis of silica nanotubes 72 for nanofluidic channels and integration with lithographically defined microfluidic channels 74, 78. In this example, silicon nanowires were synthesized and oxidized in dry O₂ at 850⁰C for 1 hour to form a 35 nm silica sheath. To avoid photoresist filling into the nanotubes, the as-made Si/Siθ2 core-sheath nanowires were left sealed instead of etching through the Si cores to form SiO₂ nanotubes until all the surface device structures had been fabricated. After dispersion of SiZSiO₂ nanowires onto quartz substrates, a 100nm Cr metal layer was sputtered onto the substrates, and subsequently etched with photolithography defined photoresist etching mask to form Cr lines above the Si/SiO2 nanowire serving as gate electrodes 82. Then a 2μm thick low temperature oxide 88 was deposited on the entire substrate 86 using low-pressure chemical vapor deposition (LPCVD) with SiH₄ chemistry and then densified by annealing in inert gas at ambient temperature. Two microfluidic channels are patterned and etched to connect to each of the ends of Si/Siθ2 nanowires. Two metal lines (e.g., Pt or Ag) were patterned on both sides of the nanowire as source and drain electrodes. It is also convenient and practical from a testing perspective to simply insert two Ag/AgCI electrodes into the access holes to serve as source/drain electrodes. Finally the silicon core of the nanowire was etched away using XeF₂ to form the silica nanotube. [00155] The resulting devices were bonded with a PDMS cover in which access holes had already been drilled. Before bonding, device chips were cleaned with oxygen plasma at 200W for one minute and immersed in Dl water to form hydrophilic surfaces in microfluidic channels which facilitate the aqueous solution injection during experiment. The device chips were taken out and dried utilizing a nitrogen gun. A piece of fresh PDMS cover with access holes was cleaned in isopropanol (IPA) for three minutes assisted by ultrasonic treatment. Finally, the PDMS cover was aligned and pressed onto the device chip to complete the bonding process. Measurements were conducted typically one to three days after bonding. [00156] 3.1 Electrical Measurements. [00157] Electrical measurements for an implementation of the device shown in FIG. 26 were carried out. By way of example and not limitation, electrical testing of the device was conducted in a Faraday cage having a common ground with all the measurement equipment. Current-voltage (I-V) characterization was carried out with a source measure unit. Gate voltage was supplied with a voltage source generating up to 100 V. The measurement system was controlled with lab-based software and data was collected through an IEEE-488 interface. A rise time of 33 μS was set for all these tests. Data collection in this test setup was carried out with a PC-based data acquisition system (e.g., in this case having a maximum sampling rate 100,000 samples/Second). Silver or silver-chloride (Ag/AgCI) electrodes were used as source and drain electrodes in these tests except for the deionized (Dl) water conductance measurements in FIG. 27 and FIG. 28 for which inert platinum (Pt) electrodes were used to avoid contamination. All the electric measurements were conducted in a clean room to avoid dust contamination. [00158] 3.2 Surface functionalization with APTES.

