Biomedical Engineering Reference
In-Depth Information
and QD fl uorescence intensity increases upon photoexcitation, possibly caused by
local environmental interactions with the QD surface. Moreover, Silver and Ou have
looked at QD fl uorescence of endocytosed QDs and have observed that intracellular
QDs exhibited “photoactivation” or “photobrightening” after ~1 min of photoactiva-
tion (Silver and Ou
2005
). QDs that were barely detectable in lightly labeled intracel-
lular structures became markedly brighter and were observed more strongly in cells in
PBS than in ethanol. While environment-dependent variations could prove problematic
for QD fl uorescence quantifi cation and tagging, further studies to carefully examine the
effect of pH, lysosomal degradation of specifi c QD surface molecules, and other local
environmental intracellular factors could provide new opportunities for development of
environmentally sensitive high resolution cellular probes.
6
Quantum Dot Surface Chemistry for Cellular Interaction
Surface chemistry that allows tethering of bioactive molecules to the QD surface is
a crucial factor in customizing QDs for specifi c biological interactions in cells. QDs
that have hydrophobic surfaces are not directly soluble in aqueous solution and
may aggregate in the presence of physiologically relevant electrolytic concentrations.
A challenge for adapting QDs for biological applications has been to fi nd suitable
surface coatings that maintain reproducible photophysical properties, stability in
aqueous solution, and chemical conjugation/manipulation in physiological condi-
tions. An additional critical factor to consider when designing QD surface coatings
is to maintain the total end diameter so that increase in QD size does not negatively
impact the mobility and accessibility of the resulting QD-conjugated biomolecules
(Jaiswal and Simon
2004
). Surprisingly rapid progress has been made in fi nding
suitable coatings for functionalizing QD surfaces to meet these above criteria in the
past few years. Initial QD coatings included mercaptoacetic acid, dihydrolipoic
acid, or modifi ed amphiphilic polymers such as polyacrylic acid. These approaches
offer the advantage of serving as very general and can be adopted for other nanoma-
terials with similar hydrophobic surfactants on their surface (Chan and Nie
1998
;
Goldman et al.
2002
; Jaiswal et al.
2003
; Wu et al.
2003
; Pinaud et al.
2006
) . Another
general approach has been to use exchange chemistries such as di-thiol ligands with
additional coatings of engineered proteins and peptides to cross-link the ligands and
thus provide a more stable coating (Pinaud et al.
2004,
2006
) .
Once QDs have hydrophilic coatings, strategies must be devised to bind ligands
and other biomolecules of interest. To date, biomolecules have been linked to water-
dispersible QDs using covalent attachment via -COOH, -SH, or NH
2
groups,
electrostatic attraction (Goldman et al.
2002
), and biomolecular protein linking sys-
tems ( Fig.
2
). The widely used avidin-biotin system has also been used to link
immunoglobulin G molecules to QD surfaces (Wu et al.
2003
) . Biomolecules such
as antibodies, biotin, oligonucleotides, peptides, and proteins have been bound to
QD surfaces using these linking systems. In further developments, Howarth et al. have
used the
Escherichia coli
enzyme biotin ligase-acceptor peptide system to label
