Abstract:
A capillary ionic transistor and method of using is disclosed. The method including providing a capillary pipette ( 100 ) having an inner surface defining a channel, and a conductive layer disposed ( 102 ) about electrode the channel; filling at least a portion of the channel with an ionic solution ( 110 ) such that an electrical double layer forms on the inner surface of the pipette; inducing an electric potential within the ionic solution sufficient to generate a longitudinal flow of ions within the channel; and inducing an electric potential in the conductive layer sufficient to alter the zeta potential of the electrical double layer and adjust the flow of ions within the ionic solution.

Description:
REFERENCE APPLICATION DATA 
       [0001]    The present application claims the benefit of U.S. provisional patent application No. 61/944,753, filed Feb. 26, 2014, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates generally to ionic transistors, and in more particular aspects, to ionic transistors employing a capillary pipette. 
       BACKGROUND 
       [0003]    The ability to control ionic and molecular transport in nanochannels is of particular interest in the fields of physics and chemistry. To this end, nanofluidic devices such as ionic diodes and ionic transistors form important elements in applications such as ionic transport regulating systems. These devices are also important in biotechnological applications such as separation sensing and drug delivery, for example, intracellular implantation. 
         [0004]    An ionic transistor is a device that enables control of ionic current through a nanochannel in both directions by a small change in gate voltage. Current ionic transistors have been fabricated using techniques such as FIB drilling, etching, lithography and nanowire growing. However, these techniques are expensive and complex. Accordingly, there is a need in the art for an ionic transistor that is less costly and relatively simpler to fabricate. 
       SUMMARY 
       [0005]    The present disclosure is directed to an inexpensive and simply fabricated ionic transistor. Accordingly, the embodiments herein are directed an ionic transistor cheaply and easily formed from a capillary pipette. In one aspect, a method for controlling the transport of ions through a channel, comprises the steps of: providing a capillary pipette having an inner surface defining a channel, and a conductive layer disposed about the channel; filling at least a portion of the channel with an ionic solution such that an electrical double layer forms on the inner surface of the pipette; inducing an electric potential within the ionic solution sufficient to generate a longitudinal flow of ions within the channel; inducing an electric potential in the conductive layer sufficient to alter the electrical double layer and adjust the flow of ions within the ionic solution. 
         [0006]    According to an embodiment, the capillary pipette is dimensioned such that the electric double layer overlaps at at least one point. 
         [0007]    According to an embodiment, the capillary pipette is dimensioned such that the electric double layer will overlap at least one point upon the application of a predetermined potential to the conductive layer. 
         [0008]    According to an embodiment, the pipette is dimensioned to narrow at at least one point. 
         [0009]    According to an embodiment, the narrow point of the pipette is minimally 10 nm in diameter and maximally 100 nm in diameter. 
         [0010]    According to an embodiment, the narrow point is at a tip of the pipette  100 . 
         [0011]    According to an embodiment, the potential applied to the conductive layer causes the electrical double layer to widen, reducing the flow of ions within the channel. 
         [0012]    According to an embodiment, the potential applied to the conductive layer causes the electrical double layer to narrow, enhancing the flow of current within the channel. 
         [0013]    According to an embodiment, the conductive layer is comprised of one of chromium, aluminum, copper, or any other material capable of retaining a charge. 
         [0014]    According to another aspect, a device for controlling the transport of ions through a channel, comprises: a capillary pipette having an inner surface defining a channel, wherein the pipette is adapted to form an electric double layer on the inner surface when filled with an ionic solution; a conductive layer positioned to exhibit an electric field within the channel when subjected to an electric charge, such that any electric double layer on the inner surface will be altered by the electric field. 
         [0015]    According to an embodiment, the device further comprises a voltage source positioned to induce a longitudinal flow of ions in any ionic fluid at least partially filling the channel. 
         [0016]    According to an embodiment, the device comprises a voltage source connected to apply a charge to the conductive layer. 
         [0017]    According to an embodiment, the capillary pipette is dimensioned to cause the electric double layer to overlap when the channel is filled with an ionic solution. 
         [0018]    According to an embodiment, the capillary pipette is dimensioned to cause the electric double layer to overlap when the channel is filled with an ionic solution and a predetermined charge is applied to the conductive layer. 
         [0019]    According to an embodiment, the capillary pipette is dimensioned to narrow at at least one point. 
         [0020]    According to an embodiment, the narrow point is at the tip of the pipette  100 . 
         [0021]    According to an embodiment, the capillary pipette is minimally 10 nm in diameter and maximally 100 nm in diameter. 
