Reagents for scientific and research use, namely, for fluorescence assay of metal ion concentration in aqueous solutions; reagents for scientific and research use, namely, for fluorescence-based assay of divalent metal ion concentration in aqueous solutions; reagent kits comprising a metal ion quantitation standard solution for fluorescence-based assay of metal ion concentration in aqueous solutions; fluorescent-labelled proteins for use as metal ion sensors for use in scientific and medical ...