nano electrospinning
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Porous structures with their high surface areas have found
applications in many different areas. Nanofibers, with their large
surface-to-volume ratio, have the potential for use in various
applications where high porosity is desirable. A porous structure
made out of nanofibers is a dynamic system where the pore size
and shape can change, unlike conventional rigid porous structures.
Nanofibers can also be linked to form a rigid structure if required.
Perhaps the most versatile process for producing nanofibers with
relatively high productivity is electrospinning. Porous, nanofiber
meshes made by electrospinning have been identified for use in
numerous applications (Fig. 1).
Electrospinning nanofibersThere are several methods of producing nanofibers, from high-volume
production methods such as melt fibrillation1, island-in-sea2, and gas
jet3 techniques, to highly precise methods like nanolithography4,5 and
self-assembly6-9. However, their usefulness is limited by combinations
of restricted material ranges, possible fiber assembly, cost, and
production rate. Here, electrospinning has an advantage with its
comparative low cost and relatively high production rate. Micron size
Nanofibers are able to form a highly porous mesh and their large
surface-to-volume ratio improves performance for many applications.
Electrospinning has the unique ability to produce nanofibers of differentmaterials in various fibrous assemblies. The relatively high production
rate and simplicity of the setup makes electrospinning highly attractive
to both academia and industry. A variety of nanofibers can be made for
applications in energy storage, healthcare, biotechnology, environmental
engineering, and defense and security.
Seeram Ramakrishna1,2,3,*, Kazutoshi Fujihara3, Wee-Eong Teo1, Thomas Yong3, Zuwei Ma1, and Ramakrishna Ramaseshan1
1Nanoscience and Nanotechnology Initiative, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
2Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore3Division of Bioengineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
*E-mail:[email protected]
ISSN:1369 7021 Elsevier Ltd 2006M ARCH 20 06 | VOLUME 9 | NUM BER 30
Electrospun nanofibers:solving global issues
Defense & security
Environmentalengineering &biotechnology
Membranes & filters
Chemical & biological protectionSensors
Energy
Solar cells & fuel cells
APPLICATIONS
Healthcare
Tissue engineering &tissue repairDrug delivery
Fig. 1 Potential applications of electrospun fibers.
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The ability to form porous fibers through electrospinning means that
the surface area of the fiber mesh can be increased tremendously. Phase
separation is proposed as the main mechanism behind the formation of
porous fibers. When more volatile solvents are used, solvent-rich regions
begin to form during electrospinning that transform into pores14.
Another method of producing porous nanofibers is the spinning of ablend of two different polymers. One of the polymers is removed after
fiber formation by dissolution in a solvent in which the other polymer is
insoluble15.
Since stretching of the solution arises from repulsive charges, the
electrospinning jet path is very chaotic and only nonwoven meshes are
produced using a typical setup. Nevertheless, more ordered assemblies
that allow the porosity of the mesh to be controlled have been produced
through clever manipulation of the setup and solution composition.
Several methods have been developed that yield aligned fibers with
various degrees of order16-19 and fiber directions20,21 for two- and three-
dimensional assemblies22-26 (Fig. 6). Such assemblies are usually
achieved through control of the electric field between the tip of the
spinneret and the collector, use of a dynamic collector such as a rotating
mandrel, or a combination of both. Li et al.20 used a pair of parallel
conducting electrodes to create an electric field such that the
electrospun fibers are preferentially aligned across the gap in between
the electrodes. Boland et al.16 used a rotating drum at a speed of
1000 rpm to collect aligned fibers. To fabricate a tubular scaffold,
electrospun fibers can be deposited on a rotating tube and the deposited
fiber layer subsequently extracted from the tube. Fiber alignment can be
controlled using auxiliary electrodes to create an electric field profile that
influences the flight of the electrospinning jet (Fig. 7).
