Potential clinical applications of quantum dots

Abstract

The use of luminescent colloidal quantum dots in biological investigations has increased dramatically over the past several years due to their unique size-dependent optical properties and recent advances in biofunctionalization. In this review, we describe the methods for generating high-quality nanocrystals and report on current and potential uses of these versatile materials. Numerous examples are provided in several key areas including cell labeling, biosensing, in vivo imaging, bimodal magnetic-luminescent imaging, and diagnostics. We also explore toxicity issues surrounding these materials and speculate about the future uses of quantum dots in a clinical setting.

Figure 1

Comparison of Rhodamine Red (RR) and DsRed2 spectra to those of representative samples of QDs. Multiple QD emission spectra fit within the same spectral window as that of an organic or genetically encoded dye. (a) Absorption and emission of xis6 different QD populations. The black line shows the absorption of the 510 nm emitting QDs. (b) Absorption and emission of RR, a common organic dye, and genetically encoded DsRed2 protein (Baird et al 2000). (c) Color photo demonstrating the size-tunable fluorescence properties and spectral range of the six QD dispersions plotted in (a) versus average CdSe core size. All samples were excited at 365 nm with a UV lamp. For the 610 nm emitting QDs, this demonstrates an effective Stokes shift of about 250 nm. (d) Comparison of QD size to a maltose binding protein (MBP) molecule. 555 nm emitting CdSe-ZnS core-shell QDs, diameter ~60 Å, surface functionalized with dihydrolipoic acid (red shell ~9–11 Å) has a diameter ~78–82 Å. Diagram depicts the homogeneous orientation MBP assumes relative to the QD (Copyright © 2004 National Academy of Sciences USA) (Medintz et al 2004). MBP is a midsize protein (Mr ~44 kDa) with dimensions of 30 × 40 × 65 Å (Medintz et al 2003). Source: Medintz IL, Uyeda HT, et al 2005. Quantum dot bioconjugates for imaging, labeling, and sensing. Nature Materials 4:435–46. Reproduced with permission.

Figure 2

Examples of surface ligands used to produce water soluble nanocrystals. Following synthesis, the hydrophobic coordinating ligands in (a) can be replaced with hydrophilic moieties through a cap exchange process. Some possibilities include: (b) mercapto n-alkyl acids (MnA; eg, mercaptoacetic acid, MAA) (Chan and Nie 1998), (c) dithiothreitol (DTT) (Pathak et al 2001), (d) dihydrolipoic acid (DHLA) (Mattoussi et al 2000), (e) peptides containing appropriate high affinity residues (e.g, Cys – C and His – H) (Pinaud et al 2004), (f) trishydroxypropyl phosphine (THPP) (Kim and Bawendi 2003), (g) dihydrolipoic acid-polyethylene oxide (DHLA-PEG) (Uyeda et al 2005), (h) dendrons (Guo et al 2003). Many of these can be processed further to crosslink the ligands, add new functional groups, and/or attach biomolecules (QD/ligands not drawn to scale).

Figure 3

(a) Emission maxima and sizes of quantum dots of different composition. Quantum dots can be synthesized from various types of semiconductor materials (II-VI: CdS, CdSe, CdTe; III-V: InP, InAs.; IV-VI: PbSe) characterized by different bulk band gap energies. The curves represent experimental data from the literature on the dependence of peak emission wavelength on QD diameter. The range of emission wavelength is 400 to 1350 nm, with size varying from 2.0 to 9.5 nm (excluding the organic passivation/solubilization layer). All spectra are typically around 30 to 50 nm (full width at half maximum). Inset: Representative emission spectra for some materials. Data for CdHgTe-ZnS have been extrapolated to the maximum emission wavelength obtained in the Weiss group. (b) Absorption (upper curves) and emission (lower curves) spectra of four CdSe-ZnS QD samples. The blue vertical line indicates the 488 nm line of an argon-ion laser, which can be used to efficiently excite all four types of QD simultaneously. (c) Size comparison of QDs and comparable probes/biomolecules. FITC, fluorescein isothiocyanate; GFP, green fluorescent protein; QD, green (4 nm, top) and red (6.5 nm, bottom) CdSe-ZnS QD; qrod, rod-shaped QD (sizes reported on Invitrogen’s web site). Three proteins – streptavidin (SAV), maltose binding protein (MBP), and immunoglobulin G (IgG) – have been used for further functionalization of QDs and add to the final size of the QD, in conjunction with the solubilization chemistry. Copyright © 2005 AAAS. Michalet X, Pinaud FF, et al 2005. Quantum dots for live cells, in vivo imaging, and diagnostics. Science, 307:538–44. Reproduced with permission.

