Versatile Nickel(II) Scaffolds as Coordination-Induced Spin-State Switches for 19 F Magnetic Resonance-Based Detection

An Fe complex for 19F magnetic resonance-based reversible redox sensing and multicolor imaging

We report a first-in-class responsive, pentafluorosulfanyl (-SF₅)-tagged ¹⁹F MRI agent capable of reversibly detecting reducing environments via an Fe^II/III redox couple. In the Fe^III form, the agent displays no ¹⁹F MR signal due to paramagnetic relaxation enhancement-induced signal broadening; however, upon rapid reduction to Fe^II with one equivalent of cysteine, the agent displays a robust ¹⁹F signal. Successive oxidation and reduction studies validate the reversibility of the agent. The -SF₅ tag in this agent enables 'multicolor imaging' in conjunction with sensors containing alternative fluorinated tags and this was demonstrated via simultaneous monitoring of the ¹⁹F MR signal of this -SF₅ agent and a hypoxia-responsive agent containing a -CF₃ group.

19 F MRI Nanotheranostics for Cancer Management: Progress and Prospects

Fluorine magnetic resonance imaging (19 F MRI) is a promising imaging technique for cancer diagnosis because of its excellent soft tissue resolution and deep tissue penetration, as well as the inherent high natural abundance, almost no endogenous interference, quantitative analysis, and wide chemical shift range of the 19 F nucleus. In recent years, scientists have synthesized various 19 F MRI contrast agents. By further integrating a wide variety of nanomaterials and cutting-edge construction strategies, magnetically equivalent 19 F atoms are super-loaded and maintain satisfactory relaxation efficiency to obtain high-intensity 19 F MRI signals. In this review, the nuclear magnetic resonance principle underlying 19 F MRI is first described. Then, the construction and performance of various fluorinated contrast agents are summarized. Finally, challenges and future prospects regarding the clinical translation of 19 F MRI nanoprobes are considered. This review will provide strategic guidance and panoramic expectations for designing new cancer theranostic regimens and realizing their clinical translation.

Enzyme Control Over Ferric Iron Magnetostructural Properties

Fe³⁺ complexes in aqueous solution can exist as discrete mononuclear species or multinuclear magnetically coupled species. Stimuli-driven change to Fe³⁺ speciation represents a powerful mechanistic basis for magnetic resonance sensor technology, but ligand design strategies to exert precision control of aqueous Fe³⁺ magnetostructural properties are entirely underexplored. In pursuit of this objective, we rationally designed a ligand to strongly favor a dinuclear μ-oxo-bridged and antiferromagnetically coupled complex, but which undergoes carboxylesterase mediated transformation to a mononuclear high-spin Fe³⁺ chelate resulting in substantial T₁ -relaxivity increase. The data communicated demonstrate proof of concept for a novel and effective strategy to exert biochemical control over aqueous Fe³⁺ magnetic, structural, and relaxometric properties.

Single Fluorinated Agent for Multiplexed 19 F-MRI with Micromolar Detectability Based on Dynamic Exchange

The weak thermal polarization of nuclear spins limits the sensitivity of MRI, even for MR-sensitive nuclei as fluorine-19. Therefore, despite being the source of inspiration for the development of background-free MRI for various applications, including for multiplexed imaging, the inability to map very low concentrations of targets using ¹⁹ F-MRI raises the need to further enhance this platform's capabilities. Here, we employ the principles of CEST-MRI in ¹⁹ F-MRI to obtain a 900-fold signal amplification of a biocompatible fluorinated agent, which can be presented in a "multicolor" fashion. Capitalizing on the dynamic interactions in host-guest supramolecular assemblies in an approach termed GEST, we demonstrate that an inhalable fluorinated anesthetic can be used as a single ¹⁹ F-probe for the concurrent detection of micromolar levels of two targets, with potential in vivo translatability. Further extending GEST with new designs could expand the applicability of ¹⁹ F-MRI to the mapping of targets that have so-far remained non-detectable.

The Design of Abnormal Microenvironment Responsive MRI Nanoprobe and Its Application

Magnetic resonance imaging (MRI) is often used to diagnose diseases due to its high spatial, temporal and soft tissue resolution. Frequently, probes or contrast agents are used to enhance the contrast in MRI to improve diagnostic accuracy. With the development of molecular imaging techniques, molecular MRI can be used to obtain 3D anatomical structure, physiology, pathology, and other relevant information regarding the lesion, which can provide an important reference for the accurate diagnosis and treatment of the disease in the early stages. Among existing contrast agents, smart or activatable nanoprobes can respond to selective stimuli, such as proving the presence of acidic pH, active enzymes, or reducing environments. The recently developed environment-responsive or smart MRI nanoprobes can specifically target cells based on differences in the cellular environment and improve the contrast between diseased tissues and normal tissues. Here, we review the design and application of these environment-responsive MRI nanoprobes.