[00159] By way of example and not limitation, the interior of the nanotube was functional ized with an APTES solution. The APTES solution can be prepared by adding 2% (vol) APTES liquid (Aldrich) to acetone, which was pre-dhed overnight using 4 - 8 mesh (4A pore size) molecular sieves. The as-fabricated device chips were cleaned with oxygen plasma and dried at 100⁰C in a convection oven for about 20 minutes prior to PDMA cover bonding. Then the chips were immersed in APTES acetone solution for a desired time with the reaction container capped to prevent introduction of moisture. After functionalization, the device chips were rinsed with dry acetone a couple of times and then left in acetone overnight. Finally, PDMS cover bonding was conducted in a similar way as described previously, although it took only 30 seconds of oxygen plasma cleaning at 100W. It will be appreciated by one of ordinary skill in that art that these process steps can be alternatively implemented in a number of different ways. [00160] 3.3 Scanning Electron Micrographs of Devices. [00161] FIG. 27 and FIG. 28 are renderings of a field emission scanning electron microscopy (FESEM) characterization of the as-fabricated nanofluidic transistor before PDMS cover bonding. These example embodiments show the structure of microfluidic channels at both sides bridged by a single silica nanotube which is embedded underneath an LPCVD SiO₂ layer. Metal gate electrodes which are also embedded in the SiO₂ layer are visible topographically in the renderings of the FESEM images. [00162] 3.4 Unipolar ionic Distribution and Transport in Silica Nanotubes. [00163] FIG. 29 through FIG. 32 illustrate ion distribution and electric potential diagrams for silica microfluidic channels and nanofluidic channels. These figures qualitatively describe the difference between microsized (FIGS. 29-30), and nanosized (FIGS. 31 -32) fluidic systems and the formation of unipolar ionic environments when shrinking channel size to approach the Debye screening length. A schematic example of ionic distribution within a microfluidic channel is depicted in FIG. 29, which contrasts in character with the ionic distribution of the nanofluidic channel depicted in FIG. 31. In comparing FIG. 31 with FIG. 29 it can be seen that the relative availability of mobile negative ions in the nanofluidic channel of FIG. 31 in comparison with FIG. 29. The nanofluidic channels thus are capable of taking advantage of EDL effects. Potential and ionic concentration are shown compared between the microfluidic channel in FIG. 30 in comparison with the use of the nanofluidic channel of FIG. 32. Through these diagrams one can clearly discern the unique characteristics of the nanofluidic channels which are beneficially utilized according to the different aspects of the present invention. [00164] FIG. 33 is a graph of a theoretical calculation of the total ionic density (cation and anion) as a function of nanotube size, based on the simple principle of charge neutrality, wherein surface charge density was assumed to be 0.01 C/m². Simulation results clearly indicate two profound trends in the data. First, decreasing nanotube diameter leads to more enhanced total ionic density inside nanotubes, which is essentially the cations which are required to balance the negative surface charge at the silica nanotube surface. Smaller nanotubes squeeze the cations and increase the real ionic density to form a unipolar carrier (ion) profile inside nanotubes. Secondly, low concentration ionic (< 0.01 M) solution tends to readily form a unipolar environment inside the nanotube, while the high concentration solution (> 0.1 M) only shows slight enhancement in very small nanotubes. The meagerness of this change is due to low ionic strength solutions having larger Debye screening length which allows significant extension of surface charge effect across entire nanotubes. [00165] FIG. 34 is a graph of experimental data of KCI salt concentration dependence of the ionic conductance in a single silica nanotube nanofluidic transistor which clearly shows deviation from microfluidic system prediction (0 C/m² line) and confirms the formation of unipolar ionic conduction at low concentration (< 1 mM). A high concentration solution in the nanotube essentially behaves like bulk salt solution in microfluidic systems due to very short Debye screening length (< Znm for [KCI] > 0.01 M). [00166] 3.5 Calculation of ζ Potential and Charge Density. [00167] The ionic concentration is given by the Boltzmann distribution:

where n₊₍_, denotes the real cation and anion density, respectively, inside a

nanotube where electrical potential is φ . The value n₀ denotes the cation or anion density at φ = 0, which equals the bulk KCI concentration, K_B is the Boltzmann constant, T is absolute temperature, e is the electron charge.

Total ionic density n (both cations and anions) is given by:

[00168] The net charge density p is •nh J ^e(P 1

J

[00169] The Poisson equation for the electric potential is:

where £"_w is the electrical permissivity of the aqueous solution.

[00170] In a symmetric system (cylindrical nanotubes), the boundary condition in the center is given by:

— = 0 which gives dx _x=0

[00171] Herein, value x is the coordinate across the center of the nanotube which is the origin. Value x is normalized with radius R of the nanotube, such that 0 < x < 1 . For simplicity, φ[x) is also nondimehzed by

[00172] Φ (0) is calculated by integration ofΦ(x) from ζ to Φ(0) , and x from 1 to 0 in Eq. (8). Then Φ(x) can be numerically solved by integration from

Φ (0) to Φ(x) while x varies from 0 to x .

