Quantum Dots: Heralding a Brighter Future for Clinical Diagnostics

Tamer M Samir; Mai MH Mansour; Steven C Kazmierczak; Hassan ME Azzazy

Nanomedicine. 2012;7(11):1755-1769.

Future Perspective

The coming few years are likely to see QDs making a distinct improvement in the field of clinical laboratory medicine. QDs have the potential to be much more than just better fluorophores and could open up new frontiers beyond conventional testing strategies. The remarkable developments in other nanostructures have further expanded the realm of analytical strategies utilizing QDs. For example, a FRET-based assay for H5N1 influenza virus has been developed using QDs and carbon nanotubes.^[63] QDs have been used for simultaneous monitoring of a drug used for the treatment of colorectal cancer and the corresponding cancer biomarker. Thus, this allows assessment of the therapeutic effect of the drug on the cancer, opening a new arena of quantitative pharmacological and pharmacotherapeutic assays in which both the drug and its effect can be quantitatively measured.^[64] Additionally, QD-peptide conjugates have been used for multiplex detection of two enzymes of different classes, a protease and kinase, in a FRET-based assay, demonstrating significant utility for drug screening and diagnosis.^[65] QDs also show promise in supporting point-of-care testing. They have been used in a lateral flow strip format for the development of a portable biosensor for detecting nitrated ceruloplasmin as a marker of a cardiovascular disease, with detection limit of 8 ng/ml.^[66]

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Next Section

Abstract
Structure & Properties of Quantum Dots
Bioconjugation of QDs
Applications of QDs in the Clinical Laboratory
Toxicity Issues
Conclusion
Future Perspective
References
Sidebar

Table 1. Summary of quantum dot–protein/peptide conjugation approaches.
Biomolecule	QD type	QD surface cap	Strategy	Application	Ref.
Transferrin protein	CdSe/ZnS	Mercaptoacetic acid	Crosslinking of QD surface carboxylic group to reactive amine groups	Labeling and imaging of cultured HeLa cells	[19]
TAT peptide	CdTe	Thiol	Crosslinking using EDC	Labeling and imaging of human hepatocellular carcinoma (QGY) cells and human breast cancer (MCF7) cells	[24]
Phytochelatin-related peptides	CdSe/ZnS	TOPO	Direct binding with thiolated peptide containing cysteine residues	Labeling and imaging of HeLa cells	[13]
Multifunctional peptides contain protease recognition/cleavage sequence for caspase-1, collagenase and chymotrypsin	CdSe/ZnS	DHLA	Self-assembly of peptides containing amino-terminal hexahistidine domain on DHLA-capped QDs	Detect proteolytic activity of caspase-1, thrombin, collagenase and chymotrypsin	[28]
Maltose binding protein	CdSe/ZnS	DHLA	Self-assembly of QD and protein due to electrostatic attraction between the negatively charged DHLA cap on the QD and positively charged leucine zipper tail of the protein	An alternative conjugation strategy for conjugating proteins to the surface of CdSe/ZnS DHLA-capped QDs other than using EDC crosslinker	[20]
Concanavalin A protein	CdTe	TGA	Crosslinking of QD surface carboxyl group to reactive amine groups of concanavalin A	FRET-based glucose biosensor	[9]

DHLA: Dihydrolipoic acid; EDC: 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide; FRET: Förster resonance energy transfer; QD: Quantum dot; TGA: Thioglycolic acid; TOPO: Trioctylphosphine oxide.

Table 2. Selected quantum dot-based diagnostic applications.
QDs	Application	Ref.
CdTe/CdS QDs conjugated to goat anti-mouse IgG secondary antibodies	In sandwich and reverse-phase immunoassay formats, cancer biomarkers CEA and AFP were detected using respective mouse antibodies. The detection was carried out on a microfluidic chip with femtomolar sensitivity for both biomarkers When applied to human serum the detection limit was approximately 2.5 pM for CEA, and 250 fM for both AFP and CEA in buffer. Commercial ELISA kits have detection limits of 5 and 7 pM for CEA and AFP, respectively Quantitative determination of CEA and AFP was simultaneously achieved (concentration range: 25 fM to 25 nM)	[67]
CdSe/ZnS/SiO₂ QDs	QDs were used to count bacterial cells (Escherichia coli) in samples instead of the old viable count culture methods The method used glutaraldehyde as a crosslinker to covalently bind QDs to the bacteria. The bacteria were then mixed with the QD–glutaraldehyde mixture and the excess QDs were removed by ultrafiltration Direct bacterial counting with a detection limit of 3 × 10² CFU/ml (10–20-times lower than that of plate counting method) was used Reduction of detection time to 2 h	[68]
CdTe QDs conjugated to oligonucleotides (molecular beacon structure)	Simultaneous detection of single-base mutations in synthetic DNA targets using QDs conjugated to oligonucleotide molecular beacons and capillary electrophoresis analysis Detection limit of 16.2 pg of DNA	[69]