Three-dimensional solution-phase Förster resonance energy transfer analysis of nanomolar quantum dot bioconjugates with subnanometer resolution

Luminescent semiconductor quantum dots (QDs) play an important role in optical biosensing and, in particular, in FRET (Forster resonance energy transfer)-based luminescent probes. The QD materials that form the basis for these probes are in actuality quite heterogeneous and consist of different types of QDs with variations in material compositions, surface coatings, and available biofunctionalization strategies. To optimize their role in active sensors that rely on FRET, extensive physicochemical characterization is required. A technique that can provide precise information about size, shape, and bioconjugation properties of different QD-biomolecule conjugates from a single sample and measurement under actual experimental biosensing conditions would therefore be highly important for advancing QDs to a next generation nanobiosensing tool. Here, we present a detailed FRET study on a large set of QD-biomolecule conjugates, which allows for a homogeneous solution-phase size, shape, and bioconjugation analysis of peptide and protein self-assembled QDs at subnanomolar concentrations and with subnanometer resolution. Direct incorporation of luminescent Tb-complexes (Tb) in the peptides or proteins leads to Tb-to-QD FRET upon assembly to the different QD surfaces. Luminescence decay times and time-gated intensities, which precisely decode the FRET interactions, provide a wealth of useful information on the underlying composite structure and even biochemical functionality. In contrast to other high-resolution techniques, which require rather sophisticated instrumentation, well-defined experimental conditions, and low sample throughput, our technique uses a commercial time-resolved fluorescence plate reader for very fast and simple data acquisition of many aqueous samples in a standard microtiter plate.