With such versatility, electrospun fibers are being explored for use in
many different applications. Currently, most tests use nonwoven fiber
meshes made out of smooth fibers. Ceramic nanofibers derived from
nonwoven electrospun fiber meshes have opened up new areas of
opportunities. Besides nonwoven meshes, testing of other fibrous
assemblies for potential applications has been limited. Nevertheless, theversatility of electrospun fibers can be seen in the established results
and on-going research in major areas like healthcare, biotechnology and
environmental engineering, defense and security, and energy storage
and generation.
Healthcare applicationsCurrent medical practice is based almost entirely on treatment regimes.
However, it is envisaged that medicine in the future will be based
heavily on early detection and prevention before disease manifestation.
Together with nanotechnology, new treatment modalities will emerge
that will significantly reduce medical costs.
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REVIEW FEATURE Electrospun nanofibers: solving global issues
Published application
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Fig. 5 Number of filed patents and patent applications in the US.
Others 12
Japan 8
Korea 23Europe 21
USA 63China 16
Fig. 4 Distribution of universities working on electrospinning around the world.
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With recent developments in electrospinning, both synthetic and
natural polymers can be produced as nanofibers with diameters ranging
from tens to hundreds of nanometers with controlled morphology and
function. The potential of these electrospun nanofibers in human
healthcare applications is promising, for example in tissue/organ repair
and regeneration, as vectors to deliver drugs and therapeutics, as
biocompatible and biodegradable medical implant devices, in medical
diagnostics and instrumentation, as protective fabrics against
environmental and infectious agents in hospitals and general
surroundings, and in cosmetic and dental applications.
Tissue/organ repair and regeneration are new avenues for potential
treatment, circumventing the need for donor tissues and organs in
transplantation and reconstructive surgery. In this approach, a scaffold is
usually required that can be fabricated from either natural or synthetic
polymers by many processing techniques including electrospinning and
phase separation.
The biocompatibility of the scaffold is usually tested ex vivobyculturing organ-specific cells on the scaffold and monitoring cell growth
and proliferation. An animal model is used to study the biocompatibility
of the scaffold in a biological system before the scaffold is introduced
into patients for tissue-regeneration applications.
Nanofiber scaffolds are well suited to tissue engineering as the
scaffold can be fabricated and shaped to fill anatomical defects; its
architecture can be designed to provide the mechanical properties
necessary to support cell growth, proliferation, differentiation, and
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Electrospun nanofibers: solving global issues REVIEW FEATURE
Fig. 7 Controlling fiber alignment on a tubular scaffold through mechanical rotation and modification of the electric field.
Fig. 6 Two- and three-dimensional structures made of electrospun fibers.
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motility; and it can be engineered to provide growth factors, drugs,
therapeutics, and genes to stimulate tissue regeneration. An inherent
property of nanofibers is that they mimic the extracellular matrices
(ECM) of tissues and organs. The ECM is a complex composite of fibrous
proteins such as collagen and fibronectin, glycoproteins, proteoglycans,
soluble proteins such as growth factors, and other bioactive molecules
that support cell adhesion and growth. Studies of cell-nanofiberinteractions have shown that cells adhere and proliferate well when
cultured on polymer nanofibers27-29.
One of our aims is to fabricate electrospun polymer nanofiber
scaffolds for engineering blood vessels, nerves, skin, and bone. We have
demonstrated that human coronary artery smooth muscle cells cultured
on synthetic nanofibrous scaffolds of the copolymer poly(L-lactic
acid)/poly(-caprolactone), or PLLA/PCL, show normal morphology andgood proliferation. The cells organize along the aligned nanofibers in a
directional manner typified by the orientation of the cytoskeletalprotein -actin (Fig. 8), suggesting that nanofiber orientation can impart
a functional development on the cells30.
On collagen-modified nanofibers, human coronary artery endothelial
cells exhibit cobble-stone morphology (Fig. 9a), typical of endothelial
cells cultured on a polystyrene surface with comparable adhesion and
proliferation rates31. On aligned PLLA nanofibers, c17.2 neural cells
adhere, elongate along the fibers, and neurites extend along the
direction of the aligned fibers (Fig. 9b)32. Human dermal fibroblasts
have been demonstrated to grow better on collagen nanofibrous
scaffolds than polystyrene tissue culture surfaces (Fig. 9c)33.
