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Journal of Nanomaterials
Volume 2011 (2011), Article ID 834139, 13 pages
Application of Quantum Dots in Biological Imaging
Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
Received 15 May 2011; Accepted 2 June 2011
Academic Editor: Xing J. Liang
Copyright © 2011 Shan Jin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Quantum dots (QDs) are a group of semiconducting nanomaterials with unique optical and electronic properties. They have distinct advantages over traditional fluorescent organic dyes in chemical and biological studies in terms of tunable emission spectra, signal brightness, photostability, and so forth. Currently, the major type of QDs is the heavy metal-containing II-IV, IV-VI, or III-V QDs. Silicon QDs and conjugated polymer dots have also been developed in order to lower the potential toxicity of the fluorescent probes for biological applications. Aqueous solubility is the common problem for all types of QDs when they are employed in the biological researches, such as in vitro and in vivo imaging. To circumvent this problem, ligand exchange and polymer coating are proven to be effective, besides synthesizing QDs in aqueous solutions directly. However, toxicity is another big concern especially for in vivo studies. Ligand protection and core/shell structure can partly solve this problem. With the rapid development of QDs research, new elements and new morphologies have been introduced to this area to fabricate more safe and efficient QDs for biological applications.
Semiconductor nanocrystals, or so-called quantum dots (QDs), show unique optical and electronic properties, including size-tunable light emission, simultaneous excitation of multiple fluorescence colors, high signal brightness, long-term photostability, and multiplex capabilities [1–4]. Such QDs have significant advantages in chemical and biological researches in contrast to traditional fluorescent organic dyes and green fluorescent proteins on account of their photobleaching, low signal intensity, and spectral overlapping [5–7]. These properties of QDs have attracted great interest in biology and medicine in recent years. At present QDs are considered to be potential candidates as luminescent probes and labels in biological applications, ranging from molecular histopathology, disease diagnosis, to biological imaging [8–10]. Numerous studies have reported the use of QDs for in vitro or in vivo imaging of sentinel lymph nodes [11–17], tumor-specific receptors [18–20], malignant tumor detectors , and tumor immune responses .
However, the major concerns about potential toxicity of II-IV QDs (such as CdTe and CdSe) have cast doubts on their practical use in biology and medicine. Indeed, several studies have reported that size, charge, coating ligands, and oxidative, photolytic, and mechanical stability, each can contribute to the cytotoxicity of cadmium-containing QDs. Another critical factor that determines the cytotoxicity of QDs is the leakage of heavy metal ions from the core caused by photolysis and oxidation [23–25]. The long-term effects of these ions enrichment and durations within the body raise concerns about the biocompatibility and safety of QDs [24, 26]. On the other hand, the rapid clearance of QDs from the circulation by the reticuloendothelial system (RES) or entrapment of QDs in the spleen and liver [27, 28] can result in the degradation of the imaging quality of the objective and the enhancement of background noise.
One promising solution to these problems, including QDs sequestered in the liver/spleen and long-term retention in the body, is to evade the recognition and uptake by the reticuloendothelial system thus extending the circulation time of the particles in the body. In addition, for biological studies, QDs should be aqueous soluble in order to adapt the biological environment. Therefore, when QDs are employed in biological imaging, many factors should be considered and the system should be properly designed in order to meet a set of requirements [29–31].
In this report, we first briefly review different types of QDs that have been fabricated so far and their synthesis methods, including surface functionalization. Next, we focus on their applications in biological imaging. Finally, the new trends of QDs imaging are discussed.
2. Types and Characteristics of QDs
Traditionally, the typical QDs consist of a II-IV, IV-VI, or III-V semiconductor core (e.g., CdTe, CdSe, PbSe, GaAs, GaN, InP, and InAs; Figure 1) [3, 32, 33], which is surrounded by a covering of wide bandgap semiconductor shell such as ZnS in order to minimize the surface deficiency and enhance the quantum yield [5, 34].The unique optical and electronic properties of QDs, including narrow light emission, wideband excitation, and photostability, provide them with significant advantages in the applications of multiplexed molecular targets detection  and as optical bioimaging probes [8–10]. One major drawback which severely hinders the application of II-IV, IV-VI, or III-V QDs for in vivo biological research is the concern about the toxicity associated with the cadmium, lead, or arsenic containing QDs. The toxic heavy metal ions can be easily leaked out into biological systems if the surfaces are not properly covered by the shells or protected by ligands.
The limitation of heavy metal-containing QDs stimulates extensive research interests in exploring alternative strategies for the design of fluorescent nanocrystals with high biocompatibility. In this case, the practical strategy is to develop highly fluorescent nanoparticles based on nontoxic elements or explore luminescent π-conjugated polymer dots.
Silicon nanoparticle is another type of QDs which possess unique properties when their sizes are reduced to below 10 nm. Similar to their predecessors, silicon QDs also have many advantages over traditional fluorescent organic dyes , including resistance to photobleaching and wide emission range from visible to infrared region with relatively high quantum yields. Moreover, owing to silicon’s nontoxic and environment-friendly nature, Si QDs are used as fluorescent probes for bioimaging . It has been reported that for in vivo applications, Si QDs mainly degrade to silicic acid which can be excreted through urine , and for in vitro use, Si QDs are considered 10-times safer than Cd-containing quantum dots (Figures 2 and 3) . However, the challenges present in employing Si QDs in bioimaging arise from their water solubility and biocompatibility. Usually, Si QDs are produced in nonpolar solvents with hydrophobic ligands (such as styrene and octene) on their surface in order to protect the Si cores. Therefore, it is a common problem that Si QDs show poor solubility and unstable photoluminescence (PL) in aqueous solutions [38–40]. PL degradation can occur gradually after modified Si QDs are transferred from organic solvents to aqueous solutions. Thus, fabricating good water-disperse Si QDs with stable optical characters is vital to their applications in bioimaging studies. Recently, Tilley and Yamamoto reported that water-soluble Si QDs covered by allylamine [41, 42] and poly(acrylic acid), respectively, had been successfully obtained and showed good performance in fixed-cell labeling .
