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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Adv Drug Deliv Rev. Author manuscript; available in PMC Aug 17, 2009.
Published in final edited form as:
PMCID: PMC2649798
NIHMSID: NIHMS62165

Bioconjugated Quantum Dots for In Vivo Molecular and Cellular Imaging

Abstract

Semiconductor quantum dots (QDs) are tiny light-emitting particles on the nanometer scale, and are emerging as a new class of fluorescent labels for biology and medicine. In comparison with organic dyes and fluorescent proteins, they have unique optical and electronic properties, with size-tunable light emission, superior signal brightness, resistance to photobleaching, and broad absorption spectra for simultaneous excitation of multiple fluorescence colors. QDs also provide a versatile nanoscale scaffold for designing multifunctional nanoparticles with both imaging and therapeutic functions. When linked with targeting ligands such as antibodies, peptides or small molecules, QDs can be used to target tumor biomarkers as well as tumor vasculatures with high affinity and specificity. Here we discuss the synthesis and development of state-of-the-art QD probes and their use for molecular and cellular imaging. We also examine key issues for in vivo imaging and therapy, such as nanoparticle biodistribution, pharmacokinetics, and toxicology.

Keywords: Quantum dots, nanocrystals, nanoparticles, nanotechnology, fluorescence, molecular imaging, cellular imaging, drug delivery, cancer, biomarkers, toxicology

1. Introduction

The development of biocompatible nanoparticles for molecular imaging and targeted therapy is an area of considerable current interest [19]. The basic rationale is that nanometer-sized particles have functional and structural properties that are not available from either discrete molecules or bulk materials [13]. When conjugated with biomolecular affinity ligands, such as antibodies, peptides or small molecules, these nanoparticles can be used to target malignant tumors with high specificity [1013]. Structurally, nanoparticles also have large surface areas for the attachment of multiple diagnostic (e.g., optical, radioisotopic, or magnetic) and therapeutic (e.g., anticancer) agents. Recent advances have led to the development of biodegradable nanostructures for drug delivery [1418], iron oxide nanocrystals for magnetic resonance imaging (MRI) [19, 20], luminescent quantum dots (QDs) for multiplexed molecular diagnosis and in vivo imaging [2125], as well as nanoscale carriers for siRNA delivery [26, 27].

Due to their novel optical and electronic properties, semiconductor QDs are being intensely studied as a new class of nanoparticle probe for molecular, cellular, and in vivo imaging [1024]. Over the past decade, researchers have generated highly monodispersed QDs encapsulated in stable polymers with versatile surface chemistries. These nanocrystals are brightly fluorescent, enabling their use as imaging probes both in vitro and in vivo. In this article, we discuss recent developments in the synthesis and modification of QD nanocrystals, and their use as imaging probes for living cells and animals. We also discuss the use of QDs as a nanoscale carrier to develop multifunctional nanoparticles for integrated imaging and therapy. In addition, we describe QD biodistribution, pharmacokinetics, toxicology, as well as the challenges and opportunities in developing nanoparticle agents for in vivo imaging and therapy.

2. QD Chemistry and Probe Development

QDs are nearly spherical semiconductor particles with diameters on the order of 2–10 nanometers, containing roughly 200–10,000 atoms. The semiconducting nature and the size-dependent fluorescence of these nanocrystals have made them very attractive for use in optoelectronic devices, biological detection, and also as fundamental prototypes for the study of colloids and the size-dependent properties of nanomaterials [28]. Bulk semiconductors are characterized by a composition-dependent bandgap energy, which is the minimum energy required to excite an electron to an energy level above its ground state, commonly through the absorption of a photon of energy greater than the bandgap energy. Relaxation of the excited electron back to its ground state may be accompanied by the fluorescent emission of a photon. Small nanocrystals of semiconductors are characterized by a bandgap energy that is dependent on the particle size, allowing the optical characteristics of a QD to be tuned by adjusting its size. Figure 1 shows the optical properties of CdSe QDs at four different sizes (2.2 nm, 2.9 nm, 4.1 nm, and 7.3 nm). In comparison with organic dyes and fluorescent proteins, QDs are about 10–100 times brighter, mainly due to their large absorption cross sections, 100–1000 times more stable against photobleaching, and show narrower and more symmetric emission spectra. In addition, a single light source can be used to excite QDs with different emission wavelengths, which can be tuned from the ultraviolet [29], throughout the visible and near-infrared spectra [3033], and even into the mid-infrared [34]. However QDs are macromolecules that are an order of magnitude larger than organic dyes, which may limit their use in applications in which the size of the fluorescent label must be minimized. Yet, this macromolecular structure allows the QD surface chemistry and biological functionality to be modified independently from its optical properties.

Figure 1
Size-dependent optical properties of cadmium selenide QDs dispersed in chloroform, illustrating quantum confinement and size tunable fluorescence emission. (a) Fluorescence image of four vials of monodisperse QDs with sizes ranging from 2.2 nm to 7.3 ...

2.1. QD Synthesis

QD synthesis was first described in 1982 by Efros and Ekimov [35, 36], who grew nanocrystals and microcrystals of semiconductors in glass matrices. Since this work, a wide variety of synthetic methods have been devised for the preparation of QDs in different media, including aqueous solution, high-temperature organic solvents, and solid substrates [28, 37, 38]. Colloidal suspensions of QDs are commonly synthesized through the introduction of semiconductor precursors under conditions that thermodynamically favor crystal growth, in the presence of semiconductor-binding agents, which function to kinetically control crystal growth and maintain their size within the quantum-confinement size regime.

The size-dependent optical properties of QDs can only be harnessed if the nanoparticles are prepared with narrow size distributions. Major progress toward this goal was made in 1993 by Bawendi and coworkers [39], with the introduction of a synthetic method for monodisperse QDs made from cadmium sulfide (CdS), cadmium selenide (CdSe), or cadmium telluride (CdTe). Following this report, the synthetic chemistry of CdSe QDs quickly advanced, generating brightly fluorescent QDs that can span the visible spectrum. As a result, CdSe has become the most common chemical composition for QD synthesis, especially for biological applications. Many techniques have been implemented to post-synthetically modify QDs for various purposes, such as coating with a protective inorganic shell [40, 41], surface modification to render colloidal stability [42, 43], and direct linkage to biologically active molecules [44, 45]. QD production has now become an elaborate molecular engineering process, best exemplified in the synthesis of polymer-encapsulated (CdSe)ZnS (core)shell QDs. In this method, CdSe cores are prepared in a nonpolar solvent, and a shell of zinc sulfide (ZnS) is grown on their surfaces. The QDs are then transferred to aqueous solution through encapsulation with an amphiphilic polymer, which can then be cross-linked to biomolecules to yield targeted molecular imaging agents.

In the design of a QD imaging probe, the selection of a QD core composition is determined by the desired wavelength of emission. For example, CdSe QDs may be size-tuned to emit in the 450–650 nm range, whereas CdTe can emit in the 500–750 nm range. QDs of this composition are then grown to the appropriate wavelength-dependent size. In a typical synthesis of CdSe, a room-temperature selenium precursor (commonly trioctylphosphine-selenide or tributylphosphine-selenide) is swiftly injected into a hot (~300°C) solution containing both a cadmium precursor (dimethylcadmium or cadmium oleate) and a coordinating ligand (trioctylphosphine oxide or hexadecylamine) under inert conditions (nitrogen or argon atmosphere). The cadmium and selenium precursors react quickly at this high temperature, forming CdSe nanocrystal nuclei. The coordinating ligands bind to metal atoms on the surfaces of the growing nanocrystals, stabilizing them colloidally in solution, and controlling their rate of growth. This injection of a cool solution quickly reduces the temperature of the reaction mixture, causing nucleation to cease. The remaining cadmium and selenium precursors then can grow on the existing nuclei at a slower rate at lower temperature (240–270°C). Once the QDs have reached the desired size and emission wavelength, the reaction mixture may be cooled to room temperature to arrest growth. The resulting QDs are coated in aliphatic coordinating ligands and are highly hydrophobic, allowing them to be purified through liquid-liquid extractions or via precipitation from a polar solvent.

