Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 72

tive approach uses small pores between electrodes coated with materials comple-
mentary to a desired molecular unit. An analyte that contains gold particles and a
suspected DNA sequence will attach the gold between the electrodes, registering
a significant change in the conductivity between the electrodes after suitable
treatment. This scheme has been reported to give sub-picomolar sensitivities. 126
Magnetic fields from submicron beads are used to detect forces between
nanoparticles with the Bead Array Counter (BARC). 127 In this method, carefully
designed target DNA materials are first patterned on top of a chip containing an
array of micron-scale GMR strips. These strips are configured to electronically
sense very small magnetic fields. When a solution containing DNA from uniden-
tified pathogens flows over the corresponding area containing complementary
target DNA on the chip, this pathogen DNA is captured on the surface. Micron-
size magnetic beads, especially designed to bind only to the pathogen DNA
molecules, are then injected into the solution and are bound to the selected site.
The GMR sensors detect these beads, and the intensity and location of the signals
indicate the concentration and identity of any pathogens present. The current
BARC chip contains a 64-element sensor array; however, with recent advances in
magnetoresistive technology developed for computer memory, chips with mil-
lions of GMR sensors will soon be commercially available. This advance will
speed the development of a chip capable of screening for thousands of analytes
simultaneously. Because each GMR sensor is capable of detecting a single mag-
netic bead, in theory, the BARC biosensor should be able to detect the presence
of a single pathogen DNA molecule.
Optical indicators are successful sensors using well-known coalescence phe-
nomena. Nanoparticle-based colorimetric detection is an effective sensing tech-
nology already in the marketplace for pregnancy detection. The principle in-
volves antibodies attached to gold nanoparticles. The presence of an antigen or
other complementary interacting agent binds the gold nanoparticles into clusters
having modified optical properties (due to the larger cluster dimension), causing
a change in color. 128 Surface plasmon detection, sensitive to local refractive
index changes, provides an important technique for monitoring analyte adsorp-
tion.
Advances in chemical, biological, and explosives sensors are expected to be
rapid, paralleling the increasing activity in nanotechnology in general. Bio-
nanotechnology is even being studied as a means of destroying chemical and
biological agents.
Self-Sensing
To ensure that the mission is completed, every part of the sensing system can
and sometimes should be monitored for performance. In the case of manned
aircraft, this includes the pilot, the crew, and their environment (e.g., CO 2 levels
in the pilot's G-suit and cockpit). In the case of satellites, the environment of the