Tunable T1 and T2 contrast abilities of manganese-engineered iron oxide nanoparticles through size control

Progress in ultrasmall ferrite nanoparticles enhanced T1 magnetic resonance angiography

Contrast-enhanced magnetic resonance angiography (CE-MRA) plays a critical role in diagnosing and monitoring various vascular diseases. Achieving high-sensitivity detection of vascular abnormalities in CE-MRA depends on the properties of contrast agents. In contrast to clinically used gadolinium-based contrast agents (GBCAs), the new generation of ultrasmall ferrite nanoparticles-based contrast agents have high relaxivity, long blood circulation time, easy surface functionalization, and high biocompatibility, hence showing promising prospects in CE-MRA. This review aims to comprehensively summarize the advancements in ultrasmall ferrite nanoparticles-enhanced MRA for detecting vascular diseases. Additionally, this review also discusses the future clinical translational potential of ultrasmall ferrite nanoparticles-based contrast agents for vascular imaging. By investigating the current status of research and clinical applications, this review attempts to outline the progress, challenges, and future directions of using ultrasmall ferrite nanoparticles to drive the field of CE-MRA into a new frontier of accuracy and diagnostic efficacy.

DNA-Mediated Magnetic-Dimer Assembly for Fault-Free Ultra-High-Field Magnetic Resonance Imaging of Tumors

Ultra-high-field (UHF) magnetic resonance imaging (MRI) stands as a pivotal cornerstone in biomedical imaging, yet the challenge of false imaging persists, constraining its full potential. Despite the development of dual-mode contrast agents improving conventional MRI, their effectiveness in UHF remains suboptimal due to the high magnetic moment, resulting in diminished T1 relaxivity and excessively enhanced T2 relaxivity. Herein, we report a DNA-mediated magnetic-dimer assembly (DMA) of iron oxide nanoparticles that harnesses UHF-tailored nanomagnetism for fault-free UHF-MRI. DMA exhibits a dually enhanced longitudinal relaxivity of 4.42 mM-1·s-1 and transverse relaxivity of 26.23 mM-1·s-1 at 9 T, demonstrating a typical T1-T2 dual-mode UHF-MRI contrast agent. Importantly, DMA leverages T1-T2 dual-modality image fusion to achieve artifact-free breast cancer visualization, effectively filtering interference from hundred-micrometer-level false-positive signals with unprecedented precision. The UHF-tailored T1-T2 dual-mode DMA contrast agents hold promise for elevating the accuracy of MR imaging in disease diagnosis.

Optical and MRI Multimodal Tracing of Stem Cells In Vivo

Stem cell therapy has shown great clinical potential in oncology, injury, inflammation, and cardiovascular disease. However, due to the technical limitations of the in vivo visualization of transplanted stem cells, the therapeutic mechanisms and biosafety of stem cells in vivo are poorly defined, which limits the speed of clinical translation. The commonly used methods for the in vivo tracing of stem cells currently include optical imaging, magnetic resonance imaging (MRI), and nuclear medicine imaging. However, nuclear medicine imaging involves radioactive materials, MRI has low resolution at the cellular level, and optical imaging has poor tissue penetration in vivo. It is difficult for a single imaging method to simultaneously achieve the high penetration, high resolution, and noninvasiveness needed for in vivo imaging. However, multimodal imaging combines the advantages of different imaging modalities to determine the fate of stem cells in vivo in a multidimensional way. This review provides an overview of various multimodal imaging technologies and labeling methods commonly used for tracing stem cells, including optical imaging, MRI, and the combination of the two, while explaining the principles involved, comparing the advantages and disadvantages of different combination schemes, and discussing the challenges and prospects of human stem cell tracking techniques.

Magnetic Nanoparticle-Based High-Performance Positive and Negative Magnetic Resonance Imaging Contrast Agents

In recent decades, magnetic nanoparticles (MNPs) have attracted considerable research interest as versatile substances for various biomedical applications, particularly as contrast agents in magnetic resonance imaging (MRI). Depending on their composition and particle size, most MNPs are either paramagnetic or superparamagnetic. The unique, advanced magnetic properties of MNPs, such as appreciable paramagnetic or strong superparamagnetic moments at room temperature, along with their large surface area, easy surface functionalization, and the ability to offer stronger contrast enhancements in MRI, make them superior to molecular MRI contrast agents. As a result, MNPs are promising candidates for various diagnostic and therapeutic applications. They can function as either positive (T₁) or negative (T₂) MRI contrast agents, producing brighter or darker MR images, respectively. In addition, they can function as dual-modal T₁ and T₂ MRI contrast agents, producing either brighter or darker MR images, depending on the operational mode. It is essential that the MNPs are grafted with hydrophilic and biocompatible ligands to maintain their nontoxicity and colloidal stability in aqueous media. The colloidal stability of MNPs is critical in order to achieve a high-performance MRI function. Most of the MNP-based MRI contrast agents reported in the literature are still in the developmental stage. With continuous progress being made in the detailed scientific research on them, their use in clinical settings may be realized in the future. In this study, we present an overview of the recent developments in the various types of MNP-based MRI contrast agents and their in vivo applications.

Nanomaterial-based contrast agents

Medical imaging, which empowers the detection of physiological and pathological processes within living subjects, has a vital role in both preclinical and clinical diagnostics. Contrast agents are often needed to accompany anatomical data with functional information or to provide phenotyping of the disease in question. Many newly emerging contrast agents are based on nanomaterials as their high payloads, unique physicochemical properties, improved sensitivity and multimodality capacity are highly desired for many advanced forms of bioimaging techniques and applications. Here, we review the developments in the field of nanomaterial-based contrast agents. We outline important nanomaterial design considerations and discuss the effect on their physicochemical attributes, contrast properties and biological behaviour. We also describe commonly used approaches for formulating, functionalizing and characterizing these nanomaterials. Key applications are highlighted by categorizing nanomaterials on the basis of their X-ray, magnetic, nuclear, optical and/or photoacoustic contrast properties. Finally, we offer our perspectives on current challenges and emerging research topics as well as expectations for future advancements in the field.

Structural and Molecular Fusion MRI Nanoprobe for Differential Diagnosis of Malignant Tumors and Follow-Up Chemodynamic Therapy

Enhanced imaging techniques using contrast agents enable high-resolution structural imaging to reveal space-occupying lesions but rarely provide detailed molecular information. To this end, we report a structural and molecular fusion magnetic resonance imaging (MRI) nanoprobe for differential diagnosis between benign and malignant tumors. This fusion nanoprobe, termed FFT NPs, follows a working mechanism involving a T₁-/T₂-weighted magnetic resonance tuning effect (MRET) between a magnetic Fe₃O₄ core and a paramagnetic Fe-tannic acid (Fe-TA) shell. The FFT NPs with an "always-on" inert T₂ signal provide structural MRI (sMRI) contrast of tumors while affording an activated T₁ signal in the presence of ATP, which is overproduced during the rapid growth of malignant tumors to enable molecular MRI (mMRI) of tumor lesions. We propose the use of the ratiometric mMRI:sMRI intensity to assist in the differential diagnosis of malignant 4T1 tumors from benign L929 fibroblast tumors. Furthermore, the dissociated FFT NPs were found to be able to catalyze H₂O₂ conversion in 4T1 tumors to generate excess reactive oxygen species (ROS) for chemodynamic therapy. The described fusion nanoprobe strategy enables the differential diagnosis of tumors from a combined spatial and molecular perspective with one-stop MRI imaging with potential applications in precision intervention.

