Dynamic coordination of natural amino acids-lanthanides to control reversible luminescent switching of hybrid hydrogels and anti-counterfeiting

A new design of two environment-friendly hydrogels containing lanthanide ions emitting green and red luminescence have been fabricated by the coordinative self-assembly of natural phenylalanine (Phe) and lanthanide ions (Tb3+ or Eu3+). The coordination bond formation was confirmed by Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis. Luminescent spectra indicated that such hydrogels exhibit reversible gel-sol transition along with the reversible luminescent ON/OFF properties through the reversible association and dissociation of coordinative bonds between Phe and lanthanides upon pH stimuli. Luminescent decay curves revealed that the lifetimes of both hydrogels (Phe-Tb and Phe-Eu) in their gel state were remarkably increased almost double in contrast to that in sol state. Shear rate sweep data indicated that the hydrogels possess shear thinning properties. Remarkably, such environment-friendly hydrogels with long luminescent lifetimes, good stability, colorless under daylight, together with shear thinning properties, could be applied directly as luminescent inks for anti-counterfeiting. This provides an alternative strategy to prepare environment-friendly luminescent inks based on lanthanides for anti-counterfeiting.