Applications of QDs

To date, a variety of methods have been developed for the efficient delivery of QDs into cells [204, 205]. These include nonspecific endocytosis [206-209] or receptor-mediated endocytosis that involves QDs decorated with transfection reagents (peptides [210-215], proteins [65, 216-219], cationic liposomes [204], dendrimers [204], polymers [220,221] or small molecules [222,223]) that were used for the intracellular delivery of QDs. Physical techniques such as, electroporation [204, 224] or microinjection [204, 225, 226] have also been employed to deliver QDs into cells. 

In the past, QDs have been used extensively for intracellular applications such as, cell imaging and the labeling of subcellular compartments, and these topics have been recently reviewed [201, 202]. The use of cell-incorporated QDs to probe drug delivery or to follow intracellular processes is, however, scarce, and the subject holds great promise for future developments. One major challenge would involve the use of 

QDs have also been used as fluorescence markers to follow intracellular metabolic pathways, with the system having been used to screen anticancer drugs [140]. When the CdSe/ZnS QDs were modified with the dye Nile blue, the chemically modified QDs could serve as optical labels for the sensing of NAD(P) cofactors (see Sections 6.3 and (6.4). As the NADH cofactor is formed upon activation of the intracellular 

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For the application of QDs to three-dimensional biological imaging, a large two-photon absorption cross section is required to avoid cell damage by light irradiation. For application to optoelectronics, QDs should have a large nonlinear refractive index as well as fast response. Two-photon absorption and the optical Kerr effect of QDs are third-order nonlinear optical effects, which can be evaluated from the third-order nonlinear susceptibility, or the nonlinear refractive index, y, and the nonlinear absorption coefficient, p. Experimentally, third-order nonlinear optical parameters have been examined by four-wave mixing and Z-scan experiments. 

Nanoparticles such as those of the heavy metals, like cadmium selenide, cadmium sulfide, lead sulfide, and cadmium telluride are potentially toxic [14,15]. The possible mechanisms by which nanoparticles cause toxicity inside cells are schematically shown in Fig. 2. They need to be coated or capped with low toxicity or nontoxic organic molecules or polymers (e.g., PEG) or with inorganic layers (e.g., ZnS and silica) for most of the biomedical applications. In fact, many biomedical imaging and detection applications of QDs encapsulated by complex molecules do not exhibit noticeable toxic effects [16]. One report shows that the tumor cells labeled with QDs survived in circulation and extravasated into tissues... 

Although very preliminary, the above studies seem to open the door to novel transduction schemes and applications of QDs for (bio)chemical analysis. 

Last, but not least, full application of QDs in chemical sensors would require the immobilization of the nanoparticles into appropriate solid supports in order to develop reliable active phases (able to provide, for instance, convenient fiber optic-based sensing applications). Although only a few reports have been published so far regarding the trapping of the QDs in solid matrices, some important steps have already started towards the realization of the potential of these technologies. There is still plenty of room for further development in all those directions. 

An interest to intraband relaxation in quantum dots (QDs) with discrete energy levels is conditioned by the application of QDs as effective active media for semiconductor lasers. 

Substantial progress in the application of QDs for optical sensing and biosensing was accomplished in the past decade, and several reviews have summarized the advances in this field [56-58]. In this chapter, the aim is to emphasize the use of QDs for following molecular and biomolecular recognition events, and to probe the dynamics of chemical or biomolecular transformations by the application of modified hybrid QDs. 

Figure 6.4 Different mechanisms for the application of QDs as optical sensors. Path (a) Competitive detection of an analyte using a labeled analyte and FRET/ET as transduction...

Figure 6.13 Application of QDs as optical labels for biorecognition events, (a) Formation of immune-complexes (b) Probing DNA hybridization.

Similarly, semiconductor QDs were integrated with proteins, such that the hybrid systems would permit the real-time analysis of catalytic transformations stimulated by the proteins [102, 192-194]. For example, the hydrolytic functions of a series of proteolytic enzymes were followed by the application of QD reporter units, using the FRET process as a readout mechanism. In this case, the QlSe QDs were modified with peptide sequences that were specific for different proteases, where the quencher units were tethered to the peptide termini. Within the QDs/fluorophore-modified hybrid assembly, the fluorescence of the QDs was quenched. Subsequent hydrolytic... 

The different applications of QDs to monitor biocatalytic processes have required the specific modification of Q Ds with capping layers spedfic for target enzymes. The synthesis of QDs with a versatile modifier capable of analyzing a broad dass of... 

The use of FI-ECL sensor was also reported for the determination of durabolin in an aqueous system based on CdTe QD films. These QD films, used as a recognizer to determine durabolin, were packed into a homemade cell. The proposed sensor works on the intensive anodic ECL emission which was achieved at a starting potential of +1.3 V (vs. Ag/AgCl) in a carbonate bicarbonate buffer solution with a pH of 9.93 at a CdTe QDs-modified ITO electrode. This approach could easily open new avenues for the applications of QDs in ECL biosensing [82]. 

The prerequisite for biomedical applications of QDs is availability of photostable, compatible, and water-soluble nanocrystals [52]. 

CdSe QDs were prepared via the method described by Peng et al. The CdSe QDs were introduced into a conventional free radical miniemulsion recipe as described by Trindade et al. A schematic representationofthevariousstepsinthepreparationofQD-containing latexes is given in Figure 6.18. In this way, the application of QDs into latex technology is extremely straightforward. ... 

After reviewing some solid state theory, we are now capable of modeling exdtons and exciton-polaritons in semiconductor nanostructures. We first study and model the exciton in general, then exdtons in QD-based nanostructures, and finally applications of QD excitons and exciton-polaritons. Details of exciton theory can be found in Dimmock (1967) and Fu and Willander (1999). 

Fig. 3 Common bio-medical applications of QDs (adapted from reference [227])...

However, cytotoxicity still remains the serious problem. Chen et al. [234] have studied the cytotoxicity of CdTe/CdS (core-shell) as well as CdTe/CdS/ZnS (core-shell-shell) structured aqueous synthesized QDs, and their results suggest that the cytotoxicity of CdTe QDs not only comes from the release of Cd ions but also intracellular distribution of QDs in cells and the associated nanoscale effects [235], Recently, clinical applications of QDs have been reviewed [233], The application areas include (1) biomarker detection in various cancers, (2) imaging and sensing of infectious diseases, and (3) other clinical therapeutic applications. QDs with intense and stable fluorescent properties could enable the detection of tens to hundreds of cancer biomarkers in blood assays, on cancer tissue biopsies, or as contrast agents for medical imaging. 

As an example of the application of QD labeling, we describe below the use of QDs to measure the diffusion of a neuronal transporter. 

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