Biocompatible NaLn(WO4)2 core–shell nanoplatelets for multimodal MRI contrast, NIR imaging, and high sensitivity infrared luminescent ratiometric thermometry

Abstract

Multifunctional nanoprobes combining magnetic resonance imaging (MRI) contrast as well as near infrared (NIR) imaging and thermometry are demonstrated by using quasi-bidimensional core–multishell nanostructures based on the scheelite-like NaLn(WO₄)₂ host (Ln = trivalent lanthanide). These nanostructures are composed of a NaHo(WO₄)₂ core, plus a first shell of Tm,Yb:NaGd(WO₄)₂, and a second shell of Nd,Yb:NaGd(WO₄)₂. Proton nuclear magnetic relaxation dispersion studies and MRI of water dispersions of nanoprobes, whose quasi-bidimensional geometries promote the interaction of Gd³⁺ with water protons, reveal behaviors evolving from a T₁-weighted MR contrast agent (CA) at 1.5 T to a highly effective T₂-weighted MR CA at ultrahigh magnetic fields of 7 T and above, and even a dual T₁/T₂-weighted CA at a clinical 3 T magnetic field. By NIR excitation (λ_EXC ∼ 803 nm) of Nd³⁺, luminescence-based thermometry was accomplished at wavelengths within the second biological transparency window (II-BW) through ratiometric analysis of ⁴F_3/2 → ⁴I_11/2 Nd³⁺ (λ = 1058 nm) and ²F_5/2 → ²F_7/2 Yb³⁺ (λ = 996 nm) emissions. Under a biologically safe excitation of 0.68 W cm⁻², a chemically stable 2 mg mL⁻¹ nanoprobe water dispersion presents absolute, S_A, and relative, S_R, thermal sensitivities as remarkable as S_A = 480 × 10⁻⁴ K⁻¹, and S_R = 0.89% K⁻¹ at 40 °C (313 K), and temperature resolution δ ≈ 0.1 K. Moreover, through efficient Nd³⁺ → Yb³⁺ → Tm³⁺ and Nd³⁺ → Yb³⁺ → Ho³⁺ energy transfers, NIR photoluminescence from Tm³⁺ at ∼1800 nm and Ho³⁺ at ∼2000 nm facilitates in depth imaging. The low nanoprobe cytotoxicity allows NIR biolabeling during cellular temperature measurement.