THE FILTRATION MEMBRANE CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application makes reference to and claims the benefit of priority of an application for "Development of Novel Silver Nanoparticle/Multi-walled Carbon Nanotube Nanohybrid Coatings on Polyacrylonitrile Hollow Fiber Membrane for Water Disinfection and Bio fouling Control" filed on June 24, 201 1, with the United States Patent and Trademark Office, and there duly assigned serial number 61/500,840. The content of said application filed on June 24, 2011, is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] The invention relates to a nanocomposite, and a filtration membrane comprising the nanocomposite. The invention also relates to methods to form the nanocomposite and the filtration membrane.

BACKGROUND

[0003] Colloidal silver at a concentration of a few ppm has been found effective in killing numerous infectious bacteria, and has been approved by the U.S. Environmental Protection Agency as a disinfectant for hospitals and medical centers. In its ionic form, the interaction of silver ions (Ag ⁺ ) with thiol groups (S-H) of cysteine and other compounds and the formation of Ag-S bonds can cause detrimental effects to cells by damaging proteins, interrupting the electron transport pathway, and dimerizing DNA.

[0004] On the other hand, the exact antibacterial mechanism of metallic silver nanoparticles remains unclear. It has been postulated that the bactericidal mechanism of silver nanoparticles takes place via a contact-inhibition mechanism. By their direct incorporation into the cell membrane and the subsequent formation of permeable pits, the silver nanoparticles may cause an increase in cell membrane permeability, thereby resulting in osmotic collapse and release of intracellular materials.

[0005] Despite the good antimicrobial activities of silver nanoparticles, direct application of colloidal silver, for example, by adding it directly to main water treatment stream for disinfecting purposes, is not viable nor publicly acceptable, based on the following considerations. Firstly, to achieve an active dosage in water, continuous supply of silver is required. Secondly, due to its potential impact on human health and ecosystems, the silver species have to be removed from water after disinfection treatment.

[0006] Pathogenic microorganisms present in natural and used water pose a serious threat to public health if not removed during water treatment. With increasing demands for higher quality water to satisfy escalating public and industrial needs, the generation of pathogen-free clean water with few byproducts has gained significant attention. Membrane filtration represents an important and advanced water purification and desalination process. However, the lack of bactericidal properties of current filtration membranes results in membrane biofouling, which necessitates frequent backflushing, chemical treatment, and even membrane replacement.

[0007] Various research groups have attempted to immobilize silver nanoparticles onto filtration membranes to develop a disinfection system with good reliability and ease of operation. Water filtration membranes based on polymers such as polysulfone incorporated with silver nanoparticles have been fabricated using conventional phase inversion technique. These composite membranes may be formed, for example, by first blending ex situ prepared silver nanoparticles into polymer solutions, followed by a phase inversion process. As the polymer and silver source were uniformly mixed in the initial blending stage, silver nanoparticles were physically entrapped and distributed in the entire structure of the formed membrane.

[0008] Under convective transport conditions, for example, during continuous filtration, it has been reported that the silver/polysulfone composite membrane did not exhibit significant difference in biofilm inhibition, when compared to the unmodified polysulfone membrane. The limited efficiency has been attributed to leaching of the silver ions from the composite membrane, where the silver ions were convectively carried away from bacteria accumulated on the membrane surface to the permeate side of the membrane. Furthermore, there is the problem of poor adhesion of silver to polymer, which aggravates leaching of the silver ions from the membrane. Consequently, the antimicrobial properties of the membrane were greatly reduced. Apart from the above, intrinsic hydrodynamic performance of the membranes may also be affected due to incorporation of the silver nanoparticles into the membranes. [0009] In view of the above, there is a need for an improved filtration membrane and a method of forming the filtration membrane that overcomes at least some of the above- mentioned problems. SUMMARY OF THE INVENTION

[0010] In a first aspect, the invention relates to a nanocomposite. The nanocomposite comprises

a) carbon nanotubes; and

b) silver nanoparticles,

wherein the silver nanoparticles are coupled to the surface of the carbon nanotubes.

[001 1] In a second aspect, the invention relates to a filtration membrane comprising

a) a porous substrate; and

b) a nanocomposite according to the first aspect,

wherein the nanocomposite is present as a layer attached to the surface of the porous substrate.

[0012] In a third aspect, the invention relates to a method of forming a nanocomposite according to the first aspect, the method comprising

a) mixing carbon nanotubes comprising a linker bound to their surface with a solution comprising a silver nanoparticle precursor, to obtain a suspension of the carbon nanotubes with the solution comprising the silver nanoparticle precursor; and b) chemically reducing the silver nanoparticle precursor using the linker on the carbon nanotubes to allow precipitation of the silver nanoparticles on the surface of the carbon nanotubes.

[0013] In a fourth aspect, the invention relates to a method of forming a filtration membrane, comprising depositing the nanocomposite according to the first aspect on a porous substrate to form a layer of nanocomposite on the porous substrate.

[0014] In a fifth aspect, the invention relates to a water disinfection system comprising a filtration membrane according to the second aspect or a filtration membrane formed by a method according to the fourth aspect.

[0015] In a sixth aspect, the invention relates to use of a filtration membrane according to the second aspect or a filtration membrane formed by a method according to the fourth aspect for the removal of microrganisms from water. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0017] Figure 1 is a schematic diagram showing a nanocomposite according to embodiments of the invention. In the embodiment shown, silver nanoparticles are coupled to the surface of a carbon nanotube.

[0018] Figure 2 is a schematic diagram showing a method of forming a nanocomposite according to embodiments of the invention. Carbon nanotubes comprising a linker bound to their surface are mixed with a solution comprising a silver nanoparticle precursor, wherein the silver nanoparticle precursor is chemically reduced using the linker on the carbon nanotubes to allow precipitation of the silver nanoparticles on the surface of the carbon nanotubes.

[0019] Figure 3 is a schematic diagram depicting aminolysis reaction between a diamine with a porous substrate. In the embodiment shown, the diamine is ethylene diamine (EDA) and the porous substrate is partially hydrolyzed polyacrylonitrile (PAN).

[0020] Figure 4 is a schematic diagram depicting formation pathway of a filtration membrane according to embodiments of the invention. Figure 4A shows a pristine carbon nanotube. The pristine carbon nanotube is contacted with acid comprising a mixture of concentrated sulfuric acid and nitric acid, at 70 °C for 3 hours. Figure 4B shows acid-treated carbon nanotubes, where carboxyl groups are bound to the surface of the carbon nanotubes. The acid-treated carbon nanotubes are treated with a linker comprising polyethylene glycol (PEG,) and 1 ,6-hexamethylene diisocyanate, to form PEG-grafted carbon nanotubes as shown in Figure 4C. The PEG-grafted carbon nanotubes are contacted with silver nitrate at a temperature of 60 °C for 3 hours. The silver nitrate is chemically reduced by the PEG to precipitate silver nanoparticles on the surface of the carbon nanotubes, to form a nanocomposite as shown in Figure 4D. Linker molecules comprising l-ethyl-3-(3- dimethylaminopropyl)carbodiimide (EDC) are contacted with the nanocomposite, such that the linker molecules chemically react with the carboxyl groups present on the nanotubes to form a EDC-modified nanocomposite as shown in Figure 4E. Figure 4F shows a EDA- modified PAN substrate. The EDA-modified PAN substrate is contacted with the EDC- modified nanocomposite to form a filtration membrane as shown in Figure 4G. In the embodiment shown in Figure 4G, the EDA-modified PAN substrate chemically reacts with the EDC-modified nanocomposite, such that the nanocomposite is covalently attached to the substrate via an amide bond.

[0021] Figures 5 A to 5D are scanning electron microscopy (SEM) images of a pristine PAN hollow fiber membrane at different resolutions. As can be seen from Figures 5A and 5B, the membrane has a typical asymmetric pore structure with long finger-like pores.

[0022] Figures 6A to 6D are scanning electron microscopy (SEM) images of PAN hollow fiber membranes modified with EDA by immersion in 100 mL of 20 vol% aqueous EDA solution at room temperature for 24 hours.

[0023] Figure 7 is a graph showing the attenuated total reflectance (ATR) spectra of (A) pristine PAN; and (B) PAN-EDA.

[0024] Figures 8A and 8B are graphs showing the Barret-Joyner-Halenda (BJH) desorption pore size distribution of (A) pristine PAN; and (B) PAN-EDA.

