Surface-Enhanced Raman Scattering-Based Bioimaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 323

hundred micrometers in living tissue) even in niR range due to highly diffusive
nature of living tissue. a significant improvement on the penetration depth achiev-
able in Raman spectroscopy of turbid media was recently accomplished by uti-
lizing the diffuse component of light. The diffuse component is of highest
importance to the spectroscopic investigations of deep layers of turbid media due
to its deepest penetration among all components of light (see fig. 11.3a). in tissue,
the diffuse component penetrates to the depths of up to several centimeters as
opposed to the hundreds micrometers or few millimeters of the ballistic and snake
components in the niR region of the spectrum. Two principal noninvasive con-
cepts, temporal and spatial approach, were developed according to the photon
migration concept [25].
The investigation of stratified layers within a turbid sample using temporal
approach is based on impulsive Raman excitation and fast temporal gating of the
Raman signal. This concept relies on the simple fact that photons emanating from
deeper layers of the measured turbid medium have to diffusely migrate larger
distances through than the photons generated at shallower depths. Temporal resolved
Raman light was first used to image an object in a turbid medium by Wu et al . [27].
Later, Matousek et al . demonstrated that the Raman spectrum of a deeply buried
layer in a turbid sample could be fully recovered [28].
an alternate method known as spatially offset Raman spectroscopy (SoRS)
involves spatially offsetting the collection of Raman scattered light from the excita-
tion regions (i.e., laser incident regions), which enables the collection of diffuse com-
ponent of scattered light as opposed to conventional ballistic and snake photons in
backscattering geometry, thus significantly enhancing the depth of probing. Recently,
this method has been integrated with SERS resulting in spatially offset SERS
(SESoRS), which enables significantly deeper probing into mammalian tissue [26,
29]. Multiplexed Raman signals emanating from SERS probes buried 25 mm deep
inside a porcine tissue were successfully collected and analyzed by offsetting the
excitation and collection by 180°, that is, transmission Raman spectroscopy, a special
case of SoRS.
11.3.3.3 Rapid Line Raman Mapping The conventional Raman point mapping
provides information of complete spectra of each pixel in the scanning area as the
focused beam is rastered across the sample area. The point scanning technique is
time-consuming, especially if longer exposure time is required for low concentration
samples. Raman line mapping dramatically decreases the time to record an image
by line focusing the laser and recording spectra across the full area of the ccD.
The high efficiency of line mapping was obtained by compromising with reduced
resolution along the laser line, which can be traded against Y binning and scanning
speed. Moreover, the power density on any one area is reduced by a factor of 40-60
times due to the defocus of laser and rapid scanning, compared to that in point
mapping. Surface-enhanced resonance Raman scattering (SERRS) mapping has
been achieved on bone marrow-derived immune cells treated with plasmonic
nanoparticles by multiple-wavelength line scanning [30]. Thanks to the high
efficiency of the SERRS method, line mapping was reported to collect up to 1000