The semiconducting properties of conjugate polymers derive from their π-electron delocalization along the polymer chain. Mobile charges are allowed to move between π and π* orbitals. Highly bright fluorescence can be stable under one-photon or two-photon excitation. Polymers with such structures are promising in the fields of LED and biosensors [43, 44]. Burroughes and coworkers first reported the luminescent conjugated polymers in 1990 . Later on, Friend and So successfully applied π-conjugated polymers to light-emitting devices [46, 47]. Recently, related works focused on fluorescent polymer encapsulation and functionalization to yield conjugated polymer dots (CP dots) for bioimaging and probes (Figure 4) [48, 49]. Compared with other fluorescent probes, CP dots are convenient for fluorescence microscopy and laser excitation because of their absorption ranging from 350 to 550 nm. Also, fluorescence quantum yields can be slightly variable around 40%. Moreover, CP dots can exhibit higher brightness than any other nanoparticles of the same size under certain conditions .
3. Synthesis Methods of QDs
3.1. Organometallic Method
Organometallic chemistry provides an efficient pathway to produce monodisperse QDs in nonpolar organic solvents, such as CdSe nanocrystals covered by tri-n-octylphosphine oxide (TOPO) ligand. In a typical preparation, Me2Cd and Bis(trimethylsilyl)selenium ((TMS)2Se) can be used as organometallic precursors. Monodisperse CdSe with uniform surface derivatization and regularity in core structure can be obtained based on the pyrolysis of organometallic reagents by injection into a hot coordinating solvent between 250°C to 300°C . The adsorption of ligands results in slow growth and annealing of the cores in the coordinating solvents. QDs with different sizes can be successfully obtained at different temperatures. The advantage of this method is obvious because this method can produce a variety of QDs with high quantum yields and the size distribution can be easily controlled by altering temperature or reaction time. Organometallic method is currently considered as the most important means to synthesize QDs.
3.2. QDs Synthesized in Aqueous Solution
Since Rajh and coworkers first reported the synthesis of thiol-capped CdTe QDs in aqueous solution, a great deal of efforts have focused on synthesizing water-dispersed QDs directly . In this process, ionic perchlorates such as Cd(ClO)4·6H2O and Al2Te3 are used as the precursors. In the presence of ligands such as 3-mercaptopropionic acid (3-MPA), glutathione (GSH), or other hydrosulfyl-containing materials, CdTe QDs can be successfully obtained in aqueous medium. The advantage of this method lies in its environment-friendly synthesis procedure and relatively low cost. QDs produced by this method can be directly applied in biological researches without ligand exchange procedure. However, compared with the QDs produced by organometallic method, thiol-capped QDs show low quantum yields, and broad full width of half-maximum (FWHM), which can be attributed to its wide size distribution and poor stability in aqueous solution.
3.3. Hydrothermal Method/Microwave-Assisted Method
In order to reduce QDs surface defects generated during the growth process, these two methods are put forward to synthesize QDs with narrow size distribution and high quantum yields. According to hydrothermal method, all reaction regents are loaded in hermetic container and then heated to supercritical temperature; high pressure produced by the temperature can efficiently reduce reaction time and surface defects of QDs . Moreover, because high reaction temperature can separate the processes of nucleus formation and crystal growth, this method has clear advantages over traditional aqueous synthesis, affording narrow size distribution QDs.
Microwave-assisted irradiation is another attractive method for QDs synthesis. This method was first introduced by Kotov group  and followed by Qian and coworkers to demonstrate a fast and simple synthesis of CdTe and CdSe-CdS QDs by microwave-assisted irradiation in 2006 . Microwave irradiation can be considered as heating source to optimize synthesis conditions, and in this system, water is an excellent solvent for its is equal to 0.127. The whole solution can be quickly heated to above 100°C to yield homogeneous QDs and the quantum yields can reach as high as 17%.
4. Toxicity, Solubilization, and Functionalization of Quantum Dots
In order to apply QDs to biological studies, safety, and biocompatibility are the issues that must be concerned. Toxicological studies have suggested that certain potential adverse human health effects may be resulted from their exposure to some novel nanomaterials, such as gold nanoparticles and QDs, but the fundamental cause-effect relationships are relatively obscure . Thus, it is necessary to figure out the interaction of QDs with biological systems and their potentially harmful side effects in cells. Several studies reveal that the toxicity of QDs depends on many factors which can summarized as inherent physicochemical properties and environmental conditions [23, 24, 26]. Not all QDs are alike, each type of QDs should be characterized individually as to its potential toxicity. QD sizes, charges, concentrations, outer coatings materials and functional groups, oxidation, and mechanical stability all have been implicated as contributing factors to QD toxicity . Derfus et al. studied the cytotoxicity of CdSe QDs and pointed that without coating, the cytotoxicity of CdSe cores correlated with the liberation of free Cd2+ ions due to deterioration of QDs lattice. And in vivo, the liver is the primary site of acute injury caused by cadmium ions even though the concentration of Cd2+ is reduced to 100–400 μM . In order to eliminate QD toxicity resulted from heavy metal ions releasing, surface coating and core/shell structure are considered to be efficient solutions to this problem. Many new types of QDs such as core/shell (CdSe/ZnS) and core/shell/shell (CdSe/ZnS/TEOS) QDs with high luminescence are developed [34, 35]. Some researchers also found that the quantum size effect which resulted from the minute size of nanomaterials could lead to cytotoxicity. Since nanoparticles within certain diameters (this value may be different according to biological environment) are of similar size to certain cellular components and proteins, they may bypass natural mechanical barriers, thus leading to adverse tissue reactions .
The application of QDs in biological imaging also requires QDs with good water solubility. Several strategies have been developed to circumvent this problem, which are summarized as follows.
The first one is to synthesize QDs in aqueous solution directly, which has been depicted in this review (vide supra). The second is to obtain QDs in organic solvents, and then transfer them to aqueous solutions (Scheme 1). Nie and Weiss groups pioneered the preparation of water-dispersed QDs by ligand exchange method (Figure 4) [5, 56]. In addition, polymer and lipid coating is another efficient way to render QDs with good water solubility [57, 58].
