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Vol. 2 (2) Apr – Jun 2011 www.ijrpbsonline.com 448

International Journal of Research in Pharmaceutical and Biomedical Sciences

ISSN: 2229-3701

_________________________________________________________________________Review Paper

Quantum Dot: Novel Carrier for Drug Delivery

Niladri Shekhar Dey* and M.E. Bhanoji Rao

Department of Pharmaceutics, Roland Institute of Pharmaceutical Sciences, Khodasingi, Berhampur, Orissa, India. ___________________________________________________________________________ ABSTRACT Quantum dots are nanoscale semiconductor crystals ranging typically between 1-10 nanometers and have capacity to glow or fluorescence brightly when excited by a light source such as a laser. Quantum dots are tiny bits of microscopic metal, thousand times smaller than width of a hair or semiconductor boxes such as cadmium selenide-zinc sulphide. Quantum dots are emerging as a new class of fluorescent probes for biomolecular and cellular imaging. The nature of this technology makes it suitable for application such as in-vivo imaging including live cell and whole animal imaging, blood cancer assay, cancer detection and treatment. With a focus on the use of quantum dots this review includes a detailed examination of quantum dot, their synthesis, properties and applications, toxic effects, use in imaging and analysis plus quantum dot drug delivery systems. Key Words: Quantum dots, semiconductor crystals, nanocrystals, fluorescent probes. INTRODUCTION Quantum dots are nanoscale semiconductor crystals ranging typically between 1-10 nanometer and have capacity to glow or fluorescence brightly when excited by a light source such as a laser. Quantum dots are tiny semiconductor particles generally no larger than 10 manometers that can be made to fluoresce in different colours depending on their size1. Quantum dots are microscopic metal or semiconductor boxes that hold a certain number of electrons and, thus, have a wide number of potential applications in electronics and photonics. Unlike molecular fluorophores, which typically have very narrow excitation spectra, semiconductor quantum dots absorb light over a very broad spectral range. This makes it possible to optically excite a broad spectrum of quantum dot colours using a single excitation laser wavelength, which enables one to simultaneously probe several markers. Although the luminescence properties of semiconductor nanocrystals have historically been sensitive to their local environment and nanocrystals surface preparation, recent core-shell geometries where the nanocrystals is enclosed in a shell of a wider band gap semiconductor have resulted in increased fluorescence quantum efficiencies (>50%) and greatly improved photochemical stability2. (Fig. 1) Nanoshells are tunable plasmonic nanoparticles ~10-300 nm diameter, quantum efficiencies ~10-4, ________________________________________ *Address for correspondence: E-mail: [email protected]

spectral range (extinction): 500(Ag)-9000nm, cross section: ~ 10-13 m2 and where as quantum dots are tunable exitonic nanoparticles ~1-10 nm diameter uncoated), quantum efficiencies ~0.1-0.5, spectral range (emission): 400(Ag)-2000nm, cross section: ~10-19 m2 (Fig. 2). Quantum Dots have the unique ability to emit light representing the entire rainbow of colours from the same elemental material based solely on the size of the particles of that material being excited. Prior to quantum dots, light emitting semiconductors, such as light emitting diodes (LED), could not emit white light. AE Quantum Dots with particle size distributions less than 500 manometers (nm) produce LED emissions in the blue range which may allow for the commercial use of solid state semiconductors to generate luminescent light 3

.Quantum dots (QDs), are emerging as a new class of fluorescent probes for bio molecular and cellular imaging. In comparison with organic dyes and fluorescent proteins, quantum dots have unique optical and electronic properties such as size-tunable light emission, improved signal brightness, resistance against photo bleaching, and simultaneous excitation of multiple fluorescence colours. These properties are most promising for improving the sensitivity of molecular imaging and quantitative cellular analysis by 1-2 orders of magnitude 4. The semiconductor nanocrystals fluoresce when excited by a light source, emitting bright colors that can identify and track properties and processes in various biological applications. They have significant advantages over traditional fluorophores



