Characterization of Nanoparticles 15231 Physicochemical Characterization

The methods used for the physicochemical characterization of nanoparticles are listed in Table 15.1. Particle size is the prominent feature of nanoparticles; the fastest and most routine methods of size analysis are photon correlation spectroscopy (PCS) and laser diffractometry (LD). The former method is useful for the determination of smaller particles,41 whereas the latter method is useful for the determination of larger particles. PCS determines the hydrodynamic diameter of the nanoparticles via Brownian motion. The electron microscopy methods also allow the exact particle determination. Scanning electron microscopy requires the coating of the dry sample with a conductive material such as gold. The gold coating usually results in an estimate of particle sizes that is slightly greater than the normal. TEM, with or without staining, is a relatively easier method to determine particle size. Some nanoparticle materials are not electron dense and thus cannot be stained; others melt and sinter when irradiated by the electron beam of the microscope and thus cannot be visualized by this method. Such nanoparticles can be visualized in TEM after freeze fracture or freeze substitution.42 This method is optimal because it also allows for the fracturing of the particles and consequently an observation of their interior. Unfortunately, the method is time-consuming and cannot be used for the routine determinations. Recently, new types of highresolution microscopes such as the atomic-force microscope, the laser-force microscope, and the scanning-tunneling microscope were developed.43-46 These microscopes are especially useful for the investigation of nanoparticle surfaces.

The molecular weight determination of nanoparticles can be performed after the particles have been dissolved in an appropriate solvent and analyzed by gel-permeation chromatography. Accurate results can be obtained only if the polymer standards have a similar structure and similar properties as the test material. The limitation of this technique, however, is that the

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