The adjacent effect between Gd(III) and Cu(II) in layered double hydroxide nanoparticles synergistically enhances T1-weighted magnetic resonance imaging contrast

Magnetic resonance imaging (MRI) is one key technology in modern diagnostic medicine. However, the development of high-relaxivity contrast agents with favorable properties for imaging applications remains a challenging task. In this work, dual Gd(III) and Cu(II) doped-layered double hydroxide (GdCu-LDH) nanoparticles show significantly higher longitudinal relaxivity compared with sole-metal-based LDH (Gd-LDH and Cu-LDH) nanoparticles. This relaxation enhancement in GdCu-LDH is also much greater than the simple addition of the relaxivity rate of the two paramagnetic ions in Gd-LDH and Cu-LDH, presumably attributed to synergistic T₁ shortening between adjacent Gd(III) and Cu(II) in the LDH host layers (adjacent effect). Moreover, our GdCu-LDH nanoparticles exhibit a pH-ultrasensitive property in MRI performance and show much clearer MR imaging for tumor tissues in mice than Gd-LDH and Cu-LDH at the equivalent doses. Thus, these novel Gd/Cu-co-doped LDH nanoparticles provide higher potential for accurate cancer diagnosis in clinic application. To the best of our knowledge, this is the first report that two paramagnetic metal ions in one nanoparticle synergistically improve the T₁-MRI contrast.

Ultrasmall Fe3O4 and Gd2O3 hybrid nanoparticles for T1-weighted MR imaging of cancer

Gadolinium-based contrast agents (GdCAs) have been the most frequently used T1-weighted magnetic resonance imaging (MRI) contrast agents for decades. However, they have serious disadvantages such as low longitudinal relaxivity value (r1) and high dose associated-nephrotoxicity that restrict their wide applications. These emphasize the need for an ideal stable and biocompatible T1-weighted CA with high contrast enhancement performance. Here, we propose a wet-chemical synthesis technique to form a nanocomposite consisting of ultrasmall iron oxide nanoparticles (US-IO) and Gd2O3 hybrid nanoparticles stabilized with dextran (FG-HNPs) for T1-weighted MR imaging. Relaxometry study showed that FG-HNPs have a high r1 value (42.28 mM−1S−1) and low relaxivity ratio (r2/r1: 1.416) at 3.0T. In vivo MRI contrast enhancement factor (ΔSNR) for FG-HNPs (257.025 ± 17.4%) was found to be 1.99-fold higher than US-IO (129.102 ± 15%) and 3.35-fold higher than Dotarem (76.71 ± 14.2%) as routinely used T1-weighted CA. The cytotoxicity assay and histological examination confirmed the biocompatibility of FG-HNPs. The biodistribution study, transmission electron microscopy (TEM) and Prussian blue (PB) staining of tumor tissue proved the effective tumor localization of FG-HNPs. Therefore, FG-HNPs can be suggested as a promising CA for T1-weighted MRI of tumors by virtue of their remarkable relaxivities and high biocompatibility.

Tailoring the Magnetic and Structural Properties of Manganese/Zinc Doped Iron Oxide Nanoparticles through Microwaves-Assisted Polyol Synthesis

Tuning the fundamental properties of iron oxide magnetic nanoparticles (MNPs) according to the required biomedical application is an unsolved challenge, as the MNPs' properties are affected by their composition, their size, the synthesis process, and so on. In this work, we studied the effect of zinc and manganese doping on the magnetic and structural properties of MNPs synthesized by the microwave-assisted polyol process, using diethylene glycol (DEG) and tetraethylene glycol (TEG) as polyols. The detailed morpho-structural and magnetic characterization showed a correspondence between the higher amounts of Mn and smaller crystal sizes of the MNPs. Such size reduction was compensated by an increase in the global magnetic moment so that it resulted in an increase of the saturation magnetization. Saturation magnetization MS values up to 91.5 emu/g and NMR transverse relaxivities r2 of 294 s-1mM-1 were obtained for Zn and Mn- doped ferrites having diameters around 10 nm, whereas Zn ferrites with diameters around 15 nm reached values of MS∼ 97.2 emu/g and of r2∼ 467 s-1mM-1, respectively. Both kinds of nanoparticles were synthesized by a simple, reproducible, and more sustainable method that makes them very interesting for diagnostic applications as MRI contrast agents.

Effect of manganese substitution of ferrite nanoparticles on particle grain structure

To investigate the influence of manganese substitution on the saturation magnetization of manganese ferrite nanoparticles, samples with various compositions (Mn x Fe3-x O4, x = 0, 0.25, 0.5, 0.75, and 1) were synthesized and characterized. The saturation magnetization of such materials was both calculated using density functional theory and measured via vibrating sample magnetometry. A discrepancy was found; the computational data demonstrated a positive correlation between manganese content and saturation magnetization, while the experimental data exhibited an inverse correlation. X-ray diffraction (XRD) and magnetometry results indicated that the crystallite diameter and the magnetic diameter decrease when adding more manganese, which could explain the loss of magnetization of the particles. For 20 nm nanoparticles, with increasing manganese substitution level, the crystallite size decreases from 10.9 nm to 6.3 nm and the magnetic diameter decreases from 15.1 nm to 3.5 nm. Further high resolution transmission electron microscopy (HRTEM) analysis confirmed the manganese substitution induced defects in the crystal lattice, which encourages us to find ways of eliminating crystalline defects to make more reliable ferrite nanoparticles.

The Metal Ion Release of Manganese Ferrite Nanoparticles: Kinetics, Effects on Magnetic Resonance Relaxivities, and Toxicity

Mn²⁺ release is particularly important for biological application of manganese-based nanomaterials. However, the Mn²⁺ release profiles of the manganese ferrite nanoparticles are under clarification. Here, we synthesized 3, 10, and 18 nm manganese ferrite nanoparticles (MFNPs) as model systems to study the Mn²⁺ release behavior, size, and pH-dependent kinetics. The Mn²⁺ release kinetic study showed that the first-order kinetic model was suitable for 3 and 10 nm MFNPs, while the Higuchi model was suitable for 18 nm MFNPs in a neutral PBS buffer (pH 7.4). In an acidic PBS buffer (pH 4.8), the Mn²⁺ release from all sizes of MFNPs follows first-order kinetics, which is possible due to the reaction between MFNPs and H⁺. The influence of Mn²⁺ release was evaluated by comparing the variations of magnetic resonance (MR) relaxation and magnetic properties before and after Mn²⁺ release of MFNPs. The results showed that the saturation magnetization (M_s), longitudinal relaxivity (r₁), and transverse relaxivity (r₂) values declined due to Mn²⁺ release, while the ratio of r₂/r₁ increased slightly, showing that all sizes of MFNPs exhibited the same MR mode as the synthesized MFNPs. More importantly, the release kinetics were employed to estimate the toxicity of the released Mn²⁺ in vivo. The potential toxicity is acceptable for MFNP administration since the calculated amount of Mn²⁺ is in the range of safe doses.

