Literature Review--Is Nanotechnology the Next Big Thing? Yufei Chang
Abstract The term ―Nanotechnology‖ was first introduced in 1974 by a Japanese engineer, Norio Taniguchi. [1] The term originally described a technology that went far more beyond conventional materials and engineering on the micrometer scale, which had dominated the twentieth century. [2] Today, nanotechnology influences almost any field and is going to change every aspect of our live. Researches in this field are being carried out all over the world, leading to a growing industry and entrepreneurial activity. It is described as an enabling technology and will drive every stream of technology in the future. This review supports the argument from current development and its potential achievement in many aspects: electronic industry associated atom technology, energy, and health care, military, manufacturing and environmental issues. Therefore emphasise the importance. Due to the multidisciplinary nature of nanotechnology, it associated with some uncertainties, which make it very difficult to prophecy any future impact. How to guide and control these uncertainties to make nanotechnology benefit human future is also concerned in this review.
1 Atom Technology Since the transistor was invented more than 50 years ago, electronic devices have been one part of our life and now have grown to one of the major industries. However, the high growth rate up to now may not be guaranteed in next 30 years. The dimensions of chip components can‘t be reduced any further limited by today‘s manufacturing techniques. Therefore, some revolutionary approach and concept is urgently needed. Due to such background, a more widely area based on nanotechnology and nanoscience (N&N), which named ―atom technology‖ emerges out. This new technology specialised in static and dynamic observation, the manipulation of materials, the materials formation at atomic or molecular levels and the handing of individual atoms. [3]
1.1Atom manipulation Beam-probe techniques and mechanical-probe techniques as well as particle trapping techniques in a 3-D free space will play a critical role for future atom manipulation. [3] Current techniques such like scanning probe microscopy (SPM) will far from the maturity for the next 30 years.
1.2 Nanoscale self-organisation Current self-organisation is characterized as‗group behaviour‘ of an entire system consisting of lots of molecules and atoms. So, large amount of time is needed to finish up and can‘t reach desired accuracy sometimes. Obviously, actual techniques can‘t fulfil the needs of atom-by-atom processes. The trends of self-organisation process in the next step should including self-assembly, self-ordering and selflimiting phenomena through which a large amount of nanostructures can be fabricated in parallel processing, with high atomic accuracies and be controlled within an acceptable time. Therefore benefit the mass productions of electronic manufacturing.
1.3 Critical-state phase control This technique is temporarily stay in concept, but will emerge out soon in the future. It supports a sort of guiding principle for new materials, new phenomena and new
processings. It may also contribute to atom manipulation and nanoscle selforganisation. This concept basically can control the macroscopic phase of the system, when the system is set up at a critical state adjacent to the phase transition, by a subtle perturbation. Various phase transition included, such as electric transition from metal to insulator, structural transformation from amorphous to crystalline etc. [3]
2 Energy Technologies Field Energy and CO2 emissions returned on political and scientific agendas worldwide last year. In the next 25 years, the total energy consumption is expected to increase by circa 2.0%/year. [4] [5] and will continue be dominated by conventional fossil fuels. N&N will play a very important role in tackling these fundamental challenges: generating and using renewable energy, leading towards to a higher energy efficiency level and storing carbon from emissions. [6,7]
2.1Solar photovoltaic Nanostructured materials allow light to be collected from a boarder range than bulk counterparts. Quantum dots (QDs) can enhance cross-section due to the momentum delocalisation of selection rules. [8] Single photo generation of multiple excitons has been widely employed in PbSe, Cdse and Pbs QDs, which impressively increased the efficiency limit. [9] Organic molecules is associated with lots of π-conjugated bonds, which is employed in organic solar cells and sensitised solar cell that allow for incorporation of efficient charge separating structures. [10] Dye-sensitised solar cell was the first solar cell that utilising nanoscale components for increasing the performance. [11] Here, light is absorbed by dye molecules anchored to the surface of nanoparticles of a boarder bandgap semiconductor, usually TiO2. To absorb more light, the TiO2 is made to be a nanoporous semiconductor (NS) layer, which have a large surface area per unit volume and