1,1-Dioctadecyl-3,3,3,3-tetramethylindocarbocyanine-5,5-diSulphonic acid [DiIC18(3)-DS]1,1-Dioctadecyl-3,3,3,3-tetramethylindocarbocyanine-5,5-diSulphonic acid [DiIC18(3)-DS] ≥95%