Quantum dots

CHAPTER

Quantum dots: dynamic tools in cancer nanomedicine

3

Surya Kant Tripathi1, Rajneet Kaur Khurana2, Gurvir Kaur1,3, Teenu2 and Bhupinder Singh2,4 1

Department of Physics, Center of Advanced Study in Physics, Panjab University, Chandigarh, India 2University Institute of Pharmaceutical Sciences, UGC Centre of Advanced Studies, Panjab University, Chandigarh, India 3Department of Physics, Sant Longowal Institute of Engineering and Technology, Longowal, India 4UGC-Centre of Excellence in Applications of Nanomaterials, Nanoparticles & Nanocomposites (Biomedical Sciences), Panjab University, Chandigarh, India

3.1 INTRODUCTION Nanotechnology has revolutionized the development of multifunctional translational nano-tools for tracking cancerous cells and delivering therapeutic molecules. Magnetic resonance imaging (MRI) (Lee et al., 2006), positron emission tomography (PET) (Rege et al., 1994), single photon emission computed tomography (SPECT) (Kao et al., 2000), computed tomography (CT) (Sozzi et al., 2013), and optical imaging are some of the imaging aids being employed for the determination of cancer stage and its precise location. However, optical imaging offers unique benefits in molecular level visualization of cancer cells. As the fluorescent probes employed for optical imaging can be designed to be “switched on” under certain conditions, advancements in cancer imaging have taken a considerable leap due to the applications of nanomolecular imaging probes, popularly known as quantum dots (QDs). QDs perform multidimensional functions to accurately diagnose, manage, and treat malignant cancer. These probes are generally engineered to emit signals only after binding to a target tissue, which significantly increases sensitivity and specificity in the detection of a disease or disorder (Rao et al., 2007). Of the imaging methods, the conventional techniques such as angiography, CT, MRI, and radionuclide imaging, rely on contrast agents like iodine, gadolinium, and radioisotopes. This chapter not only endeavors to provide an updated version in an illustrative manner about the engineering and designing of the QDs for theranostics, but also highlights its

Nanobiomaterials in Medical Imaging. DOI: http://dx.doi.org/10.1016/B978-0-323-41736-5.00003-0 © 2016 Elsevier Inc. All rights reserved.

71

72

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

superlative qualities in molecular imaging and cancer diagnosis. It also furnishes the advanced applications of QD in theranostics through selected literature examples, primarily focusing on their efficiency as carriers/vehicles for therapeutic compounds as well as for genes, either to kill or alter malignancies.

3.2 CHARACTERISTIC FEATURES OF QUANTUM DOTS QDs are nearly spherical nanocrystals (NCs) composed of semiconductor materials that bridge the gap between individual atoms and bulk semiconductor solids (Murphy and Coeffer, 2002). Due to their intermediate size, typically between 2 8 nm in diameter, that is, 100 1000 atoms, these probes acquire unique properties matching neither individual atoms nor bulk materials. They have discrete electronic energies because of the quantum confinement of exciton (electron hole) pair, which raises exclusive optical properties (Jaiswal and Simon, 2004; Baskoutas and Terzis, 2006). The word, “confinement” refers to confining the motion of a randomly moving electron to restrict its motion in specific energy levels, while the word, “quantum” reflects the atomic realm of particles. This has been the most striking property of semiconductor QDs, that is, the massive change in optical properties as a function of size, typically with diameter less than 8 nm. Therefore, as the size of a particle is decreased to the nanoscale, any reduction in confining dimension makes the energy levels discrete. This increases or widens the band gap, and ultimately the energy gap too (http://nanoawesomeworld.blogspot.in). Along with unique optical properties, QDs have a rich surface chemistry that makes them useful as probes or carriers for traceable targeted delivery and therapy applications (Cassette et al., 2013; Yong et al., 2012). QDs can be functionalized to target specific cells or tissues by conjugating them with targeting ligands which has resulted in their rapid emergence as fluorescent probes for biomolecular and cellular imaging. An important advantage of QDs, that offers opportunities to integrate nanotechnology with biology at molecular and cellular levels, is their dimensional similarity to biological molecules (Niemeyer, 2001). Moreover, the spectral properties of QDs endow them with increased sensitivity and render them suitable for multicolored quantitative imaging, making them better as compared to several organic fluorophores (Figure 3.1; True and Gao, 2007). QDs overcome the limitations of conventional fluorophores (including organic dye and protein) including the pH dependence, quenching effects, propensity to photobleaching, lack of aqueous stability, and short-lived excited states (Ferrari and Bergquist, 2007).

3.3 COMPOSITION OF QUANTUM DOTS As already stated, QDs are NCs of semiconductors. The semiconductors are primarily made up from elements, where s and p are valence electrons and include elements

3.4 Architecture of Multifunctional Quantum Dots

FIGURE 3.1 A sketch of quantum dots exhibiting hues of colors in the visible range at different wavelengths.

from groups IV, II VI, III V, or IV VI of the periodic table. All semiconductors exist in either cubic or hexagonal crystal structure. Semiconductors with cubic structure have either diamond (C, Si, Ge, α-Sn) or zinc blende, also called sphalerite phase (ZnS, GaAs), while semiconductors with hexagonal crystal structure have wurtzite phase (such as ZnO, GaN) (Shahid, 2012). Out of all the groups of the periodic tables, the most interesting groups are II VI, that is, materials such as cadmium selenide (CdSe), cadmium sulfide (CdS), cadmium telluride (CdTe), zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), and zinc telluride (ZnTe) and group IV VI compounds, such as lead selenide (PbSe) and lead sulphide (PbS), as they possess unique optical and electronic properties (Fuhrhop and Wang, 2009). These groups share large-scale utility for the production of photovoltaic devices. On the other hand, the group IV materials, such as carbon (C), silicon (Si), and germanium (Ge), are nontoxic in their bulk form and have higher crystallization temperature. These groups carry vital potential to tag biological molecules. While III V compound semiconductors are obtained by combining group III elements (essentially aluminum (Al), gallium (Ga), and indium (In)) with group V elements (essentially, nitride (N), phosphide (P), arsenic (As), and antimony (Sb)). This provides possible combinations; the most important are probably GaAs, InP, GaP, and GaN. Various permutations and combinations are constantly being tried to form quaternary and ternary compounds, from all these elements, in order to impart characteristic optical and biologically active features (Adachi, 2009).

3.4 ARCHITECTURE OF MULTIFUNCTIONAL QUANTUM DOTS Cancer treatment generally involves the sequential use of diagnostic tools as well as therapeutic modalities. QDs not only serve as contrasting agents but also assist

73

74

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

FIGURE 3.2 A model of a classical quantum dot as a theranostic carrier featuring all the plausible attachments.

in shielding the therapeutic moieties, both hydrophilic and hydrophobic. Lipophilic drugs can be embedded between the inorganic core and the amphiphilic polymer coating layer, whereas the lyophilic agents can be immobilized onto the hydrophilic side of the amphiphilic polymer, through covalent or non-covalent bonds. Another approach is to either embed these QDs in this amphiphilic polymer or attach with the help of linkers (Mudshinge et al., 2011; Yong et al., 2012). Some basic requirements for preparing theragnostic carrier formulations for targeted in vivo therapy are: (i) functionalization of the NC surface with targeting ligands for site-specific delivery to tumor cells; (ii) decreasing the size of the NCs for easy excretion from the body; (iii) the release of drug at tumor cells by triggering it externally or by local environmental factors; and (iv) the passivation of the NC surface with longlasting biocompatible polymers to prevent degradation or breakdown of the QDs (Singh and Lillard, 2009). An attempt has been made to conceptualize the functional modalities usually attached to QDs in Figure 3.2.

3.4.1 DETECTION COMPONENT (NONINVASIVE IMAGING) QDs serve as one of the extremely popular optical probes to label different components within a cell, and are capable of providing molecular information for detection purposes (Jaiswal and Simon, 2004). Conventional fluorophores (including organic dye and fluorescent protein) suffer from serious chemical and photophysical liabilities, such as pH dependence, self-quenching at high concentrations, and susceptibility to photobleaching (Ferrari and Bergquist, 2007). QDs have some key spectral properties that make these the best fluorophores for sensitive, multicolor, and quantitative imaging of histopathological sections (True

3.4 Architecture of Multifunctional Quantum Dots

and Gao, 2007). Broad absorption profiles of QDs allow simultaneous excitation of multiple colors, where the emission wavelengths can be continuously tuned by varying particle size and chemical composition (Kaur and Tripathi, 2014a,b). There are examples in the literature in which QDs (Michalet et al., 2005) have been associated with iron oxide nanoparticles (Tassa et al., 2011), carbon nanotubes (Liu et al., 2009), gold nanoparticles (Daniel and Astruc, 2004), and silica nanoparticles (Slowing et al., 2008) for detection purposes. Dual, as well as ternary, combinations of these agents can be used for multimodal imaging and are currently being employed for various applications (Song et al., 2011).

