Nanosensors - Nanotechnologies for Water Environment Applications - page 419

control the orientation of antibodies is by immobilizing the antibodies on a self-
assembled monolayer of alkanethiols. The interaction of thiol groups (-SH) on a gold
surface forms robust SAMs with different terminal group functionalities (amine,
carboxyl, methyl etc.) depending upon the chemical structure of alkanethiol molecule.
The orientation in self assembled monolayer molecules induces the similar orientation
on the immobilized antibodies (Oh et al., 2004). Immobilization of antibodies on
Langmuir-Blodgett (LB) monomolecular films or protein films are also widely applied
(Oh et al., 2003).

For nucleic acid/nucleic acid interaction, a short sequence of nucleic acid, called
“probe” is immobilized on a surface and the complementary sequence of nucleic acid,
called “target” is hybridized to the probe. Probes are signature sequences that are
specific to the target organism. Usually they are part of a virulence or marker gene that
is highly specific to the organism. Databases related to such virulence and marker genes
of pathogenic organisms (e.g., VFDB located at: http://www.mgc.ac.cn/VFs/ and
Virulence Database of Lawrence Livermore National Laboratory located at:
http://mvirdb.llnl.gov/) are slowly becoming available to help in the design of such
signature sequences. The target molecule is usually labeled with an optically detectable
compound (fluorescent dye, quantum dot, gold particle), which interacts with the
complementary probe and gives a specific signal. If assay has a low detection limit,
target amplification using polymerase chain reaction (PCR) may be necessary prior to
the labeling and hybridization step. Nucleic acid based sensors provide superior
specificity (Lian et al., 2004) as well as genotyping (Lindroos et al., 2002), and multi-
target detection capabilities.

Misorientation Overcrowding

Perf ect Orientation

Antigen

Antibody

substrate

Figure 13.3 Schematic of antibody (hollow V-shaped) and antigen (filled triangle)
interaction mechanism. As illustrated, disorientation and overcrowding of immobilized
antibody induces non-specific interactions.