Because QDs have high surface area to volume ratios, a large fraction of the constituent atoms are exposed to the surface, and therefore have atomic or molecular orbitals that are not completely bonded. These “dangling” orbitals serve as defect sites that quench QD fluorescence. For this reason, it is advantageous to grow a shell of another semiconductor with a wider bandgap on the core surface after synthesis to provide electronic insulation. The growth of a shell of ZnS on the surface of CdSe cores has been found to dramatically enhance photoluminescence efficiency [40, 41]. ZnS is also less prone to oxidation than CdSe, increasing the chemical stability of the QDs, and greatly decreasing their rate of oxidative photobleaching [46]. As well, the Zn2+ atoms on the surface of the QD bind more strongly than Cd2+ to most basic ligands, such as alkyl phosphines and alkylamines, increasing the colloidal stability of the nanoparticles [47]. In a typical shell growth of ZnS on CdSe, the purified cores are again mixed with coordinating ligands, and heated to an elevated temperature (140–240°C). Molecular precursors of the shell, usually diethylzinc and hexamethyldisilathiane dissolved in TOP, are then slowly added [40]. The (CdSe)ZnS nanocrystals may then be purified just like the cores.

More recently, it has become possible to widely engineer the fluorescence of QDs by changing the material composition while maintaining the same size. The technological advances that made this possible were the development of alloyed QDs [29, 30] and type-II heterostructures [32]. For example, homogeneously alloying the semiconductors CdTe and CdSe in different ratios allows one to prepare QDs of 5 nm diameter with emission wavelengths of 620 nm for CdSe, 700 nm for CdTe, and 800 nm for the CdSe0.34Te0.67 alloy [30]. Alternatively, type-II QDs allow one to physically separate the charge carriers (the electron and its cationic counterpart, known as the hole) into different regions of a QD by growing an appropriately chosen material on the QD as a shell [32]. For example, both the valence and conduction band energy levels of CdSe are lower in energy than those of CdTe. This means that in a heterostructure composed of CdTe and CdSe domains, electrons will segregate to the CdSe region to the lowest energy of the conduction band, whereas the hole will segregate to the CdTe region, where the valence band is highest in energy. This will effectively decrease the bandgap due to the smaller energy separating the two charge carriers, and emission will occur at a longer wavelength. By using different sizes of the core and different shell thicknesses, one can engineer QDs with the same size but different wavelengths of emission.

2.2. Surface Modification

QDs produced in nonpolar solutions using aliphatic coordinating ligands are only soluble in nonpolar organic solvents, making phase transfer an essential and nontrivial step for the QDs to be useful as biological reporters. Alternatively, QD syntheses have been performed directly in aqueous solution, generating QDs ready to use in biological environments [48], but these protocols rarely achieve the level of monodispersity, crystallinity, stability, and fluorescent efficiency as the QDs produced in high-temperature coordinating solvents. Two general strategies have been developed to render hydrophobic QDs soluble in aqueous solution: ligand exchange, and encapsulation by an amphiphilic polymer. For ligand exchange, a suspension of TOPO-coated QDs are mixed with a solution containing an excess of a heterobifunctional ligand, which has one functional group that binds to the QD surface, and another functional group that is hydrophilic. Thereby, hydrophobic TOPO ligands are displaced from the QD through mass action, as the new bifunctional ligand adsorbs to render water solubility. Using this method, (CdSe)ZnS QDs have been coated with mercaptoacetic acid and (3-mercaptopropyl) trimethoxysilane, both of which contain basic thiol groups to bind to the QD surface atoms, yielding QDs displaying carboxylic acids or silane monomers, respectively [44, 45]. These methods generate QDs that are useful for biological assays, but ligand exchange is commonly associated with decreased fluorescence efficiency and a propensity to aggregate and precipitate in biological buffers. More recently it has been shown that these problems can be alleviated by retaining the native coordinating ligands on the surface, and covering the hydrophobic QDs with amphiphilic polymers [10, 23, 49]. This encapsulation method yields QDs that can be dispersed in aqueous solution and remain stable for long periods of time due to a protective hydrophobic bilayer surrounding each QD through hydrophobic interactions. No matter what method is used to suspend the QDs in aqueous buffers, they should be purified from residual ligands and excess amphiphiles before use in biological assays, using ultracentrifugation, dialysis, or filtration. Also, when choosing a water solubilization method, it should be noted that many biological and physical properties of the QDs may be affected by the surface coating, and the overall physical dimensions of the QDs are dependent on the coating thickness. Typically the QDs are much larger when coated with amphiphiles, compared to those coated with a monolayer of ligand.

2.3. Bioconjugation

Water-soluble QDs may be cross-linked to biomolecules such antibodies, oligonucleotides, or small molecule ligands to render them specific to biological targets. This may be accomplished using standard bioconjugation protocols, such as the coupling of maleimide-activated QDs to the thiols of reduced antibodies [22]. The reactivities of many types of biomolecules have been found to remain after conjugation to nanoparticles surfaces, although possibly at a decreased binding strength. The optimization of surface immobilization of biomolecules is currently an active area of research [50, 51]. The surfaces of QDs may also be modified with bio-inert, hydrophilic molecules such as polyethylene glycol, to eliminate possible nonspecific binding, or to decrease the rate of clearance from the bloodstream following intravenous injection. QDs have also emerged as a new class of sensor, mediated by energy transfer to organic dyes (fluorescence resonance energy transfer, FRET) [5254]. It has also recently been reported that QDs can emit fluorescence without an external source of excitation when conjugated to enzymes that catalyze bioluminescent reactions, due to bioluminescence resonance energy transfer (BRET) [55].

