Polyethylenimine-Coated Ultrasmall Holmium Oxide Nanoparticles: Synthesis, Characterization, Cytotoxicities, and Water Proton Spin Relaxivities

Abstract

Water proton spin relaxivities, colloidal stability, and biocompatibility of nanoparticle magnetic resonance imaging (MRI) contrast agents depend on surface-coating ligands. In this study, hydrophilic and biocompatible polyethylenimines (PEIs) of different sizes (M_n = 1200 and 60,000 amu) were used as surface-coating ligands for ultrasmall holmium oxide (Ho₂O₃) nanoparticles. The synthesized PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles, with an average particle diameter of 2.05 and 1.90 nm, respectively, demonstrated low cellular cytotoxicities, good colloidal stability, and appreciable transverse water proton spin relaxivities (r₂) of 13.1 and 9.9 s^-1mM^-1, respectively, in a 3.0 T MR field with negligible longitudinal water proton spin relaxivities (r₁) (i.e., 0.1 s^-1mM^-1) for both samples. Consequently, for both samples, the dose-dependent contrast changes in the longitudinal (R₁) and transverse (R₂) relaxation rate map images were negligible and appreciable, respectively, indicating their potential as efficient transverse T₂ MRI contrast agents in vitro.

Keywords: Ho2O3; cytotoxicity; polyethylenimine coating; relaxivity; ultrasmall nanoparticle.

Figure 1

One-pot polyol synthesis of ultrasmall Ho₂O₃ nanoparticles coated with PEI1200 and PEI60000 (M_n = 1200 and 60,000 amu, respectively).

Figure 2

HRTEM images of (a) PEI1200- and (b) PEI60000-coated ultrasmall Ho₂O₃ nanoparticles. Dotted circles indicate individual nanoparticles. Large dotted circles in (b) indicate nanoparticles grafted together with one PEI60000. (c) Particle diameter distributions and log-normal function fits of PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles. EDS spectra of (d) PEI1200- and (e) PEI60000-coated ultrasmall Ho₂O₃ nanoparticles.

Figure 2

HRTEM images of (a) PEI1200- and (b) PEI60000-coated ultrasmall Ho₂O₃ nanoparticles. Dotted circles indicate individual nanoparticles. Large dotted circles in (b) indicate nanoparticles grafted together with one PEI60000. (c) Particle diameter distributions and log-normal function fits of PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles. EDS spectra of (d) PEI1200- and (e) PEI60000-coated ultrasmall Ho₂O₃ nanoparticles.

Figure 3

(a) Images of PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles in aqueous media (vials on the left and right-side, respectively) with a concentration of ~30 mM Ho. (b) DLS patterns of PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles in aqueous media with log-normal function fits to the observed DLS patterns to estimate d_avg. (c) The zeta potential curves of PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles in aqueous media. (d) Tyndall effect (or light scattering by the nanoparticle colloids) of PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles in aqueous media (samples on the middle and right-side, respectively), establishing the colloidal dispersion of PEI-coated nanoparticles in aqueous media; no such light scattering is observed in triple-distilled water (sample on the left). Arrows show laser light scattering by nanoparticle colloids.

Figure 4

XRD patterns before (i.e., as-prepared) and after TGA of (a) PEI1200- and (b) PEI60000-coated ultrasmall Ho₂O₃ nanoparticles. The (222), (400), (440), and (622) assignments on the XRD peaks after TGA are the (hkl) Miller indices of cubic Ho₂O₃. All peaks after TGA are assigned with the (hkl) Miller indices of cubic Ho₂O₃.

Figure 5

(a) FT-IR absorption spectra of PEI1200 and PEI60000 and PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles. Subscripts “s” and “b” indicate stretching and bending vibrations, respectively. (b) TGA curves of PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles. (c) PEI-coating structure: each nanoparticle is grafted with approximately fifteen PEI1200 polymers (left), multiple hard acid–hard base type of bondings (middle), and approximately two nanoparticles grafted with one PEI60000 polymer (left).

Figure 6

(a) In vitro cell viabilities after normalization with untreated control cells (0.0 mM Ho). 10% (v/v) DMSO was used as a positive control. (b) Optical microscopy images of the DU145 cells 48 h after incubation with PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles. (c) Optical microscopy images of the DU145 cells 48 h after incubation with 10% (v/v) DMSO and 28 μM PEI1200.

Figure 7

Mass-corrected M–H curves of the PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles at 300 K obtained using the net masses of Ho₂O₃ nanoparticles without PEI, which were estimated from the TGA curves.

Figure 8

(a) Plots of inverse relaxation times 1/T₁ and 1/T₂ as a function of Ho concentration for PEI1200- and PEI60000-coated ultrasmall Ho₂O₃ nanoparticles in aqueous media at 3.0 T and 22 °C; the slopes yield r₁ and r₂ values, respectively. (b) R₁ and R₂ map images.