NANOPARTICLES

O.V. SALATA

University of Oxford

Oxford, United Kingdom

INTRODUCTION

Nanoparticle (From a Greek word ‘‘o va´ noz’’ meaning ‘‘dwarf’’), ‘‘nano’’ is a prefix defining a billion times reduction (10 9) in magnitude. Intuitively, a nanoparticle is an object that is a ‘‘nano’’ times smaller than just a particle. The generally accepted size range for the objects to be called ‘‘nanoparticles’’ is between 1 and 100 nm in at least in two dimensions. This broad definition can be narrowed by defining a nanoparticle of a certain material as a particle that is smaller than a critical size for this material (Fig. 1). The critical size is defined as a cross-point size for the transition from bulk to sized-dependent material property or properties, for example, solubility, melting point, magnetization, light absorption, and fluorescence. The actual value of the critical size depends on the strength of the interatomic or intermolecular forces. The weaker the forces, the weaker the overall interactions between the atoms in the particle. It is known from quantum mechanics that an atom or molecule can be characterized by a set of permitted electron energy levels. Those energy levels define physical and chemical properties of atoms and molecules. When two identical atoms are brought into proximity, they would experience both repulsing and attracting forces. An equilibrium distance might exist where the two forces are balanced. A new molecule containing two atoms will have a system of energy levels that is similar to that of the initial atoms, but each level will be split into two sublevels. The energy gap between the two closest sublevels in this molecule will now be smaller than the corresponding smallest energy gap in one atom. Each time a new atom is added, a new set of energy sublevels will be formed. The energy gap will be changing in value with the number of atoms used to build a nanoparticle, and because properties of the nanoparticle will depend on the energy gap, it will also depend on its size. Then millions of the same atoms form a bulk material; these energy sublevels are saturated and broadened into the energy bands that are insensitive to the changes in the number of atoms composing the solid body. Another component affecting the critical size of the nanoparticle is its surface-to-volume ratio. The smaller the size of the particle, the higher the proportion of the total number of atoms or molecules that are positioned at the surface. The surface atoms often have unsaturated or dangling bonds, leading to the high reactivity and catalytic ability of the nanoparticles. It also results in the restructuring and rearrangement of the crystal lattice near the surface. All of this affects the size-dependent properties of nanomaterials.

Nanoparticles can be made out of any material, including metals, ceramics, semiconductors, polymers, and biomolecules. They can possess a complex structure, which might contain a combination of different materials, or have a complex shape. Nanometer-sized objects that include nanoparticles fall in the realm of nanotechnology. Nanotechnology is developing in several directions: nanomaterials, nanodevices, and nanosystems. The nanomaterials/ particles level is the most advanced currently, both in scientific knowledge and in commercial applications. Nanobiotechnology is a subfield of nanotechnology that deals with the applications of nanotechnology to biology. Understanding of biological processes on the nanoscale level is a strong driving force behind development of nanobiotechnology. Living organisms are built of cells that are typically more that 10 mm across. However, the cell-forming components are much smaller and are in the submicron size domain. Even smaller are the proteins with a typical size of just 5 nm, which is comparable with the dimensions of the smallest manmade nanoparticles. This simple size comparison gives an idea of using nanoparticles as very small probes (1) that would allow us to spy at the cellular machinery without introducing too much interference. Semiconductor nanoparticles, also known as ‘‘quantum dots’’ (2), show a strong dependence of their physical properties on their size. Just a decade ago, quantum dots were studied because of their size-dependent physical and chemical properties. One of the properties of the semiconductor nanoparticles that are changing with size is the color of their fluorescence, and now they are used as photostable fluorescent probes. As nanoparticles are rapidly taken up by all kinds of cells, they are also used in drug delivery. In pharmacology, the term ‘‘nanoparticles’’ specifically means polymer nanoparticles or, sometimes, submicron particles that carry a drug load (3). This term has been used in drug delivery for more than three decades. At about the same time, magnetic particles with submicron dimensions were employed for the first time to assist with cell separation. However, colloidal gold, which can be alternatively called ‘‘a dispersion of gold nanoparticles,’’ has been used in medicine for many decades if not centuries. Colloidal gold tinctures were used by alchemists to treat many illnesses. Colloidal gold was used as a contrast agent by the first optical microscopists as early as the 1600s. In the 1950s, work was started on the use of radioactive colloidal gold as a treatment for cancer (4). When functionalized with antibodies, gold nanoparticles are used to stain cellular organelles or membranes to create markers for the electron microscopy (5). Consequent decoration of the gold markers with silver assists in further signal magnification.

Out of a plethora of size-dependent physical properties available to someone who is interested in the practical side of nanomaterials, optical and magnetic effects are the most used for biological applications. Hybrid bionanomaterials

Figure 1. Nanoparticles are a state of matter intermediate between the bulk (size-independent properties) and atomic or molecular (fixed properties) form of the same material with its physical and chemical properties (such as melting temperature, solubility, optical absorbance and fluorescence, magnetization, catalytic activity, and specific chemical reactivity) being dependent on the particle size. The critical size for the transition from the bulk to the nano-regime can vary from tenths to just a few nanometers. A 100 nm threshold is a convenient size to use for a generalized description of the expected bulk-nanoparticle transition.

can also be applied to build novel electronic, optoelectronics, and memory devices.