Figure 2 depicts the most commonly used and technologically advanced QD probes. Biologically nonfunctional QDs may be prepared by using a variety of methods. As shown from left to right (top), QDs coated with a monolayer of hydrophilic thiols (e.g. mercaptoacetic acid) are generally stabilized ionically in solution [45]; QDs coated with a cross-linked silica shell can be readily modified with a variety of organic functionalities using well developed silane chemistry [44]; QDs encapsulated in amphiphilic polymers form highly stable, micelle-like structures [23, 49]; and any of these QDs may be modified to contain polyethylene glycol (PEG) to decrease surface charge and increase colloidal stability [56]. Also, water-soluble QDs may be covalently or electrostatically bound to a wide range of biologically active molecules to render specificity to a biological target. As shown in Figure 2 (middle), QDs conjugated to streptavidin may be readily bound to many biotinylated molecules of interest with high affinity [23]; QDs conjugated to antibodies can yield specificity for a variety of antigens, and are often prepared through the reaction between reduced antibody fragments with maleimide-PEG-activated QDs [22, 57]; QDs cross-linked to small molecule ligands, inhibitors, peptides, or aptamers can bind with high specificity to many different cellular receptors and targets [58, 59]; and QDs conjugated to cationic peptides, such as the HIV Tat peptide, can quickly associate with cells and become internalized via endocytosis [60]. Further, QDs have been used to detect the presence of biomolecules using intricate probe designs incorporating energy donors or acceptors. For example, QDs can be adapted to sense the presence of the sugar maltose by conjugating the maltose binding protein to the nanocrystal surface (Figure 2, bottom) [53]. By initially incubating the QDs with an energy-accepting dye that is conjugated to a sugar recognized by the receptor, excitation of the QD (blue) yields little fluorescence, as the energy is nonradiatively transferred (grey) to the dye. Upon addition of maltose, the quencher-sugar conjugate is displaced, restoring fluorescence (green) in a concentration-dependent manner. QDs can also be sensors for specific DNA sequences [52]. By mixing the ssDNA to be detected with (a) an acceptor fluorophores conjugated to a DNA fragment complementary to one end of the target DNA and (b) a biotinylated DNA fragment complementary to the opposite end of the target DNA, these nucleotides hybridize to yield a biotin-DNA-fluorophore conjugate. Upon mixing this conjugate with QDs, QD fluorescence (green) is quenched via nonradiative energy transfer (grey) to the fluorophore conjugate. This dye acceptor then becomes fluorescent (red), specifically and quantitatively indicating the presence of the target DNA. Finally, QDs conjugated to the luciferase enzyme can nonradiatively accept energy from the enzymatic bioluminescent oxidation of luciferins on the QD surface, exciting the QDs without the need for external illumination [55].

Figure 2
Schematic diagrams of nonfunctionalized and bioconjugated QD probes for imaging and sensing applications. See text for detailed discussion.

3. Live-Cell Imaging

Researchers have achieved considerable success in using QDs for in vitro bioassays [61, 62], labeling fixed cells [23] and tissue specimens [63, 64], and for imaging membrane proteins on living cells [58, 65]. However, only limited progress has been made in developing QD probes for imaging inside living cells. A major problem is the lack of efficient methods for delivering monodispersed (that is, single) QDs into the cytoplasms of living cells. A common observation is that QDs tend to aggregate inside cells, and are often trapped in endocytotic vesicles such as endosomes and lysosomes.

3.1. Imaging and Tracking of Membrane Receptors

QD bioconjugates have been found to be powerful imaging agents for specific recognition and tracking of plasma membrane antigens on living cells. In 2002 Lidke et al. coupled red-light emitting (CdSe)ZnS QDs to epidermal growth factor, a small protein with a specific affinity for the erbB/HER membrane receptor [58]. After addition of these conjugates to cultured human cancer cells, receptor-bound QDs could be identified at the single-molecule level (single QDs may be distinguished from aggregates because the fluorescent intensity from discrete dots is intermittent, or “blinking”). The bright, stable fluorescence emitted from these QDs allowed the continuous observation of protein diffusion on the cellular membrane, and could even be visualized after the proteins were internalized. Dahan et al. similarly reported that QDs conjugated to an antibody fragment specific for glycine receptors on the membranes of living neurons allowed tracking of single receptors [65]. These conjugates showed superior photostability, lateral resolution, and sensitivity relative to organic dyes. These applications have inspired the use QDs for monitoring other plasma membrane proteins such as integrins [50, 66], tyrosine kinases [67, 68], G-protein coupled receptors [69], and membrane lipids associated with apoptosis [70, 71]. As well, detailed procedures for receptor labeling and visualization of receptor dynamics with QDs have recently been published [72, 73], and new techniques to label plasma membrane proteins using versatile molecular biology methods have been developed [74, 75].

3.2. Intracellular Delivery of QDs

A variety of techniques have been explored to label cells internally with QDs, using passive uptake, receptor-mediated internalization, chemical transfection, and mechanical delivery. QDs have been loaded passively into cells by exploiting the innate capacity of many cell types to uptake their extracellular space through endocytosis [7678]. It has been found that the efficiency of this process may be dramatically enhanced by coupling the QDs to membrane receptors. This is likely due to the avidity-induced increase in local concentration of QDs at the surface of the cell, as well as an active enhancement caused by receptor-induced internalization [58, 77, 79]. However, these methods lead to sequestration of aggregated QDs in vesicles, showing strong colocalization with membrane dyes. Although these QDs cannot diffuse to specific intracellular targets, this is a simple way to label cells with QDs, and an easy method to fluorescently image the process of endocytosis. Nonspecific endocytosis was also utilized by Parak et al. to fluorescently monitor the motility of cells on a QD-coated substrate [78]. The path traversed by each cell became dark, and the cells increased in fluorescence as they took up more QDs. Chemically-mediated delivery enhances plasma membrane translocation with the use of cationic lipids or peptides, and was originally developed for the intracellular delivery of a wide variety of drugs and biomolecules [60, 8083]. The efficacy of these carriers for the intracellular deliver of QDs is discussed below (Section 3.3 and Section 3.4). Mechanical delivery methods include microinjection of QDs into individual cells, and electroporation of cells in the presence of QDs. Microinjection has been reported to deliver QDs homogeneously into the cytoplasms of cells [49, 83], however this method is of low statistical value, as careful manipulation of single cells prevents the use of large sample sizes. Electroporation makes use of the increased permeability of cellular membranes under pulsed electric fields to deliver QDs, but this method was reported to result in aggregation of QDs in the cytoplasm [83], and generally results in widespread cell death.

Despite the current technical challenges, QDs are garnering interest as intracellular probes due to their intense, stable fluorescence, and recent reports have demonstrated that intracellular targeting is not far off. In 2004, Derfus et al. demonstrated that QDs conjugated to organelle-targeting peptides could specifically stain either cellular mitochondria or nuclei, following microinjection into fibroblast cytoplasms [83]. Similarly, Chen et al. targeted peptide-QD conjugates to cellular nuclei, using electroporation to overcome the plasma membrane barrier [60]. These schemes have resulted in organelle-level resolution of intracellular targets for living cells, yielding fluorescent contrast of vesicles, mitochondria, and nuclei, but not the ability to visualize single molecules. Recently Courty et al. demonstrated the capacity to image individual kinesin motors in HeLa cells using QDs delivered into the cytoplasm via osmotic lysis of pinocytotic vesicles [84]. By incubating the cells in a hypertonic solution containing QDs, water efflux resulted in membrane invagination and pinocytosis, trapping extracellular QDs in endosomal vesicles. Then a brief incubation in hypotonic medium induced intracellular water influx, rupturing the newly formed vesicles, and releasing single QDs into the cytosol. All of the QDs were observed to undergo random Brownian motion in the cytoplasm. However if these QDs were first conjugated to kinesin motor proteins, a significant population of the QDs exhibited directional motion. The velocity of the directed motion and its processivity (average time before cessation of directed motion) were remarkably close to those observed for the motion of these conjugates on purified microtubules in vitro. Although this work managed to overcome the plasma membrane diffusion barrier, it highlighted a different problem fundamental to intracellular imaging of living cells, which is the impossibility of removing probes that have not found their target. In this report, the behavior of the QDs was sufficient to distinguish bound QDs from those that were not bound, but this will not be the case for the majority of other protein targets. Without the ability to wash away unbound probes, which is a crucial step for intracellular labeling of fixed, permeabilized cells, the need for activateable probes that are ‘off’ until they reach their intended target is apparent. However QDs have already found a niche for quantitative monitoring of motor protein transport and for tracking the fate of internalized receptors, allowing the study of downstream signaling pathways in real time with high signal-to-noise and high temporal and spatial resolution [58, 67, 68, 85, 86].

