Nanoparticles for Bioimaging: Analytical Characterization and Measurements - Nanotechnology for Biomedical Imaging and Diagnostics - page 172

25,00,000

20,00,000

15,00,000

10,00,000

500,000

0

400 600 800 1000 1200 700 750 800 850 900 950 1000

Wavelength (nm) Wavelength (nm)

ICG-R2658P grat 1200/500
t = 0.01s
ICG-R928P grat 1200/500
t = 0.01s

R928P R2658P

figure 6.20 Performance of two commonly used NIR PMTs: R928P and R2658P. left:
no signal is seen with R928P beyond 850 nm, while R2658P remains sensitive. Spectra are
normalized and shifted vertically for clarity. Right: NIR emission of ICg recorded with R928P
and R2658P under the same conditions.

800 nm, and beyond 900 nm, the quantum efficiency approaches zero. To further
improve the sensitivity in NIR, Hamamatsu introduced an indium gallium arsenide
(Ingaas) R2658(P) PMT in the late 1980s [45]. although it has lower quantum
efficiency in the visible range, this detector is far more sensitive in NIR (up to
1010 nm), which allows one to measure fluorescence in a previously inaccessible
region. alternatively, germanium-based (ge) NIR detectors can be used [46, 47].
Since ge detectors exhibit a higher level of thermally induced noise than an Ingaas
detector, this detector needs to be enclosed in a dewar with liquid nitrogen.

6.7.1.6 Fiber Optics Selection of fiber optics is important for a growing segment
of fiber optically based spectrophotometers and peripheral devices such as inte-
grating spheres and microscopes. fiber-optic material (plastic, glass, or plastic-glass
silica), dopings, the presence of silanol (≡Si-oH) hydroxyl groups, and cladding and
coating material can significantly attenuate the amount of light and induce autofluo-
rescence and Raman peaks [48]. High-purity glass fibers with low concentrations of
silanols normally exhibit good transmission in NIR [49]. liquid lightguides filled
with special NIR transparent liquids is another option.

6.7.2 Brightness of Nanoparticles

optical characterization of nanoparticles starts from measurements of the brightness
of a probe. The brightness of a probe is defined as a product of two major parameters:
molar absorptivity ε and fluorescence quantum yield Ф . for a nanoparticle labeled
with multiple fluorophores, the number also known as a degree of labeling, N f , is
incorporated into the equation

B =×× εΦ f N