NANOPARTICLE FABRICATION

Two general ways are available to a manufacturer to produce nanoparticles (Fig. 2). The first way is to start with a bulk material and then break it into smaller pieces using mechanical, chemical, or another form of energy (topdown). An opposite approach is to synthesize the material from atomic or molecular species via physical condensation of atomized materials (energy released), or chemical reactions (exoor endothermic), allowing the precursor particles to grow in size (bottom-up). Both approaches can be done in gas, liquid, or solid states, or under a vacuum. Both the top-down and the bottom-up processes can be happening during the formation of nanoparticles at the same time, for example, during mechano-chemical ball milling process.

The more detailed classification of the nanoparticle manufacturing techniques relies on the combination of the form of energy with the type of the controlled environment. Each technique has its advantages and disadvantages. Most manufacturers are interested in the ability to control particle size, particle shape, size distribution, particle composition, and degree of particle agglomeration. Absence of contaminants, processing residues or solvents, and sterility are often required in the case of biological and medical applications of nanomaterials. The scale-up of the production volume is also very important. Hence, the discussion of the nanoparticle production techniques is limited to some of those that are currently being pursued by the manufacturers.

Ball Milling

Ball milling is a process where large spheres of the milling media crush substantially smaller powder particles (6). Normally it is used to make fine powder blends or to reduce the degree of powder agglomeration. High-energy ball milling is a more energetic form capable of breaking ceramics into nanoparticles. It is used to create nano-structured composites, highly supersaturated solid solutions, and

amorphous phases. The drawbacks of this technique include high energy consumption and poor control over particle sizes. A variation of high-energy ball milling is called mechano-chemical processing. Chemical reactions are mechanically activated during milling, forming nanoparticles via a bottom-up process from suitable precursors. A solid diluent’s phase is used to separate the nanoparticles. In the pharmaceutical industry, wet ball milling is often used to produce nano-formulations of the drugs that are poorly soluble in their bulk form but acquire a much improved solubility when turned into nanoparticles.

Electro-Explosion

This process is used to generate 100 nm metal nanoparticles in the form of dry powders. Michael Faraday first observed it in 1773. It involves providing a very high current over a very short time through thin metallic wires, in either an inert or a reactive gas, such that extraordinary temperatures of 20,000 to 30,000 K are achieved. The wire is turned into plasma, contained, and compressed by the

Energy in

Figure 2. Two basic approaches to nanomaterials fabrication: top-down (shown here from left to the right) and bottom-up (from right to the left). Usually, energy in one of its forms (mechanical, thermal, etc.) is supplied to the bulk matter to create new surfaces. Chemical synthesis of nanomaterials from the atomic or molecular species can be either exothermic or endothermic. Condensation of atoms under vacuum results in cluster formation accompanied by the release of energy.

electromagnetic field. After that, the resistance of the wire becomes so high that the current terminates. The disappearing of the electromagnetic field makes the plasma expand very rapidly. The extremely fast cooling rate results in stabilization of otherwise meta-stable materials. The powders made by electro-explosion have greater purity and reactivity as compared with the ball-milled powders. This technique is used, for example, to produce high surface area nano-porous filtering media benefiting from the enhanced bactericidal properties of silver nanoparticles.

Laser Ablation

In laser ablation, pulsed light from an excimer laser is focused onto a solid target inside a vacuum chamber to supply thermal energy that would ‘‘boil off’’ a plume of energetic atoms of the target material. A substrate positioned to intercept the plume will receive a thin film deposit of the target material. This phenomenon was first observed with a ruby laser in the mid-1960s. Because this process then contaminated the films made with particles, little use was found for such ‘‘dirty’’ samples. The laser ablation method has the following advantages for the fabrication of nanomaterials: The fabrication parameters can be easily changed in a wide range; nanoparticles are naturally produced in the laser ablation plume so that the production rate is relatively high; and virtually all materials can be evaporated by laser ablation.

Colloidal Chemistry

Two general colloidal chemistry approaches are available to control the formation and growth of the nanoparticles. One is called an arrested precipitation; it depends on exhaustion of one of the reactants or on the introduction of the chemical that would block the reaction. Another method relies on a physical restriction of the volume available for the growth of the individual nanoparticles by using various templates (7).

Any material containing regular nanosized pores or voids can be used as a template to form nanoparticles. Examples of such templates include porous alumina, zeolites, di-block copolymers, dendrimers, proteins, and other molecules. The template does not have to be a 3D object. Artificial templates can be created on a plane surface or a gas–liquid interface by forming self-assembled monolayers. The template is usually removed by dissolving it in the solvent that is not affecting the formed nanoparticles. The main advantages of the colloidal chemistry techniques for the preparation of nanomaterials are low temperature of processing, versatility, and flexible rheology. They also offer unique opportunities for preparation of organic–inorganic hybrid materials. The most commonly used precursors for inorganic nanoparticles are oxides and alcoxides.

Aerosols

As an alternative to liquids, chemical reactions can be carried out in a gaseous media, resulting in the formation of nanoparticles aerosols (8). Aerosols can be defined as solid or liquid particles in a gas phase, where the particles can range from molecules up to 100 mm in size. Aerosol

NANOPARTICLES 3

generation is driven by the pressure differential created with the help of compressed gases, vacuum, mechanical oscillations, or electrostatic forces acting on liquid. Aerosols were used in industrial manufacturing long before the basic science and engineering of the aerosols were understood. For example, carbon black particles used in pigments and reinforced car tires are produced by hydrocarbon combustion; titania pigment for use in paints and plastics is made by oxidation of titanium tetrachloride; fumed silica and titania formed from respective tetrachlorides by flame pyrolysis; and optical fibers are manufactured by a similar process. Aerosols are also widely used as a drug delivery technique.

