Introduction

Inorganic nanomaterials are the main part of most contrast agents, and they can enhance the contrast between the patient's part and the surrounding tissue, which has attracted widespread attention from scientists in various fields such as biology, chemistry, physics and materials. There are many types of inorganic nanomaterials that can be used as contrast agents, including magnetic iron oxide materials, nano-gold particles, mesoporous silica materials, upconversion nanoparticles (UCNPs), and semiconductor quantum dot fluorescent materials.

Magnetic iron oxide materials

The magnetic iron oxide mainly includes Fe3O4 and γ-Fe2O3. Magnetic iron oxide nanoparticles can not only be biodegraded and have low toxicity, but also control the magnetic properties by adjusting the size of the particles. The bulk Fe3O4 is ferrimagnetic and has a multi-domain structure. When the size of Fe3O4 is less than 100nm, its coercive force reaches its maximum, and when the size is reduced to 20nm, the magnetization of Fe3O4 magnetic nanoparticles is transformed into superparamagnetism. Fe3O4 and γ-Fe2O3 magnetic iron oxide nanoparticles have a wide range of applications in biomedicine, including cell tracking, biosensing, imaging diagnosis, and drug delivery.

Gold nanoparticles

Gold nanoparticles refer to tiny particles of gold in three dimensions, including gold nanospheres and gold nanorods. Nano-gold particles have high electron density and have a strong attenuation effect on X-rays, and are widely used in CT angiography. Gold nanoparticles are widely used in molecular imaging because they are easy to prepare, and have the advantages of long in vivo circulation time, low toxicity, and easy binding of targeting molecules on the surface of materials. The most common application of gold nanoparticles is CT imaging. Compared to human tissue, gold nanoparticles have a strong absorption of X-rays, which is 2.7 times that of traditional iodine compounds, so it can greatly improve the contrast of CT imaging. Nano-gold particles are the most promising CT contrast agents.

Mesoporous silica materials

The mesoporous silicon oxide material is a silicon oxide with a large number of microporous structures inside. Mesoporous silica has good biocompatibility and excellent biodegradability. In recent years, it has attracted more and more attention as a drug carrier in the field of biomedicine such as drug transportation and imaging diagnosis. Compared with organic carriers, mesoporous silica has better thermal stability and chemical stability, high loading capacity, and shows unique advantages in drug delivery. With the development of nanosynthetic chemistry, various functionalized mesoporous silicas have been synthesized, and they are all obtained by compounding some magnetic materials or fluorescent materials with mesoporous silica. Mesoporous silica-based composite contrast materials are widely used in targeted MRI imaging and fluorescence imaging of tumors by adding different functional components. At present, the most studied mesoporous silica composite nano-biomaterials are mesoporous magnetic silica nanomaterials, which are obtained by compounding magnetic iron oxide nanoparticles into mesoporous silica. The composite of mesoporous silica and magnetic iron oxide nanoparticles can introduce magnetic properties into the mesoporous silica material, giving it the ability of MRI imaging contrast. If mesoporous silica and fluorescent materials are compounded together, fluorescence imaging can be achieved.

Upconversion nanoparticles (UCNPs)

Upconverted nanoparticles (UCNPs) are functional materials that can convert low-energy photons into high-energy photons, especially upconverted nanoparticles doped with lanthanum (Ln), which are currently the new generation of biological imaging contrast agents. A significant feature of upconversion luminescent materials is that the energy of the photons absorbed by the material is lower than the energy of the photons emitted by the material. Generally speaking, up-conversion nanoluminescent materials are mainly composed of a host matrix, a sensitizer and an activator. Common matrix materials include fluorides, oxides and chlorides. Up-conversion luminescent nanomaterials have excellent characteristics such as higher chemical stability, narrow band-gap emission, and good light stability. And they have good tissue penetrability under the excitation of near-infrared light without interference of background fluorescence, and have no damage to living organisms, so they have broad application prospects in medical imaging.

Semiconductor quantum dots

Semiconductor quantum dots (QD) is a nanomaterial composed of III-V and II-VI elements (such as CdTe, CdSe, ZnSe, InAs, InP, etc.). Compared with traditional fluorescent reagents, quantum dots, as fluorescent substances, have their unique advantages including wide band absorption, adjustable fluorescence emission, narrow fluorescence peaks, stable and long-lifetime fluorescence. By changing the size and composition of semiconductor quantum dots, fluorescence with a wavelength ranging from near ultraviolet to far infrared can be obtained. Modern medical research often requires multicolor imaging. For organic dyes, because of their wide emission bands, signal overlap is easy to occur, which affects their clinical application. The quantum dots have narrow and adjustable fluorescence emission, which makes multicolor imaging simple and feasible. The unique optical properties and mature synthesis make quantum dot a good fluorescent probe. As the research on semiconductor quantum dots continues, they will play an increasingly important role in imaging diagnosis.

Conclusion

For years, researchers have been exploring new and promising methods to improve the treatment efficiency of cancer and other diseases and the effectiveness of imaging studies. Imaging technology is the core driving force for the development of biomedical research. Advances in the performance of contrast materials have improved the clarity and resolution of imaging, which has enabled doctors to obtain more abundant medical imaging information and greatly promoted the development of clinical medical imaging. The multimodal imaging combined with different imaging modes and the combination of imaging and treatment will become the focus of future research on inorganic contrast materials.