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WIREs Nanomed Nanobiotechnol
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Quantum dots and nanocomposites

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Abstract Quantum dots (QDs), also known as semiconducting nanoparticles, are promising zero‐dimensional advanced materials because of their nanoscale size and because they can be engineered to suit particular applications such as nonlinear optical devices (NLO), electro‐optical devices, and computing applications. QDs can be joined to polymers in order to produce nanocomposites which can be considered a scientific revolution of the 21st century. One of the fastest moving and most exciting interfaces of nanotechnology is the use of QDs in medicine, cell and molecular biology. Recent advances in nanomaterials have produced a new class of markers and probes by conjugating semiconductor QDs with biomolecules that have affinities for binding with selected biological structures. The nanoscale of QDs ensures that they do not scatter light at visible or longer wavelengths, which is important in order to minimize optical losses in practical applications. Moreover, at this scale, quantum confinement and surface effects become very important and therefore manipulation of the dot diameter or modification of its surface allows the properties of the dot to be controlled. Quantum confinement affects the absorption and emission of photons from the dot. Thus, the absorption edge of a material can be tuned by control of the particle size. This paper reviews developments in the myriad of possibilities for the use of semiconductor QDs associated with molecules producing novel hybrid nanocomposite systems for nanomedicine and bioengineering applications. WIREs Nanomed Nanobiotechnol 2010 2 113–129 This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology

A schematic representation of the band structure in solids: (a) quantum confinement effect on changing quantum dot size; (b) surface trap sites with their electronic energy states localized within the QDs bandgap; (c) the electronic structure of a core–shell quantum dot made of two semiconductors forming a heterojunction (core surrounded by the shell of a wider bandgap).

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Cartoon illustrating the potential application of a hypothetically tailored nanostructured system aiming at simultaneous imaging involving multiple‐target sites.

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The representation of cell biomembrane with the important entities: double‐layer of lipids and proteins and specific receptor molecules; the QDs‐conjugates approximation (a); affinity ligand–cell receptor interaction at interface (b); early endosome from endocytosis (c).

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Typical process used to produce QDs for bioapplications, considering the major steps: (a) coreshell QDs (CdSe–ZnS); (b) organic colloidal stabilization [ligand tri‐n‐octylphosphine oxide (TOPO)]; (c) hydrophilic polymer attachment (PEG); (d) bioconjugated with affinity ligands as targeting molecule (immunoglobulin‐G); (e) integrated hybrid biocompatible nanocomposite (‘micellar’ structure).

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Schematic drawing representing the changes on optical behavior of nanoparticles associated with their size. Top: Electronic structure of QDs with ‘blue shift’ due to quantum confinement.

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