01/01/2018
Left panel: The European Upconversion Network; Right panel: schematic visualization of the upconversion of two (or more) lower energy, i.e., near-infrared (NIR) exciting photons into one higher energy photon.

Left panel: The European Upconversion Network; Right panel: schematic visualization of the upconversion of two (or more) lower energy, i.e., near-infrared (NIR) exciting photons into one higher energy photon. The NIR photons are sequentially absorbed by the sensitizer and transferred via sequential energy transfer processes to the activator which then emits differently colored, typically blue, green, and red light.

Source: Left panel: European Upconversion Network; Right panel: BAM, Division Biophotonics

Upconversion nanomaterials – Applications and future perspectives and challenges for European research activities

Because of their potential to convert light between different spectral regions and their unique photophysical properties, lanthanide-doped photon-upconversion nanoparticles (UCNPs) have been in the focus of many research activities in the material and life sciences. Upconversion is a process which converts light of low energy, e.g. (near) infrared light, into light of higher energy like visible light. In the case of UCNPs, that typically contain different optically active lanthanide ions like ytterbium (Yb3+) and erbium (Er3+) or thulium (Tm3+) ions, this implies light absorption by ytterbium ions (acting as sensitizer) and energy transfer to neighboring erbium or thulium ions (acting as activator). These emissive activator ions store the energy until the next energy transfer process occurs that further excites them to a higher energy level, resulting eventually in the emission of photons of higher energy as the absorbed ones.

Full exploitation of the application potential of these fascinating nanomaterials as optical reporters for assays, bioimaging studies, and superresolution microscopy, barcodes for anti-counterfeiting, new optical devices, and solar cells requires to overcome several challenges. This includes the low brightness particularly of small UCNPs with sizes < 20 nm, their controlled surface functionalization, and the reliable quantification of the excitation power density-dependent multiphotonic upconversion luminescence. With special focus on bioanalysis and the life sciences, recent developments, future trends, and representative applications of these materials are presented in two critical reviews. Part I of this series covers the synthesis of UCNPs up to different design concepts to enhance their luminescence and brightness including optical-spectroscopic tools required to quantify material performance and identify luminescence quenching processes. In part II, surface modification strategies and bioanalytical applications are addressed as well as selected examples for the use of UCNPs as reporters in different assay formats and as nanosensors.

In order to support and focus these research activities, the European network COST Action CM1403 “The European Upconversion Network: From the Design of Photon-upconverting Nanomaterials to (Biomedical) Applications” was established in 2014 with BAM participation. The goal of CM1403 is to address the current challenges of photon upconversion (UC) technology highlighted in this review series. BAM, via division Biophotonics, is tightly involved in this European research network, with Dr. U. Resch-Genger being a member of the management committee of CM1403 and leading the working group “Synthesis and Spectroscopic Characterization of Upconversion Nanomaterials”. Since 2015, division Biophotonics has hosted research visits of several graduate students from other European research groups within the framework of this COST action, who successfully utilized the unique spectroscopic measurement techniques developed at BAMfor the characterization of their materials, with more students to come next year. Division Biophotonics contributed also to a COST Training School on Upconverting Nanoparticles in Bioaffinity Assays organized by Professor T. Soukka at the University of Turku, Finland, in April 2017, sharing in different lectures its expertise on the characterization of luminescent nanomaterials and calibration of fluorescence measuring devices.

Perspectives and challenges of photon-upconversion nanoparticles - Part I: routes to brighter particles and quantitative spectroscopic studies
Ute Resch-Genger, H.H. Gorris
Analytical and Bioanalytical Chemistry, October 2017, Volume 409, Issue 25, pp 5855–5874
BAM Department Analytical Chemistry; Reference Materials, Division Biophotonics

Perspectives and challenges of photon-upconversion nanoparticles - Part II: Bioanalytical applications
Ute Resch-Genger, H.H. Gorris
Analytical and Bioanalytical Chemistry, October 2017, Volume 409, Issue 25, pp 5875–5890
BAM Department Analytical Chemistry; Reference Materials, Division Biophotonics