量子點(diǎn)（quantum dot）是納米大小的發(fā)光晶體，可以從金,、銀等金屬或者半導(dǎo)體中獲得,。由于有納米級(jí)別的大小，量子點(diǎn)具有引人注目的光學(xué)性質(zhì)：熒光強(qiáng),、穩(wěn)定耐光,、可激發(fā)多種熒光、激發(fā)光譜廣和發(fā)射熒光光譜窄等優(yōu)點(diǎn),。而且,，由于比一般的熒光標(biāo)記物質(zhì)要小得多，所以不會(huì)干擾被標(biāo)記分子的生物活性,。正是因?yàn)槿绱?，一直都是科學(xué)家熒光標(biāo)記的重要候選物。不過,，遺憾的是,，量子點(diǎn)也有“先天不足”：生理環(huán)境會(huì)損害量子點(diǎn)的光學(xué)特性，反過來,，量子點(diǎn)對(duì)機(jī)體核細(xì)胞也有毒,。
雖然研究者苦苦探索，卻一直都沒有實(shí)現(xiàn)利用量子點(diǎn)進(jìn)行活體定位標(biāo)記和成像,。
在最新的一期《Nature biotechnology》上,，Emory University的科學(xué)家利用特別的生物兼容性量子點(diǎn)平臺(tái)實(shí)現(xiàn)了這個(gè)長(zhǎng)時(shí)間來夢(mèng)寐以求的愿望。研究者利用一種ABC triblock共聚物包被由半導(dǎo)體硒化鎘（cadmium selenide）產(chǎn)生的量子點(diǎn),，并進(jìn)一步用聚乙烯包被,。這種液態(tài)包被環(huán)境既能保存其光學(xué)特性，又能夠保護(hù)不受外界極度 的酸堿鹽條件的影響,。
接下來是功能性試驗(yàn)中,，研究者將這種包被的量子點(diǎn)連接到前列腺腫瘤特異的抗體上，并注入到前列腺腫瘤的小鼠,，觀察證明它們定位于前列腺腫瘤細(xì)胞,。當(dāng)小鼠被光照，這些量子點(diǎn)能夠發(fā)光,，表明腫瘤的部位和大小,。
這項(xiàng)研究似乎揭示了量子點(diǎn)診斷的無限潛力,，我們能夠進(jìn)一步研究，似乎可以預(yù)期這樣的一種美好的前景：當(dāng)病人接受一種追蹤試劑,，然后進(jìn)行全身掃描,，就可以知道潛在的腫瘤狀況。然后,，放大腫瘤部位,，甚至可以了解細(xì)胞或者亞細(xì)胞水平的狀況。
In vivo cancer targeting and imaging with semiconductor quantum dots
We describe the development of multifunctional nanoparticle probes based on semiconductor quantum dots (QDs) for cancer targeting and imaging in living animals. The structural design involves encapsulating luminescent QDs with an ABC triblock copolymer and linking this amphiphilic polymer to tumor-targeting ligands and drug-delivery functionalities. In vivo targeting studies of human prostate cancer growing in nude mice indicate that the QD probes accumulate at tumors both by the enhanced permeability and retention of tumor sites and by antibody binding to cancer-specific cell surface biomarkers. Using both subcutaneous injection of QD-tagged cancer cells and systemic injection of multifunctional QD probes, we have achieved sensitive and multicolor fluorescence imaging of cancer cells under in vivo conditions. We have also integrated a whole-body macro-illumination system with wavelength-resolved spectral imaging for efficient background removal and precise delineation of weak spectral signatures. These results raise new possibilities for ultrasensitive and multiplexed imaging of molecular targets in vivo.
Figure 1. Schematic illustration of biconjugated QDs for in vivo cancer targeting and imaging.
(a) Structure of a multifunctional QD probe, showing the capping ligand TOPO, an encapsulating copolymer layer, tumor-targeting ligands (such as peptides, antibodies or small-molecule inhibitors) and polyethylene glycol (PEG). (b) Chemical modification of a triblock copolymer with an 8-carbon side chain. This hydrophobic side chain is directly attached to the hydrophilic acrylic acid segment and interacts strongly with the hydrophobic tails of TOPO. Dynamic light scattering shows a compact QD-polymer structure, indicating that QDs are tightly wrapped by the hydrophobic segments and hydrocarbon side chains. (c) Permeation and retention of QD probes via leaky tumor vasculatures (passive targeting) and high affinity binding of QD-antibody conjugates to tumor antigens (active targeting).
Figure 4. Spectral imaging of QD-PSMA Ab conjugates in live animals harboring C4-2 tumor xenografts.
Orange-red fluorescence signals indicate a prostate tumor growing in a live mouse (right). Control studies using a healthy mouse (no tumor) and the same amount of QD injection showed no localized fluorescence signals (left). (a) Original image; (b) unmixed autofluorescence image; (c) unmixed QD image; and (d) super-imposed image. After in vivo imaging, histological and immunocytochemical examinations confirmed that the QD signals came from an underlying tumor. Note that QDs in deep organs such as liver and spleen were not detected because of the limited penetration depth of visible light.
Figure 5. In vivo fluorescence images of tumor-bearing mice using QD probes with three different surface modifications: carboxylic acid groups (left), PEG groups (middle) and PEG-PSMA Ab conjugates (right).
For each surface modification, a color image (top), two fluorescence spectra from QD and animal skin (middle) and a spectrally resolved image (bottom) were obtained from the live mouse models bearing C4-2 human prostate tumors of similar sizes (0.5–1.0 cm in diameter). The amounts of injected QDs and the lengths of circulation were: 6 nmol and 6 h for the COOH probe; 6 nmol and 24 h for the PEG probe; and 0.4 nmol and 2 h for the PSMA probe (same as in Fig. 4). The site of QD injection was observed as a red spot on the mouse tail. The spectral feature at 700 nm (red curve, middle panel) was an artifact caused by mathematical fitting of the original QD spectrum, which has little or no effect on background removal.
Figure 6. Sensitivity and multicolor capability of QD imaging in live animals.
(a,b) Sensitivity and spectral comparison between QD-tagged and GFP-transfected cancer cells (a), and simultaneous in vivo imaging of multicolor QD-encoded microbeads (b). The right-hand images in a show QD-tagged cancer cells (orange, upper) and GFP-labeled cells (green, lower). Approximately 1,000 of the QD-labeled cells were injected on the right flank of a mouse, while the same number of GFP-labeled cells was injected on the left flank (circle) of the same animal. Similarly, the right-hand images in b show QD-encoded microbeads (0.5 m diameter) emitting green, yellow or red light. Approximately 1–2 million beads in each color were injected subcutaneously at three adjacent locations on a host animal. In both a and b, cell and animal imaging data were acquired with tungsten or mercury lamp excitation, a filter set designed for GFP fluorescence and true color digital cameras. Transfected cancer cell lines for high level expression of GFP were developed by using retroviral vectors, but the exact copy numbers of GFP per cell have not been determined quantitatively56.
