WO2009067090A1

WO2009067090A1 - Electro-membrane and method of making an electro-membrane

Info

Publication number: WO2009067090A1
Application number: PCT/SG2008/000446
Authority: WO
Inventors: Chee Seng Toh; Pui Sze Cheow; Thi Thanh Binh Nguyen; En Kai Alister Peh
Original assignee: National University Of Singapore
Priority date: 2007-11-23
Filing date: 2008-11-24
Publication date: 2009-05-28

Abstract

There is disclosed an electro-membrane that comprises a porous body having a front surface and a rear surface opposite to the front surface. A plurality of nano-channels extend through the porous body between the front and rear surfaces for allowing passage of analyte therebetween. A porous conductive coating is disposed on the front and rear surfaces of the porous body for allowing an electrical potential to be generated therebetween.

Description

ELECTRO-MEMBRANE AND METHOD OF MAKING AN ELECTRO-MEMBRANE

Technical Field

The present invention generally relates to an electro-membrane and a method of making an electro- membrane. The present invention also relates to an electro-membrane holder.

Background Chemical separations are a major cost component of most chemical, pharmaceutical and petrochemical processes. Membrane-based separations are commercially desirable as they tend to consume relatively lower amounts of energy and hence are more economical than competing separation technologies. Traditionally, membranes have been used mainly for size-based separations with high-throughput requirements but relatively low-purity requirements. Known membrane filtration systems include microfiltration, clarification, sterile filtration and ultra-filtration.

Most of the known membrane separation systems work on the principle of separating solutes utilizing molecular size- dependent sieving mechanisms. However, these known membrane separation systems which utilize molecular size- dependent sieving mechanisms are often not effective in separating similar sized molecules.

Another problem inherent in known membrane separation systems is membrane fouling in which the pores of the membrane become blocked, thereby causing a drop in the flux duty of the membrane. Yet another problem is the build-up of a concentration layer of the analytes being separated at the surface of the membrane ("concentration polarization") , which also causes a drop in membrane flux duty and is a particular problem in the separation of proteins. A drop in the flux duty of the membrane can reduce the quality and quantity of separation of desired analytes. Furthermore, severe membrane fouling may also require costly chemical cleaning or complete membrane replacement. Another commonly used analyte purification method is electrophoresis. Whilst electrophoresis is a useful technique for protein separation and purification, it is limited in scale by convective mixing. Convective mixing is disadvantageous as it leads to zone broadening, which limits the upper level on the electric field strength that can be used. Convective mixing may also cause detrimental heating of analytes.

One proposed technique to overcome problems associated with separating proteins using membranes based solely on differences in pore size is to utilize Electro- membrane filtration (EMF) . EMF combines the separation mechanisms of membrane filtration and electrophoresis and hence, proteins of similar size may be separated from each other not only by the size of the membrane pores but also by the speed at which they travel through the membrane pores when an electrical potential has been applied over the membrane. One known EMF technique has utilized gold (Au) nanotubes disposed within the pores of track-etched polycarbonate membranes to separate proteins. However, these known membranes are difficult to make because the Au nanotubes must be inserted into the pores of the membrane. Furthermore, these known electro-membranes have been known to be susceptible to membrane adsorption by proteins and hence membrane fouling. To overcome the incidence of membrane fouling, one approach has been to chemisorb thiols to the Au nanotube walls. However, this complicates the manufacture of the electro-membrane, which can increase the membrane manufacture costs. Furthermore, the presence of thiol groups on the walls of the Au nanotube may interfere with the ability of the membrane to separate certain proteins, such as for example, the thiol group may form a S-S bond with a cysteine residue of a protein being separated.

There is a need to provide membrane that is capable of separating analytes that overcome or at least ameliorate one or more of the disadvantages described above . There is a need to provide a membrane that has improved selectivity whilst maintaining high-yield characteristics .

Sntnmaτ*y According to a first aspect, there is provided an electro-membrane comprising a porous body having a front surface and a rear surface opposite to the front surface, and a plurality of nano-channels extending through the porous body between the front and rear surfaces for allowing passage of analyte therebetween; and a porous conductive coating disposed on the front and rear surfaces of the porous body for allowing an electrical potential to be generated therebetween.

Advantageously, the conductive coating on the front and rear surfaces of the porous body may allow an electric potential to be applied directly to the membrane so as to generate an electric potential between the front and rear surfaces. This may result in the electro- membrane achieving a relatively high electric field strength, that is not possible in situations where an external electric field is generated by electrodes that are placed a distance away from conventional membranes. The high field strength achieved can be about 20 kV irf¹ to about 40 kV irf¹, or about 30 kV πf¹, which may be suitable for electrokinetic transport of proteins. According to a second aspect, there is provided a method of making an electro-membrane comprising the step of applying a porous conductive coating to a front surface of a porous body and a rear surface of the porous body opposite to the front surface, the porous body having a plurality of nano-channels extending between the front and rear surfaces.

According to a third aspect, there is provided a electro-membrane holder comprising a passage for transmission of a sample solution containing analyte therein from an inlet of the passage to an outlet of the passage; means for securing a plurality of electro- membranes within the passage, the electro-membranes being configured in use to block the. passage; electrodes capable of being electrically coupled to the electro- membrane to induce an electrical potential across the membrane for adjusting the rate of transmission of analyte within the sample solution through the membrane; and a detector configured to detect the transmission of the analyte through the outlet of the passage. Advantageously, the design of the membrane holder may allow for a relatively high pressure flow of the receiver solution when connected to a High Performance Liquid Chromatography (HPLC) pump. More advantageously, the membrane holder may be adjusted in order to hold a desired number of electro-membranes in the membrane holding area. This may allow stacking of several membranes which may be crucial to achieving high efficiency, hence increasing the selectivity and specificity of the membrane system. Even more advantageously, the membrane holder may allow placement of gaskets such that the gaskets can extend beyond the membrane holding area to thereby allow direct electrical connections between the power supply and the electro- membranes. The shape of the outlet passage, as will be explained further below, may aid in reducing the problem of concentration polarization and membrane fouling.

