Supporting effect of polyethylenimine on hexarhenium hydroxo cluster complex for cellular imaging applications

Abstract

Abstract The present work introduces an insight into pH- and concentration-dependent binding of branched polyethylenimine (PEI) with hexarhenium hydroxo cluster complex [{Re 6 S 8 }(OH) 6 ] 4− aimed at enhancing the stability, luminescence intensity and cellular uptake of the cluster complex in aqueous solutions at pH 7.4. Luminescence and conductivity measurements revealed pH 7.4 and 1:1 of PEI:cluster molar ratio as specific conditions ensuring multiple electrostatic interactions of protonated amino groups of PEI with the tetra-anionic cluster units. These interactions resulted in a significant supporting effect of PEI on stability and luminescence of the cluster complex, while no effect of the cluster complex on the acid-base and aggregation properties of PEI was detected from dynamic light scattering and 1 H NMR spectroscopy measurements. The cluster luminescence remained without degradation for at least two weeks due to the supporting effect of PEI, which can facilitate the application of the cluster complex in bioimaging. The cellular uptake of [{Re 6 S 8 }(OH) 6 ] 4− and [{Re 6 S 8 }(OH) 6 ] 4− @PEI was investigated. Confocal microscopic studies showed that the pristine cluster complex [{Re 6 S 8 }(OH) 6 ] 4− did not enter cells (the human larynx carcinoma cell line (Hep-2)), whereas the supramolecular assembly [{Re 6 S 8 }(OH) 6 ] 4− @PEI internalized into the cells.