To improve in vitro and in vivo imaging quality and detection sensitivity in biological fields, multiplex capabilities QDs have been developed. Recent efforts mainly focus on the functionalization of QDs to accommodate the demands of imaging sensitivity and specificity . Nie reported multicolor QD-antibody conjugates for the rapid detection of rare Hodgkin’s and Reed-Sternberg (HRS) cells from infiltrating immune cells such as T and B lymphocytes ; Shi combined Fe3O4 with amine-terminated QDs to yield fluorescent superparamagnetic QDs for in vivo and in vitro imaging (Figure 5) .
5. QDs in Biological Imaging
In order to apply QDs in biological imaging, recent studies have focused on developing near-IR luminescent QDs which exhibit an emission wavelength ranging from 700 to 900 nm. Light within this range has its maximum depth of penetration in tissue and the interference of tissue autofluorescence (emission between 400 nm and 600 nm) is minimal [62, 63]. In addition, for the purpose of gathering more information during specific cellular processes in real time, QDs must be conjugated with molecules which have the capabilities of recognizing the target. These surface modifications can also help prevent aggregation, reduce the nonspecific binding, and are critical to achieving specific target imaging in biological studies. Further functionalization of QDs can be achieved by the modification of protecting ligands [64–66], introducing targeting groups such as apolipoproteins and peptides (Scheme 1) . In this part, we mainly introduce the recent development of conjugated QDs for in vitro and in vivo imaging; some new QDs related to bioimaging are also included.
5.1. In Vitro Imaging
Early studies of in vitro imaging used QDs to label cells. For instance, PbS and PbSe capped with carboxylic groups had been obtained in aqueous solution by ligands exchange to label cells ; CdTe capped with 3-mercaptopropionic acid (3-MPA) was used as imaging tool to label Salmonella typhimurium cells . Jaiswal et al. reported labelling HeLa cells with acid-capped CdSe/ZnS QDs . It is found that the cells can store QDs in vesicles through endocytosis after washing away the excess QDs. Further, they also demonstrated the potential application of QDs in cell tracking by using avidin-conjugated QDs to label cells.
With the development of biomarkers in cell biology, the tracking of some specific cells (such as cancer cells) becomes possible. Gac et al. have successfully detected apoptotic cells by conjugating QDs with biotinylated Annexin V, which enables the functionalized QDs to bind to phosphatidylserine (PS) moieties present on the membrane of apoptotic cells but not on healthy or necrotic cells (Figure 6) . The detection and imaging of apoptotic cells makes it possible to monitor specific photostable apoptosis. Wolcott reported silica-coated CdTe QDs decorated with functionalized groups not only for labelling proteins, but also for preventing toxic Cd2+ leaking from the core . Nie developed cell-penetrating QDs coated with polyethylene glycol (PEG) grafted polyethylenimine (PEI), which were capable of penetrating cell membranes and disrupting endosomal organelles in cells . Recently, in the demand of using biocompatible and nontoxic QDs as nanoprobes, rare earth (RE) elements are used to fabricate a new type of QDs, such as Gd-doped ZnO QDs. RE-doped QDs have distinct advantages over heavy metal-containing QDs, not only because of avoiding the increase of particle size by polymer or silica coating in synthesis procedure, but also providing a simple, green synthesis method. Liu et al. reported the development of Gd-doped ZnO QDs with enhanced yellow fluorescence, and these QDs can be used as nanoprobes for quick cell detection with very low toxicity . Other progresses include CdSe/ZnS with PEG coating and CdSe/ZnS-peptide conjugation for cytosol localization and nucleus targeting [74–77], CdS passivated by DNA to yield nontoxic QDs as new biological imaging agent , CdSe/L-cysteine QDs designed to label serum albumin (BSA) and cells , polymer QDs (PS-PEG-COOH) with conjugation to biomolecules, such as streptavidin and immunoglobulin G (IgG) to label cell surface receptors and subcellular structures in fixed cells [21, 80], and other non-Cd-containing QDs such as Ge-QDs and near-IR InAs for imaging of specific cellular proteins [81, 82]. In order to broaden QDs applications, MRI detectable QDs were demanded. Many studies have set out to explore paramagnetic QDs. Sun reported a new kind of magnetic fluorescent multifunctional nanocomposite which contained silica-coated Fe3O4 and TGA-capped CdTe QDs. This nanocomposite was successfully employed for HeLa cell labelling and imaging, as well as in magnetic separation . Mulder et al. have successfully modified CdSe/ZnS with paramagnetic coating and arginine-glycine-aspartic acid (RGD) conjugation in order to combine the functions of specific protein binding and contrast agent for molecular imaging .
The challenges encountered in cancer therapy stimulate comprehensive studies. Many efforts focus on the tracking and diagnosis of cancer cells. Cao et al. used near-IR QDs with an emission wavelength of 800 nm (QD800) to label squamous cell carcinoma cell line U14 (U14/QD800). The fluorescent images of U14 cells can be clearly obtained after 6 h by cell endocytosis . In 2007, Bagalkot et al. reported a more complex QDs-aptamer- (Apt-) doxorubicin (Dox) conjugate system [QD-Apt(Dox)] to endow QDs with the capability of targeting, imaging, therapy, and sensing the prostate cancer cells that express the prostate-specific membrane antigen (PSMA) protein (Figure 7) . Other types of QDs that have been applied in pancreatic cancer cells imaging include CdSe/CdS/ZnS using transferrin and anti-Claudin-4 as targeting ligands, InP QDs conjugated with cancer antibodies, and water-soluble Si-QDs micelles encapsulated with polymer chains [86–88]. Another important role played by QDs in biological application is transfection. Many studies reported fluorescence imaging and nucleus targeting of living cells by transfection and RNA delivery. Biju identified an insect neuropeptide, also-called allatostatin, which can transfects living NIH 3T3 and A431 human epidermoid carcinoma cells and transports quantum dots (QDs) inside the cytoplasm and even the nucleus of the cells . This method showed the conjugation of QDs with allatostatin could achieve high transection efficiency, which was promising for DNA gene delivery and cell labelling. Also, with specific conjugation with allatostatin, the QDs had another potential application in cancer, called photodynamic therapy .
5.2. In Vivo Imaging
In addition to their usage as nanoprobes and labels for in vitro imaging, QDs have also been widely used as in vivo imaging agents (Figure 8) [91–94]. Cancer-specific antibody, coupled to near-IR QDs with polymer coatings is the most popular QDs agent for tumor-targeted imaging. Progress in this field has been reviewed in one literature [94, 95].