3.3. Tat-QD Conjugates

Cell-penetrating peptides are a class of chemical transfectants that have garnered widespread interest due to the high transfection efficiency of their conjugated cargo, versatility of conjugation, and low toxicity. For this reason, cell-penetrating peptides such as polyarginine and Tat have been investigated for their capacity to deliver QDs into living cells [81, 85, 87], but the delivery mechanism and the behavior of intracellular QDs are still a matter of debate. Considerable effort has been devoted to understanding the delivery mechanism of these cationic carrier, especially the HIV-1-derived Tat peptide, which has emerged as a widely used cellular delivery vector [8893]. The delivery process was initially thought to be independent of endocytosis because of its apparent temperature-independence [8993]. However, later research showed that the earlier work failed to exclude the Tat peptide conjugated cargos bound to plasma membranes, and was largely an artifact caused by cellular fixation. More recent studies based on improved experimental methods indicate that Tat peptide-mediated delivery occurs via macropinocytosis [94], a fluid-phase endocytosis process that is initiated by the binding of Tat-QD to the cell surface [90]. These new results, however, did not shed any light on the downstream events or the intracellular behavior of the internalized cargo. This kind of detailed and mechanistic investigation would be possible with QDs, which are sufficiently bright and photostable for extended imaging and tracking of intracellular events. In addition, most previous studies on Tat peptide-mediated delivery are based on the use of small dye molecules and proteins as cargo [8993], so it is not clear whether larger nanoparticles would undergo the same processes of cellular uptake and transport. This understanding is needed for the design and development of imaging and therapeutic nanoparticles for biology and medicine.

Ruan et al. have recently used Tat peptide-conjugated QDs (Tat-QDs) as a model system to examine the cellular uptake and intracellular transport of nanoparticles in live cells [95]. The authors used a spinning-disk confocal microscope for dynamic fluorescence imaging of quantum dots in living cells at 10 frames per second. The results indicate that the peptide-conjugated QDs are internalized by macropinocytosis, in agreement with the recent work of Dowdy and coworkers [90]. It is interesting, however, that the internalized Tat-QDs are tethered to the inner surface of vesicles, and are trapped in intracellular organelles. An important finding is that the QD-loaded vesicles are actively transported by molecular machines (such as dyneins) along microtubule tracks to an asymmetric perinuclear region called the microtubule organizing center (MTOC) [96]. Furthermore, it was found that Tat-QDs strongly bind to cellular membrane structures such as filopodia, and that large QD-containing vesicles are able to pinch off from the tips of filopodia. These results not only provide new insight into the mechanisms of Tat peptide-mediated delivery, but also are important for the development of nanoparticle probes for intracellular targeting and imaging.

3.4. QDs with Endosome-Disrupting Coatings

Duan and Nie [97] developed a new class of cell-penetrating quantum dots (QDs) based on the use of multivalent and endosome-disrupting (endosomolytic) surface coatings (Figure 3). Hyperbranched copolymer ligands such as PEG-grafted polyethylenimine (PEI-g-PEG) were found to encapsulate and stabilize luminescent quantum dots in aqueous solution through direct ligand binding to the QD surface. Due to the cationic charges and a “proton sponge effect” [98100] associated with multivalent amine groups, these QDs could penetrate cell membranes and disrupt endosomal organelles in living cells. This mechanism arises from the presence of a large number of weak bases (with buffering capabilities at pH 5–6), which lead to proton absorption in acidic organelles, and an osmotic pressure buildup across the organelle membrane [100]. This osmotic pressure causes swelling and/or rupture of the acidic endosomes and a release of the trapped materials into the cytoplasm. PEI and other polycations are known to be cytotoxic, however the grafted PEG segment was found to significantly reduce the toxicity and improve the overall nanoparticle stability and biocompatibility. In comparison with previous QDs encapsulated with amphiphilic polymers, the cell-penetrating QDs were smaller in size and exceedingly stable in acidic environments [56]. Cellular uptake and imaging studies revealed that these dots were rapidly internalized by endocytosis, and the pathways of the QDs inside the cells showed dependence on the number of PEG grafts of the polymer ligands. While higher PEG content led to QD sequestration in vesicles, the QDs coated by PEI-g-PEG with fewer PEG grafts are able to escape from endosomes and release into the cytoplasm.

Figure 3
Encapsulation and solubilization of core-shell CdSe/CdS/ZnS quantum dots by using multivalent and hyperbranched copolymer ligands. (a) and (b) Chemical structures of PEI and 19 PEI-g-PEG copolymers consisting of two or four PEG chains per PEI polymer ...

Lovric et al. [101] recently reported that very small QDs (2.2 nm) coated with small molecule ligands (cysteamine) spontaneously translocated to the nuclei of murine microglial cells following cellular uptake through passive endocytosis. In contrast, larger QDs (5.5 nm) and small QDs bound to albumin remained in the cytosol only. This is fascinating because these QDs could not only escape from endocytotic vesicles, but were also subjected to an unknown type of active machinery that attracted the QDs to the nucleus. Nabiev et al. [102] studied a similar trend of size-dependent QD segregation in human macrophages, and found that small QDs may target nuclear histones and nucleoli after active transport across the nuclear membrane. They found that the size cut-off for this effect was around 3.0 nm. Larger QDs eventually ended up in vesicles in the MTOC region, although some QDs were found to be free in the cytoplasm. This group proposed that the proton sponge effect was also responsible for endosomal escape, as small carboxyl-coated QDs could buffer in the pH 5–7 range. These insights are important for the design and development of nanoparticle agents for intracellular imaging and therapeutic applications.

4. In Vivo Animal Imaging

Compared to the study of living cells in culture, different challenges arise with the increase in complexity to a multicellular organism, and with the accompanying increase in size. Unlike monolayers of cultured cells and thin tissue sections, tissue thickness becomes a major concern because biological tissue attenuates most signals used for imaging. Optical imaging, especially fluorescence imaging, has been used in living animal models, but it is still limited by the poor transmission of visible light through biological tissue. It has been suggested that there is a near-infrared optical window in most biological tissue that is the key to deep-tissue optical imaging [103]. The rationale is that Rayleigh scattering decreases with increasing wavelength, and that the major chromophores in mammals, hemoglobin and water, have local minima in absorption in this window. Few organic dyes are available that emit brightly in this spectral region, and they suffer from the same photobleaching problems as their visible counterparts, although this has not prevented their successful use as contrast agents for living organisms [104]. One of the greatest advantages of QDs for imaging in living tissue is that their emission wavelengths can be tuned throughout the near-infrared spectrum by adjusting their composition and size, resulting in photostable fluorophores that are stable in biological buffers [24].

4.1 Biodistribution of QDs

For most in vivo imaging applications using QDs and other nanoparticle contrast agents, systemic intravenous delivery into the bloodstream will be the main mode of administration. For this reason, the interaction of the nanoparticles with the components of plasma, the specific and nonspecific adsorption to blood cells and the vascular endothelium, and the eventual biodistribution in various organs are of great interest. Immediately upon exposure to blood, QDs may be quickly adsorbed by opsonins, in turn flagging them for phagocytosis. In addition, platelet coagulation may occur, the complement system may be activated, or the immune system can be stimulated or repressed (Figure 4). Although it is important for each of these potential biological effects to be addressed in detail, so far there are no studies that directly examine blood or immune system biocompatibility of QDs in vivo or ex vivo. However, a recent review article by Dobrovolskaia and McNeil addresses the immunological properties of polymeric, liposomal, carbon-based, and magnetic nanoparticles [105]. Considering the many factors that may affect systemically administered QDs, such as size, shape, charge, targeting ligands, etc., the two most important parameters that affect biodistribution are likely size and the propensity for serum protein adsorption.

