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WIREs Nanomed Nanobiotechnol
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Label‐free detection and manipulation of single biological nanoparticles

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In the past several years, there have been significant advances in the field of nanoparticle detection for various biological applications. Of considerable interest are synthetic nanoparticles being designed as potential drug delivery systems as well as naturally occurring or biological nanoparticles, including viruses and extracellular vesicles. Many infectious diseases and several human cancers are attributed to individual virions. Because these particles likely display different degrees of heterogeneity under normal physiological conditions, characterization of these natural nanoparticles with single‐particle sensitivity is necessary for elucidating information on their basic structure and function as well as revealing novel targets for therapeutic intervention. Additionally, biodefense and point‐of‐care clinical testing demand ultrasensitive detection of viral pathogens particularly with high specificity. Consequently, the ability to perform label‐free virus sensing has motivated the development of multiple electrical‐, mechanical‐, and optical‐based detection techniques, some of which may even have the potential for nanoparticle sorting and multi‐parametric analysis. For each technique, the challenges associated with label‐free detection and measurement sensitivity are discussed as are their potential contributions for future real‐world applications. WIREs Nanomed Nanobiotechnol 2016, 8:717–729. doi: 10.1002/wnan.1392 This article is categorized under: Diagnostic Tools > In Vitro Nanoparticle-Based Sensing Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease
Nanowire‐based detection of single influenza A viruses. (a) Two nanowires incubated with different antibodies to promote specific binding. (b) The corresponding changes in conductance reflect the surface charge of a virus that binds and unbinds to nanowire 2. (Reprinted with permission from Ref . Copyright 2004 National Academy of Sciences of the United States of America)
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Optical trapping virometry of HIV‐1 virions. (a) Experimental schematic for optical trapping of single HIV‐1 virions in culture fluid. HIV‐1 virions were delivered into a microfluidic chamber and trapped by the IR laser focused at the center of the chamber. The xyz dimensions are shown as indicated, with y perpendicular to the figure plane. (b) Representative two‐photon fluorescence (TPF) time courses from individually trapped HIV‐1 virions. All traces were fit with a single exponential function (red), with time constants for each trace as follows: 60.7 (square), 49.5 (cross), 51.5 (circle), and 54.1 seconds (triangle). Time zero started with the onset of TPF collection. (c) The laser deflection signal measured in real time using back‐focal‐plane interferometry (BFPI), which can be used to distinguish single HIV‐1 particles from aggregates in complete media. (d) Representative Alexa‐594 TPF time course from a single virion bound with monoclonal antibody b12 labeled with Alexa‐594, where individual photobleaching steps are indicated with arrows. (Inset) A cartoon of a HIV‐1 virion with a single envelope glycoprotein trimer. (Reprinted with permission from Ref . Copyright 2014 Nature Publishing Group)
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Schematic of the dark‐field heterodyne interferometric detection technique. Nanoparticles inside a glass nanofluidic channel are detected as they traverse an illumination spot formed using an excitation laser (Eexc). The detector signal is proportional to the product of the nanoparticle's scattering field (Es) and an additional frequency‐shifted reference field (Er). (Inset) Scanning electron microscope image of the nanofluidic channels. Scale bar = 2 µm. (Reprinted with permission from Ref . Copyright 2012 Elsevier)
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Detection of influenza A virions by a microsphere resonator. (a) Illustration of a silica microsphere immersed in aqueous solution. A tunable distributed feedback laser excites whispering gallery modes (WGMs) of a microsphere by evanescent coupling to a tapered optical fiber. (Inset) Typical transmission spectrum for a WGM mode detected while the laser wavelength is tuned. (b) The resonance wavelength shifts associated with the binding of single influenza A virions to a microsphere cavity (r = 39 mm) in PBS buffer. (Reprinted with permission from Ref . Copyright 2008 National Academy of Sciences of the United States of America)
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Surface plasmon resonance microscopy for detection of influenza A viruses. (a) Schematic of the experimental setup. (b) Images of H1N1 influenza A viruses and silica nanoparticles of varying size in PBS buffer. (Insets) Corresponding nanoparticle images produced by numerical simulation. (Reprinted with permission from Ref . Copyright 2010 National Academy of Sciences of the United States of America)
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Nanochannel design for resistive‐pulse sensing of hepatitis B virus capsids. (a) Schematic of the microfluidic chamber with two channels for delivery of biological nanoparticles. (b) Scanning electron microscope image of the two microchannels with an enlarged view of a 2.5‐µm long pore‐to‐pore channel. (c) Atomic force microscopy image of this channel incorporating pores that are 45 nm wide, 45 nm deep, and 430 nm long. (Reprinted with permission from Ref . Copyright 2014 American Chemical Society)
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Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease
Diagnostic Tools > In Vitro Nanoparticle-Based Sensing

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