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WIREs Nanomed Nanobiotechnol
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Near infrared imaging with nanoparticles

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Abstract Near infrared imaging has presented itself as a powerful diagnostic technique with potential to serve as a minimally invasive, nonionizing method for sensitive, deep tissue diagnostic imaging. This potential is further realized with the use of nanoparticle (NP)‐based near infrared (NIR) contrast agents that are not prone to the rapid photobleaching and instability of their organic counterparts. This review discusses applications that have successfully demonstrated the utility of nanoparticles for NIR imaging, including NIR‐emitting semiconductor quantum dots (QDs), resonant gold nanoshells, and dye‐encapsulating nanoparticles. NIR QDs demonstrate superior optical performance with exceptional fluorescence brightness stability. However, the heavy metal composition and high propensity for toxicity hinder future application in clinical environments. NIR resonant gold nanoshells also exhibit brilliant signal intensities and likewise have none of the photo‐ or chemical‐instabilities characteristic of organic contrast agents. However, concerns regarding ineffectual clearance and long‐term accumulation in nontarget organs are a major issue for this technology. Finally, NIR dye‐encapsulating nanoparticles synthesized from calcium phosphate (CP) also demonstrate improved optical performances by shielding the component dye from undesirable environmental influences, thereby enhancing quantum yields, emission brightness, and fluorescent lifetime. Calcium phosphate nanoparticle (CPNP) contrast agents are neither toxic, nor have issues with long‐term sequestering, as they are readily dissolved in low pH environments and ultimately absorbed into the system. Though perhaps not as optically superior as QDs or nanoshells, these are a completely nontoxic, bioresorbable option for NP‐based NIR imaging that still effectively improves the optical performance of conventional organic agents. WIREs Nanomed Nanobiotechnol 2010 2 461–477 This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices Diagnostic Tools > In Vivo Nanodiagnostics and Imaging

Near infrared (NIR) absorption spectrum of water and main tissue‐absorbing components, oxy‐ and deoxyhemoglobin. The highest optical transmission, the ‘imaging window’, between 800 and 1000 nm, is highlighted. (Modified and reprinted with permission from Ref 6. Copyright 2006 SPIE).

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Near‐IR transillumination images (excitation: 755 nm, emission: 830 nm) taken at various times track the fluorescence signals and pharmacokinetic distributions for the ICG‐CPNPs and controls delivered systemically via tail‐vein injections in nude mice implanted with subcutaneous human‐breast adenocarcinoma tumors. Hash marks next to each mouse indicate the position of the 5‐mm tumors. Two control samples, (i) carboxylate‐terminated CPNPs without ICG encapsulant and (ii) free ICG, match the particle concentration and fluorophore content (1013 particles/ml and 10−5 M, respectively) of a (iii) PEGylated ICG‐CPNP sample. (b ii) No fluorescence signal is detected from the free ICG at 24 h post injection while the PEG‐ICG‐CPNP sample (c iii) retains significant signal even after 96 h. (b iii) The fluorescence signal is unmistakably localized to the tumors 24 h after administration with PEGylated ICG‐CPNPs. (Reprinted with permission from Ref 117. Copyright 2009 SPIE).

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Comparative spectral effect of four biorelevant solvents on the emission response of (a) free ICG dye and (b) dye‐doped ICG‐CPNPs. The normalized peaks spread across 18 nm for the free fluorophore (standard deviation of 7.8 nm), while encapsulation in CPNPs has an order of magnitude smaller 1.6 nm spread (0.68 nm standard deviation). These data confirm the largely impermeable nature of the CP matrix to the surrounding environment, shielding the encapsulated dye from solvent interaction. (Reprinted with permission from Ref 89. Copyright 2008 SPIE).

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Ratios of gold concentrations by neutron activation analysis (NAA) in each organ to that in tumor (solid black line) at 24 h post‐systemic administration of PEGylated gold nanoshells. Data reveal unfavorable accumulation of nanoshells in untargeted organs. (Reprinted with permission from Ref 82. Copyright 2007 Springer).

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Imaging enhancement of SKBr3 breast cancer cells using HER2‐targeted gold nanoshells. Images show the scatter‐based darkfield imaging of HER2 expression (Row a) and silver stain assessment of nanoshell binding (Row b). Note increased contrast in cells treated with the NIR‐emitting nanoshells (a3) compared to controls (a1 & a2). Greater silver staining intensity is seen in cells exposed to anti‐HER2 nanoshells (b3) compared to controls (b1 & b2), suggesting enhanced nanoshell binding to cell surfaces overexpressing HER2. (Modified and reprinted with permission from Ref 81. Copyright 2005 American Chemical Society).

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Optical tunability demonstrated for nanoshells with a 60‐nm silica core radius and gold shells 5, 7, 10, and 20 nm thick. The plasmon resonance (extinction) of the particles red shifts with decreasing thickness of the gold shell (or an increasing core:shell ratio). Nanoshells are easily fabricated with resonance in the NIR. (Reprinted with permission from Ref 3. Copyright 2006 Springer).

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Near infrared (NIR) QD sentinel lymph node (SLN) mapping of the porcine jejunum. From top to bottom are shown the NIR images of the surgical field before injection, during injection, 1 min after injection, during image‐guided SLN resection, and during post‐resection evaluation of the sentinel (S) and negative control (N) nodal groups. Each time point is shown in color video (left), NIR fluorescence (middle), and pseudocolored merged images (right). The lymphatic channels between the injection site (arrowheads) and SLN (arrows) are clearly visible. (Reprinted with permission from Ref 31. Copyright 2006 Springer).

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Representative excitation and emission spectra for the three NIR particulate technologies reviewed illustrating the differences in Stokes shifts and emission characteristics (a) NIR‐emitting semiconductor nanocrystals exhibit an almost continuous excitation spectrum with a narrow emission response, providing a very large Stokes shift. (b) The surface plasmon resonance (SPR) response from resonant gold nanoshells spans a broad wavelength range, allowing tunability within the NIR region. (c) The optical properties of dye‐encapsulating calcium phosphate nanoparticles (CPNPs) are dependent on the internalized fluorophore, such as the NIR‐emitting indocyanine green (ICG) depicted here.

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Illustration of band gap structures for type‐I and type‐II semiconductor heterostructures. Type‐II structures have a staggered bang gap as a result of the valence and conduction bands of the material used in the core being lower (or higher) than those in the shell. This results in a spatial separations of carriers, as one carrier is mostly confined to the core, while the other is mostly confined to the shell. This in turn allows emission at energies that are smaller than the band gap of either material, which permit access to higher wavelengths in the NIR. (Modified and reprinted with permission from Ref 30. Copyright 2004).

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