Recent progress on MRI probes for metal ion detection and biological applications

Abstract

The disorder of metal ion levels can have a significant impact on the ecological environment and human health due to their crucial role in complex biological metabolism processes and the overall ecological balance. Traditional detection technologies for various metal ions have been successfully developed. However, these traditional methods have limitations when it comes to monitoring the physiological distribution of metal ions with high sensitivity. Magnetic resonance imaging (MRI), known for its ability to provide deep tissue imaging with excellent spatiotemporal resolution, has been widely adopted in biomedical and clinical diagnostics. Interestingly, numerous studies have focused on designing and developing MRI nanoprobes specifically for the detection of metal ions. In this review, we highlight the research progress, limitations, and remaining challenges in the development of metal ion detection probes based on MRI technology.

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Zhiming Deng

Zhiming Deng received his degrees (PhD in 2022) in physics from the Key Laboratory of Low-dimensional Quantum Structures and Quantum Control of Ministry of Education, Hunan Normal University. He is currently doing postdoctoral research in the College of Chemistry and Chemical Engineering at Hunan University under the supervision of Prof. Guosheng Song. His research is focused on the design and synthesis of functional nanomaterials for biomedical applications.

Dingyou Lu

Dingyou Lu is currently a postgraduate in the College of Chemistry and Chemical Engineering at Hunan University under the supervision of Prof. Guosheng Song. He received his BS degree from Hunan University in 2022. His current research focuses on designing MRI nanomaterials for bioimaging and cancer therapy.

Sulai Liu

Sulai Liu is an associate chief physician at Hunan Provincial People's Hospital (The First Affiliated Hospital of Hunan Normal University) in China. He received his PhD degree from Central South University in 2016. His research focuses on hepatocellular carcinoma, cholangiocarcinoma, and immunotherapy.

Xianzheng Tan

Dr Xian-Zheng Tan is an associate chief physician of radiology at Hunan Provincial People's Hospital, the First Affiliated Hospital of Hunan Normal University in China. His research interests are focused on artificial intelligence applications in hepatobiliary-pancreas imaging.

Guosheng Song

Guosheng Song is a professor at Hunan University in China. He received his PhD degree from Donghua University in 2014. He carried out postdoctoral research at Soochow University and then at Stanford University from 2014 to 2018. In 2018, he joined the College of Chemistry and Chemical Engineering, Hunan University. His research mainly focuses on designing activatable nanoplatforms for multimode imaging and cancer specific therapy.

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1. Introduction

Metal ions, such as Zn²⁺, Ca²⁺, K⁺, Mg²⁺, Cu²⁺, Pb²⁺, Mn²⁺, Fe^2+/3+, and Cd²⁺, play a crucial role in numerous biological processes, including cellular functions, osmotic regulation, molecular structures, and maintaining water and electrolyte balance in organisms.¹ It is essential to maintain appropriate concentrations of metal ions for the appropriate functioning of biological systems. Additionally, metal ions are distributed unevenly across various organs,² further emphasizing the importance of tightly controlling their levels to maintain normal physiological processes. Consequently, there is growing interest in non-invasive quantitative analysis of metal ion levels in vivo.

Recent advancements in life sciences have revealed associations between metabolic disorders of metal ion levels and conditions such as Wilson's disease,³ diabetes,⁴ and malignant tumors.⁵ For instance, Wilson's disease is a congenital disorder of copper metabolism, characterized by the presence of excessive free copper ions in the patient's urine. Metal-mediated oxidative stress also contributes to tumor growth and abnormal angiogenesis in malignant tumors.⁶ These findings highlight the urgent need for a novel imaging tool capable of quantitatively monitoring and spatially detecting metal ion levels in vivo. While there are various technologies available for metal ion detection, such as photoacoustic imaging,⁷ optical imaging,^8–10 and ICP-MS detection,¹¹ these methods can be costly and involve complex operational processes. Therefore, there is a pressing demand for the development of novel imaging technology specifically designed for non-invasive monitoring of metal ion levels in deep tissues.

Magnetic resonance imaging (MRI)^12–19 is a non-invasive imaging method commonly used in clinical diagnostics for detecting disorders and monitoring treatment outcomes. It has proven particularly useful in high-resolution imaging of soft tissues, making it a valuable tool in biomedical research and clinical diagnostics. In recent years, the field of MRI has made significant progress, offering capabilities for high-resolution anatomical imaging and monitoring. Several MRI contrast strategies have been developed, including well-designed probes for various purposes, such as tracking therapeutic cells, imaging enzyme activity, and regulating gene expression. Numerous MRI nanoprobes have also been designed for the detection of metal ions, including Zn²⁺,^20,21 Ca²⁺,²² K⁺,²³ Mg²⁺,²³ Cu²⁺,^24–27 and heavy metal ions²⁸ such as Pb²⁺ and Cd²⁺.

While previous reviews on metal ion monitoring have focused on optical imaging,²⁹ colorimetric techniques,³⁰ and photoacoustic imaging,³¹ there is limited research summarizing the use of MRI for systematic and comprehensive metal ion detection. This review aims to provide a detailed introduction to various metal ion detection methods based on MRI imaging technology (Scheme 1). It will cover recent progress in metal ion detection and highlight the key challenges for future applications.

Scheme 1 Schematic illustration of MRI probes for the detection of various metal ions in vitro and in vivo.

2. Design of MRI-based probes for metal ion detection

Metal ion-responsive MRI probes are developed to detect specific metal ions by altering MRI signals through the interaction between the targets and the probes. There are four main types of metal ion-responsive MRI probes.³²

1. Metal ion-identification moiety and MRI contrast agent

This approach combines a metal ion-identification moiety with an MRI contrast agent for ion-specific detection. The contrast agent interacts with water molecules when the ion receptor binds to the metal ion, leading to changes in relaxation times (Fig. 1A).

Fig. 1 Schematic illustration of MRI probes for metal ion detection. (A) The mechanism for metal ion detection based on T₁ contrast agents. (B) Sensing metal ions using a heteronuclear sensor. (C) The mechanism for metal ion detection based on SPIO NP. (D) The mechanism of MRI metal ion detection based on HSA.

2. ¹⁹F-MRI probes

Small-molecule-based ¹⁹F-MRI probes can be classified into two types based on their ¹⁹F signal readout: off–on and chemical shift change. Off–on probes use paramagnetic metal ions as quenchers to switch off the ¹⁹F signal, which can be de-quenched by cleavage or changes in the redox states of the quenchers in response to the analytes of interest (Fig. 1B).

3. Fe₃O₄ nanoparticles

Fe₃O₄ nanoparticles, also known as superparamagnetic iron oxide (SPIO) nanoparticles, can influence T₁/T₂ signal changes through a self-assembly strategy (Fig. 1C).

4. Contrast agents with increased relaxivity

Some contrast agents exhibit increased relaxivity in the presence of metal ions by forming a ternary complex with human serum albumin (HSA) (Fig. 1D).

These different approaches offer diverse mechanisms for metal ion detection and enable label-free MRI probes when non-¹H nuclei are involved. By utilizing these strategies, researchers can design metal ion-responsive MRI probes for specific detection and imaging applications.

