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 Ln(3+)-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 Tm(3+) and green and red from Er(3+) or Ho(3+). Nearly all researchers use the absorption of 980 nm light by Yb(3+) as the sensitiser for the co-doped emissive Ln(3+) ions. The focus will be on LnF(3) and MLnF(4) (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 Ln(3+) 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 Yb(3+) 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.

Deep tissue optical imaging of upconverting nanoparticles enabled by exploiting higher intrinsic quantum yield through use of millisecond single pulse excitation with high peak power

“…The present low QY thus hinders the potential of these techniques to be unleashed due to prolonged data acquisition and treatment times, and shallow applicable depths. 12,15 Although low QY to some extent can be overcome by increasing the excitation light level, such improvements are fundamentally restricted for continuous wave (CW) excitation due to risks of tissue damage, regulated by the ANSI standards. 16 Instead, the opportunity to break through the low power density limit of upconversion (UC) emission while limiting thermal effects of the excitation light is here proposed by employing pulsed excitation.…”

Application of Functionalized Lanthanide-Based Nanoparticles for the Detection of Okadaic Acid-Specific Immunoglobulin G

“…18 Further, they have high quantum yields and very narrow emission bands compared to organic fluorophores, and long-lived excited state lifetimes (~1 ms) enable both facile measurement of donor lifetime changes and elimination of short-lived background auto-fluorescence by signal collection after specific time delays. 19,20 Indeed, detection of resonance energy transfer at the single lanthanide nanoparticle level has also been reported. 21 Thus, biosensors based on nanomaterials may offer significant advantages over those currently in use in terms of sensitivity.…”

Lanthanide upconversion luminescence at the nanoscale: fundamentals and optical properties

“…Impaired upconversion intensity can be due to down-converted emission, surface quenching (from bound organic moieties, surface defects, or phonons), cross-relaxation and further excitation 171 . These properties can be influenced using several strategies, including host lattice manipulation, energy transfer modulation, surface 40 passivation, surface plasmon coupling, broadband sensitization and photonic crystal engineering, as summarized in a recent review by Liu's group 172 .…”