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The surface modification of nanometer- and micrometer-sized particles with polyethylene glycol (PEG) ligands of varying length is a very common strategy to tune their hydrophilicity and biocompatibility, minimize unspecific interactions, improve biofunctionalization efficiencies, and enhance blood circulation times. Nevertheless, simple methods for the quantification of PEG ligands are rare. This is similarly true for the spectroscopic characterization of lanthanide-doped upconverting nanoparticles (UCNPs), novel near infrared (NIR)-excitable nonlinear fluorescence reporters for bioanalysis and theranostics, providing background-free multiple narrow emission bands in the visible and NIR, excellent photostability, and long luminescence lifetimes.
We present here a simple method for the determination of the number of PEG ligands on nanomaterials and screening methods for the fast identification of nonradiative deaction pathways in UCNPs.
Fluorescent particles like nm- and m-sized polymeric beads doped or labeled with different types of fluorophores and nanocrystalline systems like quantum dots and upconversion phosphors emitting in the visible (vis), near-infrared (NIR), and IR (infrared) region are of increasing importance as fluorescent reporters for bioanalysis and medical diagnostics. The assessment and comparison of material performance and the development of rational design strategies for improved systems requires suitable spectroscopic tools for the determination of signal-relevant optical properties and analytical tools for the determination of the number of surface groups, ligands, biomolecules and /or fluorophores per bead. In this respect, suitable spectroscopic tools for the characterization of the optical properties of such materials like photoluminescence quantum yields and brightness values and the determination of their surface chemistry are introduced. This includes integrating sphere setups for absolute measurements of fluorescence quantum yields of liquid and solid, transparent and scattering materials in the wavelength region of 350 nm to 1600 nm at varying excitation power densities for the study of multi-photon processes and simple optical assays, validated by comparison with established analytical techniques relying on different detection principles. Here, different examples for the optical and analytical characterization of different types of nanoscale reporters are presented.
There is an increasing interest in optical reporters like semiconductor quantum dots and upconversion nanocrystals with emission > 800 nm for bioanalysis, medical diagnostics, and safety barcodes. Prerequisites for the comparison of material performance, the mechanistic understanding of nonradiative decay channels, and the rational design of new nanomaterials with improved properties are reliable fluorescence measurements and validated methods for the assessment of their surface chemistry. The latter is of special relevance for nanocrystalline emitters, where surface states and the accessibility of emissive states by quenchers largely control accomplishable photoluminescence quantum yields and hence, signal sizes and detection sensitivities from the reporter side. Here, we present the design of integrating sphere setups for the excitation power density-dependent absolute measurement of emission spectra and photoluminescence quantum yields in the wavelength region of 350 to 1600 nm and results from spectroscopic studies of semiconductor quantum dots and upconversion nanocrystals of different size and surface chemistries in various environments. Subsequently, examples for simple approaches to surface group and ligand analysis are presented.
Different types of optical spectroscopies are introduced with special emphasis on method-inherent limitations and reliable instrument calibration and performance validation. In addition, procedures for the determination of spectroscopic key parameters like the photoluminescence quantum yield are presented including required instrument calibrations and material-specific effects related to certain emitters.
Optical spectroscopy – Techniques, instrumentation, and typical molecular and nanoscale reporters
(2018)
Different types of optical spectroscopies are introduced with special emphasis on method-inherent limitations and reliable instrument calibration and performance validation. In addition, different classes of molecular and nanocrystalline emitters are presented and the underlying photophysical processes are briefly described.
Die Bundesanstalt für Materialforschung und -prüfung (BAM) ist eine forschende Bundesoberbehörde und Einrichtung der Ressortforschung der Bundesrepublik Deutschland. Unter ihrer Leitlinie „Sicherheit in Technik und Chemie“ ist sie zuständig für die öffentliche technische Sicherheit und für metrologische Aufgaben in der Chemie. Das Aufgabenspektrum der BAM, das sich an aktuellen Fragestellungen aus Wissenschaft, Wirtschaft, Politik und Normung orientiert, bietet sehr viele interessante Tätigkeitsfelder für Naturwissenschaftler*Innen und Ingenieur*Innen.
In the focus of division Biophotonics are the design, preparation, analytical and spectroscopic characterization, and application of molecular and nanoscale
functional materials, particularly materials with a photoluminescence in the visible, near infrared (NIR) and short-wave infrared (SWIR). This includes optical reporters for bioimaging and sensing, security and authentication barcodes, and materials for solid state lighting, energy conversion, and photovoltaics. For the identification of optimum particle structures quantitative spectroscopic studies are performed under application-relevant conditions, focusing on the key performance parameter photoluminescence quantum yield. In addition, simple, cost-efficient, and standardizable strategies for quantifying functional groups on the surface of nano- and microparticles are developed, here with a focus on optical assays and electrochemical titration methods, cross-validated by more advanced methods such as quantitative NMR. In addition, reference materials and reference products are developed for optical methods, particularly luminescence techniques, and for analytical methods utilized for the characterization of nanomaterials.
Photoluminescence Quantum Yields of Luminescent Nanocrystals and Particles in the UV/vis/NIR/SWIR
(2023)
The rational design of functional luminescent materials such as semiconductor quantum dots and lanthanide-based upconversion nanoparticles, all photophysical and mechanistic studies, and the comparison of different emitters require accurate and quantitative photoluminescence measurements. Particularly the reliable determination of the key performance parameter photoluminescence quantum yield (f), the number of emitted per absorbed photons, and the brightness are of special importance for luminescence applications in the life and material sciences and nano(bio)photonics.[1] In this context, examples for absolute measurements of the photoluminescence quantum yields of UV/vis/NIR/SWIR emissive semiconductor quantum dots and rods, made from different materials, and spectrally shifting lanthanide upconversion nanocrystals with different surface chemistries in transparent matrices are presented including excitation wavelength and power density dependent studies utilizing integration sphere spectroscopy.[2,3] In addition, procedures for the absolute determination of the photoluminescence quantum yields of scattering dispersions of larger size quantum rods and differently sized inorganic particles have been developed as well as procedures for the characterization of solid luminescent nanomaterials such as different perovskites and YAG:Cer converter materials.[4] Thereby, challenges and pitfalls of f measurements in different wavelength regions including the SWIR and material-specific effects related to certain emitter classes are addressed, achievable uncertainties are quantified, and relative and absolute measurements of photoluminescence quantum yield measurements are compared to underline limitations of the former approach. Finally, a set of novel UV/vis/NIR quantum yield standards is presented including their certification with a complete uncertainty budget.[5]