Cryo-transmission electron microscopy reveals mesoscopic H- and J-aggregates of near infrared cyanine dyes

Cryogenic transmission electron microscopy (cryo-TEM) studies were performed to probe the morphological structure of aggregates formed by the two near-infrared dyes IR-140 and Indocyanine Green (ICG) in water. Both dyes formed sheet-like monolayered J-aggregates of mesoscopic size. This structure is consistent with a stacking geometry known for other cyanine dye J-aggregates. H-aggregates, which dominate in freshly prepared aqueous ICG solutions, proved to be linear aggregates of mesoscopic size that are characterized by a broad size distribution. Besides small oligomer particles up to tens of nanometers long thread-like aggregates were detected.