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Microcantilever Sensors
18.4.2
Microelectromechanical systems can be composed of multiple micron-thick can-
tilevers (visualize miniature diving boards) that respond by bending due to changes
in mass. Appropriate coatings are applied to the cantilevers to adsorb chemicals of
interest. The adsorption of the chemical causes the microcantilever to bend, which can
be recorded either electrically or optically. This particular technology has been used
for developing infrared sensors to “see” images in darkness, but commercial devices
using microcantilever sensors to detect volatile organic chemicals are less common.
Active research in this area is being performed by Oak Ridge National Laboratory.

18.5 OPTICAL SENSORS
18.5.1 Fiber Optic Sensors

Fiber optic sensors are a class of sensors that use optical fibers to detect chemical
contaminants. Light is generated by a light source and is sent through an optical fiber.
The light then returns through the optical fiber and is captured by a photo detector.
Some optical fiber sensors use a single optical fiber while others use separate optical
fibers for the light source and for the detector. There are three general classes of fiber
optic sensors (see Figure 18.3). The first type is completely passive. A spectroscopic
method can be used to detect individual types of contaminants. This method involves
sending a light source directly through the optical fiber and analyzing the light that
is reflected or emitted by the contaminant. The refractive index of the material at
the tip of the optical fiber can be used to determine what phases (vapor, water, or
NAPL) are present. Asecond class of fiber optic sensors consist of a fiber optic sensor
with a chemically interacting thin film attached to the tip. This film is formulated to
bind with certain types of chemicals. Contaminant concentration can be found by
measuring the color of the thin film, the change in refractive index, or by measuring
the flourescing of the film. The third type of fiber optic sensors involves injecting a
reagent near the sensor. This reagent reacts either chemically or biologically with the
contaminant. The reaction products are detected to give an estimate of the contaminant
concentration.
Pros: Low power; several types have no moving parts; can detect various chemicals
at very low concentrations.
Cons: Limited ability to transmit light through the optical fiber over long distances.
Some organic pollutants are not easily differentiated using UV-visible spectroscopy.
Concentration range sensitivity may be limited. Sensors that use chemically sensitive
coatings may degrade with time.

18.5.2 Colorimetry
Pocket colorimeter test kits can be used to measure trace levels of contaminants.
They work by analyzing the color of contaminated water that has been mixed with