Recent advances in engineering iron oxide nanoparticles for effective magnetic resonance imaging

Iron oxide nanoparticle (IONP) with unique magnetic property and high biocompatibility have been widely used as magnetic resonance imaging (MRI) contrast agent (CA) for long time. However, a review which comprehensively summarizes the recent development of IONP as traditional T₂ CA and its new application for different modality of MRI, such as T₁ imaging, simultaneous T₂/T₁ or MRI/other imaging modality, and as environment responsive CA is rare. This review starts with an investigation of direction on the development of high-performance MRI CA in both T₂ and T₁ modal based on quantum mechanical outer sphere and Solomon–Bloembergen–Morgan (SBM) theory. Recent rational attempts to increase the MRI contrast of IONP by adjusting the key parameters, including magnetization, size, effective radius, inhomogeneity of surrounding generated magnetic field, crystal phase, coordination number of water, electronic relaxation time, and surface modification are summarized. Besides the strategies to improve r₂ or r₁ values, strategies to increase the in vivo contrast efficiency of IONP have been reviewed from three different aspects, those are introducing second imaging modality to increase the imaging accuracy, endowing IONP with environment response capacity to elevate the signal difference between lesion and normal tissue, and optimizing the interface structure to improve the accumulation amount of IONP in lesion. This detailed review provides a deep understanding of recent researches on the development of high-performance IONP based MRI CAs. It is hoped to trigger deep thinking for design of next generation MRI CAs for early and accurate diagnosis.

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T₂ contrast capacity of iron oxide nanoparticles (IONPs) could be improved based on quantum mechanical outer sphere theory.

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IONPs could be expand to be used as effective T₁ CAs by improving q value, extending τ_s, and optimizing interface structure.

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Environment responsive MRI CAs have been developed to improve the diagnosis accuracy.

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Introducing other imaging contrast moiety into IONPs could increase the contrast efficiency.

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Optimizing in vivo behavior of IONPs have been proved to enlarge the signal difference between normal tissue and lesion.

Self-Confirming Magnetosomes for Tumor-Targeted T1 /T2 Dual-Mode MRI and MRI-Guided Photothermal Therapy

Nanomaterials as T₁ /T₂ dual-mode magnetic resonance imaging (MRI) contrast agents have great potential in improving the accuracy of tumor diagnosis. Applications of such materials, however, are limited by the complicated chemical synthesis process and potential biosafety issues. In this study, the biosynthesis of manganese (Mn)-doped magnetosomes (MagMn) that not only can be used in T₁ /T₂ dual-mode MR imaging with self-confirmation for tumor detection, but also improve the photothermal conversion efficiency for MRI-guided photothermal therapy (PTT) is reported. The MagMn nanoparticles (NPs) are naturally produced through the biomineralization of magnetotactic bacteria by doping Mn into the ferromagnetic iron oxide crystals. In vitro and in vivo studies demonstrated that targeting peptides functionalized MagMn enhanced both T₁ and T₂ MRI signals in tumor tissue and significantly inhibited tumor growth by the further MRI-guided PTT. It is envisioned that the biosynthesized multifunctional MagMn nanoplatform may serve as a potential theranostic agent for cancer diagnosis and treatment.

Towards the design of contrast-enhanced agents: systematic Ga3+ doping on magnetite nanoparticles

The main objective of the preparation of the Fe3-xGaxO4 (0.14 ≤ x ≤ 1.35) system was to further the knowledge of the magnetic response of Ga3+-doped magnetite for application as MRI contrast agents. With this purpose, monodisperse nanoparticles between 7 and 10 nm with different amounts of gallium were prepared from an optimized protocol based on thermal decomposition of metallo-organic precursors. Thorough characterization of the sample was conducted in order to understand the influence of gallium doping on the structural, morphological and magnetic properties of the Fe3-xGaxO4 system. X-ray diffraction and X-ray absorption near-edge structure measurements have proved the progressive incorporation of Ga in the spinel structure, with different occupations in both tetrahedral and octahedral sites. Magnetization measurements as a function of field temperature have shown a clear dependence of magnetic saturation on the gallium content, reaching an Ms value of 110 Am2 kg-1 at 5 K for x = 0.14 (significantly higher than bulk magnetite) and considerably decreasing for amounts above x = 0.57 of gallium. For this reason, nanoparticles with moderate Ga quantities were water-transferred by coating them with the amphiphilic polymer PMAO to further analyse their biomedical potential. Cytotoxicity assays have demonstrated that Fe3-xGaxO4@PMAO formulations with x ≤ 0.57, which are the ones with better magnetic response, are not toxic for cells. Finally, the effect of gallium doping on relaxivities has been analysed by measuring longitudinal (T1-1) and transverse (T1-1) proton relaxation rates at 1.4 T revealing that nanoparticles with x = 0.14 Ga3+ content present remarkable T2 contrast and the nanoparticles with x = 0.26 have great potential to act as dual T1-T2 contrast agents.

Magnetic Nanostructures: Rational Design and Fabrication Strategies toward Diverse Applications

In recent years, the continuous development of magnetic nanostructures (MNSs) has tremendously promoted both fundamental scientific research and technological applications. Different from the bulk magnet, the systematic engineering on MNSs has brought a great breakthrough in some emerging fields such as the construction of MNSs, the magnetism exploration of multidimensional MNSs, and their potential translational applications. In this review, we give a detailed description of the synthetic strategies of MNSs based on the fundamental features and application potential of MNSs and discuss the recent progress of MNSs in the fields of nanomedicines, advanced nanobiotechnology, catalysis, and electromagnetic wave adsorption (EMWA), aiming to provide guidance for fabrication strategies of MNSs toward diverse applications.

A Comprehensive Updated Review on Magnetic Nanoparticles in Diagnostics

Magnetic nanoparticles (MNPs) have been studied for diagnostic purposes for decades. Their high surface-to-volume ratio, dispersibility, ability to interact with various molecules and superparamagnetic properties are at the core of what makes MNPs so promising. They have been applied in a multitude of areas in medicine, particularly Magnetic Resonance Imaging (MRI). Iron oxide nanoparticles (IONPs) are the most well-accepted based on their excellent superparamagnetic properties and low toxicity. Nevertheless, IONPs are facing many challenges that make their entry into the market difficult. To overcome these challenges, research has focused on developing MNPs with better safety profiles and enhanced magnetic properties. One particularly important strategy includes doping MNPs (particularly IONPs) with other metallic elements, such as cobalt (Co) and manganese (Mn), to reduce the iron (Fe) content released into the body resulting in the creation of multimodal nanoparticles with unique properties. Another approach includes the development of MNPs using other metals besides Fe, that possess great magnetic or other imaging properties. The future of this field seems to be the production of MNPs which can be used as multipurpose platforms that can combine different uses of MRI or different imaging techniques to design more effective and complete diagnostic tests.

Emerging 2D Nanomaterials for Biomedical Applications

Two-dimensional (2D) nanomaterials are an emerging class of biomaterials with remarkable potential for biomedical applications. The planar topography of these nanomaterials confers unique physical, chemical, electronic and optical properties, making them attractive candidates for therapeutic delivery, biosensing, bioimaging, regenerative medicine, and additive manufacturing strategies. The high surface-to-volume ratio of 2D nanomaterials promotes enhanced interactions with biomolecules and cells. A range of 2D nanomaterials, including transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), layered silicates (nanoclays), 2D metal carbides and nitrides (MXenes), metal-organic framework (MOFs), covalent organic frameworks (COFs) and polymer nanosheets have been investigated for their potential in biomedical applications. Here, we will critically evaluate recent advances of 2D nanomaterial strategies in biomedical engineering and discuss emerging approaches and current limitations associated with these nanomaterials. Due to their unique physical, chemical, and biological properties, this new class of nanomaterials has the potential to become a platform technology in regenerative medicine and other biomedical applications.

