We have come a long way since 1993 when Zinquin was used
to measure the first transition metal ion (Zn2+) in living cells. In
the past 20 years, we have witnessed an explosion in the
number of probes available, an expansion in the range of metals
that can be detected, increasing sophistication in the types of
measurements that can be performed, and remarkably creative
design platforms to ensure specificity (reaction-based probes)
and overcome traditional challenges associated with detecting
paramagnetic ions. While many existing tools could benefit
from improved brightness, higher dynamic range, and increased
specificity, the current toolbox has nevertheless provided an
unprecedented view of accessible metal pools in live cells and
organisms. These tools have been used to quantify accessible
metal ion pools, map their location, and monitor dynamics and
fluxes of metal ions.