         [0022]    According to an embodiment, the conductive layer is comprised of one of chromium, aluminum, copper, or any other material capable of retaining a charge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which: 
           [0024]      FIG. 1  shows a diagram of a capillary ionic transistor according to an embodiment of the present invention. 
           [0025]      FIGS. 2A and 2B  show a diagram of a system according to an embodiment of the present invention. 
           [0026]      FIGS. 3A, 3B, and 3C  show a diagram of a device according to separate embodiments of the present invention. 
           [0027]      FIG. 4  shows a block diagram of a method according to an embodiment of the present invention. 
           [0028]      FIG. 5A  shows a graph according to an embodiment of the present invention showing current through a pipette  100  without a voltage applied to the gate  102 . 
           [0029]      FIG. 5B  shows a graph according to an embodiment of the present invention showing current through a pipette  100  with a voltage applied to the gate  102 . 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Referring now to the drawings, wherein like reference numerals refer to like parts throughout, there is seen in  FIG. 1  a diagram of a system for controlling ionic current through a channel, according to an embodiment of the present invention.  FIG. 1  shows an ionic transistor, comprising a capillary pipette  100 , a gate  102 , a source  104 , a drain  106 , and a voltage source  108  connected to gate  102 . According to an embodiment, capillary pipette  100  defines an inner channel, with a diameter that can range from a minimum of 10 nm to a maximum of 100 nm value. In an exemplary embodiment, the inner channel is narrower at at least one point, such as the tip of the pipette  100 . In use, capillary pipette  100  may be filled with, or otherwise placed in a bath of, an ionic solution  110 . 
         [0031]    Generally speaking, in operation, source  104  and drain  106  applies an electric potential difference within ionic solution  110 . The electric potential, in turn, creates a current in the form of a flow of ions from one end of the channel to the other. Separately, ionic solution  110  naturally forms an electric double layer on the inner surface of the capillary  100 . A potential applied to gate  102  generates an electric field within capillary pipette  100 , changing the zeta potential of the electric double layer. As the zeta potential of electric double layer changes, the width of the electric double layer changes, limiting or, alternately, enhancing the flow of ions between the source and the drain  106 . 
         [0032]    Whether the current is limited or enhanced is dependent upon the polarity of the potential applied to the gate  102 . If the gate  102  voltage is positive, the positive ions accumulated on the inner surface will repel from the surface, effectively shrinking the electric double layer and creating an open channel for the current to flow. By contrast, if the gate  102  voltage is negative, the width of the electric double layer will increase as a greater number of positive ions are attracted to the surface. The wider electric double layer will effectively create a barrier which limits the flow of current. In an exemplary embodiment, the pipette  100  is dimensioned such that, without a charge applied to the gate  102 , the electric double layer will overlap at least one point, stopping or severely curtailing the flow of current through the channel. Accordingly, the flow of current will cease until a positive charge is applied to the gate  102  and the double layer recedes. In alternate embodiment, the pipette  100  is dimensioned such that the electric double layer will overlap upon the application of a negative charge to gate  102 . One of ordinary skill will recognize that other factors, such as the composition of the pipette  100 , the dimensions of gate  102 , the conductivity of gate  102 , the strength the potential applied to gate  102 , and the concentration of ions in ionic solution  110  may all effect the width of the electric double layer and each may be tailored to achieve the desired current in the channel. 
         [0033]    The capillary pipette  100  is fashioned, in the embodiment, from borosilicate glass; however, pipette  100  may be made from quartz or from any other material suitable for fabricating a pulled capillary micropipette  100 . As mentioned above, one of ordinary skill will recognize that the composition of the pipette  100  will affect the naturally forming electric double layer. Accordingly, different materials may be selected to tailor the naturally formed electric double layer for the desired configuration of the ionic transistor. 
         [0034]      FIG. 3  shows several embodiments of capillary pipette  100 .  FIG. 3A  shows capillary pipette  100 - 1  narrowing at one tip, according to one embodiment.  FIG. 3B , according to another embodiment, shows a capillary pipette  100 - 2  narrowing at both tips.  FIG. 3C  shows a capillary pipette  100 - 3  with a narrow channel in the middle. Each embodiment may be best suited for different applications. Current flow through the pipette  100  pore has a preferable direction from small opening to wide, while current in reverse direction is suppressed. Thus, pipette  100 - 1  may be best suited in applications where current in one direction needs to be suppressed. By contrast,  100 - 2 , having two tips, and  100 - 3  having the same tapering in both directions, may be best suited for applications where current should be suppressed in both directions. However a person of ordinary skill would recognize that these shapes are not exclusive, nor are the applications of the shapes limited to those suggested—the pipette  100  may be any combination of the shapes suggested as long as at least one point of capillary pipette  100  is narrow enough to cause the electric double layer to overlap when the pipette  100  is in use, and a voltage is applied to the gate  102 . In an exemplary embodiment, the diameter of at least one point in the pipette  100  is comparable to the Debye length, or having a maximum of 100 nm. 