Solvent Drying

This technique is frequently used to generate particles of soluble materials (9). Starting materials, for example, a drug and a stabilizing polymer, are dissolved in waterimmiscible organic solvent, which is used to prepare an oil- in-water microemulsion. Water can be evaporated by heating under reduced pressure, leaving behind drug-loaded nanoparticles. Both nanospheres (uniform distribution of components) and nanocapsules (polymer encapsulated core) can be created with this method. A monomer can be used instead of the polymer, if the micelle polymerization step is possible. Solvent drying can be achieved via spray-drying step, where a homogeneous solution is fed to an aerosol generator, which produces uniformly sized droplets containing equal amounts of dissolved material. Solvent evaporation from the droplets under the right conditions would result in the formation of nanoparticles with a narrow size distribution.

Electro-Spinning

An emerging technology for the manufacture of thin polymer fibers is based on the principle of spinning dilute polymer solutions in a high-voltage electric field.

Electro-spinning is a process by which a suspended drop of polymer is charged with thousands of volts. At a characteristic voltage, the droplet forms a Taylor cone (the most stable shape with an apex angle of about 578), and a fine jet of polymer releases from the surface in response to the tensile forces generated by the interaction of an applied electric field with the electrical charge carried by the jet. This produces a bundle of polymer fibers. The jet can be directed to a grounded surface and collected as a continuous web of fibers ranging in size from a few micrometers to less than 100 nm.

Self-Assembly (10)

The appropriate molecular building blocks can act as parts of a jigsaw puzzle that join in a perfect order without obvious driving force present. Various types of chemical bonding can be used to self-assemble nanoparticles. For example, electrostatic interaction between the oppositely charged polymers can be used to build multilayered nanocapsules, the difference in hydrophobicity between the different molecules in the mixture can lead to a formation of a 3D assembly, and proteins can be selected to self-assemble

4 NANOPARTICLES

into virus like nanoparticles. Another example is a use of artificially created oligonucleotides that can be designed to assemble in a variety of shapes and forms.

Nanoparticle Surface Treatment (11)

‘‘Bare’’ nanoparticles of the same material would rapidly agglomerate with each other, forming bulk material. Encapsulating nanoparticles after production helps to maintain particle size and particle size distribution by inhibiting particle growth that can be caused by evaporation/redeposition, dissolution/reprecipitation, or surface migration and/or flocculation/aggregation/agglomeration. Encapsulation quenches the particle’s reactivity and reduces degradation of either the particle or the matrix that surrounds it. The encapsulating coating may be functionalized to facilitate dispersion into organic or aqueous liquid systems. Surface treatment with functional groups enables direct interaction between nanoparticles and resins. Typical functional groups are as follows:

Acrylate

Epoxide

Amine

Vinyl

Isocyanate

The stability ofthe collected nanoparticlepowders against agglomeration, sintering, and compositional changes can be ensured by collecting the nanoparticles in liquid suspension. For semiconductor particles, stabilization of the liquid suspension has been demonstrated by the addition of polar solvent; surfactant molecules have been used to stabilize the liquid suspension of metallic nanoparticles. Alternatively, inert silica encapsulation of nanoparticles by gas-phase reaction and by oxidation in colloidal solution has been shown to be effective for metallic nanoparticles.

When nanosized powders are dispersed in water, they aggregate due to attractive van der Waals forces. By altering the dispersing conditions, repulsive forces can be introduced between the particles to eliminate these aggregates. There are two ways of stabilizing nanoparticles. First, by adjusting the pH of the system, the nanoparticle surface charge can be manipulated such that an electrical double layer is generated around the particle. Overlap of two double layers on different nanoparticles causes repulsion and hence stabilization. The magnitude of this repulsive force can be measured via the zeta potential. The second method involves the adsorption of polymers onto the nanoparticles, such that the particles are physically prevented from coming close enough for the van der Waals attractive force to dominate; this is termed steric stabilization. The combination of two mechanisms is called electrosteric stabilization; it occurs when polyelectrolytes are adsorbed on the nanoparticle surface.

APPLICATIONS (12)

The fact that nanoparticles exist in the same size domain as proteins makes nanomaterials suitable for biotagging and

Figure 3. Schematic representation of an example of a biofunctionalized nanoparticle containing a magnetic core coated with a fluorescent layer, which, in turn, is coated by a thin protecting layer (e.g., silica). Linker molecules are attached to the protective layer at one end and to various functional molecules at the other.

payload delivery. However, size is just one of many characteristics of nanoparticles that it is rarely sufficient if one is to use nanoparticles as biological tags. To interact with a biological target, a biological or molecular coating or layer acting as a bioinorganic interface should be attached to the nanoparticle. Examples of biological coatings may include antibodies, biopolymers like collagen, or monolayers of small molecules that make the nanoparticles biocompatible. In addition, as optical detection techniques are widespread in biological research, nanoparticles should either fluoresce or change their optical properties. The approaches used in constructing nano-biomaterials are schematically presented below (Fig. 3). Nanoparticle usually forms the core of nano-biomaterial. It can be used as a convenient surface for molecular assembly and may be composed of inorganic or polymeric materials. It can also be in the form of a nano-vesicle, surrounded by a membrane or a layer. The shape is more often spherical, but cylindrical, plate-like, and other shapes are possible. The size and size distribution might be important in some cases, for example, if penetration through a pore structure of a cellular membrane is required. The size and size distribution are becoming extremely critical when quantum-sized effects are used to control material properties. A tight control of the average particle size and a narrow distribution of sizes allow creation of very efficient fluorescent probes that emit narrow light in a very wide range of wavelengths. This helps with creating biomarkers with many well-distinguished colors. The core might have several layers and be multifunctional. For example, combining magnetic and luminescent layers, one can both detect and manipulate the particles. The core particle is often protected by several monolayers of inert material, for example, silica. Organic molecules that are adsorbed or chemisorbed on the surface of the particle are also used for this purpose. The same layer might act as a biocompatible material. However, more often, an additional layer of linker molecules is required to proceed with further functionalization. This linear linker molecule has reactive

