The sensitivity and reproducibility [9–11] of SERS signal strongly relies on different fabricated hot-spots, in which a vital role is played by a SERS substrate. In general, SERS substrate can be divided into two fundamental classes, random and artificial substrates . Both of them should possess enough surface area to absorb more molecules to contribute to the Raman scattering and abundant hot-spots to enhance the local electromagnetic field. However, random substrate, such as colloidal, is proved to be limited because of weak reproducibility and fractal nanoparticle aggregation, leading their enhancement factors to decrease with MM-102 manufacturer increasing
fractal size . For the artificial nanostructure, the Cilengitide price CH5424802 purchase fourth power of local electromagnetic field of the hot-spots contributes to the signals of SERS and is sensitive to the critical dimension of artificial nanostructure [5, 13]. To date, however, it is a challenge to control the nanostructures with
extremely small size. Typically, previous engineering nanostructures were resorted to lithography-based nanotechnologies, involving electron-beam lithography (EBL), nanoimprint (NIL), nanosphere lithography (NSL), electrochemical lithography , and so on. For example, some arbitrary two-dimensional (2D) dimer nanostructures with small gaps such as bowties and nano-antennas, were proposed and prepared by EBL [15–25]. Some nanostructures were fabricated by NIL such as nanograting  and nanopost  as uniform SERS hot substrate. However, the major limitation lies in the sophistication of the fabrication processes and the inevitable defect. Triangular noble nanoparticle arrays were fabricated by NSL [24, 27]. Recently, nanocrescent [28, 29] as a quasi-three-dimensional (3D) and tuning resonance SERS substrate was fabricated by NSL, which resorted by glancing angular metal deposition onto nanospheres. However, it is difficult to fabricate large-area and uniform 3D nanostructures with small Etomidate gaps between adjacent patterns because lithography-based
techniques are isotropic and the resolution is limited. Previous investigations depended on wet etching and electrochemical method, a typical example is pyramidal pits [30, 31]; these engineering structures had large pitches which are much larger than the excitation laser probe spot size and lead to SERS enhancement with poor reproducibility and sensitivity. It is of crucial importance to develop 3D metal nanostructures with controllable nanogap sizes for the generation of strongly localized field. Van Duyne  and Fang  proposed metal films over nanosphere (MFON) electrodes as SERS active substrates in order to improve the surface nanostructure stability and suppress the inherent loss, where nanocavities with hot-spots are presented.