Phosphorescent Pt(ii) complexes spatially arrayed in micellar polymeric nanoparticles providing dual readout for multimodal imaging

Abstract

In this paper we report phosphorescent Pt(ii) complexes as monomers which can be directly incorporated into growing polymers. Due to the amphiphilic nature of the polymers they can self-assemble into micellar nanoparticles, where the phosphorescent Pt(ii) complexes can arrange selectively in the core or shell of the nanoparticles. The complexes enable dual orthogonal imaging, made possible by the heavy metal, which enhances the contrast for these micelles in electron microscopy and facilitates spin-orbit coupling that turns on microsecond lifetime luminescence.

Figure 1.

Synthetic route towards polymeric NPs and their characterization. Left: The particles were prepared by assembly of copolymer amphiphiles incorporating the Pt(II)-containing luminophore. Polymerization of M_am as the hydrophobic block, followed by M_OEG as the hydrophilic block and the addition as M_Pt as the last monomer, yielded polymer P_shell (top). Polymerization of M_OEG as a first hydrophilic block, followed by M_am as the hydrophobic block and the addition as M_Pt as the last monomer, yielded polymer P_core (bottom). Dialysis into 10% PBS formed NP_shell or NP_core (top and bottom, respectively). Center: dry state and unstained TEM images of NP_shell (top) and NP_core (bottom). Both micelle populations show a diameter of approximately 30 nm, confirmed by DLS measurements as well (insets). Right: Energy Filtered TEM (EFTEM) analysis of the NPs using a three-window-method of the Pt-O-edge shows a fairly homogenous distribution of Pt atoms in NP_core while NP_shell shows an enrichment of Pt at preferentially in the shell.

Figure 2.

Normalized excitation (dashed lines, λ_em = 520 nm) and emission (solid lines, λ_ex = 320 nm) spectra of NP_shell (top) and NP_core (bottom) in diluted PBS at room temperature (0.5 mg/mL). The dotted excitation spectrum was obtained by monitoring the emission at 600 nm, the ellipses highlight the bands corresponding to the excitation into ¹MMLCT and emission from ³MMLCT states, whereas the arrows indicate the wavelengths at which the given amplitude-weighted average lifetimes have been measured (see also ESI).

Figure 3.

Top: TEM images of HeLa cells pretreated with PBS (right) or NP_shell (left) at a concentration of 0.1 mg/mL for 30 min and imaged at different magnifications. NP_shell attached to the cell membrane or in the intracellular space are shown with yellow arrows. Bottom: Two-photon laser scanning microscopy (not time-gated) of HeLa cells pretreated with PBS (right) or NP_shell (left) at a concentration of 0.1 mg/mL polymer in PBS for 60–75 min. Cells were imaged in modified Tyrode’s solution under two-photon excitation at 750 nm and fluorescence was collected through a 490–560 nm bandpass filter. NP_shell-pretreated cells show bright luminescent spots, indicated by yellow arrows.