Definitions

The following words and terms used herein shall have the meaning indicated: The term "nano-channels" as used herein refers to a channel with a dimension in the nano-range. Hence, the channel may have a dimension that is less than about 500 nm, less than about 300 run, less than about 200 ran, less than about 100 nm, or less than about 50 nm. The dimension of the nano-channel may refer to the diameter of the nano-channel which, when viewed in cross-section, is substantially circular. In embodiments where the nano-channel has an irregular shape and is not substantially circular (when viewed in the cross- section) , the term "equivalent diameter" may be used when referring to the dimension of the nano-channel. The equivalent diameter of the nano-channel is relative to a completely circular diameter such that when the nano- channel is completely circular, the equivalent diameter is equal to the actual diameter of the nano-channel. The term "conductive coating" as used herein refers to a layer of a material on a surface that is able to allow a substantially unimpeded flow of an electrically charged particles through the material to thereby carry an electric current therethrough. The term "diamond-like carbon" as used herein refers to a material that is composed dominantly of sp³ hybridized carbon atoms and hydrogen atoms.

The term "turbulent fluid flow" as used herein refers to non-streamlined fluid flow paths characterized by fluid flow lines that include a radial component, or are other than smooth, parallel, or collinear. A fluid having a "turbulent fluid flow" is typically a liquid with a Reynolds number (R_e) more than 4,0-00.

The term "transient fluid flow" as used herein refers to a fluid flow that is transitional between turbulent fluid flow and laminar fluid flow. A fluid having a "transient fluid flow" is typically a liquid with a R_e between 2,300 and 4,000.

The term "laminar fluid flow" refers to non- turbulent fluid flow and is characterized as smooth and regular motion where the direction of motion at any point remains substantially constant as if the fluid were moving in a series of layers of different velocity sliding over one another without substantial mixing. A fluid having a "laminar fluid flow" is typically a liquid with a Reynolds number (R_e) less than 2,300. The R_e is a dimensionless quantity defined as the ratio of inertial forces to viscous forces, and for a pipe or duct can be expressed as:

Re = p v d_h / μ where p is the density of the fluid (kgirf³), v is the linear velocity (m/s) , d_h is the hydraulic diameter (in) of the duct or pipe and μ is the viscosity of the fluid (m²/s) .

The word "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. Where necessary, the word "substantially" may be omitted from the definition of the invention.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/-

3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

Throughout this disclosure, certain embodiments may^¬ be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Detailed Disclosure of Embodiments

Exemplary, non-limiting embodiments of an electro- membrane will now be disclosed. The electro-membrane comprises a porous body having a front surface and a rear surface opposite to the front surface. A plurality of nano-channels extends through the porous body between the front and rear surfaces for allowing passage of analyte therebetween. A porous conductive coating is disposed on the front and rear surfaces of the porous body for allowing an electrical potential to be generated therebetween .

The porous body may be a ceramic membrane and optionally may be selected from the group consisting of an alumina membrane, a titanium dioxide membrane and a zirconia membrane.

In another embodiment, the membrane may be made of a semi-conductive material such as silicon dioxide.

In another embodiment, the porous body may comprise a silane-based membrane. The silane-based membrane may be modified by coating with nitride. The silane based membrane may be an organosilane that may have been chemically modified. In this embodiment, the organosiloxanes are attached onto the wall surface of the nano-channels . The membrane may be chemically modified by treatment with an organic compound having a carboxyl moiety. The organic compound may be attached onto the surface of the nano-channels such that the size of the nano-channels is decreased. Hence, the resultant pore size may be smaller than the pore size of an unmodified membrane. The organic compound may be positively charged, negatively charged or neutrally charged. Due to the charge on the organic compound, when the organic compound attaches onto the surface of the nano-channels, the nano-channels may acquire the same charge as the organic compound. Hence, the surfaces of the nano-channels may be substantially positively charged, negatively charged or neutrally charged. Due to the change in the size of the nano- channels and/or the charge of the surfaces of the nano- channels, the extent of analyte interaction with the nano-channels may be altered accordingly. For example, the change in the surface charges may alter the electrostatic interaction of the analyte with the surfaces of the nano-channels. Further, the presence of bulky groups attached to the wall surfaces of the nano- channels may retard the movement of larger sized analytes. Therefore, depending on the type of organic compound used in the chemical modification of the membrane and/or nano-channels, the selectivity of the separation for an analyte of interest may be altered accordingly. The organic compound may comprise a carboxyl group. The carboxyl group may be of the formula R(COOH)_n, where R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, the aliphatic hydrocarbon group or aromatic hydrocarbon group may be optionally substituted with an amine group; and n is 1 or 2. R may be an aliphatic hydrocarbon selected from the group consisting of straight or branched chain alkyls, alkenyls, alkynyls and combinations thereof. When R is an aliphatic hydrocarbon, R can be an alkyl group having 1 to 8 carbon atoms, an alkenyl group having 2 to 8 carbon atoms or alkynyl group having 2 to 8 carbon atoms. In another embodiment, R may be an aromatic hydrocarbon selected from the group consisting of benzyl, toluyl, xylyl, naphthyl and combinations thereof. The carboxylic group may be selected from the group consisting of methanoic acid, ethanoic acid, propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, benzoic acid, salicylic acid, aldaric acid, oxalic acid, malonic acid, malic acid, fumeric acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, aminoethanoic acid, aminopropanoic acid, aminobutanoic acid, aminopentanoic acid, aminohexanoic acid, aminoheptanoic acid and aminooctanoic acid. The nano-channels may have a diameter or an equivalent diameter thereof selected from the group consisting of about 1 nm to about 200 nm, about 20 nm to about 200 nm, about 40 nm to about 200 nm, about 60 nm to about 200 nm, about 80 nm to about 200 nm, about 100 nm to about 200 nm, about 120 nm to about 200 nm, about 1 nm to about 100 nm, about 1 nm to about 80 nm, about 1 nm to about 60 run, about 1 nm to about 40 run, about 1 run to about 20 run, about 1 nm to about 10 nm and about 1 nm to about 5 nm. The diameter or equivalent diameter of the nano-channels may be about 20 nm, about 100 nm or about 200 nm.

The electro-membrane may have a dimension between the front and rear surfaces of the porous body selected from the group consisting of about 10 micron to about 300 micron, about 50 micron to about 300 micron, about 100 micron to about 300 micron, about 150 micron to about 300 micron, about 200 micron to about 300 micron, about 250 micron to about 300 micron, about 10 micron to about 250 micron, about 10 micron to about 200 micron, about 10 micron to about 150 micron, about 10 micron to about 100 micron, about 10 micron to about 50 micron, about 20 micron to about 200 micron and about 50 micron to about 70 micron. The electro-membrane may have a dimension between the front and rear surfaces of the porous body of about 60 micron. The front surface and rear surface of the electro- membrane may be substantially flat and may be generally parallel to each other. Advantageously, this makes manufacture of the electro-membrane easier, particularly when applying the conductive coating and also ensures that analyte solution flows at a constant rate through the membrane body.