3. Detection of metal ions

3.1. MRI probe for Zn²⁺ ion monitoring

With its involvement in metalloenzyme catalysis, signal transduction, DNA binding, and gene expression, Zn²⁺ is crucial for the survival of organisms. The majority of Zn²⁺ ions in biological systems are firmly bound by proteins.³³ However, Zn²⁺ abnormalities have been linked to various physical growth retardations and neurological conditions, including Alzheimer's disease and cerebral ischemia. To detect Zn²⁺ levels using MR imaging techniques, several MRI probes have been designed.

For instance, Sherry and co-workers³⁴ pioneered the design of a novel probe based on GdDOTA-diBPEN (Fig. 2A). This probe has a high detection limit (about 30 μM) for Zn²⁺ ion detection in the presence of HSA. The probe's relaxivity (r₁) increased from 5.0 ± 0.1 mM⁻¹ s⁻¹ to 6.0 ± 0.1 mM⁻¹ s⁻¹ in the presence of Zn²⁺, making it suitable for detecting free Zn²⁺ ions in vivo without interfering with other biological processes involving Zn²⁺. In this case, GdDOTA-diBPEN's relaxivity rose by around three times, and the agent's binding to HSA was extremely strong. Next, in order to maximize the r₁ of the Zn²⁺-responsive probes, a series of MRI probes³⁵ (Fig. 2B) were designed to fine-tune the rate of water exchange from the inner sphere of Gd. Especially for Gd-4 and Gd-5, which were up to 50 mM⁻¹ s⁻¹ for r₁ relaxivity values upon binding to Zn(ii) and HSA.

Fig. 2 (A) Hypothetical structures and T₁-weighted phantom MR images of GdDOTA-diBPEN in the presence and absence of Zn²⁺. Reproduced from ref. 34 with permission from American Chemical Society, copyright 2009. (B) Increasing Zinc(ii) sensitivity MRI contrast agents with responsive water exchange rates. Reproduced from ref. 35 with permission from American Chemical Society, copyright 2015. (C) Proposed mechanism of a Zn²⁺-activated Gd-daa3 contrast agent. Reproduced from ref. 36 with permission from American Chemical Society, copyright 2008. Reproduced from ref. 37 with permission from NATL ACAD SCIENCES, copyright 2007. (D) GdL binding mode to Zn²⁺ and confocal fluorescence pictures in MDA-MB-231 cells after incubation with Zn²⁺. Reproduced from ref. 38 with permission from American Chemical Society, copyright 2012. (E) Optimized geometry of the SAP isomers of GdZnL²⁺ and GdHL⁺. Reproduced from ref. 39 with permission from American Chemical Society, copyright 2015. (F) Zn²⁺-induced T₁/T₂ signal change in solution. Reproduced from ref. 40 with permission from NATL ACAD SCIENCES, copyright 2007. (G) The steps involved in insulin and Zn²⁺ release from-cells during glucose-stimulated insulin secretion are depicted schematically. Reproduced from ref. 41 with permission from NATL ACAD SCIENCES, copyright 2011.

For other Zn²⁺ detection probes, Meade and co-workers^36,37(Fig. 2C) designed double kinds of Zn²⁺-responsive GdDO3A-based agents containing either an iminodiacetate or a pyridine substituent for specific Zn²⁺ binding. The advantage of those probes relies on the fact that upon Zn²⁺ binding, the Gd³⁺ center becomes accessible for water molecule exchange, resulting in increased relativity and a high detection limit for Zn²⁺ detection. To further verify the biosafety of the designed probe, two kinds of cells were treated with 0.5 mM Gd-daa3 probes, which showed no observable cell apoptosis. Cells proliferated exponentially at the expected rate, indicating low toxicity from the agent. These results provide support for potential in vivo studies in the future. Therefore, dual imaging probes are urgently being developed to improve the specificity of Zn²⁺ ion detection. Following these efforts to develop MRI based probes to monitor Zn²⁺ in biology, a Zn²⁺-responsive bimodal probe GdL based on Gd-DO3A bound to 8-sulfonamidoquinoline was proposed.³⁸ As the fluorescence increases, the r₁ of GdL also increases from 3.8 mM⁻¹ s⁻¹ to 5.9 mM⁻¹ s⁻¹ in the presence of Zn²⁺(Fig. 2D). In the physiological pH range, the Zn²⁺ responsive imaging probe exhibits favorable selectivity and tolerance over a range of biologically significant anions and metal ions for both relaxivity and luminescence. In order to improve the contrast performance of Zn²⁺ detection, a DO3A unit for Gd³⁺ chelate was linked to an NO2A moiety for Zn²⁺ binding.³⁹ Obviously, the relativity of the obtained sensor was increased by 150% at a high magnetic field B₀ (7.0 T) in the presence of Zn²⁺ (Fig. 2E). Although other strategies for improving Zn²⁺ detection sensitivity were proposed, none of the mentioned probes have been further used in vivo.

For further in vivo detection of Zn²⁺, Lippard and coworkers designed an alternative scaffold called Mn-(DPA-C2)2-TPPS3 that uses Zn²⁺-selective binding units coupled to a water-soluble porphyrin template to make it easier to utilize probes to detect fluctuating Zn²⁺ levels in biological systems.⁴⁰ As shown in Fig. 2F, the zinc-containing solution presented a weaker signal than the zinc-free sample, and the reference Mn-TPPS₄ showed no zinc-induced T₂ change. This Mn-porpyrin probe has several advantages over the Gd-based probes stated above, including the ability to be detected by both MRI and optical modalities. Recently, the high-affinity Zn²⁺ agent, GdL1, and the low-affinity Zn²⁺ agent, GdL2, were described as improved Zn²⁺-responsive MRI contrast agents⁴¹ for visualization of beta-cell function (Fig. 2G). Deep tissue monitoring of beta-cell activity in vivo during type 2 diabetes progression or following islet implantation in type I diabetic patients is an attractive prospective use for these MRI probes.

Recently, in order to distinguish Zn²⁺ from other abundant endogenous metal ions, a highly effective MRI contrast agent based on paramagnetic Gd³⁺ has been created.⁴² Its longitudinal relaxivity significantly increases as a result of the interaction with Zn²⁺, which modifies the hydration of the complex. An experiment using tube phantoms indicated the potential of this compound to function as an MRI contrast agent. Six tubes containing GdL alone or with various metal ions were photographed. As demonstrated in Fig. 3A, tubes containing GdL and Zn²⁺ present a significant increase in MR signal strength, but tubes containing Ca²⁺ or Mg²⁺ had no discernible change. They designed a novel responsive platform that is ideal for exploring Zn-related physiological processes in high magnetic fields. Bar-Shir et al. recently developed innovative probes for the detection of Zn²⁺ in the brain.⁴³ A strong ¹⁹FiCEST effect was achieved, which was depicted as a Zn²⁺ map superimposed on a ¹H-MRI anatomical image, as expected from a labile-zinc-rich ROI. In contrast, no discernible difference between the two ¹⁹F-MRI data sets could be shown, and the resulting ¹⁹F-iCEST exhibited no signal when the probe was administered to an area that was not anticipated to contain large levels of labile Zn²⁺ (Fig. 3B). As we all know, a healthy prostate has the highest concentration of Zn²⁺. Due to the fact that this level drops dramatically during the early stages of prostate cancer, in vivo detection of prostate zinc content may be used for prostate cancer diagnosis.⁴⁴ There was a distinct difference in the ¹⁹F-iCEST signal before and after D-glucose injection, indicating the presence of a normal prostate. In the future, in order to achieve more accurate Zn²⁺ ion detection, the sensitivity and specificity can be further improved. Achieving good biocompatibility is a primary goal for clinical applications of materials. Most probes are mainly organic and face issues such as poor stability. It is necessary to develop stable and specific probes that are not affected by other metal ions.