However, the depth of targets makes the QDs in vivo imaging very challenging. This special application of QDs requires them with low toxicity, high contrast, high sensitivity, and photostability. Dubertret et al. fabricated one type of CdSe/ZnS QDs which were encapsulated individually in phospholipid block-copolymer micelles. After further modification by DNA, this multifunctionalized DNA-QD-micelles can be developed to obtain Xenopus embryo fluorescent images in PBS. Moreover, after injection into embryos, the QD micelles showed high stability and nontoxicity (<5 × 109 nanocrystals per cell) . Another study reported by Michalet showed that CdSe/ZnS QDs can exhibit high contrast and imaging depth in two-photon excitation confocal microscopy by visualizing blood vessels in live mice [3, 97]. They confirmed that coatings such as PEG on the surfaces of QDs could further reduce their toxicity and accumulation in liver. Some studies also pointed out that the increase of the length of PEG coating polymer on the surface of QDs was able to slow down their extraction toward the liver, so coating polymer on the surfaces was another factor worthy of considering when QDs are employed for in vivo imaging . Other reports that used low-toxicity QDs include the development of non-Cd-containing QDs such as CuInS2/ZnS core/shell nanocrystals for in vivo imaging . In addition, sensitivity is another important factor that must be considered in QDs-related in vivo imaging. One study used nude mice for in vivo imaging after near-IR QD800-labeled BcaCD885 cells (BcaCD885/QD800) being implanted . Fluorescence signals of QDs accumulated in the tumor could be detected after 16 days of incubation at certain concentrations. It suggested that, compared with CT and MRI, QD800-based imaging could efficiently increase the sensitivity of early diagnosis of cancer cells.
With the development of QDs in recent years, many studies tried to explore the potential of QDs in wider fields. Kobayashi reported fluorescence lymphangiography by injecting five QDs with different emission spectra . Through simultaneous injection of five QDs into different sites in the middle of phalanges, the upper extremity, the ears, and the chin, different parts of the mouse body can be identified by certain fluorescence color. This is the first demonstration of simultaneous imaging of trafficking lymph nodes with QDs having different emission spectra. As biological fluorescent markers, no matter in vitro or in vivo, much attention has been concentrated on developing spherical QDs conjugations, but there are few reports on how other morphologies of QDs will interact with the biological systems. Yong et al. reported CdSe/CdS/ZnS quantum rods (QRs) coated with PEGylated phospholipids and RGD peptide for tumor targeting . This conjugation was demonstrated to be a bright, photostable, and biocompatible luminescent probe for the early diagnosis of cancer and was believed to offer new opportunities for imaging early tumor growth.
Owing to the initial success of QDs-biomolecule conjugates employed for in vitro and in vivo imaging, it is conceivable that future researches will continue to focus on the development of QDs conjugations for cell labelling and cancer diagnoses. However, for bioapplications, biocompatibility of QDs remains the major challenge which must be carefully dealt with. Many studies have showed the short-term stability and long-term breakdown of Cd-containing QDs in in vivo imaging, which raise concerns about their chronic toxicity. Some studies showed that even robust coatings on the surfaces of QDs cannot prevent their releasing of Cd2+ ions. The emerging picture of Cd-containing QDs toxicity not only suggests that QDs are required to improve safety before their wide biological applications as fluorescent nanoprobes, but also a variety of noncadmium alternatives are in urgent demand. At present, many efforts have focused on exploring new types of fluorescent nanomaterials such as upconversion materials NaYF4 and NaGaF4. In addition, with the development of new techniques and detection methods, QDs are shown to have applications in wider fields. Zhang et al. successfully introduced Kelvin force microscopy (KFM) as a method to examine the binding of QDs with DNA in vitro and in vivo ; Huang et al. demonstrated a QDs sensor for quantitative visualization of surface charges on single living cell . We believe that with the development of new QDs with low toxicity and multiplex functionalities, QDs will find more versatile applications in biological fields other than bioimaging.
The authors are grateful to the “100 Talents” program of the Chinese Academy of Sciences, and 973 Program (2010CB933600) for funding support.
- P. Alivisatos, “The use of nanocrystals in biological detection,” Nature Biotechnology, vol. 22, no. 1, pp. 47–52, 2004.
- X. H. Gao, L. L. Yang, J. A. Petros, F. F. Marshall, J. W. Simons, and S. M. Nie, “In vivo molecular and cellular imaging with quantum dots,” Current Opinion in Biotechnology, vol. 16, no. 1, pp. 63–72, 2005.
- X. Michalet, F. F. Pinaud, L. A. Bentolila et al., “Quantum dots for live cells, in vivo imaging, and diagnostics,” Science, vol. 307, no. 5709, pp. 538–544, 2005.
- A. M. Smith, H. W. Duan, A. M. Mohs, and S. M. Nie, “Bioconjugated quantum dots for in vivo molecular and cellular imaging,” Advanced Drug Delivery Reviews, vol. 60, no. 11, pp. 1226–1240, 2008.
- W. C. W. Chan and S. M. Nie, “Quantum dot bioconjugates for ultrasensitive nonisotopic detection,” Science, vol. 281, no. 5385, pp. 2016–2018, 1998.
- B. Dubertret, P. Skourides, D. J. Norris, V. Noireaux, A. H. Brivanlou, and A. Libchaber, “In vivo imaging of quantum dots encapsulated in phospholipid micelles,” Science, vol. 298, no. 5599, pp. 1759–1762, 2002.
- A. Konkar, S. Y. Lu, A. Madhukar, S. M. Hughes, and A. P. Alivisatos, “Semiconductor nanocrystal quantum dots on single crystal semiconductor substrates: high resolution transmission electron microscopy,” Nano Letters, vol. 5, no. 5, pp. 969–973, 2005.
- Y. Xing, Q. Chaudry, C. Shen et al., “Bioconjugated quantum dots for multiplexed and quantitative immunohistochemistry,” Nature Protocols, vol. 2, no. 5, pp. 1152–1165, 2007.
