Nanoparticles have been developed for a variety of biomedical applications, such as biosensor, drug delivery, therapeutic agents, and medical devices. The physical and chemical properties of nanoparticles are extremely important to their performance and their interaction with living systems. Therefore, a range of sophisticated analytical techniques are required to characterize and determine the properties of nanoparticles.
The key parameters of nanoparticles’ physical characterization include, size, shape, surface, and the morphology. For example, size is one of the principle properties of nanoparticles, which related to the surface to volume ratio and their toxicity to living systems. Moreover, zeta potential is one of the important physical related to the long-term stability of the nanoparticles in solution and suspension. The chemical composition and the intrinsic toxicological properties of the chemical are of importance. In general, there are several techniques to evaluate the physical and chemical characterizations of nanoparticles.
1. Electron Microscopy
Electron microscopy uses a beam of accelerated electrons to provide a much higher resolution, which cannot be achieved by light microscope. The most commonly used electron microscopy is transmission electron microscopy (TEM) and scanning electron microscopy (SEM).
The TEM uses a high voltage electron beam to illuminate the specimen and generate an image. The electron beam is originated from an electron gun, accelerated and then transmitted through the specimen on a conducting grid. The transmitted electron carries information about the structure of the specimen, and such information can be recorded by an imaging detector (fluorescence screen or CCD camera). The TEM normally requires extremely thin section of specimen, typically no more than 100 nm. Biological specimens, organic polymers and similar materials may need special treatment with heavy atom labels in order to obtain sufficient image contrast.
The SEM is based on the interaction of the electron beam with the specimen surface. Due to the high depth of field in SEM, a three-dimensional appearance can be displayed. If not conductive, the specimen needs to be coated with an ultrathin layer of conductive material like gold, platinum or graphite to obtain a clear image. The substrate, where the specimen is held, is typically a filter membrane or a conducting grid.
2. Dynamic Light Scattering (DLS)
Particle size can also be determined using light scattering techniques. DLS is also called quasielastic light scattering or photon correlation spectroscopy. Usually, the Brownian motion of particles cause the fluctuations and the neighboring particles may have destructive or constructive interference of the scattered light intensity in a certain direction. This particle diffusion can cause fluctuations in the scattered light, therefore, the particle size has been measured.
3. Atomic Force Microscope (AFM)
The AFM offers the capability of three dimensional visualization and material sensing to measure the size, height, morphology, surface texture, and roughness of nanoparticles. It utilizes a cantilever with a nanoscale thin tip, which oscillates over the sample surface. The piezoelectric actuators control the scanning over the surface (X and Y-axis) and the oscillating movement (Z-axis). With proper statistical analysis, AFM can provide both qualitative and quantitative information.
4. X-ray Diffraction (XRD)
XRD is the most common method utilized for determining the atomic and molecular structure of materials. The crystalline atoms lead to the diffraction of the incident X-ray beam into specific directions. So the diffraction angles and corresponding intensities can be measured and recorded, which depict a three-dimensional picture of the density of electrons within the crystal. A great number of crystalline materials can be characterized by XRD, including inorganic and organic materials, and biological molecules. The physical states of the materials for XRD measurement are flexible, and they can be loose powders, thin films, and polycrystalline and bulk materials. The advantages of XRD technique include minimum quantity of sample required, non-destructive measurement, and easy to interpret.
5. Zeta Potential Instrument
Zeta potential is the measurement for electric charge on nanoparticle surface. Particles dispersed in solution or suspension has electric charged because of their intrinsic ionic properties and other dipolar characteristics. Zeta potential represents the extent of electrostatic repulsion between all the adjacent and is the fundamental indicator of the stability of various colloidal dispersions. For nanoparticles with high absolute zeta potential, it shows an improved stability, while nanoparticles with zeta potential close zero may lead to the dispersion, aggregation or flocculation problems in solution or suspension. A various zeta potential instrument can provide data for the zeta potential and differ from company to company.
6. BET/Surface Area (Brunauer, Emmett and Teller)
The surface area of a powder can affect its behavior in many applications including pharmaceuticals. The relatively weak forces (van der Waals forces) between the adsorbent surface area of the solid and the adsorbate gas molecules are the sources of the physical adsorption. The specific surface area is based on this physical adsorption of a gas on the surface of the test powder. Then amount of adsorbate gas on the surface will be calculated. This measurement is usually conducted in liquid nitrogen at the temperature.
7. Liquid Chromatography - Mass Spectrometry ( LC-MS/MS)
Organic component analysis and inorganic elemental analysis can be evaluated by using liquid chromatography - mass spectrometry (LC-MS). It can provide the information about the identification, quantitation, and mass analysis of the materials and compounds. LC-MS first separates the test compounds in sample mixture via the chromatography based on the compound’s intrinsic affinity for stationary phase and mobile phase. Then when the separated compound passes through the mass detector, it collects the intensity for the compound as it is vaporized and atomized in the plasma compared to a reference material. The mass fraction or size of the compounds can be correlated to the measured data point can be correlated.
8. Fourier Transform Infrared Spectroscopy (FTIR)
Functional groups chemical information of nanoparticles can be measured by fourier transform infrared spectroscopy (FTIR). Since most molecules can absorb infra-red light, their absorption will create molecular fingerprints of the sample and show how the sample absorbs light at each wavelength. It is very useful for identify the side chains, functional groups, and cross-links of the nanoparticles, due to the fact that all of them have characteristic vibrational frequencies in the infra-red range.
9. Thermogravimetric Analysis (TGA)
As coated nanoparticles becomes increasingly important in a range of applications, the quantitative information of the amount of coating molecules and the relation to the surface area are necessary and key parameters need to be evaluated. Therefore, TGA can be used as the analytic method to provide information of the surface coatings, purity, and compositional data of the nanoparticles. TGA is based on the measurement of the mass/weight change (gain or loss) and the rate of weight change in relation to the change of time, temperature, and atmosphere. Several studies has been carried out to measure the surface coating, ligand binding, and grafting density for various nanoparticles, such as carbon nanotubes, SiO2, and gold nanoparticles.