A portable paper-based surface enhanced Raman scattering platform for Al³⁺ sensing

The adverse effects of Al³⁺ ions on human health necessitate the development of ultra-sensitive detection methods for Al³⁺ ions. In this regard, the compact and portable design of the detection substrate is of utmost importance for achieving in-situ and sensitive detection of Al³⁺ ions. In our study, we have successfully developed a surface-enhanced Raman scattering (SERS) platform with gold nanoparticles (Au NPs) that was modified with histidine (His) and 4-mercaptobenzoic acid (4-MBA) for the SERS detection of Al³⁺ ions. His and 4-MBA molecules are attached to the Au NPs through a co-assembly strategy. His with amino and carboxyl groups imparts the Au NPs with an aggregation effect against Al³⁺ ions, while 4-MBA provides the SERS responsiveness. The coordination effect of Al³⁺ ions leads to the aggregation of Au NPs, creating hot spots around the 4-MBA molecules and thereby enhancing the SERS signals. Based on this principle, we have constructed a SERS analysis method for detecting Al³⁺ ions in solution. Furthermore, the paper-based plasmonic strips loaded with His-4-MBA-Au NPs have been successfully employed for the SERS detection of Al³⁺ ions. These strips exhibit a significantly higher enhancement factor of 9.77 × 10⁶ compared to the solution system, thanks to the physical aggregation of the strips. The Au NPs tend to generate localized electromagnetic hot spots on the surface of strips, playing a critical role in achieving superior SERS performance. The limit of detection (LOD) of His-4-MBA-Au NPs for Al³⁺ detection is as low as 7.38 nM. The fabricated strips have also undergone repeatability testing and analysis of real samples. The established platform for the detection of Al³⁺ ions holds great promise for portable and on-the-spot analysis.