weight. [12] Therefore, one direction of the optimisation of solar cells is to improve the performance of dye/NS layer, which is seriously dependent on nanoscale fabrication and characterisation. Considering the mass production of solar cell, achieving sophisticated and affordable nanostructure on large-scale surface is also a big challenge in the future. Selfassembly or self-organising schemes and role-to-role processing are attractive concepts. [12] Nanotechnology may also enable new device concept. [12] For example, Conventional semiconductor materials have extinction depths on the Οm scale. Reducing the active layer thickness can improve the carrier transportation (electron transport becoming ballistic), hence increasing the efficiency. Several new concepts based on nanoscience are now investigated e.g. light trapping by micro-texturing the surface of Si solar cell. [13,14] The latest development is to couple the light into wave-guided modes by using plasmonic noble metal. This process is surprisingly efficient for wave-guides with thicknesses on the nanometer scale. [15-17] Furthermore, the nanotechnology may be designed to manage the electromagnetic near-field, where the field energy can be spatially confined to a photoactive region. Usually inspired by the natural photosynthetic antenna system. These approaches include: carbon naonotubes, zeolites and metallic nanoparticles. [18-21]
2.2 Solar water splitting N&N also play an important role in splitting of water to hydrogen and oxygen using solar cell. [12] This technique is crucial for the future renewable energy industry. Current method is photo-electrochemical water decomposition by using wide-band gap semiconductors, which have limited light (visible) absorption. The N&N can be used to improve light harvesting. Basically, the photovoltaics and this photo-induced water splitting only differ in how excited electrons are used. The resent N&N
approaches in light absorption described in 2.1section are applicable to watersplitting case. Current technologies for photo-electrochemical water decomposition also suffer from fast electron-hole recombination and concurrent low efficiencies. [12] Conventional way is to deposit small noble metal islands which less than 5 nm, which can enhance the photo-catalytic activity for titannia-assisted water splitting, by the charge separation across the metal-semiconductor interface. [22] Latest N&N approaches towards increasing the probability of excited charge carriers reaching the surface (where the water splitting reaction takes place). These approaches include the utilising of single-walled carbon nanotubes, nanostructured hematite films and carbon-doped titania nanotube arrays. [23-27] N&N methods provide an impressive high degree of control, which can help reducing the structural complexity of the photo-anodes, and which can improve the understanding of the critical reaction steps. [12]
2.3 Fuel Cell (FC) This is one of the most promising energy resources for the future. Nanotechnology provides lots of contributions and improvements on FC, especially in catalysis, diagnostics and electrolytes aspects. The structure of conventional FC electrodes is very complex and hard to establish a detailed and systematically investigated structure-performance relationship. [12] These electrodes are composed of finely dispersed electro-catalyst particles on a high surface area carbon support. [12] Resent N&N approaches help to fabricate planar model electro-catalysts, which allow one to control the shape, size, separation of the catalyst particles and also investigate these parameters systematically. For polymer electrolyte fuel cells (PEFCs), they typically consist of a Nafion membrane. When temperature be elevated above 100C. It shows limitations on conductivity, due to the loss of proton conductivity on Nafion based membranes at high temperature. This problem has been improved by introducing nanometer-sized inorganic fillers e.g. TiO2. [28] Resent approaches found that nanomaterials such like titania nanotubes exhibit impressive proton conducting properties at high temperature (above 100C), and tend to contribute a large improvement on membranes.
2.4 Other Options As N&N is an enabling technology, it accelerated the development of other energy technology options: Materials and phase separation techniques for nuclear fusion/fission. New catalysis for non-renewable energy development. The development of high efficiency lighting devices. New materials for low loss power transmission lines and electric power applications. [29] And emission cleaning methods for both air and water filtration purposes.
3 Health Care This could be the most revolutionary improvement that N&N may bring to us in future. It has opened up new areas and created new opportunities for disease treatments, clinical applications and medical science. And already did great contributions with its rapid development in many areas. Such as: drug and gene delivery, non-invasive diagnostic method, cardiac therapy, dental care and orthopaedic applications.
