Biological copper can be tightly bound in a protein’s active site, or it can be dynamic and loosely bound. Loosely bound copper has been tentatively linked to cell signaling, but that’s difficult to verify with imaging techniques. But now Christopher J. Chang’s lab at the University of California, Berkeley, has developed a new fluorescent probe and used it to explore copper’s roles in mouse neural circuit cultures (Proc. Natl. Acad. Sci. USA 2014, DOI: 10.1073/pnas.1409796111). Chang’s team reasoned that its prior probes were inadequate for tissue imaging because the molecules’ hydrophobicity gave them a tendency to aggregate in such environments. The researchers designed their latest probe (shown) to be more hydrophilic yet retain its brightness and selectivity for Cu+ over other ions. They then visualized loosely bound copper in neurons cultured from the hippocampus or retina of developing mice. When the team chelated copper away or genetically inactivated a copper transporter protein, the neural circuit cultures experienced more frequent spikes in calcium concentration, a surrogate for neuronal activity. The Chang lab’s probes “are a very important step toward interrogating labile copper in cells and possibly tissue,” says Martina Ralle, who studies the cell biology of metals at Oregon Health & Science University.