Nanomolar sensitive colorimetric assay for Mn²⁺ using cysteic acid-capped silver nanoparticles and theoretical investigation of its sensing mechanism

A new facile, rapid, sensitive and selective colorimetric assay is proposed for the determination of manganese ions (Mn²⁺) utilizing cysteic acid (CA)-capped silver nanoparticles (CA-AgNPs) as colorimetric probes. The CA-AgNPs were prepared by reducing AgNO₃ with NaBH₄ in the presence of CA as the capping ligand. The presence of Mn²⁺ induces the quick aggregation of CA-AgNPs, associated with notable color changes of the CA-AgNPs solution from yellow to dark green. The Mn²⁺ can form a coordinated structure with CA capping on the AgNPs and leads to formation of large particles aggregation. We also used density functional theory (DFT) to calculate the change of the Gibbs free energy (ΔG) of the interactions between the CA-AgNPs and various metal ions, which shows that CA-AgNPs have high specificity for Mn²⁺. The high sensitivity and selectivity for Mn²⁺ were achieved and the detection limit is as low as 5 nM. Furthermore, the proposed method was successfully applied in detecting Mn²⁺ in a rat model of focal ischemia and the results indicate that our proposed method has great potential for practical applications.