[0025] Figure 9 is a graph showing the X-ray diffraction pattern (XRD) of a silver nanoparticle/multi-walled carbon nanotubes (Ag/MWNT) nanocomposite according to an embodiment of the invention.

[0026] Figures 10A to 10D are transmission electron microscopy (TEM) and high- resolution transmission electron microscopy (HRTEM) images of Ag/MWNTs.

[0027] Figure 11 is a graph showing Fourier transform infrared spectroscopy (FTIR) spectra of (A) MWNTs; (B) multi-walled carbon nanotubes modified with carboxyl groups (MWNTs-COOH); and (C) multi -walled carbon nanotubes modified with PEG groups (MWNTs-PEG).

[0028] Figure 12 is a graph showing the thermal gravimetric analysis (TGA) results of (A) MWNTs; (B) MWNTs-COOH; and (C) MWNTs-PEG.

[0029] Figures 13A to 13F are scanning electron microscopy (SEM) images of (A) and (B) (silver nanoparticle/multi-walled carbon nanotubes nanocomposite)/polyacrylonitrile filtration membrane (nAg/MWNTs/PAN), (C) and (D) control sample using pristine PAN (without modification by EDA), and (E) and (F) control sample prepared in the absence of EDC. [0030] Figure 14 is a graph showing relative flux variation of pristine PAN using the medium (hydroxyethyl piperazineethanesulfonic acid (HEPES) (1 mM)/glucose (1 wt%)) as the feed water.

[0031] Figures 15A to 15C are graphs showing relative flux varation using bacterium containing feed water (medium: HEPES (1 mM)/glucose (1 wt%)) for (A) E. coli; (B) P. aeruginosa; and (C) S. aureus. Data points denoted by diamonds: pristine PAN membrane. Data points denoted by squares: nAg/MWNTs/PAN composite membrane.

[0032] Figures 16A to 16C are graphs showing the concentration of living bacterium cells in the reject water samples in the form of relative colony-forming units (CFU) (CFU at time t/initial CFU) for (A) E. coli; (B) P. aeruginosa; and (C) S. aureus. Data points denoted by diamonds: pristine PAN membrane. Data points denoted by squares: nAg/MWNTs/PAN composite membrane. Data points denoted by triangles: nAg/MWNTs/PAN composite membrane after soaking in deionized water for 14 days.

[0033] Figures 17A to 17F are SEM images of the pristine PAN membranes (A, C, and E) and nAg/MWNTs/PAN composite membranes (B, D and F) after filtration test using bacterium containing feed water, (A) and (B) E. coli, (C) and (D) P. aeruginosa, and (E) and (F) S. aureus.

[0034] Figure 18 is a schematic diagram of the bench-scale filtration test system used in the experiments. The filtration test system includes four pieces of hollow fiber membrane of 200 mm in length. The fibers were sealed at one end with epoxy glue. The wall of the module was made of acrylic glass pipe with an outer diameter (OD) of 25.4 mm and length of 300 mm.

[0035] Figure 19 is a table summarizing parameters of a PAN hollow fiber membrane.

[0036] Figure 20 is a graph showing FTIR spectrum of dry PAN powder.

[0037] Figures 21A and 21B are graphs depicting results of continuous filtration test: (A) the flux change using E. co/z ^' -containing feedwater and (B) viable E. coli cell concentration in the reject water. Data points in circles: control permeation test of pristine PAN using the medium without E. coli; data points in squares: pristine PAN; data points in triangles: Ag/MWNTs/PAN; and data points in diamonds: Ag/MWNTs/PAN after soaking in deionized water for 14 days.

[0038] Figure 22 is a schematic diagram showing a hollow fiber (A) with nanocomposite; (B) without nanocomposite. As can be seen from the diagram, the portion of hollow fiber without a layer of nanocomposite leads to biofilm built up, whereas the portion of hollow fiber that has a layer of nanocomposite leads to breakdown of microorganisms present in the feedwater. DETAILED DESCRIPTION OF THE INVENTION

[0039] In a first aspect the present invention refers to a nanocomposite. The nanocomposite of the present invention comprises carbon nanotubes and silver nanoparticles. The silver nanoparticles are coupled to the surface of the carbon nanotubes.

[0040] By depositing a layer of nanocomposite comprising carbon nanotubes and silver nanoparticles, in which the silver nanoparticles are coupled to the surface of the carbon nanotubes, on a porous substrate such as a membrane, the silver nanoparticles on the carbon nanotubes matrix may be located at the membrane/feed water interface so as to allow direct contact between the silver nanoparticles and the bacteria cells comprised in the feed water for improved disinfecting. Furthermore, the carbon nanotubes provide an open network structure that minimizes impact of the nanocomposite layer on water flux through the membrane. Both the deposition of the silver nanoparticles on the carbon nanotubes, and the coating of the nanocomposite on the porous substrate, may further include chemically reacting the silver nanoparticles with the carbon nanotubes and/or the nanocomposite with the porous substrate, to result in covalent bonding hence stronger binding among the various components of the filtration membrane. In so doing, the disinfecting efficiency as well as the life time of the filtration membrane may be improved.

[0041] As used herein, the term "nanocomposite" refers generally to a mixture of materials, where each material in the mixture has at least one dimension in the nanometer range. For example, a nanocomposite may comprise a mixture of zero dimensional materials such as nanoparticles; one dimensional materials such as nanorods, nanowires and nanotubes; and/or two dimensional materials such as nanoflakes, nanoflowers, nanodiscs and nanofilms.

[0042] The nanocomposite of the invention comprises carbon nanotubes. The term "carbon nanotube" refers to a cylindrical single- or multi-walled structure in which the at least one wall of the structure is predominantly made up of carbon. The terms "carbon nanotube" and "nanotube" are used interchangeably throughout the entire disclosure. Generally, carbon nanotubes can be formed by methods such as arc-discharge, laser ablation and chemical vapor deposition (CVD). [0043] The number of shells in a carbon nanotube can vary from one, i.e., constituting a single-walled carbon nanotube (SWNT or SWCNT), to as many as 50 shells, in which case it is termed a multi-walled carbon nanotube (MWNT or MWCNT). Single-walled carbon nanotubes may be described as a graphite plane (so called graphene) sheet rolled into a hollow cylindrical shape so that the structure is one-dimensional with axial symmetry, and in general exhibiting a spiral conformation, called chirality. In multi-walled carbon nanotubes, the shells that make up the carbon nanotubes may be concentric, with each pair of adjacent shells in such structure having a spacing between layers that is on the order of ~ 0.34 nm. Examples of carbon nanotubes that may be used in the present invention include, but are not limited to, single- walled carbon nanotubes, double-walled carbon nanotubes (DWNT or DWCNT), multi-walled carbon nanotubes, or a mixture thereof. In various embodiments of the invention, the carbon nanotubes comprise multi-walled carbon nanotubes. In some embodiments, only multi- walled carbon nanotubes are used, i.e. the carbon nanotubes consist of multi-walled carbon nanotubes.

[0044] The carbon nanotube may be a metallic carbon nanotube, or a semiconducting carbon nanotube, or a combination of both. The carbon nanotube may be of any length and diameter. For example, in embodiments in which multi-walled carbon nanotubes are used, each multi -walled carbon nanotube may have a diameter of greater than about 10 nm, such as about 10 nm to about 200 nm, about 50 nm to about 100 nm, about 100 nm to about 200 nm, or about 10 nm to about 50 nm. In various embodiments, the mean diameter of the multi- walled carbon nanotubes is greater than about 10 nm. Each multi- walled carbon nanotube can have a length of about 0.5 μιη to about 300 μηι, such as about 0.5 μιη to about 200 μηι, about 0.5 μιη to about 100 μηι, or about 0.5 μηι to about 50 μιη. Typically, multi-walled carbon nanotubes are about 10 nm to about 50 nanometers in diameter, and have a length of about 0.5 μιη to about 100 μηι. Atomic Force Microscopy (AFM) and/or Raman Scattering Spectroscopy may for instance be used to determine the dimensions of the carbon nanotubes. Generally, the longer the carbon nanotubes, the greater the tendency of the nanotubes to entangle. As a result, an entangled mass or cluster of carbon nanotubes may be formed.

