Ultrathin Two-Dimensional Metal-Organic Framework Nanosheets with Activated Ligand-Cluster Units for Enhanced SERS

Ultrathin two-dimensional (2D) metal-organic framework (MOF) nanosheets (MOFNs) comprise an emerging family of attractive materials with excellent potential for use in different catalytic, electrochemical, and sensing applications owing to their striking features such as ultrathin thickness, a large surface area, and highly ordered network structures. However, to the best of our knowledge, the ligand-cluster units activated through exfoliation into the MOFNs have rarely been realized, which is indeed crucial for surface-enhanced Raman scattering (SERS) analysis. Herein, we emphasize that the activated ligand-cluster units are based on the accessible coordination sites at the exposed cluster nodes accompanied by a complete excitation of the ligand-cluster units under incident photons, which make MOFNs highly effective SERS substrates, significantly outperforming their bulk counterparts. The SERS enhancement of MOFNs is further illustrated by an efficient integration of the inherent ligand-cluster charge-transfer (LCCT) transitions in MOFNs into interfacial charge-transfer processes through an "L"-type charge-transfer (CT) pathway, as further evidenced by an ultrahigh degree (0.98) of CT contributed to the SERS enhancement. This study provides an efficient strategy of exfoliating MOFs into ultrathin nanosheets for the design of highly efficient MOF-based SERS substrates.