Characterization Techniques of Living Cells Encapsulated with Nanomaterials - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 84

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Figure 5.2 TEM images of bare S. cerevisiae cells (a), S. cerevisiae cells coated with
AuNPs (b) and AgNPs (c).
(Reprinted with permission from ref. 11. Copyright 2009 the American
Chemical Society.)

cell surface. Further, energy-dispersive X-ray spectroscopy (EDS or EDX)
confirming the presence of NPs can be performed. Most SEM and TEM
microscopic systems are usually equipped with an EDS or EDX detector. For
example, the presence of magnetic NPs immobilized onto unicellular algae
and yeast cells was verified with EDX analysis during TEM imaging. 12,13 EDX
analysis can be performed as a line analysis or an area on the cell surface can
be mapped to obtain information about the elemental distribution, and thus
the material distribution. Figure 5.6c shows the line analysis of a yeast cell
coated with SiO 2 SH showing the presence of silicon and sulfur on the cell
surface.
In electron-based scanning microscopies such as SEM and TEM, the
density of the material to be imaged is critical. For example, most nano-
particles such as noble metals and CNTs can be visualized with TEM or SEM
but polymers and biological materials with low density may not be imaged
with these techniques since they are transparent to electrons. In such cases,
atomic force microscopy (AFM) can be used to visualize surface topography
down to nanometer resolution. Figure 5.3 shows the comparison of the AFM
images of before and after coating E. coli cells with AgNPs and AuNPs as an
example of AFM imaging. 14 A line analysis (green and red lines and cor-
responding graphs on the right side of the Figure 5.3) is performed to
measure the topographic roughness to evidence the presence of noble metal
NPs. As can be seen, in the absence of the NPs on the cell surface, both the
lines are smoother while it is coarser upon attachment of NPs.
Other materials such as proteins have also been used to coat the living
cells. In a recent report by Drachuk et al., Saccharomyces cerevisiae cells were
coated with fibroin (silk) using a layer-by layer approach. 15 It was claimed that
the cells were still viable after the coating process. The surface modification of
the cells was also verified with AFM. Another technique used in that study for
the characterization of the cell coating was fluorescence-spectroscopy-based

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