Large‐Area Ultrathin Covalent‐Organic Framework Membranes for Surface‐Enhanced Raman Scattering: Optimal Performance Through Thickness Control

Exploration and construction of novel π-conjugated organic semiconductors with low cost, small background interference, and excellent performance as surface-enhanced Raman scattering (SERS) substrates is one of the current focuses for the development of SERS technology. Based on precise control over synthesis conditions, a series of large-area tetraphenylporphyrin-based 2D covalent-organic framework membranes (2D-porphyrin-COFs) with high uniformity and precisely controllable thickness are constructed as SERS substrates. The delicate balance among the intensity of the substrate interference, the degree of π-conjugation extension, and the proportion of the edge-on channels within the total exposed region results in the optimal SERS performance of ultrathin multilayer 2D-porphyrin-COFs with the thickness between 5.0 to 9.0 nm toward MB, including the enhancement factor on the order of 10⁵ and the experimental limit of detection down to 10^-8 M, which are comparable to classic plasmonic metal substrates. This work highlights the powerful application potential of COFs in the SERS field and unveils thickness control as an effective strategy to facilitate the exploration of high-performance organic SERS substrates.