The characterization of nanoparticles is a branch of nanometrology that deals with the characterization, or measurement, of the physical and chemical properties of nanoparticles.Ranjan R., Manoj K., Manoranjan P.S. (2023). XRD analysis of nanoparticles synthesized using aqueous and alcoholic extracts of "Cuscuta reflexa". Balneo and PRM Research Journal, 14(3): 585. Nanoparticles measure less than 100 nanometers in at least one of their external dimensions, and are often engineered for their unique properties. Nanoparticles are unlike conventional chemicals in that their chemical composition and concentration are not sufficient metrics for a complete description, because they vary in other physical properties such as size, shape, surface properties, crystallinity, and dispersion state.Manoj K., Ranjan R., Sinha M. P., Raipat B. S. (2023). Different techniques utilized for characterization of metallic nanoparticles using biological agents: A Review. Balneo and PRM Research Journal, 14(1): 534.
Nanoparticles are characterized for various purposes, including nanotoxicology studies and exposure assessment in workplaces to assess their health and safety hazards, as well as manufacturing process control. There is a wide range of instrumentation to measure these properties, including microscopy and spectroscopy methods as well as particle counters. Metrology standards and reference materials for nanotechnology, while still a new discipline, are available from many organizations.
Background Nanotechnology is the manipulation of matter at the atomic scale to create materials, devices, or systems with new properties or functions. It has potential applications in energy, healthcare, industry, communications, agriculture, consumer products, and other sectors. Nanoparticles measure less than 100 nanometers in at least one of their external dimensions, and often have properties different from the bulk versions of their component materials, which make them technologically useful. This article uses a broad definition of nanoparticles which includes all free nanomaterials regardless of their shape or how many of their dimensions are nanoscale, rather than the more restrictive ISO/TS 80004 definition that only refers to round nano-objects.
Nanoparticles have different analytical requirements than conventional chemicals, for which chemical composition and concentration are sufficient metrics. Nanoparticles have other physical properties that must be measured for a complete description, such as size, shape, surface properties, crystallinity, and dispersion state. The bulk properties of nanoparticles are sensitive to small variations in these properties, which has implications for process control in their industrial use. These properties also influence the health effects of exposure to nanoparticles of a given composition.
An additional challenge is that sampling and laboratory procedures can perturb the nanoparticles' dispersion state, or bias the distribution of their other properties. In environmental contexts, many methods cannot detect low concentrations of nanoparticles that may still have an adverse effect. A high background of natural and incidental nanoparticles may interfere with detection of the target engineered nanoparticle, as it is difficult to distinguish the two. Nanoparticles may also be mixed with larger particles. For some applications, nanoparticles may be characterized in complex matrices such as water, soil, food, polymers, inks, complex mixtures of organic liquids such as in cosmetics, or blood.
Types of methods
Microscopy methods generate images of individual nanoparticles to characterize their shape, size, and location. Electron microscopy and scanning probe microscopy are the dominant methods. Because nanoparticles have a size below the diffraction limit of visible light, conventional optical microscopy is not useful. Electron microscopes can be coupled to spectroscopic methods that can perform elemental analysis. Microscopy methods are destructive, and can be prone to undesirable artifacts from sample preparation such as drying or vacuum conditions required for some methods, or from probe tip geometry in the case of scanning probe microscopy. Additionally, microscopy is based on single-particle measurements, meaning that large numbers of individual particles must be characterized to estimate their bulk properties. A newer method, enhanced dark-field microscopy with hyperspectral imaging, shows promise for imaging nanoparticles in complex matrices such as biological tissue with higher contrast and throughput.
Spectroscopy, which measures the particles' interaction with electromagnetic radiation as a function of wavelength, is useful for some classes of nanoparticles to characterize concentration, size, and shape. Semiconductor quantum dots are fluorescent and metal nanoparticles exhibit surface plasmon absorbances, making both amenable to ultraviolet–visible spectroscopy. Infrared, nuclear magnetic resonance, and X-ray spectroscopy are also used with nanoparticles. Light scattering methods using laser light, X-rays, or neutron scattering are used to determine particle size, with each method suitable for different size ranges and particle compositions.
Some miscellaneous methods are electrophoresis for surface charge, the Brunauer–Emmett–Teller method for surface area, and X-ray diffraction for crystal structure; as well as mass spectrometry for particle mass, and particle counters for particle number. Chromatography, centrifugation, and filtration techniques can be used to separate nanoparticles by size or other physical properties before or during characterization.