Ultrasmall superparamagnetic iron oxide nanoparticles: A next generation contrast agent for magnetic resonance imaging

As a research hotspot, the development of magnetic resonance imaging (MRI) contrast agents has attracted great attention over the past decades for improving the accuracy of diagnosis. Ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles with core diameter smaller than 5.0 nm are expected to become a next generation of contrast agents owing to their excellent MRI performance, long blood circulation time upon proper surface modification, renal clearance capacity, and remarkable biosafety profile. On top of these merits, USPIO nanoparticles are used for developing not only T₁ contrast agents, but also T₂ /T₁ switchable contrast agents via assembly/disassembly approaches. In recent years, as a new type of contrast agents, USPIO nanoparticles have shown considerable applications in the diagnosis of various diseases such as vascular pathological changes and inflammations apart from malignant tumors. In this review, we are focusing on the state-of-the-art developments and the latest applications of USPIO nanoparticles as MRI contrast agents to discuss their advantages and future prospects. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Tuneable manganese oxide nanoparticle based theranostic agents for potential diagnosis and drug delivery

Among various magnetic nanoparticles, manganese oxide nanoparticles are considered as established T 1 magnetic resonance imaging (MRI) contrast agents for preclinical research. The implications of their degradation properties and use as therapeutic carriers in drug delivery systems have not been explored. In addition, how the chemical composition and size of manganese oxide nanoparticles, as well as the surrounding environment, influence their degradation and MRI contrast properties (T 1 vs. T 2) have not been studied in great detail. A fundamental understanding of their characteristic properties, such as degradation, is highly desirable for developing simultaneous diagnosis and therapeutic solutions. Here, we demonstrate how the precursor type and reaction environment affect the size and chemical composition of manganese oxide nanoparticles and evaluate their influence on the nanoparticle degradability and release of the drug l-3,4-dihydroxyphenylalanine (l-dopa). The results show that the degradation rate (and the associated release of drug l-dopa molecules) of manganese oxide nanoparticles depends on their size, composition and the surrounding environment (aqueous or biometric fluid). The dependence of MRI relaxivities of manganese oxide nanoparticles on the size, chemical composition and nanoparticle degradation in water is also established. A preliminary cell viability study reveals the cytocompatible properties of l-dopa functionalized manganese oxide nanoparticles. Overall, this work provides new insights into smartly designed manganese oxide nanoparticles with multitasking capabilities to target bioimaging and therapeutic applications.

Shaping Magnetite by Hydroxyl Group Numbers of Small Molecules

Despite numerous reports on magnetite formation with the assistance of various additives, the role of hydroxyl group (-OH) numbers in small polyol molecules has not yet been understood well. We selected small molecules containing different -OH numbers, such as ethanol, ethylene glycol, propanetriol, butanetetrol, pentitol, hexanehexol, and cyclohexanehexol, as additives in coprecipitation. By increasing the -OH number in these small polyol molecules, the formation of crystallization was slowed, and the size and shape of magnetite were regulated as well possibly due to the changed complexation strength and the stability of the precursor. The increase in temperature and the Fe²⁺/Fe³⁺ ratio can reduce the complexation strength. The nucleation and growth of magnetite proceed possibly through the aggregation of polyol-stabilized amorphous complexes and two-line ferrihydrite with low crystallinity based on the -OH numbers, suggesting a nonclassical pathway. The as-prepared magnetite showed a r₂/r₁ ratio after in vitro MRI measurement as follows: Fe₃O₄@He-6OH rod < Fe₃O₄@Pr-3OH sheet < Fe₃O₄@Pe-5OH cube. The Fe₃O₄@He-6OH rod and Fe₃O₄@Pr-3OH sheet displayed T₁-T₂ dual modal contrast ability, while the Fe₃O₄@Pe-5OH cube can be T₂-dominated. This research provides a simple but an essential approach for designing MRI contrast agents.

Construction of nanomaterials as contrast agents or probes for glioma imaging

Malignant glioma remains incurable largely due to the aggressive and infiltrative nature, as well as the existence of blood-brain-barrier (BBB). Precise diagnosis of glioma, which aims to accurately delineate the tumor boundary for guiding surgical resection and provide reliable feedback of the therapeutic outcomes, is the critical step for successful treatment. Numerous imaging modalities have been developed for the efficient diagnosis of tumors from structural or functional aspects. However, the presence of BBB largely hampers the entrance of contrast agents (Cas) or probes into the brain, rendering the imaging performance highly compromised. The development of nanomaterials provides promising strategies for constructing nano-sized Cas or probes for accurate imaging of glioma owing to the BBB crossing ability and other unique advantages of nanomaterials, such as high loading capacity and stimuli-responsive properties. In this review, the recent progress of nanomaterials applied in single modal imaging modality and multimodal imaging for a comprehensive diagnosis is thoroughly summarized. Finally, the prospects and challenges are offered with the hope for its better development.

Structure-Relaxivity Mechanism of an Ultrasmall Ferrite Nanoparticle T1 MR Contrast Agent: The Impact of Dopants Controlled Crystalline Core and Surface Disordered Shell

Ultrasmall ferrite nanoparticles (UFNPs) have emerged as powerful magnetic resonance imaging (MRI) T₁ nanoprobe for noninvasive visualization of biological events. However, the structure-relaxivity relationship and regulatory mechanism of UFNPs remain elusive. Herein, we developed chemically engineered 3.8 nm Zn_xFe_3-xO₄@Zn_xMn_yFe_3-x-yO₄ (denoted as Zn_xF@Zn_xMn_yF) nanoparticles with precise dopants control in both crystalline core and disordered shell as a model system to assess the impact of dopants on the relaxometric properties of UFNPs. It is determined that the core-shell dopant architecture allows the optimal tuning of r₁ relaxivity for Zn_0.4F@Zn_0.4Mn_0.2F up to 20.22 mM^-1 s^-1, which is 5.2-fold and 6.5-fold larger than that of the original UFNPs and the clinically used Gd-DTPA. Moreover, the high-performing UFNPs nanoprobe, when conjugated with a targeting moiety AMD3100, enables the in vivo MRI detection of small lung metastasis with greatly enhanced sensitivity. Our results pave the way toward the chemical design of ultrasensitive T₁ nanoprobe for advanced molecular imaging.