3.4.2 TARGETING LIGANDS Two targeting strategies have primarily been investigated to target drugs and imaging agents to tumors: passive and active targeting. Fluorescent NPs in a certain size range could passively target macromolecules and NP carriers to solid tumors (Maeda et al., 2000). In active targeting, the large surface area of NPs and their usual core-shell structure offer a platform to encapsulate diverse drugs or imaging agents, for site-specific targeting to the receptors, which are over expressed on the surface of solid tumors vis-a`-vis the healthy tissue. For instance, several monoclonal antibodies or antibody fragments such as scFv, folic acid, growth factors, carbohydrates, peptides, glycoproteins, or receptor ligands, that are selectively overexpressed on cancer cells, serve as active targeting moieties (Peer et al., 2007). Cellpenetrating peptides are another potential class of molecule that have been exploited for passive targeting due to their nonspecific mechanism of cellular uptake that are applicable to a variety of cell types and tumor classes (MacEwan and Chilkoti, 2013). A study probed therapeutic effectiveness of motifs like Arg-Gly-Asp (RGD) and leucine-aspartic acid-valine (LDV). These were investigated to functionalize QDs, so that the QD peptide complexes selectively bind to integrins on HER-2positive cancer cells (Shi et al., 2006). In another promising study, Lieleg and associates highlighted the potential of a wide class of cyclic Arg-Gly-Asp (RGD) peptides and a biotin streptavidin linkage, coupled with QDs to treat and diagnose osteoblast cells (Lieleg et al., 2007).

3.4.3 THERAPEUTIC COMPONENTS Owing to unique surface properties and ability to track cells in vivo, these nanoprobes hold tremendous potential in nanomedicine, obliterating the need to sacrifice animals. Several approaches have been adapted to either associate or modify the surfaces of QDs by polymers, biomolecules, antibodies, and therapeutic agents to improve the biocompatibility and properties for biological applications (Adeli et al., 2011). The cytotoxic agents have been effectively loaded on QDs that allow their specific and traceable delivery to the intended cancer cell(s). QDs act as nanovehicles for different cytotoxic drugs, such as daunorubicin, doxrubicin, temozolomide, etc. (Li et al., 2006; Bagalkot et al., 2007; Wu et al., 2010). Also, QDs have served

75

76

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

as a cart for genetic silencers (oligonucleotides) to visualize the action on the target site (Qi and Gao, 2008; Bonoiu et al., 2009). CdTe QD/methylene blue hybrid system has been successfully investigated for inhibition of HepG2 and HeLa cancerous cells (Rakovich et al., 2010).

3.4.4 POLYMER ENCAPSULATION/DRUG-LOADING CAPABILITY Polymers are widely employed to coat QDs; an approach that not only tackles biocompatibility and water-solubility issues, but also provides a linker for bioconjugation. The amphiphilic nature of the coating allows solubility in aqueous solutions and rapid transfer across cell membranes, and low concentrations of the QDs for overall toxicity reduction. Additionally, the polymers can carry ionic or reactive functional groups facilitating receptor targeting and cell attachment for successful applications in the fields of bioimaging and biosensing. The derivatives of polymers, such as polyethylene glycol (PEG), poly(ethylene oxide)-containing block copolymers, chitosan, etc., usually carry ionic or reactive functional groups for incorporation of various targeting and therapeutic components (Zhang and Clapp, 2011). Hu et al. (2012) encapsulated PEG-grafted phospholipid micelles associated with near-infrared-emitting ultra-small PbS QDs, whereas, Rizvi et al. (2012) demonstrated the potential of stabilizing agents, such as polyhedral oligomeric silsesquioxane (POSS)-coated CdTe-cored QDs, with mercaptosuccinic acid and D-cysteine. The polymeric structure decides the drug-loading capabilities of the QDs (Zhang and Clapp, 2011; Hu et al., 2012). With change in the biological stimuli, like pH, light, temperature, ultrasound, enzymes, the drug molecules which earlier were conjugated to the QD surface are released (Schmaljohann, 2006). The QD drug-conjugated system reaches the desired organ or tissue, and releases the drug molecules either upon degradation of polymer particles at low pH or upon diffusion from polymer (Hoffman, 2013; Wang et al., 2007; Luo et al., 2012). The polymer coating significantly improved the optical properties of the QDs, which enhanced the photoluminescence quantum yield by about 50%. In situ immobilization of ZnSe/ZnS QDs in β-cyclodextrin and chitosan polymer loaded with suberoylanilide hydroxamic acid (SAHA) expressed long-term optical properties with anticancer effect (Chang et al., 2013).

3.5 SYNTHESIS AND FUNCTIONALIZATION OF QDs Colloidal QDs can be prepared with different emissions, varying from the ultraviolet to the infrared; though QDs need to be water-soluble for biological and theranostic purposes (Gatsouli et al., 2007). Along with stability in water, QDs need to have functional groups for bioconjugation, as it helps to attain native properties and adds biocompatibility and non-immunogenicity in living systems (Jamieson et al., 2007;

3.5 Synthesis and Functionalization of QDs

FIGURE 3.3 Schematic representation of four common approaches to hydrophilic surface modification of TOPO or TOP stabilized quantum dots. (i) TOPO replacement with heterobifunctional linker consisting of a thiol end group, a spacer, and a hydrophilic end group such as carboxylic acid (Akerman et al., 2002); (ii) TOPO replacement with a linker consisting of two thiol groups on one end and a hydrophilic end group on the other end (Mattoussi et al., 2000); (iii) TOPO replacement with a silane forming a stable shell via crosslinking (Zhu et al., 2007); (iv) stabilization of TOPO layer using amphiphilic molecules, such as PEG lipopolymers of amphiphilic diblock copolymers, that are held on the surface by hydrophobic interaction with the octyl chains of TOPO (Kang et al., 2004).

Sperling and Parak, 2010). Properties of QDs are completely dependent on the route of synthesis. Generally, chemical synthesis, wherein the growth can be controlled at atomic level, meets the required quality of QDs. Synthesis of highly luminescent QDs is possible in organic as well as aqueous medium. Conventionally, high-quality NCs are prepared in organic solvents using tri-n-octylphosphine oxide (TOPO) and trioctylphospine (TOP) by pyrolysis of organometallic precursors such as Cd and Se to generate CdSe NCs (Murray et al., 1993). These compounds provide the most controlled growth conditions, while addition of other appropriate groups (such as hexadecylamine, HDA) permits tailoring of particle morphology and emission efficiency (Talapin et al., 2001; Peng et al., 2000), and is the most popular method for obtaining good-quality QDs (Figure 3.3).

77

78

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

FIGURE 3.4 Schematic representation of the biological labeling by QDs through (i) solubilization and (ii) bioconjugation of QDs. For solubilization, the surfactant layer is replaced or coated by an additional layer introducing either electric charge or hydrophilic polymers for mediating solubility in water (iii) after coating there could be site-specific binding for the same (Mattoussi et al., 2000).

NCs of CdSe, CdS, CdTe, ZnS, and ZnSe have been widely used for biological labeling, but are not considered very efficient due to low emission efficiency. The surface of these bare NCs has dangling bonds which may act as charge carrier trap sites, and can decrease the semiconductor QD emission efficiency (Spanhel et al., 1987). The enhancement of emission efficiency requires passivation of surface defects by inorganic or organic capping. Inorganic layer passivates both anionic and cationic surface sites simultaneously which, otherwise, is hard to achieve with organic ligands (Peng et al., 1997; Figure 3.4). These NC systems show characteristic features of the core and shell materials, where the relative energy band alignment of the composite decides the nature of band transitions in composite material. The shell also acts as an insulating medium that limits electronic communication with surface ligands or the environment and enhances the photostability (Kaur and Tripathi, 2014). Colloidal core shell, QDs such as CdSe/ZnS, CdTe/ CdS, CdSe/CdS/ZnS, and InP/ZnS, are some of the materials with high emission efficiency and are of immense interest for molecular imaging and cancer therapy. Ligand exchange involves the exchange of surface ligands with a thiol group of thio-acids or polysilanes (Zhu et al., 2007). Thio-acids and silica achieve water solubility through the presence of carboxyl groups, and hydroxyl groups on its surface, respectively (Kim and Bawendi, 2003). The use of disulfide linkers leads to more stable nanoparticle capping corresponding to the ligand exchange than

3.5 Synthesis and Functionalization of QDs

FIGURE 3.5 Transmission electron micrographs of CdSe/ZnS core shell quantum dots of (a) size B4 nm dispersed in water (b) at high resolution.

the reagents which contain a single thiol group and the cyclic disulfide linkers have the maximum stability. The greater stability is likely a result of anchoring of the ligands to the nanoparticles through two sulfur atoms. Figure 3.5 shows transmission electron micrographs of CdSe/ZnS QDs, where the CdSe is the core material and ZnS is the shell material (a) at low and (b) high magnification. These QDs are synthesized directly in water using thioglycolic acid as capping reagent. In another way, one can encapsulate the hydrophobic QDs with amphiphilic macromolecules. This involves the hydrophobic interactions between TOPO/ TOP and the hydrophobic ends of the amphiphilic polymer, and its hydrophilic end providing aqueous solubility (Kang et al., 2004). A number of natural polymers, for example, chitosan, organic dendron, oligomeric ligands, poly(maleic anhydride-alt-1-tetradecene), and the phospholipid micelles, provides a second layer to the surface of QDs for aqueous solubility (Bakalova et al., 2011). Surface coating with polymer and lipids can preserve the quantum yield of QD fluorescence, but tends to increase the size of the initial QDs (Kirchner et al., 2005). The colloidal properties of solubilized QDs, such as charge and hydrodynamic status, change with the method of encapsulation and need to be tailored according to the biological system being used. The surface charge and other surface properties of thiol-capped NPs can be controlled by the choice of stabilizing agent with appropriate free functional groups (Sharma and Tripathi, 2013; Bao et al., 2012). Conjugation of inorganic NPs to biomolecules (called bioconjugation) is possible by physical adsorption, electrostatic interactions, covalent coupling, specific binding, or through crosslinkers. It is a vital requirement for

