Kinkajou : So what Nano-materials are in existence today?
Erasmus : Nano -material structures can exist in the natural environment. Clay soils can contain complex mixes of nano crystals. Opals owe their colour to interference and diffraction of light between naturally occurring nanoscale silica spheres (150 - 300 nm in diameter).
Viruses, the spatially on the bottom of the feet geckos, wax crystals on some leave such as nasturtium leaves are all examples of natural organic nano materials
- Viral capsid
- "Lotus effect", hydrophobic effect with self-cleaning ability
- Close-up of the underside of a gecko's foot as it walks on a glass wall. (Spatula: 200 × 10-15 nm).
AFM Gecko Spatula Foot
Atomic Scale Lotus Effect
Erasmus : Other nano materials have been produced artificially. Fullerenes are composed of carbon sheets (graphene) rolled into tubes forming carbon nanotubes or rolled into spheres such as buckyballs.
Silica has also been crafted into nanotube structures. Fullerenes are of interest because they form structures over 200 times stronger than steel by weight, can conduct heat and electricity and can be made transparent.
Fullerenes have been produced artificially by sending an electric current between two closely position graphite electrodes in an inert gas atmosphere. The plasma arc between the electrodes vaporises the carbon to form a sooty residue from which fullerenes can be isolated.
Here research has shown that graphite vaporized by a laser coalesces naturally into stable clusters typically of around 60 covalently bonded carbon atoms. Called buckminsterfullerenes, buckyballs or C60 structures.
Graphene and its band structure and Dirac Cones.
Rotational view of C60, one kind of fullerene
Kinkajou : Tell us more about the uses of fullerenes and graphene carbon derivatives.
Erasmus : The fullerenes are essentially all three dimensionally folded sheets of graphene carbon. They include structures such as Buckminsterfullerene C60, also known as buckyballs.
Carbon nanotubes also sometimes called Bucky tubes. Other structures include carbon nano rods, nano wires and quantum dots. The structures have electronic properties which suggest the usefulness in nano-electronics.
Nanotubes allow faster electron conduction, faster frequency oscillation of electrons (measured via the dielectric constant) and greater material stability as well as small sizes (nano dimensions).
Although carbon-based polymers are normally regarded as insulating, this changes at Nano dimensions. Carbon-based structures such as molecular graphite sheets, fullerenes or graphenes function as excellent Nano conductors.
Here the challenge is to develop methods of fabricating small carbon graphite objects which could be considered to be single molecules.
Carbon nanotubes can be made to essentially function in a similar fashion to a cathode ray tube, allowing high efficiency output to a display (high-efficiency power throughput at nano sizes allowing for brighter displays at lower powers with greater contrast and smaller pixel sizes, all of these being important in display devices).
This view of a carbon nanotube shows its 3D structure.
Carbon nanotubes have been used to produce stain resistant textiles.
Kinkajou : Tell us about other nano materials.
Printing Molecules as a Fabrication
Erasmus : Nano- particles include materials such as quantum dots, nano wires and nano rods.
Silver Nano materials have been bonded to dressings due to their antimicrobial properties.
Other nano-metals and nano-metal oxides such as vanadium oxide, aluminum oxide, silicon dioxide, titanium dioxide, zinc oxide, cerium oxide, nano platinum, nano silver, and nano zero valent iron. DTSC is also planning to include quantum dots, ocean plastics, and nano-clay into the list of chemicals of interest.
Erasmus : A quantum dot is a semiconductor crystal whose nano size causes charge carriers within it to be quantum confined in three spatial dimensions. This ability to hold charge carriers can confer unusual optical and/or electrical properties on quantum dots.
Quantum dots are nanoscale objects, which can be used, among many other things, for the construction of lasers. The advantage of a quantum dot laser over the traditional semiconductor laser is that their emitted wavelength depends on the diameter of the dot. Quantum dot lasers are cheaper and offer a higher beam quality than conventional laser diodes.
