Trends in Biotechnology
Research FocusMolecular profiling of single cells and tissue specimens with quantum dots
Section snippets
Surface chemistry
A key development is the use of amphiphilic compounds to coat the surface of hydrophobic QDs. In one method, QDs are solubilized with an octylamine-modified polyacrylic polymer [9]. The hydrophobic alkyl side chains strongly interact with tri-n-octylphosphine oxide (TOPO) on the QD surface, and the hydrophilic carboxylic acid groups face outward and render QDs water soluble. In another method, QDs are coated with a polyethylene glycol (PEG)–lipid layer, which has an amphiphilic surfactant
Labeling single cells
Cellular labeling using organic dyes and fluorescent proteins has had great success, and state-of-the-art instrumentation currently allows simultaneous measurement of up to 13 parameters on individual cells [14]. Nevertheless, traditional fluorophores suffer from several problems, such as photobleaching, spectral cross-talking and narrow excitation. QDs have the potential to overcome these problems. As demonstrated by Wu et al. [9], the QD-labeled cells are brighter and more resistant to
Clinical tissue specimens
A further application of QDs is in multiplexed labeling and molecular analysis of pathological tissue specimens. In comparison with single cells, clinical tissue specimens are often highly heterogeneous (containing different cell populations in various microenvironments) and are therefore more difficult to analyze 15, 16. QDs, coupled with spectroscopy and spectral imaging, could have an important role in mapping out the true molecular profiles associated with different diseases or different
In vivo applications
Ruoslahti and co-workers first reported the use of QD–peptide conjugates to target tumor vasculature in vivo [11]. Histological staining revealed that QDs homed to tumor vessels guided by the peptides and were able to escape the reticuloendothelial system. They also showed that multiple peptide molecules could be conjugated to a single dot, achieving enhanced binding affinities and exquisite specificities through a multivalency effect. This is particularly important for small-molecule ligands
Note added in proof
The recent ‘burst’ of activities on biological applications of QDs should also include a paper by Bruchez and colleagues [23], which demonstrated the use of QDs for multiphoton fluorescence imaging of small vasculatures in vivo, and a paper by Aida and colleagues [24], which showed that ATP molecules could trigger the release of QDs from the cavities of chaperonin proteins.
Acknowledgements
The authors are grateful to Dominic Ansari and Dr. Zhongxing Liang for technical help, and to Professors Leland Chung, Fray Marshall, John Petros, Hyunsuk Shim, Jonathan Simons, and Lily Yang for stimulating discussions on cancer research. They also acknowledge financial support from the National Institute of Health (R01 GM60562) and the Coulter Translational Research Program at Georgia Tech and Emory University.
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