Controlled aggregation of superparamagnetic iron oxide nanoparticles for the development of molecular magnetic resonance imaging probes

Recent Advances in the Development of Drug Delivery Applications of Magnetic Nanomaterials

With the predicted rise in the incidence of cancer, there is an ever-growing need for new cancer treatment strategies. Recently, magnetic nanoparticles have stood out as promising nanostructures for imaging and drug delivery systems as they possess unique properties. Moreover, magnetic nanomaterials functionalized with other compounds can lead to multicomponent nanoparticles with innovative structures and synergetic performance. The incorporation of chemotherapeutic drugs or RNA in magnetic drug delivery systems represents a promising alternative that can increase efficiency and reduce the side effects of anticancer therapy. This review presents a critical overview of the recent literature concerning the advancements in the field of magnetic nanoparticles used in drug delivery, with a focus on their classification, characteristics, synthesis and functionalization methods, limitations, and examples of magnetic drug delivery systems incorporating chemotherapeutics or RNA.

Mechanobiological Analysis of Nanoparticle Toxicity

Nanoparticles (NPs) are commonly used in healthcare and nanotherapy, but their toxicity at high concentrations is well-known. Recent research has shown that NPs can also cause toxicity at low concentrations, disrupting various cellular functions and leading to altered mechanobiological behavior. While researchers have used different methods to investigate the effects of NPs on cells, including gene expression and cell adhesion assays, the use of mechanobiological tools in this context has been underutilized. This review emphasizes the importance of further exploring the mechanobiological effects of NPs, which could reveal valuable insights into the mechanisms behind NP toxicity. To investigate these effects, different methods, including the use of polydimethylsiloxane (PDMS) pillars to study cell motility, traction force production, and rigidity sensing contractions, have been employed. Understanding how NPs affect cell cytoskeletal functions through mechanobiology could have significant implications, such as developing innovative drug delivery systems and tissue engineering techniques, and could improve the safety of NPs for biomedical applications. In summary, this review highlights the significance of incorporating mechanobiology into the study of NP toxicity and demonstrates the potential of this interdisciplinary field to advance our knowledge and practical use of NPs.

Tailored design and preparation of magnetic nanocomposite particles for the isolation of exosomes

Here, we prepared a magnetic nanocomposite system composed of a cluster of magnetite nanoparticles coated with silica shell (MSNPs) with an average diameter of 140 ± 20 nm and conjugated with CD9 antibody (AntiCD9) using different strategies including adsorption or chemical conjugation of antibody molecules to either aminated MSNPs (AMSNPs) or carboxylated MSNPs (CMSNPs). Then, MSNPs were employed to isolate exosomes from ultracentrifuge-enriched solution, PC3 cell-culture medium, or exosome-spiked simulated plasma samples. Quantitative tests using nanoparticle-tracking analysis confirmed antibody-covalently conjugated MSNPs, i.e. the AntiCD9-AMSNPs and AntiCD9-CMSNPs enabled >90% recovery of exosomes. Additionally, the exosomes isolated with AntiCD9-CMSNPs showed higher recovery efficiency compared to the AntiCD9-AMSNPs. For both nanoadsorbents, lower protein impurities amounts were obtained as compared to that of exosomes isolated by ultracentrifugation and Exocib kit. The mean diameter assessment of the isolated exosomes indicates that particles isolated by using AntiCD9-AMSNPs and AntiCD9-CMSNPs have smaller sizes (136 ± 2.64 nm and 113 ± 11.53 nm, respectively) than those obtained by UC-enriched exosomes (140.9 ± 1.6 nm) and Exocib kit (167 ± 10.53 nm). Such promising results obtained in the isolation of exosomes recommend magnetic nanocomposite as an efficient tool for the simple and fast isolation of exosomes for diagnosis applications.

Nanoparticles for Magnetic Heating: When Two (or More) Is Better Than One

The increasing use of magnetic nanoparticles as heating agents in biomedicine is driven by their proven utility in hyperthermia therapeutic treatments and heat-triggered drug delivery methods. The growing demand of efficient and versatile nanoheaters has prompted the creation of novel types of magnetic nanoparticle systems exploiting the magnetic interaction (exchange or dipolar in nature) between two or more constituent magnetic elements (magnetic phases, primary nanoparticles) to enhance and tune the heating power. This process occurred in parallel with the progress in the methods for the chemical synthesis of nanostructures and in the comprehension of magnetic phenomena at the nanoscale. Therefore, complex magnetic architectures have been realized that we classify as: (a) core/shell nanoparticles; (b) multicore nanoparticles; (c) linear aggregates; (d) hybrid systems; (e) mixed nanoparticle systems. After a general introduction to the magnetic heating phenomenology, we illustrate the different classes of nanoparticle systems and the strategic novelty they represent. We review some of the research works that have significantly contributed to clarify the relationship between the compositional and structural properties, as determined by the synthetic process, the magnetic properties and the heating mechanism.

Multivalency in CXCR4 chemokine receptor targeted iron oxide nanoparticles

The CXCR4 chemokine receptor is an important biomolecular target in cancer diagnostics and therapeutics. In a new multivalent approach, iron oxide nanoparticles were conjugated with multiple binding units of a low affinity azamacrocylic CXCR4 antagonist. The silica coated nanostructure has good suspension stability, a mode size of 72 nm and high affinity for CXCR4, showing >98% inhibition of anti-CXCR4 mAb binding in a receptor binding competition assay on Jurkat cells.

