Carbon dots were synthesized by a simple and green strategy for selective and sensitive Cu²⁺ ion detection using both down and upconversion fluorescence. These fluorescent nanosensors show low cytotoxicity and are applied for intracellular sensing and imaging of Cu²⁺ in biological systems.

Single-molecule FRET assays used to probe the conformational dynamics of ubiquitin chains reveal that conformational selection is an important mechanism of ubiquitin chain recognition. Post-translational modification of protein by ubiquitin regulates many biological processes. Ubiquitin is conjugated to its substrates either as a monomer or as polymeric chains. These chains come in various forms with distinct linkage types, for instance through the Lys 48, Lys 63 or Met 1 residues, depending on the cell function involved. In this study, Komander and colleagues use single-molecule fluorescence resonance energy transfer assays to probe the conformational dynamics of ubiquitin chains. The results suggest that conformational selection is an important mechanism of chain recognition by ubiquitin-binding domains and deubiquitinases, suggesting that factors influencing ubiquitin chain conformation and dynamics may be regulators of the ubiquitin system. Mechanisms of protein recognition have been extensively studied for single-domain proteins1, but are less well characterized for dynamic multidomain systems. Ubiquitin chains represent a biologically important multidomain system that requires recognition by structurally diverse ubiquitin-interacting proteins2,3. Ubiquitin chain conformations in isolation are often different from conformations observed in ubiquitin-interacting protein complexes, indicating either great dynamic flexibility or extensive chain remodelling upon binding. Using single-molecule fluorescence resonance energy transfer, we show that Lys 63-, Lys 48- and Met 1-linked diubiquitin exist in several distinct conformational states in solution. Lys 63- and Met 1-linked diubiquitin adopt extended ‘open’ and more compact ‘closed’ conformations, and ubiquitin-binding domains and deubiquitinases (DUBs) select pre-existing conformations. By contrast, Lys 48-linked diubiquitin adopts predominantly compact conformations. DUBs directly recognize existing conformations, but may also remodel ubiquitin chains to hydrolyse the isopeptide bond. Disruption of the Lys 48–diubiquitin interface changes conformational dynamics and affects DUB activity. Hence, conformational equilibria in ubiquitin chains provide an additional layer of regulation in the ubiquitin system, and distinct conformations observed in differently linked polyubiquitin may contribute to the specificity of ubiquitin-interacting proteins.

The first CdSe/ZnS quantum dot photoluminescence lifetime-based pH nanosensor has been developed. The average lifetime of mercaptopropionic acid-capped QD nanosensors showed a linear response in the pH range of 5.2–6.9. These nanosensors have been satisfactorily applied for pH estimation in simulated intracellular media, with high sensitivity and high selectivity toward most of the intracellular components.