groups at both ends. One group is aimed at attaching the linker to the nanoparticle surface, and the other is used to bind various moieties like biocompatibles (dextran), antibodies, and fluorophores, depending on the function required by the application.

Some current applications of nanomaterials to biology and medicine are listed as follows:

Biological tags or labels: Mainly gold colloids are used for both electron and light microscopy; also, silver and silver-coated gold nanoparticles are used. A recent addition of semiconductor nanocrystals or quantum dots is finding increasing application as a substitute for the organic fluorophores. Greater photo-stability and the single-wavelength excitation option are among the quoted benefits. Badly designed quantum dots might disintegrate, releasing toxic components (e.g., cadmium or arsenic) that could be lethal at a cellular level. A use of porous silicon nanoparticles as fluorescent tags might be a safer option. Both quantum dots and colloidal gold were used recently for the detection of pathogens, proteins, and for the DNA sequencing. Colloidal gold and, more recently, quantum dots, have also been employed in phagokinetic studies.

Drug delivery: Mainly use polymeric nanoparticles

(13)because of their stability in biological fluids. Flexibility offered by a wide choice of polymers helps to control the rates of drug release and particle biodegradation. Polymeric nanoparticles can be used for all possible administration routes. Surface modifications allow creation of ‘‘stealth’’ as well as targeted drug carriers. Two major approaches are used in their preparation: from polymers (e.g., polyesters) or from monomers (e.g., alkylcyanoacrylate). Either solid nanospheres or liquid core nanocapsules can be produced. The fabrication technologies can be based on the solvent evaporation from oil-in-water microemulsions, created with help of surfactants, or by polymer precipitation caused by the addition of the nonsolvent. Another trend is to produce nanoparticles out of the poorly soluble drugs. Size reduction (nanosizing) (14) can significantly prolong drug bioavailability, increase its dissolution rate and maximum concentration, and dramatically shorten the onset of the drug action.

MRI contrast enhancement: MRI detects the spatial distribution of the signals from water protons inside the body. These signals depend on the local amount of water and the proton relaxation times T1 and T2. The relaxation times can be affected by several factors; they can also be shortened by the presence of paramagnetic molecules or particles. The T1 shortening would result in the increased signal intensity, whereas the T2 shortening would lead to the opposite effect. These effects are nonlinear functions of the concentration of the contrasting agent.

Superparamagnetic iron oxide (SPIO), whose average particle size is 50 nm, with dextrane or siloxane coating, is used as a tissue-specific contrast agent

NANOPARTICLES 5

(Feridex, Endorem, GastroMARK, Lumirem, Resovist); ultra-small superparamagnetic oxide particles, 10 nm (Sinerem, Combidex), are used to distinguish between the metastatic and inflamed lymph nodes, and to identify arteriosclerosis plaque deposits. Intravenously injected SPIO would pass through the vascular endothelium into the interstitium. After that, the SPIO would be taken up by both healthy and inflamed lymph nodes. The uptake is followed by phagocytosis. Normal lymph nodes show a decrease in signal intensity on T2*- and T2-weighted MR images because of the effects of magnetic susceptibility and T2 shortening on the iron deposits that are a direct result of phagocytosis. However, metastatic lymph nodes are bad at phagocytosis, form no deposits, and do not show such reduction in the signal intensity. This effect can be used to distinguish between the healthy and the benign lymph nodes.

Separation and purification of biomolecules and cells: Dynabeads are highly uniform in size and superparamagnetic; depending on the antibodies present on the surface, the beads can be applied to different tasks like separation of T-cells, detection of microbes and protozoa, HLA diagnostics for organ transplantation, and various in vitro diagnostics assays. They are also used for the isolation of DNA and proteins.

Tissue engineering: Nanoparticles of hydroxyapatite are used to mimic the mineral particles occurring in the bone structure, whereas collagen is often replaced with a 3D porous scaffolding of a biodegradable polymer. This approach allows for the high mobility of the osteoblasts and, consequently, a uniform growth of the new bone. A similar strategy is used to promote the cellular growth on the surface of prosthetic implants.

Tumor destruction via heating (hyperthermia): The nanoparticle approach is currently relying on the higher metabolic rates and enhanced blood supply to the tumors. As a result, the cancerous cells are likely to be enriched in a nanoparticulate matter, introduced in the blood circulation, or directly injected into the tumor. An external electromagnetic energy source is directed toward the tumor. The nanoparticles are designed to absorb the electromagnetic energy and convert it into localized heat, which would preferentially cause the apoptosis of the malignant cells. Localized heating might also result in the increased acidosis of the cancer cells. It is also been suggested that the high density of blood vessels in and around the growth stops them from expanding as efficiently as those in the healthy tissue, leading to a higher heat retention. The range of temperatures used is typically within 39 to 43 8C. Alternating magnetic fields can be used to heat up magnetic iron oxide nanoparticles concentrated inside the tumor tissue. More recently developed nanoshells rely on the illumination with a near-infrared laser. Nanoparticles can be designed to actively target the surface receptors on the malignant cells by coating the nanoparticles with the appropriate antibodies.

