Near-infrared Bimatellic Iridium Complexes For Ratiometric Monitoring Of Viscosity Change In Living Cell

Intracellular viscosity is a critical factor governing the interactions between biomacromolecules as well as transportation of mass and signals.Indeed,it has been demonstrated as a vital indicator for atherosclerosis,diabetes,Alzheimer’s disease,and even cell malignancy.Therefore,it is significant to monitor viscosity change at cellular level.Herein,we synthesised a bimetallic iridium complex viscosity probe,termed C₁₀（[（df-ppy）2Ir（bpy）（CH2）10（bpy）Ir（btph）2]2+）,which has dual emission peaks（521 and 708 nm）for ratiometric detecting.The logarithm of emission intensity ratiometric value（I708 nm / I521 nm）exhibits good nonlinear fitting relationship（R2 = 0.982）against the logarithm of viscosity value,so C₁₀ can be applied to detect cell viscosity change.The maximum Stokes shift of C₁₀ reaches 258 nm（λex = 450 nm,λem = 708 nm）.Its phosphorescence lifetime is long,and the near infrared emission can effectively avoid the interference of biological fluorescence.C₁₀ is not susceptible to solvents,polarity and biomolecules（calf thymus DNA,human serum albumin and 18 amino acids）,so it can be used to detect viscosity specifically.C₁₀ has very low cytotoxicity,and exhibits different luminescence intensity in imaging cancer cells and normal cells,meaning that it can be applied to distinguish between normal cells and cancer cells.In addition,C₁₀ performed well in monitoring of viscosity change caused by etoposide induced cell apoptosis in real time.We synthesised C₅ and C₁₂（[（df-ppy）2Ir（bpy）（CH2）n（bpy）Ir（btph）2]2+,n=5,12）for investigating the effect of different carbon chain length on performance of bimetallic iridium complexes.In the LRET efficiency of two luminophores,C₁₀ is highest,while C₁₂ is worst.The logarithm of emission intensity ratiometric value（I713 nm / I526 nm）exhibits the best nonlinear fitting relationship（R2 = 0.997）against the viscosity value logarithm of C₅.All three complexes have long phosphorescence lifetime,and they were not susceptible to different solvents,polarity changes and biomolecules.The cytotoxicity gradually increased with the extension of carbon chain.In cell imaging,C₅ and C₁₂ also have different luminescence intensity responses in cancer cells and normal cells,and the viscosity distribution in cytoplasm can be seen by ratio images of cancer cells.In addition,C₅ and C₁₂ show the significant increase in luminescence intensity in Hela cells treated with etoposide for 30 min,meaning that the cell viscosity increasing is detected.