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as they can be predictably tuned according to their size, shape and intrinsic solid-state properties. Their flexibility means it has applications in cell biology, drug discovery, cancer research and other fields. By using the molecular photo detectors, allowing researchers to communicate with the cells using precise wavelengths of light.Quantum Dot Corporation offers novel solutions for biomolecular labeling and detection that employ Quantum Dot [Qdot(R)] semi-conductor nanocrystals, which emit bright light in a range of sharp colors. The unique properties of these nanometer-sized Qdot(R) particles include excellent photostability and narrow emission spectra and brightness, making them well suited for a wide range of applications within life sciences and beyond. Quantum dot emission frequency is dependent on the size of the quantum dot. It is available in emissions of 520nm (Adirondack Green), 600nm (Fort Orange), 620nm (Maple Red-Orange) and 680nm. EviFluors exhibit unique fluorescent lifetimes and possess optical and electronic properties to enable exploration of many next generation life science applications5.

Properties of quantum dots 1. Quantum dots “designer atoms” offer

innumerable optical and electronic properties that can work around natural limits inherent in traditional semiconductors.

2. Quantum dots are made from tiny bits of metal about thousand times smaller than width of a hair.

3. Quantum dots can be molded into different shapes and coated with a variety of biomaterials.

4. Quantum dots luminescence under UV light, with the size of the dots controlling its colour. e.g. 2nm Quantum dots luminescence bright green,5 nm Quantum dots –luminescence red

5. Fluorescent quantum dots are usually compounds from group II to VI and III to V e.g. Ag, Cd, Hg, Ln, P, Pb, Se, Te, and Zn etc.

6. As size of quantum dots decreases, the wavelength it emits turns shorter.

7. Quantum dots have a broad excitation range.

8. Quantum dots have precise emission wavelength, so the spectra doesn’t overlap in multiple fluorescent emission 6.

Synthesis of quantum dots

Calcium Oxide + Selenium----------------------------Quantum dots In surfactant solution ↓ (TOPO) Cooling the reaction after reaching desired crystal size

Surfactant is used to avoid aggregation, to make the quantum dots water-soluble; these organoleptic surface species are generally exchanged with more polar species. PEG is also added to check early elimination 6. The most studied quantum dots prepared by this method have been of cadmium selenium (CdSe). To produce these dots, researchers inject a syringe of the desired organometallic precursors into heated triocytlphosphine oxide (TOPO) that has been vigorously stirred under an inert atmosphere. The solution immediately begins to change from colourless to yellow, then orange and red/brown, as the quantum dots increase in size. When the desired size is reached, the heat is removed from the flask. Quantum dots of different sizes can be identified by placing them under a "black light" and observing the various colour differences in their fluorescence indicates the quantum dots after fluorescence in different manometer wave length 2

.After being excited the quantum dots emits blue colours at 514 nm and followed by greenish blue, green, yellowish green, yellow, yellowish orange, orange at 544,559, 571,577,581 and 610 nm respectively (Fig.3). Mode of action of quantum dots Quantum dots conjugate work by seeking out and bonding with target materials. Once bound to target, each individual quantum dot particle emits light and depending on their size, they can glow in a variety of colours 7. Advantages of quantum dots

1. Quantum dots are much more resistant to degradation than other optical imaging probes, allowing them to track cell processes for longer periods of time and shed new light on molecular interactions.



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2. As Quantum dots are nanocrystals they provide good contrast for imaging with an electron microscope as scattering increases.

3. Quantum dots have size-tunable emission(from UV to IR)

4. Fluorescence lasts for longer time as compared to conventional dyes.

5. Quantum dots have increased optical activity with innumerable avenues of applications in biotechnology and life sciences.

