Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/48—Biological material, e.g. blood, urine; Haemocytometers
  • G01N33/483—Physical analysis of biological material
  • G01N33/487—Physical analysis of biological material of liquid biological material
  • G01N33/48707—Physical analysis of biological material of liquid biological material by electrical means
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/48—Biological material, e.g. blood, urine; Haemocytometers
  • G01N33/483—Physical analysis of biological material
  • G01N33/487—Physical analysis of biological material of liquid biological material
  • G01N33/48707—Physical analysis of biological material of liquid biological material by electrical means
  • G01N33/48728—Investigating individual cells, e.g. by patch clamp, voltage clamp
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
  • G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
  • G01R27/26—Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
  • G01R27/2605—Measuring capacitance
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
  • G01N27/22—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance

Definitions

• the present invention pertains to the art of electrical sensing devices and, more particularly, to a method and apparatus for sensing a time varying electrical current passing through an ion channel located in a membrane.
• a membrane typically surrounds biologic cells, with the membrane having ion channels located therein. These ion channels are made of protein structures that regulate the passage of various ions into and out of the cell. These ion channels are important to study due to their wide variety of biological functions. In the past, such ion channels were studied by clamping a portion or patch of a cell membrane and measuring changes in its electrical properties when the membrane and ion channels were exposed to various analytes which altered the ion channel's behavior. Unfortunately, there are great difficulties in positioning a cell for such measurement. In order to make ion channel studies more efficient, people have modeled the cell membrane by forming a lipid bilayer membrane and inserting therein proteins that act as ion channels. Such an artificial membrane may then be used in experiments rather than an actual cell membrane.
• first and second electrodes in resistive electrical contact with the electrolyte.
• the first electrode is maintained in a first or bath volume of electrolyte, while the second electrode is maintained in a second or sensing volume of electrolyte.
• a voltage differential is maintained between the first and second electrodes establishing an electrical field.
• an oxidation-reduction (redox) reaction must occur for a charge to transfer between the first electrode and the solution.
• redox oxidation-reduction
• atoms of the first electrode enter the solution as cations. The cations move through the solution under the influence of diffusion and the electrical field and are deposited at the second electrode as the cations are reduced.
• the interaction between the resistive electrode and the solution produces a concentration gradient of the ions in solution near the surface of the first electrode.
• the gradient causes a half-cell potential to be established between the first electrode and the bulk of the electrolyte. If a current flows, these potentials can be altered causing an over- voltage to appear.
• the over- voltage results from an alteration in a charge distribution of the solution in contact with the first and second electrodes, creating a polarization effect.
• the resulting charge distribution can cause a significant measurement artifact if either the first or second electrodes move relative to the solution for any reason.
• prior measurement apparatuses have positioned the measurement volume of interest away from a region of variable concentration around the electrodes.
• volumes having dimensions in the order of 1 cm x 1 cm x 1 cm are typically utilized on either side of the membrane.
• a wire electrode is immersed in each volume at a distance in the order of millimeters from an active area of the channel (or pore etc.).
• scale concentration gradient effects are negligible.
• capacitive electrodes have not previously been used to measure potentials or currents in electrolytes. The reason is that existing biopotential electrodes have been adequate for experimental scale geometries utilized to date and have the benefit of being DC coupled.
• capacitive electrodes themselves become more difficult to use. That is, as the electrodes are made smaller, the capacitance of the electrode is reduced to a very small level. For example, for a 10 ⁇ m x 10 ⁇ m electrode that might be used in a chip scale sensor, the capacitance of the electrode is in the order of less than 1 pF. At low frequency, a capacitance in the order of 1 pF represents very high impedance. Coupling an amplifier efficiently to such a high impedance source while maintaining low input noise levels and removing low frequency drift is traditionally a difficult problem.
• the variable of interest was the electric potential produced within a heart, brain, or other cell.
• the potential of the cleft region between the cell outer surface and the point of measurement was, in most cases, found to obscure and dominate the cell potential.
• the potential of the cleft region is determined by a combination of capacitive coupling to the internal potential of the cell, ionic current that flows through channels in a portion of the cell that faces the cleft region, and a resistive coupling through the electrolyte in the cleft region to the bath that maintains the cell. Owing to variations in spacing the cell from the electrode (i.e. the height of the cleft region) and in the local properties of the cell membrane, there is considerable, uncontrollable variation in the coupling of the cell potential to the transistor.
• the present invention is directed to a sensing apparatus or system for sensing a time varying current passing through a membrane.
• the system includes first and second volumes that are filled with an electrolyte and separated by a membrane.
• the first volume or bath is large and can be common to multiple membranes.
• the second or sensing volume is generally smaller than the first volume.
• each sensing volume serves to localize measurement to a specific membrane of interest.
• An analyte(s)of interest is introduced into the bath.
• a particular application of the invention is to measure discrete conductivity states in channels. These states change stochastically in the presence of analytes in a manner that allows separate determination of concentration and species.