Libraries of Uniform Magnetic Multicore Nanoparticles with Tunable Dimensions for Biomedical and Photonic Applications

Multicore iron oxide nanoparticles, also known as colloidal nanocrystal clusters, are magnetic materials with diverse applications in biomedicine and photonics. Here, we examine how both of their characteristic dimensional features, the primary particle and sub-micron colloid diameters, influence their magnetic properties and performance in two different applications. The characterization of these basic size-dependent properties is enabled by a synthetic strategy that provides independent control over both the primary nanocrystal and cluster dimensions. Over a wide range of conditions, electron microscopy and X-ray diffraction reveal that the oriented attachment of smaller nanocrystals results in their crystallographic alignment throughout the entire superstructure. We apply a sulfonated polymer with high charge density to prevent cluster aggregation and conjugate molecular dyes to particle surfaces so as to visualize their collection using handheld magnets. These libraries of colloidal clusters, indexed both by primary nanocrystal dimension (d_p) and overall cluster diameter (D_c), form magnetic photonic crystals with relatively weak size-dependent properties. In contrast, their performance as MRI T₂ contrast agents is highly sensitive to cluster diameter, not primary particle size, and is optimized for materials of 50 nm diameter (r₂ = 364 mM^-1 s^-1). These results exemplify the relevance of dimensional control in developing applications for these versatile materials.

In Vivo Evaluation of Magnetic Targeting in Mice Colon Tumors with Ultra-Magnetic Liposomes Monitored by MRI

PURPOSE

The development of theranostic nanocarriers as an innovative therapy against cancer has been improved by targeting properties in order to optimize the drug delivery to safely achieve its desired therapeutic effect. The aim of this paper is to evaluate the magnetic targeting (MT) efficiency of ultra-magnetic liposomes (UML) into CT26 murine colon tumor by magnetic resonance imaging (MRI).

PROCEDURES

Dynamic susceptibility contrast MRI was applied to assess the bloodstream circulation time. A novel semi-quantitative method called %I_0.25, based on the intensity distribution in T₂^*-weighted MRI images was developed to compare the accumulation of T₂ contrast agent in tumors with or without MT. To evaluate the efficiency of magnetic targeting, the percentage of pixels under the intensity value I_0.25 (I_0.25 = 0.25(I_max - I_min)) was calculated on the intensity distribution histogram.

RESULTS

This innovative method of processing MRI images showed the MT efficiency by a %I_0.25 that was significantly higher in tumors using MT compared to passive accumulation, from 15.3 to 28.6 %. This methodology was validated by ex vivo methods with an iron concentration that is 3-fold higher in tumors using MT.

CONCLUSIONS

We have developed a method that allows a semi-quantitative evaluation of targeting efficiency in tumors, which could be applied to different T₂ contrast agents.

Clustering superparamagnetic iron oxide nanoparticles produces organ-targeted high-contrast magnetic resonance images

Aim: Superparamagnetic iron oxide nanoparticles (SPIONs) have been used as magnetic resonance imaging (MRI) contrast agents; however, a number of T2-weighted imaging SPIONs have been withdrawn due to their poor clinical contrast performance. Our aim was to significantly improve SPION T2-weighted MRI contrast by clustering SPIONs within novel chitosan amphiphiles. Methods: Clustering SPIONs was achieved by encapsulation of hydrophobic-coated SPIONs with an amphiphilic chitosan polymer (GCPQ). Results: Clustering increases the spin-spin (r₂) to spin-lattice (r₁) relaxation ratio (r₂/r₁) from 3.0 to 79.1, resulting in superior contrast. Intravenously administered clustered SPIONs accumulated only in the liver and spleen; with the reduction in T2 relaxation confined, in the liver, to the extravascular space, giving clear MRI images of the liver vasculature.

Biodistribution and Tumors MRI Contrast Enhancement of Magnetic Nanocubes, Nanoclusters, and Nanorods in Multiple Mice Models

Magnetic resonance imaging (MRI) is a powerful technique for tumor diagnostics. Iron oxide nanoparticles (IONPs) are safe and biocompatible tools that can be used for further enhancing MR tumor contrasting. Although numerous IONPs have been proposed as MRI contrast agents, low delivery rates to tumor site limit its application. IONPs accumulation in malignancies depends on both IONPs characteristics and tumor properties. In the current paper, three differently shaped Pluronic F-127-modified IONPs (nanocubes, nanoclusters, and nanorods) were compared side by side in three murine tumor models (4T1 breast cancer, B16 melanoma, and CT26 colon cancer). Orthotopic B16 tumors demonstrated more efficient IONPs uptake than heterotopic implants. Magnetic nanocubes (MNCb) had the highest r2-relaxivity in vitro (300 mM⁻¹·s⁻¹) compared with magnetic nanoclusters (MNCl, 104 mM⁻¹·s⁻¹) and magnetic nanorods (MNRd, 51 mM⁻¹·s⁻¹). As measured by atomic emission spectroscopy, MNCb also demonstrated better delivery efficiency to tumors (3.79% ID) than MNCl (2.94% ID) and MNRd (1.21% ID). Nevertheless, MNCl overperformed its counterparts in tumor imaging, providing contrast enhancement in 96% of studied malignancies, whereas MNCb and MNRd were detected by MRI in 73% and 63% of tumors, respectively. Maximum MR contrasting efficiency for MNCb and MNCl was around 6-24 hours after systemic administration, whereas for MNRd maximum contrast enhancement was found within first 30 minutes upon treatment. Presumably, MNRd poor MRI performance was due to low r2-relaxivity and rapid clearance by lungs (17.3% ID) immediately after injection. MNCb and MNCl were mainly captured by the liver and spleen without significant accumulation in the lungs, kidneys, and heart. High biocompatibility and profound accumulation in tumor tissues make MNCb and MNCl the promising platforms for MRI-based tumor diagnostics and drug delivery.

