A nanoparticle is a particle with a diameter that is much smaller than one millionth of a metre and they find widest use in many day to day products. Nanoparticles of a substance behave, quite simply, differently than large particles of the same substance. It is important to determine their size, shape and surface area, in order to improve their properties within various areas of application.
Nanoparticles are not influenced by gravity and thus they do not fall to the bottom of any liquid or gas, but spread and float throughout the space where it is kept. Their area of contact with the surrounding medium is very large due to their small size, which is the reason for many interesting properties.
Nanoparticle sources
There are several sources that result in nanoparticle formation: stationary industrial sources, such as coal fired combustion systems and incinerators; mobile sources, such as automobiles and diesel powered vehicles; and occupational environments, such as those where welding processes are prevalent and those where engineered nanoparticles are deliberately synthesized. There are several natural sources and nanoparticles of biological origin that also need significant attention. For example, pollen fragments are potential causes of allergies, and viral nanoparticles can be used as vaccines or can play a significant role in the spread of disease.
Size distribution analysis
Nanoparticles of a substance can be counted and the size distribution can be determined by dispersing the nanoparticles into a gas. But some nanoparticles tend to aggregate when the surrounding conditions change. Certain types of nanoparticles can even start to aggregate under special conditions to form gel.
Scientists at the University of Gothenburg, Sweden, have reported that it is possible to sort and count the particles, even when they have formed aggregates. The researchers have studied one such aggregating systems, colloidal silica. The gel that forms when salt is added to colloidal silica can be used, for example, to seal rock and to stabilise soil. Nanoparticles that have aggregated can be analysed individually if a colloidal silica gel, which contains these aggregated nanoparticles, is first diluted and then dispersed into the gas phase. If the samples are analysed immediately after being diluted, this method gives an accurate picture of the gel aggregate.
Principle of measurement
Nanoparticles move under Brownian motion and small particles move faster than larger particles. Diffusion Coefficient can be calculated by tracking and analysing the movement of each particle separately but simultaneously. Through application of the Stokes-Einstein equation, particle size can be calculated. Particle concentration/number can also be estimated.
Instrumental methods
In the disk centrifuging method the sample is spun inside a stack of conical disks and components are separated in the space between disks. Individual nanoparticles can be studied under the electron microscope, but this is slow and hardly practical at the routine level and these methods are not successful when the particles to detect are present in small numbers.
Coulter counter method is an indirect method. In this, there is a microscopic aperture in an insulating membrane. An electrical gradient is applied between the two sides of the membrane, which is immersed in a conducting liquid. As the membrane is an insulator, any current can flow only through the aperture, and this current is measured. Now, if there are cells or other particles in the liquid and one of them passes through the aperture, it will effectively block the current path while it is passing through, which would register as a drop in the current. Such changes in current strength are detected and counted to provide data of particle movement. The same principle is used in the nanopore, which can detect much smaller particles like DNA or protein molecules. But while these methods have been useful in the fields where they were developed, they are cumbersome and cannot provide rapid count rates, which is required in many nanoparticle characterisation applications.
Researchers at the University of California, Santa Barbara, reports on improved equipment that can detect and count nanoparticles as small as 10 nm.
NanoSight has developed a unique capability to directly size and visualize nanoscale particles in liquids, with high-resolution, in real-time and with minimal sample preparation. Using nanoparticle tracking analysis (NTA) visualization technique size, count and concentration measurements can be made to give insight into true size distributions even in complex systems.
Izon's qNano utilizes a new non-optical detection principle to enable the size, concentration, dynamics and charge of a wide range of particle types to be measured with high accuracy. Resistive Pulse Sensing combined with Size Tunable Nanopores enables accurate particle-by-particle measurement with representative information on the size and polydispersity of mixtures.
Researchers at Washington University in St. Louis have turned an acoustic phenomenon into a high-resolution nanoparticle detector. Using a ring-shaped micro-laser, the sensor can detect and count individual viruses or synthetic and biological nanoparticles with single particle resolution.
TSI's Scanning Mobility Particle Sizer Spectrometer is a high resolution nanoparticle sizer used for nanoparticle size characterization and the method is independent on the refractive index of the particle or fluid.