         [0035]    In an exemplary embodiment, the gate  102  covers the entire outer surface of the capillary pipette  100 . In an alternative embodiment, the gate  102  may only cover a portion, such as the tip or the center of the pipette  100 . In the embodiment, the gate  102  may be implemented as a film or coating over capillary pipette  100 . It will be obvious to a person of ordinary skill in the art that the thickness of the gate  102  can vary while maintaining its function of retaining a charge. In another embodiment, the gate  102  may consist of one or more wires, or a conductive surface housed in a sleeve and placed over capillary pipette  100 . In an exemplary embodiment, the gate  102  may be placed over the narrowest point of the pipette  100 . A person of ordinary skill in the art will recognize that the gate  102  may be placed in any fashion that would exhibit an electric field on the interior of the capillary pipette  100 , to strengthen, or alternately weaken, the electric double layer. 
         [0036]    If the gate  102  is located near the end of the pipette  100 , additional current suppressing behavior may arise due to a depletion barrier forming at the tip of the pipette  100 .  FIG. 2  demonstrates this behavior. Applying a negative potential to gate  102  will cause negative ions to collect around the tip of the gate  102 , repelling negative ions within the ionic bath surrounding the tip, and consequently forming a depletion barrier. As a result of the decreasing number of ions of one polarity around the pore, the concentration of ions with the opposite polarity will also decrease in order to maintain electro-neutrality. The above behavior has the net effect of further suppressing ionic current in the channel. Alternately, when a positive potential is applied to gate  102 , the zeta potential of the electric double layer and depletion barrier recedes, and the flow of ions between source  104  and drain  106  is enhanced. 
         [0037]    In an exemplary embodiment, the gate  102  may be comprised of chromium, which adheres well to glass, and forms a consistent, continuous coating. However, in alternative embodiments the gate  102  may be made out of copper, aluminum, or any other conductive or semi-conductive material that would allow the gate  102  to maintain a certain potential. 
         [0038]    Returning to  FIG. 1 , source and gate  102  may be attached to the same voltage  108  source or independent voltage sources. If the source and gate  102  are attached to the same voltage source  108 , intervening circuitry may be implemented so that the voltage applied to the source  104  and gate  102  may be adjusted independently. Furthermore, a current amplifier may be applied to source electrode  104 . 
         [0039]    Furthermore, as shown in  FIG. 1 , the capillary pipette  100  may further comprise an insulator  100  covering the gate  102  to prevent the gate  102  from shorting if the pipette  100  is immersed in bath of ionic solution. 
         [0040]      FIG. 4  shows a method of practicing the present invention, according to an illustrative embodiment. In step  400 , a capillary pipette  100  is provided, wherein at least a portion of the pipette  100  is covered in a conductive layer. The capillary pipette  100  has an inner surface defining a channel. In step  402 , at least a portion of the channel is filled with an ionic solution, forming an electric double layer within the capillary pipette  100 . In step  404 , a source and drain  106  is provided to create a potential difference within the capillary pipette  100 . The source and drain  106  may be positioned anywhere within capillary pipette  100  sufficient to create a potential difference within the ionic solution and through at least a portion of the pipette  100 . Specifically, the source and drain  106  are positioned to generate a longitudinal flow of ions through the capillary pipette  100 —in other words, to generate a current flowing through the pipette  100  which may be adjusted by applying a voltage to the gate  102  in later steps. For example, as shown in  FIG. 1 , if capillary pipette  100  is placed in a bath of ionic solution, the source could be place in the top of the pipette  100 , while the drain  106  could be located within the bath of ionic solution. In step  406 , a voltage is applied to conductive layer  102  to control ionic flow through the pipette  100  by altering the width of the electric double layer. This step may include the step of first attaching the voltage source of the conductive layer. Furthermore, as described above, different polarity voltages may be applied to alternately strengthen or weaken the electric double layer and modify the flow of current. 