• Another application of the invention is to measure channel current when conductivity is simply determined by a blocking effect produced by the presence of an analyte in an internal flow region of the channel.
• Such a measurement is the capacitive analog of the resistive pulse technique that is the basis of the Coulter counter.
• the capacitive technique has the same general benefits when going to smaller length scales as it does for stochastic sensing.
• a benefit of the capacitive method is that, because the impedances of the electrodes and driving voltage circuitry are so high as compared to the electrolyte, electrolyte resistance does not contribute to system noise over a range of typical system values.
• pipette access resistance sets a minimum detectable signal level.
• the present invention offers higher sensitivity than possible in prior art patch clamp methods.
• the channel is biased at a fixed voltage that is maintained by feedback if necessary (i.e., voltage clamping).
• DC potential can be measured directly by resistive electrodes if these are used.
• resistive electrodes In the case of a bias voltage, such electrodes are connected to a very high impedance circuit to prevent current flow that could short the potential of the sensing volume.
• the present invention preferably only utilizes capacitive coupling to the electrolyte.
• the DC potential is deduced from net ionic current that flows into the sensing volume.
• the required DC voltage can be maintained across the channel by driving the bath voltage to a given potential difference from the measured sensing volume potential.
• a desired potential difference could be created by allowing a specified amount of net charge to build up in the sensing volume and to bias the subsequent ion current to have zero average deviation about this net value.
• the output waveform can then be calculated based on capacitive coupling of the bias voltage to the bath and sensing volume.
• a benefit of capacitive sensing in the measurement configuration described by the invention is that the resulting system noise is, in many configurations, less dependent on the seal resistance between the membrane and the rest of the system than is the traditional patch clamp method.
• relatively low values of the seal resistance ( ⁇ 100 M ⁇ ) appear in general to have less of an overall effect on the system noise than the same seal resistance would for the patch clamp method.
• capacitive sensors have a further benefit for measuring ionic currents in small devices.
• a means must be provided to convert the current to a voltage. This is easily and naturally achieved by the fact that, as an ionic current flows into a confined volume, a net electrostatic charge builds up as the ion current accumulates. This build up charge leads to an increase in the electric potential in a manner analogous to the rise in potential due to the build up of charge during electron flow into a capacitor.
• this effect has the very desirable property that, as the volume in question is reduced in size, its capacitance decreases and, as a result, the voltage produced by a given net charge increases.
• Figure 1 is a schematic representation of a sensing system constructed in accordance with the present invention
• Figure 2 is a schematic representation of the sensing system of Figure 1 depicted as an electrical circuit
• Figure 3 is a graphic representation of a time varying measured voltage and amplifier input current corresponding to a channel switching between 1 G ⁇ and 300 G ⁇ produced with the sensing system of the present invention
• Figure 4 is a schematic representation of a sensing system constructed in accordance with a second embodiment of the present invention.
• Sensing system 2 includes a first volume or bath 10 within which is provided a first electrolyte 15, and a second or sensing volume 20 provided with a second electrolyte 25. Sensing volume 20 is typically smaller than bath 10. However, it should be noted that this is not a necessary requirement.
• membrane 40 separates bath 10 and sensing volume 20.
• Membrane 40 includes one or more channels, indicated generally at 50, which can be engineered or otherwise modified as desired for a specific measurement application. For convenience, we simply denote an ion channel, protein pore, nanochannel, micro aperture, or any functionally similar structure that enables ions to traverse the membrane, as channel 50. In any event, membrane 40 could be formed from a bilipid, liquid film or a solid material such as polycarbonate or polyimide.
• sensing system 2 includes a first electrode 60 positioned in bath 10 and a second electrode 65 that is positioned in sensing volume 20.
• a voltage source 70 applies a potential difference or driving potential across first and second electrodes 60 and 65 to establish an ionic current (I c ) through channel 50.
• a third or capacitive sensing electrode 80 is electrically coupled to sensing volume 20 to sense a potential of electrolyte 25.
• capacitive sensing electrode 80 is connected to a high-impedance circuit 90.
• second electrode 65 and capacitive sensing electrode 80 can be an integral unit so long as a circuit (not shown) is provided which can isolate the driving potential from high-impedance circuit 90.
• the voltage for driving ion current (I c ) could be applied by conventional resistive electrodes, e.g. electrode 60 and 65.
• a separate capacitive electrode e.g. capacitive sensing electrode 80, is used to sense the potential of electrolyte 25 in sensing volume 20.
• electrode 65 is a resistive type, then the circuit which is employed to apply voltage 70 to drive ion current (I c ) must have a high impedance level, i.e., the impedance level must be sufficient to prevent shorting of channel 50.
• Capacitance Cm and resistance Rs that couple sensing volume 20 and bath 10 represent membrane 40.
• R s represents all sources of shunt resistance, including a seal (not labeled) present between membrane 40 and a substrate 100 ( Figure 1), defects in membrane 40, and defects in sealing channel 50 to membrane 40.
