Nanoparticles: A New Technology with Wide Applications

INTRODUCTION

What are the building blocks of nanobiotechnology? The technology Springs from advancements in material science-the ability to fabricate nano-scale materials in a uniform and reliable manner, at reasonable scale and cost. In the nanometer dimension we can now manufacture crystals, particles, spheres, wires and tubes. Materials at this scale change the mechanics of molecular interactions and thus permit much greater sensitivity and permeability and enable a much more exclusive manner of interaction. As we learn to manipulate the architecture at the nanoscale, we will develop applications of unprecedented sensitivity to our internal and external environment. The following categories list some of the nanotechnology applications currently under development. Nanotechnology emerges from the physical, chemical, biological and engineering sciences where, novel techniques are being developed to probe and manipulate single atoms and molecules. In nanotechnology, a nanoparticle (10^-9 m) is defined as a small object that behaves as a whole unit in terms of its transport and properties. The science and engineering of nanosystems is one of the most challenging and fastest growing sectors of nanotechnology.

The emerging nanotechnology has turned many of our dreams true by enabling construction of micro/nanodevices. Since, the birth of nanotechnology, it has never been a single field technology. It is more preferably called nanotechnologies, as refers to a set of methods and approaches in physics and chemistry science, engineering fields, biological and medical areas. The researchers in different fields usually have different understanding towards this technology, which sometimes causes uneven development towards nanoscale. For example, while engineers and physics scientists race to shrink the size of transistors and MEMS components through nanofabrication to create the next generation of high-performance electronic devices, biologists and life scientists have just begun to employ micropatterning and to a more limited extent, nanopatterning techniques to build high-throughput detection systems for genomic and proteomic studies (Bhainsa and D’Souza, 2006; Cheon and Horace, 2009).

The burst of nanotechnologies is believed to be observed at the convergence of different research fields. Many challenges and new directions for research will be posed upon blurring of the boundaries. The most notable breakthroughs are expected at unification of nanotechnology and biotechnology, two promising research fields for the 21st century. Nowadays the study of biological science has reached down to molecular and DNA level. The interaction of these basic life components is the foundation of various macroscopic behaviours of the living organisms. Since, molecular and DNA components are usually nanometers or even smaller in size, current technologies at conventional scale seem insufficient for characterization and analysis. Nanotechnologies kick in right in time.

Early history: The concept of nanotechnology though considered to be a modern science has its history dating to as back as the 9th century. Nanoparticles of gold and silver were used by the artisans of mesopotamia to generate a glittering effect to pots. The first scientific description of the properties of nanoparticles was provided in 1857 by Michael Faraday in his famous paper Experimental relations of gold (and other metals) to light (Faraday, 1857).

In 1959, Richard Feynman gave a talk describing molecular machines built with atomic precision. This was considered the first talk on nanotechnology. This was entitled There’s plenty of space at the bottom.

The 1950’s and the 1960’s saw the world turning its focus towards the use of nanoparticles in the field of drug delivery. One of the pioneers in this field was Professor Peter Paul Speiser. His research group at first investigated polyacrylic beads for oral administration, then focused on microcapsules and in the late 1960s developed the first nanoparticles for drug delivery purposes and for vaccines. This was followed by much advancement in developing systems for drug delivery like (for e.g.) the development of systems using nanoparticles for the transport of drugs across the blood brain barrier. In Japan, Sugibayashi et al. (1977) bound 5-fluorouracil to the albumin nanoparticles and found denaturation temperature dependent differences in drug release as well as in the body distribution in mice after intravenous tail vein injection. An increase in life span was observed after intraperitoneal injection of the nanoparticles into Ehrlich Ascites Carcinoma-bearing mice (Kreuter, 2007).

The nano-revolution conceptually started in the early 1980’s with the first paper on nanotechnology being published in 1981 by K. Eric Drexler of Space Systems Laboratory, Massachuetts Institute of Technology. This was entitled An approach to the development of general capabilities for molecular manipulation.

With gradual advancements such as the invention of techniques like TEM, AFM, DLS etc., nanotechnology today has reached a stage where, it is considered as the future to all technologies.