Integration of metabolomics and transcriptomics in nanotoxicity studies

Biomedical research involving nanoparticles has produced useful products with medical applications. However, the potential toxicity of nanoparticles in biofluids, cells, tissues, and organisms is a major challenge. The '-omics' analyses provide molecular profiles of multifactorial biological systems instead of focusing on a single molecule. The 'omics' approaches are necessary to evaluate nanotoxicity because classical methods for the detection of nanotoxicity have limited ability in detecting miniscule variations within a cell and do not accurately reflect the actual levels of nanotoxicity. In addition, the 'omics' approaches allow analyses of in-depth changes and compensate for the differences associated with high-throughput technologies between actual nanotoxicity and results from traditional cytotoxic evaluations. However, compared with a single omics approach, integrated omics provides precise and sensitive information by integrating complex biological conditions. Thus, these technologies contribute to extended safety evaluations of nanotoxicity and allow the accurate diagnoses of diseases far earlier than was once possible in the nanotechnology era. Here, we review a novel approach for evaluating nanotoxicity by integrating metabolomics with metabolomic profiling and transcriptomics, which is termed "metabotranscriptomics". [BMB Reports 2018; 51(1): 14-20].

Hybrid magneto-plasmonic liposomes for multimodal image-guided and brain-targeted HIV treatment

Image-guided drug delivery is an emerging strategy in the field of nanomedicine. The addition of image guidance to a traditional drug delivery system is expected to achieve highly efficient treatment by tracking the drug carriers in the body and monitoring their effective accumulation in the targeted tissues. In this study, we developed multifunctional magneto-plasmonic liposomes (MPLs), a hybrid system combining liposomes and magneto-plasmonic nanoparticles for a triple-modality image-guided drug delivery. Tenofovir disoproxil fumarate, an antiretroviral drug used to treat human immunodeficiency virus type 1 (HIV-1), was encapsulated into the MPLs to enable the treatment in the brain microenvironment, which is inaccessible to most of the drugs. We found strong negative and positive contrasts originating from the magnetic core of MPLs in magnetic resonance imaging (MRI) and magnetic particle imaging (MPI), respectively. The gold shell of MPLs showed bright positive contrast in X-ray computed tomography (CT). MPLs achieved enhanced transmigration across an in vitro blood-brain barrier (BBB) model by magnetic targeting. Moreover, MPLs provided desired therapeutic effects against HIV infected microglia cells.

Recent Advances on Magnetic Relaxation Switching Assay-Based Nanosensors

Magnetic relaxation switching assay (MRSw)-based nanosensors respond to the changes of transverse relaxation time (T₂) of water molecules resulted from the analyte-induced aggregation and disaggregation of magnetic nanoparticles (MNPs). This strategy has been widely applied to the detections of various substrates from heavy metal ions to organic pollutants, proteins, nucleic acids, bacteria and viruses, and specific cells. Compared with other nanosensors, MRSw-based nanosensors not only are free from the background interferences, signal bleaching, and quenching but also overcome light scattering from samples without pretreatments. Therefore, MRSw-based nanosensors have been developed as real-time and on-site detection platforms for environmental protection, food safety, and risk assessment. This review summarizes the latest developments of the principles, the applicable magnetic nanoparticles, and the exploited environmental and biological applications of MRSw-based nanosensors.

Methionine-mediated synthesis of magnetic nanoparticles and functionalization with gold quantum dots for theranostic applications

Biocompatible superparamagnetic iron oxide nanoparticles (NPs) through smart chemical functionalization of their surface with fluorescent species, therapeutic proteins, antibiotics, and aptamers offer remarkable potential for diagnosis and therapy of disease sites at their initial stage of growth. Such NPs can be obtained by the creation of proper linkers between magnetic NP and fluorescent or drug probes. One of these linkers is gold, because it is chemically stable, nontoxic and capable to link various biomolecules. In this study, we present a way for a simple and reliable decoration the surface of magnetic NPs with gold quantum dots (QDs) containing more than 13.5% of Au⁺. Emphasis is put on the synthesis of magnetic NPs by co-precipitation using the amino acid methionine as NP growth-stabilizing agent capable to later reduce and attach gold species. The surface of these NPs can be further conjugated with targeting and chemotherapy agents, such as cancer stem cell-related antibodies and the anticancer drug doxorubicin, for early detection and improved treatment. In order to verify our findings, high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), FTIR spectroscopy, inductively coupled plasma mass spectroscopy (ICP-MS), and X-ray photoelectron spectroscopy (XPS) of as-formed CoFe₂O₄ NPs before and after decoration with gold QDs were applied.

Hollow mesoporous silica nanosphere-supported FePt nanoparticles for potential theranostic applications

A nanocomposite comprising FePt nanoparticles and hollow mesoporous silica nanospheres has been fabricated for MRI, NIR photothermal therapy and combined chemo-/thermotherapy.

Silica-coated magnetic nanoparticles impair proteasome activity and increase the formation of cytoplasmic inclusion bodies in vitro

The potential toxicity of nanoparticles, particularly to neurons, is a major concern. In this study, we assessed the cytotoxicity of silica-coated magnetic nanoparticles containing rhodamine B isothiocyanate dye (MNPs@SiO₂(RITC)) in HEK293 cells, SH-SY5Y cells, and rat primary cortical and dopaminergic neurons. In cells treated with 1.0 μg/μl MNPs@SiO₂(RITC), the expression of several genes related to the proteasome pathway was altered, and proteasome activity was significantly reduced, compared with control and with 0.1 μg/μl MNPs@SiO₂(RITC)-treated cells. Due to the reduction of proteasome activity, formation of cytoplasmic inclusions increased significantly in HEK293 cells over-expressing the α–synuclein interacting protein synphilin-1 as well as in primary cortical and dopaminergic neurons. Primary neurons, particularly dopaminergic neurons, were more vulnerable to MNPs@SiO₂(RITC) than SH-SY5Y cells. Cellular polyamines, which are associated with protein aggregation, were significantly altered in SH-SY5Y cells treated with MNPs@SiO₂(RITC). These findings highlight the mechanisms of neurotoxicity incurred by nanoparticles.

