Imaging probes (for references)

• Conventional organic fluorophores: emit light of a longer wavelength than they are
illuminated with. Near-infrared light is most efficiently transmitted through tissue,
and fluorophores in this range are best suited to intravital imaging (for example
Cy5.5 and Cy7).
• Fluorescent proteins: are genetically encoded and have intrinsic fluorescent
properties. Green fluorescent protein (GFP) was the first to be widely used, and
various mutants with enhanced or altered spectral properties remain popular. Red
fluorescent proteins have recently been developed, and may supercede GFP
owing to the more efficient transmission of red light through tissue.
• Luminescent proteins: are genetically encoded, and in the presence of an
appropriate injected substrate catalyse a light-generating reaction. Both firefly
and Renilla luciferase have been used, but simultaneous use in vivo is
problematic.
• Intrinsic signals: Numerous endogenous molecules have fluorescent properties, and
reflectance imaging can be used to determine information about the structure of
tissues. Second harmonic generation by fibrous collagen can also be imaged.
• Quantum dots: are highly efficient inorganic fluorophores that do not bleach, and a
large difference in their excitation and emission reduces problems with tissue
autofluorescence. Quantum dots can also be detected in electron microscopy,
thereby facilitating correlative analysis of light and electron microscopy. Quantum
dots linked to luminescent proteins will fluoresce in the absence of an external
light source.
• Affinity probes: Fluorescently-labelled antibodies can be used to probe the
localization of tumour antigens. Alternatively, molecules that bind to biomolecules
of interest can be labelled with fluorophores and used for imaging.
Examples include hydroxyapatite for bone imaging and RGD peptides for
integrin distribution.
• Protein–protein interaction probes: Fluorescence resonance energy transfer can be
used to investigate the distance between two fluorophores, and hence protein–
protein interactions and/or protein conformation. GFP genetically split into two
can be fused to two different proteins, so that when the proteins interact the two
parts of GFP are brought together and the fluorescent properties of GFP are
recreated. A related strategy uses light generated by bioluminescence to excite a
local fluorophore.

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There is a special molecule: bimodal contrast agent Gadolinium-RhodamIne Dextran [GRID] which could be detectable by both magnetic resonance imaging (MRI) and fluorescent microscopy and it’s utilizing in the field of pre-labelling neural stem cells