3.1 Drug Delivery The fundamental developments of the drug delivery system is to improve the absorption, stability, therapeutic concentration and accuracy of realise of drug at the target site, as well as improving patient comfort. Oral route is the most common method of drug delivery, due to it is non-invasive. However, because of the resistance exerted by stomach and intestine as well as its acidic conditions, the bioavailability of the drug has been reduced. Also due to
current drugs is based on small molecules with a mass less than 1000 Da. The toxicity to non-target tissues still can‘t be avoided. [30] Nanotechnology-based drug delivery system can effectively solve the drug solubility and cell permeability issues by combining tissue/organ-specific targeting with therapeutic action. This technique could also be employed in multifunctional nanodelivery systems. [31-33] Pharmaceutical development is one of the most important clinical applications of N&N. It takes the advantage of the unique properties of nanoparticles as components of drug or even as drugs, and also make it have controlled release function, drug targeting as well as salvage of drug with low bioavailability. For instance, capsule made by nanoscale polymer could release drugs at desired rate and can be break down to allow differential release at certain conditions, such as acid milieu, tumor tissues, etc. [34, 35] N&N is opening new opportunities for conventional oral formulations. Reformulations of a drug with nanoscale particles may improve oral bioavailability.[36,37] Nanoparticles formulations provide protections for agents which are susceptible to degradation under harsh PH environment, and also increase the drug releasing duration by increasing retention of the formulation through bio-adhesion.[40] Delivery of antigens for vaccination is another important application for N&N. [38, 39] Human immunity system is limited by both degradation of the vaccine and limited uptake. [40] Resent approaches demonstrated that nanoparticles and microparticles obtain good capability of enhancing immunisation. For instance, (within the mucosal immunity), it has been shown that M cells in the Peyer‘s patches of the distal small intestine can engulf large micro-particles, and recent approaches have shown the benefits of nano-encapsulaton. [40]
3.2 Gene Delivery Gene therapy can prevent the genetic disorders by correcting defective genes responsible for disease development. It has been recently introduced and the method based on the delivery of repaired genes or even replaces the incorrect ones. [41-44] The most common way is to insert a normal gene into genome with a nonspecific location, then replace a non-functional gene or swap the abnormal gene through so-called homologous recombination. [40] Genetic materials can be directly injected into tissues using so-called gene guns. Other types are viral/non-viral vectors gene delivery systems. Non-viral vector system is in the form of nanoparticles (usually 50-500 nm), liposomes or dendrimers. The current viral vectors system suffers from the inherent difficulties of effective pharmaceutical processing and serious questions about the immunogenicity. [45,46] Non-viral vectors have already shown its potential during a test for transporting plasmid DNA, and may replace the current viral vectors system by less immunogenic nanosized gene carries in the near future. [47]
3.3 Non-invasive Diagnostic Method Non-invasive imaging techniques have done a great contribution to modern medical diagnostic over the past 25 years. Current research in this field is to enhance spatial resolution and contrast agent, for instance in magnetic resonance imaging techniques. N&N could offer novel opportunities for molecular disease imaging and therapeutic intervention. [48] By associating with quantum dots (QDs), nanotechnology already achieved intracellular imaging, and could directly investigate the intracellular biochemical process without the corporation of naturally occurring fluorescent proteins. [40,48] QDs is a sort of semi-conductor nanocrystal obtains impressive electrical and optical prosperities: sharp and symmetrical emission spectra, broad absorption range, good photostability and size-dependent emission wavelength tenability. [49-52] Recent approaches combined the nanosensors with florescence resonance technique, which is by using ―QDs linked DNA probes‖ to capture DNA targets, and
can effectively detect low concentrations of DNA in a separation-free format. In addition, QDs can amplifies and concentrates the target signal by confining numbers of targets into a nanosclae domain. [53]. However, this requires excitation from external illumination sources to fluorescence, which limits the utility of existing QDs. Therefore one research in this field is to improve QDs functions without an external excitor. [54]
3.4 Cardiac Therapy Nanotechnology offers tools that can raise the cardiovascular science into cellular level. N&N-based tools can be used in tissue engineering, diagnosis as well as the imaging technique mentioned in (3.3), which could be effectively used to treat this condition. [55] Nanoscaled sensors and nanobarcodes can sense and monitor many sorts of biological signals such as inflammatory events. N&N also helps in designing medical using nanorobots and atomic-scaled machines by incorporating or imitating human biological systems at the molecular level. [40] These nano-machines will shift the treatment into a new level in the near future. Nanotechnology-based localised drug can prolong drug releasing level in the target tissues without causing toxicity and can effectively restenosis (the obstruction of an artery after interventional procedures). This technique could also be used for the prevention of cardiovascular diseases.
