Analytische Chemie
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In Part II of this Review series on lanthanide-doped photon-upconversion nanoparticles (UCNPs), we present and critically discuss the Performance and suitability of UCNPs as background-free luminescent Reporters in bioimaging and bioanalytical applications. The preparation of a biocompatible nanoparticle surface is an integral step for all life - science-related applications. UCNPs have found their way into a large number of diagnostic platforms, homogeneous and heterogeneous assay formats, and sensor applications. Many bioanalytical detection schemes involve Förster resonance energy transfert (FRET), which is still debated for UCNPs and Needs to be much improved. The Need for dedicated and standardized instruments as well as recent studies on the Dissolution and potential toxicity of UCNPs are addressed. Finally we outline future Trends and challenges in the field of upconversion.
Artificial light harvesting complexes find applications in artificial photosynthesis, photovoltaics and light harvesting chemical sensors. They are used to enhance the absorption of light of a reaction center which is often represented by a single acceptor. Here, we present different light harvesting systems on DNA origami structures and analyze systematically the light harvesting efficiency. By changing the number and arrangement of different fluorophores (FAM as donor, Cy3 as transmitter and Cy5 as acceptor molecules) the light harvesting efficiency is optimized to create a broadband absorption and to improve the antenna effect 1 (including two energy transfer steps) from 0.02 to 1.58, and the antenna effect 2 (including a single energy transfer step) from 0.04 to 8.7, i.e. the fluorescence emission of the acceptor is significantly higher when the light-harvesting antenna is excited at lower wavelength compared to direct excitation of the acceptor. The channeling of photo energy to the acceptor proceeds by Förster Resonance Energy Transfer (FRET) and we carefully analyze also the FRET efficiency of the different light harvesting systems. Accordingly, the antenna effect can be tuned by modifying the stoichiometry of donor, transmitter and acceptor dyes, whereas the FRET efficiency is mainly governed by the spectroscopic properties of dyes and their distances.
Upconversion nanoparticles (UCNPs) offer new strategies for luminescence-based sensing. The potential of UCNPs to serve as donors in Förster resonance energy transfer (FRET) applications is intensely discussed, owing to their anti-Stokes shifted narrow emission bands, chemical inertness, photostability, and long luminescence lifetimes (> 100 μs). Since FRET is distance dependent, the diameter of the particles is expected to affect the FRET efficiency. In order to identify the ideal particle architecture for FRET-based applications, we performed a systematic spectroscopic study of the influence of the UCNP size on the energy transfer using the organic dyes rose bengal and sulforhodamine B acting as model FRET acceptors for the green upconversion emission. High-quality Yb,Er-doped UCNPs with precisely controlled diameters between 10 and 43 nm were prepared using a high temperature synthesis. The monodisperse, oleate-capped particles were directly modified with the organic dyes by a two-step ligand exchange procedure, resulting in the shortest possible donor-acceptor distance. Successful FRET was demonstrated through the simultaneous drastic reduction of the luminescence intensity and the lifetime of the respective upconversion emission. In contrast to intensity measurements, time-resolved studies on both donor and acceptor luminescence allowed for the elimination of dependencies on excitation power density and particle concentration and for the discrimination between inner filter effects and FRET. The maximum FRET efficiency was observed at a particle diameter around 21 nm, which was attributed to an increasing fraction of the total amount of Er3+ donors inside the UCNPs being within Förster distance. Smaller UCNP diameters did not further improve the FRET efficiency, demonstrating the growing contribution of opposing effects, like the competition of non-radiative surface deactivation, at larger surface-to-volume ratios. This comprehensive understanding of energy transfer processes at the surface of UCNPs is essential for the rational design of upconversion FRET platforms for applications in sensing, imaging, and theranostics with improved sensitivity, reliability and comparability. The energy transfer can also be utilized to shift the luminescence emission by the choice of the organic dye in order to explore applications that require specific emission wavelengths due to interfering substances, while still making use of the advantages of near-infrared excitation.
Upconversion nanoparticles (UCNPs) are attractive candidates for energy transfer-based analytical applications. In contrast to classical donor−acceptor pairs, these particles contain many emitting lanthanide ions together with numerous acceptor dye molecules at different distances to each other, strongly depending on the particle diameter.
UCNPs with precisely controlled sizes between 10 and 43 nm were prepared and functionalized with rose bengal and sulforhodamine B by a ligand-exchange procedure. Timeresolved studies of the upconversion luminescence of the UCNP donor revealed a considerable shortening of the donor lifetime as a clear hint for Förster resonance energy transfer (FRET). FRET was most pronounced for 21 nm-sized UCNPs, yielding a FRET efficiency of 60%. At larger surface-to-volume ratios, the FRET efficiency decreased by an increasing competition of nonradiative surface deactivation. Such dye-UCNP architectures can also provide an elegant way to shift the UCNP emission color, since the fluorescence intensity of the organic dyes excited by FRET was comparable to that of the upconversion emission of smaller particles.
Lanthanide-doped up-converting nanoparticles (UCNPs) are promising reporters for medical diagnostics and bioimaging, which are excited in the near infrared (NIR) by multiphoton absorption processes, and show multiple narrow emission bands in the visible (vis) and NIR, long luminescence lifetimes in the μs range, and excellent photostability. Current limitations present their relative low absorption cross sections and low fluorescence efficiencies, with the latter being affected by particle size, surface chemistry, and microenvironment, particularly water.
Here, we present results from systematic studies of the excitation power density dependent upconversion luminescence spectra, intensities/intensity ratios of the individual emission bands, slope factors, and quantum yields of UCNPs of varying size, dopant concentration, and surface chemistry in different microenvironments as well as the up- and downconversion luminescence decay kinetics of the different emission bands. Moreover, first studies of the energy transfer from UCNPs to surface-bound organic dyes acting as fluorescence acceptors are shown. Based upon these measurements, fluorescence deactivation channels are identified and spectroscopic parameters for the screening of material performance are derived.