6. Anti-counterfering measure- their extremely small size gives them great versatility by allowing them to be injected into many environments, including liquid mixtures, fabrics, and polymer matrices 6

Future prospective of quantum dots

1. Research is ongoing for designing hydrophilic Quantum dots that are luminescent.

2. More selective and specific approach of labelling cells and biomolecules is undergoing research.

3. Work is being carried to study interference effect of Quantum dots with normal physiology.

4. NASA scientist working on QD’s as drug carrier for Mars expedition in near future6.

Single quantum dots of compound semiconductors were successfully used as a replacement of organic dyes in various bio-tagging applications. This idea has been taken one step further by combining differently sized and hence having different fluorescent colours quantum dots, and combining them in polymeric micro beads. A precise control of quantum dot ratios has been achieved. The selection of nanoparticles used in those experiments had 6 different colours as well as 10 intensities. It is enough to encode over 1 million combinations. The uniformity and reproducibility of beads was high letting for the bead identification accuracies of 99.99% 8. With a focus on the use of QDs in cancer detection and treatment, the review includes a detailed examination of Quantum Dot advantages, their synthesis, properties and applications, toxic effects, use in imaging and analysis plus Quantum Dot drug delivery systems. This technology also outlines some early success in the detection and treatment of breast cancer. These concludes with a quick look at what the future may hold for Quantum Dots in providing new insights into understanding the pathophysiology of cancer and real time imaging and screening of tumors 9.

Application QDs-As diagnostics in clinical applications The most important potential applications of Quantum dots (QDs) are for cancer diagnosis. Luminescent and stable QD bioconjugates enable visualization of cancer cells in living animals. QDs can be combined with fluorescence microscopy to follow cells at high resolution in living animals. QDs have been coated with a polyacrylate cap and covalently linked to antibodies for immunofluorescent labelling of breast cancer marker Her2 carbohydrate encapsulated QDs with detectable luminescent properties are useful for imaging of cancer. Another application of QDs is for viral diagnosis. Rapid and sensitive diagnosis of Respiratory Syncytial Virus (RSV) is important for infection control and development of antiviral drugs. Antibody- conjugated nanoparticles rapidly and sensitively detect RSV and estimate relative levels of surface protein expression. A major development is the use of dual-colour QDs or fluorescence energy transfer nanobeads that can be simultaneously excited with a single light source. A QD system can detect the presence of particles of the RSV in a matter of hours. It is also more sensitive, allowing detection of the virus earlier in the course of an infection. When an RSV virus infects lung cells, it leaves part of its coat containing F and G proteins on the cell’s surface. QDs have been linked to antibodies keyed to structures unique to the RSV coat. As a result, when QDs come in contact with either viral particles or infected cells they stick to their surface10. (Table 1) Pharmaceutical field In the pharmaceutical domain, for example, liposomes, polymer based of active ingredients with fluorescence. micro and nanoparticles have been subjects of intense research and development during the last. In the field of diagnosis, magnetic resonance imaging is one of the first and up to now the most developed application of metallic particles. But beside this application, a very new generation of biosensors based on the optical properties of colloidal gold and fluorescent nanocrystals, called quantum dots seem to ready to be implemented in diagnosis and medical imaging. Concerning therapeutic applications, the potentialities of metal nanoparticles to help fulfilling the need of time and space controlled release of drugs has been intuited fora long time. It should also be used for the detection of active ingredient with fluorescence 11. In-vivo imaging with quantum dots Non-targeted near infrared emitting quantum dot core T2-MP EviTags were tested in tumor bearing mice. Optical image was acquired after intravenous