- M. V. Yezhelyev, A. Al-Hajj, C. Morris et al., “In situ molecular profiling of breast cancer biomarkers with multicolor quantum dots,” Advanced Materials, vol. 19, no. 20, pp. 3146–3151, 2007.
- A. M. Smith, S. Dave, S. M. Nie, L. True, and X. Gao, “Multicolor quantum dots for molecular diagnostics of cancer,” Expert Review of Molecular Diagnostics, vol. 6, no. 2, pp. 231–244, 2006.
- A. Robe, E. Pic, H. P. Lassalle, L. Bezdetnaya, F. Guillemin, and F. Marchal, “Quantum dots in axillary lymph node mapping: biodistribution study in healthy mice,” BMC Cancer, vol. 8, no. 1, pp. 111–119, 2008.
- M. Takeda, H. Tada, H. Higuchi et al., “In vivo single molecular imaging and sentinel node navigation by nanotechnology for molecular targeting drug-delivery systems and tailor-made medicine,” Breast Cancer, vol. 15, no. 2, pp. 145–152, 2008.
- S. Kim, Y. T. Lim, E. G. Soltesz et al., “Near-infrared fluorescent type II quantum dots for sentinel lymph node mapping,” Nature Biotechnology, vol. 22, no. 1, pp. 93–97, 2004.
- E. G. Soltesz, S. Kim, R. G. Laurence et al., “Intraoperative sentinel lymph node mapping of the lung using near-infrared fluorescent quantum dots,” Annals of Thoracic Surgery, vol. 79, no. 1, pp. 269–277, 2005.
- E. G. Soltesz, S. Kim, S. W. Kim et al., “Sentinel lymph node mapping of the gastrointestinal tract by using invisible light,” Annals of Surgical Oncology, vol. 13, no. 3, pp. 386–396, 2006.
- C. P. Parungo, S. Ohnishi, S. W. Kim et al., “Intraoperative identification of esophageal sentinel lymph nodes with near-infrared fluorescence imaging,” Journal of Thoracic and Cardiovascular Surgery, vol. 129, no. 4, pp. 844–850, 2005.
- B. Ballou, L. A. Ernst, S. Andreko et al., “Sentinel lymph node imaging using quantum dots in mouse tumor models,” Bioconjugate Chemistry, vol. 18, no. 2, pp. 389–396, 2007.
- P. Diagaradjane, J. M. Orenstein-Cardona, N. E. Colon-Casasnovas et al., “Imaging epidermal growth factor receptor expression in vivo: pharmacokinetic and biodistribution characterization of a bioconjugated quantum dot nanoprobe,” Clinical Cancer Research, vol. 14, no. 3, pp. 731–741, 2008.
- X. H. Gao, Y. Y. Cui, R. M. Levenson, L. W. K. Chung, and S. M. Nie, “In vivo cancer targeting and imaging with semiconductor quantum dots,” Nature Biotechnology, vol. 22, no. 8, pp. 969–976, 2004.
- D. S. Lidke, P. Nagy, R. Heintzmann et al., “Quantum dot ligands provide new insights into erbB/HER receptor-mediated signal transduction,” Nature Biotechnology, vol. 22, no. 2, pp. 198–203, 2004.
- L. F. Qi and X. H. Gao, “Quantum dot—amphipol nanocomplex for intracellular delivery and real-time imaging of siRNA,” ACS Nano, vol. 2, no. 7, pp. 1403–1410, 2008.
- D. Sen, T. J. Deerinck, M. H. Ellisman, I. Parker, and M. D. Cahalan, “Quantum dots for tracking dendritic cells and priming an immune response in vitro and in vivo,” PLoS One, vol. 3, no. 9, Article ID e3290, 2008.
- A. M. Derfus, W. C. W. Chan, and S. N. Bhatia, “Probing the cytotoxicity of semiconductor quantum dots,” Nano Letters, vol. 4, no. 1, pp. 11–18, 2004.
- R. Hardman, “Toxicological review of quantum dots: toxicity depends on physicochemical and environmental factors,” Environmental Health Perspectives, vol. 114, no. 2, pp. 165–172, 2006.
- G. N. Guo, W. Liu, J. G. Liang, H. B. Xu, Z. K. He, and X. L. Yang, “Preparation and characterization of novel CdSe quantum dots modified with poly (D, L-lactide) nanoparticles,” Materials Letters, vol. 60, no. 21-22, pp. 2565–2568, 2006.
- Y. Pan, S. Neuss, A. Leifert et al., “Size-dependent cytotoxicity of gold nanoparticles,” Small, vol. 3, no. 11, pp. 1941–1949, 2007.
- M. L. Schipper, Z. Cheng, S. W. Lee et al., “MicroPET-based biodistribution of quantum dots in living mice,” Journal of Nuclear Medicine, vol. 48, no. 9, pp. 1511–1518, 2007.
- P. Diagaradjane, A. Deorukhkar, J. G. Gelovani, D. M. Maru, and S. Krishnan, “Gadolinium chloride augments tumor-specific imaging of targeted quantum dots in vivo,” ACS Nano, vol. 4, no. 7, pp. 4131–4141, 2010.
- M. Q. Chu, X. Song, D. Cheng, S. P. Liu, and J. Zhu, “Preparation of quantum dot-coated magnetic polystyrene nanospheres for cancer cell labelling and separation,” Nanotechnology, vol. 17, no. 13, pp. 3268–3273, 2006.
- F. Corsi, C. de Palma, M. Colombo et al., “Towards ideal magnetofluorescent nanoparticles for bimodal detection of breast-cancer cells,” Small, vol. 5, no. 22, pp. 2555–2564, 2009.
- A. Quarta, R. D. Corato, L. Manna, A. Ragusa, and T. Pellegrino, “Fluorescent-magnetic hybrid nanostructures: preparation, properties, and applications in biology,” IEEE Transactions on Nanobioscience, vol. 6, no. 4, pp. 298–308, 2007.
- P. N. Prasad, Biophotonics, Wiley-Interscience, Hoboken, NJ, USA, 2003.
- H. Wittcoff, B. G. Reuben, and J. S. Plotkin, Industrial organic chemicals, Wiley-Interscience, Hoboken, NJ, USA, 2004.
