A Magnetic Sensor Based on Poly(γ-Glutamic Acid)-Functionalized Iron Oxide Nanoparticles for Cr³⁺ Detection

Background: The highly sensitive and selective detection of Cr³⁺ is critical. Objective: We report the development of poly(γ-glutamic acid) (γ-PGA)-functionalized iron oxide nanoparticles (γ-PGA-Fe3O4 NPs) as a magnetic nanosensor for magnetic resonance (MR) detection of trivalent chromium (Cr³⁺) in aqueous solution. Methods: The γ-PGA-Fe3O4 NPs with a mean particle size of 7.3 nm, good colloidal stability and ultrahigh r2 relaxivity (326.8 mM⁻¹s⁻¹) were synthesized via a facile mild reduction approach in the presence of γ-PGA, and used for MR detection of Cr³⁺. Results: Upon exposure to Cr³⁺, the γ-PGA-Fe3O4 NPs aggregated into nanoclusters as verified by dynamic light scattering due to the coordination of Cr³⁺ with γ -PGA side-chain carboxyl groups, resulting in the decrease in their transverse relaxation time. This MR signal change enables detection of Cr³⁺ in a concentration range of 0.4-1 nM. We also show that the γ-PGA-Fe3O4 NPs have an excellent selectivity toward Cr³⁺ and a high recovery percentage of 83.8% or above. Conclusion: This study thus demonstrates that the developed γ-PGA-Fe3O4 NPs may be used as a nanoprobe for MR sensing of Cr³⁺ in water environment.