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Recent Developments

Tissue Engineering. Natural bone surface often contains features that are about 100 nm across. If the surface of an artificial bone implant was left smooth, the body would try to reject it. So the smooth surface is likely to cause production of a fibrous tissue covering the surface of the implant. This layer reduces the bone-implant contact, which may result in loosening of the implant and further inflammation. It was demonstrated that by creating nanosized features on the surface of the hip or knee prosthesis, one could reduce the chances of rejection as well as stimulate the production of osteoblasts. The osteoblasts are the cells responsible for the growth of the bone matrix and are found on the advancing surface of the developing bone. The effect was demonstrated with polymeric, ceramic, and more recently, metal materials. More than 90% of the human bone cells from suspension adhered to the nanostructured metal surface, but only 50% did in the control sample. In the end, this finding would allow the design of a more durable and longer lasting hip or knee replacement and reduce the chances of the implant getting loose. Titanium is a well-known bone repairing material widely used in orthopedics and dentistry. It has a high fracture resistance, ductility, and weight-to-strength ratio. Unfortunately, it suffers from the lack of bioactivity, as it does not support cell adhesion and growth well. Apatite coatings are known to be bioactive and to bond to the bone. Hence, several techniques were used to produce an apatite coating on titanium. Those coatings suffer from thickness nonuniformity, poor adhesion, and low mechanical strength. In addition, a stable porous structure is required to support the nutrients’ transport through the cell growth. It was shown that using a biomimetic approach — a slow growth of nanostructured apatite film from the simulated body fluid — resulted in the formation of a strongly adherent, uniform nanoporous layer. The layer was found to be built of 60 nm crystallites and possess a stable nanoporous structure and bioactivity. A real bone is a nanocomposite material, composed of hydroxyapatite crystallites in the organic matrix, which is mainly composed of collagen. Thanks to that, the bone is mechanically tough and plastic, so it can recover from mechanical damage. The actual nanoscale mechanism leading to this useful combination of properties is still debated. An artificial hybrid material was prepared from 15 to 18 nm ceramic nanoparticles and poly (methyl methacrylate) copolymer. Using the tribology approach, a viscoelastic behavior (healing) of the human teeth was demonstrated. An investigated hybrid material, deposited as a coating on the tooth surface, improved scratch resistance as well as possessed a healing behavior similar to that of the tooth.

Cancer Therapy. Photodynamic cancer therapy is based on the destruction of the cancer cells by laser-generated atomic oxygen, which is cytotoxic. A greater quantity of a special dye that is used to generate the atomic oxygen is taken in by the cancer cells when compared with a healthy tissue. Hence, only the cancer cells are destroyed and then exposed to a laser radiation. Unfortunately, the remaining dye molecules migrate to the skin and the eyes and make the patient very sensitive to the daylight exposure. This

effect can last for up to 6 weeks. To avoid this side effect, the hydrophobic version of the dye molecule was enclosed inside a porous nanoparticle. The dye stayed trapped inside the Ormosil nanoparticle and did not spread to the other parts of the body. At the same time, its oxygen-generating ability has not been affected and the pore size of about 1 nm freely allowed for the oxygen to diffuse out.

Another recently suggested approach is to use goldcoated nanoshells. A surface plasmon resonance effect causes an intense size-dependent absorbance of the near-infrared light by the gold shells. This wavelength of light falls into the optical transparency window of the biological tissue, which can be used to detect cancerous growth up to a certain depth. Compact, powerful, and relatively inexpensive semiconductor lasers are readily available to generate light at this wavelength. The nanoshells are injected into a blood stream and rapidly taken up by the cancerous cells, as they possess a higher metabolism rate. Laser light is absorbed by the gold shells and converted into a local heating, which only kills the cancer cells and spares the healthy tissue. Surface-modified carbon nanotubes can also be used for the same purpose as they would absorb light in the infrared region and convert it into heat.

Multicolor Optical Coding for Biological Assays. The ever increasing research in proteomics and genomic generates an escalating number of sequence data and requires development of high throughput screening technologies. Realistically, various array technologies that are currently used in parallel analysis are likely to reach saturation when several array elements exceed several millions. A three-dimensional approach, based on optical ‘‘bar coding’’ of polymer particles in solution, is limited only by the number of unique tags one can reliably produce and detect. Single quantum dots of compound semiconductors were successfully used as a replacement of organic dyes in various bio-tagging applications. This idea has been taken one step further by combining differently sized and, hence, having different fluorescent colors quantum dots, and combining them in polymeric microbeads. A precise control of quantum dot ratios has been achieved. The selection of nanoparticles used in those experiments had 6 different colors as well as 10 intensities. It is enough to encode over one million combinations. The uniformity and reproducibility of beads was high, allowing for bead identification accuracies of 99.99%.