79

80

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

immobilizing the targeting and therapeutic components for cancer diagnostics and therapeutics (Mazumder et al., 2009). QDs are usually stabilized by anionic ligands, such as carboxylic acid derivatives (citrate, tartrate, lipoic acid), so the positively charged proteins or their domains have a tendency to adsorb on the surface through electrostatic interactions. However, such interactions are sensitive toward pH of the medium and charge screening and are inherently nonspecific. It is possible to modify the QD surface chemistry by changing the charge on the end of the NP ligand and the ligand hydrophobicity for regio-specific interactions with the proteins (Kaur and Tripathi, 2015). Chemisorption of thiol derivatives of biomolecules on the QD surface is a relatively straightforward covalent bioconjugation approach (Figure 3.5). This strategy has been used to attach oligonucleotides, DNA (Mitchell et al., 1999), and bovine serum albumin (BSA) (Willard et al., 2001) to QDs. However, the linkage is dynamic due to a weak bond between Zn and thiol (Chan and Nie, 1998). A better approach could be to covalently link the water-soluble QDs with carboxylic acid, amino or thiol groups, to biological molecules through bifunctional linkers. This method is most commonly used for making biofunctionalized QDs for in vitro cell labeling and in vivo imaging purposes. EDC, (1ethyl-3-(3-dimethylaminopropyl)) is a commonly used crosslinker to link aNH2 and aCOOH groups, whereas 4-(N-maleimidomethyl)-cyclohexanecarboxylic acid N-hydroxysuccinimideester (SMCC) can be used to crosslink aSH and aNH2 groups (Aslam and Dent, 1998). The linkers have been used to associate several biomolecules including oligonucleotides, biotin, peptides, and proteins such as avidin/streptavidin, albumin, adaptor proteins (e.g., protein A and G) and antibodies (Goldman et al., 2002), with that of QDs. Also, the native functional groups (aCOOH, aNH2, or aSH) on a water-soluble QD surface can be converted to other functional groups that allow more flexible conjugation of QDs to biomolecules through site-specific conjugation. For instance, the carboxylic acids on QDs can be converted to hydrazides for the attachment of biomolecules containing sugar groups. Another strategy based on specific interaction utilizes protein tag (HaloTag) to functionalize QDs for biological imaging. The HaloTag is a modified haloalkane dehalogenase designed to covalently bind synthetic ligands that comprise a chloroalkane linker attached to a variety of useful molecules, such as fluorescent dyes, affinity handles, or solid surfaces (Tripathi et al., 2015; Zhang et al., 2006; Figure 3.6). Besides, all these approaches, affinity-based systems found in nature have attracted significant attention during recent years, where the avidin-biotin system is the most well-known. The reactive biotin/avidin are covalently linked to either surface sulfhydryl or amine functionalities, thus allowing for the biotinylation of the QD surface and subsequent binding to streptavidin (Wilchek and Bayer, 1990). The biomolecules, such as DNA oligomers, peptides, antibodies, and fluorescent dyes, readily modified with biotin or avidin (or its derivatives), are commercially available (Figure 3.7).

3.5 Synthesis and Functionalization of QDs

FIGURE 3.6 Schematic presentation of steps involved in the bioconjugation of QDs.

FIGURE 3.7 Targeted delivery of drugs to cancer cells by using quantum dots.

81

82

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

3.6 QUANTUM DOTS IN CANCER THERAPY Despite being an alternative to imaging, these engineered probes serve as excellent carriers for delivering chemotherapeutic drugs and genes for altering tumor cells (Qi and Gao, 2008). QDs hold massive potential to detect several cancer markers for sorting out the complex gene expression profiles of cancers and thereby for accurate clinical diagnosis (Smith et al., 2006). Undoubtedly, QDs have become an indispensible tool today for targeting, imaging, and performing specific therapeutic functions.

3.6.1 QUANTUM-DOT-BASED ANTICANCER DRUG DELIVERY QDs provide a versatile platform for engineering traceable drug-delivery systems with potential for improving pharmacological treatment of cancers. QDs conjugated to anticancer drugs followed by delivery of drug/QD conjugates to specific sites and successive release of the drug molecules from the QD surface in response to local biological conditions, such as pH or the presence of enzymes, is a powerful technology (Luo et al., 2012). Li et al. (2006) reported an approach to enhance the concentration of daunorubicin in leukemia K562 cells through CdS QDs. Tada et al. (2007) injected the monoclonal anti-HER2 antibody (trastuzumab) labeled with QDs into HER2-overexpressing breast cancer rodent models to analyze the key effects of its targeted delivery to the tumor. However, conjugating full-length monoclonal antibodies directly to QDs is a relatively difficult process. In contrast, single-chain antibody fragments (scFv) with relatively small size are generally amenable to be genetically and structurally manipulated and are advantageous over monoclonal antibodies as carriers of radionuclei, drugs, and nanobeads (Nelson, 2010). Tian et al. (2011) reported the drug-loaded liposome-QD-Dox (L-QD) hybrid vesicles encapsulated with TOPO into two types of lipid bilayers, namely, the “rigid” disteroylphosphatidyl-choline and a fluid-phase bilayer of egg PC (EPC:Chol:DSPEPEG 2000) with better encapsulation efficiency of hydrophilic small molecules like carboxyfluorescein. Chakravarthy et al. (2011) reported the ability of nanoconjugates of CdSe/CdS/ZnS QD and Dox to successfully target alveolar macrophages during inflammatory lung injuries. Also, Wang et al. (2012) reported a new method of screening micrometastases of lung cancer in peripheral blood by magnetic nanoparticles and QDs to achieve early diagnosis and recurrence prevention. Listed in Table 3.1 are some of the recent approaches of QDs in cancer diagnosis and treatment.

3.6.2 QUANTUM-DOT-BASED GENE DELIVERY Extracellular nucleases within the body rapidly break down oligonucleotides in the circulation and these tend to accumulate in the liver and kidneys before their eventual clearance from the body. Therefore, endeavor has been made by several researchers to

Table 3.1 Selected Literature Reports on Applications of Quantum Dots in Targeted Delivery Targeted Receptor

Cell Culture and Tumor Model for Bioimaging

Targeting Ligands

QDs

Linkage

Folate

Diethylene-triaminepenta-acetic acid gadolinium and folic acid

Graphene QDs

EDC

In vitro HeLa and HepG2 cells lines in vivo evaluation of toxicity to the embryonic development of zebrafish

Thrombin and trypsin

Cysteine, glutathione, dihydrolipoic acid, or 3-mercapto propionic acid

QD peptide complex

DHLA and MPA coatings

Protease using Förster resonance energy transfer (FRET)-based assays

Bone marrow cancer cells

Polyimidazole ligands

QD Ab conjugates

Tetrazinenorbornene cycloaddition

Cytometric imaging in mice

Study Inference

Reference

Multifunctional nanocarriers could be used as promising targeted drug-delivery vehicles for the diagnosis and image-guided chemotherapy of various cancers Clearly demonstrates the adsorption of QDs over nanoparticle interface in mediating substrate turnover and act as the strongest support The intravital imaging studies using a chronic calvarial bone window showed that QD-Ab conjugates diffuse into the entire bone marrow and efficiently label single cells belonging to rare populations of hematopoietic and progenitor cells

Huang et al. (2015)

Wu and Algar (2015)

Han et al. (2015)

(Continued)

Table 3.1 Selected Literature Reports on Applications of Quantum Dots in Targeted Delivery Continued Cell Culture and Tumor Model for Bioimaging

Targeted Receptor

Targeting Ligands

QDs

Linkage

Cancerous cell lines

Polymer coating

CdSe/ZnS

Carboxyl or amine polymer

Integrin αvβ3

cRGDfC-peptide

Polymer coated CdSe/ZnS

SMCC

Folate receptor

Folic acid

CdTe/CdS

EDC/NHS

Nude mouse, xenografted with A549 tumor in the left thigh and HepG2 tumor in the right thigh

VEGFR2

VEGF2 antibody

QD655

Streptavidin/ biotin

Colons of AOMtreated mice, salinetreated control mice

Epithelial (BEAS-2B), fibroblast (HFF-1), and lymphoblastoid (TK6) cell line Human oral squamous carcinoma cell line (BcaCD885)/ Male nude mice (BALA/c-nu/nu)

Study Inference

Reference

The system could very well diagnose cancer cells.

Manshian et al. (2015)

Greatest amount of QD800-RGD was found in liver spleen, followed by tumor and lung. No detectable signal of QD800-RGD was found in brain, heart, kidney, testis, stomach, or intestine indicating broad range applications of QDs in personalized treatment. Oligomeric/inorganic hybrid NPs provided a new type of biomaterials for tumor-targeted imaging with high selectivity. Colorectal cancer was successfully labeled in vivo.