[0045] The nanocomposite further comprises silver nanoparticles, which are coupled to the surface of the carbon nanotubes. The silver nanoparticles may comprise oxidized silver, silver, or silver oxide. In various embodiments, the silver nanoparticles consist essentially of silver. In some embodiments, the silver nanoparticles consist entirely of silver. The size of the silver nanoparticles may be characterized by their mean diameter. The term "diameter" as used herein refers to the maximal length of a straight line segment passing through the center of a figure and terminating at the periphery. The term "mean diameter" refers to an average diameter of the nanoparticles, and may be calculated by dividing the sum of the diameter of each nanoparticle by the total number of nanoparticles. Although the term "diameter" is used normally to refer to the maximal length of a line segment passing through the centre and connecting two points on the periphery of a nanosphere, it is also used herein to refer to the maximal length of a line segment passing through the centre and connecting two points on the periphery of nanoparticles having other shapes, such as a nanocube or a nanotetrahedra, or an irregular shape. The mean diameter of the silver nanoparticles may be about 2 nm to about 5 nm, such as about 2 nm, about 3 nm, about 4 nm or about 5 nm. In various embodiments, the silver nanoparticles are essentially monodisperse. Generally, for a given concentration of silver in the nanocomposite, a smaller size of the silver nanoparticles results in a larger surface area of the silver nanoparticles in the nanocomposite. The silver nanoparticles may have a regular shape, or may be irregularly shaped. Typically, <111> facet exposed silver nanoparticles, which have been found to be most effective in disinfection and which have been obtained using a method of the invention, have a triangular shape.

[0046] The silver nanoparticles may be coupled to the surface of the carbon nanotubes by non-covalent bonds. For example, the silver nanoparticles may be physically entrapped in clusters of the carbon nanotubes, or attached to the carbon nanotubes by van der Waals forces. In various embodiments, the silver nanoparticles are chemically bound to the surface of the carbon nanotubes. For example, the silver nanoparticles may be covalently bonded to the carbon nanotubes, whereby the use of a covalent bond translates into a stronger interfacial bond between the silver nanoparticles and the nanotubes.

[0047] In various embodiments, the silver nanoparticles are chemically bound to the surface of the carbon nanotubes via a linker, wherein the linker is covalently coupled to the surface of the carbon nanotubes. As used herein, the term "linker" refers to a molecule connecting the surface of the carbon nanotube with the silver nanoparticle. In various embodiments, the linker is covalently coupled to both the carbon nanotube and the silver nanoparticle.

[0048] Carbon nanotubes do not typically comprise such linkers, or may contain linkers only in a very small amount. Therefore, the carbon nanotubes may be subjected to a treatment for introducing (and binding) linkers to their surface, prior to contacting with the silver nanoparticles. A linker may comprise molecules having a functional group such as a hydroxyl group, an ester, a thiol, an amine, or a carboxyl group. It is also possible that mixtures of different linkers of the aforementioned group are used to bind nanoparticles to the surface of the carbon nanotubes. Examples of suitable linkers include, but are not limited to, porphyrine (which includes amine groups) or polyethylene glycol (PEG; also known as poly(ethylene oxide)) (which includes -OH groups).

[0049] In various embodiments, the linker comprises polyethylene glycol (PEG). The silver nanoparticles may be coupled to the carbon nanotubes by means of PEG groups covalently coupled to the nanotubes. To facilitate attachment of the PEG groups on the nanotubes, the carbon nanotubes may be subjected to an acid treatment step, for example, by contacting with a mixture of concentrated sulfuric acid and concentrated nitric acid, to introduce carboxyl groups on the surface of the nanotubes, so as to covalently couple the PEG groups to the nanotubes via the carboxyl groups. In various embodiments, a coupling agent may be used to connect the PEG groups to the carboxyl groups. Examples of suitable coupling agents include a diisocyanate. In one embodiment, the coupling agent comprises 1 ,6-hexamethylene diisocyanate.

[0050] The silver nanoparticles may be at least substantially evenly distributed on the surface of the carbon nanotubes. In some embodiments, the silver nanoparticles are evenly distributed on the surface of the carbon nanotubes. The amount of silver nanoparticles in the nanocomposite may range from about 0.5 wt% to about 5 wt%, such as about 1 wt %, 2 wt%, 3 wt% or 4 wt%. In one embodiment, the amount of silver nanoparticles in the nanocomposite is about 2.5 wt%.

[0051] It has been demonstrated herein that the nanocomposite comprising silver nanoparticles and carbon nanotubes, when used as a layer coated on the external surface of a porous substrate such as a membrane, is effective in killing microorganisms such as bacteria, and in the control of biofilm growth. In this regard, the invention relates in a second aspect, to a filtration membrane. The filtration membrane comprises a porous substrate; and a nanocomposite according to the first aspect, wherein the nanocomposite is present as a layer attached to the surface of the porous substrate.

[0052] The porous substrate may include a polymer. Generally, any polymer that may be used to form a membrane may be used. Examples of polymer that may be used to form the porous substrate include, but are not limited to, polyacrylonitrile, polysulfone, polyvinylidene fluoride, polymethacrylic acid, polyamide, polyimide, polyether imide, cellulose acetate, and mixtures thereof. In various embodiments, the porous substrate comprises polyacrylonitrile. In some embodiments, the porous substrate consists of polyacrylonitrile.

[0053] The nanocomposite may be chemically bound to the surface of the porous substrate. For example, the nanocomposite may be covalently bonded to the porous substrate. As a result of the chemical bond, a stronger attachment or binding of the nanocomposite to the porous substrate may be achieved, as compared to when the nanocomposite is physically coated on the porous substrate.

[0054] To facilitate chemical bonding of the nanocomposite to surface of the porous substrate, the porous substrate may be functionalized with a functional group, for example, by reacting with molecules having a functional group such as a hydroxyl group, an ester, a thiol, an amine, or a carboxyl group. In various embodiments, the nanocomposite is coupled to the surface of the porous substrate by means of diamine groups covalently coupled to the substrate. In one embodiment, when polyacrylonitrile is used as the porous substrate, it is modified by reaction with ethylene diamine (EDA), which is a diamine. By reacting the nanocomposite with a linker molecule such as l-ethyl-3-(3- dimethylaminopropyl)carbodiimide (EDC), a EDC-modified nanocomposite is formed. Accordingly, by allowing the EDA-modified porous substrate with the EDC-modified nanocomposite to react, the nanocomposite is chemically bound to the surface of the porous substrate via an amide bond.

[0055] In various embodiments, the average pore size of the porous substrate is smaller than the mean diameter of the carbon nanotubes comprised in the nanocomposite. By virtue of their size difference, the carbon nanotubes may remain as an external layer on the porous substrate, since the (smaller) size of the pores serve to keep out the (larger) nanotubes. Accordingly, in some embodiments, the carbon nanotubes do not infiltrate the porous substrate. This external layer of nanocomposite is advantageous, as it has been found by the inventors of the invention that the nanocomposite that is coated as a separate layer on the surface of the membrane acts as an effective barrier for killing of bacteria and preventing biofilm growth. As mentioned above, the mean diameter of the carbon nanotubes may be greater than about 10 nm. Therefore, the average pore size of the porous substrate may be smaller than 10 nm, such as about 2 nm to about 8 nm, about 4 nm to about 8 nm, or about 4 nm to about 6 nm. In various embodiments, the average pore size of the porous substrate is about 2 nm to about 6 nm.

[0056] In various embodiments, the porous substrate is a hollow fiber membrane. Hollow fiber membranes typically assume the form of hollow tubes of circular cross-section, whereby the wall of the tube may function as the membrane. The use of a hollow fiber membrane may be advantageous in that there is a higher surface to volume ratio of the membrane, which allows a greater area of the filtration membrane, hence the nanocomposite layer, to come into contact with feed water for disinfection and biofouling control purposes.

[0057] According to a third aspect, the invention relates to a method of forming a nanocomposite according to the first aspect. The method comprises mixing carbon nanotubes comprising a linker bound to their surface with a solution comprising a silver nanoparticle precursor, to obtain a suspension of the carbon nanotubes with the solution comprising the silver nanoparticle precursor.

