Liquid-liquid interfacial self-assembly of ordered CuS@cu core-shell heterostructure for synergistic SERS enhancement

Semiconductors offer advantages as surface-enhanced Raman scattering (SERS) substrates, including high stability and low cost. However, their intrinsic plasmonic resonance is inherently weak due to bandgap limitations, where valence electrons can only be excited to the conduction band under specific conditions. Consequently, their SERS enhancement mechanism primarily relies on charge transfer processes at the material-molecule interface, resulting in enhancement factors typically 1-3 orders of magnitude lower than those of conventional noble metal substrates (e.g., Au, Ag). To address this limitation, this study proposes a semiconductor-metal heterojunction synergistic enhancement strategy. A SERS substrate was designed by constructing a core-shell heterostructure of CuS and metallic Cu (CuS@Cu), aiming to combine efficient charge transfer with strong electromagnetic field enhancement. The results demonstrate that the synthesized CuS@Cu core-shell heterojunction exhibits significantly enhanced Raman activity. The optimization of SERS performance were systematically investigated by tuning precursor concentrations (e.g., CuS colloidal concentration, CuSO₄ concentration). Utilizing a liquid-liquid interfacial self-assembly technique, with n-butyl acetate as the oil phase and an aqueous colloidal dispersion of CuS@Cu nanoparticles as the aqueous phase, large-area, ordered CuS@Cu core-shell heterojunction films were fabricated. Leveraging a selective interfacial CT mechanism between the CuS semiconductor and probe molecules (e.g., Rhodamine B (RhB), Methylene Blue (MB), Malachite Green (MG)), this substrate enables specific enhancement of their SERS signals (EF ∼ 10⁵ for RhB). Quantitative limits of detection reached 1 × 10^-7 mol/L, 3.16 × 10^-7 mol/L, and 3.16 × 10^-6 mol/L for RhB, MB, and MG, respectively. Also, the substrate exhibits high reproducibility and homogeneous SERS response (RSD < 4.5 %), ensuring reliable and robust sensing performance. The combination of the controllable liquid-liquid interfacial self-assembly and the heterojunction enhancement effect highlights the significant potential of this substrate for application in portable SERS sensing devices.