- B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec et al., “(CdSe)ZnS core-shell quantum dots: synthesis and characterization of a size series of highly luminescent nanocrystallites,” Journal of Physical Chemistry B, vol. 101, no. 46, pp. 9463–9475, 1997.
- S. T. Selvan, “Silica-coated quantum dots and magnetic nanoparticles for bioimaging applications,” Biointerphases, vol. 5, no. 3, pp. FA110–FA115, 2010.
- J. H. Park, L. Gu, G. Von Maltzahn, E. Ruoslahti, S. N. Bhatia, and M. J. Sailor, “Biodegradable luminescent porous silicon nanoparticles for in vivo applications,” Nature Materials, vol. 8, no. 4, pp. 331–336, 2009.
- X. H. Gao and S. M. Nie, “Doping mesoporous materials with multicolor quantum dots,” Journal of Physical Chemistry B, vol. 107, no. 42, pp. 11575–11578, 2003.
- F. Erogbogbo, K. T. Yong, I. Roy et al., “In vivo targeted cancer imaging, sentinel lymph node mapping and multi-channel imaging with biocompatible silicon nanocrystals,” Nano Letters, vol. 5, no. 1, pp. 413–423, 2011.
- W. J. Parak, D. Gerion, D. Zanchet et al., “Conjugation of DNA to silanized colloidal semiconductor nanocrystalline quantum dots,” Chemistry of Materials, vol. 14, no. 5, pp. 2113–2119, 2002.
- S. Sato and M. T. Swihart, “Propionic-acid-terminated silicon nanoparticles: synthesis and optical characterization,” Chemistry of Materials, vol. 18, no. 17, pp. 4083–4088, 2006.
- R. D. Tilley and K. Yamamoto, “The microemulsion synthesis of hydrophobic and hydrophilic silicon nanocrystals,” Advanced Materials, vol. 18, no. 15, pp. 2053–2056, 2006.
- J. H. Warner, A. Hoshino, K. Yamamoto, and R. D. Tilley, “Water-soluble photoluminescent silicon quantum dots,” Angewandte Chemie International Edition, vol. 44, no. 29, pp. 4550–4554, 2005.
- L. H. Chen, D. W. McBranch, H. L. Wang, R. Helgeson, F. Wudl, and D. G. Whitten, “Highly sensitive biological and chemical sensors based on reversible fluorescence quenching in a conjugated polymer,” Proceedings of the National Academy of Sciences, vol. 96, no. 22, pp. 12287–12292, 1999.
- C. H. Fan, S. Wang, J. W. Hong, G. C. Bazan, K. W. Plaxco, and A. J. Heeger, “Beyond superquenching: hyper-efficient energy transfer from conjugated polymers to gold nanoparticles,” Proceedings of the National Academy of Sciences, vol. 100, no. 11, pp. 6297–6301, 2003.
- J. H. Burroughes, D. D. C. Bradley, A. R. Brown et al., “Light-emitting diodes based on conjugated polymers,” Nature, vol. 347, no. 6293, pp. 539–541, 1990.
- R. H. Friend, R. W. Gymer, A. B. Holmes et al., “Electroluminescence in conjugated polymers,” Nature, vol. 397, no. 6715, pp. 121–128, 1999.
- F. So, B. Krummacher, M. K. Mathai, D. Poplavskyy, S. A. Choulis, and V. E. Choong, “Recent progress in solution processable organic light emitting devices,” Journal of Applied Physics, vol. 102, no. 9, Article ID 091101, 2007.
- C. F. Wu, C. Szymanski, and J. McNeill, “Preparation and encapsulation of highly fluorescent conjugated polymer nanoparticles,” Langmuir, vol. 22, no. 7, pp. 2956–2960, 2006.
- C. F. Wu, C. Szymanski, Z. Cain, and J. McNeill, “Conjugated polymer dots for multiphoton fluorescence imaging,” Journal of the American Chemical Society, vol. 129, no. 43, pp. 12904–12905, 2007.
- C. F. Wu, B. Bull, C. Szymanski, K. Christensen, and J. McNeill, “Multicolor conjugated polymer dots for biological fluorescence imaging,” ACS Nano, vol. 2, no. 11, pp. 2415–2423, 2008.
- C. B. Murray, D. J. Norris, and M. G. Bawendi, “Synthesis and characterization of nearly monodisperse CdE (, Se, Te) semiconductor nanocrystallites,” Journal of the American Chemical Society, vol. 115, no. 19, pp. 8706–8715, 1993.
- T. Rajh, O. I. Micic, and A. J. Nozik, “Synthesis and characterization of surface-modified colloidal CdTe quantum dots,” Journal of Physical Chemistry, vol. 97, no. 46, pp. 11999–12003, 1993.
- C. Ding, Y. Li, and Y. Qu, “Synthesizing quantum dot with uniform grain diameter distribution in water phase comprises preparing molding board agent and cadmium sulfydryl composite precursor, producing water solution of sodium borohydride, and synthesizing quantum dot,” East China Normal University, 2010.
- M. A. Correa-Duarte, M. Giersig, N. A. Kotov, and L. M. Liz-Marzan, “Control of packing order of self-assembled monolayers of magnetite nanoparticles with and without SiO2 coating by microwave irradiation,” Langmuir, vol. 14, no. 22, pp. 6430–6435, 1998.
- H. F. Qian, X. Qiu, L. Li, and J. C. Ren, “Microwave-assisted aqueous synthesis: a rapid approach to prepare highly luminescent ZnSe(S) alloyed quantum dots,” Journal of Physical Chemistry B, vol. 110, no. 18, pp. 9034–9040, 2006.
- F. Pinaud, D. King, H. P. Moore, and S. Weiss, “Bioactivation and cell targeting of semiconductor CdSe/ZnS nanocrystals with phytochelatin-related peptides,” Journal of the American Chemical Society, vol. 126, no. 19, pp. 6115–6123, 2004.
- T. Pellegrino, L. Manna, S. Kudera et al., “Hydrophobic nanocrystals coated with an amphiphilic polymer shell: a general route to water soluble nanocrystals,” Nano Letters, vol. 4, no. 4, pp. 703–707, 2004.
- C. A. J. Lin, R. A. Sperling, J. K. Li et al., “Design of an amphiphilic polymer for nanoparticle coating and functionalization,” Small, vol. 4, no. 3, pp. 334–341, 2008.
