Development of luminescent and nontoxic Pb-free perovskite quantum dots (PQDs) for quantification of toxic/nontoxic heavy metal ions has attracted much attention recently. In this paper, blue emissive Pb-free bare and poly(ethylenimine), oleic acid stabilized methylammonium tin tribromide quantum dots (MASnBr3QDs and PEI-OA-MASnBr3QDs) are developed via modified synthetic routes with fluorescent quantum yields of (φf) of 8.7 and 14.6%, respectively. The particle size, structures, diffraction patterns, and surface potential of PQDs are investigated using a high-resolution transmission electron microscope (HR-TEM), powder X-ray diffraction (PXRD), dynamic light scattering (DLS), and zeta potential techniques. Photoluminescence (PL) investigations demonstrate agglomeration-mediated energy transfer at various precursor concentrations and water sensitivity of PQDs. At 20 μL precursor concentration in DMSO, both QDs exhibit diverse fluorescent quenching to Fe3+ and Cr6+ with linear regression between 1-500 μM and nanomolar detection limits (LODs). Estimated Stern-Volmer quenching constant values are on the order of 103-104 M-1 higher than those of other ions. PL and time-resolved PL studies confirm involvement of dynamic and static quenching in quantification of Fe3+/Cr6+ for MASnBr3QDs and PEI-OA-MASnBr3QDs, respectively. Agglomeration of PQDs, Sn2+/MA+ cationic displacement by Fe3+/Cr6+, and the existence of metal-oxide/hydroxide layer above the surface of QDs are confirmed by HR-TEM, DLS, zeta potential, X-ray photoelectron spectroscopy, and energy-dispersive spectroscopy investigations and supported by the density functional theory optimization. Biocompatibility of PQDs is authenticated by the methyl thiazolyl tetrazolium assay and IC50 interrogations with supporting results from time-dependent cellular imaging of Fe3+ and Cr6+ ions. Individual titrations of PQDs with Fe3+ and Cr6+ in tap, lake, and seawater samples display linear behavior with micro/nanomolar LODs. Fe3+ and Cr6+ in spiked real water sample experiments show exceptional PL recoveries (>100%), which agree with the inductively coupled plasma-mass analysis.