Huang et al. (2013)

Yuan et al. (2014)

Carbary-Ganz et al. (2014)

VEGF receptor

VEGF antibody

CdSe/ZnS coated with oleylamine poly (aspartate) graft poly (ethylene glycol) dodecylamine (PASP Na g PEG DDA)

EDC HCl

Human liver cancer (HepG2) cells

Matrix metalloproteinase-2

MMP-2 specific peptide substrate (GPLGVRGKGG)

CdTe QD

_

MCF-7 cells, nude mice bearing MDAMB-231

QDs were found to have satisfactory stability, strong targeting and intracellular fluorescence, together with low cytotoxicity. Successful examination of MMP2 in live cells and tumor on nude mouse which promised a wide range of applications such as the detection of different biomarkers and early diagnosis of disease.

Sun et al. (2014)

Li et al. (2014)

QDs, quantum dots; EDC, (1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide); HepG2, liver hepatocellular cells; QD-Ab, quantum dot-antibody; CdSe/ZnS, cadmium selenide/zinc sulfide core shell; (cRGDfC) peptide, cyclo(arginine-glycine-aspartic-acid-D-phenylalanine-cysteine) peptide; U87MG cell, human glioblastoma-astrocytoma cells; SMCC, succinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate; VEGF, vascular endothelial growth factor; EDC.HCl, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride; EDC/NHS, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide; HER2/neu, human epidermal growth factor receptor/neu; MCF-7 cells, human breast cancer cell line.

86

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

employ semiconductors as nucleic-acid cargo and also integrated additional functionalities to enhance gene or protein delivery to specific targets (de Aberasturi et al., 2012). Gene therapy uses DNA as a pharmaceutical agent to treat disease and was first conceptualized in 1972 (Friedmann and Roblin, 1972). The most common form involves DNA that encodes a functional, therapeutic gene to replace a mutated gene. Other forms involve direct correction of a mutation, or use DNA that encodes a therapeutic protein drug to provide treatment. The rising field of gene therapy requires multifunctional delivery platforms in order to overcome the cellular barriers. QDs provide an optically fluorescent and biocompatible surface to act as a multifunctional delivery platform for gene therapy (Walther et al., 2008). A promising strategy for in vivo delivery of siRNAs (short interfering RNA) associated with QDs allows the tracking of transfection and allocation of QD siRNA complexes in the cytoplasm. Chen et al. (2005) demonstrated that luminescent QDs could conjugate to a membrane-translocating protein, transferrin responsible for endocytosis of QDs by living cancer cells in culture. Li et al. (2012) reported CdSe/ZnS fluorescent QDs delivering small siRNAs targeting β-secretase (BACE1) for better transfection efficiency of siRNAs (Hardy and Selkoe, 2002). Several other studies revealed that siRNA targeting β-secretase (BACE1) could significantly inhibit the expression of specific BACE1 messenger RNA and could reduce the generation and accumulation of β-amyloid (Gonzalez-Alegre, 2007; Laird et al., 2005), but the problem was of the blood brain barrier (BBB). QDs can freely cross cell membranes and the BBB and can become siRNA carriers upon modification. The CdSe/ZnS QDs with the conjugation of amino-PEG were synthesized. Negatively charged siRNAs were electrostatically adsorbed onto the surface of QDs to develop QD-PEG/siRNA nanoplexes, therefore, promoting the transfection efficiency of siRNA. The biodegradable PEG polymer coating could protect QDs from being exposed to the intracellular environment and restrained the release of toxic Cd21. Therefore, the QD-PEG/siRNA nanoplexes might serve as ideal carriers for siRNAs (Harris et al., 2010). Table 3.2 furnishes the applications of QDs in nucleic-acid drug delivery.

3.7 QUANTUM-DOT-BASED PHOTODYNAMIC THERAPY (PDT) QDs, as photosensitizers (PSs) themselves or as carriers of the same, have exhibited potential in photodynamic therapy (PDT). QDs, like other PSs, can be activated by certain light and can transfer the energy to nearby oxygen molecules which damages cancer cells (Bakalova et al., 2004; Biju et al., 2010; Xie et al., 2010). Tsay et al. (2007) modified QDs with streptavidin and conjugated biotinylated pDNA with it. They found the generation of reactive oxygen intermediates (ROI) through nitroblue tetrazolium (NBT) assay and subsequent damage to purine and pyrimidine bases through assays with base excision repair enzymes.

Table 3.2 Select Literature Instances of Widely Used Chemotherapeutic Agents Delivered Through Quantum Dots Cell Line(s)/ Animal Model

Carrier

QDs

Drug

Targeting Mode

Targeting Model/ Ligand

Multifunctional nanoparticles

Horseradish peroxidise antibodyconjugated QD605

Nutlin-3a

Active

Aptamer

Poly(D, L-lactide-coglycolide)

EpCAM receptor

Liposomes

Silicacoated

Mycotoxins zearalenone and aflatoxin B1

Passive

_

Proteins

_

MCF-7, SKOV3, and ZR751. HEK293 In vitro

Quantum dots

Graphene

DA

Passive

_

Polyindole Polymers

_

In vitro

Polymeric nanoparticle

QDs

Dox

Active

Chitosan

PEG

Cellular uptake

In vitro cell lines

Linker

Targeted Receptor

Therapeutic Outcome

Reference

This multifunctional nanosystem would act as a treatment option for cancer

Das et al. (2015)

Application of silicacoated liposomes-loaded with QDs as a label in FLISA resulted in a fourfold increase in the assay sensitivity for zearalenone and a sixfold increase in the sensitivity for aflatoxin B1 determination The prepared sensor can rebind DA in dual-type with both low and high affinity The system is used to observe the surface properties effect of other polymers such as chitosan and poly (ethylene) glycol on the cellular interaction and uptake. Moreover, quantum dots can be used to study microparticle theranostic delivery

Beloglazova et al. (2015)

Zhou et al. (2015)

Win et al. (2014)

(Continued)

Table 3.2 Select Literature Instances of Widely Used Chemotherapeutic Agents Delivered Through Quantum Dots Continued Cell Line(s)/ Animal Model

Carrier

QDs

Drug

Targeting Mode

Targeting Model/ Ligand

Albumin nanoparticles

Graphene

Gemcitabine

Active

HA

Human serum albumin

CD44

Panc-1 cell lines

PEGylated QDs

GSH-CdTe

Dox

Active

Folic acid

PEG

Folate receptor

HeLa cells

PGA encapsulated QDs

CuInS2

Dox

Passive

Linker

L-cysteine

Targeted Receptor

PC3M cells and HepG2 cells

Therapeutic Outcome

Reference

The graphene QDs enhanced the efficacy of system as a drug-delivery vehicle as well as the bioimaging Multifunctional DOX-QDPEG-FA system showed great potential for tumor imaging, targeting, and therapy QDs/PGA Dox nanoparticles delivered Dox to targeted cancer cells and monitored its release based on the fluorescence “turn-on” signal of CuInS2 QDs, which could simultaneously image the cancer cells

Nigam et al. (2014)

Chen et al. (2015a,b)

Gao et al. (2014)

QDs, quantum dots; MCF-7 cells, Michigan Cancer Foundation-7 breast cancer cells; SKOV3 (human ovarian cancer; ZR751 (human breast cancer); HEK-293 (human embryonic kidney); FLISA, fluorescent labeled immunoassay; DA, dopamine; PEG, poly(ethylene glycol); HA, hyaluronic acid; CD44 antigen, cell-surface glycoprotein; GSH-CdTe, glutathione conjugated cadmium telluride; Dox, doxorubicin; PC3M cells, prostate cancer cells; PGA, poly(L-glutamic acid); HepG2, liver hepatocellular cells; CuInS2, copper indium sulfide.

3.7 Quantum-Dot-Based Photodynamic Therapy (PDT)

As compared to small-molecule chemical PSs, QDs are more chemically stable, water-soluble, and (for near-infrared fluorescent QDs) less susceptible to optical interference with biological tissues. However, the QDs are often limited by the low yield of quantum singlet oxygen (typically less than 5%), which is vastly inferior to that of classic PSs (40 60%). QDs possess important characteristics that make them potentially good PSs for PDT. This is the energy transfer process from QDs to PS drugs and further to oxygen in QD PS conjugates, and are quite efficient for ROI production. QD PS conjugates are fairly advantageous over conventional PS drugs; for example, indirect photoactivation of PS drugs by photostable QDs offers prolonged imaging and PDT without photobleaching. The large surface area of QDs allows space for conjugating multiple PS and cancer markers for efficient and targeted cancer imaging, PDT, and broad absorption band, with large twophoton absorption cross-section of QDs being advantageous for unrestricted and NIR photoactivation (Daniel and Astruc, 2004; Willard et al., 2001). Peptide-coated QD PS conjugates (PSs-rose bengal and chlorine) have been synthesized by covalent conjugation strategies on peptides that overcoat CdSe/CdS/ZnS NCs. Singlet oxygen production from the conjugate could be achieved through indirect excitation through FRET from the NCs to PSs, or by direct excitation of the PSs (Shi et al., 2006). The system can be used simultaneously for fluorescence imaging and singlet oxygen generation. Rakovich et al. (2010) have studied the photodynamic properties of a novel CdTe QD methylene blue hybrid PS. The use of methylene blue augmented the production of singlet oxygen as determined by near-infrared photoluminescence measurements. In vitro growth studies revealed that the increased efficiency of singlet oxygen production subsequently improved the efficiency of the methylene blue semiconductor NCs hybrid system in killing HepG2 and HeLa cancer cells (Rakovich et al., 2010). Tsay et al. (2007) demonstrated energy transfer from the QDs to phthalocyanines (Pc) upon photoexcitation of the QDs and observed that the nature of the carboxylic thiol stabilizer enhanced the efficiency of energy transfer. As a result of the nanoparticle and Pc mixing, the photoluminescence efficiency of the Pc moieties in the mixtures does not strictly follow the quantum yield of the bare Pcs and exhibits high singlet oxygen quantum yield. Another type of QD conjugate that is made luminescent through bioluminescence resonance energy transfer (BRET) has been reported (Hsu et al., 2010). The external excitation light source is a problem for clinical application because of the limitation of tissue-penetrating properties. During a BRET process, QDs accept energy from luciferase (Luc) catalyzed coelenterazine through nonradiation energy transfer (So et al., 2006). Bioluminescent Luc-QDs can exhibit self-illumination at 655 nm for PS activation after adding coelenterazine. Thus, HeLa cells were co-treated with QD-Luc and a clinical PS called Foscan, which can be excited at 652 nm for PDT. The results portrayed that the QD-Luc can stimulate the Foscan and significant cytotoxicity can be observed after coelenterzine addition, with no apparent cytotoxicity of PS. Table 3.3 highlights the applications of PDT in the domain of drug delivery.