A recent study carried out with human coronary endothelial cells
cultured on nanofibrous scaffolds34 indicates that nanofiber scaffolds
positively promote cell-matrix and cell-cell interactions, with the cells
having a normal phenotypic shape and gene expression. This can be
attributed to the ECM-like properties of the nanofiber scaffolds that
mimic the natural tissue environment.
Further research is required to elucidate the influence of nanofibers
on the biochemical pathways and cellular signaling mechanisms that
regulate cell morphology, growth, proliferation, differentiation,
motility, and genotype. Insight into how natural ECM components
secreted by cells replace the biodegradable polymeric scaffolds is also
needed. This complete understanding of cell-nanofiber scaffold
interactions will pave the way for successful engineering of various
tissues and organs, such as vascular grafts, nerve, skin and bone
regeneration, cornea transplants, skeletal and cardiac muscle
engineering, gastrointestinal and renal/urinary replacement therapy, andeven stem cell expansion and differentiation to specific cells types and
organ regeneration.
In the pharmaceutical and cosmetic industry, nanofibers are
promising tools for controlled delivery of drugs, therapeutics,
molecular medicines, and body-care supplements. For example, DNA
covalently attached to a patterned carbon nanofiber array and
inserted into cells by centrifuging the cells onto the array, does not
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Fig. 9 (a) Metabolic dye CMFDA staining of human coronary endothelial cells cultured on random, collagen-blended nanofibers. (b) Metabolic dye CMFDA stainingof c17.2 neural cells cultured on aligned nanofibers. (Reprinted with permission from32. 2005 Elsevier.) (c) Scanning electron micrograph of human fibroblastscultured on random, pure collagen nanofibers. Metabolic dyes are cell stains that only fluoresce or produce a color in live cells.
(a) (b) (c)
Fig. 8 Human coronary artery smooth muscle cells cultured on alignednanofibers that have been stained for-actin filaments. (Reprinted withpermission from17. 2004 Elsevier.)
(a) (b) (c)
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rejection) without a significant drop in flux performance42. No particles
were found trapped in the membrane, so the membrane could be
effectively recovered upon cleaning. This opens up new avenues of
application of electrospun membranes for the pretreatment of water
prior to reverse osmosis.In our laboratory, nanofiber membranes are also being tested as
affinity (or adsorptive) membranes. Affinity membranes are a broad
class of membranes that selectively capture specific target molecules (or
ligates) by immobilizing a specific capturing agent (or ligand) onto the
membrane surface. In biotechnology, affinity membranes have
applications in protein (such as IgG) purification and toxin (such as
endotoxin) removal from bioproducts. In the environmental industry,
affinity membranes have applications in organic waste removal and
heavy metal removal in water treatment.
To be used as affinity membranes, electrospun nanofibers must
be surface functionalized with ligands. In most cases, the ligand
molecules should be covalently attached on the membrane to preventleaching of the ligands. Cellulose nanofiber membranes have been
surface functionalized with cibacron blue for the purification of
albumin43. Cellulose nanofiber membranes functionalized with
protein A/G (a recombinant 50 449 Da protein from Pierce
Biotechnology that has an increased ability to bind IgG molecules)
shows a high ability to capture IgG molecules with a capacity of
~134 g/cm2, which is higher than that of the commercialized
membrane (~80 g/cm2).
Water pollution is now becoming a critical global issue. One
important class of inorganic pollutant of great physiological significance
is heavy metals, e.g. Hg, Pb, Cu, and Cd. The distribution of these metals
in the environment is mainly attributed to the release of metal-
containing wastewaters from industries. For example, copper smelters
may release high quantities of Cd, one of the most mobile and toxic
among the trace elements, into nearby waterways44. It is impossible to
eliminate some classes of environmental contaminants completely, such
as metals, by conventional water purification methods. Affinity
membranes will play a critical role in wastewater treatment to remove
(or recycle) heavy metals ions in the future. Polymer nanofibers
functionalized with a ceramic nanomaterial, such as hydrated
alumina/alumina hydroxide and iron oxides, could be suitable materials
for fabrication of affinity membranes for water industry applications.
