Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 234

Optical microscope
(transmission)

Electron microscope
(transmission)

Electron microscope
(scanning)

Light bulb Electron guns

Cathode ray
tube (CRT) High

Voltage
generator

Electron gun

Condenser lens

Signal
amplifier

Deflection coils

Specimen

Scan
generator

Objective lens

Fluorescent
screen of CRT
Final magn.
<100000X

Deetector

First image plane

Specimen

Projectior lens

Final image

Binoculars
or camera
Final magn.
<1000X

Fluorescent
screen or
camera
Final magn.
<1000000X

= Glass lens

= Electromagnetic lens

Figure 6.10 Schematic views of transmission and scanning electron microscopes and optical
microscope is shown for comparison. (Adapted from Wilkinson, K. J. and Lead, J. R. (2007),
Environmental Colloids and Particles: Behaviour, Structure and Characterization , John Wiley
& Sons Ltd, Chichester.)

the aquatic environment, such particles will always be outnumbered by naturally
occurring nanosized materials. The challenge in the fi rst place will be to selectively
detect the engineered NPs. Thus, microscopic techniques that allow selective
probing of specifi c properties of the engineered NPs will be most promising.

Scanning electron microscopy. In the SEM, a focused electron probe is scanned
over the surface of the sample and the detected signal is simultaneously projected
on a monitor. Different signals can be recorded in parallel (such as secondary
electrons (SE) and backscattered electrons (BSE)) providing different information
about the investigated sample. There is no direct path between the electron
beam and the image displayed and thus the signal can be electronically enhanced.
For SEM a large selection of excellent textbooks is available and a good intro-
duction into the topic can be found in Goldstein et al. , 2003. For a brief