Gold nanorods are rod-shaped gold nanoparticles with a scale from a few nanometers to hundreds of nanometers. Gold is a precious metal material with very stable chemical properties. Gold nanoparticles follow this property of their bulk materials and are therefore relatively stable, but have very rich chemical and physical properties.

The crystal growth method has become the most effective way to synthesize high-purity solvent phase gold nanorod colloidal solution by chemical methods. Cetyltrimethylammonium bromide (CTAB) is one of the most commonly used surfactants in the synthesis of gold nanorods (CTAB exists as a surfactant and stabilizer).

Optical properties of gold nanorods
Due to the unique size and physicochemical properties of gold nanorods, in addition to the similar properties of gold nanoparticles, there are the following unique characteristics.

l Surface plasmon resonance absorption characteristics
The gold nanorod has a transverse plasmon resonance absorption peak (TSPR) and a longitudinal plasmon resonance absorption peak (LSPR) corresponding to two characteristic sizes of the horizontal axis and the vertical axis, respectively, and the ratio of the length of the vertical axis to the diameter of the horizontal axis is gold nanorods. The length-to-diameter ratio can be used to prepare gold nanorods with adjustable length and aspect ratio by changing experimental conditions.

Fluorescence characteristics
The fluorescence of gold nanorods has been reported in many experimental and theoretical calculations. The enhanced fluorescence of short gold nanorods with low fluorescence quantum yield can be detected by both fluorescence emission and two-photon induced photoluminescence.

Application of gold nanorods
Photothermal therapy
GNRs have a strong adjustable absorption light between visible light and near infrared. Under the irradiation of laser light, GNRs can efficiently convert light energy into heat energy, which can increase the local temperature of the tumor, destroy the cell membrane of tumor cells, inhibit the synthesis of genes and proteins, and disintegrate the nuclear membrane. The disappearance of the kernel eventually leads to apoptosis, thereby inhibiting the growth of the tumor. At the same time, GNRs can target drug-carrying drugs, so that the drug can only be enriched in the target cells at a higher concentration; under the action of photosynthesis, the sensitivity of the cells to drugs can be increased, thereby making the drug selection for tumor cells. Sexual killing is more obvious, thus reducing damage to normal tissues and cells. The therapy not only removes the lesion tissue in the body, but also provides a new non-surgical treatment for the clinical treatment of tumors.

Anti-counterfeiting recognition technology
Gold nanorods can have a continuous wavelength response from visible (550 nm) to near infrared (1550 nm). In particular, the optical response in the near-infrared region makes gold nanorods an excellent anti-counterfeiting material. Nano-infrared barcodes can be formed by using gold nanorods that respond in different infrared bands. This indistinguishable barcode can display different digital combinations and even pattern combinations on infrared display devices for high-end anti-counterfeiting needs.



Optical imaging
Bio-optical imaging refers to the method of using special materials to absorb excitation light and image cells or tissues or even organisms into acoustic signals to obtain biological information. GNRs can be used for deep tissue imaging through optical paths by particles that are absorbed and emitted in the near-infrared region. Due to the SPR effect, GNRs can perform light absorption well and perform high-efficiency conversion between light and heat. Especially after silicon encapsulation, the cell uptake is greatly increased, the photoacoustic signal is also enhanced, and the resolution is high. With sensitivity, it has become a popular substance for photoacoustic imaging.

Supplement: Applications on optical components:
Near-infrared filter: Due to its strong absorption in the near-infrared region, gold nanorods can be used to make filters.
Nonlinear optical components: surface plasmon resonance causes the electric field strength of the gold nanorods to be greatly enhanced (up to 10e7 times). This electric field enhancement effect reduces the illumination intensity threshold required to achieve nonlinear effects. Can be used to fabricate a variety of nonlinear optical components.
Polarizer: The gold nanorod has a plasmon resonance mode parallel to the long axis direction and two degenerate perpendicularsto the long axis direction, which are called axial surface plasmon resonance mode and radial surface plasmon resonance, respectively.
Drug targeted therapy


Functionalized GNRs can be immobilized on GNRs by chemical or physical methods, and the drugs can be accurately and efficiently delivered to effector cells, and the drug can be released by photothermal control for targeted therapy of tumors. GNRs are a light-controlled release method that provides flexible control of the amount of drug released and the time of drug release. The function of GNRs to effectively deliver drugs to target cells and release under light irradiation can fully exert the synergistic effect of photothermal therapy and chemotherapy of tumors. Therefore, the modified GNRs can promote the development of intelligent and controllable drug release nanosystems, bringing new hope for the treatment of tumors.