Fig. 3 (A) The T₁-weighted MR images of GdL treated with different metal ions. Reproduced from ref. 42 with permission from Willey VCH, copyright 2021. (B) In vivo¹⁹F-iCEST maps of Zn²⁺ in the mouse brain. Reproduced from ref. 43 with permission from American Chemical Society, copyright 2021.

It should be pointed out that MRI probes for Zn²⁺ detection often lack the required sensitivity to detect low concentrations of Zn²⁺. This can limit their usefulness in applications where high sensitivity is crucial, such as in the detection of Zn²⁺ related diseases. Meanwhile, MRI probes for Zn²⁺ detection may also lack specificity, meaning they can interact with other metal ions. This can lead to false-positive results and hinder accurate Zn²⁺ detection. And some MRI organic molecule probes for Zn²⁺ detection present poor stability, especially in complex biological environments. This can result in probe degradation or loss of signal over time, reducing their reliability and longevity. Moreover, for Zn²⁺ detection, crossing cell membranes and accessing intracellular Zn²⁺ pools is another challenge in studying Zn²⁺ dynamics within cells. To address these limitations, modifying the probe structure or incorporating specific binding moieties that enhance sensitivity and specificity for Zn²⁺ detection is feasible. The development of cell-penetrating peptides or nanoparticle-based delivery systems to facilitate probe entry into cells is able to improve the cell permeability of MRI probes for Zn²⁺ detection. By addressing these challenges and implementing potential solutions, the limitations of MRI probes for Zn²⁺ detection can be overcome, leading to improved sensitivity, specificity, stability, and cell permeability for a wide range of applications.

3.2. MRI probe for Ca²⁺ ion monitoring

All types of cells use Ca²⁺ signaling as a second messenger in signaling pathways to convey information and control many biochemical processes. One of the main focuses of life science research has been to create methods to dynamically analyze the Ca²⁺ concentration in vivo.⁴⁵ The dynamic evaluation of Ca²⁺ is mainly done through calcium optical imaging, which also sparked the creation of numerous synthetic and genetically engineered calcium markers.^46,47 Although this technique is capable of imaging Ca²⁺ signals in vivo, penetration of deep tissues by photon scattering is a major obstacle to its use in large animals and humans. Due to the importance of Ca²⁺ in the setting of cellular action potentials and its critical role in neuronal activity, researchers developed a non-invasive method to monitor changes in Ca²⁺ levels in deep neural tissue of living humans. The general comprehension of neuronal activity has greatly benefited from functional MRI investigations,⁴⁸ but the indirect observation of neuronal function necessitates a direct and quantitative MRI-based method to spatially detect Ca²⁺ alternations. Therefore, it is highly important to develop an MRI probe for direct observation of Ca²⁺ levels in vivo.

As mentioned above, designing an MRI probe for monitoring Ca²⁺ levels could aid in the diagnosis of Ca²⁺-related diseases. As shown in Fig. 4A, Dhingra et al. designed a Ca²⁺-sensitive MRI contrast agent that was blocked from inner-sphere water molecules by two APTRA iminoacetates that interacted with the Gd(III) centers.⁴⁹ As a consequence, the Gd-DOPTRA presents a low relaxivity down to 3.5 mM⁻¹ s⁻¹, and the probe presents a high detection specificity for Ca²⁺. Next, the group also designed a series of MRI probes (GdL1–GdL4) by combining the DO3A-Gd(III) complex with various chelators (Fig. 4B).⁵⁰ In these designed DO3A-Gd(III) complexes, the GdL1 complexes were found to be insensitive to Ca²⁺. When similar r₁ alterations were produced by the addition of other metal ions, it was discovered that GdL2 and GdL3 were responsive to Ca²⁺ in a variety of concentration ranges with poor selectivity. Interestingly, in a relaxometric titration experiment, the r₁ of GdL₄ was dramatically raised by 63% upon binding with Ca²⁺.

Fig. 4 (A) The process in equilibrium with Ca²⁺ of Gd-DOPTRA. Reproduced from ref. 49 with permission from The Royal Society of Chemistry, copyright 2008. (B) The structures of the investigated ligand L1–L4. Reproduced from ref. 50 with permission from The Royal Society of Chemistry, copyright 2011.(C) Chemical structures of L1 and L2. Reproduced from ref. 51 with permission from The Royal Society of Chemistry, copyright 2019. (D) Chemical structures of L¹, L² and L³. Reproduced from ref. 53 with permission from American Chemical Society, copyright 2008. (E)–(G) Chemical structures of Ca²⁺-sensitive MRI contrast agents. Reproduced from ref. 56, 57 and 58 with permission from American Chemical Society, American Chemical Society and Elsevier, copyright 2002, 2013 and 2016, respectively. (H) The Ca²⁺-dependent MRI signal changes in cells. Reproduced from ref. 59 with permission from The Springer Nature, copyright 2019.

Recently, other Ca²⁺-responsive MRI probes (Fig. 4C, named GdL1 and GdL2) with different linker lengths between the MRI signaling unit and Ca²⁺-responsive moieties were developed by Angelovski et al.⁵¹ The total r₁ and r₂ relaxivity changes for GdL₂ were measured by relaxometric titrations of both probes and were determined to be 131 and 138%, respectively. Greater r₁ and r₂ relaxivity modifications in GdL2 showed that even small structural variations between the acyclic Ca²⁺ binding unit and the macrocyclic Gd(III) ligand might result in significantly different relaxation characteristics of the Gd(III) complex. Then, a dual frequency response probe (SCA) to detect Ca²⁺ by recording ¹H and ¹⁹F NMR signals based on GdL was developed, in which –NO₂ was replaced by the –CF₃ group of the benzyl moiety.⁵² After the addition of Ca²⁺, the ¹H relaxometry measurement of SCAs showed a 2.89-fold increase in r₁ relaxivity from 3.18 to 9.20 mM⁻¹ s⁻¹ in contrast to GdL. Interestingly, the signal-to-noise ratio of the ¹⁹F NMR signal decreased from 46 to 20 in the presence of Ca²⁺. Both ¹H and ¹⁹F NMR signal changes allowed SCAs for dual-frequency Ca²⁺ responses. The results show that these double-frequency tracers are extremely promising for the creation of new functional MRI techniques.