In another embodiment, the front surface and rear surface of the electro-membrane may not be flat and may not be parallel to each other. In one embodiment, the electro-membrane may have a curved surface. For example, the electro-membrane may be a tubular shaped with a lumen extending therethrough.

The porous body of the electro-membrane may have a substantially regular pore structure (nano-channel) . The standard deviation of the pore (nano-channel) diameters may be generally less than about 10% or may be about 5%. This indicates that the dimensions of the pores (or nano- channels) are generally similar to each other such that the pores form a highly uniform structure. The pore density of the porous body may be selected from the group consisting of about 10⁸ to about 10¹² pores cm^"2, about 10⁹ to about 10¹² pores cm^"2, about 10¹⁰ to about 10¹² pores cm^"2, about 10¹¹ to about 10¹² pores cm^"2, about 10⁸ to about 10⁹ pores cm^"2, about 10⁸ to about 10¹⁰ pores cm^"2 and about 10⁸ to about 10¹¹ pores cm^"2.

The porosity of the porous body may be in the range of about 8% to about 60%, about 8% to about 15% or about 25% to about 50%.

The electro-membrane -may be provided with a porous conductive coating on the front and rear surfaces of the electro-membrane. The conductive coating may comprise at least one of metal, metal alloy, metalloid, and carbon (including diamond-like carbon) . An exemplary metal alloy may be stainless steel. The metal may be selected from the group consisting of a group III metal, a transition metal, a noble metal and alloys thereof. The group III metal may be aluminum. The metal may be a transition metal selected from the group consisting of titanium, silver, copper, nickel, cobalt, chromium, zirconium, hafnium and rutherfordium. The metal may be a noble metal selected from the group consisting of gold, platinum, iridium, osmium, palladium, rhodium and ruthenium.

The conductive coating may be sputtered onto the front and rear surfaces of the electro-membrane. The conductive coating may be disposed onto the front and rear surfaces of the electro-membrane by way of evaporation. The conductive coating may be sprayed onto the front and rear surfaces of the electro-membrane. The thickness of the porous conductive coating may^¬ be selected from the group consisting of about 20 nm to about 100 nm, about 20 nm to about 80 nm, about 20 nm to about 60 nm, about 20 nm to about 40 nm, about 20 nm to about 40 nm, about 20 nm to about 60 nm, about 20 nm to about 80 nm and about 40 nm to about 60 nm.

A method of making an electro-membrane will now be disclosed. The method comprises the step of applying a porous conductive coating to a front surface of a porous body and a rear surface of the porous body opposite to the front surface, the porous body having a plurality of nano-channels extending between the front and rear surfaces .

The method may comprise the step of sputtering a metal onto the front and rear surfaces of the porous body. The sputtering step may be carried out for a time duration of about 1 minute to less than about 15 minutes, about 1 minute to about 12 minutes, about 1 minute to about 10 minutes, about 5 minutes to about 12 minutes, about 8 minutes to about 12 minutes. In one embodiment, the sputtering may be undertaken for about 10 minutes The sputtering step may be carried out at a sputtering current in the range of about 1OmA to about

5OmA, about 1OmA to about 4OmA, about 10mA to about 3OmA, about 10mA to about 2OmA and about 20 mV to about 50 mA. The sputtering current may be about 20 mA.

The method may comprise the step of evaporating a metal onto the front and rear surfaces of the porous body.

The method may comprise the step of spraying a metal solution onto the front and rear surfaces of the porous body.

An electro-membrane holder will now be disclosed. The electro-membrane holder comprises a passage for transmission of a sample solution containing analyte therein from an inlet of the passage to an outlet of the passage; means for securing a plurality of electro- membranes within the passage, the electro-membranes being configured in use to block the passage; electrodes capable of being electrically coupled to the electro- membrane to induce an electrical potential across the membrane for adjusting the rate of transmission of analyte within the sample solution through the membrane; and a detector configured to detect the transmission of the analyte through the outlet of the passage. The body of the electro-membrane holder may be made of a non-conductive material. Advantageously, the use of a non-conductive material throughout the body of the electro-membrane holder prevents short-circuiting between the electro-membranes and the body of the electro- membrane holder in use. The non-conductive material may be made of a glass or plastic material, which advantageously, may be easily molded in shape. In one embodiment, the body of the electro-membrane holder may be made of a substantially transparent material which allows the user to observe the analyte passing through the electro-membranes and to observe color differentials that may be present in analyte passing from the inlet end of the passage and the outlet end of the passage. In one embodiment, the body of the electro-membrane holder may be made of perplex glass. The electro-membrane holder may be configured to secure more than one electro-membrane. The electro- membrane holder may be capable of holding 2 to 10 electro-membranes. Hence, the electro-membrane holder may be more flexible than conventional membrane holders which are unable to hold more than one membrane.

The means for securing a plurality of electro- membranes within the passage may comprise means for adjusting the number of electro-membranes that are secured within the passage. Advantageously, this enables the operability of the electro-holder to be changed so that different analytes can be detected or to enhance the detection of different analytes. For example, if the analytes are proteins of similar size, a user of the electro-membrane may decide to utilize more membranes so as to enhance the selective separation of the proteins through the electro-membranes by using electro-membranes having different trans-membrane electric potentials and thereby better detect the different proteins. On the other hand, in another analyte detection system, the time of passage through the membranes may be too slow and hence a user may decide to use fewer membranes when operating the electro-membrane holder to thereby increase the flow of analyte through the electro-membrane and thereby decrease the detection time.

In one embodiment the electro-membrane holder may be comprised of a first part and a second part, wherein the means for adjusting the number of electro-membranes that are secured within the passage may comprise means for moving the first part and a second part of the electro- membrane holder toward and away from each other while maintaining the fluid communication of the passage. In one embodiment, the means for moving the first part and a second part of the electro-membrane holder may comprise a bolt and thread-screw arrangement which allows the respective first part and second part to be moved relative to each other and thereafter secured in place while maintaining the fluid communication of the passage to transmit the sample solution therein. In one embodiment, at least a portion of the passage of at least one of the first and second parts of the electro-membrane holder may be dimensioned to reside m the other passage of the other respective part of the first and second electro-membrane holder. In one embodiment, at least one of the passages of the first and second electro-membrane holder has a smaller diameter relative to the respective other passage so that the smaller diameter passage may move in telescopic movement relative to the larger diameter passage to thereby maintain fluid communication of the passage.