[0058] Examples of linker that may be used in the present invention have already been described above. In various embodiments, the linker comprises polyethylene glycol (PEG). As mentioned above, carbon nanotubes do not typically comprise such linkers, or only in a very small amount. Therefore, the carbon nanotubes comprised in the nanocomposite may be subjected to a treatment for introducing (and binding) linkers to their surface. In various embodiments, the carbon nanotubes comprising a linker may be formed by reacting the carbon nanotubes with an acid to form carboxyl groups on their surface, and reacting the carbon nanotubes comprising carboxyl groups with the linker in the presence of a coupling agent to graft the linker to at least some of the carboxyl groups.

[0059] Suitable acids that may be used include concentrated sulfuric acid, and/or concentrated nitric acid. In one embodiment, a mixture of concentrated sulfuric acid and concentrated nitric acid is contacted with carbon nanotubes, to form carboxyl groups on the surface of the carbon nanotubes.

[0060] Depending on the type of linkers used, a coupling agent may be required to graft the linker to the carboxyl groups. In various embodiments, the coupling agent comprises a diisocyanate. In embodiments where PEG groups is used as the linker, 1 ,6-hexamethylene diisocyanate is used as the coupling agent.

[0061] Some or most of the carboxyl groups that is present on the surface of the carbon nanotubes may be grafted with the linker. In various embodiments, such as in the fabrication of a filtration membrane comprising the nanocomposite, some of the carboxyl groups may be used to chemically bind the nanocomposite to the porous substrate. Accordingly, depending on the type of application, not all of the carboxyl groups that are present on the surface of the carbon nanotubes are grafted with the linker. In various embodiments, more than 50% of the carboxyl groups that are present on the surface of the carbon nanotubes are grafted with the linker. In so doing, the loading of silver nanoparticles on the carbon nanotubes may be controlled.

[0062] The method of forming a nanocomposite according to the first aspect comprises chemically reducing the silver nanoparticle precursor using the linker on the carbon nanotubes to allow precipitation of the silver nanoparticles on the surface of the carbon nanotubes. The silver nanoparticle precursor may be a silver salt. Examples of silver salt include silver nitrate, silver acetate, silver bromide, silver lactate, silver chloride, silver fluoride, silver iodide, silver sulfate, and silver phosphate. In various embodiments, the silver nanoparticle precursor is silver nitrate (AgN0 ₃ ). The silver nitrate may be chemically reduced by the PEG linkers present on the carbon nanotubes, thereby forming silver nanoparticles (Ag) which bind to the nanotubes via the PEG linkers.

[0063] It has been demonstrated herein that the grafted PEG on the carbon nanotubes are able to reduce silver ions to silver nanoparticles of controlled sizes, and at the same time bind them on the surface of the carbon nanotubes. In particular, through the use of a method of the invention, it has been found that silver nanoparticles with a narrow size range of about 2 nm to about 5 nm mean diameter may be formed on the surface of the carbon nanotubes.

[0064] Generally, the size of the nanoparticles formed by the process may be varied by factors such as the temperature at which chemical reduction takes place, and concentration of the silver nanoparticles precursor used. For example, the chemical reduction step for forming the silver nanoparticle may take place at about 20 °C to about 80 °C, such as about 40 °C to about 60 °C, about 60 °C, or about 60 °C to about 80 °C. In some embodiments, the chemical reduction step may take place at room temperature. The concentration of the silver nanoparticles precursor may range from about 0.001 M to about 1 M, such as about 0.001 M to about 0.01 M, or about 0.01 M to about 1 M.

[0065] On the other hand, it has been found by the inventors that the reaction time or incubation time of the carbon nanotubes with the silver nanoparticle precursor do not affect the size of the silver nanoparticles formed by the process to the same extent as that resulting from a change in the reaction temperature and concentration of the silver nanoparticles precursor used. Generally, the reaction time is between about 60 minutes to about 240 minutes, such as about 60 minutes to about 120 minutes, about 120 minutes to about 180 minutes, or about 180 minutes to about 240 minutes.

[0066] In one embodiment in which 0.01 M silver nitrate is used as the silver nanoparticle precursor, the chemical reduction of the silver nitrate to silver nanoparticles using PEG linkers bound to the carbon nanotubes takes place at a temperature of about 60 °C for about 180 minutes. As can be seen from the examples, this results in formation of silver nanoparticles with a narrow mean diameter range of about 2 nm to about 5 nm.

[0067] According to a fourth aspect, the invention relates to a method of forming a filtration membrane. The method comprises depositing the nanocomposite according to the first aspect on a porous substrate to form a layer of nanocomposite on the porous substrate. The nanocomposite may be deposited on the porous substrate using any suitable thin film coating method, such as spin coating, dip coating and painting.

[0068] In various embodiments, the method further comprises chemically reacting the nanocomposite with the porous substrate to attach the nanocomposite to the surface of the porous substrate. In various embodiments, this may be carried out, for example, by contacting the nanocomposite with linker molecules to covalently bind the linker .molecules to the carbon nanotubes, to form functionalized carbon nanotubes. The porous substrate, on the other hand, may also be contacted with molecules having a functional group, such as a diamine, to covalently bind the diamine to the substrate, to form a functionalized porous substrate. Subsequent contact of the functionalized carbon nanotubes with the functionalized porous substrate may result in a chemical reaction between the linker molecules on the carbon nanotubes with the diamine on the substrate. In so doing, the carbon nanotubes are covalently bonded to the porous substrate, thereby attaching the nanocomposite to the surface of the porous substrate. In various embodiments, the diamine comprises ethylene diamine (EDA).

[0069] In various embodiments, the linker molecules are covalently bound to the nanotubes via carboxyl groups present on the nanotubes. As described above, carbon nanotubes that are comprised in the nanocomposite may be subjected to a treatment with an acid to form carboxyl groups on their surface, as an initial step to introduce linkers on the surface of the carbon nanotubes. Residual carboxyl groups, i.e. carboxyl groups which are not grafted with the linker, may upon contact with the linker molecules, bind with the linker molecules. Linker molecules that may be used include molecules that comprise a carbodiimide group. For example, the linker molecules may comprise a compound selected from the group consisting of l-ethyl-3-(3-dimethylaminopropyl)carbodiimide and Ν,Ν'- dicyclohexylcarbodiimide. In one embodiment, the linker molecules comprise l-ethyl-3-(3- dimethylaminopropyl)carbodiimide.

[0070] According to a fifth aspect, the invention relates to a water disinfection system comprising a filtration membrane according to the second aspect or a filtration membrane formed by a method according to the fourth aspect.

[0071] In various embodiments, the filtrate membrane comprised in the water disinfection system is oriented such that the nanocomposite layer, hence the silver nanoparticles comprised in the nanocomposite layer, is located at the interface between the membrane and the feedwater. In such an orientation, the feedwater may first pass through the layer of nanocomposite, and subsequently through the porous substrate. In so doing, there is improved contact between the silver nanoparticles and the microorganisms in the feedwater. In various embodiments, there is direct contact between the silver nanoparticles and the microorganisms in the feed water. The efficacy of the silver nanoparticles in disinfecting and antifouling is thereby improved.

[0072] In a sixth aspect, the invention relates to use of a filtration membrane according to the second aspect or a filtration membrane formed by a method according to the fourth aspect for the removal of microorganisms from water. The term "microorganism" as used herein includes bacteria, fungi, protozoa, viruses and other biological entities and pathogenic species which can pollute a water source.

[0073] Bacteria may be classified as gram-positive and gram-negative bacteria. Examples of bacteria include such as Escherichia coli (E. Coli), Pseudomonas aeruginosa (P. aeruginosa), Staphylococcus aureus (S. aureus), Streptococcus faecalis, Legimella pneumophila, Yersinia enterocolitica, Klebsiella terrigena, Sphingomonas, Alcaligenes, Chryseobacterium, and Salmonella typhi. Examples of viruses include hepatitis A virus, norovirus, and adenovirus. Examples of fungi include Paecilomyces and Trichoderma, including those which are not pathogenic but are advantageously removed to improve the aesthetic properties of the water. Examples of protozoa include Enteroamoebae, Giardia, and Cryptosporidium parvum. [0074] In various embodiments, the microorganisms comprise E. Coli. It has been demonstrated in the filtration studies by the inventors using feedwater containing E. Coli that the filtration membrane comprising a layer of nanocomposite significantly enhances the antimicrobial activities and antifouling properties of the membrane, with a much lower degree of fouling observed on the filtration membrane. Consequently, the flux drop over the filtration membrane was significantly smaller due to lower levels of fouling, as compared to a membrane that does not contain the nanocomposite layer.

