we report the synthesis and photophysical 
 properties of zbr4 and zr1 , two resorufin - based ditopic probes for 
 mobile zinc . upon binding zn2 + , the sensors display 14- 
 and 41-fold enhancements of their red fluorescence emission , respectively . 
 in contrast to zr1 and other members of the zbr family , which accumulate 
 in the endoplasmic reticulum , zbr4 spontaneously localizes to the 
 mitochondria of hela cells . the modular approach in designing the 
 constructs facilitates a homologation strategy aimed at tuning the 
 zinc - binding and intracellular targeting properties of future probes .