89

Table 3.3 Select Literature Instances on Applications of Quantum Dots as Gene Carrier Carrier

Composition

Gene

Linker

Targeted Receptor

Cell-derived microparticles (MPs)

QDs

siRNA

VEGF

Fluorescence encoding microfluidic platform

Graphene QDs

HIV gene and variola virus

QDs

Carbon

Survivin siRNA

Assay

Therapeutic Outcome

Reference

Biotin

Human umbilical vein endothelial cells

Chen et al. (2015a,b)

Poly(dimethylsiloxane) (PDMS)

_

In vitro

Polyethylenimine

MGC-803

Human gastric cancer cell line

QD-labeled MPs had inherent cell-targeting and biomolecule conveying ability and were successfully employed for combined bioimaging and tumortargeted therapy. This study provides the first reliable and biofriendly strategy for transforming biogenic MPs into functionalized nanovectors This method achieves simultaneous multiplexed DNA measurements with a significantly time-saving way and without different dyelabeled probes or complex operation procedures The Cdot-based and PEIadsorbed complexes both as imaging agents and siRNA nanocarriers have been developed for Survivin siRNA delivery. The results indicate that Cdot-based nanocarriers could be utilized in a broad range of siRNA delivery systems for cancer therapy

Chen et al. (2015a,b)

Wang et al. (2014)

QDs

QD 800

Herpes simplex virus thymidine kinase gene (HSV-TK)

EDC/NHS

Human hepatocellular carcinoma cell lines (HepG2)

In vitro (HepG2 cells)/in vivo (BALB/c nu/ nu mice)

Silica QDs

QDs with a programmable DNA hybrid

DNA

PEG

MicroRNAs (miR-21)

In vitro (HeLa cells)

Real-time tracing of hepatocellular carcinoma treated with HSV-TK/GCV suicide gene system in vivo was performed by QD-based NIR fluorescence imaging, which provided useful insight toward QD-based theranostics in future cancer therapy miRNA-responsive drugdelivery model paved the way for combining chemotherapy and gene therapy to obtain an optimized therapeutic efficacy in cancer treatment

Shao et al. (2014)

Zhang et al. (2014)

QDs, quantum dots; VEGF, vascular endothelial growth factor; HIV, human immunodeficiency virus; DNA, deoxyribonucleotide; MGC-803, human gastric cancer cell growth; siRNA, short-interfering ribonucleoside; EDC/NHS, (1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide)/N-hydroxysuccinimide; HSV-TK, herpes simplex virus thymidine kinase; HepG2, liver hepatocellular cells; HeLa cells, human epithelial cells.

92

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

3.8 TOXICITY CONCERNS Although the applications of QDs in biology have emerged to be the most useful and successful application of nanotechnology in cancer detection and treatment (Hsu et al., 2010), most of these NCs, unfortunately, contain elements that are often thought to be detrimental to health and the environment (Bottrill and Green, 2011). The major components employed are Cd, Se, and Te, which are known to be highly toxic when administered in the human body. The Cd ion is a potent carcinogen that can cross the BBB, placenta, and deposit in other body tissues (Valizadeh et al., 2012; Alam and Yadav, 2013). Even low levels of Cd ions (100 400 μM) are known to reduce the viability of hepatocytes in vitro and prolonged exposure to Cd is detrimental and results in acute injury to the liver. Furthermore, at least 25% of the Cd administered to rats accumulates in the liver, which is the primary site of Cd-induced injury (Santone et al., 1982). Also, the cell culture studies indicated that CdSe QDs are highly toxic to cultured cells under UV light as it may break the chemical bond and dissolve the QDs by photolysis, ultimately leading to cellular damage. However, the chemical composition and surface modifications of the QDs determine the amount of Cd21 released inside the cell. Primarily, the oxidative imbalance of the cell causes oxidative stress, leading to generation of reactive oxygen species (ROS), such as superoxide (O22), hydroxyl radicals (HO•), peroxide radicals (ROO•), hydrogen peroxide (H2O2), and singlet oxygen, can adversely affect cellular functions (Yong et al., 2009; Hardman, 2006). The nonspecific accumulation of QDs due to RES, including in the liver, spleen, and lymphatic system, should also be considered. Several studies have shown that QDs less than 5 nm in size could be removed by the kidneys (Pelley et al., 2009). Immunogenicity or biocompatibility is the other major issue related to the QD complexes. QD complexes, including the capping materials, can be immunogenic, and result in dangerous immune reactions in subjects, or could make the QDs ineffective as a result of antibody binding (Derfus et al., 2007). Thus, considering biosafety for in vivo applications, long-term toxicological and pharmacokinetic investigations involving degradation, excretion, persistence, and generation of immune response and genotoxic effects of QDs should be systematically assessed.

3.9 FUTURE PROSPECTS Currently, continuous progression and advancement in the synthesis, biofunctionalization, and various encapsulation techniques of QDs have created tremendous interest among scientists working in the field of cancer biology and medicine to explore these probes. The immense popularity of these systems is due to their well-established optical properties and high photostability for monitoring cellular, molecular and physiological events in live cells and animals (Tino et al., 2011).

References

Furthermore, numerous surface functionalizations enable simultaneous detection, multiple imaging, and targeting. Some of the current exciting applications for which QDs are currently being researched and explored are in the field of dual-mode nanosensor(s) for detection of hormonal level within biological samples in clinical applications (Shi et al., 2014). Nonetheless, a few toxicological and pharmacological issues are still pending which need to be addressed before making any judgment on their future prospects in the clinical spectrum (Bukowski and Simmons, 2002). The need of the hour is to search for better coating materials which can limit toxicity, as well new materials like InP, SiC, CuInS2, AgInS2, and group IV (silicon or graphene) QDs need to be brought into consideration. Amongst all other materials, graphene and its derivatives seem to be the most promising materials for the future due to their properties including broad (visible to the NIR) absorption, deep-red emission, high dispersibility in aqueous solutions, superior photo- and pH stability, and biocompatibility (Ge et al., 2014; Feito et al., 2014). Therefore, explicit studies should be conducted before establishing their human uses.

3.10 CONCLUSIONS The unique properties of QDs promise innumerable applications in nanomedicine, particularly for multiplexed theranostics. Beyond the well-known optical properties of QDs, exploration for non-cadmium-based agents is yielding new advancements, such as the development of carbon dots and III V group semiconductor QDs, etc. QD nanomedicine aspires to optimize and individualize dose administration. Therefore, a large number of endeavors have been made to enhance their water solubility and polymer encapsulation. To enable QDs to be successfully used in clinics, the prime focus would be to improve their stability in the biological environment, improve drug loading, tissue targeting, transport and release, and overcome their toxicity concerns by understanding their interaction(s) with biological barriers. Only then, will it be truly possible to harness the advantageous properties of these QDs to tackle the most threatening challenges in cancer medicine.

REFERENCES Adachi, S., 2009. Properties of semiconductor alloys: group-IV, III V and II VI semiconductors. In: Capper, P., Kasap, S., Willoughby, A. (Eds.), Materials for Electronic & Optoelectronic Applications. Wiley Publication, pp. 1 422. Adeli, M., Hakimpoor, F., Parsamanesh, M., Kalantari, M., Sobhani, Z., Attyabi, F., 2011. Quantum dot-pseudopolyrotaxane supramolecules as anticancer drug delivery systems. Polymer 52, 2401 2413.