Kinkajou : You mentioned Nanowires and nano rods
Erasmus : Nano wires are of interest because the conductivity is influenced by edge effects. Atoms at the surface of the nano wire are not three-dimensionally bonded to other atoms within the wire. Electricity can only be conducted in the core of the wire due to these effects.
Also the conductivity follows quantum rules in that the energy of electrons being transmitted through nano wire can only assume a number of discrete values. This property is of value in nano-electronics.
Nano rods are used in display technologies. By applying an electric field, the orientation of the rods can be changed causing alterations in the reflectivity of the rods.
Another property relates to their thermal sensitivity.
Nano rods absorb energy in the near infrared spectrum and generate heat when exposed to infrared light. If they are chemically bonded to cancer therapeutic agents which identify tumours, they can be heated by exposure to infrared leading to heat damage to cancerous tissues. Similar applications include drug delivery and as biosensors.
Other uses include Organic solar cells or organic LEDs.
Nano rods can also be used to create stable suspensions. Because nanoparticles have a very high surface area to volume ratio, suspensions tend not to precipitate, adding to longevity of consumer products such as liposomes.
Altered properties in Nano particles create problems in nano engineering. For example, ferrous magnetic storage materials smaller than 10 nm will switch their magnetic polarity simply with exposure to room temperature thermal energy, making them useless as a memory storage device.
Nano materials when mixed and sintered (compacted into a solid mass by exposure to heat and/or pressure without melting to liquefaction) have lower sintering temperatures than normal bulk materials.
Examples of sintering include ice cubes adhering to each other in a glass and powder metallurgy for materials such as tungsten and molybdenum which have a very high melting point.
This can be important in manufacturing by allowing the avoidance of high temperatures which can damage the materials forming the factory.
Atomic Sized Dendrimer
Kinkajou : So let’s put it all together. What can all these devices do in terms of replacing electronics?
Erasmus : Nano – electronics (molecular scale electronics) uses molecules to replace traditional electronic components such as diodes, transistors, rectifiers, insulators and conductors. In nano-systems, single electrons can alter system properties in molecular function.
As molecules form the smallest stable chemical structures is hoped that molecular scale electronics can ultimately deliver molecules as circuit components.
The recent innovations include the use of carbon nanotubes to construct “Field effect transistors”. In these structures electric field controls the shape of a conducting channel and hence controls the conductivity of this channel in a semiconductor material.
Nanoscale lithographic engineering of silicon chips means that many processes need to be built with a precision of only a few atoms in width. To have reliable performance and low failure rates, demands new methods of fabrication be used.
Of the biggest problems in the electronics has been the connection of molecular components with mechanical conductive elements on silicon. Solutions have involved engineering atoms with terminal sulphur moiety is which can be attached to gold electrodes.
However small changes in interatomic geometric relationships can substantially alter the conductivity and the reproducible performance of manufactured electronics components. Fullerenes have also been proposed due to their better linkage properties to silicon conductors.
Erasmus : Current proposed molecular nano electronic structures typically feature many double atomic bonds alternating with single atomic bonds (as typified by molecule structures such as C-C=C-C). This pattern delocalizes the position of the electron within the inter-atomic orbitals making it possible for electrons to move freely.
Molecular wires designed to electrically connect different parts of molecular electronic circuit typically have structures with no functional groups. An example includes carbon nanotubes. The size of the structures makes the electrical behavior more akin to that seen in silicon semiconductors than in molecular structures.
Molecular transistors control the probability that a single electron can attach to or leave a single molecule, thereby modifying the energy of the molecular orbitals. This means that molecular nano electronic transistors function more in a binary on/off format.
Silicon transistors are in effect a gate between a source and drain electrode which controls the charge density between the two gates. Transistors based on silicon tend to have quadratic responses to gate voltage, effectively substantially altering circuit design from molecular electronic transistors.
Molecule groups have been used as a semiconducting channel in molecular transistors includes. Oligopolyphenylenevinylenes (OPVs), fullerenes, and ring-shaped molecules with structures akin to benzene.
