# Review on Silver Nanoparticles as a Novel Class of Antibacterial Solutions
## 3 Characterization Methods for Metal Nanoparticles
Moreover, similar X-ray-based methods are presented by the authors such as: X-ray absorption spectroscopy suitable for determination of the X-ray absorption coefficient, or X-ray photoelectron spectroscopy for surface chemical analysis. Transmission Electron Microscopy (TEM) is a microscopy technique that uses uniform current electron beams which either transmit or scatter the electrons from a thin sample. The interaction is dependent on the nanoparticles' size, sample density and elemental composition. This is the most used technique to analyze size and shape, because of its ability to provide an accurate estimation of nanoparticle homogeneity. It does, however, have some limitations when quantifying a large number of particles and it can produce unreliable images due to orientation effects [33]. High Resolution Transmission Electron Microscopy (HRTEM) uses both transmitted and scattered electrons to produce the images. Thanks to its high resolution, HRTEM became the most used technique used to characterize the internal structure of nanoparticles [33]. Atomic Force Microscopy (AFM) is used to create three dimensional images of surfaces. It has the advantage of being cost effective and space saving and it is mainly used to detect nanoparticles' size [33]. Both electron microscopy techniques (e.g., TEM) and AFM have demonstrated to be very useful in AgNPs characterization. While quantitative assessment of size and shape could be obtained in any of the two techniques, AFM can have the advantage of real-time assessment of interaction of silver nanoparticles with the lipid bilayer of bacterial membrane [36]. UV-VIS spectroscopy is a method that uses the intensity of reflected light from a reference material and compares it to the intensity reflected from the sample. The size, shape, concentration, agglomeration state and refractive index are optical properties that can be observed using UV-VIS [33]. UV-VIS represents an extensively used tool for silver nanoparticle characterization. Particularity of AgNPs is the close proximity of valence and conduction band, thus allowing the free movement of electrons. The absorbance surface plasmon resonance band, coming from their collective oscillation was widely observed and reported for diameters below 100 nm. Several factors may affect absorbance of AgNPs, such as chemical environment, diameter of particle, dimension [37; 38]. FTIR spectroscopy is also widely used in silver nanoparticle characterisation having the advantage of increased signal-to noise ratio as well as reduced thermal deterioration of the sample [39]. Limitations exist, however, the recent update in the technique, attenuated total reflection (ATR)-FTIR spectroscopy, provides new features for proper evaluation of chemical features at the surface of nanoparticles [40]. Nuclear Magnetic Resonance (NMR) is a technique that uses a strong magnetic field to measure the energy difference between spin-ups and downs of nuclei. This transition can be measured using electromagnetic radiation in the radio wave range. This technique has limitations when trying to characterize ferri or ferro-magnetic materials [33]. Zeta potential is used to analyze the stability of colloidal dispersions. Particles with high positive or negative charge, tend to repel each other and form stable colloidal solutions [33]. Electrophoretic mobility (EPM) is used for the surface charge of nanoparticles. High EPM values were correlated with stable nanoparticles for long periods of time while low EPM values are associated with large aggregations of iron oxide nanoparticles in relation to nanoparticle concentration [33]. Dynamic light scattering (DLS) is a method used to measure nano and sub micrometer sizes of nanoparticles in colloidal suspensions. In order to avoid multiple scattering, a low concentration of nanoparticles is used in DLS. DLS has proven very useful for real time observation of the aggregation process due to its ability to measure the previously formed cluster sizes [8].