Aggregation of cyanine dyes in lipid environment

Cyanine dyes, a wide class of fluorescent probes with unique photophysical properties, have found numerous application in different areas as optical imaging agents, active ingredients in semiconducting materials, laser dyes, photographic sensitizers, photopolymerization initiators, stains and fluorescent labels, to name only a few. These compounds are of particular interest for biomedical research and diagnostics due to their favorable spectral characteristics, namely, longwave absorption and emission, large extinction coefficient, high fluorescence quantum yield, etc. Cyanine dyes are photosensitive compounds possessing two quaternized, nitrogen-containing, heterocyclic structures, which are linked through a polymethine bridge [1]. Due to dual hydrophobic and cationic nature of these dyes, which leads to strong interaction with polyanionic DNA duplex, cyanines are mainly used as bright labels for nucleic acids in microarray-based expression analysis, DNA sequencing and DNA intercalation bioanalytical assays [2–14]. It has been demonstrated that mono-, tri- and pentamethine cyanine dyes are suitable for quantitative detection of amyloid formation and protein labeling [5]. Fluorescence intensity of cyanine dyes is greatly increased upon their binding to nucleic acids or proteins as a result of the rigidization of the fluorophores [2–5]. Furthermore, the utility of near infrared cyanine dyes demonstrates unique hydrophobic characteristics in aqueous medium that were exploited for target-specific pH probing [6]. Moreover, it is known that the central polymethine chain of cyanine dyes can be cleaved by reactive oxygen and nitrogen species, which gives the impetus for cyanine use for in vivo sensing of oxidative stress and monitoring the dynamics of redox cycles in living cells [7]. In addition, cyanine dyes showed unique properties as optical imaging probes for cancer labeling [8].