[0075] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0076] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0077] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

EXPERIMENTAL SECTION [0078] Figure 1 is a schematic diagram showing a nanocomposite according to embodiments of the invention. In the embodiment shown, silver nanoparticles are coupled to the surface of a carbon nanotube.

[0079] Figure 2 is a schematic diagram showing a method of forming a nanocomposite according to embodiments of the invention. Carbon nanotubes comprising a linker bound to their surface are mixed with a solution comprising a silver nanoparticle precursor, wherein the silver nanoparticle precursor is chemically reduced using the linker on the carbon nanotubes to allow precipitation of the silver nanoparticles on the surface of the carbon nanotubes.

[0080] Figure 3 is a schematic diagram depicting aminolysis reaction between a diamine with a porous substrate. In the embodiment shown, the diamine is ethylene diamine (EDA) and the porous substrate is partially hydrolyzed polyacrylonitrile (PAN).

[0081] Figure 4 is a schematic diagram depicting formation pathway of a filtration membrane according to embodiments of the invention. Figure 4A shows a pristine carbon nanotube. The pristine carbon nanotube is contacted with acid comprising a mixture of concentrated sulfuric acid and nitric acid, at 70 °C for 3 hours. Figure 4B shows acid-treated carbon nanotubes, where carboxyl groups are bound to the surface of the carbon nanotubes. The acid-treated carbon nanotubes are treated with a linker comprising polyethylene glycol (PEG,) and 1 ,6-hexamethylene diisocyanate, to form PEG-grafted carbon nanotubes as shown in Figure 4C. The PEG-grafted carbon nanotubes are contacted with silver nitrate at a temperature of 60 °C for 3 hours. The silver nitrate is chemically reduced by the PEG to precipitate silver nanoparticles on the surface of the carbon nanotubes, to form a nanocomposite as shown in Figure 4D. Linker molecules comprising l-ethyl-3-(3- dimethylaminopropyl)carbodiimide (EDC) are contacted with the nanocomposite, such that the linker molecules chemically react with the carboxyl groups present on the nanotubes to form a EDC-modified nanocomposite as shown in Figure 4E. Figure 4F shows a EDA- modified PAN substrate. The EDA-modified PAN substrate may be formed using the process shown in Figure 3. The EDA-modified PAN substrate is contacted with the EDC-modified nanocomposite to form a filtration membrane as shown in Figure 4G. In the embodiment shown in Figure 4G, the EDA-modified PAN substrate chemically reacts with the EDC- modified nanocomposite, such that the nanocomposite is covalently attached to the substrate via an amide bond. [0082] Example 1: Modification of polyacrylonitrile (PAN) hollow fiber membrane by ethylene diamine (EDA)

[0083] The properties of PAN hollow fiber membranes (Ultra-Flo Pte Ltd., Singapore) are listed in the table shown in Figure 19. PAN was soaked in deionized water after arrival. The amination of PAN was conducted by immersing the wet PAN membranes (250 mm in length) in 100 mL of an aqueous solution of 20 vol % of ethylene diamine (EDA, Alfa Aesar, 99%). The mixture was shaken at 100 rpm at room temperature for 24 h. The modified PAN membranes were rinsed with deionized water several times until the pH of the rinsed water reached about 7.0. The membranes denoted as PAN-EDA were kept wet for further use.

[0084] Mild conditions as detailed above were applied in order to prevent any drastic changes to the pore structure of the membrane. To elucidate the reaction mechanism, PAN was also modified at the reflux temperature while keeping other conditions the same and such modified PAN was denoted as PAN-ED A-R.

[0085] Example 2: Preparation of silver/multiwalled carbon nanotubes (Ag MWNTs) nanocomposite

[0086] Example 2.1; Carboxylation of MWNTs

[0087] MWNTs (CNano Technology Ltd., purity: greater than 95%, average length: 10 μιη, average diameter: 11 nm) were first carboxylated by treatment with concentrated sulphuric acid (H ₂ S0 ₄ ) (Merck, 98%) and nitric acid (HN0 ₃ ) (Honeywell, 69%).

[0088] Briefly, 1 g of calcined MWNTs (350 °C for 2 hours in static air with a heating rate of 5 °C/min) was utrasonicated in 150 mL of a mixture of H ₂ S0 ₄ and HN0 ₃ solutions with a volumetric ratio of 3: 1 for 1 hour. The MWNTs suspension was heated at 70 °C for 4 hours. Prior to filtration and washing, a copious amount of deionized water (about 2 L) was added to the suspension to dilute the concentrated acid solution. Filtration was carried out using vacuum filtration with nylon based filter membranes (pore size: 0.8 μηι). The collected carboxylated-MWNTs (MWNTs-COOH) were washed with deionized water for several times until pH of the filtrate reached around 4.0. The obtained solid was dried in vacuum overnight for further use.

[0089] Example 2.2: Functionalize MWNTs-COOH with polyethylene glycol (PEG)

[0090] To functionalize MWNTs-COOH with polyethylene glycol (PEG), 100 mg of dried MWNTs-COOH was ultrasonicated in 60 mL of acetone for 30 minutes followed by purging with nitrogen (N ₂ ) gas at 60 mL/min at 50 °C for another 30 minutes. Subsequently, 0.2 mL (1.25 mmol) of 1 ,6-hexamethylene diisocyanate (HDI, Fluka, 99%) was added into the MWNTs-COOH suspension, and the mixture was stirred at 50 °C for 2 hours.

[0091] Subsequently, 9 g (1.5 mmol) of PEG (molecular weight: 6000, Alfa Aesar, > 99%) was added into the suspension followed by stirring for another 2 hours at the same temperature. N ₂ purging was carried out throughout the reaction, and a water condenser was mounted on top of the flask. The resulting product (MWNTs-PEG) was first washed with acetone followed with deionized water several times, and then dried under vacuum overnight for further loading of silver nanoparticles.

[0092] Example 2.3: Loading silver nanoparticles onto MWNTs-PEG

[0093] Silver nanoparticles were loaded onto MWNTs-PEG through reduction of Ag ⁺ ions by PEG grafted on the surface of the MWNTs. Briefly, 50 mg of MWNTs-PEG was ultrasonicated in 10 mL of deionized water for 30 minutes. Then, 20 mL of 0.01 M silver nitrate (AgN0 ₃ ) (BDH, 99%) aqueous solution was added into the MWNTs-PEG suspension. The mixture was heated to 60 °C and maintained at this temperature for 3 hours under stirring. The resulting product, denoted as Ag/MWNTs, was washed twice with deionized water and then dried under vacuum overnight.

[0094] Example 3: Grafting of Ag/MWNTs on a PAN-ED A hollow fiber membrane

[0095] PAN-EDA hollow fibers were immersed in a suspension of 25 mg of Ag/MWNTs in 100 mL of deionized water. Both ends of the hollow fiber membranes were lifted above the surface of the suspension in order to prevent the entrance of Ag MWNTs into the interior space of the hollow fibers. Then, 200 mg of l-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC, Acros, 99%) was added, and the mixture was shaken gently at room temperature for 16 hours. The membranes attached with Ag/MWNTs, denoted as Ag/MWNTs/PAN, were then rinsed with deionized water and kept wet for subsequent filtration and antimicrobial testing.

[0096] Example 4: Materials characterization

[0097] The powder X-ray diffraction pattern of Ag/MWNTs was recorded in a Bruker AXS D8 X-ray diffractometer with Cu Κα (λ = 1.5406 A) radiation at 40 kV and 20 mA. Fourier transform infrared (FTIR) spectra were obtained on a Digilab FTS 3100 FTIR with a 4 cm ^"1 resolution and in the range of 400 cm ^"1 to 4000 cm ^"1 using a standard potassium bromide (KBr) disk technique. Particle size and morphology were observed using JEOL 3010 transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) was carried out by heating the dry powder samples at a rate of 10 °C/min with nitrogen flow at 200 mL/min over 25 °C to 800 °C in a TA Instruments SDT Q600.

