A selective triarylmethine-based spectroscopic probe for Zn²⁺ ion monitoring

We report the synthesis and application of a novel probe of the di-2-picolylamine (DPA)-conjugated triarylmethine (TAM) dye which has aimed at detecting Zn²⁺ ions. The 1:2 binding stoichiometry is estimated between Zn²⁺ and DPA-TAM according to Job's plot diagram and density functional theory (DFT) calculations. The increment of Zn²⁺ ion concentrations leads to the elevated SERS intensity of DPA-TAM on gold nanoparticles (AuNPs) due to the intermolecular chelation between the Zn²⁺ ion and DPA-TAM. Our synthetic spectroscopic probe shows [Zn²⁺]-correlated SERS intensities with an estimated detection limit of 50 μM. This is due to a plasmonic enhancement effect from the well-dispersed metal nanoparticles to DPA-TAM-AuNP aggregates, which is activated by Zn²⁺ ion. The prepared DPA-TAM-AuNP system can also be successfully applied to monitoring of intracellular Zn²⁺ ions both in human cervical carcinoma HeLa cells in vitro and daphnia in vivo through dark-field microscopy (DFM)-equipped Raman cellular imaging. Our DPA-TAM-AuNP system provides a new insight in potential applications of bioanalytical Zn²⁺ ion detection methodology.