Semiconductor quantum dots as fluorescent probes for in vitro and in vivo bio-molecular and cellular imaging

Abstract

Over the years, biological imaging has seen many advances, allowing scientists to unfold many of the mysteries surrounding biological processes. The ideal imaging resolution would be in nanometres, as most biological processes occur at this scale. Nanotechnology has made this possible with functionalised nanoparticles that can bind to specific targets and trace processes at the cellular and molecular level. Quantum dots (QDs) or semiconductor nanocrystals are luminescent particles that have the potential to be the next generation fluorophores. This paper is an overview of the basics of QDs and their role as fluorescent probes for various biological imaging applications. Their potential clinical applications and the limitations that need to be overcome have also been discussed.

Keywords: in vitro and in vivo imaging; quantum dots.

Fig. 1

Structure of a quantum dot.

Fig. 2

Size tunable emission.

Fig. 3

Pathway of processing quantum dots for biological application.

Fig. 4

Biological applications.

Fig. 5

Fixed cell imaging and simultaneous detection of multiple cellular targets using QD conjugates. (A) Nuclear antigens in the nuclei of human epithelial cells labelled with ANA, anti-human IgG-biotin and QD 630-streptavidin. (B) When normal human IgGs were used, no detectable stain was observed. (C) Simultaneous labelling of nuclear antigens (red) and microtubules (green) using different QD conjugates in a 3T3 cell. (D) Her2 on the surface of SK-BR-3 cells was stained green with mouse anti-Her2 antibody and QD 535-IgG (green). Nuclear antigens were labelled with ANA, anti-human IgG-biotin and QD 630-streptavidin (red). Reprinted with permission from Macmillan Publishers Ltd: Nature Biotechnology (30) copyright 2003.

Fig. 6

Detection of chromosome region 1q12 in human metaphase chromosomes by FISH using a QD-labelled probe. (A) Control (no QD conjugate); (B) streptavidin-Qdot 605 detection of chromosome 1q12 region in homologous chromosomes (vertical and horizontal arrows); bar in panel C is 10 µm. (32). Printed by permission of Oxford University Press.

Fig. 7

Detection of sentinel lymph node using NIR QDs in a mouse model. NIR QDs injected intradermally into the foot pad of a mouse migrate to the sentinel lymph node 5 min post injection. (A) Colour video image; (B) NIR fluorescence image; isosulphan blue dye colocalised to the same node (indicated by the arrows). Reprinted by permission from Macmillan Publishers Ltd: Nature Biotechnology (21), copyright 2004.

Fig. 8

Schematic diagram of a near-infrared imaging system for SLNB in breast cancer surgery.

Fig. 9

Mechanism of PDT using quantum dots. QD-PS FRET pair localises to the site of cancer and is activated by light of a specific wavelength in the presence of molecular oxygen (³O₂) to generate singlet oxygen, which is toxic to cancer cells. Free radicals are also generated directly by activation of the QD by light.