The sample solution may transmit from a feed side to a receiver side, wherein the inlet of the passage is in the feed side and the outlet of the passage is in the receiver side. The electro-membranes may form a physical barrier in the passage to thereby separate the feed side from the receiver side.

Each of the plurality of electro-membranes may be individually coupled to an electrode such that an electrical potential is generated across each electro- membrane. More than one electro-membrane may be electrically coupled to the same electrode. The electro- membranes may be configured such that the electrical potential is generated across a specific electro-membrane or electro-membranes depending on the analyte to be separated from the sample solution. A sheet of a conductive material may be provided to promote the application of the electrical potentials directly to the electro-membranes. The sheet may be a silicon rubber sheet provided with a layer of conductive aluminum tape on one side of the silicon rubber sheet such that the aluminium tape forms a ring disposed along the perimeter of the silicon rubber sheet. The silicon rubber sheet is provided with a hole, preferably disposed in the central area of the silicon rubber sheet, to allow the analytes to pass through the silicon rubber sheet. Two conductive silicon sheets may be placed on both sides of the electro-membranes and clamped together. The layer of aluminum tape is also provided on the perimeter of the electro-membrane. The silicon-electro-membrane assembly is then electrically coupled (for example via crocodile clips) to a power supply (such as a battery cell) to the aluminium tape that is exposed from the assembly to apply the electric potential to the assembly. The silicon rubber sheets can also act as gaskets to prevent leakage of the sample solution from the silicon-electro-membrane assembly.

In use, depending on the type of analyte to be separated and detected, the electro-membrane that is selected for that analyte can be "switched-on" such that an electrical potential is generated across that specific electro-membrane. Hence, by having more than one electro- membrane present in the membrane holder at the start of the analysis, it is possible to selectively separate out an analyte as desired without having to stop the analysis to ^"add in further electro-membranes.

The sample solution may flow in the passage between an inlet end in the feed side of the electro-membrane holder to an outlet end in the^" receiver side of the electro-membrane holder. The passage in the feed side of the electro-membrane holder is configured such that the flow of the sample solution is substantially orthogonal to a front surface of the first electro-membrane that is exposed to the passage in the feed side. As the selected analyte flows through the electro- membrane from the feed side to the receiver side, the analyte enters an outlet passage, which transports the analyte to a detector configured to detect the composition of the analyte therein. In one embodiment, the outlet passage is configured to inhibit turbulent fluid flow, preferably to inhibit transient fluid flow and more preferably to promote laminar fluid flow within the outlet passage. Advantageously, laminar flow of the solution within the outlet passage inhibits intimate mixing of different analytes that may be present within the outlet passage but which have passed through the electro-membrane. Laminar fluid flow within the outlet passage enables better detection of the analyte component by the detector.

Laminar fluid flow will be dependent on the dimensions and shape-configuration of the outlet passage, the viscosity of the analyte solution and the speed with which it passes through the outlet passage. In one embodiment, the analyte is received in a substantially "L-shaped" conduit that diverts the analyte from the receiver face of the electro-membrane to the detector. The "L-shaped" conduit may comprise a first passage portion that is substantially parallel to the flow of analyte from the electro-membranes and a second passage portion extending from the first passage portion which is substantially perpendicular to the first passage portion. Hence, the "L-shaped" conduit in the receiver side is capable of substantially removing the analytes rapidly from the electro-membrane surface so as to prevent membrane polarization and may aid in minimizing mixing of the desired analyte with other types of analytes that may be present in the receiver side.

At the receiver side, the analyte is carried in a suitable medium to the detector. The medium may be any- suitable solvent or solution that is capable of receiving the analyte from the electro-membrane to the detector. The medium may by an aqueous solution, an organic solvents or mixtures thereof. The medium may flow through the "L-shaped" conduit at the receiver side under substantially laminar flow conditions. Accordingly, the "L-shaped" conduit is capable of inhibiting turbulent fluid flow therein in order to minimize mixing of the analytes that have passed through the electro-membranes.

The detector may be any device which is configured to identify the analyte being separated by the electro- membranes. Exemplary, detectors include an ultra-violet light detector, a fluorescence detector, a conductivity detector, a mass spectrometer and an electro-chemical detector. In some embodiments, the detector may be configured to quantify the amount of analyte present within a sample that has passed through the electro- membrane, such as for example, using UV detectors.

Brief Description Of Drawings

The accompanying drawings illustrate a disclosed embodiment and serve to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention. Fig. 1 is a top view (a) and a cross-sectional view (b) of a platinum-coated alumina membrane.

Fig. 2 is a schematic diagram of the platinum-coated alumina membrane system showing how the electrical potential gradient was applied across the micrometer- thick membrane housed within an electrically conductive holder.

Fig. 3 is a scanning electron microscope (SEM) micrographs at 30,00Ox magnification of the surface of platinum-coated alumina membranes with (a) unsputtered, (b) 5 min, (c) 10 min, (d) 20 min of platinum coating. Fig. 4 is a graph illustrating the movement/transport of BSA across the platinum-coated alumina membrane of Fig. 1 at electrical potential of:

(a) E = 0 V; (b) E = -0.5 V; (c) E = -1.0 V; (d) E = -1.5 V; (e) E = 0.5 V; (f) E = 1.0 V; and (g) E = 1.5 V.

Fig. 5 is a graph illustrating the initial fluxes of BSA, lysozyme, and myoglobin across the platinum-coated alumina membrane of Fig. 1 at different transmembrane potentials . Fig. 6 is a graph illustrating the movement/transport of lysozyme across the platinum-coated alumina membrane of Fig. 1 at electrical potential of:

(a) E = O V; (b) E = -0.5 V; (c) E = -1.0 V; (d) E = -1.5

V; (e) E = 0.5 V; (f) E = 1.0 V; and (g) E = 1.5 V. Fig. 7 is a graph illustrating the movement/transport of myoglobin across the platinum- coated alumina membrane of Fig. 1 at electrical potential of: (a) E = O V; (b) E = -0.5 V; (c) E = -1.0 V; (d) E = -1.5 V; (e) E = 0.5 V; (f) E = 1.0 V; and (g) E = 1.5 V. Fig. 8 is a graph illustrating the percentage of BSA, myoglobin and lysozyme detected at different potentials taken after 60 min of separation process using the electro-membrane of Fig. 1.

Fig. 9 is a graph illustrating the effect of transmembrane potentials on the initial fluxes of BSA, lysozyme and myoglobin in a mixed protein experiment using the electro-membrane of Fig. 1.