History of nanotechnology in medicine: Professor Noro Taniguchi of the Tokyo University of Science coined the term nanotechnology in 1974. Nanotechnology refers to molecular devices smaller than 1 μm on the nano scale. One nanometer (nm) is one billionth or 10^-9 of a meter. The field was originally inspired by a talk There’s plenty of room at the bottom, by Richard Feynman in 1959 at the American Physical Society. Feynman suggested a number of concepts, including print font size, which would permit the Encyclopedia Britannica to fit on the head of a pin; a feat since accomplished. The broader concept was that because of their small size, nanomaterials have unique qualities that are not found in the same materials at larger sizes. Principles developed from nanotechnology research are being used to develop everything from the next generation of computer chips to fluid-handling devices that will markedly miniaturize current devices. Importantly, the field of Nano Electro Mechanical Systems (NEMS) will be important in implantable devices for a range of biological systems from stress sensors in aneurysms to neural implants. Soon, after the development of mechanical and electrical approaches in nanotechnology, biologists began to explore direct applications using this technology. Biological approaches and novel applications for molecular nanotechnology was the first scientific conference held on the topic in 1996. The initial focus was small robots that create billions of tiny factories small enough to work within a single cell, but this proved to be more dream than scientific endeavor. However, it became clear that biological systems are organized at nanoscale dimensions and synthetic nanomaterials correlated in size with biological structures such as proteins, glycolipids and DNA. Unique interactions between synthetic nanomaterials and more complex biological systems were also observed, most likely due to their size. These ranged from good (deliver of materials across the gut) to potentially dangerous (ability of nanoparticles to enter the brain). It was also discovered that the detrimental activities of some types of environmental materials, such as diesel exhaust, was due to their nanoscale dimensions. Building on these discoveries, scientists are now using nanostructures for biological applications based on their unique capabilities to traverse and interact with similarly sized biological materials. Nanotechnology now remains at the forefront of medicine and biological technologies from a research perspective.

Properties of nanoparticles: Material properties depend on structure and composition and can typically be engineered or modified by changing the relative influence of interfacial or interphase properties and the macroscopic bulk properties through the characteristic size or dimension of components and domains. This approach had already emerged centuries ago with steel alloys and has been so powerful that many engineering materials today are composites with micro to nanoscale domain sizes. Depending on the physical or chemical character of each domain, there is a complex interrelation between the structure and the composition of the material, which may relate to the bulk and surface properties of each ingredient and newly emerging properties localized at the interface. Selective chemical reactivity is quite common with nanocomposites, which gives the potential for disintegration of the material into one or the other component. Complex processes govern this behaviour, which clearly relates to nanoparticle release into the environment.

Nanoparticles: Physical and chemical properties: The principal parameters of nanoparticles are their shape (including aspect ratios where appropriate), size and the morphological sub-structure of the substance. Nanoparticles are presented as an aerosol (mostly solid or liquid phase in air), a suspension (mostly solid in liquids) or an emulsion (two liquid phases). In the presence of chemical agents (surfactants), the surface and interfacial properties may be modified. Indirectly such agents can stabilise against coagulation or aggregation by conserving particle charge and by modifying the outmost layer of the particle. Depending on the growth history and the lifetime of a nanoparticle, very complex compositions, possibly with complex mixtures of adsorbates, have to be expected. In the typical history of a combustion nanoparticle, for example, many different agents are prone to condensation on the particle while it cools down and is exposed to different ambient atmospheres. The appropriateness of existing methodologies to assess the potential risks associated with engineered and adventitious products of nanotechnologies processes are to be expected and have been identified only for a small number of particulate model systems. At the nanoparticle-liquid interface, polyelectrolytes have been utilised to modify surface properties and the interactions between particles and their environment. They have been used in a wide range of technologies, including adhesion, lubrication, stabilization and controlled flocculation of colloidal dispersions (Liufu et al., 2005). At some point between the Angstrom level and the micrometre scale, the simple picture of a nanoparticle as a ball or droplet changes. Both physical and chemical properties are derived from atomic and molecular origin in a complex way. For example, the electronic and optical properties and the chemical reactivity of small clusters are completely different from the better known property of each component in the bulk or at extended surfaces. Complex quantum mechanical models are required to predict the evolution of such properties with particle size and typically very well defined conditions are needed to compare experiments and theoretical predictions.

Classification of nanoparticles: Nanoparticles can be broadly grouped into two: namely organic and inorganic nanoparticles. Organic nanoparticles may include carbon nanoparticles (fullerenes) while some of the inorganic nanoparticles may include magnetic nanoparticles, noble metal nanoparticles (like gold and silver) and semiconductor nanoparticles (like titanium dioxide and zinc oxide).

