In clinical diagnostics, homogeneous time-resolved (TR) FRET immunoassays are used for fast and highly sensitive detection of biomarkers in serum samples. The most common immunoassay format is based on europium chelate or cryptate donors and allophycocyanin acceptors. Replacing europium donors with terbium complexes and the acceptors with QDs offers large photophysical advantages for multiplexed diagnostics, because the Tb-complex can be used as FRET donor for QD acceptors of different colors. Water-soluble and biocompatible QDs are commercially available or can be synthesized in the laboratory using many available recipes from the literature. Apart from the semiconductor material composition, an important aspect of choosing the right QD for TR-FRET assays is the thickness of the QD coating, which will influence the photophysical properties and long-term stability as well as the donor-acceptor distance and FRET efficiency. Here we present a detailed time-resolved spectroscopic study of three different QDs with an emission maximum around 605 nm for their application as FRET acceptors (using a common Tb donor) in TR-bioassays: (i) Invitrogen/Life Technologies Qdot605, (ii) eBioscience eFluorNC605 and iii) ter-polymer stabilized CdSe/CdS/ZnS QDs synthesized in our laboratories. All FRET systems are very stable and possess large Förster distances (7.4-9.1 nm), high FRET efficiencies (0.63-0.80) and low detection limits (0.06-2.0 pM) within the FRET-bioassays. Shapes, sizes and the biotin/QD ratio of the biocompatible QDs could be determined directly in the solution phase bioassays at subnanomolar concentrations. Both commercial amphiphilic polymer/lipid encapsulated QDs and self-made ligand-exchanged QDs provide extremely low detection limits for highly sensitive TR-FRET bioassays.

Ultrathin black phosphorus (BP) is one of the promising two-dimensional (2D) materials for future optoelectronic devices. Its chemical instability in ambient conditions and lack of a bottom-up approach for its synthesis necessitate efficient etching methods that generate BP films of designed thickness with stable and high-quality surfaces. Herein, reporting a photochemical etching method, we demonstrate a controlled layer-by-layer thinning of thick BP films down to a few layers or a single layer and confirm their Raman and photoluminescence characteristics. Ozone molecules generated by O2 photolysis oxidize BP, forming P2O5-like oxides. When the resulting phosphorus oxides are removed by water, the surface of BP with preset thickness is highly flat and self-protective by surface oxygen functional groups. This method provides a fabrication strategy of BP and possibly other 2D semiconductors with band gaps tuned by their thickness.

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