List of Characterization Techniques for Nanoparticles

Parameter Method of Characterization

Particle size Photon correlation spectroscopy, laser diffraction, transmission electron microscopy,

Molecular weight

Density

Crystallinity

Surface charge

Hydrophobicity

Surface characteristics

Surface element analysis scanning electron microscopy Gel permeation chromatography Helium compression pycnometer X-ray diffractometry, differential scanning calorimetry

Electrophoresis, laser Doppler anemometry, amplitude-weighted phase structuration Hydrophobic interaction chromatography, contact angle measurements Scanning electron microscopy, atomic force microscopy X-ray photoelectron spectroscopy for chemical analysis (ESCA)

molecular weight of cross-linked polymers and nanoparticles from natural macromolecules cannot be determined.

Density measurements can be performed by the helium compression pycnometry and by density gradient centrifugation. A comparison of these two methods may offer information about the internal structure of nanoparticles.

Further information about the nanoparticle structure and crystallinity may be obtained using x-ray diffraction and thermoanalytical methods such as differential scanning calorimetry, differential thermal analysis, thermogravimetry, and thermal optical analysis.47 The charge on nanoparticle surfaces is mainly determined by electrophoretic mobility, laser Doppler anemometry, and amplitude-weighted phase structuration.

Hydrophobicity of the nanoparticles can be determined by two major methods: contact angle measurements and hydrophobic interaction chromatography. The former can be performed only on flat surfaces and not directly on hydrated nanoparticles in their dispersion media. For this reason, hydrophobic interaction chromatography is the better method, wherein a differentiation between nanoparticles with different surface properties can be obtained by loading the particles on columns with alkyl-sepharose and eluting them with a Triton X-100 gradient.48

Kreuter41 performed the physicochemical characterization of polymethyl, polyethyl, and PBCA nanoparticles. Because it gives only the mean diameter and the multimodal size distributions cannot be measured, determination of nanoparticle size by Photon Correlation Spectrometry is very susceptible to errors caused by the presence of bigger particles in the dispersion. Another disadvantage is that this method measures the Brownian motion of the particles, and the particle diameter is then calculated from the measurements via the diffusion coefficient.41 Thus, the particle size determined by this method can be affected by the surrounding medium, including the adsorbed surfactants or hydration layers, and may therefore be different from the sizes obtained by electron microscopy. In case of characterization using electron microscopy, the individual particles can be analyzed and measured. According to Kopf et al.,26 the surfactants present after the separation of the dispersion medium from the nanoparticles will coat the nanoparticles and lead to the disappearance of individual nanoparticle structures. This coating significantly reduces the particle surface area, and polycyanoacrylate nanoparticles could thus not be obtained surfactant-free in a nonaggregated form. The nanoparticles were x-ray amorphous, and their diffractograms indicated no sign of crystallinity.20,21 The surface charge of colloidal particles expressed by their electro-phoreitc mobility may have an important influence on their body distribution behavior in humans and animals.49,50 The PACA nanoparticles possess negative charge. Charge measurements in serum yielded a more pronounced decrease caused by the adsorption of the serum contents. Another parameter that is of great importance for the fate of nanoparticles in the blood is the hydrophili-city/hydrophobicity of the particle surface. The latter has a profound influence on the interaction of the particles with the blood components.49,51,52 The particles' hydrophilicity/hydrophobicity can be determined by measuring the water contact angle.53 This was done by centrifugation, followed by washing with water and drying the poly(cyanoacrylate) nanoparticles. The dried material was dissolved in acetone and casted as a film with subsequent solvent evaporation.41 The water droplet contact angles for polymethyl, polyethyl, and PBCA nanoparticles were reported as 55.6°, 64.7°, and 68.9°, respectively.41

Muller et al.48 studied the physicochemical properties of alkyl cyanoacrylates of different chain lengths such as methyl, ethyl, isobutyl, and isohexyl cyanoacrylate. Zeta potential of the nanoparticles was determined as an alternative means of evaluating particle surface charge. Interaction of nanoparticles with serum proteins was assessed by the zeta-potential measurements. The nanopar-ticles were incubated with poloxamer 407 (Pluronic F-108), and the hydrophobicity determination by contact angle measurement revealed that the coating layer was very thin. The layer thickness was higher in isohexyl cyanoacrylate particles, indicating that they had the most hydrophobic nature of all the polymers used.

The molecular weight differences between polycyanoacrylate nanoparticles with different side-chain lengths as well as the molecular weight differences of polycyanoacrylate nanoparticles produced at different hydrochloric acid concentrations or surfactant concentrations appear to not be very pronounced.54 Nevertheless, an increase in side-chain length led to the increasing molecular weights, whereas increasing the hydrochloric acid concentrations reduced the molecular weights.

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