There is a growing interest in inorganic nanoparticles as they provide superior material properties with functional versatility. Due to their size features and advantages over available chemical imaging drugs agents and drugs, inorganic nanoparticles have been examined as potential tools for medical imaging as well as for treating diseases. Inorganic nanomaterials have been widely used for cellular delivery due to their versatile features like wide availability, rich functionality, good biocompatibility and capability of targeted drug delivery and controlled release of drugs (Xu et al., 2006). For example, mesoporous silica when combined with molecular machines prove to be excellent imaging and drug releasing systems. Gold nanoparticles have been used extensively in imaging, as drug carriers and in thermo therapy of biological targets (Cheon and Horace, 2009). Inorganic nanoparticles (such as metallic and semiconductor nanoparticles) exhibit intrinsic optical properties which may enhance the transparency of polymer-particle composites. For such reasons, inorganic nanoparticles have found special interest in studies devoted to optical properties in composites. For instance, size dependant colour of gold nanoparticles has been used to colour glass for centuries (Caseri, 2009).

Strategies used to synthesize nanoparticles: Previously nanoparticles were produced only by physical and chemical methods. Some of the commonly used physical and chemical methods are ion sputtering, solvothermal synthesis, reduction and sol gel technique. Basically there are two approaches for nanoparticle synthesis namely the bottom up approach and the top down approach.

In the top down approach, scientists try to formulate nanoparticles using larger ones to direct their assembly. The bottom up approach is a process that builds towards larger and more complex systems by starting at the molecular level and maintaining precise control of molecular structure.

Top-down method: Top-Down method refers to a set of fabrication technologies which fabricate by removing certain parts from a bulk material substrate. The removing methods can be mechanical, chemical, electrochemical and etc., depending on the material of the base substrate and requirement of the feature sizes.

Fig. 1:	Top down approach

Fig. 2:	Bottom-up approach

The formed structures usually share the same material with the base substrate. There are a couple of manufacturing technologies in the conventional scale which can be categorized top-down. Milling is a representative example. In the milling process, material is selectively removed from the substrate, usually a metal sheet, forming a cavity with certain geometries. The dimensions of the cavity depend on the travel path of the mill, which can be precisely controlled with the help of computer assisted numerical systems. The milling technique, along with similar methods such as drilling and grinding, is the most widely used technique in conventional manufacturing industry. People have attempted to extend top-down method into nanometer domain and supplemented the mechanical removing methods with chemical and electrochemical methods (Fig. 1).

Bottom-up method: As the opposite to top-down fabrication technologies, bottom-up methods refer to a set of technologies which fabricate by stacking materials on top of a base substrate. These methods are similar in principle to welding and riveting at the conventional scale, in which a different type of material is attached to the base component by melted solder or physical fitting. In welding and riveting, attention is mainly paid to the strength of the contact area in order to maintain the construct as a reliable component for high load application. Similarly, in bottom-up nanofabrication, the adhesion of the surface layer to the base substrate is also an important concern. There is extensive research on the surfactants to enhance adherence and avoid cracks during the subsequent processing. Research has also focused on autonomous patterning of the surface layer into nanometer scale features since, manipulation of nanoscale components is not ever an easy task as compared to that at the conventional scale (Fig. 2).

Physical and chemical methods of nanoparticle synthesis: Some of the commonly used physical and chemical methods include: Biomimetic synthesis of nanoparticles: Science, technology and applicability:

Biosynthesis of nanoparticles: The need for biosynthesis of nanoparticles rose as the physical and chemical processes were costly. So, in the search of for cheaper pathways for nanoparticle synthesis, scientists used microorganisms and then plant extracts for synthesis. Nature has devised various processes for the synthesis of nano-and micro-length scaled inorganic materials which have contributed to the development of relatively new and largely unexplored area of research based on the biosynthesis of nanomaterials (Mohanpuria et al., 2008).

Biosynthesis of nanoparticles is a kind of bottom up approach where the main reaction occurring is reduction/oxidation. The three main steps in the preparation of nanoparticles that should be evaluated from a green chemistry perspective are the choice of the solvent medium used for the synthesis, the choice of an environmentally benign reducing agent and the choice of a non toxic material for the stabilization of the nanoparticles. Most of the synthetic methods reported to date rely heavily on organic solvents. This is mainly due to the hydrophobicity of the capping agents used (Raveendran et al., 2003). Synthesis using bio-organisms is compatible with the green chemistry principles: the bio-organism is (1) eco-friendly as are (2) the reducing agent employed and (3) the capping agent in the reaction (Li et al., 2007). Often chemical synthesis methods lead to the presence of some toxic chemical species adsorbed on the surface that may have adverse effects in medical applications (Parashar et al., 2009a, b). This is not an issue when it comes to biosynthesized nanoparticles as they are eco friendly and biocompatible for pharmaceutical applications.