The polymer nanofiber membrane acts as a carrier of the reactive
nanomaterial that can attract toxic heavy metal ions, such as As, Cr, and
Pb, by adsorption/chemisorption and electrostatic attractionmechanisms.
Compared with heavy metal pollutants, overall water quality is much
more sensitive to organic pollutants. Although such organics are usually
no more than 1% of the pollution in a river, they tend to use up its
dissolved oxygen, making the water unable to sustain life. While the
transformations and pathways of metals in the environment have been
studied to some extent, much less information is available on most
commercial organic products because of their complex structures. Again,
affinity membranes provide an alternative approach for removing
organic molecules from wastewater. For example, -cyclodextrin is a
cyclic oligosaccharide comprising of seven glucose units. It has a stereo-
specific toroidal structure with a hydrophobic interior and hydrophilicexterior that can capture hydrophobic organic molecules from water by
forming an inclusion complex. -cyclodextrin has been introduced
into a poly(methyl methacrylate) nanofiber membrane using a physical
mixing method to develop an affinity membrane for organic waste
removal45.
Electrospun nanofibers have also received great attention for sensor
applications because of their unique high surface area. This is one of the
most desirable properties for improving the sensitivity of
conductometric sensors because a larger surface area will absorb more
of a gas analyte and change the sensors conductivity more significantly.
Nanofibers functionalized with a semiconductor oxide such as MoO3,
SnO2, or TiO2 show an electrical resistance that is sensitive to harmful
chemical gases like ammonia and nitroxide46. Single polypyrrole
nanofibers containing avidin were studied as biosensors for detecting
biotin-labeled biomolecules such as DNA. Specific binding of the
biomolecules to the nanofibers changes the electrical resistance of a
single nanofiber47. A fluorescent polymer, poly(acrylic acid)-poly(pyrene
methanol), or PAA-PM, was used as a sensing material for the detection
of organic and inorganic waste. The fluorescence is quenched by
adsorbed metal ions Fe3+ or Hg2+ or 2,4-dinitrotoluene (DNT) on the
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Fig. 11 An electrospun polysulphone membrane: (a) surface; (b) cross-section; and (c) magnified cross-section images.
(a) (b) (c)
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nanofiber surfaces48. In our laboratory, nylon-6 nanofiber was
functionalized with biotinylated glucose oxidase to develop a novel
biosensor for testing glucose concentration49.
Defense and security applicationsMilitary, firefighter, law enforcement, and medical personnel requirehigh-level protection when dealing with chemical and biological threats
(which include chemicals like nerve agents, mustard gas, blood agents
such as cyanides, and biological toxins such as bacterial spores, viruses,
and rickettsiae) in many environments ranging from combat to urban,
agricultural, and industrial. Current protective clothing is based on full
barrier protection such as hazardous materials (HAZMAT) suits, or
permeable adsorptive protective overgarments such as those used by
the US military. The obvious limitations of these suits are weight and
moisture retention, which prevent the user from donning them for long
periods.
Nanostructures with their small size, large surface area50, and lightweight will improve, by orders of magnitude, our capability to:
Detect chemical and biological warfare agents with sensitivity and
selectivity;
Protect through filtration and destructive decomposition of harmful
toxins; and
Provide site-specific in vivoprophylaxis.
Polymer nanofibers are considered as excellent membrane materials
for this purpose owing to their light weight, high surface area, and
breathable (porous) nature51. The high sensitivity of nanofibers toward
warfare agents makes them excellent candidates as sensing interfaces
for chemical and biological toxins in concentration levels of parts per
billion52. Governments across the world are investing in strengthening
the protection levels offered to soldiers in the battlefield53. Various
methods of modifying nanofiber surfaces to enhance their capture and
decontamination capability of warfare agents are currently under
investigation. One protection method is through chemical surface
modification and attachment of reactive groups such as oximes,
cyclodextrins, and chloramines54,55 that bind and detoxify warfare
agents.