SPION@Cu2−xS nanoclusters for highly sensitive MRI and targeted photothermal therapy of hepatocellular carcinoma

The diagnostic function of SPIONs and photo-thermal therapeutic function of CuS NPs have been incorporated into a single nanoplatform for biomedical applications.

How morphology influences relaxivity - comparative study of superparamagnetic iron oxide-polymer hybrid nanostructures

Superparamagnetic iron oxides (SPIOs) are widely used in MRI as T2 contrast agents, and interest is still growing. Here, the T2 relaxivity of three different SPIO-polymer hybrid morphologies, i.e. homogeneously distributed iron oxide within a polymer matrix, Janus-like nanoparticles and polymer nanocapsules containing iron oxides, is studied. Making use of calculations based on theory for agglomerated systems, the obtained T2 values could be predicted for all different morphologies, except for nanocapsules. Nanocapsules, in contrast to full spheres, allow for water exchange between encapsulated water and bulk water, and thus have two contributions to relaxivity. One originates from the capsules acting as a weakly magnetized cluster and the other stems from the individual SPIOs inside the capsule. Therefore, the relaxivities were also computed using an empirical equation found in the literature, which considers water exchange, resulting in a better T2 forecast for the nanocapsules. The presented study is the first example of a comparison between measured and calculated relaxivities of nanocapsules.

Ellipsoid-shaped superparamagnetic nanoclusters through emulsion electrospinning

Ellipsoid-shaped nanoclusters composed of single superparamagnetic nanoparticles and possessing a high saturation magnetization can be generated by emulsion electrospinning.

Accounting for biological aggregation in heating and imaging of magnetic nanoparticles

Aggregation is a known consequence of nanoparticle use in biology and medicine; however, nanoparticle characterization is typically performed under the pretext of well-dispersed, aqueous conditions. Here, we systematically characterize the effects of aggregation on the alternating magnetic field induced heating and magnetic resonance (MR) imaging performance of iron oxide nanoparticles (IONPs) in non-ideal biological systems. Specifically, the behavior of IONP aggregates composed of ~10 nm primary particles, but with aggregate hydrodynamic sizes ranging from 50 nm to 700 nm, was characterized in phosphate buffered saline and fetal bovine serum suspensions, as well as in gels and cells. We demonstrate up to a 50% reduction in heating, linked to the extent of aggregation. To quantify aggregate morphology, we used a combination of hydrodynamic radii distribution, intrinsic viscosity, and electron microscopy measurements to describe the aggregates as quasifractal entities with fractal dimensions in the 1.8-2.0 range. Importantly, we are able to correlate the observed decrease in magnetic field induced heating with a corresponding decrease in longitudinal relaxation rate (R1) in MR imaging, irrespective of the extent of aggregation. Finally, we show in vivo proof-of-principle use of this powerful new imaging method, providing a critical tool for predicting heating in clinical cancer hyperthermia.

Colloidal assemblies of oriented maghemite nanocrystals and their NMR relaxometric properties

¹H-NMR relaxometric experiments over an extended frequency range show that ferrimagnetic colloidal nanoclusters exhibit enhanced transverse relaxivity, r₂.

Magnetic PEGylated Pt3Co nanoparticles as a novel MR contrast agent: in vivo MR imaging and long-term toxicity study

Magnetic resonance (MR) imaging using magnetic nanoparticles as the contrast agent has been extensively explored in biomedical imaging and disease diagnosis. Herein, we develop biocompatible polymer coated ultra-small Pt3Co magnetic nanoparticles as a new T2-weighted MR imaging contrast agent. A unique class of alloy Pt3Co nanoparticles is synthesized through a thermal decomposition method. After being modified with polyethylene glycol (PEG), the obtained Pt3Co-PEG nanoparticles exhibit an extremely high T2-weighted relaxivity rate (r2) up to 451.2 mM s(-1), which is much higher than that of Resovist®, a commercial T2-MR contrast agent used in the clinic. In vitro experiments indicate no obvious cytotoxicity of Pt3Co-PEG nanoparticles to various cell lines. After intravenous injection of Pt3Co-PEG nanoparticles, in vivo T2-weighted MR imaging of tumor-bearing mice reveals strong tumor contrast, which is much higher than that offered by injecting Resovist®. We further study the long-term biodistribution and toxicology of this new type of MR contrast nanoparticles after intravenous injection into healthy mice. Despite the significant retention of Pt3Co-PEG nanoparticles in the mouse liver and spleen, no appreciable toxicity of these nanoparticles to the treated animals has been noted in our detailed histological and hematological analysis over a course of 60 days. Our work demonstrates that functionalized Pt3Co nanoparticles may be a promising new type of T2-weighted MR contrast agent potentially useful in biomedical imaging and diagnosis.

