Fabrication of long-range ordered, broccoli-like SERS arrays and application in detecting endocrine disrupting chemicals

Abstract

Periodic broccoli-shaped Au and Ag surface-enhanced Raman spectroscopy (SERS) arrays were fabricated by combining ordered SiO₂ colloidal crystal templates with the physical deposition technique. The SiO₂ colloidal crystal-assisted Au and Ag SERS substrates have a long-range, adjustable periodic structure and a clean surface without incorporating any reductants or surfactant chemicals. Different from depositing directly on the flat substrates, the colloidal crystal-assisted nanostructure array has a larger effective surface area under the same projected area of laser irradiation, which exposes more “hot spots”. An increased roughness and a larger surface area have also been created as the highly bumpy surface feature of the broccoli-shaped SERS morphologies, resulting in a greater Raman amplification than the conventional metal film over nanosphere (MFON). SERS performances by Au and Ag SERS arrays reveal that the long-range broccoli-like morphology is a promising SERS platform as it is highly sensitive, reproducible and stable. The colloidal crystal-assisted Ag SERS array has a slightly higher enhancement factor (EF) than the Au SERS array, and they both are of the order of 10⁷ enhancement. Compared to our previous work, which directly deposited noble metal nanoparticles onto flat substrates, the EF of the colloidal crystal-assisted SERS array is improved by one to two orders of magnitude. Finite-difference time-domain (FDTD) simulation was performed to estimate the electromagnetic field distribution. Finally, two endocrine disrupting chemicals (EDCs) (dioctyl phthalate (DOP) and dibutyl phthalate (DBP), homologous series) at different concentrations were successfully identified by Au and Ag SERS arrays with the detection limits of 0.24 × 10⁻⁹ M and 0.22 × 10⁻⁹ M, respectively. This study suggests that the broccoli-like Au and Ag SERS arrays are promising candidates for chemical sensing and this SERS substrate fabrication technique can be accessible to the standard industrial processes.