Fig. 10 is a graph illustrating the separation of myoglobin (a) , lysozyme (b) and BSA (c) across the platinum-coated alumina membrane of Fig. 1 under the influence of a negative electric field (E = -1.5 V) over time.

Fig. 11. is a graph illustrating the experimental and calculated fluxes of BSA and lysozyme under the influence of A) favourable and B) unfavourable membrane potentials using the electro-membrane of Fig. 1.

Fig. 12 is a schematic diagram of the electro- membrane transport and separation system using a flow system. Fig. 13 is a graph illustrating the excellent linearity observed between retention time and reciprocal of gold particle size across the electro-membrane.

Fig. 14A is a schematic diagram of the cross- sectional view of a membrane holder. Fig. 14B is a schematic diagram of the side view of the membrane holder of Fig. 14A on the receiver side.

Fig. 14C is a schematic diagram of the side view of the membrane holder of Fig. 14A on the feed side.

Fig. 14D and Fig. 14E are photographic images of the membrane holder of Fig. 14A.

Fig. 15 is a graph illustrating the effect of grafting different functional groups onto alumina membrane on the transport of proteins with positive transmembrane potential applied (E = +1.5V).

Detailed Description Of Drawings

Referring to Fig. Ia, there is shown a schematic diagram showing the top view of an electro-membrane 10 as disclosed herein. The electro-membrane 10 is composed of a porous body in the form of an alumina membrane 2 that is coated with a layer of platinum to form a platinum coating 4 via sputtering. A region of the alumina membrane 2 as shown by the region 6 is left uncoated to avoid short-circuiting when the potential is applied on both sides of the electro-membrane 10. As shown in Fig. Ia, the diameter of the electro-membrane 10 is 13 mm, with the uncoated alumina membrane 2 forming a ring of 1 mm around the perimeter of the electro-membrane 10. Fig. Ib is a cross-sectional view of the electro-membrane 10 of Fig. Ia. As seen in Fig. Ib, the front surface 2a and back surface 2b of the alumina membrane 2 are coated with a platinum coating 4. A plurality^' of nano-channels 3 is provided within the alumina membrane 2 and extends from the front surface 2a to the back surface 2b of the electro-membrane 10 such that the nano-channels 3 act as passageways to allow an analyte to permeate through the electro-membrane 10. Fig. 2 is a schematic diagram showing the experimental set-up of the electro-membrane 10 in use. In Fig. 2, the expanded view shows the alumina membrane 2 coated with a platinum coating 4 being connected to a number of platinum-coated layers 8. The electro-membrane 10 is held in place by membrane holder connectors 12 in a membrane holder 14. The membrane holder 14 was made conductive by sputter-coating micrometer thick platinum layers 8 along selective areas to maximize electrically conductive contacts with the electro-membrane 10. The platinum-coated layers 8 were connected to a potentiostat 18 via copper wires. The working electrode of the potentiostat 18 was connected to the receiver side of the electro-membrane 10 and the auxiliary and reference leads were attached to the feed side of the electro-membrane 10. The potentiostat ensures that the voltage is kept constant throughout EMF. In use, a sample solution is fed into the membrane holder 14 from a sample tube 16. As the sample solution passes into the membrane holder 14, the analytes in the sample solution separate out from each other and selectively . permeate through the electro- membrane 10 under influence of the electrical potential applied to the electro-membrane 10. As a desired analyte passes through the electro-membrane 10, the analyte is received and detected in a UV detector, such as a UV- visible spectrophotometer 20. Fig. 12 is another schematic diagram showing the experimental set-up of the electro-membrane 10 in use. The components used in the experimental set-up of Fig. 12 are similar to those used in the experimental set-up depicted in Fig. 2 and hence, like reference numerals are used to denote like components, but with a prime (') symbol. Here, the electro-membrane 10 has been grafted with 6-aminohexanoic acid such that the walls of the nano-channels in the electro-membrane 10 acquires a positive charge. A HPLC system made up of an elution buffer reservoir 24 and a HPLC pump 22 is connected to the membrane holder 14. The test analyte in the form of gold nanoparticles having varying particle sizes are injected into the feed flow at position 26 such that the gold nanoparticles in the elution buffer is introduced into the membrane holder 14. An electrical potential is then applied to the electro-membrane 10 to determine the electrophoretic mobility of the gold nanoparticles as a function of the particle size, as will be explained further below.

Referring to Fig. 14A, there is provided a cross- sectional view of the electro-membrane holder 30 as disclosed herein. The electro-membrane holder 30 comprises a feed side gasket 34 and a receiver side gasket 36 that are connected to each other and held in place by screw threads 32. For the feed side gasket 34, a hole is drilled in the center to hold the sample solution. Near the top of the feed side gasket (as seen in Fig. 14C), two holes were drilled to allow insertion of electrodes to carry out the electrochemical reactions and a third hole was added for introducing the sample solution. The feed side gasket 34 and receiver side gasket 36 meets at an electro-membrane holding area 28. The electro-membrane holding area 28 is capable of securing or holding a plurality of electro-membranes (10a, 10b, 10c) . The number of electro-membranes (10a, 10b, 10c) that can be inserted into the electro- membrane holding area 28 can be adjusted by tightening or loosening the screw threads 32. Single sided conductive silicon sheets are placed between the electro-membranes (10a, 10b, 10c) and the electro-membrane-silicon sheet layers are held tight by screw threads 32 that traverse the feed side gasket 34 and receiver side gasket 36. The single-sided conductive silicon sheets are connected to a potentiostat (not shown) to allow application of electric potentials directly to the electro-membranes (10a, 10b, 10c) . A passage for transmission of a sample solution from an inlet end 40 to an outlet end 42 is provided in the electro-membrane holder 30. The passage is separated into a feed side passage 38a and a receiver side passage 38b by the electro-membrane holding area 28. As shown in Fig. 14A, the receiver side passage 38b comprises a first passage portion that is substantially parallel to the flow of analyte from the electro- membranes (10a, 10b, 10c) and a second passage portion extending from the first passage portion which is substantially perpendicular to the first passage portion. In use, a sample solution is passed from the inlet end 40 of feed side passage 38a and a desired analyte is separated from the sample solution as it selectively permeates through the electro-membrane (10a, 10b, 10c) in the electro-membrane holding area 28 under influence of an electrical potential across an electro-membrane

(10a, 10b, 10c) that is selected for separating that particular analyte. As the desired analyte passes from the feed side 34 to the receiver side 36, an appropriate medium such as water is used to carry the analyte after it permeates through the electro-membrane (10a, 10b, 10c) via a luer lock connector 44 (with stainless steel or Teflon tubihg) to a detector (not shown) . The medium is pumped into the electro-membrane holder 30 via a second luer lock connector 48 from a HPLC pump under laminar flow conditions.

