Single‐Atom Gd Nanoprobes for Self‐Confirmative MRI with Robust Stability

Abstract

which could exacerbate nephrogenic systemic fibrosis, and deposit in the heart, kidney, skin and even in the brain, threatening the patient's health. [5] To overcome these shortcomings, Gdbased inorganic nanoparticles (NPs), such as Gd 2 O 3 , [6] NaGdF 4 , [7] Gd-doped oxide, [8] were reported on account of the higher stability of Gd 3+ ions in Gd NPs-based CAs. The unsaturated coordination properties of Gd 3+ exposed at the NPs surface can be used to increase the hydration sites, which contribute to the high T 1 relaxivity for MRI. [9] While metal ions inside NPs were not available for direct chemical exchange with protons and thus attenuated the MRI signal. Therefore, increasing the number of accessible paramagnetic metal centers would directly increase the relaxivity. Recently, single-atom materials (SAMs) have been reported and extensively utilized in catalysis, pollution treatment, biosensing, and tumor therapy. [10] However, few studies have focused on the construction of SAMs as MRI contrast agents. [11] We anticipated that SAMs such as single-atom Gd nanomaterial could provide as a good substitute for traditional Gd complex by simulating the coordination environment of paramagnetic metal with ligands at the atomic level, maximizing the exposure of Gd atom as well as possessing better structural stability, which was conducive to enhance imaging contrast. In addition, according to Solomon-Bloembergen-Morgan theory, [12] the enhanced relaxivity was mainly related to increasing the number of coordination water in the inner layer (q), shortening water exchange lifetime (τ M ), prolonging molecular rotation time (τ R ) and facilitating diffusion related time of water molecules (τ D ). [13] Therefore, the fabrication of Gd SAMs with open coordination sites, large surface area, and hydrophilic properties are highly desirable. In another aspect, due to the fluctuation of endogenous paramagnetic species as well as the concentration gradient of CAs, false-positive signals were frequently encountered. [14] As we know, T 1 /T 2 dual-mode MRI CAs have attracted great research interest, with T 1 -weighted MRI being used to assess adipose tissues or anatomical structures, while T 2 -weighted MRI for assessing waterrich structures or local inflammation. [15] Thus, self-confirmative CAs especially with T 1 /T 2 dual-modal MRI were able to provide an accurate diagnosis. [16] Herein, we constructed a highly stable single atom Gd-based MRI CA through spatial confinement and controllable etching strategy for effective T 1 /T 2 dual-mode MRI with excellent Gadolinium (Gd)-based complexes are extensively utilized as contrast agents (CAs) in magnetic resonance imaging (MRI), yet, suffer from potential safety concerns and poor tumor targeting. Herein, as a mimic of Gd complex, single-atom Gd nanoprobes with r 1 and r 2 values of 34.2 and 80.1 mM −1 s −1 (far higher than that of commercial Gd CAs) at 3 T are constructed, which possessed T 1 /T 2 dual-mode MRI with excellent stability and good tumor targeting ability. Specificall...

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