[00173] Once the potential diagram Φ (x) is solved for various zeta potentials ( ζ ), then the Boltzmann distribution can be further utilized to calculate ionic concentration for both cations and anions. The relation between surface charge density and zeta potential ( ζ ) is calculated based on the total charge neutrality.

σ = f 2πxLp(x)dx = f 2xn₍₎ sinh (Φ(x) )dx (9) [00174] For the sake of simplicity, it is assumed that the mobilities for K⁺ and CV are equal, wherein the enhancement factor is defined as the measured conductance in the nanotube over the ideal conductance in bulk solution if it is confined in the same volume (SZS₀) , given as:

SZS₀ = ^2πx(n⁺ +n^')Z(2n₀)dx = ^2πxcosh(Φ(x))dx (10)

[00175] For various zeta potentials ( ζ ), one can numerically compute a set of SZ S₀ values according to Eqs. (8) and (10). Then based on experimentally measured enhancement factor (SZS₀ ) , zeta potentials can be back-extracted. Eq. (9) represents the correlation between surface charge density σ and ζ , wherein surface charge density σ can be calculated from ζ once it is known.

[00176] Nanofluidic systems are described which incorporate inorganic nanotubes for the sensing and/or controlled transport of ions, molecular species, and biochemical species (e.g., DNA). Implementations are described for sensing the flow of DNA in response to translocation through the nanotube. In addition, fluidic transistor implementations are described which allow for controlling the movement of either or both ionic polarities in response to applied gate voltage. In either case, the nanotube interior can be modified, such as functionalized, to alter specificity of flow or ionic characteristics. Fabrication details were described by way of example and not limitation throughout the preceding specification. One of ordinary skill in the art will appreciate that numerous variations and modifications can be incorporated within these nanofluid devices without departing from the teachings herein. [00177] Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention.

Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art. In the appended claims, reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present disclosure and claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present disclosure and claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U. S. C. 112, sixth paragraph, unless the element is expressly recited using the phrase "means for."

Claims

CLAIMS What is claimed is:

1. A nanofluidic device, comprising: at least a first and second fluid supply structure configured for supplying a fluid containing chemical or bio-chemical species; a nanotube of inorganic material which fluidly couples said at least first fluid supply structure to said second fluid supply structure; at least a first and second electrode, separated from one another along the length of said nanotube, and configured for establishing electrical contact with the fluid in said nanotube; and means for detecting or controlling the motion of the chemical or bio-chemical species flowing through said nanotube in response to current flow between said first and second electrodes.

2. A nanofluidic device as recited in claim 1 , wherein said fluid supply structure comprises a nanofluidic channel structure.

3. A nanofluidic device as recited in claim 1 , wherein said means for detecting or controlling is configured for detecting a change of current passing through said nanotube between said first and second electrode.

4. A nanofluidic device as recited in claim 1 , wherein said detection of current changes, comprises: a voltage source configured for establishing a biasing current between said first and second electrode, said biasing current passing through said fluid which comprises an ionic solution containing molecules to be detected; and a current detection circuit configured for registering transient changes in the biasing current in response to the translocation of the molecules through said nanotube.

5. A nanofluidic device as recited in claim 1 : wherein said means for detecting or controlling comprises a gate electrode configured for controlling the flow of ions between said at least first and second fluid supply structure in response to the voltage applied to said gate electrode; wherein said gate electrode is retained proximal said inorganic nanotube; and wherein said nanofluidic device operates as a field-effect transistor (FET).

6. A nanofluidic sensor, comprising: a nanofluidic system configured for receiving DNA molecules retained within a fluid; an inorganic nanotube coupled to said nanofludic system through which the DNA molecules can be passed; and means for detecting transient current changes through the fluid in response to translocation of the DNA molecules passing through said nanotube.

7. A nanofluidic sensor as recited in claim 6, wherein said fluid comprises an ionic solution of a known concentration.

8. A nanofluidic sensor as recited in claim 6, wherein said nanotube is configured to stretch the DNA molecule while it is passing through said nanotube.

