Analytische Chemie
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Luminescence-encoded microbeads are important tools for many applications in the life and material sciences that utilize luminescence detection as well as multiplexing and barcoding strategies. The preparation of such beads often involves the staining of premanufactured beads with molecular luminophores using simple swelling procedures or surface functionalization with layer-by-layer (LbL) techniques. Alternatively, these luminophores are sterically incorporated during the polymerization reaction yielding the polymer beads. The favorable optical properties of semiconductor quantum dots (QDs), which present broadly excitable, size-tunable, narrow emission bands and low photobleaching sensitivity, triggered the preparation of beads stained with QDs. However, the colloidal nature and the surface chemistry of these QDs, which largely controls their luminescence properties, introduce new challenges to bead encoding that have been barely systematically assessed. To establish a straightforward approach for the bead encoding with QDs with minimized loss in luminescence, we systematically assessed the incorporation of oleic acid/oleylamine-stabilized CdSe/CdS-core/shell-QDs into 0.5–2.5 μm-sized polystyrene (PS) microspheres by a simple dispersion polymerization synthesis that was first optimized with the organic dye Nile Red. Parameters addressed for the preparation of luminophore-encoded beads include the use of a polymer-compatible ligand such as benzyldimethyloctadecylammonium chloride (OBDAC) for the QDs, and crosslinking to prevent luminophore leakage. The physico-chemical and optical properties of the resulting beads were investigated with electron microscopy, dynamic light scattering, optical spectroscopy, and fluorescence microscopy. Particle size distribution, fluorescence quantum yield of the encapsulated QDs, and QD leaking stability were used as measures for bead quality. The derived optimized bead encoding procedure enables the reproducible preparation of bright PS microbeads encoded with organic dyes as well as with CdSe/CdS-QDs. Although these beads show a reduced photoluminescence quantum yield compared to the initially very strongly luminescent QDs, with values of about 35%, their photoluminescence quantum yield is nevertheless still moderate.
The DNA origami technique has great potential for the development of brighter and more sensitive reporters for fluorescence based detection schemes such as a microbead-based assay in diagnostic applications. The nanostructures can be programmed to include multiple dye molecules to enhance the measured signal as well as multiple probe strands to increase the binding strength of the target oligonucleotide to these nanostructures. Here we present a proof-of-concept study to quantify short oligonucleotides by developing a novel DNA origami based reporter system, combined with planar microbead assays. Analysis of the assays using the VideoScan digital imaging platform showed DNA origami to be a more suitable reporter candidate for quantification of the target oligonucleotides at lower concentrations than a conventional reporter that consists of one dye molecule attached to a single stranded DNA. Efforts have been made to conduct multiplexed analysis of different targets as well as to enhance fluorescence signals obtained from the reporters. We therefore believe that the quantification of short oligonucleotides that exist in low copy numbers is achieved in a better way with the DNA origami nanostructures as reporters.
Immunochromatography and enzyme-linked immunosorbent assay (ELISA) represent selective and sensitive procedures based on solid-phases for separation/detection and quantification of anthropogenic pollutants in the aquatic environment. In contrast with batch-wise procedures, such as microplate-based platforms, automated methods reduce manual handling of reagents, thus increasing overall precision and decreasing time-to-result.
Microparticles have been shown to be an adequate support for carrying out immunoassays in meso and microfluidic systems. They offer a wide range of coupling sites for biomolecules such as antibodies, combined with specialised anti-fouling surfaces to prevent non-specific binding and high compressibility for optimum fluidics.
In this work we investigated the protein-coupling behaviour of two commercially available microsphere supports (Tentagel® polystyrene-PEG-COOH and PolyAn® PMMA beads with 3D antifouling surface) using DCC/EDC and NHS/S-NHS activation chemistry. The study of coupling conditions (pH, proportion of reagents and type of buffering system) was addressed. The success of the biomodification of the supports was demonstrated by using self-prepared fluorophore-protein conjugates (Fig. 1). Laser-scanning microscopy and flow cytometry were applied for further characterization of the functionalized particles. The applicability of the developed particles will be demonstrated through the design of suspension multiplex assays for the detection, quantification and preconcentration of bioactive substances such as caffeine and carbamazepine, using Lab-on-valve (LOV) platforms.