[0098] The functional groups of the pristine and modified membrane were studied using ATR-IR on a Perkin-Elmer Spectrum One FTIR spectrometer. The morphologies of the membranes were examined by SEM (JEOL JSM 6700F field emission) at 5kV, 10 μΑ. The surface area and pore size of the membrane were measured by nitrogen physisorption using Quantachrome Autosorb 6B by analyzing 10 pieces of hollow fiber membranes (length: 1 cm). The contact angle measurement of water on thoroughly dried membranes was conducted in a FTA 200 (First Ten Angstroms, Portsmouth, VA, USA). The percentages of silver in Ag/MWNTs and Ag/MWNTs/PAN were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) in a Perkin-Elmer ICP Optima 2000DV after dissolving silver in dilute solutions of HN0 ₃ .

[0099] Example 5: Filtration module setup and permeation test

[00100] A bench-scale filtration test system was set up to evaluate the permeation and antimicrobial properties of a Ag/MWNTs/PAN composite membrane. Figure 18 is a schematic diagram of the bench-scale filtration test system used in the experiments. The filtration test system includes four pieces of hollow fiber membrane of 200 mm in length. The fibers were sealed at one end with epoxy glue. The wall of the module was made of acrylic glass pipe with an outer diameter (OD) of 25.4 mm and length of 300 mm.

[00101] Before the permeation test, the module was pressurized at 2 bar for leakage checking. The module was operated with an outside-in mode. Deionized water was permeated through the membrane module for 30 minutes for initial flux measurement, during which the flow rate was fixed at 5 mL/min. The permeate volume was measured to calculate the flux according to Equation 1 , as follows

[00102] Q = A x flux x P Equation (l)

[00103] where Q is the volumetric flow rate, A is the total surface area of the hollow fibers, and P is the transmembrane pressure (TMP). TMP was measured by a digital pressures gauge (GE Druck DPI 104, Accuracy: 0.05% of full span).

[00104] Example 6: Permeation test of a Ag/MWNTs/PAN composite membrane using bacterial water

[00105] Example 6.1; Preparation of bacterial water [00106] Feed water samples containing the representative bacterial systems {Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus) were prepared by introducing a loop of bacteria from the stock agar plate in 10 mL of Luria-Bertani (LB) broth (Difco LB Broth, Miller; from Becton, Dickinson and Company) (Tube A). The broth was shaken in an incubator at 37 °C and 250 rpm overnight, whereby the cells were cultured to a midlog phase.

[00107] Subsequently, 250 was taken from Tube A and transferred into 25 mL of fresh LB broth (Tube B) and incubated for another 4 hours under the same condition. Subsequently, the bacterial suspension was centrifuged at 6000 rpm for 5 min, and washed with a mixed solution of HEPES buffer (10 mM, PAA the cell culture company) and glucose (1 wt%, alpha-D(+)-Glucose, 99+%, anhydrous, Acros) once. The absorbance of the final bacterial suspension in HEPES/glucose medium (20 mL) was measured at wavelength 600 nm and adjusted to an optical density of 0.6 with the same medium when necessary.

[00108] To obtain the concentration of colony forming units (CFU) at around 10 ⁶ CFU/mL, 20 mL of bacterial suspension was further added into 2 L of HEPES/glucose medium. The resulting mixture was used as the feed water for the subsequent permeation test.

[00109] Example 6.2: Permeation test

[001 10] The permeation test for the Ag/MWNTs/PAN composite membrane and the pristine PAN membrane was performed at room temperature using the bench-scale module shown in Figure 18. The flow rate of bacteria-containing feedwater was fixed at 24 L/h/m ² . The flux was measured at a time interval of 5 h for a total period of around 25 h. The flux was obtained for both membranes for comparison. The permeation of the pristine PAN membrane was also investigated using the HEPES/glucose medium as the feed to obtain the effect of the medium on the flux.

[001 1 1] Example 7: Measurement of viable bacterium concentration

[001 12] The concentration of living bacterium cells inside the filtration module (i.e. in the reject water) and in the filtrate was determined by the colony-counting method.

[00113] A serial dilution method was used before counting the viable bacterial colonies. Briefly, 100 of the reject or filtrate water sample was transferred into 900 of HEPES/glucose medium, mixed well, and denoted as Sample 10 ^"1 as it was diluted 10 times. Subsequently, 100 L was withdrawn from Sample 10 ^" ' tube and transferred into another 900 μ . medium to attain 100 times of dilution (Sample 10 ^"2 ), and so on. Three drops of 5 μΐ, of solutions from Sample 10 ^"3 or 10 ^"4 tube were dropped onto LB agar plate and incubated at 30 °C overnight. During the incubation period, living bacteria grew and formed colonies that can be counted on the following day.

[00114] The concentration of the bacteria in the water sample was calculated using Equation 2.

[001 15] C (CFU/mL) = [N x (1000 μί/5 μί)] x £>/0.1 mL Equation (2)

[00116] where N is number of colonies and D is the dilution factor (e.g. 10 ³ ).

[00117] Example 8: Measurement of leached Ag ⁺ ion concentration

[00118] The concentrations of Ag ⁺ ions in the filtrate were measured during the filtration process. Each time 10 mL of the water sample was withdrawn and added with 0.3 mL of 69 % HN0 ₃ to make a solution of around 2 % HN0 ₃ for ICP-AES analysis.

[00119] Example 9: SEM observation of the membranes after filtration

[00120] Both the pristine and nAg/MWNTs/PAN composite membranes were examined by FESEM (JEOL JSM 6700F field emission) and SEM (JEOL JSM-6390) after the filtration test. Prior to the analysis, all membrane samples were subjected to freeze drying under vacuum at -50 °C for 12 hours.

[00121] Example 10: Characteristics of Ag MWNTs

[00122] Multi walled carbon nano tubes (MWNTs) have been reported as suitable materials for immobilization of nanoparticles for various advanced applications such as sensing, catalysis, and electrocatalysis.

[00123] Silver nanoparticles immobilized on MWNTs (Ag/MWNTs) have been demonstrated herein with good antimicrobial properties against bacteria. In the experiments carried out, MWNTs were first grafted with polyethylene glycol (PEG), which played dual roles of hydrophilicity enhancement of MWNTs and reducing sites for formation of well- dispersed silver nanoparticles. The preparation of Ag/MWNTs was according to the steps shown in Figure 4. After carboxylation of MWNTs, 1,6-hexamethylene diisocyanate was used as a linker between the carboxylate group and PEG.

[00124] The pristine MWNTs, carboxylated MWNTs (MWNTs-COOH), and PEG-grafted MWNTs (MWNTs-PEG) were characterized with Fourier transform infrared (FTIR) as shown in Figure 11. Figure 11 is a graph showing Fourier transform infrared spectroscopy (FTIR) spectra of (A) MWNTs; (B) multi-walled carbon nanotubes modified with carboxyl groups (MWNTs-COOH); and (C) multi-walled carbon nanotubes modified with PEG groups (MWNTs-PEG). [00125] The pristine MWNTs exhibit bands around 1640 cm ^"1 and 3400 cm ^_1 ,which can be assigned to the bending mode and the -OH stretching vibration of adsorbed water molecules, respectively.

[00126] After oxidation with the concentrated acids, there appears a shoulder at 1720 cm ^"1 , which is attributed to the carboxylic acid group in MWNTs-COOH. The peaks at 2909 cm ^"1 and 2962 cm ^"1 in the FTIR spectrum of MWNTs-COOH can be assigned to the stretching vibration of the C-H group. Similar absorption peaks for acid-oxidized carbon nanotubes and carbon nanofibers have also been reported by other groups. The presence of CH ₂ /CH ₃ groups in nanotubes and carbon nanofibers was attributed to defects generated in the graphitic structure of carbon during their production. It is also likely that acid treatment creates such defects.

[00127] Upon grafting with PEG, the stretching mode of C-C-O is evident by the appearance of the band at 1090 cm ^"1 . Without wishing to be bound by any theory, it is believed that the grafted PEG on MWNTs in the current system plays a similar role in reduction of silver ions to silver nanoparticles of controlled sizes and at the same time binding them on the surface of MWNTs.

[00128] The X-ray diffraction (XRD) pattern of the resulting sample is shown in Figure 9. The broad peak at about 26 ° is assigned to the (002) plane of MWNTs, while the remaining peaks can be indexed to the cubic phased metallic silver (PDF no. 001-1167), as indicated by the line pattern. The strongest peak at 38.1 ° correlates to the (111) diffraction plane. As shown in the transmission electron microscopy (TEM) images (Figure 10A to C), an excellent dispersion of silver nanoparticles sized around 2 nm to 5 nm on MWNTs can be observed.

