What are Nanoparticles?
Nanoparticles are particles which have two or more dimensions in the size range of 1 to 100 nanometers. They have unique chemical and physical properties compared to solid bulk materials, as they have a large surface area and strong electronic properties.
These particles can be used in many different areas, including analytical testing, forensics, drug discovery, biological and chemical threat detection, imaging, diagnostics, biotechnology and biomedical sensing. Nanoparticles can be used as nanotags as a way of labelling and authenticating different objects such as banknotes for security, and identifying fraud during the transportation of goods.
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Nanoparticles are also commonly used in colloidal solutions to enhance Raman spectroscopy. The size and shape of nanoparticles have been shown to affect the enhancement. Nanospheres are the most common shape of nanoparticles, but other shapes such as nanostars, nanocubes, nanorods and nanowires can be produced through a polymer-mediated polyol process. Nanoparticles can also be capped or hollowed using various chemical methods. For a more accurate spread for detection, nanoparticles can be deposited or spin-coated onto multiple surfaces .
Metallic nanoparticles are nanoparticles from a variety of different metals, the nanoscale size causes electron confinement in metal nanoparticles, which results in surface plasmon resonance. Plasmonic gold and silver nanoparticles are the most commonly used nanoparticles due to their unique physical properties.
Gold Nanoparticles
Gold nanoparticles were discovered over twenty years ago, but metals such as silver and copper have also since been explored. Copper is not as popular as gold or silver because gold and silver are less reactive and more stable in air than copper. Gold nanoparticles are widely used in biotechnology and in the biomedical field due to their large surface area and high level of conductivity.
Small gold nanoparticles of roughly 30 nanometers (nm) absorb light in the blue to green range of the spectrum (450nm) and reflect red light. As particle size increases, the wavelength of surface plasmon resonance shifts to longer wavelengths with a darker red colour, meaning that blue light is reflected. When salt is added to nanoparticle solutions, the surface charge becomes neutral and causes particles to aggregate and change the colour of the solution from red to blue.
Gold nanoparticles can be synthesized by a variety of different techniques that are chemical, physical or biological. The most common method for making colloidal gold is by a chemical citrate reduction method, but gold nanoparticles can also be grown by being encapsulated and immersed in polyethylene glycol dendrimers before being reduced by formaldehyde under near infra-red treatment. Gold nanoparticles can also be produced via γ-irradiation using polysaccharide alginate as stabilizer, and photochemical reduction. A relatively new biological method can be used to make gold nanoparticles by dissolving gold in sodium chloride solution, using natural chitosan without any stabilizer and reductant.
Gold nanoparticles are non-toxic particles with large surface areas that can be modified with other molecules to be used in biomedical fields. Gold nanorods are a type of gold nanoparticle that are frequently used for invivo cell imaging. Due to their small size, it is easy to introduce the nanoparticles into tissues and cells.
Silver Nanoparticles
Silver nanoparticles are highly commercial due to properties such as good conductivity, chemical stability, catalytic activity, and their antimicrobial activity. Due to their properties, they are commonly used in medical and electrical applications.
Silver nanoparticles optical properties are also dependent on the nanoparticle size. Smaller nanospheres absorb light and have peaks near to 400 nm, and larger nanoparticles have increased scattering to gives peaks that broaden and shift towards longer wavelengths. Larger shifts into the infrared region of the electromagnetic spectrum are achieved by changing the nanoparticles shape to rods or plates.
Silver nanoparticles can also be synthesised chemically, physically or biologically. Silver nanoparticles are chemically produced using the polyol process, which uses a polyvinylpyrrolidone (PVP) polymer with AgNO3, and ethylene glycol as the reducing agent. Size and shapes of the silver nanoparticles can be altered based on the molar ratio of AgNO3 and PVP.
Physical methods such as condensation, evaporation, spark discharging and pyrolysis are used to produce silver nanoparticles, but they produce a low yield of nanoparticles and have a high energy consumption compared to other methods. The biological method of preparing silver nanoparticles uses green plants as stabilizing and reducing agents.
As a result of their antimicrobial and properties, silver nanoparticles are used in the treatment of microbes such as bacteria, fungi and viruses. Silver nanowires (a form in which silver nanoparticles are used) are being studied for their use in advanced technological applications. Silver nanoparticles can also be used for colloidal coating and in paints, and are frequently used in textiles, keyboards, wound dressings, and biomedical devices.
Characterization
Both gold and silver nanoparticles can be characterized by using microscopy, spectroscopy and X-ray crystallography. Microscopy techniques include atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electron microscopy. Spectroscopy techniques include UV–Vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR).
The characterization techniques can be used to determine the size, shape, crystallinity, and surface area of the nanoparticles. The shape and particle size can be analysed using a variety of techniques such as SEM, TEM and AFM. AFM can also be used to determine particle height and volume with three-dimensional images. Particle size distribution can be assessed by dynamic light scattering, and crystallinity determined by X-ray diffraction.
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