Small molecule optical sensors for nickel: The quest for a universal nickel receptor

Abstract Nickel is classified as an essential transition metal, facilitating important biological processes in bacteria, archaea and plants. In humans, the role of trace concentrations of nickel is less well understood, but it is known that excess exposure causes adverse health effects. As a result, being able to identify nickel in the environment and in biological systems is highly important. Small molecule optical sensors are powerful tools for detecting and quantifying metal ions in samples ranging from cell organelles to entire bodies of water. Unlike well-studied ions such as Zn(II) and Cu(I), a selective receptor group for Ni(II) is yet to be discovered, limiting the development of small molecule Ni(II) probes for diverse applications. This work will contribute to the knowledge-base of nickel sensing by: 1) outlining properties of successful optical probes; 2) summarising strategies for Ni(II) receptor group design; 3) reviewing and discussing small molecule optical probes for Ni(II) in the literature to date; and 4) drawing conclusions and giving recommendations to inform the future design of Ni(II) probes; aiding the quest for a universally-selective receptor group for Ni(II).