[00129] Although the carboxylic acid groups of acid treated MWNTs can also function as the reduction sites, their control over the particle size and dispersion is limited on the basis of the literature results and also our control experiment. The function of PEG can be clearly distinguished on the basis of our studies.

[00130] A high-resolution image (HRTEM) of an individual nanoparticle (Figure 10D) shows the lattice distance of 0.230 nm, which is in good agreement with that of the (11 1) plane distance of 0.224 nm.

[00131] The weight percentage of silver in the composite was determined to be about 2.45 % using inductively coupled plasma (ICP) analysis. The thermogravimetric analysis (TGA) results of (A) pristine MWNTs, (B) MWNTs-COOH, arid (C) MWNTs-PEG are shown in Figure 12. On the basis of the weight loss data at 600 °C, it can be estimated that roughly 16.9 % of carbon of MWNTs is oxidized to -COOH and 18.7 % of -COOH is further grafted with PEG. The FTIR spectrum of MWNTs-PEG (Figure 11, Curve C) shows consistent evidence that the carboxylic acid group is still present after PEG grafting, as indicated by the absorption at 1720 cm ^"1 . Hence, the remaining free -COOH on the surface of MWNTs-PEG is proposed to be responsible for the attachment of Ag/MWNTs to EDA modified PAN, as proposed in Figure 3.

[00132] Example 11: Properties of PAN membrane and PAN-EDA membrane

[00133] Figures 5A to D are SEM images of pristine PAN hollow fiber membrane. The pristine PAN hollow fiber membrane has a typical asymmetric pore structure with long finger-like pores (Figure 5A and 5B) and a dense and smooth outer layer (Figure 5B and 5C). Figures 6A to 6D are scanning electron microscopy (SEM) images of PAN hollow fiber membranes modified with EDA (PAN-EDA) by immersion in 100 mL of 20 vol % aqueous EDA solution at room temperature for 24 hours. After treatment by immersing in a 20 vol % aqueous solution of EDA, the outer surface of EDA-treated PAN (PAN-EDA) is no longer smooth and patches of surface defects can be observed (Figure 6C), although the large finger-like pore structure is unaffected (Figure 6D).

[00134] The chemical modification of PAN by EDA was evidenced from ATR-FTIR spectra shown in Figure 7. Figure 7 is a graph showing the attenuated total reflectance (ATR) spectra of (A) pristine PAN; and (B) PAN-EDA. In Figure 7A, the characteristic vibration of the nitrile group (C J) appears at 2244 cm ^"1 . The peaks at 2904 cm ^"1 and 2948 cm ^"1 are assigned to the stretching vibration of the CH ₂ group of PAN, while the peak at 1453 cm ^"1 is due to the bending vibration of the same group. The additional peaks at 1735 cm ^"1 in the spectrum of PAN (compared to the FTIR spectrum of the fresh PAN powder in Figure 20) can be attributed to the vibration of C=0 of the carboxylic group, indicating that PAN has been partially hydrolyzed after being soaked in water before modification with EDA. Similar results have been reported in literature.

[00135] In the spectrum of PAN-EDA (Figure 7B), the above peaks remain at similar positions. Additional vibrations can be found at 1650 cm ^"1 , 1569 cm ^"1 , and 1321 cm ^"1 . The first two can be assigned to amide I (C=0 stretching) and amide II (NH deformation), respectively. The peak at 1321 cm ^"1 is due to the bending vibration of the CH ₂ group in EDA. On the basis of such results, the proposed reaction pathway of PAN modification by EDA is via an aminolysis reaction of partially hydrolyzed PAN, as shown in Figure 3.

[00136] To evaluate the nanoscaled pore structure at the outer surface of the membrane, physisorption by liquid nitrogen was performed to measure the surface area and pore size of both pristine PAN and PAN-EDA. The pore size distribution results are shown in Figure 8.

2 2

The surface area of PAN was increased from 39.6 m /g to 46.4 m /g after EDA treatment. The pristine PAN membrane (Figure 8A) has a few distinct pore sizes centered at 2.7 nm, 3.4 nm, and 4.2 nm, while PAN-EDA exhibits a broad pore size distribution in the range 2 nm to 5 nm (Figure 8B). The results indicate that the pore structure at the outer surface layer was slightly modified after the chemical treatment by 20 vol % EDA.

[00137] Consequently, the initial flux measured using deionized water for PAN-EDA was 220 L/m ² /li/bar, which was about 25 % lower than that of the pristine PAN. Other chemical treatments using aqueous sodium hydroxide (NaOH) solution (1.0 M), hydrochloric acid (HC1) solution (1.0 M), and more concentrated EDA solutions (50 vol % and 99 vol %) were also used to modify PAN. However, all these treatments resulted in more than 90 % decrease in the initial flux, which may be attributed to a more severe influence on the pore structure.

[00138] Example 12: Properties of Ag/MWNTs/PAN Composite Membrane

[00139] Attachment of Ag MWNTs on PAN-EDA was carried out with the assistance of 1- ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) as an amide-forming agent (Figure 4). The addition of EDC facilitates the amidation reaction between the carboxylic acid on Ag/MWNTs and the amine group on PAN-EDA. Consequently, Ag/MWNTs can be stably grafted on the surface of PAN by covalent linkage.

[00140] Two control samples were prepared to investigate the effects of EDA and EDC. The first sample was obtained by reacting pristine PAN with Ag/MWNTs in the presence of EDC, and the second sample was prepared by reacting PAN-EDA with Ag/MWNTs in the absence of EDC.

[00141] The SEM images of Ag/MWNTs/PAN are shown in Figures 13A and 13B. It can be clearly observed that a significant amount of Ag/MWNTs was attached to the surface of PAN. On the other hand, there is no evidence of attachment of Ag/MWNTs in both control samples (Figures 13C to 13F). Such results support the proposed reaction schemes in which the modification of PAN by EDA and the presence of EDC as a linking agent are both important towards the formation of Ag/MWNTs/PAN composite membranes, with the external surface of the PAN membrane covalently grafted with nAg/MWNTs through the amidation reaction.

[00142] The average pore size of the dense layer of PAN-ED A was measured to be about 3 nm by the N ₂ adsorption method. Since the average diameter of MWNTs used is around 11 nm, the coating will be largely on the external surface of the membrane in a mesh-like form, as shown in Figures 13A and 13B.

[00143] Although the surface of the pristine MWNTs consisting of a graphitic carbon layer is hydrophobic, functionalization of MWNTs with carboxylic groups and PEG greatly increases the hydrophilicity of the resulting Ag/MWNTs. In fact, the Ag/MWNTs/PAN composite membrane has a similar hydrophilicity to that of the pristine PAN, with their contact angles measured to be 76.5 ⁰ and 79.9 ° respectively. The SEM images in Figures 13A and 13B indicate that nAg/MWNTs are coated in a mesh-like form on the membrane surface since MWNTs have a one-dimensional structure. The thickness of the coating should be less than about 20 nm which is limited by the diameter of MWNTs. As such, the coating does not add significant hydraulic resistance to the membrane. The initial flux of the Ag/MWNTs/PAN composite membrane drops only about 7 % to 205 L/m ² /h/bar compared with that of PAN-EDA.

[00144] Example 13: Antimicrobial properties of Ag MWNTs/PAN composite membrane using Escherichia coli

[00145] Escherichia coli (E. coli) was used as a model bacterium to investigate the antimicrobial and antifouling properties of the composite membrane due to a growing concern over the infections worldwide caused by this bacterium. A solution of HEPES (1 mM)/glucose (1 wt%) with a constant pH of 7.0 was used as the medium for the bacterium- containing feedwater during filtration. This medium can provide a nutritious environment (by glucose) to minimize bacterium death, since a high concentration of viable cells in the feedwater is necessary during filtration to access the antimicrobial and antifouling properties of the composite membrane. To obtain the effect of the medium on the flux, a control permeation test was conducted over the pristine PAN membrane with the medium itself as the feed.