In association with the Defense Science and Technology Agency
(DSTA) in Singapore, our laboratory is working on functionalizing
nanofibers to be used in facemasks for chemical and biowarfare defense
(Fig. 12). The facemask consists of two main components: a high-
efficiency particulate air (HEPA) filtering layer and an activated charcoalbed that adsorbs harmful gases and contaminants.
Nanofiber membranes may be used to replace the activated charcoal
in adsorbing toxins from the atmosphere. Active reagents can be
embedded into the nanofiber membrane by chemical functionalization,
post-spinning modification, or through using nanoparticle polymer
composites (Fig. 13). Preliminary tests using chemical warfare simulators
such as paraoxon and dimethyl methyl phosphonate on the
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Fig. 12 Schematic showing the cross section of a facemask canister used for protection from chemical and biological warfare agents.
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functionalized fibers show evidence of decontamination. Metal
nanoparticles (Ag, MgO, Ni, Ti, etc.), which have proven abilities in
decomposing warfare agents, can also be embedded in the nanofibers.
There are many avenues for future research in nanofibers from the
defense perspective. As well as serving protection and decontamination
functions, nanofiber membranes will also have to provide the durability,
washability, resistance to intrusion of all liquids, and tear strength
required of battledress fabrics.
Energy generation applicationsNatural energy resources such as crude oil, coal, natural gas, and
uranium are a necessity for everyday life. Rapid economic growth inAsia and the subsequent increase in demand for energy mean that the
rate of oil production is no longer adequate. This is evident in the
soaring price of crude oil, which has reached over $60 per barrel 56.
Large volumes of carbon dioxide emitted by the burning of fossil fuels
is also the main culprit in climate change. Thus, there is an urgent need
to identify new sources of energy that are environmentally friendly and
able to replace current supplies. Polymer batteries, fuel cells,
photovoltaic cells, wind power generators, and geothermal power
generators are some possible alternatives.
Given their high porosity and inherent large total surface area,
electrospun nanofiber membranes are being considered for polymer
batteries57-59, photovoltaic cells60-63, and polymer electrolyte
membrane fuel cells (PEMFCs).Polymer batteries have been developed for PC notebooks and cell
phones to replace conventional, bulky lithium batteries. The
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REVIEW FEATURE Electrospun nanofibers: solving global issues
PVDF nanofibrous membranewhich absorbs lithium electrolyte
LiCoO2 cathode
MCMBanode
Fig. 14 Polymer battery assembled by sandwiching PVDF nanofiber membranes between a mesocarbon microbead (MCMB) anode and a LiCoO2 cathode58,59.
Fig. 13 Schematic of the incorporation of functional groups into a polymer nanofiber mesh.
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components of polymer batteries are a carbon anode, a lithium cobalt
oxide cathode, and a polymer gel electrolyte. When a battery is
subjected to charging, Li+ ions are confined in the carbon anode. Ondischarging, the Li+ ions move to cathode. Noteworthy properties of
polymer batteries are less electrolyte leakage, high dimension
flexibility, and high energy density per weight. However, there is still a
need to improve energy density per weight of polymer batteries to
increase their market share. Choi et al.57 and Kim et al.59 have
assembled a new type of polymer battery using poly(vinylidene
fluoride), or PVDF, nanofiber membranes (Fig. 14). The porous
structure of the PVDF nanofiber membrane favors high uptake
(350 wt.%) of lithium electrolyte so that electrolyte leakage is
reduced. These factors make it possible to hold a large quantity of
lithium electrolyte in thinner battery packs. The large surface area of
the nanofibrous network also enhances ion conductivity, thus polymerbatteries comprising nanofiber membranes may improve energy
density per weight as compared with conventional polymer batteries.