Figure 4
Schematic diagram showing QD interactions with blood immune cells and plasma proteins. The probable modes of interactions include (a) QD opsonization and phagocytosis by leucocytes (e.g., monocytes), (b) non-specific QD-cell membrane interactions (electrostatic ...

The number of papers published on quantum dot pharmacokinetics and biodistribution is limited, but several common trends can be identified. It has been consistently reported that QDs are taken up nonspecifically by the reticuloendothelial system (RES), including the liver and spleen, and the lymphatic system [106108]. These findings are not necessarily intrinsic to QDs, but are strictly predicated upon the size of the QDs and their surface coatings. Ballou and coworkers reported that (CdSe)ZnS QDs were rapidly removed from the bloodstream into organs of the RES, and remained there for at least 4 months with detectable fluorescence [107]. TEM of these tissues revealed that these QDs retained their morphology, suggesting that given the proper coating, QDs are stable in vivo for very long periods of time without degradation into their potentially toxic elemental components. A complimentary work by Fischer, et al. showed that nearly 100% of albumin-coated QDs were removed from circulation and sequestered in the liver within hours after a tail vein injection, much faster than QDs that were not bound to albumin [108]. Within the liver, QDs conjugated to albumin were primarily associated with Kupffer cells (resident macrophages). From a clinical perspective, it may be possible to completely inhibit the accumulation of QDs and avoid potential toxic effects if they are within the size range of renal excretion. Recent publications have focused on this insight. Frangioni and coworkers demonstrated that the renal clearance of quantum dots is closely related to the hydrodynamic diameter of the nanoparticle and the renal filtration threshold (~5–6 nm) [109]. Of equal importance to the QD size, is that the surface does not promote protein adsorption, which could significantly increase QD size above that of the renal threshold, and promote phagocytosis. However, it is unlikely that even small QDs could be entirely eliminated from the kidneys, as it has also been found that small QDs (~9 nm) may directly extravasate out of blood vessels, into interstitial fluid [110].

For targeted imaging, specific modulation of the biodistribution of QD contrast agents is the main goal. One way to increase the probability of bioaffinity ligand-specific distribution is to increase the circulation time of the contrast agent in the bloodstream. QD structure and surface properties have been found to strongly impact the plasma half-life. It was demonstrated by Ballou et al. [107] that the lifetime of anionic, carboxylated QDs in the bloodstream of mice (4.6 minutes half-life) is significantly increased if the QDs are coated with PEG polymer chains (71 minutes half-life). This effect has also been documented for other types of nanoparticles and small molecules, in part due to decreased nonspecific adsorption of the nanoparticles, an increase in size, and decreased antigenicity [111]. In a more recent study using perfused porcine skin in vitro, Lee, et al. demonstrated that carboxylated QDs were extracted more rapidly from circulation, and had greater tissue deposition than PEG coated QDs [112]. It is important to note that a bioaffinity molecule may also be prone to RES uptake, despite a strong affinity for its intended target. For example, Jayagopal et al. reported that QD-antibody conjugates have a significantly longer circulation time if the Fc antibody regions (non-antigen binding domains) are immunologically shielded to reduce nonspecific interactions [113].

4.2. In Vivo Vascular Imaging

One of the most immediately successful applications of QDs in vivo has been their use as contrast agents for the two major circulatory systems of mammals, the cardiovascular system and the lymphatic system. In 2003, Larson et. al demonstrated that green-light emitting QDs remained fluorescent and detectable in capillaries of adipose tissue and skin of a living mouse following intravenous injection [114]. This work was aided by the use of near-infrared two-photon excitation for deeper penetration of excitation light, and by the extremely large two-photon cross-sections of QDs, 100–20,000 times that of organic dyes [115]. In other work, Lim et al. used near-infrared QDs to image the coronary vasculature of a rat heart [116], and Smith et al. imaged the blood vessels of chicken embryos with a variety of near-infrared and visible QDs [117]. The later report showed that QDs could be detected with higher sensitivity than traditionally used fluorescein-dextran conjugates, and resulted in a higher uniformity in image contrast across vessel lumena. Jayagopal et al. [113] recently demonstrated the potential for QDs to serve as molecular imaging agents for vascular imaging. Spectrally distinct QDs were conjugated to three different cell adhesion molecules (CAMs), and intravenously injected in a diabetic rat model. Fluorescence angiography of the retinal vasculature revealed CAM-specific increases in fluorescence, and allowed imaging of the inflammation-specific behavior of individual leukocytes, as they freely floated in the vessels, rolled along the endothelium, and underwent leukostasis. The unique spectral properties of QDs allowed the authors to simultaneously image up to four spectrally distinct QD tags.

For imaging of the lymphatic system, the overall size of the probe is an important parameter for determining biodistribution and clearance. For example, Kim et al. [24] intradermally injected ~16–19 nm near-infrared QDs in mice and pigs. QDs translocated to sentinel lymph nodes, likely due to a combination of passive flow in lymphatic vessels, and active migration of dendritic cells that engulfed the nanoparticles. Fluorescence contrast of these nodes could be observed up to 1 cm beneath the skin surface. It was found that if these QDs were formulated to have a smaller overall hydrodynamic size (~9 nm), they could migrate further into the lymphatic system, with up to 5 nodes showing fluorescence [110]. This technique could have great clinical impact due to the quick speed of lymphatic drainage and the ease of identification of lymph nodes, enabling surgeons to fluorescently identify and excise nodes draining from primary metastatic tumors for the staging of cancer. This technique has been used to identify lymph nodes downstream from the lungs [106, 118], esophagus [119], and from subcutaneous tumors [120]. Recently the multiplexing capabilities of QDs have been exploited for mapping lymphatic drainage networks. By injection of QDs of different color at different intradermal locations, these QDs could be fluorescently observed to drain to common nodes [121], or up to 5 different nodes in real time [122]. A current problem is that a major fraction of the QDs remain at the site of injection for an unknown length of time [123].

4.3. In Vivo Tracking of QD-Loaded Cells

Cells can also be loaded with QDs in vitro, and then administered to an organism, providing a means to identify the original cells and their progeny within the organism. This was first demonstrated on a small organism scale by microinjecting QDs into the cytoplasms of single frog embryos [49]. As the embryos grew, the cells divided, and each cell that descended from the original labeled cell retained a portion of the fluorescent cytoplasm, which could be fluorescently imaged in real time under continuous illumination. In reports by Hoshino et al. [124] and Voura et al. [82], cells loaded with QDs were injected intravenously into mice, and their distributions in the animals were later determined through tissue dissection, followed by fluorescence imaging. Also Gao et al. loaded human cancer cells with QDs, and injected these cells subcutaneously in an immune-compromised mouse [10]. The cancer cells divided to form a solid tumor, which could be visualized fluorescently through the skin of the mouse. Rosen et al. recently reported that human mesenchymal stem cells loaded with QDs could be implanted into an extracellular matrix patch for use as a regenerative implant for canine hearts with a surgically-induced defect [125]. Eight weeks following implantation, it was found that the QDs remained fluorescent within the cells, and could be used to track the locations and fates of these cells. This group also directly injected QD-labeled stem cells into the canine myocardium, and used the fluorescence signals in cardiac tissue sections to elaborately reconstruct the locations of these cells in the heart. With reports that cells may be labeled with QDs at a high degree of specificity [80, 81], it is foreseeable that multiple types of cells may be simultaneously monitored in living organisms, and also identified using their distinct optical codes.