3.5 Dental Care Nanodentistry is a new medical application of nanotechnology, which is by utilising nanomaterials, biotechnology and nanorobotics to maintain oral health. [40] One of the major approaches to nanodentisry is local anesthesia. During the operation, patient‘s gingiva has been instilled with a colloidal suspension which contains millions of active analgesic dental nanorobitc particles. [40,56] Once contacting with the surface of the crown or mucosa, these nanorobots could migrate into the gingival sulcus, then reach the dentin and pass painlessly to the target issues. Nanorobots could enter dentinal tubule holes which are in micrometer scale [56-58] and proceed toward the pulp. They are guided by a combination positional navigation, temperature differential and series of chemical gradients. All of these are controlled by an onboard nanocomputer. Apart from this, nanodentistry can also do tooth repairing. Orthodontic nanorobots can manipulate the periodontal tissues; therefore straighten the tooth, or even rotating and vertical repositioning. The whole process could last within minutes to hours and it is painless. Nanodentistry can also help maintain the natural tooth. [40]
4 Military and Defence The military is the main driving force in N&N. It already played a vital role in understanding and developing nanotechnology. These researches also included environmental pollution and health hazards. Current and potential uses of N&N may be focused on smart weapons, nanocomputers, bio-weapons and terrorism-related issues. [59]
4.1Smart Weapons Miniaturised system and chips may help robotic weapons become reality. By using nanocomputers, the sizes of weapons have been significantly reduced. Nanocompters can replace human from battle field, making warfighting less costly. With the help of nanotechnology, better sensors and computational system could be built with lower-cost production. This could make large deployment of tele-operated robotic-soldiers able to occupy territory without risk to human soldiers. [59]
4.2 Bio-weapons By utilising nanobiology, the ―next generation‖ bio-weapons could be more lethal. These include so-called ―hybrid genetic marker targeting quasi-viral components‖ and hyper-virulent prions. These artificial viruses and viral hybrids can remain dormant for a long period of time, with low of even no maintenance and undetectable
until released. N&N has already been used in many weapons and make these bioweapons extremely ―stealth‖. [59]
5 Ecological Concerns N&N provided many opportunities and solutions to the ecological system. Devices that could reorganize atoms and molecules of the biosphere, aim to adjust the unbalanced environmental relationships. [1] Nanotechnology associated ―bottom-up‖ creation could produce materials and devices without producing wasteful and dangerous by-products, as current manufacturing process. Therefore reduce the environmental effects. By utilising N&N, the higher strength-to-weight ratio materials and devices are become more widely used. These could eventually eliminated the massive need for huge infrastructural power generation systems and reduce the human ecological footprints. [60] However, due to nanotechnology is still in its infant stages, some unpredictable consequences and potential dangers may associate with it. A well-know cautionary principle stated: ―the lack of certainty, given the current scientific and technological knowledge shall not delay effective and proportionate actions to prevent hazards‖. [61] A whole spectrum of the possible harmful effects of nanotechnologies to the environment must be built, before they are introduced to the environmental and biological world. Sustainability of N&N for production must keep pace with the future development and researches. This is very like the case of genetically modified foods. [1]
6 Foresights for Nanotechnology All objects and living being consist of atoms and molecules and N&N offer the ways to understand and engineer them at a molecular level. For this reason nanotechnology is characterised by interdisciplinary. [62] (See fig 1)
Fig 1 [62] the interdisciplinary and convergence at the nanoscale of N&N
Therefore, it could be useful to note the future development of this ―convergence technology‖ can enable each other in the pursuit of a common goal. [63] Four areas, nanotechnology, biotechnology, information technology and cognitive science (see fig 2) could be merged to development and strengthen together. These four areas sometimes called NBIC is now regarded as the most promising filed over a very long timescale. [62]
Fig 2 NBIC [62]
7 Conclusions Nanotechnology has already penetrated into every aspect of the science, industry and will keep playing a vital role. Its scientific potential will provide solutions to every sector of our current problems: The energy efficiency or finding renewable energy, medical treatment and diagnostic methods, manufacturing limits and cost and the harmony between human and the environment. Despite, there are some uncertainties that can‘t be currently predicted. It can prevent the nanotechnology becomes the next big thing. A careful guidance and international cooperation are essential for this powerful technique to release its astonishing potential.
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