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injection of 100pmol of T2-MP EviTags (left) or of physiological buffer as a control (right) into the tail vein of tumor bearing mice. In this preliminary experiment, T2-MP EviTags were shown to be capable of generating a reasonable signal to noise image when compared to the control. Further, the biodistribution pattern as determined from the optical image shows favorable clearance of the non-targeted T2-MP EviTags through the lymphatics, kidneys and bladder. No uptake in the tumor was observed, suggesting the next round of imaging to be done with tumor targeted T2-MP EviTags will have minimal background signal within the tumor.The development of T2-MP EviTags as non-invasive optical molecular imaging probes will have a great impact on the early detection,diagnosis and treatment monitoring of cancer. The following image demonstrates the ability of InGaP EviTags quantum dots to be imaged in-vivo after subcutaneous injection into a mouse liver and tumor11. (Fig.4) Immunoassay An immunoassay readout method based on fluorescent imaging analysis with laser confocal scanning is described. The ZnS-coated CdSe quantum dots (ZnS/CdSe QDs) were linked to a detection antibody. Immunoassay was carried out on a glass chip using a sandwich assay approach, where antibody covalently boundtoaglass chip was allowed to capture antigen specially. Afterwards, thedetectionantibody labeled with QD was allowed to bind selectively to the captured antigen. The fluorescent signals of the sandwich conjugate were detected by a laser confocal scanner. A diode laser was used to excite efficiently the fluorescent signals while bovine serum albumin was used to eliminate nonspecific binding sites.The detection limit of this approach was up to10#8722; 9 M under current experimental conditions. The specificity of the QDs-labeled immunoglobulin (IgG) was tested by an experiment using goat IgG and human IgG samples. The result was consistent with the binding specificity in a sandwich-type assay12. Bimodal molecular imaging The synthesis of quantum dots with a water-soluble and paramagnetic micellular coating as a molecular imaging probe for both fluorescence microscopy and MRI. The quantum dots preserve their optical properties and have a very high relaxivity. Targeting ligands can be coupled to these QDs via maleimide or other functional groups. In this study, the paramagnetic quantum dots were functionalized by conjugating them with cyclic RGD peptides and were successfully targeted to human endothelial cells in vitro. We infer that this nanoparticulate bimodal contrast agent may be of great use for the detection of (tumor) angiogenesis13.

Plasmid DNA with semiconductor quantum dots Semiconductor nanocrystal quantum dots (QDs) allow long-term imaging in the cellular environment with high photo stability. QD biolabeling techniques have previously been developed for tagging proteins and peptides as well as oligonucleotides. In this contribution, QD-decorated plasmid DNA was utilized for the first time for long-term intracellular and intranuclear tracking studies. Conjugation of plasmid DNA with phospholipid-coated QDs was accomplished using a peptide nucleic acid (PNA) #8211; N-succinimidyl-3-(2-pyridylthio) propionate linker. Gel electrophoresis and confocal and atomic force microscopy (AFM) were used to confirm the structure of QD#8211; DNA conjugates. AFM imaging also revealed that multiple QDs were attached in a cluster at the PNA-reactive site of the plasmid DNA. These QD#8211;DNA conjugates were capable of expressing the reporter protein, enhanced green fluorescent protein, following transfection in Chinese hamster ovary (CHO-K1) cells with an efficiency of ca. 62%, which was comparable to the control (unconjugated) plasmid DNA14. Quantum dots offer many technical advantages over traditional fluorescent dyes and newer DNA chip technologies, which are commonly used to detect and track biological molecules. They are brighter and easier to visualize than organic dyes. They are also more flexible and yield faster results than other current technologies, such as DNA chips. In addition to their usefulness in identifying and tracking molecules in basic biomedical studies, quantum dots promise faster, more flexible, and less costly tests for on-the-spot clinical analyses such as screening for illegal drugs and diagnosing conditions ranging from HIV infection to allergies15. Quantum dots for brain tumor diagnosis The intra operative diagnosis of brain tumors and the timely evaluation of biomarkers that can guide therapy are hindered by the paucity of rapid adjunctive studies. This study evaluates the feasibility and specificity of using quantum dot-labeled antibodies for rapid visualization of epidermal growth factor receptor (EGFR) expression in human brain tumor cells and in surgical frozen section slides of glioma tissue. Streptavidin-coated quantum dots (QDs) were conjugated to anti-EGFR antibodies and incubated with target cultured tumor cells and tissues. The experiments were conducted first in human glioma tumor cell lines with elevated levels of EGFR expression (SKMG-3, U87) and then in frozen tissue sections of glioblastoma multiform and of oligo dendroglioma. The bioconjugated QDs used in the study were found to bind selectively to brain tumor cells expressing EGFR. QD complexes



