Dynamics of Silver Nanoparticles in Aqueous Solution in the Presence of Metal Ions

Using a combined UV-vis, DLS, and electrochemical approach, this work experimentally studies the physical origin of the observed colorimetric sensitivity of aqueous silver nanoparticles toward divalent metal ions. In the presence of Pb²⁺, AgNPs are slow to reversibly form agglomerates (the time scale of the reverse deagglomeration process is of the order of hours). This agglomeration is shown to be induced by complex formation between Pb²⁺ and citrate groups localized on the AgNPs, reducing surface charges (zeta-potential) and hence electrostatic repulsion between the AgNPs. Other divalent metal ions including Ca²⁺, Cd²⁺, Zn²⁺, Ni²⁺, Co²⁺, and Sn²⁺ are also studied, and the resulting sizes of the AgNPs clusters and the extents of the UV-vis spectrum red-shift in λ_max have a strong positive correlation with the metal-ligand (citrate) complex formation constant (K_f). This work thus serves as a guide for the selection of capping agents on the basis of K_f and demonstrates the correlation between sizes and spectrophotometric as well as electrochemical responses of the AgNPs clusters. Importantly, we give further physical insights into the size-dependent properties of AgNPs and emphasize the difference between theoretical and experimental values of extinction coefficients, where the latter is affected by the angle-dependent scattering intensities and the measurement technique used.