Magnetic nanoparticles: surface effects and properties related to biomedicine applications

Due to finite size effects, such as the high surface-to-volume ratio and different crystal structures, magnetic nanoparticles are found to exhibit interesting and considerably different magnetic properties than those found in their corresponding bulk materials. These nanoparticles can be synthesized in several ways (e.g., chemical and physical) with controllable sizes enabling their comparison to biological organisms from cells (10–100 μm), viruses, genes, down to proteins (3–50 nm). The optimization of the nanoparticles’ size, size distribution, agglomeration, coating, and shapes along with their unique magnetic properties prompted the application of nanoparticles of this type in diverse fields. Biomedicine is one of these fields where intensive research is currently being conducted. In this review, we will discuss the magnetic properties of nanoparticles which are directly related to their applications in biomedicine. We will focus mainly on surface effects and ferrite nanoparticles, and on one diagnostic application of magnetic nanoparticles as magnetic resonance imaging contrast agents.

Casein-coated iron oxide nanoparticles for high MRI contrast enhancement and efficient cell targeting

Surface properties, as well as inherent physicochemical properties, of the engineered nanomaterials play important roles in their interactions with the biological systems, which eventually affect their efficiency in diagnostic and therapeutic applications. Here we report a new class of MRI contrast agent based on milk casein protein-coated iron oxide nanoparticles (CNIOs) with a core size of 15 nm and hydrodynamic diameter ~30 nm. These CNIOs exhibited excellent water-solubility, colloidal stability, and biocompatibility. Importantly, CNIOs exhibited prominent T2 enhancing capability with a transverse relaxivity r2 of 273 mM(-1) s(-1) at 3 tesla. The transverse relaxivity is ~2.5-fold higher than that of iron oxide nanoparticles with the same core but an amphiphilic polymer coating. CNIOs showed pH-responsive properties, formed loose and soluble aggregates near the pI (pH ~4.0). The aggregates could be dissociated reversibly when the solution pH was adjusted away from the pI. The transverse relaxation property and MRI contrast enhancing effect of CNIOs remained unchanged in the pH range of 2.0-8.0. Further functionalization of CNIOs can be achieved via surface modification of the protein coating. Bioaffinitive ligands, such as a single chain fragment from the antibody of epidermal growth factor receptor (ScFvEGFR), could be readily conjugated onto the protein coating, enabling specific targeting to MDA-MB-231 breast cancer cells overexpressing EGFR. T2-weighted MRI of mice intravenously administered with CNIOs demonstrated strong contrast enhancement in the liver and spleen. These favorable properties suggest CNIOs as a class of biomarker targeted magnetic nanoparticles for MRI contrast enhancement and related biomedical applications.

Effects of Mesoporous Silica Coating and Post-Synthetic Treatment on the Transverse Relaxivity of Iron Oxide Nanoparticles

Mesoporous silica nanoparticles have the capacity to load and deliver therapeutic cargo and incorporate imaging modalities, making them prominent candidates for theranostic devices. One of the most widespread imaging agents utilized in this and other theranostic platforms is nanoscale superparamagnetic iron oxide. Although several core-shell magnetic mesoporous silica nanoparticles presented in the literature have provided high T₂ contrast in vitro and in vivo, there is ambiguity surrounding which parameters lead to enhanced contrast. Additionally, there is a need to understand the behavior of these imaging agents over time in biologically relevant environments. Herein, we present a systematic analysis of how the transverse relaxivity (r₂) of magnetic mesoporous silica nanoparticles is influenced by nanoparticle diameter, iron oxide nanoparticle core synthesis, and the use of a hydrothermal treatment. This work demonstrates that samples which did not undergo a hydrothermal treatment experienced a drop in r₂ (75% of original r₂ within 8 days of water storage), while samples with hydrothermal treatment maintained roughly the same r₂ for over 30 days in water. Our results suggest that iron oxide oxidation is the cause of the r₂ loss, and this oxidation can be prevented both during synthesis and storage by the use of deoxygenated conditions during nanoparticle synthesis. The hydrothermal treatment also provides colloidal stability, even in acidic and highly salted solutions, and a resistance against acid degradation of the iron oxide nanoparticle core. The results of this study show the promise of multifunctional mesoporous silica nanoparticles but will also likely inspire further investigation into multiples types of theranostic devices, taking into consideration their behavior over time and in relevant biological environments.

Optimized synthesis of 100 nm diameter magnetoliposomes with high content of maghemite particles and high MRI effect

Magnetoliposomes are liposomes surrounding an iron oxide core, which are used as contrast enhancing agents in magnetic resonance imaging (MRI). One method for producing magnetoliposomes consists of hydration of a lipid film with citrate-coated iron oxide particles followed by extrusion. Two parameters are of major importance for in vivo applications of magnetoliposomes, namely their size, which must be small, optimally around 100 nm diameter, in order to ensure their prolonged circulation in the bloodstream, and their iron content, which must be maximal for generating high MRI effect. We studied the formation of magnetoliposomes by passive encapsulation of maghemite (γ-Fe(2)O(3)) particle suspensions of varying concentrations, with the objective of producing magnetoliposomes of small size and high iron content. The iron to lipid ratio was used to determine the iron content of the magnetoliposomes after the successive purification steps and cryo-TEM was used to characterize their size, their homogeneity and the efficiency of purification. The size of citrate-coated maghemite clusters was found to be of critical importance for obtaining magnetoliposomes smaller than 200 nm. We were able to reproducibly synthesize magnetoliposomes of 100 nm diameter with high iron content -up to 77 particles per liposome (5.6 moles iron per mole lipid) - and high r(2) MRI relaxivity - up to 320 m m(-1) . s(-1) . The magnetoliposomes present improved characteristics compared with previous reports. Future research will focus on using these magnetoliposomes as drug delivery systems for in vivo diagnostics or therapeutics applications.