Application areas of nanobiotechnology
Bioanalysis: The life sciences research market continually seeks improvements in bioanalytical research tools with regard to further miniaturization, the ability to conduct experiments in parallel and improvements in sensitivity. There are limitations associated with the accuracy and resolution of fluorescent labelling methods and often the speed and cost of target amplification methods create a critical bottleneck in the design of ultrahigh-throughput bioanalytical systems. Nanoscale bioanalytical technology platforms seek to eliminate some of these limitations. These platforms include the use of nanoparticles (dots, bars, rods) as labels for biomolecules for separation and screening, as well as nanopore and nanoscale fluidic assay systems and self-assembling arrays of nanoparticles. Such applications are more amenable to ultrahigh-throughput formats and theoretically provide more sensitive and highly-specific detection and analysis capabilities. For example, current advances being made with nanoparticles promise to significantly improve signal generation and detection in high throughput, multiplexed biological assays. If successful, these developments will greatly enhance research productivity in the life sciences, significantly reduce the time, effort and expense of DNA sample preparation and analysis and find broad application in the clinical, food, agriculture and environmental markets.

Diagnostics: Nanotechnology is at the core of advances in the biosensor field through the use of novel materials, improved surface engineering and patterning techniques and systems integration. Biosensors are being developed using nanowires, nanoparticle arrays and nanofluidics systems-devices will likely include the integration of many of these components. These materials permit unprecedented sensitivity to our internal and external environment. For example, with the ability to detect proteins down to a few molecules, the field of diagnostics can be brought to the fundamental level of a single cell. And for patient monitoring and diagnosis, the assay may require only a single breath. The key to biosensing lies in the sensitivity of molecular detection, which is often determined by the method attachment of biomolecules to the sensor surface. General methods of coupling biomolecules to sensors include physical adsorption, covalent bonding, membrane entrapment and porous encapsulation. Detection can be performed optically, electrochemically, thermally or through various other techniques. The biosensor market can be broken down to three basic categories: diagnostics for clinical and research use, nutritional and consumer product safety and chemical and biological warfare defense.

Therapeutics: Of the Life Sciences, this area has taken the quickest advantage of advances in nanotechnology. While some of the earliest applications have appeared in sunscreens and cosmetics, methods have been developed for in vivo drug delivery via nanoparticles such as nanocrystals, nanospheres, nanocapsules and can also include dendrimer technologies. By the nature of their size, these nano delivery systems traverse membrane boundaries and can be readily absorbed into the bloodstream. Their surface chemistry can be modified to display high concentrations of a therapeutic drug or tissue-specific targeting molecules; alternatively the drug may be encapsulated for controlled stealth mode activity. Surface coatings can also manipulated to exhibit fast or slow release, or for higher in situ stability and shelf-life. The drug delivery market is estimated to be $20 billion in 2002 and growing because of new technologies that revive drugs with less-than-favorable oral bioavailability. Drugs may be reformulated as nanocrystals or encapsulated for more efficient uptake. Targeted nanotherapeutics suggest the promise tissue-specific delivery with a strong localized dose-requiring a lower overall concentration of the drug, while at the same time providing lower patient toxicity and side-effects. In some cases, payload delivery might be triggered by a secondary mechanism such as light activation. Nanotechnology may be able to accelerate therapeutics for protein and macromolecule drugs, infectious disease and cancer. Nanoparticle inhalation technology provides a patient-friendly alternative to injection and may permit a lower dose strategy with protein drugs like insulin. With the ability to cross the blood-brain barrier, there may be new methods to diagnose and treat neurodegenerative disease. And finally, there are promising new treatments using nanocapsules for cholesterol removal and nanostructured silicon to treat osteoporosis.

Medical devices: Nanoscale devices open up a new horizon in medical diagnostics and treatment, as technological advances in materials and biosensors become precursors for advancing medical applications. Nanomaterials will have a strong impact in the markets for MRI and X-Ray contrast agents, estimated to be close to $400 million and $3 billion per year, respectively. Current contrast agents require catheterization and have limited tissue specificity and retention rates, requiring immediate imaging. Here, nanoparticles may be useful at lower doses for tissue-specific targeting and retention. More significantly, nanoparticles have the advantage of slow diffusion out of the bloodstream, which could permit imaging of the circulatory system and blood pool-particularly useful in cases of stroke. In the area of cancer treatment, the removal of tumours is typically done through a combination of surgery, chemotherapy and radiation, to varying degrees of success but at some cost to the general health of the patient. Similar to targeted drug delivery, nanoparticles may be useful as site-specific probes for tissue destruction, using light or heat to induce thermal oblation or deposit a localized chemotherapy payload. For future applications, nanostructured silicon may prove useful as temporary scaffolding in reconstructive bone surgery and it has been demonstrated that nanoparticles can assist the generation of new bone matrix material. Prostheses can be designed with nanoporous interface to enhance integration of artificial structures and living tissue. Not too much farther down the road, devices like retinal implants can take advantage of nanoscale solar technology, where nanoporous electrodes provide a high-density interface with the nerves of the retina.

