Acid-induced degradation of widely used NIR dye DiR causes hypsochromic shift in fluorescence properties.

DiR (1,1'-dioctadecyl-3,3,3',3'-tetramethylindotricarbocyanine iodide) is one of the most widely used near-infrared dyes for in-vivo imaging due to its favorable optical properties. So far, chemical stability has been taken for granted by most investigators. However, in a recently published imaging study, we found that DiR can exert a hypsochromic shift in fluorescence in-vivo, potentially induced by low pH.