It should be noted that it was shown to be difficult to track the fluctuation of Ca²⁺ concentration in the millimolar range using conventional nanoprobes for free Ca²⁺ concentration in the micromolar range. In view of this, Mishra et al. developed three Ca²⁺-responsive Ln³⁺ complex MRI probes (L1, L2, and L3) (Fig. 4D).⁵³ In order to expand the detection range to higher Ca²⁺ values, the Ca²⁺ recognition moiety in the center of these ligands has a low affinity for Ca²⁺. These MRI probes were developed by adding Gd(III)-DO3A-type bismacrocyclic complexes to Ca²⁺ recognition molecules such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriamine pentaacetic acid (DTPA) bisamides, respectively. Measurements of relaxation titration revealed that the ligand architecture affected how sensitive their relaxivity was toward Ca²⁺. After chelation with Ca²⁺, the relaxivity of L1 and L3 increased by 15% and 32%, respectively. Due to the significant distance between the Ca²⁺ binding site and Gd(III)-DO3A, the poor Ca²⁺ sensitivity of L2 was attributed to a poorer Ca²⁺ coordination capacity.

To measure Ca²⁺ fluctuations, Angelovski and coworkers also designed two calcium-activated MRI probes (Gd₂L¹ and Gd₂L²). The probes Gd₂L¹ and Gd₂L² were created by combining two DO3A-Gd(III) complexes with the Ca²⁺-specific chelator.⁵⁴ In order to lower the Ca²⁺-association constant of EGTA while maintaining its selectivity for other competing metals, two of its carboxylate groups were converted to amides. Due to the modification of EGTA, the Ca²⁺ chelating unit demonstrated high flexibility, which is advantageous for Ca²⁺ detection. It was discovered that the initial relativity was 4.05 and 3.44 mM⁻¹ s⁻¹.

Recently, two monomeric bifunctional chelators that can act as Ca²⁺-responsive contrast agents were described by Angelovski et al.⁵⁵ According to the results of the relaxometric nuclear magnetic resonance titrations, there were no detectable changes in the r₁ signals of the probe in the presence of Ca²⁺. Due to the interaction of GdL2 with extracellular Ca²⁺, the T₁-weighted signal intensity of MRI rapidly dropped, reaching up to 5% of the difference in the detrended signal. As shown in Fig. 4E, the Ca²⁺-sensitive MRI contrast agent, DOPTA-Gd (Gd-1), was designed by Meade and colleagues.⁵⁶ Two DO3A-Gd chelates are combined to form DOPTA-Gd, which is connected by a modified BAPTA moiety, a well-known selective Ca²⁺ binder. In the absence of Ca²⁺, the iminodiacetate groups of the BAPTA linker bind to the Gd³⁺ centers, producing a relaxivity of 3.26 mM⁻¹ s⁻¹. These groups preferentially bind Ca²⁺ upon the addition of Ca²⁺, opening up the coordination sites around the Gd³⁺ centers to solvent water and causing about 80% rise in relaxivity to 5.76 mM⁻¹ s⁻¹. The selectivity and Ca²⁺ binding affinity of DOPTA-Gd make it potentially useful for monitoring intracellular Ca²⁺ in a biological setting. McMahon et al. used M²⁺-5F-BAPTA probes to detect the signal of Ca²⁺ even at low Ca²⁺ concentrations with high sensitivity (Fig. 4F).⁵⁷ With the simple Ca²⁺ sensing substance EGTA (ethylene glycol tetraacetic acid), Wu et al. developed a Ca²⁺-responsive poly (ethylene glycol) and polyethylenimine coated iron oxide nanoparticle (PEI/PEG-SPIONs) MRI contrast agents.⁵⁸ As shown in Fig. 4G, the Ca²⁺-induced aggregation of EGTA-SPIONs reduced the T₂ weighted MRI signal intensity. When the Ca²⁺ concentration dropped from 1.2 to 0.8 mM, the T₂ values dropped by 25%. Furthermore, Ca²⁺ is required for signal transduction in almost all cells, where it coordinates activities ranging from embryogenesis to brain function. To solve this issue, Jasanoff et al. designed a novel contrast agent, ManICS1-AM, for monitoring calcium levels in cells by MRI.⁵⁹ As shown in Fig. 4H, when stimulated with pharmacological reagents or photogenetic tools, ManICS1-AM loaded cells showed changes in MRI contrast. The response ability is consistent with the signal obtained using a fluorescent calcium indicator. Furthermore, depending on the repeated brain activation in response to various stimuli in vivo, Okada et al. developed a specific MRI nanoprobe for endogenous Ca²⁺ detection,⁶⁰ which is suitable for tracking extracellular Ca²⁺ signaling processes. The created probes can diagnose AD quickly in response to changes in Ca²⁺ level with a high signal-to-noise ratio.

Furthermore, depending on the repeated brain activation in response to various stimuli in vivo, Okada et al. developed a specific MRI nanoprobe for endogenous Ca²⁺ detection,⁶⁰ which is suitable for tracking extracellular Ca²⁺ signaling processes. The created probes can diagnose AD quickly in response to changes in Ca²⁺ level with a high signal-to-noise ratio. Researchers employed the MaCaReNas in combination with medial forebrain bundle electrical stimulation, a neural activation paradigm often used in addiction research, to investigate MaCaReNas’ capacity to detect more physiological neuronal activation on a quicker timeline. MaCaReNas were injected into the dorsal striatum and subsequently scanned utilizing a block-design paradigm in combination with T₂-weighted spin-echo echo-planar imaging. Image contrast alterations were found only when functional MaCaReNas were administered ipsilaterally to the activated medial forebrain bundle, not when contralateral stimulation or calcium-insensitive LCIOs were utilized. All these well-designed MRI probes provided a promising method for Ca²⁺ monitoring with high specificity. In order to directly monitor neuronal networks and activity, manganese-enhanced MRI was proposed as an alternative or complementary method to fMRI, in which manganese ions (Mn²⁺) mimic Ca²⁺ and can be used as MRI contrast agents. However, high Mn²⁺ concentrations were associated with some toxic side effects. Therefore, a robust, routinely used molecular MR imaging methodology for direct observation of Ca²⁺ levels is of interest, and because of this several strategies to rationally design and synthesize molecular probes for this purpose are meaningful. Manganese-enhanced MRI has been proposed as an alternate or supplementary technique to MRI for directly monitoring neural networks and activity, in which manganese ions imitate Ca²⁺ and can be utilized as an MRI contrast agent. High Mn²⁺ concentrations, on the other hand, were linked to several hazardous adverse effects. As a result, a robust, frequently used molecular MR imaging technology for direct observation of Ca²⁺ levels is of interest, and numerous methodologies for rationally designing and synthesizing molecular probes for this purpose are being investigated.