Metal-based environment-sensitive MRI contrast agents

Interactions of paramagnetic metal complexes with their biological environment can modulate their magnetic resonance imaging (MRI) contrast-enhancing properties in different ways, and this has been widely exploited to create responsive probes that can provide biochemical information. We survey progress in two rapidly growing areas: the MRI detection of biologically important metal ions, such as calcium, zinc, and copper, and the use of transition metal complexes as smart MRI agents. In both fields, new imaging technologies, which take advantage of other nuclei (¹⁹F) and/or paramagnetic contact shift effects, emerge beyond classical, relaxation-based applications. Most importantly, in vivo imaging is gaining ground, and the promise of molecular MRI is becoming reality, at least for preclinical research.

A Fluorescence-Detected Coordination-Induced Spin State Switch

The response of the spin state to in situ variation of the coordination number (CISSS) is a promising and viable approach to smart sensor materials, yet it suffers to date from insensitive detection. Herein, we present the synthetic access to a family of planar nickel(II) complexes, whose CISSS is sensitively followed by means of fluorescence detection. For this purpose, nickel(II) complexes with four phenazine-based Schiff base-like ligands were synthesized and characterized through solution-phase spectroscopy (NMR and UV-vis), solid-state structure analysis (single-crystal XRD), and extended theoretical modeling. All of them reveal CISSS in solution through axial ligating a range of N- and O-donors. CISSS correlates nicely with the basicity of the axial ligand and the substitution-dependent acidity of the nickel(II) coordination site. Remarkably, three out of the four nickel(II) complexes are fluorescent in noncoordinating solvents but are fluorescence-silent in the presence of axial ligands such as pyridine. As these complexes are rare examples of fluorescent nickel(II) complexes, the photophysical properties with a coordination number of 4 were studied in detail, including temperature-dependent lifetime and quantum yield determinations. Most importantly, fluorescence quenching upon adding axial ligands allows a "black or white", i.e. digital, sensoring of spin state alternation. Our studies of fluorescence-detected CISSS (FD-CISSS) revealed that absorption-based CISSS and FD-CISSS are super proportional with respect to the pyridine concentration: FD-CISSS features a higher sensitivity. Overall, our findings indicate a favored ligation of these nickel(II) complexes in the excited state in comparison to the ground state.

Rational Ligand Design Enables pH Control over Aqueous Iron Magnetostructural Dynamics and Relaxometric Properties

Complexes of Fe³⁺ engage in rich aqueous solution speciation chemistry in which discrete molecules can react with solvent water to form multinuclear μ-oxo and μ-hydroxide bridged species. Here we demonstrate how pH- and concentration-dependent equilibration between monomeric and μ-oxo-bridged dimeric Fe³⁺ complexes can be controlled through judicious ligand design. We purposed this chemistry to develop a first-in-class Fe³⁺-based MR imaging probe, Fe-PyCy2AI, that undergoes relaxivity change via pH-mediated control of monomer vs dimer speciation. The monomeric complex exists in a S = 5/2 configuration capable of inducing efficient T₁-relaxation, whereas the antiferromagnetically coupled dimeric complex is a much weaker relaxation agent. The mechanisms underpinning the pH dependence on relaxivity were interrogated by using a combination of pH potentiometry, ¹H and ¹⁷O relaxometry, electronic absorption spectroscopy, bulk magnetic susceptibility, electron paramagnetic resonance spectroscopy, and X-ray crystallography measurements. Taken together, the data demonstrate that PyCy2AI forms a ternary complex with high-spin Fe³⁺ and a rapidly exchanging water coligand, [Fe(PyCy2AI)(H₂O)]⁺ (ML), which can deprotonate to form the high-spin complex [Fe(PyCy2AI)(OH)] (ML(OH)). Under titration conditions of 7 mM Fe complex, water coligand deprotonation occurs with an apparent pK_a 6.46. Complex ML(OH) dimerizes to form the antiferromagnetically coupled dimeric complex [(Fe(PyCy2AI))₂O] ((ML)₂O) with an association constant (K_a) of 5.3 ± 2.2 mM^-1. The relaxivity of the monomeric complexes are between 7- and 18-fold greater than the antiferromagnetically coupled dimer at applied field strengths ranging between 1.4 and 11.7 T. ML(OH) and (ML)₂O interconvert rapidly within the pH 6.0-7.4 range that is relevant to human pathophysiology, resulting in substantial observed relaxivity change. Controlling Fe³⁺ μ-oxo bridging interactions through rational ligand design and in response to local chemical environment offers a robust mechanism for biochemically responsive MR signal modulation.