You may prepare Resazurin (solid) at 0.015% in PBS pH 7.2.
You may weigh 1.5mg in 10ml PBS, vortex and filter sterilize (using 0.22 um filter). You may add 20ul of Resazurin solution (0.15mg/ml) per 100ul suspension per well in 96-well plate to check cell viability.