Most conventional photovoltaic cells use single-crystalline,
polycrystalline, or amorphous Si. It is well known that a single-crystal Si
cell can achieve an energy translation efficiency of ~20%, and this value
is higher than other types of solar cells. However, the biggest
shortcoming for single-crystal Si solar cells is their high manufacturing
cost. There is also a need for a large surface area to obtain sufficient
electrical output.As an alternative, Grtzel and colleagues64 have developed dye-
sensitized solar cells. The principle here is that sensitizing dye molecules
coated onto TiO2 nanoparticles absorb photons and transfer excited
electrons through the conduction band of TiO2 to the cathode.
A nanotopographic TiO2 layer works as the electrode and enhances the
total surface area to achieve a high electrical output. Dye-sensitized
solar cells are less costly to manufacture than Si-based solar cells, but
there are issues that need to be addressed, including reducing
electrolyte leakage and improving the energy conversion efficiency
(generally ~4-10%). With respect to electrolyte leakage, an alternative
solution is to use a viscous polymer gel electrolyte. However, it is
difficult to infuse a viscous gel into a conventional TiO2
nanotopographic layer. Song et al.61-63 have solved this problem by
using TiO2 nanofiber membranes fabricated by electrospinning in
combination with sol-gel processes (Fig. 15). The viscous polymer gel
electrolyte can easily penetrate into the porous nanofiber membrane.
Their assembled TiO2 nanofiber dye-sensitized solar cells are able to
achieve an energy conversion efficiency of 6.2%63.
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Electrospun nanofibers: solving global issues REVIEW FEATURE
e-
H2
O2
H2O
H2O
Polymerelectrolyte
membrane
Anode
2H+
H2 2H
+ + 2e- 1/2 O2 + 2H+ + 2e-
Cathode
Fig. 16 Principle of electricity generation in fuel cells.
Fig. 15 Dye-sensitized solar cells assembled using TiO2 nanofiber membranes61-63.
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Electricity generation in PEMFCs is through the chemical reaction of
hydrogen at the anode and oxygen at the cathode (Fig. 16). Protons are
transmitted through an electrolyte membrane that contains distilled
water, while electrons are transmitted from the anode to the cathode.
The key properties of electrolyte membranes are high proton
conductivity and shielding of electron transport. As the membrane needs
to hold distilled water for proton conductivity, water retention of the
membrane is also important. Nafion (DuPont), a perfluorosulfonic acid
polymer film, has been widely used so far. However, Nafion membranes
are expensive at up to $800/kg. For the same membrane area,
electrospun Nafion fiber membranes require less material than
conventional Nafion fuel cell membranes, thereby reducing cost. Porous
nanofiber membranes are also able to hold distilled water, thus
enhancing proton conductivity. Therefore, such nanofiber membranes
have the potential to be used in PEMFCs.
ConclusionGiven the versatility of electrospinning for generating highly porous
nanofiber meshes made out of different materials, it is no surprise that
it has found possible uses in different fields ranging from healthcare,
biotechnology, and environmental engineering to defense and security,
and energy generation. Electrospinning may be able to produce
microengineered scaffolds for tissue engineering. Improved wound
dressings could be made out of nanofiber meshes impregnated with
drugs. Membranes for water treatment or use in biotechnology could be
made of electrospun fibers. Nanofiber clothing and filters could deal
more effectively with chemical and biological threats. In the future, we
may no longer be dependent on crude oil thanks to more efficient
conversion of other energy sources to electricity. With the ability to
mass-produce nanofibers, electrospinning may well be one of the most
significant nanotechnologies of this century.
http://www.wtec.org/nanoreports/cbre/CBRE_Detection_11_1_02_hires.pdfhttp://www.wtec.org/nanoreports/cbre/CBRE_Detection_11_1_02_hires.pdfhttp://www.sc.doe.gov/bes/reports/files/NCT_rpt.pdfhttp://www.nymex.com/index.aspxhttp://www.nymex.com/index.aspxhttp://www.nymex.com/index.aspxhttp://www.sc.doe.gov/bes/reports/files/NCT_rpt.pdfhttp://www.wtec.org/nanoreports/cbre/CBRE_Detection_11_1_02_hires.pdf