4.4. In Vivo Tumor Imaging

Imaging of tumors presents a unique challenge not only because of the urgent need for sensitive and specific imaging agents of cancer, but also because of the unique biological attributes inherent to cancerous tissue. Blood vessels are abnormally formed during tumorinduced angiogenesis, having erratic architectures and wide endothelial pores. These pores are large enough to allow the extravasation of large macromolecules up to ~400 nm in size, which accumulate in the tumor microenvironment due to a lack of effective lymphatic drainage [126129]. This “enhanced permeability and retention” effect (EPR effect) has inspired the development of a variety of nanotherapeutics and nanoparticulates for the treatment and imaging of cancer (Figure 5). Because cancerous cells are effectively exposed to the constituents of the bloodstream, their surface receptors may also be used as active targets of bioaffinity molecules. In the case of imaging probes, active targeting of cancer antigens (molecular imaging) has become an area of tremendous interest to the field of medicine because of the potential to detect early stage cancers and their metastases. QDs hold great promise for these applications mainly due to their intense fluorescent signals and multiplexing capabilities, which could allow a high degree of sensitivity and selectivity in cancer imaging with multiple antigens.

Figure 5
Schematic diagram showing QDs involved in both active and passive tumor targeting. In the passive mode, nanometer-sized particles such as quantum dots accumulate at tumor sites through an enhanced permeability and retention (EPR) effect [126 ...

The first steps toward this goal were undertaken in 2002 by Akerman et al., who conjugated QDs to peptides with affinity for various tumor cells and their vasculatures [130]. After intravenous injection of these probes into tumor-bearing mice, microscopic fluorescence imaging of tissue sections demonstrated that the QDs specifically homed to the tumor vasculature. In 2004 Gao et al. demonstrated that tumor targeting with QDs could generate tumor contrast on the scale of whole-animal imaging [10]. QDs were conjugated to an antibody against the prostate-specific membrane antigen (PSMA), and intravenously injected into mice bearing subcutaneous human prostate cancers. Tumor fluorescence was significantly greater for the actively targeted conjugates compared to nonconjugated QDs, which also accumulated passively though the EPR effect. Using similar methods, Yu et al. were able to actively target and image mouse models of human liver cancer with QDs conjugated to an antibody against alpha-fetoprotein [131], and Cai et al. showed that labeling QDs with RGD peptide significantly increased their uptake in human glioblastoma tumors [132].

The development of clinically relevant QD contrast agents for in vivo imaging is certain to encounter many roadblocks in the near future (see Section 5), however QDs can currently be used as powerful imaging agents for the study of the complex anatomy and pathophysiology of cancer in animal models. Stroh et al. [133] demonstrated that QDs greatly enhance current intravital microscopy techniques for the imaging of tumor microenvironment. The authors used QDs as fluorescent contrast agents for blood vessels using two-photon excitation, and simultaneously captured images of extracellular matrix from autofluorescent collagen, and perivascular cell contrast from fluorescent protein expression. The use of QDs allowed stark contrast between the tumor constituents due to their intense brightness, tunable wavelengths, and reduced propensity to extravasate into the tumor, compared to organic dye conjugates. In this work, the authors also used QD-tagged beads with variable sizes to model the size-dependent distribution of various nanotherapeutics in tumors. As well, the authors demonstrated that bone marrow lineage-negative cells, which are thought to be progenitors for neovascular endothelium, were labeled ex vivo with QDs and imaged in vivo as they flowed and adhered to tumor blood vessels following intravenous administration. More recently, Tada et al. used QDs to study the biological processes involved in active targeting of nanoparticles. The authors used QDs labeled with an antibody against human epidermal growth factor receptor 2 (HER2) to target human breast cancer in a mouse model [134]. Through intravital fluorescence microscopy of the tumor following systemic QD administration, the authors could distinctly observe individual QDs as they circulated in the bloodstream, extravasated into the tumor, diffused in extracellular matrix, bound to their receptors on tumor cells, and then translocated into the perinuclear region of the cells. The combination of sensitive QD probes with powerful techniques like intravital microscopy and in vivo animal imaging could soon lead to major breakthroughs in the current understanding of tumor biology, improve early detection schemes, and guide new therapeutic designs.

5. Nanoparticle Toxicity

Great concern has been raised over the use of quantum dots in living cells and animals due to their chemical composition of toxic heavy metal atoms (e.g. Cd, Hg, Pb, As, Pb). Presently the most commonly used QDs contain divalent cadmium, a nephrotoxin in its ionic form. Although this element is incorporated into a nanocrystalline core, surrounded by biologically inert zinc sulfide, and encapsulated within a stable polymer, it is still unclear if these toxic ions will impact the use of QDs as clinical contrast agents. It may be of greater concern that QDs, and many other types of nanoparticles, have been found to aggregate, bind nonspecifically to cellular membranes and intracellular proteins, and induce the formation of reactive oxygen species. As previously stated, QDs larger than the renal filtration threshold quickly accumulate in the reticuloendothelial system following intravenous administration. The eventual fate of these nanoparticles is of vital importance, but so far has yet to be elucidated.

5.1. Cadmium Toxicity

In the only long-term, quantitative study on QD biodistribution to date, Yang, et al. showed that after intravenous administration of cadmium-based QDs, the concentration of cadmium in the liver and kidneys gradually increased over the course of 28 days, as determined via ICP-MS [135]. The cadmium levels in the kidneys eventually reached nearly 10% of the injected dose, compared to 40% in the liver. Although it was not apparent if the cadmium was in the form of a free ion, or remained in the nanocrystalline form, fluorescence microscopy revealed the presence of intact QDs in both the liver and kidneys. However the redistribution of the cadmium over time may signify the degradation of QDs in vivo, since the natural accumulation sites of Cd2+ ions are the liver and kidneys [79, 136, 137]. In acute exposures, free cadmium also may be redistributed to the kidneys via hepatic production of metallothionein [138]. Whether or not this is the specific mechanism observed in this report should be the focus of detailed in vivo validation studies. Nevertheless, these findings stress that (a) QD size and nonspecific protein interaction should be minimized to allow renal filtration, or else QDs will inevitably accumulate in organs and tissues of the RES, lung, and kidney, and (b) the potential release of the elements of the QD and their distribution in specific organs, tissues, cell types, and subcellular locations must be well understood.

In general, most in vitro studies on the exposure of cells to QDs have attempted to relate cytotoxic events to the release of potentially toxic elements and/or to the size, shape, surface, and cellular uptake of QDs. Because the toxicity of Cd2+ ions is well documented, a significant body of work has focused on the intracellular release of free cadmium from the QDs. Cd2+ ions can be released through oxidative degradation of the QD, and may then bind to sulfhydryl groups on a variety of intracellular proteins, causing decreased functionality in many subcellular organelles [139]. Several groups have investigated methods to quantify the amount of free Cd2+ ions released from QDs, either intracellularly or into culture media, by ICP-MS or fluorometric assays, leading to the conclusion that Cd2+ release correlates with cytotoxic manifestations [79, 140, 141]. Derfus, et al. facilitated oxidative release of cadmium ions from the surface of CdSe QDs by exposure to air or ultraviolet irradiation [79]. Under these conditions, CdSe QD cores coated with small thiolate ligands were toxic. Capping these QDs with ZnS shells or coating with BSA rendered the QD cores less susceptible to oxidative degradation and less toxic to primary rat hepatocytes, implicating the potential role of cadmium in QDs cytotoxicity. The decrease in QD cytotoxicity of CdSe QDs with the overgrowth of a ZnS shell has since been verified in several reports [139, 142]. If it is revealed in the future that Cd2+ release is a major hindrance for the use of QDs in cells and in animals, several new types of QDs that have no heavy metals atoms may be useful for advancing this field [143, 144].