Metrics Size and dispersion
Particle size is the external dimensions of a particle, and dispersity is a measure of the range of particle sizes in a sample. If the particle is elongated or irregularly shaped, the size will differ between dimensions, although many measurement techniques yield an equivalent spherical diameter based on the surrogate property being measured. Size can be calculated from physical properties such as settling velocity, diffusion rate or coefficient, and electrical mobility. Size can also be calculated from microscope images using measured parameters such as Feret diameter, Martin diameter and projected area diameters; electron microscopy is often used for this purpose for nanoparticles. Size measurements may differ between methods because they measure different aspects of particle dimensions, they average distributions over an ensemble differently, or the preparation for or operation of the method may change the effective particle size.
For airborne nanoparticles, techniques for measuring size include cascade impactors, electrical low-pressure impactors, mobility analyzers, and time-of-flight mass spectrometers. For nanoparticles in suspension, techniques include dynamic light scattering, laser diffraction, field flow fractionation, nanoparticle tracking analysis, particle tracking velocimetry, size exclusion chromatography, centrifugal sedimentation, and atomic force microscopy. For dry materials, techniques for measuring size include electron microscopy, atomic force microscopy, and X-ray diffraction. Back-calculation from surface area measurements are commonly employed, but these are subject to error for porous materials. Additional methods include hydrodynamic chromatography, static light scattering, multiangle light scattering, nephelometry, laser-induced breakdown detection, and ultraviolet–visible spectroscopy; as well as near-field scanning optical microscopy, confocal laser scanning microscopy, capillary electrophoresis, ultracentrifugation, cross-flow filtration, small-angle X-ray scattering, and differential mobility analysis. Use of an environmental scanning electron microscope avoids morphological changes caused by the vacuum required for standard scanning electron microscopy, at the cost of resolution.
A closely related property is dispersion, a measure of the degree to which particles clump together into agglomerates or aggregates. While the two terms are often used interchangeably, according to ISO nanotechnology definitions, an agglomerate is a reversible collection of particles weakly bound, for example by van der Waals forces or physical entanglement, whereas an aggregate is composed of irreversibly bonded or fused particles, for example through covalent bonds. Dispersion is often assessed using the same techniques employed to determine size distribution, and the width of a particle size distribution is often used as a surrogate for dispersion. Dispersion is a dynamic process strongly affected by properties of the particles themselves as well as their environment such as pH and ionic strength. Some methods have difficulty distinguishing between a single large particle and a set of smaller agglomerated or aggregated particles; in this case using multiple sizing methods can help resolve the ambiguity, with microscopy being particularly useful.
Shape
Morphology refers to the physical shape of a particle, as well as its surface topography, for example, the presence of cracks, ridges, or pores. Morphology influences dispersion, functionality, and toxicity, and has similar considerations as size measurements. Evaluation of morphology requires direct visualization of the particles through techniques like scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Several metrics can be used, such as sphericity or circularity, aspect ratio, elongation, convexity, and fractal dimension. Because microscopy involves measurements of single particles, a large sample size is necessary to ensure a representative sample, and orientation and sample preparation effects must be accounted for.
Chemical composition and crystal structure
Bulk chemical composition refers to the atomic elements of which a nanoparticle is composed, and can be measured in ensemble or single-particle elemental analysis methods. Ensemble techniques include atomic absorption spectroscopy, inductively coupled plasma optical emission spectroscopy or inductively coupled plasma mass spectrometry, nuclear magnetic resonance spectroscopy, neutron activation analysis, X-ray diffraction, X-ray absorption spectroscopy, X-ray fluorescence, and thermogravimetric analysis. Single-particle techniques include time-of-flight mass spectrometry, as well as utilizing elemental detectors such as energy-dispersive X-ray analysis or electron energy loss spectroscopy while using scanning electron microscopy or transmission electron microscopy.
The arrangement of elemental atoms in a nanoparticles may be organized into a crystal structure or may be amorphous. Crystallinity is the ratio of crystalline to amorphous structure. Crystallite size, the size of the crystal unit cell, can be calculated through the Scherrer equation. Generally, crystal structure is determined using powder X-ray diffraction, or selected area electron diffraction using a transmission electron microscope, though others such as Raman spectroscopy exist. X-ray diffraction requires on the order of a gram of material, whereas electron diffraction can be done on single particles.
Surface area Surface area is an important metric for engineered nanoparticles because it influences reactivity and surface interactions with ligands. Specific surface area refers to the surface area of a powder normalized to mass or volume. Different methods measure different aspects of surface area.