• Channel 50 is represented by a resistance R c , a time-varying resistance that varies between approximately 1 G ⁇ and several hundred G ⁇ depending on the nature of channel 50. In some cases, the variation may only constitute modulation of the current I c by a factor of approximately 30%. In addition, channel 50 may have more than two discrete conduction states.
• a biasing voltage for driving ionic current (I c ) is denoted V b and is connected to electrolytes 15 and 25 by impedances Zb i and Zb 2 .
• impedances Zb i and Zb 2 As discussed above, capacitive or resistive electrodes can implement the impedances.
• Electrolyte 15 is connected to a circuit ground point 120 through impedance Z g which can be implemented by either a capacitive or resistive electrode to the electrolyte or via the free-space capacitance of voltage circuit 70 or some combination thereof.
• Capacitive sensing electrode 80 is connected to circuit ground point 120 through input capacitance Q n portion of a readout circuit 125.
• Readout circuit 125 also includes an input resistance (not shown) relative to ground 120. In any case, the input resistance is preferably established to have a negligible effect at the particular frequencies of interest. In cases wherein the resistance does have a noticeable effect, the resistance can be taken into account in the equivalent circuit.
• the capacitance of sensing volume 20 to ground is C v .
• sensing volume 20 includes a fluid path 130 to allow filling sensing volume 20 with electrolyte 25, the impedance of fluid path 130 to ground must also be controlled to prevent shorting of the potential of bath 10.
• electrolyte 25 When an ionic current I c flows into sensing volume 20, the voltage of electrolyte 25 increases in a manner similar to that of a simple capacitor in an electrical sense. Because of the relatively high conductivity, electrolyte 25 is of uniform potential and voltage appears across membrane 40, the capacitance coupling between capacitive sensing electrode 80 and electrolyte 25, readout circuit 125, and insulation 140 on walls 145 of sensing volume 20. As discussed above, a benefit of the capacitive method of measuring ionic current (I c ) is that, as sensing volume 20 is reduced, a magnitude of the voltage produced by a given ion current I c increases.
• sensing volume 20 could have an overall thickness of less than approximately 1 mm, and as thin as 1 ⁇ or even 10 nm.
• sensing volume 20 with a given layer of insulation 140 may have a capacitance of order 10 pF
• a volume 10 ⁇ m x 10 ⁇ m x 1 ⁇ m made in the same way would have a capacitance of order 0.1 pF.
• a sensing system 2 includes a membrane 40' located directly on a capacitive sensing electrode 80' and substrate 100.
• This geometry is referred to as a supported niembrane
• membrane 40 shown in Figure 1 constitutes a suspended membrane.
• Membrane 40' can be supported on a hard surface, such as capacitive sensing electrode 80 and substrate 100 by a variety of methods not shown in Figure 4 that include: ultra thin viscous films, hydrophilic polymers, soft polymer cushions, aerogels, xerogels, and tethers. With this arrangement, there is no necessity to fabricate sensing volume 20, e.g., by etching a hole in silicon.
• a sensing system 2 positions capacitive sensing electrode 80' near channel 50 either directly in contact or in the same manner as support membrane 40' shown in Figure 4.
• First electrode 60' is positioned substantially closer to channel 50 in bath 10 and includes a hole 175 to allow the ionic current to enter channel 50.
• This geometry has the capability of measuring voltage variations directly across channel 50. Locating first electrode 60' so close to channel 50 (note electrode 60' could be atop membrane 40') requires depositing a material similar to that used to support membrane 40' and fabricating a conducting surface over that material. The presence of electrode 60' could be used to corral channel 50 in the desired region of membrane 40'. While technically challenging, the resulting embodiment of the invention enables the electric field across channel 50 to be measured directly.
• the present invention applies to any modulation of current I c in channel 50 such that it leads to a signal that can be measured.
• Particular applications of the invention are to measure discrete conductivity states in ion channels, protein pores, nanochannels and the like. These states can change stochastically in the presence of analytes in a manner that allows separate determination of concentration and species.
• the invention would preferably only utilize capacitive coupling to the electrolyte. With this arrangement, the DC potential is deduced from net ionic current flowing into the sensing volume.
• the required DC voltage is preferably maintained across channel 50 by driving bath voltage at a given potential difference from the measured sensing volume potential.
• the patch clamp method employs a voltage clamp as disclosed, such a clamp is not required for all applications.
• Another application of the invention is to measure channel current I c when conductivity is simply determined by a blocking effect produced by the presence of analyte 55 in an internal flow region of channel 50. Such a measurement constitutes the capacitive analog of the resistive pulse technique that is the basis of the Coulter counter.
• the capacitive technique has the same general benefits when going to smaller length scales as it does for stochastic sensing.
• Such a measurement of the ionic current I c has a number of new aspects and features that are of particular benefit in reducing the size of the apparatus used to make the measurement, and integrating an array of such an apparatus into a chip.

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• 2005
  • 2005-07-22 US US11/658,094 patent/US7622934B2/en active Active
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