Fluorescence dye adsorption assay to quantify carboxyl groups on the surface of poly(methyl methacrylate) microbeads.

Microbead-based assays have evolved into powerful tools for the multiplex detection of biomolecules. Analytes are captured by DNA or protein capture molecules which are coupled on microbead surfaces. A homogeneous carboxylation of microbeads is essential for the optimal and reproducible coupling of capture molecules and thus a prerequisite for an optimal multiplex microbead-based assay performance. We developed a simple fluorescence dye adsorption assay for the description of microbead carboxylation and for the prediction of coupling successes of capture molecules. Using the fluorescence dye SYTO-62 it is possible to quantify the degree of carboxylation of poly(methyl methacrylate) (PMMA) microbeads within 1 h in a multiplex format by fluorescence microscopy or flow cytometry. Compared to conventional bulk assays which only provide an average degree of carboxylation the main advantage of the SYTO-62 assay is the single microbead analysis and therefore the description of the qualitative distribution of carboxylation in microbead populations. The SYTO-62 assay is sensitive enough to even determine weak carboxylation. Also, the quality of microbeads can be evaluated. To our knowledge this is the first report which applies a reversible noncovalent fluorescent dye adsorption assay to quantify the degree of carboxylation on surfaces.