3.3. MRI probe for Cu²⁺ ion monitoring

The copper ions are the third transition metal that is absolutely necessary for human health.^61,62 Additionally, copper is necessary for the growth of connective tissue, cellular respiration, and the production of bones.⁶³ However, unusual Cu²⁺ concentrations may lead to Menkes disease,⁶⁴ Wilson disease⁶⁵ and Alzheimer's disease.⁶⁶ Therefore, it is important to explore highly sensitive probes for Cu²⁺ level detection in vivo. Instrument-based analytical techniques, such as optical approaches and graphite furnace atomic absorption spectroscopy, are the standard for Cu²⁺ detection. However, due to their low sensitivity, expensive equipment costs, and need for trained staff, their use is limited during routine detection. Based on this, it is meaningful to design an MRI probe for Cu²⁺ detection.^67,68

Thus, Chang and colleagues designed a Cu²⁺-sensitive MRI probe with remarkable selectivity.⁶⁹ A Gd(III) complex was coupled to a pendant iminodiacetate Cu²⁺ binding site to create the probe. The entry of inner-sphere water was hampered by the interaction of carboxylates with the Gd(III) ion. In order to facilitate the access of inner-sphere water to the Gd(III) center, the pendant carboxylate receptor reduced steric constriction by chelating Cu²⁺. With the binding of Cu²⁺, the relaxivity of copper-gad-1 rose from 3.76 to 5.29 mM⁻¹ s⁻¹. Moreover, it was discovered that the Cu²⁺ binding to the probe was reversible, with the relaxivity returning once the probe-Cu²⁺ was treated with too much EDTA with super Cu²⁺ selectivity.

Interestingly, Chang et al. designed a series of copper-activated MRI probes (CG2–CG6) for the detection of copper at different oxidation states.⁷⁰ The probes were created by joining several thioether-based copper receptors with the Gd(III)-DO3A scaffold through a 2,6-dimethylpyridine linker (Fig. 5A). These probes demonstrated strong Cu²⁺ or Cu⁺ selectivity over competing metal ions. Due to the coordination of the pyridyl linker with Gd(III), in the absence of Cu²⁺, these probes displayed poor proton relaxivity (r₁ = 1.2–2.2 mM⁻¹ s⁻¹). As shown in Fig. 5B, the CG2–CG5 responded to Cu⁺ only, but probe CG6 responded to both Cu⁺ and Cu²⁺, leading to a striking increase in relaxivity (Dr₁ = 2.3–6.9 mM⁻¹ s⁻¹). The largest “off–on” responses (360% increasement) were seen for these probes (CG2 and CG3) after Cu⁺ binding. The Cu⁺ concentrations ranged from 0 to 100 mM, and the T₁-weighted image clearly displayed an increase in MRI signals. In order to improve the charge and hydrophilicity of the platform, the probe was created by swapping out the acetate arms with hexanedioate arms. The 2-(ethylthio)-N-(2-(ethylthio)ethyl)ethana mine substituted pyridyl copper recognition unit was joined to a Gd(III)-DO3A complex. With Cu⁺ binding, the r₁ of copper-Gad-7 increased from 2.6 to 11.4 mM⁻¹ s⁻¹ (about 340% enhancement), and this rise in relaxivity is equivalent to the r₁ increase of probe CG2. Over competing cations and anions, such as Cu²⁺, citrate, bicarbonate, phosphate, and lactate anions, the copper-gad-7 probe demonstrated remarkable specificity for Cu⁺. The probes might be utilized to capture MRI pictures and track changes in Cu⁺ levels, according to T₁-weighted images.

Fig. 5 (A) Molecular structure of CG2–CG6. Reproduced from ref. 70 with permission from The Royal Society of Chemistry, copyright 2009. (B) T₁-weighted phantom images of CG2–CG6 with and without Cuⁿ⁺. Reproduced from ref. 70 with permission from The Royal Society of Chemistry, copyright 2009. (C) Plots of 1/T₁vs. concentration of Cu²⁺ added to different probes to form complexes. Reproduced from ref. 72 with permission from American Chemical Society, copyright 2019. (D) MRI T₁-weighted images of PVIm dots aqueous solution with different concentrations of Cu²⁺. Reproduced from ref. 72 with permission from American Chemical Society, copyright 2019. (E) Imaging of free Cu²⁺in vivo. Reproduced from ref. 73 with permission from The Royal Society of Chemistry, copyright 2019.

Recently, Aoki et al. investigated a novel probe based on the nido-carborane derivative 4 bearing N,N,N′-trimethylethylenediamine for the ultrasensitive detection of Cu²⁺.⁷¹ It observed changes in the ¹¹B NMR spectra and MRI images of 4 bearing N,N,N′-trimethylethylenediamine after different concentrations of Cu²⁺ incubation. In addition, prolonged clearance times are highly expressed on the detection and treatment of serious diseases. For instance, in a single step, Hong Bi and coauthors created PVIm dots with good biocompatibility, hydrophilic properties, and a strong affinity for Cu²⁺ (Fig. 5C and D).⁷² This makes it simple to create Cu²⁺-PVIm dots complexes. It has been effectively shown in vitro that the Cu²⁺-PVIm dots complexes work well as MRI contrast agents. A novel approach for MRI testing and prolonged renal clearance time can most likely be provided by the PVIm dots. The Cu^2+/+-activatable MRI probe discussed here may serve as a promising tool for Cu^2+/+-related disease diagnosis. Interestingly, Sherry and co-workers designed a probe named GdL1, which presents high selectivity for Cu²⁺ with a 43% increase in r₁ relaxivity. As shown in Fig. 5E, the T₁-weighted imaging demonstrates that GdL1 can detect Cu²⁺in vivo.⁷³ This is an unprecedented opportunity to investigate the function of Cu²⁺ in the genesis and progression of neurological illnesses like Wilson's disease.

MRI probes for Cu²⁺ detection, like Zn²⁺ probes, may lack the sensitivity necessary to detect low amounts of Cu²⁺. This can restrict their utility in applications requiring high sensitivity, such as the detection of Cu²⁺ in biological samples. Background interference from endogenous Cu²⁺ found in biological samples can make MRI probes for Cu²⁺ detection vulnerable. This can make distinguishing between external and endogenous Cu²⁺ signals difficult. Similar to Zn²⁺ detection, signal amplification methods may be used to improve the sensitivity of Cu²⁺ detection in MRI probes. To boost the probe's capacity to detect low concentrations of Cu²⁺, amplification agents or signal enhancement techniques may be used. Methods for suppressing background interference from endogenous Cu²⁺ in biological samples can be investigated by researchers. This may entail the use of chelating or masking agents that bind to endogenous Cu²⁺ while minimizing interference with external Cu²⁺ signals. The limits of MRI probes for Cu²⁺ detection can be solved by addressing these issues and applying alternative solutions, resulting in enhanced sensitivity, specificity, stability, and reduced background interference for a wide range of applications.