Peptide modified manganese-doped iron oxide nanoparticles as a sensitive fluorescence nanosensor for non-invasive detection of trypsin activity in vitro and in vivo

Herein, a fluorescence turn-on nanosensor (MnIO@pep-FITC) has been proposed for detecting trypsin activity in vitro and in vivo through covalently immobilizing an FITC modified peptide substrate of trypsin (pep-FITC) on manganese-doped iron oxide nanoparticle (MnIO NP) surfaces via a polyethylene glycol (PEG) crosslinker. The conjugation of pep-FITC with MnIO NPs results in the quenching of FITC fluorescence. After trypsin cleavage, the FITC moiety is released from the MnIO NP surface, leading to a remarkable recovery of FITC fluorescence signal. Under the optimum experimental conditions, the recovery ratio of FITC fluorescence intensity is linearly dependent on the trypsin concentration in the range of 2 to 100 ng mL-1 in buffer and intracellular trypsin in the lysate of 5 × 102 to 1 × 104 HCT116 cells per mL, respectively. The detection limit of trypsin is 0.6 ng mL-1 in buffer or 359 cells per mL HCT116 cell lysate. The MnIO@pep-FITC is successfully employed to noninvasively monitor trypsin activity in the ultrasmall (ca. 4.9 mm3 in volume) BALB/c nude mouse-bearing HCT116 tumor by in vivo fluorescence imaging with external magnetic field assistance, demonstrating that it has excellent practicability.

Ultrasmall Fe@Fe3O4 nanoparticles as T1-T2 dual-mode MRI contrast agents for targeted tumor imaging

Targeted T₁-T₂ MRI contrast agents, which can eliminate the difficulty of image matching across multiple imaging instruments and permit specific localization of lesions, are promising candidates for more accurate diagnosis of tumors. In this study, ultrasmall Fe@Fe₃O₄ nanoparticles were designed and synthesized as T₁-T₂ dual-mode MRI contrast agents for accurate tumor imaging. First, to investigate the influence of nanoparticle size, Fe@Fe₃O₄ nanoparticles with diameters of 4, 8, and 12 nm were prepared, among which the 8 nm 3-(3,4-dihydroxyphenyl)propionic acid (DHCA)-modified nanoparticles exhibited the optimal T₁-T₂ dual-mode MRI performance. Next, to develop a tumor-targeted contrast agent, the DHCA-Fe@Fe₃O₄ nanoparticles were conjugated with the F56 peptide, which targets the vascular endothelial growth factor receptor, and the resulting F56-DHCA-Fe@Fe₃O₄ nanoparticles were found to exhibit good T₁-T₂ dual-mode imaging and tumor-targeting performance both in vitro and in vivo, indicating the nanoparticles represent a new research tool for accurate tumor diagnosis.

Colloidal polymer-coated Zn-doped iron oxide nanoparticles with high relaxivity and specific absorption rate for efficient magnetic resonance imaging and magnetic hyperthermia

Colloidally stable nanoparticles-based magnetic agents endowed with very high relaxivity and specific absorption rate are extremely desirable for efficient magnetic resonance imaging and magnetic hyperthermia, respectively. Here, we report a water dispersible magnetic agent consisting of zinc-doped superparamagnetic iron oxide nanoparticles (i.e., Zn-SPIONs) of 15 nm size with high saturation magnetization coated with an amphiphilic polymer for effective magnetic resonance imaging and magnetic hyperthermia of glioblastoma cells. These biocompatible polymer-coated Zn-SPIONs had 24 nm hydrodynamic diameter and exhibited high colloidal stability in various aqueous media, very high transverse relaxivity of 471 mM^-1 s^-1, and specific absorption rate up to 743.8 W g^-1, which perform better than most iron oxide nanoparticles reported in the literature, including commercially available agents. Therefore, using these polymer-coated Zn-SPIONs even at low concentrations, T₂-weighted magnetic resonance imaging and moderate magnetic hyperthermia of glioblastoma cells under clinically relevant magnetic field were successfully implemented. In addition, the results of this in vitro study suggest the superior potential of Zn-SPIONs as a theranostic nanosystem for brain cancer treatment, simultaneously acting as a contrast agent for magnetic resonance imaging and a heat mediator for localized magnetic hyperthermia.

Ultrasmall Ternary FePtMn Nanocrystals with Acidity-Triggered Dual-Ions Release and Hypoxia Relief for Multimodal Synergistic Chemodynamic/Photodynamic/Photothermal Cancer Therapy

Multimodal imaging-guided synergistic anticancer strategies have attracted increasing attention for efficient diagnosis and therapy of cancer. Herein, a multifunctional nanotheranostic agent FePtMn-Ce6/FA (FPMCF NPs) is constructed by covalently anchoring photosensitizer chlorin e6 (Ce6) and targeting molecule folic acid (FA) on ultrasmall homogeneous ternary FePtMn nanocrystals. Response to tumor microenvironment (TME), FPMCF NPs can release Fe²⁺ to catalyze H₂ O₂ into •OH by Fenton reaction and simultaneously catalyze hydrogen peroxide (H₂ O₂ ) into O₂ to overcome the tumor hypoxia barrier. Released O₂ is further catalyzed into ¹ O₂ under 660 nm laser irradiation with Ce6. Thus, the FPMCF NPs exhibit superior dual-ROS oxidization capability including ferroptosis chemodynamic oxidization and ¹ O₂ -based photodynamic oxidization. Interestingly, FPMCF NPs reveal strong photothermal conversion efficiency exposed to an 808 nm laser, which can assist dual-ROS oxidization to suppress solid tumor remarkably. Additionally, Mn²⁺ can be released from FPMCF NPs to enhance longitudinal relaxivity (T₁ -weighted magnetic resonance (MR) imaging) and Fe-synergistic transverse relaxivity (T₂ -weighted MR imaging), which is convenient for diagnosis of solid tumors. Meanwhile, the fluorescent/photothermal (FL/PT) imaging function of FPMCF NPs can also accurately monitor tumor location. Therefore, FPMCF NPs with multimodal MR/FL/PT imaging-guided synergistic chemodynamic/photodynamic/photothermal cancer therapy capability have potential bioapplication in bionanomedicine field.

Mn(ii) chelate-coated superparamagnetic iron oxide nanocrystals as high-efficiency magnetic resonance imaging contrast agents

In this communication, a paramagnetic bifunctional manganese(ii) chelate ([Mn(Dopa-EDTA)]2-) containing a catechol group is designed and synthesized. The catechol can bind iron ions on the surface of superparamagnetic iron oxide (SPIO) nanocrystals to form core-shell nanoparticles. Both 4 and 7 nm SPIO@[Mn(Dopa-EDTA)]2- show good water solubility, single-crystal dispersion, and low cytotoxicity. The study of the interplay between the longitudinal and transverse relaxation revealed that 4 nm SPIO@[Mn(Dopa-EDTA)]2- with lower r 2/r 1 = 1.75 at 0.5 T tends to be a perfect T 1 contrast agent while 7 nm SPIO@[Mn(Dopa-EDTA)]2- with a higher r 2/r 1 = 15.0 at 3.0 T tends to be a T 2 contrast agent. Interestingly, 4 nm SPIO@[Mn(Dopa-EDTA)]2- with an intermediate value of r 2/r 1 = 5.26 at 3.0 T could act as T 1-T 2 dual-modal contrast agent. In vivo imaging with the 4 nm SPIO@[Mn(Dopa-EDTA)]2- nanoparticle shows unique imaging features: (1) long-acting vascular imaging and different signal intensity changes between the liver parenchyma and blood vessels with the CEMRA sequence; (2) the synergistic contrast enhancement of hepatic imaging with the T 1WI and T 2WI sequence. In summary, these Fe/Mn hybrid core-shell nanoparticles, with their ease of synthesis, good biocompatibility, and synergistic contrast enhancement ability, may provide a useful method for tissue and vascular MR imaging.