Small molecules accentuate quantum mechanical behavior in the atomic and subatomic components. This may mean that other properties such as electron “spin” may be able to be incorporated in molecular circuit transistor design.
Erasmus : Molecular rectifiers typically have an asymmetric construction allowing the molecule to accept electrons at one end but not at the other. Electrons can enter at the end of the molecule designated the electron acceptor (A) and can leave at the end of the molecule called the electron donor (D).
This way, the unstable state D+ – A− will be more readily made than D− – A+. The result is that an electric current can be drawn through the molecule, if electrons are added at the acceptor (A) end.
Molecular scale electronics may also be more amenable to usage in biological applications such as implants within the body with sensor roles, (e.g. glucose monitoring).
Quantum dots and Photonic crystals are important new nano electronic constructs due to the potential for greater bandwidth and capacity then existing electrical devices.
Photonic crystals are formed from materials which periodically vary the refractive index of the lattice at a spacing approximately half the wavelength of the incident light used.
This creates a gap for the propagation of specific light wavelengths, effectively creating a semiconductor for photons instead of electrons, allowing the creation of nano photo electronic elements.
Kinkajou: I think that Instability and quantum effects make the research of molecular electronics very problematic. It may be many years before we see substantial progress in this area, with a replacement of many current electronic integrated circuits.
Nano Assembler Factory Unit
Erasmus : I will just make a mention about electronic memory.
New memory structures have been proposed from nano technology. These include crossbar switch based electronics. A horizontal and vertical grid of overlapping wires allows addressing of every crossover node. This promises the development of the very high density memory, not based on molecular magnetic interactions.
Magnetic resistance has been used as a way of interacting with electron spins. When two ferromagnetic layers are separated by a non-magnetic layer up to several nanometres in thickness (e.g. Co-Cu-Co), nodes in the ferromagnetic layers display magnetic resistance to an external field due to the spin of their electrons (in Nano materials only). This effect is called GMR (giant magneto-resistance).
A similar effect is called TMR (tunnelling magneto-resistance). Due to nanoscale constructions, very high data densities can be achieved for non-volatile computer memory, one example being magnetic random access memory (MRAM).
Nanoscale Memristors have also been proposed as a future replacement for flash memory in computer systems.
Kinkajou : Can solar be used at nano dimensions?
Erasmus : Nano materials offer the potential for solar photovoltaic cells to operate at greater efficiency. Bio electricity production from nanoscale devices could be used to power devices such as pacemakers, nano robots, biological sensors, or transmission devices.
Carbon nanotubes have been incorporated in nano radio devices.
Kinkajou : Lithography is a well-established an important technique in electronics manufacturing. So where does nano lithography take us?
Erasmus : Nanolithography is a “top down” technology for the fabrication of nanoscale structures or structured patterns (1 nm to 100 nm). Currently, it aims to allow the direct construction of complex structures less than 20 atoms wide.
Firstly, a layer of material is made via standard bulk construction technology.
Opals show colour because of the diffraction from the silicon sub particle boundaries, not due to the nature of the chemicals contained therein.
Construction methods then include:
- Imprinting (patterns are created by mechanical deformation of a basic polymer which is cured through the use of heat or UV energy.)
- masked or maskless direct writes via
- Use of optical laser beams to cut the layer, (currently extreme ultraviolet lithography EUVL uses light of approximately 13 nm wavelength). Software technology enhancements allow more precise resolution with techniques such as phase shift masks or optical proximity correction.
Photo-masks may or may not be used. Two different photon energies may be combined to induce a change in the solubility of the resistant base layer.
- Use of electron beams to etch or deposit material (unfortunately, a high resolution and low throughput technique).
- use of shield layers to protect layers from acid etching
- multiple layering used to print or edge mark features
- Use of multiple laser or electron beams in parallel to increase throughput.
