Gold Nanoparticles and Their Applications

Nanomaterials

Recent research shows that the application of nanomaterials plays an important role in the advancement of nanoscience and nanotechnology. Nanostructured materials have a wealth of physical and chemical properties compared to other small or bulk materials. Nanoparticles have unique intrinsic reactivity due to their increased surface area and are ideal for the synthesis of therapeutic drugs. The interaction between nanomaterials and biological systems depends on the type of surface functional groups of the nanoparticles, particle size, particle shape, and aggregation state, as well as cell type, uptake pathway, and targeting organelles. Among different types of nanomaterials, metal nanoparticles, especially gold nanoparticles (AuNPs), have attracted great interest from researchers in different scientific fields due to their unique properties. Since the beginning of the 20th century, scientists have done a lot of research on the existence of anisotropic gold nanoparticles, and found that the structure, optical properties, electronic properties, magnetic properties and catalytic properties of anisotropic gold nanoparticles are different from those of spherical gold nanoparticles, and are generally superior to spherical gold nanoparticles.

Advantages of gold nanoparticles

1) The X-ray absorption coefficient is high;

2) Simple synthesis operation;

3) Physicochemical properties of particles that can be precisely controlled;

4) Strong binding affinity for mercaptans, disulfides, and amines;

5) Unique adjustable optical and electronic properties;

6) Widely used in nanoelectronics, optoelectronics, catalysis, and biomedical applications.

Classification of gold nanoparticles

According to the size, AuNPs can be divided into four categories (Figure 1):

1) Zero-dimensional gold nanoparticles: quantum dots, spherical nanoparticles (Fig. 1a);

2) One-dimensional gold nanoparticles: nanorods (Fig. 1b), nanowires (Fig. 1e), nanotubes, nanobelts;

3) Two-dimensional gold nanoparticles: nanostars (Fig. 1f), nanodisks;

4) Three-dimensional gold nanoparticles: triangular nanoprisms (Fig. 1d), nanodumbbells (Fig. 1c), nanodendrites (Fig. 1g), nanocubes (Fig. 1h).

Application of gold nanoparticles

• Photodynamic therapy

Photodynamic therapy (PDT) is considered to be an important treatment for tumor diseases and certain skin or infectious diseases, using photosensitizers as light-sensitizing agents and a laser (wavelengths related to dye absorption peaks). Singlet oxygen and high activity free radicals produced by photosensitizer energy induce tumor cell apoptosis or necrosis. In addition, gold nanoconjugates are easy to combine with thiols, disulfides, and amines to promote intracellular penetration.

• Photothermal therapy

Photothermia (PTT), also known as thermal ablation or optical hyperthermia, is a minimally invasive, widely used method of cancer treatment. Gold nanoparticles have a maximum absorption peak in the visible or near-infrared region and can receive light and generate heat. High temperatures can lead to the death of malignant tumors. Spherical solid gold nanoparticles larger than 50 nm in diameter are more common in photothermal therapy due to strong absorption in the near-infrared region. In addition, gold nanoparticle-antibody conjugates are also useful in diagnostic and photothermographic therapies. Like photodynamic therapy, the strong binding properties of gold nanoparticles play a key role in their intracellular transfer.

• X-ray imaging

As an X-ray contrast agent, gold nanoparticles have attracted extensive attention due to their high X-ray absorption coefficient and easy synthesis operation, and non-toxicity and surface functionalization. Common angiographic agents such as iodinated molecules have a low molecular weight. Although these iodinated aromatic compounds have high water solubility and low toxicity characteristics, they possess short blood circulation time and are quickly excreted through the kidneys. Therefore, multiple injections of iodinated aromatic compounds are required, which may cause a risk of thyroid dysfunction. Compared to common agents, gold nanoparticles have longer vascular retention time, making them suitable for imaging applications.

• Drug delivery

As mentioned earlier, gold nanoparticles have many advantages, such as unique optical and physicochemical properties, high biocompatibility, functional flexibility, adjustable monolayer, controlled dispersion, high surface area for loading drugs, nontoxicity and stability, which makes them an effective nanocarrier in drug delivery systems (DDSs). These highly efficient nanocarriers are capable of delivering a variety of drugs such as peptides, proteins, plasmid DNA (pDNA), small interfering RNA (siRNA) and chemotherapeutic agents. In addition to spherical gold nanoparticles, researchers have recently synthesized stable colloidal gold nanorods as suitable carriers for drug delivery. PEGylated gold nanorods prevent the scavenging effects of the reticuloendothelial system (RES), providing an efficient means of drug delivery. Another nanocarrier is a gold nanocage. Targeted drug delivery is achieved by binding cancer cell receptors to the surface of nanocages surface conjugated with biologically active molecules such as antibodies. Gold nanorods are suitable drug delivery vehicles that benefit from their ability to utilize surface plasmon resonance to convert incident light energy into thermal energy, which is the basis for targeted drug delivery applications for phototherapy combined with cancer therapy.

• Sensing

One of the main applications of gold nanoparticles is chemical and biological sensing. They have been used as effective sensors for detecting different analytes, such as metal ions, anions, sugars, nucleotides, proteins and toxins. According to the sensing strategy, the gold nanoparticle sensor has various principles and different types of nanobiosensors utilize different characteristics of gold nanoparticles. For example, the basic principle of colorimetric sensors is based on the visible color change caused by the aggregation of gold nanoparticles (Fig. 2). The fluorescence sensor utilizes the fluorescence quenching characteristics of gold nanoparticles; the electrical and electrochemical sensors utilize the conductivity of gold nanoparticles, and their high surface area and catalytic properties; gold nanoparticle-based surface plasmon resonance sensors utilize the optical properties of gold nanoparticles; and gold nanoparticle-based bio-barcode assay uses the strong binding affinity of gold nanoparticles to thiol and the visible color change caused by the aggregation of gold nanoparticles to achieve detection purposes.

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