MOF nanosheets with topology enhanced ultrahigh relaxivity

Two-dimensional (2D) metal-organic framework (MOF) nanosheets have emerged as a promising class of functional materials. Nevertheless, the fabrication of MOF nanosheets with well-defined uniformity and ultrathin thickness via traditional synthetic strategies still remains a great challenge. Herein, we report that ultrasonic treatment plays a crucial regulatory role in the bottom-up assembly of MOF nanosheets. This innovative synthetic protocol enables the precise preparation of Co-Mn-TCPP nanosheets with a uniform thickness of ∼1.82 nm. Notably, the as-synthesized Co-Mn-TCPP nanosheets exhibit an ultrahigh longitudinal relaxivity (r₁) of 12.05 mM^-1s^-1. A combination of theoretical calculations and experimental characterizations reveals that the exceptional relaxivity of Co-Mn-TCPP nanosheets stems predominantly from their unique 2D square lattice structure, which imposes a strong restriction on molecular motion and thus prolongs the rotational correlation time (τᵣ). Furthermore, this single-unit-cell 2D MOF demonstrates remarkable magnetic resonance imaging (MRI) contrast enhancement efficacy across multiple tumor models. More importantly, it can effectively assist in MRI-guided precise resection of small tumors. This work not only showcases the great versatility of ultrasound as a powerful tool for the synthesis of high-quality MOF nanosheets but also provides a new strategy for the design of high-relaxivity MRI contrast agents by capitalizing on specific 2D topological features.