3.4. MRI probe for K⁺ and Mg²⁺ monitoring

Potassium (K⁺) is widely distributed in the natural world in the form of salt and is also an important component in human muscle and nerve tissue. K⁺ is able to maintain nerve health and a normal heartbeat, can prevent strokes, and helps muscles contract normally. When high sodium intake leads to hypertension, K⁺ has the effect of lowering blood pressure.⁷⁴ Suzuki's group⁷⁵ (Fig. 6A and B) designed a Gd³⁺ contrast agent whose relaxivity relies on K⁺ concentration (KMR-K1, Gd-7). After adding K⁺, the r₁ decreases from 5.05 to 4.78 mM⁻¹ s⁻¹. The Mg²⁺, an electrolyte found in significant amounts all over the body, is a crucial cofactor in more than 300 enzyme activities. In biology, interactions between Mg²⁺ and phosphate anions are common and important for the stability of DNA and RNA biomolecules.⁷⁶ And Mg²⁺ has been linked to a number of cardiovascular diseases. A Gd-DTPA derivative modified with one charged b-diketone functionality (KMR-Mg, Gd-8) (Fig. 6A and B)⁷⁵ responds to variations in Mg²⁺ concentrations. When 1 equivalent of Mg²⁺ is supplied, the KMR-r₁ of Mg falls from 4.98 to 3.95 mM⁻¹ s⁻¹. Interestingly, Ling and colleague designed a K⁺-sensitive dual-mode nanoprobe for non-invasive identification of tumor malignancy. The probes presented high-performance T₂ MRI capability. As illustrated in Fig. 6C, the invention of a K⁺-sensitive dual-mode nanoprobe improves the imaging performance from dual MRI and fluorescence imaging modalities, allowing for more accurate imaging of tumor malignancies based on ion homeostasis.⁷⁷

Fig. 6 (A) Diagrammatic representation of the suggested conformational shift of KMR-K1 and KMR-Mg in the presence and absence of K⁺ or Mg²⁺, respectively. Reproduced from ref. 75 with permission from The Royal Society of Chemistry, copyright 2007. (B) T₁-weighted MR images of KMR-K and KMR-Mg in the presence of various amounts of K⁺ and Mg²⁺. Reproduced from ref. 75 with permission from The Royal Society of Chemistry, copyright 2007. (C) Logic operation of tumor imaging in vivo after being injected with K⁺-sensitive dual-mode nanoprobe. Reproduced from ref. 77 with permission from Oxford university Press, copyright 2022.

3.5. MRI probe for Fe^2+/3+ ion monitoring

The most prevalent transition metal in humans is iron. Iron can only exist in its stable redox states of ferrous ion (Fe²⁺) and ferric ion (Fe³⁺) under physiological conditions. The majority of the iron in cells is tightly attached to enzymes and certain transport and storage proteins, while the labile iron is only present in trace amounts and is only weakly bound to anions, which are multifunctional ligands. Based on its powerful redox capability, iron plays crucial roles in many biological processes, including DNA synthesis, electron transport, and oxygen metabolism. On the other hand, iron overload results in aberrant ROS generation that leads to severe cell damage and organ malfunction. A range of conditions, including cancer, hepatitis, and neurodegenerative illnesses, have also been connected to disruption in iron homeostasis. Age-related increases in iron levels in the brain are associated with Alzheimer's.^78,79

In order to selectively detect Fe in aqueous solutions, Nagasawa and colleagues designed the F-Nox-1 ¹⁹F-MRI probe as a Fe(ii)-selective chemical switch for the chemical shift and signal intensity of ¹⁹F signals in MR spectroscopy.⁸⁰ There was no crosstalk between the ¹⁹F-MR signals of the N-oxide and its deoxygenated constituent. They attempted to detect Fe in cells using ¹⁹F-NMR for biological purposes. However, no discernible ¹⁹F signal was seen in the cells. F-Nox-1 cytotoxicity was not seen, indicating that the absence of the ¹⁹F signal was not attributable to cell death. As a result, the sensitivity of ¹⁹F-NMR spectroscopy, rather than cell-membrane permeability, would make the detection of cellular Fe significantly more challenging. The current work suggested that more efforts should be made to improve ¹⁹F signals for ¹⁹F-MR imaging of cellular labile Fe in cells and in vivo. F-Nox-1's extraordinarily low cytotoxicity and water solubility are both intriguing qualities for in vivo application. Next, double representative examples were designed.^81,82 One Feⁿ⁺ center links several Gd³⁺-chelates to build larger complexes with correspondingly longer rotational correlation periods and greater relaxivity values. These super-molecular assemblies were developed as high-relaxivity contrast agents. Even though these substances have not yet generally been described as iron-responsive MRI indications, they still serve as a suitable foundation for future sensor developments. For instance, 2,20-bipyridine (H8L) and phenanthroline (DTPA-5-amido-1,10-phenanthroline) have been developed for Fe²⁺ chelation. When 1/3 equiv. of Fe²⁺ is added to the bipyridine compound H8L, its effective mass relaxivity increases by 59% (r₁ = 20.2 to 32.2 mM⁻¹ s⁻¹). Localized hyperintensity was seen in the right liver lobe on a T₂-weighted MRI, showing that there were reperfused liver lobes after ischemia damage and focal dorsal muscle necrosis. The infarcted liver lobe was somewhat less intense or isointense on T₁-weighted MRI when compared to the healthy liver, making the diagnosis difficult. The Fe homeostasis is essential for ferroptosis, as well as macrophage differentiation and polarization, which are linked to tumor development and malignant behavior.⁸³ An activatable MR imaging nanoprobe for controlling and monitoring Fe³⁺ release was designed by Gao et al.⁸⁴ The nanoprobes may release Fe³⁺ into the weak acidic environment of tumors, allowing the T₁ MRI effect to be activated, providing a unique platform for detecting the release of exogenous Fe³⁺ for tumor treatment (Fig. 8D). MRI is an emerging technique for studying the spatial distribution of dominant iron sources in vivo. However, it is still challenging to distinguish between Fe(ii) and Fe(iii) using MRI probes regardless of the convoluted data acquisition and processing, giving prominence to develop molecular imaging probes enabling mapping of the Fe(ii) in living individuals. MRI probes for Fe²⁺/Fe³⁺ detection may indeed face challenges in terms of sensitivity, oxidation state discrimination, and interference from endogenous iron or other paramagnetic species. However, researchers can explore various strategies to overcome these limitations:

1. Enhancing sensitivity. Amplification agents or signal enhancement methods can be incorporated into the probe design to improve the detection of low concentrations of Fe²⁺/Fe³⁺.

2. Selectivity improvement. Modifying the probe structure or incorporating specific binding moieties can enhance the selectivity of the probe towards Fe²⁺/Fe³⁺.

3. Oxidation state discrimination. Developing probes with different affinities for Fe²⁺ and Fe³⁺ or utilizing redox-sensitive probes can enable tracking changes in the oxidation state of iron.

4. Suppression of background interference. Chelating agents or masking agents can be used to selectively bind to endogenous iron or interfering species, minimizing their impact on the detection of exogenous Fe²⁺/Fe³⁺.

By addressing these challenges and implementing appropriate solutions, the limitations of MRI probes for Fe²⁺/Fe³⁺ detection can be overcome, leading to improved sensitivity, selectivity, and oxidation state discrimination. This would have significant implications for applications involving tracking iron metabolism and studying iron-related processes in biological systems.