Magnetic Nanomaterials as Contrast Agents for MRI

Magnetic Resonance Imaging (MRI) is a powerful, noninvasive and nondestructive technique, capable of providing three-dimensional (3D) images of living organisms. The use of magnetic contrast agents has allowed clinical researchers and analysts to significantly increase the sensitivity and specificity of MRI, since these agents change the intrinsic properties of the tissues within a living organism, increasing the information present in the images. Advances in nanotechnology and materials science, as well as the research of new magnetic effects, have been the driving forces that are propelling forward the use of magnetic nanostructures as promising alternatives to commercial contrast agents used in MRI. This review discusses the principles associated with the use of contrast agents in MRI, as well as the most recent reports focused on nanostructured contrast agents. The potential applications of gadolinium- (Gd) and manganese- (Mn) based nanomaterials and iron oxide nanoparticles in this imaging technique are discussed as well, from their magnetic behavior to the commonly used materials and nanoarchitectures. Additionally, recent efforts to develop new types of contrast agents based on synthetic antiferromagnetic and high aspect ratio nanostructures are also addressed. Furthermore, the application of these materials in theragnosis, either as contrast agents and controlled drug release systems, contrast agents and thermal therapy materials or contrast agents and radiosensitizers, is also presented.

Iron Oxide Nanoparticles: An Alternative for Positive Contrast in Magnetic Resonance Imaging

Iron oxide nanoparticles have been extensively utilised as negative (T2) contrast agents in magnetic resonance imaging. In the past few years, researchers have also exploited their application as positive (T1) contrast agents to overcome the limitation of traditional Gd3+ contrast agents. To provide T1 contrast, these particles must present certain physicochemical properties with control over the size, morphology and surface of the particles. In this review, we summarise the reported T1 iron oxide nanoparticles and critically revise their properties, synthetic protocols and application, not only in MRI but also in multimodal imaging. In addition, we briefly summarise the most important nanoparticulate Gd and Mn agents to evaluate whether T1 iron oxide nanoparticles can reach Gd/Mn contrast capabilities.

Shape Anisotropic Iron Oxide-Based Magnetic Nanoparticles: Synthesis and Biomedical Applications

Research on iron oxide-based magnetic nanoparticles and their clinical use has been, so far, mainly focused on the spherical shape. However, efforts have been made to develop synthetic routes that produce different anisotropic shapes not only in magnetite nanoparticles, but also in other ferrites, as their magnetic behavior and biological activity can be improved by controlling the shape. Ferrite nanoparticles show several properties that arise from finite-size and surface effects, like high magnetization and superparamagnetism, which make them interesting for use in nanomedicine. Herein, we show recent developments on the synthesis of anisotropic ferrite nanoparticles and the importance of shape-dependent properties for biomedical applications, such as magnetic drug delivery, magnetic hyperthermia and magnetic resonance imaging. A brief discussion on toxicity of iron oxide nanoparticles is also included.

[T1-weighted magnetic resonance imaging contrast agents and their theranostic nanoprobes]

Magnetic resonance imaging (MRI) is an important imaging modality for clinical disease diagnosis, and nearly 50% of clinical MRI examinations require contrast agents to enhance the diagnostic sensitivity. This review provides a summary of the major MRI contrast agents and their classification, and the advantages and limits of the commercially available MRI contrast agents, and elaborates on the exceedingly small magnetic iron oxide nanoparticles (ES-MIONs), dotted core-shell iron and gadolinium hybrid nanoparticles (FeGd-HN) and exceedingly small gadolinium oxide nanoparticles (ES-GON). These nanoparticles can greatly improve the efficiency of T₁-weighted MRI due to their high r₁ value and low r₂/r₁ ratio, and are expected to be translated into clinical contrast agents for T₁-weighted MRI. The authors also review the diagnostic and therapeutic integration system that combines MRI contrast agents with various tumor therapies, such as MRI-guided ferroptosis therapy, radiosensitization therapy, and photothermal therapy, which allow efficient treatment as well as real-time monitoring of tumors and serve as potential cancer therapy strategies. The possible future research directions in the field of MRI-based multifunctional diagnostic and therapeutic formulations are also discussed.

Transparent Films Made of Highly Scattering Particles

Today, colloids are widely employed in various products from creams and coatings to electronics. The ability to control their chemical, optical, or electronic features by controlling their size and shape explains why these materials are so widely preferred. Nevertheless, altering some of these properties may also lead to some undesired side effects, one of which is an increase in optical scattering upon concentration. Here, we address this strong scattering issue in films made of binary colloidal suspensions. In particular, we focus on raspberry-type polymeric particles made of a spherical polystyrene core decorated by small hemispherical domains of acrylate with an overall positive charge, which display an unusual stability against aggregation in aqueous solutions. Their solid films display a brilliant red color due to Bragg scattering but appear completely white on account of strong scattering otherwise. To suppress the scattering and induce transparency, we prepared films by hybridizing them with oppositely charged PS particles with a size similar to that of the bumps on the raspberries. We report that the smaller PS particles prevent raspberry particle aggregation in solid films and suppress scattering by decreasing the spatial variation of the refractive index inside the film. We believe that the results presented here provide a simple strategy to suppress strong scattering of larger particles to be used in optical coatings.

Dual-targeting and excretable ultrasmall SPIONs for T1-weighted positive MR imaging of intracranial glioblastoma cells by targeting the lipoprotein receptor-related protein

A multifunctional targeted nanoprobe composed of ultrasmall superparamagnetic iron oxide nanoparticles with surface-conjugated Angiopep-2 was successfully constructed for targeted MR imaging of intracranial glioblastoma.

An extracellular pH-driven targeted multifunctional manganese arsenite delivery system for tumor imaging and therapy

Expanding the use of arsenic trioxide (ATO, As2O3) in cancer chemotherapy has received extensive attention in recent years owing to its remarkable efficacy in treating acute promyelocytic leukemia (APL). To date, the use of ATO for clinical treatment of solid tumors is still limited by its poor biocompatibility and severe toxic side effects. To address these limitations, here we developed a pH-low insertion peptide (pHLIP) modified ATO-based multifunctional drug-delivery system (DDS), which is termed MnAs@SiO2-pHLIP. With the coating of pHLIP, MnAs@SiO2-pHLIP could efficiently target the acidic tumor microenvironment, resulting in high intracellular accumulation of the DDS. As a "smart" nanoparticle (NP) platform, the DDS could controllably discharge the loaded ATO in response to acidic environments, which promotes the apoptosis of cancer cells. The features of controlled release capacity and the outstanding targeting ability contribute to better anticancer efficacy and less toxicity towards normal tissues compared with free ATO. It is worth noting that the acidic tumor microenvironment would also trigger the release of manganese ions (Mn2+) that brighten the T1 signal, which is exploited for real-time monitoring via contrast-enhanced magnetic resonance imaging (MRI). These multifunctional features, as demonstrated by both in vitro and in vivo experiments, could potentially expand the use of ATO to the treatment of solid tumors. We believe that MnAs@SiO2-pHLIP could serve as an auspicious agent for cancer theranostics and find tremendous applications in cancer management.