A universal scaling law to predict the efficiency of magnetic nanoparticles as MRI T(2)-contrast agents

Magnetic particles are very efficient magnetic resonance imaging (MRI) contrast agents. In recent years, chemists have unleashed their imagination to design multi-functional nanoprobes for biomedical applications including MRI contrast enhancement. This study is focused on the direct relationship between the size and magnetization of the particles and their nuclear magnetic resonance relaxation properties, which condition their efficiency. Experimental relaxation results with maghemite particles exhibiting a wide range of sizes and magnetizations are compared to previously published data and to well-established relaxation theories with a good agreement. This allows deriving the experimental master curve of the transverse relaxivity versus particle size and to predict the MRI contrast efficiency of any type of magnetic nanoparticles. This prediction only requires the knowledge of the size of the particles impermeable to water protons and the saturation magnetization of the corresponding volume. To predict the T(2) relaxation efficiency of magnetic single crystals, the crystal size and magnetization - obtained through a single Langevin fit of a magnetization curve - is the only information needed. For contrast agents made of several magnetic cores assembled into various geometries (dilute fractal aggregates, dense spherical clusters, core-shell micelles, hollow vesicles…), one needs to know a third parameter, namely the intra-aggregate volume fraction occupied by the magnetic materials relatively to the whole (hydrodynamic) sphere. Finally a calculation of the maximum achievable relaxation effect - and the size needed to reach this maximum - is performed for different cases: maghemite single crystals and dense clusters, core-shell particles (oxide layer around a metallic core) and zinc-manganese ferrite crystals.

AGGREGATION BEHAVIOR OF IRON OXIDE NANOPARTICLES MEASURED BY SQUID MAGNETOMETRY

The aggregation behavior of superparamagnetic iron oxide nanoparticles in an aqueous medium has been studied using SQUID magnetometry and XRD analysis. Iron oxide nanoparticles were synthesized by the coprecipitation method using iron salt and sodium hydroxide as precursors. The reactions were initially carried out at high temperature with different concentrations of iron salt. The synthesized nanoparticles were then collected for subsequent bio-functionalization. Structural characterizations were done using XRD and TEM measurements and show that the particles are magnetite (cubic spinel structure) with an average size of 9.5 nm. Magnetic measurements done using SQUID predict a magnetic particle size of 7.75 nm, which suggests a "magnetically dead layer" is present on the surface.

Ultra-fast method to synthesize mesoporous magnetite nanoclusters as highly sensitive magnetic resonance probe

An ultra-fast method to synthesize mesoporous magnetite (Fe(3)O(4)) nanoclusters is reported. These mesoporous magnetite can be used as a highly sensitive magnetic resonance imaging (MRI) probe. The nanoclusters were synthesized by reducing iron (III) acetylacetonate with hydrazine in ethylene glycol under microwave irradiation and only 5 min was needed in the synthesis. The diameter of the nanoclusters could be controlled effectively between 75 nm and 115 nm by increasing the amount of iron (III) acetylacetonate. Brunauer-Emmett-Teller (BET) results reveal a mesoporous structure and a large surface area of 72.3 m(2) g(-1). Cytotoxicity test performed in HepG2 cell line indicated that the as-prepared nanoclusters were non-cytotoxic. The nanoclusters exhibited an enhanced T(2) relaxivity value of 417.4±29.9 s(-1) mM(-1). In vitro and in vivo MRI confirmed the high sensitivity of the magnetite nanoclusters as MRI probe. The biodistribution of the nanoclusters in rat liver and spleen after intravenous injection was also investigated.

Accumulation of magnetic iron oxide nanoparticles coated with variably sized polyethylene glycol in murine tumors

Iron oxide nanoparticles have found widespread applications in different areas including cell separation, drug delivery and as contrast agents. Due to water insolubility and stability issues, nanoparticles utilized for biological applications require coatings such as the commonly employed polyethylene glycol (PEG). Despite its frequent use, the influence of PEG coatings on the physicochemical and biological properties of iron nanoparticles has hitherto not been studied in detail. To address this, we studied the effect of 333-20,000 Da PEG coatings that resulted in larger hydrodynamic size, lower surface charge, longer circulation half-life, and lower uptake in macrophage cells when the particles were coated with high molecular weight (M(w)) PEG molecules. By use of magnetic resonance imaging, we show coating-dependent in vivo uptake in murine tumors with an optimal coating M(w) of 10,000 Da.

Photocytotoxicity and magnetic relaxivity responses of dual-porous γ-Fe2O3@meso-SiO2 microspheres

Novel high magnetization microspheres with porous γ-Fe(2)O(3) core and porous SiO(2) shell were synthesized using a templating method, whereas the size of the magnetic core and the thickness of the porous shell can be controlled by tuning the experimental parameters. By way of an example, as-prepared γ-Fe(2)O(3)@meso-SiO(2) microspheres (170 nm) display excellent water-dispersity and show photonic characteristics under externally applied a magnetic field. The magnetic property of the γ-Fe(2)O(3) porous core enables the microspheres to be used as a contrast agent in magnetic resonance imaging with a high r(2) (76.5 s(-1) mM(-1) Fe) relaxivity. The biocompatible composites possess a large BET surface area (222.3 m(2)/g), demonstrating that they can be used as a bifunctional agent for both MRI and drug carrier. Because of the high substrate loading of the magnetic, dual-porous materials, only a low dosage of the substrate will be acquired for potential practical applications. Hydrophobic zinc(II) phthalocyanine (ZnPC) photosensitizing molecules have been encapsulated into the dual-porous microspheres to form γ-Fe(2)O(3)@meso-SiO(2)-ZnPC microspheres. Biosafety, cellular uptake in HT29 cells, and in vitro MRI of these nanoparticles have been demonstrated. Photocytotoxicity (λ > 610 nm) of the HT29 cells uptaken with γ-Fe(2)O(3)@meso-SiO(2)-ZnPC microspheres has been demonstrated for 20 min illumination.