5.2. Toxicity Induced by Colloidal Instability

Presently it is nearly impossible to drawing firm conclusions about the toxicity of QDs in cultured cells due to (a) the immense variety of QDs and variations of surface coatings used by different labs and (b) a technical disparity in experimental conditions, such as the duration of the nanoparticle exposure, use of relevant cell lines, media choice (e.g. with or without serum), and even the units of concentration (e.g. mg/ml versus nM). Nonetheless, the cytotoxicity of QDs reported in the literature has strongly correlated with the stability and surface coatings of these nanoparticles, which can be separated into three categories. (1) Core CdTe QDs that are synthesized in aqueous solution and stabilized by small thiolate ligands (e.g. mercaptopropionic acid or mercaptoacetic acid). These QDs have been widely used due to their ease of synthesis, low cost, and immediate utility in biological buffers. However, because these QDs are protected only by a weakly bound ligand, they are highly prone to degradation and aggregation, and their cytotoxicity toward cells in culture has been widely reported [140, 145]. (2) Core/shell CdSe/ZnS QDs synthesized in nonpolar solvents and transferred to water using thiolate ligands. CdSe is less prone to oxidation than CdTe, and ZnS is even more inert, and therefore these QDs are much more chemically stable. With direct comparison to CdTe QDs, these nanocrystals are significantly less cytotoxic, although high concentrations have been found to illicit toxic responses from cells [140]. Because these QDs are coated with a ZnS shell, the origin of this cytotoxicity is still unclear, whether it is from degradation of the shell, leading to cadmium release, or if it is caused by other effects. When coated with small ligands, these QDs have similar surface chemistries compared to aqueous CdTe QDs, burdened by significant dissociation of ligands from the QDs, rendering the nanoparticles colloidally unstable [146]. This propensity for aggregation may contribute to their cytotoxicity, even if free cadmium is not released. Importantly for the comparison between CdSe/ZnS QDs and their cadmium-only counterparts (CdSe or CdTe core QDs), thiolate ligands bind more strongly to zinc than to cadmium, which may contribute colloidal stability. (3) Core/shell CdSe/ZnS QDs synthesized in nonpolar solvents and transferred to water via encapsulation in amphiphilic polymers or cross-linked silica. These QDs have been found to be significantly more stable colloidally, chemically, and optically when compared to their counterparts coated in small ligands [56]. For this reason, they have been found to be nearly biologically inert in both living cells and living animals [10, 24, 49, 60, 79, 107, 114, 147]. Only when exposed to extreme conditions or when directly injected into cells at immensely high concentrations have these QDs been found to elicit toxic or inflammatory responses [49, 142].

It is feasible that a significant amount of toxicological data obtained for QDs thus far has been considerably influenced by the colloidal nature of these nanoparticles. The tendency for nanoparticles to aggregate, precipitate on cells in culture, nonspecifically adsorb to biomolecules, and catalyze the formation of reactive oxygen species (ROS) may be just as important as heavy metal toxicity contributions to toxicity. For example, Kircher et al. found that CdSe/ZnS QDs coated with an amphiphilic polymer shell induced the detachment of human breast cancer cells from their cell culture substrate [139]. This effect was found to also occur for biologically inert gold nanoparticles coated with the same polymer, thus ruling out the possibility of heavy metal atom poisoning. Microscopic examination of the cells revealed that the nanoparticles precipitated on the cells, causing physical harm. Indeed, carbon nanotubes, which are entirely composed of harmless carbon, have been found to be capable of impaling cells and causing major problems in the lungs of mammals [148]. Nonspecific adsorption to intracellular proteins may also impair cellular function, especially for very small QDs (3 nm and below), which can invade the cellular nucleus [101], binding to histones and nucleosomes [102], and damage DNA in vitro [149, 150]. QDs are also known to catalyze the formation of ROS [145, 151], especially when exposed to ultraviolet radiation. In fact, Cho et al. exposed cells to CdTe QDs in cell culture and determined that their cytotoxicity could only be accounted for with the effects of ROS generation, as there was no dose-dependent relationship with intracellular Cd2+ release, as determined with a cadmium-reactive dye [140]. However, protection of the surface of QDs with a thick ZnS shell may greatly reduce ROS production [152, 153]. Despite a significant surge of interest in the cytotoxicity of nanoparticles, there is still much to learn about the cytological and physiological mediators of nanoparticle toxicology. If it is determined that heavy metal composition plays a negligible role in QD toxicity, QDs will have as good of a chance as any other nanoparticle at being used as clinical contrast agents.

6. Dual-Modality QDs for Imaging and Therapy

In comparison with small organic fluorophores, QDs have large surfaces that can be modified through versatile chemistry. This makes QDs convenient scaffolds to accommodate multiple imaging (e.g., radionuclide-based or paramagnetic probes) and therapeutic agents (e.g. anticancer drugs), through chemical linkage or by simple physical immobilization. This may enable the development of a nearly limitless library of multifunctional nanostructures for multimodality imaging, as well as for integrated imaging and therapy.

6.1. Dual-Modality Imaging

The applications of QDs described above for in vivo imaging are limited by tissue penetration depth, quantification problems, and a lack of anatomic resolution and spatial information. To address these limitations, several research groups have led efforts to couple QD-based optical imaging with other imaging modalities that are not limited by penetration depth, such as MRI, positron emission tomography (PET) and single photon emission computed tomography (SPECT) [154158]. For example, Mulder et al. [154] developed a dual-modality imaging probe for both optical imaging and MRI by chemically incorporating paramagnetic gadolinium complexes in the lipid coating layer of QDs [154, 155]. In vitro experiments showed that labeling of cultured cells with these QDs led to significant T1 contrast enhancement with a brightening effect in MRI, as well as an easily detectable fluorescence signal from QDs. However, the in vivo imaging potential of this specific dual-modality contrast agent is uncertain due to the unstable nature of the lipid coating that was used. More recently, Chen and coworkers used a similar approach to attach the PET-detectable radionuclide 64Cu to the polymeric coating of QDs through a covalently bound chelation compound [158]. The use of this probe for targeted in vivo imaging of a subcutaneous mouse tumor model was achieved by also attaching αvβ3 integrin-binding RGD peptides on the QD surface. The quantification ability and ultrahigh sensitivity of PET imaging enabled the quantitative analysis of the biodistribution and targeting efficacy of this dual-modality imaging probe. However, the full potential of in vivo dual-modality imaging was not realized in this study, as fluorescence was only used as an ex vivo imaging tool to validate the in vivo results of PET imaging, primarily due to the lower sensitivity of optical imaging in comparison with PET. This imbalance in sensitivity is fundamental to the differences in the physics of these imaging modalities, and points to an inherent difficulty in designing useful multimodal imaging probes. The majority of these probes are still at an early stage of development. The clinical relevance of these nanoplatforms still needs further improvement in sensitivity and better integration of different imaging modalities, as well as validation of their biocompatibility and safety.

