Labels and Detection Methods - Microarray Technology and Its Applications - page 150

Fig. 8.3. Size- and material-dependant fluorescence emission spectra of several
surfactant-coated semiconductor nanocrystals. (Reprinted with permission from
Professor Paul Alivisatos at UC Berkeley)

one study, it was estimated that single ZnS-capped CdSe QDs are 20 times

brighter than rhodamine 6G organic dye molecules [43]. The QDs are also
highly stable against photo-bleaching.
These fluorescence properties are suitable for many biological applications
such as cellular imaging [48] and fluorescent in situ hybridization (FISH) [49]
where stable fluorophores are desired, and microarray labelling where multiple
colored probes are possible using a single excitation source [24]. However, a
limitation of QD technology to date has been the di culty in functionalizing
the particles with biomolecules for robust labelling in such applications [24].
As a result, only a few examples of direct QD labelling for biological appli-
cations have been reported. An innovative approach designed to utilize the
unique fluorescent properties of QDs for optical coding of biomolecules was
recently reported by Nie and coworkers [50]. In this approach, 1.2

∼

m poly-
meric microbeads are optically encoded by embedding different colored QDs
at defined ratios for use in 'liquid arrays', where a biomolecule attached to the
surface of the microbead is barcoded by the unique signal from a single QD
or a group of different QDs in a well-defined ratio. Beads encoded with or-
ganic fluorophores were previously reported for liquid array applications [51].
However, major benefits of QDs for barcoding include a single UV excitation
source for bead detection, and the potential for many more unique colors. The
number of codes in this approach is defined by the number of unique colors
and intensities, such that n intensities and m colors generate n m −

µ

1 codes.
The authors suggest that six spectrally unique QDs at 6 intensity levels is
feasible, generating approximately 10,000 recognizable QD codes [50]. Exper-