- J. H. Phan, R. A. Moffitt, T. H. Stokes et al., “Convergence of biomarkers, bioinformatics and nanotechnology for individualized cancer treatment,” Trends in Biotechnology, vol. 27, no. 6, pp. 350–358, 2009.
- J. Liu, S. K. Lau, V. A. Varma, B. A. Kairdolf, and S. M. Nie, “Multiplexed detection and characterization of rare tumor cells in Hodgkin's lymphoma with multicolor quantum dots,” Analytical Chemistry, vol. 82, no. 14, pp. 6237–6243, 2010.
- H. S. Cho, Z. Y. Dong, G. M. Pauletti et al., “Fluorescent, superparamagnetic nanospheres for drug storage, targeting, and imaging: a multifunctional nanocarrier system for cancer diagnosis and treatment,” ACS Nano, vol. 4, no. 9, pp. 5398–5404, 2010.
- Y. A. Cao, K. Yang, Z. G. Li, C. Zhao, C. M. Shi, and J. Yang, “Near-infrared quantum-dot-based non-invasive in vivo imaging of squamous cell carcinoma U14,” Nanotechnology, vol. 21, no. 47, Article ID 475104, 2010.
- W. Jiang, A. Singhal, J. N. Zheng, C. Wang, and W. C. W. Chan, “Optimizing the synthesis of red- to near-IR-emitting CdS-capped CdTexSe1-x alloyed quantum dots for biomedical imaging,” Chemistry of Materials, vol. 18, no. 20, pp. 4845–4854, 2006.
- W. J. M. Mulder, R. Koole, R. J. Brandwijk et al., “Quantum dots with a paramagnetic coating as a bimodal molecular imaging probe,” Nano Letters, vol. 6, no. 1, pp. 1–6, 2006.
- W. J. M. Mulder, G. J. Strijkers, G. A. F. V. Tilborg, D. P. Cormode, Z. A. Fayad, and K. Nicolay, “Nanoparticulate assemblies of amphiphiles and diagnostically active materials for multimodality imaging,” Accounts of Chemical Research, vol. 42, no. 7, pp. 904–914, 2009.
- J. H. Bang, W. H. Suh, and K. S. Suslick, “Quantum dots from chemical aerosol flow synthesis: preparation, characterization, and cellular imaging,” Chemistry of Materials, vol. 20, no. 12, pp. 4033–4038, 2008.
- B. R. Hyun, H. Y. Chen, D. A. Rey, F. W. Wise, and C. A. Batt, “Near-infrared fluorescence imaging with water-soluble lead salt quantum dots,” Journal of Physical Chemistry B, vol. 111, no. 20, pp. 5726–5730, 2007.
- H. Li, W. Y. Shih, and W. H. Shih, “Synthesis and characterization of aqueous carboxyl-capped CdS quantum dots for bioapplications,” Industrial and Engineering Chemistry Research, vol. 46, no. 7, pp. 2013–2019, 2007.
- J. K. Jaiswal, H. Mattoussi, J. M. Mauro, and S. M. Simon, “Long-term multiple color imaging of live cells using quantum dot bioconjugates,” Nature Biotechnology, vol. 21, no. 1, pp. 47–51, 2002.
- S. L. Gac, I. Vermes, and A. V. D. Berg, “Quantum dots based probes conjugated to annexin V for photostable apoptosis detection and imaging,” Nano Letters, vol. 6, no. 9, pp. 1863–1869, 2006.
- A. Wolcott, D. Gerion, M. Visconte et al., “Silica-coated CdTe quantum dots functionalized with thiols for bioconjugation to IgG proteins,” Journal of Physical Chemistry B, vol. 110, no. 11, pp. 5779–5789, 2006.
- H. W. Duan and S. M. Nie, “Cell-penetrating quantum dots based on multivalent and endosome-disrupting surface coatings,” Journal of the American Chemical Society, vol. 129, no. 11, pp. 3333–3336, 2007.
- A. Liu, S. Peng, J. C. Soo, M. Kuang, P. Chen, and H. Duan, “Quantum dots with phenylboronic acid tags for specific labeling of sialic acids on living cells,” Analytical Chemistry, vol. 83, no. 3, pp. 1124–1130, 2011.
- F. Q. Chen and D. Gerion, “Fluorescent CdSe/ZnS nanocrystal-peptide conjugates for long-term, nontoxic imaging and nuclear targeting in living cells,” Nano Letters, vol. 4, no. 10, pp. 1827–1832, 2004.
- I. Yildiz, B. McCaughan, S. F. Cruickshank, J. F. Callan, and F. M. Raymo, “Biocompatible CdSe-ZnS Core-shell quantum dots coated with hydrophilic polythiols,” Langmuir, vol. 25, no. 12, pp. 7090–7096, 2009.
- G. Ruan, A. Agrawal, A. I. Marcus, and S. M. Nie, “Imaging and tracking of Tat peptide-conjugated quantum dots in living cells: new insights into nanoparticle uptake, intracellular transport, and vesicle shedding,” Journal of the American Chemical Society, vol. 129, no. 47, pp. 14759–14766, 2007.
- R. Wilson, D. G. Spiller, A. Beckett, I. A. Prior, and V. Sée, “Highly stable dextran-coated quantum dots for biomolecular detection and cellular imaging,” Chemistry of Materials, vol. 22, no. 23, pp. 6361–6369, 2010.
- N. Ma, J. Yang, K. M. Stewart, and S. O. Kelley, “DNA-passivated CdS nanocrystals: luminescence, bioimaging, and toxicity profiles,” Langmuir, vol. 23, no. 26, pp. 12783–12787, 2007.
- P. Liu, Q. S. Wang, and X. Li, “Studies on CdSe/L-cysteine quantum dots synthesized in aqueous solution for biological labeling,” Journal of Physical Chemistry C, vol. 113, no. 18, pp. 7670–7676, 2009.
- C. F. Wu, T. Schneider, M. Zeigler et al., “Bioconjugation of ultrabright semiconducting polymer dots for specific cellular targeting,” Journal of the American Chemical Society, vol. 132, no. 43, pp. 15410–15417, 2010.