93

94

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

Akerman, M.E., Chan, W.C., Laakkonen, P., Bhatia, S.N., Ruoslahti, E., 2002. Nanocrystal targeting in vivo. Proc. Natl. Acad. Sci. USA 99, 12617 12621. Alam, F., Yadav, N., 2013. Potential applications of quantum dots in mapping sentinel lymph node and detection of micrometastases in breast carcinoma. J. Breast Cancer 16, 1 11. Aslam, M., Dent, A., 1998. Bioconjugation: Protein Coupling Techniques for the Biomedical Sciences. Grove’s Dictionaries, New York, NY, USA, Macmillan, London, UK. Bagalkot, V., Zhang, L., Levy-Nissenbaum, E., Jon, S., Kantoff, P.W., Langer, R., 2007. Quantum dot-aptamer conjugates for synchronous cancer imaging, therapy, and sensing of drug delivery based on bi-fluorescence resonance energy transfer. Nano Lett. 7, 3065 3070. Bakalova, R., Ohba, H., Zhelev, Z., Nagase, T., Jose, R., Ishikawa, M., et al., 2004. Quantum dot anti-cd conjugates: are they potential photosensitizers or potentiators of classical photosensitizing agents in photodynamic therapy of cancer? Nano Lett. 4, 1567 1573. Bakalova, R., Zhelev, Z., Kokuryo, D., Spasov, L., Aoki, I., Saga, T., 2011. Chemical nature and structure of organic coating of quantum dots is crucial for their application in imaging diagnostics. Int. J. Nanomed. 6, 1719 1732. Bao, Y., Li, J., Wang, Y., Yu, L., Wang, J., Du, W., et al., 2012. Preparation of water soluble CdSe and CdSe/CdS quantum dots and their uses in imaging of cell and blood capillary. Opt. Mater. 34, 1588 1592. Baskoutas, S., Terzis, A.F., 2006. Size-dependent band gap of colloidal quantum dots. J. Appl. Phys. 99 (013708), 1 4. Beloglazova, N.V., Goryacheva, O.A., Speranskaya, E.S., Aubert, T., Shmelin, P.S., Kurbangaleev, V.R., et al., 2015. Silica-coated liposomes loaded with quantum dots as labels for multiplex fluorescent immunoassay. Talanta 134, 120 125. Biju, V., Mundayoor, S., Omkumar, R.V., Anas, A., Ishikawa, M., 2010. Bioconjugated quantum dots for cancer research: present status, prospects and remaining issues. Biotechnol. Adv. 28, 199 213. Bonoiu, A., Mahajan, S.D., Ye, L., Kumar, R., Ding, H., Yong, K.T., 2009. MMP-9 gene silencing by a quantum Dot-siRNA nanoplex delivery to maintain the integrity of the blood brain barrier. Brain Res. 1282, 142 155. Bottrill, M., Green, M., 2011. Some aspects of quantum dot toxicity. Chem. Commun. 47, 7039 7050. Bukowski, T.J., Simmons, J.H., 2002. Quantum dot research: current state and future prospects. Crit. Rev. Solid State Mater. 27 (3 4), 119 142. Carbary-Ganz, J.L., Barton, J.K., Utzinger, U., 2014. Quantum dots targeted to vascular endothelial growth factor receptor 2 as a contrast agent for the detection of colorectal cancer. J. Biomed. Opt. 19 (8), 086003. Cassette, E., Helle, M., Bezdetnaya, L., Marchal, F., Dubertret, B., Pons, T., 2013. Design of new quantum dot materials for deep tissue infrared imaging. Adv. Drug Deliv. Rev. 65, 719 731. Chakravarthy, K.V., Davidson, B.A., Helinski, J.D., Ding, H., Law, W.C., Yong, K.T., et al., 2011. Doxorubicin-conjugated quantum dots to target alveolar macrophages and inflammation. Nanomed. Nanotech. Biol. Med. 7, 88 96. Chan, W.C.W., Nie, S., 1998. Quantum dot bioconjugates for ultrasensitive nonisotopic detection. Science 281 (5385), 2016 2018.

References

Chang, S., Ruixin, L., Jin, G., Li, D., Wenying, Z., 2013. New generation of β-cyclodextrinchitosan nanoparticles encapsulated quantum dots loaded with anticancer drug for tumor-target drug delivery and imaging of cancer cells. J. Nanopart. Res. 15, 1927. Chen, A.A., Derfus, A.M., Khetani, S.R., Bhatia, S.N., 2005. Quantum dots to monitor RNAi delivery and improve gene silencing. Nucl. Acids Res. 33, e190. Chen, G., Zhu, J.Y., Zhang, Z.L., Zhang, W., Ren, J.G., Wu, M., et al., 2015a. Transformation of cell-derived microparticles into quantum-dot-labeled nanovectors for antitumor siRNA delivery. Angew. Chem. Int. Ed. 54, 1036 1040. Chen, J., Zhou, G., Liu, Y., Ye, T., Xiang, X., Ji, X., et al., 2015b. Assembly-line manipulation of droplets in microfluidic platform for fluorescence encoding and simultaneous multiplexed DNA detection. Talanta 134, 271 277. Daniel, M.C., Astruc, D., 2004. Gold nanoparticles: assembly, supramolecular chemistry, quantum-sizerelated properties, and applications toward biology, catalysis, and nanotechnology. Chem. Rev. 104, 293 346. Das, M., Duan, W., Sahoo, S.K., 2015. Multifunctional nanoparticle-EpCAM aptamer bioconjugates: a paradigm for targeted drug delivery and imaging in cancer therapy. Nanomedicine 11, 379 389. de Aberasturi, D., Montenegro, J.M., Ruiz de Larramendi, I., Rojo, T., Klar, T.A., AlvarezPuebla, R., et al., 2012. Optical sensing of small ions with colloidal nanoparticles. Chem. Mater. 24, 738 745. Derfus, A.M., Chen, A.A., Min, D.H., Ruoslahti, E., Bhatia, S.N., 2007. Targeted quantum dot conjugates for siRNA delivery. Bioconjug. Chem. 18, 1391 1396. Feito, M.J., Vila, M., Matesanz, M.C., Linares, J., Gonc¸alves, G., Marques, P.A.A.P., et al., 2014. In vitro evaluation of graphene oxide nanosheets on immune function. J. Coll. Interface Sci. 432, 221 228. Ferrari, B.C., Bergquist, P.L., 2007. Quantum dots as alternatives to organic fluorophores for cryptosporidium detection using conventional flow cytometry and specific monoclonal antibodies: lessons learned. Cytometry A 71A, 265 271. Friedmann, T., Roblin, R., 1972. Gene therapy for human genetic disease? Science 175, 949 955. Fuhrhop, J.H., Wang, T., 2009. Metallic and molecular interactions in nanometer layers, pores and particles. In: Sankaran, R.M. (Ed.), Plasma Processing of Nanomaterials. CRC Press, pp. 52 159. Gao, X., Liu, Z., Lin, Z., Su, X., 2014. CuInS2 quantum dots/poly(l-glutamic acid)-drug conjugates for drug delivery and cell imaging. Analyst 139 (4), 831 836. Gatsouli, K.D., Pispas, S., Kamitsos, E.I., 2007. Development and optical properties of cadmium sulfide and cadmium selenide NPs in amphiphilic block copolymer micellarlike aggregates. J. Phys. Chem. C 111, 15201 15209. Ge, J., Lan, M., Zhou, B., Liu, W., Guo, L., Wang, H., et al., 2014. A graphene quantum dot photodynamic therapy agent with high singlet oxygen generation. Nat. Commun. 5, 4596. Goldman, E.R., Anderson, G.P., Tran, P.T., Mattoussi, H., Charles, P.T., Mauro, J.M., 2002. Conjugation of luminescent quantum dots with antibodies using an engineered adaptor protein to provide new reagents for fluoroimmunoassays. Anal. Chem. 74, 841 847. Gonzalez-Alegre, P., 2007. Therapeutic RNA interference for neurodegenerative diseases: from promise to progress. Pharmacol. Ther. 114, 34 55.

