Atomic Force Microscopy
The visualization in three dimensions can be achieved by AFM. Resolution in the vertical axis, or Z, is limited by the vibration environment of the instrument, whereas resolution in the horizontal axis, or X— Y, is limited by the diameter of tip utilized for scanning. AFC provides the main information about structures of nanoencapsulation systems at the atomic and molecular level by scanning the surface of samples with a sharp tip, without any contact. As explained schematically in Fig. 10.5B, mounted tip on a cantilevered arm is closely held against the surface of the sample. The ventricular movement of a tip from the surface due to the repulsive forces is detected by the optical system that senses the spacing between the probe and the sample and provides a correction signal. By producing an amplified signal of the minute deflections of the cantilever, it is possible to obtain an image of its structure (Binnig, Quate, & Gerber, 1986; Jalili & Laxminarayana, 2004).
The main advantages of AFM include providing an image from nanomaterials in both lateral and vertical positions without any sample preparation, the possibility of operation under air, or fluid in addition to both lateral and vertical resolution (Jalili & Laxminarayana, 2004; Liang, Mao, & Ng, 2004). Torres-Giner, Gimenez, and Lagaran (2008) applied AFM to study the surface topography of zein nanofibers prepared via electrospinning method. AFM image revealed that the morphology of the fibers is not homogeneous
FIGURE 10.5 Schematic diagram of confocal laser scanning microscope (CLSM) (A). CLSM images of chitosan-coated nanoemulsion (NEII) (a—c) and nanocapsules (d—f) reprinted with permission from Abbas et al. (2014). Copyright © 2016, ACS. z-Stack confocal images of NEII droplets (a) oily core (red, pictures a, c and d) marked with Nile Red, (b) chitosan layer (green, pictures b, c and e) marked with FITC. (c) Superposition of image (a) and (b), showing oily droplet inside chitosan layer. z-Stack confocal images of nanocapsules (d) oily core (red, pictures a, c and d) marked with Nile Red, (e) chitosan—CMC shell (green, pictures b, c and e) marked with FITC. (f) Superposition of image (d) and (e) (yellowish in color), showing oily droplet encapsulated inside the polymeric shell. Sematic diagram of atomic force microscope (AFM) (B). AFM height (left) and phase (right) images of zein fibers electrospun under the so-called standard conditions, reprinted with permission from Torres-Giner et al. (2008). Copyright © 2016, ACS.
or wholly tubular (Fig. 10.5g). In addition, the ribbons with two tubes at the sides were observed in AFM image.