Manipulation of Cells and Biomolecules. Fictionalized magnetic nanoparticles have found many applications, including cell separation and probing; these and other applications are discussed in a recent review. Most of the magnetic particles studied so far are spherical, which somewhat limits the possibilities to make these nanoparticles multifunctional. Alternative cylindrically shaped nanoparticles can be created by employing metal electrodeposition into a nanoporous alumina template. Depending on the properties of the template, the nanocylinder radius can be selected in the range of 5 to 500 nm while their length can be as big as 60 mm. By sequentially depositing various thicknesses of different metals, the

structure and the magnetic properties of individual cylinders can be tuned widely. As surface chemistry for functionalization of metal surfaces is well developed, different ligands can be selectively attached to different segments. For example, porphyrins with thiol or carboxyl linkers were simultaneously attached to the gold or nickel segments, respectively. Thus, it is possible to produce magnetic nanowires with spatially segregated fluorescent parts. In addition, because of the large aspect ratios, the residual magnetization of these nanowires can be high. Hence, the weaker magnetic field can be used to drive them. It has been shown that a self-assembly of magnetic nanowires in suspension can be controlled by weak external magnetic fields. This would potentially allow controlling cell assembly in different shapes and forms. Moreover, an external magnetic field can be combined with a lithographically defined magnetic pattern (‘‘magnetic trapping’’).

Protein Detection. Proteins are the important part of the cell’s language, machinery, and structure, and understanding their functionalities is extremely important for further progress in human well-being. Gold nanoparticles are widely used in immunohistochemistry to identify the protein–protein interaction. However, the multiple simultaneous detection capabilities of this technique are limited. Surface-enhanced Raman scattering spectroscopy is a wellestablished technique for detection and identification of single dye molecules. By combining both methods in a single nanoparticle probe, one can drastically improve the multiplexing capabilities of protein probes. The group of Prof. Mirkin has designed a sophisticated multifunctional probe that is built around a 13 nm gold nanoparticle. The nanoparticles are coated with hydrophilic oligonucleotides containing a Raman dye at one end and terminally capped with a small molecule recognition element (e.g., biotin). Moreover, this molecule is catalytically active and will be coated with silver in the solution of Ag(I) and hydroquinone. After the probe is attached to a small molecule or an antigen it is designed to detect, the substrate is exposed to silver and hydroquinone solution. Silver plating is happening close to the Raman dye, which allows for dye signature detection with a standard Raman microscope. Apart from being able to recognize small molecules, this probe can be modified to contain antibodies on the surface to recognize proteins. When tested in the protein array format against both small molecules and proteins, the probe has shown no cross-reactivity.

Commercial Exploration

Some companies involved in the development and commercialization of nanomaterials in biological and medical applications are listed below (Table 1). Most of the companies are small recent spinouts of various research institutions. Although not exhausting, this is a representative selection reflecting current industrial trends. Most companies are developing pharmaceutical applications, mainly for drug delivery. Several companies exploit quantum size effects in semiconductor nanocrystals for tagging biomolecules or use bioconjugated gold nanoparticles for labeling various cellular parts. Many companies are applying nano-

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ceramic materials to tissue engineering and orthopedics. Most major and established pharmaceutical companies have internal research programs on drug delivery that are on formulations or dispersions containing components down to nanosizes. Colloidal silver is widely used in antimicrobial formulations and dressings. The high reactivity of titania nanoparticles, either on their own or then illuminated with UV light, is also used for bactericidal purposes in filters. Enhancedcatalytic propertiesof surfaces ofnano-ceramicsor those of noble metals like platinum are used to destruct dangerous toxins and other hazardous organic materials.

Future Directions

As it stands, most commercial nanoparticle applications in medicine are geared toward drug delivery. In the biosciences, nanoparticles are replacing organic dyes in the applications that require high photo-stability as well as high multiplexing capabilities. There are some developments in directing and remotely controlling the functions of nanoprobes, for example, driving magnetic nanoparticles to the tumor and then making them either to release the drug load or just heating them to destroy the surrounding tissue. The major trend in further development of nanomaterials is to make them multifunctional and controllable by external signals or by local environment, thus essentially turning them into nanodevices.

HEALTH ISSUES (15)

It has been shown by several researchers that nanomaterials can enter the human body through several ports. Accidental or involuntary contact during production or use is most likely to happen via the lungs from where a rapid translocation through the blood stream is possible to other vital organs. On the cellular level, an ability to act as a gene vector has been demonstrated for nanoparticles. Carbon black nanoparticles have been implicated in interfering with cell signaling. There is work that demonstrates uses of DNA for the size separation of carbon nanotubes. The DNA strand just wraps around it if the tube diameter is right. Although excellent for separation purposes, it raises some concerns over the consequences of carbon nanotubes entering the human body.

Ports of Entry

Human skin, intestinal tract, and lungs are always in direct contact with the environment. Whereas skin acts as a barrier, lungs and intestinal tract also allow transport (passive and/or active) of various substances like water, nutrients, or oxygen. As a result, they are likely to be a first port of entry for the nanomaterials’ journey into the human body. Our knowledge in this field mainly comes from drug delivery (pharmaceutical research) and toxicology (xenobiotics) studies.

Human skin functions as a strict barrier, and no essential elements are taken up through the skin (except radiation necessary to buildup vitamin D). The lungs exchange oxygen and carbon dioxide with the environment, and some water escapes with warm exhaled air. The intestinal tract

is in close contact with all of the materials taken up orally; here all nutrients (except gases) are exchanged between the body and the environment.

The histology of the environmental contact sides of these three organs is significantly different. The skin of an adult human is roughly 1.5 m2; in area and is at most places covered with a relatively thick first barrier (10 mm), which is built of strongly keratinized dead cells. This first barrier is difficult to pass for ionic compounds as well as for water-soluble molecules.