95

96

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

Han, H.S., Niemeyer, E., Huang, Y., Kamoun, W.S., Martin, J.D., Bhaumik, J., et al., 2015. Quantum dot/antibody conjugates for in vivo cytometric imaging in mice. Proc. Natl. Acad. Sci. USA 112, 1350 1355. Hardman, R., 2006. A toxicologic review of quantum dots: toxicity depends on physicochemical and environmental factors. Environ. Health Perspect. 114, 165 172. Hardy, J., Selkoe, D.J., 2002. The amyloid hypothesis of Alzheimer’s disease: progress and problems on the road to therapeutics. Science 297, 353 356. Harris, T.J., Green, J.J., Fung, P.W., Langer, R., Anderson, D.G., Bhatia, S.N., 2010. Tissue-specific gene delivery via nanoparticle coating. Biomaterials 31, 998 1006. Hoffman, A.S., 2013. Stimuli-responsive polymers: biomedical applications and challenges for clinical translation. Adv. Drug Deliv. Rev. 65, 10 16. Hsu, C.Y., Rao, J.H., Lai, P.S., 2010. Bioluminescemt quantum dots-induced photodynamic therapy in vitro. NSTI-Nanotechnology 3, 490 493. Hu, R., Law, W.C., Lin, G., Ye, L., Liu, J., Liu, J., et al., 2012. PEGylated phospholipid micelle-encapsulated near-infrared PbS quantum dots for in vitro and in vivo bioimaging. Theranostics 2, 723 733. Huang, C.L., Huang, C.C., Mai, F.D., Yen, C.L., Tzing, S.H., Hsieh, H.T., et al., 2015. Application of paramagnetic graphene quantum dots as a platform for simultaneous dual modality bioimaging and tumor targeted drug delivery. J. Mater. Chem. B 3, 651664. Huang, H., Bai, Y.L., Yang, K., Tang, H., Wang, Y.W., 2013. Optical imaging of head and neck squamous cell carcinoma in vivo using arginine-glycine-aspartic acid peptide conjugated near-infrared quantum dots. Oncol. Targets Ther. 6, 1779 1787. Jaiswal, J.K., Simon, S.M., 2004. Potentials and pitfalls of fluorescent quantum dots for biological imaging. Trends Cell Biol. 14, 497 504. Jamieson, T., Bakhshi, R., Petrova, D., Pocock, R., Imani, M., Seifalian, A.M., 2007. Biological applications of quantum dots. Biomaterials 28, 4717 4732. Kang, E.C., Ogura, A., Kataoka, K., Nagasaki, Y., 2004. Preparation of water-soluble PEGylated semiconductor nanocrystals. Chem. Lett. 33, 840 841. Kao, C.H., Hsieh, J.F., Tsai, S.C., Ho, Y.J., Lee, J.K., 2000. Quickly predicting chemotherapy response to paclitaxel-based therapy in non-small cell lung cancer by early technetium99m methoxyisobutylisonitrile chest single-photon-emission computed tomography. Clin. Cancer Res. 6, 820 824. Kaur, G., Tripathi, S.K., 2014a. Size tuning of MAA capped CdSe and CdSe/CdS quantum dots and their stability in different pH environments. Mater. Chem. Phys. 143, 514 523. Kaur, G., Tripathi, S.K., 2014b. Probing photoluminescence dynamics of colloidal CdSe/ ZnS core/shell nanoparticles. J. Lumin. 155, 330 337. Kaur, G., Tripathi, S.K., 2015. Investigation of trypsin CdSe quantum dot interactions via spectroscopic methods and effects on enzymatic activity. Spectrochim. Acta A 134, 173 183. Kim, S., Bawendi, M.G., 2003. Oligomeric ligands for luminescent and stable nanocrystal quantum dots. J. Am. Chem. Soc. 125, 14652 14653. Kirchner, C., Liedl, T., Kudera, S., Pellegrino, T., Javier, A.M., Gaub, H.E., et al., 2005. Cytotoxicity of colloidal CdSe and CdSe/ZnS NPs. Nano. Lett. 5, 331 338. Laird, F.M., Cai, H., Savonenko, A.V., Farah, M.H., He, K., Melnikova, T., et al., 2005. BACE1, a major determinant of selective vulnerability of the brain to amyloid-β

References

amyloidogenesis, is essential for cognitive, emotional, and synaptic functions. J. Neurosci. 25, 11693 11709. Lee, J.H., Huh, Y.M., Jun, Y.W., Seo, J.W., Jang, J.T., Song, H.T., et al., 2006. Artificially engineered magnetic nanoparticles for ultra-sensitive molecular imaging. Nat. Med. 13, 95 99. Li, J., Wu, C., Gao, F., Zhang, R., Lv, G., Fu, D., 2006. In vitro study of drug accumulation in cancer cells via specific association with CdS nanoparticles. Bioorg. Med. Chem. Lett. 16, 4808 4812. Li, S., Liu, Z., Ji, F., Xiao, Z., Wang, M., Peng, Y., et al., 2012. Delivery of quantum dot-siRNA nanoplexes in SK-N-SH cells for BACE1 gene silencing and intracellular imaging. Ther. Nucl. Acids 1, e20. Li, X., Deng, D., Xue, J., Qu, L., Achilles, S., Gu, Y., 2014. Quantum dots based molecular beacons for in vitro and in vivo detection of MMP-2 on tumor. Biosens. Bioelectron. 61, 512 518. Lieleg, O., Lo´pez-Garcı´a, M., Semmrich, C., Auernheimer, J., Kessler, H., Bausch, A.R., 2007. Specific integrin labeling in living cells using functionalized nanocrystals. Small 3 (9), 1560 1565. Liu, Z., Tabakman, S.M., Chen, Z., Dai, H., 2009. Preparation of carbon nanotube bioconjugates for biomedical applications. Nat. Protoc. 4, 1372 1381. Luo, G., Long, J., Zhang, B., Liu, C., Ji, S., Xu, J., et al., 2012. Quantum dots in cancer therapy. Expert. Opin. Drug Deliv. 47 58. MacEwan, S.R., Chilkoti, A., 2013. Harnessing the power of cell-penetrating peptides: activatable carriers for targeting systemic delivery of cancer therapeutics and imaging agents. Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 5, 31 48. Maeda, H., Wu, J., Sawa, T., Matsumura, Y., Hori, K., 2000. Tumor vascular permeability and the EPR effect in macromolecular therapeutics: a review. J. Control. Release. 65, 271 284. Manshian, B.B., Soenen, S.J., AlAli, A., Brown, A., Hondow, N., Wills, J., et al., 2015. Cell type dependent changes in CdSe/ZnS quantum dot uptake and toxic endpoints. Toxicol. Sci. 144, 246 258. Mattoussi, H., Mauro, J.M., Goldman, E.R., Anderson, G.P., Sundar, V.C., Mikulec, F.V., et al., 2000. Self-assembly of CdSe-ZnS quantum dot bioconjugates using an engineered recombinant protein. J. Am. Chem. Soc. 122, 12142 12150. Mazumder, S., Dey, R., Mitra, M.K., Mukherjee, S., Das, G.C., 2009. Review: biofunctionalized quantum dots in biology and medicine. J. Nanomater. 815734, 1 17. Michalet, X., Pinaud, F.F., Bentolila, L.A., Tsay, J.M., Doose, S., Li, J.J., 2005. Quantum dots for live cells, in vivo imaging, and diagnostics. Science 307 (5709), 538 544. Mitchell, G.P., Mirkin, C.A., Letsinger, R.L., 1999. Programmed assembly of DNA functionalized quantum dots. J. Am. Chem. Soc. 121, 8122 8123. Mudshinge, S.R., Deore, A.B., Patil, S., Bhalgat, C.M., 2011. Nanoparticles: emerging carriers for drug delivery. Saudi Pharm. J. 19, 129 141. Murphy, C.J., Coeffer, J.L., 2002. Quantum dots: a primer. Appl. Spectros. 56, 16A 27A. Murray, C.B., Noms, D.J., Bawendi, M.G., 1993. Synthesis and characterization of nearly monodisperse CdE (E 5 S, Se, Te) semiconductor nanocrystallites. J. Am. Chem. Soc. 115, 8706 8715. Nelson, A.L., 2010. Antibody fragments: hope and hype. MAbs 2, 77 83.

97

98

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

Niemeyer, C.M., 2001. Nanoparticles, proteins, and nucleic acids: biotechnology meets materials science. Angew. Chem. Int. Ed. 40, 4128 4158. Nigam, P., Waghmode, S., Louis, M., Wangnoo, S., Chavan, P., Sarkar, D., 2014. Graphene quantum dots conjugated albumin nanoparticles for targeted drug delivery and imaging of pancreatic cancer. J. Mater. Chem. B 2, 3190 3195. Peer, D., Karp, J.M., Hong, S., Farokhzad, O.C., Margalit, R., Langer, R., 2007. Nanocarriers as an emerging platform for cancer therapy. Nat. Nanotechnol. 2, 751 760. Pelley, J.L., Daar, A.S., Saner, M.A., 2009. State of academic knowledge on toxicity and biological fate of quantum dots. Toxicol. Sci. 112, 276 296. Peng, X., Schlamp, M.C., Kadavanich, A.V., Alivisatos, A.P., 1997. Epitaxial growth of highly luminescent CdSe/CdS Core/Shell nanocrystals with photostability and electronic accessibility. J. Am. Chem. Soc. 119, 7019 7029. Peng, X., Manna, L., Yang, W., Wickham, J., Scher, E., Kadavanich, A., et al., 2000. Shape control of CdSe nanocrystals. Nature 404, 59 61. Qi, L., Gao, X., 2008. Quantum dot-amphipol nanocomplex for intracellular delivery and real-time imaging of siRNA. ACS Nano 2 (7), 1403 1410. Rakovich, A., Savateeva, D., Rakovich, T., Donegan, J.F., Rakovich, Y.P., Kelly, V., et al., 2010. CdTe quantum dot/dye hybrid system as photosensitizer for photodynamic therapy. Nanoscale Res. Lett. 5, 753 760. Rao, J., Dragulescu-Andrasi, A., Hequan, Y., 2007. Fluorescence imaging in vivo: recent advances. Curr. Opin. Biotechnol. 18, 17 25. Rege, S., Maass, A., Chaiken, L., Hoh, C.K., Choi, Y., Lupin, R., et al., 1994. Use of positron emission tomography with fluorodeoxyglucose in patients with extracranial head and neck cancers. Cancer 73, 3047 3058. Rizvi, S.B., Yildirimer, L., Ghaderi, S., Ramesh, B., Seifalian, A.M., Keshtgar, M., 2012. A novel POSS-coated quantum dot for biological application. Int. J. Nanomed. 7, 3915 3927. Santone, K.S., Acosta, D., Bruckner, J.V., 1982. Cadmium toxicity in primary cultures of rat hepatocytes. J. Toxicol. Environ. Health 10, 169 177. Schmaljohann, D., 2006. Thermo- and pH-responsive polymers in drug delivery. Adv. Drug Deliv. Rev. 58, 1655 1670. Shahid, R., 2012. Green chemical synthesis of II-VI semiconductor quantum dots (Doctoral thesis). Functional Materials Division, School of Information and Communication Technology. Royal Institute of Technology (KTH), Stockholm, Sweden. Shao, D., Li, J., Xiao, X., Zhang, M., Pan, Y., Li, S., et al., 2014. Real-time visualizing and tracing of HSV-TK/GCV suicide gene therapy by near-infrared fluorescent quantum dots. ACS Appl. Mater. Interfaces 6, 11082 11090. Sharma, M., Tripathi, S.K., 2013. Photoluminescence study of CdSe nanorods embedded in a PVA matrix. J. Lumin. 135, 327 334. Shi, P., Chen, H., Cho, M.R., Stroscio, M.A., 2006. Peptide-directed binding of quantum dots to integrins in human fibroblast. IEEE Trans. Nanobiosci. 5, 15 19. Shi, Y., Pan, Y., Zhang, H., Zhang, Z., Li, M.J., Yi, C., et al., 2014. A dual-mode nanosensor based on carbon quantum dots and gold nanoparticles for discriminative detection of glutathione in human plasma. Biosens. Bioelectron. 56, 39 45. Singh, R., Lillard, J.W., 2009. Nanoparticle-based targeted drug delivery. Exp. Mol. Pathol. 86, 215 223.

