Vertically aligned nanostructures for a reliable and ultrasensitive SERS-active platform: Fabrication and engineering strategies

Surface-enhanced Raman spectroscopy (SERS) is a powerful cross-disciplinary analytical technique allowing ultrasensitive detection of various analytes with a fingerprint-like characteristic, even down to the single-molecule level. One of the key issues for SERS-based trace applications is the rational design and fabrication of structurally uniform substrates with high sensitivity, stability, and good signal reproducibility. Low-dimensional SERS-active nanomaterials are increasingly facing many challenges and cannot meet these requirements. In recent years, due to developments in nanofabrication and an improved knowledge of plasmonic properties of nanomaterials, significant progress has been made in the area of complex nanostructures with ordered architectures, elemental compositions, and controlled plasmonic properties, which are critical for ultrasensitive, uniform, reliable, and reproducible measurements. In this review, the fabrication methodologies and classification of vertically structured nanoarchitectures are outlined. Subsequently, the morphological and geometrical effects on SERS performance are discussed based on the substrate materials. Insight into advanced engineering strategies towards some SERS-active nanostructures is supplied in this context. Finally, the challenges and future directions are pointed out for the development of vertically nanostructured SERS substrates and their ultrasensitive sensing applications.