What are Metal Nanoclusters
Metal nanoclusters are a kind of nanomaterials with a size of less than 2 nm and stacked by 1 to 150 metal atoms. Metal nanoclusters have a typical core-shell structure, consisting of a metal atomic core and a ligand molecular shell. Ligands are usually substances that have strong covalent interaction with metal atoms such as amino groups, sulfhydryl groups, and phosphorus groups, such as thiol compounds, dendrimers, polymers, deoxyribonucleic acid (DNA), polypeptides, and proteins. At present, gold nanoclusters (AuNCs), silver nanoclusters (AgNCs), platinum nanoclusters (PtNCs), copper nanoclusters (CuNCs) and other metal nanoclusters have been widely synthesized. In recent years, metal nanoclusters have made remarkable achievements in biomedical fields such as biomarkers, biosensing, bioimaging, and tumor treatment.
Physical and Chemical Properties of Metal Nanoclusters
Because the size is close to the Fermi wavelength of electrons (~1 nm), metal nanoclusters exhibit some special physical and chemical properties such as fluorescence, chirality, magnetism, and catalysis that are different from metal nanocrystals. Among them, fluorescence is one of the most important optical properties of metal nanoclusters. When the size of nanomaterials is gradually reduced to 2 nm, the unique plasmonic properties of metal nanoparticles disappear, the electronic energy levels are dispersed from a continuous state to a discontinuous state, and the electronic transitions between molecular energy levels become active and produce strong the light absorption causes strong fluorescence emission. The fluorescence of metal nanoclusters is adjustable, which can be customized from the visible light region to the near-infrared light region. At the same time, the fluorescence of metal nanoclusters also has the characteristics of large Stokes shift and good luminescence stability. In recent years, studies on the chirality of metal nanoclusters have found that its chirality mainly includes three aspects: metal chirality, surface ligand chirality, and shell chirality formed by metal and ligand. In addition, metal nanoclusters also have cis Magnetism. Studies have shown that the valence of metal nanoclusters directly affects its paramagnetism, so it can be controlled by redox methods. In terms of catalysis, due to the exposure of more surface metal atoms, metal nanoclusters are used in a variety of catalytic chemistry It shows better catalytic activity in the synthesis reaction. At the same time, the ligand and the number of atoms of the metal nanoclusters play a key role in its catalytic activity.
The special properties of metal nanoclusters, such as light and magnetism, make it show unique application advantages in biomedical research. Metal nanoclusters have easy surface modification and good biocompatibility. The ligands on the surface of metal nanoclusters can be further coupled with polyethylene glycol (PEG), targeting peptides or drugs, etc., which can effectively enhance the stability and dispersion of metal nanoclusters in solutions and complex biological environments, and can realize the integrated function of metal nanocluster targeting and diagnosis and treatment.
In addition, metal nanoclusters also have characteristics related to biomedical research applications such as simulated enzyme catalytic activity, high X-ray mass attenuation coefficient, near-infrared light absorption, and generation of reactive oxygen radicals. In terms of simulating enzyme catalysis, due to the high specific surface area and high surface energy, metal nanoclusters tend to show higher catalytic activity of simulating enzymes than metal nanocrystals with similar composition and surface properties. In addition, Au has a high atomic number, can effectively absorb X-rays, and has an X-ray mass attenuation coefficient that is 1.7 times higher than that of traditional iodine compound contrast agents. AuNCs have significant advantages in X-ray imaging and radiotherapy research. Recently, it has been reported that AuNCs can generate active oxygen free radicals under light. Due to the excited triplet state and the long life of the electronic excited state, the oxygen absorption sites in AuNCs exist well, and the energy is effectively transferred to molecular oxygen to generate active oxygen radicals. In addition, AuNCs have also been found to have absorption properties in the near-infrared region. It is mainly related to the type of ligand, the size of gold core and the surface charge distribution.