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High Speed Water Sterilization Using One-Dimensional Nanostructures

April 26, 2011
Recently, Schoen et. al. investigated the incorporation of silver nanowires (AgNWs) and carbon nanotubes (CNTs) within a matrix of cotton, enabling a membrane with multiscale nanostructured materials to effectively inactivate  bacteria in water. This strategy takes advantage of the chemical and mechanical robustness of cotton as the support, along with the large pore structure between cotton fibers (tens to hundreds of microns), which, much greater than the length scale of bacteria, prevents physical clogging from occurring. In this paper, a novel approach to synthesize a textile based membrane architecture incorporating nanomaterials to enable specific functionality within the membrane has been reported. The fabrication approach is both simple and scalable, and provides effective inactivation of bacterial elements in a gravity fed filtration system.


Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network

  • Schoen DT, Schoen AP, Hu L, Kim HS, Heilshorn SC, Cui Y. 2011. High Speed Water Sterilization Using One-Dimensional Nanostructures. Nano Letters 10:3628-3632. doi:10.1021/nl101944e

Various nanostructured materials have proven effective for the filtration and sterilization of water resources. Examples include silver nanoparticles (AgNPs), which can provide bulk and surface antibacterial properties, as well as carbon nanotube (CNT) membranes which enable size exclusion of both inorganic and bacterial elements. In the case of AgNPs, adhesion stability of the nanoparticles within a porous or fibrous support structure is a challenge, and ineffective dispersion within the support structure can limit electrical conductivity, which is important for electric field and current driven inactivation of bacterial elements. Nanoporous membrane structures require a high pressure drop to function, which ultimately leads to clogging and biofouling problems. In order to effectively inactivate bacteria within the water while providing a high throughput, gravity fed, biofouling resistant device, a high surface area multiscale materials network is required. The high surface area multiscale materials network provides a durable support of antibacterial nanostructures with sufficient electrical conductivity for applications in which current and field bacterial inactivation is desired.

Image
Schematic, fabrication, and structure of cotton, AgNW/CNT device. (A) Schematic of active membrane device proposed. (B) Treatment of cotton with carbon nanotubes (CNTs). (C) Treatment of device with silver nanowires (AgNWs). (D) Integration of treated cotton into funnel. (E) SEM image showing large scale structure of cotton fibers. (F) SEM image showing AgNWs. (G) SEM image showing CNTs on cotton fibers.
Recently, Schoen et. al. investigated the incorporation of silver nanowires (AgNWs) and CNTs within a matrix of cotton, enabling a membrane with multiscale nanostructured materials to effectively inactivate  bacteria in water. This strategy takes advantage of the chemical and mechanical robustness of cotton as the support, along with the large pore structure between cotton fibers (tens to hundreds of microns), which,much greater than the length scale of bacteria, prevents physical clogging from occurring. AgNWs having diameters of 40-100 nm, with lengths up to 10 µm were then incorporated as a secondary mesh between the cotton fibers. Silver was selected for its well-known bactericidal properties. The nanowire structures enabled multiple binding sites with the cotton fibers for excellent physical robustness. Additionally, the AgNWs form continuous electrical networks facilitating effective electron conduction in contrast to electron hopping networks. This feature adds the possibility of enhancing the antibacterial properties of the AgNWs when placed under an electric field. The addition of CNTs to the matrix further ensures good electrical conductivity of the textile filter device structure over the entire active area.

In order to fabricate the cotton based filtration device, AgNWs with the previously described dimensions were first synthesized by reducing AgCl in a poly(vinylpyridine) and ethylene glycol solution, after which AgNO3 is dissolved to facilitate nanowire growth. The as-grown AgNWs were then transferred and dispersed into methanol. The CNT ink was prepared using a dispersion of 1.6 mg/ml CNTs in water combined with 10 mg/ml sodium dodecylbenzenesulfonate (SDBS) surfactant. Cotton textiles were then coated with CNTs by submergence in the CNT ink and rinsing in distilled water to remove excess surfactants. The AgNWs were applied by pipetting directly from the methanol suspension followed by drying at 95°C and subsequent rinsing to remove residual surfactant and solvents. The resulting filter device has a sheet resistance of ~1 Ω/sq, and is physically robust for easy manipulation into the filter configuration.

The performance of the textile based filtration device was characterized by forming the cotton support as a plug in a simple funnel having 4 mm diameter and 2.5 cm length. Using a 100 ml solution containing E. Coli with a nominal density of 107/ml, the efficacy of the filtration system was evaluated as a function of applied voltage. Using the conductive textile as one electrode and a copper mesh electrode held at ground potential positioned in the solution approximately 1 cm above the filter, results for textile filters with and without the AgNWs were compared. The AgNW/CNT containing cotton filters exhibited bacteria inactivation of 89% at -20 V, and 77% at +20 V. The CNT cotton filters exhibited significantly less bacterial inactivation. In comparison to nanofibrous size exclusion membranes operating at ~130 psi, the gravity fed textile filtration enabled scaled flow rates on the order of 80,000 L/(h-m2) versus 1 L/(h-m2). With a power dissipation of 60 mW (3 mA @ 20 V) for the textile based membrane in comparison to ~250 mW for a nominal ultrafiltration membrane operating at 1 L/h, thereby providing substantial system energy savings.

Thus a novel approach to synthesize a textile based membrane architecture incorporating nanomaterials to enable specific functionality within the membrane has been reported. The fabrication approach is both simple and scalable, and provides effective inactivation of bacterial elements in a gravity fed filtration system. Further studies must evaluate the use of such membranes in both passive and active system designs, as well as evaluate the relationship between antibacterial properties and electric field distribution within the membrane architecture.

 

Figure reprinted with permission from Schoe DT, Schoen AP, Hu L, Kim HS, Heilshorn SC, Cui Y. 2011. High Speed Water Sterilization Using One-Dimensional Nanostructures. Nano Letters 10:3628-3632. doi:10.1021/nl101944e. Copyright 2010 American Chemical Society.

Last updated: May 04, 2011
 

DOI: 10.4053/er516-110426

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Tags: Water Sterilization, silver nanowires, Nanowires, cotton, filtration, Carbon nanotube (CNT)

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