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.

Cancer remains one of the leading causes of death worldwide, prompting extensive research into novel theranostic (combined word of diagnostic and therapeutic) strategies. Nanomedicine has emerged as a potential breakthrough in cancer theranosis, overcoming limitations of conventional approaches. Among such approaches, carbon dots (CDs) with a size smaller than 10 nm have garnered significant attention for their potential use in cancer theranosis, owing to their low toxicity, good water solubility, easy synthesis, facile surface modification, and unique optical and photothermal and photodynamic properties. Researchers have demonstrated that surface functionalization of CDs with diverse hydrophilic groups can be easily achieved by choosing proper carbon precursors in synthesis, and further surface modification of CDs with cancer-targeting ligands, photosensitizers, anticancer drugs, and genes can also be easily achieved using various methods, thereby establishing a versatile approach for cancer theranosis. This review described the various surface modification methods of CDs, in vitro and in vivo toxicity of CDs, and various cancer theranostic methods such as drug delivery, photodynamic therapy, photothermal therapy, gene therapy, sonodynamic therapy, and gas therapy. Therefore, CDs can serve as various mono and combined theranostic modalities, offering us new methods for cancer theranosis.

X-ray computed tomography (CT) contrast agents offer extremely valuable tools and techniques in diagnostics via contrast enhancements. Heavy metal-based nanoparticles (NPs) can provide high contrast in CT images due to the high density of heavy metal atoms with high X-ray attenuation coefficients that exceed that of iodine (I), which is currently used in hydrophilic organic CT contrast agents. Nontoxicity and colloidal stability are vital characteristics in designing heavy metal-based NPs as CT contrast agents. In addition, a small particle size is desirable for in vivo renal excretion. In vitro phantom imaging studies have been performed to obtain X-ray attenuation efficiency, which is a critical parameter for CT contrast agents, and the imaging performance of CT contrast agents has been demonstrated via in vivo experiments. In this review, we focus on the in vitro and in vivo studies of various heavy metal-based NPs in pure metallic or chemical forms, including Au, Pt, Pd, Ag, Ce, Gd, Dy, Ho, Yb, Ta, W, and Bi, and provide an outlook on their use as high-performance CT contrast agents.

Magnetite nanoparticles (Fe₃O₄ NPs) have been intensively investigated because of their potential biomedical applications due to their high saturation magnetization. In this study, core–shell Fe₃O₄@C NPs (core = Fe₃O₄ NPs and shell = amorphous carbons, d_avg = 35.1 nm) were synthesized in an aqueous solution. Carbon coating terminated with hydrophilic –OH and –COOH groups imparted excellent biocompatibility and hydrophilicity to the NPs, making them suitable for biomedical applications. The Fe₃O₄@C NPs exhibited ideal relaxometric properties for T₂ magnetic resonance imaging (MRI) contrast agents (i.e., high transverse and negligible longitudinal water proton spin relaxivities), making them exclusively induce only T₂ relaxation. Their T₂ MRI performance as contrast agents was confirmed in vivo by measuring T₂ MR images in mice before and after intravenous injection.

Surface-coating polymers contribute to nanoparticle-based magnetic resonance imaging (MRI) contrast agents because they can affect the relaxometric properties of the nanoparticles. In this study, polyaspartic acid (PASA)-coated ultrasmall Gd₂O₃ nanoparticles with an average particle diameter of 2.0 nm were synthesized using the one-pot polyol method. The synthesized nanoparticles exhibited r₁ and r₂ of 19.1 and = 53.7 s⁻¹mM⁻¹, respectively, (r₁ and r₂ are longitudinal and transverse water–proton spin relaxivities, respectively) at 3.0 T MR field, approximately 5 and 10 times higher than those of commercial Gd-chelate contrast agents, respectively. The T₁ and T₂ MR images could be obtained due to an appreciable r₂/r₁ ratio of 2.80, indicating their potential as a dual-modal T₁ and T₂ MRI contrast agent.

Although numerous studies have investigated gadolinium oxide (Gd₂O₃) nanoparticles (NPs) as positive (T₁) magnetic resonance imaging (MRI) contrast agents (CAs), comprehensive reviews on this topic remain scarce. Therefore, it is essential to evaluate their current status and outline prospects. Despite promising physicochemical properties such as considerably higher relaxivities compared to 3–5 s⁻¹mM⁻¹ of clinically approved Gd(III)-chelate contrast agents and encouraging results from in vivo animal studies such as highly improved contrast enhancements, drug loading, and tumor targeting, extensive in vivo toxicity assessments including long-term toxicity and formulation advancements suitable for renal excretion (d < ~3 nm) are still required for clinical translation. This review summarizes the synthesis, characterization, in vitro and in vivo toxicity, and in vivo MRI applications of surface-modified Gd₂O₃ NPs as T₁ MRI CAs. Finally, future perspectives on the development of surface-modified Gd₂O₃ NPs as potential next-generation T₁ MRI CAs are discussed.