Self-assembled vesicle and rod-like aggregates of functionalized perylene diimide: reaction-based near-IR intracellular fluorescent probe for selective detection of palladium

Herein, we report design, synthesis, and self-assembly of perylene diimide (PDI) based near-IR intracellular probe (PS-PDI). PS-PDI molecules undergo aggregation to form self-assembled nanospheres and nanorods morphology in THF : H₂O (1 : 1) and DMSO : H₂O (1 : 9), respectively. The nanospheres have an open hole on surface reminiscent of vesicle structure (with a diameter of internal void in the range of 20-25 nm) whereas lengths of the nanorods extended up to few μm range. The Pd⁰ based depropargylation leads to de-aggregation of these PS-PDI aggregates into smaller spherical aggregates as evidenced by DLS, SEM, and TEM studies. Interestingly, these aggregates of PS-PDI in solution show highly sensitive behavior in the presence of Pd⁰ showing absorbance changes in NIR region (λ_max = 710 nm) and quenching of emission at λ_em 630 nm (DMSO : H₂O, 1 : 9) with a limit of detection = 6.6 × 10^-9 M or at λ_em 564 nm (THF : H₂O, 1 : 1) with limit of detection = 2.1 × 10^-8 M. Time and concentration dependent kinetics profiles of PS-PDI aggregates revealed an impressive rate constant value of 0.4 s^-1 and 0.21 s^-1 (DMSO : H₂O, 1 : 9), respectively in fluorescence and UV-Vis spectroscopy using 2 × 10^-5 M concentration of Pd⁰. PS-PDI undergoes rapid internalization into HeLa cells with low cytotoxicity and was successfully used as an intracellular imaging reagent for Pd⁰ in live HeLa cells. For practical applications, we exploited these nano-aggregates of PS-PDI for estimation of Pd²⁺ in the presence of a NaBH₄-PPh₃ mixture, Pd⁰ in drug and environmental samples, and Pd²⁺ in urine samples with excellent selectivity and sensitivity.