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quickly to the cell membrane (less than 15min), and binding was highly specific and depended on the expression level of EGFR on the cell membrane. Tissue experiments showed that only tumor specimens expressing EGFR were labeled in less than 30min by QD complexes. These findings demonstrate that QD-labeled antibodies can provide a quick and accurate method for characterizing the presence or absence of a specific predictive biomarker16. The multiphoton microscopy techniques and transgenic mice that expressed green fluorescent protein, and combined them with the use of quantum dot preparations. That shows fluorescent semiconductor nanocrystals can be customized to concurrently image and differentiate tumor vessels from both the perivascular cells and the matrix. Moreover, it used to measure the ability of particles of different sizes to access the tumor and it has successfully monitored the recruitment of quantum dot#8722; labeled bone marrow#8722; derived precursor cells to the tumor vasculature. These examples show the versatility of quantum dots for studying tumor17. Cell tracking Quantum dots (QDs) encapsulated in phospholipid micelles were used to label individual blastomeres in xenopus embryos. These encapsulated QDs were stable in vivo, did not become aggregated and were able to label all cell types in the embryo. At the levels required for fluorescence visualization (2×109/cell) the QD-micelles were not toxic to the cells, but concentrations of 5×109/cell did produce abnormalities. The QDs were confined to the injected cell and its progeny, though unintended translocation to the nucleus was observed at a particular stage in the development of the embryo. Another group labelling Dictyostelium discoideum found that cell labeling for over a week was possible, and that QD labelling had no detectable effects on cell morphology or physiology. Differently coloured QDs could also be used to label different populations in order to investigate the effect of starvation on D. discoideum development. These cells could be tracked for long periods with no discernible fluorescence loss. Zebrafish embryo blastomeres labelled with QDs and co-injected with CFP, a traditionally used lineage marker, showed passing of QDs to daughter cells in most cases, although some cells displaying CFP fluorescence did not show QD fluorescence. This was suggested to be due to aggregation of QDs, leading to unequal inheritance by daughter cells. This is a recognized problem, with fluorescence loss and instability in the QD structure in biological solutions18.

Removal of quantum dots toxicity The usefulness of quantum dots comes from their peak emission frequency's extreme sensitivity – quantum mechanical in nature - to both the dot's size and composition. QDs have been touted as possible replacements for organic dyes in the imaging of biological systems, due to their excellent fluorescent properties, good chemical stability, broad excitation ranges and high photo bleaching thresholds. However, the main drawback of QDs is their toxicity and therefore their application is problematic. If this toxicity problem could be addressed, QDs may one day be safely utilized in many areas. For instance, cadmium telluride (CdTe - which is toxic) QD based nanocomposites can be used as fluorescent probes for biological imaging, they can also be utilized to monitor targeted drug delivery and for controlled modification of structural and functional properties of intracellular components. Scientists in Ireland have been using gelatin during the production of CdTe QDs thereby reducing the toxicity of the particles. Their approach could be useful for the development of other nanoparticle composites with low toxicity as well19. Barriers to use in vivo Firstly, QD complexes, including their capping materials may be immunogenic, which could result in both dangerous immune reactions in subjects, and could also render the QDs ineffective as a result of antibody binding. Secondly, the heavy metals contained in the core, and the materials used for capping (e.g. MPA) may be toxic to the host. Thirdly, the size of QD complexes precludes renal excretion, making clearance from the bloodstream unlikely. This will result in eventual uptake and concentration in the liver, which is particularly sensitive to cadmium toxicity. A large number of high-quality and high powered trials specifically addressing these issues will need to be undertaken before QDs can be considered for human use, and such a process is likely to be lengthy20. Detecting Cell Death By combining a quantum dot with a novel carrier of the magnetic resonance imaging (MRI) agent gadolinium, a team of investigators at the University of Maastricht, in The Netherlands, has developed a nanoparticle that can spot apoptosis, or programmed cell death, using both MRI and fluorescence imaging. Tests in animals showed that this nanoparticle can provide anatomical information using MRI and cellular level information using fluorescence imaging. Imaging programmed cell death in the body could provide an early indication that an antitumor therapy is indeed killing cancer cells. MRI experiments showed that the nanoparticle produced an imaging signal that was approximately