- Use of scanning probes to control pattern formation via addition of material (dip pen nanolithography), triggering chemical reactions (thermochemical nanolithography), and triggering oxidation reactions (local oxidation nanolithography).
Kinkajou : Lithography for integrated circuits is currently reaching the limits of atomic or molecular resolution. Many lithographic integrated circuit features are so small that engineering almost perfect structures to allow the design circuit to achieve planned performance is very difficult.
Kinkajou : So what about all the really cool stuff to heal people or fix people?
Erasmus : Nano-medicine includes systems such as developing:
- medical applications of nano materials
- nano biological devices
- nano electronic biosensors
- methods for detecting biomolecules
- biological machines
- drug delivery systems
- understanding the toxicity and environmental impact of nano materials
Nanoparticles (top), liposomes, and dendrimers
Micelle production and liposome production are an important direction for nanotechnology development in medicine.
Typically these can be produced from phospholipid bilayers which have an established and Recognised safety profile. However, micelles and liposomes can also be produced from polymers.
Biological bilayers usually contain membrane proteins which substantially alter the interactions of these containment structures, forming cells within the body. To date, there has been little use of trans-membrane or membrane receptors as either recognition markers or as functional elements such as cellular pumps.
Incorporating nano pump and nano receptor technology is in bilayers such as liposomes may well prove to be important in enhancing useless of the structures in human body.
Erasmus : Some interesting applications include using micelles or liposomes:
- To encapsulate materials
- To be used as vaccine adjuvants;
- To use as vehicles for drug encapsulation,
liposomes change the charge characteristics of pharmaceuticals by interposing the liposome surface between the pharmaceutical and carriers such as blood.
- As vehicles for storage and depot formation altering drug pharmacokinetic and elimination profiles;
Many drugs are bonded to oils and injected into the body creating a mechanism for slow-release into the body system.
Using liposomes creates a different method for release to that of chemical bonding drugs to oils)
Using a different vehicle to an oil may also allow targeting of drugs to different body regions.
Liposomes for example, are commonly used as beauty treatments, in allowing the delayed release of moisturizers to the skin.
- To allow a more stable suspension of materials in liquids, preventing layering or precipitation of heavier materials. The high surface area of nanoparticles and their atomic level interaction with molecules in solvents such as water allows particles to remain in suspension, that normally would precipitate out of a solution or suspension.
Erasmus : Some interesting applications for nanoparticles include:
- Iron nanoparticles or gold nano shells have been used to mark cancers and to carry drugs into cancer tissues.
- nano particles of drugs for example nano particle albumin bound paclitaxel, have been used to carry oncology drugs to cancers
- Nano particles such as fluorescent dye loaded silica nanoparticles used to mark tumour cells,
- micro particles developed from the cell walls of specially developed mutated bacteria used to carry anti-cancer agents to phagocytic cells
Kinkajou : That reminds me of the Austin Powers movies. I think Dr. Evil used mutated sea bass as his vessel of evil.
Erasmus: Excuse me. We are trying to do some good here.
- Polymeric nanoparticles form an alternative to phospholipid nano liposomes in drug delivery systems. Their advantages over liposomes are that they are chemically more stable and predictable in the chemical interactions.
- Targeting nanoparticles containing photosensitive agents via photodynamic therapy to abnormal tissues that bond to the nanoparticles. Heat and light generated via photo sensitizers can cause cell destruction and death, as in for example cancer tissues.
- Quantum dots containing fluorescent dyes can provide higher contrast than typical contrast media dies. Unfortunately quantum dots are often created from toxic metallic elements. By separating the action of the dye from the carrier, different cell and tissue types can be identified or studied e.g. cancer tissues may be able to be isolated throughout the body.
Other delivery systems can be based on nano materials such as Dendrimers. Dendrimers are repetitively branching molecules. They can generally be shaped.
Active materials such as dyes, reactive chemical ligands or radiopharmaceutical agents can be bonded to the surface, allowing the targeting of a generally active chemical agent to a specific tissue type able to bond to or internalize the dendrimer construct.