3.6. MRI probe for heavy metal ion monitoring

The Cd²⁺, as heavy metal ions, can harm human health as well as enter organs and tissues through the blood.⁸⁵ For the requirement of high sensitivity detection, most Cd²⁺ responsive probes have been explored.⁸⁶ Due to the buildup of Cd²⁺ in organisms, the ability to detect Cd²⁺ in living cells or tissues is essential. While the only frequently used way to detect it in vivo is optical imaging.⁸⁷ Researchers have paid close attention to MRI probes, which can be used throughout the tissue since they are not constrained by light scattering or photobleaching. Recently, numerous MRI sensors have been created for biological purposes. However, few MRI sensors have been reported for metal ion detection in biological systems. To date, many magnetic nanoprobes for Pb²⁺ detection have been reported. As shown in Fig. 7A, the iron oxide nanoparticles modified with ethyl 1-(2-(3′,4′-dihydroxyphenyl)-2-oxoethyl)-1H-1,2,3-triazole-4 carboxylate (ETC) for Cd²⁺ detection were created by Yang's teams.⁸⁸ The ETC-(Zn, Mn)Fe₃O₄ NPs were able to efficiently collect Cd²⁺ and demonstrate the sensitive and selective detection of Cd²⁺ in live cells. This technique demonstrates a straightforward, quick, and effective approach for detecting Cd²⁺, which has a wide range of potential applications in biological systems. In order to increase specificity, Yang's groups have further created 3-(3,4-dihydroxyphenyl) propionic acid (DHCA)-modified Fe/Fe₃O₄ nanoparticles (DHCA-Fe/Fe₃O₄) for the detection of Pb²⁺ with outstanding selectivity over other metal ions (Fig. 7B).⁸⁹ As shown in Fig. 7C, Xu et al. designed novel MRI DNAzyme probes for the highly sensitive detection of Pb²⁺.⁹⁰ With an increase in Pb²⁺ concentration, a decrease in the T₂ relaxation time was present. This method employs functionalized magnetic nanoparticles to detect Pb²⁺ in an easy, quick, and effective manner that can also be broadly used to detect other species. One further hazardous metal that poses a major risk to human health and the environment is mercury.^91,92 The three different types of mercury are organic compounds, inorganic salts, and elemental mercury. The most prevalent species in the environment are organic mercury compounds and inorganic mercury salts.⁹³ As shown in Fig. 7D, the mercury ion caused the T–T coupling to assemble into the T-Hg-T structure after it was initially incorporated into the water solution containing the probe.⁹⁴ Following the reaction, the scattered probes came together to form aggregates that altered the magnetic relaxation characteristics of nearby water protons and shortened the T₂ relaxation time. Organic forms of mercury, primarily methylmercury species (CH₃Hg⁺), are far more dangerous than their inorganic counterparts (Hg⁰, Hg²⁺) because of their high cellular membrane permeability and potential to accumulate in the brain, kidneys, and endocrine system. Singh et al. presented a Gd(III) complex MRI probe named o-MeHgGad⁹⁵ for non-invasive visualization of CH₃Hg⁺, which was motivated by the particular interaction of mercury with the sulphur ether. This “off–on” MRI probe was created using the dithioacetal groups’ removal as the response mechanism. Water molecules were not able to interact with the Gd(III) ion. At three equivalents of CH³Hg⁺, the relaxivity increased from 2.3 to 5.9 mM⁻¹ s⁻¹ due to the increased water molecule access to the Gd(III) ion. However, a minor relaxivity increase was also achieved in the presence of Cu²⁺. As shown in Fig. 8E, the T₁-weighted MR images showed a considerable contrasting increase in the kidney, liver, and gut of mice exposed to CH₃Hg⁺. Furthermore, the relaxivity increase was found to be highly selective to CH₃Hg⁺ over other species.

Fig. 7 (A) The T₂ values of ETC-(Zn, Mn)Fe₃O₄ NPs in the presence of different metal ions. Reproduced from ref. 88 with permission from Elsevier, copyright 2015. (B) The T₂ intensity of DHCA-Fe/Fe₃O₄ NPs in the presence of Pb²⁺ and other metal ions. Reproduced from ref. 89 with permission from Elsevier, copyright 2016. (C) and (D) A diagram of the MRI probes for the detection of Pb²⁺ and Hg²⁺. Reproduced from ref. 90 and 94 with permission from American Chemical Society and The Royal Society of Chemistry, copyright 2013 and 2011. (E) The MR images of CH₃Hg⁺-mediated probes and T₁-weighted MR images of C57BL/6JNarl mice after injection of o-MeHgGad. Reproduced from ref. 95 with permission from The Royal Society of Chemistry, copyright 2015.

Fig. 8 (A) MRI signal of the Mn²⁺ sensitive probe after treatment with 0 mM, 1 mM and 5 mM Mn²⁺. Reproduced from ref. 98 with permission from Elsevier, copyright 2017. (B) MRI brain images of patients overexposed to environmental Mn. Reproduced from ref. 99 with permission from Springer Nature, copyright 2016.

MRI probes for heavy metal ion detection should be designed with a focus on minimizing potential toxicity and improving selectivity and sensitivity. Researchers can consider the following strategies:

1. Designing selective probes. Incorporate specific binding moieties or functional groups that selectively interact with the target heavy metal ions, reducing cross-reactivity with other substances in the sample matrix.

2. Signal amplification. Use amplification agents or signal enhancement methods to improve the probe's ability to detect low concentrations of heavy metal ions, enhancing sensitivity.

3. Minimizing toxicity. Pay careful attention to the potential toxicity of the probes by exploring the use of biocompatible materials or encapsulation strategies to minimize the release of heavy metal ions and reduce potential toxicity concerns.

By addressing these challenges and implementing potential solutions, the limitations of MRI probes for heavy metal ion detection can be overcome, leading to improved selectivity, sensitivity, interference mitigation, and safety for various applications involving heavy metal ion detection.

3.7. MRI probe for Mn²⁺ ion monitoring

Manganese (Mn) is crucial for controlling a wide range of biological processes. Several enzymes use Mn²⁺ as a structural and catalytic cofactor.^96,97 According to estimates of Mn concentrations in pathological settings, the average concentration is about 160 μM in the brain, while striatal local concentrations can reach 1–5 mM. In this situation, high sensitivity detection methods are crucial for detecting Mn²⁺. For patho-physiologically significant Mn²⁺ concentrations, Datta et al.⁹⁸ designed a fluorinated macrocyclic ligand (Fig. 8A) that allows for a linear rise in ¹⁹F relaxation rates. Importantly, even at mM levels of Mn²⁺, the impacts of line widening could be seen in the ¹⁹F NMR signal intensities. For Mn²⁺ detection in the biological milieu, water solubility is an important factor. To obtain a water-soluble Mn²⁺ probe, they added arms in the macrocycle that contained an anionic oxygen donor. The ¹⁹F-MR probe may also be useful for visualizing Mn²⁺ distributions in biological material, according to T₁ and T₂ weighted ¹⁹F-MR images of phantoms. Following the anterior pituitary gland, the midbrain, pontine tegmentum, and corpus callosum, high Mn accumulation was found in the globus pallidus region of the basal ganglia, putamen, caudate, subthalamic, and dentate nuclei in the MRI images of the brains of manganism patients (Fig. 8B).⁹⁹ Moreover, because ¹H-MRI is based on NMR, it has poor sensitivity by nature and is only now helpful for highlighting areas with high mM to M levels of Mn buildup.

