Cyanines, also referred to as tetramethylindo(di)-carbocyanines are a synthetic dye family belonging to the polymethine group. Although the name derives etymologically from terms for shades of blue, the cyanine family covers the electromagnetic spectrum from near IR to UV. Chemically, cyanines are a conjugated system between two nitrogen atoms; in each resonance structure, exactly one nitrogen atom is oxidized to an iminium. Typically, they form part of a nitrogenous heterocyclic system. The main application for cyanine dyes is in biological labeling. Nevertheless, there is a wide literature on both their synthesis and uses, and cyanines are common in some CD and DVD media. Structure Cyanines have been classified in many ways: Streptocyanines or open chain cyanines: R2N+=CH[CH=CH]n-NR2 (I) Hemicyanines: Aryl=N+=CH[CH=CH]n-NR2 (II) Closed chain cyanines: Aryl=N+=CH[CH=CH]n-N=Aryl (III) Additionally, these classes are recognized: Neutrocyanines: R2N+=CH[CH=CH]n-CN and R2N+=CH[CH=CH]n-CHO Merocyanines including spiropyrans and quinophthalones. Apocyanines where two quaternary nitrogens are joined by a polymethine chain. Both nitrogens may each be independently part of a heteroaromatic moiety, such as pyrrole, imidazole, thiazole, pyridine, quinoline, indole, benzothiazole, etc. History and use in industry Cyanines were first synthesized over a century ago. They were originally used, and still are, to increase the sensitivity range of photographic emulsions, i.e., to increase the range of wavelengths which will form an image on the film, making the film panchromatic. Cyanines are also used in CD-R and DVD-R media. The ones used are mostly green or light blue colour, and are chemically unstable. For that reason, unstabilized cyanine discs are unsuitable for archival CD and DVD use. Recent cyanine discs contain stabilizers, typically a metal atom bonded to the cyanine molecule, that slow the deterioration significantly. These discs are often rated with an archival life of 75 years or more. The other dyes used in CD-Rs are phthalocyanine and azo. Use in biotechnology For applications to biotechnology, special cyanine dyes are synthesized from 2, 3, 5 or 7-methine structures with reactive groups on either one or both of the nitrogen ends so that they can be chemically linked to either nucleic acids or protein molecules. Labeling is done for visualization and quantification purposes. Biological applications include comparative genomic hybridization and gene chips, which are used in transcriptomics, and various studies in proteomics such as RNA localization, molecular interaction studies by fluorescence resonance energy transfer (FRET) and fluorescent immunoassays. Cyanine dyes are available with different modifications such as methyl, ethyl or butyl