Ln3+-doped nanoparticles for upconversion and magnetic resonance imaging: some critical notes on recent progress and some aspects to be considered

Abstract

In this feature article we will critically discuss the synthesis and characterisation aspects of Ln³⁺-doped nanoparticles (NPs) that show upconversion, upon 980 nm excitation. Upconversion is a non-linear process that converts two or more low-energy photons, often near-infrared photons, into one of higher energy, e.g. blue and 800 nm from Tm³⁺ and green and red from Er³⁺ or Ho³⁺. Nearly all researchers use the absorption of 980 nm light by Yb³⁺ as the sensitiser for the co-doped emissive Ln³⁺ ions. The focus will be on LnF₃ and MLnF₄ (M = alkali metal) as the host matrix, because most progress has been made with these. In particular we will argue that a detailed understanding of how the dopant ions and the host Ln³⁺ ions are distributed (in the core) and how (doped) shell growth occurs is not well understood. Moreover, their use as optical and magnetic resonance imaging contrast agents will be discussed. We will argue that deep-tissue imaging beyond 600 μm with retention of optical resolution, i.e. to see fine structure such as blood capillaries in brain tissues, has not yet been achieved. Three key parameters have been identified as impediments: (i) the low absorption efficiency of the Yb³⁺ sensitiser, (ii) the low quantum yield of upconversion, and (iii) the long-lived excited states. On the other hand, there are very encouraging results that suggest that these nanoparticles could be developed into very potent magnetic resonance imaging (MRI) contrast agents.