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40 times stronger than that produced by the gadolinium carrier alone. Subsequent imaging experiments were able to detect injury-induced apoptosis in mice20. Drug delivery system methods We describe the development of multifunctional nanoparticle probes based on semiconductor quantum dots (QDs) for cancer targeting and imaging in living animals. The structural design involves encapsulating luminescent QDs with an ABC triblock copolymer and linking this amphiphilic polymer to tumor-targeting ligands and drug-delivery functionalities. In vivo targeting studies of human prostate cancer growing in nude mice indicate that the QD probes accumulate at tumors both by the enhanced permeability and retention of tumor sites and by antibody binding to cancer-specific cell surface biomarkers. Using both subcutaneous injection of QD-tagged cancer cells and systemic injection of multifunctional QD probes, we have achieved sensitive and multicolor fluorescence imaging of cancer cells under in vivo conditions. We have also integrated a whole-body macro-illumination system with wavelength-resolved spectral imaging for efficient background removal and precise delineation of weak spectral signatures. These results raise new possibilities for ultra sensitive and multiplexed imaging of molecular targets in vivo21. Evaluating multiple biomarkers Quantum dots linked to biological molecules, such as antibodies, have shown promise as a new tool for detecting and quantifying a wide variety of cancer-associated molecules. In general, quantum dot preparation takes approximately one day. Clinical assays take an addition one to three days, depending on the number of biomarkers being assessed simultaneously. Because quantum dots come in a variety of colors, it is possible to use a uniquely colored quantum dot for each biomarker being assayed. Multiplexed imaging and computer-aided analysis of the resulting fluorescence emitted by the quantum dots then provides quantitative results for each biomarker21. Quantum dots in cancer research Using prostate cancer specimens, the bioconjugated quantum dots are effective in simultaneously identifying multiple molecular biomarkers in cancer tissue. The technology is a variation of immuno histochemistry, the laboratory staining process commonly used by pathologists to identify proteins in a tissue section from a cancer patient. Bioconjugated quantum dots are collections of different sized nanoparticles embedded in tiny beads made of polymer material. In a process called “multiplexing,” they can be finely tuned to a myriad of luminescent colors that can tag a

multitude of different protein biomarkers or genetic sequences in cells or tissues. Semiconductor quantum dots (QDs) are nanometer-scale, light-emitting particles with unique optical and electronic properties such as size-tunable light emission, improved signal brightness, enhanced stability of the fluorescent signal, and the ability to simultaneously excite multiple fluorescent colors. These properties are most promising for improving the sensitivity of molecular imaging and quantitative cellular analysis by 1 to 2 orders of magnitude. Nie et al first reported that it is feasible to simultaneously target and image prostate tumors in living animal models using bioconjugated, prostate membrane antigen-targeted QDs. This new class of QD conjugate contains an amphiphilic triblock copolymer layer for in vivo protection and multiple PEG molecules for improved biocompatibility and circulation, making it highly stable and able to produce bright signals. Another advantage is that QD probes emitting at different wavelengths can be used together for imaging and tracking multiple tumor markers simultaneously, potentially increasing the specificity and sensitivity of cancer detection. Recently, QDs producing NIRF signals have been developed. NIRF light penetrates much more deeply into tissues compared with visible fluorescence and allows for the detection of signals inside animals, as compared with visible fluorescent signals, which can only pass through several millimeters in the tissues. A major advantage of NIRF QDs is that their emission is well beyond the spectral range of the fluorescence signal produced by blood and tissues (autofluorescence), resulting in imaging with a high signal-to-background ratio. Detection of QD NIRF signals in sentinel lymph nodes within large animals in real time has been demonstrated. Therefore, QDs are excellent optical imaging nanoprobes for evaluating the specificity of tumor targeting ligands in vitro in tumor cells and in vivo in animal tumor models. Sensitive real-time detection of tissue distribution of targeted QDs is also possible using the NIRF optical imaging system after systemic delivery. However, since cadmium is the main component of most QDs, there is some concern over their potential toxicity, making the feasibility of using these QDs for future clinical application still undetermined22. Conjugated immunoliposomes for cancer diagnostics A multifunctional nanoparticle that combines the optical properties of luminescent quantum dots (QDs) and therapeutic modality of immunoliposomes (ILs) has been developed. Carboxylate-derivatized QDs cross-linked with amino- poly (ethylene glycol)-functionalized liposomes were modified with anti-human



