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WIREs Nanomed Nanobiotechnol
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Physiological behavior of quantum dots

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Abstract Quantum dots (QDs) have shown great potentials in biomedical applications like bioimaging, sensors and diagnostics due to various advantages such as their robust fluorescence, remarkable photostability, large absorption cross section, and tunable fluorescence emission. The fate, behavior, metabolism, and toxicities of QDs are the primary aspects to be assessed before their bio‐applications. Numerous studies concerning those aspects have been reported in the past years. However, only several reviews discussed the toxicities of QDs and various contradictory conclusions appear between these studies. In this review, the fate, metabolism, and behaviors of various QDs and crucial parameters that may determine their fate and behavior in vivo are discussed in depth. This review may provide insights for a better understanding of the biological impacts of QDs. We also propose several suggestions for how to develop QDs application in humans in the future. WIREs Nanomed Nanobiotechnol 2012, 4:620–637. doi: 10.1002/wnan.1187 This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Toxicology and Regulatory Issues in Nanomedicine > Toxicology of Nanomaterials

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Examples of in vitro and in vivo applications of Qdot probes. (a) Example of the design of a fluorescence resonance energy transfer (FRET) sensor. Shown on the left is a 530 Qdot‐MBP‐Cy3‐β‐CD‐Cy3.5 maltose sensor. Addition of more maltose releases many β‐CD‐Cy3.5 from the complex, increasing Cy3 emission. MBP, maltose binding protein; β‐CD, β‐cyclodextrin. (b) Multicolor staining of HeLa cells with red‐light‐emitting EGF‐Qdots and green‐light‐emitting Qdots. (c) In vivo labeling of a Xenopus embryo with green‐micelle‐coated Qdots, showing the migration pattern of neural crest cells; scale bar is 0.5 mm. (d) Image of Qdots targeting prostate tumor tissues in vivo in a mouse bearing a xenograft tumor. The tumor was targeted using orange‐red‐emitting Qdot probes. A control (no tumor) mouse is shown on the left.2

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Reproductive behavior and egg‐laying difficulty after long time exposure. (a) Mating behavior of hermaphrodite and male C. elegans worms. The boxed region indicates the vulva that is enlarged as shown in (g). (b, c) QDs distribution in live worms after short time exposure. The boxed region in (c) indicates the egg laying process that is enlarged as shown in (h). (d–f) QDs distribution in fresh dead worms after long time exposure (16 days). The nonuniformity in the background is due to the pictures assembly. (i) QDs confined in the digestive tract without entering into the eggs after short time exposure. (j) Damaged egg after long time QD exposure. (l, n) Corresponding fluorescence images that are illustrated for a better understanding. The autofluorescence (green) of the worm body in (l) is exaggerated 10 times to clearly show the vulva structure and QD expelling process. (o) QD transfer from intestine to reproductive system. The schematic drawing represents transverse sections through the intestine, gonad, and vulva of adult hermaphrodite mid body regions, according to image from Worm book website and TEM images from Worm atlas database. Red arrows indicate the transferring and expelling pathways.64

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In situ elemental analysis and degradation of QDs. (a) Schematic drawings of the anatomical structure of adult hermaphrodite C. elegans. (Reprinted with permission from Ref 66. Copyright 2002–2008 Wormatlas) (b) Schematic drawing of metabolic fates of MEA‐CdSe@ZnS QDs. (Reprinted with permission from Wormatlas database) (c, d) Fluorescence versus elemental distributions of two intact worms exposed to MEA‐CdSe@ZnS for 12 h (c) and 24 h (d). Fluorescence microscopic images are subsequently merged with bright field ones (BF). Microbeam X‐ray fluorescence (µ‐XRF) mappings of Se and Zn elements in QDs. The quantity of each individual element is plotted on the respective scale below. The dashed lines delimit the boundary of the worm body. Boxed regions indicate the head, body, or tail regions that are enlarged and shown in (e–h). The large white dashed lines delimit the boundary of the pharynx. Arrows point to the beam positions of µ‐XANES spectra in figure (i, j). (i, j) In situ Se K edge microbeam X‐ray absorbance near edge structure (µ‐XANES) spectra of QDs within the pharynx or intestine of the two worms in (c) and (d), respectively. Exact positions of each spectrum are displayed in (e–h) by a, b, c, d, and e. Beam sizes of µ‐XRF mappings and µ‐XANES spectra are 5 × 5 µm.64

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QD distribution in intestinal GFP‐labeled C. elegans. (a–d) Laser scanning confocal microscope images of whole body distribution of MPA‐CdTe QDs in two C. elegans worms. (e–t) Enlarged views of different body parts. Fluorescent images of GFP‐labeled intestinal endothelial cells (a, e, i, m, p) and QDs (b, f, j, n, r) are merged with DIC images subsequently (d, h, l, p, t) to show the exact QD location in the worm body. Arrowheads in (a–d) point to the original positions of enlarged regions. Arrows in (e–t) point to the colocalized positions.64

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SPECT/CT images of 125I‐labeled QDs in normal mice. SPECT/CT images showed high uptake in both the liver (L) and spleen (S), moderate uptake in the lungs and intestine at 2 h. Coronal images are ventral view.60

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The alternation after QDs have reacted with three serum proteins in vitro for 12 h. (a) QDs in PBS solution as a control; (b) QD–albumin complexes aggregated into clava‐like particles of hundreds of nanometers; (c) QDs–γ‐globulin complexes aggregated into floccule‐like particles of hundreds of nanometers; (d) QDs having reacted with transferrin appear to have no significant aggregation. (e) Excretion of QDs from mice via feces and urine over 5 days. The solid and dotted curves show the integral masses from feces and urine, respectively. The urine and feces collected at the same time were combined (n = 3), so no SD can be shown for this index. (f) Bio‐distribution histogram of QDs at different time intervals. (Reprinted with permission from Ref 36. Copyright 2008 Elsevier)

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The CD spectra of l‐ and d‐GSH (a) and l‐ and d‐GSH‐QDs (b), the chirality‐dependent cytotoxicity of QDs (c), the l‐GSH‐QDs had more profound effects on the induction of autophagy than d‐GSH‐QDs (d), and QDs decreased lysosome stability in HepG2 cells (e, f). AO in Figure 4E means acridine orange. (Reprinted with permission from Ref 32. Copyright 2012 WILEY‐VCH Verlag GmbH & Co. KGaA)

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Fluorescence images of the distribution of lysosomes and Tat‐QDs in QGY cells. (a) The distribution of lysosomes, (b) the distribution of Tat‐QDs, (c) DIC image of the cells, and (d) a merged image of a, b, and c. At the bottom and the right of (d) are the XZ and YZ profiles measured along the lines marked in the main image, showing the three‐dimensional distributions of Tat‐QDs and lysosomes. P1 is a spot in a lysosome and P2 is far from any lysosome.29

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Schematic of current Qdot surface coatings that permit Qdots to interface with biological systems and biorecognition molecules. We call these surface coatings: (a) bifunctionalization, (b) silanization, (c) hydrophobic–hydrophobic interaction, (d) electrostatic interaction, (e) micelle encapsulation, (f) amphiphilic polymer, and (g) hydroxylation. In the schematic, the thicknesses of the surface coatings with respect to the size of Qdots are not drawn to scale.2

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