References

Slowing, I.I., Vivero-Escoto, J.L., Wu, C.W., Lin, V.S.Y., 2008. Mesoporous silica nanoparticles as controlled release drug delivery and gene transfection carriers. Adv. Drug Deliv. Rev. 60, 1278 1288. Smith, A.M., Dave, S., Nie, S., True, L., Gao, X., 2006. Multicolor quantum dots for molecular diagnostics of cancer. Expert. Rev. Mol. Diagn. 6, 231 244. So, M.K., Xu, C.J., Loening, A.M., Gambhir, S.S., Rao, J.H., 2006. Self illuminating quantum dot conjugates for in vivo imaging. Nat. Biotechnol. 24, 339 343. Song, E.Q., Hu, J., Wen, C.Y., Tian, Z.Q., Yu, X., Zhang, Z.L., 2011. Fluorescentmagnetic-biotargeting multifunctional nanobioprobes for detecting and isolating multiple types of tumor cells. ACS Nano 5, 761 770. Sozzi, G., Boeri, M., Rossi, M., Verri, C., Suatoni, P., Bravi, F., et al., 2013. Clinical utility of a plasma-based mirna signature classifier within computed tomography lung cancer screening: a correlative mild trial study. J. Clin. Oncol. 50, 4357. Spanhel, L., Haase, M., Weller, H., Henglein, A., 1987. Photochemistry of colloidal semiconductors. J. Am. Chem. Soc. 109, 5649 5655. Sperling, R.A., Parak, W.J., 2010. Surface modification, functionalization and bioconjugation of colloidal inorganic NPs. Phil. Trans. R. Soc. A 368, 1333 1383. Sun, X., Huang, X., Guo, J., Zhu, W., Ding, Y., Niu, G., et al., 2014. Self-illuminating (64) Cu-doped CdSe/ZnS nanocrystals for in vivo tumor imaging. J. Am. Chem. Soc. 136, 1706 1709. Tada, H., Higuchi, H., Wanatabe, T.M., Ohuchi, N., 2007. In vivo real-time tracking of single quantum dots conjugated with monoclonal anti-HER2 antibody in tumors of mice. Cancer Res. 67, 1138 1139. Talapin, D.V., Rogach, A.L., Kornowski, A., Haase, M., Weller, H., 2001. Highly luminescent monodisperse CdSe and CdSe/ZnS nanocrystals synthesized in a hexadecylamine2 trioctylphosphine oxide 2 trioctylphospine mixture. Nano Lett. 1, 207 211. Tassa, C., Shaw, S.Y., Weissleder, R., 2011. Dextran-coated iron oxide nanoparticles: a versatile platform for targeted molecular imaging, molecular diagnostics, and therapy. Acc. Chem. Res. 44, 842 852. Tian, B., Al-Jamal, W.T., Al-Jamal, K.T., Kostarelos, K., 2011. Doxorubicin-loaded lipidquantum dot hybrids: surface topography and release properties. Int. J. Pharm. 416, 443 447. Tino, A., Ambrosone, A., Mattera, L., Marchesano, V., Susha, A., Rogach, A., et al., 2011. A new in vivo model system to assess the toxicity of semiconductor nanocrystals. Int. J. Biomater. 2011, 792854, 1 8. Tripathi, S.K., Kaur, G., Khurana, R.K., Kapoor, S., Singh, B., 2015. Quantum dots and their potential role in cancer theranostics. Crit. Rev. Ther. Drug 32, 461 502. True, L.D., Gao, X., 2007. Quantum dots for molecular pathology. J. Mol. Diagn. 9, 7 11. Tsay, J.M., Trzoss, M., Shi, L., Kong, X., Selke, M., Jung, M.E., et al., 2007. Singlet oxygen production by peptide-coated quantum dot-photosensitizer conjugates. J. Am. Chem. Soc. 129, 6865 6871. Valizadeh, A., Mikaeili, H., Samiei, M., Farkhani, S.M., Zarghami, N., Kouhi, M., et al., 2012. Quantum dots: synthesis, bioapplications, and toxicity. Nanoscale. Res. Lett. 7, 480. Walther, C., Meyer, K., Rennert, R., Neundorf, I., 2008. Quantum dot-carrier peptide conjugates suitable for imaging and delivery applications. Bioconjug. Chem. 19, 2346 2356.

99

100

CHAPTER 3 Quantum dots: dynamic tools in cancer nanomedicine

Wang, J., Yong, W.H., Sun, Y., Vernier, P.T., Koeffler, H.P., Gundersen, M.A., et al., 2007. Receptor-targeted quantum dots: fluorescent probes for brain tumor diagnosis. J. Biomed. Opt. 12, 044021. Wang, Q., Zhang, C., Shen, G., Liu, H., Fu, H., Cui, D., 2014. Fluorescent carbon dots as an efficient siRNA nanocarrier for its interference therapy in gastric cancer cells. J. Nanobiotechnol. 12 (1), 288. Wang, Y., Zhang, Y., Du, Z., Wu, M., Zhang, G., 2012. Detection of micrometastases in lung cancer with magnetic nanoparticles and quantum dots. Int. J. Nanomed. 7, 2315 2324. Wilchek, M., Bayer, E.A., 1990. Introduction to avidin-biotin technology. Meth. Enzymol. 184, 5 13. Willard, D.M., Carillo, L.L., Jung, J., Orden, A.V., 2001. CdSe-ZnS quantum dots as resonance energy transfer donors in a model protein-protein binding assay. Nano Lett. 1, 469 474. Win, K.Y., Teng, C.P., Ye, E., Low, M., Han, M.Y., 2014. Evaluation of polymeric nanoparticle formulations by effective imaging and quantitation of cellular uptake for controlled delivery of doxorubicin. Small 2014, 02073. Wu, M., Algar, W.R., 2015. Acceleration of proteolytic activity associated with selection of thiol ligand coatings on quantum dots. ACS Appl. Mater. Interfaces 7, 2535 2545. Wu, Y.Z., Chakrabortty, S., Gropeanu, R.A., Wilhelmi, J., Xu, Y., Er, K.S., 2010. pH-Responsive quantum dots via an albumin polymer surface coating. J. Am. Chem. Soc. 132, 5012 5014. Xie, J., Lee, S., Chen, X., 2010. Nanoparticle-based theranostic agents. Adv. Drug Deliv. Rev. 62, 1064 1079. Yong, K.T., Roy, I., Ding, H., Bergey, E.J., Prasad, P.N., 2009. Biocompatible near-infrared quantum dots as ultrasensitive probes for long-term in vivo imaging applications. Small 5 (17), 276 296. Yong, K.T., Wang, Y., Roy, I., Rui, H., Swihart, M.T., Law, W.C., 2012. Preparation of quantum dot/drug nanoparticle formulations for traceable targeted delivery and therapy. Theranostics 2, 681 694. Yuan, Y., Zhang, J., An, L., Cao, Q., Deng, Y., Liang, G., 2014. Oligomeric nanoparticles functionalized with NIR-emitting CdTe/CdS QDs and folate for tumor-targeted imaging. Biomaterials 35, 7881 7886. Zhang, P., Cheng, F., Zhou, R., Cao, J., Li, J., Burda, C., et al., 2014. DNA-hybrid-gated multifunctional mesoporous silica nanocarriers for dual-targeted and microRNAresponsive controlled drug delivery. Angew. Chem. Int. Ed. Engl. 53, 2371 2375. Zhang, Y., Clapp, A., 2011. Overview of stabilizing ligands for biocompatible quantum dot nanocrystals. Sensors 11, 11036 11055. Zhang, Y., So, M.K., Loening, A.M., Yao, H., Gambhir, S.S., Rao, J., 2006. HaloTag protein-mediated site-specific conjugation of bioluminescent proteins to quantum dots. Angew. Chem. Int. Ed. Engl. 45, 4936 4940. Zhou, X., Wang, A., Yu, C., Wu, S., Shen, J., 2015. Facile synthesis of molecularly imprinted graphene quantum dots for the determination of dopamine with affinityadjustable. ACS Appl. Mater. Interfaces. Zhu, M.Q., Chang, E., Sun, J., Drezek, R.A., 2007. Surface modification and functionalization of semiconductor quantum dots through reactive coating of silanes in toluene. J. Mater. Chem. 17, 800 805.