[00146] Figure 21 shows that the flux drop due to the medium is not significant, with about 8 % during the first 5 hours and another 4 % during the remaining 15 hours. The initial drop could be due to the adsorption of glucose and/or the ionic species in the medium onto the pores of PAN, which slightly affects the hydrodynamics of the membrane. The presence of E. coli in the feedwater affected the flux of the pristine PAN membrane significantly. As shown in Figure 21 A, the flux dropped 55 % from 260 L/h m ² /bar to 117 L/h/m ² /bar in 20 hours, indicating that the pristine PAN membrane can be easily fouled under the present conditions. In contrast, the Ag/MWNTs/PAN composite membrane shows a much improved fouling resistance.

[00147] The flux drop was only 6 % from 205 L/h/m ² /bar to 193 L/h/ Vbar, which could be mainly due to the effect of the medium. It is notable that no bacterium cells were found in the permeate water samples for both pristine and composite membranes, as the external surface of the PAN membrane with a pore size of a few nanometers is effective in filtering micrometer-sized bacterium cells. Hence, all cells were trapped inside the filtration module, and they could further grow or be killed depending on the properties of the membrane. The population of the living cells in the reject water was measured during filtration.

[00148] The population of the viable cells in the reject water was measured during filtration. As there were always some variations in the initial concentration of bacteria in the prepared feed water, the relative CFU (CFU at time t/initial CFU) was plotted. As shown in Figure 21B, the concentration of living E. coli cells increased to about 80-fold (starting concentration at about 2 x 10 ⁶ cfu) in the reject water at 21 hours of filtration when the pristine PAN membrane was used. In contrast, the composite membrane with the Ag/MWNTs disinfection layer can effectively inhibit the bacterial growth within the test duration of 22 h. No significant increase in living E. coli cell concentration was found, indicating that majority of the cells were killed during the filtration process.

[00149] The above results are well correlated to the biofilm inhibition properties of the membranes. As shown in Figure 17A, dense and complex biofilm was formed on the pristine PAN membrane after the filtration test in about one day. The presence of biofilm on the membrane severely affected the water flux, as shown in Figure 21A. In contrast, only scattered cells were observed on the Ag/MWNTs/PAN composite membrane (Figure 17B). Furthermore, the majority of E. coli cells found on the composite membrane were shown with damaged cell walls (as indicated by the arrows for some representative cells and the magnified image in Figure 17B inset), indicating that the cell membranes were disrupted, which is probably due to the direct contact between the cells and silver nanoparticles. Taurozzi et al. showed that under the flow test the silver-embedded polysulfone membrane was not effective in reducing biofouling by E. coli. Our previous study has shown that MW Ts-COOH exhibited a much lower efficacy in killing bacteria (50% kill for E. coli) than Ag/MWNTs (> 99%) under the same conditions. Our other work consistently showed that both MWNTs and MWNTs-PEG have low antimicrobial activities and biofilm inhibition ability. The distinctively high efficacy of our composite membrane on biofouling control in this work lies in the proper dispersion of silver nanoparticles on the surface of the membrane for direct contact with bacterial cells. After 600 L/m ² of filtration, the silver loss in the composite membrane was determined to be about 22 %. The concentration of leached Ag ⁺ ions in the permeate water was measured to be always less than 10 ppb, which is below the allowable maximum silver concentration in drinking water (around 50 ppb). Silver loss from the surface was commonly reported by others, due to their easy accessibility to the water. To test the stability of our membrane, the Ag/MWNTs/PAN composite membrane was soaked in deionized water for two weeks followed by filtration under the same test conditions.

[00150] Although about 60% of silver was lost from the membrane, their antimicrobial properties did not deteriorate much. As shown in Figure 21 A, the flux drop was increased from 6 % to 13 %, which was still much better than that of the pristine membrane. There was also no significant increase in the concentration of the living cells in the reject water, as compared to that about 80 times increase in the concentration of the living cells when the pristine PAN membrane was used (Figure 21B). More research work is underway to achieve a better durability of the system as well as effective silver regeneration.

[00151] Example 14: Antimicrobial properties of Ag/MWNTs/PAN composite membrane using P. aeruginosa and S. aureus

[00152] Two other bacteria were used to investigate the antimicrobial and antifouling properties of the composite membrane, P. aeruginosa (gram-negative) and S. aureus (gram- positive). P. aeruginosa and S. aureus, because of their virulence and antibiotic resistance, are found as the major causes of nosocomial infections. In particular, P. aeruginosa is responsible for chronic respiratory infections causing cystic fibrosis and cancer. S. aureus leads to a wide range of diseases, such as skin infections, endocarditis and toxic shock syndrome, etc

[00153] As in the case for E. coli, a solution of HEPES (1 mM)/glucose (1 wt%) with a constant pH of 7.0 was used as the medium in the bacterium containing feed water during filtration, and a similar permeation test performed. [00154] The presence of bacteria in the feed water affected the flux of the pristine PAN membrane tremendously. For P. aeruginos (Figure 15B) and S. aureus (Figure 15C), the flux dropped 39 % and 31 % at 25 hours respectively. Such results indicate that the pristine PAN membrane can be easily fouled by the bacteria. In contrast, the nAg/MWNTs/PAN composite membrane shows an improved biofouling resistance. Under the same test conditions, the flux drop was 13 %, and 18 % for P. aeruginos, and S. aureus respectively as shown in Figure 15.

[00155] Similar observations were found with P. aeruginos as shown in Figure 16B. The concentration of the living cells was increased to about 35 times and only less than 3 times in the reject water when the pristine PAN and composite PAN membrane were used respectively.

[00156] In the case of S. aureus, it was found that the cells were not stable in the current medium used. As a result, the concentration of living cells in the reject water only increased to about 9 times when the pristine PAN was used. When the composite membrane was used, the concentration of living cells was increased to 6 times. A more suitable medium will be used for this bacterium in our future work. In addition, for S. aureus, no continuous biofilm was found on the composite membrane after filtration. However, a substantial amount of scattered cells were observed (Figure 17F). These observations together with the flux data suggest that the nAg/MWNTs disinfection layer is more effective towards the gram-negative bacteria than towards the gram-positive bacteria probably due to the different membrane structures of the bacteria. The latter has a much thicker peptidoglycan layer which may impose a higher resistance to the disinfectants. Such findings are consistent to those reported in the literature about the antimicrobial activities of both supported and non-supported silver nanoparticles against different types of bacteria.

[00157] A nanocomposite coating layer based on multiwalled carbon nanotubes (MWNTs) and silver nanoparticles (Ag/MWNTs) on the surface of a commercial hollow fiber polyacrylonitrile (PAN) membrane has been fabricated as an embodiment of the invention. Besides functioning as the support for silver nanoparticles, the nanoscale diameter and one dimensional morphology of MWNTs allow an open network structure on the membrane surface to minimize the impact on water flux and also facilitate the contact of silver nanoparticles by pathogens. Both the deposition of silver nanoparticles on MWNTs and the coating of Ag/MWNTs on PAN were achieved by covalent grafting to attain strong binding among various components as depicted in Figure 4.

[00158] In various embodiments, silver nanoparticles of controlled sizes (2 nm to 5 nm) have been successfully coated on multi-walled carbon nanotubes (MWNTs) that were first modified with a linker, PEG. PEG not only acted as a reducing agent for conversion of silver ions to metallic silver nanoparticles, leading to their immobilization on MWNTs, but also increased the hydrophilicity of MWNTs for less adverse effect on the flux of the resulting composite membrane. Ag/MWNTs were covalently coated on the external surface (i.e. the dense layer) of a PAN hollow fiber membrane.

[00159] It has been shown that the Ag/MWNTs disinfection layer coated on the external surface is effective in killing bacteria and controlling biofilm growth under the filtration mode. The Ag/MWNT coating significantly enhanced the antimicrobial activities and antifouling properties of the membrane. A much lower degree of fouling was observed on the Ag/MWNTs/PAN membrane. As a result, the relative flux drop over Ag/MWNTs/PAN was significantly smaller than that over the pristine PAN. It is believed that Ag ⁺ ions released from the Ag/MWNTs layer as well as the direct contact between the silver nanoparticles and the cells caused effective killing of the bacteria and inhibition of biofouling.

[00160] In this regard, a water disinfection system comprising a nanocomposite, which includes silver nanoparticles and carbon nanotubes, coated on a porous substrate, has been developed. The unique system developed in this work could be potentially used as a disinfection system for antimicrobial water treatment. The methodology may also be extended to other disinfectant/membrane systems for wide applications.