It is also noteworthy that recent advances in the synthesis of QDs containing paramagnetic dopants, such as manganese, have led to a new class of QDs that are intrinsically fluorescent and magnetic [159, 160]. However the utility of these new probes for bioimaging application is unclear because they are currently limited to the ultraviolet and visible emission windows, and their stability (e.g., photochemical and colloidal) and biocompatibility have yet to be systematically investigated [144]. As well, inorganic heterodimers of QDs and magnetic nanoparticles have generated dual-functional nanoparticles [161, 162]. Although these new materials are of great interest, they are still in development and have only recently shown applicability in cell culture, but not yet in living animals [160, 163].

6.2. Integration of Imaging and Therapy

Drug-containing nanoparticles have shown great promise for treating tumors in animal models and even in clinical trials [157]. Both passive and active targeting of nanotherapeutics have been used to increase the local concentration of chemotherapeutics in the tumor. Due to the size and structural similarities between imaging and therapeutic nanoparticles, it is possible that their functions can be integrated to directly monitor therapeutic biodistribution, to improve treatment specificity, and to reduce side effects. This synergy has become the principle foundation for the development of multi-functional nanoparticles for integrated imaging and cancer treatment. Most studies are still at a proof-of-concept stage using cultured cancer cells, and are not immediately relevant to in vivo imaging and treatment of solid tumors. However, these studies will guide the future design and optimization of multifunctional nanoparticle agents for in vivo imaging and therapy [164167].

In one example, Farokhzad et al. reported a ternary system composed of a QD, an aptamer, and the small molecular anticancer drug doxorubicin (Dox) for in vitro targeted imaging, therapy and sensing of drug release [165]. As illustrated in Figure 6, aptamers were conjugated to QDs to serve as targeting units, and Dox was attached to the stem region of the aptamers, taking advantage of the nucleic acid binding ability of doxorubicin. Two donor-quencher pairs of fluorescence resonance energy transfer occurred in this construct, as the QD fluorescence were quenched by Dox, and Dox was quenched by the double-stranded RNA aptamers. As a result, gradual release of Dox from the conjugate was found to “turn on” the fluorescence of both QDs and Dox, providing a means to sense the release of the drug. However it is clear that the current design of this conjugate will not be sufficient for in vivo use unless the drug loading capacity can be greatly increased (currently 7–8 Dox molecules per QD).

Figure 6
Schematic illustration of QD-Aptamer-Dox FRET system and its targeted delivery through receptor-mediated endocytosis. (a) QDs-aptamer conjugates (QD-Apt) are fluorescent until they are mixed with the fluorescent drug doxorubicin (Dox), which intercalates ...

6.3. QDs for siRNA Delivery and Imaging

QDs also provide a versatile nanoscale scaffold to develop multifunctional nanoparticles for siRNA delivery and imaging. RNA interference (RNAi) is a powerful technology for sequence-specific suppression of genes, and has broad applications ranging from functional gene analysis to targeted therapy [168172]. However, these applications are limited by the same delivery problems that hinder intracellular imaging with QDs (Section 3.2), namely intracellular delivery and endosomal escape, in addition to dissociation from the delivery vehicle (i.e. unpacking), and coupling with cellular machines (such as the RNA-induced silencing complex or RISC). For cellular and in vivo siRNA delivery, a number of approaches have been developed (see ref. [168] for a review), but these methods have various shortcomings and do not allow a balanced optimization of gene silencing efficacy and toxicity. For example, previous work has used QDs and iron oxide nanoparticles for siRNA delivery and imaging [27, 166, 167, 173], but the QD probes were either mixed with conventional siRNA delivery agents [166] or an exogenous compound, such as the antimalaria drug chloroquine, was needed for endosomal rupture and gene silencing activity [173].

Gao et al. have recently fine-tuned the colloidal and chemical properties of QDs for use as delivery vehicles for siRNA, resulting in highly effective and safe RNA interference, as well as fluorescence contrast [174]. The authors balanced the proton-absorbing capacity of the QD surface in order to induce endosomal release of the siRNA through the proton sponge effect (see Section 3.4). A major finding is that this effect can be precisely controlled by partially converting the carboxylic acid groups on a QD into tertiary amines. When both are linked to the surface of nanometer-sized particles, these two functional groups provide steric and electrostatic interactions that are highly responsive to the acidic organelles, and are also well suited for siRNA binding and cellular entry. As a result, these conjugates can improve gene silencing activity by 10–20 fold, and reduce cellular toxicity by 5–6 fold, compared with current siRNA delivery agents (lipofectamine, JetPEI, and TransIT). In addition, QDs are inherently dual-modality optical and electron microscopy probes, allowing real-time tracking and ultrastructural localization of QDs during transfection.

7. Concluding Remarks

Quantum dots have been received as technological marvels with characteristics that could greatly improve biological imaging and detection. In the near future, there are a number of areas of research that are particularly promising but will require concerted effort for success:

(1) Design and development of nanoparticles with multiple functions

For cancer and other medical applications, important functions include imaging (single or dual-modality), therapy (single drug or combination of two or more drugs), and targeting (one or more ligands). With each added function, nanoparticles could be designed to have novel properties and applications. For example, binary nanoparticles with two functions could be developed for molecular imaging, targeted therapy, or for simultaneous imaging and therapy. Ternary nanoparticles with three functions could be designed for simultaneous imaging and therapy with targeting, targeted dual-modality imaging, or for targeted dual-drug therapy. Quaternary nanoparticles with four functions can be conceptualized in the future to have the capabilities of tumor targeting, dual-drug therapy and imaging.

(2) Use of multiplexed QD bioconjugates for analyzing a panel of biomarkers and for correlation with disease behavior, clinical outcome, and treatment response

This application should begin with retrospective studies of archived specimens in which the patient outcome is already known. A key hypothesis to be tested is that the analysis of a panel of tumor markers will allow more accurate correlations than single tumor markers. As well, the analysis of the relationship between gene expression from cancer cells and the host stroma may help to define important cancer subclasses, identify aggressive phenotypes of cancer, and determine the response of early stage disease to treatment (chemotherapy, radiation, or surgery).

(3) Design and development of biocompatible nanoparticles to overcome nonspecific organ uptake and RES scavenging

There is an urgent need to develop nanoparticles that are capable of escaping RES uptake, and able to target tumors by active binding mechanisms. This in vivo biodistribution barrier might be mitigated or overcome by systematically optimizing the size, shape, and surface chemistry of imaging and therapeutic nanoparticles.

(4) Penetration of imaging and therapeutic nanoparticles into solid tumors beyond the vascular endothelium

This task will likely require active pumping mechanisms such as caveolin transcytosis and receptor-mediated endocytosis, or cell-based strategies such as nanoparticle-loaded macrophages.

(5) Release of drug payloads inside targeted cells or organs

This task will likely require the development of biodegradable nanoparticle carriers that are responsive to pH, temperature, or enzymatic reactions.

(6) Nanotoxicology studies including nanoparticle distribution, excretion, metabolism, pharmacokinetics, and pharmacodynamics in animal models in vivo

These investigations will be vital for the development of nanoparticles beyond their current use as research tools, toward clinical applications in cancer imaging and therapy.

Acknowledgements

This work was supported by grants from the National Institutes of Health (P20 GM072069, R01 CA108468, and U01HL080711, U54CA119338), the US Department of Energy Genomes to Life Program, and the Georgia Cancer Coalition (GCC). One of the authors (A.M.S.) acknowledges the Whitaker Foundation for generous fellowship support.

Footnotes

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