Detection of glomerular complement C3 fragments by magnetic resonance imaging in murine lupus nephritis

One of the challenges of treating patients with glomerulonephritis is to accurately assess disease activity. As renal biopsies are routinely stained for deposits of C3 activation fragments and glomerular C3 deposits are found in most forms of glomerulonephritis, we sought to determine whether a relatively noninvasive measure of C3 fragment deposition in the kidney can serve as a good biomarker of disease onset and severity. We recently developed a magnetic resonance imaging (MRI)-based method of detecting glomerular C3 and used this to track the progression of renal disease in the MRL/lpr mouse model of lupus nephritis using superparamagnetic iron oxide nanoparticles conjugated to complement receptor type 2 as a targeting agent. Quantitative immunofluorescence showed that glomerular C3b/iC3b/C3d deposition progressively increased with disease activity, a finding replicated by the T2-weighted MRI. The T2 relaxation times decreased with disease activity in the cortex and medulla of the MRL/lpr but not in MRL/Mpj control mice. Thus, MRI contrast agents targeted to glomerular C3 fragments can be used to noninvasively monitor disease activity in glomerulonephritis. As therapeutic complement inhibitors are used in patients with renal disease, this method, should it become feasible in humans, may identify those likely to benefit from complement inhibition.

Monte Carlo simulation and theory of proton NMR transverse relaxation induced by aggregation of magnetic particles used as MRI contrast agents

Superparamagnetic particles are widely used in MRI as R2 contrast agents. In this last decade, different studies have focused on aggregation of superparamagnetic particles for important applications such as multimodal agents. A complete study--via simulations--of the influence of aggregation on the MR efficiency of these particles at high magnetic field is presented here. First, an empirical expression is proposed for R2 in the presence of uniformly distributed nanoparticles, taking into account two regimes at once (motional averaging and slow motion regimes). Three cluster shapes are simulated: Sphere, shell and line. An analytical model is proposed to understand water transverse relaxation induced by spherical and shell aggregates. Simulations lead to the conclusion that, in the motional averaging regime, the most efficient aggregate contrast agent is the densest sphere or shell.

Magnetite Nanoparticles for Medical MR Imaging

Nanotechnology has given scientists new tools for the development of advanced materials for the detection and diagnosis of disease. Iron oxide nanoparticles (SPIONs) in particular have been extensively investigated as novel magnetic resonance imaging (MRI) contrast agents due to a combination of favorable superparamagnetic properties, biodegradability, and surface properties of easy modification for improved in vivo kinetics and multifunctionality. This review discusses the basics of MR imaging, the origin of SPION's unique magnetic properties, recent developments in MRI acquisition methods for detection of SPIONs, synthesis and post-synthesis processes that improve SPION's imaging characteristics, and an outlook on the translational potential of SPIONs.

Stimuli-responsive liposome-nanoparticle assemblies

INTRODUCTION

Nanoscale assemblies are needed that achieve multiple therapeutic objectives, including cellular targeting, imaging, diagnostics and drug delivery. These must exhibit high stability, bioavailability and biocompatibility, while maintaining or enhancing the inherent activity of the therapeutic cargo. Liposome-nanoparticle assemblies (LNAs) combine the demonstrated potential of liposome-based therapies, with functional nanoparticles. Specifically, LNAs can be used to concentrate and shield the nanoparticles and, in turn, stimuli-responsive nanoparticles that respond to external fields can be used to control liposomal release. The ability to design LNAs via nanoparticle encapsulation, decoration or bilayer-embedment offers a range of configurations with different structures and functions.

AREAS COVERED

This paper reviews the current state of research and understanding of the design, characterization and performance of LNAs. A brief overview is provided on liposomes and nanoparticles for therapeutic applications, followed by a discussion of the opportunities and challenges associated with combining the two in a single assembly to achieve controlled release via light or radiofrequency stimuli.

EXPERT OPINION

LNAs offer a unique opportunity to combine the therapeutic properties of liposomes and nanoparticles. Liposomes act to concentrate small nanoparticles and shield nanoparticles from the immune system, while the nanoparticle can be used to initiate and control drug release when exposed to external stimuli. These properties provide a platform to achieve nanoparticle-controlled liposomal release. LNA design and application are still in infancy. Research concentrating on the relationships among LNA structure, function and performance is essential for the future clinical use of LNAs.

Clusters of superparamagnetic iron oxide nanoparticles encapsulated in a hydrogel: a particle architecture generating a synergistic enhancement of the T2 relaxation

Clusters of iron oxide nanoparticles encapsulated in a pH-responsive hydrogel are synthesized and studied for their ability to alter the T(2)-relaxivity of protons. Encapsulation of the clusters with the hydrophilic coating is shown to enhance the transverse relaxation rate by up to 85% compared to clusters with no coating. With the use of pH-sensitive hydrogel, difficulties inherent in comparing particle samples are eliminated and a clear increase in relaxivity as the coating swells is demonstrated. Agreement with Monte Carlo simulations indicates that the lower diffusivity of water inside the coating and near the particle surface leads to the enhancement. This demonstration of a surface-active particle structure opens new possibilities in using similar structures for nanoparticle-based diagnostics using magnetic resonance imaging.

