Red Laser-Induced Fluorescence Energy Transfer in an Immunosystem

Long-wavelength fluorescence lifetime labels

“…Compared to cyanines, the FLTs of squaraines are highly sensitive to the microenvironment. Squaraines have short lifetimes on the order of ten to several hundred picoseconds in protic and polar solvents such as water and long lifetimes at around 1.5-3 ns in aprotic solvents and when covalently or non-covalently bound to biomolecules [95][96][97][98][99]. Recently, several commercially available squaraine labels of the Square and Seta series were developed [100].…”

Long-Wavelength Probes and Labels Based on Cyanines and Squaraines

“…The lower detection limit of a representative squaraine dye in blood was estimated to be half that of a commonly used cyanine label Cy5. Sq635-b and Sq660 were also utilized as donor-acceptor pairs in combination with an HSA/anti-HSA system, in a fluorescence energy transfer (FRET)-based immunoassay [95,96]. One of the shortcomings of the above squaraine labels is the lack of watersolubility, which results in quenching of fluorescence.…”

“…Wolfbeis, Terpetschnig, and co-workers synthesized and investigated the spectral properties of two reactive long-wavelength squaraine labels Sq635-m and Sq635-b (RG-634) with either one or two NHS ester groups [94,95]. For improvement of the water solubility, two sulfonic acid groups were introduced into the heterocyclic ring systems.…”

Fluorescent Dyes Used in Polymer Carriers as Imaging Agents in Anticancer Therapy

“…However, cyanine dyes are far from perfect and have several limitations such as moderate to poor photostability, undesired reactivity with nucleophiles, and propensity to self-quench [41,90]. Therefore, different NIR dyes as squaraines [91] (i.e., bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine) that emit in the "optical window of tissue" at wavelengths above 800 nm in aqueous solution was assessed in PDT therapy. Squaraine derivatives consist of an oxocyclobutenolate core with aromatic or heterocyclic components at both ends of the molecules [92].…”