Principles of Fluorescence Spectroscopy

PRINCIPLES OF FLUORESCENCE SPECTROSCOPY 723
Figure 21.36. Donor and acceptor molecular beacons for a hybridization proximity array. Reprinted with permission from [116]. Copyright © 2003,
American Chemical Society.
Figure 21.37. Emission spectra of the donor and acceptor beacons in
Figure 21.36 in the absence and presence of target DNA. Revised and
reprinted with permission from [116]. Copyright © 2003, American
Chemical Society.
nearby acceptor. Emission from both the donor and acceptor
is only seen when both beacons bind to the same target
sequence. This type of beacon could be made even more
specific using a lanthanide donor and detection of the sensitized
acceptor emission
21.4.4. Molecular Beacons Based on Quenching
by Gold
Gold surfaces and colloids are becoming more widely used
in bioassays because of the well-developed chemistry for
linkage to the surface, the ease of colloid preparation, and
the chemical stability of the surfaces. Gold is an highly
effective quencher of fluorescence. 117–119 Quenching probably
occurs by RET to the gold surface, but other mechanisms
may also be present. Because of the strong quenching
gold can provide a large on–off ratio for molecular beacons.
120–121 A molecular beacon on a gold surface is made
by binding a labeled oligomer to the surface by a sulfhydryl
group. 121 In the absence of target DNA the rhodamine label
is quenched (Figure 21.38). In the presence of target DNA
the rhodamine moves away from the gold surface and
becomes fluorescent. Surface-bound molecular beacons
could have a different sequence at each position on an array,