NMR relaxation and magnetic properties of superparamagnetic nanoworms

Maghemite particles are used as T₂ contrast agents for magnetic resonance imaging, especially for molecular and cellular imaging. Linear clusters of particles - called nanoworms - were recently developed to enhance the targeting efficiency. In this work, the magnetic and NMR relaxation properties of these nanoworms are studied at multiple magnetic fields. After the usual saturation at 0.5 T, the magnetization of the worms is still increasing, which results in an appreciable increase of the transverse relaxivity at high magnetic fields. The obtained relaxivities are typical of superparamagnetic particles of iron oxide (SPIOs). The transverse relaxation of the worms is clearly more efficient than for the isolated grains, which is confirmed by computer simulations. At high field, the longitudinal relaxation of the worms is less pronounced than for the grains, as expected for SPIOs. The nanoworms thus constitute a promising T₂ agent for cellular and molecular imaging.

Incorporation of magnetite nanoparticle clusters in fluorescent silica nanoparticles for high-performance brain tumor delineation

Bifunctional nanoprobes with both magnetic and optical contrast have been developed for ultra-sensitive brain tumor imaging at the cellular level. The nanoprobes were synthesized by simultaneously incorporating a magnetite nanoparticle cluster and fluorescence dyes into silica encapsulation by a sol-gel approach under ultrasonic treatment. The nanoprobes maintain superparamagnetic behavior at room temperature and possess enhanced transverse relaxivity and good photostability. As a glioma targeting ligand, chlorotoxin was covalently bonded to the surface of the nanoprobes. In vitro cellular uptake assays demonstrated that the nanoprobes were highly specific, taken up by human U251-MG glioma cells via receptor-mediated endocytosis. The labeled glioma cells were readily detectable by both MR imager and confocal laser scanning microscopy.

Nanobeads highly loaded with superparamagnetic nanoparticles prepared by emulsification and seeded-emulsion polymerization

Functional superparamagnetic colloids possessing high saturation magnetization are prepared by emulsification of superparamagnetic nanoparticles (SPM NPs) and heterogeneous polymerization. The colloids consist of a core of densely packed NPs encapsulated within a thin polymer shell. The cores are made by emulsifying SPM NPs and toluene into an aqueous surfactant solution, and subsequently condensing the emulsion droplets by removal of the solvent generating clusters of SPM NPs. By tuning the emulsification condition, this approach allows for control over the size of the clusters from approximately 40 to 200 nm. The polymer shells encapsulating the clusters are made by using seeded-emulsion polymerization concepts. Control over the thickness of the shell and the incorporation of functional groups to the colloid is achieved. Characterization by thermogravimetric analysis (TGA) and magnetometry shows that these colloids have 66 wt % of magnetic material and saturation magnetization of 47 emu/g, confirming that this route generates colloids with a high loading of SPM NPs and high saturation magnetizations.

Renal inflammation: targeted iron oxide nanoparticles for molecular MR imaging in mice

PURPOSE

To determine the feasibility of T2-weighted magnetic resonance (MR) imaging in the noninvasive quantification of renal inflammation by using superparamagnetic iron oxide (SPIO) nanoparticles targeted to tissue-bound C3 activation fragments in a mouse model of lupus nephritis.

MATERIALS AND METHODS

All animal procedures were approved by the University of Colorado-Denver animal care and use committee. SPIO nanoparticles were encapsulated by using amine-functionalized phospholipids. A recombinant protein containing the C3d-binding region of complement receptor type 2 (CR2) was then conjugated to the surface of the SPIO nanoparticle. Five MRL/lpr mice (a model of lupus nephritis) and six C57BL/6 wild-type mice were assessed with T2-weighted MR imaging at baseline and after SPIO injection. The same five MRL/lpr mice and three C57BL/6 mice also underwent MR imaging after injection of CR2-targeted SPIO. A series of T2-weighted pulses with 16 echo times was used to enable precise T2 mapping and calculation of T2 relaxation times in the cortex and outer and inner medulla of the kidneys, as well as in the spleen, muscle, and fat. The effects of treatment and animal genotype on T2 relaxation times were analyzed with repeated-measures analysis of variance.

RESULTS

At baseline, the T2-weighted signal intensity in the kidneys of MRL/lpr mice was higher than that in the kidneys of wild-type mice. Injection of untargeted SPIO did not alter the T2-weighted signal in the kidneys in either strain of mice. Injection of CR2-targeted SPIO in MRL/lpr mice, however, caused a significant accumulation of targeted iron oxide with a subsequent decrease in T2 relaxation times in the cortex and outer and inner medulla of the kidneys. No changes in T2 relaxation time were observed in the wild-type mice after injection of targeted SPIO.

CONCLUSION

Injection of CR2-conjugated SPIO caused a significant reduction in T2-weighted MR imaging signal and T2 relaxation time in nephritic kidneys.

Experimental validation of proton transverse relaxivity models for superparamagnetic nanoparticle MRI contrast agents

Analytical models of proton transverse relaxation rate enhancement by magnetic nanoparticles were tested by making measurements on model experimental systems in a field of 1.4 T. Proton relaxivities were measured for five aqueous suspensions of iron oxide (maghemite) nanoparticles with nominal mean particle sizes of 6, 8, 10, 11, and 13 nm. Proton relaxivity increased with mean particle size ranging from 13 s(-1) mM Fe(-1) for the 6 nm sample, up to 254 s(-1) mM Fe(-1) for the 13 nm sample. A strong correlation between the measured and predicted values of the relaxivity was observed, with the predicted values being consistently higher than the measured values. The results indicate that the models give a reasonable agreement with experimental results and hence can be used as the basis for the design of new magnetic resonance imaging contrast and labelling agents.