Rhodamine spirolactam sensors operated by sulfur-cooperated metal complexation

New rhodamine Schiff base sensors were developed to improve selective sensing by introducing sulfide, ester, and dithiocarbonate groups, as well as using ketones coupled to rhodamine-hydrazine. Metal sensing proceeded through the 1:1 complexation of the metal ion for most sensors in the presence of Cu²⁺ and Hg²⁺. A sensor carrying a dithiocarbonate group responded selectively to Hg²⁺ showing a strong colorimetric change and intense fluorescence. The association constants of the sensors were determined from a linear plot performed at micro-molar concentrations to afford values in the range of 10⁴. Sensing was interrupted at the initial time of Hg²⁺ exposure due to the isomerization of imine and preferential metal bonding of two dithiocarbonate groups regardless of the main structure of rhodamine. The sensors exhibited the reversible and reproducible performance for Hg²⁺ sensing.