9. A nanofluidic sensor as recited in claim 6, wherein said nanotube has a sufficiently high aspect ratio to confine the entire DNA molecule during translocation.

10. A nanofluidic sensor as recited in claim 6, wherein said nanofluidic system is configured with nanopores, nanochannels, or a combination of nanopores and nanochannels for supplying a fluid containing the DNA molecules to be passed through said nanotube.

11. A nanofluidic sensor as recited in claim 6, wherein said nanofludic system comprises a nanopore structure.

12. A nanofluidic sensor as recited in claim 6, further comprising at least one access hole in said nanofluidic system through which the fluid is communicated to said nanotube.

13. A nanofluidic sensor as recited in claim 6, further comprising functionalizing of said nanotube to change its translocation characteristics.

14. A nanofluidic sensor as recited in claim 6, wherein said means for detecting transient current changes comprises: at least a first and second electrode positioned proximal said inorganic nanotube and separated from one another; said electrodes configured for establishing electrical contact with the fluid which comprises an ionic solution; biasing source configured for establishing a bias current through the ionic solution between said first and second electrodes; and transient current detector configured for detecting transient changes in bias current levels responsive to the translocation of DNA molecules through the portion of the nanotube positioned between said first and second electrodes.

15. A nanofluidic sensor as recited in claim 6, wherein said translocation of the DNA molecules is electrophoretically driven.

16. A nanofluidic transistor, comprising: a fluidic source; a fluidic drain; an inorganic nanotube coupled in fluidic communication between said fluidic source and fluidic drain; and a gate electrode retained proximal said inorganic nanotube for controlling the flow of ions between source and drain regions in response to the voltage applied to said gate electrode.

17. A transistor as recited in claim 16, wherein said nanofluidic transistor operates as a field-effect transistor (FET).

18. A transistor as recited in claim 16, wherein said nanofluidic transistor comprises a metal-oxide-solution field-effect transistor (MOSo/FET).

19. A transistor as recited in claim 16, wherein said transistor is configured to conduct either or both ionic polarities in response to voltage applied to said gate electrode.

20. A transistor as recited in claim 16, wherein said transistor comprises a p-type, n-type or ambipolar field effect transistor.

21 . A transistor as recited in claim 16, wherein said transistor exhibits rapid field-effect modulation of ionic conductance.

22. A transistor as recited in claim 16, wherein the voltage applied to the gate electrode of said transistor shifts the electrostatic potential distribution within said nanotubes.

23. A transistor as recited in claim 16, wherein said inorganic nanotube comprises a silica material.

24. A transistor as recited in claim 16, wherein said nanotube is configured to have a diameter comparable with the diffuse layer of the electrical double layer (EDL) that forms within the nanotube to screen surface potential which remains non- zero even at the center of the nanotube.

25. A transistor as recited in claim 16, wherein said nanotube is configured with a diameter of approximately 50 nm or less.

26. A method of detecting molecular species, comprising: establishing a flow path of ionic fluid through an inorganic nanotube configured with a diameter and length for translocation of single molecules of desired molecular species; conducting a current through said ionic fluid in said inorganic nanotube; and detecting current transients in response to a translocation event of the desired molecular species.

27. A method as recited in claim 26, further comprising controlling the movement of ionic species through said inorganic nanotube in response to applying a voltage to a gate disposed adjacent said nanotube.

28. A method of controlling molecular flow in a fluid, comprising: communicating ionic fluid through an inorganic nanotube having a first end and second end; establishing a bias current through said ionic fluid which passes through said inorganic nanotube; and controlling ionic movement through said nanotube in response to a level of voltage as applied to a gate electrode retained between said first and second ends of said nanotube.

29. A method as recited in claim 28, wherein said nanotube is configured with a sufficiently small diameter so that an electrical double layer (EDL) forms whose diffuse layer extends at least to the approximate center of the nanotube.

30. A method as recited in claim 28, further comprising functionalizing said nanotube to selectively control the movement of molecules through said nanotube.

31. A method as recited in claim 28, further comprising functionalizing said nanotube to selectively control the movement of anions, cations, or both anions and cations, through said nanotube.