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epidermal growth factor receptor 2 (anti- HER2) scFv, resulting in quantum dot-conjugated immunoliposomes (QD-ILs). QD-ILs exhibited highly efficient and specific internalization by HER2-overexpressing tumor cells and facilitated long-term tracking of the drug delivery systems in vitro. However, reduced uptake and increased cell surface binding were observed for some aggregated, bulkier QD-ILs. To use QD-ILs for in vivo applications, a thorough understanding of the physical properties of the nanocomplex is necessary 22. Quantum dots and neuroscience Quantum dots represent a new tool of significant potential in neuroscience research. In addition to offering an alternative to traditional immunocytochemistry, they are particularly valuable for studies of neurons. Quantum dots can be used to visualize, measure, and track individual molecular events using fluorescence microscopy, and they provide the ability to visualize and track dynamic molecular processes over extended periods (e.g., from seconds to many minutes). These properties are difficult to achieve using other techniques or approaches. For example, quantum dots are useful for experiments that are limited by

the restricted anatomy of neuronal and glial interactions, such as the small size of the synaptic cleft, or between an astrocyte process and a neuron. Because of their extremely small size and optical resolution, they are also well suited for tracking the molecular dynamics of intracellular and/or intercellular molecular processes over long time scales. However, it should be appreciated that the hydrodynamic radius of functionalized quantum dots is larger (15–20 nm) than their actual size of 5–8 nm23. Recent studies using quantum dots in neuroscience illustrate the potential of this

technology. Triller and colleagues used antibody functionalized quantum dots to track the lateral diffusion of glycine receptors in cultures of primary spinal cord neurons. They were able to track the trajectory of individual glycine receptors for tens of minutes at spatial resolutions of 5–10 nm, demonstrating that the diffusion dynamics varied depending on whether the receptors were synaptic, perisynaptic, or extrasynaptic24. Vu et al. tagged nerve growth factor (βNGF) to quantum dots and used them to promote neuronal-like differentiation

in cultured pheochromocytoma 12 (PC12) cells. Ultimately, these approaches could be used to visualize and track functional responses in neurons. However, as with any new technology, there are caveats. For example, Vu et al. reported that βNGF conjugated to quantum dots had reduced activity compared with free βNGF. Other groups are

pushing the technology forward and providing new

quantum-dot-based tools25. Brinker and colleagues developed a technique to produce biocompatible water-soluble quantum dot micelles that retain the optical properties of individual quantum dots. These micelles showed uptake and intracellular

dispersion in cultured hippocampal neurons26. Ting and colleagues are developing a modified quantum dot labeling approach that addresses the relatively large size of antibody–quantum-dot conjugates and the instability of some quantum-dot–ligand interactions. Their technique tags cell surface proteins with a specific peptide (a 15 aa

polypeptide called acceptor protein; GLNDIFEAQKIEVWHE) that can be directly biotinylated as a target for streptavidin-conjugated

quantum dots. Using this approach, they were able to specifically label and track AMPA receptors on cultured hippocampal neurons27. Ultimately, quantum dot nanotechnologies will require easy-to-use approaches that can be straightforwardly replicated in a typical

