Early disease detection through quantum dots

Michael A. Greenwood

Detecting diseases such as cancer in their initial stages can mean the difference between life and death. A system that provides such warning can give treatments — the identical methods that might be ineffective only months or even weeks later — a chance to work.

One field of study being explored by researchers focuses on protein-based recognition of various types of cancers and other medical conditions. Although approaches already exist to do this type of molecular profiling — including gel electrophoresis, Western blot and protein microarrays — these methods are too costly, time-consuming and complex to be practical in many cases.

Two researchers at Vanderbilt University in Nashville, Tenn., report that they have devised a novel detection technique that relies on conjugated quantum dots that self-assemble and agglomerate on the target protein, supplying a bright fluorescent tag that can be detected easily and quickly. The technique has the potential to offer highly sensitive and relatively inexpensive disease detection.

Quantum dots are surface-functionalized with polyclonal antibodies to respond to several different proteins (a). These conjugates are deposited into a sample well (b). The biological sample is introduced, and the proteins bridge the polyclonal antibodies, forming two-body quantum dot agglomerates (c). The agglomeration continues, allowing particles that can be characterized by light scattering and by fluorescence (d). The agglomerates are detected with flow cytometry (e). Courtesy of Todd D. Giorgio.

Researchers Todd D. Giorgio and Chinmay P. Soman used commercial quantum dots with emission wavelengths of 525, 585 and 705 nm and conjugated them with polyclonal antibodies using a streptavidin-biotin interaction. In the presence of two target proteins used during the experiments — the human cancer biomarker protein angiopoietin-2 and mouse IgG — the quantum dots first formed into two-body agglomerates and then continued to form into multibody agglomerates around the proteins.

The investigators detected the targeted proteins with flow cytometry. A BioTek microplate reader was used to measure bulk fluorescence, and dynamic light scatter measurements were taken with a device from Malvern Instruments Ltd. of Malvern, UK. Measuring the light scattering enabled easy screening of the agglomerated quantum dots from unattached quantum dots.

The target proteins were detected at sub-picomolar concentrations using the method. In the case of angiopoietin-2, concentrations of 0.5 pM were identified. The sensitivities are comparable to those achieved using existing techniques.

The quantum dot technique requires only a single-step reaction, a characteristic that might allow it to be implemented on a microfluidic device for automated molecular diagnostics.

The investigators said that they want to create a completely automated platform and that they are working with other groups at Vanderbilt to advance the technology. Further optimizing the quantum dot conjugates for detection will be the focus of future research.

Langmuir, April 15, 2008, pp. 4399-4404.