Analytische Chemie
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Biophotonics and analytics - Photoluminescence properties of nanocrystals and surface group analysis
(2018)
Correlating the photoluminescence (PL) properties of nanomaterials like semiconductor nanocrystals (QDs) and upconversion nanocrystals (UCNPs) assessed in ensemble studies and at the single particle level and studying their surface chemistry is increasingly relevant for applications of these nanomaterials in the life and material sciences. Here we present a comparison of the spectroscopic properties of ensembles and single nanocrystalline emitters and simple methods for the quantification of functional groups and ligands on particle surfaces. The overall goal of this study was to derive particle architectures and surface chemistries well suited for spectroscopic and microscopic applications.
Surface functionalization of nanomaterials is nowadays at the core of many applications of functional materials in the life and material sciences. Examples range from membranes and microarrays over bead-based assays, and next generation sequencing to nanometer-sized optical reporters, nanosensors, and magnetic and optical contrast agents. Typical function-nalization steps include silanization and grafting reactions with reactive monomers to introduce functional groups like amino or carboxylic acid groups or the attachment of ligands like polyethylene glycol (PEG) molecules and biomolecules. [1-3] This enables to tune e.g., dis-persibility, hydrophilicity and biocompatibility, minimize unspecific interactions, improve biofunctionalization efficiencies, and enhance blood circulation times and allows for the use of nanomaterials as reporters in assays or the design of targeted probes for bioimaging.
At the core of all functionalization strategies are reliable and validated methods for surface group and ligand quantification that can be preferably performed with routine laboratory instrumentation, require only small amounts of substances, and are suitable for many different types of nanomaterials. [3] There is meanwhile a considerable need to make these methods traceable. We present here versatile and simple concepts for the quantification of common functional groups, ligands, and biomolecules on different types of organic and inorganic nanomaterials, using conventional and newly developed cleavable and multimodal reporters, that can be detected with optical spectroscopy. [4-7] These reporters are chosen to enable method validation with the aid of method comparisons and mass balances. Also, strategies how to make these simple assays traceable to SI units using quantitative nuclear resonance spectroscopy (qNMR) and X-ray photoelectron spectroscopy (XPS) are derived.
Optical spectroscopic studies of the influence of size, particle architecture, and surface chemistry of different types of photoluminescent nanocrystals with emission in the vis/NIR will be presented including semiconductor quantum dots and lanthanide-based upconversion nanoparticles. This will include the photophysics of these materials assessed with steady state and time-resolved fluorometry on the ensemble and single particle level and concepts for the quantification of surface groups at nanomaterials with optical methods using cleavable probes and catch-and-release assays.
Photoluminescence applications in the life and material sciences require bright molecular and nanocrystalline emitters, stimuli-responsive optical probes, signal enhancement, multiplexing, and barcoding strategies and traceable methods to quantify the signal-relevant optical properties of luminescent materials at the ensemble and single molecule/particle level. In this context, current research at Division Biophotonics of BAM is presented ranging from dye and nanocrystal photophysics, absolute measurements of photoluminescence quantum yields in the UV/vis/NIR/SWIR, lifetime multiplexing, and the development of different types of fluorescence standards for validating optical-spectroscopic measurements.
Synthesis and Optical Quantification of Surface Groups on Organic and Inorganic Particle-Carriers
(2020)
Differently sized organic and inorganic particles are of great interest in the life and material sciences, as they can be used e.g. as drug carriers, fluorescent sensors, and multimodal labels in bioanalytical assays and imaging applications.1 Particle performance in such applications depends mainly on the sum of their intrinsic physicochemical properties. Here, the surface chemistry, i.e., the total number of surface functional groups (FG) and the number of FG accessible for subsequent modification with ligands and/or biomolecules, is one of the key parameters. Moreover, the surface chemistry of these materials controls the behavior and fate of the particles when released to the environment or taken up by cells. Nevertheless, it is still relatively rare that FG are quantified in particle safety studies. Methods for FG quantification should be simple, robust, reliable, fast, and inexpensive, and allow for the characterization of a broad variety of materials differing in size, chemical composition, and optical properties.
Aiming at the development of simple, versatile, and multimodal tools for the quantification of bioanalytically relevant FG such as amine2,3, carboxy2,3, thiol, and aldehyde4 functionalities, we designed a catch-and-release assay utilizing cleavable probes that enable the quantification of the cleaved-off reporters in the supernatant after particle separation, and thus, circumvent interferences resulting from particle light scattering and sample-inherent absorption or emission.2 The potential of our cleavable probes for the quantification of carboxy and amino groups was demonstrated for commercial and custom-made polymer and silica particles of varying FG densities, underlining the benefit of the catch-and-release assays as a versatile method for the FG quantification on all types of transparent, scattering, absorbing and/or fluorescent particles.2,3 In the future, our cleavable probe strategy can be easily adapted to other analytical techniques requiring different reporters, or to different types of linkers that can be cleaved thermally, photochemically, or by pH, utilizing well-established chemistry, e.g. from drug delivery systems. It can contribute to establish multi-method characterization strategies for particles to provide a more detailed picture of the structure-properties relationship and thus can support the design of sustainable and safe(r) materials.