New insights on the blue intrinsic fluorescence of oxidized PAMAM dendrimers considering their use as bionanomaterials
Like other bionanomaterials, dendrimers are usually labelled with fluorescent compounds in order to be optically detected within cells. However, this process can interfere with their biological properties, so that it is crucial to find other solutions for their traceability. Here, the strategy of enhancing the blue intrinsic fluorescence of amine-terminated poly(amidoamine) (PAMAM) dendrimers was followed through an oxidative treatment with ammonium persulfate (APS). The effects of dendrimer’s generation (G3, G4, and G5) and of pH on the spectroscopic behavior of both pristine and APS-treated PAMAM dendrimers were studied in aqueous solution. Overall, results pointed out that there are at least two types of emitting electron-rich hetero-atomic sub-luminophores (HASLs) confined within the dendrimer scaffold that have very close maximum emission wavelengths and whose emission properties strongly depends on pH. The APS treatment significantly enhanced the fluorescence intensity by leading to the protonation of the dendrimer’s interior. However, fluorescence intensity was not only dependent on the number of HASLs in the dendrimer scaffold (i. e., on dendrimer generation), but also on the rigidification suffered by the dendrimer due to the acidic environment (at low pH values, APS-treated G4 was indeed the most emissive species). Moreover, photoluminescence studies with lyophilized samples were done too, that confirmed the coexistence of more than one type of HASLs emitting in the dendrimer structure. The APS treatment affected these HASLs to a different extent. Time-resolved fluorescence experiments always showed higher HASLs’ average lifetimes for APS-treated dendrimers than for pristine ones, in accordance with fluorescence intensity results. On the other hand, the fraction and lifetimes of HASLs in APS-treated dendrimers were similar in solution and the lyophilized form. This behaviour was different for the pristine dendrimers that presented increased luminescence upon aggregation. Finally, the highly emissive oxidized dendrimers were shown not only to be much less cytotoxic and hemotoxic than pristine dendrimers but also detectable inside cells upon excitation with UV light.