Nanoparticles are nano-sized particles that have two or more dimensions between the size range of 1 to 100 nanometers. They have unique chemical and physical properties compared to solid bulk materials, as they have a high surface area and electronic properties. Image analysis is a set of techniques that extract meaningful information from images, and when combined with nanoparticle technology allows for the characterisation of nanoparticles. The ability to characterize structures formed by nanoparticle assembly is needed to be able to predict and engineering properties of nanocomposite materials.
Image Credits | shutterstock.com/g/anyaivanova
Nanoparticles can be split into various types depending on their shape, shape, material properties and if they are organic or inorganic nanoparticles. Organic nanoparticles include dendrimers and liposomes, whilst the bulk of nanoparticle research is focused around the inorganic quantum dots and metallic nanoparticles.
Nanoparticles possess physical properties such as uniformity, conductance and unique optical properties which make them desirable in both biology and materials science. Areas that nanoparticles are used in include 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 authentication of different objects such as banknotes for security and identifying fraud during transportation of goods.
In the medical field, nanoparticles are useful because they circulate widely throughout the body as well as entering and binding to specific cells. The properties of nanoparticles have allowed new ways to enhance images of organs, tumours and diseased tissues in the body.
Nanospheres are the most common shape for nanoparticles, but other shapes such as nanostars, nanorods, nanotubes 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 various surfaces.
A common type of nanoparticle is metallic nanoparticles in colloidal solutions. Metal nanoparticles are nanoparticles available in a various different metal types, as well as a variety of shapes and sizes. 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.
Types of imaging
Due to nanoparticles being smaller than the wavelength of light, they are undetectable with the human eye and only observable with optical microscopes in liquid samples under certain conditions. Instead, nanoparticles are analysed using electron microscopy, scanning probe microscopy and indirectly using X-Rays.
Electron microscopy uses an electron beam to create an image, with electromagnets acting as lenses. It allows for higher resolution and detail when scanning than optical microscopy due to the wavelength of the electrons being much smaller than that from a bulb or laser.
Two of the main types of nanoparticle image analysis are Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). TEM operates by sending a beam of electrons through a very thin sample and capturing the electrons that have passed through to create a detailed two-dimensional image. SEM operates by sending a beam of focused electrons to the sample and bouncing them off to create three-dimensional surface images. TEM is used to study the interior of a sample, whereas SEM is used to study the surface of a sample.
Scanning Probe Microscopy
Scanning probe microscopy scans the surface of samples with a probe to measure fine surface shapes and properties and generate an image. Types of scanning probe microscopy include atomic force microscopy (AFM), scanning tunnelling microscopy (STM) and near-field scanning optical microscopes (MSOM). AFM has a fine silicon or silicon nitride probe attached to a cantilever. STMs have metal tips with single apical atoms, with the tip being attached to a tube where the current flows. MSOM have probes consisting of a light source in an optical fibre, which is covered with a tip.
Indirect X-Ray Analysis
Types of indirect X-Ray analysis include small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), as well as the grazing incident (GI) surface-specific analogues known as GISAXS and GISANS, and X-ray or neutron reflectometry (XR/NR). The advantage of the X-Ray techniques includes being able to simultaneously sample and average large numbers of nanoparticles which do not require any sample preparation.
Nanoparticle Tracking Analysis
Using electron microscopy to analyse nanoparticles is time-consuming and expensive. A cheaper and faster alternative to microscopy is nanoparticle tracking analysis. Nanoparticle tracking analysis analyses nanoparticle suspensions by tracking light scattering nanoparticles in videos for characterization. Perpendicular scattering and observation arrangement are used by adding nanoparticle suspensions to a glass-cell are illuminated by a laser. The laser light is scattered by the particles and the generated diffraction patterns are recorded by a CCD camera. The Brownian motion of each single nanoparticle is tracked over time and for each trajectory, the mean squared displacement of the nanoparticles is estimated. The corresponding diffusion coefficient is calculated and converted into the hydrodynamic diameter and the calculated diameters are used to estimate a size distribution.
In 2015, a team at the Department of NanoEngineering at the University of California developed a new software called Particle Image Characterization Tool (PICT). PICT is a new type of quantitative image analysis software, that is used to characterize the structural properties of nanoparticle clusters from the experimental images of nanocomposites. PICT analyses SEM images taken during the assembly of surface-functionalized metal nanoparticles within a polymer matrix. It characterizes the morphology of nanoparticle clusters to provide quantitative information used to elucidate the physical mechanisms of nanoparticle assembly.
Liquid cell TEM (LTEM) is a new method being developed by the University of Cambridge as a way of overcoming current difficulties with analysing nanoparticles. Liquid cells are equipped with microfabricated thin film windows and a vacuum-tight sealing. This allows the opportunity to utilize the high spatial and temporal resolution of TEM whilst studying reactions of colloidal nanoparticles. Different types of electron beam transparent window materials including silicon nitride, silicon, and graphene are being fabricated and compared for use with the new LTEM technology.