Layered rare-earth hydroxides as multi-modal medical imaging probes: particle size optimisation and compositional exploration

Recently, layered rare-earth hydroxides (LRHs) have received growing attention in the field of theranostics. We have previously reported the hydrothermal synthesis of layered terbium hydroxide (LTbH), which exhibited high biocompatibility, reversible uptake of a range of model drugs, and release-sensitive phosphorescence. Despite these favourable properties, LTbH particles produced by the reported method suffered from poor size-uniformity (670 ± 564 nm), and are thus not suitable for therapeutic applications. To ameliorate this issue, we first derive an optimised hydrothermal synthesis method to generate LTbH particles with a high degree of homogeneity and reproducibility, within a size range appropriate for in vivo applications (152 ± 59 nm, n = 6). Subsequently, we apply this optimised method to synthesise a selected range of LRH materials (R = Pr, Nd, Gd, Dy, Er, Yb), four of which produced particles with an average size under 200 nm (Pr, Nd, Gd, and Dy) without the need for further optimisation. Finally, we incorporate Gd and Tb into LRHs in varying molar ratios (1 : 3, 1 : 1, and 3 : 1) and assess the combined magnetic relaxivity and phosphorescence properties of the resultant LRH materials. The lead formulation, LGd1.41Tb0.59H, was demonstrated to significantly shorten the T2 relaxation time of water (r2 = 52.06 mM−1 s−1), in addition to exhibiting a strong phosphorescence signal (over twice that of the other LRH formulations, including previously reported LTbH), therefore holding great promise as a potential multi-modal medical imaging probe.


Particle size optimisation
Table S1 Summary of reaction parameters generated using the JMP Pro 14 software, and precursor solution volumes for all samples.Sample names are designated as T-X-Y-Z, where T = temperature during synthesis, X = incubation time, and Y = fill volume, and Z = replicate number (if applicable).Application of optimised synthetic method to other LRHs (R = Pr, Nd, Gd, Dy, Er, Yb)

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Figure S2 Observed X-ray diffraction pattern for phase obtained at 200 °C (above), and simulated pattern for Pnma Tb(OH) 2 Cl based on the prototype Lu(OH) 2 Cl (below).Observed and simulated patterns are for Mo Kα1 radiation.The phase obtained at 200 °C can be indexed using the primitive orthorhombic cell a = 12.6008(6) Å, b = 3.6638(2) Å, c = 6.2506(3)Å.This is consistent with the Pnma phases Ln(OH) 2 Cl (Ln = Tm, Yb, Lu) reported previously. 2 This previous report identified a monoclinic form of Ln(OH) 2 Cl, but it is clear that a second form exists, orthorhombic Ln(OH) 2 Cl.

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Figure S7 Thermograms of mixed LRH materials with varying composition (R = Gd/Tb).

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Figure S8 Representative SEM image used for EDXS analysis (sample LGd 1.41 Tb 0.59 H-Cl).Gadolinium and terbium atoms detected are shown in red and green respectively; yellow indicates sites which are either Gd or Tb.

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Figure S9 Observed (crosses), calculated (upper line), and difference (lower line) profiles for Rietveld refinement ofLGdH-Cl.Tick marks show the positions of allowed reflections.

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Figure S10 Observed (crosses), calculated (upper line), and difference (lower line) profiles for Rietveld refinement of LTbH-Cl.Tick marks show the positions of allowed reflections.

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Figure S11 Layout of MR imaging phantom consisting of a dilution series ofLGd 1.41 Tb 0.59 H in water in a section of a well plate (each well is 300 µl in volume, filled with 300 µl of suspension at the indicated concentration (in mg/ml).Note: suspensions 8, 9, and 10 were excluded from analysis due to low signal change relative to water.

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Figure S12 Representative T 1 -weighted images of LGd 1.41 Tb 0.59 H phantom (shown in Figure S11) at various inversion times.

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Figure S13 Representative T 2 -weighted images of LGd 1.41 Tb 0.59 H phantom (shown in Figure S11) at various echo times.

Table S2
Summary of average particle sizes from SEM images, hydrodynamic diameters, and polydispersity indexes of materials made under different conditions.Note: none of the above were measured for samples synthesised at 200 °C, as material morphology was not suitable for theranostic applications.Hydrodynamic diameter and polydispersity measurements were conducted only for size-optimised samples.

Table S3
Statistical analysis of the particle sizes measured by SEM.One-way analysis of variance (ANOVA) was used to determine the statistical significance (at the p = 0.05 level) in the difference of means for particle size measurements.A Fisher's LSD (if sample means had unequal variance) post hoc test was used to determine which sample means differed significantly.Samples which share a group number are not statistically different.* Data from previously published work. 1 Figure S4 XRD patterns for the replicates of LTbH synthesised at the optimised conditions (90 °C, 4-hour incubation time, 18 ml fill volume).The LTbH system is indexed to the orthorhombic Pca2 1 space group.

Table S4 A
summary of (010) reflection positions and refined unit cell parameters of LRH materials.(R=Pr, Nd, Gd,  Dy, Er, Yb).Note: LYbH was not phase pure, data shown is for most abundant phase.

Table S6
Summary of observed (and calculated)values for elemental analysis and TGA data.