Figure 4

NIR QD imaging in vivo. Images showing the surgical field of a pig injected intradermally with 400 pmol of NIR QDs in the right groin. Four time points are shown from top to bottom: before injection (autofluorescence), 30 s after injection, 4 min after injection and during image-guided resection. For each time point, color video (left), NIR fluorescence (middle) and color-NIR merge (right) images are shown. Note the lymphatic vessel draining to the sentinel node from the injection site. Copyright © 2004 Nature Publishing Group. Kim S, Lim YT, Soltesz EG, et al 2004. Near-infrared fluorescent type II quantum dots for sentinel lymph node mapping. Nature Biotechnology 22:93–7. Reproduced with permission.

Figure 5

Specificity of fluorophore – streptavidin detection of biotinylated total human DNA probe in metaphase chromosomes and photostability (Mulder et al 2006). (a) Control (no fluorophore – streptavidin conjugate); (b) streptavidin – Qdot 605 detection of chromosome 1q12 region (vertical and horizontal arrows); (c) Texas Red – streptavidin detection of biotinylated DNA hybridized to 1q12 (vertical arrows) and (d) FITC – streptavidin detection of 1q12 sites (vertical arrows). Bar in panel (c) is 10 μm. (e) Signal decay upon continuous illumination with fluorescence microscope/mercury illumination in metaphase chromosome band 1q12 during 2 h continuous illumination. Red is Qdot 605, green is Texas Red, and blue is FITC. (f) Total intensity of whole interphase nuclei during 120 ms illumination (blue bars) and background (red bars). N = 3 cells in each. Copyright © 2004 Oxford University Press. Xiao Y, Barker PE. 2004. Semiconductor nanocrystal probes for human metaphase chromosomes. Nucleic Acids Research, 32:e28. Reproduced with permission.

Figure 6

Strategies for creating bifunctional luminescent-magnetic QD materials. (a) Schematic and TEM image of heterodimeric luminescent/magnetic CdS/FePt nanoparticles. Copyright © 2004 American Chemical Society. Gu H, Zheng R, et al 2004. Facile one-pot synthesis of bifunctional heterodimers of nanoparticles: A conjugate of quantum dot and mangetic nanoparticles. J Am Chem Soc 126:5664–5. Reproduced with permission. (b) Schematic of the synthesis and properties of Co/CdSe core/shell nanocomposite nanocrystals. Copyright © 2005 American Chemical Society. Kim H, Achermann M, et al 2005. Synthesis and characterization of Co/CdSe core/shell nanocomposites: Bifunctional magnetic-optical nanocrystals. J Am Chem Soc 127:544–6. Reproduced with permission. (c) Schematic representation of the preparation of QDs with a paramagnetic micellar coating. Copyright © 2006 American Chemical Society. Mulder WJM., Koole R, et al 2006. Quantum dots with a paramagnetic coating as a bimodal molecular imaging probe. Nano Letters 6:1–6. Reproduced with permission. (d) Schematic drawing of the synthesis of chitosan nanobeads that encapsulate both semiconductor QDs and paramagnetic Gd-DTPA. Copyright © 2005 Wiley-VCH. Tan WB, Zhang Y. 2005. Multifunctional quantum-dot-based magnetic chitosan nanobeads. Adv Mater, 17:2375–80. Reproduced with permission.