It has also been considered combining nano electromechanical systems with liposomes to allow for the controlled release of drugs to specific sites or the specific circumstances.
Kinkajou : Any thoughts on how you make power some of these devices?
Erasmus : The development of bio enzymatic fuel cells to power nano devices and nano sensors is critical to improve the longevity and sustainable operation of nano implants within the body.
Kinkajou : Sounds dangerous using nanoparticles to carry oncology drugs in cancer patients. Wouldn’t the nano particles add to toxicity issues?
Erasmus : Unfortunately for many applications, nano materials can have very variable pharmacokinetics between individuals. They can also have unrecognized toxicity and interaction profiles related as much to size as to chemistry. For example, the size of particles, may allow them to interact with DNA or enzyme systems, even though an understanding of their biological chemistry would not suggest such actions.
Kinkajou: Any comments on nano sensors in medicine.
Erasmus : Nano sensors are another important direction for medical biotech.
Current developments include:
- Using magnetic nanoparticles bound to antibodies to label specific molecules, structures and even microorganisms.
- Using nano pore sensors to identify nucleic acids within DNA sequences and convert this information directly to electronic format.
- Taking gold nanoparticles with short single-strand DNA segments to allow identification of like DNA segments within target samples.
- Proposed usage of nano sense test chips to detect proteins and other biomarkers arising from cancer cells in sub-regional blood circulation zones.
- Attempting to reduce the size of arthroscopes and other internal body visualization instrumentation.
- By creating nano sensors attached to magnetic nanoparticles, specific substances within blood (such as toxins or viral particles in dialysis blood), can be tagged and removed from the circulation via magnetic activated sorting mechanisms. Long-term this may allow for new processes for control of sepsis.
Dr Xxxxx : However, infections such as cellulitis are complex processes within the body and do involve blood vessel clotting and blood vessel flow restriction, both of which are likely to reduce access of bio agents to infection sites.
Kinkajou : Is there anyway of bypassing blood stream transport systems?
Erasmus : Medical nano robots may be able to hijack a ride from within the blood vessels into tissues by being carried by cells such as neutrophils leukocytes, lymphocytes, macrophages and mast cells. In such a way cells may cross the blood endothelium by transmigration within immune cell carriers.
Motion within the biological role requires a new understanding of locomotion at the nanoscale. It is not appropriate to simply build small motors. However: existing solutions are ready curve in the flagella and microtubules structures that bacteria used to propel themselves.
Kinkajou : Has there been any other work done with cellular intercellular reactions?
Erasmus : One mechanism demonstrated involved the fusion of two pieces of meat into a single piece using suspension of gold coated nano shells activated by an infrared from a laser to weld the tissues.
Cellular engineering has been proposed as a direction for medical nano research. By supporting the creation or repair of cellular scaffolds, it is thought that healing processes can be enhanced. Perhaps bone repair can be enhanced through the creation of vascular tissues and biodegradable support structures arising from graphenes or Carbon nanotubes.
Again this process is complex and requires appropriate targeting of damaged tissues, creation of scaffolding structures for the use of new biopolymers (to be developed) and to apply appropriate stimuli to damaged tissues to encourage integration of normal cells with artificially instituted repair structures .
Kinkajou : If we can get inside cells, what can we do in there?
Erasmus : More radical suggestions utilizing molecular assemblers to assemble specialized molecules within the body or nano robots to perform molecular level detection, repair or remediation activities. Nano bots are far beyond our current capabilities.
Ninja particles have been suggested to be a form of nano bot which is capable of identifying foreign bacteria and killing them.
The problem of course is that to successfully function they must perform a number of tasks: to travel to where foreign bacteria may be found, to identify the bacteria, to engage with the bacteria and to undertake a lethal cell destroying activity towards these foreign particles “bacteria”.
Other proposals for medical nanotechnology are much simpler. The usage of new types of contrast agent to be able to image abnormalities and to allow treatment to be directed towards them.