Interactions of variously coated gold and silver nanoparticles with a bis(triarylborane) photodyanmic therapy (PDT)-dye; their cellular uptake, cytotoxicity and photo-activity

Abstract

Background and purpose: Diethynylarene-linked bis(triarylborane) tetracations can be used as probes for fluorimetric and Raman sensing of biomacromolecules, as well as promising theragnostic agents. Among them, bis(triarylborane) fluorophore (TAB3), when bonded to Ag nanoparticles (NP), stood out with specific properties such as Raman signal enhancemen of the TAB3 dye in a cuvette. However, TAB3 dye - nanoparticle composites have not been studied in biological systems. For this reason, questions arose as to whether different types of metal nanoparticles (Au or Ag-based) with different coatings (negatively charged citrate or neutral PVP) could be efficiently stained with the TAB3 dye in a cuvette. The aim of this research was to examine Au and Ag nanoparticles of similar size (20-25 nm) with different stabilizers for their cellular uptake, cytotoxicity in the dark and under visible light radiation, to characterize the interactions of nanoparticles with the TAB3 fluorophore, and to study NP-TAB3 composites in cells, evaluate their intracellular staining, as well as possible photoinduced release and biological activity.

Materials and methods: The binding constants of Au- and Ag- based nanoparticles with TAB3 were determined by fluorimetric titrations. The cytotoxic effect of NPs was determined by the survival of A549 cells (MTT assay). Cellular uptake of both NP and NP-TAB3 composites were performed by live cell imaging experiments.

Results: The Au- or Ag-based NPs with different coatings bind to the TAB3 with high affinity. These NPs, as well as TAB3-NP complexes, efficiently enter living human cells, accumulating in cytoplasm with no apparent selectivity for a particular organelle. Even prolonged 3-day treatment with the NPs studied did not show any toxic effect on the cells. Bioimaging studies in cells revealed that the TAB3-NP complex does not intracellularly dissociate; the previously reported photo-bioactivity of TAB3 is completely inhibited by binding to NPs.

Conclusion: Au- and Ag NPs were non-covalently stained by TAB3, irrespective of the different coatings, with similar binding affinities. Emission from TAB3 is strongly quenched by the NPs, but not completely. Experiments on living human cells revealed that neither free NPs, nor their composites with TAB3, were toxic. Bioimaging studies by confocal microscopy revealed that all NPs efficiently enter living cells within 90 min. Colocalization experiment with simultaneous collection of data in the reflection and fluorescence modes demonstrated that the TAB3 dye remained bound to NPs inside cells. Strong irradiation of TAB3-NP inside cells with a 457 nm laser did not yield any damage to the cells, at variance with our previously shown very strong photo-bioactivity of the TAB3 dye alone. Thus, binding of a chromophore to a nanoparticle can inhibit the chromophore’s ability to undergo photo-induced singlet oxygen production, consequently blocking its photo-bioactivity.