Nanoparticles as tools in medicine: It has proved difficult to channel pharmaceuticals into the brain. A type of cell barrier protects the brain from pathogens and many harmful molecules. This blood-brain barrier also denies access to many therapeutic substances. Studies have shown that nanoparticles (diameter between 10 and 1000 nm) with distinct surface properties can overcome this barrier. At the University of Frankfurt am Main, a team headed by Prof. Dr. Jörg Kreuter is successfully working on transferring substances into the brain with the aid of microscopically small plastic spheres. Magnetic nanoparticles could also be of use in combating cancer, as shown by the so called magnetic liquid hyperthermia developed by Dr. Andreas Jordan and co-workers at the Charite Hospital in Berlin: Firstly, iron oxide particles are selectively transported into the carcinoma. Then, an alternating magnetic field heats the nanoparticles and thus the cancer cells, which are killed by overheating. The particles are produced for instance by the Leibniz Institute for New Materials in Saarbrücken (INM). Prof. Dr. Helmut Schmidt and colleagues from INM attempt to modify the surface of the nanoparticles according to the requirements of the Berlin group so that the particles can be delivered to the blood stream. Above all, only cancer cells incorporate the particles, so that healthy cells are unaffected.

Protein design for optical information processing: Bacteriorhodopsin originates from so called halobacteria using this protein to convert light energy into other suitable forms of energy. Bacteriorhodopsin changes colour from purple to yellow when it is irradiated by light. The photochromic properties can selectively be modified and stabilised with the aid of genetic techniques. This entails it is interesting as a high-performance material for optical media, especially for holographic pattern recognition and interferometry. Many other applications are also possible. For example, biofilms coated with the protein can be produced thus creating optical data memory systems with extremely high capacities. In the past few years, the necessary biotechnological tools have been established so that bacteriorhodopsin can be technologically exploited. Research is currently undertaken on integrating the new material into optical systems ready for application.

Goals of nanobiological research:

Risk assessment: For emerging technologies like nanotechnology, with their specific, new and partly unknown risks, the question has been asked whether the existing regulations for the different types of medical products are sufficient to guarantee the safe use of these technologies in practice. In order to answer this question, an assessment of the specific risks is needed. For the application of nanotechnology in medical technology the risks which are judged to need special attention are related to the toxicology of nanoparticles and nanostructures.

Biological synthesis of nanoparticles from Indian perspective: There has been considerable significant research in India in the field of biological synthesis of nanoparticles. More research has been found to be concentrated in the area of synthesis using terrestrial plants and marine medicinal plants.

Recently stable gold nanoparticles have been synthesized using the marine alga, Sargassum wightii. Nanoparticles with a size range between 8 to 12 nm were obtained using the seaweed. An important potential benefit of the method of synthesis was that the nanoparticles were quite stable in solution (Singaravelu et al., 2007).

Extracellular biosynthesis of functionalized silver nanoparticles was done by using strains of Cladosporium cladosporioides fungus (Balaji et al., 2009). Biosynthesis of zirconia nanoparticles has been done using the fungus Fusarium oxysporum (Bansal et al., 2004).

It has been observed that a novel alkalothermophilic actinomycete, Thermomonospora sp. and fungi synthesized gold nanoparticles extracellularly when exposed to gold ions under alkaline conditions (Sastry et al., 2003; Ahmad et al., 2003). The use of algae for the biosynthesis of nanoparticles is a largely unexplored area. There is very little literature supporting its use in nanoparticle formation.

Aspergillus flavus (Vigneshwaran et al., 2007) and Aspergillus fumigatus (Bhainsa and D’ Souza, 2006) has been found to accumulate silver nanoparticles on the surface of its cell wall when reacted with silver nitrate solution. Monodisperse silver nanoparticles with a size range of 8.92+/-1.61 nm and 5-25 nm were obtained using these organisms.

Recently, scientists in India have reported the green synthesis of silver nanoparticles using the leaves of the obnoxious weed, Parthenium hysterophorus and Mentha piperita leaf extract. Particles in the different size range of 10-80 nm were obtained after 10 min of reaction. The use of this noxious weed has an added advantage in that it can be used by nanotechnology processing industries (Parashar et al., 2009a, b).

Rapid synthesis of Au, Ag and bimetallic Au core-Ag shell nanoparticles using neem (Azadirachta indica) leaf broth (Shankar et al., 2004).