Surface-Enhanced Raman Scattering-Based Bioimaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 319

2. The full width at half maximum (fWHM) of Raman bands (1-2 nm) are nearly
20-30 times narrower compared to the emission bands of QDs (40-60 nm),
making spectral multiplexing an order of magnitude higher for SERS probes
compared to that attainable with QDs [19].
3. The excitation and emission of SERS probes can be easily tuned to the niR
therapeutic window (650-900 nm), where the endogenous absorption coeffi-
cient of tissue is nearly two orders magnitude lower compared to that in the
visible parts of electromagnetic (EM) spectrum [19]. furthermore, absence of
interference from water and autofluorescence of the tissues in niR is yet
another significant advantage of SERS compared to photoluminescence-based
optical imaging techniques.
4. SERS probes offer remarkable photostability compared to organic fluoro-
phores and QDs, which suffer from either photobleaching or blinking. SERS
probes, typically made of gold, are significantly less cytotoxic compared to
niR emitting QDs comprised of cdSe and cdTe.

11.3 raman inStrumentation for Bioimaging

11.3.1 overview of the instrumentation

Since the first experimental observation of Raman scattering by Sir chandrasekhara
Venkata Raman using sunlight as the excitation source and the eye as detector, there
have been remarkable advancements in Raman instrumentation to current handheld
spectrometers and advanced confocal Raman microscopes combined with comple-
mentary microscopic techniques such as atomic force microscope [19-21]. a highly
simplified schematic of micro-Raman spectrometer is shown in figure 11.1 [10]. The
design is based on a confocal optical microscope equipped with an objective with a
large numerical aperture (na) (as high as 1.4) and a pinhole in the intermediate focal
plane. an intense light source such as a laser with a narrow line width (~0.2 nm)

Video
monitor

Spectrograph

Laser

Filter

CCD

Microscope

Sample

Computer

Motorized XYZ stage

figure 11.1 Simplified schematic illustration of a setup of micro-Raman spectrometer.
(Reprinted with permission from Ref. [10]. © Elsevier.)