The lung consists of two different parts: airways (transporting the air in and out the lungs) and alveoli (gas exchange areas). Our two lungs contain about 2,300 km of airways and 300 million alveoli. The surface area of the lungs is 140 m2 in adults, as big as a tennis court. The airways are a relatively robust barrier, an active epithelium protected with a viscous layer of mucus. In the gas exchange area, the barrier between the alveolar wall and the capillaries is very thin. The air in the lumen of the alveoli is just 0.5 mm away from the blood flow. The large surface area of the alveoli and the intense air–blood contact in this region makes the alveoli less well protected against environmental damage when compared with airways.

The intestinal tract is a more complex barrier and exchange side; it is the most important portal for macro-

molecules to enter the body. From the stomach, only small molecules can diffuse through the epithelium. The epithelium of the small and large intestines is in close contact with ingested material so that nutrients can be used. A mixture of disaccharides, peptides, fatty acids, and monoglycerides generated by digestion in the small intestine are further transformed and taken in. The area of the gastrointestine tract (GIT) is estimated as 200 m2.

Lung. Most nanosized spherical solid materials will easily enter the lungs and reach the alveoli. These particles can be cleared from the lungs, as long as the clearance mechanisms are not affected by the particles or any other cause. Nanosized particles are more likely to hamper the clearance, resulting in a higher burden, possibly amplifying any possible chronic effects caused by these particles. It is also important to note that the specific particle surface area is probably a better indication for maximum tolerated exposure level than total mass. Inhaled nanofibers (diameter smaller than 100 nm) also can enter the alveoli. In addition, their clearing would depend on the length of the specific fiber. Recent publications on the pulmonary effects of carbon nanotubes confirm the intuitive fear that the nanosized fiber can induce a general nonspecific pulmonary response. Passage of solid material from the pulmonary

epithelium to the circulation seems to be restricted to nanoparticles. The issue of particle translocation still needs to be clarified: both the trans-epithelial transport in the alveoli and the transport via nerve cells. Thus, the role of factors governing particle translocation such as the way of exposure, dose, size, surface chemistry, and time course should be investigated. For instance, it would be also very important to know how and to what extent lung inflammation modulates the extrapulmonary translocation of particles. Solid inhaled particles are a risk for those who suffer from cardiovascular disease. Experimental data indicate that probably many inhaled particles can affect cardiovascular parameters, via pulmonary inflammation. Nanosized particles, after passage in the circulation, can also play a direct role in, e.g., thrombogenesis.

Intestinal Tract. Already in 1926, it was recognized by Kumagai that particles could translocate from the lumen of the intestinal tract via aggregations of intestinal lymphatic tissue [Peirel’s Patches (PP)] containing M-cells (specialized phagocytic enterocytes). Particulate uptake happens not only via the M-cells in the PP and the isolated follicles of the gut-associated lymphoid tissue, but also via the normal intestinal enterocytes. There have been several excellent reviews on the subject of intestinal uptake of particles. Uptake of inert particles has been shown to occur trans-cellular through normal enterocytes and PP via M- cells and, to a lesser extent, across paracellular pathways. Initially it was assumed that the PP did not discriminate strongly in the type and size of the absorb particles. Later it has been shown that modified characteristics, such as particle size, the surface charge of particles, attachment of ligands, or coating with surfactants, offers possibilities of site-specific targeting to different regions of the GIT including the PP.

The kinetics of particle translocation in the intestine depends on diffusion and accessibility through mucus, initial contact with enterocyte or M-cell, cellular trafficking, and post-translocation events. Cationic nanometersized particles became entrapped in the negatively charged mucus, whereas negatively charged nanoparticles can diffuse across this layer. The smaller the particle diameter, the faster they could permutate the mucus to reach the colonic enterocytes. Once in the sub-mucosal tissue, particles can enter both lymphatic and capillaries. Particles entering the lymphatic are probably important in the induction of secretory immune responses, whereas those that enter the capillaries become systemic and can reach different organs. It has been suggested that the disruption of the epithelial barrier function by apoptosis of enterocytes is a possible trigger mechanism for mucosal inflammation. The patho-physiological role of M-cells is unclear; for example, it has been found that in Crohn’s disease, M-cells are lost from the epithelium. Diseases other than of gut origin, for example, diabetes, also have marked effects on the ability of the GIT to translocate particles. In general, the intestinal uptake of particles is understood better and studied in more detail than pulmonary and skin uptake. Because of this advantage, it is maybe possible to predict the behavior of some particles in the intestines.

NANOPARTICLES 9

Skin. Skin is an important barrier, protecting against insult from the environment. The skin is structured in three layers: the epidermis, the dermis, and the subcutaneous layer. The outer layer of the epidermis, the stratum corneum (SC), covers the entire outside of the body. In the SC we find only dead cells, which are strongly keratinized. For most chemicals, the SC is the rate-limiting barrier to percutaneous absorption (penetration). The skin of most mammalian species is covered with hair.