MRI probes for Mn²⁺ detection may face challenges due to background interference from endogenous Mn²⁺ or other paramagnetic species, as well as potential toxicity concerns associated with high Mn²⁺ concentrations. To overcome these limitations, researchers can consider the following solutions:

1. Minimizing interference. Carefully design the probe structure or incorporate specific binding moieties that enhance selectivity for Mn²⁺, reducing interference from other paramagnetic species or endogenous Mn²⁺ in biological samples.

2. Enhancing sensitivity. Implement signal amplification techniques or signal enhancement methods to improve the probe's ability to detect low concentrations of Mn²⁺, increasing sensitivity in Mn²⁺ detection.

3. Addressing potential toxicity. Pay attention to the potential toxicity of the probe, especially in applications involving in vivo imaging. Modify the probe's design or explore the use of biocompatible materials or encapsulation strategies to minimize any release of Mn²⁺ and reduce potential toxicity concerns.

Conclusions

Due to its capacity to provide images with great spatial and temporal resolution from deep inside the body, MRI technology has been widely used in disease diagnosis and scientific research. There has been a lot of interest in developing responsive MRI probes for metal ion detection. The MRI probes respond to endogenous and external stimuli via a particular interaction, binding, or reaction with a targeted analyte and then produce distinctive changes in the MRI signal for the detection of metal ions. Various strategies have been suggested for the development of metal ion responsive probes, as outlined in Table 1. The most popular method for creating MRI probes is based on adjusting the amount of coordinated water molecules to modify relaxivity. Although these methods have received extensive research attention in the development of MRI probes, there is still a great need for the identification of novel response mechanisms. Future designs of more durable MRI probes for in vivo imaging will be possible thanks to the new processes. The MRI technology for metal ion detection is still limited by its sensitivity/selectivity, accurate quantification\imaging and biological safety. To speed-up the widespread adoption of MRI technology for metal ion detection in vivo and in vitro, we propose the following considerations:

Table 1 Performance of various probes in metal ion detection via MRI imaging

Metal ions	Probe name	Chelating agent	Imaging modality	Ref.
Zn^2+	GdDOTA-diBPEN	N,N-bis-(2-pyridyl-methyl) ethylene diamine	T 1	34, 35
	Gd-daa3	Iminodiacetate or pyridine	T 1	36, 37
	GdL	Sulfonamidoquinoline	T 1	38, 39
	Mn-(DPA-C2)2-TPPS3	Dipicolylamine	T 1	40
Ca^2+	Gd-DOPTRA	APTRA (o-amino-phenol-N,N,O-triacetate)	T 1	49
	GdL1-GdL4	Polyaminocarboxylates and polyetheraminocarboxylates	T 1	50
	GdL1,GdL2	EGTA	T 1,^1H,^19F	51, 52
	L1,L2,L3	Ethylenediaminetetraacetic acid (EDTA) and diethylenetriamine pentaacetic acid (DTPA)	T 1	53
	Gd2L1,Gd2L2	EGTA	T 1	54
	DOPTA-Gd (Gd-1)	BAPTA	T 1	56
	M^2+-5F-BAPTA	BAPTA	T 1	57
	PEI/PEG- SPIONs	EGTA	T 2	58
	ManICS1-AM	BAPTA	T 1	59
	MaCaReNas	C2 domains of synaptotagmin 1	T 2	60
Cu^2+	copper-gad-1	Iminodiacetate	T 1	69
	CG2-CG6	Thioether-based copper receptors	T 1	70
	Copper-Gad-7	2-(Ethylthio)-N-(2-(ethylthio)ethyl)ethana	T 1	70
	Nido-carborane derivatives 4	N,N,N′,N′-Tetramethylethylenediamine	^11B	71
	Cu^2+-PVIm	poly-N-Vinylimidazole (PVIm)	T 1	72
	GdL1	Iminodiacetate	T 1	73
K^+,Mg^2+	KMR-K1, Gd-7	Crown ether	T 1	76
	KMR-Mg Gd-8	b-diketone functionality	T 1	76
Fe^2+/3+	F-Nox-1	Tetrafluoro-p-phenylenediamine (TFPDA)	^19F	80
	H8L	TTAHA (N-tris(2-aminoethyl)amine-N′,N′,N′′,N′′,N′′′,N′′′-hexaacetate)	T 2	81, 82
	UCPN-GA		T 1	84
Heavy metal ions	ETC-(Zn,Mn)Fe3O4	Ethyl 1-(2-(3′,4′-dihydroxyphenyl)-2-oxoethyl)-1H-1,2,3-triazole-4-carboxylate	T 2	88
	DHCA-Fe/Fe3O4	3-(3,4-Dihydroxyphenyl)propionic acid(DHCA)	T 2	88
	Fe3O4 MNPs	DNAzyme	T 2	90
	Gold nanoparticle	DNA	T 2	94
	o-MeHgGad	Disulfide bond	T 1	95
Mn^2+	p-FML	Penta-aza macrocyclic ligand	^19F	98

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Enhancing detection specificity

Traditional metal ion MRI detection probes can be interfered with by various metal ions, the biological microenvironment, enzymes, and other substances, leading to reduced detection specificity. To improve specificity, it is crucial to design novel probes or identify specific metal ion response targets that offer higher selectivity and minimize interference.

Ensuring probe safety

MRI probes for metal ion detection play a vital role in biomedical imaging and diagnostics. It is essential for these probes to have minimal toxicity to ensure safe in vivo use. Extensive toxicity testing is typically performed to assess the safety profile of MRI probes. Additionally, the probes should be efficiently cleared from the body after imaging to minimize any potential long-term effects. Stability during imaging is crucial, as the probe should not degrade or release toxic byproducts.

Understanding metabolic pathways and excretion mechanisms

Upon administration, MRI probes are taken up by cells through mechanisms like passive diffusion or active transport. They may undergo metabolic transformations or enzymatic reactions that modify their structure. The specific metabolic pathways can vary depending on the probe and the targeted metal ion. After imaging, the probe or its metabolites are eliminated from the body through routes such as renal excretion or metabolism by liver enzymes. It is important to conduct extensive research and preclinical studies to understand the bio-safety and metabolic pathways of MRI probes for metal ion detection.

By addressing these considerations, researchers can overcome sensitivity/selectivity limitations, ensure accurate quantification/imaging, and enhance the biological safety of MRI technology for metal ion detection. These efforts will contribute to the wider adoption and application of MRI in the field of metal ion detection in both in vivo and in vitro settings.

Conflicts of interest

The authors have declared no conflicts of interest for this article.

Acknowledgements

This review was supported by the National Natural Science Foundation of China (grants U21A20287, 51872088, 21890744, and 21974039), the National Key R&D Program of China (2019YFA0210100), the Science and Technology Project of Hunan Province (2020RC3022 and 2020SK2014), the National Natural Science Foundation of China (22374045), Youth medical key talents foundation of Hunan Province (20230508-1034) and China Postdoctoral Science Foundation (2023M731063).

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Footnote

† Both these authors contributed equally to this work.

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