Gold sensing with rhodamine immobilized hydrogel-based colorimetric sensor.

A highly sensitive and selective optical membrane for determination of Au3+ was synthesized by immobilization of a rhodamine derivative on agarose hydrogel. The sensing dye was synthesized by solvatochromism of rhodamine B via rhodamine lactone-zwitterion equilibrium. UV-vis spectroscopy, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were employed to confirm that the rhodamine-lactone (RhoL) was incorporated into the agarose hydrogel. The results showed that the sensor was highly selective for recognizing Au3+ over other metal ions in real systems. In addition, DFT calculation results suggested that the membrane sensor formed stable complexes with Au3+ through a large number of cation-dipole and ion-ion interactions. In addition, according to changes in signaling upon adding various Au3+ concentration, the limit of detection of Arg-RhoL for Au3+ is calculated to be 5 µM. This approach may provide an easily measurable and inherently sensitive method for Au3+ ion detection in environmental and biological applications.