A sensitive Ag⁺-mediated magnetic relaxation and colorimetry dual-mode sensing platform

To address the relatively low sensitivity of current redox reagent-mediated magnetic relaxation sensing methods, we present a novel Ag⁺-mediated magnetic sensing platform that enhances the sensitivity by three orders of magnitude. The new sensing platform is based on Ag⁺-catalyzed oxidation of Mn²⁺ to KMnO₄, accompanied by a distinct color change, which facilitates colorimetric detection. In the case of insufficient Ag⁺ ions, MnO₂ is an additional oxidation product and the KMnO₄/MnO₂ ratio is dependent on the concentration of Ag⁺. When combined with a specific quantity of reducing agent, both KMnO₄ and MnO₂ are reduced to Mn²⁺ with a large relaxivity, and the concentration of Mn²⁺ in the resultant solution inversely correlates with the amount of KMnO₄ since KMnO₄ consumes more reductant during reduction. Consequently, the transverse relaxation rate of the solution exhibits a negative correlation with the Ag⁺ concentration. Thus, by coupling this Ag⁺-mediated Mn²⁺ to KMnO₄ transformation with reactions that modulate Ag⁺ concentration, a dual-mode sensing platform for magnetic relaxation and colorimetry can be realized. Herein, we take H₂O₂ as an example to verify the detection performance of this sensing platform since H₂O₂ can oxidize Ag⁰ in Ag@Fe₃O₄ nanoparticles to Ag⁺. Experimental findings demonstrate detection limits of 10 nM and 20 nM for the magnetic relaxation and colorimetry modes, respectively, affirming the excellent sensitivity and the potential practical application of this strategy.