- P. M. Allen, W. H. Liu, V. P. Chauhan et al., “InAs(ZnCdS) auantum dots optimized for biological imaging in the near-infrared,” Journal of the American Chemical Society, vol. 132, no. 2, pp. 470–471, 2010.
- S. Prabakar, A. Shiohara, S. Hanada, K. Fujioka, K. Yamamoto, and R. D. Tilley, “Size controlled synthesis of germanium nanocrystals by hydride reducing agents and their biological applications,” Chemistry of Materials, vol. 22, no. 2, pp. 482–486, 2010.
- P. Sun, H. Y. Zhang, C. Liu et al., “Preparation and characterization of Fe3O4/CdTe magnetic/fluorescent nanocomposites and their applications in immuno-labeling and fluorescent imaging of cancer cells,” Langmuir, vol. 26, no. 2, pp. 1278–1284, 2010.
- W. J. M. Mulder, R. Koole, R. J. Brandwijk et al., “Quantum dots with a paramagnetic coating as a bimodal molecular imaging probe,” Nano Letters, vol. 6, no. 1, pp. 1–6, 2006.
- V. Bagalkot, L. F. Zhang, E. Levy-Nissenbaum et al., “Quantum dot-aptamer conjugates for synchronous cancer imaging, therapy, and sensing of drug delivery based on Bi-fluorescence resonance energy transfer,” Nano Letters, vol. 7, no. 10, pp. 3065–3070, 2007.
- J. Qian, K. T. Yong, I. Roy et al., “Imaging pancreatic cancer using surface-functionalized quantum dots,” Journal of Physical Chemistry B, vol. 111, no. 25, pp. 6969–6972, 2007.
- F. Erogbogbo, K. T. Yong, I. Roy, G. X. Xu, P. N. Prasad, and M. T. Swihart, “Biocompatible luminescent silicon quantum dots for imaging of cancer cells,” ACS Nano, vol. 2, no. 5, pp. 873–878, 2008.
- K. T. Yong, H. Ding, I. Roy et al., “Imaging pancreatic cancer using bioconjugated inp quantum dots,” ACS Nano, vol. 3, no. 3, pp. 502–510, 2009.
- C. Walther, K. Meyer, R. Rennert, and I. Neundorf, “Quantum dot—carrier peptide conjugates suitable for imaging and delivery applications,” Bioconjugate Chemistry, vol. 19, no. 12, pp. 2346–2356, 2008.
- V. Biju, D. Muraleedharan, K. I. Nakayama et al., “Quantum dot-insect neuropeptide conjugates for fluorescence imaging, transfection, and nucleus targeting of living cells,” Langmuir, vol. 23, no. 20, pp. 10254–10261, 2007.
- K. C. Weng, C. O. Noble, B. Papahadjopoulos-Sternberg et al., “Targeted tumor cell internalization and imaging of multifunctional quantum dot-conjugated immunoliposomes in vitro and in vivo,” Nano Letters, vol. 8, no. 9, pp. 2851–2857, 2008.
- R. R. Smith, Z. Cheng, A. De, A. L. Koh, R. Sinclair, and S. S. Gambhir, “Real-time intravital imaging of RGD-quantum dot binding to luminal endothelium in mouse tumor neovasculature,” Nano Letters, vol. 8, no. 9, pp. 2599–2606, 2008.
- H. S. Choi, W. H. Liu, F. B. Liu et al., “Design considerations for tumour-targeted nanoparticles,” Nature Nanotechnology, vol. 5, no. 1, pp. 42–47, 2010.
- A. Papagiannaros, J. Upponi, W. Hartner, D. Mongayt, T. Levchenko, and V. Torchilin, “Quantum dot loaded immunomicelles for tumor imaging,” BMC Medical Imaging, vol. 10, no. 1, article 22, 2010.
- L. Li, T. J. Daou, I. Texier, T. T. K. Chi, N. Q. Liem, and P. Reiss, “Highly luminescent cuins 2/ZnS core-shell nanocrystals: cadmium-free quantum dots for in vivo imaging,” Chemistry of Materials, vol. 21, no. 12, pp. 2422–2429, 2009.
- B. Dubertret, P. Skourides, D. J. Norris, V. Noireaux, A. H. Brivanlou, and A. Libchaber, “In vivo imaging of quantum dots encapsulated in phospholipid micelles,” Science, vol. 298, no. 5599, pp. 1759–1762, 2002.
- B. Ballou, B. C. Lagerholm, L. A. Ernst, M. P. Bruchez, and A. S. Waggoner, “Noninvasive Imaging of quantum dots in mice,” Bioconjugate Chemistry, vol. 15, no. 1, pp. 79–86, 2004.
- T. J. Daou, L. Li, P. Reiss, V. Josserand, and I. Texier, “Effect of poly(ethylene glycol) length on the in vivo behavior of coated quantum dots,” Langmuir, vol. 25, no. 5, pp. 3040–3044, 2009.
- K. Yang, Y. A. Cao, C. Shi et al., “Quantum dot-based visual in vivo imaging for oral squamous cell carcinoma in mice,” Oral Oncology, vol. 46, no. 12, pp. 864–868, 2010.
- E. Cassette, T. Pons, C. Bouet et al., “Synthesis and characterization of near-infrared Cu-In-Se/ZnS core/shell quantum dots for in vivo imaging,” Chemistry of Materials, vol. 22, no. 22, pp. 6117–6124, 2010.
- K. T. Yong, R. Hu, I. Roy et al., “Tumor targeting and imaging in live animals with functionalized semiconductor quantum rods,” ACS Applied Materials & Interfaces, vol. 1, no. 3, pp. 710–719, 2009.
- W. Zhang, Y. Yao, and Y. S. Chen, “Imaging and quantifying the morphology and nanoelectrical properties of quantum dot nanoparticles interacting with DNA,” Journal of Physical Chemistry C, vol. 115, no. 3, pp. 599–606, 2011.
- Y. X. Huang, X. J. Zheng, L. L. Kang et al., “Quantum dots as a sensor for quantitative visualization of surface charges on single living cells with nano-scale resolution,” Biosensors and Bioelectronics, vol. 26, no. 5, pp. 2114–2118, 2011.