Fig. 14B is a diagram showing the region of the receiver side gasket 36 as viewed from the direction depicted by (a) in Fig. 14A. The connector 48 is provided with a screw thread 50 to prevent leaking of the medium. Fig. 14C is a diagram showing the region of the feed side gasket 34 as viewed from the direction depicted by (a) in Fig. 14A. The dotted regions (52a, 52c) shows the position of the two electrodes and the dotted region 52b shows the position of the feed side passage 38a.

Fig. 14D and Fig. 14E are photographic images showing a prototype of the electro-membrane holder 30 of

Fig. 14A. Fig. 14E shows the feed side gasket 34 and receiver side gasket 36 that are coated with conductive aluminium tape 50 to connect to the electro-membranes on both sides. The feed side gasket 34 and receiver side gasket 36 are held together by screw threads 32 that traverse both feed side gasket 34 and receiver side gasket 36 as shown in Fig. 14D.

Examples The invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Anodic nanoporous alumina membranes were used as the separation membranes disclosed in the following experiments. The Whatman™ Anodisc™ 13 alumina membrane has a thickness of 60 μm and various nominal pore size, including 20 nm, 100 nm and 200 nm (Whatman, Maidstone, Kent, UK) . The membrane possessed a model pore network that had a narrow pore diameter distribution around its median value, with cylindrical pores going almost straight through the symmetrical membrane.

The alumina membranes were sputter-coated with platinum on front and rear sides using a JEOL Auto Fine Coater (Model JFC-1600, of Jeol Datum Ltd, Japan) with a 57-mm-diameter platinum target (purity 99.9%). The distance between the center of the target and the substrate stage was 30 mm. A 1 mm thick ring along the outer edge of the membrane was left uncoated to avoid short-circuiting when the potential was applied on both sides of the membrane. The sputtering of platinum was conducted for different durations to get optimal balance between porosity and electrical conductivity. Different deposition times of platinum coating were carried out with 5, 10, 15 and 20 min of coating at a sputtering current of 2OmA. A schematic diagram of the platinum- coated alumina membrane is given in Fig. 1 (a) and Fig. 1 (b) . The morphology and the microstructure of the platinum-coated membranes were observed using a field emission scanning electron microscopy (SEM; FEI XL30-FEG SEM of FEI, Oregon, USA) with an energy dispersive X-ray analyzer (EDAX Phoenix XEDS system, Phoenix, USA) . The electrical conductivities of different platinum-coated alumina membranes -were determined by measuring the resistivity of the alumina membranes using the four-point probe method.

Example 1 Protein Transport and Separation of Protein

Mixture

The electrochemically controlled transport of proteins across the conductive alumina membranes were carried out using the transport cell depicted in Fig. 2.

An electrical potential was applied to the membrane using a potentiostat (eDaq EA161, Australia) to ensure that constant voltage was applied throughout the experiment. The electrical field was applied directly to the membrane and across a membrane thickness of the order of 60 mircometer .

Bovine Serum Albumin (BSA) , myoglobin and lysozyme from chicken egg white obtained from Sigma-Aldrich, USA were used and prepared as a feed solution for electrochemically controlled transport studies. Both single protein transport study and mixed protein separation experiments were carried out. The net charge of these proteins was controlled to be positive or negative depending upon the proteins' pi values and the pH of the solution (Table 1) . In deionized water, BSA is negatively charged whereas lysozyme is positively charged and myoglobin is nearly neutral.

Electrochemically controlled transport was monitored by measuring the amount of protein transported from the feed solution to the permeate solution. The amount of BSA, myoglobin and lysozyme transported from the feed solution was measured and analysed using UV-visible spectrophotometry at 280 nm. BSA was detected at 600 nm- and 280 nm, lysozyme at 280 nm and myoglobin was observed at 410 nm and 280 nm.

Table 1. pi of proteins

SEM studies on the effect of deposition time of platinum on the membrane revealed that the pore structure of the platinum-coated membranes was partially bridged or blocked by platinum particles (Fig. 3). For alumina membranes coated with 15 and 20 min of platinum, the pore structure was almost completely blocked by platinum. On the other hand, for 5 and 10 min platinum-coated alumina membranes, their permeability to ions and proteins are still retained although the pore structures are partially blocked. In addition, EDX analysis demonstrated that the atomic ratio of Pt: Al increased rapidly with an increasing amount of platinum coated on the alumina membrane and the atomic ratio of Pt: Al increased from 1:6 to 11:1 after coating for 20 min.

Furthermore, the electrical conductivities of the platinum-coated alumina membrane increased with increasing deposition time from 5 min to 20 min, ranging from -153 Sm^"1 to 12400 Sm^"1. The conductivity of the platinum-coated alumina membranes increased with increasing thickness of the platinum layers. This can probably be attributed to the presence of stronger delocalized electron clouds in increasing layers of atoms in the film that function as charge carriers across the surface of the membranes.

SEM studies also revealed that platinum coating of the alumina membrane with platinum might cause substantial effects on the alumina membrane pore structure. When coating time is about 10 min, a preferred pore size of 60 nm was achieved. Additionally, the platinum-coated membrane achieved a conductivity of approximately 103 Sm^"1, which provides reasonable electrical potential across the surface. The resulting platinum-coated membrane advantageously retains ions and proteins, whilst an external potential may- be applied to the surface, thus further influencing the transport behavior of species across the membrane.

Example 2 Single Protein Transport: under con-trolled potential conditions using static arrangement

The electrochemically controlled transport of BSA, myoglobin and lysozyme across the platinum-coated alumina membranes was investigated by controlling the potential

(Eapp) across the two faces of the membrane. Feed solutions used were BSA aqueous solution of 5000 ppm, lysozyme aqueous solution of 2000 ppm, and myoglobin aqueous solution of 2000 ppm. The permeate side used in these experiment was Milli-Q water. The transport characteristics of each protein were observed individually.

A constant potential was applied across the two conductive faces of the membrane. A 1 mm thick ring along the outer edge of the membrane was left uncoated to avoid short-circuiting when the potential was applied on both sides of the membrane.

