Conjugated Pt(II) Complexes as Luminescence-Switch-On Reporters Addressing the Microenvironment of Bacterial Biofilms

In this work, the synthesis, structural and photo-physical characterization of six phosphorescent H2O-soluble Pt(II) complexes are reported while addressing their emission maxima, photoluminescence quantum yields (Phi(L)), lifetimes (tau), aggregation tendency, and microenvironment sensitivity as a function of the substitution pattern on the main tridentate luminophore. Different ancillary ligands, namely, a trisulfonated phosphane and maltohexaose-conjugated pyridines (with or without amide bridges), were introduced and evaluated for the realization of switch-on-photoluminescent labels reporting on the microenvironment sensed in biofilms of Gram(+) and Gram(-) models, namely, Staphylococcus aureus and Escherichia coli. With the aid of confocal luminescence micro(spectro)scopy, we observed that selected complexes specifically interact with the biofilms while leaving planktonic cells unlabeled. By using photoluminescence lifetime imaging microscopy, excited-state lifetimes within S. aureus biofilms were measured. The photoluminescence intensities were drastically boosted, and the excited state lifetimes were significantly prolonged upon binding to the viscous biofilm matrix, mainly due to the suppression of radiationless deactivation pathways upon shielding from physical quenching processes, such as interactions with solvent molecules and O-3(2). The best performances were attained for non-aggregating complexes with maltohexaose targeting units and without amide bridges. Notably, in the absence of the maltodextrin, a hydrophobic adamantyl moiety suffices to attain a sizeable labeling capacity. Moreover, photoluminescence studies showed that selected complexes can also effectively interact with E. coli biofilms, where the bacterial cells are able to partially uptake the maltodextrin-based agents. In summary, the herein introduced concepts enable the development of specific biofilm reporters providing spatial resolution as well as lifetime- and spectrum-based readouts. Considering that most theragnostic agents reported so far mainly address metabolically active bacteria at the surface of biofilms but without reaching cells deeply immersed in the matrix, a new platform with a clear structure-property correlation is provided for the early detection of such bacterial arrays.

Automated summary

This work reports the synthesis, structural and photo-physical characterization of six phosphorescent H2O-soluble Pt(II) complexes with different ancillary ligands, evaluating their potential as switch-on-photoluminescent labels for specific biofilm sensing. The study reveals that selected complexes can interact specifically with biofilms, offering a novel platform for early detection of bacterial arrays in complex environments.