Molecular imaging with surface-enhanced Raman spectroscopy nanoparticle reporters

Abstract

Molecular imaging scans cellular and molecular targets in living subjects through the introduction of imaging agents that bind to these targets and report their presence through a measurable signal. The picomolar sensitivity, signal stability, and high multiplexing capacity of Raman spectroscopy satisfies important needs within the field of molecular imaging, and several groups now utilize Raman and surface-enhanced Raman spectroscopy to image molecular targets in small animal models of human disease. This article details the role of Raman spectroscopy in molecular imaging, describes some substrates and imaging agents used in animal models, and illustrates some examples.

Figure 1

Molecular imaging produces quantitative representations of (a) biological features via (b) imaging agents. For surface-enhanced Raman spectroscopy, these agents consist of a chemically linked reporter (nanoparticle) and a targeting component (ligand). The ligand concentrates the imaging agent at the site of biological interest, while the reporter component works in tandem with (c) molecular imaging hardware and software to create an image. In (b), the yellow sphere represents a gold nanoparticle, and the red outline represents a small molecule Raman dye adsorbed to the surface.

Figure 2

(a) The excitation (dotted line) and emission (solid line) spectra of Raman and fluorescence (from quantum dots [QDs]) are contrasted with the optical window (shaded red box). QDs require excitation in the visible and ultraviolet regions (outside the optical window); they also have relatively broad emission peaks, which limit multiplexing. In contrast, many different types of Raman labels can be excited with the same 785 nm laser, yet produce markedly different emission spectra based on differences in chemical structure (black dashed box). (b) Multiplexing with surface-enhanced Raman spectroscopy (SERS) shows that very different spectra are produced by changing the (c) small-molecule Raman dye. Importantly, the excitation source and underlying gold substrate remain the same. Even subtle changes from hydrogen (S420) to deuterium (S421) produce very different SERS spectra.

Figure 3

Different imaging agents used in Raman molecular imaging. (a) Core–shell nanoparticles, (b) nanorods, (c) nanospheres, (d) roughened spheroids, and (e) carbon nanotubes. The lower table plots the size and dose used to image various molecular imaging targets in different small animal models of human disease. N/A under “Target” indicates in vivo imaging of tumor or tumor boundaries without a biological ligand. BPE, 1,2-bis (4-pyridyl)-ethylene; IR792, IR-792 infrared laser dye; MBA, mercaptobenzoic acid; MGI, malachite green isothiocyanate; and EGFR, epidermal growth factor receptor. (c) Adapted with permission from Reference . © 2008 Nature Publishing Group. (d) Adapted with permission from Reference . © 2009 American Chemical Society. (e) Reprinted with permission from Reference . © 2005 National Academy of Sciences.

Figure 4

In vivo imaging. (a) A mouse xenograft ovarian tumor (T) next to muscle (M) and liver (L). This animal was intravenously injected with 200 µL of 5.4 nM surface-enhanced Raman spectroscopy (SERS) gold nanorods. Twenty-four hours later, the skin was removed and the tumor analyzed for SERS signal. (b) The spectrum at each point was raster scanned and compared to a reference spectrum of gold nanorods before injection. At each point, dynamic least squares analysis indicated the similarity between the reference spectrum and the sample spectrum—more similar spectra produce brighter pixels on the Raman map. The color scale shows the degree of dynamic least squares correlation, where 1 is a perfect match between the pixel and reference spectrum, and 0 is no match. This map highlights the use of SERS to indicate tumor margins. (c) In a model of glioblastoma brain cancer, the tumor was imaged in exposed brain similar to human tumor resection. The SERS signal (d)–(d–v) decreases as the tumor is surgically removed (c)–(c–v). The picomolar sensitivity of SERS allowed microscopic foci of the tumor (invisible to visual inspection) to be detected and removed (d-iv). (a and b) Reprinted with permission from Reference . © 2012 American Chemical Society. (c and d) Reprinted with permission from Reference . © 2012 Nature Publishing Group.