At the sites where hair follicles grow, the barrier capacity of the skin differs slightly from the ‘‘normal’’ stratified squamous epidermis. Most studies concerning penetration of materials into the skin have focused on whether drugs penetrate through the skin using different formulations containing chemicals and/or particulate materials as a vehicle. The main types of particulate materials commonly used in contact with skin are liposomes, solid poorly soluble materials such as TiO2, and polymer particulates and submicron emulsion particles such as solid lipid nanoparticles. TiO2 particles are often used in sunscreens to absorb UV light and therefore to protect skin against sunburn or genetic damage. It has been reported by Lademann et al. that micrometer-sized particles of TiO2 get through the human stratum corneum and even into some hair follicles, including their deeper parts. However, the authors did not interpret this observation as penetration into living layers of the skin. In a recent review, it was stated that ‘‘very small titanium dioxide particles (e.g. 5–20 nm) penetrate into the skin and can interact with the immune system.’’ Unfortunately, this has not been discussed any further.

Penetration of nonmetallic solid materials such as biodegradable poly(D,L-lactic-co-glycolic acid (PLGA)) microparticles, 1 to 10 mm with a mean diameter of 4.61 0.8 mm, were studied after application on to porcine skin. The number of microparticles in the skin decreased with the depth (measured from the airside toward the subcutaneous layer). At 120 mm depth (where viable dermis is present), a relative high number of particles was found; at 400 mm (dermis), some microparticles were still observed. At a depth of 500 mm, no microparticles were found. In the skin of persons, who had an impaired lymphatic drainage of the lower legs, soil microparticles, frequently 0.4-0.5 mm but as larger particles of 25 mm diameter, were found in the dermis of the foot in a patient with endemic elephantiasis. The particles are observed to be in the phagosomes of macrophages or in the cytoplasm of other cells. The failure to conduct lymph to the node produces a permanent deposit of silica in the dermal tissues (a parallel is drawn with similar deposits in the lung in pneumoconiosis). This indicates that soil particles penetrate through (damaged) skin, most probably in every person, and normally are removed via the lymphatic system.

Liposomes penetrate the skin in a size-dependent manner. Microsized, and even submicron sized, liposomes do not easily penetrate into the viable epidermis, whereas liposomes with an average diameter of 272 nm can reach into the viable epidermis and some are found in the dermis. Smaller sized liposomes of 116 and 71 nm were found in higher concentration in the dermis. Emzaloid particles, a type of submicron emulsion particle such as liposomes and nonionic surfactant vesicles (niosomes), with a diameter of

10 NANOPARTICLES

50 nm to 1 mm, were detected in the epidermis in association with the cell membranes after application to human skin. The authors suggested that single molecules, which make up the particles, might penetrate the intercellular spacesand,at certain regions in the stratum corneum, can accumulate and reformintomicrospheres.Ina subsequent experiment,itwas shown that the used formulation allowed penetration of the spheres into melanoma cells, even to the nucleus.

From the limited literature on nanoparticles penetrating the skin, some conclusions can be drawn. First, penetration of the skin barrier is size dependent, and nanosized particles are more likely to enter more deeply into the skin than larger ones. Second, different types of particles are found in the deeper layers of the skin, and currently, it is impossible to predict the behavior of a particle in the skin. Third, materials, which can dissolve or leach from a particle (e.g., metals), or break into smaller parts (e.g., Emzaloid particles), can possibly penetrate into the skin.

Currently, there is no direct indication that particles, that had penetrated the skin also entered the systemic circulation. The observation that particles in the skin can be phagocytized by macrophages, Langerhan cells, or other cells is a possible road toward skin sensitization.

Summary of Health Risks

Particles in the nanosize range can certainly enter the human body via the lungs and the intestines; penetration via the skin is less evident. It is possible that some particles can penetrate deep into the dermis. The chances of penetration would depend on the size and surface properties of the particles and on the point of contact in the lung, intestines, or skin.

After penetration, the distribution of the particles in the body is a strong function of the surface characteristics of the particle. It seems that size can restrict the free movement of particles. The target organ-tissue or cell of a nanoparticle needs to be investigated, particularly in the case of potentially hazardous compounds. Before developing an in vitro test, it is essential to know the pharmacokinetic behavior of different types of nanoparticles; therefore, it would be important to compose a database of health risks associated with different nanoparticles.

Beside the study of the health effects of the nanomaterials, investigations should also take into consideration the presence of contaminates, such as metal catalysts present in nanotubes and their role in the observed health effects.

The increased risk of cardiopulmonary diseases requires specific measures to be taken for every newly produced or used nanoparticle. There is no universal ‘‘nanoparticle’’ to fit all cases; each nanomaterial should be treated individually when health risks are expected. The tests currently used to test the safety of materials should be applicable to identify hazardous nanoparticles.

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FURTHER READING

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Moriarty P. Nanostructured materials. Rep Prog Phys 2001;64: 297–381.

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Bourgeat-Lami E. Organic-inorganic nanostructured colloids. J Nanosci Nanotechnol 2002;2:1–24.

Fendler JH. Colloid chemical approach to nanotechnology. Korean J Chem Eng 2001;18:1–13.

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1997;2:188–191.

Shimomura M, Sawadaishi T. Bottom-up strategy of materials fabrication: a new trend in nanotechnology of soft materials.

Curr Opin Colloid In 2001;6:11–16.

Tomalia DA, Wang ZG, Tirrell M. Experimental self-assembly: the many facets of self-assembly. Curr Opin Colloid In 1999;4:3–5.

Ullmann M, Friedlander SK, Schmidt-Ott A. Nanoparticle forma-

tion by laser ablation. J Nanoparticle Res 2002;4:499–509. Yu SH. Hydrothermal/solvothermal processing of advanced cera-

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Biological and Medical Applications

de la Isla A, Brostow W, Bujard B, Estevez M, Rodriguez JR, Vargas S, Castano VM. Nanohybrid scratch resistant coating