A carbon dot based theranostic platform for dual-modal imaging and free radical scavenging

Magnetofluorescent carbon dots (Cdots) doped with both P³⁺ and Mn²⁺ (abbreviated as PMn@Cdots) have been synthesized in an aqueous solution via a microwave-assisted pyrolysis method. In this system, a P³⁺ dopant was introduced to enhance the emission efficiency of the Cdots, while the presence of a Mn²⁺ dopant granted magnetic resonance imaging (MRI) capability. To the best of our knowledge, the present work is the first attempt to regulate red-emission and free radical scavenging of PMn@Cdots to serve as a dual-modal imaging nanoprobe and an antioxidant agent. Unlike most red-emitting Cdots, the as-prepared PMn@Cdots can be readily purified from unreacted precursors through antisolvent precipitation instead of by time-consuming purification methods. The whole synthetic procedure is rapid, facile, efficiently reproducible, and scalable. More importantly, further conjugation of the PMn@Cdots with hyaluronic acid (termed PMn@Cdots/HA) gives them good in vivo and in vitro biocompatibility as well as the capability to selectively target CD44-overexpressing cancer cells, as investigated by flow cytometry, fluorescence, and MRI. Meanwhile, PMn@Cdots exhibit antioxidant activity against multiple DPPH, hydroxyl, and superoxide radicals, which is comparable to that for ascorbic acid. Favorably, PMn@Cdots/HA showed a dose-dependent cytoprotective capability against H₂O₂-induced oxidative stress in B16F1, HeLa, and HEL cells. Therefore, the Cdot based theranostic platform can simultaneously function as a potential therapeutic candidate and as a dual-modal probe for enabling accurate diagnosis in future clinical applications.

MRI Detectable Polymer Microspheres Embedded With Magnetic Ferrite Nanoclusters For Embolization: In Vitro And In Vivo Evaluation

OBJECTIVE

The objective of this study was to develop magnetic embolic microspheres that could be visualized by clinical magnetic resonance imaging (MRI) scanners aiming to improve the efficiency and safety of embolotherapy.

METHODS AND DISCUSSION

Magnetic ferrite nanoclusters (FNs) were synthesized with microwave-assisted solvothermal method, and their morphology, particle size, crystalline structure, magnetic properties as well as T2 relaxivity were characterized to confirm the feasibility of FNs as an MRI probe. Magnetic polymer microspheres (FNMs) were then produced by inverse suspension polymerization with FNs embedded inside. The physicochemical and mechanical properties (including morphology, particle size, infrared spectra, elasticity, etc.) of FNMs were investigated, and the magnetic properties and MRI detectable properties of FNMs were also assayed by vibrating sample magnetometer and MRI scanners. Favorable biocompatibility and long-term MRI detectability of FNMs were then studied in mice by subcutaneous injection. FNMs were further used to embolize rabbits' kidneys to evaluate the embolic property and detectability by MRI.

CONCLUSION

FNMs could serve as a promising MRI-visualized embolic material for embolotherapy in the future.

Anisotropic nanomaterials for shape-dependent physicochemical and biomedical applications

This review contributes towards a systematic understanding of the mechanism of shape-dependent effects on nanoparticles (NPs) for elaborating and predicting their properties and applications based on the past two decades of research. Recently, the significance of shape-dependent physical chemistry and biomedicine has drawn ever increasing attention. While there has been a great deal of effort to utilize NPs with different morphologies in these fields, so far research studies are largely localized in particular materials, synthetic methods, or biomedical applications, and have ignored the interactional and interdependent relationships of these areas. This review is a comprehensive description of the NP shapes from theory, synthesis, property to application. We figure out the roles that shape plays in the properties of different kinds of nanomaterials together with physicochemical and biomedical applications. Through systematic elaboration of these shape-dependent impacts, better utilization of nanomaterials with diverse morphologies would be realized and definite strategies would be expected for breakthroughs in these fields. In addition, we have proposed some critical challenges and open problems that need to be addressed in nanotechnology.

Size-Tailored Biocompatible FePt Nanoparticles for Dual T1/T2 Magnetic Resonance Imaging Contrast Enhancement

The development of new contrast agents (CAs) for magnetic resonance imaging (MRI) is of high interest, especially because of the increased concerns of patient safety and quick clearance of clinically used gadolinium and iron oxide-based CAs, respectively. Here, a two-step synthesis of superparamagnetic water-soluble iron platinum (FePt) nanoparticles (NPs) with core sizes between 2 and 8 nm for use as CAs in MRI is reported. First, wet-chemical organometallic NPs are synthesized by thermal decomposition in the presence of stabilizing oleic acid and oleylamine. Second, the hydrophobic NPs are coated with an amphiphilic polymer and transferred into aqueous media. Their magnetization values and relaxation rates exceed those published for CAs already used for clinical application. Their saturation magnetization increases with the core size to approximately 82 A·m²/kg_Fe. For 8 nm NPs, the T₂ relaxivity of approximately 221 (mM·s)^-1 is 5 times larger than that for the ferumoxides, and for 6 nm NPs, the T₁ relaxivity of approximately 12 (mM·s)^-1 is slightly higher than that of ultrasmall gadolinium oxide NPs. The 6 nm FePt NPs are identified as excellent CAs for both T₁ and T₂ imaging. Most importantly, because of their coating, significantly low cytotoxicity is achieved. FePt NPs prove to be a promising alternative to gadolinium and iron oxide NPs showing high-quality CA characteristics for both T₁- and T₂-weighted images.

Characterization and Relaxation Properties of a Series of Monodispersed Magnetic Nanoparticles

Magnetic iron oxide nanoparticles are relatively advanced nanomaterials, and are widely used in biology, physics and medicine, especially as contrast agents for magnetic resonance imaging. Characterization of the properties of magnetic nanoparticles plays an important role in the application of magnetic particles. As a contrast agent, the relaxation rate directly affects image enhancement. We characterized a series of monodispersed magnetic nanoparticles using different methods and measured their relaxation rates using a 0.47 T low-field Nuclear Magnetic Resonance instrument. Generally speaking, the properties of magnetic nanoparticles are closely related to their particle sizes; however, neither longitudinal relaxation rate r1 nor transverse relaxation rate r2 changes monotonously with the particle size d. Therefore, size can affect the magnetism of magnetic nanoparticles, but it is not the only factor. Then, we defined the relaxation rates ri′ (i = 1 or 2) using the induced magnetization of magnetic nanoparticles, and found that the correlation relationship between r1′ relaxation rate and r1 relaxation rate is slightly worse, with a correlation coefficient of R2 = 0.8939, while the correlation relationship between r2′ relaxation rate and r2 relaxation rate is very obvious, with a correlation coefficient of R2 = 0.9983. The main reason is that r2 relaxation rate is related to the magnetic field inhomogeneity, produced by magnetic nanoparticles; however r1 relaxation rate is mainly a result of the direct interaction of hydrogen nucleus in water molecules and the metal ions in magnetic nanoparticles to shorten the T1 relaxation time, so it is not directly related to magnetic field inhomogeneity.

