Noninvasive molecular imaging of small living subjects using Raman spectroscopy

Abstract

Molecular imaging of living subjects continues to rapidly evolve with bioluminescence and fluorescence strategies, in particular being frequently used for small-animal models. This article presents noninvasive deep-tissue molecular images in a living subject with the use of Raman spectroscopy. We describe a strategy for small-animal optical imaging based on Raman spectroscopy and Raman nanoparticles. Surface-enhanced Raman scattering nanoparticles and single-wall carbon nanotubes were used to demonstrate whole-body Raman imaging, nanoparticle pharmacokinetics, multiplexing, and in vivo tumor targeting, using an imaging system adapted for small-animal Raman imaging. The imaging modality reported here holds significant potential as a strategy for biomedical imaging of living subjects.nanotubes ͉ SERS nanoparticles M olecular imaging of living subjects provides the ability to study cellular and molecular processes that have the potential to impact many facets of biomedical research and clinical patient management (1-4). Imaging of small-animal models is currently possible by using positron emission tomography (PET), single photon emission computed tomography, magnetic resonance imaging, computed tomography, optical bioluminescence and fluorescence, high frequency ultrasound, and several other emerging modalities. However, no single modality currently meets the needs of high sensitivity, high spatial and temporal resolution, high multiplexing capacity, low cost, and high-throughput.Fluorescence imaging, in particular, has significant potential for in vivo studies but is limited by several factors (5, 6), including a limited number of fluorescent molecular imaging agents available in the near infra-red (NIR) window with large spectral overlap between them, which restricts the ability to interrogate multiple targets simultaneously (multiplexing). In addition, background autofluorescence emanating from superficial tissue layers restricts the sensitivity and the depth to which fluorescence imaging can be used. Moreover, rapid photobleaching of fluorescent molecules limits their useful lifetime and prevents studies of prolonged duration. Therefore, we have attempted to develop new strategies that may solve some of the limitations of fluorescence imaging in living subjects.Raman spectroscopy can differentiate the spectral fingerprint of many molecules, resulting in very high multiplexing capabilities. Narrow spectral features are easily separated from the broadband autofluorescence, because Raman is a scattering phenomenon as opposed to absorption/emission in fluorescence, and Raman active molecules are more photostable compared with fluorophores, which are rapidly photobleached. Unfortunately, the precise mechanism for photobleaching is not well understood. However, it has been linked to a transition from the excited singlet state to the excited triplet state. Photobleaching is significantly reduced for single molecules adsorbed onto metal particles because of the rapid quenching of excited electrons by the metal surface...

Vibrational spectroscopic mapping and imaging of tissues and cells

“…The strong resonance-enhanced Raman signals from single-walled carbon nanotubes (SWNTs) also allow highly sensitive in vivo detection. As such, both SWNT nanoparticles and silica-coated biotags have been used to image small animals noninvasively using a 785-nm laser (Keren et al 2008;Schipper et al 2008).…”

Plasmons in nanoscale and atomic-scale systems

“…For example, it can be used in multiplex in vivo cancer-targeting agents [62][63][64] because of their sharp spectroscopic fingerprint [63,64], negligible long-term toxicity [64], and potential use in photothermal therapy [66]. In this biomedical application, plasmonic nanostructures are essential for increasing the Raman scattering efficiency, primarily caused by the local electromagnetic field enhancement [11] within the surface plasmon polariton mechanism, as in the case of SEIRA.…”

An x-ray fluorescence imaging system for gold nanoparticle detection

“…Techniques based on optical spectroscopy were found to be satisfactory for the quantification of GNP concentration in vivo (Zaman et al 2007, Qian et al 2008, but their use is limited to shallow sites (∼5 mm deep). Photo-acoustic imaging (Yang et al 2009) and surface-enhanced Raman spectroscopy techniques (Keren et al 2008) have also been used to image GNPs, along with multi-modal imaging techniques (Kircher et al 2012).…”