Metallic nanoparticles are, as the name suggests, nanosized metals with at least one of the dimensions within 1 to 100 nm. Under this definition one can classify metallic nanomaterials in to four main categories. They are, metallic nanoparticles (0D), metallic nanowires and rods (1D), metallic sheets and platelets (2D) metallic nanostructures (3D). The most extensively explored metallic nanoparticles are gold and silver. However, scientists have developed metal nanoparticles with almost all stable metals in the periodic table.
Metallic nanomaterials have attracted large number of scientists over the years and has grown in to a distinct stream of research. This interest is primarily due number of advantageous properties these materials have shown that may unlock new pathways in the field of nanotechnology. Among others, the field of biomedical science and engineering has heavily utilized metallic nanomaterials. This is primarily due to the fact that these materials can be synthesized and modified with appropriate functional groups that would allow them to bind with drugs, antibodies, ligands: substances of high interest in biomedical field.
The earliest application of metallic nanoparticles is probably their use in stain glass manufacturing at the medieval era that was originally used to decorate cathedral windows. The first reported scientific exploration of metallic particles with small sizes was made by Michel Faraday in1857, who investigated the stability parameters of these particles. A scientific explanation to their color properties (absorbance wavelengths) was given by Mie in 1908. In the year of 1925, Richard Zsigmondy received the Nobel Prize in chemistry for his pioneering role in the field of colloid chemistry and for his invention of ultra-microscope for his studies in gold sols.
Synthesis and stabilization of metallic nanomaterials
Reduction of metallic ions in a solution using appropriate reduction agents is the most widely used method for producing nanoparticles. This is classified as a chemical method. However, there are several synthesis methods that have been developed for generation of metallic nanoparticles, such as vapor synthesis, sputtering, photochemical, electrochemical , radiolytic (x-ray assisted synthesis).
In most of these processes, metal ions are converted in to zerovelent metal atoms in a reducing environment. In the chemical synthesis, this is achieved by a reducing agents. Usually, in chemical synthesis method other chemicals are also added to increase the stability of the produced particles. The reduction potential of the reducing agents needed to be more negative than that of the ions to produce nanoparticles. Precursor concentration, temperature, reducing agent and solvent are the most critical control parameters in the chemical synthesis method.
Metallic nanoparticles have high tendency to aggregate and make bigger structures due to large surface energy that drives thermodynamically favored coalescence process. Bigger particles have low surface which leads to low surface energy. Therefore the natural tendency of these nanoparticles is to agglomerate and make bigger structures unless the particles are stabilized. Stabilization of metallic particles in a liquid medium is achieved by two main strategies.
In the first strategy particles are stabilized by electrical double layer formed by absorption of negatively charged ions to the metallic nanoparticles. This approach is referred to as static stabilization. This charged layer can repel individual nanoparticles from each other preventing further agglomeration. The strength of this electrical double layer is measured by a parameter call “Zeta potential”. Zeta potential can be tuned by solution pH, Ionic strength, temperature, etc to obtain required stability. However, this method will only work in polar liquids like water and ethanol which can dissolve electrolytes.
The second strategy is referred to as steric stabilization and involves capping of the metallic nanoparticles with an agent like polymer, surfactant or a ligand, typically with long alkyl chains to the particle surface. The long protruding chains of these organic molecules prevents individual particles from aggregation as it prevent pushes away any particles that are coming close to each other.
Origin of color and plasmonic absorption
Unlike their bulk counterparts, nanoscale metals can be tuned to obtain spectrum of colors. For an example, gold, a shiny, bright yellow color metal in the bulk scale, give out brilliant red or violet colors in an aqueous solution when the gold particles in the nanoscale. Origin of color of metallic nanoparticles is explained with an approach called localized surface plasmon resonance.
Metallic nanoparticles have a plasma made of its own electrons around the particles. This plasmon can resonate around the particle with a natural frequency that is determined by particle parameters such as material, shape, size and the dielectric nature of the surrounding environment. Light waves; the type of electromagnetic radiation that’s responsible for making color is also electrically and magnetically active as it has oscillating electric and magnetic field parallel to each other. Since the nanoparticles are much smaller than the wavelength of the light waves (380 – 760 nm) these waves can displace electron plasma cloud relative to the nuclei of the nanoparticle. When a metallic nanoparticle meets with incoming light waves, some energy is absorbed by the nanoparticles as light causes oscillation of the plasma cloud. The energy absorption is maximum when the frequency of the incoming light radiation is in the same frequency of the natural frequency of the electron plasma. This collective absorption of light waves from nanoparticles produce a color to a nanoparticle dispersion. Different colors can be seen in nanoparticles with different shapes and sizes depending on the respective visible light absorption pattern. After absorption, the surface plasmon decays radiatively resulting light scattering or nonradiatively through conversion of absorbed light energy in to heat. This is the introduction to localized surface plasmon resonance, in the simplest form.
Gold nanoparticles are nanosized particles of gold. They are most frequently synthesized and kept as a suspension in aqueous medium which is referred to as a colloidal suspension. Gold colloidal suspension with spherical particles in water shows an intense red color. This color is due to the localized surface plasmonic resonance. The gold nanoparticles around 10-20 nm diameter have strong absorption of light which peaks around 520 nm in aqueous solutions.
Gold nanoparticle have been extensively used in biomedical research and applications due to easy modification of the gold particles with the agents like therapeutic agents, drugs, ligands, etc. It is also used in imaging applications such as cancer imaging by selectively transporting gold nanoparticles in to the cancer cells. Moreover, the use of gold nanoparticle and nanorods are used in certain therapies such as photo thermal therapy and as biomarkers in the diagnosis of critical illnesses like cancers and heart diseases. Apart from the biomedical field, gold nanostructures are heavily exploited in areas such as sensors, electronic, microscopic, solar cell and fuel cell research.
Silver nanoparticle are one of the most heavily used nanomaterial in the world. They are also widely prepared and stored as a metal particle dispersion. Colloidal suspensions of spherical silver nanoparticles are bright yellow, typically showing maximum absorption around 420 nm. They are most commonly prepared by reduction of silver salt in to zerovelent state using a reducing agent.
Silver nanoparticles have gained tremendous acceptance in number of fields due to their unique optical electrical and thermal properties and range from solar cells to chemical sensors. The most common application of silver nanoparticles are as antimicrobial coatings, textiles, plastics, wound dressing and biomedical devices. These applications depend on low level release of silver ions to the surrounding environment that can kill most of the even toughest bacteria. The applications exploiting their electric properties are conductive inks, conductive fillers and pastes. These applications take use of high electrical conductivity with the oxidation stability and low sintering temperatures of silver nanoparticles.