neurobiology lab. Unfortunately, in our experience, most quantum dot protocols intended for non-neural mammalian cells in the peer-reviewed literature and in instructions from commercial

sources do not always readily work with primary neurons, glia, and related cell lines. In the supplemental data, we present a very simple and straightforward protocol for specific labeling of both neurons and glia. Of particular note, as far as we are aware, these results represent the first use of quantum dots in glial cells. Given the unique

properties that quantum dots have to offer neuroscience research, their future looks pretty bright (and stable). Quantum Dotsdiagnosis and drug delivery The use of quantum dots for the investigation of cellular uptake 7 intracellular delivery of nanocrystals offers new possibilities in order to study sub-cellular processes, due to small, uniform size and the unique optical properties of quantum dots. Recent progress in the surface chemistry of quantum dotsexpanded their use in biological applications, reduced their cytotoxicity and rendered quantum dots a powerful tool for the investigation of dinstinct cellular processes, like uptake, receptor trafficking and intracellular delivery. In the field of nanomedicine, quantum dots can make a worthy contribution to the development a new diagnostic and delivery systems as they offer unique optical properties for highly sensitive detection are well defined in size and shape and can be modified with various targeting principles28.



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Table 1: Summary of application areas for nanoscale pharmaceuticals and medicine in diagnostics Material/technique Property Applications Timescale (to market launch)

Diagnostics Nanosized markers, i.e. the attachment of nanoparticles to molecules of interest.

Minute quantities of a substance can be detected, down to individual molecules.

E.g. detection of cancer cells to allow early treatment.

----------

‘Lab-on-a-chip’ technologies

Miniaturisation and speeding up of the analytical process

The creation of miniature, portable diagnostic laboratories for uses in the food, pharmaceutical and chemical industries; in disease prevention and control; and in environmental monitoring.

Although chips currently cost over £125 (US$2085) each to make, within three years the costs should fall dramatically, making these tools widely available.

Quantum dots

Quantum dots can be tracked very precisely when molecules are ‘bar coded’ by their unique light spectrum.

Diagnosis. In early stage of development, but there is enough interest here for some commercialization.

Fig. 1: Quantum dots in core shell coating

Nanoshells Quantum Dots

Fig. 2: Comparing nanoshells to quantum dots



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Fig. 3: Quantum dots after fluorescence in different nanometer wavelength

↓ ↓ ↓

Evidots Evitags Evifluors Fig. 4: Different types of quantum dots

CONCLUSIONS Within the next decade diagnostic devices based on nanotechnology, which can make thousands of measurements very rapidly and very inexpensively will become available. Future trends in diagnostics will continue in miniaturization of biochip technology to the nanoscale range. The most common clinical diagnostic application will be blood protein analysis. Blood is systemic circulation reflects the state of health or disease of most organs. Therefore, detection of bood molecular fingerprints will provide a sensitive assessment of health and disease. In the next decade nanobiotechnology will play important roles not only in diagnosis but also in linking diagnosis with treatment and development of personalized medicine. Due to the integration and interrelationships of several technologies involved in nanodiagnostics, those who conduct these tests or devise new tests will be

taking a more active part in decision making in the future health care systems. In this scenario, QDs are playing and will play multidimensional role with more innovative modifications. Quantum dots offer multitude optical and electronic properties that can work around natural limits inherent in traditional semiconductors. These are used by life science researches as tiny beacons or markers, allowing them to see individual genes, nucleic acids, proteins, and small molecules especially the new available unique ternary core material with a molecular plate shell. Moreover they are immense use in the field of disease diagnosis, intracellular tagging as photo sensitizer for treatment of cancer, biotechnology, bioassays and to develop advanced quantum dot based anti-counter feting materials. Future prospective unfolded by quantum dots are in developing more selective and specific approach of labelling cells



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and biomolecules and studying their interference effects with normal physiology. It seems that future technology will be based entirely on nanotechnology like quantum dots in all sphere of life. REFERENCES

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