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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.
Coating, stabilization layers, functionalization of particles or simple contamination are common variants of a core-shell system. For smaller nanoparticles this is of major importance. A particle with 16 nm diameter and a usual surface layer of 2 nm will have the same volume for the core as for the shell. In this case the material of the particle doesn’t have a clear definition. It is a common case that a particle consists of four different layers: Core, shell, stabilization layer and contamination. The properties of the particles differ according to this structure. For example silver particles might have a different dissolution rate for pure particles and for particles which are grown on top of a core.
Different solubility or defined other properties of materials is a common reason for producing core-shell systems. Gold cores are surrounded by silica to stabilize them or to get a defined distance between the cores. Silica might be surrounded by gold and the silica dissolved afterwards. This delivers hollow shells. Another important example for core-shell systems are quantum dots. A small core is surrounded by a different material for increasing the photoluminescence. Furthermore there a stabilization layer is needed. The smallest part of the final particles is the initial core. The photoluminescence is based on this core, but the shells contain much more material. Categorization should address this.
Core-shell systems are not covered by most of the existing decision trees for grouping. They are either regarded as special case or a singular layer. This disqualifies core-shell systems for grouping within the common models. There might be a very easy way to avoid this problem and even to combine some of the different decision trees. Starting the decision tree with the solubility of the outer shell and subsequently addressing the inner layers will be a pragmatic approach to solve the problem. If there is no shell, the categorization can start with a tiered approach or with the proposed “stawman” chemical categorization. If a shell is covering the surface there is a need to check if the shell is stable. If it is stable, the particle can be categorized based on this shell. If it is soluble, the ions need to be addressed as in the classic case. Furthermore the shell might increase the uptake by the cells. If the ions and the uptake are not critical the categorization can continue with the next layer.
With this not perfect but pragmatic approach, the surface layers can be addressed with very limited additional efforts. Most criteria are based on classically tabulated data. Including a rating system like the precautionary matrix approach might even address the fact that some parameters are not always Yes/No, e.g. solubility, ion toxicity and uptake.