Dual turn-on fluorescent chemosensor for Cu²⁺ and Hg²⁺ in aqueous medium based on a water-soluble polyacrylamide containing rhodamine

An acrylic monomer bearing rhodamine 6G group, salicylaldehyde rhodamine 6G hydrazone acrylate (AR6GHS), was synthesized from salicylaldehyde rhodamine 6G hydrazone (R6GHS) and acryloyl chloride in the presence of triethylamine in dry dichloromethane (CH₂Cl₂) at room temperature. The poly(acrylamide- salicylaldehyde rhodamine 6G hydrazone acrylate) (poly(AM-AR6GHS)) was synthesized by a combiation of AR6GHS and a hydrophilic comonomer acrylamide (AM) with 2,2′-azobis(2-methylpropionitrile) (AIBN) as a thermal initiator in tetrahydrofuran (THF). The resultant AR6GHS and poly(AM-AR6GHS) were identified by FTIR and ¹H NMR spectra. The fluorescent characteristic of the poly(AM-AR6GHS) in the aqueous solutions were investigated both in varied pH and in the presence of metal cations. It was found that the copolymeric sensor exhibited “off-on” recognition toward Cu²⁺ and Hg²⁺ over a wide range of tested metal ions with remarkably enhanced fluorescent intensities. The poly(AM-AR6GHS) had a good linear response between the relative fluorescence intensity and Cu²⁺ or Hg²⁺ concentration. In addition, upon the addition of Cu²⁺ or Hg²⁺ into the aqueous solution of poly(AM-AR6GHS), the color changes occurred from colorless to pale pink or pink under visible light, and the emission color of the sensor solution turned from dark to bright greenish yellow or orange yellow under the UV light at 365 nm.