Characterization of BSA transport across membrane

Since the pi of BSA is lower than the pH of aqueous water, the protein is negatively charged. Fig. 4 shows the movement of BSA through the membrane at different potentials. Fig. 5 shows the initial BSA flux through the membrane at different potentials. Both Fig. 4 and Fig. 5 demonstrate that the magnitude of the potential applied at the receiving end, both negative and positive, plays a significant role in the quantity of protein moving across the membrane. When the membrane is more positively polarized at the receiver side, greater amount of protein permeates across to the receiving solution and vice versa.

Characterization of lysozyme transport across membrane

For lysozyme, as the pi of lysozyme is higher than the pH of aqueous water, the protein is positively charged. Fig. 6 shows the movement of lysozyme through the membrane at different potentials. Fig. 5 demonstrates that lysozyme flux decreased as the transmembrane potential increased positively. When the receiver side of the membrane was positively polarized, electrostatic repulsion exists between p^'rotein and the receiver side. The electrostatic repulsion leads to a reduced protein distribution coefficient inside the pore and consequently a reduced flux.

Characterization of myoglobin transport across membrane The effect of applied potential is negligible in altering the transport behaviour of myoglobin across the membrane because myoglobin is somewhat neutral in aqueous water and there should be a reduced electrostatic interaction between the protein and the surface of the nanopore. Therefore, its movement is primarily due to diffusion. The transport flux of myoglobin shows minimal variation with applied electric field as shown in Fig. 7 and Fig. 5. Example 3 Mixed Protein Separation under controlled potential conditions using static arrangement

The transport processes taking place in the case of the mixed protein are more complex than in single protein transport. An aqueous BSA, lysozyme and myoglobin mixture was prepared and used as the feed solution. In this experiment, all proteins were observed at 280 nm. Correction for the interfering absorbance intensities by BSA and myoglobin was done to derive the concentration of lysozyme from the 280 nm peak intensity. The concentrations of the BSA and myoglobin were derived from the 610 nm and 410 nm absorbance peak directly without further corrections. The most effective applied potential to separate the protein mixture is when the receiver side of the membrane was negatively polarized as shown in Fig. 8 that the greatest difference in the fraction of proteins detected after 60 min was when E = -1.5V. However, generally, the protein fluxes followed closely the trend observed in the single protein experiments under different applied potentials between -1.5V and +1.5V (Fig. 9) .

Hence, by applying an electrical potential between the two sides of the membrane with the receiver side polarized negatively, lysozyme would transport towards the permeate side solution while BSA would be retained and myoglobin would transport towards by diffusion only.

The transport of lysozyme across the membrane was shown to be significantly faster than that of the transport of BSA (Fig. 9 and 10) . However, not all BSA was retarded by electrostatic repulsion as postulated. This may be attributed to the diffusion flux, which is responsible for the transport of BSA.

The selectivity obtainable from different electrochemical controls for BSA, myoglobin and lysozyme was considered. It is apparent from the transport curves that transport is dependent upon the polarity of the potential applied to the platinum-coated alumina membranes. Thus it can be seen that, for the platinum- coated alumina membrane, the selectivity factor for lysozyme/BSA is highest (approximately 16) when a negative potential is applied at the receiver end. Myoglobin is somewhat neutral, thus the effect of the applied electric field is negligible. Nonetheless, we do not disregard the possibility that protein clusters form (e.g. dimers, trimers) and these clusters will diffuse slower than single molecules.

Additionally, Fig. HA shows a good agreement between the experimental data and theoretical flux values for BSA and lysozyme at favourable transmembrane potentials. The mobilities of BSA and lysozyme derived from the initial experimental fluxes after consideration of the reduced applied potentials at the electrode/channel interface were 4.5 x 10^~8 and 6.0 x 10^~8 IrI²S^-1V^"1 respectively. On the other hand, whilst the electrophoretic driving force model predicted flux reversals at unfavourable transmembrane potentials, such was not observed experimentally (Fig. HB) . Thus, indicating that the disclosed system is advantageously similar to the facilitated diffusion of solutes in cell membranes in which specific molecules traverse the membrane from high concentration to low concentration, through the action of specific carrier proteins.

Example 4 Nanoparticles characterization under controlled potential conditions using flow arrangement

For the flow system, the same cell setup was employed, but connected to a Shimadzu liquid chromatograph LC-6A pump (Shimadzu, Kyoto, Japan) with a loop volume of 10 μL. Fig. 12 shows the schematic diagram of the flow system, which was employed to determine the electrophoretic mobility of the gold nanoparticles with different size. The electrophoretic mobility of gold nanoparticles with different sizes in the membrane system was investigated by using the 6- aminohexanoic acid-grafted membranes under optimized conditions. The experimentally obtained dependence of the electrophoretic mobility of the gold nanoparticles on the particle size is shown (Fig. 13) . The mobility decreased with particle radius and showed a fairly linear dependence on particle size (R² > 0.99), reflecting the dependence of the mobility on the charge-to-size ratio. Fig. 13 also demonstrated that the membrane system is capable of characterizing nanoparticles in nanometer-size regimes .

Example 5 Effect of the modified membrane on proteins separation

To increase protein selectivity, the nanoporous alumina membranes may be modified by a method as described in PCT/SG2006/000183, the contents of which are incorporated herein for reference. Briefly, alumina membrane was immersed in a halogenated carboxylic acid solution, and the solution was refluxed at 70⁰C for 2 hours. The halogenated carboxylic acid solution was prepared by dissolving 0.03 moles of trifluoroacetic acid (99 vol.%) in one litre (1000cm³) of pure 1, 2-dichloroethane . The alumina membrane was removed from the carboxylic acid solution and was rinsed with de-ionised water to remove any- residual 1, 2-dichloroethane. The alumina membrane was subsequently dried in an oven at 45°C for 4 hours.

The change in the membrane surface and channels will influence the transport of proteins across the membrane. The change in the pore size will influence the extent of wall surface-analyte interactions. For example, the change in the surface charges may alter the electrostatic interaction of the analytes with the channel wall surfaces. Also, the presence of bulky groups attached to the wall surfaces could retard the movement of larger sized analytes. Thus, careful selection of the organic acid used in the chemical modification of alumina membrane may be useful in improving the selectivity of the separation.

