Principles of Fluorescence Spectroscopy

658 FLUORESCENCE SENSING
Figure 19.67. Fluorescence imaging of a labeled somatostatin analogue
in a mouse tumor. The images were taken before (middle) and
after (bottom) injection with the labeled peptide. The upper panel
shows a schematic of the instrument. Revised from [252–253].
the extracellular matrix, and are often present at elevated
levels in cancerous tissues and for other disease states. 262–264
The mouse in Figure 19.68 had a gliosarcoma surgically
implanted in the brain.
Figure 19.68. Schematic of an instrument for three-dimensional small
animal imaging. PLS, programmable light switch. The image shows a
sagittal section of a combined MRI and fluorescence image of a
mouse injected with a cathepsin B-sensitive molecular beacon. From
[258].
The labeled peptide was cleaved by a cathepsin in the
tumor, resulting in increased intensity from Cy5.5 (color
spot in Figure 19.68). The spot was superimposed on the
MRI image of the same mouse. It is unlikely that in-vivo
fluorescence imaging will provide the high spatial resolution
available with MRI or CT. However, one can imagine a
wide variety of labeled molecules that will localize in
desired locations and be sensitive to specific enzymes. Fluorescence
in-vivo imaging can add functional or physiological
information to the images obtained using other modalities.
19.14. IMMUNOASSAYS
Immunoassays constitute a large and diverse family of
assays that are based on many of the principles described in
this book. The basic idea is to couple the association of antibody
(Ab) with antigen (Ag) to some other event that yields
an observable spectral change. Various mechanisms are
possible, including energy transfer, anisotropy, delayed lan-
Figure 19.69. Schematic of an enzyme-linked immunosorbent assay (ELISA). AP is alkaline phosphate. Ag is an antigen, and UmP is umbelliferyl
phosphate.