Although Fe₃O₄ magnetic nanoparticles (MNPs) have recently been developed as artificial enzymes in a wide range of applications, there is a debate on whether the observed peroxidase-like activity originates from the nature of the intact MNPs themselves or from the Fenton reaction of surface bound and free ferric/ferrous ions. In this work, Fe³⁺ ions serving as the peroxidase mimic towards the 3,3′,5,5′-tetramethyl benzidine (TMB)–H₂O₂ system was investigated in detail for the first time. Experimental results revealed that the peroxidase-like activity of Fe³⁺ ions is much higher than that of Fe₃O₄ MNPs. On the basis of these findings, a simple, highly sensitive and selective colorimetric sensing platform for L-cysteine (L-Cys) was developed with a limit of detection (LOD) as low as 0.97 μM.