Highly sensitive, reproducible and uniform SERS substrates with a high density of three-dimensionally distributed hotspots: gyroid-structured Au periodic metallic materials

Surface-enhanced Raman spectroscopy (SERS) is an important tool for the analytical, trace detection of many inorganic and organic materials, especially for materials involved in medical care, food safety and environmental pollution. Numerous efforts have been dedicated to exploring periodic metallic materials with a high density of hotspots. However, for most periodic metallic materials fabricated by top-down and bottom-up approaches, the distribution of hotspots is restricted to one or two dimensions. Here, for the first time, we report the successful fabrication of a bio-inspired bicontinuous gyroid-structured Au SERS substrate with a high density of three-dimensionally (3D) distributed hotspots. The as-required gyroid-structured substrates were demonstrated to be highly sensitive, reproducible and uniform, with an enhancement factor of up to 109. Finite-difference time domain (FDTD) simulations were conducted to reveal the mechanism leading to the high enhancement and we found that the interconnected helices in the gyroid structure not only increase the hotspot density but also contribute to increasing the scattering cross-section of the incident laser. The substrate was then adopted for the SERS detection of bis(2-ethylhexyl) phthalate, the most frequently used plasticizer in food, paints, house-hold items, perfumes and so on, and reached a detection limit of 1 fM, which is among the best results ever reported. Moreover, the mechanism deduced here will provide insight into the future design and selection of novel surface plasmonic resonance substrates, as many other bicontinuous interconnected systems are available. Inspired by butterfly wings, a team in China has developed by a technique that enables nanotextured gold surfaces to be reproducibly made. Metal nanostructures can enhance the interaction between light and matter, making them useful for the sensitive optical detection of molecules in a technique called surface-enhanced Raman spectroscopy (SERS). But practical application of this concept requires substrates that can be and behave uniformly across their surfaces. Now, Wang Zhang from Shanghai Jiao Tong University and co-workers has fabricated a SERS substrate using a template based on the wing scales of the green hairstreak butterfly. Its surface consists of an array of three-dimensional gyroid structures. The substrates, which were reproducible, uniform and highly sensitive — enhancing the detection signal by up to a factor of one billion — are promising for highly sensitive molecular detection. Gyroid-structured Au periodic metallic materials were fabricated by an electroless deposition method. By controlling the deposition time, we successfully acquired 3D-distributed hotspots of high density. The as-required plasmonic substrates demonstrate ultra-high SPR enhancement (~109) with superior reproducibility and uniformity. Combined with FDTD simulations, we revealed that the ultrahigh enhancement is originated from the interconnected helices, which not only increase the density of the hotspots but also increase the scattering cross-section of the incidence light. The mechanism deduced here will provide insight into the future design and selection of novel surface plasmonic resonance (SPR) substrates, as many other bicontinuous interconnected systems are available.