Superparamagnetic Nanoparticles with Phosphorescent Complexes as Hybrid Contrast Agents: Integration of MRI and PLIM

Abstract

Two different hybrid nanosystems are prepared by loading highly crystalline, monodisperse magnetite nanocubes (MNCs) with phosphorescent Pt(II) complexes (PtCxs). One involves the encapsulation of the hydrophobic PtCx1 within an amphiphilic comb polymer (MNC@poly(maleic anhydride-alt-1-octadecene) [PMAO]-PtCx1), whereas the other involves the direct binding of the hydrophilic PtCx2 to the surface of the MNC mediated by a ligand-exchange procedure (MNC@OH-PtCx2). Both systems are evaluated as potential candidates for multimodal imaging in magnetic resonance imaging (MRI) and photoluminescence lifetime imaging micro(spectro)scopy (PLIM). PLIM measurements on agarose phantoms demonstrate significantly longer excited-state lifetimes compared to the short-lived autofluorescence of biological background. Additionally, both nanosystems perform as effective MRI contrast agents (CAs): the r ₂* values are 3-4 times higher than for the commercial CA ferucarbotran. MNC@PMAO-PtCx1 particles also cause significant increases in r ₂. While the ligand exchange procedure efficiently anchors PtCxs to the MNC surface, the polymeric encapsulation ensures higher colloidal stability, contributing to differences in PLIM and MRI outcomes. In these results, the successful integration of two complementary noninvasive imaging modalities within a single nanosystem is confirmed, serving as the impetus for further investigation of such systems as advanced multimodal-multiscale imaging agents with dual orthogonal readouts.

Keywords: magnetic nanohybrids; magnetic resonance imaging (MRI); nanoparticles; phosphorescent Pt(II) complexes; photoluminescence lifetime imaging micro(spectro)scopy (PLIM); time-resolved microscopy; time-resolved spectroscopy.

Figure 1

a) Schematic synthetic procedure toward monodisperse magnetite nanocubes (MNC)@poly(maleic anhydride‐alt‐1‐octadecene) (PMAO)–PtCx1. b) Scanning transmission electron microscopy high‐angle annular dark‐field (STEM‐HAADF) images and c) energy‐dispersive X‐ray spectroscopy (EDS) elemental mapping of Fe and Pt for MNC@PMAO–PtCx1. d) Size distribution of MNCs (black) and MNC@PMAO–PtCx1 (orange) obtained by DLS. e) Hysteresis loops of MNC@PMAO–PtCx1. f) Normalized excitation spectrum of PtCx1 (black line) in dichloromethane (DCM) at RT and photoluminescence spectra in DCM at RT for PtCx1 (green solid line), for MNC@PMAO–PtCx1 in sodium borate buffer (SBB) (orange line) and for PtCx1 in the amorphous solid state (green dots). Inset: photograph of an MNC@PMAO–PtCx1 suspension in a quartz cuvette irradiated by light with a wavelength of 366 nm.

Figure 2

a) Schematic synthetic procedure toward MNC@OH–PtCx2. b) STEM‐HAADF images and c) EDS elemental mapping of Fe and Pt for MNC@OH–PtCx2. d) Size distribution of MNCs (black) and MNC@OH–PtCx2 (orange) obtained by DLS. e) Hysteresis loops of MNC@OH–PtCx2. f) Normalized excitation spectra of PtCx2 (black line) in DCM at RT and photoluminescence spectra in DCM at RT for PtCx2 (green solid line), for MNC@OH–PtCx2 in SBB buffer (orange line) and for PtCx2 in the amorphous solid state (green dots). Inset: photograph of an MNC@OH–PtCx2 suspension in a quartz cuvette irradiated by light with a wavelength of 366 nm.

Figure 3

a) Bright‐field images, b) photoluminescence microscopy image, and c) photoluminescence lifetime map of MNC@PMAO–PtCx1 in 1% agarose. d) Phase‐contrast image and e) photoluminescence microscopy image of MNC@OH–PtCx2 in 1% agarose. f) Normalized photoluminescence spectra of the particles in the phantom gels (λ _ex = 375 nm) measured using a photoluminescence spectrometer coupled to a confocal microscope (λ _ex = 375 nm, low‐pass (LP) cut‐off filter 514 LP).

Figure 4

a) Maps of relaxation times T ₁, T ₂, and T ₂*. Scale bars: 1 cm. The displayed maps are partly stitched together from multiple scans but all panels have been adjusted to the same windowing. b) T ₁ relaxation times. c) T ₂ and T ₂* relaxation times. d–f) Relaxivities r ₁, r ₂, and r ₂*, respectively. Relaxivities are represented by the slopes of the linear regressions. The exact [Fe] values in (a–c) correspond to those of Resovist (see Table S2 for further details, Supporting Information). Values are means ± standard deviation.