Fig. 15 demonstrates the effect of modifying the alumina membrane with three different organic acid: pimelic acid (COOH group) , hexanoic acid (CH₃ group) and β-aminohexanoic acid (NH₂) . Since the experiment was carried out at pH 7, the channel walls would be positively charged, negatively charged and neutral when bonded to molecules containing NH₂, COOH and CH₃ group respectively. Different potentials were applied to study the effect of the modified group on the transport of BSA, myoglobin and lysozyme across membrane. Fig. 15 shows the percentage of the permeated proteins after 60 min with an applied transmembrane potential of +1.5 V. It is clear that the membrane modified with hexanoic acid was most effective to improve the selectivity of the proteins at an applied potential of +1.5 V.

Applications

The disclosed electro-membrane may be used in chemical separation applications in most chemical, pharmaceutical and petrochemical processes.

The disclosed electro-membrane may be used to separate molecules of similar sizes and hence, does not employ the molecular size-dependent sieving mechanisms of conventional membranes.

The disclosed electro-membrane may not suffer from the problems of conventional membrane separation systems such as membrane fouling or concentration polarization. Hence', the need -for costly chemical cleaning or- complete- membrane replacement is substantially minimized.

The disclosed electro-membrane may not suffer from the problem of convective mixing that is associated with known electrophoresis systems. Accordingly, unwanted heating of the analytes may be substantially minimized. The disclosed electro-membranes may be easier to manufacture than known EMF membranes as it is not necessary to align and insert nanotubes into the pores of the membranes or to modify the nanotubes.

The disclosed electro-membranes and associated membrane holder may be used in a number of applications such as analytical protein purification, diagnostic analysis, environmental monitoring, biomedical and pharmaceutical analysis. The electro-membrane and membrane holders may be applied in routine characterization of analytes with sizes in the range of 5 to 40 nm, such as proteins and nanoparticles .

The disclosed membrane holder may be fashioned into different shapes and sizes in order to contain membranes of different dimensions. Due to the possibility of varying the pore sizes and chemical modifications of the electro-membranes, as the electro-membranes are stacked together in the membrane holder, this results in a multilayer configuration to enhance the selectivity and specificity of the system.

The membrane system can also be used in electrochemistry studies. The electro-membrane can be employed as the working electrode in a three electrode system. Electrochemistry can be carried out within the feed cell by inserting the reference and counter electrodes through additional feed inlets. The electrochemical products can be readily separated and subsequently analyzed in the working electrode. A biopotentiostat can be used to monitor the electrochemical reactions at this dual working electrode system. It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

1. An electro-membrane comprising: a porous body having a front surface and a rear surface opposite to said front surface, and a plurality of nano-channels extending through said porous body between said front and rear surfaces for allowing passage of analyte therebetween; and a porous conductive coating disposed on the front and rear surfaces of said porous body for allowing an electrical potential to be generated therebetween.

2. The electro-membrane as claimed in claim 1, wherein the porous body is at least one of a ceramic material, a semiconductive material and a polymeric material.

3. The electro-membrane as claimed in claim 2, wherein the ceramic material is selected from the group consisting of an alumina membrane, a titanium dioxide membrane and a zirconia membrane.

^'4. The electro-membrane as claimed in claim 1, wherein the porous body comprises a silica-based membrane.

5. The electro-membrane as claimed in claim 1, wherein the surfaces of the nano-channels are substantially neutrally charged.

6. The electro-membrane as claimed in claim 5, wherein the surfaces of the nano-channels are chemically modified to thereby acquire the neutral charge.

7. The electro-membrane as claimed in claim 1, wherein the nano-channels have a diameter or equivalent diameter thereof in the range of 1 nm to 200 nm.

8. The electro-membrane as claimed in claim 1, wherein the dimension between the front and rear surfaces of said porous body is in the range of 20 micron to 200 micron.

9. The electro-membrane as claimed in claim 1, wherein the front surface and rear surface are substantially flat and are generally parallel to each other.

10. The electro-membrane as claimed in claim 1, wherein the porous body has a substantially regular pore structure.

11. The electro-membrane as claimed in claim 1, wherein the pore density of the porous body is 10⁸ to 10¹² pores cm^"2.

12. The electro-membrane as claimed in claim 1, wherein the porous conductive coating is selected from the group consisting of a metal, metal alloy, a metalloid, carbon and diamond-like carbon.

13. The electro-membrane as claimed in claim 1, wherein the metal is selected from the group consisting of a group III metal, a transition metal, a noble metal and alloys thereof.

14. The electro-membrane as claimed in claim 13, wherein the metal is selected from the group consisting of aluminium, silver, copper, palladium, nickel, cobalt, chromium, stainless steel, gold, platinum, titanium, zirconium and alloys thereof.

15. The electro-membrane as claimed in claim 1, wherein the porous conductive metal coating has been sputtered onto said front and rear surfaces.

16. The electro-membrane as claimed in claim 1, wherein the thickness of the porous conductive coating is about 20 nm to 100 nm.

17. A method of making an electro-membrane comprising the step of applying a porous conductive coating to a front surface of a porous body and a rear surface of the porous body opposite to said front surface, said porous body having a plurality of nano- channels extending between said front and rear surfaces.

18. The method as claimed in claim 17, wherein the applying step comprises the step of: sputtering a metal onto said front and rear surfaces of said porous body.

19. The method as claimed in claim 18, wherein said sputtering step is carried out for a time duration of 1 min to 14 min.

20. The method as claimed in claim 18, wherein said sputtering step is carried out at a sputtering current in the range of 10mA to 5OmA.

21. The method as claimed in claim 17, wherein the applying step comprises the step of: evaporating a metal onto said front and rear surfaces of said porous body.

22. The method as claimed in claim 17, wherein the applying step comprises the step of: spraying a metal solution onto said front and rear surfaces of said porous body.

23. An electro-membrane holder comprising: a passage for transmission of a sample solution containing analyte therein from an inlet of said passage to an outlet of said passage; means for securing a plurality of electro-membranes within said passage, said electro-membranes being configured in use to block the passage; electrodes capable of being electrically coupled to the electro-membrane to induce an electrical potential across the membrane for adjusting the rate of transmission of analyte within said sample solution through said membrane; and a detector configured to detect the transmission of said analyte through the outlet of said passage.

24. An electro-membrane holder as claimed in claim 23, wherein the outlet of the passage is configured to inhibit turbulent fluid flow therein in use.

25. An electro-membrane holder as claimed in claim 24, wherein the outlet of the passage is comprised of a first passage portion that is parallel to the flow of analyte from the electro-membranes and a second passage portion extending from the first passage portion which is perpendicular to the first passage.

26. An electro-membrane holder as claimed in claim 23, wherein the means for securing a plurality of electro-membranes within said passage comprises means for adjusting the number of electro-membranes that are secured within said passage.