Synthesis and fluorescent characteristics of imidazole–indocyanine green conjugates

Abstract We have successfully synthesized imidazole-dye conjugates by linking imidazole and nitroimidazoleacetic acids to an indocyanine green (ICG) carboxylic acid derivative, using an ethanolamine linker. These dye-conjugates show absorbance peaks at 754–756 nm and fluorescence peaks at 780 nm. The dye-conjugates show a blue shift of 25 nm and 30 nm in the absorption and fluorescence spectra respectively when compared to that of standard cardiogreen. There is no change in absorption and fluorescence spectral profiles between the ICG derivative and imidazole conjugates. The extinction coefficients of new ICG derivative and imidazole conjugates are 1.8 times higher than that of standard ICG. The relative quantum yields of the new compounds are 4.5–5.5 times higher than that of the Sigma–Aldrich’s ICG. The dyes are tested for hypoxia in-vitro with 4T1 luc cell lines and it is found that the cells treated with 2-nitroimidazole ICG show a contrast of fluorescence signal of 2.5–3.0 for the cells under hypoxic to that of cells under normoxic. However pure ICG shows no significant difference between hypoxic and normoxic cells.