         [0041]    As an example of the above device and method, a capillary pipette  100  was fabricated from borosilicate glass capillaries with initial inner diameter 0.5 mm and outer diameter 1 mm. Pulling was performed with commercial puller P-2000 (“Sutter, Novato”, Calif.). Prior to pulling, capillaries were cleaned thoroughly with alcohol. The tip diameter of the nanopore and wall thickness after pulling was determined by Scanning Electron Microscopy (SEM) images, because wall thickness can vary depending on pulling settings, especially temperature. Pipettes  100  with 60 nm pore diameter were used, and each having a wall thickness about 25-30 nm at the tip. A 15 nm thick film of Chromium on outer wall of pipette  100  served as a Gate  102  electrode. Deposition of chromium was performed using the electron beam evaporation technique, with pipette  100  tip slightly tilted upwards to prevent blocking and deposition inside the pore. Different thicknesses of Cr layers were tested, from 5 nm to 50 nm. Layers thicker than 30 nm often blocked the pore, whereas, films below 10 nm were mechanically unstable. After Cr deposition, micropipette  100  tip was coated with polymer by dipping it in a photoresist solution. In order to remove excessive polymer and make the coating uniform, pipette  100  was placed into a spinner and centrifuged for 60 seconds. After that, to prevent blockade of the pore, the pipette  100  was connected to air pump with flow direction from narrow to wide opening. As a final step, to solidify photoresist, the pipette  100  was baked at 120 degrees Celsius for 2 minutes. 
         [0042]    The coated micropipette  100 , described above, was filled with sodium chloride solution and immersed in the bath with the same solution. An Ag/AgCl measurement electrode (source electrode) was placed inside pipette  100 , and reference electrode (drain electrode  106 ) immersed into the bath solution close to capillary tip. A gate  102  electrode was attached to pipette  100  coated with Cr, above the layer of photoresist. To record the source  104 -drain  106  current, an Axopatch 200B amplifier was used in voltage clamp mode with a low-pass Bessel filter at 2 or 5 kHz bandwidth. The signal was digitized by an Axon Instruments Digidata 1440A with sampling rate 250 kHz, and recorded by AxoScope 10.2 (Axon Instruments, USA). The Axopatch amplifier also serves as a voltage source between source  104  and drain  106  electrodes. The gate  102  was applied by regulated DC power supply. A picoammeter was connected in series with the gate  102  in order to measure leakage current. 
         [0043]      FIG. 5A  shows the results of the applying a voltage between the source and drain  106 , without a potential applied to the gate  102 . Positive current through the pore was established when positive voltage was applied to the Source electrode  104  with respect to grounded Drain  106  electrode. This means that positive potassium ions drifted from the wide opening of pipette  100  to small opening. The I SD -V SD  curve was measured for potentials from −1 V to 1V with interval of 100 mV, for three different KCl concentrations 1 M, 0.1 M, 0.001 M and zero Gate  102  voltage. For all concentrations pH value was buffered to 7. High concentrations show almost linear dependence, when low concentrations present rectification phenomena: allowing more current to flow in one direction than the other. Current flow through the pipette  100  pore has a preferable direction from small opening to wide, while current in reverse direction is suppressed. 
         [0044]      FIG. 5B  shows the results of the example with a gate  102  voltage applied. Gate  102  voltage (V G ) was introduced with no potential difference between Source and Drain  106  (V SD =0 v). Leakage current between Gate  102  and Source (I leak ) did not exceed 15 pA for V G =±5 V. I SD -V SD  dependence for three different Gate  102  potentials is presented on for V G =0 we observed typical I-V curve for conical capillary pore. For positive potential, which is increasing from V SD =0 V to V SD =+1 V, current through nanochannel will saturate and I SD =43 pA. While for reverse potential, which decreased from V SD =0 V to V SD =−1 V, current showed almost linear dependence and I SD =−84 pA. The rectification ratio, which can be calculated as ratio of channel conductance at V SD =+1V to channel conductance at V SD =−1 V, was equal to 0.51. 
         [0045]    For V G =−5 V even more current suppression for positive voltage was observed. Current saturated and reached value I SD =31 pA for V SD =+1 V. However, for negative Source-Drain  106  voltage current suppression was more significant and I SD  equaled −41 pas for V SD  equal to −1 V. Therefore, rectification ratio increased to the value 0.76. For V G  equal to +5 V, the opposite behavior was observed. The positive gate  102  potential strongly affected positive current, raising conductance and having almost no effect on negative voltage current. When V SD =+1 V current between Source and Drain  106  electrodes rose to I SD =79 pA, and for V SD =−1 V current was I SD  equal to 90 pA. In this case rectification ratio was 0.88.