Investigation of the photoluminescent properties, scintillation behaviour and toxicological profile of various magnesium tungstate nanoscale motifs

Abstract

We have synthesized several morphologies and crystal structures of MgWO₄ using a one-pot hydrothermal method, producing not only monoclinic stars and large nanoparticles but also triclinic wool balls and sub-10 nm nanoparticles. Herein we describe the importance of reaction parameters in demonstrating morphology control of as-prepared MgWO₄. Moreover, we correlate structure and composition with the resulting photoluminescence and radioluminescence properties. Specifically, triclinic-phase samples yielded a photoluminescence emission of 421 nm, whereas monoclinic-phase materials gave rise to an emission maximum of 515 nm. The corresponding radioluminescence data were characterized by a broad emission peak, located at 500 nm for all samples. Annealing the wool balls and sub-10 nm particles to transform the crystal structure from a triclinic to a monoclinic phase yielded a radioluminescence (RL) emission signal that was two orders of magnitude greater than that of their unannealed counterparts. Finally, to confirm the practical utility of these materials for biomedical applications, a series of sub-10 nm particles, including as-prepared and annealed samples, were functionalized with biocompatible PEG molecules, and subsequently were found to be readily taken up by various cell lines as well as primary cultured hippocampal neurons with low levels of toxicity, thereby highlighting for the first time the potential of this particular class of metal oxides as viable and readily generated platforms for a range of biomedical applications.

Keywords: bioimaging; nanomaterials; oxides; scintillation; synthesis; toxicology.

Figure 1.

SEM images and size distributions of MgWO₄ motifs consisting of (a,b) stars, (c,d) large nanoparticles and (e,f) wool balls are shown. (g,h) TEM images and the associated size distributions are presented for sub-10 nm nanoparticles.

Figure 2.

XRD patterns (with indexed peaks) of MgWO₄ motifs consisting of (from top to bottom): stars, larger nanoparticles, wool balls, and sub-10 nm nanoparticles.

Figure 3.

Probing the effects of reaction time and pH. XRD patterns of 1 : 1 EG : H₂O samples, run under different reaction conditions to produce the desired MgWO₄ motifs.

Figure 4.

PL emission plots for all samples of MgWO₄, (a) excited at 280 nm with an emission maximum located at 515 nm and (b) excited at 340 nm with an emission maximum at 421 nm. The positions of the emission maxima are indicated by the black dashed lines.

Figure 5.

PL decay plots for the unannealed triclinic samples, observed for (a) wool balls and (b) sub-10 nm particles. The excitation was 340 nm and the PL decay was measured at the 421 nm emission peak.

Figure 6.

RL emission plots for samples of MgWO₄ with different morphologies and crystal structures, including (a) stars, (b) larger nanoparticles, (c) sub-10 nm particles (unannealed), (d) sub-10 nm particles (annealed), (e) wool balls (unannealed), and (f) wool balls (annealed). All graphs are displayed in a log10 scale for easier comparison.

Figure 7.

RL time decay plots for samples of MgWO₄ characterized by different morphologies and crystal structures, including (a) stars, (b) larger nanoparticles, (c) sub-10 nm particles (unannealed), (d) sub-10 nm particles (annealed), (e) wool balls (unannealed) and (f) wool balls (annealed).

Figure 8.

Confocal images of COS7 cells (nuclei labelled blue), demonstrating uptake of FITC-labelled particles (in white). (a) Confocal image of COS7 cells incubated with particles for 2 h. (b) Higher magnification image of cells in (a), illustrating a more detailed view. (c) Confocal image of COS7 cells incubated with particles for 12 h. (d) Higher magnification image of cells in (c), highlighting a more detailed view. (e) Confocal image of COS7 cells incubated with particles for 18 h. (f) Higher magnification image of cells in (e), showing a view given in greater detail.

Figure 9.

Confocal images of neurons (nuclei labelled blue), showing the uptake of FITC-labelled particles (in white). (a) Confocal image of neurons incubated with particles for 20 h. Arrows point to the positions of neurons, shown in (b) and (c). (b,c) Higher magnification images (FITC channel only) of selected neurons, providing details of the observed particle uptake in axons/dendrites. (d,e) Additional images of neurons (FITC channel only), demonstrating particle uptake coupled with details of associated axons/dendrites.

Figure 10.

Cell viability assays, taken using MTT in both HEK and COS cell lines, incubated with annealed and non-annealed, sub-10 nm particles. Different concentrations of MgWO₄ particles were probed, including 0 μg ml⁻¹ for the control sample, followed by 2, 20 and 200 μg ml⁻¹ concentrations, respectively, of the relevant oxide particle samples.