Hydroxyl-PEG-Phosphonic Acid-Stabilized Superparamagnetic Manganese Oxide-Doped Iron Oxide Nanoparticles with Synergistic Effects for Dual-Mode MR Imaging

The T₁-T₂ dual-mode contrast agents for magnetic resonance imaging (MRI) can generate self-complementary confirmed T₂ and T₁ images, hence greatly improving the reliability. Facilely synthesizing nanoparticles with the ultrasensitive contrast property remains extremely challenging in nanoscience. Moreover, uncovering the mechanism correlating the signal enhancements and chemical constituents is vital for designing novel efficient synergistically enhanced T₁-T₂ dual-mode MRI nanoprobes. Herein, we report a one-pot facile method to synthesize the superparamagnetic manganese oxide-doped iron oxide (Fe₃O₄/MnO) nanoparticles for T₁-T₂ dual-mode MR imaging. Under external magnetic field, the local magnetic field intensities of MnO and Fe₃O₄ could be simultaneously enhanced through embedding MnO into Fe₃O₄ nanoparticles and hence can cause synergistic T₁ and T₂ contrast enhancements. Moreover, a novel and facile cost-effective method for large-scale synthesis of hydroxyl-polyethylene glycol-phosphonic acid-stabilizing ligands is designed. The facile synthetic method and surface coating strategy of superparamagnetic Fe₃O₄/MnO nanoparticles offer an idea for the chemical design and preparation of superparamagnetic nanoparticles with ultrasensitive MRI contrast abilities for disease evaluation and treatment.

Theranostic Calcium Phosphate Nanoparticles With Potential for Multimodal Imaging and Drug Delivery

Calcium phosphate (CaP) bioceramics closely resemble the natural human bone, which is a main reason for their popularity as bone substitutes. However, this compositional similarity makes it difficult to distinguish CaPs, especially in particulate form, from native bone by imaging modalities such as X-ray radiography, computed tomography (CT), and magnetic resonance imaging (MRI) to monitor the healing progress. External contrast agents can improve the imaging contrast of CaPs but can affect their physicochemical properties and can produce artifacts. In this work, we have attempted to improve the contrast of CaP nanoparticles via ion substitutions for multimodal imaging. Calcium-deficient hydroxyapatite (CDHA) nanoparticles with silver (Ag), gadolinium (Gd), and iron (Fe) substitution were prepared by a microwave-accelerated wet chemical process to improve the contrast in CT, T1 (spin–lattice), and T2 (spin–spin) MRI relaxation modes, respectively. Ag, Gd, and Fe were substituted at 0.25, 0.5, and 0.25 at.%, respectively. The ion-substituted CDHA (ICDHA) was found to be phase pure by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Transmission electron microscopy (TEM) images showed that the ICDHA nanoparticles were platelet shaped and of 52 ± 2 nm length and 6 ± 1 nm width. The ICDHA showed high contrast in X-ray and CT compared to CDHA. The vibrating sample magnetometry (VSM) studies showed the ICDHA to exhibit paramagnetic behavior compared to diamagnetic CDHA, which was further confirmed by improved contrast in T1 and T2 MRI mode. In addition, the in vitro tetracycline drug loading and release was studied to investigate the capability of these nanoparticles for antibiotic drug delivery. It was found that a burst release profile was observed for 24 h with 47–52% tetracycline drug release. The ICDHA nanoparticles also showed in vitro antibacterial activity against Staphylococcus aureus and Escherichia coli due to Ag, which was further enhanced by antibiotic loading. In vitro biocompatibility studies showed that the triple-ion-substituted ICDHA nanoparticles were cytocompatible. Thus, the ion-substituted CDHA nanoparticles can have potential theranostic applications due to their multimodal image contrast, antibacterial activity, and drug delivery potential. Future work will be conducted with actual bone samples in vitro or in animal models.

Construction of Paramagnetic Manganese-Chelated Polymeric Nanoparticles Using Pyrene-End-Modified Double-Hydrophilic Block Copolymers for Enhanced Magnetic Resonance Relaxivity: A Comparative Study with Cisplatin Pharmacophore

Cationic metal-mediated self-assembly of double-hydrophilic block copolymers (DHBCs) has been of great interest for the preparation of hybrid nanoparticles for versatile applications. Among many functional transition-metal ions, manganese (Mn^II) is a highly attractive element due to its paramagnetic property with a high coordination number. However, Mn^II does not lead to the efficient self-assembly of DHBCs because of the relatively high aqueous solubility of coordinated Mn^II. This article reports a facile method for direct conjugation of Mn^II ions inside sterically stabilized polymer assemblies, composed of pyrene-end-modified DHBCs. Nitroxide-mediated radical polymerization was used to prepare the poly(ethylene glycol)- b-poly(acrylate) DHBC precursor, followed by the end-modification with pyrene maleimide via the radical-exchange reaction. Employing the self-associated DHBC as the nanoscale template, the simple addition of Mn^II enables a large number of polyvalent Mn^II ions to be immobilized at the chelating blocks of DHBCs, which can be readily monitored by the excimeric fluorescence emission change of the terminal pyrene fluorophore. The resulting Mn^II-loaded polymeric nanoparticles (Mn^II-PNPs) possess nanogel-like scaffolds, which allow for efficient water permeation at the Mn^II-incorporated interior for enhanced magnetic resonance contrasting effect. Additionally, by comparing the coordination properties of Mn^II and cisplatin, we endeavor to understand the internal structures and the relevant physicochemical features of metal-chelated nanoparticles.

Positive magnetic resonance angiography using ultrafine ferritin-based iron oxide nanoparticles

Iron oxide nanoparticles with good biocompatibility can serve as safe magnetic resonance imaging contrast agents. Herein, we report that ultrafine ferritin-based iron oxide (hematite/maghemite) nanoparticles synthesized by controlled biomimetic mineralization using genetically recombinant human H chain ferritin can be used as a positive contrast agent in magnetic resonance angiography. The synthesized magnetoferritin with an averaged core size of 2.2 ± 0.7 nm (hereafter named M-HFn-2.2) shows a r1 value of 0.86 mM-1 s-1 and a r2/r1 ratio of 25.1 at a 7 T magnetic field. Blood pool imaging on mice using the M-HFn-2.2 nanoparticles that were injected through a tail vein by single injection at a dose of 0.54 mM Fe per kg mouse body weight enabled detecting detailed vascular nets at 3 minutes post-injection; the MR signal intensity continuously enhanced up to 2 hours post-injection, which is much longer than that of the commercial magnevist (Gd-DTPA) contrast. Moreover, biodistribution examination indicates that organs such as liver, spleen and kidney safely cleared the injected nanoparticles within one day after the injection, demonstrating no risk of iron overload in test mice. Therefore, this study sheds light on developing high-performance gadolinium free positive magnetic resonance contrast agents for biomedical applications.

Structure-Relaxivity Relationships of Magnetic Nanoparticles for Magnetic Resonance Imaging

Magnetic nanoparticles (MNPs) have been extensively explored as magnetic resonance imaging (MRI) contrast agents. With the increasing complexity in the structure of modern MNPs, the classical Solomon-Bloembergen-Morgan and the outer-sphere quantum mechanical theories established on simplistic models have encountered limitations for defining the emergent phenomena of relaxation enhancement in MRI. Recent progress in probing MRI relaxivity of MNPs based on structural features at the molecular and atomic scales is reviewed, namely, the structure-relaxivity relationships, including size, shape, crystal structure, surface modification, and assembled structure. A special emphasis is placed on bridging the gaps between classical simplistic models and modern MNPs with elegant structural complexity. In the pursuit of novel MRI contrast agents, it is hoped that this review will spur the critical thinking for design and engineering of novel MNPs for MRI applications across a broad spectrum of research fields.