An Optical Au³⁺ Sensor Based on layer-by-layer PEI/PAA-Rho thin Films on ITO

This study presents the development of a sensitive and selective gold ion (Au³⁺) sensor utilizing layer-by-layer (LbL) assembled thin films composed of polyethylenimine (PEI) and poly (acrylic acid) (PAA) conjugated with rhodamine (Rho). The first study revealed that the polymeric sensors (PAA-Rho) demonstrated significant selectivity and sensitivity in their colorimetric and fluorescence responses to Au³⁺ compared to other metal ions. In their spirolactam form, the polymeric sensors were non-fluorescent but could selectively transform into the fluorescent ring-opened amide form upon interaction with Au³⁺ ions, resulting in fluorescence enhancement and observable color changes. Common co-existing metal ions showed negligible interference in the detection of Au³⁺. The LbL sensor exhibited a linear increase in absorbance with the addition of bilayers, confirming successful film deposition. Surface morphology analysis using SEM, along with structural confirmation via ATR-FTIR and XRD, further validated the sensor’s capability to detect cation. Results demonstrated that the LbL sensor exhibited selectivity for Au³⁺ ions within the range 1 × 10⁻⁶ to 1 × 10⁻³ M. This approach offers an easily understandable and intrinsically sensitive means for detecting Au³⁺ ions in both environmental and biological applications.